Simultaneous Pre-and Post-Turbulence Compensation of
Multiple Orbital-Angular-Momentum 100-Gbit/s Data
Channels in a Bidirectional Link Using a Single Adaptive-
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Citation
Ren, Yongxiong, Hao Huang, Guodong Xie, Changjing Bao,
Long Li, Nisar Ahmed, Yan Yan, et al. “Simultaneous Pre-and
Post-Turbulence Compensation of Multiple Orbital-AngularMomentum 100-Gbit/s Data Channels in a Bidirectional Link
Using a Single Adaptive-Optics System.” Frontiers in Optics
2013 Postdeadline (2013).
As Published
http://dx.doi.org/10.1364/FIO.2013.FW6B.6
Publisher
Optical Society of America
Version
Author's final manuscript
Accessed
Thu May 26 00:52:14 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/93884
Terms of Use
Creative Commons Attribution-Noncommercial-Share Alike
Detailed Terms
http://creativecommons.org/licenses/by-nc-sa/4.0/
Simultaneous Pre-and Post-Turbulence Compensation of Multiple
Orbital-Angular-Momentum 100-Gbit/s Data Channels in a
Bidirectional Link Using a Single Adaptive-Optics System
Yongxiong Ren1, Hao Huang1, Guodong Xie1, Changjing Bao1, Long Li1, Nisar Ahmed1, Yan Yan1, Moshe Willner1,
Martin P. J. Lavery2, Moshe Tur3, Mark Neifeld4, Samuel J. Dolinar5, Miles J. Padgett2, Robert W. Boyd6,
Jeffrey H. Shapiro7, and Alan E. Willner1
1.Dept. of Electrical Engineering, Univ. of Southern California, Los Angeles, CA 90089, USA, Email: yongxior@usc.edu
2. School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
3. School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel
4. Dept. of Electrical and Computer Engineering, Univ. of Arizona, Tucson, Arizona 85721, USA
5. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
6.Dept. of Physics and Astronomy, The Institute of Optics, Univ. of Rochester, Rochester, New York 14627, USA
7.Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Abstract: The simultaneous pre-and post-turbulence compensation of multiple orbital-angular-momentum
(OAM) modes propagating through turbulent channel is experimentally demonstrated in a bidirectional freespace optical (FSO) link by using a single adaptive-optics system. ©2013 Optical Society of America
OCIS codes: (050.4865) Optical vortices; 060.2330 Fiber optics communications; (060.4230) Multiplexing.
1. Introduction
Orbital angular momentum (OAM) has gained much interest recently due to its potential for efficiently multiplexing many
independent data streams in a space-division-multiplexing (SDM) communication system [1, 2]. Light beams carrying OAM
can be described in the spatial phase form of exp(ilφ) (l=0, ±1, ±2,..), in which φ refers to the azimuth angle and l determined
the OAM value [1, 3]. One key characteristic of beams carrying OAM is that they have a helical phase-front and “doughnut”
intensity shape when l≠0, and they lie within by an unbounded mode space (i.e., l can take any integer value) with each of
them orthogonal to all of the others [2, 3]. The orthogonality enables the efficient multiplexing and demultiplexing of the
spatially overlapping multiple OAM beams in SDM-based free space optical (FSO) links.
Experimental reports have demonstrated 2.56-Tbit/s on a single wavelength with 32 modes [2] and 100-Tbit/s on 42
wavelengths each with 24 modes [4]. However, a key challenge in OAM multiplexed FSO links is the ability to compensate for
atmospheric turbulence, given that the turbulence distortions will significantly degrade OAM beams and cause crosstalk among
adjacent channels [5-7]. Recently, it has been experimentally shown that turbulence compensation of multiple OAM beams can
be achieved in an unidirectional link by using a new adaptive optics (AO) compensation scheme at the receiver [8]. Meanwhile,
the reciprocity of the atmospheric turbulence [9] implied that this AO system could potentially be employed to pre-distort the
outgoing beams emitted from the receiver simultaneously, significantly simplifying the system complexity [10, 11].
In this paper, we demonstrate that by using a single AO system, the pre-and post-turbulence compensation of multiple
OAM beams could be achieved simultaneously in a bidirectional FSO link, in which the AO system is located at only one end
of the system such that one direction experiences pre-compensation and the other direction experiences post-compensation.
Rotatable phase screen plate with phase distribution obeying Kolmogorov spectrum statistics [12] is used to emulate
atmospheric turbulence. The experimental results suggests the link with post-compensation will achieve better performance
than the pre-compensated link by about >1 dB depending on the mode-spacing ∆ of the transmitted modes (>7 dB when ∆ =2).
2. Concept and Experimental Setup
The concept of simultaneous pre-and post-compensation of the OAM-based bidirectional FSO link is illustrated in Fig. 1.
Two groups of multiplexed data-carrying OAM beams (at TX-1 and TX-2) propagate through the atmospheric turbulence with
opposite directions. A recently-proposed AO system for OAM beams [8] is placed at one end of the system (TX-2), which is
used to compensate the received OAM beams from TX-1 and pre-distort the outgoing beams from TX-2 simultaneously.
Adaptive Optics Compensator
Beam Splitter
OAM Gen.
& Mux
Blue: TX/RX-1 link for post-comp.
Red: TX/RX-2 link for pre-comp.
OAM Gen.
& Mux
Wavefront
Corrector
OAM TX-1
OAM TX-2
Corrected OAM
Beams (TX-2)
Atmospheric
Turbulence
Feedback
controller
Wavefront
Sensor
Corrected OAM
Beams (TX-1)
Fig. 1. Concept diagram of the bidirectional link. The multiplexed beams from TX-1 and TX-2 experience post- and pre compensation respectively.
The experimental setup is presented in Fig. 2.Two groups of three OAM-multiplexed beams each carrying 100-Gbit/s
QPSK signal are created by using two spatial light modulators (SLM 1-2, the surface of each SLM is divided into two different
regions, one region generates one OAM beam, the other region generates two OAM beams by loading a specially-designed
spiral phase pattern). A Gaussian beam, which acts as a probe beam for wavefront sensing in the AO system, is expanded by
using a 4-f lens system to become as wide as the widest OAM beam along the propagation path and propagates collinearly with
OAM beams from TX-1. The multiplexed beams transmitted from TX-1 and TX-2 propagate with opposite directions through
the turbulence emulator, which consists of a rotatable stage and a phase screen plate. The phase screen plate with Fried’s
parameter r0= 1mm is used to emulate a 1 km turbulence channel. A closed-loop Shack-Hartmann-based AO system is placed
at TX-2 side to compensate the turbulence-distorted OAM from TX-1 and pre-distort the outgoing beam from TX-2 by using
the same correction pattern obtained from the Gaussian probe beam. Both the multiplexed beams from TX-1 and TX-2 are then
down-converted back into Gaussian beams respectively, which is then coupled into single mode fibers and sent for coherent
detections and off-line digital signal processing after passing through both the phase screen plate and the wavefront corrector.
HWP
Rotating Phase plates
TX/RX-1 link for post-compensation
TX/RX-2 link for pre-compensation
WFS
Laser
Feedback
Controller
OAM MUX-1
OAM MUX-2
Mirror
Mirror
SLM-1
QPSK
TX-1
1x2
OC
Fiber
PC
SLM-2
Wavefront
Corrector
BS
Col.
Turbulence Emulator
Fiber
Closed-Loop AO System
OAM Demux &
Coherent RX-2
Col.
QPSK
TX-2
PC
OAM Demux &
Coherent RX-1
Fig. 2. Experimental setup. AO: adaptive optics, BS: beam-splitter, Col.: Collimator, HWP: half wave plate, OC: optical coupler, PC: polarization
controller; SLM: spatial light modulator, WFS: wavefront sensor ( HASO from Imagine optic Inc.).
3. Experiment Results and Discussion
Fig. 3 shows the intensity profile and interference pattern of the OAM beams with/without pre- and post-compensation
respectively. From the far-field images, one can see that the distorted OAM beams (b1-2, c1-2) are efficiently compensated
(b5-6, c5-6) for both TX/RX-1 and TX/RX-2 links. The interference patterns shown in (b7-8, c7-8), in comparison with (b3-4,
c3-4) verify that the OAM wavefronts has also been corrected significantly. Note that the post-compensation for TX/RX-1 link
get better compensation performance than the pre-compensation experienced by TX/RX-2 link. The received power
fluctuations when transmitting OAM l=5 for both links with/without compensation is depicted in Fig. 4(a), indicating that the
received power after compensation could remain fairly stable. Fig. 4(b) further illustrates the distribution of power coupled into
the fiber over neighboring OAM modes when transmitting OAM l=5 with/without pre- and post-compensation for a specific
turbulence realization. One can see that after compensation, the power of transmitted mode l=5 dominates in the power
spectrum. The BER performance for channel l=5 with/without compensation under turbulence realization #10 when
transmitting 3 multiplexed channels is presented in Fig. 5 (a). The corresponding recovered constellations of 100-Gbit/s QPSK
signal are also shown in Fig. 5(b1-8), respectively. Considering that crosstalk is dependent on the separation of the transmitted
OAM modes, the performance under different OAM mode-spacings (∆ =2, 4) is also investigated. It is observed that the BERs
in the cases without compensation are extremely high (not shown in Fig. 5(a)), and the TX/RX-1 link with post-compensation
could achieve better performance than the pre-compensated TX/RX-2 link by > 1 dB depending on the mode-spacing.
XT = -5.0 dB
with post-comp.
without post-comp.
with pre-comp.
without pre-comp.
-35
-35
-2
-2
10
10
-30
-30
-25
-25
-3
2
3
4
5
6
7
8
9
Turbulence
Turbulence
Realizations
Power
distribution afterRealizations
post-compensation
for l = 5
-15
-15
-20
-20
-25
Power (dBm)
Power (dBm)
1
10
W/o Post-compensation
W/ Post-compensation
W/o Pre-compensation
W/ Pre-compensation
(b)
(a)
l= 3,5,7
∆=2
-35
-40
-40
00
11
22 33 44 55 66 77 88 99
OAM Channel l
OAM Channel l
B2B
Ch l = 3,B2B
7 On, Post-comp.
l = 3,
7 On, Post-comp.
Ch l = 1,Ch
9 On,
Post-comp.
l = 1,
9 On, Post-comp.
Ch l = 3,Ch
7 On,
Pre-comp.
l = 3,
7 On, Pre-comp.
Ch l = 1,Ch
9 On,
Pre-comp.
l = 1,
On, Pre-comp.
Ch l = 1,Ch
9 On,
w/o9Pre-comp.
Ch l = 1, 9 On, w/o Pre-comp.
-5
10 -5
10
l= 1,5,9
∆=4
10
10
Fig. 4 (a) the power (coupled into fiber) of
OAM l=5 under different turbulence
realizations, and (b) Power distribution over
neighboring OAM modes when transmitting
OAM l=5 under a specific turbulence state,
with/without pre-and post-compensation.
(b2)
(b5)
(b3)
(b4)
(b7)
w/ Comp.
w/o Comp. w/ Comp.
(b1)
[2] J. Wang et. al, NP 6, 488, (2012).
[5] J. Anguita et. al, AO 47, 2414 (2008).
[8] Y. Ren et. al, ECOC' 2013, We.3.D.1 (2013).
[11] J. Barchers et al, JOSA 19, 1779 (2002).
(b6)
(b8)
w/o Comp.
Fig. 5 (a) BER for OAM channel l=5 when
transmitting 3 multiplexed channels with different
mode spacing (l=3, 5, 7 or l=1, 5, 9), under turbulence
realization #10, and (b1-b8) The corresponding
QPSK constellations recovered for channel l=5,
with/without pre-and post-compensation.
(We acknowledge the support of DARPA under InPho program and Intel Labs University Research Office.)
References
[1] G. Gibson et. al., OE 12, 5448 (2004).
[4] H. Huang et. al, OFC'13, OTh4G.5 (2013).
[7] B. Rodenburg et. al, OL 37, 3735 (2012).
[10] R. Biérent et. al, OL 38, 2367 (2013).
XT = -8.7 dB
10
10 6 8 10 12 14 16 18 20 22 24 26
6 8 10 12OSNR
14 (dB)
16 18 20 22 24 26
Pre-compensation
Post-compensation
OSNR
(dB)
-45
Fig. 3: The far-field intensity and interference images of the
OAM beams (l = 3, 5) before (upper, b1-b4, c1-c4) and
after (lower, b5-b8, c5-c8) post-and pre-compensation,
respectively. The RMS (root mean square), PV (peak-tovalley) and Strehl ratio of wavefront for the probe beam
before/after compensation are also given.
XT = -13.5 dB
XT = -17.6 dB
-6
-6
-30
-30
-50
-50
FEC Limit
-4
10 -4
10
-20
-20
-15
-15
XT = -8.9 dB
10 -3
10
(a)
BER
BER
(dBm)
Power
Power
(dBm)
-40
-40
[3] A. M. Yao et. al., AOP 3, 161 (2011).
[6] Y. Ren et. al, CLEO'13, CM2G.4 (2013).
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[12] L. Andrews et. al, SPIE Press (2005).