Basic concept of optical fiber communication.
An optical fiber is a dielectric optical waveguide, a waveguide structure that blocks light and propagates light in the axial direction.
Very fine fiber made of quartz glass, synthetic resin, etc.
Single mode fiber: core 8-10um, cladding 125um
Multimode fiber: core 51um, cladding 125um
The communication method of transmitting optical signals using optical fibers is called optical fiber communication.
Light waves belong to the category of electromagnetic waves.
The wavelength range of visible light is 390-760 nm, the portion larger than 760 nm is infrared light, and the portion smaller than 390 nm is ultraviolet light.
Light wave working window (three communication windows):
The wavelength range used in fiber-optic communication is in the near-infrared region
Short-wavelength region (visible light, which is an orange light by the naked eye) 850nm orange light
Long wavelength region (invisible light region) 1310 nm (theoretical minimum dispersion point), 1550 nm (the theoretical minimum attenuation point)
Fiber structure and classification
1.The structure of the fiber
The ideal fiber structure: core, cladding, coating, jacket.
The core and cladding are made of quartz material, and the mechanical properties are relatively fragile and easy to break. Therefore, two layers of coating layer, one resin type and one layer of nylon type are generally added, so that the flexible performance of the fiber reaches the practical application requirements of the project.
2.Classification of optical fibers
(1) The fiber is divided according to the refractive index distribution of the cross section of the fiber: it is divided into a step type fiber (uniform fiber) and a graded fiber (non-uniform fiber).
Assume that the core has a refractive index of n1 and the cladding refractive index is n2.
In order to enable the core to transmit light over long distances, the necessary condition for constructing the optical fiber is n1>n2
The refractive index distribution of a uniform fiber is a constant
The refractive index distribution law of non-uniform fiber:
Among them, △ – relative refractive index difference
Α—refractive index, α=∞—step-type refractive index distribution fiber, α=2—square-law refractive index distribution fiber (a graded fiber). This fiber is compared to other graded fibers.Mode dispersion minimum optimal.
(1) According to the number of modes transmitted in the core: divided into multimode fiber and single mode fiber
The pattern here refers to the distribution of an electromagnetic field of light transmitted in an optical fiber. Different field distributions are a different mode.
Single mode (only one mode is transmitted in the fiber), multimode (multiple modes are simultaneously transmitted in the fiber)
At present, due to the increasing requirements on the transmission rate and the increasing number of transmissions, the metropolitan area network is developing in the direction of high speed and large capacity, so most of them are single mode stepped fibers. (The transmission characteristics of itself are better than multimode fiber)
(2) Characteristics of optical fiber:
①Loss characteristics of optical fiber: Light waves are transmitted in the optical fiber, and the optical power gradually decreases as the transmission distance increases.
The causes of fiber loss include: coupling loss, absorption loss, scattering loss, and bending radiation loss.
Coupling loss is the loss caused by the coupling between the fiber and the device.
Absorption losses are caused by the absorption of light energy by fiber materials and impurities.
The scattering loss is divided into Rayleigh scattering (refractive index non-uniformity) and waveguide scattering (material unevenness).
The bending radiation loss is the loss caused by the bending of the fiber leading to the radiation mode caused by the bending of the fiber.
②Dispersion characteristics of optical fiber: Different frequency components in the signal transmitted by the optical fiber have different transmission speeds, and the physical phenomenon of distortion caused by signal pulse broadening when reaching the terminal is called dispersion.
The dispersion is divided into modal dispersion, material dispersion, and waveguide dispersion.
Basic components of optical fiber communication systems
Send part:
The pulse modulation signal output by the electric transmitter (electrical terminal) is sent to the optical transmitter (the signal sent by the program-controlled switch is processed, the waveform is shaped, the inverse of the pattern is changed… into a suitable electrical signal and sent to the optical transmitter)
The primary role of an optical transmitter is to convert an electrical signal into an optical signal that is coupled into the fiber.
Receiving part:
Converting optical signals transmitted through optical fibers into electrical signals
The processing of the electrical signal is restored to the original pulse modulated signal and sent to the electrical terminal (the electrical signal sent by the optical receiver is processed, the waveform is shaped, the inverse of the pattern is inverted… the appropriate electrical signal is sent back to the programmable switch )
Transmission part:
Single-mode fiber, optical repeater (electrical regenerative repeater (optical-electric-optical conversion amplification, transmission delay will be larger, pulse decision circuit will be used to shape the waveform, and timing), erbium-doped fiber Amplifier (completes the amplification at the optical level, without waveform shaping)
(1) Optical transmitter: It is an optical transceiver that realizes electric/optical conversion. It consists of a light source, a driver and a modulator. The function is to modulate the light wave from the electric machine to the light wave emitted by the light source to become a dimmed wave, and then couple the modulated optical signal to the optical fiber or the optical cable for transmission.
(2) Optical receiver: is an optical transceiver that realizes optical/electrical conversion. The utility model is composed of a light detecting circuit and an optical amplifier, and the function is to convert the optical signal transmitted by the optical fiber or the optical cable into an electrical signal by the optical detector, and then amplify the weak electrical signal to a sufficient level through the amplifying circuit to be sent to the signal. The receiving end of the electric machine goes.
(3) Fiber/Cable: Fiber or cable constitutes the transmission path of light. The function is to transmit the dimmed signal sent by the transmitting end to the optical detector of the receiving end after long-distance transmission through the optical fiber or the optical cable to complete the task of transmitting information.
(4) Optical repeater: consists of a photodetector, a light source, and a decision regeneration circuit. There are two functions: one is to compensate the attenuation of the optical signal transmitted in the optical fiber; the other is to shape the pulse of the waveform distortion.
(5) Passive components such as fiber optic connectors, couplers (no need to supply power separately, but the device is still lossy): Because the length of the fiber or cable is limited by the fiber drawing process and cable construction conditions, and the length of the fiber is also Limit (eg 2km). Therefore, there may be a problem that a plurality of optical fibers are connected in one optical fiber line. Therefore, the connection between optical fibers, the connection and coupling of optical fibers and optical transceivers, and the use of passive components such as optical connectors and couplers are indispensable.
The superiority of optical fiber communication
Transmission bandwidth, large communication capacity
Low transmission loss and large relay distance
Strong anti-electromagnetic interference
(Beyond wireless: wireless signals have many effects, multipath benefits, shadow effects, Rayleigh fading, Doppler effects
Compared with coaxial cable: optical signal is larger than coaxial cable and has good confidentiality)
The frequency of the light wave is very high, compared with other electromagnetic waves, the interference is small.
Disadvantages of optical cable: poor mechanical properties, easy to break, (improve mechanical performance, will have an impact on interference resistance), it takes a long time to build, and is affected by geographical conditions.