Martial Oger

Supermirror phase anisotropy measurement.

The high sensitivity of the eigenstates of a large-finesse passive Fabry-Perot cavity is used to measure the residual anisotropy of supermirrors at strictly normal incidence. An experimental demonstration leads to the measurement of phase retardances of the order of 10(-6) rad.

The Malus Fabry–Perot interferometer

Abstract We describe a general model for the behaviour of a sensor based on a Fabry–Perot interferometer placed between crossed polarizers. Compared to single-pass polarimeters, the sensitivity for the measurement of intracavity anisotropies is shown to be enhanced by a factor of the order of the square of the finesse compared to single-pass polarimeters. Our model, based on a vectorial spatial description of the cavity, predicts the response of the system to circular and/or linear intracavity anisotropies. It also gives the ultimate sensitivity and takes into account the spurious backgrounds. Experimental illustrations are presented for different types of reciprocal and non-reciprocal anisotropies. Moreover, it is shown that the insertion of an optical bias combined with a modulation of the intracavity anisotropies leads to experimental sensitivities limited only by the shot noise level, in agreement with theoretical predictions. We discuss further improvements and potential applications for the polarimeter.

Development and test of a trans-horizon communication system based on a MIMO architecture

A multiple-input multiple-output (MIMO) system for trans-horizon radio communications within the high-frequency (HF) band (3 to 30 MHz) is presented. The diversity of transmitted polarizations is proposed as an alternative to spatial diversity in order to limit the aperture of antenna arrays at both ends of the radio link. In a theoretical step providing the estimation of capacity gain for different MIMO architectures, a 2 × 2 MIMO solution transmitting two complementary circular polarizations is identified as a balanced trade-off between performance increase and complexity. The design of the corresponding system is described with a focus on antenna arrays and the kind of signal processing that should be implemented. This novel communication system has been tested on a 280-km-long radio link. The first results underline a data transfer rate reaching a value of 24.09 kbps (in a 4.2-kHz bandwidth) that significantly exceeds the current standards for HF modems.

Early results of experiments to investigate the feasibility of employing MIMO techniques in the HF band

Over the last few years, the concept of multiple input multiple output (MIMO) has become a popular area of research in the field of wireless communications with the aim of delivering increased data rates. However, to date, MIMO research has focussed primarily on communications within the VHF, UHF and SHF bands (and above). Very little experimental or modelling research has been conducted in the area of exploiting MIMO techniques within the HF radio band. This paper describes some of the preliminary experimental work that has been carried out in order to investigate the feasibility of implementing MIMO techniques within the HF band.

Frequency stability measurement on magnetic Lamb dip-stabilised lasers

Compact Zeeman lasers have been built to test the frequency stability which can be obtained using the magnetic Lamb dip. The lasers are strictly monomode. No frequency modulation is needed, the error function resulting from a Zeeman modulation of the sigma components of the laser line for a fixed laser frequency. Frequency fluctuations of better than 10-11 are obtained by Allan variance analysis for the integration period range of 0.1-100 s.

New improvements in HF Ionospheric Communication and Direction Finding Systems

This chapter presents new HF (3–30 MHz) systems dedicated to ground to ground radio links with applications to ionospheric characterisation, channel modelling, radio communications, direction finding and single site localisation. The received signals result from the vectorial addition of the multipaths generated by the ionosphere. Considering the acquisitions at the outputs of an array of identical antennas (homogeneous array), a high level of spatial and temporal correlation can be observed. Therefore, it appears relevant to additionally discriminate the incoming modes by considering their polarisations. The purposes of the different systems which are described in the following sections are the use of a heterogeneous array. This polarisation-sensitive solution for array processing is principally characterized by the spatial distribution of non identical antennas. Consequently, the applications to digital communication involve a multi channel processing in the receiver as a SIMO (single input multiple output) structures. The correlation factors depend on the polarisation characteristics of the incident wave. Moreover, the heterogeneous array is still efficient with a reduced space diversity (set up in a limited place), the differences in the polarisation parameters balancing the weak values of the differential geometrical phases. In the following developments, the suggested techniques aim to take some better advantage of the ionospheric medium in several applications.

An Ambulatory Electroencephalography System for Freely Moving Horses: An Innovating Approach

Electroencephalography (EEG) that has been extensively studied in humans presents also a large interest for studies on animal brain processes. However, since the quality of the recordings is altered by muscular activity, most EEG recordings on animals are obtained using invasive methods with deeply implanted electrodes. This requires anesthesia and can thus only be used in laboratory or clinical settings. As EEG is a very useful tool both for detecting brain alterations due to diseases or accidents and to evaluate the arousal and attentional state of the animal, it seemed crucial to develop a tool that would make such recordings possible in the horse’s home environment, with a freely moving horse. Such a tool should neither be invasive nor cause discomforts to the horse as the usual other practice which consists, after shaving the zone, in gluing the electrodes to the skin. To fulfill these requirements, we developed a novel EEG headset adapted to the horse’s head that allows an easy and fast positioning of the electrodes and that can be used in the home environment on a freely moving horse. In this study, we show that this EEG headset allows to obtain reliable recordings, and we propose an original evaluation of an animal’s “EEG profile” that allows comparisons between individuals and situations. This EEG headset opens new possibilities of investigation on horse cognition, and it can also become a useful tool for veterinarians to evaluate cerebral disorders or check the anesthesia level during a surgery.

Attentional state and brain processes: state-dependent lateralization of EEG profiles in horses

Lateralization of brain functions has been suggested to provide individuals with advantages, such as an increase of neural efficiency. The right hemisphere is likely to be specialized for processing attention for details and the left hemisphere for categorization of stimuli. Thus attentional processes actually may underlie lateralization. In the present study, we hypothesized that the attentional state of horses could be reflected in the lateralization of brain responses. We used i) a recently developed attention test to measure horses’ visual attentional responses towards a standardized stimulus and ii) a recently developed portable EEG telemetric tool to measure brain responses. A particular emphasis was given to the types of waves (EEG power profile) and their side of production when horses were either attentive towards a visual stimulus or quiet standing. The results confirmed that a higher attentional state is associated with a higher proportion of gamma waves. There was moreover an interaction between the attentional state, the hemisphere and the EEG profile: attention towards the visual stimulus was associated with a significant increase of gamma wave proportion in the right hemisphere while “inattention” was associated with more alpha and beta waves in the left hemisphere. These first results are highly promising and contribute to the large debate on functional lateralization.

Evaluation of Ionospheric HF MIMO Channels: Two complementary circular polarizations reduce correlation.

An original multiple-input, multiple-output (MIMO) system for a transhorizon transmission through the ionospheric channel is presented. This project, involving decametric wavelengths, has to cope with the constraints of reasonable array dimensions at both ends of the radio link. Therefore, this work considers a diversity in transmitted polarizations as an alternative to the classical spatial diversity and, more specifically, the generation of complementary circular polarizations. The design of the corresponding system is described with a focus on antenna arrays, waveforms, and signal processing for channel sounding. This novel communication system has been tested on a 850-km long radio link to exhibit the degree of diversity provided by the ionospheric MIMO channel. The measurements indicate a significantly low level of correlation for the four-channel transfer functions linking the transmitting and receiving ends (in a frequency dispersive context). Moreover, the ergodic or outage capacities have been calculated over several hundreds of channel estimations: the gain in the capacity of a MIMO versus a single-input, single-output (SISO) architecture reaches the value of 1.82.

Design and test of a HF MIMO system of transmission set up with compact antenna arrays

This paper proposes a MIMO 2×2 solution to increase the capacity of HF radio links through the ionospheric channel. A diversity in the transmitted polarizations replaces the classical space diversity and authorizes a compact set up in a context of decametric wavelengths. After a theoretical step of capacity estimation, an HF 2×2 MIMO system has been designed and tested on a 300 km long radio link. The data transfer rate reaches up to 24.09 kbps (in a 4.2 kHz bandwidth) with a good quality of service.