How Fiber Transmission Works

Light rays are modulated into digital pulses with a laser or LED and move along the core without penetrating the cladding. The light stays confined to the core because the cladding has a lower refractive index, which is the measure of a material’s ability to bend light.

 

This results in the phenomenon of total internal reflection (TIR), which happens when a propagating wave strikes a boundary between two mediums (in this case, the core and the cladding) at an angle larger than the fiber’s critical angle. If the refractive index is lower on the other side of the boundary (the cladding) and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected.

 

Signal Attenuation

 

Signal attenuation in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express loss as attenuation per 1 km length (dB/km). This value is multiplied by the total length of the optical fiber in kilometers to determine the fiber’s total loss in dB.

 

Light traveling in an optical fiber is not 100% efficient; there are several causes of signal attenuation. The loss of power also depends on the wavelength of the light and on the propagating material. For silica glass, the shorter wavelengths are attenuated the most. The lowest loss occurs at the 1550 nm wavelength, which is commonly used for long-distance transmissions.

 

Simplex OM3 10G 50/125 Multimode Fiber Optic Patch Cable

 

Loss Inherent to Fiber: Light loss in a fiber that cannot be eliminated during the fabrication process is due to impurities in the glass and the absorption of light at the molecular level. Loss of light due to variations in optical density, composition, and molecular structure is called Rayleigh scattering. Rays of light encountering these variations and impurities are scattered in many directions and lost.

 

The absorption of light at the molecular level in fiber is mainly due to contaminants in glass such as water molecules. The ingress of water molecules into an optical fiber is one of the main factors contributing to the fiber’s increased attenuation as it ages. Silica glass’s (Si02) molecular resonance absorption also contributes to some light loss.

 

Loss Resulting from Fiber Fabrication: Inconsistencies in the fiber manufacturing process will result in the loss of light. For example, a 0.1% change in the core diameter can result in a 10 dB loss per kilometer. Precise tolerances must be maintained throughout the manufacturing of the fiber to minimize loss.