| 摘要 |
PURPOSE:To simplify a pattern, to extend a coupling length and to give a sharp wavelength selectivity by using an optical branching type grating filter instead of a directional coupler type filter. CONSTITUTION:If another mode exists in the position to which a propagation constant is shifted in the positive or negative direction by 2pi/LAMBDA when the grating having a period LAMBDA exists in the position of a grating filter part 3, coupling with this mode is caused. If modes -betae and -betao exist in the position to which the propagation constant is shifted in the negative direction, the progressive wave of the even mode couples with the opposite-direction progressive wave of the odd mode, and the progressive wave of the odd mode is coupled with the opposite- direction progressive wave of the even mode. As a result, waves are emitted to an opposite-direction exit waveguide 2 as the superposition of both opposite- direction modes. These couplings are caused only for a wavelength lambdao which satisfies betae(lambdao)+betao(lambdao)=2pi/LAMBDA, and couplings are weakened quickly according as a wavelength goes away from the wavelength lambdao on both sides of it, and thus, a very narrow-band filter is attained where the half-amplitude level of the filter characteristic is 0.1-0.5nm as wavelength. |
