IEEE Solid-States Circuits Magazine - Summer 2021 - 29

carrier, impedance matching, RF
attenuation, and the -3-dB electrical
BW of the driver.
Figure 10 shows a traveling-wave
MZM driven by a differential driver
[2]. All
impedances are matched
to be RT, say, 50 Ω. When the input
to the differential driver turns the
transistor off in one branch, the
tail current is steered to the other
branch, and vice versa. The differential
peak-to-peak swing from
the driver to the MZM goes from
RTIT/2 to −RTIT/2, the total being
RTIT. This swing, in turn, leads to
an optical modulation swing. Let us
assume that it reaches a fraction of
V .r
How do we increase this swing
further? Recall that we want to get
closer to Vr
to increase our ER. RT is
fixed due to impedance matching.
We can increase IT at the expense
of power consumption. But there is
a limit: for a given VDD, increasing
The overall O/E BW of a traveling-wave MZM
is determined by velocity matching between the
RF and the optical carrier, impedance matching,
RF attenuation, and the -3-dB electrical BW
of the driver.
IT can drive the tail current source
into the triode region. To solve that,
we can also increase VDD with a
further power consumption penalty.
Increasing
the swing and VDD
must be done carefully as they can
lead to reliability and breakdown
concerns for the transistors. With
elevated VDD and signal swing, adding
a thick-gate, long-channel cascode
transistor protects the switching
transistors from overvoltage conditions,
allows higher gate and drainto-source
voltages, and reduces the
Miller capacitance. One can also
VDD
RT
Data Driver
I/P
Z0 = RT
CTRL
Power Measurement <8:0>
FIGURE 11: A control circuit to bias the MZM at its quadrature point. CTRL: controller;
MPD: monitor PD; I/P: input; O/P: output.
0000 011 1
Laser
Wavelength
DRV
IL
V0
ER
Optical " 0 "
Through
λ
0000 011 1
λλ0 λ1
λλ0 λ1
FIGURE 12: A microring modulator (MRM) along with its normalized transmission responses showing the tradeoff between IL and ER.
DRV: driver.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SUMMER 2021
29
Optical " 1 "
V1
IL
ER
V0
V1
Optical " 0 "
Laser
Wavelength
Optical " 1 "
RT
5% Tap
O/P
VD
MPD
ADC
RTIA
use peaking inductors to absorb the
parasitic capacitance at the drain
nodes [26].
The two arms of the MZI can exhibit
a phase offset due to fabrication tolerances.
The offset is compensated for
using a control loop consisting of an
integrated optical tap that directs a
small portion of the output light to a
monitor PD, followed by an analog-todigital
converter (ADC), a finite-state
machine, and a digital-to-analog converter
(DAC) (Figure 11). This information
is then used to adjust the currents
in the thermal phase modulators in
each of the MZI arms. The control
loop can choose the thermal phase
modulator current to keep the average
output power of the MZI at 3 dB
below its maximum output power, the
" quadrature point " of the MZI. This
mechanism assumes that the input
optical power is constant [1], but other
control algorithms can be used if the
power varies [27].
Microring Modulator
Figure 12 shows a microring modulator
(MRM). The normalized transmission
Normalized Transmission
Normalized Transmission

IEEE Solid-States Circuits Magazine - Summer 2021

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