  We report on the first singly-resonant (SR), synchronously pumped optical
parametric oscillator (OPO) based on orientation-patterned gallium arsenide
(OP-GaAs). Together with a doubly resonant (DR) degenerate OPO based on the
same OP-GaAs material, the output spectra cover 3 to 6 ${\mu}$m within ~3 dB of
relative power. The DR-OPO has the highest output power reported to date from a
femtosecond, synchronously pumped OPO based on OP-GaAs. We discovered strong
three photon absorption with a coefficient of 0.35 ${\pm}$ 0.06
cm${^3}$/GW${^2}$ for our OP-GaAs sample, which limits the output power of
these OPOs as mid-IR light sources. We present a detailed study of the three
photon loss on the performance of both the SR and DR-OPOs, and compare them to
those without this loss mechanism.
