Optical loss mechanisms in femtosecond laser-written point-by-point fibre Bragg gratings

Mattias L Åslund,John Canning,Michael J Withford,Alexander Fuerbach,Nathaniel Groothoff,Stuart D Jackson,Nemanja Nemanja,Graham D Marshall

doi:10.1364/oe.16.014248

Abstract

Fibre Bragg gratings inscribed with the point-by-point method using a Ti-sapphire femtosecond laser operating at 800 nm are shown to display strong increasing attenuation towards shorter wavelengths with a large and spectrally sharp recovery observed below 400 nm. The origin of this loss is shown to be Mie scattering, and the sharp recovery in the transmission results from wavelength dependent scattering within the numerical aperture of the core. The permanent losses from these Type II gratings have implications for high temperature sensors and fibre lasers.

Highlights

Type II fibre Bragg gratings (FBGs), i.e., those that are written above the damage threshold of glass, have been written using high intensity UV-lasers [1, 2], as well as more recently, by femtosecond lasers [3,4,5]
Fibre Bragg gratings inscribed with the point-by-point method using a Ti-sapphire femtosecond laser operating at 800 nm are shown to display strong increasing attenuation towards shorter wavelengths with a large and spectrally sharp recovery observed below 400 nm
The periodic structures analysed in this investigation give rise to large scattering losses at short wavelengths because of the large number of interfaces present

Summary

Introduction

Type II fibre Bragg gratings (FBGs), i.e., those that are written above the damage threshold of glass, have been written using high intensity UV-lasers [1, 2], as well as more recently, by femtosecond lasers [3,4,5] Both processes operate above the so-called damage threshold of the glass [6, 7], which for point-by-point (PbP) femtosecond laser written gratings is ~0.15 μJ for a ~120 fs pulse duration, as determined through microscope and annealing studies. This leads to structural changes that can withstand temperatures approaching 1000 °C [8]. A limited study of short wavelength attenuation induced by femtosecond gratings [18] and recent studies of these gratings employed in core pumped fibre lasers [19] indicate that the differences in confinement of the induced changes compared to long pulse UV Type II gratings give rise to significant differences in attenuation, either from scattering and/or defect absorption

Methods

Discussion

Conclusion