Optical Amplifiers and their Applications in Nonlinear Fiber-Optic Communication Systems

Doktorsavhandling, 1994

The work presented in this dissertation deals with the characterization and modeling of optical amplifiers as well as their applications in nonlinear transmission systems with the main motivation to enhance system capacity. In the first part of the dissertation the work is focused on two different types of amplifiers: the semiconductor laser amplifier (SLA) and the erbium-doped fiber amplifier (EDFA). The noise characteristics of EDFAs are studied in detail, since EDFAs will be employed as in-line amplifiers and will thus determine a great deal of the system performance. The multi-functional performance of an SLA and the trade-offs between different functions of the SLA were investigated, since multi-functionality may facilitate monolithic integration. Utilizing a new and accurate pulsed source technique, the gain compression dependence of the noise figure (NF) of an EDFA was examined. An algorithm to map out all possible EDFA parameter configurations, including the NF, given constraints on the gain, the gain compression, and the output signal was developed. When no solutions existed, an additional degree of freedom in form of a post amplifier loss was introduced. The inclusion of amplified spontaneous emission (ASE) was shown to be crucial to accurately predict the EDFA performance. During the course of investigation a method to extract parameters in the effective ASE banddwidth model from experimental data was devised. In the second part, schemes to compress solitons and to overcome some of the obstacles hampering soliton transmission systems are studied: suppression of instabilites due to too short amplifier distances by means of distributed amplification, and reduction of interaction between adjacent solitons by alternating-amplitude (AA) modulation. Soliton compression by propagation through fiber junctions has been investigated experimentally and theoretically. The compressed solitons contained most of the original energy with minor oscillations in the pulse widths. A numerical investigation, showing the potential to double the system length with AA solitons, was conducted using a 100 Gbit/s, 400 km long AA soliton system with realistic fiber parameters and long pseudo random word. Lumped amplification perturb soliton transmission if the amplifier spacing is smaller than the soliton period, which scales as the pulse width squared. For high bit rate soliton systems the amplifier spacing becomes impractically small. One remedy is to use distributed amplification. Soliton transmission of 10 ps solitons over more than 90 km using distributed-EDFA transmission fiber was demonstrated. Finally, the impact of cross phase modulation induced spectral and temporal distortion in a fiber-based mid- span spectral inverter, intended for chromatic dispersion compensation, has been studied.