Optical waveguide structures

Optical waveguide structures with symmetric Dranches preceded by abrupt bends were studied. Measured power-splitting ratios range from 0 to 6 dB and oscillate with the bend- brrancn separation. The power splitting also depends on the mode effective index, wnicn is affected by the polarization, wavelength and waveguide fabrication parameters. Device implications are also discussed. Earlier studies of related structures with closely spaced abrupt bends have shown that the transmission of such structures varies in a damped oscillatory manner with the length of the connecting segment.2~H This effect was attributed to coupling via the radiation field propagating in the connecting segment. The period of the oscillation was shown to be directly proportional to the optical wavelength and inversely proportional to the difference between the effective indices of the interfering modes in the connecting segment. It was also shown that tne transmission depends on the relative angular discrepancy between the constant-phase front of the field entering the second bend and the tilt of that Dend.5 The behavior of the bend-branch structure is illustrated in Figure 1. First assume that the optical power incident on the abrupt bend immediately preceding tne branch is solely in the guided mode. In order to satisfy field continuity requirements at the bend, a portion of tne guided-moae power is coupled into both symmetric and antisymmetric radiation modes, with the rest of the power being coupled into the symmetric guided mode. For small bend- branch separations, the radiation mode power is still concentrated near the waveguide at the beginning of the branch. The wider portion of the branch taper (to the right of the dashed line) can support two local guided modes. The separating branch arias can be thought of as a 5-iayer waveguide structure which also supports two local guided inodes. The symmetric local guiaed mode of the branch is excited primarily by tne syminetic guided mode of the connecting segment. The antisymmetric local guided mode is excited by antisymmetric radiation modes of the connecting segment. The final power splitting of the branch depends on the amplitudes and the phase difference of the two local guided modes at a point where the branch arms are no longer significantly coupled. Therefore the output power from each branch arm is expected to vary with the connecting-segment length, due to the interference between the single guided mode and the continuous spectrum of radiation modes propagating in the connecting segment. The branch-arm output also depends on the additional change in the branch-mode phase difference which occurs in the branching region, since the two branch modes have different propagation constants.