Why is the loss per kilometer at 1550nm less than at 1310nm?
In standard quartz single-mode optical fibers (such as the G.652 series), the attenuation coefficient at 1550\text{ nm} (typically around 0.18 \sim 0.22\text{ dB/km}) is significantly lower than at 1310\text{ nm} (typically around 0.32 \sim 0.35\text{ dB/km}). This phenomenon is primarily determined by the intrinsic physical absorption and scattering mechanisms of the fiber material, with the core reasons stemming from the combined superposition effect of Rayleigh Scattering and Infrared Multiphonon Absorption.
1. Wavelength Dependence of Rayleigh Scattering (Dominant Factor)
During the manufacturing process of quartz glass, microscopic density non-uniformities and minor refractive index fluctuations are inevitably formed during cooling from a high-temperature molten state, resulting in intrinsic Rayleigh Scattering.
The Rayleigh scattering loss coefficient \alpha_{\text{R}} is inversely proportional to the fourth power of the light wavelength:
\alpha_{\text{R}} \propto \frac{1}{\lambda^4}
- At a wavelength of \lambda = 1310\text{ nm}, due to the shorter wavelength, Rayleigh scattering loss is higher, approximately 0.30\sim0.33\text{ dB/km}.
- When the wavelength shifts to \lambda = 1550\text{ nm}, the scattering loss significantly decreases:
\frac{\alpha_{\text{R}}(1550)}{\alpha_{\text{R}}(1310)} \approx \left(\frac{1310}{1550}\right)^4 \approx 0.51
Based on Rayleigh scattering alone, the loss at 1550\text{ nm} is reduced by nearly 50\% compared to 1310\text{ nm} (dropping to approximately 0.15\text{ dB/km}).
2. Equilibrium Point of Infrared Multiphonon Intrinsic Absorption
The quartz glass molecule (Si-O bond) has specific intrinsic lattice vibration frequencies. Its strong fundamental infrared absorption is located in the mid-to-far infrared band (around 9\ \mu\text{m}), and its higher harmonics form the Infrared Absorption Edge in the near-infrared band:
- At \lambda < 1500\text{ nm}, the intrinsic infrared absorption loss is extremely small and can be almost ignored.
- As the wavelength increases (especially beyond 1600\text{ nm}), the infrared absorption loss rises exponentially and sharply.
- At \lambda = 1550\text{ nm}, infrared absorption loss begins to appear, but its absolute value is still very weak (around 0.02\sim0.03\text{ dB/km}).
3. Minimum Total Loss Window (C-Band Window)
The total intrinsic attenuation coefficient of a single-mode fiber can be expressed as the superposition of various attenuation mechanisms:
\alpha_{\text{total}}(\lambda) = \alpha_{\text{Rayleigh}}(\lambda) + \alpha_{\text{IR}}(\lambda) + \alpha_{\text{UV}}(\lambda) + \alpha_{\text{impurity}}(\lambda)
- At the 1310\text{ nm} window (O-band): It is primarily limited by strong Rayleigh Scattering, with total loss between 0.32\sim0.35\text{ dB/km} (the main advantage of this band is that the material dispersion is close to zero, i.e., the zero-dispersion wavelength).
- At the 1550\text{ nm} window (C-band): Rayleigh scattering is significantly reduced, while the infrared absorption of the quartz material has not yet increased sharply. The decreasing Rayleigh scattering curve and the increasing infrared absorption curve intersect near this band, forming a physical minimum valley in total loss (around 0.18\sim0.20\text{ dB/km}).
Summary
The fundamental physical reason why 1550\text{ nm} has a lower loss per kilometer compared to 1310\text{ nm} is that: the increase in optical wavelength halves the Rayleigh scattering loss proportional to \propto \frac{1}{\lambda^4}, while not yet reaching the steep infrared multiphonon absorption cutoff region of quartz material, thus forming the optical window with the lowest transmission loss for quartz fibers.