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Applications of all optical signal processing for advanced optical modulation formats

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Scott R Nuccio
Abstract:
Increased data traffic demands, along with a continual push to minimize cost per bit, have recently motivated a paradigm shift away from traditional on-off keying (OOK) fiber transmission links towards systems utilizing more advanced modulation formats. In particular, modulation formats that utilize the phase of the optical signal, including differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK) along with polarization multiplexing (Pol-MUX), have recently emerged as the most popular means for transmitting information over long-haul and ultra-long haul fiber transmission systems. DPSK is motivated by an increase in receiver sensitivity compared to traditional OOK. DQPSK is motivated by a doubling of the spectral efficiency, along with increased tolerance to dispersion and nonlinear distortions. Coherent communications has also emerged as a primary means of transmitting and receiving optical data due to its support of formats that utilize both phase and amplitude to further increase the spectral efficiency (bits/sec/Hz) of the optical channel, including quadrature amplitude modulation (QAM). Polarization multiplexing of channels is a straight forward method to allow two channels to share the same wavelength by propagating on orthogonal polarization axis and is easily supported in coherent systems where the polarization tracking can be performed in the digital domain. Furthermore, the forthcoming IEEE 100 Gbit/s Ethernet Standard, 802.3ba, provides greater bandwidth, higher data rates, and supports a mixture of modulation formats. In particular, Pol-MUX (D)QPSK has grown in interest as the high spectral efficiency allows for 100 Gbit/s transmission while still occupying the current 50 GHz/channel allocation of current 10 Gbit/s OOK fiber systems. In this manner, 100 Gbit/s transfer speeds using current fiber links, amplifiers, and filters may be possible. In addition to advanced modulation formats, it is expected that optical signal processing may play a role in the future development of more efficient optical transmission systems. The hope is that performing signal processing in the optical domain may reduce optical-to-electronic conversion inefficiencies, eliminate bottlenecks and take advantage of the ultrahigh bandwidth inherent in optics. While 40 to 50 Gbit/s electronic components are the peak of commercial technology and 100 Gbit/s capable RF components are still in their infancy, optical signal processing of these high-speed data signals may provide a potential solution. Furthermore, any optical processing system or sub-system must be capable of handling the wide array of data formats and data rates that networks may employ. It is also worth noting that future networks may use a combination of data-rates and formats while it has been estimated that " we may start seeing the first commercial use of Terabit Ethernets by 2015 ". -Robert Metcalfe. To this end, the work presented in this Ph.D. dissertation is aimed at addressing the issue of optical processing for advanced optical modulation formats. All optical multiplexing and demultiplexing of Pol-MUX and phase and QAM encoded signals at the 100 Gbit/s Ethernet standard is addressed. The creation and development of an extremely large continuously tunable all-optical delay capable of handling a variety of modulation formats and data rates is presented. As optical delays are viewed as a critical element to achieve efficient and reconfigurable signal processing, the presented delay line is also utilized to enable a tunable packet buffer capable of handling data packets of varying rate, varying size, and multiple modulation formats.

Table of Contents Dedication ii

Acknowledgements iii

Table of Figures vi

Abstract xiv

Chapter 1: Introduction 1

Chapter 2: Advanced Modulation Formats and Format Transparent Optical Signal Processing 5

2.1 Motivation for Advanced Modulation Formats 5

2.2 Differential Phase Shift Keying (DPSK) 6

2.3 Differential Quadrature Phase Shift Keying (DQPSK) 15

2.4 Polarization Multiplexing (Pol-MUX) 18

2.5 Coherent Transmission and Reception 21

2.6 Raman Amplification 31

2.7 Wavelength Conversion Using PPLN Waveguides 37

2.8 Four-Wave-Mixing In Highly Nonlinear Fiber 43

Chapter 3: All Optical Multiplexing and Demultiplexing of 100 Gbit/s Pol- MUX Signals 46

3.1 Introduction 46

3.2 Concept 48

3.3 Experimental Setup 48

3.4 Results and Discussion 52

Chapter 4: λ -Conversion of 160-Gbit/s PDM 16-QAM Using a Single Periodically-Poled Lithium Niobate Waveguide 55

4.1 Introduction 55

4.2 Concept 57

4.3 Experimental Setup 59

4.4 Results and Discussion 60

Chapter 5: 503 ns Continuously Tunable Delay of 40 Gbit/s OOK and DPSK with Improved Dispersion Compensation 63

v

5.1 Introduction 63

5.2 Concept 66

5.3 Experimental Setup 68

5.4 Results and Discussion 70

Chapter 6: 1.16

s Continuously Tunable Delay of 100 Gbit/s DQPSK 73

6.1 Introduction 73

6.2 Experimental Setup 75

6.3 Results and Discussion 77

Chapter 7: Higher-Order Dispersion Compensation to Enable a 3.6-

s Wavelength-Maintaining Delay of a 100-Gbit/s DQPSK Signal 79

7.1 Introduction 79

7.2 Concept 81

7.3 Experimental Setup 82

7.4 Results and Discussion 85

Chapter 8: Delay Extension to 5-

s for a 10 Gbit/s RZ-DPSK Signal 89

8.1 Experimental Setup 89

8.2 Results and Discussion 92

Chapter 9: Fine Tuning of Optical Delays Using Cascaded Acousto-Optic Frequency Shifters 95

9.1 Introduction 95

9.2 Concept 97

9.3 Experimental Setup 101

9.4 Results and Discussion 103

Chapter 10: Continuously Tunable All Optical Buffer Using Conversion Dispersion Based Delay 105

10.1 Introduction 105

10.2 Concept 108

10.3 Experimental Setup 112

10.4 Results and Discussion 117

10.5 Conclusion 130

Conclusion 131

References 134

vi

Table of Figures Figure 2-1: (a) Illustration of DPSK transmission (b) DPSK constella tion 7

Figure 2-2: Theoretical bit error rate curves for coherent and di fferential detection versions of binary and quaternary phase modulation formats. 8

Figure 2-3: Mach-Zehnder modulator configuration for DPSK modulati on [37]. 11

Figure 2-4: Mach-Zehnder modulator configuration for RZ pulse carvi ng [37]. 12

Figure 2-5: Illustration of a DPSK receiver including delay -line interferometer for differential-phase to intensity conversion and balanced detection. 13

Figure 2-6: Frequency domain response of 1-bit interferometer. T he input signal is bandpass filtered in the constructive port and notch filtered in the destructive port. Spectra are plotted with 10 dB/division and 50 GHz/division. 14

Figure 2-7: Parallel modulator for generation of optical DQPSK , along with ideal resulting constellation diagram. 16

Figure 2-8: Illustration of a typical DQPSK receiver, along with the transmission response of each DLI output port. Four ports are staggered by ¼ of the symbol rate. Constructive and destructive port of each interferometer staggered by ½ the symbol rate. 17

Figure 2-9: Potential formats for the IEEE 802.3ba 100 Gbit/s Ethe rnet Standard include quadrature-PSK, utilizing 4 phase states for 2 bits/symbol and polarization multiplexing (Pol-MUX) to achieve 4 bits/symbol. 19

Figure 2-10: Coherent transmission system (a) implementation, (b) system model. 24

Figure 2-11: Single-polarization downconverter employing a (a) het erodyne and (b) homodyne design. 25

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Figure 2-12: Raman gain of fused quartz plotted as a function of fre quency shift from an exciting line at 526 nm. The experimental point is the gain measured in the amplifier and the error bar represe nts a combination of the uncertainties both in the measurement of the gain and the spontaneous cross section [85]. 32

Figure 2-13: Level diagrams showing (a) stimulated Raman Stoke s scattering; (b) stimulated Raman anti-Stokes scattering; ( c) coherent anti-Stokes four-wave mixing; (d) multiple Stokes and anti-Stokes scattering; and (e) hyper-Raman scattering [3]. 35

Figure 2-14: Illustration of quasi-phase matching in a periodical ly poled Lithium Niobate waveguide. 39

Figure 2-15: Illustration of difference frequency generation in a periodically poled Lithium Niobate waveguide. 40

Figure 2-16: Illustration of cascaded wavelength conversion schemes in a PPLN waveguide. Second harmonic generation is followed by difference frequency generation 41

Figure 2-17: Illustration of cascaded wavelength conversion in a PPLN waveguide. Sum frequency generation is followed by difference frequency generation. 42

Figure 2-18: Illustration of four-wave-mixing processes that sat isfy the phase match condition. 45

Figure 3-1: Conceptual diagram of all optical polarization demult iplexing and polarization multiplexing. 48

Figure 3-2: Experimental setup. A 100 Gbit/s Pol-MUX signal is generated and combined with two orthogonal pumps for demultiplexing to two 50 Gbit/s WDM channels. Similarly, two 50 Gbit/s WDM channels are generated and combined with two orthogonal pumps for multiplexing into a single 100 Gbit/s Pol-MUX channel. 50

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Figure 3-3: (a) Experimental spectra for demultiplexing (t op) and multiplexing (bottom) between 100 Gbit/s Pol- MUX RZ-OOK and 2 x 50 Gbit/s RZ-OOK. (b) Experimental spectra for the demultiplexing (top) and demultiplexing (bottom) when using RZ-DPSK. (c) Back-to-Back eyes and demultiplexed/multiplexed eyes for comparison. 52

Figure 3-4: (a) Comparison of RZ-OOK performance after demulti plexing and multiplexing compared to back-to-back performance. (b) Comparison of RZ-DPSK performance after demultiplexing and multiplexing compared to back-to-back. (c) Performance of the multiplexed 100-Gbit/s Pol-MUX channel after 1km of uncompensated propagation. 53

Figure 4-1: Conceptual diagram of transparent polarization (a) and phase and amplitude (b) conversion in a PPLN waveguide. 58

Figure 4-2: Experimental setup. A 20-Gbaud PDM 16-QAM or 40-Gbaud

16-QAM signal is generated and combined with a CW pump in a bidirectional PPLN based wavelength converter. An EAM is used for 40-Gbaud to 20-Gbaud down sampling followed by coherent detection. 59

Figure 4-3: (a) Experimental constellation diagrams for 40-Gb aud single- polarization 16-QAM back-to-back (top) and after (bottom) conversion for pump powers of 16.2 dBm (left) and 21 dBm (right). (b) Experimental constellation diagrams for 20-Gbaud PDM 16-QAM back-to-back (top) and after (bottom) conversion for both polarizations. (c) Experimental spectra for 40-Gbaud single polarization (right) and 20 G-Baud PDM (left) 16-QAM. 61

Figure 4-4: Bit-error-rate (BER) measurements for (a) 40- Gbaud 16-QAM and (b) 20-Gbaud PDM 16-QAM back-to-back and after wavelength conversion. (c) Relative power penalty vs. pump power for 40-Gbaud 16-QAM. 62

Figure 5-1: Conceptual diagram of tunable delay methods. As opposed to

previous methods (a) and (b), following wavelength conversion (W/C) and delay in dispersion compensating fiber (DCF), the signal is not returned to the original wavelength for compensation (c). 66

ix

Figure 5-2: Experimental setup. A 40-Gbit/s signal is waveleng th converted and passed through the Raman pumped DCF. The signal is then phase conjugated and shifted by ~3.4 nm before passing back through the DCF for detection. 68

Figure 5-3: Measured fiber dispersion profile for (a) dispersion, ( b) compensation and (c) comparison of residual dispersion for compensation at a fixed wavelength and using the newly proposed method. Residual dispersion is reduced by >95%. 70

Figure 5-4: Measured delay versus converted wavelength and exper imental spectra of both the wavelength conversion and phase conjugation stages. 71

Figure 5-5: Bit-error-rate (BER) curves for (a) 40-Gbit/s RZ-OOK and (b) RZ-DPSK. Performance after back-to-back, the first wavele ngth conversion (Stage 1), and after the full system (Final) is compared. 40-Gbit/s RZ-OOK (c) and RZ-DPSK (d) back-to- back performance compared to the minimum, maximum, and middle delay performances. 72

Figure 6-1: Block diagram. Dispersion compensating fiber (DCF), fiber Bragg grating (FBG), bandpass filter (BPF), transmitter (TX) , receiver (RX), and highly nonlinear fiber (HNLF). 74

Figure 6-2: (a) Measured delay of 1.16

s. (b) Received 50 Gbit/s ODB signal for 10 pm changes in laser wavelength showing ~275 ps changes in delay. (c) Experimental spectra of first and sec ond wavelength conversion stages for the maximum delay value. 76

Figure 6-3: (a) 0, 0.5ps, and 1ps delay resolution of a single 40Gbit/s RZ- OOK bit. (b) RF-spectra showing optical mixing for different AOM frequency offsets. (c) Bit-error-rate curves for varyi ng delay values with and without the AOMs. 77

Figure 7-1: (a) Conceptual diagram of pre-dispersion block to enabl e 100 Gbit/s operation. (b) A 96% reduction in residual 3 rd -order dispersion is achieved using fixed fiber-Bragg-gratings (FBGs) and a tunable spatial light modulator (SLM). 81

x

Figure 7-2: Block diagram. Dispersion compensating fiber (DCF), s patial light modulator (SLM), band-pass filter (BPF), erbium-doped fiber amplifier (EDFA), receiver (Rx), and highly nonlinear fiber (HNLF). 82

Figure 7-3: (a) Measured delay of 3.6

s for 100-Gbit/s RZ-DQPSK. (b) 7- Gbit/s packets used to illustrate the full delay tuning range. 85

Figure 7-4: (a) Constellation diagrams showing the DPSK (Top) and DQPSK (Bottom) signals before (Left) and after (Right) at the middle delay value, ~1567 nm. (b) Experimental spectra of the first stage (top) and third stage (bottom) at the minimum delay value. 86

Figure 7-5: Measured bit-error-rate performance of (a) 100 (Solid), 80 (Dashed), and 20-Gbit/s (Solid) DQPSK and (b) 50 (Solid), 40 (Dashed), and 10-Gbit/s (Solid) DPSK for the minimum (Red), middle (Blue), and maximum (Green) delay values. 87

Figure 7-6: Power penalty as a function of residual 3 rd -order dispersion (ps/nm2) for 100-Gbit/s RZ-DQPSK. 88

Figure 8-1: Improved experimental setup utilizes -48 ns/nm of DCF to achieve a 5.4

s relative delay. 89

Figure 8-2: Measured residual dispersion slope after the addition of the extra DCF. At the minimum wavelength, this corresponds to ~0.8 dB penalty for a 10 Gbit/s RZ-DPSK signal. 90

Figure 8-3: Experimental measurement of the delay range was performed using a 300 Mbit/s packet. The full 5.4

s range is shown (800 ns/Div). 92

Figure 8-4: (a) Experimental spectra for the first and third wavelength conversion stages. Wavelength maintaining operation is accomplished. (b) Measured relative delay vs. the converted wavelength showing the full 5.4

s delay range. 93

Figure 8-5: Bit-error-rate measurements for the minimum, midd le, and maximum delay values compared to the back-to-back case. 94

xi

Figure 9-1: (a) A tunable laser with 1pm (125MHz) resolution is us ed to coarse tune the delay from 0 to 256ns. Cascaded acousto-optic modulators (AOMs) shift the laser center frequency with 1kHz resolution; fine tuning the delay from 0 to 25ps. (b) Measured fine and coarse tuning ranges of our system. 98

Figure 9-2: Acousto-optic frequency shifters for up shifting and down shifting a CW laser. 100

Figure 9-3: Block diagram. Wavelength Converter (W/C1), dispersio n compensating fiber (DCF), acousto-optic modulator (AOM), Mach-Zehnder modulator (MZM), receiver (RX), and highly nonlinear fiber (HNLF). 101

Figure 9-4: (a) Sampling scope trace of 40 Gbit/s RZ-OOK bits with inset showing 0, 0.5, and 1ps delay shifts. (b) Tuning range of our cascaded AOMs is shown through optical mixing measurements. 103

Figure 9-5: Bit-error-rate curves for varying delay values w ith and without the AOMs. 104

Figure 10-1: Conceptual block diagram of the demonstrated optical buffe r. Input packet stream is sent to two paths. Upper path induces the relative delay on the selected packet(s), where the lower path deletes any desired packet(s). 107

Figure 10-2: Conceptual block diagram of the conversion/dispersion technique used to generate relative delays in the optical buffer. The first wavelength conversion controls the amount of delay. The second wavelength conversion is the phase conjugation stage. After the delay, the signal is converted back to the original wavelength to have a wavelength transparent delay. 108

Figure 10-3: Illustration of reconfiguration of the optical buffer. Packets to be delayed are extracted to the corresponding wavelengths in the first wavelength conversion stage. Thus, they experience different amounts of delay in the dispersive element. The reconfiguration should take place within the guard time between the packets. Therefore, the minimum guard time without any data loss is determined by the reconfiguration speed. 110

xii

Figure 10-4: Experimental setup for the optical buffer. Modificati ons for demonstration of reconfiguration are shown with dotted lines and italic titles. MZM: Mach-Zehnder modulator; CLK: clock; TDL: tunable delay line; BPF: bandpass filter; DCF: dispersion compensating fiber; SSMF: standard single mode fiber; PPLN: periodically poled Lithium Niobate waveguide; Rx: preamplified receiver. 111

Figure 10-5: Packet-1 being buffered from time slot 1 to time sl ot 11. Eye diagrams of the signal shown are also given. (a) 40 Gbit/s input packet stream (424 bits/packet, 1 ns guard time); (b) Packet-1 after extraction to λ PKT1 (~1552.5 nm) in PPLN-1. (c) Packet-1 after double passing through the DCF and after SMF, signal is at λ PKT_C (~1556.9 nm) due to phase conjugation; (d) Output packet stream where delayed Packet-1 is converted to λ Sig and original Packet-1 is deleted from time slot 1 in the lower path. 117

Figure 10-6: Experimental spectra of the wavelength conversion proce sses in the buffer. (a) Packet extraction in the PPLN-1 with gated pump λ GP1 (1550.6 nm). PPLN-1 QPM wavelength is shown with a dotted line (~1551.6 nm); (b) Phase conjugation in the HNLF; (c) Delayed Packet-1 is wavelength converted back to λ Sig in PPLN-2. PPLN-2 QPM is shown with the dotted line and is at ~1552.7 nm. The scale is the same, 8 dB/div and 3 nm/div, for all plots. 119

Figure 10-7: (a) Relative delay achieved for the system for a 40 Gbit/s input signal; (b) Output packet stream for various buffering scena rios including zero and maximum delay. 120

Figure 10-8: Experimental spectra of the three wavelength convers ion stages for the cases of: (a) maximum (116 ns), (b) middle, and (c) zero delay. The first row shows the packet extraction in PPLN-1, the second row shows the phase conjugation in HNLF, and the third row shows the wavelength conversion of the delayed Packet-1 to the original wavelength in PPLN-2. For all plots, the center wavelength is 1552.7 nm and the scale is 3 nm/div for the horizontal and 8 dB/div for the vertical axis. 122

xiii

Figure 10-9: BER performances for several buffering scenar ios. Back-to- back performance and BER performance of the signal at the output of lower path (with Packet-1 deleted) is also given for comparison. 123

Figure 10-10: Packets-2 and -3 being buffered by three and five ti me slots in the reconfiguration experiment. (a) The input packet sequence of 8 packets. The guard time between the Packet-2 and Packet-3 is 25 ps. The inset shows the guard time; (b) An illustration of the gated pumps generated by the switch and the MZM in the packet extraction stage; (c) Extracted Packets 2 and 3; (d) Packets 2 and 3 after the second pass through the DCF; (e) Output packet sequence where Packets 2 and 3 are inserted at the corresponding time slots. 124

Figure 10-11: Experimental spectra of the wavelength conversion p rocesses in the buffer. (a) Packet extraction in the PPLN-1 with gated pump λ GP1 (1550.6 nm). PPLN-1 QPM wavelength is shown with a dotted line (~1551.6 nm); (b) Phase conjugation in the HNLF; (c) Delayed Packet-1 is wavelength converted back to λ Sig. PPLN-2 QPM is shown with the dotted line and is at ~1552.7 nm. 125

Figure 10-12: Transient response of the 2x2 Lithium Niobate switch us ed for toggling between pump lasers in packet extraction. The rise/fall time is ~25 ps for both output ports. 127

Figure 10-13: Last and first several bits of the Packets 2 and 3, respectively: (a) Packet-2 and Packet-3 from the input packet stream with a guard time (i) 25 ps, (ii) 1 ns; (b) Packet-2 and Packet-3 after

extraction in the PPLN-1. For Packet-2, Packet-3 extraction pump is turned off for demonstration purposes; (c) Packet-2 and Packet-3 after the delay (before the combination with the lower arm) when both gated pumps are on. The scale is 100 ps/div for all the plots except (a)-(ii). 128

Figure 10-14: BER performances of the buffer reconfiguration exper iment for guard times of 1 ns and 25 ps. 130

xiv

Abstract

Increased data traffic demands, along with a continual push to minimi ze cost per bit, have recently motivated a paradigm shift away from traditiona l on-off keying (OOK) fiber transmission links towards systems utilizing more advanced modulation formats. In particular, modulation formats that utilize the phase of the optical signal, including differential phase shift keying (DPSK) and differentia l quadrature phase shift keying (DQPSK) along with polarization multiplexing (Pol-M UX), have recently emerged as the most popular means for transmitting inf ormation over long- haul and ultra-long haul fiber transmission systems. DPSK is m otivated by an increase in receiver sensitivity compared to traditional OOK. DQPSK is motivated by a doubling of the spectral efficiency, along with increased t olerance to dispersion and nonlinear distortions. Coherent communications has also emerged as a primary means of transmitting and receiving optical data due to its support of formats that utilize both phase and amplitude to further increase the spectral ef ficiency (bits/sec/Hz) of the optical channel, including quadrature amplitude modulation (QAM). Polarization multiplexing of channels is a straight forwa rd method to allow two channels to share the same wavelength by propagating on or thogonal polarization axis and is easily supported in coherent systems where the polarization tracking can be performed in the digital domain. Furthermore, the f orthcoming IEEE 100 Gbit/s Ethernet Standard, 802.3ba, provides greater bandwidth, higher data ra tes,

xv

and supports a mixture of modulation formats. In particular, Pol-MUX ( D)QPSK has grown in interest as the high spectral efficiency allows for 100 Gbit/s transmission while still occupying the current 50 GHz/channel allocation of cur rent 10 Gbit/s OOK fiber systems. In this manner, 100 Gbit/s transfer speeds using current fiber links, amplifiers, and filters may be possible. In addition to advanced modulation formats, it is expected that optical signal processing may play a role in the future development of more effi cient optical transmission systems. The hope is that performing signal process ing in the optical domain may reduce optical-to-electronic conversion inefficiencie s, eliminate bottlenecks and take advantage of the ultrahigh bandwidth inherent in opt ics. While 40 to 50 Gbit/s electronic components are the peak of commercial techn ology and 100 Gbit/s capable RF components are still in their infancy, optic al signal processing of these high-speed data signals may provide a potential solution. F urthermore, any optical processing system or sub-system must be capable of hand ling the wide array of data formats and data rates that networks may employ. It i s also worth noting that future networks may use a combination of data-rates and formats w hile it has been estimated that “ we may start seeing the first commercial use of Terabit Et hernets by 2015 ”. –Robert Metcalfe. To this end, the work presented in this Ph.D. dissertation is aimed at addressing the issue of optical processing for advanced optical modulation format s. All optical

xvi

multiplexing and demultiplexing of Pol-MUX and phase and QAM encode d signals at the 100 Gbit/s Ethernet standard is addressed. The creation and deve lopment of an extremely large continuously tunable all-optical delay capable of handling a variety of modulation formats and data rates is presented. As optical dela ys are viewed as a critical element to achieve efficient and reconfigurable si gnal processing, the presented delay line is also utilized to enable a tunable packet buf fer capable of handling data packets of varying rate, varying size, and multiple modulation f ormats.

1 Chapter 1:

Introduction

Low-loss optical fiber was introduced in 1970 by Corning. The first na tional network based on fiber optic technology did not appear until the mid-1980s. The se links were first operated at an initial rate of merely 51.84 Mb/s , referred to as OC-1 under the Synchronous Optical Networks (SONET) North American sta ndard [82]. Since this time, fiber optic telecommunication has advanced tremendous ly. Currently, the majority of long-haul transmission and global networking is made possible through the use of fiber optic communication links. Deployed links

currently operate up to 10 Gbit/s (OC-192) with 40 Gbit/s links begin ning to be deployed, the 100 Gbit/s standard development nearing completion and demonstrations of single-channel rates at 1-Tb/s and beyond. In order to m eet current and future projected demands [18], mainly driven by the exponential gro wth of internet traffic, efforts are being made to transmit inform ation in a more cost- effective and efficient manner and at ever-increasingly higher rates.

Initial fiber optic links employed simple on-off keying (OOK) modulation, in which the laser intensity was directly modulated to encode digita l information. Since then, techniques for transmission and detection of optical information have gone through many phases. In the late 1970’s there was a significant i nterest in receivers employing coherent detection. Coherent detection incorporates a local oscillator (LO) laser to mix with the incoming signal, thereby generat ing an electrical beat

2 signal carrying the modulating signal. This allowed for ultimat e receiver sensitivity but required a complex receiver design, including a phase-locked-loop and a narrow linewidth LO. After the advent of optical amplifiers in the 1980’s, the original coherent techniques were abandoned, as optical amplification was able to provide more robust, cost-effective solutions with comparable sensitivity. A decade or two later, coherent communications began a revival due to t he ever increasing need for optical bandwidth and the decreasing amount of bandwi dth available. Additionally, the ability to compensate for many of the optical impairments that limit current optical systems using digital signal processing (DSP) at the receiver presents a cost-effective way of upgrading c urrent links to higher data-rates while providing more robust operation. Current commercial direction is to utilize coherent technology to both upgrade existing links and in the deplo yment of new ones to provide 100 Gbit Ethernet connectivity. With the continual demands for increased transmission lengths and data rates, research has recently focused on more advanced optical transmissi on and detection schemes, which provide some key advantages including: enhanced receive r sensitivity, increased spectral efficiency, increased single -user channel rates, and reduced complexity. Differential phase shift keying (DPSK) em erged as the first practically promising scheme for ultra-high bit-rate transm ission. While not widely deployed, DPSK installations are still being utilized. Furthermore, multi-level versions of DPSK, namely differential quaternary phase shift key ing (DQPSK) are

3 being explored as bandwidth efficient methods for meeting future de mands. In addition polarization division multiplexing (PDM) is being used as a means to further double spectral efficiency, reduce the required bandwidth of ele ctronic components and reduce sensitivities to fiber impairments. In particula r PDM- (D)QPSK, utilizing coherent receivers, has become the industry favo rite for reaching the 100 Gbit/s mark. The development of the IEEE 100 Gbit/s Ethernet S tandard, 802.3ba, has set a goal beyond what is capable with current OOK modulati on and electronic components. The standard includes provisions for the use of thes e advanced modulation formats especially multi-level and Pol-MUX for mats, due to their ability to send multiple bits per symbol. This allows for the use of lower rate electronics at the receiver and transmitter but makes midstrea m processing and routing of the data difficult. Formats that utilize both amplitude a nd phase modulation have become the most popular means for achieving the 100 Gbit/s standard and for demonstrations of future 400 Gbit/s systems [53, 95]. Quadr ature amplitude modulation (QAM) has emerged as the dominant candidate for f uture modulation formats. In addition to advanced modulation formats, the use of optical signal proc essing may play a key role in future high-speed dense WDM networks. Sig nal processing is generally considered an efficient and powerful enabler for a host of communication functions as well as a system performance enhancer. The hope is that performing signal processing in the optical domain might reduce any optical-e lectronic

4 conversion inefficiencies and take advantage of the ultrahigh bandwidt h inherent in optics [94]. It is important that optical signal processing functi ons be capable handling and operating on advanced modulation formats. In this manner, the bottleneck of midstream routing and processing may be alleviated. One of the basic building blocks to achieve efficient and reconfigurable signal proc essing is a continuously tunable optical delay line, and yet, this element has hist orically been difficult to realize. In this dissertation, novel methods for the optical processing of hig h speed, ≥ 40 Gbit/s, signals including support for both basic OOK and advanced modulation formats, are presented. The proposal is structured as follows. The next chapter introduces potential modulation formats to be used in future 100 Gbit/s sys tems, along with some potential non-linear processes that can support these formats. The subsequent chapters 3-9 present material in support of this dissertation.

5 Chapter 2:

Advanced Modulation Formats and Format Transparent Optical Signal Processing

This chapter provides a perspective of the progress in the field of opt ical modulation formats and an introduction to optical signal processing. To this end, an overview of advanced modulation formats that are favored for future 100 Gbit/s systems and nonlinear optical processes capable of supporting these format s. 2.1 Motivation for Advanced Modulation Formats The choice of transmission and detection of optical information depends on various system parameters, including link length, number of users, de sired bit rate, desired quality of service and cost. For short distance metro links, OOK may be the most suitable format. As the link length and the single-channel bit r ate increases, OOK becomes more susceptible to waveform degradations; including chrom atic dispersion (CD), polarization mode dispersion (PMD), self phase modulati on (SPM), cross phase modulation (XPM) and four-wave-mixing (FWM). In such environments, more advanced modulation formats may be more suitable. DPS K has recently emerged as the most practically promising format for future high bit rate, long-haul systems. Other potential formats include, quaternary phas e shift keying (QPSK) and quadrature amplitude modulation (QAM) and both polariza tion multiplexing (Pol-MUX) and orthogonal frequency division multiplexing (OFDM)

6 of these signals. In addition, a trade-off exists between non-ret urn-to-zero (NRZ) and return-to-zero (RZ) formats. NRZ formats tend to be more spec trally efficient, while RZ formats tend to have higher tolerance to nonlinear effects such as SPM, XP M and FWM, along with better receiver sensitivity. Since many of the se impairments are exponentially proportionally to the bandwidth of the optical signal, p olarization multiplexing (Pol-MUX) has gained much recent attention due to its inherent ability to transmit twice the data using the same optical bandwidth. This di ssertation proposal will mainly focus on the nonlinear processing of these advance d formats modulation formats, in particular DPSK, DQPSK, QAM, and polarization

multiplexing of such formats. 2.2 Differential Phase Shift Keying (DPSK) Differential phase shift keying has been used extensively in the RF domain for transmission of digital information. Although (non-differential) bina ry phase shift keying (BPSK) has superior tolerance to DPSK, the receiver st ructure for DPSK is much simpler and the penalty for differential detection is fai rly small. In DPSK, information is transmitted via the differential phase of the optica l carrier: ( ) ( ) ( ) t t t T ϕ φ φ = − + τ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ ∝ { } ( ) cos ( ) E A t t t ω ϕ = + , where A(t) represents the time-varying complex amplitude, ω is the radial frequency of the optical field and φ (t) is the time-varying optical phase of the laser. In the typical convention, a 1-bit resul ts in a change in

7 phase of 180° between the current and previous bit, while a 0-bit results in no phase change (note: the opposite convention has also been used in the past). Because an optical phase modulator typically encodes the absolute phase, an ele ctronic differential encoder is employed at the transmitter to encode t he bits prior to modulation onto the optical carrier. The precoding for DPSK follows t he relationship: 1 k k k d d b − = + , where k b represents the original bits and k d

Full document contains 159 pages
Abstract: Increased data traffic demands, along with a continual push to minimize cost per bit, have recently motivated a paradigm shift away from traditional on-off keying (OOK) fiber transmission links towards systems utilizing more advanced modulation formats. In particular, modulation formats that utilize the phase of the optical signal, including differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK) along with polarization multiplexing (Pol-MUX), have recently emerged as the most popular means for transmitting information over long-haul and ultra-long haul fiber transmission systems. DPSK is motivated by an increase in receiver sensitivity compared to traditional OOK. DQPSK is motivated by a doubling of the spectral efficiency, along with increased tolerance to dispersion and nonlinear distortions. Coherent communications has also emerged as a primary means of transmitting and receiving optical data due to its support of formats that utilize both phase and amplitude to further increase the spectral efficiency (bits/sec/Hz) of the optical channel, including quadrature amplitude modulation (QAM). Polarization multiplexing of channels is a straight forward method to allow two channels to share the same wavelength by propagating on orthogonal polarization axis and is easily supported in coherent systems where the polarization tracking can be performed in the digital domain. Furthermore, the forthcoming IEEE 100 Gbit/s Ethernet Standard, 802.3ba, provides greater bandwidth, higher data rates, and supports a mixture of modulation formats. In particular, Pol-MUX (D)QPSK has grown in interest as the high spectral efficiency allows for 100 Gbit/s transmission while still occupying the current 50 GHz/channel allocation of current 10 Gbit/s OOK fiber systems. In this manner, 100 Gbit/s transfer speeds using current fiber links, amplifiers, and filters may be possible. In addition to advanced modulation formats, it is expected that optical signal processing may play a role in the future development of more efficient optical transmission systems. The hope is that performing signal processing in the optical domain may reduce optical-to-electronic conversion inefficiencies, eliminate bottlenecks and take advantage of the ultrahigh bandwidth inherent in optics. While 40 to 50 Gbit/s electronic components are the peak of commercial technology and 100 Gbit/s capable RF components are still in their infancy, optical signal processing of these high-speed data signals may provide a potential solution. Furthermore, any optical processing system or sub-system must be capable of handling the wide array of data formats and data rates that networks may employ. It is also worth noting that future networks may use a combination of data-rates and formats while it has been estimated that " we may start seeing the first commercial use of Terabit Ethernets by 2015 ". -Robert Metcalfe. To this end, the work presented in this Ph.D. dissertation is aimed at addressing the issue of optical processing for advanced optical modulation formats. All optical multiplexing and demultiplexing of Pol-MUX and phase and QAM encoded signals at the 100 Gbit/s Ethernet standard is addressed. The creation and development of an extremely large continuously tunable all-optical delay capable of handling a variety of modulation formats and data rates is presented. As optical delays are viewed as a critical element to achieve efficient and reconfigurable signal processing, the presented delay line is also utilized to enable a tunable packet buffer capable of handling data packets of varying rate, varying size, and multiple modulation formats.