HIGH PRECISION OPERATION OF FIBER BRAGG GRATING SENSOR WITH INTENSITY-MODULATED LIGHT SOURCE

HIGH PRECISION OPERATION OF FIBER BRAGG GRATING SENSOR WITH INTENSITY-MODULATED LIGHT SOURCE Nobuaki Takahashi, Hiroki Yokosuka, Kiyoyuki Inamoto and Satoshi Tanaka Department of Communications Engineering, National Defense Academy 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686 Japan tak@nda.ac.jp ABSTRACT High precision operation of a fiber Bragg gratin (FBG) vibration sensor is examined for accurate measurement of signal phase in noisy environment by using electro-optically (EO) modulated light source and demodulation technique with two approaches: one with the lock-in detection technique and another with the envelope detection technique. The experimental demonstrations are successful and definite signal waveforms are obtained in the sensor output in real time without performing average of the signals. The advantage of the former approach is the ability of obtaining extremely definite signal waveforms while the latter provides us with wider frequency range in the sensor operation. Even with the EO modulated light source in the sensor system, shifting the source wavelength when the temperature change causes shift in the Bragg wavelength of the FBG sensing element would thermally stabilize the sensor. Introduction An optical fiber sensor is characterized by high sensitivity, wide dynamic range, distributed sensing, immunity to electromagnetic interference, resistivity to chemical corrosion, compact size, light weight and so on [1]. Because it can operate in harsh environment such as high temperature [2,3] and high pressure [4], one can use it for in-well imaging and monitoring applications [5] and expect to use it even in a down hole drilled into the earth mantle. Furthermore its small size and light weight make it possible to be buried in a material and attached to a structure without affecting their nature for real time health monitoring [6,7]. Among fiber optic sensors, a sensor using a fiber Bragg grating (FBG) as a sensing element is especially expected to play an important role in various applications because of its easy operation, quasi-point sensing and inherent wavelength-division-multiplicity. Fundamental principle of FBG sensing is based on the Bragg wavelength shift induced by physical influence applied to the FBG [8]. Interrogation methods of FBG sensor systems may be divided into two schemes: One is to determine the shift of the Bragg wavelength by measuring the reflection peak wavelength of the FBG shined by broad spectrum light [6]. Another is to measure change in intensity of the light either reflected from or transmitted through a FBG while illuminating the FBG by narrow spectrum light [9]. In the latter scheme, the wavelength of the light is tuned to the slope of either reflectance or transmittance spectrum curve of the FBG and then a FBG works as an optical intensity modulator. We adopt this method (the intensity-modulation method) because of the advantages that the interrogation system is simple and the observation is rather direct, namely, the output of the optical detection is directly proportional to the applied physical influence and one can observe amplitude and phase of the signal in real time in the case of dynamic behavior measurement. Furthermore, the sensor can be operated in either reflection or transmission mode though only the reflected light is made use of in the measurement of the peak-wavelength shift. Using the intensity-modulation method, clear signal waveforms are obtained, for example, in the underwater acoustic measurement so that the direction of the sound source can be determined by measuring phase difference between two observation points when a signal is sufficiently strong [10]. When a signal is not strong, however, the waveform of the signal becomes ambiguous due to various kinds of noises and it is difficult to perform the phase detection of the signal. In the present work, therefore, we propose a new method to clarify a signal of a FBG vibration and/or underwater acoustic sensor for precise measurement of signal phase by reducing noises involved in the system. The method is based on the use of a laser light source that is intensity-modulated at high frequency. Using two techniques, the demodulation of the signal is carried out, depending on the range of the signal frequency. In a lower frequency range the lock-in detection is conducted while the envelope detection of amplitude-modulated (AM) signal is examined in a higher frequency range. An experimental demonstration is carried out for the case of vibration sensing

Principle When one interrogates a FBG sensor with the intensity-modulation method, as has been said above, narrow spectrum light is launched into the FBG and its wavelength is tuned to the slope region of the FBG reflection spectrum curve, for example, in the reflection mode. We here suppose that the intensity of the incident light is modulated at high frequency. If the intensity of the light is given by I in(t) = I 0(1 + mcosω mt), (1) where I 0 is the average intensity, m is the modulation index and ω m is the angular frequency of the modulation, then the intensity of the reflected light is given by I r(t) = R(λ in)i 0 (1 + mcosω mt), (2) where λ in is the wavelength of the incident light, R(λ) is the reflection spectrum curve, i.e., reflectance of the FBG as a function of optical wavelength and in turn R(λ in) is the reflectance of the FBG at λ in. Suppose the FBG is under influence of vibration, for example, the FBG spectrum curve is shifted along the wavelength axis in synchronization of the vibration. When the vibration-induced strain across the FBG is uniform and not large, the shift is in proportion to the vibration and the shape of the FBG spectrum curve is kept unchanged, which is indeed the case in most of practical applications. When the wavelength shift is small, we can then rewrite the above equation as I r(t) = (R 0 - "R $ v % 0 sin&t )I 0 (1 + mcosω mt) "# = I 0R 0 "R - I 0 $ v % 0 sin&t "# + I 0(R 0 - "R $ v % 0 sin&t )mcosω mt, (3) "# where R 0 is the reflectance of the FBG when there is no vibration applied to the FBG, η v is the Bragg wavelength shift rate per unit strain due to the vibration, ξ 0 and ω are the amplitude and angular frequency of the strain [11]. The frequency of the optical source modulation is chosen in such a way that it is to be far higher than that of the vibration of interest. It is thus seen from Eq. (3) that the optical detection of the reflected light gives us a dc component, ω frequency component and ω m frequency component whose amplitude is amplitude-modulated by the strain. The third term is utilized to extract the vibration signal from the detector output. One can use a lock-in amplifier to obtain the signal because the lock-in detection of the detector output locked at the frequency ω m yields a signal proportional to the amplitude of the third term. Although this is powerful method to extract the signal in a noisy environment, the operation frequency of the method is rather limited due to the bandwidth of an available instrument. In a higher frequency range, therefore, we apply the demodulation technique for an AM signal to the interrogation of the FBG signal.. A) In lower frequency Experiments and Results When the frequency of vibration is low, the lock-in detection technique is applied to extract the signal from the output of the optical detector. Figure 1 shows the experimental setup for the proposed FBG vibration sensor. In this configuration then the sensor is in the reflection mode. Figure 2 shows the reflection spectrum of the FBG used in the experiment along with the spectrum of the optical source. The Bragg wavelength and bandwidth of the FBG are 1556.23 and 1.3 nm, respectively. The output light from the DFB laser diode is electro-optically (EO) modulated. The function generator provides the driving signal for the EO modulator and the reference signal for the lock-in detection. An optical isolator is here inserted for stabilization of the optical source operation. After transmitting through an optical circulator, the light is launched into an FBG sensing element that is under influence of mechanical vibration. A mechanical vibrator is used in the experiment and induces expansion and contraction in the FBG length. The vibration of the vibrator is monitored with a laser Doppler vibrometer. Since we assume that there is no slip between the FBG and vibrator, i.e., the strain of the FBG is equal to that of the vibrator, we can practically measure the dynamic strain of the FBG in real time. The length change and associated photoelastic effect in the FBG shifts the Bragg wavelength of the FBG periodically. Since the perturbation is usually small, as said above, the Bragg wavelength shift is proportional to the vibration amplitude applied to the FBG and the shape of the FBG reflection spectrum curve is then kept unchanged. When the wavelength of the light is tuned to the slope of the FBG reflection spectrum curve as seen in Fig. 2, the envelope of the EO modulated light intensity is modulated by the vibration-induced strain

on reflecting back from the FBG. The output of the detector therefore contains the vibration signal as an envelope of the highfrequency modulated signal. Circulator DFB Laser Diode Isolator Electro-Optic Modulator FBG Vibrator Fiber Function Generator Detector Driver Electric Wire Ref. Input RF Lock-in Amplifier OUTPUT Figure 1. Experimental Setup for FBG Vibration Sensor: Lock-in Detection Scheme 1.0 1.0 0.5 0.5 0.0 1556 1557 1558 wavelength 0.0 Figure 2. FBG Reflection Spectrum and Light Source Figure 3 shows a typical waveform of the optical detector output when 110 Hz vibration is applied to the FBG. The amplitude of the dynamic strain due to the vibration is 6.2 µε. Although one can see a sinusoidal waveform, the signal is noisy and we suffer from ambiguity in determining phase of the signal. Figure 4 shows the output of the sensor when the EO modulated light and lock-in detection technique are used with the other conditions kept identical to those in Fig. 3. The frequency of the EO modulation is 100 khz. We did not perform averaging for signal processing in observing the waveform. It is clear from the figure that the signal waveform is far more definite and the noise associated with the signal is far less than that in Fig. 3. Varying the frequency of the vibration while keeping the strain amplitude constant, the frequency characteristic of the sensor is measured. The result is shown in Fig. 5. We may say that the proposed system can be used up to about 1 khz. Since a FBG is known to operate at even more than a few MHz [12], the frequency limit of the sensor in this configuration is caused by the bandwidth of the RF lock-in amplifier used in the experiment.

Figure 3. Sensor Output Signal without Lock-In Detection Figure 4. Sensor Output Signal with Lock-In Detection Figure 5. Vibration Frequency Dependence of Sensor; Lock-in Detection Scheme

B) In higher frequency From the above discussion, the FBG sensor with the lock-in detection technique suffers from some inaccuracy if the frequency of the vibration is a few khz and it can no longer operate at the frequency higher than several khz. We then apply the envelope detection technique that is commonly used for demodulating an amplitude-modulated (AM) signal. Figure 6 shows the experimental setup for the proposed sensor. The optical detector output is here sent to the process of the AM demodulation instead of the lock-in detection. The detected signal is first filtered out by passing it through an intermediate frequency filter. The frequency of the filter is 10.7 MHz. So is the frequency of the EO modulation. The signal is then amplified by an intermediate frequency amplifier. The output of the sensor is finally extracted from the envelope of the amplified signal. DFB Laser Diode Fiber Isolator Electric Wire 10.7 MHz Electro-Optic Modulator Function Generator Circulator Detector 10.7 MHz Band-Pass Filter Fiber FBG Vibrator Driver Amplifier Output Envelope Detection Figure 6. Experimental Setup for FBG Vibration Sensor: Envelope Detection Scheme Figure 7. Sensor Output Signal without Envelope Detection

Figures 7 and 8 show typical experimental results without and with the proposed technique, respectively. The frequency of the vibration is 5 khz and strain amplitude is 25 µε. The signal is clear and its phase is definite when the envelope detection technique is applied, although without the technique it is perturbed by noise and its phase is indefinite. The output of the sensor is again measured as a function of vibration frequency. Figure 9 shows the vibration frequency dependence of the sensor output. (The measurement was done up to 30 khz because of the availability of the equipment in the laboratory. Since we could not get a signal when the frequency is 50 khz, the cut-off frequency is considered to be located between 30 and 50 khz.) The upper limit of the operation frequency is extended and higher than that with the lock-in detection scheme by more than an order. Figure 8. Sensor Output Signal with Envelope Detection Figure 9. Vibration Frequency Dependence of Sensor; Envelope Detection scheme Discussion and Summary Both the lock-in and envelope detection schemes are successfully demonstrated for precise operation of the FBG vibration sensor. The former is better in obtaining more definite signal in the noisy environment. Because its operation frequency is

rather limited and can work up to about 1 khz, however, it should be used for lower frequency application. The latter scheme provides us with wider frequency range and much more flexibility in the choice of the operation frequency at lower cost. The proper choice of an intermediate frequency filter specification would give us a proper frequency range for the sensor operation. The wider frequency bandwidth of the filter would give rise to a sensor that operates at higher frequency. However, the output waveform is not so definite as in the former scheme. It should then be used either for the case that the signal is relatively large or for the case that a wider frequency range is required. It is also better when the cost effectiveness is important. When the FBG sensor with the intensity-modulation method is to be used in temperature varying environment, we need to stabilize the sensor thermally because the temperature change causes large change in the sensor sensitivity. The dc component of the optical detector output has the information about the operation point of the sensor on the FBG reflectance spectrum curve and can be used to stabilize the sensor sensitivity. Namely, the sensor sensitivity would be kept unchanged if the wavelength of the light source were shifted when the temperature varies so that the operation point on the curve should stay at the same position [13]. All the analyses and experiments are done for the reflection mode of a FBG sensor. Because the similar analyses and techniques hold also in the transmission mode of the sensor, however, it should also be possible to repeat the same experiments in the transmission mode. The reflection mode provides us with a compact design because a lead fiber can be used as a two-way road for light propagation, whereas in the transmission mode one needs less number of optical components and the optical source power can also be made less since the loss involved in the system is smaller compared to that in the reflection mode. References 1. Culshaw, B. and Dakin, J., eds, Fiber Sensors Vol. 1-3, Artech House, Boston, 1966. 2. Zhao, W., Shen, Y., Sun, T. and Grattan, K. T. V., FBGs in Bi-Ge Co-doped Photosensitive Fibre for High Temperature Measurement up to 1000ºC, in 18 th International Fiber Sensors Conference Technical Digest ( Society of America, Washington, DC, 2006), TuE86. 3. Artyushenko, V., Ingram, J., Lobachev, V., Sakharova, T., Savitskij, D. and Wojciechowski, C., Fiber optic sensing in broad spectrum range: 0.2-1.8µm, in 18 th International Fiber Sensors Conference Technical Digest ( Society of America, Washington, DC, 2006), ThE94. 4. Takahashi, N., Saeki, T., Tetsumura, K. and Takahashi, S., Pressure and temperature dependence of fiber Bragg grating for acoustic sensing, J. Marine Acoust. Soc. Jpn., 26, 231-238 (1999). 5. Knudsent, S., Havsgard, G. B., Berg, A., Thingbo, D., Bostick, F. and Eriksrud, M., High Resolution Fiber-Optic 3-C Seismic Sensor System for in-well Imaging and Monitoring Applications, in 18 th International Fiber Sensors Conference Technical Digest ( Society of America, Washington, DC, 2006), FB2. 6. Udd, E., Structual Health Monitoring Using FBGs for Aerospace and Composite Manufacturing, in 18 th International Fiber Sensors Conference Technical Digest ( Society of America, Washington, DC, 2006), MF2. 7. Iwaki, H., Long Span Cable Stayed Bridge Monitoring Using Distributed Fiber Sensing System, in 18 th International Fiber Sensors Conference Technical Digest ( Society of America, Washington, DC, 2006), MF5. 8. Kersey, A. D., Davis, M. A., Patrick, H. J., LeBlanc, M., Koo, K. P., Askins, C. G., Putnam, M. A. and Friebele, E. J, Fiber Grating Sensors, J. Lightwave Technol., 15, 1442-1463 (1997). 9. Takahashi, N., Yoshimura, K. and Takahashi, S., FBG vibration sensor based on intensity-modulation method with incoherent light, in International Conference on Engineering for Sensing and Nanotechnology (ICOSN), ed. K. Iwata, Proceedings of SPIE Vol. 4416, 62-65 (2001). 10. Takahashi, N., Yoshimura, K., Takahashi, S. and Imamura, K., Characterization of Fiber Bragg Grating Hydrophone, IEICE Trans. Electron., E83-C, 275-281 (2000). 11 Ogawa, T., Tanaka, S. and Takahashi, N., Temperature-Independent Sensing of Vibration Using Two Fiber Bragg Gratings, Memoirs of the National Defense Academy, Japan, 43, 19-26 (2003). 12. Takahashi, N., Yoshimura, K., Takahashi, S. and Imamura, K., Development of an optical fiber hydrophone with fiber Bragg grating, Ultrasonics, 38, 581-585 (2000). 13. Takahashi, N., Thongnum, W., Ogawa, T., Tanaka, S. and Takahashi, S., Thermally Stabilized Fiber-Bragg-Grating Vibration Sensor Using Servo Motor Control, Opt. Rev., 10, 106-110 (2000).