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Dynamic range is the DB difference between the initial backscatter level and the noise level, one of the main performance indexes of OTDR, which determines the maximum measurable length of the OTDR. The larger the dynamic range, the better the curve type and the longer the measurable distance.
OTDR measuring distance refers to the maximum length of optical fiber that can be measured by OTDR. It is determined by the dynamic range of OTDR, the insertion loss of optical devices, the transmission loss of optical cables, the loss of optical fiber joints (mechanical joints, fusion joints), etc. it should be calculated and determined according to the specific situation of the project.
The Formula of measuring distance is as follows
L = The longest measurable distance of OTDR measuring system
P = The dynamic distance of OTDR (usually dynamic @1550nm is 2dB less than that @ 1310nm)
Ac = Insertion loss, refers to insertion loss sum of OTDR, optical switch, WDM, optical filter, etc.
Mc = Cable line redundancy (dB)
Ma = Measuring accuracy margin (dB)
Af = The average attenuation coefficient of optical cable (dB/km). Generally speaking, the attenuation per km of single mode fiber is around 0.33dB/km @1310nm, around 0.20dB/km @1550nm
As = The average attenuation of optical fusion joint (dB/km)
Value of variable in Formula
P is preset by OTDR system supplier
Ac, all the insertion loss of OTDR, optical switch, WDM, filter and mechanical joint shall be included.
Mc, Cable line redundancy. To ensuring the measuring accuracy, it should be valued 3dB.
Ma is 10dB.
Af is provided by optical cable supplier. If the average attenuation @ 1625nm cannot be provided, it can be replaced by that at 1550nm.
As, presuming that there is one fusion joint per 2km optical cable and the attenuation of each fusion joint is 0.08db, As should be 0.04db. (not included if there is no fusion joint).
For the calculation of OTDR measuring distance, the correct way is firstly to put all the data into the formula, and then correct them according to experience and the specific situation of the project. However, the above data is not absolute. They are closely related to the number of link nodes and the insertion loss of links.