EXFO IQS-12002B Calibration System - Manual (Page 78)

Calibration Methods
74 IQS-12002B
FTB-7000D and FTB-7000B-B OTDR Modules
Distance Measurement Specification
Distance specification depends on the location offset, distance scale
deviation, and sampling resolution.
For example, for an OTDR of the FTB-7000D family, the distance
specification is:
Note: For FTB-7000B-B, displayed resolution is 1 m for a distance between
markers greater than 1 km. Therefore, specification is to be evaluated
according to S = ±(1 + 0.0025 % x distance + 1) m.
The OTDR specification for distance depends on the range and pulse
duration used, as they will determine the sampling resolution on the trace.
Also, as the optical path changes with wavelength (because of the fiber
effective index dependence on wavelength) and due to the fact that fiber
calibration is performed using a normalized index of refraction of 1.46, the
optical distance of the reference fiber changes with the OTDR wavelength.
The equation which describes the difference in the optical length due to
the difference between the calibration wavelength and the actual
wavelength of the OTDR is:
where:
S0 is the zero dispersion slope in ps/(nm
2
* km), as given by the fiber
manufacturer.
For example, Corning gives as a typical value, for the SMF-28 fiber, an S0
coefficient of 0.086 ps/(nm
2
* km), with a maximum value of
0.092 ps/(nm
2
* km), at a λ0 of 1311.5 nm ±10 nm.
S location offset scale deviation distance sampling resolution + × + ( ) ± =
S ± 0.75 0.0025 % distance 0,04 + × + ( ) m =
δL
λ Lcal
S0
8
⋅ λ
2
λcal
2
− λ0
4
λ
2 −
λcal
2 −
−
( )
⋅ +
⎡⎣ ⎤⎦
⋅
c
N
⋅ :=