Autumn 2013 Optical Connections Magazine

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FINISAR

Spectral manipulation and analysis for advanced optical communication systems

An example of the sort of control over filter shape was demonstrated by Gringeri et al [1] where a programmable filter was configured to simulate a series of concatenated Wavelength Selective Switches (WSS) for an investigation of the robustness of 100G coherent transmission. A second imperfection which needs to be considered is the impact of Polarisation Dependent Loss (PDL) on transceiver performance. Unlike impairments such as Polarisation Mode Dispersion and Chromatic Dispersion, mitigation of PDL in the receiver DSP has proven to be more challenging. It has been observed that in real network environments PDL levels of several dB may occur. Systems must therefore be designed for low PDL and receivers tested for their sensitivity to both intra-channel and broad-band PDL. This testing requires a system which allows the PDL to be accurately controlled in both a broadband and a frequency-dependent manner. Whilst broadband-testing can be managed with a relatively-simple constant PDL device, controllable, frequency-dependent PDL has recently become available as described by Clarke et al [2]. An example of channel-to-channel PDL which can now be generated is shown in Figure 1. The ability to independently control the transmission characteristics of both polarisations can also be used for system emulation including, for example, emulating the effects of Polarization- Dependent Frequency Shift (PDFS) in filter components. At a deeper research level, there is considerable interest in rapidly programmable optical circuits which include multiple individual functions like power splitting/ combining, signal delay, routing, and attenuation. Such capability – often described as an “optical FPGA” – allows researchers to quickly emulate new device functionalities and verify their properties and performance. This capability is a development

By Simon Poole T he required capacity growth of communication networks drives the development of new transmission technologies like coherent transmission, flexible grid, tunable transceivers, and higher order modulation formats. These technologies, in turn, create new test requirements during the development and manufacturing of components, modules and systems. In particular, more sophisticated capabilities are required with regard to filtering and analysing the spectrum of optical signals. A key requirement for any transmission system is robustness to impairments due to system and transmission line imperfections. Whilst coherent transmission, in all its multiple flavours, provides an extremely high level of robustness, one issue which needs to be taken into account is the impact of spectral- narrowing due to the filtering effects of cascaded ROADMs and Wavelength Selective Switches (WSS). There are many different designs of WSS and multiple core switching technologies (e.g. LCoS, MEMS, Liquid Crystal), each of which has its own unique spectral characteristics and hence concatenation effects. To properly test a coherent transceiver therefore requires filters which are not only tunable in terms of bandwidth and centre frequency, but which also have a programmable shape to allow simulation of multiple different WSS types and concatenations.

Figure 1 - Emulation of channel to channel variation of Polarization Dependent Loss across the C-band

from the LCoS (Liquid Crystal on Silicon) technology used in many programmable optical filters and is created by introducing programmable phase delays and power-splitting algorithms between the output ports on a multi-port filter. An example of this is the recent demonstration of novel signal demodulators for Optical OFDM systems by Schroeder et al [3]. The introduction of highly spectrally efficient polarisation- multiplexed coherent transmission, and, more recently, super-channel architectures also requires a new approach to optical spectral analysis. Traditional grating-based Optical Spectrum Analyzers (OSAs) do not provide the resolution required to analyse the broadband, channel-filling signals which these new transmission formats generate and coherent OSAs are now becoming a key part of the research and test armoury. Coherent OSAs have been around for a number of years and provide extremely fine spectral resolution down to the MHz level. In recent developments, a new generation of compact, high- speed coherent OSAs are now becoming available which can sweep at multi-Hz rates with full spectral resolution and full dynamic range. These are designed for simple integration into research and production test systems with a concomitant reduction in test times and increase in throughput.

A final trend which is worth noting is that the ubiquity of computer- controlled testing is driving a change in the way such instruments are designed and build. The traditional stand-alone piece of test equipment with full front- panel control and (usually) some form of integrated display is being replaced by so-called ‘blank panel’ instruments – either as individual instruments or in some form of pluggable chassis. In these, the instrument set-up and data display aremanaged by a remote computer and the instrument display is limited to simple indicators such as power- on and connectivity. This reduction in equipment complexity, size and build cost is further helping to reduce overall testing costs. Simon Poole Optical Instrumentation Group at Finisar Corporation, Sydney, Australia Contributing authors to the article also include: Michael Roelens, Cibby Pulikkaseril, and Ralf Stolte from the Optical Instrumentation Group at Finisar Corporation. References [1] “Real-time 127-Gb/s coherent PM-QPSK transmission over 1000km NDSF with >10 cascaded 50GHz ROADMs”, Gringeri et al, Proc ECOC 2010. [2] “PDL and PMD emulation with control of amplitude and spectral dependence to a sub-channel level across the C-band”, Clarke et al, Proc OFC/NFOEC 2011. [3] “Multi-output-port spectral pulse-shaping for simulating complex interferometric structures”, Schroder et al, Proc. of Conf. on Lasers and Electro-Optics (CLEO), paper CF2l.6, 2012.

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