Calculating Light Loss and Beam Spread in Opto-Electronics Components

[Gogsey]

This is not a homework question but for a project.

We are using a laser diode, one that you would normally find in a laser pointer to transfer information to a photo-transistor for a free space optical communications system.

Now I know the power output is around 5mW, but over a distance will we lose power as the light travels towards the photo-transistor? If so, how do you calculate the power at a certain distance, say 25 m or so?

Also, could someone also tell me the formula for calculating the spread of the laser light over a distance. This is so we know how much light is missing the photo-transistor and so we can correct this by using lenses to focus the light onto the photo-transistor?

Thanks in advance.

 

[ranger]

Well it would still have to obey the inverse-square law. Doubling the distance from the source means a 1/4 reduction in power at the receiver end.

 

[huwwaters]

Sorry for the brief reply, but see http://en.wikipedia.org/wiki/Gaussian_beam#Beam_parameters

for beam divergence.