Easy question.....I hope!

rzeheb · Post by **rzeheb** » Sun Jul 05, 2009 12:03 pm

I recently upgraded my system with the purchase of a variable beamsplitter. The flexibility it gives me in controlling the light intensity between my object and reference beams is awesome. However, I noticed a problem that maybe I can get some advice on. Presumably, because the light is reflecting off of both the front and back surfaces of the beamsplitter, I now get interference fringes in both my object and reference beams. It's does not appear to be much of a problem for my object beam since the light gets diffused every which way anyway, but it does create uneven illumination in my reference beam which is a problem. The beamsplitter is thin enough and my path lengths are short so that I can't separate and block off the secondary reflections to prevent them from interacting. Any ideas?

Joe Farina · Post by **Joe Farina** » Sun Jul 05, 2009 4:45 pm

One way to solve the problem is to cement a thicker piece of glass to the backside of the beamsplitter using an optical adhesive. Then there will be enough separation between the two beams to block off the back-reflected beam. The only difficulty might be in finding a thick piece of glass. The piece I used with my Edmund variable beamsplitter was either 3/8 or 1/2 inch thick (I can't remember which) and this provides adequate separation. I'm not really sure if commonly-availbale 1/4 inch plate glass is thick enough for this purpose (maybe another holographer can help with that question). An ordinary UV-curing glass adhesive (which might be available at a hardware store for glass repair, such as Super Glue Glass Adhesive) should work well for this purpose. But the completed assembly should always be stored indoors in a reasonably low-humidity environment, or else it may begin to delaminate over time. I didn't do this with mine, and it has suffered a lot of damage over the years. (See the thread on "Casting Resin" in the General Holography section for a discussion of the same problems encountered with sealing DCG holograms in glass.

Colin Kaminski · Post by **Colin Kaminski** » Sun Jul 05, 2009 6:54 pm

It should be obvious but make sure to glue the glass to the non-silvered side. I have done this a lot and used the sun as my UV source. So far I have not had any issues. I used 1/4" plate glass but I had 6' from the beamsplitter to the first mirror and it was a 1/4" diameter mirror. I used Norland UV cure glue from Thorlabs. The secondary reflection simply missed the mirror. When I have a chance to set up again I will end up with more room.

rzeheb · Post by **rzeheb** » Mon Jul 06, 2009 11:36 am

Joe/Colin,

Thanks much for the suggestions! I suppose I'll start by seeing if I can create more working room in the arrangement of my optics that might allow me to block (or simply miss, as Colin did) the secondary reflections. Failing that, I'll give more thought to thickening the splitter. It's a circular, continuously variable splitter that appears to be some sort of OEM replacement that fits (more or less) on a Jodon 200 base. Ideally, I would want the glass that I use to match all around (i.e, to also be circular and of the same ID and OD as the splitter), and flat enough to mate well with the splitter. I confess to being a bit nervous about destroying the splitter in the process.

dave battin · Post by **dave battin** » Mon Jul 06, 2009 12:25 pm

hi all , let me add my two cents here as well. i would advise you to add the extra glass to the back as well to get that nasty banding out of the way, ive done this many times and it works well, use a clear epoxy such as "water clear"named brand, i have see these inconel attuneators before ,and was thinking the best way to tackle this would be to find (easly online) a surplus optical window, you may not get the diameters to match perfictly, but from there it would be easy to mount.you might even take the window to your local glasier and have him drill a same size hole as your beamsplitter, making it cuite easy to alighn the two when you are epoxying them together.

check this one , http://www.surplusshed.com/pages/item/pm1168.html

Joe Farina · Post by **Joe Farina** » Mon Jul 06, 2009 12:52 pm

Just to clarify something I said above about the long-term adhesion of the cement. As long as you store the finished beamsplitter indoors, it should be OK. Just don't keep it in an unheated building, a damp basement, or any place where the humidity goes over ~75% for a week or more. There's no need to be daunted by the cementing process, it's not that difficult. I would just practice a little bit before actually cementing the beamsplitter. Sometimes bubbles can be a problem, as they can get trapped in the adhesive between the glass pieces. This can usually be correcting by pressing the pieces a little more tightly together, by using your finger in the area of the bubbles and pushing them out. If that doesn't remove them, then you can slide the top piece around before the adhesive sets, and the bubbles will get "skimmed" off by the edges of the glass. Just practice a little bit first.

rzeheb · Post by **rzeheb** » Mon Jul 06, 2009 10:20 pm

Thank you one and all for the very helpful advice. Dave, the link was great. Frankly, I wouldn't have known what to search for so there was little chance that I would have found what I needed without your help. As it is, I have already placed the order

Same for Colin and Joe. The optical cement has been ordered and I feel (at least a modicum) of confidence in my ability to solve this problem. This forum is a terrific resource!

Jeffrey · Post by **Jeffrey** » Tue Jul 07, 2009 6:12 pm

are you using the BS at 45 degrees? rather than closer to incident? More spot separation that way.

rzeheb · Post by **rzeheb** » Tue Jul 07, 2009 11:22 pm

Yup, at 45 degrees. But, I recently learned that what I bought was really an attenuator, not a splitter. It's only about 3 mm thick and the laser beam is also close to that size, so the front and back reflections are actually partially superimposed and impossible to separate. I hope that gluing the attenuator to the optical window (which is about 6 mm thick) will provide me with enough separation. My three nagging concerns are (1) the ability to do a good gluing job; (2) will enough UV light (from the sun since I don't have a lamp) penetrate the 6mm window to cure the glue, and (3) the refractive index of the glue does not exactly match that of glass so there will still be some internal reflections (hopefully, very very weak).

Jeffrey · Post by **Jeffrey** » Wed Jul 08, 2009 12:16 am

Don't worry, it will be easy. Use a lot of glue, so it squooshes out the sides and leaves NO BUBBLES.