Daniel-Joseph Lougnot

Integration of micrometer-sized polymer elements at the end of optical fibers by free-radical photopolymerization

A simple method of manufacturing micrometer-sized polymer elements at the extremity of both single-mode and multimode optical fibers is reported. The procedure consists of depositing a drop of a liquid photopolymerizable formulation on a cleaved fiber and using the light that emerges from the fiber to induce the polymerization process. After exposure and rinsing a polymer tip is firmly attached to the fiber as an extension of the fiber core. It is shown that the tip geometry can be adjusted by the variation of basic parameters such as the geometry of the deposited drop and the conditions of drop illumination. When this process is applied to a multimode fiber three-dimensional molds of the fibers linearly polarized modes can be obtained. The process of polymer-tip formation was simulated by a numerical calculation that consisted of an iterative beam-propagation method in a medium whose refractive index is time varying. It is shown that this process is based on the gradual growth, just above the fiber core, of an optical waveguide in the liquid formulation. Experimental data concerning two potential uses of the tipped fibers are presented.

PEW: Photopolymerization by evanescent waves. II. Revealing dramatic inhibiting effects of oxygen at submicrometer scale

A simplistic model of photopolymerization by evanescent waves (PEW) has been introduced in a previous paper. This model predicts a linear dependence of the polymerized thickness on the logarithm of exposure time. It provides the general trend of the experimental data. The present work shows that the properties of the material and the chemistry of the system also have to be taken into account to fit the experiments more precisely. Evidence was provided that dissolved oxygen has a marked effect on gel dose. In PEW the reaction is induced in very small solution volumes, so that marked competition seems to take place between reaction and diffusion.

DRY PHOTOPOLYMER FILMS FOR COMPUTER-GENERATED INFRARED RADIATION FOCUSING ELEMENTS

A new technological approach makes fabrication of relief computer-generated focusing elements for IR radiation by use of a dry photopolymer recording material possible. The formation of a relief structure by self-development takes place in the dark, subsequent to the holographic illumination, without wet processing. Consequently these diffractive elements exhibit low surface scattering. The formation of a surface wave of the monomer along the light-darkness boundary is observed for the first time to our knowledge and confirms the previously proposed thermodynamic model of the mechanism of the hologram formation in photopolymerizable layers. Dye-sensitized polymerization of acrylamide is found to produce nonlinearity of the relief recording. At least partial compensation of this nonlinearity is attained by the introduction of appropriate corrections into the computer-generated amplitude function. A diffraction efficiency of ~ 55% is obtained for CO(2) laser radiation (λ = 10.6 µm).

Photopolymerizable material for holographic recording in the 450-550 nm domain

The photopolymerizable recording media, can be used to record holographic patterns produced by laser sources emitting from 450 to 550 nm. The polymerization takes place only in the bright areas corresponding to the regions of constructive interference and volume phase holograms are created. Diffraction efficiencies of 50% are recorded with an index modulation of 2*10-3. Optimum exposure is typically 300 mJ/cm2 and the upper limit for spatial frequencies is higher than 3000 lines/mm.

High spatial frequency evanescent wave holographic recording in photopolymers

In the present work we report the results of holographic recording with two contra-propagating evanescent waves in photopolymers. The spatial frequency of the recorded permanent grating is 6380 mm−1 (grating step 156.7 nm) and 0.015% diffraction efficiency at 514.5 nm is achieved. The angular selectivity of the recorded evanescent grating is investigated at 632.8 nm. The maximum diffraction efficiency value is obtained at a critical angle of 60.1° ± 0.2° to the boundary with the total internal reflection prism, that determines the average refractive index of the grating of 1.501 ± 0.003. This is in good agreement with independent refractometric measurements and with theoretical expectations. A simple theoretical analysis of the evanescent wave holographic recording is made taking into account weak light absorption by the recording second medium. The close analogy with the attenuated total internal spectroscopic method is emphasized.

Replication of patterns on nanometric polymer films obtained by photopolymerization by evanescent waves

In this paper, the process of thin film photopolymerization using evanescent waves is used and the adaptability of this process to pattern replication is studied. Variable frequency patterns are copied on film by an imaging process. After photopolymerization, a thin polymer image is obtained, and the shape of the polymer parts is studied. The main limitation to this process is the loss of fidelity between the light profile just after the target and after the imaging set-up. Taking into account optical considerations, the theoretical shape of the polymer parts obtained by this method is calculated and then compared with experimental results. The general shape can be predicted by this approach, but a finer analysis indicates that the response of the formulation has also to be taken into account.

Evanescent wave holographic recording in a photopolymer

We present a study of holographic recording with an evanescent reference and a homogeneous (plane) object wave. The grating step was 324 nm. The dependence of the diffraction efficiency on exposure was investigated. The maximum value obtained was 0.01% at 5 mJ cm−2 exposure. The optimal pre-exposure, needed for grating adhesion to the glass substrate, was 1 μJ cm−2.

Integration of polymer elements at the end of optical fibers by free-radical photopolymerization

In this article, we report a method of free-radical photopolymerization on the top end of a single-mode optical fiber. The process is very simple to carry out and uses the visible light guided in the fiber. It permits one to produce, on the fiber end, a micronic polymer tip which may be viewed as an extension of the fiber core and whose optical properties improve the intrinsic capability of the fiber, particularly in terms of efficiency of coupling with a laser source. Formation of the tip is explained by self-guiding of the light within the material and photophysical response of the material.

CO2 laser diffractive optics fabrication on photopolymer and performance evaluation

A new technological approach using dry photopolymer recording material makes it possible to fabricate the relief focusing elements for IR radiation. The formation of the relief structure by self-development takes place in the dark, subsequent to the illumination through the computer generated amplitude mask. No wet processing is required. These diffractive elements exhibit low surface scattering. Dye sensitized polymerization of acrylamide has been found to produce nonlinearity of the relief recording. A diffraction efficiency of about 55% has been achieved for CO2 laser radiation.

Self-processing photopolymer materials for holographic recording

Photopolymer materials have been used for recording holograms since the late sixties. A great variety of formulations and conditioning were developed with the object of meeting the requirements of specific applications in the fields of holography, information storage or optical elements. The need for materials exhibiting a self-processing character aroused the creativity of many scientists who studied the details of the recording process in this original class of sensitive systems. In fact, the process of optical information storage that results from the coupling of the photochemically induced conversion of monomers, mass transport due to diffusion and grooving due to gradients of surface free energy can be made selfdeveloping by a proper choice of the physico chemical and photochemical parameters of the recording system. Real-time, double exposure and time-averaged holographic interferometry, recording of image and computer generated holograms and chopped light recording are some of the typical applications of the self-developing materials.