445 prism pair correction - images

[Benm]

GBD was kind enough to provide me with a prism pair, and today i had my first attempt at correcting the 445 beam with them - see pictures attached.

These tests were all taken at limited power (500 mA diode current) to minimize the risk of back reflections ending up in the wrong places - including straight back into the diode.

For now, i only adjust the prism pair by adjusting them by hand, following a course diagram of how they should be placed but without actually calculating the best prism angles at all. Even with just that, the divergence it the unwanted direction dropped by about half. At about 9 meters, the stripe width shrunk from 2.0 to 1.2 centimeters (the grid in the pics is 1 cm).

I'll try to get some readings on how much laser power is lost in the prism process, but i suspect is will not be all that much: The prisms didnt seem to scatter or absorb much light, and all reflections i could see were fairly weak. I'd expect something in the order of 20% crossing the 2 prisms and 4 surfaces involved, but so far that seems to be a very good tradeoff.

 

[JLSE]

Are they coated for 445? I was losing 30% on 808 coated.. You should be seeing less than
that.

They do make a big difference in the output..

 

[Benm]

Afaik they arent coated for any specific wavelength. GBD doesn't think so, and i cant see any of the typical color effects you get with dielectric coating. There probably is a broadband AR coating (MgF2?) on them though: the reflections seem very weak compared to a plain glass surface.

See attached picutre of the prisms themselves.

 

[GBD]

Yeah, when I got these ages ago IIRC they weren't said to be coated... but broad band coating is a possibility. One thing for sure it is not coated for any specific wavelenth.
Due to you saying that the reflections are weaker then they would be on glass, then broad band coating is probable.

Have you metered the losses yet through the pair? I suppose the estimated 20% loss isn't really noticable to the eye at the power 445nm diodes typically can do (not to mention the wavelenth, even with 500mw).

 

[jeffreythe00]

^looking forward to this as well. I also want to see what the dot looks like with the prism's precisely aligned.

 

[bhwollen]

Anamorphic prism pair to suit 445nm 1w diodes | eBay

Little more expensive then I have seen.

 

[GBD]

Those aren't the ones I bought originally, I forgot were, but they were listed as noncoated and were <20$. But feeling has it they were mislabled and probably had broad band coating.

They also make acrylic prisms IIRC for cheaper, but idk how well they will handle high powers though. At the end of the day a prism is a prism. it will work, just what its made from and what coating it has will differentiate its use.

 

[Benm]

Work on these remains open here - i'll try to find and set the optimum alignment for 4:1 beam expansion on the fast axis. The power loss seems hardly significant by eye for now, but i will post some metered results once i get the alignment right.

As for the material of the prisms: just by experience i'd say the look like fairly ordinary crown glass. Even with MgF2 coating there will be some reflective losses from the prisms, but just eyeballing the reflections they are less than 1% each. I guess there may be some scatter and absorption, but nothing to worry about too much.

 

[Benm]

I've just done some measurements to gauge the losses in the prism pair, and the results are better than i expected.

- without prisms: 421 mW (running at ~500 mA)
- with prism pair: 380 mW

The total loss in both prisms is only 10%... this would make it feasible to use 2 prism pairs in a row to correct the beam even further.

 

[JLSE]

What lens are you using for the LD? I used the 405 G1's in my handheld
with Lava's cylindrical correction, squeezed as much power as I could :eg:

The 3 element should offer a better beam overall, but not offer the gains..

10% loss is not bad at all.. Now if I could only remove the coatings off the
prisms I have..

 

[Benm]

I'm using an aixiz lens coated for 445 (or 405-445 if you wish)... i doubt that matters for the loss figures though, i used the same lens for both measurements and didnt adjust its focus between them. I adjusted the focus for infinity as best as i could.

Given the distances required i'll probably need a beam contractor or mirror to make the final adjustments to the prism positions - my house isnt big enough to judge the far field otherwise... the longest path i can use is about 10 meters in length.

 

[JLSE]

I meant that setting the LD conservatively, you can squeeze more power from
the collimator, not the prisms.. The aixiz lens will give a better overall divergence
from what ive seen between the 2.. the aspheric's, more power..

I miss the days of living in a high up apartment, trying to set optics at ground level
is a PITA.. I used to have several thousand feet unobstructed at my disposal,,
The good ole days :beer:

 

[Benm]

I live on the 15th floor, but i dont want to do divergence tests with the nearby buildings as targets unless i'm sure they are deserted

Perhaps the results will be somewhat different if driving the LD at higher currents - something i will try to do later on. But it seems realistic to run the system at 1 watt output power with the prisms in place, something i consider rather attractive compared to blasting the uncorrected beam/stripe all over the place

 

[JLSE]

My use of laser tends to be indoors, so the beam of the 445's dont bug me so much.
These diodes were released after I moved unfortunately, that view would have
been great for setting my twin beam lasers.. Instead now I have to use the night
sky and safe targets in the early evening to avoid unwanted attention.

People in the city see a beam and they call the cops Wonder if there is
a 'title' for the phobia of monochromatic dots :thinking:

My old view was overlooking a fair size park and valley. I used to do my optics work
at 3-4am when the area was nice and quiet. The winter was perfect, not too many
folks out at that hour, let alone in the park. The flat roof of the arena down the
street was perfect, and even then I checked for people on it with nigh vision..

 

[Benm]

Thats the thing with the 445s... up to a few meters the dot is fine without much in correction optics at all - i think its naturally about square 1.5 meters from the laser. The 'dot' does become a stripe whenever i target something outside... i live on the 15th floor and have plenty of relatively safe things (blind walls of empty office buildings etc) to aim at.

Once i get the whole thing mounted i'll try to post some pictures of the beam shape further away (100-200 meters or so) so the difference is more clear.