External cavity mid-infrared semiconductor lasers

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Abstract
GaSb-based and InAs-based semiconductor gain media with band-edge wavelengths between 3.3 to 4 micrometers were used in grating-tuned external cavity configuration. Output wavelength was tuned up to approximately 9.5% of the center wavelength; and power from few tens of mW to 0.2-W peak, 20- mW average was achieved at 80 K operation. The tuning range is approximately 2 - 3 times wider than those of near-IR semiconductor lasers, as expected for mid-IR semiconductors which have smaller electron masses. The external cavity laser had a multimode linewidth of 1 - 2 nm, which was approximately 10 to 20 times narrower than that of a free running laser. Analysis of the gain/loss spectral properties indicates that the tuning range is still severely limited by facet anti-reflection coating and non-optimal wafer structure. Model calculation indicates a tuning range a few times larger is possible with more optimal wafer design.

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