Phase-front measurements of an injection-locked AlGaAs laser-diode array

The phase-front quality of the primary spatial lobe emitted from an injection-locked gain-guided AlGaAs laser diode array is measured by using an equal-path, phase-shifting Mach-Zehnder interferometer. Root-mean-square phase errors of 0.037 +/- 0.003 wave (lambda/27) are measured for the single spatial lobe, which contained 240-mW cw output power in a single longitudinal mode. This phase-front quality corresponds to a Strehl ratio of S = 0.947, which results in a 0.23-dB power loss from the single lobe's ideal diffraction-limited power. These values are comparable with those measured for single-stripe index-guided AlGaAs lasers.