Abstract:
A method of generating an optical signal provides a diode laser assembly including an epitaxial structure formed on a substrate. A laser and an amplifier are formed in the epitaxial structure. At least a portion of the laser and amplifier share a common waveguide. A tunable laser output is produced from the laser. The laser output is coupled into the amplifier along the common waveguide. An optical signal is generated from the amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part and claims the benefit of priority of U.S. Provisional Application Ser. No. 60/152,072, filed Sep. 2, 1999, U.S. Provisional Application Ser. No. 60/152,049, filed Sep. 2, 1999, U.S. Provisional Application Ser. 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 now U.S. Pat. No. 6,580,739,  09 / 614 , 895  (now U.S. Pat. No. 6,349,106, issued Feb. 19, 2002), Ser. Nos. 09/614,674, 09/614,378, 09/614,376 now U.S. Pat. No. 6,614,819,  09 / 614 , 195  now U.S. Pat. No. 6,574,259,  09 / 614 , 375  and 09/614,224, filed on the same date as this application, which applications are fully incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to laser assemblies, and more particularly to a widely tunable laser assembly with an integrated optical amplifier. 
     BRIEF DESCRIPTION OF THE RELATED ART 
     Thin fibers of optical materials transmit light across a very broad frequency bandwidth and therefore communications data from a light source may be transmitted over such fibers over broad frequency ranges. At any particular frequency, a laser source must have high output power, narrow laser linewidth and good transmission performance through great distances of optical fiber. 
     In higher bandwidth communications systems, where many frequencies of laser light are transmitted along a fiber, there may be one or several laser sources. While a tunable laser source would be preferred, higher data capacity systems presently use multiple laser sources operating on different frequency channels to cover the wide fiber transmission bandwidth. This is the case since appropriate laser sources are presently incapable of rapid, electronic frequency tuning without attendant deterioration of other significant figures-of-merit. 
     For example, at a fixed frequency, sampled grating distributed Bragg reflector (SGDBR) lasers have the high output power, narrow laser linewidth and good transmission performance necessary for an optical data network. While some SGDBR lasers can be rapidly tuned over more than 100 different transmission channels, two problems nevertheless prevent these devices from being employed in fiber optic communication systems. The most significant problem is the significant absorption of the mirror material. The resulting large cavity losses act to make the laser output power insufficient for the requirements of a present-day communications system. A second problem is that the output power and frequency tuning are dependent on each other. This coupling results in inadequate controllability for a present-day communications system. 
     What is needed, instead, is a device with a combination of sufficiently high output power for a high-bandwidth optical communications network and with frequency tuning controllability substantially independent of output power controllability. 
     SUMMARY 
     Accordingly, an object of the present invention is to provide an integrated laser assembly that includes a tunable solid state laser and optical amplifier where all of the elements are fabricated in a common epitaxial layer structure. 
     Another object of the present invention is to provide an integrated laser assembly that includes a tunable solid state laser and optical amplifier with an output mode conditioned for transmission in an optical fiber. 
     Another object of the present invention is to provide an integrated laser assembly that includes a tunable laser and optical amplifier reducing optical feedback from the amplifier to the laser. 
     A further object of the present invention is to provide a tunable, integrated laser assembly where laser frequency control and output power control are substantially independent. 
     These and other objects of the present invention are achieved in a laser assembly that includes an epitaxial structure formed on a substrate. A tunable laser resonator and a separately controllable optical amplifier are formed in the common epitaxial structure. The amplifier is positioned outside of the laser resonator cavity to receive and adjust an output received from the laser, however, at least a portion of the laser and amplifier share a common waveguide. 
     In different embodiments of the present invention, properties of the common waveguide such as optical properties, or centerline curvature or cross-sectional are non-uniform along the waveguide centerline or non-uniform across a normal to the centerline. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A is a block diagram of a laser assembly that illustrates different functional elements of a laser assembly. 
     FIG. 1B is a cross-sectional view of one embodiment of a widely tunable laser assembly of the present invention and the integration of materials with differing optical properties by an offset quantum well technique. 
     FIG. 2A is a cross-sectional view of one embodiment of an amplifier illustrating several layer structures and the integration of two materials with differing optical properties by a selected area growth technique. 
     FIG. 2B is a cross-sectional view of the FIG. 2 assembly illustrating one embodiment for the integration of materials with differing optical properties by a disordered well technique. 
     FIG. 2C is a cross-sectional view of one embodiment of an amplifier illustrating one embodiment for the integration of several different band gap materials by a butt joint regrowth technique. 
     FIG. 3A is a cross-sectional view of one embodiment of the FIG. 1 optical amplifier element where a portion of the waveguide is curved and an interface between an active and a passive section is oblique. 
     FIG. 3B is a cross-sectional view of one embodiment of the FIG. 1 optical amplifier element where the amplifier includes a plurality of gain sections. 
     FIG. 3C is a cross-sectional view of one embodiment of the FIG. 1 optical amplifier element where the amplifier includes a flared waveguide. 
     FIG. 3D is a cross-sectional view of one embodiment of the FIG. 1 optical amplifier element where the amplifier includes a waveguide mode adapter. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A shows a schematic of an embodiment of the invention. In FIG. 1A, laser assembly  100 , waveguide  105 , amplifier gain section  110 , front resonator mirror  120 , laser gain section  130 , laser phase control section  140 , back mirror  150  and electrical contact  160 , epitaxial structure  170 , laser  180 , optical amplifier  190  and output facet  195  are shown. 
     In FIG. 1A, laser assembly  100  comprises an integration of a laser and an optical amplifier, with the optical amplifier located external to the laser cavity. Front resonator mirror  120 , laser gain section  130 , laser phase control section  140 , and back mirror  150  form a SGDBR-type laser  180  in epitaxial structure  170 . The front and back mirrors define a laser cavity. Amplifier gain section  110  and a portion of waveguide  105  define optical amplifier  190 . 
     As shown in FIG. 1A, despite being external to the laser cavity, the optical amplifier shares a common epitaxial structure  170  with the laser. Epitaxial structure  170  is formed on a substrate (not shown) by processes well-known in the art of semiconductor fabrication. By tailoring optical properties (such as band gap) of different portions of the epitaxial structure, both optically active and optically passive sections can be fabricated in a common structure. Examples of optically active sections of the embodiment shown in FIG. 1 are gain sections  110  and  130 , phase control section  140  and mirrors  120  and  150 . An example of an optically passive section is the portion of waveguide  105  proximal to output facet  195 . 
     According to the invention, at least a portion of laser  180  and optical amplifier  190  share a common waveguide  105 . Different portions of the common waveguide may extend through optically active or passive regions. A common waveguide for the laser and optical amplifier enables the output from the laser to be directly coupled into the amplifier. 
     In the embodiment of FIG. 1A, amplifier  190  is external to the resonant cavity of laser  180  formed by mirrors  120  and  150 . Moreover, amplifier gain section  110  is separately controllable from the laser and is adjustable to increase or decrease the light intensity and output power. The SGBDR laser elements may be controlled separately from the amplifier to tune the laser frequency and otherwise control the input to the optical amplifier. By this arrangement of elements, power amplification and tuning functions are substantially uncoupled. 
     In the embodiment of FIG. 1A, optical amplifier  190  has an active section and a passive section. The active section, amplifier gain section  110 , is substantially straight. The passive section of waveguide  105  is curved and intersects output facet  195  at an oblique angle. Both waveguide curvature and the oblique intersection with the output facet act to prevent reflections at the output facet from coupling back into the optical amplifier  190  and laser  180 . 
     FIG. 1B shows a longitudinal cross section of a laser assembly  100  of FIG.  1 A. In FIG. 1B, laser assembly  100 , waveguide  105 , amplifier gain section  110 , front resonator mirror  120 , laser gain section  130 , laser phase control section  140 , back mirror  150  and electrical contact  160 , epitaxial structure  170 , laser  180 , optical amplifier  190 , output facet  195 , p type semiconductor layer  125 , n-type semiconductor layer  115 , mirror sampling period  135 , offset quantum wells  145  and stop etch layer  155  are shown. 
     In FIG. 1B waveguide  105  is formed between p-type and n-type semiconductor layers  125  and  115 , respectively. Mirrors  120  and  150  are formed by sample gratings etched in waveguide  105  with sampling period  135 , as is well-understood in the art. 
     FIG. 1B illustrates the structure resulting from an offset quantum well technique for optically active and passive section formation. According to the offset quantum well technique, the optically active sections have multiple quantum well layers  145  grown in a region offset from waveguide  105 . The multiple quantum well layers are separated from the waveguide by a thin stop etch layer  155 . Removal of quantum wells, by etching for example, forms optically passive sections. 
     FIGS. 2A-2C illustrate cross-sectional structures over a portion of laser assembly  100  (see FIG. 1) resulting from different techniques for forming optically active and passive sections and their junctions. FIG. 2A illustrates a cross-sectional structure over a portion of laser assembly  100  (see FIG. 1) resulting from a selected area regrowth technique. The selected area regrowth technique uses a dielectric mask to selectively control the growth rate and composition over different areas of the epitaxial structure. Thus, the material&#39;s bandgap can be shifted in certain sections making the material in that section passive or non-absorbing at desired wavelengths. In FIG. 2A, optically passive section  210 , optically active section  220 , bandgap-shifted quantum wells  230 , active section quantum wells  240 , and waveguide  105  (see FIGS. 1A-1B) are shown. In FIG. 2A, different portions of waveguide  105  are optically active or passive due to bandgap-shifting of the quantum wells within the waveguide. 
     FIG. 2B illustrates a cross-sectional structure over a portion of laser assembly  100  (see FIG. 1) resulting from a selected area disordering technique for forming optically active and passive sections. The selected area disordering technique uses a dielectric cap or ion implantation to introduce vacancies which can be diffused through an active region to disorder the quantum wells by intermixing them. This disordering shifts quantum well bandgaps, creating optically passive waveguide sections. 
     In FIG. 2B, optically passive section  210 , optically active section  220 , disordered wells  250 , active section multiple quantum wells  260 , and waveguide  105  (see FIGS. 1A-1B) are shown. In FIG. 2B, different portions of waveguide  105 , sections  210  and  220 , are optically active or passive due to the organization of the quantum wells within the waveguide material. 
     FIG. 2C illustrates a cross-sectional structure over a portion of laser assembly  100  (see FIG. 1) resulting from a butt joint regrowth technique for forming optically active and passive sections. According to the butt joint regrowth technique, the entire waveguide is etched away in optically passive sections and an optically passive waveguide is grown again. The newly grown portion of the waveguide is butted up against the active waveguide. In FIG. 2B, optically passive section  210 , optically active section  220 , active, butt-joint interface  270 , passive waveguide section  275 , active waveguide section  285  and waveguide  105  (see FIGS. 1A-1B) are shown. In FIG. 2B, active waveguide section  285  and passive waveguide section  275  are separated by a distinct large gradient butt-joint interface  270  as a result of the etch removal process. 
     FIGS. 3A-3D are plan views, illustrating different embodiments of optical amplifier  190  (see FIG.  1 ). In FIGS. 3A-3D optical amplifier  190 , waveguide  105 , epitaxial structure  170 , output facet  195 , active amplifier section  310 , passive amplifier section  320 , active-passive junction  330 , curved waveguide portion  340 , flared waveguide portions  350  and  355  and waveguide mode adapter  360  are shown. 
     In FIG.  3 A. optical amplifier  190  has an active amplifier section  310  combined with a passive amplifier section  320 , where the passive amplifier section includes curved waveguide portion  340 . The curved waveguide portion intersects output facet  195  at an oblique angle. Both the waveguide curvature and oblique intersection significantly reduces the amount of light reflecting from the output facet back into the amplifier and laser. Active-passive junction  330  is preferably oblique to a centerline of wave guide  105  so that any reflections from this interface coupling back into the amplifier and laser will be reduced. However, alternate embodiments may have active-passive junction  330  substantially normal to a centerline of the waveguide. 
     FIG. 3B shows an alternate embodiment where the amplifier active section has been segmented into a plurality of active sections in order to increase the amplifier output power and reduce a noise figure. In this embodiment shown in FIG. 3B, the amplifier active section is segmented into two amplifier active sections  310  that may be independently controllable. Other embodiments have more than two amplifier active sections. This segmenting of the amplifier enables the use of different bias points for the different sections. Having a plurality of amplifier stages allows higher saturated output powers to be reached with better noise performance. 
     FIG. 3C shows an alternate embodiment where a waveguide portion in the amplifier active section is flared, or tapered, to increase the saturated output power. Flared waveguide portion  350  increases the amplifier active volume as compared to the embodiment shown in FIG.  3 A and decreases the photon density. To accomplish this effectively without introducing significant fiber coupling difficulties it is preferable to use an adiabatic flare, wherein there is no energy transfer across optical modes over the flare to a wider waveguide cross-section. In a preferred embodiment, a second flared-down section  355  to a narrow waveguide cross-section is positioned in the amplifier optically passive section  320  since it is difficult to couple effectively from a wide waveguide into a single mode fiber at output facet  195 . In a preferred embodiment, such a flared-down portion is before a curved waveguide portion  340 , otherwise, higher order modes will be excited when curving the wide waveguide. In the embodiment shown in FIG. 3C, active-passive junction  330  is angled so that any reflections from this interface coupling back into the amplifier and laser will be reduced. 
     FIG. 3D shows another embodiment including a waveguide mode adapter. A waveguide mode adapter is preferred in many embodiments to enlarge the optical mode near output facet  195  so that it is more closely matched to the mode in an optical fiber that, as an element in a communications system, may carry the light away from the output facet. Including a waveguide mode adapter thus reduces the fiber coupling loss and increases the alignment tolerances between laser assembly  100  (see FIG. 1) and an optical fiber of another system. An embodiment of a waveguide mode adapter includes a section of passive waveguide wherein the waveguide&#39;s cross sectional is varied to expand the waveguide optical mode in an adiabatic manner. 
     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. 
     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.