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Ultra-High-Capacity Optical Packet Switching Networks with Coherent Polarization Division Multiplexing QPSK/16QAM Modulation Formats José Manuel Delgado Mendinueta *, Satoshi Shinada, Hideaki Furukawa and Naoya Wada Photonic Network System Laboratory, National Institute of Information and Communications Technology (NICT), 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan; [email protected] (S.S); [email protected] (H.F); [email protected] (N.W.) * Correspondence: [email protected]; Tel.: +81-42-327-6853 Received: 27 January 2017; Accepted: 31 March 2017; Published: 7 April 2017

Abstract: Optical packet switching (OPS) networks and its subsystems, like the burst-mode receiver, are an essential technology currently used in passive optical networks (PONs). Moreover, OPS may play a fundamental role on future hybrid optical circuit switching (OCS)/OPS networks and datacenter networks. This paper focuses on two fundamental subsystems of packetized optical networks: the digital coherent burst-mode receiver and the electro-optical switch. We describe and experimentally characterize a novel digital coherent burst-mode receiver that makes uses of the Stokes parametrization to rapidly estimate the state of polarization (SOP) and optimize the equalizer convergence time. This burst-mode receiver is suitable for optical packetized networks that make use of advanced modulation formats such as quadrature amplitude modulation (QAM). We study the suitability of (Pb,La)(Zr,La)O3 (PLZT) optical switches for amplitude-variable coherent polarization division multiplexing (PDM) 16QAM modulation format and demonstrate a switching capacity of 10.24 Tb/s/port. We demonstrate a full 2 × 2 OPS node with a control plane capable of solving packet contention by means of packet dropping or buffering with a switching capacity of 10.24 Tb/s/port. Finally, we demonstrate the operation of the 2 × 2 OPS node with a record capacity of 12.8 Tb/s/port plus 100 km of dispersion-compensated fiber transmission. Keywords: optical communications; optical packet-switched networks; coherent burst-mode receivers; PLZT optical switches

1. Introduction Currently deployed optical networks are able to dynamically allocate network resources with a granularity of a wavelength channel in the so-called fixed grid optical networks [1], or, more recently, with some variability on the spectral width of each wavelength channel and adapting the modulation format in the flexible/elastic optical networks [2]. All these networks follow the connection oriented paradigm [3] and hence may collectively be referred to as optical circuit switching (OCS) networks [4]. Compared to static optical networks, where no optical switching is done, the flexibility in the wavelength dimension enables a better utilization of the network resources, a reduction of the energy consumption and lowering the costs [5]. In spite of this, it is expected that the increased demand of network capacity, which is likely to grow steadily over the following years [6], will push the requirements of optical networks even further. In order to meet these challenges, further increasing of the network granularity may be the key point to enhance the network utilization, reduce the energy consumption and lower the costs. In the past, the need to increase the network granularity motivated the emergence of optical packet switching (OPS) and optical burst switching (OBS) networks [7]. However, both OPS and OBS networks never abandoned the academic circles due to its great implementation Photonics 2017, 4, 27; doi:10.3390/photonics4020027

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due to its great implementation complexity and the ability of electronical switching to effectively complexity and ability promised of electronical switchingRecently, to effectively compete with the promised compete with thethe benefits by OPS/OBS. research attention hasbenefits been focused on by OPS/OBS. Recently, research attention has been focused on hybrid optical networks [4]. These hybrid optical networks [4]. These hybrid optical networks combine OCS flexible/elastic lightpaths for hybrid optical networks combine flexible/elastic lightpaths for network servicesoptical that require network services that require high OCS availability with OPS datagrams [3] that deliver packetshigh on availability with OPS datagrams [3] that deliver optical packets on a “best effort” approach. Hybrid a “best effort” approach. Hybrid OCS/OPS networks may make use of standard single mode fibers OCS/OPS networks may make of standard mode fibers [4] or multicore fibers [8]. (SMFs) [4] or multicore fibers [8].use Moreover, OPSsingle subsystems such (SMFs) as the burst-mode receiver with Moreover, OPS subsystems such asmodulation the burst-mode receiver with non-coherent keying (OOK) non-coherent on-off keying (OOK) format are currently used in the on-off upstream of optical modulation format are currently used in the upstream of optical access networks such as passive access networks such as passive optical networks (PONs) [9] and upgraded coherent burst-mode optical networks (PONs) [9] and upgraded coherent burst-mode receivers may play an important role receivers may play an important role in future datacenter networks [10,11]. Whether for hybrid in future datacenter [10,11]. Whether for hybrid OCS/OPS networks, PONs orand datacenter OCS/OPS networks, networks PONs or datacenter networks, fast optical switching technologies digital networks,burst-mode fast optical receivers switchingare technologies digital technologies coherent burst-mode receivers are going to of be coherent going to beand essential to leverage the development essential technologies tothe leverage the development of optical networking over the next years. optical networking over next years. Previously, extensive research has been been carried carried out out in in our our group group on on OPS-related OPS-relatedsubsystems. subsystems. Previously, extensive research has Our OPS node architecture has an all-optical data plane that is made of optical switches and fiber fiber Our OPS node architecture has an all-optical data plane that is made of optical switches and delay lines (FDLs). transparency, optical packets withwith different data data rates delay (FDLs). Owing Owingtotothe theoptical opticalswitch switch transparency, optical packets different and high-order modulation formats can be handled without changing the OPS node architecture. rates and high-order modulation formats can be handled without changing the OPS node Figure 1 shows an evolution of evolution the OPS node percapacity port over time. differential architecture. Figure 1 shows an of thecapacity OPS node per port Employing over time. Employing quaternary quaternary shift keying shift (DQPSK) modulated, length, colored opticalcolored packetsoptical over 64 wavelength differential keying (DQPSK)fixed modulated, fixed length, packets over channels and a (Pb,La)(Zr,La)O (PLZT) [12] optical switch with packet buffering, a switching capacity 64 wavelength channels and a 3(Pb,La)(Zr,La)O3 (PLZT) [12] optical switch with packet buffering, a of 1.28 Tb/s/port demonstrated [13]. Using polarization division multiplexing (PDM)-DQPSK switching capacity was of 1.28 Tb/s/port was demonstrated [13]. Using polarization division multiplexing modulation format, we demonstrated a switching capacity of 1.28 Tb/s/port with 32 wavelength (PDM)-DQPSK modulation format, we demonstrated a switching capacity of 1.28 Tb/s/port with 32 channels [14] and later 2.56 Tbit/s/port with 64 wavelength channels [15,16]. wavelength channels [14] and later 2.56 Tbit/s/port with 64 wavelength channels [15,16].

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Figure Figure 1. 1. Evolution Evolutionover overtime timeof ofthe theswitching switchingcapacity capacityof ofrecent recent Optical Optical packet packet switching switching(OPS) (OPS) experiments experimentsat atthe theNational National Institute Institute of of Information Information and and Communications Communications Technology Technology (NICT). (NICT).

In this paper, we will focus on the state-of-the-art optical switching subsystems that make use In this paper, we will focus on the state-of-the-art optical switching subsystems that make use of of wavelength division multiplexing (WDM) optical packets, advanced coherent modulation wavelength division multiplexing (WDM) optical packets, advanced coherent modulation formats formats such as PDM-QPSK and PDM 16 quadrature amplitude modulation (QAM) and PLZT such as PDM-QPSK and PDM 16 quadrature amplitude modulation (QAM) and PLZT optical switches optical switches capable of switching multi-level QAM optical signals. On Section 2, we investigate capable of switching multi-level QAM optical signals. On Section 2, we investigate the requirements the requirements of digital coherent burst-mode receivers (DCBMRXs) and describe a novel digital of digital coherent burst-mode receivers (DCBMRXs) and describe a novel digital coherent burst-mode coherent burst-mode receiver that makes use of a state of polarization (SOP) estimator based on the receiver that makes use of a state of polarization (SOP) estimator based on the Stokes parameters for Stokes parameters for rapid packet demodulation [17]. This receiver is an essential subsystem used rapid packet demodulation [17]. This receiver is an essential subsystem used in all the subsequent in all the subsequent OPS demonstrations. Section 3 demonstrates a 10.24 Tb/s/port optical switch OPS demonstrations. Section 3 demonstrates a 10.24 Tb/s/port optical switch for PDM-16QAM for PDM-16QAM optical packets [18]. One of the main aspects of optical switches for higher optical packets [18]. One of the main aspects of optical switches for higher multi-level modulation multi-level modulation format is transparency. A semiconductor optical amplifier (SOA) switch, format is transparency. A semiconductor optical amplifier (SOA) switch, which was often used as a which was often used as a time-division optical switch in the past, may cause serious distortion on time-division optical switch in the past, may cause serious distortion on multi-level QAM signals due multi-level QAM signals due to the patterning effect [19,20]. To alleviate this, we employ

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to the patterning effect [19,20]. To alleviate this, we employ electro-optical (EO) phase shift switches made of PLZT [12,18]. Additionally, the PLZT switch can provide a wider switching wavelength band than the SOA switches. Section 4 describes a full 2 × 2 OPS optical switching node with a control plane capable of dealing with packet contention by means of packet dropping or packet buffering in fiber delay lines (FDLs). This 2 × 2 OPS node is first demonstrated in back-to-back with a switching capacity of 10.24 Tb/s/port [21]. Section 5 demonstrates the 2 × 2 OPS switching node with an improved transmitter and achieves 12.8 Tb/s/port in a transmission experiment consisting of two spans of 50 km of SMF and dispersion compensating fiber (DCF) [22]. Finally, the paper is summarized and the main conclusions are drawn. 2. Coherent Burst-Mode Receivers for PDM-QPSK and PDM-16QAM Modulated Payloads The vast majority of current optical networks follow the connection oriented network paradigm and hence both transmitter and receiver work in continuous-mode from the beginning until the end of the connection [3]. For example, in a wavelength routed optical network (WRON), a wavelength channel named lightpath is open and remains open for the duration of the connection [1]. In dynamic optical networks, however, the information stream is packetized at the optical level, and this introduces a series of challenges in the optical receiver compared to the continuous-mode case. A receiver capable of receiving optical packets is referred as a burst-mode receiver (BMRX) [23,24]. Current optical packet networks, such as PONs [9], make use of non-coherent, analog BMRXs with OOK modulation format [23]. The main challenges of OOK BMRXs are the detection of the presence of an incoming optical packet and the quick estimation of the amplitude of the optical signal to establish a threshold to cope with different packet amplitudes on a packet-by-packet basis. The introduction of coherent modulations requires digital coherent burst-mode receivers (DCBMRXs). Compared with analog BMRXs, DCBMRXs perform the signal processing in the digital domain and new parameters should be estimated, including the SOP, differential group delay (DGD), carrier frequency offset and carrier phase. Moreover, for PDM modulated signals, the quick estimation of the SOP is of extreme importance for correct signal demodulation and requires new algorithms to be developed for DCBMRXs. For example, in continuous-mode receivers, the constant modulus algorithm (CMA) has been proposed and widely accepted as a practical solution due to its low computational complexity [25]. However, depending on the initial equalizer kernel and the SOP of the optical packet, the CMA equalizer may converge to a situation when one of the polarizations is output to both tributaries. This situation is known as the CMA singularity [25]. This singularity problem is not critical on continuous-mode systems where it can be solved by inspecting the equalizer output and resetting the convergence process. Furthermore, the CMA equalizer may take several hundredths symbols to converge, which is not acceptable in a BMRX where the equalizer convergence should be as rapid as possible in order to increase the network throughput, i.e., reduce the optical layer packet overhead. Several DCBMRX designs have been proposed in the literature to determine the SOP in a non-singular way. Ref. [26] transmits an optical packet header with only one polarization tributary and hence the SOP can be estimated with the algorithm proposed in [27]. However, this requires a more complex transmitter capable of nullifying one of the signal polarizations and additionally the lower energy in the packet header will impair the packet detection process due to additional noise compared to having a PDM header. Ref. [28] proposes to track the slope of the modulus of the CMA error signal, an operation that can be performed in parallel. However, this solution may have a high implementation complexity. In this section, we propose and experimentally characterize a novel DCBMRX that makes use of the Stokes parametrization in order to promptly initialize the equalizer with instantaneous convergence time and in a non-singular way.

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2.1. Coherent Burst-Mode Receiver Based on the Stokes Sphere Using the Jones notation, the received optical signal can be expressed as: e0 X e0 Y

!

e X e Y

=M

!

+ N ( t ),

(1)

e are the complex envelopes of the two orthogonal transmitted polarizations, X e0 e and Y e 0 and Y where X  T ex ( t ) n ey (t) is a complex additive white Gaussian are the received complex envelopes and N (t) = n noise (AWGN) added by active electrical and optical components to both polarizations. The matrix M models the polarization rotation and can be expressed as [29,30]: M=

cos(θ )e−iϕ/2 sin(θ )e+iϕ/2

−sin(θ )e−iϕ/2 cos(θ )e+iϕ/2

! ,

(2)

where the angles θ in (−90◦ , 90◦ ) and ϕ in (−180◦ , 180◦ ) cover the whole Poincaré sphere. Note that this model does not include fiber attenuation losses, chromatic dispersion (CD), polarization dependent loss (PDL) and fiber nonlinearities [31]. The matrix M is unitary and hence its inverse is equal to its transpose conjugate. Consequently, M− 1 can be trivially computed if an estimation of the values of the SOP rotation angles θ and ϕ is known. The Stokes parameters of the received optical signal are defined as [32]: s0 = | x |2 + | y |2 , s1 = | x |2 − | y |2 ,

(3)

s2 = 2· Re( xy), s3 = 2· Im( xy).

When plotted on the Poincaré sphere and for a PDM-QAM modulated signal, the Stokes transformation defines a lens-shaped object [33]. For an optical signal digitized at two samples per bit, each of the tributary components defines an eye diagram as illustrated on Figure 2a, where the clock recovery has placed the even samples at the center of the eye (the optimum sampling point) and the odd samples define the transition points. When plotted on the Stokes space, the even samples appear as a cluster of points located close to the sphere surface and the transition samples, i.e., the odd samples appear as smaller clusters with lower absolute value inside the sphere. For a PDM-QPSK modulated signal, the even samples produce four clusters of points on the sphere surface located on the vertexes of a square. High order QAM modulation formats produce clusters of points that approach the lens-shaped object [34]. This is illustrated on Figure 2d for a PDM-16QAM modulation. Note that the lens-shaped object defines a geodesic line on the Poincaré sphere and that this geodesic line defines an intersection plane with the sphere that can be characterized by a normal orthogonal → vector n . The SOP angles θ and ϕ can be estimated by finding the best fitting plane for the set of n rotation o →

→

points S = s i , where s = (s1 , s2 , s3 ) correspond to the points in the Stokes space given by the even samples of the signal. In this work, the singular value decomposition (SVD) [35] was used to → → estimate the vector n = (n1 , n2 , n3 ) normal to this plane. With the knowledge of n , an estimation of the SOP rotation angles is given by:

θˆ =

tan−1

q

n22 + n23

n1 ϕˆ = tan−1

n3 . n2

,

(4)

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𝑠1 = |𝑥|2 − |𝑦|2 ,

(3)

𝑠2 = 2 ∙ 𝑅𝑒(𝑥𝑦̅), 𝑠3 = 2 ∙ 𝐼𝑚(𝑥𝑦̅). Photonics 2017, plotted 4, 27 When

5 of 18 on the Poincaré sphere and for a PDM-QAM modulated signal, the Stokes transformation defines a lens-shaped object [33]. For an optical signal digitized at two samples per bit, each of thehas tributary components defines an eye diagram as illustrated Figure 2a,θˆwhere the The SVD a high computational cost. However, an accurate estimation on of the angles and ϕˆ can clock recovery has placed the even samples at the center of the eye (the optimum sampling point) be obtained with only a reduced number of S points, which may reduce the overall complexity of the and estimator the odd samples define the transition points. When plotted on algorithms. the Stokes With space, the even SOP compared to alternative DCBMRX designs with iterative knowledge samples appear as a cluster of points located close to the sphere surface and the transition ˆ of θ and ϕˆ and assuming a six-tap CMA equalizer (three symbols), the initial equalizer kernel issamples, defined i.e., the odd samples appear as smaller clusters with lower absolute value inside the sphere. For a as: h i h i jϕ jϕ PDM-QPSK modulated H signal, the even samples produce four clusters of points on the sphere − xx = 0 0 0cosθe 2 0 0 , Hxy = 0 0 0sinθe 2 0 0 , surface located on the vertexeshof a square. High order QAM modulation formats produce clusters(6) of i h i jϕ jϕ −2 2 0 0 This , Hyyis=illustrated 0 0 0 cosθe 0 0 . 2d for a PDM-16QAM Hyx = 0 0 0 −object sinθe [34]. points that approach the lens-shaped on Figure modulation. Note that the lens-shaped object defines a geodesic line on the Poincaré sphere and that Note that a CMA equalizer initialized with a kernel defined in the previous equation is singularity this geodesic line defines an intersection plane with the sphere that can be characterized by a normal free and the convergence is instantaneous. orthogonal vector 𝑛⃗.

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Figure 2. (a) Eye diagram for any of the tributary components of a PDM-QPSK signal, sampled at Figure 2. (a) Eye diagram for any of the tributary components of a PDM-QPSK signal, sampled two samples-per-symbol; (b)(b) format the at two samples-per-symbol; formatofofthe theoptical opticalpacket packet header. header. Each Each number number represents represents the transmitted constellation symbol on each symbol period; (c) QPSK constellation diagram showing transmitted constellation symbol on each symbol period; (c) QPSK constellation diagram showing the constellation constellation symbol symbol mapping; mapping; and and (d) (d) example example of of the the lens-shaped lens-shaped object object for for PDM-16QAM PDM-16QAM and and aa the polarization rotation. rotation. polarization

2.2. Digital Coherent Burst-Mode Receiver DSP Stages Figure 3 shows a diagram of the digital signal processing (DSP) blocks used in the proposed DCBMRX [17]. First, a packet detector detects when an incoming optical packet arrives to the receiver and triggers the execution of the subsequent receiver stages [36]. Note that, in this work, this packet detector was not implemented and perfect packet detection was emulated with knowledge of the pattern timing from the transmitter. After the packet detector, a front-end correction and resampling stage corrects for channel skew and resamples the captured signal to four samples-per-symbol or the Nyquist rate of two samples-per-symbol. Then, the signal is normalized and the carrier frequency offset estimated and compensated by means of a fast Fourier transform (FFT). After the frequency offset has been removed, samples are aligned in a clock recovery stage using the Lee algorithm for two samples-per-symbol signals [37] or the Oerder algorithm for four samples-per-symbol signals [38]. Note that Nyquist shaped signals with small roll-off factor may require custom algorithms for clock recovery [39]. Next, the SOP angles θ and ϕ are estimated using the proposed algorithm and its values used to initialize the CMA equalizer in case of receiving a PDM-QPSK modulation or a radially directed equalizer (RDE) for PDM-16QAM modulation. After equalization, the carrier phase is estimated with the Viterbi and Viterbi algorithm [40] or a set-partitioned Viterbi and Viterbi [41] for QPSK and 16QAM modulations, respectively. Finally, bit error rate (BER) is measured by error counting.

require custom algorithms for clock recovery [39]. Next, the SOP angles 𝜃 and 𝜑 are estimated using the proposed algorithm and its values used to initialize the CMA equalizer in case of receiving a PDM-QPSK modulation or a radially directed equalizer (RDE) for PDM-16QAM modulation. After equalization, the carrier phase is estimated with the Viterbi and Viterbi algorithm [40] or a set-partitioned Photonics 2017, 4, 27 Viterbi and Viterbi [41] for QPSK and 16QAM modulations, respectively. Finally, 6 ofbit 18 error rate (BER) is measured by error counting. Stokes SOP estimation

fOffset estimation Packet detector

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Normalization

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Figure 3. 3. Digital signal processing (DSP) stages of the digital coherent burst-mode receiver. Figure Photonics 2017, 4, 27 Digital signal processing (DSP) stages of the digital coherent burst-mode receiver.

Error counter

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Digital Coherent Coherent Burst-Mode Receiver Experimental Characterization 2.3. Digital The experimental experimentalsetup setupfor forthe thecharacterization characterizationofofthe the coherent burst-mode receiver is shown The coherent burst-mode receiver is shown in in Figure output external cavitylaser laser(ECL) (ECL)was wassplit split and and modulated modulated using using two Figure 4a.4a. TheThe output of of anan external cavity dual-parallel Mach-Zehnder Mach-Zehnder modulators modulators (DPMZM). (DPMZM). These These modulators modulators were were driven driven with with four dual-parallel independent bit-streams bit-streams at 10 Gb/s each to to independently independently modulate modulate every every PDM-QPSK PDM-QPSK tributary tributary independent Gb/s each electrical phase delays and polarization X contained an optical component. Channels Channels22and and4 included 4 included electrical phase delays and polarization X contained an delay line for precise aligning of the tributary components of the optical signal. After modulation, optical delay line for precise aligning of the tributary components of the optical signal. After polarizations polarizations X and Y were X combined in a polarization beam combiner (PBC). PDM-QPSK optical modulation, and Y were combined in a polarization beamThe combiner (PBC). The signal was amplified using was an erbium-doped fiber (EDFA), and,amplifier finally, and an optical PDM-QPSK optical signal amplified using anamplifier erbium-doped fiber (EDFA), and, attenuator to set the desired the optical receiver. optical finally, andwas an used optical attenuator wasoptical used power to set before the desired optical powerThe before thereceiver optical consistedThe of an ECL receiver free-running laser of used as local oscillator laser (LO),used a polarization-diverse optical receiver. optical consisted an ECL free-running as local oscillator (LO), a hybrid plus balanced photodetectors, sampling oscilloscope and used a as adigital digitizer, having polarization-diverse optical hybrid and plusa digital balanced photodetectors, sampling an electrical used bandwidth of 16 GHz. Theanfour electrical signals after balanced photodiodes were oscilloscope as a digitizer, having electrical bandwidth of 16 the GHz. The four electrical signals digitized at 50 GS/s. after the balanced photodiodes were digitized at 50 GS/s.

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Figure 4. (a) Experimental setup for the characterization of the proposed coherent burst-mode Figure 4. (a) Experimental setup for the characterization of the proposed coherent burst-mode receiver; receiver; and (b) diagram of the generated optical packets. and (b) diagram of the generated optical packets.

Figure 4b depicts a diagram of the generated optical packets used for the characterization of the Figure 4b depictsEach a diagram the generated optical of packets used for followed the characterization of the burst-mode receiver. packetofconsisted of a header 1096 symbols by a payload of burst-mode receiver. Each packet consisted of a header of 1096 symbols followed by a payload of 15,288 symbols. The pattern used on each payload binary tributary consisted of the first 15,288 bits of 15,288 symbols. The pattern used on each payload binary tributary consisted of the first 15,288 bits of a pseudo-random binary sequence (PRBS) sequence of order 20 and the accumulated PRBSa pseudo-random binary different sequence tributaries (PRBS) sequence of order 20 and theexperimental accumulated setup PRBS decorrelation decorrelation between was 262,144 bits. In the of Figure 4a, between different tributaries was 262,144 bits. In the experimental setup of Figure 4a, no packet carver no packet carver such as an acousto-optical modulator (AOM) or SOA was used. Instead, to simulate such as an acousto-optical modulator (AOM) or SOA was used. Instead, to simulate optical packets optical packets with a desired header length, the oscilloscope was triggered at the beginning of the with a desired header length, the were oscilloscope triggered at knowledge the beginning of the pattern and then pattern and then optical packets digitallywas chopped with of the timing. The optical optical packets were digitally chopped with knowledge of the timing. The optical packet header packet header consisted of two PDM tributaries, each repeating the constellation pattern shown in consisted of two tributaries, each constellation pattern Figurepower 2b,c. This Figure 2b,c. ThisPDM constellation has therepeating propertythe that the transitions to shown nullify in optical are constellation has the property that the transitions to nullify optical power are maximized, is maximized, which is beneficial for timing recovery. Furthermore, sufficient points in thewhich Stokes beneficial for timing recovery. Furthermore, sufficient points in the Stokes space are generated for space are generated for having an accurate estimation of the SOP. having an accurate estimation of the SOP. 2.4. Methodology and Experimental Results 2.4. Methodology and Experimental Results Note that the experimental setup of Figure 4a does not include a polarization scrambler to Note that the experimental setup of Figure 4a does not include a polarization scrambler to rotate rotate the SOP of the optical packets before the receiver. Instead, a set of 128 packets having a known the SOP of the optical packets before the receiver. Instead, a set of 128 packets having a known SOP SOP were captured, and then the SOP was estimated using the whole of the optical packet in order were captured, and then the SOP was estimated using the whole of the optical packet in order to to minimize the estimation errors, and the SOP was digitally corrected. Then, each packet was chopped to the desired header length and the SOP was digitally rotated using Equation (2) with 𝜑 in the range [–180°, 180°] and 𝜃 in the range of [–90°, 90°] to simulate any arbitrary SOP rotation state over the whole Poincaré sphere. Before the digital SOP rotation, synthetic AWGN, independent for every optical packet and every SOP rotation, was added to the signal in order to produce a payload BER of roughly 10−3.

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minimize the estimation errors, and the SOP was digitally corrected. Then, each packet was chopped to the desired header length and the SOP was digitally rotated using Equation (2) with ϕ in the range [–180◦ , 180◦ ] and θ in the range of [–90◦ , 90◦ ] to simulate any arbitrary SOP rotation state over the whole Poincaré sphere. Before the digital SOP rotation, synthetic AWGN, independent for every optical packet and every SOP rotation, was added to the signal in order to produce a payload BER of roughly 10−3 . Figure 5a shows the averaged payload-BER of 128 optical packets as a function of the rotation angles θ and ϕ covering the whole Poincaré sphere surface. In this case, no synthetic noise was added Photonics 4, 27 was a standard CMA equalizer initialized with taps: 7 of 17 and the 2017, equalizer H = Hyy 𝐻xx 𝑥𝑥 = 𝐻𝑦𝑦 Hxy = Hyx 𝐻𝑥𝑦 = 𝐻𝑦𝑥

= 0 00 11 00 00], [0 0 ], = [0 = [0 0 0 0 0 0]. = [0 0 0 0 0 0].

(7) (7)

Inspection Inspection of of Figure Figure 5a 5a clearly clearly reveals reveals that that the the CMA CMA equalizer equalizer exhibits exhibits aa singularity singularity when when the the ◦ ◦ SOP rotation angle is θ = 45 , − 45 for any value of ϕ. We repeated this measurement and added ] for any value of 𝜑. We repeated this measurement SOP rotation angle is 𝜃 [+ = [+45°, −45°] and synthetic noise tonoise the experimental data with a power as the payload-BER equaled the forward added synthetic to the experimental data with asuch power such as the payload-BER equaled the −3 . Figure−35b shows that the effect of noise, compared to Figure 5a, error correction (FEC) limit of 10 forward error correction (FEC) limit of 10 . Figure 5b shows that the effect of noise, compared to is to unsmooth curves, the butcurves, the CMA is still present. Next, the proposed Figure 5a, is to the unsmooth butequalizer the CMAsingularity equalizer singularity is still present. Next, the SOP estimation and equalizer initialization algorithm were tested for header sizes of {4, 8, 16, 32,{4,64} proposed SOP estimation and equalizer initialization algorithm were tested for header sizes of 8, samples. The results of this measurement are plotted in Figure 5c. Inspection of the figure reveals that 16, 32, 64} samples. The results of this measurement are plotted in Figure 5c. Inspection of the figure 3 ], independently −3, 5 × 10−3], independently the average stays in aBER range of approximately [10−3 , 5 × 10−[10 of the headerofsize, reveals thatBER the average stays in a range of approximately the which demonstrates that the proposed algorithm solves the CMA singularity problem. header size, which demonstrates that the proposed algorithm solves the CMA singularity problem.

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Finally, we assessed the equalizer convergence time by doing a time-resolved BER measurement averaging the errors of each of the four PDM-QPSK PRBS tributaries over the 128 captured packets. Figure 6a shows the time-resolved BER in the packet payload, for the standard CMA receiver and the proposed new receiver algorithm, averaged over all SOP rotations covering the Poincaré sphere. The red line shows the average BER for all 128 packets without any SOP rotation and can be considered as the calibrated BER value. For the standard CMA receiver, all the cases where the singularity was detected were not computed in order to not artificially increase the number of errors. Figure 6b shows the curves of Figure 6a filtered with a median filter having a window size of 31 to smooth the curves. Results demonstrate that, on average, the CMA receiver takes up to 3000 symbols to converge. However, the CMA equalizer initialized with the algorithm proposed in this work converges instantly for an estimation size of 16 or greater and completely avoids the singularity problem. Under a noise loading that yields a payload BER of 10−3 , initialization of the CMA equalizer is successful with only 16 estimation points of the packet header. Figure 6. (a) time-resolved BER measurement of the packet payload; (b) same as (a), but a median filter has been applied to smooth the curves.

Finally, we assessed the equalizer convergence time by doing a time-resolved BER measurement averaging the errors of each of the four PDM-QPSK PRBS tributaries over the 128 captured packets. Figure 6a shows the time-resolved BER in the packet payload, for the standard

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parameters as (a) but for the proposed algorithm and varying-size header lengths.

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Figure 6. (a) time-resolved BER measurement of the packet payload; (b) same as (a), but a median Figure 6. (a) time-resolved BER measurement of the packet payload; (b) same as (a), but a median filter filter has been applied to smooth the curves. has been applied to smooth the curves.

Finally, we assessed the equalizer convergence time by doing a time-resolved BER 3. High-Capacity DWDM PDM-16QAM Optical Switch measurement averaging the errors of each of the four PDM-QPSK PRBS tributaries over the 128 This section theshows first experimental demonstration of apacket high-capacity wavelength captured packets.describes Figure 6a the time-resolved BER in the payload,dense for the standard division multiplexing optical packet switching node using PDM-16QAM modulated optical CMA receiver and the(DWDM) proposed new receiver algorithm, averaged over all SOP rotations covering packets and electro-optical PLZT switches. the Poincaré sphere. The red line shows the average BER for all 128 packets without any SOP rotation and can be considered as the calibrated BER value. For the standard CMA receiver, all the 3.1. Experimental Setup cases where the singularity was detected were not computed in order to not artificially increase the number of errors. Figure 6b shows the curves Figure 6a for filtered withPDM-16QAM-modulated a median filter having a Figure 7 shows the experimental setup of aoftransmitter DWDM, window size of As 31 light to smooth thewe curves. Results demonstrate that, onfrom average, CMA receiver optical packets. sources, used an array of 64 lasers, ranging 1537.0the to 1562.23 nm on takes to grid 3000spacing. symbolsThe to converge. However, initialized with thecollectively algorithm the 50 up GHz continuous waves ofthe the CMA 32 oddequalizer and 32 even channels were modulated by two independent DPMZMs, respectively. The modulators generated 20 GBd single polarization 16-QAM optical packets with an aggregated bit rate of 80 Gbit/s per channel and per polarization. To generate the optical packet train, the continuously modulated signal was gated by two MZM modulators to remove the bogus PRBS data used in the guard time between packets. Then, packets were polarization-multiplexed by a polarization beam combiner (PBC) with a decorrelation of 12 symbols between polarizations. As a result, waveband optical packets at a rate of 10.24 Tbit/s were generated. A train of eight packets of different lengths, ranging from 1000 to 4000 symbols, was generated and each packet had either a PSK label A or label B, as shown in the inset of Figure 7. For label generation, 10 GHz optical pulses generated by a mode-locked laser diode (MLLD, 1530 nm) were modulated using known timing from the coherent packets payload pattern by an intensity modulator. The pulses were input to a 200 Gchip/s multiple optical encoder (MOE) to generate optical phase-shift keying (PSK) codes. The MOE has an AWG configuration [42,43] with 16 inputs and 16 outputs, and behaves like a transversal filter to simultaneously generate 16-chip PSK codes. When a short pulse is sent into one of input ports of MOE, 16 different PSK codes are generated from each output port [43]. In this demonstration, two PSK codes were chosen and named label A and label B. After coupling of even and odd channels by a wavelength interleaver (100 GHz to 50 GHz), each packet was sandwiched by the PSK labels.

optical phase-shift keying (PSK) codes. The MOE has an AWG configuration [42,43] with 16 inputs and 16 outputs, and behaves like a transversal filter to simultaneously generate 16-chip PSK codes. When a short pulse is sent into one of input ports of MOE, 16 different PSK codes are generated from each output port [43]. In this demonstration, two PSK codes were chosen and named label A and label B.2017, After coupling of even and odd channels by a wavelength interleaver (100 GHz to 50 GHz), Photonics 4, 27 9 of 18 each packet was sandwiched by the PSK labels. Synthesizer

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Figure 7. Transmitter setup for the generation of DWDM PDM-16QAM modulated optical packets Figure 7. Transmitter setup for the generation of DWDM PDM-16QAM modulated optical packets and and 64 wavelength channels, (inset) diagram of the generated eight-packet train. 64 wavelength channels, (inset) diagram of the generated eight-packet train.

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The Thepacket packettrain trainwas wasthen thensplit splitinto intotwo twoto tosimulate simulatetwo twodifferent differenttransmitters, transmitters,named namedTX1 TX1and and TX2, as shown in Figure 8. In this experiment, we used two 1 × 2 PLZT switches for the label-switching TX2, as shown in Figure 8. In this experiment, we used two 1 × 2 PLZT switches for the (2 × 2 OPS without designedbuffer), to forward optical label A onlywith input from label-switching (2 ×buffer), 2 OPS without designed to packets forwardwith optical packets label A TX1 only and TX2. Each 1 × 2 PLZT optical switch had a PDL ranging from 0.3 to 1 dB. When multiple optical input from TX1 and TX2. Each 1 × 2 PLZT optical switch had a PDL ranging from 0.3 to 1 dB. When decoders [42,43] recognized an input label as an label A, an output gateA, signals an FPGA based multiple (MODs) optical decoders (MODs) [42,43] recognized input label as label an output gate signals buffer manager. The buffer manager sends open or close signals to two switches for TX1 and TX2. an FPGA based buffer manager. The buffer manager sends open or close signals to two switches As for aTX1 result, trainsfour-packet with label Atrains werewith output to A each port. To demonstrate packet additiona andfour-packet TX2. As a result, label were output to each port.aTo demonstrate process, a delayed four-packet trainfour-packet of TX2 was train added a four-packet of TX1. Output packet addition process, a delayed ofto TX2 was added train to a four-packet trainpackets of TX1. after the packet addition process were input to a polarization-diverse coherent receiver, shown in Output packets after the packet addition process were input to a polarization-diverse coherent Figure 8, using a free-running tunable laser as local oscillator (LO) with linewidth of approximately receiver, shown in Figure 8, using a free-running tunable laser as local oscillator (LO) with linewidth 100 The four-channel electrical signal after photo detection thenphoto digitized at 50 GS/s, of kHz. approximately 100 kHz. The four-channel electrical signalwas after detection was and then the offline processing was performed with MATLAB (version R2015b, The MathWorks, Inc., Natick, digitized at 50 GS/s, and the offline processing was performed with MATLAB (version R2015b, The MA, USA) forInc., easyNatick, prototyping. MathWorks, MA, USA) for easy prototyping.

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Figure8.8. Experimental Experimental setup setup of of the the optical optical packet packet switching switching node, node, configured configured to toforward forwardpackets packets Figure withlabel labelAAand anddrop droppackets packetswith withlabel labelB.B.(insets) (insets)Constellation Constellationdiagram diagramof ofthe thepacket packetheader headerand and with waveformof ofthe thereceived receivedeight eightpackets. packets. waveform

The inset of Figure 8 shows a diagram of the optical packet train at the receiver. Each optical packet had a header of 64 symbols (128 samples) followed by a variable-length PDM-16QAM payload. The optical packet header consisted of two PDM tributaries, each modulating the outermost ring of the 16-QAM constellation as shown in the inset of Figure 8. Optical packet detection was not considered in this experiment and known timing from the transmitter was used to

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The inset of Figure 8 shows a diagram of the optical packet train at the receiver. Each optical packet had a header of 64 symbols (128 samples) followed by a variable-length PDM-16QAM payload. The optical packet header consisted of two PDM tributaries, each modulating the outermost ring of the 16-QAM constellation as shown in the inset of Figure 8. Optical packet detection was not considered in this experiment and known timing from the transmitter was used to digitally chop the optical packets for further DSP processing, thus simulating a null packet error rate (PER). The main difference of this receiver with the one described in Section 2.2 is an extra retiming stage necessary for quickly compensating for DGD. It was found that packets P2, P4, P6 and P8, after switching, had a significant amount of DGD compared with packets P1, P3, P5 and P7. In order to digitally compensate this, SOP was reverted with a one-tap butterfly structure, and then the Lee algorithm [37] was employed to realign both polarizations before the RDE equalizer. 3.2. Experimental Results and Discussion Figure 9a–c shows the bit-error-rate (BER) for the 64 wavelength channels after the transmitter (back-to-back, measured at point A on Figure 8), after label-switching (measured at point B) and after packet addition (measured at point C), respectively, for every individual packet P1 to P8 and each measurement averaged over three packets (best three of 10 experimental trials). Discontinuities in the plots arise from error-free measurements. The observed saw-tooth shape is due to uneven biasing of the inphase-quadrature (IQ) modulator between odd and even channels and also to modulator BIAS instabilities. These plots show that PDM-16QAM payload BER is below the FEC limit of 3 . Moreover, the soft horizontal lines show the total BER averaged for all odd packets (cyan) 3 × 10−2017, Photonics 4, 27 10 of 17 and even packets (magenta), showing a very small BER penalty after label-switching. However, after the addition of packets, evenof packets in the train an additional penalty due to increased However, after the addition packets, evenpacket packets in suffer the packet train suffer an additional penalty DGD originating in the optical switch. due to increased DGD originating in the optical switch.

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Figure 9. (a) wavelength Figure 9. (a) Back-to-back Back-to-back payload payload BER BER for for optical optical packets packets P1-P8 P1-P8 as as aa function function of of the the wavelength channel measured at point A in the setup of Figure 8; (b) payload BER after optical switching and channel measured at point A in the setup of Figure 8; (b) payload BER after optical switching and label label swapping measured at B point B of Figure 8; (c) payload BER after addition packet addition measured swapping measured at point of Figure 8; (c) payload BER after packet measured at point at C point C on Figure 8. on Figure 8.

Next, we optimized the OPS transmitter by placing the packet carving MZMs in front of the Next, we optimized the OPS transmitter by placing the packet carving MZMs in front of the modulator (instead of after the modulators) and optimized the DPMZMs BIAS. The average was modulator (instead of after the modulators) and optimized the DPMZMs BIAS. The average was computed over 10 traces. Figure 10 shows a significant BER improvement compared to Figure 9 and computed over 10 traces. Figure 10 shows a significant BER improvement compared to Figure 9 and demonstrates the successful switching with a capacity of 10.24 Tbit/s/port. We confirmed that the demonstrates the successful switching with a capacity of 10.24 Tbit/s/port. We confirmed that the OPS OPS node processing using PLZT optical switches did not adversely affect the multi-level node processing using PLZT optical switches did not adversely affect the multi-level PDM-16QAM PDM-16QAM modulated optical packet signals. modulated optical packet signals.

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Next, we optimized the OPS transmitter by placing the packet carving MZMs in front of the modulator (instead of after the modulators) and optimized the DPMZMs BIAS. The average was computed over 10 traces. Figure 10 shows a significant BER improvement compared to Figure 9 and demonstrates the successful switching with a capacity of 10.24 Tbit/s/port. We confirmed that the OPS node processing using PLZT optical switches did not adversely affect the multi-level Photonics 2017, 4, 27 11 of 18 PDM-16QAM modulated optical packet signals.

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Figure 10. 10. Improved Improved BER BER measurements measurements with with respect respect to to Figure Figure 9. 9. (a) (a) Back-to-back Back-to-back payload payload BER for Figure BER for optical packets P1-P8 as a function of the wavelength channel measured at point A in the setup of optical packets P1-P8 as a function of the wavelength channel measured at point A in the setup of Figure 8; (b) payload BER after optical switching and label swapping measured at point B in Figure Figure 8; (b) payload BER after optical switching and label swapping measured at point B in Figure 8; 8; (c) payload payload BER BER after after packet packet addition addition measured measured at at point point C C in in Figure Figure 8. 8. (c)

4. High 4. High Capacity Capacity 22 × ×22OPS OPSNode Nodewith withContention ContentionResolution Resolution In this this section, section,we wecharacterize characterizea a2 × 2 ×2 2OPS OPS node, shown in Figure strategies to In node, shown in Figure 11a,11a, withwith two two strategies to deal deal with packet contention: optical buffering packet dropping. In control the control plane, with packet contention: optical buffering withwith FDLsFDLs and and packet dropping. In the plane, the the label processor consists of multiple optical decoders [42,43] that recognize an input label and label processor consists of multiple optical decoders [42,43] that recognize an input label and send send triggering to manager a buffer implemented manager implemented on a field programmable gate[44,45]. array triggering signalssignals to a buffer on a field programmable gate array (FPGA) (FPGA) [44,45]. The label generation process is described on Section 3.1. The buffer manager is able The label generation process is described on Section 3.1. The buffer manager is able to electronically to electronically control the optical switch matrix to perform packet forwarding, packet dropping or control the optical switch matrix to perform packet forwarding, packet dropping or packet buffering. packet buffering. The optically transparent oneswitch 1 × 8 on PZLT switch on 2each The optically transparent data plane uses onedata 1 × 8plane PZLTuses optical eachoptical input port. This ×2 input port. This 2 × 2 OPS node will be the key subsystem in the packet contention resolution OPS node will be the key subsystem in the packet contention resolution experiment of this section and experiment of this section and packetexperiment transmission a switching experiment of Section 5. in the packet plusina the switching ofplus Section 5. Photonics 2017, 4,transmission 27 11 of 17 31-FDL Buffer

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Figure 11. (a) architecture of the 2 × 2 optical switching node with contention resolution by packet Figure 11. (a) architecture of the 2 × 2 optical switching node with contention resolution by packet dropping and optical packet buffering; (b) picture of the 2 × 2 OPS node prototype. dropping and optical packet buffering; (b) picture of the 2 × 2 OPS node prototype.

4.1. Switching and Buffering Experimental Setup 4.1. Switching and Buffering Experimental Setup In this experiment, we used the same optical packet transmitter described in Section 3.1. After In this experiment, we used the same optical packet transmitter described in Section 3.1. After the the transmitter, the eight-packet train was split in a 1 × 2 splitter to simulate two different transmitter, the eight-packet train was split in a 1 × 2 splitter to simulate two different transmitters transmitters named TX1 and TX2, and both TX1 and TX2 were input to the optical switching node, as named TX1 and TX2, and both TX1 and TX2 were input to the optical switching node, as shown in shown in Figure 12. We reconfigured our 31-FDL buffering system, which consisted of nine 1 × 8 Figure 12. We reconfigured our 31-FDL buffering system, which consisted of nine 1 × 8 PLZT switches, PLZT switches, fiber sheet delay lines (FSDLs) and an FPGA-based buffer manager, and made a 2 × 2 OPS node system where two PLZT switches were followed by three FSDLs. The unit delay time of each FSDL was 100 ns. One of three different delays {0, 100, 200} ns was given to the packet by changing an output port of the switch. In this demonstration, only packets with label A were processed and characterized. When the multiple optical decoders [42,43] recognized an input label

4.1. Switching and Buffering Experimental Setup In this experiment, we used the same optical packet transmitter described in Section 3.1. After the transmitter, the eight-packet train was split in a 1 × 2 splitter to simulate two different transmitters named TX1 and TX2, and both TX1 and TX2 were input to the optical switching node, as Photonics 2017, 4, 27 12 of 18 shown in Figure 12. We reconfigured our 31-FDL buffering system, which consisted of nine 1 × 8 PLZT switches, fiber sheet delay lines (FSDLs) and an FPGA-based buffer manager, and made a 2 × 2 OPS sheet nodedelay system where twoand PLZT were followed by three FSDLs. time of fiber lines (FSDLs) an switches FPGA-based buffer manager, and made a 2The × 2unit OPSdelay node system each FSDL was switches 100 ns. One three different nstime wasofgiven to thewas packet by where two PLZT wereof followed by three delays FSDLs. {0, The100, unit200} delay each FSDL 100 ns. changing andifferent output delays port of{0,the demonstration, only packetsanwith label One of three 100,switch. 200} ns In wasthis given to the packet by changing output portAofwere the processed anddemonstration, characterized. only When the multiple optical decoders [42,43] an input switch. In this packets with label A were processed andrecognized characterized. Whenlabel the as label optical A, gatedecoders signals output optical packet tolabel the buffer The buffer manager can multiple [42,43] the recognized an input as labelmanager. A, gate signals output the optical resolvetoathe maximum contention 4 × 1 packets. packet buffer manager. Theofbuffer manager can resolve a maximum contention of 4 × 1 packets. 2x2 OPS from TX1

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Figure 12. (a) experimental setup of the 2 × 2 optical packet switch one drop line and three fiber sheet Figure 12. (a) experimental setup of the 2 × 2 optical packet switch one drop line and three fiber delay lines; (b) schematic of the optical packet trains at the output of each optical switch and the sheet delay lines; (b) schematic of the optical packet trains at the output of each optical switch and the aggregated packet train to be processed by the receiver. aggregated packet train to be processed by the receiver.

The aggregated output packets after the OPS node were input into a polarization-diverse The aggregated outputinpackets the OPS node were tunable input into a polarization-diverse coherent coherent receiver, shown Figureafter 8, using a free-running laser as local oscillator (LO) with receiver, shown in Figure 8, using a free-running tunable laser as local oscillator (LO) with linewidth linewidth of approximately 100 kHz (the combined linewidth of TX plus RX was approximately of approximately 100 kHz (the combined linewidth of TX plus RX was approximately ~200 kHz). The four-channel electrical signal was then digitized at 50 GS/s and the offline processing was performed with MATLAB with a DCBMRX as described in Sections 2.2 and 3.1. Figure 12b shows a diagram of the optical packets used in the experiment, consisting of an optical header of 64 symbols followed by a variable-length PDM-16QAM payload. The format of the received optical packet train and the methodology for processing them was similar to the one described on Section 3.1. 4.2. Experimental Results and Discussion In the optical switch, optical packets with label B were dropped and packet contentions between packets with label A of TX1 and TX2 were resolved. In this designed packet contentions, four packets with label A of TX1 and TX2 were given a delay of {0, 0, 0, 100} ns and {100, 0, 100, 200} ns, respectively. All packets with label A were output successfully without overlapping. Figure 13 shows the measured BER for the entire 64 wavelength channels after the OPS node, for every individual packet (P1 to P8) and averaged over 10 packets from different experimental realizations. The observed saw-tooth shape is due to uneven biasing of the DPMZMs between odd and even channels. These plot show that the payload BER is below the FEC limit of 3 × 10−3 after the OPS node and demonstrates the successful operation of the 2 × 2 OPS node including packet buffering with a capacity of 12.24 Tb/s/port.

shows the measured BER for the entire 64 wavelength channels after the OPS node, for every individual packet (P1 to P8) and averaged over 10 packets from different experimental realizations. The observed saw-tooth shape is due to uneven biasing of the DPMZMs between odd and even channels. These plot show that the payload BER is below the FEC limit of 3 × 10−3 after the OPS node and demonstrates the successful operation of the 2 × 2 OPS node including packet buffering with a Photonics 2017, 4, 27 13 of 18 capacity of 12.24 Tb/s/port. 10

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Figure 13. 13. BER BER of of the the 64 64 wavelength wavelength channels channels after after buffering buffering in in the the OPS OPS node node for for each each of of the the eight eight Figure packets with label A from TX1 and TX2, shown Figure 12b. packets with label A from TX1 and TX2, shown Figure 12b.

5. High Capacity 2 ×2 OPS Node with Contention Resolution and Fiber Transmission 5. High Capacity 2 × 2 OPS Node with Contention Resolution and Fiber Transmission This section describes the most up-to-date OPS experiment with a record capacity of 12.8 Tb/s/port. This section describes the most up-to-date OPS experiment with a record capacity of 12.8 Compared to Section 4, we used an improved transmitter setup and the optical packet signal was Tb/s/port. Compared to Section 4, we used an improved transmitter setup and the optical packet transmitted over 100 km of dispersion compensated SMF. signal was transmitted over 100 km of dispersion compensated SMF. 5.1. Transmission Transmission Experimental Experimental Setup Setup 5.1. Figure 14 14shows showsthethe experimental setup of a transmitter for DWDM PDM-16-QAM optical Figure experimental setup of a transmitter for DWDM PDM-16-QAM optical packets. packets. Compared with the setup introduced in Figure 7, the improved setup of Figure 14 Compared with the setup introduced in Figure 7, the improved setup of Figure 14 introduces two introduces two enhancements. arbitrary waveform generators used the to generate enhancements. First, arbitraryFirst, waveform generators (AWGs) were(AWGs) used towere generate driving the driving signal for the modulators as opposed to two-bit digital to analog converters (DACs) signal for the modulators as opposed to two-bit digital to analog converters (DACs) made of discrete made of discrete components. Secondly, the to AWGs used to thegenerate signal inthe order to generate components. Secondly, the AWGs were used carvewere the signal in carve order to optical packets the optical packets and the OOK gate based on an MZM was removed from the setup. and the OOK gate based on an MZM was removed from the setup. The AWGs used to drive the modulators had an electrical bandwidth of 16 GHz and a resolution of eight bits. The four-level I and Q electrical tributaries input to the modulator were equalized with a Nyquist 128-symbol square root raised cosine (SRRC) filter with a roll-off factor of 0.25, followed by a 1025-tap pre-emphasis filter to compensate for the DACs’ non-ideal frequency response. Notably, additional MZM modulators were not used in this work to carve the continuous wave (CW) signal into optical packets owing to the ability of the AWGs to set both I and Q tributaries to the null point of the DPMZMs. The data rate of the optical packets was increased from 160 Gb/s to 200 Gb/s and the decorrelation between the X and Y polarization was six symbols, in order to yield a total capacity of 12.8 Tbit/s. The back-to-back error vector magnitude of the generated PDM-16QAM signal was about 10% without any receiver equalization (with the exception of the receiver’s matched SRRC filter). The format of the optical packet train and packet labels after the transmitter was similar to the one shown on the inset of Figure 7. After the transmitter, the optical signal was split into two to simulate two transmitters, named TX1 and TX2. After the transmitter, the set of eight packets from TX1 and TX2 were propagated through different 50 km SMFs plus DCF spans and were input to both input ports of the 2 × 2 OPS node, described in Section 4. In this experiment, the OPS node was configured to forward optical packets with label A of TX1 and TX2 to the same output port in order to emulate packet contention. To resolve this packet contention, FDLs were used as an optical buffer. The buffer manager calculated the delay time required for each packet to avoid collisions and controlled a set of PLZT switching matrices to send each packet to appropriate FDLs having 0, 100 and 200 ns, respectively. On the other hand, optical packets with

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label B (only from TX2) were sent to the alternative output port without the buffering process (drop process). This whole processing is illustrated in Figure 15, which shows the waveforms processed in the data plane of the OPS node. In the label switching process, packets with label A and label B were separated and four packets with label B of TX2 were output (marked in Figure 15 as DROP). Optical packets with label A were output after the buffering process without any packet loss (labelled FORWARD Photonics 2017, on 4, 27Figure 15). 13 of 17 Label A

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Figure 14. General experimental description/setup of the packet buffering and transmission Figure 14. General experimental description/setup of the packet buffering and transmission experiment. (a) Packet transmitter; and (b) general diagram of the OPS transmission experimental experiment. (a) Packet transmitter; and (b) general diagram of the OPS transmission experimental setup. setup.

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The AWGs used to drive the modulators had an electrical bandwidth of 16 GHz and a Input Packets After label Switching Buffering Receiver resolution of eight bits. The four-level I and Q electrical tributaries input to the modulator were FDL-0 (=0 ns) FORWARD A of equalized square root raised cosine (SRRC) filter with a roll-offLabel factor from TX1with a Nyquist 128-symbol Label A 0.25,Label followed by a 1025-tap pre-emphasis filter to compensate for the DACs’ non-ideal frequency A B A B A B A B response. Notably, additional MZM modulators wereFDL-1 not (=100 usedns)in this work to carve the continuous wave (CW) signal into optical packets owing to the ability of the AWGs to set both I and Q A A’ increased A A’ A A’ Afrom A’ tributaries to the1 null point ofAthe DPMZMs. The dataFDL-2 rate(=200 of the ns) optical packets was us A A A 160 Gb/s to 200 Gb/s and the decorrelation between the X and Y polarization was six symbols, in Packet order to yield a total capacity of 12.8 Tbit/s. The back-to-back error vector magnitude of the Contention FDL-0 generated (withLabel the exception B from TX2PDM-16QAM signal Labelwas A about 10% without any receiver equalization DROP of the receiver’s matched SRRC filter). The format of the optical packet train and packet labels after A B A B A B A B FDL-17. After the transmitter, the the transmitter was similar to the one shown on the inset of Figure optical signal was split into two to simulate two transmitters, named TX1 and TX2. After the transmitter, theA set Aof eight packets from through FDL-2 TX1 and TX2 were propagated B B B B A A different 50 km SMFs plus DCF spans and were input to both input ports of the 2 × 2 OPS node, described in Section 4. In this experiment, the OPS node was configured to forward optical packets with label A of TX1 and TX2 to the same output port in order to emulate packet contention. To Figure 15. Waveforms of the packets processed in the 2 × 2 OPS node. DROP and FORWARD, along resolve this15. packet contention, FDLs were usedinasthe an2 optical The buffer manager calculated Figure Waveforms of the packets processed × 2 OPSbuffer. node. DROP and FORWARD, along with back-to-back, were the waveforms at the input of the coherent receiver (ECOC 2014). with back-to-back, were waveforms inputcollisions of the coherent receiver (ECOC the delay time required forthe each packet at tothe avoid and controlled a set2014). of PLZT switching matrices to send each packet to appropriate FDLs having 0, 100 and 200 ns, respectively. On the The optical packets were received by a polarization diverse coherent receiver followed by a otherThe hand, optical packets with label Bby (only from TX2) diverse were sent to the alternative output by port optical packets were received a polarization coherent receiver followed a digitizer as described in Section 3.1. Asynchronous optical packet detection was not considered in without the (drop process). This whole processing illustratedwas in Figure 15, which digitizer as buffering describedprocess in Section 3.1. Asynchronous optical packetisdetection not considered this experiment and known timing from the transmitter was used to digitally chop the optical shows waveforms in the from data the plane of the OPS node. Into thedigitally label switching in this the experiment andprocessed known timing transmitter was used chop theprocess, optical packets for further DSP processing, simulating a null PER. The DSP blocks were similar to the ones packets with label A and label B were separated and four packets with label B of TX2 were output described in Section 2.2, with the difference that, after normalization, the digital signal was filtered (marked in Figure 15 as DROP). Optical packets with label A were output after the buffering process with a matched SRRC filter similar to the one used in the transmitter. Then, the signal was without any packet loss (labelled FORWARD on Figure 15). upsampled to four samples-per-symbol for timing recovery using the delay multiplication algorithm [46], which is appropriated for Nyquist pulse-shaped signals. After timing recovery performed at

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packets for further DSP processing, simulating a null PER. The DSP blocks were similar to the ones described in Section 2.2, with the difference that, after normalization, the digital signal was filtered with a matched SRRC filter similar to the one used in the transmitter. Then, the signal was upsampled to four samples-per-symbol for timing recovery using the delay multiplication algorithm [46], which is appropriated for Nyquist pulse-shaped signals. After timing recovery performed at four samples-per-symbol, the signal was downsampled to two samples-per-symbol for subsequent processing. 5.2. Packet Switching, Buffering and Transmission Results and Discussion BER measurements of back-to-back, after the packet dropping process (named DROP in Figure 15) and after the packet forwarding process (named FORWARD) are shown in Figure 16a–c, respectively. The BER for every individual packet was averaged over 10 captured packets. Discontinuities in the plots of back-to-back arise from error-free measurements. Deterioration of BER after the packet dropping was caused by a large insertion loss of the PLZT switch and couplers (about 18 dB). The BER after the forwarding process was kept below the FEC limit of 3 × 10−3 . However, due to crosstalk between output ports of the optical switch, residual packets were output in the undesired delay lines. As a result of the interference between signals and residual signals after the couplers, a severe deterioration occurred. In contrast, the drop process without the buffering had no such interferences, and, consequently, BER was much lower than in the forward process. The improvement of the OPS node crosstalk characteristics is still an issue to be tackled in future work. These results successfully demonstrate the operation of a 12.8 Tbit/s optical packet switching node with buffering after 50 km of fiber transmission. Even after the signal propagation, DWDM PDM-16QAM optical packets could be correctly handled by the 2 × 2 OPS node consisting of low polarization dependent, wideband PLZT optical Photonics switches. 2017, 4, 27 15 of 17

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6. Conclusions 6. Conclusions We have reviewed the current state-of-the-art of OPS technologies using coherently modulated We have reviewed the current state-of-the-art of OPS technologies using coherently modulated PDM-QPSK/PDM-16QAM optical packets with special attention on the digital coherent burst-mode PDM-QPSK/PDM-16QAM optical packets with special attention on the digital coherent burst-mode receiver and the electro-optical PLZT optical switch subsystems and have characterized a complete 2 × 2 receiver and the electro-optical PLZT optical switch subsystems and have characterized a complete OPS node with a control plane capable of resolve packet contention by dropping or buffering. We 2 × 2 OPS node with a control plane capable of resolve packet contention by dropping or buffering. have proposed and experimentally demonstrated a novel DCBMRX that makes use of the Stokes We have proposed and experimentally demonstrated a novel DCBMRX that makes use of the Stokes parameters to promptly estimate the SOP. The estimator makes use of the SVD decomposition and parameters to promptly estimate the SOP. The estimator makes use of the SVD decomposition and an estimator of size 16 is enough to guarantee a packet payload at the FEC limit of 10−3 and this may an estimator of size 16 is enough to guarantee a packet payload at the FEC limit of 10−3 and this reduce the overall complexity of the receiver. We have also experimentally demonstrated the may reduce the overall complexity of the receiver. We have also experimentally demonstrated suitability of PLZT optical switches as a subsystem for the switching of high-order QAM modulated the suitability of PLZT optical switches as a subsystem for the switching of high-order QAM optical packets. Using a transmitter with a two-bit DAC made of discrete components to generate modulated optical packets. Using a transmitter with a two-bit DAC made of discrete components PDM-16QAM modulated optical packets, we demonstrated a switching capacity of over 10.24 Tb/s/port. to generate PDM-16QAM modulated optical packets, we demonstrated a switching capacity of over We used the PLZT optical switch as a building block for a full 2 × 2 OPS node with packet contention resolution by packet dropping or packet buffering in FDLs. We first evaluated this optical switch in a back-to-back to demonstrate the node operation with packet buffering at 10.24 Tb/s/port. Finally, we increased the switching capacity to 12.8 Tb/s/port and transmitted the OPS signal thought two spans of 50 km of dispersion compensated SMF, owing to an improved packet transmitter consisting of

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10.24 Tb/s/port. We used the PLZT optical switch as a building block for a full 2 × 2 OPS node with packet contention resolution by packet dropping or packet buffering in FDLs. We first evaluated this optical switch in a back-to-back to demonstrate the node operation with packet buffering at 10.24 Tb/s/port. Finally, we increased the switching capacity to 12.8 Tb/s/port and transmitted the OPS signal thought two spans of 50 km of dispersion compensated SMF, owing to an improved packet transmitter consisting of eight-bit AWGs, which did not require a packet carver. Acknowledgments: The authors would like to thank Takeshi Makino and Takahiro Hashimoto for their technical support during the experimental work. Author Contributions: J.M.D.M. and S.S. conceived and designed the experiments; J.M.D.M. performed the experiment described in Section 2; J.M.D.M. and S.S. performed the experiments described in Sections 3–5; all authors were involved in the discussion of the results; J.M.D.M. and S.S. wrote the manuscript and prepared the figures; and all authors reviewed the manuscript and approved it for publication. Conflicts of Interest: The authors declare no conflict of interest.

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