Multiple hot images from an obscuration in an intense laser beam through cascaded Kerr medium disks

We present a theoretical investigation on the formation of hot images in an intense laser beam through cascaded Kerr medium disks, to disclose the distribution and intensity of hot images in high-power disk amplifiers. It is shown that multiple hot images from an obscuration may be formed, instead of one hot image as reported previously in the literature. This gives a clear explanation for the curious damage pattern of hot images, namely, damage sites appearing on alternating optics in periodic trains. Further analysis demonstrates that the distribution and intensity of hot images depend closely on the number of Kerr medium disks, the distance from the obscuration to the front of the first disk downstream, the space between two neighboring disks, and the thickness and B integral of each disk. Moreover, we take two cascaded Kerr medium disks for example to detail multiple hot images from an obscuration and confirm the theoretical results by numerical simulations.     

Computer simulation study of nonlinear imaging properties for two phase-typed scatterers

Nonlinear imaging of two phase-typed scatterers in super-Gaussian laser beams is modeled and its properties obtained by computer simulation are presented. The formations of hot images and second-order hot images are verified. It is found that the in-beam locations of hot images correspond to those of the scatterers, but that there can be only one second-order hot image, which is at the middle point between the in-beam locations of the scatterers. Interestingly, the image intensity can be suppressed and it increases in an oscillating manner with some regularity as the distance between the scatterers increases. Moreover, one more scatterer makes the effect of the B integral and that of the phase shift caused by scatterers quite different from the predictions for single-scatterer case. The variation trend of hot image intensity with the B integral is not in agreement with that described by the analytical result for the single-scatterer case. The variation of phase shift caused by the scatterers can result in two peaks in the variation of hot image intensity with it, and the phase shift corresponding to the larger peak is approximately half the predicted result for the single-scatterer case. These results indicate that the number of scatterers has a significant influence on nonlinear imaging.     

Risky intensity peaks resulting from nonlinear holographic imaging

A study of the maximal intensity peaks due to nonlinear holographic images of obstacles such as obscurations or phase defects in a high-power laser system is presented. It is shown that the interference of the high-power plane wave and the converging image wave results in the formation of intensity maximums in the vicinity of the image plane, the values of which significantly exceed the intensity in the image plane itself. For round obstacles, analytical expressions that describe magnitudes and locations of the maxima depending on the radius and the type of obstacle are given. A procedure of numerical modeling that allows estimation of the influence of beam size, medium thickness, type, size, and shape of obstacles onto the properties of nonlinear images is described. It is demonstrated that for a given combination of the nonlinear medium and the high-power beam parameters, there is an intrinsic size of obstacles that is most harmful for the laser system components.     

Nonlinear holographic imaging of phase errors

Experimental measurements and computer simulations of nonlinear holographic imaging of phase errors in laser beams are presented. The computer models are found to accurately predict the results of the experiments. Comparison with similar results by use of amplitude scatterers reveals that the image location (along the propagation path) is the same for phase and amplitude scatterers. However, the intensity and fluence of the image of a phase scatterer are significantly larger, indicating that phase objects pose a larger damage threat to optical components.     

Potential and limits of holographic reconstruction algorithms

For the purpose of ultrasonic nondestructive testing of materials, holography in connection with digital reconstruction algorithms has been proposed as a modern tool to extract crack sizes from ultrasonic scattering data. Defining the typical holographic reconstruction algorithm as the application of the scalar Kirchhoff diffraction theory to backward wave propagation, we demonstrate its general incapability of reconstructing equivalent sources, and hence, geometries of scattering bodies. Only the special case of a planar measurement recording surface, that is to say, a hologram plane, and a planar crack with perfectly rigid boundary conditions parallel to the hologram plane and perpendicular to the incident field yields a nearly perfect correlation between crack size and reconstructed image; the reconstruction algorithm is then referred to as the Rayleigh-Sommerfeld formula; it therefore represents the optimal case matched to that special geometrical situation and, hence, may be interpreted as a quasi-matched spatial filter. Using integral equation theory and physical optics, we compute synthetic holographic data for a linear cracklike scatterer for both plane and spherical wave incidence, the latter case simulating a synthetic aperture impulse echo situation, thus illustrating how the Rayleigh-Sommerfeld algorithm or its Fresnel approximation increasingly fail for cracks inclined to the hologram plane and excited nonperpendicularly. Furthermore, we point out how the physical data recording process may additionally influence the reconstruction accuracy, and, finally, guidelines for a careful and serious application of these holographic reconstruction algorithms are given. The theoretical results are supported by measurements.     

Design approaches to power-over-optical local-area-network systems

This paper discusses fiber optic power and signal transmission systems considering the application of dc powering to information tools such as personal computers. We discuss system requirements and technical issues for the system components, including high-power laser diodes and photovoltaic cells. It is clarified that the conversion efficiencies of photovoltaic cells are kept constant with heat radiation and improve with extremely small series resistance. The transmittable optical powers through the optical fiber limited by a nonlinear optical effect are estimated. We also discuss the system designs for the use of single- and multi-mode optical fibers.     

The imaginary components of dispersion parameters and the Stokes mode dynamics in stimulated Raman scattering

The Stokes pulse dynamics during stimulated Raman scattering has been studied in the constant pumping approximation. The nonlinear problem of interaction between a pumping wave and the Stokes component is reduced to a linear equation describing propagation of an optical pulse in the amplifying medium with imaginary components of the first-and second-order dispersion parameters, the presence of which leads to the possibility of the pulse envelope propagation at a supraluminal velocity.     

Femtosecond snapshot imaging of propagating light itself

An ultrafast imaging technique has been developed to visualize directly a light pulse that is propagating in a medium. The method, called femtosecond time-resolved optical polarigraphy (FTOP), senses instantaneous changes in the birefringence within the medium that are induced by the propagation of an intense light. A snapshot sequence composed of each femtosecond probing the pulse delay enables ultrafast propagation dynamics of the intense femtosecond laser pulse in the medium, such as gases and liquids, to be visualized directly. Other examples include the filamentation dynamics in CS2 liquid and the propagation dynamics in air related to the interaction with laser breakdown plasma. FTOP can also be used to extract information on the optical Kerr constant and its decay time in media. This method is useful in the monitoring of the intensity distribution in the nonlinear propagation of intense light pulses, which is a frequently studied subject in the field of physics regarding nonlinear optics and laser processing.     

Intrinsic speckle noise in off-axis particle holography

In holographic imaging of particle fields, the interference among coherent wave fronts associated with particle scattering gives rise to intrinsic speckle noise, which sets a fundamental limit on the amount of information that particle holography can deliver. It has been established that the intrinsic speckle noise is especially severe in in-line holography because of superposition of virtual image waves, the direct transmitted wave, and the real image. However, at sufficiently high particle number densities, such as those typical in holographic particle image velocimetry (HPIV) applications, intrinsic speckle noise also arises in off-axis particle holography from self-interference among wave fronts that form the real image of particles. To overcome the latter problem we have constructed a mathematical model that relates the first- and second-order statistical properties of the intrinsic speckle noise to relevant holographic system parameters. Consistent with our experimental data, the model provides a direct estimate of the information capacity of particle holography. We show that the noise-limited information capacity can be expressed as the product of particle number density and the extent of the particle field along the optical axis. A large angular aperture of the hologram contributes directly to achievement of high information capacity. We also show that filtering in either digital or optical form is generally ineffective in removing the intrinsic speckle noise from the particle image as a result of the similar spectral properties of the two. These findings emphasize the importance of angular aperture in designing holographic particle imaging systems.     

Letter Phase-mismatching effects in the internal second-harmonic generation in InGaAs quantum-well laser diodes

Abstract

We have characterized the role of phase-mismatching in the spectral distribution of the internal second-harmonic generation (ISHG) in the CW operation of high-power InGaAs quantum-well laser diodes emitting at around 960 nm. The observed pairs of narrow blue-green peaks of ISHG have symmetrical spectral positions with respect to the central peak of the pure second harmonic at ～480 nm, suggesting that their generation in the laser waveguide is most probably mediated by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. The relatively strong emission of the ISHG radiation at the output mirror facet indicates that this second-order nonlinear optical process is enhanced by waveguiding in the laser active region.     

Photo-thermo-refractive glass co-doped with Nd as a new laser medium

Photo-thermo-refractive (PTR) glass demonstrates refractive index change after exposure to UV radiation followed by a thermal treatment that enables recording of high efficiency holographic optical elements. This work demonstrates feasibility of function of this material as a complex optical medium which posseses both photosensitive and luminescent properties and paves a way for creation of monolythic solid state lasers where resonator components can be holographically recorded inside of a laser medium. It was found, that incorporating of Nd³⁺ ions in PTR glass does not affect photosensitivity required for hologram recording. It was demonstrated that emission wavelength, spectral width, and cross section of Nd³⁺ luminescence in PTR glass are typical for silicate laser glasses and Nd-doped PTR glass can be considered as a promising laser medium for monolithic solid state lasers.     

Wave equation-based imaging of mode converted waves in ultrasonic NDI, with suppressed leakage from nonmode converted waves

The value of imaging techniques in ultrasonic nondestructive inspection (NDI) to find and characterize defects in steel components has already been demonstrated. The imaging techniques based on the integral representation of the wave equation, the Rayleigh integrals for wave field extrapolation, are becoming feasible and attractive due to advances in array technology and due to faster computers. Known implementations are the total focusing method (TFM), the synthetic aperture focusing method (SAFT), and the inverse wave field extrapolation method (IWEX). In principle, these techniques compensate propagation effects from sources to a scatterer such as a defect and propagation effects from the scatterer to receivers. Currently, this approach is applied to wave fronts of single modes (pure longitudinal or pure transversal). In practice, multiple wave fronts from the scatterer will be received as a result of mode conversion. These arrivals will not have the same arrival time because of the difference in sound velocity between longitudinal and transversal waves. Images of mode converted waves are obtained by choosing the appropriate sound velocity that corresponds with the mode-converted scattered wave in the imaging process. Therefore, the nonmode converted waves will image as leakage artifacts in the mode-converted images, and vice versa. This may lead to false interpretations. In this paper, such artifacts will be identified and explained with the help of an analytical example. Measurements from steel test pieces with a 4 MHz linear array transducer with 64 elements will be used to demonstrate the artifacts. Furthermore, a procedure to predict the artifacts and the subsequent suppression from the input measurements will be presented and demonstrated.     

Chaotic scattering of waves from nonlinear scatterers

Summary Two examples of the effect of nonlinearities on the scattering of nondispersive waves are presented. It is shown that nonlinear boundary conditions can lead to broadband, chaotic scattered waves when the scatterer is excited by a single wavelength plane wave. One of the examples treats a spherical scatterer in a linear elastic medium. A number of modern techniques in nonlinear analysis is used to diagnose the chaotic dynamics including Poincaré maps, fractal dimensions and Lyapunov exponents.     

Three types of computer-generated hologram synthesized from multiple angular viewpoints of a three-dimensional scene

We describe various techniques to synthesize three types of computer-generated hologram (CGH): the Fresnel-Fourier CGH, the Fresnel CGH, and the image CGH. These holograms are synthesized by fusing multiple perspective views of a computer-generated scene. An initial hologram is generated in the computer as a Fourier hologram. Then it can be converted to either a Fresnel or an image hologram by computing the desired wave propagation and imitating an interference process of optical holography. By illuminating the CGH, a 3D image of the objects is constructed. Computer simulations and experimental results underline the performance of the suggested techniques.     

Nonlinear formation of holographic images of obscurations in laser beams

Computer models are used to simulate the nonlinear formation of images of obscurations in laser beams. The predictions of the model are found to be in good agreement with measurements conducted in the nonlinear regime corresponding to a typical solid-state laser operation. In this regime, peak-to-mean fluence ratios large enough to induce damage in optical components are observed. The amplitude of the images and their location along the propagation axis are accurately predicted by the simulations. This indicates that the model is a reliable design tool for specifying component staging and optical specifications to avoid optical damage by this mechanism.     

High-efficiency metallic diffraction gratings for laser applications

The design and fabrication of large-area, high-efficiency metallic gratings for use in high-power laser systems is described. The gratings exhibit a diffraction efficiency in excess of 95% in the m = -1 order (Littrow mount) and have a high threshold for laser damage. Computations and experimental measurements are presented that illustrate the effect of grating shape and polarization on efficiency. A simple theory for optical damage to metallic diffraction gratings is developed and compared with experimental measurements of the laser-damage threshold over the pulse range from 400 fs to >1 ns.     

Low-frequency scattering pattern of an arbitrary penetrable obstacle

We consider the scattering of low-frequency sound waves by an arbitrary penetrable obstacle whose density and compressibility are different from those of the surrounding infinite medium. We present solutions both inside and outside the scatterer. The physical properties of the scatterer are implicit in the transition conditions at its surface and in the interior value of the wave length. This leads to a two-parameter boundary value problem which we convert into an integral equation valid both inside and outside the obstacle. This integral equation is then recast into an infinite system of algebraic equations which are judicially truncated to yield the required solutions.     

Phase-matching and femtosecond difference-frequency generation in the quaternary semiconductor AgGaGe5Se12

We present data on the linear (transmission, index of refraction) and nonlinear (second-order susceptibility) optical properties of the quaternary semiconductor AgGaGe5Se12 with orthorhombic symmetry--a solid solution in the AgxGaxGe1-xSe2 system with x = 0.17. The nonlinear coefficients are estimated from phase-matched second-harmonic generation near 3 microm. After numerical analysis of the phase-matching configurations for three-wave nonlinear interactions, the first experimental results on difference-frequency mixing, producing tunable (4-7.5-microm) femtosecond pulses at a 1-kHz repetition rate, are described. The pulses of only five optical cycles (FWHM = 84 fs) are generated near 5 microm with energy of 0.5 microJ. Because of its higher damage threshold, larger birefringence and bandgap, and greater variety of phase-matching schemes, AgGaGe5Se12 could become an alternative to AgGaS2 and AgGaSe2, more widely used in high-power and specific applications.     

Laser soldering with light-intensity patterns reconstructed from computer-generated holograms

We describe a laser soldering technology with diffractive optics. This technology provides efficient one-step laser illumination for components to be soldered. A phase-only computer-generated hologram set in a variable-focal-length optical configuration generates a diffraction pattern with the dimensions required for processing solder. We verified the effectiveness of the proposed scheme by sealing a ceramic package such that it could house a quartz device. The factors for successful soldering include alignment of the diffraction pattern to the work piece, the thermal properties of the materials involved, and the wavelength of the laser used to process the solder. The beam intensity across the diffraction pattern influences the process, and the 0th-order intensity should be minimized to prevent damage to the work piece.