Collimation Of Laser Diode Beams For Free Space Optical Communications

Owing to its compactness, lightness, and low cost, laser diodes (LD) play animportant role as a coherent source in various fields of technology. And it can generatecoherent light beams of high power. However, the output beam quality of a LD is poor.Because of the waveguide properties of their active areas, laser diodes generate largedivergence-angle beams with elliptically shaped intensity profiles. And the beam of LDhas astigmatism. So it is difficult to collimate LD beams effectively.Based on a far-field asymptotic expression of diode laser beams, the collimation characteristicsof a diode laser beam are investigated, and a design method for collimating optics of a laser diode isproposed. It is shown that the collimated beam divergence angles in the fast-and slow-axisdirections are different, if the beam sizes in the two orthogonal directions near the collimatingsystem are the same. It is shown also that a circular beam spot could be obtained in the extremefar-field zone, although the beam is not circularized near the collimating system.The results of this paper could be useful for much practical work.