Interferometry as a highly sensitive non-invasive optical diagnostic tool needs intrinsically a thermal and mechanical well controlled environment. In opposition to the situation on ground space applications have severe constraints concerning volume, mass, modularity, etc.. This results in a more complex structure of an optical/mechanical set-up connected with stability problems, e.g. a folded optical path passing through more than one structural element in a more or less uncontrolled thermal environment. Thermo-mechanical deformations of the set-up can lead to significant errors in the resulting interferograms, especially for long term measurements, e.g. in crystal growth experiments. These deformations are subject to active compensation and alignment techniques, respectively. In this paper an active as well as a passive system developed for the compensation of optical effects caused by thermal induced deformations in space-borne interferometers is presented. The active system is able to detect wavefront tilting and curvature errors and to compensate them by means of piezoelectric driven optical components. An interferometer concept including Holographic Interferometry, ESPI and Shearing Interferometry and the misalignment detection as well as the compensation system are realised in a breadboard based on the interferometer design which will be integrated in the Fluid Science Laboratory (FSL) of the International Space Station (ISS). Extensive tests using the integrated interferometers show the suitability of the proposed compensation technique, not only for experiments in space but also for ground applications.
The collimation of strongly diverging laser beams emitted by diode lasers is performed with micro-optical components. In order to obtain a good beam profile high-quality cylindrical micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is costly or inaccurate with respect to the required accuracy of the lens shape. In conventional interferometry the resulting interferogram has to be imaged onto the CCD-target. The imaging lenses in the set-up can lead to additional wavefront aberrations and therefore to measurement errors. Additionally, conventional techniques are using phase shifting techniques for the evaluation of the interferograms. These techniques require at least three phase shifted interferograms which leads to a higher experimental and temporal effort. Digital Holography is an advanced optical diagnostic tool mainly used for surface deformation analysis, measurement of refractive index variations and particle analysis in transparent media: Holograms are stored electronically without any imaging optics and the reconstruction is performed by numerical methods. Due to the reconstruction process a numerical representation of the recorded wavefront can be evaluated including amplitude and phase from one hologram. In this paper Digital Holography as a measurement tool for the characterization of micro-optical components is presented, which has some advantageous properties with respect to other interferometric techniques. An analysis of the resolution of digital holography is performed in order to optimize the efficiency of this technique for the characterization of microlenses. The application of Digital Holography leads to a simple and robust measurement tool for shape measurement of microlenses which is demonstrated with two exemplary experiments characterising typical microlenses.
Laser cladding is an innovative surface treatment process which has several advantageous properties like a reduced material distortion compared to conventional techniques. In this technique the cladding material is fed as a powder through the laser beam to the melt pool. For an optimisation of this process with respect to treatment time and efficiency a characterisation of powder size, distribution and velocity is crucial.
Holographic particle image velocimetry is a powerful tool for characterisation of particle distributions with respect to size, 3D-position and velocity. Due to the holographic recording principle 3D-information can be evaluated from just one hologram. Its major drawback, the time-consuming development and repositioning of the hologram plates, can be avoided using the well-known technique of digital holography. In this case the hologram is recorded by a CCD-camera and reconstructed numerically.
Common digital holographic particle measurements are performed using an inline configuration in order to minimise the experimental effort. In this case the measurements are limited to low-density particle fields due to increased noise generated by an overlap of real and virtual image in the reconstruction process.
In this paper the application of off-axis digital holographic particle velocimetry to the characterisation of powder distributions in a laser cladding process is presented. Besides the experimental realisation special emphasis is given to the numerical reconstruction of the 3D-position and velocity of the particles. In extensive tests the suitability of the proposed technique is demonstrated. In the powder measurements up to 300 particles are detected with diameters of about 100μm and characterised with respect to position in a volume of about 1cm3 from just one hologram. In addition the speed of the particles is determined by double pulse measurements.
Novel types of thin-film microoptical components have been found very advantageous for beam shaping of high-power and ultrashort-pulse lasers. Measuring techniques, nonlinear optics, materials processing, and further advanced photonic applications, will benefit from specific advantages. Compared to conventional microoptics, low dispersion and absorption as well as added degrees of freedom in structure and functionality are accessible. Single or multilayer designs, spherical and non-spherical profiles, extremely small angles, and flexible substrates enable key components for the tailoring of lasers in spatial, temporal, and spectral domain at extreme parameters. By vacuum deposition and selective etching transfer, a cost-effective fabrication of single or array-shaped refractive, reflective, or hybrid components is possible. During the last decade significant progress in this field could be achieved. Including very recent applications for spatio-temporal shaping and characterization of complex and non-stationary laser fields, the state of the art is presented here. Particular emphasis is put on the generation of localized few-cycle wavepackets from Ti:sapphire laser beams by the aid of broadband microaxicons. Special microoptics are capable of transforming vacuum ultraviolet radiation. Wavefronts of strongly divergent sources can be analyzed by advanced Shack-Hartmann sensors based on microaxicon-arrays. Single-maximum nondiffractive beams are generated by different approaches for self-apodizing systems. Prospects for future developments, like robust multichannel information processing with arrays of self-reconstructing X-pulses, are discussed.
Ultrashort-pulse single-maximum nondiffracting beams of microscopic radius and large axial depths are interesting for applications in nonlinear optics and spectroscopy, for acceleration and manipulation of particles, measuring techniques, materials treatment or information processing. Here we report on the experimental generation of such beams by self-apodized truncation of Bessel and pseudo-Bessel beams from a Ti:sapphire oscillator. Small angle operation was enabled by thin-film structures. To obtain self-apodization, the diameter of the truncating diaphragm was adapted to the first minima of Bessel distribution. The propagation of (a) Bessel beams of meter-range axial extension shaped by axicon mirrors, and (b) microscopic pseudo-Bessel beams of millimeter-range extension shaped by Gaussian-shaped microaxicon lenses was studied. In case (a), single-maximum beams of > 20 cm depth were produced. To generate comparable focal zones from Gaussian beams, a much larger distance (10x) is necessary, and axial stretching of spectrum destructs the temporal structure. In case (b), the focal zone length was increased by a factor of >5 compared to a Gaussian beam. Arrays of truncated Bessel beams were generated as well. The experimental results indicate that truncated Bessel beams enable more compact setups than corresponding Gaussian beams and are in particular advantageous for ultrashort pulses. Further improvements are possible by combining coherent addition in resonators with truncation outcoupling.
For spatiotemporal transformation and processing of ultrashort-pulse laser beams, serious design constraints arise from dispersion and diffraction. At pulse durations in 10-fs range, temporal and spatial parameters of propagating wave packets are coupled and significant inhomogeneities appear. To enable a controlled shaping or encoding and a reliable detection or decoding with 2-D spatial resolution, specific advantages of thin-film micro-optical arrays can be exploited. Transmitting and reflecting components of extremely small conical angles are used to generate multiple nondiffracting beams and self-imaging phase patterns. With novel-type metal-dielectric microaxicons, low-dispersion reflective devices are realized. Beam propagation is simulated numerically with Rayleigh-Sommerfeld diffraction theory. For ultrafast time-space conversion, matrix processors consisting of dielectric thin-film microaxicons are tested. Transversally resolving linear and nonlinear autocorrelation techniques are applied to characterize the space-time structure of localized few-cycle wave packets shaped from Ti:sapphire laser beams at pulse durations down to 8 fs. Bessel-like X waves are generated and their propagation is studied. In combination with autocorrelation, wavefront analysis of ultrashort-pulse lasers with Bessel-Shack-Hartmann sensors operated in reflection setup is demonstrated.
Different interferometric techniques are required to cover most of the scientific needs in the field of fluid dynamics science in microgravity research. The Fluid Science Laboratory (FSL), currently under upgrade for the Columbus Orbital Facility of the International Space Station (ISS), shall provide Holographic Interferometry, Digital Holography, Electronic Speckle Pattern Interferometry (ESPI) and Shearing Interferometry among other diagnostic tools. On earth, these highly sensitive interferometers are operated in a thermal and mechanical controlled environment. In opposition to the situation on ground the multi-user facility of the FSL has severe constraints for what concerns volume, mass, modularity, operational needs and its environment. This results in a three-dimensional modular drawer structure for the design of the optical-mechanical set-up, where performance limitations must be expected compared to systems on ground. In a rather uncontrolled thermal environment onboard the ISS this leads to misalignment due to thermo-mechanical changes of the Aluminum structure during experiment runs which finally result in interferogram distortions and therefore to significant measurement errors. In this paper we report about a misalignment detection- and active compensation concept developed on the basis of a thermo-mechanical and optical analysis of the set-up. The detection system is based on a simplified Hartmann-Sensor. It is able to separate wave front tilt and curvature errors due to misalignments of the interferometers itself from the effects caused by the experiment. The closed-loop compensation system uses optical components of the set-up driven by piezoelectric actuators. Due to its active approach this concept allows for the real time accessibility of the experimental effects in the framework of “Telescience.”
Extensive functional tests as well as representative thermal tests show the suitability of the proposed technique to compensate interferogram distortions due to thermal-mechanical deformations. Thus, it is able to ensure interferometric measurements with sub-wavelength accuracy onboard the ISS.
With increasing globalization many enterprises decide to produce the components of their products at different locations all over the world. Consequently, new technologies and strategies for quality control are required. In this context the remote comparison of objects with regard to their shape or response on certain loads is getting more and more important for a variety of applications. For such a task the novel method of comparative digital holography is a suitable tool with interferometric sensitivity. With this technique the comparison in shape or deformation of two objects does not require the presence of both objects at the same place. In contrast to the well known incoherent techniques based on inverse fringe projection this new approach uses a coherent mask for the illumination of the sample object. The coherent mask is created by digital holography to enable the instant access to the complete optical information of the master object at any wanted place. The reconstruction of the mask is done by a spatial light modulator (SLM). The transmission of the digital master hologram to the place of comparison can be done via digital telecommunication networks. Contrary to other interferometric techniques this method enables the comparison of objects with different microstructure. In continuation of earlier reports our investigations are focused here on the analysis of the constraints of the setup with respect to the quality of the hologram reconstruction with a spatial light modulator. For successful measurements the selection of the appropriate reconstruction method and the adequate optical set-up is mandatory. In addition, the use of a SLM for the reconstruction requires the knowledge of its properties for the accomplishment of this method. The investigation results for the display properties such as display curvature, phase shift and the consequences for the technique will be presented. The optimization and the calibration of the set-up and its components lead to improved results in comparative digital holography with respect to the resolution. Examples of measurements before and after the optimization and calibration will be presented.
Spatially resolved wavefront sensing and time-resolved autocorrelation measurement of ultrashort pulses are usually separated procedures. For few-cycle pulses with significant spatial inhomogeneities and poor beam quality, a fully spatio-temporal beam characterization is necessary. Here we report on a new concept for a joint two-dimensional mapping of local temporal coherence and local wavefront tilt based on the combination of collinear autocorrelation and Shack-Hartmann wavefront sensing. Essentially for this "wavefront autocorrelation" is a splitting of the beam into a matrix of Bessel-like sub-beams by an array of thin-film microaxicons. The sub-beams are further processed by a two-dimensional collinear autocorrelation setup. The second harmonic distribution of sub-beams at a defined distance is imaged onto a CCD camera. The nondiffractive sub-beams ensure an extended depth of focus and a low sensitivity towards angular misalignment or axial displacement. With low-dispersion small-angle refractive-reflective shapers, wavefront-sensing of Ti:sapphire laser wavepackets was demonstrated experimentally for the first time.
Recent progress in laser beam shaping and characterization with novel-type thin-film microoptics is presented. These novel microoptical devices offer several distinctive advantages, such as a short optical path, small angles, low roughness or multilayer design. These features allow shaping of laser beams at extreme parameters with respect to spectrum, angular distribution, intensity, or pulse duration. Particular emphasis is laid on (i) hybrid components for high-power diode laser collimation, (ii) spatio-temporal shaping of localized few-cycle wavepackets, and (iii) microoptics for the vacuum ultraviolet. For the fabrication of thin-film structures, vapor deposition with shading masks was used. To improve the efficiency of diode laser collimation, spatially variable AR coatings and integrated arrays of cylindrical microlenses were developed. Arrays of Bessel-like beams were generated from sub-10-fs Ti:sapphire laser pulses by refractive and reflective microaxicons. We further demonstrated the use of microaxicon arrays for spatially resolved autocorrelation of ultrashort pulses. Deposition and etching transfer of flat VUV-structures was studied. Finally, the generation of single-maximum nondiffracting beams by self-apodizing system design is discussed.
For spatio-temporal processing of ultrashort-pulse laser beams, design constraints arise from dispersion and diffraction. In sub-10-fs region, temporal and spatial coordinates of propagating wavepackets get non-separable. To enable controlled shaping and detection with spatial resolution, specific advantages of thin-film microoptical arrays are exploited. Transmitting and reflecting components of extremely small conical angles were used to generate multiple nondiffracting beams and self imaging patterns. With novel-type metal-dielectric microaxicons, low-dispersion reflective devices were realized. Beam propagation was simulated with Rayleigh-Sommerfeld diffraction theory. For time-space conversion, matrix processors consisting of thin-film microaxicons were tested. Transversally resolving linear and nonlinear autocorrelation techniques were applied to characterize the space-time-structure of localized few-cycle wavepackets shaped from Ti:sapphire laser beams at pulse durations down to 8 fs. Bessel-like X-waves were shaped and their propagation was studied. In combination with autocorrelation, wavefront analysis of ultrashort-pulse lasers with Bessel-Shack-Hartmann sensors operated in reflection setup was demonstrated.
Spectral interference caused by structured thin-film components has been used for shaping and characterization of few-cycle femtosecond laser beams. Array structures enable spatially resolved measurements of coherence and wavefront. The generation of spatially and temporally localized optical wavepackets with reflective and refractive axicons was demonstrated in theory and experiment.
The collimation of strongly diverging laser beams emitted by diode lasers is performed with aspherical micro-optical components. In order to obtain a good beam profile high-quality micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is not suitable, costly or inaccurate with respect to the required accuracy of the lens shape. Digital Holography as a measurement tool for the characterization of micro-optical components offers several advantageous properties with respect to other interferometric techniques, such as avoidance of aberrations introduced by imaging and magnification optics. The large numerical aperture of the microlenses under test leads to high fringe densities in the holograms which can not be resolved by CCD-detectors. In order to avoid this problem digital holography is combined with multiple wavelength and speckle techniques. A diffusing screen is placed directly behind the microlens in order to destroy the large divergence and at least two measurements with different wavelengths are performed for the recovery of the wavefront information. The speckle pattern in the numerical reconstruction of the wavefront reduces the accuracy of the resulting difference phase significantly. In this paper a technique for the reduction of speckle noise is proposed which is not based on classical filtering techniques such as median filters. Several holograms of the same object under test are recorded with different speckle patterns. A proper averaging taking into account the properties of the wrapped phases leads to a improvement of the accuracy up to 1/60 of the wavelength. Results of the characterization of aspherical microlenses using the new technique are presented.
The collimation of strongly diverging laser beams emitted by diode lasers is performed with aspherical micro-optical components. In order to obtain a good beam profile high- quality micro-lenses with a large numerical aperture compared to conventional lenses have to be applied. The characterization of these components using conventional interferometric techniques is not suitable, costly or inaccurate with respect to the required accuracy of the lens shape. In this paper Digital Holography as a measurement tool for the characterization of micro-optical components is presented, which has some advantageous properties with respect to other interferometric techniques. Results of the characterization of single cylindrical microlenses as well as microlens-arrays used for laser beam collimation and shaping are presented.
Micro-optical components such as microlenses and microlens arrays are of growing interest in the fields of laser beam shaping, optical processing and similar applications. The characterization of these components require a fast and robust measurement technique, especially for simultaneous inspection of lenses manufactured in array structures e.g. on silicon wafers.
In this paper a general geometric description of the optical methods for 3D coordinate measurement is presented. Similar to holographic interferometry this new approach is based on the concept of measuring sensitivity. As a special case the derived basic relation is applied to the fringe projection technique using a physical model of this measurement method. Moreover a geometric 3D model that contributes to a dramatic reduction of systematic distortions of measured 3D coordinates is presented. On the one hand this model is sufficiently general but on the other hand still easy to handle. It permits an explicit and direct determination of 3D coordinates from primary measuring data as well as a calibration of the measuring set-up using linear identification methods mainly. The described 3D model can be applied also with advantage to multiview registration tasks.