Annealing dynamics of waveguide Bragg gratings: evidence of femtosecond laser induced colour centres

P Dekker,M J Withford,D J Little,M Ams,G D Marshall

doi:10.1364/oe.18.003274

Abstract

There is still significant speculation regarding the nature of femtosecond laser induced index change in bulk glasses with colour centre formation and densification the main candidates. In the work presented here, we fabricated waveguide Bragg gratings in doped and undoped phosphate glasses and use these as a diagnostic for monitoring subtle changes in the induced refractive index during photo- and thermal annealing experiments. Reductions in grating strengths during such experiments were attributed to the annihilation of colour centres.

Highlights

Femtosecond laser direct writing of waveguide devices is a significant enabling technology in the fields of sensing, communications, defence and integrated quantum optics [1, 2]
The results demonstrate that the waveguide Bragg grating (WBG) used in our distributed feedback (DFB) waveguide lasers are thermally robust up to 70 oC and could be ‘aged’ in the same way as fibre Bragg gratings (FBGs) used in the telecommunications industry to provide long term stability
White light transmission spectra/discussion In Sections 4 and 5 we show that both photobleaching and thermal annealing can affect the performance of femtosecond laser written WBGs in doped and undoped Kigre ‘QX’ phosphate glass

Summary

Introduction

Femtosecond laser direct writing of waveguide devices is a significant enabling technology in the fields of sensing, communications, defence and integrated quantum optics [1, 2]. We recently demonstrated distributed feedback (DFB) monolithic waveguide lasers (WGLs) fabricated in Yb:phosphate [8] by directly writing a waveguide and grating in a single processing step. In this case we obtained output powers of 100 mW and optical conversion efficiencies of 17% with stable laser operation for periods of approximately 50 hours. In the case of the Er, Yb WGL structure [9], laser operation was sustained for only one hour We believe that these changes in operation are due to annealing of the waveguide Bragg grating (WBG) structure akin to that observed in photosensitive fibres [10]. To better understand this phenomenon and in order to create WGLs with long term stability we investigate the effects of photo- and thermal annealing of ultrafast laser written waveguide and WBG structures

Background

Photo-annealing

Thermal annealing

Findings

Conclusion