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Cylindrical Plano-Convex vs Rod Lens for Laser Line Generation?

Alaskan

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I've been searching, maybe asking the wrong questions, but so far I have not been able to find an answer regarding why some laser line generators use cylindrical plano-convex lenses and others use rod lenses.

Can someone help clear this up? I can see that a rod lens will have a focal point on both sides, well, anywhere around it since the curvature is constant, but other than that, don't understand why one or the other is chosen in a line generation laser assembly.

Thanks.



Here's my best guess, for the purpose of line generation, cylindrical plano-convex lenses are used for un-collimated light, rods for collimated, correct? Is that it? I imagine they could be used either way, but in general, are cylindrical plano-convex lenses used because their flat aperture on one side is without a focal point allowing them to be a better choice for the raw output of a laser diode when desiring a line output?

If this is true, why use rod lenses for collimated beams? Does it produce a better line or is it because a wide aperture isn't needed if a small collimated dot? Maybe because a rod lens, although having a focal length, doesn't care how far the lens is from the output of a collimated laser, it works the same at any distance (as long as the beam is small enough) to produce a line output, without the need to adjust the distance between the laser and the rod?

Did I just successfully reason these two questions out on my own?
 
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Cyparagon

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I'd venture a guess that one simply provides a larger "line spread" than the other. I think of one as plano-convex, and the other as double-convex.
 
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I'd venture a guess that one simply provides a larger "line spread" than the other. I think of one as plano-convex, and the other as double-convex.
Yeah, that is what I was thinking. A cross
section of a biconvex lens is like a
flattened rod. Was that the droids you
were looking for?
 

Alaskan

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I have small rods which provide different amounts of beam spread of the line and it isn't the length that does it, its the ratio of the width to the length.
 

Alaskan

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I'm looking at a long cylinder lens on ebay I am interested in making an offer on, but the glass is green. What do you all think of that? Green? Huh? Is this unsuitable for 445nm laser?

Optical Cylindrical Convex Lens 57x2000 mm FL 87mm Laser Optics as Is Bin 8x B 7 | eBay

Edit: They took my offer of 40 dollars, hope I didn't buy something with too much loss due to the green tinted glass. Also, it's not 2000mm, it's 200mm as can be viewed in the photo, they appear to have made a typo in the listing header.
 
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Alaskan

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Anyone know if there is such a thing as "line divergence" through a cylinder lens? Divergence in the opposite plane from the line? If I take the raw output of a laser diode and use this lens on it, do I get any benefit from the beam expanding to reduce the divergence before being formed into a line? I have had answers both ways, yes and no but not sure which is the correct answer.
 
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Divergence will still occur on that axis.
In fact, imperfections in the cylinder lens
will actually make it worse, but hopefully
not much worse. I can go into more detail,
but I need as much of my brain as possible
right now. (Building a pulse driver)
 

Alaskan

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There's got to be a way to reduce the divergence of a line by expanding it into a thicker one, I would think. Open to ideas, out of the box thinkers welcome to comment, I've been stuck in this box too long.
 
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Use a b/exp ahead of the cylinder lens,
then the cylinder last in the optical
train. If you think about it, all the
cylinder does is expand the beam in one
dimension. The other dimensions are
(mostly) left alone. So if you have a 3mm
dot on a wall and add a cylinder to turn it
into a line, you will end up with a 3mm
wide line, plus whatever imperfections
there are in the cylinder.
 

Alaskan

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I was thinking I could just allow the normal expansion of the laser beam straight out of the diode and then collimate it into a line with a cylinder lens would be equivalent to using a beam expander, it will work with a normal lens that way to reduce divergence. Seem reasonable to you?
 
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I see what you're saying. Yes, that will
work if the cylinder has the right focal
length to give the desired line width and
divergence.
 
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Alaskan

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With the raw output of the laser diode I expect the amount of expansion is the size of the diodes aperture in um X the width of the beam when it hits the glass lens, but I'm not at all convinced this will reduce the divergence when using a cylinder lens because the beam is not remaining expanded, it's collapsing in one plane to form the line. In a normal lens, when the beam collapses down to form a small beam the divergence increases, so I don't see how it can have a lower divergence when the beam forms a line, regardless of how much it expanded before going through the lens to do so.

Yet, I still hope there must be a way to have a low divergence line, at 90 degrees to the line. I'm missing something elementary here & so far no one is able to give me the full answer I seek which is whether this is possible as well as how. It seems I need a professor of optics, even the technical people at Edumond Optics who answer their phones can't help me, they get lost and don't know how to handle the question, it's like the question isn't valid, is the question just wrong from the outset, that laser lines have no divergence, yet I know they do, I can see the beam get thicker at distance when it falls upon something. Maybe I need another lens to expand the line thickness (not width) to reduce it's divergence, but what would that be? Another cylinder lens, but a negative one and then another large cylinder lens? That doesn't make sense because I've already expanded the beam as it traveled forward as the raw laser light output from the diode. Seems I'm stuck in an infinity loop with this now.

If I have indeed reduced the divergence of the line by just letting the light expand as it leaves the laser diode and then collimating it into a line, it's terrible, the line width is huge after a few hundred feet. Expanding the beam of a normal laser spot using a concave and convex lens doesn't do that, the beam remains fairly small for a long distance with very little expansion, but the line I'm making doesn't hold to a tight divergence like a spot does. I am sure that I'm focusing the line to "infinity" properly because the line becomes sharpest when done correctly, but beyond that I don't know what I can do.
 
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