Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. The layers adjacent to the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In an edge emitting Fabry-Perot type semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide feedback reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge emitting or surface emitting, utilizes distributed feedback structures rather than conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index or the gain or both of the semiconductor laser structure.
High output power diode lasers with wavelengths in the 730-780 nm range are needed for a variety of applications ranging from laser printing and optical recording to cancer treatments such as photodynamic therapy. For wavelengths less than 840 nm, typical laser structures use AlGaAs in the active region, which can result in long-term reliability problems. For devices intended to operate with emission wavelengths less than 780 nm, the high aluminum content of the Al.sub.x Ga.sub.1-x As active layer (x&gt;0.1) required to obtain light emission at such wavelengths makes reliable high-power operation increasingly difficult to achieve. The high surface recombination velocity for AlGaAs leads to catastrophic optical mirror damage (COMD) at relatively low power densities. Even small Al concentrations (e.g., x.apprxeq.0.07-0.10) lead to significant reductions in internal power densities at COMD. Using a specially processed oxygen gettered aluminum source for the metal-organic chemical-vapor deposition (MOCVD) growth process, AlGaAs active-layer devices (100 .mu.m-wide emission aperture) have been reported with continuous wave (cw) output powers of 540 mW at an emission wavelength of 715 nm. P. L. Tihanyi, F. C. Jain, M. J. Robinson, J. E. Dixon, J. E. Williams, K. Meehan, M. S. O'Neill, L. S. Heath, and D. M. Beyea, IEEE Photonics Technol. Lett. 6, 775 (1994). More recently, compressively strained AlGaInAs active-layer lasers have been reported in the 730 nm wavelength range demonstrating 2.2 W cw output powers from broad-stripe (100 .mu.m-wide) devices. M. A. Emanuel, J. A. Skidmore, M. Jansen and R. Nabiev, IEEE Photonics Technol. Lett. 9, 1451 (1997). Although high output powers have been obtained from the AlGa(In)As active-layer devices, long-term reliability is still an open question because, even if the mirror facets are passivated, since defects in the bulk of the active region material cause device degradation. For such devices emitting at wavelengths in the range of 700 nm to 780 nm there is no extensive lifetest data and high power devices are not commercially available.
The lower surface recombination velocity of InGaAsP compared with AlGaAs leads to a corresponding reduction in facet-temperature rise during high-power cw operation. D. Z. Garbuzov, N. Yu. Antonishkis, A. D. Bondarev, A. B. Gulakov, S. N. Zhigulin, N. I, Katsavets, A. V. Kochergin, and E. V. Rafailov, IEEE J. Quantum Electron. QE-27, 1531 (1991). Tensile-strained (In) GaAsP active-layer lasers have been reported operating in the 700-800 nm wavelength range. D. P. Bour, D. W. Treat, K. J. Beernink, R. L. Thornton, T. L. Paoli, and R. D. Bringans, IEEE Photonics Technol. Lett. 6, 1283 (1994). However, little is known about the properties of compressively strained quantum-well lasers in this wavelength region. Compressively strained active layers have been reported with emission at 980 nm, D. F. Welch, W. Streifer, C. F. Schaus, S. Sun and P. L. Gourley, Appl. Phys. Lett. 56, 10 (1990); at 1.3 .mu.m, P. J. A. Thijs, L. F. Tiemeijer, J. J. M. Binsma and T. van Dongen, IEEE J. Quantum Electron. QE-30, 477 (1994); and at 1.55 .mu.m, A. Mathur and P. Dapkus, IEEE J. Quantum Electron. QE-32, 3223 (1996).