## 3 Results  
Figure 3b shows the measurements of the retardance, R, as a function of the laser pulse energy for both investigated fibers. We focused on the Type II fs-IR threshold where the optical properties of the irradiated regions are modified. A strong linear birefringence originating from the formation of nanogratings can be detected when the energy exceeds the threshold. Illustrated in Figure 3b, the 3D printed fiber shows a lower Type II threshold  
Figure 3: (**a**): Experimental laser writing design: irradiation of fs laser inside fiber core and cladding of an optical fiber using different pulse energies. (**b**) Retardance, R, as a function of pulse energy for 3D printed fiber (squares) and SMF28 fiber (circles). Experimental conditions are: \(\lambda=1030\) nm, rr = 100 kHz, v = 100 \(\upmu\)m/s, NA = 0.6, t = 250 fs, laser polarization \(\perp\) and // to writing direction. (**c**) Optical microscope image of for 3D printed fiber using crossed polarizer and analyzer with the Yx configuration (\(\perp\)). (**d**) Optical microscope image of 3D printed fiber using crossed polarizer and analyzer and a full retardation waveplate inserted at 45\({}^{\circ}\) indicating the orientation of the slow axis. than conventional fiber. A well-defined birefringence response appears when the pulse energy is above E\({}_{\text{P}}\)\(\sim\)0.1 uJ for 3D-printed fiber. For the SMF 28 fiber, the Type II threshold is observed at a laser pulse energy E\({}_{\text{P}}\)\(\sim\)0.2 uJ. Such variations in Type II threshold could be explained by the higher Ge concentration that leads to higher interface stresses and therefore lower thresholds. Under crossed polarizer and analyzer, the lines appear birefringent when E\({}_{\text{P}}\) > 0.2 uJ for 3D printed fiber as shown in Figure 3c. Furthermore, the first order (full wave) plate technique [18] was used to detect the slow/fast axis orientation of the photo-induced birefringence (Figure 3d). When the inscribed line slow axis is parallel (perpendicular) to the slow axis of the full wave, the line appears blue (yellow). This is consistent with birefringence that we have observed previously that is related to the formation of nanogratings [19; 20], and where the fast axis is oriented parallel to the laser polarization. Although within error there are small differences between threshold values, R increases as a function of pulse energy in a similar way for both fibers (both in shape and magnitude) as shown in Figure 3b. R values rapidly increased after exceeding a threshold and began to plateau after E\({}_{\text{P}}\)\(\sim\)0.4 uJ. It should be noted that the retardance is lower for the Yx configuration relative to the Yy configuration. Such polarization dependence is explained by boundary conditions and varying spatial-temporal properties of the ultrashort pulse laser beam. This was quantified with pulse front tilting, leading to anisotropic photosensitivity [21; 22]. These results indicate that these 3D printed fibers exhibit analogous photosensitivity compared with SMF28, consistent with similar quality germanosilicate glass. They also suggest that similarly to conventional fibers, applications such as FBGs are possible using 3D printed preform derived optical fibers. This comparative study suggests material quality is comparable despite the large differences in initial preform fabrication. This is a promising result pointing towards a feasible disruption of conventional fiber fabrication using additive manufacturing. In order to better understand and confirm the origin of the polarization dependent birefringence detected in Figure 3, the fiber segment was simply cleaved by the ceramic fiber scribe and the cross sectional area with the laser polarization \(\perp\) to the writing direction was directly investigated using an SEM (Field-Emission Gun Scanning Electron Microscope, ZEISS SUPRA 55 VP, 1 kV accelerating voltage, ZEISS, Jena, Germany). Figure 4c shows that the trajectory of the laser perfectly passes through the center of the fiber.