## 3 Results
To further investigate the thermal stability of the written Type II modifications in 3D-printed fibers at different pulse energies, and once again to compare them with conventional CVD fibers, we annealed the irradiated samples isochronously from T = 400 \({}^{\circ}\)C to 1100 \({}^{\circ}\)C, with a constant time increment of 30 min at each temperature step. Figure 6 shows the  
Figure 5: Thermal stability with normalized retardance, Rnorm, as a function of annealing temperature, T, for drawn 3D printed fiber (in blue) and SMF 28 fiber (in red), for // configurations. The pulse energy is E = 1 \(\mu\)J and the measurements were performed at room temperature after each annealing step (\(\Delta\)t = 30 min). evolution of normalized retardance (Rnorm) as a function of T for lines irradiated with different energy (E). With different pulse energies, the two fibers exhibit similar trends during heat treatment. There are at least two distinct features in the retardance response, one at low temperature and one at high temperature. At low pulse energy (3D printed fibers E\({}_{\text{p}}\leq\) 0.7 uJ and SMF 28 fiber E\({}_{\text{p}}\leq\) 0.9 uJ) there is immediate retardance decay after the start of annealing. Typically, Rnorm decreases until T = 600 \({}^{\circ}\)C corresponding to a 20-60% decrease in magnitude. This effect is more pronounced for the lowest pulse energies. Next, R increases in the range of T = (600-800) \({}^{\circ}\)C and is also more pronounced in the low energy regime. In the literature the appearance of this unusual increase in retardance can be related to annealing out of the residual part of the Type I modifications [26] between the nanolayer, that is mostly attributed to densification and related stress field (and thus stress-induced birefringence). This leaves a higher birefringence or FBG reflectivity [11]. Type I modifications require a lower deposited energy than Type II during the irradiation. This means the contribution of Type I response on the measured retardance is likely higher during lower energy Type II modification. The more significant increase in retardance Rnorm of low energy Type II changes arise after the residual effects of Type I are \"erased\" or relaxed. This behavior has been observed across various conventional gratings written by UV (Type I, Type IA and Type II and Type IIA), with high temperature annealing, reflecting intrinsic amorphous material properties rather than laser writing [26]. As the annealing temperature increases beyond T = 800 \({}^{\circ}\)C, the Rnorm for all lines and at any pulse energy starts to decline, likely due to gradual stress relaxation that takes place below the glass transition temperature (\(\sim\)0.9 T\({}_{\text{g}}\) for 30 min). Finally, the erasure of the porous nanolayers is observed over T = (950-1080) \({}^{\circ}\)C range as a function of the laser pulse energy [16]. Indeed, the higher the laser pulse energy, the higher the annealing temperature needed to fully erase Type II modifications. This can be explained by noting that as the energy increases, the non-linear writing light dosage increases. As the energy increases in the Type II range, the thermal stability of the modification is enhanced for both the processed 3D printed fiber and the SMF28 fiber. This is a typical case described in Reference [23] where there is a link between writing and erasure. When the pulse energy increases, the demarcation energy increases and more stable sites, or deeper traps, are accessed, and more stable species produced. The subsequent modifications become more stable. The  
Figure 6: Normalized retardance (Rnorm, relative to its initial value at room T) as a function of temperature for **(a)** SMF 28 fiber and **(b)** 3D printed fiber; different pulse energies were used. Experimental conditions are: \(\lambda\) = 1030 nm, rr = 100 kHz, v = 100 \(\upmu\)m/s, 103 pulses/\(\upmu\)m, NA = 0.6, pulse duration t = 250 fs, laser polarization // to writing direction (Yy).