We study the shift of a long period grating's resonance wavelength with UV induced refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high-index rods is modified through UV exposure, and we observe that the long period grating's resonance shifts with the bandgaps. With a sensitivity of 21,000 nanometers per refractive index unit and a 8.8 nm resonance width changes of refractive index of 3 x 10(-6) are in principle detectable Crown Copyrigh

Eggleton, B J

Knight, Jonathan C

Kuhlmey, B T

Lazaro, J M

Lopez-Higuera, J M

OPUS

        Citation for published version:Lazaro, JM, Kuhlmey, BT, Knight, JC, Lopez-Higuera, JM & Eggleton, BJ 2009, 'Ultrasensitive UV-tunablegrating in all-solid photonic bandgap fibers', Optics Communications, vol. 282, no. 12, pp. 2358-2361.https://doi.org/10.1016/j.optcom.2009.03.014DOI:10.1016/j.optcom.2009.03.014Publication date:2009Document VersionPeer reviewed versionLink to publicationNOTICE: this is the author’s version of a work that was accepted for publication in Optics Communications.Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting,and other quality control mechanisms may not be reflected in this document. Changes may have been made tothis work since it was submitted for publication. A definitive version was subsequently published in OpticsCommunications, vol 282, issue 12, 2009, DOI: 10.1016/j.optcom.2009.03.014University of BathGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 12. May. 2019Optics Communications  1. Introduction  Solid core photonic band gap fibers (SC-PBGFs) are photonic crystal fibers [1] in which a solid core of refractive index nlow is surrounded by a cladding consisting of a periodic arrangement of inclusions having a refractive index (nhigh) higher than that of the background (nlow) (Fig.1). SC-PBGFs can be made either by infiltrating high-index fluids into holey fibers [2], or by incorporating high-index Germanium doped silica inclusions in a lower index silica background at the preform stage [3], leading after drawing to all-solid SC-PBGFs. The light guidance for SC-PBGFs can equivalently be explained in terms of antiresonant reflecting optical waveguide (ARROW) or bandgap effects [4-8]. In particular, SC-PBGFs have high and low transmission wavelength bands, which are delimited by the cutoff of the high index inclusions’ modes [4,5], and are thus sensitive to the refractive index of the high-index inclusions [9]. This sensitivity, in particular when the inclusions are high-index fluids, can be used to create refractive index sensors [10] or temperature tunable filters [9]. While the wavelength shift of transmission bands can be quite dramatic with refractive index changes, the edges of the bands aren’t very sharp, so that the smallest detectable refractive index changes aren’t necessarily competitive with other sensing techniques [11]. By introducing sharper features in the transmission spectrum of SC-PBGFs this limitation could be overcome, under the condition that these features also shift with the transmission bands of the SC-PBGF with changes in refractive index.  Steinvurzel et al showed that long period gratings (LPG) can be used for that purpose [12]. LPGs are corrugations of the refractive index or geometry of a fiber with a period much longer than the wavelength, of the order of hundreds of micrometers [13]. LPGs couple light between co-propagating modes, typically between a core mode and a cladding mode. When in a SC-PBGF, resonant wavelengths at which such coupling occur have a sensitivity to refractive index changes similar to that of the SC-PBGF’s transmission bands themselves. It had been predicted that this could lead to LPGs having extreme refractive index sensitivities when nhigh-nlow is small [12]. Here, we study the sensitivity to refractive index changes of a microbend LPG in a low index-contrast all-solid SC-PBGF. We demonstrate a sensitivity of 2.1.104 nm per refractive index unit (nm/RIU), leading to a detection limit of the order of  3.10-6 RIU [11].  2. Experimental setup  We use an all-solid SC-PBGF. The cladding of this fiber consists of a hexagonal array of high-index inclusions made out of 20% Germanium doped silica, in a silica background (Fig. 1). The fiber has a centre to centre distance between inclusions of Λ≃6.7μm, and rod diameter d≃3.2μm.  Since Ge-doped silica is photosensitive, the refractive index of the high index Contents lists available at ScienceDirect  Optics Communications  journal  homepage:  www.elsevier.com/locate/optcom ARTICLE INFO  Article  history:  ABSTRACT We demonstrate ultra-high sensitivity of a long period grating’s resonance wavelength to refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high index rods is modified through UV exposure, and we observe a sensitivity of 21,000 nanometers per refractive index unit with a 8.8nm resonance width, leading to detectable changes of refractive index of 3⋅10-6.  ©  2008  Elsevier  B.V.  All  rights  reserved.  PACS:  42.70.Qs 42.81.Pa42.79.Dj  Keywords:  Photonic crystal fibers  Long period gratings Tunable filters Refractive index sensing Optical fiber characterization   ULTRASENSITIVE UV-TUNABLE GRATING IN ALL SOLID PHOTONIC BANDGAP FIBERS  JM Lázaroa, B.T. Kuhlmeyb, Jonathan C. Knightc, JM Lopez-Higueraa, B.J. Eggletonb   a Grupo de Ingeniería Fotónica, Dpto. TEISA, Universidad de Cantabria, Avda. Castros S/N, 39005 Santander, Spain  b Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS) and School of Physics, University of Sydney, New South Wales 2006, Australia  c Centre for Photonic and Photonics Materials, Department of Physics, University of Bath, Bath BA2 7AY, U.K.  Optics Communications regions can be tuned after fabrication by exposing the fiber to UV light [14]. Each Ge-doped inclusion has a cylindrical graded refractive index distribution described by  osilicaooGIsilicarrnrrrrnnrn))/(1(1()( where r is the distance from rod’s center, α  ≈ 4.7, ΔnGI ≈ 0.0203 and ro ≈ 1.6μm . The transmission spectrum of the fiber before any UV exposure is shown in Fig. 2, and displays the typical high- and low-transmission windows of bandgap fibers. In the remainder of the article we will concentrate on the lowest order high transmission window, above 1200nm.  Fig. 1: Optical micrograph of the SC-PBGF used in the experiment. Dimensions are given in the text.   Fig. 2: Transmission spectrum of the SC-PBGF  In order to improve the photosensitivity of the Ge-doped inclusions, the SC-PBGF was hydrogen loaded [15,16]. Two experiments were carried out: In a first experiment, we simply demonstrated our ability to modify the refractive index of the Ge-doped rods through exposure to UV light, which was observed through the resulting wavelength shift in the transmission bands. For this, we exposed a hydrogen-loaded SC-PBGF to UV light using a frequency doubled continuous-wave Argon-ion laser (244nm, ~80mW). The laser light was focused with a cylindrical lens on the fiber so as to have a spot size of the same size as the fiber diameter. The laser spot was then swept a number of times over a length of 10.5cm of the fiber using a translation stage. Total exposure after each sweep was calculated using the sweep time, measured laser power and measured spot size. In a second experiment, a new piece of hydrogenated SC-PBGF was placed between a stainless steel threaded rod and an aluminium frame, and butt-coupled to a supercontinuum source and an optical spectrum analyzer (Fig. 3). The threaded rod creates a 5cm long microbend LPG with 0.7 mm periodicity. The SC-PBGF was then exposed to UV using the same method as in the first experiment, but with sweeps covering the length of the LPG only.     Fig. 3: Schematic of the experimental setup. Bottom left, detail of the threaded rod applied to the SC-PBGF to generate the microbend LPG.  3. Results  Figure 4 shows the results for our first experiment, in which we measure the evolution of the edge of the lowest order transmission window with total UV exposure, without an LPG. Each sweep lasts 23 min, with an estimated deposited energy of  ~55 J/cm2 per sweep. A shift of ~119 nm is obtained for a total UV energy of ~500 J/cm2 after 9 sweeps. The shift in bandgap is due to the UV-induced refractive index change in SC-PBGF, and is consistent with previous results [14], demonstrating our setup can successfully tune the refractive index of the Ge-doped rods.  Fig. 4: Shift of transmission bands of the SC-PBGF after successive exposures to UV light.   -30-25-20-15-10-50600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700Wavelength(nm)Power(dB)Optics Communications Once our ability to tune bandgaps of the SC-PBGF was demonstrated, we proceeded to our second experiment.  Figure 5 shows the transmission spectrum of the SC-PBGF with the microbend LPG, before any UV exposure and after successive sweeps of ~41 min duration, each corresponding to an energy of approximately 75 J/cm2 per sweep. The dip in transmission due to the LPG shifts in wavelength with successive sweeps, at a rate comparable to that of the edge of the band. However, the feature of the LPG is much sharper, allowing greater precision in detecting shifts. A shift of 53 nm has been obtained after a total UV energy of 300 J/cm2. We also note that the depth of the resonance dip decreases, which we attribute to imperfections in the uniformity of the UV exposure, mostly as a result of the difficulty to maintain the fiber at the same position in the UV beam during sweeps.     Fig. 5: Shift of the resonance of our microbend SC-PBGF LPG with succesive UV exposures.  4. Sensitivity  High-transmission bands of SC-PBGFs are delimited by the cutoffs of high index inclusions [9]. Approximating the graded index profile by a step index rod, the lowest band is delimited by the cutoff of the lowest order rod modes, having normalized frequency given by     2   √        ≃ 2.    Where nh=nsilica(1+ΔnGI) and nl=nsilica. To first order, a change δλ of the wavelength of the edge of the band thus corresponds to a change δn in nh given by   n ≃                With δλ=53nm, we obtain δn≃ .  2 , or alternatively a sensitivity of 2.1⋅104nm/RUI. Considering that the LPG resonance dip has a 3-dB width of 8.8nm, the detectable change of refractive index assuming a signal to noise ratio of 60dB would be 3.10-6 [11]. This is comparable to the best published fiber based refractive index sensing devices, and can be improved in particular by designing the LPG to have narrower resonances. Our results represent an order of magnitude improvement in sensitivity compared to previous studies of LPGs in SC-PBGFs [12], due to the use of smaller index-contrast high-index inclusions.   5. Conclusions  We have realized a tunable LPG in an SC-PBGF, and have demonstrated its resonant wavelength is extremely sensitive to changes in refractive index of the high-index inclusions. Such an LPG could be used as a UV adjustable notch filter as-is. In such a case uniform exposure to maintain the depth and width of the LPG notch will be primordial. The most promising application remains however in sensing. We have demonstrated that our setup allows in principle to detect changes in refractive index of 3⋅10-6. If the all-solid fiber is replaced by a fluid filled SC-PBGF, ultra sensitive fluid refractive index sensors can be achieved. The small detectable change results from the combination of the large sensitivity of the bandgaps with the narrow feature of the long period grating.    AcknowledgementsThis work was produced with the assistance of the Australian Research Council under the ARC Centre of Excellence program and the discovery project program (DP0881528). CUDOS is an ARC Centre of Excellence. J.M. Lázaro is grateful for financial support from the Spanish Government's Ministry of Science and Technology through the projects TEC'2005-08218-C02-02 and TEC'2007-67987-C02-01 allowing him to spend three months with CUDOS at University of Sydney. References [1] Ph. Russell, “Photonic Crystal Fibers”, Science, Vol. 299, No. 17 January, pp. 358-362 (2003) [2] R.T. Bise, R.S. Windeler, K.S. Kranz, C. Kerbage, B.J. Eggleton, D.J. Trevor, in OSA Trends in Optics and Photonics (TOPS) vol. 70, Optical Fiber Communication Conference, Technical Digest, post conference edition (Optical Society of America, Washington DC, 2002), p 466 [3] F. Luan, A. K. George, T. D. Hedley, G. J. Pearce, D. M. Bird, J. C. Knight, P. St. J. Russell, “All-solid photonic band gap fiber,” Opt. Lett. 29, 2369-2371 (2004).  [13] A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long Period Fiber Gratings as Band Rejection Filters,” J. Lightwave Technol. 14, 58–65 (1996). [4] N. M. Litchinitser, A. K. Abeeluck, C. Headley, and B. J. Eggleton, “Antiresonant reflecting photonic crystal optical waveguides”, Opt. Lett., Vol. 27, No. 18,  pp. 1592–1594 (2002). -45-40-35-30-25-20-15-10-501200 1250 1300 1350 1400 1450 1500 1550 1600Wavelength(nm)Power(dB)initial LPGsweep 1sweep 2sweep 3sweep 4Optics Communications [5] T. P. White, R. C. McPhedran, C. M. de Sterke, N. M. Litchinitser, and B. J. Eggleton, “Resonance and scattering in microstructured optical fibers” Opt. Lett., Vol. 27, No. 22, pp. 1977–1979 (2002). [6] P. Steinvurzel, C. M. de Sterke, M. J. Steel, B. T. Kuhlmey, and B. J. Eggleton, “Single scatterer Fano resonances in solid core photonic band gap fibers” Opt. Express, Vol. 14, No. 19, pp. 8797–8811 (2006). [7] J. Laegsgaard, ”Gap formation and guided modes in photonic bandgap fibres awith high-index rods,” Journal of Optics A-Pure and Applied Optics, Vol. 6, pp. 798–804 (2004). [8] T. A. Birks, J. A. Pearce, and D. M. Bird, “Approximate band structure calculation for photonic bandgap fibres” Opt. Express, Vol. 14, No. 20, pp. 9483–9490 (2006). [9] N. M. Litchinitser, S. C. Dunn, P. E. Steinvurzel, B. J. Eggleton, T. P. White, R. C. McPhedran, and C. M. de Sterke, “Application of an Arrow model for designing tunable photonic devices” Opt. Express, Vol. 12, No. 8, pp. 1540–1550 (2004). [10] N.M. Litchinitser and E. Poliakov, “Antiresonant guiding microstructured optical fibers for sensing applications,” Appl. Phys. B 81, 347–351 (2005)  [11] I. M. White and X. Fan, "On the performance quantification of resonant refractive index sensors," Opt. Express 16, 1020-1028 (2008) [12] P. Steinvurzel, E.D. Mägi, and B.J. Eggleton, “Tuning properties of long period gratings in photonic bandgap fibers”, Optics Letters, vol. 31, No. 14, pp. 2103-2105,  2006.  [14] Long Jin, Zhi Wang, Qiang Fang, Yange Liu, Bo Liu, Guiyun Kai, and Xiaoyi Dong, “Spectral characteristics and bend response of Bragg gratings inscribed in all-solid bandgap fibers”, Optics Express, vol. 15, No. 23, pp. 15555, (2007). [15] Albert, J., Malo, B; Bilodeau, F; Johnson, D C; Hill, K O; Hibino, Y; Kawachi, M “Photosensitivity in Ge-doped silica optical waveguides and fibers with 193-nm light from an ArF excimer laser” Optics Letters, Vol.19,1994, pp. 387-389. [16] Xie, W.X., P. Niay, P. Bernage, M. Douay, J.F. Bayon, T. Georges, M. Monerie, B. Poumellec, “Experimental evidence of two types of photorefractive effects occurring during photoinscriptions of Bragg gratings written within germanosilicate fibres” Optics Communications, Vol. 104, 1993, pp. 185-195.  

Ultrasensitive UV-tunable grating in all-solid photonic bandgap fibers

        Citation for published version:Lazaro, JM, Kuhlmey, BT, Knight, JC, Lopez-Higuera, JM & Eggleton, BJ 2009, 'Ultrasensitive UV-tunablegrating in all-solid photonic bandgap fibers', Optics Communications, vol. 282, no. 12, pp. 2358-2361.https://doi.org/10.1016/j.optcom.2009.03.014DOI:10.1016/j.optcom.2009.03.014Publication date:2009Document VersionPeer reviewed versionLink to publicationNOTICE: this is the author’s version of a work that was accepted for publication in Optics Communications.Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting,and other quality control mechanisms may not be reflected in this document. Changes may have been made tothis work since it was submitted for publication. A definitive version was subsequently published in OpticsCommunications, vol 282, issue 12, 2009, DOI: 10.1016/j.optcom.2009.03.014University of BathGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 07. Dec. 2019Optics Communications  1. Introduction  Solid core photonic band gap fibers (SC-PBGFs) are photonic crystal fibers [1] in which a solid core of refractive index nlow is surrounded by a cladding consisting of a periodic arrangement of inclusions having a refractive index (nhigh) higher than that of the background (nlow) (Fig.1). SC-PBGFs can be made either by infiltrating high-index fluids into holey fibers [2], or by incorporating high-index Germanium doped silica inclusions in a lower index silica background at the preform stage [3], leading after drawing to all-solid SC-PBGFs. The light guidance for SC-PBGFs can equivalently be explained in terms of antiresonant reflecting optical waveguide (ARROW) or bandgap effects [4-8]. In particular, SC-PBGFs have high and low transmission wavelength bands, which are delimited by the cutoff of the high index inclusions’ modes [4,5], and are thus sensitive to the refractive index of the high-index inclusions [9]. This sensitivity, in particular when the inclusions are high-index fluids, can be used to create refractive index sensors [10] or temperature tunable filters [9]. While the wavelength shift of transmission bands can be quite dramatic with refractive index changes, the edges of the bands aren’t very sharp, so that the smallest detectable refractive index changes aren’t necessarily competitive with other sensing techniques [11]. By introducing sharper features in the transmission spectrum of SC-PBGFs this limitation could be overcome, under the condition that these features also shift with the transmission bands of the SC-PBGF with changes in refractive index.  Steinvurzel et al showed that long period gratings (LPG) can be used for that purpose [12]. LPGs are corrugations of the refractive index or geometry of a fiber with a period much longer than the wavelength, of the order of hundreds of micrometers [13]. LPGs couple light between co-propagating modes, typically between a core mode and a cladding mode. When in a SC-PBGF, resonant wavelengths at which such coupling occur have a sensitivity to refractive index changes similar to that of the SC-PBGF’s transmission bands themselves. It had been predicted that this could lead to LPGs having extreme refractive index sensitivities when nhigh-nlow is small [12]. Here, we study the sensitivity to refractive index changes of a microbend LPG in a low index-contrast all-solid SC-PBGF. We demonstrate a sensitivity of 2.1.104 nm per refractive index unit (nm/RIU), leading to a detection limit of the order of  3.10-6 RIU [11].  2. Experimental setup  We use an all-solid SC-PBGF. The cladding of this fiber consists of a hexagonal array of high-index inclusions made out of 20% Germanium doped silica, in a silica background (Fig. 1). The fiber has a centre to centre distance between inclusions of Λ≃6.7μm, and rod diameter d≃3.2μm.  Since Ge-doped silica is photosensitive, the refractive index of the high index Contents lists available at ScienceDirect  Optics Communications  journal  homepage:  www.elsevier.com/locate/optcom ARTICLE INFO  Article  history:  ABSTRACT We demonstrate ultra-high sensitivity of a long period grating’s resonance wavelength to refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high index rods is modified through UV exposure, and we observe a sensitivity of 21,000 nanometers per refractive index unit with a 8.8nm resonance width, leading to detectable changes of refractive index of 3⋅10-6.  ©  2008  Elsevier  B.V.  All  rights  reserved.  PACS:  42.70.Qs 42.81.Pa42.79.Dj  Keywords:  Photonic crystal fibers  Long period gratings Tunable filters Refractive index sensing Optical fiber characterization   ULTRASENSITIVE UV-TUNABLE GRATING IN ALL SOLID PHOTONIC BANDGAP FIBERS  JM Lázaroa, B.T. Kuhlmeyb, Jonathan C. Knightc, JM Lopez-Higueraa, B.J. Eggletonb   a Grupo de Ingeniería Fotónica, Dpto. TEISA, Universidad de Cantabria, Avda. Castros S/N, 39005 Santander, Spain  b Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS) and School of Physics, University of Sydney, New South Wales 2006, Australia  c Centre for Photonic and Photonics Materials, Department of Physics, University of Bath, Bath BA2 7AY, U.K.  Optics Communications regions can be tuned after fabrication by exposing the fiber to UV light [14]. Each Ge-doped inclusion has a cylindrical graded refractive index distribution described by  osilicaooGIsilicarrnrrrrnnrn))/(1(1()( where r is the distance from rod’s center, α  ≈ 4.7, ΔnGI ≈ 0.0203 and ro ≈ 1.6μm . The transmission spectrum of the fiber before any UV exposure is shown in Fig. 2, and displays the typical high- and low-transmission windows of bandgap fibers. In the remainder of the article we will concentrate on the lowest order high transmission window, above 1200nm.  Fig. 1: Optical micrograph of the SC-PBGF used in the experiment. Dimensions are given in the text.   Fig. 2: Transmission spectrum of the SC-PBGF  In order to improve the photosensitivity of the Ge-doped inclusions, the SC-PBGF was hydrogen loaded [15,16]. Two experiments were carried out: In a first experiment, we simply demonstrated our ability to modify the refractive index of the Ge-doped rods through exposure to UV light, which was observed through the resulting wavelength shift in the transmission bands. For this, we exposed a hydrogen-loaded SC-PBGF to UV light using a frequency doubled continuous-wave Argon-ion laser (244nm, ~80mW). The laser light was focused with a cylindrical lens on the fiber so as to have a spot size of the same size as the fiber diameter. The laser spot was then swept a number of times over a length of 10.5cm of the fiber using a translation stage. Total exposure after each sweep was calculated using the sweep time, measured laser power and measured spot size. In a second experiment, a new piece of hydrogenated SC-PBGF was placed between a stainless steel threaded rod and an aluminium frame, and butt-coupled to a supercontinuum source and an optical spectrum analyzer (Fig. 3). The threaded rod creates a 5cm long microbend LPG with 0.7 mm periodicity. The SC-PBGF was then exposed to UV using the same method as in the first experiment, but with sweeps covering the length of the LPG only.     Fig. 3: Schematic of the experimental setup. Bottom left, detail of the threaded rod applied to the SC-PBGF to generate the microbend LPG.  3. Results  Figure 4 shows the results for our first experiment, in which we measure the evolution of the edge of the lowest order transmission window with total UV exposure, without an LPG. Each sweep lasts 23 min, with an estimated deposited energy of  ~55 J/cm2 per sweep. A shift of ~119 nm is obtained for a total UV energy of ~500 J/cm2 after 9 sweeps. The shift in bandgap is due to the UV-induced refractive index change in SC-PBGF, and is consistent with previous results [14], demonstrating our setup can successfully tune the refractive index of the Ge-doped rods.  Fig. 4: Shift of transmission bands of the SC-PBGF after successive exposures to UV light.   -30-25-20-15-10-50600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700Wavelength(nm)Power(dB)Optics Communications Once our ability to tune bandgaps of the SC-PBGF was demonstrated, we proceeded to our second experiment.  Figure 5 shows the transmission spectrum of the SC-PBGF with the microbend LPG, before any UV exposure and after successive sweeps of ~41 min duration, each corresponding to an energy of approximately 75 J/cm2 per sweep. The dip in transmission due to the LPG shifts in wavelength with successive sweeps, at a rate comparable to that of the edge of the band. However, the feature of the LPG is much sharper, allowing greater precision in detecting shifts. A shift of 53 nm has been obtained after a total UV energy of 300 J/cm2. We also note that the depth of the resonance dip decreases, which we attribute to imperfections in the uniformity of the UV exposure, mostly as a result of the difficulty to maintain the fiber at the same position in the UV beam during sweeps.     Fig. 5: Shift of the resonance of our microbend SC-PBGF LPG with succesive UV exposures.  4. Sensitivity  High-transmission bands of SC-PBGFs are delimited by the cutoffs of high index inclusions [9]. Approximating the graded index profile by a step index rod, the lowest band is delimited by the cutoff of the lowest order rod modes, having normalized frequency given by     2   √        ≃ 2.    Where nh=nsilica(1+ΔnGI) and nl=nsilica. To first order, a change δλ of the wavelength of the edge of the band thus corresponds to a change δn in nh given by   n ≃                With δλ=53nm, we obtain δn≃ .  2 , or alternatively a sensitivity of 2.1⋅104nm/RUI. Considering that the LPG resonance dip has a 3-dB width of 8.8nm, the detectable change of refractive index assuming a signal to noise ratio of 60dB would be 3.10-6 [11]. This is comparable to the best published fiber based refractive index sensing devices, and can be improved in particular by designing the LPG to have narrower resonances. Our results represent an order of magnitude improvement in sensitivity compared to previous studies of LPGs in SC-PBGFs [12], due to the use of smaller index-contrast high-index inclusions.   5. Conclusions  We have realized a tunable LPG in an SC-PBGF, and have demonstrated its resonant wavelength is extremely sensitive to changes in refractive index of the high-index inclusions. Such an LPG could be used as a UV adjustable notch filter as-is. In such a case uniform exposure to maintain the depth and width of the LPG notch will be primordial. The most promising application remains however in sensing. We have demonstrated that our setup allows in principle to detect changes in refractive index of 3⋅10-6. If the all-solid fiber is replaced by a fluid filled SC-PBGF, ultra sensitive fluid refractive index sensors can be achieved. The small detectable change results from the combination of the large sensitivity of the bandgaps with the narrow feature of the long period grating.    AcknowledgementsThis work was produced with the assistance of the Australian Research Council under the ARC Centre of Excellence program and the discovery project program (DP0881528). CUDOS is an ARC Centre of Excellence. J.M. Lázaro is grateful for financial support from the Spanish Government's Ministry of Science and Technology through the projects TEC'2005-08218-C02-02 and TEC'2007-67987-C02-01 allowing him to spend three months with CUDOS at University of Sydney. References [1] Ph. Russell, “Photonic Crystal Fibers”, Science, Vol. 299, No. 17 January, pp. 358-362 (2003) [2] R.T. Bise, R.S. Windeler, K.S. Kranz, C. Kerbage, B.J. Eggleton, D.J. Trevor, in OSA Trends in Optics and Photonics (TOPS) vol. 70, Optical Fiber Communication Conference, Technical Digest, post conference edition (Optical Society of America, Washington DC, 2002), p 466 [3] F. Luan, A. K. George, T. D. Hedley, G. J. Pearce, D. M. Bird, J. C. Knight, P. St. J. Russell, “All-solid photonic band gap fiber,” Opt. Lett. 29, 2369-2371 (2004).  [13] A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long Period Fiber Gratings as Band Rejection Filters,” J. Lightwave Technol. 14, 58–65 (1996). [4] N. M. Litchinitser, A. K. Abeeluck, C. Headley, and B. J. Eggleton, “Antiresonant reflecting photonic crystal optical waveguides”, Opt. Lett., Vol. 27, No. 18,  pp. 1592–1594 (2002). -45-40-35-30-25-20-15-10-501200 1250 1300 1350 1400 1450 1500 1550 1600Wavelength(nm)Power(dB)initial LPGsweep 1sweep 2sweep 3sweep 4Optics Communications [5] T. P. White, R. C. McPhedran, C. M. de Sterke, N. M. Litchinitser, and B. J. Eggleton, “Resonance and scattering in microstructured optical fibers” Opt. Lett., Vol. 27, No. 22, pp. 1977–1979 (2002). [6] P. Steinvurzel, C. M. de Sterke, M. J. Steel, B. T. Kuhlmey, and B. J. Eggleton, “Single scatterer Fano resonances in solid core photonic band gap fibers” Opt. Express, Vol. 14, No. 19, pp. 8797–8811 (2006). [7] J. Laegsgaard, ”Gap formation and guided modes in photonic bandgap fibres awith high-index rods,” Journal of Optics A-Pure and Applied Optics, Vol. 6, pp. 798–804 (2004). [8] T. A. Birks, J. A. Pearce, and D. M. Bird, “Approximate band structure calculation for photonic bandgap fibres” Opt. Express, Vol. 14, No. 20, pp. 9483–9490 (2006). [9] N. M. Litchinitser, S. C. Dunn, P. E. Steinvurzel, B. J. Eggleton, T. P. White, R. C. McPhedran, and C. M. de Sterke, “Application of an Arrow model for designing tunable photonic devices” Opt. Express, Vol. 12, No. 8, pp. 1540–1550 (2004). [10] N.M. Litchinitser and E. 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Ultrasensitive UV-tunable grating in all-solid photonic bandgap fibers

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