Optical system performs white-light Fourier transforms
By combining a specially designed lens system and liquid-crystal phase-shifter array, researchers in Colorado have been able to demonstrate wide-angle white-light beam steering.
In the past, this kind of interference-based system has only been practical for narrow wavelengths; for broader band applications, steering was confined to extremely limited angles because of chromatic dispersion.
The new system compensates for this. Applications of the white-light system include optical processing and adaptive optics, and the technology is already being applied to broadband IR sensing.
Liquid-crystal light modulators have long been investigated for use as beam steering devices. The idea is that the modulator is structured as a set of prisms lying on their sides, each one increasing the optical path by 0 at its closed end and 2 at the open end. When the beams traveling through each of these interfere, the effect is like that of a single large prism deflecting the beam (with the added complication of light lost through zero and high diffractive orders).
For a broadband light beam, there are three problems. First, the optical path difference introduced by the light modulator is inherently wavelength dependent (because 2 is defined by the wavelength). Second, the liquid crystal material itself is chromatically dispersive when used in a prism structure (as a conventional prism would be). Finally, the diffraction grating itself is dispersive because the angle through which the beam is steered is wavelength dependent; for a different angle with the same optical path difference you need a different grating period.
Researchers at Boulder Nonlinear Systems, Inc. (Layfayette, CO) and the Optoelectronic Computing Systems Center at the University of Colorado (Boulder, CO), have found answers to two of these problems through a collaboration with researchers at Rochester Photonics Corporation (Rochester, NY). The first step was to design a less-dispersive liquid crystal modulator, which they did by moving to a chiral smectic material. Unlike many other LCs, it is not the refractive index that changes as the voltage varies. Rather it is the optic axis orientation, or polarization, that is affected, and the process is wavelength independent.
In their experiment (see figure 2), Jay Stockley and his colleagues used a tungsten filament source from which the IR component was filtered, leaving just the visible light.1 After collimation this light is polarized, passes through an achromatic half- wave plate, is reflected by a binary chiral smectic LC-SLM, and then passes back through the achromatic half-wave plate. At this point the main sources of chromatic dispersion are the period of the grating structure in the SLM -- the different colors separate out to form diffraction patterns of different sizes -- and the dispersion from the liquid crystal material itself.
Next, the light is transformed using an achromatic Fourier transform lens system. The multilens system effectively uses the wavelength to change the size of the diffraction pattern: wavelength-dependent magnification. This is chromatic dispersion again, but this time working in the opposite direction as that caused by the grating. As a result, it makes all the colors look the same in terms of spatial frequency. After being Fourier transformed, all colors are all mapped to the same place producing a white-light spot (figure 1).
So far, the wide-angle steering has not been that wide: 0.81 deg. However, this is an order of magnitude larger than other broadband steerers, which can only operate over milliradians. So far the new technology has been limited by the number of electrodes that can fit on a single SLM device, but this is slowly improving. Also, though the initial experiments have been performed using binary gratings, researchers say that the chiral smectic material can be used in analog mode. This will increase the diffraction efficiency of the device. The first application is being developed for the U.S. Air Force as a means of changing the field of view of a broadband IR sensor,2 and researchers hope that the technology will also find its way into adaptive optics and optical processing.
1. J. E. Stockley, S. A. Serati, D. Subacious, K. J. McIntyre and K. F. Walsh, Broadband Phase Modulating System for White-Light Fourier Transformations, Proc. SPIE 3633, 1999.
2. J. E. Stockley, S. A. Serati, G. D. Sharp, P. Wang, K. F. Walsh, and K. M Johnson, Broadband Beam Steering, Proc. SPIE 3131, pp. 111-123, 1997.