Abstract

A mode-locked laser provides a sequence of pulses at regular time intervals. The Fourier transform of such a sequence being a regular comb in the frequency domain, a mode-locked laser is a frequency ruler which can measure the frequency interval between optical sources.^1,2,3 The comb’s m^th mode frequency is given by v_m = (Δϕ/2π+m)/τ_RJ where τ_RT is the round-trip time of the pulse in the laser, and Δϕ is the pulse to pulse carrier-envelope phase slip⁴ or “offset”. An optical frequency standard based on a mode- locked laser requires a stabilization scheme that (i) locks the repetition rate 1/τ_RT to a stable reference and (ii) fixes the relationship between repetition rate and optical frequency. We have shown⁵ that both parameters Δϕ and τ_RT can be fixed by stabilization to a long (equal to the laser cavity length) Fabry-Pcrot reference cavity. Contrary to other techniques,⁶’⁷ our stabilization scheme does not depend on an independent, stable radio frequency (RF) source to lock the laser repetition rate.

© 2002 Optical Society of America