Wavelength converters based on four-wave mixing in semiconductor optical amplifiers for advanced modulation formats

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Wavelength converters based on four-wave mixing in

semiconductor optical amplifiers for advanced

modulation formats

Thèse

Benoît Filion

Doctorat en génie électrique

Philosophiae doctor (Ph. D.)

Québec, Canada

© Benoît Filion, 2016

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Résumé

Les convertisseurs de longueur d’onde sont essentiels pour la réalisation de réseaux de communications optiques à routage en longueur d’onde. Dans la littérature, les convertisseurs de longueur d’onde basés sur le mélange à quatre ondes dans les amplificateurs optiques à semi-conducteur constituent une solution extrêmement intéressante, et ce, en raison de leurs nombreuses caractéristiques nécessaires à l’implémentation de tels réseaux de communications. Avec l’émergence des systèmes commerciaux de détection cohérente, ainsi qu’avec les récentes avancées dans le domaine du traitement de signal numérique, il est impératif d’évaluer la performance des convertisseurs de longueur d’onde, et ce, dans le contexte des formats de modulation avancés. Les objectifs de cette thèse sont : 1) d’étudier la faisabilité des convertisseurs de longueur d’onde basés sur le mélange à quatre ondes dans les amplificateurs optiques à semi-conducteur pour les formats de modulation avancés et 2) de proposer une technique basée sur le traitement de signal numérique afin d’améliorer leur performance.

En premier lieu, une étude expérimentale de la conversion de longueur d’onde de formats de modulation d’amplitude en quadrature (quadrature amplitude modulation - QAM) est réalisée. En particulier, la conversion de longueur d’onde de signaux 16-QAM à 16 Gbaud et 64-QAM à 5 Gbaud dans un amplificateur optique à semi-conducteur commercial est réalisée sur toute la bande C. Les résultats démontrent qu’en raison des distorsions non-linéaires induites sur le signal converti, le point d’opération optimal du convertisseur de longueur d’onde est différent de celui obtenu lors de la conversion de longueur d’onde de formats de modulation en intensité. En effet, dans le contexte des formats de modulation avancés, c’est le compromis entre la puissance du signal converti et les non-linéarités induites qui détermine le point d’opération optimal du convertisseur de longueur d’onde.

Les récepteurs cohérents permettent l’utilisation de techniques de traitement de signal numérique afin de compenser la détérioration du signal transmis suite à sa détection. Afin de mettre à profit les nouvelles possibilités offertes par le traitement de signal numérique, une technique numérique de post-compensation des distorsions induites sur le signal converti, basée sur une analyse petit-signal des équations gouvernant la dynamique du gain à l’intérieur des amplificateurs optiques à semi-conducteur, est développée. L’efficacité de cette technique est démontrée à l’aide de simulations numériques et de mesures expérimentales de conversion de longueur d’onde de signaux 16-QAM à 10 Gbaud et 64-QAM à 5 Gbaud. Cette méthode permet d’améliorer de façon significative les performances du convertisseur de longueur d’onde, et ce, principalement pour les formats de modulation avancés d’ordre supérieur tel que 64-QAM.

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Finalement, une étude expérimentale exhaustive de la technique de post-compensation des distorsions induites sur le signal converti est effectuée pour des signaux 64-QAM. Les résultats démontrent que, même en présence d’un signal à bruité à l’entrée du convertisseur de longueur d’onde, la technique proposée améliore toujours la qualité du signal reçu. De plus, une étude du point d’opération optimal du convertisseur de longueur d’onde est effectuée et démontre que celui-ci varie en fonction des pertes optiques suivant la conversion de longueur d’onde. Dans un réseau de communication optique à routage en longueur d’onde, le signal est susceptible de passer par plusieurs étages de conversion de longueur d’onde. Pour cette raison, l’efficacité de la technique de post-compensation est démontrée, et ce pour la première fois dans la littérature, pour deux étages successifs de conversion de longueur d’onde de signaux 64-QAM à 5 Gbaud.

Les résultats de cette thèse montrent que les convertisseurs de longueur d’ondes basés sur le mélange à quatre ondes dans les amplificateurs optiques à semi-conducteur, utilisés en conjonction avec des techniques de traitement de signal numérique, constituent une technologie extrêmement prometteuse pour les réseaux de communications optiques modernes à routage en longueur d’onde.

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Abstract

Wavelength converters are essential building blocks of future all-optical wavelength-routed optical networks. Wavelength converters based on four-wave mixing in semiconductor optical amplifiers have been proposed since they possess many of the features required for the implementation of such networks. In the literature, they have been extensively studied in the context of direct detection optical communication systems. With the recent emergence of commercial coherent systems together with the advances in digital signal processing, there is an imperative need to study the performance of wavelength converters for advanced modulation formats. The objective of this thesis is to investigate the feasibility of wavelength converters based on four-wave mixing in semiconductor optical amplifiers for advanced modulation formats and subsequently propose a digital signal processing technique that aims to improve their performance.

An experimental investigation of m-ary quadrature amplitude modulation (QAM) wavelength conversion is first performed. In particular, wideband wavelength conversion of 10 Gbaud 16-QAM and, for the first time, 5 Gbaud 64-QAM signals is successfully achieved over the whole C-band using a commercial semiconductor optical amplifier. The results show that the induced nonlinear distortions on the wavelength converted signal lead to a different optimal wavelength converter operating condition compared to wavelength conversion of intensity modulation formats. For wavelength conversion of advanced modulation formats, the tradeoff between the wavelength converted signal power and the nonlinearities dictates the optimal wavelength converters operating condition.

The digital signal processing capabilities of coherent receivers renders possible the compensation of the signal impairments in the digital domain after detection. Making use of this, a digital post-compensation technique based on a small-signal analysis of the equations governing the gain dynamics of semiconductor optical amplifiers is developed in order to mitigate the nonlinear distortions imposed on the wavelength converted signal. The proof of concept is realized both via numerical simulations and experimental measurements of 10 Gbaud 16-QAM and 5 Gbaud 64-QAM: the post-compensation technique significantly improve the wavelength converter especially in the case of high-order advanced modulation formats such as 64-QAM.

Finally, a thorough experimental investigation of the post-compensation technique performance for 5 Gbaud 64-QAM wavelength conversion is done. The results indicate that the signal quality is still improved even in the presence of a noisy signal at the wavelength converter input. Furthermore, it is shown that the optimal wavelength converter operating condition varies upon the amount of optical loss that follows. In a wavelength-routed optical network, the signal will likely pass through multiple wavelength conversion stages. The experimental investigation is extended to cascaded wavelength conversion stages and, for the first time, dual

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stage wavelength conversion of 5 Gbaud 64-QAM signal is performed. In this context, the proposed post-compensation technique still successfully improves the wavelength converter performance.

The results of this thesis strongly suggest that wavelength converters based on four-wave mixing in semiconductor optical amplifiers, together with the use of digital signal processing techniques, is a promising candidate for advanced modulation formats wavelength conversion in future wavelength routed optical networks.

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List of Tables

Table 1. OFAs and SOAs typical performance [47] ... 20

Table 2. Conversion efficiency ... 53

Table 3. Conversion efficiency and power penalty ... 55

Table 4. SOA parameters used for the numerical simulations ... 67

Table 5. Extracted SOA parameters for 10 Gbaud 16-QAM ... 110

Table 6. Extracted SOA parameters for 5 Gbaud 64-QAM ... 110

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List of Figures

Fig. 1. A typical OXC with an ESF ... 3

Fig. 2. Optical core networks underlying technology evolution: a) early WDM systems b) WDM systems with OADMs c) WDM systems with ROADMs and OXCs ... 3

Fig. 3. A transparent OXC ... 4

Fig. 4. Wavelength-routed network architecture ... 6

Fig. 5. OBS concept ... 7

Fig. 6. a) OPS concept b) packet with parallel optical label and c) packet with a serial optical label ... 9

Fig. 7. A typical OPS switch architecture ... 10

Fig. 8. Block diagrams of basic IM-DD ... 11

Fig. 9. Signal constellations (in-phase (I) and quadrature (Q) components) of OOK, 8-PSK, 16-QAM and 64-QAM ... 12

Fig. 10. Typical digital coherent receiver architecture (only one polarization is shown). ... 13

Fig. 11. Optical spectrum assignment with a) a fixed 50-GHz grid and b) a flexible grid ... 15

Fig. 12. Typical SOA applications [47] ... 20

Fig. 13. Typical SOA structure ... 21

Fig. 14. Simplified band diagram of a direct band gap semiconductor ... 22

Fig. 15. Radiative recombination processes: a) absorption b) spontaneous emission and c) stimulated emission ... 23

Fig. 16. Auger recombination of type CCHC ... 24

Fig. 17. Energy against the density of states of the conduction and valence bands [52] ... 25

Fig. 18. Gain as a function of energy (in electron-volt) for different carrier densities [55] ... 26

Fig. 19. Waveform distortion and pattern effects in a SOA: a) with signal repetition rate ≪ 1𝜏_𝑠 and b) with signal repetition rate ~1𝜏𝑠... 30

Fig. 20. WC configurations based on XGM in a SOA: a) co-propagating and b) counter-propagating [47]... 32

Fig. 21. Typical CE as a function of the modulation frequency of the input pump of a WC based on XGM for different values of internal loss. The CE calculations are based on a small-signal theory using the following parameter: 𝑒Γ𝑔₀𝐿 = 30 dB, the input probe power is -3 dBm, the input pump power is 0 dBm, 𝑃_𝑠𝑎𝑡 = 10 dBm, 𝜏_𝑠 = .1 ns. The solid line is for 𝛼_𝑖𝑛𝑡𝛤𝑔₀= 0, the dashed line is for 𝛼_𝑖𝑛𝑡𝛤𝑔₀= 0.1, the dotted line if for 𝛼_𝑖𝑛𝑡𝛤𝑔₀= 0.3 and the dot-dashed line is for 𝛼_𝑖𝑛𝑡Γ𝑔₀= 0.5 [65] ... 33

Fig. 22. 10 Gb/s wavelength conversion based on XGM: a) received BER for a down-wavelength shift from 1546 nm to 1531 nm b) received BER for an up-wavelength shift from 1535 nm to 1552 nm and c) portion of the wavelength shifted pattern and eye diagram for an up-wavelength shift from 1546 nm to 1552 nm [63] ... 34

Fig. 23. Eye diagrams of wavelength conversion for a down-wavelength shift from 1546 nm to 1531 nm after transmission through 20 km of single-mode fiber: a) 2.5 Gb/s b) 5 b/s c) 8 Gb/s and d) 10 Gb/s [67].35 Fig. 24. Measured linewidth enhancement factor as a function of the bias current (normalized to the threshold current) in a bulk SOA for 𝜆 = 1500 nm (circles), 𝜆 = 1510 nm (triangles) and 𝜆 = 1530 nm (squares). Solid and dashed curves show theoretical results according to [68] ... 36

Fig. 25. WCs based on XPM in SOAs: a) asymmetric MZI configuration and b) symmetric MZI configuration [47] ... 36

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Fig. 26. Typical response of the output probe power (at 1560 nm) as a function of the input pump power (at 1550 nm) of a WC based on XPM with a symmetric MZI configuration [72] ... 37 Fig. 27. FWM in a SOA ... 38 Fig. 28. CE as a function of the frequency detuning between the pump and the signal for SOAs with different

active region lengths (with -2 dBm input pump power and -14 dBm input signal power) for a) frequency up-conversion and b) frequency down-conversion ... 39 Fig. 29. CE and SBR as a function of the input pump power for frequency up-conversion of 250 GHz for SOAs

with different active region lengths (the ratio of the pump power to the signal power has been maintained constant at 12 dB) [74] ... 39 Fig. 30 BER as a function of the received power for RZ-DQPSK wavelength conversion over 2 nm for different

pump power to signal power ratios at the SOA input [80] ... 40 Fig. 31. Wideband wavelength conversion with two orthogonally polarized pumps ... 41 Fig. 32. Wideband wavelength conversion with two parallel polarized pumps ... 42 Fig. 33 Polarization independent wavelength conversion with a) two orthogonally polarized pumps and b) two

parallel polarized pumps... 43 Fig. 34. Polarization diversity wavelength conversion scheme ... 44 Fig. 35. BER as a function of the received power of wavelength conversion with two parallel polarized pump

for a) 40 Gb/s and 80 Gb/s OOK/PM-OOK and b) 40 Gb/s and 80 Gb/s DPSK/PM-DPSK [89] ... 44 Fig. 36. Density of states as a function of energy for a bulk SOA and a QW-SOA [24] ... 45 Fig. 37. Conjugate power as a function of the frequency detuning for frequency up-conversion (closed circles)

and down-conversion (open circles) for a) QD-SOA [99] and b) bulk SOA [100] ... 47 Fig. 38. BER as a function of the received OSNR 10 Gbaud wavelength conversion in a QD-SOA for a) QPSK

b) 8-PSK and c) 16-QAM [98] ... 48 Fig. 39. CE as a function of the frequency detuning for frequency up-conversion and down-conversion in the

QD-SOA used in [98] ... 48 Fig. 40. Received constellations: a) Signal power optimized to maximize CE b) Signal power optimized to

minimize EVM ... 51 Fig. 41. Experimental setup ... 52 Fig. 42. BER as a function of received power of 16-QAM with 40 dB input OSNR for converted wavelengths

spanning over the C-band and constellation diagrams for 𝜆_𝑐 = 1527.7 nm ... 53 Fig. 43. BER as a function of received power of 16-QAM for converted wavelengths spanning over the C-band

and constellation diagrams at 1529.7 nm with dual-pump configuration with a) 30dB input OSNR and b) 20dB input OSNR ... 54 Fig. 44. BER as a function of received power of 64-QAM with 36 dB input OSNR for converted wavelengths

spanning over the C-band and constellation diagrams for 26 dBm received power ... 56 Fig. 45. BER as a function of received power of 64-QAM with a single pump configuration for 26 dB and 36

dB input OSNR and constellation diagrams ... 56 Fig. 46. Simplified block diagram of the DFBP scheme for: a) linear amplification b) wavelength conversion.

The dashed boxes represent DSP steps, the prime notation indicates that the post-compensation has been applied and EST stands for estimate ... 61 Fig. 47. Block diagram of the DFBP scheme applied with the knowledge of 𝐸_𝑐: the first block encompass the

approximation of 𝐸_𝑠 (51), the second block is the digital filter (58) and the third one is the actual post-compensation (61) ... 66 Fig. 48. Block diagram of the simulation setup ... 67

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Fig. 49. Simulated wavelength conversion of 10 Gbaud 16-QAM at the SOA output and after application of DFBP: a) EVM as a function of the signal power for a pump power of 5 dBm and 10 dBm b) received constellations examples ... 68 Fig. 50. Simulated wavelength conversion of 5 Gbaud 64-QAM at the SOA output and after application of

DFBP: a) EVM as a function of the signal power for a pump power of 5 dBm and 10 dBm b) received constellations examples ... 69 Fig. 51. EVM after wavelength conversion for 10 Gbaud 16-QAM as a function of the number of samples per

symbol without DFBP (solid lines) and with DFBP (dashed lines) with received constellations examples for a pump power of 5 dBm and a signal power of -6 dBm ... 70 Fig. 52. EVM after wavelength conversion for 10 Gbaud 16-QAM as a function of the number of samples per

symbol without DFBP (solid lines) and with DFBP (dashed lines) with received constellations examples for a pump power of 10 dBm and a signal power of 0 dBm ... 71 Fig. 53. EVM after wavelength conversion for 5 Gbaud 64-QAM as a function of the number of samples per

symbol without DFBP (solid lines) and with DFBP (dashed lines) with received constellations examples for a pump power of 5 dBm and a signal power of -6 dBm ... 72 Fig. 54. EVM after wavelength conversion for 5 Gbaud 64-QAM as a function of the number of samples per

symbol without DFBP (solid lines) and with DFBP (dashed lines) with received constellations examples for a pump power of 10 dBm and a signal power of -2 dBm ... 72 Fig. 55. Block diagram of the experimental setup ... 73 Fig. 56. Typical optical spectrum of 16-QAM and 64-QAM wavelength conversion ... 74 Fig. 57. 10 Gbaud 16-QAM wavelength conversion experimental measurements: a) CE b) OSNR c) EVM d)

BER e) received constellations examples ... 75 Fig. 58. 5 Gbaud 64-QAM wavelength conversion experimental measurements: a) CE b) OSNR c) EVM d)

BER e) received constellations examples ... 76 Fig. 59. Block diagram of a) the experimental setup and b) the wavelength converter ... 83 Fig. 60. Typical optical spectra at the input (solid line) and at the output (dashed line) of a single stage

wavelength conversion ... 85 Fig. 61. Typical spectra for dual stage wavelength conversion experiments at the input (solid line) and at the

output (dashed line): a) of the first wavelength conversion stage and b) of the second wavelength conversion stage ... 85 Fig. 62. Measured differential EVM (𝛥𝐸𝑉𝑀) and BER, before DFBP (solid lines) and after DFBP (dashed

line), as a function of the received OSNR of 5 Gbaud 64-QAM single stage wavelength conversion for an input OSNR of 35dB with received constellations examples: a) 𝛥𝐸𝑉𝑀 for a pump power of 5 dBm b) 𝛥𝐸𝑉𝑀 for a pump power of 10 dBm c) BER for a pump power of 5 dBm and d) BER for a pump power of 10 dBm ... 87 Fig. 63. Constellation examples before DFBP and after DFBP of 5 Gbaud 64-QAM single stage wavelength

conversion for an input OSNR of 35dB: a) for a pump power of 5 dBm and b) for a pump power of 10 dBm ... 88 Fig. 64. Measured differential EVM (𝛥𝐸𝑉𝑀) and BER, before DFBP (solid lines) and after DFBP (dashed

line), as a function of the received OSNR of 5 Gbaud 64-QAM single stage wavelength conversion for an input OSNR of 25dB with received constellations examples: a) 𝛥𝐸𝑉𝑀 for a pump power of 5 dBm b) 𝛥𝐸𝑉𝑀 for a pump power of 10 dBm c) BER for a pump power of 5 dBm and d) BER for a pump power of 10 dBm ... 88

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Fig. 65. Constellation examples before DFBP and after DFBP of 5 Gbaud 64-QAM single stage wavelength conversion for an input OSNR of 25dB: a) for a pump power of 5 dBm and b) for a pump power of 10 dBm ... 89 Fig. 66. Measured conversion efficiency of 5 Gbaud 64-QAM single stage wavelength conversion with 35 dB

input OSNR. The inset shows the conversion efficiency as a function of the input pump power for an input signal power of -4 dBm ... 90 Fig. 67. Equivalent experiment setup for the EVM and BER measurements as a function of the loss after

wavelength conversion ... 91 Fig. 68. Measured EVM as a function of the input signal power of 5 Gbaud 64-QAM single stage wavelength

conversion with 35 dB input OSNR for a link loss of a) 0 dB b) 7 dB and c) 14 dB. The EVM is shown before (solid lines) and after (dotted lines) application of the DFBP post-compensation technique .... 91 Fig. 69. Measured EVM with an optimal input signal power as a function of the link loss of 5 Gbaud 64QAM

single stage wavelength conversion before and after application of the DFBP post-compensation technique with (a) 35 dB input OSNR and (b) 25 dB input OSNR ... 92 Fig. 70. Measured BER as a function of the link loss of 5 Gbaud 64QAM single stage wavelength conversion

with (a) 35 dB input OSNR and (b) 25dB input OSNR ... 93 Fig. 71. a) Measured EVM with an optimal input signal power as a function of the link loss of 5 Gbaud 64QAM

dual stage wavelength conversion with 35 dB input OSNR. Measured EVM as a function of the input signal power for a link loss of a) 0 dB b) 4 dB and c) 8 dB. The EVM is shown before (solid lines) and after (dotted lines) application of the DFBP post-compensation technique ... 94 Fig. 72. Measured BER as a function of the link loss of 5 Gbaud 64-QAM dual stage wavelength conversion ... with 35 dB input OSNR ... 95 Fig. 73. Measured and least-squares fit of the static gain versus SOA input power... 109

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List of Symbols

This section contains the list of symbols used in this thesis: their definition is valid throughout the thesis.

Symbol Definition For sorting _only!

𝑐 Light velocity 1c1

𝑑 Active region height 1d1

𝐷 Diffusion coefficient 1d2

𝐸 Electrical field slowly-varying envelope 1e1

𝐸⃗ Electrical field vector 1e2

𝐸_𝑐 Output conjugate slowly-varying amplitude envelope 1e3

𝐸̂_𝑐 Post-compensated output conjugate slowly-varying amplitude envelope 1e4

𝐸_𝑐,𝑣 Electron and hole energy 1e5

𝐸_{𝑓𝑐,𝑓𝑣} Conduction band and valence band quasi Fermi level 1e6

𝐸𝑔 Band gap energy 1e7

𝐸_{𝑖𝑛,𝑜𝑢𝑡} Input and output electrical field slowly-varying envelope 1e8

𝐸_𝑝,𝑠 Input pump and signal slowly-varying amplitude envelope 1e9

𝐸̂_𝑠 Estimate of the input signal slowly-varying amplitude envelope 1e91

𝑓_𝑐,𝑣 Conduction band and valence band Fermi level 1f1

𝐹 Parameter including the frequency detuning dependence of the conversion _efficiency 1f2

𝑔 Gain 1g1

𝑔0 Small-signal gain 1g2

𝐺₀ Unsaturated gain 1g3

𝑔_𝑁 Differential gain 1g4

ℎ Planck constant 1h1

ℏ Reduced Planck constant 1h2

ℎ_𝑏𝑏 Baseband term of the integrated gain 1h3

ℎ̅𝑏𝑏 Mean of the baseband term of the integrated gain 1h4

ℎ𝐶𝐻 Integrated gain contribution from carrier heating 1h5

ℎ_𝑁 Integrated gain contribution from the carrier density 1h6

ℎ_𝑠𝑏 Sideband term of the integrated gain 1h7

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ℎ𝑡𝑜𝑡 Total integrated gain 1h9

𝐼 Injection current 1i1

𝑘 Wavenumber 1k1

𝐾 Single-pole low-pass filter constant 1k2

𝐾′ Updated single-pole low-pass filter constant 1k3

𝐿 Active region length 1l1

𝑚𝑐,𝑣 Electron and hole effective mass 1m1

𝑛 Carrier density 1n1

𝑛0 Carrier density at transparency 1n2

𝑛_𝑏 Background refractive index 1n3

𝑛𝑐,𝑣 Electron and hole density 1n4

𝑛𝑒𝑓𝑓 Effective index 1n5

𝑛_𝑔 Group index 1n6

𝑝 Particle momentum 1p1

𝑝̅_𝑐 Normalized mean of the output conjugate slowly-varying amplitude envelope 1p2

𝑝̅_𝑝,𝑠 Normalized mean of the input pump and signal slowly-varying amplitude envelope 1p3

𝑃𝑠𝑎𝑡 Saturation power 1p4

𝑃 Parameter related to the 4 levels energy band structure 1p5

𝑞 Electron charge 1q1

𝑟 Single-pole low-pass filter impulse response 1r1

𝑟′ Updated single-pole low-pass filter impulse response 1r2

〈𝑅𝑐𝑣〉2 Squared average dipole moment 1r3

𝑈 Waveguide mode transverse distribution 1u1

𝑉 Active region volume 1v1

𝑤 Active region width 1w1

𝑥 Polarization unit vector 1x1

𝛼 Linewidth enhancement factor 2a1

𝛼_𝑐 Parameter related to the 4 levels energy band structure 2a2

𝛼_𝑖𝑛𝑡 Internal loss coefficient 2a3

𝛿 Dirac delta function 2d1

𝛿ℎ_𝑏𝑏 Zero-average term of the baseband term of the integrated gain 2d2

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𝛿𝑝𝑠 Normalized zero-average term of the input signal slowly-varying amplitude _envelope 2d4

Δ𝑡 Sampling time 2d5

Δ𝑡_𝑠𝑖𝑚 Simulation sampling time 2d6

𝜀 Dielectric permittivity 2e1

𝜖𝐶𝐻 Carrier heating nonlinear suppression factor 2e2

𝜖_𝑆𝐻𝐵 Spectral hole-burning nonlinear suppression factor 2e3

Γ Confinement factor 2g1

𝜆 Wavelength 2l1

𝜆_𝐵 De Broglie wavelength 2l2

𝜌 Density of states 2r

𝜏₁ Carrier-carrier scattering time 2t1

𝜏_𝑒𝑓𝑓 Single-pole low-pass filter time constant 2t2

𝜏′𝑒𝑓𝑓 Updated single-pole low-pass filter time constant 2t3

𝜏_ℎ Carrier-phonon scattering time 2t4

𝜏𝑖𝑛 Intraband relaxation time 2t5

𝜏_𝑠 Carrier lifetime 2t6

𝜇 Magnetic permeability 2u1

𝜐𝑔 Group velocity 2v1

𝜔 Angular frequency 2w1

Ω Pump and signal frequency detuning 2w2

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List of Acronyms

This section contains the list of acronyms used in this thesis in alphabetical order: their definition is valid throughout the thesis.

Acronym Definition

ASE Amplified Spontaneous Emission AWG Arbitrary Waveform Generator

BCP Burst Control Packet BER Bit Error Rate BPG Bit Pattern Generator

CE Conversion Efficiency CH Carrier Heating

CQD-SOA Columnar Quantum-Dot Semiconductor Optical amplifier CRx Coherent Receiver

CW Continuous Wave

DAC Digital to Analog Converter DC Data Center

DFBP Digital Filter-Based Back-Propagation DPSK Differential Phase-Shift Keying

DSP Digital Signal Processing EDFA Erbium-Doped Fiber Amplifier

ER Extinction ratio

ESF Electrical Switch Fabric EVM Error Vector Magnitude EXC Electrical Cross-Connect FDL Fiber Delay Line

FOC Frequency Offset Compensation FWC Fixed Wavelength Converter FWM Four-Wave Mixing

HDFWM Highly Degenerate Four-Wave Mixing

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IM-DD Intensity-Modulated Direct-Detection

IQ-MZM In-Phase/Quadrature Mach-Zehnder Modulator LAN Local Area Network

LO Local Oscillator

MAN Metropolitan Area Network

MQW-SOA Multi-Quantum-Well Semiconductor Optical Amplifier MZI Mach-Zehnder Interferometer

NDFWM Nearly Degenerate Four-Wave Mixing NGDC Next Generation Data Center

NPLN Nonlinear Phase Noise OADM Optical Add-Drop Multiplexer

OBS Optical Burst Switching OCS Optical Circuit Switching OEO Optical-to-Electrical-to-Optical

OF Optical Filter

OFA Optical Fiber Amplifier OOK On-Off-Keying

OPLL Optical Phase-Locked Loop OPS Optical Packet Switching OPU Optical Payload Unit OSF Optical Switch Fabric

OSNR Optical-to-Signal-to-Noise-Ratio OXC Optical Cross-Connect

PBS Polarization Beam Splitter PC Polarization Controller

PDM Polarization-Division Multiplexing PM Polarization-Multiplexed

PMD Polarization Mode Dispersion

PM-DPSK Polarization-Multiplexed Differential Phase-Shift Keying PM-OOK Polarization-Multiplexed On-Off Keying

PM-QAM Polarization-Multiplexed Quadrature Amplitude Modulation PM-QPSK Polarization-Multiplexed Quadrature Phase-Shift Keying

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PRBS Pseudo-Random Binary Sequence PSK Phase-Shift Keying

Q Quadrature

QAM Quadrature Amplitude Modulation

QD-SOA Quantum-Dot Semiconductor Optical Amplifier QW-SOA Quantum-Well Semiconductor Optical Amplifier

RAM Random Access Memory RK4 4th_{order Runge-Kutta}

ROADM Reconfigurable Optical Add-Drop Multiplexer RTO Real-Time Oscilloscope

SBR Signal-to-Background Ratio SHB Spectral Hole-Burning

SOA Semiconductor Optical Amplifier TWC Tunable Wavelength Converter

UL-SOA Ultra-Long Semiconductor Optical amplifier VOA Variable Optical Attenuator

WC Wavelength Converter

WDM Wavelength-Division Multiplexing WHDD-EQ Wiener-Hopf Decision-Direct Equalizer

WSS Wavelength Switch Selective XGM Cross-Gain Modulation XPM Cross-Phase Modulation

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Remerciements

Je tiens tout d’abord à remercier ma directrice de thèse, la professeure Sophie LaRochelle. Tu as su me guider et me pousser à me réaliser dans le stimulant domaine des communications optiques. Merci pour ta collaboration, ton support et finalement, d’avoir cru en moi et de m’avoir fait confiance. Merci pour tout! J’ai aussi eu la chance d’avoir une co-directrice présente et aidante, la professeure Leslie Ann Rusch. Nos rencontres ont été nombreuses et tout aussi enrichissantes que stimulantes. Ce fut un plaisir de discuter à la fois de sciences, ainsi que de livres de science-fiction et de séries télévisées. Un gros merci!

Depuis, les cinq dernières années, j’ai côtoyé des étudiants qui sont devenus mes amis et mes confidents. Tout d’abord, un immense merci à mon ami Alexandre Delisle Simard. Il a été d’une aide et d’un support précieux de par nos nombreuses discussions autant sur le plan scientifique que personnel. Un merci spécial aussi à

Amirhossein Ghazisaeidi, qui a été en quelque sorte un mentor pour moi à mon arrivée dans le laboratoire et

est aussi devenu un ami proche. Malheureusement, il est parti pour la France trop tôt, mais j’ai eu la chance de le revoir à plusieurs reprises lors de mon stage au III-V Lab, en France.

Le laboratoire n’aurait sans doute pas été le même sans l’excellent travail des professionnels de recherche,

Chul Soo Park et, en particulier, An T. Nguyen, qui m’a aidé pour la prise de mesures expérimentales tout au

long de mon doctorat.

Un merci spécial à tous les membres du laboratoire de communications optiques. Les nombreuses réunions de groupe nous ont permis d'échanger nos idées et de critiquer constructivement notre travail.

Je tiens aussi à mentionner le plaisir que j'ai eu à travailler au sein du Centre d’Optique, Photonique et Laser

(COPL), et j'en remercie ici tous les membres. J’aimerais en particulier souligner la présence de Diane Déziel

avec qui j’ai pu partager ma passion pour le tennis.

Un sincère remerciement aux membres du jury de ma thèse de doctorat et je pense ainsi aux docteurs Wei Shi,

Trevor J. Hall, Frédéric Grillot, Leslie Ann Rusch et Sophie LaRochelle. Je dois aussi un énorme merci aux

professeures Sophie LaRochelle et Leslie Ann Rusch pour avoir lu et corrigé cette thèse avant le dépôt initial. Le soutien financier des organismes subventionnaires mérite d'être souligné et je pense ainsi au Fonds de

recherche du Québec - Nature et technologies (FRQNT), ainsi qu’au Bureau international de l’Université

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À mes parents, je donne la plus grande des reconnaissances. Merci pour votre aide dans les moments difficiles, pour vos encouragements et vos félicitations dans les moments de joies. Vous êtes indispensables à ma vie et à ma réussite. Je suis maintenant à cette étape de ma vie grâce à vous!

Enfin, je souhaite remercier ma femme Nancy, qui a changé ma vie et sans qui ces dernières années n’auraient pas été pareilles. Merci pour ton amour, ta sensibilité, ton support, ton aide et ton réconfort. Tu es toujours si attentive à mes besoins. Tu es la plus merveilleuse, je t’aime et milles fois merci!

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Preface

The main chapters of this thesis are reproductions of published papers, aside from some minor modifications made to improve the thesis uniformity. In this section, as required by the “Faculté des Études Supérieures et

Postdoctorales de l’Université Laval”, a short description of the contribution of the respective authors is made. Chapter 3 has been published in

“B. Filion, W. C. Ng, A. T. Nguyen, L. A. Rusch, and S. Larochelle, “Wideband wavelength conversion of 16 Gbaud 16-QAM and 5 Gbaud 64-QAM signals in a semiconductor optical amplifier,” Opt. Express, vol. 21, no. 17, pp. 19825–33, Aug. 2013.”

The author of this thesis performed the experimental measurements, the experimental data processing and analysis, and the redaction of the manuscript. A. T. Nguyen helped with the experimental measurements and, with W.C. Ng, provided some numerical algorithm scripts for the experimental data processing. L. A. Rusch and S. LaRochelle contributed to the overall definition of the working plan, and to the work with many suggestions that significantly improved the quality and completeness of the results. They also both actively participated in the editing of the manuscript. This work was partly presented in

“B. Filion, A. T. Nguyen, S. Amiralizadeh, L. A. Rusch, and S. LaRochelle, “Wideband wavelength conversion of 16 Gbaud 16-QAM signals in a semiconductor optical amplifier,” in Optical Fiber Communication Conference, 2013, p. OTh1C.5 ”

“W. C. Ng, B. Filion, L. A. Rusch, A. T. Nguyen, and S. LaRochelle, “Wideband Wavelength Conversion of 5 Gbaud 64-QAM Signals in a Semiconductor Optical Amplifier,” in European Conference on Optical Communication, 2013, pp. 1002–1004.”

Chapter 4 has been published in

“B. Filion, A. Nguyen, L. Rusch, and S. LaRochelle, “Digital Post-Compensation of Nonlinear Distortions in Wavelength Conversion Based on Four-Wave-Mixing in a Semiconductor Optical Amplifier,” J. Lightw. Technol., vol. 33, no. 15, pp. 3254–3264, 2015.”

The author of this thesis contributed to the idea, developed and implemented the digital post-compensation technique, performed the experimental measurements, data processing and analysis, and the redaction of the manuscript. A. T. Nguyen helped with the transmitter setup. L. A. Rusch and S. LaRochelle contributed to the

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overall definition of the working plan, and to the work with many suggestions that significantly improved the quality and completeness of the results. They also both actively participated in the editing of the manuscript.

Chapter 5 has been published in

“B. Filion, A. Nguyen, L. Rusch, and S. LaRochelle, “Post-Compensation of Nonlinear Distortions of 64-QAM Signals in a Semiconductor Based Wavelength Converter,” J. Lightw. Technol., 2016.”

The author of this thesis contributed to the idea, developed and implemented the digital post-compensation technique, performed the experimental measurements, data processing and analysis, and the redaction of the manuscript. A. T. Nguyen helped with the transmitter setup. L. A. Rusch and S. LaRochelle contributed to the overall definition of the working plan, and to the work with many suggestions that significantly improved the quality and completeness of the results. They also both actively participated in the editing of the manuscript.

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Chapter 1: Introduction

In the early 1990s, the introduction of wavelength-division-multiplexing (WDM) systems in core optical networks provided an economical and practical solution to meet the increasing demand for communication bandwidth [1]. WDM systems utilize many wavelengths, each carrying separate data, on the same optical fiber. Rather than increasing the bit rate, WDM systems take advantage of the wide optical fiber bandwidth thus avoiding the cost associated with faster optoelectronic components such as lasers, modulators and photodetectors. The key factor that rendered WDM systems possible was the development of erbium-doped fiber amplifiers (EDFAs) [2]. The high gain multichannel amplification over a wide bandwidth (~5 THz) provided by EDFAs significantly reduced the cost of long haul optical networks by replacing the expansive electronic 3R regenerators, which combines reamplification, retiming and resampling. Nevertheless, the need for 3R regenerators was not completely eliminated since EDFAs only provide signal reamplification.

This chapter briefly surveys the evolution of WDM-based optical networks. Their evolution is characterized by the replacement of electrical technologies in favor of optical technologies to realize network functionalities; a model to be continued in the future. All-optical wavelength converters (WCs) are expected to play an important role on the road to a true all-optical optical network by avoiding wavelength blocking in wavelength routed networks. WCs will increase the flexibility and capacity of optical networks and their role will only increase as we move toward future all-optical networks. In section 1.1, a brief survey of WDM-based optical network technology is presented: the importance of WCs is highlighted and their role is discussed in detail. In section 1.2, the emergence of optical coherent detection along with advanced modulation formats are discussed and, finally, the structures and the objectives of this thesis are presented in section 1.3.

1.1. Evolution of WDM-based optical networks

The first generation optical networks based on WDM technology consisted of high bandwidth point-to-point links providing circuit-switched connections to its users. At the time, all routing, forwarding and switching were implemented by electronic routers requiring optical-to-electrical-to-optical (OEO) conversion. As WDM technology improved, i.e., optical amplifiers, multiplexers and demultiplexers, optical filters, signal monitoring and optical fibers, WDM systems capacity in terms of number of wavelengths and reach was expanded to 40-120 wavelengths and a few thousands of kilometers at bit rates of 2.5 to 10 Gb/s [3]. Consequently, there came the need to access optical wavelengths at intermediate locations between major nodes. In order to provide intermediate access, optical add-drop multiplexers (OADMs) were developed and incorporated to WDM systems. In an OADM, wavelengths are either locally generated from an electrical signal (added), converted into the electrical domain for processing and switching (dropped), or simply passed through to other nodes in the network. A typical first generation OADM consists of multiplexers and demultiplexers with fiber patch cords,

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providing a static optical network since the fiber patch cords connections have to be manually modified in order to implement changes in the wavelength add-drop patterns.

The expansion of network functionalities into the optical domain led to the second generation optical networks in which reconfigurability is available directly in the optical layer of the network nodes without the need of OEO conversion [4]. The direct reconfigurability of the optical layer led to significant operating cost reduction as the network installation and planning were greatly simplified. Indeed, better optical network availability and scalability were also obtained as the need to manually modify the fiber patch cords connections was eliminated: the process of adding new wavelengths to the optical network was greatly simplified. Format and protocol transparency was also now supported since electronic routers intrinsically have to process every bits in order to perform the desired reconfigurability of the optical lightpaths. These important features were made possible by the deployment of reconfigurable optical add drop multiplexers (ROADMs) that allow remote dynamic provisioning of wavelengths, and optical cross-connects (OXCs) that allow switching of wavelengths between different optical fibers. The fundamental building block of ROADMs is the wavelength selective switch (WSS) [5], composed of a diffracting element for parallel demultiplexing of all the wavelengths on the input optical fiber in conjunction with a switching engine that accomplishes the wavelength routing function. The switching engine can be implemented via various technologies, such as microelectromechanical systems mirror arrays, liquid crystals arrays, liquid crystal on silicon and integrated planar lightwave circuit devices.

OXCs are used to manage bandwidth across the core network and also support the add-drop functionality of (R)OADMs [6]. As of today, a common OXC utilizes an electrical switch fabric (ESF) with OEO conversion stages to perform the routing, forwarding and switching of the incident optical wavelengths: a typical OXC is shown in Fig. 1. Wavelength conversion, 3R signal regeneration and performance monitoring are also available via the OEO conversion stages. The wavelength conversion capability of OXCs provides the correct set of wavelengths for traffic multiplexing at the output optical fibers and also makes them non-blocking in the wavelength dimension. On the negative side, OEO conversion stages typically dominate the OXCs cost and their inherent bit rate and protocol dependence makes an increase of the bit rate impossible without upgrade, and also leads to an inability to interconnect wavelengths between two fibers with different bit rate and protocol. In addition, OXCs with OEO conversion stages will be size limited for bit rates higher than 10 Gb/s since the electrical signals need to be broken down into multiple streams with lower bit rate, typically 2.5 Gb/s, for processing. Finally, the electrical cross-connect (EXC) in Fig. 1 is used for traffic termination and grooming.

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O/E/O O/E/O ESF O/E O/E O/E/O O/E/O O/E O/E O/E/O O/E/O E/O E/O O/E/O O/E/O E/O E/O EXC

...

Drop Add Demux Mux

...

Demux

...

Mux Users

Fig. 1. A typical OXC with an ESF

c) Transponders ROADM WSS _couplerOptical a) b)

...

Transponders OADM

...

OXC OXC

...

EDFA EDFA EDFA EDFA EDFA EDFA

Fig. 2. Optical core networks underlying technology evolution: a) early WDM systems b) WDM systems with OADMs c) WDM systems with ROADMs and OXCs

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Fig. 2 depicts the evolution of the optical core networks underlying technology until today: the introduction of WDM systems with EDFAs is shown in Fig. 2a, the increase of the transmission reach that led to the introduction of OADMs is shown in Fig. 2b and finally, the additional reconfigurability of the optical layer with ROADMs and OXCs is shown in Fig. 2c.

In the future, bit rate and protocol transparency would greatly simplify the wavelength provisioning and management across the optical networks. Such features would be available with transparent OXCs capable of switching wavelengths directly in the optical domain, without the need of the expensive OEO conversion stages. Furthermore, high capacity OXCs in a small-footprint, compared to their electrical counterpart, would be possible and the response to the increases of traffic demand would be much simpler.

Fig. 3 illustrates an example of a transparent OXC architecture with an optical switch fabric (OSF). For an OXC to be fully transparent, the optical payload unit (OPU) needs to perform all-optical wavelength conversion [7], [8], all-optical 3R regeneration [9] as well as all-optical performance monitoring [10]. As of now, the technological limitations associated with the optical implementation of these features, which are currently provided by the OEO conversion stages, prevent this solution to be viable.

EXC OSF Drop Add

...

OPU OPU

...

OPU OPU

...

..

.

OPU OPU

..

.

OPU OPU

..

.

Transponders Users

...

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Two types of networks were developed to support the transport and routing functions of WDM systems that do not require OEO conversion, namely the broadcast-and-select network and the wavelength-routed network [11]. The former is typically based on optical couplers which interconnect the network nodes. A constraint common to broadcast-and-select networks resides in the fact that all the network nodes must transmit a distinct wavelength, since all lightpaths must navigate through all the optical couplers of the network. While the broadcast-and-select architecture is naturally suited for a multicasting operation, the splitting of the signals that inherently occurs in the optical couplers severely limit the network scalability. For this reason, broadcast-and-select networks are practical only for metropolitan area networks (MANs) and local area networks (LANs) for which a small number of network nodes is required.

A more dynamic and scalable solution is offered by the wavelength-routed optical network architecture. Because of their generality and flexibility, wavelength-routed networks are not only suitable for core optical networks, but can also be deployed in LANs and MANs. A fundamental advantage of wavelength-routed networks over broadcast-and-select networks is the support wavelength reuse in different parts of the network, which improves the scalability as a large number of nodes can be served using relatively few wavelengths [12].

A typical wavelength-routed network is composed of network nodes and OXCs. As seen in Fig. 4, assuming no wavelength conversion capability, wavelength reuse is still possible; hence the lightpath between node 7 and node 2, and the one between node 3 and node 4 share the same wavelength. However, if different lightpaths share one or more links, different wavelengths must be used; hence the lightpath between node 5 and node 2 in Fig. 4 is assigned another wavelength. In the case where wavelength conversion is not available, the wavelength assigned to each lightpath must be the same across all links between two end nodes. Higher throughput and efficient usage of the available bandwidth is achieved when wavelength conversion capability is present at the intermediate network nodes. In this scenario, a lightpath can be set up assigning a different wavelength on each link providing better network capacity. Furthermore, the lack of wavelength conversion capability hampers the application of shared mesh protection schemes [13]. In the case of a network fault, a lightpath may need to be transferred to an alternative, backup route. If the signal wavelength is not free in the desired backup route, data loss is inevitable: wavelength conversion capability ensures data integrity and minimizes the number of backup paths.

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OXC OXC OXC OXC

...

..

.

...

..

.

...

1 4 3 2 5 6 7 8

Fig. 4. Wavelength-routed network architecture

Today, most WDM systems provide circuit switched type connections, i.e., optical circuit switching (OCS), across the optical networks [14]. In OCS networks, static connections are established either offline or online. In the former, once the ideal optical route and equipment configuration are determined, the connections are configured manually over a timescale of weeks or even months. This slow provisioning implies that a new connection will be justified only when it is needed for a long period of time. Furthermore, the connection set-up is also susceptible to human mistakes and a monitoring period is also necessary to ensure adequate performance. Conversely, when request for a new connection is submitted online, configuration of the appropriate network nodes is done via a signaling protocol that informs intermediate nodes of the necessary actions.

OCS networks are currently moving toward the deployment of fast and reconfigurable nodes with switching granularity at the wavelength level. This concept is represented by the automatically switched optical network architecture [15]. Fast, automated wavelength provisioning within minutes or seconds would make for a more efficient utilization of the available resources possible, allowing a dynamic allocation of lightpaths and network nodes as well as a rapid response to the changing needs of the network users.

Beyond optical switching at the wavelength level, the concepts of optical burst switching (OBS) and optical packet switching (OPS) recently emerged as new optical networking concepts based on the separation of the

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control plane and the data plane to offer optical switching at sub-wavelength granularity [16]. Due to the current domination of IP-centric traffic, the switching of data bursts or packets directly in the optical layer will potentially lead to a third generation of optical networks that integrates data networking and optical networking on a WDM platform [16], [17].

The OBS concept is illustrated in Fig. 5. In OBS networks, the data bursts are first assembled at the network edge from one or multiple data streams. Prior to a data burst transmission, a burst control packet (BCP), is created and sent toward the destination in order to reserve the necessary network resources. The BCP is typically out-of-band over a separate wavelength and processed at all the intermediate nodes in its path. Upon reception of the BCP, each intermediate nodes sets up a lightpath for the corresponding data burst. Once the BCP leaves the inner edge network node, the burst follows without waiting for an acknowledgement that the required resources have been reserved. If all-optical OXCs are used at the intermediate network nodes, the data bursts are completely transparent to protocol, bit rate and modulation format. In this scenario, the OXC switching speed is an extremely important parameter since it will determined the size of burst that can be handled, and must be of the order of microseconds. Because the network resources are reserved only for the duration of the data burst, OBS networks yield a more efficient utilization of the optical bandwidth compared to OCS networks. Contention resolution is a critical aspect of optical switching at sub-wavelength granularity such as in the OBS and OPS concepts. Contention resolution techniques are required when two data bursts or packets that possess the same wavelength are simultaneously destined to the same OXC output port. Three domains that can be

OBS Controller O/E E/O Data burst Control packet Time offset OSF

...

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used in any combinations are available to avoid such data collisions, namely, the wavelength domain, the time domain and the space domain. Wavelength-based contention resolution techniques employ wavelength conversion and assign a different wavelength to the contending data burst or packet: this is the most effective technique as it does not incur additional latency while maintaining the shortest path or minimum hop distance. Alternatively, optical time buffers such as fiber delay lines (FDLs) can be used. However, such optical time buffers are strict first-in-first-out queues with fixed delays and are considerably less effective compared to their electronic counterparts using random access memories (RAMs). Finally, space domain deflection is achieved by simply redirecting the contending data burst or packet to an alternative output port, but doing so also involves additional latency and furthermore may cause the data bursts or packets to arrive out of sequence at the outer edge of the network.

Even though the unified contention resolution in the three domains would be optimal, the desire to keep the OBS networks nodes as simple as possible, without wavelength conversion and optical time buffers, led to studies of OBS networks using only data bursts deflection [18], or burst scheduling [19]. Even if this type of buferless OBS networks seems practical, the challenges in efficient scheduling of variable size data bursts leads to a poor utilization of the network resources and make this a serious constraint on network design. Hybrid schemes involving partial wavelength conversion and optical time buffers have also been the subject of investigations [20], [21].

The next step leading to a future internet providing a unified optical networking platform that supports voice, data, and multimedia applications would be the implementation of switching and routing of IP packets directly in the optical layer. This idea naturally leads to the concept of OPS, enabling a true IP-over-WDM architecture. The OPS concept is illustrated in Fig. 6a. In OPS networks, the packet is composed of an optical payload, consisting of an IP-header followed by an IP-payload, and an optical label. The optical label contains the routing information of the optical packet and is placed either in parallel (Fig. 6b) or in series (Fig. 6c), at a lower line rate compared to the data rate, and is typically out of band with a different modulation format. Furthermore, the optical label should be easily detached and attached from the optical payload preferably using all-optical technology. At each OPS network nodes input, the optical labels are tapped via an optical coupler, extracted from the packet and processed electronically following OEO conversion. The acquired routing information from each optical labels is then used to configure the OSF in order to transparently route the optical payload to the established output port. At the node output, updated versions of the optical labels are attached to the optical payloads.

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a)

IP Payload IP Header Optical label

b) c) Optical label extraction/processing Optical header erasure/insertion OSF

...

Guard band

Fig. 6. a) OPS concept b) packet with parallel optical label and c) packet with a serial optical label

An OPS switch uses the wavelength as a primary domain for contention resolution, since it does not involve additional latency, in conjunction with optical time buffers to avoid packet blocking. Compared to the complexity of the high-speed control plane necessary to control many optical buffer stages in the store-and-forward approach of an electronic packet switch, the OPS based on all-optical contention resolution keeps the control plane very simple with very high energy efficiency as it significantly reduces buffers or signaling required for the actual packet switching. In store-and-forward based electronic packet switches, the switching fabric can afford to switch slower than the bit time without losing a bit in a packet. Because no viable optical RAM technology is available today, an OPS switch on the other hand requires ultrafast switching of the order of nanoseconds in order to accommodate for the optical packet length. Additionally, OPS networks favors efficient routing schemes such as wavelength routing for scalability and also to avoid the inherent power losses of broadcast-and-select architecture.

A typical OPS switch architecture is shown in Fig. 7. At the OPS switch input, the optical labels of each incoming wavelengths is first detached, extracted, and then converted into the electrical domain for processing. The switch controller uses the routing information contained in the extracted optical labels to configure the WCs. The WCs are either tunable wavelength converters (TWCs), with a tunable input and output, or fixed wavelength

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AWG Label extraction TWC TWC TWC TWC O/E Label processing Switch controller FWC FWC FWC FWC

...

Header erasure/insertion Electrical Optical

...

Fig. 7. A typical OPS switch architecture

converters (FWCs), with a tunable input and a fixed output. The WCs in conjunction with a uniform loss and cyclic arrayed waveguide grating and recirculating FDLs, for contention resolution, form the OSF. The switch controller sends control signals at the WCs to set the appropriate wavelength of each optical packets in order to ensure proper routing through the arrayed waveguide grating. The switching speed of the OSF is determined by the time needed to configure the WCs.

The main OPS challenges are the implementation of the optical label extraction, processing and reinsertion mechanisms, the development of an intelligent switch controller, the realization of ultrafast switching in nanoseconds timescale and the exploitation of buffering mechanisms to reduce packet blocking [11]. Because of the small size of data packets, OPS networks offer bandwidth allocation at sub-wavelength granularity that guarantees a high degree of statistical multiplexing and offers excellent scalability [16]. Furthermore, photonic integration is expected to improve reliability, reduce size as well as power consumption [22]–[24]. For these reasons, OPS is often viewed as the ultimate goal in optical networks evolution [11].

Next generation data centers (NGDCs) based on OPS technology have been proposed in order to sustain the increasing need of bandwidth needed for emerging applications such as cloud computing, video on demand, real-time video processing as well as financial services [25], as well as to respond to the global energy efficiency

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concerns. Data centers (DCs) consist a large number servers, the number of servers is approaching 100 000 in the largest DCs [25], with storage units interconnected by a communication network designed to use the available resources in an efficient manner. Large DCs with several tens of thousands of servers are currently limited in term of scalability due to the large amount of power that they require: a cutting-edge electronic packet switched router consumes approximately 1 MW [17]. NGDCs have to be able to interconnect hundreds of thousands of servers, must be scalable and rapidly reconfigurable. The use of OPS technology in NGDCs is expected to provide higher capacity and better energy efficiency, and this, at lower cost and smaller footprint.

1.2. Coherent detection and advanced modulation formats

Communication systems can be classified in two categories: non-coherent and coherent. In non-coherent systems, the information is encoded only in the intensity of the optical carrier. In such systems, the receiver is only sensitive to the signal intensity. The signal is then directly detected, hence we refer to this strategy as intensity-modulated direct-detection (IM-DD). A schematic of a basic IM-DD receiver is shown in Fig. 8. In this scheme, the intensity modulated optical carrier is first detected by a photodetector, amplified and fed through a decision circuit that determines if a mark or a space was received.

Fig. 8. Block diagram of a basic IM-DD communication system

Current conventional WDM systems are based on direct detection schemes that operate at a maximum bit rate of 40 Gb/s with 50 GHz channel spacing [26]. To answer the increasing need of communication bandwidth, coherent detection with digital signal processing (DSP) has been proposed as a cost-effective solution [27]. In a coherent system, the beating of the signal with a local oscillator (LO) enables the detection of both the intensity of the optical carrier and its phase (or frequency), or alternatively, both its in-phase (I) and quadrature (Q) components. The term coherent refers to the use of a LO, even if only the intensity of the optical carrier is detected. Compared to non-coherent systems, coherent systems offer much greater receiver sensitivity and selectivity. Furthermore, the knowledge of the amplitude and the phase of the detected signal enables the use of more spectrally efficient modulation formats, i.e., advanced modulation formats, in conjunction with polarization division multiplexing (PDM). In the optical communications society, the term “advanced” refers to all modulation formats that go beyond on-off-keying (OOK). Fig. 9 shows the signal constellation of OOK and some

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OOK 8-PSK 16-QAM 64-QAM

Q Q Q Q

I I I I

Fig. 9. Signal constellations (in-phase (I) and quadrature (Q) components) of OOK, 8-PSK, 16-QAM and 64-QAM

advanced modulation formats examples, namely, 8 phase-shift keying (8-PSK), 16 quadrature-amplitude modulation (16-QAM) and 64-QAM. The use of spectrally efficient advanced modulation formats increases the channel capacity while satisfying the 50 GHz channel constraint of the current WDM systems. In addition, they are also used to tradeoff noise resilience, fiber propagation impairments, and robustness to narrowband optical filtering due to multiple passes through (R)OADMs [28].

In a coherent receiver, the phase of the optical carrier can be recovered either with an optical phase-locked loop (OPLL), that locks the LO phase to the optical signal phase, or by means of DSP. While OPLL are still difficult to implement [29], the recent development of high-speed DSP has offered the possibility of recovering the signal carrier phase in a very simple and efficient manner. In addition, the very fast tracking of the signal carrier phase greatly improves the system stability compared to coherent receivers with OPLLs.

Fig. 10 illustrates such a digital coherent receiver based on homodyne detection, where the carrier frequencies of the optical signal and LO coincide, with the typical hardware implementation (only for one polarization) and DSP subsystems. In Fig. 10, both the optical signal and LO first go into the optical front-end, that in this case consist of a 90 optical hybrid with balanced photodetectors, whose role is to linearly map the optical signal into a set of electrical signals, i.e., the in-phase (I(t)) and quadrature (Q(t)) components of the optical signal. The analog-to-digital converters then convert the electrical signals into a discrete set of quantized signals at the receiver sampling rate (In and Qn). The DSP-based digital demodulator generates a set of digital samples at the symbol rate representing the best estimate of the transmitted symbols (Sk) and finally performs symbol decoding and subsequent bit decisions.

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LO Optical signal 90° Optical hybrid

−

ADC DSP decoding Symbol

I(t)

Q(t)

In

Qn

Sk

Deskew & orthonormalization Channel equalization Interpolation & timing recovery

Frequency offset estimation Carrier phase estimation

Fig. 10. Typical digital coherent receiver architecture (only one polarization is shown).

As seen Fig. 10, the first DSP subsystem, i.e., deskew and orthonormalization, role is to compensate for the imperfections in the optical front end: signal deskew synchronizes the I and Q components in time with respect to one another by compensating for the path length mismatches of the 90 optical hybrid while the orthonormalization process compensates for the amplitude imbalances and for nonideal 90 optical hybrid as the I and Q components may not be exactly in quadrature. In a coherent system, the optical signal is linearly mapped into the digital domain making possible the compensation of transmission impairments such as chromatic dispersion and polarization mode dispersion (PMD). Typically, the problem is partitioned into static and dynamic equalization since the former requires a set of large static filters while the latter requires a set of relatively short adaptive filters to compensate for time-varying effects, for instance, polarization rotation and PMD. Once the transmission impairments have been compensated, the interpolation and timing recovery subsystem determines the optimum sampling point by compensating for the difference between the symbol clock and the ADCs sampling rate. Next, the frequency offset estimation subsystem compensate for the frequency mismatch between the optical signal and the LO while the carrier phase recovery subsystem estimates the residual carrier phase.

Although it is not shown in Fig. 10, polarization demultiplexing is possible in a digital coherent receiver based on homodyne detection using a polarization-diversity scheme [30]. In that situation, polarization alignment is done

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in the dynamic equalization DSP subsystem. Because advanced modulation formats together with PDM requires a significant amount of DSP, transmissions beyond 40 Gb/s in laboratories are typically carried out using off-line processing after capturing samples of the received optical signal using a real-time oscilloscope (RTO). However, with the current improvement in real-time processing, for instance, using dedicated application-specific integrated circuit or, at lower speeds, using field-programmable gate array FPGAs, real-time beyond 40 Gb/s is possible. For instance, real-time processing for 600 Gb/s operation has been demonstrated in [31]. After being extensively studied in optical communication laboratories, coherent systems recently enter the market of long-haul communication systems. Along with the advances in optical amplification, these technologies have enabled long-distance dense wavelength-division multiplexed transmission with channel bit rates up to 100 Gb/s [32]. It is also envisioned that coherent detection will make its entrance into the MANs and LANs markets as the shorter reach of such networks enables the use of larger constellations which are even more spectrally efficient. The advent of coherent systems has also made the concept of flexible optical networks possible. As the capacity of optical links is approaching a fundamental limit, the emergence of bandwidth hungry and highly dynamic services calls for more advanced networking solutions that exploit the available fiber bandwidth in a more efficient manner. Flexible optical networking concepts have been recently introduced in order to improve the capacity and efficiency of existing WDM systems that offer rigid connections, i.e., with a fixed bit rate and modulation format on a fixed 50 GHz channel spacing grid [32]–[35]. Flexibility in traditional fixed WDM systems is limited to by tunable lasers and limited reconfigurability of the OXCs. In the flexible optical networking paradigm, additional flexibility is obtained due to the ability of the network to dynamically adjust its resources, for example bit rate, modulation format and channel spacing. In addition to the recent advances in coherent orthogonal frequency division multiplexing, Nyquist WDM and optical arbitrary waveform generation that enables the formation of spectrum efficient densely packed super channels, these technologies renders possible the design of a fully flexible optical network [26], [36], [37].

The use of high-order advanced modulation formats yields several benefits in a flexible optical networks. For instance, connection length may vary from a few hundred to several thousands of kilometers: flexible transceivers could tradeoff reach for bandwidth efficiency using high-order advanced modulation formats. Polarization-multiplexed quadrature phase-shift keying (PM-QPSK) has proved to be the optimal modulation format in long haul coherent systems with a transmission reach of several thousands of kilometers [38]. On the other hand, highly efficient advanced modulation formats such as 16-QAM and 64-QAM constitute good potential candidates for connections that cover distances of several hundreds of kilometers. In the scenario of a flexible optical network with adaptable symbol rate and modulation format, the use highly efficient advanced modulation formats in conjunction with a flexible grid will maximize the bandwidth occupation.

Wavelength converters based on four-wave mixing in semiconductor optical amplifiers for advanced modulation formats