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We observe that the cascaded typical Y junctions will introduce unwanted periodic fringes over the spectrum in practical systems when they link with multimode waveguides. To solve the problem, we design and experimentally demonstrate a wavelength insensitive multimode interferometer (MMI) based 3-dB splitter which has all the merits of Y-splitters such as polarization insensitivity and ultra-compactness. The splitter has a footprint of 1.5 × 1.8 µm(2), nearly one order smaller than the previously reported MMI splitters. The measured excess losses for TE and TM modes at telecom wavelength are as low as -0.11 dB and -0.18 dB respectively.

Silicon-rich oxide/silicon dioxide ͑SiO x /SiO 2 ͒, x = 1.2, superlattice thin films, deposited by the radio-frequency magnetronsputtering technique, was rapid thermal annealed at temperatures ranging from 500 to 1000°C. The photoluminescence ͑PL͒ spectra of samples after rapid thermal annealing ͑RTA͒ at 1000°C consists of oxygen-deficiency-related PL bands around ϳ400 and ϳ515 nm, and silicon ͑Si͒ nanocrystal-related PL bands around ϳ768 nm. Absorption bands at 364 nm ͑3.41 eV͒ and 302 nm ͑4.1 eV͒ are considered to be related to Si nanocrystals. On the basis of PL and PL excitation spectra, an energy-band diagram of a RTA-treated SiO x /SiO 2 superlattice sample was constructed.

Grating couplers with double surface corrugation are proposed to adjust the resonance behavior of both TE and TM modes simultaneously. Through proper selection of parameters, we designed a polarization-independent grating coupler operating at 1.55-µm telecommunication wavelength. Efficiency is achieved at around 55% for both TE and TM modes, and the polarization-dependent loss is <0.05 dB within the spectral window from 1544 to 1561 nm. Furthermore, a double-channel grating coupler is proposed to excite the TE and TM modes at 1.31 and 1.49 µm, respectively. The achievable efficiency is around 60% for both channels with good uniformity.

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