Broadly wavelength-tunable external cavity mid-infrared quantum cascade lasers

Wavelength tuning over 120 run for a grating-coupled 5.1-mum quantum-cascade type-l laser was studied for temperature from 80 to 243 K (-30degreesC). Both the Littman-Metcalf and first-order grating direct feedback cavity configurations were used with similar tuning results. The goal is to achieve broad tunability, and the result is a combined grating and temperature tuning of 245 nm, from 5.040 to 5.285 mum. The laser was designed for predominantly single-mode or at most, two-longitudinal mode operation. The instrument-limited laser linewidth was less than the cavity longitudinal mode spacing. Stepping-motor control of the grating allowed 0.4-GHz wavelength increments (35 pm) to be realized with high reproducibility. A current-induced wavelength shift of similar to2-3 GHz was observed, corresponding to an effective refractive index change of similar to10(-3). Analysis indicates that single-mode, continuously tunable operation is feasible with a more optimal device and cavity.