What Is Wavelength Selective Switch–WSS?

Here is how the design works.

This MEMS based switch can switch as many as 128 wavelengths with 50 GHz spacing. The total insertion loss is less than 6 dB. It uses a 100mm focal length mirror and a 1100 lines/mm grating. The micromirrors can be actuated by +/- 8° using a voltage of <115V and the switch can be used as variable attenuator by detuning the tilt angle of the micromirrors.

The optical design we discussed in the previous section is based on MEMS micromirrors. Here we will discuss several more switching engine technologies.

Micromirror array is fabricated in silicon, using wafer scale lithographic processes leveraged from semiconductor industry.

When voltage is applied to electrode, it causes mirror to tilt due to electrostatic attraction. Attenuation is provided by tilting to offset the beam slightly at the output fiber. A angle-to-offset lens converts beam tilt to beam displacement at an input/output fiber array.

The advantage is that offset perpendicular to wavelength dispersion direction gives attenuation with no change in channel shape as shown below.

The tradeoffs are between mechanical resonance frequency (hinge stiffness), driving voltage, and tilt angle often result in high driving voltages.

Liquid crystal cell selectively controls the polarization state of transmitted light by application of a control voltage as shown below.

For the switching process to work, the liquid crystal cell (LC) must be followed by a polarization dependent optical element such as a PBS (Polarization Beam Splitter) to change the path of the transmitted light based on their polarization.

Random polarized input must be separated into two orthogonal polarizations.

In a binary switching configuration, N liquid crystal (LC) cells can select among 2^N output ports. And an extra liquid crystal (LC) cell and polarizer can be used to provide attenuation.

The following two graphics show Liquid Crystal on Silicon (LCoS) technology and the optical design of an wavelength selective switch based on LCoS switching technology.

A LCoS-based switch engine built uses an array of phase controlled pixels to implement beam steering by creating a linear optical phase retardation in the direction of the intended deflection.

LCOS is a display technology which combines Liquid Crystal and semiconductor technologies to create a high resolution, solid-state display engine. In WSS design, the LCoS is used to control the phase of light at each pixel to produce an electrically-programmable grating. This can control the beam deflection in a vertical direction by varying either the pitch or blaze of the grating whilst the width of the channel is determined by the number of pixel columns selected in the horizontal direction.

In the WSS design, it incorporates polarization diversity, control of mode size and a 4-f wavelength optical imaging in the dispersive axis of the LCoS providing integrated switching and optical power control.

In operation, the light passes from a fiber array through the polarization diversity optics which both separates and aligns the orthogonal polarization states to be in the high efficiency s-polarization state of the diffraction grating. The light from the input fiber is reflected from the imaging mirror and then angularly dispersed by the grating, reflecting the light back to the cylindrical mirror which directs each optical frequency (wavelength) to a different portion of the LCoS. The path for each wavelength is then retraced upon reflection from the LCoS, with the beam-steering image applied on the LCoS directing the light to a particular port of the fiber array.

Some switching engine technologies require a minimum beam size to function properly, and thus place a limit on the minimum optical system length for a give channel passband width, channel spacing, and dispersive element.

The advantages of a small optical system size include a smaller overall module footprint, greater functional density, lower packing cost, and greater tolerance to mechanical shock and environmental conditions.

The following figure shows the minimum spot size for various switch engine technologies.