Sébastien Bette

Transverse Strain Measurements Using the Birefringence Effect in Fiber Bragg Gratings

Birefringence in fiber Bragg gratings (FBGs) leads to polarization-dependent properties such as polarization-dependent loss (PDL). Although this property is not desired for telecommunications applications, we demonstrate that the PDL generated by uniform FBGs written into standard single-mode fiber can be advantageously used for transverse strain measurements, which is not directly possible through amplitude spectral measurements.

Spectral characterization of differential group delay in uniform fiber Bragg gratings

In this paper, we completely study the wavelength dependency of differential group delay (DGD) in uniform fiber Bragg gratings (FBG) exhibiting birefringence. An analytical expression of DGD is established. We analyze the impact of grating parameters (physical length, index modulation and apodization profile) on the wavelength dependency of DGD. Experimental results complete the paper. A very good agreement between theory and experience is reported.

Influence of the Grating Parameters on the Polarization Properties of Fiber Bragg Gratings

Due to the lateral inscription process, photo-induced birefringence is present in fiber Bragg gratings (FBGs) written into photosensitive single mode fiber. The birefringence value is generally too small to be perceived in the grating spectral response but it can lead to significant polarization dependent loss (PDL) and differential group delay (DGD) evolutions. In this paper, we first theoretically analyze the evolution with wavelength of PDL and DGD as a function of the grating parameters and the birefringence value. We demonstrate that the PDL and DGD evolutions with wavelength can be strongly enhanced by a modification of the grating parameters. Simulations carried out using the coupled mode theory and the Jones formalism are then confirmed by experiments conducted on FBGs written into photosensitive single mode fiber. Our work brings a complete characterization of polarization related phenomena in FBGs and presents a great interest for the evaluation of system performances and the design of gratings for specific applications, either for telecommunications or sensing purposes. In addition, based on the comparison between experimental and simulated evolutions, we are able to verify that the birefringence value is strongly dependent on the fluence of the laser used for the grating inscription.

Tilted Fiber Bragg Grating Refractometer Using Polarization-Dependent Loss Measurement

In this letter, a novel demodulation technique based on the monitoring of the polarization-dependent loss in a 1-nm wavelength range is proposed to measure the surrounding refractive index by means of weakly tilted fiber Bragg gratings. A 110-3 refractive index resolution as well as a temperature-insensitive behaviour are reported.

All-fiber tunable optical delay line

We present a tunable optical delay line based on the use of a single chirped fiber Bragg grating written into a standard single mode optical fiber. In the proposed scheme, the delay is induced through the Bragg grating differential group delay curve. This is achieved by launching orthogonally polarized optical pulses in both directions into the Bragg grating and by controlling its local birefringence. This bidirectional propagation allows to compensate the second-order dispersion. The setup is suitable to delay pulses with a spectral width just less than the grating reflection bandwidth, which is particularly useful in the context of forthcoming wavelength division multiplexing ultra-high bit rate lightwave systems. In this work, the performances of the setup are investigated using a pulsed laser delivering 6.3 ps Fourier transform limited pulses at 1548 nm. A maximum delay of 120 ps (about 20 times the pulse width) is reported experimentally.

Analysis of the MAC performances in 802.11g radio-over-fiber systems

This paper investigates the performance when the IEEE 802.11g standard is used in a radio-over-fiber (RoF) network. We report that the IEEE 802.11g throughput may be dramatically deteriorated in RoF systems. We then show the possibility to adapt the slottime parameter in order to improve the IEEE 802.11g performances in RoF networks.

Distribution profiling of a transverse load using the DGD spectrum of chirped FBGs

In this paper, we propose a new method to determine the longitudinal distribution of a non-uniform transverse force applied to an optical fiber. For that purpose, we use a chirped fiber Bragg grating (CFBG) for which we monitor the polarization parameters in reflection. In particular, we demonstrate that the differential group delay (DGD) spectrum of the CFBG is an imprint of the load profile so that it can be used for the shape determination of an applied load. Thereafter, we discuss the influence of the CFBG parameters on the achievable accuracy and resolution of our technique. An experimental validation is finally reported with two 48 mm long CFBGs subject to step transverse load profiles.

Reduction of polarization related effects in superimposed fiber Bragg gratings

Fiber Bragg gratings, and more specifically, superimposed fiber Bragg gratings (SIFBGs), are attractive commercial solutions for several multiband telecommunication applications. However, as a part of a telecommunication system, the polarization dependent properties present in SIFBGs due to the fabrication process dramatically limit their possible implementation in high bit rate optical communications. The development of techniques for the reduction of differential group delay (DGD) and the polarization dependent loss (PDL) in system components is then crucial. We present a simple method to reduce the DGD and the PDL induced during the fabrication of SIFBGs. The proposed fabrication method consists of irradiating the fiber core from different well controlled directions depending on the total number of expositions. We theoretically predict and experimentally demonstrate a reduction of the DGD and the PDL after each illumination.

Transverse force sensor exploiting the birefringence effect in uniform fibre Bragg gratings

Birefringence in fiber Bragg gratings (FBGs) can result from two distinct effects that combine with the intrinsic fiber birefringence: the birefringence induced by the UV photo-writing and the birefringence due to a transversal load. In both cases, it leads to polarization dependent loss (PDL) and differential group delay (DGD) inside gratings. Although these parameters are not desired for optical telecommunications applications, we demonstrate here that they can be advantageously used to obtain temperature-insensitive transverse strain measurements with uniform FBGs written into standard single mode fiber, which is not possible through conventional amplitude spectral measurements. Simulated results obtained by means of the coupled mode theory and the Jones formalism as well as experimental results measured by means of a tunable laser source and a polarimeter are presented.

Analysis of the twisting parameters in the DVB-T2 column-twist interleaver

The Bit-Interleaved Coded Modulation (BICM) block implemented in the second generation digital terrestrial television broadcasting system (DVB-T2) introduces a modified block interleaver: the column-twist interleaver. The aim of this paper is to study how the properties, dispersion, s-parameter and memory of this interleaver are influenced by the choice of the twisting parameters. Our challenge is to put forward a set of twisting parameters that could make this interleaver more efficient, regarding the listed properties, than the ones used in the DVB-T2 system.