## 3 Comparative Study
### Experiment Introduction  
#### 3.1.1 The PZT/FBG Hybrid Sensing System  
The complete experimental setup is shown in Figure 4. A five-cycle sinusoidal tone of 250 kHz burst modulated by the Hanning window is generated (Figure 5) by the arbitrary waveform generator module of the computer-based central terminal instrument (PXI-5412, produced by National Instruments, Austin, TX, USA), which outputs digitalized signals to the power amplifier (HVA-400-A, produced by Ciprian, Grenoble, France). The signals are then amplified 200 times and the maximum voltage reaches 200 V. The amplified signals are sent to a PZT disc (diameter: 8 mm, thickness: 0.5 mm) to generate ultrasonic waves. The PZT is attached to the rail surface with adhesives.  
The above descriptions have concluded the actuation part of the system. As for the transducing and sensing part of the system, the main instruments involve the balanced photodetector, the FBG sensor, and the laser source (TLB-6700, produced by Newport, Irvine, CA, USA) and its controller. The laser controller ensures that the narrow-bandwidth light transmitted from the source is of the fixed wavelength and fixed emitting power. In this study, the laser wavelength is set to 1549.95 nm (3 dB position on the right-hand side of the FBG spectrum) and the output current is 40 mA. The laser is evenly split via the 50%/50% coupler (F-CPL, produced by Newport, Irvine, CA, USA), with half transmitting to the FBG and the other half to the balanced photodetector (2117-FC, produced by Newport, Irvine, CA, USA). The grating length of the FBG is 10 mm, which is approximately the wavelength of the UGWs, to obtain as many faultless waveforms as possible. Lastly, the received optical signals are converted into digital signals by the balanced photodetector and are then sent back to the data acquisition and oscilloscope modules of the computer-based instrument. The sampling frequency is 10 MHz in this experiment and the duration of each test is 1 ms.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Features** & **Bulk Wave Inspection** & **Guided Wave Inspection** \\ \hline Efficiency & Laborious and time-consuming & Fast and convenient \\ Accuracy & Point-by-point scan (accurate rectangular grid scan) & Global in nature (approximate line scan) \\ Reliability & Unreliable (can miss points) & Reliable (volumetric coverage) \\ Complexity & High-level training required for inspection & Minimal training \\ Distance & Fixed distance from reflector required & Any reasonable distance from reflector acceptable \\ Identification & The reflector must be accessible and seen & The reflector can be hidden \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison between bulk wave inspection and guided wave inspection [43].  
The whole procedure, including the excitation, laser emission, and data acquisition, is controlled by preprogrammed software based on LabVIEW, which is installed on the computer-based instrument. This program guarantees that the excitation signals and the laser power remain the same in separate tests.  
Figure 4: The experimental setup: (**a**) the schematic; (**b**) the laboratory setup.  
Figure 5: The excitation signals in (**a**) time domain; and (**b**) frequency domain.