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The influence of surface conditions on chevron structures and zig-zag defects in a SmC liquid crystall cell is studied both experimentally and theoretically. In order to gain insight into basic properties of the system the Landau-Ginzburg type theory is used in terms of the nematic director field and the smectic complex order parameter. On a simple model system transitions in the C1 and C2 chevron structures are studied as functions of the surface pretilt angle. The threshold conditions are calculated. The width of the straight element of the domain wall (running parallel to smectic layers) and the costs for its formation are estimated. In the experimental part of the work the effect of the confining substrate with controlled variation of the surface slope on the formation of zig-zag defects is analysed. Observed results are in line with theoretical expectations Preučevali smo vpliv površinskih pogojev na strukturo ševronov in zig-zag efektov v SmC tekočem kristalu, omenjenem v planparalelni celici. Kvalitativno obnašanje sistema smo ocenili izhajajoč iz Lindau-Ginzburgove proste energije, izražene z nematičnim direktorskim poljem in smektičnim kompleksnim ureditvenim parametrom. Na poenostavljenem modelu smo proučevali prehode med ševronskima strukturama C1 i C2 v odvisnosti od nagiba molekul ob površini celice. Izračunali smo kritični pogoj prehoda. Ocenili smo širino in potrebno energijo za tvorbo domenske stene med področjima različne orientacije ševrona. V eksperimentalnem delu smo preučevali vpliv površine s kontrolirano variacijo površinskega nagiba na nastanek zig-zag defektov. Opaženi rezultati se kvalitativno ujemajo s teoretičnimi napovedmi

Delo predstavlja pojav prosto plavajočih nanomehurčkov. Za generiranje nanomehurčkov je bila izbrana metoda presvetlitve vode s kolimiranim laserskim snopom. Opazovanje je bilo osnovano na uporabi podtlačnega udarnega vala, ki je nanomehurčke razširil do velikosti primerne za opazovanje. Meritve so bile opravljene s pomočjo hitre kamere. Čas, ko so mehurčki vidni je relativno kratek, s trajanjem v območju mikrosekund. V sklopu analize je bil narejen algoritem, ki iz posnetka v eno metriko združi število in velikost nanomehurčkov. Ugotovljeno je bilo, da se ob povečevanju energije pulza presvetlitvenega laserja gostota nanomehurčkov najprej povečuje in nato doseže zasičenje. Prav tako je bilo ugotovljeno, da lahko življenjski čas nanomehurčkov v vodi znaša tudi do več deset sekund. This master's thesis describes a phenomenon involving bulk nanobubbles. A method where a collimated laser beam is shined through the water, was chosen for seeding bulk nanobubbles. The observations were made using a rarefaction wave that expanded nanobubbles to observable sizes. Observations were made with a high-speed camera. Time, when one can observe expanded nanobubbles, is relatively short, ranging to a few microseconds. We devised an algorithm that combines number and size of nanobubbles into a single metric. We discovered that increasing the energy of a collimated laser beam results in an increase of the density of nanobubbles, until it reaches the saturation zone. We also discovered that nanobubbles can survive for tens of seconds in water.

Zaradi prehitrega potovanja taline ob tlačnem litju, se na odlitku pojavi defekt imenovan dvoplastnost. Za upočasnitev hitrosti potovanja taline bi gravure hrapavljali oziroma naredili hrapave. S primerjavo različnih metod hrapavljenja sem izbral obdelavo z laserskim žarkom, ki se je izkazala za najbolj optimalno. S testnim litjem smo dokazali vpliv hrapavljenja gravurnih oblik na zmanjšanje dvoplastnosti na odlitku, na preizkuševališču z laserjem pa sem določil optimalne parametre laserske obdelave vzorcev za doseganje hrapavosti Rz 45 in izdelal možno obliko za nadaljnje hrapavljenje gravurnih oblik. Dodatno sem izdelal CAD model za uporabo na 3D laserskem sistemu. Due to the rapid travel of the melt during die casting, a defect called delamination occurs on the casting. To reduce the speed of traveling of the melt, the engravings would be roughened. By comparing different roughing methods, I chose the laser beam treatment that proved to be the most optimal. With test casting, we proved the influence of roughness of engraving shapes on the reduction of delamination on the casting, and at the laser test site I determined the optimal parameters of laser processing of samples for roughness Rz 45 and made a possible shape for future roughing of engraving shapes. In addition I made a CAD model for a 3D laser system.

V tem doktorskem delu je predstavljen hibridni laser za visoko natančne obdelave v industriji, ki omogoča delovanje v načinu bliskov na zahtevo. Za visoko natančne obdelave potrebujemo laserski sistem z ultrakratkimi bliski, ki pa mora biti za doseganje velikih obdelovalnih hitrosti združljiv tudi s poligonskimi oz. resonančnimi skenirnimi sistemi. Zaradi uporabe v industrijskem okolju mora biti tak sistem tudi čim bolj kompakten. Težavo pri takem laserju predstavljajo predvsem nelinearni optični pojavi, ki vplivajo tako na časovno kot na spektralno obliko bliskov. Da bi zmanjšali njihov vpliv in hkrati zagotovili kompaktnost sistema, smo raziskali hibridni laser, ki temelji na kombinaciji vlakenskega in trdninskega ojačevalnika v MOPA postavitvi in uporablja t. i. CPA metodo za ojačenje ultrakratkih bliskov. Zaradi kombinacije vlakenskega in trdninskega ojačevalnika pri takem sistemu naletimo na dodatne težave pri tvorbi bliskov na zahtevo. Te smo rešili s kontrolo valovne dolžine jalovih bliskov, s katerimi nadziramo obrnjeno zasedenost oz. ojačenje pri tvorbi bliskov na zahtevo. Z uporabo omenjenih metod nam je tako v sklopu doktorske naloge uspelo raziskati hibridni laser, ki lahko deluje v načinu bliskov na zahtevo in lahko generira ultrakratke laserske bliske s časom trajanja 500 fs ter največjo energijo 300 µJ. This thesis presents a hybrid laser with pulse on demand operation for high precision industrial processing. For high precision materials processing the laser system must produce ultrashort laser pulses and needs to be compatible with polygon or resonant scanners in order to achieve high processing speeds. Additionally, a compact system is required in order to be used in an industrial environment. Nonlinear optical phenomena are the main cause of problems in such system, as they influence both temporal and spectral pulse shape. To reduce their influence and in the same time ensure a compact design, a hybrid laser was investigated. It is based on a combination of a fiber and solid state amplifier in a MOPA configuration and uses a so called CPA method for ultrashort pulse amplification. Such systems exhibit additional problems when producing pulses on demand, due to the combination of a fiber and a solid-state amplifier. We solved this problem with wavelength control of the idler pulses, which are used for gain control in pulse on demand operation. Using the above methods, a hybrid laser capable of pulse on demand operation that can generate ultrashort laser pulses with 500 fs pulse duration and 300 µJ pulse energy, was developed.

Naloga obravnava problematiko postavitve za slikanje lasersko povzročenih udarnih valov v tekočini. Slikanje teh valov zaradi njihove velike hitrosti ni enostavno, razlike v lomnem količniku, ki jih ti povzročijo, pa ne moremo zaznati s klasičnimi metodami slikanja. Metodi schlieren in senčne fotografije omogočata zaznavanje majhnih sprememb lomnega količnika. Na podlagi zahtev obeh metod smo v programu Solidworks izdelali model sistema, ki lahko z večkratno osvetlitvijo senzorja tekom časa ekspozicije zajame ostre slike udarnih valov. Prilagojen je svetlobi z valovno dolžino 800 nm in lahko zajame vidno polje premera 24 mm. Vključuje zaslonko z ostrim robom za potrebe schliren fotografije, z njeno odstranitvijo pa lahko sistem uporabljamo tudi po metodi senčne fotografije. S sistemom lahko slikamo tudi druge pojave, ki spremenijo lomni količnik v transparentnem mediju, kot so na primer kavitacijski mehurčki, mešanje fluidov in konvektivni prenos toplote. Komponente sistema so modularne in jih lahko prilagodimo našim potrebam. The thesis deals with the problem of setup for imaging laser-induced shock waves in a fluid. Imaging of these waves is difficult due to their high speed, and the differences in the refractive index they cause cannot be detected by classical imaging methods. The schlieren and shadowgraphy methods enable us to detect small changes of the refractive index. Based on the requirements of both methods, we have built a Solidworks model of an optical system that can capture sharp images of shock waves, by repeatedly illuminating the sensor during its exposure time. It is designed for the light with the wavelength of 800 nm and can capture a field of view with a diameter of 24 mm. It includes a knife-edge insert for the needs of schlieren photography, and by removing it, the system can also be used for shadowgraphy. The system can also be used to image other phenomena that change the refractive index in a transparent medium, such as cavitation bubbles, fluid mixing and convective heat transfer. The components of the system are modular and can be adapted to our needs.

Merjenje polarizacije svetlobe predstavlja pomembno področje karakterizacije laserske svetlobe. V magistrskem delu je skonstruiran polarimeter, ki omogoča točno merjenje vseh polarizacijskih stanj svetlobe v realnem času. Za zasnovani optomehatronski sistem je prikazana izdelava merilne in krmilne enote, ter izbira optičnih, mehanskih in elektronskih komponent. Izdelano je električno vezje, ki omogoča lažje povezovanje med posameznimi komponentami. Na njem je integriran mikrokrmilnik, ki predstavlja logično enoto izdelanega polarimetra. Na mikrokrmilniku je implementiran krmilni algoritem, ki omogoča merjenje in komunikacijo z uporabnikom. Polarimeter je zasnovan na principu popisa rotirajoče retardacijske ploščice in fiksnega polarizatorja s Stokes-Muller-jevo formulacijo. V primeru zamenjave optičnih elementov je uporabljena tehnika primerna za različne valovne dolžine. Za zamenjavo optičnih elementov je zasnovan postopek kalibracije. Uporabnik z izdelanim polarimetrom upravlja preko LCD zaslona ali osebnega računalnika. Za ta namen je izdelan uporabniški vmesnik, ki omogoča krmiljenje, kalibracijo in analizo meritev. Za kontrolo pravilnosti delovanja izdelanega polarimetra je zasnovan postopek določitve negotovosti. Pri tem je uporabljen laserski izvor z valovno dolžino $lambda = 1064$ nm, frekvenco bliskov 1 Mhz in dolžino bliskov t$_text{FWHM}$ = 1 ns. Z izdelanim polarimetrom lahko merimo Stokes-ove parametre z razširjeno merilno negotovostjo 0,02 in maksimalno frekvenco merjenja 3 vzorce/s. Measuring light polarization is important for the characterization of the laser light. In the thesis, we devise a polarimeter that enables accurate measurement of light polarization states in real time. For the devised optomechatronical system we show the realization of the measuring and control units, and the choice of optical, mechanical and electronic components. We devised an electrical circuit that facilitates the connections among individual components. On the electrical circuit, we integrated a microcontroller that represents a logical unit of the devised polarimeter. A control algorithm is implemented on the microcontroller which enables data acquisition and communication with the user. The polarimeter uses the principle of rotating optical waveplates and a fixed polarizer with Strokes-Muller formulation. Measuring at different wavelengths can be achieved by a simple substitution of the optical elements. For this we devised a calibration procedure. A user can control the polarimeter via LCD screen or a personal computer. For this purpose, a user interface was written that allows for control, calibration and analysis of measurements. We established a procedure for determining the measurement uncertainty in order to control for the correct functioning of the devised polarimeter. To achieve that, we used a laser source with $lambda = 1064$ nm wavelength, mbox{1 Mhz} laser beam frequency and t$_text{FWHM}$ = 1 ns beam length. With the devised polarimeter, we can measure Strokes parameters with an expanded measurement uncertainty of 0.02 and a maximum frequency measurement of 3 samples/s.