**1. Introduction**

Having its origin in the 1960s as well as the silica glass fiber, the polymer optical fiber (POF) stayed long in the shadow of the huge development and success of glass fiber communications. However, the advances in POF technology and the growing need for high-speed short-range communication networks make POF nowadays gain more and more importance. The key advantage of POF is a large core diameter. It makes POF tolerant to the fiber facet damages and relaxes the alignment tolerances, thus also reducing the installation costs. Furthermore, POF is pliable, durable, and inexpensive; offers small weight and short bend radius; allows easy installation, simple termination, and quick troubleshooting; and also provides the immunity to electromagnetic interference. Due to its diverse advantages, in shortrange applications POF established itself as a reasonable alternative to the traditional data communication media such as glass fibers, copper cables, and wireless systems (see **Table 1**).


**Table 1.**

*Comparison of different transmission media. Characteristics between very bad (−−) and particularly good (++) [1].*

Today, POF is produced with different core materials, core diameters, and index profiles. A comprehensive overview on various POFs is given in [1]. Two major POF types are made of polymethyl methacrylate (PMMA) and perfluorinated (PF) materials. The parameters of the common PMMA and PF POFs are specified in the IEC Standard 60793-2-40, which defines eight different POF classes [2]. The PMMA POF is produced with both step-index (SI) and graded-index (GI) profile, whereas the PF POF offers only GI profile. The GI profile of the core ensures high modal bandwidth exceeding 1.5 GHz × 100 m for the PMMA POF and 300 MHz × 1 km for the PF POF. However, the implementation of the PMMA GI-POF is confined to 500–680 nm wavelength range due to the high optical attenuation at other wavelengths (>400 dB/km). In contrast, the PMMA SI-POF suffers from intermodal dispersion limiting the bandwidth-length product to around 50 MHz × 100 m but also provides several attenuation windows in the visible spectrum (400–700 nm). Due to its advantages over the other POF types such as technological maturity, ease and cost of production, and high numerical aperture (NA), the standard 1 mm PMMA SI-POF (POF class A4a.2 according to IEC 60793-2-40) is the best known and by far the most widely employed type of POF. This is also the fiber this work concentrates on.

In vehicles SI-POF displaces copper in the network structure of a passenger cabin for multimedia data services. The infotainment communication system known as Media Oriented System Transport (MOST) connects different multimedia components in the SI-POF-based ring topology [3], as illustrated in **Figure 1**. The current (third) version of the MOST system (MOST150) supports the data transfer at 150 Mb/s over link lengths of about 10 m.

Another sector where SI-POF displaces traditional communication media are short-range networks in houses and offices. As an in-house extension of a broadband access network (e.g., VDSL, HFC, FTTB), the typical application of POF technology is the delivery of triple-play services (combination of broadcasting, telecommunication, and the Internet) to the end user. The Fast Ethernet transceivers (100 Mb/s) and since 2013 also the Gigabit Ethernet transceivers (1 Gb/s) are available on the market enabling the transmission of broadband services over 50 m SI-POF. The Gigabit solutions from KD-POF employing the multilevel signaling and from Teleconnect based on the multicarrier modulation are accompanied by the technical standards ETSI TS 105175-1-2 [4] and ITU-T G.9960, Annex F [5], respectively.

The commercial communication systems with SI-POF use a single channel for data transmission. However, the transmission performances of SI-POF are impaired by strong intermodal dispersion and high optical attenuation. Stimulated by the growing bandwidth demands (e.g., 10–40–100 Gb/s Ethernet speed), various concepts to overcome the low-pass characteristic of SI-POF have been successfully demonstrated over the last few years. The simplest solutions utilized passive

*Optoelectronic Key Elements for Polymeric Fiber Transmission Systems DOI: http://dx.doi.org/10.5772/intechopen.86423*

**Figure 1.** *SI-POF-based ring topology of a MOST system in a car.*

equalization implemented as an analog high-pass filter that increased the electrical −3 dB bandwidth of a channel [6]. A major focus was also placed on the digital signal processing techniques, which were mostly implemented offline due to the lack of commercial components. Both the non-return-to-zero (NRZ) and the spectrally efficient multilevel signaling were combined with the digital receiver equalization to increase the data rates over SI-POF [7, 8]. The sophisticated spectrally efficient multicarrier modulation formats were also successfully implemented to combat the highly dispersive SI-POF channel [9, 10].

Complying with any of the hitherto developments, utilization of several optical carriers for parallel transmission of data channels over a single fiber represents another alternative to increase the transmission capacity of SI-POF. The technique is well-known as wavelength division multiplexing (WDM). The principle of WDM is shown in **Figure 2**. Since different wavelengths λ1–λN do not interfere with each other in a linear medium, they can be used to simultaneously carry the data signals over a single fiber. Thereby, the capacity of a fiber, i.e., of an optical communication system, increases almost proportionally with the number of wavelength channels.

Two components are essential for WDM, a wavelength multiplexer and demultiplexer. The multiplexer combines the signals at different wavelengths, coming from different transmitters, onto a single fiber. On the opposite side of the optical link, the demultiplexer performs an inverse function, separating the wavelength channels to be detected by separate receivers.

**Figure 2.** *Principle of WDM: MUX, multiplexer; DEMUX, demultiplexer.*

The existing WDM components developed for single-mode glass fibers in the infrared region, such as Mach-Zehnder interferometers, arrayed waveguide gratings or fiber Bragg gratings, cannot be reused for a highly multimode SI-POF. On the other hand, the operating principles of demultiplexers based on thin-film interference filters and on a diffraction grating can be applied for POF. In spite of some other demultiplexing solutions (e.g., employing dispersion prisms), these two demultiplexing techniques have been recognized as the most promising for SI-POF. However, because of the difference in the operating wavelength range, fiber diameter, NA, etc., compared to the glass fibers, such demultiplexers must be newly designed for SI-POF communication. An overview of the state-of the-art thin-film interference filter-SI-POF demultiplexers will be given. The first aim of this work is to further investigate experimentally these demultiplexing techniques for SI-POF. Accordingly, the aim of this work is to demonstrate experimentally high-speed POF WDM data transmission offering capacity increase compared to the single-channel systems.
