Q1.Briefly describe the composition of the fiber.

A: The fiber consists of two basic parts: a core and a cladding, a coating made of a transparent optical material.

Q2.What are the basic parameters describing the transmission characteristics of optical fiber lines?

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

Q3. What is the cause of the fiber attenuation?

A: Fiber attenuation refers to the reduction in optical power between two cross sections of an optical fiber, which is wavelength dependent. The main causes of attenuation are scattering, absorption, and light loss due to connectors and connectors.

Q4.How is the fiber attenuation coefficient defined?

A: Defined by the attenuation per unit length of a uniform fiber in the steady state (dB/km).

Q5.What is the insertion loss?

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

Q6.What is the bandwidth of optical fiber related to?

A: The bandwidth of an optical fiber refers to the modulation frequency at which the amplitude of the optical power is reduced by 50% or 3 dB over the amplitude of the zero frequency 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 length of the bandwidth is a constant.

Q7.How many kinds of dispersion are there? What is it related to?

A: Fiber dispersion refers to the broadening of group delay in an optical fiber, including mode dispersion, material dispersion and structural dispersion. It depends on the characteristics of light source and optical fiber.

Q8.How to describe the dispersion characteristics of signals propagating in optical fibers?

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

Q9.What is the cut-off wavelength?

A: It refers to the shortest wavelength of the fiber that can only conduct the fundamental mode. For a single mode fiber, the cutoff wavelength must be shorter than the wavelength of the conducted light.

Q10.What effect does the dispersion of optical fibers have on the performance of optical communication systems?

A: The dispersion of the fiber will cause the light pulse to broaden during transmission in the fiber. ffect the size of the bit error rate, the length of the transmission distance， and system rate.

Q11.What is backscattering?

A: Backscattering is a method for measuring attenuation along the length of optical fibers. Most of the optical power in optical fibers propagates forward, But a small part of it backscatters to the illuminator. By observing the backscattering time curve at the emitter, we can not only measure the length and attenuation of the uniform optical fibers, but also measure the local irregularity, breakpoint and the optical power loss caused by the connectors and connectors.

Q12.What is the testing principle of Optical Time Domain Reflectometer (OTDR)? What are the functions?

A: The OTDR is based on the principle of light backscattering and Fresnel reflection. The backscattered light generated by the propagation of light in the fiber is used to obtain the attenuation information, which can be used to measure fiber attenuation, joint loss, fiber fault location and fiber. The distribution of loss along the length is an indispensable tool in the construction, maintenance and monitoring of optical cables. Its main indicator parameters include: dynamic range, sensitivity, resolution, measurement time and blind area.

Q13.What is the blind area of OTDR? What impact will it have on testing? How to deal with the blind area in the actual test?

A: A series of "blind spots" caused by saturation of OTDR receiving ends caused by reflections of characteristic points such as movable connectors and mechanical joints are called blind areas.

The blindness in the fiber is divided into two types: the event blind areas and the attenuation blind areas: the reflection peak caused by the intervention of the active connector, the length distance from the start point of the reflection peak to the receiver saturation peak is called the event blind areas; The interventional activity connector causes a reflection peak, which is called the attenuation blind areas, from the starting point of the reflection peak to the distance between other event points.

For OTDR, the smaller the blind area, the better. Blind area will increase with the increase of pulse width. Increasing pulse width increases the measurement length, but also increases the measurement blind area. Therefore, narrow pulse should be used for OTDR accessory fiber and adjacent event points, while wide pulse should be used for remote measurement.

Q14.Can OTDR measure different types of fibers?

A: If a single-mode OTDR module is used to measure multimode fiber, or a multimode OTDR module is used to measure a single mode fiber such as a core diameter of 62.5 mm, fiber length measurements are not affected, but such as fiber loss, light The result of joint loss and return loss is incorrect. Therefore, when measuring the fiber, it is necessary to select the OTDR that matches the fiber to be measured for measurement, so as to get the correct results of all performance indicators.

Q15.What does “1310nm” or “1550nm” refer to in common light test instruments?

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

Q16.In the current commercial fiber, what wavelength of light has the smallest dispersion? What wavelength of light has the least loss?

A: Light with 1310 nm wavelength has the minimum dispersion and light with 1550 nm wavelength has the minimum loss.

Q17.According to the change of refractive index of fiber core, how to classify fiber?

A: 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 graded fiber has a wide bandwidth and is suitable for medium- and large-capacity communication.

Q18.According to the different modes of light wave transmission in optical fibers, how to classify optical fibers?

A: Can be divided into single mode fiber and multimode fiber. The single-mode fiber has a core diameter of about 1 to 10 μm, and transmits only a single fundamental mode at a given operating wavelength, which is suitable for a large-capacity long-distance communication system. Multimode fiber can transmit multiple modes of light waves with a core diameter of about 50-60 μm, and the transmission performance is worse than that of a single-mode fiber.

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

Q19.What is the significance of the numerical aperture passage (NA) of step index optical fibers?

A: NUMERICAL HOLE TRANSMISSION (NA) denotes the optical fiber's ability to collect light. The larger the NA, the stronger the optical fiber's ability to collect light.

Q20.What is the birefringence of a single-mode fiber?

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

Q21.How many of the most common cable structures are there?

A: There are two types: layered strand type and skeleton type.

Q22.What are the main components of optical cables?

A: It mainly consists of fiber core, optical fiber ointment, sheath material, PBT (polybutylene terephthalate) and other materials.

Q23.What is the armor of optical cable?

A: It refers to the protective elements (usually steel wires or strips) used in special purpose optical cables (such as submarine optical cables). The armor is attached to the inner sheath of the cable.

Q24.What material is used for optical cable sheath?

A: The cable jacket or sheath is typically constructed of polyethylene (PE) and polyvinyl chloride (PVC) materials to protect the core from external influences.

Q25.List special fiber optic cables used in power systems.

A: There are mainly three kinds of special optical cables:

Ground wire composite optical cable (OPGW), the optical fiber is placed in the power line of the ladle aluminum stranded structure. The application of OPGW fiber optic cable plays the dual function of ground wire and communication, effectively improving the utilization rate of power tower.

Wrap-around fiber optic cable (GWWOP), which is wrapped or suspended on the ground wire where existing transmission lines are present.

Self-supporting optical cable (ADSS) has a strong tensile capacity and can be hung directly between two power towers with a maximum span of 1000m.

Q26.How many applications structures of OPGW cable are there?

A: 1) The structure of plastic pipe layer strand + aluminium pipe; 2) The structure of central plastic pipe + aluminium pipe; 3) Aluminum skeleton structure; 4) Spiral aluminium tube structure; 5) Single-layer stainless steel pipe structure (central stainless steel pipe structure, stainless steel pipe strand structure); 6) Composite stainless steel pipe structure (central stainless steel pipe structure, stainless steel pipe layer strand structure).

Q27.What are the main components of the stranded wire outside the core of OPGW cable?

A: It consists of AA wire (aluminium alloy wire) and AS wire (aluminium clad steel wire).

Q28.What are the technical conditions to be selected for the OPGW cable model?

A: 1) The nominal tensile strength (RTS) of the OPGW cable (kN); 2) the number of fiber cores (SM) of the OPGW cable; 3) short circuit current (kA); 4) short circuit time (s); 5) Temperature range (°C).

Q29.How does the degree of bending of optical cables limit?

A: The bending radius of the cable should be no less than 20 times of the outer diameter of the cable and no less than 30 times of the outer diameter of the cable during construction (in non-stationary state).

Q30.What should we pay attention to in ADSS cable engineering?

A: There are three key technologies: mechanical design of optical cable, determination of suspension point and selection and installation of matching fittings.

Q31.What are the main optical cable armour clamp?

A: Fiber optic cable armour clamp refer to the hardware used in the installation of fibre optic cables, mainly including tension clamp, suspension clamp, anti-vibration device and so on.

Q32.Fiber optic connectors have two basic performance parameters, what are they?

A: Optical fiber connectors are commonly known as live connectors. For the optical performance requirements of single fiber connectors, the focus is on the two most basic performance parameters of interference loss and return loss.

Q33.How many types of fiber optic connectors are commonly used?

A: According to different classification methods, optical fiber connectors can be divided into different types, according to the different transmission media can be divided into single-mode optical fiber connectors and multi-mode optical fiber connectors; according to the different structure can be divided into FC, SC, ST, D4, DIN, Biconic, MU, LC, MT and other types; according to the connector pin end surface can be divided into FC, PC (UPC). And APC. Commonly used optical fiber connectors: FC/PC type optical fiber connector, SC type optical fiber connector, LC type optical fiber connector.

Q34.In optical fiber communication systems, the following items are common. Please indicate their names.

A: AFC, FC type adapter ST type adapter SC type adapter FC/APC, FC/PC type connector SC type connector ST type connector LC type jumper MU type jumper single mode or multi mode jumper

Q35.What is the insertion loss (or insertion loss) of a fiber optic connector?

A: It refers to the amount of effective power reduction of transmission line caused by the involvement of connectors. For users, the smaller the value, the better. ITU-T stipulates that its value should not exceed 0.5dB.

Q36.What is the return loss (or reflection attenuation, return loss, return loss) of optical fiber connectors?

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

Q37.What is the most striking difference between light emitting diodes and semiconductor lasers?

A: Light produced by light emitting diodes is incoherent light with wide spectrum; light produced by lasers is coherent light with narrow spectrum.

Q38.What is the most obvious difference between the working characteristics of light emitting diodes (LED) and semiconductor lasers (LD)?

A: LED has no threshold, LD has a threshold. Laser will be generated only when injection current exceeds the threshold.

Q39.Which two kinds of single longitudinal mode semiconductor lasers are commonly used?

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

Q40.What are the two main types of light receiving devices?

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

Q41.What are the factors causing noise in optical fiber communication systems?

A: There are noise caused by unqualified extinction ratio, noise caused by random variation of light intensity, noise caused by time jitter, point noise and thermal noise of receiver, mode noise of optical fiber, noise caused by pulse broadening caused by dispersion, noise caused by mode assignment of LD, noise caused by frequency chirp of LD and noise generated by reflection.

Q42.What are the main optical fibers currently used for transmission network construction? What are its main characteristics?

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

The G.652 single-mode fiber has a large dispersion in the C-band 1530～1565nm and the L-band 1565～1625nm, generally 17～22psnm•km. When the system speed reaches 2.5Gbit/s or above, dispersion compensation is needed. At 10Gbit/s, the cost of dispersion compensation is high. It is the most common type of fibers laid in the transmission network at present.

The dispersion of G.653 dispersion-shifted fiber in C-band and L-band is - 1-3.5psnm km, and zero dispersion at 1550nm. The system speed can reach 20Gbit/s and 40Gbit/s. It is the best single-wavelength ultra-long-distance transmission fiber. However, due to its zero dispersion characteristics, when using DWDM to expand capacity, there will be a non-linear effect, leading to signal crosstalk, resulting in four-wave mixing FWM, so it is not suitable to use DWDM.

G.655 non-zero dispersion-shifted fiber: G.655 non-zero dispersion-shifted fiber has a dispersion of 1 to 6 psnm•km in the C-band, 6 to 10 psnm•km in the L-band, and the dispersion is small, avoiding zero dispersion region. It not only suppresses four-wave mixing FWM, but also can be used in DWDM expansion, but also can open high-speed systems. The new G. 655 optical fiber can enlarge the effective area to 1.5-2 times that of ordinary optical fibers. Large effective area can reduce the power density and the non-linear effect of optical fibers.

Q43.What is the nonlinearity of optical fibers?

A: When the input optical power exceeds a certain value, the refractive index of the optical fiber will be nonlinearly correlated with the optical power, and Raman scattering and Brillouin scattering will occur, which will change the frequency of the incident light.

Q44.What effect does optical fiber nonlinearity have on transmission?

A: Nonlinear effects will cause some additional losses and disturbances, which will deteriorate the performance of the system. WDM system has large optical power and long distance transmission along the optical fiber, so it produces non-linear distortion. Nonlinear distortion includes stimulated scattering and nonlinear refraction. The stimulated scattering includes Raman scattering and Brillouin scattering. These two kinds of scattering reduce the energy of incident light and cause loss. When the input power is small, it can be neglected.

Q45.What is PON?

A: PON is an optical fiber loop network in local user access network. It is based on passive optical devices, such as couplers and splitters.