Theses 2012

Abstract: The novel low-cost biologically derived substrates made of biogenic materials such as high-density fiberboards, felt and paper based compositions are investigated by using microscopy techniques. Plates from these materials were carbonized or graphitized, pressed and diced. In order to seal the surface of the substrates they were processed through different post treatments such as siliconization by capillary effect or pack cementation, alternatively they were covered by a silicon carbide (SiC) layer by chemical vapor deposition (CVD). The substrates were studied regarding their surface properties by using conventional and confocal white-light microscopy. The study revealed different roughness characteristics as well as micro cracks in some of the substrates. Different crystalline silicon thin-film layers deposited on these substrates for solar cell fabrication were investigated by employing scanning electron microscopy (SEM) of layer cross-sections.

Conclusions: This preliminary study demonstrated that Raman spectroscopy could be used effectively to differentiate between different types of tissue. We have accomplished work in the development of ex vivo Raman spectroscopic system, which includes a hardware instrumentation and software implementation.

The hardware instrumentations consisted of a tunable Lithrow-configuration diode laser emitting at 785 nm, high efficiency spectrometer (QE 65000), lenses and mirrors, and a combination of other optical components. Our Raman setup was successfully used to excite and collect signal from different types of pig tissues as fat, nerve, skin, and muscle. In contrast, the software contained a three main VIs for computerized motor control, acquired spectra from QE65000 spectrometer, and to do shift excitation difference spectroscopy. Our customized software was also successfully used for the control of the laser and the specotrimeter, and to collect Raman spectra.

It was found that the optimal set of parameters which can give the bet results for Raman signal detection were excitation wavelength 785 nm, for incident laser power of P = 200 mW at irradiation time t=0, and acquisition time = 2s.

Raman spectra were analyzed by shifted excitation Raman difference spectroscopy (SERDS) method and pllynomial fit method. SERDS method is based on the difference of the two spectra obtained with different excitation wavelenghts. Our SERDS spectra showed significant difference between different tissue types by simple visual inspection except for the fat/nerve tissue. A simple algorithm was used to reconstruct Raman spectra from its SERDS derivatives spectra.

Fluorescence rejection from Raman spectra by means of polynoial fit method is based on the principle that plynomial fit to the background tiessue fulorescence signal, and then a polynomial subtraction from the measured signal. A successful background removal from different types of tissue has been reported. These results showed significant diffierences between different tissue types by simple visual inspection.

The obtained results showed a good agreement between SERDS method and polynomial fit mehtod especially in fat and nerve tissues. However, it is clear that there are significant differences between the two methods, which are related to peak shapes and peak height. This confirmed what has been described previously, that it is not necessary to reconstruct the Raman spectra form its SERDS derivatives, and it is better to avoid further spectral treatment that may distort band intensity irregularly. SERDS method showed a better removal fulorescence background while the plynomial fit method showed a high noise level especially in skin and muscle tissues. Therefor, in order to compare which of these two methods can give better resutls, we must compare between SERDS derivative spectra and reconstructed Raman spectra from polynomial fit method. This consequently requires a more effective method than a simple visual inspection, which can be achieved by applying the statistical analysis (principal componentn analyis).

For future work, the effect of varying the laser line shift Δλ needs to be studied in more detail. Trying to reduce excitation laser power and signal collection time is an imoortant area for future development. Further development of the instrumentation and software needs to be carried out for real time clinical application. Instrumentation dvelopoment includes the use of optical fiber to excite the samples and collect the signal, while the software development includes signal collection and data analysis.

Within this thesis several optical properties of different types of compicate structure of hybrid crystal were studied.

Extraordinary optical transmission (EOT) was observed in the sturctures with corrugated silver films mostly due to the coupling of surface plasmon polaritons from both side of the film which resemble the enhanced transmission spectra of sub-wavelenght periodic array of perforations in flat metal film. The location of these EOTs in the transmission and reflection spectra was proven to be function of lattice constant that replicates the periodicity of 2D heagonal lattice of silica spheres on which the silver film is deposited.

Another feature that was observed it the splitting of the EOT peak in case of double and triple metal film. This corresponds to interaction of evanescent SPPs of different metal films. In case of two silver film the peak splits into two and in triple case into three.

The reasonance mechanisms distinguished in this work are the tools for tailoring the optical properties of photonic crystals. These resonances can be tunend individually by changig the geometry of the structures such as changing the lattice periodicity or differing the order of layers of metal and dielectric. Future development of such crystals can be seen in designing more complex heterogeneous archtitectures like hybrid crystals with a corrugated metal layer inside the 3D opal film as well as using more high ordered metal particle besides corrugated metal film in the hybrid structure in order to strengthen the localized plasmonic effects.

Because of all mentioned properties and exclusive optical properties of fabricated nanostructured sample accompanied by surface plasmon polaritons this type of hybrid photonic-plasmonic crystal can be and in some field already are exploited in application ranging from low threshold lasers to surface plasmon resonance bio-sensors and light tarpping among many other applications.

Conclusions: In this thesis, a sensor based on the self-mixing effect has been introduced for displacement and vibration measurements during laser welding. Furthermore, the possibility of onlince detection of the penetration depth has been investigated by positioning the LD sensor beam into the keyhole.

The self-mixing interferometer was calibrated with the help of the comercial fiber distance sensor and sound speaker as the source of oscillations. Two signal processing methods have been applied in order to reconstruct the initial displacement of the workpiece. According to the calibration measurements, the fringe counting method and the phase-unwrapping approach have uncertainty of +/- 446 nm and +/- 298 nm respectively. During the calibration measurements, we studied the influence of the angle, between the sensor and a target, on the signal quality of the interferometer. The self-mixing signal was found to be still sufficient for the signal processing algorithys until 4 degrees tilt. The quality of the signal was also perturbed by the high amount of the back-reflected light which destabilizes the LD outpout power. This was overcome by using a gray filter. The electronics of the sensor determine the resolution of the interferometer, i.e. the sensor is band limited.

During the displacement and the vibration measurments of the robot arm several effects have been identified that complcate the application of the self-mixing interferometer for mointoring the laser weding process.

First the self-mixing siganal suffers from the speckle effect which represents the intensitiy pattern of dark and bright spots on a diffusive surface of metal. Second, the complexity of the laser welding process fluctuations of the keyhole shape and also interaction of the LD beam with the metal vapor plume make it impossible to perform any displacement measurements. It was discovered that the influence of the vapor pllume emsission, also the reflections fo the disk laser are especcially strong at full penetration of the metal by the disk laser. Hence, these intensitis are not completely surpressed by the bandpass filter and interfere with the signal. The high temperature in the resgions close to the vapor plume changes the refractive index of the air that the LD beam is refracted.

The future improvement of the sensor might be the implementation of the speckle tracking technique which overcomes the amplitude modulation from the speckles by keeping the LD beam on the brightest spot on a target surface. The  usage of the sensor during the laser cutting might be promising since it does not have the complexity as the keyhole behaviour and the vapor plume influence which could give a sufficient signal for further signal processing. Faster electronics will provide better resolution.

The thesis summarizes basic information about palsmon resonance conditions and goes into detail for spehrical gold nanoparticles. It could be shown, that the resoncance position is influenced by the particle size and shape as well as the surrounding material. A temperature gradient affects especially on the longitudinal surface plasmon nanorod peak.

The wavelength dependencie could be immediatly acquired from the spectroscopic measurements. In addition, at the example of liquid tissue phantoms could be approved that the absorption with organic material can be enhanced by gold nanosphere. Magnifications up to 78% were registered. Hence, an absorpiton amplification effect could be approved.

However, the nanosphere are not the bet choice to work as absrobing material. Nanorods show even higher absorption effects. Thus, it would be interesting to repeat the measurement with nanorods and maybe also other material.

Further effort should be spent to investigate the nanoparticle behaviour in case of high irradiances, like femto-second lasers. Also experiments with tissue should be taken into account to investigate for the given nanoparticles in a better way.

The goal of this thesis is to develop a novel method to localize the centers of EEG electrodes in 3D space by employing an optical sensor based on „Flying Triangulation“.

Accurate electrode coordinates enable an activity source analysis of EED data and yield valuable information for presurgical evaluation and plannig of epilepsy surgery. The electrode lcalizations combined with the activies recorded by an EEG sensor, together with magnetoencephalography (MEG) and magnet resoncance imaging (MRI) measurement, yield a nerological examination which can be sued to detect sources in a human brain responsible for epileptic seizures. These regions can then be removed by a surgeon in order to improve the patient’s health. Seizures happen when the electrical system of the brain malfunctions. In this work the focus lays on the localization of EED electrode positions.

EEG records spontaneous electrical activity of the brain over a short period of time, detected by multiple electrodes placed on a scalp. The electrodes are usually joined in an EEG cap placed on the patient’s head. In neurology, epileptic activity can create abnormalities in an EEG study. The 3D electrode positions are captured to determine the potential activity sources within the brain, in combination with recorded EEG signals.

The state-of-the-art method for digitizing the localizations of the EEG electrodes is based on employing a digitizing pen called „Polhemus“. The procedure is as follows: The tip of the Polhemus pen is inserted into the center of each electrode of the EEG cap placed on a patient’s head. The position is captured by pressing a button, the Plhemus pen is moved, and positioned at the next electrode center and so on. The order of the electrodes to be localized is fixed and needs to be adhered to. However, the sate-of-the-art method has several dawbacks: Commonly, an EED cap consists of more than 60 electrodes. This makes a tactile capturing of the electrodes a time consuming taks. It usally takes more than 20 minutes to acquire the positions of 68 electrodes. Further, interactive methods are user dependent which means that the positions may differ when captured by different persons or at different times.

For these reasons, we presented a novel EEG electrode localization method which employs an optical 3D sensor based on the measurment principle called „Flying Triangulation“. Flying Triangulation enables a freely hand-guided measurement of objects with real-time visualization of the current measurement result. The single 3D views are algned and visualized in real-time, allowing the user to get immediate feedback about non-measured object parts. Further, the sensor has been optimized to show minimal measuerment of uncertainty of ~150 µm. Hence, employing an optical 3D sensor based on Flying Triangulation promises to simplify the localization procedure and to improve the repeatability and accuracy of the localization.

The state-of-the-art method employing the digitizing pen „Polhemus“ is compared to the novel method employing a Flying Trinagulation sensor. For this purpose, a 3d-printed head model (made of plaster-like material) of a person wearing an EEG cap has been created from a 3D point cloud obtined with an optical 3D sensor based on the widely applied „Fringe Projection“ measurement rpinciple. The electrode positions determined from the 3D point cloud serve as ground truth for the comparison.

The physical heas model was then used to measure the electrode positons with both methods, employing Polhemus and Flying Triangulation sensors. From all trhee resulting data sets (ground truth, Polhemus, Flying Triangulation) the electrode positions were extracted for comparison. The data sets have been registered in order to be in one common coordinate system. The Eculidean distances of the corresponding positions of electrodes and their standard deviation  have been calculated . The standard deviation of Fringe to Polhemus data is σ = 3.39 mm and the deviation of Fringe to Flying Triangulation data is σ = 0.97 mm. Thus, employing the novel method based on Flying Triangulation improved the accuracy of electrode localization by a factor of 3.5.

A reason for the large deviation of the Polhemus data to the ground truth might be caused by the movement of the head model during the 20 minute measurement inside the MEG chamber. Capturing the head with Flying Triangulation took less than a minute. While the post-processing steps necessary for the extraction of the electrode positions are still time consuming, they promise to be automatable and to be speeded up.

Abstract: A geometric filter method is developed for the robust detection of point correspondences in endoscope images acquired with the axial motion stereo geometry. Displacements of the detected point correspondences due to system deviations from the axial stereo geometry are analysed and corrected. A comprehensive evaluation including a 3D reconstruction of simulated image sequences, a 3D reconstruction of bore holes from metal parts including camera calibration, and a developed method for estimating parameters for image dewarping prove the robustness and precision of the point correspondences even for weakly structured surfaces. The reconstruction of the bore holes shows very promising results with a at least ten times higher precision compared to preceding approaches.

The integration calculation method that was first introduced for Makyoh data evaluation is proven to have high error and bad performance on noisy data. It ws shown that especially for the reconstruction of the Nanotophography on semiconductor wafer surfaces, it exhibits faulty data. New algorithms were introduced based on Fourier and Cosine transforms, they are Frankot-Chellappa, Poisson Solver, α-surface, M-estimator, Regularization and Diffusion.

Frankot-Chellappa is favored, since it has only less than 20 lines of program code, it is simple and fast. It is based on the Fourier Transform. hence, the reconstructed image filters out of the larger spatial wavelenght regions. It is also shown that it is not robust against noise and cannot preserve the edge.

Poisson is an alternative option and proves to be very good in reconstructing the syntheti topographies. It is also robust against noise. For real nanotophography measurement data, this algorithm show good calculation results in peak-to-valley variation profile.

M-estimator is almost similar with Poisson solver. It is robust against noise and shows good calculation results. But the compuational process is iterative and complicated. Regularization and Diffusion is not robust against noise but work very well in reconstructing smooth surface. α-surface reconstructs noisy image very well but the deviation from the ground truth is large.

Foir the validity of the reconstructed Makyoh data, further attempts need to be done on performing reference measurements. That is the nanotopography profile reconstructed by algorithms introduced in this thesis needs to be compared with results obtained from other measurment methods, i.e. by Stylus Profiler or Interferometer.

After a short introduction about expected ablation processes and thoroughly literature research on the results of previous publications and the preliminary work about ablation of IOL materials first estimations are made. Under considertion of the absorption of different materials in IOL several processing wavelenghts are chosen. CI26 as well as CI26Y are studied.

As short chemical view on polymer ablation is presented. The results of this overview show, that – in case of carbonization – oxygen as process assisting gas could lead to well improvements. Since no carbonization could be detected at adequate processing paramters this had not yet been implemented. For more explicit information, a detailed chemical analysis of the produced cuts has to be done.

To make further experiments possible an adaptable sample holder has been designed and built. It does not only allow reproducible tests, but also can be used a process atmosphere chamber with different process gas flows or vacuum processing.

Over 200 documented single cutting tests, where only the most important are mentioned in this thesis, were exectued and analyzed. Several test series have been studied to achieve bst possbile working parameters for fs and ps laser ablation of IOL materials.

In 800 mm fs laser ablation applicable results for opal edges are presented. Due to the long fabrication time of over 30 seconds it is reommended to use ps laser blation. For completely clear transparent cutting edeges the fs laser can be used at 266 nm. Unfortunately this results in the given setup at extremly long fabrication times per lens of over 15 minutes.

Pico second laser ablation has been studied more detailed, what is also a result of the options provided by the laser systems. It can be concluded that ablation at nearby no pulse overlap leads to the most homogeneous edges. By applying multiple cutting techniques and optimizing the lateral displacement of the cuts, perfectly opal edges can be produced. The cutting angle can be reduced by adequate cutting repetitions and displacement size. The theoretical fabrication time for one intra ocular lens can be driven down to 2 seconds. Due to software compatibility issues no completely produced lens is being presented. Though, a partial practicability is shown by cutting out the end of the haptics.

As option processing possibility surface fusing is being studied. Applying experiences of the fast feed motion processing, the plasma bulge can be minimized and a macroscopic transparent fused surface is produced.

As next steps research on low cost ps laser systems could be advidable to quantify economic feasibility. Similarly there should be future research on fs laser ablation of IOL materials in focus on high quality lenses.

Abstract: The main goal of this study was to identify different tissue types during laser ablation which will provide a real time feedback mechanism for clinical Laser surgery applications to significantly improve the application of the technique. Laser Induced Breakdown Spectroscopy (LIBS) by monitoring the plasma plume created during laser surgery processes is used to qualitatively differentiate tissues. Experiments on tissue samples of bone, fat, muscle, nerve and skin were performed using a high power laser capable of generating plasma from the samples. Atomic emissions from the plasma generated were collected using a spectrometer. The collected spectra were analyzed using statistical analysis methods, Principal Component Analysis (PCA), Linear Discriminant Analysis (LDA) and Receiver Operating Characteristic (ROC), for the classification of the spectra to their respective tissues. Successful tissue differentiation results were obtained promising the application of the technique in providing real time qualitative tissue information for a laser surgery feedback system.

The thesis i mostly a study of whispering gallery mode resonators with various edge shapes. Also there is an attempt to fabricate small crystalline resonators via diamond turning.

The simulation of whispering gallery modes are done with COMSOL 4.2 using its RF module. It is observed that the whispering gallery modes can be simulated using the 2D-axissymmetric model. Some effects of modes on the edge shape is studied. Plasmonic whispering gallery modes are also observed when the resonator is coated with a layer of metal. Some interesting phenomena are seen such as the existence of whispering galleries on negative curvatures and the localization of plasmonic modes on conical shapes. Such localization also results in a white light resonator.

For transmission of optical signals between units that are rotatable relative to each other fiber optic rotary joints (FORJs) are used both for single channel and multichannel operation. In applications where polarization state of the input light must be preserved a FORJ is replaced by polarization maintaining fiber optic rotary joint (PM FORJ). For the multichannel PM FORJ the basic constitutents of the device are image rotators like Dove prism and plarization retarderts like quarter wave plates (QWPs). The device is meant to be operated in both directions interchangeably. In this tehsis I have tudied the polarization transforming properties of the Dove prism and experimentally verified the resutls for the light of 1548nm. Further, I have carried out a complete theoretical as well as experimental study of the assembly which comprises two quarter wave plates seperated by a Dove prism such that it preserves circular states of polarization. This assembly is central to the functioning of the PM FORJ. In particular I have worked out the polarization transforming properties of this assembly with respect to an elliptic input light and azimuthal misalignments of the QWPs and experimentally verfied it. Finally, a complete model of the PM FORJ is developped based on the knowledge of the individual components comprising it. The experimental verfication of the behavior of free space part of the PM FORJ is carried out by studying an equivalent system.

We applied a novel laser-based measurement strategy to extract information during supercritical processes. In supercritical antisolvent (SAS) method for instance where particles of tailored characteristics can be produced, the mass transfer behaviour between droplets and compressed antisolvent environment is widely considered to play a vital role in particle morphology. This parameter is not yet fully studied in detail and cannot be instantly treated with traditional mass transfer measurement techniques due to complexity. The generation of a semi-stationary hanging droplet allows us to mimic similar interactions that occur throughout the SAS process. With the utilization of the depth of focus, our strategy on linear one-dimensional rama scattering revealed for the first time to the best of our knowledge, with high spatioal resolution and accuracy, the compositions of droplets and even mixtures inside the capillary during various dropping events. These key results bring us further to a promising breakthrough to a successful mass transfer measurement which is the next step of this work

Development and Testing of DSP Based Format Flexible Coherent Optical Receiver 

Digital Backward Propagation (DBP) technique is gaining imporance as it allwos the joint electronic compensation of linear and non-linear impairments in the optical fiber. Effect of root raised cosine (RRC) pulse shapng on the performance of DBP algorithm is investigated for 112 Gbits/sec DP-QPSK transmission system and performance of DBP for non return to zero (NRZ), return to zero (RZ) and RRC pulse shape is compared. A new concept of optizmized RRC pulses is proposed. It is also shown that the optimized-RRC pulses are more tolerant to the non-linear effects as compared to simple RRC pulses. In additon to this, a comparative study is also performed for different fiber types and multi-span DBP which shows 80% reduction in computational efforts of DBP algorithm. From these results somple model of DBP is derived and the performance of different transmission scenarios is also investigated. These investigations show the adaptivity of DBP algorithm to the misinformation about transmission link design parameters specially the errors in total transmission length.

Optimized Pulse Shaping for Efficitent Performance of Digital Backward Propagation in 112 Gb/s DP-QPSK transmissions

Digital Backward Propagation (DBP) technique is gaining imporance as it allwos the joint electronic compensation of linear and non-linear impairments in the optical fiber. Effect of root raised cosine (RRC) pulse shapng on the performance of DBP algorithm is investigated for 112 Gbits/sec DP-QPSK transmission system and performance of DBP for non return to zero (NRZ), return to zero (RZ) and RRC pulse shape is compared. A new concept of optizmized RRC pulses is proposed. It is also shown that the optimized-RRC pulses are more tolerant to the non-linear effects as compared to simple RRC pulses. In additon to this, a comparative study is also performed for different fiber types and multi-span DBP which shows 80% reduction in computational efforts of DBP algorithm. From these results somple model of DBP is derived and the performance of different transmission scenarios is also investigated. These investigations show the adaptivity of DBP algorithm to the misinformation about transmission link design parameters specially the errors in total transmission length.

Abstract: The supercontinuum generation (SCG) in photonic crystal fiber has become a popular topic for several years. Recently, SCG with the synchronized feedback in a loop cavity configuration stgarts to attract a lot of interest inside nonlinear optics community, because the synchronized feedback is considered to play alos a very important role in SCG and introduced other nonlinear dynamics like period doubling into the system. This thesis will present the establishment and investigation of the nonlinear dynamics of the SCG with synchronized feedback in a photonic crystal fiber loop cavity configuration. The nonlinear dynamics of the system are observed with an optical specturm analyzer and an oscilloscope. The typical spectral broadening is observed, meanwhile three unique regimes (steady, period-2 and chaotic stte) can be distinguished from the results of the temporal dynamics of the system. Since these two results are both obtained after the system is already in equilibrium state, an Acousto-Optic Modulator (AO) is introduced and implemented as a fast pulse picker to access the transient regime of the system. By increasing the number of the round trip propagations of the pulse inside the loop cavity, those three unique features start to be established and clearer, besides the intensity of the pulses also become larger.

Summary and conclusions: In this work different error sources which have an impact onto PMD measurements have been investigated. The focus lied on systematic error sources which are affecting the global accuracy the most.

With the help of a technical solution the image elevation could be totally eliminated. This is rather practical since it is a cheap solution and it can be applied for almost all kinds of PMD systems. The problem has been solved by using a volume scattering foil which is glued directly on the projector (TFT); thereby a new light source is generated. This light source is image elevation free and has very good lambertian properties. The disadvantage os this foils is, that some luminance of the TFT and some contrast of the sinusodial pattern is lost. The contrast lost ist almost negligible since measurements with the same quality can be carried out. Apart from this, the advantages are clearly dominating. The foil totally depolarizes the light of the TFT due to the mulitple scattering processes. No polarization issues like the Brewster angle problem are aparent. Furthermore, it makes the linerization of the light source angle independent. Therebiy no expensive medical TFT screen, which have an internal linearization correction, have to be used anymore. A perfect linearization can be done with the foil even for simple TFT screens.

A mathematical algorithm which is able to solve the highly systematic error source of the circle of the confusion has been presented. This alogrithm is based on a reconstruction for the whole measurements process including the surface cuvature. For each single pixel a circle of confusion is generted and it behaviour and influence during the measurment is analyzed. This permits to compensate the introduced error of the circle of confusion.

Furthermore the TFT as a light source for PMD measurments has been reviewed. The sub-pixel design of different TFT technologies has been investigated and the influence on PMD measurements has been assessed. This analysis showed that the foil which is eliminating the image elevation also solves this problem. The contrast loss which is introduced due to the foils low pass character is averaging out these sub-pixel effects.

In addition a screen calibration of a TFT was done to see how strong the sagging effect due to the gravity is. For the big eye glass setup this has already been done because it was obvious there that the screen is sagging. For TFT this has been untended since the size is quite small. However, the results showed that even for PMD setups with small TFT’s a screen calibration should be carried out since the sagging is in the mm range.

In addition the running in characteristics of different PMD setup components have been investigated. This was done to get a rule of thumb how long the required warm-up time for precise PMD measurements is.

Modulated stray light as an error source was introduced in this thesis. The rpojected pattern of the screen is back reflected by the object itself onto the screen again. Thereby the gray values of the screen are superimposed and phase errors are introduced since the back reflection is modulated light again. This effect is a fundamental problem. Therfore a recipe has been described how this error source can be reduced as much as possible.

Finally different error sources which are introduced during the data evaluation process were discussed. Moreover an improved algorithm for the evolution of patched machine integrated measurements was developed. This algorithm makes use of filgering the measured normals.

To sum up a measurment with the machin integrated setup is compared with the results where all the effects which were introdcued in this work were considered.