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Description
Duobinary data encoding is a form of correlative coding in partial response
signaling. The modulator drive signal can be produced by adding one-bit-delayed data to the present
data bit to give levels 0, 1, and 2. An identical effect can be achieved by applying a low-pass
filter to the ideal binary data signal. Optical duobinary modulation is achieved by 100% overdriving
a Mach-Zehnder modulator with the Duobinary encoded electrical signal. In this case, level 0 and 2
produce 100% transmission with opposite optical phases, and level 1 produces 0% transmission. The
correlated three-level signal can be demodulated into a binary signal again using an optical direct
detection receiver.
The advantage of this correlative electrical data encoding is that the duobinary modulated optical
signal has a narrower bandwidth compared to the binary NRZ modulated signal. As a consequence, the
effect of fiber dispersion is reduced and ultradense WDM systems applications are feasible.
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Typical Results
The simulation setup is displayed in
.
It demonstrates that a 10-Gb/s duobinary modulated signal can be transmitted over more than 200 km of
standard SMF without dispersion compensation. The system performance is evaluated every 40 km and
compared to the results obtained from NRZ modulation.
shows the optical spectra after the transmitter for an OSNR of 20 dB and the eye diagrams of the
received electrical signals for the back-to-back case. The main lobe of the duobinary optical spectrum
is narrower than that of the NRZ signal, and side lobes are drastically suppressed. The eye diagrams
show, however, that duobinary encoding results in a back-to-back penalty compared to NRZ encoding.
shows the received eye diagrams after 120 km and 240 km. The initial penalty of the duobinary encoding
is recovered after 120 km. Even after propagating over 240 km SSMF without CD compensation, data can be
detected successfully, while the NRZ encoded signal is completely dispersed.
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Further Information
Keywords: Optical Duobinary Modulation, Chromatic Dispersion, Mach-Zehnder Modulator (MZM)
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
[1] A. J. Lowery, J. Armstrong, OPTICS EXPRESS, Vol. 14, No. 6 (2006).
