**5. WDM approach using Polymer Optical Fiber Bragg Gratings (POFBGs)**

As previously stated, one solution to increase the POF's bandwidth, and thus its capacity, is the transmission of information over more than just a single wavelength. This is what is known the WDM approach. This architecture not only has been proved to be suitable for transmission information purposes but also has been demonstrated to be fully compatible with the inter‐ rogation of multiple remotely located intensity-based optical fiber sensors, thus taking advantage of the power loss reduction as well as the high scalability provided by the use of WDM devices.

The basis of WDM systems is the spectral characteristics of the optical multiplexers and demultiplexers which are used in the fiber plant instead of optical power splitters. Moreover, within these approaches, FBGs are usually employed both for monitoring purposes or providing an effective and compact strategy to operate in reflective configuration [77, 78]. These devices are a wavelength-selective filter fabricated inside the core of an optical fiber for which the reflected wavelength changes under the influence of external perturbations [79]. First FBGs were traditionally manufactured on silica optical fiber. More recently, FBGs have been inscribed into SI-POF [80] and microstructured Polymer Optical Fiber (mPOF) [81] based on PMMA, leading to what is called polymer optical fiber Bragg gratings (POFBGs). The reason for this development is the exploitation of polymer benefits such as larger elastic limit, higher maximum strain limit, larger temperature and humidity responses and low cost compared to silica, while maintaining the benefits of FBGs. And this fact is also true when considering FBGs as optical sensing elements [82, 83]. In addition, polymer reveals to be intrinsically more biocompatible than silica for as it may be used for *in vivo* biomedical applications where the use of glass is inappropriate due to danger from breakages. Nevertheless, limited effort has been directed towards synergizing biocompatible POF-based photonic sensing with the WDM interrogation method that allows multiplexing by the use of FBGs, with just a few exceptions [84]. The main underlying reason behind this lack of development is the mismatch between the optimum operating wavelength regions of POFs and the optical devices exploited for telecommunications purposes as aforementioned at the beginning of this chapter.

In this section, we intend to bridge the gap between a WDM compatible topology and the use of novel POFBGs by analyzing the feasibility of a hybrid silica-POF WDM network for remotely addressing multiple intensity-based self-referenced fiber-optic sensors. The proposed topol‐ ogy is compatible with the target of developing a single optical broadband network architec‐ ture which is capable of carrying many types of services without mutual interference nor design compromises. It may include the access network domain (e.g. FTTx) as well as up to the indoor scenario with the aim of a full converged network solution. This solution will open up the path for the development of converged WDM POF communication networks in the near future. Moreover, potential medical environments and biomedical applications based on all-optical POF-based solutions are also targeted taking advantage of the intrinsic POF biocompatible characteristics.

The proposed self-referenced hybrid topology above described is illustrated in Fig. 17. The novelties of this configuration in comparison with previous works [78, 85] are: a) the combi‐ nation of silica- and polymer- FBGs (the latter may be used for *in vivo* scenarios or just simply at the patient's vicinity if the target is a biocompatible optical system for medical applications); b) the usage of a single reference FBG; and c) an improved centralized monitoring unit (that can be remotely located up to units of km) which includes virtual instrumentation techniques and data processing. A broadband light source (BLS) is, either internally or externally, modulated at a single frequency ( *f* ). This modulated signal is launched into the remote sensing points via a broadband circulator and a Coarse Wavelength-Division Multiplexer (CWDM). Each remote sensing point located consists of a sensing POFBG placed after the fiber-optic sensor (FOS). A single silica FBG is located before the CWDM acting as a reference channel for the topology. Let assume the central wavelengths of the reference and sensing FBGs to be λSi and λPOF, respectively. The broadband optical circulator receives the reflected multiplexing signals from the reference and the sensor channels, in which the sensor information is encoded. At the remote monitoring unit, the optical signal is demultiplexed by a CWDM device and distributed to an array of photodetectors (PD) by means of a data acquisition board (DAQ) together with a band-pass filter (BPF), used to eliminate noise from all signals at frequencies outside the system frequency. Then, a phase-shift is applied to the reference and sensor digital signals. Finally, a virtual lock-in amplifier is used to interrogate all available sensor channels. A measurement parameter can be defined, *φ<sup>K</sup>* , corresponding to the output phase of the signal for different phase-shifts (virtual delays) at the reception stage, see Eq. 1.

rogation of multiple remotely located intensity-based optical fiber sensors, thus taking advantage of the power loss reduction as well as the high scalability provided by the use of

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

The basis of WDM systems is the spectral characteristics of the optical multiplexers and demultiplexers which are used in the fiber plant instead of optical power splitters. Moreover, within these approaches, FBGs are usually employed both for monitoring purposes or providing an effective and compact strategy to operate in reflective configuration [77, 78]. These devices are a wavelength-selective filter fabricated inside the core of an optical fiber for which the reflected wavelength changes under the influence of external perturbations [79]. First FBGs were traditionally manufactured on silica optical fiber. More recently, FBGs have been inscribed into SI-POF [80] and microstructured Polymer Optical Fiber (mPOF) [81] based on PMMA, leading to what is called polymer optical fiber Bragg gratings (POFBGs). The reason for this development is the exploitation of polymer benefits such as larger elastic limit, higher maximum strain limit, larger temperature and humidity responses and low cost compared to silica, while maintaining the benefits of FBGs. And this fact is also true when considering FBGs as optical sensing elements [82, 83]. In addition, polymer reveals to be intrinsically more biocompatible than silica for as it may be used for *in vivo* biomedical applications where the use of glass is inappropriate due to danger from breakages. Nevertheless, limited effort has been directed towards synergizing biocompatible POF-based photonic sensing with the WDM interrogation method that allows multiplexing by the use of FBGs, with just a few exceptions [84]. The main underlying reason behind this lack of development is the mismatch between the optimum operating wavelength regions of POFs and the optical devices exploited for

telecommunications purposes as aforementioned at the beginning of this chapter.

In this section, we intend to bridge the gap between a WDM compatible topology and the use of novel POFBGs by analyzing the feasibility of a hybrid silica-POF WDM network for remotely addressing multiple intensity-based self-referenced fiber-optic sensors. The proposed topol‐ ogy is compatible with the target of developing a single optical broadband network architec‐ ture which is capable of carrying many types of services without mutual interference nor design compromises. It may include the access network domain (e.g. FTTx) as well as up to the indoor scenario with the aim of a full converged network solution. This solution will open up the path for the development of converged WDM POF communication networks in the near future. Moreover, potential medical environments and biomedical applications based on all-optical POF-based solutions are also targeted taking advantage of the intrinsic POF

The proposed self-referenced hybrid topology above described is illustrated in Fig. 17. The novelties of this configuration in comparison with previous works [78, 85] are: a) the combi‐ nation of silica- and polymer- FBGs (the latter may be used for *in vivo* scenarios or just simply at the patient's vicinity if the target is a biocompatible optical system for medical applications); b) the usage of a single reference FBG; and c) an improved centralized monitoring unit (that can be remotely located up to units of km) which includes virtual instrumentation techniques and data processing. A broadband light source (BLS) is, either internally or externally, modulated at a single frequency ( *f* ). This modulated signal is launched into the remote sensing

WDM devices.

biocompatible characteristics.

$$\varphi\_K = \tan^{-1} \left[ \frac{\cdot \left( \sin \, \theta\_{Si} \star \beta\_k \bullet \sin \, \theta\_{POF\_k} \right)}{\cos \, \theta\_{Si} + \beta\_k \bullet \cos \, \theta\_{POF\_k}} \right] \tag{1}$$

where *θSi* and *θPOF <sup>k</sup>* are, respectively, the phase shifts for the reference and each sensor signal *k*, and *β<sup>k</sup>* is relates to the optical power received at the remote central unit being a function of the modulation index, the reflectivity of the silica FBG and the photodetector responsivity (for both sensing and reference wavelengths), the sensor power loss modulation, the insertion loss of the CWDM device, and the insertion loss related to the reflectivity, intrinsic attenuation and connectorization of the POFBGs. Further details of the mathematical framework can be seen in the works reported in [78, 86]. Parameter *φK* is insensitive to power fluctuations except for the sensor modulation thus performing as a self-reference measurement parameter. Its performance, i.e. linear behavior, maximum sensitivity, etc., is directly related to the digital phase-shifts applied at reception.

To test the feasibility of the topology shown in Fig. 17, a 2-sensor network was analyzed by modulating the BLS at f=1kHz by an acousto-optic modulator. The optical power was launched into the configuration via a broadband circulator. One silica FBG was used for reference purpose, being placed before the CWDM mux/demux. Its central wavelength and reflectivity were λSi=1550nm and 49%, respectively. A POFBG in 150μm cladding diameter few-moded mPOF was used for each remote sensing point, with central wavelengths λPOF1 =1525.2nm for FOS1 and λPOF2 =1567.0nm for FOS2. Their reflectivities were 27 % and 36 %, respectively. A singlemode VOA was used to emulate the sensor response and for calibration purposes. The reflected signals were demultiplexed by a CWDM and detected by three amplified InGaAs detectors. The amplifier gain was fixed at 70 dB for all measurements. A 14-bit low-cost DAQ was used to convert the electrical signals from the photodetectors to digital signals. Virtual instrumentation techniques were developed to implement the bandpass filter, the phase-shifts and the lock-in amplifiers at the reception stage.

**Figure 17.** Hybrid silica-POF WDM self-referenced topology for remotely addresing generic remote sensing points lo‐ cated at the patient's vicinity. Fiber-optic sensor (FOS).

The self-reference property was tested inducing power fluctuations in the modulated optical source through a VOA. Fig. 18 showed no changes in *φK* values after inducing 10 dB of power attenuation. It is worth mentioning that the proposed topology performs no noticeable crosstalk between adjacent channels. This means that both (or more) sensors could be inter‐ rogated simultaneously without mutual interference because of the high channel isolation of the CWDM demultiplexer. Experiments were carried out for the following phase shifts at the reception stage: *θSi* =0.83*π*, *θPOF* <sup>1</sup> =0.33*π*.

The performance of the proposed topology was further investigated obtaining resolution values far below than that of provided by most of the POF intensity based sensing solution reported in literature, and particularly for biomedical applications. Another interesting point is computing the power budget, which provides information about the maximum remote interrogation reaching distance and/or the maximum insertion losses only for sensing pur‐ poses. At the most restrictive sensing wavelength (in terms of reaching distance), a maximum length of 11 km could be obtained. For this calculation, a FOS power variation of 6 dB was considered, high enough to cover any biomedical input magnitude span. However, this reach distance can be easily improved by launching more optical power into the system, using optical devices with better insertion loss performance or using a more efficient technique to connect POFBGs. The aforementioned reaching distance could provide a remote monitoring service unit fully compliant for both short-reach networks (typically less than 1 km), i.e. LANs and inbuilding/in-hospital networks as well as suitable for medium reach-distances (typically up to 10 km). Furthermore, the latter value ensures applications in inter-hospital networks or to provide a convergent all-optical and straightforward connection between patient's homes and

a general practice service for telemedicine purposes. It should be mentioned that the above reaching distances are unbeatable if an all-POF-based optical network is intended to be deployed and a hybrid approach should be considered. Following this analysis, it can be concluded that the proposed topology do not provide limitations thus serving as the bottleneck of a multiple remote sensing scheme. 14 and straightforward connection between patient's homes and a general practice service for 15 telemedicine purposes. It should be mentioned that the above reaching distances are 16 unbeatable if an all-POF-based optical network is intended to be deployed and a hybrid 17 approach should be considered. Following this analysis, it can be concluded that the 18 proposed topology do not provide limitations thus serving as the bottleneck of a multiple

13 value ensures applications in inter-hospital networks or to provide a convergent all-optical

22 Optical Fiber

1 reported in literature, and particularly for biomedical applications. Another interesting 2 point is computing the power budget, which provides information about the maximum 3 remote interrogation reaching distance and/or the maximum insertion losses only for 4 sensing purposes. At the most restrictive sensing wavelength (in terms of reaching distance), 5 a maximum length of 11 km could be obtained. For this calculation, a FOS power variation 6 of 6 dB was considered, high enough to cover any biomedical input magnitude span. 7 However, this reach distance can be easily improved by launching more optical power into 8 the system, using optical devices with better insertion loss performance or using a more 9 efficient technique to connect POFBGs. The aforementioned reaching distance could provide 10 a remote monitoring service unit fully compliant for both short-reach networks (typically

21 Fig. 18. Output phase � self-reference test versus power fluctuations for different values of 22 sensor losses at the remote sensing point addressed by λPOF 1. **Figure 18.** Output phase *φ*<sup>1</sup> self-reference test versus power fluctuations for different values of sensor losses at the re‐ mote sensing point addressed by λPOF 1.

#### 23 6. WDM extension over PF GIPOF links **6. WDM extension over PF GIPOF links**

19 remote sensing scheme.

20

The self-reference property was tested inducing power fluctuations in the modulated optical source through a VOA. Fig. 18 showed no changes in *φK* values after inducing 10 dB of power attenuation. It is worth mentioning that the proposed topology performs no noticeable crosstalk between adjacent channels. This means that both (or more) sensors could be inter‐ rogated simultaneously without mutual interference because of the high channel isolation of the CWDM demultiplexer. Experiments were carried out for the following phase shifts at the

**Figure 17.** Hybrid silica-POF WDM self-referenced topology for remotely addresing generic remote sensing points lo‐

DPOF k

DSi

**<sup>+</sup>** LOCK-IN AMPLIFIER

λSi=1550 nm

Optical Electrical Virtual

CWDM

CWDM

PD λPOF k

ADQ

DIGITAL PROCESSING

λPOF 1 λSi

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

PD

DPOF k DSi

VPOF 1 VPOF k VSi

PD

**FOS**<sup>k</sup>

**FOS**<sup>1</sup>

φPOF k

**REMOTE CENTRAL UNIT**

λPOF k=1567 nm

λPOF 1=1525 nm

**REMOTE SENSING POINTS**

The performance of the proposed topology was further investigated obtaining resolution values far below than that of provided by most of the POF intensity based sensing solution reported in literature, and particularly for biomedical applications. Another interesting point is computing the power budget, which provides information about the maximum remote interrogation reaching distance and/or the maximum insertion losses only for sensing pur‐ poses. At the most restrictive sensing wavelength (in terms of reaching distance), a maximum length of 11 km could be obtained. For this calculation, a FOS power variation of 6 dB was considered, high enough to cover any biomedical input magnitude span. However, this reach distance can be easily improved by launching more optical power into the system, using optical devices with better insertion loss performance or using a more efficient technique to connect POFBGs. The aforementioned reaching distance could provide a remote monitoring service unit fully compliant for both short-reach networks (typically less than 1 km), i.e. LANs and inbuilding/in-hospital networks as well as suitable for medium reach-distances (typically up to 10 km). Furthermore, the latter value ensures applications in inter-hospital networks or to provide a convergent all-optical and straightforward connection between patient's homes and

reception stage: *θSi* =0.83*π*, *θPOF* <sup>1</sup> =0.33*π*.

cated at the patient's vicinity. Fiber-optic sensor (FOS).

VRef

BLS AOM

VRef

Pin

24 In this section a comparison between the achievable capacity over a single fiber channel and 25 over a WDM PF-GIPOF-based approach will be presented. It has been previously stated that 26 a typical WDM optical communication link requires both a multiplexer and a demultiplexer. 27 However, the addition of POF-WDM multiplexer and demultiplexer devices, results in a 28 bit-rate penalty as the available optical power on the system decreases due to their insertion In this section a comparison between the achievable capacity over a single fiber channel and over a WDM PF-GIPOF-based approach will be presented. It has been previously stated that a typical WDM optical communication link requires both a multiplexer and a demultiplexer. However, the addition of POF-WDM multiplexer and demultiplexer devices, results in a bitrate penalty as the available optical power on the system decreases due to their insertion losses. To establish the channel capacity comparative, a bit loading algorithm for DMT modulation format over PF-GIPOF has been considered. New power margin resulting from the additional losses considered in the system due to the WDM over POF approach are analyzed demon‐ strating the feasibility of PF-GIPOF WDM systems.

> The resulting theoretical Shannon capacity of an optical fiber channel can be calculated if its *f*3dB is known [14], modeled as a Gaussian low-pass filter. Therefore, from measure‐ ments of frequency response of different PF-GIPOF lengths, 3dB bandwidths can be obtained, and so their theoretical capacity limits operating in a single channel. This type of

fiber has been demonstrated to enable robust 2GbE (GbE, Gigabit Ethernet) and 10GbE baseband transmission over short reach distances ranging from 25m up to 100m for different link scenarios [87], even at OverFilled Launching (OFL) condition. These values are considered as an underneath estimation of the transmission limit of PF-GIPOFs as com‐ plex modulation formats, restricted mode launching schemes, equalization techniques or simultaneous data transmission over high-order latent PF-GIPOF passbands can be applied to enhance its aggregated capacity [88-90]. 14 transmission over high-order latent PF-GIPOF passbands can be applied to enhance its 15 aggregated capacity [88-90]. 16 Previous works have studied, analyzed and modeled the PF-GIPOF frequency response 17 taking into account most of the parameters that affect the latter. Some noteworthy PF-GIPOF

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

1 losses. To establish the channel capacity comparative, a bit loading algorithm for discrete

2 multitone (DMT) modulation format over PF-GIPOF has been considered. New power

3 margin resulting from the additional losses considered in the system due to the WDM over

5 The resulting theoretical Shannon capacity of an optical fiber channel can be calculated if its

6 f3dB is known [14], modeled as a Gaussian low-pass filter. Therefore, from measurements of

7 frequency response of different PF-GIPOF lengths, 3dB bandwidths can be obtained, and so

8 their theoretical capacity limits operating in a single channel. This type of fiber has been

9 demonstrated to enable robust 2GbE (GbE, Gigabit Ethernet) and 10GbE baseband

10 transmission over short reach distances ranging from 25m up to 100m for different link

11 scenarios [87], even at OverFilled Launching (OFL) condition. These values are considered

13 formats, restricted mode launching schemes, equalization techniques or simultaneous data

4 POF approach are analyzed demonstrating the feasibility of PF-GIPOF WDM systems.

23

Previous works have studied, analyzed and modeled the PF-GIPOF frequency response taking into account most of the parameters that affect the latter. Some noteworthy PF-GIPOF frequency response measurements are shown in Fig. 19, in which a good agreement between experimental results and the theoretical curves predicted by the model is observed. This figure shows the measured and theoretical frequency responses for a 50m, 75m and 100m-long 62.5μm core diameter PF-GIPOF link with OFL condition and employing a Fabry-Perot laser source operating at 1300nm. Further details of the mathematical framework and experiments are reported on [91] for the benefit of the readers. 18 frequency response measurements are shown in Fig. 19, in which a good agreement 19 between experimental results and the theoretical curves predicted by the model is observed. 20 This figure shows the measured and theoretical frequency responses for a 50m, 75m and 21 100m-long 62.5µm core diameter PF-GIPOF link with OFL condition and employing a 22 Fabry-Perot laser source operating at 1300nm. Further details of the mathematical 23 framework and experiments are reported on [91] for the benefit of the readers.

25 Fig. 19. Measured (solid line) and theoretical (dashed line) electrical responses for different 26 62.5 µm core diameter PF-GIPOF lengths. **Figure 19.** Measured (solid line) and theoretical (dashed line) electrical responses for different 62.5 μm core diameter PF-GIPOF lengths.

24

27 From frequency response measurements, as shown in the above figure, the 3dB 28 baseband bandwidth can be easily identified and, therefore, the channel capacity can be 29 calculated. The PF-GIPOFs used are commercially available from Chromis Fiber with an From frequency response measurements, as shown in the above figure, the 3dB baseband bandwidth can be easily identified and, therefore, the channel capacity can be calculated. The PF-GIPOFs used are commercially available from Chromis Fiber with an attenuation of 55dB/km at 1300nm. For the frequency response measurements, a FP laser diode used as transmitter was externally AM modulated with a RF sinusoidal signal (up to 20GHz of

30 attenuation of 55dB/km at 1300nm. For the frequency response measurements, a FP laser

31 diode used as transmitter was externally AM modulated with a RF sinusoidal signal (up to

32 20GHz of modulation bandwidth) by means of an electro-optic (E/O) Mach-Zehnder

33 modulator (16GHz bandwidth). An InGaAs-photodetector (22GHz bandwidth) is used as

14 transmission over high-order latent PF-GIPOF passbands can be applied to enhance its 16 Previous works have studied, analyzed and modeled the PF-GIPOF frequency response 17 taking into account most of the parameters that affect the latter. Some noteworthy PF-GIPOF 18 frequency response measurements are shown in Fig. 19, in which a good agreement modulation bandwidth) by means of an E/O Mach-Zehnder modulator (16GHz band‐ width). An InGaAs-photodetector (22GHz bandwidth) is used as receiver. Bandwidth limitation from both transmitter and receiver can be neglected for links >50m. The channel capacity for each length, is calculated based on the transmission characteristics listed below, and is displayed in Table 1. For some applications, e.g. home network Ethernet transceiv‐ ers, eye safety operation is required and a limited averaged transmitted optical power of 0dBm has been considered.


**Table 1.** Calculated theoretical capacity over 62.5μm core diameter PF-GIPOF, at 1300nm.

23

fiber has been demonstrated to enable robust 2GbE (GbE, Gigabit Ethernet) and 10GbE baseband transmission over short reach distances ranging from 25m up to 100m for different link scenarios [87], even at OverFilled Launching (OFL) condition. These values are considered as an underneath estimation of the transmission limit of PF-GIPOFs as com‐ plex modulation formats, restricted mode launching schemes, equalization techniques or simultaneous data transmission over high-order latent PF-GIPOF passbands can be applied

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

1 losses. To establish the channel capacity comparative, a bit loading algorithm for discrete

2 multitone (DMT) modulation format over PF-GIPOF has been considered. New power

3 margin resulting from the additional losses considered in the system due to the WDM over

5 The resulting theoretical Shannon capacity of an optical fiber channel can be calculated if its

6 f3dB is known [14], modeled as a Gaussian low-pass filter. Therefore, from measurements of

7 frequency response of different PF-GIPOF lengths, 3dB bandwidths can be obtained, and so

8 their theoretical capacity limits operating in a single channel. This type of fiber has been

9 demonstrated to enable robust 2GbE (GbE, Gigabit Ethernet) and 10GbE baseband

10 transmission over short reach distances ranging from 25m up to 100m for different link

11 scenarios [87], even at OverFilled Launching (OFL) condition. These values are considered

12 as an underneath estimation of the transmission limit of PF-GIPOFs as complex modulation

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

13 formats, restricted mode launching schemes, equalization techniques or simultaneous data

30 attenuation of 55dB/km at 1300nm. For the frequency response measurements, a FP laser

<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>18</sup> <sup>20</sup> -18

**Figure 19.** Measured (solid line) and theoretical (dashed line) electrical responses for different 62.5 μm core diameter

From frequency response measurements, as shown in the above figure, the 3dB baseband bandwidth can be easily identified and, therefore, the channel capacity can be calculated. The PF-GIPOFs used are commercially available from Chromis Fiber with an attenuation of 55dB/km at 1300nm. For the frequency response measurements, a FP laser diode used as transmitter was externally AM modulated with a RF sinusoidal signal (up to 20GHz of

Frequency (GHz)

L=75m L=100m

L=50m

31 diode used as transmitter was externally AM modulated with a RF sinusoidal signal (up to

32 20GHz of modulation bandwidth) by means of an electro-optic (E/O) Mach-Zehnder

33 modulator (16GHz bandwidth). An InGaAs-photodetector (22GHz bandwidth) is used as

4 POF approach are analyzed demonstrating the feasibility of PF-GIPOF WDM systems.

Previous works have studied, analyzed and modeled the PF-GIPOF frequency response taking into account most of the parameters that affect the latter. Some noteworthy PF-GIPOF frequency response measurements are shown in Fig. 19, in which a good agreement between experimental results and the theoretical curves predicted by the model is observed. This figure shows the measured and theoretical frequency responses for a 50m, 75m and 100m-long 62.5μm core diameter PF-GIPOF link with OFL condition and employing a Fabry-Perot laser source operating at 1300nm. Further details of the mathematical framework and experiments

to enhance its aggregated capacity [88-90].

15 aggregated capacity [88-90].

26 62.5 µm core diameter PF-GIPOF lengths.

24



Normalized

PF-GIPOF lengths.

frequency

response, dBe






Theoretical Experimental However, this capacity analysis from the frequency response may result in large discrepancies at frequencies beyond the 3dB point and the PF-GIPOF has some latent high-order passbands [88]. Consequently, the Gaussian low-pass approximation reveals itself as a pessimistic approximation of the PF-GIPOF channel capacity, and expected capacity values of PF-GIPOF can be larger than those calculated in Table 1, even more if Restricted Mode Launching (RML) schemes are applied to the injection of light into the fiber.

25 Fig. 19. Measured (solid line) and theoretical (dashed line) electrical responses for different 27 From frequency response measurements, as shown in the above figure, the 3dB 28 baseband bandwidth can be easily identified and, therefore, the channel capacity can be 29 calculated. The PF-GIPOFs used are commercially available from Chromis Fiber with an On the other hand, DMT allows the possibility to allocate the number of bits and energy per subcarrier according to its corresponding signal-to-noise ratio (SNR), typically known as bitloading. To compute rate-adaptive bit-loading for the DMT over PF-GIPOF consideration Chow's algorithm has been implemented [92]. Initially, all subchannels were loaded with 4 information bits each. Table 2 shows the theoretical results on capacity when applying DMT over a single channel, based on the measured frequency response values up to 150m-long PF-GIPOFs. Compared to the results given in Table 1, it can be seen that for the shortest length (25m), the numerically computed capacity value is lower than theoretical counterpart. This result from bandwidth limitation of the external modulator bandwidth, considered in the computation. From lengths > 50m, the computed capacity is larger because the PF-GIPOF frequency response dominates over other bandwidth limitation factors. Due to the bandwidth limitation of the PF-GIPOF link itself, the signal-to-noise-ratio decreases for higher frequencies. Bit allocation resulting from the bit loading is shown in Fig. 20 for a 100m- and 150m-long PF-GIPOF single channel link, respectively.


Recent Advances in Wavelength-Division-Multiplexing Plastic Op **Table 2.** Theoretical DMT capacity over 62.5μm core diameter PF-GIPOF, at 1300nm. tical Fiber Technologies

Table 2.- Theoretical DMT capacity over 62.5µm core diameter PF-GIPOF, at 1300nm.

> 25m 577.1 50m 380.0 75m 276.9 100m 164.1 150m 77.1

> > Note: targeted BER=10-3

Numerical DMT over PF-GIPOF Capacity (Gbps)

Length (m)

8 capacity.

24 approach.

Fig. 20. Bit allocation per subchannel, resulting from bit-**Figure 20.** Bit allocation per subchannel, resulting from bit-loading at 100m and 150m.

5 POF WDM multiplexers and demultiplexers limits the available optical power budget 6 within the fiber link thus resulting in a bit-rate penalty. This is due to the fact that the OSNR 7 (Optical Signal-to-Noise ratio) of the system is being reduced, and so the fiber transmission

9 To establish a comparison between the PF-GIPOF single channel operation and its WDM 10 extension a 4-λ WDM approach has been considered. Regarding the latter, the PF-GIPOF 11 transmission capacity must be recalculated from: a) the new bit loading resulting from the 12 DMT modulation scheme and, b) the restriction on power margin resulting from the new 13 losses considered in the system due to the addition of the mux/demux devices in the optical 14 link. An insertion loss for a future PF-GIPOF 4-λ multiplexer/demultiplexer device of 15 around 2dB per channel [93, 94] is considered. Such a performance is better in terms of 16 insertion loss with respect to PMMA-GIPOF based splitters which can provide insertion 17 losses greater than 6dB (in the symmetric-case) [95]. Consequently, the power budget of the 18 WDM system, consisting of one PF-GIPOF based multiplexer or demultiplexer device at 19 each side of the optical fiber link, results in a 5dB power reduction per channel, if an optical 20 crosstalk of 1dB is also considered. It is worth mentioning that some authors have evaluated 21 power penalties close to 2.4dB when combining a 62.5µm core diameter PF-GIPOF and 22 WDM devices based on 50µm core diameter MMF [90]. Set of Fig. 21 shows the theoretical 23 bit loading including the aforementioned restriction in power budget due to the 4- λ WDM

loading at 100m and 150m. 1 It has been previously stated that for flexible high capacity GIPOF optical networks, 2 applying the WDM approach seems to be necessary. Apart from the physical transmission 3 characteristics of the PF-GIPOF, it is equally important to consider the optical components 4 introduced to deploy advanced WDM-based optical architectures. The addition of these It has been previously stated that for flexible high capacity GIPOF optical networks, applying the WDM approach seems to be necessary. Apart from the physical transmission characteris‐ tics of the PF-GIPOF, it is equally important to consider the optical components introduced to deploy advanced WDM-based optical architectures. The addition of these POF WDM multi‐ plexers and demultiplexers limits the available optical power budget within the fiber link thus resulting in a bit-rate penalty. This is due to the fact that the OSNR (Optical Signal-to-Noise ratio) of the system is being reduced, and so the fiber transmission capacity.

To establish a comparison between the PF-GIPOF single channel operation and its WDM extension a 4-λ WDM approach has been considered. Regarding the latter, the PF-GIPOF transmission capacity must be recalculated from: a) the new bit loading resulting from the DMT modulation scheme and, b) the restriction on power margin resulting from the new losses considered in the system due to the addition of the mux/demux devices in the optical link. An insertion loss for a future PF-GIPOF 4-λ multiplexer/demultiplexer device of around 2dB per channel [93, 94] is considered. Such a performance is better in terms of insertion loss with respect to PMMA-GIPOF based splitters which can provide insertion losses greater than 6dB (in the symmetric-case) [95]. Consequently, the power budget of the WDM system, consisting of one PF-GIPOF based multiplexer or demultiplexer device at each side of the optical fiber link, results in a 5dB power reduction per channel, if an optical crosstalk of 1dB is also considered. It is worth mentioning that some authors have evaluated power penalties close to 2.4dB when combining a 62.5μm core diameter PF-GIPOF and WDM devices based on 50μm core diameter MMF [90]. Set of Fig. 21 shows the theoretical bit loading including the afore‐ mentioned restriction in power budget due to the 4- λ WDM approach. 26 Optical Fiber

**Length (m) Numerical DMT over PF-GIPOF Capacity (Gbps)**

Fig. 20. Bit allocation per subchannel, resulting from bitloading at 100m and 150m.

It has been previously stated that for flexible high capacity GIPOF optical networks, applying the WDM approach seems to be necessary. Apart from the physical transmission characteris‐ tics of the PF-GIPOF, it is equally important to consider the optical components introduced to deploy advanced WDM-based optical architectures. The addition of these POF WDM multi‐ plexers and demultiplexers limits the available optical power budget within the fiber link thus resulting in a bit-rate penalty. This is due to the fact that the OSNR (Optical Signal-to-Noise

<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)

<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

0 50 100 150 200 250

Subcarrier number

25

**25m** 577.1 **50m** 380.0 **75m** 276.9 **100m** 164.1 **150m** 77.1

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

Note: targeted BER=10-3

Table 2.- Theoretical DMT capacity over 62.5µm core diameter PF-GIPOF, at 1300nm.

> 25m 577.1 50m 380.0 75m 276.9 100m 164.1 150m 77.1

> > Note: targeted BER=10-3

Numerical DMT over PF-GIPOF Capacity (Gbps)

Length (m)

8 capacity.

24 approach.

Recent Advances in Wavelength-Division-Multiplexing Plastic Op **Table 2.** Theoretical DMT capacity over 62.5μm core diameter PF-GIPOF, at 1300nm. tical Fiber Technologies

5

6

L=100m

L=150m

**Figure 20.** Bit allocation per subchannel, resulting from bit-loading at 100m and 150m.

ratio) of the system is being reduced, and so the fiber transmission capacity.

Bit allocation

 (bits)

7

8

9

1 It has been previously stated that for flexible high capacity GIPOF optical networks, 2 applying the WDM approach seems to be necessary. Apart from the physical transmission 3 characteristics of the PF-GIPOF, it is equally important to consider the optical components 4 introduced to deploy advanced WDM-based optical architectures. The addition of these 5 POF WDM multiplexers and demultiplexers limits the available optical power budget 6 within the fiber link thus resulting in a bit-rate penalty. This is due to the fact that the OSNR 7 (Optical Signal-to-Noise ratio) of the system is being reduced, and so the fiber transmission

5 6

Bit allocation

7 8 9

 (bits)

9 To establish a comparison between the PF-GIPOF single channel operation and its WDM 10 extension a 4-λ WDM approach has been considered. Regarding the latter, the PF-GIPOF 11 transmission capacity must be recalculated from: a) the new bit loading resulting from the 12 DMT modulation scheme and, b) the restriction on power margin resulting from the new 13 losses considered in the system due to the addition of the mux/demux devices in the optical 14 link. An insertion loss for a future PF-GIPOF 4-λ multiplexer/demultiplexer device of 15 around 2dB per channel [93, 94] is considered. Such a performance is better in terms of 16 insertion loss with respect to PMMA-GIPOF based splitters which can provide insertion 17 losses greater than 6dB (in the symmetric-case) [95]. Consequently, the power budget of the 18 WDM system, consisting of one PF-GIPOF based multiplexer or demultiplexer device at 19 each side of the optical fiber link, results in a 5dB power reduction per channel, if an optical 20 crosstalk of 1dB is also considered. It is worth mentioning that some authors have evaluated 21 power penalties close to 2.4dB when combining a 62.5µm core diameter PF-GIPOF and 22 WDM devices based on 50µm core diameter MMF [90]. Set of Fig. 21 shows the theoretical 23 bit loading including the aforementioned restriction in power budget due to the 4- λ WDM

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

2 150m. **Figure 21.** Theoretical bit loading for the 4- λ WDM approach over PF-GIPOF. (a) 100m ; (b) 150m.

3

16

**BIT**

**RATE**

**(Gbps)**

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

12345

**25m 50m 75m 100m 150m**

**Single channel WDM extension** <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

2 150m.

L=100m

Bit allocation

 (bits)

15 resulting transmission capacity.

3

16

3

9

8 23.

26 Optical Fiber

(a) (b)

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

L=150m

Bit allocation

 (bits)

0 2 4 6 8 10 12 14 16

Frequency (GHz)

27

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

**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

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. 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.

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, for different core diameter PF-GIPOFs.

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.

12 7. Discussion and Conclusions

24 (WDM) approach.
