1. What the future holds for few-mode fiber
transmission?
William Shieh
Centre for Energy-Efficient Telecommunications
National ICT Australia
Department of Electrical and Electronic Engineering
The University of Melbourne, Melbourne, Australia

2. PhD Program at Melbourne Uni
澳大利亚墨尔本大学电机与电子工程系教授Prof. Shieh目前正在招收优秀博士
生（本科或者研究生平均分达到80分及以上， 以及排名30% 以上），可以选择的
研究方向包括
（1）正交频分复用技术(OFDM)在光或者无线网络中的应用
（2）相干光通信，光信号处理以及信道均衡
（3）波导设计和非线性的特性描述
（4）全光数据包交换，波长转换以及新型光网络结构
（5）射频(RF)光子技术，包括RF信号的产生、特性描述、传输以及处理
William Shieh教授是澳洲杰出青年（Australian Future fellow）获得
者以及美国光学协会院士(Fellow
of Optical Society of America)。他的个人主页是
http://people.eng.unimelb.edu.au/shiehw/
如果您对以上研究方向感兴趣，请与2012年1月15日之前递交申请。欢迎您访问如
下网页了解申请信息以及步骤
http://www.ee.unimelb.edu.au/future_students/PhD_in_Enginee
ring.html
或者直接通过电子邮件跟Shieh教授联系，他的邮箱地址是
shiehw@unimelb.edu.au。

3. Centre for Energy Efficient Telecommunications (CEET)

4. Outline
• Capacity limit in the current SMF fiber
 Analytical expression of fiber capacity
• Two-mode fiber (TMF) based transmission
 LP10 / LP11 transmission
 Two degenerate LP11 modes transmission
• Challenges in FMF fiber based systems
• Conclusion

5. Motivation
 Capacity crunch
H. Kogelnik
•A. R. Chraplyvy, “The coming capacity crunch,” ECOC’09 Plenary Talk, 2009
•R. W. Tkach, “Scaling optical communications for the next decade and beyond,” Bell
Labs Tech. J. vol. 14, pp. 3-10, 2010
•M. Nakazawa, “Hardware paradigm shifts in the optical communication infrastructure
with three “M technologies” OECC’2010

6. Degrees of Freedom for Multiplexing and Modulation
(1) Time
(2) Frequency
(3) Complex constellation or I/Q modulation
(4) Polarization
(5) Space

7. Nonlinearity Noise Density in SMF Fibers
The maximum capacity can be achieved by filling the
spectrum with signals
f
B
Assume that the input signal density is I, what is the INL?
 I 
2
1 8πα β 2 α
=  I Ic ≡ , B0 =
I NL
 Ic  γ 3N s he ln ( B / B0 ) 2π 2 β B
2( N s − 1 + e −αζ LN s − N s e −αζ L )e −αζ L
he ≡ −αζ L
+1
N s (e − 1) 2
•X. Chen, and W. Shieh, Opt. Express 18, 19039-19054 (2010).
•W. Shieh and X. Chen, IEEE Photon. Journal, vol. 3, 158 – 173, (2011).

8. Information Spectral Efficiency in Presence of
Fiber Nonlinearity
Shannon Capacity Theory:
 I 
S = log 2 (1 + SNR ) ≅ log 2 1 + 
 n + I (I / I ) 
2
 0 c 
 1 2/3 
= log 2 1 + ( I c / n0 ) 
S
 3 
 Ns 
(8πα β 2 ) ( 3γ N h ln ( B / B0 ) )
1/3 −1/3
≅ log 2  2 2
0 e 
 3 
Spectral Efficiency Parameter Dependence:
To increase 1 bit/s/Hz:
(i) Reach is to be reduced to half
(ii) Nonlinearity coefficient is to be reduced by 2.8
(iii) Chromatic dispersion is to be increased by 8
•X. Chen, and W. Shieh, Opt. Express 18, 19039-19054 (2010).
•W. Shieh and X. Chen, IEEE Photon. Journal, vol. 3, 158 – 173, (2011).

9. Why Few-mode Fiber or Two-mode Fiber?
[Hij]
Tx1 Rx1 In theory, for N-mode fibers, if we have N
transmitters and N receivers, we only need
Tx2 Rx2 compute NxN H matrix and perform the
matrix inversion H-1. Complexity of H-1
TxN RxN scales faster than N2
Complex optical design & electronic DSP design.
It is sensible to start with the two mode fiber (TMF).
TMF fiber contains three spatial modes including LP01 mode and two
degenerate LP11 modes.
B.Y. Kim, et al., Opt. Lett., 11, 389-391 (1986).
B.Y. Kim, et al., Opt. Lett., 12, 729 (1987)
Nevertheless, this still leads to 3 times bandwidth of a SMF fiber.

10. Some Scenarios of Mode Multiplexed Systems
(1) Short-reach or interconnect
High differential-mode-delay (DMD)
low mode crosstalk, optical demulitpelxing
F. Yaman, et al., Opt. Express, 18, 21342 (2010).
N. Hanzawa ‘Demonstration of mode-division multiplexing transmission over 10 km two-
mode fiber with mode coupler’, OFC’2011, Paper TWA4.
(2) Long-reach
Low DGD, high mode crosstalk, electronic demulitpelxing
(3) Higher-order constellation such as 16-QAM and beyond
Electronic demulitpelxing

11. TMF Fiber Parameters
Refractive Index for core and cladding
n _ core = 1.4518
n _ clad = 1.4440
Refractive Index Difference
n _ core - n _ clad
= ∆n = 0.0054
n _ core
Numerical Aperture
NA = 0.1505
n _ core 2 - n _ clad 2
V= ( 2π a/λ ) ⋅ NA= ( 2π × 5.935 /1.55 ) × 0.1505 =
3.6218
For Corning's SMF-28e® SMF, V = 2.0396
For Corning's Infinicor® MMF, V =~ 20.8453.

12. Two-mode or Three-mode Fiber?
1.460
core
Modal Index n eff LP01
1.450 LP11
cladding
1.440
1.430
cutoff
LP11 LP01 2323nm
1.420
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Wavelength (μm)
2 LP modes: LP01 and LP11
a b
3 Spatial modes: LP01 and 2 degenerate LP11: LP11 and LP11
6 Fiber modes: 2 (polarization) x 3 (spatial )

13. Two-mode LP10 / LP11 Transmission Systems
Polarization Polarization
Multiplexing Controlled coupling to LP01 mode
Demultiplexing
Tx 1 LP01-LP11 Dual-mode Rx1
amplifiers
Tx 2 Rx2
SMF SMF
Tx 3 Rx3
TMF
Tx 4 Rx4
LP01-LP11 Mode LP01-LP11 Mode
Multiplexing Demultiplexing

14. Mode Converter
LB = 2π /( β 01 - β11 )
Grating period = Beating length
= 500 µ m
Deformation
Mode converter 1 SMF TMF
nominal 50%
conversion ratio LP01 LP01+LP11
Mode stripper
Deformation
Mode converter 2 LP11+LP01 SMF
nominal 100% TMF
conversion ratio. LP +LP LP01
01 11
Mode stripper
R. C. Youngquist, J. L. Brooks, and H. J. Shaw, "Two-mode fiber modal coupler," Opt.
Lett. 9, 177-179 (1984)

15. CO-OFDM Experiment Setup
One-symbol delay
Optical OFDM Tx Center
50:50 PBC Splicing MS1 MC1
Band::
Band 1 2 3
λ
4.5 km TMF
EDFA
50 % MC
LP11 Rx
PD ADC Offline
MS2 Offline
MC2 EDFA Polarization
Polarization PD ADC 2x2 MIMO
2x2 MIMO
Diversity
Diversity OFDM
PD ADC OFDM
90° hybrid
90° hybrid detection
detection
PD ADC
100 % MC
LO
LO
LP01 Rx
MS3 PD ADC
Offline
Offline
EDFA Polarization
Polarization PD ADC 2x2 MIMO
2x2 MIMO
Diversity
Diversity OFDM
PD ADC OFDM
PBC: Polarization-beam-combiner 90° hybrid
90° hybrid detection
detection
MS: Mode stripper MC: Mode converter PD ADC
LO: Local oscillator TMF: Two-mode fiber LO
LO
Tx/Rx: Transmitter/Receiver
107 Gb/s dual-mode dual polarization transmission over 4.5-km TMF fiber.
‘X’ indicates controlled coupling between LP01 modes of SMF and TMF by
center splicing.

16. Transmission Parameters
Parameters Value for OFDM Transmission Unit
Polarization 2
Band 3
Mode 2 (LP01+LP11)
Bit Rate (raw) 25 (per pol/band/mode) * 6 Gbit/s
Bit Rate (net) 107 Gbit/s
Symbol Period 7.2 ns
Bandwidth 6.5625 (per band) GHz
No. of Subcarriers 64
Total No. of Symbols 500
No. of Training Symbols 20
Cyclic Prefix (CP) 1/8 of observation window
Modulation Format QPSK
Fiber Length 4.5 km
Launch Power 5.5 dBm
Receive Power -0.5(LP01) / -5.3(LP11) dBm
Y. Ma, Y. Tang, and W. Shieh, "107 Gbit/s transmission over multimode fibre …" Electron.
Lett. 45, 848-849 (2009).

17. Experiment Results
Optical Spectra
LP01 0.1nm/div LP11 0.1nm/div
10dB
10dB
19.69GHz 19.69GHz
1549.21nm 1549.21nm
Constellation
LP01 LP11
Band1 Band2 Band3 Band1 Band2 Band3

18. Q Factors for 12 Tributaries
LP01 Band1 Band2 Band3 Avg.
pol-x 19.5 18.4 18.1 18.7
pol-y 18.5 18.3 17.9 18.3
Avg. 19.0 18.4 18.0 18.5
LP11 Band1 Band2 Band3 Avg.
pol-x 15.2 18.6 16.2 16.9
pol-y 14.7 17.0 16.5 16.2
Avg. 15.0 17.8 16.4 16.5
No error was measured out of 100,590 bits for
each band, polarization and mode measured.

19. Two Degenerate LP11 Modes Transmission
(a) 0.9 mm jacket Rotating FC
connector
SMF TMF
Core-center Mode stripper Mode converter
aligned splice coiling V-groove
CL
TMF
CL
CL
BS
SMF TMF
IR beam
profiler
(b) TMF V-groove
LP11a
BS
CL
TMF
LP11b

20. ‘Dream’ System of Multimode Fiber Link
Narrow Linewidth (<10 ~ 100 MMF with low
KHz) laser Array loss: ~ 0.2
dB/km MMF OADM
MMF MMF
MMF MUX Amplifier DeMUX

21. Review of Progress of Few-mode Transmission
Low-speed Short-reach Systems
•J. Sakai, et. al., Trans. Micro. Theory & Techn. 26, 658-665 (1978).
•K. Kitayama, et. Al., IEEE J. Quantum Electron. (Lett.), vol.QE-15, pp. 6-8, 1979.
High-speed Long-reach systems using Conventional MMF
•Z. Tong, et. al., OECC’2008, paper PDP5.
•Z. Tong, et. al., Electronics Letters, vol. 44, pp. 1373-1375, 2008.
High-speed Long-reach systems using FMF fiber
OFC’2011, Postdeadline papers
•A. Li et al, Proc. OFC, 2011, p.PDPB8.
•M. Salsi et al., Proc. OFC, 2011, p.PDPB9.
•R. Ryf et al, Proc. OFC, 2011, p.PDPB10
ECOC’2011, 8 more postdeadline papers on few-mode/core fibers

22. Exponential Internet Traffic Growth
R. W. Tkach,
Bell Labs Tech.
J., vol. 14, 2010
Bandwidth needs to scale up ~ 30 dB for the next two decades!!

23. Implications of 30 dB of More Bandwidth
Transponders Fiber cables ROADMs Optical Amplifiers
System Complexity
Implications
Power
Space

24. Trade off between Spectral and Energy Efficiency
512 QAM
Spectral Efficiency [b/s/Hz]
n 256
nno
Sha 64
64QAM
512QAM
101 36QAM
16
OFDM/64QAM
16QAM
QPSK
4
2010
(PDM)
100
0 5 10 15 20 25 30 35
Required SNR per bit (dB)
(i) Strive for high spectral efficiency with low energy efficiency
(ii) Strive for high energy efficiency with low spectral efficiency
But few-mode transmission can achieve both high spectral and
energy efficiency

25. Spatial Mode Multiplexing: a Promise or a Curse?
•Will the system be too complex and never be practical?
•Need to be mindful of DSP complexity for MIMO processing
• Electronics is getting better, but not a
panacea; Could be energy-hog.

26. Conclusion
• Spatial mode division multiplexing (SMDM) has
recently been demonstrated to be an additional
degree of freedom for achieving ultrahigh capacity
beyond that of SSMF fiber.
• Should be always mindful of complexity involved
when proposing devices and subsystems for SMDM
based systems.
• But FMF or SMDM is a fertile ground for innovation.