Agilent Technologies 86140B Series, User Manual

Page: 58 / 290

2-17
Using the Instrument
Calibrating Wavelength Measurements
External Multipoint Wavelength Calibration
An external multipoint wavelength calibration can be performed over any
specified wavelength range, up to and including the full wavelength range of
the OSA (600 nm to 1700 nm). Narrow measurement spans can be chosen to
provide greater accuracy over a selected range. Calibrating the wavelength
every 10 nm within the desired wavelength range is usually sufficient to
improve wavelength accuracy. After a multipoint wavelength calibration,
wavelength accuracy will be ±10 pm within 10 nm of each calibration wave-
length.
Refer to “Calibrator Multi-Pt Align” on page 3-18 .
N o t e For a full explanation of external multipoint wavelength calibration, along with a
sample program to perform the calibration, refer to Product Note 86140-2,
Wavelength Calibration for the 86140X Series Optical Spectrum Analyzers
(Literature part number 5980-0043E).
The following steps outline one method for an external multipoint wavelength
calibration routine. This assumes a program executed on a external PC con-
troller. The steps outlined are those written in the program.
1 A signal is sent from a tunable laser source into a multi-wavelength meter and
the OSA simultaneously.
2 The wavelength of the input signal is measured on both instruments.
3 The two measured values are compared.
4 Taking the multi-wavelength meter readings as actual, the software calculates
the error offsets at each wavelength using the equation:
WL Error = (OSA indicated WL) – (multi-wavelength meter actual WL)
5 The previous steps are repeated over the entire wavelength range.
6 The data is averaged over narrow wavelength spans to provide a suitable
correction for each span. The example below demonstrates this technique.
Once the instrument is calibrated, the new wavelength accuracy can be main-
tained for many hours without recalibration, assuming a stable temperature
environment.
T i p If the OSA is turned off, the multipoint data will be retained at the next power-
on, but the internal thermal shift can introduce inaccuracies to the calibration
data. To help compensate for this, a single point calibration using the Offset
feature in the Wavelength Calibration Setup panel can be used to adjust the
multipoint data. Access this feature by selecting System > Calibration >