Aberrations in (3+1)-dimensional Bragg diffraction using pulsed Laguerre-Gaussian laser beams

The design and simulation of ultrashort pulse shaping systems require pulse propagation methods that take the combined effects of dispersion, diffraction, and system aberrations into account. In the conventional pulse propagation methods based on the spectrum of plane wave, usually large number of sampling points are needed for the correct Fourier transform operations due to the fast oscillating phase of the complex pulse field. In this work, I have developed an alternative pulse propagation method, based on the Gaussian pulsed beam decomposition, as an extension of the monochromatic Gaussian beam propagation method. Methods for the decomposition of an input pulse, with arbitrary spatial and temporal (spectral) profiles, into a set of elementary Gaussian pulsed beams are proposed. Algorithms for computing the spatio-temporal and spatio-spectral profiles of the propagated pulse as the phase correct superposition of individual Gaussian pulsed beams are developed. The proposed decomposition method allows the elementary Gaussian pulsed beams to have different parameters depending on the local spatial and spectral phase of the given input pulse which reduces the number of Gaussian pulsed beams required to decompose an input pulsed beam with a given accuracy. Furthermore, a new kind of beam called the truncated Gaussian beam, is introduced and combined with the conventional Gaussian beam decomposition method to enable decomposition of fields after hard apertures. The analytical propagation equation of the truncated Gaussian beam through a paraxial optical system is derived. Additionally, the application of the Gaussian beam decomposition method is extended to handle the propagation of vectorial fields. Several example calculations are presented to validate the proposed methods and show their application in propagating fields through optical systems which are rather complicated to model using the conventional methods.

The far-field anomalous spectral behaviours of a space–time-dependent Gaussian pulsed beam passing through a rectangular aperture are studied. By expanding a hard aperture function into a finite sum of complex Gaussian functions and starting from the Fresnel diffraction integral, the approximate analytical expression for the spectral intensity of a space–time-dependent Gaussian pulsed beam passing through a rectangular aperture is derived. Meanwhile, the corresponding closed-forms for the slit and the unapertured cases are also given as special cases of the general results. The red and blue shifts and the spectral intensity distribution are studied and illustrated with numerical calculations. Specifically, it is shown that the spectral switch takes place when the truncation parameter is equal to particular values or the observation position is at the critical diffraction angle. The possibility of tunable spectral switching in the far field with an apertured pulsed beam by varying the size of the rectangular aperture is highlighted.

Theoretically, we investigated the optical force exerted on a nano-dielectric sphere illuminated by a pulsed Laguerre-Gaussian beam. In the frame of the dipole approximation, analytical expressions for the optical force were derived. Based on these analytical expressions, the effects of pulse duration τ and beam mode order (l,p) on the optical force were studied. It is recognized that the optical force values and the trapping regions are remarkably affected by changing pulse duration and mode parameters. Our results show good agreement with the results obtained by other authors for the use of a continuous Laguerre-Gaussian beam and pulsed Gaussian beam.

The trapping process of the biological micro-particle in the tweezers using CW laser beam has been investigated in series of our works. In the CW regime, the dynamics of polystyrene microsphere linking to λ-phage DNA molecule depending on the optical force, Brown force and its elastic force are simulated and discussed. In this paper we present the finite difference Langevin equation (FDLE) describing the dynamics of trapped bead linking to DNA molecules in an optical tweezers. Using FDLE the dynamics, i.e., the position-time characteristics of polystyrene microsphere linking to λ-phage DNA molecule are simulated in the pulse regime. The obtained results show that all principle parameters as the peak intensity, beam waist radius, and duration of pulsed Gaussian laser beam, well as the setting-up position of DNA molecule influence on the dynamics, especially on the “pulling” and “stability” times of trapped bead. The conditions for λ-phage DNA molecule in stretching state are discussed for experimence.

Thermal analysis of magneto-optical thin films under laser irradiation

The gain-switching properties of an InAs-InP (113)B quantum dot laser based on multi-population rate equations are examined theoretically in detail to generate shorter pulses with the application of a Gaussian pulse beam to the laser excited state. The numerical results demonstrated that as the homogeneous and the inhomogeneous broadening increase, the differential gain, the gain compression factor, and the threshold current of the excited state decrease while the threshold current of the ground state increases. It was also observed that the contribution of the excited state to gain-switched output pulses depend on not only the value of the inhomogeneous broadening but also the magnitude of the applied current. Additionally, it was found that without an optical beam, the output pulse has a long pulse width due to ground state emissions and the change in the parameters strongly affecting the output. However, under the optical beam, narrow pulses around 26 ps have high peak power owing to the excited state emission generated even at low currents and also the change in the laser parameters having a negligible effect. Finally, the gain-switching characteristics with and without a Gaussian pulse beam are shown to be similar for liner-gain and nonlinear-gain cases except that higher peak power and narrower output pulses are obtained for the linear-gain case.

By using the angular spectrum and complex analytic signal representations, the analytic propagation expression and its Fourier spectrum for ultrashort Gaussian pulsed beams with constant waist width in the far field are derived. The far-field properties of few- and single-cycle ultrashort Gaussian pulsed beams with constant waist width are studied, and compared with those of ultrashort Gaussian pulsed beams with constant diffraction length. The properties of pulse broadening, spectrum narrowing, off-axis spectrum redshift and on-axis spectrum blueshift are found for ultrashort Gaussian pulsed beams with constant waist width in the far field.

Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies needed to cut these materials.Profile cutting of thin sheet bright metals can be achieved in a cost effective manner through the use of a pulsed fiber laser in conjunction with a galvanometric scanhead without co-axial assist gas. This is attractive where many batches of distinct small parts are manufactured in volume, for instance within the jewellery and the micro-electronics industries.In this paper the capabilities of a pulsed fiber laser are considered in detail when processing bright metals such as brass, copper and silver. The effect of key laser parameters such as peak power, temporal pulse shape and laser pulse frequency will be considered along with several process parameters such as pulse overlap and beam wobbling techniques. This is with the aim of producing parts at high speed and/or at high quality and to determine the maximum thickness that can be cut at effective speeds. The results will be compared and contrasted with other metals such as stainless steel and aluminium to determine the range of processing strategies ne...

Influence of turbulence on the spectrum of diffracted chirped Gaussian pulsed beams

Spectral anomalies of pulsed Hermite–cosh–Gaussian beams focused by an aperture lens

Invite paper: Self-imaging in multimode graded-index fibers and its impact on the nonlinear phenomena

Photoacoustic tomography has been developed for in vivo functional, metabolic, molecular, and histologic imaging by physically combining optical and ultrasonic waves. Broad applications include early-cancer detection and brain imaging. High-resolution optical imaging—such as confocal microscopy, two-photon microscopy, and optical coherence tomography—is limited to superficial imaging within the optical diffusion limit (~1 mm in the skin) of the surface of scattering tissue. By synergistically combining light and sound, photoacoustic tomography conquers the optical diffusion limit and provides deep penetration at high ultrasonic resolution and high optical contrast. Photoacoustic tomography has two major embodiments: photoacoustic computed tomography and photoacoustic microscopy. In photoacoustic computed tomography, a pulsed broad laser beam illuminates the biological tissue to generate a small but rapid temperature rise, which leads to emission of ultrasonic waves due to thermoelastic expansion. The unscattered pulsed ultrasonic waves are then detected by ultrasonic transducers. High-resolution tomographic images of optical contrast are then formed through image reconstruction. In photoacoustic microscopy, a pulsed laser beam is delivered into the biological tissue to generate ultrasonic waves, which are then detected with a focused ultrasonic transducer to form a depth resolved 1D image. Raster scanning yields 3D high-resolution tomographic images. Super-depths beyond the optical diffusion limit have been reached with high spatial resolution. The annual conference on PAT has grown exponentially since early 2000 and become the largest in SPIE’s 20,000-attendee Photonics West since 2010.

A recent study has shown that the double-pass method provides a good estimate of the ocular modulation transfer function (MTF) but that it does not yield the phase transfer function (PTF) [J. Opt. Soc. Am. A 12, 195 (1995)]. Therefore, one cannot recover the true retinal point-spread function (PSF). We present a modification of the double-pass method to overcome this problem. The key is to break the symmetry between the two passes. By using an unexpanded Gaussian input beam, we produce a diffraction-limited PSF for the first passes. Then, by using a large exit pupil, we get an aberrated PSF for the second pass. The double-pass aerial image is the cross correlation of both PSF's, so that the Fourier transform of such an aerial image directly provides the true retinal PTF, up to the cutoff frequency of the effective (small), diffraction-limited entrance pupil. The resulting double-pass aerial image is a blurred version of the true retinal PSF. Thus it shows the effect not only of even symmetric aberrations but also of odd and irregular aberrations such as coma. We have explored two different ways to retrieve the true retinal PSF: (a) deblurring of the aerial image and (b) PSF reconstruction combining PTF data with conventional double-pass MTF. We present promising initial results with both artificial and real eyes.

The use of laser beams made up of ultrafast pulses for the processing of materials can be bothered by the consequences of optical Kerr effect (OKE) cumulated by the propagation through optical devices (windows, laser crystal, prisms, lenses, …). The latter are mainly a reduction of the intensity in the focal plane accompanied by a distortion of the temporal and spatial pulse shape. We present a comparative study on such distortions for Gaussian, super-Gaussian and LG10 (one central peak surrounded by a ring) beams. It is demonstrated that the LG10 beam shows sensitivity to OKE which is smaller than that of the Gaussian LG00, and super-Gaussian beams. As a result, the focusing performances of the LG10 beam are quite superior to that observed with Gaussian or super-Gaussian beam: a higher on-axis intensity, a narrower intensity pattern, and a temporal shape and an energy fluence almost undistorted.

In this investigations, heat transfer mechanism of laser drilling process is analyzed by using Matlab program 8.5. Calculations of the heat impacted zone were also done. The moving boundary condition generates phase shift and influences heat transfer during the laser drilling process. The classical approach is used to study heat conduction in solids, with modifications made to account for the boundary condition transition from Stefan to continuous heat flow. According to the suggested model, for a given laser beam intensity and pulse time, the drilling hole profiles in a certain material will be quite near to one another. Robot placement errors are complicated since there are many different sources for them, programming in the python language examined. The positioning precision of the robot is improved, and its application capabilities are increased, to reinforced machine learning. The suggested methodology offers a simple and direct method for real-time robot position modification in industrial settings during production setup or readjustment situations. a deep learning approach used to increase the operational positioning precision of an articulated robot. Approximately 300 iterations later, the placement accuracy had significantly improved.