Theses 2019

Abstract: In high power thin disk lasers the achievable inversion is limited by ASE. In this thesis, a simulation model is presented in order to investigate the effect of ASE. Special attention is paid to spatial distribution of the upper laser level population. The developed simulation model is based on a Monte-Carlo ray tracing approach to solve the radiation transport within the thin disk. In order to get location depended and time resolved amplification and absorption of the radiation the disk is divided up in cells. In each cell the laser rate equations are solved. First, the evolution of the spatial distribution of the population inversion is examined with given initial distributions. It could be shown that ASE has a stronger influence on the edge of the exited area than in the center due to the longer average path lengths. This distorts the population inversion profile noticeably and grows with increasing inversion. The same effect could be also shown if the pump effect is taken into account. To benchmark the simulation, the results are compared to experimental data. Especially the maximum reachable single pass gain is in good agreement, but also the temporal evolution of the gain shows great similarities.

Within this thesis project, a simulation has been made of the measurement system for ocular wavefront aberrations that is used within the diagnostic device InnovEyes Sightmap, developed by WaveLight GmbH. The InnovEyes Sightmap can perform multiple measurements on the eye, including but not limited to, the tomography of the cornea and the wavefront aberrations of the eye.
Measurements with InnovEyes Sightmap are made and compared to the simulations made with Zemax.
One measurement and simulation measures only astigmatic eyes from -6D up to +6D, while the other takes a combination of negative cylinder values with varying degrees of defocus. These measurements were made with a reference eye; no real eyes were used for any of the measurements.
The results that were sought after are spherical errors, cylinder errors, and SEQ values. To get these results from the simulation, the Zernike polynomials are calculated from which the sphere, cylinder, and SEQ values can be derived. These are then compared to the theoretical (expected) values and should have a maximum deviation of 0.16D as required by WaveLight GmbH.
The results that InnovEyes Sightmap gives for these measurements show that while it performs within the requirements for spherical errors (defocus), the cylinder values are not within the requirements. The results also show that the simulation made in Zemax needs to be improved as the cylinder values gained from the simulations are also not within the requirements of WaveLight GmbH.

Brain shift is one of the critical challenges that faces neurosurgeons during complex operation. This phenomenon caused by many factors such as gravity, tissue manipulation and tumor size and it affects the accuracy of neuronavigation during tumor resection. Stereo vision is one of Image-Guided Neurosurgical Systems that used in calculating the deformation of the cortical brain surface. In this thesis, two stereo vision modalities, RGB and infrared, are directly connected to a neurosurgical microscope. The calibration of these modalities is challenging due to multifocal lens of the microscope. Therefore, single and stereo camera calibration are performed as well as dynamic zoom calibration. Each calibrated modality can estimate the depth of an object locates within the microscope field of view. These depth measurements are validated for different calibration methods by using an object that has a pre-defined 3D structure. The maximum error in relative depth measurements of that 3D structure is 4 mm while the minimum error is below 1 mm. Additionally, a depth map of brain phantom is calculated before and after deformation. Both modalities were able of showing and estimating the direction of that deformation.

Since the laser was invented, it has been found in many applications. The laser is a very useful tool for a wide variety of clinical diagnostic and therapeutic procedures. In ophthalmology, the laser is involved into solving problems with both anterior segments and posterior segments of the eye.

Currently, a laser procedure called LASIK (Laser – Assisted In Situ Keratomileusis) is the most popular refractive surgery in the laser vision correction. Like other types of refractive surgery, the LASIK procedure reshapes the cornea to enable light to be properly focused onto the retina. Before changing the cornea curvature, a cut of the first corneal layer should be performed. Creation of the, so called, Flap, which is lifted after this cut enables the Excimer laser radiation applied directly onto the stroma. In LASIK, originally, the Flap is made using a microkeratome.

Rapid development of new laser technologies enabled the application of ultra-short lasers in refractive surgery. Focused ultra-short laser pulses in near-infrared spectral range can generate laser induced breakdown (LIB) in the cornea, which will disrupt the tissue. Refractive surgery which allows reshaping of the cornea without the mechanical impact but by using femtosecond (FS) pulses is called FS-LASIK. The cutting depth and position can be established by varying the laser focus position.

The cutting process remains an area for development. Improving the efficiency, precision, and safety continues to present a challenge and requires better knowledge of the cutting mechanisms.

This thesis describes a way of improvement of the Flap performed by Wavelight® FS200 by reshaping of the beam profile from a simple Gaussian to a donut-shaped beam.

Abstract: Quantum mechanics holds the promise of a second technological revolution, one driven by a special type of quantum correlations not present in the classical framework, denoted by entanglement. Optical parametric oscillators are a type of nonlinear optical devices that constitute the most reliable source of entangled light. As such, they are of great importance for the fields of quantum information, quantum optics, and quantum technologies. This thesis focuses on the quantum theoretical description of a special type of optical parametric oscillators, dubbed actively-phase-locked, which have the distinctive property of undergoing limit-cycle motion in a certain parameter regime. In regions where they reach stationary behavior, the quantum properties of these devices were studied through the usual linearization technique, which assumes that quantum fluctuations around the classical states are small, a generally good approximation for nonlinear optical cavities. In contrast, the nontrivial periodic time dynamics associated to limit-cycle motion offers a big challenge, because the fluctuations cannot be considered small along the closed trajectory drawn by the cycle in phase space. In this thesis we apply a recently developed technique that extends the linearized approach to such situations. The technique allows us to approximate the asymptotic quantum state by a mixture of Gaussian states centered at the points of the cycle’s trajectory. Analyzing in detail the quantum correlations of these Gaussian states, we show that actively-phase-locked optical parametric oscillators produce light within the cavity with large entanglement levels, even in the regions where they undergo limit-cycle motion. Therefore, the results found in this thesis pave the way for an experimental observation of entanglement in such a dynamically nontrivial scenario.”


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