## 1 Introduction  
Polymer optical fibers (POFs) are fibers whose cores and claddings are made of plastic materials, of which poly(methyl methacrylate) (PMMA) and amorphous fluoropolymers are the most used [1, 2]. These fibers have found a number of applications over the years, in which their use is advantageous compared to other media, including sensing and communications in short-range scenarios [3, 4]. In particular, the use of POFs as a transmission medium in communication systems is strongly favored by their transmission properties, which are notably different for the fibers made of the two aforementioned materials. POFs made of amorphous fluoropolymers, also called perfluorinated POFs, have core diameters from 50 to 250 \(\upmu\)m, and their transmission performance can even surpass that of multi-mode glass optical fibers [5]. One of the most common materials for this type of fiber is polypropylene-butenylvinylether (commercially known as CYTOP(r)), which gives rise to high performance fibers that, however, partially lose some of the advantages of standard large-core PMMA POFs. These fibers have larger core diameters (typically 980 \(\upmu\)m) and numerical apertures (NAs) of up to 0.6 \(\upmu\)m. They can be manufactured with a single core (SC) or a number of smaller cores (multi-core, MC) with different index profiles: step-index (SI), multi-step-index (MSI) or graded-index (GI) profiles [6]. In general, large-core PMMA POFs, which we refer to as standard POFs, have narrower bandwidth and higher attenuation than glass fibers, but are still adequate to meet the demands of short-range networks in automotive, industrial, domestic and more recently, avionics environments [7]. The automotive industry led the way, introducing them as the transmission media in car data links [8], and more recently, they have also succeeded in domestic networks, usually combined with Wi-Fi [9]. Nowadays, the manufacturing of PMMA fibers with thermoset polymer jackets able to sustain temperatures up to 105 \({}^{\circ}\)C, has raised the interest of the aeronautics industry in POFs [10]. In addition to the advantages of all optical fibers (electromagnetic immunity and electric passiveness), POFs' specific characteristics make them very attractive. They are lightweight, flexible and resistant to impacts. Moreover, PMMA fibers experience minimum attenuation in the visible range of the spectrum (530, 570 and 650 nm), which facilitates failure detection and avoids safety issues [2]. Their most attractive features, however, are their large cores and high numerical apertures that enable easy connections using simple termination tools. In consequence, standard POFs can be easily manipulated and do not require expert handling. This \"do-it-yourself\" feature is responsible for POFs' success in domestic applications [11], and their reduced deployment and maintenance costs constitute a key factor for cost-effective applications [12; 13; 14]. On the other hand, standard POFs' large sizes and high apertures allow transmission of millions of modes, giving rise to high modal dispersion that decreases bandwidth, and thus limits POF systems' capacities. In addition, PMMA has high diffusion coefficients that promote strong mode coupling with a negative impact on transmission properties [15]. Although the absolute minimum of PMMA's spectral attenuation is near 530 nm, POF links are often based in laser diodes emitting in the red region of the visible spectrum because of the wider choice of commercially available devices. The use of sources at other wavelengths for data transmission was reviewed in [6]. Those transmitters were mainly based on green emitting LEDs that profited from the lower attenuation of POFs in this region to increase the reach of relatively low-capacity links. More recently, the application of spectrally efficient modulation formats has boosted system capacity, which can be further increased with wavelength division multiplexing (WDM) schemes that transmit in green light, among other wavelengths [16; 17].