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Old 11-16-2013, 07:50 AM #1
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Default zero aperature....airy disk?

I have a project, and I am waiting for my parts to come in. My intention is to use a beam expander to take a 405nm beam to appx 12mm diameter, then pass it through a zero aperture iris. Beam power is less important to my particular project than maintaining and controlling beam diameter along the entire beam.

I am no optics ninja, in fact other than teasing cats and sighting my gun, Im a complete noob. So I just discovered this Airy Disk thing, I am curious to know 1st does a zero aperture iris cause this same distortion pattern? If so, what is necessary to eliminate it? If not, just how fine a beam do you think is possible given the arrangement described?

I dont mind you dumbing it down if necessary...Ill even read a bunch of links if you feel it necessary for me to understand.

Thanks in advance
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Old 11-16-2013, 04:13 PM #2
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Default Re: zero aperature....airy disk?

Why is the expansion necessary?

"passing it through a zero aperture" is contradictory, since an aperture of zero blocks all light.

All you should need is a pointer, and some sort of dark material with a tiny, tiny hole in it.
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Old 11-16-2013, 10:47 PM #3
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Default Re: zero aperature....airy disk?

From what I was able to gather, the zero aperture iris blocks all light (as Cyparagon correctly mentioned). The Airy Disk is a pattern produced by even the most tight focus point achievable by any means. There are ways to minimize it, but you can't completely eliminate it.

The pinhole approach may be as good as any other to achieve a small point, although most of the light may be wasted. A good aspherical lens may be the easiest solution. At least you'll get to use all the light. I presume this is why you want the beam expanded? You shouldn't need it to achieve focus though, but I'm also curious about the beam expander use, since it seems to be often used in optics setups. Anyone?

Ref. links from a simple google search:
Airy disk - Wikipedia, the free encyclopedia
The Airy Disk
http://www.edmundoptics.com/optomech...iaphragms/1373
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Old 11-17-2013, 01:40 PM #4
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Default Re: zero aperature....airy disk?

to answer both of you....
The beam is first expanded to 12mm then passed through the zero aperture iris which varies from 12mm fully open, allowing the full expanded beam to pass, to the zero position fully blocking the beam.

My concern and the reason for this question being posted, having learned that a laser passed through a pinhole can exhibit this "airy disk" phenomena and not fully understanding it, I am wondering firstly, if this will occur at a very tight setting of the iris, and if so what can be done to block or reduce it.

Wasting light is of no concern for my project. Minimal divergence along beam length regardless of effective beam diameter is my main concern, uniform power density at varying beam widths being secondary. The expander and iris should accomplish this, but if at the finest beam diameter, as the iris first opens, this airy disk happens I need to work to counter that effect as it would be highly undesirable.

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Old 11-17-2013, 04:18 PM #5
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Default Re: zero aperature....airy disk?

Yes, it will occur at a low aperture. This is because of diffraction - a law of physics - so there's not much you can do about it. Just keep it above 1mm or so and you shouldn't see it become a problem.


As for the expansion, most people will tell you to use option one or two, but I prefer option 3 myself. Choosing the lens(es) just requires a bit of geometry work.

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Old 11-18-2013, 06:52 AM #6
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Default Re: zero aperature....airy disk?

1mm? lol yeah thats not a pinhole thats a portal. Id be better off using a combination of a variable expander/condenser and power control if that ends up being the point of minimizing diffraction scatter.

I spent the day studying the maths involved in computing airy disk diameter. I have a much stronger understanding of the issues at hand now and have a few more expensive than I had planned on options already.

Im still hoping that Ill find someone here familiar with this issue that could recommend some form of collimating filter or other such simple means of blocking the diffracted portions of the beam.

I appreciate your attempts to assist but I think we are on two entirely different pages. Ill be sure to both check in for others input, and when I have completed my assembly update this post with some images and explanations of the whys and hows.

Thanks again.
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Old 11-19-2013, 05:46 PM #7
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Default Re: zero aperature....airy disk?

try multiple pinholes, with large distance, and output pinhole larger than 1mm? same idea as blocking the flood on a flashlight using a paper towel tube.

I think the problem is that your pinhole is too small. Perhaps changing wavelength will reduce airy disk enough for your application?
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Old 11-19-2013, 05:50 PM #8
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Default Re: zero aperature....airy disk?

How about looking into CD-ROM optics? They get that spot super small, don't they?
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Old 11-21-2013, 04:05 AM #9
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Default Re: zero aperature....airy disk?

Bshan, I get what your saying. Once my order arrives and I get everything benched up I should have a much better idea of what or how significant the issue really is. Switching the laser wouldnt really be an option, Im using 405nm and the expense of a shorter wavelength is pretty significant. A second pinhole would have to be tuned to the diameter of the disk so it would be adding an iris....its something Im considering. I had really hoped the solution would be as simple as a filter of some sort but....it doesnt seem to be quite that simple.

Of course, as most things theoretical, I may be worrying over nothing. The effects of the disc may be negligible once I am actually testing the design.

Sly, Cdrom optics are convergent. They dont use a pinhole so they arent effected by this phenomena. Focusing at a fine point is a very easy and practical way to get a tight spot at a known focal length. I need to maintain that diameter regardless of the plane of intersection, so I would have to dynamically track the target and adjust the optics on the fly...which is far more complicated a solution....of course there may well be a reason that it is the usual solution as well....the airy disc may be the why behind the complexity I am trying to avoid by using this aperture iris method.
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