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Detection of external refractive index change with high sensitivity using long-period gratings in photonic crystal fiber
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Sensors and Actuators B 131 (2008) 265–269
Detection of external refractive index change with high sensitivity using long-period gratings in photonic crystal fiber Yinian Zhu ∗ , Zonghu He, Henry Du Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, United States Received 18 May 2007; received in revised form 28 October 2007; accepted 12 November 2007 Available online 28 November 2007
Abstract We report that the proposed long-period gratings (LPGs), which were fabricated in an endlessly single-mode photonic crystal fiber (ESM-PCF) via point-by-point residual stress relaxation utilizing focused CO2 laser irradiation, exhibit a high sensitivity to variations in the external refractive index, with an identified shifting of attenuation band as large as 4.4 pm for 1 × 10−5 change in the surrounding refractive index over the range from 1.42 to 1.45. Contrary to LPGs inscribed in conventional all-solid optical fiber under similar condition, the mode-coupling resonance band of the ESM-PCF-LPG moves towards longer wavelength with increasing refractive index in the surrounding medium of the fiber cladding. The resonance wavelength has a low-temperature coefficient of 2.2 pm/◦ C over 30–200 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Long-period gratings; Optical fiber sensors; Photonic crystal fiber; Refractometry
1. Introduction Photonic crystal fibers (PCFs, also termed microstructured fibers or holey fibers), have attracted great attention in recent years [1]. Index-guiding PCFs have a solid core in the form of a missing air channel in the fiber center while the cladding consists of a microstructured array of air channels running along the fiber axis. The design possibilities of PCFs with variation of air channel geometry and lattice structure offer a large degree of freedom for tailoring the optical properties that can not be realized in conventional optical fibers. The confinement of light to the core by modified total internal refraction in PCFs signifies many novel implementations in the field of fiber-optic sensing. There is a growing interest in exploring PCFs for advanced sensor components and devices [2]. The ability to fabricate fibers with unique dispersion profiles and the strong overlap of the optical field with the open-air channels provides potential opportunities for evanescent field sensing and robust device applications. A PCF-based sensor using evanescent effect for detection of biomolecules in aqueous solutions was reported [3].
∗
Corresponding author. Tel.: +1 201 216 8309; fax: +1 201 216 8306. E-mail address: [email protected] (Y. Zhu).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.11.040
The results show that the enhancement of sensitivity is attributed to the possibility of achieving long-effective interaction length in compact fashion while only submicroliter sample volumes are needed. An evanescent-wave gas sensor was also demonstrated by infiltrating analyte into air channels and interrogating its absorption spectroscopy [4]. Unlike in [3,4], where PCFs were used as a sensing platform for evanescent field maximizing overlap with measurand, a new pressure sensor has been recently developed using the polarization maintaining PCF [5]. The capability of this fiber to maintain the light polarization state unchanged during transmission had deliberately enhanced sensitivity to pressure and significantly diminished sensitivity to temperature, which can be utilized for measurement of physical parameters without bulk temperature compensator. A long-period gratings (LPG) couples the fundamental core mode to co-propagating cladding modes in a single-mode fiber, resulting a sequence of attenuation resonances in the transmission spectrum [6]. The LPG is an intrinsic and passive device that can be induced by a periodic refractive index change in the fiber with a typical period of few hundred micrometers. The phase-matching condition for LPGs is expressed by βcore − βclad(i) = 2π/Λ, where βcore and βclad(i) are the propagation constants of the fundamental core mode and the ith cladding mode, respectively. Λ is the grating periodicity, and i is the
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order of cladding modes. Likewise, the resonance condition eff for mode coupling is given by λi = (neff core − nclad(i) )Λ, where λi is the resonance wavelength of ith cladding mode coupled eff by the fundamental core mode, and neff core and nclad(i) are the effective refractive indices of the fundamental core mode and the ith cladding mode. The LPGs within standard silica-based optical fibers have provided a transducer for environmental sensing such as temperature, strain, and external refractive index [7]. Fiber-optic grating based refractometers are particularly attractive from applications standpoint due to their immunity to electromagnetic interference, fast response, and network compatibility. It is desirable to incorporate grating structures into PCFs to evaluate if PCF designs provide enhanced sensitivity. Indeed, Dobb et al. showed that the sensitivity of PCF-LPG to index variation in the ambient is correlated to dimension and lattice structure of the PCF cladding air channels [8]. By injecting liquid measurand into the air channels of the PCF where a titled fiber Bragg grating is inscribed, Huy et al. found that a highorder coupled cladding mode improved index sensitivity through strong evanescent field/liquid measurand interaction [9]. Theoretical analysis on sensitivity of PCF-LPG formed by an electric arc, being performed by Petrovic et al., revealed that such grating is sensitive to strain and surrounding refractive index by very weak sensitive to temperature [10]. As a refractive index transducer, a PCF-LPG that has an refractive index unit (RIU) sensitivity of approximately 10−4 has been used for measurement of thickness of DNA that is immobilized as a monolayer on surface of air channels [11]. Compared with conventional LPGs, the advantages of PCF-LPG refractive index sensor are high sensitivity, fast response time, and robustness. Previous methods for fabricating PCF-based LPGs include UV imprinting [12,13], acoustic wave propagation [14], mechanical microbending [15], and periodic deformation of a PCF by heat treatment with either a CO2 laser [16] or an arc discharge [17], among others. Several PCF designs are comprised of pure silica and therefore do not possess a photosensitive core required for UV lithography-based gratings. Furthermore, the air channels within the cladding structure limit the flux of UV light into the core and here non UV-lithograph-based methods are generally preferable for generation of LPGs within PCF. The investigation into the effect of external refractive index change on resonance wavelengths of LPGs written in PCFs has been limited. Here, we report a study that entails the point-bypoint fabrication of endlessly single-mode PCF-LPGs (referred to as ESM-PCF-LPGs thereafter) using a focused CO2 laser to induce periodic stress relaxation and the characterization of refractive index as well as temperature responses of the resultant ESM-PCF-LPGs.
excellent thermal stability, compared with LPGs written in SMF28 conventional optical fiber by the same technique that we have employed. The ESM-PCF used in our experiments was fabricated at OFS Laboratories using sol–gel casting method [18]. It consists of an 8.8 m diameter pure silica core surrounded by five rings of hexagonally patterned air channels in a silica matrix. The center-to-center space between adjacent air channels is about 5.3 m and the average air-channel diameter is 2.2 m. The out diameter of the fiber is 125 m. The ratio of air-channel diameter to distance between two air channels is about 0.42, which is consistent with the fiber single-mode propagation at all wavelengths for which fused silica is transparent [19]. The SMF-28 standard fiber was obtained from Corning Inc. This fiber has a numerical aperture (NA) of 0.14, refractive index difference of 0.36%, core and cladding diameters of 8.2 and 125 m, respectively. The steps for grating fabrication were similar to those described in Refs. [20,21]. 3. Results and discussion Shown in Fig. 1 are the transmission spectra of the ESM-PCFLPG for different numbers of period. The total grating length and the periodicity are fixed at 36 periods and 635 m, respectively. Two notches in the transmission spectrum correspond respectively to coupling from the fundamental HE11 mode to the TE02 and HE22 core–cladding modes. The core mode area is more likely penetrated by the field of even cladding mode, which is azimuthally symmetric mode that is of odd one. The mode field profiles of TE02 - and HE22 -like cladding modes in the ESM-PCF overlap strongly with that of the fundamental core mode, giving rise to deep notches in transmission spectrum. The ESM-PCFLPG with 36 periods inscribed exhibited the lowest transmission intensity values at wavelengths of 1476.7 and 1557.26 nm. The insertion loss is about 2 dB. The LPG in SMF-28 fiber was writ-
2. Experimental There is no sign of deformation in the cladding structure after laser irradiation. The insets ‘a’ and ‘b’ of Fig. 1 are the scanning electron microscope (SEM) micrographs of the cleaved end of the ESM-PCF before and after an LPG inscribed by CO2 laser. Our investigation has shown that the ESM-PCF-LPGs have very high sensitivity to changes in external index of refraction and
Fig. 1. Evolution of transmission spectra of ESM-PCF-LPGs for different numbers of period (from 16 to 36 periods). Insets: (a) SEM micrograph of fiber cross-section before the writing of an LPG by CO2 laser exposure and (b) SEM micrograph of fiber cross-section after the writing of an LPG by CO2 laser exposure. Images of inside insets are one air channel before and after CO2 laser irradiation, respectively.
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ten using the same procedure as for the ESM-PCF-LPG with a total number of 17 periods and a grating length of 508 m. The resonance wavelength is located at 1557.74 nm. No visible deformation of the fiber was observed. For an ESM-PCF with relatively small cladding airfilling fraction, the differential effective refractive index eff can be expressed as [neff core (λi , nsur ) − nclad(i) (λi , nsur )], where eff eff [ncore (λi , nsur )] and [nclad(i) (λi , nsur )] are the effective indices of the fundamental core mode and the ith order of cladding mode, respectively, and nsur is the refractive index of the surrounding medium. The symmetrical profiles of higher orders of cladding modes are coupled to the fundamental core mode. The integral overlap of high-order modes coupled to the fundamental core mode is transversely distributed to the surface of the fiber where a significant amount of core–cladding coupling power is located. The influence of variations in the refractive index of medium surrounding the cladding of an LPG can be written by ∂neff ∂λi ∂neff clad(i) (λi , nsur ) core (λi , nsur ) Λ (1) = − ∂nsur ∂nsur ∂nsur The term ∂neff clad(i) (λi , nsur )/∂nsur is distinct for each cladding mode. The spectral response of LPG thus depends strongly on the order of the coupled cladding mode. In order to determine the response to changes in the refractive index of surroundings, we measured the transmittances of both the ESM-PCF-LPG and the SMF-28-LPG which were submersed in glycerine–water mixture of glycerine concentration ranging from 0 to 85 wt.% at room temperature. Pure glycerine has a refractive index of 1.462 at wavelength of 1550 nm and 25 ◦ C [22]. Plotted in Fig. 2a and b are the transmission spectra from both an ESM-PCF-LPG and an SMF-28-LPG at glycerine concentrations of deionized water (DI-water), 40, 50, 60, 70, and 80 wt.%, respectively. Light from a super-luminescent diode (SLD) illuminated the fiber using single-mode fiber patch-cord and bare fiber adaptor. An optical spectrum analyzer (OSA) with a resolution of 0.005 nm was used measure the spectrum of transmitted light. A number of gratings were tested under similar conditions. The relationship between HE11 and HE22 mode-coupling resonance and refractive index change for the ESM-PCF-LPG and the SMF-28-LPG were determined after normalization of transmission spectra. Shown in Figs. 3 and 4 are respective wavelength shift and power intensity in the corresponding systems. The wavelength sensitivity coefficient by wavelength interrogation of the ESM-PCF-LPG is 440 nm/RIU over a range of 1.42–1.45. Assuming ∼1 pm resolution of a typical OSA [23], the PCF-LPG fabricated here can exhibit an index sensitivity of 4.4 pm/10−5 RIU. This is the highest value reported for grating-based refractive index sensors to our best knowledge [24]. The transmission power sensitivity coefficient of the ESM-PCF-LPG is −4.82 dB/RIU at refractive index range from 1.33 to 1.44. In addition, it is also seen from Fig. 3 that, as an opposite phenomena from the SMF-28-LPG, the resonance wavelength moved to longer wavelength for almost 20 nm when the refractive index changes from refractive index of 1.33 to 1.45. The resonance wavelength shift is proportional
Fig. 2. (a) Transmission spectra from ESM-PCF-LPGs at glycerine concentrations of 40, 50, 60, 70, 80 wt.% and DI-water. (b) Transmission spectra from SMF-28-LPGs at glycerine concentrations of 40, 50, 60, 70, 80 wt.% and DI-water.
to the difference of effective refractive indices between the guided mode and the ith cladding mode. The positive wavelength dependence of the resonance wavelength with external refractive index suggests that phase-match condition is dominated by the dispersion of the core and cladding modes with eff ∂neff core (λi , nsur )/∂λi > ∂nclad(i) (λi , nsur )/∂λi . Interestingly, as is seen in Fig. 4, even when the external refractive index exceeds
Fig. 3. Dependence of resonance wavelength on refractive index of the glycerine–water mixture for ESM-PCF-LPGs and SMF-28-LPGs.
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Fig. 4. Dependence of transmission power on refractive index of the glycerine–water mixture for ESM-PCF-LPGs and SMF-28-LPGs.
PCF-LPG fabricated via relaxation of residual stress using a focused beam of CO2 laser. In contrast to LPGs written in conventional optical fibers such as the SMF-28, ESM-PCF-LPGs are less sensitive to temperature variation in the environment. The resonance wavelength for ESM-PCF-LPGs shows a positive wavelength dependence shift as a function of external refractive index, compared to the negative wavelength dependence for SMF-28-LPGs. The positive wavelength dependence for ESM-PCF-LPGs indicates that the core mode dispersion, ∂neff core (λi , nsur )/∂λi , is significantly larger than the cladding mode dispersion, ∂neff clad(i) (λi , nsur )/∂λi . As ESM-PCF-LPGs can be used as gas or liquid transmission cells, their high sensitivity to changes in index of refraction makes them attractive candidates for chemical and biological sensing applications. They may also be suited for in situ process monitoring in microfluidic/microreactor systems. Acknowledgements The authors would like to thank Ryan T. Bise, Denis J. Trevor, and David J. Digiovanni in OFS Laboratories for the fabrication of ESM-PCF. This work was supported by the National Science Foundation under grant number ECS-0404002. References
Fig. 5. Response of resonance wavelength shift and transmission power to temperature change from 30 to 200 ◦ C for ESM-PCF-LPGs and SMF-28-LPGs.
that of silica, the wavelength dependence is still positive and there is no noticeable change in the spectral shape or intensity of the LPG resonance, suggesting that the external refractive index does not cause a large modification of the cladding mode index. In contrast, for the SMF-28-LPG, the resonance wavelength shifts towards shorter wavelength and the peak attenuation is reduced when the external index exceeds the index of silica. The temperature responses of the ESM-PCF-LPG and the SMF-28-LPG were measured using a heating assembly over 30–200 ◦ C with a heating rate of 4 ◦ C/min. The transmission spectra are presented in Fig. 5. A temperature coefficient of dλi /dT as low as 2.2 pm/◦ C was deduced for the ESM-PCFLPG. In contrast, a temperature coefficient of 78.2 pm/◦ C was estimated for the SMF-28-LPG. For the temperature dependence of loss, the SMF-28-LPG exhibits that a temperature coefficient of dIi /dT (Ii is the resonance of the ith cladding mode coupled with the core mode) is −7.1 × 10−3 dB/◦ C, whereas this coefficient of the ESM-PCF-LPG was shown to be only half as much. 4. Conclusion In conclusion, we have demonstrated a high sensitivity to change in the external index of refraction using the ESM-
[1] T. Birks, J.C. Knight, P.St.J. Russell, Endlessly single-mode photonic crystal fiber, Opt. Lett. 22 (1997) 961–963. [2] J.M. Fini, Microstructure fibers for optical sensing in gases and liquids, Meas. Sci. Technol. 15 (2004) 1120–1128. [3] J.B. Jensen, L.H. Pedersen, P.E. Høiby, L.B. Nielsen, T.P. Hansen, J.R. Folkenberg, J. Riishede, D. Noordegraaf, K. Nielsen, A. Carlsen, A. Bjarklev, Opt. Lett. 29 (2004) 1974–1976. [4] G. Pickrell, W. Peng, A. Wang, Random-hole optical fiber evanescent-wave gas sensing, Opt. Lett. 29 (2004) 1476–1478. [5] W.J. Bock, J. Chen, T. Eftimov, W. Urbanczyk, A photonic crystal fiber sensor for pressure measurements, IEEE Trans. Instrum. Meas. 55 (2006) 1119–1123. [6] A.M. Vengsarkar, P. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, Long-period fiber gratings as band-rejection filters, J. Lightw. Technol. 14 (1996) 58–65. [7] V. Bhatia, A.M. Vengsarkar, Optical fiber long-period grating sensors, Opt. Lett. 21 (1996) 692–694. [8] H. Dobb, K. Kalli, D.J. Webb, Measured sensitivity of arc-induced longperiod grating sensors in photonic crystal fiber, Opt. Commun. 260 (2006) 184–191. [9] M.C. Phan Huy, G. Laffont, Y. Frignac, V. Dewynter-Marty, P. Ferdinand, P. Roy, J.-M. Blondy, D. Pagnoux, W. Blanc, B. Dussardier, Fiber Bragg grating photowriting in microstructured optical fibers for refractive index measurement, Meas. Sci. Technol. 17 (2006) 992–997. [10] J.S. Petrovic, H. Dobb, V.K. Mezentsev, K. Kalli, D.J. Webb, I. Bennion, Sensitivity of LPGs in PCFs fabeicated by an electric arc to temperature, strain, and external refractive index, J. Lightw. Technol. 25 (2007) 1306–1312. [11] L. Rindor, J.B. Jensen, M. Dufva, L.H. Pedersen, P.E. Høiby, O. Bang, Photonic crystal fiber long-period gratings for biochemical sensing, Opt. Express 14 (2006) 8224–8231. [12] B.J. Eggleton, P.S. Westbrook, R.S. Windeler, S. Splter, T.A. Strasser, Grating resonances in air silica microstructured optical fiber, Opt. Lett. 24 (1999) 1460–1462. [13] R.P. Espindola, R.S. Windeler, A.A. Abramov, B.J. Eggleton, T.A. Strasser, D.J. DiGiovanni, External refractive index insensitive air-clad long-period fiber grating, Electron. Lett. 35 (1999) 327–328.
Y. Zhu et al. / Sensors and Actuators B 131 (2008) 265–269 [14] A. Diez, T.A. Birks, W.H. Reeves, B.J. Mangan, P.St.J. Russell, Excitation of cladding modes in photonic crystal fibers by flexural acoustic waves, Opt. Lett. 25 (2000) 1499–1501. [15] J.H. Lim, K.S. Lee, J.C. Kim, B.H. Lee, Tunable fiber gratings fabricated in photonic crystal fiber by use of mechanical pressure, Opt. Lett. 29 (2004) 331–333. [16] G. Kakarantzas, A. Ortigosa-Mlanch, T. Birks, P.St.J. Russell, Structured long-period gratings in photonic crystal fiber, Opt. Lett. 27 (2002) 1013–1015. [17] G. Humbert, A. Malki, S. Fevrier, P. Roy, D. Pagnoux, Characterizations at high temperatures of long-period gratings written in germanium-free silica microstructure fiber, Opt. Lett. 29 (2004) 38–40. [18] R.T. Bise, D.J. Trevor, Sol–gel derived microstructured fiber: fabrication and characterization, in: Proceedings of Optical Fiber Communications Conference (OFC2005), invited paper OWL6, OSA, March 6–11, 2005. [19] J.C. Knight, T.A. Birks, P.St.J. Russell, D.M. Atkins, All-silica singlemode optical fiber with a photonic crystal cladding, Opt. Lett. 21 (1996) 1547–1549. [20] Y. Zhu, P. Shum, J.-H. Chong, M.K. Rao, C. Lu, Deep-notch, ultracompact long-period grating in a large-mode-area photonic crystal fiber, Opt. Lett. 28 (2003) 2467–2469. [21] Y. Zhu, P. Shum, H.-W. Bay, M. Yan, X. Yu, J. Hu, J. Hao, C. Lu, Straininsensitive and high-temperature long-period gratings inscribed in photonic crystal fiber, Opt. Lett. 30 (2005) 367–369. [22] J. Hetrick, Cargille, Characteristic Sheet of OHGL, New Jersey, USA, 2006. [23] S.D. Dyer, P.A. Williams, R.J. Espejo, J.D. Kofler, S.M. Etzel, Fundamental limits in fiber Bragg grating peak wavelength measurements, in:
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Biographies Yinian Zhu has been a Senior Research Associate at Stevens Institute of Technology since 2007. He received BS from Zhongshan University in China in 1982, and MPhil and PhD from University of Johannesburg in South Africa in 1999 and 2003, respectively. His research interests includes fiber Bragg gratings, long-period gratings, photonic crystal fibers and their applications in sensors and telecommunications. Zonghu He studied Materials Science and Engineering and received his BS and MS in 2000 and 2003 at Beihang University and Beijing University of Chemical Technology, China, respectively. He is currently pursuing PhD degree at Stevens Institute of Technology. His doctoral dissertation is on design, fabrication, and implementation of photonic crystal fiber based long-period gratings for sensing applications. Henry Du is Professor of Materials Engineering at Stevens Institute of Technology. He received his BS degree from South China University of Technology in 1982 and his PhD degree from The Pennsylvania State University in 1988. His primary research areas include novel sensing platform, molecular and nanoscale surface functionalization, chemical and biological sensing and imaging.
Sensors and Actuators B 131 (2008) 265–269
Detection of external refractive index change with high sensitivity using long-period gratings in photonic crystal fiber Yinian Zhu ∗ , Zonghu He, Henry Du Department of Chemical, Biomedical, and Materials Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, United States Received 18 May 2007; received in revised form 28 October 2007; accepted 12 November 2007 Available online 28 November 2007
Abstract We report that the proposed long-period gratings (LPGs), which were fabricated in an endlessly single-mode photonic crystal fiber (ESM-PCF) via point-by-point residual stress relaxation utilizing focused CO2 laser irradiation, exhibit a high sensitivity to variations in the external refractive index, with an identified shifting of attenuation band as large as 4.4 pm for 1 × 10−5 change in the surrounding refractive index over the range from 1.42 to 1.45. Contrary to LPGs inscribed in conventional all-solid optical fiber under similar condition, the mode-coupling resonance band of the ESM-PCF-LPG moves towards longer wavelength with increasing refractive index in the surrounding medium of the fiber cladding. The resonance wavelength has a low-temperature coefficient of 2.2 pm/◦ C over 30–200 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Long-period gratings; Optical fiber sensors; Photonic crystal fiber; Refractometry
1. Introduction Photonic crystal fibers (PCFs, also termed microstructured fibers or holey fibers), have attracted great attention in recent years [1]. Index-guiding PCFs have a solid core in the form of a missing air channel in the fiber center while the cladding consists of a microstructured array of air channels running along the fiber axis. The design possibilities of PCFs with variation of air channel geometry and lattice structure offer a large degree of freedom for tailoring the optical properties that can not be realized in conventional optical fibers. The confinement of light to the core by modified total internal refraction in PCFs signifies many novel implementations in the field of fiber-optic sensing. There is a growing interest in exploring PCFs for advanced sensor components and devices [2]. The ability to fabricate fibers with unique dispersion profiles and the strong overlap of the optical field with the open-air channels provides potential opportunities for evanescent field sensing and robust device applications. A PCF-based sensor using evanescent effect for detection of biomolecules in aqueous solutions was reported [3].
∗
Corresponding author. Tel.: +1 201 216 8309; fax: +1 201 216 8306. E-mail address: [email protected] (Y. Zhu).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.11.040
The results show that the enhancement of sensitivity is attributed to the possibility of achieving long-effective interaction length in compact fashion while only submicroliter sample volumes are needed. An evanescent-wave gas sensor was also demonstrated by infiltrating analyte into air channels and interrogating its absorption spectroscopy [4]. Unlike in [3,4], where PCFs were used as a sensing platform for evanescent field maximizing overlap with measurand, a new pressure sensor has been recently developed using the polarization maintaining PCF [5]. The capability of this fiber to maintain the light polarization state unchanged during transmission had deliberately enhanced sensitivity to pressure and significantly diminished sensitivity to temperature, which can be utilized for measurement of physical parameters without bulk temperature compensator. A long-period gratings (LPG) couples the fundamental core mode to co-propagating cladding modes in a single-mode fiber, resulting a sequence of attenuation resonances in the transmission spectrum [6]. The LPG is an intrinsic and passive device that can be induced by a periodic refractive index change in the fiber with a typical period of few hundred micrometers. The phase-matching condition for LPGs is expressed by βcore − βclad(i) = 2π/Λ, where βcore and βclad(i) are the propagation constants of the fundamental core mode and the ith cladding mode, respectively. Λ is the grating periodicity, and i is the
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order of cladding modes. Likewise, the resonance condition eff for mode coupling is given by λi = (neff core − nclad(i) )Λ, where λi is the resonance wavelength of ith cladding mode coupled eff by the fundamental core mode, and neff core and nclad(i) are the effective refractive indices of the fundamental core mode and the ith cladding mode. The LPGs within standard silica-based optical fibers have provided a transducer for environmental sensing such as temperature, strain, and external refractive index [7]. Fiber-optic grating based refractometers are particularly attractive from applications standpoint due to their immunity to electromagnetic interference, fast response, and network compatibility. It is desirable to incorporate grating structures into PCFs to evaluate if PCF designs provide enhanced sensitivity. Indeed, Dobb et al. showed that the sensitivity of PCF-LPG to index variation in the ambient is correlated to dimension and lattice structure of the PCF cladding air channels [8]. By injecting liquid measurand into the air channels of the PCF where a titled fiber Bragg grating is inscribed, Huy et al. found that a highorder coupled cladding mode improved index sensitivity through strong evanescent field/liquid measurand interaction [9]. Theoretical analysis on sensitivity of PCF-LPG formed by an electric arc, being performed by Petrovic et al., revealed that such grating is sensitive to strain and surrounding refractive index by very weak sensitive to temperature [10]. As a refractive index transducer, a PCF-LPG that has an refractive index unit (RIU) sensitivity of approximately 10−4 has been used for measurement of thickness of DNA that is immobilized as a monolayer on surface of air channels [11]. Compared with conventional LPGs, the advantages of PCF-LPG refractive index sensor are high sensitivity, fast response time, and robustness. Previous methods for fabricating PCF-based LPGs include UV imprinting [12,13], acoustic wave propagation [14], mechanical microbending [15], and periodic deformation of a PCF by heat treatment with either a CO2 laser [16] or an arc discharge [17], among others. Several PCF designs are comprised of pure silica and therefore do not possess a photosensitive core required for UV lithography-based gratings. Furthermore, the air channels within the cladding structure limit the flux of UV light into the core and here non UV-lithograph-based methods are generally preferable for generation of LPGs within PCF. The investigation into the effect of external refractive index change on resonance wavelengths of LPGs written in PCFs has been limited. Here, we report a study that entails the point-bypoint fabrication of endlessly single-mode PCF-LPGs (referred to as ESM-PCF-LPGs thereafter) using a focused CO2 laser to induce periodic stress relaxation and the characterization of refractive index as well as temperature responses of the resultant ESM-PCF-LPGs.
excellent thermal stability, compared with LPGs written in SMF28 conventional optical fiber by the same technique that we have employed. The ESM-PCF used in our experiments was fabricated at OFS Laboratories using sol–gel casting method [18]. It consists of an 8.8 m diameter pure silica core surrounded by five rings of hexagonally patterned air channels in a silica matrix. The center-to-center space between adjacent air channels is about 5.3 m and the average air-channel diameter is 2.2 m. The out diameter of the fiber is 125 m. The ratio of air-channel diameter to distance between two air channels is about 0.42, which is consistent with the fiber single-mode propagation at all wavelengths for which fused silica is transparent [19]. The SMF-28 standard fiber was obtained from Corning Inc. This fiber has a numerical aperture (NA) of 0.14, refractive index difference of 0.36%, core and cladding diameters of 8.2 and 125 m, respectively. The steps for grating fabrication were similar to those described in Refs. [20,21]. 3. Results and discussion Shown in Fig. 1 are the transmission spectra of the ESM-PCFLPG for different numbers of period. The total grating length and the periodicity are fixed at 36 periods and 635 m, respectively. Two notches in the transmission spectrum correspond respectively to coupling from the fundamental HE11 mode to the TE02 and HE22 core–cladding modes. The core mode area is more likely penetrated by the field of even cladding mode, which is azimuthally symmetric mode that is of odd one. The mode field profiles of TE02 - and HE22 -like cladding modes in the ESM-PCF overlap strongly with that of the fundamental core mode, giving rise to deep notches in transmission spectrum. The ESM-PCFLPG with 36 periods inscribed exhibited the lowest transmission intensity values at wavelengths of 1476.7 and 1557.26 nm. The insertion loss is about 2 dB. The LPG in SMF-28 fiber was writ-
2. Experimental There is no sign of deformation in the cladding structure after laser irradiation. The insets ‘a’ and ‘b’ of Fig. 1 are the scanning electron microscope (SEM) micrographs of the cleaved end of the ESM-PCF before and after an LPG inscribed by CO2 laser. Our investigation has shown that the ESM-PCF-LPGs have very high sensitivity to changes in external index of refraction and
Fig. 1. Evolution of transmission spectra of ESM-PCF-LPGs for different numbers of period (from 16 to 36 periods). Insets: (a) SEM micrograph of fiber cross-section before the writing of an LPG by CO2 laser exposure and (b) SEM micrograph of fiber cross-section after the writing of an LPG by CO2 laser exposure. Images of inside insets are one air channel before and after CO2 laser irradiation, respectively.
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ten using the same procedure as for the ESM-PCF-LPG with a total number of 17 periods and a grating length of 508 m. The resonance wavelength is located at 1557.74 nm. No visible deformation of the fiber was observed. For an ESM-PCF with relatively small cladding airfilling fraction, the differential effective refractive index eff can be expressed as [neff core (λi , nsur ) − nclad(i) (λi , nsur )], where eff eff [ncore (λi , nsur )] and [nclad(i) (λi , nsur )] are the effective indices of the fundamental core mode and the ith order of cladding mode, respectively, and nsur is the refractive index of the surrounding medium. The symmetrical profiles of higher orders of cladding modes are coupled to the fundamental core mode. The integral overlap of high-order modes coupled to the fundamental core mode is transversely distributed to the surface of the fiber where a significant amount of core–cladding coupling power is located. The influence of variations in the refractive index of medium surrounding the cladding of an LPG can be written by ∂neff ∂λi ∂neff clad(i) (λi , nsur ) core (λi , nsur ) Λ (1) = − ∂nsur ∂nsur ∂nsur The term ∂neff clad(i) (λi , nsur )/∂nsur is distinct for each cladding mode. The spectral response of LPG thus depends strongly on the order of the coupled cladding mode. In order to determine the response to changes in the refractive index of surroundings, we measured the transmittances of both the ESM-PCF-LPG and the SMF-28-LPG which were submersed in glycerine–water mixture of glycerine concentration ranging from 0 to 85 wt.% at room temperature. Pure glycerine has a refractive index of 1.462 at wavelength of 1550 nm and 25 ◦ C [22]. Plotted in Fig. 2a and b are the transmission spectra from both an ESM-PCF-LPG and an SMF-28-LPG at glycerine concentrations of deionized water (DI-water), 40, 50, 60, 70, and 80 wt.%, respectively. Light from a super-luminescent diode (SLD) illuminated the fiber using single-mode fiber patch-cord and bare fiber adaptor. An optical spectrum analyzer (OSA) with a resolution of 0.005 nm was used measure the spectrum of transmitted light. A number of gratings were tested under similar conditions. The relationship between HE11 and HE22 mode-coupling resonance and refractive index change for the ESM-PCF-LPG and the SMF-28-LPG were determined after normalization of transmission spectra. Shown in Figs. 3 and 4 are respective wavelength shift and power intensity in the corresponding systems. The wavelength sensitivity coefficient by wavelength interrogation of the ESM-PCF-LPG is 440 nm/RIU over a range of 1.42–1.45. Assuming ∼1 pm resolution of a typical OSA [23], the PCF-LPG fabricated here can exhibit an index sensitivity of 4.4 pm/10−5 RIU. This is the highest value reported for grating-based refractive index sensors to our best knowledge [24]. The transmission power sensitivity coefficient of the ESM-PCF-LPG is −4.82 dB/RIU at refractive index range from 1.33 to 1.44. In addition, it is also seen from Fig. 3 that, as an opposite phenomena from the SMF-28-LPG, the resonance wavelength moved to longer wavelength for almost 20 nm when the refractive index changes from refractive index of 1.33 to 1.45. The resonance wavelength shift is proportional
Fig. 2. (a) Transmission spectra from ESM-PCF-LPGs at glycerine concentrations of 40, 50, 60, 70, 80 wt.% and DI-water. (b) Transmission spectra from SMF-28-LPGs at glycerine concentrations of 40, 50, 60, 70, 80 wt.% and DI-water.
to the difference of effective refractive indices between the guided mode and the ith cladding mode. The positive wavelength dependence of the resonance wavelength with external refractive index suggests that phase-match condition is dominated by the dispersion of the core and cladding modes with eff ∂neff core (λi , nsur )/∂λi > ∂nclad(i) (λi , nsur )/∂λi . Interestingly, as is seen in Fig. 4, even when the external refractive index exceeds
Fig. 3. Dependence of resonance wavelength on refractive index of the glycerine–water mixture for ESM-PCF-LPGs and SMF-28-LPGs.
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Fig. 4. Dependence of transmission power on refractive index of the glycerine–water mixture for ESM-PCF-LPGs and SMF-28-LPGs.
PCF-LPG fabricated via relaxation of residual stress using a focused beam of CO2 laser. In contrast to LPGs written in conventional optical fibers such as the SMF-28, ESM-PCF-LPGs are less sensitive to temperature variation in the environment. The resonance wavelength for ESM-PCF-LPGs shows a positive wavelength dependence shift as a function of external refractive index, compared to the negative wavelength dependence for SMF-28-LPGs. The positive wavelength dependence for ESM-PCF-LPGs indicates that the core mode dispersion, ∂neff core (λi , nsur )/∂λi , is significantly larger than the cladding mode dispersion, ∂neff clad(i) (λi , nsur )/∂λi . As ESM-PCF-LPGs can be used as gas or liquid transmission cells, their high sensitivity to changes in index of refraction makes them attractive candidates for chemical and biological sensing applications. They may also be suited for in situ process monitoring in microfluidic/microreactor systems. Acknowledgements The authors would like to thank Ryan T. Bise, Denis J. Trevor, and David J. Digiovanni in OFS Laboratories for the fabrication of ESM-PCF. This work was supported by the National Science Foundation under grant number ECS-0404002. References
Fig. 5. Response of resonance wavelength shift and transmission power to temperature change from 30 to 200 ◦ C for ESM-PCF-LPGs and SMF-28-LPGs.
that of silica, the wavelength dependence is still positive and there is no noticeable change in the spectral shape or intensity of the LPG resonance, suggesting that the external refractive index does not cause a large modification of the cladding mode index. In contrast, for the SMF-28-LPG, the resonance wavelength shifts towards shorter wavelength and the peak attenuation is reduced when the external index exceeds the index of silica. The temperature responses of the ESM-PCF-LPG and the SMF-28-LPG were measured using a heating assembly over 30–200 ◦ C with a heating rate of 4 ◦ C/min. The transmission spectra are presented in Fig. 5. A temperature coefficient of dλi /dT as low as 2.2 pm/◦ C was deduced for the ESM-PCFLPG. In contrast, a temperature coefficient of 78.2 pm/◦ C was estimated for the SMF-28-LPG. For the temperature dependence of loss, the SMF-28-LPG exhibits that a temperature coefficient of dIi /dT (Ii is the resonance of the ith cladding mode coupled with the core mode) is −7.1 × 10−3 dB/◦ C, whereas this coefficient of the ESM-PCF-LPG was shown to be only half as much. 4. Conclusion In conclusion, we have demonstrated a high sensitivity to change in the external index of refraction using the ESM-
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Biographies Yinian Zhu has been a Senior Research Associate at Stevens Institute of Technology since 2007. He received BS from Zhongshan University in China in 1982, and MPhil and PhD from University of Johannesburg in South Africa in 1999 and 2003, respectively. His research interests includes fiber Bragg gratings, long-period gratings, photonic crystal fibers and their applications in sensors and telecommunications. Zonghu He studied Materials Science and Engineering and received his BS and MS in 2000 and 2003 at Beihang University and Beijing University of Chemical Technology, China, respectively. He is currently pursuing PhD degree at Stevens Institute of Technology. His doctoral dissertation is on design, fabrication, and implementation of photonic crystal fiber based long-period gratings for sensing applications. Henry Du is Professor of Materials Engineering at Stevens Institute of Technology. He received his BS degree from South China University of Technology in 1982 and his PhD degree from The Pennsylvania State University in 1988. His primary research areas include novel sensing platform, molecular and nanoscale surface functionalization, chemical and biological sensing and imaging.