# Experimental Research on In Situ Uniaxial Tensile Response of Silica-Based PLC Optical Splitters
## 3 Results and Discussion
### Observed Failure Mode
In Figure 5, the \(\Delta\)IL dropped sharply to about \(-15\) dB, indicating that the damage at the adhesively-bonded joint has accumulated to the point that the adhesive layer loses its load-bearing capacity and partially or completely fractured. The failure characteristics  
Figure 5: Typical \(\Delta\)IL-time curves at 1.55 \(\upmu\)m wavelength for eight output ports in a \(1\times 8\) PLC optical splitter module in displacement control mode. Figure 6: Typical \(\Delta\)IL-time curves at 1.55 \(\upmu\)m wavelength for eight output ports in a \(1\times 8\) PLC optical splitter module in load control mode. are similar to brittle fractures. The second steep drop of \(\Delta\Pi\)L, that is, the \(\Delta\Pi\)L of eight ports dropped sharply to about \(-60\) dB almost at the same time, indicating that the distance between the end face of the PLC chip and the end face of the fiber array are separated to completely lose the optical coupling ability. Figure 6 shows the \(\Delta\Pi\)L-time curves of a sample from stable standing to the beginning of uniaxial tensile loading to fracture in load-controlled mode. Similarly, the \(\Delta\Pi\)L-time curves can be divided into three stages: stable standing Stage I, uniaxial tensile loading Stage II, and post-fracture Stage III. As shown in Figure 6, in Stage II, the \(\Delta\Pi\)L of each port under uniaxial tensile loading has a similar trend with time, that is, it starts to slowly decrease relative to Stage I, and then the \(\Delta\Pi\)L of each port gradually decreases to approximately \(-10\) dB, then quickly dropped below \(-20\) dB. The \(\Delta\Pi\)L gradually decreases to about \(-10\) dB over time, which reflects the gradual damage process of the adhesive layer. The \(\Delta\Pi\)L of each port drops sharply below \(-20\) dB, indicating that the bonding interface has been completely separated. Comparing Figures 5 and 6, it can be observed that the two show significant differences in Stage II. Compared with Figure 5, the \(\Delta\Pi\)L of the eight ports in Figure 6 shows greater dispersion, and this dispersion becomes more significant with time. This can reflect that before the fracture, the internal damage of the specimen corresponding to Figure 6 was more serious than that of the specimen corresponding to Figure 5. It can be seen from Figures 5 and 6 that after \(\Delta\Pi<-5\) dB, the optical power transmission of each port becomes more unstable, and this phenomenon can be attributed to the generation and further expansion of more micro-cracks at the bonding interface. As shown in Figures 5 and 6, depending on the state of the sample, the \(\Delta\Pi\)L response in the \(\Delta\Pi\)L-time curve can be divided into three stages. In particular, in Stage II, the changing trend of the \(\Delta\Pi\)L-time curve can well reflect the deformation, damage, and debonding process of the adhesive layer under uniaxial tensile loading, revealing the use of measurable optical performance parameter \(\Pi\)L can realize the in situ monitoring of the damage evolution process of the PLC optical splitter. Based on the mechanical and optical performance response that was obtained from the experiment, through comparison and analysis, the corresponding relationship between the \(\Delta\Pi\)L and the damage degree of the PLC optical splitter is established, as shown in Table 2. In Table 2, the mechanical damage degree of the PLC optical splitter is divided into three categories: light damage, moderate damage, and severe damage, and the corresponding \(\Delta\Pi\)L values are (\(-5\) dB; \(0\) dB), (\(-15\) dB; \(-5\) dB), and (\(-\infty\), \(-15\) dB), respectively. It should be pointed out that based on this correspondence, it can be used to preliminarily evaluate the damage degree of the weak area of the PLC optical splitter. However, due to the lack of a stress/strain field description, the characterization of the specific damage in the weak area needs to be further studied.