EXFO IQS-12002B Calibration System - Manual (Page 67)

Calibration Methods
Calibration System 63
Power Meter
However, before performing the linearity calibration, both branch of the
coupler have to be balanced (i.e. have the same output power). Balancing
the linearity setup is the basis of this method. It requires a subscript coupler
to split the power in two and a shutter on both branches in order to block
light or let it through. Power is then recombined using a second coupler
before sending the light to the power meter under test. A variable
attenuator on each path is very useful in order to match the output power
of each branch of the coupler. Finally, the power coming from each
individual branch (P
a
and P
b
) and also from both branches (P
total
) is
measured on the photo detector. The deviation of P
total
from P
a
+ P
b
is the
local nonlinearity (NLL).
By using an attenuator between the source and the first splitter, the total
power is attenuated by a factor of 2 (~3 dB) with respect to the previous
step. Adding the local nonlinearities and referencing to the reference
power, we obtain the total linearity error (NLglobal), starting from the
reference power level where the nonlinearity is zero. This process is
repeated throughout the desired range.
The following is the procedure to measure nonlinearity using the
superposition method.