This PDF file contains the front matter associated with SPIE Proceedings Volume 7194, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing

Advanced laser crystallization of Si films for large flat panel displays requires a narrow very homogeneous focus with at least 235 mm length and high depth of focus. Earlier we have reported on the development and application of an ultranarrow (5-9 μm) homogeneous line-shaped laser focus of 60 mm length for sequential lateral solidification (SLS) of Si. Key element of our line shaping system is an anisotropic mode transformation of the 2nd green harmonic of a Nd:YAG laser beam and its following homogenization for the long focus axis. The design and built-up of a much longer "green line" requires innovative optical approaches and very high precision optical manufacturing. We analyze in detail different process requirements, their physical compatibility (e.g. line width vs. depth of focus) and practical feasibility. To reach high energy densities in the long lines we design optical schemas bundling up to 8 beams of separate lasers.

Many laser applications need a homogeneous - so called flat hat - light distribution in the application area. However, many laser emit Gaussian shaped light. The technology of diffractive optical elements (DOE) can be used to shape the Gaussian beam into a flat hat beam at a compact length. SCHOTT presents a DOE design of a flat hat DOE beam shaper made out of optical glass. Here the material glass has the significant advantage of high laser durability, low scattering losses, high resistance to temperature, moisture, and chemicals compared to polymer DOEs. Simulations and measurements on different DOEs for different wavelength, laser beam width, and laser divergence are presented. Surprisingly the flat hat DOE beam shaper depends only weakly on wavelength and beam width but strongly on laser divergence. Based on the good agreement between simulation and measurement an improved flat hat DOE beam shaper is also presented.

Fly's eye condensers are commonly used for the beam shaping of an arbitrary input intensity distribution into a top hat. The setup usually consists of a Fourier lens and two identical regular microlens arrays - often referred to as tandem lens array - where the second one is placed in the focal plane of the first microlenses. The output intensity distribution is modulated by equidistantly located sharp intensity peaks due to the periodic structure of the regular arrays. In a chirped array, the inflexibility of a regular structure has been overcome. Hence, an array can be formed which is non-periodic and consequently the equidistantly located intensity peaks can be suppressed. A far field speckle pattern results with more densely and irregularly located intensity peaks leading to an improved homogeneity of the intensity distribution. Additionally, a stochastic array which features a nonperiodic layout can be used in order to avoid the equidistantly located intensity peaks. Again, a far field speckle pattern results with more densely and irregularly located intensity peaks leading to an improved homogeneity of the intensity distribution. We compare fly's eye condensers based on regular, chirped and stochastic tandem microlens arrays in terms of achievable intensity homogenization, design and fabrication methods and their advantages and disadvantages in their applications.

Shaping laser beams within the laser cavity can improve the laser power efficiency and allows the generation of predetermined beam structures suitable for various applications such as metrology, lithography and particle manipulation. In this work we study a resonator in which one of the mirrors is replaced by a conical reflector and derive resonator parameters for required beam selections. For example, we show that choosing a negative cone angle can lead to oscillation on pure high-order modes corresponding to the first and second order Bessel functions in the resonator integral equation kernels. These modes possess a distinct zero intensity on the axis that can be exploited for applications such as singular beam metrology and particle manipulation. To generate the specific beam structures we found optimal cone angles and mirror sizes for stable resonators with two configurations, close to flat and to concentric. These optimal conditions were derived by requiring that the selected mode has minimal diffraction loss. It is shown that the selection of these higher-order modes is more efficient in the concentric configuration.

A new type of electro-optical (E-O) deflector which combines microoptical laser beam manipulations and electro-optical light wave phase control is presented. It consists of two stages, which include E-O arrays of LiNbO₃ as key components. The first stage forms a moveable "comb" of interference beamlets at the entrance to the second one. The second stage recombines the beamlets, reconstructs a plane wavefront and converts the translational movement of the comb to an angular deflection of the unified beam. Advantages of the concept as compared to other deflector types will be discussed. The laboratory results with He-Ne lasers are presented. The demonstrator is designed to provide a 63 mrad deflection with a diffraction limited resolution of 0.025 mrad. The technique is applicable for material processing with highrepetition- rate lasers, for laser projection, lidars and in other fields where high speeds and robustness are necessary or sources of vibration need to be avoided.

The different propagating modes in an optical fiber determine the properties of the light emerging the fiber. Therefore an exact knowledge of the modal content is a key to understand the underlying physical effects. Different approaches exist to measure the modal content of an optical fiber, such as interferometry, M² measurement or phase retrieval methods with either high experimental complexity or ambiguity relating to the modal content. In this paper we present a method for measuring the modal content by the aid of optical correlation analysis. We demonstrate this with measurements on a passive step-index LMA fiber at a wavelength of 1064 nm.

Direct laser patterning of various materials is today widely used in several micro-system production lines like inkjet printing, solar cell technology, flat-panel display production and medicine. Typically single-mode solid state lasers and their higher harmonics are used especially for machining of holes and grooves. The most prominent lasers are pulsed Nd:YAG lasers and their harmonics @ 266, 355 and 532 nm. Recently, the striking advantages of flat top intensity distributions for the efficiency and quality of these processes were demonstrated. The use of LIMO's compact Gaussian-to- top-hat converter enables the creation of steeper and sharper edges. Additionally, the higher energy efficiency of rectangular top hat profiles compared to smooth, circular Gaussian shapes allows for faster patterning. A standard method to reduce process times is the use of optical scanning systems. Yet, the application of Gaussian-to-top-hat converters in combination with a scanner was hindered by distortions of the top hat introduced by the F-Theta focussing lens of the scanners even at very small deflection angles (<2°). We solved this challenge by implementing an alternative scanning approach (patent pending). Scanning results obtained with a 50x50μm² top hat field (homogeneity down to <10%) in a scan area of 156x156mm² will be presented. The minimal distortions of the top hat observed within the scan area make LIMO's compact Gaussian-to-top-hat converter excellently suited for industrial scanning applications, e.g. for the processing of solar panels.

The problem of wavefront measurements is one of the most important tasks in the modern optics and laser physics. There are lots of different tools for wavefront measurement. And the user might choose any type of instrument which could solve certain task. The devices for wavefront analysis differ from each other not only by setup and by processing algorithms but also dynamic range and accuracy should be taken into account. The most popular devices are Shack- Hartmann wavefront sensor and various types of interferometers. In this paper we compare Shack-Hartmann wavefront sensor with Fizeau interferometer; classical analysis of interferograms (based on measurement of the fringe centers position) and phase-shifting methods are also compared. Advantages and disadvantages of three methods are discussed.

Modal decomposition by means of correlation filters has been proved as a key for real online laser beam analysis. To compare that method with the "standard" M² method, we generated series of different laser beams (1064 nm), applied both methods to one and the same beam and evaluated achieved results. An adjustable Nd:YAG laser served as transversal mode generator, delivering diverse "pure" Gauss-Hermite modes and superpositions of modes, respectively. In the case of incoherent superposition of modes, their particular contribution to the general M² value should be proportional to their relative strength, whereas in the case of coherent superposition M² is distinctly influenced by phase differences between the discrete modal components. Achieved experimental findings are well confirmed by computer simulation.

We experimentally demonstrate a compact self-aligned external cavity 25W high power laser diode bar at 976 nm tunable over 0.5 nm with a bandwidth of 0.25 nm. The external cavity is based on double diffraction from a glass reflective volume holographic grating (VHG). The wavelength tuning is performed by rotation of the VHG. Passive alignment and fine wavelength tuning of the proposed external cavity are attractive new features over the state-of-the-art active alignment method and fixed wavelength limitation currently used in wavelength stabilized high power laser diode bars.

Multiple tunable laser lines were obtained by pumping solution of Rh6G in ethanol (1mM) by five pairs of the second harmonic of a passively Q. switched and mode locked Nd:YAG laser. The time delays among the excitation pulses were varied within coherence length of 1cm. Twenty one equally spaced lines were obtained by pumping dye solution with ten pairs of excitation beams derived from the same source. It was possible to tune the wavelengths by a microcontroller based mirror mounted stage. Number of lasing lines varied from minimum five to maximum twenty one. The wavelength of output lines varied from 540 to 590nm. The pulse lengths were measured, using Hadland Streak Camera, to vary from minimum 10 to maximum 30ps. The experimental results have lead to maturity of a 21-lines model of a distributed feedback dye laser. The dye cell was excited by the 2nd harmonic of a laboratory built passively Q. switched and mode-locked Nd:YAG laser to induce simultaneous temperature phase grating in the dye solution. This work on distributed feedback dye laser is in agreement with most of the published results on semiconductor DFB lasers. Simultaneous operation of 21-lines of slightly varying wavelengths opens a new era of research in biosensors, multiphoton ignition and measurements. This multi-wavelength operation of DFDL is based on mutual couplings of five overwritten dynamic gratings.

Wavelength around 940 nm lasing can be obtained by quasi-three-level operation of Nd doped laser crystal. Diode end pumping provides the necessary high pump intensity in the laser crystal. In order to get high efficiency of the end pumped laser system, the overlap coefficient between pump beam and laser beam should be optimized. Thermal lens coefficient is one of the most important parameters to design the laser cavity structure. The time dependent heat conduction equation is solved numerically in order to study the thermal lens effect in pulsed pumping laser crystal. Calculated results showed that the thermal lens coefficients change with different pump frequencies. Experiments are done with Nd:YAG and Nd:GSAG laser rod. The thermal lens coefficient of Nd:YAG at pump frequency 50 Hz with pump beam diameter 1.5 mm is 10.2 Wm, while the thermal lens coefficient of Nd:GSAG at pump frequency 50 Hz with pump beam diameter 1.75 mm is 5.9 Wm.

The goal of this study is to demonstrate how a planar ensemble of Gaussian beams (TEM₀₀) can be modeled to create flattop beams for laser applications. We employed the Gaussian Beam Propagation Method (GBPM) in the ASAP (BRO, Tucson AZ) computing and visualization environment in this endeavor. Our study revealed that ensemble individual beam's widths must be set at 1.6 mm, half-angle divergence cannot exceed .5 degrees. In addition, individual laser positions in the ensemble must be such that all ensemble Gaussian beams reach the target/detector in phase. Such beam manipulations are critical for many applications as they strive to customize industry de facto Gaussian laser beams into flattops. In addition, our study has demonstrated that beam flatness is achievable without the aid of dissipative optical entities.

The complex physical optics behavior in modern solid state lasers can only approximately be described using common simulation techniques, such as a Gaussian mode analysis or beam propagation methods. For this reason we present a new 3-dimensional, time-dependent method to model the laser beam in a resonator in a more comprehensive way. We transform the wave equation by a special ansatz and solve the resulting equation, which is similar to a Schroedinger equation, by a finite element analysis. In this paper we explain our new approach and present first numerical results for a simple laser cavity. The results are compared with those of a dynamic multimode analysis using Gaussian eigenmodes.

An actively Q-switched Nd:GSAG laser with 942nm wavelength was frequency doubled in a critically type-I phase-matched LBO. Maximum pulse energy of 8mJ with 300ns pulse duration at 471 nm was obtained with 19mJ incident radiation at 10Hz. The corresponding conversion efficiency was 42%. The frequency doubling of a focused Gaussian beam involves spatially dependent phase mismatching due to beam divergence. It decreases the conversion efficiency and deteriorates the beam quality. According to the theoretical calculation, elliptical focusing was used to improve the second harmonic beam quality and slightly increase the conversion efficiency.

We show that the stimulated Raman process impairs multiple applications of ultra high Q whispering gallery mode resonators and discuss possible ways of its suppression. One of the most promising ways is the coating of the resonators with films containing spectrally selective absorbers able to reduce optical quality factor at Raman Stokes frequencies and to leave without changes the quality factor of the modes of interest.

An optical waveguide consisting of coupled identical resonators in a linear array can slow down the propagation of light and act as a delay line. However, such a slow-wave structure offers only a modest improvement in delay per unit length over a spool of optical fiber, as its performance rapidly degrades if the resonators or their spacing are not exactly identical. Here we show that the same degree of functionality can be achieved in a more compact and disorder-immune structure, formed by nesting one resonator inside another, and thereby folding the light path back onto itself.

We present the theory of operation along with detailed device designs and initial experimental results of a new class of uncooled thermal detectors. The detectors, termed microphotonic thermal detectors, are based on the thermo-optic effect in high quality factor (Q) micrometer-scale optical resonators. Microphotonic thermal detectors do not suffer from Johnson noise, do not require metallic connections to the sensing element, do not suffer from charge trapping effects, and have responsivities orders of magnitude larger than microbolometer-based thermal detectors. For these reasons, microphotonic thermal detectors have the potential to reach thermal phonon noise limited performance.

Strong coupling between whispering gallery modes (WGMs) is studied in polystyrene bispheres by using spatially resolved spectroscopy. The supermonodispersive pairs of spheres (size deviation <0.03%) were selected using their uncoupled WGM peaks' positions. By using novel geometries of capturing light by the imaging spectrometer we observed clear spectral signatures of strong coupling between WGMs for bispheres with mean sizes from 2.9 to 10 μm. In a special geometry with the collection of light along the axis of bisphere and with the slit of the spectrometer oriented perpendicular to the substrate we observed unusual and characteristic kites in the spectral images of such photonic molecules. We showed that such kites allow unambiguous relating of the split components (antibonding and bonding modes) in the spectral image to their WGM eigenstates in the uncoupled cavities. In many cases such kites simplify the interpretation of the dense spectral images of bispheres. Using various geometries of the experiments we quantified the dependence of coupling constant on the sizes of spheres for maximally coupled fundamental modes located in the equatorial planes of spheres on the substrate. The results show the feasibility of achieving a coherent WGM-based optical transport in microsphere resonator circuits.

Using the equilibrium configuration of an imprinted cholesteric elastomer film with a twist defect, we have found the solution of the boundary value problem for the reflection and transmission of normal incident circularly polarized light for different values of chiral order parameter. We have found resonant modes for both polarizations in smaller values of chiral order parameter and in parameters related with elastic energy.