Photoreceptors are living optical waveguides and play important roles in vision. The mode field diameter (MFD) is a very important parameter of a single-mode waveguide, and it is related to many important optical characteristics of a single-mode waveguide. In this paper, we theoretically calculate the MFD of outer fibers of rod photoreceptor cells. We find that when the geometric diameter approaches 0.5 μm, the MFD increases sharply and is far larger than its geometric diameter. Anatomically, the inner segment is connected to the nucleus by the relatively long and narrow outer fiber with geometric diameter of 0.45 μm. So the MFD of outer fiber of rods is as large as 206 μm. The MFD of rods is far larger than that of cones. Considering the average density of rod cells in the human retina, there are thousands of rods within the MFD of outer fiber. It is conjectured that the rod cell can behave like a blurred ground glass. When the incident light is weak, the light with ultralarge MFD can stimulate a part of thousands of rods simultaneously. When the incident light is strong, the ultra-large MFD will protect the rod from damage by distributing most of the light power outside the rod. Our findings of ultra-large MFD of rods may give a new insight on the signal pathway from rods to ganglion cells. Additionally, we may reveal a new optical mechanism for explaining the response of rods across a wide range of light intensities.

© 2017 Optical Society of America

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