In an optical communication switching system, two input signals may contend for a single output fiber.  Hence, it is imperative to store the information from one input channel for a short period of time while the data in the other is loaded into the output channel.  This memory could be achieved by converting the input optical signal into electrical form, storing it, and retransmitting it, but modern electronics are limited to about 40 Gb/s and must be designed for a specific data rate.  The memory can instead be implemented with a loop of optical fiber around which the signal recirculates.  This storage mechanism is transparent to the bit rate, which can exceed 100 Gb/s.  This optical memory loop (OML) must allow data to be switched in, recirculated with little distortion, and switched out.

There are several sources of loss in any fiber loop: fiber attenuation (~0.2 dB/km at 1550 nm), insertion loss of the switch, and the loss associated with the other elements in the loop, which may include an optical isolator, a bandpass filter, a polarization controller, and a monitoring tap. To compensate for these losses, an optical amplifier such as an erbium-doped fiber amplifier (EDFA), can be used. Unfortunately, the EDFA amplifies noise as well as signal. Thus, the signal power must be kept well above the noise power, so that the signal will take all of the gain provided by the amplifier.

For more information, contact Michael Gross at michael.gross@ece.gatech.edu.