Coupled modes in fiber optics refer to the phenomenon whereby two or more optical modes share energy, and propagate together along an optical fiber cable. This phenomenon occurs when light waves of different wavelengths, frequencies, and polarizations interact within the cable, resulting in the formation of a composite wave. The distribution of energy among the coupled modes can change over the length of the cable, depending on the properties of the cable and the modes themselves.
Coupled modes occur when light travels in a waveguide, such as an optical fiber, that has a finite length and a finite number of modes. Light entering the waveguide will be split into multiple modes based on the properties of the fiber, such as the core diameter, numerical aperture, and refractive index. Each of these modes will then propagate along the fiber, and their individual optical fields will overlap and interact with each other. As they interact, the modes will exchange energy, which results in each of the modes becoming “coupled” and propagating together.
The degree to which the modes are coupled will depend on the length of the fiber and the type of coupling that occurs. Different types of coupling can occur, such as cross-polarization coupling and mode coupling, which can result in different distributions of energy between the modes. The coupling between modes can also be affected by temperature changes and other environmental factors, which can alter the distribution of energy among the coupled modes.
Overall, coupled modes in fiber optics refer to the phenomenon whereby two or more optical modes share energy, and propagate together along an optical fiber cable. This phenomenon is important for understanding the behavior of light in optical fibers, and can have a significant impact on the performance of the cable.