Nicolas Passilly

Simple interferometric technique for generation of a radially polarized light beam

We present a theoretical and experimental investigation of an interferometric technique for converting a linearly polarized Gaussian beam into a radially polarized doughnut beam. The experimental setup accomplishes the coherent summation of two orthogonally polarized TEM01 and TEM10 beams that are obtained from the transformation of a TEM00 beam by use of a simple binary diffractive optical element. We have shown that the degree of radial polarization is maximum at a given distance from the interferometer output port that depends on the diameter of the incident beam at the interferometer input port.

Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques

We report a novel process technology of hemispherical shaped microlenses, using isotropic wet etching of silicon in an acid solution to produce the microlenses molds. Governed by process parameters such as temperature and etchant concentration, the isotropic wet etching is controlled to minimize various defects that appear during the molding creation. From the molds, microlenses are fabricated in polymer by conventional replication techniques such as hot embossing and UV-molding. The characterization of molds and measurements of replicated microlenses demonstrate high smoothness of the surfaces, excellent repeatability of mold fabrication and good optical properties. Using the proposed method, a wide range of lens geometries and lens arrays can be achieved.

Nanosecond pulse generation in a passively Q-switched Yb-doped fiber laser by Cr/sup 4+/:YAG saturable absorber

We explored efficient nanosecond pulse generation in a passively Q-switched cladding-pumped Yb-doped fiber laser by use of an external-cavity configuration containing a Cr/sup 4+/:YAG saturable absorber crystal. By exploiting passive Q-switched regime and stimulated Brillouin scattering, pulses of durations as short as 2.7 ns, corresponding to a peak power of /spl sim/9 kW, have been achieved. The laser was tunable over 70 nm and green output power generation was obtained by external frequency doubling in a KTP crystal.

Beam-shaping longitudinal range of a binary diffractive optical element

An experimental and theoretical investigation of laser beam shaping using a simple binary diffractive optic is presented. Beam tailoring has been characterized by the experimental determination of two relevant parameters: beam propagation factor M(2) and the beam-shaping longitudinal range, which represents the propagating distance for which the tailored beam remains nearly unchanged.

Micromachined array-type Mirau interferometer for parallel inspection of MEMS

We present the development of an array type of micromachined Mirau interferometers, operating in the regime of low coherence interferometry (LCI) and adapted for massively parallel inspection of MEMS. The system is a combination of free-space micro-optical technologies and silicon micromachining, based on the vertical assembly of two glass wafers. The probing wafer carries an array of refractive microlenses, diffractive gratings to correct chromatic and spherical aberrations and reference micro-mirrors. The semitransparent beam splitter plate is based on the deposition of a dielectric multilayer, sandwiched between two glass wafers. The interferometer matrix is the key element of a novel inspection system aimed to perform parallel inspection of MEMS. The fabricated demonstrator, including 5 × 5 channels, allows consequently decreasing the measurement time by a factor of 25. In the following, the details of fabrication processes of the micro-optical components and their assembly are described. The feasibility of the LCI is demonstrated for the measurement of a wafer of MEMS sensors.

Low-cost fabrication of form-birefringent quarter-wave plates

An extensive study of the single-step replication of form-birefringent quarter-wave plates is presented. Using rigorous diffraction theory, the fabrication parameters and tolerances are carefully studied in order to obtain ideal conditions for successful replication. The design considerations are then applied to fabricate a master element by electron-beam lithography, and to replicate quarter-wave plates using the UV-moulding process. The measurements show that the replicas behave as high-performance quarter-wave plates for the design wavelength.

Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings

Subwavelength metallic and dielectric diffraction gratings which rotate the linear polarization of incident light by 90 degrees are examined. Using rigorous diffraction theory in total-internal-reflection configuration, it is shown that full conversion from incident transverse electric field to transverse magnetic zero-order field can be achieved with both dielectric and metallic elements, but dielectric gratings provide higher efficiency and are thus preferable. The fabrication aspects and constraints are discussed in detail and the behavior of the gratings over broad wavelength bands is presented.

Experimental and theoretical investigation of a rapidly varying nonlinear lensing effect observed in a Cr 3+ :LiSAF laser

An experiment has been performed to find evidence and analyze a nonlinear diverging lensing effect occurring in a flash-lamp-pumped Cr3+:LiSAF laser. The effect is assigned to a refractive-index change of the material that is proportional to the Cr3+-excited ion population, the corresponding constant of proportionality being determined from the time variation of the laser-pulse far-field divergence.

A simple method for quality evaluation of micro-optical components based on 3D IPSF measurement.

This paper presents a simple method based on the measurement of the 3D intensity point spread function for the quality evaluation of high numerical aperture micro-optical components. The different slices of the focal volume are imaged thanks to a microscope objective and a standard camera. Depending on the optical architecture, it allows characterizing both transmissive and reflective components, for which either the imaging part or the component itself are moved along the optical axis, respectively. This method can be used to measure focal length, Strehl ratio, resolution and overall wavefront RMS and to estimate optical aberrations. The measurement setup and its implementation are detailed and its advantages are demonstrated with micro-ball lenses and micro-mirrors. This intuitive method is adapted for optimization of micro-optical components fabrication processes, especially because heavy equipments and/or data analysis are not required.

Exciting higher-order radial Laguerre–Gaussian modes in a diode-pumped solid-state laser resonator

In this paper we experimentally demonstrate the intracavity generation of selected Laguerre-Gaussian modes of variable radial order, from 0 to 5. Our technique requires only an amplitude mask made up of absorbing rings to be placed inside the cavity, with the ring radii selected to coincide with the zeros of the desired Laguerre-Gaussian mode. We demonstrate high mode purity and a mode volume proportional to the order of the mode. Our results suggest a possible route to high brightness diode-pumped solid-state laser sources.