# High Resolution Brillouin Sensing of Micro-Scale Structures
## 5 Sensing of Micro-Scale Structures
### Distributed Backward Brillouin Measurement  
The longitudinal map of the opto-acoustic responses of a PCF based on BOTDA was reported by [PERSON] et al. [106], where the BFS and the full-width half-maximum (FWHM) of the Brillouin spectrum along the PCF was resolved with a spatial resolution of 2 m. Figure 9a shows the mapping of the Brillouin responses based on BEDS reported by [PERSON] et al. [38], where the effect of the strain and the waveguide irregularities on the local backward SBS responses of the PCF with the spatial resolution of 30 cm was studied. [PERSON] et al. [100] mapped the uniformity of a micro-fibre by capturing the distributed backward SBS responses using phase-coded BOCDA measurement. Figure 9b shows the local Brillouin spectrum along the 13 cm-long microwire with the spatial resolution of 9 mm. Later, [PERSON] et al. demonstrated a local excitation and capturing of the surface acoustic waves along a microwire [50]. Their setup allows for the local excitation and monitoring of the core diameter by analysing the surface opto-acoustic responses along the waveguide based on the phase-coded BOCDA technique with a spatial resolution of 5.2 cm. Distinct acoustic resonances along the tapered fibre are shown in Figure 10.  
Figure 10: Brillouin spectrum of the tapered fibre using phase-coded BOCDA measurement at different frequency ranges (**a–f**) [50] (reprinted with permission from [50], OSA). Figure 9: Longitudinal map of (**a**) a PCF using Brillouin echo distributed sensing (BEDS) techniques with a spatial resolution of 30 cm [38] (reprinted with permission from [38], OSA) and (**b**) a microfibre based on phase-coded BOCDA [100] (reprinted with permission from [100], OSA). The high spatial resolution offered by BOCDA also enables monitoring of opto-acoustic responses along the cm- and mm-scale on-chip waveguides. [PERSON] et al. [46] studied the effect of bends on the backward SBS responses in a silica planar waveguide using BOCDA. In this experiment, a frequency-modulated BOCDA measurement with a spatial resolution of 5.9 mm was demonstrated. In Figure 11a, the fluctuations in the BFS determine the positions of the bends along the waveguide. Following the introduction of the ASE-based BOCDA measurement by [47], which offers simplicity and high spatial resolution, the local backward SBS responses of a short chalcogenide \(As_{2}S_{3}\) photonic waveguide were measured as shown in Figure 11b [48]. This experiment achieved 800-\(\upmu\)m spatial resolution thanks to the improved SNR compared to the optical fibre. The enhancement in the SNR was the result of a phase-sensitive detection technique, the large gain coefficient, and small opto-acoustic interaction area in the \(As_{2}S_{3}\) waveguide. In order to confirm the on-chip spatial resolution, a 6-mm waveguide with a 2 mm-long longitudinal feature in the form of a width variation was designed in a hybrid silicon-chalcogenide platform [49]. The Brillouin back-scattering from different sections of this waveguide was detected, as shown in Figure 12. A follow up of this work demonstrated a jump in the detection capability by detecting a 200-\(\upmu\)m longitudinal feature in the form of a width variation on a chip by further optimizing the filtering between the back-reflected pump and the amplified probe. Figure 13 shows the detection of 1-mm, 500-\(\upmu\)m, and 200-\(\upmu\)m features on hybrid silicon-chalcogenide waveguides [97]. In this experiment, both pump and probe waves are driven from a polarized ASE source with a bandwidth of 80 GHz centred at a 1550-nm wavelength. More detail about the experimental setup can be found in [97]. Figure 11: Mapping of (**a**) a silica planar waveguide with a spatial resolution of 5.9 mm using frequency-modulated BOCDA [46] (reprinted with permission from [46], SPIE) and (**b**) a chalcogenide photonic waveguide with a spatial resolution of 2.5 mm using ASE-based BOCDA measurement [48] (reprinted from [48] under a Creative Commons Attribution 4.0 International Public License).