In this paper, a novel 1×2 MMI coupler is proposed in the 3 m thick-silicon platform, which is based on rib input/output waveguide and strip multimode core waveguide. The hybrid waveguide structure can achieve a low-loss optical transmission with 0.017 dB at 1550 nm which is a comparable performance of traditional strip MMI coupler. In addition, the proposed structure can directly join the power splitter with other optical element such as ultra-low-loss single-mode rib waveguide and high-speed electro-optical Mach-Zehnder interferometers, which is a strong candidate for the conventional waveguide transformer between strip coupler and rib modulator.
A compact polarization demultiplexer (P-DeMux) is proposed and characterized to enable wavelength-divisionmultiplexing and mode-division-multiplexing simultaneously. The proposed structure is composed of a microring resonator in ultrathin waveguide and two bus channels in the novel silicon nitride silica silicon horizontal slot waveguides. In the slot waveguide, the transverse electric (TE) mode propagates through the silicon layer, while the transverse magnetic (TM) mode is confined in the slot region. In the designed ultra-thin waveguide, the TM mode is cutoff. The effective index of the TE modes for ultrathin and slot waveguides have comparable values. Thanks for these distinguishing features, the input TE mode can be efficiently filtered through the ultra-thin microring at the resonant wavelength, while the TM mode can directly output from the through port. Simulation Results show that the extinction ratio of the proposed P-DEMUX for TE and TM modes are ∼36.5 and 31.27 dB, and the insertion losses are ∼0.22 and 0.249 dB respectively.
An interpretation of optical unitary transformation is proposed for general non-overlapping-image multimode interference (MMI) couplers with any input and output ports based on the matrix mechanics. The light transformation in the MMI couplers can be considered as the optical field matrix acting on the input light column vector. We investigate the general phase principles of output light images. The complete proof of nonoverlapping-image MMI coupler’s optical unitarity is provided along with the phase analysis of matrix element. Based on a two-dimensional finite-difference time-domain simulation, the unitary transformation is obtained for a 4×4 non-overlapping-image MMI coupler within the deviation of 4×10-2 for orthogonal invariance among the C-band spectral range.
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