I want to couple light (785nm,~80mw) into a polarization maintaining fiber PMJ-3HPC3A-780-5/125-3AS-1-5-1 (5nm core diameter) with a coupler installed on the laser head (Sacher TEC 400 laser) .The coupler has three tilt knobs and literal moving lens. I tried my best to couple maximum light but can't exceed coupling efficiency more than 0.1%. I am now frustrated and out of ideas. I also tried company instruction but again no luck. can you please suggest or help me find a way to resolve this problem. The below pictures are the instruction provided by company for coupling.
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$\begingroup$ good video here: shop.ozoptics.com/pmj-3u3u-850-5125-3-5-1 $\endgroup$– PhysicsDaveCommented Oct 21, 2018 at 17:28
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2 Answers
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Do you have the means to free space couple a fiber with laser light at 780nm? If so you could remove the coupler from the laser and send light backwards into a fiber and have it exit out of the coupler. You could then adjust the sliding lens adjuster (using the tool provided) to ensure the fiber coupler is roughly collimated. A red fiber checker could work for this purpose. It won't be exactly right because red isn't the same as 780 nm but it's close enough for 1st pass alignments.
Make sure you properly understand the function of the 3 screws indicated with red arrows in the diagrams above. These screws provide counter-force against the alignment screws and hold the coupler in place but they can make adjustment of the adjustment screws (marked in green) a bit tricky.
Finally make sure the lens in the collimator slides easily when you adjust it with the slider tool (fig 3). There are also set screws (on the side body of the collimator) which lock the lens in the place so that the sliding mechanism does not work. If you try to adjust the lens when these set screws are locked you will end up stripping out the notch that the tool grabs onto in such a way that you can't really adjust the lens easily anymore. I've made this mistake. Make sure the set screws are loose enough that the lens slide but that tight enough that you have some control to reliably position the coupling lens.
0.1% is very low coupling efficiency. It seems like something is wrong. Make sure the fiber you are using is appropriately matched to the collimator. For example, a PC fiber should not be used with an APC fiber collimator/coupler and vice-versa. APC vs. PC.
If your problems persist I recommend contacting the company who provided the laser and/or coupler.
answered Oct 21, 2018 at 17:07
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Thanks @jgerber for your comment. Did you have any experience on these type of couplers?. To me it seem like i am unable to hit the main focusing peak and wondering in some local maximas or tales of the maximas. I also tried to couple in an other single mode 785nm fiber but the efficiency remains more or less the same. Moreover the output of the laser after focusing lens is not symmetric in horizontal and vertical planes. When I paced a beam profier at approximately 60mm from the lens. The horizontal beam waist was 1972.1um while the vertical beam waist is 6027.3um but although its not symmetric I would expect two focusing points where the intensity shoots up. If your have any experience with such couplers how one can first find the main peak at first place for which the tilt mechanism then will ensure the maximum coupling.