## 2 Principles of New Measurement Methods  
### DSS-Based Deformation Measurement  
The DSS-based measurement system is a modern method to sense the deformation of civil infrastructures. The technique is a derivative of time domain reflectometry (TDR), which is widely used in the telecommunication industry to detect the location of a troubled/damaged point in a telecommunication cable. The TDR identifies the existence of the trouble/damaged points, because the existence of the reflected wave indicates the existence of a partial or complete discontinuity in the cable, as shown in Figure 1.  
When the technique is used with the travel time (distance) ranging method, it can locate the position of the defect (distance from the measure). Then, the technique essentially serves the role of a radar. When the technique is applied to optical laser light and fiber optic cable, it is called optical time domain reflectometry (OTDR). Fiber optic cables have additional characteristics such as Brillouin scatter and Ramen scatter. Brillouin scatter is the scatter of laser light created by the strain caused in the cable, while the Ramen scatter is the scatter of laser light created by the temperature anomaly caused in the cable. Therefore, combining radar-like distance ranging capability, strain detecting capability, and temperature detecting capability, the DSS system can be used for monitoring the deformation and temperature in structures with a high spatial resolution. Typically, the frequency shift due to Brillouin scatter is in the 9 to 11 GHz range, while that, due to the Raman scatter, is higher, reaching typically up to the 13 GHz range ([PERSON] [14]). Therefore, it seems that the deformation and temperature may be obtained without mutual interference. However, the DSS system is not faultless, as addressed below. It is noted that the temperature fluctuation may cause thermal strain in the cable and cause the additional Brillouin frequency shift. [PERSON] et al. [15] reported the effect to be approximately 2.03 \(\mu\varepsilon/^{\circ}\)C temperature-driven strain shift or 1.03 MHz/\({}^{\circ}\)C frequency  
Figure 1: Time domain reflectometry (TDR) (Revised plot based on [PERSON] [13]). shift for SiO\({}_{2}\) core. However, this magnitude can be slightly different depending on the properties and thickness of core and cladding materials, as reported by [PERSON] et al. [16] and [PERSON] et al. [17]. Therefore, it is logical to presume that a low-level strain (10 to 20 \(\mu\epsilon\)) measured from OTDR may not always indicate the mechanical deformation or strain of a target structures. It could be just the result of the temperature fluctuation. In addition, the strain-dependent Brillion frequency shift is not just by the axial strain. It can be affected by the shear or twisting strains ([PERSON] et al. [18]) because it is affected by any mechanism causing changes in the lattice structure of a material ([PERSON] et al. [19]). Therefore, a careless handling (e.g., unwinding) of the fiber optic cable (without utilizing an unwinding roller) may cause an additional frequency shift, requiring attention from the installers/operators until quantitative research clarifies the twisting effect. To further facilitate the separation of mechanical strain and thermal strain, a cable with tight buffered wire and loose tubed wire, as shown in Figure 2. This cable system is also used to obtain the mechanical strain by subtracting the thermal strain in the strain relief loose tube cable from the mechanical + thermal strain in the tight buffered cable. However, accurate separation of mechanical strain and thermal strain is still a challenging task, as the cable in the loose tube has a vinyl or polymer jacket that may interfere with the thermal stain in the fiber optic cable. Therefore, further research is needed to accurately separate these two different strains. Further details of the OTDR-based DSS system are found in a study by [PERSON] et al. [20].