| The linewidth, or the coherence length of a lase is an important indicator, which limits its aplication in the fields of optical frequency transmission, optical fiber sensing, precision spectroscopy, laser radar, range finding or remote sensing, etc.Low-noise performance, measuring distance, accuracy and sensitivity of these systems all depend on it. How to precisely measure the linwidth of the applied laser is always a key issue.Currently, self heterodyne detection is the most promising one for the spectral linewidth measurement and laser frequency noise characterization which does not require another ultra-narrow and more stable light source as a reference, and the output spectrum of the laser can be observed directly. Meanwhile,the measurement results directly from its RF frequency spectrum without complex frequency conversion. The use of only a single laser for accurate measurement of the extremely narrow laser linewidths is not only today but also in the future.However, in the existing self-heterodyne detection techniques, the resolution of delayed self-heterodyne interferometer(DSHI) is extremely limited, which could only be applied to laser linewidth of less than 1kHz. Although the conventional loss compensated recirculating way(LC-RDSHI) improved the resolution of the delayed self-heterodyne interferometer. Its measurement accuracy is still constrained and the system adjustment is too hard to be applied in the real case. In order to overcome the deficiencies of the traditional self-heterodyne detection methods, in this paper the delayed self-heterodyne mathematical model is studied to find the causation of the accuracy limitation of the conventional LC-RDSHI, based on the basic principles of self-heterodyne detection. Furthmore, the Parameters-Insensitive LC-RDSHI and the Michelson interferometeric LC-RDSHI are proposed. Compare to the previous studies,the output power spectrum is insensitive to system parameters of the two methods improve the overall accuracy as well as an easier application. The software simulations shows the impact of the variation of system parameters and the delay time to the power spectrum profile of the beat notes. In the meantime, the multi-interferences effects of the relative intensity noise, vibration frequency and fiber birefringence of LC-RDSHI system are also discussed in this thesis.Then an experimental equipment is set up according to the theoretical analysist,to measure the system power spectrum of the three methods. Based on the measured power spectrum, the linwidth measurement accuracies of the three methods are compared. Experimental results show that the conventional LC-RDSHI is constrained in three aspects: firstly, the conventional LC-RDSHI is more complicated to manipulate because of the complex system parameters regulating. Secondly, the conventional LC-RDSHI shows a strong multi-interference, and a low precision.Thirdly, it is sensitive to environment and instruments. Meanwhile, results from the Parameters-Insensitive LC-RDSHI and Michelson interferometric LC-RDSHI show that these two methods enhance the linewidth measurement accuracy since the system parameters variation has a smaller effect on the output power spectrum. However,since the structure of the Parameters-Insensitive LC-RDSHI is the Mach-Zehnder interferometeric, the system polarization state is very unstable which needs timely adjustments and the signal to noise ratio of high-orders of beat notes is quite low. In the case that Faraday mirrors and bi-EDFA are available, the Michelson interferometric LC-RDSHI should be the most efficient self-heterodyne detection technique for characterising the ultra-narrow laser linewidth and its frequency noises. |
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