Download 512 QAM (54 Gbit/s) Coherent Optical Transmission over 150 km

512 QAM (54 Gbit/s) Coherent Optical Transmission over
150 km with an Optical Bandwidth of 4.1 GHz
Seiji Okamoto, Kazushi Toyoda, Tatsunori Omiya, Keisuke Kasai, Masato Yoshida and
Masataka Nakazawa
Research Institute of Electrical Communication, Tohoku University
2-1-1, Katahira, Aoba-ku, Sendai-shi 980-8577, Japan E-mail: [email protected]
Abstract We report the first demonstration of 512 QAM coherent optical transmission by using an
optical PLL. A polarisation-multiplexed 54 Gbit/s data signal was successfully transmitted at 3
Gsymbol/s with an optical bandwidth of 4.1 GHz including a tone signal.
SE exceeding 10 bit/s/Hz by using a frequencystabilised fibre laser and an optical phase locked
7,8
loop (OPLL) . In this paper, we report the first
512 QAM coherent transmission experiment, in
which
we
successfully
transmitted
a
polarisation-multiplexed 54 Gbit/s signal over
150 km within an optical bandwidth of 4.1 GHz.
Introduction
Recently, to cope with the rapid growth of
information
capacity,
coherent
optical
transmission with a high spectral efficiency (SE)
using a multi-level format has been intensively
studied1,2. Of the many multi-level formats,
quadrature amplitude modulation (QAM), in
which information is encoded on both amplitude
and phase, has attracted a lot of attention
because of its high SE compared with other
formats3,4. QAM has been applied to WDM
transmission with a record-breaking capacity, in
which 69.1 and 64 Tbit/s transmissions were
achieved using 16 and 36 QAM with SEs as
high as 6.4 and 8 bit/s/Hz, respectively.
To achieve further increases in the
transmission capacity, QAM with a higher
multiplicity could play an important role by taking
full advantage of the finite bandwidth over the Cand L-bands. Previously, we demonstrated 128
and 256 QAM transmissions that achieved an
Pilot
Intensity
Arbitrary
Waveform
Generator
Experimental
set-up
for
polarisation
multiplexed 512 QAM coherent optical
transmission
Figure 1 shows the experimental set-up for
polarisation-multiplexed 512 QAM transmission.
As a coherent light source, we used a 1.5 µm
acetylene frequency-stabilised fibre ring laser
9
with a linewidth of 4 kHz . The output of the
laser was divided into two paths, and one was
coupled to an IQ modulator. A 3 Gsymbol/s 512
QAM baseband signal was produced by an
arbitrary waveform generator (AWG) operating
at 12 Gsample/s and fed into the IQ modulator.
Amplifier
12 GSample/s
QAM Signal
2.03 GHz Optical Frequency
IQ Mod.
C2H2 FrequencyStabilised Fibre
Laser
PC
Att
EDFA
⊥ ～
Optical Filter
( 3 nm)
Att
OFS
PBC
(fOFS=2.03 GHz)
-4 dBm
75 km SLA
FBG (6 GHz)
75 km SLA
PBS
Raman Pump
(1.44 µm)
Att
Prec
Baseband Signal
Polariser
˚
90 Optical
Optical
Hybrid
Filter
PD
OFS: Optical Frequency Shifter
PBC: Polarisation Beam Combiner
FBG: Fibre Bragg Grating
SLA: Super Large Area Fibre
B-PD: Balanced Photo-Detector
DBM: Double Balanced Mixer
B-PD
A/D
B-PD
A/D
DBM
Digital Signal
Processor
Feedback
Circuit
Synthesiser
fsyn=2.03 GHz
Optical Filter (
～2.5 nm)
Local
Oscillator
(Tuneable Fibre Laser)
Fig.1: Experimental set-up for polarisation-multiplexed 512 QAM coherent optical transmission.
Transmission results
Figure 2 (a) and (b) show the optical spectra
of the QAM signal before and after 150 km
transmission, respectively. By using Raman
amplifiers, the OSNR degradation after
Resolution: 0.1 nm
0
-10
P ow er [dBm ]
-20
-30
Resolution: 0.1 nm
0
-10
40.5 dB
-40
-50
-60
-20
-30
36 dB
-40
-50
-60
-70
1538.2 1538.4 1538.6 1538.8
1539
-70
1539.2 1538.2 1538.4 1538.6 1538.8
Wavelength [nm]
1539
1539.2
Wavelength [nm]
(a)
(b)
Fig. 2: Optical spectra of the QAM signal before
and after 150 km transmission (a), (b).
Demodulation bandwidth
=4.05 GHz
20
10
0
Power [dB]
Pow er [dBm ]
We employed a root raised-cosine Nyquist
10
filter with a roll-off factor of 0.35 at the AWG as
well as in the digital signal processor (DSP) at
the receiver using software, so that the
bandwidth of the QAM signal was reduced to
4.05 GHz. In addition, a pre-equalisation
process was adopted to compensate for the
distortions caused by individual components
such as AWG and the IQ modulator by using a
finite impulse response (FIR) filter with 99
complex-valued taps. We also pre-compensated
individually for the nonlinear phase rotation
caused by self phase modulation (SPM) and for
the waveform distortion caused by chromatic
dispersion (CD) during transmission. The optical
QAM signal generated by the IQ modulator was
then orthogonally polarisation-multiplexed with a
polarisation beam combiner (PBC). The other
path from the laser output was led to an optical
frequency shifter (OFS), which fed a frequency
downshift of 2.03 GHz against the carrier
frequency as shown in the inset of Fig. 1. This
signal was used as the pilot tone signal required
for optical phase tracking of the local oscillator
(LO) under OPLL operation. The polarisation of
the pilot signal was aligned with one of the two
polarisation axes of the QAM signal. These
signals were combined and launched into a 150
km fibre link with a transmission power of -4
dBm. The lowest BER was obtained with this
transmission power, which is the optimum value
for minimizing impairments caused by S/N and
nonlinearity. The fibre link was composed of two
75 km spans of super large area (SLA) fibre, a
backward Raman amplifier, and an EDFA. The
SLA fibre had an average fibre loss of less than
2
0.19 dB/km, an effective area of 106 µm , a
dispersion of 20 ps/nm/km, and a dispersion
2
slope of 0.06 ps/nm /km. The Raman amplifier
provided 10 dB gain to the signal at each span.
The power of the pilot tone signal was set -25
dB lower than the QAM signal power.
At the receiver, the QAM signal was
homodyne detected at a 90-degree optical
hybrid after passing through a fibre Bragg
grating (FBG) optical filter with a 6 GHz
bandwidth and a polariser for ASE reduction. As
an LO, we used a frequency-tuneable fibre laser
whose phase was locked to the transmitted pilot
tone signal via the OPLL. After detection with
balanced photodiodes, the data were A/D
converted at 40 Gsample/s. Then, the QAM data
were demodulated with a DSP in an off-line
condition.
Pilot tone signal
-10
-20
-30
-40
-50
-60
-3
-2
-1
0
1
2
3
RF frequency [GHz]
Fig. 3: Electrical spectrum of the demodulated
signal at the DSP.
transmission was only 4.5 dB, and a value as
high as 36 dB was obtained. Figure 3 shows the
electrical spectrum of the demodulated signal at
the DSP. The demodulation bandwidth was set
at 4.05 GHz due to the adoption of a Nyquist
filter. The 54 Gbit/s data could be transmitted
within an optical bandwidth of 4.06 GHz
including the tone signal. To realise a QAM
transmission with a higher multiplicity, it is
essential to reduce the phase noise of the IF
signal. The phase noise of the signal was
estimated by integrating the single side band
(SSB) noise power spectrum of a beat signal
between the tone signal (fc - 2.03 GHz) and an
LO (fc) under OPLL operation. Under a back-toback condition, the phase noise was 0.3 degree,
whereas it increased to 0.75 degree after a 150
km transmission because of the OSNR
degradation. These values are less than half of
the tolerable phase noise of 1.8 degree for 512
QAM, which is determined by the phase
difference between the nearest symbols.
Figure 4 shows the BER characteristics of a
polarisation-multiplexed, 3 Gsymbol/s, 512 QAM
(54 Gbit/s) transmission. Figure 5 (a) and (b)
show the respective constellations of the 512
QAM signal before and after transmission at the
maximum received power. The power penalty at
Pol-MUX bb(⊥)
-1
10
Pol-MUX bb(//)
Pol-MUX 150 km (-4 dBm) (⊥)
Pol-MUX 150 km (-4 dBm) (//)
Theoretical bb curve (phase noise 0.3 degree)
-2
10
BER
FEC
threshold
-3
10
-4
10
-5
10
-35
-30
-25
-20
-15
Received Power [dBm]
(a)
Fig. 4: BER characteristis of poralisation-multiplexed,
3 Gsymbol/s, 512 QAM transmission over 150 km.
the BER of the forward error correction (FEC)
threshold (2 x 10-3) was about 8 dB for both
polarisations, but it was possible to achieve a
BER lower than the FEC threshold for both
polarisations. Without Raman amplifiers, the
maximum received power was limited to -23
-3
dBm, where the BER was as large as 3.9 x 10 .
The solid curve shows the theoretical BER
under a back-to-back condition including the
distortions caused by imperfect implementation
of the hardware and the phase noise of the IF
signal of 0.3 degree. The magnitude of the
distortion produced by the hardware was
calculated from the distributions of the
constellations shown in Fig. 5 (a). The
theoretical curve fits the experimental result well.
The power penalty after transmission is
attributed to cross phase modulation (XPM)
between the two polarisations, and imperfect
compensation for the interaction between SPM
and CD. The IF phase noise is also responsible
for the power penalty after a 150 km
transmission.
Conclusion
We described the first demonstration of a 512
QAM coherent optical transmission, where 54
Gbit/s data were successfully transmitted over
150 km with an optical bandwidth of 4.06 GHz
including a tone signal. The adoption of Raman
amplifiers contributed greatly to the realisation of
this high QAM multiplicity. The result indicates
the possibility of realising an ultrahigh SE of
12.4 bit/s/Hz in a multichannel transmission
even when taking account of the 7 % FEC
overhead.
(b)
Fig. 5: Constellations (32768 symbols) before (a) and
after 150 km transmission (b).
References
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