Orange surface waveguide laser in Pr:LiYF4 produced by a femtosecond laser writing.

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.

Red Sm:KGd(WO4)2 laser at 649†nm.

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.

Interferometric images of irregular rough particles in the presence of time-dependent index inhomogeneities in front of the imaging system.

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.

Extended focus depth for Gaussian beam using binary phase diffractive optical elements.

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.

Generation of Laguerre-Gaussian LGp0 beams using binary phase diffractive optical elements.

In recent years, considerable attention has been devoted to laser beams with specific intensity profile, i.e., non-Gaussian. In this work, we present a novel technique to generate high-radial-order Laguerre-Gaussian beams LG(p0) based on the use of a binary phase diffractive optical element (BPDOE). The latter is a phase plate made up of annular zones introducing alternatively a phase shift equal to 0 or π modeled on positions which do not coincide with the position of the zeros of the desired LG(p0) beam. The LG(p0) beams are obtained by transforming a fundamental Gaussian beam through an appropriate BPDOE. The design of the latter is based on the calculation of the Fresnel-Kirchhoff integral, and the diffracted intensity at the focus plane of a lens has been modeled analytically for the first time. The numerical simulations and experiment demonstrate a good beam quality transformation. Obtained LG(p0) are suitable for atom trap and pumping solid state laser applications.

Spatial properties of coaxial superposition of two coherent Gaussian beams.

In this paper, we explore theoretically and experimentally the laser beam shaping ability resulting from the coaxial superposition of two coherent Gaussian beams (GBs). This technique is classified under interferometric laser beam shaping techniques contrasting with the usual ones based on diffraction. The experimental setup does not involve the use of some two-wave interferometer but uses a spatial light modulator for the generation of the necessary interference term. This allows one to avoid the thermal drift occurring in interferometers and gives a total flexibility of the key parameter setting the beam transformation. In particular, we demonstrate the reshaping of a GB into a bottle beam or top-hat beam in the focal plane of a focusing lens.

Reconstruction-free sensitive wavefront sensor based on continuous position sensitive detectors.

We propose a new device that is able to perform highly sensitive wavefront measurements based on the use of continuous position sensitive detectors and without resorting to any reconstruction process. We demonstrate experimentally its ability to measure small wavefront distortions through the characterization of pump-induced refractive index changes in laser material. In addition, it is shown using computer-generated holograms that this device can detect phase discontinuities as well as improve the quality of sharp phase variations measurements. Results are compared to reference Shack-Hartmann measurements, and dramatic enhancements are obtained.

Transverse superresolution technique involving rectified Laguerre-Gaussian LG(p)° beams.

A promising technique has been proposed recently [Opt. Commun. 284, 1331 (2011), Opt. Commun. 284, 4107 (2011)] for breaking the diffraction limit of light. This technique consists of transforming a symmetrical Laguerre-Gaussian LG(p)° beam into a near-Gaussian beam at the focal plane of a thin converging lens thanks to a binary diffractive optical element (DOE) having a transmittance alternatively equal to -1 or +1, transversely. The effect of the DOE is to convert the alternately out-of-phase rings of the LG(p)° beam into a unified phase front. The benefits of the rectified beam at the lens focal plane are a short Rayleigh range, which is very useful for many laser applications, and a focal volume much smaller than that obtained with a Gaussian beam. In this paper, we demonstrate numerically that the central lobe's radius of the rectified beam at the lens focal plane depends exclusively on the dimensionless radial intensity vanishing factor of the incident beam. Consequently, this value can be easily predicted.

Baryscan: a sensitive and user-friendly alternative to Z scan for weak nonlinearities measurements.

A new and efficient technique for measuring weak optical nonlinearities is reported. Like Z scan, its implementation is basic, both experimentally and theoretically, but leads to an improved sensitivity of λ/5.104, which represents, to date, one of the highest observed enhancements. With this technique, which is based upon the use of a position sensitive detector, nonlinear properties are deduced by monitoring the barycentric position of a truncated pump-probe laser beam as the sample is moved along the optical axis. The technique is experimentally validated by measuring the pump-induced refractive index change and the underlying polarizability variation resulting from the excitation of the Cr(3+) ions in ruby.

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.

Cascades of pi-phase plates: a transparent diffractive focusing system.

Typically, refractive lenses are used to focus rays of light, but an alternative way can be found by exploiting diffraction of light. It is well known that cascades of hard-edge apertures are able to focus light but with the great drawback of absorption losses. In this paper, we demonstrate that replacing hard-edge apertures with pi-phase plates within a cascade greatly improves the focusing of collimated Gaussian beams. In addition, we propose a simple model to design this cascade, in particular to find the locations and the radii of the different optics once the focal length has been chosen. This model deduced from numerical simulation is useful for sizing cascades consisting of a high number of components and characterized by a strong focusing ability, without requiring a time-consuming optimization process.

Modeling the propagation of apertured high-order Laguerre-Gaussian beams by a user-friendly version of the mode expansion method.

The mode expansion method (MEM) models the propagation of an apertured beam by expressing the diffracted field as a finite series of Laguerre-Gaussian or Hermite-Gaussian modes. An optimal expansion parameter set (beam waist of the modes and its location) reduces the number of modes, which is difficult to derive, especially for high-order incident beams. We propose a user-friendly version of the MEM in which the expansion parameter set and the suitable number of modes are simply deduced from the approximation of the apertured incident beam.

Adaptive Laguerre-Gaussian variant of the Gaussian beam expansion method.

A variant of the Gaussian beam expansion method consists in expanding the Bessel function J0 appearing in the Fresnel-Kirchhoff integral into a finite sum of complex Gaussian functions to derive an analytical expression for a Laguerre-Gaussian beam diffracted through a hard-edge aperture. However, the validity range of the approximation depends on the number of expansion coefficients that are obtained by optimization-computation directly. We propose another solution consisting in expanding J0 onto a set of collimated Laguerre-Gaussian functions whose waist depends on their number and then, depending on its argument, predicting the suitable number of expansion functions to calculate the integral recursively.

Improvement of the self-Q-switching behavior of a Cr:LiSrAlF6 laser by use of binary diffractive optics.

It has been shown experimentally and theoretically that Q-switching behavior is possible in a flashlamp-pumped Cr-doped LiSrAlF6 (Cr3+:LiSAF) laser that consists only of two mirrors, a laser crystal, and a diaphragm. We demonstrate that insertion into a laser of a binary diffractive optical element can speed up the dynamics of the self-Q-switched laser such that the output pulse is shortened (from 60 to 33 ns) and its energy is increased (from 36 to 54 mJ). The self-Q-switching behavior of the laser has the ability to produce a laser pulse with a duration that one can adjust continuously from 60 to 700 ns just by opening the diaphragm.