Modulation Formats

Back to
Applications
Overview

Dispersion-Tolerant Optical Transmission Using Duobinary Modulation

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 consequence, the effect of fiber dispersion is reduced and ultra-dense WDM systems applications are feasible.

Typical Results

The simulation setup is displayed in Figure 1. It demonstrates that a 10-Gb/s Duobinary modulated signal can be transmitted over more than 200km of standard SMF without dispersion compensation. The system performance is evaluated every 40km, and compared to results from NRZ modulation. Figure 2 shows the optical spectra after the transmitter for an OSNR of 20dB, 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. Figure 3 shows the received eye diagrams after 120km and 240km. The initial penalty of the Duobinary encoding is recovered after 120km. Even after propagation over 240km SSMF without CD compensation data can be detected successfully, while the NRZ encoded signal is completely dispersed.

Keywords

Optical Duobinary Modulation, Chromatic Dispersion, Mach-Zehnder Modulator (MZM)