Abstract
Noise models for both single-pulse and coded Brillouin optical time-domain analyzers (BOTDA) are established to quantify the actual signal-to-noise ratio (SNR) enhancement provided by pulse coding at any fiber position and in any operating condition. Simulation and experimental results show that the polarization noise and spontaneous Brillouin scattering (SpBS) to signal beating noise could highly penalize the performance of coded-BOTDA, depending on the code type and the interrogated fiber position. The models also serve as a useful tool to optimize the SNR improvement by trading off the accumulated Brillouin gain and optical noises.
Highlights
Distributed optical fiber sensors based on Brillouin optical time-domain analysis (BOTDA) have attracted considerable attentions in recent years, owing to its capability to inform on the spatial distribution of targeted quantities over a long optical fiber (> 25 km) [1,2,3,4]
Based on the noise models, the reported analysis points out that the Brillouin-gain-dependent optical noises, such as polarization noise and spontaneous Brillouin scattering (SpBS)-signal beating noise, could highly compromise the attained coding gain that is theoretically defined by assuming that the noise level of the coded BOTDA system remains unchanged with respect to the single-pulse BOTDA
The impact of such noises differs depending on the code type: 1) For aperiodic codes, the coding gain is always largely compromised at the fiber near-end due to the large Brillouin gain, while at the fiber far-end the local Brillouin gain reduces due to the fiber attenuation, so that the penalty of coding gain due to optical noises partially or entirely vanishes depending on the fiber length
Summary
Distributed optical fiber sensors based on Brillouin optical time-domain analysis (BOTDA) have attracted considerable attentions in recent years, owing to its capability to inform on the spatial distribution of targeted quantities over a long optical fiber (> 25 km) [1,2,3,4]. The SNR improvement (so-called coding gain) provided by coding techniques is the ratio of the noise levels before and after decoding, which is calculated to be proportional to square-root of the coding length [12,13,14,15,16,17,18,19] Very importantly, such a coding gain can be fully achieved only if the noise level remains unchanged between coded- and single-pulse BOTDA schemes, generally meaning that only if the photo-detection thermal noise dominates the measurements. Polarization noise and spontaneous Brillouin scattering (SpBS) to signal beating noise [20,21], which may be considerably enhanced from single-pulse BOTDA to coded BOTDA due to the largely increased pump energy, penalizing the theoretical coding gain. For periodic codes, optical noises remain globally high and unchanged along the measured trace, affecting the entire sensing fiber length and penalizing the coding gain at all fiber positions
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