Design considerations for the structural optimization of a single-mode fiber

Design considerations are made for the structural optimization of single-mode fibers used in high-bit-rate and long-haul transmission systems in the long-wavelength region. As the basic fiber parameters, a combination of the spot size W
0
and the effective cutoff wavelength λ
ce
is newly chosen, because the combination is found to suitably describe various actual index profiles which deviate from an ideal step-index profile. A procedure to specify the usable range of W
0
and λ
ce
is established, whereby the overall transmission-line loss in one repeater section is calculated using simple expressions for fiber intrinsic loss, excess loss in the cabling process, and splice loss, etc. The optimum values for a 400 Mbit/s transmission system operating at 1.3 μm with a repeater spacing of 20 km are obtained as
W_{0} = 5.0 \pm 0.5\mu
m and 1.1 μm
\leq\lambda_{ce}\leq 1.28 \mu
m taking into consideration the additional requirement for the possible use at
\lambda=1.55 \mu
m