Page 11 - Fiber Optic Communications Fund

x Contents

5.3.2 pin Photodetector (pin-PD) 203
5.3.3 Schottky Barrier Photodetector 204
5.3.4 Metal–Semiconductor–Metal Photodetector 204
5.3.5 Photoconductive Detector 206
5.3.6 Phototransistor 206
5.3.7 Avalanche Photodetectors 207
5.3.8 Advanced Photodetectors ∗ 212
5.4 Direct Detection Receivers 219
5.4.1 Optical Receiver ICs 220
5.5 Receiver Noise 224
5.5.1 Shot Noise 224
5.5.2 Thermal Noise 226
5.5.3 Signal-to-Noise Ratio, SNR 227
5.6 Coherent Receivers 227
5.6.1 Single-Branch Coherent Receiver 228
5.6.2 Balanced Coherent Receiver 232
5.6.3 Single-Branch IQ Coherent Receiver 234
5.6.4 Balanced IQ Receiver 237
5.6.5 Polarization Effects 239
Exercises 242
References 244

6 Optical Amplifiers 247
6.1 Introduction 247
6.2 Optical Amplifier Model 247
6.3 Amplified Spontaneous Emission in Two-Level Systems 248
6.4 Low-Pass Representation of ASE Noise 249
6.5 System Impact of ASE 251
6.5.1 Signal–ASE Beat Noise 253
6.5.2 ASE–ASE Beat Noise 256
6.5.3 Total Mean and Variance 256
6.5.4 Polarization Effects 258
6.5.5 Amplifier Noise Figure 260
6.5.6 Optical Signal-to Noise Ratio 262
6.6 Semiconductor Optical Amplifiers 263
6.6.1 Cavity-Type Semiconductor Optical Amplifiers 264
6.6.2 Traveling-Wave Amplifiers 268
6.6.3 AR Coating 270
6.6.4 Gain Saturation 271
6.7 Erbium-Doped Fiber Amplifier 274
6.7.1 Gain Spectrum 274
6.7.2 Rate Equations ∗ 275
6.7.3 Amplified Spontaneous Emission 280
6.7.4 Comparison of EDFA and SOA 281
6.8 Raman Amplifiers 282
6.8.1 Governing Equations 283