J.-P. Meunier

Strain and Temperature Sensing Characteristics of Single-Mode–Multimode–Single-Mode Structures

We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa.

A general approach to the numerical determination of modal propagation constants and field distributions of optical fibres

A new approach to modal propagation for optical fibres with arbitrary refractive index profile, based on the Bubnov-Galerkin method, is developed. The theory furnishes an accurate description of modal propagation constants and field distributions. When tested for step-index and truncated parabolic profiles the method is found to agree with the known exact results.

Proton- and Gamma-Induced Effects on Erbium-Doped Optical Fibers

We characterized the responses of three erbium-doped fibers with slightly different concentrations of rare-earth ions (240-290 ppm) and Al2O3 (7-10 wt.%) during proton and gamma-ray exposures. We have simultaneously measured the radiation-induced attenuation (RIA) around the Er3+ ion pumping wavelength (980 nm) and the associated changes of the Er3+ emission around 1530 nm. The three erbium-doped fibers show similar radiation responses. All fibers exhibit RIA levels between 9 times 10-3 and 1.7 times 10-2 dB m-1 Gy-1 at 980 nm and between 4 times 10-3 and 1.1 times 10-2 dB m-1 Gy-1 at 1530 nm. Protons and gamma-rays lead to similar radiation damages, with small differences between the protons of different energies (50 MeV and 105 MeV). Furthermore, we have performed online measurements of the spectral dependence of RIA from 600 to 1600 nm and offline measurements from 1200 to 2400 nm. The three fibers exhibit the same spectral response. Losses decrease monotonically from the visible to the infrared part of the spectrum. We have performed spectral decomposition of these RIA curves with the help of absorption bands previously associated with radiation-induced point defects. Our analysis shows that the main part of the RIA (600-1700 nm) in erbium-doped glass can be explained by the generation of Al-related point defects. The other defects related to the germanium and phosphorus doping of the silica seem to have a lower contribution to the induced losses. The Er3+ ion properties seem to be mainly unaffected by proton exposure, suggesting a solvation shell around the Er3+ ion formed by Al2O3 species.

The SOPHIE spectrograph: design and technical key-points for high throughput and high stability

SOPHIE is a new fiber-fed echelle spectrograph in operation since October 2006 at the 1.93-m telescope of Observatoire de Haute-Provence. Benefiting from experience acquired on HARPS (3.6-m ESO), SOPHIE was designed to obtain accurate radial velocities (~3 m/s over several months) with much higher optical throughput than ELODIE (by a factor of 10). These enhanced capabilities have actually been achieved and have proved invaluable in asteroseismology and exoplanetology. We present here the optical concept, a double-pass Schmidt echelle spectrograph associated with a high efficiency coupling fiber system, and including simultaneous wavelength calibration. Stability of the projected spectrum has been obtained by the encapsulation of the dispersive components in a constant pressure tank. The main characteristics of the instrument are described. We also give some technical details used in reaching this high level of performance.

/spl gamma/-rays and pulsed X-ray radiation responses of germanosilicate single-mode optical fibers: influence of cladding codopants

The radiation-induced attenuation (RIA) in germanosilicate single-mode optical fibers was measured at 1.55 and 1.31 mm after a pulsed X-ray irradiation and at 1.55 mm during and after a steady-state g-ray irradiation. The influence of codoping the fiber cladding with germanium (Ge), phosphorus (P), and fluorine (F) on the sensitivity of Ge-doped core fibers was characterized. P-codoping makes it possible to decrease the RIA for short times (10/sup *6/ s*10/sup *3/ s) post- pulse. However, P-codoped fibers exhibit larger values of permanent RIA than P-free fibers after transient exposure and are inadequate for a steady-state environment. The impact of F-codoping depends on the other codopants incorporated in the fiber cladding, but its addition seems to be deleterious for the radiation hardening of the germanosilicate fiber at the two tested wavelengths. Ge-codoping increases the sensitivity of P-, F-codoped fibers under X-rays and steady-state g-ray irradiation, whereas it decreases the RIA in F-doped ones. Some hypotheses on the creation mechanisms and properties of the color centers related to these three codopants are proposed.

Side-Polished Optical Fiber Grating-Based Refractive Index Sensors Utilizing the Pure Surface Plasmon Polariton

In this paper, we present the ambient refractive index (ARI) sensing characteristics of metal-coated side-polished optical fiber gratings based on the excitation of the pure surface plasmon polariton. The resonance wavelength shift as a function of the ARI and the grating lengths required for a fixed minimum transmittivity (30%) for the TM-like mode are obtained for different metal and residual cladding thicknesses. It is found that a long-period grating (LPG)-based sensor is about 5-20 times more sensitive to the change in the ARI and requires much shorter grating lengths for a given sensitivity than the one based on fiber Bragg grating (FBG). Further, unlike a FBG-based device, an LPG-based sensor is found to have maximum sensitivity at an optimum value of metal thickness, the reason for which is also explained. This paper should find application in the design of sensitive fiber-optic bio/chemical sensors.

A numerical technique for the determination of propagation characteristics of inhomogeneous planar optical waveguides

A numerical method, using the Ritz-Galerkin approach has been applied to the problem of determining the propagation characteristics of inhomogeneous planar optical waveguides. It is shown that very accurate results are obtained for mode spectra and field distributions when compared with the predictions of other exact or approximate methods.

Radiation Effects on Ytterbium- and Ytterbium/Erbium-Doped Double-Clad Optical Fibers

We characterize by different spectroscopic techniques the radiation effects on ytterbium- (Yb) and ytterbium/erbium (Yb/Er)-doped optical fibers. Their vulnerability to the environment of outer space is evaluated through passive radiation-induced attenuation (RIA) measurements during and after exposure to 10 keV X-rays, 1 MeV ¿-rays, and 105 MeV protons. These fibers present higher levels of RIA (1000×) than telecommunication-type fibers. Measured RIA is comparable for ¿-rays and protons and is on the order of 1 dB/m at 1.55 ¿ m after a few tenths of a kilorad. Their host matrix codoped with aluminum (Al) and/or phosphorus (P) is mainly responsible for their enhanced radiation sensitivity. Thanks to the major improvements of the Er-doped glass spectroscopic properties in case of Yb-codoping, Yb/Er-doped fibers appear as very promising candidates for outer space applications. In the infrared part of the spectrum, losses in P-codoped Yb-doped fibers are due to the P1 center that absorbs around 1.6 ¿ m and are very detrimental for the operation of Er-codoped devices in a harsh environment. The negative impact of this defect seems reduced in the case of Al and P-codoping.

Vulnerability analysis of optical fibers for laser megajoule facility: preliminary studies

The Laser Megajoule project is a major component of the French simulation program to study nuclear fusion by inertial confinement. The future Laser Megajoule facility requires control-command systems that will operate in a harsh radiative environment. Commercial off-the-shelf optical fiber data links are envisaged as a radiation tolerant solution for this application. In this paper, we present our preliminary study of their vulnerability. For this, we firstly have used an original method consisting of ultraviolet (/spl sim/5 eV) exposures of the fibers to identify the different germanosilicate optical fibers containing phosphorus, which leads them unacceptable for both steady state /spl gamma/-rays and successive pulsed X-ray irradiations. We have demonstrated the validity of the /spl gamma/-UV comparison by spectroscopic measurements. After this first selection, we have tested under pulsed X-rays (dose rate >10 MGy/s dose <0.5 kGy) the resistance of the P-free optical fibers at 1310 nm for the shortest times after an ionization pulse (10/sup -9/ to 10/sup -1/ s). Based on these results, we discuss the validity of the optical fiber data links for the control-command applications in LMJ facility.

Properties of phosphorus-related defects induced by γ-rays and pulsed X-ray irradiation in germanosilicate optical fibers

Abstract Time dependent radiation-induced attenuation changes of germanosilicate (∼13 wt%) optical fibers have been measured under steady-state γ-ray and pulsed X-ray irradiations. Particularities of phosphorus (P)-codoped cladding fibers for these environments are presented. For each irradiation type, spectral attenuation measurements show that permanent radiation-induced losses at 1.55 μm are due to the infrared absorption band of P 1 centers. We assume that particular kinetics of recovery and an increase of attenuation measured for P-codoped fibers, after pulsed X-ray irradiation, are induced by a charge retrapping phenomenon from phosphorous oxygen hole (POH) centers to P 1 ones. With this mechanism, we explain the P-codoped fiber characteristics after both types of irradiation. Our study shows that the effect of P-related color centers is increased when the P-codoped cladding (∼0.3 wt%) contains also germanium (∼0.3 wt%).