Introduction

Phase-locked arrays of diode lasers have been studied extensively over the last 15 years. Such devices have been pursued in the quest to achieve high coherent powers (> 100 mW diffraction limited) for applications such as space communications, blue-light generation via frequency doubling, optical interconnects, parallel optical-signal processing, high-speed, high-resolution laser printing and end-pumping solid-state lasers. Conventional, narrow-stripe (3–4 μm wide), single-mode lasers provide, at most, 100 mW reliably, as limited by the optical power density at the laser facet. For reliable operation at watt-range power levels, large-aperture (≥100 µm) sources are necessary. Thus, the challenge has been to obtain single-spatial-mode operation from large-aperture devices, and maintain stable, diffraction-limited-beam behavior to high power levels (0.5–1.0 W).

By comparison with other types of high-power coherent sources (master oscillator power amplifier (MOPA), unstable resonators), phase-locked arrays have some unique advantages: graceful degradation; no need for internal or external isolators; no need for external optics to compensate for phasefront aberrations due to thermal- and/or carrier-induced variations in the dielectric constant; and, foremost, beam stability with drive level due to a strong, built-in, real-index profile. The consequence is that, in the long run, phase-locked arrays are bound to be more reliable than either MOPAs or unstable resonators.