The Research Of Linewidth Measurement System Of Narrow Linewidth Laser

With the rapid development of optical fiber communications, narrow linewidth semiconductor laser has increasingly been extensive applied. For the narrow linewidth laser light, the line width(FWHM) is a very important index. In certain fields which need high measuring accuracy, such as optical fiber interference speed measurement, optical fiber temperature measurement and long-distance coherent detection, the requirement for semiconductor laser linewidth becomes stricter. What’s more, the peculiarity of laser linewidth plays a decisive role for the performance of the whole system. Therefore, it is of great significance to study a high-precision method of measuring the linewidth of narrow linewidth semiconductor laser in terms of the optical fiber communication system and the optical measurement.At present, the measuring methods of the linewidth of narrow linewidth semiconductor laser mainly include nonequilibrium optical fiber interferometer measurement method, the beat frequency method, delayed self-homodyne method, delayed self-heterodyne method and loop gain compensation delayed self-heterodyne method, etc. Among those approaches, delayed self-heterodyne method is the most commonly used way. However, this method has some deficiencies, such as its low accuracy and the overlong delayed fiber length. To solve these problems, this paper presents an improved delayed self-heterodyne method. Focusing on this method, the author had carried out a series of theoretical and experimental studies. The research contents and main work of this paper are’as follows:1) We investigated the technical background of linewidth measurement, presented the overseas and Domestic Research Status, and deduced the formula of the delayed self-heterodyne method. Also, we had a research on several factors affecting the laser linewidth. Besides, we analyzed and compared the principles, advantages and disadvantages of the five measurement methods of the linewidth.2) We studied the principle of delayed self-heterodyne method and made theoretical analysis. Then the theoretical analysis was verified based on the simulation platform. What’s more, The basic principle of short fiber delayed self-heterodyne method was analyzed and the mathematical simulation was carried out. The theoretical analysis and simulation results show that it at least demands120km delayed optical fiber to get relatively accurate measured value when measuring10kHz linewidth. Besides, we got the fitting formula of the linewidth when the delayed optical fiber was25km. It suggests that the two approaches above both have the deficiencies of low accuracy and overlong delayed fiber length.3) To deal with these problems mentioned above, we proposed a modified delayed self-heterodyne method. This new method adds the usage of Faraday Rotation Mirror, through which we can increase the stability of polarization of light, and reduce the length of fiber, so that the precision is enhanced. Meanwhile, based on the new method we built an experimental system, and tested the feasibility of that method. Finally it helped us measure the laser linewidth which is about140kHz. Compared with the traditional methods, this approach can be applied to complete excellent measure work aimed at the linewidth with kHz. It not only enhances the accuracy, but also shortens the length of the used fiber which reduce about50%compared with the existing technology.4) We analysis the results of linewidth measurement and point out the advantage of the delayed self heterodyne method compared with the traditional measurement method.According to the error occurring in the experiment, we investigated the factors that affect experimental results, such as fiber dispersion, photoelectric detector, and laser frequency deviation.