Influences And Suppression Techniques Of Nonlinear Effects On Long-haul Interferometric Fiber Sensing Systems

With the developments of optoelectronics and fiber sensing technologies,interferometric fiber sensing systems such as fiber hydrophone have been widely usedin the fields including submarine detection, oil and natural gas prospecting, andearthquake inspection. Recently, with the developments of erbium-doped fiber amplifier(EDFA) and fiber Raman amplifier (FRA), interferometric fiber sensing systemsdevelop towards the long-haul direction. Although the sensing distance is increased,fiber nonlinear effects also become significant. The foregone researches focus on fibercommunication system or nonlinear effects themselves, while pay little attention tononlinear effects in interferometric fiber sensing systems. Compared with fibercommunication systems which treats bit error rate (BER) as its research focus,interferometric fiber sensing systems pay great attention to phase noise, which decidesthe detection sensitivity of the system. Considering that a variety of nonlinear effectssuch as stimulated Brillouin scattering (SBS), four-wave mixing (FWM), modulationinstability (MI), self-phase modulation (SPM) and cross phase modulation (XPM)introduce phase noise and lead to system performance degradation, influences andsuppression techniques of nonlinear effects are key technologies for the long-haulinterferometric fiber sensing systems.Based on the structure and principle of the long-haul interferometric fiber sensingsystem, the phase noise structure of the system considering a variety of nonlinear effectsis analyzed for the first time. The phase noise results from two sources i.e. phase noisetransferred from intensity noise and phase noise induced by laser linewidth, originatingfrom intensity fluctuation and frequency dithering, respectively. The former is alsocalled GM noise with its linear part introduced by SBS and FWM and its nonlinear partresulting from optical Kerr effect i.e. SPM and XPM. SBS, FWM and MI can causelinewidth broadening, leading to the increase of phase noise. The proposition of phasenoise structure provides guidance for the design and application of long-haulinterferometric fiber sensing systems.The SBS localized fluctuating model is used for investigating the intensity noise ofthe forward output light and the backscattered light. It is found that, for the forwardoutput light, the intensity noise is small at first, increases dramatically when reachingthe SBS threshold, and then tends to stabilize gradually. For the backscattered light,after the SBS threshold, the intensity noise decreases fast at first and then slowly, andbecomes stable at last. The above results agree with the measured variations of theforward and backward intensity noise. The forward output phase noise is also measuredand its variation is accordant with that of the corresponding intensity noise, whichverifies that phase noise can be transferred from intensity noise. The linewidth with and without SBS are measured, which confirms that SBS can cause linewidth broadeningand introduce phase noise. Frequency modulation and phase modulation are used tosuppress SBS. The suppression effect of the former method is very limited, while thelatter method can improve the SBS threshold by7dB. However, phase modulation isbased on laser linewidth broadening. Although SBS as well as its induced phase noise issuppressed, the phase noise related to linewidth is induced. Therefore, in practicalapplications, both SBS suppression and laser linewidth broadening should be considered,and the measured phase noise is used for finding the optimum balance of the twoopposite effects, which has been not referred in former researches. Furthermore, theinfluence of SBS on FRA is investigated, and the Brillouin bandwidth is measured to be50MHz using SBS slow light technique.The relations between FWM efficiency as well as its intensity noise and thechannel spacing and fiber length are numerically simulated, and the results areexplained using FWM quasi-phase-matching condition. Taking two-wave transmissionfor example, FWM induced intensity noise is measured and it is found that the intensitynoise caused by the energy exchange instability between FWM generated light andoriginal light can be ignored, and what should be concerned most is SBS rather thanFWM. Taking three-wave and four-wave transmission for examples, FWM inducedphase noise is measured and it is found that the influence of FWM on phase noise canalso be ignored, which is because the channels are not perfectly equally spaced and thebeat frequency between the generated light and original light is larger than thebandwidth of the narrowband photodetector used in the interferometric fiber sensingsystem, leading to that the beat noise is filtered. Based on this, suppressing FWMinduced phase noise with narrowband photodetector is proposed, which has not beendiscussed in the former researches. Furthermore, the pump echo technique is used tostudy the case when FWM and SBS are combined, and the result is explained from theFWM phase-match point of view.Three kinds of physical mechanisms of MI are introduced. MI is numericallysimulated using nonlinear Schordinger equation which describes pulse transmission inthe fiber. Spontaneous MI and induced MI are observed on the background ofbroadband amplified spontaneous emission (ASE) caused by EDFA. The relationsbetween the MI threshold and the input spectrum and fiber length are investigated and itis found that broadband spectrum has a higher MI threshold than monochromaticspectrum, and MI threshold is inversely proportional to effective fiber length. Thedifference between MI and SBS is analyzed and it is found that MI responds to pulsepeak power while SBS responds to average power, which is due to that the respondingtime of MI and SBS are10fs and10ns, respectively. Considering that MI causes amass of phase noise, the methods of pulse peak power control and narrowband fibergrating filtering are used to suppress MI. The modulation instability resonance (MIR) introduced by the combination of MI and FWM is investigated and explained fromFWM phase-match point of view. A method of selecting FWM sidebands is proposedfor the first time, which is based on the simultaneous utilization of a CW channel and apulse channel.Influences of nonlinear effects on system performance are discussed from fouraspects including fiber segment number of optical amplifier link, input power, fiberlength and channel number. For long-haul transmission over50km, the influence ofSBS should be considered when input average power is above4mW, and the influenceof MI should be considered when input peak power is above110mW, and the influenceof XPM should be considered when channel number is more than20, while theinfluence of FWM need not to be considered due to the narrowband detectioncharacteristic of interferometric fiber sensing system. Using SBS and MI thresholds andconsidering both the time-domain multiplexing (TDM) efficiency and transmissiondistance, pulse duty cycle is optimized. When phase modulator is used for SBSsuppression, for50km fiber and25MHz modulation frequency, the optimummodulation indices are0.57π,0.71π and0.86π when the input power is between3mWand5mW, between6mW and10mW, between12mW and14mW, respectively. Theabove conclusions provide good guidance for the parameter selection in the practicalapplications of interferometric fiber sensing systems.