Has anyone known the reason of the burn out in the middle of a rare-earth doped fiber? I used a polarization maintaining fiber for laser experiment, however, it was burned out in the middle of it. Before it was burned out, the pump power is about 30W and the pump power is just 0.65W. Has anyone ever met this situation? The two end facets of the fiber were both well cleaved. So how to prevent it from being burned out in the middle of the fiber before it happened.
I guess the 30 W is the output power from the pump laser and 0.65 W is the power coupled into the doped fiber. As your description, the coupling scheme is batt coupling?
If it does not burn at the input facet, I guess the fiber has some imperfections somewhere. Did you implement a pump dump?
Hi Xinyang, it is probably due to thermal Lensing effects, i have not experienced this effect myself, yet i think ist the most certain reason for exeeding the damage threshold of the fiber.
Best Regards
It's possible that you exceeded the damage threshold of the fibre. Many fibres probably can't tolerate 30W of power under optimal circumstances
Generally, you should be aligning your fibre at low power. You shouldn't be pumping with 30W if you're only getting 0.65W out. Pump at 100 mW or less first, and make sure you're getting optimal coupling. You shouldn't be losing more than a few percent in a single-mode fibre, maybe as much as 50% if it's a multimode fibre. 2% transmission is very low, and almost certainly means that there's a problem with your alignment or your fibre, and this should be corrected before using the high-powered beam. Otherwise, you are risking further damaging your fibre by dumping the bulk of your laser power into heating the fibre.
There is an effect called parasitic lasing that can occur in rare-earth fibres. Without knowing more about your experimental setup, I can't say that this is the cause, but it's something to look into.
In an un-doped fibre, I would have guessed that it was some variant of the "fibre fuse" effect. Local heating raises the local absorption coefficient, so that more energy is absorbed and the temperature rises exponentially. This can lead to catastrophic failure, in which a damage zone glows visibly and propagates back towards the pump laser. This is typically initiated by damage or contamination of the exit facet of the fibre, or by a tight radius bend which dumps energy into the polymer coating and heats the fibre indirectly.
In rare earth doped fibres there are additional mechanisms including excited state absorption, cooperative up-conversion, fluorescence quenching and cross-relaxation which can transfer pump energy to lattice vibrations. I don't have direct experience of these at the power levels you are using, but this could induce damage directly, or via local heating and the fibre fuse described above.
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