Research On Systems Of Monitoring Parameters In Optical Fiber Based On Analysis Methods In Frequency Domain

With the rapid development of fiber optic communication systems,fiber optic communication networks have become more complex and large.Under such circumstances,the management of fiber optic communication links is crucial to ensure the normal operation of data services and the emergency response capability for unexpected events in the links.Therefore,in this paper,the coherent optical time domain reflectormeter(C-OTDR)technology in single-mode fiber scenario and the differential modal group delay(DMGD)characterization technology based on frequency-modulated continuous wave(FMCW)in few-mode fiber scenario are studied with the parameter monitoring technology in link management as the starting point.Around the above mentioned techniques,the research content of this paper and its results are shown as follows:(1)The theoretical models and parameters of the existing C-OTDR technique and DMGD characterization technique based on FMCW are analyzed,and the theoretical models of the dual modulator cascaded C-OTDR system based on short time fourier transform(STFT)noise reduction algorithm and the spectrum recovery algorithm(SRA)-based DMGD characterization system using externally modulated FMCW are designed.Based on the theoretical models,the effects of parameters such as signal pulse width and mode coupling on the system performance are simulated for different systems,respectively.(2)A C-OTDR system with dual modulator cascade and STFT-based noise reduction processing algorithm are designed.The designed scheme enhances the transmit signal extinction ratio by cascading a Mach-Zehnder modulator(MZM)with an acoustic-optical modulator,and separates part of the phase noise in the received signal.Finally,the STFT noise reduction algorithm is combined with to enhance the system dynamic range and accelerate the speed of signal processing.Experimental measurements were performed on commercial standard single-mode fibers with a total length of about 200 km,and the system and algorithm can improve the signal-to-noise ratio by 5.3 d B and 3.7 d B,respectively,compared with a conventional single-modulator C-OTDR system and a fixed band-pass filtering averaging noise reduction algorithm.The solution achieves parameter monitoring for 150km of standard single-mode fiber in only 19s,and it can achieve 30.47d B one-way dynamic range,1km spatial resolution,and detection of Fresnel reflection events at 202km.(3)A scheme of DMGD characterization based on externally modulated FMCW and the SRA under this system is designed,which avoids the sweeping nonlinearity of the sweeping laser by using the MZM for the external modulation of FMCW,and improves the DMGD measurement precision and simplifies the measurement system by combining the SRA to recover part of the spectral details of the periodic signal.The experimental measurement of wavelength-dependent DMGD between LP₀₁ and LP₁₁ modes in a graded-index two-mode fiber with trench configuration with a length of about 23 km is completed with an experimental precision of 0.007 ps/m.The measurement results are both experimentally validated and simulated by the time-of-flight based DMGD measurement system.and simulation validation,while the theoretical precision of the system is 0.00014ps/m.