Reduction of modal noise by using reduced spot excitation

The paper describes a computational model to estimate the reduction of nonlinear distortions due to a fiber-fiber joint by properly focusing the optical beam from a single transverse-mode semiconductor laser source to the input end of the first fiber. Mode mixing effects are included in the model. Material dispersion is found to make a negligible contribution in comparison with a typical source coherence time. In a typical situation, where the fiber joint coupling efficiency eta is better than 0.7, all harmonic distortion components produced in the joint could be reduced by ~10 dB by halving the input beam spot radius from the value equal to the fiber core radius.