Compact Silicon IQ Modulator for High-Speed Coherent Communications

Introduction

The ever-increasing demand for higher data rates and more energy-efficient transceivers in data centers has driven the development of compact and high-performance optical modulators. Silicon photonics (SiPh) has emerged as a promising platform for integrating various photonic components onto a single chip, enabling compact and cost-effective solutions. In this article, we will explore a novel carrier-suppressed silicon IQ modulator based on germanium-silicon electro-absorption modulators (GeSi-EAMs), capable of operating at 75 Gbaud.

Device Design and Characteristics

The proposed IQ modulator features a compact 3-arm structure, as shown in Fig. 1(a). It consists of three GeSi-EAMs and three thermo-optic phase shifters arranged in a symmetrical configuration. The input light is coupled into the chip through a grating coupler (GC) and split equally into three paths using a 1x3 multimode interferometer (MMI). After passing through the modulators and phase shifters, the light is recombined by another 1x3 MMI and coupled into a single-mode fiber (SSMF).

Figure 1: (a) Microscope image of the fabricated IQ modulator, (b)-(d) EO S21, extinction ratio spectrum, transmission of single GeSi-EAM, (e) schematic representation of the IQ modulator and the corresponding optical phase of the phase shifters, (f) carrier-suppression representation in complex plane.

The GeSi-EAMs exhibit a wide electro-optic (EO) bandwidth, as shown in Fig. 1(b), where the S21 parameter of a single GeSi-EAM test structure is measured using a 67 GHz lightwave component analyzer (LCA). Figures 1(c) and 1(d) depict the static extinction ratio (ER) spectrum at different DC voltages and the transmission at 1555 nm, respectively.

The key feature of this design is the ability to suppress the optical carrier by tuning the integrated phase shifter in the center arm, as illustrated in Fig. 1(e). The top and bottom arms have a π/2 phase difference for complex modulation, while the center arm has a -3π/4 phase. This configuration allows for destructive interference of the carrier, as represented in the complex plane shown in Fig. 1(f).

Experimental Setup and Results

The high-speed experimental setup is depicted in Fig. 2(a). An arbitrary waveform generator (Keysight M8194A) is used as the signal source, and two 60 GHz phase-matched RF amplifiers with integrated bias tees are employed as modulator drivers. The GeSi-EAMs are biased at -2.5 V, and phase-matched RF cables are used to minimize electrical phase mismatch between the I and Q channels.

Figure 2: (a) High-speed experimental setup, (b) carrier suppression at 70 Gbaud, (c) BER versus datarate, (d) constellation at 70 Gbaud.

A commercial external cavity laser (ECL) source with a 100 kHz linewidth is used as the optical carrier at 1555 nm and 12 dBm. After modulation, the optical signal is amplified using an erbium-doped fiber amplifier (EDFA) to compensate for on-chip coupling losses and modulator insertion losses.

At the receiver side, an optical spectrum analyzer (OSA) monitors the signal spectrum and carrier suppression, as shown in Fig. 2(b) for a 70 Gbaud signal. The signal is received using a dual-polarization coherent receiver, consisting of a 90-degree optical hybrid and four 40 GHz balanced photodiodes, connected to a 33 GHz, 80 GSa/s real-time oscilloscope (RTO) (Keysight DSOZ634A). A second 100 kHz linewidth ECL source serves as the local oscillator (LO). Since the transmitter operates in a single polarization, only two electrical channels are used for analog-to-digital conversion (ADC). The data is recorded at the RTO and processed using off-line digital signal processing (DSP).

Using QPSK modulation format, the IQ modulator is tested from 40 Gbaud up to 75 Gbaud, as shown in Fig. 2(c). The results demonstrate up to 140 Gb/s under 7% hard-decision forward error correction (HD-FEC) and 150 Gb/s under 20% soft-decision FEC (SD-FEC). The constellation diagram at 70 Gbaud is presented in Fig. 2(d). The results are limited by the oscilloscope bandwidth of 33 GHz, corresponding to a signal bandwidth of approximately 66 Gbaud.

As shown in Fig. 2(b), the 3-arm structure can effectively suppress the optical carrier with an extinction of over 30 dB. Instead of fully suppressing the carrier, this structure can also be used for receivers where a carrier tone is required for signal recovery, such as the Kramers-Kronig (KK) receiver. The carrier can be tuned using the center arm phase shifter to achieve a desired carrier-to-sideband ratio (CSR).

Advantages and Applications

The proposed silicon IQ modulator offers several advantages compared to traditional Mach-Zehnder modulators (MZMs) and other SiPh IQ modulators. First, its compact footprint, more than 10 times smaller than MZM-based IQ modulators (excluding bondpads), enables higher integration density and reduced chip area. Second, the lumped electrode design eliminates the need for termination resistors, reducing device capacitance and energy-per-bit. Third, the carrier suppression capability minimizes the power transmitted, further enhancing energy efficiency.

Additionally, the wide optical bandwidth of the GeSi-EAMs (over 30 nm) simplifies the design by eliminating the need for multi-lane feedback control circuits and processors required to stabilize and synchronize the resonances of microring modulators (MRMs).

This compact and efficient IQ modulator is well-suited for next-generation, large channel count, and small form-factor coherent transceivers in data centers, enabling higher capacity and more energy-efficient optical communications.

Conclusion

The carrier-suppressed silicon IQ modulator demonstrates promising performance, achieving data rates up to 150 Gb/s under 20% SD-FEC. Its compact 3-arm structure, based on GeSi-EAMs, offers significant advantages in terms of footprint, energy efficiency, and design simplicity. With its ability to suppress or tune the optical carrier, this modulator can be integrated with coherent detection and Kramers-Kronig (KK) detection schemes for multi-lane compact coherent transceivers. The demonstrated results pave the way for highly integrated and efficient optical transceivers to meet the ever-increasing demand for high-capacity data communications in data centers and beyond.

Reference

[1] D. W. U. Chan, X. Wu, Y. Tong, C.-W. Chow, C. Lu, A. P. T. Lau, and H. K. Tsang, "Carrier-suppressed Silicon IQ Modulator at 75 Gbaud," Dept. of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong; Photonics Research Institute, Dept. of Electrical and Electronic Engineering, The Hong Kong Polytechnic University, Hong Kong; Microelectronics Thrust, The Hong Kong University of Science and Technology (Guangzhou), Guangdong, China; Dept. of Photonics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.