General Information
Mach-Zehnder intensity modulators
Lithium niobate (LiNbO3) Mach-Zehnder intensity modulators are external modulators
that have been widely used since the mid-nineties, mostly in long haul optical networks for
their ability to modulate optical signal from 1300 nm and 1550 nm laser diodes with short
transition times and without chirp. Thanks to their broad wavelength performance, they
currently also offer modulation solutions to 1064 nm and other near infrared lasers.
Mach-Zehnder modulators are two-wave interferometric devices and exhibit a sine
transfer function (Figure 1). Although highly stable components, their operating point can
suffer slow drift due to variations of external conditions and that drift may result in variations
of key performances like extinction ratio.
In order to adjust the operating point of Mach-Zehnder modulators independently from
the high frequency modulation signal applied, they can be designed with two sets of
electrodes: one set of electrodes, the RF Electrodes, is used to apply the RF signal (PRBS
data stream, analog signal, short pulses…). The second set of electrodes, the DC Bias
Electrodes, is generally used to adjust with a controlled DC voltage the working point of the
modulator. Figure 2 illustrate the typical electrode architecture of an IQ modulator with its
internal structure. We will now give more details on the IQ modulator.
IQ modulator
An IQ modulator is basically a dual parallel Mach-Zehnder (DPMZ) modulator composed of
four waveguides. The two first waveguides are linked together with input and output Y-
junctions forming a first sub-Mach-Zehnder modulator (MZ1). The two last waveguides are
linked together with Y-junctions as well, forming a second sub-Mach-Zehnder modulator
(MZ2). Each output port of the Y-junctions is linked with a larger Y-junction. In conclusion, the
IQ modulator structure is a Mach-Zehnder modulator with one sub-Mach-Zehnder modulator
nested on each of his arms.
Electrodes are placed parallel to the waveguide arms. Two sets of high speed electrodes
allow high speed modulation of the optical signal (one set per signal quadrature). Two sets of
DC electrodes allow setting the nested modulator’s polarization. A last set of DC electrodes
allows adjusting the phase delay between the I & Q signals. In (D) QPSK / SSB / OFDM /
QAM modulation formats, each DC voltage (DC1 and DC2) of the sub-MZ's is adjusted in
order to introduce a phase shift equal to π, while the voltage applied to the phase difference
between the two sub-MZ's is adjusted to introduce a phase shift of π/2.
