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Remote temperature sensor employing erbium-doped silica fiber
Infrared Physics & Technology 43 (2002) 219–222 www.elsevier.com/locate/infrared
Remote temperature sensor employing erbium-doped silica fiber Jesus Castrellon, Gonzalo Paez, Marija Strojnik
*
Centro de Investigaciones en Optica, Apartado Postal 1-948, 37000 Le on, Gto., Mexico
Abstract We present experimental results demonstrating the performance of the erbium-doped silica fiber as a remote temperature sensor in the temperature interval 26–60 °C. It uses the fluorescence intensity ratio with energy levels 2 H11=2 and 4 S3=2 . With the measured fluorescence data and incorporating a fiber optic link, its temperature resolution is better than 0.06 °C, and the sensitivity is 0.06 °C1 . Ó 2002 Published by Elsevier Science B.V. Keywords: Temperature sensor; Erbium-doped silica; Fiber optic sensor; Emission fluorescence
1. Principle of sensor operation The fiber optic sensing devices are particularly appropriate for the operation in thermally and chemically hazardous, and electromagnetically unprotected environments, and fire detection in such spaces, where the line of sight may be obstructed. Rare-earth-doped silica fibers have previously been investigated for the possibility of developing new temperature sensors [1,2], utilizing the fluorescence intensity-ratio technique [3,4]. The temperature dependence of the fluorescence emission spectrum of erbium-doped silica arises due to the temperature dependence of the population of the energy levels and the homogeneous broadening of the emission line widths. When the erbium-doped fiber is pumped with the photon energy of 2:484 1019 J, (k ¼ 800 *
Corresponding author. Tel.: +5247-731017; fax: +5247175000. E-mail address: [email protected] (M. Strojnik).
nm), the 4 I9=2 erbium level is excited and the 4 I13=2 metastable level is quasi-instantaneously populated due to the non-radiative transition. The 4 I13=2 level absorbs pump photons resulting in the excitation of levels 2 H11=2 and 4 S3=2 , responsible for the emission around 520 nm. The latter levels are said to be in quasi-thermal equilibrium, because of the small energy gap between them (1:59 1020 J), in comparison with the relatively large energy difference between them and the next lower level (5:9636 1020 J). In silica, a fast thermal coupling between these two levels has been studied theoretically and observed experimentally [5]. The ratio (R) of intensities (I), resulting from the transition between two levels 2 H11=2 and 4 S3=2 , I1 (2 H11=2 )/I2 (4 S3=2 ) is temperature dependent. 2. Sensor concept Figs. 1 and 2 show the optical schematic and the photo of the key components of the erbiumdoped fiber optic sensor for remote temperature
1350-4495/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 1 3 5 0 - 4 4 9 5 ( 0 2 ) 0 0 1 4 2 - 1
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J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
Fig. 1. Principle of operation of the erbium-doped fiber optic sensor for remote temperature measurements, employing fluorescence emission ratio in the wavelength interval 515–570 nm.
Fig. 2. Photo of the experimental setup to evaluate the performance of the erbium-doped fiber optic temperature sensor.
measurements, employing fluorescence emission ratio in the wavelength interval 515–570 nm.
The laser beam radiating at 785 nm passes through a dichroic mirror into a standard telecommunication fiber (monomodal at 1.55 lm). The erbium-doped (960 ppm) fiber of 20 cm length and the core diameter of 3.2 lm is located inside an enclosure whose temperature T is additionally monitored with a thermocouple. The radiative power of the fluorescence is 10 nW at 60 °C for a 60-mW pump power. We use the interference filters with a 10-nm spectral width transmission, centered on the transition peaks. The dichroic mirror transmits the pumping infrared laser radiation and it reflects the green fluorescence radiation. Another dichroic mirror (WDM 50/50) is used to separate different spectral lines in the emission spectrum. Again, each beam is filtered with a 10nm spectral transmission filter to isolate spectral bands. Finally, beams are detected and divided to give the intensity ratio.
J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
221
A photodiode satisfies the principal requirements of the detection system for high sensitivity in the spectral interval 515–570 nm, low dark current, and compatibility with the optical fiber. In addition to the optical transmission losses at the beam-splitters, filters, and lenses, the noise sources also decrease the signal-to-noise ratio of the sensor.
3. Experimental results We analyzed the sensitivity of the emission bands to the temperature changes. Fig. 3 shows the normalized fluorescence spectrum as a function of wavelength for 2 H11=2 and 4 S3=2 , displayed as the ratio of spectral power at an elevated temperature, Pout ðk; T þ DT Þ, to the spectral power at the reference temperature of 26 °C, Pout ðk; T Þ. The transition 2 H11=2 is much more sensitive to the temperature than transition 4 S3=2 , with linear dependence.
4. Sensor figures-of-merit Fig. 4 shows the ratio of the change in the fiber power output DPout to the change in temperature DTfiber , DPout =DTfiber , as a function of temperature. It is approximately 0.3 nW/°C For the spectral band at 520–530 and 525–535 nm. Similarly, the
Fig. 3. The temperature dependence of the fluorescence spectrum, defined as the ratio of the spectral signal Pout ðk; T þ DT Þ to the spectral signal at the reference temperature of 26 °C, Pout ðk; T Þ, as a function of wavelength.
Fig. 4. The ratio of increase in power output DPout to the increase in temperature change DTfiber , as a function of temperature for several spectral bands.
signal-to-noise ratio is also the highest for these spectral bands. The temperature resolution, seen in Fig. 5, is the smallest temperature change DTmin detected by the sensor. The resolution is <0.05 °C for two spectral bands sensitive to temperature changes. The sensitivity S of the sensor, defined as the ratio of the increase in intensity ratio DRðI1 =I2 Þ to the increase in temperature DTfiber , is depicted in Fig. 6 for the ratio of powers integrated over spectral intervals 515–525, 555–565, and 525–535, 550–560 nm to be approximately 0.03 °C1 , with the value nearly doubled for the other two ratios.
Fig. 5. Temperature resolution DTmin as a function of temperature for several spectral bands.
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J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
The spectral bands highly sensitive to temperature changes are 515–525, 520–530, and 525–535 nm.
References
Fig. 6. Sensitivity of the sensor, evaluated as the ratio of the increase in intensity ratio DRðI1 =I2 Þ to the temperature increase DTfiber , as a function of temperature.
5. Summary We presented experimental results on the radiometric performance of erbium-doped silicafiber optic sensor using the fluorescence intensityratio technique, in temperature interval 26–60 °C.
[1] T. Sun, Z.Y. Zhang, K.T.V. Grattan, A.W. Palmer, Ytterbium-based luorescence decay time fiber optic temperature sensor systems, Rev. Sci. Instrum. 69 (1998) 4179–4185. [2] E. Maurice, S.A. Wade, S.F. Collins, G. Monnom, G.W. Baxter, Self-referenced point temperature sensor based on a fluorescence intensity ratio in Yb3þ -doped silica fiber, Appl. Opt. 36 (31) (1997) 8264–8269. [3] J. Castrellon, G. Paez, M. Strojnik, Radiometric analysis of a fiber optic sensor, Opt. Eng. 41 (6) (2002). [4] P.V. Dos Santos, M.T. Araujo, A.S. Gouveia-Neto, J.A. Madeiros Neto, A.S.B. Sombra, Optical temperature sensing using upconversion fluorescence emission in Er3þ /Yb3þ codoped chalcogenide glass, Appl. Phys. Lett. 73 (1998) 578–580. [5] E. Maurice, G. Monnom, A. Saissy, D.B. Ostrowsky, G.W. Baxter, Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers, Opt. Lett. 19 (1994) 990–992.
Remote temperature sensor employing erbium-doped silica fiber Jesus Castrellon, Gonzalo Paez, Marija Strojnik
*
Centro de Investigaciones en Optica, Apartado Postal 1-948, 37000 Le on, Gto., Mexico
Abstract We present experimental results demonstrating the performance of the erbium-doped silica fiber as a remote temperature sensor in the temperature interval 26–60 °C. It uses the fluorescence intensity ratio with energy levels 2 H11=2 and 4 S3=2 . With the measured fluorescence data and incorporating a fiber optic link, its temperature resolution is better than 0.06 °C, and the sensitivity is 0.06 °C1 . Ó 2002 Published by Elsevier Science B.V. Keywords: Temperature sensor; Erbium-doped silica; Fiber optic sensor; Emission fluorescence
1. Principle of sensor operation The fiber optic sensing devices are particularly appropriate for the operation in thermally and chemically hazardous, and electromagnetically unprotected environments, and fire detection in such spaces, where the line of sight may be obstructed. Rare-earth-doped silica fibers have previously been investigated for the possibility of developing new temperature sensors [1,2], utilizing the fluorescence intensity-ratio technique [3,4]. The temperature dependence of the fluorescence emission spectrum of erbium-doped silica arises due to the temperature dependence of the population of the energy levels and the homogeneous broadening of the emission line widths. When the erbium-doped fiber is pumped with the photon energy of 2:484 1019 J, (k ¼ 800 *
Corresponding author. Tel.: +5247-731017; fax: +5247175000. E-mail address: [email protected] (M. Strojnik).
nm), the 4 I9=2 erbium level is excited and the 4 I13=2 metastable level is quasi-instantaneously populated due to the non-radiative transition. The 4 I13=2 level absorbs pump photons resulting in the excitation of levels 2 H11=2 and 4 S3=2 , responsible for the emission around 520 nm. The latter levels are said to be in quasi-thermal equilibrium, because of the small energy gap between them (1:59 1020 J), in comparison with the relatively large energy difference between them and the next lower level (5:9636 1020 J). In silica, a fast thermal coupling between these two levels has been studied theoretically and observed experimentally [5]. The ratio (R) of intensities (I), resulting from the transition between two levels 2 H11=2 and 4 S3=2 , I1 (2 H11=2 )/I2 (4 S3=2 ) is temperature dependent. 2. Sensor concept Figs. 1 and 2 show the optical schematic and the photo of the key components of the erbiumdoped fiber optic sensor for remote temperature
1350-4495/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 1 3 5 0 - 4 4 9 5 ( 0 2 ) 0 0 1 4 2 - 1
220
J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
Fig. 1. Principle of operation of the erbium-doped fiber optic sensor for remote temperature measurements, employing fluorescence emission ratio in the wavelength interval 515–570 nm.
Fig. 2. Photo of the experimental setup to evaluate the performance of the erbium-doped fiber optic temperature sensor.
measurements, employing fluorescence emission ratio in the wavelength interval 515–570 nm.
The laser beam radiating at 785 nm passes through a dichroic mirror into a standard telecommunication fiber (monomodal at 1.55 lm). The erbium-doped (960 ppm) fiber of 20 cm length and the core diameter of 3.2 lm is located inside an enclosure whose temperature T is additionally monitored with a thermocouple. The radiative power of the fluorescence is 10 nW at 60 °C for a 60-mW pump power. We use the interference filters with a 10-nm spectral width transmission, centered on the transition peaks. The dichroic mirror transmits the pumping infrared laser radiation and it reflects the green fluorescence radiation. Another dichroic mirror (WDM 50/50) is used to separate different spectral lines in the emission spectrum. Again, each beam is filtered with a 10nm spectral transmission filter to isolate spectral bands. Finally, beams are detected and divided to give the intensity ratio.
J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
221
A photodiode satisfies the principal requirements of the detection system for high sensitivity in the spectral interval 515–570 nm, low dark current, and compatibility with the optical fiber. In addition to the optical transmission losses at the beam-splitters, filters, and lenses, the noise sources also decrease the signal-to-noise ratio of the sensor.
3. Experimental results We analyzed the sensitivity of the emission bands to the temperature changes. Fig. 3 shows the normalized fluorescence spectrum as a function of wavelength for 2 H11=2 and 4 S3=2 , displayed as the ratio of spectral power at an elevated temperature, Pout ðk; T þ DT Þ, to the spectral power at the reference temperature of 26 °C, Pout ðk; T Þ. The transition 2 H11=2 is much more sensitive to the temperature than transition 4 S3=2 , with linear dependence.
4. Sensor figures-of-merit Fig. 4 shows the ratio of the change in the fiber power output DPout to the change in temperature DTfiber , DPout =DTfiber , as a function of temperature. It is approximately 0.3 nW/°C For the spectral band at 520–530 and 525–535 nm. Similarly, the
Fig. 3. The temperature dependence of the fluorescence spectrum, defined as the ratio of the spectral signal Pout ðk; T þ DT Þ to the spectral signal at the reference temperature of 26 °C, Pout ðk; T Þ, as a function of wavelength.
Fig. 4. The ratio of increase in power output DPout to the increase in temperature change DTfiber , as a function of temperature for several spectral bands.
signal-to-noise ratio is also the highest for these spectral bands. The temperature resolution, seen in Fig. 5, is the smallest temperature change DTmin detected by the sensor. The resolution is <0.05 °C for two spectral bands sensitive to temperature changes. The sensitivity S of the sensor, defined as the ratio of the increase in intensity ratio DRðI1 =I2 Þ to the increase in temperature DTfiber , is depicted in Fig. 6 for the ratio of powers integrated over spectral intervals 515–525, 555–565, and 525–535, 550–560 nm to be approximately 0.03 °C1 , with the value nearly doubled for the other two ratios.
Fig. 5. Temperature resolution DTmin as a function of temperature for several spectral bands.
222
J. Castrellon et al. / Infrared Physics & Technology 43 (2002) 219–222
The spectral bands highly sensitive to temperature changes are 515–525, 520–530, and 525–535 nm.
References
Fig. 6. Sensitivity of the sensor, evaluated as the ratio of the increase in intensity ratio DRðI1 =I2 Þ to the temperature increase DTfiber , as a function of temperature.
5. Summary We presented experimental results on the radiometric performance of erbium-doped silicafiber optic sensor using the fluorescence intensityratio technique, in temperature interval 26–60 °C.
[1] T. Sun, Z.Y. Zhang, K.T.V. Grattan, A.W. Palmer, Ytterbium-based luorescence decay time fiber optic temperature sensor systems, Rev. Sci. Instrum. 69 (1998) 4179–4185. [2] E. Maurice, S.A. Wade, S.F. Collins, G. Monnom, G.W. Baxter, Self-referenced point temperature sensor based on a fluorescence intensity ratio in Yb3þ -doped silica fiber, Appl. Opt. 36 (31) (1997) 8264–8269. [3] J. Castrellon, G. Paez, M. Strojnik, Radiometric analysis of a fiber optic sensor, Opt. Eng. 41 (6) (2002). [4] P.V. Dos Santos, M.T. Araujo, A.S. Gouveia-Neto, J.A. Madeiros Neto, A.S.B. Sombra, Optical temperature sensing using upconversion fluorescence emission in Er3þ /Yb3þ codoped chalcogenide glass, Appl. Phys. Lett. 73 (1998) 578–580. [5] E. Maurice, G. Monnom, A. Saissy, D.B. Ostrowsky, G.W. Baxter, Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers, Opt. Lett. 19 (1994) 990–992.