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Basic Knowledge Points Of Fiber Optic Cable(1)

Jun. 04, 2019

The coverage of fiber optic cable is very wide, and we can only better apply it if we fully understand this knowledge.

  1. Briefly describe the composition of the fiber:

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

    Basic Knowledge Points Of Fiber Optic Cable

  2. Basic parameters of optical fiber transmission characteristics:

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

  3. Causes of fiber attenuation:

      The optical power in the fiber gradually decreases along the vertical axis. The reduction in optical power is related to the wavelength. In optical fiber links, the main cause of optical power reduction is scattering, absorption, and optical power loss caused by connectors and fusion splices. The unit of attenuation is dB.

       Basic Knowledge Points Of Fiber Optic Cable

Causes: There are many reasons for the attenuation of the fiber, including: absorption attenuation, including impurity absorption and intrinsic absorption; scattering attenuation, including linear scattering, nonlinear scattering and structural incomplete scattering; other attenuation, including microbending attenuation, etc. . The most important of these is the absorption of impurities causing attenuation.

Basic Knowledge Points Of Fiber Optic Cable

Fiber attenuation coefficient:The attenuation of the optical signal power per kilometer of fiber. Unit: dB/km.

Fiber bending loss

The fiber is very sensitive to bending, excessive bending = light overflow. If the bend radius is <20x outside diameter, most of the light will overflow from the coating. Single mode fiber optic cables are more sensitive to bending losses than multimode fiber optic cables.

Light loss occurs in both bends:Macrobend & Microbend


Basic Knowledge Points Of Fiber Optic Cable

When Macrobend Bend is corrected, it can be recovered.

Basic Knowledge Points Of Fiber Optic Cable


Basic Knowledge Points Of Fiber Optic Cable

Microbend cannot be recovered, such as caused by tight cable ties.

Basic Knowledge Points Of Fiber Optic Cable

4.How is the fiber attenuation coefficient defined?

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

5.What is the insertion loss?

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

6.What is the bandwidth of the fiber?

   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.

7.What are the chromatic dispersions of optical fibers? What is it about?

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

Basic Knowledge Points Of Fiber Optic Cable

A phenomenon in which an optical pulse is broadened by a different group velocity of different wavelengths in a spectral component of a light source.

Basic Knowledge Points Of Fiber Optic CableNormal pulse

Basic Knowledge Points Of Fiber Optic CablePulse broadening

Material dispersion

The refractive index of the fiberglass quartz glass has different values for different wavelengths of transmitted light. It is a proof that many different wavelengths of sunlight can be divided into seven different colors after passing through the prism. For the above reasons, the phenomenon that the refractive index of the material changes with the wavelength of light to cause pulse broadening is called material dispersion.

Basic Knowledge Points Of Fiber Optic Cable

Waveguide dispersion

Since the difference in refractive index between the core and the cladding of the optical fiber is small, when total reflection occurs at the interface, a part of the light may enter the cladding. After this part of the light is transmitted within a certain distance in the cladding, it may return to the core to continue the transmission. The intensity of this portion of the light entering the cladding is related to the wavelength of the light, which is equivalent to the length of the optical transmission path depending on the wavelength of the light.

Basic Knowledge Points Of Fiber Optic Cable

When a light pulse emitted from a light source having a certain spectral width is incident on the optical fiber, since the optical transmission paths of different wavelengths are not completely the same, the time to reach the end point is also different, and pulse broadening occurs. Specifically, the longer the wavelength of the incident light, the greater the proportion of light entering the cladding, and the longer the distance the light travels. This dispersion is caused by the optical waveguide in the fiber, and the resulting pulse broadening phenomenon is called waveguide dispersion. 

Refractive index distribution of fiber:

Basic Knowledge Points Of Fiber Optic Cable

8.How to describe the dispersion characteristics of a signal propagating in an optical fiber?

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

Basic Knowledge Points Of Fiber Optic Cable

9.What is the cutoff wavelength?

    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.

10.What effect does the dispersion of the fiber have on the performance of the fiber-optic communication system?

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

11.What is backscattering?

    Backscatter is a method of measuring attenuation along the length of a fiber. Most of the optical power in an optical fiber is forward propagating, but a small portion is backscattered toward the illuminator. The time curve of backscattering is observed by the spectroscope at the illuminator, and the length and attenuation of the connected uniform fiber can be measured from one end, and local irregularities, breakpoints and joints and connectors are detected. Optical power loss.

Basic Knowledge Points Of Fiber Optic Cable

The OTDR uses backscatter to measure the loss, length, etc. of the cable line.

Basic Knowledge Points Of Fiber Optic Cable

Rayleigh scattering and Fresnel reflection occur when light propagates through the fiber. OTDR is a high-tech, high-precision photoelectric light that utilizes the characteristics of light to collect backscattering and reflection of light pulses in the path. Integrated instrumentation.

12.What is the testing principle of OTDR? What is the function?

    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 point location and optical 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 dead zone.

Basic Knowledge Points Of Fiber Optic Cable

13.What is the blind spot of the OTDR? What is the impact on the test? How to deal with blind spots in actual tests?

     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 generally referred to as dead zones.

The blindness in the fiber is divided into two types: the event dead zone and the attenuation dead zone: 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 dead zone; The interventional activity connector causes a reflection peak, which is called the attenuation dead zone, from the starting point of the reflection peak to the distance between other event points.

For OTDR, the smaller the blind zone, the better. The dead zone increases with 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 fiber, the measurement of the fiber and adjacent event points of the OTDR accessory is performed. Use a narrow pulse and use a wide pulse when making measurements on the far end of the fiber.

14.Can OTDR measure different types of fiber?

     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 that the performance results are correct.

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

     Refers to the wavelength of the optical signal. Optical fiber communication uses a wavelength range in the near-infrared region with a wavelength between 800 nm and 1700 nm. It is often divided into short-wavelength bands and long-wavelength bands, the former referring to the 850 nm wavelength and the latter to the 1310 nm and 1550 nm.

The working wavelength of fiber-optic communication lies in the near-infrared region, and the bands are:

O-band: 1260nm to 1310nm

E-band: 1360nm to 1460nm

S-band: 1460 nm to 1530 nm

C-band: 1535nm to 1565nm

L-band: 1565nm to 1625nm

U-band: 1640nm to 1675nm

Basic Knowledge Points Of Fiber Optic Cable

Single mode fibers typically operate at 1310 nm, 1550 nm, and 1625 nm.

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

    Light at a wavelength of 1310 nm has a minimum dispersion, and light at a wavelength of 1550 nm has a minimum loss.