（5）60 essential facts about optical fiber cables, save them and keep them with you!

34. In fiber optic communication systems, the following items are commonly found. Please indicate their names.

AFC, FC Adapter ST adapter SC adapter FC/APC, FC/PC connector SC connector ST connector LC patch cord MU patch cord Single-mode or multi-mode patch cord

35.What is the insertion loss (or insertion loss) of a fiber optic connector?

A: It refers to the value of the reduction in effective power of the tranSMission line caused by the intervention of the connector. For users, the smaller the value, the better. ITU-T stipulates that its value should not exceed 0.5dB.

36. What is the return loss (also known as reflection attenuation, return loss, return loss) of the optical fiber connector?

A: It is a measure of the input power component that is reflected from the connector and returned along the input channel. Its typical value should be no less than 25dB.

37.What is the most prominent difference between the light emitted by light-emitting diodes and semiconductor lasers?

Answer: The light produced by the light-emitting diode is incoherent light with a wide spectrum; the light produced by the laser is coherent light with a very narrow spectrum.

38.What is the most obvious difference in the operating characteristics of light-emitting diodes (LEDs) and semiconductor lasers (LDs)?

Answer: LED has no threshold, but LD has a threshold. Laser will only be generated when the injected current exceeds the threshold.

39.What are the two commonly used single longitudinal mode semiconductor lasers?

Answer: Both DFB laser and DBR laser are distributed feedback lasers, and their optical feedback is provided by the distributed feedback Bragg grating in the optical cavity.

40.What are the two main types of optical receiving devices?

Answer: There are mainly photodiodes (PIN tubes) and avalanche photodiodes (APDs).

41.What are the factors that produce noise in fiber-optic communication systems?

Answer: There are noises caused by unqualified extinction ratio, noises caused by random changes in light intensity, noises caused by time jitter, point noises and thermal noises of the receiver, mode noises of the optical fiber, noises caused by pulse broadening due to dispersion, mode distribution noises of LD, noises caused by frequency chirp of LD and noises caused by reflection.

42.What are the main optical fibers currently used in transmission network construction? What are their main features?

Answer: There are mainly three types, namely G.652 conventional single-mode fiber, G.653 dispersion-shifted single-mode fiber and G.655 non-zero dispersion-shifted fiber.

G.652 single-mode optical fiber has a large dispersion in the C-band 1530-1565nm and the L-band 1565-1625nm, generally 17-22psnm•km. When the system rate reaches 2.5Gbit/s or above, dispersion compensation is required. At 10Gbit/s, the system dispersion compensation cost is relatively high. It is the most commonly laid optical fiber in the current transmission network.

The dispersion of G.653 dispersion-shifted fiber in C-band and L-band is generally -1 to 3.5psnm•km, and it is zero dispersion at 1550nm. The system rate can reach 20Gbit/s and 40Gbit/s, making it the best fiber for single-wavelength ultra-long-distance transmission. However, due to its zero dispersion characteristics, when DWDM is used for capacity expansion, nonlinear effects will occur, resulting in signal crosstalk and four-wave mixing FWM, so it is not suitable for DWDM.

G.655 non-zero dispersion-shifted fiber: The dispersion of G.655 non-zero dispersion-shifted fiber in the C band is 1 to 6 psnm•km, and the dispersion in the L band is generally 6 to 10 psnm•km. The dispersion is small, avoiding the zero dispersion area, suppressing four-wave mixing FWM, and can be used for DWDM capacity expansion and high-speed system opening. The new G.655 fiber can expand the effective area to 1.5 to 2 times that of ordinary optical fiber. The large effective area can reduce the power density and reduce the nonlinear effect of the optical fiber.