Ulf Krause

High-power laser projection displays

In this paper recent research on high-power laser displays for large display applications is reported. We discuss our compact high-power red-green-blue laser system for use as a light source based on a passively mode-locked solid-state laser with an optical parametric oscillator and sum- frequency mixing with very high efficiency and 19 W of white light. Concepts for power scaling to 35 W of white laser light are presented. A direct laser beam scanning technology is described which results in high resolution displays. Multi channel concepts as required in flight simulator applications are discussed. The laser light is modulated at 32 MHz using acousto-optic modulators with 75% diffraction efficiency. A high contrast ratio of 1:150,000 is achieved and has been successfully tested in novel planetarium applications. We also present performance and lifetime data of initial commercial laser display systems in the field.

Progress report on Safe VISITOR: approaching a practical instrument for terahertz security screening

As reported before,1, 2 Safe VISITOR (Safe VISible, Infrared and Terahertz Object recognition) is a German project to build a passive security camera which visualizes sub-mm wavelengths using cooled bolometer arrays. This camera could be used for a variety of application scenarios, such as airport screenings or to protect military camps. In all cases, a practical instrument requires ease of use, in particular a flexible installation and a straightforward usage by the security personnel. Here we present a new generation of Safe VISITOR designed to meet these requirements. The main condition for an effective operation is a high frame rate of the imager. Safe VISITOR is able to record videos up to 10 Hz, using a small array of superconducting bolometers in combination with an opto-mechanical scanner. The required cooling of the detector array is provided by a commercial pulse tube cooler with a second, self-contained cooling stage. The cooling cycle is completely automated; after 10 hours of initial cooling from room temperature the system can operate quasi-continuously. For imaging, a 50 cm diameter optics is used which is able to provide an object resolution of approximately 1.5 cm at 8 m distance. For a flexible installation, the object distance can be tuned manually between 7 and 10 m. Additionally, video streams from two commercial cameras are fused with the sub-mm stream: a CCD for visible light and a microbolometer for far infrared (14 μm). This combines the ability of identification of the person under test with the unprecedented temperature resolution at infrared and the almost perfect transmission at sub-mm. To assist a security official, all image data are displayed in various graphic renditions by a unified system software.

4.6-mJ-diffraction limited solid-state laser for industrial micromaterials processing

A novel Nd:YAG micro slab laser amplifier is presented and experimental results for Q-switched operation are discussed. The laser crystals are directly pumped by conduction cooled laser diodes with an ISO/FDIS 17526 extrapolated standard lifetime above 30,000 h. The diffraction limited cw output power of the amplifier is 50 W. In pulsed operation, the laser shows a pulse energy of 4.65 mJ and a pulse length of 12 ns FWHM at a pulse repetition rate of 6 kHz. Beam quality has been measured to be M2 = 1.1. The beam pointing stability was 10 μrad and the pulse-to-pulse stability was 1.5% rms in a linear-polarised beam.