Stefan Altmeyer

Non-interferometric, non-iterative phase retrieval by Green’s functions

In this paper a non-interferometric, non-iterative method for phase retrieval by Greens functions is presented. The theory is based on the parabolic wave equation that describes propagation of light in the Fresnel approximation in homogeneous media. Greens first identity will be used to derive an algorithm for phase retrieval considering different boundary conditions. Finally it will be shown that a commonly used solution of the transport-of-intensity equation can be obtained as a special case of the more general Greens function formulation derived here.

Refractive index determination of transparent samples by noniterative phase retrieval

We present a simple method to determine the refractive indices of transparent specimens. The refractive index of an object under investigation is received by evaluating the optical path difference introduced by the object, while taking into account geometric parameters. The optical path difference that corresponds to the phase distribution is obtained by a noninterferometric, noniterative phase retrieval method based on Greens functions. It will be shown that this technique is a highly accurate and quantitative method for refractive index determination.

Optics and information technology

Information and communication is the backbone of modern society. The underlying technology can be organised in four major tasks: processing and generation of information, transfer of information, visualisation of information and storage of information.

Quantitative determination of the optical properties of phase objects by using a real-time phase retrieval technique

The transport-of-intensity equation (TIE) describes a deterministic relation between the intensity distribution in different focal planes and the corresponding phase distribution. A Greens function solution of the TIE is used to retrieve the phase distribution of an object considering specific boundary conditions. This leads to an accurate reconstruction of the properties of phase objects, e.g. the refractive indices and thus the numerical aperture (NA) of optical fibers. The required intensity distributions are captured simultaneously by the use of a multi-camera microscope. The TIE is solved using a computer algorithm, which can be massively parallelized. This offers the application of general purpose computation on graphics processing units (GPGPU). Therefore real-time reconstruction of the phase distribution is possible.

Characterization of high power multimode combiners

In this investigation, a 1.5 W Nd:YAG laser with good beam quality is utilized to characterize the output numerical aperture (NA) of the multimode combiners with reference to the input NA. The transmission of the combiners, being one of the factors to define the brightness, is also measured when the light is coupled in with a 0.15 NA. Hence, commercially available couplers are characterized for the first time according to brightness conservation and tested up to a power level of more than 580 W of coupled power.

Mechano-optical modulator based on a rotating optical flat applied to simultaneous holographic multiplexing of gratings

Simultaneous holographic multiplexing methods are used to store multiple holograms concurrently into one holographic volume. Compared to serial exposure, one advantage is the higher processing speed. Additionally, for parallel exposure the different gratings develop in a linear relation to one another, although the developing process itself is nonlinear caused by the holographic photopolymer. A mechano-optical modulator using a rotating optical flat is introduced to enable economic single-shot exposure of centimeter to decimeter-sized holograms utilizing only one laser. The modulator specifically suppresses undesired interference, which occurs with this type of simultaneous exposure. A device concept of the modulator is presented. The application to simultaneous holographic multiplexing is theoretically discussed and experimentally demonstrated. The experimental results are compared to simulations and are evaluated. The functionality and practicability are proven, giving rise to a new design concept and a wider range of applications.

Fringe visibility degeneration utilizing a rotating optical flat for holographic multiplexing purposes

A device concept utilizing a rotating optical flat to degenerate the fringe visibility for simultaneously holographic multiplexing purposes is presented. The device basically consists of a rotating slanted optical flat. The rotation induces a periodically varying phase shift to a transmitting wave, which causes a specific degeneration of the maximum achievable time-average visibility. This property can be used to expose independent gratings simultaneously into one photopolymer film with a single coherent light source. Theoretical investigations of the resulting time-average visibility are carried out and presented in detail. Experimental results are compared to the theoretical findings and discussed. The presented device is capable of decreasing the visibility to the desired degree. Thus, it is well suited for holographic multiplexing purposes. A conceptual setup utilizing the device for holographic angular multiplexing is suggested.