Dual-wavelength digital holographic interferometry for technical applications

A method of dual-wavelength digital holographic interferometry for analyzing and controlling surface shape for technical applications, including surfaces exposed to high-temperature plasma in fusion reactors, is presented. The capability of applying the method both using miniature vertical-emitting diodes (VCSEL) and conventional wavelength-tunable lasers is shown. The research method is based on dual-wavelength digital holographic interferometry, in which the phases of wave fronts reflected from the object detected at different wavelengths are compared to provide information about the shape of the object. Moreover, the sensitivity of the method is determined by the value of synthetic wavelength, which depends on the difference of wavelengths used for acquisition of digital holograms. The method used following wavelengths 854.000–854.082 nm and 779.900–779.870 nm. Implementation of vertical-emitting diodes for dual-wave holographic interferometry methods is shown. It is found that such diodes have a coherence length of about 20 cm and similar to He-Ne laser. The dependence of the emission wavelength of such sources on the current has been examined and it is determined that the output wavelength deviates less than 1 % during 24 hours. The application of the holographic method has been demonstrated for measuring the shape of objects used in various technical applications (a car body part and a shielding element of the internal wall of the Tokamak fusion reactor). The results of the research illustrate the opportunity to apply the technique of dual-wavelength holographic interferometry to measure a shape of the technical objects surface of various types. It should be noted that other method such as fringe projection can also be used to solve such problems, but it does not work sufficiently on low-scattering surfaces, in our case the lacquered surface of the car body, or specular reflecting surfaces. In addition, the LIDAR technique requires scanning the surface over time, which can lead to measurement errors if the object is unstable due to mechanical movements or vibrations. In dualwavelength holographic interferometry such drawbacks can be reduced by short camera exposure times of milliseconds/ microseconds, or by using pulse laser with pulse durations of 10 ns. However, the major disadvantage of the method is the dependence of the mutual correlation of speckle structures of holograms on the wavelength difference. In order to increase the sensitivity of the method, it is necessary to increase this wavelength difference, and this can significantly reduce the signal-to-noise ratio and decrease the accuracy of the obtained data.