Frequency Comb-based High-Speed Data Transmission with Integrated Tunable Dispersion Compensation

Introduction

The ever-increasing demand for data necessitates continuous scaling of high-speed data transmission line rates while reducing cost and energy consumption. Utilizing a frequency comb generated from a single laser source for intensity modulated direct detection (IMDD) data transmission is a promising approach to address these needs. Frequency comb-based transmission is limited in fiber reach due to intensity and phase noise, as well as dispersion impairments caused by the varying velocities of different frequency components traversing the optical fiber. This tutorial discusses the use of an integrated silicon nitride (SiN) platform for frequency comb generation and tunable dispersion compensation to enable high data rates over significant fiber distances.

Frequency Comb Generation

A microring resonator with a high Q-factor of 1.9 x 10^6 and 225 GHz free spectral range is used to generate primary comb states from two pump resonances at 1545 nm and 1560 nm wavelengths, as shown in Fig. 1a. Specific comb lines (indicated by dashed lines) are filtered and modulated with IMDD data before transmission through various fiber lengths.

Figure 1 shows a micro-ring resonator generating frequency combs (a). Specific comb lines (red and blue) are filtered and modulated before transmission through a fiber. The DC grating device can be thermally tuned (b) to access the right dispersion mode (TE or TM) for compensating the chosen comb line (red or blue).

Tunable Dispersion Compensation

Dispersion compensation is crucial to mitigate signal degradation from inter-symbol interference and spectral crosstalk caused by chromatic dispersion in the fiber. A grating-based dispersion compensator (DC) device with dimensions of 4.2 mm length, 1.5 μm width, and 800 nm height provides tunable normal dispersion to compensate for the anomalous dispersion accumulated in the fiber.

The required normal dispersion magnitude is accessed by thermally tuning the DC grating device, where an increase in temperature causes a red-shift in the group delay spectrum, as depicted in Fig. 1b. Normal dispersion can be accessed on both the blue and red sides of the band gap, compensating for up to 25 km and 43 km of fiber when utilizing the TE and TM modes, respectively.

High-Speed Measurement Setup

The schematic of the high-speed measurement setup is shown in Fig. 2a. A tunable laser source (TLS) is used to scan across the microring resonator resonance, generating the frequency comb lines. A Mach-Zehnder modulator driven by a bit error rate tester (BERT) modulates a filtered comb line with 25 Gb/s NRZ and 25 GBd/s (50 Gb/s) PAM4 signals. A polarization controller (PC) is used before the DC grating device to permutate between the TE and TM modes, effectively doubling the channel capacity.

Fig. 2 shows the setup for high-speed transmission (a) and the Bit-Error Rate (BER) for different received powers after dispersion compensation using TE (b) and TM (c) modes of the DC grating device (25Gb/s NRZ signals). It also compares eye diagrams (d, e) and TDECQ values (f) before and after compensation for NRZ (d) and PAM4 (d, f) data signals.

Results and Discussion

Figs. 2b and 2c demonstrate the improvement in bit error rate (BER) after dispersion compensation for both TE and TM modes, respectively, when transmitting 25 Gb/s NRZ signals over 20 km of fiber. The dispersion penalty for a BER of 10^-3 is significantly reduced from 8.45 dBm to 1.4 dBm in the TE mode and from 8.9 dBm to 5.5 dBm in the TM mode after compensation.

The eye diagrams in Figs. 2d and 2e further illustrate the improvement in signal quality after dispersion compensation for both NRZ and PAM4 signals. The Transmitter and Dispersion Eye Closure Quaternary (TDECQ) metric, inversely proportional to signal degradation at the receiver, is used to quantify the PAM4 signal quality.

As shown in Fig. 2f, severe dispersion impairments are present after transmitting the frequency comb line with PAM4 data signals over 6-20 km of fiber without compensation. However, applying dispersion compensation results in significantly lower TDECQ values, with over 14 dB improvement at 50 Gb/s for various fiber lengths.

Conclusion

This work demonstrates frequency comb-based transmission of 50 Gb/s high-speed data over fiber reaches of up to 20 km, enabled by efficient, low-loss amelioration of dispersion impairments using a tunable on-chip dispersion compensation device. This integrated solution showcases the potential for fully dispersion-managed, frequency comb-based systems to power higher capacity wavelength-multiplexed data center communications.

Reference

[2] K. Y. K. Ong et al., "Frequency Comb-Based High-Speed Data Transmission with Integrated Tunable Dispersion Compensation," IEEE Journal of Selected Topics in Quantum Electronics, vol. 30, no. 6, pp. 4100507, Nov. 2023, doi: 10.1109/JSTQE.2023.3338804