What Is an Optical Switch?

Paul Reed

An optical switch is a device that transfers light signals between different channels in communications networks. Optical fiber networks were developed in the 20th century to carry higher quantities of data than were possible with earlier copper wire systems. Increasing use of the Internet and expanding cellular telephone and television offerings required greater quantities of data to be managed by communications networks.

Woman holding a disc
Woman holding a disc

When a fiber optic network carries a light signal from one phone or computer to another, it may be required to move the signal between different fiber paths. To accomplish this, a switch is required that can transfer the signal with a minimum loss of voice or data quality. When fiber optics were first developed, this was accomplished with an electro-optical switch that changed the light signal to an electrical signal, performed the switch function, and converted the signal back to a light form. This system was acceptable for early fiber optics systems, but problems developed as transmission speeds increased.

Electrical switches have some limitations on switching speed compared to the speed of light used in fiber transmissions. As data requirements grew, the electrical part of the electro-optical switch created limits for how much data could be transmitted. More advanced optical switch technologies were needed, particularly to remove the electrical conversion when switching light signals.

A large improvement came with the development of microelectromechanical systems (MEMS), which use tiny mirrors to transfer light signals. MEMS were an advantage over electro-optical switches because the conversion to and from electrical signals was not needed. The light transmissions were transferred directly between different fibers in a MEMS device, allowing transmission speeds equivalent to the fiber optics limits up to a point.

MEMS devices transfer signals by reflecting the light signals from an incoming fiber cable to a different fiber with tiny movable mirrors. A computer controller determines where the call or data communication is going and which outgoing fiber is needed to complete the connection. Each incoming optical fiber has a mirror next to the end of the fiber that is controlled by a small electrical motor. When the light signal exits the fiber, it reflects off the mirror and into the end of the outgoing fiber that the computer determines is needed. These switches operate very rapidly, allowing a large amount of data to be sent across fiber networks.

Problems with MEMS designs occurred when fiber optics companies continued to expand their transmission systems. As fiber optics cables became larger to accommodate more data, MEMS began to cause signal losses because the mirrors were transferring light signals to many more connections. Signal quality began to degrade as the distances between the fibers became longer. One improvement was to create three-dimensional (3D) MEMS devices, where a series of switches were stacked on each other, permitting each switch to handle less signals using short switching distances.

Another type of optical switch that has no moving parts is a digital switch, using silicon crystals to control the light. In these switches, a solid silicon crystal is placed between pairs of optical fibers. The refractive index, or amount that light is bent as it passes through the crystal, will change if heat is applied. Small heaters are placed in positions along the crystal, and are activated when light signals enter. As the refractive index changes, the light signal can be directed to different output fibers, without the need for mirrors or other moving parts. Signal quality can also be improved over MEMS devices, because mirrors cause small losses not seen with digital switches.

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