Compensation of beam ellipticity and astigmatism based on holographic diffractive elements recorded onto spherical substrates

Abstract

Edge emitting diode lasers with their divergent, highly elliptical and astigmatic beams in the visible spectrum are widely used in all branches of photonics. Usually the beams must be transformed into circular anastigmatic beams for the majority of applications. Holographic diffractive elements on spherical substrates are devised for transformation of beams to circular collimated beams. An off-axis holographic set-up is used to record diffractive elements into a thin photoresist layer as shallow surface-relief gratings working in reflection mode with curved and chirped grooves. The elements are destined for the diode lasers emitting at a suitable wavelength and with appropriate ellipticity and astigmatism. The performance of the elements is tested on the basis of intensity patterns and the elements produced at a focal plane on their illumination with a collimated expanded beam of a HeNe laser.     

Trident: a versatile high-power Nd:glass laser facility for inertial confinement fusion experiments

The Trident Nd:glass laser system operates as an experimental facility supporting the national Inertial Confinement Fusion program at Los Alamos. The laser has two identical main beam lines with 14-cm-disk final amplifiers. The beams are frequency doubled, expanded to 19.2 cm, and focused on target with a variety of focusing optics. A third beam with 10-cm disk final amplifiers is also frequency doubled and used as a target-shooting or diagnostic beam simultaneously with the other two beams.

The facility provides a flexible combination of energy, pulse-shaping capabilities, and diagnostic tools for laser-target interaction experiments.

     

Optical image encryption scheme with multiple light paths based on compressive ghost imaging

An optical image encryption method with multiple light paths is proposed based on compressive ghost imaging. In the encryption process, M random phase-only masks (POMs) are generated by means of logistic map algorithm, and these masks are then uploaded to the spatial light modulator (SLM). The collimated laser light is divided into several beams by beam splitters as it passes through the SLM, and the light beams illuminate the secret images, which are converted into sparse images by discrete wavelet transform beforehand. Thus, the secret images are simultaneously encrypted into intensity vectors by ghost imaging. The distances between the SLM and secret images vary and can be used as the main keys with original POM and the logistic map algorithm coefficient in the decryption process. In the proposed method, the storage space can be significantly decreased and the security of the system can be improved. The feasibility, security and robustness of the method are further analysed through computer simulations.     

Splitting of femtosecond laser pulses by using a Dammann grating and compensation gratings

When a Dammann grating is used to split a beam of femtosecond laser pulses into multiple equal-intensity beams, chromatic dispersion will occur in beams of each order of diffraction and with different scale of angular dispersion because the incident ultrashort pulse contains a broad range of spectral bandwidths. We propose a novel method in which the angular dispersion can be compensated by positioning an m-time-density grating to collimate the mth-order beam that has been split, producing an array of beams that are free of angular dispersion. The increased width of the compensated output pulses and the spectral walk-off effect are discussed. We have verified this approach theoretically and validated it through experiments. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points.     

Oblique incidence of gaussian beams on waveguide gratings with the four-wave coupled-mode theory

Diffraction of obliquely incident Gaussian beams on waveguide gratings with infinitely long grooves is described. The analysis is based on the angular spectrum of the beam and on the grating response derived from the four-wave coupled-mode theory, which simultaneously considers all four (approximately) synchronous waves. Results concerning the shapes of the emerging beams and their directions, the power content, and the possibility of beam splitting could differ significantly from those obtained by means of two-wave coupling. The analysis method is general and can be performed on many kinds of realistic beam shapes and applications.     

An interferometric method for simultaneously determination the phase retardation and fast-axis azimuth angle of a wave plate

In this study, an interferometric method was proposed for the simultaneous measurement of phase retardation and fast-axis azimuth angle of a wave plate by using a Mach–Zehnder interferometer. The wave plate to be tested is placed in one of the light passages in the interferometer, and two analyzers with transmission axes at horizontal and vertical orientations are arranged at the two output regions. When a linearly polarized laser light beam is passed through the interferometer, two interference light beams are simultaneously generated. Through an analysis of the intensities of the two light beams, the phase retardation and fast-axis azimuth angle of the tested wave plate can be simultaneously determined using specially derived equations. The feasibility of the proposed method was demonstrated using measurements of zero-order half-wave and zero-order quarter-wave plates. The proposed method is easy to operate, enables rapid measurement, and has high stability and accuracy.     

Raman scattering in air: four-dimensional analysis

Inertial confinement fusion requires propagation of high-intensity, pulse-shaped IR and UV laser beams through long air paths. Such beams are subject to energy losses and decreased beam quality as a result by stimulated rotational Raman scattering (SRRS). In this paper we describe how quantum fluctuations, stimulated Raman amplification, diffraction propagation, and optical aberrations interact during the propagation of short, high-power laser pulses using a four-dimensional (4-D) model of the optical beams and the medium. The 4-D model has been incorporated into a general optical-propagation computer program that allows the entire optical system to be modeled and that is implemented on high-end personal computers, workstations, and supercomputers. The numerical model is used to illustrate important phenomena in the evolution of the optical beams. In addition, the OMEGA Upgrade laser system is used as a design case to illustrate the various considerations for inertial confinement fusion laser design.     

Effects of optical feedback in a birefringence-Zeeman dual frequency laser at high optical feedback levels

Optical feedback effects are studied in a birefringence-Zeeman dual frequency laser at high optical feedback levels. The intensity modulation features of the two orthogonally polarized lights are investigated in both isotropic optical feedback (IOF) and polarized optical feedback (POF). In IOF, the intensities of both beams are modulated simultaneously, and four zones, i.e., the e-light zone, the o-light and e-light zone, the o-light zone, and the no-light zone, are formed in a period corresponding to a half laser wavelength displacement of the feedback mirror. In POF, the two orthogonally polarized lights will oscillate alternately. Strong mode competition can be observed, and it affects the phase difference between the two beams greatly. The theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.     

多晶硅梁的加速疲劳实验

MEMS器件在循环振动载荷作用下,器件可能会发生断裂、软化等疲劳失效现象.本文中选取了以表面工艺加工的多晶硅结构—固支梁与悬臂梁作为实验研究对象,并在微结构梁上利用光刻的方法对两个被测结构分别引入了凹槽和切口两种缺陷形式,并在其上加载循环静电载荷,进行加速疲劳实验.实验利用激光多普勒测振仪测量谐振频率的变化,来表征微梁结构等效弹性模量的改变.实验结果表明,无论固支梁或者悬臂梁,其谐振频率都发生了明显的偏移:固支梁结构初始频率为170.749 kHz,实验后谐振频率增大,偏移量达到15.618 kHz,其相对变化量为9.15%,而悬臂梁结构初始频率为112.357 kHz,实验后谐振频率变小,减小量达到1.342 kHz,相对偏移为1.34%,器件性能发生明显退化.     

High cw power using an external cavity for spectral beam combining of diode laser-bar emission

In extension to known concepts of wavelength-multiplexing diode laser arrays, a new external cavity is presented. The setup simultaneously improves the beam quality of each single emitter of a standard 25 emitter broad-area stripe laser bar and spectrally superimposes the 25 beams into one. By using this external resonator in an "off-axis" arrangement, beam qualities of Mslow2<14 and Mfast2<3 with optical powers in excess of 10 W in cw operation are obtained.     

Astigmatic bessel laser beams

Abstract

The oblique incidence of a He-Ne laser beam onto a phase-only diffractive optical element (DOE) that simultaneously produces several unimode different-order Bessel beams propagating at various angles with respect to the optical axis is studied theoretically and experimentally. It is shown that, under obliquely incident illumination of a DOE that forms Bessel beams, the resulting astigmatic diffraction pattern can be used to unambiguously identify the direction of the Bessel beam's phase rotation and the order of the Bessel mode.     

Photobleaching of a solid photochromic medium by a Gaussian laser beam

Equations that describe the photobleaching of photochromic layers illuminated by Gaussian laser beams are given. The photochromic samples are made of thionine, triethanolamine, and polyvinyl alcohol and follow simple kinetics law. Spatial absorbance and time-dependent power transmittance are well described by the developed equations. The model is used to calculate the Gaussian dimension of helium-neon laser beams from kinetics data.     

Apodizing holographic gratings for dual-wavelength operation of broad-area semiconductor lasers

Apodizing holographic gratings are designed to have a Gaussian reflectivity profile in the -1 order and a complementary reflectivity profile in the specular (0) order. They are obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. These gratings are intended to be used as the coupler of the external cavity of a broad-area semiconductor laser. When the grating is oriented to get the -1 order counterpropagating with respect to the incident beam, single-longitudinal- and single-lateral-mode operation is obtained. We report on the fabrication technique of an apodizing holographic grating designed to allow lasing on two wavelengths simultaneously, with a preselected wavelength separation. The results obtained with a commercial broad-area laser are presented.     

The Tube to Tube Through Transmission Technique for Inspecting Finned Heat Exchanger Tubes

Finned tubes are used in chemical processing and air conditioner heat exchangers. Conventional electromagnetic methods cannot be used to inspect the finned tubes for flaws because of the interference caused by the fins. We describe here tests of a recently developed method, the Tube to Tube Through Transmission (T4) technique which uses coplanar exciter and detector coils in neighboring tubes. As the method is new there is, as yet, little theoretical or experimental explanation for it. We report first, therefore, studies of the magnetic fields and the induced eddy currents. Since the coupling path between exciter and detector coils is primarily diffusive, skin effect equations can be applied to give useful approximations for signal analysis. Signals from machined full circumferential grooves in finned heat exchanger tubes were analyzed using these equations, and it was found that the depth of the grooves or general thinning could be estimated to within ±10%.     

Fabrication of Periodic Nanostructures Using AFM Tip-Based Nanomachining: Combining Groove and Material Pile-Up Topographies

This paper presents an atomic force microscopy (AFM) tip-based nanomachining method to fabricate periodic nanostructures. This method relies on combining the topography generated by machined grooves with the topography resulting from accumulated pile-up material on the side of these grooves. It is shown that controlling the distance between adjacent and parallel grooves is the key factor in ensuring the quality of the resulting nanostructures. The presented experimental data show that periodic patterns with good quality can be achieved when the feed value between adjacent scratching paths is equal to the width between the two peaks of material pile-up on the sides of a single groove. The quality of the periodicity of the obtained nanostructures is evaluated by applying one- and two-dimensional fast Fourier transform (FFT) algorithms. The ratio of the area of the peak part to the total area in the normalized amplitude–frequency characteristic diagram of the cross-section of the measured AFM image is employed to quantitatively analyze the periodic nanostructures. Finally, the optical effect induced by the use of machined periodic nanostructures for surface colorization is investigated for potential applications in the fields of anti-counterfeiting and metal sensing.     

Scanning lidar with a coupled radar safety system

A small scanning three-wavelength lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make atmospheric measurements on stratospheric and upper tropospheric aerosols and on the evolution of aircraft exhaust plumes. Many of these measurements have been made away from the zenith, and, to reduce the hazard to air traffic produced by the laser beam, a radar safety device has been installed. The radar application is original in that the radar beam is made collinear with the laser beam by use of a dichroic mirror that transmits the laser radiation and reflects the microwaves. This mirror is inserted into the outgoing optical path prior to the radiation from both the radar and the laser passing through the independent scanning unit. Tests of the complete system show that the lidar and radar beams remain collocated as they are scanned and that the radar can be used to inhibit the laser prior to an aircraft passing through the beam.     

Gaussian-reflectivity mirror resonator for a high-power transverse-flow CO2 laser

A Gaussian-reflectivity mirror resonator is proposed to achieve high-quality laser beams. To analyze the laser fields in a Gaussian-reflectivity mirror resonator, the diffraction integral equations of a Gaussian-reflectivity mirror resonator are converted to the finite-sum matrix equations. Consequently, according to the Fox-Li laser self-reproducing principle, we describe the mode fields and their losses in the proposed resonator as eigenvectors and eigenvalues of a transfer matrix. The conclusion can be drawn from the numerical results that, if a Gaussian-reflectivity mirror is adopted for a plano-concave resonator, a fundamental mode can easily be obtained from a transverse-flow CO2 laser and high-quality laser beams can be expected.     

Shear-wave focusing with a laser-ultrasound phased-array

Pulsed laser arrays have been used to improve the beam parameters of laser generated ultrasound. The focusing and steering of shear-acoustic-wave beams generated by an array of thermoelastic sources has been investigated. A modeling of the impulse diffraction response of a single line source was derived from the point source model (Surface Center Of Expansion) of the thermoelastic source. The acoustic field launched by an array of point- or line-sources has been computed. Experimentally, the impulse response of a line source has been measured by using a heterodyne interferometer. It was shown with a 16-beam YAG laser, that shear-acoustic-wave beams can be focused and steered inside a duraluminum sample with a great efficiency     

Decorrelation with wavelength of laser intensity fluctuations after passage through a thermal plume

Abstract

Experiments are described in which two laser beams of different wavelength (mean λ=1550 nm, with variable detuning Δλ up to ±4%) are simultaneously transmitted through a thermal phase screen created by a convection heater. The work is motivated by applications such as differential absorption lidar (DIAL) in which beams of multiple wavelength are transmitted through the atmosphere. The experiment is optical fibre based with common transmit and receive optics being used for the two wavelengths, thus ensuring near-identical spatial modes. The far-field case is examined by placing the receiver at a focus of the beam, and reflective optics are used to avoid problems of chromatic aberration. A heterodyne detection scheme measures the intensity fluctuations at a point in the centre of the beam for each wavelength, and the degree of correlation between the two resulting intensity time series is determined as a function of detuning. The decorrelation increases as the square of the detuning, and the results are consistent with the predictions of theory in which it is assumed that the phase screen undergoes Gaussian phase fluctuations, and off-axis terms are ignored.     

Design of interference using coherent beams configured as a six-sided pyramid

Various interference pattern formations have been investigated using six countering laser beams that form a six-sided pyramid. Phase shift and amplitude variations among the interfering beams are very useful for designing interference patterns summarized here. The interference patterns are categorized into 32 unit figures, and structural changes with different thresholds are investigated in detail.