Laser Spectral Linewidth Suppression Mechanism And Technique For Coherent Detection

Advanced modulation formats and coherent detection is the mainstream solution forhigh-capacity long-haul optical fiber communication system. The narrow-linewidth、lownoise laser as local oscillator in coherent detection has become one of research hot spots.This dissertation launched the research of the laser linewidth suppression mechanism andtechnique, for the needs of coherent detection. The main contents are as the following:A spectral linewidth suppression device is designed, which is composed of an Ultralong cavity, an active ultra-narrow band filter based on stimulated brillouin scattering and adouble mode selection of combined optical fiber ring and unpumped erbium doped fiber. Theoperation principles of three unit: stimulated brillouin scattering medium, the combinedoptical fiber ring and unpumped erbium doped fiber are analyzed. The effaces of unitparameters to overall performance of the device are discussed.A method based on heterodyne detection for stimulated brillouin scattering gainspectrum measurement is presented. The changing rules of gain spectrum characterization inhighly nonlinear optical fiber with the wavelength, power and polarization of pump areexperimental studied using this method. And on this basis, the structural parameters andworking conditions of the device are determined.The experimental study on the device’s performance is made. Performances such aslongitudinal mode linewidth suppression, spontaneous emission noise suppression,wavelength continuity and stability, relative intensity noise and conversion efficiency aretested and analyzed. Experimental results show that the device achieves measured linewidthsuppression from3MHz to15kHz and improves a significant signal to source spontaneousemission ratio of15dB over30nm range in C-band. The measured relative intensity noise isless than-135dB/Hz. The output power is more than10dBm.A back-to-back coherent optical communication system simulation platform is builtbased VPI software. The effects of local oscillator’s linewidth, relative intensity noise andoutput power to performance of coherent detection are analyzed. Verify the narrow-linewidth,low noise laser is important for coherent optical communication system.