Inscription Conditions Research Articles

Radiation Effects on Fiber Bragg Gratings: Vulnerability and Hardening Studies

Fiber Bragg gratings (FBGs) are point optical fiber sensors that allow the monitoring of a diversity of environmental parameters, e.g., temperature or strain. Several research groups have studied radiation effects on the grating response, as they are implemented in harsh environments: high energy physics, space, and nuclear facilities. We report here the advances made to date in studies regarding the vulnerability and hardening of this sensor under radiation. First, we introduce its principle of operation. Second, the different grating inscription techniques are briefly illustrated as well as the differences among the various types. Then, we focus on the radiation effects induced on different FBGs. Radiation induces a shift in their Bragg wavelengths, which is a property serving to measure environmental parameters. This radiation-induced Bragg wavelength shift (RI-BWS) leads to a measurement error, whose amplitude and kinetics depend on many parameters: inscription conditions, fiber type, pre- or post-treatments, and irradiation conditions (nature, dose, dose rate, and temperature). Indeed, the radiation hardness of an FBG is not directly related to that of the fiber where it has been photo-inscribed by a laser. We review the influence of all these parameters and discuss how it is possible to manufacture FBGs with limited RI-BWS, opening the way to their implementation in radiation-rich environments.

Refractive index sensor in eccentric fiber Bragg gratings using a point-by-point IR femtosecond laser.

In this paper, we demonstrate point-by-point inscription of eccentric fiber Bragg grating (EFBG) in conventional single-mode fiber (Corning SMF-28) with second-order Bragg resonances within the C-band using an IR femtosecond laser. Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. EFBGs are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. The inscription conditions to optimize the grating fabrication have been explored to achieve second-order resonances of good spectral quality. Potential applications of EFBG in refractometry sensing are further illustrated. Two interrogation techniques for EFBG in refractometry are reported and experimentally tested.

Femtosecond laser induced selective etching in fused silica: optimization of the inscription conditions with a high-repetition-rate laser source.

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.

Study of Fiber Bragg Grating Samples Exposed to High Fast Neutron Fluences

Fiber Bragg grating (FBG) sensors are expected to provide valuable data in extreme radiation environments associated with nuclear research reactors. However, when the fast neutron fluence reaches 1018–1019 n/cm2, the radiation-induced changes in the material density and refractive index may drastically bias the measurements. This paper evaluates the radiation effect on the FBG performances by comparing their properties before and after their exposure to fast neutron fluences exceeding 1019 n/cm2 ( $E > 1$ MeV). We studied the responses of FBGs manufactured by three different laboratories in the same single-mode optical fiber but using different inscription conditions. The Bragg wavelength and the reflectivity were measured before and after irradiation thanks to a dedicated mounting. For nearly all FBGs, the Bragg peak remains visible after the irradiation while the radiation-induced Bragg wavelength shifts (RI-BWSs) vary from a few picometers (equivalent temperature error <1 °C) to nearly 1 nm (~100 °C error) depending of the FBG inscription conditions. Such high RI-BWSs can be explained by the huge refractive-index variation and compaction observed for bare fiber samples through other experimental techniques. Our results show that by using specific hardening techniques, the FBG-based temperature measurements in a nuclear research reactor experiment may become feasible.

Radiation Vulnerability of Fiber Bragg Gratings in Harsh Environments

The difficulties encountered in the implementation of a temperature or strain sensor based on fiber Bragg grating (FBG) in a harsh radiative environment are introduced. We present the choices made to select both a radiation-resistant fiber in terms of transmission and also the grating inscription conditions necessary to write radiation tolerant FBGs in such fibers with a femtosecond laser. The radiation response of these gratings was also studied under radiation at dose up to 1 MGy. The comparison between Ge-free and Ge-doped fibers was highlighted.

Feeding the Transcendent Body

Feeding the Transcendent Body George Yudice To eat is to appropriate by destruction; it is at the same time to be filled up with a certain being.... When we eat we do not limit ourselves to knowing certain qualities of this being through taste; by tasting them we appropriate them. Taste is assimilation.... The synthetic intuition of food is in itself an assimilative destruction. It reveals to me the being which I am going to make my flesh. Henceforth, what I accept or what I reject with disgust is the very being of that existent.... It is not a matter of indifference whether we like oysters or clams, snails or shrimp, if only we know how to unravel the existential signification of these foods. Generally speaking there is no irreducible taste or inclination. They all represent a certain appropriate choice of being. Jean-Paul Sartre1 At first glance, it seems unlikely that contemporary U.S. culture can offer a gastrosophy to match that of other civilizations. Brillat-Savarin’s (and Feuerbach’s) adage, “You are what you eat,” does not throb today with metaphysical significance as it did scarcely two generations ago for Sartre. In the United States, it is indeed a matter of indifference “whether we like oysters or clams, snails or shrimp”; much of the lower priced seafood today is made from other processed fish. Consequently, the differences between particular foods are less important; what really matters is taste itself, laboratory produced flavor. Food as substance gives way to the simulacrum of flavor, which is something that “science” recombines in ever new ways to seduce us to this or that convenience food. As synthetic food replaces Sartre’s “synthetic intuition of food,” we find it impossible to transcend the brute “facticity” of eating, which is ironically as fake as it is real. We eat substances (the “real”) yet we do not know them as such but as simulations (the “fake”). The portrait I’ve drawn here obviously calls for a reference to Baudrillard, which will come in due time. First, however, it is necessary to reflect a bit more on the changes wrought by the transition to simulation in our (seemingly) most immediate experience: eating. Anthropologists have explained in great detail how entire civilizations defined themselves allegorically through their eating practices. Inclusion or exclusion, symbolic and material exchange, body boundaries, gender, and other identity factors are systematically and most deeply inscribed in the members of a given group through eating practices. Consequently, the metaphysics of most groups is conveyed by these practices. This inscription conditions, for example, how people understand divinity. For the Greeks of Hesiod’s Theogony, the rituals of sacrificial cooking and eating, paralleled in agricultural, funereal and nuptial practices, establish a communication between mortals and immortals which paradoxically expresses their incommensurability. In contrast, the Orphic anthropogony makes possible the mystical transcendence of the barrier between gods and humans by rejecting the sacrifice of the official religion. By refusing this sacrifice, by forbidding the bloodshed of any animal, by turning away from fleshy food to dedicate themselves to a totally “pure” ascetic life—a life also completely alien to the social and religious norms of the city—men would shed all the Titanic elements of their nature. In Dionysus they would be able to restore that part of themselves that is divine.2 Since mystical transcendence usually involves some relation to eating—or not eating, as in the Orphic cult—it is interesting to ask what are the possibilities of such transcendence in an age of fake fat and microwavable synthetic meals. The mystic engages in a struggle whose reward is nourishing grace. As Saint Teresa says, the soul “finds everything cooked and eaten for it; it has only to enjoy its nourishment.”3 In our consumer culture, however, such convenience food comes to most of us without the struggle. Unlike the mystic—who is “like a man who has had no schooling...and [yet] finds himself, without any study, in possession of all living knowledge”—we are not graced by any special knowledge. Without negativity—Sartre’s “appropria[tion] by destruction”— there is no transcendence. And negativity is precisely what gives the Orphics and...