Advantages of Optical Fiber Communication

●Large communication capacity ●Long relay distance ●No electromagnetic interference ●Abundant resources ●Light weight and small size of optical fiber

A Brief History of Optical Communication Development

More than 2000 years ago, the beacon tower – lights, semaphore 1880 optical telephone – wireless optical communication 1970 optical fiber communication ● In 1966, “the father of optical fiber” Dr. Kao Kun first proposed the idea of ​​optical fiber communication. ● In 1970, Lin Yanxiong of Bell Research Institute developed a semiconductor laser that could work continuously at room temperature. ● In 1970, Corning’s Kapron produced optical fiber with a loss of 20dB/km. ● 1977 Chicago’s first 45Mb/s commercial line.

Electromagnetic Spectrum

Communication Band Division and Corresponding Transmission Media

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 substance, refraction and reflection will occur at the interface of the two substances. Also, the angle of the refracted light varies with the angle of the incident light. When the angle of the 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 for the same wavelength of light (that is, different substances have different refractive indices of light), and the same substance has different refraction angles for different wavelengths of light. Optical fiber communication is formed based on the above principles. Reflectance distribution: An important parameter to characterize optical materials is the refractive index, which is represented by N. The ratio of the speed of light C in vacuum to the speed of light V in the material is the refractive index of the material. The refractive index of silica glass for N=C/V optical fiber communication is about 1.44

The development process of optical communication

Basic knowledge of light

Fiber Structure

The bare fiber is generally divided into three layers: The first layer: the central high refractive index glass core (the core diameter is generally 9-10μm, (single mode) 50 or 62.5 (multimode). The second layer: the middle is low refractive index silicon Glass cladding (generally 125μm in diameter).The third layer: the outermost is the resin coating for reinforcement.

1) Core: high refractive index, used to transmit light; 2) cladding coaTIng: low refractive index, together with the core to form a total reflection condition; 3) protective jacket: high strength, can withstand greater impact , protect the fiber. 3mm fiber optic cable Orange MM Multimode Yellow SM Singlemode

Fiber size

The outer diameter is generally 125um (an average hair is 100um) Inner diameter: single-mode 9um, multi-mode 50/62.5um

Numerical aperture

The light incident on the end face of the fiber cannot be all transmitted by the fiber, only the incident light within a certain angle range can. This angle is called the numerical aperture of the fiber. A larger numerical aperture of the optical fiber is advantageous for the butt-joining of the optical fiber. Optical fibers produced by different manufacturers have different numerical apertures

Types of optical fibers

According to the transmission mode of light in the fiber, it can be divided into:

Multi-mode (MulTI-Mode) (abbreviation: MM) Single-mode (Single-Mode) (abbreviation: SM) Multi-mode fiber: The central glass core is thicker (50 or 62.5μm), which can transmit light in multiple modes. But its intermodal dispersion is large, which limits the frequency of transmitting digital signals, and it will be more serious with the increase of distance. For example: 600MB/KM fiber has only 300MB bandwidth at 2KM. Therefore, the distance of multimode fiber transmission is relatively short, generally only a few kilometers. Single-mode fiber: The central glass core is relatively thin (the core diameter is generally 9 or 10 μm), and only one mode of light can be transmitted. In fact, it is a kind of step-type fiber, but the core diameter is very small. In theory, only straight light from a single propagation path is allowed to enter the fiber and propagate in a straight line in the core. Fiber pulses are barely broadened. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication, but its chromatic dispersion plays a major role, so the single-mode fiber has higher requirements on the spectral width and stability of the light source, that is, the spectral width should be narrower and the stability should be better. .

Classification of optical fibers

Classification by material: Glass fiber: Both the core and the cladding are glass, with low loss, long transmission distance, and high cost; Silicone fiber: The core is glass and the cladding is plastic, with similar properties to glass fiber, but with lower cost ; Plastic optical fiber: The core and cladding are plastic, the loss is large, the transmission distance is very short, and the price is very low. It is mostly used for home appliances, audio, and short-distance image transmission. According to the optimal transmission frequency window: conventional single-mode fiber and dispersion-shifted single-mode fiber. Conventional type: The optical fiber manufacturer optimizes the optical fiber transmission frequency on a single wavelength of light, such as 1310nm. Dispersion-shifted type: Optical fiber manufacturers optimize the optical fiber transmission frequency at two wavelengths of light, such as: 1310nm and 1550nm. Abrupt type: The refractive index from the central core of the fiber to the glass cladding is abrupt. It has low cost and high intermodal dispersion. It is suitable for short-distance low-speed communication, such as industrial control. However, due to the small intermodal dispersion of single-mode fiber, the single-mode fiber adopts abrupt type. Gradient fiber: The refractive index from the center core of the fiber to the glass cladding gradually decreases, so that the high-mode light can propagate in a sinusoidal form, which can reduce the intermodal dispersion, improve the fiber bandwidth, and increase the transmission distance, but the cost is high. Mode fibers are mostly graded fibers.

Common Fiber Specifications

Fiber size: 1) Single mode core diameter: 9/125μm, 10/125μm 2) Cladding outer diameter (2D) = 125μm 3) Coating outer diameter = 250μm 4) Pigtail: 300μm 5) Multimode: 50 /125μm, European standard 62.5/125μm, American standard 6) Industrial, medical and low speed network: 100/140μm, 200/230μm 7) Plastic: 98/1000μm for automotive control

Fiber attenuation

The main factors that cause fiber attenuation are: intrinsic, bending, extrusion, impurities, non-uniformity and butt joint. Intrinsic: It is the inherent loss of the fiber, including: Rayleigh scattering, intrinsic absorption, etc. Bending: When the fiber is bent, part of the light in the fiber will be lost due to scattering, resulting in loss. Squeeze: Loss caused by tiny bends in an optical fiber when it is squeezed. Impurities: Losses caused by impurities in the fiber absorbing and scattering light propagating in the fiber. Non-uniformity: Loss caused by non-uniform refractive index of the fiber material. Docking: The loss generated when the optical fiber is docked, such as: different axes (the coaxiality of single-mode fiber is required to be less than 0.8 μm), the end face is not perpendicular to the axis, the end face is not flat, the butt core diameter does not match, and the fusion quality is poor.

Types of fiber optic cables

1) According to the laying method, there are: self-supporting overhead optical cable, pipeline optical cable, armored buried optical cable and submarine optical cable. 2) According to the optical cable structure, it is divided into: bundled optical cable, layered optical cable, tightly hugged optical cable, ribbon optical cable, non-metallic optical cable and branchable optical cable. 3) According to the use, it is divided into: optical cable for long-distance communication, short-distance outdoor optical cable, hybrid optical cable and optical cable for building.

Fiber optic cable splicing and termination

The connection and termination of optical cables are the basic skills that the maintenance personnel of optical cable lines must master. The splicing technology of optical cable is classified: 1) The splicing technology of optical fiber and the splicing technology of optical cable are two parts. 2) The end of the optical cable is similar to the connection of the optical cable, but the operation should be different due to the different joint materials.

Types of fiber optic connections

Optical cable splices can generally be divided into two categories: 1) Fixed splices of optical fibers (commonly known as dead joints). Generally, optical fiber fusion splicer is used; it is used for the direct head of optical cable. 2) The flexible connector of the optical fiber (commonly known as the live connector). Connect with a detachable connector (commonly known as a union). It is used for optical fiber jumpers, equipment connection, etc. Due to the incompleteness of the fiber end face and the uneven pressure of the fiber end face, the splice loss of the optical fiber spliced ​​by one-time discharge is relatively large, and the secondary discharge fusion method is now used. First, preheat and discharge the fiber end face, shape the end face, remove dust and sundries, and at the same time make the fiber end face pressure uniform through preheating.

Monitoring method of optical fiber connection loss

There are three monitoring methods for fiber connection loss: 1. Monitor on the fusion splicer. 2. Monitoring of light source and optical power meter. 3. OTDR measurement method

How to operate fiber optic connection

The optical fiber splicing operation is generally divided into: 1. The processing of the optical fiber end face. 2. Optical fiber connection installation. 3. Fiber fusion. 4. Protection of fiber optic connectors. 5. There are five steps to leave the residual fiber. Usually the entire optical cable is connected according to the following steps: The first step: a large number of good lengths, strip the optical cable, and remove the cable sheath; the second step: clean and remove the oil filling paste in the optical cable. Step 3: Bundle the optical fibers. Step 4: Check the number of optical fiber cores, check the number of optical fibers, and check whether the color code of the optical fiber is wrong; Step 5: Strengthen the core connection; Step 6: Various auxiliary wire pairs, including business wire pairs, control wire pairs, and shielding Connect the ground wire, etc. (if there are the above-mentioned pairs. Step 7: Connect the optical fiber. Step 8: Protect the optical fiber connector; Continue; Step 11: Protection of optical cable connectors.

fiber loss

1310 nm : 0.35 ~ 0.5 dB/Km 1550 nm : 0.2 ~ 0.3dB/Km 850 nm : 2.3 ~ 3.4 dB/Km Fiber splice point loss: 0.08dB/point Fiber splice point 1 point/2km

Common fiber nouns

1) Attenuation attenuation: energy loss when light is transmitted in the optical fiber Single-mode fiber 1310nm 0.4~0.6dB/km 1550nm 0.2~0.3dB/km Plastic multi-mode fiber 300dB/km

2) Dispersion: Dispersion: the widening of the bandwidth caused by the optical pulse traveling a certain distance along the fiber. It is the main factor limiting the transfer rate. Intermodal Dispersion: Occurs only in multimode fibers because different modes of light travel along different paths. Material Dispersion: Light travels at different speeds at different wavelengths. Waveguide Dispersion: Occurs because light energy travels at slightly different speeds as it travels through the core and cladding. In single-mode fiber, it is very important to change the dispersion of the fiber by changing the internal structure of the fiber. Fiber Type G.652 Zero Dispersion Point Around 1300nm G.653 Zero Dispersion Point Around 1550nm G.654 Negative Dispersion Fiber G.655 Dispersion Shifted Fiber Full Wave Fiber loss, the light transmission no longer has good directionality.

Fiber System Basics

The structure of the basic optical fiber system and its function introduction: 1. Transmitting unit: converts electrical signals into optical signals; 2. Transmission unit: medium for carrying optical signals; 3. Receiving unit: receives optical signals and converts them into electrical signals; 4 .Connecting devices: connecting optical fibers to light sources, light detection and other optical fibers.

Common Connector Types

Connector end face type

coupler

Main function redistribution of optical signals Important application in optical fiber network, especially in local area network, application in wavelength division multiplexing device, basic structure coupler is a bidirectional passive device. The splitter is represented graphically

wavelength division multiplexer

WDM—Wavelength Division Multiplexer transmits multiple optical signals in one optical fiber, and these optical signals have different frequencies and different colors. The wavelength division multiplexer is to couple multiple optical signals into the same fiber; the demultiplexer is to distinguish multiple optical signals from one fiber. Wavelength Division Multiplexer (Legend)

sending unit

receiving unit

amplifier

Optical digital communication

Definition of pulse in digital system:

1. Amplitude: The height of the pulse represents the optical power energy in a fiber optic system.

2. Rise time: The time it takes for a pulse to rise from 10% to 90% of its maximum amplitude.

3. Fall Time: The time it takes for a pulse to fall from 90% to 10% of its amplitude.

4. Pulse width: the width of the pulse at 50% amplitude, expressed in time.

5. Cycle: The specific time of the pulse is the working time required to complete a cycle.

6. Extinction ratio: the ratio of 1 signal optical power to 0 signal optical power

Editor: Huang Fei

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