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We implement a new approach for generating broadband mid-infrared frequency combs via degenerate optical
parametric oscillation. This technique efficiently transfers the desirable properties of shorter wavelength mode-locked
sources to the mid-IR. Our OPO resonator is a 3m ring cavity composed of one pair of concave mirrors with R=50mm
and four flat mirrors, all but one of which are gold coated with > 99% reflection. A single dielectric mirror is used to
introduce the 1560nm pump (Menlo Systems C-fiber, 100 MHz, 70 fs, 350 mW or Toptica Photonics FemtoFiber Pro, 80
MHz, 85 fs, 380 mW). The dielectric mirror is transmissive for the pump and reflective in the 2.5- 4 micron range.
Broadband parametric gain around 3.1-micron subharmonic is provided by short (0.2-0.5mm) periodically poled lithium
niobate (MgO:PPLN) at Brewster angle. Crystals were cut from Crystal Technology Inc. material having QPM period of
34.8 microns for type 0 (e=e+e) phase matching at t=32 deg. C. The enormous acceptance bandwidth at degeneracy,
typical for OPOs with type 0 (or type I) phase-matching, gives broad bandwidth and makes temperature tuning
insignificant. Broadband oscillation is achieved when signal/idler are brought into degenerate resonance by fine-tuning
the cavity length with a mirror on a piezo stage. Using an 8% reflective pellicle, we outcouple a frequency comb of more
than 1000nm bandwidth, centered around 3.1 microns. A 1mm or 2.5mm thick ZnSe plate at Brewster angle provides
2nd-order group velocity dispersion compensation, improving the OPO bandwidth. The OPO threshold was measured to
be < 30mW. When locked, the OPO outputs 60 mW of average power centered at 3.1 microns. With proper intracavity
dispersion management including chirped mirrors, we expect to extend the spectral width to an octave or more.
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