High-temperature stable fiber Bragg gratings with ultrastrong cladding modes written using the phase mask technique and an infrared femtosecond laser

Bragg gratings with ultrastrong cladding modes are inscribed in a standard telecom optical fiber (i.e., SMF-28) using the phase mask technique and an infrared femtosecond laser. 1000°C-resistant fiber Bragg gratings (FBGs) whose cladding modes exceed 30 dB in transmission and spectrally span more than 250 nm can be produced with only a few (i.e., less than five) laser pulses, provided that the light intensity in the fiber core is kept at the level of ∼ 6 × 10 13 W / c m 2 . The use of FBGs fabricated under the above conditions for refractometry is demonstrated.