The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.

Noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasiphase matching (QPM) technique are analyzed experimentally. In the experiment, a0.78 μm second-harmonic (SH) wave was generated when a 1.56 μm fundamental wave passed through a PPLN crystal (bulk or waveguide). The time-domain and frequency-domain noise characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics of SHG were further analyzed when a visible light (532 nm) and an infrared light (1090 nm) copropagated with the fundamental light, respectively. The noise characterizations were also investigated at different temperatures. It is found that for the bulk and waveguide PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped PPLN. The 532 nm irradiation can lead to higher noise in PPLN than the 1090 nm irradiation. In addition, increasing temperature of device can alleviate the problem of noise in conjunction with the photorefractive effect incurred by the irradiation light. This is more significant in undoped PPLN than in MgO-doped one.

No abstract

No abstract

The implementation of wavelength conversion around 1.55 µm in quasi-phase-matched (QPM) periodically poled LiNbO 3 (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.