Abstract
Logarithmic detectors (LogDs) have been used in coherent Brillouin optical time-domain analysis (BOTDA) sensors to reduce the effect of phase fluctuation, demodulation complexities, and measurement time. However, because of the inherent properties of LogDs, a DC component at the level of hundreds of millivolts that prohibits high-gain signal amplification (SA) could be generated, resulting in unacceptable data acquisition (DAQ) inaccuracies and decoding errors in the process of prototype integration. By generating a reference light at a level similar to the probe light, differential detection can be applied to remove the DC component automatically using a differential amplifier before the DAQ process. Therefore, high-gain SA can be employed to reduce quantization errors. The signal-to-noise ratio of the weak Brillouin gain signal is improved from ∼11.5 to ∼21.8 dB. A BOTDA prototype is implemented based on the proposed scheme. The experimental results show that the measurement accuracy of the Brillouin frequency shift (BFS) is improved from ±1.9 to ±0.8 MHz at the end of a 40-km sensing fiber.
Highlights
At each sweep frequency point, a 4000 times average was implemented by the data acquisition (DAQ) card to improve the signal-to-noise ratio (SNR)
A differential detection method based on SSB probe light was proposed to remove the DC component dynamically and improve the SNR before DAQ in a Brillouin optical timedomain analysis (BOTDA) system
The “pure” Brillouin gain signal is further amplified by the signal amplification (SA) high gain circuit with around 40 times amplification
Summary
Distributed optical fiber sensors based on Brillouin scattering have attracted widespread attention because of their characteristic abilities to achieve high-performance strain and temperature measurements over long distances.[1,2,3] Among these Brillouin fiber sensor techniques, the Brillouin optical timedomain analysis (BOTDA) based on stimulated Brillouin scattering (SBS) exploits the dependence of the BFS parameter on strain and temperature, achieving highly accurate measurements over distances of more than several tens of kilometers.[4,5] Aiming for higher measurement accuracy, longer sensing distance, and higher spatial resolution, many efforts such as pulse coding, Raman amplification, and frequency comb have been devoted to improve the signal-to-noise ratio (SNR) and reduce measurement time of the BOTDA sensors.[6,7,8,9] Recently, coherent detection as a new method has been proposed, in which a local light beat and a probe light beam propagate through a photodetector (PD) by generating a GHz carrier to carry the Brillouin gain spectrum (BGS), resulting in significant SNR improvement (>10 dB typically) because of strong baseband noise perturbation reduction.[10]. To simultaneously generate local light and probe light in a coherent BOTDA sensor, double-sideband (DSB), phase modulation (PM), and intensity modulation (IM) are normally used. DSB modulation is very sensitive to chromatic dispersion (CD) of the sensing fiber, causing PM–IM conversion and power fading, and further introducing crosstalk in the measured BGS.[11,12,13,14]
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