Abstract:
A spatially modulated waveguide Bragg grating mirror is suspended over a substrate by plurality of fingers extending laterally away from the waveguide centerline. The positions of the fingers are coordinated with the positions of crests and valleys of amplitude or phase modulation of the Bragg grating, to avoid disturbing the Bragg grating when it is tuned by heating. When the Bragg grating is heated, the heat flows through the fingers creating a quasi-periodic refractive index variation along the Bragg grating due to quasi-periodic temperature variation created by the heat flow from the grating through the supporting fingers. Due to coordination of the positions of supporting fingers with positions of the crests and valleys of modulation, the optical phase coherence is maintained along the Bragg grating, so that the spectral lineshape or filtering property of the Bragg grating is substantially preserved.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to optical waveguides and lasers, and in particular to structures and methods for tuning optical waveguide gratings in lasers. 
       BACKGROUND OF THE INVENTION 
       [0002]    A laser diode includes a p-n junction between a pair of mirrors for creating optical feedback for light generated and amplified at the p-n junction when a forward current is applied to the p-n junction. To provide wavelength tenability, the mirrors are made wavelength selective, and a reflection wavelength of at least one of the mirrors is tuned. 
         [0003]    In waveguide laser diodes, waveguide gratings are frequently used as wavelength selective mirrors. In a waveguide grating, periodic perturbations of the effective refractive index of the waveguide are created to selectively reflect light at a wavelength corresponding to the spatial frequency of the periodic refractive index perturbations. A waveguide grating can be tuned by heating or, for waveguide gratings formed at a p-n junction, by providing a direct current to the p-n junction, which changes its overall refractive index through carrier injection. 
         [0004]    Current-tunable p-n junction waveguide gratings have drawbacks. Supplying direct current to a waveguide grating can induce optical loss, which negatively impacts laser light generation efficiency and broadens the emission spectral linewidth of the laser. Thermally tuned gratings are generally free from these drawbacks. However, thermal tuning requires considerable amounts of heat applied to the waveguide grating to change its temperature, which can also impact the temperature of the lasing p-n junction. This is because waveguide gratings are typically fabricated integrated with the lasing p-n junction, which must be heat sunk very well to prevent overheating of the laser diode during normal operation. By way of example, Ishii et al. in an article entitled “Narrow spectral linewidth under wavelength tuning in thermally tunable super-structure grating (SSG) DBR lasers”, published in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2 (1995), pp. 401-407, disclose a super-structure grating distributed Bragg reflector laser, which can be thermally tuned over 40 nm by thermally tuning SSG reflectors. In the Ishii device, the max thermal tuning power dissipation per unit length of mirror to achieve full tenability was 1.3 mW per 1 micrometer of length, which for the front and back mirror section lengths used of 400 and 600 micrometers, respectively, corresponds to a prohibitively-high total power dissipation of 1300 mW. The tuning 1/e time constant is about 1.6 milliseconds, which is relatively slow. 
         [0005]    Attempts have been made in the prior art to utilize thermal tuning more efficiently by thermally decoupling the waveguide grating from the common substrate with the lasing p-n junction. By way of example, Cunningham et al. in U.S. Pat. No. 7,848,599 disclose a thermally tunable waveguide that is free standing above a substrate to increase thermal resistance between the waveguide and the environment. Matsui et al. in U.S. Pat. No. 7,778,295 disclose a Distributed Bragg Reflector (DBR) laser, in which the DBR section of the laser is suspended over the substrate to increase the thermal resistance between the DBR section and the substrate. 
         [0006]    Detrimentally, waveguides suspended over a substrate without additional structural support are prone to a mechanical failure. Multiple legs were used in a Cunningham device to support the suspended waveguide along their length, but these can result in an overly complex waveguide structure and/or interfere with the optical function of the waveguide. 
         [0007]    It is therefore a goal of the invention to provide a tunable waveguide grating that could be tuned quickly and efficiently, substantially without degradation of spectral properties, while providing an adequate structural support for the waveguide. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the invention, a waveguide Bragg grating is suspended over a substrate by plurality of fingers extending laterally away from the waveguide centerline, resulting in a simple and easily manufacturable structure. The Bragg grating can be in the form of a sampled grating, which consists of periodically spaced uniform grating bursts separated by blanked regions without gratings, and is characterized by a grating period of a high spatial frequency and a burst period of a low spatial frequency. More generally, the Bragg grating can consist of a slow spatial modulation of the grating strength or the grating phase along the waveguide centerline or optical axis, forming crests and valleys of modulation. The crests and valleys can be of a square shape, such as in a sampled grating, or of a smooth, wave-like varying shape. 
         [0009]    According to the invention, the positions of the fingers are coordinated with the positions of the crests and valleys of modulation, to avoid disturbing the Bragg grating upon thermal tuning of the grating. When the Bragg grating is heated, the heat flows through the fingers, creating a quasi-periodic refractive index variation along the Bragg grating optical axis due to a quasi-periodic temperature variation created by the heat flow from the grating through the supporting fingers. Since the positions of the supporting fingers are coordinated with the positions of the grating modulation crests and valleys, the optical phase coherence is maintained between the grating modulation crests, so that the Bragg grating is not disturbed by the heating. As a result, smooth and continuous tuning of the Bragg grating is possible substantially without perturbing the reflection bandshape. 
         [0010]    In accordance with the invention, there is provided a tunable Bragg grating comprising: 
         [0011]    a first substrate section; 
         [0012]    first and second spaced apart support bars extending upwardly from the first substrate section; 
         [0013]    a first waveguide for guiding light therein, wherein the first waveguide has an optical axis and is supported by the first and second support bars above the first substrate section, so that a first gap exists between the first substrate section and the first waveguide, 
         [0014]    wherein an effective refractive index of the first waveguide is spatially modulated along the optical axis, forming a grating for reflecting an optical frequency component of the light guided by the first waveguide to propagate back therein, wherein at least one of phase or amplitude of the spatial modulation of the effective refractive index is varying along the optical axis, forming modulation crests and valleys, wherein the crests are spaced apart at a first spatial frequency along the optical axis; and 
         [0015]    a first resistive heater disposed on the first waveguide, for heating the first waveguide for tuning an optical frequency of the reflected optical frequency component; 
         [0016]    wherein the first waveguide has first and second arrays of openings extending therethrough and into the first gap, the openings of the first and second arrays running along the optical axis of the first waveguide on respective opposite first and second sides of the optical axis, the first and second arrays of openings defining first and second arrays of heat conducting fingers, respectively, extending between the optical axis and the first and second support bars, respectively, 
         [0017]    wherein positions of the heat conducting fingers along the optical axis are coordinated with positions of the modulation crests and valleys along the optical axis, 
         [0018]    whereby, when heat is applied by the first resistive heater to the first waveguide, spatial refractive index variations, caused by spatial temperature variations along the optical axis due to heat flow through the heat conducting fingers to the first and second support bars, are spatially coordinated with the modulation crests and valleys. 
         [0019]    In one embodiment, the spatial frequency of the heat conducting fingers along the optical axis is an integer multiple of the first spatial frequency. 
         [0020]    In accordance with another aspect of the invention, there is further provided a tunable laser diode comprising: 
         [0021]    the tunable Bragg grating as described above, 
         [0022]    a substrate comprising the first substrate section; 
         [0023]    a spacer layer supported by the substrate, the spacer layer comprising the first and second support bars; 
         [0024]    an active waveguide for amplifying the light, optically coupled to the tunable Bragg grating and disposed in mechanical, thermal, and electrical contact with the spacer layer; and 
         [0025]    an electrode disposed on the active waveguide, for providing electrical current thereto. 
         [0026]    Preferably, the active waveguide and the first waveguide comprise a single monolithically fabricated shallow-ridge waveguide, which results in a particularly simple and efficient structure. Bulk micromachining from the back of the substrate can be used to have the first gap to extend completely through the first substrate section. 
         [0027]    In accordance with yet another aspect of the invention, there is further provided a method for tuning a laser diode having an active waveguide, the method comprising: 
         [0028]    (a) providing a tunable Bragg grating having a substrate, first and second spaced apart support bars extending upwardly from the substrate, and a first waveguide optically coupled to the active waveguide, wherein the first waveguide has an optical axis and is supported by the first and second support bars above the substrate, forming a gap between the substrate and the first waveguide, wherein an effective refractive index of the first waveguide is spatially modulated along the optical axis, forming a grating for reflecting an optical frequency component of the light guided by the first waveguide to propagate back therein, wherein at least one of phase or amplitude of the spatial modulation of the effective refractive index is varying along the optical axis, forming modulation crests and valleys spaced apart at a first spatial frequency along the optical axis; 
         [0029]    (b) providing first and second arrays of openings extending through the first waveguide and into the gap, the openings of the first and second arrays of openings running on respective opposite first and second sides of the optical axis, the first and second arrays of openings defining first and second arrays of heat conducting fingers, respectively, extending from the optical axis towards the first and second support bars, respectively; 
         [0030]    wherein step (b) comprises disposing the openings so that positions of the heat conducting fingers along the optical axis are coordinated with positions of the modulation crests and valleys along the optical axis; and 
         [0031]    (c) heating the first waveguide for tuning the optical frequency of the optical frequency component, thereby tuning the laser diode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Exemplary embodiments will now be described in conjunction with the drawings, in which: 
           [0033]      FIG. 1A  is a top schematic view of a Sampled Grating Distributed Bragg Reflector (SG-DBR) laser of the invention including a tunable Bragg grating of the invention; 
           [0034]      FIG. 1B  is a magnified view of the DBR of  FIG. 1A  superposed with a corresponding effective refractive index plot of the Bragg grating, showing burst modulation of the grating; 
           [0035]      FIG. 1C  is a plan view of a variant of the Bragg grating having a continuous variation of modulation depth, superposed with a corresponding refractive index plot; 
           [0036]      FIGS. 2A ,  2 B, and  2 C are cross-sectional views taken along lines A-A, B-B, and C-C, respectively, of  FIG. 1A ; 
           [0037]      FIGS. 3A and 3B  are a plan and side cross-sectional views, respectively, of an embodiment of the SG-DBR laser of  FIG. 1A , comprising an optical amplifier section; 
           [0038]      FIG. 4  is a three-dimensional view of an embodiment of a tunable Bragg grating having a uniform top heater, showing a simulated temperature distribution of the surface of the Bragg grating; 
           [0039]      FIGS. 5A and 5B  are cross-sectional views of the temperature distribution of the tunable Bragg grating of  FIG. 4  taken along lines A-A and B-B, respectively, of  FIG. 4 ; 
           [0040]      FIG. 6  is a longitudinal distribution of the waveguide temperature of the tunable Bragg grating of  FIG. 4 ; 
           [0041]      FIG. 7  is a simulated temporal plot of the temperature variation upon application of a heat pulse to the Bragg grating of  FIG. 4 ; 
           [0042]      FIG. 8  is a three-dimensional view of an embodiment of a tunable Bragg grating having a plurality of electrically connected, jointly driven individual heaters; 
           [0043]      FIG. 9A  is a SG-DBR laser including the tunable Bragg grating of  FIG. 8 ; 
           [0044]      FIG. 9B  is a schematic top view of the connection of the resistive heaters of the tunable Bragg grating of  FIG. 8 ; 
           [0045]      FIG. 10A  is a cross-sectional view of the SG-DBR laser of  FIG. 1A ; 
           [0046]      FIG. 10B  is a magnified cross-sectional view of  FIG. 10A ; and 
           [0047]      FIGS. 11A and 11B  are side and bottom views, respectively, of a SG-DBR laser, in which the tunable Bragg gratings are suspended over the substrate by using bulk micromachining. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]    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, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0049]    Referring to  FIG. 1A  and  FIGS. 2A to 2C , a SG-DBR laser  120  of the invention includes front and back tunable Bragg gratings (DBR)  100 , a gain section  126 , and an optional phase section  160 . In the embodiment shown, the Bragg gratings  100 , the gain section  126 , and the phase section  160  are sections of a ridge waveguide structure having a common top ridge  115  for guiding a light mode  108 . 
         [0050]    The Bragg gratings  100  include a first substrate section  102 , first and second spaced apart support bars  104  extending upwardly from the first substrate section  102 , a first ridge waveguide  106 , and a first resistive heater  117 . The first ridge waveguide  106  is supported by the first and second support bars  104  above the first substrate section  102 , forming a first gap  105  between the first substrate section  102  and the first ridge waveguide  106 . The first ridge waveguide  106  includes a stack of: a bottom cladding layer  110  supported by the first and second support bars  104 , a core layer  112  disposed on the bottom cladding layer  110 , and a top cladding layer  114  disposed on the core layer  112 . The top cladding layer  114  has the ridge  115  on top, for guiding the light mode  108  along the ridge  115 . 
         [0051]    The refractive index of the core layer  112  is higher than refractive indices of the top  114  and bottom  110  cladding layers, for confining the light  108  guided by the ridge  115  substantially to the core layer  112 . The effective refractive index of the first ridge waveguide  106  is spatially modulated, forming a grating for reflecting an optical frequency component  109  of the light  108  guided by the first ridge waveguide  106  to propagate back therein. For example, the refractive index of the core layer  112  can be spatially modulated, or the ridge  115  can be laterally corrugated to create the spatial modulation of the effective refractive index. At least one of phase or amplitude of the spatial modulation of the effective refractive index is varying along the optical axis, forming modulation crests  116 - 1  and valleys  116 - 2 . By way of example, the grating period can be about 0.24 micrometers, the length of a sampled grating burst (modulation crest  116 - 1 ) can be 3 micrometers, the burst period, or distance between neighboring crests  116 - 1  can be 50 micrometers. By way of example, there can be 7 to 11 crests  116 - 1  per DBR  100 . The first resistive heater  117  is disposed on an insulating dielectric layer, not shown, which is deposited on the top cladding layer  114 . 
         [0052]    The first ridge waveguide  106  has first and second arrays of openings  118 A and  118 B, respectively, extending through the first ridge waveguide  106  and into the first gap  105 . As seen in  FIG. 1A , the openings  118 A and  118 B run on opposite sides of the ridge  115 , defining first and second arrays of heat conducting fingers  119 A and  119 B, respectively, extending from the ridge  115  towards the first and second support bars  104 . Preferably, first and last openings  118 C of the first array of the openings  118 A are longer than the remainder of the openings  118 A. Similarly, the first and last openings  118 D of the first array of the openings  118 B are preferably longer than the remainder of the openings  118 B. The longer openings  118 C and  118 D facilitate creation of a more uniform temperature distribution upon heating the DBR  100  by the first resistive heater  117 . Both length and width of first and last openings  118 C and  118 D can be adjusted to improve the temperature uniformity. 
         [0053]    Referring now to  FIGS. 1B and 1C , the first ridge waveguide  106  can be burst-modulated ( FIG. 1B ) or smoothly modulated ( FIG. 1C ), or modulated in a more complicated manner. The modulation can include amplitude or phase modulation or both. The modulation can be periodic or quasi-periodic. In  FIG. 1B , the modulation crests  116 - 1  comprise bursts of a sampled Bragg grating, having substantially no modulation in the valleys  116 - 2  between the bursts  116 - 1 . In  FIG. 1C , the modulation is more smooth, so that the modulation valleys  116 - 2  have some refractive index modulation. The modulation crests  116 - 1  are spaced apart at a first spatial frequency f 1  along an optical axis  107  of the first ridge waveguide  106 . As shown in  FIGS. 1B and 1C  with dashed lines  150 , positions of the heat conducting fingers  119 A along the optical axis  107  are coordinated with positions of the modulation crests  116 - 1  and valleys  116 - 2  along the optical axis  107 . As a result, when heat is applied by the first resistive heater  117  to the first ridge waveguide  106 , spatial refractive index variations, caused by spatial temperature variations along the optical axis  107  due to heat flow through the heat conducting fingers  119 A and  119 B to the first and second support bars  104 , are spatially coordinated with the modulation crests  116 - 1  and valleys  116 - 2 . The fingers  119 B are omitted in  FIGS. 1B and 1C  for clarity, although they are also coordinated with the modulation crests  116 - 1  and valleys  116 - 2 . 
         [0054]    In a preferred embodiment, the second spatial frequency f 2  of the heat conducting fingers  119 A and  119 B along the ridge  115  is an integer multiple of the first spatial frequency f 1 . For instance, in  FIG. 1B , f 2 =2f 1 ; and in  FIG. 1C , the frequencies f 1  and f 2  are equal. 
         [0055]    Referring back to  FIGS. 1A and 2C , the gain section  126  includes a substrate  122 , which is preferably a common substrate with the first substrate section  102 , a spacer layer  124  supported by the substrate  122 , an active waveguide  126  for amplifying the light mode  108 , and a first electrode  137  for providing electrical current to the active waveguide  126 . As best seen by comparing  FIGS. 2A ,  2 B, and  2 C, the spacer layer  124  includes the first and second support bars  104  of the tunable DBR section, or Bragg grating  100 . The active waveguide  126  is optically coupled to the tunable DBR section  100  and disposed in mechanical, thermal, and electrical contact with the spacer layer  124 . The active waveguide layer  126  includes the stack of: a first conductivity type layer  130  supported by the spacer layer  124  and integrally formed with the bottom cladding layer  110 ; a junction layer  132  supported by the first conductivity type layer  130  and integrally formed with the core layer  112 ; and a second conductivity type layer  134  supported by the junction layer  132  and integrally formed with the top cladding layer. The ridge  115  runs through the first ridge waveguide  106  and the active waveguide  126  for providing optical coupling therebetween. In one embodiment, the first and second conductivity type layers  130  and  134 , respectively, include n- and p-doped InP layers, respectively, the junction layer  132  includes InGaAsP quantum wells, and the spacer layer  124  includes an InGaAs layer. 
         [0056]    Still referring to  FIG. 1A , the phase section  160  is a variant of the structure of the DBR section  100 , lacking the spatial modulation of the refractive index. The phase section  160  includes a second substrate section  142 , third and fourth support bars  144  extending upwardly from the first substrate section  142 , and a second ridge waveguide  146  for guiding the light mode  108 , supported by the first and second support bars  104  above the first substrate section  102 , forming a second gap  145  between the second substrate section  142  and the second ridge waveguide  146 . The second ridge waveguide  146  includes the same stack as the first ridge waveguide  106 , with the difference that the effective refractive index of the second ridge waveguide  146  is not spatially modulated. A second resistive heater  157  is disposed on the top cladding layer  114 , for providing heating of the second ridge waveguide  146  for tuning optical phase of the light mode  108  propagating therein. 
         [0057]    In operation, the guided light mode  108  generated in the gain section  126  propagates along the ridge  115 . An optical frequency component  109  of the guided light mode  108  is reflected to propagate back along the ridge  115  towards the gain section  126 , thus providing an optical feedback to the laser  120 . The reflected optical frequency component  109  has a wavelength corresponding to the spatial frequency of the effective refractive index modulation of the first ridge waveguide  106 . The first resistive heater  117  provides heating to the first ridge waveguide  106 , for tuning the optical frequency of the reflected optical frequency component  109 . When heat is applied by the first resistive heater  117  to the first ridge waveguide  106 , spatial refractive index variations, caused by spatial temperature variations along the ridge  115  due to heat flow through the heat conducting fingers  119 A and  119 B to the first and second support bars  104 , are spatially coordinated with the modulation crests  116 - 1 . As a result, the heating by the first resistive heater  117  substantially does not disturb or modify the reflected frequency spectrum beyond simply tuning of the center frequency of the reflected frequency spectrum. This allows one to reduce a bandwidth variation as the laser  120  is tuned in optical frequency or wavelength. 
         [0058]    The first and second gaps  105  and  145  can be filled with a chemically inert gas such as xenon, argon, or nitrogen. Also, the first and second gaps  105  and  145  can form a single gap, although the latter structure will have somewhat increased thermal crosstalk between the tunable DBR and phase sections  100  and  160 , respectively; this is why two separate gaps  105  and  145  are preferred. The gap  105  can be formed by lateral selective undercut etching of the sacrificial spacer layer  124 . The gap  145  can be formed by selective etching of the spacer layer through the openings  118 A,  118 B,  118 C, and  118 D. These etching techniques are generally referred to as “micromachining”, a term adopted from micro-electro-mechanical systems (MEMS) manufacturing. 
         [0059]    The first and second ridge waveguides  106  and  146 , respectively, and the active waveguide  126  preferably form a single monolithic shallow-ridge waveguide structure having the active section  126 , the tunable Bragg grating or DBR section  100 , and the phase section  160 . The openings  118 A- 118 D and the gap  105  are particularly easy to form in shallow-ridge waveguides, ensuring ease of overall manufacture. However, it is to be understood that the ridge type waveguides are only example embodiments of waveguides of the invention. Other waveguide types, known to a person skilled in the art, can also be used in the tunable Bragg grating  100 , the gain section  126 , and/or the phase section  160 . Similarly to the ridge type waveguide  106 , the other waveguide types must be suspended over the substrate  102  by the support bars  104 , and the at least one of phase or amplitude of the spatial modulation of the effective refractive index of the Bragg grating  100  has to have a plurality of crests  116 - 1  and valleys  116 - 2 . The positions of the heat conducting fingers  119 A and  119 B along the optical axis  107  have to be coordinated with the positions of the crests  116 - 1  and valleys  116 - 2  along the optical axis  107 , to lessen the bandshape variation of the reflected optical frequency component  109 . 
         [0060]    Turning now to  FIGS. 3A and 3B  with further reference to  FIGS. 1A and 2A  to  2 C, an embodiment  320  of the SG-DBR laser  120  of  FIG. 1A  includes two tunable DBR sections  100 , the phase section  160 , the gain section  126 , and an amplifier section  300  formed within a common shallow-ridge waveguide  306  suspended over the common substrate  122  at the tunable DBR sections  100  and the phase section  160 . A back-facet absorber section  301  is provided for absorbing laser light at the left-hand side of the waveguide  306  in  FIG. 3A , to prevent light reflected from a left facet  310  of the shallow-ridge waveguide  306  to interfere with light selectively reflected by the left tunable Bragg grating  100 . A common backplane electrode  302  is electrically coupled to a back side of the substrate  122 . The gain section  126  is powered with the first electrode  137 , and the amplifier section  300  is powered by a second electrode  337  or providing electrical current to the amplifier section  300 . The first and second electrodes  137  and  337 , and the heaters  117  and  157  are omitted in  FIG. 3A , so as not to hide the underlying structures. The function of the amplifier section  300  is to amplify the light  108  generated in the gain section  126 , to provide a constant output power as the emission wavelength of the laser  320  is tuned by the synchronously tuned Bragg gratings  100 . The output power can be measured by an integrated photodetector  314 . 
         [0061]    In a preferred embodiment, the resistive heaters  117  of the tunable Bragg gratings  100  are uniform thin-film resistive heaters applied to the ridge  115  of the ridge waveguide  106 , with current passing along the length of the heater  117  between two contact pads  117 A. A passivating layer of dielectric is disposed between the thin-film heater  117  and the underlying ridge waveguide  106 . Referring to  FIGS. 4 ,  5 A, and  5 B, with further reference to  FIGS. 1A ,  2 A to  2 C,  3 A, and  3 B, a numerical simulation has been performed for an embodiment  400  of the tunable Bragg grating  100  having the uniform heater  117  (not shown in  FIG. 4 ) running along the ridge  115 . Positions, lengths, and widths of the openings  118 A to  118 D are selected so as to create a substantially uniform temperature distribution along the ridge  115 . The positions, lengths, and widths of the openings  118 A to  118 D define lengths and widths of the heat conducting fingers  119 A and  119 B. As seen in  FIG. 4 , the openings  118 A and  118 B define a mesa  410  therebetween, and this mesa  410  limits the optical interaction between the optical mode  108  of the ridge waveguide  106  and the refractive index discontinuity resulting from the openings  118 A,  118 B. The length and width of the mesa  410  including end mesa sections  411  have an impact on the resulting temperature distribution. In the simulation of  FIGS. 4 ,  5 A, and  5 B, the ridge  115  is 2 micrometers tall and 2 micrometers wide. The mesa width (distance between the openings  118 A and  118 B of the first and second arrays, respectively) is 20 micrometers, the thickness of the mesa sections  410 ,  411  is 2 micrometers, and the height of the gap  105  (vertical dimension of the gap  105  in  FIGS. 2A and 2B ) is 2 micrometers. The total length of the mesa  410  is 400 micrometers. The material is InP. When 25 mW of uniform heat flux is applied to the mesa  410 , a temperature distribution is formed. In  FIG. 5A , the simulated temperature distribution of the mesa  410  between the heat conducting fingers  119 A and  119 B is shown. In  FIG. 5B , the temperature distribution is along the heat conducting fingers  119 A and  119 B. 
         [0062]    Turning to  FIG. 6  with further reference to  FIGS. 1A ,  4 ,  5 A, and  5 B, a longitudinal distribution  600  of the temperature of the waveguide sections  410 ,  411  is shown. The temperature varies from approximately 303° K to 334° K. One can see from  FIG. 6  that the temperature along the mesa sections  410 ,  411  oscillates at approximately 3° K peak-to-peak amplitude, the peaks of the temperature distribution being denoted at  602 . The peak temperatures have been made substantially uniform by optimizing the geometry of the end openings  118 C and  118 D and the end mesa sections  411 , such that heat generation along the end mesa section  411  is balanced by heat conduction through the end mesa section  411  to the spacer layer  124  and the substrate  122 . Since the positions of the heat conducting fingers  119 A and  119 B along the ridge  115  are coordinated with the positions of the modulation crests  116 - 1  along the ridge  115 , the temperature oscillation peaks  602  are also coordinated with the positions of the modulation crests  116 - 1  (not shown in  FIGS. 4 ,  5 A,  5 B), thus maintaining optical phase coherence of the optical signal  108  between the modulation crests  116 - 1 . When the phase coherence is maintained and the temperature peaks  602  are substantially uniform, the spectral filtering properties of the Bragg grating  100  are very similar to those with an ideally even temperature profile. It is to be understood that the end mesa sections  411 , where the temperature is varying from approximately the temperature of the substrate  102  at the ends to the peak temperature at the first fingers  119 A and  119 B, can minimally include gratings because of the highly non-uniform temperature profile. 
         [0063]    Referring now to  FIG. 7 , a simulated time trace  700  of temperature rise upon a quasi-instantaneous application of the 25 mW of heater power shows that the 27.5° K temperature increase is achieved in less than 100 microseconds. The cooling-down time is also below 100 microseconds, which is more than 16 times faster than the tuning time reported by Ishii et al. in the above-mentioned paper entitled “Narrow spectral linewidth under wavelength tuning in thermally tunable super-structure grating (SSG) DBR lasers”, published in IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2 (1995), pp. 401-407. Only 25 mW of heater power is required to create 27.5° K temperature increase, corresponding to approximately 2.7 nm of tuning the wavelength of the reflected component  109 . Full tenability of &gt;−5 nm, needed for typical peak spacings in sampled or modulated grating DBRs, requires only 50 mW heater power, which is approximately a 10-fold reduction compared to that reported by Ishii. This illustrates the capability of the tunable Bragg grating  100  to quickly and efficiently tune the wavelength of the SG-DBR laser  120 . 
         [0064]    Referring to  FIG. 8  with further reference to  FIG. 1A , a tunable Bragg grating  800  is a variant of the tunable Bragg grating  100  of  FIG. 1A . In the tunable Bragg grating  800  of  FIG. 8 , the first resistive heater  117  includes a plurality of electrically coupled individual thin-film heaters  817  running on top of the heat conducting fingers  119 A and  119 B. Additional heater elements  817 A are disposed on the ends  411  of the mesa  410 , to further improve uniformity of the temperature distribution. Turning to  FIG. 9A , a SG-DBR laser  820  is a variant of the SG-DBR laser  320  of  FIG. 3A . In the SG-DBR laser  920  of  FIG. 9 , the tunable Bragg gratings  100  of  FIGS. 1A ,  2 A, and  2 B have been replaced with the tunable Bragg gratings  800  of  FIG. 8 . Referring now to  FIG. 9B , the individual thin-film heaters  817 , having the electrical resistance r, are connected in four serial groups of four parallel heaters  817 , resulting in a total resistance between V− and V+ electrodes R=4r/4=r. A comparable longitudinal heater, for example a tantalum nitride (TaN) thin film of a same thickness, would have many times higher resistance. This connection of the individual heaters  817  can better match the impedance of driving electronics, not shown. Of course, other connections are possible, to tailor the total resistance R to a requirement of the driving electronics. To provide an adequate heating, the individual thin-film heaters  117  must run on top of at least some of the heat conducting fingers  119 A and  119 B. By placing heating elements  117  only on the heat conducting fingers  119 A and  119 B and on the ends  411  of the mesa  410 , a substantial reduction in the longitudinal peak-to-peak temperature variation along the mesa  410  can be achieved. 
         [0065]    Turning to  FIG. 10A  with further reference to  FIGS. 1A and 2A  to  2 C, the spacer layer  124  can include a sacrificial InGaAs layer. 1-2 micrometer thick sacrificial InGaAs layer  124  provides a good electrical contact of the active waveguide  126  to the substrate  122 . Detrimentally, such a thick sacrificial InGaAs layer  124  can be difficult to grow and subsequently micromachine to manufacture the support bars  104 , and its thermal impedance will have a deleterious effect on active section performance. Reducing thickness of the sacrificial InGaAs layer  124  will result in a reduced height of the support bars  104 , leading to heat conduction across the gap  105 , which is detrimental to the thermal tuning efficiency. To overcome this tradeoff, the spacer layer  124  can be made in form of a stack including several different layers. Referring to  FIG. 10B , the spacer layer  124  includes in sequence from bottom to top a 20 nm thick InGaAsP bottom etch stop layer  1002  having a bandgap wavelength of 1.2 um, a 200 nm thick InP bottom second stage sacrificial layer  1004 , a 200 nm thick first stage InGaAs sacrificial layer  1006 , a 2000 nm thick InP top second stage sacrificial layer  1008 , and a 20 nm thick InGaAsP top etch stop layer  1010 . InP can be grown much more easily to a larger thickness than InGaAs. As a result, the thickness of the multilayer stack spacer layer  124  can be increased, the thickness of at least one of the InP sacrificial layers  1004  and  1008  being larger than the thickness of the InGaAs sacrificial layer  1006 , and the total thickness of the InGaAs or InGaAsP material can be reduced. The spacer layer  124  of  FIG. 10B  can then be etched, or micromachined, in two etching steps, one for the InGaAs layer  1006  to achieve lateral undercut, and one for the InP layers  1004  and  1008  to etch vertically to achieve the thicker gap  105 . 
         [0066]    Referring now to  FIGS. 11A and 11B  with a supplementary reference to  FIGS. 3A and 3B , a SG-DBR laser  1120  is an embodiment of the SG-DBR laser  320  of  FIG. 3 . In the SG-DBR laser  1120  of  FIGS. 11A and 11B , the first and second gaps  105  and  145  of the tunable Bragg grating sections  100  and the phase section  160 , respectively, extend completely through the substrate  122 . This can be achieved by using bulk micromachining techniques known from MEMS technologies, by etching through the bottom of the substrate  122 . 
         [0067]    It is to be understood that the invention as described above is not limited to particular types of waveguide structures and/or to particular material systems. In general, any laser diode having an active waveguide for laser light generation, for example the active waveguide  126  of  FIG. 1A , can be tuned according to the invention by following the three steps A, B, and C. 
         [0068]    Step A includes providing a tunable Bragg grating such as the tunable DBR  100 , having the first substrate section  102 , the first and second spaced apart support bars  104  extending upwardly from the substrate section  102 , and a first waveguide, for example the ridge waveguide  106  optically coupled to the active waveguide  126 . The first waveguide  106  is supported by the support bars  104  above the first substrate section  102 , forming the gap  105  between the first substrate section  102  and the first waveguide  106 . The effective refractive index of the first waveguide is spatially modulated along the optical axis  107 , forming a grating for reflecting the optical frequency component  109  of the light  108  guided by the ridge waveguide  106  to propagate back. At least one of phase or amplitude of the spatial modulation of the effective refractive index is varying along the optical axis, forming the modulation crests  116 - 1  and valleys  116 - 2  (best seen in  FIGS. 1B and 1C ) spaced apart at a first spatial frequency f 1  along the optical axis  107 . 
         [0069]    Step B includes providing first and second arrays of the openings  118 A to  118 D extending through the first waveguide  106  and into the gap  105 , the openings  118 A to  118 D running on respective opposite first and second sides of the optical axis  107  as shown. The openings ( 118 A,  118 C) and ( 118 B,  118 D) define first and second arrays of heat conducting fingers  119 A and  119 B, respectively, extending from the optical axis  107  towards the support bars  104 . Step B includes disposing the openings  118 A to  118 D so that positions of the heat conducting fingers  119 A and  119 B along the optical axis  107  are coordinated with the positions of the modulation crests  116 - 1  along the optical axis  107 . Step B can also include disposing the first and last, or end openings  118 C and  118 D to achieve a substantially uniform longitudinal temperature profile along the length of the first waveguide  106  containing the grating. 
         [0070]    Step C includes heating the first waveguide  106  for tuning the optical frequency of the optical frequency component, thereby tuning the laser diode. Due to coordination of the position of the heat conducting fingers  119 A and  119 B with the modulation crests  116 - 1 , the optical phase coherence of the light mode  108  is maintained between the modulation crests  116 - 1 , so that the spectral bandshape of the light  109  reflected by the tunable Bragg grating  100  is not disturbed, or at least disturbed less, in the process of tuning. 
         [0071]    In one embodiment, Step B comprises disposing the openings  118 A to  118 D so that the first and second arrays of the respective heat conducting fingers  119 A and  119 B are disposed at a second spatial frequency along the optical axis  107 , wherein the second spatial frequency is an integer multiple of the first spatial frequency. 
         [0072]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.