###### Abstract
\"Wet\" fibers are characterized by a strong RIA in the visible domain caused by the NBOHCs and their 1.97 eV (\(\sim\)630 nm) absorption band that, in addition to the process described by (2), can be created by other processes whose efficiencies also depend on the glass stoichiometry [21,22]:  
\[\equiv\mathrm{Si-O-H}\xrightarrow{ radiations}\equiv\mathrm{Si-O\bullet}+\ \bullet\mathrm{H}^{0} \tag{3}\]\[\equiv\mathrm{Si}-\mathrm{O}-\mathrm{O}-\mathrm{Si}\equiv\overset{ radiations}{\to} \equiv\mathrm{Si}-\mathrm{O}\bullet\ +\ \bullet\mathrm{O}-\mathrm{Si}\equiv \tag{4}\]  
In the \"dry\" fibers case, the NBOHC contribution is strongly reduced as fewer precursor sites are available for their generation upon exposure, but an increasing contribution from the absorption bands of Cl-related defects is clearly observed. These bands are located around 3.78 eV (328 nm) and 2.3 eV (539 nm) for the Cl\({}_{2}\), 3.26 eV (380 nm) and 3.65 eV (340 nm) for the Cl\({}^{0}\) species [23; 24]. For the optical fibers presenting both low-OH and low-Cl levels, it was shown that if NBOHC and Cl-defect concentrations are efficiently reduced, self-trapped holes (STHs) can be generated, strongly decreasing the fiber transmission in the visible domain through their absorption bands around 2.61 eV, 2.16 eV, 1.88 eV and 1.63 eV [14; 25]. For this specific defect class, it was shown that the glass structure plays a key role, in particular, its fictive temperature \(T_{g}\)[26] regarding the STH creation: larger \(T_{g}\) leads to larger STH-related absorption. As a consequence, no perfect PSC composition exists for operation in steady-state environments. A possibility to further improve the radiation resistance of PSC fiber consists in its loading with molecular hydrogen [15]. Indeed, hydrogen allows efficient passivation of most of the radiation-induced point defects absorbing in the visible domain. This is the case, for example, of NBOHCs that can be passivated through the following process also resulting in a simultaneous increase of the IR-absorption around 1380 nm [5]:  
\[2\times\equiv\mathrm{Si}-\mathrm{O}\bullet\ +\mathrm{H}_{2}\to 2\times \equiv\mathrm{Si}-\mathrm{O}-\mathrm{H} \tag{5}\]  
However, while the hydrogen can be easily loaded at room temperature in silica-based fibers, it will also quickly diffuse out [27] rendering the treatment efficiency dependent on the time delay between the treatment and the irradiation. From an applicative point of view, this strongly limits the interest of the technique for fusion-devoted facilities considering their long lifetimes. To maintain the hydrogen into the fibers, several techniques can be suggested implying the use of metallic coatings instead of polymer ones [7; 8] or the addition of a hermetic thin carbon layer between the cladding and the coating [11; 28]. In these cases, the hydrogen loading should be done at high temperatures to facilitate the hydrogen diffusion into the fiber core or a hole-assisted structure has to be used [28]. It should be noted that these particular structures could also lead to larger intrinsic attenuation, caused for example by some additional micro-bending losses. In this paper, we investigated how the intrinsic and RIA attenuations of MM PSC fibers, in the visible-near IR domain, are controlled by their OH amounts. We characterized three different fiber samples: one \"wet\" and one \"dry\" fiber and also one sample with an intermediate OH content (_medium_-OH, 27 ppm). Furthermore, we discuss the H\({}_{2}\) pre-treatment efficiency of the _medium_-OH fiber to permanently improve both its intrinsic losses in the visible domain and its steady-state radiation response up to 300 kGy dose levels. This procedure allows achieving optical fibers fulfilling the requirements of ITER plasma diagnostics as the ones described in [11] without using carbon-layers or metallic coatings.