Currently, I am putting up a 2 micron mode-locked fiber laser oscillator based on SESAM. However, I am encountering several problems with my experiment, My questions are as follows,
First of all, I know most groups are always using single mode fibers when realizing a passively mode-locked fiber oscillator. I wonder why not use an LMA(large-mode-area,such as Nufern-TDF-25/400) fiber as a gain media. Furthermore, almost all the group doing fiber laser research in the world tend to use the single mode fiber(9/125 or 10/130) when realizing an oscillator. Someone said that an LMA fiber owns lower nonliearity, so once a higher pulse energy and average output power will be got than a single mode fiber when the same SESAM is used in the optical path. Do you think is it right?
Secondly, how many powers can a two-micron passively mode-locked fiber laser reach without any dispersion compensating? Through reading papers, currently I think the maximum of the pulse energy of a two micron oscillator is just 5 nJ, and the average output power only reach 150 mW. Could anyone please give me your idea on this question? Once an LMA fiber is used as the gain media, will the average output power and pulse energy be enchanced largely? Or it will also limited to several nanojoules?
Dear Xinyang,
this is a frequent misconception. Mode locked lasers have to be operated at single longitudinal mode as it makes use of the equal spacing of laser modes. In free space lasers higher order modes experience a Gouy phase shift changing their frequency. This phase shift would rip apart the circulating pulses within a few roundtrips. In multimode fibers the dispersion for higher order modes is even stronger and in addition more complex.
Sorry, the Nufern's LMA fiber is not a single transverse mode fiber. It contains several high-order transverse modes, does this matter when realizing mode-locking?
Operation of a mode locked laser in a higher transversal order is not possible.
Hi, Vincent Lecoeuche. I am sorry, I made a mistake, I always think that LMA-TDF-25P_400-HE is a multimode fiber. If it is really a sigle-mode fiber, may be higher average power and pulse energy would be realized. However, I do not see any group use this fiber with SESAM.
If you are using a 25 μm fiber core as the gain media and operating it in the dissipative soliton regime I believe you can get pulse energy up to tens of nanojoule. If you just operate the oscillator in the typical soliton regime, you I'm afraid you can only get energy no more than several nanojoule. But if the obtained pulse is not a noise-like pulse rather than a perfect mode-locking status, you probably can get energy up to μJ-level.
Hi, Dr. Zhao, I do not know the sentence "if the obtained pulse is not a noise-like pulse rather than a perfect mode-locking status, you probably can get energy up to μJ-level", what is a noise-like pulse? Do you mean “pulse splitting”? At that time, mode-locking does not exist anymore.... And I think "μJ-level" can be realized in a Q-switched state(in the condition of an oscillator).
I'm sorry for a typing error that the "not" before "a noise-like pulse" should be deleted. Typically a noise-like pulse exhibits an ultra-short pulse duration (fs or several ps) around the peak part but a much wider pedestal.
There is no fundamental reason that you cannot use a multimode fibre as the gain medium, provided that you ensure that the cavity as a whole has sufficiently low round-trip gain at all modes except the desired one. But be aware that the internal gain of off-axis modes will normally be higher than that of the lowest-order mode*, so the method can carry all sorts of risks
*Assuming you have some control over the fibre, this effect can be avoided using a lossy near-index-matched cladding or (better still) a "w" fibre with significant penetration of unwanted modes into a lossy outer cladding. These designs can be functionally equivalent to single mode fibres, even though they carry multiple core modes. N.B. that the cladding modes of single-mode fibres are essentially low-gain modes, so the system effect is not all that much different.
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