Ph. Robert

Brillouin gain spectrum characterization in single-mode optical fibers

A novel method for Brillouin gain spectrum measurements in optical fibers is presented. It is based on the pump and probe technique with the specificity to use a single laser source together with an external modulator to generate the interacting lightwaves. The high accuracy and inherent stability of the technique makes it suitable for calibration and reference measurements. Different fibers with different refractive index profiles have been tested and characterized. The problem of the evolution of the polarization of the interacting waves is addressed in the article and a polarization insensitive determination of the actual Brillouin gain coefficient is made possible through two successive measurements with different polarizations. The effects of strain and temperature on the Brillouin gain spectrum are also fully characterized.

Spatio-temporal parameters of gait measured by an ambulatory system using miniature gyroscopes

In this study we describe an ambulatory system for estimation of spatio-temporal parameters during long periods of walking. This original method based on wavelet analysis is proposed to compute the values of temporal gait parameters from the angular velocity of lower limbs. Based on a mechanical model, the medio-lateral rotation of the lower limbs during stance and swing, the stride length and velocity are estimated by integration of the angular velocity. Measurements accuracy was assessed using as a criterion standard the information provided by foot pressure sensors. To assess the accuracy of the method on a broad range of performance for each gait parameter, we gathered data from young and elderly subjects. No significant error was observed for toe-off detection, while a slight systematic delay (10 ms on average) existed between heelstrike obtained from gyroscopes and footswitch. There was no significant difference between actual spatial parameters (stride length and velocity) and their estimated values. Errors for velocity and stride length estimations were 0.06 m/s and 0.07 m, respectively. This system is light, portable, inexpensive and does not provoke any discomfort to subjects. It can be carried for long periods of time, thus providing new longitudinal information such as stride-to-stride variability of gait. Several clinical applications can be proposed such as outcome evaluation after total knee or hip replacement, external prosthesis adjustment for amputees, monitoring of rehabilitation progress, gait analysis in neurological diseases, and fall risk estimation in elderly.

Physical activity monitoring based on accelerometry: validation and comparison with video observation

The objective of this feasibility study is to evaluate the use of the ‘Physilog’ device, an ambulatory physical-activity recorder based on acceleration measurement, for the monitoring of daily physical activities. Accelerations measured at the level of the chest and the thigh are recorded by Physilog over a period of 1 h in five normal subjects. A specially designed studio-like room allowing the performance of most usual activities of everyday life is used. A video film synchronised with the Physilog is obtained for each subject to check the accuracy of the data derived from Physilog. Based on the analysis of the average and the deviation of the acceleration signal, an algorithm is developed to classify the activities in four categories, i.e. lying, sitting, standing and locomotion. Compared with the video observations, the results from the algorithm show an overall misclassification of 10.7%, which is mainly due to confusion between dynamic activities and the standing posture. In contrast, the misclassification between postures is negligible. It is concluded that Physilog can be used in the clinical setting for the reliable measurement and long-term recording of most-usual physical activities.

Temporal feature estimation during walking using miniature accelerometers: an analysis of gait improvement after hip arthroplasty

A new method for the detection of gait cycle phases using only two miniature accelerometers together with a light, portable digital recorder is proposed. Each subject is asked to walk on a walkway at his/her own preferred speed. Gait analysis was performed using an original method of computing the values of temporal parameters from accelerometer signals. First, to validate the accelerometric method, measurements are taken on a group of healthy subjects. No significant differences are observed between the results thus obtained and those from pressure sensors attached under the foot. Then, measurements using only accelerometers are performed on a group of 12 patients with unilateral hip osteo-arthritis. The gait analysis is carried out just before hip arthroplasty and again, three, six and nine months afterwards. A mean decrease of 88% of asymmetry of stance time and especially a mean decrease of 250% of asymmetry of double support time are observed, nine months after the operation. These results confirm the validity of the proposed method for healthy subjects and its efficiency for functional evaluation of gait improvement after arthroplasty.

Evaluation of an ambulatory system for gait analysis in hip osteoarthritis and after total hip replacement

Spatial and temporal parameters of gait have clinical relevance in the assessment of motor pathologies, particularly in orthopaedics. A new gait analysis system is proposed which consists of (a) an ambulatory device (Physilog) including a set of miniature gyroscopes and a portable datalogger, and (b) an algorithm for gait analysis. The aim of this study was the validation of this system, for accuracy and clinical applicability. Eleven patients with coxarthrosis, eight patients with total hip arthroplasty and nine control subjects were studied using this portable system and also a reference motion analyzer and force plate. The small differences in the stance period (19 +/- ms), stride length and velocity (0.4 +/- 9.6 cm and 2.5 +/- 8.3 cm/s, respectively), as well as thigh and shank rotations (2.4 +/- 4.3 degrees and 0.3 +/- 3.3 degrees, respectively), confirmed good agreement of the proposed system with the reference system. In addition, nearly the same accuracy was obtained for all three groups. Gait analysis based on Physilog was also in agreement with their Harris Hip Scores (HHS): the subjects with lower scores had a greater limp, a slower walking speed and a shorter stride. This ambulatory gait analysis system provides an easy, reproducible and objective method of quantifying changes in gait after joint replacement surgery for coxarthrosis.

Distributed sensing using stimulated Brillouin scattering : towards ultimate resolution

In this paper, we discuss the fundamental limitations of the SBS analysis as a distributed sensing method when the spatial resolution is in the meter range. We also present a novel experimental configuration that reaches the best performances achievable for this kind of sensors.

M&NEMS: A new approach for ultra-low cost 3D inertial sensor

We are presenting a novel approach for very low cost 3D inertial sensor. The idea consists in using on same device MEMS and NEMS technologies. The MEMS part is used for the mass to keep sufficient inertial force, and the NEMS is used as a very sensitive sub-μm suspended stress gauge. This way enables the detection of in-plane and out-of-plane acceleration on a same device and with differential detection means (to reduce thermal sensitivity). Technological realization and first characterizations of the accelerometer have been achieved and are detailed in this paper. Accelerometer area is around 0.1mm2 per axis, which means at least 3 to 5 times smaller than conventional MEMS accelerometers. Moreover, the performances are compatible with typical consumer market accelerometer specifications.

Novel measurement scheme for injection-locking experiments

A novel experimental setup for injection-locking experiments is presented. The single-mode-fiber-based configuration allows one to precisely control the power and the polarization state of the light injected from the master laser into the slave laser cavity. Different behaviors typical for injection locking with single-mode semiconductor lasers (e.g., stable injection locking, undamped relaxation oscillations, nearly degenerate four-wave mixing, period doubling, chaotic behavior) are experimentally observed and theoretically verified using a rate-equation-based model. Measurements and calculations are entirely linked analytically and thoroughly compared by means of the corresponding power spectra. The good quantitative agreement between measurements and model validates the model, the analytical approach, and the experimental setup.

Stabilised current sensor using Sagnac interferometer

A current sensor based on a stabilised fibre-optic Sagnac interferometer has been constructed and preliminary tests have been carried out. Measurements made with this system are independent of variations in the following parameters: laser intensity, light intensity injected into the optical fibre and birefringence within the fibre. The stabilised output is obtained by electronically combining the output signal from the interferometer with samples of the signal which have traversed the optical fibre in opposite directions. The mathematical basis for the stabilisation technique and construction details for the instrument are given.

Temperature dependence of bend- and twist-induced birefringence in a low-birefringence fiber

The temperature dependence of the bend- and twist-induced birefringence in LB600 low-birefringence fiber has been measured at lambda = 0.633 microm. The relative temperature dependence B = (1/beta)dbeta/dT of the bend-induced birefringence beta was found to be (5.7 +/- 0.2) x 10(-4) K(-1) over a temperature range from -5 to 70 degrees C. The relative temperature dependence A = (1/g)dg/dT of the twist-induced birefringence g was found to be (4.95 +/- 0.25) x 10(-4) K(-1) (between 22 and 150 degrees C). The measured values of A and B are in accordance with a relative temperature dependence (1/C)dC/dT of the stress-optic coefficient C equal to (4.2 +/- 0.2) x 10(-4) K(-1).