| Abstract [eng] |
In laser systems, the laser beam always experiences greater or lesser optical aberrations due to the optical elements used. They can typically be corrected using additional optical elements, but when attempting to focus or pass a laser beam on transparent media with higher refractive indices, optical aberrations occur in the system which are depended on the laser beam angle, wavelength, media refractive index, and beam focusing depth in the material. In this case, aberration compensation becomes cumbersome because of the need for a correction method that can dynamically change the aberration correction function when the beam position is changed. For this reason, it is advantageous to use spatial light modulators with which the fiber wavefront can be formed, thus compensating for the wavefront distortions caused by aberrations. These spatial light modulators are also useful in beam formation. By varying the phase delay in different areas of the laser beam wavefront, the beam can be formed as bottle-beam or flat-top beam, thus expanding the applications of the available laser. In this work, aberration correction diffraction masks were developed, which were modeled from Zernike polynomials describing the aberrated wavefront. Polynomials are calculated by the beam tracing method, replacing a real lens with an ideal one in an optical system model. The developed diffraction masks were tested by focusing the laser beam on quartz glass. This method of aberration correction helped to create a Gaussian beam with a 0.8 Strehl* coefficient. After applying the correction, the beam intensity increases by 5.5 times. After that, calculated polynomials were valued for their sensitivity to system changes. The generated Zernike polynomials were found to correspond to the values of the wavefront polynomials of a real optical system. It is estimated that a change in focus of 0.2 mm from the initial position results in an 10% reduction in the Strehl* coefficient. An alternative design method for forming bottle-beam and flat-top beams using aperture-shaped π-phase delay diffraction masks was also tested. It has been found that these diffractive masks effectively create bottle-beams and by selecting the appropriate aperture diameter, flat-top beams are obtained. The resulting beam withstand 6 mm of space propagation without distorting. It has been determined that in order to obtain a flat-top beam profile, the aperture diameter must be 1.13 times larger than the width of the Gaussian fiber used. Compared to the flat profile fibers of the classical sinc function method, the beams of the phase delay aperture diffraction mask are obtained with smaller diameter and higher intensity. |
