Weijiang Zhao

Heterodyne Doppler velocity measurement of moving targets by mode-locked pulse laser

In this study, heterodyne detection is adopted to measure the velocity of a target simulated by a rapidly rotating plate by using a mode-locked pulse laser as the resource. The coherent beat frequency of the signal light reflected by target and local oscillation light occurred on the surface of the detector. Then the waveform of beat frequency was processed by filtering to obtain the Doppler frequency shift of the signal light induced by target. With this frequency shift, the velocity of target could be obtained by calculation. Results indicate that the measurement has a high precision. The error on average is within 0.4 m/s.

Simple lidar detecting wake profiles

A simple lidar to detect wake profiles based on laser backscattering by bubbles at 180° is reported. The monostatic optical geometry is adopted in this lidar and a theoretical model for measuring light scattered backwards by bubbles at 180° is given. The profiles of the wakes produced by a two-blade propeller with a diameter of 46 mm at 6000 rpm (revolutions per minute) and 8000 rpm are achieved. Thus the wake region is identified and the fine structure within wakes is detected. Also, the repeatability of the simple lidar is tested through many experiments. Results show that it is possible to detect wakes using laser backscattering by bubbles at 180°.

High energy, widely tunable Si-prism-array coupled terahertz-wave parametric oscillator with a deformed pump and optimal crystal location for angle tuning

A high energy, widely tunable Si-prism-array coupled terahertz-wave parametric oscillator (TPO) has been demonstrated by using a deformed pump. The deformed pump is cut from a beam spot of 2 mm in diameter by a 1-mm-wide slit. In comparison with a small pump spot (1-mm diameter), the THz-wave coupling area for the deformed pump is increased without limitation to the low-frequency end of the tuning range. Besides, the crystal location is specially designed to eliminate the alteration of the output position of the pump during angle tuning, so the initially adjusted nearest pumped region to the THz-wave exit surface is maintained throughout the tuning range. The tuning range is 0.58-2.5 THz for the deformed pump, while its low frequency end is limited at approximately 1.2 THz for the undeformed pump with 2 mm diameter. The highest THz-wave output of 2 μJ, which is 2.25 times as large as that from the pump of 1 mm in diameter, is obtained at 1.15 THz under 38 mJ (300  MW/cm2) pumping. The energy conversion efficiency is 5.3×10-5.

Scattering properties of ultrashort laser pulses by air bubbles in the sea water

The Generalized Lorenz-Mie theory(GLMT), which describes the interaction between spherical particles and laser pulses, is firstly introduced in this paper. Then scattering properties of ultrashort laser pulses by air bubbles in the sea water are studied, including the scattering efficiency factor, extinction efficiency factor, phase function and degree of polarization, and influences of the bubble size, pulse duration and scattering angle on pulse scattering properties are discussed. Finally, pulse scattering properties of bubbles are compared with those of suspended particles in the sea water. The results show that laser pulses have the smoothing effect, which strengthens with decrease of the pulse duration, but have no influences on the critical angle scattering and Brewster angle scattering. So they could be measured to distinguish between air bubbles and suspended particles and thus to detect air bubbles. This will have certain significances on pulsed laser detection for wake bubbles. The results also show that small bubbles could enhance the backscattering of the sea water, which is advantageous for pulsed laser detection for wake bubbles.

Passively Q-switched laser with single longitudinal mode based on the frequency selection of grating and F-P etalon in twisted-mode folded cavity

A passively Q-switched side-pumped laser with folded resonator is specially constructed for singlelongitudinal-mode smooth pulse output. Nd:YAG is chosen as the laser active medium and Crlsupg4+l/supg:YAG as the saturable absorber medium. Additionally, the method of frequency selection by grating with 1200 line/mm and Fabry-Perot (F-P) etalon is used in the twisted-mode cavity. The single-frequency smooth pulses are produced with 10-Hz repetition rate, 20-ns pulse width, and 1.064-\mu m wavelength. The probability of single-frequency laser output measured is over 99% by using the methods of Fourier analysis and F-P etalon multiple-beam interferometry at the threshold voltage. The measured near-field and far-field angles of divergence are 1.442 and 1.315 mrad, respectively. The values of Mlsupg2l/supg are 1.32 and 1.31 separately with the knife-edge method. Single pulse at 1.064 \mu m with the energy of 8.8 mJ is achieved in TEMlsubg00l/subg mode.

Si-prism-array coupled terahertz-wave parametric oscillator with pump light totally reflected at the terahertz-wave exit surface.

A Si-prism-array coupled terahertz (THz)-wave parametric oscillator with the pump totally reflected at the THz-wave exit surface (PR-Si-TPO) is demonstrated by manufacturing an 800 nm air gap between the crystal and the Si-prism array. Influence on the total reflection of the pump from the Si prisms is eliminated and efficient coupling of the THz wave is ensured by using this air gap. When the THz-wave frequency varies from 1.8 to 2.3 THz, compared with a Si-prism-array coupled TPO (Si-TPO) with the pump transmitting through the crystal directly, the THz-wave output energy is enhanced by 20-50 times, and the oscillating threshold is reduced by 10%-35%. Furthermore, the high end of the THz-wave frequency tuning range of the PR-Si-TPO is expanded to 3.66 THz compared with 2.5 THz for the Si-TPO.

Research on heterodyne detection of a mode-locked pulse laser based on an acousto-optic frequency shift

Heterodyne detection research is carried out on a mode-locked pulse laser using an acousto-optic frequency shifter (AOFS). Theoretical calculation and numerical simulation of the beat frequency of the mode-locked pulse were carried out, and the results show that the frequency offset can be calculated according to the beat period of the pulse waveform, and the offset is the frequency difference between the two adjacent longitudinal modes. Experiments were implemented with a self-built mode-locked laser using an AOFS to simulate the frequency shift of the detection laser induced by the object, the coherent beat frequency of the signal light and local oscillation light that were observed on the surface of the detector. The beat frequency wave was processed by filtering to obtain the frequency shift of the signal light. The experimental results are in good agreement with the numerical simulation.

An improved method of new image decomposition technique: two-dimensional EMD

An improved algorithm of 2-D Empirical Mode Decomposition (EMD) in image processing has been presented. It contains selecting extrema of the pixels and interpolation of them in the course of EMD, A variance phenomenon has been discovered in the interpolation of boundary pixels. Delaunay Triangulation has been used to partition the selected extrema, a pixel that not contained in the Delaunay polygon can be processed through symmetry principle, It can restrain the variance phenomenon that appeared in the cubic spline interpolation. An image has been processed with the improved algorithm, The calculation result of standard deviation between the original image and the reconstructed image is 6.667×10-6 . A slight fluctuation can be seen from the calculation result. The reconstructed image is consistent with the original image. It demonstrates that the improved algorithm been presented is accurate and feasible. The method of EMD is used in image compression and de-noising more and more popular, therefore, the improved algorithm in the paper will be useful in the course of calculation speed enhancement of image processing based on EMD.

Passive frequency stabilization in Nd:YAG pulse laser using reflective volume Bragg grating

We demonstrate a stable Q-switched single-longitudinal-mode (SLM) Nd:YAG laser using a volume Bragg grating as the output coupler. The reflective volume Bragg grating, serving as a longitudinal selector and passive frequency stabilizer, effectively eliminates the mode hopping effect of the laser. The maximum output energy of the SLM obtained from the current experimental setup is 18.5 mJ. The maximum separation of frequencies is significantly less than the longitudinal mode separation, indicating that a stable SLM laser is achieved.

Laser linewidth measurement based on image processing and non-air gap F-P etalon

Laser linewidth measurement has been realized in the paper through image processing and on non-air gap F-P . Firstly , The expression of linewidth measurement founded on non-air gap F-P has been obtained from the interference theory of multi-beam of light. Secondly, The practical linewidth of pulse Nd:YAG laser has been measured with the method above. An interference pattern produced by CCD was used for digital image processing. In the course of processing, Canny operator was applied for the sake of picking-up the edge of interference circle, then the inner and external radius of the interference circle can be acquired in the form of pixels. The actual physical size can be calculated through relative transformation according to the radius of the interference circle. At last, Nd: YAG laser with 28ns pulsed-width was used as the emission source, The experimental results based on the method above is 36.8 MHz. This data is in agreement with 34.2 MHz through Discrete Fourier Transform(DFT), As we all know, the frequency resolving power of DFT is depend on the effective acquisition points, So the cubic spline interpolation was introduced after the course of DFT, and the better result has been achieved.