Abstract:
A laser, including a grating structure having two or more gratings having a plurality of different wavelength peaks for reflection of optical radiation therefrom. The laser further includes a semiconductor device, having an active region which is operative to amplify the optical radiation, and a reflective region, which is adapted to reflect the optical radiation at a tunable resonant wavelength of the reflective region, the device being optically coupled to the grating structure so as to define a laser cavity having a single cavity mode defined by tuning the resonant wavelength of the reflective region to overlap with one of the wavelength peaks of the grating structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/177,405, filed Jan. 20, 2000, which is assigned to the assignee of the present patent application and is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to lasers, and specifically to stabilization of lasers operating in multiple modes.  
         BACKGROUND OF THE INVENTION  
         [0003]    [0003]FIG. 1 is a schematic diagram showing operation of a lasing system  18 , as is known in the art. System  18  comprises two mirrors  20  and  22  separated by a distance L. In order for system  18  to lase i.e., to resonate, at a wavelength λ, a medium  24  between mirrors  20  and  22  must provide gain, and an effective optical path length L eff  between the mirrors must be an integral number of half-wavelengths. Quantitatively,  
           L eff =nL  (1a)  
           [0004]    so that  
             m·λ/ 2= nL   (1b)  
           [0005]    or  
             f=m·c/ (2 nL )  (1c)  
           [0006]    wherein m is a positive integer, n is a refractive index of medium  24 , f is the frequency corresponding to the wavelength λ, and c is the speed of light.  
           [0007]    From equation (1c), a separation Δf of lasing frequencies is given by  
           Δ f=c/ (2 nL )  (2)  
           [0008]    Each such lasing frequency corresponds to a longitudinal cavity mode. Since f=c/λ, Δf≈−c·Δλ/λ 2  so that equation (2) can be rewritten to give a separation Δλ of lasing wavelengths:  
           Δλ≈λ 2 /(2 nL )  (3)  
           [0009]    [0009]FIG. 2 is a graph of intensity I vs. wavelength k illustrating cavity modes for system  18 , as is known in the art. A curve  30  represents an overall gain of medium  24  in system  18 . Peaks  32 A and  32 B, with separation AX, show the cavity modes present in system  18 , each node corresponding to a different value of m. As is evident from FIG. 2, there are many possible cavity modes for system  18 .  
           [0010]    Optical communications within fiber optic links require that the laser carrier have as small a frequency spread as possible, particularly when multiple wavelengths are to be multiplexed on a single fiber. Thus, for efficient communication only one cavity mode should be used, and optimally the frequency spread within the mode should be minimized. Typically, methods for stabilizing the frequency of the laser include utilizing distributed feedback (DFB) lasers and/or distributed Bragg reflector (DBR) lasers. DFB lasers have a frequency-selection grating built into the laser chip, the grating being physically congruent with the gain medium. The grating in a DBR laser is external to the gain medium. The gratings in DFB and DBR lasers are part of the semiconductor material, which is unstable. DFB and DBR lasers are therefore typically externally stabilized utilizing an external wavelength reference in order to achieve good stability.  
           [0011]    [0011]FIG. 3 shows the effect of adding a tuning element such as a fiber grating to system  18 , as is known in the art. A curve  34  shows the resonance curve of the fiber grating, which has a bandwidth Δλ G  of the same order as Δλ, the separation between the longitudinal cavity modes. If the grating is optically coupled to system  18 , then mode  32 A is present, and other modes such as mode  32 B, are suppressed.  
           [0012]    [0012]FIG. 4 is a schematic diagram showing a gain medium  38  coupled to a fiber grating  50 , as is known in the art. Gain medium  38  is formed from a semiconductor gain element  44  having a laser gain region  42 . Light from region  42  exits from a facet  56  of region  42  to a medium  46 , and traverses medium  46  so that a lens  48  collects the light into a fiber optic  52 . Fiber grating  50  is mounted in fiber optic  52 , which grating reflects light corresponding to curve  34  of FIG. 3 back to region  42 . The mirrors of the laser cavity comprise a rear mirror which in this example is a back facet  57  of the semiconductor gain element, and an output coupling mirror which in this example is fiber grating  50 . The rear and output coupling mirrors could also be reversed. In the reversed configuration the rear mirror would be the fiber grating and the output coupling mirror would be back facet  57  of the semiconductor gain element.  
           [0013]    It is desirable to eliminate parasitic reflections due to surfaces and interfaces internal to the cavity. To eliminate parasitic reflection from the facet of the semiconductor closest to the fiber grating, in this case facet  56 , that facet is usually anti reflection coated. It is also useful to anti reflection coat a tip  49  of the fiber closest to the semiconductor gain element to again reduce parasitic reflections. Preferably, grating  50  is written directly at the end of the fiber optic facing the laser. Alternatively, a length L f  of a fiber  63  is interposed between lens  48  and fiber optic grating  50 . Thus region  42 , medium  46 , fiber optic  63  and grating  50  form a resonant system  60  corresponding to region  24  of FIG. 1. This architecture is generally known in the art as an external cavity laser or more specifically as a fiber grating laser (FGL).  
           [0014]    System  60  has an effective optical path length L eff  given by:  
             L   eff=   n   1   ·L   1   +n   0   ·L   0   +n   f   ·L   f   +n   g   ·L   gef   (4)  
           [0015]    wherein  
           [0016]    n 1  is a refractive index of region  42 ;  
           [0017]    L 1  is a length of region  42 ;  
           [0018]    n 0  is a refractive index of medium  46 ;  
           [0019]    L 0  is a length of medium  46 ;  
           [0020]    n f  is a refractive index of fiber  63 ;  
           [0021]    L f  is the length of fiber  63 .  
           [0022]    n g  is a refractive index of grating  50 ; and  
           [0023]    L gef  is an effective length of grating  50 .  
           [0024]    Replacing the optical path length nL of equation (1b) by that given by equation (4) leads to the following equation giving cavity modes for the system of FIG. 3:  
             m λ/ 2=( n   1   ·L   1   +n   0   ·L   0   +n   f   ·L   f   +n   g   ·L   gef )  (5)  
           [0025]    In constructing system  60 , it is necessary to adjust and maintain the positions of curve  32 A and  34  (FIG. 3) to have their peaks at the same wavelength. Changes in temperature and/or changes in injection current into region  42  and/or mechanical changes affect one or more parameters of the optical path length given by equation (4). Such changes can thus cause mode hopping, which refers to the phenomena whereby mode  32 A shifts underneath resonance curve  34  of the fiber grating. When that shift is large enough, an adjacent mode will at some point experience a larger gain and start to lase. These mode hops occur underneath the resonance curve of the fiber grating (curve  34  in FIG. 3) resulting in wavelength shifts and intensity noise when the mode hops. For example, referring to FIG. 3, mode  32 B could shift within resonance curve  34  of the fiber grating and resonate instead of mode  32 A.  
           [0026]    An article titled “1.5-1.6 μm dynamic-single-mode (DSM) lasers with distributed Bragg reflector,” by Koyama et al., in the Vol. 19 (1983) issue of  IEEE Journal of Quantum Electronics , which is incorporated herein by reference, describes a method for tuning a DBR by injecting current.  
           [0027]    Super structure grating (SSGs) are known in the art as structures which comprise a plurality of gratings distributed along a fiber in a manner such as to provide a spectral response with several peaks. An article titled “Long periodic superstructure Bragg gratings in optical fibers,” by Eggleton et al., in the Vol. 30 (1994) issue of Electronics Letters, which is incorporated herein by reference, describes one such SSG. An SSG in a fiber provides a system having a plurality of relatively highly stable fixed wavelengths.  
           [0028]    SSG systems have been implemented in semiconductor devices, to take advantage of the fact that when implemented therein the cavities so formed are tunable. An article titled “Theory, design and performance of extended tuning range semiconductor lasers with sampled gratings,” by Jayaraman et al., in the Vol. 29 (1993) issue of  IEEE Journal of Quantum Electronics , which is incorporated herein by reference, describes two such systems which are tuned in a vernier-like manner. Unfortunately, due to the inherent characteristics of the semiconductor, SSG devices implemented in semiconductors are relatively unstable, as is also true for single wavelength DBRs implemented therein.  
           [0029]    The information carrying capacity of a single lasing line can be increased by increasing the frequency of modulation of the laser. However such increased frequency widens the laser line width through chirp, which in turn reduces the transmission range due to dispersion. In order to circumvent the effects of dispersion, a method known in the art is to use wavelength division multiplexing (WDM) wherein a plurality of laser lines are each modulated at an intermediate bandwidth. The total bandwidth is then the number of laser lines multiplied by the intermediate bandwidth. For example, instead of modulating one line at 10 Gbit/s, four lines can each be modulated at 2.5 Gbit/s to provide the same information carrying capacity.  
           [0030]    Standard ITU-T G. 692 of the International Telecommunications Union (ITU), Place des Nations CH-1211, Geneva 20, Switzerland, defines allowable wavelengths for WDM systems, so that systems implemented by different manufacturers will be compatible with each other. Systems known in the art for implementing WDM use a plurality of lasers, each having a different fixed wavelength corresponding to the ITU standard.  
         SUMMARY OF THE INVENTION  
         [0031]    It is an object of some aspects of the present invention to provide improved methods and apparatus for generating a plurality of laser wavelengths.  
           [0032]    In preferred embodiments of the present invention, a semiconductor device comprises an active gain region and a distributed Bragg reflector (DBR), which acts as a first, highly-reflecting mirror at one end of a laser cavity that contains the gain region. The DBR is tuned by, most preferably, varying a current injected into the DBR. The active gain region is coupled at the side opposite the DBR to a fiber optic comprising a super structure grating (FO-SSG) having a plurality of relatively highly stable resonant peaks, the peaks most preferably being separated substantially equidistantly. The FO-SSG acts as a second, partially-reflecting mirror at the other end of the laser cavity. An output of the cavity is derived from light transmitted by the FO-SSG into the fiber optic.  
           [0033]    To operate the laser, the gain region is activated, and the DBR is tuned so that a resonant peak of the DBR is aligned with one of the resonant peaks of the FO-SSG. Thus, the laser resonates in a single cavity mode defined by the two resonant peaks, and all other modes are substantially suppressed. By scanning the tuning of the DBR over the range of the FO-SSG, all the different resonant peaks of the FO-SSG may be selected at will, producing corresponding single cavity modes. Coupling a DBR with an FO-SSG combines the advantages of tunability associated with the DBR and stability associated with the FO-SSG for all modes of the cavity. Preferred embodiments of the present invention thus enable a single laser to be used as a generator in a WDM system.  
           [0034]    In some preferred embodiments of the present invention, the DBR is written as a super structure grating (DBR-SSG) within the semiconductor device. The spacing of peaks of the DBR-SSG is implemented to be slightly different from the spacing of the peaks of the FO-SSG. The cavity produced by the combination of the DBR-SSG with the FO-SSG is then tuned in a vernier-like manner, by setting one of the peaks of the DBR-SSG to align with one of the peaks of the FO-SSG. Because of the vernier-like spacing relationship between the two SSGs, all other peaks, apart from the aligned pair, are misaligned, so that only one mode defined by the aligned pair resonates and all other modes are suppressed. All the resonant peaks of the FO-SSG may be selected by scanning the DBR-SSG over a range that is substantially the same as the spacing of two peaks of the DBR-SSG.  
           [0035]    In some preferred embodiments of the present invention, the laser cavity is stabilized by thermally modulating one or more optical elements, and/or parameters thereof, so as to vary an effective length of the cavity. A method of thermal modulation is described in detail in PCT patent application PCT/IL00/00401 which is assigned to the assignee of the present invention and which is incorporated herein by reference. The modulation generates an error signal which is dependent on a relationship of the oscillating mode with resonant frequency of the cavity, and the error signal is used in a negative feedback loop to ensure that the mode and resonant frequency substantially coincide.  
           [0036]    There is therefore provided, according to a preferred embodiment of the present invention, a laser, including:  
           [0037]    a grating structure, including two or more gratings having a plurality of different wavelength peaks for reflection of optical radiation therefrom; and  
           [0038]    a semiconductor device, including an active region, which is operative to amplify the optical radiation, and a reflective region, which is adapted to reflect the optical radiation at a tunable resonant wavelength of the reflective region, the device being optically coupled to the grating structure so as to define a laser cavity having a single cavity mode defined by tuning the resonant wavelength of the reflective region to overlap with one of the wavelength peaks of the grating structure.  
           [0039]    Preferably, the grating structure includes a super structure grating (SSG) written in a fiber optic.  
           [0040]    Preferably, fiber optic includes a lens which focuses optical radiation from the semiconductor device to the grating structure.  
           [0041]    Preferably, the two or more gratings are adapted to partially transmit optical radiation at the different wavelengths, so as to provide output optical radiation.  
           [0042]    Preferably, the reflective region includes a Distributed Bragg Reflector (DBR) written onto the semiconductor device, wherein the resonant wavelength of the reflective region is tuned by a current injected into the DBR.  
           [0043]    Preferably, the plurality of different wavelength peaks are substantially equidistantly spaced by a first separation, wherein the reflective region includes a Distributed Bragg Reflector with a super structure grating (DBR-SSG) having a plurality of different wavelength peaks substantially equidistantly spaced by a second separation different from the first separation, so that the first separation is related to the second separation in a vernier-like manner and so that the single cavity mode is defined when one of the grating structure wavelength peaks overlaps with one of the DBR-SSG wavelength peaks  
           [0044]    Preferably, the laser includes:  
           [0045]    an optical length changer which varies an optical length of at least one of a group of optical elements including the grating structure the active region and the reflective region, so as to vary accordingly an optical length of the laser cavity;  
           [0046]    a detector which is adapted to monitor a level of the optical radiation responsive to the variation in the optical length of the at least one of the group; and  
           [0047]    a stabilizer which responsive to the measured output from the detector supplies a control signal to the optical length changer to control an optical length of at least one of the group, so that the laser cavity resonates stably in the single cavity mode.  
           [0048]    Further preferably, the optical length changer includes at least one of a thermally active group comprising a heater and a thermoelectric cooler, wherein the at least one of the thermally active group is adapted to alter a temperature of at least one of the group of optical elements.  
           [0049]    There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including:  
           [0050]    providing a grating structure having a plurality of different wavelength peaks for reflection of optical radiation therefrom;  
           [0051]    optically coupling a semiconductor device to the structure so as to define a laser cavity between the structure and a reflective region of the device, which is adapted to reflect the optical radiation at a tunable resonant wavelength of the reflective region; and  
           [0052]    tuning the resonant wavelength of the reflective region to overlap with one of the wavelength peaks of the grating structure so as to generate a laser output in a single cavity mode defined by the overlap.  
           [0053]    Preferably, providing the grating structure includes writing a super structure grating (SSG) in a fiber optic.  
           [0054]    Preferably, providing the grating structure includes providing two or more gratings which are adapted to partially transmit optical radiation at the different wavelength peaks, so as to provide output optical radiation.  
           [0055]    Preferably, the reflective region includes a Distributed Bragg Reflector (DBR) written onto the semiconductor device, wherein tuning the resonant wavelength of the reflective region includes injecting a current into the DBR.  
           [0056]    Preferably, the reflective region includes a Distributed Bragg Reflector with a super structure grating (DER-SSG) having a plurality of different DBR-SSG wavelength peaks substantially equidistantly spaced by a first separation, wherein providing the grating structure includes substantially equidistantly spacing the plurality of different wavelength peaks by a second separation different from the first separation and related to the first separation in a vernier-like manner, and wherein tuning the resonant wavelength includes overlapping one of the grating structure wavelength peaks with one of the DBR-SSG wavelength peaks  
           [0057]    Preferably the method includes:  
           [0058]    varying with an optical length changer an optical length of at least one of a group of optical elements including the grating structure, an active region of the semiconductor device, and the reflective region, so as to vary accordingly an optical length of the laser cavity;  
           [0059]    monitoring a level of the optical radiation responsive to the variation in the optical length of the at least one of the group; and  
           [0060]    supplying a control signal to the optical length changer responsive to the monitored level so as to control an optical length of at least one of the group, so that the laser cavity resonates stably in the single cavity mode.  
           [0061]    Further preferably, varying the optical length includes altering a temperature of at least one of the group of optical elements using at least one of a thermally active group comprising a heater and a thermoelectric cooler.  
           [0062]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAW S  
       [0063]    [0063]FIG. 1 is a schematic diagram showing operation of a laser system, as is known in the art;  
         [0064]    [0064]FIG. 2 is a graph of intensity vs. wavelength, illustrating cavity modes for the system of FIG. 1, as is known in the art;  
         [0065]    [0065]FIG. 3 shows the effect of adding a spectrally selective element such as a fiber grating to the system of FIG. 1, as is known in the art;  
         [0066]    [0066]FIG. 4 is a schematic diagram showing a semiconductor gain medium coupled to a fiber grating forming a fiber grating laser (FGL), as is known in the art;  
         [0067]    [0067]FIG. 5 is a schematic diagram of a laser system, according to a preferred embodiment of the present invention;  
         [0068]    [0068]FIG. 6 shows schematic graphs of intensity vs. wavelength relationships for different elements of the system of FIG. 5, according to a preferred embodiment of the present invention;  
         [0069]    [0069]FIG. 7 is a schematic diagram of a laser system, according to an alternative preferred embodiment of the present invention;  
         [0070]    [0070]FIG. 8 shows schematic graphs of intensity vs. wavelength relationships for different elements of the system of FIG. 7, according to a preferred embodiment of the present invention; and  
         [0071]    [0071]FIG. 9 is a schematic diagram of apparatus for locking a longitudinal mode of the system of FIG. 5 to a super structure grating peak, according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0072]    Reference is now made to FIG. 5, which is a schematic diagram of a laser system  70 , according to a preferred embodiment of the present invention. A semiconductor device  72  comprises a gain medium  74 , within which is formed a laser gain region  76  which acts as an active region amplifying optical radiation. Device  72  is generally box-shaped, and has front and back facets  78  and  80  which are most preferably anti-reflection coated in order to eliminate parasitic reflections within region  76 . Adjacent to facet  80 , region  76  comprises a Distributed Bragg Reflector (DBR) section  82 , which is written onto device  72  by a photolithographic technique, as is known in the art. Distributed Bragg Reflector (DBR) section  82  has a relatively broad spectral response curve. The relationship between the curve of DBR  82  and resonant curves of other elements of system  70  is described below with reference to FIG. 6. During operation of system  70 , DBR section  82  acts as a reflective region which is a substantially fully reflecting first mirror of a resonant cavity  84 .  
         [0073]    Cavity  84  also comprises a fiber optic  86 , the fiber optic most preferably supporting single mode transmission. Fiber optic  86  comprises a super structure grating (SSG)  88  which is written in the fiber optic. SSG  88  acts as a grating structure which is implemented to have a plurality of gratings  88 A,  88 B,  88 C, . . . , with dead zones in between. The gratings may be of different lengths as may be the dead zones. The plurality of gratings  88 A,  88 B,  88 C, . . . and dead zones of SSG  88  provide a device with several spectral reflection features as will be described below. If system  70  is to be used in a wavelength division multiplexing (WDM) system, these spectral reflection features are most preferably written to correspond to wavelengths of the WDM system. Fiber optic  86  is coupled to device  72  by methods known in the art, for example, by using a lens  90  between the fiber optic and device  72  or by butting the fiber optic to facet  78 . Preferably, if the fiber optic is butted to device  72 , the device comprises a mode converter for better collection efficiency between the semiconductor and the fiber. The mode converter is most preferably implemented in the semiconductor, preferably by an architecture known in the art as a taper. SSG  88  acts as a partially reflecting second mirror of cavity  84 , so that light transmitted through the SSG into fiber optic  86  is output light of system  70 .  
         [0074]    Device  72  comprises a first upper electrode  96  and a second upper electrode  97 . Electrode  96  and lower electrode  94  are used to inject current into region  74  in order to cause region  76  to lase. Current injected via electrode  97  is varied so as to tune DBR section  82 , by methods known in the art.  
         [0075]    [0075]FIG. 6 shows schematic graphs of intensity vs. wavelength relationships for different elements of system  70 , according to a preferred embodiment of the present invention. A graph  100  corresponds to a fundamental gain curve of device  72 , according to the composition of the device. As described above in the Background of the Invention with reference to equations (1b), (2), and (3), cavity  84  has longitudinal cavity modes  102 A,  102 B,  102 C, . . . separated by Δλ, with wavelengths which are a function of an optical length between the mirrors of the cavity and the number of half-wavelengths comprising the mode. A graph  104  corresponds to the overall spectral reflection features of SSG  88 , wherein each peak  104 A,  104 B,  104 C, . . . of the graph is a relatively narrow resonant curve. Most preferably, each resonant curve is sufficiently narrow so that substantially only one longitudinal mode  102 A,  102 B,  102 C, . . . can resonate.  
         [0076]    A graph  106  corresponds to a resonant curve of DBR section  82 . DBR section  82  is preferably written in device  72  so that its resonant curve substantially encloses only one of the peaks of graph  104 . Thus in FIG. 6, longitudinal cavity mode  102 K will resonate since it is within the resonant curve of the SSG, at λ B , and section  82  is tuned to this wavelength. Modes such as  102 J,  102 H, and  102 L will be substantially suppressed since they are on the wings of graph  106 . As described above, DBR section  82  is tunable, so that for mode  102 L to resonate the section is tuned to lower wavelength λ A . Similarly, for modes  102 H,  102 F, and  102 C to resonate, section  82  is respectively tuned to higher wavelengths λ C , λ D , λ E . Thus system  70  can be effectively scanned from λ A  to λ E  by tuning DBR section  82  across the same wavelength range.  
         [0077]    [0077]FIG. 7 is a schematic diagram of a laser system  120 , according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of system  120  is generally similar to that of system  70  (FIG. 5), so that elements indicated by the same reference numerals in both systems  70  and  120  are generally identical in construction and in operation. Adjacent to facet  80  of device  72 , region  76  comprises an SSG  124  implemented in a DBR section  122 . SSG  124  comprises a plurality, preferably the same plurality as SSG  88 , of gratings  124 A,  124 B,  124 C, . . . Most preferably, the separation between adjacent resonant peaks of SSG  124  is different from the separation between adjacent resonant peaks of SSG  88 , and the two separations are related in a vernier-like manner, as is described in more detail hereinbelow with reference to FIG. 8. Setting the separations to be different enables system  120  to be scanned over the whole range of wavelengths of SSG  88  using a reduced wavelength range scan of DBR section  122 .  
         [0078]    [0078]FIG. 8 shows schematic graphs of intensity vs. wavelength relationships for different elements of system  120 , according to a preferred embodiment of the present invention. A graph  136  corresponds to a resonant reflection curve of DBR section  124 . For clarity, graph  136  is shown separate, i.e., not overlaid, from graphs  100  and  104 . Peaks  136 A,  136 B,  136 C, . . . of section  124  are assumed to be separated by a spacing Δ DBR , and peaks  104 A,  104 B,  104 C, . . . of SSG  88  are assumed to be separated by a different spacing Δ 80   F . In general, if SSG  88  comprises j gratings each having a resonant wavelength separated by Δλ F , and DBR section  124  comprises j spectral reflection peaks separated by Δλ DBR , then the separations should be implemented so as to satisfy:  
               Δλ   DBR     =         (       j   -   1     j     )     ·   Δ                     λ   F               (   6   )                               
 
         [0079]    Setting Δλ F  and Δλ DBR  to be related according to equation (6) allows a vernier effect to be used to accomplish the tuning, as explained hereinbelow.  
         [0080]    In the situation represented by FIG. 8, wherein by way of example j=5, mode  102 L will resonate, and other modes of system  102  will be substantially suppressed,  25  since peak  136 A is substantially aligned with peak  104 A, and no other peaks of graphs  104  and  136  align. To tune system  120  to mode  102 K, corresponding to peak  104 B, curve  136  needs to move right by δλ, where  
         δλ=Δλ F −Δλ DBR   (7)  
         [0081]    so that peak  136 B substantially aligns with peak  104 B, and the other peaks of graphs  104  and  136  do not align.  
         [0082]    Similarly, to tune system  120  to modes corresponding with peaks  104 C,  104 D, . . . , curve  136  needs to move right by 2δλ, 3δλ, . . . . The resonant peaks of graph  136  should have narrow enough widths so that substantially only one longitudinal mode of system  120  lases at each of the alignments of curves  104  and  136 . Thus, as illustrated by FIG. 8, system  12 C can be effectively scanned from λ A  to λ E  by tuning section  124  by a total of 4δλ.  
         [0083]    [0083]FIG. 9 is a schematic diagram of a system  150  for locking a longitudinal mode to an SSG peak, according to a preferred embodiment of the present invention. In implementing system  120  and system  70 , it is necessary to adjust respective DBR sections  124  and  82  so that all modes in the respective SSG are suppressed apart from one. Most preferably, the position of the relevant cavity mode should be adjusted to correspond with the aligned peaks of the SSG and DBR section. Once adjusted, it is necessary to maintain the mode in position. It will be appreciated that effects such as temperature change, change in injected current, and mechanical changes will tend to move the peak of the mode relative to the peaks of the SSG and the DBR, causing mode hopping. System  150  is implemented for system  70 , but the principles of system  150  apply also to system  120 . The implementation of a system substantially similar to system  150  is described in detail in the above-referenced PCT patent application.  
         [0084]    In system  150 , system  70  is mounted on a substrate  154 , beneath which is coupled a thermoelectric cooler (TEC)  152 . Above electrode  96  of system  70  is mounted a heater  156 . Heater  156  and TEC  152  may be adjusted either separately or together to alter temperatures of elements of cavity  84 , and so change optical lengths of elements of the cavity. System  150  further comprises a detector  160  which measures a parameter of light output from cavity  84 . Preferably, detector  160  measures the output from facet  80 . Alternatively, detector  160  is positioned at another point in system  150  where it is able to measure the parameter without substantially interfering with the operation of the system. Detector  160  supplies an error signal, generated responsive to the parameter output, to a wavelength stabilizer  158 , which acts as a controller of heater  156  and TEC  152 .  
         [0085]    Referring back to FIG. 5, an effective length of cavity  84  is given by an equation:  
           L   eff   =n   DBR   ·L   DBR   +n   g   ·L   g   +n   SSG   ·L   SSG   (8)  
         [0086]    wherein  
         [0087]    n DBR  is a refractive index of DBR region  82 ;  
         [0088]    L DBR  is a length of region  82 ;  
         [0089]    n g  is a refractive index of gain region  76 ;  
         [0090]    L g  is a length of region  76 ;  
         [0091]    n SSG  is a refractive index of SSG  88 ; and  
         [0092]    L SSG  is an effective length of SSG  88 .  
         [0093]    Stabilizer  158  preferably modulates the temperature of one or more of DBR region  82 , gain region  76 , and SSG  88 , so that their lengths and/or refractive indices, and  25  thus the effective length L eff  of cavity  84 , are modulated. The modulation is performed by stabilizer  158  sending modulation signals to heater  156  and/or TEC  152 . The parameter measured by detector  160  is a function of the modulation, such as a phase of the output compared to  30  the modulating input, in which case the phase is used by detector  160  to generate the error signal. The error signal is used by stabilizer  158  as a negative feedback control so as to shift, as required, a mode of cavity  84  to an SSG  88  peak.  
         [0094]    It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.