Common knowledge of fiber optic cable


The raw material of optical fiber is mainly glass, so the manufacturing cost is relatively low. Optical fiber communication has good characteristics, such as: confidentiality, high capacity, high speed. Therefore, the application of optical fiber is extremely wide, and there are roughly the following categories:


1. Backbone transmission network (SDH/SONET), such as submarine optical cables between major cities and ocean bottoms, etc.;


2. Ethernet (GBE), including the current fiber-to-the-home (FTTH), to the building (FTTB), to the community, etc., mainly our home and office networks;


3. Data network (Fiber channel), various storage devices, databases, including the developing cloud computing service system;

FTTH cable

4. Cable TV transmission (PIN reception);


5. Other special purpose transmissions, such as fighter jets and ships.


1. Briefly describe the composition of optical fibers

A: An optical fiber consists of two basic parts: the core and cladding made of transparent optical material, and the coating.


2. What are the basic parameters that describe the transmission characteristics of optical fiber lines?

A: Including loss, dispersion, bandwidth, cutoff wavelength, mode field diameter, etc.


3. What is the cause of fiber attenuation?

Answer: The optical power in the fiber gradually decreases along the vertical axis. The reduction in optical power is wavelength dependent. In optical fiber links, the main reasons for the reduction of optical power are scattering, absorption, and optical power loss caused by connectors and splices. The unit of attenuation is dB.


Causes: There are many reasons for the attenuation of optical fibers, mainly including: absorption attenuation, including impurity absorption and intrinsic absorption; scattering attenuation, including linear scattering, nonlinear scattering, and structural incomplete scattering; other attenuations, including microbending attenuation, etc. . Chief among them is the attenuation caused by impurity absorption.


Fiber attenuation coefficient: the attenuation value of optical signal power per kilometer of fiber. Unit: dB/km.


Fiber Bend Loss

Fiber optics are very sensitive to bending, excessive bending = light spillage. If the bend radius is <20x outer diameter, most of the light will escape from the coating. Singlemode fiber optic cables are more sensitive to bending losses than multimode fiber optic cables.


Optical loss occurs in two types of bends: Macrobend and Microbend.



When the Macrobend bend is corrected, it can be restored.


Microbend cannot recover, for example, caused by cables being bundled too tightly.


4. How is the fiber attenuation coefficient defined?

Answer: Defined by the attenuation (dB/km) per unit length of a uniform fiber in steady state.


5. What is insertion loss?

Answer: It refers to the attenuation caused by inserting optical components (such as inserting connectors or couplers) in the optical transmission line.


6. What is the bandwidth of optical fiber related to?

Answer: The bandwidth of the fiber refers to the modulation frequency when the amplitude of the optical power is 50% or 3dB lower than the amplitude of the zero frequency in the transfer function of the fiber. The bandwidth of an optical fiber is approximately inversely proportional to its length, and the product of the bandwidth length is a constant.


7. How many kinds of dispersion are there in optical fiber? What is it about?

Answer: The dispersion of an optical fiber refers to the expansion of the group delay in a fiber, including modal dispersion, material dispersion and structural dispersion. Depends on the characteristics of both the light source and the fiber.


The phenomenon of optical pulse broadening in optical fibers caused by different group velocities of different wavelengths in the spectral composition of the light source.


material dispersion

The refractive index of the optical fiber material quartz glass has different values ​​for different wavelengths of transmitted light. It is a proof that many different wavelengths of sunlight can be divided into seven different colors after passing through the prism. For the above reasons, the phenomenon that the refractive index of a material varies with the wavelength of light, causing pulse broadening, is called material dispersion.


waveguide dispersion

Since the refractive index difference between the core and the cladding of the optical fiber is very small, when total reflection occurs at the interface, a part of the light may enter the cladding. After this part of the light travels a certain distance in the cladding, it may return to the core to continue transmission. The intensity of this part of the light entering the cladding is related to the wavelength of the light, which means that the length of the optical transmission path varies with the wavelength of the light.


After the light pulse emitted by a light source with a certain spectral width is injected into the fiber, since the optical transmission paths of different wavelengths are not exactly the same, the time to reach the end point is also different, resulting in pulse broadening. Specifically, the longer the wavelength of the incident light, the greater the proportion of light intensity entering the cladding, and the longer the distance traveled by this part of the light. This dispersion is caused by the optical waveguide in the fiber, and the resulting pulse broadening phenomenon is called waveguide dispersion.

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