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Details on the
measured signal :
This is a Differential Phase
Shift Keying (DPSK) measurement realized with the
AP2440A optical complex spectrum analyzer.
The blue
trace represents the temporal intensity variations
and the red trace represents
the temporal phase variations.
In this example, the DPSK signal was generated with a LiNbO3 Mach-Zhender
type intensity modulator biased at the null in its transmission curve
and driven at twice the switching voltage by a 10Gb/s NRZ 4 bits
pattern (1 1 0 0).
For every 180° phase transition, we can notice an intensity variation
caused by the intensity
modulator instead of a phase modulator.
AP2440A
benefits :
To our knowledge,the AP2440A is the only instrument able to
directly measure the temporal phase variations (no need for a DPSK demodulator).
So, it is now possible to measure DPSK modulation without the need of a DPSK
demodulator.
DPSK or other phase modulations can be measured at very high bit rates with the AP2440 : 10Gb/s, 40Gb/s, 80Gb/s, 160Gb/s, 320Gb/s and 640Gb/s.
Technical
information on DPSK modulation :
"In the early days of
optical communications, because of the immaturity of semiconductor
laser sources, the optical phase was not stable enough for phase-based
modulation schemes. Recent rapid improvement of single-frequency laser
sources and the application of active optical phase-locking make PSK
feasible in practical optical systems. Specifically, differential-phase-shift-keying
(DPSK) is the most often used format.
A digital “1” is represented by a phase change between the consecutive
data bits in the optical carrier, while there is no phase change between the
consecutive data bits in the optical carrier for a digital “0”."
Reference : "Advanced Optical Modulation Formats in High-speed Lightwave System by Sen Zhang"