Mode-Locked Lasers: Lower Average Powers in Shorter Pulses

Posted on 2008-03-26. Permanent link: http://www.rp-photonics.com/spotlight_2008_03_26.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on mode-locked lasers, ultrafast lasers, ultrashort pulses

It is very common that those mode-locked lasers generating the shortest pulses tend to offer lower average output powers and pulse energies. This could be surprising, since there is no direct relation between the average power capability and the pulse duration. However, there are various reasons behind the mentioned correlation. Here are some examples:

• Shorter pulses have a larger optical bandwidth, so that a gain media with a larger gain bandwidth are required. Unfortunately, such media tend to have a lower thermal conductivity. This is not surprising if they are glasses: the random order in a glass increases spectral broadening of electronic transitions, while also disturbing the flow of heat (phonons). Even for crystalline media, a broader gain bandwidth very often comes together with poor thermal properties.
• An increased emission bandwidth of a gain medium also usually implies lower peak cross sections. This means that the sigma-tau product is reduced, which tends to reduce the power efficiency. Furthermore, lower cross sections lead to a higher tendency for Q-switching instabilities, and measures used to keep those under control can often further degrade the power efficiency.
• Shorter pulses lead to higher peak powers (for a given pulse energy) and thus to stronger nonlinear effects. This can limit the pulse energy and peak power.

These are just some typical issues, which introduce trade-offs between different performance parameters of a laser design. Any laser design involves many different aspects with many more or less obvious relations between those, and any dropped requirement (such as a particularly short pulse duration) may allow the designer to shift certain properties so that better performance in other aspects can be achieved. And of course the opposite holds as well: demanding more on one side can compromise other features – at least when the overall design is optimized, so that no room for general improvement is wasted anywhere.