Detrimental effect of self-phase modulation on the performance of Brillouin distributed fiber sensors

Abstract

We show that the spectral broadening of the pump pulse through self-phase modulation in a time-domain distributed Brillouin sensor has a considerably detrimental effect in the measurement, especially in the case of long distances and high-resolution pulses. Using 30 ns pump pulses with peak power of 276 mW, self-phase modulation leads to a doubling of the effective gain linewidth after some 20 km, which is equivalent to a contrast loss of 2 dB in the measurement. The impact is higher for shorter pulses (higher resolution). The theoretical modeling is fully confirmed by experimental results. © 2011 Optical Society of America OCIS codes: 290.5900, 190.2640, 060.2370 For at least two decades, Brillouin fiber sensors have attracted great interest in the fiber-sensing community for their temperature and strain-monitoring capability [1,2]. In time-domain-distributed Brillouin sensors, pulses are used to interrogate the local interaction in the fiber. The accuracy on the measurand is scaled by the spectral spreading of the effective gain, which, at its turn, is given by the convolution between the pulse spectrum and the natural Brillouin gain spectrum (BGS). According to standard time-bandwidth relations, the Gaussian pulse is presumably the best candidate for this interrogation when compared to other profiles (rectangular, triangular). However, we show here that this is not the best choice when addressing long ranges, because this pulse shape leads to a significant spectral broadening of the BGS along the fiber. An observed broadening of the BGS was suspected to be caused by self-phase modulation (SPM) in an early work by Lecoeuche et al. [3] and by Izumita et al.[4] in a coherent optical time-domain reflectometer system. SPM leads to small phase chirps during intensity transitions in the pump pulse (leading and trailing edges) that eventually become important in long fibers. The frequency broadening associated with this phase modulation leads to a reduced peak gain and uncertainties in the determination of the Brillouin shift ν B , but it leaves the temporal intensity distribution of the pump pulse unchanged, and hence the spatial resolution is preserved. Although the former [3][4][5] works showed a correct intuition addressing qualitatively the issue, either no theoretical model was given or the model was incomplete [5].We present here a quantitative model of the detrimental impact of SPM supported by a clear experimental demonstration. Two optical pulses with different temporal profiles were judiciously chosen (rectangular and Gaussian) showing the same FWHM and carrying the same energy, to evenly study and compare the SPM impact on their spectrum. Then we clearly experimentally demonstrate the spectral broadening of the BGS due to SPM in a Brillouin distributed sensor in various conditions in terms of pump pulse temporal profiles, power, and width. The results are compared with a theoretical model showing good agreement. SPM is a consequence of the nonlinear Kerr effect in the fiber that results in an intensity-...

Phase-sensitive OTDR probe pulse shapes robust against modulation-instability fading

“…The behavioral pattern observed from the simulation results states that the further is the pulse envelope shape from rectangular, the smoother is the reduction of visibility due to FPU recurrence of the modulation unstable probe pulses for the same energy and same FWHM. We have verified through simulations that the effect of SPM of the pulses, contrary to the case of Brillouin-based sensors [6], barely affects the final power trace as long as the spectral broadening of the pulse lies within the photodetector bandwidth. Otherwise, the smooth visibility decay of the traces will have an additional, reduced component due to the power filtered out by the photodetector.…”

“…Most of the φOTDR sensing systems employ rectangularlike probe pulses with widths of tens or hundreds of nanoseconds and peak powers under 1 W, which provide power traces of several kilometers with acceptable SNR. In the literature, the effect of the probe pulse shape in the sensor performance has been investigated in the case of Brillouin-based distributed sensors [6]. This study has determined that the use of rectangular pulses is more convenient than other narrower-bandwidth shapes (e.g., Gaussian-like) for Brillouin sensors, since rectangular-shaped pulses suffer less spectral broadening due to self-phase modulation (SPM), leading to a better determination of the Brillouin gain spectrum along the fiber.…”

Raman-assisted Brillouin optical time-domain analysis with sub-meter resolution over 100 km

“…This is known as the resolution-uncertainty trade-off [5]. In long-range systems, this trade-off is worsened by the additional detrimental effect of self-phase modulation, which introduces an extra spectral broadening of the pulses as they travel along the fiber [6].…”

Non-Local Effects in Brillouin Optical Time-Domain Analysis Sensors

“…The SNR of the probe wave detected in a BOTDA depends on the power of the pump and probe waves that can be injected in the fiber under test. The pump pulse power is fundamentally limited by the onset of nonlinear effects such as modulation instability (MI), self-phase modulation and Raman scattering [6,7]. As for the probe wave, its power is first limited by the onset of so-called non-local effects.…”