The present invention discloses a core of an optical patch cord comprising a first end with a refraction index varying in a continuous manner along the radial direction, a second end including a blocking region at a radial center thereof, and a graded region extending from the first end to a predetermined position between the first end and the second end. The graded region has a refraction index distribution varying from the refraction index distribution of the first end to the refraction index distribution of the second end. The present optical signal transmission system comprises a multimode fiber and an optical patch cord including a first segment including a blocking region positioned at a radial center thereof and a second segment connected to the first segment for guiding a light beam into a region outside the blocking region of the first segment.