Spectral characterization of Fabry-Perot etalon operating in Short Wave Infra-Red (SWIR) region

A promising technique for precise spectral characterization of solid Etalon operating in SWIR wavelength is demonstrated using tunable laser with a minimum wavelength scan step interval of 1 picometer. Measurement is carried out for 1640nm - 1660nm, an important region containing greenhouse gas absorption spectra. The etalon considered for characterization is made of fused silica. The thickness is nearly 346 micron with physical diameter of 25.4 mm and clear aperture of 23 mm. The etalon has a finesse of 7.25 and extremely narrow bandwidth like 0.4 nm or 400 picometer. The measurement was as well carried out with an Optical Spectrum Analyzer (OSA). The conventional OSA instrument had a resolution of 80 picometer limiting the measurement capability to detect a spectral shift as fine as 50 picometer. Also the measured bandwidth had an error of ± 40 picometer over a spectral bandwidth of 400 picometer which is nearly ±10 %. This leads to the use of a tunable laser exhibiting a provision to vary the wavelength at a step of 1 picometer. This method provides measurement accuracy at least 4 times better than the earlier method. Accuracy may be further improved by making the wavelength scan step interval even finer. The absorption due to presence of greenhouse gas like methane is higher for the etalons with narrower bandwidth. This method can provide very good accuracy for characterization of etalons with spectral bandwidth as narrow as 50 picometer.