Visualizing the invisible: Diode lasers shine light on tricky detection problems

November 07, 2000

There are solid reasons for wanting to monitor colorless gases such as carbon dioxide and ammonia. Trouble is, a lot of those gases are not merely invisible to the human eye, but located in places a human eye would not want to be -- for instance, a combustion chamber cooking away at a balmy 3,000 degrees Fahrenheit. Fortunately, with the right instruments and methods you can find out all kinds of things without poking your head in the furnace.

At a June workshop on the Stanford campus, Mechanical Engineering Department Chair Ron Hanson and his colleagues traded news with representatives from industry on the burgeoning use of diode-laser­based sensors to measure the properties of otherwise difficult-to-monitor gases in industrial processes. Hanson, the Clarence J. and Patricia R. Woodward Professor of Mechanical Engineering; a few of his graduate students; and senior research engineer Jay Jeffries updated the assembled participants and conducted a tour of the High Temperature Gasdynamics Laboratory, where Hanson's diode-laser sensor group engages in pioneering research on laser-based diagnostics for monitoring combustion, propulsion and industrial processes.

The workshop was sponsored by the Alliance for Innovative Manufacturing at Stanford (AIMS) and organized by Hanson, Jeffries and graduate student Michael Webber. AIMS is a campus-based joint venture initiated by Stanford's Graduate School of Business and School of Engineering and corporate partners to foster the rapid, two-way exchange of technical ideas and techniques between academia and industry.

A gas molecule can absorb light -- but only when the frequency of the light wave matches the difference between two of the specific frequencies at which the molecule's constituent atoms knock into and rotate around one another. Methane, carbon dioxide, ammonia and other industrially significant gases absorb radiation at distinct frequencies in the near-infrared range, conferring on each a characteristic absorption "signature."

Diode lasers -- so named because they use semiconductor diodes as their light source -- can be "tuned" to emit specific frequencies of infrared light. The lasers can be used to figure out which kinds of molecules, and how much of each, are present. A laser beam shines through a collection of gases in a chamber and a sensor on the far side of the chamber detects the frequencies at which some of the emitted light gets absorbed in transit. Applications of this capability are spreading rapidly from basic research labs to industry.

One such application, according to graduate student Jian Wang, is in reducing the build-up of carbon monoxide, a nasty pollutant that, in urban areas, comes almost entirely from cars and other vehicles. Smog checks don't necessarily accurately assess carbon monoxide output in real-world driving conditions, Wang told the group, because "some people mechanically alter their engines after passing their inspection and then cruise around with their cars performing better but polluting more."

The ideal method of detection would monitor the gas in the act of spewing from a moving car's exhaust pipe. Wang's measurements demonstrate that, despite the problems inherent in roadside monitoring of passing cars (for example, changes in temperature or wind direction), carbon monoxide readings reliable to within 1 percent using remotely placed diode-laser sensors are possible.

Another industrial gas amenable to diode-laser detection techniques is ammonia, according to Webber. Ammonia is widely used as a refrigerant, a raw material for fertilizers and, paradoxically, an additive in fuel mixes to reduce the production of other pollutants (but you have to add just the right amount). Webber reported that, using diode-laser sensors, he and his colleagues were able to accurately measure ammonia densities as low as 5 parts per million. With further work, "we could probably get down to one-tenth of that," he added.

It's often important to know a burning mixture's temperature at any given moment -- say, in a jet engine. But combustion may generate temperatures far greater than any ordinary thermometer can handle. Diode-laser sensors make it possible to use a key combustion product, water, as a thermometer, said graduate student Scott Sanders. A water molecule can absorb light at more than one frequency, he explained, but how much it absorbs at each frequency depends on its temperature. Using a diode-laser beam to compare absorption at two different carefully chosen frequencies lets you infer the water's temperature, which approximates that of the entire gas mix. Sanders has demonstrated temperature measurements up to 5,000 degrees Fahrenheit in certain engines.

Still another use of diode-laser sensors is in keeping an incinerator burning both hot and clean simultaneously. That's far from easy when the "fuel" consists of a constantly varying mix of things like hair clippings, chicken and old clothes. Webber recounted how a Stanford team packed an incinerator with sensors that tracked carbon dioxide, carbon monoxide and water concentrations as well as temperature. Aided by their ability to closely monitor combustion in the incinerator, the researchers were able to fine-tune the burn by adding just enough exogenous fuel at just the right time. And, added Jeffries, efficiently operating incinerators don't just work better -- they smell better, too.

Stanford University

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