# High Resolution Brillouin Sensing of Micro-Scale Structures
## 3 Distributed Brillouin Sensing Techniques  
In this section, we provide a brief overview of various Brillouin-based distributed sensing techniques. Brillouin optical time domain analysis (BOTDA) employs a pulsed pump and a cw probe to limit the SBS interaction length to the pump pulse duration. The first BOTDA demonstration in optical fibre using counter-propagating pulsed pump and cw probe waves was reported by [PERSON] et al. [72]. This approach has improved the signal to noise ratio (SNR) and the sensitivity in comparison to the Brillouin time domain reflectometry (BOTDR) method, which is based on spontaneous Brillouin scattering [73]. The first distributed temperature measurement using BOTDA was reported by [PERSON] et al., where the temperature distribution along a silica single-mode optical fibre with a spatial resolution of 100 m was measured [39]. This demonstration was followed by the works of [PERSON] et al. [40] reporting a distributed temperature measurement in an optical fibre with 5 m spatial resolution and [PERSON] et al. [41] measuring distributed strain and temperature with a stabilized probe using a single laser source. The spatial resolution of the BOTDA technique, however, is limited to 1 m. This is due to the fact that for pump pulses shorter than the the phonon lifetime (approximately 10 ns in silica fibre), the Brillouin spectrum experiences a broadening, which makes the measurement inaccurate [42]. Different techniques including dark pulses [74] and phase pulses, which were based on the initial work of [PERSON] et al. [75; 76], were introduced to overcome the spatial resolution limit of BOTDA [77; 78; 79]. These techniques are based on a sudden change in the phase pulse like Brillouin echo distributed sensing (BEDS) [53] or a sudden drop in amplitude of the pump wave (dark pulses), which affects the Brillouin response and can be detected by a fast detection scheme [53; 80]. Spatial resolutions of 5 cm and 2 cm were reported using phase pulse [81] and dark pulse [80], respectively. Another technique, called pre-pump Brillouin optical time domain analysis (PPP-BOTDA), is based on the long pump pulse to excite the phonon followed by a short pump pulse to enable high spatial resolution measurement, while the probe is in the form of a continuous wave, counter propagating the pump [82; 83; 84; 85]. Spatial resolution down to 2 cm was demonstrated using this technique [83]. Other distributed Brillouin measurement techniques include Brillouin optical frequency domain analysis (BOFDA) and Brillouin dynamic grating (BDG). BOFDA is based on the intensity modulation of the pump with a sinusoidal wave such that by sweeping the modulation frequency, a distributed SBS response can be obtained [86]. Although this technique does not have the spatial resolution limitations of the time domain and a spatial resolution of 3 cm was demonstrated using this technique, it requires heavy post-processing to recover the Brillouin gain spectrum, and therefore, the measurement time is relatively long for cm-long spatial resolutions [86]. BDG employs the refractive index grating generatedin one polarization to Bragg reflect the pump wave in the orthogonal polarization [87]. This technique could achieve sub-cm spatial resolution (5.5 mm) [88]; however, it requires polarization-maintaining fibre and more than one pump laser to realize SBS in two orthogonal directions. Although time domain, frequency domain, and dynamic gratings have been improved significantly in terms of spatial resolution and performance, the record spatial resolution in Brillouin distributed sensing is achieved based on a technique called Brillouin optical correlation domain analysis (BOCDA), which is described in detail in the next section. Figure 2 compares the spatial resolution evolution of the time domain techniques versus correlation domain techniques. The solid blue line in the plot shows the overall trend of the spatial resolution in time domain techniques, which has improved from 100 m to a few cm.