In this paper, a spectral characteristic of two ring-three fiber mutually coupling filter has quantitatively analysed by assuming a total-amplitude coupling coefficient in the weakly and strongly coup0ling sections of the fused biconical structure to be equal to the amplitude coupling coefficient in the filtering network equations. The measured passband profile of the experimehntal filter is basically in agreement with the theoretical one. The measured center wavelength, regraction, half-power bandwidth and insertion loss of the investigated filter are 1.27 μm, >1.3 dB, 72 μm and 3.9 dB, respectively.Dark soliton pairs propagating in the birefringent fiber with the normal dispersion exchange the energy periodically by coupling between them. The energy of one dark soliton may be transformed to another, so the birefringent fiber can be used to make the optical switching. The switching condition for the birefringent fiber with the ellipticity angle 35° is discussed, and switching phenomenon only appears when the pulse is input with a special polarization angle. The formula related to the length of fiber, difference of phase and initial relative energy difference between the soliton paris are presented.