In recent years, with the rapidly growing transmission capacity of global optical communication networks, silicon photonics (SiP), featuring compatibility with CMOS and high integration, has attracted great attention in optical communication systems, in which the modulator is an important part of the optical communication link. Mach-Zehnder (MZ) modulator based on traveling-wave electrode (TWE) carrier depletion has been widely employed in practical communication equipment because of its thermal stability and high robustness. However, the large junction capacitance and low modulation efficiency of the traditional ridge waveguide modulator limit its performance. In recent years, academic circles have made innovations in junction structures. Heteromorphic structures such as interleaved pn junction increase the interaction area between the light field and carrier, and improve the phase shifting efficiency. However, this is achieved at the expense of junction capacitance and bandwidth. Additionally, the TWE of the modulator must be well designed to ensure the matching between the refractive indexes of the RF signal and the optical signal, and the matching in electrode impedance to reduce the reflection at the source and the terminal to ensure the modulation depth. This paper aims at the large pn junction capacitance and low modulation efficiency of the silicon-based modulator with a single-drive push-pull scheme. We propose a hetero-doped silicon-based slot waveguide modulator to increase the bandwidth on the basis of ensuring modulation efficiency.

The whole design is divided into two parts of pn junction structure and electrode design. pn junction is formed by ion doping into the silicon waveguide, in which the boron atom is doped into the silicon waveguide to form the p region of pn junction, and the phosphorus atom is doped into the n region of pn junction. The top and bottom of the silicon waveguide are coated with silicon dioxide cladding. The two L-doped pn junctions and the intermediate heavily doped n⁺⁺ region are reversely connected as the main part of the MZ modulator phase shift region. The DC bias voltage is applied to the n⁺⁺ doped region, which makes both pn junctions work in the reverse bias state. The hetero-doped structure increases the interaction region between the optical field and the charge carrier, but also increases the junction capacitance. Therefore, the ridge waveguide structure is etched with slit regions to reduce the dielectric constant in them and reduce the junction capacitance. To maximize the modulation depth, this paper designs the TWE structure, adopts the T-shaped capacitive load electrode structure, and optimizes the parameters to ensure that the refractive index of the electrode is the same as that of the light in the waveguide, and the electrode impedance is matched by 50 Ω. Finally, the proposed structure at the link level is simulated to prove its high-speed modulation performance.

pn junction adopts the hetero-doped slot waveguide structure. This paper simulates the loss and phase shift efficiency of the proposed structure and the traditional ridge waveguide structure. The results show that the proposed structure does not introduce excessive optical loss and ensures phase shift efficiency (Fig. 5). In addition, a decimation comparison of the junction capacitance values is conducted, and the proposed structure leads to a 24% reduction in junction capacitance compared with the conventional ridge waveguide (Fig. 6). Then, the electrode design is optimized by the ABCD matrix method, which ensures the matching of the refractive index and impedance of the electrode. The bandwidth of the traditional ridge waveguide and the proposed structure modulator are compared through simulations. Under the bias voltage of 4 V, the bandwidth of the proposed structure modulator reaches 42 GHz, which is 32% higher than the bandwidth of the traditional ridge waveguide (Fig. 10). Finally, the modulation performance of the hetero-doped slot waveguide structure in a 70 Gbit/s high-speed link is demonstrated, and the extinction ratio of eye diagram reaches 5.2 dB.

This paper proposes an all-silicon modulator design for hetero-doped slot waveguides with single-drive push-pull TWEs, thereby increasing the interaction between the carrier depletion region and the optical field and ensuring modulation efficiency. Compared with the conventional ridge waveguide, the junction capacitance is reduced by 24%, and the bandwidth is increased by 32%. The hetero-doping ensures sound modulation efficiency. Under DC bias voltage, the modulation efficiency of 1-4 V is 1.8-2.5 V·cm, and the waveguide loss in the active region caused by carrier doping remains below 0.1 dB/cm. The slot waveguide structure reduces the dielectric constant in the etched area of the waveguide, reduces the junction capacitance, and increases the bandwidth. At the same time, the T-shaped track electrode is designed based on the waveguide structure design, and the excellent impedance matching and refractive index matching of the electrode are carried out by the transmission line equivalent circuit model. Under 4 V DC bias voltage, the electro-optic 3 dB bandwidth of the modulator reaches 42 GHz. Finally, the OOK signal modulation eye diagram of the hetero-doped modulator with slot waveguide is obtained at a low peak voltage of 2 V_pp of 70 Gbit/s, and the extinction ratio of the eye diagram reaches 5.2 dB, which proves that the designed modulator has a good high-speed modulation performance.