Advances in Optical Modulators for Analog Communications Paul K. L. Yu*, Kangwei Wang, Steve Pappert, Y. Fainman and C.K. Sun** Department of Electrical and Computer Engineering University of California, San Diego * Email: pyu@ucsd.edu; ** VEO, Corporation Presentation to IEEE SSCS Joint Chapter at San Diego August 14, 2018 Sponsor acknowledgement: ONR (N00014-18-1-2027); DARPA (W911NF-16-2-0152), and Multidisciplinary University Research Initiative (N00014-13-1-0678)
Outline Introduction: Analog/Digital Applications External Optical Modulator for fiber-optic link and performance figure of merit Technological trend for high performance MZM modulators Summary
The Wideband Optical Modulator Analog Optical Transmission • The lack of a highly efficient (<1 V V π =half-wave voltage), wideband (>100 GHz) optical modulator has long frustrated microwave photonics for use in military antenna applications Digital Optical Transmission • Commercial network capacity scaling (data center/cloud) is driving the need for 100G+ data links with minimal power consumption • The non-existence of a highly efficient (<1 V V π ), wideband (>100 GHz) optical modulator now thwarts commercial optical networks as well 3
The Market Place Commercial Metro and Data Center networking: IEEE P802.3bs 400 Gb/s Ethernet Task Force • $1.6B GbE/10/40/100G Data transceiver in 2014, 10%CAGR Center for 2014-2018 • Expanding rapidly • $4B metro 100G by Metro 2018; <$500M in 2014, 100G inflection point in 2015 Finisar quarterly report: growth due to 40/100Gb transceivers for data center and wireless applications Military Platform RF Networking & Signal Processing: Small (<< $1B) but critically important US military market to provide wideband antenna/sensor interconnects and mixed-signal networking aboard air, land and sea platforms. Modulator performance will bring dramatic change to these RF systems. 4
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Analog RF Photonics ● RF Signal Distribution – Antenna Remoting ● Front-End RF Signal Processing Optical Technology Advantages ● Distance-bandwidth product improves RF performance for broadband transmission - Low loss (0.2 dB/km), low frequency dependence - Design freedom in antenna location, cable routing, receiver location Main Takeaways: ● Cabling size and weight - Reduced cable weight, diameter, bend radius High Performance RF Photonic Links ● Signal Isolation Reduces the dependence on Front-End - No cross talk between cables / EMI resistant Electronics ● Design flexibility/scalability E-O modulator is the key for achieving - Change sensor/transmitter/receiver without changing fiber Low Noise Figure, High Dynamic - Wavelength Division multiplexing Range, Wideband Photonic Links ● Unprecedented Time-Bandwidth Product (TBWP) Processors common in these applications 6
RF over Fiber (RFoF) Links & Signal Processing Outlook RF Antenna • Power Handling • Bandwidth • Drive-Voltage Optical Domain RF Processing • Responsivity LNA • Bandwidth • Linearity Filtering • Insertion Loss Channelization (tunable • Linearity (wideband) O/E Receiver wideband) E/O ADC/DSP Frequency ADC conversion pre-processing Linearization • Power Laser • Noise • Linewidth • Gradual Laser, Modulator & Detector Improvement for Analog Operation • Modulator limits RF link Noise Figure (NF) and Spurious Free Dynamic Range (SFDR) Yesterday Today Tomorrow Amp-less RFoF Link Amp-less RFoF Link Amp-less RFoF Link 20 GHz 50 GHz 100 GHz NF: >30 dB NF: ~15 dB NF: <5 dB SFDR: 110 dB-Hz 2/3 SFDR: 120 dB-Hz 2/3 SFDR: 130 dB-Hz 2/3 7
Important Analog Link Parameters RF Antenna • Power Handling • Bandwidth • Drive-Voltage Optical Domain RF Processing • Responsivity • Bandwidth LNA • Linearity • Insertion Loss Filtering Channelization (tunable • Linearity (wideband) Receiver O/E wideband) E/O ADC/DSP Frequency ADC conversion pre-processing Linearization • Power Laser • Noise • Linewidth 1. RF link Gain: RF power output RF power input 2. Noise Figure: Input SNR Output SNR 3. Spurious free dynamic range: RF Power Range (dB) above noise and inter-modulation distortions 8
Intensity Modulation/Direct Detection Analog Fiber Optic Link* LINK DEFINITION Direct in Detection S N S N Optical out Source Intensity RF opt RF Modulation S , S out in a RF / Optical Photo- Modulator Detector RF RF Input Output Optical Fiber S N S , where N in kT, and T = 290°K * NF 10 log in out G S N S in , a out S out f 2 S out f 1 S out ( f 1 ) SFDR in for which S out (2 f 2 – f 1 ) N out SFDR at S S out (2 f 2 – f 1 ) N out 1 f 2 S out 2 f 2 f 1 S out 2 f * “IRE Standards on Methods of Measuring Noise in Linear Twoports, 1959,” Proc. IRE , 48 , No. 1 (Jan 1960), pp. 60-68. Courtesy of Charles Cox
Outline Introduction: Analog/Digital Applications External Optical Modulator for fiber-optic link and performance figure of merit Technological trend for high performance MZM modulators Summary
External Optical Modulator Requirements Ideal Modulator : low V p (< 1 V); high linearity; low optical insertion loss; high saturation power; low polarization sensitivity; reliable. p 2 2 t R 2 2 Link Gain: ff in 2 G P L R R opt f d out 2 V p where: t ff = fiber-to-fiber optical insertion loss of the modulator R in = the modulator drive impedance, V p = p /(2 dT/dV) L f = optical loss in the fiber R d = the photodetector responsivity R out = the detector load impedance
Optical Modulator Technology Choices Popular Modulator Types Material Choices Electrode Choices • Lithium Niobate • Lumped Electrode Y-Branch Interferrometric Mach-Zehnder Modulator (MZM) • III-V semiconductor • Traveling Wave Electrode • 50 W Terminated • Silicon • Polymer • Unterminated • Graphene; Nanowires • Plasmonic Directional Coupler Modulator (DCM) Performance Parameters of Interest • Modulation Bandwidth • Optical Bandwidth • Sensitivity (V p ) • Power Handling Ring Resonator Modulator • Optical Insertion Loss • Thermal Stability • Extinction Ratio • Form Factor The goal is to develop low-loss E-O modulators with a performance Electroabsorption Modulator (EAM/EML) parameter: M PF = BW(GHz, 3dBe)/ V π (V) 12
Analog Fiber Link: Gain Limitation of EAM modulator Small-signal Equivalent circuit of EA Modulator: Effect of Modulator Photocurrent G. Betts et al, PTL 2007
MZM Based E-O Modulator MZM Optical Transfer Curve Electrooptic Modulator 1 Optical Transmission 0.8 V p RF Input 0.6 0.4 Optical Input Optical Output 0.2 0 -10 -5 0 5 10 Bias Voltage (V) Popular materials for Electro-optic Modulator : (a) Lithium Niobate (b) Semiconductor (c) Polymer (large electro-optic coefficients)
E-O Modulator Technology Outlook Lack of high performance wideband optical modulator • Drive Voltage: ~2V today (sub volt is highly desired or required) • Bandwidth: 30-40GHz today (~100GHz is desired for > 100Gb/s) Transmitter driver power: P V π • 2 , dominating power consumption V π BW: more bandwidth results in higher drive voltage • 10 15.3 GigOptix polymer MZM OFC2010 8.6 Academic Sugiyama Fujitsu 15.2 Mitsubishi EML IEEE meeting 2014 LiNbO3 OFC 2006 24.7 Lange InP MZM OFC 2015 Commercial Watts Silicon ring 1 Current M PF SOTA: 51.9 modulator 2014 ✕ 32 42 Dagli MQW MZM OL 2014 ~15 commercial & ~50 academic Sebastian SOH MZM 2015 V π Performance factor (M PF ) : BW(GHz, 3dBe)/ V π (V) 0.1 BW(GHz, 3dBe) 10 100 SYSTEM IMPACTS: Small M PF modulators result in: • High noise figure broadband analog links • High power consumption digital data centers • Reduced resolution and efficiency LIDAR 15
Performance Limitation Analysis Exemplary MZM modulators with respect to M PF 𝑓𝑝 2 0 𝜌 𝑡 𝑓𝑝 3 - + InAlAs polymer InP, 0.77V/40GHz 6.2V/95GHz Polymer-MZM, M PF =15.3 InP-MZM, (Dagli et al) M PF =51.9 • High r : 100pm/V ( r : relevant EO coef. ) • Low r : 8.2pm/V • Large V π : 6.2V, l =0.37cm, d=6.3μm • Long device length: 0.77V, l =1cm, d=2μm • n eo : 1.7 • n eo : 3.3 • Moderate C s : > 2 x 0.058 pF • Large C s : 2 x 1.2pF • Traveling-wave for speed • Traveling-wave for speed Commercial (M PF ~15) Today’s Best Laboratory (M PF ~50) 16
Outline Introduction: Analog/Digital Applications External Optical Modulator for fiber-optic link and performance figure of merit Technological trend for high performance MZM modulators Summary
GaN Nanowire MZM Modulator Motivation for using Nanowires l d V p 1. Potential for small V p 3 r XX n e l 2. High second harmonic susceptibility: ( 2 ) I 2 2 r 31 1200 31 4 4 2 n n E r GaN, NW 0 0 1000 fitting' =2.3r LiNbO3 measured'data EFFECTIVE'X(2)'(PM/V) 800 Nanowire 150 200 250 300 350 400 diameter 600 (nm) r GaN, NW =r LiNbO3 400 200 SEM 0 0 50 100 150 200 250 300 350 400 NANOWIRE'DIAMETER'(NM)
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