Fiber optic extrinsic Fabry-Perot interferometric sensor for high blast pressure measurement

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 53, September 2015, pp Fiber optic extrinsic Fabry-Perot interferometric sensor for high blast pressure measurement G C Poddar, Asha Kumar, Shilpa Das, Deepa Srivastava & G S Singh CSIR-Central Scientific Instruments Organisation, Sector-30-C, Chandigarh gopal287@rediffmail.com Received 22 September 2014; revised 20 January 2015; accepted 21 May 2015 Optical fiber based Extrinsic Fabry-Perot Interferometric (EFPI) sensor significantly emerged as an advanced optical sensor for the health monitoring of civil, mechanical and aeronautical structures. This sensor responds its well capability also towards the measurement of acoustic/shock wave pressure. This paper presents an approach for high blast (acoustic/shock wave) pressure measurement using low cost indigenous rugged type optical fiber EFPI sensor. Optical fiber EFPI sensor works on the two fundamental principles; Fresnel reflection and Fabry-Perot Interferometry by interconnecting single mode optical fiber within the silica capillary which represents Fabry-Perot cavity (or air cavity) as sensing elements. This sensor consists of dielectric materials and is immune to Electromagnetic Interference (EMI), Radio Frequency Interference (RFI) and less sensitive to noise. This paper highlights the fabrication as well as packaging techniques of EFPI sensor which are experimentally more reliable in blast pressure. Keywords: High blast pressure, Shock wave, Fresnel reflection, Fabry Perot interferometry, Optical fiber EFPI sensor, Single mode fiber 1 Introduction A blast or explosion in free air and underwater causes a shock wave which fluctuates/disturbs the atmospheric pressure rapidly and reacts to restore the equilibrium state. The pressure field generated by an explosion is propagated from the source by a shock wave-refraction wave system. The field is described by the ambient conditions and by three variablespressure, time and distance from the source 1. In this state, high amplitude of shock wave generated rapidly that results in damages. Shock wave (SW) is characterized as a large pressure fluctuation that typically lasts only a few seconds. This may lead to not only blast-induced traumatic brain injuries 2, killing the human beings but also damaging the precious and huge infrastructures. Therefore, a mechanism for measuring blast pressure is important and its design should be on appropriate structure which is able to withstand blast impacts and save damages. In the recent years, Extrinsic Fabry Perot Interferometric (EFPI) sensor has emerged as an important technology for monitoring blast (acoustic/shock wave) pressure owing to their various attractive features over the existing conventional techniques. In this direction, various applications of Optical fiber EFPI sensor are realized. For underwater shock wave sensor based on polymer film optical fiber Fabry-Perot cavity manufactured by vacuum deposition technology 3 in which the acoustic stress wave that changes the thickness of the polymer film, thereby giving rise to a phase shift. An experimental study shows that using EFPI sensor, the simultaneous measurement of temperature and pressure can be detected, respectively by using CO 2 laser to create a loss between them to balance their reflection power levels 4. In the airport ground traffic monitoring EFPI, sensor has been used for acoustic emission and mechanical interaction between the vehicle and ground 5. Although EFPI sensor leads with strain phenomenon which presents high sensitivity with the development of pressure, strain and temperature beyond the safe limit in the civil, mechanical and aerospace structures generates the adverse conditions. To detect these hazardous parameters developed in the structures, different types of sensor technologies have been reported globally. In this trend, extrinsic Fabry-Perot interferometer pressure sensor based on a high quality nanothick silver diaphragm 6 may be reliable and safe but it is more beneficiary towards the high blast pressure applications. This paper presents the rugged type EFPI sensor which has been designed and fabricated by integrating it with a suitable packaging technique for the high blast pressure measurement.

2 574 INDIAN J PURE & APPL PHYS, VOL 53, SEPTEMBER 2015 Fiber Optic Sensor technology has emerged with special characteristics as a new sensor technology which is immune to Electro Magnetic Interference (EMI) and Radio Frequency Interference (RFI). Mostly, fiber optic sensors are based on the intensity, wavelength and refractive index modulations. Particularly, EFPI sensor is based on phase modulation, which gives the best performance in the detection of crack, deformation, strain, pressure and temperature at various structures. 2 Theory EFPI sensor consists of one silica capillary tube and two single mode fibers. In EFPI sensor, two single mode fibers are adjusted in a silica capillary tube using high thermal bonding adhesive 7 in such a manner that determining the sensing guage length (L) and a certain length of an air cavity (S) typically of 55 µm is created between them. A fiber can be either of single mode or multi mode called as reflector fiber and opposite of it a single mode fiber called as detector fiber or source fiber is simply connectorized with a Ferrule Connector (FC) on its other end. The faces of both the fibers placed in the silica capillary tube opened in the air cavity act as partial mirrors. These mirrors respond Fresnel Reflection of light beam and Air Cavity called as Fabry Perot (FP) cavity. This responds Fabry Perot Interference between two beams of infrared light having 830 nm wavelength occurred on the face of detector due to phase difference or change in optical path length. The source fiber is connected with IR source and detector using 3dB coupler. Optical fiber EFPI sensor works on the two fundamental principles; Fresnel Reflection and Fabry-Perot Interferometer by interconnecting single mode optical fiber within the silica capillary which represents Fabry-Perot Cavity (or Air Gap) as sensing elements. EFPI sensor is interfaced with an IR source and a detector using 3 db coupler in-built in the Optical Processor. 3 db fiber coupler splits 50 per cent IR light of wavelength 830 nm transmitting from the source and responds to coupling it into sensor s domain. Here, characteristic of light obeys the Fresnel Reflection from the two partial mirrors, respectively between the FP cavity and later transmits both the reflected beams together into source fiber towards the detector connected with the coupler. Both the beams of IR light interfere with each other and generate the interference pattern due to change in optical path length or phase difference on the face of detector. FP cavity represents a short interval between the two reflectance from the partial mirrors which cause phase difference. Phase difference or optical path length changes occurred correspondingly to the change in the length of FP cavity of the sensor. Number of interference pattern or fringe pattern depends upon the physical change in the length of FP cavity. Figure 1(a) shows the working principle and Fig. 1(b) shows the model of EFPI sensor fabricated indigenously in our laboratory. As reported, the EFPI sensor has been designed with the diaphragm based EFPI pressure sensors have been successfully used for low pressure and acoustic wave detection. Fabry-Perot cavity is usually formed between the fiber end face and the inner surface of the diaphragm 8. The increase in pressure causes the diaphragm to deflect, changing the cavity length of the EFPI. By measuring the cavity length, the pressure can be determined 9. In other option, the optical gap varies with silicon diaphragm deflection, which in turn varies with applied pressure 10. Some of EFPI sensors have also been (a) (b) Fig. 1 (a) Schematic of EFPI sensor perspective and (b)-model of EFPI sensor as strain gage

3 PODDAR et al.: FIBER OPTIC EXTRINSIC FABRY-PEROT INTERFEROMETRIC SENSOR 575 fabricated and tested made of metal diaphragm and a single mode optical fiber 11 for the measurement of static pressure. These types of diaphragms are very reliable and durable in the applications of optical fiber EFPI Sensor for low acoustic wave/shock wave pressure measurement. This paper differs the above diaphragm based applications and explains the existing technique for widely capturing high shock wave generated from the explosion in the face of rugged type EFPI sensor embedded in the centre of 60 rigid cone grooved into 30 kg metallic block. It has been observed during an experiment that indigenous EFPI sensor has high sensing ability in the open air for the measurement of high blast (acoustic/shock wave) pressures in the case of maintaining the proper packaging process. In the event of blast pressure, the sensor quickly operates into smart mode and plots the graph of amplitude of strain ( ) versus applied blast pressure physically with acquired data on the computer monitor. EFPI sensor responds its sensitivity by changing in length of its FP cavity ( dp) correspondingly change of optical spectrum (fringe pattern). The sensitive gauge length (L g ) of the sensor can be determined at the time of fabrication. So, the EFPI formula can be established as: dp = (1) L g Hence, dp = Lg (2) Value of strain ( ) obtained in micro strain unit on applying certain blast pressure which causes change in the FP cavity length ( d p ) of sensor. We have predetermined gauge length (L g ) between the two thermal bonding points. Therefore, we can easily obtain the change in FP cavity length ( d p ) or change in optical path length from Eq. (2). Now, from the principle of mechanics 12, change in FP cavity length ( d p ) varies with air pressure (p) as per Eq. (3): 2 PLg ro p 2 2 E ro ri d = (1 2 µ ) ( ) (3) where r o is the outer diameter and r i is the inner diameter of silica capillary tube used in the fabrication of EFPI sensor. Young s modulus of the silica capillary materials is E and Poisson ratio is µ. By eliminating d p and L g using Eqs (2) and (3), the values of strain and applied pressure obtained, are shown in Eqs (4) and (5), respectively. 2 Pro 2 2 o ri = (1 2 µ ) E( r ) 2 2 o ri 2 o E( r ) P = r (1 2 µ ) (4) (5) The value of strain ( ) can be obtained from Eq. (4) or directly by optical signal processing and shockwave pressure (P) from Eq. (5), which present the response of EFPI sensor. 3 Experimental Details In our Advanced Materials and Sensors Laboratory, an application of EFPI sensor has been developed indigenously for shock wave measurement. The EFPI sensor is packaged in a 75 mm long steel needle of 18 gauge diameter and embedded into a bolt (see Fig. 2). This bolt was tighten longitudinally with a rigid metallic structure by keeping the EFPI face open in the air towards the blast source. In the set-up, a cubic iron block of 30 kg is used for rigidly holding the sensor to bear the blast pressure. The block has a large cone cut on the surface to capture the wide shock Fig. 2 EFPI packaging fixture for blast pressure sensing

4 576 INDIAN J PURE & APPL PHYS, VOL 53, SEPTEMBER 2015 waves generated by the source, on its centre where the face of embedded EFPI sensor in a packaging bolt is opened (Fig. 2). The cubic structure was mounted with nut-and-bolts on the smooth surface of concrete structure at the blast site. The sensor is attached with 125 meters fiber patch cord and interfaced with the optical signal processor in the blast process laboratory which was 100 meter away from the blast region. The patch cord was hidden in the ground so that it could be well protected from the effect of shock wave generated by the explosive material. Later 100 g of detonating/explosive material has been tied off with the arm of iron stand and sustained it with a thread in the air at 1.5 m height from the face of EFPI sensor (Fig. 3). On completion of experimental set-up, the signal processing unit was switched on to run and compile the software on computer monitor. At the initial stage, IR light of 830 nm wavelength is launched from the inbuilt source of optical processor. We have recorded the initial spectrum of complete optical path length (Phase difference) due to two beam interference pattern extrinsically formed on the face of detector causing by the intrinsically occurrence of phase modulation in the sensor. Figure 4(a) shows the spectrum of initial optical path length. Then data acquired by the signal processing unit showing the change in initial path length (phase change) of light corresponding to change in FP cavity due to the detonation of explosive causing shockwave impact on the face of EFPI sensor longitudinally and compressively. Figure 4(b) shows the change in the spectrum of path length after applying blast pressure on the sensor face. This results into high amplitude of optical signal with respect to developed strain (0.0016µ ) because of the applied pressure ( MPa) of shockwave in the sensor s domain as shown in Fig Results and Discussion The experimental set-up was prepared in the laboratory and first trial was initiated with the detonating material of 100 g for withstanding low impact of blast pressure upon EFPI sensor. Low impact of blast pressure from the distance of source 1.5 m was observed more effectively. In this trial, sensor was found to be safe and reliable for next attempt. Then explosion was repeated one by one with the same kind of detonating material and data with graphical expressions were recorded accordingly. Sensor was tested with another set of 200 g detonating material. In both the trials, it is observed that the performance of EFPI sensor depends upon its phase shifts which correspondingly change in the length of FP cavity as µm and µm, respectively due to high impacts of shockwave. Figure 4(c) shows the online strain-performance with respect to time by signal processing unit of well packaged optical fiber EFPI sensor to withstand shockwave impact upon it from a certain distance. Characteristic of EFPI strain graph tallies almost the ballistic-pressure graph within a time duration of shock wave effect. Then analyzing the recorded data by putting the values of strainamplitude obtained from the signal processing in the given shockwave pressure-eq. (5), we found that the strain was developed at µ in EFPI Sensor on applied blast pressure load of MPa by detonating 100 g explosive from a distance 1.5 m. Another set of data presented strain-amplitude of µ on applied blast pressure load of MPa by detonating 200 g explosive from the same distance. Optical fiber EFPI sensor responds within a fraction of sharp time duration of 60 µs and Fig. 3 Schematic of experimental set-up for shockwave measurement

5 PODDAR et al.: FIBER OPTIC EXTRINSIC FABRY-PEROT INTERFEROMETRIC SENSOR 577 (a) (b) Fig. 4 (a) Spectrum-1(Total Optical Path Length) and (b) Spectrum-2 (Change in Total Optical Path length due to effect of shockwave Fig. 5 Recorded EFPI strain amplitude graph on generating shockwave effect Table 1 Results obtained as per acquiring data from the signal processing Quantity taken of Explosive (Gram) Height & Distance of Detonating material from EFPI Sensor (Meter) / /100 Strain developed during experiment (µ ) Average Strain (µ ) Blast Pressure (MPa) µs approximately and withstands the high shockwave in both the trials by taking s for its thermal bonding adjustment due to raising temperature in the structure during blast impact. EFPI sensor performs also its characteristics according to the structure behaviour in which this is embedded. Therefore, it is concluded that optical fiber EFPI sensor presents a high strain-amplitude peak at the shortest fraction of micro second in the event of high blast pressure. Once EFPI sensor excited due to shockwave impact, it takes so long in resuming the rest due to adjusting the thermal bonding as well as temperature after withstanding the shockwave. The experiment has been conducted using the EFPI sensor fabricated with the UV cured high thermal bonding adhesive in the laboratory. Absolute thermal bonding is a major challenge in fabrication of EFPI sensor which can be improved by using carbon dioxide (CO 2 ) gas laser 12 technique. The results have been obtained as per acquiring data from the signal processing as presented in Table 1 which give the future trend in blast pressure measurement using optical fiber EFPI sensor. Acknowledgement The authors acknowledge the financial support provided by CSIR-Central Scientific Instruments

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