What are the characteristic parameters of optical fibers?

Optical fiber parameters can be categorized into three main types: geometric, optical, and transmission characteristics, including:Attenuation (Loss Coefficient)、Dispersion and others.

Attenuation (Loss Coefficient)

Attenuation is one of the most critical parameters for both multimode (MMF) and single-mode fibers (SMF), significantly influencing the maximum transmission distance.

Definition & Formula

Attenuation coefficient (α) measures power loss per kilometer (dB/km):

α=10/L * lg (pi/po) (dB/km)

Pi: Input power (W)

Po​: Output power (W)

L: Fiber transmission distance (km)

Example: If α = 3 dB/km, after 1 km, power reduces to half (10^0.3≈2).

Modern SMF Performance

Low-loss SMF: ≤ 0.18 dB/km (e.g., Corning® SMF-28® Ultra).

System Example:

EDFA output: +5 dBm

Receiver sensitivity: -28 dBm

Max theoretical distance: 33 dB       

distance (Max theoretical distance/Attenuation coefficient):

33/0.18 dB/km≈183 km (practical distance is shorter due to aging margin).

Causes of Attenuation

Absorption Loss

Impurity absorption (e.g., OH⁻ ions, metal ions).

Intrinsic absorption (SiO₂ molecular vibrations).

Scattering Loss

Rayleigh scattering (linear).

Nonlinear scattering (e.g., Brillouin, Raman).

Structural Imperfections

Microbending, macrobending losses.

Key Mitigation: Ultra-pure silica (impurities < 1 ppb).

Dispersion

Dispersion broadens optical pulses, limiting bandwidth and transmission distance.

Types of Dispersion

Modal Dispersion (MMF only):

Caused by multiple propagation paths (modes) with different speeds.

Dominates in MMF (e.g., OM3: ~1 GHz·km bandwidth).

Material Dispersion:

Wavelength-dependent refractive index of SiO₂.

Waveguide Dispersion:

Due to fiber core/cladding geometry.

Dispersion Coefficient D(λ)

Definition: Pulse broadening per km per nm spectral width (ps/nm·km).

Formula:

Δτ=D(λ)⋅L⋅δλ

$\Delta \tau$: Pulse broadening.

L: Fiber transmission distance

$\delta \lambda$: Laser spectral width.

ITU-T Fiber Standards

Fiber Type	Zero-Dispersion λ	Application
G.652	1310 nm	Standard SMF (needs dispersion compensation at 1550 nm for 10G+).
G.653	1550 nm	10G SMF (unsuitable for DWDM due to FWM).
G.655	~1550 nm (low)	Optimized for DWDM/SDH.

Key Rule: Zero dispersion → High nonlinearities (e.g., four-wave mixing (FWM) in DWDM).

 Bandwidth (Bc​), NA, MFD, and Cutoff Wavelength (λc​)

Bandwidth (Bc)

Definition: Frequency at which output power drops to 50% (3 dB bandwidth).

Relation to Dispersion:

Bc≈0.44Δτ​

SMF vs. MMF:

SMF: > 25 GHz·km (dominated by chromatic dispersion).

MMF: < 1 GHz·km (modal dispersion-limited).

Numerical Aperture (NA)

Definition: Light-gathering ability (NA=sin⁡θc​, where θc​ is acceptance angle).

Trade-off: Higher NA → More light capture but worse modal dispersion.

Standard Range: 0.18–0.24 (ITU-T recommendation).

Mode Field Diameter (MFD)

Definition: Effective diameter of light propagation in SMF (~9–10 µm at 1550 nm).

Approximation: Close to core diameter (SMF: ~8–10 µm).

Cutoff Wavelength (λc​)

Definition: Minimum wavelength for single-mode operation (e.g., G.652: λc≤1260 nm).

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