Problem: We want to couple the energy of a metal vapor laser beam of wavelength (lambda=628nm) into a fiber optic cable. The beam of the laser has a cross-sectional area of 10mm2 and all the energy from the beam has to be coupled into a diameter of 2.5um (fiber core). A lens is used to couple the laser beam into the fiber core.
Part A: What should be the focal length f of the lens to be able to couple the beam into the fiber core?
I need help to find the focal length f of the lens to be able to couple the beam into the fiber core.