The thermal stress damage of optical elements always restrict the development of high power laser system. We studied the thermal damage mechanism of the optical elements with contaminants induced by high power continuous wave (CW) lasers. An experiment was carried out by a self-build optical element testing platform and a model based on the temperature field theory and thermodynamic theory was set up. We recorded the thermal stress damage process based on a 10 kW/cm<sup>2</sup> level mid-infrared continuous wave laser. Then we calculated the thermal damage process of optical elements. The calculated results are in agreement with our experimental record. The results showed the success of modeling calculation in the thermal damage mechanism caused by contaminants.
In order to measure the absorption coefficient and performance degradation characteristics of optical components which
used in high power laser system, an intra-cavity device was established based on a discharge-drived CW chemical laser.
Two pieces of 45 degree reflecting mirrors were tested. Each mirror was tested for more than twenty times, and high
power laser irradiation on the testing mirrors lasted 100 seconds continuously in each test. The dependence of absorption
coefficients on irradiation times was acquired. The testing results of both reflecting mirrors showed that, the differences
between the experimental absorption coefficients and their fitting curve were up to 30.7% and 21.6% respectively, and
the differences were independent of irradiation condition, such as irradiation energy, irradiation power and beam crosssectional
area. The uncertainty of absorption coefficient was composed of two parts. For one thing, the uncertainty of the
direct measurement results, such as the temperature rise of optical components, can cause the uncertainty of absorption
coefficients. This part of uncertainty was about 11.3%. For another, the resonant cavity need to be adjusted again when
other optical components were replaced, which lead to the change of the incident angle of the optical components to be
measured. A typical film system of 24 layers (12 pairs) was calculated by Thin Film Design Software called TFCalc,
which showed the absorption coefficients increased with the increase of incident angle. When the angle of incidence was
0.5 degree from the design value, there would be -60~71ppm difference of absorption coefficient from the original one,
and the uncertainty was 14.5%. When there was a deviation of 1 degree, the difference of absorption coefficient and the
uncertainty were -112~155ppm and 31.7% respectively. This results showed that, the deviation of incident angle was
between 0.5~1 degree in the test. In order to reduce the testing uncertainty of absorption coefficients, the deviation
between the incident angle of optical components and the design value should be reduced as much as possible. This
provides guidance for measuring the absorption coefficients of optical components with an intra-cavity device.
In this paper, the physical properties of multimode beam are analysed by using the theory of partially coherent light. Based on the spatial coherence measurement results of a multimode fiber laser, we provide a theoretical basis for aberration correction for multimode beams. To improve the beam quality of multimode lasers, phase correction of multimode laser based on a dual-phase-only liquid-crystal spatial light modulator is presented which is used as aberration correction device. The phase distribution was optimized by the stochastic parallel gradient descent algorithm. In this paper the power in the bucket of the far field was used as the evaluation function and the multimode beam included multiple higher order Laguerre-Gaussian beam modes. The real-time aberration correction of Multimode beam by stochastic parallel gradient descent Algorithm is simulated studied and experimental analysed respectively. According to the results the parameters of stochastic parallel gradient descent algorithm can be adjusted and the efficiency and practicability of the algorithm are determined.
Light-spots centroid positions detection is one of the major error sources in the application of Hartmann-Shack (H-S) wavefront sensors. The double images (ghosting) in part of sub-aperture, caused by multiple reflections from optical elements or paraxial stray light from operating environment, result in centroid detection errors and that the precisions of wavefront aberration reconstruction decline. The conventional threshold method often leads to the loss of available information of light-spots, though, by which ghosting can been brought under effective control. In this paper, an improved method and procedure, aiming at ghosting detection and local removal automatically, is proposed by combining several algorithms which include nonlinear processing, autocorrelation, convolution, and local filtering, and so on. Then, the corresponding research has been carried out by a numerical simulation platform established by ourselves, and the results can show that this method and procedure is effective.
In order to study the damage characteristic of the contaminated resonating mirror in high power continuous wave (cw) laser system, we established a theoretical model based on the optical transmission theory with a gain medium. The optical propagation in the cavity is calculated utilizing a Fast Fourier Transform (FFT) repeatedly until the convergence of the calculations tend to a steady-state oscillation mode pattern. The influence of the contaminant size, the contaminant number and the cavity structure on the damage characteristic of the resonating mirror is studied in the theoretical model.
The performance parameters of reflecting mirrors such as absorption coefficient or thermal distortion determine the beam quality of the output laser, so the quality of mirrors is one of the most important factors affecting the capability of the whole laser system. At the present time, there was obviously insufficient in test methods for the mirrors performance. The reflection coefficient, absorption coefficient and scattering coefficient of mirrors could be measured by a lot of test methods such as cavity ring-down method, photothermal deflection method, surface thermal lens method and laser calorimetry. But these methods could not test under high power density radiation. So the test data and results could not indicate the real performance in a real laser system exactly. Testing in a real laser system would be expensive and time consuming. Therefore, the test sequence and data would not be sufficient to analyze and realize the performance of mirrors. To examine the performance of mirrors under high power density radiation, the working principle of intra-cavity was introduced in this paper. Utilizing an output mirror with a low output coupling ratio, an intra-cavity could produce high-power density laser in the resonant cavity on the basis of a relatively small scale of gain medium, and the consumption and cost were very low relatively. Based on a discharge-drived CW DF chemical laser, an intra-cavity device was established. A laser beam of 3kw/cm<sup>2</sup> was achieved in the resonant cavity. Two pieces of 22.5 degree mirrors and two pieces of 45 degree mirrors could be tested simultaneously. Absorption coefficient and thermal distortion were measured by calorimetry and Hartmann wavefront sensor respectively. This device was simple, convenient, low-maintenance, and could work for a long time. The test results would provide support for process improvement of mirrors.
In high power solid state lasers, thermal lens effect always give rise to the multi-modes oscillation in the resonator. The beam quality will deteriorate with the increase of output power. In this paper, an intra-cavity beam shaper is introduced to actively compensate the thermal lens in the laser resonator. One round trip ABCD matrix of the resonator with an intra-cavity beam shaper and thermal lens is calculated. The design parameters with wide stable zone are concluded through the ABCD matrix. The mode size and stability diagram of the resonator are calculated under different focal length of the thermal lens. The relationship between the adjustment of the intra-cavity beam shaper and the mode size under different thermal lenses are concluded, and general method to actively control the modes contents by adjusting the intra-cavity beam shaper is introduced. The effectiveness and performance of active mode control with the intra-cavity beam shaper are verified by simulations of the output modes of resonators. It shows that the M2 factor is well maintained below 1.6 even the focal length of the thermal lens changes from 5m to 0.5m.