All-optical phase locking of two femtosecond Ti:Sapphire lasers: A passive coupling mechanism beyond the slowly varying amplitude approximation

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.