Multi-GHz repetition rate, tunable ultrafast mid-IR source

High repetition rate ultrafast optical radiation across the electromagnetic spectrum is of paramount interest due to a variety of applications. Typically, the repetition rate of an oscillator is determined by the round-trip optical length of the cavity, necessitating ultra-compact microcavities to achieve GHz repetition rates. However, increasing the repetition rate of ultrafast sources by shortening the cavity length of oscillators is limited by mechanical constraints. Here, we report a novel experimental scheme, avoiding such limitations, generating tunable, ultrafast mid-infrared (mid-IR) radiation at multi-GHz repetition rates using a synchronously-pumped, singly resonant optical parametric oscillator (OPO) in a fractional cavity configuration. Using two identical MgO:PPLN crystals at the foci of a carefully designed composite ring cavity, we demonstrate the generation of mid-infrared (mid-IR) pulses with tunable repetition rates up to as high as 1250th harmonic of the pump laser repetition rate, corresponding to the mid-IR pulse repetition rate of 100 GHz. The OPO output is tunable across 3280–3394 nm, with mid-IR average power exceeding 20 mW, and high-quality 𝑇𝐸𝑀00 spatial profile (𝑀2 𝑥 ∼ 1.1, 𝑀2 𝑦 ∼ 1.3). This generic approach provides a novel, to our knowledge, route for generating high repetition rate ultrafast radiation across broad spectral regions.