Ray-transfer-matrix model for accurate pulsed cavity ring-down measurement in the mismatching case

A theoretical model based on the ray-transfer matrix is developed for the pulsed cavity ring-down (CRD) technique to numerically investigate the influence of the geometric parameters of the pulsed-CRD arrangement on the CRD signal. By fitting the spatial distribution of the pulsed laser beam to that of the TEM(00) cavity mode, the geometric parameters are optimized to obtain perfect matching between the laser beam and the ring-down cavity. It is indicated by the numerical simulations that as long as the laser power exiting the ring-down cavity is fully collected, a single exponential-decay signal, identical to the perfectly-matched CRD signal, is obtained in the mismatching case to determine accurately the cavity decay time. Intensity fluctuations appear in the mismatched CRD signal if the laser power exiting the ring-down cavity is not fully collected. Both the conventional exponential decay fitting approach and a linear fitting procedure are employed to analyze these mismatched CRD signals and the latter is recommended to make an accurate pulsed-CRD measurement.     

Generation of a flat-top laser beam for gravitational wave detectors by means of a nonspherical Fabry-Perot resonator

We have tested a new kind of Fabry-Perot long-baseline optical resonator proposed to reduce the thermal noise sensitivity of gravitational wave interferometric detectors--the "mesa beam" cavity--whose flat top beam shape is achieved by means of an aspherical end mirror. We present the fundamental mode intensity pattern for this cavity and its distortion due to surface imperfections and tilt misalignments, and contrast the higher order mode patterns to the Gauss-Laguerre modes of a spherical mirror cavity. We discuss the effects of mirror tilts on cavity alignment and locking and present measurements of the mesa beam tilt sensitivity.     

Optical-axis perturbation in triaxial ring resonator

We report several findings on the optical-axis perturbation of monolithic triaxial ring resonator. A criterion, C, which represents the mismatching error of the monolithic triaxial ring resonator, has been found out and it cannot be decreased by modifying the angles of the terminal surfaces or the terminal mirrors of the resonator. When C not equal 0, an optimization method to share the mismatching error C in some specific directions equally and simultaneously has been proposed. The interesting findings are important to cavity design, cavity improvement, and alignment of the monolithic triaxial ring resonator.     

Cavity with a deformable mirror for tailoring the shape of the eigenmode

We demonstrate an optical cavity that supports an eigenmode with a flattop spatial profile--a profile that has been proposed for the cavities in the Advanced Laser Interferometer Gravitational Wave Observatory, the second-generation laser interferometric gravitational wave observatory--because it provides better averaging of the spatially dependent displacement noise on the surface of the mirror than a Gaussian beam. We describe the deformable mirror that we fabricated to tailor the shape of the eigenmode of the cavity and show that this cavity is a factor of 2 more sensitive to misalignments than a comparable cavity with spherical mirrors supporting an eigenmode with a Gaussian profile.     

Scattering of a transversely confined Neumann beam by a spherical particle

Various properties of an electromagnetic wave whose spherical multipole expansion contains only Riccati-Neumann functions are examined. In particular, the novel behavior of the beam phase during diffractive spreading is discussed. When a Neumann beam is scattered by a spherical particle, the diffraction and external reflection portions of the scattering amplitude constructively interfere for large partial waves. As a result, a set of rapidly decreasing beam shape coefficients is required to cut off the partial wave sum in the scattering amplitudes. Because of its strong singularity at the origin, a Neumann beam can be produced by a point source of radiation at the center of a spherical cavity in a high conductivity metal, and Neumann beam scattering by a spherical particle can occur for certain partial waves if the sphere is placed at the center of the cavity as well.     

Polarization characteristics of a Nd:YAG laser side pumped by diode laser bars

A detailed experimental investigation of the polarization characteristics of a diode laser side-pumped Nd:YAG laser has been performed. A Brewster plate inserted into the cavity resulted in a decrease of 50% of the output power that is due to depolarization. A thin-film polarizer (TFP) was introduced into the cavity and the depolarization loss served as the output coupling. By introduction of a quarter-wave plate (QWP) the output coupling rate and the beam shape were improved. An analytical expression for the output coupling rate of a cavity containing a TFP and a QWP has been derived. The experimental results are in good agreement with the simplified theoretical analysis.     

Generation of doughnutlike vortex beam with tunable orbital angular momentum from lasers with controlled Hermite-Gaussian modes

This study demonstrates successive higher-order Hermite-Gaussian (HG(0,m)) mode operations in a microchip solid-state laser with a controlled off-axis laser diode (LD) pumping and generation of the corresponding doughnutlike laser beam of tunable ring diameter and orbital angular momentum, by experimentally focusing a Hermite-Gaussian mode (HGM) lasing beam into an astigmatic mode converter (AMC) with a mode-matching lens. Based on the successful generation of stable doughnutlike vortex beams by combining the LD off-axis pumping of microchip lasers and an AMC, this study proposes a design for a compact, solid doughnutlike vortex laser beam generator that combines three elements (i.e., laser cavity, mode-matching lens, and AMC) into one practical device. The desired doughnutlike vortex beam with different orbital angular momentum is easily generated by simply controlling the lateral off-axis pump position and pump beam shape on the laser crystal by numerical simulation.     

Cross-polarized photorefractive four-wave mixing with extraordinary writing beams and an ordinary reading beam

In parallel-polarized photorefractive four-wave mixing the phase-matching condition can be achieved automatically if the pump beams are allowed to propagate in directions opposite to each other. However, in cross-polarized photorefractive four-wave mixing, the incident angles of the beams should be adjusted to maintain the phase-matching condition. We analyze the phase-matching properties of cross-polarized four-wave mixing with extraordinary writing beams and an ordinary reading beam and obtain the phase-conjugate reflectivity in consideration of phase mismatching. We also perform an experiment in which a phase-conjugate beam is generated in cross-polarized four-wave mixing and confirm the analyses.     

A regularization approach for the determination of the time offlight distribution in the cesium beam standards

In this paper, a new method is presented to directly determine from the shape of a Ramsey pattern the real microwave level present in the cavity of a beam frequency standard. Then a new method to evaluate the atomic time of flight distribution using a regularization approach is presented. In opposition to existing methods, this approach uses only one Ramsey measurement because it allows the incorporation of the a priori information about the time of flight distribution. The validity of this method is confirmed by experimental results     

Evaluating the effect of transmissive optic thermal lensing on laser beam quality with a shack-hartmann wave-front sensor

We examine wave-front distortion caused by high-power lasers on transmissive optics using a Shack-Hartmann wave-front sensor. The coupling coefficient for a thermally aberrated Gaussian beam to the TEM(00) mode of a cavity was determined as a function of magnitude of the thermally induced aberration. One wave of thermally induced phase aberration between the Gaussian intensity peak and the 1/e(2) radius of the intensity profile reduces the power-coupling coefficient to the TEM(00) mode of the cavity to 4.5% with no compensation. With optimal focus compensation the power coupling is increased to 79%. The theoretical shape of the thermally induced optical phase aberration is compared with measurements made in a neutral-density filter glass, Faraday glass, and lithium niobate. The agreement between the theoretical and the measured thermal aberration profiles is within the rms wave-front measurement sensitivity of the Shack-Hartmann wave-front sensor, which is a few nanometers.     

A new cavity configuration for cesium beam primary frequencystandards

In the design of cesium beam frequency standards, the presence of distributed cavity phase shifts (associated with residual running waves) in the microwave cavity, due to the small losses in the cavity walls, can become a significant source of error. To minimize such errors in future standards, it has been proposed that the long Ramsey excitation structure be terminated with ring-shaped cavities in place of the conventional shorted waveguide. The ring cavity will minimize distributed cavity phase variations at the position of the atomic beam, provides only that the two sides of the ring and the T-junction feeding the ring are symmetric. A model is developed to investigate the validity of this concept in the presence of the small asymmetries that inevitably accompany the fabrication of such a cavity. The model, partially verified by laboratory tests, predicts that normal tolerances will allow the frequency shifts due to distributed cavity phase variations to be held at the 10^-15 level for a beam tube with a Q of 10⁸     

High-intensity multi-bunch beam generation by a photo-cathode RF gun

A multi-bunch photo-cathode RF gun system has been developed as an electron source for the production of intense quasi-monochromatic X-rays based on inverse Compton scattering. The desired multi-bunch beam is 100 bunches/pulse with a total charge of 500 nC and a bunch spacing of 2.8 ns. We modified the gun cavity of a ‘BNL-type IV’ RF gun to allow a CsTe cathode plug in the end plate. The system uses a four-dipole chicane beam line to allow the injection of laser light normal to the cathode surface. We compensate the gun cavity beam loading caused by the high-intensity multi-bunch electron beam by injecting the laser pulse before RF power has filled the cavity. We have achieved a total intensity of 220 nC in 100 bunches with a bunch-to-bunch energy spread under 1.3% (peak-to-peak). This paper concentrates on experiments to generate the high-intensity multi-bunch beam with compensation of the bunch-to-bunch energy spread due to heavy beam loading.     

Automatic alignment-control system for a suspended Fabry-Perot cavity

In interferometric gravitational-wave detectors, most of the optical components are suspended by wires so that they are isolated from all kinds of forces except gravity. The requirement for the alignment of optical components to the laser beam is crucial. We have demonstrated a servo system developed for a Fabry-Perot cavity whose mirrors are suspended independently. We use mechanical modulation and a lock-in detection method to detect any misalignment. This system directly detects the relation between the axis of the laser beam and the axis of the cavity and automatically aligns the cavity to the laser beam. We confirmed that the intensity of the reflected light from the suspended Fabry-Perot cavity can be minimized with this system. Automated control of the alignment of the large-scale detectors is also discussed.     

Sensitivity of a three-mirror cavity to thermal and nonlinear lensing: Gaussian-beam analysis

We consider a compact three-mirror cavity consisting of a flat output coupler, a curved folding mirror, and an active medium with one facet cut at the Brewster angle and the other facet coated for unit reflectivity. We examine the sensitivity to thermal lensing and to self-focusing in the active medium of the Gaussian beam that is circulating in that cavity. We use a simple thin-lens model; the astigmatism of the beam that is circulating in the cavity and the nonlinear coupling between the field distributions along the two orthogonal axes are taken into account. We find configurations in which beam ellipticity is compensated for at either end of the cavity in the presence of thermal lensing. We have derived an analytical criterion that predicts the sensitivity of the beam size to nonlinear lensing. The ability of the cavity to favor self-mode locking is found to be sensitive to the strength of thermal lensing. In the absence of thermal lensing, cavities operated as telescopic systems (C = 0) or self-imaging systems (B = 0) are most appropriate for achieving self-mode locking, with nonlinear mode selection accomplished through saturation of the spatially varying laser gain. We identify conditions for which self-mode locking can be produced by variable-reflectivity output couplers with either maximum or minimum reflectivity at the center of the coupler. We use our model to estimate the nonlinear gain produced in laser cavities equipped with such output couplers. We identify a cavity configuration for which nonlinear lensing can simultaneously produce mode locking and correction of beam ellipticity at the output coupler.     

Numerical Analysis of the Effect of Diffusion and Creep Flow on Cavity Growth

In this paper, intergranular cavity growth in regimes, where both surface diffusion and deformation enhanced grain boundary diffusion are important, is studied. In order to continuously simulate the cavity shape evolution and cavity growth rate, a fully-coupled numerical method is proposed. Based on the fully-coupled numerical method, a gradual cavity shape change is predicted and this leads to the adverse effect on the cavity growth rate. As the portion of the cavity volume growth due to jacking and viscoplastic deformation in the total cavity volume growth increases, spherical cavity evolves to V-shaped cavity. The obtained numerical results are physically more realistic compared to results in the previous works. The present numerical results suggest that the cavity shape evolution and cavity growth rate based on the assumed cavity shape, spherical or crack-like, simply cannot be used in this regime.     

Off-Axis Electron Holography of Nearly-Spherical Faceted Voids in Self-Annealed Implanted Silicon

Nearly spherical voids, with a size on the order of some tens of nanometers, are defects that have recently attracted a renewed interest, due to their capability to getter impurities and point defects in silicon. High-resolution electron holography is employed here to study the three-dimensional configuration of nearly spherical cavities obtained by 100keV P+ ion bombardment of a silicon wafer using an ion beam with a power density of about 40 W/cm² for 4 sec. Reconstructed phase maps have been used to obtain the qualitative topography of the cavity shape as well as quantitative measurements of the depth variations. Faceting of the nearly spherical voids is discussed in detail.     

Design of interaction cavity of a 170-GHz, 1-MW gyrotron for ECRH application

The design of the interaction cavity of a 170 GHz gyrotron operating in the TE_34,10 mode is presented in this article. An in-house developed code GCOMS and Particle-in-Cell (PIC) code MAGIC are used for the mode selection and beam–wave interaction simulations, respectively. The cold cavity analysis and beam–wave interaction computation are carried out to analyze the eigenmode and output power, respectively. A thorough parametric analysis of the interaction cavity geometry and electron beam parameters is also carried out with respect to the output power and frequency. The results show the capability of the interaction cavity, designed for the TE_34,10 mode, to produce more than 1 MW of RF power of a gyrotron at the operating frequency of 170.03 GHz.     

Generation of radially polarized beams with an image-rotating resonator

We show how a passive image-rotating optical resonator can be used to convert a linearly polarised, lowest-order Gaussian beam into a radially polarized beam. The image and polarization rotation of the cavity removes the frequency degeneracy of the modes, making it possible to select the radially polarized mode by cavity tuning. With the addition of gain, the same cavity should operate as a radially polarized laser when injection seeded at the proper wavelength.     

Frequency-Impulse Modulation as a Means of Attaining Accuracy in Cesium Atomic Clocks

An electronic system has been developed which corrects for cavity-detuning errors in cesium atomic-beam frequency standards. The RF signal applied to the beam tube is square-wave phase modulated, i.e., frequency-impulse modulated. The transient response of the beam tube to these phase steps is used as a control signal. When the positive and negative transients are both equal in area and identical in shape, it will be shown that the applied RF signal must be exactly at cesium frequency (for the given magnetic field) and the RF cavities must be exactly in tune. Two feedback loops are needed; one for correcting the crystal oscillator, the other for adjusting the relative phase of the RF cavities. With cavity detuning error greatly reduced by this system, the remaining source of inaccuracy is the uncertainty of the magnetic field in the drift region of the beam tube. An experiment is described which may permit setting a given cesium-beam standard to a frequency that differs by a precisely known amount from the zero-field cesium-resonance frequency.     

Physical and Thermal Processes During Electron Beam Welding

Complicated processes of the seam formation during electron beam welding are described. The physical processes in the weld pool and in the vapor-plasma mixture emitted in the cavity into the liquid metal created by intensive high energy beam are discussed. The dynamics of the liquid metal in the weld pool are interconnected with the energy distribution of the electron beam in the cavity which influences the weld depth and width and controls the defect formation in the weld such as pores (blow holes), non-uniformity of the weld root (spiking) and rippled weld surface. The moving heating source, working in the weld pool of metal samples is steady only as a first approximation due to the molten pool dynamics. A more exact heat model of the electron beam welding must take into account the unstable nature of beam transport within vapors and gas plasma in the cavity as well as the variations of energy dissipation on the crater walls and of heat transfer through molten metal of the weld pool.