James R. Leger

Focus shaping using cylindrical vector beams

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.

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

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.

Efficient array illuminator using binary-optics phase plates at fractional-Talbot planes

We describe a phase plate based on the fractional-Talbot effect that converts a single expanded laser beam into a regular array of uniformly illuminated apertures with virtually 100% efficiency. The size, spacing, and fill factor of the illuminated aperture grid can be freely chosen to interface with a variety of electro-optic devices. A binaryoptics phase plate is demonstrated that converts uniform illumination into an array of square illumination cells with a fill factor of 1/16.

Coherent laser addition using binary phase gratings

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.

Geometrical transformation of linear diode-laser arrays for longitudinal pumping of solid-state lasers

A 200-stripe linear diode-laser array is geometrically transformed into a two-dimensional, symmetric virtual source with symmetric divergence to end pump a Nd:YAG laser. The geometrical transformation is performed by two planes of diffractive optical elements separated by a 2.6-cm gap. Discounting optical losses, a TEM/sub 00/ mode slope efficiency of 56% is demonstrated. Methods of increasing the throughput efficiency of the diffractive elements (currently approximately 50% per element,) are explored. A theoretical model for estimating the maximum useful pump array size in longitudinally pumped rod and fiber lasers shows that this pump geometry is close to optimum. >

Lateral mode control of an AlGaAs laser array in a Talbot cavity

The lateral mode thresholds of an external cavity AlGaAs laser diode array are measured as a function of cavity length. It is found that certain cavity lengths induce the array to lase in either the fundamental or highest order lateral mode. The results are explained using the theory of Talbot self‐imaging in fractional Talbot planes

Thin-sheet laser imaging microscopy for optical sectioning of thick tissues

We report the development of a modular and optimized thin-sheet laser imaging microscope (TSLIM) for nondestructive optical sectioning of organisms and thick tissues such as the mouse cochlea, zebrafish brain/inner ear, and rat brain at a resolution that is comparable to wide-field fluorescence microscopy. TSLIM optically sections tissue using a thin sheet of light by inducing a plane of fluorescence in transparent or fixed and cleared tissues. Moving the specimen through the thinnest portion of the light sheet and stitching these image columns together results in optimal resolution and focus across the width of a large specimen. Dual light sheets and aberration-corrected objectives provide uniform section illumination and reduce absorption artifacts that are common in light-sheet microscopy. Construction details are provided for duplication of a TSLIM device by other investigators in order to encourage further use and development of this important technology.

Microellipsometer with radial symmetry

We report on a novel microellipsometer that uses a spatially filtered high-numerical-aperture (NA) lens for large-angle ellipsometric illumination and high spatial resolution. A radially symmetric ellipsometric signal is achieved with two half-wave plates to produce a pure polarization rotation and a birefringent cube as a radial analyzer. This radial symmetry offers a better signal-to-noise ratio compared with other microellipsometer techniques. Ellipsometric measurement with a spatial resolution of 0.5 microm is performed with a He-Ne (632.8-nm) laser source and an objective lens with an NA of 0.8. Experimental data on SiO2 samples with different thicknesses are in good agreement with spectroscopic ellipsometer results. We acquired ellipsometric images of photoresist microstructure through scanning the sample. Surface profiles of the photoresist microstructure are derived from the ellipsometric data and compared with the results from a stylus profiler.

Experimental measurement of longitudinal component in the vicinity of focused radially polarized beam

Experiments were carried out to study the focusing properties of radially polarized light. By direct recording of the focal pattern in photoresist, the intensity distribution in the vicinity of the beam focus was measured, and the non-propagating longitudinal component (z-component) was clearly demonstrated. Comparison with corresponding theory shows good agreement.