The formation of two grating types in SMF-28 fiber by focusing 125 fs, 0.5-2 mJ pulses through a phase mask onto a fiber sample is studied. The first type, specified as type I-IR, occurs below the damage threshold of the medium. The scaling behavior of the type I-IR gratings with field intensity and annealing properties suggests that their formation is related to nonlinear absorption processes, possibly resulting in color center formation. The second type, denoted as type II-IR, occurs coincidentally with white light generation within the fiber. These type II-IR gratings are stable at temperatures in excess of 1000 masculineC and are most likely a consequence of damage to the medium following ionization.

High-quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm. Induced index modulations of 1.9 x 10(-3) were achieved with peak power intensities of 1.2 x 10(13) W/cm2 without any fiber sensitization. The fiber gratings are stable and did not erase after 2 weeks at 300 degrees C. The primary mechanism of induced index change results from a structural modification to the fiber core.

Using an ultrafast Ti:sapphire 800 nm laser and a phase mask, fibre Bragg gratings (FBGs) with high thermal stability were fabricated in Ge-doped SMF-28 fibre for sensor applications and were subjected to long annealing tests at 1000 °C. FBGs that maintained more than 99.95% reflectivity after several hundred hours at this temperature are demonstrated. The gratings perform well in cycling experiments up to 1000 °C, and hysteresis in the wavelength response was not detected. At a temperature of 1050 °C, a permanent drift of the central wavelength is observed which is associated with a reduction of the grating strength. The capability of this new type of FBG to be used for high temperature sensors is discussed.

No abstract

No abstract

No abstract

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.

Fiber Bragg gratings were fabricated in all-silica core fiber by focusing 125-fs 800-nm pulses with an 80-mm lens through a phase mask with 4.28-microm pitch onto a fiber sample. When the phase-mask-fiber separation was 5 mm the observed structure was clearly the result of two-beam interference between the +/- 1 orders. The elimination of the remaining 9 orders is a consequence of the walk-off experienced by the mask orders and the short duration of the femtosecond pulse. This effect is unique to the fabrication of Bragg gratings with femtosecond sources and would not be observed with a longer pulse duration or incoherent UV sources.