Attenuation Analysis of Polymer Optic Fibres (POF) Manufactured with Different Materials

Polymer optical fibres (POF) are light-guiding monofilaments that are utilized mostly for short-distance data transmission, as fibre-optic sensors, and in illumination applications. POF are typically bicomponent circular fibres with a core-cladding structure. The core is the light-transmitting part of the optical fibre, whereas the cladding is required to have a reflection interface with a consistent ratio of refractive indices so that total internal reflection can come into existence [1].

There are numerous methods for producing POFs, each with advantages and disadvantages. Extensive process knowledge is crucial for selecting the most cost-effective manufacturing method for the optical fibre required [2].

Melt spinning is the technique most used for producing commercial synthetic fibres. Variation of fibre morphology by bicomponent (conjugated) spinning is a trend in polymer melt spinning. A bicomponent fibre is formed from two polymers with distinct chemical (e.g. composition, additives) and/or physical (e.g. average molecular weight, crystallinity) properties, extruded from a single spinneret. The primary purpose of bicomponent melt spinning is to leverage capabilities not present in either polymer alone by combining the beneficial mechanical, physical, or chemical qualities of two polymers into a single fibre, thus broadening the variety of applications [3].

This method's primary benefits are its high haul-off speed and, consequently, its high productivity. The procedure's disadvantages are the high acquisition costs for the melt spinning equipment and the higher attenuation due to impurities in the polymer granulate [2]. The continuous melt-spinning process is illustrated in Figure 1 [1].

Fig. 1.

Continuous melt spinning process

When a light ray crosses from one optically distinct medium to another, the light is refracted and reflected at the interface between the two. Refraction is dependent on the material or medium and a particular material property known as the refractive index. The refractive index is illimitable and is defined as the ratio between the speed of light in a vacuum and the speed of light propagation in the medium [4].

The majority of optical fibre types, particularly polymer-optical fibres (POF), rely on the total internal reflection (TIR) process. By arranging the index of refraction of the cladding material to be lower than that of the core, one may ensure that light emitted into the inner core region cannot pass to the outside cladding. At the interface of two regions with differing refractive indices, light will be reflected [1]. The majority of POF integrated into textile structures feature poly(methyl methacrylate) (PMMA) cores and fluoropolymers as cladding materials. They are resistant to alkalis, weak acids, gasoline, mineral and turpentine oils [4, 5].

Attenuation is the most significant process light faces as it travels through a fibre. When light travels through an optical fibre of length L, its intensity decreases.

Figure 2 visualizes the input power and output power of a POF [1]. P_L and P₀ are the power of light after travelling through a fibre of length L in kilometers and at the front end of the fibre, respectively. The attenuation coefficient is denoted as α. Equation 1 shows the attenuation measurement. Depending on the type of optical fibre, attenuation is measured either in terms of dB/km (pure glass fibre and plastic-clad silica fibre, or PCS) or dB/m. POF is increasingly utilized for fairly small transmission distances in the visible spectrum. Lastly, PMMA can also be utilized for mm-scale waveguide structures. In addition, a rise in crystallinity is undesirable for optical fibres because it generates abnormalities and density variations in the material, resulting in higher light scattering and attenuation [1, 5, 6]. (1) α=10ΔL⋅log(P0PL) \alpha = {{10} \over {\Delta {\rm{L}}}} \cdot \log ({{{{\rm{P}}_0}} \over {{{\rm{P}}_{\rm{L}}}}})

Fig. 2.

Visualisation of the input and output power of a POF

The aim of the study is to shed light on the best material selection for the use of POF in textile applications and to provide a guide for future studies on the historical development and application areas of POF according to its intended use. It is intended to provide a guide for ease of manufacture and selection in subsequent studies by providing POF production with different materials, testing POFs for attenuation properties, which is the most important criterion in textile applications, and providing an information guide on the usability and manufacturability of materials in textile applications. At present, fluoropolymers are used as cladding material for POF. POFs are therefore contaminated with PFAS (per- and polyfluoroalkyl substances). PFAS are a class of over 4.730 substances [7]. PFAS are very persistent, some of which are proven to be toxic [8, 9, 10, 11]. The entire class of substances is therefore at risk of being banned by the EU [12]. Therefore, the possibility of using PFAS-free cladding materials is being investigated. Thus, specifically non-fluorinated polymers as cladding materials were tried for this study.

In the second half of the 19th century, the concept of lighting glass rods was of tremendous research interest because dentists required light-guiding technologies to illuminate the entire mouth or for other medical uses like imaging, spectroscopy, endoscopy, tissue pathology, blood flow monitoring, light therapy, biosensing, biostimulation, laser surgery, dentistry, dermatology, and health status monitoring. The initial medical application of optical fibres enabled the illumination of inside organs during endoscopic treatments. Manfred Borner created the first optical-data transmission system based on optical fibres in 1965 at the Telefunken Research Laboratories in Ulm, Germany. The technology was patented in 1966. The invention of the first laser system in 1960 by Theodore Maiman was another revolutionary development [2, 13].

The development of long-distance functioning glass fibres and POF can be said to have begun with the advent of the laser. In the late 1960s, research on light-guiding fibres split between glass optical fibres and the newly created polymer-based optical fibres. In the later decades of the 20th century and at the start of the 21st century, fabrication technology vastly improved, and fibre losses decreased drastically. POF applications are suitable for data transfer, sensors, and the integration of fibres into functional textiles, and have high market potential. Especially, the innovative characteristics of current material selections, intelligent fabrics, and the market review should contribute to the value of previously published works on POF [2].

Due to the insufficient purity of the monomer materials utilized, the attenuation was still close to 1.000 dB/km. In the 1970s, losses for PMMA-based POF were lowered nearly to the theoretical limit of around 125 dB/km at wave lengths of 650 nm. At that time, huge amounts of glass fibres with losses significantly below 1 dB/km at 1,300 nm/1,550 nm were available, although at a premium price. After the complete digitalization of the data communication for long-distance transmission in the 1990s, the major development of digital systems for individual users commenced. The history of the international conference for polymer optical fibres and applications, which has been held yearly since 1992 and is the most prominent scientific event in this specialized sector, is indicative of this progress [6].

Figure 3 visualizes the comparison of glass fibers and POF [1]. The 1.0 mm POF is the standard for data transmission. The smaller diameters of 0.5 mm and 0.25 mm are used for textile/weaving applications. POF offers significant advantages over their glass alternatives. Specifically, their large diameter (usually 0.25 to 1 mm) enables the use of low precision plastic connections, which decreases the system's total cost. In addition, POF is distinguished by its increased flexibility, resilience to shocks and vibrations, and light coupling from the light source to the fibre. Due to these benefits, several POF applications have been developed and marketed [14].

Fig. 3.

Aperture angle and cross section of glass optical fibres and polymer optical fibres, the first number refers the core diameter, whereas the second number refers the fiber diameter (core and cladding)

The historical development of SI-POF can be seen in Table 1. The majority of POF literature centres on communication, but the applications of polymer optical fibres extend beyond data transmission. One of the largest markets is the illumination. POF is ideally suited for being woven into fabrics. Then, these fibres may be utilized for illumination, as wearing sensors, or for communication [2].

A comparison of the measurements of the fibres on the same bobbin was made to check their accuracy and precision through the fibre, which can be seen in Figure 7 [1]. Two specimens on the same bobbin for both bobbins show different accuracy. Precision/stabilization through the fibre was not suitable for the whole bobbin in the same production. The PLA fibre has a very flat curve in its second trial, which is considered suitable for stability. The fibre produced with a PMMA core and PP cladding material does not show power measurement with a length of 2 meters to 1 meter. Hence, the fibre with PMMA-PP is measured at a length of 1 meter by shortening the fibre by 0.1 meters each time.

Fig. 7.

Comparison of the two different attenuation measurements of two specimens on the same bobbin with respect to their each measurement length

As a result of manufacturing and attenuation test measurement, it is seen that the bobbins with materials PMMA-PVDF and PMMA-PMP show a lower attenuation measurement overall. The accuracy and precision of the attenuation measurement with their two specimens on the same bobbins are nearly close to each other. The bobbin with materials PMMA-PLA show a moderate attenuation measurement compared to others. However, the accuracy and precision of the attenuation measurement with their two specimens on the same bobbins are far from each other. PMMA-PP shows the worst attenuation measurement result, with an average of 30 dB/m, nearly 5 or 6 times that of the other bobbin combinations. This result was considered unsuitable due to its high capacity to lose light intensity. Also, measurements could not be performed on PMMA-PP with a fibre length of 2 meters long. The attenuation integrating sphere with a light detector captured no light intensity or power until it was cut to a 1 metre length. Measurement of PMMA-PP fibre with a length of 1 metre showed the highest attenuation through the fibre. The accuracy and precision of two specimens on the same bobbins were not close. The overall mean attenuation measurement result is seen in Figure 8 [1].

Fig. 8.

Overall mean attenuation measurement of fibres for two specimens on the same bobbin for 1 mm

Homogeneous distribution of light and attenuation was aimed to show a stable and steady change at each measurement length for all groups. PMMA-PP bobbins also show very imprecise results with respect to standard deviation when compared to other groups of bobbins. This means these bobbins have different power values through the fibre, which is unsuitable for most applications. Standard deviations of the PMMA-PP bobbins also show a massive difference between two specimens of the same bobbin. PMMA-PLA bobbins also show imprecision when comparing two specimens of the same bobbin. The PMMA-PVDF bobbins are nearly below 1 dB/m with respect to standard deviation. It is nearly stabilized by the two specimens in the same bobbin. These bobbins can be considered stable, and the attenuation throughout the length is changed gradually and proportionally. Also, PMMA-PMP bobbins are considered moderate because their standard deviation values behave like PMMA-PVDF. PP performs badly due to its opaqueness and lower transparency. PMP performs well during the trials but its high melting temperature. which is higher than that of PMMA, can cause thermal damage to the light guiding PMMA core or their core-cladding interface. PVDF performs well in terms of standard deviation but shows worse than PMP in terms of absolute attenuation. This could be due to its high crystallinity.

The fibre of PMMA-PP showed the highest attenuation test result at 30 dB/m, with an attenuation 5–6 times higher than that of other groups of bobbins. The attenuation of fibres of PMMA-PLA was four times greater than that of the combination of PMMA-PMP and PMMA-PVDF. Also, the bobbins are imprecise, as shown by two specimens of same bobbin.

Bobbins with PMMA-PVDF and PMMA-PMP showed the lowest attenuation and more precision compared to the other combinations of bobbins. Hence, material selection was reduced to PMMA-PVDF-PMP for future trials.