Transient response in high-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time

Abstract

We perform numerical simulations on a model describing a Brillouin-based temperature and strain sensor, testing its response when it is probed with relatively short pulses. Experimental results were recently published [e.g., Opt. Lett. 24, 510 (1999)] that showed a broadening of the Brillouin loss curve when the probe pulse duration is reduced, followed by a sudden and rather surprising reduction of the linewidth when the pulse duration gets shorter than the acoustic relaxation time. Our study reveals the processes responsible for this behavior. We give a clear physical insight into the problem, allowing us to define the best experimental conditions required for one to take the advantage of this effect.  2000 Optical Society of America OCIS codes: 060.2370, 190.5890, 290.5900. Stimulated Brillouin scattering couples two counterpropagating optical waves, the pump and the Stokes waves, and an acoustic wave. Interaction is maximum for a precise value of the frequency shift between the two optical waves, which itself depends on the temperature and strain conditions in the material. Distributed sensing in an optical fiber is commonly obtained by use of a cw pump and a Stokes-shifted probe pulse. One measures the intensity of the transmitted pump field as a function of the frequency shift to resolve the Brillouin gain spectrum. Positional information is obtained through a time-domain analysis; shorter probe pulses increase the spatial resolution. Unfortunately, but expectedly, the Brillouin gain spectral width broadens as the pulse duration is reduced. 1,2This broadening leads to diff iculties in the determination of the resonant Brillouin shift and therefore to a larger error in the measurement for submeter spatial resolution. However, it was found recently that reducing the pulse duration to values smaller than the acoustic relaxation time entails a sudden decrease of the spectral width. 3,4 In those studies it was suggested that a spatial resolution of 10 cm was achievable, with a relatively good strain or temperature accuracy.In this Letter we present the results of numerical simulations highlighting the mechanisms responsible for the observed spectral width variation. These theoretical findings show good agreement with the experimental results described in Ref.3. The present study also reveals the crucial inf luence of a totally unsuspected parameter, that is, the extinction ratio of the electro-optic modulator (EOM) used to generate the pulses. We show that a modest extinction ratio (20-30 dB) results in spectral narrowing with ultrashort pulses, as reported in Ref. 3, whereas for a high (infinite) extinction ratio the gain linewidth broadens, as expected from a standard Fourier analysis of the system. The theoretical model used to simulate the interactions in the sensing fiber is the following 4,5 :where E p , E s , and E a are the normalized pump, Stokes, and acoustic fields, respectively. The fiber attenuation, which is not relevant for lengths in the meter range, is neglected. The acoustic-wave convecti...

Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives

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