Research into the spatial reshaping of monochromatic laser beams grew significantly in the late 1990s due to improvements in the fabrication of diffractive optical elements. Nowadays, some applications, such as optical coherence tomography, necessitate the use of broadband light beams with a spectral width of hundreds of nanometers. The difficulty in reshaping such spectrally broadened beams lies in the wavelength dependence of the beam shaping process. This paper presents a numerical study of the wavelength dependence of two beam shaping techniques that allow a Gaussian beam to be transformed into a flat-top or doughnut intensity profile in the focal plane of a focusing lens. The first technique is based on the diffraction of an incident Gaussian beam passing through a simple binary diffractive optical element. The second technique can be described as an interferometric method, as it involves the coaxial superposition of two Gaussian beams emerging from a Michelson interferometer. We compared the stability of these two techniques’ ability to reshape the beam versus the spectral bandwidth of the incident Gaussian beam. We showed that the interferometric method is more resilient than the diffractive method to changes in the spectral bandwidth of the Gaussian beam. We also considered the case of a quasi-monochromatic beam delivered by a widely tunable laser and reshaped using the interferometric method, where the dispersion of beam reshaping could be mitigated by two programmable liquid lenses that enable control of the curvature of the Michelson interferometer mirrors.

The research on high-order transverse modes in lasers is a subject as old as the laser itself and has been largely abandoned. However, recently several studies have demonstrated an interest in using, instead of the usual Gaussian beam, a radial Laguerre–Gauss LGp0 beam, as, for instance, one can observe a strong improvement, for a given power, in the longitudinal and radial forces in optical tweezers illuminated by a LGp0 beam instead of the usual Gaussian beam. Since in most commercial lasers, the delivered laser beam is Gaussian, we therefore think it opportune to consider the problems of forcing a laser to oscillate individually on a higher-order transverse LGp0 mode. We propose a comprehensive analysis of the effects of an intra-cavity phase or amplitude mask on the fundamental mode of a plano-concave cavity. In particular, we discuss the best choice of parameters favouring the fundamental mode of a pure radial Laguerre–Gauss LGp0 model.

We report on the first laser operation of a Sm3+-doped monoclinic KGd(WO4)2 double tungstate crystal in the red spectral range. Pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) at 479.1 nm, the 0.8 at. % Sm:KGd(WO4)2 laser generated an output power up to 17.6 mW at 649.1 nm (the 4G5/2 → 6H9/2 transition) with a slope efficiency of 16.9%, a laser threshold down to 29 mW and a linear polarization. The laser exhibited a self-pulsing behavior, delivering µs-long pulses with a repetition rate of a few kHz. The polarized spectroscopic properties of Sm3+ ions were determined as well.

Depressed-cladding surface channel waveguides were inscribed in a 0.5 at.% Pr:LiYF4 crystal by femtosecond Direct Laser Writing. The waveguides consisted of a half-ring cladding (inner diameter: 17 µm) and side structures ("ears") improving the mode confinement. The waveguide propagation loss was as low as 0.14 ± 0.05 dB/cm. The orange waveguide laser operating in the fundamental mode delivered 274 mW at 604.3 nm with 28.4% slope efficiency, a laser threshold of only 29 mW and linear polarization (π), representing record-high performance for orange Pr waveguide lasers.

We report on polarized spectroscopy and first laser operation of Sm:KGd(WO4)2 crystal. Red Samarium laser generated 17.6 mW at 649.1 nm with 16.9% slope efficiency, a threshold down to 29 mW and a linear polarization.

Depressed-cladding low-loss (0.14 dB/cm) surface channel waveguides were fabricated in bulk Pr:LiYF4 by Ultrafast Laser Inscription. An orange waveguide laser generated 274 mW at 604.3 nm with 28.4% slope efficiency and 29 mW laser threshold.

We report on a polarization-resolved spectroscopic study of Sm 3+ -doped monoclinic KGd(WO 4 ) 2 crystals. The transition probabilities for Sm 3+ ions were calculated using a modified Judd-Ofelt theory. For the 4 G 5/2 → 6 H 9/2 transition in the red spectral range, the stimulated-emission cross-section is 5.59×10 -21 cm² at 649.0 nm (for light polarization E || N p ) and the luminescence lifetime of the 4 G 5/2 state is 719 μs (0.4 at.% Sm 3+ -doping). Sm:KGd(WO 4 ) 2 is promising for orange and red lasers.

Using a Digital Micromirror Device (DMD), the 3D-reconstruction of programmable rough particles (centrosymmetric or non-centrosymmetric) are done from a set of 120 interferometric images. This can be done using the error-reduction (ER) algorithm for the 2D shape reconstructions and the filtered back-projection for the 3D tomographic reconstruction. (C) 2022 Elsevier Ltd. All rights reserved.

The Error-Reduction algorithm is tested to reconstruct the exact 2D-shape of irregular rough particles from their experimental interferometric images. The particles tested are "programmable"particles generated with a digital micromirror device. The method is first applied to centrosymmetric particles, and then to non-centrosymmetric particles where the twin image problem brings additional difficulty.

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.

The possibility to perform the tomography of irregularly-shaped rough particles in a flow using multi-view interferometric imaging is investigated. Combining three perpendicular angles of views, we reconstruct a family of possible 3D-shapes from speckle patterns. The estimation of an error parameter enables the elimination of erroneous 3D-shapes to obtain a more accurate estimation of the particle's volume. The principle is tested and confirmed experimentally by analyzing a set of three interferometric images of "programmable"particles generated by a digital micromirror device.

A digital micromirrors device is used to reproduce the speckle-like interferometric images that would produce rough particles. Time-dependent index inhomogeneities induced by a flame are added between the particle and the imaging system. The size measurements deduced from 2D-Fourier analysis of the interferometric patterns show a less than 10% error when the programmed object is fixed, and a less than 20% error when a scintillation of the object is programmed.

The usual parabolic approximation used for modelling the focusing properties of a Kerr lens induced by a Gaussian laser beam, through optical Kerr effect, consists to roughly approximate a Gaussian profile by a parabola. The Kerr focal length deduced from the ABCD formalism is found to not correctly provide the focal plane position. The latter has been numerically determined from a numerical diffraction analysis, and we have empirically found that the right focal length is obtained by multiplying, by a factor equal to 3, the focal length given by the usual parabolic approximation. The expression of the focal length associated with the Kerr lens that we have obtained is in a good agreement with the expression of the focal length determined on the basis of a Zernike decomposition. In addition, it is demonstrated that the emerging beam from the Kerr lens is no longer Gaussian since its propagation factor M-2 is larger than unity. It is found that the M-2 factor increases linearly with the on axis phase shift.

A novel technique to improve the focus depth of a Gaussian beam is presented in this paper. The improvement is based on two-step beam shaping using a cascade of binary phase diffractive optical elements (BPDOEs). The first BPDOE transforms the incident Gaussian beam into a high-order radial Laguerre-Gaussian beam (LGp0). Then the second BPDOE rectifies the obtained LGp0 beam and gives rise to a quasi-Gaussian one in the focal plane of a converging lens. This resulting quasi-Gaussian beam exhibits a lower divergence and larger focus depth compared to the pure Gaussian beam having the same beam waist. These results open new possibilities in laser beam manufacturing and micromachining, and in applications that need an extended focus depth.

The set of experimental and numerical tools that have been developed to perform interferometric out-of-focus images of ice particles is presented. The different experimental results that have been obtained and that validate the measurement method are presented, analyzed and discussed.

The electronic refractive index variation is associated with the difference in the polarizabilities (Delta alpha(p)) of the Cr3+ ion in its ground and excited states. In order to further address the physical origin of Amp in a Cr3+-doped YAG crystal, time-resolved Z-scan measurements were performed and analyzed at lambda = 457 nm by using a chopped Ar+ ion laser. It is found a nonlinear refractive index with the real and imaginary parts n(2)' = 2.2 x 10(-8) cm(2)/W and n(2)" = 2.8 x 10(-1)0 cm(2)/W, respectively. The real part is associated with a polarizability difference Delta alpha(p) = 2.2 x 10(-25) cm(3). The imaginary part indicates that excited state absorption (ESA) occurs and that Cr:YAG behaves as a saturable absorber. The transient response of the Z-scan signal decreases with the laser intensity as tau(-1)= tau(-1)(0)(1+I/Is), where To is the excited state lifetime and I-s the saturation intensity. By measuring this transient response at different laser intensities, it was possible to confirm the tau(0) value which can be derived from fluorescence measurements and to determine a Is value of 8.3 kW/cm(2). (C) 2018 Elsevier B.V. All rights reserved.

Many laser applications utilise a focused laser beam having a single-lobed intensity profile in the focal plane, ideally with the highest possible on-axis intensity. Conventionally, this is achieved with the lowest-order Laguerre-Gaussian mode (LG(00)), the Gaussian beam, in a tight focusing configuration. However, tight focusing often involves significant spherical aberration due to the high numerical aperture of the systems involved, thus degrading the focal quality. Here, we demonstrate that a high-order radial LG(p0) mode can be tailored to meet and in some instances exceed the performance of the Gaussian. We achieve this by phase rectification of the mode using a simple binary diffractive optic. By way of example, we show that the focusing of a rectified LG(50) beam is almost insensitive to a spherical aberration coefficient of over three wavelengths, in contrast with the usual Gaussian beam for which the intensity of the focal spot is reduced by a factor of two. This work paves the way towards enhanced focal spots using structured light.