Mode Hope??

Dichromated Gelatin.
Tony DCG
Posts: 147
Joined: Thu Jan 08, 2015 1:47 pm

Mode Hope??

Post by Tony DCG »

I am sure but not 100% sure that my MG 457nm laser is mode hoping. I would like to sanity check with you guys and see if it make sense.

I was making masters last night. I started with a yellowish hologram which look great. When I shifted it blue it showed a sort of slice bread look, meaning as I tilted the plate in the laser light the image would get bright in some areas and not in others. It seemed the center would be bright then as I tilt it, the center goes dim the the outer image gets brighter and so on.

Does this make since? Would a more Broadband image not show rings then show up when it is shifted to narrow band?

I've been talking to Sam and Phil who suggested a spectrum analyser. I am seeing if I can use one here at work.
holomaker
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Mode Hope??

Post by holomaker »

Tony DCG wrote:Does this make since? Would a more Broadband image not show rings then show up when it is shifted to narrow band?
Tony DCG wrote:Does this make since? Would a more Broadband image not show rings then show up when it is shifted to narrow band?
yes I think you are correct in your thinking as the yellow is allowing more colors to reflect back (wider band)and somewhat overlap as where the blue is reflecting a much narrower band of light showing you the true nature of what’s going on with your laser, try to adjust the output slightly ? an analyzer will show you for sure what’s happening in it's little head! Set up a Fizeau interferometer and see if its jumping.

Very easy to see, expand beam , bounce beam off glass slide and watch interferometer bands seen on wall................

Image
142laser
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Location: Tampa, FL

Mode Hope??

Post by 142laser »

Hi Dave,
Thanks for helping. You are not saying you can test for single frequency with a microscope slide are you? :) If the laser was really bad maybe you could see something (like it was pure noise) but really to test for single frequency you need a laser spectrum analyzer I believe.

Tony, what do you have at work?
holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Mode Hope??

Post by holomaker »

Yes Phil that is what I'm saying you can see if it's a single Frequency if you have looked carefully!
142laser
Posts: 453
Joined: Wed Jan 07, 2015 10:14 am
Location: Tampa, FL

Mode Hope??

Post by 142laser »

OK the microscope slide is at most 2 mm thick. So the PLD an an interferometer is short...some light reflects off the front, some off the back, an interference pettern forms).

How can you see if the laser has one longitudinal mode? A Michelson interferometer I can see testing coherence length with but explain how a microscope slide can serve as a high resolution interferometer? I don't understand that at all.
holomaker
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Joined: Wed Jan 07, 2015 8:01 am

Mode Hope??

Post by holomaker »

Hi Phil , I was more thinking about the mode hopping question, if the glass is flat enough and is possible to get a few fringes across the FOV one can easily see if the Frequency is hopping/changing. Yes ? If noise is present you will lose contrast of fringes ......... :roll:
Tony DCG
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Joined: Thu Jan 08, 2015 1:47 pm

Mode Hope??

Post by Tony DCG »

holomaker wrote:Very easy to see, expand beam , bounce beam off glass slide and watch interferometer bands seen on wall................
Many thanks Dave interesting method.

Can you eloborate a bit more.

Say my beam is expanded, do I place a piece of hopefully paralell grass at an incident angle and reflect the image on the wall?? Sound right? The just watch it?

Can I do it real time?
While shoot a hologram? Have my dennisyuk beam as normal, place a piece of glass in there, expose plate, watch.

Tony
BobH
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Location: Mesa, AZ

Mode Hope??

Post by BobH »

Yes to all questions Tony. You're looking for a nice high contrast set of fringes. If the laser starts going unstable, the contrast will degrade. If the mode jumps' the fringes will shift. There's a unique set of fringes for each frequency emitted by the laser. They will be close together.
Tony DCG
Posts: 147
Joined: Thu Jan 08, 2015 1:47 pm

Mode Hope??

Post by Tony DCG »

Hey all,

Thanks for all the input..

So I tryed as Dave suggection and saw a very nice set of fringes.
I watched it for 10 minutes and saw no change in either the fringes moving or drifting.
I will try it real time while I shoot film tomorrow

So this shows me that my laser is running in Single Longitudinal Mode I am guessing.
Anyone case to guess what else might be happening?
holomaker
Posts: 772
Joined: Wed Jan 07, 2015 8:01 am

Mode Hope??

Post by holomaker »

When you turn the laser on before it runs stable are you watching it mode hop, also watch for them to drift too as this is a wavelength variation as well, that is why you only want to see s few fringes expanded and not ten or tweny as this shuttle change is small
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