A New Type of Etched Fiber Grating Hydrophone
A New Type of Etched Fiber Grating Hydrophone
Liu, Wen-Fung;Li, Jia-Guan;Chang, Hung-Ying;Fu, Ming-Yue;Chen, Chi-Fang
2022-04-11 00:00:00
hv photonics Article 1 , 1 1 2 3 Wen-Fung Liu *, Jia-Guan Li , Hung-Ying Chang , Ming-Yue Fu and Chi-Fang Chen Department of Electrical Engineering, Feng-Chia University, Taichung 40724, Taiwan; smallfamiport@gmail.com (J.-G.L.); hungying.chang@gmail.com (H.-Y.C.) Center for General Education, Air Force Academy, Kaohsiung 82047, Taiwan; fumy@cc.cafa.edu.tw Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei 10216, Taiwan; chifang@ntu.edu.tw * Correspondence: wfliu@fcu.edu.tw Abstract: We propose a new type of fiber hydrophone composed of an etched fiber Bragg grating and a special packaging structure for detecting acoustic waves in the low-frequency band under water. The operating mechanism is based on the mechanical vibration of the fiber Bragg grating from the induced vibrating stress of acoustic pressure. The induced pressure of acoustic waves pushes the silicone rubber thin film, causing its vibration and then stretching the fiber Bragg grating, thus resulting in the grating wavelength shift which is overlapped with a tunable laser. The variation in the overlapped light intensity is transferred to an electrical signal by using a photodetector. From the experimental results, we can determine that the smaller the fiber diameter, the higher the sensitivity and frequency response. In order to confirm that this FBG hydrophone has the ability to work in high- frequency acoustic waves, this fiber grating hydrophone and a standard piezoelectric hydrophone are experimentally compared to in the same test conditions in the frequency range from 4 to 10 kHz. According to the experimental results, the fiber grating hydrophone has better responsivity than that of the conventional hydrophone. Due to the unique sensing structure design, this wide-band fiber hydrophone can be useful in long-term continuous monitoring of acoustic waves. Keywords: fiber Bragg grating; fiber hydrophone; acoustic wave sensor; silicone diaphragm Citation: Liu, W.-F.; Li, J.-G.; Chang, H.-Y.; Fu, M.-Y.; Chen, C.-F. A New 1. Introduction Type of Etched Fiber Grating Fiber Bragg gratings (FBGs) [1–3] have recently received much attention for applica- Hydrophone. Photonics 2022, 9, 255. https://doi.org/10.3390/ tions in fiber sensors and fiber lasers. Due to their compact size, light weight, high sensi- photonics9040255 tivity, immunity to electromagnetic interference, and corrosion resistance, fiber gratings sensors have become a preferred choice over conventional electrical sensors. Nowadays, Received: 23 March 2022 by means of different structure designs and packages, they are used in various sensing Accepted: 8 April 2022 applications [4–11]. Published: 11 April 2022 For sensing and communication systems under water, electromagnetic wave signals Publisher’s Note: MDPI stays neutral are difficult to transmit for measuring function due to the large attenuation caused mainly with regard to jurisdictional claims in by water molecule absorption. In order to overcome this problem, a hydrophone based published maps and institutional affil- on acoustic waves can be utilized for measuring the object distance under water. Conven- iations. tional hydrophones are based on a piezoelectric transducer generating an electric signal by sensing a pressure change, with the disadvantages of having a large size, being liable to corrosion and short circuiting by seawater, etc. Thus, for solving these problems, dif- ferent kinds of fiber hydrophones are proposed. In general, there are five different kinds Copyright: © 2022 by the authors. of fiber hydrophones: the Eisenmenger fiber hydrophone [12], Fabry Perot polymer film Licensee MDPI, Basel, Switzerland. hydrophone [13], multilayer dielectric fiber hydrophone [14], interferometric displacement This article is an open access article fiber hydrophone [15], and fiber grating hydrophone [16–21]. The operation mechanism of distributed under the terms and the Eisenmenger fiber hydrophone [8] is based on the detection of pressure variation to conditions of the Creative Commons cause the refractive index mismatch between the tip of an optical fiber and water. The Fabry Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Perot polymer film hydrophone [13,22] is based on the interferometric detection of acousti- 4.0/). cally induced changes in the optical thicknesses of a thin film sensing structure deposited Photonics 2022, 9, 255. https://doi.org/10.3390/photonics9040255 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 255 2 of 11 on the tip of a single-mode fiber. The multilayer dielectric fiber hydrophone [14,15,23] is based on the interferometric detection of acoustically induced changes in the optical thickness of a thin-film multi-layer structure which consists of a series of thin dielectric films of alternating high and low indices to be sputtered onto the single-mode fiber tip. The interferometric displacement fiber hydrophone is based on detecting acoustically induced displacements of the tip of an optical fiber, in which the fiber forms one arm to be an interferometer for detecting its length change [15]. FBG-based hydrophones [16–21] have the advantages of high sensitivity, large dy- namic range, flexibility, and multiplexing capability, and also have several potential appli- cations including in building structure security monitoring, marine detecting, and medical measurements. For acoustic signal detection, Fisher et al. [16] proposed in-fiber Bragg grating for ultrasonic medical applications in 1997. The fiber Bragg grating hydrophone [17] was proposed by N. Takahashi et al. based on the intensity modulation of laser light in an FBG under the influence of sound pressure to show the linearity with the dynamic range of about 70 dB, and is operated in the range of 1 kHz to 3 MHz for the acoustic frequency. A. Cusano et al. [18] proposed an optical fiber hydrophone using polymer-coated fiber Bragg grating, in which an appropriate coating material was selected with an elastic modu- lus much lower than that of fiber. M. Moccia et al. [19] proposed a resonant hydrophone based on coated fiber Bragg gratings with ring-shaped polymers of different sizes and mechanical properties for measuring the acoustic frequency range from 4 to 35 kHz with greatly enhanced responsivity. B. O. Guan et al. [20] proposed a dual polarization fiber grating laser hydrophone by integrating a dual polarization fiber laser and an elastic di- aphragm. By using a diaphragm, the acoustic wave pressure is transformed into the fiber axis force, acting on the laser cavity to change the fiber birefringence and the beat frequency between the two polarization lines. A. R. Karas et al. [21] proposed a passive optical fiber hydrophone array by utilizing fiber Bragg gratings which are interrogated using a single solid-state spectrometer to reduce the cost of the deployed system with lower ambient ocean acoustic noise. In this study, we propose a new type of fiber grating hydrophone by using etched fiber Bragg gratings and a specially designed packaging structure for measuring the low- frequency band of acoustic waves. The HF-etched fiber technique serves to reduce the fiber diameter for increasing the sensing sensitivity [24,25]. We confirmed that this fiber hydrophone has the capability to measure high-frequency acoustic waves and that it performs better than conventional hydrophones in the frequency range of 4 to 10 kHz. 2. Sensing Principle For this fiber grating hydrophone, the fiber Bragg grating is the key component which is fabricated by using an KrF excimer laser with the wavelength of 248 nm, combining a phase mask to create a period interference beam pattern, and then laterally exposing the core of single-mode fiber. This exposure induces the refractive index change in the fiber core to create a period index modulation along the fiber axis for forming a fiber Bragg grating. The grating wavelength l is obtained as follows: l = 2n ^ (1) e f f where n is the grating effective refractive index and L is the grating period. The grating eff reflectivity (or intensity) is dependent on the magnitude of index modulation. For increas- ing the fiber photosensitivity, the germanium-doped SMF-28 fiber is hydrogen-loaded in the pressure of 1500 psi for one week. According to Equation (1), the strain from external applied stress and pressure or a temperature change can cause variations in both the grating period and effective index. Therefore, the shifted grating wavelength is proportional to an applied stress along the fiber axis and a temperature variation. Thus, the operating mechanism of sensing the acoustic waves under water is based on the grating wavelength shift caused by the acoustic pressure. Photonics 2022, 9, x FOR PEER REVIEW 3 of 12 is proportional to an applied stress along the fiber axis and a temperature variation. Thus, the operating mechanism of sensing the acoustic waves under water is based on Photonics 2022, 9, 255 3 of 11 the grating wavelength shift caused by the acoustic pressure. The fiber sensing head is composed of a silicone rubber thin film inserted with an FBG, an acrylic ring, and a hollow cylinder, as shown in Figure 1. For the procedure of The fiber sensing head is composed of a silicone rubber thin film inserted with an fabricating the sensing head, the FBG is firstly fixed on the groove of acrylic ring with an FBG, an acrylic ring, and a hollow cylinder, as shown in Figure 1. For the procedure of axial stress. The specifications of the FBG fabricated for the fiber hydrophone are 20 mm fabricating the sensing head, the FBG is firstly fixed on the groove of acrylic ring with an in length, FWHM bandwidth of around 0.3 nm, reflectivity of 95%, and grating center axial stress. The specifications of the FBG fabricated for the fiber hydrophone are 20 mm wavelength of 1554.6 nm (grating pitch of about 537.5 nm). Then, the spin coater is used in length, FWHM bandwidth of around 0.3 nm, reflectivity of 95%, and grating center to coat a silicone rubber thin film, which can transform the acoustic pressure into the wavelength of 1554.6 nm (grating pitch of about 537.5 nm). Then, the spin coater is used to fiber axial stress to cause the grating wavelength shift. Then, the silicone rubber thin film coat a silicone rubber thin film, which can transform the acoustic pressure into the fiber is axial set on str the e ess to nd-f cause ace o the f the grating hollow wavelength cylinder. shift. Then, the silicone rubber thin film is set on the end-face of the hollow cylinder. L = 50mm 34 mm Acrylonitrile butadiene styrene (ABS) (a) (b) Silicone rubber thin film Fiber Fiber Bragg grating (Length = 20mm) (c) Figure 1. The FBG hydrophone configuration including (a) the silicon diaphragm, (b) the frame of Figure 1. The FBG hydrophone configuration including (a) the silicon diaphragm, (b) the frame of acrylonitrile butadiene styrene, and (c) the FBG hydrophone composed of (a,b). acrylonitrile butadiene styrene, and (c) the FBG hydrophone composed of (a,b). When acoustic pressure causes stress on the thin film, the FBG is strained to create When acoustic pressure causes stress on the thin film, the FBG is strained to create the the grating wavelength shift. The thin film is subjected to a deformation proportional to grating wavelength shift. The thin film is subjected to a deformation proportional to the the amount amoun oft str of s ess. tres The s. The lateral later strain al st ofra the in of the film w f (rilm ) canw be (r) c described an be describe throughd the thr following ough the fol equations lowing eq [8 ua ]: tions [8]: pR w(r) = (2) 𝐰𝐫 = 64D[1 ( ) ] R 𝟐 (2) Et D= (3) 12(1 v ) 𝐄𝒕 where w(r) is the transverse displacement in the deformation axis, p is the acoustic pressure, (3) 𝐃= 𝟏𝟐 𝟏− 𝒗 R is the radius of the thin film, t is the thickness of the thin film, E is the Young’s modulus coefficient of the thin film material, and v is the Poisson ratio of the thin film material. where w(r) is the transverse displacement in the deformation axis, p is the acoustic When the acoustic pressure causes the film to produce a shape deformation, the FBG is pressure, R is the radius of the thin film, t is the thickness of the thin film, E is the stretched by stress ", and then the grating center wavelength l is shifted. The relationship Young’s modulus coefficient of the thin film material, and v is the Poisson ratio of the thin film material. 𝟏 𝟔𝟒𝐃 𝐩𝑹 Photonics 2022, 9, 255 4 of 11 between transverse displacement and axial strain w(r) from the acoustic pressure is as shown in the following formula [12]: 48w(r)y " = (4) 2L where " is the axial stress of the fiber Bragg grating, y is the distance between the center point of deformation and the FBG, and L is the film length. The grating wavelength shift Dl is positively correlated with the variation in both stress and temperature, which can be represented by the following formula [11]: Dl = l (K # + K T) (5) B B T where K and K are pressure and temperature constants, respectively. When the tempera- ture remains constant, the relationship between the grating wavelength shift and strain can be rewritten into: 48K w(r)y Dl = l (6) B B 2L However, a good hydrophone should not only be able to clearly distinguish the frequency but also have the same sensitivity at different frequencies. Thus, the high- frequency transducer will generate a sound wave as the sound pressure level (SPL) in a frequency. The sound pressure level will cause hydrophones to detect voltage signals, which relates to the sensitivity of hydrophone and amplify gain. The relationship equation is as follows: VL(dB) SEN(dB) Gain(dB) = SPL(dB) (7) where SPL is a sound pressure level, VL is a voltage level of measuring, Gain is an amplify gain, and the VL can be expressed with the following formulas: VL = 20log Vrms (8) p p V = p (9) rms 2 2 where V is the root mean square of voltage and V is the measured peak-to-peak rms p p voltage. With Equations (7) and (8) substituted into Equation (6), the converted relation can be described by the following equation: P P 20log SEN Gain = SPL (10) 2 2 Because the acoustic wave source is the same, the relationship between the standard hydrophone and the FBG hydrophone may be expressed as V V 1P P 2P P 20log p SEN = 20log p SEN Gain (11) 1 2 2 2 2 2 where V is the peak-to-peak voltage of the standard hydrophone, V is the peak- 2P P 1P P to-peak voltage of the hydrophone, SEN is the sensitivity of the standard hydrophone obtained at the factory, and SEN is the sensitivity of the FBG hydrophone. By detecting both the V and V , the sensitivity of the FBG hydrophone can be expressed as 1P P 2P P V V 1P P 2P P SEN = SEN 20log p + 20log p Gain (12) 2 1 2 2 2 2 Photonics 2022, 9, x FOR PEER REVIEW 5 of 12 Photonics 2022, 9, 255 5 of 11 3. Experimental Setup and Results 3. Experimental Setup and Results The experimental setup to measure low-frequency acoustic waves by using the FBG The experimental setup to measure low-frequency acoustic waves by using the FBG hydrophane includes a signal generator, a power amplifier, a loudspeaker, an FBG hydrophane includes a signal generator, a power amplifier, a loudspeaker, an FBG hy- hydrophone, a rectangular water tank (2 × 1 × 1 m), an optical circulator, a tunable laser, drophone, a rectangular water tank (2 1 1 m), an optical circulator, a tunable laser, a a photodetector, and an oscilloscope, as shown in Figure 2. photodetector, and an oscilloscope, as shown in Figure 2. Figure 2. The experimental setup of using FBG hydrophone to detect low-frequency acoustic waves. Figure 2. The experimental setup of using FBG hydrophone to detect low-frequency acoustic From Figure 2, we can see that the loudspeaker and the FBG hydrophone are placed waves. inside the water tank that is filled with water. Owing to the simple insulated loudspeaker, it is difficult to generate high-frequency acoustic waves under water. Thus, acoustic waves of From Figure 2, we can see that the loudspeaker and the FBG hydrophone are placed the low-frequency band are only generated by the loudspeaker, which is driven by a signal inside the water tank that is filled with water. Owing to the simple insulated generator and a power amplifier. The FBG hydrophone is positioned in front of the speaker loudspeaker, it is difficult to generate high-frequency acoustic waves under water. Thus, with the separation of around several centimeters. The wavelength of the tunable laser is acoustic waves of the low-frequency band are only generated by the loudspeaker, which tuned to be identical to the wavelength of the FBG hydrophone to obtain the maximum is driven by a signal generator and a power amplifier. The FBG hydrophone is overlapped optical power, as shown in Figure 3. When the acoustic pressure is applied to positioned in front of the speaker with the separation of around several centimeters. The cause the sensing grating wavelength shift, the variation frequency of the overlapped light wavelength of the tunable laser is tuned to be identical to the wavelength of the FBG power between the grating and the tunable laser will be synchronized with the acoustic hydrophone to obtain the maximum overlapped optical power, as shown in Figure 3. wave frequency. From the experimental results, the variation frequency of overlapped light power is confirmed to be identical to the acoustic frequency. The schematic configuration When the acoustic pressure is applied to cause the sensing grating wavelength shift, the of measuring acoustic frequency is illustrated in Figure 4. variation frequency of the overlapped light power between the grating and the tunable The acoustic wave generated from the loudspeaker is measured by the FBG hy- laser will be synchronized with the acoustic wave frequency. From the experimental drophone in the format of a light signal created by the overlap between the tunable laser results, the variation frequency of overlapped light power is confirmed to be identical to output power and the reflection spectrum of the sensing grating. The overlapped light the acoustic frequency. The schematic configuration of measuring acoustic frequency is power variation from the output port of the circulator is launched into the photodetector illustrated in Figure 4. (PD) for obtaining the transferred electrical signal, which is sent to an oscilloscope for comparing the original acoustic signal. One of the two output ports of the signal gener- ator is connected to the input of the signal amplifier to attain the larger electrical signal to drive the loudspeaker for obtaining the acoustic wave. The other output port of the signal generator is connected to the input channel 1 (above curve) of the oscilloscope to Photonics 2022, 9, 255 6 of 11 compare the measured acoustic signal of channel 2 (below curve) from the output terminal of the photodetector to confirm that the two signals have identical frequency, as shown in Photonics 2022, 9, x FOR PEER REVIEW 6 of 12 Figure 4. From this figure, we can see that the signals in the two channels have the same Photonics 2022, 9, x FOR PEER REVIEW 6 of 12 frequency of 130.73 Hz. Moreover, by means of changing the frequency or amplitude of the signal generator, we can also see that the two channel signals in the oscilloscope are simultaneously varied but with a slight phase shift. -20 -20 Original Original Stretched Stretched -30 -30 Tunable light source Tunable light source -40 -40 -50 -50 -60 -60 -70 -70 -80 -80 -90 -90 1550 1552 1554 1556 1558 Wavelength (nm) 1550 1552 1554 1556 1558 Wavelength (nm) Figure 3. The spectra both of laser and grating wavelength shift for detecting the acoustic Figure 3. The spectra both of laser and grating wavelength shift for detecting the acoustic frequency. frequency. Figure 3. The spectra both of laser and grating wavelength shift for detecting the acoustic frequency. Figure 4. The comparison between the original electrical signal and the FBG hydrophone detecting Figure 4. The comparison between the original electrical signal and the FBG hydrophone detecting signal shown in an oscilloscope. signal shown in an oscilloscope. By using the HF-etching fiber technique to increase the sensing sensitivity, three FBGs The acoustic wave generated from the loudspeaker is measured by the FBG with different fiber diameters of 33, 71, and 97 m are obtained for three FBG hydrophones. Figure 4. The comparison between the original electrical signal and the FBG hydrophone detecting hydrophone in the format of a light signal created by the overlap between the tunable The experimental results of measuring the low-frequency acoustic waves are shown in signal shown in an oscilloscope. laser output power and the reflection spectrum of the sensing grating. The overlapped Figure 5. From this figure, the fiber diameter of 33 m has the best sensitivity, showing that light power variation from the output port of the circulator is launched into the the thinner the fiber diameter, the better the sensitivity and response. Acoustic signals above photodetector (PD) for obtaining the transferred electrical signal, which is sent to an The acoustic wave generated from the loudspeaker is measured by the FBG the frequency of 200 Hz cannot be detected, as the acoustic wave power in the frequency oscilloscope for comparing the original acoustic signal. One of the two output ports of hydrophone in the format of a light signal created by the overlap between the tunable above 200 Hz becomes very small and difficult to measure with this FBG hydrophone, as the signal generator is connected to the input of the signal amplifier to attain the larger shown in Figure 5. laser output power and the reflection spectrum of the sensing grating. The overlapped electrical signal to drive the loudspeaker for obtaining the acoustic wave. The other light power variation from the output port of the circulator is launched into the output port of the signal generator is connected to the input channel 1 (above curve) of photodetector (PD) for obtaining the transferred electrical signal, which is sent to an the oscilloscope to compare the measured acoustic signal of channel 2 (below curve) oscilloscope for comparing the original acoustic signal. One of the two output ports of from the output terminal of the photodetector to confirm that the two signals have the signal generator is connected to the input of the signal amplifier to attain the larger identical frequency, as shown in Figure 4. From this figure, we can see that the signals in electtric he t al wo channel signal to drive s have the sa the loud me spea frequency of ker for obt 130.73 aining the aco Hz. Moreover, by ustic wmea ave.n The s of other changing the frequency or amplitude of the signal generator, we can also see that the output port of the signal generator is connected to the input channel 1 (above curve) of two channel signals in the oscilloscope are simultaneously varied but with a slight phase the oscilloscope to compare the measured acoustic signal of channel 2 (below curve) shift. from the output terminal of the photodetector to confirm that the two signals have By using the HF-etching fiber technique to increase the sensing sensitivity, three identical frequency, as shown in Figure 4. From this figure, we can see that the signals in FBGs with different fiber diameters of 33, 71, and 97 μm are obtained for three FBG the two channels have the same frequency of 130.73 Hz. Moreover, by means of hydrophones. The experimental results of measuring the low-frequency acoustic waves changing the frequency or amplitude of the signal generator, we can also see that the are shown in Figure 5. From this figure, the fiber diameter of 33 μm has the best two channel signals in the oscilloscope are simultaneously varied but with a slight phase sensitivity, showing that the thinner the fiber diameter, the better the sensitivity and shift. By using the HF-etching fiber technique to increase the sensing sensitivity, three FBGs with different fiber diameters of 33, 71, and 97 μm are obtained for three FBG hydrophones. The experimental results of measuring the low-frequency acoustic waves are shown in Figure 5. From this figure, the fiber diameter of 33 μm has the best sensitivity, showing that the thinner the fiber diameter, the better the sensitivity and Refractive power (dBm) Refractive power (dBm) Photonics 2022, 9, x FOR PEER REVIEW 7 of 12 Photonics 2022, 9, x FOR PEER REVIEW 7 of 12 response. Acoustic signals above the frequency of 200 Hz cannot be detected, as the acoustic wave power in the frequency above 200 Hz becomes very small and difficult to measure with this FBG hydrophone, as shown in Figure 5. response. Acoustic signals above the frequency of 200 Hz cannot be detected, as the Photonics 2022, 9, 255 7 of 11 acoustic wave power in the frequency above 200 Hz becomes very small and difficult to measure with this FBG hydrophone, as shown in Figure 5. 98μm 71μm 98μm 33um 71μm 33um 0 50 100 150 200 250 300 Frequency(Hz) 0 50 100 150 200 250 300 Frequency(Hz) Figure 5. The detecting electrical signal versus different fiber diameters. Figure 5. The detecting electrical signal versus different fiber diameters. To confirm that this FBG hydrophone has the ability to detect high-frequency Figure 5. The detecting electrical signal versus different fiber diameters. To confirm that this FBG hydrophone has the ability to detect high-frequency acoustic acoustic waves, the experimental setup used to detect acoustic waves with the FBG waves, the experimental setup used to detect acoustic waves with the FBG hydrophone, hydrophone, except for the signal generator, amplifier, loudspeaker, and water tank, To confirm that this FBG hydrophone has the ability to detect high-frequency except for the signal generator, amplifier, loudspeaker, and water tank, was moved to was moved to the Underwater Acoustic Laboratory of the Department of Marine acoustic waves, the experimental setup used to detect acoustic waves with the FBG the Underwater Acoustic Laboratory of the Department of Marine Engineering of Taiwan Engineering of Taiwan University to measure acoustic waves in the frequency range University to measure acoustic waves in the frequency range from 4 to 10 kHz. The hydrophone, except for from 4 to 10 kHz. The Underwa the signal ter Acou generator, stic La boratory amplif has the mo ier, loudspeaker st stand , and ard water tank, Underwater Acoustic Laboratory has the most standard measurement facility for detecting measurement facility for detecting acoustic waves under water, including a signal was moved to the Underwater Acoustic Laboratory of the Department of Marine acoustic waves under water, including a signal processor, an amplifier, a conventional processor, an amplifier, a conventional standard hydrophone, an acoustic wave source, Engineering of Taiwan University to measure acoustic waves in the frequency range standard hydrophone, an acoustic wave source, and a large water pool with length of and a large water pool with length of 120 m, width of 8 m, and depth of 4 m, as shown in from 4 t 120 m, width o 10 of kHz 8 m, and . The Underwa depth of 4 m, as shown ter Acou in Figur stic L e 6. This aboratory conventional has the mo standard st standard Figure 6. This conventional standard hydrophone is based on a piezoelectric transducer, hydrophone is based on a piezoelectric transducer, transferring the detected acoustic waves meas tran uremen sferringt fac the deiltect ity ed for acous detec tic wav tin eg s toac the ous votlic w tage sia gna ves l in under w the high-freq ate uer, inc ncy balnd uding a signal to the voltage signal in the high-frequency band to be shown in an oscilloscope. The to be shown in an oscilloscope. The FBG hydrophone is based on the photodetector, processor, an amplifier, a conventional standard hydrophone, an acoustic wave source, FBG hydrophone is based on the photodetector, which converts the optical signal into the which converts the optical signal into the electrical signal to be shown in an oscilloscope. and a large water pool with length of 120 m, width of 8 m, and depth of 4 m, as shown in electrical signal to be shown in an oscilloscope. The experimental results of both the FBG The experimental results of both the FBG hydrophone and the standard hydrophone can hydrophone and the standard hydrophone can be compared to determine which kind of Figure 6. This conventional standard hydrophone is based on a piezoelectric transducer, be compared to determine which kind of hydrophone demonstrates better sensing hydrophone demonstrates better sensing performance. performance. transferring the detected acoustic waves to the voltage signal in the high-frequency band to be shown in an oscilloscope. The FBG hydrophone is based on the photodetector, which converts the optical signal into the electrical signal to be shown in an oscilloscope. The experimental results of both the FBG hydrophone and the standard hydrophone can be compared to determine which kind of hydrophone demonstrates better sensing performance. Figure 6. The standard measurement system of detecting acoustic waves in the Underwater Figure 6. The standard measurement system of detecting acoustic waves in the Underwater Acoustic Acoustic Laboratory of the Department of Marine Engineering of Taiwan University. Laboratory of the Department of Marine Engineering of Taiwan University. After the standard hydrophone is calibrated, the FBG hydrophone and standard hydrophone are located 2 m away from the acoustic wave source for receiving the same level of acoustic wave pressure. The separation between the FBG hydrophone and standard hydrophone is 1 m to avoid mutual interference, as shown Figure 7. Finally, the sensitivity Figure 6. The standard measurement system of detecting acoustic waves in the Underwater Acoustic Laboratory of the Department of Marine Engineering of Taiwan University. Volts(mV) Volts(mV) Photonics 2022, 9, x FOR PEER REVIEW 8 of 12 After the standard hydrophone is calibrated, the FBG hydrophone and standard hydrophone are located 2 m away from the acoustic wave source for receiving the same level of acoustic wave pressure. The separation between the FBG hydrophone and Photonics 2022, 9, 255 8 of 11 standard hydrophone is 1 m to avoid mutual interference, as shown Figure 7. Finally, the sensitivity of the FBG hydrophone is calculated by measuring the sound pressure level (SPL) and peak-to-peak voltage value (VL). of the FBG hydrophone is calculated by measuring the sound pressure level (SPL) and peak-to-peak voltage value (VL). Figure 7. Experimental platform by using the FBG hydrophone and the standard piezoelectric hydrophone. Figure 7. Experimental platform by using the FBG hydrophone and the standard piezoelectric hydrophone. A frequency transducer (TR-208A) is used to generate the acoustic wave with the frequency of 4 kHz and 6 pulses per signal package. From Figure 8, the top blue signal is the A fre original quency tran outputsd signal ucer (T from R-20 the 8A) is us generator ed to gener to be output ate the aco to drive ustthe ic wav transducer e with the (TR- freq 208A), uencthe y ofmiddle 4 kHz and black 6 p signal ulsesis pe the r sig acoustic nal pacwave kage. From measur Fed igu by re 8, the th standar e top bd lu hydr e sign ophone al is the or (8103, igin B&K), al ouand tputthe sign bottom al from signal the gen is the erato acoustic r to bewave outpu detected t to driv by e the the tr FBG ansd hydr ucer (T ophone. R- 20Fr 8A om ), the the comparison middle blac ofk s these ignal experimental is the acou rs esults, tic wav wee m caneas seeured that b the y the proposed standar FBG d Photonics 2022, 9, x FOR PEER REVIEW 9 of 12 hydrop hydrophone hone (8 has 103better , B&K) sensitivity , and the b than ottom that sign of the al istandar s the aco d piezoelectric ustic wave de hydr tectophone. ed by the For detecting the original acoustic wave of 6 pulses, there are 20 pulses to be measured by the FBG hydrophone. From the comparison of these experimental results, we can see that two hydrophones. This is because the echo caused by the acoustic wave is reflected back to the proposed FBG hydrophone has better sensitivity than that of the standard the hydrophone. piezoelectric hydrophone. For detecting the original acoustic wave of 6 pulses, there are 20 pulses to be measured by the two hydrophones. This is because the echo caused by -0.002 0.000 0.002 0.004 the acoustic wave is reflected back to the hydrophone. TR-208A 0.25 0.00 -0.25 -0.50 Standard hydrophone 0.12 4 5 6 7 1 8 10 11 13 14 15 16 12 17 19 20 0.00 -0.12 0.46 Fiber grating hydrophone 5 6 0.23 8 9 16 11 13 17 12 18 0.00 -0.23 -0.002 0.000 0.002 0.004 Time domain (sec) Figure 8. Electrical signals for a frequency of 4 kHz. Figure 8. Electrical signals for a frequency of 4 kHz. Moreover, for changing the acoustic frequency from 4 to 10 kHz with 1 kHz per step, the experimental results of using the FBG hydrophone with the fiber diameter of 25 μm are shown in Figure 9. According to this figure, the sensitivity or responsiveness of the FBG hydrophone is inversely proportional to the acoustic frequency. This is attributed to three factors, namely, the silicone diaphragm size, the FBG length, and the packaging materials. Different sensing areas and different lengths of FBG for detecting acoustic waves are similar to different lengths of guitar strings with different frequency responses. For the third factor, the hardness, elasticity, and ductility of the packaging material will affect the sensing performance. In the future, the sensing size and packaging materials will be changed to obtain better performance. Voltage (V) Photonics 2022, 9, 255 9 of 11 Moreover, for changing the acoustic frequency from 4 to 10 kHz with 1 kHz per step, the experimental results of using the FBG hydrophone with the fiber diameter of 25 m are shown in Figure 9. According to this figure, the sensitivity or responsiveness of the FBG hydrophone is inversely proportional to the acoustic frequency. This is attributed to three factors, namely, the silicone diaphragm size, the FBG length, and the packaging materials. Different sensing areas and different lengths of FBG for detecting acoustic waves Photonics 2022, 9, x FOR PEER REVIEW 10 of 12 are similar to different lengths of guitar strings with different frequency responses. For the third factor, the hardness, elasticity, and ductility of the packaging material will affect the sensing performance. In the future, the sensing size and packaging materials will be changed to obtain better performance. 0.000 0.002 0.004 0.24 10 kHz 0.12 0.00 -0.12 0.13 9 kHz 0.00 -0.13 -0.26 0.28 8 kHz 0.14 0.00 -0.14 0.36 7 kHz 0.18 0.00 -0.18 0.34 6 kHz 0.17 0.00 -0.17 0.34 5 kHz 0.17 0.00 -0.17 0.36 4 kHz 0.18 0.00 -0.18 0.000 0.002 0.004 Time domain (sec) Figure 9. The electrical signals of a fiber Bragg grating hydrophone for different frequencies in the range of 4 to 10 kHz. Figure 9. The electrical signals of a fiber Bragg grating hydrophone for different frequencies in the range of 4 to 10 kHz. Figure 10 shows the sensitivity curves of the FBG hydrophone versus the frequency of acoustic waves with three different fiber diameters of 80, 60, and 25 m. From this figure, Figure 10 shows the sensitivity curves of the FBG hydrophone versus the frequency we can observe that the FBG hydrophone of 25 m has the highest sensitivity, because the smaller the fiber diameter, the easier the fiber bending. The experimental results reveal that of acoustic waves with three different fiber diameters of 80, 60, and 25 μm. From this the FBG hydrophone has a suitable performance to measure acoustic signals under water. figure, we can observe that the FBG hydrophone of 25 μm has the highest sensitivity, Although the frequency response of the FBG hydrophone is not wide-band in comparison because the smaller the fiber diameter, the easier the fiber bending. The experimental with the other types of hydrophones, the sensing head can be redesigned to improve the results reveal that the FBG hydrophone has a suitable performance to measure acoustic performance in the future. signals under water. Although the frequency response of the FBG hydrophone is not wide-band in comparison with the other types of hydrophones, the sensing head can be redesigned to improve the performance in the future. Voltage (V) Photonics 2022, 9, x FOR PEER REVIEW 11 of 12 Photonics 2022, 9, 255 10 of 11 -190 80μm -195 60μm -200 25μm -205 -210 -215 -220 -225 -230 -235 -240 -245 -250 4 567 89 10 Frequency(kHz) Figure 10. The sensitivity curves of fiber grating hydrophone versus the acoustic wave frequency in three different fiber diameters. Figure 10. The sensitivity curves of fiber grating hydrophone versus the acoustic wave frequency in three different fiber diameters. 4. Conclusions In this study, the proposed hydrophone based on FBGs is a simple and new type of 4. Conclusions fiber hydrophone. The experimental results demonstrate that the FBG hydrophone has better sensitivity than that of the commercial standard hydrophone, illustrating that the In this study, the proposed hydrophone based on FBGs is a simple and new type of smaller the fiber diameter, the higher the sensitivity and frequency response. The fre- fiber hydrophone. The experimental results demonstrate that the FBG hydrophone has quency response and sensitivity can be improved by reducing the fiber diameter. Moreover, better sensitivity than that of the commercial standard hydrophone, illustrating that the the performance of this FBG hydrophone can be optimized by changing several parame- ters, including the fiber axial stress, fiber diameter, silicone thin film thickness, and thin smaller the fiber diameter, the higher the sensitivity and frequency response. The film radius. frequency response and sensitivity can be improved by reducing the fiber diameter. Moreover, the performance of this FBG hydrophone can be optimized by changing Author Contributions: Conceptualization, W.-F.L.; methodology, W.-F.L., J.-G.L. and H.-Y.C.; formal analysis, W.-F.L. and J.-G.L.; investigation, W.-F.L., J.-G.L., H.-Y.C. and M.-Y.F.; resources W.-F.L. and several parameters, including the fiber axial stress, fiber diameter, silicone thin film C.-F.C.; data curation, W.-F.L. and J.-G.L.; writing—original draft preparation, W.-F.L., J.-G.L., H.-Y.C. thickness, and thin film radius. and M.-Y.F.; writing—review and editing, W.-F.L., H.-Y.C., M.-Y.F. and C.-F.C.; supervision, W.-F.L.; project administration, W.-F.L.; funding acquisition, W.-F.L. All authors have read and agreed to the published version of the manuscript. Author Contributions: Conceptualization, W.-F.L.; methodology, W.-F.L., J.-G.L. and H.-Y.C.; formal analysis, W.-F.L. and J.-G.L.; investigation, W.-F.L., J.-G.L., H.-Y.C. and M.-Y.F.; resources Funding: This research was funded by the Ministry of Science and Technology, Taiwan (contract no. W.-F MOST .L. an 110-2221-E-035-055). d C.-F.C.; data curation, W.-F.L. and J.-G.L.; writing—original draft preparation, W.- F.L., J.-G.L., H.-Y.C. and M.-Y.F.; writing—review and editing, W.-F.L., H.-Y.C., M.-Y.F. and C.- Institutional Review Board Statement: Not applicable. F.C.; supervision, W.-F.L.; project administration, W.-F.L.; funding acquisition, W.-F.L. All authors Informed Consent Statement: Not applicable. have read and agreed to the published version of the manuscript. Data Availability Statement: Not applicable. Funding: This research was funded by the Ministry of Science and Technology, Taiwan (contract Acknowledgments: The authors would like to thank the Ministry of Science and Technology, Taiwan, no. MOST 110-2221-E-035-055). for sponsoring this research under contract no. MOST 108-2221-E-035-075-MY2 and no. MOST 110-2221-E-035-055-MY2. Institutional Review Board Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Informed Consent Statement: Not applicable. References Data Availability Statement: Not applicable. 1. Hill, K.O.; Fujii, Y.; Johnson, D.D.; Kawasaki, B.S. Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication. Appl. Phys. Lett. 1978, 32, 647–649. [CrossRef] Acknowledgments: The authors would like to thank the Ministry of Science and Technology, 2. Meltz, G.; Morey, W.W.; Glenn, W.H. Formation of Bragg gratings in optical fibers by a transverse holographic method. Opt. Lett. Taiwan, for sponsoring this research under contract no. MOST 108-2221-E-035-075-MY2 and no. 1989, 14, 823–825. [CrossRef] [PubMed] MOST 110-2221-E-035-055-MY2. Conflicts of Interest: The authors declare no conflict of interest. References 1. Hill, K.O.; Fujii, Y.; Johnson, D.D.; Kawasaki, B.S. Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication. Appl. Phys. Lett. 1978, 32, 647–649. 2. Meltz, G.; Morey, W.W.; Glenn, W.H. Formation of Bragg gratings in optical fibers by a transverse holographic method. Opt. Lett. 1989, 14, 823–825. 3. Hill, K.O.; Malo, B.; Bilodeau, F.; Thériault, S.; Johnson, D.C.; Albert, J. Variable-spectral-response optical waveguide Bragg grating filter for optical signal processing. Opt. Lett. 1995, 20, 1438–1440. Sen sitivty(d B re 1 Vμ Pa ) Photonics 2022, 9, 255 11 of 11 3. Hill, K.O.; Malo, B.; Bilodeau, F.; Thériault, S.; Johnson, D.C.; Albert, J. Variable-spectral-response optical waveguide Bragg grating filter for optical signal processing. Opt. Lett. 1995, 20, 1438–1440. [CrossRef] [PubMed] 4. Kersey, A.D.; Davis, M.A.; Patrick, H.J.; LeBlanc, M.; Koo, K.P.; Askins, C.G.; Putnam, M.A.; Friebele, E.J. Fiber Grating Sensors. J. Lightwave Technol. 1997, 15, 1442–1463. [CrossRef] 5. Munendhar, P.; Aneesh, R.; Khijwania, S.K. Development of an all-optical temperature insensitive nonpendulum-type tilt sensor employing fiber Bragg gratings. Appl. Opt. 2014, 53, 3574–3580. [CrossRef] [PubMed] 6. Liu, T.; Chen, Y.; Liu, Q.H.F.; Yao, Y. Sensor based on macrobent fiber Bragg grating structure for simultaneous measurement of refractive index and temperature. Appl. Opt. 2016, 55, 791–795. [CrossRef] 7. Liao, C.; Wang, Q.; Xu, L.; Liu, S.; He, J.; Zhao, J.; Li, Z.Y.; Wang, Y.P. D-shaped fiber grating refractive index sensor induced by an ultrashort pulse laser. Appl. Opt. 2016, 55, 1525–1529. [CrossRef] 8. Ahmad, H.; Chong, W.Y.; Zulklifi, M.Z.; Poopalan, P.; Harun, S.W. High Sensitivity Fiber Bragg Grating Pressure Sensor Using Thin Metal Diaphragm. IEEE Sens. 2009, 9, 1654–1659. [CrossRef] 9. Liaw, S.K.; Hung, K.L.; Lin, Y.T.; Chiang, C.C.; Shin, C.S. C-band Continuously Tunable Fiber Lasers Using Fiber Bragg Gratings. Opt. Lasers Eng. 2017, 39, 1214–1217. [CrossRef] 10. Rao, T.J. Recent Process in Applications Of In Fiber Bragg Gratings Sensors. Opt. Lasers Eng. 1999, 31, 297–324. [CrossRef] 11. Lee, C.L.; Tsai, Y.N.; Chen, G.H.; Xiao, Y.J.; Hsu, J.M.; Horng, J.S. Refined Bridging of Microfiber Plugs in Hollow Core Fiber for Sensing Acoustic Vibrations. IEEE Photonic Technol. Lett. 2015, 27, 2403–2406. [CrossRef] 12. Staudenraus, J.; Eisenmenger, W. Fibre-optic probe hydrophone for ultrasonicand shock-wave measurements in water. Ultrasonics 1993, 31, 267–273. [CrossRef] 13. Beard, P.C.; Mills, T. Miniature optical fiber ultrasonic hydrophone using a Fabry-Perot polymer film interferometer. Electron. Lett. 1997, 33, 801–803. [CrossRef] 14. Koch, C. Coated fiber-optic hydrophone for ultrasonic measurement. Ultrasonics 1996, 34, 687–689. [CrossRef] 15. Koch, C.; Molkenstruck, W.; Reibold, R. Shock-wave measurement using a calibrated interferometric fiber-tip sensor. Ultrasound Med. Biol. 1997, 23, 1259–1266. [CrossRef] 16. Fisher, N.E.; Surowiec, J.; Webb, D.J.; Jackson, D.A.; Gavrilov, L.R.; Hand, J.W.; Zhang, L.; Bennion, I. In-fiber Bragg grating for ultrasonic medical applications. Meas. Sci. Technol. 1997, 8, 1050–1054. [CrossRef] 17. Takahashi, N.; Yoshimura, K.; Takahashi, S.; Imamura, K. Development of an optical fiber hydrophone with fiber Bragg grating. Ultrasonics 2000, 38, 581–585. [CrossRef] 18. Cusano, A.; Campopiano, S.; Addio, S.D.; Balbi, M.; Balzarini, S.; Giordano, M.; Cutolo, A. Optical Fiber Hydrophone using Poltmer-Coated Fiber Bragg Grating. In Optical Fiber Sensors; Optical Society of America: Washington, DC, USA, 2006; p. ThE85. 19. Moccia, M.; Consales, M.; Iadicicco, A.; Pisco, M.; Cutolo, A.; Galdi, V.; Cusano, A. Resonant Hydrophone Based on Coated Fiber Bragg Gratings. J. Lightwave Technol. 2012, 30, 2472–2481. [CrossRef] 20. Guan, B.O.; Tan, Y.N.; Tam, H.Y. Dual polarization fiber grating laser hydrophone. Opt. Express 2009, 17, 19544–19550. [CrossRef] 21. Karas, A.R.; Papageorgiou, A.W.; Cook, P.R.; Arkwrigh, J.W. A passive optical fibre hydrophone array utilising fibre Bragg grating sensors. In Proceedings of the SPIE of the Photonic Instrumentation Engineering V 105390H, San Francisco, CA, USA, 22 January 2018. 22. Beard, P.C.; Hurrell, A.M.; Mills, T.N. Characterization of a polymer film optical fiber hydrophone for use in the range 1 to 20 MHz: A comparison with PVDF needle and membrane hydrophones. IEEE Trans. Ultra. Ferro. Freq. Control 2000, 47, 256–264. [CrossRef] 23. Koch, C.; Ludwig, G.; Molkenstruck, W. Calibration of a fiber tip ultrasonic sensor up to 50 MHz and the application to shock wave measurement. Ultrasonics 1998, 36, 721–725. [CrossRef] 24. Sypabekova, M.; Korganbayev, S.; Gonzalez-Vila, A.; Caucheteur, C.; Shaimerdenova, M.; Ayupova, T.; Bekmurzayeva, A.; Vangelista, L.; Tosi, D. Functionalized etched tilted fiber Bragg grating aptasensor for label-free protein detection. Biosens. Bioelectron. 2019, 146, 111765. [CrossRef] [PubMed] 25. Rui Min, R.; Liu, Z.; Pereira, L.; Yang, C.; Sui, Q.; Marques, C. Optical fiber sensing for marine environment and marine structural healthmonitoring: A review. Opt. Laser Technol. 2021, 140, 107082. [CrossRef]
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