A Big Boost for Online Optical Analysis

E-mail Ray Peacock

Until just recently, cavity ring down spectroscopy (CRDS) has been a tantalizing, but ultimately disappointing, prospect. Yes, CRDS enables trace moisture analysis to a parts-per-trillion resolution and creates a whole new class of standard techniques in moisture measurement of gases. But—darnit— it's been limited to specific laser wavelengths.

Thankfully, the federal agency National Institute of Standards and Technology (NIST) and a commercial venture, Tiger Optics, have been researching applications for CRDS and developing new methods of using it. So have other organizations around the world.

How And Why
CRDS is a form of laser absorption spectroscopy, and in fact is also known as cavity ring-down laser absorption spectroscopy (CRLAS). The technique traps a laser pulse in a highly reflective (typically R > 99.9%) detection cavity. "The intensity of the trapped pulse decreases by a fixed percentage during each round trip within the cell due to both absorption by the medium within the cell and reflectivity loss. The intensity of light within the cavity is then determined as an exponential function of time," explains Wikipedia.

Last year's Cavity Ring-Down User Meeting (September 2005) showcased many exciting uses. And at the Sandia Labs site you can find a paper that portrays CRLAS as an important complement to the technique of laser-induced fluorescence (LIF)—because it detects species, such as singlet methylene, that LIF cannot. "A combustion intermediate that cannot be trapped because of its high reactivity, singlet methylene also cannot be measured by molecular beam spectrometry because its ionization potential is so close to its ground triplet state," the paper says.

Radical Breakthrough + Broad Application = Huge Impact
CRDS has now been demonstrated using spectrally broad light sources. In broadband CRDS, aka BBCRDS, the sky's the limit. The frequency limit has been eliminated by the ability of lasers to generate hundreds of thousands of wavelengths simultaneously: New optical frequency control and tuning devices based on "frequency comb" technology have elevated CRDS to a whole new realm.

One industry observer has likened the pairing of CRDS and frequency comb to the effect the telephone had on communications, and the impact of oscilloscopes on AC electrical circuits analysis. The combination of methods represents a radical breakthrough in sensing and measurement at the molecular level. Now near-real time devices can monitor species and concentrations of a wide range of chemicals with unprecedented speed and resolution.

The Joint Institute for Laboratory Astrophysics (JILA, operated by the University of Colorado and NIST) has developed a highly sensitive chemical analyzer that could be used in security screenings for explosive or biochemical devices, or to test a patient's breath for disease. The device also could provide data at environmental monitoring stations or reveal the precise composition of Mars' atmospheric gases, said Jun Ye, lead researcher of the project, which was discussed recently in Science magazine.

"It's like being able to see every single tree of an entire forest," Ye says. "This is something that could have tremendous industrial and commercial value." According to Todd Neff, writer for the University of Colorado's Daily Camera, Jun Ye's research group has brought to the brink of widespread commercial use the technology behind last year's Nobel prize in physics. (That technology is the optical frequency comb technique, invented by John Hall--Ye's doctoral advisor.)

Already In Use
Will this technology advance to commercial status soon? One of the researchers at JILA estimates it will be in serious commercial use within two years. Tiger Optics' equipment is already in widespread use throughout the world and another fairly new company, Los Gatos Research, is already selling a range of research and pre-production instrumentation based on the technology. In addition, frequency comb is yielding other new measurement methods.

An animation of the JILA frequency comb spectroscopy technique, as the combined technology is being called, is available at the NIST site. More detail and links to other information is in the NIST press release.