OTDR

OTDR

An Optical Time Domain Reflectometer (OTDR) analyzes the light loss in an optical fiber network. The OTDR injects a short laser pulse into the optical fiber and then measures the backscatter and reflection of the light as a function of time. The characteristics of the light are analyzed to determine the locations of any breaks or splice losses in the fiber cable.

The OTDR is an impressive and costly piece of equipment.

You can use an OTDR to locate a break or other problem in a cable run, and to take a snapshot of the cable fiber network before turning the installation over to a customer. This snapshot gives a permanent record of the state of the fiber at any point in time.

From the OTDR we can quickly determine the following characteristics of the fiber link under test.

  • The length of the fiber
  • The attenuation in dB of the whole fiber link and the attenuation of the   separate sections of fibers (if any)
  • The attenuation characteristics of the basic fiber itself.
  • The location of connectors joints and faults in the cable.

OTDR is a one person bi-directional instrument which operates on the principle of Rayleigh back scatter and is one of the most useful installation and diagnostic tool available today.

OTDR specifications

  • Dynamic range. This is the combination of the total pulse power of the laser source and the sensitivity of the sensor.
  • Dead zone. As mentioned above, the dead zone is the space on a fiber trace following a fresnel reflection, in which the high return level of the reflection covers up the lower level of backscatter. This space is directly related to the pulse width of the laser source; a short pulse yields a relatively small dead zone, and a long pulse yields a relatively large dead zone.
  • Resolution. This is the ability of the OTDR to distinguish between the levels of power it receives. It may also refer to spatial resolution, which is how close the individual pieces of data are spaced in time.
  • Level accuracy and linearity. These are measurements of how closely the electrical current output corresponds to the input optical power. This is expressed as a plus-or-minus (+/-) dB amount or a percentage of the power level.
  • Distance accuracy. Accuracy is dependent upon clock stability, data point spacing, and the level of uncertainty of the index of refraction.
  • Operating an OTDR. Operating an OTDR is not especially difficult, but it does require familiarity with the particulars of the make and model you are using. To properly operate an OTDR, you generally have to make the following settings:
  • Fiber type. Singlemode or multimode.
  • Wavelength. Singlemode is set for 1310 nm or 1550 nm, and multimode is set for 850 nm or 1300 nm.
  • Measurement parameters. The typical parameters to be set are distance range, resolution, and pulse width.
  • Event threshold. This determines how much loss or change will be tagged as an event.
  • Index of refraction. This is the speed of light in that fiber. You can obtain this figure from the fiber manufacturer. In most cases you can take it directly from a standard spec sheet.
  • Display units. These are usually labeled in feet or meters.
  • Storage memory. This should be cleared so a new figure can be saved and/or stored.
  • Dead zone jumper. You must connect this fiber, which should be sufficiently long, between the OTDR and the fiber under test. Sometimes you may have to connect it at the far end of the cable, as well.
  • Measurement problems. At times you’ll encounter some obstacles you can’t overcome. The following events will put your troubleshooting skills to the test.
  • Nonreflective break. This occurs when a fiber has been shattered or immersed in liquid. In both cases, very little light reflects back to the OTDR, and it’s difficult to identify the break.
  • Gainer. A gainer is a splice in a fiber that shows up as a gain in power. A passive device like a splice cannot generate light and cannot cause a gain in light. But if there is a mismatch in the fibers that are spliced, it may appear to the OTDR as a gain. For example, if the splice goes from a 50-micron fiber to a 62.5-micron fiber, the difference in backscatter coefficients (the 62.5-micron core being larger) appears to the OTDR as a gain in light.
  • Ghosts. Ghosts are repetitions of a trace or portion of a trace. They are caused by a large reflection in a short fiber, causing light to bounce back and forth.

How does an OTDR work?

Due to the Raleigh scattering, some of the energy return back along the fiber towards the light source called back scatter. This back scatter is a fixed proportion of the incoming power and as the losses along the fiber take their toll from the incoming power, so the returned power also diminishes. We can say that OTDR uses a system rather like a radar set. It sends out a pulse of light and ‘listen’ for the echoes from the fiber.

Thus if it know the speed of the light and can measure the time taken for the light to travel along the fiber. It will be then easy job to calculate the length of the fiber.

Unlike sources and power meters, which measure the loss of the fibre optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fibre optic transmission link, so the measurement correlates well with actual system loss. The OTDR, however, uses unique phenomena of fibre to imply loss.

The biggest factor in optical fibre loss is scattering. It is like billiard balls bouncing off each other, but occurs on an atomic level between photons (particles of light) and atoms or molecules. If you have ever noticed the beam of a flashlight shining through foggy or smokey air, you have seen scattering. Scattering is very sensitive to the colour of the light, so as the wavelength of the light gets longer, toward the red end of the spectrum, the scattering gets less. Very much less in fact, by a factor of the wavelength to the fourth power - that's squared-squared. Double the wavelength and you cut the scattering by sixteen times!

You can see this wavelength sensitivity by going outside on a sunny day and looking up. The sky is blue because the sunlight filtering through the atmosphere scatters like light in a fibre. Since the blue light scatters more, the sky takes on a hazy blue cast.

Figure 1 Scattering in an Optical Fibre

In the fibre, light is scattered in all directions, including back toward the source as shown in Figure 1. The OTDR uses this "backscattered light" to make its measurements. It sends out a very high power pulse and measures the light coming back. At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fibre. Think of the OTDR pulse as being a "virtual source" that is testing all the fibre between itself and the OTDR as it moves down the fibre. Since it is possible to calibrate the speed of the pulse as it passes down the fibre, the OTDR can correlate what it sees in backscattered light with an actual location in the fibre. Thus it can create a display of the amount of backscattered light at any point in the fibre.

Figure 2 OTDR Display

There are some calculations involved. Remember the light has to go out and come back, so you have to factor that into the time calculations, cutting the time in half and the loss calculations, since the light sees loss both ways. The power loss is a logarithmic function, so the power is measured in dB.

The amount of light scattered back to the OTDR is proportional to the backscatter of the fibre, peak power of the OTDR test pulse and the length of the pulse sent out. If you need more backscattered light to get good measurements, you can increase the pulse peak power or pulse width as shown below.

Figure 3 Increasing the pulse width increases the backscatter level

Note on the display shown in Figure 2, some events like connectors show a big pulse above the backscatter trace. That is a reflection from a connector, splice or the end of the fibre. They can be used to mark distances or even calculate the "back reflection" of connectors or splices, another parameter we want to test in singlemode systems.

OTDR Basic Components

These are the basic components of OTDR.

  • Pulse Generator: It produces a voltage pulse which is used to start the timing process in the display at the same moment as the laser is  activated.
  • Pulsed Laser: The laser is switch on for a brief moment. The “ON” time being between 1nsec and 10µsec. The wavelength of the laser can be switched to suit the system to be investigated straight through.
  • Circulator: It allows the laser light to pass straight through the fiber under test. The backscatter from the whole length of the fiber approaches the circulator from the opposite direction and diverted to APD. After that light has been converted into an electrical signal.
  • Boxcar Average: The electrical signal from the APD is very weak and requires amplification before it can be displayed.
  • Display: The amplified signals are passed onto the display. The display is either a cathode ray tube (CRT) like an oscilloscope or a computer monitor or a liquid crystal as in calculators and laptop computers.
  • Data handling: An internal memory of a floppy disk drive can store the data for later analysis. The output is also available via an RS232 link for downloading to a computer. Some OTDR have an onboard printer to provide hard copies.

OTDR limitations

The limited distance resolution of the OTDR makes it very hard to use in a LAN or building environment where cables are usually only a few hundred meters long. The OTDR has a great deal of difficulty resolving features in the short cables of a LAN and is likely to show "ghosts" from reflections at connectors, more often than not simply confusing the user.

OTDR Applications

  • Measuring fiber length
  • measuring distance to faults, splices, connector and stresses placed on fiber.
  • measures loss as dB/Km
  • measures splices loss and connector
  • present the ture picture of fiber

Using the OTDR

When using an OTDR, there are a few cautions that will make testing easier and more understandable. First always use a long launch cable, which allows the OTDR to settle down after the initial pulse and provides a reference cable for testing the first connector on the cable. Always start with the OTDR set for the shortest pulse width for best resolution and a range at least twice the length of the cable you are testing. Make an initial trace and see how you need to change the parameters to get better results.

 

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