Increasing the divergence of a laser.

[will171]

I am slightly unsure about how the divergence can be increased by the use of either bi-concave or plano-concave lenses. I understand the general theory behind it but am having trouble putting numbers to it. e.g. if you have a laser beam with a diameter of 2mm and a divergence of 2mrad what focal length lens would be needed to increase its divergence to 5mrad/10mrad/etc? Any help would be greatly appreciated especially if it explains the calculations needed to work these things out.

 

[Andy Resnick]

A gaussian beam (most laser beams) have a really nice property- the product of divergence and beam waist is a constant. So, if the initial beam has a waist of 2 mm and divergence of 2 mrad and you want a divergence of 10 mrad, you only need focus the beam to a waist of 0.4 mm.