Haining Yang

Demonstration of Multi-Casting in a 1 × 9 LCOS Wavelength Selective Switch

A multi-functional 1 × 9 wavelength selective switch based on liquid crystal on silicon (LCOS) spatial light modulator technology and anamorphic optics was tested at a channel spacing of 100 and 200 GHz, including dynamic data measurements on both single beam deflection and multi-casting to two ports. The multi-casting holograms were optimized using a modified Gerchberg-Saxton routine to design the core hologram, followed by a simulated annealing routine to reduce crosstalk at non-switched ports. The effect of clamping the magnitude of phase changes between neighboring pixels during optimization was investigated, with experimental results for multi-casting to two ports resulting in a signal insertion loss of -7.6 dB normalized to single port deflection, a uniformity of ±0.6%, and a worst case crosstalk of -19.4 dB, which can all be improved further by using a better anti-reflection coating on the LCOS SLM coverplate and other measures.

Diffraction based phase compensation method for phase-only liquid crystal on silicon devices in operation

A method to measure the optical response across the surface of a phase-only liquid crystal on silicon device using binary phase gratings is described together with a procedure to compensate its spatial optical phase variation. As a result, the residual power between zero and the minima of the first diffraction order for a binary grating can be reduced by more than 10 dB, from -15.98 dB to -26.29 dB. This phase compensation method is also shown to be useful in nonbinary cases. A reduction in the worst crosstalk by 5.32 dB can be achieved when quantized blazed gratings are used.

Origin of Transient Crosstalk and Its Reduction in Phase-only LCOS Wavelength Selective Switches

The transient crosstalk in a phase-only liquid crystal on silicon (LCOS) based wavelength selective switch using a Fourier transform setup was investigated. Its origin was identified using an in situ test procedure and found to be related to the transient phase patterns displayed by the LCOS device during the switching. Two different methods were proposed to reduce the transient crosstalk without the need to modify the optics or electronics in use. Experimental results show both methods are able to reduce the worst-case transient crosstalk by at least 5 dB.

Reduction of crosstalk in a colourless multicasting LCOS-based wavelength selective switch by the application of wavefront encoding

To optimise the design of a wavelength selective switch based on a phase-only liquid crystal on silicon spatial light modulator and mitigate crosstalk, we propose using a technique referred to as wavefront encoding that involves purposefully building a wavefront error into the optical system. Experimental results taken at 674nm are presented that show wavefront encoding based on defocus can reduce the worst case crosstalk by >10dB compared to a standard Fourier transform set-up. In the case of the WSS we propose using wavefront encoding based on astigmatism.

Application of the fractional Fourier transform to the design of LCOS based optical interconnects and fiber switches

It is shown that reflective liquid crystal on silicon (LCOS) spatial light modulator (SLM) based interconnects or fiber switches that use defocus to reduce crosstalk can be evaluated and optimized using a fractional Fourier transform if certain optical symmetry conditions are met. Theoretically the maximum allowable linear hologram phase error compared to a Fourier switch is increased by a factor of six before the target crosstalk for telecom applications of -40 dB is exceeded. A Gerchberg-Saxton algorithm incorporating a fractional Fourier transform modified for use with a reflective LCOS SLM is used to optimize multi-casting holograms in a prototype telecom switch. Experiments are in close agreement to predicted performance.

Small phase pattern 2D beam steering and a single LCOS design of 40 1 × 12 stacked wavelength selective switches

Two-dimensional beam steering by small, square, phase patterns as small as 50 × 50 pixels on a phase-only liquid crystal on silicon (LCOS) device is experimentally verified as suitable for the application of wavelength selective switches (WSSs), in terms of the diffraction efficiency and steering accuracy. This enables a proposed highly functional and versatile stacked switch architecture, where 40 independent 1 × 12 WSSs can be realised on a single 4k LCOS device. They can be configured to support a 1 × N WSSs with N≤144, or an N × N wavelength crossconnect with N≤12.

Low-cost CDC ROADM architecture based on stacked wavelength selective switches

A highly flexible, stacked, switch module is proposed, wherein multiple independent 1 × N wavelength selective switches (WSSs) can be realized on a single 4k liquid crystal on silicon device. The stacked WSS module can be configured in different ways for application at either the transit side or the add/drop side of a colorless, directionless, and contentionless (CDC) reconfigurable optical add/drop multiplexer (ROADM). TwoROADMarchitectures are proposed based on the stacked WSS modules. Their costs are analyzed for both a 4-deg network node and a larger 8-deg node. The first proposed ROADM architecture with full CDC features is shown to realize a cost reduction of at least 35% in these two test network nodes, when compared with the conventional CDC ROADMarchitecture based on the standalone WSSs and multicasting switches (MCSs). The second ROADM architecture proposed has a small probability of wavelength contention, which could be prevented by a local wavelength assignment algorithm. According to our cost estimation, we are able to aggressively reduce the number of components at the add/drop side and make an overall cost reduction of >70% and >80% in the 4- and 8-deg network nodes, respectively.

Transient crosstalk in LCOS based WSS and a method to suppress the crosstalk levels

The origin of the transient crosstalk (TC) in a phase-only LCOS based WSS using a Fourier transform setup was investigated and identified. Two methods were proposed to reduce the TC by at least 5dB without the need to modify the optics or electronics in use.

The use of wavefront encoding to reduce crosstalk in a multicasting fiber telecom switch

A multicasting fiber optic switch employing a liquid crystal on silicon spatial light modulator is used to demonstrate wavefront encoding, a novel technique for crosstalk mitigation. Experimentally we reduce worst case crosstalk by 7.5dB.