A novel wavelength converter using LiNbO<sub>3</sub> waveguide ring resonator is proposed and simulated.
In the design, a new wavelength is generated by combining the second harmonic generation (SHG) and the
difference frequency generation (DFG). The quasi-phase-matched condition is achieved by periodically
domain inverted lithium niobate crystal. A waveguide ring and four 1x2 beam splitters/combiners are used
to link the SHG and SFG structures. The 1x2 beam splitter/combiners are carefully designed so that only
the SHG signal at λ=0.77μm can stay inside the ring while the pumping signal at λ=1.54μm, idler signal at
λ=1.55μm, and the converted signal at λ=1.53μm can coupled into and out of the ring. Therefore, the SHG
signal, served as intermediate pumping source for the DFG, keeps circulating inside the ring and form
resonance to gain higher power and to achieve higher conversion rate. The detail design is described and
the design concept is proved by the simulation results.
Quasiphase matched (QPM) wavelength converters have attracted much attention due to their versatile applications, such as unltrashort-pulse generation at short wavelength. The behavior of pulse propagation in QPM waveguides has been an interesting research topic around the world. In this report, numerical study of second harmonic generation (SHG) in QPM waveguides is, for the first time, performed systematically for the fundamental pump pulses from nanosecond to femtosecond. In the proposed theoretical approach, all the dispersion effects are considered. Furthermore, our simulations take into account not only the SHG effect but also the sum frequency generation (SFG) effect on the nonlinear interaction when a pulsed pump light is used. Group-velocity mismatch (GVM) and phase-velocity mismatch are also considered in the study. Therefore, the proposed simulation model is suitable for analyzing the SHG of ultrashort-pulse since the spectral full width at half maximum (FWHM) increases with the decrease of pump pulse width. It is shown that conversion efficiencies are strongly dependent on the fundamental pulse width since the walk-off effect gradually dominates with the decrease of the fundamental pulse width. Furthermore, the temporal FWHM of converted pulses is determined by the fundamental pulse width as well as the effective interaction length related to the GVM. Under the condition of large GVM, large distortion exhibits in converted pulses. The dependence of the conversion efficiency on pump energy is also studied. The results show that the conversion efficiency saturates when the pump energy increases. The simulation results provide a guideline of device design and applications of the QPM wavelength converters in ultrashort-pulse region.
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.
Cascaded second-order nonlinear interaction (&chi<sup>(2)</sup>);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 LiNbO<sub>3</sub> 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.