Modulator Drives Record-Fast Transmission in Face of Data Traffic

Although silicon photonics holds potential as a platform for optical transceivers, owing largely to its CMOS compatibility, the approach is limited in its electro-optic bandwidth. It also carries high driving voltage requirements. The qualities can hinder its use in optical communications network scaling, which is needed to accommodate the rapid growth of data traffic.

Using standard chip technology and standard data encoding algorithms, researchers at McGill University and Ericsson Canada demonstrated optical communication at 105 Gbaud with net 1 Tbit/s transmission. The team designed the system using a CMOS-compatible silicon photonic modulator, and claimed a record-breaking data rate of 1 Tbit/s. This speed is fast enough to support 800 Gbit/s (800G), the next-generation standard for telecommunications.

CMOS devices, such as the modulator conceptualized and developed by the researchers, have high noise immunity and low power consumption, and they are relatively low-cost to produce. The work demonstrates the potential of silicon photonics as a platform for 800G applications.

Further, according to the researchers, the silicon photonic modulator based on their design could be commercialized for telecommunications systems built to run on the 800G standard.

The modulator operates in the conventional C-band of wavelengths between 1530 and 1565 nm. The researchers designed and characterized two single-segment, silicon photonic, C-band in-phase quadrature modulators (IQMs) with phase shifter lengths of 3 and 4 mm. They analyzed the transmission performance of each IQM, as well as the design trade-offs, with an eye toward possible future commercialization. The IQMs modulate both the amplitude and phase of light and support polarization multiplexing.

Using an advanced modulation format, the researchers tested both IQMs to evaluate the speed and quality of the data communication. They used a standard quadrature amplitude modulation (QAM) algorithm to encode multiple bits of data onto a single light pulse to better support a high rate of symbol transmission. Currently, the researchers use a dual-polarization 16QAM format for the coding of information.

The researchers documented the performance of both single-polarization and dual-polarization transmission results and the optical signal-to-noise ratio (SNR) performance. The large-signal transmission experiments conducted by the team revealed that the long IQM supported higher data transmission rates. Due to its higher phase-shifting efficiency, the long IQM outperformed the short one despite the long IQM’s inferior electro-optic bandwidth. The measurements for optical SNR showed that the long IQM had a higher optical SNR penalty. However, the long IQM could achieve a higher optical SNR than the short IQM, resulting in better bit-error-rate performance.

Using the long IQM and all-electronic equalization, the researchers transmitted 95-Gbaud 32QAM over 80 km of standard single-mode fiber at a bit-error-rate below the 1.25 × 10⁻² concatenated forward error correction (C-FEC) threshold, which corresponded to a net rate of 413 Gbit/s.

Researchers at McGill University and Ericsson Canada achieved a significant milestone with the successful demonstration of what they claimed to be the first net 1-Tbit/s transmission using a CMOS-compatible silicon photonic modulator. Courtesy of E. Berikaa et al., Silicon Photonic Single-Segment IQ Modulator for Net 1 Tbps/λ Transmission Using All-Electronic Equalization, doi: 10.1109/JLT.2022.3191244.

Using nonlinear pre-distortion and dual-polarization emulation, the team transmitted 95-Gbaud dual-polarization-32QAM and 115-Gbaud dual-polarization-16QAM over 80 km under the 1.25 × 10⁻² C-FEC threshold. These transmissions demonstrated net rates of 827 and 800 Gbit/s, respectively.

Moreover, the researchers demonstrated the transmission of 105-Gbaud dual-polarization-64QAM over 80 km below the 5 × 10⁻² soft-decision forward error correction (SD-FEC) bit-error-rate threshold using all-electronic equalization in a conventional coherent setup, to achieve a record net rate of 1 Tbit/s (line rate of 1.26 Tbit/s).

The researchers were able to transmit ultrahigh symbol rate signals despite the modest bandwidth of the silicon photonic IQM. To the best of the team’s knowledge, the transmission rates realized in this work are the highest reported at all the considered FEC thresholds using an all-silicon IQM.

The use of only electronic equalization and single-segment IQM allowed the researchers to preserve the conventional architecture of coherent networks and transceivers. Still, the work highlights single-segment silicon photonic IQMs as a candidate technology for next-generation coherent networks.

The research was published in Journal of Lightwave Technology (https://ieeexplore.ieee.org/document/9829913).