Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes

hv photonics

Article Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes

Laura Martín González 1,* ID , Sjoerd van der Heide 2, Xuwei Xue 2, John van Weerdenburg 2, Nicola Calabretta 2, Chigo Okonkwo 2, Josep M. Fàbrega 1 and Michela Svaluto Moreolo 1,*

1 Centre Tecnològic de Telecomunicacions de Cataluya (CTTC/CERCA), 08860 Castelldefels, Barcelona, Spain; [email protected] 2 Electrical Engineering Department, Technische Universiteit Eindhoven (TU/e), 5600 MB Eindhoven, The Netherlands; [email protected] (S.v.d.H.); [email protected] (X.X.); [email protected] (J.v.W.); [email protected] (N.C.); [email protected] (C.O.) * Correspondence: [email protected] (L.M.G.); [email protected] (M.S.M.)

Received: 28 June 2018; Accepted: 8 August 2018; Published: 13 August 2018

Abstract: Adaptive Sliceable-Bandwidth Variable Transceivers (S-BVTs) are key enablers for future optical networks. In particular, those based on Discrete MultiTone (DMT) modulation and Direct Detection (DD) can be considered a flexible solution suitable to address the cost efficiency requirement of optical metro networks. In this paper, we propose to use a cost-effective S-BVT option/implementation in optical metro networks adopting switching nodes based on Semiconductor Optical Amplifier (SOA) technology. Bit loading (BL) and power loading (PL) algorithms are applied to the Digital Signal Processing (DSP) modules, to maximize the performance and/or the capacity as well as enhance the flexibility and adaptability of the system. Our analysis considers switching nodes based on SOAs with and without filtering elements and fiber spans of 25 km. We present the results up to 100 km, with and without SOA-based nodes. Firstly, we analyze the adaptive BVT transmission using the Margin Adaptive (MA) BL/PL algorithm at a fixed bit rate of 28 Gb/s. The possibility of controlling the SOAs current is a key factor to face the transmission impairments due to the fiber and the filtering elements. We also analyze the system considering Rate Adaptive (RA) transmission at a fixed target Bit Error Rate (BER) of 3.8 × 10−3, showing that a maximum capacity above 34 Gb/s can be achieved for a single span of 25 km. Although the cascading of filtering elements still constitutes a limiting factor, we show that an improvement of the net bit rate performance can be obtained thanks to the combined use of BVT and SOA technology at the switching nodes, resulting in a promising approach for designing future optical metro networks.

Keywords: optical metro networks; Bandwidth Variable Transceivers (BVT); Orthogonal Frequency Division Multiplexing (OFDM); Semiconductor Optical Ampliﬁer (SOA)

1. Introduction New challenges are envisioned for future optical networks to cope with the exponential and uncertain growth of internet traffic. In particular, the metro segment is emerging as the most challenging, due to the stringent requirements of cost efficiency and reduced power consumption, while offering very high capacity and dynamicity. To address these challenges, optical metro networks need to be equipped with flexible, adaptive, and programmable transmission and switching systems able to efficiently manage the available resources as well as the high peak of traffic and adaptive bit rates with cost- and power-efficient solutions [1,2].

Photonics 2018, 5, 24; doi:10.3390/photonics5030024 www.mdpi.com/journal/photonics Photonics 2018, 5, 24 2 of 13

In particular, to overcome the increasing traffic demand, programmable adaptive transceivers have been proposed as a suitable transmission technology [3]. In dynamic future optical networks, adaptability, flexibility, and programmability are of great importance, especially if deployed fiber infrastructure wants to be exploited. These requirements can be dealt with using Sliceable-Bandwidth Variable Transceivers (S-BVTs) [3]. They allow the generation of multiple flows while suitably and flexibly adapting the transmission in terms of bit rate, modulation formats, bandwidth, and reach. Multiple parameters and S-BVT components can be suitably configured on demand by means of an integration of the transceiver in a Software-Defined Networking (SDN) control plane, following the SDN principles [4]. In particular, an S-BVT based on Orthogonal Frequency Division Multiplexing (OFDM) offers the possibility to manage the capacity/spectrum at the subcarrier level [3,5,6]. The S-BVT architecture can be suitably tailored for the metro network segment [7]. For a cost-effective design Discrete MultiTone (DMT) modulation is preferred as the simplest OFDM implementation [5,6]. Optical networking based on Wavelength Selective Switch (WSS) technology has been studied in [8] for the future adoption of an elastic optical network. In fact, using flexi-WSS at the network nodes the optical spectrum can be used more flexibly and efficiently [8]. However, a significant source of penalty is caused by the transmission through a cascade of nodes. This effect, known as the filter narrowing effect, has been analyzed in [8,9]. In the context of elastic optical networks, Semiconductor Optical Amplifier (SOA) technology can be advantageously used, particularly for the metro segment, where the cost and power consumption are critical issues. In fact, the main advantages of using SOAs are the low power consumption, low cost, small size, and the possibility to be integrated with other optical components [10]. On the other hand, SOAs have high noise value compared to Erbium Doped Fiber Amplifier (EDFA) and have residual polarization-dependent operation <1 dB [11]. However, some studies focus on the reduction of these impairments, as in [12]. SOAs can be designed to deploy building blocks for optical switching as well as a booster amplifier, inline amplifier, or preamplifier [13,14]. By carefully adjusting the current injected into the SOA, it can be used as an optical amplifier; meanwhile, by turning on/off the electrical current, the SOA can act as a fast optical gate. Furthermore, SOAs can be used for wavelength selective applications by combining them with wavelength-filtering technologies [10]. In this work, we focus on the role of SOAs as building blocks in optical switching technologies [13]. In particular, we consider an SOA-based switching node with and without filtering elements. In the last case, the switching functionalities are performed by the SOAs and can also find application in filterless optical networks [15]. Filterless optical networks are simple network architectures based on passive splitters and combiners avoiding optical filters. Thus, they are more cost-effective than networks adopting filters; however, some functionalities are limited, such as wavelength reuse or capacity at high utilization rates [8,15]. In [15], a comparison between the filterless option and active switching, in terms of cost and performance, is provided. In the case where SOAs are combined with wavelength-filtering technologies, we consider SOA-based wavelength selectors as part of the Optical Add/Drop (OAD) nodes. These nodes have the capacity to insert or drop traffic to the optical network, by means of a SOA array with a splitter, a combiner and filtering elements based on Wavelength Division Multiplexing (WDM) technology [13]. WDM technology allows the multiplexing of multiple signals with different optical carrier wavelengths. In visible light communications, the performance of WDM technology is limited due to transmission losses [16,17]. Some studies propose the design of a novel 1 × 4 optical demultiplexer based on multimode interference [16] or the design of 1 × 4 silicon-alumina wavelength demultiplexer based on multimode interference [17], to overcome this problem. In the range of C band for optical communications, an 8-channel wavelength multimode demultiplexer is demonstrated to work with low cross-talk [18]. WDM cross-connect switches combined with SOA technology have been assessed in [10,19] for optical data center networks. The system is validated considering three different types of modulation formats, such as Non-Return Zero-On Off Keying (NRZ-OOK), Pulse Amplitude Modulation (PAM) and DMT. Potential lossless operation and low cross-talk (<−30 dB) has been Photonics 2018, 5, 24 3 of 13 demonstrated. Using BVTs, the bandwidth and bit rate can be dynamically adjusted by adopting a modulation format per each subcarrier. Therefore, more flexibility and adaptability can be obtained [7]. In this paper, we propose a combined use of BVT and SOA technologies to design flexible and cost-effective solutions for future optical metro networks. Bit loading (BL) and power loading (PL) algorithms can be implemented in the transceiver Digital Signal Processing (DSP) module, to enhance the capacity, adaptability, and the resilience towards transmission impairments [5,6]. We will present preliminary results on the use of BVT in the context of elastic optical metro networks with switching nodes based on SOA technology, particularly analyzing the case of a cost-effective implementation of the BVT. The paper is organized as follows. In Section2, the S-BVT and its cost-effective implementation are described. Then, in Section3 further details on optical metro network adopting switching nodes based on SOA technology are provided. In Section4, we present the proposed system, adopting BVT and SOA technology. The experimental setup and system optimization are presented. In Section5 the results obtained are discussed for the different analyzed scenarios and finally the conclusions are drawn in Section6.

2. S-BVT Architectures Tailored for Optical Metro Networks The S-BVT is a key element in elastic optical networks since it supports programmable functions and multi-adaptive, software-defined optical transmission. In fact, a multi-rate, multi-format, multi-reach, and multi-flow transmission is enabled on demand with sub- and super-wavelength granularity thanks to the SDN programmability and S-BVT functionalities/capabilities [6]. The slice-ability concept confers more flexibility to the BVT since it implies the capability of aggregating and distributing different flows. Furthermore, the adoption of OFDM at the DSP offers the possibility to load the subcarriers individually, according to the channel profile [3,5,6]. Hence, the spectrum can be optimized according to the traffic demand. Figure1 shows a modular S-BVT architecture composed by N BVT modules. The multiple flows are aggregated/distributed at the output of the array of the N BVTs by an additional element using, for example, a bandwidth variable WSS, which can be implemented in Liquid Crystal on Silicon (LCoS) technology or can be a simple and cost-effective passive element, implemented for example in Planar Ligthwave Circuit (PLC) [6,20]. To design the optoelectronic front-ends for an S-BVT, we can consider simple architectures using Direct Detection (DD) receivers. Several possibilities considering the DMT modulation can be used including Single Side Band (SSB) or Vestigial Side Band (VSB) modulations [5,21] On the other hand, we can adopt more complex S-BVT using coherent detection, also combined with simple DMT transmission, as it has been demonstrated in [7]. For the experiments, we consider a single BVT. For a cost-effective solution, the BVT proposed is based on DMT, as the simplest version of the OFDM, and simple DD. Despite the simplicity of DMT, the Chromatic Dispersion (CD) can limit the system performance due to the transmission over the fiber [5]. At the increase of the fiber length, the impact of the CD is higher. As a result, various subcarriers, corresponding with certain frequencies, are highly attenuated. The frequencies were the attenuation peak appears, depends on the speed of light, the center wavelength, the fiber length, or the dispersion parameter as it is explained in Section4. To mitigate this effect loading algorithms can be implemented to adapt the modulation format to the channel profile [5]. In DMT system, Hermitian Symmetry (HS) is forced on the input of the Inverse Fast Fourier Transform (IFFT) obtaining a real signal [5]. Thus, the complexity of the DSP module is reduced. The main building blocks of a DMT-based BVT are illustrated in Figure1. At the Transmitter (Tx), the input data is parallelized. After that, the signal is mapped adapting loading using Binary Phase-Shift Keying (BPSK) and M-ary Quadrature Amplitude Modulation (M-QAM) or with uniform loading, using 4-QAM. According to the Signal-to-Noise Ratio (SNR) profile, estimated at the Receiver (Rx), with a uniform loaded probe signal, the DMT subcarriers are modulated at the transmitter side using BL and PL algorithms. The algorithms can be implemented under two criteria. The Margin Photonics 2018, 5, 24 4 of 13

Photonics 2018, 5, 24 4 of 13 Adaptive (MA) criterion considers ﬁxed bit rate and energy minimization and the Rate Adaptive (RA) criterion considers a ﬁxed energy for bit rate maximization [5,22].

Metro node

SOA-based switching node Metro node

Optical Metro Network

Figure 1. Figure 1. SS-BVT-BVT architecture architecture based based on on adaptive adaptive DSP DSP using using DMT DMT for optical for optical metro metro networks networks with SOA with- SOA-based switching nodes. The S-BVT is programmable and composed by an array of BVT modules. based switching nodes. The S-BVT is programmable and composed by an array of BVT modules.

Training SymbolsSymbols (TS)(TS) are are added added for for zero-forcing zero-forcing equalization equalization atthe at receiverthe receiver side. side. Then, Then, the IFFT the isIFFT performed is performed forcing forcing the HS, the the HS, Cyclic the Cyclic Prefix Prefix (CP) is (CP) included, is included, the signal the issignal serialized is serialized (P/S) and (P/S) finally and itfinally is symmetrically it is symmetrically clipped clipped (CLIP) (CLIP) [5]. After [5]. that, After the that, digital the digital signal issignal converted is converted from digital from todigital analog to byanalog means by means of a Digital of a Digital to Analog to An Converteralog Converter (DAC). (DAC). The TxThe optoelectronic Tx optoelectronic front-end front-end consists consists of anof externalan external modulator modulator and and a Tunable a Tunable Laser Laser Source Source (TLS), (TLS), for for arbitrary arbitrary wavelength wavelength selection. selection. At theAt the Rx side,Rx side, the the signal signal is photo-detectedis photo-detected by by a DDa DD optoelectronic optoelectronic front-end front-end and and convertedconverted fromfrom analoganalog to digital by an Analog to Digital Converter (ADC). Finally, the signal is processed by the receiver DSP module. Thereby, the signal isis firstlyfirstly parallelizedparallelized (S/P),(S/P), then then the the CP is removed. After that, the Fast Fourier Transfer (FFT)(FFT) is performed consideringconsidering that the signal has HS, it is equalized, and the TS are removed. Finally, thethe signalsignal isis demodulateddemodulated andand serializedserialized toto obtainobtain thethe originaloriginal data.data.

3. SOA Technology for Optical MetroMetro NetworkNetwork NodesNodes SOA technology can be adopted for implementingimplementing switchingswitching devices. There are other optical switching technologies based on Electro-Optic,Electro-Optic, Acousto-Optic,Acousto-Optic, Thermo-OpticThermo-Optic or Opto-MechanicalOpto-Mechanical switching. The main advantage advantagess of SOAs with respect to these other switching technologies are the loss compensation,compensation, the the high high speed, speed, and and the the scalability scalability [13 ].[13]. When When several several switches switches are in are cascade, in cascade, SOAs canSOAs control can control the gain the with gain the injectedwith the current injected and, current depending and, on depending the node onarchitecture, the node overcome architecture, the limitationovercome due the to limitation the power due decay. to the As losses power due decay. to the As network losses elements due to the can network be compensated elements thanks can be to thecompensated presence of thanks SOAs, tothis the switching presence option of SOAs, can be this envisioned switching also option for applications can be envisioned in filterless also optical for networks.applications This in filterless would represent optical networks. a cost-effective This would solution represent [15], as a active cost-effective reconfigurable solution components [15], as active are eliminatedreconfigurable or minimized components using are passiveeliminated optical or minimized elements as using combiners passive or/and optical splitters elements to as interconnect combiners theor/and fiber splitters links and to interconnect to add or drop the channel fiber links wavelengths and to add at or the drop nodes. channel Some wavelengths advantages at are the expected nodes. forSome these advantages networks such are asexpected simplified for maintenancethese networks or reconfigurability. such as simplified However, maintenance in a filterless or opticalreconfigurability. network we However, must consider in a filterless the related optical drawbacks network andwe must limitations. consider In the this related scenario, drawbacks the use ofand SOA-based limitations. switching In this nodes scenario, combined the use with of SOA BVT- technologybased switching can be nodes advantageous. combined SOA-based with BVT switchingtechnology nodes can be can advantageous. be useful to compensate SOA-based the switching attenuation, nodes due can to be the useful passive to elements compensate and the transmissionattenuation, due over to the the fiber, passive and elements BVT with and loading the transmission capabilities, toover combat the fiber, the CD.and BVT with loading capabilities,On the otherto combat hand, the a genericCD. filtered optical mesh network is equipped with OAD multiplexer, placedOn at the the other nodes, hand, enabling a generic to dynamically filtered optical configure mesh the network dropping is orequipped adding ofwith different OAD wavelengthsmultiplexer, placed at the nodes, enabling to dynamically configure the dropping or adding of different wavelengths through the fiber, remotely. Typically, the reconfigurable OAD nodes adopt WSS technology. Particularly, in the context of elastic optical networks, flexible WSS are needed to deploy

Photonics 2018, 5, 24 5 of 13

Photonics 2018, 5, 24 5 of 13 through the fiber, remotely. Typically, the reconfigurable OAD nodes adopt WSS technology. a Particularly,more flexible in and the bandwidth context of elasticefficient optical network networks, [8]. The flexiblemain disadvantage WSS are needed of this to solution deploy ais morethe penaltiesflexible andin signal bandwidth degradation efficient due network to the [filter8]. The narrowing main disadvantage effect caused of thisby the solution crossing is the through penalties a cascadein signal of degradation WSS [8], as due it was to the mentioned filter narrowing in Section effect 1. caused The use by of the S- crossingBVT can through flexibly a adapt cascade the of transmission.WSS [8], as it was mentioned in Section1. The use of S-BVT can flexibly adapt the transmission. InIn this this work, work, SOA SOA-based-based wavelength wavelength selectors selectors are are considered considered as as the the key key element element in in OAD OAD nodes nodes toto drop drop and and add add traffic traffic inin thethe networknetwork [ 13[13,23,23].]. WDM WDM optical optical cross-connect cross-connect based based on on SOA SOA has has also also been beenpresented presented in [10 in] for[10] interconnecting for interconnecting network network elements, elements, computing, computing, or storage or storage resources resources in a metroin a metronetwork network architecture. architecture. It has been It has experimentally been experimentally demonstrated demonstrated the potential the lossless, potential low lossless, cross-talk low or cross-talk or multicasting operation of these network elements. multicasting operation of these network elements. In our study, we analyze the performance when the OAD nodes with SOA-based wavelength In our study, we analyze the performance when the OAD nodes with SOA-based wavelength selectors are included, being the target the metro segment. The OAD node consists of one splitter, selectors are included, being the target the metro segment. The OAD node consists of one splitter, one combiner, the SOA array for gating one or more wavelengths and two wavelength selectors. One one combiner, the SOA array for gating one or more wavelengths and two wavelength selectors. acting as a demultiplexer and the other acting as a multiplexer. In the proposed system, the filter One acting as a demultiplexer and the other acting as a multiplexer. In the proposed system, narrowing effect is also present due to the concatenation of the wavelength selectors [8]. The the filter narrowing effect is also present due to the concatenation of the wavelength selectors [8]. architecture of the SOA-based OAD node can be seen in Figure 2. In this case, losses are generated The architecture of the SOA-based OAD node can be seen in Figure2. In this case, losses are generated by the transmission over the fiber links and the filtering elements as well. Adjusting the SOA bias by the transmission over the fiber links and the filtering elements as well. Adjusting the SOA bias current, the filtering effects mainly due to the wavelength selectors and the losses due to the fiber, current, the filtering effects mainly due to the wavelength selectors and the losses due to the fiber, can be compensated. can be compensated.

FigureFigure 2. 2. ArchitectureArchitecture of of a aSOA SOA-based-based OAD OAD node. node.

4.4. BVT BVT with with Switching Switching Nodes Nodes Adopting Adopting SOA SOA Technology Technology:: Experimental Experimental Set Setupup and and Optimization Optimization We propose to adopt the S-BVT described in Section2 for optical metro networks adopting SOA-basedWe propose switching to adopt nodes. the In S-BVT particular, described for the in experimentsSection 2 for the optical simplest metro BVT networks architecture adopting using SOADMT-based and DDswitching is considered nodes. attractiveIn particular, for a for cost-effective the experiments implementation the simplest and, BVT thus, architecture it is envisioned using to DMTbe used and when DD is SOA considered technology attractive is adopted for a forcost the-effective switching implementation nodes. and, thus, it is envisioned to be usedTo analyze when SOA the performance technology is of adopted this approach, for thetwo switching scenarios nodes. are considered as shown in Figure3. BothTo scenarios analyze considerthe performance spans of 25 of km this of fiber approach, and different two scenarios cascading are nodes; considered scenario as (a) shown envisions in Figurethe use 3. ofBoth a simple scenarios SOA consider acting as switchingspans of 25 node, km whileof fiber scenario and different (b) introduces cascading OAD nodes; nodes scenario based on (a)SOA envisions technology, the use as of specified a simple in SOA Figure acting2 and as Section switching3. As node, a reference, while scenario the case ( ofb) multipleintroduces ( up OAD to 4 ) nodesfiber spoolsbased ofon25 SOA km, technology, without any as SOA specified is analyzed in Figure as well. 2 and The Sec scenariotion 3. withoutAs a reference, considering the case filtering of multipleelements (up (filterless) to 4) fiber is analyzed spools of to 25 study km, thewithout impact any on SOA the system is analyzed performance as well. at The different scenario transmission without consideringdistances with filtering and elements without SOAs.(filterless) Then, is analyzed we analyze to thestudy case the of impact SOA-based on the OAD system nodes performance after each atspan different of fiber. transmission To evaluate distances the proposed with and system, without the setupSOAs. of Then, Figure we2 hasanalyze been the considered. case of SOA The-based WDM OADconsists nodes of after 32 channels each span with of afiber. channel To evaluate spacing the of 100proposed GHz. However,system, the for setup the experimentsof Figure 2 has only been the considered.1550.12 nm The channel WDM has consists been of studied. 32 channels It is markedwith a channel by the redspacing color of in 100 Figure GHz.3. However, Thus, the for signal the experimentstraverses the only first the WDM 1550.12 acting nm as channel a demultiplexer, has been thestudied. SOA andIt is finallymarked the by signal the red is multiplexedcolor in Figure by the3. Thus,second the WDM. signal traverses the first WDM acting as a demultiplexer, the SOA and finally the signal is multiplexed by the second WDM. To experimentally analyze the proposed system, the setup of Figure 3 has been considered. A single BVT, based on DMT and DD, transmits over the cascading of fiber spools of 25 km and SOA- based switching nodes according to scenario (a) or scenario (b).

Photonics 2018, 5, 24 6 of 13

To experimentally analyze the proposed system, the setup of Figure3 has been considered. APhotonics single 2018 BVT,, 5, 2 based4 on DMT and DD, transmits over the cascading of ﬁber spools of 25 km6 and of 13 SOA-based switching nodes according to scenario (a) or scenario (b). Python software has been used at the DSP module. The IFFT with 512 subcarriers modulates the Python software has been used at the DSP module. The IFFT with 512 subcarriers modulates the mapped sequence. Due to the HS, only half of the IFFT subcarriers supports data. The total number mapped sequence. Due to the HS, only half of the IFFT subcarriers supports data. The total number of of frames is 125 being 5 of them TS. The CP is 1.9% and the Forward Error Correction (FEC) frames is 125 being 5 of them TS. The CP is 1.9% and the Forward Error Correction (FEC) considered is considered is 7%. As it is well-known, the clipping factor can cause distortions and the degradation 7%. As it is well-known, the clipping factor can cause distortions and the degradation of the system of the system performance. For this reason, the best clipping level should be selected according to the performance. For this reason, the best clipping level should be selected according to the adopted adopted constellation format. To estimate the channel profile, uniform loading is adopted, in constellation format. To estimate the channel proﬁle, uniform loading is adopted, in particular 4-QAM. particular 4-QAM. The clipping level recommended for this modulation format is 7 dB, which The clipping level recommended for this modulation format is 7 dB, which corresponds to a clipping corresponds to a clipping factor of 2.24. When other modulation formats are used, 7 dB can be not factor of 2.24. When other modulation formats are used, 7 dB can be not enough. The clipping level enough. The clipping level must be selected according to the highest modulation format, when BL is must be selected according to the highest modulation format, when BL is used. In our case, the highest used. In our case, the highest modulation format used in this experiment is 16-QAM. Accordingly, modulation format used in this experiment is 16-QAM. Accordingly, we have determined that the best we have determined that the best clipping level is 8.5 dB giving a clipping factor of 2.6. The sample clipping level is 8.5 dB giving a clipping factor of 2.6. The sample rate of the DAC is 28 GS/s. rate of the DAC is 28 GS/s.

Figure 3. BVT architecture and experimental setup. OM: optical power monitoring, Att: Attenuator, Figure 3. BVT architecture and experimental setup. OM: optical power monitoring, Att: Attenuator, OSA: Optical Spectrum Analyzer, EQ: Equalizer. The different analyzed scenarios for the optical OSA: Optical Spectrum Analyzer, EQ: Equalizer. The different analyzed scenarios for the optical metro network are indicated: (a) with SOAs and 25 km Single Standard Mode Fiber (SSMF) spools, metro network are indicated: (a) with SOAs and 25 km Single Standard Mode Fiber (SSMF) spools, and (b) with OAD nodes based on SOAs. and (b) with OAD nodes based on SOAs. The obtained electrical signal is the input of a Dual Drive Mach-Zehnder Modulator (DD-MZM). The TheDD- obtainedMZM is working electrical in signal the push is the-pull input operation. of a Dual The Drive laser Mach-Zehnder driving the DD Modulator-MZM is (DD-MZM). centered at The1550.12 DD-MZM nm with is working13 dBm of in output the push-pull power. As operation. the DD-MZM The laser is polarization driving the dependent, DD-MZM isa Polarization centered at 1550.12Controller nm with(PC) 13is needed dBm of to output obtain power. the maximum As the DD-MZM power of is the polarization laser driving dependent, the DD-MZM. a Polarization Another ControllerPC is at the (PC) output is needed of the to DD obtain-MZM. the With maximum those powerPCs we of can the control laser driving the power the DD-MZM. at the input Another of the PCEDFA, is at theobtaining output a of power the DD-MZM. value of 1 With.5 dB thosem at the PCs input we can and control 13 dBm the at power the output at the of input the ofEDFA. the EDFA, Then, obtainingthe power a powerlaunched value to ofthe 1.5 network dBm at is the measured input and by 13 an dBm optical at the power output monitor of the EDFA. (OM). Then, At the the receiver, power launchedan attenuator to the followed network by is measured an EDFA byis used an optical to vary power the Optical monitor Signal (OM).-to At-Noise the receiver, Ratio ( anOSNR attenuator). After followedthat, another by an OM EDFA is placed is used for to being vary theable Optical to ensure Signal-to-Noise a constant power Ratio at (OSNR). the input After of the that, photodetector. another OM The sample rate of the ADC is 80 GS/s. The OSNR is measured within 0.1 nm and the target BER is set to 3.8 × 10−3. The fiber spans are Single Standard Mode Fiber (SSMF) of 25 km each, with 0.2 dB/km of attenuation.

Photonics 2018, 5, 24 7 of 13 is placedPhotonics for 2018 being, 5, 2 able4 to ensure a constant power at the input of the photodetector. The sample rate7 of 13 of the ADC is 80 GS/s. The OSNR is measured within 0.1 nm and the target BER is set to 3.8 × 10−3. A probe signal modulated with uniform loading (4-QAM) is sent to the channel to estimate the The ﬁber spans are Single Standard Mode Fiber (SSMF) of 25 km each, with 0.2 dB/km of attenuation. SNR profile at the receiver side. To calculate the SNR, a sliding window method is used. We name A probe signal modulated with uniform loading (4-QAM) is sent to the channel to estimate the the number of samples taken after or before the subcarrier considered, as Window Size (WS). The SNR proﬁle at the receiver side. To calculate the SNR, a sliding window method is used. We name the total number of the subcarriers considered for calculating the mean is given by TW = WS 2 + 1 where number of samples taken after or before the subcarrier considered, as Window Size (WS). The total TW is the total size of the window. Figure 4 shows the optimization for 75 km (3 spans of 25 km of number of the subcarriers considered for calculating the mean is given by TW = WS 2 + 1 where TW is SSMF). We can see that the minimum BER is achieved by a WS of 4 (meaning 4 subcarriers taken after the total size of the window. Figure4 shows the optimization for 75 km (3 spans of 25 km of SSMF). and 4 subcarriers taken before the subcarrier considered) that corresponds to a TW size of 9. We can see that the minimum BER is achieved by a WS of 4 (meaning 4 subcarriers taken after and 4 subcarriers taken before the subcarrier considered) that corresponds to a TW size of 9.

Figure 4. BER versus WS for 75 km (3 spans of 25 km of SSMF) and OSNR of 36 dB without considering Figure 4. BER versus WS for 75 km (3 spans of 25 km of SSMF) and OSNR of 36 dB without considering any SOA placed inline. any SOA placed inline.

As anAs example, an example, the SNR the SNR estimation estimation for the for different the different scenarios scenarios with with and withoutand without SOAs SOAs and withand with SOA-basedSOA-based OAD OAD is shown is shown in Figure in Figure5a considering 5a considering two spanstwo spans of 25 of km 25 for km a for total a total of 50 of km 50 of km fiber. of fiber. ThereThere it is possible it is possible to observe to observe a degradation a degradation of the of SNR the SNR around around the 156th the 156th subcarrier, subcarrier, which which is due is todue to the CD.the ThisCD. This attenuation attenuation depends depends on the on fiber the fiber length length and appearsand appears at certain at certain frequencies frequencies that arethat given are given by theby expression the expression s c(2n−1) n 2λ2 푐(2푛−1) f = 2 (1) 푓푛 LD= √ 2휆 , (1) 퐿퐷 being c the speed of light, λ the center wavelength, L the fiber length, D the dispersion parameter and n the n-th attenuation peak (positive integer). being 푐 the speed of light, 휆 the center wavelength, 퐿 the fiber length, 퐷 the dispersion When the SOAs are included, this peak suffers a shift of around 10 subcarriers towards the lower parameter and 푛 the 푛-th attenuation peak (positive integer). frequencies. The maximum obtained SNR decreases 1 dB, but the SNR profile is similar. This frequency shift, withWhen respect the to SOAs the theoreticalare included, frequency, this peak is suffers due to a theshift variation of around of 10 the subcarriers injected current towards of the the lower SOAsfrequencies. to find the best The working maximum point. obtained In our SNR case decreases study, the 1 best dB, conditions but the SNR are achieved profile is for similar. a SOA This inputfrequency power of shift, -8 dBm with and respect an injected to the currenttheoretical within frequency, 60 mA andis due 80 to mA. the variation of the injected current Whenof the SOAs the SNR to find profile the isbes estimatedt working for point. the caseIn our using case SOA-basedstudy, the best OAD conditions nodes, the are maximum achieved for a SNRSOA decreases input down power to of 14 -8 dB, dBm and and the an fading injected frequency current peakwithin is shifted60 mA and to higher 80 mA. frequencies, where the minimumWhen SNR the isSNR about profile 6.5 dB. is Inestimated this case, for the the cascading case using of networkSOA-based elements, OAD nodes, in particular the maximum the presenceSNR ofdecreases filtering down elements, to 14 causes dB, and the the reduction fading frequency of the obtained peak is SNRshifted for to all higher the subcarriers frequencies, and where the shiftthe minimum of the fading SNR peak. is about Indeed, 6.5 fordB. thisIn this case, case, the the peak cascading fading is of not network so pronounced elements, as in in particular the case the presence of filtering elements, causes the reduction of the obtained SNR for all the subcarriers and the shift of the fading peak. Indeed, for this case, the peak fading is not so pronounced as in the case without filtering elements and it covers a greater number of subcarriers (from the 120th to the 200th

Photonics 2018, 5, 24 8 of 13 Photonics 2018, 5, 24 8 of 13 withoutapproximately). filtering elementsAccording and to itthe covers SNR profile, a greater the number subcarriers of subcarriers for each scenario (from the are 120th modulated to the 200th at the approximately).transmitter sideAccording using BL/PL to thealgorithms. SNR profile, Figure the 5b subcarriers shows the for bit each distribution scenario areaccording modulated to the at SNR the transmitterestimation sidein Figure using 5a BL/PL and considering algorithms. Figurethe MA5b criterion shows the with bit distributiona fixed gross according bit rate of to 2 the8 G SNRb/s at estimation34 dB of OSNR. in Figure It can5a and be seen considering that the the modulation MA criterion format with order a fixed decreases gross bit with rate ofthe 28 reduction Gb/s at 34 of dB the ofSNR OSNR. values It canand be increases seen that with the the modulation increase of format the SNR order values. decreases For the with OAD the scenario, reduction since of the the SNR SNR valuesprofile and is moreincreases affected with theby theincrease fiber of imp theairments SNR values. (CD) For and the the OAD filtering scenario, elements, since the lower SNR bits profile per issymbol more affected are loaded by theonto fiber the subcarriers. impairments In (CD) Figure and 5c, the the filtering PL distribution elements, according lower bits to perthe symbolSNR profile are loadedis presented. onto the It subcarriers.can be seen Inthat Figure the 5subcarriersc, the PL distribution corresponding according to the to lower the SNR SNR profile values is for presented. the case Itwith can be SOAs seen do that not the have subcarriers power corresponding assigned. For the to the OAD lower scenario, SNR values all the for subcarriers the case with have SOAs power do notassigned have powerand the assigned. power variation For the corresponds OAD scenario, to the all change the subcarriers from one have modulation power assignedformat to andanother. the powerIn Figure variation 5d, the corresponds BER per subcarrier to the changefor the different from one analyze modulationd cases format is reported. to another. We can In observe Figure 5thatd, thefor BERthe OAD per subcarrier scenario, forthe thesubcarriers different affected analyzed by cases CD present is reported. high Wenumber can observe of errors. that for the OAD scenario, the subcarriers affected by CD present high number of errors.

(a) (b)

Figure 5. (a) SNR profile, (b) BL assignment, (c) PL assignment and (d) BER per DMT subcarrier Figure 5. (a) SNR proﬁle, (b) BL assignment, (c) PL assignment and (d) BER per DMT subcarrier considering MA transmission at 28 Gb/s over two SSMF spans of 25 km, with and without SOAs and considering MA transmission at 28 Gb/s over two SSMF spans of 25 km, with and without SOAs and for SOA-based OAD, at 34 dB of OSNR. for SOA-based OAD, at 34 dB of OSNR.

5. Results and Discussion 5. Results and Discussion As shown in Figure 3 and detailed in Section 4, the experimental analysis considers two As shown in Figure3 and detailed in Section4, the experimental analysis considers two scenarios: scenarios: the first using simple SOAs acting as switching nodes and the second adopting OAD nodes the ﬁrst using simple SOAs acting as switching nodes and the second adopting OAD nodes based on based on SOA technology. Fiber spans of 25 km until a maximum of 4, for a total path of 100 km, are SOA technology. Fiber spans of 25 km until a maximum of 4, for a total path of 100 km, are considered. considered. In particular, to observe the impact of adopting the SOAs in the system, we place an SOA In particular, to observe the impact of adopting the SOAs in the system, we place an SOA after each after each span of fiber and compare the results with the transmission over the fiber link without them. To analyze the second scenario, we consider SOA-based switching nodes based on the OAD node architecture of Figure 2. For both scenarios, the current injected into the SOA is properly varied

Photonics 2018, 5, 24 9 of 13 span of fiber and compare the results with the transmission over the fiber link without them. To analyze thePhotonics second 2018 scenario,, 5, 24 we consider SOA-based switching nodes based on the OAD node architecture9 of 13 of Figure2. For both scenarios, the current injected into the SOA is properly varied to compensate theto compensate losses due to the the losses fiber impairments due to the fiberor/and impairments the different or/and elements the (splitters, different combiners, elements (splitters, filtering elements)combiners, of filtering the OAD elements) node. of the OAD node. First,First, we we analyze analyze the the case case at fixed at fixedbit rate bi (28t rate Gb/s) (28 using Gb/s) the using MA BL/PL the MA algorithm BL/PL for algorithm maximizing for themaximizing performance. the performance. The BER performance The BER performance at the varying at of the the varying OSNR isof presented the OSNR in is Figure presented6. It is in interestingFigure 6. It tois observeinteresting that to a observe similar behaviorthat a similar is obtained behavior for is 75 obtained km (3 spans) for 75 of k fiberm (3 withoutspans) of SOAs, fiber 50without km (2 SOAs, spans) 5 of0 k fiberm (2 withspans) an of SOA fiber after with each an SOA span after and oneeach fiber span span and ofone 25 fiber km withspan anof OAD25 km node, with −3 beingan OAD the difference node, being between the difference these cases between lower than these 1 dB cases at the lower target thanBER of1 3.8dB× at10 the. Similartarget BER results of are3.8 obtained× 10−3. Similar when results we compare are obtained the BER wh curvesen we compare for 100 km the (4 BER spans) curves of fiber, for 10 750 kmkm (4 (3 spans) of fiber, fiber with75 km an (3 SOA spans) per of each fiber span with and an 50 SOA km (2per spans) each ofspan fiber and including 50 km (2 2 OADspans) nodes of fiber (one including after each 2 span),OAD beingnodes as (one well after the each penalty span), less being than 1as dB, well at the targetpenalty BER. less Up than to 1 100 dB, km at (4the fiber target spans) BER. can Up be to reached 100 km by(4 fiber cascading spans) 4 can SOAs be withreached an OSNRby cascading value of 4 33SOAs dB. with It was an not OSNR possible value to of retrieve 33 dB. resultsIt was not after possible 75 km (3to spans)retrieve with results the OAD after nodes 75 km because (3 spans) of thewith losses the accumulation,OAD nodes because mainly dueof the to thelosses filtering accumulation, elements, andmainly the due noise to introduced the filtering by elements, the SOAs. and The the introduction noise introduced of OAD by nodesthe SOAs. involves The introduction losses around of 4 OAD dBm pernodes each involve WDMs elementlosses around and 1 dBm4 dBm per per each each combiner/splitter. WDM element and 1 dBm per each combiner/splitter.

Figure 6. BER versus OSNR (with and without SOAs and including SOA-based OAD nodes) using Figure 6. BER versus OSNR (with and without SOAs and including SOA-based OAD nodes) using BL BL and PL under the MA criteria for a fixed gross bit rate of 28 Gb/s. and PL under the MA criteria for a fixed gross bit rate of 28 Gb/s. Then, we have studied the maximum achievable net bit rate, adopting the RA BL/PL algorithm, for differentThen, we transmission have studied distances the maximum and scenarios achievable considering net bit rate, at adoptinga fixed target the RA BER BL/PL of 3.8 algorithm, × 10−3. In −3 forFigure different 7, the transmissionresults at 33 d distancesB of OSNR, and are scenarios presented. considering We can see at that, a fixed as the target fiber BER length of 3.8 increases,× 10 . Inthe Figure maximum7, the results net bit at rate 33 dB decreases, of OSNR, being are presented. more pronounced We can see for that, the as case the fiber of fiber length transmission increases, thewithout maximum SOAs. net In bitfact, rate for decreases, the 50 km being (2 spans) more of pronounced fiber, the maximum for the case net of bit fiber rate transmission is obtained withoutwhen 2 SOAs.SOAs or In 2 fact, SOA for-based the 50 OAD km (2nodes spans) are of included fiber, the in maximum the optical net path, bit obtaining rate is obtained 26.5 G whenb/s. The 2 SOAsvalue orof 2the SOA-based net bit rate OAD when nodes only are 2 includedspans of in fiber the opticalare placed path, inline, obtaining is lower, 26.5 Gb/s. being The its value value 26. of1 the G netb/s. bitTherefore, rate when the onlyintroduction 2 spans ofof fiberthe SOAs, are placed as switching inline, isnodes lower, themselves being its valueor either 26.1 as Gb/s. part of Therefore, the OAD thenode, introduction improves the of net the bit SOAs, rate performance as switching at nodes least themselvesfor fiber links or higher either than as part 50 k ofm. the In fact, OAD there node, is improvesan improvement the net of bit 2 rate Gb/s performance when 75 km at of least fiber for links fiber and links 3 SOAs higher are than included, 50 km. with In fact, respect there to is the an case when the fiber links do not include the SOAs. This is due to the possibility of better optimizing the transmission by controlling the current injected into the SOAs, in combination to the transceiver adaptability by means of the BL/PL algorithm. According to the obtained results, a 26 Gb/s connection can be supported up to 50 km considering 2 cascading SOA-based OAD nodes and a maximum of 27 Gb/s for 75 km considering 3 cascading SOAs without filtering elements.

Photonics 2018, 5, 24 10 of 13 improvement of 2 Gb/s when 75 km of fiber links and 3 SOAs are included, with respect to the case when the fiber links do not include the SOAs. This is due to the possibility of better optimizing the transmission by controlling the current injected into the SOAs, in combination to the transceiver adaptability by means of the BL/PL algorithm. According to the obtained results, a 26 Gb/s connection can be supported up to 50 km considering 2 cascading SOA-based OAD nodes and a maximum of Photonics27 Gb/s 2018 for, 5 75, 24 km considering 3 cascading SOAs without filtering elements. 10 of 13

FiFiguregure 7. 7. NetNet bit bit rate rate versus versus ﬁber fiber link link (with (with and and without without SOAs SOAs and including and including SOA-based SOA- OADbased nodes), OAD −3 nodes),using RA using BL/PL RA algorithm,BL/PL algorithm, for an OSNRfor an ofOSNR 33 dB of at 33 a d targetB at a BER target of 3.8BER× of10 3.8−3 .× 10 .

Table 1 shows the maximum achievable net data rate at different transmission distances for the Table1 shows the maximum achievable net data rate at different transmission distances for the considered scenarios adopting RA BL/PL. Please note that the maximum net bit rate of 37.32 Gb/s is considered scenarios adopting RA BL/PL. Please note that the maximum net bit rate of 37.32 Gb/s achieved for 25 km of fiber when the SOAs are not included. On the other hand, the net bit rate is achieved for 25 km of fiber when the SOAs are not included. On the other hand, the net bit rate penalty between the cases of 25 km of fiber and 100 km of fiber is greater (12.25 Gb/s of difference), penalty between the cases of 25 km of fiber and 100 km of fiber is greater (12.25 Gb/s of difference), when the SOAs are not included than when the SOAs are added (10.7 Gb/s of difference). Therefore, when the SOAs are not included than when the SOAs are added (10.7 Gb/s of difference). Therefore, there is a reduction of the penalties, when SOAs are included. Therefore, also in this case, the current there is a reduction of the penalties, when SOAs are included. Therefore, also in this case, the current injected into the SOAs is a key factor to control the penalties when multiple fiber links and SOAs are injected into the SOAs is a key factor to control the penalties when multiple fiber links and SOAs are in cascade. in cascade.

Table 1.1. Achievable net bit rate and maximum OSNR for differentdifferent transmission distances using RA BL/PL algorithm, algorithm, for for different different fiber ﬁber link link with with and and without without SOAs SOAs and and considering considering SOA SOA-based-based OAD. OAD. Fiber Link w/o SOAs Fiber Link with SOAs Fiber Link with OAD Fiber Link w/o SOAs Fiber Link with SOAs Fiber Link with OAD Reach (km) Net Bit Rate (Gb/s) Net Bit Rate (Gb/s) Net Bit Rate (Gb/s) Reach (km) Net Bit Rate (Gb/s) Net Bit Rate (Gb/s) Net Bit Rate (Gb/s) 25 37.32 34.57 34.77 50 2533 37.32 34.5731.78 34.7727.97 50 33 31.78 27.97 75 31.4 27.68 - 75 31.4 27.68 - 100100 25.07 25.07 23.87 - -

In case of considering SOA-based OAD nodes, the net bit rate decrease is greater than the case In case of considering SOA-based OAD nodes, the net bit rate decrease is greater than the case of using only SOAs. However, the maximum net bit rate achievable after 25 km of fiber is similar: of using only SOAs. However, the maximum net bit rate achievable after 25 km of fiber is similar: 34.57 Gb/s and 34.77 Gb/s, with a simple additional SOA or including an OAD node, respectively. 34.57 Gb/s and 34.77 Gb/s, with a simple additional SOA or including an OAD node, respectively. When 2 OAD nodes and 50 km of fiber links are included, the maximum net bit rate is 27.97 Gb/s that When 2 OAD nodes and 50 km of fiber links are included, the maximum net bit rate is 27.97 Gb/s that is similar to the maximum net bit rate obtained for 75 km of fiber link and 3 SOAs (the difference is is similar to the maximum net bit rate obtained for 75 km of fiber link and 3 SOAs (the difference is 0.29 dB). 0.29 dB). In Figure 8a,b the net bit rate versus the OSNR for 25 km and 50 km of fiber are presented. It can In Figure8a,b the net bit rate versus the OSNR for 25 km and 50 km of fiber are presented. It can be observed that the overlapping between the results of the different scenarios is higher when 2 spans be observed that the overlapping between the results of the different scenarios is higher when 2 spans (50 km) of fiber are considered. This is due to the increase of the number of elements in the optical (50 km) of fiber are considered. This is due to the increase of the number of elements in the optical network, either SOAs acting as switching nodes or OAD nodes. In fact, this enables the possibility to network, either SOAs acting as switching nodes or OAD nodes. In fact, this enables the possibility to suitably manage (and carefully adjust) the SOAs bias current. Consequently, the system performance can be better optimized. For 50 km transmission distance, the penalties when the SOAs/OAD nodes are included, are minimum (<1 dB). Unfortunately, in this experiment, it has not been possible to further increase the cascading of more OAD nodes. Due to the different filtering elements and the associated noise accumulation, the maximum achievable OSNR was limited [21]. Thus, it was not possible to retrieve results after 75 km with the OAD nodes. However, to target metro network distances, the system can be improved considering other alternatives such as DMT with SSB or VSB modulation [20,21] with DD. In [24], a multi-band SSB-DD system has been studied for a metro- regional network. With this system, 545 km of transmission distance has been demonstrated.

Photonics 2018, 5, 24 11 of 13 suitably manage (and carefully adjust) the SOAs bias current. Consequently, the system performance can be better optimized. For 50 km transmission distance, the penalties when the SOAs/OAD nodes are included, are minimum (<1 dB). Unfortunately, in this experiment, it has not been possible to further increase the cascading of more OAD nodes. Due to the different filtering elements and the associated noise accumulation, the maximum achievable OSNR was limited [21]. Thus, it was not possible to retrieve results after 75 km with the OAD nodes. However, to target metro network Photonicsdistances, 2018 the, 5, 2 system4 can be improved considering other alternatives such as DMT with SSB or11 VSBof 13 modulation [20,21] with DD. In [24], a multi-band SSB-DD system has been studied for a metro-regional Furthermore,network. With DD this can system, be replaced 545 km with of transmission coherent detection distance for has improving been demonstrated. the performance. Furthermore, In [25], DMTDD can transmission be replaced with and coherent coherent detection detection for ha improvings been demonstrated the performance. in flexgrid In [25], DMT metro transmission networks coveringand coherent distances detection up to has150 k beenm. More demonstrated recently, in in [26] flexgrid two different metro networks transceiver covering configurations distances have up beento 150 experimentally km. More recently, validated. in [26 ] One two is different based on transceiver intensity configurations modulation with have DD been and experimentally the other on amplitudevalidated. One modu is basedlation on with intensity coherent modulation reception with for DD the and metro the other networks. on amplitude Thus, in modulation future work, with differentcoherent configurations reception for the would metro be networks.addressed Thus, to improve in future the work,performance. different configurations would be addressed to improve the performance.

(a) (b)

Figure 8. Net bit rate versus OSNR for (a) 25 km and (b) 50 km for the different scenarios at the Figure 8. Net bit rate versus OSNR for (a) 25 km and (b) 50 km for the different scenarios at the target BER. target BER. 6. Conclusions 6. Conclusions In this paper, the use of S-BVTs, based on DMT modulation and DD with adaptive loading In this paper, the use of S-BVTs, based on DMT modulation and DD with adaptive loading capabilities has been proposed as a flexible and adaptive cost-effective solution for future optical capabilities has been proposed as a flexible and adaptive cost-effective solution for future optical metro metro networks adopting SOA-based switching nodes. A preliminary assessment of this system has networks adopting SOA-based switching nodes. A preliminary assessment of this system has been been analyzed considering two scenarios: with or without filtering elements. Using loading analyzed considering two scenarios: with or without filtering elements. Using loading algorithms for algorithms for both scenarios, the limitations due to transmission impairments (caused by the CD, both scenarios, the limitations due to transmission impairments (caused by the CD, DAC bandwidth DAC bandwidth or the network elements) can be mitigated enabling higher transmission rates or the network elements) can be mitigated enabling higher transmission rates and reach. and reach. When BL and PL algorithms are implemented using the MA criterion, the results show that the When BL and PL algorithms are implemented using the MA criterion, the results show that the BER performance obtained for a 25 km of fiber link with an SOA-based OAD node has similar BER BER performance obtained for a 25 km of fiber link with an SOA-based OAD node has similar BER performance than the case of 50 km (2 spans of 25 km) of fiber with 2 SOAs acting as switching nodes, performance than the case of 50 km (2 spans of 25 km) of fiber with 2 SOAs acting as switching nodes, or the case of 75 km (3 spans) of fiber. Thus, the introduction of the SOAs or the OAD node, in terms of or the case of 75 km (3 spans) of fiber. Thus, the introduction of the SOAs or the OAD node, in terms BER performance, results in an additional span of 25 km of fiber is included. of BER performance, results in an additional span of 25 km of fiber is included. On the other hand, using the RA criterion, the net bit rate performance improves including SOA On the other hand, using the RA criterion, the net bit rate performance improves including SOA elements in the transmission system, compared to the case of simply adding fiber spans. This is elements in the transmission system, compared to the case of simply adding fiber spans. This is due due to the possibility of controlling the SOA current to face the transmission impairments. These to the possibility of controlling the SOA current to face the transmission impairments. These impairments include the losses due to the fiber links, the SOAs as well as the WDM elements or the impairments include the losses due to the fiber links, the SOAs as well as the WDM elements or the splitter/combiners used in the experiments [21]. These losses can be up to 4 dBm for each WDM splitter/combiners used in the experiments [21]. These losses can be up to 4 dBm for each WDM element. Thus, as the number of SOA-based switching nodes increases, more SOA elements can be element. Thus, as the number of SOA-based switching nodes increases, more SOA elements can be controlled, to ensure a better optimization of the system and an improvement of the performance controlled, to ensure a better optimization of the system and an improvement of the performance overcoming the power losses due to the different elements in cascade. However, adding SOAs or/and overcoming the power losses due to the different elements in cascade. However, adding SOAs or/and filtering elements limits the maximum achievable OSNR and thus the number of cascading nodes that filtering elements limits the maximum achievable OSNR and thus the number of cascading nodes that can be traversed by the adaptive signal generated by the S-BVT. In particular, 27 Gb/s connections can be supported up to 75 km including 3 SOAs acting as switching nodes without any filtering element, and while 26 Gb/s for 50 km is the maximum achievable reach including 2 OAD nodes. The experimental assessment demonstrates that thanks to the possibility of controlling the injected current to the SOAs and the application of loading schemes at the adaptive transceivers, high flexibility, scalability, and adaptability can be obtained. However, despite the possibility of compensating the losses thanks to the SOAs, the maximum transmission distance with 2 OAD nodes was 50 km, due to the introduction of the WDM elements and the splitters/combiners. These losses

Photonics 2018, 5, 24 12 of 13 can be traversed by the adaptive signal generated by the S-BVT. In particular, 27 Gb/s connections can be supported up to 75 km including 3 SOAs acting as switching nodes without any filtering element, and while 26 Gb/s for 50 km is the maximum achievable reach including 2 OAD nodes. The experimental assessment demonstrates that thanks to the possibility of controlling the injected current to the SOAs and the application of loading schemes at the adaptive transceivers, high flexibility, scalability, and adaptability can be obtained. However, despite the possibility of compensating the losses thanks to the SOAs, the maximum transmission distance with 2 OAD nodes was 50 km, due to the introduction of the WDM elements and the splitters/combiners. These losses are around 4 dBm for each WDM element and 1 dBm considering each splitter/combiner, meaning that the current injected into the SOA needs to be efficiently controlled to avoid them. Different alternatives must be exploited for this setup to improve the attainable distance including the OAD nodes. OFDM based on SSB or VSB can be adopted with DD to simply maintain architecture [7,20,21,24]. To enhance the impairment tolerance and the achievable distance, coherent detection can also be used as in [25,26]. Monitoring techniques and the SDN paradigm play a key role in enabling these features. The combination of cost-effective implementation of programmable (SDN-enabled) BVT and SOA-based switching nodes seems to be promising for further investigation. In fact, future work in this direction will be carried out to improve the proposed system and target future optical metro networks.

Author Contributions: Investigation, L.M.G.; Methodology, L.M.G., S.v.d.H. and X.X.; Resources, S.v.d.H., X.X., J.v.W., N.C. and C.O.; Software, L.M.G. and J.M.F.; Supervision, N.C., C.O., M.S.M. and J.M.F.; Validation, L.M.G., S.v.d.H., J.v.W., N.C. and C.O.; Visualization, S.v.d.H., J.v.W., N.C. and C.O.; Writing—original draft, L.M.G.; Writing—review & editing, L.M.G. and M.S.M. Funding: This work has been partially funded by the European Commission through the H2020-ICT-2016-2 METROHAUL project (G.A. 761727) the Spanish MINECO project DESTELLO (TEC2015-69256-R) and the FPI research scholarship grant BES-2013-064397. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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