Notch filtering using a multiple passband AOTF in the SWIR region.

A notch filtering operation was accomplished using a TeO2 acousto-optic tunable filter (AOTF) with 16 simultaneous overlapping passbands in the shortwave infrared wavelength region. By switching off specific radio frequency signals applied to the AOTF, laser wavelengths corresponding to the inactive passbands are rejected, providing see-through capability with the remaining wavelengths. The rejection level was determined by leakage through the sidelobes of adjacent passbands, as was shown by theory and corresponding measurements. By switching off multiple passbands near the laser wavelength, the rejection level can be increased at the expense of reduced see-through capability. The AOTF imaging system used telecentric confocal optics that compensate for AOTF aberrations, which are severe at high sidelobe operation.

Acousto-optic tunable filter imaging spectrometer with full Stokes polarimetric capability.

Polarization is an important addition to spectral imaging in detecting and identifying objects of interest, and simple linear polarization measurements are often inadequate. Full polarization analysis can give additional information for discrimination where the polarization state is completely described by the Stokes parameters. An acousto-optic tunable filter (AOTF) imaging system was built incorporating two liquid-crystal variable retarders (LCVRs) that can provide complete spectral-polarimetric analysis, and it is believed to be the first demonstration of a full Stokes polarimetric AOTF spectral imaging system with no moving parts. It is also shown that a single LCVR cannot provide all the Stokes parameters.

Acousto-optic tunable filter sidelobe analysis and reduction with telecentric confocal optics.

The acousto-optic tunable filter (AOTF) has optical sidelobes that are due to the acoustic field produced by the transducer. These sidelobes were analyzed by wave-vector phase matching between the optical and acoustic fields, which correlated with measurements made with a TeO2 AOTF. A white-light point source was filtered and imaged, showing reasonably large and slowly decreasing sidelobes covering a large spectral range. This effect reduces the image quality of an AOTF system by producing faint secondary images of bright objects. The image quality can be improved with a telecentric confocal optical arrangement in which the angular shift of the sidelobes is greatly reduced, producing a much sharper image. This effect was also demonstrated experimentally with the point source.

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

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 10(6) 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.