Research For Dispersion Measurement Based On Optical Fiber Laser Beat Frequency

Dispersion measurement is an important link in improving the communication quality of optical fiber communication system.As a very important optical parameter of the fiber,the dispersion will cause the optical signal transmitted in the fiber to develop a pulse broadening,causing the error phenomenon,resulting in optical signal transmission distortion,seriously affecting the normal operation of information transmission.The dispersion has become the biggest obstacle to expand the communication capacity and increase the large relay communication distance.Dispersion compensation methods are usually used to compensate for the dispersion that occurs during fiber transmission.However,the premise of dispersion compensation is measurement of fiber dispersion accurately,so as to contribute to the effective management of dispersion in optical communication.This paper puts forward a method of measuring the dispersion of the fiber based on the demodulation of the multi-longitudinal mode.Compared to other traditional dispersion measurement methods,this method uses the beat frequency signal as the demodulation signal and converts optical signals into electrical signals for processing.The operation is simpler and the instruments used to detect are only photo detectors and spectrum analyzers.Affordable,this method is with good prospects for development.First,a number of common methods for measuring fiber dispersion are described in detail,and their advantages and disadvantages are analyzed.This paper reviews the application history of the beat frequency demodulation technology in the field of optical fiber sensing,and analyzes the advantages of the beat frequency demodulation.Then,the development background and unique advantages of fiber laser are introduced.Compared with traditional lasers,fiber lasers are composed of three parts: gain medium,optical resonator and pump source.But the smaller size,the simpler structure,better beam quality,higher pump efficiency,easy to achieve single-mode single-frequency operation and the more affordable cost and so the advantages make fiber laser applications very extensive..In this paper,the structure of fiber lasers,the three-level structure of erbium-doped fiber and the working principle of laser generation are introduced in detail.Then,the concept and the process of the longitudinal mode in the fiber resonator are introduced.The mechanismof the multi-longitudinal mode of the laser is analyzed.Finally,the influencing factors of the stabilized laser beat frequency are summarized.Finally,combined with background knowledge dispersion compensating optical fiber and single mode fiber,an experimental design is proposed to measure the dispersion of optical fiber by using beat frequency based on the fiber laser.The two mirrors of fiber resonator are: A Sagnac ring composed of a optical fiber coupler through one end of the two branches of each other to form a loop;A fiber optic ring mirror consisting of a fiber optic ring and a Fiber Fabry-Perot Tunable Filter.In this paper,the basic principle of dispersion measurement is analyzed in detail,and the optimal frequency of observation is obtained by calculating the accuracy of measurement.Then,the 500 m dispersion compensating fiber and the 3.636 km ordinary single-mode fiber are used as the fiber to be measured.The process of the experiment is introduced in detail.The dispersion coefficients of dispersion compensating fiber and ordinary single mode fiber are obtained by data processing respectively,which is basically consistent with reference value provided by the manufacturers.Finally,the experimental results are analyzed in many aspects.In this paper,the feasibility of measuring the fiber dispersion scheme based on laser beat frequency is proved by the experiment.The measurement method is simple and the cost of the test system is low,which provides some reference value for the measurement of dispersion.