###### Abstract
Ultrasonic guided waves (UGWs) are especially suitable for railway systems [18; 19; 20]. UGWs can transmit for a long distance in the longitudinal direction of the rail web. Once the UGW signals are obtained, further analysis of the wave reflection, mode conversion, and energy loss during the transmission can be conducted accordingly [1; 9; 18], serving as the damage indices to determine whether defects exist. For example, [PERSON] et al. [1] analyzed the energy attenuation and waveform distortion resulting from the existence of surface cracks, indicating the feasibility and prospects of the mass application of guided wave-based defect detection methods. However, the performance of UGW monitoring is closely related to the utilization of different types of sensors. AE sensors are generally sensitive to ultrasonic waves, and thus can be utilized as precise transducers. Most AE sensors are lead-zirconate-titanate (PZT) transducers, and the working principle for such sensors is basically to transform the deformation into electric signals via piezoelectric materials. The main drawbacks of these piezoelectric AE sensors are that they are not yet verified to work efficiently on railway sites where strong electromagnetic interference (EMI) should be considered. Furthermore, they require stabilizing electricity power supplies to be functional on-site, let alone the  
Figure 1: On-site photos taken on the East Rail Line: (**a**) crack on the rail near Fo Tan Station in 2017; (**b**) crack on the rail near Hung Hom Station in 2019.  
cost of AE sensors is very likely to be unacceptable for mass transit systems. Last, long-distance electric wires can be fragile and redundant in complex rail systems. Because of the above reasons, optical fiber-based sensing technologies have been proposed to replace the traditional PZTs as transducers to be installed on rail tracks [6; 7; 8; 21; 22; 23]. The non-electric working principle ensures that the sensors can be immune to EMI. Moreover, the optical fiber can last for several kilometers, enabling long-distance monitoring. The cost for bare fibers is considerably lower than most electronic transducers. The high-temperature resistance and the multiplexing feature of optical fibers make them the better choice for on-site installation. Although optical fiber-based sensors are suitable for rail monitoring, it is still challenging to demodulate the optical signals caused by ultrasonic vibrations. The normal dynamic interrogators can achieve the sampling rate of 2 kHz, which is far from adequate considering the dominant frequency of most ultrasonic waves in solid materials exceeds 20 kHz. To detect minor defects UGWs with shorter wavelengths are required, meaning the actuated frequency may exceed 200 kHz. Therefore, fiber Bragg grating (FBG) sensors have been studied and utilized to monitor ultrasonic signals using a laser source to demodulate the optical spectrum. An FBG would only reflect the light with a specific wavelength (referred to as the Bragg wavelength) so that the change in that wavelength would indicate the effect of the external force or temperature. The specific methodology of using FBG to receive UGWs will be introduced in Section 2. As for current related research, Lamb wave-based detection and source location were studied by [PERSON] et al. [24], followed by [PERSON] et al. [21] who investigated the directional sensitivity of FBGs to receive Lamb waves through experimental studies. An improved adhesive was proposed by [PERSON] et al. [25] by implementing laser ultrasonic visualization. [PERSON] et al. [26] utilized adaptive phased array imaging approach to detect damage using FBGs. Those researchers have mostly conducted UGW inspections in laboratories based on Lamb or Rayleigh waves. However, rail tracks are generally more complex, thus it can be extremely difficult to apply commonly used indices to find defects through UGW signals. In previous studies, [PERSON] et al.