Mono-Layer Graphene Saturable Absorbers with Greatly Enhanced Graphene-Evanescent Field Interaction for Stable Passive Mode-Locking of All-Fibre Laser

Ultrafast all-fibre lasers have been intensively studied with several advantages including alignment-free structure, excellent spatial beam quality and compactness over solid-state lasers, potentially finding the applications in diverse fields. Generally saturable absorption elements are necessarily used in a passively mode-locked fibre laser system for environmentally stable and self-starting operation. Recently novel saturable absorbers (SAs) based on low dimensional nano-materials have been investigated as an alternative of semiconductor saturable absorber mirrors (SESAMs). Among them, graphene exhibits outstanding optical properties such as huge Kerr-nonlinearity, broad spectral operation range and ultrafast recovery time due to its unique bandgap structure [1, 2]. However, low optical absorption of 2.3% in a mono-layer graphene often limits its application to various fibre laser systems. Although there have been several attempts to increase graphene-light interaction based on lateral interaction scheme with evanescent field [3], realization of efficient mode-locker by evanescent-field interaction with a mono-layer graphene is still challenging due to its weak graphene-light interaction.