Scattering loss in optical fibers is generally caused by microscopic variations in the material density and non-uniform concentrations of components such as SiO2, GeO2, and P2O5. These non-uniformities create localized regions with irregular refractive index distributions, leading to light scattering and power loss as a portion of the light is scattered out of the fiber. Additionally, during the manufacturing process, defects, bubbles, and imperfections can occur at the interface between the fiber core and cladding. These geometric defects, much larger than the optical wavelength, cause wavelength-independent scattering losses and shift the entire fiber loss spectrum upwards, although this type of scattering loss is significantly smaller compared to the previous one.
Linear Scattering Loss
No optical fiber waveguide can be perfect, and defects or non-uniformities in materials, dimensions, shapes, and refractive index distributions can lead to scattering losses in the propagating modes. Since the power loss caused by this type of scattering is linearly related to the mode power, it is referred to as linear scattering loss.
(1) Rayleigh Scattering Rayleigh scattering is a fundamental scattering process and is considered an intrinsic scattering mechanism. During the fiber manufacturing process, thermal agitation causes atomic compression or rarefaction, resulting in density and refractive index non-uniformities. These non-uniformities, much smaller than the optical wavelength, are frozen in during the cooling process. When the propagating light in the fiber encounters these non-uniform microscopic particles, it is scattered in various directions. This scattering, where the particle size is much smaller than the wavelength, is known as Rayleigh scattering. In optical fibers, some of the scattered light is influenced by the waveguide effect and can propagate forward or backward, while other scattered light deviates from the propagation direction and becomes radiation modes. This process reduces the forward-propagating light power, resulting in loss. The loss caused by Rayleigh scattering is proportional to λ^-4, meaning that it decreases rapidly with increasing wavelength. For short-wavelength fibers, the loss is primarily determined by Rayleigh scattering loss. It is important to note that Rayleigh scattering loss is an intrinsic loss mechanism, and together with intrinsic absorption loss, it forms the theoretical limit of optical fiber loss.
(2) Scattering Loss Due to Imperfect Fiber Structure (Waveguide Scattering Loss) During the fiber manufacturing process, defects such as incomplete core-cladding interfaces, core diameter variations, non-uniform circularity, residual bubbles, and cracks can occur due to various factors, including process and technical issues, as well as random factors. These structural imperfections, with dimensions much larger than the optical wavelength, cause wavelength-independent scattering losses. In this case, scattering is a descriptive term. In reality, it is caused by mode conversion or mode coupling due to structural imperfections, as illustrated in Figure 2. When the core-cladding interface is not a straight line but has irregularities, the incident angle of the propagating light changes, leading to a mode conversion due to the change in the angle θ. If a lower-order mode converts to a higher-order mode, the propagation path increases, resulting in increased attenuation. If the changed θ no longer satisfies the total internal reflection condition, the light will radiate into the cladding, forming a radiation mode. Consequently, these scattered light rays cannot propagate over long distances in the fiber, causing a loss in the transmitted optical power. This shifts the entire fiber loss spectrum upwards. However, with improvements in manufacturing processes, structural defect-induced losses can generally be reduced to the range of 0.01 to 0.05 dB/km.
Non-linear Scattering Loss
There are two types of non-linear scattering in optical fibers, both related to the vibrational excited states of the silica fiber: stimulated Raman scattering and stimulated Brillouin scattering. At high power transmissions, stimulated Raman scattering and stimulated Brillouin scattering can lead to significant losses. Once the incident optical power exceeds a threshold, the scattered light intensity will increase exponentially. These two types of scattering losses must be considered when employing wavelength division multiplexing and erbium-doped fiber amplifiers (EDFAs) in the system.
