Opt. Lett. 36, 4443–4445 (2011)

Femtosecond mode-locked erbium-doped fibre (Er-fibre) lasers operating in the telecommunications wavelength range of 1.5–1.6 μm are a valuable light source for optical communications, such as long-distance clock distribution via fibre-optic links. So far, the best timing jitter performance demonstrated for free-running Er-fibre lasers is 2.6 fs. Tae Keun Kim and co-workers from the Korea Advanced Institute of Science and Technology have now demonstrated optical pulse trains with timing jitters of less than 100 as from two nearly identical 78 MHz free-running mode-locked Er-fibre lasers. Both lasers had a central wavelength of 1,582 nm and a pulse width of ∼60 fs. One of the lasers contained a sigma cavity with a piezoelectric transducer mounted mirror in its free-space section to enable repetition-rate mode-locking between the two lasers. The net cavity dispersion was set to −0.002 ± 0.001 ps2 at the central wavelength by balancing the positive dispersion of the Er-fibre and the negative dispersion of standard single-mode fibre. At high offset frequencies of above 8 MHz, the timing jitter spectral density was around 10−12 fs2. The integrated root-mean-square timing jitter for offset frequencies of 10 kHz to 38.8 MHz was 70 as, which is, according to the team, the lowest timing jitter ever achieved for free-running mode-locked fibre lasers.