Study On Phase Noise Measurement Technology Of Narrow Linewidth Laser

Semiconductor laser is one of the most widely used optoelectronic devices.It plays an important role in many application fields,such as long-distance optical fiber communication,high-precision laser radar,and high-sensitivity optical sensing due to its low noise,narrow linewidth and high coherence.Narrow linewidth and low phase noise of the laser source are required.Therefore,the research on the linewidth and phase noise of the laser source is of critical importance.In this thesis,theoretical analysis and experimental demonstration on the linewidth and phase noise technology of narrowlinewidth semiconductor lasers are carried out.Furthermore,a novel phase noise measurement method based on self-homodyne structure with a Faraday rotating mirror(FRM)and an optical coherent receiver is proposed,by which the Frequency modulation(FM)noise spectrum and linewidth of the lasers can be obtained simultaneously.The main contents of the thesis are as follows:(1)In the theoretical analysis section,the generation mechanism and distribution characteristics of the linewidth,phase noise and FM noise of semiconductor lasers are presented by analyzing the working principle of semiconductor lasers.The influence of various factors on the output laser linewidth and phase noise are analyzed,including the internal and external temperature,the length of the resonator cavity,the spontaneous emission from the active region,and the carrier concentration of the semiconductor lasers.Several traditional linewidth measurement methods are introduced and discussed about the linewidth characterization for semiconductor lasers.Only the linewidth characteristics can be measured by the traditional linewidth measurement technologies.Therefore,the phase noise measurement technology of self-homodyne optical coherent receiver is analyzed in detail.Not only the FM noise spectrum curve and the laser noise distribution characteristics can be obtained by utilizing this method,the linewidth information of the lasers can also be deduced.(2)In the experimental demonstration section,a linewidth measurement system based on traditional Delayed Self-heterodyne Interferometer(DSHI)is constructed.It is found that DSHI is not suitable for measurement of narrow linewidth below 200 kHz and wide-tuning semiconductor lasers according to the research results combining with theoretical analysis and mode simulation.Then,a phase noise measurement system based on the delayed self-homodyne optical coherent receiver is also built.Combined with the offline digital signal processing(DSP)technology,the FM noise spectrum of laser can be obtained,whose FM white noise part can be used to caculate the laser linewidth.The stability of output center frequency can also be observed through low-frequency noise from FM noise spectrum curve.Therefore the performance of the semiconductor lasers can be judged though this technology.The results show that the delayed self-homodyne optical coherent receiver technology can be used for measurement of the phase noise of the semiconductor lasers within linewidth below 10 kHz,and can be applicable to many types of semiconductor lasers.(3)Based on the theoretical analysis and experimental demonstration above,a coherent receiver structure with FRM is proposed in this thesis.Compared with the traditional coherent method,the length of delay fiber required in this proposed sheme is halved.The optical signal transmission is more stable.By comparing with the measurement results of the delayed self-homodyne optical coherent receiver method and the commercial linewidth/noise analyzer,the optical coherent receiver method with FRM can characterize the phase noise and linewidth of semiconductor lasers with linewidth below 10 kHz accurately.