Monitoring the propagation of mechanical waves using an optical fiber distributed and dynamic strain sensor based on BOTDA

Abstract

We report a Brillouin-based fully distributed and dynamic monitoring of the strain induced by a propagating mechanical wave along a 20 m long composite strip, to which surface a single-mode optical fiber was glued. Employing a simplified version of the Slope-Assisted Brillouin Optical Time Domain Analysis (SA-BOTDA) technique, the whole length of the strip was interrogated every 10 ms (strip sampling rate of 100 Hz) with a spatial resolution of the order of 1m. A dynamic spatially and temporally continuous map of the strain was obtained, whose temporal behavior at four discrete locations was verified against co-located fiber Bragg gratings. With a trade-off among sampling rate, range and signal to noise ratio, kHz sampling rates and hundreds of meters of range can be obtained with resolution down to a few centimeters.

Single-shot distributed temperature and strain tracking using direct detection phase-sensitive OTDR with chirped pulses

“…Note that the total frequency shift correspondent to 300 με is ≈45.4 GHz (Eq. (6), and that the total integrated strain variation applied was 1200 με (≈181.4 GHz), which largely exceeds the pulse spectral content (2.32 GHz). However, the calculated strain was observed to return to zero when the applied strain was returned to zero after 160 s. This again clearly demonstrates the long term stability of the sensor and that large measurement ranges (which largely exceed the equivalent to the pulse spectral content) are achievable.…”

“…With traditional Brillouin Optical Time Domain Analysis (BOTDA), spatial/temperature resolutions of 2m/1.2K over 100 km have been demonstrated [3], but, due to the requirement of a large of number averages and a frequency sweep, the measurement times are still typically of several minutes. Recently, schemes using Brillouin based sensing have been proposed for dynamic strain sensing [4][5][6][7]. In 2011, samplings of 20 Hz were demonstrated in a version of Brillouin optical correlation-domain analysis (BOCDA) [4].…”

“…With a fast implementation of BOTDA [5], samplings of 10 kHz with a standard deviation of strain of 5με were demonstrated, although the technique was best suited for short fibers (100 meters in [5]) and a low number of averages, as a frequency sweep was still required. The same group later demonstrated a practical utilization of the slope-assisted BOTDA to allow for strain measurements without requiring a frequency sweep [6]. In this case, the dynamic response is achieved by working tuned to the slope of the Brillouin gain response of the fiber.…”

Single-Shot True Distributed Strain Variation Measurements Over >10 km Using Phase-Sensitive OTDR with Chirped Pulses

“…While the local strain can be retrieved by comparing the relative phase variations between closely-spaced points of the fiber, the overall detection system is more complex. The measurement of true dynamic strain has also been successfully demonstrated using slope-assisted Brillouin Optical Time Domain Analysis (BOTDA) [6]. Since the measurement does not require a frequency sweep, high bandwidths of detection can be achieved, although in this case the measurable strain range is limited by the Brillouin gain curve.…”

Experimental modal analysis of an aluminum rectangular plate by use of the slope-assisted BOTDA method

“…Also, because the strain is directly related to the stiffness of the structure, it is more sensitive to structural defects with respect to displacement or velocity [7]. In previous work [8], a Brillouin optical time-domain analysis (BOTDA) configuration operated in the slope-assisted mode [9,10] was employed in order to retrieve the natural frequencies and modal shapes of an aluminum cantilever beam. In this work, the same method is applied to measure the natural frequencies and modal shapes of a rectangular aluminum plate in different clamped conditions.…”