Novel Technique Monitors Carbonyl Sulfide
Researchers at Universität Düsseldorf in Germany have developed a technique that they call cavity leak-out spectroscopy, a variation of cavity ringdown spectroscopy (see “Through the Looking Glass and What Cavity Ringdown Found There,” Photonics Spectra, October 2004, page 76), and have used it in the 5-µm spectral region to detect carbonyl sulfide with a detection limit of 7 parts per trillion. The demonstration is especially interesting because carbonyl sulfide and other diatomic and triatomic molecules with vibrational transitions in the 5-µm region, such as NO, CO and CS2, are of significant biomedical and environmental interest.
Figure 1. The CO laser could oscillate on more than 100 vibrational-rotationaltransitions between 4.75 and 5.5 µm, but the grating acting as the output coupler reflected only one wavelength back into the resonator, forcing the laser to oscillate on that transition alone.
The researchers used a frequency-stabilized CO laser with a liquid-nitrogen-cooled gain tube as their 5-µm source (Figure 1). A piezoelectric-mounted silver mirror served as the laser’s back mirror, and an echelette grating, which selected a single vibrational-rotational line from the CO spectrum, was the output coupler. The output power varied between 50 and 300 mW as the grating tuned the laser over its spectral range between 4.75 and 5.5 µm. The spectral width of the single-line output was typically less than 100 kHz over a period of one second.
The researchers added sidebands of 8 to 18 GHz to the narrow laser output with a CdTe electro-optic modulator, which they drove with about 20 W of RF power. The power in a single sideband was about 10 to 40 µW for 100 mW of incoming laser power. The investigators separated the sidebands from the carrier with a polarizer following the modulator and matched one of the sidebands into the fundamental transverse mode of a ringdown cavity. The ringdown cavity’s mirrors had a reflectivity of 99.995 percent at 4.75 µm, and its frequency was electronically locked to that of the laser.
A signal generator swept the sideband’s frequency back and forth across the ringdown cavity’s resonant frequency at 345 Hz, so that the incoming signal resonated with the cavity twice per cycle; i.e., 690 times per second. Each time it did, a new bunch of photons was injected into the cavity, and the researchers monitored the leak-out of these photons with an InSb photodetector. They calculated the 1/e delay time with a fast exponential fitting algorithm.
Figure 2. The investigators measured leak-out times from the external cavity during an observation period of 160 s. Each point represents the mean value of 100 consecutive measurements acquired in approximately 145 ms.
To determine the detection limit of their system, the scientists measured the leak-out time of the empty ringdown cavity. For an observation period of 160 s, they measured an average time of about 32.1 µs, corresponding to an optical distance of 10 km and a relative standard deviation of the mean of 5 X 10−6 (Figure 2). This deviation corresponds to a noise-equivalent absorption of 7 × 10−11 cm−1 Hz−1/2, which they believe is more than 300 times the best sensitivity previously reported in the 5-μm spectral range.
In a subsequent measurement of carbonyl sulfide in ambient air, the researchers determined that its concentration was 463 ±7 parts per trillion. In a sample of exhaled breath, it was 438 ±9 parts per trillion.
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