Abstract
The signal-to-noise ratio (SNR) of Brillouin optical time-domain analyzers (BOTDA) is modelled and experimentally validated, using direct detection with and without the use of optical pre-amplification. The behavior of SNR as a function of the Brillouin gain and the probe power reaching the photo detection is analyzed in depth using this developed model and checked using two photodetectors with different specifications. It proves that a pre-amplification associated to a good-quality photodetector and a well-matched post-processing filtering can secure the highest SNR for direct-detection BOTDA. Such an optimal SNR presents only a 2.3 dB penalty compared to the ideal shot-noise-limited case that can only be reached using rather sophisticated configurations. In addition, the model here established predicts the SNR at any fiber position in any given experimental condition.
Highlights
Distributed optical fiber sensors have proven to be excellent tools providing unique benefits for health monitoring of large structures [1]
Studies in literature conclude that all critical specifications of Brillouin optical time-domain analysis (BOTDA) are scaled and traded-off by the signal-to-noise ratio (SNR), which represents the parameter of uttermost importance defining the overall sensing performance [5]
The signal power cannot be infinitely increased in classical BOTDA, since the pulse duration is fixed by the required spatial resolution, whilst the maximum powers of the pulse and of the CW probe are limited by the onset of nonlinear effects [6,7,8,9]
Summary
Distributed optical fiber sensors have proven to be excellent tools providing unique benefits for health monitoring of large structures [1]. With the increasing demand for better performance of distributed Brillouin sensors, sophisticated techniques have been proposed to further improve the SNR by enhancing the equivalent signal power, such as distributed Raman/Brillouin amplification [11,12,13,14] and/or optical pulse coding [15,16,17,18,19,20]. Analytical models for the SNR in the two detection schemes are established, and experimentally verified using two distinct photodetectors with 3-dB bandwidths of 75 MHz and 350 MHz. Somehow counterintuitively, results show that the simple use of direct detection associated with a basic optical pre-amplification can lead to a very decent SNR, being marginally lower than the shot-noise-limited SNR usually obtained by rather sophisticated configurations
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