Abstract
We report on two previously unknown non-local effects that have been found to impair Brillouin optical time-domain analysis (BOTDA) sensors that deploy limited extinction ratio (ER) pump pulses. The first one originates in the increased depletion of the pedestal of the pump pulses by the amplified probe wave, which in turn entails a reduced amplification of the probe and a measurement distortion. The second effect is due to the interplay between the transient response of the erbium-doped fiber amplifiers (EDFA) that are normally deployed to amplify the pump and the pedestal of the pump pulses. The EDFA amplification modifies the pedestal that follows the pulses in such a way that it also leads to a distortion of the measured gain spectra after normalization. Both effects are shown to lead to non-local effects in the measurements that have similar characteristics to those induced by pump pulse depletion. In fact, the total depletion factor for calculations of the Brillouin frequency shift (BFS) error in BOTDA sensors is shown to be the addition of the depletion factors linked to the pump pulse as well as the pedestal. A theoretical model is developed to analyze both effects by numerical simulation. Furthermore, the effects are investigated experimentally in long-range BOTDA sensors. The pedestal depletion effect is shown to severely constrain the probe power as well as the minimum ER of the pulses that can be deployed in BOTDA sensors. For instance, it is shown that, in a long-range dual-probe BOTDA, an ER higher that 32-dB, which is above that provided by standard electro-optic modulators (EOM), is necessary to be able to deploy a probe power of -3 dBm, which is the theoretical limit for that type of sensors. Even more severe can be the limitation due to the depletion effect induced by the EDFA transient response. It is found that the impairments brought by this effect are independent of the probe power, hence setting an ultimate limit for the BOTDA sensor performance. Experimentally, a long-range BOTDA deploying a 26-dB ER EOM and a conventional EDFA is shown to exhibit a BFS error higher than 1 MHz even for very small probe power.
Highlights
During the last few decades, distributed fiber optic sensors based on the Brillouin scattering nonlinear effect have been thoroughly researched mainly due to their ability to provide high precision distributed temperature and strain measurements over extremely large structures
One of the most fundamental limitations in a Brillouin optical time-domain analysis (BOTDA) comes from the optical power of the pump and probe waves that can be injected into the sensing fiber, because this defines the signal-to-noise ratio (SNR) of the detected sensor response, which is the quantity that constrains the performance of the sensor [1]
We focus here on another limiting factor for BOTDA sensors that has been largely overlooked to date: the pedestal of the pump pulses deployed in the sensor [5]
Summary
During the last few decades, distributed fiber optic sensors based on the Brillouin scattering nonlinear effect have been thoroughly researched mainly due to their ability to provide high precision distributed temperature and strain measurements over extremely large structures. Our group showed that the extra amplification of the probe wave by the pump pedestal could lead to an exacerbation of the pump pulse depletion and non-local effects [6] Another negative impact of limited ER pump pulses comes from the worsening of the SNR of the sensor due to laser phase noise [7]. A rather conservative and somewhat arbitrary requirement on the ER of the pulses in a BOTDA sensor was established in the context of the modeling of non-local effects due to pump depletion [4] This requirement stated that the ER should be sufficient to guarantee that the Brillouin amplification of the probe provided by the pedestal of the pulse along the full sensing length had to be smaller than the local amplification by the pulse itself. Both effects are studied theoretically, through numerical calculations, and experimentally
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