# 20 essential knowledge of optical fiber and cable (1)

0 TUMTEC 2019-11-19 13:55:05

1.Briefly describe the composition of optical fiber?

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

2. What are the basic parameters to describe the transmission characteristics of optical fiber line?

Including loss, dispersion, bandwidth, cut-off wavelength, mode field diameter, etc.

3. What causes optical fiber attenuation?

A: optical fiber attenuation refers to the reduction of optical power between two cross sections of an optical fiber, which is related to wavelength. The main causes of attenuation are scattering, absorption and light loss caused by connectors and joints.

4. How is the attenuation coefficient of optical fiber defined?

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

5. What is insertion loss?

A: the attenuation caused by the insertion of optical components (such as connectors or couplers) in optical transmission lines.

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

A: the bandwidth of optical fiber refers to the modulation frequency when the amplitude of optical power is 50% or 3dB lower than the amplitude of zero frequency in the transmission function of optical fiber. The bandwidth of an optical fiber is approximately inversely proportional to its length.

7, there are several kinds of optical fiber dispersion? What is it about?

A: optical fiber dispersion refers to the expansion of the group delay in an optical fiber, including mode 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 width, optical fiber bandwidth and optical fiber dispersion coefficient.

9. What is cut-off wavelength?

Answer: it is the shortest wavelength that can only conduct fundamental mode in optical fiber. For single-mode fiber, the cut-off wavelength must be shorter than the wavelength of conduction light.

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

A: the dispersion of the optical fiber will make the optical pulse widen during the transmission in the optical fiber. Affects the size of bit error rate, and the length of the transmission distance, as well as the size of the system speed.

The phenomenon of optical pulse broadening caused by different group velocities of different wavelength in optical fiber.

11. What is backscattering?

A: backscattering is a method of measuring attenuation along the length of the optical fiber. Most of the optical power in optical fiber is forward propagation, but very little is backscattering towards luminescent. The time curve of backscattering can be observed by using a splitter at the luminator, and the length and attenuation of the connected uniform fiber can be measured from one end.

OTDR USES backscattering to measure loss and length of optical cable.

12. What is the test principle of the optical time domain reflectometer (OTDR)? What does it do?

Answer: OTDR based on the backscattering light and Fresnel reflection principle, when the use of light propagation in optical fiber attenuation of backscatter light to get information, can be used for measuring optical attenuation, splicing loss, fiber optic fault point positioning and understand the status of loss distribution along the length of optical fiber, etc., is an essential part of fiber optic cable construction, maintenance and monitoring tools. The main parameters include dynamic range, sensitivity, resolution, measurement time and blind area.

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

A: a series of "blind spots" caused by OTDR receiver saturation caused by reflection of feature points such as active connectors and mechanical joints are generally referred to as blind spots.

The blind area in optical fiber is divided into event blind area and attenuation blind area. The length distance between the initial point of the reflection peak and the saturated peak of the receiver caused by the interference of the active connector is called event blind area. The distance between the initial point of the reflection peak and other event points that can be identified is called attenuation blind area.

For OTDR, the smaller the blind spot, the better. The blind area will increase with the increase of the width of the pulse width. Although the increase of the pulse width increases the measurement length, it also increases the measurement blind area. Therefore, when testing the optical fiber, narrow pulse should be used to measure the optical fiber of OTDR attachment and adjacent event points, while wide pulse should be used to measure the far end of the optical fiber.

14. Can OTDR measure different types of optical fibers?

Answer: if single-mode OTDR module is used to measure multi-mode fiber, or a single-mode OTDR module is used to measure single-mode fiber with a core diameter of 62 or 5mm, the measurement results of fiber length will not be affected, but the results of fiber loss, optical joint loss and return loss are incorrect. Therefore, when measuring optical fiber, it is necessary to select OTDR matching with the tested optical fiber for measurement, so as to obtain correct results of all performance indexes.

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

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

16. In the current commercial optical fiber, what wavelength of light has the minimum dispersion? What wavelength of light has a minimum loss?

A: light of 1310nm wavelength has the minimum dispersion, and light of 1550nm wavelength has the minimum loss.

17. How is the fiber classified according to the refractive index of the fiber core?

Answer: it can be divided into step fiber and gradient fiber. The bandwidth of step fiber is narrow, which is suitable for short distance communication with small capacity. Tapered fiber has wide bandwidth and is suitable for medium - and large-capacity communication.

18. How are optical fibers classified according to the modes of transmission in optical fibers?

A: can be divided into single-mode fiber and multi-mode fiber. Single mode fiber core diameter is about 1 ~ 10 microns, in a given working wavelength, only a single fundamental mode transmission, suitable for large capacity long distance communication system. Multimode fiber can transmit multiple modes of light wave, the core diameter is about 50 ~ 60 microns, the transmission performance is worse than single-mode fiber.

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

19. What is the significance of the NA of step index fiber?

A: the value hole (NA) represents the optical fiber's light collecting ability. The larger the NA, the stronger the optical fiber's light collecting ability.

20. What is birefringence of single-mode fiber?

Answer: there are two orthogonal polarization modes in single-mode fiber. When the fiber is not completely circular column symmetric, the two orthogonal polarization modes are not degenerate, and the absolute value of the difference of the mode refractive index of the two orthogonal polarization is birefringence.