Hi everyone,
DISCLAIMER: do not open a laser case, and if you do make sure you have the right protective equipment, my question is not meant to be a guide on how to fix any particular laser. Do not reproduce the steps I am detailing below.
TL;DR: does anyone know how to optimize fiber coupling efficiency on an OPUS 1064 (image below)?
Probably a bit of a long shot, but I got my hands onto an old 4W (CW) OPUS 1064 laser. Similar to this product from Novanta. Except, the laser I got is branded Laser Quantum Ltd and is dated APR-2018.
The laser, controller, and two optical fibers came in a box. I checked both fibers and they were badly damaged on one side (I’m assuming it was on the laser side, but I will never now since they did not came attached to the laser). I measured the laser power at the fiber port without fiber (I was not willing to risk burning the fiber), and the output power is still comparable to the set value on the screen. One of the fiber had a label: FG025LJA. Lucky for me, it is a fiber from Thorlabs (0.10 NA, 25-um diameter core) and I could get a new one. Novanta also confirmed that this was the original fiber type that was supplied with this laser and mentioned a coupling efficiency >80%. However, when I measured the coupling efficiency, it was more in the order of 50%.
Now, I’d like to get this laser up and running for a heating experiment on a Zeiss microscope. But I don’t feel comfortable knowing the relatively low coupling efficiency. Based on the fact that the laser I got came with two damaged fibers, I’m assuming this is a long-standing problem with this particular laser. Anyway, I have a bit of experience with laser coupling (including single-mode fibers) and thought I could give it a go. The fiber adapter is a sort of bulkhead adapter secured with three screws and nothing to adjust it really. I tried loosening it and move it around with whatever play is in the clearance holes but it is finicky at best and I didn’t get the coupling efficiency to improve much. Then, I broke the warranty seal, and opened the case (picture below).
I won’t pretend like I know about what’s happening inside the laser. But I noticed the focusing lens was sort of cemented to the case (top-left). Therefore, not much to be done on this front. Then, I noticed two 45-deg (dielectric?) mirrors on the top-right and bottom right of the case in the picture. Those mirrors are just secured by a single Torx screw each. Therefore, one could possibly only change the rotation about an axis orthogonal to the mounting plane of the laser. I used laser safety goggles and tried to lightly unscrew the mirrors (one after the other) and push them a bit to induce a little rotation. I used little laser power (40mW on screen) not to damage my new fiber, but of course I am aware that within the case there are much higher powers at play. I noticed the power changes a lot with little displacement of the mirror, which I expected. What I did not expect is that the power is somehow regulated. If I make a tiny mirror displacement (this happens for both), the power might increase quite a bit, say 80mW, but then it slowly goes back to something around 20mW (my original 50% coupling). I’m assuming there’s some sort of feedback mechanism at play, but it made the alignment nearly impossible and in the end, I’m still at 50% coupling efficiency.
My question is: has anyone done that before on such similar laser architectures, and what advice can you give me to improve the coupling efficiency? I really don’t have much budget for this project unfortunately, so sending it back to the company is not an option. I was considering using the laser in free space, but the fiber is preferable.
Thank you for your help and take care,
Omni