The performance of laser weapon system depends among others on harnessing or mitigation of transient thermal optics effects (trans-TOE’s) occurring inside High Energy Laser (HEL) and in laser optics beam forming train as well. In the developed in last year at MUT laser effector based on of 10-kW fiber HEL the optical train consists of about ten lenses, mirrors and windows including the most critical fiber endcap. We have measured the transient 2D temperature distributions in these optical elements under 10-kW laser beam exposition and compared results to numerical modeling in COMSOL Multiphysics. Applying such experimental / numerical approach the effective absorption in dielectric layers of typical mirrors and in volume of transmissive elements under high laser power were determined. The layer absorption was determined to 20 − 50 ppm for High Reflective (HR) mirrors and less than 10 ppm for Anti Reflecting (AR) coatings. The idea of dynamic self-compensation of trans-TOE’s by means of tailored design of the following transmissive and reflecting elements was proposed. The numerical model of this concept for the simplest combination consisted of 2 HR mirrors and single AR coated lens was presented.
The performance of laser weapon system based on Coherent Beam Combining (CBC) depends on its propagation properties in atmosphere. The appropriate semi-analytical model based on partial coherent beam combination for assumed coherence coefficients between beams in CBC lattice was developed. To simulate atmospheric turbulences the Kolmogorov-Fried model of partial coherence and Hufnagel-Valley model of Cn2 dependence on atmosphere parameters were implemented. The approximated formula on CBC performance in dependence on Fried radius was proposed. The results of CBC modeling were compared to known analytical solutions of Gaussian beam propagation in turbulent atmosphere. The dependence of CBC performance on Cn2 parameter, elevation angle and range was analyzed. The general conclusion is that application of CBC has not practical sense for propagation on medium and long ranges without effective adaptive optics system.
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.