Gaussian Beam Optics Research Articles

Design and Installation of a New Electron Cyclotron Emission Diagnostic Antenna in LHD

A new electron cyclotron emission (ECE) antenna was designed and installed outside of torus, when the heating and diagnostics systems were reconfigured in LHD. Its design is based on Gaussian beam optics and consists of two plane and two concave mirrors. The mirror surfaces are defined using the concept of constant phase to improve matching to the corrugated waveguide. The new ECE antenna was used in a 2010 experiment to measure the electron temperature profile and its fluctuations.

Position detection and vibration monitoring system using quad-cell optical beam power distribution

In this paper, we study the accuracy of an optical vibration sensing system using Gaussian beam analysis. The proposed optical system uses a He–Ne laser source whose Gaussian beam impinges on a quad-cell photodetector array. The normalized optical power for each cell is obtained through numerical simulation based on the well-known Gaussian beam optics formulation, as the beam scans the plane of the photodetectors due to vibration. The system detection principle lies upon finding a relationship between the power distribution of the quad-cell and the position of the beam centroid, which allows us to identify the characteristics of the vibration applied, such as direction, frequency and strength. The aspects of practical implementation and experimental limitations on the power distribution accuracy are discussed and the discrepancies with the theoretical results are presented.

Analysis of the optical force dependency on beam polarization: dielectric/metallic spherical shell in a Gaussian beam

We carry out a detailed numerical simulation study to investigate the dependency of optical force on beam polarization. Using the well-known finite-difference time domain method and Maxwell's stress tensor, we consider general dielectric (e.g., glass) or metallic (e.g., gold) spherical shells immersed in a Gaussian optical beam. Our results show that TE and TM polarized Gaussian beams exert different amounts of optical force depending on the shell dimensions and material properties. We specifically show that purely dielectric shells do not experience different optical forces due to polarization differences but TM polarized beams exert higher optical force on metallic shells than equivalent TM polarized beams.

Optical Gaussian beam interaction with one-dimensional thermal wave in the Raman-Nath configuration

Optical Gaussian beam interaction with a one-dimensional temperature field in the form of a thermal wave in the Raman-Nath configuration is analyzed. For the description of the Gaussian beam propagation through the nonstationary temperature field the complex geometric optics method was used. The influence of the refractive coefficient modulation by thermal wave on the complex ray phase, path, and amplitude was taken into account. It was assumed that for detection of the modulated Gaussian beam parameters two types of detector can be used: quadrant photodiodes or centroidal photodiodes. The influence of such parameters as the size and position of the Gaussian beam waist, the laser-screen (detector) distance, the thermal wave beam position and width, as well as thermal wave frequency and the distance between the probing optical beam axis and source of thermal waves on the so-called normal signal was taken into account.

A prototype quantitative film scanner for radiochromic film dosimetry

We have developed a high resolution, quantitative, two-dimensional optical film scanner for use with a commercial high sensitivity radiochromic film (RCF) for measuring single fraction external-beam radiotherapy dose distributions. The film scanner was designed to eliminate artifacts commonly observed in RCF dosimetry. The scanner employed a stationary light source and detector with a moving antireflective glass film platen attached to a high precision computerized X-Y translation stage. An ultrabright red light emitting diode (LED) with a peak output at 633 nm and full width at half maximum (FWHM) of 16 nm was selected as the scanner light source to match the RCF absorption peak. A dual detector system was created using two silicon photodiode detectors to simultaneously measure incident and transmitted light. The LED light output was focused to a submillimeter (FWHM 0.67 mm) spot size, which was determined from a scanning knife-edge technique for measuring Gaussian optical beams. Data acquisition was performed with a 16-bit A/D card in conjunction with commercial software. The linearity of the measured densities on the scanner was tested using a calibrated neutral-density step filter. Sensitometric curves and three IMRT field scans were acquired with a spatial resolution of 1 mm for both radiographic film and RCF. The results were compared with measurements taken with a commercial diode array under identical delivery conditions. The RCF was rotated by 90 deg and rescanned to study orientation effects. Comparison between the RCF and the diode array measurements using percent dose difference and distance-to-agreement criteria produced average passing rates of 99.0% using 3%/3 mm criteria and 96.7% using 2%/2 mm criteria. The same comparison between the radiographic film and diode array measurements resulted in average passing rates 96.6% and 91.6% for the above two criteria, respectively. No measurable light-scatter or interference scanner artifacts were observed. The RCF rotated by 90 deg showed no measurable orientation effect. A scan of a 15 x 15 cm2 area with 1 mm resolution required 22 min to acquire. The LED densitometer provides an accurate film dosimetry system with 1 mm or better resolution. The scanner eliminates the orientation dependence of RCF dosimetry that was previously reported with commercial flatbed scanners.

Complex degree of coherence for partially coherent general beams in atmospheric turbulence

With the use of the general beam formulation, the modulus of the complex degree of coherence for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian, annular and higher-order Gaussian optical beams is evaluated in atmospheric turbulence. For different propagation lengths in horizontal atmospheric links, the moduli of the complex degree of coherence at the source and receiver planes are examined when reference points are taken on the receiver axis and off-axis. In the on-axis case, it is observed that in propagation, the moduli of the complex degree of coherence are symmetrical and look like the intensity profile of the related coherent beam propagating in a turbulent atmosphere. For all the beams considered, the moduli of the complex degree of coherence profiles turn into Gaussian shapes beyond certain propagation lengths. In the off-axis case, the moduli of complex degree of coherence patterns become drifted at the earlier propagation lengths. Among the beams investigated, the cos-Gaussian beam is found to be almost independent of the changes in the source partial coherence parameter, and the annular beam seems to be affected the most against the variations of the source partial coherence parameter.

Study of nonlinear hot image effect of Gaussian optical beams

In high-power lasers, nonlinear hot image is likely to damage expensive optics. The location and intensity of hot image of plane light wave has widely been studied. However, in the operating lasers, the light beam possesses a limited spatial width. Based on the aberrationless model of self-focusing, the location and intensity of hot image of Guassian optical beams is investigated. It is shown that the location and intensity of hot image greatly differ from those of plane light wave when the Guassian beam has a small beam waist, while they approximately equal those of plane light wave for a very big beam waist.

On the scintillation index of a multiwavelength Gaussian beam in a turbulent free-space optical communications channel

The scintillation statistics of a multiwavelength Gaussian optical beam are characterized when the beam is subjected to a turbulent optical channel. It is assumed that the level of turbulence in the atmosphere ensures a weak-turbulence scenario and that fluctuations in the signal intensity are due to variations in the refractive index of the medium, which in turn are caused by regional temperature variations due to atmospheric turbulence. Furthermore, it is assumed that the propagation path is nearly horizontal and that the heights of the transmitter and receiver justify a near-ground propagation assumption. The Rytov approximation is used to arrive at the desired results. Furthermore, it is assumed that the first- as well as second-order perturbation terms are present in modeling the impact of atmosphere-induced scintillation. Numerical results are presented to shed light on the performance of multiwavelength optical radiation in weak turbulence and to underscore the benefits of the proposed approach as compared with its single-wavelength counterpart in combating the effect of turbulence. Furthermore, it is shown that if the separation of wavelengths used is sufficiently large, wavelength separation affects the scintillation index in a measurable way.

Gaussian optical beam propagation in thermal wave field – the ray theory, the complex ray theory and experimental results

The comparison of the ray theory, the complex ray theory with the experimental results was studied in the work. The least-squares procedure was used in multiparameter fitting of experimental data and theoretical result. Some parameters of experimental setup were also determined. There was proved in the work that the complex ray theory better describes the experimental results than the ray theory. Moreover, the complex ray theory takes into account some effects that do not occur in the ray theory at all.

Ray Model of an Incident Gaussian Beam for Analyzing a Micro-Optical Communication Device with a GRIN Lens System

For the design of reliable micro-optical devices with a high coupling efficiency, a source-modeling method necessary for ray tracing is newly proposed. This method is based on the ray-division of an incident wavefront estimated by Gaussian beam optics. The simulation result obtained using the proposed method was compared with those of the conventional methods, and its accuracy was confirmed by the experimental results. Finally, the method was used to design a micro-optical device with two GRIN lenses and a thin-film filter.

Modelling and optimization of a Fabry–Perot microcavity for sensing applications

In this work, a generalized study has been carried out on the transmission characteristics of a Fabry–Perot microcavity illuminated by a laser beam. The analytical expressions are obtained based on Gaussian beam optics and Smith's method. A novel Fabry–Perot microcavity optimization model is then established to achieve both the best sensing performance and least measurement tolerance for high yield of sensors during batch fabrication. The perturbations of Fabry–Perot pressure sensor performance induced by other common issues, such as non-normal incidence angle and microcavity mirror imperfections, are also discussed.

Diffraction of optical communication Gaussian beams by volume gratings: comparison of simulations and experimental results.

The diffraction effects induced by a thick holographic grating on the propagation of a finite Gaussian beam are theoretically analyzed by means of the coupled-wave theory and the beam propagation method. Distortion of the transmitted and diffracted beams is simulated as a function of the grating parameters. Theoretical results are verified by experimentation realized by use of LiNbO3 volume gratings read out by a 1550-nm Gaussian beam, typical of optical fiber communications. This analysis can be implemented as a useful tool to aid with the design of volume grating-based devices employed in optical communications.

Hybrid GB and PO analysis of electromagnetic radiation/scattering from large reflector antennas with tapered impedance surfaces

AbstractA hybrid scheme combining Gaussian beam (GB) and physical optics (PO) approaches is presented for an efficient analysis of reflector antennas with tapered‐impedance edge boundaries. The reflecting surface is divided into an interior portion with uniform impedance and a smooth boundary, where the GB approach can be employed, and an exterior portion of tapered impedance, which is analyzed by the PO approach. The high efficiency of the GB analysis potentially accelerates the entire analysis, as compared with the case in which only PO is employed. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 34–37, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20198

Tunable Optical Transversal Filters Based on a Gires–Tournois Interferometer With MEMS Phase Shifters

We present tunable optical filters based on a modified Gires-Tournois interferometer. The back reflection plane of the interferometer is replaced with a one-dimensional micromirror array for phase modulation. Using Gaussian beam optics, we show that the transfer function of the device has the form of the transversal filter in digital signal processing. The design and analysis techniques of conventional digital filters can, therefore, be adapted to tunable optical filters. Both the amplitude and phase of the transfer function can be controlled by reconfiguring the micromirrors. Application examples in dispersion compensation and variable bandwidth bandpass filtering are discussed and experimentally demonstrated.

Effect of spatial phase modulation on propagation of photovoltaic spatial solitons

Based on the technology of spatial phase modulation, a simple scheme to steer optical Gaussian beams in photovoltaic media is presented. Numerical simulations show that we can realize the self-deflection, soliton like propagation, divergence and splitting of Gaussian beams in photovoltaic media. The relationship between the intensity profile of the incident beam and the modulation parameters is also discussed.

Two-dimensional correlation measurements of electron cyclotron emission fluctuations on the stellarator Wendelstein 7-AS

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level.

DISPLACEMENT AND DEFLECTION OF OPTICAL BEAMS BY NONLINEAR PLANAR WAVEGUIDE

In this paper we investigate the propagation characteristics of an end fired Gaussian optical beam through a planar nonlinear waveguide. Using variational formalism we have derived three coupled second order dynamical equations for the beam widths and centre of gravity of the propagating optical beam. Using these equations, we have investigated numerically the beam deflection, displacement and their possible control by nonlinearity. We have shown that intensity dependent control is possible only when the waveguide core possesses nonlinearity and the substrate is linear. In addition, we have demonstrated that such control is not possible in bulk media. We have further established that setting an appropriate length of the waveguide one can achieve maximum displacement with minimum deflection or minimum displacement with maximum deflection. Possible application of our investigation in optical communication and signal processing are highlighted.

Euclidean Light: High-School Geometry to Solve Problems In Gaussian Beam Optics

It is possible to describe the propagation of a gaussian beam using diagrams based only on Euclidean geometry. All that is needed to solve problems is a ruler and a pair of compasses.

Wave and ray spatial dynamics of the light field in the generation, evolution, and annihilation of phase dislocations

The spatial dynamics of the light field during the generation, evolution, and annihilation of screw phase dislocations in Gaussian optical beams is analyzed. The regularities of transformation of an aberration wave front of a beam into a singular front are exposed. The structure of the field of directions of the energy-flux density vector is analyzed on the basis of a solution of a parabolic wave equation for the complex amplitude of a monochromatic light field with the construction of streamlines and investigation of the full set of critical points in the corresponding dynamic system. It is established that the energy redistribution in the beam and appearance of dislocations occur in accordance with the transformation of singularities: local focusing of a streamline in the vicinity of a node is a precursor of a generated pair of screw dislocations, and the streamline takes on a helix shape near an unstable focus.

Application of Novel Focusing Mirrors in Gyrotron Transmission Lines

A novel focusing mirror based on the shifting of the beam phase according to the Gaussian beam optics enabled us to realize a focusing elements with different focal lengths in different directions and to convert the gyrotron output into a Gaussian-like beam. In this paper, we compare the quality of beams produced by the new system (which includes novel mirrors) and a conventional system consisting of a quasi-optical antenna, an ellipsoidal mirror and two parabolic cylinder mirrors.