OPTICAL DEVICE

Japanese Patent JP2017134290

A

PROBLEM TO BE SOLVED: To provide an optical device with which it is possible to suppress variation in the manner of light propagation.SOLUTION: The optical device comprises a plurality of cores 13, and a clad 15 enclosing the outer circumference of the cores and having lower refractive index than those of the cores, each core having a large diameter part 21, a tapered part 22, and a diameter reduced part 23 along the longitudinal direction, the refractive index being gradually increased from the outer circumference toward the center. In each core, a distance r [μm] in the radial direction from the center, a refractive index n(r) at the distance r, a relative refractive index difference Δ[%] of the center to the clad, a radius r[μm], and a constant α satisfy expressions (1) through (4), and a wavelength λ(nm) of a light propagated to the core diameter R of the cores before diameter reduction relative to the diameter after diameter reduction satisfy expressions (5) and (6). n(r)=Δ×{1-(r/r)}(0≤r≤r)(1),0.9<Δ<1.2(2), 22.5