Midinfrared (MIR) wavelength conversion based on degenerate four-wave mixing is theoretically investigated in hydrogenated amorphous silicon (a-Si:H) waveguides. The broadband phase mismatch is achieved in the normal group-velocity dispersion regime. The conversion bandwidth is extended to 900 nm, and conversion efficiency of up to −14 dB with a pump power of 70 mW in a 2-mm long a-Si:H rib waveguides is obtained. This low-power on-chip wavelength converter will have potential for application in a wide range of MIR nonlinear optic devices.
Terahertz (THz) wave generation via four-wave mixing (FWM) in silicon membrane waveguides is investigated with mid-infrared pump. The silicon membrane waveguides with width of 12 μm and heights varied from 14 μm to 17 μm, which can confine the THzwave ranging from 7.5 THz to 10 THz due to the large refractive index contrast of the waveguide core and cladding, are designed to realize the collinear phase matching for THz-wave generation via FWM. Compared with the conventional parametric amplification or wavelength conversion based on FWM in silicon waveguides, which needs a pump wavelength located in the anomalous groupvelocity dispersion (GVD) regime to realize broad phase matching, the pump wavelength located in the normal GVD regime is required to realize phase matching because of the large signal-pump frequency detuning. Phase matching for a tunable THz-wave ranging from 8.57 THz to 10 THz can be realized by tuning the pump wavelength from 4.2 μm to 4.4 μm in the silicon waveguide with rib height of 15 μm. Whilst, the phase matching bandwidth of THz-wave ranging from 7.7 THz to 10 THz can be achieved by tailoring the waveguide height from 14 μm to 17 μm when the pump wavelength is 4.3μm. Moreover, the conversion efficiency of the THz-wave generation is studied with different pump wavelengths and waveguide heights, the maximum conversion efficiency of 1.25 % at 9.2 THz can be obtained in a 6-mm long silicon waveguide when the pump wavelength is 4.3 μm and the waveguide height is 15 μm.
A terahertz (THz)-wave parametric oscillator (TPO) pumped by a stable and single-longitudinal mode Q-switched Nd:YAG laser under various room temperatures is demonstrated. It is based on a cavity TPO architecture allowing stable single-resonance operation and low oscillation threshold. The output results, including the effects of the room temperature on this wavelength-agile TPO with a MgO:LiNbO3 crystal, indicate that the performance of TPO under the lower temperature is better. We obtain a widely tunable THz-wave source in the range 104 to 226 μm via tuning the cavity flexibly under different room temperatures. The peak power of the THz wave reaches 220 mW at the wavelength of 146.2 μm when the room temperature is 20°C. The peak power of the THz wave decreases to 48 mW when the room temperature rises from 20°C to 25°C.
We present a new method to generate broadband cascaded four-wave mixing (FWM) products in the silicon-on-insulator (SOI) waveguide. A simulation model of the nonlinear Schrödinger equation is used to describe the cascaded FWM in the SOI waveguide, which consists of launching two strong pump waves near the zero-dispersion wavelength of the very short (just a few millimeters) SOI waveguide. The numerical results based on the split step Fourier method have demonstrated that the output cascaded FWM products represent bandwidth of more than 1000 nm (range from C-band to IR-band). We also analyze the remarkable influences of parameters of the waveguide length, the pump power, and the dispersion slope on the cascaded FWM products in the SOI waveguide.
The modulation instability and the phase-matching condition around the zero dispersion wavelength of the high nonlinear photonic crystal fiber (PCF) is numerically analyzed and simulated. The ultrawide tunable range can be obtained from the fiber optical parametric oscillator and the ultrashort pulse can be generated through choosing the parameters of dispersion coefficient, nonlinear coefficient, and pump power appropriately. The numerical simulation results show that a tunable range as wide as 318 nm has been obtained from the femtosecond PCF optical parametric oscillator, around 1.5 μm.
A fiber tunable high-power picosecond laser system has been demonstrated. A gain switch semiconductor laser diode was introduced as seed source, and a multi-stage single mode Yb-doped fiber amplifier was combined with a single mode double-clad Yb-doped fiber amplifier and a PCF amplifier to construct the amplification system. High stability and good beam quality pulses with 1MHz tunable repetition rate, 6.8W average power, 90ps pulse duration, and central wavelength tunable from 1053 nm to 1073 nm were generated. The completely fiber-integrated approach has the potential to be scaled to significantly higher average powers.
In a photoconductive semiconductor switch (PCSS) ultrawideband (UWB) array system, optical fiber bundles with several hundreds or more fibers are employed to distribute laser beam energy and yield multiple laser beams with the same energy and coherence. Ideally, if each fiber of the bundle is the same length, the laser beams transmit in the fiber and may arrive to the PCSS simultaneously in the UWB array system. In this work, the coupling rate of the fiber bundle is estimated. The experimental result of dividing laser energy shows that multiple lasers can be obtained by using a fiber bundle. Specifically, the experiments on the 17-PCSS antenna array radiation indicate that the pulses generated by PCSS give up only several picoseconds of time jitters, thus demonstrating that laser distribution by fiber bundle can be used for UWB coherent combination.
The problem of phase-mismatch-compensation for ultra-broadband OPCPA is first studied theoretically by us, and a scheme for phase-mismatch-compensation is proposed, which is based on the matching of both the group velocity and pulse front between signal and idler by the combination of the no collinear-phase-match and pulse-front-tilt that is accomplished by angular dispersion of the interacting rays. By this Scheme, the phase mismatch to first order in frequency shift can be completely compensated, and then an ultra-broadband OPCPA is realized. It is shown that the phase mismatch to both first and second order can be completely compensated simultaneously in some cases, and this leads to an extremely broadband spectral width. Therefore the important criterion for constructing an ultra-broadband OPCPA that both pulse-front and group-velocity between signal and idler are exactly matched simultaneously. Finally, specific numerical calculations and simulations are presented for BBO-OPCPA with type-I noncollinear angularly dispersed geometry.
The texture synthesis techniques have wide applications in the fields of computer graphics and imaginary objects creation in virtual environment. In this paper, we have a preliminary study on the scheme of 'steerable stochastic' texture synthesis. The schemes use the narrow-band noise (long-created wave) model to generate the macro structure of stochastic texture. By such modeling, the properties of texture are controlled by seven parameters. In order to acquire the reality in vision, random perturbations produced by fractional Brownian motion (fBm) were lead into the model to generate more details of the textural image. Experience results of typical textures are given.
The edge of an image is one of the important features in image analysis and processing. The edge often corresponds to the sharp variation points of intensities that carry most of the image information. In practice, the implementation of edge detection is often performed by some multiscale operators. Wavelet transform is closely related to multiscale edge detection. This paper suggests a regularization method for determining scales for wavelet transform adaptively for each site in image plane. An energy function was introduced to obtain a set of optimal scales by minimizing this function. The multiscale wavelet transform filter was derived from this energy function. The maxima of wavelet transform modulus at these scales acquired from the local site of the image were detected to form the edge map. Experiments for real image shows that both step and differ edges can be detected by this method.
This paper develops a steerable multiscale analysis theory. A number of models based on the wavelet theory are proposed for multiscale TV scene analysis and image processing, including image representation, image edge detection, and noise analysis and removal. A multiscale interpretation method is discussed that makes full use of multiresolution images and edge feature. In order to employ multiple information and all relative information reasonably and effectively, information fusion has been investigated. The idea of geometric reasoning has been also proposed for interpreting objects in TV scenes. Finally, some experiments have illustrated that the proposals described above are feasible.