FEATURE Optical IQ modulators
system applications. The operation
of a LiNbO3 modulator is based
on a linear electro-optic effect.
The modulation bias point is set
by a control voltage on each MZ
arm, either via a bias-tee through
the RF port or a separated phase
electrode. InP modulator phase
control is accomplished via either
reverse or forward biased phase
electrodes to adjust the operating
points. As with all InP-based lasers
or photodiodes, proper attention
is required for the voltage and
current limits of the control circuits.
It’s well known that the strong
thermal drift of LiNbO3 material
requires a very fast bias control to
stabilize the operation point in a
system. The fast phase change can
be compensated by applying a fast
control signal to a phase electrode.
For InP material, this fast
thermal drift is absent, leading to
a lower speed, simpler control
loop. For InP devices, the material
characteristics still need to be
stabilized using a thermo-electric
cooler (TEC) to ensure constant
operation over the environmental
temperature range. To maintain
the suppressed carrier at null
bias point over the operational
lifetime, a slow control loop
will be needed to compensate
for the device’s aging.
Low-Vπ, high bandwidth
InP IQ modulator
An InP modulator based on QCSE
requires a DC bias to provide the
necessary pn-junction electric
modulators enable the use of lower-
voltage drivers, decreasing the
complexity of the amplifier design
and reducing the number of gain
chips required in a package, thus
leading to a potential cost benefit.
Linearity is a key requirement
for 200G and 400G applications,
where more advanced modulation
formats will be needed. To provide
a linear output, driver amplifier
design requires an increased
voltage supply level to compensate
for the distortion at higher output
voltages. A smaller modulator Vπ
naturally reduces this requirement,
enabling a more efficient amplifier
design with a lower supply voltage.
The ER of each child and parent
MZ is defined as the ratio between
the maximum and minimum optical
intensities measured at the same
port. Poor ERs and any imbalance
between the two MZ arms will induce
chirp in the optical signal. Chirp is
the optical phase variation due to
relative variation of optical intensity.
The presence of chirp in a
transmitted signal will distort the
transitions between constellation
points and increase the minimum
required OSNR for the system.
With closely spaced constellation
points, higher order modulation
formats such as 16QAM will require
better ERs than the values defined
in current 100G standards.
Although the DP-QPSK modulation
format for 100G is common among
system vendors, there are many
approaches for future 400G systems.
(This fact has led some to draw
parallels to the modulation format
debates that surrounded 40G about a
decade ago.) Regardless, modulators
with higher bandwidth will provide
better linearity and spectral efficiency
in such next gen coherent systems.
As the Table illustrates, recent
advances in InP-based traveling-wave MZ modulator have shown
improved bandwidth that can lead
to several system benefits.
of InP IQ modulators
New and improved technologies
often bring different requirements to
TABLE: System benefits vs. key parameters of InP IQ modulators
Key parameters InP IQ modulator System benefits
Drive voltage, Vπ
1. 5 V Lower power dissipation, lower driver cost, improved linearity
37 mm Higher port density, smaller module size
Extinction ratio 25 dB Improved OSNR performance
33 GHz Enables higher order modulation formats for 400G and 1T
Note: Typical values shown here are for a packaged modulator.