After light signals are transmitted through optical fibers, their optical power decreases due to factors such as absorption and scattering. Fiber optic loss is a critical performance metric for fiber transmission, directly influencing the maximum transmission distance of fiber-optic communication systems.
An important issue in achieving fiber-optic communication is minimizing fiber optic loss as much as possible.
Fiber Optic Loss
So-called attenuation refers to the reduction in optical signal strength per unit length of fiber, measured in dB/km. The level of fiber attenuation directly influences the transmission distance or the spacing between repeater stations. Therefore, understanding and minimizing fiber loss is critically important for practical fiber-optic communication systems. Although light waves possess an incredibly wide bandwidth, from 1961 to 1970, researchers primarily focused on using the atmosphere as a medium to transmit optical signals. However, practical experiments revealed that atmospheric conditions severely hindered reliable communication. As a result, among the alternative transmission media being explored, optical fibers—made from quartz glass—emerged as the most promising solution for guiding light signals. At the time, though, conventional quartz glass materials exhibited an astonishingly high attenuation rate of up to 1,000 dB/km, severely limiting their effective transmission range. In July 1966, Dr. K.C. Kao, a Chinese-British scientist at the Standard Telecommunication Laboratories in the UK, along with Dr. G.A. Hockham, drew upon dielectric waveguide theory to demonstrate that the high attenuation of optical fibers wasn’t an inherent property of the material itself—but rather a consequence of impurities embedded within it. They boldly predicted that by significantly reducing these impurities, fiber attenuation could be lowered to as little as 20 dB/km, or even lower. True to their vision, in 1970, Corning Glass Works in the United States successfully developed low-loss quartz optical fibers with an attenuation of just 20 dB/km. This breakthrough not only proved that optical fibers were fully capable of serving as an efficient medium for transmitting light waves but also ushered in a new era of fiber-optic communication technology.
Classification
1. Absorption Loss in Optical Fibers
This is caused by the absorption of light energy by the fiber material and impurities, which dissipate the optical energy as heat within the fiber. It represents a significant type of loss in optical fibers. Absorption losses can be categorized into the following types:
(1) UV Absorption Fiber Loss
Fiber optic materials absorb the energy of incident light, causing electrons to transition to higher energy levels—and simultaneously leading to energy loss in the incoming light, typically occurring in the short-wavelength range.
(2) Infrared Absorption Fiber Loss
Light waves interacting with the fiber-optic lattice transfer some of their energy to the lattice, causing its vibrations to intensify—and consequently leading to losses.
(3) Intrinsic Absorption Curve
The absorption of impurities is primarily caused by ions such as iron, copper, and chromium, as well as OH⁻ ions, present in the fiber material. The higher the concentration of these metal ions, the greater the resulting loss—but by strictly controlling their levels, we can significantly reduce the associated attenuation. Notably, these impurities have a more pronounced impact on shorter wavelengths, while their effect diminishes at longer wavelengths. OH⁻ ions exhibit distinct absorption peaks at two key wavelengths: 1.38 μm and 0.95 μm, with the most severe absorption occurring precisely at 1.38 μm. Interestingly, a minor absorption peak also appears at 1.25 μm. However, if the OH⁻ ion concentration is reduced to below one part per billion, the absorption loss at 1.38 μm becomes virtually negligible, effectively transforming the entire long-wavelength region into a nearly flat, absorption-free zone (as illustrated by the 1980 curve in the figure).
Atomic defect absorption refers to the loss caused by atomic defects generated in optical fibers during manufacturing when the glass is subjected to thermal excitation or intense radiation.
2. Scattering Losses in Optical Fibers
Scattering loss occurs when fluctuations in the atomic density of the fiber material components or defects in the fiber waveguide structure cause optical power to couple out of or leak beyond the core.
Intrinsic scattering is the most significant type of scattering in materials, and its power loss is linearly proportional to the power of the propagating mode. This phenomenon arises from imperfections in the material's atoms, molecules, or structural inhomogeneities, which introduce microscopic variations in the refractive index, thereby causing scattering of the transmitted light waves. As an inherent property of the material, intrinsic scattering cannot be eliminated and represents the fundamental lower limit of fiber optic losses—Rayleigh scattering falls squarely into this category. Notably, Rayleigh scattering loss is inversely proportional to the fourth power of the wavelength, meaning that operating at longer wavelengths can significantly reduce fiber attenuation.
Another type of intrinsic scattering arises from doping non-uniformities. During fiber manufacturing, certain oxides are added to alter the glass’s refractive index. When the concentration of these oxides becomes uneven or fluctuates, it leads to this kind of scattering.
Nonlinear scattering includes stimulated Brillouin scattering and stimulated Raman scattering. When a medium is exposed to high-intensity light, inelastic collisions between incident photons and the medium's molecules generate phonons. Specifically, when light is scattered by propagating acoustic phonons, it’s known as Brillouin scattering; and when light scatters due to molecular vibrations or optical phonons, it’s called Raman scattering. Both of these stimulated scattering processes have a threshold power—occurring only when the input power exceeds this critical value. In typical optical communication systems, the optical power fed into the fiber is usually quite low, so nonlinear scattering effects are generally negligible.
3. Irregular Losses Due to Fiber Structure
Irregular structural losses arise from minor structural fluctuations at the core-cladding interface and from inhomogeneities within the fiber's waveguide structure. When the fiber's geometry becomes irregular, mode conversion occurs, causing some of the transmitted energy to leak out of the core and propagate as radiating modes, thereby increasing overall loss. Fortunately, these losses can be minimized by advancing manufacturing techniques.
4. Bending Losses in Optical Fibers
Bending loss refers to the attenuation caused by curvature in the fiber optic axis. Any visible deviation of the fiber axis from a straight line—whether large enough to be detected by the naked eye—is classified as either macrobending or simply bending. When a fiber bends, it induces coupling between different modes propagating within the fiber. If energy from a guided mode couples into radiative or leaky modes, this results in bending loss. Notably, this type of loss increases exponentially as the radius of curvature decreases. Another common type of loss occurs when the fiber axis experiences random, micrometer-scale lateral displacements—this is known as microbending loss. Microbending typically arises during processes such as coating, cabling, jacketing, or installation, where the fiber is subjected to excessive, uneven lateral pressure or longitudinal stress. Alternatively, it can also result after fiber manufacturing due to mismatches in thermal expansion coefficients between the coating layer and the fiber itself, or between the coating and the outer jacket.
Fiber optic loss
Absorption Losses Caused by Dopants and Impurity Ions
Fiber optic loss
Fiber optic materials contain transition metals such as iron, copper, and chromium, each of which has its own distinct absorption peaks and bands that vary depending on their oxidation states. The fiber loss caused by these transition-metal ions is directly proportional to their concentration. Additionally, the presence of OH⁻ ions also leads to absorption losses, with OH⁻ exhibiting a primary absorption peak around 2.7 μm and an absorption band spanning the 0.5–1.0 μm range. For pure quartz fibers, the impact of impurity-induced losses can typically be neglected.
Solution:
(1) Chemical purification of fiber optic materials, achieving purities as high as 99.9999999%.
(2) Improvements in manufacturing processes, such as avoiding the use of oxyhydrogen flame heating (vapor-phase axial deposition method)
Atomic defect absorption loss
Fiber optic materials can develop atomic defects when exposed to heat or intense radiation, leading to light absorption and resulting in signal loss. However, under normal conditions, this effect is typically minimal.
Factors causing fiber optic loss
The main factors contributing to fiber optic loss include absorption loss, scattering loss, and other types of attenuation. These losses can further be categorized into intrinsic loss, manufacturing loss, and additional loss, among others.
Intrinsic Loss
Intrinsic loss refers to a type of loss inherent in the optical fiber material itself—it’s unavoidable and essentially sets the fundamental limit for fiber attenuation. For quartz fibers, intrinsic loss consists of two main components: intrinsic absorption and Rayleigh scattering. Intrinsic absorption arises from the material properties of quartz itself, encompassing both infrared and ultraviolet absorption. Infrared absorption is caused by molecular vibrations and becomes significant in the 1500–1700 nm wavelength range, impacting fiber-optic communication. Meanwhile, ultraviolet absorption results from electronic transitions and affects signals in the 700–1100 nm range. On the other hand, Rayleigh scattering occurs due to microscopic fluctuations in the refractive index along the fiber—these random variations cause light waves to scatter as they travel through the material. Rayleigh scattering notably influences fiber communication across the 600–1600 nm spectral band.
Fiber Optic Manufacturing Losses
Fiber optic manufacturing losses arise during the fiber production process, primarily due to absorption caused by impure components within the fiber (impurity absorption) and structural defects in the fiber itself. Among these impurities, transition metal ions and OH⁻ ions significantly contribute to optical loss. Notably, OH⁻ ions have a particularly pronounced impact, with absorption peaks at 950 nm, 1240 nm, and 1390 nm—especially the latter, which poses a major challenge for fiber-optic communication systems. Fortunately, as fiber manufacturing techniques continue to advance, the influence of transition metals has become less significant. State-of-the-art processes can now reduce OH⁻-induced losses at 1390 nm down to as low as 0.04 dB/km, nearly negligible in practical applications. Additionally, imperfections in the fiber's structural design can also lead to scattering losses.
Additional loss occurs after the optical fiber is cabled, primarily due to light leakage caused by bending or microbending of the fiber, which results in optical signal attenuation.
In addition to the three types of losses mentioned above, fiber optic systems also experience connection losses and coupling losses. Furthermore, when the incident optical power in the fiber exceeds a certain threshold, nonlinear effects can lead to scattering losses.
The loss characteristic curve of optical fiber—loss spectrum
Adding up the three types of losses mentioned above gives you the total loss, which forms a curve that varies with wavelength—this is known as the fiber's loss characteristic curve, or simply the loss spectrum.
From the loss spectrum curve of quartz optical fiber, we can identify the three low-loss "windows"—or rather, three low-loss valleys—used in fiber-optic communication: the 850 nm band (short-wavelength region), the 1310 nm band, and the 1550 nm band (long-wavelength region). Currently, fiber-optic communication systems primarily operate in the 1310 nm and 1550 nm bands. Notably, the 1550 nm band is especially favored, as most long-distance, high-capacity fiber-optic communication systems are designed to function within this wavelength range.
Factors related to the attenuation coefficient
The fiber optic loss spectrum vividly illustrates the relationship between the attenuation coefficient and wavelength. As shown in the fiber loss spectrum, the attenuation coefficient decreases as the wavelength increases; the peak of the loss is primarily associated with OH⁻ ions. Additionally, at wavelengths longer than 1600 nm, the increased loss is attributed to absorption losses in quartz glass and micro- (or macro-)scopic bending losses. Currently, advanced fiber manufacturing techniques have successfully eliminated the absorption peak caused by OH⁻ ions near 1385 nm, enabling fibers to maintain extremely low loss levels across the entire 1300–1600 nm wavelength range.
Measurement
Overall, as the optical signal propagates through the fiber, its power decays exponentially with increasing distance L: fiber attenuation.
So, the evaluation of fiber optic loss characteristics can be measured using the attenuation coefficient. The attenuation coefficient of an optical fiber is defined as: fiber loss
Where L is the fiber length, and Pin and Pout represent the input and output optical powers, respectively. Typically, standard single-mode fibers have an attenuation coefficient of 0.2 dB/km at 1550 nm.