The laboratory model of 10-kW class laser effector was designed and assembled. The special laboratory setup for characterization of its parameters and research on interaction with materials was developed. As a result of dynamic thermaloptic phenomena inside laser source and measurement setup the observed laser beam distributions in far field present features of 4D spatial-temporal, non-stationary stochastic process, thus averaging in given plane over long exposures times was not justified. Measurement of laser beam parameters directly in far field and Wavefront Sensing Measurement by Shack-Hartmann method were applied in experiment. To analyze the experimental data of distorted wavefront measurements Wigner Transform method was applied. Beam quality and brightness determined via Wigner approach was changed in the same way as the direct measurements of beam parameters in far field. The deterministic aberration as a result of dynamic thermal-optic effects depending on averaged laser power was found, which can explain non-Gaussian profiles in the vicinity of focal plane.
An analysis of beam combining quality and the influence of beam profile on tilt and piston error tolerances is presented. We define beam combining performance metrics in terms of powers contained within a specific radius. It is shown that the selection of this radius has a significant effect on the obtained tolerance values. We have taken the tolerance limit as a decrease in intensity of 20%, for piston and tilt error. In addition, for the tilt error, as tolerance limit, we have taken a pointing error equal to the diffraction limit. Our analysis demonstrates that the beam combining performance metric based on the diffraction-limited radius functions best for describing the impact of aberrations on the coherent combined laser array optical system. Our results lead to two important conclusions. First, the tilt error has a greater impact on the degradation of beam quality. Second, a Gaussian beam has greater tolerance for both errors than a top-hat beam.
The aim of work was the development of semi-analytical model for evaluation of coherent combining optical systems. The far-field intensity distributions were calculated based on coherent summation of individual Fourier images. To define measures of combining efficiency, Strehl Ratio and Power in Bucket (PIB) distribution were calculated for each case. In such a way we can determine maximal intensity and power content in main diffraction lobe, the horizontal-PIB to define beam diameter at certain level (e.g. 86.5% PIB) and power content for a given beam diameter (vertical-PIB). The effects of the individual tilts and phase errors on the effectiveness of the combining, Strehl Ratio and PIB were investigated.
The semi-analytical model for evaluation of partial coherent combining of 2D laser beams was developed. The 2D arrays of laser beams ordered in rectangular or hexagonal lattice architecture were analyzed. The far field intensity distributions were calculated based on partial coherent summation of individual Fourier images. The partial coherence coefficients matrix based on the geometry of the array and Gaussian-Schell coherence function with a priori defined coherence radius was implemented. To define metrics of combining efficiency, Power In Bucket (PIB) distributions were calculated for each case. The more dense hexagonal geometry has shown the advantages over rectangular one, mainly because of better filling factor. The two opposite cases (fully coherent combing vs incoherent combining) were analyzed in the first steps. It was found that taking the criterion of 86.5% of PIB we obtained the same beam diameter in both cases for rectangular geometry. In a case of hexagonal geometry more than 2x beam area in far field was obtained for the incoherent combining w.r.t coherent combining for ‘top-hat’ beam evidencing the important role of the compactation and beam profile shaping. The worst case of profile is the untruncated Gaussian one for which the power content in main diffraction lobe is below 40% and more than 60% bigger beam area at 86.5% PIB comparing to ‘top-hat’ beam array with the same lattice architecture.