An FBG has periodical variation in the refractive index within the core of an optical fiber and acts as a narrowband reflection filter. The central wavelength of light reflected from an FBG is called the Bragg wavelength λ_B and is given by the following equation [11]: (1) λ B = 2 n Λwhere n and Λ are the effective refractive index of the fiber core and the grating period, respectively. Under a constant temperature condition, the relative shift in the Bragg wavelength Δλ_B is in direct proportion to applied strain ε along the fiber axis [11]. The shift in the Bragg wavelength is positive when the FBG expands. Conversely, the Bragg wavelength shifts to negative when the FBG shrinks.

A tunable laser source was used in the present study for guided wave detection. As shown in Figure 1, a PXI-6115 (National Instruments Co., USA) simultaneously functions as an incident wave generator and wave acquisition device. The generated incident wave was amplified by a Piezo-Amplifier (M-2643, MESS-TEK Co., Japan) and was emitted into the specimen by a PRYY-0929 PZT actuator (Physik Instrumente GmbH & Co. KG, Germany) to excite Lamb guided waves. FBG receives guided waves as the strain change of the laminate depending on time and, therefore, center the wavelength of the reflected light from the FBG changes. The wavelength shift is detected using the high speed optical wavelength interrogation system. Finally, the PXI-6115 acquires guided waves filtered by the filter FV-628B (NF Corporation, Japan). FBGs (Fujikura Ltd., Japan, gauge length: 3 mm, wavelength: circa 1,550 nm, full width at half maximum (FWHM): circa 0.5 nm, and reflectivity: > 90%) were used in the present study.

In the ultrasonic detection system with a tunable laser source, the intensity of the light reflected from FBG sensor directly corresponds to ultrasonic response of host structures. A schematic of the high-speed optical wavelength interrogation system, based on tunable laser source, is shown in Figure 2(a). The laser emission wavelength of ‘Tunable Laser’ in Figure 2(a) (Agilent 8164A, Agilent Technologies, USA) is set to λ_out where the reflectivity of the sensor at strain free is reduced by half as shown in Figure 2(b). In this system the optical circulator, photodetector and the low noise amplifier are YC-1100-155 (FDK Corp., Japan), PDA10CS (Thorlabs, USA), and SA-230F5 (NF Corp., Japan), respectively. When FBG expands, the Bragg wavelength shifts to a longer wavelength [‘FBG-Shift’-marked curve in Figure 2(b)] and hence the reflectivity of FBG at the lasing wavelength decreases, and vice versa. In the experiments, gain of the photodetector and optical power of the laser source were 4.75 × 10³V/A ± 2% (10dB setting) and 2.375 mW, respectively.

The principle of the FOD sensor is based on the Doppler effect of light wave transmission in optical fiber. Consider the light wave, with frequency f₀, transmission in an optical fiber with refractive index n and length L. When an accident, such as strain rate in host structure of the optical fiber, causes the length of the fiber to change from L to L + dL in an infinitesimal time dt, the Doppler frequency shift f_D can be obtained by: (2) f D = − n λ 0 ⋅ d L d twhere λ₀ is the light wavelength in the vacuum, and λ₀/n is the light wavelength in the optical fiber.

In the previous studies [12,22], three kinds of FOD sensors with different shapes were proposed. The common shape of these FOD sensors is their circular part, as depicted in Figure 3(a), where points A and B denote the light source and observer, respectively. The theoretical Doppler frequency shift f_D of the circular loop FOD sensor is obtained by [22]: (3) f D = − π R n e q 2 λ 0 ( ε ˙ x + ε ˙ y )where ε̇_x and ε̇_y are the strain rates on x- and y- directions, respectively, R and D are the radius and diameter of the circular part of the FOD sensor, respectively, n_eq is the equivalent refractive index of the waveguide and λ₀/n_eq is the equivalent length of light wave in the waveguide. The Doppler frequency shift f_D (or sensitivity of FOD sensor) is directly proportional to the sensing length L = πD of the FOD sensor. In this study, a spiral shape was selected for the FOD sensor to make it easy to glue it on the surface of CFRP laminate for guided wave detection. A sketch and picture of the spiral FOD sensor are shown in Figures 3(b) and (c), respectively. Heat-resistant optical fiber HEATOP^® (Totoku Electric Co., Ltd., Japan) was used to make the spiral FOD sensor in the present study. It is evident the spiral FOD sensor is omnidirectional in ultrasonic detection, which will be discussed in detail in Section 3. Moreover, the FOD sensor can be made manually using only optical fiber and therefore the cost of FOD sensor is considerably smaller than that of FBG.

A laser Doppler velocimeter (LDV) was used to detect the frequency shift of the FOD sensor, in which extension/compression of the optical fiber was measured. The Doppler frequency shift, f_D, between reflected light from the fiber end and the reference plane, relates to the relative displacement rate (strain rate) ν = dL/dt. During the detection process, frequency of the reflected laser beam changes from f₀ to f₀ + f_D. The sensitivity depends on the length of the optical fiber bonded on the specimen however the other part of the optical fiber keeps stress free. Setup of the ultrasonic detection system using LDV and the spiral FOD sensor is schematically shown in Figure 4, in which the light source is He-Ne laser (output power: 1 mW, wavelength λ₀: 632.8 nm) and heterodyne interference technique is applied to the measurement. An acoustooptical modulator (AOM) changes the frequency of the reference light source from f₀ to f₀ + f_M (f_M = 80MHz) in order to produce beating signals with frequency of f₀ + f_M. A frequency-voltage convertor in the detector is used to offer voltage output for direct ultrasonic acquisition, detailed in Figure 4. Outer diameter, D, and inner diameter, d, of the spiral FOD sensor used in this study are 21.2 mm and 8.0 mm, respectively. All the FOD sensors used in this study were made by the authors at the University of Tokyo, Japan [Figure 3(c)].

The spiral FOD-based ultrasonic detection system is the same as the FBG-based system except that the high-speed optical wavelength interrogation system in the FBG-based system is substituted by the laser Doppler velocimeter in Figure 4. Moreover, the filter in the FBG-based system (as shown in Figure 1) is necessary because the signal-to-noise ratio (SNR) of acquired guided wave signals using FBG are relatively low [11] and, in contrast, was taken out in the FOD-based ultrasonic detection system [22], which will be discussed in detail in Section 3. Therefore, the cost of the FOD-based system is smaller than that of the FBG-based system without the filter and, moreover, the tunable laser source.

For illustration and without losing generality, quasi-isotropic CFRP laminates are used as host structures for fiber optic sensors-based guided wave and damage detection in the present study. The quasi-isotropic laminates were stacked in accordance with [45/0/-45/90]_2s using Pyrofil™ carbon fiber (TR30S, Mitsubishi Rayon Co., Ltd., Japan) with technical properties listed in Table 1. Laminates were fabricated with the dimension of L 500 mm × W 30 mm × TH 1.92 mm. Two types of CFRP laminates were applied in the present study, which are intact and delaminated laminates. Sketch and picture of the specimen are shown in Figures 5(a) and (b), respectively, with the locations of piezoelectric (PZT) actuator, fiber optic sensors and delaminations. In the experiments, the left and the right ends of the specimens were respectively fixed with length of circa 2 mm. For the delaminated CFRP laminates, single- and double-damage scenarios were introduced by inserting one and two thin Teflon^® films between the 8^th and the 9^th laminas of the laminate, respectively. The two delaminations are named D_L and D_R, respectively. For the single-damage laminate, only the D_L exists. On the other hand, both the D_L and D_R are present in the CFRP laminate with two delaminations. To ensure that the guided wave signals are acquired under the same excitation circumstance for the same specimen, experiments for one specimen were done under three procedures: 1) acquiring guided wave signal using FOD sensor; 2) removing the FOD sensor from the specimen and bonding FBG sensor at the same position as the FOD sensor; 3) acquiring guided wave signal using the FBG sensor.

In the literature, reflected/transmitted waves were usually used for guided wave-based damage detection [23-25]. In the case of composite structures, delamination could only reflect very faint energy [26] or the reflection could only happen when delamination was introduced in certain plies of the laminates [27]. For example, in the case of semi-infinite delaminated damage, the maximum reflected energy ratio of delamination was less than 0.012, which is relatively small and therefore complicates feature extraction and damage detection using the reflected wave because of noise and the inevitable dispersion. Moreover, when structure is damaged in at least two positions, the problem becomes decidedly more complex. So far relatively few researchers have addressed the multiple damage assessment for structures [28,29]. In the present study, transmitted guided wave is selected for the purpose of fiber optic sensors-based guided wave and damage detection.

As concluded above, both FBG and FOD sensors can be used for guided wave and damage detection in CFRP laminates. FOD sensor excels FBG sensor in damage detection because it is sensitive to the delamination-induced SH₀ guided wave that the FBG sensor cannot detect. Based on the principle of ultrasonic detection using FBG in Figure 2(b), it can be concluded that FBG has directivity in ultrasonic detection [5]. In contrast, according to Figure 3, it is evident that FOD sensor is omnidirectional [22] in ultrasonic detection because of its spiral shape and therefore can detect all the propagating guided wave modes in the CFRP laminates.

Moreover, signal-to-noise ratio (SNR) of acquired signal can also indicate performance of a kind of sensor. SNRs of guided wave signals acquired from the intact CFRP laminate using FBG and FOD sensors are listed in Table 2. It is clear that SNR of FBG-based original signal, 26.36, is almost half of that, 52.60, of FOD-based original signal, despite the fact that an extra filter was used in the FBG-based system (as shown in Figure 1). Further, SNR of the average of FBG-based 10 original signals is also much smaller than that of the FOD-based signals, and SNR of the average of FBG-based 60 original signals is even smaller than that of the FOD-based originally acquired signal, viz. 43.68 versus 52.60. Therefore, in comparison with the FBG sensor, the spiral FOD sensor can offer higher SNR in guided wave detection.

Despite the fact that the FOD sensor exceeds the FBG sensor in omnidirectional properties and ability to offer higher SNR of the acquired guided wave signals, the FBG sensor also has its own particular advantages such as multiplexing. Wavelength division multiplexing (WDM) is one of the major multiplexing arrangements. For illustration, Figure 11 shows a wavelength division multiplexing (WDM) scheme, such as is used with a series of FBGs, each written at a slightly different wavelength, with care being taken to avoid an overlap of the wavelength of one fully perturbed sensor with the spectral envelope of the next. In this way, WDM-based architecture can be the basis of systems where multiple channels may be used to create true multi-sensor systems [38]. In contrast, so far multiplexing technique for FOD sensor has not yet been invented.