Abstract:
A tunable transmission optical filter is optically coupled between a laser section and semiconductor optical amplifier (SOA) section of a tunable laser device. The optical filter may be tuned to provide a high transmission near the lasing peak while suppressing a significant portion of back-propagating amplified spontaneous emission (ASE) of the SOA section. Without the optical filter, the laser output spectrum may develop side lobes of higher intensity after the ASE is amplified and reflected in the forward direction by the laser gain and mirror sections. While lessening the side lobes, the optical filter simultaneously transmits the laser peak for amplification by the SOA section.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to light sources, and in particular, to tunable laser sources. 
       BACKGROUND 
       [0002]    In a wavelength division multiplexed (WDM) optical network, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined and transmitted through a series of spans of optical fiber. At a receiver end, the wavelength channels are separated and detected by optical receivers. 
         [0003]    The optical signals to be encoded are usually provided by laser diodes, one laser diode per one wavelength channel. It is desirable to provide backup laser diodes for redundancy purposes. In view of multiple wavelengths used in a dense WDM (DWDM) transmission, tens and even hundreds of wavelengths in some cases, providing a separate backup laser diode for each wavelength may become prohibitively expensive. Tunable laser sources help solve this problem. 
         [0004]    Tunable laser sources also prove valuable in reconfigurable WDM optical networks, in which new wavelength channels are added as a network load increases. Adding and dropping wavelength channels in such a “wavelength-agile” network may be done dynamically, in response to fluctuating data bandwidth requirements between various network nodes. From the network architecture standpoint, it may be preferable to have laser sources tunable to any desired wavelength. Such sources have to be widely tunable, provide sufficient output optical power, and have strong side mode suppression to avoid coherent crosstalk with other wavelength channels. 
         [0005]    Referring to  FIG. 1A , an exemplary prior-art tunable laser source  100  is shown. A similar laser source is described, for example, in U.S. Pat. No. 5,325,392 by Tohmori et al. The laser source  100  includes optically serially coupled a rear mirror  102 , again section  104 , a phase section  106 , and a front mirror  108 . The front  108  and rear  102  mirrors include optical gratings having a periodic wavelength dependence of reflectivity. Turning to  FIG. 1B , an example wavelength dependence  112  of the rear mirror  102  reflectivity has a period of 5.6 nm. A wavelength dependence  118  of the front mirror  108  reflectivity has a larger period of 6.3 nm. Peaks  112 A,  118 A of the wavelength dependencies  112  and  118  overlap at 1550 nm. As a result, a product wavelength dependence  130 , obtained by multiplying the rear  112  and front  118  wavelength dependences, has its biggest peak  132  at 1550 nm. The product wavelength dependence  130  is shown in  FIG. 1B  magnified by a factor of four. The product wavelength dependence  130  is proportional to a round trip optical gain for light circulating between the front  108  and rear  102  mirrors of the laser source  100  ( FIG. 1A ). The product wavelength dependence  130  ( FIG. 1B ) determines wavelength emission properties of the laser source  100 . Three longitudinal resonator modes  121 ,  122 , and  123 , denoted with cross (“+”) signs superimposed on the product reflectivity trace  130 , are disposed within the 1550 nm peaks  112 A,  118 A. Additional modes  134 ,  136 , and  138  are present near 1544 nm ( 134 ) and 1556 nm ( 136 ,  138 ). Of these modes  121 ,  122 , and  123 ,  134 ,  136 , and  138 , only the central mode  122  results in generation of a laser beam  109  of substantial optical power due to its much higher round trip gain; emission at the side mode  122 ,  123 ,  134 ,  136 , and  138  wavelengths occurs at much lower optical power level. 
         [0006]    The laser source  100  is tuned by shifting the wavelength dependencies  112  and  118  in opposite directions. When two other peaks of the wavelength dependencies  112  and  118  overlap at another wavelength, lasing occurs at one of longitudinal modes at that wavelength. In essence, the lasing wavelength is tuned using a Vernier effect over wavelength range that is much wider than a wavelength range of tuning the individual mirrors  102 ,  108  themselves. The wavelength tuning occurs in stepwise fashion. A proper selection of longitudinal mode spacing and reflectivity periods of the back  102  and front  108  mirrors allows one to define a desired magnitude of the wavelength step. 
         [0007]    Referring now to  FIG. 1C  with further reference to  FIG. 1A , an exemplary prior-art amplified laser source  150  is shown. A similar laser source is described, for example, in U.S. Pat. No. 6,788,719 by Crowder. The amplified laser source  150  includes the laser source  100  of  FIG. 1A  and an integrated semiconductor optical amplifier (SOA)  130  serially optically coupled lo the front mirror  108 . The addition of the SOA  130  allows one to boost the output power of the laser beam  109  to much higher levels than those achievable in the laser source  100  of  FIG. 1A . However, the SOA  130  generates additional spontaneous emission noise. Furthermore, the amplification by the SOA  130  is not spectrally uniform across an amplification band due to so-called gain tilt. As a result, the SOA  130  may amplify side modes of the laser beam  109  more than the fundamental mode, reducing side mode suppression ratio (SMSR). For example, the SMSR may be reduced from 50 dB in the laser source  100  to less than 40 dB in the amplified laser source  150  for lasing wavelengths away from the gain spectrum peak. The SMSR degradation may be unacceptable in many applications including a tunable laser source application for a wavelength-agile optical network. A tradeoff exists in the prior art between output optical power and spectral purity of an amplified widely tunable laser source. 
       SUMMARY 
       [0008]    In accordance with one embodiment, a tunable transmission optical filter is optically coupled between a laser section and an SOA section of a tunable laser device. The optical filter may be tuned to have high transmission near the lasing wavelength, and it may be configured for low transmission proximate the gain peak tor lasing wavelengths substantially detuned from the gain peak wavelength. This suppresses back-propagating amplified spontaneous emission (ASE) of the SOA near the filter stop band, which would otherwise be reflected forward by the laser mirrors and amplified by laser active section. This back-reflected ASE may be a major source of SMSR degradation. In general, ASE-induced degradation of SMSR is most extreme at the shortest and/or longest wavelengths of the laser tuning range when the lasing wavelength is detuned farthest from the peak gain. In effect, the placement of the tunable transmission optical filter between the laser section and the SOA doubles the ASE suppression, resulting in a corresponding increase of the SMSR. Preferably, the laser section, the tunable transmission optical filter, and the SOA section are monolithically formed as a single structure, simplifying overall construction and eliminating reflections between components. 
         [0009]    In accordance with an embodiment, there is provided a tunable laser device comprising:
       a tunable laser section configured to generate light at a lasing wavelength, wherein the tunable loser comprises an optical cavity for tuning the lasing wavelength within a tuning range spanning from a first wavelength to a second wavelength, wherein the second wavelength is longer than the first wavelength;   a tunable transmission optical filter disposed outside of the optical cavity and downstream of the tunable laser section, wherein the tunable transmission optical filter comprises:   a passband configured to transmit light at the lasing wavelength, and   a stopband configured to attenuate light at a sidelobe wavelength of the tunable laser section, wherein the sidelobe wavelength is different from the lasing wavelength, and wherein the lasing wavelength and the sidelobe wavelength are within the tuning range; and   a semiconductor optical amplifier (SOA) section optically coupled to and downstream of the tunable transmission optical filter, wherein the semiconductor optical amplifier section has an amplification band that comprises the tuning range.       
 
         [0015]    In one exemplary embodiment, the tunable transmission optical filter comprises an asymmetric Mach-Zehnder waveguide interferometer, formed monolithically with the tunable laser and SOA sections. The Mach-Zehnder waveguide interferometer is tunable to have a transmission maximum at the lasing wavelength, or a transmission minimum, e.g. a center of the stopband, at the sidelobe wavelength. 
         [0016]    In accordance with one embodiment, there is further provided a laser source comprising the above tunable laser device and a controller operationally coupled to the tunable laser section, the tunable transmission optical filter, and the semiconductor optical amplifier, wherein the controller is configured to:
       tune the losing wavelength of the tunable laser section; and   tune a center wavelength of the passband of the tunable transmission optical filter by adjusting a first tuning parameter thereof to correspond to the lasing wavelength.       
 
         [0019]    In accordance with an embodiment, there is further provided a method for calibrating a tunable laser device comprising coupled in sequence a tunable laser section, a tunable transmission optical filter, and a semiconductor optical amplifier section, the method comprising:
       (a) tuning a lasing wavelength of the tunable laser section to a calibration wavelength within a tuning range of the tunable laser section;   (b) upon completion of step (a), scanning a center wavelength of a passband of the tunable transmission optical filter;   (c) while performing step (b), determining an output optical power or a side mode suppression ratio of the laser source;   (d) selecting a value of the center wavelength scanned in step (b) corresponding to a maximum output optical power or a maximum side mode suppression ratio determined in step (c); and   (e) associating the value of the center wavelength selected in step (d) with the calibration wavelength tuned to in step (a).       
 
         [0025]    In accordance with another aspect, there is further provided a method for generating light comprising:
       (a) providing a tunable laser device comprising coupled in sequence a tunable laser section, a tunable transmission optical filter, and a semiconductor optical amplifier section;   (b) energizing the tunable laser section and tuning a losing wavelength thereof to a first working wavelength within a tuning range of the tunable laser section;   (c) tuning a passband center wavelength of the tunable transmission optical filter so as to increase a side mode suppression ratio at the first working wavelength; and   (d) energizing the semiconductor optical amplifier section.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Exemplary embodiments will now be described in conjunction with the drawings, in which: 
           [0031]      FIG. 1A  illustrates a schematic block diagram of a prior-art tunable laser source; 
           [0032]      FIG. 1B  illustrates mirror reflection spectra, the product spectrum at 4× scale, and longitudinal mode positions of the laser source of  FIG. 1A ; 
           [0033]      FIG. 1C  illustrates a schematic block diagram of a prior-art amplified tunable laser source; 
           [0034]      FIG. 2A  illustrates a typical emission spectrum of the laser source of  FIG. 1A ; 
           [0035]      FIG. 2B  illustrates a typical emission spectrum of the laser source of  FIG. 1C ; 
           [0036]      FIG. 3  illustrates a schematic block diagram of a tunable laser device having a tunable filter; 
           [0037]      FIG. 4A  illustrates an implementation of the tunable laser device of  FIG. 3 , wherein the tunable filter includes an asymmetric Mach-Zehnder (MZ) interferometer; 
           [0038]      FIG. 4B  illustrates a transmission spectrum of the asymmetric MZ interferometer of  FIG. 4A  superimposed with an emission spectrum of the amplified laser source of  FIG. 4A  if the asymmetric MZ interferometer were omitted from the amplified laser source; 
           [0039]      FIG. 4C  illustrates an emission spectrum of the tunable laser device of  FIG. 4A  including the asymmetric MZ interferometer, showing suppression of side peaks in comparison with  FIG. 4B ; 
           [0040]      FIG. 5  illustrates a laser source according to one embodiment; 
           [0041]      FIG. 6  illustrates an embodiment of a tunable laser device, having a cascaded MZ interferometer; 
           [0042]      FIG. 7  illustrates an exemplary method for calibrating a laser source of  FIGS. 3, 4A, 5, and 6 ; and 
           [0043]      FIG. 8  illustrates an exemplary method for generating light using e.g. a laser source of  FIGS. 3, 4A, 5, and 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
         [0045]    The source of SMSR degradation caused by the addition of an SOA to a Vernier-tunable laser diode will be considered first. Turning to  FIG. 2A , an example emission spectrum  200 A of the laser source  100  of  FIG. 1A  is shown. The emission spectrum  200 A was measured by the inventors. The emission spectrum  200 A has a main lasing peak  129 ; back mirror reflection side peaks  125 ; and front mirror reflection side peaks  126 . In  FIG. 2A , the laser peak  129  is located near the short wavelength edge. e.g. 1530 nm, of the tuning range spanning e.g. between 1530 and 1570 nm, resulting in an overall SMSR of about 50 dB. 
         [0046]    Turning to  FIG. 2B , an emission spectrum  200 B of the amplified laser source  150  of  FIG. 1C  is shown. The emission spectrum  200 B was measured by the inventors under a similar short-wavelength tuning condition. Side peaks  135  are caused by the ASE from the SOA  130  propagating back through the gain section  104  towards the rear mirror  102 , reflecting from the rear mirror  102 , propagating again through the gain section  104  and the SOA  130 . This double-pass amplification of the ASE in the gain section  104  and, at least partially, in the SOA  130 , results in a reduction of the SMSR to a value of only 40 dB. The SMSR value of 40 dB may be insufficient in wavelength-agile applications. 
         [0047]    Referring now to  FIG. 3 , a tunable laser device  300  may be provided as described below. For example, the tunable laser device  300  embodiment includes optically coupled (in sequence) a tunable laser section  302 , a tunable transmission optical filter  304 , and a SOA section  306 . The tunable laser section  302  may include an optical cavity  303  for tuning a lasing wavelength λ output  within a tuning range Δλ spanning from a first wavelength λ 1  to a second wavelength λ 2 &gt;λ 1 . The optical cavity  303  may include from  311  and rear  332  mirrors. The tunable transmission optical filter  304  is disposed outside of the optical cavity  303  and downstream of the tunable laser section  302 . The tunable transmission optical filter  304  has a passband for transmitting light at the lasing wavelength Xλ output , and a stopband for attenuating light at a sidelobe wavelength λ S  of the tunable laser section, different from the lasing wavelength λ output . Both the lasing λ output  and sidelobe λ S  wavelengths are within the tuning range Δλ. The SOA section  306  is disposed downstream of the tunable transmission optical filter  304 . The SOA section  306  has an amplification band including the tuning range Δλ. 
         [0048]    In operation, the tunable laser section  302  generates light at the lasing wavelength λ output . The tunable transmission optical filter  304  transmits the light at the lasing wavelength λ output  while attenuating light at a sidelobe wavelength λ S . The SOA  306  may amplify the laser light, producing an output laser beam  309 . ASE  308  at the sidelobe wavelength λ S  generated by the SOA section  306  may propagate through the tunable transmission optical filter  304 , gets attenuated by the tunable transmission optical filter  304 , reflects from the rear mirror  312 , propagates again through the tunable transmission optical filter  304 , and gets attenuated again. In accordance with one embodiment, the double attenuation of the ASE  108  at the sidelobe wavelength λ S  by the tunable transmission optical filter  304  may result in a considerable SMSR improvement. Of course, not only one sidelobe wavelength λ S , but many such wavelengths different from the lasing wavelength λ output  within the stopband may be attenuated by the tunable transmission optical filter  304 , depending on wavelength selective properties of the optical cavity  303  and a spectral shape of the tunable transmission optical filter  304 . 
         [0049]    Turning to  FIG. 4A , a tunable monolithic laser device  400  is a preferred embodiment of the tunable monolithic laser device  300  of  FIG. 3 . An optical cavity  403  of the tunable monolithic laser device  400  of  FIG. 4  includes front  411  and rear  412  tunable sampled grating minors having different tunable periods of corresponding reflection wavelengths for tuning the lasing wavelength λ output  via Vernier effect. The tunable laser section  402  includes a gain section  405  and a phase section  407  optically coupled between the front  411  and rear  412  tunable sampled grating mirror. A main function of the gain section  405  is to provide optical gain at the lasing wavelength λ output . A main function of the phase section  407  is to adjust the optical path length of the optical cavity  403  to provide an efficient wavelength tuning. The tunable monolithic laser device  400  further includes a tunable transmission optical filter  404  and an SOA section  406 . The tunable laser section  402 , the tunable transmission optical filter  404 , and the SOA section  406  form a monolithic structure. By way of example, me tunable laser section  402 , the tunable transmission optical filter  404 , and the SOA section  406  may be disposed, and monolithically formed, on a common semiconductor substrate, not shown. 
         [0050]    In the embodiment of  FIG. 4A , the tunable transmission optical filter  404  is implemented as an asymmetric Mach-Zehnder waveguide interferometer  404 A including an input port  421  optically coupled to the front tunable sampled grating mirror  411 , an output port  422  optically coupled to the SOA section  406 , first  431  and second  432  branch waveguides having different optical path lengths, an input coupler  441  configured to optically couple the input port  421  to the first  431  and second  432  branch waveguides, and an output coupler  442  configured to optically couple the first  431  and second  432  branch waveguides to the output port  422 . For tuning, the asymmetric tunable Mach-Zehnder waveguide interferometer  404 A includes phase adjusters  433  and  434 , configured to adjust an optical path length difference between the first  431  and second  432  branch waveguides. At least one phase adjuster  433  or  434  may be provided. 
         [0051]    In operation, the front  411  and rear  412  tunable sampled grating mirrors are tuned to have a reflection overlap at a particular desired lasing wavelength λ output . The gain section  405  provides sufficient optical gain to overcome losses in the optical cavity  403 . The phase section  407  may be tuned to place a longitudinal mode of the optical cavity  403  at a maximum reflection wavelength of the overlapping reflection peaks of the front  411  and rear  412  tunable sampled grating mirrors. Laser light  409  propagates through the asymmetric Mach-Zehnder waveguide interferometer  404 A and may be amplified by the SOA section  406 . 
         [0052]    Referring to  FIG. 4B  with further reference to  FIG. 4A , the asymmetric Mach-Zehnder waveguide interferometer  404 A ( FIG. 4A ) may be tuned by the phase adjusters  433  and  434  to have a transmission minimum  471  ( FIG. 4B ), corresponding to a center wavelength of a stopband  471 A, at the sidelobe wavelength λ S . It is also possible, and in fact more practical, to tune a transmission maximum  472  of the asymmetric Mach-Zehnder waveguide interferometer  404 A to the lasing wavelength λ output . These two conditions can be simultaneously achieved when the free spectral range of the asymmetric Mach-Zehnder waveguide interferometer  404 A is twice the separation between λ output  and λ S . Since SMSR is typically most degraded at the short wavelength side of the tuning range of the monolithic laser device  400 , it is desirable to select the free spectral range to be twice the separation between the shortest required lasing wavelength and λ S  as shown in  FIG. 4B . 
         [0053]    For λ output  corresponding to worst-case SMSR condition ( FIG. 4B ), the minimum transmission point  471  is preferably tuned to be close to a peak gain point of the SOA section  406  ( FIG. 4A ). The SOA section  406  typically has an approximately parabolic gain spectrum described by a peak gain at a center wavelength, falling off at other wavelengths with a roughly parabolic dependence. Because of this, an emission spectrum  480  of the tunable monolithic laser device  400 , in the absence of the asymmetric Mach-Zehnder waveguide interferometer  404 A, would include not only the Using wavelength λ output , but also a plurality of side peaks  435  due to reflection of back-propagated ASE from the back mirror  412 , as explained above. Referring to  FIG. 4C  with further reference to  FIGS. 4A and 4B , the asymmetric Mach-Zehnder waveguide interferometer  404 A ( FIG. 4A ) suppresses the side peaks  435  ( FIG. 4C ), especially those in the vicinity of the minimum transmission point  471  ( FIGS. 4B, 4C ), corresponding to the stopband  471 A center wavelength. As seen by comparing  FIGS. 4B and 4C , the SMSR improves from 40 dB to 50 dB, that is, by 10 dB. 
         [0054]    From the perspective of spectral purity, a tunable transmission optical fitter should have a narrow single-peak pass-band, less than the back mirror peak spacing of the laser device  300  or  400 . A sharp transmission roll-off, low transmission in the stop band, and wide tunability across the entire amplification band of the SOA section  306  or  406  band are also desired. However, narrowband transmission optical filters are usually large in size. In contrast, broadband filters may be made more compact, simplifying monolithic integration of the tunable monolithic laser device  400  on a substrate, not shown. As an example, filters having a passband at 3 dB level of at least 40% of the free spectral range may be used. An asymmetric Mach Zehnder waveguide filter has a sinusoidal transmission spectrum with a 3 dB transmission bandwidth of half its free spectral range. Preferably, the free spectral range approximately equals twice the maximum detuning between the laser wavelength and the gain peak wavelength. This amounts to 50˜60 nm for a full-band tunable laser. 
         [0055]    Other types of tunable transmission optical filters may be monolithically integrated into the tunable monolithic laser device  300  of  FIG. 3 and 400  of  FIG. 4 . By way of a non-limiting example, the tunable transmission optical tillers  304  and  404  may include a grating-assisted co-directional coupler or a tunable multimode interference coupler. 
         [0056]    Referring now to  FIG. 5 , a laser source  550  includes a tunable monolithic laser device  500  coupled to a controller  555 . The tunable monolithic laser device  500  is a variant of the tunable monolithic laser device  400  of  FIG. 4A . The tunable monolithic laser device  500  of  FIG. 5  may include the tunable laser section  402 , an asymmetric tunable Mach-Zehnder waveguide interferometer  504 A coupled to the tunable laser section  402 , and the SOA section  406  coupled to the asymmetric tunable Mach-Zehnder waveguide interferometer  504 A. The asymmetric tunable Mach-Zehnder waveguide interferometer  504 A may include input  541  and output  542  couplers connected by a pair of branch waveguides  531  and  532 . Preferably, one of, or both input  541  and output  542  couplets are 2×2 couplers, e.g. directional or 2×2 multimode interference couplers, so that optional first  561  and second  562  photodetectors may be coupled to free waveguides of the respective input  541  and output  542  2×2 couplers. The controller  555  may be operationally coupled to the tunable laser section  402 , the Mach-Zehnder waveguide interferometer  504 A, the SOA section  406 , and the optional photodetectors  561  and  562 . The controller  555  may be configured, e.g. programmed, to tune the lasing wavelength λ output  of the tunable laser section  402 , and to tune the center wavelength of the passband of the Mach-Zehnder waveguide interferometer  504 A to correspond to the lasing wavelength λ output . The Mach-Zehnder waveguide interferometer  504 A may also be tuned to suppress the side peaks  435  ( FIGS. 4B, 4C ) to increase the SMSR. When the tree spectral range of the Mach-Zehnder waveguide interferometer  504 A is properly selected, the conditions of sufficiently high transmission and sufficiently high SMSR may be satisfied simultaneously in most cases. 
         [0057]    The Mach-Zehnder waveguide interferometer  504 A is typically tuned by adjusting a tuning parameter such as the optical path length difference between the branch waveguides  531  and  532 . As noted above, it may be more practical to merely maximize the output optical power at the lasing wavelength λ output . To that end, the controller  555  may be configured to lessen an optical power level of light detected by the second photodetector  562 . When the optical power level is minimized, all generated optical power is coupled to the SOA section  406 , thus maximizing the transmission of the asymmetric tunable Mach-Zehnder waveguide interferometer  504 A at the lasing wavelength λ output . The controller  555  may also monitor the forward voltage of the SOA section  406 , or the reverse photocurrent of the SOA section  406  (when the SOA section  406  is temporarily operated under reverse bias to function as a photodetector), to determine the Mach-Zehnder waveguide interferometer  504 A tuning condition for maximum optical transmission. 
         [0058]    Different tunable filter geometries may be used to suppress back-propagating ASE from the SOA section  406 . Turning to  FIG. 6 , a tunable monolithic laser device  600  is a variant of the tunable monolithic laser device  300  of  FIG. 3, 400  of  FIG. 4 , or  500  of  FIG. 5 . The tunable monolithic laser device  600  of  FIG. 6  includes the tunable laser section  402 , an asymmetric cascaded tunable Mach-Zehnder waveguide interferometer  604 A coupled to the tunable laser section  402 , and the SOA section  406  coupled to the asymmetric cascaded tunable Mach-Zehnder waveguide interferometer  604 A. The asymmetric cascaded Mach-Zehnder waveguide interferometer  604 A may include, for example, first  681  and second  682  Mach-Zehnder stages. The cascaded Mach-Zehnder waveguide interferometer  604 A may have a wider suppression spectral band than a single Mach-Zehnder interferometer, and thus it may provide a better SMSR. More than two stages, for example two, three and four stages, may be used. 
         [0059]    To provide a high level of transmission at the lasing wavelength λ output  while suppressing the side peaks  425  ( FIGS. 4B and 4C ), a free spectral range of the asymmetric tunable Mach-Zehnder waveguide interferometers  404 A of  FIG. 4A, 504A  of  FIG. 5, and 604A  of  FIG. 6  may be selected to be substantially equal to the tuning range Δλ. Another guideline may be to have the free spectral range substantially equal to twice a separation between the first wavelength λ 1  and a center of the amplification band of the SOA section  406 . This allows one to maximize transmission at the lasing wavelength λ output , while suppressing the side peaks  425  where the side peaks  425  are the strongest—see, for example,  FIG. 4C . 
         [0060]    Method of calibration and operation of a tunable laser device comprising coupled in sequence a tunable laser section, a tunable transmission optical filter, and a semiconductor optical amplifier section e.g. the laser device  300  of  FIG. 3, 400  of  FIG. 4 , the laser source  550  of  FIG. 5 , or the laser device  600  of  FIG. 6  will now be considered. Referring to  FIG. 7 , a method  700  for calibrating e.g. the laser source  550  of  FIG. 5  starts at  701 . In a step  702 , the lasing wavelength λ output  may be tuned to a calibration wavelength λ C  within the inning range Δλ. When the step  702  is completed, then in a next step  704 , the passband center wavelength of the asymmetric Mach-Zehnder interferometer  504 A may be scanned in a step  704  by adjusting a tuning parameter, such as a tuning current or voltage applied to the phase adjusters  433  and/or  434 . As the passband center wavelength is scanned, the controller  555  determines the current output optical power and/or the current SMSR in a step  706 . When the scanning is complete, then the controller  555  selects in a step  708  a value of the tuning parameter scanned in the scanning step  704 , and/or a value of the center wavelength scanned in the step  704 , corresponding to a maximum output optical power and/or maximum SMSR determined in the SMSR calculation step  706 . Then, in a step  710 , the value of the tuning parameter and/or the value of the center wavelength selected in the step  708  may be associated with the calibration wavelength tuned to in the first step  702 . The steps  702  to  710  may be repeated in a step  712  for a grid of calibration wavelengths λ G . For a wavelength within the tuning range Δλ but not equal to any of the grid calibration wavelengths λ G  of the step  712 , a value of the corresponding tuning parameter and/or the value of the center wavelength may be determined in a step  714  by interpolation between two nearest calibration wavelengths λ G  of the grid. The method  700  ends at  715 . As explained above, when the free spectral range of the asymmetric Mach-Zehnder interferometer  504 A is properly selected, determining maximum output optical power (step  706 ) may be sufficient for optimizing SMSR. 
         [0061]    Turning to  FIG. 8 , an embodiment of a method  800  for generating light includes a step  802  of providing a tunable laser device comprising coupled in sequence a tunable laser section, a tunable transmission optical filter, and a semiconductor optical amplifier section, e.g. the laser device  300  of  FIG. 3, 400  of  FIG. 4 , or  500  of  FIG. 5 ;  600  of  FIG. 6 ; or providing the laser source  550  of  FIG. 5 . The laser devices  300 ,  400 ,  500 ,  550 , or  600  may be calibrated in a step  804  using the method  700  of  FIG. 7 . In a next step  806 , the laser may be energized; and the lasing wavelength λ output  may be tuned to a first working wavelength within the tuning range Δλ. Then, in a step  808 , the center wavelength of the stopband of the tunable transmission optical filter  304  or  404  may be tuned so as to increase the output optical power and/or the SMSR at the first working wavelength. The SOA section  406  may be energized in a step  810 . In an embodiment where tuning the tunable transmission optical filter  304  or  404  in the step  808  causes a maximum transmission wavelength to not be equal to the first working wavelength, resulting in an extra optical loss in the tunable transmission optical filter  304  or  404 , the SOA section  406  may be energized in the step  810  to a level of amplification sufficient to compensate for the extra optical loss. Should a closed-loop control be required to compensate for aging-induced drifts of the laser devices  300 ,  400 ,  500 ,  550 , or  600 , a dither of a phase of the tunable transmission optical filter  304 ,  404  about the bias condition can be applied to stay locked to a local minimum or maximum of a parameter being monitored, such as output optical power, SOA current, SMSR, etc. 
         [0062]    The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gale array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
         [0063]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.