Design and development of a versatile acousto-optic based filterscope for fusion plasma diagnosis

A new filterscope concept is proposed leveraging an acousto-optic tunable filter (AOTF) in-lieu of conventional interference filters. The AOTF operates through Bragg diffraction of RF-driven acoustic wave allowing the central wavelength passing through the filter to be varied in microseconds. The adoption of an AOTF allows fast wavelength hopping to study of different emission lines from a single sightline within the same plasma discharge. A prototype is designed and developed based on an AOTF operating in the visible spectrum from 400–650 nm, with wavelength resolution of 1–2 nm across the range. Two optical outputs allow simultaneous measurement of the filtered light through a photodiode and the rejected spectrum with a dedicated spectrometer. A python-based codebase has been developed to program the AOTF operation by specifying the desired complex time series of wavelength interrogation, as well as to control the oscilloscope/digitizer to store the measured signal. Gas light sources are used to characterize the AOTF’s spectral response to acoustic frequencies before proof of concept tests on DIII-D.