Modern optical communications rely heavily on dense wavelength-division multiplexing (DWDM) technology because of its capability of significantly increasing transmission channels. Here, we demonstrate, for the first time to the best of our knowledge, a compact photonic chip for DWDM transmitters on lithium-niobate-on-insulator (LNOI) by introducing the array of 2×2 Fabry–Perot (FP) cavity electro-optic (EO) modulators. A four-channel LNOI photonic chip for DWDM is designed and realized with a channel spacing of 1.6 nm (which is the narrowest one reported until now for LNOI optical transmitters), exhibiting a total excess loss of 1.3 dB and high 3-dB EO bandwidths of >67 GHz for all channels. Specifically, these four 2×2 FP cavities are designed with broadened LNOI photonic waveguides in the cavity sections, and they are placed very closely on the chip so that their resonance wavelengths are aligned precisely with the desired channel-spacing of ∼1.6 nm. Finally, the generation of 4×80-Gbps on–off keying and 4×100-Gbps PAM4 signals is demonstrated successfully with four channels, and the power consumption is as low as ∼5.1 fJ/bit. The present photonic chip has a compact footprint of about 0.78 mm×0.58 mm, showing great potential to work with more than four channels and to be very useful for future large-capacity optical links.
A high-performance silicon arrayed-waveguide grating (AWG) with 0.4-nm channel spacing for dense wavelength-division multiplexing systems is designed and realized successfully. The device design involves broadening the arrayed waveguides far beyond the single-mode regime, which minimizes random phase errors and propagation loss without requiring any additional fabrication steps. To further enhance performance, Euler bends have been incorporated into the arrayed waveguides to reduce the device’s physical footprint and suppress the excitation of higher modes. In addition, shallowly etched transition regions are introduced at the junctions between the free-propagation regions and the arrayed waveguides to minimize mode mismatch losses. As an example, a 32×32 AWG (de)multiplexer with a compact size of 900 μm×2200 μm is designed and demonstrated with a narrow channel spacing of 0.4 nm by utilizing 220-nm-thick silicon photonic waveguides. The measured excess loss for the central channel is ∼0.65 dB, the channel nonuniformity is around 2.5 dB, while the adjacent-channel crosstalk of the central output port is −21.4 dB. To the best of our knowledge, this AWG (de)multiplexer is the best one among silicon-based implementations currently available, offering both dense channel spacing and a large number of channels.
Group velocity and impedance matches are prerequisites for high-speed Mach-Zehnder electro-optic modulators. However, not all platforms can realize match conditions, restricting high-speed modulation in many practical platforms. Here we propose and demonstrate a general method to satisfy the group velocity and characteristic impedance matches by cascading fast-wave and slow-wave traveling wave electrodes on a thin-film lithium-niobate-on-insulator platform with a silica cladding. The effective group velocity can be flexibly adjusted by changing the ratio of fast-wave and slow-wave traveling wave electrodes. The radio frequency signal insertion loss at the boundary of the slow-wave and fast-wave electrodes is less than 0.1 dB. In addition, for a modulator of 6000 μm length, our simulation indicates an electro-optic response of over 100 GHz, surpassing what can be achieved with purely slow-wave or fast-wave electrodes that lack matching conditions. Our results will open many opportunities for high-speed electro-optic modulators in various platforms.
The management of polarization state is crucial for silicon photonics, however, it is often compromised by weak light-matter interactions, leading to the need for extending footprints of on-chip devices and huge power cost. In this paper, we propose a tunable silicon photonic polarizer designed to separate and manipulate polarization states based on selective silicon asymmetric directional couplers (ADCs) assisted with phase change material (PCM)[4]. The proposed polarizer includes a polarized beam splitter, a TE mode selective ADC assisted with PCM, a TM mode selective ADC assisted with PCM and a polarized light combiner. By tuning the GST of the TE/TM light selective ADC into crystalline state, phase matching occurs in the directional coupler between the hybrid waveguide and the bus waveguide, then the TE/TM modes can be efficiently excluded from the polarizer. On the other hand, by tuning the GST of the TE/TM light selective ADC into amorphous state, there is a phase mismatch between the hybrid waveguide and the bus waveguide, then the TE/TM light can pass through the bus waveguide and output from the polarized beam combiner. Simulation results indicate that this selective silicon photonic polarizer has a high extinction ratio over 37 dB for the TM mode and over 31 dB for the TE mode, with a minimal insertion loss of 1.2 dB for the input light.
Optical tunable filters play a key role in silicon photonic integrated circuits. Highly energy-efficient tunability and a wide continuous tuning range are strongly desired for silicon photonics filters. All-optically thermo-optic (TO) tunable devices based on the light absorbers integrated close to the silicon structure as localized heaters have attracted increasing attention because optical heaters, compared with electrical ones, can greatly reduce thermal loads and heat leakage for the device. They provide a new approach to implementing high-efficiency TO tuning with a fast response. In this work, we propose and experimentally demonstrate an on-chip all-optically tunable filter based on a suspended silicon microdisk resonator with an ultra-compact optical heater, which is a platinum absorber deposited directly on the top of the ridge waveguide. Attributed to the novel optical pumping scheme, ultra-small device size, and suspended waveguide structure, an ultra-high tuning efficiency of 37.70 nm/mW is achieved. Only 1.405 mW pump power is required to tune the single-resonance filter over a wide spectral range of ∼54.5 nm. The demonstrated tunable optical filter has the advantages of high tuning efficiency, compact footprint, and simple fabrication processes, which has significant applications for on-chip all-optical systems.
Silicon photonic switches are recognized as a key element in the applications of telecommunication networks, data center and high-throughput computing due to the low losses, low power consumption, large bandwidth and high integrated density. In this paper our recent works on silicon photonic switches for reconfigurable photonic integrated devices and circuits used in wavelength-division-multiplexing (WDM), mode-division-multiplexing (MDM), as well as hybrid WDM-MDM systems. First, high-performance Mach-Zehnder switches with an ultra-broad bandwidth, polarization-insensitivity, and low phase errors are reviewed. Second, wavelength-selective photonic switches based on MRRs are discussed. Finally, the progresses of multi-channel reconfigurable optical add-drop multiplexers are reviewed.
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