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Lens surface accuracy question; 1/2, 1/4 or 1/10 wavelength?

Alaskan

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There is a China manufacturer for large diameter PCX lenses which they specify for magnification use, with an accuracy of 1/2 lambda. Can someone tell me if we are limited to 1/4 accuracy (or greater) with lasers, or whether a lens with half wave accuracy is useful as a beam expander without too much distortion? I'm seeing some optic manufacturers specify lambda/10 ($$$).

Here's their web site, but for the life of me I cannot seem to dig that spec out anywhere now, maybe I'm remembering some other manufacturers spec: Large Lens,Diameter 300mm To 600mm Spherical Optical Glass Plano Convex Lens For Optics Instruments,Bk7,K9,B270 - Buy Large Lens,Large Convex Lens,300mm Convex Lens Product on Alibaba.com

Chris
 
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RedCowboy

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Re: Lens surface accuracy question; 1/4, 1/2 or 1/10 wavelength?

Good question, I know at 75 feet I can see the de focused bar jitter, not from physical movement but from the diodes emitter shifting it's output ever so slightly, the oscillation is up and down, never side to side, it's in the diodes emitter output and only seen under blown up conditions, it is interesting and I will try to get a video of it, but the lenses show the dominate wavelength in the center and the side bars, now the ripple in the lines may be lens imperfections.

I wish I had a high quality camera, but I will try to get a video in medium daylight so we can see it unfiltered.

Looking through my 3X expander you can see that alignment counts, but the center seems dimensionally stable yet the edges are curved, but white light vs single wavelength laser is much more difficult to reproduce.

I expect physical alignment and emitter source quality of multi lode diodes will be a limiting factor before 1/4 or 1/10 wavelength lens grind accuracy, but under extreme magnification at long distance it could be a factor, I don't think it's worth a much higher price though. :beer:

These pics are my cheap camera through one of my 3x from Podo.

CAMERA ONLY


Expander held in front of camera.



For what we do it's probably a non issue, if running a drive in movie theater the projector lens would need to be very accurate.
This is a tree through the expander in a light rain.


Now these I expect would need to be very accurate as they are used for super long distance alignment, it makes the multiple beam approach more understandable as any distortions are comparable, thus one beam could give a inaccurate alignment if a distortion in the sodium layer existed.
 

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Alaskan

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Thanks for the photo's, I can see the distortion at the edges. When I get back home I will try the same test with some of my expanders, good idea. Still don't know what is acceptable surface accuracy for a beam expander lens, not sure many in this group concern themselves with such, we normally just buy them pre-engineered and assembled. I bet Cyp. knows, if he will chime in.

I found this information in regard to telescopes, complex subject, more than just surface errors in angle, but roughness and other factors too:

http://www.telescope-optics.net/fabrication.htm - From this, I'm guessing 1/2 lambda precision (or error, depending on how you look at it) isn't good enough for professional lasers, 1/4 minimum but 1/10 or better desired. I know with RF reflectors one forth wavelength error is their maximum but for a cheap beam expander I'm thinking 1/2 wavelength precision is good enough, but is it? Anyone willing to point out issues with doing so, please post a response.
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Alaskan

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Found this:

https://www.newport.com/optical-surfaces

Figure Cost Applications

λ/2 Low Used where wavefront distortion is not as important as cost
λ/4 Moderate Excellent for most general laser and imaging applications where low wavefront performance must be balanced with cost
λ/8 High For laser and imaging applications requiring low wavefront distortion, especially in systems with multiple elements

* Unless otherwise stated, surface irregularity for Newport lenses is peak-to-valley, per surface.
 

RedCowboy

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Interesting, I want to learn all the lens terminology and how to calculate what I need, there are efficient ways to do what we want even if we can't use a FAC at the emitter we can still build HH size optical trains to get good results, SANWU has shown us how easy it can be, just the G7 and 3X expander compete with 6X cylindrical correction on a NUBM44 diode and it's a lot easier, I still have a lot to learn and as diodes change we will need new optics.
 

Alaskan

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Looks like my old friend is "spamming" the thread but he isn't very skilled at it, most of it didn't work.

Here's a link to a ebay sale for an old Russian anamorphic projector lens, something like this looks like a good way to both expand the output of a multimode laser diode while at the same time adding some correction.

Lot 2pcs LOMO Anamorphic Big Lens 35 Nap 2 3M 35KP 1 8 120 | eBay
 

RedCowboy

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Cool, have you tested that type of lens before?
It would be nice if it will work and provide adjustability too.
Maybe we could have such lenses make with modern coated optics, I heard 0.2% loss about some coated Edmunds lenses recently.

Something like that could be ideal.
p.s. Don't know why I can't rep yet, strange.
 
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Alaskan

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I have one like it, but a fixed expander with cylinder lenses in it which acts the same way. Being it has adjustable focus, I imagine the output of a laser diode could be made to focus to infinity with it, if spaced correctly from the lenses inside. I asked the seller if the focus turned smoothly, he says it is very tight, probably needs some refreshing of the lubricant.
 

steve001

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There is a China manufacturer for large diameter PCX lenses which they specify for magnification use, with an accuracy of 1/2 lambda. Can someone tell me if we are limited to 1/4 accuracy (or greater) with lasers, or whether a lens with half wave accuracy is useful as a beam expander without too much distortion? I'm seeing some optic manufacturers specify lambda/10 ($$$).

Here's their web site, but for the life of me I cannot seem to dig that spec out anywhere now, maybe I'm remembering some other manufacturers spec: Large Lens,Diameter 300mm To 600mm Spherical Optical Glass Plano Convex Lens For Optics Instruments,Bk7,K9,B270 - Buy Large Lens,Large Convex Lens,300mm Convex Lens Product on Alibaba.com

Chris
Did you notice there's a minimum order amount?

I would not worry to much about surface accuracy, but I would be aware that about 9% of the light transmitted through these lenses is lost not including the light lost due to reflection because these lenses are not AR coated. Look at the chart.

For experimentation I'd shop here http://www.edmundoptics.com/clearance-products/ and I'd take a look at Ebay. The nice things about Edmund is they have lots of info and if you still don't understand something you can call and talk with a person.
 
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Alaskan

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Yea, I saw the minimum order. I asked for them to contact me if they do a run for someone else to add me to it as a single lens purchase which they agreed to do. I am not too concerned about 10% loss due to reflection, it's the ultra low divergence I'd be after with a 20+ inch diameter lens, not too bad of a hit considering the amount of light delivered at distance from the low divergence.

Edit: Edmunds doesn't do lenses as large as I want so I gave up on looking there. Since glass is so heavy, I've been looking around for large diameter PMMA or acrylic lenses, so far, can't find large ones. I've tried the flat Fresnel lenses, they distort the beam a lot, don't like them. If I can't find a huge 20 inch lens, I have a 12 inch and some 10 inch PCX lenses I could use, probably ought to just stick with those, doubling from 12 inches to close to 24 doesn't reduce the divergence all that much more. I have a 14 inch diameter glass lens with some bad scratches on it (bought it that way) I'd like to use if I can repair it.
 
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steve001

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Yea, I saw the minimum order. I asked for them to contact me if they do a run for someone else to add me to it as a single lens purchase which they agreed to do. I am not too concerned about 10% loss due to reflection, it's the ultra low divergence I'd be after with a 20+ inch diameter lens, not too bad of a hit considering the amount of light delivered at distance from the low divergence.

Edit: Edmunds doesn't do lenses as large as I want so I gave up on looking there. Since glass is so heavy, I've been looking around for large diameter PMMA or acrylic lenses, so far, can't find large ones. I've tried the flat Fresnel lenses, they distort the beam a lot, don't like them. If I can't find a huge 20 inch lens, I have a 12 inch and some 10 inch PCX lenses I could use, probably ought to just stick with those, doubling from 12 inches to close to 24 doesn't reduce the divergence all that much more. I have a 14 inch diameter glass lens with some bad scratches on it (bought it that way) I'd like to use if I can repair it.
They do carry experimental lenses up to 91mm diameter and Plano convex up to 100mm diameter. Google "200mm diameter Plano convex lenses" to see what shows up.
 
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Alaskan

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I've got a dozen 100-180mm diameter PCX lenses in my collection now, none AR coated though.
 

Alaskan

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If I remember correctly, the focal lengths are close to 3-4 inches for my 1 to 2 inch diameter PCX lenses, some of my larger 4 to 6 inch diameter lenses have focal lengths from 8 to 14 inches. This may seem unusual, but I rarely measure divergence, what I do is check with the FAA and see what the cloud deck heights are and see if I can put a spot on one after dark. A 1.5mRad 532nm green at 100+mw can easily do so at 5,000 feet and if I squint, up to 7,000 feet. A four inch lens on a 1400mw 520nm green can allow me to see a spot up to 11,000 feet with the naked eye, if I look carefully, but difficult to see that high. If I use a 1 inch diameter lens with a PL520 single mode green laser diode I can also see a spot at 5,000 feet, but the amount of power at 80mw, when pushing one hard, makes the ability to see a spot much higher difficult. I believe the mRad for that diode with that size of lens is much less than 1 mRad, but I haven't actually measured it.

To estimate the divergence with the big lenses, I just take the normal divergence such and such a laser with a known divergence has and divide that figure into the diameter of the larger lens, as a larger ratio. i.e, a laser with a known divergence at 532nm using a 6mm diameter lens producing about a 4mm wide beam expanded to 40mm would be ten times less divergence. I probably should take the time to measure, but I live in a small apartment with a back yard shared by three other apartments, so don't do much outside work and shooting across the living room isn't much room. I will shoot into the sky if I don't see or hear aircraft, no problem, but low across the ground, not.

If you are crazy about low divergence, and want to build a huge beam expander with at least six inches of clear aperture, this lens on ebay is uber high quality and made by Edmund Optics, I bought a bunch of them, he only has three remaining:

http://www.ebay.com/itm/291696319108

No AR coating, but a very nice lens, the photo on ebay does not do it justice, and it comes nicely wrapped in the original box. I can imagine this lens cost two to three hundred or more new, maybe more than I think. Only problem is the focal length is like 18 inches or so (if I remember correctly, don't hold me to it), the raw output of a 1 watt 520nm green laser diode causes about 10% overshoot at its focal length for infinity focus.
 
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malford

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Alaskan, I love what you are doing. I always admire anytime someone takes an idea to the fullest extension possible. In your case, beam expansion and low divergence.

I recently ordered some beam expansion optics from Thor Labs and am waiting for them to arrive. They should give me a beam profile of 20x28mm. I'd like to ask you, with all your experience in beam expanders, what would you say is the largest diameter I could use while still maintaining a clearly visible beam at night? I currently have a 1.4W 520nm and will be getting a 4-6W blue diode soon.
 

Alaskan

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With over a watt of 520nm you can use a 12 inch diameter lens and have clearly visible beam, but I think the beam looks better at 3 to 4 inches of diameter, at least, close up. The photo of the big lens on the end of a kitchen colendar in my avatar photographs has about a 8 inch diameter lens and you can see it produces a nice beam at 1.4 watts of 520nm. The thing about laser pointers and divergence (opinion; if at 250mw or higher) is even though the beam is expanded to several inches, it will continue to be a highly visible beam for much further than a pointer which doesn't have an expander on it. For example, a fairly common divergence for many pointers at 532nm is 1.5 mRad and after 1000 feet the beam will expand to about 18 inches, but if expanding the same laser pointers beam to 3 inches at the output, when focused to infinity, the divergence is reduced so much the beam diameter will be less than 5 inches after 1000 feet. So, which beam is more visible at distance?
 
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