## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Application Trials of BOCDA  
Besides the studies on the basic principle and the performance improvement schemes [68; 69], application trials have also been performed for the BOCDA systems.  
Utilizing a centimeter-order spatial resolution, the strain distribution measurement on a surface of a 15-cm diameter pipe was successfully demonstrated, with just one-cm spatial resolution [69]. These trial experiments were done considering a BOCDA application to urban gas pipeline monitoring, which is set under the ground. The measured strain distribution agreed well with a theoretical one [69].  
A tiny crack caused on a surface of a steel-reinforced concrete specimen was detected with the centimeter-order spatial resolution of a BOCDA system [70]. Even when the clock width was around 20 \(\upmu\)m, the crack was recognized through the distributed measurement of a tensile strain with around one-cm spatial resolution. If the resolution is around one meter, the measured strain is not a tensile one, but rather a compressive one. Then, a tiny crack cannot be detected with low spatial resolution systems.  
Utilizing the random accessibility of BOCDA, dynamic strain measurement was performed at multiple points in a tall building model under an earthquake waveform vibration [71]. Even when the strain change between the situations before and after the earthquake vibration was small, the dynamic strain measurement with a sampling rate of 10 samples/s for one point told us that the material had a large strain applied beyond the yielding strain amount several times. This means that the building was almost destroyed by the earthquake waveform vibration.  
In the structural health monitoring (SHM) for aircraft wings and bodies, a strain-distributed measurement along the long-length fiber nerve set within the aircraft full body is required. Besides such distributed measurements, dynamic strain measurements at specific selected points are also required. The specific points include, for example, portions around the wheels in landing situations, and connection portions between the wings and the body in flight situations. The random accessibility of the BOCDA technique is fit to such requests. Then, we have been conducting collaboration studies with an aircraft maker on the SHM application of BOCDA systems to aircraft bodies and wings. We have already fabricated several BOCDA prototype models, and performed in-flight tests of the models [72; 73; 74]. Using a business jet plain, in which the fiber nerve was set on the body material, a strain distribution change between the in-flight and on-ground situation has been successfully measured. The random accessibility has given us the data for simultaneous dynamic strain measurements at arbitrarily selected multiple points along the fiber nerve under the acrobatic flight conditions.  
In a civil engineering company, BOCDA systems have already been used to monitor the strain distribution along large real structures, such as an asphalt-faced slope at the bottom of a man-made large pond. The crack detection by the fiber nerve with a centimeter spatial resolution was also performed in a pre-stressed steel-reinforced concrete structure, which is used for a pedestrian deck in practice.  
Utilizing the high spatial resolution, we have applied the BOCDA technique to evaluate photonic-integrated circuits. A silica-based planar lightwave circuit (PLC) that has a total waveguide length of 46.8 cm has been evaluated by measuring the BFS distribution with a 5.9-mm spatial resolution [75]. The BFS shift distribution, which corresponds to the waveguide route shape, has successfully been measured [75]. Recently, a BOCDA setup was used to evaluate a silicon-chalcogenide waveguide at the University of Sydney [76].