Fengjie Xi

Near-diffraction-limited annular flattop beam shaping with dual phase only liquid crystal spatial light modulators

We demonstrate the annular flattop beam shaping technique with dual phase only liquid crystal spatial light modulators (LC-SLM) based on the refractive laser beam shaping systems. One LC-SLM redistributes the intensity distribution, and the other restores the initial underlying wave front. Differing from the conventional annular beam shaping technique, the wave front of the output beam can be maintained. The influences of deviations of beam waist and beam shape on the output beam profile are discussed in detail. Experimental results show that approximate 71% of the power is enclosed in a region with less than 7% rms intensity variation. The 4.1mm diameter near-diffraction-limited beam retains an annular flattop intensity distribution without significant diffraction peaks for a working distance of more than 24cm in the near field.

Simultaneous displacement and angular drift measurement based on defocus grating.

We propose and demonstrate a displacement and angular drift simultaneous measurement technique based on a defocus grating. The displacement and angular drift of the incident beam can be detected by monitoring the movements of +/-1 diffraction order spots of the defocus grating. The relationship between drift of the incident beam and movements of +/-1 diffraction order spots is studied in detail. Compared with other methods, this technique eliminates the requirement of two or more detecting systems for measuring displacement and angular drift simultaneously. The proof-of-principle experiment shows that the root-mean-square errors of displacement and angular drift measurements are less than 0.5 microm and 0.84 microrad, respectively.

Simulation for thermal blooming of the axial pipe flow

For the thermal blooming of the beam path indoor, solving the coupling equations of optical field and fluid field completely is a meaningful and important subject. In this paper a numerical emulation platform for solving the coupling equations was established. The laser beam coupled with the fluid field by the method of User Defined Function which was offered by the CFD software. Thermal blooming effects in the beam path indoor of the line pipe are modeled by the established numerical emulation platform. In order to testify the rightness of the numerical emulation results, steady-state thermal blooming effects in the axial pipe flow are calculated by the theoretical methods, and corresponding experiments are also carried out. The results indicate that the numerical emulation platform is creditable in simulating the thermal blooming of axial pipe flow.

Shack-Hartmann wavefront sensor measurement for dynamic temperature profiles in heat-capacity laser rods

A Shack-Hartmann sensor nonintrusive measurement for the temperature profile in a heat-capacity neodymium-doped glass rod is proposed. This technique is possible because the optical path length of the rod changes with temperature linearly over a wide range. The temperature change of the solid-state laser rod is often recorded by using a thermocouple, thermal camera, or phase-shifting interferometer. Based on an analysis of temperature-induced changes in length and index of refraction, we can get the temperature profiles from the wavefront reconstructions in real time. The results suggest the Shack-Hartmann sensors could replace microbolometer-based thermal cameras and phase-shifting interferometers for dynamic temperature profiles in heat-capacity laser rods with particular advantages. A strange temperature chaos of the Nd:glass rod just after the pump cycle is discovered.

Theoretical analysis for scaling law of thermal blooming based on optical phase deference

In order to explore the laser propagation influence of thermal blooming effect of pipe flow and to analysis the influencing factors, scaling law theoretical analysis of the thermal blooming effects in pipe flow are carry out in detail based on the optical path difference caused by thermal blooming effects in pipe flow. Firstly, by solving the energy coupling equation of laser beam propagation, the temperature of the flow is obtained, and then the optical path difference caused by the thermal blooming is deduced. Through the analysis of the influence of pipe size, flow field and laser parameters on the optical path difference, energy scaling parameters Ne=nTαLPR2/(ρεCpπR02) and geometric scaling parameters Nc=νR2/(εL) of thermal blooming for the pipe flow are derived. Secondly, for the direct solution method, the energy coupled equations have analytic solutions only for the straight tube with Gauss beam. Considering the limitation of directly solving the coupled equations, the dimensionless analysis method is adopted, the analysis is also based on the change of optical path difference, same scaling parameters for the pipe flow thermal blooming are derived, which makes energy scaling parameters Ne and geometric scaling parameters Nc have good universality. The research results indicate that when the laser power and the laser beam diameter are changed, thermal blooming effects of the pipeline axial flow caused by optical path difference will not change, as long as you keep energy scaling parameters constant. When diameter or length of the pipe changes, just keep the geometric scaling parameters constant, the pipeline axial flow gas thermal blooming effects caused by optical path difference distribution will not change. That is to say, when the pipe size and laser parameters change, if keeping two scaling parameters with constant, the pipeline axial flow thermal blooming effects caused by the optical path difference will not change. Therefore, the energy scaling parameters and the geometric scaling parameters can really describe the gas thermal blooming effect in the axial pipe flow. These conclusions can give a good reference for the construction of the thermal blooming test system of laser system. Contrasted with the thermal blooming scaling parameters of the Bradley-Hermann distortion number ND and Fresnel number NF, which were derived based on the change of far field beam intensity distortion, the scaling parameters of pipe flow thermal blooming deduced from the optical path deference variation are very suitable for the optical system with short laser propagation distance, large Fresnel number and obviously changed optical path deference.

Analysis for angle anisoplanatic effect of the steady thermal blooming

The effect of beacon Anisoplanatism needs to be considered in analyzing the error of the adaptive optical system. Therefore, thermal blooming anisoplanatic effect of the Gaussian beam is analyzed numerically and theoretically. Wavefront distortion of the Gaussian beam caused by thermal blooming anisoplanatic effect is expanded by the Zernike polynomials. The Zernike coefficient and the fitting error are obtained by numerical calculations. The comparisons between the Zernike coefficients indicate that the defocus item is the most important to the angular anisoplanatic error. Based on the Wave-front distortion caused by the thermal blooming angular anisoplanatic effect, the defocus coefficient of the Zernike polynomials is obtained theoretically. The result of the angular anisoplanatic error calculated by theoretical formula is consistent with the outcome of the numerical calculation, and the result also indicates that the angular anisoplanatic error is the function of the caliber size and varies as the square of the anisoplanatic angle. The square relation of angle anisoplanatism is consistent with the result obtained by the turbulence angular anisoplanatic effect.

Laser beam collimation testing with defocus grating

A novel method for testing the collimation of laser beam with defocus grating is presented. Defocus grating combined with lens of short focus length allows simultaneous imaging of two defocused spots in the ±1 diffraction order spot. When the well collimated beam passes through the defocus grating, the diameter of the ±1 diffraction order spot is equal to each other. The relationship between the collimation of the incident beam and diameters of the ±1 diffraction order spots are analyzed in detail. The position of the pinhole and pointing errors of the collimation system can be detected by the defocus grating. The feasibility of this method is verified experimentally on the collimation of the He-Ne laser. The method which has a simple data processing algorithm does not require any referencing or fringe analysis and can be implemented full-optically.