What Is Optical Switching?

Optical switching is the technique of redirecting optical signals from one path to another within a network or system without converting those signals to electrical form for routing decisions. By keeping data in the optical domain throughout the switching process, the approach eliminates the power and latency overhead associated with optical-electrical-optical (OEO) conversion at each network node. The field draws from photonics, network engineering, and signal processing, and encompasses both the physical-layer devices that perform the redirection and the network protocols and control planes that determine when and where signals should be routed. Optical switching is fundamental to building the high-capacity transport networks that carry internet backbone traffic, interconnect data center clusters, and support long-haul wavelength-division multiplexed systems.

Wavelength Routing and Circuit Switching

In wavelength-division multiplexed (WDM) networks, individual wavelength channels each carry a separate data stream, and optical circuit switching selects which wavelength is routed to which output port. Reconfigurable optical add-drop multiplexers (ROADMs) and optical cross-connects (OXCs) perform this function at network nodes, using wavelength-selective switches based on liquid crystal on silicon (LCoS), MEMS micro-mirrors, or arrayed waveguide gratings to route individual channels independently. Circuit switching establishes a dedicated optical path for the duration of a traffic demand, offering deterministic latency and guaranteed bandwidth at the cost of leaving capacity unused when the circuit is idle. The scalability of wavelength-routing architectures has been examined in many IEEE publications on optical transport, including discussions of the colorless, directionless, and contentionless (CDC) ROADM architectures that underpin modern flexible-grid metro and core networks.

Packet and Burst Switching

Optical packet switching (OPS) and optical burst switching (OBS) aim to use wavelength capacity more efficiently by sharing it among traffic demands on shorter timescales than circuit switching allows. In OBS, a burst control packet is sent ahead of the data burst over a separate channel, reserving resources along the route just in time for the data to arrive without requiring the burst itself to wait. This just-in-time reservation reduces the buffering requirements that make full optical packet switching technically difficult, since optical random-access memory remains an unsolved engineering problem. Burst switching accommodates the statistical multiplexing gains of packet-oriented traffic while operating on timescales of microseconds to milliseconds, bridging the gap between circuit and true packet granularity.

OFDM-Enabled Sub-Wavelength Switching

Orthogonal frequency-division multiplexing (OFDM) applied to optical channels creates structured multi-carrier super-channels in which individual subcarriers, each occupying as little as a few gigahertz, can be independently addressed and switched. Research published in IEEE Journal of Lightwave Technology on multi-band OFDM transmission demonstrated that optical add-drop operations can be performed on sub-bands as narrow as 8 GHz within a 100 Gbps signal, providing sub-wavelength switching granularity without splitting the super-channel electrically. This approach allows a transport network to allocate just the bandwidth a demand requires rather than a full wavelength, improving spectral efficiency on lightly loaded spans. Optical switches compatible with both wavelength-division multiplexing and mode-division multiplexing have been integrated on silicon photonic chips, as described in a PubMed-indexed study on photonic network-on-chip switch fabrics supporting multiple spatial modes with broad optical bandwidth. Network implementations combining super-channels and sub-wavelength switching are described in analyses of multi-layer transport at science.gov.

Applications

Optical switching has applications in a wide range of fields, including:

• Internet backbone and long-haul networks: dynamic wavelength routing across hundreds of nodes via ROADM-equipped optical transport platforms
• Data center interconnects: optical circuit or burst switches providing high-bandwidth, low-latency connectivity between server racks and storage systems
• Metro and regional networks: flexible-grid switching that reallocates spectrum to match time-varying traffic demands
• Research networks: programmable optical switching fabric in testbeds used to experiment with new network architectures and protocols