Optical Laser Beam Research Articles

A Movable Thomson Scattering System Based on Optical Fibers (TS-Probe)

This paper proposes a movable compact Thomson scattering (TS) system based on optical fibers (TS-probe). The TS-probe consists of a probe head, optical fibers, a laser diode, a polychromator, and lock-in amplifiers. Laser beam optics and light collection optics are mounted rigidly on a probe head with a fixed scattering position. Laser light and scattered light are transmitted by flexible optical fibers, enabling us to move the TS-probe head freely during plasma discharge. The light signal scattered from an amplitude-modulated laser is detected against the plasma light based on the principle of the lock-in amplifier. With a modulated laser power of 300 W, the scattered signal from a sheet plasma 15 mm deep with ne ∼1019 m−3 can be measured with 10 % accuracy by setting the integrating time to 0.1 s. The TS-probe head is similar to a 1/20 model of the currently operating Large Helical Device-TS.

Dynamical Schwinger effect and high-intensity lasers. Realising nonperturbative QED

We consider the possibility of experimental verification of vacuum e+e- pair creation at the focus of two counter-propagating optical laser beams with intensities 1020–10 $^{22}~{\rm W/cm}^2$ , achievable with present-day petawatt lasers, and approaching the Schwinger limit: 10 $^{29}~{\rm W/cm}^2$ to be reached at ELI. Our approach is based on the collisionless kinetic equation for the evolution of the e+ and e- distribution functions governed by a non-Markovian source term for pair production. As possible experimental signals of vacuum pair production we consider e+e- annihilation into γ-pairs and the refraction of a high-frequency probe laser beam by the produced e+e- plasma. We discuss the dependence of the dynamical pair production process on laser wavelength, with special emphasis on applications in the X-ray domain (X-FEL), as well as the prospects for μ+μ- and π+π- pair creation at high-intensity lasers. We investigate perspectives for using high-intensity lasers as “boosters” of ion beams in the few-GeV per nucleon range, which is relevant, e.g., to the exploration of the QCD phase transition in laboratory experiments.

The Gaussian free space optical MIMO channel with Q-ary pulse position modulation

The main drawback in communicating via the free space optical (FSO) channel is the detrimental effect the atmosphere has on a propagating laser beam. Atmospheric turbulence causes random fluctuations in the irradiance of the received optical laser beam, commonly referred to as scintillation. This paper investigates the use of multiple lasers and multiple apertures to mitigate the effects of scintillation. In particular, the FSO multiple-input multiple-output (MIMO) channel with Q-ary pulse position modulation (QPPM) and transmit repetition under the assumption of non-ideal photodetection is analyzed in terms of its uncoded bit error rate (BER) and ergodic channel capacity. This analysis is based, in part, on the use of irradiance fluctuation samples that were obtained from a laser range experiment that was conducted in the presence of moderate turbulence conditions. Expressions are derived for the log-likelihood ratio (LLR) of a received bit, uncoded BER and ergodic capacity. Using these results it is shown that large gains are possible with the use of MIMO combined with strong coding techniques.

Effects of atmospheric turbulence on the scintillation and fade probability of flattened Gaussian beams

In an attempt to mitigate the effects of the atmosphere on the coherence of an optical (laser) beam, interest has recently been shown in changing the beam shape to determine if a different power distribution at the transmitter will reduce the effects of random fluctuations in the refractive index. Here, a model is developed for the field of a flattened Gaussian beam as it propagates through atmospheric turbulence, and the resulting effects on the scintillation of the beam and beam wander are determined. A comparison of these results is made with the like effects on a standard TEM00 Gaussian beam. The theoretical results are verified by comparison with a computer simulation model for the flattened Gaussian beam (FGB). Further, a determination of the probability of fade and of mean fade time under weak fluctuation conditions is determined using the widely accepted lognormal model. Although this model has been shown to be somewhat optimistic when compared to results obtained in field tests, it has value here in allowing us to compare the effects of atmospheric conditions on the fade statistics of the FGB with those of the lowest order Gaussian beam.

Magnetic characterization for cryogenic permanent-magnet undulators: a first result

The cryogenic permanent-magnet undulator (CPMU) is a novel insertion device recently proposed at SPring-8, in which permanent magnets (PMs) are cooled to cryogenic temperatures to improve the magnetic performances, such as remanence and coercivity. A new measurement system for carrying out high-precision magnetic field mapping using a Hall probe has been developed in order to characterize the magnetic field generated by PM arrays at cryogenic temperatures. In this system, alignment of the Hall probe was dynamically performed by means of detecting the variation in its transverse position using optical laser beams introduced into the vacuum chamber. Magnetic measurements of a CPMU prototype were made at different temperatures in order to investigate variations of the magnetic performances owing to temperature. The maximum remanence deduced from the average peak value of the field profile was found to be close to that obtained from a former measurement with a single piece of the same PM material. In addition, the error components in the field profile were found to be insensitive to temperature in terms of the electron trajectory and phase error. This result suggests that the field correction of CPMUs can be performed based on the field profile measured at room temperature, which considerably reduces the task and time necessary for construction of CPMUs.

Fourier treatment of near-field synchrotron radiation theory

In this paper, we couple synchrotron radiation (SR) theory with a branch of physical optics, namely laser beam optics. We show that the theory of laser beams is successful in characterizing radiation fields associated with any SR source. Both radiation beam generated by an ultra-relativistic electron in a magnetic device and laser beam are solutions of the wave equation based on paraxial approximation. It follows that they are similar in all aspects. In the space–frequency domain SR beams appear as laser beams whose transverse extents are large compared with the wavelength. In practical situations (e.g. undulator, bending magnet sources), radiation beams exhibit a virtual “waist” where the wavefront is often plane. Remarkably, the field distribution of a SR beam across the waist turns out to be strictly related with the inverse Fourier transform of the far-field angle distribution. Then, we take advantage of standard Fourier Optics techniques and apply the Fresnel propagation formula to characterize the SR beam. Altogether, we show that it is possible to reconstruct the near-field distribution of the SR beam outside the magnetic setup from the knowledge of the far-field pattern. The general theory of SR in the near-zone developed in this paper is illustrated for the special cases of undulator radiation. Using known analytical formulas for the far-field pattern and its inverse Fourier transform we find analytical expressions for near-field distributions in terms of far-field distributions.

Colloidal printer based on an optical micropump

The author demonstrate a colloidal printer based on optical forces. A focused optical laser beam transports paramagnetic particles from a reservoir and firmly pushes them against a substrate where they attach. The substrate is either functionalized with suitable chemical groups or contain magnetic patterns which can trap the particles.

Observable manifestation of an electron-positron plasma created by the field of an optical laser

Electron-positron vacuum pair creation in the quasi-periodic electric field of a standing wave is investigated for field strengths and frequencies corresponding to modern optical lasers using a quantum kinetic equation with a non-markovian source term. For a field E ∼ 5 · 1011 V/cm, the instantaneous quasiparticle electron-positron plasma is created which maximum density of order 1020cm−3. The mean value of the pair density per period <n> exceeds by many orders of magnitude the residual density nr which is taken over an integer number of field periods. The value <n> is proportional to the squared field intensity and does not depend on the frequency at ν ≪ m. Under conditions of presently available optical lasers with intensities exceeding I ∼ < 1020W/cm2, the mean number of created pairs in the volume of a wavelength cubed is about 107 which corresponds to 5 - 10 two-photon annihilation events per one laser pulse. The generated γ-quanta are suggested to be an observable signal of e+ e− pair vacuum creation in experiments with counter-propagating optical laser beams.

Calibrating laser beam deflection systems for use in atomic force microscopes and cantilever sensors

Most atomic force microscopes and cantilever-based sensors use an optical laser beam detection system to monitor cantilever deflections. We have developed a working model that accurately describes the way in which a position sensitive photodetector interprets the deflection of a cantilever in these instruments. This model exactly predicts the numerical relationship between the measured photodetector signal and the actual cantilever deflection. In addition, the model is used to optimize the geometry of such laser deflection systems, which greatly simplifies the use of any cantilever-based instrument that uses a laser beam detection system.

Ultracold quantum gases in optical lattices

Artificial crystals of light, consisting of hundreds of thousands of optical microtraps, are routinely created by interfering optical laser beams. These so-called optical lattices act as versatile potential landscapes to trap ultracold quantum gases of bosons and fermions. They form powerful model systems of quantum many-body systems in periodic potentials for probing nonlinear wave dynamics and strongly correlated quantum phases, building fundamental quantum gates or observing Fermi surfaces in periodic potentials. Optical lattices represent a fast-paced modern and interdisciplinary field of research.

Knotted threads of darkness

Destructive interference may lead to complete cancellation when light waves travelling in different directions cross, and in three-dimensional space this occurs along lines that are vortices of electromagnetic energy flow. Here we confirm theoretical predictions by experimentally creating combinations of optical laser beams in which these dark threads form stable loops that are linked and knotted.

Photo-injector study for the ELETTRA linac FEL

A new injector is one of the necessary upgrades to be made in the existing ELETTRA linac to improve its performance to the level required for FEL operation. To this end, the design of an RF photo-injector is in progress at ELETTRA. Several types of RF guns are currently under consideration. The drive laser system, beam optics, diagnostics, and other relevant issues are also being addressed.

Femto-second X-ray generation by the tera-watt laser slice method in the SPring-8 storage ring

Feasibility study of the generation of ultra-short pulse X-rays in the SPring-8 storage ring with a laser slice method is discussed. The electron beam energy is modulated by applying a tera-watt laser with a length of 100 fs in parallel to the electron beam running through a wiggler. Pulsed X-rays, emitted from a bending magnet, are separated from the X-rays due to the beam bunch with a slit. Parameters of the system are optimized based on a formalism with respect to the laser beam interaction in a wiggler, beam dynamics and laser beam optics. Numerical calculations of the interaction are carried out taking into account a Gaussian distribution of electron and laser beams. The expected flux is 8.1×10 4 photons/s/mrad/0.1% bandwidth at a bunch beam current of 10 mA with a pulse length of 240 fs in full width. If the lattice functions are modified such that an undulator is used as a radiator in a negative dispersion section, we can expect the pulsed X-rays with a flux of 2.9×10 7 photons/s/0.1% bw and a pulse length 180 fs. Because of a high beam energy of 8 GeV, a very high laser field is required, which induces limitations in the laser beam optics. Discussions are extended to the New SUBARU storage ring of 1.5 GeV.

Laser signals’ nonlinear change in fatty acids

Previous works showed that thin layers of fatty acids and fatty acid–cholesterol mixtures behaved as optical liquid crystals, even at low incident laser power. The paper presents an experimental and computer study of laser signals, emergent from such samples, in presence of fluctuations. The optical emergent laser beams’ features at different incident parameters were experimentally determined for different type (c.w. and pulsed) lasers, as for example helium–neon and Nd 3+ glass lasers. The results were correlated with the amount of cholesterol in mixtures and with their response in external electric field. These measurements are in all cases affected by fluctuations. We developed some computer-based procedures, by using the TableCurve3D from Jandel Scientific software and equations Runge–Kutta in MATLAB for taking into account these fluctuations.

Structure of a nonparaxial gaussian beam near the focus: III. Stability, eigenmodes, and vortices

Exact analytical structurally stable solutions of the Maxwell equations for singular mode beams propagating in free space or a uniform isotropic medium are obtained. Approximate boundary conditions are chosen in the form of the requirement that in the paraxial approximation the fields of nonparaxial mode beams in the waist plane are transformed into the fields of eigenmodes and vortices of weakly guiding optical fibers with the axial symmetry of refractive index. It is shown that optical vortices, in spite of a rather complex structure of field distribution, do not experience substantial changes in the beam form and reproduce, in general features, the field of paraxial vortices. Linear perturbations of the characteristic parameters of mode beams do not change the structure of their electromagnetic field. Nonparaxial singular beams have one more important property, in addition to the fact that the structure of these beams in the paraxial approximation is similar to the structure of the fields of eigenmodes in a fiber. The propagation constants of eigenmodes of a fiber exactly coincide (in the first approximation of perturbation theory) with the projection of the wave vector of a mode beam on the optical axis (an analog of the propagation constant). The possibility of the paraxial transition for nonparaxial mode beams with arbitrary values of azimuthal and radial indices is shown. The properties of nonparaxial modes are illustrated by numerous examples. The solutions obtained and the results of their analysis can be used for exact matching optical fibers and laser beams in various applications.

Numerical Analysis of CW Laser Propulsion.

Laser propulsion powered by a CW laser has been studied. Thruster performance and energy balance in the thruster were numerically computed. Laser beam optics, inverse-bremsstrahlung absorption, ionization/recombination reactions, radiation, heat conduction, and convection have been modeled. Computational stiffness resulting from the very small flow speed has been overcome by using a flux vector splitting implicit scheme with a large CFL number. The computed positions of the Laser Sustained Plasma (LSP) in the thruster show good agreement with the measured ones. The estimated energy conversion efficiency was 23%, and the rest of the input power was lost as radiation from the LSP and also carried by the laser beam passing through the LSP.

U.S. National Missile Defense: Looking at the Whole Package

The recent technical objections to the U.S. National Missile Defense system by a study group of the Union of Concerned Scientists (UCS) ([1][1]) (ScienceScope, “Hit or missile?”, 14 Apr., p. [243][2]) and by several independent researchers (News Focus, “Researchers target flaws in ballistic missile defense plan” by D. Malakoff and A. Cho, 16 June, p. 1940) are missing the point: The planned system will not operate just on its own, but will rely for warhead/decoy discrimination on information supplied by a number of complementary systems and use the interceptor's on-board sensors mainly for homing on a designated target. It is critical to discuss these complementary systems as well, not just the limited discrimination capability of the system under consideration for approval by President Bill Clinton this fall. Four major passive or active discrimination techniques—radar and infrared detectors, and laser and particle beams—were evaluated in a report from the American Physical Society (APS) study group on the status of the science and technology of directed energy weapons ([2][3]). According to the APS study, the best potential for discrimination is offered by interactive high-energy beam techniques, techniques that are not even mentioned in the UCS report. In this area, one of the most important advances in the past 10 years has been the development of superlasers ([3][4], [4][5]); that is, the invention of “chirped pulse amplification,” which increases the instantaneous power of lasers by a factor of 1 million. Such power enables tabletop lasers to produce nuclear reactions directly ([4][5]) (see also Science , 2 Apr. 1999, p. [35][6]). Superlaser beams combine the ease in steering and focusing of optical laser beams with the capacity of particle beams to generate high-energy secondary particles in distant targets, which gives them the ability to “x-ray” remote objects and discriminate as to whether they are warheads or decoys. The political implications of ballistic missile defense are of such a magnitude that it is important that any technical assessment should be as accurate and comprehensive as possible—including a serious evaluation of the prospect of all existing and emerging warhead/decoy discrimination techniques. 1. [↵][7]1. A. M. Sessler 2. et al. , Countermeasures Union of Concerned Scientists, Cambridge, MA, (2000). 2. [↵][8]1. N. Bloembergen 2. et al. , Rev. Mod. Phys. 59, S1 (1987). [OpenUrl][9][CrossRef][10] 3. [↵][11]1. A.J. Gsponer, 2. P. Hurni , Fourth Generation Nuclear Weapons IANUS, Darmstadt Univ. of Technology, Germany, (1999). 4. [↵][12]1. G. A. Mourou 2. et al. , Phys. Today 22, ((January 1999)); 1. J.-M. Hopkins 2. et al. , Sci. Am. 54 (Sept. 2000). [1]: #ref-1 [2]: /lookup/doi/10.1126/science.288.5464.243b [3]: #ref-2 [4]: #ref-3 [5]: #ref-4 [6]: /lookup/doi/10.1126/science.284.5411.35a [7]: #xref-ref-1-1 View reference 1 in text [8]: #xref-ref-2-1 View reference 2 in text [9]: {openurl}?query=rft.jtitle%253DRev.%2BMod.%2BPhys.%26rft.volume%253D59%26rft.spage%253DS1%26rft.atitle%253DREV%2BMOD%2BPHYS%26rft_id%253Dinfo%253Adoi%252F10.1103%252FRevModPhys.59.S1%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [10]: /lookup/external-ref?access_num=10.1103/RevModPhys.59.S1&link_type=DOI [11]: #xref-ref-3-1 View reference 3 in text [12]: #xref-ref-4-1 View reference 4 in text

Photothermal stability of encapsulated Si solar cells and encapsulation materials upon accelerated exposures

We have conducted a series of accelerated exposure test (AET) studies for various samples of crystalline-Si (c-Si) and amorphous-Si (a-Si) cells that were encapsulated with different superstrates, pottants, and substrates. Transmittance, fluorescence, color indices, impedance spectroscopy, laser optical beam induced current (OBIC), and light and dark current–voltage ( I– V) measurements were used to characterize the samples. Nonuniform browning patterns of ethylene vinyl acetate (EVA) pottants were observed for glass/EVA/glass-encapsulated c-Si cell samples under solar simulator exposures at elevated temperatures. The EVA discolored gradually into a yellow–brown color when exposed under ∼7.5 ultraviolet (UV) suns at a black panel temperature (BPT) of ∼85°C, and very rapidly into a dark brown color when exposed under ∼9.0 UV suns at a BPT of ∼145°C. The latter conditions also caused extensive delamination of a polyolefin-based thermoplastic pottant laminated between two borosilicate plates. No visible color change of EVA could be observed for the c-Si cell samples heated in an oven at 85°C or exposed under ∼1.2 UV suns at BPT=60–65°C or 80–85°C. Under ∼1.2 UV suns exposure at a BPT of 60–65°C, the thin layers of EVA or silicone in the lightweight c-Si cell samples, which were laminated in a polymer/solar cell/polymer configuration with Tedlar or Tefzel films, did not discolor because of photobleaching reactions. In comparison, the polyester or nylon superstrate films turned yellow on laminates of polymer/a-Si/polymer minimodules, and significant delamination was observed for the polyester/EVA layers. For all c-Si cell samples tested, irregular changes in the I– V parameters were observed that could not be attributed only to transmittance changes of the superstrate/pottant layers. Under UV-transmitting polymer superstrate films, current/efficiency losses were greater for the c-Si and a-Si cells that were laminated with silicone-type adhesives than with EVA or polyethylene pottants.

Process optimization controller for robotic laser machining

This work details the performance assessment of a high-bandwidth optical range sensor for local stand-off control of a robotically manipulated optical fiber laser beam delivery system. Consistent, repeatable high quality materials processing, using fiber optically delivered Nd3+-YAG laser radiation requires precise local control of stand-off distance. Unless specifically designed for laser processing applications, many robots produce a jerking incremental trajectory and usually suffer from position dependent displacement of the end effector. With the addition of a focal height servomechanism performance of the system can be considerably improved. The high-bandwidth optical range sensor described herein allows the use of such robots for fiber optically delivered laser beams enabling high quality laser cutting and welding operations using equipment that may already exist. Radiation from a low power laser diode is focused onto a workpiece surface; light reflected from the surface is collected through a main lens and directed into an astigmatic lens, which focuses the signal onto a quadrant photodiode. A differential amplifier is used to generate an output signal that determines the magnitude and direction of any workpiece displacement. The system facilitates a measuring range of ±5 mm; its effectiveness in controlling the stand-off distance for laser materials processing applications is assessed.

Two-dimensional Z scan for arbitrary beam shape and sample thickness

A two-dimensional Z-scan technique has been developed using a two-dimensional charge-coupled device (CCD) detector to study the nonlinear optical properties and beam profile evolution within optical limiting devices. Using the split step beam propagation method, the far field pattern at each Z position can be calculated precisely for any arbitrary beam shape and sample thickness. A two-dimensional far field pattern is recorded by the CCD detector, and the evolution of the beam distribution inside the nonlinear optical medium can be obtained directly. This technique has been applied to study the nonlinear optical parameters and laser beam profiles in nonlinear liquids. We have used thin and thick samples and Gaussian, top-hat and split Gaussian beam distributions, and the experimental results agreed very well with the calculation. This technique also offers a simple and accurate means for optimizing the design of optical limiting devices.