Polarization and differential-group-delay insensitive digital timing error detection for polarization-multiplexed coherent optical systems
View Patent ↗Digital timing error detection systems and techniques are described. The described techniques are independent of polarization and differential-group-delay and are used to perform timing recovery of polarization-multiplexed coherent optical systems.
1. A method for timing error detection in a polarization-multiplexed coherent optical receiver comprising:
in a timing error detector of the receiver, forming a timing error signal matrix:
[
TES
XX
TES
XY
TES
YX
TES
YY
]
each element of the matrix being the imaginary part of a corresponding receiver spectral correlation matrix;
acquiring initial timing error information from TES XX ; and,
tracking timing error information, starting from the acquired initial timing error information, at least in part by evaluating a timing error detector signal:
TED=cos( A )(TES XX −TES YY )−sin(A)(TES XY +TES YX )+(TES XX +TES YY ),
where A is a polarization rotation angle obtained from an adaptive equalizer of the receiver.
2. The method of claim 1 , further comprising acquiring the initial timing error information from TES XY when a result of the acquisition of the initial timing error information from TES XX is zero or undefined.
3. The method of claim 1 , further comprising timing an analog-to-digital converter of the receiver with a clock that is controlled based on the timing error detector signal.
4. The method of claim 1 , further comprising
correcting data timing errors of a data signal based on the timing error detector signal to obtain a corrected data signal; and
sending the corrected data signal to an adaptive equalizer of the receiver.
5. The method of claim 1 , wherein the elements of the timing error signal matrix comprise:
TES_xx
=
∑
n
=
0
N
2
-
1
R
X
(
2
n
)
·
[
R
X
(
2
n
+
1
)
-
R
X
(
2
n
-
1
)
]
*
,
TES_xy
=
∑
n
=
0
N
2
-
1
R
X
(
2
n
)
·
[
R
Y
(
2
n
+
1
)
-
R
Y
(
2
n
-
1
)
]
*
.
TES_yx
=
∑
n
=
0
N
2
-
1
R
Y
(
2
n
)
·
[
R
X
(
2
n
+
1
)
-
R
X
(
2
n
-
1
)
]
*
,
and
TES_yy
=
∑
n
=
0
N
2
-
1
R
Y
(
2
n
)
·
[
R
Y
(
2
n
+
1
)
-
R
Y
(
2
n
-
1
)
]
*
,
where R X and R Y correspond to data signals corresponding to X and Y polarizations, respectively.
6. A polarization-multiplexed coherent optical receiver comprising:
a coherent mixer, photodetector and analog-to-digital converter that convert incoming polarization-multiplexed optical data signals into electrical data signals;
a data receiver for demodulating and decoding the electrical data signals, the data receiver comprising
an adaptive equalizer that monitors a degree of polarization mixing in the incoming optical data signals; and,
a timing error detector operable in acquisition and tracking modes, the acquisition mode comprising acquisition of initial timing error detection signal from TES XX , and the tracking mode comprising evaluation of a timing error detector signal
TED=cos( A )(TES XX −TES YY )−sin( A )(TES XY +TES YX )+(TES XX +TES YY ),
where TES XX , TES XY , TES YX , TES YY , are imaginary parts of a corresponding receiver spectral correlation matrix, and A is a polarization rotation angle obtained from the adaptive equalizer as the degree of the polarization mixing.
7. The receiver of claim 6 , further comprising:
a clock that provides timing to the analog-to-digital converter, and
a timing correction block that corrects data timing errors before the data is sent to the adaptive equalizer, the timing correction block receiving a timing error signal from the timing error detector.
8. The receiver of claim 6 , further comprising a clock that provides timing to the analog-to-digital converter, the clock receiving a clock control signal from the timing error detector.
9. The receiver of claim 6 , where the acquisition mode further comprises acquisition of the initial timing error detector signal from TES XY in the event that a result of the acquisition of timing error detector signal from TES XX is zero or undefined.
10. The receiver of claim 6 , where the analog-to-digital converter and the data receiver including the adaptive equalizer and the timing error detector are implemented in a field programmable gate array.
11. The receiver of claim 6 , where the analog-to-digital converter and the data receiver including the adaptive equalizer and the timing error detector are implemented in an application specific integrated circuit.
12. The receiver of claim 6 , where the imaginary parts of the corresponding receiver spectral correlation matrix comprise:
TES_xx
=
∑
n
=
0
N
2
-
1
R
X
(
2
n
)
·
[
R
X
(
2
n
+
1
)
-
R
X
(
2
n
-
1
)
]
*
,
TES_xy
=
∑
n
=
0
N
2
-
1
R
X
(
2
n
)
·
[
R
Y
(
2
n
+
1
)
-
R
Y
(
2
n
-
1
)
]
*
.
TES_yx
=
∑
n
=
0
N
2
-
1
R
Y
(
2
n
)
·
[
R
X
(
2
n
+
1
)
-
R
X
(
2
n
-
1
)
]
*
,
and
TES_yy
=
∑
n
=
0
N
2
-
1
R
Y
(
2
n
)
·
[
R
Y
(
2
n
+
1
)
-
R
Y
(
2
n
-
1
)
]
*
,
where R X and R Y correspond to data signals corresponding to X and Y polarizations, respectively.