Stimulated Brillouin Scattering (SBS) is a non-linear effect which can couple between two optical waves along standard optical fibers. In SBS, a relatively intense pump wave interacts with a counter-propagating, typically weaker signal wave, which is detuned in frequency. Effective coupling, however, requires that the difference between the two optical frequencies should closely match a particular, fiber-dependent value known as the Brillouin frequency shift n_B ~ 11 GHz. The value of n_B varies with both temperature and mechanical strain. Hence, a mapping of the local Brillouin gain spectrum along standard fibers is being used in distributed sensing of both quantities for 25 years. The most widely employed configuration for such measurements is known as Brillouin optical time domain analysis (B-OTDA), in which pump pulses are used to amplify continuous-wave (CW) signals and the output signal power is monitored as a function of time. The measurement range of B-OTDAs could reach 100 km. However, the duration of pulses in the fundamental B-OTDA scheme is restricted to the acoustic lifetime t ~ 5 ns or longer, which corresponds to a spatial resolution limitation on the order of 1 m.

The strength of the Brillouin interaction at a given fiber location is closely related to the inner product between the complex envelopes of the pump and signal waves. The spatial pattern of the interaction strength is therefore directly associated with the temporal cross-correlation between the envelopes of the counter-propagating waves. Starting in the late 90's, the modulation of the two waves has been proposed, so that their envelopes are correlated at discrete points of interest only, referred to as correlation peaks. The technique came to be known as Brillouin optical correlation domain analysis, or B-OCDA. While providing mm-scale resolution, initial B-OCDA was restricted to an unambiguous measurement range of a few hundreds of resolution points only, limited by the separation between neighboring periodic peaks. The measurement range was since extended using more elaborate frequency modulation profiles.

Over the last two years, we have employed a joint phase modulation of the SBS pump and signal waves by a common, high-rate pseudo-random bit sequence (PRBS). The modulation effectively confines the SBS interaction to correlation peaks whose spatial extent corresponds to that of a single coding symbol. In using phase modulation at a rate of 12 Gbit/s, for example, a spatial resolution of 9 mm had been achieved. At the same time, the separation between neighboring peaks is governed by the length of the code, which can be arbitrarily long. PRBS phase modulation therefore effectively decouples between range and resolution in B-OCDA. In a recent work

Recently, we proposed and demonstrated a combined B-OTDA / B-OCDA technique, which reduces the acquisition time of the measurements by over two orders of magnitude. In this method, an amplitude-pulsed pump and a CW signal are both phase modulated by a joint sequence, following the B-OCDA principle. However, two significant advances are introduced: First, a short, perfect Golomb code is used in the phase modulation of the pump and signal waves instead of a long PRBS. The special correlation properties of this sequence help reduce the noise due to residual, off-peak Brillouin interactions. Second, due to the short length of the code, a large number of correlation peaks are generated during the propagation of the pump wave pulse. With careful choice of the pump pulse duration with respect to the Golomb code period and the Brillouin lifetime, the SBS amplification which takes place at the different peaks can be temporally resolved in measurements of the output signal power, much like in a B-OTDA. Using this method, the number of scans per choice of optical frequencies that is necessary for mapping the Brillouin gain spectrum over the entire fiber equals the length of the Golomb code only, which is 127 bits-long in our case. This number of scans is orders-of-magnitude smaller than that of an equivalent PRBS-coded B-OCDA, and it does not increase with the number of resolution points. The experimental mapping of the Brillouin gain spectrum over a 400 m-long fiber with 2 cm resolution, or 20,000 resolution points, is reported below. A 5 cm-long hot spot is properly recognized and localized in the measurements. The technique might provide the breakthrough that is necessary to make high-resolution, long-range Brillouin-sensing more practical.