DIGITAL PHASE MODULATED FIBER OPTICAL COMMUNICATION SYSTEM

The purpose of this research is to investigate the effect of the optical fiber dispersion characteristics on a subcarrier phase-modulated with minimum-shift keyed modulation format. An optical fiber system hardware including a laser-fiber coupler is constructed to demonstrate experimentally the feasibility of using a MSK modulation format in a long haul fiber optic system.;Two optical fiber systems are built. System No. 1 uses a laser transmitter, 100 meters long of graded index fiber and an optical receiver. The subcarrier frequency is centered at 53.8 MHz with a data rate of 6.95 Mb/s. System No. 2 has a different laser transmitter, 2200 meters long of graded index fiber and a different optical receiver. It has a 10.7 MHz subcarrier frequency and a data rate of 1.34 Mb/s. The system No. 2 is necessary due to the inconclusive result of the system No. 1. System No. 1 has an experimental two-lens laser-fiber coupler.;The work done in this dissertation has resulted in a feasible laser-fiber coupler using lenses, and an intensity-modulated optical system with a phase-modulated subcarrier. The laser-fiber coupler yielded a fairly good result. However, because of its critical alignment problem, this type of coupler is not suitable for field application. The digital phase modulated optical fiber system with a MSK modulation format has proved to be superior than the ON-OFF keyed technique when the subcarrier frequency is below the optical fiber cutoff frequency. Beyond the cutoff frequency of the fiber, the MSK modulation scheme, or the digital phase-modulation format, is not suitable for transmissions over a dispersive optical fiber. Thus in a long haul fiber optic system where the fiber dispersion is cumulative, the angle modulation cannot be used effectively.;The MSK modulation format has the following properties: most of its energy is contained in a narrow spectral width, its phase transition is linear. With these two important properties, MSK signal is less affected by the dispersion of the transmission medium than any other common angle modulation scheme.