Hans Dieter Tholl

Determination of the mean refractive index and thickness of dichromated gelatin holographic films using the thin film resonance method

The precise knowledge of the mean refractive index and the thickness of holographic films is important in applications such as polarization-sensitive holographic optical elements or substrate-mode holography. We present results of the measurements of the mean refractive index and the thickness of dichromated gelatin films before exposure, uniformly exposed films, and holograms. The measurements are based on the thin film resonance method. The interference between the two waves reflected at the air-film surface and the film-substrate interface modulates the reflectivity of the holographic film as a function of the angle of incidence. The frequency and the amplitude of this modulation are analyzed in order to determine the optical parameters. With the measured mean refractive index and the thickness as input we simulated the angular response of volume gratings and compared the results of the modelling with experimental data. The excellent agreement between the simulated and measured diffraction efficiencies confirm the applicability of the method to holographic films.

Development, manufacturing, and integration of holographic optical elements for laser Doppler velocimetry applications

Techniques specifically developed for the design and manufacturing of specialized holographic optical elements (HOEs) that are used in the fabrication of integrated optical systems for laser- Doppler velocimetry (LDV) applications are presented. The specialized HOEs needed for the construction of an integrated LDV-optics are: beam-splitters, lenses and/or lens arrays, phase correction plates and the corresponding waveguide structures. The HOEs are designed by means of computer programs that facilitate the optimization of the recording geometry and provide information for the correction procedures needed for optimized performance. The HOEs are recorded in dichromated gelatin films and are subsequently subjected to chemical and thermal after-treatment. These procedures guarantee the realization of HOEs with high diffraction efficiency and low scattering losses. The optimized holographic process has been previously described. In this report we discuss the following specialized HOEs and their specific characteristics as required by the LDV applications.

Two-wave coupled-wave theory of the polarizing properties of volume phase gratings

The subject matter of this report is the analysis of the dependence of the diffraction efficiency of volume phase gratings on the state of polarization of the read-out wave. I revise the derivation of the two-wave coupled-wave theory starting from Maxwells equations. The new formulation of the theory differs from the well-known results of Kogelniks theory mainly in three aspects: (1) The new theory incorporates the effect of form birefringence caused by the layered structure of the refractive index profile. (2) The dephasing parameter which describes the dephasing between the read-out wave and the signal wave has a definite parity. Consequently, the new theory is invariant under the reciprocity transformation of optics. (3) The grating strength for the p-polarization depends not only on the read-out geometry but on the profile of the refractive index as well.

Infrared reflectometry of skin: Analysis of backscattered light from different skin layers

We have recently reported infrared spectroscopy of human skin in vivo using quantum cascade laser excitation and photoacoustic or photothermal detection for non-invasive glucose measurement . Here, we analyze the IR light diffusely reflected from skin layers for spectral contributions of glucose. Excitation of human skin by an external cavity tunable quantum cascade laser in the spectral region from 1000 to 1245cm-1, where glucose exhibits a fingerprint absorption, yields reflectance spectra with some contributions from glucose molecules. A simple three-layer model of skin was used to calculate the scattering intensities from the surface and from shallow and deeper layers using the Boltzmann radiation transfer equation. Backscattering of light at wavelengths around 10μm from the living skin occurs mostly from the Stratum corneum top layers and the shallow layers of the living epidermis. The analysis of the polarization of the backscattered light confirms this calculation. Polarization is essentially unchanged; only a very small fraction (<3%) is depolarized at 90° with respect to the laser polarization set at 0°. Based on these findings, we propose that the predominant part of the backscattered light is due to specular reflectance and to scattering from layers close to the surface. Diffusely reflected light from deeper layers undergoing one or more scattering processes would appear with significantly altered polarization. We thus conclude that a non-invasive glucose measurement based on backscattering of IR light from skin would have the drawback that only shallow layers containing some glucose at concentrations only weakly related to blood glucose are monitored.

A quantum cascade laser-based goniometer for the determination of tissue optical properties in the mid-infrared

A detailed knowledge of optical properties of tissue is important for the development of non-invasive monitoring systems for clinical practice. However, tissue optical properties are rarely known in the mid-infrared wavelength range. Goniometry offers an opportunity to determine in particular the scattering properties of tissue. Here, a custom-built setup is presented for goniometric measurements in the mid-infrared based on quantum cascade lasers.

Stacked volume holograms as light-directing elements

Holographic optical elements are utilized in daylighting systems as light directing elements. The holograms can be fabricated on thin foils which are laminated between glass panes. The function of the holograms is limited by dispersion. Especially for large angles of incidence only a small portion of the solar spectrum is diffracted by a single hologram. Thus the redirected sunlight changes color. In this paper we show how the color changes can be minimized by using a stack of volume holograms. Each hologram diffracts a different portion of the solar spectrum into the same direction. The diffracted waves are superimposed in order to generate white light according to the additive color theory. The case of two holograms operating in the blue and red portion of the visible spectrum is analyzed theoretically and realized experimentally. Measurements of the diffraction efficiency as a function of wavelength are presented for different angles of incidence. From these measurements the color performance and the angular sensitivity of the stack is inferred.

Efficiency and bandwidth analysis of holographic glazing materials in the conical diffraction configuration

The redirection of light by holographic glazing is based on diffraction. The intensity and the color of the redirected light depend strongly on the direction of the diffracted waves. The paper presents a model for the design of volume holograms as light directing elements which takes into account the 3D character of the diffraction process. Formulas for the Bragg- wavelength, the bandwidth, and the diffraction efficiency are given. It is shown how to use conventional photometric formulas in order to calculate the illuminance and the color distribution on surfaces illuminated by the diffracted light. Finally, the design method is demonstrated for a single volume grating incorporated into a window as a light directing element.