Performance characteristics of an electronically tunable acousto-optic filter for fast scanning spectrophotometry

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Abstract
A commercially available CaMoO4 tunable acousto‐optic filter has been evaluated for use as a monochromator and modulator for incorporation into a rapid scanning spectrophotometer for the wavelength range 450–750 nm. The construction of a suitable rf driver is described. The bandwidth of light transmitted by the filter was found to be between 2 and 6 Å, permitting excellent spectral resolution. The contrast ratio was greater than 1400 over the major portion of the tuning range, and the skirt rejection was 35–40 dB for 1‐nm‐wide bands 10–100 nm distant from the center wavelength. The transmission of the filter can be switched in 20 μsec, and modulation frequencies up to 25 kHz are attainable without appreciable loss of transmission. With a conventional light source the output intensity of monochromatic light is comparable to grating monochromators. Stray light can be effectively compensated for by transmission modulation and phase‐sensitive detection of the output. We show that the use of the filter as a monochromator is in many respects superior to the other devices which have been used previously for rapid scanning. The filter may also be used as a light modulator or multiple‐beam spectrophotometer and, by virture of its electronic control and ease of interfacing with digital computers, can be considered to be a programmable light source.

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