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Measurements show that single-mode waveguides can be constructed which have radiation losses of about 7 dB/km—very close to the intrinsic material scattering loss. These waveguides, with total losses between 60 and 70 dB/km, have permitted the measurement of bending losses which become dominant at radii of curvature of a few centimeters. An approximate theory based on bending a rectangular guide appears to represent the circular waveguide results very well.
Kapron et al. (Sun,) studied this question.