For high-speed coherent communication compact silicon-based optoelectronic IQ modulator

Compact silicon-based optoelectronic IQ modulator for high-speed coherent communication
The increasing demand for higher data transmission rates and more energy-efficient transceivers in data centers has driven the development of compact high-performance optical modulators. Silicon based optoelectronic technology (SiPh) has become a promising platform for integrating various photonic components onto a single chip, enabling compact and cost-effective solutions. This article will explore a novel carrier suppressed silicon IQ modulator based on GeSi EAMs, which can operate at a frequency of up to 75 Gbaud.
Device design and characteristics
The proposed IQ modulator adopts a compact three arm structure, as shown in Figure 1 (a). Composed of three GeSi EAM and three thermo optical phase shifters, adopting a symmetrical configuration. The input light is coupled into the chip through a grating coupler (GC) and evenly divided into three paths through a 1×3 multimode interferometer (MMI). After passing through the modulator and phase shifter, the light is recombined by another 1×3 MMI and then coupled to a single-mode fiber (SSMF).

Figure 1: (a) Microscopic image of IQ modulator; (b) – (d) EO S21, extinction ratio spectrum, and transmittance of a single GeSi EAM; (e) Schematic diagram of IQ modulator and corresponding optical phase of phase shifter; (f) Carrier suppression representation on the complex plane. As shown in Figure 1 (b), GeSi EAM has a wide electro-optic bandwidth. Figure 1 (b) measured the S21 parameter of a single GeSi EAM test structure using a 67 GHz optical component analyzer (LCA). Figures 1 (c) and 1 (d) respectively depict the static extinction ratio (ER) spectra at different DC voltages and the transmission at a wavelength of 1555 nanometers.
As shown in Figure 1 (e), the main feature of this design is the ability to suppress optical carriers by adjusting the integrated phase shifter in the middle arm. The phase difference between the upper and lower arms is π/2, used for complex tuning, while the phase difference between the middle arm is -3 π/4. This configuration allows for destructive interference to the carrier, as shown in the complex plane of Figure 1 (f).
Experimental setup and results
The high-speed experimental setup is shown in Figure 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 used as modulator drivers. The bias voltage of GeSi EAM is -2.5 V, and a phase matched RF cable is 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) Error rate and data rate, (d) Constellation at 70 Gbaud. Use a commercial external cavity laser (ECL) with a linewidth of 100 kHz, wavelength of 1555 nm, and power of 12 dBm as the optical carrier. 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 receiving end, an Optical Spectrum Analyzer (OSA) monitors the signal spectrum and carrier suppression, as shown in Figure 2 (b) for a 70 Gbaud signal. Use a dual polarization coherent receiver to receive signals, which consists of a 90 degree optical mixer and four 40 GHz balanced photodiodes, and is connected to a 33 GHz, 80 GSa/s real-time oscilloscope (RTO) (Keysight DSOZ634A). The second ECL source with a linewidth of 100 kHz is used as a local oscillator (LO). Due to the transmitter operating under single polarization conditions, only two electronic channels are used for analog-to-digital conversion (ADC). The data is recorded on RTO and processed using an offline digital signal processor (DSP).
As shown in Figure 2 (c), the IQ modulator was tested using QPSK modulation format from 40 Gbaud to 75 Gbaud. The results indicate that under 7% hard decision forward error correction (HD-FEC) conditions, the rate can reach 140 Gb/s; Under the condition of 20% soft decision forward error correction (SD-FEC), the speed can reach 150 Gb/s. The constellation diagram at 70 Gbaud is shown in Figure 2 (d). The result is limited by the oscilloscope bandwidth of 33 GHz, which is equivalent to a signal bandwidth of approximately 66 Gbaud.

As shown in Figure 2 (b), the three arm structure can effectively suppress optical carriers with a blanking rate exceeding 30 dB. This structure does not require complete suppression of the carrier and can also be used in receivers that require carrier tones to recover signals, such as Kramer Kronig (KK) receivers. The carrier can be adjusted through a central arm phase shifter to achieve the desired carrier to sideband ratio (CSR).
Advantages and Applications
Compared with traditional Mach Zehnder modulators (MZM modulators) and other silicon-based optoelectronic IQ modulators, the proposed silicon IQ modulator has multiple advantages. Firstly, it is compact in size, more than 10 times smaller than IQ modulators based on Mach Zehnder modulators (excluding bonding pads), thus increasing integration density and reducing chip area. Secondly, the stacked electrode design does not require the use of terminal resistors, thereby reducing device capacitance and energy per bit. Thirdly, the carrier suppression capability maximizes the reduction of transmission power, further improving energy efficiency.
In addition, the optical bandwidth of GeSi EAM is very wide (over 30 nanometers), eliminating the need for multi-channel feedback control circuits and processors to stabilize and synchronize the resonance of microwave modulators (MRMs), thereby simplifying the design.
This compact and efficient IQ modulator is highly suitable for next-generation, high channel count, and small coherent transceivers in data centers, enabling higher capacity and more energy-efficient optical communication.
The carrier suppressed silicon IQ modulator exhibits excellent performance, with a data transmission rate of up to 150 Gb/s under 20% SD-FEC conditions. Its compact 3-arm structure based on GeSi EAM has significant advantages in terms of footprint, energy efficiency, and design simplicity. This modulator has the ability to suppress or adjust the optical carrier and can be integrated with coherent detection and Kramer Kronig (KK) detection schemes for multi line compact coherent transceivers. The demonstrated achievements drive the realization of highly integrated and efficient optical transceivers to meet the growing demand for high-capacity data communication in data centers and other fields.