  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.
