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# Need help selecting a plano-convex cylindrical lens (beam correction)...

#### LazyBeam

##### 0
I want to to correct the fast axis of a diode to slow it down and match the divergence of the slow axis. I basically want both divergences to be close 12 degrees so I can create a sort of handheld laser "flashlight" that has a mostly circular/square shape. I've spent a day and a half reading about lenses and I found many articles and equations and information on collimating or focusing down to a point or onto a CCD, but nothing on just reducing the angle a little bit. So I'll come right and say it... I have no idea what is going on here. I don't even know what side of the lens to shine the light into (flat or convex). I feel more lost than when I started. Can someone help me find a suitable cylindrical lens?

Diode I want to use: http://insaneboard.de/blog/wp-content/uploads/2015/06/HLD980200H4T.pdf
Diode Package: TO-18
Fast Axis Divergence: 35 degrees
Slow Axis Divergence: 12 degrees

I want to take the fast axis from 35 degrees to about 12 degrees... so I'm pretty sure I can accomplish this with a single plano-convex lens. Thor labs sells 650-1050 AR coated lenses like this, but I have no idea what focal length to buy. Although I think the lens must sit 0-5mm from the front of the TO-18 can for the diode and optics can fit into the host. If it matters, it looks like the emitter sits about 1mm behind the front of the can. The can has a 1mm aperture opening. I have no idea how big the emitter is, but I think 50um (0.05mm) is pretty common.

So far I've managed to find this:
y2 = θ1 * f
y1 = θ2 * f

So I select a 10mm focal length and plug in my numbers:
(I have no idea if I'm doing this correct)...

f = 10mm
y1 = aperture = 1mm
θ1 = fast axis half angle = 17.5deg = .3054rad
θ2 = ? ... but I know I want a 6 degree half-angle.
Y2 = ? ... Do I care what y2 is? Because i'm not trying to focus/project an image. I just want a 12 degree light angle.

Working out the math:
θ2 = .100rad = 5.7deg ... looks good.
y2 = 3.054mm ... I'm not sure what this means or if it matters to me.

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Here's an illustration of my problem... because everything's better with an MS Paint Diagram:

Generally nobody plays around with 980nm for several reasons--is not an LPM topic for any reason but once in a blue moon, correcting a 980nm beam axis has never come up, so...

See: https://www.ebay.com/itm/Cylindrical-Mirro-Group-for-High-Power-780nm-980nm-IR-Diode-Beam-Correction/142322875097?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20131003132420&meid=ae6e374f88e94604b421eab041ace754&pid=100005&rk=6&rkt=12&sd=202736829410&itm=142322875097&pg=2047675&_trksid=p2047675.c100005.m1851

Quite a bit of 980nm offerings on ebay, see: https://www.ebay.com/sch/i.html?_fr...0.X980nm+laser.TRS0&_nkw=980nm+laser&_sacat=0

You could see what Edmund Optics offer in lenses coated to 980nm see: https://www.edmundoptics.com/f/beam-shaping-pcv-cylinder-lenses/33141/

Might be a better idea, less aggravation and perhaps cost to get a high quality hand held 980nm for JetLasers rather than experimenting see: https://www.jetlasers.org/index.php?id_product=21&controller=product

Is there any real world reason you have to have only what you are imagining you can put together somehow and only 980nm?

One problem with using a single c-lens is you need to put it very close to the emitter so you can square your raw beam then you will need your collimating lens, or you can collimate first and then use a c-lens pair, basically a beam expander that acts upon the aggressive ( FAST ) axis to square your beam.

After some experiments with round light beams through a round plano-convex lenses at home and doing more reading, I think I had a bit of a mental breakthrough... I realize my initial assumptions above were wrong. I need a plano-convex cylindrical lens and I need to place the LD emitter INSIDE the focal point approximately 2/3rds the focal length's distance away from the lens. This should should create a 3X virtual magnified image which essentially means the a 1/3 decrease in divergence of the beam through the lens... taking my fast axis from around 35 down to around 12 - matching the slow axis divergence!

You could trying buying a lens like this and shift it around until the beam axes roughly match https://www.ebay.com/itm/263682747045
Disclaimer: I am no optics expert
Looks good, I'll definitely consider it; although being only 3mm wide, I'm hoping the unmodified divergence can get through the lens before being clipped by the narrow sides. My initial optic choice was ThorLabs LJ1874L1-B ... but at \$66.98 it's a pricey pill to swallow. However the geometry is ideal because it has a 4mm radius of curvature meaning a plain 5/16" ball nose endmill can easily make an great fitting lens mount. But that tiny ebay lens you posted is WAY WAY cheaper. I didn't even think to check ebay.

That lens you found is cheap enough that I could get it and a cheap weaker \$10 980nm premade mounted diode from laserlands, then ditch the collimating lens, and then see what foacal lengths and distances I will actually need to achieve a 3X beam reduction on the high power build before making the real high-power diode and lens mount.

....

Is there any real world reason you have to have only what you are imagining you can put together somehow and only 980nm?
The reason for 980nm is that this is one part of a two part project.

Part one is to make a picatinny rail adapter mount for a SiOnyx Aurora night vision camera.

Part two is to make a laser illuminator to compliment the night vision.
Rather than buy an off the shelf laser for this purpose, I want the laser host to have an integral 1913 picatinny rail mount and be able to be remotely switched. I have already bought and disassembled a (clone) Surefure M600 light. It comes with a mount, remote switch, holds two CR123A or one 16650 battery and has plenty of space in the head for a driver/diode/lens.

It's notable that a commercial high power laser illuminators costs well over \$1000 alone and civilians can't buy the full power units anyways. They're typically LE/military only and civilians get stuck with buying illuminators that are either LED (lots of spill and not good throw) or the laser illuminators are limited to only like 30mW... and these LED or weaker units are STILL several hundred dollars.

So where does the "980nm" come into play? If I'm going to make my OWN illuminator, it will be in a wavelength that benefits ME. I want a wavelength higher than can be easily detected by gen 3 image intensifier tubes (typically limited light below 950nm), but I also want a wavelength that is also still able to seen by the newer digital night vision tech that's hitting the market using black-silicon SWIR sensors. A \$350 cameras like the SiOnyx Aurora is sensitive to light upwards of 1200nm while still being 1/8 the price of a \$3000 PVS-14 night vision unit. This means if I can make an illuminator in the 950-1200nm range for <\$100, I could have night vision and illuminator setup that has the advantage of being relatively undetectable by the best traditional NVGs costing thousands more.

One problem with using a single c-lens is you need to put it very close to the emitter so you can square your raw beam then you will need your collimating lens, or you can collimate first and then use a c-lens pair, basically a beam expander that acts upon the aggressive ( FAST ) axis to square your beam.
I'm not trying to collimate the beam. I only want it to be relatively round or square instead of a bar. I'm going for more a "flashlight" with a tight beam for excellent throw. Ideally, I want the beam to have a divergence of around 5-16 degrees. The \$22 diode I'm looking at seems pretty perfect because the slow axis is already 12 degrees and needs no correction. Once the fast axis is slowed to 12 degrees, I should have a spot size of 2m wide @ 10m, 10m wide at 50m, and 20m wide at 100m.

You don't need to do anything fancy to get 12 degrees of divergence, just get an adjustable focus laser and de-focus the collimation lens a whole lot, although I don't know if most adjustable laser pointers can be de-focused that much, something to look into. Most diodes produce a rectangle output, you just can't see it is a rectangle once collimated, unless looking closely, or seeing the spot it produces on something at a far enough distance. Some though, produce very wide stripes, most IR or infrared laser diodes produce very wide stripes, unless fairly low power single mode diode.

You can use a cylinder pair to correct the stripe shaped output of many IR laser diodes to be more square, those IR cylinder pairs might square up the output of your diode, but it really depends on the emitter geometry of your diode, if it isn't too wide requiring a higher correction factor, or if the lens corrects too much, but I doubt that will be the case. The Techhood listing indicates those lenses have an AR coating, but I have my doubts, he sells those same lenses for visible spectrum too, they may, or may not have AR coating, or the wrong coating. This is how they sell stuff so much, "mistakes" in their listings, with apologies, if you complain, or can even know they are wrong. This happens all of the time as a matter of regular business with many China laser parts vendors.

Warning though, I don't think using a de-focused laser diode output is safe for spotting because if you shine it into someones face, due to how small the light source is, or the laser diode emitter, the lenses in their eyes can focus the beam to a uber tiny spot, much smaller than a normal flashlight would produce and potentially burn a hole in someones retina. You could use it for a lot of things, but I wouldn't shine it at anyone unless you have done a lot of calculations to make sure the power density into someones eye at distance is very low. Of course, the more you spread or widen the beam, and the further someone is from the beam, the lower the power density would be.

Coherent light coming from a tiny point source is far more hazardous to our eyes than non-coherent light coming from a broad emitter source. This is why there are radiation hazard warnings for bare laser diodes without a lens too, the lens in our eyes can focus the wide beam very easily into a very tiny retina burning spot. This is one of the reasons some laser diodes burn so well, due to the source of the coherent light being so tiny, you can also focus the output to a very tiny spot too. If the emitter was large, you could not focus it down to a tiny spot.

That said, if you use only a 300 mw laser diode and do indeed spread the beam a lot and don't shine it at anyone very close to you, maybe that kind of power is safe. I'd crunch the numbers to determine the power into someones pupil just to be sure though. Still, accidents can happen, if a flash into your own face. Although, even if the power density is safe for the diameter of a human pupil, the lens of our eyes can focus the beam to such a tiny spot, the power density could be too high and cause damage anyway.

I tried to find specifications showing the emitter geometry of the hld980300h4t laser diode, but didn't find it. Maybe better to consider a LED IR source which has a much larger emitter? Would something like this be useful? https://www.ebay.com/itm/5W-970-980...ared-IR-Led-Light-Emitting-Diode/372321441626

Here's a IR laser pointer which might due, if you can de-focus it enough. They specify under 5 mw, that's likely BS, probably much more, they list a duty cycle of 45 seconds on, 15 seconds off, there is no 5 mw laser pointer I know of with limited duty cycle: https://www.ebay.com/itm/Powerful-F...ter-Pen-LED-Torch-980T-150-14500/221624535820

Maybe consider this with beam expander on it which you can de-focus to do what you want:

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You don't need to do anything fancy to get 12 degrees of divergence, just get an adjustable focus laser and de-focus the collimation lens a whole lot, although I don't know if most adjustable laser pointers can be de-focused that much, something to look into. Most diodes produce a rectangle output, you just can't see it is a rectangle once collimated, unless looking closely, or seeing the spot it produces on something at a far enough distance. Some though, produce very wide stripes, most IR or infrared laser diodes produce very wide stripes, unless fairly low power single mode diode.

You can use a cylinder pair to correct the stripe shaped output of many IR laser diodes to be more square, those IR cylinder pairs might square up the output of your diode, but it really depends on the emitter geometry of your diode, if it isn't too wide requiring a higher correction factor, or if the lens corrects too much, but I doubt that will be the case. The Techhood listing indicates those lenses have an AR coating, but I have my doubts, he sells those same lenses for visible spectrum too, they may, or may not have AR coating, or the wrong coating. This is how they sell stuff so much, "mistakes" in their listings, with apologies, if you complain, or can even know they are wrong. This happens all of the time as a matter of regular business with many China laser parts vendors.

Warning though, I don't think using a de-focused laser diode output is safe for spotting because if you shine it into someones face, due to how small the light source is, or the laser diode emitter, the lenses in their eyes can focus the beam to a uber tiny spot, much smaller than a normal flashlight would produce and potentially burn a hole in someones retina. You could use it for a lot of things, but I wouldn't shine it at anyone unless you have done a lot of calculations to make sure the power density into someones eye at distance is very low. Of course, the more you spread or widen the beam, and the further someone is from the beam, the lower the power density would be.

Coherent light coming from a tiny point source is far more hazardous to our eyes than non-coherent light coming from a broad emitter source. This is why there are radiation hazard warnings for bare laser diodes without a lens too, the lens in our eyes can focus the wide beam very easily into a very tiny retina burning spot. This is one of the reasons some laser diodes burn so well, due to the source of the coherent light being so tiny, you can also focus the output to a very tiny spot too. If the emitter was large, you could not focus it down to a tiny spot.

That said, if you use only a 300 mw laser diode and do indeed spread the beam a lot and don't shine it at anyone very close to you, maybe that kind of power is safe. I'd crunch the numbers to determine the power into someones pupil just to be sure though. Still, accidents can happen, if a flash into your own face. Although, even if the power density is safe for the diameter of a human pupil, the lens of our eyes can focus the beam to such a tiny spot, the power density could be too high and cause damage anyway.

I tried to find specifications showing the emitter geometry of the hld980300h4t laser diode, but didn't find it. Maybe better to consider a LED IR source which has a much larger emitter? Would something like this be useful? https://www.ebay.com/itm/5W-970-980...ared-IR-Led-Light-Emitting-Diode/372321441626

Here's a IR laser pointer which might due, if you can de-focus it enough. They specify under 5 mw, that's likely BS, probably much more, they list a duty cycle of 45 seconds on, 15 seconds off, there is no 5 mw laser pointer I know of with limited duty cycle: https://www.ebay.com/itm/Powerful-F...ter-Pen-LED-Torch-980T-150-14500/221624535820

Maybe consider this with beam expander on it which you can de-focus to do what you want:

Simply defocusing the collimating lens on an adjustable laser with 3:1 ratio in axis divergence will only produce a giant bar. I don't want a bar because that is less useful for use as an illuminator. If I can get the shape much closer to a circular or square output, that will be ideal... it doesn't have to be perfect either. Then, if I have enough room in the host, I can always add an AR coated collimating lens at the output if I want more or less than 12 degrees. But I suspect 12 degrees will be good for viewing 300mW out to 150m or so.

What your referring to is NOHD ratings (eye-safe distances). A fairly common emitter dimension in a TO18 diode is anywhere from 1-15um (.001-.015mm). Using the worst case scenario of 1um with a power of 300mW and a divergence of 12 degrees (209mrad); a few different NOHD calculators list the eye-safe distance as less than 1 meter. Even if this illuminator divergence were as low as 3 degrees the NOHD distance is still only about 2.5 meters. So I'm not too concerned about eye damage with this one because it's so unfocused.

I considered an LED build at first, but decided against it. Even with a quality reflector or TIR lens they have alot of spill and limited throw. It takes ALOT more LED power to equal the long distance performance of a laser illuminator.

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Those calculations sure settles the questioning, I just don't have the energy to chaise that down for you, but great you already have all of that worked out.

Those circular cylindrical lenses must be turned so they align with the emitter of your diode and be a prescribed distance apart. Keep that in mind before buying them. It is fairly easy to align them with a visible beam. Less so in IR.

If you get those cylinder lenses, please report back whether they really do have AR coating and if you believe for IR or visible spectrum, if IR they will likely have a blue tint when held at an angle.

you can see iridescence on the curved surface of the lenses in the light, so it looks like they are coated with something. I'm not sure how to tell which without a spectrophotometer, or something similar.

I'd be willing to bet that the ebay lenses are these exact lenses. Ignore the aliexpress photo, it's a stock photo. But the specs are identical... 3x5x3.2mm cylinder plano-convex with a 5mm FL, both with 700-1100nm coating and both are BK7/K9 glass and both ship direct from china. I'm willing to bet it's the same lenses being sold two different places.