Study On Ultra-long BOTDA Sensors Based On Distributed Brillouin Amplification

This paper introduces the theory and research advances for Brillouin optical time domain analysis(BOTDA)sensors based on stimulated Brillouin scattering(SBS).In DBA(distributed Brillouin amplification)-BOTDA system with constant Brillouin response,when the sensing distance is far more than 50 km,the required pump power increases exponentially with the length of the fiber to be measured.To solve this problem,a new scheme of pump modulation with multiple constant bandwidths was proposed and demonstrated.Furthermore,a new idea of direct detection of injection locked(IL)dual-bandwidths pump was proposed,which not only compensates for the loss of ultra-long fiber perfectly,but also avoids the influence of amplitude modulation(AM)noise by introducing IL.The direct demodulation of frequency modulation(FM)pump effectively suppresses the non-local effect.In addition,fast measurement of Brillouin phase spectrum(BPS)for long-distance DBA-BOTDA based on parallel demodulation of frequency-comb pump was realized.(1)A novel pump modulation scheme with multi-constant bandwidths was proposed.For Brillouin optical time-domain analysis(BOTDA)based on distributed Brillouin amplification(DBA),conventional uniform bandwidth pump can lead to a large area with lower signal-to-noise ratio(SNR);constant Brillouin response achieved by the exponentially variable bandwidth/intensity pump modulation suffers from the much lower pumping efficiency for long-range sensing,which counterbalances the merit of DBA.To solve this problem,pump modulation by multiple constant bandwidths was proposed and demonstrated.~98.9 km sensing with ~5 m spatial resolution and no use of optical pulse coding(OPC)was achieved by ~8 dBm Brillouin pump,which is lower by ~9 dB in theory by comparison with exponentially increased bandwidth modulation.Compared with traditional DBABOTDA,signal-to-noise ratio(SNR)enhancement with > 4.6 dB was obtained.The flattened standard deviation(STD)of Brillouin frequency shift(BFS)(less than ~2 MHz)along the whole fiber was demonstrated.This method can efficiently extend the sensor range of DBA-BOTDA without sacrificing the pumping efficiency.(2)Parasitical AM noise of pump is one of the crucial factors affecting the SNR of multiple-bandwidths modulation DBA-BOTDA.Here,a novel tailored DBABOTDA by direct demodulation of injection-locked dual-bandwidths pump was proposed and presented.Injection locking(IL)can perfectly suppress the impact of AM noise;dual-bandwidths pump enables the self-adaptive pulse loss compensation,thereby extremely intensifying the SNR flatness along ultra-long fiber;direct pump demodulation can overcome non-local effects and allows ~20.4 dB enhancement of probe input power.As a result,by using only 100 times averages,~148.3 km sensing and ~5 m spatial resolution were achieved with less than ~1 MHz STD of BFS over a quite broad range(~131.7 km).The reduction of averaging times was more than 10 times relative to that of previous long-range BOTDAs.Such performances were achieved without resorting to any time-consuming or post-processing techniques,such as OPC and image denoising.(3)A new configuration of Brillouin phase spectrum(BPS)fast measurement was proposed and demonstrated for vector DBA-BOTDA,in which the parallel demodulation of frequency-comb pump can be realized.In order to eliminate the problem of phase noise resulting from microwave source at the receiver end in our previous work,the frequency of local oscillator(LO)light was designed to be close to the single side-band(SSB)frequency-comb pump.Combining with the 251 bits cyclic coding for diminishing the impact of Brillouin pump depletion and log linearization for suppression of nonlinear amplification,~ 98.9 km sensing distance with ~ 6.5 m spatial resolution was demonstrated.The STD of BFS can be decreased to be within ~ 2 MHz by the average of Brillouin gain spectrum(BGS)and BPS.