Coupling length dependence on operable wavelength λ for d₁ (a) 0.6, 0.8, 1.0 μm with the other parameters d₂ = 0.8 μm, Λ = 2.0 μm, and d₂ (b) 0.6, 0.8, 1.0 μm with the other parameters d₁ = 0.8 μm, Λ = 2.0 μm. The coupling length has a peak due to the saltus of effective refractive index of odd mode at the resonance wavelength. The coupling length of X-polarization light does not have a peak when d₂ is 1.0 μm; we think it is because the calculation precision is not high enough.