The development of Mid-infrared fibre lasers has attracted the researchers’ interest due to its potential outbreaking application in several topics, for instance in biological and chemical molecular characterization, environmental sensing, micromachining, laser surgery and LIDARs. To use the advantages of all-fibre configurations, fiberized components, such as laser cavity mirrors, filters, output couplers and polarisers have to be available for Mid-IR wavelength range. Therefore, photoinduced fibre-devices have been collaborating with the all-fibre laser systems development.

While the technology of Bragg grating inscription in soft glass optical fibres currently relies majorly on point-by-point femtosecond laser methodologies, researchers have explored new possibilities in direct inscription first-order Bragg gratings in InF3 fibre using UV and VIS fs-laser and phase mask interferometer. The suggested inscription method has enabled fabrication of FBGs with high-reflectivity, side lobes suppression, very low losses over a wide spectral band (covering relevant spectral regions for laser pumping and emission) and high thermal sensitivity. Therefore, the work also opens up several new exciting perspectives not only for Mid-IR fibre laser development but could also impact numerous fibre-sensing applications.

In particular, FBGs in fluoride fibres have high potential to enhance strain and temperature sensors. Thus, conventional FBG sensors based on silica fibres become much less accurate in the cryogenic temperature range. The temperature sensitivity of a standard FBG is nonlinear and is known to be five times weaker at the boiling point of liquid nitrogen, −196°C (77 K). The techniques for significant improvement of cryogenic temperature sensitivity are coming at the cost of increasing the footprint and complexity of the design, and slowing the thermal response of the sensor. FBGs in fluoride fibres, due to lower phonon energy, have demonstrated shift of the Bragg wavelength down to 3 K (boiling point of liquid helium). Furthermore, the thermal response of FBGs in fluoride fibres was checked during fibre heating to ~373 K, showing sensitivity of ~20 pm/K which is twice higher than of FBGs in silica fibres. Therefore, FBGs in fluoride fibres are expected to overperform current state of the art thermal sensors.

Projekt Partners:
Otto Schott Institute of Materials Research, Friedrich Schiller University, Jena

In the image above:
Principle of the FBG interferometric inscription setup