Seeing the light from the side is difficult when it travels through a uniform, clear medium. Even if the medium is hazy, like air with a lot of dust particles, we can trace the beam’s trajectory by looking at it from the side. This is because the heterogeneity of the material causes light to spread in every direction. Scattering is the term for this phenomenon. Scattering loss is a type of loss that causes light energy to be radiated away from the optical cable.
Scattering is the loss of optical energy due to imperfections in the fiber and from the basic structure of the fiber.
- The light is no longer directional due to scattering.
- Scattering results in attenuation (in the form of radiation) as the scattered light may not continue to satisfy the total internal reflection in the fiber core.
The below image illustrates the scattering of light inside an optical core.
Table of Contents
Main types of scattering in optical fibers
We can classify mainly two types of scattering loss in optical fiber.
- Linear scattering
- Nonlinear scattering
Linear scattering
The frequency of the scattered light does not change due to linear scattering. There are two different types of linear scattering Rayleigh scattering and Mie scattering.
Nonlinear scattering
Light’s frequency changes when nonlinear scattering occurs. Examples of nonlinear scattering include Brillouin scattering and Raman scattering. Due to nonlinear scattering, a lot of energy is lost in all directions.
The following chart illustrates the types of scattering in optical fiber.
Rayleigh scattering in optical fiber
Rayleigh scattering is a type of light scattering where the center of the scattered light is much smaller than the wavelength. Rayleigh scattering is the most common optical effect named after Earl Rayleigh, a British physicist. It is a linear scattering of light with a center of scattering much smaller than the wavelength of the light.
Rayleigh scattering is caused by non-homogeneities on a small scale that is formed during the fiber construction process. In-homogeneities include:
- Variations in the glass composition
- Variations in density
The main features of Rayleigh scattering
- The scattering light intensity is inversely proportional to the incident wavelength’s fourth power (1/λ4).
- The scattering light intensity varies with the observation direction. The scattering light intensity varies in different observation directions.
- Scattered light has polarization, and its polarization degree depends on the angle between the scattered light and the direction of the coupling moment.
Rayleigh scattering law is applied to tiny particles whose linearity is less than one-tenth of the wavelength.
About 96% of the loss in an optical fiber is caused by Rayleigh scattering. Since scattering is inversely proportional to the fourth power of the wavelength (1/λ4), it decreases quickly as the wavelength gets longer.
As light moves through the core, it makes contact with the silica molecules there. Rayleigh scattering happens when the light wave and the silica molecules hit each other in a way that is elastic.
If the light is scattered at an angle that doesn’t allow it to keep going forward, the light is sent away from the center, which is called “attenuation.” There is no attenuation if the scattered light stays at an angle that lets it move forward in the core.
Depending on the angle at which the light comes in, some of it goes forward, and some go off the path and out of the fiber’s core. Some of the light that is spread out is sent back toward the source.
Mie scattering in optical fiber
Mie scattering, named after the German physicist Gustav Mie, is a theory that explains how particles larger than 10% of a wavelength can scatter electromagnetic radiation. Mie scattering happens in irregularities in glass fibers.
What causes Mie scattering in optical fiber
- The refractive index of the core and cladding changes along the length of the fiber.
- Impurities at the interface between the core and cladding.
- Bubbles or strains in the fiber.
- Changes in fiber diameter.
Manufacturers can lessen Mie scattering by carefully cleaning the glass and removing any flaws. Manufacturers should carefully monitor the quality and cleanliness of the manufacturing process. The effects of Mie scattering in commercial fibers are negligible.
Brillouin scattering in optical fiber
The nonlinearity of the medium results in Brillouin scattering. Brillouin scattering in glass fibers manifests as a light modulation caused by the thermal energy of the material.
An incident photon can be transformed into a phonon and a scattered photon. Photons have slightly lower energies and often move in the opposite direction. Phonons have vibrational energy.
The frequency of the reflected beam is a little bit lower than the frequency of the incident beam. The difference in frequencies is equal to the frequency of the emitted phonons. This is called a Brillouin Frequency Shift.
This phenomenon has been utilized in the development of fiber optic sensor applications.
Even at low optical powers, Brillouin scattering can occur naturally.
This differs from Stimulated Brillouin Scattering, which can only happen if the optical power is high enough to meet a certain threshold.
When the power of an incident beam is above a certain threshold, stimulated Brillouin scattering can reflect most of the beam’s power.
Stimulated Raman scattering in optical fiber
Stimulated Raman scattering in optical fibers is one of the most important research topics in the field of nonlinear fiber optics. It was discovered in 1928 by Chandrasekhara Venkata Raman and is the inelastic scattering of light induced by molecules of matter.
Stimulated Raman scattering is the nonlinear response of glass fibers to the optical intensity of light. This is caused by the vibrations of the crystal (or glass) lattice.
Brillouin scattering makes a low-frequency acoustic phonon and a scattered photon, while stimulated Raman scattering makes a high-frequency optical phonon.
When two laser beams with different wavelengths move through a Raman-active medium together, the longer wavelength beam can experience optical amplification while the shorter wavelength beam loses power. Raman amplifiers and Raman lasers use this effect.
Most of the scattering in stimulated Raman scattering happens in the forward direction. So the receiver doesn’t lose any of the power. To happen, stimulated Raman scattering also needs optical power to be higher than a threshold.
Summary
Light scattering disperses power within fiber due to imperfections. Heterogeneity causes angular redirection, attenuating signal strength over distances. Key scatter mechanisms include Rayleigh, Mie, Brillouin, and stimulated Raman interactions.
Rayleigh, proportional to inverse wavelength, arises from sub-wavelength defects. Mie impacts larger impurities. Brillouin manifests as induced phonons, lowering reflected photon frequency.
Stimulated Raman, a nonlinear response, spawns optical phonons. Rayleigh constitutes most loss. Mitigations prioritize manufacturing precision to eliminate inhomogeneities.
At low intensities, scattering negligibly impacts transmission. But above thresholds, stimulated varieties redirect heavy power backward. Understanding scattering origins informs resisting degradation through optimizing materials processing and structure. With losses curbed, dissipation preserves stronger signals even over lengthened spans.
FAQ
How can we reduce scattering in optical fiber?
To reduce the scattering in optical fiber, we can improve the purity of fiber, increase wavelength, use graded index fiber, maintain proper bend radius, and Improve the quality of the connectors.
What is the difference between fluorescence and scattering?
Fluorescence occurs when the material absorbs the light energy and excites an electron to a higher energy level. When the electron returns to its original energy level, it emits a photon of light at a longer wavelength. Scattering is a process in which light is deflected in different directions as it interacts with small particles or irregularities in a material.
What is difference between refraction and scattering?
Refraction is the bending of light as it passes through a medium with a different refractive index. This can occur when light travels from one material to another, such as from air to glass or water. Scattering is the process by which light is deflected in various directions as it interacts with particles or irregularities in a material.
What is scattering vs dispersion?
Scattering refers to the random redirection of light as it interacts with the material in the fiber, resulting in a loss of signal power. Dispersion refers to the spreading of light pulses as they propagate along the fiber. This occurs because different wavelengths of light travel at different speeds in the fiber due to differences in refractive index.
What is the Rayleigh scattering proportional to in optical fibers?
Rayleigh scattering is proportional to the fourth power of the wavelength of the light used in optical fibers. This means that shorter wavelengths are scattered more than longer wavelengths, which is why optical fibers are designed to operate in the infrared range, where attenuation due to scattering is much lower.
