Fiber Theory and Fiber Structure
ⅠLight and the Features of Light
1. Light is a Kind of Electromagnetic Wave.
The range of visible wavelength is: 390 ~ 760nm. The part more than 760nm is infrared light, while the part less than 390nm is ultraviolet light. 850nm, 1300nm, 1310nm, 1550nm are applied in optical fiber. The long wave we often refer to is a laser with wavelength of 1310, 1550nm. And the shortwave refers to is a laser wavelength of 850nm.
2. The Refraction, Reflection and Total Reflection of the Light
Since the speed of light propagation is different in different materials, when light irradiate from a substance to another, this two materials will produce refraction and reflection in the interface. Besides, the angle of refraction changes with the angle of incident light. When the angle of incident light reaches or exceeds a certain angle, the refracted light will disappear, and all the incident light is reflected back, this is the total reflection of light. Different substances have different refractive index of the same wavelength light (i.e. different materials have different refractive index), and the same material has different refraction angles towards the same wavelength of light. Optical fiber communication is based on the above principle.
Ⅱ Fiber Structure and Fiber Types
1. Fiber Structure
Bare fiber is generally divided into three layers. First layer is the center high refractive index glass core (diameter is generally 50 or 62.5 m). The second layer lies the low refractive index silicon glass cladding (diameter is generally 125 m) and the external layer is resin coating used for strengthening.
2. The Numerical Aperture
The incident light can not all be transmitted by the optical fiber, only in a certain range can it be transmitted. This angle is called the numerical aperture of the fiber. The bigger the numerical aperture, the more favorable to the optical fiber, and optical fibers manufactured by different manufactures have different numerical aperture.(AT&TCORNING)
3. Fiber Types
A. Classified by transmission modes, optical fiber patch cable can be divided into common single-mode optical fiber and multimode optical fiber.
Multimode optical Fiber
Multimode optical fiber can transmit several types light, and its center glass core is relatively thick (50 or 62.5μm). However, the dispersion of the mode is large, which limits the transmission of digital signal frequency; furthermore, with the increase of the distance it will be more severe. For example: 600MB/KM's optical fiber is only 300MB of broadband in 2KM. Therefore, multimode fiber transmission distance is relatively close, generally only a few kilometers.
Single-mode Optical Fiber
Single-mode optical fiber’s glass core is relatively thin, (core diameter is generally 9 or 10 m), but it can only transmit a single kind of light. Therefore, the dispersion of the model is very small, and it is suitable for long-distance communication. The chromatic dispersion of single-mode optical fiber plays a critic role, so that the spectral width and the stability of the single-mode fiber has a higher requirement to the light source, that is, the narrower the spectral width, the better the stability.
B. Classified by the optimal transmission frequency window, optical fibers can be divided into the common single-mode fiber and dispersion shifted single-mode fiber.
Fiber optic manufacturers will optimize optical fiber transmission frequency in a single wavelength of light, such as 1300 M.
Dispersion Shift Type
Fiber optic manufacturers will optimize optical fiber transmission frequency in two wavelengths of light, such as: 1300 m and 1550 M.
C. Classified by the distribution of refractive index, optical fiber can be divided into the mutant and graded fiber.
The refractive index of the fiber core to the glass cladding is mutant. It is low cost and high dispersion, which is suitable for industrial control and other short distance communication. However, because of the small mode dispersion, all single-mode fiber adopts mutant type.
Graded Index Fiber
From the enter of the fiber core to the glass cladding refractive index is gradually become smaller, the high light mode spread in sinusoidal form, which can reduce the inter modal dispersion, while increasing the boardband of optical fiber and transmitting distance. But is is high cost. Now, most of the multimode fibers are graded index fiber.
Common Optical Fiber Specifications:
Single-mode: 8/125μm,9/125μm, 10/125μm
European standard: 62.5/125μm
American, industrial, medical, and low speed networks
standard: 100/140μm， 200/230μm
Plastic: 98/1000μm, used for automobile controlling.
Ⅲ Optical Fiber Fabrication and Attenuation
1. Optical Fiber Fabrication
Major optical fiber fabrication methods are: CVD (chemical vapor deposition) method, rod internal CVD method, PCVD (plasma chemical vapor deposition) method and VAD (axial vapor deposition) method.
2. Optical Fiber Attenuation
The main factors causing the attenuation of optical fiber are: the inhesion, bending, extruding, impurity, inhomogeneity and docking.
Intrinsic loss of the fiber, including Rayleigh scattering, self-absorption and so on.
Bending: the light in the fiber is lost due to scattering, which can result in the loss of light.
Extruding: the loss caused by a slight bending of the fiber.
Impurities: the loss caused by absorption and scattering of light in the fiber.
Inhomoheneities: the loss caused by the inhomogeneous refractive index of the fiber material.
Docking: the loss caused by the optical fiber connection, such as: the different axis (single-mode fiber coaxial degree requirement is less than 0.8 m), and the end of the axis is not perpendicular to the end, the end face is uneven, and the butt joint diameter is not matching and welding quality.
Ⅳ Advantages of Optical Fiber
1. The passband of the optical fiber is very wide. It can reach 30 trillion Hertz in theory.
2. No relay segment length tens to more than 100 kilometers, copper only a few hundred meters.
3. No affection from electromagnetic field and electromagnetic radiation.
4. Light weight, small size. For example: on the 21000 word path of the 900 pairs of twisted pair, its diameter is 3 inches, the weight of 8 tons /KM. The communication volume is ten times the diameter of the fiber optic cable is 0.5 inches, the weight of 450P/KM.
5. Optical fiber communication is not charged, it can be used in flammable, explosive area safely.
6. Wide range of ambient temperature
7. Chemical corrosion, long service life