A telecentric confocal optical arrangement is presented that greatly reduces the diffraction aberrations of the acousto-optic tunable filter (AOTF). Analytical expressions for the aberrations were identified based on the fundamental properties of Bragg diffraction, and additional aberrations due to focusing through the AOTF were also included. The analysis was verified by use of a geometrical ray trace optical code, and an experimental AOTF system was analyzed. Considerable improvement in the potential spatial resolution is predicted with confocal optics, which could accommodate large pixel-limited image fields of greater than 106 pixels. When the image quality of the experimental system was assessed, the resolution was found to be improved by the confocal optics and was diffraction limited. Higher resolution could have been obtained with the use of larger optics to increase the throughput before being limited by the aberrations.

Telecentric confocal optics for aberration correction of acousto-optic tunable filters

White-light imaging was accomplished by operation of a TeO(2) acousto-optic tunable filter (AOTF) with 40 simultaneous overlapping passbands from 400 to 700 nm. The AOTF was chromatically compensated by a wedge applied to the output surface of the AOTF, and the measured spatial resolution correlated well with predictions. Switching off specific rf's applied to the AOTF produced optical rejection corresponding to the inactive passbands. A rejection ratio of 30 dB was demonstrated, and the rejection level was found to be controlled by leakage through the sidelobes of adjacent passbands.

White-light imaging by use of a multiple passband acousto-optic tunable filter.

Acousto-optic devices. Optical elements for spectroscopy.

Acousto-Optic Devices

A Tl3AsSe3 (TAS) acousto-optic tunable filter (AOTF) hyperspectral imaging system is designed and demonstrated that enables real-time imaging of ambient temperature scenes at wavelengths around 10 µm. The predicted sensitivity improvement is a factor of 46 compared to previous TAS AOTF systems, and is accomplished by increasing the spectral passband, improving the AOTF diffraction efficiency, and using a higher sensitivity camera. Optical system designs are also developed and tested for reducing AOTF aberrations and producing pixel-limited resolution.

Long-wave infrared spectral imager with an 8-cm−1 passband acousto-optic tunable filter

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 A, 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.

John Biggins

Papers

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