Thomas Hellmuth

Simultaneous Measurement of Dispersion, Spectrum, and Distance with a Fourier Transform Spectrometer

A Fourier transform spectrometer is used to simultaneously measure distance, dispersion and spectrum. It is shown that short coherence interferometry has the potential to measure the three-dimensional distribution of the spatial structure of a sample with a resolution determined by the coherence length of the light source, absorption spectrum with a resolution of 1 cm-1 and a dispersion with a resolution of up to 10-5.

High-speed low-coherence interferometer for optical coherence tomography

Optical coherence tomography (OCT) is a noninvasive imaging technology, which provides subsurface imaging of biological tissue with a resolution in the micrometer range. OCT sensors either work in the time or Fourier domain. We present a new interferometer setup based on a fiber double pinhole arrangement. Two fibers are placed in parallel similar to Young’s two-pinhole interference experiment with spatial coherent and temporal incoherent light. The interference pattern is observed on a linear CCD-array. A complete A-scan can be derived from a single readout of the CCD-array. The experimental setup is described in detail. The main parameters of the setup are derived theoretically and compared with experiments. First images of technical and biological samples are presented.

Noncontact measurement of the optical imaging quality of an eye

The optical imaging properties of the eye are both determined by optical aberrations and deviations of the eye length. We report on a device for simultaneous measurement of aberrations and eye length combining a Shack Hartmann sensor and a short coherence interferometer. The short measurement duration minimizes disturbing influences like tear film or eye length changes. The low irradiance of the atients eye minimizes exposure time.

Contrast and resolution in optical coherence tomography

Optical coherence tomography (OCT) is an interferometric scanning technique. The instrument core is a Michelson interferometer comprising a moving reference mirror and a backscattering object representing the second mirror. The light source coherence length determines the depth resolution of this imaging method. OCT provides images with improved contrast and depth resolution compared to confocal microscopy in the case of turbid objects. Here, the imaging performance and contrast mechanisms of OCT are analyzed in terms of linear system theory. It is shown that OCT has a band pass filter behavior. The system transfer function is compared to the transfer function of a coherent confocal microscope.

Spherical Aberration In Confocal Microscopy

Scanning confocal microscopy has become an important tool in the field of optical sectioning and 3D-reconstruction of biological objects. Usually these samples are embedded in dispersive media, introducing depth dependent spherical aberration. Depth and lateral resolution are therefore impaired. We discuss this problem theoretically and show experimental results.

3-dimensional scanning of grinded optical surfaces based on optical coherence tomography

In the manufacturing process of aspheric glass lenses the grinding step plays a key role both in respect of the final quality of the polished lens as well as in respect of manufacturing costs. Therefore, the form of the grinded surface must be measured with high precision. The typically used tactile measuring machines provide sufficient precision regarding depth resolution but suffer from limited lateral resolution. In particular it is not possible to detect surface and sub-surface damages which essentially influence the duration of the subsequent polishing process. In order to detect these damages we set up and tested a scanning short-coherence interferometer very similar to optical coherence tomography. The aspheric lens under test is mounted on a rotation stage which can be translated in the lateral direction. The sensor beam of the interferometer is focused onto the sample and can be moved along the axial direction. The precision of the depth measurement is 0.25μm, the lateral positioning precision is 2μm. The system is used to optimize the grinding process for aspheric lenses to minimze sub-surface damages and therefore to maximize processing speed.

Simulation of focus invariant optical systems

Normally, depth of focus and resolution of an optical system are complementary parameters. According to an established technique known as wavefront coding the depth of focus can be enhanced by inserting a phaseplate with a cubic surface function into the exit pupil plane of an optical system. Although contrast is reduced the image quality can be restored very efficiently by inverse filtering because the modulation transfer function (MTF) almost does not change when the system is defocused. In addition the MTF has no zero crossings. Therefore inverse filtering has no singularities. Thus, three dimensional objects can be imaged with microscopes with a large depth of focus. The waveoptical performance of a commercial microscope equipped with a cubic phase plate has been simulated. The simulation results are discussed and compared with experimental data. The system has been analyzed both regarding effects specifically related to the phase plate design and regarding effects related to the optical properties which result from the combination of the phase plate with the microscope system. A variable phase plate design is also presented which can be adapted to specific objects and optical systems.

Multiphoton Ionization of Atoms

The alkali atoms are especially attractive for the investigation of multi-photon processes. The ionization potentials are small so that the energy of only a few visible photons is sufficient to ionize the atoms. Therefore, dye lasers can be used for the experiments and, in particular, make it possible to investigate, among other things, the wavelength dependence of the photoionization cross sections. On the other hand, the simplicity of the alkali energy level scheme is an advantage for the theoretical understanding and interpretation of the results.

Research on fabrication of aspheres at the Center of Optics Technology (University of Applied Science in Aalen)

The Center of Optics Technology at the University of Applied Science, founded in 2003, is part of the School of Optics and Mechatronics. It completes the existing optical engineering department with a full optical fabrication and metrology chain and serves in parallel as a technology transfer center, to provide area industries with the most up-to-date technology in optical fabrication and engineering. Two examples of research work will be presented. The first example is the optimizing of the grinding process for high precision aspheres, the other is generating and polishing of a freeform optical element which is used as a phase plate.

Optimisation and characterisation of parabolic membrane mirrors

MOEMS-based thin silicon membrane mirrors with a useable diameter of 5mm and fast (up to 1kHz) tunable focal length (80 mm to 1m) have been realized. A ring shaped counter electrode is used to achieve a parabolic membrane deformation by electrostatic forces. A circular kerf at the outer perimeter of the membrane provides a soft suspension to the rim and thus reduces the needed driving voltage. FEM has been used for optimisation of the design, especially of the soft suspension, which is realized by a controlled thinning of the outer rim of the Si-membrane. A critical issue for demanding applications is the membrane distortion induced by material stress and the fabrication process. Membrane residual stress reduction has been obtained by using SOI-technology (c-silicon) and by optimisation of the Al deposition process (Al-coated Si-membrane). For dynamic tests of the optical mirror properties a stroboscopic interferometer has been realized. A pulsed laser diode with a pulse duration of 10μs is used as a light source which is synchronized with the modulated electrical field driving the membrane mirror. The interference pattern is recorded with a CCD and evaluated with conventional phaseshift techniques. The geometry is similar to a Mach-Zehnder interferometer. The reference path length can be varied with a piezoceramic to induce the phase shift.