Excitation of a doubly clad optical fiber by misaligned Gaussian beams.

The modal excitation efficiency of a doubly clad optical fiber is determined by using an approximate method which utilizes a series representation of the incident beam in terms of the orthogonal waveguide modes. The dependence of the excitation efficiency on the refractive index difference between the outer and inner cladding, the beam radius of curvature and spot size, as well as a tilt or offset misalignment is investigated. These factors have a pronounced effect on the efficiency and practical tolerances for efficient mode launching may be obtained from the results.