EXFO IQS-12002B Calibration System - Manual (Page 66)

Calibration Methods
62 IQS-12002B
Power Meter
Linearity Calibration
The nonlinearity of the power meter should be calibrated to ensure
accurate measurements at power levels different from the calibration level
and for relative measurements such as loss and gain measurements.
Nonlinearity is the relative difference between the response at a given
power (P) and the reference power (P
0
):
The calibration should be made by increasing and decreasing the power
level to detect nonlinearities over each amplifier range and, whenever
possible, at both sides of each amplifier boundary in order to include
nonlinearities at the switching between scales. Detector nonlinearity is
dependent on the wavelength. For example, an InGaAs detector linear at
1310 nm and 1550 nm may be nonlinear at 850 nm.
The method privileged by the international standard on power meter
calibration (IEC 61315 Ed. 2.0 b:2005) is the superposition method. It is the
most accurate and does not require a reference standard as it is a
self-calibrating (ratio-type) method. It consists in verifying that the sum of
the power read in each branch of a 1 × 2 coupler is equal to the total
power read when measuring the two branches simultaneously. This is
performed over the useful dynamic range of the DUT.
1
) (
) (
0
−
ℜ
ℜ
=
P
P
NL