Hugues Giovannini

Study of the resonant behaviour of waveguide gratings: increasing the angular tolerance of guided-mode filters

This paper is devoted to the study of resonant waveguide grating filters. It is shown that doubly periodic structures permit one to increase the angular tolerance of the filters without modifying the frequency linewidth. A theoretical study of such structures is presented and their response to an incident Gaussian beam is analysed. It is also shown that doubly periodic structures can be used to reduce absorption losses.

Dual-wavelenth passive homodyne detection unit for fiber-coupled white-light interferometers

This paper is devoted to the description of a prototype Emission/Detection Unit, the ACCORD® Module, developed for the accurate demodulation of fiber optic sensors using spectral modulation encoding techniques. This prototype contains a static polarimetric interferometer with 4 ports to provide the four phase signals needed by the passive homodyne scheme, and uses a dual-wavelength arrangement to extend the measurement range by removing the 2ic ambiguity in the phase determination. Resolution as high as A/40000 has been achieved with 1 Hz bandwith and Optical Path Difference mismatch in a 20 microns range.

Strong extraction coefficient for sources located inside waveguide grating structures

An electromagnetic theory for sources radiating into crossed waveguide gratings is presented and implemented numerically. In the framework of spontaneous emission control, we present a structure which can couple out the total guided-mode power emitted by a point source dipole located in the corrugated region. Numerical calculations show that most of the total emitted light is confined in specific arc shape directions.

Enhanced absorption in very rough overcoated black surfaces

We show how to use dielectric thin films in order to reduce scattering and maximize broad-band absorption in arbitrary rough overcoated black surfaces. A rigorous calculation is developed thanks to an improved differential method, and the results are compared to experiment. Scattering is reduced by a factor 10 (down to 5.10-3), though the starting bare surface is a standard low-cost black paint (5.10-2 scattering). Absorption can therefore be optimized up to 0.995.

Interest of hybrid structures for thin-film design: multilayered subwavelength microgratings

The basic principles of guided mode resonance filters constituted by a high index modulated coating deposited on a classical glass substrate are presented. A rigorous method used to compute the reflection coefficients of this kind of structures is described. We show that in some cases, computation can be made by using an approximate method. These structures, which permit one to design narrowband inverse filters, are compared to solutions based on classical multidielectric coatings.

Mechanical properties of dielectric thin films

Stress in thin films deposited by Reactive Low-Voltage Ion Plating is studied in air and at room temperature. A multilayer stack, composed of tantalum pentoxide and silicon dioxide layers, is considered and the interactions layer to layer turn out to have no effect as regards to the final bending. Evolution in stress after annealing shows the possibility to reduce the stress as well as the absorption for tantalum pentoxide thin films. Finally, ion implementation, such as helium and xenon, at high energy, prove to be also a way to vary and diminish the stress in thin films.

Roughness-induced absorption, scattering ellipsometry, and multidielectric resonances for laser damage investigation

We present new improvements that were achieved at LOSCM Marseilles for a better characterization of optical thin films. Roughness-induced absorption, angular ellipsometry of light scattering and multidielectric resonances are discussed in multilayers. Theoretical and experimental results are given and new applications are emphasized.

USE OF A GRATING INTERFEROMETER AS A PARALLEL RECEIVER FOR FIBRE OPTICS WHITE LIGHT QUASI DISTRIBUTED SENSORS

We show that a grating interferometer can be used as a receiver for quasi-distributed fiber optic sensors using white light interferometry. We report the results obtained when the sensor is based on an embedded fiber optic polarimetric interferometer for strain measurement of carbon/epoxy composites.

Temperature measurement with a tapered monomode fiber by coherence multiplexing

We describe a temperature sensor based on a tapered monomode fiber. The sensor is demodulated by a static interferometer designed to measure a phase by the coherence multiplexing technique. A comparison between experimental results, theoretical analysis, and numerical simulation is presented. The performances of the sensing device are also given.

Fiber optic magnetic field sensor using spectral modulation encoding

This paper is devoted to the description of a new type of fiber optic magnetic field sensor based on spectral modulation encoding techniques. This device uses a Faraday magneto-optic element as transducer and converts the rotation of the plane of polarization of the light induced by the magnetic field into a small change in the large optical path difference provided by a birefringent coding plate. The use of a broadband source and of an interferometric detector, with an optical path difference matched to the sensor one, allows to reach high resolution and perfect down-lead insensitivity. A dual-wavelength arrangement is proposed to determine the temperature and magnetic fied values with the same probe.