This week’s blog post is provided by featured author, Dr. Simon Poole.
Following on from last months’ blog on alternative uses for LCoS (Liquid Crystal on Silicon) technology, I’d like to return this month to some of the advanced research that’s being done using the LCoS that’s in our WSS and WaveShaper products.
One of the advantages of LCoS over most other approaches to optical switching is the ability to not only switch incoming light between different output ports but also, if properly programmed, to split the incoming light between multiple output ports. We have, for many years, supported a basic power sharing capability(1) in our DWP 100 range of Wavelength Selective Switches, in which optical power can be shared between an express port and an arbitrary drop port for use in drop-and-continue network architectures. This architectural approach can have advantages from both a traffic management perspective and also from an energy-efficiency perspective (2).
The implementation of optical power sharing in a WSS is, by necessity, limited to a very small subset of what is technically possible due to the need for robustness and simplicity of operation. However, this limitation does not apply when considering other potential uses of optical power splitting in R&D applications. Furthermore, it should be possible to implement wavelength-dependent splitting functions while retaining the phase and attenuation control which is present in our WaveShaper range of Programmable Optical Processors. We have therefore been working with the research team at CUDOS, Sydney University to investigate how such functionality might be implemented and some of the potential applications of the technique (3,4).
In general, splitting to different output ports is possible by a generating a superposition of phase patterns on the LCoS. As the splitting can be performed for individual pixel columns of the LCoS-array, it is possible to vary the splitting and phase as a function of wavelength, which enables reconfigurable implementation of complex interferometric structures.
The researchers have demonstrated the capabilities of the technique by creating various complex structures, including a Mach-Zehnder Interferometer (MZI), two interleaved MZIs for the demodulation of differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals, as well as an all-optical implementation of a discrete Fourier Transform (DFT) Filter for demultiplexing optical orthogonal frequency-division multiplexing (OFDM) signals. The results of these are shown in the Figure below.
Figure 1: (a) Insertion loss and phase response of the constructive port of a DPSK Demodulator with an FSR of 43 GHz and an 80 GHz bandwidth; (b) Insertion loss and phase response of the four output ports of a DQPSK demodulator with 40 GHz FSR and 100 GHz bandwidth; (c) Insertion loss and phase response of the three drop ports and one continue port of an all-optical DFT filter with 15GHz channel spacing. In these results, only the phase response of one of the filter channels is shown for clarity.
The results obtained show good agreement with the expected transfer functions of the different devices. In particular, the extinction ratio of the DPSK demodulator is excellent at above 20 dB and the DFT filter shows a sinc response with the maximum in one channel aligning with the nulls of all other channels as expected.
For me, what is particularly exciting about this work is that we have now demonstrated the ability to generate, literally ‘on-the-fly’, multi-port interferometric optical devices with arbitrary transfer functions. This capability should prove a boon to researchers everywhere who need to rapidly prototype demodulators, demultiplexers and other arbitrary interferometric filters.
We will be demonstrating the ability of the WaveShaper to generate these interferometric devices at the ECOC 2012 exhibition in Amsterdam, September 17-19. Feel free to drop by the Finisar booth #500 (you can’t miss it right at the exhibition entrance) any time to see what’s possible!
1 “High performance ‘Drop and Continue’ functionality in a Wavelength Selective Switch”, S Frisken et al, Proc OFC 2006, Paper PDP
2 “Energy-efficiency of Drop-and-Continue Traffic Grooming”, F. Farahmand et al , Proc OFC 2011, Paper OTuR6
3 “LCOS-based WaveShaper technology for optical signal processing and performance monitoring”, J Schroeder et al, Proc OECC, July 2012
4 “Multi-output-port spectral pulse-shaping for simulating complex interferometric structures”, J Schroeder et al, Proc CLEO, June 2012