Soliton Mode Locking | <<< | >>> | Feedback |
Definition: a mechanism for laser mode locking based on soliton pulses
For the generation of femtosecond pulses, soliton mode locking is a frequently used technique, in the context both of mode-locked bulk and fiber lasers. Here, the total intracavity chromatic dispersion is made anomalous e.g. by inserting a prism pair. For a suitable balance of dispersion and Kerr nonlinearity, quasi-soliton pulses can be generated. For true soliton mode locking, these soliton shaping effects play a dominant role, and the pulse duration is nearly independent of other parameters. A saturable absorber is still required for starting and stabilizing the mode locking, but the pulse duration may be significantly shorter than the response time of the absorber.
Compared with the regime of mode locking with near-zero chromatic dispersion in the laser resonator, soliton mode locking allows for significantly stronger nonlinear phase shifts due to the Kerr nonlinearity, which could otherwise make the pulses unstable. The optimum nonlinear phase shift per resonator round trip is normally between some tens and a few hundred milliradians:
- Too low values of the nonlinear phase shift lead to a stronger effect of other influences, such as pulse shaping details of the saturable absorber. This can happen for mode-locked solid-state bulk lasers operating with relatively long pulses; in this regime, inconveniently large amounts of anomalous intracavity dispersion are often needed.
- On the other hand, too strong nonlinear phase shifts can make the pulses unstable due to the periodic disturbance on a length scale which is not much shorter than the soliton period. In this regime, which is frequently encountered in mode-locked fiber lasers, Kelly sidebands often occur in the optical spectrum. The necessary limitation of nonlinear phase shifts, combined with the fundamental soliton condition, implies certain scaling laws for soliton mode-locked lasers. For example, quadrupling the chromatic dispersion in the laser resonator allows the pulse energy to be doubled, while the pulse duration is also doubled, so that the peak power remains unchanged.
For mode-locked bulk lasers, soliton mode locking usually works best for pulse durations below 1 ps. For pulse durations well above 1 ps, impractically large amounts of anomalous dispersion and also possibly elements for an enhanced total nonlinearity would be required. Only for pulse durations below 10 fs, nonlinear phase shifts usually become so strong that the stability of the circulating solitons requires a very strong saturable absorber. For mode-locked fiber lasers, on the other hand, the nonlinear phase shifts are strong enough in the regime of multiple picoseconds and typically become too strong for pulse durations well below 1 ps.
When used in the appropriate regime of nonlinear phase shifts, soliton mode locking normally allows for very high pulse quality, i.e. for well-shaped close to bandwidth-limited ultrashort pulses with low chirp. In the case of fiber lasers, soliton mode locking in the picosecond regime works well, but usually limits the pulse energy achievable to a few picojoules, whereas Kelly sidebands occur in the femtosecond regime.
Bibliography
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See also: solitons, soliton period, mode locking, dispersion compensation, Kelly sidebands, mode-locked fiber lasers
