25 Feb 2009
Optics.org speaks to the team pioneering supercontinuum cavity ring-down spectroscopy.
In a break with convention, researchers in Germany and Poland have developed a cavity ring-down spectroscopy (CRDS) technique that uses broadband illumination in the form of a white-light supercontinuum. The method is already protected by a patent and the team is now looking for industrial collaborators. (Applied Physics B 94 369)
"The new aspect of our work is the light source and in particular its broad spectral content," Kamil Stelmaszczyk of the Free University of Berlin told optics.org. "The supercontinuum we used spread from 350 to 1100 nm and allowed us to get rid of a time-consuming wavelength scan. By applying the supercontinuum and, on the detection side, a spectrograph with gated iCCD camera we were able to follow the decay of 128 wavelengths at the same time."
Most CRDS systems use a narrow linewidth laser, however the team believes that its broadband approach offers significant advantages. These include a simple cavity design and the ability to monitor several absorbers simultaneously.
In its most recent work, the group says it is the first to apply supercontinuum CRDS to a real-world sample: a cavity filled with NO2 gas. The extrapolated sensitivity of the set-up is said to be around 5 parts per billion. (Optics Express 17 3673)
Stelmaszczyk and colleagues start by firing pulses from a Ti:sapphire laser operating at 800 nm into a quartz block, which gives rise to around 200 filaments. They then take the supercontinuum light generated by these filaments and pass it into an optical cavity comprising two spherical mirrors.
"In the case of NO2, we used a filter so that only the range from 424 to 453 nm passed into the cavity," explained Stelmaszczyk. "The iCCD camera gate was continuously delayed in discrete 0.3 microsecond steps with respect to the laser pulse to gain the time resolution and the typical signal duration was 45 microseconds. This gate is at least several times longer than the cavity decay times. Our measurements take approximately 10 minutes."
Following this successful demonstration using NO2, the next steps are to detect multiple absorbers and miniaturize the light source. "We want to replace our bulky Ti:sapphire and the quartz block with a laser-pumped photonic crystal fibre," said Stelmaszczyk. "We are considering using commercially available supercontinuum sources consisting of an integrated pump and optical fibre as these are practical and turnkey sources."
Jacqueline Hewett is editor of Optics & Laser Europe magazine.