ADVANCES in SENSORS, SIGNALS and MATERIALS
Photonic Crystal Fibres for Sensing Applications ORLANDO FRAZÃO INESC Porto Rua do campo Alegre, 687, 4169-007 Porto PORTUGAL
[email protected] Abstract: - In this paper, it is presented a brief review of the sensing applications using photonic crystal fibres. Since the first publication, several authors have studied their properties and possible applications in engineering. Due to their geometry, the great potential in optical sensing is in bio and gas sensing. Recently, with the possibility of inscribing Bragg grating structures on these fibres, it is possible to use them for other applications. Finally, this paper reviews the recent works with different types of photonic crystal fibres for interferometry. Key-Words: - Optical fibre sensors, photonic Crystal fibres, interferometry. shape, nonlinearity, dispersion and birefringence can be tuned to reach values that are not achievable with conventional fibres. This fibre structure was first demonstrated in 1995 by Birks et al [4], and later works of Knight et al [5] and Cregan et al [6] established the guidance mechanisms and the main properties of these fibres. Due to the presence of a hollow core, the potential of these fibres for liquid and gas sensing, or even for photonic switching if the hollow core is filled with a liquid crystal with transmittance properties dependent of an applied external voltage (Du et al [7]) was clear from the beginning. A review the progress in optical sensing based on PCF is presented
1 Introduction Since the first publication by Knight et al in 1996 on photonic crystal fibres (PCF) [1], the optical fibre community has been continuously engaged on R&D activity around these new fibres. Indeed, the fibre structure with lattice of air holes running along its length shows remarkable properties that support a large variety of novel optical fibre devices that can be used both in communications and sensing systems. A commonly accepted classification of PCF divides them into two main classes: index-guiding PCF and photonic bandgap PCF (Figure 1). The index-guiding PCF basic structure is a solid core surrounded by a microstructured cladding. Due to the presence of air holes, the effective refractive index of the cladding is below that of the core and the light is guided along the core by the principle of total internal reflection. The application of this type of fibres for sensing has been extensively researched, as outlined by Monro et al [2] and Eggleton et al [3]. The second type of PCF has a hollow core and the light guidance mechanism is the result of the presence of a photonic bandgap in the cladding region for a specific range of wavelengths. This can be understood if it is imagine a multi-layer mirror that, for certain angles and optical wavelengths, coherently adds up reflections from each layer, transforming the cladding into an almost perfect 2-D mirror, keeping light confined in the lower index core of the fiber. This virtually lossfree mirror is called a photonic band gap, and it is created by a periodic wavelength-scale lattice of microscopic holes in the cladding glass – a photonic crystal – that inherently has certain angles and wavelengths (stop bands) for which light is strongly reflected. The big attraction is that by varying the size and location of the cladding holes and/or the core diameter, the fibre transmission spectrum, mode
ISSN: 1792-6211 / ISSN: 1792-6238
2 Index guiding PCF In this section, two different type´s of index guiding PCF is presented. In Figure 1 shows the conventional index guiding PCF.
Figure 1. index-guiding PCF. a)
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ADVANCES in SENSORS, SIGNALS and MATERIALS
demonstrate an FLM-based curvature sensor with residual temperature and strain sensitivity (Frazão et al [14]).
2.1 Bragg Grating in PCF Bragg gratings written in Hi-Bi PCF have been proposed as a temperature-insensitive strain sensor [8]. Figure 2b) shows the cross section layout of the PCF used in the experiment. For a range of 2000 , a strain resolution of ± 7 was achieved. In this work, FBGs written in Hi-Bi PCF and in standard single mode fibres were also combined to obtain different sensitivities to those physical parameters. The temperature sensitivity in an FBG written in PCF depends on the core size and presents a lower value when compared with the one obtained with a standard FBG. For simultaneous measurement of strain and temperature, the obtained resolutions were ± 1.5 ºC and ± 10 over a measurement range of 100 ºC and 2000 , respectively. fast slow
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Figure 3. Hi-Bi PCF loop mirror.
Several authors have presented different geometries for the PCF geometry and one of them is the suspended-core structure, where relatively large holes surround the fibre core that looks suspended along the fibre axis by small width silica walls. It was proposed initially by Monro et al [2] and recently Webb et al [15] demonstrated a simple technique for fabrication of these structures. See Figure 4.
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Figure 2. Optical spectra of the FBG written in the PCF.
2.2 High-birefringent PCF One configuration that has been widely explored consists of a structure where a length of Hi-Bi PCF (with a cross section as the one shown in Figure 3) is inserted in a fibre loop mirror (FLM). The works of Zhao et al [9] and Kim et al [10] follow this line and, as expected, a low temperature sensitivity (0.29 c) pm/ºC) was reported, not only because of the mentioned differential operation, but also considering the fact that it is not necessary to dope the core in this type of fibre. Therefore, based on this configuration, the development of a temperature-insensitive strain sensor was natural (Dong et al [11], Frazão et al [12]). However, it is important to mention that the temperature insensitive property can only be observed when the Hi-Bi PCF used in the FLM is uncoated. With the same type of fibre, a pressure sensor was demonstrated (Fu et al [13]), with the measurement of a large wavelength–pressure coefficient of 3,42 nm/MPa using a fibre length of 58 cm. Recently, with the Hi-Bi PCF structure with asymmetric hole region, it was possible to
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d) Figure 4. Suspended core fibre.
3.1 Fabry-Perot cavity In this section, a new geometry of a Fabry-Perot (FP) structure based on a suspended-core fibre with three and four holes is presented. The sensors’ characteristics for strain and temperature measurements are reported. Figure 5 presents the experimental setup for characterizing the sensing head. The temperature characterization of the sensing heads was conducted at a constant strain. The sensors were placed in an oven where the temperature was set from room temperature up to 90 ºC, with an error smaller than 0.1 ºC.
ISBN: 978-960-474-248-6
ADVANCES in SENSORS, SIGNALS and MATERIALS
Fabry-Perot Cavity (Suspended core)
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[4] T. A. Birks, P. J. Roberts, P. St. J. Russell, D. M. Atkin, T. J. Shepherd, “Full 2-D photonic bandgaps in silica/air structures”, Electronics Letters, Vol. 31, 1995, 1941-1942. [5] J. C. Knight, J. Broeng, T. A. Birks, P. St. J. Russell, “Photonic band gap guidance of light in optical fibers,” Science, Vol. 282, 1998,1476-1478. [6] R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, D. C. Allen, “Single-mode photonic bandgap guidance of light in air,” Science, vol. 285, 1999, 1537-1539. [7] F. Du, Y-Q. Lu, S-T. Wu, “Electrically tunable liquid-crystal photonic crystal fiber”, Applied Physics Letters, vol. 85, 2004, 2181-2183. [8] O. Frazão, J. P. Carvalho, L. A. Ferreira, F. M. Araújo, J. L. Santos, “Discrimination of strain and temperature using Bragg gratings in microstructured and standard optical fibres”, Measurement Science and Technology. Vol. 16, 2005, 2109-2113. [9] C-L. Zhao, X. Yang, C Lu, W. Jin, M.S. Demokan, “Temperature insensitive interfero-meter using a highly birefringent photonic crystal fiber loop mirror”, IEEE Photonics Technology Letters, 2004, 2535-2357. [10] D-H. Kim, J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity”, Optics Express, Vol. 12, 2004, 44904495. [11] X. Dong, H.Y. Tam, P. Shum, “Temperatureinsensitive strain sensor with polarizationmaintaining photonics crystal fiber based on Sagnac interferometer”, Applied Physics Letters, Vol. 90, 2007, 151113. [12] O. Frazão, J. M. Baptista, J. L. Santos, “Temperature-independent strain sensor based on a Hi-Bi photonic crystal fibre loop mirror”, IEEE Journal Sensors, Vol. 7, 2007, 1453-1455. [13] H. Y. Fu, H. Y. Tam, Li-Yang Shao, Xinyong Dong, P. K. A. Wai, C. Lu, and Sunil K. Khijwania, “Pressure sensor realized with polarizationmaintaining photonic crystal fiber-based Sagnac interferometer”, Applied Optics, 2008, 2835-2839. [14] O. Frazão, J. M. Baptista, J. L. Santos, P. Roy, "Curvature sensor using a highly birefringent photonic crystal fiber with two asymmetric hole regions in a Sagnac interferometer”, Applied Optics, Vol. 47, 2008, 2520-2523, (). [15] A. S. Webb, F. Poletti, D. J. Richardson, J. K. Sahu, Suspended-core holey fiber for evanescentfield sensing, Optical Engineering, Vol. 46, 2007, 010503.
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OSA Figure 5. Experimental setup with the sensing head (photos of the suspended-core fiber with 3 and 4 holes are shown).
The sensitivity coefficients found were 7.65 pm/ºC and 8.89 pm/ºC for the fibres with three and four holes, respectively. The ratio between these two coefficients is ≈ 1.16, which is approximately the ratio between the two cavity lengths (1.17), as it should be considering that the temperature sensitivity is essentially determined by the thermo-optic coefficient of silica and, therefore, proportional to the cavity length (assuming that the two fibres have approximately the same modal characteristics). The strain responses of the sensors were also studied. As mentioned before, a length HCF was spliced to the end of suspended-core fibres allowing strain measurements. A linear response to strain was observed, with slopes of 1.32 pm/με and 1.16 pm/με for the cases of the suspended-core fibres with three and four holes, respectively.
4 Conclusion The level of the reported work and the corresponding results demonstrate that PCFs are no longer a promise for the fibre optic sensing field, but a technology with a potential that is now fully demonstrated. It will therefore be no surprise if some of the ideas presented in this paper begin to follow the path from the optical tables in laboratories to production lines in start-up companies, and thus become the base of innovative commercial products in a near future. References: [1] J. C. Knight, “Photonic crystal fibres”, Nature, vol. 424, 2003, 847-851. [2] T. M. Monro, W. Belardi, K. Furusawa, J. C. Baggett, N .G. R. Broderick, D. J. Richardson, “Sensing with microstructured optical fibres”, Measurement Science and Technology, vol. 12, 2001, 854-858. [3] B. J. Eggleton, C. Kerbage, P. S. Westbrook, R. S. Windeler, A. Hale, “Microstructured optical fibers devices”, Optics Express, vol. 9, 2001, 698-713.
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