Tunable Diode Laser Absorption Spectroscopy (TDLAS) Theory

Tunable Diode Laser Absorption Spectroscopy instruments rely on well-known spectroscopic principles and sensitive detection techniques, coupled with advanced diode lasers and optical fibers developed by the telecommunications industry. The principles are straightforward: Gas molecules absorb energy at specific wavelengths in the electromagnetic spectrum. At wavelengths slightly different than these absorption lines, there is essentially no absorption. By (1) transmitting a beam of light through a gas mixture sample containing a (usually trace) quantity of the target gas, and (2) tuning the beam's wavelength to one of the target gas's absorption lines, and (3) accurately measuring the absorption of that beam, one can deduce the concentration of target gas molecules integrated over the beam's path length. This measurement is usually expressed in units of ppm-m.

In the robust near-infrared (NIR) TDLAS instruments built by PSI, a distributed feedback (DFB) diode laser serves as a spectrally bright light source having a well-defined but adjustable wavelength. The structure of a DFB laser includes a grating-like optical element that forces the laser to resonate in a single electromagnetic mode. The laser emits near-infrared radiation (1.2 - 2.5 µm, or 4000 - 8500 cm^-1) with a linewidth less than 0.003 cm^-1, which is considerably narrower than molecular absorption linewidths (typically 0.1 cm^-1 at atmospheric pressure). By accurately controlling the laser temperature and the electrical current that powers the laser, the laser wavelength may be tuned precisely to a specific molecular absorption line that can be selected to be free of interfering absorption from other molecules. Typically, each TDL system is built using a DFB laser having a specific design wavelength chosen to optimize the sensitivity to a particular target gas.

In most TDLAS instruments, the laser's fast tuning capability is exploited to rapidly and repeatedly scan the wavelength across the selected gas absorption line. While this scanning occurs, the fraction of emitted laser power that is transmitted through the gas mixture is monitored with a photodetector. When the wavelength is tuned to be off of the absorption line, the transmitted power is higher than when it is on the line. Measurement of the relative amplitudes of off-line to on-line transmission yields a precise and highly sensitive measure of the target gas concentration along the path transited by the laser beam. Furthermore, techniques known as frequency or wavelength modulation spectroscopy (WMS) and Balanced Ratiometric Detection (BRD) are frequently employed in TDLAS instruments to make them exquisitely sensitive to even very weak absorption of the laser power. Thus, TDLAS instruments offer a combination of high sensitivity to trace concentrations of many gases, freedom from cross-species and external interference, and fast response. Examples of gases that can be sensed with TDLAS, and typical minimum detectable path-integrated concentrations, are listed in Table 1.

In many current implementations of TDLAS, the diode laser beam is coupled to optical fibers. This enables separating the laser, associated electronics, and microprocessor from the measurement head. This capability has led to the evolution of a TDLAS system architecture, utilized by many of today's practitioners for monitoring and controlling various chemical, combustion, or manufacturing processes, that has made the technology attractive for use in harsh, hazardous, or difficult to access industrial environments. The instrument is separated into two distinct yet interconnected components called the System Console and the Measurement Path, illustrated by Figure 1. The laser beam originates within the System Console, which is usually installed in a general-purpose environment. An optical fiber conducts the beam to the Measurement Path, which is built to withstand the harsh installation environment. At the beginning of the Measurement Path, the laser beam exits the fiber and is transmitted into the gas sample. At the opposite end of the Measurement Path, the laser beam impinges upon the photodetector which converts the beam, and the information it carries, into an electrical signal that is returned to the System Console via electrical cable. At the console, the signal is processed and the path-integrated target gas concentration is reported. Noise reduction in sensitive BRD and WMS detection techniques is accomplished by use of an optical or electronic reference signal that connects directly from the Laser Transmitter module to the Signal Processor module.

Figure 1. TDLAS gas detector system.
 

Related References

Cooper, D.E. and Martinelli, R.U., "Near-infrared diode lasers monitor molecular species," Laser Focus World, November 1992.

Bomse, D.S., "Diode Lasers: Finding Trace Gases in the Lab and the Plant," Photonics Spectra, 29(6), 1995.

Bjorklund, G.C., "Frequency modulation spectroscopy: a new method for measuring weak absorptions and dispersions", Opt. Lett. , 5, 1980.

Bomse, D.S., Stanton, A.C., and Silver, J.A., "Frequency Modulation and Wavelength Modulation Spectroscopies: Comparison of Experimental Methods Using a Lead-Salt Diode Laser", Applied Optics, 31, 1992.

Schiff, H.I., Bechara, J., Pisano, J.T. and Mackay, G.I., "Measurements of CF₄ and C₂F₆ in the emissions from aluminum smelters by Tunable Diode Laser Absorption Spectroscopy", in Optical Sensing for Environmental Monitoring, Air and Waste Management Association Volume SP-89 (1994).