Abstract

The launching efficiency (η) from semiconductor lasers into plane-ended (PE) multimode fibers is low(~10 20%). We polished PE into rooftop (RT) ends, having a RT angle of 65° to the fiber axis, and observed a substantial increase in η; 27% (PE) to 51 % (RT). This was achieved (η_max) after optimizing the apex position of RT fiber with respect to the stripe; the two have to be parallel. When they were perpendicular, η_min = 29% was obtained. The effect of longitudinal, lateral, and angular misalignments was compared for the PE and RT ends; the latter were always more critical. We further investigated, for the first time, the effect on the intermodal pulse dispersion and obtained the following results: (i) output FWHM after 1 m having PE or RT end was 0.57 ns, (ii) FWHM after 1 km with PE input was 0.70 ns, giving pulse dispersion (t_m) of 0.41 ns, (iii) FWHM after 1 km with RT input and r/max was 0.61 ns giving t_m = 0.22 ns, (iv) FWHM after 1 km with RT end and η_min was 0.65 ns giving t_m = 0.31 ns. The material dispersion was negligible. Hence comparing (ii) and (iii) above, t_m was reduced by a factor of 2. This was explained by alteration of the modal power distribution in fiber core and confirmed by output far-field measurements.

© 1985 Optical Society of America