Or at least it feels that way...
A while ago I got my FAP800 up and running and managed to melt some powdered Aluminium and Stainless steel with it - happy days. However, I did this with the collimator I purchased from Ebay. At the time what I didn't realize was that my collimator was set 2 focal lengths away from the point source hence it was behaving as a focusing lens. When my focal lens actually arrived and I tried to setup my collimator properly I ran into some issues. The beam was still highly divergent when setup 1 focal length away from the point source and therefore the focusing lens (plano-convex f=100mm) wouldn't do its job. It would narrow the beam and produce a clear image of the source (the beam projected by the collimator) but was far from what it needs to be (spot size of 8-10mm). I realize why this is but now am in the depths of figuring out what it is exactly that I need to correct this issue.
Now I know from reading on these forums that FAPs have terrible beam quality and therefore require some 'heavy' beam shaping to get them usable. I've read Alaskan's posts and spoken to him personally but still need help making the right decisions and building a clear understanding for myself. I've also been rummaging around the internet to try and gain a better understanding of the parameters at play but can't rid myself of an intuition that I know to be wrong, but not as to why.
My issue relates to selecting the right aspheric condenser for my setup. I've roughly measured the half angle at FWHM which turns out to be 14.5 degrees ~ NA 0.26. Yikes. I notice that aspheric condensers state their maximum NA and it's matching this that I'm having issues with. My intuition tells me that the outer rays (marginal?) cast from the point source need to be incident on the outer edge (90% or so) of the plano side of the condenser as the steepest radius of curvature exists there and therefore will have the best ability to straighten the beam assuming that it doesn't pass the critical angle . Therefore the NA of the condenser needs to be roughly the same as the point source (slightly larger) to ensure that the projected cone covers most of the lens.
But then I saw this on the Thorlabs collimation tutorial:
This doesn't fit in with my understanding at all and therefore my knowledge is lacking. Can somebody please help me to better understand this problem?
A while ago I got my FAP800 up and running and managed to melt some powdered Aluminium and Stainless steel with it - happy days. However, I did this with the collimator I purchased from Ebay. At the time what I didn't realize was that my collimator was set 2 focal lengths away from the point source hence it was behaving as a focusing lens. When my focal lens actually arrived and I tried to setup my collimator properly I ran into some issues. The beam was still highly divergent when setup 1 focal length away from the point source and therefore the focusing lens (plano-convex f=100mm) wouldn't do its job. It would narrow the beam and produce a clear image of the source (the beam projected by the collimator) but was far from what it needs to be (spot size of 8-10mm). I realize why this is but now am in the depths of figuring out what it is exactly that I need to correct this issue.
Now I know from reading on these forums that FAPs have terrible beam quality and therefore require some 'heavy' beam shaping to get them usable. I've read Alaskan's posts and spoken to him personally but still need help making the right decisions and building a clear understanding for myself. I've also been rummaging around the internet to try and gain a better understanding of the parameters at play but can't rid myself of an intuition that I know to be wrong, but not as to why.
My issue relates to selecting the right aspheric condenser for my setup. I've roughly measured the half angle at FWHM which turns out to be 14.5 degrees ~ NA 0.26. Yikes. I notice that aspheric condensers state their maximum NA and it's matching this that I'm having issues with. My intuition tells me that the outer rays (marginal?) cast from the point source need to be incident on the outer edge (90% or so) of the plano side of the condenser as the steepest radius of curvature exists there and therefore will have the best ability to straighten the beam assuming that it doesn't pass the critical angle . Therefore the NA of the condenser needs to be roughly the same as the point source (slightly larger) to ensure that the projected cone covers most of the lens.
But then I saw this on the Thorlabs collimation tutorial:
A good rule of thumb is to pick a lens with an NA twice of the NA of the laser diode. For example, either the A390-B or the A390TM-B could be used as these lenses each have an NA of 0.53, which is more than twice the approximate NA of our laser diode (0.26). Note that these lenses each have a focal length of 4.6 mm, resulting in an approximate major beam diameter of 2.5 mm
This doesn't fit in with my understanding at all and therefore my knowledge is lacking. Can somebody please help me to better understand this problem?
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