Coherent backscattering

Coherent backscattering is a phenomenon observed in the field of optics and wave propagation. It refers to the enhancement of backscattered light in a medium with multiple scattering events. This effect occurs when waves interfere constructively upon being scattered back towards the source, resulting in a higher intensity of light in the backward direction.

Overview

Coherent backscattering was first discovered by the physicist Akira Ishimaru in the 1970s. It has since been extensively studied and has found applications in various fields, including remote sensing, biomedical imaging, and material characterization.

The phenomenon of coherent backscattering arises due to the interference of multiple scattering paths. When light interacts with a medium containing numerous scattering centers, it undergoes multiple scattering events before being scattered back towards the source. These scattering events cause the incident wavefront to become distorted and spread out in various directions.

However, due to the wave nature of light, the scattered waves can interfere with each other. In the case of coherent backscattering, the interference is constructive in the backward direction, leading to an increase in the intensity of the backscattered light. This enhancement is particularly pronounced at small scattering angles, where the interference effects are most significant.

Mechanism

The mechanism behind coherent backscattering can be understood through the concept of time-reversed paths. When a wave is scattered by a medium, it follows a specific scattering path. In the case of coherent backscattering, the time-reversed path, which is the same path but traversed in the opposite direction, also contributes to the interference.

The interference between the forward and time-reversed paths depends on the coherence properties of the incident light. Coherent light, such as that produced by lasers, exhibits a well-defined phase relationship between different parts of the wavefront. This coherence allows for the constructive interference necessary for coherent backscattering to occur.

Applications

Coherent backscattering has several practical applications in various fields. In remote sensing, it can be used to study the properties of atmospheric aerosols, such as their size distribution and refractive index. By analyzing the backscattered light, researchers can gain valuable insights into the composition and characteristics of these aerosols.

In biomedical imaging, coherent backscattering has been utilized to improve the resolution and sensitivity of optical coherence tomography (OCT) systems. By exploiting the enhanced backscattered signal, OCT can provide detailed imaging of biological tissues, aiding in the diagnosis and monitoring of various diseases.

Coherent backscattering has also found applications in material characterization. By analyzing the backscattered light from a material, researchers can extract information about its optical properties, such as its scattering coefficient and anisotropy factor. This information is crucial for understanding the behavior of light in complex media and designing optical devices.

See Also

• Multiple scattering
• Interference
• Optical coherence tomography
• Remote sensing