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Procedia Engineering 5 (2010) 1095–1098 Procedia Engineering 00 (2010) 000–000
Procedia Engineering www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
Intensity referencing in an extrinsic optical fiber temperature sensor Andreas Apelsmeiera, Ramona Gleixnera, Matthias Mayera, Mikhail Shamonina*, Bernhard Schmaussb a
SOL - Laboratory for Sensor Technology, University of Applied Sciences, Postfach 12 03 27, D-93025 Regensburg, Germany b Lehrstuhl für Hochfrequenztechnik, Universität Erlangen-Nürnberg, Cauerstraße 9, D-91058 Erlangen, Germany
Abstract Optical fiber sensors based on intensity measurement require some form of intensity referencing to avoid errors arising from parasitic losses. Known techniques of referencing such as balanced bridge, divided beam systems or two-wavelength referencing are not suitable for low-cost applications since they use relatively complicated optical components such as multiple LED sources, couplers, filters etc. In this work a novel method of referencing in an extrinsic optical fiber sensor system utilizing temperature dependence of absorption edge in a semiconductor crystal is described. The sensor system comprises a single LED source and no optical fiber junctions. The emission spectrum of an LED depends on its temperature. The reference is provided by controlling the temperature of an LED source and transmission measurements with different emission spectra. The entire process is controlled by a microprocessor unit. Performance of a sensor system is investigated and it is shown that the losses in connectors may be compensated for.
c© 2010
2009 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Keywords: extrinsic optical fiber sensor; intensity referencing; temperature measurement; absorption edge; semiconductor; hot-spot measurement
1. Introduction In some specific applications e.g. power transformer monitoring [1] or magnetic resonance imaging [2] fiber optic sensors are used for temperature measurements since they do not interfere with close proximity electromagnetic fields. Conventional fiber-optic temperature sensors are based on one of three methods: fluorescence decay time, Fabry-Perot interferometry or the shift of the absorption edge in semiconductor crystals [3]. These systems are not low cost, since they comprise relatively complicated optical components like interferometer, couplers, spectrometer, filter etc. The motivation of this work is to realize a low-cost fiber-optic temperature sensor for the above mentioned applications. Optical fiber sensors based on intensity measurement require some form of intensity referencing to avoid errors arising from parasitic losses [4]. We demonstrate a novel method of referencing in an extrinsic optical
* Corresponding author. Tel.: +49-941-9431105; fax: +49-941-9431424. E-mail address:
[email protected].
c 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. 1877-7058 doi:10.1016/j.proeng.2010.09.301
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fiber system utilizing temperature dependence of absorption edge in a semiconductor crystal. The system comprises a single LED source and no optical fiber junctions. Nomenclature I0
forward drive current of the LED
Ȝ
wavelength
ȜP
peak wavelength of the LED spectrum
dP/dȜ
spectral power distribution of the LED
R
responsivity of the photo-detector
tInP
transmission factor of the sensitive element (indium phosphide prism)
Ta
ambient temperature to be measured
TB
temperature of the diode function block
TLED
temperature of the LED
2. Working principle The concept of a sensor [5] is shown in Figure 1. Light from an LED (ȜP § 950 nm at room temperature) is guided through the input optical fiber into the sensor head. There it is coupled into an indium phosphide (InP) prism, deflected twice and coupled into the output fiber. The transmitted light power is reduced with the growing temperature of the InP-prism Ta. Due to the tiny prism dimensions a novel manufacturing method bridging fine mechanics and microsystems technology is required [6]. As this fiber-optic sensor is based on intensity measurement, parasitic losses must be compensated for. The block diagram of a sensor system is shown in Figure 2. The temperature of the diode function block is maintained a constant level above the surrounding temperature. This is achieved by heating the diode function block with the power transisitor. In the stationary regime I0 is constant, TB and TLED are equal. The monitoring photodiode controls the intensity emitted by the LED. 3. Compensation of parasitic losses The emission spectrum of an LED source depends on its temperature TLED. It is known that ȜP increases with the growing TLED [7]. For the LED used in this work ǻȜP/ǻTa = 0,22 nm/K. The ratio R1/2 of two photocurrents taken at two different temperatures TB1, TB2 is independent of losses d: f
R1/ 2 ( Ta )
³ R( O ,T
B1
) t InP ( O ,Ta ) > dP( TLED1 ) d O @ d O
³ R( O ,T
B2
) t InP ( O ,Ta ) > dP( TLED 2 ) d O @ d O
0 f
.
(1)
0
It is assumed that the losses in a fiber-optic system are not significantly wavelength dependent and may be taken into account by damping coefficient d.
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Fig. 1. Working principle of a sensor head.
3
Fig. 2. Block diagram of a sensor system.
Figure 3a compares the results of measurements in the stationary regime with modeling. Notice that for given values of TB1 and TB2, R1/2 is not a monotonic function of Ta in the required range of temperatures (-50°C dTa d 150°C). 4. Transient operation regime Non-monotonic behaviour of R1/2 with Ta is a disadvantage, since it limits the range where the temperature may be determined unambiguously. Modelling shows that for given material parameters one has to increase the temperature of the LED in order to get a monotonic behaviour of R1/2 in the required temperature range. Continuous operation of an LED at elevated temperatures decreases its expected operating life-time. To overcome this limitation transient operation regime is introduced. The LED is loaded by higher current pulse I0(t) and heated itself due to power dissipation. The measurements have been performed as follows. The initial LED temperature was set to 70°C. Then the LED was self heated by sending a current pulse of Io = 500 mA during 2 seconds. The amplified output voltages of a photo-diode 1 Uph and a monitoring diode Umon have been measured at the beginning and the end of the current pulse. In order to eliminate transient oscillations averaged values as given below were calculated: 30
¦U U out 2
194
¦U
ph,i
i 6
25
; U out1
i 170
25
30
¦U
ph,i
; U mon 2
194
¦U
mon,i
i 6
25
; U mon1
mon,i
i 170
25
.
(2)
The sampling rate was 100 Hz. The resulting ratio R1/2,meas is given by
R1/ 2 ,meas
U out1 U mon1 . U out 2 U mon 2
(3)
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1.55
1.55 Measurement
1.45
1.45 Simulation
Measurement Simulation
1.35 R1/2
R1/2
1.35 1.25
1.25
1.15
1.15
1.05
-40
0
40 80 T / °C a
(a)
120
160
1.05
-40
0
40 80 T / °C
120
160
a
(b)
Fig. 3. Comparison between experiment and simulation: (a) stationary regime, I0= 80 mA,TB1=60°C, TB2= 30°C ; (b) transient regime. Parameters of measurements and simulations are described in the text.
The transient LED temperatures TLED1 and TLED2 cannot be measured directly. Figure 3b compares the measurement results with simulation. A good agreement is seen. The fit is achieved for TLED1 = 160°C and TLED2 = 140°C. The obtained dependence R1/2(Ta) is monotonic in the entire working range. Performance of the sensor system has been investigated under realistic conditions. The losses into fiber connectors have been introduced by pulling the connecting ferrules out of the diode block. Compensation for the connector losses (as large as 4 dB) has been proven experimentally. Finally we verified the results of temperature measurement on a surface of a standard working 50 KVA transformer with an infrared camera. Excellent agreement has been demonstrated and no interference with electromagnetic fields has been observed.
Acknowledgements Financial support by the Bavarian State Ministry of Sciences, Research and the Arts within the priority research program “Miniaturized Sensor Systems with Emphasize on Applications in Medical Engineering, Biotechnology, Automotive and Automation Engineering” is gratefully acknowledged. References [1] G. Betta, A. Pietrosanto, A. Scaglione. An Enhanced Fiber-Optic Temperature Sensor for Power Transformer Monitoring. IEEE Trans Instrum Meas 2001;50:1138-44. [2] R.W. Martin, K.W. Zilm. Variable Temperature System Using Vortex Tube Cooling And Fiber Optic Temperature Mesurement for Low Temperature Magic Angle Spinning NMR. J Magn Reson 2004:168:202-9. [3] J. Tang. Fiber-Optic Measurement Systems: Microwave and Radio Frequency Heating Applications. In: Heldman DR, editor. Encyclopedia of Agricultural, Food, and Biological Engineering, London: Taylor & Francis; 2007. [4] B. Culshaw, J. Dakin, eds. Optical Fiber Sensors: Systems and Applications. Volume 2 Boston: Artech House; 1989, p. 446-9. [5] B. Schmauß, M. März, J. Ernst. A fiber-optic sensor for microwave field measurements. Rev Sci Instrum 1995:66:4031-33. [6] R. Trautner, B. Schmauss, M. Shamonin. Manufacture and Characterization of an Extrinsic Elementary Fiber-Optical Sensor for Temperature Measurement tm – Technisches Messen 2008;75 :565–70. [7] E.F. Schubert. Light-Emitting Diodes, Cambridge: Cambridge University Press; 2005.
