Fabrication And Characteristics Of Multipeaks Resonant Fiber Gratings By Femtosecond Laser Pulses

The periods of long-period fiber gratings (LPFGs) are approximately tens ofmicrometers to hundreds of micrometers, while the periods of multipeaks resonantfiber gratings (MRFGs) go up to several millimeters. MRFGs both have theconventional advantages of LPFGs and some unique properties. So MRFGs areexpected to have better applications in fiber communication and sensing.We investigated the coupled-mode theory for LPFGs and MRFGs, simulated theirtransmission spectra, discussed the dependence of transmission spectra on the indexchange, the period number and the period length respectively, and deduced theresonant wavelength shift sensitivities for temperature, stretch and surroundedmedium index.Then we fabricated the LPFG with period of 500μm and MRFGs with periods of1mm, 2mm and 4mm by femtosecond laser pulses using the point-by-point writingtechnique, and investigated the dependence of transmission spectra on periodnumber, pulses power, period length, temperature and surrounded medium indexrespectively in our experiments.Compared to LPFGs, MRFGs with larger periods display more resonant peaks.These peaks result mainly from the coupling of the fundamental guided core modeto the high-diffraction-order cladding modes, and every peak has unique sensitivityto different environmental parameters. Besides, MRFGs fabricated by femtosecondlaser pulses have better temperature tolerance and temperature sensitivity. So thefemtosecond laser induced MRFGs can be widely used as high sensitivity sensorsunder high temperature in the filed of multi-parameter measurement.