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1. What Is a Wavelength Selective Switch (WSS)?
WSS stands for Wavelength Selective Switch. WSS has become the central heart of modern DWDM reconfigurable Agile Optical Network (AOC).
WSS can dynamically route, block and attenuate all DWDM wavelengths within a network node. The following figure shows WSS’s functionality.
The above figure shows that a WSS consists of a single common optical port and N opposing multi-wavelength ports where each DWDM wavelength input from the common port can be switched (routed) to any one of the N multi-wavelength ports, independent of how all other wavelength channels are routed.
This wavelength switching (routing) process can be dynamically changed through a electronic communication control interface on the WSS. So in essence, WSS switches DWDM channels or wavelengths.
There are also variable attenuation mechanism in WSS for each wavelength. So each wavelength can be independently attenuated for channel power control and equalization.
2. How Does a WSS Work?
There are several WSS switching engine technologies on the market today, here we will demonstrate a MEMS based design. The different switching technologies will be discussed in the next section.
A) 1X2 Configuration
The following figure shows a diffraction grating and MEMS based 1×2 wavelength selective switch.
The light from a fiber is collimated by a lens with focal length f and demultiplexed by diffraction off the grating.
The direction of the beam after the grating will depend on the wavelength λ0 of the beam. The diffracted beams then pass through the lens for a second time, and the spectrally resolved light is focused on the reflective linear MEMS device, which is also referred as 1D (1 dimension) MEMS device. The MEMS device then either changes the amplitude (attenuate) or the direction of the beam.
The reflected light passes through the lens and is wavelength-multiplexed by diffraction off the grating, and finally the lens couples the light back into the fiber. The output light is separated from the input light by a circulator.
B) 1XN Wavelength Selective Switch
The 1XN switch can be considered as a generalization of the 1×2 switch. Because every wavelength in the 1XN switch can be switched to any one of the N output ports, this switch can be used in a fully flexible OADM (Optical Add Drop Multiplexer) with multiple add/drop fiber ports, each of which carries single or multiple wavelengths.
1XN switches can be cascaded to form larger architectures, and NxN wavelength selective matrix can be built by interconnecting back-to-back 1XN switches.
Let’s look at the optical design of the 1XN wavelength selective switch (WSS).
In the 1xN switch design, it uses an additional lens in Fourier transform configuration to perform a space to angle conversion in the first stage of the switch. Also the 1xN switch will require tilt mirrors with N different tilt angles. These are usually implemented as analog mirrors.
Here is how the design works.
- The common input fiber enters the switch at point A where light is collimated by a microlens.
- The following lens image the collimated beam on the diffraction grating at point C.
- The wavelength dispersed beams fall then onto the MEMS device plane D
- On MEMS device plane D, the beams are reflected with certain tilt angle depending on micromirrors’ setting.
- All reflected beams are focused on point B again, where the angle to space conversion section will image the beam on the output fiber. Each output corresponds to a specific tilt angle of the micromirrors.
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.
3. WSS Switching Engine Technologies
The optical design we discussed in the previous section is based on MEMS micromirrors. Here we will discuss several more switching engine technologies.
A. MEMS Switching Engine
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.
B. Liquid Crystal (LC) Switching Engine Principle
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 2N output ports. And an extra liquid crystal (LC) cell and polarizer can be used to provide attenuation.
C. LCoS (Liquid Crystal on Silicon)
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.
4. Switch Engine Technologies and Minimum Achievable Spot Sizes
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.
