Mode Hope??

Tony DCG · Post by **Tony DCG** » Sun Sep 29, 2013 10:18 am

Hi Dave

Yes when I first turn on the laser I can see the fringes jump around for the first 10-20 seconds then solid

Then I watch the large fringes and see if the drift
so far so good

I will do it with film in place next

Tony

142laser · Post by **142laser** » Mon Sep 30, 2013 7:59 am

The microscope slide interferometer does not have enough resolution to tell much if anything about longitudinal modes; it may work for checking a diode laser but not a crystal laser; I bet a stable HeNe looks great and has 5 or more modes with a 1GHz linewidth. I wish it was this easy to check SLM lasers. If theye were a second mode at say 30% of the main mode height you would get the stripes on the holograms but the image from the slide would look fine. You can only tell gross malfunctions I believe. Who do you know in CA with a scanning interferometer?

Fringes shift for other reasons...loose optic? Heat source added to table recently causing slow thermal expansion? Check everything else you can think of in case the problem is not the laser at all.

Jeffrey Weil · Post by **Jeffrey Weil** » Mon Sep 30, 2013 9:28 am

How about just doing a very simple Deni with a flat, very solid object. Like Ed W's waffle iron. A setup like that can barely move but you'll see the bread slices if your hopping.

142laser · Post by **142laser** » Mon Sep 30, 2013 10:19 am

The problem with watching for mode hops like this is they happen fast...50-100 mS and the new mode is there....it is just a blink. If you have just one mode lasing and the laser feedback can't maintain it there will be a blink but it is fast. Also you caoulf not tell if there are two or more stable modes...I bet a HeNe looks great right?

Tony do you have a standard oscilloscope? Sam can send a scanning interferometer system for you to borrow for a week to test the laser if you can't trace the problem. This would be a system we have for sale but you have no obligation to buy; just pay shipping/insurance. Let us know.

Phil

Ed Wesly · Post by **Ed Wesly** » Mon Sep 30, 2013 3:56 pm

Have you tried another hologram since the one described above? Did it come out OK? My guess, for what it's worth, might just simply be plate movement.

142laser · Post by **142laser** » Mon Sep 30, 2013 6:59 pm

Thanks Ed, that is easier to fix than the laser and it could well be.

You have the same laser and more than a little experience making holograms...

Thanks for helping!

Tony DCG · Post by **Tony DCG** » Mon Sep 30, 2013 11:19 pm

So I know one way to check for mode hopping is to take a hologram of a deep object (I think thats what I heard). One should see rings if it mode hops

I decided to take a holo of a flat object. After processing I see rings a big one right in the center
The glass is thick so I dont think Im drooping

The set is a Denisyuk and the plate is sitting on three ball bearrings
I let is set for 20 minutes
1 minute exposure

Yes Ed I tryed several shots with the glass plate in place. No movement rings on my object when I go narrow band.
Does that offer any clues?

Tony DCG · Post by **Tony DCG** » Mon Sep 30, 2013 11:32 pm

Here is what one looks like broad band

holomaker · Post by **holomaker** » Tue Oct 01, 2013 8:30 am

That one picture sure looks like there's bowing during exposure

Tony DCG · Post by **Tony DCG** » Tue Oct 01, 2013 9:43 am

Thanks Dave
The glass is like 1/8" only 5x6" so I doublt the glass is bowing
The object could be it since it is PCB plasic

If its a flat object mode hopping shouldn't effect it right?