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ArcticMyst Security by Avery

Lens AR questions / Green AR coating.

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Nov 2, 2012
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I've been thinking about AR coatings lately and here is what I am curious about:

Is the 405nm coating designed for peak transmission at 405nm, or is it a "broad spectrum" type AR coating, e.g. 400nm to 600nm as I have seen on the G2-type lenses on Thorlabs' site. (At least, they are essentially the same as the G2 in terms of diameter, focal length, NA, etc.) I've seen it posted that the G2-405 lens has good transmission up to yellow/orange wavelengths, which would agree with a 400nm to 600nm transmission band. This particular AR coating on Thorlabs' site has a peak transmission at about 525nm.

It makes me wonder if the Thorlabs G2-type lenses would be superior for 520nm builds, given that 525nm transmission maximum. If the G2-405 is coated for peak transmission at 405nm, isn't 520nm a bit far away from that? How well does the 405nm AR coating transmit 520nm green?

Green AR lenses might be good for 532nm builds as well, although perhaps not a G2-type lens, given the narrower DPSS beam profile.

With the new 1W 520nm multimode diodes, does anyone know of, or plan to offer, optics that are AR coated specifically for greens, in the same manner as the G2-635 is for red wavelengths?
 





Joined
Nov 2, 2012
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I'm really surprised this didn't get any responses, given this emerging era of high-wattage green diode lasers. My apologies for making a second post, but maybe I should have provided more information:

This page is the Thorlabs page I found the lenses in question on. The one that caught my eye the most was item 352340-A. The the effective focal length for this lens is comparable to the G2-405 on Laser 66's page. The numerical aperture (NA) is slightly larger (0.62 vs 0.6). According to the graph at the top of the page, the coating on this lens is designed for 400 to 600nm, and at 520nm the reflectance should be ~0.4%.

I haven't been able to find transmission spectrum data on the G2-405, at least, not on Google or the forums.

The price for Thorlabs' lens is not cheap; the bare lens (352340-A, no mounting) costs $82 (USD) plus shipping. Does that price reflect performance/quality, or simply Thorlabs' markup?

"worth the money" is of course a separate question, but let's say I try this lens in a PL520 build. The other option is the G2-405. Due to the higher NA, I think that would capture and focus some extra milliwatts on its own. Probably only see those mW on an LPM, but I imagine they'd be there. But what about Thorlabs' AR coating; I really wonder if it's superior to the G2-405 coating, particularly at 520nm rather than 405nm?

Thorlabs' coating would almost have to be multi-layer to have such a broad transmission range, wouldn't it? AFAIK the G2-405 is a single-layer coating. If optimized for 405nm, then isn't 520nm a bit far off target for good transmission efficiency?

I hope these questions aren't bad; I am genuinely curious but I am striking out doing searches on this stuff. I figure the newness and expense of green diodes play a big part. My plan is that I want to do a PL520 build when funds are available, and when they do become available, I might be willing to splurge on Thorlabs' lens, provided it would give me an extra ~10 to 20mW or thereabouts.

I guess I could contact them, but I feel kind of funny asking about using their lens in a "hobbyist laser project" or a "handheld" or similar.

Any thoughts or insight would be welcome....
 
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One reason there might not be as much response about green-AR coated lenses is that the eye's response is so much better for green than it is for other wavelengths that achieving maximum output really isn't much of a priority. Another reason is that the optics for green lasers have traditionally been for the DPSS 532nm greens, where it was more a priority to filter out the IR rather than pass as much green as possible.
 
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Marco;

The most common AR coating is magnesium fluoride, MgF2 .

It has an index of 1.38, and is commonly used to make an "A" coating which is good from 400 to 700nm.

This is the coating I used to specify for lenses I sold here on LPF.

The 0.6NA lens worked very well on 405nm and 650nm LD's.

The lens testing on this thread shows the benefits of a good lens w/AR coating at several wavelengths:

Updated-g9-vs-g2-vs-3-element-vs-acrylic-study by andrewb

G9 lens seems like it has the coating you are looking for.

LarryDFW
 
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May 14, 2013
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I agree with Bionic on this. I just finished a 520nm build and considering the parts cost me $150 it didn't seem worth it to spend the extra money for a G series lens for the few extra mw from what is already a low power laser. It is plenty bright as it is. The more powerful the laser the more likely I would be to buy an expensive lens, however this is an interesting question and someone should do some testing of different lenses with 520nm.

Alan
 
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indeed i've gotten my best results for 520 with the G9 and an A390 lens. the 3 element 405 lens isn't too bad as well.
 
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Has anyone seen a general guide for the different colored coatings? What is red generally used for in wavelength, blue, green?
 
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Varies depending on the manufacturer, but usually the lens will show the opposite color to the color the lens is AR coated for. So if it was AR coated for green the lens should reflect every color except green-aka a reddish tint if you hold it up against a light source. But it can vary
 




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