Abstract:
A Semiconductor Optical Amplifier (SOA) with extended bandwidth and the method of fabricating the same are disclosed. In one embodiment, the SOA includes a Multiple Quantum Well (MQW) stacks whose thickness varies along the length of the device. The SOA has a flatter and broader gain spectrum than SOAs with substantially flat MQW stacks.

Description:
The present application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/357,072, filed Feb. 12, 2002, which is herein incorporated by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates generally to an extended bandwidth semiconductor optical amplifier. 
     BACKGROUND OF THE INVENTION 
     In optical communication systems, optical amplifiers play an important role in compensating for losses in multiplexing, splitting, switching, or wavelength converting. Semiconductor optical amplifiers (SOA&#39;s) are used widely to meet these needs. The use of SOA&#39;s is advantageous because they are less expensive and smaller than fiber amplifiers. One problem associated with conventional SOA&#39;s, however, is that their gain spectrum is narrow, typically around thirty to forty nanometers wide. Because of their narrow gain spectrum, the usefulness of conventional SOA&#39;s is somewhat limited in WDM (Wavelength Divisional Multiplexing) applications. 
     Accordingly, there exists a need for an optical amplifier whose gain spectrum is wider so as to cover as much of the useful fiber-optic communications spectrum as possible. 
     SUMMARY OF THE INVENTION 
     The present invention provides a semiconductor optical amplifier (SOA) with a wide gain spectrum. In one embodiment, the SOA has Multiple Quantum Well (MQW) stacks whose thickness varies along the length of the device. The gain spectrum of this embodiment is flatter and broader than conventional SOA&#39;s that are constructed using MQW stacks of uniform thickness. 
     Many semiconductor fabrication techniques can be used to fabricate the extended bandwidth SOA. Selective area growth is a technique that allows the growth rate to be varied as a function of position due to the proximity of a mask. When the gap between the masked areas is small the growth rate is highest. This results in thicker MQW stacks that have a gain peak at shorter wavelengths. In some embodiments where the MQW stacks are thicker at one end than at another, the cross-sectional area of the MQW stacks perpendicular to the propagation direction of optical signals remains largely constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the present invention will be more readily apparent from the following description and appended claims when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a semiconductor optical amplifier (SOA) according to an embodiment of the present invention; 
     FIG. 2 is a top view of the SOA shown in FIG. 1; 
     FIG. 3 is a side view of the SOA shown in FIGS. 1 and 2; 
     FIG. 4 is a cross-sectional view of the SOA of FIGS. 1-3 near one of its ends; 
     FIG. 5 is a cross-sectional view of the SOA of FIGS. 1-3 near another one of its ends; 
     FIG. 6 is a graph illustrating the gain spectrum of an SOA according to an embodiment of the present invention; 
     FIG. 7 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention; 
     FIG. 8 illustrates a side view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention; 
     FIG. 9 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention; and 
     FIG. 10 illustrates a top view of a Semiconductor Optical Amplifier in accordance with an alternate embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     FIGS. 1,  2  and  3  depict a perspective view, a top view and a side view of a semiconductor optical amplifier (SOA)  50  according to an embodiment of the present invention. In FIGS. 1-3, the SOA  50  has a substrate  10  and a mesa structure  12 . Masks  14  are disposed on the substrate  10  along the sides of the mesa structure  12 . In the present embodiment, the substrate  10  is made of III-V semiconductor materials such as InP, and the masks  14  are made of an amorphous semiconductor material, such as amorphous silicon dioxide. 
     In the embodiment illustrated in FIGS. 1,  2  and  3 , the masks  14  are wider at a first end  16  of the mesa structure  12  than they are at a second end  17 , and the mesa structure  12  is thicker at the first end  16  than it is at the second end  17 . In this embodiment, light can propagate from the first end  16  to the second end  17 , or vice versa, along the length of the mesa structure  12 . 
     With reference again to FIGS. 1,  2  and  3 , the mesa structure  12  has an active layer of Multiple Quantum Well (MQW) stacks  18  that include alternating layers of barriers and quantum wells. Both the barriers and quantum wells are formed using appropriately selected III-V semiconductor materials. When properly biased, the MQW stacks  18  amplify the optical signal received at one end  16  and emit amplified optical signal at the other end. In the present embodiment, electrodes (including an electrode  21  covering the top of the mesa structure  12  and another electrode  22  covering the bottom of the substrate  10 ) are used to provide the proper biasing of the SOA. 
     The thickness of the MQW stacks  18  varies along the light propagation path of the SOA  50 . The thickness of the MQW stacks  18 , however, does not vary linearly from the first end  16  to the second end  17 . Rather, the thickness varies in a non-linear fashion. In some portions of the SOA  50 , the thickness of the MQW stacks  18  remains the same. The thicker MQW stacks preferably include thicker layers of quantum wells. According to the present invention, the gain spectrum is dependent on the thickness of the quantum wells. Therefore, at the thicker end  16 , the gain is peaked at longer wavelengths due to the thicker MQW stacks. At the thinner end  17 , the gain is peaked at shorter wavelengths. One may consider the MQW stacks  18  to be analogous to many SOA&#39;s of small gain connected in series, each SOA having a slightly different gain peak. Optical signals with a flat and broad gain spectrum, which is shown in FIG. 6 as an example, can thus be produced. The gain spectrum shown in FIG. 6 is wider than that of conventional semiconductor optical amplifiers. 
     Attention now turns to FIGS. 4-5 and a fabrication processes for fabricating the SOA  50 . FIG. 4 illustrates the cross section of SOA  50  across line A—A, and FIG. 5 illustrates the cross section across line B—B of FIG.  3 . This fabrication process utilizes a technique known as Selective Area Growth (SAG), which is a technique that allows the growth rate to be varied as a function of position due to the proximity of a mask. The SAG technique involves, in the present embodiment, forming masks  14  over the semiconductor substrate  10  so as to permit vapor phase growth of semiconductor crystals in unmasked area. The masks  14  are preferably formed by depositing an amorphous material not conducive to epitaxial growth of semiconductor materials on the substrate  10  and patterning the amorphous material to expose the unmasked area. The width of the gap between the masks  14  varies along the path in which light propagates. In this particular embodiment, the gap between the masks  14  is smaller at the first end  16  than the gap at the second end  17 . As a result, the growth rate is higher at the first end  16 . In other embodiments, the gap between the masks is larger at the light-receiving end than at the light-emitting end. 
     With reference still to FIGS. 4-5, a Metal-Organic Chemical Vapor Deposition (MOCVD) process is carried out to deposit semiconductor materials, including semiconductor materials for forming the MQW stacks  18 , on the unmasked area. The thickness of the semiconductor materials grown on the exposed area varies according to the width of the exposed area. In particularly, where the gap between the masks  14  is small, the growth rate is higher, resulting in a thicker layer of semiconductor materials. Conversely, where the gap between the masks  14  is large, a thinner layer of semiconductor materials will be deposited. A product of the MOCVD process is the mesa structure  12  where the MQW stacks  18  are thicker near the first end  16  and near at the second end  17 . 
     In some embodiments, although the MQW stacks  18  are thicker at one end than at another, the process can be controlled such that the cross-sectional area of the MQW stacks perpendicular to the propagation direction of optical signals remains substantially constant. Furthermore, in the embodiment shown in FIGS. 4-5, the mesa structure  12  has a rectangular cross-sectional area. In other embodiments, the mesa structure has a trapezoidal cross-sectional area. In yet other embodiments, the mesa structure has an inverted trapezoidal cross-sectional area. 
     Other processes, such as the formation of electrodes and application of anti-reflective coatings to the first end  16  and the second end  17 , are carried out so as to complete the process of manufacturing the SOA  50 . 
     FIGS. 7 and 8 illustrate a top view and a side view, respectively, of a Semiconductor Optical Amplifier  70  in accordance with an alternate embodiment of the present invention. The SOA  70  is formed by cascading two SOA&#39;s  50 . In the embodiment shown in FIGS. 7 and 8, the thickness of the MQW stacks  18  varies along the direction of light propagation from one end to the other. As shown, a middle portion of the MQW stacks  18  is thicker than the end portions. The thicker MQW stacks  18  preferably include thicker layers of quantum wells. This embodiment of the present invention also has a flat and broad gain spectrum. 
     FIGS. 9 and 10 illustrate a top view and a side view, respectively, of a Semiconductor Optical Amplifier  80  in accordance with an alternate embodiment of the present invention. The SOA  80  is formed by cascading two SOA&#39;s  50 . In the embodiment shown in FIGS. 9 and 10, the thickness of the MQW stacks  18  varies along the direction of light propagation from one end to the other. Particularly, the end portions of the MOW stacks  18  is thicker than the middle portion. The thicker MOW stacks  18  preferably include thicker layers of quantum wells. Similar to the embodiments described above, this embodiment of the present invention also has a flat and broad gain spectrum. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein. For instance, multiple SOA&#39;s of the present invention can be cascaded with SOA&#39;s with substantially flat MQW stacks to form SOA&#39;s with a broad gain spectrum. The present invention can be used in many different types of apparatuses as well. For example, SOA&#39;s of the present invention can be integrated on a chip with other active components to either boost output in the case of lasers or electroabsorption lasers. SOA&#39;s of the present invention can also be used as optical amplifiers at the input stage of an optical receiver.