For saving energy and healthy lighting, many researches focus on the sunlight illumination system. A Natural Light
Guiding System can be separated into collecting, transmitting, and lighting parts. With a cascadable concentrator in the
collecting part, the transmitting part will use large number of fibers. It means the most of cost is on the transmitting part.
With an N to 1 coupler, the number of lightpipe can be reduced quickly. In general, the optical coupler is tapered
structure. According to the Etendue principle, however, the product of beam angle and area is contact. The beam angle of
coupled sunlight will increase that isn't easy coupled again and transmitted with long distance. The total energy of the
exit beam from the N to 1 coupler should be bigger than the energy of one incident beam. In this paper, we use stepped
structure to design an optical coupler for coupling N to 1. In the research, the Natural Light Guiding System with the
optical coupler is simulated, and we evaluate the parameters of the stepped structure. Finally, we analyze the coupled
efficiency of the coupler.