Bragg gratings in a few mode microstructured polymer optical fiber in less than 30 seconds Ricardo Oliveira,* Lúcia Bilro, and Rogério Nogueira Instituto de Telecomunicações, Pólo de Aveiro, 3810-193 Aveiro, Portugal * [email protected]
Abstract: We report the inscription of a Bragg grating in an undoped polymethylmethacrylate based microstructured fiber in a time record. The fiber has been irradiated with a 248 nm ultraviolet radiation, through the phase mask technique using low fluence and low repetition rate. The experimental conditions were chosen to modify the core refractive index of the fiber at the incubation regime and avoiding polymer ablation. The peak reflection of the Bragg grating was centered in the infrared region with 20 dB reflection and 0.16 nm bandwidth. These spectral properties are well attractive for sensors and communications applications. ©2015 Optical Society of America OCIS codes: (060.3738) Fiber Bragg gratings, photosensitivity; (060.4005) Microstructured fibers; (160.5470) Polymers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
D. Webb and K. Kalli, “Polymer Fiber Bragg Gratings” in Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, A. Cusano, A. Cutolo, and J. Albert, eds. (Bentham Science Publishers, 2012), pp. 292–312. O. Rodríguez, F. Fornasiero, A. Arce, C. J. Radke, and J. M. Prausnitz, “Solubilities and diffusivities of water vapor in poly(methylmethacrylate), poly(2-hydroxyethylmethacrylate), poly(N-vinyl-2-pyrrolidone) and poly(acrylonitrile),” Polymer (Guildf.) 44(20), 6323 (2003). D. Saez-Rodriguez, J. L. Cruz, I. Johnson, D. J. Webb, M. C. J. Large, and A. Argyros, “Water diffusion into Uv inscripted long period grating in microstructured polymer fiber,” IEEE Sens. J. 10(7), 1169 (2010). C. Markos, W. Yuan, K. Vlachos, G. E. Town, and O. Bang, “Label-free biosensing with high sensitivity in dual-core microstructured polymer optical fibers,” Opt. Express 19(8), 7790–7798 (2011). Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, “Highly tunable Bragg gratings in single-mode polymer optical fibers,” Photon. Technol. Lett. 11(3), 352 (1999). C. Markos, I. Kubat, and O. Bang, “Hybrid polymer photonic crystal fiber with integrated chalcogenide glass nanofilms,” Sci. Rep. 4, 6057 (2014). M. J. Bowden, E. A. Chandross, and I. P. Kaminow, “Mechanism of the photoinduced refractive index increase in polymethyl methacrylate,” Appl. Opt. 13(1), 112–117 (1974). D. Sáez-Rodríguez, K. Nielsen, O. Bang, and D. J. Webb, “Photosensitivity mechanism of undoped poly(methyl methacrylate) under UV radiation at 325 nm and its spatial resolution limit,” Opt. Lett. 39(12), 3421–3424 (2014). H. Y. Liu, G. D. Peng, and P. L. Chu, “Polymer fiber Bragg gratings with 28-dB transmission rejection,” Photon. Technol. Lett. 14(7), 935 (2002). C. A. F. Marques, L. Bilro, N. J. Alberto, D. J. Webb, and R. Nogueira, “Narrow bandwidth Bragg gratings imprinted in polymer optical fibers for different spectral windows,” Opt. Commun. 307, 57–61 (2013). C. Markos, A. Stefani, K. Nielsen, H. K. Rasmussen, W. Yuan, and O. Bang, “High-Tg TOPAS microstructured polymer optical fiber for fiber Bragg grating strain sensing at 110 degrees,” Opt. Express 21(4), 4758–4765 (2013). G. D. Peng, Z. Xiong, and P. L. Chu, “Photosensitivity and gratings in dye-doped polymer optical fibers,” Opt. Fiber Technol. 5(2), 242 (1999). H. Dobb, D. J. Webb, K. Kalli, A. Argyros, M. C. J. Large, and M. A. van Eijkelenborg, “Continuous wave ultraviolet light-induced fiber Bragg gratings in few- and single-mode microstructured polymer optical fibers,” Opt. Lett. 30(24), 3296–3298 (2005). I. L. Bundalo, K. Nielsen, C. Markos, and O. Bang, “Bragg grating writing in PMMA microstructured polymer optical fibers in less than 7 minutes,” Opt. Express 22(5), 5270–5276 (2014). R. Oliveira, C. A. F. Marques, L. Bilro, and R. N. Nogueira, “Production and characterization of Bragg gratings in polymer optical fibers for sensors and optical communications,” Proc. SPIE 9157, 915794 (2014). S. Küper and M. Stuke, “UV-excimer-laser ablation of polymethylmethacrylate at 248 nm: characterization of incubation sites with Fourier transform IR-and UV-spectroscopy,” Appl. Phys., A Mater. Sci. Process. 49, 211– 215 (1989).
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10181
17. R. Srinivasan, B. Braren, R. W. Dreyfus, L. Hadel, and D. E. Seeger, “Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies,” J. Opt. Soc. Am. B 3(5), 785 (1986). 18. A. K. Baker and P. E. Dyer, “Refractive-index modification of polymethylmethacrylate (PMMA) thin films by KrF-laser irradiation,” Appl. Phys., A Mater. Sci. Process. 57(6), 543 (1993). 19. C. Wochnowski, S. Metev, and G. Sepold, “UV–laser-assisted modification of the optical properties of polymethylmethacrylate,” Appl. Surf. Sci. 154–155, 706–711 (2000). 20. H. Y. Liu, H. B. Liu, G. D. Peng, and P. L. Chu, “Polymer optical fibre Bragg gratings based fibre laser,” Opt. Commun. 266(1), 132 (2006). 21. H. Y. Liu, H. B. Liu, G. D. Peng, and P. L. Chu, “Observation of type I and type II gratings behavior in polymer optical fiber,” Opt. Commun. 220(4-6), 337 (2003). 22. R. Kashyap, Fiber Bragg Gratings (Academic, 2010).
1. Introduction Bragg gratings in polymer optical fibers (PFBGs) have been pointed as a promising technology due the advantages of polymers over silica. In fact, polymer optical fibers (POFs) are mainly composed of polymethylmethacrylate (PMMA), a material with a Young modulus thirty times lower than silica [1]. Together with this feature, PMMA has higher tensile strength, higher thermo-optic coefficient [1], high water absorption capabilities [2,3] and biological compatibility [4]. These advantages are well attractive especially when large strains can be imposed in POFs without breaking the fiber [5]. Additionally the possibility of combining microstructured polymer optical fibers (mPOFs) with soft glass for nonlinear applications is also being subject of interest [6]. The first PFBG was reported in 1999 using a 325 nm laser source [5], being the preferred wavelength for PFBG inscription in the scientific community, since then. Their results were done in a multimode dye-doped POF and the reflection spectrum of the Bragg grating formed was due to the periodic refractive index change (RIC) associated with the photosensitivity at that wavelength. Bowden et al. reported the mechanisms involved on the PMMA photosensitivity at 325 nm [7]. They concluded that the increase of the PMMA refractive index in the irradiated region was due to the photoinduced polymerization of the unreacted monomer (1-2%) resulting in an increase of molecular density (and hence refractive index) [7]. Recently it was proven that the main mechanism behind the photosensitivity in an undoped PMMA based mPOF under 325 nm ultraviolet (UV) light, is a competitive process between photodegradation and polymerization [8]. To date, PFBGs, have been written in different spectral windows in doped and undoped step-index (SI) (i.e [8–10].), microstructured [10–14] and graded-index (GI) POFs [15]with writing wavelengths near 325 nm. Nevertheless, it is known that the inscription of PFBGs is a time consuming process, where tens of minutes can take place [9, 13]. Until now the record inscription time was obtained in less than 7 minutes [14], which is well below than the previously reported works. It should be noted that when tens of minutes are underway in the inscription process, implies in some way practical challenges, due to the stability needed in the inscription setup. In the present work this problem was overcome with the use of a 248 nm excimer (KrF) laser. Until now, this UV light was not considered suitable for PFBG inscription because, according to Peng et al., the fiber absorption at this wavelength was very high [5, 12]. This conclusion was obtained since the Bragg grating created on a polymer preform was a surface relief grating [5]. This result may perhaps be explained by the use of high fluences together with high exposure time, prompting to a periodic removal and ablation of the polymer preform surface. The PMMA photosensitivity to the UV light is still not well understood in the current days. Indeed, in addition to the PMMA production process, the wavelength and power of the light source are key factors for the photosensitive mechanisms, leading to different chemical reactions inside the PMMA chains. Küper and Stuke [16] performed infrared and UV spectroscopic experiments with PMMA films irradiated with 248 nm light sources, among others. Results allowed the explanation of an incubation process on PMMA at 248 nm well below the reported PMMA ablation threshold [17]. Using low repetition rates (R < 6Hz) and
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10182
low fluences (I = [35-40] mJ/cm2), the UV and infrared spectra presented new absorptions bands (ester bands) concluding the formation of photoproducts in the exposed areas including methyl formate. These results were confirmed by a subsequent study done by Srinivasan et al. [17]. Baker et al. [18], in a study based on the work done by Küper and Stuke, showed a small but significant change in the refractive index of a PMMA film using a KrF laser irradiation together with low repetition rates and also low fluences (R < 5 Hz and I = 40 mJ/cm2) in order to minimize the ablative removal of PMMA. The RIC increased with the increasing number of pulses presenting a value of 10−2 after 2000 pulses [18]. Another interesting detailed work regarding the refractive index modification with 248 nm laser light at low fluences (I = 17 mJ/cm2 with R = 5 Hz) was presented by Wochnowski et al. [19]. In this work it was concluded that the RIC was due to a Norrish Type I photochemical reaction. This means that a complete separation of the side chain from the PMMA molecule take place, promoting a volume contraction by the Van der Waals interactions and consequently increase of the RIC [19]. In this work, to the author’s best knowledge, a 248 nm UV light with suited laser parameters was used for the first time to inscribe a high quality Bragg grating in an undoped mPOF in few seconds. The fast growing of the PFBG was due to the strong absorption of the PMMA together with the high photosensitivity at this wavelength. The energy density was set to be below the threshold for the PMMA ablation. At the same time R was also set to its minimum (1 Hz), in order to give time to reduce the temperature that is raised after each irradiation pulse and thus, avoiding the ablative process related with temperature. With this new approach, a Bragg grating in an mPOF was successfully inscribed in a time record of 20 seconds. The spectral properties of the grating were not compromised, presenting a rejection band of more than 20 dB as well as a bandwidth of about 0.16 nm at 3 dB. 2. Inscription setup For the Bragg gratings inscription a KrF Bragg Star TM Industrial-LN excimer laser operating at 248 nm was used. The laser has a beam spot of 6 mm in width and 1.5 mm in height, with pulse duration of 15 ns. The fiber used in this work is composed of an undoped PMMA and it is a few mode (FM) mPOF (Kiriama Pty Ltd). The fiber has 250 μm cladding, 18 μm core, 6 hole layers spaced by a pitch ( Λ ) of 6.2 μm and the holes diameter is 3.2 μm. The estimated attenuation is around 3 dB/cm in the infrared region. Prior to the inscription process, both ends of the 15 cm mPOF were cleaved by hand with a hot blade and then polished with suited tools to give a smooth flat end face (see Fig. 1).
Fig. 1. End face of the polished FM mPOF used for the inscription process.
The Bragg grating was inscribed with the phase mask technique. A cylindrical lens, followed by a slit with 4.5 mm width, shapes the beam before it arrives to the phase mask, designed for 248 nm operation, with a uniform period of 1023 nm. The fiber was placed within two clamps and kept in strain to avoid undesired curvatures. A temporary connection was made using a SM silica pigtail fiber (9/125 μm) cleaved at 8° angle and the FM mPOF. A small amount of index matching gel was used in the coupling to reduce Fresnel reflections and at the same time lowering the background noise. Furthermore, to ensure that the mode is propagated into the core of the mPOF, light was injected into the silica pigtail fiber and, at the end of the polished mPOF, a 20X
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10183
magnification lens followed by a beam profiler was placed to check if the modes were being propagated in the fiber core. After correct alignment of the silica pigtail fiber related to the mPOF, an interrogation system with wavelength accuracy of 1 pm was used to monitor the grating growing in real time. Thus the saturation time is easily known, which avoids too much exposure time. The entire setup can be seen in Fig. 2.
Fig. 2. Setup used for the inscription of Bragg gratings in real time in an mPOF by the phase mask technique.
It should be noted that both R and energy where tailored to low values in order to avoid ablation of the PMMA material. 3. Results and discussion Before the inscription process, the parameters as repetition rate and fluence were investigated, ensuring that the surface of the polymer fiber was not damaged/ablated. Thus, the POF was fixed on the inscription setup, and R was set to 50 Hz, keeping I = 33 mJ/cm2 (below the reported threshold [17]), during a few seconds. With that trial, it was concluded that R was too high because the surface appears to be damaged by naked eye. The same conclusion was obtained for the same fluence but for R = 5 Hz during 60 seconds. It was observed that there was still damage of the fiber surface as shown in Fig. 3(a) and this was mainly due to the rise of temperature on the fiber. Therefore, the laser parameters were set to its minimum, with R = 1 Hz, and pulse energy of 3 mJ, giving a fluence of I = 33 mJ/cm2, during the same period of time. By doing that, the cumulative heating on the fiber is minimized to avoid overheating and thus the occurrence of the ablative process. As can be seen in Fig. 3(b), no ablation on the fiber surface was observed, proving clear evidences that those parameters were suited for the trials related with Bragg gratings inscription.
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10184
Fig. 3. Microscope images of the FM mPOF surface, exposed to 248 nm UV light with the phase mask technique, when R was changed from 5 Hz (a) to 1 Hz (b), with the same fluence (I = 33 mJ/cm2, and exposure time (t = 60 seconds). The image in (a), clearly shows surface ablation.
During the inscription process each spectrum acquired from the interrogation system was recorded for further analysis. Additionally, care was taken to know when the saturation time was achieved. For that, the peak power was seen in real time by the interrogator allowing to know when the UV laser should be stopped. The results for the grating inscribed in a FM mPOF, 3 cm away from the coupling to the silica pigtail fiber are shown in Fig. 4. The acquired PFBG spectra shows an asymmetry in its shape, this is probably due the nonuniform distribution of the laser intensity as already reported [20] or even due the orientation of the microstructure against the laser beam, that can lead to a non-uniform grating [13].
Fig. 4. Inscription of a Bragg grating in a FM mPOF during time, (each spectra was taken with a period of 1s). Inscription parameters: L = 4.5 mm, Λ = 1023 nm, R = 1 Hz, I = 33 mJ/cm2, and N ≈ 20 pulses.
The peak power and the reflection Bragg wavelength evolution can be seen in Fig. 5. The optimum irradiation time was estimated to be around 20 seconds meaning that only 20 pulses where needed to produce the best RIC for the Bragg condition. Figure 5(b) shows that the Bragg wavelength was blue shifted during the writing process maintaining its value after the irradiation process. As known, PMMA has a negative thermo-optic coefficient and thus it
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10185
could explain the blue shift. However there was no red shift after the irradiation was stopped, leading us to exclude this possibility. Thus we believe that we are in the presence of a type I PFBG, as already reported by Liu et al. [21], where the gratings are formed mainly due to the refractive index change in the core. The core mean RIC can be estimated by [22]:
Δneff =
neff .δλ
2.λB0
(1)
where neff is the effective refractive index of the core, δλ is the wavelength shift for the grating with maximum reflectivity, λB0 is the nominal Bragg wavelength when the refractive index modulation is not considered. The calculated value was 2.38 x 10−4, which is one order of magnitude lower than the values reported in the literature for PFBGs at 325 nm using SI [21] or mPOFs with a few hole layers [8]. This low value is due to the low visibility of the interference fringes created on the mPOF core, which is directly related with the backscattered light on the microstructure. A low RIC in a 4.5 mm grating allows to have a high reflectivity with a high peak to side lobe ratio in the reflection spectrum.
Fig. 5. Evolution of the, (a) peak wavelength and (b) peak power, of the grating inscribed 3 cm away from the polymer fiber end face, with L = 4.5 mm, Λ = 1023 nm, R = 1 Hz, I = 33 mJ/cm2, and N ≈ 20 pulses.
After the PFBG have been produced, a strain characterization was performed in order to show the spectral dependence of the Bragg reflection peak with strain. The results are shown in Fig. 6 and as it can be seen the Bragg wavelength shift (δλ) was linearly red shifted 35 nm with 2% deformation. After adjusting a linear regression model, the sensitivity obtained was 1.65 pm/με, which is similar to the values already reported for PFBGs written with 325 nm laser source [15, 20].
Fig. 6. Bragg wavelength shift obtained from the inscribed PFBG under different strains.
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10186
4. Conclusion
In this work, the successful inscription of a Bragg grating in an mPOF in a few seconds was presented. The inscription process is fast and feasible, avoiding constrains related with the stability of the mechanical setup during the inscription process. The fast growing achieved is related with the high absorption/high photosensitivity of the PMMA under 248 nm UV light. The key factors for the success were the use of low fluence, low repetition rate and also low number of pulses in contrast with the reported in literature. Thus the RIC was due to an incubation phenomenon well below the PMMA ablation threshold. This RIC could be caused by the complete side chain removal of the PMMA molecule leading to a densification [19]. The Van der Waals forces pull neighboring molecules closer together promoting a volume contraction. Inherently there is a RIC increase in the exposed regions, as already reported by others [19]. The PFBG was recorded in the core of a FM mPOF and presents a rejection band of more than 20 dB and a bandwidth of 0.16 nm at 3 dB. The spectral dependence of the PFBG with strain is linearly defined in the elastic regime with a sensitivity of 1.65 pm/με. The current system used for the inscription of a Bragg grating in a FM mPOF, can also be used to write gratings in other types of POF. Additionally this work shows that common Bragg grating systems designed for silica fibers can be used to inscribe PFBGs, paving the way for a more intense use of PFBGs in both telecommunications and sensors industries. Acknowledgments
This work was funded by National Funds through FCT - Fundação para a Ciência e ecnologia under the projects POFCOM (PTDC/EEATEL/122792/2010), UID/EEA/50008/2013, PEstOE/EEI/LA0008/2013; scholarships (SFRH/BD/88472/2012 and SFRH/BPD/78205/2011).
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10187
Abstract: We report the inscription of a Bragg grating in an undoped polymethylmethacrylate based microstructured fiber in a time record. The fiber has been irradiated with a 248 nm ultraviolet radiation, through the phase mask technique using low fluence and low repetition rate. The experimental conditions were chosen to modify the core refractive index of the fiber at the incubation regime and avoiding polymer ablation. The peak reflection of the Bragg grating was centered in the infrared region with 20 dB reflection and 0.16 nm bandwidth. These spectral properties are well attractive for sensors and communications applications. ©2015 Optical Society of America OCIS codes: (060.3738) Fiber Bragg gratings, photosensitivity; (060.4005) Microstructured fibers; (160.5470) Polymers.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
D. Webb and K. Kalli, “Polymer Fiber Bragg Gratings” in Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, A. Cusano, A. Cutolo, and J. Albert, eds. (Bentham Science Publishers, 2012), pp. 292–312. O. Rodríguez, F. Fornasiero, A. Arce, C. J. Radke, and J. M. Prausnitz, “Solubilities and diffusivities of water vapor in poly(methylmethacrylate), poly(2-hydroxyethylmethacrylate), poly(N-vinyl-2-pyrrolidone) and poly(acrylonitrile),” Polymer (Guildf.) 44(20), 6323 (2003). D. Saez-Rodriguez, J. L. Cruz, I. Johnson, D. J. Webb, M. C. J. Large, and A. Argyros, “Water diffusion into Uv inscripted long period grating in microstructured polymer fiber,” IEEE Sens. J. 10(7), 1169 (2010). C. Markos, W. Yuan, K. Vlachos, G. E. Town, and O. Bang, “Label-free biosensing with high sensitivity in dual-core microstructured polymer optical fibers,” Opt. Express 19(8), 7790–7798 (2011). Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, “Highly tunable Bragg gratings in single-mode polymer optical fibers,” Photon. Technol. Lett. 11(3), 352 (1999). C. Markos, I. Kubat, and O. Bang, “Hybrid polymer photonic crystal fiber with integrated chalcogenide glass nanofilms,” Sci. Rep. 4, 6057 (2014). M. J. Bowden, E. A. Chandross, and I. P. Kaminow, “Mechanism of the photoinduced refractive index increase in polymethyl methacrylate,” Appl. Opt. 13(1), 112–117 (1974). D. Sáez-Rodríguez, K. Nielsen, O. Bang, and D. J. Webb, “Photosensitivity mechanism of undoped poly(methyl methacrylate) under UV radiation at 325 nm and its spatial resolution limit,” Opt. Lett. 39(12), 3421–3424 (2014). H. Y. Liu, G. D. Peng, and P. L. Chu, “Polymer fiber Bragg gratings with 28-dB transmission rejection,” Photon. Technol. Lett. 14(7), 935 (2002). C. A. F. Marques, L. Bilro, N. J. Alberto, D. J. Webb, and R. Nogueira, “Narrow bandwidth Bragg gratings imprinted in polymer optical fibers for different spectral windows,” Opt. Commun. 307, 57–61 (2013). C. Markos, A. Stefani, K. Nielsen, H. K. Rasmussen, W. Yuan, and O. Bang, “High-Tg TOPAS microstructured polymer optical fiber for fiber Bragg grating strain sensing at 110 degrees,” Opt. Express 21(4), 4758–4765 (2013). G. D. Peng, Z. Xiong, and P. L. Chu, “Photosensitivity and gratings in dye-doped polymer optical fibers,” Opt. Fiber Technol. 5(2), 242 (1999). H. Dobb, D. J. Webb, K. Kalli, A. Argyros, M. C. J. Large, and M. A. van Eijkelenborg, “Continuous wave ultraviolet light-induced fiber Bragg gratings in few- and single-mode microstructured polymer optical fibers,” Opt. Lett. 30(24), 3296–3298 (2005). I. L. Bundalo, K. Nielsen, C. Markos, and O. Bang, “Bragg grating writing in PMMA microstructured polymer optical fibers in less than 7 minutes,” Opt. Express 22(5), 5270–5276 (2014). R. Oliveira, C. A. F. Marques, L. Bilro, and R. N. Nogueira, “Production and characterization of Bragg gratings in polymer optical fibers for sensors and optical communications,” Proc. SPIE 9157, 915794 (2014). S. Küper and M. Stuke, “UV-excimer-laser ablation of polymethylmethacrylate at 248 nm: characterization of incubation sites with Fourier transform IR-and UV-spectroscopy,” Appl. Phys., A Mater. Sci. Process. 49, 211– 215 (1989).
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10181
17. R. Srinivasan, B. Braren, R. W. Dreyfus, L. Hadel, and D. E. Seeger, “Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies,” J. Opt. Soc. Am. B 3(5), 785 (1986). 18. A. K. Baker and P. E. Dyer, “Refractive-index modification of polymethylmethacrylate (PMMA) thin films by KrF-laser irradiation,” Appl. Phys., A Mater. Sci. Process. 57(6), 543 (1993). 19. C. Wochnowski, S. Metev, and G. Sepold, “UV–laser-assisted modification of the optical properties of polymethylmethacrylate,” Appl. Surf. Sci. 154–155, 706–711 (2000). 20. H. Y. Liu, H. B. Liu, G. D. Peng, and P. L. Chu, “Polymer optical fibre Bragg gratings based fibre laser,” Opt. Commun. 266(1), 132 (2006). 21. H. Y. Liu, H. B. Liu, G. D. Peng, and P. L. Chu, “Observation of type I and type II gratings behavior in polymer optical fiber,” Opt. Commun. 220(4-6), 337 (2003). 22. R. Kashyap, Fiber Bragg Gratings (Academic, 2010).
1. Introduction Bragg gratings in polymer optical fibers (PFBGs) have been pointed as a promising technology due the advantages of polymers over silica. In fact, polymer optical fibers (POFs) are mainly composed of polymethylmethacrylate (PMMA), a material with a Young modulus thirty times lower than silica [1]. Together with this feature, PMMA has higher tensile strength, higher thermo-optic coefficient [1], high water absorption capabilities [2,3] and biological compatibility [4]. These advantages are well attractive especially when large strains can be imposed in POFs without breaking the fiber [5]. Additionally the possibility of combining microstructured polymer optical fibers (mPOFs) with soft glass for nonlinear applications is also being subject of interest [6]. The first PFBG was reported in 1999 using a 325 nm laser source [5], being the preferred wavelength for PFBG inscription in the scientific community, since then. Their results were done in a multimode dye-doped POF and the reflection spectrum of the Bragg grating formed was due to the periodic refractive index change (RIC) associated with the photosensitivity at that wavelength. Bowden et al. reported the mechanisms involved on the PMMA photosensitivity at 325 nm [7]. They concluded that the increase of the PMMA refractive index in the irradiated region was due to the photoinduced polymerization of the unreacted monomer (1-2%) resulting in an increase of molecular density (and hence refractive index) [7]. Recently it was proven that the main mechanism behind the photosensitivity in an undoped PMMA based mPOF under 325 nm ultraviolet (UV) light, is a competitive process between photodegradation and polymerization [8]. To date, PFBGs, have been written in different spectral windows in doped and undoped step-index (SI) (i.e [8–10].), microstructured [10–14] and graded-index (GI) POFs [15]with writing wavelengths near 325 nm. Nevertheless, it is known that the inscription of PFBGs is a time consuming process, where tens of minutes can take place [9, 13]. Until now the record inscription time was obtained in less than 7 minutes [14], which is well below than the previously reported works. It should be noted that when tens of minutes are underway in the inscription process, implies in some way practical challenges, due to the stability needed in the inscription setup. In the present work this problem was overcome with the use of a 248 nm excimer (KrF) laser. Until now, this UV light was not considered suitable for PFBG inscription because, according to Peng et al., the fiber absorption at this wavelength was very high [5, 12]. This conclusion was obtained since the Bragg grating created on a polymer preform was a surface relief grating [5]. This result may perhaps be explained by the use of high fluences together with high exposure time, prompting to a periodic removal and ablation of the polymer preform surface. The PMMA photosensitivity to the UV light is still not well understood in the current days. Indeed, in addition to the PMMA production process, the wavelength and power of the light source are key factors for the photosensitive mechanisms, leading to different chemical reactions inside the PMMA chains. Küper and Stuke [16] performed infrared and UV spectroscopic experiments with PMMA films irradiated with 248 nm light sources, among others. Results allowed the explanation of an incubation process on PMMA at 248 nm well below the reported PMMA ablation threshold [17]. Using low repetition rates (R < 6Hz) and
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10182
low fluences (I = [35-40] mJ/cm2), the UV and infrared spectra presented new absorptions bands (ester bands) concluding the formation of photoproducts in the exposed areas including methyl formate. These results were confirmed by a subsequent study done by Srinivasan et al. [17]. Baker et al. [18], in a study based on the work done by Küper and Stuke, showed a small but significant change in the refractive index of a PMMA film using a KrF laser irradiation together with low repetition rates and also low fluences (R < 5 Hz and I = 40 mJ/cm2) in order to minimize the ablative removal of PMMA. The RIC increased with the increasing number of pulses presenting a value of 10−2 after 2000 pulses [18]. Another interesting detailed work regarding the refractive index modification with 248 nm laser light at low fluences (I = 17 mJ/cm2 with R = 5 Hz) was presented by Wochnowski et al. [19]. In this work it was concluded that the RIC was due to a Norrish Type I photochemical reaction. This means that a complete separation of the side chain from the PMMA molecule take place, promoting a volume contraction by the Van der Waals interactions and consequently increase of the RIC [19]. In this work, to the author’s best knowledge, a 248 nm UV light with suited laser parameters was used for the first time to inscribe a high quality Bragg grating in an undoped mPOF in few seconds. The fast growing of the PFBG was due to the strong absorption of the PMMA together with the high photosensitivity at this wavelength. The energy density was set to be below the threshold for the PMMA ablation. At the same time R was also set to its minimum (1 Hz), in order to give time to reduce the temperature that is raised after each irradiation pulse and thus, avoiding the ablative process related with temperature. With this new approach, a Bragg grating in an mPOF was successfully inscribed in a time record of 20 seconds. The spectral properties of the grating were not compromised, presenting a rejection band of more than 20 dB as well as a bandwidth of about 0.16 nm at 3 dB. 2. Inscription setup For the Bragg gratings inscription a KrF Bragg Star TM Industrial-LN excimer laser operating at 248 nm was used. The laser has a beam spot of 6 mm in width and 1.5 mm in height, with pulse duration of 15 ns. The fiber used in this work is composed of an undoped PMMA and it is a few mode (FM) mPOF (Kiriama Pty Ltd). The fiber has 250 μm cladding, 18 μm core, 6 hole layers spaced by a pitch ( Λ ) of 6.2 μm and the holes diameter is 3.2 μm. The estimated attenuation is around 3 dB/cm in the infrared region. Prior to the inscription process, both ends of the 15 cm mPOF were cleaved by hand with a hot blade and then polished with suited tools to give a smooth flat end face (see Fig. 1).
Fig. 1. End face of the polished FM mPOF used for the inscription process.
The Bragg grating was inscribed with the phase mask technique. A cylindrical lens, followed by a slit with 4.5 mm width, shapes the beam before it arrives to the phase mask, designed for 248 nm operation, with a uniform period of 1023 nm. The fiber was placed within two clamps and kept in strain to avoid undesired curvatures. A temporary connection was made using a SM silica pigtail fiber (9/125 μm) cleaved at 8° angle and the FM mPOF. A small amount of index matching gel was used in the coupling to reduce Fresnel reflections and at the same time lowering the background noise. Furthermore, to ensure that the mode is propagated into the core of the mPOF, light was injected into the silica pigtail fiber and, at the end of the polished mPOF, a 20X
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10183
magnification lens followed by a beam profiler was placed to check if the modes were being propagated in the fiber core. After correct alignment of the silica pigtail fiber related to the mPOF, an interrogation system with wavelength accuracy of 1 pm was used to monitor the grating growing in real time. Thus the saturation time is easily known, which avoids too much exposure time. The entire setup can be seen in Fig. 2.
Fig. 2. Setup used for the inscription of Bragg gratings in real time in an mPOF by the phase mask technique.
It should be noted that both R and energy where tailored to low values in order to avoid ablation of the PMMA material. 3. Results and discussion Before the inscription process, the parameters as repetition rate and fluence were investigated, ensuring that the surface of the polymer fiber was not damaged/ablated. Thus, the POF was fixed on the inscription setup, and R was set to 50 Hz, keeping I = 33 mJ/cm2 (below the reported threshold [17]), during a few seconds. With that trial, it was concluded that R was too high because the surface appears to be damaged by naked eye. The same conclusion was obtained for the same fluence but for R = 5 Hz during 60 seconds. It was observed that there was still damage of the fiber surface as shown in Fig. 3(a) and this was mainly due to the rise of temperature on the fiber. Therefore, the laser parameters were set to its minimum, with R = 1 Hz, and pulse energy of 3 mJ, giving a fluence of I = 33 mJ/cm2, during the same period of time. By doing that, the cumulative heating on the fiber is minimized to avoid overheating and thus the occurrence of the ablative process. As can be seen in Fig. 3(b), no ablation on the fiber surface was observed, proving clear evidences that those parameters were suited for the trials related with Bragg gratings inscription.
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10184
Fig. 3. Microscope images of the FM mPOF surface, exposed to 248 nm UV light with the phase mask technique, when R was changed from 5 Hz (a) to 1 Hz (b), with the same fluence (I = 33 mJ/cm2, and exposure time (t = 60 seconds). The image in (a), clearly shows surface ablation.
During the inscription process each spectrum acquired from the interrogation system was recorded for further analysis. Additionally, care was taken to know when the saturation time was achieved. For that, the peak power was seen in real time by the interrogator allowing to know when the UV laser should be stopped. The results for the grating inscribed in a FM mPOF, 3 cm away from the coupling to the silica pigtail fiber are shown in Fig. 4. The acquired PFBG spectra shows an asymmetry in its shape, this is probably due the nonuniform distribution of the laser intensity as already reported [20] or even due the orientation of the microstructure against the laser beam, that can lead to a non-uniform grating [13].
Fig. 4. Inscription of a Bragg grating in a FM mPOF during time, (each spectra was taken with a period of 1s). Inscription parameters: L = 4.5 mm, Λ = 1023 nm, R = 1 Hz, I = 33 mJ/cm2, and N ≈ 20 pulses.
The peak power and the reflection Bragg wavelength evolution can be seen in Fig. 5. The optimum irradiation time was estimated to be around 20 seconds meaning that only 20 pulses where needed to produce the best RIC for the Bragg condition. Figure 5(b) shows that the Bragg wavelength was blue shifted during the writing process maintaining its value after the irradiation process. As known, PMMA has a negative thermo-optic coefficient and thus it
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10185
could explain the blue shift. However there was no red shift after the irradiation was stopped, leading us to exclude this possibility. Thus we believe that we are in the presence of a type I PFBG, as already reported by Liu et al. [21], where the gratings are formed mainly due to the refractive index change in the core. The core mean RIC can be estimated by [22]:
Δneff =
neff .δλ
2.λB0
(1)
where neff is the effective refractive index of the core, δλ is the wavelength shift for the grating with maximum reflectivity, λB0 is the nominal Bragg wavelength when the refractive index modulation is not considered. The calculated value was 2.38 x 10−4, which is one order of magnitude lower than the values reported in the literature for PFBGs at 325 nm using SI [21] or mPOFs with a few hole layers [8]. This low value is due to the low visibility of the interference fringes created on the mPOF core, which is directly related with the backscattered light on the microstructure. A low RIC in a 4.5 mm grating allows to have a high reflectivity with a high peak to side lobe ratio in the reflection spectrum.
Fig. 5. Evolution of the, (a) peak wavelength and (b) peak power, of the grating inscribed 3 cm away from the polymer fiber end face, with L = 4.5 mm, Λ = 1023 nm, R = 1 Hz, I = 33 mJ/cm2, and N ≈ 20 pulses.
After the PFBG have been produced, a strain characterization was performed in order to show the spectral dependence of the Bragg reflection peak with strain. The results are shown in Fig. 6 and as it can be seen the Bragg wavelength shift (δλ) was linearly red shifted 35 nm with 2% deformation. After adjusting a linear regression model, the sensitivity obtained was 1.65 pm/με, which is similar to the values already reported for PFBGs written with 325 nm laser source [15, 20].
Fig. 6. Bragg wavelength shift obtained from the inscribed PFBG under different strains.
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10186
4. Conclusion
In this work, the successful inscription of a Bragg grating in an mPOF in a few seconds was presented. The inscription process is fast and feasible, avoiding constrains related with the stability of the mechanical setup during the inscription process. The fast growing achieved is related with the high absorption/high photosensitivity of the PMMA under 248 nm UV light. The key factors for the success were the use of low fluence, low repetition rate and also low number of pulses in contrast with the reported in literature. Thus the RIC was due to an incubation phenomenon well below the PMMA ablation threshold. This RIC could be caused by the complete side chain removal of the PMMA molecule leading to a densification [19]. The Van der Waals forces pull neighboring molecules closer together promoting a volume contraction. Inherently there is a RIC increase in the exposed regions, as already reported by others [19]. The PFBG was recorded in the core of a FM mPOF and presents a rejection band of more than 20 dB and a bandwidth of 0.16 nm at 3 dB. The spectral dependence of the PFBG with strain is linearly defined in the elastic regime with a sensitivity of 1.65 pm/με. The current system used for the inscription of a Bragg grating in a FM mPOF, can also be used to write gratings in other types of POF. Additionally this work shows that common Bragg grating systems designed for silica fibers can be used to inscribe PFBGs, paving the way for a more intense use of PFBGs in both telecommunications and sensors industries. Acknowledgments
This work was funded by National Funds through FCT - Fundação para a Ciência e ecnologia under the projects POFCOM (PTDC/EEATEL/122792/2010), UID/EEA/50008/2013, PEstOE/EEI/LA0008/2013; scholarships (SFRH/BD/88472/2012 and SFRH/BPD/78205/2011).
#231171 - $15.00 USD (C) 2015 OSA
Received 26 Dec 2014; revised 20 Feb 2015; accepted 23 Feb 2015; published 13 Apr 2015 20 Apr 2015 | Vol. 23, No. 8 | DOI:10.1364/OE.23.010181 | OPTICS EXPRESS 10187
