beam divergence

firefly

Newly Enlightened
Joined
Feb 18, 2005
Messages
73
Location
France
Hi everybody !!!

i would like to reduce the beam divergence of my 5mW leadlight, but i don't know what's the lens i need to change and with other i can remplaced.
it is the biconcave lens or the plano convex after the MCA?
and what the focal, etc, etc .... i need to reduce my beam divergence?

Firefly
 
There are 2 lenses that are used in green pointers one lens is a Plano-concave or double-concave[PCV or DCV I do not know which], focal length is unknown but my guess is it is around -6mm and the other is a Plano-convex[PCX] lens, effective focal length [EFL] about 18mm. This type of beam expander is called a Galilean beam expander.There is also another type of beam expander which is called a Keplerian beam expander. Stick with the first type. For the first type, purchase a longer focal length PCX lens that has been Anti-Reflective coated [AR coated]. Keep in mind that a longer focal length lens may require you to extend the length of the pointer's casing. Changing the PCX lens is the easyest way to change beam divergence.
I think that a PCV lens would be the best lens followed by a DCV lens. You could ask which lens would be best by posting at Google's groups sci.optics.
There is a company in France called Melles Griot you must take a look at their info. Beam expander power is the ratio of the two focal lengths of both lenses, for instance
18mm EFL / -6mm = a 3x beam expander
or
60mm EFL / -6 = a 10x beam expander.
gaussian beam

There is a recent post called Focus...Focus read that also.
 
Place a monocular (or a pair of binocular) in front of your LP with the eyepiece facing the LP, the divergence will be greatly reduced. Ideally, the reduction factor of the divergence equals to the power of the monocular/binocular. e.g., if the divergence of your LP is 1.4 mrad and the power of the monocular/binocular is 7, then you get a 0.2 mrad divergence combined.
 
So if we attached sniper scopes to the ends of our lasers, we could essentially reduce the divergence to nil?
 
Yeah you could do that and it would work. Centering would need to be precise and stay precise but, as far as looks go it would be sloppy I think.
Introducing more lenses into a system also results in loss of transmitted light output if the lenses are not AR coated and type of glass also affects transmission. I've read somewhere that as much as 4% loss per lens surface can result from reflections.
 
It's nice to know that the physics behind it exists though. Would be something to consider with a custom laser design in the future.
 
Yes, the easiest modification to reduce the divergence is to replace the front lens with a long focus one. The front lens of the LP and the eyepiece cancel themseves each other. Therefore, you can take off the front lens and the eyepiece togeter, resulting a single object lens which will do the job better because we have less glasses.
 
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