EXFO PSO-200 Optical Modulation Analyzer - Manual (Page 35)

Setting Up the Optical Modulation Analyzer
Optical Modulation Analyzer 27
Using an External Local Oscillator
Using an External Local Oscillator
The External local oscillator (LO) option allows you to use your own laser
as a source for the local oscillator instead of the PSO-200 internal laser. This
is useful in the following cases:

When your signal source has an extremely low phase noise, such that
the overall phase noise becomes dominated by the internal LO source.
If you use as LO a laser source with equivalent (or better) phase noise
characteristics, you can reduce the total phase noise in acquisitions.

To accomplish self-homodyne measurements by using the same laser
source split between the signal and LO, offering identical phase noise
characteristics on both.
In order to cancel the influence of the source phase noise on the
measurements, and thus not requiring phase tracking, you must
ensure that the delays of the signal and LO paths will match precisely
(even more when your laser source has wider linewidth).
The external laser source must meet the specified power range (see
Technical Specifications on page 131) and must use
polarization-maintaining fiber (PMF).
When the external LO source is distinct from the signal source:

The ITU Channel parameter also applies to the external LO source, so
both sources must have the same frequency/wavelength.

The Linewidth parameter should be set to the wider laser source.