1-GHz harmonically pumped femtosecond optical parametric oscillator frequency comb.

Area of Science:

• Laser Physics
• Quantum Optics
• Spectroscopy

Background:

• Femtosecond optical parametric oscillators (OPOs) are crucial for generating broadband coherent light.
• Frequency combs enable high-precision measurements by providing a spectrum of discrete, equally spaced laser lines.
• Harmonic pumping offers potential advantages in OPO operation but requires careful analysis of comb properties.

Purpose of the Study:

• To demonstrate the first harmonically-pumped femtosecond OPO frequency comb.
• To achieve a stabilized signal comb with 1-GHz mode spacing in the 1.1-1.6-µm range.
• To evaluate the frequency stability and phase noise performance of the harmonically pumped comb.

Main Methods:

• Utilized a 333-MHz Ti:sapphire laser for harmonic pumping of a femtosecond OPO.
• Implemented simultaneous electronic locking of the carrier-envelope offset (CEO) and repetition frequencies.
• Measured frequency uncertainties and phase-noise power spectral density.

Main Results:

• Achieved a stabilized signal comb at 1-GHz mode spacing in the 1.1-1.6-µm band.
• Obtained frequency locking uncertainties of 0.27 Hz (CEO) and 5 mHz (repetition frequency) over 1 s.
• Phase-noise performance was comparable to fundamental pumping, with stability limited by the Rb atomic frequency reference.

Conclusions:

• Harmonic pumping of femtosecond OPO frequency combs is feasible and does not substantially degrade frequency stability.
• The achieved performance is competitive with fundamental pumping schemes.
• The Rb atomic frequency reference represents the primary limitation for further improvements in comb stability.