Help Needed With mRad Calculations....

[Accutronitis]

You only need to know that if you want to know the beam expander power. If not don't concern yourself.

You said your are using a lens from a JL expander. That implies you took one apart. I have one too. How did you ?

I don't real care what the BE power is....

Ya I took my apart BUT I destroyed the housing that contains the output lens assy, Which was ok with me because I realized I wasn't ever going to work in the way I bought it for so I basically replaced 85% of the BE with custom parts.....

For any beam expander.
If you know the focal lengths of the lenses then divide the longer focal length by the shorter focal length will give the expander power.

I don't know any of the lens spects like focal length for the lenses I'm using right now in my BE

It doesn't need measuring for divergence calculations, but it's good to know anyway. If it's a very large expansion that'd explain your rather low divergence.

 

[steve001]

I don't real care what the BE power is....

Ya I took my apart BUT I destroyed the housing that contains the output lens assy, Which was ok with me because I realized I wasn't ever going to work in the way I bought it for so I basically replaced 85% of the BE with custom parts.....


I don't know any of the lens spects like focal length for the lenses I'm using right now in my BE

Here. https://youtu.be/lS_CJ5B3o6E

 

[Accutronitis]

Here. https://youtu.be/lS_CJ5B3o6E

Then the focal point would depend how the beam expander is focused But I just now found a mistake I was making with the distance from the laser to the wall, I was using 175 feet but that was wrong, It's actually 155 feet.

So now the new numbers are as follows......

<Dot Size & Distance>

Focused to infinity at zero feet it's 12.5mm fast axis 10mm slow axis.

Focused to infinity at 155 feet it's 49mm fast axis 19mm slow axis.

<Calculated mRad>

Slow Axis mRad Is 0.1905003786963173 focused to infinity.

Fast Axis mRad Is 0.7725847247877537 focused to infinity .

 

[steve001]

Then the focal point would depend how the beam expander is focused But I just now found a mistake I was making with the distance from the laser to the wall, I was using 175 feet but that was wrong, It's actually 155 feet.

So now the new numbers are as follows......

<Dot Size & Distance>

Focused to infinity at zero feet it's 12.5mm fast axis 10mm slow axis.

Focused to infinity at 155 feet it's 49mm fast axis 19mm slow axis.

<Calculated mRad>

Slow Axis mRad Is 0.1905003786963173 focused to infinity.

Fast Axis mRad Is 0.7725847247877537 focused to infinity .

That vid is a demonstration how to find the focal length of a single negative lens. That method can also be used for a positive lens.

 

[Accutronitis]

That vid is a demonstration how to find the focal length of a single negative lens. That method can also be used for a positive lens.

With my laser focused to infinity my lasers beam is diverging so slowly the focal point would have to be a very long distance behind the output lens.

Take the fast axis for example, it starts at the output lens 12.5mm wide and after 155 feet it has only diverged to 49mm wide so the focal point would have to be greater than 155 feet behind the output lens, so behind the output lens is -24mm at 155 feet so at 155 feet behind the output lens and assuming that the rate of convergence behind the output lens is constant and equal to the rate of divergence in front of the output lens the beam still has 12.5mm worth of convergence to go before reaching the virtual focal point behind the output lens.......

Unless there is something I'm missing ?

 

[RedCowboy]

Are you measuring without the expander?

If so, I would stop worrying about infinity and measure the spot foot by foot so we can calculate what it's doing, there are many variables.

I was saying the other day that with a NUBM44 and a DTR G2 with that G2 focused sharp at 15 feet then my 6x cyl pair set 35mm apart I got a thin ribbon about 7mm wide at the aperture, that beam ran negative to 7 feet where it was about 5mm wide then at 14 feet it was 7mm wide again and at 32 feet it was around 15mm wide.

Now if I calculated divergence from aperture to 32 feet with no other variables that divergence would be wrong. If I calculated from aperture to 14 feet it seems to be perfect but it's a trick I used to get better burning at my up close working range, it's not really holding 7mm wide over 14 feet, but with just 2 points to calculate it looks like it is.

However with a sanwu G2 my beam does not run negative and it also works better in the 3X expander for some reason, shorter FL I believe.

Anyway give spot measurements foot by foot, you can't go by what it looks like and how infinity looks is near useless except for pleasing aesthetics, so make real measurements foot by foot from aperture to 30 feet and you can get an idea if your divergence is non linear for some reason.

You can almost see in this pic how the beam gets thinner but it expands again from 7 feet.
p.s. This is something DTR's G2 did with the 06 diode as well, the further I set my G2 focus the further the bottom out until divergence ran positive again, but if you set your G2 to infinity then you may well be running slightly negative a long way, this will work well, but you can't calculate straight forward divergence this way, I had posted some videos showing the difference between the DTR and Sanwu single element G2 and I could not get the DTR G2 to move the tightest spot back to the lens face as I could with the Sanuw.

Anyway I will bet you are running slightly negative divergence for quite some distance.

 

[Accutronitis]

Are you measuring without the expander?

No with the beam expander focused to infinity....

I was saying the other day that with a NUBM44 and a DTR G2 with that G2 focused sharp at 15 feet then my 6x cyl pair set 35mm apart I got a thin ribbon about 7mm wide at the aperture, that beam ran negative to 7 feet where it was about 5mm wide then at 14 feet it was 7mm wide again and at 32 feet it was around 15mm wide.

Anyway give spot measurements foot by foot, you can't go by what it looks like and how infinity looks is near useless except for pleasing aesthetics, so make real measurements foot by foot from aperture to 30 feet and you can get an idea if your divergence is non linear for some reason.

There is nothing like that going on as i walked the entire length of the beam carefully looking at it and by blocking the beam with a sheet of white paper at different points along the way and checking the beams cross section, The beam is just very slowly diverging from the time it leave the output lens to the point the beam hits the wall, The fast axis is diverging slightly more than the slow axis but there is no funny business going on, Just a very constantly and very slowly diverging beam ....

 

[steve001]

No with the beam expander focused to infinity....

There is nothing like that going on as i walked the entire length of the beam carefully looking at it and by blocking the beam with a sheet of white paper at different points along the way and checking the beams cross section, The beam is just very slowly diverging from the time it leave the output lens to the point the beam hits the wall, The fast axis is diverging slightly more than the slow axis but there is no funny business going on, Just a very constantly and very slowly diverging beam ....

Technically you are correct, but the beam does not expand at the same rate long it's entire length. Within the Rayleigh Length (RL) the beam expands 1.4 times its diameter. Measure the beam diameter at the beam expander. Then at *10 meters measure again. 10 more meters measure again. At some point starting at the end of the RL the beam will expand or behave light a flashlight beam linearly. It appears to me you have a long RL.

* The first measurement should be 10m. Measurements after that can be in whatever increments you want to fine tune the RL distance.

Is what I've explained clear to you?

 

[Accutronitis]

Technically you are correct, but the beam does not expand at the same rate long it's entire length. Within the Rayleigh Length (RL) the beam expands 1.4 times its diameter. Measure the beam diameter at the beam expander. Then at *10 meters measure again. 10 more meters measure again. At some point starting at the end of the RL the beam will expand or behave light a flashlight beam linearly. It appears to me you have a long RL.

* The first measurement should be 10m. Measurements after that can be in whatever increments you want to fine tune the RL distance.

Is what I've explained clear to you?

Will do when I get it back together, I bought a survival lasers G-2 lens during their 4th of july sale and it will be here today and I'm going to see how it works compared to DTR's G-2, Plus while I'm waiting for the "SL G-2" I was messing around with the Sanwu Lasers G-7 that I bought awhile ago.....

 

[steve001]

Will do when I get it back together, I bought a survival lasers G-2 lens during their 4th of july sale and it will be here today and I'm going to see how it works compared to DTR's G-2, Plus while I'm waiting for the "SL G-2" I was messing around with the Sanwu Lasers G-7 that I bought awhile ago.....

Always keep in mind when creating a low diverging beam one lens always has to have a short focal length while the other must have a longer focal length.