A Quantitative Investigation on the Impact of Microring Modulator Coupling States to 112 Gbps PAM4 Signal Transmission

In this paper, we conduct a comprehensive quantitative analysis on the impact of silicon microring modulators (Si-MRM) coupling states to 112 Gbps PAM4 transmission system based on a system-level model of Si-MRM, which include a dynamic ring resonator and a linearized equivalent electrical circuit. The simulation results reveal that among three of the coupling states, the critical coupling state yields the most optimal extinction ratio (ER) and bit error rate (BER) performances. Within the range of 0.70 to 0.85 for round-trip loss
$a$
and self-coupling coefficient
$t\ (a=t)$
, an optimal system performance is attainable. In the cases of overcoupling and undercoupling, favorable performance can be achieved when
$t$
is approximately 0.75 (∼ 0.05 smaller than
$a)$
and 0.79 (∼0.04 larger than
$a$
, respectively. At BER of
$3.8\times 10^{-3}$
, the power penalty for the critical coupling state under
$a=t= 0.70$
to
$a=t=0.94$
is ∼6.61 dB. And there's ∼7.13 dB power penalty for the overcoupling state at
$t= 0.55\ (a=0.80)$
) compared to the critical coupling benchmark when
$t=0.80$
at BER of
$1.5\times 10^{-2}$
. For undercoupling, the power penalty for the state of
$t=0.95 (a=0.75)$
is ∼ 7.07 dB compared to
$t=0.75$
at BER of
$3.8\times 10^{-3}$
. These results clearly reveal the influence of coupling states of MRM to the system performance, offering insights that applicable to modulator design, compensating for fabrication errors, and employing Si-MRM at the system level.