Synthetic Spectrum Approach for Brillouin Optical Time-Domain Reflectometry

Abstract

We propose a novel method to improve the spatial resolution of Brillouin optical time-domain reflectometry (BOTDR), referred to as synthetic BOTDR (S-BOTDR), and experimentally verify the resolution improvements. Due to the uncertainty relation between position and frequency, the spatial resolution of a conventional BOTDR system has been limited to about one meter. In S-BOTDR, a synthetic spectrum is obtained by combining four Brillouin spectrums measured with different composite pump lights and different composite low-pass filters. We mathematically show that the resolution limit, in principle, for conventional BOTDR can be surpassed by S-BOTDR and experimentally prove that S-BOTDR attained a 10-cm spatial resolution. To the best of our knowledge, this has never been achieved or reported.

Performance improvement of synthetic Brillouin optical time-domain reflectometer employing Golay coding

“…Probe light f (j) (t) is injected into an optical fiber, and the Brillouin backscattered light is heterodyned with a reference light to yield heterodyned signal X (j) (t). In a previous paper [4], a Brillouin spectrum was obtained by synthesizing four spectrums of X (j) (t), j = 1, 2, 3, 4. However, it is simpler to synthesize cross spectrums instead of the spectrums, as was previously adopted [5].…”

“…3. The real part of cross spectrum Y 11 Y 22 [4] becomes the ideal Brillouin spectrum if the pulse width of the short pulse is set to the desired spatial resolution and that of the long pulse is set to be sufficiently longer than the phonon lifetime. The cross spectrum for probe light f (j) is written as…”

“…Unfortunately, its spatial resolution has been limited to 1 meter [2], which is insufficient for many applications. To overcome this spatial resolution limit, the double-pulse BOTDR (DP-BOTDR) [3], the synthetic BOTDR (S-BOTDR) [4], and the phase shift pulse BOTDR (PSP-BOTDR) [5] have been proposed. Instead of a single pulse of a conventional BOTDR, the DP-BOTDR uses two short pulses, and both S-BOTDR and PSP-BOTDR use composite pulses composed of short and long pulses.…”

Improving performance of phase shift pulse BOTDR

“…More recently, it has been realized that we can use coherence characteristics of spontaneous Brillouin scatterings to achieve the high spatial resolution BOTDRs [8,9,10]; then measurements are possible by accessing just one end of the fiber while BOTDAs need two-end access. Double pulse BOTDR (DP-BOTDR) [8] is the first BOTDR that realized a spatial resolution of 20 cm, and needs no major changes in the configuration of the conventional BOTDR.…”

“…However, the spectrum of the measured Brillouin backscattered light has a plurality of peaks with slightly different intensities, so that a high signal-to-noise ratio is required to accurately measure the BFS. Another high spatial resolution BOTDR is synthetic BOTDR (S-BOTDR) [9]. S-BOTDR uses four types of optical pulse probe generated by superimposing a short-duration and large-amplitude pulse and a long-duration and small-amplitude pulse with quadrature phase shift keying modulation.…”