Single laser beam of spatial coherence from an array of GaAs lasers: Free-running mode

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Abstract
Spatially coherent radiation from a monolythic array of three GaAs lasers in a free‐running mode is reported. The lasers, with their mirror faces antireflection coated, are operated in an external optical cavity built of spherical lenses and plane mirrors. The spatially coherent beam formation makes use of the Fourier transformation property of the internal lenses. The transverse mode control is accomplished by a spatial filter. The optical cavity is similar to that used for the phase‐controlled mode of spatially coherent beam formation, which had been reported; only the spatial filters are different. The spatial filter in the free‐running mode controls the fields of the lasers to be in the lowest‐order transverse mode, but it does not control their phase angles. Random amplitude and phase variations of the longitudinal modes prevent the formation of a stationary interference waveform. In the far field (when restored by an external lens), the intensities of the lasers in the array are concentrated in a single laser beam of spatial coherence, without any grating lobes. The far‐field distribution is not influenced by the spacings among the lasers. The far‐field distribution of the laser array in the free‐running mode differs significantly from the interference pattern of the phase‐controlled mode. The modulation characteristics of the optical waveforms of the two modes are also quite different. This is because the modulation is related to the interaction of the spatial filter with the longitudinal modes of the laser array with the optical cavity. The modulation of the optical waveform of the free‐running mode is nonperiodic, confirming that the fluctuations of the optical fields of the lasers are random. Experiments were performed at room temperature using a monolythic array of three GaAs lasers. The far‐field distribution of the laser array and the optical waveform were investigated.