The subject of the invention is a fiber optic coupler applying microstructured multi-core optical fibers.

Power couplers are the one of the basic components used in telecommunication lines based on the application of optical fibers. Developing towards the increase of data transmission density, the telecommunication market must account for the requirements of data recipients in terms of focusing on increasing data transmission density, improving functionality and reducing new system installation costs. The purpose of fiber optic couplers is to transfer the power from one or several input optical fibers to one or several output optical fibers. Couplers can be executed in any configuration of inputs and outputs. The most popular coupler types include type X couplers (2 inputs and 2 outputs) and type Y couplers (1 input and 2 outputs / 2 inputs and 1 output). Such couplers occur in both the symmetrical and asymmetrical version. However, market demands have determined the need to manufacture couplers with a larger number of channels. Depending on the needs, a coupler can have any number of inputs and outputs, whereas the primary limitations are the coupler fabrication capacities. In extraordinary cases, when the coupler has less inputs than outputs (particularly when it has one input and N outputs), it becomes a power splitter. With N inputs and one output, we are dealing with an optical signal combiner.

To ensure full development of state of the art access networks, particularly those defined as FTTx networks (Fiber To The x, e.g. FTTH - Fiber To The Home), properly integrated power splitters and combiners are required.

FTTx networks are usually constructed in the PON (Passive Optical Network) technology. These are point-to-multipoint networks executed in the logical star topology. Physical topologies depend primarily on the distribution of subscribers: with single-family housing, the possible topology is the bus network, whereas in the case of multi-family housing, the most popular solution is the tree topology. In each case, the central point consists in optical couplers dividing the signal from the distribution device, the so-called OLT (Optical Line Termination) to network terminals with the recipients - the so called ONTs (Optical Network Termination).

Latest commercially available signal coupling elements applied in telecommunication are generally fabricated with the use of two technologies: optical fiber fusing (FBT - Fused Bionical Tapering) and the planar technology (PLC - Planar Lightwave Circuit). In the case of the bus network topology, the optical couplers applied are usually executed in the FBT technology. In the case of the tree network topology, which requires a large number of output ports, the predominant solution applied are PLC couplers.

FBT couplers are created by placing two optical fibers adjacent to one another, then fusing them together and tapering them to create a single waveguide. In this structure, properly approximated cores can no longer be treated as separate telecommunication channels. A signal entering one of the coupler's arms passes through to the tapered area, where by considerable decrease of optical fiber dimensions, the core can lose the capacity to transfer light and thus the light is conducted by the entire glass surface, and air takes the role of the cladding. When expanding the tapered optical fibers, the cores, whose diameters are also increased, restore their light conduction capacity. In such layout, Maxwell' equations are solved for the entire structure, and so-called supermodes propagating simultaneously in certain or in all cores occur. Depending on the configuration, light propagation in such structure can be used to construct a power combiners or splitters.

Coupler fabrication process through fusing, according to the FBT technology, has been included in patent ref. US 4550974, which describes the aforementioned couplers and their fabrication methods. The coupler presented is a symmetrical 2x2 coupler, however, asymmetrical couplers, i.e. couplers with uneven power division, are also possible to fabricate. Such coupler can operate in the 1x2 power splitter configuration, if only one of the inputs is used. It is characterized by high resistance to changing conditions in external networks, low insertion loss and insignificant reverse reflections. One of the disadvantages of such couplers is that the maximum number of ports is four, as an equal division of power is difficult to achieve with a larger number of ports.

Contrary to FBT, the PLC planar technology allows for fabricating couplers with a larger number of input ports (from 4 to 128), guaranteeing small dimensions of the product itself as well as high operational stability in full 1260 - 1650 nm spectral range. Such structures apply, among others, to integrated optics elements. Devices featuring such structure are characterized by relatively high losses resulting from the need to implement mode transformation, as well as high internal losses, which results in the total losses reaching several dB. In addition, the technology for assembling and jointing integrated optics and optical fibers requires advanced, expensive methods. The structure of PLC couplers and their fabrication methods are described, among others, in patent ref. US 5745619 and patent application ref. US 20030091289 Al.

The basic parameters of currently applied optical couplers pose a severe obstacle to the popularization of access networks, particularly those defined as FTTx networks assuming optical fiber use at all transmission stages. Therefore, the purpose of the invention consisting in a fiber optic coupler applying microstructured multi-core optical fibers, particularly applied as a power combiner and splitter, was to develop a device serving as an element guaranteeing optimal distribution of power to any number of channels. Whereas the fabrication technology constituting the essence of the patent assumes the fabrication of the coupler from optical fibers, which is more beneficial, compared to the planar technology, which is difficult to integrate with telecommunication systems based on optical fibers. Thanks to the fabrication technology applied, the device according to the invention guarantees the required division of power while maintaining low losses. Both criteria - obtaining a specific power division and low losses during such division, are key in terms of common application. The fully fiber optic coupler developed allows for lowering optical power losses in division to below 0.5 dB, and theoretical losses can be close to zero. In addition, it is possible to apply the devices described in a wide scope of temperatures. An essential advantage of the invention is the possibility of operating the device in any configuration, i.e. as both a power splitter and a power combiner, as well as an MxM coupler. In addition, the possibility of operating as a splitter/combiner/coupler creates a possibility of using the device as an optical switch.

With the invention of photonic-crystal fibers, also referred to as microstructured fibers, the possibilities of shaping modes in optical fibers have considerably expanded. Thanks to the possibility of differentiating geometry, which, in the case of microstructured fibers, entails the manipulation of arrangements and properties of structural holes, we can produce fiber properties which are impossible to achieve with the use of conventional optical fibers. These include e.g. single-mode operation in a very broad spectral range, high birefringence, increased sensitivity to pressure, elongation, and many others. The effect of air holes to the characteristics of the modes can be insignificant in terms of light propagation without the impact of external factors, but can significantly improve fiber performance in the occurrence of additional external factors. An example of such application can be e.g. bending-insensitive fibers, whose optical fiber contains a core surrounded by air holes. Such an optical fiber was presented in an article titled "Low Bending Loss Single-Mode Hole-Assisted Fiber", written by Toshiki Taru et. al., published in the SEI Technical Review 75 (2012). The advantages of this type of fiber are revealed when it is bent - in the case of an optical fiber whose core is not surrounded by air holes, there are considerable losses at a bent. In the case of hole-assisted insulation, there is a possibility of "mode outflow" and mode power radiation to the cladding is practically impossible thanks to the occurrence of a large structure refractive index pitch (the refractive index for the hole area is assumed as the refractive index for air, although the holes can be filled with various substances). Therefore, the presence of holes does not have a significant effect on such properties as dispersion or attenuation, as they are positioned relatively remotely from the core, but can affect the character of propagation with external factors.

Air holes in microstructured optical, without their significant participation in propagation (which significant participation is observed in the case of LMA-8 optical fibers), fibers are also used to construct multi-core optical fibers. Thanks to the air hole environment of the cores, power permeation between particular cores is practically eliminated - the so-called crosstalk phenomenon does not occur. Furthermore, the holes can insulate the cores, which means that power propagation in each core is practically independent. Cores can be also surrounded by holes in a manner that modes cannot be assigned to particular cores - such cores are referred to as "coupled" cores, and the supermodes are propagated in the structure. Whether we are dealing with insulated or coupled cores depends on the material and geometrical parameters of the structure. In the majority of cases, decreasing holes and bringing cores closer facilitates the propagation of supermodes and thus the transfer of power between the cores.

Widespread application of microstructured optical fibers also results from the possibility of using their properties by modifying their parameters, e.g.: hole collapsing, fiber tapering, hole filling. A change of propagation conditions after the optical fiber is fabricated is thus possible.

While the technology of hole collapsing is known, controlled application of this phenomenon in the achievement of optical fiber coupling between particular multi-core optical fiber cores has not been developed yet.

For instance, patent ref. US 66 31234 describes the possibility of processing optical fibers by heating and tapering to obtain couplers based on photonic crystal fibers. The coupler is constructed with the use of at least two photonic crystal fibers. Single-core photonic crystal fibers only are considered. The phenomenon of core collapsing is described as "weakening or destroying the differential refractive indexes between the cladding and the core". Furthermore, controlled differentiation of the size of air holes and change of the optical fiber diameter can be used to modify the birefringence of the optical fiber.

Hole collapsing in photonic crystal fibers is also used in splicing microstructured optical fibers. The hole collapsing phenomenon is perceived as a problem, as it generates losses on the splice. The phenomenon can be taken advantage of (e.g. by gradient hole collapsing) or eliminated. In the majority of cases, however, the splicing technology is considered for single-core photonic crystal fibers.

For instance, in patent application ref. US 20080037939, the inventors have patented the process of tapering single-core photonic crystal fibers applying gradient hole collapsing to reduce losses in the joint (splice).

In turn, patent application ref. 20060067632 presents a method of splicing single-core photonic crystal fibers characterized by small cores, in which the splice execution method focuses on the least hole collapsing as possible, to produce the lowest losses possible.

A method of splicing single-core photonic crystal fibers is also described in patent ref. US 7609928 B2, in which hole collapsing is explicitly indicated as the cause of losses.

The use of hole collapsing in the processing (splicing, tapering etc.) microstructured multi- core optical fibers can be used as a desired phenomenon when constructing various types of sensors.

For instance, patent application ref. 20090052852 Al presents a method of hole collapsing on single-core microstructured optical fiber tapers. This invention aims at the complete collapsing of holes, thanks to which modes (conducted in the core and the cladding) can interfere with one another. A specific Mach-Zehnder interferometer is created this way.

An analogous solution, in which splice areas are fused and the cladding modes and the core mode are conducted in two interferometer arms, is described in patent ref. EP 1939659 Bl.

What is more, modern telecommunication networks based on standard single-mode single-core optical fibers will soon be no longer sufficient due to their limited capacity. One of the strategies to solve this problem is Mode-Division Multiplexing, which uses optical few-mode fibers for transmission, where each mode is used as an independent transmission channel. To be able to build a transmission network based on few-mode fibers it is necessary to use special components for multiplexing and demultiplexing modes. Multiplexing modes consists of combining the signals from the N standard singlemode fibers and introducing them as N independent channels into multi-mode fiber (few-mode). For this purpose, firstly there is a signal conversion from a standard single-mode fiber to the specific mode, and then all channels are put to the few-mode fiber. During demultiplexing there is a reverse process, where N modes in the few-mode fiber which are several (N) independent channels are divided into N outputs. Accordingly, there is a need for a device allowing multiplexing or demultiplexing channels in an optical few-mode fiber. In addition, it is necessary for that this type of elements to be characterized by low loss and high mode selectivity.

One of the methods of selectively modes exciting uses phase-plates or SLM. In both cases the light beam (usually a fundamental mode) hits the phase structure - transparent element having a predetermined refractive index distribution, which in a result gives particular higher- order mode after a certain distance behind the plate. Similarly, one can use the SLM, which introduces inflicted appropriate phase delay. In the case of using the SLM it is possible to program the delay phase, which will result in any shape of higher order mode, so on using SLM is very versatile. Both of these methods use bulk optics at the light beam input and output of few-mode fiber. Unfortunately, this results in the fact that size of this type of equipment, often consisting of several elements, is large. At the same time the price resulting from the use of devices with high precision is high. Although this method is considered to be the simplest, it is fraught with big loss. The article by . Ryf, et al., "Mode-Division Multiplexing over 96 km of few-mode fiber using coherent 6x6 MIMO processing" J. Lightwave Technol. 30, 2012 presents multiplexing transmission using few-mode multiplexing for the transmission by six independent channels. Achieved transmission speed was equal to 640 Gb/s over a distance of 96 km and loss less than 1.2 dB. A system for signal transmission in few-mode optical fibers, wherein an element shaping the beam into the desired mode was SLM, was also shown. In fiber optics networks, the use of bulk optics is associated with the introduction loss what is inevitably associated with the transition from bulk optics to optical fibers.

Another method called "photonic lantern" is based on the tapering of several single-core fibers with required parameters, which are generally different (the size of the core, the refractive index of the core) until the core ceases to independently conduct light and what results in the formation of few-mode fiber with outer capillary tube made out of glass with lowered refractive index, which acts as a fiber cladding. By appropriate selection of parameters of single-core fibers on the input one can obtain particular mode at the output. At the same time, depending on to which of the input single-core fiber signal introduced, it is possible to stimulate another mode at the output. The advantage of this method is the introduction of very low loss and using only optical fiber technology (all-fiber). However, the challenge is to maintain low crosstalk between the modes. Currently, for transmission using this type of multiplexers, due to the high coupling between the modes, during demultiplexing it is necessary to apply electronic signal processing.

For selective modes excitation it is also possible to use integrated optics. There are, for example converters based on the long-period fiber grating. The article I. Giles et al., "Fiber LPG mode converters and mode selection for multimode SDM technique," IEEE Photonics Technology Letters 24, 2012 presents the idea of such a device based on the long-period fiber grating and the method for its examination. Another method of using integrated optics is the use of symmetric or non-symmetric couplers. Yet another method for the selective modes stimulation (excitation) are non-symmetric planar structures planar Y-shaped (Y-junction). In the article by J.D. Love and N. iesen, "Single-, few-, and multimode Y-junctions," J. Lightwave Technol. 30, 304-309, 2012 the results of simulations of non-symmetric structures of this type used to stimulate higher order modes have been presented.

There are examples, among others, in publication by Sung Hyok Chang et al., "The models and wavelength-division multiplexed transmission using all-fiber multiplexer mode based on the mode selective couplers," Opt. Express 23 (2015), using fiber couplers for multiplexing and demultiplexing the three modes - the solution relies on a cascade of conventional couplers. The solution does not contain a multi-core optical fibers or microstructured fibers, and what is more using this method of multiplexing does not scale easily to more modes.

In turn, patent EP 2336813 relates to a multi-core fiber for transmission using mode multiplexing in which there is no selective and above all, a precise addressing modes. Modes in the fiber are coupled in groups and the cores structure does not have insulation. In the description of the invention the possibility of multiplexing and demultiplexing is only mentioned, but it is not disclosed how such an operation is carried out. Indirectly, the content of the description and drawings can be concluded that this operation is carried out by planar phase- plate or by something similar. Thus, multiplexing and demultiplexing is not involved in the indicated embodiment by using solely fiber optics and it requires the use of additional components to meet the objectives of the invention.

Similarly, the solution EP2706387 applies to the optical fiber for spatial multiplexing. In this embodiment, as in the previously described, multiplexing or demultiplexing phenomenon can only occur in the outer element (outside optical fiber), and only converted signal is introduced into the fiber dedicated for transmission.

The elements which enable the signal multiplexing are usually devices based on bulk optics, for example, a device disclosed in US 6332050 which relates to the design of the bulk multiplexer. Those types of solutions are expensive and inefficient, which was one of the reasons for the start of work on the presented invention.

On the other hand, US 2013039627 relates to the use of fiber with coupled cores for transmission basing on mode multiplexing, but it does not disclose how addressing modes is done, what requiring as it might seem on the basis of a description of additional experimental work.

In turn, US 2015188659 discloses a method for multiplexing and demultiplexing using a ring resonator. This solution is characterized by a huge degree of complexity, and considering that his design is not purely fiber-optic, it does not allow for simple inclusion into fiber infrastructure. Known solutions are characterized by, amongst others, significant degree of complexity. Thus, the aim of the invention was to provide an element which will remove the disadvantages in the prior art consisting of a highly problematic control of what modes and to what extent are stimulated (interchangeable: excited, addressed), so with the use of which modes signals can be carried out effectively and independently. By using the invention it is possible to realize transmission using several modes in one optical fiber (mode multiplexing). With the use of the invention it is also possible to realize add-drop multiplexer/demultiplexer. In addition, the object of the invention was to develop a structure entirely in an optical fiber technology with which it will be possible to avoid jointing planar/bulk optics with optical fibers. According to the invention, it is possible to addressing modes: fundamental modes, higher order modes and polarization modes. Addressing polarization modes can be effectively utilized in the construction, among others, fiber optic polarizer, fiber optic polarization divider (splitter) and fiber optic coupler which maintain polarization. The construction of such elements is also demanded by the market.

The power coupler according to the invention allows effective optical power coupling thanks to the application of controlled hole collapsing in the multi-core optical fiber structure. Multi-core optical fiber with beneficially insulated cores is applied as the basic medium, whereas it is assumed that insulated cores are construed as cores with -lOdB or preferably lower crosstalk between one another, which is guaranteed by the occurrence of areas in the neighborhood of the core, which are characterized by lowered refractive index, compared to the refractive index of the coating. In a beneficial embodiment, the zones with lowered refractive index take the form of holes, preferably filled with air. Such holes provide i.e. crosstalk minimization, and their effect on signal propagation parameters (losses, dispersion) is beneficially insignificant. Furthermore, thanks to the holes, it is possible to enter light to the coupler, theoretically, without any losses generated.

In its basic configuration, the fiber optic coupler according to the invention contains a multi-core optical fiber connected, on the one side, with a single optical fiber, preferably a standard single-mode one, and with at least two single optical fibers, preferably standard single- mode ones, which can be placed in a capillary, but can be also etched and/or tapered to align their cores with the cores of the multicore optical fiber, and at least one fragment of the multicore optical fiber is tapered at a section of 300 μιη and more and/or the holes in this section are collapsed. Whereas, the power distribution is performed in a manner that the signal from the single optical fiber passes to one of the cores of the microstructured multicore optical fiber, preferably the central one, and due to the insulation of the cores, signal is conducted through this core until the region where the holes of the microstructured multicore optical fiber are collapsed and/or tapered. In the region where the holes of the microstructured multicore optical fiber are collapsed and/or tapered, core insulation is reduced in a controlled manner by increasing crosstalk between the cores, as performed by suitably selecting the parameters of the hole tapering and/or collapsing process, which can be carried out e.g. preferably on a fusion splicer. The length of multicore optical fiber sections outside the taper waist region and transition regions and/or hole collapsing regions has less effect on coupler performance and effective division of power.

Cores are not insulated and supermodes occur in the region where the holes of the microstructured multicore optical fiber are collapsed and/or tapered. Therefore, thanks to reducing the insulation of cores as a result of hole collapsing and /or thanks to bringing the cores close to one another, which is the case of a tapered section, and since the cores brought together are coupled (crosstalk between particular cores increases), the optical fiber passes from insulated core operation to coupled core operation.

In result, the power conducted in one core is divided, preferably, to all cores. In a beneficial embodiment, the taper length is 300 μιη and longer and/or the holes in this taper section are collapsed, and the taper length and taper ratio and/or level of hole collapsing determines the degree of power division; whereas a taper length and taper ratio and/or hole collapsing level can be found, for which the division of power to all cores will be beneficially equal.

The taper ratio is construed as percentage decrease of the cross-section of the fiber in taper waist region, whereas in a beneficial embodiment, this cross-section reduced in a uniform manner. The taper length is selected preferably by experiment aimed at the desired division of power.

Depending on the design of the microstructured multicore optical fiber, any MxN division can be achieved. Complete collapsing of the cores is not necessary for achieving various types of effects. In addition, diversified division of power to particular cores can be executed by introducing external interaction from: temperature, stresses (stretching, compressing, twisting, bending and other), pressure and other. The design of the multicore fiber has direct effect on the division of power as a result of tapering and/or hole collapsing.

In another embodiment, the fiber optic coupler according to the invention contains at least one input optical fiber, preferably single-mode, and an N number of output optical fibers and at least N-core multicore optical fiber with insulated cores. The signal propagated in a standard single-mode optical fiber/optical fibers is passed to the core/cores of the multicore microstructured optical fiber.

The input optical fiber/fibers is spliced to the multicore optical fiber. After passing through to the multicore optical fiber, the signal is propagated in the tapered section and/or without hole collapsing, further on in this core/cores, to which it has been entered until the tapered region (a transition taper region and then the taper waist region) and/or the hole collapsing region.

In the transition taper region, the cross-section of the optical fiber is reduced: the coating, core and hole diameters, in a manner that the tapering operation is conducted until the designed taper waist diameter is obtained. Hole diameter reduction and approximation of particular cores and core diameter reduction induce a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring power from any core/cores to the remaining cores thanks to the creation of supermodes. In the taper waist region, the holes are completely collapsed or their diameters remain constant.

Depending on the desired division of power, there is a possibility of adjusting the division by selecting the diameters of the reduced holes and selecting the length of the tapered sections (both transition taper regions and taper waist region) and the taper waist ratio. For this optical fiber design, there is a combination of parameters which allows for equal division of power. Fixation of the division of power obtained in the taper section results from the "freezing" of the mode structure in the passage through the transition section to the section with insulated cores. Low losses achieved in the invention are the result of constant stimulation of supermodes, which is performed by the characteristic tapering of the multicore fiber and/or hole collapsing. Hole collapsing on the splice produces a device acting as a splitter, but the losses are usually higher.

The device according to the invention can be also used in the following manner. When, for example, two wavelengths are propagated in the input optical fiber, a different path exists for each of them, following each a given wavelength will be located completely in one of two cores of a multicore optical fiber, particularly the dual-core one. Therefore, tapering and/or hole collapsing parameters can be selected to enable the separation of two wavelengths propagated in the input fiber into separate cores of the multicore fiber, and successively into output optical fibers. The same principle of separating wavelengths into particular cores can be applied in a larger number of cores than the two cores set forth in this example, and in a larger number of wavelengths.

In particular applications, in a beneficial embodiment of the invention, the signal is collected from one output only. Such situation occurs when e.g. certain wavelengths must be filtered out instead of wavelengths being merely separated. Here, the principle of operation of the device does not change, but the purpose of the element does. When filtering out wavelengths, one or several output optical fibers are used, leaving the remaining fibers unused, or one fiber can be spliced to the output. Splicing one fiber to the output is beneficial from the technological point of view. Effective wavelength filtering from the spectrum can be also performed by applying serially connected taper adapters with varying taper parameters.

It is also possible to adapt the taper parameters to achieve controlled power % in the output compared to input power - in this case, the device operates in the attenuator function. Such element can be applied in the optical cavities for adjusting the Q. factor of the optical cavity (operation as a Q-switch).

The above specified coupler operation principle can be beneficially reversed in the following manner. A beam of optical signals can be entered to optical fibers, which are spliced to a multicore fiber with insulated cores. In the non-tapered section, propagation does not change the character in relation to the propagation conducted in the input fibers. In the tapered and/or hole collapsed region, propagation passes from insulated core operation to coupled core operation. Supermodes are created, and in result the signals, which have been running independently, are now merged. Al least one single-core optical fiber is spliced to the multi-core optical fiber. This way, the merged signal from input cores, with adequate power loss, is now propagated in the output optical fiber/fibers. It is therefore a "mirror" configuration in relation to the splitter configuration, hereinafter referred as the combiner, which does not require any structural and/or technological modifications of the system, but introducing a modification in terms of input-output arrangement. Application of this type of configuration can be the following. When a different wavelength is propagated in each of the fibers comprising input optical fibers, mixed signals are obtained in the output in the output optical fiber/fibers, and signal with several wavelengths is propagated in one optical fiber.

In another embodiment, the coupler according to the invention enables the construction of element for the controlled addressing modes and it includes a multi-core optical fiber with cores which are insulated by zones (spaces) with lowered refractive index. Preferably, at least one of the cores of multi-core optical fiber is few-mode or birefringent (it has separated polarization modes), which means that its modes can be independently addressed (interchangeable: stimulated, excited, multiplexed). It is preferably, when each core of addressing cores and addressed core is surrounded by an insulating structure which insulates the one core from the other cores, preferably when the insulating structure has the form of zones with lowered refractive index, particularly made out of holes, preferably filled with air or other gas, solid or liquid. Particularly, the holes can be filled with the material of fiber cladding, then the whole cladding acts as a zone with lowered refractive index which insulates the cores. Insulation serves to maintain the low-efficiency of supermodes creation (building) on the cores - on the area of the non-decreased insulation, supermodes build in the manner that the maximum observed crosstalk between any pair of cores is less than -lOdB.

Addressed core, otherwise multiplexed core (one of which modes are addressed /multiplexed) is a few-mode and/or birefringent and it has separated effective refractive indices of the particular modes. The birefringence of the core is achieved by any known method, e.g. ellipticity of the cores or state of stress around the cores. The term of 'multi-mode' and 'few- mode' , is understood as any case, wherein the core fiber has at least two modes, including a separated polarization modes at the used wavelengths. In the literature there is no well-defined difference between the multi-mode fiber and few-mode fiber hence in the later part of the patent, these terms are used interchangeably.

In the neighbourhood of the addressed core there is at least one, preferably a single- mode addressing core, otherwise multiplexing core (one, whose mode is used to excite/address a particular mode in the addressed/multiplexed core) which has the effective refractive index selected so that it is matched to the effective the refractive index of one of the modes in the addressed core. By "stimulation/excitement of specific mode" it is meant its addressing, and hence the ability to effectively building the supermode (and in effect observing the crosstalk) on the cores, which is conditioned by adjusting the effective refractive indices of the modes, when separate considerations of the cores. After reducing the insulation, the cores are coupled, so one do not talk already about individual modes but about supermode created on both cores.

Selection of the effective refractive indices of the modes in such a way that the individual cores have separated effective refractive indices and others have adjusted effective refractive indices plays two roles. Firstly, it reduces crosstalk between the addressing cores (inefficient building of supermodes by separating the effective refractive indices of each pair of modes), and, secondly, by phase matching, it enables selective crosstalk between the addressing core and the addressed core (effective building of supermodes). The ability of various shaping of supermodes in the areas with decreased (reduced) insulation is possible due to the selection of the effective refractive indices of the modes in the structure with the insulated cores - the closer the refractive indices are in particular modes, the more efficiently supermode will build on them and thus power present in the addressed core in the form of addressed mode (fundamental mode, higher- order mode, polarization mode) to the power present in the addressing core in the form of addressing mode will be bigger.

The at least dual-core multi-core fiber with insulated cores is connected at least one, at least single-core input fiber and on the opposite side of the multi-core fiber at least one, at least single-core output fiber is attached to, and input fibers and output fibers attached to the optical multi-core fiber can be placed in the capillary, and may be etched and/or tapered so that their cores are preferably aligned to the cores of multi-core fiber (fan-in-fan-out element type). In at least one fragment of multi-core fiber the insulation of the cores is reduced (decreased) by reducing (decreasing) the size of the zones with lowered refractive index in the neighbourhood of the core and/or by collapsing their structures. In the area of reduced cores insulation, supermodes build on particular addressing core/cores and addressed core/cores - their shape determines the crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than - 3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than -14dB. High efficiency of supermodes building (defined as the final crosstalk) is achieved by adjusting the effective refractive indices of each pair of modes before reducing insulation (after reducing the isolation one talk already about supermodes).

The structure of multi-core fiber is preferably tapered and/or the holes in the structure are collapsed, preferably at least in one place, at least on the section which allows the observation of the appearance of power in the form of a addressed mode in the addressed core. Preferably, the minimum length of the taper and/or collapsing of holes is equal to 300 μιη. The length of the taper and/or collapsing of the holes have preferably a maximum length equal to the length of the used section of multi-core fiber. Preferably, the taper ratio is between 0-95%. The taper ratio is construed as percentage decrease of the cross-section of the fiber in taper waist region, whereas in a beneficial embodiment, this cross-section reduced in a uniform manner. Preferably, the optical fibers have coatings.

In a preferred embodiment, input single-core optical fibers, single mode at the utilized wavelengths, preferably seven of them, are spliced to particular cores of multi-core fiber, preferably seven-core fiber. Profiles of the refractive indices and diameters of addressing cores of multi-core fiber are selected so that all addressing modes of multi-core fiber have different effective refractive indices. The addressed core is few-mode and its refractive indices of mode are chosen so that are suited to each effective refractive indices of modes in the addressing cores. In the area of the non-decreased insulation and supermodes build in the manner that the maximum observed crosstalk between any pair of cores is less than -lOdB.

In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, supermody build resulting in increased crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non- addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than -14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi- core fiber.

To the output end of the multi-core fiber, the few-mode fiber is preferably spliced, particularly single-core, with the same or similar properties as the addressed core of multi-core fiber. Thus, in the few-mode fiber, transmission is realized using mode multiplexing. The above- described multi-core fiber with controlled reduction of cores insulation is therefore an element (coupler), enabling addressing modes in the few-mode fiber. In this configuration, in particular seven-core fiber, six fundamental modes of the addressing cores address six higher order modes in the addressed core. Fundamental mode in the addressed core is excited by initially splicing the fiber to this core.

Similarly, the signal in the few-mode fiber can be demultiplexed using coupler according to the invention - information transmitted in a few-mode fiber, encoded in the individual modes, can be separated into individual cores, and further into separate optical fibers.

In another preferred embodiment, input single-core optical fibers, single mode at the utilized wavelengths, preferably three of them, are spliced to particular cores of multi-core fibre, preferably four-core fiber. Profiles of the refractive indices and diameters of addressing cores of multi-core fiber are selected so that all addressing modes of multi-core fiber have different effective refractive indices. The addressed core is few-mode and its refractive indices of modes are chosen so that are suited to each effective refractive indices of modes in the addressing cores. In the area of the non- decreased insulation, supermodes build in the manner that the maximum observed crosstalk between any pair of cores is less than -lOdB.

In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, and supermodes build resulting in increased crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi- core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than - 14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

To the output end of the multi-core fiber, the few-mode fiber is preferably spliced, particularly single-core, with the same or similar properties as the addressed core of multi-core fiber. Thus, in the few-mode fiber, transmission is realized using mode multiplexing. The above- described multi-core fiber with controlled reduction of cores insulation is therefore an element (coupler), enabling addressing modes in the few-mode fiber. In this configuration, three fundamental modes of the addressing cores address one fundamental mode and two higher order modes in the addressed core.

Similarly, the signal in the few-mode fiber can be demultiplexed using coupler according to the invention - information transmitted in a few-mode fiber, encoded in the individual modes, can be separated into individual cores, and further into separate optical fibers.

In another preferred embodiment single-core optical fiber is spliced to the addressing core of multi-core optical fiber, preferably dual-core, in which the signal propagates in addressing core. Addressed core of multi-core fiber has a high birefringence - present polarization modes have separated effective refractive indices. Addressing core of multi-core fiber has low birefringence - present polarizing modes have equal effective refractive indices of the modes, what allow to call it colloquially single-mode core. One of the polarization modes in addressed core (polarization mode x) has an effective refractive index equal to an effective refractive index of the mode in addressing core of multi-core fiber.

In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, and supermodes build resulting in increased crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi- core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than - 14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

One polarization mode is propagating in the addressed core with high birefringence. After the section with collapsed holes signal is propagating in the core with high birefringence, in which only one polarization mode is effectively excited. To the core with high birefringence, a preferably birefringent, polarization maintaining fiber is spliced. This way of addressing polarization modes enables efficient addressing one polarization mode, so constructing a coupler with a functionality of fiber optic polarizer.

In another preferred embodiment single-core optical fiber is spliced to the addressing core of multi-core optical fiber, preferably three-core. Addressing core of multi-core fiber has low birefringence - present polarizing modes have equal effective refractive indices of the modes, what allow to call it colloquially single-mode core. Addressed cores of multi-core fiber in the neighbourhood of addressing core have high birefringence - present polarization modes have separated effective refractive indices. One of the polarization modes in first addressed core (polarization mode x) has an effective refractive index equal to an effective refractive index of the mode in addressing core of multi-core fiber and the second addressed core (polarization mode y) has an effective refractive index equal to an effective refractive index of the mode in addressing core of multi-core fiber. In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, and supermodes build resulting in increased crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi- core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than - 14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

After the section with collapsed holes signal is propagating in the cores with high birefringence, in which only one polarization modes are effectively excited - polarization mode (x) in one of the addressing core and polarization mode (y) in the second of the addressing core. To the cores with high birefringence, a preferably birefringent, polarization maintaining fibers are spliced. This way of addressing polarization modes enables efficient addressing particular polarization modes, so constructing a coupler with a functionality of fiber optic polarization divider.

In another preferred embodiment birefringent single-core optical fiber is spliced to the addressing core of multi-core optical fiber, preferably dual-core. Addressing cores of multi-core fiber has high birefringence - present polarization modes have separated effective refractive indices. Addressed core of multi-core fiber in the neighbourhood of addressing core has also high birefringence - present polarization modes have separated effective refractive indices. Preferably, addressing core and addressed core are homogenous.

Addressed core has polarization modes (x) and (y), which modes have effective refractive indices matched to the refractive indices of polarization modes (x) and (y) in addressing core.

In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, and supermodes build resulting in increased crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output of multi- core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is preferably bigger than -5dB and preferably lower than -3dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi- core fiber.

After the section with collapsed holes signal is propagating in the cores with high birefringence, in which polarization modes are excited - polarization mod (x) and (y). To the cores with high birefringence, a preferably birefringent, polarization maintaining fibers are spliced. This way of addressing polarization modes enables efficient addressing particular polarization modes, so constructing a coupler (splitter) which maintain polarization.

In another preferred embodiment which enables realization of add-drop multiplexing consists in adding/releasing one of a channel to/from the signals which are propagating in one core, the multi-core optical fiber is used, preferably dual-core fiber with cores insulated by zones with lowered refractive index. Multi-core fiber has preferably at least one single-mode core, and preferably at least one few-mode core. The effective refractive index of the mode in a single- mode core is matched to the effective refractive index of one of the modes in the few-mode core. To both sides of the multi-core fiber, preferably dual-core fiber, two optical single-core fibers are attached, preferably with cores matched to the core of dual-core fiber. In the area of the non- decreased insulation, supermodes build in the manner that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build inefficiently).

In certain section, preferably at least 300 μιη, zones with lowered refractive index, which are separating the cores, are collapsed or their dimensions are reduced (eg. by tapering), which results in reduced cores insulation, and supermodes build resulting in increased crosstalk. The ratio of power present in the addressed core in a form of addressed mode at the output of multi- core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -5dB, preferably bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -lOdB, preferably less than - 14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber. Because a mode in single-mode core and one of the modes in a few-mode core have matched refractive indices before reducing isolation, supermodes build in an area of reduced insulation. Thus, mode from single-mode core addresses a mode in a few-mode core and mode from few-mode core addresses a mode in a single-mode core. Thus, it is possible to realize a type of multiplexer/demultiplexer called an add-drop multiplexer/demultiplexer. Then, both the single- mode core and few-mode core are both addressing and addressed ones.

In a beneficial embodiment of the invention, the optical fiber is wound or mounted on an element deforming the optical fiber or altering its temperature, particularly on a piezoelectric or a mechanic device deforming the optical fiber, which allows for changing the taper length, the tension in the tapering, causing a switch of the signal between particular cores, while the device operated in the same optical switch function.

The subject of the invention was presented in detail in examples and figures, which do not exhaust the possible configurations of the invention, resulting from the operation principle set forth herein.

Fig. 1 presents a beneficial embodiment of the invention: a view of a joint of a standard single-mode optical fiber (1) with visible cross-section (1A-1A) with a microstructured multicore optical fiber (2), in which a tapered region is visible, and the optical fiber (2) is then spliced to a bundle of output optical fibers placed in a capillary (3) with visible cross-section (3A-3A) - figure without proportions maintained.

Fig. 2 presents a close-up to the taper of microstructured multicore optical fiber, also without its proportions maintained, in which cross-section A-A refers to the non-tapered microstructured multicore optical fiber, and cross-section B-B refers to the transition taper region of microstructured multicore optical fiber with partially collapsed holes, and cross-section C-C illustrates the taper waist region of microstructured multicore optical fiber with completely collapsed holes.

Fig. 3 presents a tapering of a multicore optical fiber, where section (a) is the non-tapered fiber area of (dl) in total diameter, and sections (b) are the transition taper regions of decreasing/increasing taper diameter, and section c is the taper waist region characterized by taper waist diameter (d2).

Fig. 4 presents the cross-sections of a standard single-mode optical fiber (1) with a core

(4) of diameter (d3) and a cladding (5) of diameter (d4).

Fig. 5 presents a model microstructured seven-core optical fiber without a taper with cores (4) of diameters (d5), a cladding (5) with diameter (d6) and air holes (6) with diameters (d7), which can be used to construct the invention.

Fig. 6 presents a microstructured multicore optical fiber with partially collapsed holes (6) with diameters (d8), with reduced cores (4) of diameters (d9) and reduced cladding (5) with diameter (dlO).

Fig. 7 presents a microstructured multicore optical fiber with completely collapsed holes, characterized by reduced cores (4) with diameters (dll) and reduced cladding (5) with diameter (dl2).

Fig. 8 presents a bundle of standard single-mode optical fibers placed in a capillary (7), in which the cores (4) of single-mode optical fibers with diameters (dl3), the claddings (5) of the single-mode optical fibers with diameters (dl4), the internal diameter of the capillary (dl4) and the external diameter of the capillary (dl5) are visible.

Fig. 9 presents a beneficial embodiment of the invention operating as an NxN optical fiber coupler.

Fig. 10 presents a beneficial embodiment of the invention with two inputs (outputs) and seven inputs (outputs), on which in cross-section 4A-4A the glass rods (8) serving as the geometric filling of the capillary (7) are marked.

Fig. 11 presents a model microstructured dual-core optical fiber without a taper, with cores (4) of diameters (d5), a cladding (5) of diameter (d6) and air holes (6) with diameters (d7), which can be used to construct the invention.

Fig. 12 presents a beneficial embodiment of the invention with two inputs (outputs) and one output (input), where (1) is a single-mode optical fiber, (2) is a multi-core optical fiber.

Fig. 13 presents a beneficial embodiment of the invention from example 6 - a view of a joint of a bundle of standard single-mode optical fibers (1) placed in capillary (3) with visible cross- section (3A-3A) with microstructured multi-core fiber (2), with visible tapered section, which fiber (2) is then jointed to a few-mode fiber (9) with visible cross-section (9A-9A) - figure without proportions maintained.

Fig. 14 presents the cross-sections of a few-mode optical fiber (9) from example 6, with a core (10) of diameter (dl7) and a cladding (5) of diameter (dl6).

Fig. 15 presents an exemplary multi-core fiber from example 6 without tapering and without holes collapsing, with single-mode cores (4.1-4.6) of diameters (d5.1-d5.6), few-mode core (10) of diameter (dl7), lattice constant (Λ), cladding (5) of diameter (d6) and air-holes (6) with diameters (d7), which can be used for the invention construction.

Fig. 16 presents a close-up to the taper of microstructured multicore optical fiber from example 6, also without its proportions maintained, in which cross-section A-A refers to the non- tapered microstructured multicore optical fiber, and cross-section B-B refers to the transition taper region of microstructured multicore optical fiber with partially collapsed holes, and cross- section C-C illustrates the taper waist region of microstructured multicore optical fiber with completely collapsed holes.

Fig. 17 presents a transmission system based on spatial multiplexing which consists of a coupler according to the invention from example 6 to construction of multiplexer and demultiplexer correspondingly at the beginning and in the end of few-mode fiber (9) - figure without proportions maintained.

Fig. 18 presents a beneficial embodiment of the invention from example 7 - a view of a joint of a bundle of standard single-mode optical fibers (1) placed in capillary (3) with microstructured four-core fiber (2), with visible tapered section, which fiber (2) is then jointed to a few-mode fiber (9) with visible cross-section (9A-9A) - figure without proportions maintained.

Fig. 19 presents an exemplary multi-core fiber from example 7 without tapering and without holes collapsing, with single-mode cores (4.1-4.3) of diameters (d5.1-d5.3), few-mode core (10) of diameter (dl7), lattice constant (Λ), cladding (5) of diameter (d6) and air-holes (6) with diameters (d7), which can be used for the invention construction.

Fig. 20 presents a beneficial embodiment of the invention from example 8 - a view of a joint of a single-core optical fiber (1) with microstructured multi-core fiber (2), with visible tapered section, which fiber (2) is then jointed to polarization maintaining birefringent fiber (11) - figure without proportions maintained.

Fig. 21 presents an exemplary dual-core microstructured fiber from example 8 without tapering and without holes collapsing, with single-mode core (4) of diameters (d5), birefringent core (12) of short axis (dl8) and longer axis (dl9), lattice constant (Λ), cladding (5) of diameter (d6) and air-hole (6) with diameter (d7), which can be used for the invention construction.

Fig. 22 presents a beneficial embodiment of the invention from example 9 - a view of a joint of a single-core optical fiber (1) with microstructured multi-core fiber (2), with visible tapered section, which fiber (2) is then jointed to the polarization maintaining birefringent fibers (11) - figure without proportions maintained.

Fig. 23 presents an exemplary dual-core microstructured fiber from example 9 without tapering and without holes collapsing, with single-mode core (4) of diameters (d5), birefringent cores (12.1 and 12.2) of short axes (dl8.1 and dl8.2) and longer axes (dl9.1 and 19.2), lattice constant (Λ), cladding (5) of diameter (d6) and air-holes (6) with diameters (d7), which can be used for the invention construction.

Fig. 24 presents a beneficial embodiment of the invention from example 10 - a view of a joint of a single-core birefringent fiber (13) with microstructured multi-core fiber (2), with visible tapered section, which fiber (2) is then jointed to the polarization maintaining birefringent fibers (11) - figure without proportions maintained.

Fig. 25 presents an exemplary dual-core microstructured fiber from example 10 without tapering and without holes collapsing, with birefringent cores (12.1 and 12.2) of short axes (dl8.1 and dl8.2) and longer axes (dl9.1 and 19.2), lattice constant (Λ), cladding (5) of diameter (d6) and air-hole (6) with diameter (d7), which can be used for the invention construction.

Fig. 26 presents a beneficial embodiment of the invention from example 11 - a view of a joint of a single-core fibers (1) and (9) with microstructured multi-core fiber (2), with visible tapered section, which fiber (2) is then jointed to the output single-core fibers (1) and (9) - figure without proportions maintained.

Fig. 27 presents an exemplary dual-core microstructured fiber from example 11 without tapering and without holes collapsing, with single-mode core (4) of diameter (d5) and few-mode core (10) of diameter (dl7), lattice constant (Λ), cladding (5) of diameter (d6) and air-hole (6) with diameter (d7), which can be used for the invention construction.

Example 1

A coupler according to the invention contains an input optical fiber (1), which is then spliced to an optical fiber (2) in the form of a microstructured seven-core optical fiber, which is spliced to a bundle of seven output optical fibers (3).

The signal is propagated in the standard single-mode optical fiber (1), SMF-28e+ by Corning, and then passed to the central core of the microstructured multicore optical fiber (2).

The single-core optical fiber (1) is spliced to the multi-core optical fiber (2) preferably by fusion splicing, preferably using a fusion splicing apparatus (Vytran GPX-3400 or Fujikura FSM70).

After passing to the multi-core optical fiber (2), signal is propagated in section (a) still in the central core, until it reaches a taper (b, c). In the taper (b) section of preferably 5 mm and more, the cross-section of the optical fiber is reduced: the diameters of the cladding (5), the cores (4) and holes (6).

Reduction of hole cross-section (6) causes a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring power from any core/cores to the remaining cores thanks to the creation of supermodes. Therefore, propagation passes from insulated core operation to coupled core operation. Equal division of power into all cores is performed primarily in section (c), in which the holes are completely collapsed, as well as in section (b), in which hole diameters decrease/increase. The length of section (c) is 5 mm. After section (c) comes a transition zone of tapering (b). Power in each core continues to propagate independently, thanks to the occurrence of holes (6). In section (a), after passing through the taper, there are seven cores (4), in which a proportional amount of power is propagated. Thanks to the occurrence of air holes (6), signal from one core does not affect the propagation of signal from other cores, and we are therefore dealing with insulated-core propagation yet again. The multicore optical fiber whose seven cores propagate the signal is spliced to a bundle of single single-mode optical fibers (3). The splice is created in the process of fusion splicing, and the fabrication of such device and its splice with the multicore optical fiber has been described in detailed in applicable literature. After passing through the bundle of single-mode optical fibers (3), we obtain seven signals in independent single-mode optical fibers, which can be redirected to particular recipients while originating in one single-mode optical fiber (1).

Tapered sections (b, c) obtain values b = 5 mm and c = 5 mm, taper ratio of multicore fiber is equal to 20% (up to 100 μιη in waist diameter in section (c) ) with the parameters of multi-core fiber: core diameter d5= 6.5 μιη, cladding diameter d6= 125 μιη, hole diameter d7= 5.8 μιη, lattice constant Λ= 8.2 μιη.

Example 2

In a beneficial embodiment, a coupler according to the invention is used to divide power from two input optical fibers (1) to a bundle of seven optical fibers (3), and the example applies a microstructured multicore optical fiber (2). The single-mode optical fibers (1) are SM F-28e+ fibers by Corning.

Two single-core optical fibers (1) are spliced to the multi-core optical fiber (2) by fusion splicing, using a fusion splicing apparatus (Vytran GPX-3400 or Fujikura FSM70). Whereas each of the single-core optical fiber in a bundle is spliced to a different core of the multicore optical fiber. After passing to the multi-core optical fiber (2), signal is propagated in section (a) still in the two cores, until it reaches a taper (b, c). In the tapered (b) section of preferably 5 mm and more, the cross-section of the optical fiber is reduced: the diameters of the cladding (5), the cores (4) and holes (6).

Reduction of hole cross-section (6) causes a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring power from any initially actuated two cores to the remaining cores thanks to the creation of supermodes. Therefore, propagation passes from insulated core operation to coupled core operation. Equal division of power into all cores is performed primarily in section (c), in which the holes are completely collapsed, as well as in section (b), in which hole diameters decrease/increase.

The length of section (c) is 7 mm. After section (c) comes a transition zone of tapering (b). Power in each core continues to propagate independently, thanks to the occurrence of holes (6). In section (a), after passing through the taper, there are seven cores (4), in which a proportional amount of power is propagated. Thanks to the occurrence of air holes (6), signal from one core does not affect the propagation of signal from other cores, and we are therefore dealing with insulated-core propagation yet again. The multicore optical fiber whose seven cores propagate the signal is spliced to a bundle of single single-mode optical fibers (3). The splice is created in the process of fusion splicing, and the fabrication of such device and its splice with the multicore optical fiber has been described in detailed in applicable literature. After passing through the bundle of single-mode optical fibers (3), we obtain seven signals in independent single-mode optical fibers, which can be redirected to particular recipients while originating in two single- mode optical fibers (1).

Tapered sections (b, c) obtain values b = 5 mm and c = 5 mm, taper ratio of multicore fiber is equal to 20% (up to 100 μιη in waist diameter in section (c) ) with the parameters of multi-core fiber: core diameter d5= 6.5 μιη, cladding diameter d6= 125 μιη, hole diameter d7= 5.8 μιη, lattice constant Λ= 8.2 μιη.

Example 3

In a beneficial embodiment of the invention, presented in Fig. 9, in which the invention is used to couple the power from each of seven input optical fibers (1) to each of the seven output optical fibers (3) in the bundle, a microstructured multicore optical fiber (2) is applied. Signal is propagated in standard single-mode optical fibers (1), SM F-28e+ by Corning, and then passed to the cores of the microstructured multicore optical fiber (2).

The single-core optical fibers (1) are spliced to the multi-core optical fiber (2) by fusion splicing, using a fusion splicing apparatus (Vytran GPX-3400 or Fujikura FSM70). After passing to the multi-core optical fiber (2), signals are independently propagated in section (a) still in seven cores of the multicore fiber, until they reach a tapering (b, c). In the tapering section (b) of preferably 5 mm and more, the cross-section of the optical fiber is reduced : the diameters of the cladding (5), the cores (4) and holes (6). Reduction of hole cross-section (6) causes a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring signal among the cores thanks to the creation of supermodes. Therefore, propagation passes from insulated core operation to coupled core operation. M ixing of the signals among the cores is performed primarily in section (c), in which the holes are completely collapsed, as well as in section (b), in which hole diameters decrease/increase. The length of section (c) is 7 mm. After section (c) comes another transition zone of tapering (b). Power in each core continues to propagate independently, thanks to the occurrence of holes (6). In section (a), after passing through the tapering, there are seven cores (4), in which a proportional amount of power is propagated. Thanks to the occurrence of air holes (6), signal from one core does not affect the propagation of signal from other cores, and we are therefore dealing with insulated- core propagation yet again. The multicore optical fiber whose seven cores propagate the signal is spliced to a bundle of single single-mode optical fibers (3). The splice is created in the process of fusion splicing, and the fabrication of such device and its splice with the multicore optical fiber has been described in detailed in applicable literature. After passing through the bundle of single- mode optical fibers (3), we obtain seven signals in independent single-mode optical fibers, which can be redirected to particular recipients while containing information from all of the seven input optical fibers.

Tapered sections (b, c) obtain values b = 5 mm and c = 7 mm, taper ratio of multicore fiber is equal to 20% (up to 100 μιη in waist diameter in section (c) ) with the parameters of multi-core fiber: core diameter d5= 6.5 μιη, cladding diameter d6= 125 μιη, hole diameter d7= 5.8 μιη, lattice constant Λ= 8.2 μιη.

Example 4

In another embodiment of the invention, presented in Fig. 12, in which the invention is used to switch power between two output optical fibers, a microstructured multicore optical fiber (2) is applied. In this embodiment, this can be the dual-core optical fiber presented in Fig. 11. Signal propagated in the standard single-mode optical fiber (1), SM F-28e+ by Corning, is then passed to one of the cores of the microstructured multicore optical fiber (2).

The single-core optical fiber (1) is spliced to the multi-core optical fiber (2) by fusion splicing, using a fusion splicing apparatus (Vytran GPX-3400 or Fujikura FSM70). After passing to the multi-core optical fiber (2), signal is propagated in section (a) still in the core, to which the multi-core optical fiber is spliced until it reaches a taper (b, c). In the tapered section (b) of preferably 6 mm and more, the cross-section of the optical fiber is reduced : the diameters of the cladding (5), the cores (4) and holes (6). Reduction of hole cross-section (6) causes a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring power from the initial core carrying the signal to the second core, thanks to the creation of supermodes. Therefore, propagation passes from insulated core operation to coupled core operation. Depending on the length of the taper, the signal can be separated between the cores in any ratio. For a tapering (taper waist region) of 10 mm, signal will be propagated only in the core, to which the single-core fiber was spliced. For a tapering (taper waist region) extended by 8m£, the signal will be completely passed to the neighboring core. Intermediate values correspond to a situation, where power is propagated in both cores in various relations. With a piezoelectric, on the multicore optical fiber (2) is wound or mounted, we can change the length of the taper, causing a switch of the signal between particular cores, while the device operates in the same optical switch function.

Tapered sections (b, c) obtain values b = 5 mm and c = 10 mm, taper ratio of multicore fiber is equal to 30% with the parameters of multi-core fiber: core diameter d5= 6.6 μιη, cladding diameter d6= 125 μιη, hole diameter d7= 6.6 μιη, lattice constant Λ= 7.6 μιη.

Example 5

In a beneficial embodiment of the invention, presented in Fig. 12, in which the invention is used to separate two input wavelengths into particular output optical fibers, a microstructured multicore optical fiber (2) is applied. In this embodiment, this can be the dual-core optical fiber presented in Fig. 11. Signal propagated in the standard single-mode optical fiber (1), SM F-28e+ by Corning, is passed to one of the cores of the microstructured multicore optical fiber (2). Two wavelengths are propagated in the input optical fiber, in this embodiment, of 1550nm and 1310nm. The single-core optical fiber (1) is spliced to the multi-core optical fiber (2) by fusion splicing, using a fusion splicing apparatus (Vytran GPX-3400 or Fujikura FSM70). After passing to the multi-core optical fiber (2), the signal is propagated in section (a) still in the core, to which the multicore fiber was spliced, until it reaches a taper (b, c). In the tapering section (b) of preferably 3 mm and more, the cross-section of the optical fiber is reduced: the diameters of the cladding (5), the cores (4) and holes (6). Reduction of hole cross-section (6) causes a change in the character of propagation, the so-called core insulation reduction, which results in the possibility of transferring signal from the initial signal-carrying core to the second core thanks to the creation of supermodes. Therefore, propagation passes from insulated core operation to coupled core operation. Depending on the length of the taper, the signal can be separated between the cores in any ratio. For a tapering (taper waist region) of 6 mm, 1550 nm wavelength will be propagated only in the core, to which the single-core fiber was spliced, and 1310 nm wavelength will be propagated in the neighboring core only. For a tapering (taper waist region) extended by 8m£, other wavelengths will be effectively separated between the cores, in this embodiment, of 1550 nm and 980 nm. Being able to extend the fiber, we can alter the length of the tapering, thus producing various configurations of wavelength separation, and for the wavelengths specified herein, there is a taper length which allows for separating two wavelengths into particular telecommunication channels. Such an application of the invention is the implementation of the concept of a WDM (wavelength-division multiplexing) coupler.

Tapered sections (b, c) obtain values b = 3 mm and c = 6 mm, taper ratio of multicore fiber is equal to 30% with the parameters of multi-core fiber: core diameter d5= 6.6 μιη, cladding diameter d6= 125 μιη, hole diameter d7= 6.6 μιη, lattice constant Λ= 7.6 μιη.

Example 6

The coupler according to the invention enables the construction of element for the controlled addressing modes comprises a multi-core (seven-core) optical fiber (2) with cores insulated by areas with lowered refractive index 6 in the form of holes filled with air and single mode cores at a wavelength of 1550 nm (4.1, 4.2, 4.3, 4.4, 4.5 , 4.6), which cores have step refractive indices profiles and different effective refractive indices of the fundamental modes. To the seven-core fiber (2) with insulated cores, using the element of type of fan-in/fan-out, seven input single-core fibers (1) are placed in the capillary (3), and on the opposite side of the multi- core fiber, single-core few-mode output fiber (9) is attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating holes 6. Geometric midpoints of the microstructure elements (holes and cores) are arranged on a hexagonal lattice, wherein the lattice constant (Λ) is equal to 20μιη, and the insulating holes have diameters of ΙΟμιη, the core (10) - the addressed core multi-core has a step refractive index profile, it is few- mode and it has separated effective refractive indices of the particular modes. In the neighbourhood of the addressed core ( 10), there are addressing cores (4.1 - 4.6) with step refractive index profiles, wherein the effective refractive indices of the modes are selected such that they are matched to the effective refractive indices of the respective modes in the addressed core (10).

In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently). The structure of multi-core fiber (2) is modified in a section so that the holes (6) in its structure are collapsed with, and the section has a sufficient length to effectively supermode building on addressing cores and addressed core. The length of holes (6) collapsing is 5 mm and the tapering ratio is 0.5%.

In the section (c) = 5 mm holes separating the cores are collapsed with result in reducing (decreasing) cores insulation formed, supermodes are building resulting in increased crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output of multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

The output end of the multi-core fiber (2) single-core few-mode optical fiber (9) is spliced, with the same or similar properties as the addressed core (10) multi-core fiber (2). Thus, in the single-core few-mode fiber (9) mode multiplexing is performed with the use of seven modes. The above-described coupler construction in the multi-core fiber with controlled core insulation decrease (reduction) is therefore an element (coupler), enabling addressing modes in the few- mode fiber.

Similarly, the signal in a few-mode fiber can be demultiplexed with coupler according to the invention - information transmitted in the few-mode optical fiber encoded in the particular modes can be separated into individual cores, and further into separate optical fibers. The use of the coupler in the configuration of the multiplexer and demultiplexer with connection to the few- mode optical fiber allows the construction of a transmission system using mode multiplexing (Fig. 17).

Few-mode fiber (9) dimensions: cladding diameter dl6 = 125 μιη; few-mode core (10) - diameter dl7 = 20 μιτι, Si0 ₂ doped with 5,8 mol % Ge0 ₂

Multi-core fiber (2) dimensions:

- few-mode core (10) - diameter dl7 = 20 μιη, Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.1) - diameter d5.1 = 12,6 μη% Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.2) - diameter d5.2 = 8 μιη, Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.3) - diameter d5.3 = 6,4 μιη, Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.4) - diameter d5.4 = 5,4 μη% Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.5) - diameter d5.5 = 4,4 μη% Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- core (4.6) - diameter d5.6 = 2 μιη, Si0 ₂ doped with 5,8 mol % Ge0 ₂ ;

- cladding (5) - diameter d6 = 250 μιη, Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass)

- lattice constant (Λ) = 20 μιη

Holes (6) dimensions: diameters d7 = 10 μιη Taper parameters:

- section (b) = 0 mm

- section (c) = 5 mm

- diameter dl = d2 = 250 μιη

- taper ratio = 0,5% (no other taper than resulting from holes collapsing)

The invention in Example 6 is shown in Fig. 3, Fig. 13, Fig. 14, Fig. 15, Fig. 16 and Fig. 17. In this configuration, six fundamental modes of the addressing cores address the six higher orders modes of the addressed core. Fundamental mode in the addressed core is excited by initially splice the fiber to the this core.

Example 7

The coupler according to the invention enables the construction of element for the controlled addressing modes comprises a multi-core optical fiber (2) with cores insulated by areas with lowered refractive index (6) in the form of holes filled with air and single mode cores at wavelength of 1550 nm (4.1, 4.2, 4.3), which cores have step refractive indices profiles and different effective refractive indices of the fundamental modes. To the multi-core - four-core fiber (2) with insulated cores, using the element of type of fan-in/fan-out, three input single-core fibers (1) are placed in the capillary (3), and on the opposite side of the multi-core fiber, single- core few-mode output fiber (9) is attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating holes (6). Geometric midpoints of the microstructure elements (holes and cores) are arranged on a hexagonal lattice, wherein the lattice constant (Λ) is equal to 20μιη, and the insulating holes have diameters of ΙΟμιη, the core (10) - the addressed core multi-core has a step refractive index profile, it is few-mode and it has separated effective refractive indices of the particular modes. In the neighbourhood of the addressed core (10), there are addressing cores (4.1 - 4.3) with step refractive index profiles, wherein the effective refractive indices of the modes are selected such that they are matched to the effective refractive indices of the respective modes in the addressed core (10). In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently).

The structure of multi-core fiber (2) is modified in a section so that the holes (6) in its structure are collapsed, and the section has a sufficient length to effectively supermod building on addressing cores and addressed core. The length of holes (6) collapsing is (c) = 5 mm, transition taper region (b) = 2 mm and the tapering ratio is 10%.

In the section (c) = 5 mm holes separating the cores are collapsed with result in reducing (decreasing) cores insulation formed, and supermodes are building resulting in increased crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

The output end of the multi-core fiber (2) single-core few-mode optical fiber (9) is spliced, with the same or similar properties as the addressed core (10) multi-core fiber (2). Thus, in the single-core few-mode fiber (9) mode multiplexing is performed with the use of three modes. The above-described coupler construction in the multi-core fiber with controlled core insulation decrease (reduction) is therefore an element (coupler), enabling addressing modes in the few- mode fiber.

Similarly, the signal in a few-mode fiber can be demultiplexed with coupler according to the invention - information transmitted in the few-mode optical fiber encoded in the particular modes can be separated into individual cores, and further into separate optical fibers. The use of the coupler in the configuration of the multiplexer and demultiplexer with connection to the few- mode optical fiber allows the construction of a transmission system using mode multiplexing. Few-mode fiber 9 dimensions: cladding diameter dl6 = 125 μιη; few-mode core (10) - diameter dl7 = 10 μιτι, Si0 ₂ doped with 9 mol % Ge0 ₂

Multi-core fiber 2 dimensions:

- few-mode core (10) - diameter dl7 = 10 μιη, Si0 ₂ doped with 9 mol % Ge0 ₂ ;

- core (4.1) - diameter d5.1 = 10 μη% Si0 ₂ doped with 2 mol % Ge0 ₂ ;

- core (4.2) - diameter d5.2 = 8 μη% Si0 ₂ doped with 11.3 mol % Ge0 ₂ ;

- core (4.3) - diameter d5.3 = 10 μη% Si0 ₂ doped with 6.1 mol % Ge0 ₂ ;

- cladding (5) - diameter d6 = 250 μιη, Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass) - lattice constant (Λ) = 16 μιη

Holes (6) dimensions: diameters d7 = 8 μιη

Taper parameters:

- section (b) = 2 mm - section (c) = 5 mm

- diameter dl = 250 μιη

- taper ratio = 10% (d2=225 μηι)

The invention in Example 7 is shown in Fig. 3, Fig. 18, Fig. 19. In this configuration, three fundamental modes of the addressing cores address fundamental mode and two higher orders modes of the addressed core.

Example 8

The coupler according to the invention enables the construction of element for the controlled addressing polarization modes comprises a multi-core optical fiber (2) with cores have different effective refractive indices of the modes and one of the cores of multi-core fiber (2) - core (12) - is birefringent. In the neighbourhood of addressing core there is a single-mode core (4) at a wavelength of 1550 nm which have step refractive index chosen so it is matched to the effective refractive index of one of the polarization modes (polarization mode x) in addressing core (12). To the multi-core fiber (2) with insulated cores, input single-core fiber (1) is attached and to the opposite side of multi-core fiber a single-core birefringent polarization maintaining fiber (11) is attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating holes (6) and tapering the fiber. Addressing core (12) of multi-core fiber (2) has step refractive index, it is birefringent and have separated effective refractive indices of particular polarization modes. In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently).

The structure of multi-core fiber (2) is modified in a section so that the hole (6) in its structure is collapsed. The length of hole (6) collapsing is (c) = 3 mm, transition taper region (b) = 2 mm and the tapering ratio is 20%.

In the section (c) = 5 mm hole separating the cores is collapsed with result in reducing

(decreasing) cores insulation and resulting in increased crosstalk. Decreasing insulation result in building supermode which in turn result in increase of crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output of multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -4dB. Furthermore, the ratio of power present at the output of multi-core in the form of non-addressed mode in addressed core to the power in the form of addressing mode in the initially at the input of multi-core is smaller than -12dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

After the section with collapsed hole signal is propagating in the high birefringent core, in which only one polarization mode is effectively excited. The output fiber, which is birefringent and polarization maintaining is spliced to the core with high birefringence.

The above-described coupler construction in the multi-core fiber with controlled core insulation decrease (reduction) is therefore an element (coupler), enabling addressing polarization modes, so on construction fiber optic polarizer. It is also possible to use coupler in the opposite configuration.

Multi-core fiber (2) dimensions:

- core (4) - diameter d5 = 8.2 μη% Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- core (12) - short axis dl8 = 6 μιη, longer axis dl9 = 12.4 μιη, Si0 ₂ doped with 3.5 mol %

Ge0 ₂ ;

- cladding (5) - diameter d6 = 125 μιη, d Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass)

- lattice constant (Λ) = 16 μιη

Hole (6) dimensions: diameters d7 = 10 μιη

Taper parameters:

- section (b) = 2 mm

- section (c)= 3 mm

- diameter dl = 125 μιη

- taper ratio = 20% (d2=100 μηι)

The invention in Example 8 is shown in Fig. 3, Fig. 20, Fig. 21. In this configuration, fundamental mode of the addressing core (4) address polarization mode in the addressed core (12).

Example 9

The coupler according to the invention enables the construction of element for the controlled addressing polarization modes comprises a multi-core - three-core optical fiber 2 with cores having different effective refractive indices of the modes and two of the cores of multi-core fiber (2) - cores (12.1) and (12.2) - are birefringent. In the neighbourhood of addressed cores there is a single-mode core (4) at a wavelength of 1550 nm which have step refractive index, which effective refractive index is chosen so it is matched to the effective refractive indices of the polarization modes in addressed cores (12.1) and (12.2). To the multi-core fiber (2) with insulated cores, input single-core fiber (1) is attached and to the opposite side of multi-core fiber a single-core birefringent polarization maintaining fibers (11) are attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating holes 6 and tapering the fiber. Addressing cores (12.1) and (12.2) of multi-core fiber (2) has step refractive index, they are birefringent and they have separated effective refractive indices of particular polarization modes. In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently).

The structure of multi-core fiber (2) is modified in a section so that the holes (6) in its structure are collapsed. The length of hole 6 collapsing is (c) = 5 mm, transition taper region (b) = 2 mm and the tapering ratio is 10%.

In the section (c) = 5 mm holes separating the cores are collapsed with result in reducing

(decreasing) cores insulation and resulting in increased crosstalk. Decreasing insulation result in building supermode which in turn result in increase of crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output of multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -4dB. Furthermore, the ratio of power present at the output of multi-core in the form of non-addressed mode in particular addressed cores to the power in the form of addressing mode in the initially at the input of multi-core is smaller than -12dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

After the section with collapsed hole signal is propagating in the high birefringent core, in which only particular polarization modes are effectively excited. The output fibers, which are birefringent and polarization maintaining are spliced to the cores with high birefringence.

The above-described coupler construction in the multi-core fiber with controlled core insulation decrease (reduction) is therefore an element (coupler), enabling addressing polarization modes, so on construction fiber optic polarizer which divide polarization states. In the opposite configuration coupler can be used as polarization combiner.

Multi-core fiber (2) dimensions:

- core (4) - diameter d5 = 8.2 μιη, Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- core (12.1) - short axis dl8.1 = 6 μη% longer axis dl9.1 = 12.2 μη% Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- core (12.2) - short axis dl8.2 = 6 μιη, longer axis dl9.2 = 12.2 μιη, Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- cladding (5) - diameter d6 = 125 μιη, d Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass) - lattice constant (Λ) = 16 μιη

Hole 6 dimensions: diameters d7 = 10 μιη

Taper parameters:

- section (b) = 5 mm

- section (c) = 5 mm

- diameter dl = 125 μιη

- taper ratio = 10% (d2=112.5 μηι)

The invention in Example 9 is shown in Fig. 3, Fig. 22, Fig. 23. In this configuration, fundamental mode of the addressing core 4 address polarization modes in the addressed cores 12.1 and 12.2.

Example 10

The coupler according to the invention enables the construction of element for the controlled addressing polarization modes comprises a multi-core - dual-core optical fiber (2) with cores (12.1) and (12.2), which are birefringent. Cores are homogenous. To the multi-core fiber (2) with insulated cores, input single-core birefringent polarization maintaining fibers (13) are attached and to the opposite side of multi-core fiber a single-core birefringent polarization maintaining fibers (11) are attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating hole (6) and tapering the fiber.

having different effective refractive indices of the modes and two of the cores of multi- core fiber (2) - cores (12.1) and (12.2) - are birefringent. In the neighbourhood of addressed cores there is a single-mode core (4) at a wavelength of 1550 nm which have step refractive index, which effective refractive index is chosen so it is matched to the effective refractive indices of the polarization modes in addressed cores (12.1) and (12.2). To the multi-core fiber (2) with insulated cores, input single-core birefringent polarization maintaining fiber (13) is attached and to the opposite side of multi-core fiber, output single-core birefringent polarization maintaining fibers (11) are attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating hole (6) and tapering the fiber. Cores of multi-core fiber (2) have step refractive indices and they have separated effective refractive indices of particular polarization modes. In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently).

The structure of multi-core fiber (2) is modified in a section so that the hole (6) in its structure is collapsed. The length of hole (6) collapsing is (c) = 5 mm, transition taper region (b) = 2 mm and the tapering ratio is 10%.

In the section (c) = 5 mm hole separating the cores is collapsed with result in reducing (decreasing) cores insulation and resulting in increased crosstalk. Decreasing insulation result in building supermode which in turn result in increase of crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -3dB.

After the section with collapsed hole signal is propagating in the high birefringent cores, in which only particular polarization modes are effectively excited - (x) and (y). The output fibers (11), which are birefringent and polarization maintaining are spliced to the cores with high birefringence.

The above-described coupler construction in the multi-core fiber with controlled core insulation decrease (reduction) is therefore an element (coupler), enabling addressing polarization modes, so on construction fiber optic coupler (splitter) polarization maintaining - signal present at the beginning in the core (12.1) is divided into cores (12.1) and (12.2) and the polarization state is maintained. In the opposite configuration coupler can be used as polarization combiner.

Multi-core fiber (2) dimensions:

- core (12.1) - short axis dl8.1 = 6 μη% longer axis dl9.1 = 12.4 μη% Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- core (12.2) - short axis dl8.2 = 6 μιη, longer axis dl9.2 = 12.4 μιη, Si0 ₂ doped with 3.5 mol % Ge0 ₂ ;

- cladding (5) - diameter d6 = 125 μιη, d Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass)

- lattice constant (Λ) = 16 μιη

Hole (6) dimensions: diameters d7 = 10 μιη

Taper parameters:

- section (b) = 5 mm

- section (c) = 5 mm

- diameter dl = 125 μιη

- taper ratio = 10% (d2=112.5 μηι)

The invention in Example 10 is shown in Fig. 3, Fig. 24, Fig. 25. In this configuration, polarization modes of the addressing core 12.1 address polarization modes in the addressed cores 12.2. Example 11

The coupler according to the invention enables the construction of element for the controlled addressing modes comprises a multi-core optical fiber (2) with cores insulated by areas with lowered refractive index (6) in the form of hole filled with air. One of the cores (4.3) is single mode at wavelength of 1550 nm and it has step refractive indices profile and the second core (10) is few-mode and it has also step refractive index. To the multi-core - dual-core fiber (2) with insulated cores, two input single-core fibers (1) and (9) are attached and on the opposite side of the multi-core fiber two output single-core fibers (1) and (9) are attached and on a section of the multi-core optical fiber (2) insulation is reduced by collapsing the structures of insulating holes (6). Distance of the microstructure elements (holee and cores) is equal to lattice constant (Λ) = 20μιη, and the insulating holes have diameters of ΙΟμιη, the core (10) - the addressed core multi-core has a step refractive index profile, it is few-mode and it has separated effective refractive indices of the particular modes. In the neighbourhood of the addressed core (10), there is an addressing core (4.3) with step refractive index profile, wherein the effective refractive index of the mode is selected such that it is matched to the effective refractive index of the one of the modes (third higher order mode) in the addressed core (10). Because mode of the core (4.3) and one of the mode of the core (10) have matched effective refractive indices, before insulation decrease, thus in the section with insulation decrease, supermodes build. Therefore, mode of the core (4.3) address the mode of the core (10), and the mode of the core (10) address the mode of the core (4.3). In turn, there is a possibility to realize add-drop multiplexer/demultiplexer. Both cores (4.3) and (10) are simultaneously addressing cores and addressed cores.

In the area with no insulation reduced supermodes build so that the maximum observed crosstalk between any pair of cores is less than -lOdB (they build up inefficiently).

The structure of multi-core fiber (2) is modified in a section so that the hole (6) in its structure are collapsed in the length which is sufficient to build supermode on the cores (10) and (4.3). The length of hole (6) collapsing is (c) = 5 mm, transition taper region (b) = 2 mm and the tapering ratio is 10%.

In the section (c) = 5 mm hole separating the cores is collapsed with result in reducing (decreasing) cores insulation and resulting in increased crosstalk. Decreasing insulation result in building supermode which in turn result in increase of crosstalk - the ratio of power present in the addressed core in a form of addressed mode at the output multi-core fiber to the power present in the addressing core in the form of an addressing mode at the input of multi-core fiber is bigger than -3dB. In the case of exciting only one addressing core, the ratio of power present at the output of multi-core in the form of non-addressed mode (by the initially excited addressing mode) in a particular addressing core/cores and addressed core/cores to the power in the form of addressing mode in the initially excited addressing core at the input of multi-core is smaller than -14dB. Distribution of power level in each of the addressed modes emerging at the end of the modified section is "frozen" and this condition is transported further by unmodified section of the multi-core fiber.

Few-mode fiber dimensions (9): cladding diameter dl6 = 125 μιη, core diameter dl7 = 20 μιη, Si0 ₂ doped with 5.8 mol % Ge0 ₂ ;

Multi-core fiber 2 dimensions:

- few mode core (10) - diameter dl7 = 20 μιη, Si0 ₂ doped with 5.8 mol % Ge0 ₂ ;

- core (4.3), diameter d5.3 = 6,4 μιη Si0 ₂ doped with 5.8 mol % Ge0 ₂ ;

- cladding (5) - diameter d6 = 250 μιη, d Si0 ₂ doped with 0 mol % Ge0 ₂ (silica glass)

- lattice constant (Λ) = 20 μιη

Hole (6) dimensions: diameters d7 = 10 μιη

Taper parameters:

- section (b) = 2 mm

- section (c) = 5 mm

- diameter dl = 250 μιη

- taper ratio = 10% (d2=225 μηι)

The invention in this is shown in Fig. 3, Fig. 26, Fig. 27. In this configuration (multiplexer), mode of the core 4.3 address third higher order mode of the core and the third higher order mode from the core 10 address the mode of the core 4.3. Moreover, in few-mode core, other modes are propagating because of the initial excitation. In this configuration it is possible to add additional signal to the core 10 in which other modes are propagating and it is possible to drop the one signal from other signals propagating in this core. Example concerns realization of add- drop multiplexing consists in adding/releasing one of a channel to/from the signals which are propagating in one core.