Laser-generated broadband antireflection structures for freeform silicon lenses at terahertz frequencies.

We present a flexible technology to generate broadband antireflection (AR) structures for the terahertz (THz) frequency range on planar and curved surfaces of silicon optics. Ultrashort laser pulses are used to ablate the surface to form a pattern of conical pillars with a period of 30 µm. These subwavelength structures act as an effective medium with gradual transition of the refractive index from air to silicon, which reduces the Fresnel reflection losses. The characterization with the THz time-domain spectroscopy system shows an AR effect for a frequency range of 0.1-1.5 THz with a maximum enhancement of the spectral amplitude by ca. 32% at 0.4 THz for planar surfaces. In addition, we demonstrate laser-generated AR structures on convex silicon lenses of both photoconductive emitter and detector devices. Here, the THz pulse amplitude can be increased by about 28%, and single frequencies even show an improvement of the spectral amplitude up to 58%.

Resolution limitations for tailored picture-generating freeform surfaces.

Picture-generating freeform surfaces are able to generate a picture in a defined plane by incoherent beam shaping comparable to illumination purposes. No classical imaging is performed. Therefore the classical Rayleigh criterion of the diffraction limit cannot be applied. In this paper, we investigate the physical light formation of picture-generating freeform surfaces using Fresnel-Huygens-based simulations. A criterion for the diffraction limit was found. The resolution of such surfaces is significantly inferior to the resolution of classical imaging systems. However, in many cases, such systems are limited by the geometrical resolution. The influence of those two limitations were examined and a maximum of resolution, being limited by diffraction and by geometrical parameters can be found.

Terahertz line detection by a microlens array coupled photoconductive antenna array.

We present THz ultrashort pulse detection by a photoconductive antenna array consisting of 16 photoconductive antennas. The efficient excitation of the photoconductive antennas has been realized by a microlens array which generates 16 single spots from the exciting fs-laser beam. This combination of optoelectronics and microoptics improves the detection efficiency by an order of magnitude in comparison to an excitation by a line focus.

Multichannel balanced electro-optic detection for Terahertz imaging.

We present THz imaging with 1D electro-optic sampling of ultrashort THz pulses by multichannel balanced detection. Using a lock-in technique, it combines the advantage of a high signal to noise ratio along with the fast acquisition time of multichannel detection. The object is probed by a line focus and the resolution can be adjusted. The performance of the system is demonstrated exemplarily by imaging two objects.

G-scan--mobile multiview 3-D measuring system for the analysis of the face.

The development of optical 3-D measuring techniques and their use in industrial quality assurance, in design, and for rapid prototyping has experienced strong growth. A large number of optical 3-D measuring methods and systems are on the market in dentistry. CAD/CAM production has become firmly established in dental medicine, not least due to the systematic introduction of the Cerec technique and the digiDent method. The scanners on which these technologies are based are designed for a relatively small measuring area. To be able to measure and three-dimensionally assess the face--and the numerous changes in the face/forehead/neck region--it was necessary to design and develop a self-calibrating measuring system with gray code for clinical use: the G-Scan measuring system. Objects up to a size of 500 x 500 x 400 mm can be acquired three-dimensionally with it, with a measuring inaccuracy of 10 to 70 microm in a typical measuring time of 15 s. The present article describes the measuring principle, the system parameters, and the features of the new measuring system, and illustrates the measuring results on 3-D displays of the face in static occlusion and in functional occlusion positions.

Resolution enhancement in a reflection-type confocal microscope with a phase-conjugate mirror.

Theoretical and experimental results for a new reflection-type confocal microscope with a phase-conjugate mirror (PCM) are presented. The microscope achieves better lateral and axial resolution than the conventional confocal microscope. The observation volume is reduced considerably (approximately 63%). Owing to the properties of the PCM, the system is self-aligning.

Investigation of a double-pass confocal scanning microscope with a self-pumped phase-conjugate mirror.

Investigations of a double-pass scanning microscope with a self-pumped phase-conjugate mirror are presented. The microscope achieves lateral- and axial-resolution enhancements compared with the conventional confocal transmission microscope and has the advantages of self-alignment and aberration compensation owing to the properties of a phase-conjugate mirror. Using a self-pumped phase-conjugate mirror makes it possible to achieve a high scan rate, which is essential to observing objects by a confocal microscope.

Optimum lens aperture in phase-shifting speckle interferometric setups for maximum accuracy of phase measurement.

Negative exponentially distributed intensities of speckle fields seem unfavorable in terms of precision metrology, if interferometric setups are involved with a saturable photodetector and an analog-to-digital converter that imposes a finite resolution. By spatial integration, extended detector apertures modify the intensity distribution toward a less awkward function. However, because the detector aperture also integrates over points of rapidly changing speckle phases, this is done at the expense of a lower modulation of measured intensity during phase shift. An optimum set of parameters is calculated here, consisting of values for the lens aperture, the mean speckle intensity, and the beam ratio. The remaining phase-measurement error assumes its minimum of 10.6 mrad when the space-bandwidth product of the lens-detector system (thus concerning the lens aperture) is 0.31, the mean speckle intensity is 1/11 of the saturation intensity, and the reference intensity is four times higher than the mean speckle intensity. The 90 degrees phase-shift algorithms with either three, four, or five frames turned out to be quite powerful, even with interference signals of rather poor modulation. Not needing a very small lens aperture is interesting, because stopping down the lens is a trade-off with the limited power of the laser.