Abstract:
Apparatus for stabilizing an output wavelength of a laser assembly ( 80 ), including a plurality of optical elements ( 88, 92, 97, 96 ) coupled together so as to form a laser cavity resonating in a single mode dependent upon an optical length of the cavity, and an optical length changer ( 86 ) which varies an optical length of at least one of the optical elements so as to vary accordingly the optical length of the cavity. The apparatus further includes a detector ( 91 ) which monitors the output of the laser assembly responsive to the variation in the optical length of the at least one of the optical elements. There is also included a stabilizer ( 93 ) 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 optical elements, so that the cavity resonates stably at the output wavelength in the single mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application 60/142,677, filed Jul. 7, 1999, which is assigned to the assignee of the present patent application and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to lasers, and specifically to stabilization of lasers operating in a single mode. 
     BACKGROUND OF THE INVENTION 
     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 laser 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) 
       
     
     so that 
     
       
           m·λ/ 2= nL   (1b) 
       
     
     or 
     
       
           f=m·c /(2 nL )  (1c) 
       
     
     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. 
     From equation (1c), a separation Δf of lasing frequencies is given by 
     
       
         Δ f=c /(2 nL)   (2) 
       
     
     Each such lasing frequency corresponds to a longitudinal cavity mode. Since f=c/λ, Δf≅λ−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) 
       
     
     FIG. 2 is a graph of intensity I vs. wavelength λ illustrating cavity modes for system  18 , as is known in the art. A curve  30  represents an overall gain of medium  74  in system  18 . Peaks  32 A and  32 B, with separation Δλ, 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  1 X. 
     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 reflectors (DBR). 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 were therefore typically externally stabilized utilizing an external wavelength reference in order to achieve good stability. 
     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. 
     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 semiconducting 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 the fiber grating. 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. In the reversed configuration the detector would preferably be positioned behind the fiber grating. 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 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). 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) 
       
     
     wherein n 1  is a refractive index of region  42 ; 
     L 1  is a length of region  42 ; 
     n 0  is a refractive index of medium  46 ; 
     L 0  is a length of medium  46 ; 
     n f  is a refractive index of fiber  63 ; 
     L f  is the length of fiber  63 . 
     n g  is a refractive index of grating  50 ; and 
     L gef  is an effective length of grating  50 . 
     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) 
       
     
     In constructing system  60 , it is necessary to adjust and maintain the positions of curve  32 A and  34  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 A. 
     U.S. Pat. No. 4,786,132 to Gordon, whose disclosure is incorporated herein by reference, describes a semiconductor laser diode coupled to a single mode optical fiber. The fiber comprises a built-in Bragg reflector grating which reflects of the order of 50% of the light from the laser back to the laser. The reflected light provides feedback to the laser so that the laser produces a single frequency output. 
     U.S. Pat. No. 5,077,816 to Glomb et al., whose disclosure is incorporated herein by reference, describes a narrowband laser source, a portion of the light from which is supplied to a resonant grating region in a fiberoptic, external to the laser. The current through the laser is dithered, causing the frequency of the laser to dither. The correspond in dithered light intensity transmitted by the grating is used in order to adjust the current through the laser so as to maintain the frequency of the laser at the resonant frequency of the grating. 
     U.S. Pat. No. 5,706,301 to Lagerstrom, whose disclosure is incorporated herein by reference, shows a laser control system which uses a fiber optic grating as a resonant control element. A difference in light intensity between laser light passing through the grating, and light which does not pass through the grating is measured, and the difference is used in order to vary the temperature of a laser generating the light, so as to maintain the frequency of the laser at the resonant frequency of the grating. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide improved methods and apparatus for stabilization of the oscillating frequency of a laser. 
     In preferred embodiments of the present invention, a laser assembly comprises a semiconducting laser, a fiber grating, and an optical path coupling the laser and the grating. In order to stabilize the output of the laser assembly in a single cavity mode, the effective length of an optical cavity of the laser assembly is modulated about a man value by varying the optical length of at least one of the elements forming the laser assembly. A corresponding modulation of an intensity of the laser output is measured and is coupled in a feedback loop to control the optical length of the element in the laser assembly so as to provide the desired mode stabilization. 
     Most preferably, the laser and the grating are assembled on an optical bench. The fiber grating is tuned so that only a single resonating mode of the laser assembly is capable of sustaining oscillation, and an output of the single mode is provided via the fiber grating. The laser assembly acts as a resonant cavity, and the fiber grating acts as a wavelength reference within the resonant cavity. The effective cavity optical length is a function of an optical length of the semiconducting laser, an effective optical length of the fiber grating, and an optical length of the optical path coupling the grating and the laser. One or more of these lengths are controlled in order to stabilize the output of the laser assembly. 
     A difference between the modulation of the effective cavity optical length and the modulation in intensity, preferably a difference in phase, is used as an indicator of where the cavity mode of the laser assembly is oscillating relative to the resonant curve of the fiber grating. The indicator is used within the feedback loop to maintain the oscillation at the peak of the resonant curve of the fiber grating, by varying the mean value of the effective cavity optical length of the laser assembly. Choosing at least one optical length forming the effective cavity optical length and varying the chosen optical length in order to stabilize the laser output is an adaptable and accurate way to stabilize the laser. 
     In some preferred embodiments of the present invention, the effective optical length of the laser assembly is modulated by periodically varying a temperature of one of the elements of the assembly about a mean temperature, thereby causing the assembly to expand and contract. The mean value of the effective optical length is varied by varying the mean temperature of the element. 
     In some preferred embodiments of the present invention, the semiconducting laser is mounted on a thermally insulating element, and an electric heating element is placed between the laser and the insulating element. The insulating element has the effect of ensuring that a maximal temperature increase in the laser is attained for a given input electrical power to the heating element. Thus the heating element may be used to modulate the temperature and to change the mean temperature of the laser (or of one or more other elements within the laser assembly) in a controlled manner, and thus to modulate and change the mean value of the optical length of the one or more elements. 
     In some preferred embodiments of the present invention, at least some of the elements comprising the laser assembly are coupled to a thermoelectric cooler, which enables the temperature of the coupled elements to be changed. Changing the temperature of the fiber grating enables its resonant wavelength to be adjusted in a controlled manner. 
     There is therefore provided, according to a preferred embodiment of the present invention, apparatus for stabilizing an output wavelength of a laser assembly, including: 
     a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode dependent upon an optical length of the cavity; 
     an optical length changer which varies an optical length of at least one of the optical elements so as to vary accordingly the optical length of the cavity; 
     a detector which monitors the output of the laser assembly responsive to the variation in the optical length of the at least one of the optical elements; and 
     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 optical elements, so that the cavity resonates stably at the output wavelength in the single mode. 
     Preferably, the optical length changer includes a heating element which varies a temperature of at least one of the optical elements, thereby varying the optical length of the at least one of the optical elements. 
     Preferably, the heating element includes an electric heating element, which is supplied by a direct current component and an alternating current component in order to alter and modulate a mean temperature of at least one of the optical components. 
     Further preferably, the heating element dissipates a modulated power having a peak value less than or equal to about 200 mW. 
     Preferably, the heating element includes a heat insulating element, which selectively directs heat to the at least one of the optical elements. 
     Preferably, the heat insulating element includes silicon dioxide. 
     Preferably, the plurality of optical elements includes a semiconductor gain region and a fiber grating having a resonant wavelength. 
     Preferably, the at least one of the optical elements whose length is varied by the optical length changer includes the semiconductor gain region. 
     Preferably, the plurality of optical elements includes a medium optically coupling the semiconductor gain region and the fiber grating, and the at least one of the optical elements whose length is varied by the optical length changer includes the medium. 
     Preferably, the optical length of the cavity is varied to substantially lock the single mode of the cavity to the resonant wavelength. 
     Preferably, the optical length changer varies the optical length of the at least one of the optical elements so as to correspond to the resonant wavelength. 
     Further preferably, the apparatus includes a thermal transfer element which varies a temperature of at least one of the optical elements, thereby varying the optical length of the cavity. 
     Preferably, the thermal transfer element includes a cooling element, which is thermally coupled to the laser assembly and which extracts heat from the laser assembly so as to reduce an overall temperature of at least one of the plurality of optical elements. 
     Preferably, the cooling element is operated by the stabilizer, and the cooling element extracts heat from the laser assembly responsive to the measured output from the detector. 
     There is further provided, according to a preferred embodiment of the present invention, a method for stabilizing a laser assembly, the assembly including a plurality of elements each having a respective effective optical length, the plurality of elements forming a cavity resonating at a lasing wavelength in a single mode and having an effective cavity length, the method including: 
     modulating at least one of the effective optical lengths; 
     monitoring a radiation output of the assembly responsive to the modulation; and 
     adjusting the effective cavity length responsive to the output and to the modulation, so as to maintain the cavity resonating at the wavelength in the single mode. 
     Preferably, modulating the at least one of the effective lengths includes modulating a temperature of at least one of the plurality of elements. 
     Preferably, modulating the temperature includes providing a heating element which heats at least one of the plurality of elements so as to change the effective length of the at least one of the plurality of elements. 
     Further preferably, modulating the temperature includes providing a cooling element which cools at least one of the plurality of elements so as to change the effective length of the at least one of the plurality of elements. 
     Preferably, adjusting the effective cavity length includes adjusting a temperature of at least one of the plurality of elements. 
     Preferably, adjusting the effective cavity length includes adjusting at least one of the effective optical lengths. 
     Preferably, modulating the at least one of the effective optical lengths includes measuring a phase of a modulation of the effective optical length, monitoring the radiation output includes monitoring a radiation output phase and evaluating a comparison of the phase of the modulation of the effective optical length with the radiation output phase, and adjusting the effective cavity length includes adjusting at least one of the effective optical lengths responsive to the comparison. 
     Preferably, adjusting the effective cavity length includes adjusting the length responsive to the monitored radiation output substantially without reliance on an external wavelength reference. 
     Preferably, the method includes varying a resonant wavelength of at least one of the plurality of elements responsive to the single mode of the cavity. 
     There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including: 
     a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and 
     a thermal transfer element which is adapted to vary a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode. 
     Preferably, the one of the plurality of elements includes a fiber grating. 
     There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including: 
     a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and 
     a thermal transfer element which is adapted to vary a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode. 
     Preferably, the one of the plurality of elements includes a fiber grating. 
     There is further provided, according to a preferred embodiment of the present invention, laser apparatus, including: 
     a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, a first one of the plurality of elements having a resonant wavelength; and 
     a thermal transfer element which is adapted to vary a temperature of at least a second one of the plurality of elements, so as to tune the single mode to correspond with the resonant wavelength. 
     Preferably, the first one of the plurality of elements includes a fiber grating. 
     There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including: 
     coupling a plurality of optical elements together so as to form a laser cavity resonating in a single mode, one of the plurality of elements having a tunable resonant wavelength; and 
     varying a temperature of the one of the plurality of elements so as to tune the resonant wavelength to correspond with the single mode. 
     Preferably, the one of the plurality of elements includes a fiber grating. 
     There is further provided, according to a preferred embodiment of the present invention, a method for generating a laser output, including: 
     a plurality of optical elements coupled together so as to form a laser cavity resonating in a single mode, a first one of the plurality of elements having a resonant wavelength; and 
     a thermal transfer element which is adapted to vary a temperature of at least a second one of the plurality of elements, so as to tune the single mode to correspond with the resonant wavelength. 
     Preferably, the first one of the plurality of elements includes a fiber grating. 
     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 DRAWINGS 
     FIG. 1 is a schematic diagram showing operation of a lasing system, as is known in the art; 
     FIG. 2 is a graph of intensity vs. wavelength, illustrating cavity modes for the system of FIG. 1, as is known in the art; 
     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; 
     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; 
     FIG. 5 is a schematic illustration showing a stabilized fiber grating laser system, according to a preferred embodiment of the present invention; 
     FIG. 6 is a schematic perspective diagram of a diode assembly comprised in the system of FIG. 5, according to a preferred embodiment of the present invention; 
     FIGS. 7A,  7 B, and  7 C are temperature vs. time graphs for different points in the diode assembly shown in FIG. 6 for different thicknesses of a heat insulator, according to a preferred embodiment of the present invention; 
     FIG. 8 is a graph showing schematically the effect of modulation of an optical length on the intensity of radiation emitted by the system of FIG. 5, according to a preferred embodiment of the present invention; and 
     FIG. 9 is a schematic diagram of a stabilized fiber grating laser system, according to an alternative preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 5, which is a schematic illustration showing a stabilized fiber grating laser system  80 , according to a preferred embodiment of the present invention. A semiconducting gain region  88  is formed in a gain medium  83 , between non-lasing semiconductor regions  90  of the medium, as is known in the art. Region  88  has a length represented by L 1 , and has a refractive index n 1 . Preferably, semiconductor gain medium  83  is any industry-standard laser diode, such as an indium phosphide diode, having a generally box-like shape, and having a facet  89  anti reflection coated. Hereinbelow medium  83  is assumed to be a diode  81  comprising the above characteristics. Diode  81  is mounted on a heating element  86 , which is mot preferably a thin film resistor and which acts as a thermal transfer element. Heating element  86  is in turn mounted on a thermal insulator  84 , preferably formed from silicon dioxide, although any other thermal insulator could be used. 
     A substrate  82 , which is preferably a good heat conductor, such as silicon, supports insulator  84 . Substrate  82  also supports a fiber optic support  100 , which holds a fiber optic  98 . Preferably, substrate  82  is a good electrical insulator. Alternatively, if the substrate is an electrical conductor and element  86  comprises a resistor such as a thin film resistor, the resistor is electrically isolated from the substrate. In the event that silicon dioxide is used as thermal insulator  84  it also acts as an electrical isolator. In order to provide electrical contact to the diode, two additional layers are required for operation of the laser. An upper layer  87  is a contact layer to the diode, providing a path for current injected into the diode. A lower layer  95  between layer  87  and element  86  is an electrical isolation layer between the contact layer and the element. 
     Fiber optic  98  is optically coupled to a fiber grating  96 , which has an effective length L gef , a refractive index n g , and a tuned wavelength λ f . Fiber grating  96  is in turn optically coupled to a lens  94 . A length L f  of fiber  97  with index n f  is interposed between the lens and the fiber grating. A medium  92 , which may be air or vacuum or any other optically transparent medium, separates lens  94  from semiconductor gain medium  83 . Medium  92  has a length L 0  and a refractive index n 0 . 
     As described in the Background of the Invention, system  80  will resonate at a wavelength λ when: 
     
       
           m·λ/ 2= L   eff =( n   1   ·L   1   +n   0   ·L   0   +n   f   L   f   +n   g   ·L   gef )  (5) 
       
     
     wherein L eff  is the total effective optical length of system  80 : 
     n 1 ·L 1  corresponds to an optical length of region  88 ; 
     n 0 =L 0  corresponds to an optical length of region  92 ; 
     n f =L f  corresponds to an optical length of region  97 ; and 
     n g ·L gef  corresponds to an optical length of region  96 . 
     As described herein, the wavelength of system  80  is stabilized to the tuned wavelength of the peak of the resonance curve of the fiber grating, λ f , by adjusting the effective length L eff . 
     In order to stabilize system  80 , a wavelength stabilizer  93  supplies an electric current of heating element  86 . Most preferably, the current comprises a direct current component and an alternating current component, the levels of which components are separately adjustable by stabilizer  93 . Preferably, the frequency of the alternating current is set to be less than about 5 kHz. The current supplied by stabilizer  93  has the effect on both raising the mean temperature of diode  81 , and of varying the temperature about the mean temperature with a frequency equal to that of the applied alternating current. As described in more detail below, heating element  86  and insulator  84  act as an optical length changer by changing the temperature of diode  81 . The changes in temperature alter the optical length of system  80 , which in turn changes the intensity of the laser radiation emitted by region  88  of diode  81 , and the changes in emitted intensity are used in a feedback loop to stabilize the wavelength emitted by the system. 
     A portion of the laser radiation from region  88  is captured by a detector  91 . Detector  91  is preferably any industry-standard optical radiation detector, for example comprising InGaAs, which is able to measure the intensity of the radiation incident on the detector. Changes of radiation intensity, as measured by detector  91 , are fed back to stabilizer  93 , and the measured changes are used by the stabilizer to vary the level of the direct current supplied to element  86 . The level of the direct current is adjusted by stabilizer  93  in a feedback loop so as to maintain the wavelength of system  80  at a substantially fixed value determined by the resonance curve of the fiber grating. 
     FIG. 6 is a schematic perspective diagram of diode assembly  81 , showing different positions in the assembly, according to a preferred embodiment of the present invention. FIGS. 7A,  7 B, and  7 C are temperature vs. time graphs for the positions shown in FIG. 6 using different thicknesses of heating insulator  84 , according to a preferred embodiment of the present invention. Electrical contact pad  87  to the diode  81  and electrical insulation layer  95  between the contact pad and heating element  86  are not shown since they only have a marginal effect on the performance of thermal insulating layer  84 . FIGS. 7A,  7 B, and  7 C are derived by simulating heating diode assembly  81  with respective thicknesses 0 μm, 1 μm, and 2 μm, for insulator  84  (i.e., in FIG. 7A, insulator  84  is not present). The graphs are generated by applying to diode assembly  81 , for an ambient temperature of 22° C., the time-dependent heat diffusion equation:                    ∇   2        T     +       q   .     k       =       1   α                       ∂   T       ∂   t                 (   6   )                                
     wherein T is the temperature, q is the rate of heat generation, k is the heat conduction coefficient, a is the thermal diffusivity, and t is time. 
     Most preferably, the simulation is performed on an industry-standard software package, such as the ANSYS finite element software program distributed by ANSYS, Inc. of Southpointe, Canonsburg, Pa. 
     Diode assembly  81  has dimensions of length×width×height approximately equal to 300 μm×300 μm×100 μm. Positions  102  and  104  correspond respectively to top and bottom corners of assembly  81 . A position  106  corresponds to a point on substrate  82  directly beneath corner  104  and insulator  84 . Positions  108  and  110  correspond to points on the substrate respectively distant 300 μm and 1000 μm from point  106 . 
     To generate graphs for FIGS. 7A,  7 B, and  7 C, a sinusoidal power modulation having a peak substantially equal to 50 mW is applied to heating element  86 , and laser assembly  81  is assumed to dissipate a further substantially constant power equal to 100 mW. 
     Referring to FIG. 7A, graph A 5  corresponds to result obtained for position  110 , graph A 4  corresponds to results obtained for position  108 , and graph A 3  corresponds substantially to results obtained for positions  102 ,  104  and  106 . The graphs show that, when steady-state conditions are achieved, at position  110  the temperature is substantially equal to the ambient temperature, and at position  108  the temperature is approximately 0.2° C. above the ambient temperature. At position  110  and position  108  the respective temperatures have substantially zero temperature modulation. At positions  102 ,  104 , and  106  the mean temperature is 22.5° C. and there is a peak-peak temperature modulation of 0.15° C. Thus region  88  (between positions  102  and  104 ) has a mean temperature substantially equal to 22.5° C. and a peak-peak temperature modulation substantially equal to 0.15° C., when no insulator is present. 
     Graphs B 5 , B 4 , and B 3  (FIG. 7B) correspond respectively to results obtained for positions  110 ,  108 , and  106 . Graph B 1  corresponds to results obtained for position  104 , and also substantially to results for position  102 . The results shown by graphs B 5 , B 4 , and B 3  are respectively substantially as described above for graphs A 5 , A 4 , and A 3 . Graph B 1  shows that at positions  102  and  104 , the mean temperature is 24° C. and there is a peak-peak temperature modulation of 0.75° C., so that these mean and peak-peak values correspond to the values for region  88  when a 1 μm insulator is present. 
     Graphs C 5 , C 4 , and C 3  (FIG. 7C) correspond respectively to results obtained for positions  110 ,  108 , and  106 . Graph C 1  corresponds to results obtained for position  104  and also substantially to results for position  102 . The results shown by graphs C 5 , C 4 , and C 3  are respectively substantially as described above for graphs A 5 , A 4 , and A 3 . Graph C 1  shows that at positions  102  and  104 , the mean temperature is 25.25° C. and there is a peak-peak temperature modulation of 1.25° C., so that these mean and peak-peak values correspond to the values for region  88  when a 2 μm insulator is present. 
     Comparison of graphs A 3 , B 1 , and C 1 , shows that as the thickness of insulator  84  is increased from 0 μm to 1 μm to 2 μm, the peak-peak temperature modulation of semiconductor regions  90  increases from 0.15° C. to 0.75° C. to 1.25° C., and the mean temperature increases from 22.5° C. to 24° C. to 25.25° C. Thus, when insulator  84  has a thickness of 2 μm, there is an effective gain of temperature modulation equal to 1.25/0.15 i.e., a gain of approximately 8. When insulator  84  has a thickness of 1 μm, the effective gain is 0.75/0.15=5. 
     Modulating the temperature of regions  90  correspondingly modulates the physical length L 1  of diode  81 , due to thermal expansion and contraction of the diode, and also modulates the refractive index n 1  of the diode. Thus the optical length L eff  of assembly  81  is modulated in phase with the modulation in temperature. 
     FIG. 8 is a schematic graph showing the effect of modulation of optical length L eff  on the intensity of radiation emitted by system  80 , according to a preferred embodiment of the present invention. A graph  116  represents the graph of intensity I vs. wavelength λ for a longitudinal cavity mode at which system  80  is lasing, wherein the resonant wavelength of the mode is λ L , and wherein the effective length of system  80  is L eff(L) . At a point  118  on graph  116  system  80  has an effective length L eff(L−) , less than L eff(L) , and the wavelength produced by the assembly is λ L− , less than λ L . At a point  112  on graph  116  system  80  has an effective length L eff(L+) , greater than L eff(L) , and the wavelength produced by the system is λ L+ , greater than λ L . 
     Graphs  120  and  122  are graphs of modulation of L eff  vs. time t, at respective mean lengths L eff(L−)  and L eff(L+) . Graphs  120  and  122  correspond to one of the temperature vs. time modulation graphs A 3 , B 1 , or C 1 , depending on the thickness of insulator  84 . At mean length L eff(L−) , corresponding to system  80  operating at wavelength λ L− , a graph  124  represents the intensity I vs. time t graph produced by the system. Comparison of graphs  120  and  124  shows that the modulation of optical length L eff  is substantially in phase with the intensity I produced by system  80 . At mean length L eff(L+) , corresponding to system  80  operating at wavelength λ L+ , a graph  126  represents the intensity I vs. time t graph produced by the assembly. Comparison of graphs  122  and  126  shows that the modulation of L eff  is substantially 180° out of phase with the intensity I produced by system  80 . At mean length L eff(L) , i.e., when system  80  produces resonant wavelength λ L , the modulation of L eff  produces an intensity I by system  80  which is modulated at a frequency which is twice the effective length modulation frequency. Thus, comparison of modulation parameters such as the phase and/or frequency of the modulation of optical length L eff  with the phase and/or frequency of the output intensity I can be used as a measure indicating where on curve  116  system  80  is operating. 
     Returning to FIG. 5, modulating optical length L eff  by varying the temperature of diode  81  causes a corresponding modulation in radiation intensity produced by assembly  80 . The modulation in radiation intensity is registered by detector  91 , and the modulation in temperature is set to be as small as possible, but large enough so that the modulation in radiation intensity is large enough to provide a single to noise ratio sufficient to stabilize the wavelength utilizing the control loop. The temperature modulation also causes heat dissipation which raises the mean temperature of diode  81 . However, a rise in mean temperature of diode  81  reduces the mean radiation intensity emitted by the diode. Thus, a thickness of insulator  84  and the DC and AC components of the current supplied by stabilizer  93  are chosen so that the peak-peak temperature modulation of diode  81  is high enough to provide the required effective length modulation, while the mean temperature rise of the diode is as small as possible. Most preferably, the thickness of insulator  84  when the insulator is silicon dioxide is set to be of the order of 1 μm, and the AC component of the current is set to supply a peak-peak power of the order of 50 mW to heater  86 . Further most preferably, the DC temperature level due to the DC current provided to element  86  is set by the control loop to adjust the cavity mode to the center of resonance of the fiber grating. Those skilled in the art will be able to evaluate a corresponding thickness of insulator  84  when the structure of the cavity is different from that described hereinabove, for example, when the insulator, substrate, or laser, are of different materials. 
     Signals from detector  91 , responsive to the modulation in radiation intensity, are compared in stabilizer  93  to the alternating current signals applied to heater  86 . Most preferably, the phases of the detector signals and the alternating current are compared to determine a position on a resonant curve of assembly  80  where the assembly is operating, as described above with reference to FIG.  8 . Alternatively or additionally, other measures are used to determine the position on the resonant curve of assembly  80 . For example, one measure is the slope of the change of the laser intensity due to a change in wavelength, caused by an effective length change due to a temperature change. A positive slope is associated with a mean length L eff(L−) , corresponding to system  80  operating at wavelength λ L− . A negative slope is associated with a mean length L eff(L+) , corresponding to system  80  operating at wavelength λ L+ . A substantially zero slope is associated with mean length L eff(L) , corresponding to system  80  operating at wavelength λ L  which is the peak wavelength of the resonance curve of the fiber grating. Stabilizer  93  uses the determined position in order to set a level of the direct current applied to heater  86 , thereby altering the mean temperature of diode  81 , so as to maintain assembly  80  resonating at a substantially constant resonating wavelength λ L . 
     FIG. 9 is a schematic diagram of a stabilized fiber grating laser system  150 , according to an alternative preferred embodiment of the present invention. Apart from the differences described hereinbelow, the operation of assembly  150  is generally similar to that of assembly  80 , so that elements indicated by the same reference numerals in both assembly  150  and assembly  80  are generally identical in construction and operation. Substrate  82  is mounted on a thermoelectric cooler  120 , such as a model SP1020 produced by Marlow Industries, Inc., of Dallas, Tex., although any other standard or custom-built thermoelectric cooler may be used. Cooler  120  is a thermal transfer element which is generally utilized to control the mean temperature of lasers of different varieties and can also be used for the same purpose in embodiments of the present invention. As is known in the art, the mean temperature of a laser is affected by intrinsic effects like laser power dissipation or extrinsic effects like environmental effects. Cooler  120  is preferably powered by a power source external to assembly  150 . Alternatively cooler  120  is powered by a power source internal to assembly  150 , for example stabilizer  93  may act as a power source. 
     Cooler  120  can be used to reduce the mean temperature rise of diode  81  caused by modulation of element  86 , required to produce a modulation of the effective length. Most preferably, cooler  120  is operated so as to allow insulator  84  to be thicker than in system  80 , by extracting heat from substrate  82 , thereby reducing the mean temperature of diode  81  and/or assembly  150  and allowing a larger peak to peak temperature modulation to be applied to the diode and/or the assembly for the same modulation input power to element  86 . 
     Most preferably, stabilizer  93  varies a mean temperature of diode  81  and/or other elements of assembly  150  as described hereinabove for system  80 . Stabilizer  93  can change the mean temperature by applying power to element  86  or cooler  120 . Preferably, cooler  120  is operated so as to extract heat from substrate  82  at a substantially constant rate in order to control the mean temperature of system  150 . The mean temperature varies slowly due to intrinsic effects like laser power dissipation or extrinsic effects like environmental effects. 
     Alternatively, cooler  120  can be operated so as to vary the mean temperature of system  150  or any of its component parts, the variation being adjusted by stabilizer  93  together with the other parameters varied by the stabilizer, as described hereinabove, so that the wavelength radiated by system  150  is maintained at a substantially constant value. Further alternatively, cooler  120  is operated so as modulate the temperature of system  150 , the modulation being adjusted by stabilizer  93  together with the other parameters varied by the stabilizer, as described hereinabove, so that the wavelength radiated by system  150  is maintained at a substantially constant value. 
     It will be appreciated that element  86  and cooler  120  can take on together or separately the duties of producing the modulation to the effective length, of adjusting the wavelength of the peak of the resonance of the fiber grating, and/or related duties. While element  86  can only heat, cooler  120  can heat and cool. For example, current accuracy for writing a fiber grating is about +/−0.05 nm of the nominal resonant wavelength, and a temperature sensitivity of the resonant wavelength of the fiber grating is of the order of 0.01 nm/° C. To obtain a better wavelength accuracy for a laser system such as system  80 , a grating in the system is temperature tuned. In the above example, varying the temperature by of the order of +/−5° C., using a heater such as element  86  and/or a cooler such as cooler  120 , brings the resonant wavelength of the fiber grating to its nominal resonant wavelength. The stabilization method described hereinabove can then be used to lock the cavity mode to the peak of the resonance curve of the fiber grating. Alternatively or additionally, the effective length of the cavity can be changed by varying the temperature of the diode, for example, by using a heater such as element  86 . 
     It will thus 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.