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Summary form only given. Graded index (GRIN) rod lenses with their refractive index which varies gradually from the center axis to the periphery are widely used. We report a GRIN rod lens system for coupling parallel light beams to a single mode fiber. The system is composed of two pieces of GRIN rod lenses with both their lengths and separation between them optimized. Maximum coupling efficiency is achieved by choosing the lengths and separation so that the beam parameters, i.e., the divergent angle and the focus spot are matched to the numerical aperture and the core diameter of the single mode optical fiber respectively, after passing the coupling system.