[show abstract][hide abstract] ABSTRACT: Quasi-phase-matched (QPM) wavelength conversions based on the second-order nonlinear interaction, such as second-harmonic generation (SHG), difference-frequency generation (DFG) and sum-frequency generation (SFG), in periodically poled lithium niobate (PPLN)) have attracted much attention due to their excellent conversion properties such as broad bandwidth, high efficiency, and low noise. To achieve an efficient conversion, high-power pump light is always desired. However, PPLN crystals (either bulk or waveguide) are somehow vulnerable to high-power irradiating light, especially at a short wavelength due to the photorefractive effect (PRE), known as a refractive index change induced by an intense light illumination. The PRE may deteriorate device performance significantly. To suppress the PRE, PPLN crystals usually have to be operated at high temperatures (over one hundred degrees Celsius) or to be specially doped. Despite the significant impacts of the PRE on device performance, to date, there is limited information in the literature on the PRE mechanism in QPM PPLN. In this work, we adopt the pump-probe method and characterize the PRE in undoped and 5-mol% MgO-doped PPLN crystals. Especially, we compare the PRE in bulk and annealed proton-exchanged (APE) waveguides of PPLN. A broadband light source at 1.55 mum was used as the probe, and narrowband light at two wavelengths of 0.5 and 1.1 mum was alternatively taken as the pump. The period of crystals were selected to meet the QPM condition of SHG. It is shown that the decay property and temperature dependence of PRE, the wavelength and amplitude changes of the SHG tuning curve are distinct for the undoped and MgO-doped PPLN, as well as for the bulk and waveguide, which implies a few competing interactions in the crystals, such as the PRE, thermal-optic, photogalvanic and two-photon absorption effect, etc.
[show abstract][hide abstract] ABSTRACT: In this paper, noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasi-phase-matching (QPM) technique are studied experimentally. In the experiment, a 0.78-mum second-harmonic (SH) wave was generated when a 1.55-mum fundamental wave passed through a PPLN crystal (bulk or annealed proton-exchanged waveguide). The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. The time-domain and frequency-domain characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics were studied further when 532-nm irradiation light co-propagated with the 1.55-mum fundamental light in the PPLN crystals. It is found that for the bulk and waveguides of 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. In addition, the photorefractive effect incurred by the irradiation light can influence the SHG noise.
[show abstract][hide abstract] ABSTRACT: Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied 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 pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data.
Journal of Lightwave Technology 11/2006; 24(10):3698-3708. · 2.56 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Second harmonic generation (SHG) in a periodically poled MgO-doped lithium niobate (PPMGLN) waveguide is studied using a tunable pulsed pump source composed of a mode-locked fiber ring laser and two tunable filters. In the experiment, the lasing wavelength can be tuned from 1530 to 1579 nm, and the pulse width can be tuned from 2 to 7 picoseconds at 40 GHz. Second-harmonic pulses are generated when the picosecond pump pulses pass through the PPMGLN waveguide. SHG conversion efficiency versus pump pulse width, pump power, and pump wavelength is investigated experimentally. Propagation behaviors of both pump and SHG pulses are then numerically simulated. Based on the temporal and spectral characteristics of conversion, a quantitative analysis on SHG efficiency is presented. The simulation results are in good agreement with the experimental data.