CRDS makes the leap out of the laboratory

21 Sep 2004

Picarro plans to launch a portable CRDS gas sensor in 2005. James Tyrrell talks to the US-Canadian firm about the field testing of its robust, suitcase-sized technology.

From Opto & Laser Europe October 2004

Field-deployable cavity ring-down spectroscopy (CRDS), providing parts-per-billion sensitivity in a unit the size of a suitcase - a far-fetched scenario? Not according to US-Canadian company Picarro, which is busy commercializing the technology (for an explanation of CRDS see below).

Named after the type of sword carried by the Duke of Alba, a 16th-century Spanish maverick with a keen eye for technology and a sense of daring on the battlefield, Picarro plans to bring its cutting-edge technology to the market-place next year.

Formed by leading Stanford scientists in 2002, the company has an R&D base in Sunnyvale, California, and a manufacturing facility in Ottawa, Ontario. Its first product, a cyan laser, went into full production in January.

The company's chief technology officer, Barbara Paldus, is confident that CRDS has the power to take on its well established optical rivals: Fourier transform infrared (FTIR) spectroscopy, non-dispersive infrared (NDIR) spectroscopy and tunable diode-laser adsorption (TDLA) spectroscopy.

"The primary advantage of CRDS is that it measures the absolute optical loss or absorption inside the cavity," explained Paldus. "FTIR, NDIR and TDLA all require calibration because they're a relative measurement of the absorption."

She continued: "Because CRDS uses a coherent, narrow-linewidth light source, you can identify spectral lines that FTIR and NDIR can't resolve. Therefore you can distinguish the fingerprints of different molecular species with much higher precision."

Capitalizing on this, Picarro has built CRDS instruments to monitor the absolute concentration of carbon dioxide in the atmosphere to better than 1 part in 3000 in less than 1min. "That's really important," emphasized Paldus. "Because the way they measure CO₂ right now is a relative measurement, they have to use calibration bottles."

The company feels that such a "calibration-free" device, which begins field trials next summer, could have an impact on a global scale. "It would be a significant step towards being able to practically implement the Kyoto protocol, for environmental monitoring and carbon-cycle research, as well as the credits and debits of emission for each country," said Paldus.

Real-time measurement Working with Dave Bowling and his team at the University of Utah, Picarro is testing a field-deployable CRDS instrument that measures, in real time, the carbon-13:carbon-12 isotope ratio of CO₂ in ambient air with a precision of better than 0.2 parts per thousand.

"The ratio of the stable carbon isotopes in carbon dioxide [13 CO₂:12 CO₂] is an accessible variable that allows scientists to draw inferences about the carbon cycle on local and global scales," Eric Crosson, Picarro's vice-president of R&D, told Opto & Laser Europe. "The specificity of cavity ring-down is very good, so we can separate the isotopes of CO₂."

According to Crosson, most of those measurements are currently being made with mass spectrometry, which is difficult to deploy in the field. Scientists have to use bags to collect the samples, then hike them back to the laboratory for analysis.

The big benefit of Picarro's device is that it is robust enough to be stationed in a monitoring hut. "There has to be [electrical] power in the field," he explained, "although the enclosure doesn't have to be very well environmentally monitored or controlled."

The firm is also field testing its units across a variety of other applications ranging from monitoring ethylene at parts-per-billion levels (for fruit-ripening applications) through to moisture detection in semiconductor fabs (to protect the growth rates of oxides).

In order to service so many applications Picarro's focus is on developing an "optical engine" for analytical-instrument OEMs. "It's basically the heart and the brain of an analytical instrument," explained Paldus. "The optical engine contains the cavity, the optical components such as the laser and the detector, CRDS-specific control and processing electronics, plus a minimum of gas-handling equipment."

This modular design philosophy has helped the firm overcome challenges such as mirror alignment and cleaning. "It's very difficult from a service and training point of view to let the user replace the mirrors," said Paldus. "It's much easier to have a monolithic cavity that you can pop in and pop out." Making the cavity as a single unit also has operational advantages, simplifying beam alignment and vibration isolation.

To boost their chances of success in the field, Picarro has been working hard on transforming what began as a highly sensitive, lab-based, optical-bench set-up into a rugged product. "The team at Sunnyvale has actually produced an instrument that's been flown in a plane and used in trials," Bill Gignac, Picarro's vice-president of operations, told Opto & Laser Europe. "So you're not talking about something that can't be made robust."

However, does the firm have a business model to match the strength of its product? By taking the OEM route Picarro believes it can penetrate applications faster and more effectively. "Our OEM customers have very specific market-domain knowledge and excellent channels into those markets," said Paldus. "They can provide a level of service and technical support to customers that is more on a 24/7 global scale than we as a small company could afford to do."

Picarro says that its CRDS development cycle is about 85% complete. "We've shrunk the size so it fits into a standard 19inch rack mount and we've fabricated prototypes that customers are using in their facilities," explained Gignac.

"We see cavity ring-down coming into its own as a commercial product in volume in the 2006 timeframe," added Paldus. "I'd say the official product release will be in the early part of 2005, leading to full production release sometime late in that year." The pricing of the instruments is likely to range from $20,000 up to $70,000 (€16,500 to €58,000), depending on specific product configurations and performance requirements.

"The other thing we will be launching sometime in the next year is a laboratory instrument for research, and that's very exciting to us," enthused Paldus. Targeting key labs and staff, Picarro wants to put its lab product in the hands of creative researchers focused on developing revolutionary CRDS applications - medical diagnosis, for example. "We believe that there are applications for isotope detection as an enabling tool for medical diagnostics."

At the moment Picarro uses telecommunications DFB lasers which tune over a fairly limited range and limit detection to one or at most two species per laser. However, Paldus reveals that the new research instrument will be more broadly tunable.

Researchers have recently thrown open the laboratory doors to a wider range of CRDS applications. Working with Richard Zare and Kate Bechtel of Stanford University, Picarro has just demonstrated what it claims is world-record sensitivity for the detection of liquids (6.7x10^-8 absorbance units). "The technique will eventually be extended to thin films and solids," said Paldus. "It will be very interesting to see what the high sensitivity of cavity ring-down can bring to those other materials and applications."

What is CRDS? Cavity ring-down spectroscopy (CRDS) uses a high-finesse optical cavity, or resonator, constructed from two or three mirrors (Picarro has a patented triangular design). The first step is to inject light into the cavity, which has gas ports to manage sample handling.

Once enough light has built up in the optical cavity, you extinguish the source and measure the energy decay using a detector placed behind one of the cavity mirrors. The time constant of this exponentially decaying signal is known as the decay constant or ring-down time, and characterizes the sample gas. Its value is inversely proportional to the concentration of molecular species in the cavity.

With a 20cm-long cavity having an effective path length of 20km, CRDS offers a tremendous enhancement in sensitivity over tunable diode laser adsorption (TDLA) spectroscopy. Additionally, the small resonator volume (25ml) of CRDS units compared with TDLA (1l) and FTIR/NDIR (0.2-1.0l) is said to give the technique a shorter response time (also known as "dry-down time").

Mirror reflectivity is a key performance parameter of CRDS - the higher the reflectivity, the longer the decay constant and the greater the instrument sensitivity. However, higher reflectivity makes the decaying signal harder to detect as there is less and less light falling on the photodetector.