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Journal of the Optical Society of America B

Intracavity wavelength conversions employing a MgO-doped LiNbO_3 quasi-phase-matched waveguide and an erbium-doped fiber amplifier

Chang‐Qing Xu, John Bracken, Bo Chen

Published Oct 1, 2003• 13 citations
Publisher

Abstract

Intracavity sum-frequency generation and cascaded χ(2) wavelength conversions in a fiber ring resonator using a MgO-doped LiNbO3 quasi-phase-matched waveguide and an erbium-doped fiber amplifier were proposed and demonstrated. In the proposed configuration, the resonator enhances the pump light, and efficient wavelength conversion is realized. Dependence of converted signal power upon the position of a coupler used to couple a signal into the resonator and upon the round-trip loss of the resonator was studied in detail. The proposed configuration provides efficient wavelength conversion over a wide range of input signal power and resonator round-trip loss. The configuration provides a cost-effective solution to enhance pump power, thus increasing wavelength-conversion efficiency for practical system applications.

DOI
10.1364/josab.20.002142

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This thesis describes the implementation of the all-optical wavelength converters based on quasi-phase-matched (QPM) lithium niobate (LiNbO3) crystals and some related optical nonlinear materials for optical fiber communication systems and other applications. Such devices would be the enabling technology to realize high-capacity and transparent communication networks required in wavelength-division-multiplexing systems and highly efficient laser sources at desired wavelengths. The wavelength converters discussed in this thesis are based on the three-wave mixing processes attributed to the second-order nonlinear interactions (x2,). The main challenge with a LiNbO3 QPM wavelength converter is to implement a highly efficient device, which is capable of operating at a power of hundreds of mW. For this purpose, several wavelength conversion schemes have been demonstrated and theoretically investigated, including the fiber ring resonators and a novel cascaded χ(2).∖ The former based on the conventional cascaded x(2) contains erbium-doped fiber amplifiers and other passive components exclusively as pump sources (no additional pump source is required) with excellent characteristics such as high black-box efficiency. While the encouraging properties of the latter are those pump wavelengths can be set outside the communication window, and same efficiency can be achieved by employing two pump sources with lower output power as compared with the conventional cascaded one. By taking into account the higher-order dispersion effects, pulse propagation in QPM second harmonic generation waveguides is also analyzed. In addition, the photorefractive effect (PRE) in QPM waveguides is systematically studied by using a pump-probe method, since the LiNbO3 crystals are extremely vulnerable to high-power shorter-wavelength light illumination. Both temperature and wavelength dependent results are helpful in further understanding the physical insight of the PRE, and providing a guideline for practical application of the QPM LiNbO3 waveguides.
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.
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.
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.
Different frequency generation (DFG) and the cascaded second-order nonlinear interaction () based wavelength conversions in 5-mol MgO doped LiNbO3 QPM waveguides are studied and compared both experimentally and theoretically for the first time. It is shown that DFG based wavelength conversions have a higher conversion efficiency; while the cascaded based wavelength conversions have a wider 3-dB signal conversion bandwidth. It is also exhibited that the simulations are consistent well with the experimental results. Tolerance of pump wavelength for the cascaded based wavelength conversion is twice as large as that of DFG based devices. These results are very helpful in choosing suitable wavelength converters for practical optical communication systems according to the system requirements.
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.