Improve performance of long period fiber gratings (LPFGs) fabricated by femtosecond laser

Abstract

Long-period fiber gratings (LPFGs) have found wide applications in optical communications and smart sensing. Among the various methods for fabrication of LPFGs, infrared femtosecond laser writing is one of the most attractive methods in terms of flexibility, being applicable to both photo sensitive and insensitive fibers, and no needs for any pre-designed masks. Although fabricating LPFGs by infrared femtosecond laser has been demonstrated for more than ten years, the reported maximum rejection band depth of LPFGs fabricated with this method for standard telecommunication fiber (SMF-28) without hydrogen loading is no more than 10 dB, and in the meanwhile its out-of-band loss is rather high. In this report, LPFGs with major resonant attenuation over -16 dB near 1300 nm are produced in standard single mode fibers with two dimension visional femtosecond laser manufacturing system. The contrast of the resonant rejection band resulting from core-cladding mode coupling can be significantly increased by applying a proper amount of axial stress along fiber during laser writing. With higher laser energy irradiation, LPFGs of multiresonance peaks plus larger out-of-band loss are frequently made. Such out-of-band loss is mainly caused by Mie scattering, and it can be restrained by properly selecting grating duty cycle.