3.3. Grating Filters
Resonant grating filter: These novel filters are inspired by the diffractive colours in many insects, and constitute dielectric gratings with three-dimensional, sub-micron microstructure. The zeroth order reflection exhibits a broad to intermediate bandwidth ( ~ 20), is highly polarized and maintains useful efficiency over a ± 30° tilt (or rotation) range. Gale (1998) presents one grating design that produces a roughly self-similar bandpass from 450 to 850 nm over this tilt range. Grating filters, and their close relatives, evanescent gratings, show great promise but most have yet to leave the drawing board. However, since much of the research is driven by bank note security, we anticipate rapid progress. Volume phase holographic gratings - in reflection - can produce a highly efficient grating filter through Bragg diffraction.
Acousto-optic filter (AOTF): These are electronically tunable filters that make use of acousto-optic (either collinear, or more usefully, non-collinear) diffraction in an optically anisotropic medium. AOTFs are formed by bonding piezo-electric transducers such as lithium niobate to an anisotropic birefringent medium. The medium has traditionally been a crystal, but polymers have been developed recently with variable and controllable birefringence. When the transducers are excited to 10-250 MHz (radio) frequencies, the ultrasonic waves vibrate the crystal lattice to form a moving phase pattern that acts as a diffraction grating. The diffraction angle (and therefore wavelength) can be tuned by changing the radio frequency. These devices are often water cooled to assist the thermal dissipation, although this is less important in the UV where AOTFs are particularly useful. The largest devices are 2.5 cm in diameter since it proves to be difficult to maintain a uniform acoustic standing wave over larger areas. An additional problem is the 15 µm structure in the LiNO2 crystal which is not always optimal for good image quality. But the acceptance angle of the AOTF is generally larger than the Fabry-Perot.