Optic Tutorial?

Hi All,

(thought I'd posted this already but can't find it. If it was removed for being a stupid suggestion, I apologise for re-posting. )

I've been into photography for a while so when I buy new lenses for my SLR, I go on the specs, (70-200mm, f2.4, etc). As yet I haven't been able to find any for the lenses I have bought with standard Aixiz modules. Has anyone got these specs? Looking through my new Edmunds Optics catalogue, there are many options to choose from but not knowing the numbers, (back focal length, effective focal length and the rest), I have no idea what might and might not work. Are there any rules of thumb to follow? Are there any tutorials for optics? (I have used the search button a fair bit so now have to ask. )

I know there are those with the brains, and skills I don't have, that may be able to help out.

Any clues?

Always a pleasure, never a chore,

M

The lenses that are sold for "standard" optical applications have different design criteria. That is because in optics used for imaging, you have to deal with things like chromatic aberration and focal plane curvature which are irrelevant for laser collimation optics. Here, the more significant numbers would be focal length and numerical aperture (NA=1/f-number). But these lenses are generally aspherical (to compensate for the high sperical aberration of high NA/low fl lenses) - that's why acrylic is used, it's easy to stamp into the proper shape for mass production. I'm not aware of any "quality indicator" that's useful here. Yet another factor is AR coating.

I wouldn't expect any standard lens to be very useful in a diode laser, unless it's explicitely offered as a "collimation lens".

Thanks to you both dr-ebert and LarryDFW,

dr-ebert,
Where I mentioned, "standard", lens, I meant the lens you get that's fitted inside the Aixiz module. They must have been choosen because they are a collimating lens. That's why I wondered if anyone had any figures on these specific ones. (not pooh, poohing the help, it's much appreciated, just clarifying my meaning ) What makes a collimating lens special? (apart from the fact it collimates!). I mean, are they used in other applications? Could you fluke a collimating lens from optics that are not laser specific?

This is clearly, (no pun intended ;D), a complex subject. I guess what I can take from this is: Every diode has an optimum lens and a good compromise is all we can hope for if it's used with multiple diodes, (further tailoring costs!!!). Am I right in thinking that the closer you can get a lens to the diode, the better? This will reduce the diameter of the lens and catch almost all the available light? AR coatings are there to prevent optical damage from reflection, (thus sacrificing output). Compensating for the diode window is just about refraction and will this mean an, "open can", diode, (of similar wavelength), will perform less well with the same lens?

This is all of interest because I've had a word with a local, independant optician who makes small lenses for rifle sights. He has said he will be able to give some measurements on one of the lenses I have, (which will be useful, if only for the excercise.). Although he hasn't had experience with laser optics he is interested enough to see if contact lens technology could be useful. I guess I will await his reckoning.

More to this at a later date I think....

Maybe others will chip in too....

Thanks again,

M

There's nothing special in a collimating lens. It's just for a very specific purpose, which dictates its properties: it has to convert a cone of light being emitted by a very nearly point source into a parallel beam (actually I think that the standard red and BR diode emitting areas are more a thin strip maybe a micrometer high by a few dozen micrometers wide). I don't think much besides diameter, focal length, material and maybe AR coating is known about the AixiZ lens (and others). There isn't that much more to know, unless you want an exact cross section profile.

Yes, the shorter the focal length, the closer you can get to the diode and the smaller the exit beam (simple geometry). I don't think the diode window has a big effect apart from very slightly increasing the beam divergence.

I think I've got a clearer picture in my head now, thanks.

Quote : "Yes, the shorter the focal length, the closer you can get to the diode and the smaller the exit beam (simple geometry)."

I recently ordered a couple of diodes from Roithner, (delivery about 6weeks!!! :'(), that have a collimating lens mounted directly onto the diode. No heat sink as they are only 3mW, (I think), but when I inevitably kill one of them it'll be interesting to see if I can mod the lens onto one of our higher power LDs. I'm looking forward to seeing what the beam diameter will be.

Yours,

waiting for the postman,

M

(P.S. Nice shot of Hale-Bopp if that's one of yours dr-ebert! ;D)

Yes, it's a self-made picture (long time ago).

Roithner has a nice assortment of collimating lenses. It would maybe be an interesting task for someone with a LPM to compare them to AixiZ or the jayrob 405-g-1 superlens.

Maybe you can post some pictures and observations once you get your diodes. Your planned mod will certainly require some pretty nifty mechanical work.

Patience isn't always my strong point but I like working small! (Got a couple of ideas in the, "Thought Lab", at the moment)

I agree with you on the comparison test. I don't have the kit else I'd give it a go.

I'll certainly post something on the tiny diodes when I get them. Incidentally, they also have an APC, (Auto Power Control), chip inside which I'm guessing means no need for a driver circuit. Just power up with 3V! They won't burn a thing but damn compact bit of kit. (It comes up first in their, "Laser Module", section if you fancy checking them out.)

I feel a tiny build coming on!!!

M

For me the intent of what you want to do isn't clear enough.
keep in mind a short focal length positive lens creates higher divergence compared to longer focal lengths
Useful apps. click 'optical calculators' http://lightmachinery.com/

Hi steve001,

Thanks for the link, I shall peruse... It's another resource for info that I didn't know of.

There isn't an intent in mind except the one of learning. ;D The small diode I mentioned just happened to come into the conversation and I'd expect, (from reading what you've said), that the divergence will be large. No matter, as it was just a cute, cheap diode, (pics in the future).

In your second line it seems to be contrary to dr-ebert? From what I've tried to take in from this thread so far...

The shorter the focal length; the smaller the exit beam diameter but... The shorter the focal length; the great the divergence. Are these two statements correct?

I wrestle with this stuff because I'm trying to incorporate this new knowledge of laser optics into what I already know about photography. For instance, a long focal length in photography can give excellent depth of focus, (ignoring f/stops), but a narrow field of view. A macro lens, or wide angle lens gives close-up detail but at the sacrifice of depth of focus, (please chip in dr-ebert if I've got my terminology wrong as I'm not sure I've chosen a good analogy! ;D)

In short, I was looking for the experts to tell what's what, so keep the info coming...

Thanks all,

M

Hi M,

I have to disagree with some things you said

A long-focal lens (tele) has a short depth-of-field. You have to focus it carefully on the object you want to photograph; objects a little bit closer or farther (in relative terms) will be out-of-focus (fuzzy/blurred). On the other hand, a wide-angle lens can give you nice crisp pictures from a few meters to infinity (large depth-of-field).

This is essentially because of image scale. A long FL lens will magnify everything greatly, including blurriness, so it's easily visible. Whereas with a wide angle lens, this blurriness (which exists just the same) is lost because it's too small. If you could enlarge the wide-angle picture to the scale of the long FL one, it would look identical. However, in practice that is impossible because of the limited resolution (graininess) of the image-recording film or chip.

The resolution power of a lens depends on the diameter (and on the wavelength). The larger the lens, the larger the resolution power. The limit is due to the wave nature of light: the rim of the lens causes light bending (diffraction) which spreads out the image of a light point into a small disk with hazy edges. The larger the lens, the smaller the effect, as there is comparatively "less rim" than interior.

For astronomical telescopes, which are diffraction-limited, the rule-of-thumb says you get a resolution of 1" (1 second of arc=1/3600 of a degree) for a diameter of about 12cm. Going downwards, that means a 1mm-dia lens has a resolution of about 2' (2 mins of arc=1/30 of a degree).

For a collimating lens, resolution would become divergence. A 1mm lens has a theoretical optimum divergence of about 1mrad, give or take a factor of 2. For an exact determination, you'd have to take the wavelength into account (a red beam will have ~1.7 times the divergence of a bluray one).

So it's the diameter of the lens which determines the divergence; that's why you use beam expanders if you want to use lasers for e.g. telecommunication - it improves=reduces divergence. It's not the FL, it may only look that way, because generally longer FL lenses have bigger diameters as well (comparable NA or f-number).

The beam exiting from a BR diode like the PHR has a divergence of about 10[sup]o[/sup]x20[sup]o[/sup] (without lens). A 5mm FL lens should put this into a beam with a diameter of less than 2mm, large enough for a divergence of better than 1mrad, ignoring effects from the finite size of the emitting surface.

You understand, all this is just an in-my-head calculation to give an idea of what you can expect. Feel free to dig up the exact formulas.

dr-ebert said:

Hi M,

I have to disagree with some things you said  











So it's the diameter of the lens which determines the divergence; that's why you use beam expanders if you want to use lasers for e.g. telecommunication - it improves=reduces divergence. It's not the FL, it may only look that way, because generally longer FL lenses have bigger diameters as well (comparable NA or f-number).
You understand, all this is just an in-my-head calculation to give an idea of what you can expect. Feel free to dig up the exact formulas.

Click to expand...

That seems contrary to my experience. I had an older 532nm pointer and decided to run it off AA batteries and improve the divergence. I had a new housing machined plus threaded lens mount. Anyway experimenting with PCX lenses with longer focal lengths then the original lens. The longer the focal length the lower the beam divergence even though the lens diameters didn't change. The lenses I used were -6x6mm EFL PCV and PCX 12 mm dia x 30, 36, 40 and 48 EFL. Each increase in focal length decreased divergence.

I would say both of you guys are close to being right, but are thiking of different case.

What affects beam divergence if the beam profile (or quality, m^2, or how Gaussian the beam is, kind of stuff) and the beam diameter.  The beam profile is pretty fixed from the diode, and for the singlemode stuff that we use its pretty good and can usually be assumed perfect.  The bigger effect comes from the beam diameter, which can be changed by using a collimating lens with a different fl, or using a beam expander.  The longer the fl, obviously the larger the beam diameter, which will give a beam with better divergence.  The outside diameter of the optic has nothing to do with what we do, and only matters in the case where you are filling the optic with light (as would be the case with a telescope, camera, etc), and similarly the na of the lens doesn't really mean anything since we are only using a small portion of the lens (so the effective na is different than lenses na).    In fact it is my understanding that for a given lens material (acrylic vs glas would change things due do difference in difraction index) the na is more or less fixed, and the only advantage to an expensive asphere is that it is compensated for spherical aberration, possibly the window, etc.  Of course the quality of the lens (transparency, surface polish, ar coating, etc) will affect how much light actually makes it through the lens and how the spot looks.

.... Right, that's it! You've all helped to bring on a massive headache!!! ;D Thanks anyway though... !

This is going to take some time to sink in I think. I'll have to do some experimenting of my own too, (when I can get up some cash for frivilous spending!). However, I will refer back to this thread with fresh eyes in a few days, (things often seem clearer that way), and see if anything has clicked.

Any other sources to check out will be useful too. Optics seems to be the one area that is a little devoid of readily available info, (other than PBS and Dichro usage). Maybe, if we can compile a small, "resources", section, it could make it an easier subject to get to grips with. What do you think? Or am I making a big thing of nothing?

M

Morgan said:

.... Right, that's it! You've all helped to bring on a massive headache!!!  ;D Thanks anyway though... !

This is going to take some time to sink in I think. I'll have to do some experimenting of my own too, (when I can get up some cash for frivilous spending!). However, I will refer back to this thread with fresh eyes in a few days, (things often seem clearer that way), and see if anything has clicked.

Any other sources to check out will be useful too. Optics seems to be the one area that is a little devoid of readily available info, (other than PBS and Dichro usage). Maybe, if we can compile a small, "resources", section, it could make it an easier subject to get to grips with. What do you think? Or am I making a big thing of nothing?

M  

Click to expand...

Edmund has a sub-divsion called AnchorOptics.com cheaper lenses for experimentation.
There's not much you can do with a laser as is except pop balloons and burn stuff, but with optics you can have a bit more fun like this: laser contact to france http://www.lasercomms.org.uk/france.htm