OTDR, why we use them?
OTDR stands for an optical time-domain reflectometer. It is an instrument used to test an optical fiber. OTDR equipment works by injecting a series of optical pulses into the fiber and extracts, from the same end, the light that is scattered (Rayleigh backscatter) or reflected back (Fresnel reflections) from points along the fiber. OTDR technology uses the scattered or reflected light that is gathered to portray the optical fiber.
Rayleigh scattering is the result of variations in fiber density, for instance, inconsistencies in the index of refraction. This may cause small amounts of light to reflect from each point in the fiber back toward the transmitter.
Fresnel reflections may occur at connections or breaks where an air gap exists and the light traveling down the fiber encounters changes in material density. A large amount of light, compared with the Rayleigh scattering, is reflected.
The benefits of an OTDR are:
• OTDR equipment can locate defects and faults, and can also determine the amount of signal loss at any point in a fiber optic run.
• OTDR machines only need to have access to one end of a fiber to make their measurements.
• OTDR technology allows the machine to take thousands of measurements along a fiber. Using this it shows a snapshot of the cable run that is displayed on the screen as a line sloping down from left to right, showing distance on the horizontal line and signal level on the vertical line.
• OTDR machines are widely used in all phases of a fiber system’s life from installation, maintenance, fault locating to restoration. If you are installing an outside plant network you will want an OTDR to check that the slices along the run and that the fiber itself is good. OTDR machines locate splices after they are made and confirm the performance of the joint.
An OTDR can also find any stress fractures in the cables caused by improper handlings, such as exceeding the bend radius, during installation. If you are doing any restoration because a cable has been cut, an OTDR will help find where it has been cut and assist to confirm the quality of the splice after restoration. On single-mode fibers where connector reflections are of a concern, an OTDR will pinpoint any issue very easily.
Testing Optical fiber for loss
OTDR technology should not be used to measure cable plant loss. That is the job of the Light source and Power meter, which is able to duplicate the actual fiber optic link. The loss measured by a light source and power meter will not be the same as an OTDR as this device cannot show the actual cable plant loss that the system will see.
For that reason, many standards will not allow you to measure the loss of an installed cable with an OTDR. The best way to measure total attenuation in a fiber is to inject a known level of light in one end and measure the level when it is returned to the other end. The difference, measured in decibels (dB) is the end-to-end attenuation or IL (insertion loss).
However, this does not locate the exact location of the attenuation problem along the cable run?
This is where an OTDR comes into its own as it can provide a plot of distance versus signal and will show where there is a problem in the fiber along the run.
• An OTDR is an invaluable test instrument when it comes to installation, maintenance, fault locating and restoration. Many standards will not allow you to measure the loss of an installed cable with an OTDR. The best way to measure total attenuation in a fiber remains with a calibrated light source and power meter.