Y. I. Malashko, A. N. Kleymenov, I. B. Potemkin

The main task of beam control segment in mobile-based laser systems is the laser beam waist detection. Focus shift to the required distance is just a simple technical task. In this paper we present the results of the experimental study of method of laser beam waist accuracy detection by wavefront double frequency spherical modulation [1, 2]. The main idea of this method is that only near a focal plane and only in a near-axial part of a beam waist the wavefront spherical modulation results in double frequency respond signal. In a limited range of linear axial waist shifts and near-axial beam dimensions, steady undistorted double frequency signal is observed. Limitation of double frequency generating volume causes threshold nature of this method that leads to a high accuracy. Generation of double frequency signal required intersection of waist volume and focal plane. Most powerful double frequency signal and absence of nonlinear distortions can be achieved at almost symmetrical waist position relative to focal plane. In this case maximal coefficient of conversion power to double frequency achieved. In case of higher wavefront spherical modulation amplitudes, the double frequency signal amplitude increase, but nonlinear distortions increase too. Estimations show that maximum of undistorted double frequency signal achieved by observing three conditions: 1 - Centre of the beam waist coincides with observing plane; 2 - Observed near-axis waist part diameter d satisfies the condition , where F is collimator focal length, φ is laser beam angular divergence by 50% power level at the entrance of collimator. 3 - Waist shift amplitude is equal to a half of waist length relative to observing plane. Results of experimental research lead to the next conclusions: 1. The method of double frequency has obviously threshold nature because double frequency signal is generated by small beam volume placed near beam waist. 2. The method of double frequency shows high accuracy of focus plane detection (focusing detection error value less then 10-5 dioptrе) in case of 1 meter beam aperture and 3,5 ang. sec. (2.10-5 radian) angular beam divergence. 3. With above mentioned beam aperture and angular beam divergence values, coefficient of transformation of power to double frequency was 12%.