Abstract:
A method of converting an optical wavelength includes providing a wavelength converter assembly with a photodetector and a laser with a common epitaxial structure. The expitaxial structure has areas of differing bandgap. An optical input having a first wavelength at the wavelength converter assembly is absorbed. A first electrical signal is generated from the photodetector in response to the optical input. The first electrical signal is conditioned to produce a conditioned first electrical signal. A second electrical signal is generated from the conditioned first electrical signal. A laser output is generated from a gain medium of the laser at a second wavelength in response to the second electrical signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part and claims the benefit of priority of U.S. Provisional Application Serial No. 60/152,072, filed Sep. 2, 1999, U.S. Provisional Application Serial No. 60/152,049, filed Sep. 2, 1999, U.S. Provisional Application Serial No. 60/152,038, filed Sep. 2, 1999, which applications are fully incorporated by reference herein. This application is also a continuation-in-part of U.S. Ser. Nos. 09/614/377, 09/614,895, both filed Jul. 12, 2000 (now U.S. Pat. No. 6,349,106, issued Feb. 19, 2002), 09/614,674, 09/614,378, 09/614,195, 09/614,375, 09/614,665 and 09/614,224, filed on Jul. 12, 2000, the same date as this application, which applications are fully incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to laser assemblies, and more particularly to a method of converting an optical wavelength using a widely tunable laser assembly with an integrated modulator. 
     2. Brief Description of the Related Art 
     A laser transmitter for fiber optic networks must provide signals at a given stable wavelength, modulated at a desired rate with low chirp and an appropriate power launched into optical fiber. Current networks have as many as 100 wavelength channels with one laser devoted to each channel, and each laser having an external modulator. Significantly greater efficiencies could be realized with a laser transmitter and a modulator included on a chip, wherein the modulated laser is capable of being tuned to cover every channel of a system. Photonic integration can be used to provide a laser transmitter on a chip, as is well understood in the art. FIG. 1 shows a block diagram of a structure that can be used to accomplish this. While photonic integration is well known in the art, prior art efforts have been focused on the integration of lasers that are not widely tunable . Kobayashi, N.; Noda, A.; Watanabe, T.; Miura, S.; Odagawa, T.; Ogita, S. “2.5-Gb/s-1200-km transmission of electroabsorption modulator integrated DFB laser with quarter-wavelength-shifted corrugation,” IEEE Photonics Technology Letters, vol. 11, (no. 8), IEEE, August 1999. p. 1039-41; Delprat, D.; Ramdane, A.; Silvestre, L.; Ougazzaden, A.; Delorme, F.; Slempkes, S. “20-Gb/s integrated DBR laser-EA modulator by selective area growth for 1.55- mu m WDM applications,” IEEE Photonics Technology Letters, vol. 9, no. 7, IEEE, July 1997. p. 898-900. Large tuning ranges make achieving adequate performance of these functional blocks non-obvious with respect to the teachings of the prior art in general, and the prior art related to narrowly tunable devices in particular. What is needed is photonic integration techniques to construct a widely tunable laser apparatus including an integrated modulator. 
     SUMMARY 
     Accordingly, an object of the present invention is to provide a method of modulating an optical wavelength using a laser assembly where all of the elements are fabricated on a single wafer. 
     Another object of the present invention is to provide a method of modulating an optical wavelength using a diode laser assembly with the elements derived from a common epitaxial layer structure. 
     A further object of the present invention is to provide a method of modulating an optical wavelength using a widely tunable diode laser assembly with an integrated modulator. 
     Yet another object of the present invention is to provide a method of modulating an optical wavelength using a diode laser assembly with the elements fabricated on a single wafer by common process steps. 
     A further object of the present invention is to provide a method of modulating an optical wavelength using a monolithically integrated diode laser assembly made with fabrication steps that tailor optical properties of selected regions to a desired electro-optic function. 
     Another object of the present invention is to provide a method of making a monolithically integrated diode laser assembly that uses common fabrication process steps to form the elements of the assembly that are compatible with photonic device fabrication processes presently used in the lightwave industry. 
     These and other objects of the present invention are achieved in a method of converting an optical wavelength that provides a wavelength converter assembly with a photodetector and a laser with a common epitaxial structure. The expitaxial structure has areas of differing bandgap. An optical input having a first wavelength at the wavelength converter assembly is absorbed. A first electrical signal is generated from the photodetector in response to the optical input. The first electrical signal is conditioned to produce a conditioned first electrical signal. A second electrical signal is generated from the conditioned first electrical signal. A laser output is generated from a gain medium of the laser at a second wavelength in response to the second electrical signal. 
     In another embodiment of the present invention, a method of converting an optical wavelength provides a wavelength converter assembly having an epitaxial structure with areas of differing bandgap. A waveguide layer is positioned between first and second semiconductor layers of the epitaxial structure. An optically active gain medium is positioned between first and second reflectors that define a resonant cavity. The wavelength converter assembly also includes a photodetector. An optical input is detected at the photodetector. A laser output is generated from the wavelength converter assembly in, response to the optical input. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram of a laser assembly that illustrates different functional elements of a laser assembly. 
     FIG. 2 is a cross-sectional view of one embodiment of a widely tunable laser assembly of the present invention. 
     FIG.  3 ( a ) is a cross sectional view of the FIG. 2 assembly illustrating several layer structures and the integration of two different band gap materials by an offset quantum well technique. 
     FIG.  3 ( b ) is a cross sectional view of the FIG. 2 assembly illustrating several layer structures and the integration of two different band gap materials by butt-joint regrowth. 
     FIG.  3 ( c ) is a cross sectional view of the FIG. 2 assembly that illustrates one embodiment for the integration of several different band gap materials by selective area growth (SAG). 
     FIG.  3 ( d ) is a cross sectional view of the FIG. 2 assembly that illustrates one embodiment for the integration of several different band gap materials by quantum well intermixing. 
     FIG.  4 ( a ) illustrates one embodiment of the modulator element of FIG. 2 with a single section modulator that uses the same bandgap material as the front mirror. 
     FIG.  4 ( b ) illustrates a tandem embodiment of the FIG. 2 modulator element that uses the same bandgap material as the front mirror in order to provide better chirp and linearity performance. 
     FIG.  4 ( c ) illustrates a single section modulator embodiment of the FIG. 2 modulator element that uses a bandgap material chosen to provide the best chirp, drive voltage and on/off ratio over a particular wavelength range. 
     FIG.  4 ( d ) illustrates a tandem modulator embodiment of the FIG. 2 modulator element with bandgap materials chosen to provide the best chirp, drive voltage and on/off ratio performance, for the composite modulator, over a wider wavelength range than achievable by a single modulator section. 
     FIG.  5 ( a ) is a cross-sectional view of one embodiment of the FIG. 2 modulator element that includes post EAM amplification with a single section amplifier to provide highest output power at the expense of reduced extinction due to ASE. 
     FIG.  5 ( b ) is a cross-sectional view of one embodiment of the FIG. 2 modulator element with an amplifier preceding the EAM to prevent extinction ratio degradation at the expense of lower output power due to modulator insertion loss. 
     FIG.  5 ( c ) is a cross-sectional view of one embodiment of the FIG. 2 modulator element that uses a combination of pre and post-amplification to achieve the highest output power with a minimum of extinction ratio degradation. 
     FIG.  6 ( a ) is a cross-sectional view of one embodiment of the FIG. 2 output coupler element where a thickness of the waveguide is tapered to allow the output mode to be defined by an underlying waveguide layer. 
     FIG.  6 ( b ) is a top view of one embodiment of the FIG. 2 output coupler element illustrating that the waveguide&#39;s width and angle of incidence upon the facet has been changed to promote high coupling efficiency and low modal reflectivity. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, one embodiment of the present invention is a widely tunable laser assembly  10  with an epitaxial structure formed on a substrate. For purposes of this specification, a widely tunable laser is defined as a laser whose output wavelength can be tuned over a wider wavelength range than achievable by conventional index tuning, i.e. Δλ/λ&gt;Δn/n, and whose wavelength selective elements are within the same optical waveguide, i.e. not a parallel array of DFB lasers. 
     A laser element  12  and a modulator element  14  are formed in the epitaxial structure. Also formed in the epitaxial structure is an output coupler element  16  positioned to receive and adjust an output received from modulator  14 . The various elements are fabricated by common process steps. 
     Laser  12  includes front and back mirrors  18  and  20  which can be distributed Bragg reflectors (“DBR&#39;s”). A gain section  22  is positioned in laser  12  as is a mode selection section  24 . Mode selection section  24  can be a lateral mode selection element, a longitudinal mode selection element, a controllable phase shifting element, and the like. 
     Modulator  14  can include a semiconductor optical amplifier  26  (“SOA  26 ”), a first electro-absorption modulator (“EAM  28 ”) and a second EAM  30 . 
     Laser  12  is preferably widely tunable to produce laser emission with the desired spectral properties, e.g. linewidth, SMSR, wavelength, over the entire wavelength band, or at least a significant fraction, to be used in a WDM optical communication system. In one embodiment, the wavelength bands of interest lie within 1300-1600 nm range and typically have a bandwidth determined by the gain characteristics of optical fiber amplifiers. 
     In one embodiment, laser  12  is an SG/SSG-DBR laser that includes two SG/SSG-DBR mirrors  18  and  20 , gain section  22 , and phase section  24 . Jayaraman, V.; Chuang, Z. M.; Coldren, L. A. “Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings.”, IEEE Journal of Quantum Electronics, vol. 29, (no. 6), June 1993, p. 1824-34. In this embodiment, the bandgap of the gain section  22  is chosen to provide gain over the wavelength band of interest. The bandgap of SG/SSG-DBR mirrors  18  and  20  and phase section  24  is selected to provide wavelength coverage over the desired wavelength band with the lowest loss and tuning currents. 
     Other embodiments that can be used for laser  12  include but are not limited to the GCSR laser in which the output is taken from the SG/SSG-DBR mirror side, allowing the integration of the other elements as illustrated in FIG.  1 . Oberg, M.; Nilsson, S.; Streubel, K.; Wallin, J.; Backbom, L.; Klinga, T. “74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector,” IEEE Photonics Technology Letters, vol. 5, (no. 7), July 1993, p. 735-7. Another embodiment for the widely tunable laser  12  is a series connected, gain coupled DFB laser. Hong, J.; Kim, H.; Shepherd, F.; Rogers, C.; Baulcomb, B.; Clements, S., “Matrix-grating strongly gain-coupled (MC-SGC) DFB lasers with 34-nm continuous wavelength tuning range,” IEEE Photonics Technology Letters, vol. 11, (no. 5), IEEE, May 1999 p. 515-17. Both of these embodiments can be integrated with the other elements of FIG.  2 . 
     Modulator  14  encodes data onto the optical carrier produced by widely tunable laser  12 . The characteristics of the modulation that are desired are: suitable on/off ratio, control of the instantaneous wavelength such as chirp, low drive voltage, and high saturation power. For analog modulation it is desirable to have very linear response, as well. 
     Modulator  14  can be an electro-absorption modulator and include a plurality of electro-absorption modulators. In one embodiment, modulator  14  has the same bandgap as front mirror section  18 . In another embodiment, modulator  14  has a different bandgap than front mirror section  18 . In another embodiment, modulator includes a plurality of modulator sections that have differing bandgaps. Modulator  14  can include non-radiative carrier traps to reduce carrier lifetime of the modulator material. Further, modulator  14  can include an optical amplifier configured to receive an output of the electro-absorption modulator  14 . In another embodiment, modulator  14  includes an optical amplifier configured to produce an output incident on the electro-absorption modulator. An electro-absorption modulator can be positioned between first and second optical amplifiers. 
     Output coupler  16  is used to increase the coupling efficiency and alignment tolerance to whatever optical assembly follows assembly  10 , including but not limited to an optical fiber or lenses preceding an optical fiber. Output coupler  16  reduces a modal reflectivity at an output facet of output coupler  16  and modifies an output mode shape of laser  12 . 
     An important aspect of achieving the structure of FIG. 2 is the use of areas that have different band gaps to accomplish their specialized tasks. A way of specifying this band gap is to give the wavelength peak of the photoluminescence emitted from these sections. Gain section of laser  12  and SOA  26  have band gaps that are chosen to provide gain in the wavelength range over which laser  12  is to operate. Front and back mirrors  18  and  20  have a band gap chosen to provide an index change, with a minimum of optical loss, needed to tune a lasing wavelength between adjacent peaks of a sampled grating mirror over the entire wavelength range. The band gap and length of modulator  14  is chosen to give the required extinction of the lasing wavelength at a reverse bias that is easily obtainable for a given modulation speed. 
     The monolithic integration of optically dissimilar elements of assembly  10  is accomplished by a method of fabrication that tailors optical properties of selected regions to a desired electro-optic function. Tailored optical properties, including the band gap, result in optically active and passive regions on the same wafer beginning from a common epitaxial layer structure. Further, the common fabrication process steps required for forming the apparatus elements are compatible with photonic device fabrication processes presently used in the lightwave industry. Thus, the apparatus of the present invention is readily manufacturable. 
     In a particular embodiment, the fabrication methods to selectively tailor the band gaps of regions of the wafer include the steps of implantation of impurities by low energy ions, for example less than about 200 eV, in a portion of a selected wafer region near the wafer surface. The wafer is then annealed. This allows the impurities and vacancies implanted near the wafer surface to diffuse throughout the selected region and tailor the region&#39;s band gap to a desired electro-optic function. 
     For example, in the passive waveguide regions of the phase shift and mirror sections of assembly  10 , the effective bandgap should be somewhat larger (e.g., &gt;0.1 eV) than the operating lightwave energy, which is only slightly larger (typically˜0.01-0.05 eV) than the effective bandgap of the active layers in gain section  22 . Integrated external modulator elements  14  may have sections with the same larger bandgap as the other passive regions, or a bandgap intermediate between that of the active and passive sections for some desired functionality such as chirp reduction or improved linearity. Integrated external amplifier elements may have the same bandgap as the active gain section or a slightly modified bandgap for some functionality, such as increased saturation power or improved chirp of modulator/amplifier combinations. 
     In many embodiments, the passive regions are created by selective removal of the lowest bandgap layers responsible for gain in the active regions within the same sequence as some other processing steps, such as grating formation in the mirror regions, are being carried out. In these cases ion-implantation process is not necessary but can be utilized to better tailor other regions such as in integrated modulators and/or amplifier elements. This sequence is followed by a regrowth of the upper cladding layers required for the top portion of the optical waveguide. 
     There are several layer structures, well known to those skilled in the art, which allow the integration of areas having different band gaps. 
     FIGS.  3 ( a ) through  3 ( d ) illustrate several of these structures. The simplest approach is to grow a mixed quantum well (“MQW  30 ”) gain section on top of passive waveguide layer  32  as illustrated in FIG.  3 ( a ). 
     An advantage of the FIG.  3 ( a ) embodiment is simplicity, plus the band gap and geometry of each section can be somewhat optimized for the task the given area is to perform. 
     In another embodiment, illustrated in FIG.  3 ( b ), MQW  30  is formed by butt joint growth. This embodiment allows fully independent optimization of the different band gap regions. Butt-joint regrowth involves etching away the layers in one area of the device and selectively regrowing layers with the desired band gap. Wallin, J.; Landgren, G.; Strubel, K.; Nilsson, S.; Oberg, M. “Selective area regrowth of butt-joint coupled waveguides in multi-section DBR lasers.” Journal of Crystal Growth, vol. 124, no. 1-4, November, 1992, p. 741-6. 
     More sophisticated structures for achieving multiple band gap regions are illustrated in FIGS.  3 ( c ) and  3 ( d ). In FIG.  3 ( c ), selective area growth (“SAG”) involves growing the desired layer structure on a substrate patterned with dielectric masks. Aoki, M.; Suzuki, M.; Sano, H.; Taniwatari, T.; Tsutsui, T.; Kawano, T., “Quantum energy control of multiple-quantum-well structures by selective area MOCVD and its application to photonic integrated devices,” Electronics and Communications in Japan, Part 2 (Electronics), vol. 77, (no. 10), October 1994, p. 33-44. The presence of the masks perturbs the growth and enhances the growth rate near the masks. MQW regions  30 , with different band gaps, can subsequently be grown using masks of different widths to grow quantum well having different thickness. In FIG.  3 ( d ) a MQW region  30  is grown with the lowest of the desired band gaps. By inter-mixing the quantum well and barrier the band gap of the structure is blue-shifted. Hofstetter, D.; Maisenholder, B.; Zappe, H. P., “Quantum-well intermixing for fabrication of lasers and photonic integrated circuits,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 4, no. 4, IEEE, July-August, 1998, p. 794-802. The amount of this blue shift is determined by the initial compositions of the well and barrier, and the amount of intermixing. By spatially controlling the amount of inter-mixing several regions having different band gaps can be created. 
     One material system of a preferred embodiment of the present invention is InGaAsP/InP, in which citing the bandgap of the particular lattice matched alloy specifies the bulk material desired, as well as the alloys required to obtain the quantum well material whose PL peak corresponds to the cited band gap. The desired properties can also be achieved by using alloys in the InGaAlAs/InP system having the specified bandgap. 
     When different band gaps are chosen or specified for the embodiments, it is implied that one or more of the techniques illustrated in FIGS.  3 ( a ) through  3 ( d ) have been used appropriately to create the band gaps specified for all of the elements of assembly  10 . These techniques, and their combinations, are known to those skilled in the art and are capable of achieving the specifications of all the following embodiments. Delprat, D.; Ramdane, A.; Silvestre, L.; Ougazzaden, A.; Delorme, F.; Slempkes, S. “20-Gb/s integrated DBR laser-EA modulator by selective area growth for 1.55-mu m WDM applications,” IEEE Photonics Technology Letters, vol. 9, no. 7, IEEE, July 1997. p. 898-900; Hansen, P. B.; Raybon, G.; Koren, U.; Miller, B. I.; Young, M. G.; Newkirk, M. A.; Chien, M.-D.; Tell, B.; Burrus, C. A., “Monolithic semiconductor soliton transmitter,” Journal of Lightwave Technology, vol. 13, (no. 2), February 1995. p. 297-301; Wallin, J.; Landgren, G.; Strubel, K.; Nilsson, S.; Oberg, M. “Selective area regrowth of butt-joint coupled waveguides in multi-section DBR lasers.” Journal of Crystal Growth, vol. 124, no. 1-4, November 1992, p. 741-6; Aoki, M.; Suzuki, M.; Sano, H.; Taniwatari, T.; Tsutsui, T.; Kawano, T., “Quantum energy control of multiple-quantum-well structures by selective area MOCVD and its application to photonic integrated devices,” Electronics and Communications in Japan, Part 2 (Electronics), vol. 77, (no. 10), October 1994, p. 33-44; Hofstetter, D.; Maisenholder, B.; Zappe, H. P., “Quantum-well intermixing for fabrication of lasers and photonic integrated circuits,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 4, no. 4, IEEE, July-August 1998, p. 794-802. It is known to those skilled in the art that certain of these techniques require quantum well regions to allow band gap tuning. The specified band gaps of the embodiments refer to the PL peak of the sections subsequent to all band gap tuning efforts, regardless whether they are bulk or quantum well. 
     Laser  12 , modulator  14  and output coupler  16  are electrically isolated from each other, such that the operation of one does not interfere electrically with any adjacent section, within or among the elements of FIG.  2 . 
     FIGS.  4 ( a ) through  4 ( d ) illustrate several embodiments of modulator  14  in which electro-absorption is used to create the modulation. In FIG.  4 ( a ), EAM section  28  uses the same bandgap material as SG/SSG-DBR  18 . In order to optimize the modulation characteristics the DC bias can be adjusted for each wavelength produced by laser  12 . Separate optimization of the band gap may be desired for EAM section  28  and SG/SSG-DBR  18  in which one of the FIG.  3 ( a ) through  3 ( d ) embodiments is used to produce the desired bandgaps. The result is shown in FIG.  4 ( c ). 
     Due to the widely tunable nature of laser  12  it is difficult to achieve optimal performance for all the parameters listed above at every wavelength using a single modulator section. By splitting the contact one or more times, as shown in FIG.  4 ( b ) with first and second EAM sections  28  and  30 , separate biases can be applied to each EAM section  28  and  30  with the identical modulation or modulation with a phase/amplitude shift between EAM sections  28  and  30 . This tandem modulation scheme is advantageous for the control of chirp and achieving higher linearity. 
     The embodiment illustrated in FIG.  4 ( b ) can be further extended by also adjusting the bandgap of each of the modulator sections, as illustrated in FIG.  4 ( d ). The bandgap of each EAM section  28  and  30  is chosen so its absorption is tuned to provide a suitable on/off ratio, drive voltage and chirp for a subset of wavelengths within the range emitted by laser  12 . By properly biasing each EAM section  28  and  30 , the composite modulator  14  can produce these optimal characteristics over the entire wavelength range of laser  12 . Furthermore, the modulation can be applied appropriately to one or more of EAM sections  28  and  30  with or without a phase/amplitude shift, to enhance the chirp or linearity over what can be achieved through modulating only a single EAM section. 
     To achieve higher saturation powers the carrier lifetime of the material composing modulator  14  is reduced. Suitable quantum well structures with reduced carrier lifetimes can be used to achieve this purpose. Czajkowski, I. K.; Gibbon, M. A.; Thompson, G. H. B.; Greene, P. D.; Smiteh, A. D.; Silver, M. Strain-compensated MQW electroabsorption modulator for increased optical power handling. Electronics Letters, vol. 30, (no. 11), 26 May 1994., p. 900-1. It is important that these quantum well structures are achieved subsequent to all bandgap tuning steps. Alternatively, traps can be introduced via an implantation step for example, to reduce the carrier lifetime through non-radiative processes. Woodward, T. K.; Knox, W. H.; Tell, B.; Vinattieri, A.; Asom, M. T., “Experimental studies of proton-implanted GaAs-AlGaAs multiple-quantum-well modulators for low-photocurrent applications,” IEEE Journal of Quantum Electronics, vol. 30, (no. 12), December 1994, p. 2854-65. Due to the insertion loss suffered in EAM sections  28  and  30 , it may be advantageous, but not necessary, to add amplification with SOA  2  to modulator  14  element. 
     FIGS.  5 ( a ) through  5 ( d ) illustrate several embodiments of modulator  14  that contain amplification. While only one embodiment for the modulation part of the modulation/amplification element is illustrated, FIG.  4 ( d ), it will be appreciated that any of the FIGS.  4 ( a ) through  4 ( c ) embodiments can also be used. 
     FIG.  5 ( a ) illustrates an embodiment in which SOA  26  follows modulator  14 . This embodiment is advantageous for producing the highest output power given a particular saturation power of SOA  26 . An additional advantage is the ability to use the nonlinearity in SOA  26  to compensate for positive chirp in the EAM section  28 . Woodward, T. K.; Knox, W. H.; Tell, B.; Vinattieri, A.; Asom, M. T., “Experimental studies of proton-implanted GaAs-AlGaAs multiple-quantum-well modulators for low-photocurrent applications,” IEEE Journal of Quantum Electronics, vol. 30, (no. 12), December 1994, p. 2854-65; Watanabe, T.; Sakaida, N.; Yasaka, H.; Koga, M.,“Chirp control of an optical signal using phase modulation in a semiconductor optical amplifier,” IEEE Photonics Technology Letters, vol. 10, (no. 7), IEEE, July 1998, p. 1027-9. 
     FIG.  5 ( b ) illustrates an embodiment in which SOA  26  precedes modulator ( 14 ). This embodiment is advantageous for preserving the extinction ratio of modulator  14  and preventing the introduction of unmodulated ASE into the network. A disadvantage of this embodiment is the reduction of output power achievable due to the saturation of SOA  26  and insertion loss of EAM  28 . It is also no longer possible to compensate any positive chirp that may be produced in EAM section  28  with SOA  26 . 
     FIG.  5 ( c ) illustrates an embodiment in which SOA  28  and SOA  34  both precede and follow EAM section  30 . This embodiment allows the maximum achievable output power to be increased over the purely preceding SOA  28  of the FIG.  5 ( b ) embodiment with less noise and extinction ratio degradation than the purely following SOA  26  of FIG.  5 ( a ). Additionally, this embodiment also uses the nonlinearity in SOA  26  to compensate for positive chirp in EAM section  28 . 
     Furthermore, it may be advantageous to use a tandem amplification scheme in which separate biases are applied to a split contact amplifier to independently control the noise and gain saturation properties of the composite amplifier. All of the FIGS.  5 ( a ),  5 ( b ) and  5 ( c ) embodiments can be implemented using such a tandem amplification scheme. 
     Another way of controlling the saturated output power of an SOA is by adjusting its width and/or the bandgap of the gain material to increase the carrier lifetime for higher optical powers. Any adjustment in the width performed adiabatically to insure low optical transition loss. 
     FIG.  6 ( a ) illustrates one embodiment of output coupler  16  with an output coupler waveguide  36 . In this embodiment, the thickness of output coupler waveguide  34  is tapered to allow the output mode to be defined by an underlying layer  38 . The coupling efficiency and alignment tolerance are simultaneously increased by converting the optical mode that is used in every other element of the FIG. 2 embodiment, the photonic circuit (“PIC” mode), to a larger optical mode, the fiber matched (“FM” mode), more closely matching that of a lensed optical fiber. 
     The methods for transforming the mode size are well known to those skilled in the art, and usually involve an adiabatic tapering of the core that determines the PIC optical mode. This tapering can be performed laterally such as by tapering the width of the core, or vertically by tapering the thickness of the core. Kawano, K.; Kohtoku, M.; Okamoto, H.; Itaya, Y.; Naganuma, M., “Comparison of coupling characteristics for several spotsize-converter-integrated laser diodes in the 1.3- mu m-wavelength region,” IEEE Photonics Technology, vol. 9, (no. 4), IEEE, April 1997. p. 428-30. These tapers are preferably performed in a nonlinear manner to reduce the mode transformation loss for shorter taper lengths. J. D. Love, “Application of a low-loss criterion to optical waveguides and devices,” IEE Proceedings J (Optoelectronics), vol. 136, pp. 225-8, 1989. 
     Underlying waveguide layer  38  can be used to define the FM mode. This is advantageous because the FM mode shape is very sensitive to the dimensions of the tapered down PIC waveguide core which is used to define it, making it difficult to reproducible fabricate the desired FM mode shape. By including underlying waveguide layer  38  the PIC waveguide core can be completely removed allowing the FM mode to be determined solely by underlying waveguide layer  38 . Furthermore, the refractive index of underlying waveguide layer  38  is diluted such that the presence of layer  38  does not affect the PIC mode shape and the FM mode shape is more tolerant to the underlying layer dimensions. 
     Output coupler waveguide  36  can be tapered non-linearly, tapered in a direction substantially parallel to an optical axis of laser  12  or tapered in a direction substantially normal to an optical axis of laser  12 . Tapering of output coupler waveguide  36  means that a sectional area “X” of the waveguide varies along waveguide  36 . Output coupler waveguide  36  can be oriented along a crystallographic axis of the epitaxial structure of assembly  10 . 
     Another use of output coupler  16  is to lower the modal reflectivity at an output facet  40  to less than 10 −5 . There are several methods well known to those skilled in the art to accomplish this. In one embodiment, this is achieved by depositing a dielectric AR coating  42  to achieve this reflectivity. 
     FIG.  6 ( b ) illustrates an embodiment in which the angle of incidence of output coupler waveguide  36  at output facet  40  is chosen to lower the modal reflectivity to 10 −4 , allowing a simpler, broadband, AR coating to be used to reach 10 −5  reflectivity. Output coupler waveguide  36  is curved to reach the desired angle. The curvature is controlled so that higher order modes are not significantly excited. Curving of waveguide  36  to reach the desired angle permits waveguide  36  to be oriented along a crystallographic direction on the remainder of assembly  10 . This is advantageous because of the crystallographic nature, well know to those skilled in the art, of many process steps required for the fabrication of assembly  10 . Additionally, due to the long lengths of assembly  10 , angling waveguide  36  along the entire length of assembly  10  may cause the die that is used in the process to be excessively wide. 
     In one embodiment, output coupler  16  includes at least one active region and at least one passive region. An interface between the active and passive regions can be substantially normal or oblique to a centerline of output coupler waveguide  36 . Reflections at the interface between the active and passive sections can be reduced by adiabataically tapering output coupler waveguide  36 . Tapered sections of output coupler waveguide  36  can be in the passive or active sections. Output coupler waveguide  36  extends through the active and passive regions and can be truncated prior to reaching an output facet of output coupler  16 . Additionally, an end of output coupler waveguide  36  can terminate at an oblique angle to the output facet of output coupler  36 . 
     In another embodiment, output coupler  16  can include two or more active regions and a passive region. The active regions can be independently controllable and separated by a passive region. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.