Patent ID: 7546014

Claim:
An optical directional coupler comprising: two straight optical waveguides having respective cores extending longitudinally close to each other to form an optical coupler portion, each core of each straight optical waveguide having a centerline; and a tapered optical waveguide having a core connected to the core of at least one of the straight optical waveguides; wherein a width of the core of the tapered optical waveguide becomes narrow toward the at least one of the straight optical waveguides and a profile of the tapered waveguide is asymmetric relative to the centerline of the core of the at least one of the straight optical waveguides, wherein a width of the core of the straight optical waveguide and a distance between the cores of the two straight optical waveguides are determined by a V-parameter V and a coupling coefficient K, wherein the V-parameter V is represented by; V = 2 ⁢ π λ ⁢ n 1 ⁢ ( W ⁢ ⁢ 2 ⁢ a ) 2 ⁢ 2 ⁢ Δ ; ( equation ⁢ ⁢ 1 ) wherein the coupling coefficient K is represented by K = κ 2 β ⁢ 1 γ ⁢ ( W ⁢ ⁢ 2 ⁢ a ) 2 ⁢ exp ⁡ ( - γ ⁢ ⁢ LL ) 1 + ( κ γ ) 3 ; ( equation ⁢ ⁢ 2 ) wherein K is represented by κ=√{square root over ( k 0 2 n 1 2 −β 2 )} (equation 3); wherein γ is represented by γ=√{square root over (β 2 −k 0 2 n 3 2 )} (equation 4); and wherein λ is a wavelength of light to be transmitted, n1 is a value of refractive index of the cores, W 2 a is a width of the core of the straight optical waveguide, Δ is a value of relative index difference, β is a value of mode propagation constant when it is assumed that one of the two straight optical waveguides exists alone, LL is a distance between the cores of the two straight optical waveguides, k 0 =2π/λ, and n 3 is a value of refractive index of a clad intervened between the cores of the two straight optical waveguides.