Liquid crystal on silicon (LCOS) devices for phase-only holography with applications in video projection and communications.
Holography based on phase-only liquid crystal on silicon (LCoS) devices opens up a wide range of new display applications, including automotive head-up displays (HUD) and 3D displays. LCoS technology was initially developed for microdisplays for rear projection video displays, to compete with active matrix liquid crystal displays (AMLCD), and picoprojectors.
Unlike LCD projectors which use transmissive technology, where light is modulated as it passes through the liquid crystal cells, LCoS devices are reflective. In an LCoS device the liquid crystal layer is applied directly to the surface of a silicon CMOS chip and is addressed via arrays of several million pixel electrodes formed in the top layer metallization of the chip, each with its own drive electronics. Typically LCoS devices using amplified modulation are used in video projectors.
Though phase-only LCoS holographic devices are more challenging to manufacture they open up exciting new applications such as the potential for dynamic computer-generated holography. Unlike amplitude modulators, phase-only LCoS holographic devices do not absorb or block incident light but instead “redirect” it all into the image. This reduces power consumption and projector size and, more importantly, allows images to be displayed on non-planar surfaces and in 3D.
A hologram may be created in the liquid crystal layer by applying a spatially varying voltage pattern on the pixel electrodes, which when illuminated by laser light, acts to project an image by diffraction. This voltage pattern may be computer-generated in real time and written to the LCoS chip, to produce a video image, creating a high quality display.
The Photonics and Sensors Group within the University of Cambridge’s Department of Engineering had established itself as a leader in the development of LCoS technologies and associated manufacturing processes under the leadership of Emeritus Professor Bill Crossland. CIKC funding for the project PASSBACK enabled state-of-the-art facilities for assembling and testing phase-only LCoS devices to be installed at the University. These include equipment for semi-automated cell assembly and for the examination of backplanes and completed LCoS devices.
The funding enables the group to produce high quality LCoS devices at pre-production scale, with repeatability and reliability that is necessary for prototyping commercial products. The main technical challenges to overcome in the assembly process are to ensure that the backplane and the cover-glass are physically parallel and as flat as possible and then to design equipment to perform phase profiling over the chip surface so that any remaining deviations can be compensated for in software. Currently, the assembly process achieves better than 50 nm cell gap uniformity over 25 mm die.
As a consequence of the equipment investment made possible by the CIKC funding, the group – now under the leadership of Professor Daping Chu – has developed strong expertise in the custom design of prototype devices and LC materials for phase-only holography, and for telecoms applications as well as displays.
‘This has led to several interesting early-stage commercial developments,’ says Chu. ‘In one instance we supplied LCoS devices for an industry partner’s prototype modules for holographic HUDs for the automotive industry, tested by an automotive OEM.’
As a result, a UK partner, technology start-up Two Trees Photonics based in Milton Keynes, has acquired the rights to use the technology in an HUD application for the automotive market.
The group has also fabricated LCoS devices for proof-of-principle prototypes of an advanced optical communications switch and patents have been filed. The unique capabilities of phase-only LCoS to provide software-controllable wavelength separation, beam steering, multicasting and aggregation suggest the technology could play a pivotal role in realising the next generation of optical network components in the telecommunications industry, such as reconfigurable add/drop multiplexers (ROADMs). Transition to higher data rates, up to 40-Gb/s and ultimately 100-Gb/s, will further propel demand for phase-only LCoS. Through its continued research programme the group is investigating several other promising applications for phase-only LCoS. In 2013 a spin-off company ROADMap Systems was founded to commercialize this research. The company raised £500k in its seed round and a further $1.7m in series A investment in 2016 (roadmap-systems-secures-1-7m-in-series-a-investment).