Alignment tolerances of short-external-cavity InGaAsP diode lasers for use as tunable single-mode sources
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The side-mode suppression ratio (SMSR) and single-mode continuous tuning range were measured for planar short-external-cavity semiconductor diode lasers to examine the positional and angular alignment tolerances of the external cavity mirror relative to the laser facet. For inverted rib waveguide lasers, the SMSR was found to be = -25 dB for external cavity lengths of 40-200 microm and = -23 dB for rotational misalignments of +/-8 degrees . For parallel alignment of the external cavity mirror and laser facet, continuous single-mode tuning ranges of 1.0 nm, or 110% of a mode spacing, were obtained. At external cavity lengths of <100 microm, the total amount of continuous single-mode tuning, summed over all modes, was ~72 cm(-1), which corresponds to ~75% spectral coverage.
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