High-resolution long-reach distributed Brillouin sensing based on combined time-domain and correlation-domain analysis

Abstract

A new scheme for distributed Brillouin sensing of strain and temperature in optical fibers is proposed, analyzed and demonstrated experimentally. The technique combines between time-domain and correlation-domain analysis. Both Brillouin pump and signal waves are repeatedly co-modulated by a relatively short, high-rate phase sequence, which introduces Brillouin interactions in a large number of discrete correlation peaks. In addition, the pump wave is also modulated by a single amplitude pulse, which leads to a temporal separation between the generation of different peaks. The Brillouin amplification of the signal wave at individual peak locations is resolved in the time domain. The technique provides the high spatial resolution and long range of unambiguous measurement offered by correlation-domain Brillouin analysis, together with reduced acquisition time through the simultaneous interrogation of a large number of resolution points. In addition, perfect Golomb codes are used in the phase modulation of the two waves instead of random sequences, in order to reduce noise due to residual, off-peak Brillouin interactions. The principle of the method is supported by extensive numerical simulations. Using the proposed scheme, the Brillouin gain spectrum is mapped experimentally along a 400 m-long fiber under test with a spatial resolution of 2 cm, or 20,000 resolution points, with only 127 scans per choice of frequency offset between pump and signal. Compared with corresponding phase-coded, Brillouin correlation domain analysis schemes with equal range and resolution, the acquisition time is reduced by a factor of over 150. A 5 cm-long hot spot, located towards the output end of the pump wave, is properly identified in the measurements. The method represents a significant advance towards practical high-resolution and long range Brillouin sensing systems.

Brillouin analysis with 8.8 km range and 2 cm resolution

“…Due to the rapid phase modulation , the pump and signal envelopes are in correlation only within multiple, periodic and short segments of spatial extent Δ = 2 ⁄ , that are separated by a distance = Δ 8,13 .…”

“…We recently introduced a new protocol for Brillouin sensing, in which both pump and signal are jointly-modulated by a repeating, high-rate binary phase sequence 13 . In addition, the pump wave is also amplitude-modulated by a long, lowerrate code 14 .…”

Multi-point dynamic strain sensing using external modulation-based Brillouin optical correlation domain analysis

“…This can be addressed by enabling simultaneous measurements at multiple locations. Although several configurations have been demonstrated to monitor multiple locations such as random access BOCDA [14] and temporal gating BOCDA [15,16], multiple locations are monitored sequentially in all such configurations, thereby limiting the measurement speed. We recently proposed external phase modulation-based BOCDA as a technique to monitor multiple independent locations simultaneously [17].…”

Brillouin optical correlation domain analysis with 4 millimeter resolution based on amplified spontaneous emission

“…The measurement range was later extended through more elaborate frequency modulation patterns [14]. Recent advances in B-OCDA include phase modulation by long, binary sequences for an extended measurement range [16]; time gating for improved signal-to-noise ratios (SNRs) [14,17]; use of special phase codes for reduced off-peak residual interactions and improved SNR [18]; and the combination between B-OTDA and B-OCDA for the simultaneous interrogation of a large number of peaks [18]. State-of-the-art B-OCDA setups provide few-km range alongside cm-scale resolution [17][18][19].…”