The realization of a high dynamic extinction ratio (ER) and optical modulation
amplitude (OMA) while keeping the optical and radio-frequency (RF)
signal losses low is a major issue for carrier-depletion Mach–Zehnder
(MZ) silicon optical modulators. However, there is still room to
improve modulator performance by applying the information gained from
recent advanced testing technology to the modulator design. In this
study, the extrinsic OMA (E-OMA) enhancement effect, which was
discovered through the evaluation process and by revisiting the
physics of the MZ interferometer (MZI), is investigated. First, we
raise the issue of a periodic ripple observed on an MZI spectrum that
has previously been overlooked but can affect modulator performance
and attribute it to optical resonance between the multi-mode
interferometers that compose an MZI. We show that, although having the
effect of reducing the dynamic ER in the push-pull regime, as
demonstrated experimentally, this resonance can take them beyond the
realm of modulation efficiency and generate an E-OMA enhancement
effect in the single-arm-drive regime without involving any optical
and RF signal losses. By comparing two modulator structures that
generate resonance internally, we successfully identify the factors
that are responsible for increasing the E-OMA enhancement effect. We
reveal that theoretically the OMA can easily be increased by
0.45 dB or more.