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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 13 — Jun. 23, 2008
  • pp: 9765–9780
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Spectral behavior of thin film coated cascaded tapered long period gratings in multiple configurations

P. Pilla, P. Foglia Manzillo, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano  »View Author Affiliations


Optics Express, Vol. 16, Issue 13, pp. 9765-9780 (2008)
http://dx.doi.org/10.1364/OE.16.009765


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Abstract

In this work the spectral response of cascaded tapered long period gratings coated by nano-sized polymeric films has been investigated as function of the surrounding medium refractive index (SRI). The investigation was aimed to identify the best configuration in terms of coated/not coated areas in order to fully benefit of the SRI sensitivity enhancement due to the modal transition mechanism of nano-coated long period gratings while preserving the fringes visibility.

© 2008 Optical Society of America

1. Introduction

In the last years Long Period Gratings (LPGs) have been widely investigated for sensing applications [1

1. S. W. James and R. P. Tatam, “Optical fibre long-period grating sensors: characteristics and application,” Meas. Sci. Technol. 14, R49–R61 (2003). [CrossRef]

]. More recently modification of LPGs spectral properties through deposition of nano-structured coatings onto the grating is attracting an increasingly high interest [2

2. S. W. James and R. P. Tatam, “Fibre Optic Sensors with Nano-Structured Coatings,” J. Opt. A 8, S430–S444 (2006). [CrossRef]

]. Different techniques such as Langmuir-Blodgett (LB), electrostatic self assembly (ESA) and dip-coating have been exploited for the deposition of nano-sized overlays which have shown to produce significant influence on LPGs transmission spectrum and sensitivity characteristics [3–6

3. S. W. James, N. D. Rees, G. J. Ashwell, and R. P. Tatam, “Optical fibre long period gratings with Langmuir Blodgett thin film overlays,” Opt. Lett. 9, 686–688 (2002).

]. In particular, when azimuthally symmetric nano-scale high refractive index (HRI) coatings are deposited onto LPGs, a significant modification of the cladding modes distribution occurs, depending on the layer features (refractive index and thickness) and on the surrounding medium refractive index (SRI). If layer parameters are properly chosen and the SRI is increased in a certain range, the transition of the lowest order cladding mode into an overlay mode occurs. As a consequence, a cladding modes re-organization takes place leading to relevant improvements in the SRI sensitivity in terms of wavelength shift and amplitude variations of LPGs attenuation bands [5

5. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, “Mode transition in high refractive index coated long period gratings,” Opt. Express 14, 19–34 (2006). [CrossRef] [PubMed]

]. The coated devices have found application as refractive index sensors, chemical sensors and tunable filters [7–9

7. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, “Cladding mode reorganization in high-refractive-index-coated long-period gratings: effects on the refractive-index sensitivity,” Opt. Lett. 30, 2536–2538 (2005). [CrossRef] [PubMed]

]. Unfortunately LB and ESA techniques, while providing high control of the overlay thickness produce high loss overlays as well, which are in turn responsible for the fading of the attenuation bands exactly in the most sensitive region of the device [10

10. I. D. Villar, I. R. Matias, F. J. Arregui, and M. Achaerandio, “Nanodeposition of Materials With Complex Refractive Index in Long-Period Fiber Gratings,” J. Lightwave Technol. 23, 4192- (2005). [CrossRef]

]. On the contrary the dip-coating technique permits to deposit low absorption overlays thus preserving rather well the attenuation bands from the disappearance in the transition region.

Recently, James et al. [21

21. S. W. James, I. Ishaq, G. J. Ashwell, and R. P. Tatam, “Cascaded long-period gratings with nanostructured coatings,” Opt. Lett. 30, 2197–2199 (2005). [CrossRef] [PubMed]

] investigated the effect of LB overlay deposition onto cascaded UV-written LPGs by changing the overlay thickness. They analyzed two configurations: overlay deposited onto the fiber separation length between the two LPGs and overlay deposited along the entire length of the device. In the first case, as the overlay thickness was increased, a blue wavelength shift of the interference fringes minima was observed along with a reduction of the fringes visibility, while the fringe envelope remained unchanged. In the second case, in addition to the change in the phase and visibility of the fringes, a change in the central wavelength and amplitude of the attenuation bands was recorded.

To the purposes of this work a computer-assisted arc-discharge system was used to fabricate cascaded LPGs. Each grating was made of periodic tapers in single-mode Corning SMF-28 optical fibers. Such cascaded tapered LPGs (C-TLPGs) were coated with thin films of Syndiotactic Polystyrene (sPS) by using the dip-coating method. Multiple configurations in terms of coated/not coated areas were realized and a spectral characterization of the devices to the SRI changes was performed. The aim was to extend the experimental observations made in [21

21. S. W. James, I. Ishaq, G. J. Ashwell, and R. P. Tatam, “Cascaded long-period gratings with nanostructured coatings,” Opt. Lett. 30, 2197–2199 (2005). [CrossRef] [PubMed]

] exploiting the low absorption sPS overlay and, at the same time, to show the good spectral features of C-TLPGs.

2. Theoretical background

The spectral characteristics of single LPGs are determined by coupling between the fundamental guided core mode LP01 and the co-propagating cladding modes LP0i when the core propagating light encounters a periodic perturbation of fiber refractive index. From the coupled mode theory the resonant wavelengths at which the transmission spectra present the minima are approximately given by the well known first order Bragg condition [1

1. S. W. James and R. P. Tatam, “Optical fibre long-period grating sensors: characteristics and application,” Meas. Sci. Technol. 14, R49–R61 (2003). [CrossRef]

]:

λres,0i=(neff,coneff,cl0i)·Λ
(1)

Where neff,co and n 0i eff,cl are the core and ith cladding mode effective indices respectively, Λ is the grating period.

In TLPGs the refractive index modulation is achieved by tapering the optical fiber. Although the mechanism of the grating formation is still under dispute [22

22. G. Rego, O. V. Ivanov, P. V.S. Marques, and J. L. Santos “Investigation of Formation Mechanisms of Arc-Induced Long-Period Fiber Gratings,” in Proc. of 18th Int. Conf.on Optical Fiber Sensors, Cancun, Mexico, paper TuE84 (2006).

] some observations enforce the idea that for our gratings the coupling mechanism is prevalently determined by the effective refractive index modulation due to geometric deformation. For example, from SEM analysis it was possible to estimate a bare fiber cross section reduction in the taper waist of about 18% in the case of single gratings. By assuming a constant core/cladding diameter ratio throughout the taper it is possible to infer the core diameter in the waist of the tapered region. The use of the concept of the local normal modes [23

23. D. Marcuse, Theory of Dielectric Optical Waveguides (Academic Press, New York, 1974).

] and the solution of the scalar wave equation (linearly polarized modes approximation) [5

5. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, “Mode transition in high refractive index coated long period gratings,” Opt. Express 14, 19–34 (2006). [CrossRef] [PubMed]

] in the two different sections of the device permits to evaluate the propagation constants of the core mode in the unperturbed fiber (8.2µm diameter) and in the waist of the taper. Finally, considering that the propagation constants are linked to the effective refractive index by the relation β=(2π/λ)neff, it was possible to estimate an effective refractive index modulation amplitude of about of 6.8*10-4 (averaged in the spectral range 1400-1700 nm). This is consistent with typical values of the refractive index modulation amplitude in classic UV-written gratings and hence strong enough to induce the mode coupling. However a co-participation of glass stress relaxation and dopant diffusion can not be excluded.

When two LPGs are connected in series separated by a short distance of few centimeters of unperturbed fiber we obtain a device also known as cascaded LPGs whose behavior can be described in analogy to a Mach-Zender interferometer [18

18. E. M. Dianov, S. A. Vasiliev, A. S. Kurkov, O. J. Medvedkov, and V. N. Protopopov, “In-fiber Mach-Zehnder interferometer based on a pair of long-period gratings,” in Proc. European conf. Optical Communication, 65–68 (1996).

]. The first LPG couples light to cladding modes which experience a different optical path with respect to the core mode. At the second LPG the cladding modes are coupled back into the core interfering with the core mode. The transmission spectrum of cascaded LPGs exhibits an envelope corresponding to the attenuation bands normally present for a single LPG and which is sinusoidally modulated by numerous interference fringes. It can be mathematically described by [24

24. B. H. Lee, Y.-J. Kim, Y. Chung, W.-T. Han, and U.-C. Paek, “Fibre modal index measurements based on fibre gratings,” Fiber Integr. Opt. 20, 443–455 (2001).

]:

I=Icore+α·Iclad,i+2·α·Icore·Iclad,i·cosθ
(2)

where Icore and Iclad,i are the intensities of the core mode and the ith cladding mode respectively, α is the attenuation of the cladding mode, θ is the phase difference between the two modes and include a term due to the phase delay introduced by the grating and a term due to the different propagation constants of core mode and cladding modes propagating toward the second grating through the separation length.

The spectral response of a single LPG to SRI changes has been extensively studied [25

25. H. J. Patrick, A. D. Kersey, and F. Bucholtz, “Analysis of the response of long period fiber gratings to external index of refraction,” J. Lightwave Technol. 16, 1606–1612 (1998). [CrossRef]

]. In particular, the effective refractive index of the cladding modes increases as the SRI increases determining a blue shift of the attenuation bands until the SRI reaches the cladding refractive index. At this point there are no longer guided cladding modes and broadband radiation mode coupling occurs. In the case of C-LPGs, the dependence of the cladding modes effective index from the SRI will affect both the position of the envelope of the transmission spectrum and the position of the interference fringes within the envelope. In fact the change of the cladding modes effective index will change also the phase delay cumulated through the interferometer cavity determining an additional blue shift of the interference fringes [26

26. D. W. Kim, Y. Zhang, K. L. Cooper, and A. Wang, “In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement,” Appl. Opt. 44, 5368–5373 (2005). [CrossRef] [PubMed]

].

When overlays with higher refractive index compared to the cladding one are deposited along the grating region, refraction-reflection regime at the cladding-overlay interface occurs and the new cladding modes become bounded within the structure comprising the core, the cladding and the overlay. In particular, the overlay deposition leads to a lowering of the cladding mode power bounded within the core and cladding layers, while part of the light power carried by the cladding modes moves toward the HRI overlay determining also the enhancing of the evanescent wave interaction with the surrounding medium and a consequent increased sensitivity of the device to the SRI changes [5

5. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, “Mode transition in high refractive index coated long period gratings,” Opt. Express 14, 19–34 (2006). [CrossRef] [PubMed]

]. Moreover the thin HRI overlay itself constitutes a waveguide and allows modes propagation depending on its thickness, refractive index and SRI. In HRI coated LPGs as the SRI increases the lowest order cladding mode is subject to the transition from a cladding mode to an overlay mode with a consequent re-organization of higher order cladding modes. The high sensitivity region of the coated device can be tuned over the desired SRI range by acting on the overlay thickness [6

6. A. Cusano, A. Iadicicco, P. Pilla, A. Cutolo, M. Giordano, and S. Campopiano, “Sensitivity characteristics in nanosized coated long period gratings,” Appl. Phys. Lett. 89,201116- (2006). [CrossRef]

,15

15. P. Pilla, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano, “Sensitivity Characteristics Tuning in Tapered Long-Period Gratings by Nanocoatings,” IEEE Photon. Technol. Lett. 19, 1517–1519 (2007). [CrossRef]

].

The numerical analysis of coated UV-LPGs was already reported [5

5. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, “Mode transition in high refractive index coated long period gratings,” Opt. Express 14, 19–34 (2006). [CrossRef] [PubMed]

, 27

27. I. Del Villar, I. R. Matias, F. J. Arregui, and P. Lalanne, “Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition,” Opt. Exp. 13, 56–69 (2005). [CrossRef]

] and has been shown to qualitatively hold in the case of coated TLPGs [15

15. P. Pilla, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano, “Sensitivity Characteristics Tuning in Tapered Long-Period Gratings by Nanocoatings,” IEEE Photon. Technol. Lett. 19, 1517–1519 (2007). [CrossRef]

]. The aim of this work is to exploit the enhanced sensitivity of coated LPGs combined with the higher resolution offered by the finer details present in the spectrum of C-LPGs fabricated with the flexible and low cost technique of the arc-discharge.

3. Device fabrication and overlay deposition

The C-TLPGs for our experiments were manufactured from Corning SMF-28 optical fibers, using a computer-assisted precision arc-discharge apparatus. The method is based on periodic melting of the fiber, while a pulling weight stretches it, to determine a periodically tapered fiber. A SEM image of a bare TLPG is reported in Fig. 1 (in the inset) along with the arcdischarge apparatus scheme. The grating period Λ was about 750 µm and was mainly determined by the moving step of the translation stage controlled by a computer, and by some other factors such as arc volume, arc intensity, arc duration time, and pulling weight [14

14. W. J. Bock, J. Chen, P. Mikulic, T. Eftimov, and M. Korwin-Pawlowski, “Pressure sensing using periodically tapered long-period gratings written in photonic crystal fibers,” Meas. Sci. Technol. 18, 3098–3102 (2007). [CrossRef]

]. After the realization of the first TLPG, whose length was about 2.5 cm, a separation length of about 4 cm was left before the realization of a second grating. On line monitoring of the writing process allows to stop the writing of the second grating once good spectral properties of the C-TLPG are obtained.

Fig. 1. Computer-assisted arc-discharge apparatus.

The dip-coating technique was used to deposit thin films of sPS (Questra 101 supplied by Dow Chemical Co.), whose refractive index is 1.578 [28

28. M. Giordano, M. Russo, A. Cusano, and G. Mensitieri, “An high sensitivity optical sensor for chloroform vapours detection based on nanometric film of δ-form syndiotactic polystyrene,” Sens. Actuators B 107, 140–147 (2005). [CrossRef]

], onto the device. This deposition technique consists mainly of immersing the fiber-substrate into a chloroform solution of the polymer and then of withdrawing it with a well controlled speed. The film thickness depends upon many parameters such as the withdrawal speed, the solid content and the viscosity of the liquid. If the withdrawal speed is chosen such that the shear rates keep the system in the Newtonian regime, then the coating thickness depends upon the aforementioned parameters by the Landau-Levich equation [29

29. L. E. Scriven, “Physics And Applications of Dip Coating And Spin Coating,” Mater. Res. Soc. Symp. Proc. 121, 717–729 (1988). [CrossRef]

]. The deposition operations were completely automated by a computerized control system which permitted a constant withdrawal speed of 10 cm/min, thus ensuring the formation of a conformal coating of uniform thickness on the device. A 9% solution by weight of sPS in chloroform was used for the deposition of the overlays onto the C-TLPG. In Fig. 2(a) is reported an optical microscope image (objective magnification 10×) of a coated TLPG. From this image is possible to note the presence of the overlay on the device. In Fig. 2(b) a lower objective magnification (5×) was used to give a better overview of the coated device, in this case the focus is on the surface of the fiber in order to highlight the presence of the overlay, instead the image in Fig. 2(c) is focused on the edges of the TLPG to show that no macroscopic amassment of polymeric material is present in the waist of the tapered region. In order to perform a direct measurement of the overlay thickness by Atomic Force Microscopy (AFM), a sharp blade was used to scratch the overlay. In Fig. 2(d) is reported another optical image (objective magnification 10×) of the scratched overlay. Also from this last image there is no evidence of overlay thickness disuniformity.

Fig. 2. (a) Coated TLPG (10× objective magnification);(b) overview of the coated TLPG (5× objective magnification, focus on the surface); (c) overview of the coated TLPG (5× objective magnification, focus on the edges); (d) scratched overlay for following AFM measurements (10× objective magnification).

Fig. 3. AFM topography image (12×12 µm2) of the scratched overlay onto the TLPG. Line and markers are referred to the cross section reported in Fig. 4.
Fig. 4. Cross section of the measured topography. Overlay thickness is measured to be about 320 nm in this case.

Three different configurations in terms of coated/not coated areas of the C-TLPG were studied: 1) all coated device, both gratings and separation length; 2) coated separation length and bare gratings; 3) coated gratings and bare separation length. They are summarized in Fig. 5. In particular the last two configurations were realized by coating separately the different components and then splicing them to obtain the compound structure.

Fig. 5. Summary of the three configurations studied in this work (not to scale): 1) all coated device; 2) coated separation length and bare gratings; 3) coated gratings and bare separation length.

4. Experimental results

The investigation of the thin film coated C-TLPG under test consisted of recording its transmission spectra for different SRI values in each of the aforementioned configurations starting from the bare device. The optoelectronic set-up used for SRI characterization comprises a white light source of 400-1800 nm wavelength range and an optical spectrum analyzer. The analysis was focused on the spectral range 1450–1700 nm. The SRI was changed by using aqueous glycerol solutions whose refractive indices were measured by an Abbe refractometer at 589 nm.

The measurements were performed at room temperature with an air conditioning system as the only mean to keep the temperature as constant as possible. Each experiment lasted less than 2 hours so that a couple of °C of variation in the temperature can not be excluded. Therefore a temperature-induced error in the wavelength shifts reported in this work has to be presumed. However this work was not intended to be an accurate measurement of the SRIs, but instead to suggest a way to improve the performance of cascaded LPGs when used as refractometer or as chemical sensor. When an accurate measurement has to be performed it is necessary to use an accurate control of the temperature or suitable compensation methods.

4.1 Bare C-TLPG

The SRI characterization carried out for the bare C-TLPG is presented in Fig. 6. The spectra present three main attenuation bands due to coupling of the fundamental core mode (LP01) with low order cladding modes (LP0i, i=2-4) performed by the first grating. Those attenuation bands are the envelope of interference fringes due to re-coupling of cladding light into the core, determined by the second grating, and to the consequent interference with the core light. It is observable in Fig. 6(a) that as the SRI value increases, both the envelopes and each interference fringe within them experience a small blue shift with a slight reduction of the fringe visibility.

Fig. 6. Spectral characterization to SRI changes of the bare C-TLPG under test: (a) guided cladding modes; (b) broadband radiation modes; (c) leaky modes; (d) wavelength shift of the interference fringes related to cladding mode LP03.

4.2 All coated C-TLPG

The spectral behaviour of the C-TLPG under test coated with a polymeric overlay deposited on both gratings and on the separation length between them was investigated. As already mentioned, the overlay was obtained from a 9% solution of sPS in chloroform, and the overlay thickness was evaluated to be approximately 315 nm. When an HRI overlay is deposited onto the grating the effective refractive index of the cladding modes is increased, as a consequence both the attenuation bands and interference fringes experience a blue shift. This is clearly observed in Fig. 7. where the attention has been focused on the LP03 interference fringes in the spectral range 1530–1600 nm.

Fig. 7. Shift of the interference fringes related to the cladding mode LP03 caused by the deposition of an sPS overlay of about 315 nm.

Fig. 8. Spectral characterization of the all coated C-TLPGs in different points of the SRI induced modal transition: (a) beginning; (b) half-way; (c) toward completion; (d) comparison of the LP03 interference fringes minima wavelength shift in the bare and all coated C-TLPGs.

4.3 Coated separation length, bare TLPGs

Fig. 9. Spectral characterization of the C-TLPGs with coated separation length and bare gratings in different points of the SRI induced modal transition in the separation length itself: (a) beginning; (b) half-way; (c) toward completion; (d) broadband radiation modes coupling by the bare gratings.

Moreover for SRI=1.4600 (Fig. 9(d)) the attenuation bands are positioned at the same wavelengths as in the corresponding SRI case for the bare device (Fig. 6(b)), but here interference fringes are slightly visible. This happens because the HRI overlay prevent the radiation mode coupling for this SRI in the separation length. In this way the cladding modes power is not totally dissipated and they can arrive on the second grating to interfere with core mode. Also, the coupling efficiency is greater than the bare case as witnessed by the deeper attenuation bands.

4.4 Coated TLPGs, bare separation length

Fig. 10. Spectral characterization of the C-TLPGs with coated gratings and bare separation length in different points of the SRI induced modal transition in the gratings themself: (a) beginning; (b) half-way; (c) toward completion; (d) comparison of the LP03 interference fringes minima wavelength shift in the all coated C-TLPGs and in the device with coated gratings and bare separation length; (e) comparison of the fringes visibility in the same cases as in (d).

4.5 Comments on the temperature sensitivity of the presented results

The temperature sensitivity of a bare LPG is function of the thermo-optic coefficient difference of the core and cladding materials, of the coupled cladding mode and of the wavelength at which the specific mode is fed by the core power, or in other words of the grating period [30

30. X. Shu, L. Zhang, and I. Bennion, “Sensitivity Characteristics of Long-Period Fiber Gratings,” J. Lightwave Technol. 20, 255- (2002). [CrossRef]

]. The phenomenon is even more complex when the LPG is coated by a thin overlay and the surrounding medium in not air. For these reasons a detailed analysis of the temperature sensitivity of the proposed device is complex and is behind the scope of this work. However rough estimations of the temperature sensitivity can still be given. For low order cladding modes of a LPG written in SMF-28 fibers, with air as surrounding medium, temperature sensitivities ranging from 0.05 to 0.1 nm/°C have been reported in literature [31

31. G. Humbert and A. Malki, “Electric-arc-induced gratings in non-hydrogenated fibres: fabrication and high temperature characterizations,” J. Opt. A 4, 194–198 (2002). [CrossRef]

]. When a bare LPG is used as refractometer, the non-linear behaviour of its sensitivity to SRI changes and the thermo-optic coefficient of the surrounding medium should be considered. In fact for low SRIs the intrinsic temperature sensitivity of the bare LPG is prevailing, whilst for SRIs reaching the refractive index of the cladding material the temperature-induced refractive index change of the surrounding medium becomes more important [32

32. R. Falate, G. R. C. Possetti, R. C. Kamikawachi, J. L. Fabris, and H. J. Kalinowski, “Temperature influence of an air conditioner in refractive index measurements using long-period fiber gratings,” Third European Workshop on Optical Fibre Sensors, Proc. of the SPIE 6619, 66193W (2007).

].

A SRI of about 1.46 can be obtained with a water/glycerine mixture in a weight percentage of about 10/90 %. The SRI sensitivity around this index for the LP03 of a bare TLPG is between 500 and 1000 nm/refractive index unit (RIU) [15

15. P. Pilla, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano, “Sensitivity Characteristics Tuning in Tapered Long-Period Gratings by Nanocoatings,” IEEE Photon. Technol. Lett. 19, 1517–1519 (2007). [CrossRef]

]. Assuming a thermo-optic coefficient of 2.2*10-4 for the glycerine and 1*10-4 for the water [33

33. M. J. Weber, Handbook of optical materials (CRC, New York, 2003).

], a thermo-optic coefficient of the surrounding medium of about 2*10-4 is obtained. This means a temperature sensitivity of 0.1-0.2 nm/°C plus the intrinsic grating temperature sensitivity.

In this situation a temperature sensitivity, due to the temperature-induced overlay index change, of about 1.8 nm/°C can be calculated. A SRI of about 1.36 can be obtained with a water/glycerine mixture in a weight percentage of about 80/20 % so that a thermo-optic coefficient of about 1.2*10-4 can be assumed for the surrounding medium. This means that the total temperature sensitivity in the middle of the transition region can be as high as 2 nm/°C. However it is worth to note that a material for chemical sensing applications such as the sPS can undergo an index change of the order of 10-2 in response to the presence of few ppm of analyte in the surrounding medium [8

8. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, and G. Guerra, “Coated Long-Period Fiber Gratings as High-Sensitivity Optochemical Sensors,” J. Lightwave Technol. 24, 1776- (2006). [CrossRef]

,28

28. M. Giordano, M. Russo, A. Cusano, and G. Mensitieri, “An high sensitivity optical sensor for chloroform vapours detection based on nanometric film of δ-form syndiotactic polystyrene,” Sens. Actuators B 107, 140–147 (2005). [CrossRef]

]. In this case the temperature-induced error in the measurement of the wavelength shift would be of about 1%/°C.

Considering cascaded LPGs also the optical delay line plays a role in the temperature sensitivity. However we consider just the third case (coated LPGs and bare separation length) being the most effective for sensing purposes. In this case the temperature sensitivity plus due to the bare interferometer cavity is two orders of magnitude less than that of the coated LPG [34

34. Y. -J. Kim, U. -C. Paek, and B. H. Lee, “Measurement of refractive-index variation with temperature by use of long-period fiber gratings,” Opt. Lett. 27, 1297–1299 (2002). [CrossRef]

], so that the major role is just played by the latter.

5. Conclusions

Acknowledgments

Canadian authors gratefully acknowledge Canada’s NSERC and CFI for financial support. The C-TLPGs were fabricated by Mr. P. Mikulic at the Centre de recherche en photonique, UQO.

References and links

1.

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2.

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3.

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4.

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7.

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8.

A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, and G. Guerra, “Coated Long-Period Fiber Gratings as High-Sensitivity Optochemical Sensors,” J. Lightwave Technol. 24, 1776- (2006). [CrossRef]

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D. D. Davis, T. K. Gaylord, E. N. Glytis, and S. C. Mettler, “CO2 laser-induced long-period fibre gratings: spectral characteristics, cladding modes and polarization independence,” Electron. Lett. 34, 1414–1417 (1998). [CrossRef]

13.

G. Rego, O. Okhotnikov, E. Dianov, and V. Sulimov, “High-temperature stability of long period fiber gratings produced using an electric arc,” J. Lightwave Technol. 19, 1574–1579 (2001). [CrossRef]

14.

W. J. Bock, J. Chen, P. Mikulic, T. Eftimov, and M. Korwin-Pawlowski, “Pressure sensing using periodically tapered long-period gratings written in photonic crystal fibers,” Meas. Sci. Technol. 18, 3098–3102 (2007). [CrossRef]

15.

P. Pilla, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano, “Sensitivity Characteristics Tuning in Tapered Long-Period Gratings by Nanocoatings,” IEEE Photon. Technol. Lett. 19, 1517–1519 (2007). [CrossRef]

16.

Y.-G. Han, B. H. Lee, W.-T. Han, U.-C. Paek, and Y. Chung, “Fibre-optic sensing applications of a pair of long-period fibre gratings,” Meas. Sci. Technol. 12, 778–781 (2001). [CrossRef]

17.

R. P. Murphy, S. W. James, and R. P. Tatam, “Multiplexing of Fiber-Optic Long-Period Grating-Based Interferometric Sensors,” J. Lightwave Technol. 25, 825–829 (2007). [CrossRef]

18.

E. M. Dianov, S. A. Vasiliev, A. S. Kurkov, O. J. Medvedkov, and V. N. Protopopov, “In-fiber Mach-Zehnder interferometer based on a pair of long-period gratings,” in Proc. European conf. Optical Communication, 65–68 (1996).

19.

A. Cusano, D. Paladino, A. Cutolo, I. Del Villar, I. R. Matias, and F. J. Arregui, “Spectral characteristics in long-period fiber gratings with nonuniform symmetrically ring shaped coatings,” appl. Phys. Lett. 90, 141105- (2007). [CrossRef]

20.

I. Del Villar, F. J. Arregui, I. R. Matias, A. Cusano, D. Paladino, and A. Cutolo, “Fringe generation with non-uniformly coated long-period fiber gratings,” Opt. Express 15, 9326–9340 (2007). [CrossRef] [PubMed]

21.

S. W. James, I. Ishaq, G. J. Ashwell, and R. P. Tatam, “Cascaded long-period gratings with nanostructured coatings,” Opt. Lett. 30, 2197–2199 (2005). [CrossRef] [PubMed]

22.

G. Rego, O. V. Ivanov, P. V.S. Marques, and J. L. Santos “Investigation of Formation Mechanisms of Arc-Induced Long-Period Fiber Gratings,” in Proc. of 18th Int. Conf.on Optical Fiber Sensors, Cancun, Mexico, paper TuE84 (2006).

23.

D. Marcuse, Theory of Dielectric Optical Waveguides (Academic Press, New York, 1974).

24.

B. H. Lee, Y.-J. Kim, Y. Chung, W.-T. Han, and U.-C. Paek, “Fibre modal index measurements based on fibre gratings,” Fiber Integr. Opt. 20, 443–455 (2001).

25.

H. J. Patrick, A. D. Kersey, and F. Bucholtz, “Analysis of the response of long period fiber gratings to external index of refraction,” J. Lightwave Technol. 16, 1606–1612 (1998). [CrossRef]

26.

D. W. Kim, Y. Zhang, K. L. Cooper, and A. Wang, “In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement,” Appl. Opt. 44, 5368–5373 (2005). [CrossRef] [PubMed]

27.

I. Del Villar, I. R. Matias, F. J. Arregui, and P. Lalanne, “Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition,” Opt. Exp. 13, 56–69 (2005). [CrossRef]

28.

M. Giordano, M. Russo, A. Cusano, and G. Mensitieri, “An high sensitivity optical sensor for chloroform vapours detection based on nanometric film of δ-form syndiotactic polystyrene,” Sens. Actuators B 107, 140–147 (2005). [CrossRef]

29.

L. E. Scriven, “Physics And Applications of Dip Coating And Spin Coating,” Mater. Res. Soc. Symp. Proc. 121, 717–729 (1988). [CrossRef]

30.

X. Shu, L. Zhang, and I. Bennion, “Sensitivity Characteristics of Long-Period Fiber Gratings,” J. Lightwave Technol. 20, 255- (2002). [CrossRef]

31.

G. Humbert and A. Malki, “Electric-arc-induced gratings in non-hydrogenated fibres: fabrication and high temperature characterizations,” J. Opt. A 4, 194–198 (2002). [CrossRef]

32.

R. Falate, G. R. C. Possetti, R. C. Kamikawachi, J. L. Fabris, and H. J. Kalinowski, “Temperature influence of an air conditioner in refractive index measurements using long-period fiber gratings,” Third European Workshop on Optical Fibre Sensors, Proc. of the SPIE 6619, 66193W (2007).

33.

M. J. Weber, Handbook of optical materials (CRC, New York, 2003).

34.

Y. -J. Kim, U. -C. Paek, and B. H. Lee, “Measurement of refractive-index variation with temperature by use of long-period fiber gratings,” Opt. Lett. 27, 1297–1299 (2002). [CrossRef]

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(120.0280) Instrumentation, measurement, and metrology : Remote sensing and sensors
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(310.1860) Thin films : Deposition and fabrication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 15, 2008
Revised Manuscript: March 13, 2008
Manuscript Accepted: March 15, 2008
Published: June 18, 2008

Virtual Issues
Vol. 3, Iss. 7 Virtual Journal for Biomedical Optics

Citation
P. Pilla, P. Foglia Manzillo, M. Giordano, M. L. Korwin-Pawlowski, W. J. Bock, and A. Cusano, "Spectral behavior of thin film coated cascaded tapered long period gratings in multiple configurations," Opt. Express 16, 9765-9780 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-13-9765


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References

  1. S. W. James and R. P. Tatam, "Optical fibre long-period grating sensors: characteristics and application," Meas. Sci. Technol. 14, R49-R61 (2003). [CrossRef]
  2. S. W. James and R. P. Tatam, "Fibre Optic Sensors with Nano-Structured Coatings," J. Opt. A 8, S430-S444 (2006). [CrossRef]
  3. S. W. James, N. D. Rees, G. J. Ashwell, and R. P. Tatam, "Optical fibre long period gratings with Langmuir Blodgett thin film overlays," Opt. Lett. 9, 686-688 (2002).
  4. I. Del Villar, M. Achaerandio, I. R. Matías, and F. J. Arregui, "Deposition of an Overlay with Electrostactic Self-Assembly Method in Long Period Fiber Gratings," Opt. Lett. 30, 720-722 (2005). [CrossRef] [PubMed]
  5. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, "Mode transition in high refractive index coated long period gratings," Opt. Express 14, 19-34 (2006). [CrossRef] [PubMed]
  6. A. Cusano, A. Iadicicco, P. Pilla, A. Cutolo, M. Giordano, and S. Campopiano, "Sensitivity characteristics in nanosized coated long period gratings," Appl. Phys. Lett. 89, 201116- (2006). [CrossRef]
  7. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, "Cladding mode reorganization in high-refractive-index-coated long-period gratings: effects on the refractive-index sensitivity," Opt. Lett. 30, 2536-2538 (2005). [CrossRef] [PubMed]
  8. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, and G. Guerra, "Coated Long-Period Fiber Gratings as High-Sensitivity Optochemical Sensors," J. Lightwave Technol. 24, 1776-1786 (2006). [CrossRef]
  9. J. Lee, Q. Chen, Q. Zhang, K. Reichard, D. Ditto, J. Mazurowski, M. Hackert, and S. Yin, "Enhancing the tuning range of a single resonant band long period grating while maintaining the resonant peak depth by using an optimized high index indium tin oxide overlay," Appl. Opt. 46, 6984-6989 (2007). [CrossRef] [PubMed]
  10. I. D. Villar, I. R. Matias, F. J. Arregui, and M. Achaerandio, "Nanodeposition of Materials With Complex Refractive Index in Long-Period Fiber Gratings," J. Lightwave Technol. 23, 4192-4199 (2005). [CrossRef]
  11. I. K. Hwang, S. H. Yun, and B. Y. Kim, "Long-period fiber gratings based on periodic microbends," Opt. Lett. 24, 1263-1265 (1999). [CrossRef]
  12. D. D. Davis, T. K. Gaylord, E. N. Glytis, and S. C. Mettler, "CO2 laser-induced long-period fibre gratings: spectral characteristics, cladding modes and polarization independence," Electron. Lett. 34, 1414-1417 (1998). [CrossRef]
  13. G. Rego, O. Okhotnikov, E. Dianov, and V. Sulimov, "High-temperature stability of long period fiber gratings produced using an electric arc," J. Lightwave Technol. 19, 1574-1579 (2001). [CrossRef]
  14. W. J. Bock, J. Chen, P. Mikulic, T. Eftimov, and M. Korwin-Pawlowski, "Pressure sensing using periodically tapered long-period gratings written in photonic crystal fibers," Meas. Sci. Technol. 18, 3098-3102 (2007). [CrossRef]
  15. P. Pilla, M. Giordano, M. L. Korwin-Pawlowski, W. J.  Bock, and A.  Cusano, "Sensitivity Characteristics Tuning in Tapered Long-Period Gratings by Nanocoatings, " IEEE Photon. Technol. Lett. 19, 1517-1519 (2007). [CrossRef]
  16. Y.-G. Han, B. H. Lee, W.-T. Han, U.-C. Paek, and Y. Chung, "Fibre-optic sensing applications of a pair of long-period fibre gratings," Meas. Sci. Technol. 12, 778-781 (2001). [CrossRef]
  17. R. P. Murphy, S. W. James, and R. P. Tatam, "Multiplexing of Fiber-Optic Long-Period Grating-Based Interferometric Sensors," J. Lightwave Technol. 25, 825-829 (2007). [CrossRef]
  18. E. M. Dianov, S. A. Vasiliev, A. S. Kurkov, O. J. Medvedkov, and V. N. Protopopov, "In-fiber Mach-Zehnder interferometer based on a pair of long-period gratings," in Proc. European conf.Optical Communication, 65-68 (1996).
  19. A. Cusano, D. Paladino, A. Cutolo, I. Del Villar, I. R. Matias, and F. J. Arregui, "Spectral characteristics in long-period fiber gratings with nonuniform symmetrically ring shaped coatings," Appl. Phys. Lett. 90, 141105- (2007). [CrossRef]
  20. I. Del Villar, F. J. Arregui, I. R. Matias, A. Cusano, D. Paladino, and A. Cutolo, "Fringe generation with non-uniformly coated long-period fiber gratings," Opt. Express 15, 9326-9340 (2007). [CrossRef] [PubMed]
  21. S. W. James, I. Ishaq, G. J. Ashwell, and R. P. Tatam, "Cascaded long-period gratings with nanostructured coatings," Opt. Lett. 30, 2197-2199 (2005). [CrossRef] [PubMed]
  22. G. Rego, O. V. Ivanov, P. V.S. Marques, and J. L. Santos "Investigation of Formation Mechanisms of Arc-Induced Long-Period Fiber Gratings," in Proc. of 18th Int. Conf. on Optical Fiber Sensors, Cancun, Mexico, paper TuE84 (2006).
  23. D.  Marcuse, Theory of Dielectric Optical Waveguides (Academic Press, New York, 1974).
  24. B. H. Lee, Y.-J. Kim, Y. Chung, W.-T. Han, and U.-C. Paek, "Fibre modal index measurements based on fibre gratings," Fiber Integr. Opt. 20, 443-455 (2001).
  25. H. J. Patrick, A. D. Kersey, and F. Bucholtz, "Analysis of the response of long period fiber gratings to external index of refraction," J. Lightwave Technol. 16, 1606-1612 (1998). [CrossRef]
  26. D. W. Kim, Y. Zhang, K. L. Cooper, and A. Wang, "In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement," Appl. Opt. 44, 5368-5373 (2005). [CrossRef] [PubMed]
  27. I. Del Villar, I. R. Matias, F. J. Arregui, and P. Lalanne, "Optimization of sensitivity in Long Period Fiber Gratings with overlay deposition," Opt. Express 13, 56-69 (2005). [CrossRef]
  28. M. Giordano, M. Russo, A. Cusano, and G. Mensitieri, "An high sensitivity optical sensor for chloroform vapours detection based on nanometric film of ?-form syndiotactic polystyrene," Sens. Actuators B 107, 140-147 (2005). [CrossRef]
  29. L. E. Scriven, "Physics And Applications of Dip Coating And Spin Coating," Mater. Res. Soc. Symp. Proc. 121, 717-729 (1988). [CrossRef]
  30. X. Shu, L. Zhang, and I. Bennion, "Sensitivity Characteristics of Long-Period Fiber Gratings," J. Lightwave Technol. 20, 255- (2002). [CrossRef]
  31. G. Humbert and A. Malki, " Electric-arc-induced gratings in non-hydrogenated fibres: fabrication and high temperature characterizations," J. Opt. A 4, 194-198 (2002). [CrossRef]
  32. R. Falate, G. R. C. Possetti, R. C. Kamikawachi, J. L. Fabris, and H. J. Kalinowski, "Temperature influence of an air conditioner in refractive index measurements using long-period fiber gratings," Third European Workshop on Optical Fibre Sensors, Proc. of the SPIE 6619, 66193W (2007).
  33. M. J.  Weber, Handbook of optical materials (CRC, New York, 2003).
  34. Y. -J. Kim, U. -C. Paek, and B. H. Lee, "Measurement of refractive-index variation with temperature by use of long-period fiber gratings," Opt. Lett. 27, 1297-1299 (2002). [CrossRef]

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