Practical few-cycle degenerate optical parametric oscillator with a plane-mirror cavity

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Abstract
Typical degenerate femtosecond optical parametric oscillators adopt an astigmatism-compensating resonator employing few-100 µm, Brewster-cut periodically-poled crystals to minimize intracavity dispersion and maximize the parametric gain bandwidth, but these crystals are fragile and hard to manufacture, especially when multiple gratings are required. Here, we show how the opposite approach—using a mm-thick crystal and considerable intracavity material—can still achieve few-cycle pulses and multi-100-nm spectral bandwidths. Using plane mirrors to eliminate astigmatism, our design balances the normal dispersion of the gain crystal with anomalous dispersion from thick intracavity lenses. By confining poled regions to less than the physical crystal length, we achieve a broad parametric gain bandwidth while allowing the crystal length to become an independent parameter for engineering the intracavity dispersion. We experimentally demonstrate the concept in an MgO:PPLN optical parametric oscillator, synchronously pumped by a 500 MHz Ti:sapphire laser, resulting in 42 fs pulses at 1564 nm with a -20 dB bandwidth of 300 nm and an average output power of 112 mW. The plane-mirror design is compact, robust, stable, easily aligned, and readily adapted to a wide range of repetition rates. Degenerate optical parametric oscillators based on this approach offer a route to simple generation of phase-coherent light when pumped by a femtosecond frequency comb.
Funding Information
  • Science and Technology Facilities Council (ST/V000403/1)
  • Science and Technology Facilities Council (ST/Y001273/1)
  • Science and Technology Facilities Council (ST/X002306/1)

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