Silt-Beam Shaping Method for Femtosecond Laser Point-by-Point Inscription of Highly Localized Fiber Bragg Gratings with Enhanced Cladding Modes

We propose and experimentally demonstrate a slit beam shaping method for fabricating Point-by-Point (PbP) highly localized fiber Bragg gratings (FBGs). The influences of the pulse energy and the slit width were studied, and after optimizing these parameters, the enlarged refractive index modulation (RIM) was produced. These RIMs were precisely assembled in a fiber core, generating highly localized FBGs with a comb of enhanced cladding mode resonances (30 dB) and a low insertion loss (0.3 dB). Note that the cladding mode resonance has a wide span of 240 nm in transmission spectrum. Furthermore, highly localized tilted fiber Bragg gratings (TFBGs) with various tilted angle can be fabricated to obtain a comb of selective enhanced cladding modes, via adjusting the tilted angle of the slit. Moreover, we investigated the thermal characteristics of the highly localized FBGs. The results show that such a device can withstand a temperature of 1000 °C.