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Basic knowledge of fiber optic cable (1)

Update:16-10-2020
Summary:

   1. Briefly describe the composition of the optical f […]


   1. Briefly describe the composition of the optical fiber.

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

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

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

   3. What are the reasons for fiber attenuation?

  Answer: The attenuation of an optical fiber refers to the decrease of the optical power between two cross sections of an optical fiber, which is related to the wavelength. The main causes of attenuation are scattering, absorption and optical loss due to connectors and joints.

   4. How is the fiber attenuation coefficient defined?

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

   5. What is the insertion loss?

  Answer: Refers to the attenuation caused by the insertion of optical components (such as connectors or couplers) in the optical transmission line.

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

   Answer: The bandwidth of an optical fiber refers to the modulation frequency when the optical power amplitude is 50% or 3dB lower than the zero frequency amplitude in the transfer function of the optical 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 optical fiber dispersion are there? What is it related to?

  Answer: The dispersion of an optical fiber refers to the broadening of the group delay within an optical fiber, including modal dispersion, material dispersion and structural dispersion. It depends on the characteristics of both the light source and the optical fiber.

   8. How to describe the dispersion characteristics of signal propagation in optical fiber?

  Answer: It can be described by three physical quantities: pulse broadening, fiber bandwidth, and fiber dispersion coefficient.

   9. What is the cutoff wavelength?

  Answer: It refers to the shortest wavelength that can only transmit the fundamental mode in the optical fiber. For single-mode fiber, its cut-off wavelength must be shorter than the wavelength of the transmitted light.

   10. What effect will the dispersion of the optical fiber have on the performance of the optical fiber communication system?

  Answer: The dispersion of the optical fiber will cause the light pulse to expand during the transmission process in the optical fiber. Affect the size of the bit error rate, the length of the transmission distance, and the size of the system rate.

   11. What is the backscatter method?

  Answer: The backscatter method is a method of measuring attenuation along the length of an optical fiber. Most of the optical power in the optical fiber propagates in the forward direction, but a small part is scattered back toward the emitter. Use a spectroscope to observe the time curve of backscatter at the illuminator. From one end, not only can the length and attenuation of the uniform optical fiber connected, but also local irregularities, breakpoints, and joints and connectors can be measured. Optical power loss.

  12. What is the testing principle of optical time domain reflectometer (OTDR)? What is the function?

Answer: OTDR is made based on the principle of light backscatter and Fresnel reflection. It uses the backscattered light generated when light propagates in the optical fiber to obtain attenuation information. It can be used to measure optical fiber attenuation, splice loss, fiber fault location and Understanding the loss distribution of optical fibers along the length is an indispensable tool in the construction, maintenance and monitoring of optical cables. Its main index parameters include: dynamic range, sensitivity, resolution, measurement time and blind zone.

   13. What is the dead zone of OTDR? How will it affect testing? How to deal with the blind zone in the actual test?

  Answer: A series of "blind spots" caused by the saturation of the OTDR receiving end caused by the reflection of characteristic points such as movable connectors and mechanical joints are usually called blind spots.

There are two types of blindness in optical fiber: event blind zone and attenuation blind zone: the reflection peak caused by the intervention of the movable connector, and the length distance from the starting point of the reflection peak to the saturation peak of the receiver is called event blind zone; The intervening movable connector causes the reflection peak, and the distance from the starting point of the reflection peak to the point where other events can be identified is called the attenuation dead zone.

   For OTDR, the smaller the blind zone, the better. The blind area will increase with the increase of the pulse width. Although increasing the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing the optical fiber, measure the optical fiber of the OTDR accessory and the adjacent event point. Use narrow pulses, and use wide pulses when measuring the far end of the fiber.

  14. Can OTDR measure different types of optical fibers?

Answer: If you use a single-mode OTDR module to measure a multimode fiber, or use a multimode OTDR module to measure a single-mode fiber with a core diameter of 62.5mm, the measurement result of the fiber length will not be affected, but the fiber loss The results of optical connector loss and return loss are incorrect. Therefore, when measuring optical fibers, an OTDR that matches the optical fiber under test must be selected for measurement, so that all performance indicators are correct.

   15. What does "1310nm" or "1550nm" in common optical test instruments mean?

   Answer: It refers to the wavelength of the optical signal. The wavelength range used for optical fiber communication is in the near-infrared region, and the wavelength is between 800nm ​​and 1700nm. It is often divided into short-wavelength bands and long-wavelength bands. The former refers to 850nm wavelength, and the latter refers to 1310nm and 1550nm.

For more information:visit https://www.nb-fibercome.com/

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