(no success) removing red diodes wings (pics inside)

[Krutz]

so, after some sidenotes about this idea in other threads, i now tried to remove the red LOC diodes wings with apertures.

i compared these apertures, all with the exact same aixiz acrylic lens:

(virtualvillage, back open) (for comparison)
5 mm, back opened
2 mm, stock
1.5mm
"stock aperture flipped" (more later)
1 mm

to tighten the aperture, i glued aluminum foil (from a "tea-candle") over the back, painted it black, punched a needle through it, and then widened it to 1 mm and then 1.5 mm with a tiny drillbit.

output-power, all with the same laser and battery:

(virtualvillage, back open) 360mw
5 mm, back opened 330mw
2 mm, stock 325mw
1.5mm 244mw
"stock aperture flipped" 242mw
1 mm 160mw

by flipping the aperture"cap", the pinhole is closer to the lens, which reduces the NA just the same as having a smaller aperture.

so, what did it look like? i only used one laser, so i couldnt compare it side-by-side. while testing, it all looked promising, i made a lot of measurements and pics and different apertures. then, loading the pics on my computer and comparing, wth? i couldnt even distinguish the wings! even the 1mm aperture produced wings which looked just like the non-aperture wings, at least on the pics! i dont bother uploading dozens of identical pics, one with the wings will do.. damn, bummer!

so, since it makes almost no difference, i may as well open up the back of the lenses to gain 1% more power.. *sigh*

manuel

 

[Tech_Junkie]

How about putting it in front of the lens?

 

[k1kb0t]

I'm using the Meredith Lens mod from Jayrob which has the back open and the wings are prominent.

Jon

 

[Krutz]

an aperture in front of the lens.. in theory, it would work, i guess. but if you put a sheet of paper right onto the lens, to see the dot (from the other side of the paper), you see a perfectly round dot of 5mm diameter (the lens' diameter!), evenly bright. it takes quite some distance until you see the wings "coming" out. if you put the aperture like one meter away of the lens, it would only remove the wings ;-)
and still, after 10 meters, there would be "new" wings coming "out of the dot"..

i think i give up with that. for now, that is!

manuel

 

[ElektroFreak]

You're correct. In order to successfully remove the wings, you'd need to place the aperture a ways down the beam path. The reason for this is that the wings diverge faster than the main beam so if you are too close to the aperture, the wings are indistinguishable from the beam itself. At the aperture they might protrude by a matter of microns, so you'll have real issues blocking them at that scale. Further out from the aperture the wings will be wider, and thus much more easily blocked. Even this will not be perfect. The wings originate from the same point source as the beam itself, so blocking them fully both near- and far-field will be extremely difficult.

 

[dr-ebert]

The wings are light emitting from the top of the diode while the actual primary laser beam is only emitted from an effectively dot-sized point in the middle. In one of the threads about the 405-G-1 BR lens, there's a picture where you can see even more of the diode's inside: the central bright dot, the weaker line which is the top of the crystal, the even weaker rest of the front of the diode crystal, and then even the metal on which the diode is mounted.

So, in order to block this, you'd have to put a baffle either quite a distance away from the front of the lens, or very close to the LD crystal itself.

I rather like the line, it's a good focus control.

 

[Krutz]

I rather like the line, it's a good focus control.

thats true, i watch the wings too when collimating! and, anyway, its kinda more interesting than just a plain dot. just like a led with an aspheric in front of it, projecting a square (image of the die) for no obvious reason ;-)

manuel

 

[Darkarmyofone]

didnt we figure his out before? those aperture caps from my "can splash be removed" thread, remember. those didnt work for you?

 

[dr-ebert]

Link

Most of the ideas in that thread are wrong, however. You CAN'T remove the line by placing an aperture close to the lens, this just effectively replaces the lens by a smaller-diameter one. I think people have the idea that if you place a piece of paper close to the lens, you'll get a tiny image of a dot with a line - not so, you just get a uniform red dot (the fully out-of-focus picture of the line & dot). Trace the light path and see.

The wings have a divergence of about 1:80 (10cm after 8m). This means they're 1mm out in 8cm distance. At that point, however, the dot still has about 3mm diameter, so it's clear that an effective aperture would have to be much farther away. You might see an effect, but it would be to a great part because you're reducing the intensity of the beam, making the line just less visible.

You can test that by holding a piece of paper into the beam about 1m distant - you can clearly see the wings as they're over 1cm long. If you move the paper closer to the laser, the wings "retract", but the dot stays about the same size, a couple of mm diameter. At a few dozen cm distance, you have a fat dot with just two stubby winglets sticking out - only that part could be blocked with a baffle at that distance.

 

[Krutz]

thanks for pointing that out in all clarity, dr-ebert!
seems like we should look for a different red, different sled. if one is really bothered by those wings, anyway.

manuel

 

[dr-ebert]

One thing I've often wondered is why you get such a clearly defined line with a LOC and just a sort of blurry fuzzy hint with a PHR. Not that I'm losing sleep over it!

 

[ElektroFreak]

^Die structure. Not all laser diode dies are constructed the same, so their respective beam profiles will also be different

 

[Benm]

The wings have a divergence of about 1:80 (10cm after 8m). This means they're 1mm out in 8cm distance. At that point, however, the dot still has about 3mm diameter, so it's clear that an effective aperture would have to be much farther away.

Considering that, you could place a 10cm long tube in front of the output with a diameter that fits the beam but not the wings. As long as the tube is matte on the inside, that should work. If its shiny you just get light boucing off the tube causing artefacts (probably halos).

Personally i don't mind the wings on those reds - they make focus easier, but arent usually irritatingly bright.

 

[dr-ebert]

Um, not quite. Remember that the cross section of the beam is not a small copy of the image you get on a wall a few meters away - it's a small, unfocused copy of this image. So assuming that the dot diameter is 2mm and that you're focused to infinity or to a distance much larger than 10 cm, you would get at 10 cm distance (calculating with 1mm wingspan at this point for simplicity):

1. a 2mm bright circle which is the (unfocused) image of the laser dot
2. a 2x3mm faint oval which is the (unfocused) image of the left 1mm wing line, centered 0.5mm to the left of circle #1
3. a 2x3mm faint oval which is the (unfocused) image of the right 1mm wing line, centered 0.5mm to the right of circle #1

Superimposed, you get a 2x4mm oval with a bright 2x2mm center and the outside fading into nothing. By placing a baffle carefully at this point, you could obstruct the outer "half moons" - if you obstruct more, you take power away from the central dot. However, that would not remove the wings and not even half of them as one might think next - it would just cause the brightness of them to decrease from max at the center to somewhat less than half at the (previous) tip. You could actually reduce the "wingspan" only at a point where the total image size is considerably larger than the dot image size (the PSF, or point spread function).

Maybe it's easier to visualize if you consider that the light that makes up the left tip of the wing at 10m is arriving there via all parts of the lens, i.e. it forms a long, narrow cone with a base that is the full lens and tilted slightly with respect to the center line. At 10 cm, this cone and the one making up the laser dot (on the center line) still overlap to a good degree - the image drawn above is just a cross section through all involved light cones.

 

[ElektroFreak]

I'm as bad a nitpicker as anyone, but the output from these diodes are all single-mode which makes them very useful, wings or not. The only time that the wings matter are if you're holding the beam completely stationary. If you plan to use these diodes for a laser show system (for which they are ideal), you'd have a hard time finding a cheaper source of 200+mW of coherent red photons. They would work fairly well for holography as well since they are single-mode and very stable. Also, you'd be hard pressed to find a high-powered red single mode diode that doesn't produce these wings, since the basic die structure and materials are the same regardless of manufacturer. Wings or no wings, these DVD burner diodes are a sweet deal all around..

 

[dr-ebert]

I agree; it's just my intention of showing why you can't clip the wings by placing an aperture stop over or close to the lens, which is the usual proposal when someone complains about the wings (it's a natural first reaction).

Have you got any numbers for the coherence length of the red diode? A quick search only turned up a post saying that the one for the PHR should be above 12cm.