Abstract:
A laser has a spatially varying absorption spectrum formed in a monolithic InGaAsP structure whose quantum well active structure has modified effective bandgap properties. The spatially varying emission spectrum allows emission at multiple wavelengths or emission in a broad band. The effective bandgap properties can be modified by rapid thermal annealing to cause the diffusion of defects from one or two InP defect layers into the quantum well active structure. The laser can be implemented variously as a Fabry-Perot laser and a laser array.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to semiconductor lasers and more specifically to multi-wavelength and wideband lasers formed in a monolithic structure. 
     DESCRIPTION OF RELATED ART 
     Currently, multi-wavelength networks rely on a parallel array of many light sources (LED&#39;s or lasers) to generate many colors (wavelengths) of light for many-wavelength communication. (wavelength-division multiplexing, or WDM). These multiple signals are then connected into the input ports of multiplexer (e.g., an arrayed waveguide grating). In the absence of optical integration, this means a complex assembly process, e.g., N device-fiber coupling operations, where N is the number of wavelength channels. This solution shows poor scaling. 
     Tunable lasers are urgently needed in optical communications networks. Inventory of many different types of lasers, one for each needed wavelength, is costly. Furthermore, dynamic networks based on the use of different wavelengths to express different destinations for data depend on tunability. Today, tunable lasers are costly and are not so widely tunable as desired. Tunable lasers are required which can range over the set of all wavelengths of interest in future fiber optical communication networks. This range is much greater than what is conveniently available today. The fundamental limitation: the bandwidth over which optical gain can be provided is constrained if a single active region material is employed. 
     The realization of cost-effective local-area, access, enterprise, and data center fiber-optic networks will rely on finding ways to implement wavelength-division multiplexing with a few simple, low-cost components. In contrast, today&#39;s WDM systems are complicated multi-component systems with narrow tolerances and complex assembly. Their bandwidth is intrinsically limited by the fundamental statistical properties of electrons at room temperature. 
     U.S. patent application Ser. No. 09/833,078 to Thompson et al, filed Apr. 12, 2001, entitled “A method for locally modifying the effective bandgap energy in indium gallium arsenide phosphide (InGaAsP) quantum well structures,” and published on Mar. 14, 2002, as U.S. Ser. No. 2002/0030185 A1, whose entire disclosure is hereby incorporated by reference into the present disclosure, teaches a method for locally modifying the effective bandgap energy of indium gallium arsenide phosphide (InGaAsP) quantum well structures. That method allows the integration of multiple optoelectronic devices within a single structure, each comprising a quantum well structure. 
     In one embodiment, as shown in FIG. 1A, an InGaAsP multiple quantum well structure  104  formed on a substrate  102  is overlaid by an InP (indium phosphide) defect layer  106  having point defects  108 , which are donor-like phosphorus antisites or acceptor-like indium vacancies. Rapid thermal annealing (RTA) is carried out under a flowing nitrogen ambient, using a halogen lamp rapid thermal annealing system. During the rapid thermal annealing, the point defects  108  in the defect layer  106  diffuse into the active region of the quantum well structure  104  and modify its composite structure. The controlled inter-diffusion process causes a large increase in the bandgap energy of the quantum well active region, called a wavelength blue shift. 
     Another embodiment, as shown in FIG. 1B, uses two defect types, one to generate a wavelength blue shift and the other to decrease carrier lifetime. A first InP defect layer  110  contains slowly diffusing vacancy defects  114 , while a second InP defect layer  112  includes rapidly diffusing group V interstitial defects  116 . Rapid thermal annealing causes both types of defects to diffuse into the quantum well active region. 
     As will be familiar both from the above-cited application and from general skill in the art, a semiconductor quantum well structure has at least quantum well layer  120  bounded by barrier layers  122 ,  124 . 
     However, a solution has not yet been found to the problems described above for tunable lasers. 
     SUMMARY OF THE INVENTION 
     In light of the above, it will be readily apparent that a need exists in the art to achieve two ends that may appear to be at odds with each other, namely, increased bandwidth and decreased size, complexity and expense. It is therefore an object of the invention to provide greater integration of lasers having multiple wavelengths or a wide emission band. 
     To achieve the above and other objects, the present invention is directed to a technique for producing multiple lasers in a single semiconductor device, using the techniques of the above-cited Thompson et al patent application or any other suitable intermixing techniques. 
     A first preferred embodiment of the invention concerns realization of a spatially serial multichannel transmitter with a single fiber coupling requirement for the realization of low-cost transceivers. The process of Thompson et al offers a method of creating LED&#39;s and, ultimately, lasers in which a number of wavelengths of optical emission may be chosen independently within a single device. This single, many-wavelength-producing device can then be coupled, through a single device-fiber coupling operation, into single-mode fiber. The method is potentially low-cost and exhibits excellent scaling with increased wavelength channel count, especially if assembly and packaging dominate total component cost. 
     A second preferred embodiment of the invention concerns realization of an ultra-wideband tunable Fabry-Perot laser in a single integrated device, achievable through wide tuning of the gain peak spectral location using multiple degrees of freedom coming through multiple spectrally shifted wavelength-tuning sections. The resulting device can serve as fast-wavelength-hopping transmitter. The process of the Thompson et al patent application permits realization of lasers with differentiated sections of active region, each with a different spectrum of light production. Independent control over the excitation of the various sections will permit the optical gain to be maximized at a wide range of possible wavelengths, selectable by electronic control. The resulting gain spectrum will determine the wavelength at which light will be produced. The resulting lasers will thus be widely tunable, greatly beyond the bandwidth available to devices made according to existing technology. 
     A third preferred embodiment of the invention concerns realization of an ultra-low-cost broadband, spectrally flattenable light source at, e.g., 1.55 μm for subsequent demultiplexing, modulation, and remultiplexing. The Thompson et al patent application provides a basis for realization of broadband light emitters which can address hundreds of nanometers of wavelength span, in contrast with current devices which can access tens of nanometers. That technology thus provides a basis for addressing a much greater bandwidth than in today&#39;s components, but in a way that is intrinsically integrated and prospectively cost-effective. The technology does this by allowing integrated realization of many independent sections of the device, each producing light over a relatively narrow (conventional) bandwidth, but together adding up to produce a controllably broad spectrum. 
     A fourth preferred embodiment of the invention realizes an array of lasers, either vertical-cavity or edge-emitting, on a single substrate having a single epitaxially-grown active region. Epitaxy-based spatially selective intermixing is used to shift the effective bandgap of the material differently in different regions. An array in space of lasers subsequently fabricated using this quantum well material will have different emission wavelengths by virtue of a combination of spectral gain peak shifting and (real) refractive index shifting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be set forth in detail with reference to the drawings, in which: 
     FIGS. 1A and 1B show two embodiments of the technique of the above-cited Thompson et al patent application; 
     FIG. 2 shows a schematic diagram of the first preferred embodiment of the present invention; 
     FIG. 3 shows a schematic diagram of the second preferred embodiment of the present invention; 
     FIG. 4 shows a schematic diagram of the third preferred embodiment of the present invention; and 
     FIG. 5 shows a schematic diagram of the fourth preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements or operational steps throughout. 
     FIG. 2 shows a schematic diagram of a first preferred embodiment of the present invention. The first preferred embodiment provides a many-wavelength-producing device in which multiple lasers producing different wavelengths are formed as separate active quantum well regions in a single semiconductor quantum well structure by the techniques of the above-cited Thompson et al patent application. 
     The many-wavelength-producing device  200  is formed as a semiconductor quantum well structure  202  having three (or another suitable number) sections  204 - 1 ,  204 - 2 ,  204 - 3 . Each of the sections includes a quantum well active region  206 - 1 ,  206 - 2 ,  206 - 3  which functions as a laser at a different frequency when a current I 1 , I 2 , I 3  applied through an electrode  208 - 1 ,  208 - 2 ,  208 - 3 . The light output by the quantum well active regions  206 - 1 ,  206 - 2 ,  206 - 3  is emitted in the left-to-right direction in FIG.  2  and is output through an output fiber  210 . 
     The quantum well active regions  206 - 1 ,  206 - 2 ,  206 - 3  are formed by the techniques of the above-cited Thompson et al patent application or any other suitable intermixing techniques. Thus, multiple such quantum well active regions can be formed in a single monolithic structure  202 . 
     The quantum well active regions  206 - 1 ,  206 - 2 ,  206 - 3  are tuned to output light at different frequencies. That may be done by any suitable technique, e.g., by varying the compositions or thicknesses of the quantum well active regions. For example, the quantum well active region  206 - 1  may be non-intermixed, while the quantum well active region  206 - 2  may be moderately intermixed and the quantum well active region  206 - 3  may be strongly intermixed. Each quantum well active region is characterized, relative to those regions to the left of it in FIG. 2, by a larger transition energy, higher-photon-energy light, and transparency to the lower-photon-energy light produced by the quantum well active regions to the left. The relative intensities of the light output by the various quantum well active regions can be controlled by varying the currents applied through the electrodes. 
     FIG. 3 shows a schematic diagram of a second preferred embodiment of the present invention. The second preferred embodiment provides an ultra-wideband tunable Fabry-Perot laser in a single integrated device. The Fabry-Perot laser  300  of FIG. 3 can be structured like the many-wavelength-producing device  200  of FIG. 2, except that mirrors  312  and  314  are added to implement a Fabry-Perot resonant cavity. The quantum well active regions  206 - 1 ,  206 - 2 ,  206 - 3  provide regions with different spectra of light production. Independent control over the excitation of the various regions through the application of independently controllable currents to the electrodes permits the optical gain to be maximized over a wide range of possible wavelengths, selectable by electronic control. The resulting gain spectrum determines the wavelength at which the light is produced. The resulting Fabry-Perot laser  300  is thus widely tunable, well beyond the bandwidth possible in Fabry-Perot lasers of the prior art. The mirrors  312  and  314  can be of any suitable structure, e.g., silvered mirrors or multi-layer interference stacks. Existing techniques for enhancing and controlling tunability, such as the addition of further gratings or external cavities, may effectively be used in combination with this structure. 
     FIG. 4 shows a schematic diagram of a third preferred embodiment of the present invention. The third preferred embodiment provides a wideband, spectrally flattenable light source for subsequent demultiplexing, modulation and remultiplexing. The spectrum can be centered on any suitable value, e.g., 1.55 μm. The light source  400  can be structured like the many-wavelength-producing device  200  of FIG. 2, except that antireflective coatings  416  and  418 , which can be multi-layer interference coatings, are added. Each section provides light over a relatively narrow bandwidth; however, the multiple sections, independently controllable through the application of independently controllable currents through the electrodes, together provide a controllably broad spectrum. 
     FIG. 5 shows a schematic diagram of a fourth preferred embodiment of the present invention. The fourth preferred embodiment provides an array of lasers, either vertical-cavity or edge-emitting, on a single substrate formed as a single epitaxially grown active region. Epitaxy-based spatially-selective intermixing is used to shift the effective bandgap of the material differently in different regions. An array in space of lasers subsequently fabricated using this quantum well material will have different emission wavelengths by virtue of a combination of spectral gain peak shifting and (real) refractive index shifting. 
     The array  500 , as seen from above in FIG. 5, is formed monolithically on a substrate  502 . The lasers include non-intermixed regions  504 - 1  lasing at a wavelength λ 1 , moderately intermixed regions  504 - 2  lasing at a wavelength λ 2 , and strongly intermixed regions  504 - 3  lasing at a wavelength λ 3 . The lasers in the array can be controlled by any suitable technique, e.g., a grid of electrodes. The principles of any of the first through third preferred embodiments can be incorporated as desired. 
     While four preferred embodiments have been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, any suitable output waveguide waveguides or configuration of electrodes could be used. Therefore, the present invention should be construed as limited only by the appended claims.