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Old 05-20-2015, 01:07 AM #1
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Default Beam expander

Is it possible to make a beam expander out of magnifying glasses? Just wanted to know thanks all


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Old 05-20-2015, 01:45 AM #2
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Default Re: Beam expander

If you are starting out with a thin collimated laser beam it needs to be expanded first, a lens which does that is a concave lens, magnifying glasses are always made from convex lenses and also usually made from bi-convex lenses which are rounded on both sides.

You could make a beam expander with just one lens using the raw output of a laser diode paired with one of your bi-convex lenses (I think, but not sure) or use two or more magnifying glass lenses with different focal lengths to make a Keplerian beam expander. If wanting to make a beam expander which takes the collimated output of a laser pointer and increases the diameter of the beam for reduced divergence, for ease of construction, you probably would want to use a expander/concave input lens paired with a plano convex/PCX output lens to make things easier, this is known as a Galilean beam expander.

If you want to get a low divergence fat beam out of your laser you could do so using just one large diameter PCX lens to collimate the raw output of your laser diode (removing the lens from the pointer and setting it aside), but it won't actually be a beam expander when doing this. However, it will allow you to produce a fat beam with reduced divergence the same as a beam expander can but does so a different way, you just need the correct diameter and focal length PCX lens. When I say the "correct" focal length PCX lens, I mean a lens which is large enough when placed at its focal length away from the diode you capture all of the beam coming from the laser diode, so that none is wasted by overshooting the edges. If the lens has too short of a focal length that is another problem, then the spot from the laser diode would use too little of the lens and thus a much smaller diameter output. If you want to do this, try to get a lens big enough so that 80 percent of it's diameter is used but not much less, relative to the diameter of the beam coming from the laser diode, when at its focal length distance from the diode.

Note: Most laser diodes put out a beam which expands from 30 to 45 degrees in the fast axis plane, that's a fairly fast rate of expansion, it only takes a few inches before the output becomes fairly wide. I believe this is one of the reasons manufacturers build their lasers using lenses with short focal lengths, well... for two reasons, one is the beam fans out so fast after an inch it would require a relatively large diameter lens to collimate it making the pointer much bigger, another is to keep the beam small or thin so you can see the beam due to the high power density in such a small diameter beam. To have a thin beamwidth you want to collimate the diode output within a few millimeters of leaving the laser diode which doesn't require a big lens. The commonly found 6 mm diameter lenses in most laser pointers with short focal lengths are tiny, but even then the beam has expanded a large amount from the size it was when the beam exited the face of the laser diode chip which is perhaps a few tens or up to a couple hundreds of microns across, that's a large ratio, divide 100 microns into 2 mm which is the output beam diameter of many laser pointers and you will see the beam has already expanded quite a bit before collimation due to the few millimeters of gap between the laser diode and the lens. You could have a smaller diameter beam if the focal point of your lens was even shorter to allow a closer placement to the diode, but at the cost of increased divergence.

None of the laser diodes I've been able to find for pointers put out a round spot, always a rectangular spot, even the single mode laser diodes do that. You can expand the beam and collimate with a PCX lens but then you can easily see it really has a rectangle shape producing a ribbon like output instead of appearing to be round but no fear, you will still have reduced divergence regardless of the shape you see. If a round spot is desired you could use a cylinder lens and expand the beam in the skinny sided plane to make it more rounded, or use a small pair of prisms to shape the beam to be more rounded first before expanding it more, but I believe a laser pointer beam doesn't really need to be round, it can be any shape, who cares? Main thing is that the divergence is low enough, if it isn't at least 1.5 mRad or lower, in my opinion, it's a waste because too much of the beam spreads out over a distance for my likes. After all, low divergence is what makes a laser what we think of being a laser, otherwise you might as well have a flashlight. Some pointers which use multimode laser diodes have mRad outputs at close to 3.0 or even more and they still look cool, just that your beam spreads out after a mile so much you might not be able to see a spot on low altitude clouds when trying to do so without a beam expander on them.

More info about Keplerian and Galilean beam expanders here: http://assets.newport.com/webdocumen...expander_5.pdf
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God, I actually worked on this silly post for two hours to try to get everything right. Hope someone gets something out of this and I'm not wasting my time, it took me a long time to understand this stuff.
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Old 05-20-2015, 07:14 PM #3
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Default Re: Beam expander

I use a small monocular as a beam expander. It's on my telescope and it's easy to take off and use.
Also, very interesting material Alaskan. As my "beam expander" turns your view up side down it would be a Keplerian one right?
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Old 05-20-2015, 08:31 PM #4
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Default Re: Beam expander

Yea, I'm not an optics person, just have been playing with beam expanders and reading about them enough to have some basic understanding. I had to search for the answer for whether a Keplerian telescope used as a beam expander would invert the image and found this page https://depts.washington.edu/hs***ec...s_galileo.html which confirmed it does invert.
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