Abstract
Correlation-domain analysis has enabled distributed measurements of Brillouin gain spectra along optical fibers with high spatial resolution, up to millimeter-scale. The method relies on the joint modulation of counter-propagating Brillouin pump and signal waves so that their complex envelopes are correlated in select positions only. Brillouin optical correlation-domain analysis was first proposed nearly 20 years ago based on frequency modulation of the two waves. This paper reviews two more recent variants of the concept. In the first, the Brillouin pump and signal waves are co-modulated by high-rate binary phase sequences. The scheme eliminates restricting trade-offs between the spatial resolution and the range of unambiguous measurements, and may also suppress noise due to residual Brillouin interactions outside the correlation peak. Sensor setups based on phase coding addressed 440,000 high-resolution points and showed potential for reaching over 2 million such points. The second approach relies on the amplified spontaneous emission of optical amplifiers, rather than the modulation of an optical carrier, as the source of Brillouin pump and signal waves. Noise-based correlation-domain analysis reaches sub-millimeter spatial resolution. The application of both techniques to tapered micro-fibers and planar waveguides is addressed as well.
Highlights
Distributed analysis of Brillouin gain spectra along optical fibers was first proposed as a means for temperature and strain sensing nearly 30 years ago [1,2]
The first and most widely employed architectures of stimulated Brillouin scattering (SBS) sensors are based on time-domain analysis: a Brillouin pump pulse is launched from one end of the fiber, and the amplification of a counter-propagating Brillouin signal wave is being monitored as a function of time [1,2]
This review addressed two variants of the basic concept, which are meant to increase the flexibility in the was first proposed 20 years ago to provide random-access measurements of strain synthesis of the correlation function
Summary
Distributed analysis of Brillouin gain spectra along optical fibers was first proposed as a means for temperature and strain sensing nearly 30 years ago [1,2]. These include the employment of carefully constructed phase codes for stronger suppression of the off-peak SBS interactions [21]; the overlay of pump pulses for addressing a large number of resolution points in a single trace [20,22]; use of double-pulse acquisition [23]; elimination of frequency scanning through transient analysis of Brillouin gain [24]; and the combination of amplitude codes alongside phase sequences in the same protocol [19,25].
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