Emissions-sensing tool: tunable diode laser
Tunable diode-laser spectroscopy can be used to measure gas concentrations in stacks and ducts, along long paths, or in extractive continuous monitoring applications.
By Harold I. Schiff
Tunable diode laser spectroscopy (TDLAS) for gas measurements—first reported in 1983—has rapidly gained popularity for environmental and industrial applications because of its high sensitivity, rapid response time, and freedom from interferences from particles and other gases.
Like Fourier Transform Infrared (FTIR) spectroscopy, TDLAS operates on the principal of light absorption in the infrared region. However, Tunable Diode Laser Absorption Spectrometry uses a very narrow-band light sources and its modulation to provide the required spectral information. In contrast, FTIR and Differential Optical Absorption Spectroscopy (DOAS) use relatively simple broadband light sources and rely on rather sophisticated spectral fitting and analysis at the detector end of the absorption path.
The detector end of the light path requires no spectrometer or complex analytical software, and the spectral resolution is so high that TDLAS is by far the most interference-free analytical method available today.
Consider: Tunable Laser Absorption Spectroscopy
The heart of a TDLAS system is the tunable diode laser. Tunable diode lasers are small (1 mm2) solid-state devices that emit highly monochromatic laser radiation when an electric current passes through them.
There are two main classes of tunable diode lasers. One uses crystals of Pb alloyed with other elements such as Sn, Te, and Se, which emit monochromatic light in the mid-infrared region between 2 and 20 microns. The other uses crystals of alloys of Ge, As, P, and Sb, which emit in the near infrared region between 0.78 and 2 microns.
By selecting the exact proportion of these elements, diodes can be made to emit at any prescribed wavelength in these spectral regions. It therefore is possible to select a laser to match an absorption line of the measured gas that is virtually guaranteed to be free of interference from any other gas.
Benefits: Tunability/selectivity/response
Another important feature of these devices is that the emitted wavelength also can be tuned over a narrow spectral range by changing either the temperature of the device or the current being passed through it.
This makes it possible to scan across the selected absorption line of the gas as well as the region where the gas does not absorb at all. Scanning the absorption line of the gas molecule provides the information to give an accurate determination of the concentration of the selected gas. In this way the laser frequency can be scanned over a single, selected absorption feature of the gas to be analyzed.
The emission line width of a tunable diode laser's being much smaller than the rotational line width of any absorbing molecules allows measurement of a gas to be made with virtually no interferences from other gases.
Another advantage of the TDLAS method is its fast response time. Measurements can be made in less than a hundredth of a second. Time response, of course, can be exchanged for sensitivity, and, for most applications, time responses of less than one second are readily achievable. The application of high-frequency modulation permits sensitivities in the parts per trillion in the mid-infrared to parts per billion in the near infrared for path lengths of several meters.
Better Pick: Near Infrared
Of the two types of diode lasers, the near-infrared devices are more suitable for environmental and industrial applications for a number of reasons.
Mid-infrared Pb salt diodes operate at temperatures between 20 K and 100 K which requires cryostatic cooling, making the systems rather heavy and expensive. In addition, gases viewed at atmospheric pressure have broadened the spectral lines. For this reason most mid-infrared TDLAS systems sample the gas in a multi-path cell at reduced pressure, which restricts it mostly to extractive sampling applications.
Near-infrared diodes are not as subject to these limitations. They operate at or near ambient temperature. High-quality diodes working at relatively high power are commercially available.
Also, the problem of pressure broadening is not as severe in the near infrared. Remote sensing instruments for ambient air and industrial stack monitoring systems can therefore operate at atmospheric pressure.
Bonus: Near IR Can Carry over Optical Cable
One of the most important features of the near-infrared diode laser is its beams amenability to being transported over long distances of kilometers by fiber optic cables.
The instrument may be placed in any convenient and environmentally friendly location and the measurement performed at the desired location. This also makes the unit readily accessible for servicing.
Also, because the beam is carried to and from the actual measurement location by optical fibers, harsh, hazardous, and explosive environments can be safely monitored with only the fibers exposed to these environments.
Another important benefit of operating in the near IR, is the avaibality of established multiplexing techniques to make simultaneous measurements at a number of remote stations using a single instrument with multiple optical paths.
Multiplexing can also be used to operate several lasers simultaneously with the same instrument, increasing a systems cost effectiveness.
About the author: Harold I. Schiff is with Unisearch Associates Inc., 96 Bradwick Road, Concord ON, Canada, L4K 1K8.
The previous article was adapted from an excerpt of the presentation, "Optical Sensing by Tunable Diode Laser Spectroscopy," as published in the Proceedings of the Air &Waste Management Association's 93rd Annual Conference & Exhibition, which transpired at the Salt Lake Palace in Salt Lake City, June 18 to 22, 2000.