Taper coupling of laser diode to singlemode fibre: influence of fibre field shape

The coupling efficiency of laser diodes to lensed down-tapered singlemode fibres was measured at lambda =1.318 mu m and compared to theoretical results, taking into account the spherical aberration of the taper lens. Good agreement was found irrespective of the details of the fibre near-field function. However, the coupling distance between laser and fibre for maximum efficiency depends strongly on the shape of the fibre mode, and was measured to be (25+or-5) mu m for the configuration presented. Assuming a Bessel field function, an optimum distance of 29 mu m is predicted as compared to 39 mu m for a Gaussian.