1. light and its characteristics:
1. light is an electromagnetic wave
the wavelength range of visible light is: 39~76nm (nm). The part above 76nm is infrared light, and the part below 39nm is ultraviolet light. There are three kinds of applications in optical fiber: 85, 13 and 155.
2. refraction, reflection and total reflection of light.
because the propagation speed of light in different substances is different, when light is emitted from one substance to another, it will be refracted and reflected at the interface between the two substances. Moreover, the angle of refracted light will change 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 will be reflected back, which is the total reflection of light. Different substances have different refraction angles to light with the same wavelength (that is, different substances have different refractive indexes), and the same substance has different refraction angles to light with different wavelengths. Optical fiber communication is based on the above principles.
2. Optical fiber structure and types:
1. Optical fiber structure:
Bare optical fiber is generally divided into three layers: central high refractive index glass core (core diameter is generally 5 or 62.5μm), middle low refractive index silica glass cladding (diameter is generally 125μm), and outermost resin coating for reinforcement.
2. numerical aperture:
not all the light incident on the end face of the optical fiber can be transmitted by the optical fiber, but only the incident light within a certain angle range. This angle is called the numerical aperture of the optical fiber. The larger numerical aperture of optical fiber is beneficial to the docking of optical fiber. The numerical aperture of optical fibers produced by different manufacturers is different (at&; T CORNING）。
3. Types of optical fiber:
A. According to the transmission mode of light in optical fiber, it can be divided into single-mode optical fiber and multimode optical fiber.
multimode fiber: the center glass core is thick (5 or 62.5μm), which can transmit multiple modes of light. However, its intermodal dispersion is large, which limits the frequency of transmitting digital signals, and it will become more serious with the increase of distance. For example, a 6MB/KM optical fiber has only 3MB bandwidth at 2KM. Therefore, the transmission distance of multimode fiber is relatively short, generally only a few kilometers. Single-mode fiber: The glass core in the center is thin (the core diameter is generally 9 or 1μm) and can only transmit one mode of light. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication, but its chromatic dispersion plays a major role, so single-mode fiber has higher requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good.
B. According to the optimal transmission frequency window, it can be divided into conventional single-mode fiber and dispersion-shifted single-mode fiber.
conventional type: optical fiber manufacturers optimize the transmission frequency of optical fiber at a single wavelength, such as 13nm.
dispersion shift type: optical fiber manufacturers optimize the transmission frequency of optical fiber at two wavelengths, such as 13nm and 155nm.
C. according to the refractive index distribution, it can be divided into abrupt fiber and graded fiber.
abrupt type: the refractive index from the central core of the optical fiber to the glass cladding is abrupt. It has low cost and high intermodal dispersion. Suitable for short-distance low-speed communication, such as industrial control. However, due to the small dispersion between modes, single-mode fibers all adopt abrupt type.
graded fiber: the refractive index from the central core of the fiber to the glass cladding gradually decreases, which can make the high-mode light propagate in sinusoidal form, which can reduce the intermodal dispersion, increase the bandwidth of the fiber and increase the transmission distance, but the cost is higher. Nowadays, multimode fibers are mostly graded fibers.
4. Common optical fiber specifications:
single mode: 8/125μm, 9/125μm, 1/125μm
multimode: 5/125μm, European standard
62.5/125μm, American standard
industrial, medical and low-speed network: 1. Used for automobile control
III. Optical fiber manufacturing and attenuation:
1. Optical fiber manufacturing:
At present, the main optical fiber manufacturing methods are: in-tube CVD (chemical vapor deposition), in-rod CVD, PCVD (plasma chemical vapor deposition) and VAD (axial vapor deposition).
2. Optical fiber attenuation:
The main factors causing optical fiber attenuation are intrinsic, bending, extrusion, impurities, unevenness and butt joint.
Intrinsic: refers to the inherent loss of optical fiber, including Rayleigh scattering and inherent absorption.
bending: when the optical fiber is bent, some light in the optical fiber will be lost due to scattering, resulting in loss.
squeezing: the loss caused by slight bending when the optical fiber is squeezed.
impurities: the impurities in the optical fiber absorb and scatter the light propagating in the optical fiber, resulting in losses.
non-uniformity: loss caused by non-uniformity of refractive index of optical fiber materials.
butt joint: losses caused by fiber butt joint, such as different axes (the coaxiality of single-mode fiber is required to be less than .8μm), the end face is not perpendicular to the axis, the end face is uneven, the butt joint diameter is not matched, and the welding quality is poor.
4. Advantages of optical fiber:
1. The passband of optical fiber is very wide. Theoretically, it can reach 3 billion MHz.
2. The section without relay is long, ranging from tens to more than 1 kilometers, and the copper wire is only a few hundred meters.
3. It is not affected by electromagnetic fields and electromagnetic radiation.
4. light weight and small size. For example, 9 pairs of twisted pairs with 21, telephone lines have a diameter of 3 inches and a weight of 8 tons /KM. The fiber optic cable with ten times the communication volume is .5 inch in diameter and weighs 45P/KM.
5. Optical fiber communication is uncharged and safe to use. It can be used in flammable and explosive places.
6. The temperature range of the use environment is wide.
7. chemical corrosion, long service life.