Abstract

Fluoride glass fibers are ideal candidates for all-fiber active devices such as up-conversion lasers, IR amplifiers and sensors. However, the development of the fluoride glass fiber-based devices has been hindered by the lack of a photosensitive mechanism allowing the writing of strong intracore Bragg gratings in those fibers. Among the fluoride glass compositions, the fluorozirconates and especially, the ZBLAx glass family have been mostly studied because of their qualities as good fiber materials. A recent approach was demonstrated to glass photosensitivity based on the use of non-resonant infrared (800 nm) femtosecond (fs) pulses which involves multiphoton absorption. It was originally demonstrated by Davis et al¹ that a refractive index change could be induced by focusing fs pulses in various bulk glasses including ZBLAN. However, to the best of our knowledge, no report has been made on the use of fs pulse- induced photosensitivity for writing intracore FBGs in fluoride fibers. Yet, it was previously shown that FBGs could be written in silica fibers with IR² fs pulses using a phase mask. In this communication, we report on the first FBGs written in non-photosensitized ZBLAN fibers using fs laser pulses. Index modulations almost two orders of magnitude larger than those previously reported in a fluoride fiber (10⁻³ vs 10⁻⁵) were achieved providing grating reflectivity greater than 99%. Figure 1 shows the measured transmission and reflection spectra of a FBG written in a single-mode Tm³⁺-doped ZBLAN fiber. We further demonstrate, via both a spectral analysis and the measurement of the index modulation across the FBG section, that the grating formation is based on a negative index change and that the writing process appears to be essentially insensitive to the presence of active ions. The grating’s thermal annealing behavior is investigated and demonstrates the potential application of those FBGs as intracavity reflectors for a stable operation of fluoride-based fiber lasers. Figure 2 shows the variation of the index modulation with annealing temperature for a grating written in an undoped ZBLAN fiber. The inset shows the variation of the index modulation as a function of annealing time at 150 °C. The refractive index modulation decreases exponentially versus annealing time to reach an asymptotic value after ~30 min confirming that the gratings can be made stable at room temperature after an annealing treatment. Finally, we show preliminary results of an all-fiber Tm³⁺-doped ZBLAN laser pumped at 1070nm and operating at 1480nm based on a FBG as an input coupler.

© 2007 IEEE