Dennis R. Suhre

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

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 rfs 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.

Spatial resolution of imaging noncollinear acousto-optic tunable filters

The spatial resolution limit of noncollinear acousto-optic tunable filters used for imaging with incoherent light is derived based on phase-matching requirements. It is shown that the noncollinear filter can achieve near diffraction limited imaging. This conclusion was verified experimentally.

Crystal growth, fabrication, and design of mercurous bromide acousto-optic tunable filters

Device-quality single crystals of mercurous bromide were grown by the physical vapor transport method. Crystals transmitted light wavelengths up to 30 µm and did not show any absorption bands. Detailed x-ray Laue and x-ray diffraction studies were used to characterize and orient the crystals. Optical evaluation was performed by fabricating slabs of crystals. A design was developed to fabricate acousto-optic tunable filters with 10-deg off-axis orientation operating in the mid- and long-wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 16-mm optical aperture for the 10-deg design. A theoretical tuning curve for a mercurous bromide crystal-based AOTF using this design was also computed for the first time. Experimentally measured data on frequency matching agreed well with the theoretical predictions, and the transducer thickness was suitable for filtering 7.58 µm with the fabricated AOTF.

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.

Properties of nonlinear optical crystals in the terahertz wavelength region

High-quality crystals of gallium selenide (GaSe) and thallium arsenic selenide (Tl3AsSe3) were successfully grown. The refractive indices were measured in the subterahertz spectral region using time-domain spectroscopy. GaSe has a refractive index of 3.2 and an absorption coefficient of 1 cm–1, along with an absorption peak at 0.6 THz. Tl3AsSe3 clearly shows birefringence, where the refractive indices are 5.0 and 5.4 along the fast and slow axes, respectively. The absorption coefficient is over 3 cm–1 at 0.3 THz, increasing steadily with frequency.

Operational characteristics of a long-wavelength IR multispectral imager based on an acousto-optic tunable filter

We have experimentally demonstrated and report on the results of crystal growth, fabrication, design, development, and performance for the long-wavelength infrared (LWIR) hyperspectral imager based on an acousto-optic tunable filter (AOTF) utilizing an efficient crystal, thallium arsenic selenide (Ti3AsSe3 TAS). Results on the growth of 40-mm-diameter, 15-cm-long crystal boules, to fabricate 4.0-cm-long AOTF devices, and on the system design and performance are presented. To achieve an 8-cm−1-resolution AOTF, we developed a design utilizing growth at 10.6 deg off from the c axis of the crystal and achieved >37% efficiency. A system concept was developed with high efficiency, resolution, and throughput utilizing this TAS AOTF. The test setup consisted of an LWIR camera (microbolometer), the AOTF, and a blackbody radiative source (hot filament), and represents the first time AOTF imaging has been achieved with a microbolometer camera. The filament was placed 25 cm in front of the AOTF, and the camera was aligned to the first-order diffracted beam of the AOTF. The AOTF was tuned to 10.6-µm wavelength by applying a 13.9-MHz rf signal to the transducer. Preliminary experimental results obtained for SF6 gas utilizing this system are reported.

Design and fabrication of mercurous bromide acousto-optic tunable filters

Mercurous bromide crystals with very good optical transparency were grown by the physical vapor transport method. A design was developed to fabricate 10-degree orientation acousto-optic tunable filters operating in the mid and long wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 13-15 mm diameter. A theoretical tuning curve for a mercurous bromide crystal based AOTF using this design was also computed for the first time.

Comparison of acousto-optic tunable filters and acousto-optic dispersive filters for hyperspectral imaging

The acousto-optic tunable filter (AOTF) has a narrow passband and a large angular acceptance angle, which allows for imaging at a given wavelength without having to assemble an image cube, as with grating based imagers. It is also possible to use an acousto-optic dispersive filter (AODF), which has a small acceptance angle and a broad spectral passband to form spatial images similar to the grating imager. Although the processing is more complex, the advantages of the AODF are that pixel registration and temporal fluctuations of the spectra are greatly reduced compared to the AOTF. Both the AOTF and AODF can operate in a birefringent mode, allowing for the use of high efficiency materials such as Tl3AsSe3 (TAS) in the infrared region. They can both be multiplexed to increase the sensitivity, and reduce the spectral fluctuation problem of the AOTF. The AODF can also operate in an isotropic mode, which allows for the use of deflector materials such as Ge. The issues of complexity, fluctuations, efficiency, and multiplexing are compared for AOTFs and AODFs operating in the infrared. A comparison is also made for both systems using TAS, along with AODFs using Ge.

Ternary halides: novel NLO compounds for LWIR

We carried out studies to identify, synthesize, purify and grow crystals of a novel class of halides for nonlinear optical applications. Tl3PbBr5, Tl4PbI6, Tl4HgI6 and Tl3PbI5, were synthesized by reacting binary halides and crystals were grown. Optical quality was evaluated by fabricating cm size crystals. The homogeneity of bulk crystal was evaluated by studying transparency, etchpit and X-ray rocking curve and 2θ-ω scans. These halides have transparency from visible to far-IR wavelength region. The material of the compounds of Tl3PbBr5 composition showed self-poling during the growth.

Noncollinear optical parametric oscillator design for walk-off reduction in GaSe crystals

We grow and fabricate GaSe crystals for nonlinear optical applications. One possible application for this material is an optical parametric oscillator (OPO), which could be pumped with a Nd:YAG laser, but due to the large walk-off, is generally impractical. We present a noncollinear OPO design that significantly reduces the effect of walk-off, which could make an OPO possible with GaSe. The pump, signal, and idler propagation directions are arranged to form nearly collinear rays of power flow. The design minimizes the overlap angle between the rays, and as much as an order of magnitude improvement is obtained compared to the walk-off angle of a collinear OPO. With a 1.06-μm pump, an overlap angle as small as 0.35 deg can be achieved, which is half that of a conventional OPO using ZnGeP 2 .