Qibing Sun

Ultrabroadband flat dispersion tailoring of dual-slot silicon waveguides

We propose a new strip/slot hybrid waveguide with double slots, which exhibits a flat and low dispersion over a 1098-nm bandwidth with four zero-dispersion wavelengths. Dispersion of dual-slot silicon waveguide is mainly determined by mode transition from a strip mode to a slot mode rather than by material dispersion. Dispersion tailoring is investigated by tuning different structural parameters of waveguides. Moreover, nonlinear coefficient of dual-slot silicon waveguide and phase-matching condition in FWM are both explored in detail. The dual-slot waveguide can be used to generate supercontinuum with bandwidth extending up to 1630 nm pumped by femtosecond pulses. This waveguide will have a great potential for ultrabroadband signal processing applications from near-infrared region to mid-infrared region.

Efficient terahertz-wave generation via four-wave mixing in silicon membrane waveguides

Terahertz (THz) wave generation via four-wave mixing (FWM) in silicon membrane waveguides is theoretically investigated with mid-infrared laser pulses. Compared with the conventional parametric amplification or wavelength conversion based on FWM in silicon waveguides, which needs a pump wavelength located in the anomalous group-velocity dispersion (GVD) regime to realize broad phase matching, the pump wavelength located in the normal GVD regime is required to realize collinear phase matching for the THz-wave generation via FWM. The pump wavelength and rib height of the silicon membrane waveguide can be tuned to obtain a broadband phase matching. Moreover, the conversion efficiency of the THz-wave generation is studied with different pump wavelengths and rib heights of the silicon membrane waveguides, and broadband THz-wave can be obtained with high efficiency exceeding 1%.

Impact of dispersion profiles of silicon waveguides on optical parametric amplification in the femtosecond regime

The impact of dispersion profiles of silicon waveguides on femtosecond optical parametric amplification (OPA) is theoretically investigated. It is found that flat quasi-phase-matching, smooth temporal profiles and separable spectra for 200 fs pulses can be obtained by tailoring the cross-section of silicon rib waveguide. We achieve on-chip parametric gain as high as 26.8 dB and idler conversion gain of 25.6 dB for a low pump peak power over a flat bandwidth of 400 nm in a 10-mm-long dispersion engineered silicon waveguide. Our on-chip OPA can find important potential applications in highly integrated optical circuits for all-optical ultrafast signal processing.

Wavelength conversion in highly nonlinear silicon–organic hybrid slot waveguides

Wavelength conversion based on four-wave mixing (FWM) in a silicon-organic hybrid slot waveguide is theoretically investigated in the telecommunication bands. Compared with vertical slot waveguides, the horizontal slot waveguide structure exhibits much flatter dispersion. The maximum nonlinearity coefficient γ of 1.5×10⁷ W⁻¹ km⁻¹ and the minimum effective mode area A(eff) of 0.065  μm² are obtained in a horizontal slot waveguide with a 20-nm-thick optically nonlinear layer by controlling the geometric parameters. The wavelength conversion efficiency of 7.45 dB with a pump power of 100 mW in a 4-mm-long horizontal slot waveguide is obtained. This low power on-chip wavelength convertor will have potential applications in highly integrated optical circuits.

Widely tunable femtosecond optical parametric oscillator based on silicon-on-insulator waveguides

A femtosecond optical parametric oscillator (OPO) based on silicon-on-insulator (SOI) waveguide is proposed and analyzed numerically. By utilizing split-step Fourier method (SSFM), it is demonstrated that ultra-wide tunable wavelength femtosecond pulse can be realized under the phase matching condition. Due to the interaction between nonlinearity and flexible dispersion design, the output signal wavelength can be tuned from 1645 to 1805 nm and the idler wavelength can be tuned from 1350 to 1456 nm. Moreover, the peak power of the output signal pulse exceeds 10 W from 1700 to 1770 nm with the pump peak power 50 W. The proposed OPO exhibits compact configuration and can find important applications in integrated broadband optical source.

Influence of spectral broadening on femtosecond wavelength conversion based on four-wave mixing in silicon waveguides

Femtosecond wavelength conversion in the telecommunication bands via four-wave mixing in a 1.5 mm long silicon rib waveguide is theoretically investigated. Compared with picosecond pulses, the spectra are greatly broadened for the femtosecond pulses due to self-phase modulation and cross-phase modulation in the four-wave mixing process, and it is difficult to achieve a wavelength converter when the pump and signal pulse widths are close to or less than 100 fs in the telecommunication bands because of the spectral overlap. The influence of the spectral broadening on the conversion efficiency is also investigated. The conversion bandwidth of 220 nm and peak conversion efficiency of -8 dB are demonstrated by using 500 fs pulses with higher efficiency than the picosecond pulse-pumped efficiency when the repetition rate is 100 GHz.

A Tunable Terahertz Photonic Crystal Narrow-Band Filter

We theoretically propose and investigate a magnetically tunable narrow-band terahertz filter based on a triangular lattice silicon photonic crystal with a point and two line defects. The optical properties of the filter have been analyzed in detail. It is found that a single resonant peak with the central frequency of ~1 THz is existed in the transmission spectrum, which has a narrow full width at half maximum of <;2 GHz. Moreover, under the control of an external magnetic field, transmission frequency and width of passband are adjustable, which reveals that the 2-D silicon photonic crystal waveguide with point and line defects can serve as a continuously tunable bandpass filter at the terahertz waveband.

Influence of three-photon absorption on Mid-infrared cross-phase modulation in silicon-on-sapphire waveguides

The influence of three-photon absorption (3PA) on cross-phase modulation (XPM) effect in the mid-infrared (IR) region is theoretically investigated in silicon-on-sapphire (SOS) waveguides. It is found that the 3PA-induced nonlinear losses in the SOS waveguide will be considerable for the pulse propagation in the wavelength region of 2300 nm-3300 nm when the pump peak intensity is high enough. For the XPM process, the 3PA and 3PA-induced free-carrier effects can affect the spectrum and temporal profiles of the pump and signal pulses for sufficiently high pump peak intensities. Moreover, the XPM-induced frequency shift of signal spectrum is also discussed with different pump peak intensities, and the XPM-induced blue and red shifts are reduced due to 3PA.

Influences of ASE on the performances of Q-switched ytterbium-doped fiber lasers

The output characteristics of actively Q-switched ytterbium-doped fiber lasers (YDFL) are investigated in this paper. Our experimental results show that, the combined effect between the short switching time and the gain transient property of doped fiber causes the initial amplified spontaneous emission (ASE) power fluctuation, forming the multi-peak structure in the output pulse for either ring or linear cavity Q-switched YDFL. The pulse buildup time decreases with the rising of the pump. Moreover, since the broad-band ASE generated in the YDF is very high, it may saturate the doped fiber once the switch has been opened, making it difficult to achieve Q-switched laser oscillation for both fiber lasers. By using ASE filter to suppress the initial ASE, the gain supplied by the YDF can be greatly enhanced, which can not only decrease the threshold, but also greatly decrease the duration, and enhance the peak power of the Q-switched laser pulse as well. By using such an ASE filter, the Q-switched laser pulse with the peak power of 40.7 W and the duration of 30 ns has been achieved for the linear cavity YDFL pumped with 160 mW.

Reconstruction of pulse noisy images via stochastic resonance

We investigate a practical technology for reconstructing nanosecond pulse noisy images via stochastic resonance, which is based on the modulation instability. A theoretical model of this method for optical pulse signal is built to effectively recover the pulse image. The nanosecond noise-hidden images grow at the expense of noise during the stochastic resonance process in a photorefractive medium. The properties of output images are mainly determined by the input signal-to-noise intensity ratio, the applied voltage across the medium, and the correlation length of noise background. A high cross-correlation gain is obtained by optimizing these parameters. This provides a potential method for detecting low-level or hidden pulse images in various imaging applications.