Abstract
Generation of optical millimeter-wave (mm-wave) signal using
a Mach–Zehnder modulator (MZM) based on double-sideband (DSB), single-sideband
(SSB), and double-sideband with carrier suppression (DSBCS) modulation schemes
have been demonstrated for various applications, such as broadband wireless
signals or optical up-conversion for wavelength-division-multiplexing (WDM)
radio-over-fiber (RoF) network, wideband surveillance, spread spectrum, and
software-defined radio. Among these schemes, DSBCS modulation offers the best
receiver sensitivity, lowest spectral occupancy, the least stringent requirement
of electrical bandwidth, and the smallest receiving power penalty after long
transmission distance. Nonetheless, the inherent nonlinear E/O (electrical/optical)
conversion response of a MZM is such that the signal quality of the optical
mm-wave suffers. Fabrication tolerances make a balanced 50/50 splitting ratio
of the MZM's y-splitter particularly difficult to achieve. As a result, imbalanced
MZMs have a finite extinction ratio (ER) and degrade the optical carrier suppression
ratio (OCSR) using DSBCS modulation. In this paper, the effect of the MZM
nonlinearity and imbalanced y-splitter on optical mm-wave generation by DSBCS
modulation is theoretically and experimentally investigated. A novel approach
with better performance and greater cost-effectiveness than dual-electrode
MZM (DD-MZM) is presented to realize a DSBCS modulation scheme based on a
single-electrode MZM (SD-MZM).
© 2008 IEEE
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