Shulou(Shulou.com)11/24 Report--

The invention of optical fiber has led to a revolution in the field of communication. if there is no optical fiber to provide high-capacity high-speed channels, the Internet can only stay at the stage of theoretical imagination. If the 20th century is the age of electricity, then the 21st century is the age of light. How on earth can light communicate? Let's learn the basics of optical communication with the editor.

Part1. The basic knowledge of light propagation understands that light wave is actually a kind of electromagnetic wave. In free space, the wavelength of electromagnetic wave is inversely proportional to the frequency, and the product of the two is equal to the speed of light, that is:

The wavelengths or frequencies of electromagnetic waves are arranged in order to form the electromagnetic wave spectrum. according to the different wavelengths or frequencies, electromagnetic waves can be divided into ray region, ultraviolet region, visible light region, infrared region, microwave region, radio wave region and long wave region. The bands used for communication are mainly infrared, microwave and radio wave areas. the following picture allows everyone to understand the division of communication bands and the corresponding propagation media.

The protagonist of this paper, "Optical Fiber Communication", uses infrared light waves. When it comes to this, you may wonder, why must it be the infrared band? This problem is related to the optical transmission loss of the optical fiber material, that is, silica glass, and then we need to understand how the optical fiber transmits light.

When the refraction, reflection and total reflection of light from one substance to another, refraction and reflection will occur at the interface of the two substances, and the refraction angle increases with the increase of the angle of incident light. ①→② is shown in the figure below. When the incident angle reaches or exceeds a certain angle, the refracted light disappears and all the incident light is reflected back, which is the total reflection of light, such as ②→③ in the following image.

The refractive index of different materials is different, so the propagation rate of light in different media is different. The refractive index is expressed by n, the velocity of vacuum is the velocity of the vacuum, and the velocity of propagation in the medium is v. The medium with higher refractive index is called optical dense medium, and the medium with lower refractive index is called optical sparse medium. The two conditions for total reflection are:

Transmission from optical dense medium to optical sparse medium

The incident angle is greater than or equal to the critical angle of total reflection

In order to avoid optical signal leakage and reduce transmission loss, optical transmission in optical fiber occurs under the condition of total reflection.

Part2. Optical propagation medium (optical fiber) introduction of optical fiber structure with the basic knowledge of total reflection light propagation, it is easy to understand the design structure of optical fiber. The bare optical fiber is divided into three layers:

The first layer of fiber core: located in the center of the optical fiber, the composition is high-purity silica that is glass. The core diameter is generally 9-10 microns (single mode), 50 or 62.5 microns (multimode). The fiber core has a high refractive index and is used to transmit light.

The second cladding: located around the fiber core, the composition is also silica glass (usually 125 microns in diameter). The refractive index of the cladding is low, which forms the total reflection condition together with the fiber core.

The third coating: the outermost layer is a resin coating for reinforcement. The protective layer material has high strength, can withstand large impact, and protects the optical fiber from water vapor erosion and mechanical abrasion.

Optical transmission loss optical fiber transmission loss is a very important factor affecting the quality of optical fiber communication. The main factors causing optical signal attenuation are the absorption loss of the material, the scattering loss during transmission and the loss caused by other factors, including optical fiber bending, extrusion, docking loss and so on.

With different wavelengths of light, the transmission loss in optical fiber is also different. in order to minimize the loss and ensure the transmission effect, scientists have been trying to find the most suitable light. For the light in the 1260nm~1360nm wavelength range, the signal distortion caused by dispersion is the smallest and the absorption loss is the lowest. In the early days, this wavelength range was adopted as the optical communication band. Later, after a long exploration and practice, experts gradually summed up a low loss wavelength (1260nm~1625nm) region, which is the most suitable for optical fiber transmission. Therefore, the light waves used in optical fiber communication are generally in the infrared band.

Fiber classified multimode fiber: it transmits multiple modes, but the dispersion between modes is relatively large, which limits the frequency of transmitting digital signals, and this limitation will become more serious with the increase of transmission distance. Therefore, the transmission distance of multimode optical fiber is relatively close, usually only a few kilometers.

Single-mode fiber: the diameter of the fiber is very small and can only be transmitted in one mode in theory, so it is suitable for remote communication.

Contrast item

Multimode optical fiber

single mode fiber

Optical fiber cost

High cost

Lower cost

Transmission equipment requirements

Low equipment requirements and low equipment cost

High requirements for equipment and light source

Attenuation

High

Low

Transmission wavelength

850nm-1300nm

1260nm-1640nm

Ease of use

The core diameter is large and easy to handle

Using connections is more complex

Transmission distance

Local network

(less than 2km)

Access network / medium and long distance network

(greater than 200km)

Bandwidth

Limited bandwidth

The bandwidth is almost unlimited

Conclusion

The cost of optical fiber is higher, but the cost of network opening is relatively low.

Higher performance, but higher cost to set up the network

Part3. The working principle of optical fiber communication system Mobile phones, computers and other communication products commonly used in optical fiber communication systems, the information sent is in the form of electrical signals. In optical communication, we should first convert the electrical signal into optical signal, and then convert the optical signal into electrical signal after transmission through optical fiber cable, so as to achieve the purpose of information transmission. The basic optical communication system consists of an optical transmitter, an optical receiver and an optical fiber loop that transmits light. In order to ensure the quality of long-distance signal transmission and improve the transmission bandwidth, optical repeaters and multiplexers are generally used.

Here is a brief introduction to the working principle of each device in the optical fiber communication system.

Optical transmitter: converts electrical signals into optical signals, which is mainly composed of a signal modulator and a light source.

Signal multiplexer: multiple optical carrier signals of different wavelengths are coupled to the same optical fiber for transmission to achieve the effect of doubling the transmission capacity.

Optical repeater: in the process of transmission, the waveform and intensity of the signal will deteriorate, so it is necessary to restore the waveform to the neat waveform of the original signal and increase the light intensity.

Signal demultiplexer: decomposes the multiplexed signal into the original separate signal.

Optical receiver: converts the received optical signal into electrical signal, which is mainly composed of photodetector and demodulator.

Part4. Advantages and applications of optical communication long relay distance, economy and energy saving

Suppose 10 Gbps of information is transmitted (10 billion zeros or 1 signals per second). If electrical communication is used, the signal will be relayed every few hundred meters to adjust the signal. In comparison, if optical communication is used, the relay distance can reach more than 100 kilometers. The less the times of adjusting the signal, the lower the cost. On the other hand, the material of optical fiber is silicon dioxide, which is rich in reserves and much lower in cost than copper wire, so optical communication has the effect of economy and energy saving.

Fast information transmission and high communication quality

For example, when talking to foreign friends or chatting on the Internet, the voice does not lag behind as it used to be. In the era of electric communication, international communication is mainly transmitted through man-made satellite as a relay, the transmission path will be longer and the signal will arrive slowly. With the help of submarine optical cable, optical communication shortens the transmission distance, so the transmission of information is faster. Therefore, the use of optical communication can achieve more smooth communication with overseas.

Strong anti-interference ability and good confidentiality

Electrical communication will make errors due to electromagnetic interference, which will lead to the decline of communication quality. However, optical communication will not be affected by electrical noise, so it is more secure and reliable. And because of the working principle of total reflection, the signal is completely bound to be transmitted in the optical fiber, so the confidentiality is good.

Large transmission capacity

General electrical communication can only transmit the amount of information of 10Gbps (10 billion 0 or 1 signals per second). Compared with this, optical communication can transmit the amount of 1Tbps (1 trillion 0 or 1 signals).

The application of optical communication has many advantages. So far, optical communication has been integrated into every corner of our life.

Internet-enabled devices such as mobile phones, computers and IP phones connect everyone to their region, their entire country, and even to the global communication network. For example, signals from computers and mobile phones are clustered in the base stations and network provider equipment of local communications operators and then transmitted to all parts of the world through fiber optics in undersea cables.

Video calls, online shopping, video games, drama chase and other daily activities are inseparable from the support behind it. The emergence of optical network makes our life more comfortable and convenient.

There are many application scenarios of optical communication in daily life, and the editor does not enumerate them one by one here. Through the popular science of the editor, we can also think about where optical communication is used in daily life.

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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