BACKGROUND OF THE DISCLOSURE

Field of the disclosure

[0001] The present disclosure relates to tunable laser for optical fiber signal communication systems. More particularly, this disclosure relates to external cavity diode laser (ECDL) implemented with a volume brag grating (VBG) aided with a wavelength Vernier-based etalon to achieve mode hopping free broad range tuning

Prior Art

[0002] The development of tunable single frequency diode lasers has been driven by the evolution of coherent optical telecommunication systems. Most of the tunable telecom lasers are based on distributed feedback (DFB) technology (i. e., a resonator medium with a periodic structure) and wavelength tuning that is accomplished by varying the temperature and/or the operating current. Distributed Bragg reflector (DBR) lasers, in turn, use a gain medium sandwiched between Bragg grating sections. Vertical-cavity surface-emitting lasers (VCSELs) include micro-electro-mechanical system (MEMS)-based tuning elements. An alternative to the above configurations includes ECDLs - the subject matter of the current application. All of these configurations, however, are plagued by limitations on the available wavelength range, tunability, and/or achievable linewidth. In particular, all types of single-frequency tunable laser diodes may exhibit a phenomenon called“mode-hopping” well known to one of ordinary skill in the tunable laser arts. In accordance with this phenomenon, the laser output frequency discontinuously hops from one value to another during tuning of the laser wavelength.

[0003] A typical tunable ECDL is typically configured with a semiconductor gain element or chip and an end reflector, such as mirror delimiting the external cavity on the output end. In tunable sources of this general type, in which feedback is provided by reflection,“mode hopping” can arise. This is due to the fact that there will be more than one resonant longitudinal mode for the electromagnetic radiation along the optical path in which oscillation is occurring.

[0004] To prevent mode-hopping, tuning without interruption of the phase of oscillation, or so- called phase-continuous tuning, should be achieved. A way to do that is to keep the number of wavelengths per-round trip or half-wavelengths per half the round trip whole in the external cavity as the wavelength is tuned. Thus adjusting the optical cavity length so that it equals the whole number of wavelengths or half-wavelengths is the requirement for hop-free tuning. The device for adjusting the cavity length may have different configurations and generally is referred to as a phase modulator. The latter however renders the ECDL more complex and costly.

[0005] The optical path length of the cavity or cavity length of the ECDL is defined between between a highly reflective rear faucet of the gain element and the mirror. As known, a cavity may receive various optical components each of which has its path length contributing to the length between the opposite reflectors. Accordingly, the effective cavity length is the sum of the path lengths through each optical element and the transmission medium times the coefficient of refraction for that element and/or medium.

[0006] The ECDL further includes a frequency filter configured to attenuate all cavity modes except for the desired one. The illustrated frequency filter may have a pair of Fabry-Perot (FP) etalons using the Vernier effect. The etalons may have the same finesses and slightly different free spectral range (FSR) - space between adjacent transmission peaks corresponding to a set of channel frequencies defined by the selected grid. In this aligned position the etalons suppress all of the resonant frequencies except for the desired one. The alignment may be realized by controlling the etalons by respective heaters. The aligned peaks are typically adjusted to comply with the International Telecommunications Union (ITU) standard grid. As a result ECDLs emits a narrow line radiation at the desired frequency.

[0007] However, tuning the the etalons requires a sophisticated controlling mechanism.

Furthermore, the etalons are very temperature sensitive in operation which may cause a drift of the desired single frequency, which is unacceptable and add an additional layer of complexity to the controlling mechanism which complicates the overall structure.

[0008] Therefore, a need still exists in the art for ECDLs to provide a new and improved system, component configurations and designs to overcome the above-mentioned technical difficulties and limitations. SUMMARY OF THE DISCLOSURE

[0009J Meeting the above-identified need, the present disclosure teaches a tunable ECDL laser having several aspects. However, each of the disclosed aspects includes different features that may stand on their own or be combined with one another or with the features of other aspects.

[0010] In accordance with one aspect of the disclosure, the inventive ECDL tunable to a plurality of channels of any selected grid is configured with a complex laser cavity. The cavity is defined by a broadband reflective mirror at one end, and a chirped volume grating-based reflective or semi-reflective Bragg mirror (CVBM) at the other end. The system also includes a gain element with its rear faucet defining the broadband reflecting mirror, and a FP etalon element.

[0011] According to one salient feature of this aspect, the disclosed CVBM is configured with such a chirp of the grating period that a set of planes or zones within the mirror corresponds to a set of channels of the selected grid. Such a configuration means that each newly selected wavelength has a corresponding cavity length between the rear faucet and a designated zone of the Brag mirror without a help from the phase modulator. In other words, when the ECDL is tuned to operate at a different wavelength, the cavity length is equal the whole number of wavelengths per round trip (or half-wavelengths per single pass) which provides a phase- continuous tuning operation eliminating thus the mode-hopping phenomenon.

[0012] The FP etalons use the Vernier effect to provide a wavelength filter since the CVBM chirped reflect all wavelength within the gain bandwidth of the laser. The FP etalons are individually tuned to provide alignment between one narrow transmission peak of one etalon and one transmissive peak of the other etalon which, in turn, are spectrally aligned with the selected zone of the chirped mirror and complying with the desired frequency channel of the selected grid. The disclosed ECDL outputs narrow line emission at a frequency corresponding to one of the frequencies of the selected channel. The line width does not exceeding 100 kHz, but preferably is about 50 KHz.

[0013] A further feature of the same aspect helps improve a stable operation of the ECDL at the desired single frequency and fine wavelength tuning. The channels frequencies of the selected grid each are pretty broad. For example, each ITU has a spectral width of 0.4 nm. The wavelength selection performed by the etalons in combination with the disclosed CVBM is rather coarse. It is impossible to know whether the emitted frequency corresponds to a central frequency of the selected channel having, for example, a central wavelength at 1500 nm, or some other frequency within the channel, such as a 1501 nm or 1502 nm or another wavelength. Also, the etalons are temperature sensitive which may detrimentally affect the operational stability of the ECDL at the selected frequency which is prone to drift.

[0014] To overcome the above-mentioned difficulties, a photo-thermo refractive (PTR) glass is treated with UV light to have not only the above-disclosed CVBM, but also a volume slanted gratings (VSG) with discreet lines or wavelength reflective peaks each of which may be written to correspond to the desired frequency of the selected channel. With a pair of transmission peaks of respective etalons spectrally aligned with a corresponding narrow reflective peak of the slanted grating, the external cavity is reliably locked at the desired frequency since the PTR material is substantially less temperature sensitive than the etalons which minimizes the risk of the desired frequency’s drift during the ECDL operation.

[0015] Still a further feature of the disclosed structure provided with the CVBM and VSG includes a photodetector mounted on the slab of PTR material. While a mode hop-free operation is realized by the CVBM, and the lasing stability at the desired frequency is controlled by the VSG, the photodetector optically connected to the VSGs indicates the intensity of the tapped light. If the desired frequency, such as the central frequency of the selected channel, starts drifting for some reason, the intensity of the tapped light decreases which is immediately detected by the detector.

[0016] A feedback circuit provides for the output signal of the photodetector to be received by a central processing unit (CPU) which, in tur, is operatively connected to the heaters of respective FP etalons. If the signal intensity from the photodetector mismatches a reference value corresponding to a maximum intensity at the desired lasing frequency, such as a central frequency of the selected frequency channel, the CPU outputs a control signal to the drivers. The latter adjust the etalons such that the measured intensity of the tapped light again matches the reference value.

[0017] Another possibility is to use a dither signal on the FP etalons and monitor by a photodiode (PD) to minimize first harmonics signal for the automatic alignment between FP etalons and the reference VBG channel. This will generate an absolute feedback signal which allows the wavelength locker to work properly. [0018] In accordance with a further aspect, the ECDL includes a gain element, such as the CW FP gain element similar to the previous aspect, a slab of PTR glass with written inside its multiple volume Bragg gratings (VBG). The VBGs have different periods and overlay one another within the PTR glass to form a plurality of narrowband prerecorded reflective peaks corresponding to respective desired frequencies of the channels of the grid, which is typically a central frequency. It is possible to determine the periods of respective VBGs so that the cavity length meets the condition for the mode hop-free tunability. This would be relatively easy if the gratings were slightly chirped eliminating the need for a phase modulator. Otherwise, this aspect may include the latter,

[0019] The switching of frequencies is realized by a single FP etalon configured with a plurality of transmitting peaks with the spacing therebetween which correspond to that one between frequency channels of the selected grid. The tunable FP shifts in a position in which one of its transmission peaks is aligned with a corresponding reflective peaks within the PTR material using the Vernier effect. This configuration allows not only the effective wavelength selection but also the stability of the ECDL operation at the selected wavelength because of a relatively temperature-independent PTR glass. While the number of reflective peaks can be made to correspond to full 40 - 80 channels of the ITU grid, which is the case in accordance with the previous aspect, it would be rather difficult to have as many transmissive peaks in a single etalon due to a high density of the peaks. However, this structure can effectively operate at 10 - 20 channels.

[0020] In accordance with still another aspect of the disclosure, the ECDL is configured with a directly modulated gain element, a PTR slab with multiple VBMs, a single etalon and a dispersion pre-compensation optical arrangement incorporating a CVBM. The selection of frequencies is realized in accordance with the operation of the previous aspect.

[0021 ] The packets of light propagating in the external cavity between the directly modulated gain element and the VBG is broadband and tend to spectrally broadened, i.e., dispersed, as known to one of ordinary skill in the art. This dispersion is positive. In a fiber telecom industry, the fibers are often manufactured with a negative dispersion. Accordingly, the sign of the dispersion at the output of the VBG should be reversed. This is realized by a chirped mirror (CVBM) located outside the external cavity and downstream from the VBM. BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features will be better understood in conjunction with the following drawings, in which:

[0022] FIG. 1 is a configuration of the disclosed tunable ECDL including a CVBM in accordance with one aspect of the invention;

[0023] FIG. 2 schematically illustrates the principle of operation of the external cavity delimited at the output end by the disclosed CVBM of the ECDL of FIG. 1 ;

[0024] FIG. 3 illustrates a transmission characteristic of both FP filters of the ECBM of FIG.1 ;

[0025] FIG. 4 illustrates a transmission characteristic in a state where the oscillating frequency is selected by both FP etalons;

[0026] FIG. 5 is a configuration of the disclosed tunable ECDL in accordance with a further feature of the first aspect including the monolithic slab of PTR material which is inscribed with the CVBM of FIG. 1 and a slanted volume grating;

[0027] FIGs. 6A and 6B illustrate the configuration of the slab of FIG. 5;

[0028] FIG. 7 is a configuration of the disclosed tunable ECDL in accordance with a fusther aspect of the disclosure including a plurality of volume Bragg gratings within a monolithic slab of PTR material;

[0029] FIG. 8 is a configuration of the disclosed tunable ECDL with a dispersion pre- compensation optical schematic using a CVBM; and

[0030] FIG. 9 illustrates the principle of operation of the dispersion pre-compensation optical schematic of FIG. 8.

SPECIFIC DESCRIPTION

[0031] Reference will now be made in detail to the disclosed ECDL. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form being far from precise scale. For purposes of convenience and clarity only, the terms“connect,”“couple,” and similar terms with their inflectional morphemes do not necessarily denote direct and immediate connections, but also include connections through mediate elements or devices. [0032] Referring to FIGS, land 2, a tunable ECDL 20 is provided with a CW FP gain element (chip) 22 having a rear faucet 24 which produces a broadband reflecting mirror, and antireflection coated front faucet 26, from which the cavity is extended into a chirped volume Bragg mirror 30 (CVBM). Thus, CVBM 30 is configured with a set of prerecorded reflecting zones 28 (FIG. 2) in a slab of PTR material corresponding to respective frequency channels of the selected grid, such as the ITU grid including up to 80 channels.

[0033] The zones 28 are defined by a chirp of grating period selected to compensate a phase drift every time as CVBM 20 is switched to different wavelengths. The zones 28 thus define respective different optical lengths Ll ... Ln of the cavity, each equal the whole number of wavelengths ll ... ln (FIG. 2) upon switching the ECDL to a new operating wavelength. As a result, every cavity length has a different mode-locking frequency Vi ... Vn that eliminates the need for an external phase modulator adjusting the optical cavity length for each operating wavelength.

[0034] Referring to FIG. 3 and 4, in order to emit light at a single operating wavelength aligned with a corresponding frequency channel grid, ECDL 20 is further configured with a transmission wavelength filter or channel selector 35. The filter 35 includes two FP etalons 32, 34 respectively, each having a set of transmission peaks 36, 38 (FIG. 3). The transmission peaks of respective etalons 36, 38 having slightly different periods or free spectral ranges, i.e., the sets of respective peaks are offset relative to one another, and similar finesses. The etalons 32, 34 are configured as respective thin plates made from the same or different material, such as but not limited to silica.

[0035] The filter 35 utilizes a Vernier tuning mechanism which is performed by shifting the sets of transmission peaks 36, 38 relative to one another to align the single transmission peaks of each of the FP etalons at the desired resonant frequency. The alignment of two exemplary peaks 36’, 38’ is illustrated in FIG. 3.

[0036] In use, when ECDL 20 is required to switch to a new operating wavelength, FP etalons 32, 34 are independently tunable by a temperature induced change in their thickness by respective heaters 40, 42 which alter the optical length of the cavity. It is possible to control both the thickness and respective refractive indices of the etalons. Being aligned with one another, peaks 36’ and 38’ are invariably aligned with a corresponding resonant cavity frequency of CVBM 30 which provides lasing of ECDL 20 at the desired oscillating wavelength corresponding to the selected frequency channel of the grid. As all peaks are aligned, the optical cavity length is adjusted to provide oscillation of the whole number of the desired wavelengths or half-wavelengths.

[0037] In addition, ECDL 20 includes a carrier 44 supporting CW gain element or chip 22. The beam output by gain element 22 is collimated by a lens 46. Outside the cavity, a beam splitter 52 is coupled to a photodetector 48 measuring the intensity of light output at the desired

wavelength. The highest intensity is associated with with, for example, the central frequency of the selected frequency channel of the grid. The isolator 50 located downstream from

photodetector 50 minimizes propagation of backreflected light in a counter-propagating direction towards the chip. The whole configuration is mounted on an optical bench 54 and is operatively connected to a row of thermoelectric cooler (TEC) pallets56 spaced from one another along the entire length of the illustrated structure. The TEC base 58 provides the support for pellets 56.

The TEC is needed here to maintain the entire structure at the desired constant temperature determined during the calibration of CVBM 30.

[0038] FIGs. 5, 6A and 6B illustrate a simplified structure of ECDL 20 including CVBM 30. As mentioned before, etalons 32 and 34 are difficult to control since they are very temperature sensitive and are easily shifted in response to even a slight temperature change which causes the drift of the selected lasing wavelength. In contrast, PTR material 65 is substantially less sensitive to temperature instabilities. Hence in addition of CVBM 30, PTR material 65 is also inscribed with a slanted Bragg grating 62 (SBG) (FIG. 6A) which provides an array of discreet lines 64 (FIG. 6B) located within respective reflective zones of CVBM 30 and corresponding to respective desired frequencies of the gird channels. The discreet lines are of course narrower than respective zones of CVBM and practically not affected to the temperature changes during lasing at the desired wavelength. In other words, the tilted grating is designed to even further narrow the line width of the ECDL’s output.

[0039] The tuning of ECDL 20 is generally performed analogously to the ECDL of FIG. 1 by controlling a mutual position of two FP etalons 32, 34 respectively which, when aligned, are also aligned with the desired zone of CVBM 30. In this position, the length of the cavity equals to the whole number of desired wavelengths as disclosed above. But the concrete position of diffracted beam portion I - 1 at the desired wavelength within the corresponding zone is not well determined despite the etalons. It is not known if beam II-II output from CVBM 30 is, for example, at 1500 nm center wavelength of the selected frequency channel or at 1501 or 1502 and etc., of the same channel. To avoid this uncertainty, one of the lines of SBG 62 is written within the desired zone of CVBM 30 to exactly correspond to the required wavelength of the grid. The diffracted beam I - 1 is now output as at the desire wavelength, and the cavity is locked on this wavelength providing a stable output beam II - II at the single desired wavelength.

[0040] If however certain misalignment occurs, which can happen for a variety of reasons, SBG 62 also functions as a beam splitter tapping a portion IV - IV of input beam III - III which is coupled into photodetector 48. If the measured intensity matches the reference value, which is stored in a CPU 66 and corresponds to, for example, the central wavelength of the selected grid, the operation continues. If however, the wavelength drifts, no signal is coupled into

photodetector since each line of SBG 62 reflects only on the predetermined wavelength. The control signal is then output from CPU 66 to heaters 40, 42 which continue to adjust respective etalons 32, 34 until the measured intensity of tapped off portion IV - IV matches the reference value which indicates that the system is tuned on the desired wavelength.

[0041] Instead of comparing with reference that can change over time (GC and components degradation), it may be advantageous to use a dither signal on the FP etalons to align them with the VBG recorded reference channel. This will generate an absolute feedback signal which allows the wavelength locker to work properly.

[0042] Referring to FIG. 7, ECDL 75 is configured with a monolithic slab ofPTR material 70 provided with a plurality of VBGs overlaying and overlapping one another while linearly inscribed in slab 20. In other words, several VBGs written in the same place but with slightly different grating periods so that the VBGs do not interfere with one another as long as the refractive index of the PTR material does not reach its saturation level.

[0043] Due to multiple VBGs with different grating periods, a series of discreet reflective lines or reflective peaks 72 is formed within slab 70. The periods of respective VBG can be so selected that peaks 72 would correspond to respective desired frequencies of the selected grid channels and be spaced from one another so as to prevent mode hopping upon switching from one wavelength to another. The latter can be realized particularly effectively if VBGs are all chirped. Alternatively, a cavity length modulator may be employed in the illustrated configuration.

[0044] In contrast to the above-disclosed configurations, ECDL 75 has only one FP etalon 68 which is controlled by a heater 73. Similarly to the previously disclosed configurations, the Vernier effect is used here between FP etalon 68 and each reflective peak 72. The number of lines may correspond to all 40 - 80 channels of the ITU grid, but technologically to have that many transmission peaks in etalon 68 is rather difficult. Of course, there is always a possibility of tuning slab 70 increasing thus a number of transmission peaks, but as mentioned before, the temperature needed to affect slab 70 is incomparably higher than that needed for etalon 68 to shift. Hence the illustrated configuration may be ideal for the grid comprising up to 20 channels.

[0045] The rest of ECDL 75 structure includes CW FP gain element 22 emitting a broad band of frequencies. The lens 46 is configured to climate the output beam. The beam splitter 52 located outside the cavity taps off a portion of the output beam at the desired wavelength which is coupled into photodetector 42. The isolator 50 prevent backreflected light from propagating into the cavity, and the whole structure is maintained at a constant temperature, which is determined during the calibration of the device, by a plurality of pallets 56 positioned between bench 54 and and TEC base 58.

[0046] FIGs. 8 and 9 illustrate a tunable ECDL 90 configured with a directly modulated gain element or chip 92. As known, the performance of chip 92 is limited by a wavelength positive chirp manifested by spectral broadening of light packets. This phenomenon is one of the main factors limiting the bit rate in medium and long-haul transmission systems based on standard single-mode fiber in 1.55-mih which has a negative dispersion.

[0047] The ECDL 90 is configured with collimating lens 46 collimating a beam from chip 92 which is further incident on a single etalon 68. The slab ofPTR material 70 is inscribed with multiple VBGs which are formed to overlap one another within PTR material 70. The VBGs have respective periods differing from one another so as to provide a row of discreet lines 72 within slab 90. The lines correspond to respective desired wavelengths each located within a selected frequency channel of the etalon grid. As can be easily understood a wavelength tuning operation of ECDL 90 is identical to that one of ECDL 75 of FIG. 7 discussed detail above. [0048] The schematic shown here is operative to reverse the dispersion sign of light before it is coupled into a fiber, not shown here. The chirp reversal is realized by a CVBM 100. When a signal at the desired wavelength is output from slab 70, it is incident on a polarizer 94. The polarized light at the desired wavelength is further incident on a beam splitter 96 which guides the main portion thereof on a 45% Faraday rotator 98. Thereafter the polarized light is reflected from the CVBM 100 where the polarized beams reverses the sign of dispersion and propagates in a backreflected direction through Faraday rotator 98 and beam splitter 96. The latter here functions as an optical circulator preventing the backreflected light from propagating towards chip 92. Thereafter the reflected light is directed to a further beam splitter which is coupled with a photodetector 104. In the end, the correctly chirped reflected light at the desired wavelength is coupled into a fiber which is not shown here.

[0049] Those skilled in the art will recognize that the foregoing embodiments are presented by way of example only and that within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described. The present disclosure is directed to each individual feature, system, material and/or operation described herein. In addition, any combination of two or more such features, systems, materials and/or methods, if such features, systems, materials and/or operation are not mutually inconsistent, is included within the scope of the present invention.