List of contributors ......................................... xiii
Preface ........................................................ xv
Part I. Introduction ............................................ l
1 Fabless silicon photonics .................................. 3
1.1 Introduction ............................................... 3
1.2 Silicon photonics: the next fabless semiconductor
industry ................................................... 5
1.2.1 Historical context - Photonics ...................... 6
1.3 Applications ............................................... 7
1.3.1 Data communication .................................. 8
1.4 Technical challenges and the state of the art ............. 10
1.4.1 Waveguides and passive components .................. 10
1.4.2 Modulators ......................................... 12
1.4.3 Photodetectors ..................................... 13
1.4.4 Light sources ...................................... 14
1.4.5 Approaches to photonic-electronic integration ...... 15
Monolithic integration ............................. 15
Multi-chip integration ............................. 16
1.5 Opportunities ............................................. 17
1.5.1 Device engineering ................................. 17
1.5.2 Photonic system engineering ........................ 17
A transition from devicesto systems ................ 18
1.5.3 Tools and support infrastructure ................... 19
Electronic-photonic co-design ...................... 19
DFM and yield management ........................... 20
1.5.4 Basic science ...................................... 20
1.5.5 Process standardization and a history of MPW
services ........................................... 20
ePIXfab and Europractice ........................... 21
IME ................................................ 21
OpSIS .............................................. 21
CMC Microsystems ................................... 22
Other organizations ................................ 22
References ......................................... 22
2 Modelling and design approaches ........................... 28
2.1 Optical waveguide mode solver ............................. 28
2.2 Wave propagation .......................................... 31
2.2.1 3DFDTD ............................................. 31
FDTD modelling procedure ........................... 32
2.2.2 2D FDTD ............................................ 35
2.2.3 Additional propagation methods ..................... 36
2D FDTD with Effective Index Method ................ 36
Beam Propagation Method (BPM) ...................... 37
Eigenmode Expansion Method (EME) ................... 37
Coupled Mode Theory (CMT) .......................... 38
Transfer Matrix Method (TMM) ....................... 38
2.2.4 Passive optical components ......................... 38
2.3 Optoelectronic models ..................................... 39
2.4 Microwave modelling ....................................... 39
2.5 Thermal modelling ......................................... 40
2.6 Photonic circuit modelling ................................ 40
2.7 Physical layout ........................................... 41
2.8 Software tools integration ................................ 42
References ................................................ 43
Part II. Passive components .................................... 47
3 Optical materials and waveguides .......................... 49
3.1 Silicon-on-insulator ...................................... 49
3.1.1 Silicon ............................................ 49
Silicon - wavelength dependence .................... 50
Silicon - temperature dependence ................... 50
3.1.2 Silicon dioxide .................................... 51
3.2 Waveguides ................................................ 51
3.2.1 Waveguide design ................................... 53
3.2.2 1D slab waveguide - analytic method ................ 53
3.2.3 Numerical modelling of waveguides .................. 53
3.2.4 1D slab - numerical ................................ 54
Convergence tests .................................. 55
Parameter sweep - slab thickness ................... 57
3.2.5 Effective Index Method ............................. 57
3.2.6 Effective Index Method - analytic .................. 59
3.2.7 Waveguide mode profiles - 2D calculations .......... 60
3.2.8 Waveguide width - effective index .................. 63
3.2.9 Wavelength dependence .............................. 65
3.2.10 Compact models for waveguides ...................... 66
3.2.11 Waveguide loss ..................................... 69
3.3 Bent waveguides ........................................... 69
3.3.1 3D FDTD bend simulations ........................... 70
3.3.2 Eigenmode bend simulations ......................... 73
3.4 Problems .................................................. 75
3.5 Code listings ............................................. 77
References ................................................ 89
4 Fundamental building blocks ............................... 92
4.1 Directional couplers ...................................... 92
4.1.1 Waveguide mode solver approach ..................... 93
Coupler-gap dependence .................................... 94
Coupler-length dependence ................................. 95
Wavelength dependence ..................................... 95
4.1.2 Phase .............................................. 96
4.1.3 Experimental data .................................. 99
4.1.4 FDTD modelling .................................... 102
FDTD versus mode solver .................................. 102
4.1.5 Sensitivity to fabrication ........................ 103
4.1.6 Strip waveguide directional couplers .............. 105
4.1.7 Parasitic coupling ................................ 106
Delta beta coupling ...................................... 108
4.2 Y-branch ................................................. 110
4.3 Mach-Zehnder interferometer .............................. 113
4.4 Ring resonators .......................................... 115
4.4.1 Optical transfer function ......................... 115
4.4.2 Ring resonator experimental results ............... 117
4.5 Waveguide Bragg grating filters .......................... 117
4.5.1 Theory ............................................ 117
Grating coupling coefficient ...................... 120
4.5.2 Design ............................................ 120
Transfer Matrix Method ............................ 121
Grating physical structure design ................. 123
Modelling gratings using FDTD ..................... 125
4.5.3 Experimental Bragg gratings ....................... 126
Strip waveguide gratings .......................... 127
Rib waveguide gratings ............................ 128
Grating period .................................... 129
4.5.4 Empirical models for fabricated gratings .......... 130
Computation lithography models .................... 134
Additional fabrication considerations ............. 136
4.5.5 Spiral Bragg gratings ............................. 137
Thermal sensitivity ............................... 138
4.5.6 Phase-shifted Bragg gratings ...................... 138
4.5.7 Multi-period Bragg gratings ....................... 140
4.5.8 Grating-assisted contra-directional couplers ...... 141
4.6 Problems ................................................. 143
4.7 Code listings ............................................ 144
References ............................................... 159
5 Optical I/O .............................................. 162
5.1 The challenge of optical coupling to silicon photonic
chips .................................................... 162
5.2 Grating coupler .......................................... 163
5.2.1 Performance ....................................... 164
5.2.2 Theory ............................................ 165
5.2.3 Design methodology ................................ 168
Analytic grating coupler design ................... 169
Design using 2D FDTD simulations .................. 170
Results ........................................... 172
Design parameters ................................. 173
Cladding and buried oxide ......................... 177
Compact design - focusing ......................... 179
Mask layout ....................................... 180
3D simulation ..................................... 181
5.2.4 Experimental results .............................. 181
5.3 Edge coupler ............................................. 182
5.3.1 Nano-taper edge coupler ........................... 183
Mode overlap calculation approach ................. 183
FDTD approach ..................................... 187
5.3.2 Edge coupler with overlay waveguide ............... 189
Eigenmode expansion method ............................... 189
5.4 Polarization ............................................. 190
5.5 Problems ................................................. 193
5.6 Code listings ............................................ 193
References ............................................... 211
Part III. Active components ................................... 215
6 Modulators ............................................... 217
6.1 Plasma dispersion effect ................................. 217
6.1.1 Silicon, carrier density dependence .............. 217
6.2 pn-Junction phase shifter ................................ 218
6.2.1 pn-Junction carrier distribution .................. 218
6.2.2 Optical phase response ............................ 221
6.2.3 Small-signal response ............................. 223
6.2.4 Numerical ТСAD modelling of pn-junctions .......... 224
6.3 Micro-ring modulators .................................... 226
6.3.1 Ring tuneability .................................. 227
6.3.2 Small-signal modulation response .................. 228
6.3.3 Ring modulator design ............................. 231
6.4 Forward-biased PIN junction .............................. 232
6.4.1 Variable optical attenuator ...................... 232
6.5 Active tuning ............................................ 234
6.5.1 PIN phase shifter ................................. 235
6.5.2 Thermal phase shifter ............................. 236
6.6 Thermo-optic switch ...................................... 240
6.7 Problems ................................................. 241
6.8 Code listings ............................................ 242
References ............................................... 257
7 Detectors ................................................ 259
7.1 Performance parameters ................................... 259
7.1.1 Responsivity ...................................... 259
7.1.2 Bandwidth ......................................... 260
Transit time ...................................... 260
RC response ....................................... 261
Dark current ...................................... 262
7.2 Fabrication .............................................. 264
7.3 Types of detectors ....................................... 266
7.3.1 Photoconductive detector .......................... 266
7.3.2 PIN detector ...................................... 267
7.3.3 Avalanche detector ................................ 268
Charge region design ..................................... 270
7.4 Design considerations .................................... 271
7.4.1 PIN junction orientation .......................... 271
7.4.2 Detector geometry ................................. 272
Detector length ................................... 272
Detector width .................................... 272
Detector height ................................... 272
7.4.3 Contacts .......................................... 273
Contact material .................................. 273
Contact geometry .................................. 274
7.4.4 External load on the detector ..................... 275
7.5 Detector modelling ....................................... 275
7.5.1 3D FDTD optical simulations ....................... 276
7.5.2 Electronic simulations ............................ 279
7.6 Problems ................................................. 282
7.7 Code listings ....................................... 283
References ............................................... 292
8 Lasers ................................................... 295
8.1 External lasers .......................................... 295
8.2 Laser modelling .......................................... 296
8.3 Co-packaging ............................................. 299
8.3.1 Pre-made laser .................................... 299
8.3.2 External cavity lasers ............................ 300
8.3.3 Etched-pit embedded epitaxy ....................... 301
8.4 Hybrid silicon lasers .................................... 301
8.5 Monolithic lasers ........................................ 303
8.5.1 III-V Monolithic growth ........................... 303
8.5.2 Germanium lasers .................................. 304
8.6 Alternative light sources ................................ 306
8.7 Problem .................................................. 307
References ............................................... 307
Part IV. System design ........................................ 311
9 Photonic circuit modelling ............................... 313
9.1 Need for photonic circuit modelling ...................... 313
9.2 Components for system design ............................. 314
9.3 Compact models ........................................... 314
9.3.1 Empirical or equivalent circuit models ............ 316
9.3.2 S-parameters ...................................... 317
9.4 Directional coupler - compact model ...................... 318
9.4.1 FDTD simulations .................................. 318
9.4.2 FDTD S-parameters ................................. 320
Directional coupler S-parameters .................. 321
9.4.3 Empirical model - polynomial ...................... 323
9.4.4 S-parameter model passivity ....................... 324
Passivity assessment .............................. 324
Passivity enforcement ............................. 325
9.5 Ring modulator - circuit model ........................... 330
9.6 Grating coupler - S-parameters ........................... 330
9.6.1 Grating coupler circuits .......................... 333
9.7 Code listings ............................................ 333
References ............................................... 348
10 Tools and techniques ..................................... 349
10.1 Process design kit (PDK) ................................. 349
10.1.1 Fabrication process parameters .................... 352
Silicon thickness and etch ........................ 352
GDS layer map ..................................... 352
Design rules ...................................... 352
10.1.2 Library ........................................... 352
10.1.3 Schematic capture ................................. 353
10.1.4 Circuit export .................................... 355
10.1.5 Schematic-driven layout ........................... 356
10.1.6 Design rule checking .............................. 360
10.1.7 Layout versus schematic ........................... 361
10.2 Mask layout .............................................. 362
10.2.1 Components ........................................ 362
10.2.2 Layout for electrical and optical testing ......... 362
10.2.3 Approaches for fast GDS layout .................... 364
10.2.4 Approaches for space-efficient GDS layout ......... 364
References ............................................... 366
11 Fabrication .............................................. 368
11.1 Fabrication non-uniformity ............................... 368
11.1.1 Lithography process contours ...................... 369
11.1.2 Corner analysts ................................... 370
11.1.3 On-chip non-uniformity, experimental results ...... 372
Ring resonators ................................... 373
Grating couplers .................................. 377
11.2 Problems ................................................. 379
References ............................................... 380
12 Testing and packaging .................................... 381
12.1 Electrical and optical interfacing ....................... 381
12.1.1 Optical interfaces ................................ 381
Grating couplers .................................. 381
Edge couplers ..................................... 382
Individual fibres ................................. 382
Spot-size converter ............................... 383
Fibre array ....................................... 384
Free-space coupling ............................... 385
Fibre taper coupling .............................. 386
12.1.2 Electrical interfaces ............................ 386
Bond pads ......................................... 386
Probing ........................................... 387
Wire bonding ...................................... 388
Flip-chip bonding ................................. 388
12.2 Automated optical probe stations ......................... 389
12.2.1 Parts ............................................. 391
Sample stage ...................................... 391
Fibre array probe ................................. 392
Electrical probes ................................. 393
Microscopes ....................................... 393
12.2.2 Software .......................................... 393
12.2.3 Operation ......................................... 394
Loading and aligning a chip/wafer ................. 395
Aligning the fibre array .......................... 395
Chip registration ................................. 396
Automated device testing .......................... 396
12.2.4 Optical test equipment ........................... 397
12.3 Design for test .......................................... 398
12.3.1 Optical power budgets ............................. 400
12.3.2 Layout considerations ............................. 401
12.3.3 Design review and checklist ....................... 402
References ............................................... 404
13 Silicon photonic system example .......................... 406
13.1 Wavelength division multiplexed transmitter .............. 406
13.1.1 Ring-based WDM transmitter architectures .......... 406
13.1.2 Common-bus WDM transmitter ........................ 408
13.1.3 Mod-Mux WDM transmitter ........................... 410
13.1.4 Conclusion ........................................ 411
References ............................................... 412
Index ......................................................... 414
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