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IEEE Computer Society LMU Event
Tbit/s Optical Communications using Orbital Angular Momentum
Alan E. Willner
University of Southern California, Los Angeles, CA 90089, USA
We acknowledge the support of DARPA
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Thank You!!!!
… to Dr. Mehrdad Sharbaf for his kind invitation. … to all my wonderful students and colleagues.
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USC’s OCLab Family
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Ø Wavelength-division multiplexing (WDM)
Ø Time-division-multiplexing (TDM)
Ø Polarization-division multiplexing (PDM)
Ø Space-division multiplexing (SDM)
Ø Free-space communication links using orbital angular
momentum (OAM) modes 1. OAM eigenstates are orthogonal 2. Potential for high spectral efficiency and capacity
Motivation: OAM Modes
4
Miscellaneous Multiplexing Approaches
Mode-Division Multiplexing
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High capacity communications using OAM beams
v Increase system capacity using beams carrying OAM v Use multiple orthogonal approaches
High Capacity
Polarization- division-
multiplexing
Wavelength- division-
multiplexing
OAM mode-division- multiplexing
OAM with +/- charge multiplexing
5
Space-division-multiplexing
(Concentric rings)
Higher-order modulation formats
(multiple bits/symbol)
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Outline
1. OAM Beam (De)multiplexing
2. Optical Communications using OAM
- Pol-muxing, WDM
- MIMO
3. Turbulence Emulation and Compensation
4. OAM in Vortex “Ring” Optical Fibers
5. OAM-Based Networking Functions
- Exchange, ROADMs
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7
-10 -8 -6 -4 -2 0 2 4 6 8
x 10-4
-10
-8
-6
-4
-2
0
2
4
6
8
x 10-4
[m]
[m]
-10 -8 -6 -4 -2 0 2 4 6 8
x 10-4
-10
-8
-6
-4
-2
0
2
4
6
8
x 10-4
[m]
[m
]
-10 -8 -6 -4 -2 0 2 4 6 8
x 10-4
-10
-8
-6
-4
-2
0
2
4
6
8
x 10-4
-10 -8 -6 -4 -2 0 2 4 6 8
x 10-4
-10
-8
-6
-4
-2
0
2
4
6
8
x 10-4
l = 0 l = 1
l = 2 l = 3
# of states possible = infinite! , …. ( theoretically)
l = …. -3, -2, -1 ,+1, +2, +3 ….
o Intensity null at the center o Phase spirals ‘l’ times over distance of one wavelength
‘No OAM’
Orbital Angular Momentum – LG Beams : Concept
# of states possible = 2
and
# of states possible = 2
and
Linear Polarized Light Circularly Polarized Light
Orbital Angular Momentum
Alison M. Yao, et al. Adv. in Opt. & Phot., 2011
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8
Generation and Detection of OAM (LG beams) : Concept
Incoming Gaussian beam
Converted into LG beam, carrying OAM
Incoming LG beam
Re-converted into Gaussian
Spatially de-multiplexed
@ Transmitter
@ Receiver
Holographic phase filter
Holographic phase filter
Desired spatial phase function
Grating structure
Alison M. Yao, et al. Adv. in Opt. & Phot., 2011
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General Concept of OAM Link
A. Willner, Science, Aug. 10, 2012
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OAM (0,-8)
OAM (0,+10)
OAM (0,+12)
OAM (0,-14)
Beam Splitter-Based
Multiplexing
SuperImposed OAMs
OAM (0,-8)
OAM (0,+12)
OAM (0,-14) Data 1
Data 3
Data 2
Data 4 OAM (0,+10)
Intensity Phase
Intensity Phase
Intensity Phase
Intensity Phase
Intensity
Charge “-8”
Charge “+10”
Charge “+12”
Charge “-14”
Data 1
Data 2
Data 3
Data 4
Phase Pattern Applied to Gaussian
Gaussian
Gaussian
Gaussian
Gaussian
SLM-Based OAM Generation
Concept: Generation and Multiplexing of Multiple OAMs
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OAM Demuxing by Geometrical Transformation: Mode Sorter
• Geometric transformation produces a spot, the lateral position of which is proportional to the input OAM state.
• Two optical components, (b) and (c), are required to carry out this transformation.
M. Lavery and M. Padgett
-1, -3, 0, +1, +3 “Loss-less”
Demux 0
-3-to-0
+1-to-0
-1-to-0
+3-to-0
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Multi-level Modulaton Formats in Optics
Benefits from coherent detection: • More effective for pol-demuxing • Digital processing for mitigation
1 bit/symbol 2 bits/symbol 4 bits/symbol
~112 Gbaud OOK DPSK
DB/PSBT
~56 Gbaud DQPSK (4-ASK)
~28 Gbaud PDM-(D)QPSK
(16-DPSK)
Im{Ex}
Re{Ex} ( )
Im{Ex}
Re{Ex}
Im{Ex}
Re{Ex}
Im{Ey}
Re{Ey}
8 bits/symbol
~14 Gbaud 16-QAM
8-PSK/2-ASK
Im{Ex}
Re{Ex}
Im{Ey}
Re{Ey}
Reference: R. Essiambre and P. Winzer, Alcatel/Lucent
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12.8-bit/s/Hz Spectral Efficiency"Phase Patterns Loaded to SLM1-SLM4"
Charge “+8”" Charge “+10”" Charge “+12”" Charge “+14”"
SLM1" SLM2" SLM3" SLM4"
Intensity Profiles of OAM Modes"
OAM (0,-8)" OAM (0,+12)" OAM (0,-14)"OAM (0,+10)"Superimposed
Four OAM Modes"OAM(0,-8) Demultiplexing"
Only SLM1 On"Without crosstalk"
Only SLM1 Off"Crosstalk only"
SLM1-4 On"With crosstalk"
Only SLM4 On"Without crosstalk"
Only SLM4 Off"Crosstalk only"
SLM1-4 On"With crosstalk"
OAM(0,-14) Demultiplexing"
13 J. Wang, Nature Photonics, June, 2012
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Optical Spectra"
12.8-bit/s/Hz Spectral Efficiency"
BER Performance"Four OAM Modes: OAM(0,-8), OAM(0,+10), OAM(0,+12), OAM(0,-14) "
q Without crosstalk: only the desired OAM mode is present while others are off."q With crosstalk: all OAM modes are on when demultiplexing the desired OAM mode."q The crosstalk of demultiplexing for OAM(0,-8), OAM(0,+10), OAM(0,+12), and " OAM(0,-14) is measured to be -24.7, -24.0, -20.4 and -24.0 dB, respectively."q Less than 1.2 dB OSNR (optical signal-to-noise ratio) penalty is measured at a BER " of 2e-3 without crosstalk."q A total OSNR penalty of less than 2.2 dB at a BER of 2e-3 is observed with crosstalk. "14 J. Wang, Nature Photonics, June, 2012
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12.8-bit/s/Hz Spectral Efficiency"
Constellation: Demultiplexed OAM Modes (Without Crosstalk)"
OAM(0,-8)" OAM(0,+10)" OAM(0,+12)" OAM(0,-14)"
Constellation: Demultiplexed OAM Modes (With Crosstalk)"
OAM(0,-8)" OAM(0,+10)" OAM(0,+12)" OAM(0,-14)"EVM: error vector magnitude!15
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Diagram: Generation/Mux/Demux of Pol-Muxed Multiple OAMs"
q Spatial light modulators (SLM) convert input Gaussian mode to OAM modes."q Inverted phase pattern is used to demultiplex OAM modes."q OAM+8 and OAM-8 have the same size of intensity profiles."
16
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q We measure the crosstalk of OAM demultiplexing from superposed" pol-muxed OAM modes, which is assessed to be less than -18 dB."
17
Crosstalk Measurement"
Pol-Muxed Four OAM Modes: OAM+4, OAM+8, OAM-8, OAM+16"
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Spatial DE-MUX
Beam splitter
Spatial-filter
Concept: Spatial-Multiplexed OAMs (Concentric Rings)
Multiple OAMs
Multiple OAMs
Same charge, different data
q Different data streams can be encoded on two group of OAM beams, with the same
charge but different ring radius.
q A spatial filter (or a specially designed grating) can be used to demux the inner ring and
the outer ring
Outer ring
Inner ring
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Lens 400mm Lens
(a)
(b)
EDFA
BS 1x4 OC
SLM-‐1 HWP Col
EDFA BPF A> EDFA
SLM-‐5 (Demux)
I/Q Mod.
Laser BPF PC
16QAM GeneraIon
Fiber
SLM-‐2
SLM-‐3
SLM-‐4 Mirror
Camera
PolarizaIon Diversity 90
0-‐Hybrid
ADC
ADC
ADC
ADC
Off-‐line DSP
EDFA
LO
Col
QPSK 16QAM
OSA 1% tap
2 x 20 Gbit/s
BS
BS
6dB a>en.
Φ
BS BS
BS BS
PBS PBS
Lens Lens 1:2
expansion
3:1 compression
Lens
SpaIal filter
HWP
DE-‐MUX
Pol.
Coherent DetecIon
HWP
MR MR MR MR MR
Mode-‐MUX +/-‐ Charge Pol-‐MUX Space-‐MUX
MUX
Ø 32 data channels carried by OAM beams are multiplexed in two concentric rings. Ø Each channel uses 20 Gbaud/s 16-QAM signal. Ø The total capacity is 2.4 Tbit/s, with a spectrum efficiency of 95.7 bit/s/Hz considering 7% FEC overhead (102.4 bit/s/Hz without considering 7% FEC overhead)
Experimental Setup of 32 OAM Channels Multiplexing Toward Spectral Efficiency of ~95 bits/sec/Hz
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OAM +10 OAM +12 OAM +14 OAM +16
4 modes
Inner ring a_er demux Out ring a_er demux
MUX/DEMUX Images
8 modes 16 modes 32 modes
Pol-‐muxed OAM ±10~ ±16
OAM ±10~ ±16 OAM +10~+16 Two rings of Pol-‐muxed OAM ±10~ ±16
Ø Two concentric rings are demultiplexed using a spatial filter.
SpaIal filter SpaIal filter
J. Wang, Nature Photonics, June, 2012
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1549.4 1549.6 1549.8 1550.0 1550.2 1550.4 1550.6-‐60
-‐50
-‐40
-‐30
-‐20
-‐10 w/ c ros s talk w/o c ros s talk
Power
(dBm)
Waveleng th (nm)
30dB
25GHz
Optical Spectrum of the 20 Gbaud/s16-QAM Signal after Demultiplexing
Ø Considering 20Gbaud 16-QAM signal on 32 OAM channels with 7% FEC overhead, an aggregate SE of 95.7 bit/s/Hz (80*32/25/1.07) is achieved. (SE= 102.4 bit/s/Hz without considering 7% FEC overhead) Ø Total capacity of 2.56Tbit/s on a single wavelength.
J. Wang, Nature Photonics, June, 2012
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(d) EVM 9.6(b) EVM 8.68 (c) EVM 9.5
(a) EVM 7.5
Outer ring x-‐pol
OAM +10
(f) EVM 8.5(e) EVM 8.4 (g) EVM 8.8Inner ring y-‐pol
OAM -‐16
Back to back
No crosstalk Single ring Two rings
Recovered Constellations of Two Typical Channels (Inner ring OAM -16 & Outer ring OAM +10)
Ø Recovered Constellations shows a very little penalty due to the concentric ring multiplexing.
J. Wang, Nature Photonics, June, 2012
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23 23
High capacity data link using OAM and WDM
OAM value
Wavelength Independent channels
Concept of combining OAM and WDM
Ø OAM multiplexing and WDM are in two different domains (spatial and wavelength), and they are compatible in a transmission link."Ø Combining OAM and WDM increases available independent channels and consequently the transmission capacity."
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24 24
Procedure of generating 24 OAM beams
OAM value
+4 +7 +10 +13 +16 +19
+4 +7 +10 +13 +16 +19 -‐4 -‐7 -‐10 -‐13 -‐16 -‐19
Y-‐pol
X-‐pol
OAM value
+4 +7 +10 +13 +16 +19 -‐4 -‐7 -‐10 -‐13 -‐16 -‐19
+7 +13 +19
+4 +10 +16 OAM value
OAM value
OAM value
SLM1
SLM2
(a)
(b)
(c)
Ø Each SLM generate 3 OAM modes that are far way from each other, so that their internal crosstalk to each other is <-32dB."Ø Mirrors and half-wave plates are used to generate reversed charged and polarization multiplexed OAM beams."
6 modes
12 modes
24 modes
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25
-‐50
-‐40
-‐30
-‐20
-‐10
0
Power
(dBm)
P ower of des ired OAM c hannel P ower from all other OAM c hannels
-19 -13 -7 +7 +13 +19 X-pol Y-pol OAM value
-19 -13 -7 +7 +13 +19
-16 -10 -4 +4 +10 +16 -16 -10 -4 +4 +10 +16
Measured power distribution of each OAM channel.
XTalk
Ø The difference between the power of desired OAM channel and the power from all other OAM channels indicates the external crosstalk."Ø The measured largest crosstalk is -16.2 dB.
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1530 1540 1550 1560 1570-100
-80
-60
-40
-20
Wavelength (nm)
Powe
r (dB
m)
demux OAM+10 from OAM+4,+10,+16
demux OAM+13 from OAM+4,+10,+16
XTalk
Optical spectrum after demux (x-pol, OAM+10)
The wavelength dependence for the crosstalk is very small over the wavelength range from 1530-‐1570 nm.
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BER and OSNR for all 1008 channels
0 100 200 300 400 5008
7
6
5
4
3
2
1
B C
C hannel Number
-‐log10(B
ER)
E F E C thres hod
10
15
20
25
30
35
40
OSNR (d
B)
500 600 700 800 900 10008
7
6
5
4
3
2
1
B C
C hannel Number
-‐log10(B
ER)
E F E C thres hod
10
15
20
25
30
35
40
OSNR (d
B)
BER
OSNR
BER
OSNR
FEC threshold FEC threshold
X-‐pol Y-‐pol
Ø The measured BER and OSNR for all 1008 (42×24) channels. "Ø All of the channels can achieve a BER of < 3.8×10−3 (the limit for 7% overhead FEC)
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FEC Codes in OAM Mode Multiplexed System
Motivation: LDPC Codes — Capacity Approaching Codes
Ø LDPC coding can help system performance.
Experiment Block Diagram
LDPC Coded 100-Gbit/s
QPSK"
4 OAM modes multiplexing using SLMs
OAM demulitplexing"
using SLM"
Off-line DSP and
LDPC decoding"
λ1 Free-space
transmission"
q Data sequence is encoded through LDPC encoder and fed into 100-Gbit/s transmitter"q OAM multiplexing: 4 OAM modes are multiplexed using 4 SLMs"q Demultiplexing: An SLM converts one of the multiplexed OAM modes back to the Gaussian beam"q The converted Gaussian beam is coupled into fiber for detection and followed by off-lined processing and decoding"
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Experimental Results: BER Vs OSNR for OAM+12 and OAM+14 Channels
6 8 10 12 14 16 181E -‐6
1E -‐5
1E -‐4
1E -‐3
0.01
0.1 +12, no c rk, no c d +12, no c rk, w/ c d +12, w/ c rk, no c d +12, w/ c rk, w/c d +14, no c rk, no c d +14, no c rk, w/ c d +14, w/ c rk, no c d +14, w/ c rk, w/c d
BER
O S NR (dB )
q The BER performance for demuxed OAM+12/OAM+14 with and without crosstalk."q The coding gain of LDPC(8547, 6922) is about 5.8 dB, 7.0 dB/5.7 dB, 7.3 dB at BER= 10-5 for OAM+12 / OAM+14 without and with crosstalk. "
Coding Gain
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4×4 MIMO Equalization for OAM crosstalk Mitigation
Distorted OAM l1
Distorted OAM l2
Distorted OAM l3
Distorted OAM l4
OAM l1
OAM l2
OAM l3
OAM l4
Received power
MIM
O
Equalization
OAM mode
OAM model1 l1 l2 l3l4l5
Power Power
Pure OAM mode Distorted OAM mode
(a) (b)
OAM l1
OAM l2
OAM l3
OAM l4
MUX
Propagate
Transmitted power
Concept of MIMO for OAM multiplexed system
Distorted OAM l1
Distorted OAM l2
Distorted OAM l3
Distorted OAM l4
OAM l1
OAM l2
OAM l3
OAM l4
Received power
MIM
O
Equalization
OAM mode
OAM model1 l1 l2 l3l4l5
Power Power
Pure OAM mode Distorted OAM mode
(a) (b)
OAM l1
OAM l2
OAM l3
OAM l4
MUX
Propagate
Transmitted power
H. Hao et al., ECOC 2013
EDFA
I/QMod.
Laser
BPFPC
QPSK Generation
QPSK
2 x 10 Gbit/s
OAM+2 Genera. OAM+4Genera.
OAM+6Genera.
OAM+8 Genera.
DEMUXOAM+2 DEMUX OAM+4
DEMUX OAM+6 DEMUX OAM+8
ADC
ADC
ADC
ADC
Offline processing
LOOAM Generation/MUX/DEMUX
OAM
MU
X
Free-spacetransmission
OC
OC
OC
OC
Heterodyne Detection
SMF
SMF
SMF
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Experimental Results —MIMO Equalization
CH1 CH2. CH3 CH4
CH1 CH2. CH3 CH4Without MIMO Equalization
With MIMO Equalization
21 22 23 24 25 26 276
5
4
3
2
1
w/o MIMO w/ MIMO , 7 taps w/ MIMO , 21 taps
-‐log10
(BER)
O S NR (dB )
F E C thres hold
Recovered constell. with and w/o MIMO BER Vs. OSNR under different taps
q The crosstalk on each channel is ~-19 dB, ~-12 dB, ~-11dB and ~-7 dB.
H. Hao et al., ECOC 2013
Measured crosstalk for this channel is ~ 12.5 dB.
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Outline
1. OAM Beam (De)multiplexing
2. Optical Communications using OAM
- Pol-muxing, WDM
- Coding, MIMO
3. Turbulence Emulation and Compensation
4. OAM in Vortex “Ring” Optical Fibers
5. OAM-Based Networking Functions
- Exchange, ROADMs
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OAM Propagation in Atmospheric Turbulence
Turbulence Emulator
Atmospheric Turbulence
Rotating Phase Plate
Transmitted OAM Distorted OAM
q OAM beam will experience turbulence-induced distortion, which will result in channel crosstalk and system power penalty q The atmospheric turbulence is emulated in the lab environment by using rotating phase plates, obeying Kolmogorov spectrum statistics
Y. Ren, CLEO2013, Invited
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Experimental Results —Crosstalk Measurement
= 2.5×10-16 = 0.0049
D/r0 = 0.239 F= 6.71
= 2.5×10-16
D/r0 = 0.438
= 0.0049
F= 11.35
F= 6.71
= 2.2×10-14 D/r0 = 3.94
= 0.46
F=11.35
= 2.2×10-14 D/r0 = 6.37
= 0.46
Weak Turbulence Strong Turbulence
Average crosstalk under different turbulence strength
10 mV/div
Average BER
Average BER Y. Ren, CLEO2013, Invited
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35
OAM Turbulence Compensation — Concept
Beam separator
Feedback controller
WavefrontCorrector
Beam splitter
WavefrontCorrector
WavefrontSensor
Corrected OAM beams
Corrected Gaussian beam
Gaussian beam
OAM beams
Turbulence Emulator
Free Space Propagation Adaptive Optics Compensator
p Use Gaussian beam as a pilot beam to detect wavefront distorCon of Gaussian beam by using conversional WFS.
p ConvenConal adapCve opCcs approach could not work for OAM beam
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36
Experiment Results— Far Field Images
Far Field Images
q By using the correction pattern obtained from Gaussian pilot beam in AO system, the distorted OAM beams up to OAM l=9 are efficiently compensated.
(b2) (b3) (b4) (b5) (b6)
(a1) (a2) (a3) (a4) (a5) (a6)
OAM+1 OAM+3 OAM+5 OAM+7 OAM+9Gaussian Beam
(b1)
RMS 0.613 PV 2.562 SR: 0.231
RMS 0.092 PV 0.649 SR: 0.924
Y. Ren et al., ECOC 2013
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Experimental Results — BER Performance
BER for channel OAM l = 5 before/after compensation
8 10 12 14 16 18 20 22 24 26
10-5
10-4
10-3
10-2
10-1
BER
OSNR (dB)
B2B Only Ch l = 5 After Comp. Only Ch l = 5 Before Comp. Ch l = 3, 7 On, After Comp. Ch l = 3, 7 On, Before Comp.
XT = -9.51 dB
FEC Limit
With Comp. W/o Comp.
Only l=5 -‐27.85 dBm -‐35.00 dBm
Ch l=3,7 on -‐47.80 dBm -‐44.51 dBmY. Ren et al., ECOC 2013
Received power of OAM channel l=5"
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Outline
1. OAM Beam (De)multiplexing
2. Optical Communications using OAM
- Pol-muxing, WDM
- MIMO
3. Turbulence Emulation and Compensation
4. OAM in Vortex “Ring” Optical Fibers
5. OAM-Based Networking Functions
- Exchange, ROADMs
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OAM Mode Propagation Property in a Ring Fiber
N. Bozinovic et al., CLEO 2011"
Radially Polarized Beams in Ring Fiber
S. Ramachandran et al., OL 2009"
~1-km Ring Fiber Transmission of OAM Mode
Ø ~1-km OAM mode transmission in ring fiber has been demonstrated. Ø Increasing the supported OAM modes is highly desired to improve the spectral efficiency and capacity of the optical communications system.
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N. Bozinovic, Science, vol.340, pp.1545-1548 (2013)
400 Gbit/s 4 Modes Data Transmission
B2B
DeMux BER
50Gbaud NRZ-QPSK, λ = 1550nm
Tx Mux
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OAM + WDM Multiplexing: 1.6 Tbit/s Capacity
20 Gbaud/s 16-QAM + 10 Wavelengths over 2 OAM modes
N. Bozinovic, Science, vol.340, pp.1545-1548 (2013)
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Experimental Results: 1.6 Tbit/s Capacity
q OAM+ and OAM- have <-17dB crosstalk over 10 WDM channels with 0.8 nm channel spacing.
q All of the channels achieved BER below the FEC limit of 3.8×10−3 with 20 Gbaud/s 16-QAM. q A total of 1.6 Tb/s data transmission carried by OAM modes is achieved over 1.1 km vortex fiber.
N. Bozinovic, Science, vol.340, pp.1545-1548 (2013)
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Outline
1. OAM Beam (De)multiplexing
2. Optical Communications using OAM
- Pol-muxing, WDM
- MIMO
3. Turbulence Emulation and Compensation
4. OAM in Vortex “Ring” Optical Fibers
5. OAM-Based Networking Functions
- Exchange, ROADMs
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44 44
OAM-based Reconfigurable Optical Networking Functions
p Static point-to-point data transmission links
Space Division Multiplexing (SDM)
p Reconfigurable data pathsp Wavelength multiplexed and
time multiplexed networks
Networking Methods
p Channel Add/Dropp Data Exchangep Channel Multicasting
OAM based Optical Networking
Ø Design and demonstrate several reconfigurable networking functions: p A reconfigurable OAM channel add/drop multiplexer p Multiple pairs of OAM channel data exchange p Power equalized data multicasting in OAM systems
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45 45
Add/drop function in OAM-multiplexed data link
...λ1 λ2 λnλ3 ...λ1 λ2 λnλ3
λ2
λ2/
/
Add
Drop
Ø In WDM networks, OADM has been demonstrated to selectively drop and add "a given wavelength channel without interrupting passthrough channels."
Ø A similar scheme can be implemented to selectively drop and add a given "OAM beam using SLM and spatial filters."
Wavelength Add/Drop Multiplexer Spatial Add/Drop Multiplexer for
OAM beams
Motivation
OAM charge"Wavelength" Wavelength"
Add/"drop"
OAM charge"
l1 l2 l3 lN l1 l2' l3 lN
l2'
l2
add"
drop"
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46 46
Scheme concept of OAM Mode Add/Drop
Add/drop
Down-conversion Up-conversion
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47 47
Principle of Add/drop multiplexer for OAM
To be added
Dropped beam
To be added
Dropped beam
grating
Ø The OAM mode to be dropped is down-converted to OAM0 which is in located in the beam center, while the rest OAM modes are rings that has no energy in the center."Ø The rings and the down-converted beam are reflected by different gratings, so that they can be separated without losing power.
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48 48
Input: OAM-5,2,8 OAM+2 OAM-5 OAM+8
Down conversion
Up conversion
Drop OAM+2
Drop
The dropped channel
Add
Experimental results of add/drop: images
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- Multiplexing/demultiplexing of many OAM modes can be used as a technique for potentially increasing capacity and spectral efficiency.
- OAM is compatible with other optical communications techniques, such as WDM.
- OAM modes can be manipulated to achieve various networking functions.
Summary