Wavelength Selective Switches WSS – Enabling the Growth of Optical Networks

Discover how a WSS can switch selected wavelengths or spectrum slices from an input fiber to desired output fibers.

Optical networks form the backbone for global internet traffic, carrying vast amounts of data over fibre optic cables using light waves. A key technology that has transformed optical networks is the reconfigurable optical add-drop multiplexer (ROADM). ROADMs allow individual wavelengths of light to be dynamically switched between fibres without needing to convert to electrical signals. This enables automatic rerouting of traffic and restoration of failed links while keeping signals entirely in the optical domain.

At the heart of all commercial ROADM systems is the wavelength selective switch (WSS). As the name suggests, a WSS can switch selected wavelengths or spectrum slices from an input fibre to desired output fibres. The recent progress in WSS devices, outlined in this research post, has been a crucial factor in enabling the growth and flexibility of modern optical networks.

Typical optical metro network architecture with WSS/ROADM applied.
Typical optical metro network architecture with WSS/ROADM applied.

How Wavelength Selective Switches Work

Conceptually, a WSS separates the different wavelengths using dispersion optics like a diffraction grating. This spreads the light over a spatial axis according to frequency. A switching engine made of an array of individually controllable elements then redirects each wavelength slice to the desired output. Beam shaping and polarization control optics bookend this main switching section.

The key technology for the switching array in most WSS devices today is liquid crystal on silicon (LCoS). LCoS uses a silicon chip with an array of reflective pixel electrodes under a liquid crystal layer. By applying a voltage pattern, the liquid crystal tilt can be programmed to steer light angles coming from the dispersive optics. LCoS based WSS emerged as superior for handling many ports flexibly.

Early WSS devices supported only fixed wavelength grids, but LCoS enables arbitrarily configurable passbands. This paved the way for flexgrid systems that allocate spectrum in customizable small slices rather than fixed channels. Flexgrid is essential for handling emerging multi-terabit coherent systems using advanced modulation formats and subcarriers.

Share of LCoS zone for twin WSS.
Share of LCoS zone for twin WSS.

Driving Factors for WSS Evolution

Several factors continue pushing WSS technology to evolve. A big one is port count, driven by metro network growth and more interconnected architectures. Early WSS had 9 ports, current generation support 34 ports or more.

Isolation is another critical parameter, measuring unwanted leakage between ports. Different network architectures lead to different isolation needs. Broadcast-select ROADMs, often used when fibre degrees are low, require very high 40dB+ isolation. But route-select designs divide the isolation requirement between paired WSS units.

Spectral range is also expanding. Extended C-band WSS covering 1527-1567nm are common. Products now exist over the full C+L band from 1530-1625nm, doubling the usable spectrum. This helps satisfy the relentless thirst for more fibre capacity.

Wavelength range of C and L band

Finally, customers want smaller and cheaper WSS. This leads to more integration like dual “twin” and quad WSS units in one package. But again, size constraints make it harder to maintain performance. The ways these challenges are addressed show the progression of WSS technology.

Pushing the Limits with Latest Research

As outlined in this post, the state of the art for commercial WSS is impressive but research continues pushing limits. By using ultra-high resolution optics and meticulously calibrating corrective software, the team achieved improved passbands and isolation even for large port counts like 36 ports.

Their novel quad WSS squeezes 4 units into one package. This can pack a ROADM node into a single blade or support networks needing diverse fibre pairs. Despite having 4x units, the quad WSS improved reliability versus 4 separate units thanks to shared components. Performance was also competitive with dual WSS.

Extending the spectral coverage presents challenges too. Maintaining uniform performance across very wide wavelength ranges requires special attention. But the paper shows current devices can achieve similar insertion loss and isolation across both C and L bands.

Future Outlook

WSS technology has clearly come a long way from fixed grids and 9 ports initially to fully flexible multi-terabit spectra and 34 ports today. Supporting the enormous growth in optical networks over the last 20 years would not have been possible without these advances.

Yet it’s clear there is still plenty of room for more progress. The quest for better performance, lower cost, smaller footprints, and greater functionality continues. New network and user needs will always arise to drive new WSS capabilities and innovations.

For example, free-space switching with MEMS arrays could reduce device size. 3D stacking could integrate selector chips with switch arrays. Machine learning might help speed up calibration and corrections.andi tf itmes uch lmaker vere

The flood of data demanding transport will certainly continue growing. New high-baud rate systems like 400G/1T try to cram ever more capacity per wavelength. But these are starved without adequate spectrum to populate multiple channels.

Fortunately, WSS provides a flexible way to utilize fibre capacity. Not being limited to fixed grids or shapes allows systems to optimize channel spacing and placement. Metro networks are now adopting WSS for unlocked agility. There also seems to be untapped potential in using WSS for mux/demux, modulation, and spectral shaping.

It’s an exciting time to see how reconfigurable optical networks keep evolving. And the progress in wavelength selective switch technology will undoubtedly continue playing a pivotal role in that growth story.

Quantum Soul
Quantum Soul

Science evangelist, Art lover

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