Fabrication of Long-Period Fiber Gratings by CO 2 Laser Irradiations for High Temperature Applications Tao Wei a, John Montoya a, Jian Zhang b,junhang Dong b, Hai Xiao a* a Department of Electrical and Computer Engineering, University of Missouri-Rolla, 187 Miner Circle, Rolla, MO 6549; b Department of Chemical and Materials Engineering, University of Cincinnati, 26 Clifton Ave. Cincinnati, OH 45221 ABSTRACT We report in this paper the fabrication of high performance thermal LPFGs by point-by-point CO 2 laser irradiations. These thermal LPFGs have shown much better temperature tolerance and promised applications in high temperature harsh environments. The computer-controlled fabrication system with in situ signal monitoring capability is described. The fabricated LPFGs survived high temperatures up to 8 C. Long term stability test at 55 C for 2 hours indicated that thermal shock at a higher temperature could significantly reduce the drift. Keywords: Long period fiber gratings, CO 2 laser, Fiber sensors, Gas sensing 1. INTRODUCTION Long Period Fiber Gratings (LPFGs) have many advantages such as small insertion loss, low retro-reflection, good sensory property and low cost. They have found many applications in both optical fiber communication and sensing. 1, 2 LPFGs are typically fabricated by inducing periodic refractive index modulation in the fiber core through transverse irradiation with ultraviolet light. 3, 4 However, the UV induced refractive index changes cannot survive high temperature, which has triggered an increasing interest in finding alternative methods to fabricate LPFGs for applications in high temperature harsh environment. 5 It has been reported that LPFGs fabricated by CO 2 laser irradiation or electric arc exposure can survive a temperature up to 12 o C. 6, 7,8, 9 In these two methods, the localized rapid heating and subsequent cooling of the optical fiber result in the stress relief-induced refractive index changes inside the fiber core. Meanwhile, it has been reported that LPFG coated with chemical sensitive material can serve as effective chemical sensor. 1, 11 The sensor operates based on the mechanism that the gas concentration change causes a refractive index (RI) change of coated chemical sensitive material which, consequently, changes the boundary condition of cladding modes of the LPFG. As a result, the resonant wavelength of the LPFG shifts accordingly. By measuring the resonant wavelength shift, the amount of gas presented in the surrounding environment can be detected. 12 In this paper, we report the fabrication of high performance thermal LPFGs by point-by-point CO 2 laser irradiations. These thermal LPFGs have shown high temperature tolerance and promised applications in high temperature harsh environments. Furthermore, we found that by applying thermal shock at an elevated temperature, the thermal stability of the LPFG could be significantly improved. 2. FABRICATION The LPFG inscription method we used is similar to that described in Ref. 6 and Ref. 7. As shown in Fig.1, a CO 2 laser (SYNRAD, Inc.) with a free space wavelength of 1.6µm was used in our system. A ZnSe cylindrical lens with a focal length of 5mm was used to shape the CO 2 laser beam into a narrow line with a width of 22µm. The focused laser beam was transversely loaded onto the single mode optical fiber (Corning SMF-28) mounted on a three-dimensional (3D) translation stage. Controlled by a computer, the translation stage moves the fiber at fixed stop for laser exposure, resulting in a periodic refractive index modulation in the fiber core. The laser power was set to be 8W with an exposure * Further author information: (Send correspondence to Dr. Hai Xiao) Hai Xiao: E-mail: xiaoha@umr.edu, Telephone: 1 573 341 6887 Sensors for Harsh Environments III, edited by Hai Xiao, Anbo Wang Proc. of SPIE Vol. 6757, 67578, (27) 277-786X/7/$18 doi: 1.1117/12.731372 Proc. of SPIE Vol. 6757 67578-1
time of 1ms during LPFG fabrication. The period used was around 5µm and the length of the grating was about 5cm. During grating fabrication, a tunable laser (HP81642A) and a power meter (HP 81618A) were also used to monitor the grating transmission spectrum. qp iagl? obp C2 CO 2 jjq J2 couiojj CL JfIU;pJ J12GL 1J LLGC!2!C 1G COUL()JJGL LOIGL JJJG COmb Figure 1. Schematic setup of the CO 2 laser based LPFG fabrication system 3. EXPERIMENTS AND RESULTS 3.1 LPFG Spectrum The transmission characteristics can be measured by the setup drawn in Fig.1. The tunable laser served as the light source at one end of the fiber and at the other end an optical power meter (Agilent 81618A) was used to detect the power transmitted through the grating. The wavelength stepping of the laser and the power detection of the power meter were coordinated by a computer. The typical transmission spectrum of a LPFG with a period of 535µm is shown in Fig.2. Typical transmission spectra of the fabricated LPFGs indicated a low loss (<1dB) and a high resonance strength (>25dB) of the device. The curve with black square dots is the resonance peak corresponding to the energy coupling from the fundamental core mode to the 5 th cladding mode, while the curve with circles is the numerical simulation calculated based on an analytical modal following Ref 13. Fig.2 shows that the measured transmission spectrum is in a good agreement with the simulated. In -5-1 S I. Measured Simulated -25 152 154 156 158 16 162 164 Figure 2. Measured and simulated transmission spectra of a LPFG with a period of 535µm Proc. of SPIE Vol. 6757 67578-2
3.2 High Temperature Tests To test the temperature sensitivity of the fabricated LPFGs, we installed the LPFGs into an electric furnace and increased the temperature from 1 C to 8 C. The LPFGs used in the experience were fabricated with period of 5µm in Corning SMF-28 fiber. Its resonant wavelength was about 156nm at room temperature. We held the temperature for one hour and measured the transmission spectrum of the LPFG at each 1 C increment as shown in Figure 3(a). The attenuation peak of the LPFG moved to the long wavelength region when the temperature increased. Figure 3(b) shows the resonant wavelength of the LPFG as a function of temperature. The test results clearly demonstrated that the LPFG successfully survived high temperatures up to 8 C. The results also indicated that the resonant wavelength was a linear function of temperature. 164 Transimission (db) -15-3 1C 3C 4C 5C 6C 7C 8C Increasing Temperature -45 152 155 158 161 164 Peak 162 16 158 156 2 4 6 8 Temperature (Degree C) Figure 3. Test of LPFG at various temperatures. (a) LPFG transmission spectra at different temperatures. (b) the resonance wavelength as a function of temperature. 3.3 Thermo Stability and Improvement The stability of the LPFG is a concern when it is designed and fabricated for high temperature applications. It is highly desired that the resonance peak of the grating does not drift when the grating is exposed to a high temperature environment for a long period of time. However, due to the fact that the refractive index change is caused by thermal treatment and the inevitable thermal relaxation of the treated glass, it is expected that the resonance wavelength will have a non-reversible drift, especially in a high temperature environment. 1696 1698 1686 1688 8 1 1676 1678 1666 ur( -15 F- -2 2.9 nm 1591.nm 1593.9nm -25 151 154 157 16 163 1668 1666 E VT 1553.5nm 25.3 nm =2h 1578.8nm 8 28 48 68 88 188 128 148 168 188 288 151 154 157 16 163 Time (Hour) Figure 4. Thermo stability tests and improvement Proc. of SPIE Vol. 6757 67578-3
To evaluate the thermal stability of the fabricated LPFG, a CO 2 laser fabricated LPFG was placed in electric furniture at a temperature of 55 o C. One end of the fiber was fixed to the test chamber (a ¼ inch stainless steel tube) while the other one was set loose so that the thermo expansion of the container itself would not affect the LPFG. As shown in Fig. 4, the lower curve with circle is the resonant wavelength as a function of time. We observed a wavelength shift of 25.3nm after 2 hours. We can also see that the wavelength change was faster at the beginning and getting slower as the experiment went along, indicating that the thermo stability of the LPFG improved after annealing. However, the improvement was slow and would take a long time to stabilize. In the second experiment, a LPFG was heated up to 7 o C for 2 hours, and then, we cooled it down to 55 o C and maintained for 2 hours. As shown in Fig. 4 (the upper curve), the resonant wavelength still shows a continuous drift towards a shorter wavelength but with a much smaller shift of 2.9nm after 2 hours annealing. The experiment indicates that thermo shock indeed improved the thermo stability of LPFG at high temperatures. 3.4 Response to Refractive Index Changes A LPFG was immersed in different liquids to evaluate its response to the refractive index change of the surrounding environment. The liquids used in the experiments included water (n=1.33), acetone (n=1.359) and isopropanol (n=1.3736). As shown in the insert of Fig.5, the resonant wavelength shifted towards the short wavelength region as the environmental refractive index increased. Fig. 5 also plots the simulated resonance wavelength shift, with respect to that in air, as a function of the refractive index of the surrounding environment. We charted the three measurement data points on the simulated curve and found that the measured data were in good agreement with the numerical simulations. Simulation Wavelength shift (nm -1-2 -3 Transmission (db) Measurements -4-8 -12-16 Water Air IPA Acetone -2 1577 1579 1581 1583 1585-4 1 1.1 1.2 1.3 1.4 Refractive index Figure 5. LPFG in response to refractive index changes 4. CONCLUSION High performance LPFGs were successfully fabricated using point-by-point CO 2 laser irradiation method for high temperature applications. Typical transmission spectra of the fabricated LPFGs indicated a low device loss and high resonance strength. The measured spectrum was in a good agreement with the simulated. The LPFG survived high temperatures up to 8 C and showed a linear relation between the resonant wavelength and the temperature. We also showed that thermo shock at an elevated temperature could significantly improve the thermo stability of the LPFG. The fabricated LPFGs have also shown high sensitivity for refractive index measurement. Proc. of SPIE Vol. 6757 67578-4
5. ACKOWLEDGEMENT This research was supported by the U.S. Department of Energy through the National Energy Technology Laboratory under contract number DE-FEC26-5NT42439 and the Office of Naval Research through the Young Investigator Program under the contract number (N14-7-1-8). 6. REFERENCES 1. A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T.Erdogan, J.E. Sipe, Long-period fibre grating as bandrejection filters, J. Lightwave Technol. 14, pp. 58-64, 1996. 2. A.M. Vengsarkar, J.R. Pedrazzani, J.B. Judkins, P.J. Bergano and C.R. Davidson, Long-period fibre-grating based on gain equalizer, Opt. Lett. 21, pp. 336-338, 1996. 3. B. Bhatia and A.M. Vengsarkar, Optical fibre-long period grating sensors, Opt. Lett., 21, pp. 692-694, 1996. 4. V. Grubsky and J. Feinberg, Long-period fiber gratings with variable coupling for real-time sensing applications, Opt. Lett., 25, pp. 23-25, 2. 5. L. Dong and W.F. Liu, Thermal decay of fiber Bragg gratings of positive and negative index change formed at 193 nm in a boron-codoped germanosilicate fibre, Appl. Opt., 36, p. 8222, 1997. 6. D.D. Davis, T.K. Gaylord, E.N. Glytsis, S.G. Kosinski, S.C. Mettler and A.M. Vengsarkar, Long-period fibre grating fabrication with focused CO 2 laser pulses, Electron. Lett., 34, pp. 32-33, 1998. 7. Yun-Jiang Rao, Yi-Ping Wang, Zeng-Ling Ran, and Tao Zhu, Novel Fiber-Optic Sensors Based on Long-Period Fiber Gratings Written by High-Frequency CO2 Laser Pulses, J. Lightwave Techno. 21, pp. 132-1327, 23. 8. D.D. Davis, T.K. Gaylord, E.N. Glytsis and S.C. Mettler, Very-high-temperature stable CO 2 -laser-induced longperiod fibre gratings, Electron. Letter. 35, pp. 74-742, 1999. 9. G. Rego, O. Okhotnikov, E. Dianov and V. Sulimov, High-Temperature Satbility of Long-Period Fiber Gratings Produced Using an Electric Arc, J. Lightwave Techno. 19, pp. 1574-1579, 21. 1. J.M. Corres,, I.R. Matias, I. del Villar and F.J. Arregui, Design of ph Sensors in Long-Period Fiber Gratings Using Polymeric Nanocoatings, IEEE Sensors., 7, pp. 455-463, 27. 11. M. Pasturel, M. Slaman, H. Schreuders, J. H. Rector, D. M. Borsa, B. Dam, and R. Griessen, Hydrogen absorption kinetics and optical properties of Pd-doped Mg thin films, J. Applied physics. 1, 23515, 26. 12. Heather J. Patrick, Alan D. Kersey and Frank Bucholtz, Analysis of the Response of Long Period Fiber Gratings to External Index of Refraction, J. Lightwave Techno. 16, pp.166-1612, 1998. 13. Anemogiannis E, Glytsis EN and Gaylord TK, Transmission characteristics of long-period fiber gratings having arbitrary azimuthal/radial refractive index variations, J. Lightwave Techno. 21, pp. 218-227, 23. Proc. of SPIE Vol. 6757 67578-5