Sylvain Lecler

Properties of a three-dimensional photonic jet

By focusing light with a sphere several wavelengths in diameter, we can obtain a photonic nanojet [Opt. Express 13, 526 (2005)]: if light is focused on the surface of the sphere, the width of the beam stays smaller than the wavelength along a distance of propagation of approximately two wavelengths and reaches a high intensity. We use the rigorous Mie theory to analyze the basic properties of the photonic jet in the general three-dimensional polarized case. This fast algorithm allows us to determine the influence of the radius and the refractive index of the sphere on the photonic jet. The polarization response is also studied. We observe that high-intensity concentrations and subwavelength focusing are two different effects. Their basic properties are analyzed, and explanations are proposed.

Mode couplings and elasto-optic effects study in a proposed mechanical microperturbed multimode optical fiber sensor.

A step index multimode optical fiber with a perturbation on a micrometer scale, inducing a periodic deformation of the fiber section along its propagation axis, is theoretically investigated. The studied microperturbation is mechanically achieved using two microstructured jaws squeezing the straight fiber. As opposed to optical fiber microbend sensors, the optical axis of the proposed transducer is not bended; only the optical fiber section is deformed. Further, the strain applied on the fiber produces a periodical elliptical modification of the core and a modulation of the index of refraction. As a consequence of the micrometer scale perturbation period, the resulting mode coupling occurs directly between guided and radiated modes. To simulate the transmission induced by these kinds of perturbations, simplified models considering only total mode couplings are often used. In order to investigate the range of validity of this approximation, results are compared to the electromagnetic mode couplings rigorously computed for the first time, to our knowledge, with a large multimode fiber (more than 6000 linear polarized modes) using the Marcuse model. In addition, in order to have a more complete modeling of the proposed transducer, the anisotropic elasto-optic effects in the stressed multimode fiber are considered. In this way, the transmission of the microperturbed optical fiber transmission and, therefore, the behavior of the transducer are physically explained and its applications as a future stretching sensor are discussed.

Single and Dual Photonic Jets and Corresponding Backscattering Enhancement With Tipped Waveguides: Direct Observation at Microwave Frequencies

This paper reports experiments on single and dual electromagnetic jets (also called photonic jets) emerging from a weakly multimode waveguide excited by a plane wave in the microwaves frequencies (30 GHz), ended with a tip with a special shape. The measurements are performed with a two-dimensional spatial field mapping system. This system acquires the electric field distribution at the end of the tipped waveguide. Qualitatively, the electric field maps confirm the presence of photonic jets that were recently predicted by a computational model based on boundary integral equations. Backscattering induced by the interaction of a particle with the electromagnetic jet is also measured; particles with a size smaller than the incident wavelength can be easily detected. The high sensitivity feature of the backscattering can be used to develop a near-field technique for microwave or optical imaging.

Single and Dual Photonic Jets With Tipped Waveguides: An Integral Approach

Single and dual photonic jets generated by weak multimode waveguides can be studied out rigorously by means of an integral equation. The latter can be obtained on the basis of a principle that resembles one of Mautz and Harringtons although the context is different. Comparison with finite difference time domain results showed overall good agreement, and because the method is fast, it can be used for optimization purposes. In particular, it permitted us to obtain dual photonic jets with beam separation as small as a wavelength.

Modeling a multimode Sagnac interferometer: Application for an embarked fiber optic gyroscope.

In this paper, we proposes a general model for describing and analysing the operation of the multimode fiber optic gyroscope (MFOG). In this model, each fiber gyroscope components is modeled by an electrical field transfer matrix. The output intensity is expressed in terms of the rotation rate and of the components parameters. The simulation results show that performances enhancement can be obtained by employing a symmetrical multimode coupler and low coherent light source.

3D Super-Resolution Optical Profiling Using Microsphere Enhanced Mirau Interferometry

We present quantitative three dimensional images of grooves on a writable Blu-ray Disc based on a single objective Mirau type interferometric microscope, enhanced with a microsphere which is considered as a photonic nanojet source. Along the optical axis the resolution of this microsphere assisted interferometry system is a few nanometers while the lateral resolution is around 112 nm. To understand the physical phenomena involved in this kind of imaging we have modelled the interaction between the photonic jet and the complex disc surface. Agreement between simulation and experimental results is demonstrated. We underline that although the ability of the microsphere to generate a photonic nanojet does not alone explain the resolution of the interferometer, the nanojet can be used to try to understand the imaging process. To partly explain the lateral super-resolution, the potential role of coherence is illustrated. The presented modality may have a large impact on many fields from bio-medicine to nanotechnology.

Intrinsic Optical Fiber Sensor

The use of more and more complex systems in transportation, bio medical devices, defence, production, systems, etc., that are more and more automated, having to comply with many norms related to low energy consumption, miniaturization, insensivity to modifications of the surrounding conditions, security, etc. induce the need of new sensors that have to be reliable, low cost, easy to produce, if possible without clean rooms, avoiding too demanding calibration process and using no poisonous components or materials.

Optimizing the optical components choice for performances improvement of multimode fiber gyroscope

In this work, we report a theoretical study for optimizing the optical components choice to design a new low cost and high performances multimode fiber optic gyroscope (MFOG). This study shows that high performances can be obtained by using all optical components with the same SI 50/125 multimode optical fiber associated with low coherence light source, such as a LED, and a photodetector with large active area. For further improvement of MFOG performances, we present a detailed analysis of the photo-detection circuits design considerations.

Microsphere-assisted phase-shifting profilometry

In the present work, we have investigated the combination of a superresolution microsphere-assisted 2D imaging technique with low-coherence phase-shifting interference microscopy. The imaging performance of this technique is studied by numerical simulation in terms of the magnification and the lateral resolution as a function of the geometrical and optical parameters. The results of simulations are compared with the experimental measurements of reference gratings using a Linnik interference configuration. Additional measurements are also shown on nanostructures. An improvement by a factor of 4.7 in the lateral resolution is demonstrated in air, thus giving a more isotropic nanometric resolution for full-field surface profilometry in the far field.

Photonic jet to improve the lateral resolution of laser etching

The techniques applying laser beams or optical systems are limited by the diffraction limit of the optical heads used. We demonstrate theoretically and experimentally that the use of the photonic jet allows an improvement in the optical resolution to achieve smaller etching without reducing the wavelength of the source. The potential of the photonic jet using a nanosecond pulsed near-infrared laser for micro-fabrication is also demonstrated. These lasers are the most common type of laser used in industrial processes because of their price and the fact that well-packaged sources are available. Their typical spatial resolution in laser etching is limited by the spot size of their focus point at around 25-70 μm. This is the reason why a photonic jet, a high spatial concentration onto a half-wavelength spot of a beam that emerges in the vicinity of a dielectric microsphere, is of great interest. In our experiments, micro-scale glass (ns = 1.5) and BaTiO3 spheres (ns = 1.9) have been used to achieve photonic jets. The etching process has been tested on two substrates: silicon wafers, which have a significant absorption at 1064 nm, and glass plates, which have a lower absorption at this wavelength. The smallest marking achieved on silicon has an average diameter of 1.3 μm and despite the low absorption, micrometric etchings have also been achieved on glass using larger microspheres.