Emerging LCOS Applications

The LCOS methodology has made spatial light modulators practical enough that they can be considered for a variety of market applications, which include free-space optical interconnects, holographic data storage, and holographic displays. In 1991, researchers at Displaytech showed that FLCOS SLMs could diffract unpolarized light (O'Callaghan, Opt. Lett. 16), as seen in Figure 20. At left is the diffraction pattern from a 1 × 150 pixel transmissive FLC SLM illuminated with unpolarized white light (top) with all the pixels in the same state, and (bottom) with alternate pixels switched oppositely. With the grating pattern on, the central spot is greatly attenuated as the incident light is diffracted to higher orders (some blue light remains in the central order since the device thickness was tuned for half-wave retardance in the red). Similarly, a holographic pattern like that shown in (b) displayed on a 64 × 64 FLCOS device generates the far-field image (diffraction pattern) shown in (c).

Optical interconnect. These capabilities of LCOS devices can be exploited to make optical crossbar switches. In a result from 1996, Figure 21(a) shows an image of a 256 × 256 FLCOS SLM written with a pattern of 16 beam-steering "patches" (O'Brien, Ferroelectrics vol. 181). With the SLM inserted into an optical system as shown in (b), each patch steers the output of one of a 4 × 4 array of vertical-cavity laser sources to a chosen one of 4 × 4 array of detectors or output channels, thus implementing a 16-channel switch. Replay of the holographic pattern produces the spot array shown in (c). The individual SLM patch patterns have been compensated to correct for slight curvature of the SLM backplane, improving the regularity of the spot positions in the desired upper order at the expense of regularity in the unused lower order.

A complete fiber-optic implementation of a 1 × 8 switch was implemented and characterized in 2000 as part of the "ROSES" collaboration (Crossland, J. Lightwave Technol. vol. 18) using a 1 × 540 FLCOS SLM. Although FLCOS devices offer polarization-independent gratings they require periodic pixel inversion to ensure DC balance. This can be accomplished with minimal perturbation of switch operation using innovative "ripple" updating of a scrolling grating pattern, as shown in Figure 22 (Heggarty, Ferroelectrics vol. 312).

Here, a grating pattern is scrolled from left to right by updating the SLM from top to bottom. With only a small band of SLM pixels in transition at any given instant the SLM diffraction efficiency is constant in time.

Holographic data storage. Another non-display application for LCOS spatial light modulators is holographic data storage. In this application the SLM serves as the "write head," transforming input electronic data to optical images to be recorded in the holographic media. Early Displaytech 256 × 256 devices were used in 1994 in the Tamarck Multistore drive prototype (Redfield, SPIE 2514). As part of the DARPA-funded HDSS consortium, IBM reported in 1998 development of a 1024 × 1024 SLM specifically for HDS application. Figure 23 (Sanford, IBM J. R&D vol. 42) shows their SLM mounted to a circuit board, a close-up of the 15.6 µm pixels written with a checkerboard pattern, and histograms of 0's and 1's recovered from a stored hologram. The SLM performance was sufficient that only 0.8% of the raw data fall in the overlap of the 0/1 distributions.

In 2006 Displaytech released the Mojave product, a 1216 × 1216 SLM intended for HDS applications.

Holographic display. Rather than imaging the pixels on an LCOS device to display a picture, it is possible to create the picture by diffracting light off a computer generated hologram of the desired picture written the LCOS device. This approach has been taken by Light Blue Optics, Ltd. Using a commercial SXGA FLCOS device from Forth Dimension Displays, sequentially illuminated by red, green, and blue lasers, they produced the image shown in Figure 24 (Krueerke, 2006 SID Digest).