Miniature Absolute Optical Pressure Sensor at a Fiber Tip for High Temperature Applications

Miniature Absolute Optical Pressure Sensor at a Fiber Tip for High Temperature Applications

Available online at www.sciencedirect.com Procedia Engineering 47 (2012) 698 – 701 Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland Min...

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Available online at www.sciencedirect.com

Procedia Engineering 47 (2012) 698 – 701

Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland

Miniature absolute optical pressure sensor at a fiber tip for high temperature applications Grim Keulemans*, Frederik Ceyssens, Robert Puersa KU Leuven, ESAT-MICAS, Kasteelpark Arenberg 10, 3001 Heverlee, Belgium

Abstract Fiber optic pressure sensors have attracted considerable attention because of their small size, high sensitivity and immunity to electromagnetic interference. Here, a miniature absolute pressure sensor at the edge of a fiber is presented. Contradictory to earlier pressure sensors with membranes at the fiber tip (in a co-axial configuration), we present a sensor in a cross-axial configuration. It is optimized for aerodynamic pressure measurements in turbomachinery. The sensor is fabricated by thin film deposition techniques and focused ion beam (FIB) microfabrication and uses multilayer Fabry-Perot (FP) interferometry as sensing principle. The reflected power intensity of the prototype is experimentally verified to be 0.77%/bar. © Authors. Published by Elsevier ©2012 2012The Published by Elsevier Ltd. Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. Keywords: optical fiber; pressure sensor; focused ion beam; Fabry-Perot interferometry; aerodynamics

1. Introduction Accurate probing of important flow field parameters in turbo-machines (total and static pressure, flow angle, level of turbulence and temperature) is still a considerable engineering challenge. The strong unsteady three-dimensional flow inside the different stages of a turbo-engine requires a high spatial and frequency resolution for pressure measurement. The frequency response should be higher than 100 kHz. Absolute pressure levels between 2 and 20 bars occur frequently inside civil turbojet engines. Inside cold flow test setups in research institutes, temperatures up to 600 ȗC are reached [1]. To meet these requirements, optical fiber pressure transducers based on fused silica [2] and sapphire [3] are considered as they present small overall dimensions, high sensitivity and can withstand high

* Corresponding author. Tel.: +32 16 321105 ; fax: +32 16 321975. E-mail address: [email protected]

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. doi:10.1016/j.proeng.2012.09.243

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Grim Keulemans et al. / Procedia Engineering 47 (2012) 698 – 701

temperatures due to their monolithic nature which minimizes thermal mismatch between different materials at elevated temperatures. However pressure sensors fabricated directly on the tip of a fiber present a non-ideal configuration as for optimal total pressure measurement the membrane should be perpendicular to the direction of the flow. As there is no room to bend the optical fiber between the stator and the rotor rows in a turbo-engine (Fig. 1.a), a configuration where the membrane is situated at the edge of the optical fiber is necessary as depicted in Fig. 1.b (cross-axial configuration).

(a)

(b)

Fig. 1. (a) Positioning of an aerodynamic probe (optical fiber) between the stator and rotor rows of an axial flow compressor; (b) Comparison of the original Fabry-Perot cavity pressure sensor at the tip of an optical fiber (co-axial configuration) with the novel pressure sensor at the edge of an optical fiber, optimized for fluid aerodynamic measurements (cross-axial configuration)

2. Design The sensor dimensions are optimized for optimal linear and monotone sensor response based on a multilayer Fabry-Perot optical model. The different layers are modeled using their complex permittivity İi and corresponding impedance ηi = μi /ε i . The total reflected power Pr = |ī1|2 can be calculated by recursively applying the formulae for optical impendence transformation through the different layers as described in [2] (where γ i = jω μiε i represents the propagation constant and di the thickness of layer i),

Γ n −1 =

ηn − ηn −1 ηn + ηn −1 η n −1 (1 + Γn −1e 2γ

η n −1→n =

1 − Γn −1e

(1) n −1d n−1

2γ n−1d n−1

)

(2)

Based on this model, the membrane is calculated to be allowed to deflect maximally 3ʄ/20 (ʄ = 1310 nm) in order to guarantee monotone sensor response (Fig. 2). This condition is fulfilled up to an absolute pressure of 10 bars in the case of a silica (1310 nm) – molybdenum (80 nm) – silica (900 nm) membrane having a diameter of 60 ʅm according to finite-element modeling.

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Fig. 2. Simulated dependence of the normalized total power reflection as function of the Fabry-Perot cavity length in the case of a silica (1310 nm) – molybdenum (80 nm) – silica (900 nm) thin film membrane

3. Experimental The fabrication process starts with FIB milling to cut a single-mode (SM) optical fiber (core = 8 μm, cladding = 125 μm) under an angle of 45ȗ to mirror the optical axis by 90ȗ (Fig. 3). Afterwards FIB machining and thin film deposition are used to micromachine a multilayer FP cavity at the edge of the optical fiber. Molybdenum is used as sacrificial layer. The membrane consists of two silica sputter deposited thin films (1310 and 900 nm, respectively) surrounding a molybdenum reflector (80 nm).

(a)

(b)

Fig. 3. (a) FIB image of a single mode optical fiber cut under a 45 degree angle by FIB milling; (b) Absolute pressure sensor fabricated at the edge of an optical fiber

4. Results Figure 4.a depicts the measurement setup which is used to characterize the optical sensor and consists of a 1 mW solid state laser (ʄ = 1310 nm), a single-mode optical beam splitter, an optical power meter (Exfo IQ-203), a pressure controlled chamber and a reference pressure sensor (Druck DPI 104) for calibration purposes. The reflected power intensity is experimentally verified to drop 0.77%/bar (Fig. 4.b). Measurement errors are identified to be mainly contributed by laser power output drift and by

Grim Keulemans et al. / Procedia Engineering 47 (2012) 698 – 701

transient chances in optical transmission inside the optical fiber FC connectors due to environmental vibrations.

(a)

(b)

Fig. 4. (a) Overview of the measurement setup; (b) Measured average reflected power (μW) as function of the absolute applied pressure (bar). Pressure is applied in steps of 0.5 bars. (R2 = 0.9701)

5. Conclusion The feasibility of an absolute optical pressure sensor at the edge of a fiber tip (cross-axial configuration) is presented. The sensor is optimized for aerodynamic pressure measurements in turbomachinery. The reflected power intensity is experimentally verified to drop 0.77%/bar. This rugged monolithic silica-based absolute pressure sensor offers possibilities for high temperature applications. Future work will focus on the compensation of the observed drift and noise of the laser source used by differential optical read-out and the investigation of the high-temperature and dynamic capabilities of the pressure sensor. Acknowledgements Research funded by a Ph.D. grant of the Agency for Innovation by Science and Technology (IWT). References [1] J.F. Brouckaert. Fast response aerodynamic probes for measurement in turbomachines. Proc. Of Institution of Mech. E ng., Part A: J. of Power and Energy 2007;221:811–813. [2] F. Ceyssens, M. Driesen, R. Puers. An optical absolute pressure sensor for high-temperature applications, fabricated directly on a fiber. J. Micromech. MIcroEng. 2009;19:115017. [3] J. Yi, E. Lally, A. Wang, Y. Xu. Demonstration of an all-sapphire Fabry-Perot cavity for pressure sensing. IEEE Photonics Tech Letters, 23:9-11.

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