Ultrashort laser pulse beam shaping

We calculated the temporal and spatial characteristics of an ultrashort laser pulse propagating through a diffractive beam-shaping system that converts a Gaussian beam into a beam with a uniform irradiance profile that was originally designed for continuous waves [Proc. SPIE 2863, 237(1996)]. The pulse front is found to be considerably curved for a 10-fs pulse, resulting in a temporal broadening of the pulse that increases with increasing radius. The spatial intensity distribution deviates significantly from a top-hat profile, whereas the fluence shows a homogeneous radial distribution.     

Asymmetrical prism for beam shaping of laser diode stacks

A beam-shaping scheme for a laser diode stack to obtain a flattop output intensity profile is proposed. The shaping element consists of an asymmetrical glass prism. The large divergence-angle compression in the direction perpendicular to the junction plane and the small divergence-angle expansion in the parallel direction are performed simultaneously by a single shaping element. The transformation characteristics are presented, and the optimization performance is investigated based on the ray-tracing method. Analysis shows that a flattop intensity profile can be obtained. This beam-shaping system can be fabricated easily and has a large alignment tolerance.     

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.     

Polarization beam shaping

Spatial engineering of polarization is proposed as a novel method of beam shaping. It is shown that a flat-top-shaped focus can be obtained in the far field by changing the polarization in the pupil plane in a spatially inhomogeneous manner. Experiments have been carried out to verify the validity of this method in one dimension. By comparison with traditional beam shaping methods, polarization beam shaping yields the smallest flat-top focus while maintaining high efficiency.     

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.     

Variant of the method of Fox and Li dedicated to intracavity laser beam shaping

We present a variant of the method of Fox and Li [Bell Syst. Tech. J. 40, 453 (1961); Proc. IEEE 51, 80 (1963)] dedicated to intracavity laser beam shaping for resonators containing an arbitrary number of amplitude and phase diffractive optics. Contrary to Fox and Li, the starting point is the desired field. The latter is injected into the usual sequence of lenses representing just a single round trip, and the optimization process iterates until the input and the output fields match as much as possible. We illustrate this technique by deriving a simple model for generating single cylindrical TEM(p0) modes, thanks to a π-phase plate placed inside a plano-concave cavity. The experimental validation attests an excellent agreement with numerical predictions.     

Laser beam shaping profiles and propagation

We consider four families of functions--the super-Gaussian, flattened Gaussian, Fermi-Dirac, and super-Lorentzian--that have been used to describe flattened irradiance profiles. We determine the shape and width parameters of the different distributions, when each flattened profile has the same radius and slope of the irradiance at its half-height point, and then we evaluate the implicit functional relationship between the shape and width parameters for matched profiles, which provides a quantitative way to compare profiles described by different families of functions. We conclude from an analysis of each profile with matched parameters using Kirchhoff-Fresnel diffraction theory and M2 analysis that the diffraction patterns as they propagate differ by small amounts, which may not be distinguished experimentally. Thus, beam shaping optics is designed to produce either of these four flattened output irradiance distributions with matched parameters will yield similar irradiance distributions as the beam propagates.     

Controlled supercontinua via spatial beam shaping

Abstract

Recently, optimization techniques have had a significant impact in a variety of fields, leading to a higher signal-to-noise and more streamlined techniques. We consider the possibility for using programmable phase-only spatial optimization of the pump beam to influence the supercontinuum generation process. Preliminary results show that significant broadening and rough control of the supercontinuum spectrum in the visible region are possible without loss of input energy. This serves as a proof-of-concept demonstration that spatial effects can controllably influence the supercontinuum spectrum, leading to possibilities for utilizing supercontinuum power more efficiently and achieving excellent spectral control.     

Ion beam shaping and downsizing of nanostructures

We report a new approach for progressive and well-controlled downsizing of nanostructures below the 10?nm scale. A low energetic ion beam (Ar(+)) is used for gentle surface erosion, progressively shrinking the dimensions with ～1?nm accuracy. The method enables shaping of the nanostructure geometry and polishing of the surface. The process is clean room/high vacuum compatible being suitable for various applications. Apart from technological advantages, the method enables the study of various size phenomena on the same sample between sessions of ion beam treatment.     

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.     

Geometrical-transformation approach to optical two-dimensional beam shaping

The use of geometrical transformations in the design of an optical beam shaper (OBS) is described. Elements based on this approach can transform light distributions into almost any arbitrary distribution. An example OBS is analyzed numerically.     

Diffractive shaping of excimer laser beams

Abstract

We address the problem of shaping the intensity distribution of a highly directional partially coherent field, such as an excimer laser beam, by means of diffractive optics. Our theoretical analysis is based on modelling the multi-transverse-mode laser beam as a Gaussian Schell-model beam. It is shown numerically that a periodic element, which is unsuitable for the shaping of a coherent laser beam, works well with an excimer laser beam because of its partial spatial coherence. The conversion of an approximately Gaussian excimer laser beam into a flat-top beam in the Fourier plane of a lens is demonstrated with a diffractive beam shaper fabricated as a multilevel profile in SiOl by electron-beam lithography and proportional reactive-ion etching.     

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.     

Controlled focus shaping with generalized cylindrical vector beam

An optical system is proposed for the controlled focus shaping of generalized cylindrical vector beams by a high-numerical-aperture lens. A segmented electro-optical filter with four concentric belts is introduced and works with a double-λ/2-plate. By controlling the rotational angle of the plate and the voltage applied to the filter with proper azimuth angle, a three-dimensional engineered focusing field, such as an electrically controlled axial-shifted focus, extended depth of focus, and a diffraction-limited optical tube, can be achieved. The main advantage of this focusing system is that the focused field can be adjusted and formed by the applied voltage and the rotational angle of the half-wave plates. It can provide a new focus shaping technique with free adjustability and flexibility.     

Automatic symmetrical iterative Fourier-transform algorithm for the design of diffractive optical elements for highly precise laser beam shaping

A symmetrical iterative Fourier-transform algorithm (IFTA) using a combination of phase and amplitude freedom for the design of diffractive optical elements for highly precise laser beam shaping is presented. We compare this method with the basic IFTA and the symmetrical IFTA exclusively using phase freedom, and the basic IFTA using both phase and amplitude freedom, by employing these methods for super-Gaussian beam shaping. While the latter three methods fail to produce satisfactory solutions, the first method results in a beam non-uniformity error of 0.44% and a theoretical efficiency of 97.2%. Moreover, the new approach avoids the use of the trial-and-error method for finding the appropriate modified Fourier-domain constraints during the iteration, which can be difficult for some beam-shaping problems.     

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam

In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.     

Simultaneous frequency conversion and beam shaping for optical-tweezers applications

Abstract

We review a method which allows the simultaneous frequency-conversion and shaping of a light beam into a variety of patterns. The method is based on the controllable dynamics of vortices nested in the light beams. We outline the theoretical idea behind the technique and discuss experimental observations conducted in SH generation in lithium triborate which demonstrate its feasibility. The technique might find applications in specialized optical tweezers aimed at the wavelength-optimized assembly, fabrication and manipulation of complex microstructures and nanostructures.