A miniature temperature high germanium doped PCF interferometer sensor F. C. Favero,* R. Spittel, F. Just, J. Kobelke, M. Rothhardt and H. Bartelt Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, 07745, Jena, Germany *[email protected]
Abstract: We report in this paper a high thermal sensitivity (78 pm/°C) modal interferometer using a very short Photonic Crystal Fiber stub with a shaped Germanium doped core. The Photonic Crystal Fiber is spliced between two standard fibers. The splice regions allow the excitation of the core and cladding modes in the PCF and perform an interferometric interaction of such modes. The device is proposed for sensitive temperature measurements in transmission, as well as in reflection operation mode with the same high temperature sensitivity. ©2013 Optical Society of America OCIS codes: (060.4005) Microstructured fibers; (060.5295) Photonic crystal fibers; (060.2370) Fiber optics sensors; (120.3180) Interferometry; (280.4788) Optical sensing and sensors; (280.6780) Temperature; (160.0160) Materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
O. B. Frazao, J. M. Baptista, and J. L. Santos, “Temperature-independent strain sensor based on a Hi-Bi Photonic Crystal Fiber Loop Mirror,” Sens. Journal 7(10), 1453–1455 (2007). B. Larrión, M. Hernánez, F. J. Arregui, J. Goicoechea, J. Bravo, and I. R. Matías, “Photonic crystal fiber temperature sensor based on quantum dot nanocoatings,” J. Sens. 2009, 932471 (2009). D. K. C. Wu, B. T. Kuhlmey, and B. J. Eggleton, “Ultrasensitive photonic crystal fiber refractive index sensor,” Opt. Lett. 34(3), 322–324 (2009). O. B. Frazao, C. Jesus, J. M. Baptista, J. L. Santos, and P. Roy, “Fiber-optic interferometric torsion sensor based on a two-LP-mode operation in birefringent Fiber,” IEEE Photon. Technol. Lett. 21(17), 1277–1279 (2009). Y. Jung, S. Kim, D. Lee, and K. Oh, “Compact three segmented multimode fibre modal interferometer for high sensitivity refractive-index measurement,” Meas. Sci. Technol. 17(5), 1129–1133 (2006). J. Villatoro, V. Finazzi, V. P. Minkovich, V. Pruneri, and G. Badenes, “Temperature-insensitive photonic crystal fiber interferometer for absolute strain sensing,” Appl. Phys. Lett. 91(9), 091109 (2007). H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal Interferometer using a photonic crystal fiber,” J. of Lightw. Tech. 27(17), 3933–3939 (2009). H. Y. Choi, K. S. Park, S. J. Park, U. C. Paek, B. H. Lee, and E. S. Choi, “Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer,” Opt. Lett. 33(21), 2455–2457 (2008). L. V. Nguyen, D. Hwang, S. Moon, D. S. Moon, and Y. Chung, “High temperature fiber sensor with high sensitivity based on core diameter mismatch,” Opt. Express 16(15), 11369–11375 (2008). M. J. Kim, K. S. Park, H. Y. Choi, S.-J. Baik, K. Im, and B. H. Lee, “High temperature sensor based on a photonic crystal fiber interferometer,” Proc. SPIE 7004, 700407 (2008). G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000◦C,” Opt. Express 17(24), 21551–21559 (2009). S. M. Nalawade and H. V. Thakur, “Photonic crystal fiber strain-independent temperature sensing based on modal interferometer,” IEEE Photon. Technol. Lett. 23(21), 1600–1602 (2011). C. L. Zhao, C. C. Chan, L. Hu, T. Li, W. C. Wong, P. Zu, and X. Dong, “Temperature sensing based on ethanol-filled photonic crystal fiber modal interferometer,” Sens. Journal 12(8), 2593–2597 (2012). Y. Cui, P. P. Shum, D. J. J. Hu, G. Wang, G. Humbert, and X.-Q. Dinh, “Temperature sensor by using selectively filled photonic crystal fiber Sagnac interferometer,” IEEE Photonics Journal 4(5), 1801–1808 (2012). A. Bozolan, R. M. Gerosa, C. J. S. de Matos, and M. A. Romero, “Temperature sensing using colloidalcore photonic crystal fiber,” Sens. Journal 12(1), 195–200 (2012). B. Dong, D. P. Zhou, L. Wei, W. K. Liu, and J. W. Lit, “Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer,” Opt. Express 16(23), 19291–19296 (2008). www.comsol.com J. M. Pottage, D. M. Bird, T. D. Hedley, J. Knight, T. Birks, P. St. J. Russell, and P. Roberts, “Robust photonic band gaps for hollow core guidance in PCF made from high index glass,” Opt. Express 11(22), 2854–2861 (2003).
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30266
20. J. C. Flanagan, R. Amezcua, F. Poletti, J. R. Hayes, N. G. R. Broderick, and D. J. Richardson, “The effect of periodicity on the defect modes of large mode area microstructured fibers,” Opt. Express 16(23), 18631– 18645 (2008). 21. B. J. Eggleton, P. S. Westbrook, R. S. Windeler, S. Spälter, and T. A. Strasser, “Grating resonances in airsilica microstructured optical fibers,” Opt. Lett. 24(21), 1460–1462 (1999). 22. B. J. Eggleton, P. S. Westbrook, C. A. White, C. Kerbage, R. S. Windeler, and G. L. Burdge, “Claddingmode-resonances in air-silica microstructure optical fibers,” J. Lightw. Techn. 18(8), 1084–1100 (2000). 23 F. Just, H.-R. Müller, S. Unger, J.Kirchhof, V. Reichel, and H. Bartelt, “Ytterbium-doping related stresses in preforms for high-power fiber lasers,” J. Lightw. Techn. 27(12), 2111–2116 (2009). 24. Y.-J. Kim, U. C. Paek, and B. H. Lee, “Measurement of refractive-index variation with temperature by use of long-period fiber gratings,” Opt. Lett. 27(15), 1297–1299 (2002). 25. Y. Geng, X. Li, X. Tan, Y. Deng, and Y. Yu, “Sensitivity-enhanced high-temperature sensing using allsolid photonic bandgap fiber modal interference,” Appl. Opt. 50(4), 468–472 (2011).
1. Introduction In the past years a wide range of fiber optic sensors using modal interferometry have been developed. Especially, many different applications have been explored using modal interferometry with microstructured fibers or Photonic Crystal Fibers (PCF). The PCF fiber sensors were used to measure different parameters such as strain [1], temperature [2], refractive index [3], pressure and torsion [4]. Fiber optic sensors in general provide considerable advantages in application such as compact size and immunity to electromagnetic fields and to microwave radiation. Y Jung et al demonstrated in 2006 [5] a new interferometer arrangement using a coreless fiber spliced between two multimode fibers to measure refractive index. Recently, a new technique to create a modal interferometer with PCF fiber was demonstrated consisting of splicing a piece of PCF between standard mode fibers (SMF), where in the collapsed region of the PCF the voids are fully collapsed [6]. PCFs were also exploited as strain sensors in contrast to sensors avoiding a temperature cross [7,8]. Such devices were studied as strain sensors with lower temperature dependence. H. Y Choi showed in 2008 a hybrid sensor with a PCF fiber and a Fabry Perot Interferometer to measure temperature [9]. However, the low thermal sensitivity (~1.2 pm/°C) limited the application of such a sensor concept. Another temperature sensor was described using two short stubs of a multimode fiber (MMF) for excitation of the modes in a SMF fiber [10]. The device was used especially to measure in the high temperature range. At the same time, M. J. Kim showed an interferometer using the same principle [11] with a PCF fiber to measure temperature. The low sensitivity and the transmission operation made this sensor unviable for practical applications. Villatoro et al demonstrated in 2009 [12] a sensor using a commercial PCF with two collapsed regions to measure temperature, with a medium thermal sensitivity (~8 pm/°C). The device operated in transmission and required a long thermal treatment. In 2009 B. Larrion et al [2] demonstrated a sensitive temperature sensor using a PCF fiber with a very high sensitivity (~0.14 nm/°C). This device, however, requires a metal deposition in PCF and is limited to applications in low temperature regions (nFSM). Furthermore, a distinct higher-order mode (neff R2
(4)
where R1 and R2 is 0.65 µm and 1.95 µm, respectively. The general interference of two modes in a fiber can be described very easily using the equation I = I1 + I 2 + 2 I1 I 2 cos(θ ),
(5)
where the phase θ is given by
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30271
θ = ( β1 − β 2 ) ⋅ L =
2π
λ
Δn eff (λ , T ) ⋅ L(T ).
(6)
Here β1 and β2 are the propagation constants, Δneff is the refractive index difference between the core and cladding modes, λ is the wavelength and L(T) is the temperature dependent length of the PCF which is given by: L(T ) = L(T0 )(1 + αΔT ),
(7)
where α is the linear expansion coefficient of the silica which is dependent on the Germanium concentration. Since the expansion of the fiber length plays only a minor role in the change of the magnitude of θ, we neglect the effect of the variation of the length expansion. Δneff is the refractive index difference of the interfering modes and depends on the wavelength. But the most interesting feature of this fiber is the temperature dependence of Δneff which can be obtained from Eq. (2) The refractive index difference between the modes can be assumed to be constant over the wavelength range for a constant temperature. It can be calculated using the distance between two consecutives peaks in the transmission spectrum at the wavelengths λ1 and λ1: 1 λ2 ⋅ λ1 (8) . L λ2 − λ1 The wavelength shift of a single transmission dip can be written as a function of the refractive index variation of the temperature: Δneff =
δλ1 = 2 L ⋅
λ2 − λ1 δΔneff ⋅ ΔT . λ2 + λ1 δ T
(9)
ΔT is the temperature variation and ∂Δneff/∂ΔT is the variation of the effective refractive index difference of the guide modes with temperature. The influence of the thermally induced length change is equal for both modes and therefore has no influence on the wavelength shift. From these data a high wavelength sensitivity of 94 pm/°C can be expected for such a fiber sensor. A temperature difference of 500 °C would therefore result in an interference pattern shift of about 46 nm.
5. Experimental results and discussion The experimental setup consists of a broadband source (ASE) and an Optical Spectrum Analyzer (ANDO AQ6317). The total loss measure in the device when operated in transmission was 5.8 dB. The first test involved heating the device up to 500° C and measuring the thermal characteristics in transmission. The sensor head was placed in an oven and heated at a rate of 10°C /min. The temperature was kept constant at 500°C for 10 min, then cooled down to room temperature. A thermocouple PT100 was placed beside the PCF inside the oven to monitor the temperature.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30272
Fig. 6. Transmission spectrum signal. The black line is measured before the thermal cycle. The green line is measured after the thermal cycle.
Figure 6 shows the interference pattern of the transmission spectrum of the device before the thermal cycle (black line) and after the thermal cycle (green line). The wavelength shift came back to the same point after heating and cooling with almost no hysteresis. In the spectrum it is possible to see the high value of fringe contrast in the interference pattern of more than 27 dB. The transmission signal of our device has a free wavelength range of
b)
a)
Fig. 7. The temperature characteristics of the fiber sensor head. Green dots represent the measurements under heating conditions and red triangles represent the measurements under cooling conditions. The left graph shows results from transmission measurements and the right graph shows results from reflection measurements.
~60 nm, which is large enough for a temperature range of about 600°C. The thermal sensitivity was measured in a thermal cycle with temperature steps of 50°C. The result of the temperature cycle is shown in Fig. 7 (left). The equation of fitting for the temperature dependent wavelength shift (in nm) is: −1.29 + 0.0434*T + 6.94x10−5*T2, with a very good correlation factor of R2 = 0.99996. The wavelength shift of interference patterns for a temperature variation of 500°C for the best linear fit is found to be Δλ/ΔΤ = 78 pm/K. This is similar to the result shown in [13], however applicable for much shorter PCF length (~0.5% of the length), and the value found is little less than the estimated result of 94 pm/K. The device has less temperature sensitivity when compare with liquid filled interferometers [14,15], but has the advantage of be easier to manufacture, a temperature range of operation bigger, and probably has more robustness. This sensor device could be also arranged for a reflection measurement in a simple way. To this end a thin film of gold is deposited at the end of standard fiber (SMF28). The film has a thickness of about 100 nm. When compared with the signal of the Fresnel reflection only at the end fiber of about 4% in the silica-air interface, the signal is increased by 13 dB.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30273
Fig. 8. Experimental setup in reflection. ASE is the broadband light source, OSA is Optical Spectrum Analyzer, FOC is Fiber Optic Circulator, a) head sensor, b) microscopic image of the gold film in the end fiber.
The modified experimental setup of the device operating in reflection is shown in Fig. 8, where ASE (HP83438A) is a broadband source light, OSA is the Optical Spectrum Analyzer, and FOC is the Fiber Optical Circulator. The spectrum in reflection maintains the same interference pattern, only the value of fringe contrast increases (>40 dB) when compared with the signal in transmission (Fig. 6). A thermal cycle was repeated for this configuration and the results are shown in Fig. 7 (right). The measured sensitivity curve can be described with the following fit parameters −1.29 + 0.0451*T + 6.39*10−5*T2 and with a correlation factor of R2 = 0.99991. As expected, the wavelength shift of 78 pm/°C of the interference pattern with temperature variation for the best linear fit is equal to the value measured in transmission. We believe that such a sensor would be especially interesting in the reflection arrangement for local high temperature measurements, due to its small length in combination with its high temperature sensitivity.
6. Conclusion We described in this paper the mode interference in a PCF fiber interferometer for a fiber with high Ge-core concentration and a structured cladding. An explanation about the density of states in the cladding of that fiber was explored, and the modes which can propagate and which may interfere with the core mode were calculated. The fabrication and characterization of a very short PCF interferometer temperature sensor based on such a highly doped core fiber has been demonstrated. The fabrication of the device is simple and requires only a few steps. A short PCF stub was spliced between standard mode fibers. It was demonstrated that the device can operate in reflection and transmission without changing the thermal sensitivity. A high thermal sensitivity was measured due to the use of highly Ge doped silica (>78pm/°C). A great free spectral range was achieved even with a relatively short length with a specially shaped core structure. A high fringe contrast was measured of > 27dB in transmission and > 40 dB in reflection. We believe that the device demonstrated could have a great potential for application, since the device is very short and operates in transmission and reflection.
Acknowledgments Funding by the Thuringian Ministry of Education, Science and Culture and the European EFRE program is gratefully acknowledged.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30274
Abstract: We report in this paper a high thermal sensitivity (78 pm/°C) modal interferometer using a very short Photonic Crystal Fiber stub with a shaped Germanium doped core. The Photonic Crystal Fiber is spliced between two standard fibers. The splice regions allow the excitation of the core and cladding modes in the PCF and perform an interferometric interaction of such modes. The device is proposed for sensitive temperature measurements in transmission, as well as in reflection operation mode with the same high temperature sensitivity. ©2013 Optical Society of America OCIS codes: (060.4005) Microstructured fibers; (060.5295) Photonic crystal fibers; (060.2370) Fiber optics sensors; (120.3180) Interferometry; (280.4788) Optical sensing and sensors; (280.6780) Temperature; (160.0160) Materials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
O. B. Frazao, J. M. Baptista, and J. L. Santos, “Temperature-independent strain sensor based on a Hi-Bi Photonic Crystal Fiber Loop Mirror,” Sens. Journal 7(10), 1453–1455 (2007). B. Larrión, M. Hernánez, F. J. Arregui, J. Goicoechea, J. Bravo, and I. R. Matías, “Photonic crystal fiber temperature sensor based on quantum dot nanocoatings,” J. Sens. 2009, 932471 (2009). D. K. C. Wu, B. T. Kuhlmey, and B. J. Eggleton, “Ultrasensitive photonic crystal fiber refractive index sensor,” Opt. Lett. 34(3), 322–324 (2009). O. B. Frazao, C. Jesus, J. M. Baptista, J. L. Santos, and P. Roy, “Fiber-optic interferometric torsion sensor based on a two-LP-mode operation in birefringent Fiber,” IEEE Photon. Technol. Lett. 21(17), 1277–1279 (2009). Y. Jung, S. Kim, D. Lee, and K. Oh, “Compact three segmented multimode fibre modal interferometer for high sensitivity refractive-index measurement,” Meas. Sci. Technol. 17(5), 1129–1133 (2006). J. Villatoro, V. Finazzi, V. P. Minkovich, V. Pruneri, and G. Badenes, “Temperature-insensitive photonic crystal fiber interferometer for absolute strain sensing,” Appl. Phys. Lett. 91(9), 091109 (2007). H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal Interferometer using a photonic crystal fiber,” J. of Lightw. Tech. 27(17), 3933–3939 (2009). H. Y. Choi, K. S. Park, S. J. Park, U. C. Paek, B. H. Lee, and E. S. Choi, “Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer,” Opt. Lett. 33(21), 2455–2457 (2008). L. V. Nguyen, D. Hwang, S. Moon, D. S. Moon, and Y. Chung, “High temperature fiber sensor with high sensitivity based on core diameter mismatch,” Opt. Express 16(15), 11369–11375 (2008). M. J. Kim, K. S. Park, H. Y. Choi, S.-J. Baik, K. Im, and B. H. Lee, “High temperature sensor based on a photonic crystal fiber interferometer,” Proc. SPIE 7004, 700407 (2008). G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000◦C,” Opt. Express 17(24), 21551–21559 (2009). S. M. Nalawade and H. V. Thakur, “Photonic crystal fiber strain-independent temperature sensing based on modal interferometer,” IEEE Photon. Technol. Lett. 23(21), 1600–1602 (2011). C. L. Zhao, C. C. Chan, L. Hu, T. Li, W. C. Wong, P. Zu, and X. Dong, “Temperature sensing based on ethanol-filled photonic crystal fiber modal interferometer,” Sens. Journal 12(8), 2593–2597 (2012). Y. Cui, P. P. Shum, D. J. J. Hu, G. Wang, G. Humbert, and X.-Q. Dinh, “Temperature sensor by using selectively filled photonic crystal fiber Sagnac interferometer,” IEEE Photonics Journal 4(5), 1801–1808 (2012). A. Bozolan, R. M. Gerosa, C. J. S. de Matos, and M. A. Romero, “Temperature sensing using colloidalcore photonic crystal fiber,” Sens. Journal 12(1), 195–200 (2012). B. Dong, D. P. Zhou, L. Wei, W. K. Liu, and J. W. Lit, “Temperature- and phase-independent lateral force sensor based on a core-offset multi-mode fiber interferometer,” Opt. Express 16(23), 19291–19296 (2008). www.comsol.com J. M. Pottage, D. M. Bird, T. D. Hedley, J. Knight, T. Birks, P. St. J. Russell, and P. Roberts, “Robust photonic band gaps for hollow core guidance in PCF made from high index glass,” Opt. Express 11(22), 2854–2861 (2003).
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30266
20. J. C. Flanagan, R. Amezcua, F. Poletti, J. R. Hayes, N. G. R. Broderick, and D. J. Richardson, “The effect of periodicity on the defect modes of large mode area microstructured fibers,” Opt. Express 16(23), 18631– 18645 (2008). 21. B. J. Eggleton, P. S. Westbrook, R. S. Windeler, S. Spälter, and T. A. Strasser, “Grating resonances in airsilica microstructured optical fibers,” Opt. Lett. 24(21), 1460–1462 (1999). 22. B. J. Eggleton, P. S. Westbrook, C. A. White, C. Kerbage, R. S. Windeler, and G. L. Burdge, “Claddingmode-resonances in air-silica microstructure optical fibers,” J. Lightw. Techn. 18(8), 1084–1100 (2000). 23 F. Just, H.-R. Müller, S. Unger, J.Kirchhof, V. Reichel, and H. Bartelt, “Ytterbium-doping related stresses in preforms for high-power fiber lasers,” J. Lightw. Techn. 27(12), 2111–2116 (2009). 24. Y.-J. Kim, U. C. Paek, and B. H. Lee, “Measurement of refractive-index variation with temperature by use of long-period fiber gratings,” Opt. Lett. 27(15), 1297–1299 (2002). 25. Y. Geng, X. Li, X. Tan, Y. Deng, and Y. Yu, “Sensitivity-enhanced high-temperature sensing using allsolid photonic bandgap fiber modal interference,” Appl. Opt. 50(4), 468–472 (2011).
1. Introduction In the past years a wide range of fiber optic sensors using modal interferometry have been developed. Especially, many different applications have been explored using modal interferometry with microstructured fibers or Photonic Crystal Fibers (PCF). The PCF fiber sensors were used to measure different parameters such as strain [1], temperature [2], refractive index [3], pressure and torsion [4]. Fiber optic sensors in general provide considerable advantages in application such as compact size and immunity to electromagnetic fields and to microwave radiation. Y Jung et al demonstrated in 2006 [5] a new interferometer arrangement using a coreless fiber spliced between two multimode fibers to measure refractive index. Recently, a new technique to create a modal interferometer with PCF fiber was demonstrated consisting of splicing a piece of PCF between standard mode fibers (SMF), where in the collapsed region of the PCF the voids are fully collapsed [6]. PCFs were also exploited as strain sensors in contrast to sensors avoiding a temperature cross [7,8]. Such devices were studied as strain sensors with lower temperature dependence. H. Y Choi showed in 2008 a hybrid sensor with a PCF fiber and a Fabry Perot Interferometer to measure temperature [9]. However, the low thermal sensitivity (~1.2 pm/°C) limited the application of such a sensor concept. Another temperature sensor was described using two short stubs of a multimode fiber (MMF) for excitation of the modes in a SMF fiber [10]. The device was used especially to measure in the high temperature range. At the same time, M. J. Kim showed an interferometer using the same principle [11] with a PCF fiber to measure temperature. The low sensitivity and the transmission operation made this sensor unviable for practical applications. Villatoro et al demonstrated in 2009 [12] a sensor using a commercial PCF with two collapsed regions to measure temperature, with a medium thermal sensitivity (~8 pm/°C). The device operated in transmission and required a long thermal treatment. In 2009 B. Larrion et al [2] demonstrated a sensitive temperature sensor using a PCF fiber with a very high sensitivity (~0.14 nm/°C). This device, however, requires a metal deposition in PCF and is limited to applications in low temperature regions (nFSM). Furthermore, a distinct higher-order mode (neff R2
(4)
where R1 and R2 is 0.65 µm and 1.95 µm, respectively. The general interference of two modes in a fiber can be described very easily using the equation I = I1 + I 2 + 2 I1 I 2 cos(θ ),
(5)
where the phase θ is given by
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30271
θ = ( β1 − β 2 ) ⋅ L =
2π
λ
Δn eff (λ , T ) ⋅ L(T ).
(6)
Here β1 and β2 are the propagation constants, Δneff is the refractive index difference between the core and cladding modes, λ is the wavelength and L(T) is the temperature dependent length of the PCF which is given by: L(T ) = L(T0 )(1 + αΔT ),
(7)
where α is the linear expansion coefficient of the silica which is dependent on the Germanium concentration. Since the expansion of the fiber length plays only a minor role in the change of the magnitude of θ, we neglect the effect of the variation of the length expansion. Δneff is the refractive index difference of the interfering modes and depends on the wavelength. But the most interesting feature of this fiber is the temperature dependence of Δneff which can be obtained from Eq. (2) The refractive index difference between the modes can be assumed to be constant over the wavelength range for a constant temperature. It can be calculated using the distance between two consecutives peaks in the transmission spectrum at the wavelengths λ1 and λ1: 1 λ2 ⋅ λ1 (8) . L λ2 − λ1 The wavelength shift of a single transmission dip can be written as a function of the refractive index variation of the temperature: Δneff =
δλ1 = 2 L ⋅
λ2 − λ1 δΔneff ⋅ ΔT . λ2 + λ1 δ T
(9)
ΔT is the temperature variation and ∂Δneff/∂ΔT is the variation of the effective refractive index difference of the guide modes with temperature. The influence of the thermally induced length change is equal for both modes and therefore has no influence on the wavelength shift. From these data a high wavelength sensitivity of 94 pm/°C can be expected for such a fiber sensor. A temperature difference of 500 °C would therefore result in an interference pattern shift of about 46 nm.
5. Experimental results and discussion The experimental setup consists of a broadband source (ASE) and an Optical Spectrum Analyzer (ANDO AQ6317). The total loss measure in the device when operated in transmission was 5.8 dB. The first test involved heating the device up to 500° C and measuring the thermal characteristics in transmission. The sensor head was placed in an oven and heated at a rate of 10°C /min. The temperature was kept constant at 500°C for 10 min, then cooled down to room temperature. A thermocouple PT100 was placed beside the PCF inside the oven to monitor the temperature.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30272
Fig. 6. Transmission spectrum signal. The black line is measured before the thermal cycle. The green line is measured after the thermal cycle.
Figure 6 shows the interference pattern of the transmission spectrum of the device before the thermal cycle (black line) and after the thermal cycle (green line). The wavelength shift came back to the same point after heating and cooling with almost no hysteresis. In the spectrum it is possible to see the high value of fringe contrast in the interference pattern of more than 27 dB. The transmission signal of our device has a free wavelength range of
b)
a)
Fig. 7. The temperature characteristics of the fiber sensor head. Green dots represent the measurements under heating conditions and red triangles represent the measurements under cooling conditions. The left graph shows results from transmission measurements and the right graph shows results from reflection measurements.
~60 nm, which is large enough for a temperature range of about 600°C. The thermal sensitivity was measured in a thermal cycle with temperature steps of 50°C. The result of the temperature cycle is shown in Fig. 7 (left). The equation of fitting for the temperature dependent wavelength shift (in nm) is: −1.29 + 0.0434*T + 6.94x10−5*T2, with a very good correlation factor of R2 = 0.99996. The wavelength shift of interference patterns for a temperature variation of 500°C for the best linear fit is found to be Δλ/ΔΤ = 78 pm/K. This is similar to the result shown in [13], however applicable for much shorter PCF length (~0.5% of the length), and the value found is little less than the estimated result of 94 pm/K. The device has less temperature sensitivity when compare with liquid filled interferometers [14,15], but has the advantage of be easier to manufacture, a temperature range of operation bigger, and probably has more robustness. This sensor device could be also arranged for a reflection measurement in a simple way. To this end a thin film of gold is deposited at the end of standard fiber (SMF28). The film has a thickness of about 100 nm. When compared with the signal of the Fresnel reflection only at the end fiber of about 4% in the silica-air interface, the signal is increased by 13 dB.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30273
Fig. 8. Experimental setup in reflection. ASE is the broadband light source, OSA is Optical Spectrum Analyzer, FOC is Fiber Optic Circulator, a) head sensor, b) microscopic image of the gold film in the end fiber.
The modified experimental setup of the device operating in reflection is shown in Fig. 8, where ASE (HP83438A) is a broadband source light, OSA is the Optical Spectrum Analyzer, and FOC is the Fiber Optical Circulator. The spectrum in reflection maintains the same interference pattern, only the value of fringe contrast increases (>40 dB) when compared with the signal in transmission (Fig. 6). A thermal cycle was repeated for this configuration and the results are shown in Fig. 7 (right). The measured sensitivity curve can be described with the following fit parameters −1.29 + 0.0451*T + 6.39*10−5*T2 and with a correlation factor of R2 = 0.99991. As expected, the wavelength shift of 78 pm/°C of the interference pattern with temperature variation for the best linear fit is equal to the value measured in transmission. We believe that such a sensor would be especially interesting in the reflection arrangement for local high temperature measurements, due to its small length in combination with its high temperature sensitivity.
6. Conclusion We described in this paper the mode interference in a PCF fiber interferometer for a fiber with high Ge-core concentration and a structured cladding. An explanation about the density of states in the cladding of that fiber was explored, and the modes which can propagate and which may interfere with the core mode were calculated. The fabrication and characterization of a very short PCF interferometer temperature sensor based on such a highly doped core fiber has been demonstrated. The fabrication of the device is simple and requires only a few steps. A short PCF stub was spliced between standard mode fibers. It was demonstrated that the device can operate in reflection and transmission without changing the thermal sensitivity. A high thermal sensitivity was measured due to the use of highly Ge doped silica (>78pm/°C). A great free spectral range was achieved even with a relatively short length with a specially shaped core structure. A high fringe contrast was measured of > 27dB in transmission and > 40 dB in reflection. We believe that the device demonstrated could have a great potential for application, since the device is very short and operates in transmission and reflection.
Acknowledgments Funding by the Thuringian Ministry of Education, Science and Culture and the European EFRE program is gratefully acknowledged.
#199365 - $15.00 USD Received 14 Oct 2013; revised 15 Nov 2013; accepted 15 Nov 2013; published 3 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030266 | OPTICS EXPRESS 30274
