Coherent Modulation Transmitter | IQTX
Quantifi Photonics Ltd.
Version
1.04
69
Modulator adjustments
The modulator adjustments provide manual adjustment of the In-phase (I), Quadrature (Q) and optical
output signals. With the auto-bias control option, these biases will be automatically varied to
compensate for the errors below and if the user wishes to manually control the bias voltages, they can
do so with CohesionUI.
13.3.1
Modulator bias-I and bias-Q
The bias-I and bias-Q dials are
used to set the bias offset (
ΔA
)
for either the In-phase (I) or
Quadrature (Q) components of
the optical signal.
Changing the bias offset for a given component (I, Q) is related to reducing bias error within the
constellation, represented by
ΔI
and
ΔQ
in section 13.3.2 I & Q optical phase offset.
The bias offset is a measure of how far the center of the constellation is with respect to the ideal center
point. For QPSK and QAM modulation formats, the ideal center point is [0, 0] (I, Q). It is more convenient
to represent the bias errors in percentages to make it independent of the average optical power.
Additionally, the bias error can be separated
into orthogonal components (I and Q) as shown
in the equation below, to help identify the origin
of the bias error. Since the electrical to optical
transfer function of optical modulators is
typically non-linear, a 5% bias offset as
measured in the optical domain may not
necessarily correspond to a 5% error in bias
voltage.
Δ
I
Δ
Q
Bias (Amplitude)
offset
Δ
A