52 Gbps PAM4 receiver sensitivity study for 400GBase-LR8 system using directly modulated laser.

Area of Science:

• Optical communications
• High-speed data transmission
• Photodetector technology

Background:

• Directly modulated lasers (DMLs) are crucial for cost-effective optical transceivers.
• Signal integrity in high-speed optical links is challenged by chromatic dispersion (CD).
• Avalanche photodetectors (APDs) and PIN photodetectors (PDs) are key receiver components with different performance characteristics.

Purpose of the Study:

• To demonstrate real-time 52 Gbps PAM4 transmission over single mode fiber (SMF).
• To compare the receiver sensitivity of PIN photodetectors and avalanche photodetectors under varying chromatic dispersion.
• To evaluate the impact of DML bias current on receiver performance.

Main Methods:

• Implemented a 52 Gbps Pulse Amplitude Modulation with 4 levels (PAM4) system using a DML and a PHY chip.
• Measured inner eye optical modulation amplitude (OMA) receiver sensitivities using both PIN PD and APD.
• Tested performance across the maximum and minimum chromatic dispersions (CDs) relevant to 400GBase-LR8 links.
• Assessed the effect of DML bias currents (40 mA and 60 mA) on OMA.

Main Results:

• Achieved 52 Gbps PAM4 transmission over SMF.
• PIN PD sensitivities were -17.8 dBm (10 ps/nm CD) and -18.8 dBm (-58 ps/nm CD) at a 2x10^-4 bit error rate (BER) threshold.
• APD sensitivity improved to -21.0 dBm at -58 ps/nm CD.
• Observed negligible OMA penalty (< 0.4 dB) across different DML bias currents and CD conditions.

Conclusions:

• Directly modulated lasers coupled with APDs provide superior receiver sensitivity for 52 Gbps PAM4 transmission, especially in high CD environments.
• PIN PDs offer viable performance for 52 Gbps PAM4 links, particularly at lower CD values.
• DML bias current has minimal impact on the OMA penalty, simplifying system design.