Femtosecond frequency combs stabilized with a He-Ne/CH4 laser: Toward a femtosecond optical clock

A concept of high-precision optical frequency measurements involving the use of femtosecond frequency combs with a bandwidths exceeding the frequency of a reference laser source is discussed. This relation between the frequency of the reference laser source and the bandwidth of the frequency comb allows the difference frequency of two modes in a femtosecond frequency comb to be phase-locked to the reference laser. The radio-frequency interval between two adjacent modes in the femtosecond frequency comb is also stabilized in this case, allowing high-precision frequency measurements to be performed in a very simple way. A laser system allowing this principle of high-precision frequency measurements to be implemented includes a mode-locked Ti:sapphire laser, a methane-stabilized He-Ne laser, and a tapered fiber. Special measures have been taken to improve stability characteristics of the laser system. The process of spectral broadening of femtosecond pulse trains in holey and tapered fibers is considered in terms of the Zakharov equation as a result of four-wave mixing of phase-locked equidistant spectral components. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of this technique are discussed. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.