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Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Er-doped photosensitive fibers
Optics Communications 237 (2004) 301–308 www.elsevier.com/locate/optcom
Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Er-doped photosensitive fibers Y.H. Shen a,b, S. Pal a, J. Mandal a, T. Sun a, K.T.V. Grattan a,*, S.A. Wade S.F. Collins c, G.W. Baxter c, B. Dussardier e, G. Monnom e
c,d
,
a
c
School of Engineering and Mathematical Sciences, City University, Northampton Square, London EC1V 0HB, UK b Department of Physics, Zhejiang University, Hangzhou 310027, China Optical Technology Research Laboratory, Victoria University, P.O. Box 14428, Melbourne City MC, Victoria 8001, Australia d Department of Mechanical Engineering, Monash University, Victoria 3800, Australia e Laboratoire de Physique de la Matiere Condensee CNRS, Universite de Nice-Sophia Antipolis, 06108 Nice Cedex2, France Received 10 October 2003; accepted 1 April 2004
Abstract Three different types of Er doped photosensitive fibers, germanium/erbium (Ge/Er) fiber, tin/germanium/erbium fiber (Sn/Er) and antimony/germanium/erbium fiber (Sb/Er) have been manufactured and studied for use in optical sensor systems. Their characteristics of photosensitivity, the temperature sustainability of fiber Bragg gratings (FBGs) written into these fibers and the fluorescence emission from the Er dopant were investigated and compared. It has been shown in this work that these fibers all show a satisfactory degree of photosensitivity to enable the fabrication of FBGs and a significant level of fluorescence emission within the 1550 nm band for sensor use. The high temperature sustainability of the FBGs written into these fibers was investigated and seen to be quite significant at temperatures as high as 850 °C, in particular for the Sn/Er and Sb/Er fibers. A fiber laser using the Sb/Er fiber as the gain medium was demonstrated, giving evidence of the strong fluorescence emission from the Er dopant. These fibers are all suitable for use in a variety of sensing applications for the simultaneous measurement of temperature and strain by means of monitoring both the fluorescence characteristics and the peak wavelength shift of the FBGs formed in fiber laser sensor application. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Photosensitive fiber; Fiber Bragg grating; Fluorescence
1. Introduction *
Corresponding author. Tel.: +44-2070408120; fax: +442070408121. E-mail address: [email protected] (K.T.V. Grattan).
UV-induced fiber Bragg gratings (FBGs) have been shown to be very important components for use in both the telecommunications and fiber optic
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.04.004
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sensing fields, especially over the past 10 years. A number of comprehensive reviews of the principles, performance and applications of these devices have been published recently [1,2]. One of the most important applications of FBGs in measurement is in the sensing of temperature or strain by interrogating the peak wavelength change of the reflectance spectrum from these gratings. However, it is impossible to use a single measurement of the Bragg wavelength shift to differentiate the effects of changes in strain and temperature, as they both cause the wavelength of the FBG to change. To solve this problem; several different approaches have been reported, these include the use of two co-located gratings [3], two different sensor wavelengths [4] and a grating written into the splice of two fibers with different diameters [5]. In addition to these, the concepts of using the fluorescence characteristic of a doped fiber, such as the lifetime and the intensity ratio for temperature sensing and using wavelength interrogation for temperature/strain sensing have been shown to be useful approaches [6–8]. This arises due to the fact that the effect of strain on the fluorescence emission is negligibly small and thus the measurement of temperature by using the fluorescence intensity ratio approach is essentially independent of the strain exerted. This greatly reduces the complexity of the data processing scheme involved in the sensing system. To meet the requirement of the sensing scheme discussed above, photosensitive fiber doped with a rare-earth ion is preferred as it enables a compact structure to be used for the sensor probe and thus to increase the spatial resolution and the accuracy of the system thus created. Writing an FBG directly into the fluorescent fiber also enables an intrinsic sensing system to be developed with fewer splicing joints, which is important especially in the distributed situation when several probes and multiple sensors may be multiplexed on a single cable. In this paper, three types of Er ion doped photosensitive fibers were investigated and characterized, these being high germanium/erbium (Ge/Er) fiber, tin/erbium/germanium (Sn/Er) fiber, and antimony/erbium/germanium (Sb/Er) fiber. The issues of photosensitivity, temperature sus-
tainability of the FBGs written into the fiber, and the fluorescence characteristics of these fibers were studied comprehensively and the results of the investigation are discussed here in detail.
2. Photosensitivity of optical fiber Photosensitivity of optical fiber is essential to the fabrication of the FBGs. In addition to achieving photosensitivity in the fiber by means of hydrogen loading [9], many kinds of photosensitive fibers have been developed for FBG fabrication, these mainly including using boron/ germanium (B/Ge) co-doping of the silica fiber [10], also incorporating rare-earth ions (such as Ce3þ and/or Er3þ ) into doped silica fiber [11,12] and using high germanium (Ge) doped silica fiber. The fibers resulting are quite photosensitive in comparison with standard telecommunicationstype fiber and are able to show a large refractive index change after exposure to light at a range of wavelengths in the UV including the commonly used 193, 244 and 248 nm. This work required the design and fabrication of fiber co-doped with Ge, Sn and Sb ions, to increase the photosensitivity of the fiber. As a high temperature sensing application was a major objective of this work, the doping of these ions has mainly been considered in light of the effect on the temperature sustainability of FBGs written into these fibers. Previous work using both tin codoping [13,14] and antimony co-doping in fibers recently reported by the authors [15,16] has shown good temperature sustainability (higher than 800 °C) of the FBGs written into these fibers. The photosensitivity of the fibers was compared by writing FBGs directly into the aforementioned fibers and studying their characteristics. To do so, the FBGs were fabricated by exposing the fibers to UV emission from a KrF excimer laser (Braggstar500 supplied by Tuilaser AG) with a repetition rate of 100 Hz and an energy of 12 mJ per pulse at 248 nm, through a phase-mask (pitch period: 1060 nm, supplied by OE-Land Inc., Canada). A cylindrical plano-convex lens (focal length 20 cm) was used to converge the laser beam onto the photosensitive fiber, where the phase-mask was placed immedi-
Y.H. Shen et al. / Optics Communications 237 (2004) 301–308
ately in front of the fiber to form a light interference pattern between the plus and minus first order of the diffraction pattern. The fibers used were specially produced and included high germanium Ge/Er fiber (type A021, fabricated at the University of Nice, France with a nominal Er concentration of 2700 ppm and a GeO2 concentration of 20 mole%) and Sn/Er high germanium fiber (type A022, made also at the University of Nice, France, with nominal doping concentrations of 1000 ppm Er, 10 mole% GeO2 and 0.2 mole% SnO2 ) and Sb/Er high germanium fiber (made at the China Building Materials Academy, with nominal doping concentrations of 500 ppm Er, 15 wt% GeO2 , and 5000 ppm Sb2 O3 ). All the fibers from the University of Nice contained the same amount of aluminum (about 4000 mol ppm). For the Sb/Er fiber made in China, however, no aluminum was included in the fiber composition. The writing times of the FBGs were controlled for each sample until the FBG achieved a high level of reflectivity (typically >95%) and this was monitored using an optical spectrum analyzer (OSA-Agilent HP86140A). For comparison, some B/Ge fiber (fiber type: PS1250/1500, supplied by Fibercore Ltd., UK) was also used, and about 50 s was required for the FBG to reach its highest reflectivity. For the Ge/Er fiber, the exposure time needed was less than 2 min to reach the highest value of over 99% before it saturated and then decreased in reflectivity (a type IIA grating could be fabricated with this fiber when a longer writing time was used). For the Sn/Er fiber, it took an exposure of about 11 min to achieve the maximum reflectivity of 96%. Finally, for the Sb/Er fiber, around 12 min was required to achieve the highest reflectivity of 99.6%. No decrease in the reflectivity was observed with Sn/Er and Sb/Er fibers when an increased writing time was used after the gratings had saturated. When a grating length of 6.5 mm was used, it could be deduced that the corresponding refractive index modulations were 2.85 104 and 1.82 104 for the Sb/Er fiber and the Sn/Er fiber respectively. In Fig. 1(a), the increases of the reflectivity with time for the FBGs written into these three types of fibers can be clearly seen. The corresponding refractive index modulations were also determined
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Fig. 1. Reflectivity and refractive index modulation increase, when exposed to UV radiation, for FBGs written into three types of Er doped photosensitive fibers under the same experimental conditions. (a) Reflectivity vs. UV exposure time and (b) refractive index modulation vs. UV exposure time.
by considering the known grating length of 6.5 mm and these are shown in Fig. 1(b) as well. It is clear that the fibers behave quite differently in terms of their photosensitivity, this difference being reflected in the value of the highest reflectivity (or the value of refractive index modulation) achieved with the FBGs and the UV exposure time required. It can be concluded from the data in Fig. 1 that of the three types of Er doped photosensitive fibers studied, the Ge/Er fiber has the highest photosensitivity, which seems likely to result from the especially high Ge concentration. Considering the Sb/Er fiber and the Sn/Er fiber, the former has a much higher photosensitivity which may be
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attributed to the different doping ions of Sb and Sn, as similar Ge concentrations were involved in the compositions of these two fibers. This was estimated from the similar numerical aperture values of 0.23 for the Sb/Er fiber and 0.22 for the Sn/Er fiber, and also from the similar peak wavelength of the FBGs written into the Sb/Er and the Sn/Er fibers.
3. Temperature sustainability of the FBGs The temperature sustainability of the FBGs is an important factor for a variety of sensing applications where high temperature measurements are required, e.g. in furnace lining monitoring [17] and in fire alarm systems [18]. It has been made clear from the literature that FBGs made using the B/Ge co-doped fiber will be easily destroyed at temperatures over 350 °C [19] and thus are not suitable for sensing applications at such high temperatures – a maximum of 250 °C would give a reasonable lifetime for gratings written in this fiber. In order to compare their performance, the FBGs fabricated from the three Er doped fibers were placed loosely in a silica tube and put into a Carbolite tube-oven to calibrate their thermal decay characteristics over a period of time, taking results in a series of isothermal steps starting from 100 °C, with increments in temperature of initially 100 °C, and then 50 °C as the temperature became higher. At each temperature, the grating was annealed for 24 h, and a fast decay of the grating reflectivity was seen, followed by what is a comparatively much slower decay that is observed. At temperatures defined as ‘low’ (far from the highest sustainable temperature – normally 200 °C lower than that which the gratings could sustain), this decay was so slow that there was almost no further change observed in the reflectivity, and thus of the modulation of the refractive index of the FBG after annealing had been carried out for 24 h. At higher temperatures, this decay will continue after a period of 24 h for annealing, and thus a fixed annealing interval of 24 h at each temperature was used for comparability of the results. The decays in the reflectivity of the FBGs studied, after annealing, are illustrated in Fig. 2.
Fig. 2. Annealing results for FBGs written into the Sb/Er, Sn/ Er and Ge/Er fibers with temperature, in an oven operating from room temperature to 900 °C. For each dot in the curve, the annealing period is 24 h at a temperature below 850 °C and 4 h at 900 °C. As a comparison, the decay characteristics of an FBG written into the B/Ge co-doped fiber are also presented.
It is obvious from the data produced that the highest sustainable temperatures for the different kinds of FBGs studied are quite different one from another. The grating effectively disappeared on reaching a temperature of 700 °C for the Ge/Er fiber, but the equivalent temperature for the loss of grating reflectivity (to <10%) was over 850 °C for both the Sn/Er and Sb/Er fibers and this is a significant result for a variety of sensor applications. The experimental results for the annealing tests on the FBGs shown above clearly indicated that the gratings fabricated in the Sb/Er and Sn/Er fibers had a similar high temperature sustainability. They could still retain a considerable value of reflectivity after annealing at 850 °C for 24 h and at 900 °C for 4 h (no further annealing tests were performed beyond 900 °C). The reflection spectrum of the FBG written into the Sb/Er fiber after annealing at 900 °C is illustrated in Fig. 3. It is clear from this figure that the peak wavelength of this spectrum can be readily determined and thus be used as a measurand-sensitive feature in sensor applications. The FBG samples, which had been annealed at 900 °C for 4 h, were then cooled to room temperature in the oven and then re-heated step by step, until a temperature of 850 °C was reached. The peak wavelengths of the reflective spectra were
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nm being used). The peak wavelength shifts of the reflection spectra with temperature are presented in Fig. 4 for Sb/Er fiber and Sn/Er fiber respectively after they were annealed for 24 h at 850 °C and 4 h at 900 °C. It can clearly be seen that the graph is not fully linear over the whole temperature range, as is often the situation for other fibers. It may be concluded that, for the FBGs written into the Sb/Er and the Sn/Er fibers, the temperature sensitivity of the peak wavelength shift is almost the same for each fiber, at about 12 pm/°C at room temperature and 18 pm/°C at around 800 °C.
Fig. 3. Reflectance spectrum of an FBG written into the Sb/Er fiber at 900 °C, after long term annealing from room temperature to 850 °C. The time period of the annealing is 24 h at each 100 °C point below 850 °C.
4. Fluorescence characteristics
recorded after the temperature was stabilized at each multiple of 100 °C. This process was repeated several times to test the reproducibility of the peak wavelength determined and thus the fiber performance. The results show an excellent repeatability of the peak wavelength in the reflection spectra with a deviation of less than 0.02 nm, which was equivalent to less than 2 °C of temperature change if this was used as the basis of a single temperature sensor. This deviation is within the normal temperature fluctuation in the oven and the resolution limit of the OSA (with a highest resolution of 0.01
The fluorescence characteristics of the Er doped fibers were investigated to determine their suitability as either high temperature fluorescencebased [20] or laser-based [21] sensors. These include studying the excitation of the fluorescence on the band around 1550 nm, and investigating the fiber laser performance. The familiar fluorescence spectrum on the 1550 nm band was clearly seen for all these Er-doped fibers when excited by light from a fiber pigtailed laser diode (LD) with an output power of 20 mW at 980 nm. A typical fluorescence emission spectrum is presented in Fig. 5 for the Sb/Er fiber. However, the situation is a little different when the fibers were
Fig. 4. Dependence of the peak wavelength with temperature in the reflection spectrum of FBGs written into the Sb/Er and Sn/ Er fibers.
Fig. 5. Fluorescence emission of Sb/Er fiber in the wavelength region of 1500–1580 nm band when excited by a LD operating at 980 nm.
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excited by emission from a fiber pigtailed LD working at 1480 nm. The fluorescence peaks could still be seen for the Sb/Er fiber, as illustrated in Fig. 6, but not in the case of Sn/Er and Ge/Er fibers where the fluorescence was almost wholly hidden by the emission tail of the LD. These experiments showed that the fluorescence conversion efficiency and thus the fluorescence intensity achieved under the same pump conditions are much higher for the Sb/Er fiber sample tested than for the Sn/Er and Ge/ Er fiber samples used. A similar conclusion was also obtained when using the Er doped fibers to demonstrate laser operation. Recent work on FBG-based laser sensors has shown their value for high temperature measurement [21]. In this work, laser operation could be readily realized with the Sb/Er fiber but failed with Sn/Er fiber when a similar configuration was employed (with pumping at 1480 nm) and work is continuing to investigate this further. It is interesting to note that there was no aluminum in the composition of the Sb/Er fiber, while its inclusion is generally regarded as a key element in deterring the clustering of the rare-earth fluorescent ions and thus enhancing the fluorescence efficiency [22,23]. It is possible that the doping of the Sb ion in the fiber may have played a similar role to that of aluminum, as a similar effect was also reported with the co-doping of phosphorus in glass [24].
Fluorescence emission at high temperatures was also investigated. It is clear that, for the Sn/Er and Sb/Er fibers, there was still an observable fluorescence signal even at a temperature as high as 800 °C, when excited by the 980 nm LD, which is illustrated in Fig. 7. When a high resolution measurement is required, however, the working temperature of the fiber should be below 700 °C for a strong signal and a high signal to noise ratio. However, this is an important result for high temperature sensor schemes. For reference, in Fig. 8 the fluorescence spectrum taken at a temperature of 600 °C from a sample of Er-doped Sb/ Er fiber, of length 10 cm, with an FBG written into the fiber, is shown.
Fig. 7. Fluorescence spectrum of Sb/Er fiber at 800 °C when excited by a LD working at 980 nm.
Fig. 6. Fluorescence emission of Sb/Er fiber and Sn/Er fiber observed in transmission mode when excited by a LD operating at 1480 nm. The fluorescence peaks from the Sn/Er fiber could not be seen due to the strong tail of the emission from the LD on the 1530 nm band.
Fig. 8. Fluorescence spectrum of a piece of Sb/Er fiber, with an FBG written into the fiber, at 600 °C.
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5. Discussion In all, three kinds of Er doped photosensitive fibers have been manufactured and investigated for the purpose of high temperature measurement, for example for sensing temperature and strain simultaneously in a configuration similar to that reported by some of the authors previously [7,25]. Their characteristics of photosensitivity, the high temperature sustainability of the FBGs written into these fibers, and their fluorescence characteristics were investigated and cross-compared. The photosensitivity of the Er doped fibers was found to be strongly dependent on both the doping compositions and the doping concentrations used for these fibers. It is clear that with a very high Ge concentration, as in the case of Ge/Er fiber where the Ge concentration is greater than 20 mol%, the photosensitivity is very satisfactory and well suited to FBG fabrication. The FBGs written into this fiber could quickly reach a high value of reflectivity (of greater than 99%) within 2 min of exposure at 248 nm under the conditions specified. For the Sb/Er fiber and Sn/Er fiber, the FBGs achieved their highest reflectivity of 99.6% and 96% over a slightly longer time of 11–12 min, which corresponds to a highest refractive index modulation of 2.85 104 and 1.82 104 respectively. As a similar Ge concentration was involved in the fabrication of the Sb/Er and Sn/Er fibers, the difference in their photosensitivity may be attributed mainly to the different doping of Sb or Sn also included in the fiber. It is clear that the co-doping with Sb made the fiber more photosensitive. The temperature sustainability of the FBGs inscribed in the three Er doped fibers studied is also different in each sample. It has been shown in Fig. 2 that the FBGs written into both the Sn and Sb doped fibers have a very strong temperature sustainability, of around 850 °C, which is of great significance in the application of these FBGs in high temperature measurements. Compared with this, the highest sustainable temperature for the FBG written into the Ge/Er fiber is lower comparatively than that of the Sn/Er and Sb/Er fibers, at 700 °C, but still offering a very useful working range. Such a difference may mainly be attributed
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to the doping of Sb and Sn ions. For Sb and Sn, the ion sizes are 76 and 71 pm in diameter respectively [26] and these large sizes are believed to be an important contributor to the high temperature sustainability of the FBGs considered [16]. The fluorescence emission from the Sn/Er and Ge/Er fibers was not as high as could have been expected. This may be due to the comparatively low Al doping concentration in the fiber and possibly the high Er doping concentration may have caused concentration quenching. However, it is quite interesting to note that the fluorescence in the Sb/Er fiber (without any Al doping) is very strong. Although the effect of Sb doping on the fluorescence of Er in the 1550 nm band cannot readily be determined, it is clear that such doping is beneficial in producing a higher fluorescence efficiency. The results from the fiber laser, and the presence of the fluorescence peaks around 1550 nm all support this viewpoint. In summary, three kinds of Er doped photosensitive fibers have been fabricated for the purpose of FBG writing and fluorescence detection for sensor applications. Their characteristics of photosensitivity, temperature sustainability, and their fluorescence features were investigated and cross-compared. The results discussed above suggest that the Sb/Er fiber is very promising for the creation of a wide range of high temperature sensors using a combination of either fluorescence or laser characteristics, and FBGs. Work is continuing on several sensor schemes of this type.
Acknowledgements This work was supported through several schemes from the Engineering and Physical Sciences Research Council (EPSRC), the Natural Science Foundation of China (NSFC, Project No. 10377016) and the Australian Research Council (ARC). S. Pal is grateful to the Commonwealth Scholarship Commission of the Association of Commonwealth Universities in the UK for providing a Commonwealth Scholarship and the Central Electronics Engineering Research Institute (CEERI), Pilani, India, for the study-leave abroad.
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References [1] A. Othonos, K. Kalli, Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing, Artech House, Boston, 1999. [2] R. Kashyap, Fiber Bragg Gratings, Academic Press, San Diego, 1999. [3] F. Farahi, D.J. Webb, J.D.C. Jones, D.A. Jackson, J. Lightwave Technol. 8 (1990) 138. [4] M.G. Xu, J.L. Archambault, L. Reekie, J.P. Dakin, Electron. Lett. 30 (1994) 1085. [5] S.W. James, M.L. Dockney, R.P. Tatam, Electron. Lett. 32 (1996) 1133. [6] D.I. Forsyth, S.A. Wade, T. Sun, X.M. Chen, K.T.V. Grattan, Appl. Opt. 41 (2002) 6585. [7] S. Pal, T. Sun, K.T.V. Grattan, et al., Rev. Sci. Instrum. 74 (2003) 4858. [8] S. Trpkovski, S.A. Wade, G.W. Baxter, S.F. Collins, Rev. Sci. Instrum. 74 (2003) 2880. [9] P.J. Lemaire, R.M. Atkins, V. Mizrahi, W.A. Reed, Electron. Lett. 29 (1993) 1191. [10] D.L. Williams, B.J. Ainslie, J.R. Armitage, J.R. Kashyap, R. Campbell, Electron. Lett. 29 (1993) 45. [11] M.M. Broer, R.L. Cone, J.R. Simpson, Opt. Lett. 16 (1991) 1391. [12] F. Bilodeau, D.C. Johnson, B. Malo, K.A. Vineberg, K.O. Hill, T.F. Morse, A. Kilian, L. Reinhart, Opt. Lett. 15 (1990) 1138. [13] L. Dong, J.L. Cruz, J.A. Tucknott, L. Reekie, D.N. Payne, Opt. Lett. 20 (1995) 1982.
[14] G. Brambilla, V. Pruneri, L. Reekie, Appl. Phys. Lett. 76 (2000) 807. [15] Y. Shen, T. Sun, K.T.V. Grattan, M. Sun, Opt. Lett. 28 (2003) 2025. [16] Y. Shen, J. He, T. Sun, K.T.V. Grattan, Opt. Lett. 29 (6) (2004) 554. [17] W. Zhao, Y.H. Shen, S. Pal, J. Mandel, T. Sun, K.T.V. Grattan, W.D.N. Pritchard, in: On-line measurements for quality in the metal industry, London, UK, IOM Communication Ltd, London, 2003. [18] T. Sun, K.T.V. Grattan, W.M. Sun, S.A. Wade, B.D. Powell, Rev. Sci. Instrum. 74 (2003) 250. [19] S.R. Baker, H.N. Rourke, V. Baker, D. Goodchild, J. Lightwave Technol. 15 (1997) 1470. [20] Z.Y. Zhang, K.T.V. Grattan, A.W. Palmer, B.T. Meggitt, T. Sun, Rev. Sci. Instrum. 69 (1998) 2924. [21] J. Mandal, S. Pal, T. Sun, et al., IEEE Photon. Technol. Lett. 16 (2004) 218. [22] R.S. Quimby, W.J. Miniscalco, B. Thompson, J. Appl. Phys. 76 (1994) 4472. [23] E. Desurvire, Erbium-doped fiber amplifiers, John Wiley & Sons Inc., New York, 1994. [24] K. Arai, H. Namikawa, K. Kumata, T. Honda, Y. Ishii, T. Handa, J. Appl. Phys. 59 (1986) 3430. [25] S.A. Wade, D.I. Forsyth, K.T.V. Grattan, Q. Guofu, Rev. Sci. Instrum. 72 (2001) 3186. [26] A.M. James, M.P. Lord, Macmillan’s Chemical and Physical Data, The Macmillan Press Ltd., New York, 1992.
Investigation of the photosensitivity, temperature sustainability and fluorescence characteristics of several Er-doped photosensitive fibers Y.H. Shen a,b, S. Pal a, J. Mandal a, T. Sun a, K.T.V. Grattan a,*, S.A. Wade S.F. Collins c, G.W. Baxter c, B. Dussardier e, G. Monnom e
c,d
,
a
c
School of Engineering and Mathematical Sciences, City University, Northampton Square, London EC1V 0HB, UK b Department of Physics, Zhejiang University, Hangzhou 310027, China Optical Technology Research Laboratory, Victoria University, P.O. Box 14428, Melbourne City MC, Victoria 8001, Australia d Department of Mechanical Engineering, Monash University, Victoria 3800, Australia e Laboratoire de Physique de la Matiere Condensee CNRS, Universite de Nice-Sophia Antipolis, 06108 Nice Cedex2, France Received 10 October 2003; accepted 1 April 2004
Abstract Three different types of Er doped photosensitive fibers, germanium/erbium (Ge/Er) fiber, tin/germanium/erbium fiber (Sn/Er) and antimony/germanium/erbium fiber (Sb/Er) have been manufactured and studied for use in optical sensor systems. Their characteristics of photosensitivity, the temperature sustainability of fiber Bragg gratings (FBGs) written into these fibers and the fluorescence emission from the Er dopant were investigated and compared. It has been shown in this work that these fibers all show a satisfactory degree of photosensitivity to enable the fabrication of FBGs and a significant level of fluorescence emission within the 1550 nm band for sensor use. The high temperature sustainability of the FBGs written into these fibers was investigated and seen to be quite significant at temperatures as high as 850 °C, in particular for the Sn/Er and Sb/Er fibers. A fiber laser using the Sb/Er fiber as the gain medium was demonstrated, giving evidence of the strong fluorescence emission from the Er dopant. These fibers are all suitable for use in a variety of sensing applications for the simultaneous measurement of temperature and strain by means of monitoring both the fluorescence characteristics and the peak wavelength shift of the FBGs formed in fiber laser sensor application. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Photosensitive fiber; Fiber Bragg grating; Fluorescence
1. Introduction *
Corresponding author. Tel.: +44-2070408120; fax: +442070408121. E-mail address: [email protected] (K.T.V. Grattan).
UV-induced fiber Bragg gratings (FBGs) have been shown to be very important components for use in both the telecommunications and fiber optic
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.04.004
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sensing fields, especially over the past 10 years. A number of comprehensive reviews of the principles, performance and applications of these devices have been published recently [1,2]. One of the most important applications of FBGs in measurement is in the sensing of temperature or strain by interrogating the peak wavelength change of the reflectance spectrum from these gratings. However, it is impossible to use a single measurement of the Bragg wavelength shift to differentiate the effects of changes in strain and temperature, as they both cause the wavelength of the FBG to change. To solve this problem; several different approaches have been reported, these include the use of two co-located gratings [3], two different sensor wavelengths [4] and a grating written into the splice of two fibers with different diameters [5]. In addition to these, the concepts of using the fluorescence characteristic of a doped fiber, such as the lifetime and the intensity ratio for temperature sensing and using wavelength interrogation for temperature/strain sensing have been shown to be useful approaches [6–8]. This arises due to the fact that the effect of strain on the fluorescence emission is negligibly small and thus the measurement of temperature by using the fluorescence intensity ratio approach is essentially independent of the strain exerted. This greatly reduces the complexity of the data processing scheme involved in the sensing system. To meet the requirement of the sensing scheme discussed above, photosensitive fiber doped with a rare-earth ion is preferred as it enables a compact structure to be used for the sensor probe and thus to increase the spatial resolution and the accuracy of the system thus created. Writing an FBG directly into the fluorescent fiber also enables an intrinsic sensing system to be developed with fewer splicing joints, which is important especially in the distributed situation when several probes and multiple sensors may be multiplexed on a single cable. In this paper, three types of Er ion doped photosensitive fibers were investigated and characterized, these being high germanium/erbium (Ge/Er) fiber, tin/erbium/germanium (Sn/Er) fiber, and antimony/erbium/germanium (Sb/Er) fiber. The issues of photosensitivity, temperature sus-
tainability of the FBGs written into the fiber, and the fluorescence characteristics of these fibers were studied comprehensively and the results of the investigation are discussed here in detail.
2. Photosensitivity of optical fiber Photosensitivity of optical fiber is essential to the fabrication of the FBGs. In addition to achieving photosensitivity in the fiber by means of hydrogen loading [9], many kinds of photosensitive fibers have been developed for FBG fabrication, these mainly including using boron/ germanium (B/Ge) co-doping of the silica fiber [10], also incorporating rare-earth ions (such as Ce3þ and/or Er3þ ) into doped silica fiber [11,12] and using high germanium (Ge) doped silica fiber. The fibers resulting are quite photosensitive in comparison with standard telecommunicationstype fiber and are able to show a large refractive index change after exposure to light at a range of wavelengths in the UV including the commonly used 193, 244 and 248 nm. This work required the design and fabrication of fiber co-doped with Ge, Sn and Sb ions, to increase the photosensitivity of the fiber. As a high temperature sensing application was a major objective of this work, the doping of these ions has mainly been considered in light of the effect on the temperature sustainability of FBGs written into these fibers. Previous work using both tin codoping [13,14] and antimony co-doping in fibers recently reported by the authors [15,16] has shown good temperature sustainability (higher than 800 °C) of the FBGs written into these fibers. The photosensitivity of the fibers was compared by writing FBGs directly into the aforementioned fibers and studying their characteristics. To do so, the FBGs were fabricated by exposing the fibers to UV emission from a KrF excimer laser (Braggstar500 supplied by Tuilaser AG) with a repetition rate of 100 Hz and an energy of 12 mJ per pulse at 248 nm, through a phase-mask (pitch period: 1060 nm, supplied by OE-Land Inc., Canada). A cylindrical plano-convex lens (focal length 20 cm) was used to converge the laser beam onto the photosensitive fiber, where the phase-mask was placed immedi-
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ately in front of the fiber to form a light interference pattern between the plus and minus first order of the diffraction pattern. The fibers used were specially produced and included high germanium Ge/Er fiber (type A021, fabricated at the University of Nice, France with a nominal Er concentration of 2700 ppm and a GeO2 concentration of 20 mole%) and Sn/Er high germanium fiber (type A022, made also at the University of Nice, France, with nominal doping concentrations of 1000 ppm Er, 10 mole% GeO2 and 0.2 mole% SnO2 ) and Sb/Er high germanium fiber (made at the China Building Materials Academy, with nominal doping concentrations of 500 ppm Er, 15 wt% GeO2 , and 5000 ppm Sb2 O3 ). All the fibers from the University of Nice contained the same amount of aluminum (about 4000 mol ppm). For the Sb/Er fiber made in China, however, no aluminum was included in the fiber composition. The writing times of the FBGs were controlled for each sample until the FBG achieved a high level of reflectivity (typically >95%) and this was monitored using an optical spectrum analyzer (OSA-Agilent HP86140A). For comparison, some B/Ge fiber (fiber type: PS1250/1500, supplied by Fibercore Ltd., UK) was also used, and about 50 s was required for the FBG to reach its highest reflectivity. For the Ge/Er fiber, the exposure time needed was less than 2 min to reach the highest value of over 99% before it saturated and then decreased in reflectivity (a type IIA grating could be fabricated with this fiber when a longer writing time was used). For the Sn/Er fiber, it took an exposure of about 11 min to achieve the maximum reflectivity of 96%. Finally, for the Sb/Er fiber, around 12 min was required to achieve the highest reflectivity of 99.6%. No decrease in the reflectivity was observed with Sn/Er and Sb/Er fibers when an increased writing time was used after the gratings had saturated. When a grating length of 6.5 mm was used, it could be deduced that the corresponding refractive index modulations were 2.85 104 and 1.82 104 for the Sb/Er fiber and the Sn/Er fiber respectively. In Fig. 1(a), the increases of the reflectivity with time for the FBGs written into these three types of fibers can be clearly seen. The corresponding refractive index modulations were also determined
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Fig. 1. Reflectivity and refractive index modulation increase, when exposed to UV radiation, for FBGs written into three types of Er doped photosensitive fibers under the same experimental conditions. (a) Reflectivity vs. UV exposure time and (b) refractive index modulation vs. UV exposure time.
by considering the known grating length of 6.5 mm and these are shown in Fig. 1(b) as well. It is clear that the fibers behave quite differently in terms of their photosensitivity, this difference being reflected in the value of the highest reflectivity (or the value of refractive index modulation) achieved with the FBGs and the UV exposure time required. It can be concluded from the data in Fig. 1 that of the three types of Er doped photosensitive fibers studied, the Ge/Er fiber has the highest photosensitivity, which seems likely to result from the especially high Ge concentration. Considering the Sb/Er fiber and the Sn/Er fiber, the former has a much higher photosensitivity which may be
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attributed to the different doping ions of Sb and Sn, as similar Ge concentrations were involved in the compositions of these two fibers. This was estimated from the similar numerical aperture values of 0.23 for the Sb/Er fiber and 0.22 for the Sn/Er fiber, and also from the similar peak wavelength of the FBGs written into the Sb/Er and the Sn/Er fibers.
3. Temperature sustainability of the FBGs The temperature sustainability of the FBGs is an important factor for a variety of sensing applications where high temperature measurements are required, e.g. in furnace lining monitoring [17] and in fire alarm systems [18]. It has been made clear from the literature that FBGs made using the B/Ge co-doped fiber will be easily destroyed at temperatures over 350 °C [19] and thus are not suitable for sensing applications at such high temperatures – a maximum of 250 °C would give a reasonable lifetime for gratings written in this fiber. In order to compare their performance, the FBGs fabricated from the three Er doped fibers were placed loosely in a silica tube and put into a Carbolite tube-oven to calibrate their thermal decay characteristics over a period of time, taking results in a series of isothermal steps starting from 100 °C, with increments in temperature of initially 100 °C, and then 50 °C as the temperature became higher. At each temperature, the grating was annealed for 24 h, and a fast decay of the grating reflectivity was seen, followed by what is a comparatively much slower decay that is observed. At temperatures defined as ‘low’ (far from the highest sustainable temperature – normally 200 °C lower than that which the gratings could sustain), this decay was so slow that there was almost no further change observed in the reflectivity, and thus of the modulation of the refractive index of the FBG after annealing had been carried out for 24 h. At higher temperatures, this decay will continue after a period of 24 h for annealing, and thus a fixed annealing interval of 24 h at each temperature was used for comparability of the results. The decays in the reflectivity of the FBGs studied, after annealing, are illustrated in Fig. 2.
Fig. 2. Annealing results for FBGs written into the Sb/Er, Sn/ Er and Ge/Er fibers with temperature, in an oven operating from room temperature to 900 °C. For each dot in the curve, the annealing period is 24 h at a temperature below 850 °C and 4 h at 900 °C. As a comparison, the decay characteristics of an FBG written into the B/Ge co-doped fiber are also presented.
It is obvious from the data produced that the highest sustainable temperatures for the different kinds of FBGs studied are quite different one from another. The grating effectively disappeared on reaching a temperature of 700 °C for the Ge/Er fiber, but the equivalent temperature for the loss of grating reflectivity (to <10%) was over 850 °C for both the Sn/Er and Sb/Er fibers and this is a significant result for a variety of sensor applications. The experimental results for the annealing tests on the FBGs shown above clearly indicated that the gratings fabricated in the Sb/Er and Sn/Er fibers had a similar high temperature sustainability. They could still retain a considerable value of reflectivity after annealing at 850 °C for 24 h and at 900 °C for 4 h (no further annealing tests were performed beyond 900 °C). The reflection spectrum of the FBG written into the Sb/Er fiber after annealing at 900 °C is illustrated in Fig. 3. It is clear from this figure that the peak wavelength of this spectrum can be readily determined and thus be used as a measurand-sensitive feature in sensor applications. The FBG samples, which had been annealed at 900 °C for 4 h, were then cooled to room temperature in the oven and then re-heated step by step, until a temperature of 850 °C was reached. The peak wavelengths of the reflective spectra were
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nm being used). The peak wavelength shifts of the reflection spectra with temperature are presented in Fig. 4 for Sb/Er fiber and Sn/Er fiber respectively after they were annealed for 24 h at 850 °C and 4 h at 900 °C. It can clearly be seen that the graph is not fully linear over the whole temperature range, as is often the situation for other fibers. It may be concluded that, for the FBGs written into the Sb/Er and the Sn/Er fibers, the temperature sensitivity of the peak wavelength shift is almost the same for each fiber, at about 12 pm/°C at room temperature and 18 pm/°C at around 800 °C.
Fig. 3. Reflectance spectrum of an FBG written into the Sb/Er fiber at 900 °C, after long term annealing from room temperature to 850 °C. The time period of the annealing is 24 h at each 100 °C point below 850 °C.
4. Fluorescence characteristics
recorded after the temperature was stabilized at each multiple of 100 °C. This process was repeated several times to test the reproducibility of the peak wavelength determined and thus the fiber performance. The results show an excellent repeatability of the peak wavelength in the reflection spectra with a deviation of less than 0.02 nm, which was equivalent to less than 2 °C of temperature change if this was used as the basis of a single temperature sensor. This deviation is within the normal temperature fluctuation in the oven and the resolution limit of the OSA (with a highest resolution of 0.01
The fluorescence characteristics of the Er doped fibers were investigated to determine their suitability as either high temperature fluorescencebased [20] or laser-based [21] sensors. These include studying the excitation of the fluorescence on the band around 1550 nm, and investigating the fiber laser performance. The familiar fluorescence spectrum on the 1550 nm band was clearly seen for all these Er-doped fibers when excited by light from a fiber pigtailed laser diode (LD) with an output power of 20 mW at 980 nm. A typical fluorescence emission spectrum is presented in Fig. 5 for the Sb/Er fiber. However, the situation is a little different when the fibers were
Fig. 4. Dependence of the peak wavelength with temperature in the reflection spectrum of FBGs written into the Sb/Er and Sn/ Er fibers.
Fig. 5. Fluorescence emission of Sb/Er fiber in the wavelength region of 1500–1580 nm band when excited by a LD operating at 980 nm.
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excited by emission from a fiber pigtailed LD working at 1480 nm. The fluorescence peaks could still be seen for the Sb/Er fiber, as illustrated in Fig. 6, but not in the case of Sn/Er and Ge/Er fibers where the fluorescence was almost wholly hidden by the emission tail of the LD. These experiments showed that the fluorescence conversion efficiency and thus the fluorescence intensity achieved under the same pump conditions are much higher for the Sb/Er fiber sample tested than for the Sn/Er and Ge/ Er fiber samples used. A similar conclusion was also obtained when using the Er doped fibers to demonstrate laser operation. Recent work on FBG-based laser sensors has shown their value for high temperature measurement [21]. In this work, laser operation could be readily realized with the Sb/Er fiber but failed with Sn/Er fiber when a similar configuration was employed (with pumping at 1480 nm) and work is continuing to investigate this further. It is interesting to note that there was no aluminum in the composition of the Sb/Er fiber, while its inclusion is generally regarded as a key element in deterring the clustering of the rare-earth fluorescent ions and thus enhancing the fluorescence efficiency [22,23]. It is possible that the doping of the Sb ion in the fiber may have played a similar role to that of aluminum, as a similar effect was also reported with the co-doping of phosphorus in glass [24].
Fluorescence emission at high temperatures was also investigated. It is clear that, for the Sn/Er and Sb/Er fibers, there was still an observable fluorescence signal even at a temperature as high as 800 °C, when excited by the 980 nm LD, which is illustrated in Fig. 7. When a high resolution measurement is required, however, the working temperature of the fiber should be below 700 °C for a strong signal and a high signal to noise ratio. However, this is an important result for high temperature sensor schemes. For reference, in Fig. 8 the fluorescence spectrum taken at a temperature of 600 °C from a sample of Er-doped Sb/ Er fiber, of length 10 cm, with an FBG written into the fiber, is shown.
Fig. 7. Fluorescence spectrum of Sb/Er fiber at 800 °C when excited by a LD working at 980 nm.
Fig. 6. Fluorescence emission of Sb/Er fiber and Sn/Er fiber observed in transmission mode when excited by a LD operating at 1480 nm. The fluorescence peaks from the Sn/Er fiber could not be seen due to the strong tail of the emission from the LD on the 1530 nm band.
Fig. 8. Fluorescence spectrum of a piece of Sb/Er fiber, with an FBG written into the fiber, at 600 °C.
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5. Discussion In all, three kinds of Er doped photosensitive fibers have been manufactured and investigated for the purpose of high temperature measurement, for example for sensing temperature and strain simultaneously in a configuration similar to that reported by some of the authors previously [7,25]. Their characteristics of photosensitivity, the high temperature sustainability of the FBGs written into these fibers, and their fluorescence characteristics were investigated and cross-compared. The photosensitivity of the Er doped fibers was found to be strongly dependent on both the doping compositions and the doping concentrations used for these fibers. It is clear that with a very high Ge concentration, as in the case of Ge/Er fiber where the Ge concentration is greater than 20 mol%, the photosensitivity is very satisfactory and well suited to FBG fabrication. The FBGs written into this fiber could quickly reach a high value of reflectivity (of greater than 99%) within 2 min of exposure at 248 nm under the conditions specified. For the Sb/Er fiber and Sn/Er fiber, the FBGs achieved their highest reflectivity of 99.6% and 96% over a slightly longer time of 11–12 min, which corresponds to a highest refractive index modulation of 2.85 104 and 1.82 104 respectively. As a similar Ge concentration was involved in the fabrication of the Sb/Er and Sn/Er fibers, the difference in their photosensitivity may be attributed mainly to the different doping of Sb or Sn also included in the fiber. It is clear that the co-doping with Sb made the fiber more photosensitive. The temperature sustainability of the FBGs inscribed in the three Er doped fibers studied is also different in each sample. It has been shown in Fig. 2 that the FBGs written into both the Sn and Sb doped fibers have a very strong temperature sustainability, of around 850 °C, which is of great significance in the application of these FBGs in high temperature measurements. Compared with this, the highest sustainable temperature for the FBG written into the Ge/Er fiber is lower comparatively than that of the Sn/Er and Sb/Er fibers, at 700 °C, but still offering a very useful working range. Such a difference may mainly be attributed
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to the doping of Sb and Sn ions. For Sb and Sn, the ion sizes are 76 and 71 pm in diameter respectively [26] and these large sizes are believed to be an important contributor to the high temperature sustainability of the FBGs considered [16]. The fluorescence emission from the Sn/Er and Ge/Er fibers was not as high as could have been expected. This may be due to the comparatively low Al doping concentration in the fiber and possibly the high Er doping concentration may have caused concentration quenching. However, it is quite interesting to note that the fluorescence in the Sb/Er fiber (without any Al doping) is very strong. Although the effect of Sb doping on the fluorescence of Er in the 1550 nm band cannot readily be determined, it is clear that such doping is beneficial in producing a higher fluorescence efficiency. The results from the fiber laser, and the presence of the fluorescence peaks around 1550 nm all support this viewpoint. In summary, three kinds of Er doped photosensitive fibers have been fabricated for the purpose of FBG writing and fluorescence detection for sensor applications. Their characteristics of photosensitivity, temperature sustainability, and their fluorescence features were investigated and cross-compared. The results discussed above suggest that the Sb/Er fiber is very promising for the creation of a wide range of high temperature sensors using a combination of either fluorescence or laser characteristics, and FBGs. Work is continuing on several sensor schemes of this type.
Acknowledgements This work was supported through several schemes from the Engineering and Physical Sciences Research Council (EPSRC), the Natural Science Foundation of China (NSFC, Project No. 10377016) and the Australian Research Council (ARC). S. Pal is grateful to the Commonwealth Scholarship Commission of the Association of Commonwealth Universities in the UK for providing a Commonwealth Scholarship and the Central Electronics Engineering Research Institute (CEERI), Pilani, India, for the study-leave abroad.
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