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Basic knowledge and common problems of optical fibers and cables

Basic knowledge and common problems of optical fibers and cables

2022-08-19

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

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.


2. What is the dispersion of optical fibers?

Different frequency components or different mode components of the optical signal (pulse) transmitted in the fiber propagate at different speeds, and will inevitably produce signal distortion (pulse broadening) after reaching a certain distance. The generation of optical fiber dispersion is based on two factors. One is that the optical signal entering the optical fiber is not monochromatic light (the light emitted by the light source is not monochromatic or the modulated signal has a certain bandwidth); the other is the dispersion effect of the optical fiber on the optical signal. .


3. How to describe the dispersion characteristics of signal propagation in optical fibers?

It can be described by pulse broadening, fiber bandwidth and fiber dispersion coefficient.

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4. What is the cutoff wavelength?

It means that a single-mode fiber usually has a certain wavelength. When the transmitted light wavelength exceeds this wavelength, the fiber can only propagate light in one mode (fundamental mode), and under this wavelength, the fiber can propagate multiple modes ( light containing higher-order modes).


5. What effect does the dispersion of optical fiber have on the performance of optical fiber communication system?

The dispersion of the fiber will broaden the transmission of the light pulse in the fiber. Affects bit error rate, transmission distance and system rate.


6. What is backscatter?

Backscattering is a method of measuring attenuation along the length of the fiber. Most of the optical power in the fiber is forward propagating, but there is also a small amount of optical power that is scattered back toward the illuminator. The time curve of backscattering can be observed using a beamsplitter. Starting at one end, it is possible to measure not only the length and attenuation of a uniform fiber, but also local irregularities, breaks and optical power loss at splices and connectors.


7. What is the testing principle of OTDR? What is the function?

OTDR is made according to the principle of backscattering and Fresnel reflection of light. It uses the backscattered light that is produced when light propagates in an optical fiber to obtain attenuation information. It can be used to measure the attenuation of the fiber, splice loss, the location of the fiber failure point, and understand the loss distribution along the fiber length. It is an indispensable tool in fiber optic cable construction, maintenance and monitoring. Its main parameters include: dynamic range, sensitivity, resolution, measurement time and dead zone.


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