Abstract
High power ultrafast laser sources at 2 μm are highly desirable as primary sources in many fields such as mid-IR generation, remote sensing, Lidar systems, and medical use. Ho:YAG is an excellent gain crystal for such kind of sources due to its low quantum defect, good crystal quality and broad emission bandwidth. To date, Ho:YAG has been utilized in q-switched and actively mode-locked systems [1, 2]. Very recently passive mode locking has also been demonstrated using semiconductor saturable absorber mirrors (SESAMs) [3, 4]. However, the output power and pulse duration were limited to only several hundred milliwatts and few picoseconds, respectively. Since the invention of thin disk technology, great progress has been made in power and energy scaling of thin disk lasers mode-locked by both SESAM and Kerr-lens mode locking (KLM) [5, 6]. However, KLM shows great advantages compared to SESAM mode-locking in generating shorter pulses with high power due to its fast response time, broad bandwidth operation and higher damage threshold. Here we present a KLM Ho:YAG thin disk oscillator working at 2.1 μm for the first time. It delivers 220 fs pulses with average power up to 20 W, which is, to the best of our knowledge, the shortest pulse duration ever obtained in a Ho:YAG oscillator and highest average power of any mode locked ultrafast oscillator in 2 μm range.
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