Common Fiber Instruments Lesson: Optical Time Domain Reflectometer (OTDR)

  The optical time domain reflectometer is a commonly used test instrument in the construction and maintenance of optical cable lines. It can test the location of fiber fault points, fiber attenuation, fiber connector loss and fiber length, and visually display it on the screen in the form of a graphical curve. The OTDR can also automatically store test results and bring its own printer. Since it can be measured in one section of the fiber, it is very convenient to use.

Basic working principle of OTDR

  The OTDR sends a light pulse into the fiber, measures the time required for the light pulse to be reflected back to the OTDR, and the reflected power. The reflected light signal passes through the directional coupler to the receiver of the OTDR and is converted into an electrical signal, which is finally on the screen. The result curve is displayed. The following figure is a block diagram of the composition of the optical time domain reflectometer. The main clock generates a standard clock signal, and the control pulse generator generates pulses, that is, controls the timing at which the light source emits the light pulse, and simultaneously controls the synchronization of the signal processing with the light source; the directional coupler turns the light source The emitted light is coupled to the fiber under test and couples the reflected light signal to the photodetector; the amplifier amplifies and shapes the electrical signal sent by the photodetector; the signal processing portion compares the reflected optical signal with the transmitted pulse to calculate the relevant data. .

a block diagram of the composition of the OTDR

OTDR Test Methods

  The OTDR test method is as follows:

  1. Cut the fiber optic cable, expose the fiber to be tested to a length of about 2 m, and clean the fiber to make the end face flat.
  2. Connect the fiber under test to the OTDR through a pigtail or fiber jumper.
  3. Select the wavelength and mode of the OTDR, which should be the same as the working mode of the fiber under test.
  4. According to the length and loss of the fiber, select the appropriate range and other parameters, and input the refractive index of the fiber under test.
  5. Measure the loss of reflection events (connectors, connectors, breakpoints in the fiber) and the total loss and attenuation of the fiber.
  6. Store or print the measurement results and the fiber attenuation spectrum.

Attenuation spectrum analysis

  The figure below shows the OTDR measurement versus fiber attenuation spectrum, the abscissa indicates the length of the fiber in km, and the ordinate indicates the fiber and reflection event loss in dB. The figure visually reflects the events occurring along the length of the fiber. The analysis of the points and segments of the curve is as follows. Near the beginning of the curve, that is, the fiber segment being measured near the OTDR, has a convex shape. This fiber region is called a dead zone, which is caused by the reflection generated by the connector before the OTDR. In this area, events occurring on the fiber cannot be observed in order to eliminate the effects of the blind spot. A blind fiber (approximately 1km) can be added between the OTDR and the fiber under test.

Optical attenuation spectrum measured by OTDR

  It can be seen from the figure that there is a significant steep drop in the curve at point A, indicating that there is a joint or some defect that leads to an increase in loss. Whether it is a joint or a defect can be judged by the line construction design data, and the position is known from the attenuation spectrum. The range in which the curve falls is corresponding to the loss value of the contact or defect on the vertical axis. The curve at point B suddenly rises, indicating that the reflection or scattering at this point is strong, it may be caused by a connector or fiber, type mismatch. Although there is a joint at point C, it produces a “gain” phenomenon, which is caused by a mismatch in the fiber types on both sides of the joint. At this time, the Rayleigh scattering of the fibers on both sides is different. When the light is transmitted to the fiber with a large degree of scattering by the less-scattered fiber, an upward gain occurs. Point d is the Fresnel reflection produced by the end face of the fiber, and the bottom figure shows the corresponding curve of the shape of the ends of the two fibers. Figure (a) shows the case where the end face is flat, and the degree of reflection is large; In the case where the end face is not flat, if it is not the end, the fiber has been broken.

  The OTDR determines the transmission distance on the attenuation spectrum by measuring the time difference between the incident light pulse and the reflected light signal. If the refractive index of the fiber connected to the OTDR is n, the fiber distance L1 at the event is reached.

L1=(c×Δt)/2n

  Where c is the speed of light; Δt is the time difference. Since the optical fiber is twisted in the loose tube of the optical cable, and the loose tube is spirally wound around the central reinforcing member, the measuring distance (actual length) of the optical fiber is different from the length L2 of the optical cable. The amount of excess fiber (%) of the fiber in the cable provided by the cable manufacturer reflects the relationship between them, ie

L2=L1/(1+a)

  In the formula, “a” is the excess amount.

Fiber end shape Influence on (OTDR) curve pairs

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