Integrated diffractive andrefractive elements for spectrum shaping

Diffractive elements can be designed for spectrum shaping in the Fourier or Fresnel plane by iterative methods. It is necessary to use a Fourier lens and the wavelength for which the diffractive elements were designed to get the required spectrum shaping at the Fourier plane. Using a different wavelength will cause chromatic aberration. We deal with the combination of refractive and diffractive elements and two or more different diffractive elements on the same element to get appropriate beam shaping of light sources with a multiple spectral output. Simulations are preformed that transform the profile of a He-Ne laser with a Nd:YAG laser source, and shape the trapezoidal beam profile of an excimer laser into a Gaussian beam is also considered.     

Diffractive phase elements for beam shaping: a new design method

A design method based on the Yang-Gu algorithm [Appl. Opt. 33, 209 (1994)] is proposed for computing the phase distributions of an optical system composed of diffractive phase elements that achieve beam shaping with a high transfer efficiency in energy. Simulation computations are detailed for rotationally symmetric beam shaping in which a laser beam with a radially symmetric Gaussian intensity distribution is converted into a uniform beam with a circular region of support. To present a comparison of the efficiency and the performance of the designed diffractive phase elements by use of the geometrical transformation technique, the Gerchberg-Saxton algorithm and the Yang-Gu algorithm for beam shaping, we carry out in detail simulation calculations for a specific one-dimensional beam-shaping example.     

Generation of partially coherent laser beam directly from spatial-temporal phase modulated optical resonators

Abstract

The generation of a partially coherent laser beam directly from a spatial-temporal phase modulated optical resonator is investigated both experimentally and theoretically. The laser material used in the experiment is Nd:YAG rod pumped by Krypton lamps working in continuous wave mode. The phase modulation is fulfilled by an intra-cavity LiNbO₃ electro-optic crystal driven by high voltage. The experimental results show that intracavity phase modulation is an effective way to generate partially coherent laser beams. The theoretical analysis and numerical simulation shows that the output beam can be characterized by Gaussian Schell-model (GSM) beams. The two-slit interference experiment confirms that the output beam is partially coherent.     

Functional integrated optical elements for beam shaping with coherence scrambling property, realized by interference lithography

Many applications, such as semiconductor lithography and material processing, require the shaping of laser beams to provide a homogenous field illumination. We present the conception, implementation, and experimental verification of a combined single-element homogenizer. Additionally, for excimer laser applications, the concept is associated with a coherence scrambling capability. We used the technique of holographic interference lithography to integrate the multifunctional properties in a diffractive optical element. The wavelength difference between the recording process (457.9 nm) and the application (193 nm) results in a change of the imaging properties and requires a geometrical adaptation of the optical setup. The coherence scrambling effect of the setup is based on an off-axis design, including the beam shaping diffractive structure.     

Focusing of Novosibirsk Free Electron Laser (NovoFEL) radiation into paraxial segment

Abstract

We demonstrate results of studies of a silicon binary diffractive optical element (DOE) focusing a terahertz laser Gaussian beam into a paraxial segment. The characteristics of the DOE were examined on a Novosibirsk Free Electron Laser beam of 141-μm wavelength.     

A Semi-analytic Approach for Coherent Laser Radar System Efficiency

Abstract

A nearest Gaussian approximation (NGA) is proposed to approximate any shape for a single mode laser beam by a Gaussian shape. The application considered is a determination of the system efficiency in heterodyne coherent laser radar (HCLR). For an actual beam its NGA is defined by three parameters: the waist spot size and location, and an amplitude coefficient. These parameters are computed by a maximization of the norm of the scalar product written for the actual and Gaussian beams. In the case of the truncated Gaussian beam, particularly relevant to HCLR, the waist location can be analytically calculated, and only two parameters remain unknown: the waist spot size and amplitude coefficient. Using numerical applications, it is shown the NGA is in good agreement with Fresnel integral solution. The NGA combines a good accuracy and capability of analytical solutions. It can treat a variation in system efficiency owing to a misalignment angle between the transmitter and local oscillator.     

Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing

A diffractive beam homogenizer, based on an array of square, off-axis, continuous-relief diffractive microlenses, for use with an excimer laser has been studied. We originally fabricated the homogenizer by direct-write electron-beam lithography, from which we made replicas in UV-grade fused silica by hot embossing and reactive ion etching. Atomic force microscopy measurements of original and replicated elements showed the accuracy of the replication fidelity. One of the replicated homogenizers was evaluated together with a KrF excimer laser. The homogenized beam had a flat-top profile with 31% of the beam energy contained within an area where the beam intensity was above a threshold level of 90% of the maximum intensity.     

Beam shaping in the nonparaxial domain of diffractive optics

We address the problem of shaping the radiant intensity distribution of a highly nonparaxial coherent field by means of a diffractive element located in the plane of the beam waist. To be capable of wide-angle energy redistribution the element must necessarily contain wavelength-scale transverse features, and consequently it must be designed on the basis of rigorous diffraction theory. We consider, in particular, wide-angle Gaussian to flat-top beam shaping in one dimension. Scalar designs are provided and their validity is evaluated by rigorous diffraction theory, which is also used for optimization deep inside the nonparaxial domain, where the scalar designs fail. Experimental verification is provided by means of electron-beam lithography.     

Diffractive light attenuators with variable transmission

Abstract

A new application of diffractive optical elements (DOEs) for continuous or multistage adjustment of optical radiation intensity is described. The diffractive attenuators are linear or circular gratings (amplitude or phase) with constant period and diffraction efficiency that varies across the grating. The zero order of diffraction is used as the output and transmitted through the grating without angular deviation. The diffractive attenuators, in distinction to conventional analogues, allow one to change the intensity of the light beam according to predetermined function and have no limitations for power of the regulated light beam. These elements can be used in optical systems as a beam splitter with adjusted splitting coefficient. The experimental results on a circular diffractive attenuator fabricated by direct laser writing on a chromium film are presented. The range of transmission variation was 20 times within a 340° angle of attenuator turn. The possibility to use a phase diffractive attenuator as a light radiation modulator for a powerful technological laser is discussed.     

Subwavelength-resolvable focused non-gaussian beam shaped with a binary diffractive optical element.

The diffraction-limited spot size limits the optical disk storage capacity and microscopic resolution. We describe a technique to shape a focused Gaussian beam into a superresolving beam by using a diffractive optical element fabricated by laser-assisted chemical etching. The focused shaped beam has a smaller width and a longer depth of focus than a similarly focused Gaussian beam. Using the diffraction-limited shaped beam along with threshold writing, we achieved a written pit size of less than 0.33 mum at a 695-nm laser wavelength, compared with a 0.7-mum focused Gaussian spot size (full width at e(-2) of the peak) with the same focusing lens. The energy conversion efficiency for the beam shaping was ~81%.     

Superresolution laser beam shaping

The superresolution technique is well known for its ability to compress the central diffractive spot that is smaller than the Airy diffractive spot. In this paper, we extend the superresolution technique for different laser beam shaping. A complete set of superresolution diffractive elements is developed for the flat-top beam shaping, the single-circle beam shaping, and the novel circular Dammann grating. Five phase plates, corresponding to each of its applications, have been made by use of micro-optics technology. Experiments that are presented are in good agreement with the theoretical results. The superresolution technique presented in this paper should be highly interesting for the wide applications of laser beam shaping.     

Acousto-optic Conversion of Laser Beams Into Flat-top Beams

Abstract

We employ computer-optimized synthetic acousto-optic holograms to convert a laser beam (not necessarily Gaussian) into a nearly uniform-intensity partially coherent field in the Fourier plane. The width of the flat-top region can be controlled electronically in real time.     

Acoustooptic 2-D profile shaping of a Gaussian laser beam

Acoustooptic 2-D profile shaping of a Gaussian laser beam has been achieved by two plane ultrasonic waves progressing in orthogonal directions. The spot size W of the Gaussian laser beam must be considerable less than the wavelength lambda of the ultrasonic wave at the acoustooptic interaction region. The ultrasonic cell is dealt with as a Raman-Nath 2-D phase grating but serves as a 2-D beam deflector in time for the interaction scheme of interest. The wave front of the Gaussian laser beam must be almost plane in the interaction region. The profile shaping condition is 0.15 < or = (W/lambda) < or = 0.30 only when the Raman-Nath parameter dependent on the ultrasonic power has values between v = 1.0 and 2.0.     

Comparison of resonator-originated and external beam shaping

The spatial shaping of laser beams is a subject of research in modern optics. Recently the introduction of diffractive elements in laser resonators has offered an alternative to external beam-shaping optics by mode shaping within the resonator. We describe the specification of the laser resonator mirrors to obtain by means of internal mode shaping a desired beam outside the resonator. Modal discrimination of the modified resonator and the mirror alignment sensitivity is discussed. Basic features of resonator-originated and external beam shaping are compared.     

Spatial beam shaping of ultrashort laser pulses: theory and experiment

We studied the spatial intensity profile of an ultrashort laser pulse passing through a laser beam shaping system, which uses diffractive optical elements to reshape a Gaussian beam profile into a flat-topped distribution. Both dispersion and nonlinear self-phase modulation are included in the theoretical model. Our calculation shows that this system works well for ultrashort pulses (approximately 100 fs) when the pulse peak intensity is less than 5 x 10(11) W/cm2. Experimental results are presented for 136 fs pulses at 800 nm wavelength from a Ti:sapphire laser with a 6 nJ pulse energy. We also studied the effects of lateral misalignment, beam-size deviation, and defocusing on the energy fluence profile.     

Laser beam shaping with polarization-selective diffractive phase elements

A new scheme for converting a Gaussian irradiance profile beam on the input plane into a uniform irradiance profile beam on the output plane is presented based on polarization-selective diffractive phase elements. The relevant elements were designed by use of the simulated annealing method. The simulation design shows that the shaping quality is substantially improved and is much better than that obtained with traditional diffractive phase elements.     

Generation and selection of laser beams represented by a superposition of two angular harmonics

Abstract

The properties of fields generated by diffractive phase-only optical elements that generate combinations of two angular harmonic fields with different harmonic indices in Fraunhofer and Fresnel regions are investigated theoretically and experimentally. Camomile shaped diffraction patterns are predicted and observed. It is shown that multi-order diffractive phase elements can be used to both generate these beams and to identify the weights of different angular harmonics in a given incident laser beam.     

Fraunhofer diffraction of a Laguerre–Gaussian laser beam by fork-shaped grating

In this article we present a theoretical study for Fraunhofer diffraction of a Laguerre–Gaussian laser beam with zeroth radial mode number and azimuthal mode number l by a diffractive grating with embedded fork-shaped dislocations of integer order p. Analytical expressions describing the diffracted wave field amplitude and intensity distributions in the Fourier plane are deduced and analyzed. They are also followed by the vortex radii expressions.     

准分子激光前端光束形态控制技术研究

在高功率准分子激光系统中,前端光束形态直接决定着系统输出的光束形态,并直接与靶物理需求密切相关.主要介绍了窄脉冲准分子激光前端光束形态控制的实验研究进展,基于散射法获得了满足系统要求的部分相干源,直接利用三束激光脉冲堆积获得了平顶整形脉冲,并结合放大实验结果对上述实现方法进行了评价分析.     

Evolution properties of partially coherent four-petal Gaussian beams in oceanic turbulence

Based on the extended Huygens–Fresnel integral formula, the analytical expressions for partially coherent four-petal Gaussian beam propagating in oceanic turbulence are derived, and the influences of coherence length, beam order N and the parameters of oceanic turbulence (the rate of dissipation of turbulent kinetic energy per unit mass of fluid, the rate of dissipation of mean square temperature and the relative strength of temperature and salinity fluctuations) on average intensity properties are investigated using numerical examples in detail. The results show that the beam with the higher beam order N or coherence length will lose its initial four-petal profiles slower. It is also indicated that the beam will evolve into a Gauss-like beam more rapidly with increasing oceanic turbulence strength. The results have the potential application in underwater laser communication using a partially coherent four-petal Gaussian beam.