Patent ID: 8483531

Claim:
An optical fiber imparted with a twist, which is an elastic torsion with stress, wherein a twist rate, which is the number of rotations per unit length by the twist, is represented by f(z) as a function of a position z in a longitudinal direction of the optical fiber, the f(z) having a predetermined twist cycle and being defined such that a twist rotation angle in one direction and a twist rotation angle in the opposite direction become equal to each other in the twist cycle, and wherein, when a proportionality coefficient representing circular birefringence per twist rate is denoted by g, an angular frequency is denoted by ω, external linear birefringence due to a bending and a lateral pressure is denoted by β e , and relational expressions (1a) to (1f) are satisfied, a twist condition for the optical fiber satisfies a first condition that a twist error amount A defined by a relational expression (1g) with a twist cycle denoted by L is 8π (rad) or less k= 1,2,3, . . . (1a) {circumflex over (q)} k (ρ): k-th zero point x from the smallest one among { F ⁡ ( x ) = gx 4 + ( d e d t ⁢ ρ 4 + 0.42 e ⁢ ( 1 - ρ 4 ) - 1 ) ⁢ tan ⁢ ⁢ gx 4 ⁢ ( x > 0 ) } ( 1 ⁢ b ) d e = ω β e ⁢ ∂ β e ∂ ω ( 1 ⁢ c ) d t = ω g ⁢ ∂ g ∂ ω ( 1 ⁢ d ) γ av = 〈  f ⁡ ( z )  〉 = 1 L ⁢ ∮  f ⁡ ( z )  ⁢ ⅆ z ( 1 ⁢ e ) ρ = ⁣ ⁣ 〈  f ⁡ ( z )  〉 / max ⁢  f ⁡ ( z )  ( 1 ⁢ f ) A = min k ⁢  γ av ⁢ ⁢ L - q ^ k ⁡ ( ρ ) / 1.014  ( 1 ⁢ g ) L opt =|L−{circumflex over (q)} k (ρ)/(1.014γ av )| (1h)