Francis Berghmans

High total dose radiation effects on temperature sensing fiber Bragg gratings

Fiber Bragg gratings (FBGs) written in a 10 mol.% Ge-doped core silica fiber using a phase mask were exposed to /spl gamma/-radiation. The transmission and reflection spectra were recorded during irradiation up to doses in excess of 1 MGy. There was no detectable change of the Bragg peak amplitude and the grating temperature sensitivity. The radiation-induced shift of the Bragg wavelength saturated at a dose of 0.1 MGy at a level less than 25 pm, which could still be decreased by optimization of the grating parameters. Our results confirm that FBGs are good candidates for sensing applications in radiation environments.

Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors

In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing ca- pabilities of fiber Bragg grating (FBG) temperature sensors. Our experi- ments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an air- cooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the differ- ence saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 10 19 neutron/cm 2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can sur- vive in such radiation conditions, but at 90°C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under well- specified conditions.

Bragg Grating Inscription in GeO

We present KrF excimer laser-induced dynamics of Bragg grating growths in GeO2 doped microstructured optical fibers. The studied fibers all have 6 rings of airholes in a hexagonal lattice and a GeO2 doped region in the center of the microstructure. We compare the growth rates of fiber Bragg gratings in the different microstructured fibers with UV grating inscription. The influence of the doping level, the airhole filling factor, the airhole pitch distance and the fiber orientation are investigated. We expand the range of microstructured optical fibers in which Bragg gratings can be inscribed, achieving reflection strengths that are useable for FBG-based sensing applications, even for doped regions with GeO2 concentrations as low as 1.36 mol% and 0.45 mol%.

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We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.

-Doped Microstructured Optical Fibers

We report on a photonic crystal fiber with a large mode area designed for compact high power fiber lasers and amplifiers. The fiber suppresses higher order modes when bent around a 10-cm radius and enables single mode operation in small footprint laser and amplifier architectures. We experimentally confirm the peculiar bending properties of this fiber in its passive version, by reporting on the measurement results of fundamental mode loss in bent and straight fibers, and of the influence of the bending plane orientation on this fiber loss.

Control Over the Pressure Sensitivity of Bragg Grating-Based Sensors in Highly Birefringent Microstructured Optical Fibers

We have studied the effect of gamma-radiation on the small signal gain and on the noise figure of several EDFAs with similar characteristics and based on special highly Er-doped fibers. Those fibers were fabricated using direct nanoparticle deposition technology, with different Er 3+ doping levels. The results show that the use of this technology allows improving the EDFAs radiation tolerance by increasing the concentration of Er3+ ions provided this is not accompanied with higher concentrations of other dopants, which influence the radiation sensitivity of the fiber.

Photonic Crystal Fiber With Large Mode Area and Characteristic Bending Properties

We review exceptional properties of the photonic crystal fibres enabling sensing applications of this new class of fibres. First, the sensing capabilities of highly birefringent index guided fibres are discussed. This includes dispersion characteristics of phase and group modal birefringence in different fibre structures, and sensitivity of these parameters to hydrostatic pressures and temperature. We demonstrate that index guided and photonic bandgap holey fibres of specific construction can be used as wide-band fibre-optic polarizer. We also show that combining of geometrical and stress effects makes it possible to design the holey fibres with either zero phase or group modal birefringence at virtually any given wavelength. Finally, different designs and performance of PCFs suitable for gas sensing are overviewed.

Radiation Sensitivity of EDFAs Based on Highly Er-Doped Fibers

We report on low-loss patch cords composed of a highly reproducible low-loss fusion splicing of photonic crystal fibers (PCFs) with a standard single mode fiber (SMF). Distinct from other papers in this area we report on the results for different types of PCFs, including LMA fibers with similar to a SMF core size and a mode field diameter (MFD), as well as polarization maintaining (PM) Bow-Tie PCF with an elliptical GeO 2 doped core and also a suspended-core (SC) PCF with a tiny core. We show that all studied splices between SMF and PCFs exhibit dispersive and non-reciprocal, in view of a light propagation direction, transmission losses. Defined as a larger decrease of a transmitted optical power comparing both propagation directions, achieved splicing losses, are equal to 0.46 ±0.03 dB, 0.34±0.01 dB, 0.67 ±0.17 dB and 3.09 ±0.36 dB at 1550 nm for LMA8, LMA10, Bow-Tie PCF and SC PCFs, respectively. Moreover, additionally to developed low-loss splicing, we report on low-loss patch cords for all mentioned above PCFs spliced on both ends with SMF pigtails ended with FC/APC connectors achieving the total losses at 1550 nm equal to 1.07 ±0.07 dB for LMA8, 0.72 ±0.20 dB for LMA10, 1.30 ±0.06 dB for Bow-Tie PCF, and 9.36 ±0.20 dB for SC PCF.

Photonic crystal fibers: new opportunities for sensing

We studied the influence of the fiber orientation on the growth of fiber Bragg gratings (FBGs) in pure silica microstructured optical fibers (MOFs) during femtosecond UV laser inscription. To do so we simulated the transverse coupling efficiency as a function of the relative angle between the inscribing laser beam and the internal microstructure for hexagonal lattice and highly birefringent MOFs by a finite-difference time-domain (FDTD) method. The orientation was predicted to play a far more important role in the highly birefringent MOF. We confirmed these simulation results with the fabrication of wavelength-multiplexed FBG arrays in pure silica core MOFs under different fiber orientations with 266-nm femtosecond laser pulses and a Talbot interferometer configuration.

Low-Loss Patch Cords by Effective Splicing of Various Photonic Crystal Fibers With Standard Single Mode Fiber

In-core temperature measurment is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing capabilities of FThre Bragg Grating (FBG) temperature sensors to perform the in-core temperature measurements. We first report on our irradiation experiments on multiplexed FBGs, written in different fibres, employed to measure the in-pile temperature of an air-cooled graphite-moderated nuclear reactor. For some FBGs the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the difference saturated after 30 hours of reactor operation at about 5°C. Afterwards, we irradiated multiplexed FBG sensors in our material testing nuclear reactor, evidencing the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care and in well-specified conditions.