Research Of Silicon Based On-Chip Multiplexing Devices

Because optical devices based on the silicon-on-insulator (SOI) platform are compatible with commercial Complementary Metal Oxide Semiconductor (CMOS) processes, and they have small sizes due to the large refractive index contrast provided by the silicon and the cladding/buried SiO2, the silicon photonics is an excellent technology for integrated optical circuits. Multiplexing can greatly improve the bandwidth utilization, but the silicon-based on-chip multiplexing is not mature. This paper focuses on the design, fabrication and test of the silicon multiplex devices, including polarization beam splitter (PBS), polarization rotator (PR), and wavelength division multiplexer (WDM). The details are as follows.PBS:Firstly, designing and fabricating a MMI structure PBS. The device is greatly shortened due to the few modes waveguide. The fabricated device shows the extinction ratio (ER) of about 10 dB and the insert loss (IL) of about 2 dB in the C band. Secondly, designing and fabricating a bridged waveguide directional coupler (DC) PBS. The fabricated device shows the ER of larger than 19 dB and the IL of less than 1 dB in a 90 nm wavelength range. Lastly, proposing a novel PBS based on a reversely tapered DC. This structure significantly increases the bandwidth of the PBS based on traditional DC. The fabricated device shows the ER of larger than 16 dB and the IL of less than 0.4 dB in a 100 nm wavelength range. And the fabrication tolerance of the device is large. Thus, this device is highly practical.PR:Firstly, designing and analyzing the PR based on the optical axis rotation and the PR based on mode evolution. The two kinds of structures can directly realize the polarization rotation between the TEo mode and TMo mode, but the fabrication tolerances of them are low. Secondly, designing and demonstrating the novel silicon optical PRs based on the TM0-TEn-TE0 mode conversions inside the waveguide. Employing the beam shaping method, three novel taper-structure mode order converters are designed to convert the TEo mode to TE1, TE2 and TE3 mode, respectively. Employing the mode evolution principle, three tapers are designed to convert the TM0 mode to TE1, TE2 and TE3 mode, respectively. The L-shaped cross section waveguide is proposed to convert TMo mode to high order even TE mode. Both of the above two kinds of mode converters have high fabrication tolerances. The PRs based on the TMo-TEn-TEo mode conversions are composed of those two kinds of mode converters. The minimum fabrication resolutions of the three PRs are 500 nm. By using the TE1, TE2 and TE3 mode as the transitional mode, the fabricated PRs show the IL of less than 0.4 dB,0.5 dB and 1 dB, respectively. The corresponding polarization ERs are larger than 21 dB,18 dB and 23 dB, over a wavelength range of 100 nm. This result indicates those PRs will find important applications in silicon-based optical processing and multiplexing.Polarization Rotator-Splitter (PRS):Proposing a fabrication tolerant PRS based on a waveguide with the L-shaped cross section. The L-waveguide converts the input TMo mode into the TE1-like mode and keeps the polarization and order of the input TE0 mode meanwhile. The Y-junction splits the TE1-like mode and TEo mode, and converts the TE1-like mode into the TEo mode meanwhile. The longer PRS with total lengths of 127 ?m shows less than 0.25 dB IL and larger than 14 dB polarization ER over a wavelength range from 1200 nm to 1700 nm. The shortened PRS with total lengths of 84 ?m shows less than 0.27 dB IL and larger than 14 dB polarization ER in the corresponding wavelength range. To the best of our knowledge, the PRS working from O band to U band is proposed for the first time.WDM: Analyzing, designing and fabricating the O-band WDM based on the lattice-form filter. The channel spacing is 20 nm. Except one DC, all of the couplers are 2 X 2 MMIs. This reduces the influence of the dispersion. The delay waveguide with the width of 3 ?m and a compact mode size converter based on the tapered MMI are used to improve the fabrication tolerance. At last, the tested transmission spectrum of the fabricated device is very close to the simulated spectrum. The 1 dB channel bandwidth of the multiplexer is about 10 nm and the lowest IL of each channel is less than 0.8 dB.