Contents Introduction
Recent experimental demonstrations of multi-terabit transmission have been mainly based on wavelength division multiplexing (WDM) using 40-Gb/s line rates. A channel bit rate of 40 Gb/s requires fewer channels and improves spectral efficiency. High-speed transmission at 40 Gb/s or higher through long-distance fiber, however, will be limited by fiber dispersion and nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM). To overcome these problems, we developed the carrier-suppressed return-to-zero (CS-RZ) modulation format[1]. This signal has an optical phase inversion between neighbouring pulses, hence interactions between pulses can be reduced. CS-RZ pulses have been generated by using a push-pull type LiNbO3 Mach-Zehnder (LN-MZ) modulator[1]. Recently, we demonstrated anti-phase pulse generation by using a mode-locked laser [2]. Dual-mode operation of mode-locked lasers provides anti-phase pulses, which are essentially equivalent to the CS-RZ pulses. The advantages of this method are compactness and low insertion 1oss.
