Objective As the package size of high-speed optical communication devices becomes smaller, and the performance requirements get higher, it is not easy for traditional electro-optic modules with separated components to meet the demands of modern communication. Currently, electro-optic devices are developing toward highly integrated miniaturization. There have been many studies on photonic-crystal-based electro-optic modulators and silicon-based mode division multiplexers. However, there are few studies on the integration of the two devices. During the evolution of optical communications in the future, the demands for small size, large-capacity, and high integration have become increasingly prominent. Thus, we propose a silicon-based integrated device for electro-optic modulation assembly with mode division multiplexing, which has low insertion loss, low channel crosstalk, high extinction ratio, and large modulation depth. The integrated device has a compact structure and excellent performance. It has broad application prospects in the field of high-speed and large-capacity optical communication and optoelectronic integration.

Methods The proposed integrated device consists of electro-optic modulation and mode division multiplexing modules. The electro-optic modulation module consists of silicon-based photonic crystal waveguides and a width-modulated (WM) resonant cavity. The mode division multiplexing module consists of silicon-based asymmetric parallel nanowire waveguides. A tapered structure is used at the junction of photonic crystal and nanowire waveguides to reduce the cascade loss between the two waveguides. We used the finite-different time-domain(FDTD) and device methods in the commercial optical simulation software Lumerical for simulation analysis. According to the time-domain coupled-mode theory and plasma dispersion effect, WM-type resonant cavity and PN doping structure are used to achieve the on-off modulation of TE0 mode. According to the transverse coupled-mode theory, asymmetric parallel nanowire waveguides are used to achieve the conversion from TE0 to TE1 modes. The integrated device can achieve a narrow-band on-off modulation and mode multiplexing function of TE0 and TE1 modes with a center wavelength of 1553.91 nm.

Results and Discussions The integrated device has excellent performance. To achieve low insertion loss, a tapered structure with c=9 is selected using simulation analysis to cascade photonic crystal slab and nanowire waveguides. The proposed structure can achieve narrow-band on-off modulation and mode division multiplexing for TE0 and TE1 modes with a center wavelength of 1553.91 nm. When the modulation voltage is 1.24 V, the variation of electron concentration ΔN_e reaches 4.55×10 ¹⁸ cm ^-3, and the variation of hole concentration ΔN_h reaches 5×10 ¹⁸ cm ^-3 (Fig.12). The transmittance of the integrated device under “off” and “on” states are T=0.010255% and T=96.34%, respectively (Fig.13), and the Q value reaches 1.5×10 ⁴. The extinction ratio of the integrated device is 19.73 dB, and the modulation depth is 0.9894. Through detection on ports A, B, 3, and 4, the performance parameters of each mode at output ports are measured. The calculation results showed that the minimum and maximum insertion losses of the integrated device are 0.05 dB and 0.46 dB, respectively. The minimum and maximum channel crosstalk are -34.33 dB and -14.66 dB, respectively. Besides, after comparing the performance of the photonic crystal electro-optic modulators and two-mode silicon-based mode division multiplexers proposed in the existing references (Table 1), the proposed device can integrate the electro-optic modulation and mode division multiplexing modules. Besides, the proposed device has excellent performance.

Conclusions This study proposed a silicon-based integrated device for electro-optic modulation assembly with mode division multiplexing. WM-type photonic crystal resonator and silicon-based asymmetric parallel nanowire waveguides can be used to achieve on-off modulation and mode division multiplexing. The tapered structure can reduce the cascading loss between photonic crystal and nanowire waveguides. The proposed integrated device can achieve a narrow-band on-off modulation and multiplexing function of TE0 and TE1 modes with a center wavelength of 1553.91 nm. When the modulation voltage is 1.24 V, it can achieve low insertion loss, low channel crosstalk, high extinction ratio, large modulation depth, and high Q value. The extinction ratio of the integrated device is 19.73 dB, and the modulation depth is 0.9894. The minimum and maximum insertion losses of the integrated device are 0.05 dB and 0.46 dB, respectively. The minimum and maximum channel crosstalk are -34.33 dB and -14.66 dB, respectively. It has a compact size of ~54 μm×22 μm. It can be applied to high-speed and large-capacity optical communication systems and has significant value for improving system integration.