Extremely Long Mode-locked Fiber Laser

Posted on 2009-01-05 as a part of the Photonics Spotlight.

Permanent link: http://www.rp-photonics.com/spotlight_2009_01_05.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: S. Kobtsev et al., “Ultra-low repetition rate mode-locked fiber laser with high-energy pulses”, Opt. Express 16 (26), 21936 (2008)

The above referenced paper describes mode-locked fiber lasers with a resonator length of several kilometers, leading to a pulse repetition rate as low as 77 kHz and a rather high pulse energy of 3.9 μJ. These characteristics are indeed very usual for a mode-locked fiber laser – they would be much more typical for a Q-switched laser.

The impact of nonlinearities and chromatic dispersion in the very long fiber is of course rather strong. In the normal dispersion regime, such a laser can generate strongly up-chirped pulses, which can often be temporally compressed to obtain high-quality picosecond or femtosecond pulses. In the present paper, however, this was demonstrated only for a short version of that laser, having a much higher repetition rate of 22 MHz, but not for the long laser as described above. A challenge is certainly to make a dispersive compressor with a huge amount of dispersion, related to a group delay variation of several nanoseconds within the spectral width of 0.35 nm. Therefore, this laser is currently not a practical source for high-energy picosecond or femtosecond pulses, and in fact it is not even proven that those could be obtained with a suitable compressor.

Anyway, I think it is a good idea to explore such curious parameter regimes, and there may be further progress.