View Full Version : Need help aligning optics in a Coherent yag laser head

10-17-2016, 16:29
Hello everyone, I'm hoping I can find some help with the issue in facing. I'm fairly new to lasers so any help is greatly appreciated, I have a coherent yag laser head which connects to the diode / power supply through fiber cable, the head had one of the mirrors and the lens broken off, the lens indicated by a green line in the pic I was able to glue back on using the marks from where it was previously mounted, the mirror on the far side of the resonator was broken off and the one closest to the lens broke off when I was checking the rest of the optics both indicated by the red line, at the moment both mirrors are adhered by 2 way tape which allows for some amount of turning. The biggest problem I'm facing is the fact that the diode pumps IR light into the resonator making it impossible to make any adjustments live, and yes I do have the proper eye protection. So my questions are, how to align the two mirrors, how to overcome the IR light issue, can the mirrors be aligned with the yag crystal? In place, do the mirrors have to be facing in any particular direction to reflect or pass the beam through.

10-17-2016, 20:54
A CCD camera will help you "see" what's going on with the IR. Make sure to use one you don't mind sacrificing (an old cell phone or camcorder) since it could damage the camera depending on the power of the pump source.

10-20-2016, 07:06
I'd bolt the head to an optical table, and direct a green probe beam (level to the table and square with the head) backwards into the head. With the probe beam set up so it could be translated horizontally and vertically without changing its tilts, I'd find the best path through the system by translating the probe and positioning the beamsplitter (near the lens) so that the beam goes through the crystal cleanly and centered. Then, the rear mirror can be added and positioned to retroreflect the probe. Way easier said than done, especially if you don't have a bunch of multi-axis translators.

10-20-2016, 09:17
Thank you both. I actually have been using an old Sony camera with night vision to help me see, but that's less than ideal since even with eye protection I still don't feel all that comfortable looking at the optics directly with the laser on. Another thought that I had was to buy one of the cheap fiber fault detectors from eBay which is a 30mw red laser and pump that through the fiber cable just to see how the beam is hitting the crystal, which brings me to another newbie question, does green or red really matter in this case? But that combined with feeding the beam backwards might actually do the trick. Also since I started this post I took the culminating lens back off being that it's asymmetric I'm starting to question the position of it and the beam did not look all that tight at the crystal.

10-21-2016, 12:23
With the eye protection in place, you should have no fear running the laser. I want to help, but your description is confusing to me. Can you start from the beginning and confirm that you have a nice round IR beam exiting the fiber before it even begins to enter the head? If this is the case, then if it were me, I would start by launching this beam into the head and confirm that the steering mirror that sends this beam to the YAG crystal, does in fact center the beam as well as I can determine.

I would not try to align, even roughly with loose or sketchy materials such as tape. This is not because of any safety concern, but rather that the step wise pattern of alignment will be very frustrating if a subsequent step is jeopardized by play at some, who knows what, previous step. This can seem tedious, but if you want to be confident that you are going to get to the finish line AND STAY THERE, you can't skip around.

Focusing the pump beam at the crystal will be the next step and you will not actually want this focus to occur at the entrance face, but some distance inside and this position is pretty important as it will determine how closely the pump energy distribution matches the cavity mode(s). A very tight focus before some of the energy has been absorbed can fracture the crystal.

The cavity optics are actually not difficult to align. You can often start with a simple visual alignment (they look parallel) and move each back and forth with the optical mounts and get lasing to start. From there, you go back and forth with each mirror to circularize the output and maximize the power. With a high gain laser like a YAG, I have never needed to use a secondary, alignment laser.

10-23-2016, 13:51
Thank you for your reply Planters, if you look at the latest picture the 3 labeled optics were broken off, 1 the culminating lens and 2&3 turning mirrors, all 3 were soldered to the pcb board so there is no adjustment, that was the reason for temporarily using the tape. The beam coming in appears to be good, but at the crystal directly behind the 1 and 2 optics it does not appear very tight, one reason for it might be what you mentioned about fracturing the crystal, flipping the lens produces no significant difference that I can see. As of right now I'm getting a lot of light scattered throughout the cavity and no significant beam that I can see


10-25-2016, 13:55
The pads under the optics mounts are often electrical heaters on these sorts of ceramic based lasers. Apply just enough heat to melt the solder... Get the idea?