December 15, 2014 / Vol. 39, No. 24 / OPTICS LETTERS
6835
Tilted Bragg gratings in step-index polymer optical fiber Xuehao Hu,1,* Chi-Fung Jeff Pun,2 Hwa-Yaw Tam,2 Patrice Mégret,1 and Christophe Caucheteur1 1
Electromagnetism and Telecommunication Department, Université de Mons, 31 Boulevard Dolez, Mons 7000, Belgium 2 Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China *Corresponding author: [email protected] Received September 25, 2014; accepted November 2, 2014; posted November 10, 2014 (Doc. ID 223821); published December 9, 2014 We report the first slightly tilted Bragg gratings photo-inscription in polymer optical fiber (POF). For this, we make use of trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. Tilted fiber Bragg gratings (TFBGs) are inscribed in the near-infrared wavelength range using the scanning phase mask technique with a tilted phase mask in the plane perpendicular to the laser beam direction. The transmitted amplitude spectrum evolution of a 3° TFBG is analyzed as a function of the surrounding refractive index. A maximum sensitivity close to 13 nm∕RIU (refractive index unit) is obtained in the range 1.42–1.49. © 2014 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.39.006835
Fiber Bragg gratings (FBGs) were first photo-inscribed in polymer optical fibers (POFs) in 1999 [1]. Since then, through the use of different fiber types and inscription processes, a constant progress has been obtained toward the manufacturing of high-quality gratings. Besides some point-by-point photo-inscriptions (fourth order gratings) [2], polymer optical FBGs have been mainly produced with the straightforward and robust phase mask technique, either under static exposition [3–7] or through a laser beam scanning along the phase mask [8,9]. Tilted fiber Bragg gratings (TFBGs) belong to the short period grating family (grating period close to 500 nm for use in the C+L bands) and possess a refractive index modulation slightly angled with respect to the parallel to the optical fiber propagation axis. They provide two kinds of couplings: the self-backward coupling of the core mode and numerous backward couplings between the core mode and different cladding modes [10]. TFBGs produced in silica single-mode optical fiber present comb-like transmitted amplitude spectra composed of several tens of narrow-band resonances (full width at half-maximum (FWHM) ∼100 pm). These unique spectral features make possible a wealth of applications including interrogators [11], gain flattened erbium fiber amplifiers [12], optical fiber filters [13], polarimeters [14], polarization-dependent loss equalizers [15], and physical and (bio)chemical sensors [16]. The last application is possible since each cladding mode resonance presents its own sensitivity to the surrounding refractive index (SRI) [16–18]. In refractometric sensing, the presence of the Bragg wavelength provides the additional benefit of yielding a convenient temperature reference. For the aforementioned applications, POFs bring additional benefits, such as robustness, cost-effectiveness, ease of installation and improved biocompatibility. Also, photo-inscription in POFs has matured to such a degree that it now becomes feasible to investigate TFBGs fabrication, which requires more efficiency than uniform FBGs. For the first time, thanks to an optimized set-up compared to [19], we report TFBGs photo-inscription at different tilt angles in photosensitive POFs. The 0146-9592/14/246835-04$15.00/0
refractometric sensitivity of a 3° TFBG was then analyzed by immersion in calibrated liquids. Two approaches were followed to track the transmitted amplitude spectrum evolution as a function of the SRI value: the global evolution of the area delimited by the cladding modes and the local shift of a cladding mode close to the cut-off. A ∼13 nm∕RIU (refractive index unit) maximum sensitivity is obtained in the range 1.42–1.49 of SRI values by tracking the modal wavelength shift. The fibers used in this work were manufactured at the Hong Kong Polytechnic University. They are characterized by a core diameter of 8.2 μm and a cladding diameter of 150 μm. The cladding is in pure PMMA, while the core is composed of PMMA doped with diphenyl sulfide (5% mole) and trans-4-stilbenemethanol (1% w.t.). Their refractive indices were calculated at 589 nm equal to 1.5086 and 1.4904, respectively. Around 1550 nm, it is expected that the corresponding values are around 1.498 and 1.480. The laser used to photo-inscribe TFBGs is a He–Cd laser (Kimmon IK5751I-G) with an output power of 30 mW at 325 nm. The output beam width of the laser is 1.2 mm. Figure 1 depicts the experimental set-up. The inox tube positioned above the laser allows evacuating the air turbulences caused by the cooling system, avoiding them to
Fig. 1. Experimental setup used to photo-inscribe TFBGs in step-index POFs (inset: close-up on the phase mask positioning). © 2014 Optical Society of America
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perturb the photo-writing process. The UV beam emitted by the laser was reflected by four mirrors (even number of reflections to preserve the output state of polarization) toward the POF, which was held in a V-groove plate. A uniform phase mask (Coherent) with a period of 1044 nm was deposited above the fiber on the V-groove plate. A 15-cm focal length cylindrical lens was used in front of the phase mask to focus the UV beam on the fiber core. It was privileged over a 10-cm focal length cylindrical lens as used before to optimize the power density at the fiber location and guarantee a good stability of the photo-writing conditions. The movement of the third mirror was controlled by a motor-driven translation stage, which was used to scan the laser along the fiber over a maximum length of 20 mm. To ensure a uniform refractive index modulation profile, a cache whose aperture is equal to the desired grating length was placed above the fiber, and the laser beam scan was started and stopped over the cache. To produce TFBGs, the phase mask was slightly angled with respect to the perpendicular to the optical fiber axis, as depicted in the inset of Fig. 1. The phase mask positioning was geometrically calibrated on the support, which was easily obtained thanks to the rectangular shape of the substrate. The photo-inscription of TFBGs generally requires much more laser power than uniform FBGs. This arises from the fact that the coupling efficiency of the grating decreases due to its angled fringes, as studied in [10]. Therefore, to mitigate the power limitation of the laser used in this work, POF sections were etched in a mixture of acetone and ethanol (ratio 2∶1), as done in [19], for the photo-inscription of highly reflective gratings in the same fiber type. This etching process allows decreasing the absorption of PMMA and hence producing high-contrast fringes in the fiber core. Fiber sections used in this work were set to 100 μm in diameter at the grating location. We have empirically found that this value remains the best trade-off in terms of highquality TFBGs production and fiber handling. Thinner fibers are indeed much more difficult to use in practice. Reflected and transmitted amplitude spectra were measured with an FBG interrogator (FS2200SA from FiberSensing), presenting a wavelength resolution equal to 1 pm. For this, POF sections were UV-glued (Norland 78) to two 8° angled silica single-mode fiber pigtails at both sides of POFs. The index matching glue avoids Fabry–Perot cavity effects, while angled fibers reduce Fresnel reflections. Figure 2 depicts the reflected and transmitted amplitude spectra of a 6-mm-long 3° TFBG photo-inscribed by scanning the laser beam along the phase mask at a velocity of 3 μm/s. The reflected amplitude spectrum displays several narrow bands between ∼1548 nm and ∼1552 nm, because POFs used in this experiment are not single-mode but few-mode fibers. The transmitted amplitude spectrum presents the same fundamental core mode resonances, which now appear as dips in the spectrum. Several tens of cladding mode resonances also appear at lower wavelengths extending from ∼1530 nm to ∼1548 nm. According to the TFBG phase matching conditions, each cladding mode resonance presents its own effective refractive index value [10]. Their FWHM does not exceed 100 pm, and the wavelength spacing between
Fig. 2. Reflected and transmitted amplitude spectra of a 6-mmlong 3° TFBG photo-written in slightly etched PMMA step-index POF (cladding diameter: 100 μm).
adjacent resonances lies in the range 0.4–0.7 nm (highorder modes are more spread than low-order ones). Figure 3 presents the transmitted amplitude spectra of three TFBGs with tilt angles of 1.5°, 3.0°, and 4.5°, respectively. Conclusions similar to the case of silica TFBGs can be drawn. For higher tilt angles, the spectral content increases thanks to coupling to higher cladding mode resonances, while the Bragg wavelength is red-shifted and the core mode resonance reflectivity decreases. An arrow below each curve identifies it. The reflectivity of the Bragg resonance was measured equal to 16% for the 1.5° TFBG, 12% for the 3.0° TFBG and 2% for the 4.5° TFBG. The experimental wavelength shift agrees well with the phase matching condition for the fundamental mode (λBragg 2neff Λ∕ cos θ, where neff is the effective refractive index, Λ is the grating period and θ is the tilt angle). It can also be noticed that the peak-to-peak amplitude of the cladding modes decreases with the tilt angle increasing, in accordance with [10]. The transmitted amplitude spectrum evolution of the 3° TFBG was then measured by immersing the TFBG in different calibrated liquids presenting well-known
Fig. 3. Transmitted amplitude spectra of 6-mm-long TFBGs photo-written at different tilt angles in slightly etched PMMA step-index POF (cladding diameter: 100 μm).
December 15, 2014 / Vol. 39, No. 24 / OPTICS LETTERS
refractive index values (Cargille oils). The grating was cleaned in buffer liquid before immersion in a new refractive index liquid. The grating response was instantaneous. Figure 4 shows the transmitted spectrum evolution as a function of the SRI in the range between 1.42 and 1.49. This range was considered according to the phase matching conditions and taking into account that cladding mode resonances extend ∼25 nm below the Bragg wavelength for the 3° TFBG. The last value is above the refractive index of the fiber cladding and is therefore sufficient to have all cladding modes radiated. For the readability of the figure, transmitted spectra are displayed with offsets in the vertical axis. It can be seen that the transmitted spectrum measured in n 1.42 liquid is similar to that measured in air, and then cladding mode resonances begin to disappear from the left to the right of the spectrum when the SRI value increases. As TFBGs produced in silica optical fibers [16–18], this behavior results from the fact that when the SRI reaches the effective refractive index of a cladding mode, the latter is no longer reflected at the cladding-surrounding medium interface but becomes radiated. Hence, it results in a perfect smoothing of the transmitted amplitude spectrum when SRI is not lower than the effective refractive index of a cladding mode. From the raw data displayed in Fig. 4, the refractometric sensitivity of the 3° TFBG was computed using two common approaches for silica TFBGs [16]. We first computed the area delimited by the cladding modes in the TFBG-transmitted amplitude spectrum. Figure 5 shows its evolution as a function of the SRI value. A monotonic decreasing trend is observed for all the investigated SRI range, with a maximum sensitivity between 1.45 and 1.48. It is precisely for this SRI range that TFBGs photo-written in silica optical fibers become insensitive, because the refractive index of the fiber cladding is 1.45, and all the cladding modes are therefore radiated. The inset of Fig. 5 presents the principle of the area computation, which is based on the determination of the upper and lower envelopes of the spectrum of the cladding modes. The second approach was based on the determination of the wavelength shift of selected cladding mode resonances as a function of the SRI. For this, the minimum
Fig. 4. Transmitted amplitude spectrum evolution (vertical scale with offset) as a function of the SRI value in the range 1.42–1.49.
6837
Fig. 5. Evolution of the area delimited by the cladding modes in the transmitted amplitude spectrum as a function of the SRI value (inset: principle of the computation of the area).
value of the resonance was tracked for different SRI values, and the corresponding wavelength was recorded. Figure 6 shows the corresponding results from three resonances of the 3° TFBG, respectively, around 1535, 1540, and 1545 nm. The wavelength shifts were normalized with respect to the resonance wavelength measured at 1.42. Figure 6 clearly confirms that each cladding mode presents its own refractometric sensitivity in a given SRI range, which depends on its effective refractive index. The maximum sensitivity is obtained at the upper limit of the sensitive region, when the mode reaches the cut-off wavelength (point at which the mode is no longer guided and becomes radiated). For the grating tested in our experiments, it is equal to ∼13 nm∕RIU, which is similar to the refractometric sensitivity reported for TFBGs in silica single-mode optical fibers [18]. The lower limit of SRI range can be further decreased by increasing the tilt angle, allowing the coupling to
Fig. 6. Relative wavelength shift of selected cladding modes as a function of the SRI value.
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higher order cladding modes, as provided by the 4.5° TFBG. Up to now, we have not been able to produce quality gratings with tilt angles higher than 5°. With continuous improvement of POF photosensitivity and optimized photo-writing processes, we believe that this limitation should be soon overcome. In conclusion, we have achieved the very first tilted Bragg gratings photo-inscribed in POFs. Taking into the limited power budget contrasting with the need for a high dose to manufacture TFBGs, POF sections were slightly etched prior to exposure to the UV writing beam. A 3° TFBG was characterized by immersion in calibrated refractive liquids, and a sensitivity reaching ∼13 nm∕RIU was reported in the SRI range 1.42–1.49. These first results pave the way to further developments of TFBGs in POFs and to their use for (bio)chemical sensing where POFs could preferably replace their silica counterparts, with improved bending tolerance and biocompatibility. This research was supported by the Belgian F.R.S.FNRS (Associate Research grant of C. Caucheteur), the European Research Council (Sarting grant of C. Caucheteur–Grant agreement N° 280161) and the Actions de la Recherche concertées programme (Prediction project). References 1. Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, IEEE Photon. Technol. Lett. 11, 352 (1999). 2. A. Stefani, M. Stecher, G. E. Town, and O. Bang, IEEE Photon. Technol. Lett. 24, 1148 (2012). 3. D. J. Webb, K. Kalli, K. Caroll, C. Zhang, M. Komodromos, A. Argyros, M. Large, G. Emiliyanov, O. Bang, and E. Kjaer, Proc. SPIE 6830, 683002 (2007).
4. G. Statkiewicz-Barabach, K. Tarnowski, D. Kowal, P. Mergo, and W. Urbanczyk, Opt. Express 21, 8521 (2013). 5. H. Dobb, D. J. Webb, K. Kalli, A. Argyros, M. Large, and M. A. van Eijkelenborg, Opt. Lett. 30, 3296 (2005). 6. G. Rajan, M. Ramakrishnan, Y. Semenova, E. Ambikairajah, G. Farrell, and G. D. Peng, J. Lightw. Technol. 32, 1726 (2014). 7. I. L. Bundalo, K. Nielsen, C. Markos, and O. Bang, Opt. Express 22, 5270 (2014). 8. X. Chen, C. Zhang, D. J. Webb, and K. Kalli, Meas. Sci. Technol. 21, 094005 (2010). 9. C. A. F. Marques, L. Bilro, N. Alberto, D. J. Webb, and R. Nogueira, J. Opt. 15, 075404 (2013). 10. T. Erdogan and J. E. Sipe, J. Opt. Soc. Am. A 13, 296 (1996). 11. K. S. Feder, P. S. Westbrook, J. Ging, and P. I. Reyes, IEEE Photon. Technol. Lett. 15, 933 (2003). 12. R. Kashyap, R. Wyatt, and R. J. Campbell, IEEE Electron. Lett. 29, 154 (1993). 13. J. M. Castro, D. F. Geraghty, B. R. West, and S. Honkanen, Appl. Opt. 43, 6166 (2004). 14. J. Peupelmann, E. Krause, A. Bandemer, and C. Schaffer, IEEE Electron. Lett. 38, 1248 (2002). 15. S. J. Mihailov, R. B. Walker, T. J. Stocki, and D. C. Johnson, IEEE Electron. Lett. 37, 284 (2001). 16. J. Albert, L. Y. Shao, and C. Caucheteur, Laser Photon. Rev. 7, 83 (2013). 17. C. Caucheteur and P. Mégret, IEEE Photon. Technol. Lett. 17, 2703 (2005). 18. C. F. Chan, C. Chen, A. Jafari, A. Laronche, D. J. Thomson, and J. Albert, Appl. Opt. 46, 1142 (2007). 19. X. Hu, C. F. J. Pun, H.-Y. Tam, P. Mégret, and C. Caucheteur, Opt. Express 22, 18807 (2014).
6835
Tilted Bragg gratings in step-index polymer optical fiber Xuehao Hu,1,* Chi-Fung Jeff Pun,2 Hwa-Yaw Tam,2 Patrice Mégret,1 and Christophe Caucheteur1 1
Electromagnetism and Telecommunication Department, Université de Mons, 31 Boulevard Dolez, Mons 7000, Belgium 2 Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China *Corresponding author: [email protected] Received September 25, 2014; accepted November 2, 2014; posted November 10, 2014 (Doc. ID 223821); published December 9, 2014 We report the first slightly tilted Bragg gratings photo-inscription in polymer optical fiber (POF). For this, we make use of trans-4-stilbenemethanol-doped photosensitive step-index poly(methyl methacrylate) (PMMA) POFs. Tilted fiber Bragg gratings (TFBGs) are inscribed in the near-infrared wavelength range using the scanning phase mask technique with a tilted phase mask in the plane perpendicular to the laser beam direction. The transmitted amplitude spectrum evolution of a 3° TFBG is analyzed as a function of the surrounding refractive index. A maximum sensitivity close to 13 nm∕RIU (refractive index unit) is obtained in the range 1.42–1.49. © 2014 Optical Society of America OCIS codes: (060.3735) Fiber Bragg gratings; (060.2370) Fiber optics sensors. http://dx.doi.org/10.1364/OL.39.006835
Fiber Bragg gratings (FBGs) were first photo-inscribed in polymer optical fibers (POFs) in 1999 [1]. Since then, through the use of different fiber types and inscription processes, a constant progress has been obtained toward the manufacturing of high-quality gratings. Besides some point-by-point photo-inscriptions (fourth order gratings) [2], polymer optical FBGs have been mainly produced with the straightforward and robust phase mask technique, either under static exposition [3–7] or through a laser beam scanning along the phase mask [8,9]. Tilted fiber Bragg gratings (TFBGs) belong to the short period grating family (grating period close to 500 nm for use in the C+L bands) and possess a refractive index modulation slightly angled with respect to the parallel to the optical fiber propagation axis. They provide two kinds of couplings: the self-backward coupling of the core mode and numerous backward couplings between the core mode and different cladding modes [10]. TFBGs produced in silica single-mode optical fiber present comb-like transmitted amplitude spectra composed of several tens of narrow-band resonances (full width at half-maximum (FWHM) ∼100 pm). These unique spectral features make possible a wealth of applications including interrogators [11], gain flattened erbium fiber amplifiers [12], optical fiber filters [13], polarimeters [14], polarization-dependent loss equalizers [15], and physical and (bio)chemical sensors [16]. The last application is possible since each cladding mode resonance presents its own sensitivity to the surrounding refractive index (SRI) [16–18]. In refractometric sensing, the presence of the Bragg wavelength provides the additional benefit of yielding a convenient temperature reference. For the aforementioned applications, POFs bring additional benefits, such as robustness, cost-effectiveness, ease of installation and improved biocompatibility. Also, photo-inscription in POFs has matured to such a degree that it now becomes feasible to investigate TFBGs fabrication, which requires more efficiency than uniform FBGs. For the first time, thanks to an optimized set-up compared to [19], we report TFBGs photo-inscription at different tilt angles in photosensitive POFs. The 0146-9592/14/246835-04$15.00/0
refractometric sensitivity of a 3° TFBG was then analyzed by immersion in calibrated liquids. Two approaches were followed to track the transmitted amplitude spectrum evolution as a function of the SRI value: the global evolution of the area delimited by the cladding modes and the local shift of a cladding mode close to the cut-off. A ∼13 nm∕RIU (refractive index unit) maximum sensitivity is obtained in the range 1.42–1.49 of SRI values by tracking the modal wavelength shift. The fibers used in this work were manufactured at the Hong Kong Polytechnic University. They are characterized by a core diameter of 8.2 μm and a cladding diameter of 150 μm. The cladding is in pure PMMA, while the core is composed of PMMA doped with diphenyl sulfide (5% mole) and trans-4-stilbenemethanol (1% w.t.). Their refractive indices were calculated at 589 nm equal to 1.5086 and 1.4904, respectively. Around 1550 nm, it is expected that the corresponding values are around 1.498 and 1.480. The laser used to photo-inscribe TFBGs is a He–Cd laser (Kimmon IK5751I-G) with an output power of 30 mW at 325 nm. The output beam width of the laser is 1.2 mm. Figure 1 depicts the experimental set-up. The inox tube positioned above the laser allows evacuating the air turbulences caused by the cooling system, avoiding them to
Fig. 1. Experimental setup used to photo-inscribe TFBGs in step-index POFs (inset: close-up on the phase mask positioning). © 2014 Optical Society of America
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perturb the photo-writing process. The UV beam emitted by the laser was reflected by four mirrors (even number of reflections to preserve the output state of polarization) toward the POF, which was held in a V-groove plate. A uniform phase mask (Coherent) with a period of 1044 nm was deposited above the fiber on the V-groove plate. A 15-cm focal length cylindrical lens was used in front of the phase mask to focus the UV beam on the fiber core. It was privileged over a 10-cm focal length cylindrical lens as used before to optimize the power density at the fiber location and guarantee a good stability of the photo-writing conditions. The movement of the third mirror was controlled by a motor-driven translation stage, which was used to scan the laser along the fiber over a maximum length of 20 mm. To ensure a uniform refractive index modulation profile, a cache whose aperture is equal to the desired grating length was placed above the fiber, and the laser beam scan was started and stopped over the cache. To produce TFBGs, the phase mask was slightly angled with respect to the perpendicular to the optical fiber axis, as depicted in the inset of Fig. 1. The phase mask positioning was geometrically calibrated on the support, which was easily obtained thanks to the rectangular shape of the substrate. The photo-inscription of TFBGs generally requires much more laser power than uniform FBGs. This arises from the fact that the coupling efficiency of the grating decreases due to its angled fringes, as studied in [10]. Therefore, to mitigate the power limitation of the laser used in this work, POF sections were etched in a mixture of acetone and ethanol (ratio 2∶1), as done in [19], for the photo-inscription of highly reflective gratings in the same fiber type. This etching process allows decreasing the absorption of PMMA and hence producing high-contrast fringes in the fiber core. Fiber sections used in this work were set to 100 μm in diameter at the grating location. We have empirically found that this value remains the best trade-off in terms of highquality TFBGs production and fiber handling. Thinner fibers are indeed much more difficult to use in practice. Reflected and transmitted amplitude spectra were measured with an FBG interrogator (FS2200SA from FiberSensing), presenting a wavelength resolution equal to 1 pm. For this, POF sections were UV-glued (Norland 78) to two 8° angled silica single-mode fiber pigtails at both sides of POFs. The index matching glue avoids Fabry–Perot cavity effects, while angled fibers reduce Fresnel reflections. Figure 2 depicts the reflected and transmitted amplitude spectra of a 6-mm-long 3° TFBG photo-inscribed by scanning the laser beam along the phase mask at a velocity of 3 μm/s. The reflected amplitude spectrum displays several narrow bands between ∼1548 nm and ∼1552 nm, because POFs used in this experiment are not single-mode but few-mode fibers. The transmitted amplitude spectrum presents the same fundamental core mode resonances, which now appear as dips in the spectrum. Several tens of cladding mode resonances also appear at lower wavelengths extending from ∼1530 nm to ∼1548 nm. According to the TFBG phase matching conditions, each cladding mode resonance presents its own effective refractive index value [10]. Their FWHM does not exceed 100 pm, and the wavelength spacing between
Fig. 2. Reflected and transmitted amplitude spectra of a 6-mmlong 3° TFBG photo-written in slightly etched PMMA step-index POF (cladding diameter: 100 μm).
adjacent resonances lies in the range 0.4–0.7 nm (highorder modes are more spread than low-order ones). Figure 3 presents the transmitted amplitude spectra of three TFBGs with tilt angles of 1.5°, 3.0°, and 4.5°, respectively. Conclusions similar to the case of silica TFBGs can be drawn. For higher tilt angles, the spectral content increases thanks to coupling to higher cladding mode resonances, while the Bragg wavelength is red-shifted and the core mode resonance reflectivity decreases. An arrow below each curve identifies it. The reflectivity of the Bragg resonance was measured equal to 16% for the 1.5° TFBG, 12% for the 3.0° TFBG and 2% for the 4.5° TFBG. The experimental wavelength shift agrees well with the phase matching condition for the fundamental mode (λBragg 2neff Λ∕ cos θ, where neff is the effective refractive index, Λ is the grating period and θ is the tilt angle). It can also be noticed that the peak-to-peak amplitude of the cladding modes decreases with the tilt angle increasing, in accordance with [10]. The transmitted amplitude spectrum evolution of the 3° TFBG was then measured by immersing the TFBG in different calibrated liquids presenting well-known
Fig. 3. Transmitted amplitude spectra of 6-mm-long TFBGs photo-written at different tilt angles in slightly etched PMMA step-index POF (cladding diameter: 100 μm).
December 15, 2014 / Vol. 39, No. 24 / OPTICS LETTERS
refractive index values (Cargille oils). The grating was cleaned in buffer liquid before immersion in a new refractive index liquid. The grating response was instantaneous. Figure 4 shows the transmitted spectrum evolution as a function of the SRI in the range between 1.42 and 1.49. This range was considered according to the phase matching conditions and taking into account that cladding mode resonances extend ∼25 nm below the Bragg wavelength for the 3° TFBG. The last value is above the refractive index of the fiber cladding and is therefore sufficient to have all cladding modes radiated. For the readability of the figure, transmitted spectra are displayed with offsets in the vertical axis. It can be seen that the transmitted spectrum measured in n 1.42 liquid is similar to that measured in air, and then cladding mode resonances begin to disappear from the left to the right of the spectrum when the SRI value increases. As TFBGs produced in silica optical fibers [16–18], this behavior results from the fact that when the SRI reaches the effective refractive index of a cladding mode, the latter is no longer reflected at the cladding-surrounding medium interface but becomes radiated. Hence, it results in a perfect smoothing of the transmitted amplitude spectrum when SRI is not lower than the effective refractive index of a cladding mode. From the raw data displayed in Fig. 4, the refractometric sensitivity of the 3° TFBG was computed using two common approaches for silica TFBGs [16]. We first computed the area delimited by the cladding modes in the TFBG-transmitted amplitude spectrum. Figure 5 shows its evolution as a function of the SRI value. A monotonic decreasing trend is observed for all the investigated SRI range, with a maximum sensitivity between 1.45 and 1.48. It is precisely for this SRI range that TFBGs photo-written in silica optical fibers become insensitive, because the refractive index of the fiber cladding is 1.45, and all the cladding modes are therefore radiated. The inset of Fig. 5 presents the principle of the area computation, which is based on the determination of the upper and lower envelopes of the spectrum of the cladding modes. The second approach was based on the determination of the wavelength shift of selected cladding mode resonances as a function of the SRI. For this, the minimum
Fig. 4. Transmitted amplitude spectrum evolution (vertical scale with offset) as a function of the SRI value in the range 1.42–1.49.
6837
Fig. 5. Evolution of the area delimited by the cladding modes in the transmitted amplitude spectrum as a function of the SRI value (inset: principle of the computation of the area).
value of the resonance was tracked for different SRI values, and the corresponding wavelength was recorded. Figure 6 shows the corresponding results from three resonances of the 3° TFBG, respectively, around 1535, 1540, and 1545 nm. The wavelength shifts were normalized with respect to the resonance wavelength measured at 1.42. Figure 6 clearly confirms that each cladding mode presents its own refractometric sensitivity in a given SRI range, which depends on its effective refractive index. The maximum sensitivity is obtained at the upper limit of the sensitive region, when the mode reaches the cut-off wavelength (point at which the mode is no longer guided and becomes radiated). For the grating tested in our experiments, it is equal to ∼13 nm∕RIU, which is similar to the refractometric sensitivity reported for TFBGs in silica single-mode optical fibers [18]. The lower limit of SRI range can be further decreased by increasing the tilt angle, allowing the coupling to
Fig. 6. Relative wavelength shift of selected cladding modes as a function of the SRI value.
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OPTICS LETTERS / Vol. 39, No. 24 / December 15, 2014
higher order cladding modes, as provided by the 4.5° TFBG. Up to now, we have not been able to produce quality gratings with tilt angles higher than 5°. With continuous improvement of POF photosensitivity and optimized photo-writing processes, we believe that this limitation should be soon overcome. In conclusion, we have achieved the very first tilted Bragg gratings photo-inscribed in POFs. Taking into the limited power budget contrasting with the need for a high dose to manufacture TFBGs, POF sections were slightly etched prior to exposure to the UV writing beam. A 3° TFBG was characterized by immersion in calibrated refractive liquids, and a sensitivity reaching ∼13 nm∕RIU was reported in the SRI range 1.42–1.49. These first results pave the way to further developments of TFBGs in POFs and to their use for (bio)chemical sensing where POFs could preferably replace their silica counterparts, with improved bending tolerance and biocompatibility. This research was supported by the Belgian F.R.S.FNRS (Associate Research grant of C. Caucheteur), the European Research Council (Sarting grant of C. Caucheteur–Grant agreement N° 280161) and the Actions de la Recherche concertées programme (Prediction project). References 1. Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, IEEE Photon. Technol. Lett. 11, 352 (1999). 2. A. Stefani, M. Stecher, G. E. Town, and O. Bang, IEEE Photon. Technol. Lett. 24, 1148 (2012). 3. D. J. Webb, K. Kalli, K. Caroll, C. Zhang, M. Komodromos, A. Argyros, M. Large, G. Emiliyanov, O. Bang, and E. Kjaer, Proc. SPIE 6830, 683002 (2007).
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