Abstract
Brillouin optical correlation-domain analysis (B-OCDA) allows for distributed measurements of strain and temperature with sub-cm resolution. Time-multiplexing techniques have previously extended B-OCDA to the monitoring of many km of fiber and two million resolution points. Thus far, however, the number of scans of correlation peaks positions, necessary to cover the fiber under test, was restricted to the order of 100 or more. In this work we report a B-OCDA protocol that is able to address an entire fiber using only 11 pairs of position scans per choice of frequency. The measurements protocol relies on a merger between B-OCDA principles and double-pulse-pair analysis, previously incorporated in time-domain Brillouin sensors. Phase coding of the pump and signal waves with a repeating, short and high-rate code stimulates Brillouin interactions in a large number of narrow correlation peaks, with substantial temporal overlap. Unambiguous measurements are achieved by repeating each experiment twice, using a pair of pump pulses of different durations, and subtracting the two output traces. The principle is demonstrated in the analysis of a 43 m-long fiber with 2.7 cm resolution. Several local hot-spots are properly identified in the measurements. The experimental uncertainty in the measurement of the local Brillouin frequency shift is estimated as ± 1.9 MHz. The proposed method requires broader detection bandwidth and a larger number of averages than those of previous time-gated B-OCDA setups. Hence the overall number of measurements is similar to that of previous setups.
Highlights
Stimulated Brillouin scattering (SBS) in optical fibers is a nonlinear interaction between counter-propagating pump and signal waves [1]
Amplification, is highly narrow-band: it requires that the difference between the optical frequencies of pump and signal should closely match the Brillouin frequency shift (BFS) of the fiber, which is on the order of 11 GHz for standard fiber at 1550 nm wavelength
Since the BFS varies with both temperature and mechanical strain, measurements of the local Brillouin gain spectrum (BGS) serve in the distributed monitoring of both quantities for over 25 years [2,3]
Summary
Stimulated Brillouin scattering (SBS) in optical fibers is a nonlinear interaction between counter-propagating pump and signal waves [1]. The most commonly-employed measurement protocol, known as Brillouin optical time-domain analysis (B-OTDA), relies on the amplification of a continuous signal wave by a pump pulse [2,3]. Considerable reduction in the necessary number of position scans has been achieved using hybrid B-OTDA / B-OCDA (or time-multiplexed B-OCDA) approaches [13,14,15,16,17,18] In these schemes the pump and signal are jointly and repeatedly phase-modulated by a comparatively short, high-rate code, and the amplitude of the pump wave is modulated by a single pulse. The interaction in each peak is restricted to the duration of the pulse, and multiple Brillouin gain events may be resolved by direct temporal analysis of the output signal wave [14,15,17,18].
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