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.
Dealing with the increase in data workloads and network complexity requires efficient selective manipulation of any channels in hybrid mode-/wavelength-division multiplexing (MDM/WDM) systems. A reconfigurable optical add-drop multiplexer (ROADM) using special modal field redistribution is proposed and demonstrated to enable the selective access of any mode-/wavelength-channels. With the assistance of the subwavelength grating structures, the launched modes are redistributed to be the supermodes localized at different regions of the multimode bus waveguide. Microring resonators are placed at the corresponding side of the bus waveguide to have specific evanescent coupling of the redistributed supermodes, so that any mode-/wavelength-channel can be added/dropped by thermally tuning the resonant wavelength. As an example, a ROADM for the case with three mode-channels is designed with low excess losses of <0.6, 0.7, and 1.3 dB as well as low cross talks of < − 26.3, −28.5, and −39.3 dB for the TE0, TE1, and TE2 modes, respectively, around the central wavelength of 1550 nm. The data transmission of 30 Gbps / channel is also demonstrated successfully. The present ROADM provides a promising route for data switching/routing in hybrid MDM/WDM systems.
Currently it is becoming very important to enhance the capacity of single-wavelength carrier by introducing multiplexed channels carried by multiple guided-modes as well as dual polarizations in optical interconnects. For the realization of mode-division-multiplexing (MDM) and polarization-division-multiplexing (PDM), one of the keys is the realization of efficient mode/polarization-manipulation. Accordingly, it is desired to develop various high-performance photonic integrated devices for mode/polarization-manipulation-on-chip. Silicon photonics provides an attractive option for realizing ultra-compact photonics integrated devices and has been developed very successfully. Great progresses has been made on the development of silicon photonic devices for mode/polarization-manipulation-on-chip, which is reviewed in this paper.
The coupling in a directional coupler (DC) has attracted lots of attention as a basic block for photonic integrated circuits. Most of previous work was focused on symmetric DCs consisting of two identical optical waveguides. In this case light power can be transferred from one waveguide to the other one completely when choosing the length L of the coupling region appropriately. Recently, an asymmetric DC (ADC) consisting of non-identical waveguides in the coupling region has been attracting more and more attention because of the versatility for various useful applications. ADCs can be formed by combining two ore more waveguides with different dimensions, shapes as well as bending radii for the core regions. In particular, silicon nanophotonics developed in the recent years provides a very good platform to make ADCs very useful and interesting. In this paper, we give a review for recent progresses of versatile ADCs on silicon, including the following three parts: (1) ADCs for power splitter used in microring resonators and Mach-Zehnder interferometers; (2) ADCs for realizing ultracompact and broadband PBSs; (3) ADCs for realizing mode (de)multiplexers.
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