Miniature fiber optic pressure and temperature sensors

Miniature fiber optic pressure and temperature sensors Juncheng Xu 1, Xingwei Wang, Kristie L Cooper, Gary R. Pickrell, and Anbo Wang Center for Photonics Technology Bradley Department of Electrical and Computer Engineering Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA ABSTRACT New miniature extrinsic Fabry-Perot interferometric (MEFPI) optical fiber sensors with a size of 125µm in diameter are presented, which are ideal for applications where the operation space is highly restricted. The temperature sensor can work up to 800 C with a sensitivity of 0.46nm/ C. The pressure sensor exhibited a sensitivity of about 0.36nm/psi. The sensitivities of the pressure and temperature sensor can be controlled with high precision during fabrication. In addition, their Fabry-Perot cavity lengths can be controlled with a resolution of several nanometers, which provides excellent flexibility in sensor design and signal demodulation. The sensors are composed entirely of fused silica, which is very reliable, biocompatible, corrosion resistant and immune to electromagnetic interference (EMI). Keywords: fiber optic sensor, Fabry-Perot, pressure sensor, temperature sensor 1. INTRODUCTION Miniature pressure and temperature sensors are widely used in industrial and biomedical applications where operation spaces are very limited or a high spatial resolution is required. Optical fiber sensors can be made very small, with diameters typically in the range of a few hundred microns. As one example, Fabry-Perot (FP) fiber optic sensors are easy to fabricate with low cost, and can be classified as intrinsic Fabry-Perot interferometers (IFPI) [1] or extrinsic Fabry-Perot interferometers (EFPI) [2][3]. Generally, the sensing element of an IFPI sensor is a short section of fiber sandwiched between the two FP mirrors. For a conventional EFPI sensor, the lead-in and reflecting fibers and the tubing holding them are the sensing elements. IFPI sensors are often more sensitive to temperature changes and a little smaller than EFPI sensors while the latter make it easier to adjust the FP cavity length and often have higher pressure sensitivities. In addition to the sensors mentioned, an in-line fiber etalon (ILFE) sensor has also been reported [4], in which the sensing element is a short section of hollow fiber. We present novel miniature extrinsic Fabry-Perot interferometric (MEFPI) optical fiber pressure and temperature sensors, which have the same diameter as the fiber itself. The structure of the miniature sensor is shown in Fig. 1. The length of the FP cavity (air-gap) will change by the net result of length changes in the hollow fiber and reflecting fiber along with variations of temperature or pressure. Light is injected into the lead-in optical fiber and partially reflected (~4%) by the end faces of the two fibers. Then the two reflections propagate back through the same lead-in fiber and generate interference fringes, which will be demodulated to determine the air-gap thickness. The FP cavity length can be adjusted with high precision in a wide range, giving a great deal of flexibility in the choice of light source or the signal demodulation scheme selection. 1 juxu1@vt.edu Fiber Optic Sensor Technology and Applications IV edited by Michael A. Marcus, Brian Culshaw, John P. Dakin Proc. of SPIE Vol. 6004, 600403, (2005) 0277-786X/05/$15 doi: 10.1117/12.630910 Proc. of SPIE Vol. 6004 600403-1

Lead-in fiber Hollow fiber Reflecting fiber Light Bonding Points Fig. 1. Miniature fiber optic EFPI sensors 2. SENSOR DESIGN AND FABRICATION A cleaved silica optical fiber (125µm diameter) was spliced with an approximately 3.5mm long hollow fiber with inside diameter (I.D.) of 75µm and outside diameter (O.D.) the same as the optical fiber by using a fusion splicer (Sumitomo, Type-36). Another optical fiber with O.D. of 60-65µm was inserted into the hollow fiber from the other end as a reflecting fiber to form a FP cavity between these two fiber end faces. The thinner fiber can be obtained by etching or drawing a standard 125µm O.D. optical fiber. The lead-in fiber was connected to a white light interferometer system. The cavity length was pre-adjusted by the splicer stage holding the reflecting fiber tail, and monitored on-line by the white light system. The air-gap can be set from zero to millimeters depending on the coherence length of the light source and application requirements. Because the fiber fusion splicer has precisely controlled alignment and movement, the sensor fabrication is simple, convenient and fast. Once the air-gap was adjusted to near the desired value, the electric arc was applied to bond the reflection fiber with the hollow fiber end. In addition, the sensor air-gap can be adjusted with a resolution as high as 3nm by properly controlling the power, duration and number of electric arcs. This thermally induced air-gap control technique was recently reported [5]. Pressure Sensor The sensor s pressure sensitivity is determined by its gauge length. Generally, the hollow fiber was cleaved with a length near the desired gauge length to minimize the sensor size. In addition, compared with the fiber-tip-diaphragm based sensors [6][7], this shrinking cylinder based pressure sensor is much easier to fabricate, resistant to scratches and has much larger pressure measurement range. Since the hollow fiber is uniform along its axis, the air-gap change G under applied pressure P can be calculated by [8] : G L(1 2 µ ) D = P E D d 2 2 2 ( ) (1) where d and D are the I.D. and O.D. of the hollow fiber, L is the sensor gauge length, µ is the Poisson s ratio, E is the Young s modulus of the hollow fiber. Once the sensor gauge length is determined, the sensor s pressure sensitivity is fixed. The variation in sensitivity is within 0.05nm/psi based on the eight sensors prepared and tested. By choosing different gauge lengths from 0.2mm to Proc. of SPIE Vol. 6004 600403-2

15mm the sensor can be designed for pressure measurement up to 15kpsi (103.4 MPa) with sensitivities up to 1.5 nm/psi. The fused silica hollow fiber has a coefficient of thermal expansion (CTE) of approximately α h =5.5~5.8 10-7 / C. The single mode optical fiber used was a communication grade fiber (Corning SMF-28), with a CTE of 5.6 10-7 / C, which is very close to that of fused silica. The total temperature dependence of the hermetically sealed MEFPI sensor can be expressed as: G P = ( α α ) L + α G + S T T h r r h P (2) where G is the air-gap thickness, α r and L r is the reflecting fiber CTE and length, S P is the sensor pressure sensitivity (nm/psi), and P is the residue air pressure inside the hollow fiber at temperature T (K). By optimizing these parameters, the sensor s temperature dependence can be minimized for a specific temperature range over which the CTEs are valid. Temperature Sensor For the MEFPI temperature sensor, the temperature sensitivity can be optimized by maximizing the CTE mismatch between the reflecting fiber and hollow fiber, or increasing the gauge length. We chose a multi-mode fiber as the reflecting fiber with a considerably higher CTE than the hollow fiber, which is about 7.0~7.7 10-7 / C. In general, the more germanium doping in the optical fiber, the higher the CTE. Moreover, fibers made of other materials can be used as the reflecting fiber such as borosilicate fiber, crystal fiber or even metal fiber. 3. SENSOR INTERROGATION SYSTEM The basic configuration of the sensor interrogation system is shown in Fig. 2. The tunable laser of a Component Test System (CTS, Micron Optics) was coupled into the fiber sensor through a 2x2 coupler and the light was partially reflected (~4%) by the end faces of the lead-in and reflecting fiber. Both the reflections were routed back through the same coupler to the receiver of the CTS. To obtain an interference spectrum with high fringe visibility, the air-gap must be within the coherence length of the light source used in the system. In this system, the coherence length is dependent on the spectral resolution of the CTS, the air-gap and the quality of the MEFPI sensor head. The interference spectrum measured by the spectrometer is given by [9] : where I ( λ) 4π G I() λ = 2 Is ( λ) (1+ γcos( + ϕ0)) (3) λ is the spectral power distribution of the light source with wavelength λ, γ is the visibility of the interference s spectrum, ϕ 0 is the arbitrary initial phase difference, G is the air-gap, which is determined by the physical parameters (such as pressure or temperature) to be measured. Normalizing equation (3) respect to the Gaussian spectrum of the light source, the normalized interference output can be expressed as 4πG λ ) = 2(1 + γ cos( + ϕ )) λ I n (4) ( 0 Proc. of SPIE Vol. 6004 600403-3

The value of the air-gap G can be calculated from equation (4) and then be used to demodulate the physical parameter of interest. A typical spectrum from a MEFPI sensor is shown in Fig. 3. A scan of the tunable laser produced the spectrum information from 1520 nm to 1570 nm with a 2.5 pm resolution. The two reflections generated interference fringes and a valley trace method [10] was used to process the spectrum curve to demodulate the air-gap. CTS Tunable Laser 3dB 2x2 Coupler Single Mode Fiber MEFPI Sensor Receiver Anti-Reflection End Fig. 2. Schematic of the sensor interrogation system. 5 0 Intensity (db) -5-10 -15-20 1520 1530 1540 1550 1560 1570 Wavelength (nm) Fig.3. The typical spectrum from a MEFPI sensor 4. EXPERIMENT RESULTS The air-gap measurement of the MEFPI sensors exhibited a resolution of about 0.02~0.04nm. The pressure sensor was calibrated by using a pressure gauge calibration system (Model 9035, Pressure Systems), while the temperature sensor was tested in a furnace (47900, Thermolyne Co.). The pressure sensor showed a linear response to static pressure 0-200 psi at room temperature as shown in Fig. 4. The sensor pressure sensitivity was 0.36 nm/psi and the sensor system exhibited a pressure measurement resolution of about 0.1psi (689Pa). The pressure sensor s temperature dependence was about 0.057nm/ C and the result is shown in Fig. 5. An enhanced temperature compensation method [11] can be used to Proc. of SPIE Vol. 6004 600403-4

reduce the temperature dependence greatly. The temperature test result for the MEFPI temperature sensor is shown in Fig. 6. Its temperature sensitivity is about 0.46nm/ C and the measurement resolution is about 0.05 C. When temperature increased, the sensor air-gap change was dominated by the elongation of the multi-mode fiber. If the sensor is to be used in an environment with large pressure variations, a pressure isolation scheme [12] can be chosen in sensor fabrication. Air-Gap (µm) 35.63 35.62 35.61 35.60 35.59 35.58 35.57 35.56 35.55 35.54 0 25 50 75 100 125 150 175 200 225 Pressure (psi) Air-Gap (µm) 35.630 35.628 35.626 35.624 35.622 35.620 35.618 35.616 35.614 0 50 100 150 200 250 Temperature ( o C) Fig. 4. Pressure sensor pressure response at 25 C Fig. 5. Pressure sensor temperature response at atmospheric pressure Air-Gap (µm) 23.60 23.58 23.56 23.54 23.52 23.50 23.48 0 50 100 150 200 250 Temperature ( o C) Fig. 6. Temperature sensor temperature response at atmospheric pressure. Proc. of SPIE Vol. 6004 600403-5

5. CONCLUSIONS Novel miniature pressure and temperature fiber optic EFPI sensors have been described. These MEFPI sensors have the same diameter as the fiber itself and preserve advantages of conventional EFPI sensors. Simple fabrication processes have been developed to produce hermetic fusion bonds. The fabrication not only can control the sensor gauge length but also adjust the Fabry-Perot cavity with nanometers resolution. These all-silica sensors are ideal for both biomedical or industrial applications. ACKNOWLEDGEMENTS This work was supported by the US Department of Energy (DOE) under contract DE-FC36-01GO11050. REFERENCES [1] C. E. Lee, W. N. Gibler, R. A. Atkins, and H. F. Taylor, In-Line Fiber Fabry -Perot Interferometer with High- Reflectance Internal Mirrors, J. Lightwave Technol., Vol. 10, pp.1376-1379, 1992. [2] K. A. Murphy, M. F. Gunther, A. M. Vengsarkar, and R. O. Claus, Quadrature phase shifted extrinsic Fabry-Perot optical fiber sensors," Opt. Lett., Vol. 16, pp. 273-275, Feb. 1991. [3] A. Wang, H. Xiao, J. Wang, Z. Wang, W. Zhao, and R. G. May, Self-Calibrated Interferometric-Intensity-Based Optical Fiber Sensors, J. Lightwave Technol., Vol. 19, pp. 1495-1501, Oct. 2001. [4] J. S. Sirkis, D. D. Brennan, M. A. Putman, T. A. Berkoff, A. D. Kersey, E. J. Friebele, In-line fiber etalon for strain measurement, Opt. Lett., Vol. 18, pp. 1973-1975, Nov. 1993. [5] J. Xu, G. R. Pickrell, K. L Cooper, P. Zhang, and A. Wang, Precise Cavity Length Control in Fiber Optic Extrinsic Fabry-Perot Interferometers, Conference on Lasers and Electro-Optics (CLEO), Baltimore, MD, May 2005. [6] E. Cibula, D. Donlagic, and C. Stropnik, Miniature fiber optic pressure sensor for medical applications, Sensors, 2002. Proceedings of IEEE, 1, 711-714, June 2002. [7] K. Totsu, Y. Haga, and M. Esashi, Vacuum sealed ultra miniature fiber-optic pressure sensor using white light interferometry, Transducers, Solid-State Sensors, Actuators and Microsystems, 12th International Conference, 1, 931-934, 2003. [8] B. Qi, G. Pickrell, P. Zhang, Y. Duan, W. Peng, J. Xu, Z. Huang, J. Deng, H. Xiao, Z. Wang, W. Huo, R.G. May, and A. Wang, Fiber optical pressure and temperature sensors for oil down hole applications, Proceedings of SPIE, 4578, 182-190, 2002. [9] J. Dakin and B. Culshaw, Optical Fiber Sensors: Principles and Components, Artech House, Inc., 1988. [10] B. Qi, G. R. Pickrell, J. Xu, P. Zhang, Y. Duan, W. Peng, Z. Huang, W. Huo, H. Xiao, R. G. May, and A. Wang, Novel data processing techniques for dispersive white light interferometer, Opt. Eng., Vol. 42, pp. 3165-3171, Nov. 2003. [11] J. Xu, G. Pickrell, X. Wang, W. Peng, K. L Cooper and A. Wang, A Novel Temperature Insensitive Optical Fiber Pressure Sensor For Harsh Environments, IEEE Photon. Technol. Lett. Vol. 17, 870-872 2005. [12] J. Xu, G. R. Pickrell, Z. Huang, B. Qi, P. Zhang, Y. Duan, and A. Wang, Double-Tubing Encapsulated Fiber Optic Temperature Sensor, in 8th Temperature Symposium, AIP Conference Proceedings, Vol. 684, pp. 1021-1026, Sept. 2003. Proc. of SPIE Vol. 6004 600403-6