High-precision applications of multiwavelength holography typically requires stable laboratory-like environments, which is hard to achieve in industrial applications. The influence of schlieren is crucial, especially in large fieldof-view applications, where long working distances can result in optical paths up to one meter. Schlieren is a well-known effect in interferometry and can be seen in the reconstructed object wavefronts. Small perturbations in temperature change the refractive index of air resulting in local variations of the optical path. Proper encapsulation or vacuum techniques are typically employed to compensate for this. In this work, we investigate the impact of schlieren on multiwavelength holography and propose a compensation method. The sensor used is a Mach-Zehnder-based interferometer with a field-of-view of 17.9 mm × 13.4 mm and a camera with 9344 px×7000 px. A mirror was positioned at a distance of 1 m in front of the sensor. We performed holographic and interferometric measurements with and without an encapsulating pipe around the beam to investigate the influence of schlieren. The deviations of the phase shifts of the holographic data were laterally resolved using a modified version of the algorithm proposed by Cai et al. The fringe patterns of the interferometric data captured with different exposure times and frame rates were analyzed using a sinusoidal fit and discrete Fourier transformations (DFT) to show lateral frequency deviations. Both methods show that encapsulation leads to improved measurements. A potential compensation method is proposed.
Multi-wavelength holography is a convenient method for the inline inspection of industrially machined parts. Compared with the single-wavelength approach, the unambiguity range can be extended by orders of magnitude. This is typically achieved by using multiple lasers. The smaller their wavelength difference, the larger is the unambiguity range. Reaching the centimeter or even meter range is difficult with individual lasers because of their relative wavelength drift. Here, we demonstrate multi-wavelength holography with 37.5 cm unambiguity range using only one single laser. The wavelength shifts are achieved with acousto-optic modulators driven at 200 MHz and 1 GHz. This provides unambiguity ranges of 37.5 and 7.5 cm respectively. Importantly, the perturbation caused by a possible long-term drift of the laser is significantly reduced. For a proof-of-concept demonstration, we determine the shape of a metallic object comprising height differences between 1 and 100 mm. The scheme can be extended to larger frequency shifts, i.e. better axial resolution, by using electro-optic modulators. This would enable to conveniently select the measurement range between some millimeters and meters although only one laser is used.
We present digital multi-wavelength holography measurements generated inside a machine tool. The digital holographic sensor HoloTop NX used in the application has a field of view of 12.5 mm × 12.5 mm, each field of view consists of 3008 px × 3008 px. Three separately stabilized diode lasers connected to a fiber switch serve as light sources. The unambiguous measurement range of the holographic sensor defined by the laser wavelengths is approximately 140 μm and the axial reproducibility is ~0.4 μm. The sensor is mounted to a standard 5-axis machine tool Hermle C 32U. An automatically created numerical control (NC) program enables the machine tool to meander the sensor over larger test objects, here 280 mm × 94 mm. For each field of view, a height map with 9 million data points is generated using digital holography. The images are acquired with 2 mm lateral overlap and are then stitched together to achieve a height map of the complete sample with more than 1.4 billion data points. For comparison the shape of the sample was measured with a state-of-the-art coordinate measurement machine (CMM). The datasets from the CMM and the holographic measurement are semi-automatically referenced to each other. The holographic height data are averaged over a rectangular area with radius 50 μm around each of the 360 measurement locations where the tactile CMM measurement data have been acquired. This allows us to evaluate deviations between the holographic and the CMM dataset. It turns out that the root mean square (RMS) error between the two data sets is smaller than 0.4 μm.
Digital holography enables high-precision quality control in machining production and has already been introduced to several multi-axis systems1, 2. To meet the demanding measurement tasks in the quality control of complex components, accuracies in the sub-micrometer range with measurement ranges larger than several centimeters are required. Previous measurements have shown the potential of multiwavelength digital holography to allow unambiguous ranges of few millimeters3 . We present multiwavelength digital holographic measurements using synthetic wavelengths with two meters down to a few micrometers, potentially enabling measurements with meter-scale unambiguity at sub-micrometer accuracy. Measurements on a 10 cm step-height sample have been conducted using the compact digital-holographic sensor HoloTop NX for various multi-axis systems, supplied by an Ondax LMFC single frequency diode laser at 632.852 nm and the tunable laser Hübner C-Wave used in the wavelength range of 480.786 nm – 632.852 nm. The latter offers a frequency stability of 150 MHz on a time scale of several hours. The maximum laser drift during data acquisition was observed to be 0.02 pm. Thus, at the 2 m synthetic wavelength, this results in a maximum synthetic wavelength error of 200 mm. Random noise of 20 mm at the largest used synthetic wavelength of 2 m requires multiple synthetic wavelengths to get down to micrometer precision: Eight nested synthetic wavelengths from 2 µm to 2 m and numerical refocusing of the hologram were used to evaluate a milled sample with multiple step heights, machined on a Hermle C32U machine tool. Ten repetitive measurements confirm a machining uncertainty of 9 µm for this sample at its maximum step height of 10 cm.
Adiabatic frequency conversion (AFC) in microresonators comes without phasematching restrictions and does not depend on light intensity, i.e. it can reach 100 % conversion efficiency even at the single-photon level. The AFC is experimentally achieved in various configurations since 2007. However, compared with their nonlinear-optical counterparts, they still lead a life on the edge of obscurity. Despite of some impressive proof-of-concept demonstrations, there seems to be only little interest to employ adiabatic frequency converters for real-world applications. We demonstrate an electro-optically driven adiabatic frequency converter based on a millimeter-sized whispering gallery resonator made out of a lithium niobate crystal. The electric field is applied with a self-built ultra-fast high-voltage pulse generator. It consists of a push-pull stage with two fast-switching 600-V GaN power transistors and a control unit. This enables us to generate pulses with voltages of up to 600 V, slew rates of up to 150 V/ns and repetition rates reaching 1 MHz. Considering 100 µm resonator thickness, this enables electrically-controlled frequency shifts of up to 100 GHz. We combine this frequency converter with a system for multi-wavelength digital holography. Here, interferograms are recorded at slightly different laser frequencies. Calculating the difference phase of the interferograms numerically, interferograms at the beat frequency of the respective wavelength pairs can be created that correspond to phase data at the difference frequency. Cascading this process, a large unambiguity range paired with a high axial resolution becomes possible. A single laser combined with an adiabatic frequency converter is very appealing to provide sequentially the many, exactly spaced laser frequencies needed here, replacing a series of stabilized fixed-frequency lasers.
The electronics industry is creating complex miniaturized devices with steadily higher power density. The increase of maximum operating temperatures affects the thermo-mechanical load and imposes greater requirements on the quality of electronic packages. Fast and reliable methods for inspecting the quality of electronic components can help to improve production quality and to reduce waste and environmental burden. We present a compact optical sensor based on Electronic Speckle Pattern Interferometry (ESPI) that provides a possibility to carry out such control in a fast, precise and non-contact manner and can be integrated directly in a production line. Analysing thermo-mechanical deformations of objects under study, the system is capable of identifying common defects in electronic modules, such as die attachment delamination.
Hybrid manufacturing processes, high level of automation, short product service life and decreasing vertical range of manufacture in production request for increasing flexibility and speed of quality control. With HoloCut we previously introduced the world’s first wireless digital-holographic sensor system prototype for fast and precise measurements inside a machine tool. With the experience gained so far, we now present an improved, even more compact sensor system, for the use on various multi-axis systems such as coordinate measuring machines (CMM), robots and machine tools and show first results with different handling systems. Besides improved mechanical stability, a size and weight reduction resulted from a new design approach: The arrangement of components around a central "core" made it possible to create a very compact design with a diameter of 125 mm, a height of ~180 mm and a weight of ~2 kg. The system features a 12.5 × 12.5 mm² measuring field with a lateral sampling of 4 μm. An NVIDIA Xavier embedded system enables pre-evaluations of the recorded measurement data in order to allow re-recording them, even before the complete data transmission (up to 160 MB with 2 Hz measuring rate) and evaluation. This is especially important for the use in vibration-prone environments such as multi-axis systems. Various handling systems such as a HERMLE C32U machine tool, an undamped LEITZ Reference HP 15.9.7 CMM and a UNIVERSAL ROBOT UR16e are examined with regard to vibrations. In future work, the behavior of the system under higher vibration amplitudes will be characterized.
Digital multi-wavelength holography is an emerging technology for very precise and fast 3D measurement. Here, we present a novel digital holographic system that uses a 65-Megapixel camera to achieve high resolution measurements on an 18 × 14 mm² field of view resulting in a lateral sampling of ~2 μm in x- and y-direction. Using three single frequency lasers for illumination in a temporal phase shifting scheme, we achieve data acquisition times below 150 ms for full 65- Megapixel 3D-measurements. The choice of the three lasers enables an unambiguous axial measurement range of 400 μm. On a calibrated height standard with a 20 μm step repeatability of <0.01 μm (1 standard deviation) is demonstrated. More challenging and of high interest for industrial applications are measurement samples that consist of surfaces with varying surface roughness, reflectivity or material. These kinds of samples require a sensor with a high dynamic range and pose several geometrical optical challenges: Light from differently reflecting or scattering surfaces travels through the optical system on different paths. Without compensation, this results in small, yet non-neglectable errors in the measured height values. We have applied approaches well described for single-point interferometers to the full-field imaging system used in the presented optical setup. Without a-priori knowledge about surface quality of the sample, we can compensate for these errors. Thus, the presented digital holographic sensor is able to achieve repeatability of ~0.1 μm (1 standard deviation) for height features consisting of rough and specular surfaces.
With state-of-the-art 3D measurement systems, short-wave structures such as tool marks cannot be resolved directly inside a machine tool chamber. Up to now, measurements had to be performed outside the machine tool. We present an interferometric sensor that carries out such measurements inside the machine tool, which saves time-consuming and expensive setup procedures. Our sensor HoloCut uses digital holography as measurement principle. By the use of multiple wavelengths, we get a large unambiguous axial measurement range of up to 2 mm and achieve micron repeatability, even in the presence of laser speckles. With a lateral resolution of 7 μm across the entire 20 x 20 mm2 field of view, both macro- and microstructures (such as tool marks) are measured with an axial resolution of 1 μm. Consequently, this qualifies HoloCut for in-situ measurements and integration in a machine tool. In this paper, the boundary conditions of integrating interferometers inside a machine tool are evaluated. Occurring vibrations and limited available space are particularly challenging constraints: The optical and mechanical design of HoloCut is introduced along with numerical correction algorithms: A piezo-stage setup is used to induce known displacements. Using these algorithms, measurements even with a closed-loop control of the machine tool head activated are demonstrated on a coin measurement. The use of HoloCut is motivated on the base of the daily operation of a 5-axis machine tool: We present an evaluation of an exemplary ISO 25178 parameter Sq using HoloCut measurements and compare those with reference, yet not inline-capable systems.
In this paper we present a miniaturized digital holographic sensor (HoloCut) for operation inside a machine tool.
With state-of-the-art 3D measurement systems, short-range structures such as tool marks cannot be resolved inside a machine tool chamber. Up to now, measurements had to be conducted outside the machine tool and thus processing data are generated offline.
The sensor presented here uses digital multiwavelength holography to get 3D-shape-information of the machined sample. By using three wavelengths, we get a large artificial wavelength with a large unambiguous measurement range of 0.5mm and achieve micron repeatability even in the presence of laser speckles on rough surfaces. In addition, a digital refocusing algorithm based on phase noise is implemented to extend the measurement range beyond the limits of the artificial wavelength and geometrical depth-of-focus. With complex wave field propagation, the focus plane can be shifted after the camera images have been taken and a sharp image with extended depth of focus is constructed consequently.
With 20mm x 20mm field of view the sensor enables measurement of both macro- and micro-structure (such as tool marks) with an axial resolution of 1 µm, lateral resolution of 7 µm and consequently allows processing data to be generated online which in turn qualifies it as a machine tool control.
To make HoloCut compact enough for operation inside a machining center, the beams are arranged in two planes: The beams are split into reference beam and object beam in the bottom plane and combined onto the camera in the top plane later on. Using a mechanical standard interface according to DIN 69893 and having a very compact size of 235mm x 140mm x 215mm (WxHxD) and a weight of 7.5 kg, HoloCut can be easily integrated into different machine tools and extends no more in height than a typical processing tool.
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