Transmission bandwidth evaluation based on optical pulse circulation

The optical pulse circulation technique is applied to a graded-index polymer-clad silica-core (GI-PCS) fiber at 647-nm wavelength. An optical pulse that circulates a 100-m-long GI-PCS fiber 11 times is successfully observed. Fiber-length dependence of optical pulse spreading is measured for coaxial and off-axis alignment of the test fiber input, and the resultant 3-dB transmission bandwidth is quantitatively evaluated. In order to assess the effect of multimode dispersion in the GI-PCS fiber, an optical pulse that circulates a 110-m-long single-mode fiber (cutoff wavelength of 535 nm) seven times is also observed. The experimental results for both the GI-PCS fiber and single-mode fiber suggest that chromatic dispersion is the main determiner of optical pulse spreading in GI-PCS fiber. This means that the refractive index-profile is almost fully optimized for the GI-PCS fiber examined here.