Intermittent operation of QC-lasers for mid-IR spectroscopy with low heat dissipation: tuning characteristics and driving electronics

Abstract

Intermittent scanning for continuous-wave quantum cascade lasers is proposed along with a custom-built laser driver optimized for such operation. This approach lowers the overall heat dissipation of the laser by dropping its drive current to zero between individual scans and holding a longer pause between scans. This allows packaging cw-QCLs in TO–3 housings with built-in collimating optics, thus reducing cost and footprint of the device. The fully integrated, largely analog, yet flexible laser driver eliminates the need for any external electronics for current modulation, lowers the demands on power supply performance, and allows shaping of the tuning current in a wide range. Optimized ramp shape selection leads to large and nearly linear frequency tuning (>1.5 cm−1). Experimental characterization of the proposed scheme with a QCL emitting at 7.7 μm gave a frequency stability of 3.2×10−5 cm−1 for the laser emission, while a temperature dependence of 2.3×10−4 cm−1/K was observed when the driver electronics was exposed to sudden temperature changes. We show that these characteristics make the driver suitable for high precision trace gas measurements by analyzing methane absorption lines in the respective spectral region.

Dual-Section DFB-QCLs for Multi-Species Trace Gas Analysis

“…The laser tuning rate during the current pulses was determined by inserting a solid 2ʺ Ge etalon (free spectral range: 0.0244 cm −1 ) in the beam path and using the resulting fringe-pattern to convert the time scale to wavelength. Both DFB sections were tunable over a range of more than 2 cm −1 , which is in full agreement with our previous findings using an iCW-driving scheme for standard single-DFB QCLs [15].…”

“…The laser was operated in iCW mode using a custom-built driver described previously [15]. The driver was modified for dual-section DFB-QCL operation.…”

“…A more convenient use of a dual-wavelength QCL, however, is in intermittent continuous wave (iCW) operation, where relatively simple driving electronics provide laser pulses in the order of tens to hundreds of microseconds, which significantly lowers the heat dissipation and inherently allows for sequential laser operation. The thermal chirp during these pulses can then be used as frequency tuning for spectroscopy [15].…”

Spectroscopic real-time monitoring of NO<sub>2</sub> for city scale modelling

“…The QCL is operated at room temperature and emits 3 mW at 1600 cm -1 . It is driven in intermittent continuous-wave (iCW) mode (Fischer et al, 2014) with a pulse duration of 160 μs and a duty cycle of 50 %. Thereby, the heat dissipation of the laser is lowered such that a fan is sufficient to cool the laser housing, and thus no additional cooling liquid circulation is needed.…”

Applying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics

“…From the middle of the last decade QCLAS has been successfully implemented for a variety of plasma diagnostics purposes (see [3] and references therein), although specific obstacles such as pulse-to-pulse fluctuations inherent to pulsed operation had to be overcome. Recently, the so called intermittent operation of cw QCLs, using a rectangular pulse for tuning, has been proposed [73].…”