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In this paper, a simple and effective method is proposed for measuring the focal length of a weak negative thermallyinduced lens. Generally, it is very difficult to measure the focal length larger than 1000 mm of a weak thermally-induced lens by utilizing the traditional procedures. In our experiment, we planned to construct a Yb:KGW laser system almost without the thermally-induced lens in which the focal length of the laser crystal should be measured precisely. With respect to the optical features of Yb:KGW crystal, the thermally-induced characteristics look like something of a negative lens with weak effects. The steps of measuring the focal length of a thermally-induced lens of the laser medium have been adopted as follows. First, the relationship between the focal length f1 of a positive assistant lens as well as the position of the assistant lens and the focal length fT of a thermally-induced lens were carefully analyzed and the experimental setup were designed through the theoretical simulation. Secondly, the variation of the spot size and post position for a He-Ne probe laser have been experimentally investigated after the probe laser beam passed through a thermally-induced lens (fT) and an assistant lens (f1) with the different drive currents of a pump LD with the wavelength of 980 nm. Then, the post position for a He-Ne laser beam can be obtained by use of a least square method, and then the focal length of a weak thermally-induced lens can be deduced with an indirect detection method. In this paper, we introduce a new technique for the measurement of the focal length with the absolute value large than 1000 mm of a negative lens, which has not been reported until now. The results might be useful for the evaluation of a weak thermally-induced lens of almost all solid-state lasers (SSLs).
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