Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask

High reflectivity fiber Bragg gratings (FBG) with strong cladding mode suppression were written in standard telecom fiber (SMF-28) with femtosecond 800-nm laser pulses and a phase mask. No special fiber photosensitization or fiber design was required. Induced index modulations were created by two-beam interference from the phase mask in both the core and cladding region, which resulted in cladding-mode coupling loss <0.1 dB for a high reflecting -30 dB in transmission FBG.