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Numerical Simulation of the coupling efficiency for the light emitting from a commercially available ball-lens capped TO-can laser diode into a single-mode fiber (SMF) is reported. The alignment sensitivity of the coupling efficiency for this laser-to-lens-to-fiber optical configuration is investigated in detail. The result is useful for defining the design tolerances and assembly specifications of economic SMF pigtailed To-can laser diode packages. An optical simulation software package is used to compute the laser-to-lens-to-fiber coupling efficiency. By adjusting the nominal system parameters of the given optical configuration, the maximum coupling efficiency calculated from the best fitted simulation result being equal to 17.3% is obtained, which very well agrees with experimental result of ∼16%. However, it is a bit lower than that of previously reported. It is noticed that in the presence of the ball lens, the axial alignment sensitivity on the coupling efficiency is much relieved as compared to the butt-coupling configuration. However, the lateral alignment sensitivity is still very stringent. An experimental measurement is done on this optical system The alignment sensitivities of the coupling efficiency on both directions are obtained. The results are compared with that from the simulation. A very good fit of the data onto simulated curve can be obtained by adjusting some of the critical system parameters.