Broadband Phase Modulating System for White-Light Fourier Transformations

We discuss a white-light processing system that produces a dynamic, achromatic Fourier transformation over the visible spectrum. The system includes an achromatic Fourier transform lens system and a lowdispersion spatial light modulator. A programmable phase mask can only write patterns with a spatial frequency appropriate for one wavelength. However, this problem is resolved by scaling broadband light from a point source to a common spatial frequency using an achromatic Fourier transformer. Then, the programmable phase mask must produce the same phase profile for all wavelengths. Using a chiral smectic liquid crystal (CSLC) spatial light modulator can minimize the wavelength dependence of the phase shifting elements. Phase modulation is accomplished by re-orientation of the optic axis in a plane transverse to the direction of propagation in a manner similar to mechanical rotation of a waveplate. The position of the optic axis is the same for all wavelengths and ideally so is the induced phase shift. We present experimental far field diffraction patterns due to a CSLC spatial light modulator that produces a binary broadband phase mask and an achromatic Fourier transform lens system. An analog modulator is also introduced. Applications for this technology include optical processing, beam steering and adaptive optics.