FiberOptic Communication Technology Chapter 2 Optical Fibers Total

Fiber-Optic Communication Technology Chapter 2 Optical Fibers

• • • • • Total internal reflection 全内反射 • Step-index fiber 阶跃折射率光纤 • Graded-index fiber 渐变折射率光纤 • Core-cladding interface 芯层和包层界面 • Modal dispersion 模式色散 • Numerical aperture 数值孔径 • Impulse 冲激 • Meridional rays 子午光线 • Parabolic-index fiber 抛物线折射率分布光 • 纤 • Refractive index 折射率 • Absorption coefficient 吸收系数 • Chromatic dispersion 色度色散 Optical mode 光模式 • Mode index 模式折射率 • Effective index 有效折射率 • Cutoff condition 截止条件 • Normalized frequency 归一化频率 • Single-mode condition 单模条件 • Eigenvalue equation 本征值方程 • Birefringence双折射 • 2020/12/7 OE, HUST PMD 偏振模色散 Confinement factor 限制因子 Field radius 模场半径 Attenuation coefficient 衰减系数 Impurity 杂质 Intrinsic absorption 本征吸收 Rayleigh scattering 瑞丽散射 Mie scattering 米氏散射 Waveguide imperfection 波导不完善 Macrobending 宏弯 Microbending 微弯 GVD: 群速度色散 Intramodal dispersion: 模内色散 Intermodal dispersion：模间色散 Dispersion parameter：色散参数 Material dispersion：材料色散 Waveguide dispersion：波导色散 Sellmeier equation：塞米尔方程 Zero-dispersion wavelength：零色散波长 Dispersion-shifted fiber：色散位移光纤 2

• • • • • Dispersion-flatten fiber：色散平坦光纤 • Dispersion decreasing fiber：色散渐减光纤 • Dispersion compensation fiber：色散补偿光纤 • Dispersion slope：色散斜率 • Differential-dispersion parameter：微分色散参数 • Polarization mode dispersion：偏振模色散 • Pulse broadening 脉冲展宽 • Chirped Gaussian pulse 啁啾高斯脉冲 • FWHM: 半高全宽 • Raman scattering 喇曼散射 • Brillouin scattering 布里渊散射 • SRS 受激喇曼散射 • SBS 受激布里渊散射 • Electrostriction 电致伸缩效应 • Brillouin shift 布里渊频移 • Acoustic phonon 声学声子 • Inhomogeneous非均匀的 • Germania 锗 • Isotropic 各向同性 • Self-phase modulation 自相位调制 • 2020/12/7 OE, HUST Cross-phase modulation 交叉相位调制 Large effective-area fiber 大有效面积发光 Phase-matching condition 相位匹配条件 Optical phase conjugation 光相位共轭 Elastic scattering 弹性散射 Inelastic scattering 非弹性散射 Fiber Manufacturing：光纤制作 Doubly clad：双包层 Depressed-cladding fiber: 凹陷包层光纤 Cylindrical preform：预制棒 MCVD：改进的化学汽相沉积 OVD：轴外汽相沉积 VAD：轴向沉积 PCVD：等离子体化学汽相沉积 Flame hydrolysis 火焰裂解 Sintering ：烧结 Light-duty cable 轻型光缆 Heavy-duty cable 重型光缆 Connector 连接头 Furnace 熔炉 3

Chapter 2. Optical Fibers n Geometrical Optics & Wave Optics in Fiber n Loss n Dispersion n Nonlinearity 2020/12/7 OE, HUST 4

2. 1 Geometrical Optics & Wave Optics in Fiber 2. 1. 1 Geometrical Optics suited for a >> λ Multimode Step-Index Optical Fiber weak-waveguided fiber 2020/12/7 OE, HUST 5

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Multimode Graded-Index Optical Fiber 2020/12/7 OE, HUST 8

0. 25 pitch：on axis collimator 2020/12/7 OE, HUST 9

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2. 1. 2 Wave Optics suited for a ~λ 2020/12/7 OE, HUST 11

2. Basic Concepts Normalize Frequency Mode Index n 1 n 2 2020/12/7 n 1 > OE, HUST n 2 12

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Birefringence 双折射 Ø Uniform: HE 11 → two orthogonally polarized fiber modes x, y Ø Nonuniform: y x 2020/12/7 n linear → elliptical → linear n modal birefringence beat length OE, HUST 14

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n PMF: Polarization-Maintaining Fiber (SMF: ) Reduce small random birefringence fluctuations which can affect the light polarization significantly. Spot Size w: field radius 1. 2<V<2. 4 Confinement Factor V=2, Γ=75% V=1, Γ=20% 2020/12/7 OE, HUST 16

Chapter 2. Optical Fibers n Geometrical Optics & Wave Optics in Fiber n Loss n Dispersion n Nonlinearity 2020/12/7 OE, HUST 17

2. 2 Fiber Loss 2. 2. 1 Attenuation Coefficient Pin Pout integrating 2020/12/7 OE, HUST 18

2. 2. 2 Loss Mechanism 1. Absorption (light energy → heat energy, 能量转换) Intrinsic absorption Ø Infrared (λ>7μm): vibrational resonances Ø ultraviolet (λ< 0. 4μm): electronic resonances 0. 8~1. 6 μm : < 0. 1 d. B/km Impurity absorption Ø 400 nm Ø Transitional metal (Fe, Cu, Co, Ni): 2020/12/7 OE, HUST 19

2. Scattering (方向偏折，能量不转换) Ø Rayleigh Scattering → Intrinsic, on a scale smaller than λ Ø Mie scattering → Waveguide Imperfection, on a scale larger than λ 3. Bending (模式泄漏，能量不转换) Ø Macro－bending Installing Ø Micro－bending Cabling 2020/12/7 OE, HUST 20

Chapter 2. Optical Fibers n Geometrical Optics & Wave Optics in Fiber n Loss n Dispersion n Nonlinearity 2020/12/7 OE, HUST 21

2. 3 Dispersion in SMF Bit 2 Bit 1 SMF Inter-symbol interference !!! 2020/12/7 OE, HUST 22

n Chromatic Dispersion (CD) n Group Velocity Dispersion (GVD) n Intra-modal Dispersion n High-order Dispersion (Dispersion Slope) n Polarization Mode Dispersion (PMD) 2020/12/7 OE, HUST 23

2. 3. 1 Group Velocity Dispersion 1. Dispersion parameter L T= L /Vg 2020/12/7 OE, HUST 24

2. D ~ λ 2020/12/7 OE, HUST 26

n 2 g: the group index of the cladding material 2020/12/7 OE, HUST 27

2. 3. 2 Material Dispersion Sellmeier equation: 2020/12/7 OE, HUST 28

2. 3. 3 Waveguide Dispersion 2020/12/7 OE, HUST 30

2. 3. 4 Dispersion Shift and Compensation Ø Ø p p Realized by redesign the waveguide structure of fiber to enlarge 2020/12/7 OE, HUST 32

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Ø ① SMF D=16 ps/(nm ·km) ② DCF L 1=50 km A B llong D’=-100 ps/(nm ·km) L 2=? C lshort ①: ② : Ø 2020/12/7 OE, HUST 36

llong lshort Chirped Bragg grating G. 652 • • • Dispersion Compensation by Using Chirped Bragg Grating 2020/12/7 OE, HUST 37

2. 3. 5 High－Order Dispersion Really? Ø Dispersion Slope (High-Order Dispersion) 2020/12/7 OE, HUST 38

2. 3. 6 Polarization Mode Dispersion Ey nx Ex ny Pulse as it enters the fiber 2020/12/7 Spreaded pulse as it leaves the fiber OE, HUST 39

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A limiting factor for high bitrate lightwave systems 2020/12/7 OE, HUST 41

2. 4 Dispersion－Introduced Limitations Optical pulse: T 0: half-width at 1/e intensity point 2020/12/7 OE, HUST C: frequency chirp 42

When the source spectrum is Gaussian with the RMS spectral width : dispersion-introduced broadening of Gaussian input pulses L: fiber length 2020/12/7 σ0: RMS width of the input Gaussian pulse OE, HUST 43

LED 1. Optical Sources with a Large Spectral Width, ex. non-zero-dispersion wavelength 2020/12/7 OE, HUST 44

zero-dispersion wavelength LD 2. Optical Sources with a Small Spectral Width , ex. non-zero-dispersion wavelength 2020/12/7 OE, HUST 45

zero-dispersion wavelength 2020/12/7 OE, HUST 46

2. 5 Nonlinearities in Optical Fiber 2. 5. 1 Stimulated light scattering Inelastic scattering Ø Rayleigh Scattering : not to generate new frequency Elastic scattering Ø Raman Scattering : photon → stokes photon + optical phonon both directions, frequency shift is large Ø Brillouin Scattering: photon → stokes photon + acoustic phonon backward direction, frequency shift is small 2020/12/7 OE, HUST 47

Tilt in frequency domain Inter-channel interference in time domain 2020/12/7 OE, HUST 48

1. SRS For common use, Pin<10 m. W, SRS can be neglected 2020/12/7 OE, HUST 49

Can optical amplifier or fiber sensing utilize Raman scattering ? Stokes photon Figure 2. 18: (a) Raman gain spectrum of fused silica at λp = 1μm and (b) energy levels participating in the SRS process. 2020/12/7 OE, HUST 50

Simulation 1 2020/12/7 OE, HUST 51

2. SBS Note: frequency shift < 10 GHz, channel spacing ~ 100 GHz , not to generate crosstalk, but the loss of signal power 更精确的公式：功率阈值与光源线宽有关，光源线宽越窄，功率阈值越低。 How does AM-VSB CATV increase threshold of Brillouin scattering? 2020/12/7 OE, HUST 52

Figure 2. 17: Brillouin-gain spectra measured using a 1. 525 -μm pump for three fibers with different germanium doping: (a) silica-core fiber; (b) depressed-cladding fiber; (c) dispersion-shifted fiber. Vertical scale is arbitrary. 2020/12/7 OE, HUST 53

$2. 5. 2 Phase Modulation Power dependence of refractive index! 1. SPM nonlinear refraction$

2. 5. 2 Phase Modulation Power dependence of refractive index! 1. SPM nonlinear refraction propagation constant 2020/12/7 OE, HUST 54

nonlinear phase shift 2020/12/7 OE, HUST 55

Simulation 2 2020/12/7 OE, HUST 57

2. XPM assuming equal channel power SPM → distortion of optical pulse & spectral broadening XPM → intensity noise 2020/12/7 OE, HUST 58

2. 5. 3 Four-Wave Mixing Also originates from the three-order nonlinear susceptibility Easily to realize phase-matching at zero-dispersion wavelength ! How to solve all the above problems in DWDM application? large effective area non-zero dispersion-shifted fiber is available!!! 2020/12/7 OE, HUST 59

Simulation 3 2020/12/7 OE, HUST 60

2. 5. 4 Fiber evolution G. 651 G. 652 Dry fiber G. 653 DCF RDCF G. 654 G. 655 LEAF G. 656 C+L+S fiber 2020/12/7 DCF Module OE, HUST 61

Loss Spectrum Attenuation (d. B/km) 2. 5 2. 0 C-Band: 1530– 1565 nm O-Band: 1260– 1360 nm S-Band: 1460– 1530 nm 1. 5 1. 0 L-Band: 1565– 1625 nm E-Band: 1360– 1460 nm 1 2 3 0. 5 900 1100 1300 1500 1700 Wavelength (nm) Dry Fiber(~400 nm)! 2020/12/7 OE, HUST 62