Webinar Q&A
How to Test and Characterize Conventional and Specialty Optical Fibers
October 14, 2020
How do you take Insertion Loss and Return Loss measurements with OBR 4600 (and 6225)?
Insertion Loss (IL) measurements are made by integrating over the Rayleigh scatter in separate parts
of the network under test.
Each peak has an associated return loss (RL) that is calculated by integrating the reflectivity under
the red and green highlighted areas. These areas can be made either very small to isolate only a
single event or large enough to cover the entire fiber assembly. The RL can then be associated with
either a single event or the entire link.
What polarization measurements can the OBR 4600 do?
The OBR 4600 uses a polarization diverse receiver that detects the power returned in two
orthogonal polarization states. The power in these two states can be summed to form the total
power returned from a particular location in the fiber. This summation is the blue curve displayed in
the linear and logarithmic reflection amplitude plots. (See Figure below.) In some cases, particularly
where birefringence is present, it is useful to look at the power returned in each of the polarization
states. The polarization graph shows these two power levels in red and yellow, as well as the sum of
the powers in blue.
In this case, the period of the white and red traces is equivalent to one half the beat length of the
birefringence induced by coiling the fiber (half, because in reflection the light traverses the
birefringence twice).
Is it possible to detect or measure temperature changes along the fiber with the OBR or other
system?
Yes, the OBR 4600 is available with a Distributed Sensing option, which provides measurements of
the following time-domain parameters:
▪ Spectral Shift
▪ Spectral Shift Quality Factor
▪ Temperature Change
▪ Strain
▪ Temporal Shift
With the Distributed Sensing option, the instrument can compare a reference and measurement
state scan of a fiber’s Rayleigh scatter signature and compute the spectral shift. The spectral shift
can be converted to either a temperature or strain change with user specified calibration
coefficients.
The user can also define the maximum detectable spectral shift range via the choice of wavelength
range of the reference and measurement scans, and the spatial resolution of the measurement can
be defined in the Data Processing controls, as shown below.
Luna also offers a dedicated fiber optic sensing system, the ODiSI, that uses the same operating
principle as the OBR but provides very accurate and repeatable measurement of distributed
temperature or strain.