Wave-optical modeling beyond the thin-element-approximation.

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corresponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

Reflectivity enhanced refractive index sensor based on a fiber-integrated Fabry-Perot microresonator.

We discuss a fiber-integrated refractive index sensor with strongly improved detection performance. The resonator has been implemented by means of focused-ion beam milling of a step index fiber and shows a sensitivity of about 1.15µm/RIU. Coating the resonator walls led to a strongly improved mirror reflectivity by a factor of about 26. Design rules for device optimization and a detailed mathematical analysis are discussed, revealing that the sensor operates as an optimized Fabry-Perot resonator. We also show that the performance of such kind of Fabry-Perot sensors is, in general, limited by the detection limit function - a quantity depending on the cavitiy's finesse and on the measurement capabilities used.

Fiber Bragg gratings in hole-assisted multicore fiber for space division multiplexing.

In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and temperature sensitivities of all of the Bragg peaks is also included.

A miniature temperature high germanium doped PCF interferometer sensor.

We report in this paper a high thermal sensitivity (78 pm/°C) modal interferometer using a very short Photonic Crystal Fiber stub with a shaped Germanium doped core. The Photonic Crystal Fiber is spliced between two standard fibers. The splice regions allow the excitation of the core and cladding modes in the PCF and perform an interferometric interaction of such modes. The device is proposed for sensitive temperature measurements in transmission, as well as in reflection operation mode with the same high temperature sensitivity.

Three-dimensional light bullets in arrays of waveguides.

We report the first experimental observation of three-dimensional light bullets, excited by femtosecond pulses in a system featuring quasi-instantaneous cubic nonlinearity and a periodic, transversally modulated refractive index. Stringent evidence of the excitation of light bullets is based on time-gated images and spectra which perfectly match our numerical simulations. Furthermore, we reveal a novel evolution mechanism forcing the light bullets to follow varying dispersion or diffraction conditions, until they leave their existence range and decay.

Generation of flattop pump pulses for OPCPA by coherent pulse stacking with fiber Bragg gratings.

We present a simple and robust pulse shaping device based on coherent pulse stacking. The device is embedded in a polarisation maintaining step index fiber. An input pulse is sent through a fiber optical circulator. Up to four pulse replicas are reflected by fiber Bragg gratings and interfere at the output. Temperature control allows tuning of the relative pulse phases of the sub-pulses. Additionally fine tuning of the sub-pulse amplitudes is demonstrated. We experimentally generated 235 ps and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse durations up to several nanoseconds.

Photonic crystal fiber with a dual-frequency addressable liquid crystal: behavior in the visible wavelength range.

Wave-guiding in the visible spectral range is investigated for a micro-structured crystal fiber filled with a dual-frequency addressable nematic liquid crystal mixture. The fiber exhibits a solid core surrounded by just 4 rings of cylindrical holes. Control of the liquid crystal alignment by anchoring agents permits relatively low attenuation. Samples with different anchoring conditions at the interface of the silica glass and the liquid crystal show different transmission properties and switching behavior. Polarization dependent and independent fiber optic switching is observed. Due to a dualfrequency addressing scheme, active switching to both states with enhanced and reduced transmission becomes possible for planar anchoring. Even a non-perfect fiber shows reasonable transmission and a variety of interesting effects.

Two-dimensional high-precision fiber waveguide arrays for coherent light propagation.

Fiber waveguide arrays can be applied as a very useful tool for the investigation of effects in discrete optics. The observation of coherent propagation in such discrete waveguide arrays requires, however, high structural precision and great material homogeneity. The fabrication of such a fiber array with close tolerances compared to conventional fiber technology is discussed. Linear propagation effects are modeled for an ideal fiber waveguide array and are compared with experimental results. The good agreement of these results with each other indicates the applicability of such fiber waveguide arrays in studying linear and non-linear properties in discrete optics.

Computation of local directivity, speed of sound and attenuation from ultrasonic reflection tomography data.

Ultrasound reflection tomography based on the compound scan principle, allows one to produce reflectivity images with high quality and reproducibility. In this paper, methods are discussed on how to extract additional physical parameters from the same set of reflection data for medical applications. Experimental results from a phantom object and in-vivo measurements illustrate the capabilities of such tomographic reflection systems.

Improving binary phase correlation filters using iterative techniques.

An iterative technique has been used to improve the design and performance of the binary phase version of a tandem-component correlation filter. The results are compared to a regular matched filter, a phase-only filter (POF), and a binary phase POF, in terms of optical efficiency, SNR, and peak correlation intensity.

Two-bit correlation.

We examine simplified correlation through the use of binary data. For the sake of economy we represent the real input u(x) by one amplitude bit per pixel and the correlation filter by one phase bit per Fourier pixel. With such a crude representation of input and reference, it is possible indeed to obtain fairly good correlation results.

High-efficiency hybrid computer-generated holograms.

A hybrid hologram is constructed which combines the flexibility of a computer-generated hologram (CGH) with the high-diffraction efficiency of volume phase holograms. The new hologram is constructed by using a spatially filtered wave front from a conventional 246- x 256-cell CGH as the object wave for the recording of a volume hologram. Theoretical and experimental results show that simultaneous high-diffraction efficiency and reconstruction fidelity are possible.

Beam switching with binary single-order diffractive gratings.

A new type of a micro-optomechanical dielectric switch in free-space configuration for visible light is proposed and experimentally demonstrated. The combination of scalar and rigorous diffraction analysis of Gaussian beams predicts high high switching efficiency with a single/noise ratio of better than 40 dB.

Image formation by inversion of scattered field data: experiments and computational simulation.

The 3-D scattering potential of microscopic objects is reconstructed as well as the 2-D equivalent object from amplitude and phase data of the scattered field. The experimental setup and results are presented and discussed alongside corresponding computer simulations. In the computational simulation, several small coated spheres were assumed, and the scatter field data were determined by the Mie diffraction theory. In the experiment, a small sphere of 40-microm diam was used, and the scattered field was measured by interferometric methods. Two alternative methods were used to record the scattered field data; their sensitivity to phase quantization is discussed. Since in the experimental situation the scattered field data can only be determined in a very restricted range, a distinct smear of the PSF of this imaging process results. We describe the consequences of this drawback in terms of this PSF. On the other hand, we show that this allows a reduction of computation time in the digital inversion of the scattered field data.

[Computer-assisted tissue analysis for cancer detection in breast ultrasound--significance of reference values]. / Rechnergestützte Gewebsanalyse zur Karzinomerkennung in der Mammasonographie--Bedeutung von Referenzmessungen.

Due to the inhomogeneous and very variable structure of healthy tissue of the female breast it is often very difficult to detect a carcinoma via computer-aided sonography. The present study based on preoperative measurements in 29 woman patients with carcinoma of the breast aimed at finding out to what extent it would be possible to circumvent this problem by means of additional reference measurements in healthy tissue. For this purpose, relative image parameters were determined besides absolute ones from measurements in tumorous and healthy reference tissue. These relative image parameters are much more suitable for subsequent decision analysis on account of their narrower range of variation. The results obtained in this manner could be improved further by additional optimization or extension (such as analysis of other combinations of parameters, by enlarging the field of measurement and hence the data available per tissue sample, use of higher or even different ultrasound frequencies).