Characterization of an Optical Self-Homodyne DPSK Receiver.

摘要

The goal of this project was to create a digital optical demodulator capable of processing information propagating at high data rates in an optical fiber. By processing data at higher rates the information capacity of the channel can be increased. Most demodulators in commercial applications utilize electrical systems. When an optical signal propagating in a fiber optic channel arrives at the receiver, the optical to electrical signal conversion can significantly slow down the processing speed. By optically processing the signal just before the electrical receiver, information can be transmitted at higher data rates. Many methods of building an optical demodulator have been tried using different digital signal processing techniques. In this project a demodulator was designed for differential phase shift keyed (DPSK) data using self-homodyne coherent detection, without using a local oscillator. Local oscillators are often incorporated into coherent receivers to provide a phase reference for incoming data. By replacing the local oscillator with a 3 dB coupler and a delay line, the design of this receiver was simplified. In this configuration the incoming signal uses itself as the reference. Past work on this topic has been mostly theoretical. This project went further by successfully testing a hardware model. Much of the experimental work concentrated on ensuring that constructive and destructive interference occurred in 3 dB couplers. The receiver setup was extremely sensitive to vibration. Experiments had to be performed on a floating table and the receiver had to be enclosed to give proper environmental isolation. In addition to hardware testing of this receiver, computer simulation of the model was performed to determine required component parameters. It was found that the dominant limiting factor of the communication system was the line width of the laser.