Optical time domain Reflectometer is the popular and widely used equipment to measure and monitor the performance of an opticalfiber cable plant. Known in the industry as OTDR in short, the equipment is user-friendly and produces almost reliable results.
Note that we use the word ‘almost’. An OTDR uses the back-scattering principle to measure the loss of a forward propagating lightsignal. It computes the attenuation of a forward propagating signal from the back-scattered light from different lengths of the optical link and shows theresults in a graphical format. Power level is plotted against the distance in thegraph displayed on the OTDR. Distance is calculated from the refractive indexvalue set in the OTDR. Refractive index value can be changed by the testing personnel and that shall be set as per the value given by the fiber cable supplier. Note that refractive index value changes from fiber type to fibertype within the same supplier and also from supplier to supplier.
Distance is calculated using the refractive index value and the time taken to travel along the fiber and come back to the detector of the OTDR. The time taken by the light signal has to be divided by 2 in order to get the length of the fiber, since the OTDR uses back-scattered light that are returned to the detector by travelling through the same fiberback.
When we say almost reliable results, we mean an OTDRis still acceptable for the field measurements, but for really accurate measurements and in case of disputes, results shall be verified by using other methods such as cut-back method. International standards have the same opinion about measurement results using OTDR. The reason is that an OTDR use one way measurement method. The events shown in an OTDR can not be authentic unless it is measured from the opposite end and take the average of both the readings.
OTDR is used to measure the splice losses in an optical fiber cable link. Average of bi-directional measurement is required to find out the actual splice loss. ITU-T G.650.3 for the test standards for accepting the single-mode optical fiber cable links recommends bi-directional measurement. A link shall be tested from the end A and end B and then take average of both the measurements to get the actual splice loss as shown in the formula.
Note that we use the word ‘almost’. An OTDR uses the back-scattering principle to measure the loss of a forward propagating lightsignal. It computes the attenuation of a forward propagating signal from the back-scattered light from different lengths of the optical link and shows theresults in a graphical format. Power level is plotted against the distance in thegraph displayed on the OTDR. Distance is calculated from the refractive indexvalue set in the OTDR. Refractive index value can be changed by the testing personnel and that shall be set as per the value given by the fiber cable supplier. Note that refractive index value changes from fiber type to fibertype within the same supplier and also from supplier to supplier.
Distance is calculated using the refractive index value and the time taken to travel along the fiber and come back to the detector of the OTDR. The time taken by the light signal has to be divided by 2 in order to get the length of the fiber, since the OTDR uses back-scattered light that are returned to the detector by travelling through the same fiberback.
When we say almost reliable results, we mean an OTDRis still acceptable for the field measurements, but for really accurate measurements and in case of disputes, results shall be verified by using other methods such as cut-back method. International standards have the same opinion about measurement results using OTDR. The reason is that an OTDR use one way measurement method. The events shown in an OTDR can not be authentic unless it is measured from the opposite end and take the average of both the readings.
OTDR is used to measure the splice losses in an optical fiber cable link. Average of bi-directional measurement is required to find out the actual splice loss. ITU-T G.650.3 for the test standards for accepting the single-mode optical fiber cable links recommends bi-directional measurement. A link shall be tested from the end A and end B and then take average of both the measurements to get the actual splice loss as shown in the formula.
Real Splice loss = ½ *(Aa-b + Ab-a)
Where Aa-b: OTDR displayed splice loss value from end a to b
Ab-a: OTDR displayed splice loss value from end b to a
Aa-b is not the real splice loss. Ab-a is also not the real loss. The above equation says that to get the real splice loss, we need to take bi-directional measurement and then take the average.
0 comments:
Post a Comment