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All Fiber-Optic Sensor for Monitoring Pressure Fluctuations in ON / OFF State Pabitra Nath, Sidhartha K. Neog, Rajib Biswas, and Amarjyoti Choudhury
Abstract— We report a simple, cost-efficient fiber-optic sensor for monitoring pressure fluctuations in ON / OFF state. The working principle of the proposed sensor is based on light intensity modulation of a reflected signal when external pressure fluctuations cause coupling optical signal variation between two multimode optical fibers placed side by side in front of a plane reflecting mirror attached on a pressure-sensitive diaphragm. The proposed sensing technique is found to be suitable for monitoring both periodic and nonperiodic forms of pressure variations in ON / OFF state. With our proposed sensor design, pressure variation as small as 1.5×10−5 N/cm2 can be measured with accuracy and repeatability. Index Terms— Fiber-optic sensors, intensity modulation, state, pressure fluctuations.
ON / OFF
I. I NTRODUCTION
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ITH the ready availability of cost-efficient optoelectronic components in the market, a large number of fiber-optic sensors and instruments have been developed and demonstrated in the recent past [1]–[7]. Fiber optic sensors based on intensity modulation and interferrometric techniques have largely been studied over last two decades [6], [8], [9]. It is established that optical fiber based sensors offer several important advantages over their electronic counterparts such as immunity to electromagnetic noise, geometrical flexibility, remote sensing capability and provides multiplex sensing facility. Intensity modulation based fiber optic sensors have largely been studied for measurement of various physical and chemical parameters such as liquid level, humidity, pressure, refractive index of liquid medium, temperature etc. [10]–[14]. Chaudhari et al., [15] reported a multi wavelength fiber optic refractometer sensor by measuring the intensity of reflected light signal when variation of refractive index of a medium causes fluctuation in coupling optical power between transmitting (T−) and a receiving (R−) optical fiber, placed side by side in front of a plane reflecting mirror. The amount of light signal coupling from T-fiber to R-fiber can also be varied Manuscript received June 11, 2012; revised October 12, 2012; accepted October 30, 2012. Date of publication November 16, 2012; date of current version February 5, 2013. The work of P. Nath was supported by Tezpur University, Napaam, India, under a start-up research grant. The associate editor coordinating the review of this paper and approving it for publication was Dr. Patrick Ruther. P. Nath, R. Biswas, and A. Choudhury are with the Department of Physics, Tezpur University, Napaam 784028, India (e-mail: pnath@ tezu.ernet.in;
[email protected]). S. K. Neog is with the Department of Energy, Tezpur University, Napaan 784028, India (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2012.2227711
Fig. 1. Schematic representation for light signal coupling between transmitting (T-) and the receiving (R-) fiber via reflection from the mirror surface.
by changing the spacing between the fibers and the reflecting surface. In the present work, we exploit this parameter for the development of a new fiber optic pressure sensor which has the ability to measure both periodic and non-periodic nature of pressure fluctuations and by using simple electronic circuit, the output response is converted into ON/OFF state. The advantages of the proposed technique are its simplicity, costefficiency and has an ability to measure pressure variations as low as 1.5 × 10−5 N/cm2 . Present technique is a variant of widely known intensity modulation fiber optic pressure sensor, where it yields pressure fluctuations responses in ON/OFF mode. II. M EASUREMENT P RINCIPLE As shown in Fig. 1, two multimode optical fibers are placed side by side in front of a plane reflecting mirror. End faces of both the fibers are kept aligned so that axial spacing between the fibers and the reflecting surface are same. Light signal is introduced through input port of the T-fiber and output light from this fiber is coupled to the R-fiber through reflection from the mirror surface. The amount of light signal coupling from T- to R-fiber would depend on following parameters. 1) Numerical aperture (NA) of the T-fiber. 2) Distance of the mirror surface from the fibers’ ends (x). 3) Refractive index (n) of the medium surrounding optical fibers and the reflecting surface. 4) Spacing between the two fibers (d).
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For fixed spacing between the fibers, the effect of d on the amount of light signal coupling from T-to R-fiber can be ignored. Reflected signal from the mirror surface is in the form of cone and the angle of cone of emittance depends on 1) refractive index of the medium surrounding the fibers ends and the mirror surface and 2) numerical aperture of the T fiber and is given by the following relation [15] −1 N A (1) θ = sin n where NA is the numerical aperture of the T-fiber and n is the refractive index of the medium. Again, radius of the reflected cone q is given by q = a + (2x + h) tan(θ ).
(2)
Here, ‘a’ is the T-fiber core radius and ‘h’ is the offset between the two fibers. Output light intensity from the R-fiber would depend on the amount of light signal coupling through overlapping of the cone of emittance from T to R- fiber. As the spacing (x) between the fibers ends and the reflecting surface is varied with the displacement of the mirror surface, the radius of cone of emittance changes and consequently would modulate the light signal intensity at the output of the R-fiber. Thus, variation of external pressure which displaces the reflecting surface can be measured in terms of change in intensity of the optical output power in the R-fiber. III. E XPERIMENTAL S ETUP Schematic of the experimental set-up of our proposed fiber optic pressure sensor is shown in Fig. 2. Light signal from an optical source (white LED, 2 mW output) is coupled to the input port of a multi-mode optical fiber (PMMA fiber, diameter: 980/1000 μm and NA = 0.5). The other end of this fiber is positioned normally in front of a plane reflecting surface. Another optical fiber (R-fiber) of same material, dimension and NA is placed in parallel position with the T-fiber and output port of this fiber is coupled to a detector (Light Dependent resistor (LDR) PGM5616D, epoxy resin packaged). The reflecting surface is of thickness ∼50 μm and radius 5 mm is pasted on inner face of a diaphragm. The flat diaphragm consists of a thin nitrile rubber membrane (Reglin rubber) of thickness ∼200 μm wrapped on a hollow plastic cylinder of diameter ∼1 cm. Nitrile rubber material gives good dimensional stability in presence of air, water vapour, and aromatics. It has low permeability and ability to withstand high pressure and has a good flex life. Detail mechanical properties for 200 μm nitrile rubber diaphragm can be obtained from the technical data sheet of Reglin rubber product [16]. Although, nitrile rubber of different thickness are commercially available it is beyond scope for this paper to investigate detail sensor performance for different thickness of rubber diaphragm material. The inset photograph in Fig. 2 shows the diaphragm along with the sensing region of the sensor. The optically coupled light signal from T- to R-fiber through reflection from the mirror surface would be received by the detector. The detector is biased at a fixed voltage of 5 V and the received signal from R-fiber is fed to the
Fig. 2. Schematic diagram of the experimental setup for proposed fiber optic pressure sensor.
inverting terminal of a Schmitt trigger. The non-inverting terminal is maintained at a fixed bias voltage approximately to the value appearing at the inverting terminal of the Schmitt trigger. For instance, if the inverting signal voltage is 1.2 V, the non-inverting terminal voltage is maintained at 1.195 V. The fine control on non-inverting terminal voltage can be obtained by using low resistance trimmer (2.2 K) connected in series with a constant power supply to it. The variation in the output of the sensor is monitored using a digital storage oscilloscope (DSO) (100 MHz, 1 G sample per second). Prior to study the sensor characteristics, all light coupling junctions viz. 1) optical source to T-fiber, 2) sensing region, and 3) R-fiber to detector were shielded so that stray light cannot perturb the sensor readings. Utmost care has been taken while studying sensor characteristics in a vibration-free environment. Vibration of the diaphragm due to exertion of pressure from external source would only cause intensity modulation in the received signal. IV. R ESULTS AND D ISCUSSION At first, the axial response of the sensor is studied by varying the spacing between the fibers end and mirror surface. Fig. 3 shows the characteristic curve of the sensor when both T- and R-fibers were displaced towards the reflecting mirror. For 3 mm separation between the fibers and mirror surface, a maximum signal voltage of 1.23 V has been detected Thus, optimum light signal coupling from T to R- fiber has occurred corresponding to this separation. Also, from this characteristic curve, it is evident that as both the fibers approach the diaphragm beyond optimum coupling position (< 3 mm), the detector signal voltage drops approximately by 5 mV for each 0.5 mm decrement in spacing. This amount of signal voltage change is sufficient to flip the threshold level of the Schmitt trigger from positive saturation to negative saturation level. Initially, we maintain this optimum coupling position between the fibers and the mirror surface. Both the fibers have been mounted on a solid block and attached firmly on it inside a hollow plastic cylindrical tube where end face of the tube is wrapped with the sensing diaphragm.
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Fig. 5. Characteristic response of the sensor when periodic force is exerted on the diaphragm. Fig. 3. Characteristic of the proposed sensor when both the fibers are displaced towards the mirror surface.
Fig. 6. Time response characteristic of the sensor when nonuniform and nonperiodic pressure is applied on the diaphragm. Red curve: analog pressure applied on the diaphragm. In the present case, the maximum pressure applied was ∼5.62 ×10−5 N/cm 2 and the minimum pressure applied was 1.5 × 10−5 N/cm2 . Fig. 4. Calibration curve of sensor signal versus applied pressure. Straight line: fittted plot for the proposed sensor.
After getting the optimum coupling condition, we calibrate the sensor signal for different amount of pressure applied on the sensing region. Different amount of pressure on the sensing membrane can be exerted by placing different loads on it. Loads were obtained from laboratoy manual balance. Fig. 4 shows the normalized characterisitc curve for this situation. For 1.5 × 10−5 N/cm2 of pressure corresponding to applied load of 0.2 gmwt, the sensor shows detectable change in the signal voltage. We then apply a periodic force on the sensing region which would cause periodic displacement of the reflecting surface towards the fibers end. This results a periodic modulation of light intensity in the R-fiber. The periodic pressure on the diaphragm is generated by placing a load on the diaphragm at a regular time interval. Fig. 5 shows the ON/OFF response of the sensor for applied periodic pressure of 2.5 × 10−5 N/cm2 . We repeat the investigation for different amount of pressure applied on the sensing region and it has been observed that minimum pressure to which our present sensor is responsive was 1.5 × 10−5 N/cm2 corresponding to the applied load of 0.2 gmwt force on the sensing area. This level of sensitivity
is sufficient for monitoring physiological responses of many living organism such as frog, dog, goat etc. It may be noted here that the sensor is responsive only when external pressure causes a displacement of the diaphragm towards the fiber-optic probes. In the absence of any external pressure, the sensor output remains in the negative saturation state. To investigate the sensor response for non-periodic and non-uniform pressure fluctuation, we exert different amount of forces on the membrane for different time duration and noted the analog pressures. Fig. 6 illustrates the sensor characteristics for these analog pressure applied on the diaphragm. The response curve clearly shows that for pressure exceeding 1.5× 10−5 N/cm2 corresponding to the applied force of 0.2 gmwt on the sensing area the output response flips from negative to positive saturation state and remains in the same state until the applied pressure drops again below 1.5 × 10−5 N/cm2 . To validate the claim that the designed sensor has the ability to monitor physiological responses of living organisms, we choose an Indian frog (Bufo Melanostictus) which is abundant during rainy season in this region. The epiglottis region of the frog (also called larynx) expands and compresses in a nonperiodical manner. The reason of vibration of the larynx is for calling by males to attract a mate [17]. While producing a call,
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wrapped with the cylindrical tube. In the present work the membrane has been wrapped on the tube using a scotch tape and care has been taken so that it is under minimum stress due to wrapping on the tube. The sensor responses for both periodic and non-periodic pressure fluctuations have been studied for two weeks and we observe repeatable readings for each case. Again, reliability of the designed sensor depends on the nature of the material used as a sensing membrane. Using nitrile rubber as sensing membrane in the present sensor we obtain high reliability in the measured signal. V. C ONCLUSION
Fig. 7. Time response display of the sensor when the sensing region is brought into contact with the epiglottis region of the frog.
the larynx swells which is easily visible to our naked eye. The easy visibility of regular compression and expansion of larynx of the frog suits our purpose for monitoring its physiological responses with our present sensing set-up. The sensing region of the sensor has been placed gently on the larynx of the frog and the corresponding sensor response of is shown in Fig. 7. The average time duration of compression and expansion of the larynx region of the frog is found to be different. The average duration of compression was found to be ∼600 msec and the expansion period was ∼900 msec. In our proposed sensor, the delay time of the detector is governed by following two factors: 1) Time duration required by the detector ( LDR in the present set-up) to respond to change in intensity of the incident light signal ( ∼30 ms). 2) Time required by the Schmitt trigger to flip from negative saturation to positive saturation level ( −5 V to +5 Volt) ∼10 μsec. Thus, delay in response time of the detector section is found to be much smaller compared to the average duration of compression and expansion of organs of a typical living specimen. This justifies the applicability of our designed sensor for the purpose of monitoring both periodic and non-periodic nature of pressure variation in ON/OFF state as generally seen in heartbeat, pulse rate and other organs in living organisms. The sensing region developed for the present work consists of a flat nitrile rubber material which is not suitable for monitoring physiological responses like heart-beat, pulse rate of human body. In order to monitor these responses, we plan to integrate the proposed sensing scheme on the diaphragm of a standard stethoscope and to observe its acoustic pressure variation in digital mode. This will be done in the future course of work. The repeatability behavior of the proposed sensor depends on elastic nature of the rubber membrane and how it is
In summary we demonstrate the working of a fiber optic pressure sensor capable of monitoring periodic and nonperiodic nature of pressure variations in ON/OFF state. Pressure fluctuations as small as 1.5 × 10−5 N/cm2 could be measured accurately with the present sensing set-up. In-spite of the use of low-cost detector like LDR and general purpose laboratory ICs (741) the sensor performs reliably and good degree of repeatability has been observed in it. Based on the results obtained so far, authors propose to design similar kind of fiber optic pressure sensor which will be more robust and sensitive to pressure fluctuations in future course of time. The proposed technique has a future scope of being developed as a digital fiber-optic stethoscope. R EFERENCES [1] R. A. De Blasi, G. Conti, M. Antonelli, M. Bufi, and A. Gasparetto, “A fiber optics system for the evaluation of airway pressure in mechanically ventilated patients,” Intensive Care Med., vol. 18, no. 7, pp. 405–409, Jul. 1992. [2] T. S. Myllyla, A. A. Elseoud, H. S. S. Sorvoja, R. A. Myllyla, J. M. Harja, J. Nikkinen, O. Tervonen, and V. Kiviniemi “Fibre optic sensor for non-invasive monitoring of blood pressure during MRI scanning,” J. Biophoton., vol. 4, nos. 1–2, pp. 98–107, Jan. 2011. [3] W. H. Wang, Q. X. Yu, F. Li, X. L. Zhou, and X. S. Jiang, “Temperatureinsensitive pressure sensor based on all-fused-silica extrinsic Fabry– Pérot optical fiber interferometer,” IEEE Sensors J., vol. 12, no. 7, pp. 2425–2429, Jul. 2012. [4] E. Cibula, S. Pevec, B. Lenardic, E. Pinet, and D. Donlagic, “Miniature all-glass robust pressure sensor,” Opt. Exp., vol. 17, no. 7, pp. 5098–5106, Mar. 2009. [5] E. Cibula and D. Donlagic, “Miniature fiber-optic pressure sensor with a polymer diaphragm,” Appl. Opt., vol. 44, no. 14, pp. 2736–2744, May 2005. [6] W. B. Spillman, “Multimode fiber-optic pressure sensor based on the photoelastic effect,” Opt. Lett., vol. 7, no. 8, pp. 388–390, Aug. 1982. [7] G. C. Constantin, G. Perrone, S. Abrate, and N. N. Puscas, “Fabrication and characterization of low-cost polarimetric fiber-optic pressure sensor,” J. Optoelectron. Adv. Mater., vol. 8, no. 4, pp. 1635–1638, Aug. 2006. [8] W. B. Spillman, “Multimode fiber-optic pressure sensor based on the photoelastic effect,” Opt. Lett., vol. 7, no. 8, pp. 388–390, Aug. 1982. [9] T. Bae, R. A. Atkins, H. F. Taylor, and W. N. Gibler, “Interferometric fiber-optic sensor embedded in a spark plug for in-cylinder pressure measurement in engines,” Appl. Opt., vol. 42, no. 6, pp. 1003–1007, Feb. 2003. [10] J. Feng, Y. Zhao, S. S. Li, X. W. Lin, F. Xu, and Y. Q. Lu, “Fiberoptic pressure sensor based on tunable liquid crystal technology,” IEEE Photon. J., vol. 2, no. 3, pp. 292–298, Jun. 2010. [11] I. K. Ilev and R. W. Waynant, “All-fiber-optic sensor for liquid level measurement,” Rev. Sci. Instrum., vol. 70, no. 5, pp. 2551–2554, May 1999.
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[12] S. K. Khijwania, K. L. Srinivasan, and J. P. Singh, “An evanescent-wave optical fiber relative humidity sensor with enhanced sensitivity,” Sensors Actuat. B, Chem., vol. 104, no. 2, pp. 217–222, Jan. 2005. [13] P. Nath, H. K. Singh, R. Datta, and K. C. Sarma, “All-fiber optic sensor for measurement of liquid refractive index,” Sensors Actuat. A, Phys., vol. 148, no. 1, pp. 16–18, Nov. 2008. [14] S. M. Chandani and N. A. F. Jaeger, “Fiber-optic temperature sensor using evanescent fields in D fibers,” IEEE Photon. Technol. Lett., vol. 17, no. 12, pp. 2706–2708, Dec. 2005. [15] A. L. Chaudhari and A. D. Shaligram, “Multi-wavelength optical fiber liquid refractometry based on intensity modulation,” Sensors Actuat. A, Phys., vol. 100, nos. 2–3, pp. 160–164, Sep. 2002. [16] Available: http://www.reglinrubber.com/products/technicaldatasheet/ Reglin%20Data%20Sheet%20-%20Nitrile%20Diaphragm%200.2.pdf’ [17] Frog. (2012) [Online]. Available: http://en.wikipedia.org/wiki/Frog
Pabitra Nath received the M.Sc degree in physics from Tezpur University, Napaam, India, and the Ph.D. degree in electronics science from Gauhati University, Guwahati, India, in 2000 and 2009, respectively. He is currently an Associate Professor with the Department of Physics, Tezpur University. Recently he has visited Micro and Nano Technology Laboratory (MNTL), Department of Electrical and Computer Engineering, University of Illinois at UrbanaChampaign, Urbana, through BOYSCAST fellowship program 2010–2011 sponsored by the Department of Science and Technology, Government of India. His current research interests include fiber optics sensors and system, bio-medical instrumentations, and optical biosensing systems.
IEEE SENSORS JOURNAL, VOL. 13, NO. 4, APRIL 2013
Sidhartha K. Neog received the M.Sc. degree in physics from Tezpur University, Napaam, India, in 2011, where he is currently pursuing the M.Tech. degree in energy technology. His M.Tech project work relates to the Center For Wind Energy Technology.
Rajib Biswas received the B.Sc. degree from the Department of Physics and the M.Sc. degree from the DHSK College, Dibrugarh University, Dibrugarh, India, in 2004 and 2007, respectively. He was a Junior Research Fellow with the C.S.I.RNorth East Institute of Science & Technology, Jorhat, India, from 2007 to 2010, and pursued his doctoral work in the same laboratory. He has been serving as an Assistant Professor with the Physics Department, Tezpur University, Napaam, India, since 2010.
Amarjyoti Choudhury received the M.Sc. degree in physics from Delhi University, New Delhi, India, and the Ph.D. degree from Oxford University, Oxford, U.K., in 1972 and 1977, respectively. He is currently a Senior Professor with the Department of Physics, Tezpur University, Napaam, India, and the Pro-Vice Chancellor with the same university. Previously, he had served as a Vice-Chancellor with Gauhati University, Guwahati, India. He has authored over 100 publications in peer reviewed journals and has produced 12 Ph.D.’s under his supervision. His current research interests include condensed matter physics, photonics, nanotechnology, optoelectronic instrumentation.
