| Optically-pumped type-II distributed-feedback (DFB) lasers have been developed to provide continuous-wave (CW) single-longitudinal-mode (SLM) operation, narrow spectral linewidth, high output power, good beam quality, and large wavelength tunability for spectroscopic applications in the 3- to 5-mum midwave-infrared (mid-IR) atmospheric transmission window. Results from two stages of development are provided.;In the first set of experiments, uniform index-coupled DFB gratings were fabricated using interferometric lithography (IL) and plasma etching in the top clad of the laser, which incorporates 14 InAs/InGaSb/InAs type-II quantum wells imbedded in an InGaAsSb waveguide. A 110-mum-wide stripe from a 1.9-mum CW fiber laser provides both optical pumping and gain guiding. We report a record output power of 560 mW per facet, a narrow linewidth of 0.17 nm, and a side mode suppression ratio (SMSR) of 33 dB in CW SLM operation at 80 K. The lasing wavelength of ∼3.6 mum was tunable over a 9-nm range by varying the liquid nitrogen dewar temperature from 80 to 100 K. The Bragg stop band and two degenerate DFB modes were observed at certain pump powers.;In the second set of experiments, period-chirped DFB gratings were fabricated using IL with spherical wavefronts to provide large continuous wavelength tunability. Tuning was achieved by moving the optical pump stripe relative to the device with a chirped DFB grating. We report a ∼65 nm quasi-continuous wavelength tuning range from a 3.5-mm-long device in CW SLM operation at ∼3.2 mum. Spectroscopy experiments were conducted using the chirped-grating DFB laser as a tunable light source. Methane transmittance spectra that closely match HITRAN simulations were obtained. |
