Cavity-Enhanced Spectroscopy and Sensing by Gianluca Gagliardi & Hans-Peter Loock

Author:Gianluca Gagliardi & Hans-Peter Loock
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

7.2.1 Near-Infrared

To date, the vast majority of NICE-OHMS experimental setups have worked in the near-IR spectral region. Near-infrared optical components are generally less expensive, more readily available, and have better performance than the corresponding components designed for the mid-infrared, in part due to the large amount of research and development invested in the field by the telecommunications industry. Component selection is particularly important when it comes to the optical components that need to work at radio frequencies of ≳100 MHz, namely a high-speed detector and a method of modulating the laser frequency for heterodyne spectroscopy. Dielectric coatings for cavity mirrors also tend to be better-developed and higher performing in the near-infrared than in the mid-infrared, with losses of up to an order of magnitude lower than the best mid-infrared coatings available.

One disadvantage of working in the near-IR is that no fundamental vibrational bands for molecules lie in this region, so NICE-OHMS spectrometers in this region need to observe overtones or combination bands, which tend to be much weaker than fundamental bands. Since NICE-OHMS is such a sensitive technique, these detections are still possible, but it makes the detection limit in terms of quantity of analyte required for detection significantly larger than it would be for a spectrometer of the same sensitivity observing fundamental band transitions.

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