| High-power, coherent radiation from semiconductor lasers is attractive for such diverse applications as free-space communication, optical data storage, and microsurgery. However, several factors conspire to prevent near-ideal performance from broad area devices and laser arrays. Waveguides wider than a few microns support many lateral modes with poor gain discrimination. Consequently, such modes are easily "mixed" by perturbations in gain and refractive index caused by gain saturation, thermal gradients, and inhomogeneities due to imperfect crystal growth. This causes spatially localized modes, multimode operation, and reduced spatial coherence, all of which lead to farfield patterns broader than the "diffraction limit."; In this thesis, we have investigated the influence of gain saturation on the lateral modes of broad area structures and laser arrays. Analytical and numerical techniques have been developed to solve self-consistently for mode shapes and propagation constants as a function of injected current density above threshold. In gain-guided, quantum well lasers, the nonlinear broad area modes are observed to oscillate into narrow, single-lobed farfields which broaden only slightly with increased power output up to the 500 mW level. In contrast, we have found the lateral modes of laser arrays to be unstable with increased current injection. Waveguides which are phase-matched below threshold become detuned under the influence of gain saturation, so that interguide power transfer is reduced. This diminishes the injection-locking bandwidth, and ultimately, the spatial coherence.; Finally, we have considered marrying the high-power, coherent output of broad area and array lasers with the broadband tunability possible in quantum well lasers. Experimentally, we have tuned uncoated, single quantum well stripe lasers in a grating-coupled external cavity over a range {dollar}>{dollar}125 nm centered about 800 nm. Similarly tuned broad area lasers output in excess of 200 mW into a single longitudinal mode over 80 nm. We expect that in the future, such semiconductor devices could provide a compact, rugged, more efficient alternative to dye lasers. |
