# High-speed PAM4-based Optical SDM Interconnects with Directly Modulated Long-wavelength VCSEL

## TL;DR

This paper demonstrates high-speed PAM-4 optical interconnects using a 1.5-μm VCSEL over multicore fiber, achieving 700 Gbps total data throughput with effective pre- and post-equalization techniques.

## Contribution

It presents the first demonstration of high-speed PAM-4 transmission over multicore fiber using a long-wavelength VCSEL with digital equalization to mitigate dispersion and bandwidth limitations.

## Key findings

- Achieved 70 Gbaud PAM-4 signals in back-to-back configuration.
- Transmitted 50 Gbaud PAM-4 over 1 km and 10 km fibers with acceptable BER.
- Demonstrated 700 Gbps total throughput over 7-core fiber.

## Abstract

This paper reports the demonstration of high-speed PAM-4 transmission using a 1.5-{\mu}m single-mode vertical cavity surface emitting laser (SM-VCSEL) over multicore fiber with 7 cores over different distances. We have successfully generated up to 70 Gbaud 4-level pulse amplitude modulation (PAM-4) signals with a VCSEL in optical back-to-back, and transmitted 50 Gbaud PAM-4 signals over both 1-km dispersion-uncompensated and 10-km dispersion-compensated in each core, enabling a total data throughput of 700 Gbps over the 7-core fiber. Moreover, 56 Gbaud PAM-4 over 1-km has also been shown, whereby unfortunately not all cores provide the required 3.8 $\times$ 10 $^{-3}$ bit error rate (BER) for the 7% overhead-hard decision forward error correction (7% OH HDFEC). The limited bandwidth of the VCSEL and the adverse chromatic dispersion of the fiber are suppressed with pre-equalization based on accurate end-to-end channel characterizations. With a digital post-equalization, BER performance below the 7% OH-HDFEC limit is achieved over all cores. The demonstrated results show a great potential to realize high-capacity and compact short-reach optical interconnects for data centers.