Abstract

Previously we have demonstrated that combined time/frequency active stabilization allows one to tightly synchronize two separate, passively mode- locked femtosecond.¹ This capability has allowed phase locking between the carrier waves of the two synchronized Ti:sapphire lasers.² However, since the characteristic timing jitter of 5 fs (160 Hz bandwidth) was larger than the optical cycle period, phase locking could only occur for a few tens of microseconds at a time. Coherent optical pulse synthesis from these lasers was subsequently demonstrated.³ With substantial improvements to the passive stability of both lasers and implementation of a fast servo loop to stabilize the laser repetition frequency, it is now possible to obtain a sub-femtosecond level of timing jitter at any pre-determined timing offset between two laser pulse trains. This reliable setting of the timing offset can be electronically controlled at a high speed and with an excellent repeatability. Another possible approach for achieving very low timing jitter uses cross-phase modulation to passively synchronize two mode- locked lasers that share the same intra-cavity gain medium.⁴ However, the requirement of sharing an intra-cavity element limits the flexibility and general applicability of this technique.

© 2002 Optical Society of America