[2] The incoming light is broken into a spectrum by a diffraction grating (shown at RHS of Figure) and each wavelength channel then focuses on a separate MEMS mirror.
MEMS based WSS typically produce good extinction ratios, but poor open loop performance for setting a given attenuation level.
Liquid crystal switching avoids both the high cost of small volume MEMS fabrication and potentially some of its fixed channel limitations.
Furthermore, each grid must be replicated for each of the switching stages creating the requirement of individually controlling thousands of pixels on different substrates so the advantages of this technology in terms of simplicity are negated as the wavelength resolution becomes finer.
Keeping the optical beam tightly focused over this depth is difficult and has, so far, limited the ability of high port count WSS to achieve very fine (12.5 GHz or less) granularity.
[4] LCoS technology has enabled the introduction of more flexible wavelength grids which help to unlock the full spectral capacity of optical fibers.
LCoS-based WSS also permit dynamic control of channel centre frequency and bandwidth through on-the-fly modification of the pixel arrays via embedded software.
A further array-based switch engine uses an array of individual reflective MEMS mirrors to perform the necessary beam steering (Figure 5[5] (to be uploaded).
It is likely that in future two WSS could use the same optical module utilizing different wavelength processing regions of a single matrix switch such as LCoS,[6][7] provided that the issues associated with device isolation are able to be appropriately addressed.
This provides cost and performance benefits for next generation colorless, directionless, contentionless (CDC) reconfigurable optical add-drop multiplexer (ROADM) networks, resulting from improved scalability of add/drop ports and removal of erbium-doped fiber amplifier (EDFA) arrays (which are required to overcome splitting losses in multicast switches).
[8] The technical maturity of spatial light modulators based on consumer driven applications has been highly advantageous to their adoption in the telecommunications arena.
For example, the design principles developed for LCoS could be applied to other phase-controllable arrays in a straightforward fashion if a suitable phase stroke (greater than 2π at 1550 nm) can be achieved.