Ilias G. Manolis

Holographic optical switching: the "ROSES" demonstrator

The design, assembly, and performance of a prototype 1/spl times/8 free-space switch demonstrator using reconfigurable holograms are reported. Central to the switch fabric is a ferroelectric liquid crystal (FLC) on silicon spatial light modulator (SLM) deposited with a 540/spl times/1 array of highly reflective and planar mirror strips. The input and output ports of the switch are fabricated as a linear array of silica planar waveguides connected to single-mode fibers, and the holographic beam-steerer operates without the need for adjustment or dynamic alignment. The waveguide array and the single Fourier transform lens for the 2f holographic replay system are housed in an opto-mechanical mount to provide stability. The switch operates at 1.55 /spl mu/m wavelength and has a designed optical bandwidth of >60 nm. The first measured insertion loss and crosstalk figures are 16.9 dB and -19.1 dB, respectively. Improvements in SLM performance, the use of new addressing schemes and the introduction of better alignment techniques are expected to improve these figures considerably. The preliminary performance of a 3/spl times/3 optical crossconnect is also presented to show that this technology is scalable to N/spl times/N switching fabrics.

Reconfigurable multilevel phase holograms for optical switches

A way of dynamically encoding polarization-insensitive multilevel phase holograms using nematic liquid crystals is presented. The method, using a quarter wave plate and a mirror, allows the optical efficiency of a free space single-mode fiber-to-fiber switch to be greatly enhanced. The validity of the approach has been verified experimentally by observing the power diffracted in the /spl plusmn/1 orders of a nematic liquid crystal grating as a function of input polarization and applied voltage.

Dynamic holography for optical interconnections. II. Routing holograms with predictable location and intensity of each diffraction order

An analysis of dynamic phase-only holograms, described by fractional notation and recorded onto a pixelated spatial light modulator (SLM) in a reconfigurable optical beam-steering switch, is presented. The phase quantization and arrangement of the phase states and the SLM pixelation and dead-space effects are decoupled, expressed analytically, and simulated numerically. The phase analysis with a skip-rotate rule reveals the location and intensity of each diffraction order at the digital replay stage. The optical reconstruction of the holograms recorded onto SLMs with rectangular pixel apertures entails sinc-squared scaling, which further reduces the intensity of each diffraction order. With these two factors taken into account, the highest values of the nonuniform first-order diffraction efficiencies are expected to be 33%, 66%, and 77% for two-, four-, and and eight-level one-dimensional holograms with a 90% linear pixel fill factor. The variation of the first-order diffraction efficiency and the relative replay intensities were verified to within 1 dB by performing the optical reconstruction of binary phase-only holograms recorded onto a ferroelectric liquid crystal on a silicon SLM.

Adaptive beam steering implemented in a ferroelectric liquid-crystal spatial-light-modulator free-space, fiber-optic switch

Active alignment of a 1 x 8 free-space optical switch was studied experimentally. Optical signals, carried on single-mode fibers, were switched by a ferroelectric liquid-crystal-on-silicon spatial light modulator. Continuous measurement of the in-coupled power to the fibers provided feedback for the switch control. The switch automatically located and locked to the output fibers. An advantage with adaptive switches of a similar kind is relaxed geometrical tolerances in the switch assembly. Further, such switches can adapt to possible geometrical changes and light wavelength drift during operation.

Phase-modulating bistable optically addressed spatial light modulators using wide-switching-angle ferroelectric liquid crystal layer

We report the use of an experimental ferroelectric liquid crystal material called CDRR8 in bistable optically addressed spatial light modulators for both amplitude- and phase-modulating devices. First, the methods used to improve the alignment and obtain truly bistable switching with CDRR8 are described. The diffraction efficiency and switching characteristics of a bistable CDRR8 optically addressed spatial light modulator used as an amplitude-modulating device and then as a phase-modulating device for encoding high-resolution patterns are compared. The CDRR8 devices exhibit bistability when driven by alternating monopolar pulses. The results show that the use of a device as a phase-modulating rather than an amplitude-modulating device, increases the diffraction efficiency by 10 times. Resolution is better than 100 lp/mm, as measured by the fall in diffraction efficiency by 50% compared with the diffraction efficiency at low spatial frequencies.

Characterisation of Photoalignment Materials for Photonic Applications at Visible and Infrared Wavelengths

ABSTRACT In this paper we discuss the processing and characterisation of Linearly Polymerisable Polymers to align Liquid Crystal Prepolymers for use in photonic application. Preliminary results on uniform retardation waveplates for use in free space optics are given.

Liquid crystals in telecommunications systems

The first liquid crystal devices have recently been installed in the fibre optic networks that provide the backbone of the modern telecommunications system. Most optical network devices are concerned with the manipulation of the amplitude and phase of the optical signal. Liquid crystals have the highest figure of merit for field addressed electro-optic response and can have excellent transparency in the optical telecommunications window. Here we consider the importance of liquid crystals in controlling the phase and the state of polarisation of light in these systems. We also consider arrays of liquid crystal phase modulators, fabricated using LCOS technology, in holographic switches and multifunction devices.

Control of the electro-optic bistability of some ferroelectric liquid crystals useful for binary phase optical modulators

Abstract We report measurements on the bistability of the electrooptic response of a commercially available material (CS2005) and an experimental liquid crystal material (CDRR8) based on an organosiloxane structure. Both materials have a first order transition to the SmC* phase. CS2005 gave the normal monostable behaviour with layers tilted with respect to the rubbing direction. However, the second material, CDRR8, exhibited true bistability and at the same time its smectic layers were found to lie perpendicularly to the rubbing direction. We were able to control the bistable behaviour of the CDRR8 material by means of layer rotation using asymmetrical triangular pulses. As expected the sample shows a monostable response each time the layers are rotated so that one of the two switched states approaches the rubbing direction.

Integrated optics using smectic and nematic liquid crystals

In this paper two practical technological approaches are presented to demonstrate that liquid crystals can be employed to make low cost integrated optic devices with characteristics required for optical communication systems. In a first approach ferroelectric and nematic liquid crystals are combined with ion-exchanged glass waveguides to produce high performing optical switches. A polarization independent configuration of such switches is also shown. In a second approach nematic liquid crystals are embedded in SiO2/Si V-grooves to produce channel waveguides. Modal and polarization properties of such novel liquid crystal waveguides are experimentally demonstrated.

Computer-controlled, adaptive beam steering, implemented in a FLC-SLM free-space optical switch

We have used a dynamic diffractive spatial light modulator for adaptive beam steering. With adaptive beam steering it is possible to substantially reduce the alignment accuracy needed for the assembly of a free-space optical switch and to make it adaptive to changes in the environment during operation.