Holographic interferometry is a well-established field of science and optical engineering. It has a half-century history of successful implementation as the solution to numerous technical tasks and problems. However, fast progress in digital and computer holography has promoted it to a new level of possibilities and has opened brand new fields of its application. In this review paper, we consider some such new techniques and applications.

Subject of study. A digital holographic system for layer-by-layer control of component quality in additive manufacturing able to determine the size and depth of defects on the surface of each layer was investigated. Method. The digital holographic system for layer-by-layer control of component quality in additive manufacturing operates based on a two-wavelength holographic interferometry method. The digital holographic system is based on a Michelson interferometer. The considered method involves recording holograms of the surface of each component layer at two adjacent wavelengths. As a result of hologram reconstruction, the complex amplitudes of the object wave in the reconstruction plane are determined, which, in turn, define the phases of the waves and their difference. The wave phase difference allows the depth of the defect on the component layer surface to be determined. A tunable diode laser and camera based on a CCD matrix are used as the radiation source and detector, respectively. Main results. The results of the investigation of the surfaces of additively manufactured components using the two-wavelength holographic interferometry method are presented. The recognition of large irregularities is possible through expansion of the phase pattern. The detection of surface defects with the size of 25 µm is demonstrated. The reconstructed surface of a component manufactured by laser sintering of successive layers using the M250 additive setup for selective laser sintering and the reconstructed surface of a millimeter-scale macroscopic object are presented. Practical significance. The possible use of the digital holographic system for component quality control in industrial settings, including those applying additive technologies, is demonstrated. The component quality criterion is the absence of defects (cavities or protrusions) with size larger than 25 µm.

Non-contact, non-destructive testing of surface microgeometry plays a key role in such industries as microelectronics, additive manufacturing, and precision engineering. This paper presents the development and experimental testing of a digital holographic system based on a low-coherence laser diode operating at two close wavelengths, designed to measure height differences in the micrometer range. The method is based on a Michelson interferometer and reconstruction of the complex amplitude of the object wave, which allows phase measurements with subsequent phase conversion into heights. The tests were carried out on micrometer roughness standards with a trapezoidal profile with a groove depth from 24.5 μm to 100 μm and a profile width from 65 μm to 150 μm, as well as on reference strokes with a width from 25 to 200 μm. The obtained data demonstrate the possibility of three-dimensional and two-dimensional visualization of the objects under study with a relative error in height from 5.3% to 11.6% and in width up to 18.6%. It is shown that the system allows reliable measurement of defects of metal surfaces in the range from 25 to 100 μm both vertically and horizontally. Thus, the developed method can be used for high-precision, non-destructive testing in a wide range of technological tasks.

В статье дан краткий обзор методов записи объемных голографических решеток Брэгга используемых в качестве оптических элементов для управления спектром излучения мощных импульсных твердотельных лазеров. Рассмотрены регистрирующие среды голографических решеток Брэгга (галогенидосеребряные эмульсии, слои хромированного желатина, кристалл ниобата лития (LiNbO3) и фототерморефрактивное стекло). Представлены схемы записи Ллойда, Майкельсона и Тальбота. Описаны требования, предъявляемые к пространственной стабильности лазерного излучения, и приведена методика их контроля. Приведены экспериментальные результаты процесса записи пропускающих решеток Брэгга в различных твердотельных материалах. Определен порог поверхностной лучевой стойкости фототерморефрактивного стекла путем регистрации пробоя поверхности решетки. Доказана возможность использования решетки в мощных импульсных лазерах.