Solution to adapt a 15 mm diameter PCX lens to a standard G2 lens tube

[Wakrah]

The diode used in this pointer is hard to find right now, but it is a full 1.5 watt rated output NUGM04 520 nm diode I bought on ebay which has had a FAC lens inserted inside the diode and recapped. The FAC lens makes the output a square shape instead of the rectangle it would otherwise be, but adding that FAC also changes the focal dynamics of the diode producing a very thin high divergence beam which expands far too fast for pointer use.

When collimated by a standard laser pointer G8 lens, the a beam diameter is very tiny at less than 2 mm diameter, to fix this issue I needed to increase the diameter of the collimated beam through using a longer focal length lens but at the same time I didn't want to make a bulky large collimation tube or beam expander add-on or use an additional concave lens to expand the beam, so I used the lens barrel from a G2 lens so I could thread my lens diameter adapter into a DTR diode module with a 12.7 mm to 8 mm copper pipe (very small tubing) adapter to mount a single large 15 mm x 30 mm FL Edmund Optics lens in front of the diode.







Even with a 30 mm focal length lens, the beam is still very small exiting the pointer at a diameter of about 6.5 mm which could potentially allow me to use a standard 6.3 mm diameter lens, although with poor beam characteristics as normally the beam diameter should be no larger than 80-90% of the lens diameter so it doesn't fully fill the lens diameter. However, I cannot use a standard diameter 6 mm lens with a long focal length, not one which has a FL of 30 mm! This is because I have been unable to find 9 mm lens barrels for a DTR diode mount which are 30+ mm long to work with that kind of lens, so gave up on that idea. However, if I could get a lens barrel that long, I would want to use a ~25 mm FL lens instead to keep the beam from truncating. A small 6 mm diameter PCX lens with a focal length that long can be had from edmundoptics.com for about 40 dollars, but due to the lens barrel problem, nah... this solution was simple.

I tried this same lens with a 405 nm 1.5 watt multimode laser diode in another host, but the beam from the diode appears to be truncated, or too large to fully exit the lens diameter without some amount of blockage, causing a portion of the beam power to be cut off and unable to get out, so this long focal length and lens diameter may not work for many of our multimode diodes, but for this particular FAC corrected diode, worked well enough but a 10-12 mm diameter PCX lens with a 50 mm focal length may work better with this diode to both increase the beam diameter for even lower divergence, while reducing the lens diameter at the same time, since so much of the diameter of this 15 mm lens is wasted.

The divergence is now acceptable, but if I can find a 10 mm diameter PCX lens with a 50 mm focal length, I am going to replace it for even lower divergence.

 

[RA_pierce]

Nice work.
I have used diamond abrasives to re-form lenses that are too large for some applications.
I cut out the shape I want out of masking tape, apply it to both sides of the lens, and then use the abrasives (wet) to carefully grind off the excess material. This works best when only a small amount of glass needs to be removed, of course. I most recently did this with a wedge prism used to correct the beam deviation of a 589 nm module. For a lens like this, maintaining centering will be important. It isn't so important for a wedge prism.

Although you did mention you didn't want to use a concave lens to expand the beam, I do think that this would make for the most compact solution.

Have you measured the divergence? I have a diode like this but haven't gotten around to setting it up yet. I'm curious to know what the divergence is like with the G-8 lens and with your 30 mm FL lens.

 

[Wakrah]

The thought occurred to me I could grind the lens down some so its edges don't protrude out from the side of the mount or can fit inside it, but I just don't have the right equipment to do so and keep it perfectly centered producing a nice clean accurate curved edge around the glass. I could work the copper to flair it, or find a more suitable piece, but I wanted to see how well this adaptation would work without waiting to get parts in the mail, so pushed forward.

I will check the divergence through a G8 in the next day or so, but as a pure guess, above 2 mRad with the G8 lens. I asked the company which adds the FAC this same question before buying, they could only answer what the divergence is when using a 12 mm EFL lens, I suppose we can work backwards from that, Sabrina responded with the following:

We used a collimating lens with 12mm EFL and got a divergence of 1mrad (spot size about 3×3mm at 1m) on both axis.

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That result isn't hugely off from my WAG, the size of the collimated beam coming through a G8 is so damn tiny, I'd expect worse.

 

[Wakrah]

Using a Techhood G8 lens, which may be different than a DTR G8, I measured the beam diameter when set to infinity focus (first going outside and shooting into the night sky to set to infinity) at about 27 mm (long side, it was a rectangle) at 5.35 meters away. This crunches to a divergence of about 5 mRad, slightly under. So, my guess for 2 mRad was more than two times too low. Also, when collimated, the beam is no longer a square, it is a rectangle again.

Below, the collimated beam output from a Techhood G8 lens, as you can see, the beam diameter is very small from this lens at about 1.5 mm:



Below, the beam shape as viewed through laser goggles 17.5 feet away, no longer the square shape produced when uncollimated, not at this distance:

 

[Wakrah]

With the 30 mm focal length lens, I am measuring a fair amount less than half mRad of divergence, this isn't too far out from tracking with what the seller specified using a 12 mm FL lens at close to 1 mRad.

I have a correction regarding the lens diameter leaving the larger longer FL lens, it is closer to 5 mm, at 5.2 meters distance the beam expands to be about 7 mm wide, as my eye sees it. If someone else measured it they might decide one less mm diameter on the wall, so this is a conservative measurement. Earlier I reported the beam diameter leaving this lens at 6.5 mm, wrong..... that is what it looks like on the glass, but measuring the wide side of beam itself, it is 5 mm from the 30 mm FL lens.

Edit: not to be confused with the G8 lens...