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Low-Cost Tunable Adaptive Optics
Craig Arnold, Assistant Professor, [email protected]
The tunable acoustic gradient index (TAG) lens is capable of producing rapidly tunable
complex optical beams and patterns from a single source beam. The lens uses
piezoelectric transducers to modify the density and refractive index within a fluid to
generate lensing behavior. The type of beam that the lens generates depends on the
electrical driving signal applied to the piezoelectric transducers. We have developed an
experimental prototype and verified its ability to generate basic and complex optical
beams, however work is ongoing in determining the full potential of the TAG lens.
The design and manufacture of the TAG lens is simple and inexpensive. The only
required elements are a piezoelectric transducer, a filling fluid, two glass windows, and a
chassis. The only component with significant associated cost is the single piezoelectric
actuator. Because all the components are macroscopic, no complicated techniques are
required for assembly. When compared to the intricacies and expenses of other adaptive
optics devices such as spatial light modulators and deformable mirrors, the cost and ease
of construction of a TAG lens is a great advantage. In addition, the size of the TAG lens
is easily scalable without a significant impact on cost or performance.
A simple sinusoidal driving signal can be used to generate a tunable multiscale Bessel
beam. Bessel beams are nondiffracting and self-healing. Because of these properties,
they have uses in optical micromanipulation, where they can form two dimensional traps
and transport microscopic particles over long distances. Bessel beams are also used in
laser materials processing to process uneven surfaces and to create high aspect ratio
structures within bulk transparent media. In addition, Bessel beams’ properties make
them attractive for use in free-space long-range communications. The TAG-generated
multiscale Bessel beam can be tuned by altering the amplitude and frequency of the
electrical driving signal.
By using a more complicated, non-sinusoidal driving signal, it is possible to make the
TAG lens into a variable focal-length converging or diverging lens. The focal length can
be tuned by simply varying the amplitude of the driving signal. This has applications in
many fields including machine and weapon vision where it is an advantage to have a
single compact and versatile optical element that can be electrically adjusted without any
moving parts. Similarly, a variable focal-length device has applications in microscopy.
By using still more complicated driving signals, it is possible to form arbitrary patterns.
The driving signals used to produce a desired pattern can be computed from knowledge
of the TAG lens parameters. This allows a TAG lens to be used in any application
requiring adaptive optics, such as projection displays, optical micromanipulation, atom
optics, astronomy, or maskless lithography. Currently, the most commonly used
competitive adaptive optic is the spatial light modulator (SLM). The SLM is a pixellated
liquid-crystal array, where each pixel can introduce a specific optical phase delay. TAG
lenses have advantages over SLMs in that they can change patterns at a faster rate, have
no pixellation effects, have a higher incident power laser damage threshold, are lowercost, contain easily replaceable optical elements, are scalable to both larger and much
smaller devices, and have better performance with large bandwidth (white) light.