The paper presents the development and investigation of distributed and a quasi-distributed fiber optic sensors for the real-time monitoring of radiations during cancer treatments. Both sensors rely on ad-hoc developed nanoparticle-doped optical fibers with enhanced sensitivity to radiation. The distributed sensor is interrogated with an OFDR-based instrument and allows the reconstruction of the spatial dose distribution along the fiber. The quasi-distributed sensor is implemented through fiber Bragg gratings inscribed with a femtosecond laser in the few-mode section of a single mode-multi mode-single mode interferometer.
Enhanced Rayleigh backscattering optical fibers, interrogated by an optical frequency domain reflectometer, are used to perform remote real-time measurements of X-ray irradiation profiles, with possible application as dosimeters in radiotherapy treatments. The enhanced Rayleigh backscattering is obtained by proper engineering of the composition of fiber core, either by introduction of Aluminum or Magnesium silicate nanoparticles as radiation-sensitive dopants. A detectable radiation-induced refractive index change can be spatially resolved through the measurement of the frequency shift of the Rayleigh backscattering along the fiber. It is experimentally demonstrated that two mechanisms of radiation-induced refractive index change take place. At doses nearly compatible with those delivered in radiotherapy, a negative refractive index is induced, whereas at high doses the change is positive. This behavior is also confirmed by the shift of Bragg wavelength of a fiber Bragg grating inscribed in the nanoparticles-doped fiber and used as a reference.
This work describes a procedure based on a set of thermally stimulated luminescence measurements coupled to an original theoretical analysis which allows estimating the distribution in energy of carrier-trapped states developing in the bandgap of silica-based optical fiber glasses under ionizing irradiation. This procedure is applied to undoped, aluminum-, phosphorus- and rare-earth-doped silica samples from tailormade optical fiber preforms, after irradiations in two very different conditions. The extracted Densities Of Trapped States (DOTS) always relate to distributions of trapped holes. Within a 1-1.5 eV energy range above the valence band, these DOTS contain the energy levels of well-known intrinsic or dopant-related color centers recognized as major contributors to the radiation-induced attenuation in silica fibers. Long irradiation times strongly impact the DOTS by depleting shallow states and favoring the “condensation” of holes in deep levels. This enhances the density of color centers (deeper than 1 eV) and explains part of the RIA increase with the dose.
Rare-earth (RE) doped optical fibers are extensively used in lasers and optical amplifier devices. These key applications rely on the qualities of silica glass: mechanical and chemical stability, high optical damage threshold, low cost, etc. However, silica glass has certain characteristics which may make it less efficient compared to other types of glass, particularly in some potential applications using RE ions: high phonon energy, low solubility of RE ions, etc. To overcome these limitations, one recent strategy consists of developing a fabrication method which triggers RE encapsulation in phase-separated nanoparticles. The development of this family of optical fibers was driven by this requirement: the particles must be as small as possible to avoid light scattering. However, recent studies discussed in this article tend to disapprove this doxa. First, we present the fabrication process of such fibers, emphasizing the drawing step as a process to control the shape and size of the nanoparticles. Then, we discuss on the characterization of the composition of these nanoparticles at the nm-scale. To reach this goal, we took advantage of a recent technology: Atom Probe Tomography. These results will be compared with molecular dynamics simulations. We demonstrate that the phase-separated nanoparticle composition and therefore the chemical environment of the encapsulated RE ions is nanoparticles size dependent. As a consequence, the smallest nanoparticles, promoted by the doxa, would offer limited alteration of the luminescent properties. Finally, light scattering is not only an issue but is also an opportunity to develop new temperature, strain, refractive index multiplexed optical fiber sensors.
SnO2-based glass-ceramics activated by rare earth ions have been extensively investigated because of the need to develop reliable fabrication protocols and clarify some interesting optical, structural, and spectroscopic features of the system. There is one important weakness in glass photonics when the rare earth ions are employed as luminescent sources. This is the low absorption cross section of the electronic states of the rare earth ions. A sensitizer is therefore requested. In the last years, we demonstrated that SiO2-SnO2 glass ceramics, presenting a strong absorption cross section in the UV range due to the SnO2 nanocrystal, are effective rare earth ions sensitizers. Another interesting property of the SiO2-SnO2 system is its photorefractivity. The high photorefractivity of sol-gel-derived SnO2-SiO2 glass-ceramic waveguides has been demonstrated in several papers published by our consortium. It has been shown that the UV irradiation induces refractive index change allowing the direct writing of both channel waveguides and Bragg gratings.
The results presented in this communication not only demonstrate the viability and outstanding properties of the SiO2- SnO2 glass-ceramics for photonic applications but also put the basis for the fabrication of solid state and integrated lasers. The next steps of the research are the fabrication of the channels and mirrors exploiting the photorefractivity as well as to draw glass ceramic fiber, checking the lasing action and corresponding functional characteristics. Finally, it is worth noting that the dynamic of the energy transfer from the nanocrystals to the rare earth ions is still an exciting open question.
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