Development Of A Cryogenic Infrared Acousto-Optic Tunable Spectral Filter

AbstractThis paper describes the development of advanced Acousto -Optic Tunable spectral Filter(AOTF) for infrared imaging applications. The device is made of Te02 and operates at 80Kand covers a range from 2 to 5.2 pm. The filter bandpass width is 1% in this region.IntroductionThis paper will describe the development of an advanced acousto -optic tunable spectralfilter (AOTF) for infrared imaging applications. This development was motivated by theneed for active optical devices of this type to enhance the performance of future opticalsystems. Most of the emphasis on smart sensor development has been placed on increasedsophistication of detectors and signal processing electronics. It must be realized thatsome of this sophistication can be circumvented by appropriate preprocessing of the incom-ing signal. Indeed, many applications, such as those that require spectral bandcomparisons or large amounts of spectral data cannot be performed at all without anelectrically tunable spectral filter. The alternative is to sacrifice some othercapability of the system, such as spatial resolution.The development described here has addressed one of the most difficult areas of tunablespectral filter application, that of a low power, cryogenic devices capable of operatingwith advanced mosaic focal planes. In addition to these requirements, most applicationsdemand high in -band transmission, high out -of -band rejection, wide tuning range, largegeometric aperture, wide angular aperture, rapid tuning time and narrow or variablespectral passband width.In this paper we will briefly describe our approach to these issues and show data whichillustrates the performance of a device we have developed. Details of the associatedtechnology efforts will be described elsewhere.Device design and fabricationThe device we have developed utilizes a non - collinear acousto -optical interaction firstdescribed by Yano et al.1 and I. C. Chang.2 Figure 1 shows schematically how the device isconstructed. In this interaction, the incoming broadband light is initially polarized, andlight at the selected spectral wavelength is scattered by the acoustic wave within thecrystal to the orthogonal polarization. Spectral selection is accomplished by changing theacoustic wavelength within the bulk acousto -optic material. An analyzer placed at the exitface of the crystal passes only that spectral component which has been polarization con-verted and reflects the remaining broadband signal out of the system. Detectors placedbehind this analyzer then will sense only that spectral component of the incoming lightwhich has been polarization converted.The key components of the tunable filter subsystem are the acousto -optic crystal, thepolarizer and analyzer, the acoustic transducer, the drive electronics and the mountingstructure.The material chosen for the acousto -optical material is Te02. This material has alarge acousto -optic figure -of- merit, it is transparent from the visible to beyond 5micrometers and is commercially available in high transparency single crystals. Aphotograph of a fabricated crystal is shown in Fig. 2.In the particular configuration shown here. the acoustic group velocity within thecrystal is perpendicular to the center ray of the optical system. This design makesoptimum use of the crystal material, since the acoustic strain fills the entire volume ofthe crystal. The angles at the acoustic faces are uniquely determined by the designcriteria For optimizing bandwidth and angular aperture. The value of the angle is just thecomplement of the walk -off between the acoustic phase velocity and the acoustic groupvelocity. Figure 3 shows the calculated slowness curves of Te02 at 70K and 300K. For thecrystal cut shown in Fig. 2, the shear wave acoustic phase velocity is directed at 18" fromthe (110) direction. Note that for this material, the walkoff is substantial (nearly 60 ")resulting in rather acute angles in the crystal.