Fiber Bragg grating (FBG) is advantageous owing to its compact size, light weight, anti-electromagnetic interference, high-temperature resistance, and series multiplexing, and it is increasingly adopted in several fields such as aerospace, petrochemical industry, national defense, and military. The measurement and analysis of FBG reflection or transmission spectra yield the magnitude of the physical parameter to be measured (Fig.1). Extracting characteristic information from FBG spectral signals, i.e. FBG signal demodulation, is the basis for FBG sensing measurement applications. Most of the current FBG sensing-based systems measure at frequency below 1 kHz and are mainly used in the measurement of slowly changing physical quantities such as temperature and strain. For scenarios requiring high-speed dynamic measurements, such as high-speed vibration and explosive shock, the signal demodulation scheme based on FBG sensing must satisfy the measurement speed requirements for effective application. Depending on the measurement parameters, the FBG signal high-speed demodulation methods can be divided into four categories: spectral, optical-intensity, phase, and microwave spectrum analyses (Fig.2).

The spectral analysis method (Section 3) relies on the full FBG spectrum for measurement analysis, which can be achieved via dispersive and scanning spectroscopies. Spectral measurement based on the principle of dispersion (Section 3.1) can be divided into spatial- and time-dispersion methods. Spectral measurement using the principle of spatial dispersion is the main method for FBG demodulation. The test speed depends on the scanning speed of the spectral acquisition device such as the charged-couple device (CCD). Spectral measurement systems using the principle of time dispersion require ultra-high-speed acquisition equipment. To obtain sufficient time delay, the length of the dispersion element is quite long. Hence, reducing the size of the dispersion element and guaranteeing a sufficient amount of dispersion is the key to its application. Using tunable light sources such as distributed Bragg reflector (DBR), distributed-feedback laser (DFB), Fourier domain mode-locked laser (FDML), and high-speed detectors can also obtain FBG characteristic spectrum, i.e., scanning spectroscopy (Section 3.2). The scanning speed of the tunable light source is the main factor that determines the testing speed of the system. Light intensity analysis (Section 4) can be achieved by single- (Section 4.1) or double-edge filtering (Section 4.2). The corresponding measurement system has no mechanical structure, and the measurement speed can easily reach the megahertz level. Its measurement range, sensitivity, and linearity are determined by the performance of the filter device. Presently, the optical filter components available are long-period fiber grating (LPFG), array waveguide grating (AWG), Fabry-Perot (FP) cavity, etc. The phase analysis method (Section 5) demodulates the interference signal by measuring its phase. Common systems include the Mach-Zehnder (Section 5.1), Michelson (Section 5.2), and Sagnac (Section 5.3) interference structures. Although precision is high, the demodulation range is small, and demodulation speed of several hundred kilohertz can be achieved. Microwave spectrum analysis (Section 6) converts optical domain signals into microwave frequency domain for signal analysis. The system structure of this method is very flexible, the sensitivity is high, and the speed generally depends on the back-end data acquisition system.Each of these demodulation methods has its own advantages and disadvantages. The actual application needs to be selected according to different requirements. For the measurement of vibration and dynamic strain of large structures such as bridges and rails, the application environment is relatively stable, and the measurement speed and range requirements may be relatively low. Multi-point monitoring with distributed sensing can improve efficiency and cost-effectiveness. Long-term use has high requirements for measurement accuracy, system stability, and actual data output. Scanning spectrum measurement methods or microwave spectrum analysis methods are optimal choices. However, for the instantaneous monitoring of the shock power of the explosion shock wave and the target load response, the single-point and high-speed acquisition methods are more suitable. Such transient measurements require high speed and range, the measurement time is short and signal can be collected first and then processed. Accordingly, the light intensity analysis method can be adopted. We can maximize the unique advantages of various demodulation methods and achieve efficient and accurate measurements only by selecting different high-speed demodulation methods according to different scenarios.

The spatial dispersion method is relatively mature. The Wasatch Cobra-S 800 spectrometer has a sampling rate of 250 kHz (Table 1). The measurement frequency of the systems based on the principle of time dispersion can up to 264 MHz (Table 1). In the scanning spectrometry measurement method, Tatsuya Yamaguchi et al. achieved a measurement frequency of 202.8 kHz driven by a conventional FDML laser with a scanning frequency of 50.7 kHz by processing the light source (Table 1). In the light intensity analysis method, Ding Z C et al. of Beijing Jiaotong University employed a cross-Sagnac loop as an edge filter to achieve a demodulation frequency of 200 kHz (Table 2). In the phase analysis method, Oton C et al. of the University of Florence achieved a demodulation speed of 100 kHz using an electro-optical modulator as a tunable retarder in the Sagnac loop (Table 3). In the microwave spectrum analysis method, Zhou Lei et al. used a cross-scan period to form a beat frequency, doubling the measurement frequency to 40 kHz. It has also been reported that the acquisition speed of this method can reach the megahertz level (Table 4).

This paper summarizes the main advantages and disadvantages of each method, the adaptation scenarios, and whether multi-grating measurement can be performed (Table 5); in addition, it sorts out some scenarios involving high-speed dynamic measurements and the corresponding high-speed demodulation methods (Fig.24), and looks forward to the future research direction of high-speed FBG demodulation methods. First, it is important to continue to improve the FBG signal demodulation speed, which requires further improvement of the speeds of signal acquisition and processing. The methods involve increasing the rate level of light sources, line array detectors, and other devices, and optimizing the demodulation algorithms. Second, it is important to improve the accuracy and demodulation range. The spectral range and resolution of the light source need to be increased. In addition, it is important to increase the FBG demodulation capacity. While improving the speed of spectral measurement, further expanding its spectral bandwidth is the basis for the development of quasi-distributed large-capacity FBG high-speed demodulation technology.