The invention relates to the laser technology and may find application in the communication between different spatially remote objects, including flying vehicles and satellites and also the communication in deep space, etc.

The free space optical communication (FSO-communication) is characterized by extremely wide bandwidth, license-exempt position of spectral frequencies, low power consumption and high security of an optical channel. The FSO-communication advantages include a rapid deployability and applicability when the use of fiber optic cables is not possible.

The efficiency of the FSO-communication is defined by the luminous power loss rate and degree of information distortion on the optical signal propagation path. Main causes responsible for these losses are grouped into four types: 1) atmospheric losses, including light absorption, scattering and also scintillations (flickering) and decays (fading); 2) sighting losses related to the accuracy of mutual alignment of optical axes of transmitting and receiving modules and attributed to mutual vibrations thereof, power-supply flutter; 3) optical signal (light field) diffraction scattering depending on the distance, wavelength and quality of the original laser beam, 4) losses related to application of nonoptimal schemes of combining laser beams with antennas.

Frequently implemented in practice, the method of reduction of losses related to the first two types means the confinement of the light field within an input aperture of the receiving module. The method is based on the increase in the laser beam angular divergence up to the value exceeding the angular uncertainty caused by vibrations [1] However, the application of diffraction-limited laser beams is prospective in the long-distance optical communication systems considering the increase in the luminous power losses as the beam divergence increases. In its turn, the application of diffraction-limited light beams leads to the increase in power density at the receiver input aperture, however, it requires a respective increase in accuracy of pointing and stabilization of the communication channel. The existing stabilization mechanisms in the FSO-communication systems include the pointing of the transmitter towards the receiver and maintaining a stable laser communication by tracking changes of the position of the receiving module in space. To maintain the FSO-communication stability, it is critical both to increase the level of an optical signal and to minimize fading thereof. Under turbulent atmospheric conditions, the dominant cause of fading is a non- stationary field speckle structure arising from an initially homogeneous beam. The most similar device to the claimed one in terms of technical essence is the free space laser communication system proposed in [2] In this particular case, the fading minimization method was applied with the use of a maximal intensity speckle-tracking algorithm followed by feeding the speckles to a receiving optical fiber. An optical laser communication system in a simplified form consists of optically connected receiving and transmitting modules equipped with additional devices for pointing and stabilizing a communication channel.

The receiving module of the laser communication system is configured to receive a carrier signal from the transmitting module and stabilize a sighting line for the purpose of improving energy efficiency and preventing communication outage. The receiving module optical scheme comprises three optical channels: a main communication channel including an input telescope of a relatively large diameter, an additional passive channel and a light-emitting diode transmitting channel. The configuration and mode of operation of the receiving module is similar to those of the transmitting module except for the main channel where a refractor telescope as a rule is used to receive the carrier signal, while the signal itself is recorded by a special infrared radiation receive. The bidirectional communication requires on one side transmitting and receiving modules, while similarity of configuration thereof allows them to be combined into a receiver-transmitter system. Hereinafter, we shall restrict ourselves to the description of the configuration and operation of only an optical part of the transmitting module without discussing the carrier signal modulation-demodulation system as it remains a conventional one.

The transmitting module of the laser communication system performs the following functions: generates a monochromatic radiation beam for subsequent modulation with the data stream; reduces the beam divergence in case of long-distance signal transmission, with a telescope having a relatively large diameter being used for this purpose; stabilizes the sighting line of the transmitter to compensate vibration-induced random walk.

A core part of the transmitting module optical scheme of the prototype laser communication system [2] comprises three optical channels: a laser transmitting channel, a light- emitting diode transmitting channel and a passive receiving channel. The laser transmitting channel includes an infrared laser source, a rotating beam splitter with 100% transmission in the range of light-emitting diode channel wavelength radiation and with total reflection at the wavelength of the laser transmitting channel, a beam corrector (telescope) for matching apertures of optical elements, an active tilt corrector, an ocular and an output reflecting telescope. Radiation from the infrared laser source reflected from the beam splitter is directed onto the beam corrector and then on the active tilt corrector which directs a laser beam via the ocular to the reflecting telescope sighting mirror. The light-emitting diode transmitting channel is a reference source and is configured to stabilize the receiving module input channel. It comprises a powerful light-emitting diode (beacon) and respective forming optics. The passive receiving channel comprises an object lens and a four-quadrant photoreceiver or a high-speed CCD- camera (tilt sensor) and is configured to receive a sighting line stabilization signal from the receiving module reference source and to control the active tilt corrector. The passive receiving channel operates as described hereinafter. A signal from the beacon disposed on the receiving module enters the transmitting module reflecting telescope and via a sighting mirror thereof is directed to the active tilt corrector and then to the beam corrector and to the rotating beam splitter. The beacon signal passes through the rotating beam splitter and without changing a direction enters the high-speed CCD-camera object lens. An electric signal from the high-speed CCD-camera being proportional to the displacement of a laser beam focal spot relative to the center of the receiver area of the photoreceiver is supplied to the control electronics unit and is processed by the processor according to the preset algorithm. Finally, a control signal is supplied to the motor-driven suspension of the laser beam pointing mechanism (coarse tuning) and to active tilt corrector (fine tuning). This allows maintaining and stabilizing the position of a laser beam in the central part of the photoreceiver sensitive area. Thus, the pointing and stabilization devices of the communication channel automatically maintain the center of the laser carrier beam at the input aperture of the photoreceiver and further focusing thereof.

It should be noted that application of Gaussian beams (GB) in laser communication systems in a free space is not an optimal solution even in the absence of atmospheric turbulence. To more fully use the aperture of optical elements, the GB external diameter is intentionally limited, while an internal GB intensive central part is additionally shadowed by the reflector secondary mirror in reflecting telescopes. To solve the problem of a light field input into reflecting telescopes, recently a number of methods for converting an initial GB into an annular field (AF) has been proposed. For example, to eliminate shadowing in the reflecting telescope, a method for splitting the GB into four separate parts jointly forming a discreet AF in the given plane was proposed in [3] To solve the shadowing problem, [4] proposed that the AF should be generated by using either hollow optical waveguides or micro-axicons. A GB-to-annular beam converter was applied in [5] using two similar axicons to build a laser lidar optical system on the basis of a Cassegrain telescope. Despite the fact that transition to annular fields allowed rather efficiently solving the problem of input of the laser GB into mirror telescopes, optical schemes proposed in this field have a number of disadvantages. Due to the fact that a laser light spot from the transmitter at the input aperture of the receiving telescope has the intensity profile with a maximum on the optical axis, to achieve a maximum energy efficiency, as a rule, a receiving telescope without a central shadowing, i.e. a refractor, is required. The general disadvantage of these schemes resides in the fact that the methods and schemes used for generating a laser beam are not optimized to achieve a maximum field intensity in the photoreceiver plane, even when a rather intensive maximum is formed on the optical axis. It is generally assumed [5] that this field itself having an intensive central maximum is a zero-order Bessel-like beam (BLB) characterized by a nondiffracting property. However, that is not the case, since the Fourier-spectrum of this field is not annular that is typical for BLB. [6] for the first time proposed a scheme for generating conical light beams (CLB) using two sequentially positioned axicons with cone angles y _x and y ₂

, respectively, for which Ag = g _{2 -} g _i > 0 and it was shown that an initially generated light beam within the annular field belongs to the so-called z-dependent Bessel-like beam. As it propagates, the CLB converts into the field with intensive axial maximum, the axial intensity of which depends on the longitudinal coordinate z, while the position is defined by the cone angle Ay of a conical telescope. The differences in a CLB transverse structure fields and abovementioned AF, wave angle to the axis of which equals zero, are manifested in the far diffraction zone. Ordinary AFs form a field with the axial maximum in the far zone, the CLB, as it propagates, also initially forms a field with the axial maximum, but then it again converts into the annular beam with a wave tilt opposite in sign to the optical axis. This feature of the CLB allows one and the same telescope type to be used in the receiving and transmitting modules.

A technical objective of the invention is to increase the power of an optical carrier signal at the input aperture of the receiving device with the constant output aperture and power of the transmitter and, as a result, to increase the communication distance. For the purpose of a stable functioning of the laser communication in a turbulent atmosphere, the communication device is equipped with an adaptive optical system. The claimed optical communication system allows for implementation of a maximum axial intensity in the receiver plane by using one and the same type of the telescope for reception and transmission. This objective is achieved due to the fact that in the system of free space laser communication based on Bessel-like beams with long quasi-non-diffraction propagation distance consisting of an optically connected transmitting and receiving modules equipped with transmitter and receiver stabilization assemblies, the Bessel- like beam generation optical unit comprising a tandem of two axicons movable relative to each other along the axis is disposed behind the transmitting module laser source, with a cone angle of the second axicon exceeding a cone angle of the first axicon. In this case, the optical unit for generating a Bessel-like beam may include as follows:

- two axicons with similar cone angles, while the second axicon is made of an optically denser material than the first one; - two similar axicons submerged in immersion liquids with lower refraction indices than those of axicon material, while the immersion refractive index for the first axicon is higher than the immersion refractive index for the second one;

- two similar axicons submerged in immersion liquids at different temperatures, while the immersion temperature for the first axicon is lower that the immersion temperature for the second axicon;

- two similar axicons submerged in immersion liquids at different temperatures, while to continuously control the difference in the cone angles of two axicons, the difference in temperatures of immersion liquids is controlled by the Peltier elements and stabilized by an electronic unit.

Fig. 1 shows a simplified block diagram of a laser communication in a free space with the Bessel-like beam generation system. It comprises optically connected a transmitting module 1 and a receiving module 2 equipped with additional devices for pointing and stabilizing the communication channel. The transmitting module 1 accommodates an infrared laser source 3 of the transmitting communication channel. A Bessel-like beam generation assembly 4 is disposed in the transmitting module 1 directly behind the laser source 3 and comprises two axicons 5 and 6

Fig. 2 illustrates an optical scheme of the Bessel-like beam generation assembly configured to form a special type of light fields, namely, conical light beams (CLB) and explaining the principle of operation of the claimed device. To match apertures of the infrared laser source 3 and of the Bessel-like beam generation assembly 4, a beam expander (not identified by a reference character in the Figure) is additionally mounted in the path of the light. In the simplified version, the Bessel-like beam generation assembly 4 consists of two refractive axicons 5 and 6 movable relative to each other along the optical axis. In more complex schemes, controllable immersion axicons described hereinafter may be used. The Bessel-like beam generation scheme operates as described below. The Gaussian carrier laser beam from a laser source 3 of the transmitting module 1 is directed to the first axicon 5 with the cone angle gi, which converts the incident GB into the divergent AF. An annular field entering the second axicon 6 with cone angle g ₂ forms an annular CLB with cone angle Ag = g ₂ - g _c . Then, the annular CLB with corrected propagation direction is supplied along the initial path to an output reflecting telescope of the transmitting module 1. The annular CLB with the diameter of d ₂ = m'd forms at the telescope output, where rri is the magnification coefficient. In this case, the beam convergence angle decreases rri times. A diameter of the annular beam which may be controlled by changing the distance between axicons and the convergence angle Ag > 0 define an effective distance for the information transmission.

Fig. 3 illustrates the influence of a non-zero difference in axicon cone angles Ag > 0 on the axial intensity of the field to be generated on long distances Z. Herein, the cone angle of the first axicon is selected to be equal to gi = 1°, axicon diameters - 1 inch. A curve 7 shows the dependence of relative axial intensity on the BLB propagation distance, with the axicon cone angles relationship being y ₂ = 1.005, while a curve 8 shows the dependence with the relationship being 1.003. For comparison, herein a curve 9 for AF is shown with Ag = 0 which is characteristic of the analogues [4,5] It follows from Fig. 3 that transition from AF optical generation scheme with Ag = 0 to the CLB generation scheme with selection of an optimal value of Ag > 0 allows the axial intensity of light beams used for communication to be substantially increased.

Therefore, installation of the optical Bessel-like beam generation assembly in the laser communication system results in the generation of a specific type of the light field mutually transforming while propagating from the CLB into BLB with a high light intensity in a paraxial region. Fig. 4a illustrates a 3D structure of such a field in the paraxial region representing an intensive central maximum 10 surrounded by a small (two-three) number of rings 11. To estimate intensity of central maximum of the beam in Fig.4, 6, transverse profile thereof (curve 12) is shown. The experimental studies demonstrated that the energy content in the central maximum of such a beam is about 50%. In this case, the CLB in the plane of the reflector sighting mirror has an annular structure, thereby providing an effective input/output thereof without power losses. An important feature of the conical telescope-generated CLB is the availability of a sharp external boundary of the beam intensity distribution which allows the axicon aperture to be fully used without the need of external limitation, as it takes place for GB. The key difference of the claimed FSO-communication optical scheme from the prototype and analogues resides in the use of conical light beams for communication which allow an effective input of radiation into reflecting telescopes in the plane of the transmitter and also are characterized by the cone angle Ag > 0 being optimal for achieving axial intensity maximum in the reception plane. In addition, the scheme may be provided with the capability of tuning the angle Ag , and, as a result, controlling the optimal distance of the communication line. The use of the BLB for communication under the atmospheric turbulent conditions provide specific advantages related to the fact that in the process of propagation in the BLB the light beam in the communication channel transforms, while the latter, as is known, has the property of self recovery of the profile behind the obstacles. In this case, it is also worth mentioning a specific behavior of speckles for initial Bessel beams, the so-called quasi-nondiffracting speckles [7], i.e. speckles of substantially larger length than those arising from the initial GB. Taking into account the fact that the light beam is a Bessel beam in the photoreceiver plane, splitting thereof into long speckles when propagating through the atmosphere will substantially facilitate the procedure of recording thereof.

A scheme of the conical telescope with small angle difference Ag requires the manufacture of refracting axicons with extremely small difference in cone angles (~ KG ³ deg ). This task is technically difficult to implement directly. To obtain small cone angles, the use of an immersion liquid is prospective. The idea of using the immersion as applicable to axicons is known [8] However, it was considered applicable to only single axicon and without the exploration of possibilities of retuning a resulting cone angle, and also the influence of this retuning on the annular field and the Bessel-like field for a long distance. Using the immersion liquid in the claimed communication system reduces the effective cone angle of the system consisting of two axicons (conical telescope) compared to the cone angle of one particular axicon. In this case, the quality of the immersion axi con-generated BLB substantially improves, since the relationship of refraction indices for the glass-immersion interface is significantly lower than for the glass-air interface. This, in its turn, reduces requirements for the quality of mechanically manufactured refracting axicons. The effective cone angle is easily retuned by changing the solution concentration due to which the BLB focusing at a set distance is achieved. In addition, the use of more fine temperature tuning of the effective cone angle Ay is prospective.

Fig. 5 shows the immersion axicon-based optical Bessel-like beam generation assembly. It comprises two similar devices, namely, controllable immersion axicons consisting of a body 13 being inert to the immersion liquid 14, a refracting axicon 5 and a plane-parallel optical transparent plate 15. A cone angle of conical beam generated by this axicon at incidence of the collimated Gaussian beam thereon may be calculated by the method of geometrical optics.

Ά = sin ¹ ( n ₂ sin 0 ₂ ) = sin ^-1 (n ₂ sin(0, -q _ac )) . (1) where B _t = sin ¹ (« ₁ si n(0 _¥ )/« ₂ ) , 0 _{ax -} angle at the axicon base, n _{l 2 -} refraction indices of the axicon material and liquid.

One can see from Eq. (1) that using the immersion liquid substantially changes the axicon cone angle. As an example, let us calculate cone angles of the immersion axicon-based optical Bessel-like beam generation assembly. Let the axicons are made of N-BK ₇ glass or fused quartz S1O2 (« _bk 7 = 1.5195, ¾ _o 2 = 1.4607 for l = 532 nm). Let us select the angle at the base of the first axicon as 6 = 10°, for the axicon made of N-BK7 the immersion liquid refractive index is th = 1.417, for the axicon made of S1O2 - n ₂ = 1.370. It should be noted that the values of immersion liquids’ refraction indices used in calculation were taken from the Cardille Labs’ site, immersion liquid manufacturing company. Then, the calculation by the Eq. (1) gives a cone angle of the to be generated BLB for the N-BK ₇ axicon equaling gi = 1.037°, and for S1O2 yi = 0.917°. Let the second axicon be used without immersion with an angle at the base being q _ac = 2° . Then, using Eq. (1) at n ₂ =l it is deduced that the cone angle of the BLB generated by this N-BK ₇ -axicon is equal to g ₇ = 1.04°, and by the S1O2 axicon g ₂ = 0.992°. The data resulting from the performed calculations give the ratio of the cone angles of the second and first axicons made of the N-BK ₇ - and S1O2 glass as 1.005 and 1.003, respectively (see Fig. 3).

The immersion axicon may be implemented in the form of a cylindrical liquid cell, with an input window being a base of the refracting axicon, while an output window being a plane- parallel glass plate. The cone angle of this cell may be made controlled by selecting a liquid with a preset refractive index. For example, the use of an ethyl alcohol solution (C2H ₆ 0) with properly removed impurities as an immersion liquid is promising. Selection of ethyl alcohol as the immersion liquid is conditioned by a number of reasons. Firstly, it is possible to control the liquid refractive index within the range from n ₂₀ = 1.3439 to // _s = 1.3646 , i.e. to provide

An ₂ = 0.0207 by changing the alcohol solution concentration from 20% to 80%. Secondly, ethyl alcohol is colorless and clear liquid, easily mixes with water, and the solution is a physically homogeneous substance. Thirdly, the light is actually not absorbed in the visible wavelength range. Yet another reason is the stability of properties over time and relatively high unreactiveness of an alcohol solution with contacting materials.

The use of ethyl alcohol as an immersion liquid is also justified by ability to control the cone angle of the to-be-generated BLB by changing the temperature thereof. For this purpose, Peltier elements were used and a respective electronic part (not shown in Fig. 5) configured to stabilize heating and cooling of the immersion liquid with an accuracy up to 0 ,TC . For the ethyl alcohol solution, the change in the refractive index by TC is from 2,0 x l0 ⁴ at the solution concentration of 25% to 4,0 x l0 ⁴ at the solution concentration of 75% . In this case, the change of the glass refractive index (« I,0 x l0 ⁵ ) may be neglected. The temperature method for controlling the BLB cone angle allows for continuously varying the alcohol solution refractive index, and within the temperature variation limits of AT = 25° C allows for achieving the change in the index from An ₂ = 0,005 to V? ₂ = 0,01 when changing the solution concentration from 20 to 80%, respectively.

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