1. Introduction
The first application of a polymer optical fiber (POF), developed in the sixties, was focused on data transmission [
1]. Despite POFs’ high attenuation compared to silica fibers, they are used to transmit data over short distances [
2]. Furthermore, with the advance in technologies and new materials, recent works accomplish that POFs have different sensors applications for mechanical and chemical parameters [
3,
4]. As compared to silica optical fibers, POF has higher strain limits, making it attractive for sensor applications [
3]. In addition, POF’s properties can be tailored according to the application, as different raw polymers with different characteristics can be used to produce them [
5].
The different manufacturing techniques for POFs depend on the process, where the techniques can be categorized as continuous or discontinuous [
6]. In the first technique, all processes are simultaneous, which enables the production of large quantities of filament. The second technique involves, at least, two steps, limiting the length of produced filament compared to continuous techniques [
7]. A few examples of continuous processes are continuous extrusion [
1], photochemical polymerization [
7], co-extrusion [
8], dry spinning [
9], and melt spinning [
1]. In terms of discontinuous processes, some examples include preform production [
4], heat-drawing process [
3], and batch extrusion [
10].
Innovative technologies to enhance POFs material features involve combining different material characteristics, resulting in a multimaterial POF for the enhancement of their mechanical and optical properties, e.g., the heat-resistant POF in [
11]. Furthermore, it is possible to enhance the sensitivity of the POF as a function of different quantities by combining different materials. Such materials with different Young’s modulus result in multimaterial optical fibers, which can also be made relatively insensitive to undesired parameters such as humidity and temperature [
12].
Structures with complex geometries can be fabricated from a variety of materials (such as biomaterials, ceramics, composites, polymers, metal alloys) using 3D printing, an additive manufacturing (AM) process [
13]. The simplicity, relatively low cost, less waste, freedom of design, and automation of this technology have made it widely explored [
14]. Rapid prototyping, printing large structures, and printing complex structures at high resolution all contributed to the development of some AM techniques, which enable its application in some fields, such as biomedical, mechanical, civil construction, and microelectronic device manufacturing [
14].
Fused deposition modeling (FDM) is one of the main fabrication methods of AM [
6]. In this process, thermoplastic polymers are heated to a semi-liquid state and then extruded onto a platform or on top of previously extruded layers [
14]. A key property of polymers is their thermoplasticity, which allows extruded polymers to fuse together and then solidify at room temperature [
15]. The mechanical properties of the printed part are strongly affected by some parameters such as: layer thickness, width and orientation of filaments and air gap [
16]. While FDM has some advantages, including low cost, high speed, and simplicity, it also has some limitations, including poor mechanical properties, poor surface quality, and layer-by-layer appearance [
17].
Other AM methods include powder bed fusion, inkjet printing and contour crafting, stereolithography (SLA), and direct energy deposition [
14]. There are advantages and limitations to each of these methods, and they need to be taken into consideration with other parameters such as the size of the part, the surface quality required, and the time it will take to fabricate the part [
18].
The progress in the AM methods, especially in 3D printing resulted in many advances in polymer processing, where different printing parameters were thoroughly investigated for conventional materials in 3D printing, e.g., PLA and PETG [
19]. The investigation of such parameters not only leads to the possibility of optimization in mechanical properties of 3D printing devices [
20], but also in novel shape memory devices [
21] and hybrid composites [
22]. Such developments lead to the possibility of 4D printing of polymer-based devices [
23] for shape memory performance in mechanical and thermal parameters [
24]. As another possibility of the AM method in transparent materials, the development of waveguides was explored not only using plastics [
25], but also in glass structures [
26]. In this scenario, the development of optical devices using polymers [
27] and, especially, POFs [
28] result in an additional possibility in optical fiber technology. These POFs are generally used in the development of sensors, where the integration of AM methods results in multiparameter sensing [
29] as well as the performance enhancement in temperature [
30] and force [
31] sensors. The practical applications of such fibers also include the development of smart textiles [
32], where such textiles can be used for angle measurement [
33] and activity monitoring with smart clothing [
34].
This work is focused on the fabrication and sensor applications of a POF using the AM process. Mechanical and optical characterization methods are employed for curvature and temperature sensing applications. Additionally, the fabricated POFs are used in the development of a modal interferometer using the 3D-printed POF in-between single-mode fibers for strain and temperature assessment. The progress in the 3D-printing fabrication of POFs involves the use of different optical materials and methods for extrusion as summarized in [
28]. In addition, the application of multimaterial approaches were also investigated using temperature-sensitive resins [
35] and even 3D-printing approaches [
36]. However, the application of such 3D-printed devices in interferometric approaches for sensing was not investigated in the literature yet. Thus, the main contribution of this work is the low cost fabrication of 3D-printed fibers directly extruded from the 3D printer (instead of preforms as discussed in [
28]) in conjunction with the sensors applications using not only the intensity variation principle, but also using modal interferometers. The novelty of this approach is not only the customization of the optical fiber fabrication, especially on the cladding fabrication, but also in the novel approach for interferometer development using 3D-printed POFs. The proposed sensor device can be used in different applications ranging from biosensors [
37], structural monitoring [
38] and yarn-integrated devices [
39].
2. Fabrication Method
POF’s fabrication process begins with the selection of the materials for its core and cladding. From the 3D printing filaments analyzed, the main characteristic of the fabrication process is transparency. The first analysis is the refractive index measurement of four different filaments: HDglass (a commercial PET-G filament with additives for higher transparency from FormFutura), PET-G (from 3D-Fila, Brazil), PLA (from 3D-Fila, Brazil) and Tritan (a copolyester commercial filament from 3D-Fila, Brazil). The performance of a second test is necessary to measure their transmittance. These filaments were chosen due to their wide commercial availability and the differences in their optical and mechanical properties as well as the differences in their printing parameters such as the nozzle and bed temperatures. The HDglass is also basically made of PET-G compounds. However, it has commercial grade additives for higher transparency, which are the main difference between the HDglass and PET-G filaments. The PLA filament has a semi-crystalline structure, resulting in smaller transparency than the other filaments. This filament was used for comparison purposes, since it is a commonly employed filament in 3D-printing processes. Moreover, the Tritan filament is a commercial compound made of copolyester aiming at a higher mechanical resistance and non-brittle behavior.
The first step in the POF fabrication is the 3D printing of its core, where the core is directly fabricated from the 3D printer nozzle. For the 3D printing of the optical fiber’s core, the Ender-3 V2 (Shenzhen Creality 3D Technology Co., Ltd., Shenzhen, China) 3D printer was used. In this case, the printing parameters were empirically determined in order to provide the same parameters for all filaments for comparison purposes of the different materials. It is important to mention that the parameters such as printing speed, nozzle temperature/cooling rate directly affect the pre-strain as well as the residual stress on the 3D printing of the polymer, especially the PET-G as depicted in [
40]. However, the pre-strain analysis of the POFs is not in the scope of this paper, since the first step is to define a suitable filament for the 3D printing of the optical fiber. The parameters used on the 3D printing include a printing speed of 40 mm/s, a bed temperature of 80 °C and a nozzle temperature of 235 °C. Furthermore, the nozzle diameters of 1.75 mm and 0.4 mm were used to evaluate different diameters of the optical fiber core. For the fabrication of characterization samples, the layer thickness was set as 0.4 mm, and the fan speed set as 30%.
After the results of the refractive index characterization, the optical fiber core and cladding were fabricated. To be part of the core, the refraction index of the filament must be higher than the cladding. In addition, to present a better signal transmission, the values of transmittance must be close as possible to 100%. The filament that best fits these requirements is selected to make part of the core. The extrusion of the core is performed using the 3D printer nozzle. The filament and nozzle diameters used were 1.75 mm and 0.4 mm, respectively, and the extrusion temperature was set to 235 °C. Based on empirical testing, this temperature was chosen in order to minimize the production of bubbles during the fabrication of the core. The core diameter is controlled by using constant pulling force and printing speed. In addition, the fiber diameters are measured using a caliper every 10 mm of the 3D-printed POF.
To fabricate the cladding, we used two methods. The first is the dip-coating [
5] using three different solutions: methyl acetate, sodium fluoride, and N,N Dimethylformamide. To study the cladding fabrication using this method, three immersion times were used for each solution: 6, 60 and 600 s. The purpose of this procedure is to make the ends of the core dissolve reducing its refractive index. The selected core was immersed at three different times, as shown in
Table 1. The refractive indices were measured using a benchtop Abbe-type refractometer at around 585 nm.
The second technique to fabricate the cladding uses the resin Norland Optical Adhesive 88 (Norland Products Inc., East Windsor, NJ, USA) and a polytetrafluoroethylene (PTFE) tube with 0.6 mm inner and 0.8 mm outer diameters. After filling the tube with resin using a syringe, the core is inserted into the tube, immersing it. Then, the resin must be cured using ultraviolet light.