Aixton Classroom: Application of OTDR

August 25, 2022
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11. What is the backscattering method?

Answer: 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 a small portion is backscattered towards the emitter. Using a spectroscope at the illuminator to observe the time curve of backscattering, from one end can not only measure the length and attenuation of the uniform fiber that is connected, but also measure the local irregularities, breakpoints, and damage caused by joints and connectors. Optical power loss.

OTDR uses backscattering to measure the loss, length, etc. of optical cable lines.

12. What is the test principle of Optical Time Domain Reflectometer (OTDR)? What is the function?

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

13. What is the blind spot of OTDR? What will be the impact on testing? How to deal with the blind spot in the actual test?

A: Usually, a series of "blind spots" caused by the saturation of the OTDR receiving end caused by reflection from characteristic points such as movable connectors and mechanical joints are called blind zones.

The blind zone in the optical fiber is divided into two types: the event blind zone and the attenuation blind zone: the reflection peak caused by the intervention of the active connector, the length distance from the starting point of the reflection peak to the receiver saturation peak, is called the event blind zone; Intervention of the active connector causes a reflection peak, the distance from the origin of the reflection peak to the point at which other events can be identified, known as the attenuation dead zone.

For OTDRs, the smaller the blind zone, the better. The dead zone will increase with the increase of the width of the pulse broadening. Although increasing the pulse width increases the measurement length, it also increases the measurement dead zone. Therefore, when testing the optical fiber, the measurement of the optical fiber of the OTDR accessory and the adjacent event points Use narrow pulses and wide pulses when making measurements at the far end of the fiber.

14. Can an OTDR measure different types of fibers?

Answer: If a single-mode OTDR module is used to measure a multi-mode fiber, or a multi-mode OTDR module is used to measure a single-mode fiber such as a 62.5mm core diameter, the fiber length measurement results will not be affected, but factors such as fiber loss , optical connector loss, and return loss results are incorrect. Therefore, when measuring the optical fiber, be sure to select the OTDR that matches the measured optical fiber for measurement, so as to obtain the correct results of all performance indicators.

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

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

16. In current commercial fibers, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss?

Answer: The light with the wavelength of 1310nm has the minimum dispersion, and the light with the wavelength of 1550nm has the minimum loss.

17. According to the change of the refractive index of the fiber core, how to classify the fiber?

Answer: It can be divided into step fiber and graded fiber. The step fiber has a narrow bandwidth and is suitable for small-capacity short-distance communication; the gradient fiber has a wider bandwidth and is suitable for medium and large-capacity communication.

18. According to the different modes of light waves transmitted in the fiber, how are the fibers classified?

Answer: It can be divided into single-mode fiber and multi-mode fiber. The core diameter of a single-mode fiber is about 1 to 10 μm. At a given operating wavelength, only a single fundamental mode is transmitted, which is suitable for large-capacity long-distance communication systems. Multimode fiber can transmit light waves of multiple modes, and the core diameter is about 50-60 μm, and the transmission performance is worse than that of single-mode fiber.

When transmitting the current differential protection of multiplexing protection, multi-mode optical fibers are often used between the photoelectric conversion device installed in the communication room of the substation and the protection device installed in the main control room.

19. What is the significance of the numerical aperture (NA) of a step index fiber?

Answer: Numerical Aperture (NA) indicates the light-receiving ability of the fiber. The larger the NA, the stronger the light-collecting ability of the fiber.

20. What is the birefringence of single mode fiber?

Answer: There are two orthogonal polarization modes in a single-mode fiber. When the fiber is not completely cylindrically symmetric, the two orthogonal polarization modes are not degenerate. The absolute value of the difference between the modes of the two orthogonal polarizations is for birefringence.