Abstract
Long-period fiber gratings (LPFGs) are useful optical devices with many advantages, such as easy to be fabricated, low insertion loss, low back reflection rate, and polarization independent, etc. Thus they are widely used in optical communication systems and various kinds of sensing systems. Conventionally, LPFGs are fabricated by use of photosensitive fibers and ultra violet lasers. Recently, non-ultraviolet (UV) based fabrication methods, such as arc discharge or CO2 laser, have attracted more and more attentions due to its much lower cost. By heating the fibers, the strain in them can be strengthened or relaxed so that refractive index changes are induced. There are many advantages to fabricate LPFGs by use of a CO2 laser, such as high and stable output power, easy to be controlled, no degradation problems of the inscribed gratings, and avoiding the need of expensive UV lights and photosensitive fibers. Unlike the fabrication methods of others, we use the CO2 laser based point-by-point ablation technique to enhance the local index difference. If the LPFGs are fabricated by heating local areas of the fibers, the coupling efficiency is limited. Usually, the .grating length will be more than 2 cm, 20 ~ 50 periods, and the highest extinction ratio will be less than 30 dB. By use of the ablation technique, the grating is inscribed in the core and cladding simultaneously and its length is about 7.8 mm, 12 periods, with highest extinction ratio up to 32.2 dB. In this article, we improve the fabrication method and obtain high coupling efficiency LPFGs. Surrounding a LPFG with high refractive index material and bending it, the extinction ratio of the grating can be kept as a constant while the resonance wavelength shifts. Changing the ambient temperature of a LPFG can change the resonance wavelength. The sensitivity of the LPFG is about 0.26 nm/℃. The sensitivity can be highly enhanced by tuning the refractive index of the covered polymer OCF-446 to slightly lower than that of the cladding. It is observed that the resonance wavelength shifts over 115nm within 9℃ and the sensitivity is estimated to be around 15nm/℃ which is much higher than that of other LPFGs. According to the above mentioned characteristics, our LPFGs can be bend-insensitive and tuning-efficient which makes them excellent devices to be applied to optical communication systems and sensing fields.