11-μJ Picosecond-Pulsed Hollow-Core-Fibre-Feedback Optical Parametric Oscillator

High-energy, short-pulsed mid-infrared (MIR) lasers have many important applications such as spectroscopy, materials processing, and medical treatments. Optical parametric oscillators (OPOs), converting readily available near-infrared pulses to the MIR, are often used to service these applications. Temporal synchronization of pulses between the pump and the resonant signals is required in ultrashort-pulsed MIR OPOs. This becomes to be problematic for realising high-energy picosecond (ps) pulses at few-MHz repetition rates from the OPO because cavities with an optical length of hundreds of meters are impractical to implement. This problem can be mitigated using compact fibre-feedback OPO designs that incorporate a length of flexible optical fibre in the cavity. We have previously demonstrated a novel 1-MHz, -4-µJ-level, ps-pulsed OPO based on a fibre-laser-pumped periodically poled lithium niobate (PPLN) crystal cavity consisting of a 298-m-long hollow-core fibre (HCF) as a feedback fibre [1]. Use of HCF allows for significantly improved power scaling capability compared to a traditional solid-core-fibre-feedback OPO due to its vastly lower nonlinearity. Our previous results showed that the maximum achievable pulse energy from the HCF-feedback OPO was limited by the fibre master oscillator power amplifier (MOPA) pump source due to self-phase modulation (SPM) induced spectral broadening that exceeds the acceptance bandwidth of the PPLN crystal at high peak powers. Here we report the power scaling of a fibre MOPA source that provides 130-ps pulses at 1040 nm with a maximum peak power of 267 kW and present an HCF-feedback OPO that operates at 1-MHz repetition rate and generates pulse energies up to 11 µJ. These results represent, to the best of our knowledge, the highest pulse energy yet reported from a ps OPO.