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OW1I.2.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Combined SDM and WDM transmission over
700-km Few-Mode Fiber
R. Ryf(1) , M. A. Mestre(1) , S. Randel(1) , X. Palou(1) , A. H. Gnauck(1) , R. Delbue(2) ,
P. Pupalaikis(2) , A. Sureka(2) , Y. Sun(3) , X. Jiang(3) , and R. Lingle, Jr.(3)
1 Bell
Laboratories, Alcatel-Lucent, 791 Holmdel-Keyport Rd, Holmdel, NJ, 07733, USA.
Corporation, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977, USA
3 OFS, 2000 Northeast Expressway, Norcross, GA 30071, USA.
Roland.Ryf@alcatel-lucent.com
2 LeCroy
Abstract: We experimentally demonstrate multiple-input multiple-output transmission over
a 700-km few-mode fiber of a combined 3-space-, 2-polarization-, and 34-wavelength-division
multiplex, using low-loss 3-spot mode couplers.
OCIS codes: 060.4510, 060.1660, 060.2280, 040.1880, 060.4230.
1.
Introduction
Space-division multiplexing (SDM) over a single fiber is currently under investigation for its potential to overcome the
capacity limit of single-mode fibers (SMFs). Few-mode fibers (FMFs) [1–4] have recently been proposed to overcome
the SMF shortcomings. In this work we experimentally demonstrate SDM transmission of 34 WDM channels spaced
at 50 GHz over 700-km FMF based on a 70-km span. To our knowledge this is the longest combined WDM/SDM
transmission distance reported yet in FMFs, more than an order of magnitude longer than the result reported in [2].
The record distance is achieved by using low-loss spot-based mode multiplexers (SMUX) [3] in combination with
a DGD compensated span. In our experiment, 20-Gbaud/s QPSK signals are multiplexed over 3 spatial modes, 2
polarizations, and 34 wavelengths, resulting in an aggregate capacity of 6.5 Tbit/s over a bandwidth of 1.7 THz.
The signals are recovered after 700 km transmission by off-line multiple-input multiple-output (MIMO) digital signal
processing (DSP) and we obtain a BER < 10−2 for all SDM and WDM channels.
Further, the impulse response and the mode dependent loss (MDL) of the 6 × 6 MIMO channel is analysed and
compared to previous single channel results [4] obtained with phase-plate-based mode multiplexer (PPB-MMUX),
showing that the system wide MDL is < 2.1 dB even after 700-km transmission.
2.
Three-spot based mode coupler
The spot based mode-coupler was first demonstrated for a FMF supporting 6 spatial and polarization modes (FMF6) in
[3]. The coupler consists in an equilateral triangular spot arrangement centered around the FMF core. This simple spot
arrangement can be used to sample the light coupled out from the FMF, and each spot will contain linear combination
of the fields (phase and amplitude) and polarization of the fiber modes. If the distance between the spots is chosen
correctly, it can be shown [3], that the linear transformation between the fields and polarizations in the spots and
the fiber modes is unitary, up to a common loss term. Unitary transformation can be undone completely by using
MIMO DSP and will therefore not cause any penalty for MIMO transmission. By choosing optimal spot diameter,
also the insertion loss of the coupler can be reduced and minimal insertion loss < 2dB is theoretically possible. Even
smaller insertion losses can be achieved if the spots are brought together by tapered fiber bundles so called “photonic
lanterns” [5,6]. In this experiment the SMUXs were implemented using 3 collimators, which are brought close together
by using three clipping mirrors (M1, M2, and M3), and the resulting spot pattern is then demagnified and imaged on
the FMF6 facet using a lens pair ( f1 = 100 mm and f2 = 3 mm) in double telecentric configuration (see Fig. 1 b).
Using individual collimators offers the advantage that spot coupler can be optimized to produce low loss and have a
transfer function that is very close to a unitary function. We used a phase-plate based coupler on the second end of the
FMF to analyze the modal content of the light coupled by each spot. For a correctly aligned coupler for FMF6, all spots
will have the same coupling efficiency and will all couple the same amount of power in all fiber modes (LP01 , LP11a ,
and LP11b ). The resulting experimental insertion loss of the spot couplers was 4.1 dB and 3.9 dB for the multiplexing
SMUX and the demultiplexing SMUX, respectively.
3.
DGD compensated FMF span
The FMF span with a total length of 70 km was realized by using a graded-index (GI)-FMF that supports exactly 6
spatial and polarization modes. The fiber characteristics are the following: The effective area was 64 µm2 and 67 µm2
for LP01 and LP11 modes, respectively, the absorption was 0.226 dB/km and 0.32 dB/km at a wavelength of 1.55 µm
and 1.45 µm, respectively, and the chromatic dispersion is 18.5 ps/(nm km) at 1550 nm. The span was built by splicing
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
6 × 6 coherent MIMO experiment
OW1I.2.pdf
97 ns
…
DFB
ECL
DFB
DN-MZM
Q
I
…
2ch – PPG
20 GBaud
Q
I
DN-MZM
DFB
49 ns
25 ns
O
Interleaver
E
Delay
generator
PD-CRX 0
ECL
LO
SMUX
LN-SW 1
BPF
PD-CRX 2
LeCroy 12 ch, 20 GHz, 40 GS/s
modular oscilloscope
BPF
0
SMUX
1
1
2
2
3x
Loop loading
signal
BPF
PD-CRX 1
70 km
FMF
0
LN-SW 2
PBS
Pattern sync
a)
LN-SW 0
Blocker
3 x 10:90
DFB
OFC/NFOEC Technical Digest © 2013 OSA
b) SMUX
SMF M2
port 1
SMF
port 2
SMF Periscope
port 0
M4
M3
M1
FMF
Side view
f2
f1
Lenses
Fig. 1. a) Experimental setup for coherent MIMO transmission. PBS: Polarization beam splitter.
BPF: Band-pass filter. Erbium-doped fiber amplifiers are denoted by triangles. b) Experimental setup
for a 3-spot mode multiplexer.
fiber spools with the following lengths (DGDs): 12.5 km (-2 ns), 25 km (1.64 ns), 12.5 km (-1.17 ns), and 20 km
(1.52 ns). The order of the spools was chosen to continuously compensate most of the DGD after each spool pair.
The maximal excursion in DGD occurring within theAll whole
span
is therefore
reduced to 2 ns, and the resulting total
Rights Reserved
© Alcatel-Lucent
201
DGD for the entire span is less than 200 ps based on the accuracy of our DGD measurement method. The DGD was
measured using a phase-plate-based mode coupler and a 100 ps test pulse that was selectively launched and detected
into different modes. The fiber spools were spliced together using a commercial fusion splicer and resulting span loss
including the SMUXs was 24 dB.
4. Coherent MIMO Transmission experiment
The MIMO transmission measurement was performed using the setup shown in Fig. 1 a). The wavelength channels on
a 50-GHz grid were generated by independently modulating odd and even wavelength from 34 distributed feedback
lasers (DFBs). Two double-nested LiNbO3 modulators (DN-MZM), driven by a two-channel programmable pattern
generator (PPG) were used to generated a 20-GBaud QPSK signal, and two independent De Bruijn bit sequences
(DBBS) of length 212 were used for the in-phase (I) and quadrature (Q) components of the QPSK signal, respectively.
We used an external cavity laser (ECL) with a line width of 100 KHz as the light source for the channel under test, and a
second ECL was used as a local oscillator (LO) in intradyne configuration. After traversing a polarization multiplexing
stage, that adds a orthogonally polarized copy of the signal delayed by 25-ns, the polarization multiplex QPSK signal
is divided into 3 paths with a relative delay of 49 and 97 ns, respectively.
The three delayed copies are then fed into a 3-fold recirculating loop. Three LiNbO3 switches (LN-SWs) are used to
control the loading and the closing of the loop. The loop consists of a pair of mode multiplexers (SMUXs) connecting
the 70 km compensated FMF span, and 3 two-stage Erbium-doped fiber amplifiers (EDFAs), where a multichannel
blocker is inserted between amplification stages in order to spectrally equalize the power in the loop. Note that the
three paths of the 3-fold loop have to be accurately aligned in order to avoid introduction of delays between loop paths.
The relative length of the loop was accurately tuned to within < 200 ps using optical delay lines.
Finally, three 10:90 couplers are used to extract the signals from the loop which are detected, after traversing three
EDFAs and three 0.6 nm bandpass filters, by three polarization-diverse coherent receivers (PD-CRX). The 12 electrical
signals from the PD-CRXs were captured by a modular digital storage oscilloscope (DSO) (LeCroy LabMaster 9zi)
operating at 40 GS/s with a bandwidth of 20 GHz. The waveforms captured by the DSO were processed off-line using
the MIMO DSP algorithm described in [7], which consists of a network of 6 × 6 feed-forward equalizers (FFEs) with
400 taps each. The FFE coefficients are determined according to the least-mean-square algorithm (LMS), and initial
convergence is obtained by data aided operation.
The resulting BER as function of the distance are shown for all SDM and WDM channels for an input power of
-3 dBm per mode-, wavelength-, and polarization in Fig. 2 c). After 700 km transmission the maximum observed BER
is < 10−2 , which can be compensated with a state-of-the-art forward error correction (FEC) with a 20% overhead.
| h | 2 (dB)
-20 70 km
| h | 2 (dB)
-20 140 km
| h | 2 (dB)
-20 350 km
-20 700 km
OFC/NFOEC Technical Digest © 2013 OSA
4
b)
MDL (dB)
-40
-60
-40
-40
-60
-4
SMUX
1
0
100
200
300
400
L (km)
500
600
700
-3
-4
10
-60
-5
2
-2
c)
-40
PPB-MMUX
3
0
-60
log (BER)
a)
| h | 2 (dB)
OW1I.2.pdf
-3
-2
-1
0
1
2
3
4
5
t (ns)
-5
-6
140 km
210 km
350 km
560 km
700 km
Fig. 2. a) Impulse response |h|2 of a DGD compensated FMF span at a transmission distance of 70,
140, 350, and 700 km. b) Mode dependent loss as function of the distance for spot-based and phaseplate based MMUX transmission experiments. c) BER as function of the transmission distance for
all 6 SDM and 34 WDM channels.
The impulse response as function of the distance was calculated according to a least-square error (LSE) channel
estimator. For a 3-spot SMUX, all of the 6 × 6 impulse responses
look
qualitatively
similar and only one representative
All Rights Reserved
© Alcatel-Lucent
201
impulse response is reported in Fig. 2 a). After 70 km a strong central double peak produced by the LP01 and LP11
modes is clearly visible at the center of the impulse response, surrounded by a 5-ns wide plateau generated by mode
coupling occurring in the fiber and at various splice locations along propagation in the DGD compensated FMF span.
For longer transmission distances, the impulse response becomes wider and becomes more Gaussian-like. In particular,
after 700 km, the impulse response starts to fill-up the whole 10 ns windows of Fig. 2 a), which also corresponds to
the maximum delay that the 400 tap equalizer is capable of compensating. This explains the reduced transmission
performance after 700 km. In particular, at that distance, the spread of the impulse response, is clearly wider than for
the single channel experiment [4] performed with the same kind of fiber and using phase-plate based mode couplers.
Possible explanation for the faster broadening of the impulse response in this experiment are the fact that the 70 km
span is not as well compensated as the 30 km span measured in [4]. In fact, after the first loop the central peak has
already a spread of ≈ 300 ps. Also it is more difficult to precisely align the relative loop delays using the SMUX,
because the impulse response offers fewer sharp feature that can be used to derive a precise delay measurement. The
width of the impulse response is one criterion to anticipate the performance of the MIMO channel. A second relevant
quantity is the MDL, which is obtained performing a singular value decomposition of the estimated channel matrix.
The MDL is then defined as the ratio between the largest and the smallest squared singular values. We have evaluated
the MDL for the SMUX and for the PPB-MMUX based transmission experiments, the results are reported in Fig. 2
b). The experiment based on the SMUX has initially a low MDL of 0.7 dB confirming the excellent alignment of both
SMUXs. As a function of the distance, both experiments accumulate approximately the same amount of MDL, of
about 0.2 dB/100km, which could indicate that the MDL is caused by the fiber, note however that in the PPB-MMUX
experiments the couplers are traversed more than twice more frequently because of the shorter 30 km span used.
In conclusion we have demonstrated a combined WDM/SDM transmission in few-mode fiber over a record distance
of 700 km. We transmitted 34 wavelength and 6 spatial and polarization modes resulting in an aggregate capacity of
6.5 Tbit/s over a bandwidth of 1.7 THz.
References
1.
2.
3.
4.
5.
6.
7.
R. Ryf et al., J. Lightwave Technol., 30 (4), 521 (2012).
N. Bai et al., Opt. Express, 20 (3), 2668 (2012).
R. Ryf et al., Proc. OFC 2012, PDP5B.5 (2012).
S. Randel et al., Proc. OFC 2012, PDP5C.5 (2012).
N. K. Fontaine et. al., Proc. ECOC 2012, Th.2.D.6 (2012).
S. G. Leon-Saval et. al., Opt. Express, 18 (8), 8430 (2010).
S. Randel et al., Optics Express, 19 (17), 16 697 (2011).
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