PRACTICAL 210GBIT/S POLARIZATION
DIVISION MULTIPLEX TRANSMISSION
SYSTEM
D. Sandel, S. Hinz, R. Noé, F. Wüst
Univ. Paderborn, Electrical Engineering and Information Technology, Optical Communication and HighFrequency Engineering, Warburger Str. 100, 33098 Paderborn, Germany (noe@uni-paderborn.de)
Abstract: Polarization division multiplex allows to upgrade existing trunk lines with limited amplification
bandwidth. Endless polarization tracking by electrooptic polarization transformers is demonstrated in the presence
of polarization-dependent loss.
Introduction
For capacity doubling of existing trunk lines with given
optical bandwidth, polarization division multiplex
(PolDM) /1/ is advantageous /2/ due to its superior
sensitivity, good polarization mode dispersion (PMD)
tolerance and terminal equipment simplicity. In previous
work /1/ both clock and optical frequencies differed
between the two channels, neither of which is allowed in
practice. Also, polarization tracking was not demonstrated
to be endless in that particular experiment. We have
eliminated the shortcomings and demonstrate PolDM
transmission in the presence of polarization-dependent loss
(PDL) /3/ and endless polarization fluctuations.
Transmission setup
For generation of a 210Gbit/s PolDM signal, one optical
10Gbit/s signal from transmitter TX was split 1:1 and
recombined in a polarization beamsplitter (PBS) with
orthogonal polarizations, after delaying one branch signal
by 50ns (Fig. 1). The signal passed a motorized
polarization transformer (MPT) with 4 endlessly rotating
fiberoptic /4 plates. It was transmitted through attenuators
(not shown), EDFAs and a bandpass filter. At the receiver
the signal was split 1:1. Each branch contained a
commercial electrooptic polarization transformer (EPT), a
fiberoptic polarizer (POL) and a 10Gbit/s photoreceiver.
Clock recovery was also implemented.
polarized signals at the TX, and  is a retardation angle
representing subsequent mode conversion.
Ignore the dotted lines of Fig. 1 for the time being.
Crosscorrelation in a multiplier of one photocurrent i1,i2
with the information bit b2 , b1  0,1 available at the other
decision circuit output allows to obtain error signals
b2 i1 , b1i 2 where  signifies averaging. Each EPT is
adjusted so as to minimize one crosscorrelation product.
This eliminates interchannel crosstalk,   0 .
Results
As an example, Fig. 2 shows sections of the PRBS
sequence at the two decision circuit inputs. Since
polarizations are misaligned and control is off, massive
interchannel interference occurs during b1  b2  1 . Angle
 was modulated in the TX by applying a small laser
frequency modulation, like for SBS suppression. Measured
DOP was 0.02. Here FM was converted to differential PM
in the 50 ns delay line, but differential PM would also exist
if two independent laser sources were used for the two
polarizations.
Figure 2: Detected signals, polarization control off
Figure 1: 210Gbit/s PolDM transmission setup
POL
MPT
PBS
EPT
EPT
TX
POL
50 ns
In the case of orthogonal
photocurrents are
analyzed
polarizations,
i1  b1 cos  2  b2 sin  2  2b1b2 sin  sin 
2
2
,
i2  b1 sin 2  2  b2 cos 2  2  2b1b2 sin  sin 
where  is a phase difference between the orthogonally
Signal acquisition was straightforward. Polarizations were
correctly set even with unlocked clock PLL since decisions
at random times contained enough useful information at the
multiplier inputs. Polarization lock was always acquired in
fractions of a second. The /4 plates were rotated with
different speeds and directions, resulting in 1rad/s
polarization change speeds on the Poincaré sphere. Clear
signals were obtained (Fig.3) since each electrooptical
polarization transformer tracked the minimum of a
crosscorrelation product. Eye diagrams are shown in Fig. 4.
Increased noise in the upper traces is due to a sampling
head with wider bandwidth. MPT-induced “breathing” of
one channel at the expense of the other allowed to estimate
a PDL of 1.8dB, mostly due to the EDFAs. Transmission
was nearly error-free (1 error in 45 min in a 2.5Gbit/s
eliminated, and if one microcontroller were provided per
channel.
PolDM detection is also possible if each receiver branch
maximizes the autocorrelation product b1i1 , b2 i 2
between its decision circuit input and output signals (dotted
lines in Fig. 1). Corresponding signal traces and eye
diagrams are shown in Fig. 6 and are virtually identical to
the left half of Fig. 3 and Fig. 4 right, respectively.
demultiplexed data stream, i.e. BER = 1.5  10 13 ).
Figure 6: Signals with maximized autocorrelation
rather than minimized crosscorrelation products
Figure 3: Detected signals, polarization control on,
while tracking endless polarization changes
Figure 4: Eye diagrams corresponding to Fig. 2 (left)
and Fig. 3 (right)
The discussed schemes have their pros and cons: Assume
one PolDM channel fails at the transmitter side. If
autocorrelation products are used both receiver branches
will receive the surviving channel, but identical channel
locking happens with probability 1/2 also when both
PolDM channels are present. On the other hand,
crosscorrelation products warrant that the receiver branches
lock onto different channels (which could be identified by
overhead bits) but detection of one surviving channel is not
assured. These statements have also been verified
experimentally.
Conclusions
A decision-circuit threshold was scanned. Extrapolation of
measured Q values 3 yielded Q factors of 11.2 back-toback and 10 with slowly rotating MPT, respectively
(Fig. 5).
Figure 5: Q factors back-to-back (top), and with
rotating MPT (bottom; vertically offset)
12
10
13
Tracking
polarization
fluctuations
Fixed
polarizations
11
210Gbit/s PolDM transmission has been demonstrated
with equal optical and clock frequencies in the two
channels, the first time to our knowledge. Signals are
acquired using correlation products. PDL (1.8 dB) and
endless polarization changes (1rad/s speed) in the
transmission fiber are supported.
Acknowledgement
We would like to thank Siemens ICN for providing
decision circuits and funding work on polarization control.
Deutsche Forschungsgemeinschaft is acknowledged for
funding work on PolDM.
References
/1/
8
9
6
7
4
5
2
3
/2/
/3/
threshold
Each polarization transformer was accessed in intervals of
1.2ms. A 10fold speed increase could be obtained without
shortening the measurement intervals if data transfer times
to an external PC (presently 80% of the time) were
F. Heismann et al., Automatic Polarization
Demultiplexer
for
Polarization-Multiplexed
Transmission Systems, WeP9.3, pp. 401-404, ECOC
1993, Montreux, Switzerland.
Noé, R., Sandel, D., Wüst, F., Optical Fiber
Communications Conference 2000, Baltimore,
Maryland, USA, WL4, 5-10 March 2000
L.J. Cimini, I.M.I. Habbab, R.K. John, A.A.M. Saleh,
"Preservation of polarization orthogonality through a
linear optical system", Electronics Letters, Vol. 23,
1987, No. 25, pp. 1365-1366