A compact, portable, mid-infrared, laser-based instrument that operates at room temperature has been developed for use in detecting trace concentrations of CO or any of several other gases in air. The instrument utilizes infrared absorption spectroscopy in a sample cell which either holds an air sample or is exposed to an airflow. The laser beam that interrogates the cell is formed by difference-frequency generation (DFG) in a bulk nonlinear optical medium excited by two laser beams.
The figure shows the optical layout of the instrument. One of the laser injection sources is a diode-pumped, nonplanar, monolithic ring neodymium: yttrium aluminum garnet (Nd:YAG) laser with 750-mW output power at a wavelength of 1,064 nm. In the terminology of nonlinear optics, it is the 'signal' laser. Another injection source is a 100-mW GaAlAs diode laser that operates at a wavelength of 865 nm, commonly referred to as the "pump." The laser diode is packaged with a thermistor, a thermoelectric cooler, and a monitor photodiode. This choice of lasers was motivated primarily by their capability of reliable single-frequency, low-noise operation over long times in the presence of vibration and of changes in temperature and humidity.
The output of the diode laser is collimated by a multielement lens. Both the diode laser and the lens are mounted on fixtures that ensure precise long-term alignment with minimum sensitivity to vibration. After collimation, the diode-laser beam (pumping beam) passes through a compact 40-dB isolator, then through a half-wave plate, then through an anamorphic prism pair, emerging as a vertically polarized beam that is nearly mode-matched to the signal laser beam for efficient DFG.
The pump and signal beams are combined by a dichroic beam splitter and focused by a lens, at nearly normal incidence, into the nonlinear optical medium, which is an uncoated crystal of periodically poled lithium niobate (PPLN). The crystal is z-cut and contains eight strips with domain grating periods ranging from 22.4 µm to 23.1 µm in 0.1-µm steps. For difference-frequency mixing to produce an idler beam at a wavelength of 4.6 µm, the optimum period was found to be 22.9 µm at room temperature. The idler beam is the one that is used to probe the sample cell.
The idler beam is collimated by an uncoated CaF2 lens, then separated from the pump and signal beams by a germanium filter. The idler beam is then directed into the sample cell, which is a multipass cell aligned for 92 passes between mirrors separated by 20 cm to obtain an effective path length of about 18 m. The multiple passes are used to increase the measured optical absorption by the trace gas of interest, so that the trace-gas-measurement signal can substantially exceed detector noise and optical interference. An off-axis parabolic mirror collects the idler beam at the output of the cell, and focuses it onto an HgCdTe photodetector maintained at a temperature of 65 °C by a three-stage Peltier cooler. In the output of the photodetector, absorption signals take the form of amplitude modulation. The output of the detector is digitized by a data-acquisition card that serves as an interface to a laptop computer. The card is also used to control a small mechanical shutter to block the signal beam for 10 seconds every three minutes for taking "dark" readings.
The spectroscopic information collected by the computer consists of the photodetector output voltage as a function the laser-frequency-control voltage (the signal used to control the current supplied to the diode laser and thereby control the pump wavelength). A time trace of the photodetector output voltage is a function of time averaged over 10 to 1,000 sweeps minus dark voltage. The time trace can be converted to a wavelength trace, once the pump frequency as a function of time is known. Depending on the number of sweeps used to generate the average, the spectrum can be updated at intervals that range from 1 to 20 seconds. At any point in the frequency scan, the concentration of the absorbing gas species of interest can be calculated by applying the Beer-Lambert law.
The entire instrument including power supplies and electronics is contained in an aluminum case that measures 31 by 31 by 65 cm. The instrument draws less than 50 W of electric power and can be operated from car batteries. The instrument has been shown to be capable of detecting CO in air at atmospheric pressure at mole fractions as low as 100 parts per billion, with a precision of 1 part per billion and an accuracy of 0.6 percent at an averaging time of 10 seconds. The use of a diode laser equipped with an external cavity could enable tuning over a wave-number range of 700 cm -1, making it possible to detect several trace gases, including N2O, CO2, SO2, H2CO, and CH4.
This work was done by Konstantin P. Petrov, Robert F. Curl, and Frank K. Tittel of Rice University for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Johnson Space Center
Refer to MSC-22864.