The principle of fiber optic transmission communication is achieved based on the total reflection of optical signals inside the fiber optic.
The principle behind it is that due to the different propagation speeds of light in different substances, when light is directed from one substance to another, refraction and reflection occur at the interface between the two substances. Moreover, the angle of refracted light varies with the angle of incident light. When the angle of incident light reaches or exceeds a certain angle, the refracted light disappears and all incident light is reflected back, which is called total reflection of light.
So what is the structure of optical fibers to ensure total reflection of optical signals, and can all optical signals be fully reflected? Let's learn about the structure and types of optical fibers.
Fiber optic structure and types
Fiber optic structure: Fiber optic bare fibers are generally divided into three layers: a central high refractive index glass core (core diameter generally 50 or 62.5 μ m) , with a low refractive index silicon glass cladding in the middle (usually with a diameter of 125 μ m) The outermost layer is the coating layer used for reinforcement.
N.A.: The light incident on the fiber end face cannot be transmitted entirely by the fiber, only the incident light within a certain angle range can be transmitted. This angle is called the numerical aperture of the optical fiber. A larger numerical aperture of optical fibers is beneficial for fiber docking. The numerical aperture of optical fibers produced by different manufacturers varies.
Types of optical fibers:
A.According to the transmission mode of light in optical fibers, it can be divided into single-mode optical fibers and multi-mode optical fibers.
B.Divided by the optimal transmission frequency window: conventional single-mode fiber and dispersion shifted single-mode fiber.
C.According to the distribution of refractive index, it can be divided into mutation type and gradient type optical fibers.
What are single-mode fiber and multi-mode fiber?
Multi-mode fiber: The center glass core is thicker (50 or 62.5) μ m) It can transmit multiple modes of light. But its intermodal dispersion is relatively large, which limits the frequency of transmitting digital signals and becomes more severe with increasing distance. For example, a 600MB/KM fiber has only a bandwidth of 300MB at 2KM. Therefore, the distance for multi-mode fiber optic transmission is relatively close, usually only a few kilometers.
Single-mode optical fiber: The center glass core is relatively thin (core diameter is generally 9 or 10) μ m) Only one mode of light can be transmitted. Therefore, its intermodal dispersion is small and suitable for remote communication, but its chromatic dispersion plays a major role. Therefore, single-mode fibers have high 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.
What are conventional single-mode fibers and dispersion shifted single-mode fibers?
Conventional type: Fiber optic manufacturers optimize the transmission frequency of optical fibers on a single wavelength of light, such as 1300nm.
Dispersion displacement type: Fiber optic manufacturers optimize the transmission frequency of optical fibers to two wavelengths of light, such as 1300nm and 1550nm.
What are mutation type and gradient type optical fibers?
Mutant type: The refractive index from the center core of the optical fiber to the glass cladding is abrupt. It has low cost and high inter mode dispersion. Suitable for short distance low-speed communication, such as industrial control. However, due to the small intermodal dispersion, single-mode fibers all adopt a mutation type.
Gradient type fiber: The refractive index from the center core of the fiber to the glass cladding gradually decreases, allowing high mode light to propagate in a sinusoidal manner, reducing inter mode dispersion, increasing fiber bandwidth, and increasing transmission distance. However, the cost is relatively high. Nowadays, multimode fibers are mostly gradient type fibers.
So,why do we choose fiber optic transmission instead of cable transmission? Let's talk about the advantages of fiber optic:
- The passband of optical fibers is very wide. Theoretically, it can reach 3 billion megahertz.
- The non relay section is several tens to over 100 kilometers long, and the copper wire is only a few hundred meters long.
- Not affected by electromagnetic fields and radiation.
- Lightweight and small in size. For example, 900 pairs of twisted pair cables with a diameter of 3 inches and a weight of 8 tons/KM can connect to 21000 lines. And the optical cable with a communication volume ten times that has a diameter of 0.5 inches and a weight of 450P/KM.
- Fiber optic communication is not electrified and can be used safely in flammable and explosive places.
- Wide range of ambient temperature for use.
- Chemical corrosion resistance and long service life.