Rayleigh scattering (original) (raw)
Author: the photonics expert (RP)
Definition: scattering of light at scattering centers which are much smaller than the wavelength
general optics- optical effects
- scattering
* Rayleigh scattering
* Brillouin scattering
* Raman scattering
* hyper Raman scattering
- scattering
Related: fibersRaman scatteringwavelength
Page views in 12 months: 2918
DOI: 10.61835/p2i Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
Contents
Scattering Losses in Optical Fibers
Summary:
Rayleigh scattering is a linear optical scattering process occurring at particles much smaller than the light's wavelength. Its strength scales with the inverse fourth power of the wavelength, explaining phenomena like the blue color of the sky.
In optical fibers, Rayleigh scattering from microscopic density fluctuations in the glass structure represents a fundamental lower limit for propagation losses, particularly in the near-infrared region used for optical communications. It is distinguished from Mie scattering, which applies to larger particles, and nonlinear processes like Raman and Brillouin scattering.
(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)
What is Rayleigh scattering?
Rayleigh scattering is a common scattering optical phenomenon, named after the British physicist Lord Rayleigh. It is linear scattering of light at scattering centers which are much smaller than the wavelength of the light. Under such circumstances, the scattering occurs with intensities which are
- proportional to the incoming optical intensity,
- to the fourth power of the inverse wavelength, and
- to ($1 + \cos^2 \theta$), where ($\theta$) is the scattering angle.
Forward and backward scattering (($\theta = 0$) and ($\theta = \pi$), respectively) are equally strong.
The strong wavelength dependence of Rayleigh scattering is well-known to be the reason for the blue sky: Shorter-wavelength (blue-violet) components of sunlight are far more scattered than longer-wavelength ones.
Scattering centers for Rayleigh scattering can be individual atoms or molecules. However, one can also describe Rayleigh scattering in the atmosphere, for example, as resulting from microscopic density fluctuations, which are caused by the random distribution of molecules in the air.
Note that for scattering at multiple particles or scattering centers, one cannot simply add the powers scattered by individual centers, as there are interference effects: amplitudes must be added. As a result, there would be no Rayleigh scattering of light in a perfectly pure and regular crystal. Also, Rayleigh scattering in air is possible only due to the above-mentioned random density fluctuations.
If the scattered light is assumed to be lost, the scattering effectively contributes to propagation losses. For example, in the case of a single-mode fiber any scattered light will end up in cladding modes and will effectively be lost.
Scattering at larger centers can be described by Mie scattering theory (named after Gustav Mie). Here, the characteristics are different; for example, the scattering amplitudes are stronger for forward scattering, and the wavelength dependence is different.
Scattering Losses in Optical Fibers
In amorphous optical materials such as silica glass, there are always random density fluctuations due to the irregular microscopic structure. These are even substantially stronger than they would normally be at room temperature because during fiber fabrication, the density fluctuations which occurred for the fiber near the glass softening temperature are “frozen in”. Only to a limited extent, the fluctuations can be reduced by an annealing process.
Rayleigh scattering sets a lower limit to the propagation losses in optical fibers. Of course, additional losses can result e.g. from an irregular core/cladding interface (particularly if the refractive index contrast is high), from scattering and absorption by impurities, and from macroscopic and microscopic bending. Silica fibers which have been optimized for long-distance optical fiber communications have very low propagation losses, approaching the limit given by Rayleigh scattering. For wavelengths substantially below the often used 1.5-μm region, Rayleigh scattering alone would be higher than the actual losses of these fibers at 1.5 μm wavelength. At substantially longer wavelengths, Rayleigh scattering would be weaker, but the infrared absorption of silica sets in.
In principle, one could have mid-infrared fibers made of other glasses (e.g., fluoride fibers), which could have even lower losses, but in practice silica fibers have reached the best figures.
Most of the Rayleigh-scattered light in a fiber exits the fiber on the side. Only a small portion of the scattered light is scattered back such that it is again guided in the fiber core. Therefore, the return loss of fiber devices is generally very high; the overall return loss of a fiber setup is more often caused by reflections at interfaces such as fiber ends, mechanical splices or fiber connectors.
Due to the high optical intensities which often occur in optical fibers, nonlinear scattering processes like Raman scattering and Brillouin scattering can also occur. Rayleigh scattering, being a linear process, is equally important at low light intensities.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is Rayleigh scattering?
Rayleigh scattering is the linear scattering of light at centers which are much smaller than the light's wavelength. The scattered intensity is proportional to the inverse fourth power of the wavelength, making it much stronger for shorter wavelengths.
What causes Rayleigh scattering in optical fibers?
In optical materials like silica glass, Rayleigh scattering is caused by microscopic random density fluctuations. These fluctuations are 'frozen in' from the high temperatures experienced during the fiber fabrication process.
How does Rayleigh scattering limit the performance of optical fibers?
Rayleigh scattering sets a fundamental lower limit for the propagation losses in optical fibers. This is because any light scattered out of the fiber core is effectively lost, contributing to signal attenuation.
How does Rayleigh scattering differ from Mie scattering?
Rayleigh scattering occurs with particles much smaller than the light's wavelength, while Mie scattering describes scattering from larger particles. This results in different characteristics, such as the wavelength dependence and the angular distribution of scattered light.
Is the back-scattered light in a fiber significant?
Only a small portion of Rayleigh-scattered light is guided backward in the fiber core. Therefore, the return loss from scattering is very high (meaning the back-scattered power is low), and reflections from fiber ends or connectors are usually a more significant cause of return signals.
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.