Method and arrangement for transmitting an optical transmission signal with reduced polarisation-dependent loss
The invention describes a method and an arrangement for transmitting an optical transmission signal with reduced polarization-dependent loss. A first transmission signal component and a second orthogonal transmission signal component of the optical transmission signal are transmitted with a time difference between said transmission signal components.
1. Method for transmitting an optical transmission signal Z(t) with reduced polarisation-dependent loss comprising:
generating a first transmission signal component Z U (t) and a second transmission signal component Z V (t) of the optical transmission signal Z(t) wherein Z V (t) and Z U (t) are orthogonal,
using the following pre-delaying step
(
Zu
(
t
)
Zv
(
t
)
)
=
(
u
(
t
)
v
(
t
-
D
)
)
,
wherein D is a time difference between said first and second transmission signal components Z U (t), Z V (t),
wherein u(t) and v(t) are derived or derivable from orthogonal signals S X (t) and S Y (t) by a step of rotation according to the following relationship:
(
u
(
t
)
v
(
t
)
)
=
R
(
Sx
(
t
)
Sy
(
t
)
)
where S X (t) and S Y (t) are signals that are orthogonally polarisation multiplexed and modulated according to first and second data signals, respectively, and R is a rotation matrix representing a rotation by a fixed angle,
wherein said rotation and said pre-delaying are carried out either in the optical domain, or
in the electrical or digital domain, by corresponding differential group delay predistortion component modulation signals for generating said transmission signal components Z U (t), Z V (t) directly by modulating allocated carriers, and
transmitting the optical transmission signal Z(t) with the time difference of D between Z U (t) and Z V (t).
2. The method according to claim 1 , wherein the first and second transmission signal components of the transmission signal are transmitted with equal power.
3. The method according to claim 1 , wherein the first and second transmission signal components are transmitted with a time delay of one half of a modulation section or one half of a modulation section and an integer multiple of a modulation section between the orthogonal transmission signal components.
4. The method according to claim 1 , wherein the optical transmission signal is amplitude, quadrature amplitude, frequency or phase modulated, or a combination thereof, or an orthogonal frequency division multiplex signal.
5. The method according to claim 1 , wherein
the first transmission signal component and the second transmission signal component of the transmission signal are generated by differential group delay (DGD) predistorted electrical component modulation signals derived from component modulation signals.
6. The method according to claim 1 , wherein
the method further comprises coding a first data signal to yield a first pair of values of a first complex value and coding a second data signal to yield a second pair of values of a second complex value;
the complex values are converted into (differential group delay) DGD pre-distorted component modulation values,
wherein said DGD pre-distorted component modulation values are converted into DGD pre-distorted electrical component modulation signals, and
the first transmission signal component and the second transmission signal component of the optical transmission signal are generated by said electrical DGD pre-distorted component modulation signals.
7. The method according to claim 1 , wherein
at a receiver the time difference between orthogonal transmission signal components of a received optical polarisation multiplex transmission signal is compensated to achieve reconstructed polarisation multiplex signal components.
8. The method according to claim 7 , wherein
the received polarisation multiplex signal transmission signal is coherently demodulated and sampled, and
the time difference of the orthogonal signal components represented by complex data signal values is compensated.
9. The method according to claim 8 , wherein
a butterfly filter is used for compensation of the time difference between the orthogonal transmission signal components of the transmission signal and for reconstructing transmitted data signals.
10. The method of claim 1 , wherein said fixed rotation angle is an angle of 45°, and said rotation matrix R is defined as
R
=
1
C
(
1
1
-
1
1
)
,
wherein C is a constant.
11. An arrangement for transmitting an optical signal with reduced polarisation-dependent loss comprising:
a transmission signal component generator for generating a first transmission signal component Z U (t) and a second transmission signal component Z V (t) of the optical transmission signal Z(t) wherein Z V (t) and Z U (t) are orthogonal,
using the following pre-delaying step
(
Zu
(
t
)
Zv
(
t
)
)
=
(
u
(
t
)
v
(
t
-
D
)
)
,
wherein D is a time difference between said first and second transmission signal components Z U (t), Z V (t),
wherein u(t) and v(t) are derived or derivable from orthogonal signals S X (t) and S Y (t) by a step of rotation according to the following relationship:
(
u
(
t
)
v
(
t
)
)
=
R
(
Sx
(
t
)
Sy
(
t
)
)
where S X (t) and S Y (t) are signals that are orthogonally polarisation multiplexed and modulated according to first and second data signals, respectively, and R is a rotation matrix representing a rotation by a fixed angle,
wherein said rotation and said pre-delaying are carried out either in the optical domain, or
in the electrical or digital domain, by corresponding differential group delay predistortion component modulation signals for generating said transmission signal components Z U (t), Z V (t) directly by modulating allocated carriers, and
a combiner for combining the first transmission signal component Z U (t) and the second transmission signal component Z V (t) to produce the optical transmission signal Z(t) with the time difference D between Z U (t) and Z V (t).
12. An arrangement for receiving a signal Z(t) transmitted by the arrangement of claim 11 , comprising:
means for constructing data values; and
means for reconstructing said first and second data signals.