Absorption

Absorption is a key contributor to signal loss in optical fibers. It refers to the attenuation that occurs when optical power is converted into another form of energy, such as heat. This type of loss is influenced by both the material composition and the manufacturing process of the fiber. Absorption happens when light is absorbed by the molecules in the fiber core and cladding, causing it to be converted into heat. The primary factors behind absorption are intrinsic and extrinsic material properties, which will be explored in more detail in this article.

Intrinsic Absorption in Optical Fiber

Intrinsic absorption results from the fundamental properties of the fiber material itself. In an ideal scenario, where an optical fiber is perfectly pure with no imperfections or impurities, all absorption would be intrinsic, which represents the lowest possible level of energy loss. Silica (pure glass) fibers are most commonly used in fiber optics because they have a very low intrinsic absorption at the operational wavelengths.

There are two mechanisms that lead to the intrinsic absorption of light at optical wavelengths. The primary cause of intrinsic absorption in the infrared range is the natural vibration frequency of atomic bonds. In silica glass, this absorption occurs due to the vibrations of silicon-oxygen (Si-O) bonds. When these bonds vibrate, they interact with the electromagnetic field of the optical signal, leading to intrinsic absorption. During this interaction, light energy is transferred from the electromagnetic field to the vibrating bond.

Second, intrinsic absorption in the ultraviolet region occurs due to electronic absorption bands. This happens when a photon of light interacts with an electron, exciting it to a higher energy level.

Extrinsic Absorption in Optical Fiber

Extrinsic absorption is caused by impurities introduced into the fiber material during its manufacture. Trace metal impurities, such as iron, nickel, and chromium, can enter the fiber during the fabrication process. This type of absorption occurs when these metal ions undergo electronic transitions between energy levels.

Extrinsic absorption also results from the presence of hydroxyl ions (OH-) in the fiber. When water is absorbed by silica glass, it forms a silicon-hydroxyl (Si-OH) bond, which has a fundamental absorption at 2700 nm. The harmonics or overtones of this absorption can occur at operating wavelengths, increasing extrinsic absorption at 1383 nm, 1250 nm, and 950 nm.

To minimize fiber attenuation from extrinsic absorption, the level of water (OH-) impurities should be kept to less than a few parts per billion. Reducing the amount of these impurities in the fiber will help lower the overall attenuation caused by extrinsic absorption.

How to Reduce Absorption in Optical Fibers

Reducing absorption in optical fibers is crucial for maintaining signal quality and minimizing signal loss. Here are the primary methods for achieving this:

1. 1.Using Purified Materials：Employing highly purified silica helps reduce absorption loss in optical fibers. By lowering the concentration of impurities, this method minimizes extrinsic absorption, which is directly related to the level of contaminants in the fiber.

2. 2.Dopant Engineering：Introducing specific dopants into the fiber can help decrease intrinsic absorption. For example, adding germanium to silica fibers can reduce infrared light absorption.

3. 3.Wavelength Selection：Choosing wavelengths that are less prone to absorption can help reduce overall signal attenuation. For instance, the 1550 nm wavelength is often preferred for long-distance communication because it experiences less absorption compared to other wavelengths.

4. 4.Erbium-Doped Fiber Amplifiers (EDFAs)：EDFAs can be used to amplify signals that have been weakened by absorption. Erbium, a rare-earth element, has a high absorption coefficient at 1550 nm, making it an ideal material for optical amplifiers.

By employing these strategies, absorption losses in optical fibers can be greatly reduced, leading to more efficient data transmission over longer distances.