Through reviewing papers about passively mode-locking realized at 2 micron, almost everyone uses single mode fiber(such as Nufern SM_TDF_10P_130_HE) to realize mode locking? The problem is why multimode fiber(such as PLMA_TDF_25P_400_HE_spec_20130130) can not be used for passive mode locking for build an oscillator? Actually I also once tried this fiber with a SESAM from Batop, I failed to realize mode locking but I watched some unstable Q-switching on oscilloscope. Could anyone explain this phenomenon in theory?
Actually this question has been asked by myself more than one year ago(see https://www.researchgate.net/post/Why_not_use_an_LMAlarge-mode-area_fiber_as_the_gain_media_when_putting_up_a_thulium-doped_fiber_oscillator), I am still confused though many people answered that. This time I want to ask it more clearly maybe a better answer will be got.
The short answer is that the different modes do not have the same group velocity. A short pulse (100 fs) having different modal components would thus have a tendency to spread out and split. This can prevent the formation of a stable pulse in the laser cavity.
However, Logan Wright from Frank Wise group at Cornell has shown that modelocking in multimode fibre laser is possible if one is using graded-index fibres (the intermodal dispersion is then smaller). This approach is really promising to générateur more energetic pulses.
Sorry, Vincent. The fiber PLMA_TDF_25P_400_HE is not single-mode, I am pretty sure about this. You can calculate the V number of it. If V is above 2.4, it should not be single mode. And the sentence "Robust single-mode beam quality" does not represent it is a single mode fiber, we can just say it has a very high beam quality.
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