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Hi Physics StackExchange community,

Today I had to re-do the entire optical path for the delivery of a 830 nm laser from our laser table to the experiment table. For the fiber coupling, I usually use a back-coupling device to quickly adjust the collimator. Since I didn't want to touch the fiber collimator on the experiment table, as everything is precisely aligned, I put the back-coupling device on a test fiber, with nominally the same core diameter (5 um) as the fiber going to the experiment. With this I got ~80% efficiency.

Once this is done, I pull out the test fiber, re-insert the experiment fiber to the fiber coupling setup, and tweak the mirrors and the collimator. After much beam-walking and adjustments, I still couldn't get the coupling efficiency above 60%. Now the test fiber and the experiment fiber are from different manufacturers (the former from Coastal Connections, the latter from OzOptics), it could be that tolerances are different and hence the actual core diameters as well. But I still do not understand why the adjustment of the collimator won't solve that problem. Any ideas on what could cause the difference in efficiency?

Fiber coupling setup: there are two mirrors for beam walking. The collimator (aspheric lens) is on a XY translation stage on cage rods. The fiber adapter is on a Z translation stage on cage rods.

EDIT: checked the experiment fiber, and it was a bit dirty. Cleaned it, but there is still a "stuff" on one of the stress rods, near the fiber core: enter image description here

Either it's a strongly charged dust that I couldn't remove with methanol, or some surface damage. But since it's not directly on the fiber core, would this explain the drop in coupling efficiency?

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  • $\begingroup$ 1. Can you explain more what a "back-coupling device" is? A link to a product description or datasheet would be helpful. 2. AFAIK Oz Optics doesn't manufacture any fiber (fiber drawing equipment is pretty expensive), whatever you bought from them was manufactured by somebody else. I haven't used Coastal Connectoins but they're probably the same. 3. Without knowing the specs of your two fibers and the details of your optical setup there's probably no way for anybody but you to answer this. $\endgroup$ – The Photon Jul 26 '18 at 4:47
  • $\begingroup$ For example, do the 2 fibers have the same NA? Does one or the other have some kind of AR coating on the facet? Have you inspected both facets for dust and/or scratches? Also, coupling into 5 um fiber is just fussy --- go back tomorrow and try again from the start you might get better results. $\endgroup$ – The Photon Jul 26 '18 at 4:50
  • $\begingroup$ @The Photon, 1) "back-coupling device" I meant a device that could be fiber coupled and emit light. Attach this to the collimator in question, and the light propagates in the opposite direction as the incoming beam that needs to be coupled. 2) Yes, I think the manufactures just deal with the connectorization, not fiber pulling. FC/APC and AR coated for NIR. 3) I agree...I did try to find the NA for the fibers, but I couldn't find any relevant data sheet. Let me try again tomorrow. Misc) I didn't check for dust on the cap, I should... $\endgroup$ – wcc Jul 26 '18 at 5:24
  • $\begingroup$ @The Photon, regarding NA, at least the Coastal Connection one is 0.10, and I don't expect the OzOptics one to be substantially different. Since the NA of the aspheric lens (0.68) is much larger, would a slight difference matter, as long as you can adjust collimator position? $\endgroup$ – wcc Jul 26 '18 at 5:26

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