Research Of High-performance Optical Frequency Domain Reflectometry And Its Applications

Optical reflectometry is a type of non-invasive monitoring technology for fiber links,which plays an important role for ensuring the normal work of the fiber communication system.Fiber sensing technology arises with the development of the fiber communication technology.In the fiber sensor,a fiber is employed as the perceiving and transferring medium.Compared with the traditional electrical sensor,fiber sensor has the advantages of small size,low weight,corrosion resistance,electromagnetic immunity and easy to networking.Since the appearance of the fiber sensing technology,optical reflectometry becomes an important branch for its capacity of distributed or quasi-distributed measurement.Compared with other implementation methods of optical reflectometry,OFDR has advantages of measurement range and spatial resolution,therefore,attracts more attentions from the academic and industrial circles.Nowadays,external-modulation based OFDR system can achieve an ideal measurement range and spatial resolution.However,some limitations still exist in OFDR system: firstly,a limited frequency-swept range of external modulation method hinders the spatial resolution improvement of OFDR;secondly,a limited demodulation efficiency hinders its application in some scenarios where high measurement speed is needed.And in the OFDR based sensing system,limited measurement range,insufficient measurement resolution and frequency response are ubiquitous.This article focuses on the improvement of the key indicators of OFDR and its performance in sensing application.Besides,the new application fields of OFDR are discussed,which provides some new ideas for deploying OFDR technology.The main work and novelties of this article are listed below:1)We propose two schemes for improving the performance of OFDR based fiber links monitoring system.One is a new type of hardware-adaptive algorithm for PNC-OFDR,which solves the problem of data processing inefficiency in PNC-OFDR.In the new algorithm,the complexity reduction has been achieved,thus improving the processing efficiency.Based on the new algorithm,the hardware circuit,namely the FPGA based data processing system is designed.Benefitting from the parallel pipeline processing of FPGA,a real-time demodulation system for PNC-OFDR is achieved.The other one is a dual-comb source based OFDR system,which solves the problem of limited frequency-swept range of external-modulation based OFDR system.In this system,we employ a frequency comb source as the probe and a dual comb system for signal reception so as to avoid frequency aliasing between each comb line.By doing some post-processing,a phase-continuous OFDR signal can be obtained with an enlarged frequency-swept range and without frequency-swept linearity deterioration.In the proof-of-concept experiment,120 GHz frequency-swept range is achieved and the OFDR spatial resolution is 1.86 mm.2)We propose two schemes for improving the performance of OFDR based sensing system.One is for the limited performance of the OFDR based distributed strain sensing system.We propose a mathematical model based on the one-dimensional scattering model of single-mode fiber.And based on this model,the main noise sources of the sensing system are thoroughly analyzed.In the experiment,by eliminating the main noise in the system,a distributed static strain sensing system is achieved with long measurement range(25km)and high strain resolution(<100n).And on this basis,combining with high-order sideband modulation and injection locking technology,distributed strain sensing with a high spatial resolution(0.83m)is achieved.The other one is a quasi-distributed vibration sensing system which employs the phase-sensitive OFDR to interrogate the weak reflector array,for solving the trade-off relation between the sensing range and the frequency response bandwidth in conventional pulse-wave based vibration sensing system.Benefitting from the continuous-wave probe in OFDR system,different status of the FUT under vibration can be recorded continuously,which makes it possible to break the trade-off relation between the measurement range and the frequency response bandwidth.In the experiment,a vibration sensing system with 100-km sensing range and 20-k Hz frequency response is achieved.3)We propose a wavelength change measurement scheme based on the Rayleigh backscattering,which enlarges the application scope of OFDR.Based on the high-sensitivity wavelength-dependence Rayleigh pattern,we could obtain the wavelength change by analyzing the change of the Rayleigh pattern.Benefitting from an analytical solution between the wavelength and the Rayleigh pattern in OFDR-based scheme,the self-reference demodulation can be employed for avoiding the database creation process,which simplifies the processing procedure and increases the convenience.In the experiment,64-am spectral resolution is achieved for wavelength change measurement.This scheme provides a new scope for OFDR application.In conclusion,this article is aiming at improving the overall performance and practicability of OFDR system for fiber link monitoring and sensing application by analyzing and solving the existed problems.Besides,a new OFDR application scheme is proposed for enlarging its application scope.