We first recall that Bessel beams (or `diffraction-free beams') can be produced by a Fourier optics setup where a mask with concentric transmission rings is placed at the focus of a lens. We then describe how the coherent superposition of Bessel beams with proper spatial frequencies leads to a self-imaging phenomenon. Such a behavior can be exploited to measure the curvature of the wavefront incident on the mask used to generate the Bessel beams. It is shown, theoretically and experimentally, that a parabolic phase variation along the radial coordinate for the beam incident on the mask translates into an image shift along the propagation axis. That result can be exploited for the measurement of surface deformation and for the characterization of the optical nonlinearities of materials. We also report on a procedure to optimize the sensitivity of the method.
We report on the measurements of the nonlinear susceptibility of ZnSe at 783 nm with 110 fs pulses of various intensities. We point out limitations for the measurement of (chi) (5) nonlinear susceptibility using Z- scan technique. We also present a new technique using self- imaging (SIZ-scan) which could provide more sensitive measurements of nonlinear refractive effects.
The holographic and photoelastic method have been extensively applied for the analysis of the transient vibrations. Shearography as a relative new technique compared with holography and photoelasticity, provides an alternative method for the measurement of the transient vibration. The special appeal of the shearography is that (a) it can measure strain waves directly on the object surface to be studied (compared with photoelasticity and (b) it is relatively immune to ambient interruptions and provides a wider and more controllable range of sensitivity (compared with holography). Although the ambient interruption is not a critical problem for the transient vibration measurement due to the application of the double pulse technique, the wider and more controllable range of sensitivity makes it possible to measure the relative large vibration which is beyond the measuring range of holography. In this paper, these measuring methods are compared and the developmental steps of shearography for the transient vibration measurement are represented.
Optical measurement methods and especially interferometric methods suffer from a great drawback. The beam that illuminates the investigated object travels through the air. When the air is moving due to thermal or other perturbation, the object beam propagates through a turbulent flow of air. It deviates from its expected optical path due to air refractive index changes produced by the turbulent flow. Those perturbations become very critical when interferometric measurements are achieved in industrial environment. We have developed an original solution to address that drawback of interferometers by introducing a phase conjugation mirror into the interferometer. First we record the shape of the perturbed object beam by means of interference with a reference (pump one) beam in a non- linear photosensitive polymer allowing phase conjugation. The object beam travels back and forth between the visualization part of the interferometric set-up and the object (perturbed zone). Then the interference pattern is read by another reference beam (pump two) antiparallel to the first pump beam to produce the phase conjugate of the object beam. The conjugate beam travels also back and forth the perturbed zone but in opposite direction in regard with the object beam. At the end of the perturbed zone the conjugate beam is unaffected by the optical path modifications due to refractive index changes. Finally the conjugate beam is used as object beam (in classical sense) and interferes with a reference beam (classical part of the interferometer). The complete interferometric set-up is only sensitive to optical path changes due to object displacements and not to optical path changes due to perturbed air.
We propose a single-beam interferometer where a doubly refracting crystal is used to create the two interferometer's arms. The scheme uses only one mirror for these two interferometer's arms. This allows to reduce significantly the influence of mechanical and thermal fluctuations. A simple imaging system is used to avoid complex theoretical interpretations.
The quality parameter M2 has been accepted as an useful averaged magnitude for comparing and classify laser beams with respect to their behavior in their propagation. Its definition is based on the product of two magnitudes: (the spatial size of the laser beam) X (the angular size of the laser beam). This product resembles very much a characteristic magnitude used in radiometry: the throughput, or etendue. In this work we will relate both concepts in order to identify one to the other. From a radiometry point of view the laser beam propagation can be seen as the transportation of light flux from a given source plane to a receiving plane. In most of the cases the practical situation involving laser beam propagation requires this kind of radiometric calculation for safety and energy delivery purposes. On the other hand the radiance of a laser source has been formally related with the Wigner distribution what show up some close relations between moment parametrization of laser beams and radiometric magnitudes. The description of the laser beam in terms of the moments of its amplitude distribution works very well in the formalism but it finds some difficulties to be reached in an experimental setup. Otherwise, the measurement of the energy of the beam can be easily obtained by several methods, such as the knife edge technique and some other related procedures. Our goal is find out the intrinsic relations between the easy to measure radiometric quantities and the easy to calculate generalized parameters. We will focus our attention in the relation between quality factor and throughput.
Since pulsed Nd:YAG lasers are used more and more in industrial production, there is a need for the fast and reliable beam characterization of these devices. This is even more important for flash-lamp-pumped laser systems which show strong fluctuations in the energy density distribution of their single pulses resulting in quality or performance variations and poor reproducibility in material processing. Within this paper the requirements for a diagnostic device used for observation and measurement of pulsed laser radiation are defined with respect to the needs of the laser drilling process. This device must be able to observe the shot to shot stability of the main beam parameters. Unfortunately, the most common beam measurement devices are designed mainly for cw-beam diagnostics. For this reason, we have investigated a measurement procedure and developed a prototype for laser beam diagnostics adapted to the needs of laser material processing. The special demands of the process, the measurement principle as well as the prototype and first experimental results will be discussed.
We propose a new technique of temporal measurements based on the determination of the radiation field's autocorrelation function. The technique does not request any non-linear process, and is applicable in the large spectral, temporal and energy range. The principle of the technique is based on the changing of radiation polarization in single-axe crystal. The resulting polarization allows determining the pulse duration, in case of given pulse shape. A single-axe crystal between two crossed polarizers is placed, oriented by 45 degree(s)-angle with respect to the main planes of them. The energy transition coefficient of the system is registered and the pulse duration is calculated considering the temporal delay between the two orthogonal components of the radiation and the pulse shape. The measurement's precision dramatically depends on the radiation wavelength, crystal length and refraction indexes, direct measurements of which is practically impossible. We used an additional optical channel to exclude the influence of mentioned factors on the measurement's precision. The presented technique is tested experimentally. The duration of Nd:YAG laser's picosecond pulses, as well as compressed pulses of the fiber-optic compressor are measured.
The main regularities of induced change of light polarization in the field of powerful ultrashort pulses in the resonant gaseous medium are investigated. Frequency- tunable ultrashort pulses allows to observe the polarization plane rotation at large values of detuning, in the different schemes of single-photon and two-photon interaction. On the other hand, the high intensity of these pulses allows to investigate the nonresonant interaction in the crystalline and solid transparent media. Our experiments carried out permit to propose the polarization technique for forming the given parameters pulses or train of pulses. In the observing medium placed between crossed polarizers, at counter propagation, powerful pump and linearly polarized probe pulses interact, because of which the probe pulse polarization changes. The polarization rotation angle as a time function is determined by the intensity of ultrashort pulse, the parameters of medium and frequencies of interacting waves. At the definite choice of the experimental conditions, requiring a probe wave after a crossed polarizer we can obtain the pulse which duration is determined by the length of medium, where interaction occurs. Using a nonresonant transparent media we have the possibility of forming the pulses of given duration tunable in a wide spectral region.
We demonstrate a new technique for the direct real-time pico-femtosecond scale temporal measurements based on the nonlinear-optic process of Fourier transformation (NOFT): the conversion of the temporal information to the spectral. This performance is implemented in the fiber-optic spectral compressor, first stretching and up-chirping the pulses in dispersive delay, and after quenching the induced chirp by means of self- or cross-phase modulation in the single-mode fiber. The accurate quenching of the induced chirp brings to the spectral imaging of pulse temporal profile, reducing the problem of the high-resolution temporal measurements to the standard spectrometry. The problem of spectroscopy becomes complicated for ultrafast processes because of the low temporal resolution of electronic oscilloscopes, and often the complex techniques of phase measurements are developed. NOFT with a spectrometer can serve as an ultrafast optical oscilloscope: the resolution is limited only by the temporal response of the Kerr effect, which is approximately 5 fs for silica. In our pico- and femtosecond scale experiments, with the Nd:YAG and Ti:Sapphire lasers, we shaped the multi-peak pulses, implement the radiation spectral compression, and check the given shapes of pulses by means of the spectrometer in order to demonstrate the above mentioned `oscilloscopic' performance.
Spectroscopic and holographic excitation techniques are used to study D2 azo dye doped nematic liquid crystals. Characteristic times of dye relaxation are measured. Strong dependence of the excitation upon the probe and pump light polarization is observed. The efficiency of the holographic excitation and the corresponding relaxation time depend also upon the period and orientation of the light interference pattern with respect to the initial molecular orientation. The comparison of two groups of experimental data allows the identification of the principal mechanisms of excitation of the guest-host system. The diffusion of dye molecules is, in particular, suggested to play an important role in these phenomena.
The beam propagation in optics is not only a fundamental but a practical problem. The commonly used approach is the paraxial approximation. It is natural in some situations such as the catastrophic beam collapse in self-focusing media to go beyond the paraxial approximation. Indeed since the late eighties and now more recently the problem of going beyond the paraxial approximation has been revisited numerically and analytically by several groups. In most of these approaches the refractive index variation associated with Kerr nonlinearity is incorporated but they do not take into account the vectorial effects and consequently fail to satisfy the divergence equation. More recently there have been attempts to incorporate the vectorial nature by considering the interaction between propagation and polarization. In particular the interaction between propagation and polarization was considered in a guiding structure for the description of intrafiber geometric rotation of polarization. Recently Crosignani et al. have proposed a different approach based on the coupled mode theory to deal with the problem of nonparaxial propagation. The purpose and motivation of this work is to examine the general equation for linear and nonlinear optical propagation beyond the paraxial approximation in the context of the coupled mode approach. The complete set of equations incorporating the backward propagating modes are written out. The relation between self-focusing and nonparaxiality is discussed. It is well-known that the model equation for propagation of a laser beam in a nonlinear Kerr media is the nonlinear Schrodinger equation (NLS). The singularities of NLS equation near the self-focusing region are looked at from the point of view of the general equation for propagation. In particular we attempt to examine the region of validity of NLS and compare the self-focusing region in NLS and the general propagation equation. It is interesting to look at the power in the paraxial and non-paraxial parts.
A theoretical modeling of the dynamics of mode locking taking into account the effects of dispersion and detuning applied to the study of pulse evolution inside the FM mode locked Ti:sapphire laser is presented. We note that a considerable change in the pulsewidth and pulse evolution time is brought about by dispersion and detuning. Typically pulsewidths of around ten's of picoseconds is conceivable in systems where dispersion and detuning are well within desired values. The pulse evolution time varies depending on the values of the dispersion and detuning and is typically found to be around 200 microsecond(s) ec.
The photoluminescence (PL) temperature dependence of wurtzite n-type GaN thin films grown on (0001) sapphire substrates by Magnetron sputter epitaxy is reported. Samples were non-intentionally doped, lightly and highly Si-doped. The PL of non-intentionally doped samples consist of the near band edge emission and a broad yellow band (YB) near 2.2 eV. This yellow emission is equally present in spectra of all Si-doped samples. The bound exciton (D0-X) at 3.488 eV and (A0-X) at 3.456 eV are present only in the lightly Si-doped samples. The evolution of the energy position of the (D0-X) is the same as the band gap temperature variation, but the (A0-X) transition is anormally independent of the temperature in the range studied here. In both Si-doped GaN samples a peak at 3.318 eV and transitions between 3.36 and 3.39 eV are observed. The temperature dependence of the latter shows a fine structure composed of four peaks at 3.364 eV, 3.368 eV, 3.375 eV and 3.383 eV. They are tentatively attributed to the superposition of two donor-acceptor and band-acceptor transitions. This interpretation implies the presence of two donors (D1,D2) and two acceptors (A1,A2). From the energy position of the band-acceptor and the energy gap of GaN at 20 K, an acceptor ionization energy of 120 and 135 meV respectively is obtained. Assuming 10 meV for a Coulomb interaction energy of the ionized donor-acceptor pairs, a donor ionization energy of 14 and 18 meV respectively is obtained from the energy difference between the donor-acceptor and the band-acceptor positions. An activation energy of 10.8 meV is deduced from the temperature dependence of the YB. The shallow donor (about 10 meV) contributes to the mechanism of the YB.