Pulsed digital holographic interferometry for dynamic measurement of rotating objects with an optical derotator

Alignment strategy for an optomechanical image derotator using a laser Doppler vibrometer

When monitoring the condition of technical components, it is of particular interest to obtain information of the current condition of the component during operation. The main objectives are safety and machine efficiency. A special extension in this area is the derotator, since the derotator can be used to extend measuring methods designed for static components to rotating systems. In this paper, we present a new, to the best of our knowledge, approach to align a derotator using a laser Doppler vibrometer (LDV). When aligning the derotator, the optical axis must be coaxial with the rotating object to be measured. Current conventional approaches only ensure that the optical axis and the rotational axis intersect at the center of the measuring object. In our approach, a LDV is used to determine the angular deviation between the two axes, which cannot be determined by common methods. Our approach is easy to implement because the LDV is often already used in combination with the derotator as a measuring device. Experiments are carried out to show that this approach is feasible. In addition, several investigations are being performed to analyze how different parameters affect the result, creating reproducible conditions and deriving an alignment approach. By combining the approach developed in this work for determining a rotational deviation with common image-based approaches for determining a translational deviation of the axes, the derotator can be calibrated entirely. In this way, the quality of the measurement with the derotator can be further increased, as calibration errors can be reduced.

From speckle pattern photography to digital holographic interferometry [Invited]

Speckles are inherently an interference phenomenon produced when an optically rough surface or a turbulent medium introduces some degree of randomness to a reflected or a transmitted electromagnetic field. Speckles are often nuisance in coherent image formation. Speckle patterns are however a useful tool for displacement and deformation as well as vibration and stress analysis. The development of speckle photography to speckle interferometry and digital holographic interferometry is described in this paper.

Separate recording of rationally related vibration frequencies using digital stroboscopic holographic interferometry

A method for separate recording of rationally related vibration frequencies is presented. To record and measure the mode shape of vibrations, a synchronized stroboscopic CCD camera is used. Synchronization and control of the camera acquisition for recording stroboscopic holographic sequence has been realized. The phase for different states of the object vibration is calculated using the Fourier-transform method. Experimental results are presented, and the advantages and disadvantages of the proposed method are discussed.

Digital holographic interferometry for simultaneous orthogonal radial vibration measurements along rotating shafts

A digital holographic interferometry setup used to measure radial vibrations along a rotating shaft is presented. A continuous Nd:YAG laser and a high-speed digital camera are used for recording the holograms. The shaft was polished optically smooth to avoid speckle noise from the rotating surface. The light reflected from the shaft was directed onto a diffuser which in turn was imaged by the holographic system. Simultaneous measurements with a laser vibrometer were performed at one point and comparisons between the signals showed good agreement. It is shown that different vibration components of a rotating shaft can be simultaneously measured with this technique.

On-line surveillance of a dynamic process by a moving system based on pulsed digital holographic interferometry

A method based on pulsed digital holographic interferometry for the measurement of dynamic deformations of a surface by using a moving system is presented. The measuring system may move with a speed of several meters per minute and can measure deformation of the surface with an accuracy of better than 50 nm. The deformation is obtained by comparison of the wavefronts recorded at different times with different laser pulses produced by a Nd:YAG laser. The effect due to the movement of the measuring system is compensated for by digital processing of the different holograms. The system is well suited for on-line surveillance of a dynamic process such as laser welding and friction stir welding. Experimental results are presented, and the advantages of the method are discussed.

Jones matrix for image-rotation prisms

The polarization-transforming properties of rotational prisms are analyzed with polarized light by using the Jones calculus and the exact ray-trace. A general expression of the Jones matrix for a rotational prism is derived, incorporating an explicit dependence on the image-rotation angle or the wave-front-rotation angle. The Jones matrix for the Pechan, Dove, Reversion, and Delta prisms is derived where the explicit dependence on the angle of rotation of the image is given. An experiment with a rotating Dove prism is also conducted to determine the output states of polarization for incident linearly polarized light. Experimental results agree with theoretical expectations.

Temporal phase unwrapping of digital hologram sequences

A method for recording and evaluating digital image-plane holograms is presented. Hundreds of holograms of an object that has been subjected to dynamic deformation (e.g., vibrations) are recorded. The phase of the wave front is calculated from the recorded holograms by use of a two-dimensional digital Fourier-transform method. By temporal phase unwrapping it is possible to determine the absolute deformation (included the direction of motion) of the object. Experimental results are presented, and the advantages of temporal phase unwrapping compared with spatial phase unwrapping are discussed.