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I have been reading about fiber optic cables for the past few months, specifically about bend losses. It is often mentioned that bend losses are higher at longer wavelength light and indeed this is the case, but I am curious as to why this is so. In my research, I have not been able to find any physical explanation for this phenomenon.

If anyone knows the answer it would be of great help.

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An intuitive explanation for why this occurs is as follows. If you think of the wavelength of the light as the limiting length scale for it to resolve changes in the guiding structure then as the wavelength increases resolution decreases. Since the resolution has decreased the bend in the guide appears to become more abrupt and causes a larger perturbation in the propagating mode resulting in more radiated power from the optical fiber.

Consider how the radius of curvature (R) would appear to change as the wavelength increases. To do this we can normalize (R) to the resolving ability of the light, the wavelength ($\lambda$). Looking at $R/\lambda$ it is apparent that this normalized version of the radius of curvature becomes smaller at larger wavelengths.

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