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We generate broadband mid-infrared frequency combs via degenerate optical parametric oscillation in a subharmonic
OPO. This technique efficiently transfers the desirable properties of shorter wavelength mode-locked sources to the mid-
IR. Our OPO resonator is a 3m or 4m 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
pump (2.05 micron from IMRA America, 75 MHz, 80 fs, 600mW or 1.55 micron from Menlo Systems C-fiber, 100
MHz, 70 fs, 350 mW or 1.56 micron from Toptica Photonics FemtoFiber Pro, 80 MHz, 85 fs, 380 mW). The dielectric
mirror is transmissive for the pump and reflective in a 2.5- 4 micron or 3- 6 micron (for 2 micron pump) range.
Broadband parametric gain around the 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. With the 2-micron pump, orientation patterned
gallium arsenide from BAE systems is used as the non-linear material In both systems, 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 from the Er/PPLN system. 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 the Tm/OP-GaAs system we achieve octave-spanning output from 3- 6 micron
using a mix of YAG and CaF for dispersion compensation and output powers over 30 mW.
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