**7. Discussion and conclusions**

27

26 Optical Fiber

(a) (b)

1 Fig. 21. Theoretical bit loading for the 4- λ WDM approach over PF-GIPOF. (a) 100m ; (b)

12345

**Figure 22.** Comparison of single channel operation and WDM extension over 62.5μm core diameter PF-GIPOFs.

4 On the other hand, capacity values for a 50µm core diameter PF-GIPOF following the same 5 procedure and under the same constraints are also shown (from its frequency response 6 measurements). Greater capacities can be achieved as increasing the core diameter due to 7 the presence of strong mode coupling effects and less modal noise effect, as shown in Fig.

> SC: Single channel WDM: WDM operation

10 Fig. 23. Comparison of single channel operation and WDM extension over a 100m- and

**Figure 23.** Comparison of single channel operation and WDM extension over a 100m- and 150m-long link, at 1300nm,

50µm

SC WDM

1 2 3 4

62.5µm

SC WDM

100m 150m

13 Applying WDM can further enhance the transmission capacity via plastic optical fiber (POF) 14 systems. This chapter is intented to bridge the gap of WDM POF-based networks for in-15 home deployments, where POFs have become a competitive and low-cost solution as a 16 physical medium infrastructure. In-home link lengths are relatively short thus leading to a 17 more relaxed requirements regarding bandwidth x length product and attenuation per unit 18 length, respectively. However, due to the continuous increase of bit-rate demands from end-19 users for multimedia services new techniques oriented to overcome the POF bandwidth 20 limitation are being required. Beyond complex modulation formats in which the main goal 21 is to provide a single channel communication link with a high spectral efficient (i.e. bit/Hz), 22 one potential solution to expand the usable bandwidth of POF systems is to perform 23 multiple channels over a single POF. This is known as the wavelength division multiplexing

25 Nowadays, WDM is well-established in the infrared transmission windows for silica optical 26 fibers, but this technique needs to be adapated to VIS for POFs due to their spectral 27 attenuation behavior. And novel WDM POF devices and network topologies are necessary 28 to a final success of POF in-home penetration. These devices include POF

11 150m-long link, at 1300nm, for different core diameter PF-GIPOFs.

0

25

50

75

100

BIT RATE (Gbps)

for different core diameter PF-GIPOFs.

125

150

175

12 7. Discussion and Conclusions

24 (WDM) approach.

1 Fig. 22. Comparison of single channel operation and WDM extension over 62.5µm core

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

On the other hand, capacity values for a 50μm core diameter PF-GIPOF following the same procedure and under the same constraints are also shown (from its frequency response measurements). Greater capacities can be achieved as increasing the core diameter due to the presence of strong mode coupling effects and less modal noise effect, as shown in Fig. 23.

25m 50m 75m 100m 150m

Single channel WDM extension

L=150m

Bit allocation

 (bits)

0 2 4 6 8 10 12 14 16

Frequency (GHz)

0 50 100 150 200 250

Subcarrier number

4 The corresponding aggregated WDM capacity is summarized in Fig. 22 and compared to 5 the single channel operation. The achievable capacity of a single-λ WDM system does not 6 reach the best single channel results. For a single channel operation more than twice the 7 capacity compared to the single-λ capacity in the WDM approach. Therefore, assuming a 4-λ 8 WDM system using the full available optical power and with similar bit rate transmission 9 performances in each channel the total achievable capacity would overcome the OSNR and 10 bit-rate limitation due to the optical losses introduced in the power budget of the system. It 11 is also noticed that for longer PF-GIPOF lengths the ratio between transmission capacities 12 for single channel and single- λ operation diminishes. This fact is attributed to differential 13 mode attenuation (DMA) together with mode coupling effects in PF-GIPOF that leads to a 14 sub-linear increase dependency of the fiber bandwidth regarding its length. This favours the

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

2 150m.

L=100m

Bit allocation

 (bits)

15 resulting transmission capacity.

BIT RATE (Gbps)

<sup>0</sup> <sup>2</sup> <sup>4</sup> <sup>6</sup> <sup>8</sup> <sup>10</sup> <sup>12</sup> <sup>14</sup> <sup>16</sup> <sup>4</sup>

Frequency (GHz)

0 50 100 150 200 250

Subcarrier number

3

16

3

9

8 23.

2 diameter PF-GIPOFs.

Applying WDM can further enhance the transmission capacity via POF systems. This chapter is intended to bridge the gap of WDM POF-based networks for in-home deployments, where POFs have become a competitive and low-cost solution as a physical medium infrastructure. In-home link lengths are relatively short thus leading to a more relaxed requirements regarding bandwidth x length product and attenuation per unit length, respectively. However, due to the continuous increase of bit-rate demands from end-users for multimedia services new techniques oriented to overcome the POF bandwidth limitation are being required. Beyond complex modulation formats in which the main goal is to provide a single channel commu‐ nication link with a high spectral efficient (i.e. bit/Hz), one potential solution to expand the usable bandwidth of POF systems is to perform multiple channels over a single POF. This is known as the WDM approach.

Nowadays, WDM is well-established in the infrared transmission windows for silica optical fibers, but this technique needs to be adapted to VIS for POFs due to their spectral attenuation behavior. And novel WDM POF devices and network topologies are necessary to a final success of POF in-home penetration. These devices include POF multiplexers/demultiplexers, variable optical attenuators, interleavers, switches, POFBGs and/or optical filters to separate and to route the different transmitted wavelengths. And an easy-reconfigurable performance can be an additional feature among all the spectrum of future designed and manufactured devices for the WDM POF solution, with the aim of increasing the flexibility of multiplexing, demul‐ tiplexing, switching and routing optical signals as well as modifying the optical network if required. Moreover, devices that can perform different functionalities are interesting in terms of reducing the power consumption and insertion losses. It is also important to follow the progression in stable and low cost light sources in the visible range, apart from 650nm, to successfully achieve the WDM POF implementation. And new devices, benefiting from nanoparticles principles to reach novel plasmonic switches among others.

Anyway, progresses in these POF devices for the WDM approach have been discussed. They can operate in the visible range as POFs do. Among the different technologies, within this chapter two multiplexers based on TN-LC have been introduced. The use of this technology has been demonstrated to provide several advantages as they do not have mobile parts, need low excitation voltages and have a low power comsuption. In addition, a reconfigurable optical multiplexer based on PDLC has been presented which can also acts as a variable optical attenuator. It has also been designed switches operating in a broadband range with a uniform spectral response and low thermal dependence. Birefringent structures are proved to be a versatile solution for designing devices for POF WDM networks due to their reconfiguration capacity, since they can be easily manufactured with LC technology, and flexibility, since any FIR filters synthesis method can be used.

On the other hand, the capabilities of novel POFBG devices to be compatible with WDM topologies for both sensing and communication schemes have been addressed. High scalability and power budget enhancement in comparison with all POF based network solutions is achieved due to the use of off-the shelf silica WDM devices in combination with POFBGs. Reaching distances of tens of km can be easily achieved with the proposed topology fully compliant with both short-reach networks (typically less than 1 km), i.e. LANs, inbuilding/in-home networks etc. as well as medium-reach distances (typically up to 10km) covering the access domain. The above reaching distances are unbeatable if an all-POF-based WDM optical network is intended to be deployed and a hybrid approach should be considered.

Finally, to mitigate both the impact of the high attenuation as well as the limited bandwidth of standard POFs recently developed PF-GIPOFs should be also considered. This fiber type outperforms these two features compared to their Step-Index and PMMA-based counterparts, respectively. Nevertheless its achievable capacity under the WDM-GIPOF approach must be analyzed. A future 4-λ WDM-GIPOF deployment is studied showing that its total achievable capacity can overcome the OSNR and the bit-rate limitation due to the optical losses introduced in the power budget of the system due to the addition of future GIPOF-based WDM devices.

We believe the results reported in this chapter may encourage the development of WDM-POF networks and low insertion loss POF-based WDM devices opening up the path for future inhome systems at very high bit rates. However, further improvements on mux/demux manu‐ facturing would make WDM-POF systems a future-proof and feasible solution for in-home/ building networks to attend end-users' high-speed demands.
