Abstract

Passively modelocked fiber lasers^[1-3] are a potentially very attractive source of ultrashort pulses for ultra-high bit rate communications. However, despite the large variety of suggested cavity configurations, passively modelocked fiber lasers have to date been limited to operation at low repetition rates. The reason is the low gain cross section of rare-earth doped fibers and their tendency to energy loss via cross relaxations at high rare-earth doping levels. As a result useful active fiber lengths are typically not shorter than about 1 m for the case of erbium ions⁴ (neodymium fibers can be substantially shorter, but fiber lengths shorter than 1 m are still uncommon due to handling difficulties). Thus passive mode-locking in fibers at the fundamental cavity frequency is typically limited to about 100 MHz. Higher repetition rates may in principle be obtained by the insertion of sub-cavities⁵; however, the laser then becomes sensitive to phase fluctuations between the major and the sub-cavity, which makes stable operation nearly impossible. Therefore, to date, higher-harmonic modelocking was only possible by resorting to active modulating schemes, which are phase insensitive and hence can provide stable pulse trains at repetition rates up to a few GHz.

© 1993 Optical Society of America