Femtosecond laser induced low propagation loss waveguides in a lead-germanate glass for efficient lasing in near to mid-IR

Mamoona Khalid,Heike Ebendorff-Heidepreim,David G Lancaster,George Y Chen

doi:10.1038/s41598-021-90249-9

Mamoona Khalid, Heike Ebendorff-Heidepreim + Show 2 more

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https://doi.org/10.1038/s41598-021-90249-9

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Abstract

To support the growing landscape of near to mid-IR laser applications we demonstrate a range of low propagation loss femtosecond laser (FSL) written waveguides (WGs) that have achieved guided-mode laser operation in a rare earth (RE) doped lead-germanate glass. The WGs are fabricated in both the athermal and thermal FSL writing regimes using three different pulse repetition frequencies (PRF): 100 kHz (athermal); 1 MHz; and 5 MHz (thermal). The lasing capability of Yb3+ doped lead-germanate waveguides is verified in the near-IR. The refractive index contrast (∆n) for 100 kHz WGs is ~ 1 × 10–4, while for 5 MHz, ∆n increases to ~ 5 × 10–4. The WGs in the thermal regime are less effected by self-focusing and are larger in dimensions with reduced propagation losses. For the 1 MHz repetition rate thermal writing regime we report a low propagation loss WG (0.2 dB/cm) and demonstrate laser operation with slope efficiencies of up to ~ 28%.

Highlights

To support the growing landscape of near to mid-IR laser applications we demonstrate a range of low propagation loss femtosecond laser (FSL) written waveguides (WGs) that have achieved guided-mode laser operation in a rare earth (RE) doped lead-germanate glass
The experimentally determined Fresnel reflection loss (FL) is in close approximation to the theoretical value of FL = 0.74 dB which is calculated from the refractive index of GPGN glass (n = 1.82) using FL = 10 × log ((n2 + 1)/2n)
We fabricated a range of waveguides in a RE-doped lead-germanate GPGN glass using FSL in three different pulse repetition frequencies (PRF) i.e. 100 kHz, 1 MHz and 5 MHz, and verified their lasing capability in a near-IR region

Summary

Introduction

To support the growing landscape of near to mid-IR laser applications we demonstrate a range of low propagation loss femtosecond laser (FSL) written waveguides (WGs) that have achieved guided-mode laser operation in a rare earth (RE) doped lead-germanate glass. Germanate glasses are fascinating hosts to achieve new laser operating regimes as this glass provides a good balance of properties required for efficient laser operation in the short to mid infrared r egion[3,4,5,6,7] This includes longer wavelength transmission of germanates into the infrared region, competitive thermal, chemical and mechanical strength, medium phonon energy (~ 800 cm−1), and high refractive index compared to the widely researched silicates[8] and fluorides[9] (where very low propagation loss WGs have already been reported). Irradiating FSL pulses can exceed the selffocusing critical power limit of the glass which can occur even at low average laser power in conjunction with other FSL parameters[15] This combination typically results in non-spherical and elongated structures. There are no commercially available high-index matching oils and objective lenses; limiting the parameter space that can be explored

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