High-Speed Silicon Photonic Modulators for Coherent Optical Transmission

Introduction

Silicon photonics is becoming the preferred platform for compact coherent optical transceivers used in data center interconnects. While passive silicon photonic components and germanium photodetectors have demonstrated compelling performance, silicon modulators have suffered from a trade-off between efficiency and bandwidth, limiting overall transceiver capacity. Novel photonic structures like segmented traveling-wave modulators and microring-assisted modulators can help overcome this limitation.

Segmented Traveling-Wave Modulators

Segmenting the phase shifters in a Mach-Zehnder modulator (MZM) can effectively improve the electro-optic bandwidth while maintaining or even reducing the switching voltage Vπ. The basic concept is illustrated in Figure 1. By using shorter phase shifter segments driven in parallel, a higher bandwidth can be achieved compared to a single long phase shifter driven at the same voltage.

Figure 1. IQ-modulator using STW-MZM: Layout schematics.

A key advantage of this segmented traveling-wave MZM (STW-MZM) approach is that it offers a simple way to significantly increase bandwidth using conventional manufacturing processes. A three-segment device with 67 GHz bandwidth and Vπ of only 5V has been demonstrated.

However, realizing the full bandwidth potential requires careful design to minimize delay mismatch between the segments. A two-segment configuration can provide an excellent trade-off between increased bandwidth and driving complexity. As shown in Figure 1(b), a two-segment MZM can be driven using a simple differential drive scheme with a single differential driver.

Figure 2. IQ-modulator using STW-MZM: RC circuit for differential drive; off-chip signal in blue, S (signal) and G (ground) are intentionally mapped as S of Seg1 is +data and S of Seg2 is -data.

Using a two-segment STW-MZM with probabilistic shaping, a net rate of 1.07 Tb/s over 80 km of fiber has been achieved at 116 Gbaud with 64-QAM modulation, as plotted in Figure 3. The advantage of segmentation is clearly visible at higher symbol rates and constellation orders.

Figure 3. IQ-modulator using STW-MZM: BER and OSNR performance at 120GBaud for 16-QAM and 64-QAM.

Microring-Assisted Mach-Zehnder Modulators

Another approach leverages optical resonators like microrings to enhance the modulation efficiency and reduce device footprint. The basic structure is a microring resonator nested in one arm of a Mach-Zehnder interferometer, forming a microring-assisted MZM (MRA-MZM) as depicted in Figure 2(a).

Figure 4. IQ modulator using MR-assisted MZMs: Layout schematic.

While microrings offer excellent compactness and low power consumption, resonance also brings challenges like frequency chirp and nonlinear phase distortion that limit bandwidth. By operating the MRA-MZM in push-pull mode using a quadrature configuration, these impairments can be suppressed.

An ultra-compact IQ MRA-MZM modulator with just 100 μm width has demonstrated 67 GHz bandwidth. As shown in Figure 2(b), using coherent detection, a remarkable net rate of 1 Tb/s was achieved over 40 km of fiber despite using only linear signal processing. This corresponds to an exceptional bandwidth density exceeding 5 Tb/s/mm.

Figure 5. IQ modulator using MR-assisted MZMs: Coherent optical transmission performance

The compact footprint and low power consumption of the MRA-MZM make it very appealing for highly integrated wavelength division multiplexing and space division multiplexing transceivers.

Bandwidth and SNR Scaling

The bandwidth enhancement from segmentation comes from using shorter phase shifter segments driven in parallel at a lower voltage per segment. While doubling the number of segments doubles the bandwidth, the maximum achievable signal-to-noise ratio (SNR) is reduced since the voltage swing per segment is lower.

However, in practice the SNR increase is less than theoretical due to implementation penalties from factors like impedance mismatch and signal reflections. As plotted in Figure 15(c), measurements show the OSNR improvement diminishes as more segments are added, particularly transitioning from two to three segments.

Alternatively, the enhanced SNR from segmentation can be exploited by driving all segments at the maximum voltage of a single long phase shifter. In this case, the bandwidth is set by the chosen short segment length, while tiling additional parallel segments increases the SNR until reaching diminishing returns from factors like reduced Vπ efficiency.

High-Speed Transmission Results

Using advanced signal processing like probabilistic constellation shaping, these high-bandwidth silicon modulators have enabled impressive high-speed transmission results. Dual-polarization coherent transmission exceeding 1 Tb/s net rate has been demonstrated over 80 km with the STW-MZM and 40 km with the MRA-MZM despite using only linear signal processing.

The current bottleneck is the test equipment like high-speed DACs and oscilloscopes rather than the photonic devices themselves. Implementing more complex nonlinear signal processing could extend the reach to even longer distances for future ultra-high bandwidth links.

Conclusion

Novel segmented and resonant silicon photonic modulators have overcome the traditional bandwidth limitations of this platform. Devices with over 67 GHz electro-optic bandwidth have enabled groundbreaking coherent transmission exceeding 1 Tb/s using advanced silicon modulators, despite their ultra-compact footprints. Further improvements in driving electronics and signal processing could unlock even higher capacities for future ultra-high bandwidth interconnects.

Reference

[1] W. Shi, Z. Zheng, A. Mohammadi, A. Geravand, S. Levasseur, and L. Rusch, "High-speed silicon photonic modulators for coherent optical transmission," IEEE SiPhotonics, April. 15, 2024.