Optical coupling between single-mode semiconductor lasers and strip waveguides

This analysis describes the optimum conditions for coupling a single-mode semiconductor laser to a single-mode strip waveguide, when both the laser and the waveguide have asymmetric optical beam profiles. The outputs are approximated by two Gaussian fields. Two waveguide cases are treated: 1) the case where the two orthogonal waveguide waists are independent and 2) where the ratio of these waists is fixed. The effect on the coupling efficiency of a spherical surface on the waveguide input face is discussed. The results demonstrate the importance of coupling in the laser far-field and indicate the advantage of having relatively large waveguide modes. An intrinsic minimum in the optimum coupling efficiency was found to exist in the near-field for asymmetric laser/waveguide systems. Expressions describing the optimum waveguide waists and the focal length of the lensed waveguide are given for the waveguide parameters that yield the best coupling conditions.