Abstract:
A semiconductor quantum well laser having separate lateral confinement of injected carriers and the optical mode. A ridge waveguide is used to confine the optical mode. A buried heterostructure confines injected carriers. A preferred embodiment laser of the invention is a layered semiconductor structure including optical confinement layers. A buried heterojunction quantum well within the optical confinement layers is dimensioned and arranged to confine injected carriers during laser operation. A ridge waveguide outside the optical confinement layers is dimensioned and arranged with respect to the buried heterojunction to confine an optical mode during laser operation. An index step created by the buried heterojunction is substantially removed from the optical mode.

Description:
FIELD OF THE INVENTION 
     The field of the invention is semiconductor quantum well lasers. 
     BACKGROUND OF THE INVENTION 
     Semiconductor lasers are the fundamental building block in compact optic and optoelectronic devices. Formed from Group III-V semiconductors, the semiconductor lasers emit laser light in response to electrical stimulation as electrons relax back to lower energy states and emit photons. Two conventional types of semiconductor lasers are buried heterostructure lasers (BH) and ridge waveguide (RW) lasers. 
     BH lasers are extremely effective at confining carriers. The lateral heterostructure of a BH also creates a large index step, which strongly confines the optical mode of the laser. Another result of the large index step, though, is the support of higher order modes. The higher order modes can give rise to beam instability, a large diffraction angle, and poor fiber coupling efficiency. A physically narrow BH device can defeat propagation of higher order modes, but provides a smaller available gain volume, higher optical power density at its facets, and larger diffraction angle of its emitted beam. Manufacturing narrower BH lasers also poses more difficult manufacture process control problems compared to otherwise similar wider devices. In general, the BH lasers are low threshold but high performance devices. 
     RW lasers can be made with a comparably smaller index step. The index step of an RW laser is controlled by controlling the depth of the ridge etch. RW lasers are easier to manufacture than BH lasers since the RW lasers require only a single crystal growth step. However, the RW lasers are less efficient than BH lasers. Due to unconfined spreading of carriers, a region outside of the ridge in an RW laser is also pumped leading to gain which is not effectively utilized. As a result, the threshold current of a RW laser can be twice as high as a comparable BH laser. 
     SUMMARY OF THE INVENTION 
     A semiconductor quantum well laser of the invention utilizes separate lateral confinement of injected carriers and the optical mode. A ridge waveguide is used to confine the optical mode. A buried heterostructure confines injected carriers. A preferred embodiment laser of the invention is a layered semiconductor structure including optical confinement layers. A buried heterojunction within the optical confinement layers is defined in a quantum well layer, and is dimensioned and arranged to confine injected carriers during laser operation. A ridge waveguide outside the optical confinement layers is dimensioned and arranged with respect to the buried heterojunction to confine an optical mode during laser operation. An index step created by the buried heterojunction is substantially removed from the optical mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features, objects and advantages of the invention will be apparent to artisans from the detailed description and drawings, of which: 
     FIG. 1 a  is front side view of a laser in accordance with a preferred embodiment of the invention; and 
     FIG. 1 b  is an enlarged portion of FIG. 1 a , showing the buried heterojunction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A semiconductor quantum well laser of the invention utilizes separate lateral confinement of injected carriers and the optical mode. The separate lateral confinement is achieved by a ridge waveguide (RW) aligned over a buried heterostructure (BH) defined in a quantum well (QW). Various ones of the Group III-V material systems can be used to produce lasers in accordance with the invention. The preferred example of FIGS.  1 ( a ) and  1 ( b ) includes particular cladding and barrier layer materials, but the invention is not limited thereto. Instead, the inventive principles are found in the general structure of the preferred embodiment and, particularly, the separate lateral confinement achieved by that structure. 
     In FIGS.  1 ( a ) and  1 ( b ), the preferred semiconductor quantum well laser structure  10  includes Al 0.2 Ga 0.8 As cladding layers  12 ,  14  formed on a n-GaAs buffer layer  15  and around intervening GaAs barrier layers  16 ,  18 . The laser of the invention can be made in many other materials systems so the Al 0.2 Ga 0.8 As material system is not limiting, but is just a preferred example. A quantum well In 0.28 Ga 0.72 As BH  22  is formed at the interface of barrier layers  16  and  18 . The QW BH layer is thin (a quantum well) to begin with and a BH formation step truncates it in the lateral direction. The result is a layer that is, for example, 4 or 5 μm wide but much thinner (0.005-0.01 μm). A RW  24  is defined in an upper part of the cladding layer  14  and a P+GaAs cap layer  26  caps the cladding layer  14 . The etched RW  24 , is aligned over, and preferably centered over, the wider BH quantum well  22 . The RW  24  confines the laser&#39;s optical mode while the BH  22  formed in the quantum well confines injected carriers. 
     The index step created by the BH  22  is ideally minimal, but is at least substantially removed from the optical mode such that it has only a small effect on the index guide of the laser  10 . The BH is nonetheless sufficiently narrow to limit lateral diffusion of the injected carriers, which have an effective diffusion length of about 2 μm. The inventors have calculated effective index of the FIGS.  1 ( a ) and  1 ( b ) structure for a 3.5 μm RW (W RW ) and a 6 μm BH (W BH ) as the lateral dimension varies along with the calculated intensity of the resulting optical mode. Both the RW  24  and the BH  22  create steps in the effective index. The RW  24  creates an inner index step and the BH  22  creates an outer (ideally minimal) index step. The weaker index guide created by the etched RW  24  can accommodate a wide optical mode and still maintain single lateral mode operation. 
     Although the threshold current of the laser of the invention will be slightly higher than a comparable BH laser due to the necessity of pumping a larger active volume, an offset in the required increased threshold current occurs because the entire width of the optical mode can propagate in a region of gain. The evanescent tail of the mode in the BH  22  may contain a significant fraction of the optical power, but it cannot contribute to stimulated emission since there is no gain outside of the index guide. This is accounted for in the standard equation for threshold current density J th  in quantum well lasers by introducing a lateral gain confinement factor Γ lat  that reduces the modal gain:          J   th     =         J   o       η   i            exp   [       α   -       1   L        ln        1   R             Γ   tr          Γ   lat          J   o        β       ]                              
     In equation (1), J o , η i , α, β, Γ tr , Γ lat , and R are respectively the transparency current density, internal quantum efficiency, distributed loss, gain coefficient, cavity length, transverse confinement factor, lateral confinement factor, and facet reflectivity. 
     Prototype lasers according to the FIGS.  1 ( a ) and  1 ( b ) structure have been fabricated and tested. The prototypes were grown by a two-step metal organic chemical vapor deposition (MOCVD). Growths were done at atmospheric pressure at growth temperatures of 720° for AlGaAs cladding layers, 625° C. for an InGaAs quantum well buried heterostructure and GaAs barrier layers and 650° C. for a p+GaAs cap layer. The first growth consisted of an n-GaAs buffer layer (100 nm), an Al 0.2 Ga 0.8 As lower cladding (1 μm), a GaAs barrier (200 nm), an In 0.28 Ga 0.72 As quantum well (7 nm), and part of the upper GaAs barrier (30 nm). The prototypes were then patterned using standard photolithography in stripes ranging in width from 1 to 30 μm and etched through the quantum well to form a buried heterostructure. The etch depth was made as shallow as possible to minimize the index step created by the BH. The samples then underwent a surface preparation, were reloaded in the MOCVD reactor, and overgrown to form a BH similar to that created by selective area epitaxy in Cockerill et al., “Strained-layer InGaAs-GaAs-AlGaAs Buried-Heterostructure Quantum Well Lasers by Three-Step Selective-Area MOCVD,” IEEE J. of Q. Elect., vol. 30, no 2, pp. 441-45 (February 1994), which is incorporated by reference herein. The final prototype structures have a 200 nm GaAs upper barrier, a 0.6 μm Al 0.2 Ga 0.8 As upper cladding and a 0.1 μm p+ GaAs cap. A 3.5 μm wide optical RW centered over the BH is then formed by wet etching to a depth of ˜0.5 μm. A 200 nm layer of plasma enhanced chemical vapor deposition SiO 2  was then deposited on prototypes, contact windows were opened on top of the RW, and Ti/Pt/Au metal contacts were deposited. Samples were then lapped and polished, Ge/Au contacts were deposited on the bottom of the wafer and alloyed at 400° C., and cleaved into 1 mm cavity lengths. 
     The index step created by the RW etch is calculated to be ˜0.005. This was found experimentally to be the minimum index step necessary to defeat the anti-guiding effects of injected carriers and yield stable laser operation. Prototype samples were tested mounted p-side down in a clip and pulsed for 2 μs at 1.5 kHz. The lasing wavelength was near 1.03 μm. Lasers according the FIGS.  1 ( a ) and  1 ( b ) inventive structure and having a wide BH had the same threshold as conventional single grow RW lasers, indicating that the two step fabrication process is a high quality process. A clear reduction in threshold current was apparent for devices with BH widths of less than 10 μm. Near field images of the facets showed that the lasing mode was confined within thee etched RW, and the far field was single lobed. The lateral mode was confined within the etched RW of prototype devices, and the far field was single lobed. The lateral FWHM (full width half maximum) divergence angle for wide prototype BH (10-30 μm) was ˜10°. The FWHM increases smoothly as the BH narrows, to approximately˜14° at a BH width of 4 μm. This occurs as the index step from the BH (Δn=˜0.005) comes in closer proximity to the optical mode and contributes to index guiding. 
     General usefulness is thus demonstrated by the prototypes. Artisans will appreciate, however, the layer widths and materials may also be different from those in the prototypes. The prototypes show that the RW and BH may vary in width, and knowledge in the art also provides for different materials and layer thicknesses. Generally, those properties may vary with known conventional RW and known convention BH structures. 
     Accordingly, while a specific embodiment of the present invention has been shown and others described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the appended claims.