Abstract

We report on the fabrication of, to the best of our knowledge, the first highly reflective fiber Bragg gratings for the 4 µm wavelength range. A second-order grating with a coupling coefficient ($\kappa$) of $\ 230\;{{\rm m}^{- 1}}$, losses ${\lt}{0.25}\;{\rm dB/cm}$, and a bandwidth of approximately 3 nm was inscribed into the core of a passive indium fluoride (${{\rm InF}_3}$) fiber using a femtosecond (fs) laser. Thermal annealing of this grating at a temperature of 150°C for 90 min resulted in the enhancement of $\kappa$ to $275\;{{\rm m}^{- 1}}$. Further, we show that ${{\rm InF}_3}$ fibers respond very differently to irradiation with fs laser pulses as compared to ZBLAN fibers and that this difference manifests itself in a significantly larger process window for inscription and in the formation of a more complex refractive index profile that is believed to be caused by the larger nonlinearity of ${{\rm InF}_3}$. This Letter paves the way to the development of new wavelength stabilized all-fiber mid-infrared lasers beyond 4 µm.

© 2020 Optical Society of America

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