This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2421350, IEEE Photonics Technology Letters
Fiber Sensor Based on Interferometer and Bragg Grating for Multi-parameter Detection Qi Wu, Yoji Okabe, and Jianghai Wo Abstract—An integrated in-fiber Mach–Zehnder interferometer (MZI) and fiber Bragg grating (FBG) sensor that can simultaneously detect localized temperature, strain, and ultrasonics is proposed and demonstrated. Using a 60-mm single-mode fiber with a 12-mm FBG spliced to two 3-mm multi-mode fibers and lead-in and lead-out single-mode fibers, the sensor can discriminate temperature and strain from the responses of the MZI and the FBG to the two static parameters. Ultrasonic signals are directly recorded as high-frequency voltage vibration by demodulation of Bragg wavelength shift using a corresponding sensing system. The overall system shows high practical potential for use in nondestructive testing. Index Terms—Gratings, interferometry, strain, temperature, acoustic detectors, nondestructive testing.
I. INTRODUCTION
S
tatic and dynamic parameters are both very important in the field of non-destructive testing. While strain, temperature, and ultrasonic signals can usually be detected using a strain gage, thermometer, and lead-zirconate-titanate (PZT) sensor, respectively, the ease and precision of detection can be improved by developing multifunctional sensors that can simultaneously detect all three parameters at a given position. Many different types of optical fiber sensor have been developed as alternative means of detecting static parameters (e.g., temperature and strain). For example, the Bragg wavelength shift of a fiber Bragg grating (FBG) sensor can be used to monitor either temperature or strain with high sensitivity [1, 2], although a single FBG sensor cannot simultaneously discriminate these two parameters. To do this, an FBG can be integrated with an optical fiber sensor such as a long-period fiber grating [3]. In this technique, the two types of sensor are cascaded, which means that the results are not detectable at a single position. In another method, a Mach– Zehnder interferometer (MZI) is used to detect physical parameters by monitoring the phase change between the sensing and reference light paths. Recently, researchers have proposed an in-fiber MZI with a simple structure and low cost based on multi-modal interference [4, 5]. Dynamic parameters, such as ultrasonic signals with high frequency and low energy, can also be detected using an FBG Q. Wu and Y. Okabe are with the University of Tokyo, Tokyo, 153-8505 Japan (e-mail:
[email protected];
[email protected]). J. Wo, is with Huazhong University of Science and Technology, Wuhan, 430074 China (e-mail:
[email protected]).
or an in-fiber interferometer [6, 7]. However, the simple static detection method of monitoring the Bragg wavelength shift using an optical spectrum analyzer is unsuited to such applications owing to low functional speeds, making it necessary to use a demodulation system with high sensitivity and speed. Regardless of whether an FBG or interferometer is employed, it is usually necessary to use a feedback controller to stabilize the demodulation system to the influence of the static parameters; this in turn degrades temperature and strain discrimination [8, 9]. For the reasons detailed above, it is difficult to detect temperature, strain, and ultrasonic signals at a single position using a single sensor. Although Rao et al. and Kang et al. proposed two different multifunctional sensors [10, 11] based on a Fabry–Perot interferometer and FBG, both had complex demodulation systems and their reported bandwidths of 2-kHz were not wide enough to detect ultrasonics [11]. In this paper, we propose a novel sensor and corresponding demodulation system that can detect these three parameters at a given position simultaneously and with high accuracy and speed. II. EXPERIMENT The proposed sensor is based on an in-fiber MZI and FBG, as shown in Fig. 1. To fabricate it, a 12-mm FBG is written onto a single-mode fiber (SMF), which is then cut to 60 mm and spliced to two 3-mm multi-mode fibers (MMFs) before being spliced to lead-in and lead-out SMFs. The SMFs and MMFs have core/cladding diameters of 10/125 μm and 105/125 μm, respectively. This integrated design provides the sensor with the characteristics of both interferometer and FBG. In the first section of the sensor (Fig. 1(a)), cladding modes (or a mode) are generated by the mismatch in core diameters between the SMF and MMF. Both the core and cladding modes then propagate in the 60-mm sensing SMF. The FBG reflects back modes within its reflection spectrum while maintaining propagation of the other modes, as shown in Fig. 1(b). The FBG has a relatively large impact on the core mode but little impact on the cladding modes [12]. The core mode reflected by the FBG will also transfer into partial cladding modes; however, this reflected light will cause very little interference and the cladding modes will attenuate or leak out from the lead-in SMF after travelling a certain distance. These phenomena cause the sensor reflectivity to have a spectrum shaped similarly to — but much lower in magnitude than — that of a single FBG. When the remaining modes reach the second section (Fig. 1(c)), the differing effective optical path lengths of the modes and the mismatched
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2421350, IEEE Photonics Technology Letters
core diameters cause interference to occur. The transmittance of the sensor essentially follows a periodic curve, albeit with little deformation from the FBG; i.e., the transmission property resembles that of an in-fiber MZI. Because the FBG is written in the sensing SMF part of the in-fiber MZI, multiple parameters can be detected at a single position. Furthermore, the sensor is simple to manufacture and the fiber is cheap.
Fig. 1. Schematic and principles of the sensor: (a) multiple modes are generated in the first MMF; (b) FBG reflects the core mode, showing single peak; (c) the core mode and cladding modes interfere at the second MMF, producing periodic transmittance.
in the theory above. Fig. 2(a) also shows several small side lobes caused by interference. Owing to the small reflectivity, these do not influence the performance of the sensor. The inset of Fig. 2(b) shows frequency spectra obtained by applying a Fourier Transform to the sensor’s transmittance. The intensity peaks at zero and at specific high frequency correspond to the core mode and cladding mode, respectively, the interference occurs between the core mode and the dominating first order cladding mode. The displaced spectra shown in blue and red in Fig. 2 were measured at 25°/0 με and 75°/496 με, respectively demonstrating that the spectra will shift when the environmental temperature and strain change. This phenomenon can be explained from theory. The change in the grating period and the strain-optic induced change in the refractive index n when the FBG is influenced by static strain causes a linear change in the Bragg wavelength B 2n . Thermal expansion-induced changes in and n also induce linear changes in B , although with a different coefficient [1]. The transmittance of the in-fiber MZI can be simply described in terms of interference between two modes (the core and dominating cladding modes), as shown in (1) [5]. I I core I clad 2 I core I clad cos 2neff L (1) where I core and I clad are the optical intensity in the core and cladding of the sensing SMF, respectively, L is the length of the sensing SMF, and neff is the difference in refractive index between the core and cladding. The output intensity shows a minimum value at the wavelength given in (2). (2) m 2neff L 2m 1 where m is a positive integer. We conducted an experiment in which we selected the minimum point in Fig. 2(b) of around 1542 nm as the observation point. Changes in the environmental static parameters will change neff and L ,
Fig. 2. Sensor spectrum changes and calibration results. (a) the reflectivity of the sensor shows a single peak determined by the FBG; (b) transmittance of the sensor shows a periodic curve determined by interference; (c) spectra change to temperature; (d) spectra change to strain; (e) wavelength shifts of the FBG and the MZI to temperature; (f) wavelength shifts of the FBG and the MZI to strain.
Fig. 2(a) and (b) illustrate the reflectivity and transmittance of the manufactured sensor, respectively. The spectra of the reflected and transmitted light both agree with those described
leading to a spectral shift according to (2). Fig. 2(c) and (d) show the independent spectrum changes to temperature and strain, respectively; Fig. 2(e) and (f) are the corresponding calibrated results. Temperature calibration was conducted by placing the FBG in an oven whose temperature was measured using a thermocouple. Strain calibration was conducted through tensile testing, wherein the entire fiber sensor was glued onto a plastic plate, and the reference strain was measured using a foil strain gage attached close to the sensor. According to Fig. (2), although the MZI is more sensitive to temperature than the FBG, it is relatively insensitive to strain (producing a small inverse-proportional response). Because the thermo-optic coefficient of the sensing SMF core is higher than that of the cladding, increasing neff results in red shift of the observation point of the MZI when temperature increases. However, when strain increases, the decrease in neff caused by the decrease in the ratio of refractive indexes of the fiber core and cladding is more pronounced than the increase in the fiber length, causing a very weak blue shift in the observation point of the MZI (Fig. 2(f)). We also noticed that the strain
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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2421350, IEEE Photonics Technology Letters
sensitivity of the FBG is lower than the value reported in [2], a result that should be studied further in subsequent research. Lengths of the SMF, MMF, and FBG which greatly influence the performance of the sensor prototype were selected based on the following considerations. In static measurement, the relative long length of the sensing SMF increases the period of the interference curve shown in Fig. 2(b), leading to easy determination of MZI . The long length of the MMF generates more energy in cladding mode, leading to high fringe visibility. However, longer SMFs and MMFs tend to reduce the robustness of the sensor. For example, the transmittance and reflectivity of the sensor is slightly instable when the temperature and strain increase, as shown in Fig. 2. This is because the ratio between I core and I clad will change in the MMF section when the sensor is influenced by a static parameter, leading to the reflectivity change according to (1). This effect should be mitigated in a well-designed demodulation system. In ultrasonic detection, the lengths of the SMF and FBG are also important. Longitudinal ultrasonic signals can be treated as micro strains with periodic distribution along the fiber (although Lamb waves were generated in our following experiment, the phenomenon is similar.) According to theory [13], an FBG detects ultrasonic signals because the Bragg wavelength shifts when the ultrasonic micro strain compresses or stretches the grating. However, an MZI experiences this effect with less intensity because compression counteracts stretching when the sensing SMF is longer than the wavelength of the ultrasonic signals. Furthermore, the low sensitivity of the MZI, as indicated by the gentle slope of its transmittance, makes it unsuitable for detection of ultrasonic signals. A demodulation system for the detection of multiple parameters, particularly ultrasonics with high frequency and low energy, was designed and developed based on the system in reference [14] (Fig. 3). After a broadband amplified spontaneous emission light source (FiberLabs, FL7050) illuminates the sensor, the transmitted light is monitored by an optical spectrum analyzer (Anritsu MS9710C). Because of the low reflectivity of the sensor, an erbium doped fiber amplifier (FiberLabs, AMP-FL8013) with a pump power of 225 mW is used to enhance the signal. The overlapping regions between the reflection of the sensor and the transmittances of the two neighboring channels (N and N+1) of an arrayed waveguide grating are detected by a balanced photo-detector (New Focus, 2117). When a high-frequency ultrasonic signal slightly shifts the Bragg wavelength, the two overlapping regions change with opposite phase, leading to a double AC change (ultrasonics) in the balanced photo-detector [15]. The other advantage of this method is that it is unaffected by the reflectivity instability shown in Fig 2; no matter how the reflectivity changes, the DC signal in the balanced photo-detector is zero after subtracting the optical powers of the neighboring channels of the arrayed-waveguide grating. These characteristics render the demodulation system quite suitable for use with the sensor. The output voltage from a low-pass filter is used to adjust a temperature controller (a peltier located beneath the
arrayed-waveguide grating filter). When static change causes a sufficiently large shift in the Bragg wavelength of the sensor, the controller can adjust the wavelength of the arrayed-waveguide grating filter to follow the sensor. With this arrangement, it is possible to simultaneously read three data sets directly: the wavelength shift MZI observed from the optical spectrum analyzer; the temperature change TTEC in the temperature controller; and the high-frequency signal sent through the filter. Because the wavelength shift AWG of the arrayed-waveguide grating is linearly proportional to temperature change with a slope of 10.34 pm/K, the Bragg wavelength shift of the sensor can be calculated as FBG AWG 10.34TTEC . Related with the calibration results in Fig. 2(e) and (f), the temperate change T and strain change could be deduced using (3), after detection of TTEC and MZI .
10.34TTEC FBG 10.21 0.689 T MZI MZI 45.93 -0.036
(3)
Fig. 3. Demodulation system and principles: (a) reflected light is demodulated by an arrayed-waveguide grating filter with controller and then detected by a balanced photo-detector - the transmitted light is monitored by an optical spectrum analyzer; (b) detection of change in the overlap area between sensor and arrayed-waveguide grating filter caused by ultrasonic signals; (c) using thermal expansion of a steel plate to change environmental temperature and strain in order to demonstrate the discrimination ability of the sensor. Ultrasonics are also detected in different conditions.
Fig. 4. Temperature and strain measured by the proposed sensor (red dots) closely fit values measured with conventional equipment (blue dots).
In order to demonstrate its performance, the proposed sensor was attached to the middle of a steel plate, as shown in Fig. 3(c). An actuator located near the edge of the plate generated
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ultrasonic signals. A commercial PZT sensor (Fuji Ceramics, 1045S) was placed near the sensor to provide an ultrasonic reference. We used a strain gage and thermometer to monitor the actual strain and temperature change in the experimental process. When the environmental temperature was increased through oven heating, the strain of the plate also increased owing to its thermal expansion.
a traditional PZT sensor, as shown in the inset of Fig. 5(d). After analysis of the arrival time delay and the waveform change, we could evaluate the mechanical properties and dimensional changes of the material. III. CONCLUSION In this paper, we proposed a novel sensor based on in-fiber MZI and FBG. We have designed a demodulation system to achieve multifunctional sensing. The wavelength shifts in the reflectivity and the transmittance of this sensor show differing responses to temperature and strain, allowing for the discrimination of the two parameters. Ultrasonic signals can be detected directly from voltage vibration. Through this work, we have designed and developed a sensor that can obtain three important parameters simultaneously and at a single position, which should make it extremely useful in ultrasonic nondestructive testing. REFERENCES [1]
Fig. 5. Responses to various ultrasonic signals; (a) response of the sensor to continuous signal shows sensor’s broad bandwidth; (b) real-time detection of simulated acoustic emission signals. (c–d) Under different environmental conditions in pitch-catch detection, the sensor detects ultrasonic signals with different arrival times. Inset shows the reference signals provided by the traditional PZT sensor.
Fig. 4 shows the strain and temperature detected using commercial reference sensors and our sensing system (blue and red dots, respectively). The results coincide closely, demonstrating the high accuracy and fidelity of our sensing system. Furthermore, the fact that the measurements were successfully taken over a 7-hour period demonstrates the high reliability of the sensor. The initial and final spectra of the sensor are shown in Fig. 2(a) and (b). The resolution and accuracy of this sensing system to static parameters is about 0.1° and 1.5 με, mainly restricted by the resolution of the optical spectrum analyzer (0.05 nm), and the peltier controller (0.1°). Fig. 5 shows various ultrasonic detection results. Fig. 5(a) shows that the sensor has responds accurately to a 1-MHz signal, demonstrating its broad bandwidth under ultrasonic nondestructive testing. It is believed that further improvements in sensitivity and bandwidth could be achieved by using a shorter FBG with a higher refractive index or by using a phase-shifted FBG [15]. The sensor can also detect simulated acoustic emission signals from a pencil lead break in real time, as shown in Fig. 5(b). The sensor is even better at performing the ultrasonic pitch-catch method. In this case, the input signal was a three-cycle sinusoidal wave with a Hamming window, and its middle frequency is 300 kHz. Fig. 5(c) shows the average of 4,096 signal detection events. The sensor detected ultrasonic signals both before and after heating (change from 25°/0 με to 75°/496 με) and the arrival time of the detected signal was delayed by 4 μs, as shown in Fig. 5(d). This result fits the actual arrival time delay detected using
[2] [3]
[4]
[5] [6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
A. Kersey et al., “Fiber grating sensors,” J. Lightwave Technol., vol. 15, no. 8, pp. 1442–1463, 1997. A. Othonos, “Fiber Bragg gratings,” Rev. Sci. Instrum., vol. 68, no. 12, pp. 4309–4341, 1997. H. Patrick et al., “Hybrid fiber Bragg grating/long period fiber grating sensor for strain/temperature discrimination,” IEEE Photonic. Tech. L., vol. 8, no. 9, pp. 1223–1225, 1996. Q. Wang, and G. Farrell, “All-fiber multimode-interference-based refractometer sensor: proposal and design,” Opt. Lett., vol. 31, no. 3, pp. 317–319, 2006. L. Li et al., “All-fiber Mach-Zehnder interferometers for sensing applications,” Opt. Express, vol. 20, no. 10, pp. 11109–11120, 2012. G. Wild, and S. Hinckley, “Acousto-ultrasonic optical fiber sensors: overview and state-of-the-art,” IEEE Sens. J., vol. 8, no. 7, pp. 1184–1193, 2008. D. Wang, S. Wang, and P. Jia, “In-line silica capillary tube all-silica fiber-optic Fabry–Perot interferometric sensor for detecting high intensity focused ultrasound fields,” Opt. Lett., vol. 37, no. 11, pp. 2046–2048, 2012. B. Lissak, A. Arie, and M. Tur, “Highly sensitive dynamic strain measurements by locking lasers to fiber Bragg gratings,” Opt. Lett., vol. 23, no. 24, pp. 1930–1932, 1998. J. F. Dorighi, S. Krishnaswamy, and J. D. Achenbach, “Stabilization of an embedded fiber optic Fabry-Perot sensor for ultrasound detection,” IEEE T. Ultrason. Ferr., vol. 42, no. 5, pp. 820–824, 1995. Y. J. Rao et al., “Simultaneous strain, temperature and vibration measurement using a multiplexed in-fibre-Bragg-grating/fibre-FabryPerot sensor system,” Electron. Lett., vol. 33, no. 24, pp. 2063–2064, 1997. H. K. Kang et al., “Simultaneous measurement of strain, temperature and vibration frequency using a fibre optic sensor,” Meas. Sci. Technol., vol. 13, no. 8, pp. 1191–1196, 2002. Q. Wu et al., “Fiber refractometer based on a fiber Bragg grating and single-mode–multimode–single-mode fiber structure,” Opt. Lett., vol. 36, no. 12, pp. 2197–2199, 2011. A. Minardo et al., “Response of fiber Bragg gratings to longitudinal ultrasonic waves,” IEEE T. Ultrason. Ferr., vol. 52, no. 2, pp. 304–312, 2005. S. Kojima et al., “High-speed optical wavelength interrogator using a PLC-type optical filter for fiber Bragg grating sensors,” SPIE 5384, Bellingham, WA, 2004, pp. 241. Q. Wu, and Y. Okabe, “High-sensitivity ultrasonic phase-shifted fiber Bragg grating balanced sensing system,” Opt. Express., vol. 20, no. 27, pp. 28353–28362, 2012.
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