We demonstrate a technique for tunable single-mode operation of high-power laser-diode arrays. A spatially filtered part of the far field from a gain-guided GaAlAs laser-diode array is directed to a diffractive grating and is coupled to a phase-conjugating self-pumped barium titanate crystal. The interaction among the grating, the dynamic gratings in the photorefractive crystal, and the spatial filter forces the laser-diode array with poor spatial and temporal coherence to oscillate in a single spatial and single longitudinal mode. At a drive current of two times the threshold, the array operates in a single-lobed far-field pattern that is only 1.5 times the diffraction limit. The bandwidth of the enhanced output beam is measured to be less than 0.03 nm, and the coherence length of the output is increased 45 times, to at least 16 mm. Once single-mode operation has been obtained, one can continuously tune the frequency over a range of 5 nm around a center wavelength of 811 nm by tilting the grating. More than 50% of the radiated energy of the free-running laser is coupled out of the system. The significantly enhanced coherence properties of the output may lead to a number of new or improved applications in areas such as second-harmonic generation, coupling into single-mode fiber, and pumping of solid-state lasers.
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