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We present a novel approach for detecting the absolute position of surfaces in multi-wavelength holography. We extent the depth of field from ~0.08 mm to ~3 mm and get the absolute position with accuracies better than10 µm.

Using digital holography in camera-based interferometers, 3D surface topography can be measured extremely quickly and with sub-wavelength precision. Using spatial phase-shifting, a single camera image is sufficient to reconstruct complexvalued wavefronts for multiple wavelengths. Recently, measurements on moving objects were demonstrated using setups with 1× magnification. Increasing the lateral resolution by implementing larger magnification in a microscopic setup would open up new application fields, but the larger numerical apertures (NA) of microscope objectives make the acquisition even more sensitive to motion. In this work, we show the first microscopic setup, measuring objects moving at several mm/s using two-wavelength holography. Despite the large NA of 0.42 of the 10×-objective in our setup, measurements can be acquired at 75 mm/s and beyond. Using two lasers emitting slightly different wavelengths (637.76 nm and 632.87 nm), a maximum height difference of 41.3 μm can be detected unambiguously. One single image covers a lineshaped measurement area of 3.7 mm × 0.2 mm with a lateral pixel pitch of 0.47 μm. In order to inspect larger objects, single frames can be stitched together, permitting an infinite measurement area in the direction of motion. Gap-free stitched measurements are limited to 75 mm/s due to the framerate of the camera. Measurements of the groove depth averaged over several pixels on a groove standard show a repeatability exceeding 10 nm at 35 mm/s and 20 nm at 75 mm/s.

Digital-holographische Messsysteme erlauben eine echte 100-Prozent-Qualitätskontrolle – inline in der Fertigungslinie und sogar direkt in der Werkzeugmaschine. Die hochgenauen und sehr schnellen 3D-Sensoren erfassen dabei makroskopische Topografien mit Genauigkeiten bis in den Sub-Mikrometerbereich.

3D-printed mounts and off-the-shelf components result in a digital holographic microscope for 200 € – including camera and computer. Short source code and a simple design put noninvasive quantitative bio-imaging within reach of secondary schools.

Multiwavelength digital holography enables precise and fast 3D height measurements of rough surfaces. To inspect objects during motion would enlarge the range of applications enormously. In this work the limits of this technique with respect to velocity and inclination angles are studied for linearly moving as well as for rotating objects. We demonstrate measurements on surfaces with inclination angles of up to 40° , moving linearly with a velocity of 2 mm/s, providing 2 μm accuracy, and on a rotating cylinder with circumferential speed of 10 mm/s, we achieve 1.1 μm precision. All measurements are conducted with less than 1 mW of continuous-wave laser light, so the object moves several micrometers during exposure time.

Digital holographic measurements on planar moving objects are investigated. We discuss the dependence of the interference contrast on velocity and aperture width for both diffusely and specularly reflecting objects. Using spatial phase shifting, the experimental results for motion in parallel and perpendicular to the optical axis are in good agreement with the theoretical considerations. Measurements with object velocities of up to 100 mm/s are conducted using only less than 1 mW of continuous-wave laser light. These considerations are used to determine the optimal angle between the direction of motion and the illuminating beam, resulting in the lowest decrease in contrast with increasing velocity.

Multiwavelength digital holography on moving objects enables fast and precise inline-measurements of surface pro files. Due to the use of multiple wavelengths, optically rough surfaces with structure heights in the micrometer range can be mapped unambiguously. In this work we explore the influence of the object velocity on height measurements on inclined surfaces. We show measurements using spatial-phase-shifting holography employing two wavelengths and object velocities of up to 90 mm/s with eye-safe cw-lasers with less than 1 mW of laser light. Despite motion blur exceeding the mean speckle size, reliable height measurements can be conducted at these velocities. The height map of a metal cone with two different slope angles (1° , 10° ) is measured at an exposure time of 2 ms. Using line shaped illumination, each frame yields a height map of approximately 2 x 17 mm2. The overlap between the frames allows averaging as the image is put together, improving data quality. The mean repeatability of the height information in the investigated setup is better than 4.5 µm at a synthetic wavelength of 214 µm.

Interferometric measurements of rotating objects face an axial motion component if the optical axis of the measurement system is not pointing towards the axis of rotation. In a typical interferometer, axial motion of half a wavelength reduces the interference contrast to zero. Our setup compensates for this axial component by an adapted variation of the reference path length during exposure utilizing a piezoelectric actuator. We present off-center measurements on a cylinder, rotating with different angular velocities. The repeatability of these measurements is dominated by motion blur, which demonstrates that the compensation of the axial motion works accurately.

Wavelength tuning of conventional mirror-based optical parametric oscillators (OPOs) exhibits parabolically-shaped tuning curves (type-0 and type-I phase matching) or tuning branches that cross each other with a finite slope (type-II phase matching). We predict and experimentally prove that whispering gallery OPOs based on type-0 phase matching show both tuning behaviors, depending on whether the mode numbers of the generated waves coincide or differ. We investigate the wavelength tuning of optical parametric oscillation in a millimeter-sized radially-poled lithium niobate disk pumped at 1 μm wavelength generating signal and idler waves between 1.7 and 2.6 μm wavelength. Our experimental findings excellently coincide with the theoretical predictions. The investigated whispering gallery optical parametric oscillator combines the employment of the highest nonlinear-optical coefficient of the material with a controlled type-II-like wavelength tuning and with the possibility of self-phase locking.

Broadband infrared spectroscopy employing optical parametric oscillation in bow-tie cavities, including a periodically- poled lithium niobate (PPLN) crystal, is well known. We demonstrate, however, that such spectroscopy is also possible using 2-mm-size monolithic whispering gallery resonators (WGRs). This is achieved in a radially-poled WGR by controlling wavelength tuning despite triple resonance of pump, signal, and idler light. Simulated and measured tuning characteristics of the Type-0 OPOs, pumped at about 1 μm wavelength, coincide. Tuning branches, which are crossed or curved at degeneracy, are present over a spectral range of up to 0.9 µm. As a proof-of-principle experiment, we show that all spectroscopic features of ethanol can be resolved using the idler light between 2.2 and 2.55 μm.

It was not possible so far to characterize moving objects using interferometric techniques. We present an implementation where the 3D profiles of objects moving at velocities up to 100 mm/s are obtained by digital holography.