In optical communication, the optical waveguide required for long-distance transmission of optical signals is a cylindrical dielectric waveguide called an optical fiber (or simply optical fiber). Optical fiber is a dielectric waveguide that operates at optical frequencies, guiding light energy to propagate along a direction parallel to its axis.
Structure and classification of optical fibers
△The guiding principle of optical fiber
Structure of optical fiber:
Optical fiber (OF) is a transparent dielectric fiber used to guide light. A practical optical fiber is composed of multiple transparent dielectric layers. The typical structure of an optical fiber, as shown in Figure 2-1, can be divided into three layers: the core with a higher refractive index, the cladding with a lower refractive index, and the outer coating. The structure of the core and cladding meets the requirements for light guidance, controlling the propagation of light waves along the core; the coating mainly serves a protective function (since it does not guide light, it can be dyed in various colors).
(Figure 2-1 Structure of a typical optical fiber)
(1) Fiber Core The fiber core is located at the center of the optical fiber (diameter 5~80µm). Its composition is high-purity silicon dioxide, with trace amounts of dopants such as germanium dioxide and phosphorus pentoxide added. The purpose of adding these small amounts of dopants is to appropriately increase the refractive index (n) of the fiber core. For communication optical fibers, the core diameter is 5~10µm (single-mode fiber) or 50~80µm (multimode fiber).
(2) Cladding: The cladding is located around the fiber core (its diameter is approximately 125 μm), and its composition is also high-purity silicon dioxide containing a very small amount of dopant. The role of the dopant (such as boron trioxide) is to appropriately reduce the optical refractive index (n2) of the cladding, making it slightly lower than the refractive index of the fiber core. To meet different light guiding requirements, the cladding can be made as a single layer or multiple layers.
(3) The outermost layer of the coated optical fiber is a coating composed of acrylate, silicone rubber, and nylon, which increases the mechanical strength and flexibility of the optical fiber. The coating is generally divided into a primary coating and a secondary coating. The secondary coating is an additional layer of thermoplastic material applied over the primary coating, hence it is also called a cladding. The outer diameter of the coated optical fiber is generally about 1.5 cm.
The thickness of the fiber core, the refractive index distribution of the core material, and the refractive index of the cladding material play a decisive role in the transmission characteristics of the optical fiber. The cladding material is usually a homogeneous material with a constant refractive index. If there are multiple cladding layers, the refractive indices of each cladding layer are different. The refractive index of the fiber core can be uniform or it can vary along the core radius r. Therefore, the refractive index distribution function n(r) along the radius is commonly used to characterize the change in the core refractive index.
Classification of optical fibers：
Here's the English translation of the text from the image:
"At present, there are many types of optical fibers, but their classification methods are generally divided into 4 categories: classification by fiber refractive index distribution, classification by transmission mode, classification by working wavelength, and classification by jacket and cladding material. In addition, according to the composition of optical fiber components, in addition to the most commonly used silica optical fiber, there are also fluoride optical fiber and plastic optical fiber.
(1) Classification by fiber refractive index distribution: can be divided into Step Index Fiber (SIF) and Graded Index Fiber (GIF).
1.Step index optical fiber: refers to the fiber core and cladding region where the refractive index distribution is uniform, the value is a constant, and the refractive index distribution presents a step-like layered structure. The variation of refractive index is step-like. The refractive index distribution of step index optical fiber is shown in Figure 2-2.
Its refractive index distribution expression is:
n(r) = {n₁ (r ≤ a₁)
{n₂ (a₁ < r ≤ a₂)
Step index optical fiber is an early structural form of optical fiber. Later, in multimode optical fiber, it gradually became replaced by graded index optical fiber (because graded index optical fiber can greatly reduce the modal color dispersion that multimode optical fiber has). However, it is still relatively common to use it to transmit pulsed light in optical fibers. Currently, when single-mode optical fiber gradually replaces multimode optical fiber as the main product of commercial optical fiber, step index optical fiber structure has become the only structural form of single-mode optical fiber - it must be step-like.
2.Graded index optical fiber: refers to optical fiber whose refractive index distribution varies with radius r. As the distance from the center increases and gradually decreases, the radius gradually becomes smaller. Its variation rule generally conforms to the power exponential rule. When reaching the fiber core and cladding interface, it is truncated to the values corresponding to the cladding; in the cladding region, its refractive index distribution is uniform, that is, n₂. The refractive index distribution of graded index optical fiber is shown in Figure 2-3."
Its refractive index distribution is expressed as follows:
"In the equation, g is the refractive index distribution number; it represents different values at different refractive index distributions; n₁ is the refractive index at the center of the fiber core; n₂ is the refractive index of the cladding; a₁ is the core radius; Δ is the relative refractive index difference, Δ = (n₁² - n₂²)/2n₁² = (n₁ - n₂)/n₁.
The main reason for the reduced intermodal dispersion of graded index optical fiber is that it reduces modal dispersion, extends the transmission distance, and increases the transmission capacity.
(2) Classification by transmission mode: Can be divided into Multi-Mode Fiber (MMF) and Single Mode Fiber (SMF). As the name suggests, multimode optical fiber can transmit multiple modes, while single-mode optical fiber can only transmit fundamental mode and electric field modes. It is generally believed that the new generation of transmission solutions should be dominated by single-mode optical fiber because it can transmit much farther than multimode optical fiber. When the transmission medium's loss and dispersion are the same, the information carrying capacity after single-mode modulation is much higher than that after multimode modulation.
Under certain working wavelength conditions, there are many transmission modes in optical fiber, and these fiber modes are multimode optical fibers. The modal refractive index of multimode optical fiber is approximately the same as the refractive index of the fiber core, and the number of modes is approximately proportional to the square of V (normalized frequency). Therefore, it is also called graded multimode optical fiber. Later, it gradually became graded index optical fiber.
Under certain working wavelength conditions, if there is only one transmission mode in the optical fiber, it is called single-mode optical fiber. Single-mode optical fiber can only transmit the fundamental mode (axial mode), and there is no intermodal dispersion when transmitting in this mode. Compared to multimode optical fiber with a large number of higher-order modes, this is very useful for high-speed optical fiber communication systems.
(3) Classification by working wavelength: Can be divided into short-wavelength optical fiber and long-wavelength optical fiber.
1.Short-wavelength optical fiber: In the initial stage of optical fiber communication development, the commonly used wavelength was between 0.6 ~ 0.9 μm. The main reason at that time was that semiconductor laser light sources and detectors operating in this wavelength band were relatively mature, and short-wavelength optical fiber was the main product. Currently, it is rarely used.
2.Long-wavelength optical fiber: As research work continues, when entering the wavelength bands of 1.31 μm and 1.55 μm, these two wavelength bands have shown low loss, zero dispersion, and minimum bending loss characteristics. Therefore, research work has gradually shifted towards these two wavelength bands, and optical fibers with better performance have emerged. Practices have proved that at wavelengths of 1.0 ~ 2.0 μm, optical fibers have lower loss compared to short-wavelength optical fibers.
(4)Long-wavelength optical fibers are particularly suitable for long-distance, high-capacity optical fiber communication due to their advantages such as low attenuation and wide bandwidth.
1.Conventional optical fiber: refers to optical fiber whose fiber core is doped with germanium, cladding, and core refractive index distribution are combined in a certain ratio. Since this type of optical fiber has good characteristics and is relatively easy to produce, it has passed several generations of improvements.
This is due to the high expansion coefficient of the material with germanium as raw material. At low temperatures, it will shrink and crack. Stress birefringence will occur, adding asymmetry to the optical fiber.
2.Dispersion-shifted optical fiber: Refers to optical fiber that undergoes heat treatment after doping with germanium, moving the zero-dispersion point into one wavelength, not three or three times the wavelength.
The manufacturing process of this type of optical fiber is relatively complicated. Among them, the core diameter must match with the degree of doping to optimize the optical fiber. Therefore, it has not been widely used yet."
