Pinghui Wu

Condition for invariant spectrum of an electromagnetic wave scattered from an anisotropic random media

Within the accuracy of the first-order Born approximation, sufficient conditions are derived for the invariance of spectrum of an electromagnetic wave, which is generated by the scattering of an electromagnetic plane wave from an anisotropic random media. We show that the following restrictions on properties of incident fields and the anisotropic media must be simultaneously satisfied: 1) the elements of the dielectric susceptibility matrix of the media must obey the scaling law; 2) the spectral components of the incident field are proportional to each other; 3) the second moments of the elements of the dielectric susceptibility matrix of the media are inversely proportional to the frequency.

Spectral Properties of Near-Field Evanescent Waves Scattered From a Particulate Medium With Adjustable Boundaries

Within the accuracy of the first-order Born approximation, expressions are derived for the spectrum of near-zone evanescent waves, which scatter from a spatially deterministic particulate medium with semisoft edges. By assuming that the scattering potential of the medium suffices the multi-Gaussian profile, it is shown that the degree of red-shifted spectrum of scattered field can be modulated by changing the summation index and effective radius of the scatterer. Moreover, the spectral switch, which transforms from the red shifts to the blue shifts of the spectrum, can emerge when the effective radius is sufficiently small compared with the incident wavelength. In addition, the summation index is another factor influencing the degree of spectral switches of near-zone scattered fields. These results are of potential applications to the near-field biomedical imaging and determination of scattering potentials of tiny particles.

High Power Yb Fiber Laser With Picosecond Bursts and the Quasi-Synchronously Pumping for Efficient Midinfrared Laser Generation in Optical Parametric Oscillator

A fiber laser-pumped high-power burst-mode-operated picosecond mid-infrared (IR) laser at 3.8 μm is reported. A gain-switched distributed Bragg reflector laser diode with a pulse repetition rate (PRR) of 138 MHz and pulse duration around 200 ps was applied as the seed laser of a master oscillator power amplifier (MOPA)-structured Yb fiber laser. The PRR of the MOPA was increased to about 1.1 GHz through a pulse multiplier consisting of four cascaded 2 × 2 fiber couplers. A fiber-pigtailed acousto-optic modulator was used to carve the pulse train into pulse bursts so that the peak power of the final linearly polarized fiber laser output could be optimized by adjusting the duty cycle of the pulse bursts correspondingly. The output of the fiber laser was directed to pump a periodically poled magnesium-oxide-doped lithium niobate-based optical parametric oscillator through the quasi-synchronized pump scheme. Efficient parametric conversion was realized with a maximum average power output of 7.3 W at 3.8 μm under pump power of 45 W at 1.064 μm with pump-to-idler conversion efficiency exceeding 16%.

Compact gain-switched Tm:YAP laser pumped by a high power laser diode around 790 nm

We report, to the best of our knowledge, the first demonstration of stable pulse generation from a gain-switched Tm:YAP laser directly pumped with a modulated high power laser diode around 790 nm. A piece of Tm:YAP crystal (3% Tm3+ doping) 5 mm in diameter and 12 mm in length was used as the gain medium. To realize effective gain-switching pulse operation, the laser diode was modulated with suitable pulse repetition rates and corresponding pulse durations. Stable single pulse output was achieved with repetition rates from 1 to 20 kHz. By adjusting the cavity parameters, a maximum laser output power of 4.6 W was obtained under a pump power of 16 W with corresponding overall optical-to-optical conversion efficiency of 28.7%.

Formation of focused laser beams with a hollow metal cone

A hollow metal cone is designed to focus a laser beam into a tiny highly localized beam spot. The finite difference time domain method has been introduced to investigate the beam focusing effect along the propagation direction. Without considering the laser–plasma nonlinear interaction, the numerical calculation results show that a focal spot with a full width at half maximum of ~0.7λ at greatly enhanced intensity and a depth of focus of ~3λ can be achieved. In addition, the influences of cone angle, cone tip size, metal materials, sidewall thickness and incident wavelength on the focusing properties are analyzed in detail.