Near-Infrared Spectroscopy - Rob Morris

Near-infrared spectroscopy is a common analytical technique for chemistry and process control, where typical applications include identification of species and determination of water and fat content. New detector and optical bench options make it possible to configure miniature fibre optic near-infrared spectrometer setups for high-resolution applications such as laser and optical fibre characterisation.

Laser Analysis using NIR spectroscopy

Characterisation of laser lines – examples include solid state lasers at 1064 nm and at wavelengths from 1020-1050 nm, as well as semiconductor lasers with response in the 900-1800 nm range – often require a high optical resolution. Ocean Optics has tested the optical resolution performance of the NIRQuest512-2.2 spectrometer. The NIRQuest512-2.2 spectrometer has a Hamamatsu G9206-512W InGaAs-array detector and is responsive from 900-2200 nm. For the test a xenon source used for spectrometer wavelength calibration was measured. The low-pressure gas-discharge source has a number of closely aligned emission lines in the region from ~820-2000 nm. The NIRQuest512-2.2 was configured with a 100 lines/mm grating set to 900-2050 nm, with a 25 μm slit and gold-coated collimating and focusing mirrors for enhanced reflectivity. A 50 μm VIS-NIR optical fibre was used to collect signal from the xenon source. The integration time was set to 350 ms and spectral averaging set to 5.

The spectrum from the xenon calibration source (Figure 1) illustrates that optical resolution of ~4.6 nm (FWHM) is possible with the NIRQuest512-2.2 in its standard configuration (Figure 1). What’s more, even better optical resolution is possible in a NIRQuest512-2.2 configured with a grating that has a narrower spectral bandwidth. For example, a NIRQuest512-2.2 with a 600 l/mm grating set over a 100- nanometer bandwidth and configured with a 25 μm slit would yield optical resolution of <0.5 nm (FWHM). Resolution would improve even more with a 10 μm slit, but at the expense of throughput. For most laser applications, that’s likely to be an acceptable trade-off.