Red + Green = no Yellow?

[lloyd33]

This is my first post to this forum, so be gentle..

Maybe I am just missing something, but combining the red and the
green beams together from a Lasever LSR532NL and LSR635NL using
a dichroic filter does not give me a yellow beam. It merely looks like
a green beam traveling inside of a larger diameter red beam.

The spot produced when the combined beams hit something is,
in fact, quite yellow. And you can see a few yellow photons are
being given off around the area of the dichro mirror, but I have
yet to see a single yellow photon from the beam itself.

I do not believe this to be an optics / laser alignment issue.
It should not be *that* difficult to get the beams traveling
along the same path (especially considering that I'm using
an optical breadboard and kinematic mounts)? So I assume
the issue has to do with the beam diameters being different.

Would running the red laser backwards through a 4x beam
expander help any? Could cheap dichros be the problem?
I am about to throw the entire thing in the trash can. =(

I will try to attach some photos.

Any help would be greatly appreciated.

[StarryEyed]

Your Green has too much power or your red is too weak.
How many mW do the lasers have?

StarryEyed

[dilbert]

As above, its not the beam diameter - your red looks way underpowered - what wavelength & power is it to the green?

[lloyd33]

The red is 635 nm @ 258 mW.
The green is 532 nm @ 246 mW.

Both lasers have analog modulation.

[Solarfire]

It looks like it's a case of both (red to big, green to much or red to low on power). It's best when beam diameter and divergence match up as good as possible. I take it the red is 635nm and it appears the beam is at least twice the size of the green.

[lloyd33]

Yeah, the red laser has a *horrible* beam profile. I really hate it.
The output beam is 6mm x 6mm SQUARE (I assume they use some
sort of diode bar / array? It looks like a horizontal line falling onto
the final optic(s) right before the output). Divergence is ~ 2mrad.

The green laser has a much more normal beam profile:
2mm diameter (round) at exit aperture with divergence ~ 1mrad.

[White-Light]

I know you said you didn't think it was an alignment issue, but have you checked your far field alignment?

Looking at the 2nd picture- it could just be the way the camera has recorded the red surrounding the green and the angle of viewing, but towards the projector you see yellow, but towards the camera (foreground), there appears to be a clear separation of red and green beams with the red beam displaced to the right.




What you've described seems a little unusual to me as I used to have a LW projector with a fat 650nm red, and with that you still had a yellow beam but with a red halo in fans etc. The fact that you're not seeing yellow except at the spot makes me wonder if there is a separation of the beams far field especially having seen the 2nd picture. Have you tried walking around the beams at the far end and seeing if you can see any separation from different angles?

[Solarfire]

A 2mm beam @ 1mrad within a 6mm beam @ 2mrad is just not going to cut it no matter how you align it. The power densities of the two beam intersections is just to far off with a 9:1 ratio and getting worse in the far field with the divergence ratio of 2:1.

[lloyd33]

Thanks everybody that replied!

@ White-Light, the picture does appear as though there is a misalignment,
but if you look closely you can see red to the left side as well. I went as far
out as 100 ft. to check for any separation. The beams are spot on (pun intended).

@ Solarfire, is there any simple / cheap way to correct the beam profile of the
red laser or should I just sell it? Do you know a specific laser manufacturer that
has 635nm or 650 nm red with beam specs closer to that of the DPSS green?
And if the beam profiles *were* similar, do I still need more power in the red?

I found these RGB ratios elsewhere on this site. I don't know if they are good
values to use or if somebody just pulled them out of their shoe though..

Using 635nm/532nm/473nm:

635 nm - 42.48% of total desired output
532 nm - 25.09% of total desired output
473 nm - 32.42% of total desired output

Using 650nm/532nm/473nm:

650 nm - 70.38% of total desired output
532 nm - 13.18% of total desired output
473 nm - 16.44% of total desired output

[LaNeK779]

unfortunately, the red beam properties cannot be helped
you can add lenses that reduce beam size, but these increase the divergence.

a red diode build would (or could be made to) match your green