A collinear self-referencing set-up for control of the carrier-envelope offset frequency in Ti: Sapphire femtosecond laser frequency combs

We demonstrate a simplified set-up for control of the carrier-envelope offset frequency (f_ceo) in a Ti : sapphire femtosecond laser frequency comb. A periodically poled KTiOPO₄ crystal is used for second harmonic generation with zero walk-off angle, which enables collinear propagation of beating beams in the self-referencing set-up. A beat signal with a signal-to-noise ratio of more than 40 dB can be obtained within a 300 kHz bandwidth. Using this signal, f_ceo could be tracked to a frequency synthesizer within a few millihertz for an averaging time of 1 s, which contributed a relative stability of 8 × 10^-18 in the visible region around 532 nm.