Unveiling the Frequency Response of Silicon Ring Modulators for Coherent Transmitters

Introduction

Silicon photonics has emerged as a promising technology for enabling high-performance, cost-effective, and compact photonic integrated circuits (PICs). While silicon ring modulators (RMs) have been widely employed for intensity modulation and direct detection (IM/DD) applications, such as data center interconnects, there is a growing interest in leveraging these devices for coherent modulation schemes. Coherent techniques offer higher transmission capacity and can potentially address the ever-increasing demands of hyperscale data centers. However, realizing compact yet high-performance in-phase and quadrature (I/Q) modulators in the silicon photonics platform presents technical challenges, one of which is thoroughly understanding the phase modulation characteristics of silicon RMs.

This article discusses the characterization of the complex electro-optic (E/O) frequency response of a silicon RM, encompassing both magnitude and phase domains. By accurately measuring and modeling these responses, designers can optimize the performance of RM-based coherent transmitters.

Device Description

The device under investigation is a silicon RM fabricated using the IHP silicon photonics technology. As shown in Fig. 1(a), the RM has a 16-μm radius, a 220-nm coupling gap, and a rib waveguide structure with a 220-nm thickness, 500-nm width, and 100-nm slab thickness.

Fig. 1. (a) Chip photograph of a fabricated Si RM, (b) phase modulation operation point of over-coupled RM.

The RM is designed to operate in an over-coupling condition, which provides a 2π phase shift around the resonance wavelength (λres). This allows for π-phase modulation at the operation wavelength (λin) while maintaining the same optical intensity, as illustrated in Fig. 1(b). The fabricated RM has a 10.0-dB insertion loss and a Vπ (voltage required for π phase shift) of 5.7 V peak-to-peak at λin.

Complex E/O Response Characterization

The complex E/O frequency response of the π-phase-modulated silicon RM is characterized using a heterodyne coherent reception technique. Figure 2(a) depicts the measurement setup, which includes a laser source, an erbium-doped fiber amplifier (EDFA), a commercial coherent receiver (CoRx), and a real-time oscilloscope (RTO) for data acquisition and digital signal processing (DSP).

Fig. 2. (a) Measurement setup for characterization of complex E/O responses of the RM and (b) measured and simulated normalized magnitude and phase responses of the RM.

The electrical signal is supplied by an RF signal source, amplified, and delivered to the RM through a bias-T. The modulated optical signal from the RM is amplified by the EDFA and received by the CoRx, which uses a local oscillator (LO) laser for heterodyne reception. The CoRx output signals are acquired by the RTO and processed offline by the DSP to obtain the complex E/O response.

Additionally, the complex E/O frequency response is simulated using a coupled-mode theory (CMT) model, which calculates the time-domain responses of the RM based on measured optical transmission spectra and electrical reflection coefficients. Fourier transformation of these time-domain responses yields the simulated complex E/O frequency response.

Figure 2(b) presents the measured and simulated magnitude and phase frequency responses. Although some measurement errors are present, likely due to incomplete de-embedding of the components used, the overall measurement results agree well with the simulations.

In Fig. 2(b), the 3-dB drop in the magnitude response occurs at 18.5 GHz, and at this frequency, the phase response increases by approximately +0.25π compared to the low-frequency value. This coincidence of a 3-dB magnitude drop and a 0.25π phase increase at the same frequency suggests that the silicon RM phase modulation can be modeled as a simple one-pole system, which is confirmed by the RM small-signal model presented in the literature.

Conclusion

This article discusses the characterization of the complex E/O frequency responses of a silicon ring modulator, both in magnitude and phase domains. The measured responses are validated by simulations based on a coupled-mode theory model. The presented characterization technique provides a powerful tool for optimizing silicon ring modulators for desired coherent transmitter performance, enabling further advancements in high-capacity, short-reach coherent optical communications.

Reference

[1] Y. Jo, Y. Ji, H.-K. Kim, S. Lischke, C. Mai, L. Zimmermann, and W.-Y. Choi, "Complex Electro-Optic Frequency-Response Characterization of a Si Ring Modulator," Yonsei University and IHP – Leibniz-Institut für innovative Mikroelektronik, Seoul, South Korea, and Frankfurt, Germany, 2024.