Improved dispersion tolerance of an optical duobinary transmitter based on a Mach–Zehnder modulator and an optical delay interferometer
Introduction
Optical duobinary modulation is an attractive modulation scheme, because it provides a high tolerance for chromatic dispersion and narrow spectral width compared to conventional non return-to-zero (NRZ) modulation. Therefore, many ways of generating optical duobinary signals have been developed [1], [2], [3], [4], [5], [6], [7], [8], [9]. Optical duobinary signals can be formed from several different binary modulation formats such as carrier-suppressed return-to-zero (CS-RZ), alternate-mark-inversion (AMI), and differential-phase-shift keying (DPSK) [1]. In addition, optical DPSK signals generated by a dual-arm Mach–Zehnder (MZ) modulator can be converted into duobinary signals using a one-bit-delay-and-add operation in an optical interferometer [9]. This method requires no high-speed electronics to provide three-level electrical signals or electrical low-pass filters to convert two-level electrical signals into three-level electrical signals before applying these signals to the MZ modulator. Because this method does not use three-level electrical signals, it avoids the dependence on word lengths due to the imperfection of electrical low-pass filters and drivers. Although this is a simple method of generating duobinary signals, the signals produced do not provide high tolerance for chromatic dispersion. A partial bit delay correlative modulation method in an optical interferometer was proposed to improve the dispersion tolerance [10].
In this paper, we investigate the theoretical impact of the time delay of the optical interferometer and the driving voltage to the MZ modulator on the dispersion tolerance of the duobinary transmitter. The joint effect of these two factors is responsible for the optimized dispersion tolerance of the duobinary transmitter. Using a numerical simulation of 10-Gb/s duobinary transmission systems, we show that two parameters control the spectral width of the generated duobinary signals and the amplitude of ripples at the zero level that maintain the amplitudes of zero bits as low as possible after the accumulated chromatic dispersion [4], [11]. We present a simple way to optimize chromatic dispersion tolerance for the duobinary transmitter based on this investigation.
Section snippets
Optical duobinary transmitter based on a MZ modulator and an optical interferometer
Fig. 1 shows the schematic diagram of an optical duobinary transmitter using a dual-arm MZ modulator and an optical interferometer [9]. A differential encoder is used to precode the NRZ data to drive the MZ modulator. This encoder, composed of an inverter, an EXOR gate, and a feedback tap with a 1-bit delay, is exactly the same as the conventional duobinary precoder. The peak-to-peak driving voltage on each arm of the MZ modulator is Vpp (⩽Vπ), and the bias voltage of one electrode of the MZ
Dispersion tolerance optimization
We compared dispersion tolerance of the duobinary transmitter by varying the delay time of the optical interferometer and the driving voltage of the MZ modulator. We neglected the attenuation and nonlinear effect of the optical fiber to focus on dispersion tolerance.
Firstly, we adjusted the delay time of the optical interferometer, while keeping the driving voltage of the MZ modulator fixed at Vπ. Fig. 3 shows receiver sensitivities at a BER of 10−9 as functions of the accumulated dispersion
Conclusion
We demonstrated a method of improving the dispersion tolerance of an optical duobinary transmitter based on a dual-arm MZ modulator and an optical delay interferometer. We showed that the driving voltages of the MZ modulator and the bit delay time of the optical delay interferometer control the spectrum of the optical duobinary signals and the amplitude of the ripples at the zero levels. We can determine the best dispersion tolerance of the optical duobinary transmitter by optimizing these two
Acknowledgments
This work was supported in part by a Korea University Grant, the Second Brain Korea 21 project, and WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-2008-000-10008-0).
References (12)
- et al.
Opt. Commun.
(2003) - et al.
Opt. Commun.
(2005) - et al.
Opt. Commun.
(2000) - et al.
Opt. Express
(2008) - et al.
IEEE Photon. Technol. Lett.
(1997) J. Opt. Commun.
(2002)
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Investigation on transmission characteristics of 40 GB/s optical networks based on duobinary modulation
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