**Recent Advances in Wavelength-Division-Multiplexing Plastic Optical Fiber Technologies**

David Sánchez Montero, Isabel Pérez Garcilópez, Carmen Vázquez García, Pedro Contreras Lallana, Alberto Tapetado Moraleda and Plinio Jesús Pinzón Castillo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/59518

**1. Introduction**

[33] S.P. Jung et al., "Demonstration of RSOA-based WDM PON employing self-homo‐ dyne receiver with high reflection tolerance", Optical Fiber Communication Confer‐

[34] S. Straullu et al., "Compatibility between coherent reflective burst-mode PON and TWDM-PON physical layers", OPEX 22, Issue 1, pp. 9-14 (2014), http://dx.doi.org/

[35] K. Y. Cho et al., "10-Gb/s operation of RSOA for WDM PON," IEEE PHOTON.

[36] S. J. Savory, "Digital filters for coherent optical receivers," OPEX 16, 2, 804-817 (2008). [37] F. Vacondio et al., "Experimental demonstration of a PDM QPSK real-time burst mode coherent receiver in a packet switched network," ECOC, Tu.3.A.1, (2012). [38] F. Vacondio, et al. "Flexible TDMA Access Optical Networks Enabled by Burst-Mode Software Defined Coherent Transponders", ECOC, We.1.F.2, London, UK, (2013). [39] S. Straullu et al., "TWDM-PON-compatible 10 Gbps Burst-mode coherent reflective ONU achieving 31 dB ODN loss using DFB lasers", ECOC, Cannes, France (2014). [40] A. Naughton et al., "Optimisation of SOA-REAMs for hybrid DWDM-TDMA PON

[41] S. Straullu et al., "Reflective FDMA-PON with 32 Gbps upstream capacity per wave‐

[42] S. Abrate et. al., "FDMA-PON architecture according to the FABULOUS European Project" Proc. SPIE 8645, Broadband Access Communication Technologies VII,

[43] S. Menezo et al., "Reflective silicon Mach-Zehnder modulator with Faraday rotator

length and more than 32 dB ODN loss", ECOC, Cannes, France (2014).

mirror effect for self-coherent transmission", OFC/NFOEC 2013.

ence, OFC/NFOEC (2009).

TECHNOL. LETTER, 20, 18, 1533 – 1535 (2008).

386 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

applications", Optics Express, 19, 26 (2011).

10.1364/OE.22.000009

864504 (2013).

Growing research interests are focused on the high-speed telecommunications and data communications networks with increasing demand for accessing even from the home, due to the huge successes during the last decade of new multimedia services (high-definition (HD), three-dimensional visual information (3D) or remote "face-to-face communication") which forecast requirements for data transmission speed more than 40Gbps by 2020, which can be achievable only with optical network [1]. Regarding this data transmission capability, Polymer Optical Fiber (POF) technology has emerged as a useful medium for short-reach distances scenarios such as Local Area Networks (LANs), in-home and office networks, automotive and avionic multimedia buses or data center connections among others. However, its potential capacity for communication needs a greater exploitation to meet user requirements for higherdata rates.

The strong increase of bandwidth demand presents an increasing challenge for service operators to delivery their high-quality service to the end user's device. At this moment, commercially available progressive service plans range between the 50-100Mbps while premium services typically range around 100-150Mbps. And it should be reminded that the bandwidth in the local loop is forecasted to grow with an average of 20-50% annually. Recent interests are focused on gigabit-order data transmission, being desirable, at the same time, to introduce optical fiber networks even to the customer´s premises for covering more than 10Gbps in the near future, introducing the concept of FTTx (Fiber To The Home/Node/

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Building/Curb) deployments. There is a worldwide consensus that the optical fiber solution provides enough bandwidth to attend user's demand at the required transmission distances in the short-reach domain (typically up to 200m).

In this optical fiber network deployment scenario, POF offers several advantages over conventional silica multimode optical fiber over short distances. Such fiber type can provide an effective solution as its great advantage is the even potential lower cost associated with its easiness of installation, splicing and connecting. This is due to the fact that POF have higher dimensions, larger numerical aperture (NA) and larger critical curvature radius in comparison with glass optical fibers [2]. Moreover, it is more flexible and ductile, making it easier to handle. Consequently, POF termination can be realized not only faster but also cheaper than in the case of multimode silica optical fiber [3]. To summarize, POFs have multiple applications in sensor systems at low or competitive cost compared to the well–established conventional technologies [4].

To date, most used POF type is the step index POF (SI-POF) but many variants have been manufactured and tested showing different performances between them [5]. SI-POF is made of polymethyl-methacrylate (PMMA), (also called standard POF) and it has 980 μm core diameter, 10 μm cladding thickness and 0.5 NA. However, SI-POF suffers from high modal dispersion, which reduces the usable bandwidth to typically 50 MHz × 100 m [6]. And it is only used in the visible spectrum range (VIS), where it can provide acceptable attenuation (e.g. 100 dB/Km at 650 nm) [5]. This is because of the large attenuation due to the high harmonic absorption loss by carbon-hydrogen (C-H) vibration (C-H overtone). However, improvements in the bandwidth of POF fiber can be obtained by grading the refractive index, thus introducing the so-called Graded-Index POFs (GIPOFs). Although firstly developed PMMA-GIPOFs were demonstrated to obtain very high transmission bandwidth compared to that of SI counterparts [7], the use of PMMA is not still attractive due to its strong absorption at the near-infrared (near-IR) to infrared (IR) regions. As a result, PMMA-based GIPOFs can only be used at a few wavelengths in the visible portion of the spectrum. Today, unfortunately, almost all gigabit optical sources operate in the near-infrared (typically 850nm or 1300nm), where PMMA and similar polymers are essentially opaque. Reduction of loss has been achieved by using amorphous perfluorinated polymers for the core material. This new type of POF has been named perfluorinated GIPOF (PF GIPOF), and has a relative low loss wavelength region ranging from 650nm to 1300nm (even theoretically in the third transmission window) [8]. Consequently, available off-the-shelf light sources for silica fiber based systems can be used with PF-GIPOF systems. In 1998, the PF-based GIPOF had an attenuation of around 30dB/km at 1310nm. Attenuation around 20dB/km was achieved only three years after and lower and lower values of attenuation are being achieved. The theoretical limit of PF-based GIPOFs is ~0.5 dB/km at 1250-1390nm [9]. Nevertheless, although these losses are coming down steadily due to ongoing improvements in the production processes of this still young technology, the higher than silica attenuation inhibits their use in relative long link applications, being mainly driven for covering in-building optical networks link lengths for in-building/home optical networks (with link lengths less than 1 km), and thus the loss per unit length is of less importance. In addition, PF-GIPOF can provide a bandwidth per length product ~400MHz x km at both 850nm and 1300nm, respectively, and can support bit rates of 40Gbps up to 200m for any launch condition [10]. This fact is due to the PF-GIPOF low material dispersion characteristics (even lower compared to silica multimode optical fibers) [11].

Building/Curb) deployments. There is a worldwide consensus that the optical fiber solution provides enough bandwidth to attend user's demand at the required transmission distances

In this optical fiber network deployment scenario, POF offers several advantages over conventional silica multimode optical fiber over short distances. Such fiber type can provide an effective solution as its great advantage is the even potential lower cost associated with its easiness of installation, splicing and connecting. This is due to the fact that POF have higher dimensions, larger numerical aperture (NA) and larger critical curvature radius in comparison with glass optical fibers [2]. Moreover, it is more flexible and ductile, making it easier to handle. Consequently, POF termination can be realized not only faster but also cheaper than in the case of multimode silica optical fiber [3]. To summarize, POFs have multiple applications in sensor systems at low or competitive cost compared to the well–established conventional

To date, most used POF type is the step index POF (SI-POF) but many variants have been manufactured and tested showing different performances between them [5]. SI-POF is made of polymethyl-methacrylate (PMMA), (also called standard POF) and it has 980 μm core diameter, 10 μm cladding thickness and 0.5 NA. However, SI-POF suffers from high modal dispersion, which reduces the usable bandwidth to typically 50 MHz × 100 m [6]. And it is only used in the visible spectrum range (VIS), where it can provide acceptable attenuation (e.g. 100 dB/Km at 650 nm) [5]. This is because of the large attenuation due to the high harmonic absorption loss by carbon-hydrogen (C-H) vibration (C-H overtone). However, improvements in the bandwidth of POF fiber can be obtained by grading the refractive index, thus introducing the so-called Graded-Index POFs (GIPOFs). Although firstly developed PMMA-GIPOFs were demonstrated to obtain very high transmission bandwidth compared to that of SI counterparts [7], the use of PMMA is not still attractive due to its strong absorption at the near-infrared (near-IR) to infrared (IR) regions. As a result, PMMA-based GIPOFs can only be used at a few wavelengths in the visible portion of the spectrum. Today, unfortunately, almost all gigabit optical sources operate in the near-infrared (typically 850nm or 1300nm), where PMMA and similar polymers are essentially opaque. Reduction of loss has been achieved by using amorphous perfluorinated polymers for the core material. This new type of POF has been named perfluorinated GIPOF (PF GIPOF), and has a relative low loss wavelength region ranging from 650nm to 1300nm (even theoretically in the third transmission window) [8]. Consequently, available off-the-shelf light sources for silica fiber based systems can be used with PF-GIPOF systems. In 1998, the PF-based GIPOF had an attenuation of around 30dB/km at 1310nm. Attenuation around 20dB/km was achieved only three years after and lower and lower values of attenuation are being achieved. The theoretical limit of PF-based GIPOFs is ~0.5 dB/km at 1250-1390nm [9]. Nevertheless, although these losses are coming down steadily due to ongoing improvements in the production processes of this still young technology, the higher than silica attenuation inhibits their use in relative long link applications, being mainly driven for covering in-building optical networks link lengths for in-building/home optical networks (with link lengths less than 1 km), and thus the loss per unit length is of less importance. In addition, PF-GIPOF can provide a bandwidth per length product ~400MHz x

in the short-reach domain (typically up to 200m).

388 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

technologies [4].

Although POFs reveal a cost effective solution for short-reach optical deployments, their bandwidth characteristics still limit the reach distances and the capacity to attend future end users' transmission requirements. These facts hamper the desired integration of multiple broadband services into a common multimode fiber access or in-building/home network. Overcoming the bandwidth limitation of such fibers requires the development of techniques oriented to extend the capabilities of POF networks to attend the consumer's demand for multimedia services. Different efficient and advanced modulation formats and/or adaptive electrical equalization schemes can alternatively be applied. Considering the industry's extensive experience and the large economies of scale, orthogonal frequency division multi‐ plexing (OFDM) [12], subcarrier multiplexing (SCM) [13] and discrete multitone modulation (DMT) [14] are seen as promising technologies for low-cost, reliable, and robust Gigabit transmission through hundreds of meters of POF. Particularly, DMT modulation has been demonstrated to achieve near-optimum performance and to enable highly spectral efficient transmission at high bit-rates over silica multimode fibers (MMFs) and POFs [15, 16]. Initially, transmission with SI–POF has been realized with only one channel, typically at 650 nm, reaching data rates of 100 Mb/s over links of 275 m [17]; even multi−gigabit transmission over links of 50 m has been reached [18]. Commercial systems with data rates of 1 Gbit/s via up to 50 m of SI−POF with a single channel have also been reported [19]. Moreover, theoretical simulations with data rates of 1.25 Gbit/s, 2.1 Gbit/s [20] and 6.2 Gbit/s [21], via up to 50 m of SI-POF using a single channel with NRZ, CAP-64 and QAM512 modulations, respectively, have been demonstrated.

After exploiting the capabilities of a single channel, the next step to increase the capacity of an individual POF is to use multiple channels over a single fiber what is well known as wave‐ length division multiplexing (WDM). In the last years, WDM techniques over POFs are being proposed to expand the usable bandwidth of POF-based systems. For instance, Jončić *et al.* [22] firstly reported a 10 Gbit/s transmission over 25 m of SI−POF using offline-processed NRZ modulation. Beyond that, same authors achieved data rates up to 14.77 Gbit/s, with 4 channels via up to 50 m of a SI−POF link using offline-processed discrete multitone modulation [23]. In the same way, many POF based sensors implement self–referencing schemes by transmitting different wavelengths over a single fiber [4]. However, there are some constrains that must be addressed in order to perform the same capabilities as in the case of silica-based WDM approaches. In the WDM technique, different wavelengths which are jointly transmitted over the fiber must be separated to regain all information. Therefore, for a typical WDM optical communication link two key-elements are, at the very least, indispensable and have to be introduced, a multiplexer and a demultiplexer. The former is placed before the single fiber to integrate every wavelength to a single waveguide. The latter is placed behind the fiber lead to regain every discrete wavelength. These two components have long been established for silicabased infrared telecom systems, bust must be developed completely new for POF-based WDM applications. The most common and grave disadvantage almost all of these approaches exhibit is their costly production, which makes them unsuitable for today's price sensitive mass markets. The underlying reason behind this lack of development is the mismatch between the optimum operating wavelength regions of POFs and the optical devices exploited for tele‐ communications purposes. The latter are developed for a wavelength region (C- and L-bands) totally unsuitable for POF-based transmission over medium-distances (hundreds of meters or greater) due to the high attenuation of PMMA based POF of around 1dB/cm@1550nm. A similar conclusion can be obtained for PF-GIPOFs which attenuation characteristics are not at par with that of standard silica based fibers, but still superior to that of copper based technol‐ ogies and PMMA-POF fibers. Another question to be addressed is the large POF dimensions and NA, which produce beams with high divergence thus being difficult to be routed. In addition, multiplexed systems operating in VIS range for POF networks may need reconfigu‐ ration because they do not have standard channels as well as provide flexibility in the networks to be developed.

In this framework, this chapter is intended to be a progress report and it will focus on the stateof-the art, description and experimental validation of different POF-based key devices that provide an easy-reconfigurable performance for WDM applications. Novel multiplexers/ demultiplexers, variable optical attenuators, interleavers, switches and optical filters to separate and to route the different transmitted wavelengths are described. The main target is to bridge the gap of the WDM POF-based network deployment bottleneck in the final leg of delivering. In addition, a hybrid silica-POF WDM-PON network is analyzed showing the capabilities of novel Fiber Bragg Gratings (FBG) inscribed on microestructured POF devices to be compatible with WDM topologies for both sensing and communication schemes. Moreover, the theoretical capacity for a future WDM-GIPOF deployment is addressed taking advantage of the performance of this recent fiber type. Finally the main conclusions are presented.
