Direct femtosecond laser writing fiber Bragg gratings in double-D cladding chalcogenide glass infrared fibers.

• Direct femtosecond laser writing chalcogenide glass infrared fiber Bragg gratings of high reflectivity and narrow bandwidth. • Discussions and solutions to current problems in chalcogenide glass fiber's fabrication using direct femtosecond laser writing. • Foundation for fabricating FBG with arbitrary periods in ChG fibers obtained. Chalcogenide glass (ChG) fiber Bragg grating (FBG) is one of the key components in monolithic infrared fiber lasers and sensors. However, its effective fabrication using direct femtosecond laser writing is still seldom reported, owing to complications attributable to ChG. In this study, a double-D shaped ChG fiber was used to fabricate FBGs with high reflectivity (a 20.5 dB dip in the transmittance) and narrow 3-dB bandwidth (164 pm) using direct femtosecond laser writing. The high sensitivity to laser powers of the direct writing of the ChG FBG was revealed and understood from the perspective of the low laser-induced damage threshold of ChGs. Lower-order reflectance of the directly written FBGs at the mid-infrared wavelength region were quantitively predicted to have higher reflectivity. These results provide a robust method of writing FBGs with arbitrary periods in ChG fibers, establishing a solid foundation for the development of infrared fiber lasers and fiber sensors. [ABSTRACT FROM AUTHOR]