Collimation of laser diode beams for free space optical communications

Based on a far-field asymptotic expression of diode laser beams, the collimation characteristics of a diode laser beam are investigated. In this paper we propose a method for designing a collimating system of a laser diode and do experiment to confirm it. It is shown that the collimated beam divergence angles in the fast- and slow-axis directions are different, if the beam sizes in the two orthogonal directions near the collimating system are the same. It is also shown that a circular beam spot could be obtained in the far field zone, only if the beam divergence angles have the same value in the two orthogonal directions, although the beam is not circularized near the collimating system.