## 1 Introduction  
Silica optical fiber is the backbone of optical fiber communication networks, permitting signal transmission over long distances at high bandwidth. Since the 1970s numerous methods were explored in the development of low loss silica telecom fibers, most involving the fabrication of preforms from which fiber is subsequently drawn [1]. The most successful processes have been variations of chemical vapor deposition (CVD) [2]. With the development of optical fiber technology, the focus has gradually included increasing specialization of the fiber for components such as fiber Bragg grating (FBG) sensors, lasers, and amplifiers. Ideally optical fiber fabrication has to remain compatible with standard low-loss fiber as well as opening the door to novel functionalities. Nevertheless, conventional fabrication methods such as CVD and rod-in-tube stacking, have limited capability in both material and structure flexibility for diverse and custom-designed functionalities [3, 4, 5]. To address these limitations a new alternative is the use of additive manufacturing in the fabrication of preforms from which fiber is drawn [6, 7]. The early work in this field was on 3D printing plastic fibers [6], followed by 3D printed silica-based fiber preform fabrication [7]. 3D silica printing allows greater freedom in both structure and material design and choice. This fabrication process will be a major disruption more broadly because it reduces the skill requirements typically required in CVD processes and reduces overall costs whilst enabling newer designs. Therefore, it is important to better understand and compare these new fibers against their equivalent counterpart produced by conventional means. One important comparison that provides insights into the quality of glass in these fibers is assessing photosensitivity. This can be done by creating a refractive index grating in the core of an optical fiber using optical irradiation, a process that underpins fiber Bragg gratings (FBG) inscription and application, demonstrating both scientific and practical significance [8]. Among various types of FBGs, Type II-fs-IR-FBGs, inscribed by femtosecond near-infrared (fs-IR) irradiation, have drawn great attention, due to their notable thermal stability at T-1000 \({}^{\circ}\)C for at least several hours and even 100 h at this temperature after stabilization in SMF28(single mode fiber) fibers [9; 10]. In addition, it has been shown that the thermal stability of these Type II modifications is affected by writing and fabrication conditions. For example, by increasing the writing speed, the reflectivity of Type II fs-IR FBGs is found to be more resistant to higher temperatures [11]. It has also been reported that the thermal performance of FBGs can be influenced by varying pulse energy or repetition rate [12]. Unlike Type I-fs-IR-FBGs where the refractive index changes are positive with respect to the non-irradiated region and could only be used up to T-800 \({}^{\circ}\)C [13], Type II fs-IR FBGs, associated with a high level of anisotropic index change, are attributed to the formation of self-organized nanogratings [14] made through glass oxide decomposition [15]. These nanostructures are composed of regularly spaced porous nanolayers with nanopores typically a few tens of nm in size [15; 16]. Due to their periodic subwavelength nanostructure, the Type II modifications are birefringent, and therefore the optical retardance R can be monitored to investigate the thermal stability of our object. Thus, to explore the photosensitivity of optical fibers fabricated from preforms produced by additive manufacture, an fs-IR laser was used to create changes within the fiber, henceforth termed \"3D printed fiber\". The laser-induced birefringence is measured at different pulse energies. Then the thermal stability of these fs-IR laser inscribed structures (Type II modifications) was evaluated through multiple isochronal annealing cycles.