High-speed, thermally insensitive, and low-driving voltage silicon photonic modulators are essential element for broadband data transmission, low power consumption, high integration density, and stable operation in optical interconnection systems. In this paper, we propose and experimentally demonstrate a racetrack-type silicon-based microring electro-optic modulator that utilizes a coupling modulation mechanism, achieving 112 Gbit/s four-level pulse amplitude modulation (PAM4) optical signal generation under a peak-to-peak driving voltage of less than 1.2 V. Moreover, by employing a silicon nitride waveguide as the resonant cavity, the modulator exhibits a measured thermal drift coefficient of less than 2.5 pm/K, showing excellent thermal stability and verifying stable electro-optic modulation of 112 Gbit/s PAM4 within a temperature range of 20 ℃ to 45 ℃. This device scheme is expected to significantly reduce the power consumption of optical interconnection systems and enhance their thermal stability.

The modulator consists of a 2×2 Mach-Zehnder modulator (MZM) and a silicon nitride loop waveguide that connects two ports of the MZM. Two inverse tapers facilitate the transition of the optical mode from the silicon waveguide to the silicon nitride waveguide. By tuning the MZM, the coupling ratio of the racetrack ring resonator is modified, enabling the modulation of the optical signal’s intensity. The sharp resonance spectrum of the racetrack ring resonator allows for intensity modulation with minimal driving voltages. Meanwhile, due to the low thermo-optic coefficient (TOC) of silicon nitride (2.45×10^-5 K^-1), the silicon nitride loop waveguide significantly suppresses the thermal sensitivity. Two grating couplers connecting with the other two ports of the MZM are utilized to couple optical signals in/out of the chip. The modulator is fabricated in a commercial 200 mm complementary metal oxide semiconductor (CMOS) foundry. A 2×2 MZM with length of 2.5 mm is used to modulate coupling coefficients of the racetrack ring resonator.

The optical characterization of the modulator is evaluated. A tunable laser (Santec, TSL570) is used as the source, and an optical power meter (Santec, MPM-210H) is used to record the power data during continuous scanning of the laser wavelength. At an applied voltage of 0 on the pn junction, the ER exceeds 30 dB at the resonant wavelength. As the applied voltage varies from 0 to 1.8 V, the splitting ratio between the two ports of the MZM changes. Hence, the light is isolated from the resonator and propagates through the bus waveguide, leading to a low extinction ratio (ER) (<5 dB) (Fig. 3). Then, we characterize the electro-optic (EO) response of the proposed modulator with a 67 GHz Keysight lightwave component analyzer (LCA, Keysight N5277B). The 3 dB EO bandwidth is 31 GHz with a reverse bias of 4 V (Fig. 4). The optical-eye diagrams with driving voltages of 0.4-1.2 Vpp are demonstrated (Fig. 5). The proposed modulator can operate up to 112 Gbit/s PAM4 with driving voltage of 0.4, 0.6, 1.0 and 1.2 Vpp. And the ER are 0.570 dB, 0.750 dB, 1.340 dB and 3.516 dB, respectively. It is a remarkable fact that transmitter dispersion eye closure quaternary (TDECQ) of 4.4 dB is obtained with driving voltage of 1.2 Vpp. Meanwhile, the proposed modulator can support a robust operation at 112 Gbit/s PAM4, with ER>4.5 dB and TDECQ<3.00 dB over 25 K (Fig.7).

In conclusion, a silicon photonics modulator based on the coupling modulation of a racetrack ring resonator is experimentally demonstrated. The modulator is capable of transmitting a 112 Gbit/s PAM4 with a peak-to-peak driving voltage of less than 1.2 V. Furthermore, the modulator uses a silicon nitride waveguide as the resonant cavity, which boasts a thermal sensitivity of less than 2.5 pm/K. This feature enables the modulator to operate robustly at 112 Gbit/s PAM4 from 20 ℃ to 45 ℃. The modulator can help reduce power consumption and improve thermal stability in optical interconnects.