Heterodyne slope-assisted Brillouin optical time-domain analysis for dynamic strain measurements

Abstract

We propose a slope-assisted Brillouin sensing scheme based on the interaction between a phase modulated probe and a pulsed pump. The modulation frequency of the probe is chosen so that both first-order modulation sidebands lie within the Brillouin gain spectrum generated by the pump field. Owing to stimulated Brillouin scattering (SBS), a phase modulation to intensity modulation conversion occurs in the probe field upon interaction with the pump, producing a beat note whose amplitude grows with detuning from the SBS resonance. We show that the ratio between the amplitude of the heterodyne signal and the direct component of the probe gain provides a means for performing pump-power independent dynamic strain measurements, therefore improving the conventional slope-assisted brillouin optical time-domain analysis.

Operation of slope-assisted Brillouin optical correlation-domain reflectometry: comparison of system output with actual frequency shift distribution

“…Since the basic BOCDR system was first proposed in 2008 [23], numerous configurations have been implemented in order to improve its performance [24][25][26][27][28][29][30][31][32][33][34]. For example, the measurement range was elongated using temporal gating [24] and double modulation [25]; millimeter-order spatial resolution was achieved by use of a tellurite glass fiber [26] and by apodization [27]; and the strain dynamic range was extended using polymer optical fibers [28,29].…”

“…In addition, one of the latest breakthroughs is phase-detection-based BOCDR [30], which enables a high sampling rate of >100 kHz per point. We have recently developed another high-speed configuration, motivated by previous implementations in the time domain [31][32][33], called slope-assisted (SA-) BOCDR [34], in which the spectral power of the Brillouin gain spectrum (BGS) is exploited to deduce the Brillouin frequency shift (BFS). This configuration can potentially allow for higher-speed operation than the phase-detection-based method, and for detection of not only changes in strain and temperature, but also optical losses, if necessary.…”

Recent Advances in Brillouin Optical Correlation-Domain Reflectometry

“…In contrast, the slope-assisted BOCDR provides the BFS information using the BGS slope. This idea has been already proposed for BOTDA [62][63][64]. As depicted in Figure 5a, the BFS is in one-to-one correspondence with the spectral power P B0 at a certain frequency ν B0 , which is set at the high-frequency point in the linear region (lower-frequency side) of the BGS slope.…”

Slope-assisted BOTDA based on vector SBS and frequency-agile technique for wide-strain-range dynamic measurements

“…However, the demodulation of BFS through complex curve-fitting algorithms is also time-consuming and subsequently not suitable for real time on-line monitoring. On the other hand, a slopeassisted BOTDA (SA-BOTDA) for fast strain variations measurement is proposed by only single pump pulse and using a single-slope of the local Brillouin gain spectrum [35][36][37][38].The strain vibration frequency of this system is only determined by the fiber length and average number, but its maximum strain vibration range is limited to the narrow-slope of BGS (~30MHz for single-mode fiber). A double slope-assisted BOTDA is proposed to be immune to pump pulse power variations by using both side slopes of the BGS [39].…”