Qisheng Lu

Propagation of apertured Bessel beams

The propagation features of several apertured Bessel beams are numerically calculated. The calculations show that the relations of axial intensity versus propagation distance are similar to the radial distribution of the aperture functions, which may be helpful in choosing the proper aperture functions in experiments.

Relative phase shifts of apertured Gaussian beams and transformation of a Gaussian beam through a phase aperture.

When a Gaussian beam is apertured, it undergoes a focal shift as well as a phase shift. The focal shift has been investigated extensively although the phase shift has seldom been discussed. We analyze the phase shift of the apertured Gaussian beam. Furthermore we point out that the phase aperture may be used to transform the intensity distribution of the Gaussian beam to obtain a more concentrated beam, to realize uniformity of the Gaussian beam, and to obtain a ring beam.

Revising the formula of thermal focal length in a side-pumped laser rod by experiments

The correct formula of the thermal focal length in a side-pumped Nd:YAG laser rod is discussed and confirmed by experimental results. It is shown that thermally induced stresses that cause a distortion of flatness occur within the region of the whole rod. The presented calculations are in agreement with the experimental observations. The results reveal that the temperature-dependent variation of the refractive index and the distortion caused by the thermally induced stresses constitute the major contributions to thermal lensing.

Sensitivity of the fractional Fourier transform to parameters and its application in optical measurement

The fractional Fourier transform (FRT) is becoming important in optics and can be used as a new tool to analyze many optical problems. However, we point out that the FRT might be much more sensitive to parameters than the conventional Fourier transform. This sensitivity leads to higher requirements on the optical implementation. On the other hand, high parametric sensitivity can be used in optical diffraction measurements. We give the first proposal, to our knowledge, of the FRTs applications in optical measurement.

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.

WAVELET FILTER : PURE-INTENSITY SPATIAL FILTERS THAT IMPLEMENT WAVELET TRANSFORMS

Saloma [Opt. Lett. 20, 1943 (1995)] proposed the concept of mirrors with point-spread functions that exhibit wavelet-related characteristics. We propose novel filters with wavelet point-spread functions. The mirrors are suggested to reform not only the phases of optical waves, but also the filters for amplitude. The transmittance functions of the filters, which are real and positive under some conditions, are given. Optical wavelet transforms can easily be made with these filters, and computer simulations for edge and corner extractions are given.

Model of thermally induced wavefront distortion and birefringence in side-pumped Nd-doped YAG and phosphate glass heat capacity rod lasers

We develop an analytic model to describe the dynamic average thermal distortion and phase difference between the two principal polarizations in side-pumped Nd:YAG and Nd:glass heat capacity rod lasers. It can be predicted that the average thermal distortion is proportional to the temperature profile on the cross section from the analytic expression and, therefore, it is feasible to measure the temperature profile by wavefront sensing. In addition, temperature-dependent variation of the refractive index constitutes the major contribution of the thermal lensing for Nd:YAG rod lasers. Temperature- and stress-dependent variation of the refractive index constitute the major contributions of the thermal lensing for Nd:glass rod lasers. In the case of the same pumping and cooling conditions, there are the same orders of depolarization loss for Nd-doped YAG, LG-680, LG-750, LG-760, and LG-770 glass rod lasers.

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.

Thermal and mechanical effects of shells with inner pressure exposed to continuous-wave laser

The thermal and mechanical effects of shells with inner pressure that are exposed to CW laser are investigated in this paper. In the first step, we try to apply an analytical procedure to obtain temperature response from energy conservation equation and temperature field from the heat conduction equation, then thermal damage threshold in vacuum is obtained. When it comes to mechanical effects, numerical method is applied to study laser heating thermal stresses and analytic solution is applied for stress distribution induced by inner pressure. At last, the whole picture of damage due to combined effect is given and damage threshold is evaluated.