## 3 Results
As observed in Figure 4a,b, the formation of the nanogratings can be clearly identified in both the cladding and the core zone of the 3D printed fiber. In similar bulk compositions [15], such periodic array of porous nanoplanes separated by non-porous glass leads to the formation of form birefringence [15; 20]. The thermal stability of the fibers was investigated using \(\Delta\)t = 30 min step isochronal annealing up to temperatures T = 1100 \({}^{\circ}\)C. In Figure 5, retardance values R\({}_{\text{norm}}\)(\(\Delta\)t, T) (normalized with respect to their initial room temperature values R(t = 0, and T = 20 \({}^{\circ}\)C)) as a function of annealing temperature for both SMF28 and the 3D printed fiber are  
Figure 4: Scanning electron micrographs in the 3D printed fiber, both at the cladding (**a**) and the core (**b**) area, for orthogonal laser polarization direction with respect to the writing direction, which permits sideways visualization of the nanogratings. (**c**) Optical microscope image of the cross section of 3D printed fiber. Experimental conditions are: \(\lambda\) = 1030 nm, rr = 100 kHz, v = 100 μm/s, NA = 0.6, t = 250 fs, E = 0.7 μJ/pulse. displayed. In this figure the comparison between the two samples is set for E = 1 \(\mu\)J/pulse and//irradiation condition for which the measured R values are among the highest (see Figure 3). It is worth pointing out that in order for each data point to be independent to the other, the criterion (\(\Delta\)t \(\times\) k\({}_{0}\))\({}^{\text{(AT/Tmax)}}\)\(<<\) 1 must be fulfilled [23] (with k\({}_{0}\) estimated to be around 5 \(\times\) 10\({}^{5}\) to 5 \(\times\) 10\({}^{7}\) s\({}^{-1}\) for nanogratings erasure in silica [24]). In this work, this criterion is not satisfied for T > 900 \({}^{\circ}\)C and therefore the thermal stability is slightly underestimated with respect to a stability curve having independent data points. In Figure 5 some characteristic features are visible. These reveal different erasure mechanisms. First, in the low-temperature (T < 600 \({}^{\circ}\)C) range, compared with the initial value, the two fibers exhibit a similar trend, where Rnorm slightly increases up to T = 600 \({}^{\circ}\)C. Such a feature could be attributed to central defects being erased at low temperatures (T < 600 \({}^{\circ}\)C) [20], resulting in an increase in nanogratings refractive index contrast (hence retardance). Beyond 600 \({}^{\circ}\)C, Rnorm decreases up to T = 850 \({}^{\circ}\)C for both fibers, immediately followed by a very slight rise within T = 800 to 900 \({}^{\circ}\)C. This observed growth could be related to changes in the stress field. Isochronal annealing leads to an increase in residual stresses, resulting in an increase in the magnitude of birefringence. More details with respect to this can be found in References [19; 25]. Turning to SiO\({}_{2}\) glass structure, it has been shown that changes in Si-O three and four-fold rings anneal out over T = 700-900 \({}^{\circ}\)C. They are well correlated with the structural relaxation of densified silica formed within the nanogratings regime [23]. As the annealing temperature increases, the induced stress relaxes gradually, lessening the birefringence and further decreasing retardance. Finally, above T = 925 \({}^{\circ}\)C, the normalized retardance, Rnorm, for both fiber samples diminishes drastically. The latter could not be detected above T\(\sim\)1080 \({}^{\circ}\)C. Overall, the two fibers exhibit very similar thermal stability based on their performance during the isochronal annealing. It should be noted that the concentration of GeO\({}_{2}\) in the two fibers is different and the mode overlaps different. Hence, the results presented here are in agreement with Reference [11] where it was shown that the variation of GeO\({}_{2}\) concentration in the fiber core only has a minor impact on the overall thermal behavior.