C coherent detection and sampling methods, Coherent detection – EXFO PSO-200 Optical Modulation Analyzer User Manual
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Optical Modulation Analyzer
239
C Coherent Detection and
Sampling Methods
Coherent Detection
Coherent receivers are sensitive to the optical field of the detected (data)
signal, and hence can be utilized to receive signals modulated in
amplitude, phase, and/or frequency. Coherent receivers include a local
oscillator (LO), which is a CW reference signal that beats with the data
signal on the photodetector.
The LO can either have the same frequency as the optical carrier, known
as homodyne detection, or a frequency offset from the carrier. If the
frequency offset is less than the signal bandwidth, it is known as intradyne
detection, whereas a frequency offset larger than the signal bandwidth is
referred to as heterodyne detection. In the PSO-200, an internal LO is set
near the signal carrier, resulting in intradyne detection.
By using an optical 90° hybrid, where the signal and LO are mixed, together
with balanced detectors, the in-phase (I) and quadrature-phase (Q)
components are given directly by
where:
E
s
(t) and E
LO
are the signal and LO optical fields
IF
=
s
–
LO
is the signal and LO intermediate frequency (IF)
s
(t) is the modulated signal phase
LO
is the relative phase of the LO
I t
( )
4 E
s
t
( )
E
LO
IF
t
s
t
( )
LO
t
( )
–
+
cos
=
Q t
4 E
s
t
( )
E
LO
IF
t
s
t
( )
LO
t
( )
–
+
sin
=