Objective With the advent of 5G communication, the demands for communication systems with increased capacity and speed are growing rapidly. Unit-device technology has basically matured, and the only way to break through the bottleneck of information systems to achieve high-speed, low power consumption, and small size is to use silicon-based optoelectronic integration. Various structures have been proposed for electro-optical modulators and wavelength-division multiplexers, and research on these two types of individual devices has become increasingly mature. However, there have been few studies which integrate the two devices in a single device to achieve multiple functions. Because silicon-based optoelectronic integration has the characteristics of low packaging cost, small size, and high integration, we propose here an on-chip integrated device for electro-optic modulation and coarse wavelength-division multiplexing. Integrating these two functions on the same silicon-based photonic-crystal plate yields a device with the desirable characteristics of small size, low insertion loss, small modulation voltage, large modulation depth, and low channel crosstalk. We therefore anticipate that this integrated-device study will be helpful for the rapid development of information technology in the near future.
Methods The integrated device described in this paper consists of two modules, a photonic-crystal electro-optic modulator and a coarse wavelength-division multiplexer. The integrated device is a photonic crystal with a heterogeneous structure, which achieves “on” and “off” state modulation of different wavelengths in the electro-optical modulation module, and the download function for the different wavelengths is provided by the coarse wavelength-division multiplexing module. This article is based on a simulation analysis using the FDTD and DEVICE modules in the commercial optical-simulation software Lumerical. First, an L3 resonant cavity and PN junction employ plasma dispersion to modulate the wavelength of the input signal. Then, using direct coupling between the microcavity and waveguide, the device utilizes the L3 resonant-cavity structure to obtain coarse wavelength-division multiplexing, and complete the design of the coarse wavelength division multiplexing module. Finally, the two modules are cascaded together, and the modulated input port and the wavelength-division-multiplexing output port utilize tapered waveguides to reduce the impact of mode mismatch between the photonic-crystal plate and the nanowire waveguide. Cascading the two modules shifts the resonant wavelength of the L3 cavity, so it is fine-tuned to complete the electro-optic modulation and coarse wavelength-division multiplexing at 1530 nm and 1550 nm design wavelengths.
Results and Discussions The integrated device has excellent performance, and the modulation voltage is small. When the modulation voltage is 1.505 V, the change in electron concentration reaches ΔNe=6.31×10 18cm3, and the change in hole concentration reaches ΔNh=7.94×10 18cm3 (Fig. 6), which modulates the wavelength (Fig. 7). According to the simulation, the transmittances of the integrated device at 1530 nm and 1550 nm in the “on” state are respectively 85.10% and 80.38%, while the corresponding transmittances in the “off” state are 0.68% and 0.5%, respectively. We calculate that the total insertion loss is less than 0.95 dB, the extinction ratio is greater than 20 dB, the modulation depths D are both 0.99, and the channel crosstalk values are all less than -27.59 dB (Table 1), which indicates good device performance for both modulation and wavelength division.
Conclusions This paper presents an on-chip integrated device for electro-optical modulation and coarse wavelength-division multiplexing. The L3 resonant cavity of the photonic crystal and the silicon-based photonic-crystal waveguide are coupled together, and the modulation and wavelength-division multiplexing functions are obtained by adjusting the width of the waveguide and the resonant cavity. The tapered structure of the input/output waveguide reduces the coupling loss and increases the transmittance. The simulation results show that the integrated device can complete the modulation and multiplexing of two wavelengths. Compared with previous research results, this is no longer a single-function device. The insertion loss of the device at the working wavelengths of 1530 nm and 1550 nm is 0.70 dB and 0.95 dB, respectively, the extinction ratios are 20.97 dB and 22.05 dB, the modulation depths are both 0.99, the channel crosstalk is -29.05 dB and -27.59 dB, and the device size is only 17.83 μm×17.3 μm×0.22 μm. The compact structure of the modulator makes it easy to integrate into applications for high-speed optical-communication systems. We therefore believe that this device has huge application potential for the development of integrated optical devices and high-speed large-capacity communication systems.
