External cavity mid-infrared semiconductor lasers

GaSb-based and InAs-based semiconductor gain media with band-edge wavelengths between 3.3 to 4 mu m were used in grating-tuned external cavity configuration. Output wavelength was tuned up to similar to 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 similar to 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 similar to 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.