External cavity quantum cascade lasers without anti-reflection coating with intracavity and extracavity acoustic-optic frequency shifter for fast standoff detection

Abstract

Two new tunable external cavity quantum cascade lasers (EC-QCLs) without anti-reflection coating with intracavity and extracavity acoustic-optic frequency shifter (AOFS) are reported. In the extracavity AOFS configuration, a broadly coarse tuning range of 256 cm−1 (540 nm) for the spectral range between 2309 cm−1 (4330 nm) and 2053 cm−1 (4870 nm) in pulsed operation was achieved by rotating the diffraction grating, whereas a fine tuning range of 0.3 cm−1 within 6 ms was obtained by modulating the injection current. In this setup, tunable double-mode operation was achieved using the extracavity AOFS and high-resolution gas phase spectroscopy at a sensing distance of up to 50 m with a retro-reflector (mirror) was possible. In the intracavity AOFS setup, the wavelength could be tuned rapidly in a broad tuning bandwidth of 33 cm−1 (65 nm) within 50 μs by varying the acoustic frequency from 65 MHz to 89 MHz, in addition to controlling the injection current and grating position. Due to the broader linewidth in the intracavity AOFS-setup, high-resolution gas spectroscopy was not possible. However, solid and liquid spectroscopy was feasible. Using this setup, detection of the metformin hydrochloride tablet at a distance of 1 meter without a retro-reflector was achieved.