Abstract:
Embodiments of wavelength tunable lasers are disclosed. The wavelength tunable lasers include thermo-optic organic material that has an index of refraction that can quickly vary in response to changes in temperature. By controlling the temperature in the thermo-optic organic material through the use of heaters or coolers, the wavelength tunable lasers and the integrated optical components can be quickly and selectively tuned over a broad range of wavelengths with high spectral selectivity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to the field of optical devices that manipulate optical energy of tightly controlled optical wavelength, particularly for use in communication applications. More particularly, the invention relates to lasers which produce optical energy of a specified wavelength and which can be tuned or switched to other specified wavelengths by thermal means.  
           [0003]    2. Description of the Related Art  
           [0004]    Over the past several years, there has been ever-increased interest in tunable lasers for use in optical communication systems, in general, and for use in dense wavelength division multiplexing (DWDM) applications, in particular. DWDM allows high bandwidth use of existing optical fibers, but requires components that have a broad tunable range and a high spectral selectivity. Such components include tunable lasers that should be able to access a large number of wavelengths within the S-band (1490-1525 nanometers), the C-band (1528-1563 nanometers), and the L-band (1570-1605 nanometers), each different wavelength separated from adjacent wavelengths by a frequency separation of 100 MHz, 50 MHz, or perhaps even 25 MHz.  
           [0005]    The distributed Bragg reflector (DBR) laser was the first such tunable laser used in optical communication. The DBR laser consisted of a semiconductor amplifier medium, defining an active section, and an optical waveguide. The optical waveguide included a portion without a grating that defined a phase control section and a portion in which a single grating of typically constant pitch (Λ) was formed which constituted a distributed Bragg reflector or, more simply, the Bragg section that reflected light at the Bragg wavelength λ B . Wavelength tuning of such a DBR laser was performed by transferring heat into the phase control section, the Bragg section, or both. The optical waveguide was defined by an organic layer which constituted a core with another organic confinement layer disposed both above and below the core. Wavelength tuning of such a DBR laser was performed by either injecting current or transferring heat into the phase control section, the Bragg section, or both. Injecting minority carriers made it possible to vary the refractive index of the waveguide and thus control the Bragg wavelength λ B  by the equation λ B =2n eff  Λ where Λ is the pitch of the grating and n eff  is the effective refractive index of the waveguide. Alternatively, a pair of heating resistance strips was disposed on opposite outer surfaces of the laser component for the phase control section, the Bragg section, or both. By independently controlling the voltages to the heating resistance strips, the temperature and hence the index of refraction of the organic layers that form the optical waveguide was controlled via the thermo-optical effect. Tuning by injecting current had the disadvantage of increasing optical loss and adding optical noise. Tuning by heating had the disadvantage of increasing optical loss and adding optical noise. Both options induce long-term drift in the Bragg wavelength thereby reducing reliability. For a more detailed discussion of a wavelength tunable DBR laser by heating, please refer to U.S. Pat. No. 5,732,102 by Bouadma entitled “Laser Component Having A Bragg Reflector of Organic Material, And Method of Marking It” which is hereby incorporated by reference.  
           [0006]    A super structure grating distributed Bragg reflector (SSG-DBR) laser was another type of tunable laser that held great promise. The InGaAsP—InP SSG-DBR laser was comprised of a semiconductor amplifier medium with an InGaAsP/InGaAsP multiple quantum wells active region, an SSG-DBR section on both sides of the semiconductor amplifier medium, and a phase control section between one of the SSG-DBR sections and the semiconductor amplifier medium. Thin film Pt heaters were formed on the top surface and corresponding electrodes were formed on the bottom surface of each SSG-DBR section and the phase control section. The two SSG-DBR sections were used as mirrors with different sampling periods giving different peak separations and different reflective combs in the reflectivity-wavelength spectrum. In the reflectivity-wavelength spectrum, only one reflective peak associated with each SSG-DBR section coincided and where these reflective peaks coincided at a cavity mode, that cavity mode was selected for lasing. Wavelength tuning of the SSG-DBR laser was performed by injection current into or heating of either SSG-DBR section or the phase control section. Current injection into or heating of the SSG-DBR sections changed the refractive index of each waveguide and shifted the reflection spectrum of each SSG-DBR section. Similarly, current injection into or heating the phase control section shifted the cavity modes. While providing a broad tuning range, wavelength tuning by injection current caused considerable spectrum line width broadening and a decrease in emitted power, both important criteria in DWDM applications. Further, the long term affects of wavelength tuning by injection currents on SSG-DBR laser performance remains unknown. In addition, current SSG-DBR lasers are monolithic devices fabricated from InGaAsP/InP and the manufacture of such SSG-DBR lasers results in low yield because of the immaturity of the InP or GaAs based processing technology. For a more detailed discussion of a wavelength tunable SSG-DBR laser by injection current, please refer to a paper by Ishii et al. entitled “Narrow Spectral Linewidth Under Wavelength Tuning in Thermally Tunable Super-Structure-Grating (SSG) DBR Lasers,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, Pages 401-407, June 1995, which is hereby incorporated by reference.  
           [0007]    For a more detailed discussion of the state of the art on widely tunable lasers, please refer to a paper by Rigole et al. entitled “State-of-the-art: Widely Tunable Lasers,” SPIE, Vol. 3001, Pages 382-393, 1997, which is hereby incorporated by reference.  
         SUMMARY OF THE INVENTION  
         [0008]    Embodiments of novel tunable lasers are disclosed which can quickly and repeatedly access a broad range of relevant wavelengths with high spectral selectivity yet without the problems associated with the prior art.  
           [0009]    A first embodiment of the novel tunable laser includes a substrate fabricated of a first material that supports a gain means, a first waveguide, and a second waveguide. The gain means is fabricated of a second material and includes an active emission layer that generates optical energy. The active emission layer includes a first and a second facet. The first waveguide includes a first core and a first end on the first core, which may include a first taper, is adjacent to the first facet to receive the optical energy. The first core is fabricated from an inorganic material and the first waveguide is fabricated from both inorganic and thermo-optical organic material. A first reflector receives the optical energy propagating along the first waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. The second waveguide includes a second core and a first end on the second core, which may include a taper, is adjacent to the second facet and receives optical energy. The second core is fabricated from an inorganic material and the second waveguide is fabricated from both inorganic and thermo-optical organic material. A second reflector receives the optical energy propagating along the second waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of second wavelengths. Between the first end of the first reflector and the first reflector along a reflector free-portion of the first waveguide, there may be a phase control section which can slightly shift the Fabry-Perot resonant cavity modes associated with the tunable laser. Thermo-optical organic material is disposed to shift the plurality of first reflection wavelengths, the plurality of second reflection wavelengths, and the Fabry-Perot resonant cavity modes in response to changes in the temperature in the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material associated with the first reflector, the second reflector, the phase control portion, or combinations thereof, a broad wavelength tuning range with high spectral selectivity is possible.  
           [0010]    A second embodiment of the novel tunable laser includes a substrate fabricated of a first material that supports a gain means and a waveguide. The gain means is fabricated of a second material and includes an active emission section, which generates optical energy, and includes a facet. The waveguide includes a core and an end on the core, which may include a taper, is adjacent to the facet to receive optical energy. The core is fabricated from an inorganic material and the waveguide is fabricated from both inorganic and thermo-optical organic material. A first reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. A second reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of second wavelengths. Between the end and the first reflector and between the first and second reflectors, both along a reflector free-portion of the waveguide, there may be phase control sections which can slightly shift the Fabry-Perot resonant cavity modes associated with the tunable laser and an etalon formed by the first and the second reflectors. Thermo-optical organic material is disposed to shift the plurality of first reflection wavelengths, the plurality of second reflection wavelengths, and the Fabry-Perot resonant cavity modes in response to changes in the temperature of the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material associated with the first reflector, the second reflector, the phase control portions, or combinations thereof, a broad wavelength tuning range with high spectral selectivity is possible.  
           [0011]    A third embodiment of the novel tunable laser includes a substrate that supports a gain means and a waveguide. The gain means includes an active emission layer, which generates optical energy, and includes a facet. The waveguide includes a core and an end on the core, which may include a taper, is adjacent to the facet and receives the optical energy. The core is fabricated from inorganic material and the waveguide is fabricated from both inorganic and thermo-optical organic material. A reflector receives the optical energy propagating along the waveguide and reflects the optical energy if the optical energy has a wavelength that is one of a plurality of first reflection wavelengths. Thermo-optical organic material is disposed to shift the plurality of reflection wavelengths in response to changes in the temperature in the thermo-optical organic material. Tuning of the laser may be achieved by changing the temperature in the thermo-optical organic material which has an index of refraction that varies in response to changes in temperature. By varying the temperature of heaters or coolers in the thermo-optical organic material, a broad wavelength tuning range with high spectral selectivity is possible.  
           [0012]    The thermo-optical organic material of the tunable laser is preferably selected so as to have a high coefficient of variation in refractive index as a function of temperature, the magnitude of which should be preferably greater than 1×10 −4 /° C. Examples of thermo-optical organic material used in the tunable laser and that exhibit these characteristics include polymers derived from methacrylate, siloxane, carbonate, styrene, cyclic olefin, or norbornene.  
           [0013]    An integrated optical component is also disclosed for the second embodiment of the tunable laser above. The integrated optical component includes all the functional elements associated with the respective embodiment of the tunable laser, but does not include the gain means that is typically fabricated from a different material than the S integrated optical component.  
           [0014]    It should be observed that, except for the gain means, the tunable laser is fabricated using Si processing technology and only the gain means is of GaAs, InP, InGaAsP, or other exotic semiconductor materials which requires complex and sensitive processing technology, such as epitaxial growth and cleaving. The gain means is independently fabricated with a minimum of structure. Accordingly, the tunable laser is easy to manufacture, cost effective to manufacture, and results in high yield.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:  
         [0016]    [0016]FIG. 1A is a right side elevational view of a first embodiment of a thermally wavelength tunable laser accordance with the principals of this invention;  
         [0017]    [0017]FIG. 1B is a right side elevational view along line  1 B- 1 B in FIG. 1A;  
         [0018]    [0018]FIG. 1C is a detailed front view of the laser diode along the line  1 C- 1 C shown in FIG. 1A;  
         [0019]    [0019]FIG. 1D is a diagrammatic representation of a top view along the line  1 D- 1 D in FIG. 1A which follows the optical path of the thermally wavelength tunable laser;  
         [0020]    [0020]FIG. 1E is a front view of a first embodiment of the heaters and the waveguide along the line  1 E- 1 E in FIG. 1A;  
         [0021]    [0021]FIG. 1F is a front view of a second embodiment of the heaters and the waveguide along the line  1 E-E in FIG. 1A;  
         [0022]    [0022]FIG. 2A is a right side elevational view of a second embodiment of a thermally wavelength tunable laser accordance with the principals of this invention;  
         [0023]    [0023]FIG. 2B is a right side elevational view along the line  2 B- 2 B in FIG. 2A;  
         [0024]    [0024]FIG. 2C is a diagrammatic representation of a top view along the line  2 C- 2 C in FIG. 2A which follows the optical path of the thermally wavelength tunable laser;  
         [0025]    [0025]FIG. 2D is a front view of a first embodiment of the heaters and the waveguide along the line  2 D- 2 D in FIG. 2A;  
         [0026]    [0026]FIG. 2E is a front view of a second embodiment of the heaters and the waveguide along the line  2 D- 2 D in FIG. 2A;  
         [0027]    [0027]FIG. 3A is a right side elevational view of a third embodiment of a thermally wavelength tunable laser according with the principals of this invention;  
         [0028]    [0028]FIG. 3B is a side view along the line  3 B- 3 B in FIG. 3A;  
         [0029]    [0029]FIG. 3C is a diagrammatic representation of a top view along the line  3 C- 3 C in FIG. 3A which follows the optical path of the thermally wavelength tunable laser;  
         [0030]    [0030]FIG. 4 is a diagrammatic representation of the Fabry-Perot resonant cavity modes and the gain envelop associated with the tunable laser shown in FIG. 2;  
         [0031]    [0031]FIG. 5A is a diagrammatic representation of the reflection spectrum associated with the first reflector in the thermally wavelength tunable laser shown in FIG. 2;  
         [0032]    [0032]FIG. 5B is a diagrammatic representation of the reflection spectrum associated with the second reflector in the thermally wavelength tunable laser shown in FIG. 2;  
         [0033]    [0033]FIG. 6A is a diagrammatic representation of the product of the reflection spectrums associated with the first reflector shown in FIG. 5A and with the second reflector shown in FIG. 5B, the selected reflection peaks showing the Fabry-Perot resonant cavity modes located therein;  
         [0034]    [0034]FIG. 6B is a diagrammatic representation of the selected reflection peaks shown in FIG. 6A having been shifted in wavelength due to thermal tuning of the first and second reflectors;  
         [0035]    [0035]FIG. 6C is a diagrammatic representation of the selected Fabry-Perot resonant cavity modes shown in FIG. 6B having been shifted due to thermal tuning in the phase control section;  
         [0036]    [0036]FIG. 7A is a diagrammatic representation of the reflection spectrum associated with the first reflector in the thermally wavelength tunable laser shown in FIG. 3;  
         [0037]    [0037]FIG. 7B is a diagrammatic representation of the reflection spectrum associated with the second reflector in the thermally wavelength tunable laser shown in FIG. 3;  
         [0038]    [0038]FIG. 8A is a diagrammatic representation of the coherent addition of the reflection spectrums associated with the first reflector shown in FIG. 7A and the second reflector shown in FIG. 7B, all reflection peaks showing the Fabry-Perot resonant cavity modes located therein; and  
         [0039]    [0039]FIG. 8B is a diagrammatic representation of select Fabry-Perot resonant cavity modes within the composite reflection spectrum shown in FIG. 8A having been shifted due to thermal tuning in the appropriate phase control sections. 
     
    
       [0040]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    Referring now to FIG. 1A, there is illustrated a first embodiment of a laser  10  that is highly wavelength tunable and has high spectral selectivity. The tunable laser  10  includes a gain means  12  which provides optical energy and a passive section  14  which processes the optical energy received from the gain means  12 . The passive section  14  includes a taper section  16  which couples the optical energy to a first waveguide, a phase control section  18  which slightly shifts the Fabry-Perot resonant cavity modes associated with the tunable laser  10 , and a reflector section  20  which may reflect optical energy dependent upon the wavelength of the optical energy.  
         [0042]    Most generally, the gain means  12  provides sufficient optical energy to overcome the losses associated with the components that make up the tunable laser  10  and to create oscillation within the tunable laser  10 . The gain means  18  includes a first facet  21  (FIG. 1D) and a second facet  22 . The second facet  22 , most preferably, has a highly reflective (HR) coating thereon, but may also have a partially reflective and partially transmissive coating thereon, depending on whether optical energy will be outputted from this facet.  
         [0043]    Referring now to FIG. 1C, the gain means  12  has been flipped over and flip chip bonded to a cladding layer  24  which is disposed on a substrate  26  of the tunable laser  10 . The gain means  12  is a solid-state laser which is preferably a semiconductor diode laser. The gain means  12  may be a ridge laser or a buried hetro-structure with or without multiple quantum wells. As shown, the gain means  12  is a ridge laser that is preferably fabricated on InP so as to emit in the 1550 nm region or the 1310 nm region. Alternatively, the gain means  12  may be fabricated on other convenient substrates such as sapphire or gallium arsenide. The gain means  12  includes a substrate  28  of n-type InP and sequentially deposited on a major surface of the substrate  28  is an adhesion layer  30  typically formed of titanium, a diffusion barrier layer  32  typically formed of platinum, and a bonding layer  34  typically formed of gold. A first clad layer  37  is formed on the other major surface of the substrate  28 . An active emission layer  36  of a semiconductor material, such as InGaAsP or InGaAlP that preferably includes strained quantum wells, is formed on a major surface of the first clad layer  37  and provides the optical energy of the tunable laser  10 . The dimensions of the active emission layer  36  are variable, but may typically be a fraction of a micron in the y direction (thickness) and at least a couple of microns in the x direction (width). The optical energy produced by the active emission layer  36  is typically a single transverse mode with a mode size at full width half maximum (FWHM) of approximately 0.6 microns in the y direction (height) and approximately 3 microns in the x direction (width). A second clad layer  38  is disposed on the other major surface of the active layer  36 . Both clad layers  37  and  38  are formed of a lower refractive index semiconductor material than the active emission layer  36 . On the surface of clad layer  38 , a contact layer  40  which provides low electrical resistance is grown. All of these layers may be structured into sublayers as is known in the art.  
         [0044]    A plurality of solder balls  42 ,  44 , and  46  connect the gain means  12  to the cladding layer  24 . Each solder ball includes a first adhesion layer  48  typically formed of titanium, a diffusion barrier layer  50  typically formed of platinum, a bonding layer  52  typically formed of gold, a solder ball  54  typically formed of 80% gold and 20% tin, a bonding layer  56  typically formed of gold, and a second adhesion layer  58  typically formed of chromium. Many alternative solder, barrier, adhesion, and dewetting materials may also be used, to enable processing at different temperatures, as may be alternative metal layers. An external electrical contact (not shown) exists on the bonding layer  34  and the bonding layer  56  of the central solder ball  44  thereby enabling the active emission layer  36  to be fed with amplification current in the region of the optical mode. Trenches  60  and  62  are etched through the contact layer  40  and through most of the cap layer  38  on both sides of the central solder ball  44  which effectively bounds most of the generated optical energy to the active emission layer  36  between the trenches  60  and  62 .  
         [0045]    During operation, the gain means  12  generates several hundred milli-watts of thermal power, the majority of which is generated in the active emission layer  36  between the trenches  60  and  62 , and this thermal power is dissipated through an efficient thermal flow through and beneath the gain means  12 . Thermal power generated in the active emission layer  36  is dissipated through the plurality of solder balls  42 ,  44 , and  46 , through the cladding layer  24  that is typically formed of silica, and into the substrate  26  that is typically formed of silicon, but may be also formed of sapphire, gallium arsenide, indium phosphide, metal, glass, or ceramic. The substrate  26  is substantially thicker than the cladding layer  24  and acts as a heat reservoir for the tunable laser by maintaining a relatively constant temperature with a low thermal gradient. Preferably, the rise above ambient temperature in the gain means  12  is kept beneath 50° C. and, more preferably, beneath 10° C. A single or a plurality of heat sinks (not shown) may be disposed beneath the substrate  26  in order to aid in dissipating thermal power from, most importantly, the gain means  12 , but also from the passive section  14 , as the cladding layer  24  and the substrate  26  are common to the gain means and the passive section  14 . A thermal sensor (not shown) may also be disposed near the gain means  12  to control the heat sinks (not shown) and to thereby regulate the temperature in the gain means  12 , the cladding layer  24 , and the substrate  26 .  
         [0046]    Referring now to FIG. 1E, a first embodiment of the heaters and the waveguide associated with the reflector section  20  are shown. Specifically, the cladding layer  24  is etched to produce pedestal regions  64 ,  66 , and  68  which have a height of approximately 1.5 μm. The pedestal region  64  has a width of approximately 3 μm and the pedestal regions  66  and  68  have a width of approximately 7 μm. A germanosilicate (GeSiO 2 ) layer is deposited on the cladding layer  24  and etched which defines a first core  70  on the pedestal region  64 , the first core  70  forming part of a waveguide described below. As best shown in FIG. 1B, the first core  70  includes a first end  72  which has an anti reflection (AR) coating thereon to prevent back reflection into the first core  70  and a second end  74  which may have either an AR coating or a partially reflective and partially transmissive coating, depending on whether optical energy exits the tunable laser  10  via the second end  74 . The first core  70  is between 1 to 3 μm thick, between 2 to 20 mm long, and can also be doped silica with germanium, nitrogen, lead, tin, phosphorous, boron, or combinations thereof. As best shown in FIG. 1B, a first taper  76  is formed on the top surface of the first core  70  and a first surface  78  of the first taper  76  is aligned with that portion of the active emission layer  36  that is between the trenches  60  and  62  so as to couple as much of the optical energy produced by the active emission layer  36  as possible into the first core  70 . Alternatively, the taper  76  can be directly incorporated into the gain means  12 , rather than into the first core  70 . In this situation, the first end  72  is no longer AR coated and is also aligned with that portion of the active emission layer  36  that is between the trenches  60  and  62 . Alternatively, the gain means  12  and the first waveguide  90 , defined below, can be designed so that the size of the optical mode propagating from the active emission layer  36 , into and then along the first core  70  remains constant and with minimal optical loss. In this situation, a taper would not be necessary, the first end  72  would no longer be AR coated, and the first end  72  would be aligned with that portion of the active layer  36  that is between the trenches  60  and  62 .  
         [0047]    Referring again to FIG. 1E, heaters  80  and  82  are disposed upon pedestal regions  66  and  68 . The heaters  80  and  82  are chromium (Cr) in this embodiment, but may be of any conductive material including NiCr, Ti and W. The heaters  80  and  82  are approximately 7 μm wide, between 0.05 to 0.1 μm thick, and approximately 1 mm in length. These heater dimensions are chosen to produce the desired heat output per unit length, and may be adjusted as is well known in the art to change the material, the heat production, and the longevity of the heaters. Electrical contacts and wires (both not shown) are provided to apply a potential to or for passing current through each heater. The total distance from the heaters  80  and  82  to the first core  70  is the distance (d). The distance (d) is chosen so that (a) the optical mode experiences minimal absorption loss caused by the material of the heaters and (b) the temperature of the thermo-optical organic material, discussed below, disposed adjacent to the first core can be quickly and efficiently changed. In FIGS. 1E and 1F, the distance (d) is at least 12 μm. The heaters shown in FIG. 1 are resistive heaters, but this invention contemplates the use of thermoelectric heaters or coolers that employ the Peltier effect. Specifically, thermoelectric heaters or coolers that employ the Peltier effect may be disposed on the pedestal region  66  and  68 , the cladding layer  24 , or the substrate  26 . Stated as simply as possible, thermoelectric heaters and coolers that employ the Peltier effect are semiconductor materials with dissimilar characteristics that are connected electrically in series and thermally in parallel so that two junctions are created, namely, a hot and a cold junction. If operating as a thermoelectric cooler, the cold junction should be located near the core while the hot junction should be as close to the heat sinks (not shown) as possible. Similarly, if operating as a thermoelectric heater, the hot junction should be located near the waveguides while the cold junction should be as close to the heat sinks (not shown) as possible.  
         [0048]    The phase control section  18  (FIG. 1A) has the same heater and waveguide structure (not shown) as that shown in FIG. 1E. A pair of heaters  84  and  86  associated with the phase control section are shown in FIG. 1D. Referring again to FIG. 1D, a thermo-optical organic material  88  is applied preferably by spinning onto and over the heaters  80 ,  82 ,  84 , and  86 , the first core  70 , and onto the cladding layer  24 . The thermo-optical organic material  88  has a high coefficient of variation in its&#39; refractive index as a function of temperature, the magnitude of which is preferably greater than 1×10 −4 /° C. The index of refraction of the thermo-optical organic material  88  is preferably close to or equal to the index of the cladding layer  24  at the normal operating temperature of the tunable laser  10 , namely, the temperature from which the heaters must start heating the thermo-optical organic material  88 . Specific materials may be selected for the thermo-optical organic material including, but not limited to, methacrylates, siloxanes, carbonates, styrenes, cyclic olefins, and norbornenes. It is useful to adjust the index of refraction of these materials by fluorination (replacing hydrogen molecules with fluorine molecules in the molecular formula of some of the polymer repeat units) as this has the added benefit of reducing the optical loss in the infrared region. Many of these materials meet the optical specifications for the thermo-optical organic material  88 . A specific material may be chosen according to an optimization process of the secondary characteristics such as minimizing birefringence, residual stress, and chemical reactivity, while maximizing wetting, adhesion, working lifetime, and thermal resistance. The thickness of the thermo-optical organic material  88  is chosen such the thermo-optical organic material-air interface adds only minimal and preferably no optical loss to the optical performance of the tunable laser  10 .  
         [0049]    Referring now to FIG. 1E, a first waveguide  90  includes the first core  70 , portions of the thermo-optical organic material  88  adjacent to the first core  70 , and portions of the cladding layer  24  beneath the first core  70 , as shown in FIGS. 1E and 1F. The diameter of the first waveguide  90  encompasses essentially all the optical mode. The mode size and shape is dependant upon the temperature of the thermo-optical organic material  88  adjacent to the first core  70 . At room temperature and with the heaters off, the index of refraction of the thermo-optical organic material  88  is at its highest, but is lower than the index of the first core  70 . The optical mode under these conditions will be in the first core  70  and will be partially in both the thermo-optical organic material  88  and the cladding layer  24 . With the heaters  80  and  82  (FIG. 1E) on, the temperature of the thermo-optical organic material  88  adjacent to the first core  70  increases and the index of refraction of the thermo-optical organic material  88  adjacent to the first core  70  decreases. Under these conditions, the optical mode will have appeared to have “sunk” towards the cladding layer  24  when compared to the location of the optical mode at room temperature. As portions of the thermo-optical organic material  88  adjacent to the first core  70  and along the z-axis (FIG. 1E) are heated while other portions remain at room temperature, the size and shape of the optical mode along the z direction of the first core  70  changes, but the change is preferably gradual, adiabatic, and therefore with minimal optical loss.  
         [0050]    [0050]FIG. 1F shows a second embodiment of the heaters and the waveguide associated with the reflector and the phase control sections. The second embodiment differs from the first embodiment in that the second embodiment does not include the pedestal regions  64 ,  66 , and  68  which are found in the first embodiment. The removal of the pedestal regions makes the fabrication process for the tunable laser of the second embodiment simpler but also reduces the effective tuning range when compared to that of the first embodiment.  
         [0051]    Referring now to FIG. 1B, the gain means  12  (FIG. 1A) is flip chip bonded to the cladding layer  24  so as to couple as much optical energy from the active emission layer  36  and into the first taper  76 , as possible, but without introducing any parasitic reflections. Most preferably, the active emission layer  36  between the trenches  60  and  62  is aligned with the first taper  76  along the y (thickness) and x (width) directions and a gap  92  (FIG. 1D) between the first facet  21  of the gain means  12  and the first end  78  of the first taper  76  is minimized in order to minimize the divergence of the optical energy as the optical energy propagates between the first facet  21  and the first end  78 . Typically, the gap  92  along the z direction (FIG. 1D) is on the order of 5 microns. After the gain means  12  is flip chip bonded to the cladding layer  24 , the thermo-optical organic material  88  is applied to provide coverage without incorporating voids or bubbles. The thermo-optical organic material  88  fills the gap  92  between the first facet  21  of the gain means  12  and the first end  78  of the first taper  76 , providing an advantageous index matching effect. To enable electrical connection to the n-contact  34  of the gain means  12 , a portion of the thermo-optical organic material  88  is removed, preferably by reactive ion etching through a lithographically patterned mask, from a region above the gain means  12  which leaves a slot  94 .  
         [0052]    A first reflector  96  is fabricated in the first core  70  by using ultraviolet exposure of a portion of the first core  70  to form a periodic or structured reflector. If the optical energy is outputted from the second end  74 , the first reflector  96  is preferably partially transmitting and partially reflecting at the operating wavelength. If the optical energy is outputted from the second facet  22 , the first reflector  96  is preferably highly reflecting with 90% or more reflectivity. The first reflector  96  typically is a specialized Bragg grating of base periodicity from 0.2 to 0.6 microns, but with additional phase and amplitude structure periodically repeated with a period (As) from 50 to 500 μm, dependant on the material being written on or into, and the reflection spectrum desired. For example, if the material of the first core were silica, then the base periodicity of the first reflector  96  would be approximately 530 nanometers. Similarly, if the material of the first core were silicon, then the base periodicity of the first reflector  96  would be approximately 200 nanometers. Due to the periodic structure, the optical spectrum of the first reflector  96  exhibits multiple reflection peaks, known as a comb of peaks, in the wavelength domain of individually defined amplitude and wavelength spacing. The separation between adjacent peaks in the comb, dλ, is given by:  
           d λ=λ 2 /[2 n   g Λ s ] 
         [0053]    where n g  is the effective group index. Basically, the separation between adjacent peaks in the comb is controlled by the period Λ s  while the envelope containing the peaks depends on the grating modulation function inside one sampling period. The first reflector  96  may alternatively be a UV written or an etched grating and located either in, on, or adjacent to the first core  70  so long as the optical spectrum of the reflector exhibits the comb of peaks discussed above. The heaters  80  and  82  (FIG. 1D) associated with the reflector section  20  are also disposed on both sides of the first reflector  96  and the length of the heaters  80  and  82  exceeds the length of the first reflector  96  so that the entire length of the first reflector  96  can be maintained at a uniform temperature. The heaters  84  and  86  (FIG. 1D) associated with the phase control section  18  are also disposed on both sides of a reflector free portion of the first core  70  between the first taper  76  and the first reflector  96 .  
         [0054]    Referring now to FIG. 1D, the heaters  80 ,  82 ,  84 , and  86  must generate sufficient thermal power so that the thermo-optical organic material  88  can modify the optical performance of the portion of the first core  70  between the heaters  84  and  86  and the first reflector  96  (collectively, Optical Elements) within a few milliseconds. The thickness of the thermo-optical organic material  88  disposed around the Optical Elements is important in determining the response time. If too thick, additional thermal power must be generated to change the index of refraction of the remaining thermo-optical organic material which does not modify the optical performance of the Optical Elements, but which increases the response time. If too thin, the thermo-optical organic material may overlap a smaller portion of the optical mode and may not be able to sufficiently modify an effective index in the Optical Elements. In this embodiment, the thickness of the thermo-optical organic material  88  disposed around the Optical Elements is at least 20 microns. Similarly, the cladding layer  24  beneath the heaters provides a degree of thermal isolation between the substrate and the heaters so that a larger fraction of the thermal power generated by the heaters modifies the optical performance of the Optical Elements rather than dissipates to the substrate  26 . Given the different purposes, the cladding layer  24  preferably has different thicknesses beneath the gain means and beneath the heaters.  
         [0055]    Referring again to FIG. 1D, the optical path of the tunable laser  10  is shown. In FIG. 1, optical energy propagates along the active emission layer  36  between the trenches  60  and  62 , the first taper  76 , and the first core  70  which collectively define an optical axis  98  within the tunable laser  10 . The optical axis  98  is angled near the first facet  21  of the gain means  12  and near the second end  74  of the first core  70  so that the optical axis  98  traverses the intracavity interfaces such as  21  and  78  and the extracavity interfaces such as  74  at a non-normal angle so as to prevent parasitic reflections from degrading the performance of the tunable laser  10 . The optical axis  98  is curved within the active emission layer  36  so that the second facet  22  of the gain means  18  forms one end and the reflector  96  (FIG. 1B) forms the other end of the laser cavity associated with the tunable laser  10 . In order for the tunable laser  10  to lase, the gain associated with active emission layer  36  must be greater than losses associated with the gain means and the laser cavity. The losses associated with the laser cavity include, but are not limited to: the coupling losses between the first facet  21  and the first end  78 , the coupling losses associated with the first taper  76 , the losses propagating through the thermo-optical organic material  88  and the first waveguide  90 , the reflection losses associated with the first reflector  96 , the reflections at the interfaces  21  and  78 , and any other parasitic reflections. Each loss element in the laser cavity shown should be no larger than a few decibels (dB) and preferably smaller than 0.5 dB so that the collective single pass loss along the optical axis  98  of the laser cavity is no larger than about 5 to 20 decibels.  
         [0056]    Referring now to FIGS. 2A and 2B where like elements are designated with like numerals, there is illustrated a second embodiment of the tunable laser  100 . The tunable laser  100  includes the tunable laser  10  and, adjacent to the second facet  22  which is now AR coated, a second passive section  102 . The second passive section  102  includes a second taper section  104  and a second reflector section  108  which are similar to those corresponding sections and elements within those sections in the passive section  14 , described above.  
         [0057]    Referring now to FIGS. 2D and 2E, embodiments of the heaters and the waveguide associated with the second reflector section  108  are shown and are similar to the one shown in FIGS. 1E and 1F. Referring now to FIG. 2D, the cladding layer  24  is etched to produce pedestals  110 ,  112 , and  114  and upon these pedestals are, respectively, deposited a second core  116  and heaters  118  and  120 . As best shown in FIG. 2B, the second core  116  includes a first end  122  with an AR coating to prevent back reflection and a second end  124  which may have an AR coating or a partially reflective and partially transmissive coating thereon depending on whether optical energy exits the tunable laser  100  via the second end  124  or the second end  74 . A second taper  126  is formed on the top surface of the second core  116  and a first surface  128  of the second taper  126  is aligned with that portion of the gain means  12  that is between the trenches  60  and  62  so as to couple as much optical energy produced by the gain means  12  as possible into the second core  116 . Most preferably, the first surface  128  is aligned with the active emission layer  36  between trenches  60  and  62  along the y (thickness) and x (width) directions and a gap  142  (FIG. 2C) between the second facet  22  and the first end  128  is minimized in order to minimize the divergence of the optical energy as the optical energy propagates between the second facet  22  and the first end  128 . Typically, the gap  142  along the z direction is on the order of 5 microns. Alternatively, if the second taper  128  is incorporated into the gain means  12  or the gain means  12  and the second waveguide  138 , discussed below, are such that the size of the optical mode propagating from the active emission layer  36  and into and then along the second waveguide  138  remains constant and with minimal optical loss, then the first end  122  would no longer be AR coated and would be aligned with that portion of the active layer  36  that is between the trenches  60  and  62 .  
         [0058]    Referring once again to FIG. 2D, a thermo-optical organic material  134  which is the same as that described for the thermo-optical organic material  88 , is applied preferably by spinning onto and over the heaters  118  and  120 , the second core  116 , and onto the cladding layer  24 . Referring now to FIG. 2C, the thermo-optical organic material  134  fills the gap  142  between the gain means  12  and the first end  128 , providing an advantageous index matching effect. Referring now to FIG. 2B, reactive ion etching through a lithographically patterned mask is preferably used to remove a region of the thermo-optical organic material  134  near the gain means  12  thereby leaving a slot  136  to enable electrical connection to n-contact  34  of the gain means  12 .  
         [0059]    Referring now to FIG. 2D, a second waveguide  138  includes the second core  116 , portions of the thermo-optical organic material  134  adjacent to the second core  116 , and portions of the cladding layer  24  beneath the second core  116 , as shown in FIGS. 2D and 2E. The diameter of the second waveguide  138  encompasses essentially all the optical mode. The mode size and shape is dependant upon the temperature of the thermo-optical organic material  134  adjacent to the second core  116 . At room temperature and with the heaters off, the index of refraction of the thermo-optical organic material  134  is at its highest, but is lower than the index of the second core  116 . The optical mode under these conditions will be in the second core  116  and will be partially in both the thermo-optical organic material  134  and the cladding layer  24 . With the heaters  118  and  120  (FIG. 2D) on, the temperature of the thermo-optical organic material  134  adjacent to the second core  116  increases and the index of refraction of the thermo-optical organic material  134  adjacent to the second core  116  decreases. Under these conditions, the optical mode will have “sunk” towards the cladding layer  24  when compared to the location of the optical mode at room temperature. As portions of the thermo-optical organic material  134  adjacent to the second core  116  and along the z-axis (FIG. 2D) are heated while other portions remain at room temperature, the size and shape of the optical mode along the z direction of the second core  116  changes, but the change is preferably gradual, adiabatic, and therefore with minimal optical loss.  
         [0060]    Referring now to FIG. 2B, a second reflector  140  is fabricated in the second core  116  by using ultraviolet exposure of a portion of the second core  116 . The second reflector  116  typically is a specialized Bragg grating of base periodicity from 0.2 to 0.6 microns, but with additional phase and amplitude structure periodically repeated with a period (Λ s ) from 50 to 500 μm, dependant on the material being written on or into, and the reflection wavelength desired. Due to the periodic structure, the optical spectrum of the second reflector  116  exhibits multiple reflection peaks, known as a comb of peaks, in the wavelength domain of individually defined amplitude and wavelength spacing. The separation between adjacent peaks in the comb, dλ, is given by:  
           dλ=λ   2 /[2 n   g Λ s ] 
         [0061]    where n g  is the effective group index. Basically, the separation between adjacent peaks in the comb is controlled by the period Λ s  while the envelope containing the peaks depends on the grating modulation function inside one sampling period. The heaters  118  and  120  (FIG. 2C) associated with the second reflector section  108  are also disposed on both sides of the second reflector  140  and the length of the heaters  118  and  120  exceed the length of the second reflector  140  so that the entire length of the second reflector  140  can be maintained at a uniform temperature.  
         [0062]    Referring now to FIG. 2C, the optical path of the tunable laser  100  is shown. In FIG. 2, the optical energy propagates along the second taper  126 , the second core  116 , the active layer  36  between the trenches  60  and  62 , the first taper  76 , and the first core  70  which collectively define an optical axis  144 . The optical axis  144  is angled in the tunable laser  10  as described above. In the second passive section  102  of the tunable laser  100 , the optical axis  144  is angled near the second facet  22  of the gain means  12  and near the second end  124  of the second core  116  so that the optical axis  144  transverses the intracavity interfaces such as  22  and  124  and the extracavity interfaces such as  128  at non normal angles so as to prevent parasitic reflections from degrading the performance of the tunable laser  100 . The second reflector  140  (FIG. 2B) forms one end and the first reflector  96  (FIG. 2B) forms the other end of the laser cavity associated with the tunable laser  100 . In order for the tunable laser  100  to lase, the gain associated with the active emission layer  36  must be greater than the losses associated with gain means  12  and the laser cavity, namely, the passive section  14  and the second passive section  102 . Each loss element in the laser cavity shown should be no larger than a few decibels (dB) and preferably smaller than 0.5 dB so that the collective single pass loss along the optical axis  144  of the laser cavity is no larger than about 5-20 decibels.  
         [0063]    Referring now to FIGS. 3A and 3B where like elements are designated with like numerals, a third embodiment of the tunable laser  146  is shown. The third embodiment of the tunable laser  146  is substantially similar to the tunable laser  10  shown in FIG. 1, but includes several additional elements not found in the tunable laser  10 . Referring to FIG. 3B, the tunable laser  146  includes a second reflector  148  fabricated in or on the first core  70  between the first taper  76  and the first reflector  96 . The second reflector  148  is fabricated into the first core  70  using ultraviolet exposure and the second reflector  148  is typically a specialized Bragg grating of base periodicity from 0.2 to 0.6 microns, but with additional phase and amplitude structure periodically repeated with a period (Λ s ) from 50 to 500 μm, dependant on the material being written on or into, and the reflection wavelength desired. Due to the periodic structure, the optical spectrum of the second reflector  148  exhibits multiple reflection peaks, known as a comb of peaks, in the wavelength domain of individually defined amplitude and wavelength spacing. The separation between adjacent peaks in the comb, dλ, is given by:  
           dλ=λ   2 /[2 n   g Λ s ] 
         [0064]    where n g  is the effective group index. Basically, the separation between adjacent peaks in the comb is controlled by the period Λ s  while the envelope containing the peaks depends on the grating modulation function inside one sampling period. A pair of heaters  150  and  152  (FIG. 3C) associated with the second reflector  148  are also disposed on both sides of the second reflector  148  and the length of the heaters  152  and  152  exceed the length of the second reflector  148  so that the entire length of the second reflector  148  can be maintained at a uniform temperature. The first phase control section  18  is located between the first reflector  96  and the second reflector  148  and the heaters  84  and  86  (FIG. 3C) associated with the first phase control section  18  are disposed on both sides of a reflector free portion of the first core  70  between the first and second reflectors. A second phase control section  154  is located between the first taper  76  and the second reflector  148  and heaters  156  and  158  (FIG. 3C) associated with the second phase control section  154  are disposed on both sides of a portion of a reflector free portion of the first core  70  between the first taper and the second reflector.  
         [0065]    The dynamic operation of the tunable laser  100  shown in FIG. 2 shall now be discussed. Amplification current supplied to the bonding layer  34  (FIG. 1C) and the portion of the bonding layer  56  (FIG. 1C) in electrical contact with the active layer  36  between the trenches  60  and  62  (FIG. 1C) causes population inversion in the active layer  36  (FIG. 1C) and gain in the laser cavity of the tunable laser  100 . When the round trip gain in the laser cavity of the tunable laser  100  exceeds the round trip losses, the tunable laser  100  will lase along the optical axis  144 . The laser cavity of the tunable laser  100  will have a gain curve  160  and the Fabry-Perot resonant cavity modes  162 ,  164 , and  166  shown schematically in FIG. 4. Optical energy will propagate from the active layer  36  between trenches  60  and  62 , through the gaps  98  and  142 , through the first and second tapers  76  and  126 , and into the first and second cores  70  and  116 . Since the optical path  144  near the first and second facets  21  and  22  and the front surfaces  78  and  128  are angled, parasitic reflections should be minimized and most of the optical energy should propagate into the first and second cores  70  and  116 . The first and second tapers  76  and  126  will optically transmit optical energy into the respective first and second cores  70  and  116  and to the respective first and second reflector  96  and  140 . The first reflector  96  reflects the optical energy if the wavelength associated with the optical energy is one of a first plurality of reflection wavelengths and passes all other optical energy. The reflection spectrum of the first reflector  96  is shown in FIG. 5A and, as shown in FIG. 5A, the first reflector  96  generates the “comb of peaks,” namely, a comb shaped reflective spectrum  168  with a reflection peak  172 ,  174 , and  176  at a separate wavelength. Similarly, the second reflector  140  reflects the optical energy if the wavelength associated with the optical energy is one of a second plurality of reflection wavelengths and passes all other optical energy. The reflection spectrum of the second reflector  140  is shown in FIG. 5B and, as shown in FIG. 5B, the second reflector  140  generates a comb shaped reflective spectrum  178  with a reflection peak  182 ,  184 , and  186  at a separate wavelength.  
         [0066]    Referring now to FIG. 6A which is the product of the reflection spectrum in FIG. 5A and the reflection spectrum in FIG. 5B with the Fabry-Perot resonant cavity modes from FIG. 4, the overlap of reflection peaks  172 ,  174 , and  176  associated with the first reflector  96  with the reflection peaks  182 ,  184 , and  186  associated with the second reflector  140  is shown. Specifically, the reflection peaks  172  and  176  partially overlap with the reflection peaks  182  and  186  producing the composite reflection peaks  188  and  190  and the reflection peak  174  substantially overlaps with the reflection peak  184  producing the composite reflection peak  192 . The tunable laser  100  may lase if there is a Fabry-Perot resonant cavity mode located within the range of wavelengths associated with the composite reflection peaks  188 ,  190 , and  192  and the proper gain conditions exist. As shown in FIG. 6A, the Fabry-Perot resonant cavity mode  164  is located within the range of wavelengths associated with the composite reflection peak  192 , the Fabry-Perot resonant cavity mode  162  is located within the range of wavelengths associated with the composite reflection peak  188 , and the Fabry-Perot resonant cavity mode  166  is located within the range of wavelengths associated with the composite reflection peak  190 . Since the magnitude of the reflectivity of the composite reflection peaks  188  and  190  are significantly smaller than that of the composite reflection peak  192 , the tunable laser  100  will lase preferentially at the wavelength associated with Fabry-Perot resonant cavity mode  162 . There will be a small amount of light generated at the Fabry-Perot resonant cavity modes  162  and  166 , but the intensity of this light is suppressed by the mode suppression ratio which is related to the ratio of the round trip losses at the respective wavelengths and the laser design. Accordingly, the tunable laser  100  will lase at the wavelength associated with the Fabry-Perot resonant cavity mode  164 .  
         [0067]    To change the lasing wavelength of the tunable laser  100  and as shown in FIG. 6B, current is supplied to the heaters  80  and  82  in the reflector section  20  and to the heaters  118  and  120  in the second reflector section  108 , thereby heating some or all of these heaters. This heats and changes the index of refraction of the thermo-optical organic material  88  and  134  adjacent to the first and second reflectors  96  and  140  thereby shifting the respective reflection spectrums  168  and  178  such that reflection peaks  172  and  182  substantially overlap thereby substantially increasing the magnitude of the reflectivity of the composite reflection peak  188 , reflection peaks  174  and  184  partially overlap thereby substantially decreasing the magnitude of the reflectivity of the composite reflection peak  192 , and reflection peaks  176  and  186  still partially overlap thereby maintaining the magnitude of the reflectivity of the composite reflection peak  190 . However, the wavelength associated with the Fabry-Perot resonant cavity mode  162  and the composite reflection peak  188  is not optimal to produce maximum optical intensity of the output optical energy of the tunable laser  100 . To solve this problem and as shown in FIG. 6C, current is supplied to some or all of the heaters  84  and  86  in the phase control section  18  thereby heating the heaters and heating and changing the index of refraction of the thermo-optical organic material  88  and  134  in and near the first and second cores  70  and  116  thereby shifting the wavelengths associated with the Fabry-Perot resonant cavity modes. The tunable laser  100  will then lase at the wavelength associated with the Fabry-Perot resonant cavity mode  162 .  
         [0068]    The dynamic operation of the tunable laser  146  shown in FIG. 3 shall now be discussed. Amplification current supplied to the bonding layer  34  (FIG. 1C) and the portion of the bonding layer  56  (FIG. 1C) in electrical contact with the optical axis  98  in the active emission layer  36  (FIG. 1C) causes population inversion in the active emission layer  36  (FIG. 1C) and gain in the laser cavity of the tunable laser  146 . When the round trip gain in the laser cavity of the tunable laser  146  exceeds the round trip losses, the tunable laser  146  will lase along the optical axis  98  (FIG. 3C). The laser cavity of the tunable laser  146  will have a gain curve (not shown). Optical energy will propagate from the active emission layer  36  between trenches  60  and  62 , through the gap  98 , through the first taper  76 , and into the first waveguide  90 . Since the optical path  98  (FIG. 3C) near the first facet  21  and the front surface  78  is angled, parasitic reflections should be minimized and most of the optical energy should propagate into the first core  70 . The first taper  76  will optically transmit optical energy into the first core  70  and to the first and second reflector  96  and  148  (FIG. 3B). The first reflector  96  reflects the optical energy if the wavelength associated with the optical energy is one of a first plurality of reflection wavelengths and passes all other optical energy. The reflection spectrum of the first reflector  96  is shown in FIG. 7A and, as shown in FIG. 7A, the first reflector  96  generates a comb shaped reflective spectrum  194  with reflection peaks  196 ,  198 ,  200 ,  202 , and  204  at different wavelengths. Similarly, the second reflector  148  reflects the optical energy if the wavelength associated with the optical energy is one of a first plurality of reflection wavelengths and passes all other optical energy. The reflection spectrum of the second reflector  148  is shown in FIG. 7B and, as shown in FIG. 7B, the second reflector  148  generates a comb shaped reflective spectrum  206  with reflection peaks  208  and  210  at different wavelengths.  
         [0069]    Referring now to FIG. 8A, a composite reflection spectrum is shown and which is obtained from the coherent addition of the reflection spectrum associated with the first reflector  96  shown in FIG. 7A with the reflection spectrum associated with the second reflector  148  shown in FIG. 7B. A composite reflection peak  212  is formed by the optimally phased coherent addition of light waves reflected from the first and second reflectors whose spectral amplitudes are described by the reflection peaks  196  and  204 . The composite reflection peak  212  is given by the square of the sum of the square roots of the reflection peaks  196  (FIG. 7A) and  208  (FIG. 7B) while composite reflection peaks  214 ,  216 ,  218 ,  220 , and  222  correspond, respectively, to the simple reflection associated with reflection peaks  198 ,  200 ,  210 ,  202 , and  204 . The laser cavity of the tunable laser  146  will have Fabry-Perot resonant cavity modes  224 ,  226 ,  228 ,  230 ,  232 ,  234 ,  236 ,  238 ,  240 ,  242 ,  244 ,  246 , and  248 . The coherent sum of the two reflected waves is shifted in phase relative to each other so that the positions of the Fabry-Perot resonant cavity modes  224  and  226  are shifted relative to the positions of all the other Fabry-Perot resonant cavity modes. In order for the tunable laser  146  to lase and as shown in FIG. 8B, current is supplied to some or all of the heaters  80  and  82  in the reflector section  20 , to the heaters  118  and  120  in the second reflector section  108 , to the heaters  84  and  86  in the phase control section  18 , and to the heaters  156  and  158  in the second phase control section  154  thereby aligning the Fabry-Perot resonant cavity mode  224  to the peak of the composite reflection peak  212 . The tunable laser  146  will then lase at the wavelength associated with the Fabry-Perot resonant cavity mode  224 .  
         [0070]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.