A 6 GHz low power differential VCO

Abstract

where n core and n clad are the effective index of core and cladding modes, respectively. ⌳ and m are the grating period and the cladding mode order, respectively. Since the hole-assisted fiber with large size of air holes has small area of cladding region, it has small effective index of cladding modes compared with that with small size of air holes. Accordingly, the wavelength spacing increases as the air hole size increases as seen in (1), which was measured to be changed in the range from 2.0 to 2.28 nm corresponding to air hole size. As seen in Figure 2, the number of peaks due to the cladding mode coupling was extremely reduced due to the reduction of the number of cladding modes. CONCLUSIONWe investigated the suppression of cladding mode coupling loss of FBGs fabricated with hole-assisted fibers, which has a germaniumdoped core and one-layered air holes in the silica cladding. Since the confinement of cladding modes becomes weak due to the reduction of the cladding diameter induced by air holes, higher order cladding modes cannot be supported by hole-assisted fibers because of air holes and the interaction between core and cladding modes through FBGs is reduced. Accordingly, air holes of FBGs fabricated with hole-assisted fibers could reduce the cladding mode coupling loss remarkably. The suppression of cladding mode coupling of less than ϳ0.33 dB and a 3 dB bandwidth of less than ϳ0.13 nm have been achieved. The wavelengths spacing between the main peak due to the core mode coupling and the first side peak due to the first order cladding mode coupling was changed in the range from 2.0 to 2.28 nm as the air holes size increases since the effective index of cladding mode is reduced by the air hole size within the fiber. The number of side peaks in the shorter wavelength induced by cladding mode coupling was also reduced.