Mid-infrared 333-MHz frequency comb continuously tunable from 1.95 – 4.0 µm

We report a 333-MHz femtosecond optical parametric oscillator in which carrier-envelope offset stabilization was implemented by using a versatile locking technique that allowed the idler comb to be tuned continuously over a mid-infrared range from 1.95 µm to 4.0 µm. A specially designed multi-section and multi-grating periodically-poled KTP crystal provided simultaneously phasematched parametric down-conversion and pump + idler sum-frequency generation, enabling strong heterodyne signals with the pump super-continuum (employed for locking) to be obtained across the tuning range of the device. The idler comb offset was stabilized to a 10-MHz reference frequency with a cumulative phase noise from 1 Hz–64 kHz of <1.3 rad maintained across the entire operating range, and average idler output powers up to 50 mW.