Temperature sensor using a Long Period Fiber Grating Fabricated by800 nm Femtosecond Laser PulsesYongqin Yu1,2, Shuangchen Ruan1*, Haili Yang1, Chenlin Du1, Jiarong Zheng11 Shenzhen Key Laboratory of Laser Engineering, College of Electronic Science and Technolog,Shenzhen University, Shenzhen, Guangdong P.R.China 518060；2 College of Physical Science and Technology, Shenzhen University, Guangdong P.R.China518060；ABSTRACTIn this letter, LPFGs in standard telecommunication fibers without hydrogen loading were fabricated in air using laser direct writing method, by femtosecond laser pulses with pulse duration of 200 fs and output wavelength of 800 nm. The loss peak of 1430 nm, the transmission loss of 22.86 dB and the FWHM of 6.6 nm were obtained. Temperature dependence of wavelength shift in air was measured by placing the LPFG in a temperature chamber that is temperature controlled in the range of 70 –150 ℃. The temperature sensitivities (∆λ/∆T) are estimated by using linearly regression fits, which was 43.2 pm/℃. The linearity of the temperature sensitivities is high and the R-squared values for ∆λ /∆T is larger than 0.9979.Keywords: Femtosecond laser pulse, micromachining, long-period fiber gratings, temperature sensor1. INTRODUCTIONIn recent years, long-period gratings (LPFGs) have been increasingly used in a wide variety of telecommunications and sensing applications. The long period grating is a fiber device that consists of a periodic change in the refractive index or the fiber geometry along the fiber length, with the typical period of several hundred micrometers. The LPFG couples the light from the core mode to the resonant copropagational cladding modes that are absorbed by the coating causing the characteristic attenuation bands in the transmission spectrum. In comparison with fiber Bragg gratings (FBGs), long-period gratings (LPFGs) offer a number of additional advantages including easy fabrication, low level of back-reflection and high temperature sensitivity. These advantages render the LPFG desirable for some special applications. They have been used as band rejection filters1,2 in gain equalizers and as sensors of temperature, strain, bend and refractive index, 3,4 gain-flattening filters5 for Er-doped fiber amplifiers, for spectral shaping and dispersion compensation6 and as chemical and mechanical sensors.7 One usually fabricates LPFGs by periodically changing the refractive index of a photosensitive fiber core with side exposure to a UV beam8 at wavelengths which are within the absorption bands of the Ge-doped fiber core. Irradiation with UV light causes an increase of the refractive index of these fibers owing to formation of Ge-related glass defects. Typical sources of UV radiation for fiber grating fabrication are the second harmonic of CW argon gas lasers (244 nm) and nanosecond pulsed excimer lasers (248-nm KrF and 193-nm ArF). UV absorption and the photoinduced refractive index change in low GeO content standard telecommunication fibers are relatively small. Extra processes such as hydrogen loading at high pressure or doping of photosensitive ions are usually applied to enhance the inherently weak photosensitivity response of telecommunication fibers. Moreover, LPFGs fabricated by UV-light irradiation have a problem with an aging stability because the index change relaxes even at lower temperature14. Later, many alternative methods of making LPFGs by use of fibers with nonphotosensitive cores, based on refractive index changes in the fiber core induced by thermal heating were developed. They include the use of a CO2 laser for fiber irradiation,9,10 focused ion-beam radiation,11 and electric arc12,13. However, in practice, the photochemical approach is most frequently employed for LPFG fabrication. It has been found that focused irradiation of infrared femtosecond laser pulses causes a permanent refractive-index increase in various glasses14 owing to nonlinear multi-photon absorption. Y. Kondo14 demonstrated a LPFG with high thermal 5th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies, edited by Li Yang, Yoshiharu Namba, David D. Walker, Shengyi Li, Proc. of SPIE Vol. 7655,76550Q · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.865346Proc. of SPIE Vol. 7655 76550Q-1Downloaded from SPIE Digital Library on 29 Aug 2011 to 222.29.45.179. Terms of Use: http://spiedl.org/terms
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