Wireless Terahertz Communications: Optoelectronic Devices and Signal Processing Zur Erlangung des akademischen Grades eines DOKTORS DER INGENIEURWISSENSCHAFTEN (Dr.-Ing.) von der KIT-Fakultät für Elektrotechnik und Informationstechnik des Karlsruher Instituts für Technologie (KIT) angenommene Dissertation von Tobias Harter, M. Sc. geboren in Villingen-Schwenningen, Deutschland Mündliche Prüfung: 28. November 2019 Hauptreferent: Prof. Dr. Christian Koos Korreferenten: Prof. Dr. Dr. h. c. Wolfgang Freude Prof. Dr. Guillermo Carpintero-del-Barrio Table of Contents Kurzfassung ....................................................................................................... v Preface .............................................................................................................. ix Achievements of the present work ................................................................. xi 1 Introduction ................................................................................................. 1 2 THz communications and devices ............................................................. 5 2.1 Fundamentals of T-wave communications ........................................ 6 2.1.1 Baseband and bandpass signals .............................................. 6 2.1.2 Mixer and balanced mixer ...................................................... 9 2.1.3 Digital communications ....................................................... 10 2.1.4 T-wave communication channel .......................................... 12 2.2 T-wave transmitters: State of the art ................................................ 15 2.2.1 Electronic transmitters ......................................................... 16 2.2.2 Optoelectronic transmitters .................................................. 17 2.3 T-wave receivers: State of the art ..................................................... 19 2.3.1 Electronic receivers .............................................................. 19 2.3.2 Optoelectronic receivers ...................................................... 20 2.4 T-wave communications: State of the art ......................................... 21 3 Kramers-Kronig receiver for coherent THz communications ............. 23 3.1 Introduction ...................................................................................... 24 3.2 Future wireless backbone networks ................................................. 25 3.3 Generalized Kramers-Kronig processing ......................................... 26 3.4 Experimental setup and results ......................................................... 30 3.5 Summary........................................................................................... 36 4 Wireless THz link with optoelectronic transmitter and receiver ......... 39 4.1 Introduction and background ........................................................... 40 4.2 Implementation of optoelectronic receiver ...................................... 43 4.3 Demonstration of wireless T-wave links .......................................... 45 4.3.1 Single-channel transmission and wideband tunability ......... 46 4.3.2 Multi-channel transmission .................................................. 50 i Table of Contents 4.3.3 Potential for further improvements ...................................... 52 4.4 Summary........................................................................................... 54 5 Silicon-plasmonic integrated circuits for THz signal generation and coherent detection .............................................................................. 55 5.1 Introduction ...................................................................................... 56 5.2 Silicon-plasmonic T-wave systems .................................................. 58 5.3 PIPED for optoelectronic T-wave processing .................................. 60 5.4 Demonstration of T-wave generation and detection ........................ 65 5.5 Monolithically integrated T-wave system ........................................ 72 5.6 Summary and outlook ...................................................................... 74 6 Summary and Outlook ............................................................................. 77 6.1 Summary........................................................................................... 77 6.2 Outlook and future work .................................................................. 78 Appendices ....................................................................................................... 81 A. Important antenna parameters ............................................................... 83 A.1 Aperture of an isotropic antenna ...................................................... 84 A.2 Free-space path loss .......................................................................... 85 B. Photodiode implementations .................................................................... 86 C. Kramers-Kronig receiver ......................................................................... 89 C.1 Methods ............................................................................................ 89 C.2 Experimental setup ........................................................................... 95 C.3 Model of the Schottky-barrier diode receiver .................................. 97 C.4 Characterization of SBD receiver .................................................. 100 C.5 THz link budget and noise analysis................................................ 104 C.6 Spectral broadening by non-quadratic SBD characteristics ........... 107 C.7 Oversampling requirements in the DSP ......................................... 108 C.8 32QAM measurements ................................................................... 112 C.9 Comparison of generalized Kramers-Kronig pro-cessing and heterodyne detection with guard band ........................................... 113 C.10 Comparison of EVM for QPSK and 16QAM ................................ 115 C.11 Outdoor performance ..................................................................... 116 C.12 Comparison of phase retrieval algorithms ..................................... 117 ii Table of Contents D. Optoelectronic THz receiver .................................................................. 121 D.1 Mathematical model ....................................................................... 121 D.2 T-wave receiver .............................................................................. 123 D.3 Wireless T-wave communication link ........................................... 129 D.4 T-wave amplifiers and UTC-PD .................................................... 134 D.5 Link budget and noise analysis ...................................................... 138 E. Silicon-plasmonic integrated circuits .................................................... 143 E.1 Methods .......................................................................................... 143 E.2 Detailed derivation of formulae ..................................................... 149 E.3 Calibration of T-wave Tx and Rx reference .................................. 152 E.4 Antenna-coupled Tx and Rx .......................................................... 153 E.5 T-wave Rx ...................................................................................... 154 E.6 Damage threshold ........................................................................... 156 E.7 Operating conditions of Rx ............................................................ 157 E.8 Equivalent-circuit of T-wave systems ............................................ 158 E.9 Equivalent-circuit frequency roll-off ............................................. 166 E.10 Simulation of frequency-dependent radiated Tx power ................. 168 E.11 Electric field distribution of integrated T-wave system ................. 170 E.12 Conversion from S-parameters to Z-parameters ............................ 171 E.13 Performance improvement ............................................................. 172 F. Bibliography ............................................................................................ 174 G. Glossary .................................................................................................... 191 G.1 List of abbreviations ....................................................................... 191 G.2 List of mathematical symbols ........................................................ 195 Greek symbols .......................................................................................... 195 Latin symbols ............................................................................................ 195 Danksagung ................................................................................................... 199 List of publications ....................................................................................... 202 Journal publications .................................................................................. 202 Conference publications ............................................................................ 203 iii Kurzfassung Telekommunikation ist ein Grundpfeiler der heutigen Informationsgesellschaft und hat signifikanten Einfluss auf alle Bereiche des sozialen Lebens. Der stetig ansteigende Bedarf an Vernetzung erfordert eine digitale Infrastruktur, mit welcher hohe Übertragungsraten erzielt werden können. Durch die Verwendung von Lichtwellenleitern können