Research On High-resolution Optical Spectral Analyzing Method Based On Fiber Rayleigh Scattering

With the deepening understanding of the interaction between light and matter,new photonic devices such as optical whispering gallery mode(WGM)resonator and femtosecond optical frequency comb(OFC)have been widely used.In the fields of optical sensing,material analysis,medical diagnosis,and environmental monitoring,subtle alters(e.g.binding kinetics processes of micro/nano particles,and physical/chemical changes of trace gases)could be characterized by spectral responses of sub-fm-order.However,the existing spectrum analyzing methods are either limited by the modulation /processing accuracy of the dispersive elements,or by the construction principle of the virtual filters,its spectral resolution can not reach fm-or even sub-fm-order,which is far from meeting the resolution needs of new photonic devices.In this paper,focusing on the sub-fm-order spectral resolution of new photonic devices,and aiming at the new principle of spectrum analyzing based on fiber Rayleigh scattering,theoretical research and experimental verification are carried out from three aspects: research on physical models,research on the suppression method for fiber Brillouin scattering,and research on high-precision real-time wavelength calibration.The main contents of this paper are as follows:Firstly,the coupling evolution model between fiber Rayleigh scattering and fiber Brillouin scattering is studied.Starting from Boyd's model,the influence mechanism of optical fiber(as a waveguide)on entropy fluctuation and acoustic phonon evolution is revealed by abstracting the cylindrical geometry of optical fiber into the boundary conditions of Navier Stokes equation,heat conduction equation,and continuity equation;the origin of the frequency selection characteristics of fiber Rayleigh scattering is analyzed through combining entropy fluctuation evolution equation,acoustic phonon evolution equation,and nonlinear wave equation;the coupling principle between fiber Rayleigh scattering and fiber Brillouin scattering is researched by introducing pump depletion mechanism.The results illustrate that fiber Rayleigh scattering has stronger frequency selectivity than fiber Brillouin scattering,which is suitable for constructing narrow-linewidth equivalent filter and applied in high-resolution optical spectrum analysis.Secondly,aiming at the problem that Rayleigh scattering is easy to be submerged by Brillouin scattering,the suppression method for Brillouin scattering,as well as the separation method for Rayleigh signal are studied.Based on the analyses of the acoustic-and optical-fields inside cylindrical normal waveguides,combined with the process characteristics of the fiber tapering rig based on ring-like graphite heater,the tapered fiber is divided into several segments of cylindrical normal waveguides,whose geometry and acoustic field are controlled to suppress the accumulation of Brillouin scattering along the fiber axis.The results demonstrate that Rayleigh gain can be increased by 6% when the diameter and duty cycle of the tapered region are 75 ?m and 0.02,respectively.Furthermore,using the characteristic that the Rayleigh gain spectrum is differ from Brillouin gain spectrum with a frequency bias of one Brillouin frequency shift(BFS),the information contained in two scatterings is shifted from the optical frequency domain into radio-frequency/microwave frequency band by a heterodyne interferometer,Rayleigh scattering signal is then extracted by a low-pass filter.Thirdly,aiming at the accuracy degradation or even failure of the wavelength calibration method based on Mach–Zehnder interferometer,which is due to the tuning nonlinearity,a high-accuracy real-time wavelength calibration method is studied.The closed-loop resonator structure is introduced to replace the open-loop Mach Zehnder interferometer in the original method,forcing the beam under test pass through the delay line repeatedly,and therefore transforming the sensitivity to phase difference into the sensitivity to absolute frequency;the vernier is composed of two resonators with slightly different cavity length,and the envelope structure and fine structure of the transmission spectrum are used to calibrate the wavelength of the beam under test simultaneously,and thus compensate the high-order and low-order nonlinearities.The results indicate that the proposed method can be used to calibrate the wavelength of nonlinear tuned signals in real-time.Finally,system integration and verification experiments are carried out.The coupling evolution model of optical fiber light scattering,Brillouin scattering suppression method based on tapered fiber and spectrum analyzing method based on Rayleigh scattering are verified by experiments.The results of the performance tests show that the proposed method has a spectral resolution better than 10 k Hz(corresponding to 0.08 fm at 1550 nm)and a dynamic range of about 20 d B,which can meet the spectral measurement requirements of optical sensors,material analysis,and other fields.