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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Comparison of wavelength conversions based on cascaded second-harmonic generation/difference-frequency generation under continuous-wave and pulsed pumping

Yong Wang, Jorge Fonseca-Campos, Chang-Qing Xu, Shiquan Yang, Evgueni A. Ponomarev, Xiaoyi Bao

Published Sep 11, 2006• 0 citations
Publisher

Abstract

The implementation of wavelength conversion around 1.55 μm in quasi-phase-matched (QPM) periodically poled LiNbO3 (PPLN) waveguides is mostly based on the cascaded second-harmonic generation/difference-frequency generation (SHG/DFG) process. As usual, a continuous wave (CW) and a pulsed wave are injected into a PPLN waveguide. Of them, the pulsed wave is regarded as the information carrier, and the CW is taken as the control. To transfer the information of optical codes from one wavelength to another, the codes can be applied to either the signal (CW-pumped scheme) or the pump (pulse-pumped scheme). In this work, the temporal and spectral properties of wavelength conversions during pulse propagation as well as the conversion efficiency in the two pumping schemes were compared experimentally and theoretically under different conditions of input pulse width, pump power and pump central wavelength. In the experiments, we adopted an MgO-doped PPLN waveguide, and a 40-GHz tunable picosecond-pulse source. The conversion characteristics were systematically investigated when the CW and the pulsed wave were alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In the theory, we solved the coupled-mode equations and explained the physical insights for the numerical results and experimental observations. The conversion properties of the two pumping schemes were quantitatively compared. The simulated results agree well with the experimental data, and the obtained results provide some guidelines for the design and application of QPM waveguides in wavelength conversion.

DOI
10.1117/12.707762

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The implementation of wavelength conversion around 1.55 μm in quasi-phase-matched (QPM) periodically poled LiNbO3 (PPLN) waveguides is mostly based on the cascaded second-harmonic generation/difference-frequency generation (SHG/DFG) process. As usual, a continuous wave (CW) and a pulsed wave are injected into a PPLN waveguide. Of them, the pulsed wave is regarded as the information carrier, and the CW is taken as the control. To transfer the information of optical codes from one wavelength to another, the codes can be applied to either the signal (CW-pumped scheme) or the pump (pulse-pumped scheme). In this work, the temporal and spectral properties of wavelength conversions during pulse propagation as well as the conversion efficiency in the two pumping schemes were compared experimentally and theoretically under different conditions of input pulse width, pump power and pump central wavelength. In the experiments, we adopted an MgO-doped PPLN waveguide, and a 40-GHz tunable picosecond-pulse source. The conversion characteristics were systematically investigated when the CW and the pulsed wave were alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In the theory, we solved the coupled-mode equations and explained the physical insights for the numerical results and experimental observations. The conversion properties of the two pumping schemes were quantitatively compared. The simulated results agree well with the experimental data, and the obtained results provide some guidelines for the design and application of QPM waveguides in wavelength conversion.
Wavelength conversion based on the cascaded sum-frequency generation and difference-frequency generation (cSFG+DFG) process in quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) waveguides has the advantage of no pump occupation in the communication band, compared to the traditional cascaded second-harmonic generation (SHG) and DFG process. In cSFG+DFG-based wavelength conversion, two pump waves and a signal wave are required. The input pulse train, as a carrier of information, can be applied to either the signal or one of the two pump waves. Though the converted wave is in the form of pulses, its temporal and spectral characteristics are dependent on the arrangement of the pulsed wave. In this work, we target the wavelength conversion of 40-GHz picosecond pulse trains, and systematically analyze the temporal and spectral characteristics of the pump, signal, converted, and sum-frequency waves during their propagations in PPLN waveguides by using numerical simulation. In particular, pulse shapes, optical spectra, phases and conversion efficiency are compared when the picosecond pulse train is used as the signal and pump waves, respectively.
Cascaded second-order nonlinear interaction (&chi(2));based wavelength conversion technique has attracted much attention due to its unique characteristics such as low noise and broadband, which are critical in fiber communication networks. In this report, wavelength conversions based on the newly proposed SFG-DFG (sum frequency generation - difference frequency generation) and conventional SHG-DFG (second harmonic generation - difference frequency generation) are studied and compared both experimentally and theoretically in a LiNbO3 quasi-phase matched (QPM) waveguide. It is shown that the same conversion efficiency can be achieved by employing two pump sources with only half power each (P1, P2) in the SFG-DFG scheme as compared with the SHG-DFG scheme with a single higher power pump beam (P=P1+P2). It is shown that the cascaded SFG-DFG based wavelength conversion has a larger 3-dB pump tolerance bandwidth. The theoretical results are consistent well with the experimental ones. It is found that the pump wavelength difference can be separated by a span as large as 75 nm, while 3-dB signal conversion efficiency is retained in a 45 mm-long device. It is also exhibited that tolerance of temperature for the cascaded SFG-DFG remains the same as that of the cascaded SHG-DFG based devices. The results show that the SFG-DFG wavelength conversion scheme is very attractive for practical applications.
Based on cascaded second-harmonic generation and difference frequency generation (SHG+DFG) in periodically poled LiNbO3 (PPLN) waveguides,tunable wavelength conversion as well as single-to-dual channel wavelength conversion are proposed and experimentally investigated.A picosecond pulsed signal with 40 GHz repetition rate and 1.57 ps pulse width is adopted.The continues wave (CW) pump is provided by use of a fiber ring laser (FRL).Different from the conventional SHG+DFG-based conversion schemes,here the center wavelength of the signal is set at the quasi-phase matching (QPM) wavelength for the SHG process.By adjusting the pump wavelength,a tunable converted output idler wave is achieved.Moreover,single-to-dual channel wavelength conversion is also successfully observed by employing two pumps.
Difference-frequency generation (DFG) and cascaded second-order nonlinear interaction (χ(2)) based wavelength conversions in LiNbO3 quasi-phase-matched waveguides were studied systematically. The characteristics of the two conversion methods, such as conversion efficiency, conversion bandwidth, pump-wavelength tolerance, and temperature stability, were compared with both experimental and theoretical results of MgO-doped quasi-phase-matched waveguides. It was found that for shorter device length and lower pump power, DFG based wavelength converters had higher conversion efficiency, and the cascaded χ(2) based wavelength conversions had slightly wider 3-dB signal-conversion bandwidth. The 3-dB signal wavelength-conversion bandwidth decreased exponentially with an increase in device length. For long devices and high pump power, however, the efficiency difference between the cascaded χ(2) based conversions and DFG based ones was minor. Tolerance of pump wavelength for the cascaded χ(2) based wavelength conversions was approximately two times that of DFG-based ones, which decreases with the increase of device length. In both cases, the tolerance of pump wavelength exponentially decreased with an increase of device length. It was found that the temperature stability of the cascaded χ(2) based conversions was identical to that of DFG based conversions. These results are very helpful in choosing suitable wavelength converters for practical optical communication systems.
The principle of all-optical wavelength conversion based on the cascaded sum and difference frequency generation (SFG+DFG) in quasi-phase-matched (QPM) periodically poled LiNbO3 (PPLN) waveguides is introduced. The conversion efficiency of the wavelength converter is calculated. The influences of the powers and span of two pumps on the conversion efficiency are analyzed. Bandwidth of one pump wavelength is discussed. The tuning is about 0.5 nm. The signal pulse width is compressed about 0.68 times.
A novel wavelength conversion scheme based on cascaded sum-frequency generation (SFG) and difference-frequency generation (DFG) is proposed and demonstrated in a MgO-doped LiNbO3 quasi-phase-matched waveguide. In this scheme, two pump wavelengths are set outside the communication band. It is shown that the same conversion efficiency can be achieved by use of two pump sources with lower output power (P1, P2) in this scheme compared with the conventional cascaded wavelength conversion technique based on second-harmonic generation and DFG with a single higher-power pump beam (P = P1 + P2) that is due to the use of a larger SFG nonlinear coefficient. The results significantly influence the selection of a suitable nonlinear interaction scheme for practical applications.