- Email: [email protected]
Wide measurement-range fiber-optic temperature sensor based on ZnO thin film
Optics and Lasers in Engineering 60 (2014) 49–53
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wide measurement-range fiber-optic temperature sensor based on ZnO thin film Helin Wang n, Aijun Yang n, Ling Tang College of Science, Zhejiang University of Technology, Hangzhou, Zhejiang 310023, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2013 Received in revised form 7 March 2014 Accepted 17 March 2014
Based on the spectral absorption effect of ZnO thin film, a wide measurement-range fiber optical temperature sensor with a sensitivity of 0.06 m/1C is proposed. By optimally designing the sensing head structure and using the light sources with different bandwidths (8 nm, 30 nm and 47 nm), the temperature-dependent transmission spectra from the sensor are measured and investigated experimentally. The results show that the spectral transmittance curves become smoother with the increased bandwidth of the incident light, and the red shift of the absorption edge is almost the same when the temperature is increased by 50 K. Using the annealed ZnO thin film and the optimized light source (47 nm), the temperature resolution of the designed sensor reaches approximately 1 1C in a wide temperature range 373–873 K. Besides a wide measurement range, the temperature sensor has some other advantages such as a simple optical structure, a low cost and a good stability. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Temperature sensor Thin film Fiber
Fiber-optic temperature sensors have attracted the attention of researchers due to some special properties such as small size, light weight, anti-electromagnetic interference, good dynamic response, high sensitivity, high stability, high resolution and high measurement accuracy. Based on these advantages, they can be applied into biologically, chemically and electromagnetically hazardous environments, which include highly corrosive media, aerospace vehicles, electrical power stations, oil refineries, coal mines and fire security [1–8]. In terms of the temperature sensing principle, the radiation temperature sensor based on Planck Radiation law [9], the fluorescence temperature sensor on the fluorescence life-span [10], the optical absorption temperature sensor with GaSe chip as a sensing element [11], the macro-bend temperature sensor based on the temperature dependence of the refractive index of the core and cladding, the numerical aperture of polymer optical fiber [12], the temperature sensor based on all-fiber multimode–singlemode– multimode interference [13] are the common types. However, the above-mentioned temperature sensors have some disadvantages. For example, some are only suitable for the high temperature measurement, not for the low temperature detection, while others have a high cost or a narrow measurement range. Therefore, it is necessary to develop a novel fiber-optic temperature sensor with a wide measurement range, a simple optical structure, a low cost
n
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (A. Yang). http://dx.doi.org/10.1016/j.optlaseng.2014.03.008 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
and a better stability, and it can be used to both the high and low temperature fields. As one of the temperature-dependent semiconductor materials [14–17], ZnO material is a good choice because its absorption spectrum is very sensitive to temperature. So it can be used to fabricate a wide measurement-range temperature sensor with high sensitivity [18]. In our experiment, the probe temperature is set in a range from 300 K to 900 K due to the furnace's high temperature limit. The high-quality ZnO thin film, as an optic sensing element, is first deposited on sapphire glass surface at 523 K by electron beam evaporation technology (EBET). Here the sapphire glass is chosen as the substrate of ZnO thin film because its transmission spectra can maintain a very good stability in a wide temperature range from 323 K to 723 K (see Fig. 1(b)). Then with atomic force microscope (AFM) and x-ray diffraction (XRD) methods, the surface roughness of ZnO thin film is measured to be about 7.323 nm and the particle size is between 100 and 200 nm (see Fig. 1(a)). Moreover, the ZnO has a strong diffraction peak (0 0 0 2) at the scan angle 2θ ¼ 34.5880 (see Fig. 1(c)), which indicates ZnO thin film has a strong preferred orientation along the c-axis. As a whole, the smooth surface morphology of ZnO thin film results in a high transmittance in the visible light region, and the diffraction peak position corresponds to the optical absorption measurement described later. For the temperature sensor, the sensing head is one of the most important components, so the proper mechanical design and the related optical properties must be studied. With the ZnO thin film deposited on the sapphire glass, the temperature sensing head can be designed to be a transmission- or reflection-type.
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H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
N/m
2.60 2.65 E-2(N/m)
0.02566
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5 4 3 0
0 1
1 2
2
μm 2 1
3
3 4
4 5
5 μm
0 0
1
2
μm
3
4
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Fig. 1. (a) The AFM micrograph and XRD image of ZnO thin film deposited on the sapphire glass; (b) the transmission spectra of the sapphire glass when the temperature varies from 323 K to 723 K; (c) the XRD image of ZnO thin film.
The transmission-type fiber temperature sensor has been fabricated successfully in our previous works [19], however it is more suitable for the single-point temperature measurement and does not meet the needs of the more-point temperature detection. Therefore, in order to build a distributed fiber-optic temperature sensor with a wide measurement range in the future, the reflection-type temperature sensing head is chosen, designed and fabricated in this work. The reflection-type fiber-optic temperature sensing head is made of a coupling fiber, a convex lens, a metal tube and a cone-type sapphire prism (see Fig. 2). The whole length L of the sensing head is about 150 mm. In the sensing head, the focal length f of the convex lens is about 15 mm, and the thickness of ZnO thin film is chosen to be about 500 nm for improving the crystallinity and grain size of thin film, enhancing the stability of the system and increasing the useful life of sensor [20]. The combined effect of the convex lens and the coupling fiber is equivalent to a fiber collimator, which improves the coupling efficiency of the reflected light and enhances the stability of the sensor. It is worth noting that, besides temperature, ZnO thin film is also sensitive to the other environmental noises, such as pressure and electric field, which can also result in the shift of optical absorption edge [21–25]. So, in order to eliminate the effects of those noises, all optical components of the sensing head are sealed in a stainless-steel thermal-resistance metal tube, because the sealed steel tube can withstand the high pressure and shield the external electric field. In fact, this design not only
Fig. 2. The sensing head of reflection-type optical temperature sensor.
improves the system sensitivity and stability, but also avoids the contamination from the external environment, and increases the lifetime of the sensing head. With the above designed temperature sensing head, a reflective fiber-optic temperature sensor is set up (see Fig. 3), which includes a tunable LED light source, a fiber coupler, a temperature sensing head, a ceramic heating furnace, a spectral detector, a signal processing unit and a temperature display device. The inset in Fig. 3 is a home-made integrated optical temperature sensing head. The LED light with
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
51
Fig. 3. Schematic diagram of fiber-optic temperature sensor system.
a wavelength range of 350–450 nm is first injected into the optical temperature sensing head through the fiber coupler. Then its reflective light passes through the fiber coupler again, and travels along another branch of the fiber coupler to the fiber optical spectroscope for the spectral detection. The detected signal light then is demodulated in the signal processing unit and the bandgap values at different temperatures are calculated. Finally, using the linear relation between the bandgap of semiconductor materials (ZnO) and the temperature, the measured temperature values are obtained and displayed on a screen or monitor. In temperature measurement, the sensing head is mounted in a ceramic heating furnace, and a digital thermometer with PT100 resistance is used as the temperature monitor for reference and calibration. From our previous works [19], it is known that the transmission spectrum of ZnO thin film depends on the temperature, and its optical-absorption edge generates a red-shift with the increase of temperature. However, it is not clear whether the bandwidth of the incident light has an effect to the red shift of the absorption edge. Fig. 4(b)–(d) show the measured absorption spectra versus the incident light with the different bandwidths (8 nm, 30 nm and 47 nm, see Fig. 4(a)), respectively. By carefully analyzing these absorption spectra, it is easy to find that, the red shift of the absorption edge becomes more uniform with the increase of the bandwidth of the incident light when the measured temperature increases. For example, comparing the region 1# and 2# surrounded by circle in Fig. 4(c) and (d), one can see that, the slopes of the spectral curves in the region 1# decrease with the increase of temperature, while the slopes of the spectral curves in the region 2# are almost the same. It indicates that, using the broadband incident light (47 nm), the absorption edges of the transmission spectra nearly move in parallel with the increasing temperature. This point is very important for the designation of temperature sensor with high sensitivity and high resolution, and it further shows that ZnO thin film is a more appropriate material for temperature detection. However, when the detected temperature is increased by 50 degrees at each step, the red-shift amount of the absorption edge among them still exists some minor differences, and the improvement will be illustrated later. Thus, the spectral bandwidth of the input light must also be optimized during setting up the fiber-optic temperature sensor, and here it is chosen to be 47 nm. Besides the optimal sensing head structure and the broadband light source, the annealing treatment of thin films has very important effects on the absorption edges of the signal spectra. For ZnO thin film, the referenced annealing temperature is usually 573–673 K in air [26,27], while much higher temperatures can be applied in vacuum [28] or, in the case of rapid thermal annealing, nitrogen [29]. Comparing with the un-annealed ZnO thin film, the crystal quality of the annealed ZnO film can be improved
remarkably, and its spectral transmittance curves become smoother, while the average transmittance in the visible region slightly decreases [19,30]. In our system, for further improving the absorption edge of transmission spectrum and optimizing the sensor performance, ZnO thin film is annealed in air at 673 K. Fig. 5 shows the transmission spectra of the annealed ZnO thin film at temperatures from 373 K to 873 K. Comparing with Fig. 4 (d), one can see from Fig. 5 that, the transmission spectra in the wavelength range 350–450 nm become much smoother, and the red-shift amount of the absorption edge becomes more uniform with the increase of temperature. Although the spectral transmittance in the visible region slightly decreases for the annealed ZnO thin film, it maintains above 70% due to the smooth surface and the good crystalline quality of the ZnO thin film. In addition, the sharp absorption edge shifts to the longer wavelength region (red-shift) as the temperature increases, which indicates that the band gap of ZnO thin film depends on the temperature and embodies the band gap renormalization effect caused by the increase of temperature. The experimental results reveal that the temperature-dependent red-shift of the transmission spectrum from sensor can be optimized efficiently by designing the appropriate sensing head structure, using a broadband incident light source (47 nm) and adopting the annealed ZnO thin film. Since the transmission spectra of ZnO (a direct, wide band-gap semiconductor material) thin film vary with the measured temperature, the temperature signal can be demodulated from the output transmission spectra of sensor according to the absorption properties of semiconductor materials. In fact, the optical absorption coefficient α of semiconductor materials can be expressed by the Lambert law: I(λ,d)¼ I(λ,0)exp[ α(λ)d], where I (λ,d) and I(λ,0) are the intensities of the transmitted and incident light with the operation wavelength λ, respectively. d is the thickness of the thin film (see Fig. 2). Assume that the transmittance TR(λ) of sensing head with ZnO thin film is defined as I(λ,d)/I(λ,0), one can obtain α(λ)¼ln[TR(λ)]/d, and then the temperaturedependent band gap Eg(T) of ZnO thin film can be obtained by αhν ¼A[hν Eg(T)], where A is a constant related to ZnO material, and hν is the photon energy [31]. Fig. 6 gives the band gap energy Eg of ZnO thin film versus the temperature T. The fitting result indicates that Eg and T are satisfied with a good linear relationship, and they can be expressed by Eg(T)¼ Eg(0)þ rT. Here the fitting parameter Eg(0)¼ 3.39249 eV, and the negative temperature coefficient r¼ 0.000482857 eV K 1, which is close to r¼ 0.0003 eV K 1 in Wang M S and Hong K J's work [31,32]. It's worth noting that, the fitting parameters Eg(0) and r need some slight correction according to the actual detection environment for calibrating the measured temperature value during the temperature demodulation. The linear relation between Eg and T further proves ZnO thin film can be used to a high-temperature
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H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
Fig. 4. The variation of the transmission spectra from sensor with temperature. (a) is the incident spectra with different bandwidths: 8 nm, 30 nm and 47 nm; (b) (c) and (d) are the output red-shift spectra corresponding to the incident spectral 1 (8 nm) 2 (30 nm) and 3 (47 nm), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Spectral transmittance of the annealed ZnO thin film under different temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sensing element, and certifies the designed fiber-optic temperature sensor is reliable. Moreover, the band gap energy Eg decreases from 3.21 eV to 2.97 eV when the temperature is increased from 373 K to 873 K, and it illustrates the absorption edge of transmission spectrum generates a 30 nm linear red-shift (see Fig. 5). Thus, the temperature sensitivity of our designed fiber-optic temperature sensor reaches 0.06 nm/1C.
Fig. 6. Experiment results and fitting line.
In summary, based on the ZnO thin film with a thickness of about 500 nm, a wide measurement-range fiber-optic temperature sensor with a sensitivity of 0.06 nm/1C and a resolution of 1 1C is established and optimized successfully. The absorption edges of the optimized transmission spectra become much smoother and more uniform with the increase of the test temperature (from 373 K to 873 K) when the incident light bandwidth is chosen to be 47 nm and ZnO thin film is annealed in air at 673 K. The linear
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
red-shift amount of the absorption edge reaches 30 nm and the corresponding band gap energy decreases linearly from 3.21 eV to 2.97 eV as the temperature is increased by 500 K (from 373 K to 873 K). Although the highest test temperature is only 873 K due to the furnace's high temperature limit in our experiment, the theoretical measurement range can extend from 10 K to 1800 K. So the designed optical temperature sensor has a function of high and low temperature testing, which is much higher than the measurement range of previous reported optical temperature sensors, such in Refs. [33,34] Besides the above mentioned perfect properties, the temperature sensor has some other advantages such as a simple optical structure, a low cost and a good stability, and it can also be developed into a distribution sensing structure. More importantly, it can be applied to some extremely environments, such as aircraft, nuclear power station and power transmission system and so on, due to a wide measurement range. Acknowledgments This work is supported by the National Nature Scientific Foundation of China (No. 11226148 and No. 11304279), the Scientific Research Foundation of Zhejiang Province (No. LY12F05006), the Education Department Foundation of Zhejiang Province (No. Y201121906). References [1] Hartog AH. J Lightwave Technol LT-1 1983:498. [2] Theocharous E. Proc. Soc. Photo-Opt. Instrum. Eng. 514, 199 (1984). [3] Artyushenko VG, Voitsekhoskii VV, Zubov IV, Masychev VI, Sysoev VK. Sov J Quantum Electron 1985;15:579. [4] Snitzer E, Morey WW, Glenn WH. in: Proceedings first international conference on optical fiber sensors (IEE, London, 1983), p. 79. [5] Farahi F, Jones JDC, Jackson DA. Opt Lett 1991;16:1800.
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[6] Choi HS, Taylor HF, Lee CE. Opt Lett 1997;22:1814. [7] Farries MC, Fermann ME, Laming LI, Poole SB, Payne DN, Leach AP. Electron Lett 1986;22:418. [8] dos Santos PV, de Araujo MT, Gouveia-Neto AS. Appl Phys Lett 1998;73:578. [9] Dils RR. J Appl Phys 1983;54:1198. [10] Sholes RR, Small JG. Rev Sci Instrum 1980;51:882. [11] Alden M, Omrane A, Richter M, Sarner G. Prog Energy Combust Sci 2011;37:422. [12] Moraleda AT, García CV, Zaballa JZ, Arrue J. Sensors 2013;13:13076. [13] Jianghai Wo Qizhen Sun, Liu Hai, Li Xiaolei, Zhang Jiejun, Deming Liu, Shum Perry Ping. Opt Fiber Technol 2013;19:289. [14] Pelletier JMJ, Altrkorn R. Appl Sectrosc 2000;54:1837. [15] Chistensen DA. J Bioeng 1997;1:541. [16] da Silva CJ, de Araujo MT, Gouvenia EA. Appl Phys 2000;70:185. [17] Zhou H, Sui CH, Chen NB, Xu XJ, Wei GY, Cai PG. J Funct Mater Devices 2008;14:37. [18] Sui CH, Chen NB, Xu XJ. Thin Solid Films 2008;516:1137. [19] Cai PG, Zheng D, Xu XJ, Liua YL, Chen NB, Wei GR, Sui CH. Mater Sci Eng, B 2010;171:116. [20] Chang JF, Kuo HH, Leu IC, Hon MH. Sens Actuators, B 2002;84:258–64. [21] Choopun S, Vispute RD, Noch W, Balsamo A, Sharma RP, Venkatesan T, Lliadis A, Look DC. Appl Phys Lett 1999;75:3947. [22] Sun CW, Liu ZW, Qin FW, Zhang QY, Liu K, Wu SF. Acta Phys Sin 2006;55:1390. [23] Ko HJ, Chen YF, Zhu Z, Yao T. Appl Phys Lett 2000;76:1905. [24] Ogata K, Sakurai K, Fujita S, Fujita S, Matsushige K. Cryst Growth 2000;214:312. [25] Fu ZX, Lin BX, He YP, Liao GH. Chin J Lumin 2002;23:559. [26] Minami T, Oohashi K, Takata S, Mouri T, Ogawa N. Thin Solid Films 1990;193– 194:721. [27] Chang JF, Lin WC, Hon MH. Appl Surf Sci 2001;183:18. [28] Igasaki Y, Ishikawa M, Shimaoka G. Appl Surf Sci 1998;33–34:926. [29] Kim KK, Niki S, Oh JY, Song JO, Seong TY, Park SJ, Fujita S, Kim SW. J Appl Phys 2005;97:066103. [30] Uthanna S, Subramanyam TK, Srinivasulu Baidu B, Rao G Mohan. Opt Mater 2002;19:461. [31] Wang MS, Kim EJ, Chung JS. Phys Status Solid A 2006;203:2418. [32] Hong KJ, Jeong TS. J Cryst Growth 2005;208:545. [33] Ran Wang Jianquan Yao, Miao Yinping, Lu Ying, Xu Degang, Luan Nannan, Musideke Mayilamu, Duan Liangcheng, Hao Congjing. Sensors 2013;13:7916–25. [34] Xu Wei, Gao Xiaoyang, Zheng Longjiang, Zhang Zhiguo, Cao Wenwu. Sens Actuators, B 2012;173:250–3.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Wide measurement-range fiber-optic temperature sensor based on ZnO thin film Helin Wang n, Aijun Yang n, Ling Tang College of Science, Zhejiang University of Technology, Hangzhou, Zhejiang 310023, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 December 2013 Received in revised form 7 March 2014 Accepted 17 March 2014
Based on the spectral absorption effect of ZnO thin film, a wide measurement-range fiber optical temperature sensor with a sensitivity of 0.06 m/1C is proposed. By optimally designing the sensing head structure and using the light sources with different bandwidths (8 nm, 30 nm and 47 nm), the temperature-dependent transmission spectra from the sensor are measured and investigated experimentally. The results show that the spectral transmittance curves become smoother with the increased bandwidth of the incident light, and the red shift of the absorption edge is almost the same when the temperature is increased by 50 K. Using the annealed ZnO thin film and the optimized light source (47 nm), the temperature resolution of the designed sensor reaches approximately 1 1C in a wide temperature range 373–873 K. Besides a wide measurement range, the temperature sensor has some other advantages such as a simple optical structure, a low cost and a good stability. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Temperature sensor Thin film Fiber
Fiber-optic temperature sensors have attracted the attention of researchers due to some special properties such as small size, light weight, anti-electromagnetic interference, good dynamic response, high sensitivity, high stability, high resolution and high measurement accuracy. Based on these advantages, they can be applied into biologically, chemically and electromagnetically hazardous environments, which include highly corrosive media, aerospace vehicles, electrical power stations, oil refineries, coal mines and fire security [1–8]. In terms of the temperature sensing principle, the radiation temperature sensor based on Planck Radiation law [9], the fluorescence temperature sensor on the fluorescence life-span [10], the optical absorption temperature sensor with GaSe chip as a sensing element [11], the macro-bend temperature sensor based on the temperature dependence of the refractive index of the core and cladding, the numerical aperture of polymer optical fiber [12], the temperature sensor based on all-fiber multimode–singlemode– multimode interference [13] are the common types. However, the above-mentioned temperature sensors have some disadvantages. For example, some are only suitable for the high temperature measurement, not for the low temperature detection, while others have a high cost or a narrow measurement range. Therefore, it is necessary to develop a novel fiber-optic temperature sensor with a wide measurement range, a simple optical structure, a low cost
n
Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (A. Yang). http://dx.doi.org/10.1016/j.optlaseng.2014.03.008 0143-8166/& 2014 Elsevier Ltd. All rights reserved.
and a better stability, and it can be used to both the high and low temperature fields. As one of the temperature-dependent semiconductor materials [14–17], ZnO material is a good choice because its absorption spectrum is very sensitive to temperature. So it can be used to fabricate a wide measurement-range temperature sensor with high sensitivity [18]. In our experiment, the probe temperature is set in a range from 300 K to 900 K due to the furnace's high temperature limit. The high-quality ZnO thin film, as an optic sensing element, is first deposited on sapphire glass surface at 523 K by electron beam evaporation technology (EBET). Here the sapphire glass is chosen as the substrate of ZnO thin film because its transmission spectra can maintain a very good stability in a wide temperature range from 323 K to 723 K (see Fig. 1(b)). Then with atomic force microscope (AFM) and x-ray diffraction (XRD) methods, the surface roughness of ZnO thin film is measured to be about 7.323 nm and the particle size is between 100 and 200 nm (see Fig. 1(a)). Moreover, the ZnO has a strong diffraction peak (0 0 0 2) at the scan angle 2θ ¼ 34.5880 (see Fig. 1(c)), which indicates ZnO thin film has a strong preferred orientation along the c-axis. As a whole, the smooth surface morphology of ZnO thin film results in a high transmittance in the visible light region, and the diffraction peak position corresponds to the optical absorption measurement described later. For the temperature sensor, the sensing head is one of the most important components, so the proper mechanical design and the related optical properties must be studied. With the ZnO thin film deposited on the sapphire glass, the temperature sensing head can be designed to be a transmission- or reflection-type.
50
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
N/m
2.60 2.65 E-2(N/m)
0.02566
0.02653
5 4 3 0
0 1
1 2
2
μm 2 1
3
3 4
4 5
5 μm
0 0
1
2
μm
3
4
5
Fig. 1. (a) The AFM micrograph and XRD image of ZnO thin film deposited on the sapphire glass; (b) the transmission spectra of the sapphire glass when the temperature varies from 323 K to 723 K; (c) the XRD image of ZnO thin film.
The transmission-type fiber temperature sensor has been fabricated successfully in our previous works [19], however it is more suitable for the single-point temperature measurement and does not meet the needs of the more-point temperature detection. Therefore, in order to build a distributed fiber-optic temperature sensor with a wide measurement range in the future, the reflection-type temperature sensing head is chosen, designed and fabricated in this work. The reflection-type fiber-optic temperature sensing head is made of a coupling fiber, a convex lens, a metal tube and a cone-type sapphire prism (see Fig. 2). The whole length L of the sensing head is about 150 mm. In the sensing head, the focal length f of the convex lens is about 15 mm, and the thickness of ZnO thin film is chosen to be about 500 nm for improving the crystallinity and grain size of thin film, enhancing the stability of the system and increasing the useful life of sensor [20]. The combined effect of the convex lens and the coupling fiber is equivalent to a fiber collimator, which improves the coupling efficiency of the reflected light and enhances the stability of the sensor. It is worth noting that, besides temperature, ZnO thin film is also sensitive to the other environmental noises, such as pressure and electric field, which can also result in the shift of optical absorption edge [21–25]. So, in order to eliminate the effects of those noises, all optical components of the sensing head are sealed in a stainless-steel thermal-resistance metal tube, because the sealed steel tube can withstand the high pressure and shield the external electric field. In fact, this design not only
Fig. 2. The sensing head of reflection-type optical temperature sensor.
improves the system sensitivity and stability, but also avoids the contamination from the external environment, and increases the lifetime of the sensing head. With the above designed temperature sensing head, a reflective fiber-optic temperature sensor is set up (see Fig. 3), which includes a tunable LED light source, a fiber coupler, a temperature sensing head, a ceramic heating furnace, a spectral detector, a signal processing unit and a temperature display device. The inset in Fig. 3 is a home-made integrated optical temperature sensing head. The LED light with
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
51
Fig. 3. Schematic diagram of fiber-optic temperature sensor system.
a wavelength range of 350–450 nm is first injected into the optical temperature sensing head through the fiber coupler. Then its reflective light passes through the fiber coupler again, and travels along another branch of the fiber coupler to the fiber optical spectroscope for the spectral detection. The detected signal light then is demodulated in the signal processing unit and the bandgap values at different temperatures are calculated. Finally, using the linear relation between the bandgap of semiconductor materials (ZnO) and the temperature, the measured temperature values are obtained and displayed on a screen or monitor. In temperature measurement, the sensing head is mounted in a ceramic heating furnace, and a digital thermometer with PT100 resistance is used as the temperature monitor for reference and calibration. From our previous works [19], it is known that the transmission spectrum of ZnO thin film depends on the temperature, and its optical-absorption edge generates a red-shift with the increase of temperature. However, it is not clear whether the bandwidth of the incident light has an effect to the red shift of the absorption edge. Fig. 4(b)–(d) show the measured absorption spectra versus the incident light with the different bandwidths (8 nm, 30 nm and 47 nm, see Fig. 4(a)), respectively. By carefully analyzing these absorption spectra, it is easy to find that, the red shift of the absorption edge becomes more uniform with the increase of the bandwidth of the incident light when the measured temperature increases. For example, comparing the region 1# and 2# surrounded by circle in Fig. 4(c) and (d), one can see that, the slopes of the spectral curves in the region 1# decrease with the increase of temperature, while the slopes of the spectral curves in the region 2# are almost the same. It indicates that, using the broadband incident light (47 nm), the absorption edges of the transmission spectra nearly move in parallel with the increasing temperature. This point is very important for the designation of temperature sensor with high sensitivity and high resolution, and it further shows that ZnO thin film is a more appropriate material for temperature detection. However, when the detected temperature is increased by 50 degrees at each step, the red-shift amount of the absorption edge among them still exists some minor differences, and the improvement will be illustrated later. Thus, the spectral bandwidth of the input light must also be optimized during setting up the fiber-optic temperature sensor, and here it is chosen to be 47 nm. Besides the optimal sensing head structure and the broadband light source, the annealing treatment of thin films has very important effects on the absorption edges of the signal spectra. For ZnO thin film, the referenced annealing temperature is usually 573–673 K in air [26,27], while much higher temperatures can be applied in vacuum [28] or, in the case of rapid thermal annealing, nitrogen [29]. Comparing with the un-annealed ZnO thin film, the crystal quality of the annealed ZnO film can be improved
remarkably, and its spectral transmittance curves become smoother, while the average transmittance in the visible region slightly decreases [19,30]. In our system, for further improving the absorption edge of transmission spectrum and optimizing the sensor performance, ZnO thin film is annealed in air at 673 K. Fig. 5 shows the transmission spectra of the annealed ZnO thin film at temperatures from 373 K to 873 K. Comparing with Fig. 4 (d), one can see from Fig. 5 that, the transmission spectra in the wavelength range 350–450 nm become much smoother, and the red-shift amount of the absorption edge becomes more uniform with the increase of temperature. Although the spectral transmittance in the visible region slightly decreases for the annealed ZnO thin film, it maintains above 70% due to the smooth surface and the good crystalline quality of the ZnO thin film. In addition, the sharp absorption edge shifts to the longer wavelength region (red-shift) as the temperature increases, which indicates that the band gap of ZnO thin film depends on the temperature and embodies the band gap renormalization effect caused by the increase of temperature. The experimental results reveal that the temperature-dependent red-shift of the transmission spectrum from sensor can be optimized efficiently by designing the appropriate sensing head structure, using a broadband incident light source (47 nm) and adopting the annealed ZnO thin film. Since the transmission spectra of ZnO (a direct, wide band-gap semiconductor material) thin film vary with the measured temperature, the temperature signal can be demodulated from the output transmission spectra of sensor according to the absorption properties of semiconductor materials. In fact, the optical absorption coefficient α of semiconductor materials can be expressed by the Lambert law: I(λ,d)¼ I(λ,0)exp[ α(λ)d], where I (λ,d) and I(λ,0) are the intensities of the transmitted and incident light with the operation wavelength λ, respectively. d is the thickness of the thin film (see Fig. 2). Assume that the transmittance TR(λ) of sensing head with ZnO thin film is defined as I(λ,d)/I(λ,0), one can obtain α(λ)¼ln[TR(λ)]/d, and then the temperaturedependent band gap Eg(T) of ZnO thin film can be obtained by αhν ¼A[hν Eg(T)], where A is a constant related to ZnO material, and hν is the photon energy [31]. Fig. 6 gives the band gap energy Eg of ZnO thin film versus the temperature T. The fitting result indicates that Eg and T are satisfied with a good linear relationship, and they can be expressed by Eg(T)¼ Eg(0)þ rT. Here the fitting parameter Eg(0)¼ 3.39249 eV, and the negative temperature coefficient r¼ 0.000482857 eV K 1, which is close to r¼ 0.0003 eV K 1 in Wang M S and Hong K J's work [31,32]. It's worth noting that, the fitting parameters Eg(0) and r need some slight correction according to the actual detection environment for calibrating the measured temperature value during the temperature demodulation. The linear relation between Eg and T further proves ZnO thin film can be used to a high-temperature
52
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
Fig. 4. The variation of the transmission spectra from sensor with temperature. (a) is the incident spectra with different bandwidths: 8 nm, 30 nm and 47 nm; (b) (c) and (d) are the output red-shift spectra corresponding to the incident spectral 1 (8 nm) 2 (30 nm) and 3 (47 nm), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Spectral transmittance of the annealed ZnO thin film under different temperatures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sensing element, and certifies the designed fiber-optic temperature sensor is reliable. Moreover, the band gap energy Eg decreases from 3.21 eV to 2.97 eV when the temperature is increased from 373 K to 873 K, and it illustrates the absorption edge of transmission spectrum generates a 30 nm linear red-shift (see Fig. 5). Thus, the temperature sensitivity of our designed fiber-optic temperature sensor reaches 0.06 nm/1C.
Fig. 6. Experiment results and fitting line.
In summary, based on the ZnO thin film with a thickness of about 500 nm, a wide measurement-range fiber-optic temperature sensor with a sensitivity of 0.06 nm/1C and a resolution of 1 1C is established and optimized successfully. The absorption edges of the optimized transmission spectra become much smoother and more uniform with the increase of the test temperature (from 373 K to 873 K) when the incident light bandwidth is chosen to be 47 nm and ZnO thin film is annealed in air at 673 K. The linear
H. Wang et al. / Optics and Lasers in Engineering 60 (2014) 49–53
red-shift amount of the absorption edge reaches 30 nm and the corresponding band gap energy decreases linearly from 3.21 eV to 2.97 eV as the temperature is increased by 500 K (from 373 K to 873 K). Although the highest test temperature is only 873 K due to the furnace's high temperature limit in our experiment, the theoretical measurement range can extend from 10 K to 1800 K. So the designed optical temperature sensor has a function of high and low temperature testing, which is much higher than the measurement range of previous reported optical temperature sensors, such in Refs. [33,34] Besides the above mentioned perfect properties, the temperature sensor has some other advantages such as a simple optical structure, a low cost and a good stability, and it can also be developed into a distribution sensing structure. More importantly, it can be applied to some extremely environments, such as aircraft, nuclear power station and power transmission system and so on, due to a wide measurement range. Acknowledgments This work is supported by the National Nature Scientific Foundation of China (No. 11226148 and No. 11304279), the Scientific Research Foundation of Zhejiang Province (No. LY12F05006), the Education Department Foundation of Zhejiang Province (No. Y201121906). References [1] Hartog AH. J Lightwave Technol LT-1 1983:498. [2] Theocharous E. Proc. Soc. Photo-Opt. Instrum. Eng. 514, 199 (1984). [3] Artyushenko VG, Voitsekhoskii VV, Zubov IV, Masychev VI, Sysoev VK. Sov J Quantum Electron 1985;15:579. [4] Snitzer E, Morey WW, Glenn WH. in: Proceedings first international conference on optical fiber sensors (IEE, London, 1983), p. 79. [5] Farahi F, Jones JDC, Jackson DA. Opt Lett 1991;16:1800.
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