RECIRCULATING LOOP

Long-Haul Test Bed


The main tool that we use to test transmission of data over very long distances is a loop as shown above. Data is encoded on an optical carrier at the transmitter and is introduced to the loop when the Load Switch is closed. While this switch is closed the Loop Switch is open. Once the loop is filled with optical data the switches state is changed as shown in the figure and data can circulate in the loop. Each time the optical signal passes the coupler a fraction (usually 50%) of the light is coupled out into the receiver arm for detection and measurement. The optical amplifiers in the loop compensate for the loss in the optical fiber and the coupler. As the data propagates through successive round trips it accumulates noise, dispersion distortion, and nonlinear distortion as it would through the same length of fiber (with optical amplifiers) in an installed system. In this way one can simulate transmission over thousands of kilometers of optical fiber.