Principles of Optical Fiber Communication


What is fiber optics?

Optical fiber, short for optical fiber, is a fiber made of glass or plastic that acts as a light-transmitting tool. The transmission principle is 'total reflection of light'. Kao Kun, the former president of the Chinese University of Hong Kong, and George A. Hockham first proposed the idea that optical fibers can be used for communication transmission, for which Kao won the 2009 Nobel Prize in Physics.


The tiny fiber is encased in a plastic jacket that allows it to bend without breaking. Generally, a light-emitting diode (LED) or a laser beam is used to transmit light pulses to the optical fiber by the transmitting device at one end of the optical fiber, and the receiving device at the other end of the optical fiber uses a photosensitive element to detect the pulses. In daily life, optical fibers are used for long-distance information transmission because the conduction loss of light in optical fibers is much lower than that of electricity in wires.


Usually the terms optical fiber and optical cable are confused. Most optical fibers must be coated with several layers of protective structures before use, and the coated cable is called a fiber optic cable. The protective layer and insulating layer of the outer layer of the optical fiber can prevent the damage of the optical fiber from the surrounding environment, such as water, fire, electric shock, etc. Optical cable is divided into: cable sheath, aramid fiber, buffer layer and optical fiber. Fiber optic and coaxial cables are similar, just without the mesh shield. At the center is the glass core where light travels.

fiber optic cable

Optical Fiber Communication

How does fiber transmit data?

How does optical fiber transmit signals? The principle of optical fiber communication

Optical fiber communication is a communication method that uses light waves to transmit information in optical fibers. Due to the significant advantages of lasers such as high directivity, high coherence, and high monochromaticity, the light wave in optical fiber communication is mainly laser, so it is also called laser-fiber communication.


The principle of optical fiber communication is: at the transmitting end, the transmitted information (such as voice) is first converted into an electrical signal, and then modulated onto the laser beam emitted by the laser, so that the intensity of the light changes with the amplitude (frequency) of the electrical signal. And sent out through the optical fiber; at the receiving end, the detector converts the optical signal into an electrical signal after receiving it, and restores the original information after demodulation.


The light-emitting diode LED or injection laser diode ILD emits the light signal and propagates along the optical medium, and at the other end, there is a PIN or APD photodiode as a detector to receive the signal. The modulation of the optical carrier is the amplitude shift keying method, also known as the brightness modulation (Intensity ModulaTIon). A typical approach is to represent two binary numbers with the appearance and disappearance of light at a given frequency. Both the light-emitting diode LED and the injection laser diode ILD signal can be modulated in this way, and the PIN and ILD detectors respond directly to the luminance modulation.


Power amplification: The optical amplifier is placed before the optical transmitting end to increase the optical power of the incoming fiber. The optical power of the entire line system is improved. Online relay amplification: when the building group is large or the distance between buildings is long, it can play the role of relay amplification and improve the optical power. Pre-amplification: Amplify the micro-signal after the photodetector at the receiving end to improve the receiving capacity.


Optical cables are not easy to branch, because they transmit optical signals, so they are generally used for point-to-point connections. Experimental multipoint systems with optical bus topologies have been built, but are still too expensive. In principle, due to the small power loss and attenuation of optical fiber, there is a greater potential for bandwidth. Therefore, the number of connectors that can be supported by general optical fiber is much more than that of twisted pair or coaxial cable. The low-cost and reliable transmitter is a 0.85um wavelength light-emitting diode LED, which can support a transmission rate of 100Mbps and a local area network within the range of 1.5 to 2KM. Laser diode transmitters are expensive and cannot meet the million-hour lifetime requirement. The LED detector PIN operating at 0.85um wavelength is also a low-cost receiver.


The signal gain of the avalanche photodiode is larger than that of the PIN, but it needs a power supply of 20 to 50V, while the PIN detector only needs a 5V power supply. If you want to achieve a longer distance and a higher speed, you can use a 1.3um wavelength system, which has very little attenuation, but is more expensive than a 0.85um wavelength system. In addition, the matching optical fiber connectors are also very important, requiring each connector to have a connection loss of less than 25dB, easy installation and low price.


The larger the core and aperture of the fiber, the more light it receives from the light-emitting diode (LED), and the better its performance. Fibers with a core diameter of 100um and a cladding diameter of 140um provide fairly good performance. The received light energy is 4dB more than that of 62.5/125um fiber and 8.5dB more than that of 50/125um fiber. The attenuation of fiber running at 0.8um wavelength is 6dB/Km, and the attenuation of fiber running at 1.3um wavelength is 4dB/Km. The 0.8um fiber bandwidth is 150MHz/Km, and the 1.3um fiber bandwidth is 500MHz/Km.


In the integrated wiring system, it is very suitable and necessary to use optical fiber as the transmission medium for the main line.


Using a light wavelength division multiplexing technology WDM (WAVELENGTH DIVISION MULTI-PLEXING), multiple bits can be multiplexed, sent, and transmitted on a line, generally transmitted in parallel by one byte and eight bits, and each bit stream uses different wavelength, so it requires support circuitry to operate at low rates. The optical fiber link of WDM is suitable for the device interface of byte width and is a new data transmission system.


Optical fiber communication is a communication method in which light waves are used as information carriers and optical fibers are used as transmission media. In principle, the basic material elements that constitute optical fiber communication are optical fibers, light sources and photodetectors.


It can be said that the current communication technology is all related to waves, whether it is radio waves (propagating in wires), electromagnetic waves (propagating in the air), or light waves (propagating in optical fibers) or even the sound of our speech, it is also a kind of wave (sound wave). Different amplitude frequencies contain different information.


The principle of optical fiber communication is: at the transmitting end, the transmitted information is first converted into an electrical signal, and then modulated onto the laser beam emitted by the laser, so that the intensity of the light changes with the amplitude (frequency) of the electrical signal, and is sent through the optical fiber. Go out; at the receiving end, the detector converts the optical signal into an electrical signal after receiving it, and restores the original information after demodulation.


What is the difference between optical fiber and general broadband?

The difference between fiber optic and broadband adsl

This refers to broadband access. There are multiple access methods for broadband to the user, the most commonly used are ADSL, vdsl, FTTB+LAN, and fiber access, as well as wireless access and power cat access, etc. The access method is not directly related to the bandwidth applied for, that is to say, the broadband application for 2M can be achieved no matter which method is used, and the speed can reach 2M.


It's just a different way to implement it. Just like from a certain place to a certain place, you can get there by walking, by car, by plane or by train. Because you are applying for 2M, no matter what kind of access you use, you can meet your requirements. If the network speed is increased to gigabit in the future, then ADSL, VDSL, fttb+lan, and wireless methods will not meet the needs. The optical fiber can meet the requirements. It is only necessary to replace the equipment on both sides of the terminal to replace the 100 megabit equipment with 1000 megabits. That's it. Broadband refers to a network access method with high transmission speed. Optical fiber is the transmission medium, not the same concept. It can be understood that optical fiber is one of the ways to realize broadband transmission.


Today's broadband is basically divided into optical fiber access and ADSL access. ADSL currently only has a maximum transmission volume of 10M. Optical fiber does not have this limitation. Optical fiber and broadband are actually two different concepts. Optical fiber transmits signals in the form of light pulses, and uses glass or plexiglass as the network transmission medium. It is a transmission medium, just like twisted pair, thick cable, thin cable, etc., but its transmission speed is much faster than ordinary media, which can reach more than gigabit per second.


The broadband we are talking about refers to the speed of our Internet access. The speed of dial-up Internet access has increased from 14.4Kbps to 56Kbps. However, limited by the quality of the telephone line, 56Kbps should be the limit of a general Modem. In order to obtain a faster Internet speed, it is bound to have to split the stream, so in recent years, various broadband access services have appeared one after another. So, what is broadband? In fact, there is no very strict definition. Generally, the upper limit of the current dial-up Internet speed is 56Kbps as the boundary, and the access of 56Kbps and below is called "narrowband", and the access method above is classified as on "Broadband".


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