FEATURE Optical IQ modulators
electrode design, where broadband
matching of the RF and optical wave
group velocities can be achieved.
Figure 2 illustrates the basic
device concept for a dual-polarization
traveling-wave IQ modulator.
Recent advances have produced
commercially available InP IQ
modulators with low drive voltage
and high bandwidth.
4 The devices
are inherently small in size and
ideally suited for integration with
other InP-based devices such as
tunable lasers and high speed receivers. This size advantage will be
critical to enable compact coherent
optics modules like CFP and CFP2.
The key modulator parameters for
next gen coherent systems are the
drive voltage required to induce
a π phase shift (Vπ), linearity, ER,
and modulation bandwidth.
The drive voltage directly
affects the power consumption
of the module or line card being
integrated into the coherent system.
Modulators with large drive
proven reliability while keeping
the insertion loss to an acceptable
value. Polymer- or semiconductor-
based modulator technologies
might offer such small size and low
drive voltage. But while research
on polymer modulators has shown
promising results1, the stability
of the polymer material over the
system’s life is an important concern
that limits broad deployment.
Meanwhile, recent interest in silicon
photonics has led to many silicon-based modulator developments.
However, ER and insertion loss
could be limiting factors for
long haul systems. Although an
optical amplifier can be used to
overcome such insertion loss, the
increased power consumption and
added noise are undesirable.
InP has paved the way for major
advances in high speed optical-
fiber communications. The ability
to epitaxially tailor the material
properties in III-V semiconductors
has benefited tunable lasers
and high speed receivers while
maintaining the proven reliability of
InP devices. Wafer-scale fabrication
with precise process controls
combined with low cost packaging
has dramatically reduced the cost of
components, enabling a lower cost
per transmitted bit. These benefits
make InP material an attractive
candidate to create a modulator
for next gen coherent systems.
A high speed MZ modulator
that’s small in size and with a low
drive voltage requires a material
with a large phase shift per unit
length. Ternary and quaternary
alloy materials grown epitaxially
on InP can be bandgap engineered
to alter the characteristics of the
material to suit a particular device
application. Using Quantum Confined
Stark Effect (QCSE) in an InGa AsP
alloy multiple quantum well (MQW)
structure lattice matched to InP
can create a substantial phase
shift per unit length.
modulators with a high bandwidth can
be achieved with a traveling-wave