Degenerate four-wave mixing spectroscopy excited by ultrashort laser pulses

A new method is described for spectroscopic diagnostics of combustion and explosion products, which makes it possible to study processes occurring at short times in hot and dense molecular gases. Pis’ma Zh. Tekh. Fiz. 23, 16–19 (March 26, 1997)     

超短激光脉冲腔外光谱展宽与压缩技术

超短激光脉冲是超快光学和强场激光物理研究领域的重要驱动光源,通常可以通过脉冲后压缩方法获得。本文详细介绍了超短脉冲后压缩的发展现状、原理和相关技术,包括块状固体材料压缩、薄片组压缩、多通腔压缩、中空波导压缩以及光子晶体光纤压缩等。并通过对现有的脉冲后压缩技术进行总结,为未来的研究与发展方向提供了理论指导。     

Research on optical damage to sodium chloride by ultrashort laser pulses

Thresholds of optical damage to sodium chloride by ultrashort laser pulses with a duration of about 40 fs are determined. Experiments were carried out using a terawatt titanium–sapphire laser device. p-polarized laser radiation at a wavelength of 800 nm fell on the specimen surface at an angle of 60°. Optical damage to the surface was observed when the critical electric field strength attained 94 MV/cm.     

Speckle reduction in laser projection systems by diffractive optical elements

In laser projection systems the observer in the far field of the image points on the screen will recognize serious speckle noise. There are many methods to reduce or eliminate speckles in the near field by reducing or eliminating temporal or spatial coherence of the laser. But for the far field it is hardly possible to change the coherence properties of laser sources so that speckles will disappear. We propose a new method for eliminating speckles in the far field by using a diffractive optical element. The intensity modulation depth in the far-field speckle pattern can be reduced to a few percent while good beam quality is preserved.     

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.     

Fabrication of diffractive optical elements inside polymers by femtosecond laser irradiation

Bragg-type gratings were prepared by irradiation inside a series of optical polymers with femtosecond laser pulses and the preparation conditions of the grating were examined. Repeated scanning irradiation with femtosecond laser pulses formed gratings due to refractive index changes inside polymers. Among the polymers examined in the present study, polymethylpentene (PMP) showed the highest diffraction efficiency, which was an order of magnitude higher than those of other optical polymers. The density of PMP was the lowest among the polymers evaluated in the present study, and the large volume contraction based on its low density was responsible for the larger refractive index change of PMP. Furthermore, we fabricated large-area diffractive optical elements (DOEs) in PMP measuring 15 × 25 mm² by widening the scanning area.     

Time dependence of Fresnel diffraction of ultrashort laser pulses by a circular aperture

We report on the time dependences of the Fresnel diffraction of ultrashort laser pulses by a circular aperture. The diffraction leads to a pulse delay, time shape change, pulse broadening, and peak power decrease. These effects may have to be taken into account whenever critical thresholds are encountered as, for example, in nonlinear optics or laser fusion.     

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.     

Generating inhomogeneously polarized higher-order laser beams by use of diffractive optical elements

We propose an improved version of the earlier developed optical arrangement for generating inhomogeneously polarized laser light modes with the aid of a diffractive optical element (DOE) with carrier frequency. By eliminating lenses from the optical arrangement, we achieve the miniaturization, reduced light losses, a smaller number of parameters being matched, and a simpler system adjustment procedure. Note that all the capabilities of the previous version, namely, the universality and simple readjustment to different polarization types, are fully retained. The numerical modeling of the polarization mode combiner has made it possible to analyze its performance and capabilities. In the experiments, the quality of the resulting beams is shown to be improved. For generating higher-order cylindrical beams, a lower-order mode at the output of the polarization mode combiner is additionally transformed with a DOE that operates in the zero diffraction order, introducing radial phase changes.     

Generation of large-diameter diffractive elements with laser pattern generation

We describe the design and construction of a high-precision laser writing machine for the direct generation of large-diameter rotationally symmetric diffractive optics with continuous profiles in photoresist. The photoresist profile can be used as a replication master surface or etched into a silica substrate. Machine design methodology, as well as qualification of performance, is provided. Test results for an f/2 100-mm clear-aperture diffractive lens directly etched into a silica substrate are presented. Diffraction efficiency as a function of zone spacing and wave-front performance are given.     

Rapid fabrication of diffractive optical elements by use of image-based excimer laser ablation

We describe a method of fabricating multilevel diffractive optics by excimer laser ablation. A portion of a chrome mask containing many patterns is illuminated by 193-nm laser light and imaged by an objective lens onto a poly(imide) substrate. Ablation of an entire single pattern is achieved in a single laser pulse. Multiple pulses are used to vary the ablation depth, and multiple patterns are used to create a variety of multilevel optics. We have successfully fabricated arrays of eight-level diffractive microlenses with varying focal lengths and decenters. The optics performed with diffraction-limited focusing and near-theoretical diffraction efficiency (92%).     

Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength

Glass welding by ultrashort laser pulses allows joining without the need of an absorber or a preheating and postheating process. However, cracks generated during the welding process substantially impair the joining strength of the welding seams. In this paper a sample preparation method is described that prevents the formation of cracks. The measured joining strength of samples prepared by this method is substantially higher than previously reported values.     

Excimer laser micromachining for rapid fabrication of diffractive optical elements

We demonstrate the fabrication of binary, multilevel, and blazed diffractive structures by a fast and flexible direct-write process by using an excimer-laser-based tabletop micromachining workstation with an integrated optical surface profiler.     

Design of polarization-selective diffractive phase elements in a laser cavity

We report a new design for a polarization-selective laser cavity with birefringent diffractive phase elements. This laser cavity can create two modes with different polarizations and profiles launched separately from two end mirrors. The numerical simulation results show that the constructed laser cavity can successfully generate two orthogonal polarization modes with a uniform circular shaped pattern output from one end mirror and a uniform square-shaped pattern output from another end mirror.     

Encryption of nonlinear optical signals in ZnO:Al thin films by ultrashort laser pulses

Described herein is the effect of optical annealing on the third-order non-linear optical properties exhibited by nanostructured Al-doped ZnO thin films. The samples were synthetized by an ultrasonic spray pyrolysis method. The optical annealing process was carried out by laser pulses at 532, 835 and 1064 nm wavelengths with, ps, fs and ps pulse duration, respectively. The optical non-linearity of the films was measured by the z-scan method with three different irradiations of excitation: 100 fs at 835 nm, 120 ps at 532 nm, and 150 ps at 1064 nm. The as-grown samples showed a saturable optical absorption that evolves into two-photon absorption transitions by a picosecond optical annealing phenomenon induced at 532 nm wavelength. Potential applications for developing optical encryption functions were considered.     

Zone-boundary optimization for direct laser writing of continuous-relief diffractive optical elements

Enhancing the diffraction efficiency of continuous-relief diffractive optical elements fabricated by direct laser writing is discussed. A new method of zone-boundary optimization is proposed to correct exposure data only in narrow areas along the boundaries of diffractive zones. The optimization decreases the loss of diffraction efficiency related to convolution of a desired phase profile with a writing-beam intensity distribution. A simplified stepped transition function that describes optimized exposure data near zone boundaries can be made universal for a wide range of zone periods. The approach permits a similar increase in the diffraction efficiency as an individual-pixel optimization but with fewer computation efforts. Computer simulations demonstrated that the zone-boundary optimization for a 6 microm period grating increases the efficiency by 7% and 14.5% for 0.6 microm and 1.65 microm writing-spot diameters, respectively. The diffraction efficiency of as much as 65%-90% for 4-10 microm zone periods was obtained experimentally with this method.     

Focusing and spectral characteristics of periodic diffractive optical elements with circular symmetry under femtosecond pulsed illumination

The analytical solution is derived, within the Rayleigh-Sommerfeld formulation of diffraction, for the on-axis spectral irradiance of a broadband source after diffracting through a circular symmetric hard aperture. By using this solution, and within the paraxial approximation, we investigate several diffraction-induced effects originated by binary diffractive optical elements made up of a set of annular apertures with equal areas and periodic in the squared radial coordinate. In particular, the ability to focus femtosecond pulses is investigated. In addition, the analysis of the spectral modifier function associated with these elements allows us to simulate spectral shifts at focus positions. Finally, we introduce a relatively simple and low-cost technique to slice the spectrum of a broadband source in order to generate narrow bands or wavelength channels.     

Transition between diffractive and refractive micro-optical components

Optical components are usually classified into diffractive and refractive elements. In this classification, refractive components are defined as elements that are sufficiently described by geometrical optics. For micro-optics this distinction is very often not applicable. Our goal is to understand which parameters control the transition from elements that can be interpreted as refractive to those elements that are called diffractive. We investigate the linear blazed grating and focus on the wavelength dependence of its properties. For this we adopt an approach well known from the theory of echelette gratings. Our results can easily be transferred to other blazed components, such as Fresnel lenses.     

Generation of high-power ultrashort optical pulses by semiconductor lasers

Fiber-coupled semiconductor lasers have been studied when pumped by high-power short electrical pulses of 5 ns width and leading front duration below 1 ns. In this pumping regime, it is possible to ensure significant sharpening of output pulses, the duration of which decreases below 80 ps for a single-mode laser and below 120 ps for a broad aperture multimode laser at an output peak optical power as high as 1.5 and 27 W, respectively.     

Design concept for diffractive elements shaping partially coherent laser beams.

A new two-step design algorithm for the calculation of a diffractive phase element (DPE) for use with partially coherent laser beams is presented. The optical reconstruction of the DPE is modeled by the convolution of a coherent diffraction pattern and the far-field intensity distribution of a partially coherent laser beam. Numerical deconvolution is applied to derive a suitable amplitude pattern as signal input to a standard iterative Fourier transform algorithm (IFTA). Theory and numerical results are presented. Compared with a single-step IFTA design, this new approach yields nearly equal diffraction efficiencies and a relative improvement of 15% in signal reconstruction error.