Spectroscopic Calibration of AOTF for Space Applications

Abstract

This work is aimed at the spectroscopic calibration of an Acousto-Optic Tunable Filter (AOTF) in the Near-Infrared (NIR) wavelength range of 1.0-1.7 μm. AOTF is an electro-optical device that functions as an electronically tunable filter to select the wavelength of light from the broadband input. Based on the principle of 1st-order Bragg diffraction, it acts as a narrow-band tunable filter by scanning RF frequency corresponding to the wavelength. The AOTF used in this work is based on Tellurium-dioxide (TeO2) crystal that functions as a dispersing medium and is driven by an external Radio Frequency (RF) signal. The spectroscopic calibration of dual-beam AOTF is based on the wavelength tuning frequency relation and spectral bandwidth as a function of wavelength through the AOTF transfer function. For transfer function estimation, I scanned the intensity profile of a halogen lamp as a function of RF frequency for a monochromatic input to the AOTF. Theoretically, the AOTF transfer function may be modelled as a sinc² function. However, data processing demonstrated that a sinc² function does not correctly reproduce the AOTF function. So, the bandpass function is modelled as the sum of several sinc² functions. Different sinc2 models (sum of multiple sinc2 functions) where used to construct the AOTF bandpass function from the observed transfer functions at different observation wavelengths. Then an interpolation model was used to draw trends in the fitted parameters in the function of wavelength for each of the sinc2 models so that the transfer function could be constructed for any given central wavelength. After determining the transfer function, it is convolved with Earth’s observed O2 absorption lines to demonstrate the response of the AOTF to the atmospheric oxygen spectra.