Focal length of 2 lenses to reduce laser beam diameter

[RandomGuy88]

I need to reduce the beam diameter of a laser I am using. I plan to do this by first passing the beam through a positive spherical lens with focal length f1 and then a negative spherical lens with focal length f2 and f2 < f1. By placing the lenses so that the two focal points overlap I will have a new beam diameter this is reduced by the factor f2/f1.

I know what I want f2/f1 to be but I can accomplish this with different combinations of f1 and f2. For example if I want f2/f1 = 0.5 then I could use f1,f2 = 200mm and 100 mm or 400 mm and 200 mm...

So is there some other criteria I can use to decide what f1 and f2 should be. I know space is a consideration. I only have so much physical space so f1 and f2 can not be extremely large. But is there anything else to consider that will affect the beam quality?

 

[Quantum Defect]

I need to reduce the beam diameter of a laser I am using. I plan to do this by first passing the beam through a positive spherical lens with focal length f1 and then a negative spherical lens with focal length f2 and f2 < f1. By placing the lenses so that the two focal points overlap I will have a new beam diameter this is reduced by the factor f2/f1.

I know what I want f2/f1 to be but I can accomplish this with different combinations of f1 and f2. For example if I want f2/f1 = 0.5 then I could use f1,f2 = 200mm and 100 mm or 400 mm and 200 mm...

So is there some other criteria I can use to decide what f1 and f2 should be. I know space is a consideration. I only have so much physical space so f1 and f2 can not be extremely large. But is there anything else to consider that will affect the beam quality?

What kind of laser?

 

[Khashishi]

Generally, pick the smallest lenses you need to save on cost. The etendue of the system is fixed so you want to choose lenses such that the entendue of the lens is just slightly larger than that of your laser, so you don't lose light. For a fixed lens diameter, a larger focal length will be thinner and cheaper, but have smaller etendue. Also, the space needed will be larger--this sounds like it might be your limiting constraint.

http://en.wikipedia.org/wiki/Etendue