Tunable laser absorption spectrometer (TLAS) sensors enable gas monitoring with high accuracy and gas specificity, and can be optimized for continuous, maintenance-free operation on long-duration manned spacecraft missions. This innovation is a portable, five-channel TLAS instrument designed to continuously monitor ambient concentrations of carbon monoxide, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, and carbon dioxide, with low-level detection limits below the standard spacecraft maximum allowable concentrations. Monitoring of these particular hazardous compounds allows tracking of ambient conditions and enables detection of fires associated with electrical wiring and electronics packaging materials.

The layout of the five-channel combustion product monitor, showing the laser/detector bank, two-pass absorption cell, and electronics enclosure.
To achieve maximum sensitivity with a small, robust design, the five-channel sensor uses a separate single-mode mid-infrared laser for each gas, with each laser wavelength selected to target strong absorption lines while avoiding interference from other molecules potentially found in manned spacecraft environments. Distributed-feedback laser sources were fabricated for this specific configuration using various semiconductor technologies, including a quantum cascade (QC) intersubband laser based on InP-matched semiconductor quantum well structures, an interband cascade (IC) laser based on type-II band-to-band transitions in GaSb-matched quantum wells, and GaSb-based type-I quantum well diode lasers. A heat-sink assembly accepts the five sets of dedicated lasers and matched detectors, and sits 25 cm from a mirror bank to create the 50-cm two-pass optical path. Dedicated electronics are used to simultaneously control the temperature of each source and detector with integrated thermoelectric coolers, and the ambient temperature and pressure are measured with onboard sensors to correct for absorption line broadening.

The emission wavelength of each laser is swept across target molecular absorption lines of each compound, enabling measurements of both on- and off-resonance absorption and, ultimately, a quantitative measurement of the abundance of the target compounds. By interrogating strong mid-infrared absorption lines, the instrument can detect each gas with part-per-million resolution despite the relatively short absorption path. The open absorption cell design is intended to operate at the same pressure and temperature as the ambient environment, and requires no additional pumps, concentrators, or carrier gases.

This work was done by Ryan M. Briggs, Siamak Forouhar, Clifford F. Frez, Carl E. Borgentun, and Mahmood Bagheri of Caltech; and Randy D. May of Port City Instruments LLC for NASA’s Jet Propulsion Laboratory. NPO-49505