The wavelength selective crossconnect (WXC) is a key component of the reconfigurable optical add/drop multiplexer (ROADM). Waveguide type WXC is difficult to increase the number of ports and channels, and free-space type WXC has a low switching speed of milliseconds. To solve these problems, we have proposed a hybrid type WXC. It has microsecond switching speed, where switching is performed by silicon optical circuit and wavelength division (de)multiplexing is performed by free-space optical system. In this paper, we designed the free-space optics and simulated the transmission spectra of a 16-channel 2×2 hybrid-type WXC using CodeV optical simulator. The thickness and the position of the microlens array to be attached to the silicon optical circuit has been designed. The angle of incidence on the grating coupler was 9 degrees, and the thickness of the lens was 0.53 mm. The center of the microlens array was offset by 60.1 μm from the center of the grating coupler. The distance between the lenses in the free-space optics was optimized for the x-z and y-z planes, respectively. The loss spectra with the light emitted from the grating coupler were simulated for each of the 16 channels. The loss at the center frequency of each channel varies from -0.89 dB to -2.87 dB. The loss can be reduced by optimizing the grating coupler design to be -0.89 dB to -0.91 dB.
We propose a hybrid wavelength selective switch (WSS) using silica and silicon waveguides and describe its design. In the proposed design, silica arrayed waveguide gratings (AWGs) are connected to a silicon photonics switch array by free space optics. The wavelength multiplexed (WDM) signal input to the silica waveguide is coupled to a polarization beam splitter for polarization diversity, and one of the output signals is rotated by a half-wave plate so that both output signals had the same polarization. Each output signal is guided to an AWG and spectrally decomposed onto grating couplers in the silicon photonics chip. The two AWGs are identical and cover the whole C-band. The free space optics has cylindrical lenses to adjust the size of the light profiles to match the size of the grating couplers. The grating couplers are aligned seamlessly to realize a flexible-grid WSS. The optical signals from the grating couplers are guided to a 1 × N Mach-Zehnder interferometer (MZI) switch, and to N output grating couplers. In this design, N was set to 2, limited by the size of the silicon photonics chip. The optical signals emitted from the output grating couplers are again coupled to the corresponding AWG. Finally, the two polarized light signals are combined and output from the silica waveguide.
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