Research Of The Nonlocal Effects In Brillouin Optical Time Domain Analysis Sensing Technology

Optical fiber sensor is widely used in intrusion detection of border fences,security detection of oil and gas pipelines,and state inspection of structures,bridges,dams,train tracks,aircraft structures,and other fields.Optical fibers are outstanding candidates to serve as sensors due to their unique properties:1)they can both sense and transmit the communicated information by the same waveguide;2)they offer a low-loss link and thus can be used for long-range sensing;3)they are dielectric and as such are not affected by electro-magnetic interference nor generating one;4)they are very thin(～0.1mm)and low weight,easily mount to other materials.The principle of distributed optical fiber sensing technology is to demodulate scattered light(including one or more of Rayleigh scattering,Raman scattering,and Brillouin scattering)signals in the time domain or frequency domain to measure the temperature,stress,and other physical quantities.The Brillouin optical time-domain analysis(BOTDA)technology based on the stimulated Brillouin scattering(SBS)effect has the advantages of long-range(hundreds of kilometers),high-spatial-resolution,and high precision.At the same time,BOTDA also faces a critical limitation: non-local effect(NLE).The NLE is defined as:Brillouin gain spectrum(BGS)is compressed,stretched,and distorted due to the uneven amplification(including Brillouin amplification,Raman amplification,EDFA amplification,etc.),which results in measurement errors.For example: transient effects(TE)of Erbium Doped Fiber Amplifier(EDFA)cause NLE,pump depletion directly results in NLE,frequency-modulated probe(FMP)scheme can effectively reduce NLE but it has the problem of acoustic wave crosstalk.We have carried out corresponding research on these three aspects and the detailed work is as follows:(1)A coherent detection technique is proposed to solve the TE problem of the optical amplifier(i.e.,EDFA)in the pulse-coded BOTDA sensor.When the EDFA is used to preamplify the probe,measurement errors are introduced by the TE.After eliminating the phase jitter caused by the fiber,the coherent detection technology is successfully employed in the Golay-coded BOTDA sensor.Furthermore,the Brillouin phase spectrum(BPS)is used instead of the BGS for sensing,which eliminates the ～5.4 MHz BFS measurement error caused by the TE.(Chapter 3)(2)A distributed depletion mapping(DDM)method based on direct detection is proposed to overcome the TE.The DDM can map the EDFA transient effect of the probe light in real time,and use the map light to compensate the probe light to overcome the TE.The proposed method is experimentally demonstrated along with 120-km sensing fiber with 2-m spatial resolution.Experimental results show that the DDM method compensates for ～7.69-MHz(at tail alignment)and ～12 MHz(at head normalization)measurement errors,thereby improving the measurement accuracy.(Chapter 3)(3)A depletion model(DM)is proposed to compensate the NLE caused by the pump depletion.The DM is based on the energy conservation relationship between the pulsed pump and the continuous probe.It can accurately calculate the distributed depletion along with the fiber.Further,the BGS is compensated with the calculated depletion to eliminate the NLE.In the single-sideband probe configuration,the DM can increase the probe power from-14 d Bm to +2 d Bm,thereby improving the signal-to-noise ratio(SNR)of about 6.9 d B and eliminating the BFS error of ～14.2 MHz.(Chapter 4)(4)A bidirectional frequency-modulated probe(BFMP)technology is proposed.BFMP not only inherits the advantages of FMP to reduce non-local effects,but also overcomes the problem of acoustic crosstalk caused by frequency agility.The BFMP uses bidirectional frequency modulation to generate acoustic crosstalk with the same magnitude and opposite direction,and then uses them to compensate each other to overcomes the acoustic crosstalk.Numerical simulation and experimental results indicate that the BFMP can narrow the pulse interval of the coded sequence to ～500 ns and eliminate BFS estimation errors(～2.2 MHz)over 117.46-km sensor link simultaneously.(Chapter 5)...