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Chirpy laser detects leaky aerosols at high speed

An innovative infra-red laser system can identify leaky aerosols, even on a high-speed filling line, a gas analysis specialist has claimed.

The CT2210 leak detection system developed by Cascade Technologies uses an innovative laser plus mid-infrared optical absorption spectroscopy to spot leaking gas as the cans pass through an archway. If the can under analysis exceeds a set threshold, a signal is sent to a reject mechanism to remove it from the belt.

“The system allows rapid measurement of propellant gases and is able to continuously scan in excess of 500 cans per minute. It is easy to install and highly accurate, with very low false positive and false negative rates,” said Richard Cooper, Cascade's commercial director. He said that the system recently successfully completed a year-long trial, and would allow manufacturers to dispense with the water baths currently used for leak detection. 

Its speed is in part due to the use of quantum cascade lasers, which can emit a short but high-powered 'chirp' across a range of wavelengths, allowing different gases to be detected by their different absorption characteristics, Cooper said. He added that Cascade has also used quantum lasers for the real-time detection, measurement and monitoring of gases in industrial processes, industrial emissions and IED/explosives.

“The devices we use are monochromatic but we use them with a long pulse technique,” he explained. “When sending a current through the laser over a few hundreds of nanoseconds, the semiconductor layers composing the laser gradually heat up, changing their physical structure and causing the light to shift to adjacent wavelengths. This allows the laser to chirp over a few wavenumbers during a pulse.

“The laser line-width remains narrow, hence very specific and spectrally pure, and allows for high resolution spectroscopy to be achieved at the wavelength of interest. By using this technique, Cascade can reach ppb to ppt [parts per billion/trillion] levels of detection and remain unaffected by cross interfering gases. It can also allow for multiple gas measurements with one laser, should absorption lines of different gases be present during one pulse.”

He added: “Near-infrared lasers have been applied to gas sensing for a number of years, but it is the first time that mid-IR devices are being used in gas analysis. Measuring in the mid-IR has a significant advantage over near-IR, because most of the gases of interest leave their primary absorption lines in the mid-IR. What can be seen in the near-IR are weaker ripples that are harder to detect and subject to stronger cross-interferences.”

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