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Top 10 Key Knowledge Points of Optical Fiber Communication (Part 1)

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10 Key Knowledge Points from the 21 Fundamentals of Optical Fiber Communication (Part 1)

Optical fiber, abbreviated as fiber, is a fiber made of glass or plastic that can be used as a light transmission tool. The transmission principle is "total reflection of light". Optical fiber communication has excellent characteristics such as confidentiality, high capacity, and high speed, making it widely applicable in various fields:

  1. Backbone transmission networks (SDH/SONET), such as intercity and transoceanic submarine cables.

  2. Ethernet (GBE), including Fiber to the Home (FTTH), Fiber to the Building (FTTB), and Fiber to the Community, mainly for home and office networks.

  3. Data networks (Fiber channel), including storage devices and databases, as well as emerging cloud computing service systems.

  4. Cable television transmission (PIN reception).

  5. Special transmission applications, such as fighter jets and ships.

Below is a summary of 21 fundamental knowledge points about optical fiber communication.

1. Composition of Optical Fiber

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


2. Basic Parameters Describing Optical Fiber Line Transmission Characteristics

Answer: These include loss, dispersion, bandwidth, cutoff wavelength, and mode field diameter. 

3. Causes of Optical Fiber Attenuation

Answer: Optical fiber attenuation occurs when the optical power decreases along the length of the fiber, primarily due to scattering, absorption, and losses at connectors and splices. The unit of attenuation is dB. The main causes include absorption attenuation (impurity absorption and intrinsic absorption), scattering attenuation (linear scattering, nonlinear scattering, and structural irregularity scattering), and other attenuations such as micro-bending attenuation. 



4. What is Optical Fiber Bandwidth Related To?

Answer: Optical fiber bandwidth refers to the modulation frequency at which the optical power in the fiber's transfer function drops to 50% or 3 dB of the zero frequency amplitude. The bandwidth is approximately inversely proportional to the length of the fiber, with the product of bandwidth and length being a constant. 



5. How is the Dispersion Characteristic of Signal Transmission in Optical Fiber Described?

Answer: It can be described using pulse broadening, fiber bandwidth, and fiber dispersion coefficient. 


6. What is Cutoff Wavelength?

Answer: It is the shortest wavelength at which only the fundamental mode can propagate in the fiber. For single-mode fibers, the cutoff wavelength must be shorter than the transmission wavelength. 

7. How Does Fiber Dispersion Affect the Performance of Optical Fiber Communication Systems?

Answer: Fiber dispersion causes pulse broadening during transmission, affecting error rates, transmission distances, and system speed. 

8. What is the Test Principle of Optical Time Domain Reflectometer (OTDR) and Its Function?

Answer: OTDR is based on the principle of optical backscattering and Fresnel reflection, utilizing the backscattered light generated during fiber transmission to obtain attenuation information. It is used to measure fiber attenuation, joint loss, fault location, and the distribution of loss along the fiber length. It is essential for cable construction, maintenance, and monitoring. 


9. What Do "1310nm" or "1550nm" in Common Optical Testing Instruments Refer To?

Answer: They refer to the wavelengths of optical signals. Optical fiber communication operates in the near-infrared range (800nm to 1700nm), with common wavelengths being 850nm (short-wavelength) and 1310nm and 1550nm (long-wavelength). The operating wavelengths of single-mode fibers are typically 1310nm, 1550nm, and 1625nm. 


10. What Wavelengths Have the Minimum Dispersion and Minimum Loss in Commercial Optical Fibers?

Answer: 1310nm has the minimum dispersion, and 1550nm has the minimum loss.

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