Method development for vibration characteristics registration of optically-transparent elements

Abstract

A method for investigation of natural vibration modes of the large-sized, optically-transparent products of optical systems has been developed. To measure the vibration displacements of a flat optical surface with the deposited, easily-removable, light-scattering coating, an optical scheme based on the noise-proof speckle pattern interferometer with CW-laser has been proposed. The optimal, optical transmission of the coating has been determined to provide the registration of contrast speckle-interferograms. It was shown that the maximum visibility of interference fringes in speckle interferograms was achieved with a transmission of the light-scattering layer in the range of τ_с = 43–63 % and decreased beyond the range. In addition, it was noted that use of the optically-dense, the light-scattering coatings with τ_с < 43 % affected on the number of the recorded interference fringes in speckle-interferograms, although the excitation level of object was constant. The phenomenon was simultaneously occurring with the transition from the mixed reflection to diffuse-scattering when the optically-dense light-scattering coating was deposited. At the same time, it was shown that the effect could not be caused by the detuning of the excited object from resonance due to a gain in the weight of the applied coating. For a diffusely scattering element, as optically-transparent component of the speckle-interferometer, the set of vibration modes corresponding to the calculated data of the numerical model was recorded. It was shown that the mismatch between experimental and calculation data in the magnitudes of resonant frequencies did not exceed 1 % in the range of 20–200 Hz, while for the range of 200–900 Hz it was of 5–7 %.