PLD-grown Yb:Y2O3 waveguide laser Q-switched by a graphene saturable absorber

In this paper, we present an Yb-doped Y2O3 waveguide laser fabricated by pulsed laser deposition (PLD) that is Q-switched using a mono-layer graphene saturable absorber. 140 ns pulses at a repetition rate of 1.21 MHz and pulse energy of 14 nJ are achieved at an absorbed pump power of 528 mW. Introduction: Pulsed lasers have a range of applications in areas such as medicine, material processing, telecommunications and defence. Waveguide-based pulsed laser systems have the additional potential to be integrated, compact, mass-producible and efficient, making them exciting prospects for future industrial applications. We have recently demonstrated mode-locked femtosecond waveguide lasers at multi-GHz repetition rates using semiconductor saturable absorber mirrors (SESAMs). However, SESAMs are expensive, complicated to fabricate and have a narrow operating wavelength range. In the last decade, graphene saturable absorbers have become very popular because of the ease of fabrication and their broadband operation, with Q-switched waveguide lasers and Q-switched mode-locked waveguide lasers being demonstrated. Yttria (Y2O3) is a promising laser host due to good thermal properties, ease of rare-earth doping and waveguide fabrication by pulsed laser deposition (PLD). In this paper, we demonstrate a PLD-grown Yb:Y2O3 waveguide laser that has been Q-switched using a mono-layer graphene saturable absorber. Experimental details and results: A multi-layer clad waveguide structure, consisting of 3m-thick Y2O3 lower and upper cladding layers and a 6m-thick Yb:Y2O3 core layer, was grown on a YAG substrate by PLD and end-polished to a final device length of 8 mm. A fibre-coupled laser diode operating at 975 nm was used as the pump source for the initial continuous wave (CW) characterisation of the Yb:Y2O3 waveguide laser. The pump output was collimated by an aspheric f = 8 mm lens and launched into the waveguide using a further aspheric lens with f = 8 mm, making a spot size of wo ~3.3 m at the input facet. A half-wave plate and a Faraday isolator were installed in the beam path to block any back reflections from the waveguide and to protect the laser diode. The laser cavity was formed by end-butting a 200m-thick high reflectivity (HR) mirror at the inputfacet, and 200m-thick output coupling (OC) mirrors with transmission (@1030 nm) of 0.8%, 2%, 5%, 14% and 19.6%, respectively at the output facet. CW laser action was observed at 1030 nm at a threshold power of 123 mW for the 0.8% OC and a slope efficiency as high as 25% was observed for the 19.6% OC. The laser mode radius was measured to be 3.2 m in the guided plane and 170 m in the un-guided plane. The losses were estimated to be 1.6 dB/cm, based on the measured slope efficiency for each output coupler and using Caird analysis. In order to obtain pulsed operation, single-layer graphene was used as a saturable absorber. It was grown on large-domain ultra-flat copper using atmospheric-pressure chemical vapour deposition (APCVD), and was then transferred onto a fused silica substrate using PMMA. The single-layer was subsequently transferred to the waveguide when the substrate was pressed against the output end-facet of the waveguide. A counter-propagating laser cavity was formed by putting the HR mirror on the output facet of the waveguide and aligning a mirror (2% transmission at 1030 nm, 99.9% transmission at 975 nm) on the input facet. A dichroic mirror (T=99% @ 975 nm, R=97% @1030 nm) was installed before the Tu 3a R5