Mathias Hain

Generation of doughnut laser beams by use of a liquid-crystal cell with a conversion efficiency near 100%

We present a novel technique for producing a doughnut laser beam by use of a liquid-crystal cell. It is demonstrated that the liquid-crystal cell exhibits an efficiency in energy conversion near 100%. One of the main advantages of this method is its capability of dynamic switching between a Gaussian mode and a doughnut mode of different topological charges. The liquid-crystal cell is also dynamically tunable over the visible and near-infrared wavelength range. These advantages make the device appealing for laser trapping methods used in single-molecule biomechanics and for optical guiding of cold atoms.

Fast switching liquid crystal lenses for a dual focus digital versatile disc pickup

We investigated two types of liquid crystal (LC) lenses, the convex LC lens and the adaptive LC lens. Both lens types were designed and fabricated for integration in a novel pickup head for dual layer digital versatile discs (DVDs). The polarization dependent focal length of LC lenses was used to read both DVD information layers simultaneously. For fast and near diffraction limited performance, a new circular electrode structure with 64 ring electrodes was studied. Short response times of 50 ms for blazed Fresnel lens profiles and less than 10 ms for binary lens profiles could be realized using the transient nematic effect. Both lens types were characterized in terms of diffraction limited operation, focus efficiency and switching time.

3D integral imaging using diffractive Fresnel lens arrays

We present experimental results with binary amplitude Fresnel lens arrays and binary phase Fresnel lens arrays used to implement integral imaging systems. Their optical performance is compared with high quality refractive microlens arrays and pinhole arrays in terms of image quality, color distortion and contrast. Additionally, we show the first experimental results of lens arrays with different focal lengths in integral imaging, and discuss their ability to simultaneously increase both the depth of focus and the field of view.

Effective spherical aberration compensation by use of a nematic liquid-crystal device

The next generation of optical data storage system beyond DVDs will use blue laser light and an objective lens with a high numerical aperture of 0.85 to increase storage capacity. Such high numerical aperture systems have an inherent higher sensitivity to aberrations. In particular, the spherical aberration caused by cover layer thickness tolerances and--more obvious--by dual-layer disks with a typical separation of approximately 20 microm between the two layers must be compensated. We propose a novel transmissive nematic liquid-crystal device, which is capable of compensating spherical aberration that occurs during the operation of optical pickup systems.

Integrated optical pickup for optical disks

We report on a new integrated optical pickup for double layer DVDs. The optics is almost integrated by means of diffractive optical elements. Dual focus as well as focal control is done by a liquid crystal cell.

Aberration Compensation Using Nematic Liquid Crystals

We have developed a novel transmissive nematic liquid crystal device which is capable of compensating spherical wavefront aberration that occurs during the operation of optical pickup systems. In order to increase the storage capacity, next generation optical data storage systems beyond CD and DVD will use according to the Blu-Ray specification (BD) blue laser light and an objective lens with high numerical aperture (N.A.) of 0.85. However, such high N.A. systems have an inherent higher sensitivity on aberrations. For example spherical aberration is inversely proportional to the wavelength and grows with the fourth power of N.A. of the objective lens. In an optical pickup system there are two sources for spherical aberration: The first one is the variation of the substrate thickness due to manufacturing tolerances under mass production conditions. The second one concerns disks with multiple data-layers, which cause spherical aberration when layers are switched, as the objective lens can only be optimized for a single layer thickness. We report a method for effective compensation of spherical aberration by utilizing a novel liquid crystal device, which generates a parabolic wavefront profile. This particular shape makes the device highly tolerant against lateral movement. A sophisticated electrode design allows us to reduce the number of driving electrodes down to two by using the method of conductive ladder mashing. Further evaluation in a blue-DVD test drive has been carried out with good results. By placing the device into an optical pick-up we were able to readout a dual-layer ROM disk with a total capacity of 50 gigabytes (GB). A data-to-clock jitter of 6.9% for the 80 μm and of 8.0% for the 100 μm cover layer could be realized.

Fast electro-optical lenses using liquid crystals

Summary form only. In many optical designs, movable optics are used to modulate, focus or redirect light fields. Solutions that allow more compact and shock resistant designs are needed. We present two electro-optical liquid crystal lenses which satisfy these requirements when focus tracking without any mechanical components.

Dual layer digital versatile disk pickup integrating opto-electronic and diffractive elements

Summary form only. A pickup using an optoelectronic, nematic LC lens and two diffractive optical elements (DOEs) was designed and a demonstrator model built. It was successfully demonstrated that the LC lens could focus onto both layers of a dual layer DVD. Focus switching of below 10ms were achieved. The working principle and the fabrication methods of the DOEs and the design and working of the LC lens are described. Measurements of the integrated pickup using the astigmatism method are presented.