spot size

[Niklas Bjornestal]

Can someone explain why the spot size is smaller with a 1.5" lens compared with a 4" lens? From what i understand of optics a "perfect" lens would focus the beam to one infinite small point.

[Dan Hintz]

A perfect lens still has to focus a finite wavelength beam of light. The wavelength determines the theoretical minimum spot size (about 0.4mils with CO2 lasers).

A 1.5" FL lens typically has a smaller focus point compared to a 4" FL lens simply because the lens is not perfect, and the imperfections in design stack up over a longer distance.

[Rodne Gold]

The beam entering the lens has a large diameter , the reason the spot is larger with longer focal lengths is that they are flatter and thus bend the light entering less to their focal point and "magnify" or "reduce" (dunno which term to use) the image less.

[Dan Hintz]

The beam entering the lens has a large diameter , the reason the spot is larger with longer focal lengths is that they are flatter and thus bend the light entering less to their focal point and "magnify" or "reduce" (dunno which term to use) the image less.

Rodney,

That just means the focal point is farther/closer to the lens, but the OP is specifically asking about the focal point itself. The longer the FL of the lens, the wider the focus spot... if the lenses could be made to an extremely tight tolerance (much tighter than they are now), we could get a 1mil focal point with a 4" FL lens.

[Rodne Gold]

Dan , an image of finite size is projected thru the lens (the original spot size prior to being focussed) any lens can only magnify or reduce the image
The focal point is the area where the image is in focus , not a pinpoint. Magnification is a function of focal point . It doesnt have much to do with lens tolerances - the perfect lens would still have a fixed magnification but stuff like spherical abberation , edge quality , chromatic abberation etc would be less of an issue leading to a better quality image at the focal point.
The "point" is proved by the fact that the longer the focal length of a laserlens , the larger the spot size , and to grind a flatter lens without abberation (longer focal point)is a lot easier than a shorter one with more of a curvature.

[Dan Hintz]

Dan , an image of finite size is projected thru the lens (the original spot size prior to being focussed) any lens can only magnify or reduce the image
The focal point is the area where the image is in focus , not a pinpoint. Magnification is a function of focal point . It doesnt have much to do with lens tolerances - the perfect lens would still have a fixed magnification but stuff like spherical abberation , edge quality , chromatic abberation etc would be less of an issue leading to a better quality image at the focal point.
The "point" is proved by the fact that the longer the focal length of a laserlens , the larger the spot size , and to grind a flatter lens without abberation (longer focal point)is a lot easier than a shorter one with more of a curvature.

A lens changes the direction of an incident beam. In the case of an image, that's magnification/reduction in the typical sense. For a laser, being "in focus" means getting the highest power density possible, which is a pinpoint (or as close as you can get to it). The perfect "focus" for a perfect lens is one in which the pinpoint is no larger than the wavelength of the light (in our case, about 0.4 mils). In your previous post you stated:

...the reason the spot is larger with longer focal lengths is that they are flatter and thus bend the light entering less to their focal point and "magnify" or "reduce" (dunno which term to use) the image less.

Yes, a flatter lens will bend the light rays less quickly, but that only means our desired focal pinpoint is farther away, not that it doesn't exist. If the spot is larger, you're not truly at the desired focal point (the one with the highest possible energy density).

I have a disturbing feeling we may be talking about the same thing from slightly different viewpoints, but just in case we're not...

To put a name to it, the killer for us is transverse spherical abberation. Pick a point on a wall at the focal length of a lens. A lightray through the exact center of the lens hits the wall at point 'X'. The farther from the center of the lens a lightray hits, the farther away from 'X' it will hit (a perfect lens would allow that ray to always hit 'X', no matter where on the lens it entered). This leads to a wider "focal point", and shifts from being a perfect pinpoint from a perfect lens to a less-than-perfect smudge from a real lens.

The amount of abberation is dependent upon (among other things) lens shape and material (giving us an index of refraction). The material is fixed to ZnSe for reasons mostly irrelevant here, fixing the index of refraction, so we're left with shape. If the lens shape is made aspherical, spherical abberation completely disappears, but by doing so we are forced to deal with other downsides not easily dismissed with a general-purpose laser. On top of that, the manufacturing process for glass aspherical lenses is not cheap, which is why our systems typically use plano-convex lenses.

So, I come back to my earlier statement that lens tolerances are (generally) the cause. As the focal length grows, the tolerances required to keep spherical abberation in check (not eliminated) grow significantly, though I admit that tolerance is not the complete issue.

I would be interested in seeing what an aspheric for our machines would cost, as if properly designed may hold similat performance characteristics to the HPDFO optics without the requirement of a beam spreader.

[Dan Hintz]

Ouch, just took a quick peek at aspheres from Edmunds (admittedly not the cheapest source)... a 1" FL lens is $1,250. Nothing is listed for longer FLs, though I'm sure they'll custom cut one for a sufficiently high price.

Still, cheaper than HPDFO...

[Rodne Gold]

If you take a zoom lens in photography, as you vary the focal length the image magnification changes , the focus mechanism is independant of this, the quality of the lens grind gives rise to other problems - - works the same in lasers.