camera lens as beam expander?

Can camera lenses be used as beam expanders for laser beams?
I've red somewhere about using monoculars.
Do most lenses have the AR coating laser beams need?

Camera lenses are designed to focus light onto film or a sensor. They're not really made for beam expansion. You might be able to use them as general lenses though, depending on how you have things set up.

For AR coatings, it'll depend on how the lens was manufactured and whether an AR coating was applied. Most AR coatings on cameras are broadband, which is better than nothing.

I've been looking at some telephoto lenses on ebay which are going for cheap to use as a beam expander, so far, the couple I've purchased won't work as is. I believe they can be used if you replace one or more of the lenses, depending on what you have, but as is, mine aren't working for me. That said, they appear to make a great host for such purpose, they will move the lens back and forth to find the focal point if you get the right lenses on them.

The true beam expander uses at least 2 lenses. The first expands the beam, the second collimates it.

Telephoto (a long focal length lens >100 mm) needs an additional short focal length lens like a prime 50mm or shorter. The result will be a huge expander, and not easy to carry or hold.

If your aim is to make a low divergence laser, you could just use a laser diode in a heat sink without a expansion lens in front of it because the natural expansion of the beam as it travels forward can be used to advantage, if you can match up the right tube and collimation lens for it. To do so, first you have to measure how big the beam expands at a given distance to know both the length of the tube you need as well as match the diameter of the lens and its focal length. A bigger spot means a longer tube as well as a longer focal length, but these aren't hard to determine with a visible spectrum laser, just spot it on a wall and take measurements. From what I've seen suggested, the diameter of the beam should not be greater than about 80% as wide as the lens used to collimate it.

When building the holder for the lens, or if using a suitable telephoto camera lens assembly, of course, you want some wiggle room to adjust the focus. The lens should be set in a holder which allows adjustment in and out to find the right spot for infinity focus. To find the lens placement and range of movement, you can experimentally determine this by moving the lens you intend to use to and fro in front of the beam while spotting the beam on a close wall and then on a far wall to make sure the diameter doesn't change, that should get you close to the center of the focal range you need to have, but allow an adjustment range of up to an inch either side of that and you should be good, half of that ought to work well, but more adjustment range is what I'd design for, just to be sure.

If you start out with a single mode laser diode with only a 5μm chip output aperture (.000005 meter) and it expands to just 50mm (.050 meter) that's a huge amount of expansion, unfortunately, there is a limit of mRad reduction due to a diffraction limit related to lens quality and size, smaller worse (which I am still trying to understand), but regardless still a uber low divergence. Only thing is, most diodes have such wide fast axis outputs, you need a wide shroud/tube to mount the collimation lens on which expands at about a 30+ degree angle for most diodes, up to 45 degrees for some of the multimode diodes. Because of this, the beam from the diode expands too fast and gets clipped inside a normal telephoto tube when using the raw output from a laser diode. Of course, the rate of expansion could be controlled with the right expander lens, but my thought was to build an expander without the extra input lens to reduce the total losses. Example of cheap ebay telephoto lens: Vivitar 200mm 1 3 5 Auto Telephoto Camera Lens 62OPENING Screw Mount | eBay

Back to what you were asking, as stated above by lazeristasUVISIR, with the right expander lens on the input (concave or bi-concave) and a big plano-convex/PCX output lens (replacing the original bi-convex lens) you could conceivably use the housing from a camera telephoto lens. I suppose it is possible to do so without replacing the output lens too, depends on some factors I can't speculate upon without having the particular lens assembly to study, and the help of a few of my friends, I still have lots of questions myself.

I just bought a 6 inch/150mm diameter 160X beam expander, now that's heavy! Gave up on trying to build my own due to the difficulty of finding a nice shroud. I don't have to concern myself with diffraction limited lenses and accurate mountings now, this has better than 1/2 wave accuracy at 532nm. Thank you ebay, could never afford this at the price of a new one.

Holy cow!! Now THAT'S one hell of a beam expander, Alaskan. Are you aware if it's entirely custom or if a company built it? I'd like to know where I could get one/similar.
I have interest in many things already, but this inspires me to get into beam expanders so much. It still amazes me how absolutely KNOWLEDGEABLE some of you are, even while still having questions.

LOL, tongue in cheek comment? I'm a novice at this stuff but there are some truly knowledgeable people in this forum who can answer questions, I ask more than I answer. I wrote my above response because I wanted to give some practical methods of figuring out what he might need to build his own expander, since I've recently looked into using camera lenses myself. Someone who really knows the subject can probably do so using 1/4 the words.

For example, another question:

The big 160X beam expander is made by Special Optics, they are telling me this particular unit was probably a custom order from about 20 years ago but can't tell me more than that due to its age. Using this expander I'm calculating a 1.2 mRad beam through it can go a hundred miles without expanding more than five feet. I'm not sure I believe these figures, that this beam expander can lower the divergence to .0075 mRad from the expansion, is this really possible? Anyone know?

When using a 150mm diameter lens in the expander the diffraction limit is not reached when that big & the error is less than 1/2 wavelength, according to Special Optics. I'm just thinking there is math and there is real world, can this expander be reasonably expected to match the math close enough to produce that low of a divergence?

No, not tongue in cheek at all haha. I was actually referring to all the knowledge on this forum; at the time I was reading an analysis pullbangdead did about modes, and it made me think of the huge amounts of knowledge on this forum. (He actually wrote some pretty good information, here is the link: http://laserpointerforums.com/f54/single-mode-multi-mode-laser-diodes-53695.html)

Hmm, that is a good point. Special Optics seems to have their current highest beam expander at only 40x, so it does seem kind of far-fetched. But maybe it isn't. Kind of disappointing to think about the probability, but it still seems like a very cool expander!

Alaskan said:

I just bought a 6 inch/150mm diameter 160X beam expander, now that's heavy

Click to expand...

Did you write more about this monster?

I wrote something about the 6 inch 160X expander over at the other laser forum, maybe you have seen something on it over there. I was using it last night with my LG Aries and JetLasers 532nm pointers and found the input aperture was just big enough for them. It is is awesome to see a thick cone of light become a tiny point in the distance, you can't see that with a tiny thin beam, it looks the same width all the way out. The expander was built to allow about 1mm input beamwidth, but when looking at the expanded spot from a 1.2 to 1.5mm wide beam it doesn't appear to be clipped. I'm guessing the majority of the energy, the most concentrated portion of the beam is going through unobstructed, so if it is clipped, by the looks of it more than 90% of the light is making it through. It's a handful, well, arm full when using this beam expander, too heavy to use that way for very long.

Photo of the monster beam expander, as received. Has a lot of sticker residue and tape on it, but otherwise in very nice condition. The laser you see temporarily taped to the input is a 532nm JetLasers. I need to get a coupling collar machined for it, otherwise, while holding the unit I have to move the laser to keep it perfectly flat with the input axis of the expander for the light to go through properly.



What I'd like to have for this unit is a Az-El mount which can roll around on some fat little tires outside. Also, couple it with a 10 inch diameter parabolic mirror telescope which is perfectly in line with its output, we have a mountain over 20K tall about 150 miles NNW of me I could shoot at and see the spot, maybe.

That thing is a monster. Maybe I missed it but what did it cost? Never saw one that powerful before, congrats.

It was listed on ebay for 1500, I paid about half that for it all together with shipping. Seller claimed the original price somewhere upwards of 20K in 1995. Here's another one with a much larger input aperture from a different seller:

Special Optics' 1 20 6" 160mm Beam Expander for Argon HeNe Visible Kinematic | eBay

i'll post pics of my 532nm that has a single lens beam expander. basically just took off the small final lens and replaced it with a very large high focal length lens.