James R. Leger

Bio: James R. Leger is an academic researcher from University of Minnesota. The author has contributed to research in topics: Laser & Laser beam quality. The author has an hindex of 33, co-authored 193 publications receiving 4269 citations. Previous affiliations of James R. Leger include Massachusetts Institute of Technology & Uppsala University.

Focus shaping using cylindrical vector beams

Abstract: We report a focus shaping technique using generalized cylindrical vector beams. A generalized cylindrical vector beam can be decomposed into radially polarized and azimuthally polarized components. Such a generalized cylindrical beam can be generated from a radially polarized or an azimuthally polarized light using a two-half-wave-plate polarization rotator. The intensity pattern at the focus can be tailored by appropriately adjusting the rotation angle. Peak-centered, donut and flattop focal shapes can be obtained using this technique.

Diffractive optical element for mode shaping of a Nd: YAG laser

Abstract: A diffractive laser cavity mirror is described that can customize the amplitude and phase of a laser mode. The design of this diffractive element is shown for a square, flat-topped fundamental mode. The laser cavity has a theoretical fundamental mode loss of only 0.08% and a second-order mode loss of 48.2%, resulting in high modal discrimination. The fabricated mirror is tested in a Nd:YAG laser system. The resulting square flat-topped mode has an rms variation of 1.5% over the two-dimensional flat-topped region and a large discrimination against higher-order modes.

Coherent addition of AlGaAs lasers using microlenses and diffractive coupling

Abstract: A near single‐lobed far‐field pattern was obtained from coherent operation of a nonevanescently coupled AlGaAs laser diode array. A diffractive microlens array collimated the individual beams to approximate a plane wave, and diffractive coupling from an external cavity mirror provided mutual coherence. A diffraction‐limited far‐field pattern was observed with 82% of the power contained in the central lobe. The method is directly applicable to two‐dimensional laser arrays and can be implemented as a single thin optical element.

Coherent laser addition using binary phase gratings

Abstract: Binary phase diffraction gratings are shown to couple light coherently from a laser array into a single on-axis beam. The diffraction grating, designed to split a single beam into a specific number of equal intensity diffraction orders, is placed inside the cavity formed by the laser array and a common output mirror. The grating superimposes the light beams from the lasers in the array and produces a far-field pattern with the same divergence as that of a single laser. Six GaAlAs lasers from an antireflection-coated linear array were combined with a coupling efficiency of 68.4%. The far field of the combined GaAlAs lasers consisted of a single on-axis Gaussian beam.

Flat Optics With Designer Metasurfaces

Abstract: Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Cylindrical vector beams: from mathematical concepts to applications

Abstract: An overview of the recent developments in the field of cylindrical vector beams is provided. As one class of spatially variant polarization, cylindrical vector beams are the axially symmetric beam solution to the full vector electromagnetic wave equation. These beams can be generated via different active and passive methods. Techniques for manipulating these beams while maintaining the polarization symmetry have also been developed. Their special polarization symmetry gives rise to unique high-numerical-aperture focusing properties that find important applications in nanoscale optical imaging and manipulation. The prospects for cylindrical vector beams and their applications in other fields are also briefly discussed.

Non-Hermitian physics and PT symmetry

Abstract: In recent years, notions drawn from non-Hermitian physics and parity–time (PT) symmetry have attracted considerable attention. In particular, the realization that the interplay between gain and loss can lead to entirely new and unexpected features has initiated an intense research effort to explore non-Hermitian systems both theoretically and experimentally. Here we review recent progress in this emerging field, and provide an outlook to future directions and developments. This Review Article outlines the exploration of the interplay between parity–time symmetry and non-Hermitian physics in optics, plasmonics and optomechanics.

High power fiber lasers: current status and future perspectives [Invited]

Abstract: The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding performance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operating wavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry, which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient conversion from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper we review the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-doped fiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-power performance. We then review the current status and challenges of extending the technology to other rare-earth dopants and associated wavelengths of operation. Throughout we identify the key factors currently limiting fiber laser performance in different operating regimes—in particular thermal management, optical nonlinearity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design and fabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implications they have on fiber laser performance and industrial/scientific adoption.

Laser beam combining for high-power, high-radiance sources

Abstract: Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.