We develop the technology to drive the next generation of highly-integrated optical wavelength selective switches (WSSs).
Fibre-optic communications data is often sent in wavelength-division multiplexed (WDM) format, to increase the capacity of each fibre. WSSs are used to switch the optical wavelength channels between the fibres in an optical network. A 1xN WSS can send any wavelength channel from the input fibre to any of the N output fibres, as shown in Figure 1. Switching is conducted optically, without the need for power-intensive optical-electrical-optical conversion, and retaining data format transparency.
Figure 1 – WSS functionality.
WSSs are widely used in long-haul telecommunications networks, and are increasingly common in datacentre optical networks. Rapidly growing internet traffic is increasing the demands on these networks, and in turn on the performance of the WSSs. Increased port counts, improved optical performance and reduced cost are key to further network expansion.
Channel switching within a WSS is performed in free-space. This operation is summarised in Figure 2. The input and output ports are an array of fibre optic cables. Light from the input fibre is launched into the system, and collimated by a lenslet matched to the fibre. A de-multiplexing element then spatially separates the wavelength channels, which are relayed onto different regions across the spatial light modulator (SLM). A small angular deflection is imparted by the SLM to each beam, which then propagate back through the system. The beams are then coupled into the output fibres by an array of corresponding lenslets. The angle imparted to each wavelength channel by the SLM determines which output port the light is steered to. The optical layout of a typical WSS is shown in Figure 3.
Figure 2 – WSS operation schematic.
Figure 3 – WSS optical layout.
The latest generation of WSSs use liquid crystal on silicon (LCoS) SLMs, which are compatible with flexible spectrum allocation in the optical network. The LCoS SLMs display pixelated phase-modulation holograms to steer the incident beams. A range of 1×4, 1×9 and 1×20 LCoS-based WSSs are commercially available; the current market leader is a module containing a pair of 1×35 WSSs. The limit to further integration is the size of the LCoS devices, as their development is driven by the display industry.
Conventional LCoS-based WSSs elongate the beam for each wavelength channel perpendicular to the dispersion axis, in order to cover more pixels, to enable steering to more output ports. Our advanced hologram design techniques enable us to steer with a smaller number of pixels. We therefore do not need to elongate the beams, so can steer non-preferentially in two directions. Instead of being restricted to a linear array of output ports, we can therefore steer to a two-dimensional array of fibre ports. Figure 4 illustrates how this increases the number of output ports we can access, while only using a small area of the LCoS device.
Figure 4 - Advantages of 2D beam steering.
This efficient use of the LCoS SLM means that we can share one device between multiple switches, by stacking them vertically on the LCoS plane. Each row corresponds to an independent WSS. The WSSs share the same optical system, LCoS device, control electronics and packaging. This highly-integrated switch module therefore substantially reduces the cost, power consumption and physical footprint of each individual WSS.