Dennisyuk Question

[Dutchelm05]

I have been troubled by my set up lately. Hope you all can help out.

I am using a single beam set up.
I use a concave mirror instead of a lens and spatial filter.
The beam is fairly divergent.
I get some newton rings and just sort of lived with it for now.

John showed a video of a Single beam set up in which he moved the mirror around and everything was OK.
When i do this I get rings (looks like mode hops!!) I move my curved mirror
I only move it a mm or so.

Is there something screwy with using a concave mirror? In my head I can see why it would mess up. You are moving the angle of the reference beam.
When doing this I am guessing that John was moving a flat mirror.

Can anyone confirm my thoughts?
or is it back to :wall: :wall: :wall: :wall: :wall:

Thanks guys!!

[Colin Kaminski]

I think the idea of moving the mirror is to find a dust free spot to bounce the laser beam. The rings are likely a single spec of dust.

[dave battin]

you should be able to move the the mirror or the beam of the laser itself with no problem, i shoot like this all the time with my wife working in the kitchen and the mirror is directly under her feet! I would suspect the there is some movement somewhere in the object/film...............?

[Dutchelm05]

Thanks Dave,

So my goal is to use the mirror to smear out the rings and such.
Each time I've tryed this it seems to give me problems.
As I imagine this as a ray trace excersise as I move a curved mirror back in forth (hither and dither for you folks in the UK) am I not also changing the angle of the reference beam? As well as the angle reflecting back from the object?

Perhaps your wifes footsteps are a bit more subtle than me pushing on the mirror with my might finger.


Actually Colin, I think the rings are caused by the curved optic itself and not from a piece of dust.

[JohnFP]

Hey Tony.
I am a little confused. Your beam exits the laser raw and collimated just like it exits most lasers and is a few mm in diamter. Then you bouce it off a concave mirror to expand it, right? and you see newton's rings? That should not be the case. You should not see any rings off of simply bouncing a raw beam off a concave mirror. Is it possible the mirror is coated on the back side instead of the front side? That would give you rings and it would behave as you mention. If not, is there a coating or clear protective covering over the concave mirror if it is front surface?

That mirror, if it is front surfact should act exactly like a lens (except reflecting instead of difracting). If you get no rings from the lens, then you should get none from the mirror. Curvature alone will not produce this. Something is changing the phase of some of the light in order to get it to destructively interfere with the rest of the beam to see those rings.

Do you have another concave mirror to try?

Also, to send a raw beam into a concave mirror and have it "failry divergent' must indicate this mirror has a pretty tight radius, not that that should matter.

[dave battin]

And Tony is this the blue laser or green? A neat trick i have done it to shoot the hologram twice moving only the laser beam slightly between exposuers, helping to clean up a lasers bad profile/optical sworls ............

[Joe Farina]

Tony, I'm a bit confused also. You're using the blue dpss laser? Do the rings show up when you put the beam through a good, clean expanding lens?

[Dutchelm05]

Tony, I'm a bit confused also. You're using the blue dpss laser? Do the rings show up when you put the beam through a good, clean expanding lens?

Then you bouce it off a concave mirror to expand it, right? and you see newton's rings? That should not be the case. You should not see any rings off of simply bouncing a raw beam off a concave mirror. Is it possible the mirror is coated on the back side instead of the front side? That would give you rings and it would behave as you mention. If not, is there a coating or clear protective covering over the concave mirror if it is front surface?

I am very sorry guys. Newton Rings is not what I have although the result on my hologram looks like them (I should have made that more clear) I believe they are defractive rings caused by the interaction of the laser and the concave mirror. I hope you can see the photo.

Dave are you suggesting that I shoot half the time at one angle and the move it slightly?? I was doing moving it during the exposure.

Joe, yes the rings go away when I use a lens or a weaker concave mirror.

Defraction Rings.jpg (30.89 KiB) Viewed 2812 times

[Joe Farina]

Thanks for posting the photo Tony. Offhand, I don't know what's causing the rings.

For those people not familiar with the output beam from a Melles Griot 458nm laser, it's rather divergent, and oval or rectangular in shape. This is in contrast to the output from a 315M, a HeNe, or an argon laser, which are more well behaved and circular in shape. Gas lasers especially have very nice beam quality, I think dpss lasers in general tend to be more divergent.

Maybe something is happening when the large, elliptical beam from the Melles laser is reflected from that particular concave mirror? It might be interesting to try a HeNe and see if the rings appear.

I guess your objective, Tony, is to get rid of beam imperfections that show up in the final hologram. I know this can indeed be a pain when dealing with the Melles laser. Spatial filtering is complicated by the elliptical profile. But it should be correctable by using two cylinder lenses (before reaching the spatial filter).

It might be interesting to try focusing the dpss beam with a positive lens, to a small spot on the concave mirror, to see if that has any effect. It might defeat the purpose, though, to put the beam through another optic.

If you can't get anything simpler to work, I would just concentrate on using two cylinder lenses and a spatial filter. In the long run, I think that will be your best best.

[John Klayer]

Thanks dutchelm05 for starting this thread on concave mirrors and Newton fringes. I just started trying to use a spherical mirror for the reference. I'm busy making a fine adjustment transfer mirror holder and a fine adjustment spherical mirror holder adapter using Thor Lab positioners. I hope to use the two adjustments to find a "sweet spot" that gives a nice uniform reference on the plate.