Figure 1. Optical fiber operating wavelengths

Bend Loss

Bend loss occurs at fiber cable bends that are tighter than the cable's minimum bend radius. Bending loss can also occur on a smaller scale from such factors as:

• Sharp curves of the fiber core
• Displacements of a few millimeters or less, caused by buffer or jacket imperfections
• Poor installation practice

This light power loss, called microbending, can add up to a significant amount over a long distance.

Splice and Connector Loss

Splice loss occurs at all splice locations. Mechanical splices usually have the highest loss, commonly ranging from 0.2 to over 1.0 dB, depending on the type of splice. Fusion splices have lower losses, usually less than 0.1 dB. A loss of 0.05 dB or less is usually achieved with good equipment and an experienced splicing crew. High loss can be attributed to a number of factors, including:

• Poor cleave
• Misalignment of fiber cores
• An air gap
• Contamination
• Index-of-refraction mismatch
• Core diameter mismatch

Losses at fiber optic connectors commonly range from 0.25 to over 1.5 dB and depend greatly on the type of connector used. Other factors that contribute to the connection loss include:

• Dirt or contaminants on the connector (very common)
• Improper connector installation
• A damaged connector face
• Poor scribe (cleave)
• Mismatched fiber cores
• Misaligned fiber cores
• Index-of-refraction mismatch

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 a fiber is mainly due to contaminants in glass such as water molecules (OH-). The ingress of OUT molecules into an optical fiber is one of the main factors contributing to the fiber's increased attenuation in aging. Silica glass's (Si02) molecular resonance absorption also contributes to some light loss.

Figure 1 shows the net attenuation of a silica glass fiber and the three fiber operating windows at 850, 1310, and 1550 nm. For long-distance transmissions, 1310 or 1550nm windows are used. The 1550-nm window has slightly less attenuation than 1310nm. The 850nm communication is common in shorter-distance, lower cost installations.

Loss Resulting from Fiber Fabrication

Irregularities during the manufacturing process can result in the loss of light rays. For example, a 0.1 percent change in the core diameter can result in a 10dB loss per kilometer. Precision tolerance must be maintained throughout the manufacturing of the fiber to minimize losses.

Fresnel Reflection

Fresnel reflection occurs at any medium boundary where the refractive index changes, causing a portion of the incident light ray to be reflected back into the first medium. The fiber end is a good example of this occurrence. Light, traveling from air to the fiber core, is refracted into the core. However, some of the light, about 4 percent, is reflected back into the air. The amount being reflected can be estimated using the following formula shown in figure 2: