160-Gb/s all-optical OTDM demultiplexing and pulse reshaping by using cascaded wavelength conversion in PPLN waveguides

Abstract

Ultra-fast all-optical demultiplexer is required in future high-speed optical time-division-multiplexed (OTDM) transmission systems [1]. As an important nonlinear interaction, the cascaded second-harmonic generation (SHG) and difference frequency generation (DFG) wavelength conversion in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) waveguide has many advantages, such as ultra-fast response, low noise, high efficiency, broad band width, high dynamic range, and integration compatibility [2][3][4]. Recently, by using the SHG-DFG-based wavelength conversion technique, all-optical demultiplexing from 40 Gb/s to 10 Gb/s [5], from 100 Gb/s to 10 Gb/s [6] and from 160 Gb/s to 20 Gb/s [7] has been experimentally demonstrated, and some numerical analyses have also been reported [8,9]. In the OTDM demultiplexing based on the SHG-DFG-based wavelength conversion, two input pulse trains, i.e., multiplexed signal and demultiplexing clock, are injected into a QPM PPLN waveguide, and taken as the pump and control waves. In a typical demultiplexing case shown in Fig. 1, the converted wave from the PPLN waveguide has the same bit rate as the clock (10 Gb/s), and carries the code information demultiplexed from the required channel of the 160-Gb/s signal. As depicted in Fig. 2, there are two schemes to arrange 160-and 10-Gb/s pulses with respect to their wavelengths and the device QPM wavelength. In Scheme I, the 10-Gb/s clock is set at the QPM wavelength (pump wave), and the 160-Gb/s signal is regarded as the control wave. Vice versa in Scheme II. The demultiplexed signal is hence different in the two schemes. In this work, typical OTDM demultiplexing from 160 to 10 Gb/s in the two schemes are theoretically analyzed. In particular, the characteristics of conversion efficiency, pulse reshaping and time delay of the demultiplexed pulses are emphasized. Fig. 1. Schematic of 160-to 10-Gb/s OTDM Fig. 2. Arrangement of pump and control waves in two schemes. demultiplexing in a QPM PPLN waveguide. λ p , λ s and λ c refer to the pump, control and converted waves.The device used in the simulation has similar properties to those used in the previous experiments [5,10]. The poling period is ~18.5 μm, and the exact QPM wavelength of pump is 1543.0 nm. The waveguide length of 20 mm, and its normalized SHG efficiency is ~190 %/W at 100-mW pump power. Based on the derived coupled-mode equations [10], the pulse propagation and nonlinear interactions among the pump, SH, signal and converted waves in the PPLN waveguide were obtained numerically. Here, we took 15-dBm injected average power and 30-dB signal-to-nose ratio for the both 160-and 10-Gb/s input pulses (2.0-ps Gaussian pulses).The 10-Gb/s converted pulses and the output optical spectra in the two schemes are compared in Fig. 3, and the input 160-Gb/s signal and the 10-Gb/s clock, as well as the SH wave are also given. All pulses are plotted in a reference time frame moving with the 160-Gb/s pulses. In Scheme I, the clock pulse is well synchronized to the signal pulse. I...