Focus issue introduction: Advanced Solid-State Lasers (ASSL) 2015

The editors introduce the focus issue on "Advanced Solid-State Lasers (ASSL) 2015", which is based on the topics presented at a congress of the same name held in Berlin, Germany, from October 4 to October 9, 2015. This focus issue, jointly prepared by Optics Express and Optical Materials Express, includes 23 contributed papers (17 for Optics Express and 6 for Optical Materials Express) selected from the voluntary submissions from attendees who presented at the congress and have extended their work into complete research articles. We hope this focus issue offers a good snapshot of a variety of topical discussions held at the congress and will contribute to the further expansion of the associated research areas.