Marc Douay

Study of in fiber Mach–Zehnder interferometer based on two spaced 3-dB long period gratings surrounded by a refractive index higher than that of silica

Abstract The paper deals with the response of a Mach–Zehnder interferometer composed by two 3-dB long period gratings (LPG) with different external refractive indices higher than that of silica (leaky configuration). A rough analysis of the cladding propagation arising in leaky configuration has allowed us to establish a model that is sufficient to describe the interferometer couplings evolution at the resonance. In particular, we have shown that the interferometer sensitivity depends strongly on its phase-shift Δ ϕ and is optimized for Δ ϕ equal to π [2π] or 3π/4 [2π]. An experimental study was undertaken and allowed us to validate our model. Finally, a comparison with a single LPG demonstrated an increased sensitivity to the external index, suggesting the conception of a new fiber sensor operating with external refractive indices higher than that of silica.

Photonic crystal fiber design by means of a genetic algorithm

A Genetic Algorithm (GA) is used to design photonic crystal fiber structures with user-defined chromatic dispersion properties. This GA is combined with a full vectorial finite element method in order to determine the effective index of propagation of the modes and then, the chromatic dispersion of structures generated by GA. This method proves to be a powerful tool for solving this inverse problem.

Demonstration of long-period-grating efficient couplings with an external medium of a refractive index higher than that of silica

We discuss the behavior of the transmission spectra of a long-period grating (LPG) with an external medium of refractive index higher than that of silica. We studied the evolution of the features of LPGs surrounded by several liquid media for several kinds of fiber. The study demonstrated that the behavior depends strongly on the fiber type. Efficient couplings (efficiency greater than 50%) have been obtained with an external medium of a refractive index larger than that of silica. This result indicates that a LPG can operate with surrounding materials of a refractive index higher than that of silica, promising new applications for LPGs as active optical filters.

Tomographic measurement of femtosecond-laser induced stress changes in optical fibers

The tomographic measurement of the residual stress profile in femtosecond-laser irradiated standard SMF-28 germanium-doped telecommunication fiber is demonstrated. The fiber is irradiated with weakly focused pulses to realize long-period fiber gratings. In the irradiated grating regions, an asymmetrical increase in axial core stress up to 6.2 kg/mm2 is found. The increase in stress is attributed to a densification of the irradiated glass matrix. The stress-induced anisotropic index distribution is calculated and related to the absolute index change in the irradiated regions.

Analysis of color-center-related contribution to Bragg grating formation in Ge:SiO 2 fiber based on a local Kramers–Kronig transformation of excess loss spectra

UV-induced excess losses have been measured at various pulse energy densities and exposure times in germanosilicate optical fiber preform cores. The corresponding refractive-index changes have been determined through a Kramers -Kronig analysis. Because of the nonlinear behavior of the excess losses as a function of both exposure time and fluence per pulse, one should be careful when comparing the refractive-index modulation deduced from such measurements with that obtained from Bragg grating reflectivity. Indeed nonlinear effects such as saturation imply that it is necessary to take into account the local character of the change in absorption to calculate the evolution of the refractive-index modulation accurately as a function of the exposure time. Implications of these results are discussed.

Comparison of UV optical absorption and UV excited luminescence behaviours in Ge doped silica under H2 loading or CW UV laser irradiation

Abstract We compare the CW 244 nm laser induced absorption and photoluminescence excitation changes in Ge and Ge–P doped silica glass according to H2 loading. Under H2 loading only, the absorption does not change but the luminescence excitation intensity decreases by 60%. For samples not H2 loaded, luminescence excited in the UV range is annihilated by the laser irradiation but not the absorption in the same UV range. During UV irradiation of H2 loaded samples most of the luminescence excited in the UV range disappears whereas UV absorption behaves differently. In that way, we show that defects giving rise to luminescence exhibit a weak contribution to the absorption spectrum. In particular, there are two different oxygen deficient centres: one absorbing at 5 eV and another one giving rise to luminescence under 5 eV excitation. Due to some correlations existing sometimes between absorption and luminescence, we suggest that luminescence is quenched by trapping of electrons produced by 5 eV absorbing defects. In addition, the use of a CW laser allows us to observe two successive one-photon absorption steps for accounting for the absorption change: a singlet to singlet transition and then an ionisation.

Experimental synthesis of fiber Bragg gratings using optical low coherence reflectometry

We apply a combination of optical low coherence reflectometry and a layer-peeling method to the synthesis of a fiber grating. The index modulation amplitude we obtain is compared with a Krug measurement result and the dc refractive index is derived from the phase measurement.

Does photosensitivity pave the way towards the fabrication of miniature coherent light sources in inorganic glass waveguides

Abstract Photosensitivity designates the ability to permanently change the refractive index of a glass by optical processing. The phenomenon allows the fabrication of numerous phase structures, the simplest of which is the Bragg grating obtained by photoimprinting a periodic index modulation within the material. Bragg gratings have changed the way in which optical fibre (or planar waveguide) lasers are now used. Distributed Bragg Reflector (DBR) or Distributed Feed Back (DFB) lasers, when intracore Bragg gratings are used for linear cavity feedback, are commonly fabricated in rare earth doped optical silica-based fibres. On the other hand, photosensitivity can also result in effects which can prove to be detrimental to the fabrication of miniature coherent light sources. The paper will cover some of the advances that have been made in improving the photosensitivity of inorganic glasses, in searching new photosensitive materials and in characterizing Bragg grating properties relevant to laser applications.

Performance analysis of quadratic congruence codes using superstructured fiber Bragg gratings for a flexible data rate coherent DS-OCDMA system

The performance of a coherent direct sequence optical code division multiple access (DS-OCDMA) system using quadratic congruence codes implemented with superstructured fiber Bragg gratings is experimentally analyzed. We point out that nonperiodic and sparse quadratic congruence codes reduce multipath beat noise and intersymbol interference, respectively. These two properties of the structure of the quadratic congruence codes are shown to improve the bit-error-rate (BER) performance of the coherent system. As a result, we demonstrate that the performance enhancement of the coherent DS-OCDMA system allows the data bit rate of the system to be varied from 1 Gbit/sto2.5 Gbits/s and the receiver bandwidth to be decreased from 15 GHzto5 GHz respecting the low-cost requirement of the optical access network. These data bit rate and receiver bandwidth performances are achieved with moderate BER penalty.

Suppression of discrete cladding mode resonances in fibre slanted Bragg gratings for gain equalisation

In a slanted Bragg grating, coupling between the fundamental guided mode and the counterpropagative cladding modes result in discrete resonances in the transmission spectrum. These resonances are a drawback when Slanted Bragg Gratings are used for gain flattening of fibres amplifiers. A new method based on a chemical etching of the cladding is proposed leading to an overlap of the resonances and a reduction of the amplitude of the modulation. This method can be applied for any value of photo induced modulation amplitude in the SBG.