Jun-ichi Sakai

Bending loss of propagation modes in arbitrary-index profile optical fibers

A bending loss formula for optical fibers with an axially symmetric arbitrary-index profile is derived by approximating the refractive-index profile with a staircase function. The permissible bending radius R* defined for a given value of bending loss is derived. It is deduced that R* is nearly proportional to wavelength lambda when the normalized frequency nu and the refractive-index difference Delta are fixed. The ratio of R* at two different values of nu depends only on the ratio of nu. The influence of an index dip and profile smoothing on R* is numerically evaluated.

Existence of eigen polarization modes in anisotropic single-mode optical fibers

The existence of eigen polarization modes in a twisted single-mode fiber with an elliptical core is verified theoretically and experimentally. The eigen polarization modes are transmitted without degradation in the degree of polarization. Ellipticity and the principal axis orientation of the eigen polarizations are determined by fiber-core ellipticity and fiber twist. The ellipticity of the eigen polarization modes remains unchanged for a fixed core ellipticity and twist effect, even after long-distance transmission, whereas the orientation of the principal axis varies according to the fiber twist angle. For a sufficiently large twist rate, all the incident polarizations maintain their ellipticity of polarization.

Splicing and bending losses of single-mode optical fibers

Splicing and bending losses are compared among step, power-law, W, ring-shaped, and M.W index singlemode optical fibers. A product of normalized offset and tilt misalignments D(N)Phi(N)is introduced to compare the permissible offset and tilt misalignments among these fibers. Permissible offset misalignment can be evaluated by D(N)/B(?) under constant bending loss condition. Here, B denotes the bending loss parameter. Numerical calculations on these parameters reveal that the required splicing accuracy is nearly identical for step, power-law, and W fibers. An index dip at the core center seems to cause deleterious effects on the splicing loss, provided that the fiber bending loss is kept constant. The effects of profile parameters on D(N)Phi(N) and D(N)/B(?) are investigated in detail.

Splice loss evaluation for optical fibers with arbitrary-index profile.

A method is presented for calculating offset and tilt losses for fiber splices with axially symmetric arbitrary-index profiles by approximating the profile with a staircase function. This method is applied to a large-core dual-mode fiber with zero intermodal dispersion as well as to single-mode fibers with step- and parabolic-index profiles. When a splice loss of 0.2 dB is permitted, the normalized offset misalignment is found to be D(N) = 0.635 for the dual-mode fiber at normalized frequency v = 4.605 and a power-law exponent alpha = 4.5. The D(N) value compares favorably with the values 0.560 and 0.614 for conventional step- and parabolic-index single-mode fibers, respectively. The dual-mode fiber is superior to the step- and parabolic-index fibers with respect to permissible splice offset tolerances.

Large-core, broadband optical fiber

A broadband optical fiber with larger core diameter than that of a conventional single-mode fiber is proposed. By appropriately choosing normalized frequency v = 4.6 and core profile parameter alpha = 4.5, a core diameter as large as 16.3 microm with relative index difference Delta = 0.3% at the 1.25-microm wavelength is attainable. Fabrication tolerances, bandwidth, and bending loss of the fiber are discussed.