Göran Manneberg

Polychromatic phase conjugation with noncollinearly phase-matched difference-frequency generation

A new geometry for third-order nonlinear, noncollinear difference-frequency generation is presented and experimentally verified. It is shown to be equal to a polychromatic phase conjugation. The coupled-wave equations are solved for steady-state cw pumping and for transient pumping without reconjugation. Applications to image processing and to tunable generation of coherent light are discussed.

Simulation of specular microscopy images of corneal endothelium, a tool for control of measurement errors

Purpose:  We aimed at developing simulation software capable of producing images of corneal endothelium close to identical to images captured by clinical specular microscopy with defined morphometrical characteristics. It was further planned to demonstrate the usefulness of the simulator by analysing measurement errors associated with a trained operator using a commercially available semi‐automatic algorithm for analysis of simulated images.

Image formation in polychromatic phase conjugation

The image-formation process in polychromatic phase conjugation is examined. The image-size reducing (or enlarging) property is discussed, and the spatial-filtering characteristics are developed, showing that a narrow-bandpass filter can be made with this technique. Possible applications involving frequency conversion of images from the infrared to the visual part of the spectrum are also discussed briefly.

Fundamental-mode fiber-to-fiber coupling at high-power

Fiber-to-fiber coupling between two different fibers is a state of the art technology. Products are available on the market where multimode fibers can be coupled with very low power loss, at very high powers (multi-kilowatt). We have, however, always been forced to accept a certain loss in beam quality, manifesting as an increase in the Beam Parameter Product (BPP). In fundamental-mode fiber-to-fiber coupling no beam quality is lost. We instead expect to have a certain power loss in the coupling. This paper addresses the problems in free-space fundamental-mode fiber-to-fiber coupling, including theoretical estimations of expected power loss, estimated demands on the stability of the optics as well as measured values on a fundamental mode fiber-to-fiber coupler. The theoretical calculations of the sensitivity of the coupling efficiency due to radial misalignment and defocus (longitudinal displacement) have been confirmed experimentally. Experimental results at 100 W laser power include 88% coupling efficiency using a large mode area fiber with mode-field diameter (MFD) of 18 μm and 75 % coupling efficiency using a single-mode fiber with MFD of 6.4 μm.

Image transfer using six-wave mixing with large-wavelength conversion

A new scheme for nonlinear image transfer with large-wavelength conversion and its possible applications for fine-line lithography are presented. The process uses a higher-order nonlinearity, and the coupled wave equations are solved in the lowest approximation that retains this nonlinearity.

Polychromatic phase conjugation: a study of the nonlinear problem

The nonlinear problem of polychromatic phase conjugation is described by one compact differential equation, which is solved numerically. The result verifies the possibility of tunable oscillation and also predicts multistability on phase-conjugate output.

Fully automated corneal endothelial morphometry of images captured by clinical specular microscopy

The corneal endothelium serves as the posterior barrier of the cornea. Factors such as clarity and refractive properties of the cornea are in direct relationship to the quality of the endothelium. The endothelial cell density is considered the most important morphological factor of the corneal endothelium. Pathological conditions and physical trauma may threaten the endothelial cell density to such an extent that the optical property of the cornea and thus clear eyesight is threatened. Diagnosis of the corneal endothelium through morphometry is an important part of several clinical applications. Morphometry of the corneal endothelium is presently carried out by semi automated analysis of pictures captured by a Clinical Specular Microscope (CSM). Because of the occasional need of operator involvement, this process can be tedious, having a negative impact on sampling size. This study was dedicated to the development and use of fully automated analysis of a very large range of images of the corneal endothelium, captured by CSM, using Fourier analysis. Software was developed in the mathematical programming language Matlab. Pictures of the corneal endothelium, captured by CSM, were read into the analysis software. The software automatically performed digital enhancement of the images, normalizing lights and contrasts. The digitally enhanced images of the corneal endothelium were Fourier transformed, using the fast Fourier transform (FFT) and stored as new images. Tools were developed and applied for identification and analysis of relevant characteristics of the Fourier transformed images. The data obtained from each Fourier transformed image was used to calculate the mean cell density of its corresponding corneal endothelium. The calculation was based on well known diffraction theory. Results in form of estimated cell density of the corneal endothelium were obtained, using fully automated analysis software on 292 images captured by CSM. The cell density obtained by the fully automated analysis was compared to the cell density obtained from classical, semi-automated analysis and a relatively large correlation was found.

A model for corneal endothelial morphometry by diffraction

As a part of an ongoing project on corneal endothelium morphometry by diffraction, a model for corneal endothelium simulation has been developed. The model has been developed in the mathematical programming language Matlab. Images of corneal endothelium were simulated and the diffraction pattern of the image was calculated. The diffraction pattern was calculated for a series of endothelial images while varying important variables in the simulated image. This rendered the theoretical relationships between values of variables in the diffraction pattern and values of morphometric variables in the image. At this stage, the analysis focused on the expression of endothelial mean cell size and coefficient of variation in the diffraction pattern, respectively. As expected from diffraction theory, it was found that there is a direct linear relationship between mean cell size and distance between periodic variations in the diffraction pattern. We further found that the ratio between the intensity in the central maximum and the intensity in the first harmonic of the diffraction pattern was functionally depending on the variation in cell size. The current findings demonstrate that it is possible to theoretically determine average cell size and coefficient of variation of cell size in the diffraction pattern.

Image formation in phase conjugation with large wavelength conversion

The imaging properties of wave-front reversal with nonlinear probe dependence and large frequency conversion are investigated. A calculation is made of the field generated by an arbitrary probe field distribution. It is found that a phase-matching problem, similar to that in nondegenerate four-wave mixing, limits the spatial resolution. Lensless imaging is shown to be impossible because of the nonlinearity of the process. The use of the process as a frequency-converting mirror in a conventional imaging system is studied with diffraction theory. Aberrations in such a system cannot be compensated by a phase-conjugate mirror with large frequency conversion.

Synthesis and analysis of short pulses with polychromatic phase conjugation

A new phase-matching geometry for third-order difference-frequency generation called polychromatic phase conjugation is demonstrated to have the ability to compress or elongate optical pulses in the picosecond region in such a way that the geometry is just scaled in time. The coupled wave equations for polychromatic phase conjugation are examined for the case of pulsed probe waves and pulsed pump waves. A possible method for experimental verification is also discussed.