For my project I use the end of a single mode fiber as a "transmitter". I need to set the divergence to 20 micro radians. Is there an equation how to calculate the divergence and the necessary optics to it?
Edit: I have two fiber ends at 100m distance from each other. Each should transmit light towards the other with 20 microradians divergence. I understand that there will be significant loss when coupling the received signal to the fiber. Once in the fiber, signal is separated with isolator. The question is how to specify the divergence for the output. I would like to have some equations since I want to use different wavelengths. At the moment I have 650nm and 1550nm fiber coupled lasers. (I checked the Thorlabs collimators, but they offer ~100 microradian divergence and the price is a little bit high for me. I would prefer to buy single lenses and adjust myself.)