Class 1M Laser
| | Re: Focusing at long distance
I'd just like to point out that there appears to be some confusion in some of the middle responses to this post. The primary purpose of the beam expander is to increase the diameter of the beam to reduce divergence. While this does lower the power density of a beam focused to infinity, it also allows a smaller waist at greater distances from the laser. If the OP was interested in burning things at a distance of greater than 1m or so then a beam expander can improve his chances of success. If you do some calculations assuming a Gaussian beam with some quality (this can be tied to the size of the emitter when neglecting degradation in the optics) and an initial diameter, you will find some minimum spot size that the beam can be focused to at a given distance. For distances measured in meters, you will find that this minimum spot size (= higher power density at the target) gets smaller when the initial beam diameter is larger. So, yes, you can use a beam expander to burn things at a greater distance than would be possible with the default output of the laser. That being said, you will lose power because of the added optical elements in the expander, and introduce additional aberrations, so there is a trade-off here. I can think of 2 options for focusing the laser/expander combination. The first is to adjust the colimating lens on the laser. This is not usually ideal, because it changes the diameter of the beam going into the expander such that you may not get the entire beam through your optics, but it is the only option I can see if your expander has a fixed focus. The other option would be to do at Steve has suggested and adjust the relative spacing of the expander lenses (the monocular that was referenced has an adjustable focus). I don't see any reason to add more lenses to the mix as long as some of the ones you've already got are movable.