Novel Optical Fibers for High-Capacity Transmission System
Mingjun Li1, Hao Chen2
(1.Science and Technology Division, Corning Incorporated, SP-AR-02-2, Corning, NY 14831, United States; 2.Corning Optical
Communication China, Shanghai 200233, China)
ABSTRACT
The rapid growth of multi-media and data rich applications has driven the bandwidth demand for long-haul fiberoptic links at unprecedented rates. At the same time, growing capacity demands also imposes challenges on bandwidth
and interconnects in data centers. Optical fibers are continuing improving their performance to meet the bandwidth
demand in both long haul and short reach applications. While continuing improvements in conventional fiber optic
technologies will increase the system capacity further in a short term, recent studies show that the transmission capacity
over single-mode optical fibers is rapidly approaching its fundamental Shannon limit. To overcome this limit, new
technologies using space division multiplexing are needed to provide a solution to the future capacity growth.
In this paper, we discuss novel optical fibers for increasing capacity for transmission systems. For conventional
fibers for long haul transmission, because transmission impairments such as chromatic dispersion, polarization-mode
dispersion can be perfectly compensated for by digital signal processing, the only fiber parameters that can be optimized
further are fiber attenuation and effective area. We will discuss system figure of merit for the two parameters and present
recent results on ultra-low loss and large effective area fibers. For short reach applications, we will review the current
efforts in improving multimode fiber bandwidth and discuss new trends in multimode fibers to meet future bandwidth
demand. For next generation fibers, we will focus on multicore and few mode fibers for space division multiplexing,
which has the potential to increase the capacity by an order of magnitude. We will present fiber design considerations
and review recent progress on multicore and few mode fibers. Finally, we will discuss major challenges in space division
multiplexing applications using multicore and few mode fibers.
KEY WORDS
ultra-low loss fiber, large effective area fiber, space division multiplexing, multicore fiber, few mode fiber
I. Introduction
Since the first demonstration of optical fiber of less than 20 dB/km in 1970 [1], optical fibers and transmission
systems have evolved rapidly during the past more than four decades. The advances of fiber, component and system
technologies have increased the transmission capacity tremendously. Historic data show that the transmission capacity of
a single fiber has increased by a factor of approximately 10 every four years [2].
The long haul transmission system has gone through four generations. The first generation of optical fiber
communication system utilized multimode optical fibers and LED sources operated in the 850 nm wavelength region [3].
The advantage of the multimode fiber is that it has a large core and high numerical aperture; therefore, coupling of the
light from the source into the fiber, splicing and connectorization are not difficult. However, the multimode fiber has a
fundamental bandwidth limitation due to intermodal dispersion.
One approach to eliminate intermodal dispersion is to utilize a single-mode optical fiber instead of a multimode
optical fiber. With the development of semiconductor lasers [4,5] and the opening of long wavelength transmission
windows [6,7] as well as the advance in single-mode fiber splicing technology [8,9] in the later 1970s, single-mode fiber
transmission systems became possible. The second generation of optical fiber communication system employed standard
single mode fiber and single mode lasers at 1310 nm. When the attenuation of optical fiber is considered, the attenuation
in the 1310 nm wavelength region is less than that in 850 nm. In addition, a standard single-mode fiber exhibits nearly
zero dispersion in this wavelength region [10-14].
The attenuation of a single-mode optical fiber is the lowest in the 1550 nm wavelength window. However, the
chromatic dispersion of a standard single-mode fiber in this wavelength window is very large (+17 ps/km/nm), which
was a limitation for high data rate systems. In order to take the advantage of lowest attenuation at this wavelength
window, a new type of fiber, known as a dispersion-shifted optical fiber (DSF) whose chromatic dispersion is minimum
at the 1550 nm wavelength region was developed [15-21]. This in effect allows the use of conventional lasers exhibiting
relatively large spectral width of several nm, which enabled the third generation optical fiber transmission system at
1550 nm.
The dispersion shifted fiber was optimized for single channel transmission at 1550 nm. Before it was widely
deployed in actual telecommunication systems in the late 1980s, new advances in erbium doped amplifiers (EDFA)
[22,23] and wavelength-division multiplexing (WDM) [24] technology made the multiple channel transmission viable
for the fourth generation high capacity optical fiber transmission system. It was found soon that the dispersion shifted
fiber with dispersion value of around zero at 1550 nm was not suitable for WDM transmission [25]. This is because the
nonlinear effect of four-wave mixing is the strongest when the dispersion is zero [26], which causes significant crosstalk
between two neighboring channels. To reduce the four-wave mixing effect, it is advantageous to have a certain amount
of dispersion. On the other hand, the dispersion should be small to minimize the dispersion penalty. Therefore the
concept of non-zero dispersion-shifted fiber (NZDSF) was proposed [27-30]. Typical dispersion value for NZDSFs is in
the range of 3-8 ps/nm/km at 1550 nm with an effective area of about 50 µm2. Because the nonlinear effects are
inversely proportional to the effective area of fiber, increasing the effective area will reduce further the nonlinear effects.
To increase the effective area, profiles designs with large effective area of about 72 µm2 were developed [31-34].
NZDSFs have been widely deployed worldwide for high capacity WDM networks.
The WDM technique has opened a new dimension to increase the transmission capacity by increasing the number of
wavelength channels. In parallel to the WDM development, the channel rate has improved to meet the increasing
bandwidth demand. The channel rate has increased from 2.5 Gb/s to 10 Gb/s and then to 40 Gb/s using intensity
modulation and direct detection. With the continuous increase in channel rate, coherent technologies have attracted a
large interest over the recent years [35]. Coherent detection allows information to be encoded with two degrees of
freedom, which increases the amount of information per channel. It also allows integrating digital signal processing
(DSP) function into a coherent receiver to make a digital receiver. With coherent detection techniques, advanced
modulation formats [36] such as BPSK, QPSK, 8PSK, 16QAM and higher levels of modulation formats have been
proposed, which have pushed the channel capacity to beyond 100 Gb/s. Coherent detection in conjunction with DSP can
handle transmission impairments from chromatic dispersion and polarization mode dispersion. This new system
technology has changed the fiber design direction towards fibers with lower loss and larger effective area to deal with
nonlinear transmission impairments.
While continuing improvements in conventional fiber optic technologies will increase the system capacity further
in a short term, recent studies show that the transmission capacity over single-mode optical fibers is rapidly approaching
its fundamental Shannon limit [37]. To overcome this limit, new technologies using space division multiplexing (SDM)
are needed to provide a solution to the future capacity growth [2]. There are two approaches for space division
multiplexing, one is to use multicore fiber (MCF) and the other is to use few-mode fiber (FMF). SDM adds a new
dimension to the fiber transmission system, which has the potential to increase the capacity by an order of magnitude.
MCF and FMF for SDM are current hot topics in optical fiber research.
Although multimode fiber (MMF) was replaced by single-mode fiber for long haul applications [38], MMF is still the
fiber of choice for short distance data network applications because it offers efficient coupling from light sources, low
cost splices and connectors between fibers. In the last 10 years, MMF and short-wavelength 850 nm VCSELs have
emerged as dominant technologies for short-reach high data rate networks [39, 40]. MMF is used in local area networks
(LAN) and data centers where data rates are higher or reach lengths are longer than can be met with copper networks,
and the use of VCSELs makes the overall system cost effective to enable widespread adoption. The fast growth is driven
by the demand for higher data rates for computer connections, data storage, and local communication including linking
to internet traffic. Server virtualization, cloud computing, and higher speed ports are now driving networks to 40/100
Gb/s and eventually higher speeds (400Gb/s) in data centers. To meet higher data rate and longer distance demands,
MMF has been evolving to improve its bandwidth capability and performance.
In this paper, we review recent progress on novel optical fibers for increasing capacity for both long haul and short
length transmission systems. We discuss first ultra-low loss and large effective area fibers for applications in high
capacity long-haul WDM systems. Then we will review new developments in MMF for high capacity short reach
applications. Finally we will discuss new emerging MCF and FMF for increasing the fiber capacity using SDM
technology.
II. Ultralow loss and large effective area fiber for long haul systems
With the new developments in coherent detection and digital signal processing technologies, transmission
impairments from fiber dispersion effects are no longer an issue. This simplifies greatly the fiber design. The only
important fiber parameters for high capacity long-haul transmission are the fiber attenuation and effective area.
The total attenuation of an optical fiber is the addition of intrinsic loss factors such as Rayleigh scattering α RS ,
infrared absorption α IR and ultraviolet absorption α UV , and extrinsic loss factors such as absorption due to transition
metals α TM , absorption due OH ions α OH , scattering due to waveguide imperfections α IM and loss due to fiber bending
effects α BL :
(1)
α = α RS + α IR + αUV + α TM + α OH + α IM + α BL
The contaminants due to transition metals can be eliminated practically in the fiber preform manufacturing processes by
using chemical vapor deposition techniques with high pure chemical row materials. The OH concentration can be
reduced to minimum level using chlorine drying. Waveguide imperfection loss is caused by the geometry fluctuation at
the core and cladding boundary. The boundary fluctuation is mainly due to the residual stress which is induced during
the manufacturing process. The residual stress depends on the magnitude of the viscosity difference between the core
and cladding and the fiber drawing tension. The stress can be reduced by matching the viscosity of the core and cladding
[41]. For the intrinsic factors, the most important one is the Rayleigh scattering loss. The Rayleigh scattering loss can be
expressed by the sum of two contributions from density and concentration fluctuations [42]:
(2)
α RS = α ρ + α c
The density fluctuation to the scattering coefficient depends on the fictive temperature, T f , which is determined as the
temperature where the glass structure is the same as that of the supercooled liquid:
8π 3 8 2
α ρ = 4 n p β T k BT f
3λ
(3)
 ∂n 
α c ~   ∆C 2 T f
 ∂C 
(4)
where λ is the wavelength of incident light, p the photoelastic coefficient, n the refractive index, k B the Boltzmann
constant, β T the isothermal compressibility. The concentration fluctuation is proportional to:
2
Because the Rayleigh scattering is mainly caused by frozen-in density fluctuation, and is proportional to the fictive
temperature, T f . Therefore, it is necessary for suppressing the Rayleigh scattering to reduce T f as much as possible to
increase structural relaxation. To reduce the concentration fluctuation, it is advantageous to reduce GeO 2 dopant level in
the core because the Rayleigh scattering loss is proportional the GeO 2 concentration. For this reason, it is better to use
pure silica material in the core [43, 44].
The fiber macro- and micro-bending losses are important factors for fiber attenuation, especially for fibers with large
effective areas. To increase the effective area while keeping good bending performance, the core refractive index profile
needs to be carefully designed. The optical properties that need to be considered include the effective area, cable cutoff
wavelength and bending losses. Fig. 1 shows three profile designs that can be used for low loss and large effective area
fibers. The relationship between these attributes can be understood by considering a simple step index design, which is
shown schematically in Fig. 1(a). A step index profile is characterized by two parameters: the relative refractive index
change or the core delta and the core radius. To increase the effective area, the core radius needs to be increased but core
delta needs to be reduced to keep the cable cutoff wavelength below the minimum wavelength of the application window,
e.g. 1530 nm for 1550 nm window. Due to the cable cutoff wavelength limitation, the bending losses will increase when
the effective area gets larger. The macrobend loss usually has a specified upper limit, e.g. less than 0.5 dB for 100 turns
at a bend diameter of 60 mm for practical applications. For a step index design, with the cable cutoff and macrobend loss
constraints, the maximum effective area that can be achieved is approximately 110 µm2.
(a)
(b)
(c)
Fig. 1: Profiles designs for large effective area fibers.
To increase further the effective area, we can add a depressed cladding layer or a low index trench as shown in Fig.
1(b) and (c) to suppress macrobending losses while keeping the cable cutoff wavelength below 1530 nm. For example,
Fig. 2 shows measured bend loss as a function of effective area for design (b) at 1550 nm. For 30 mm bend diameter, the
bend loss start to increase rapidly when effective area is greater than 130 µm2, while for 40 mm bend diameter, the
effective area can be as large as 175 µm2.
Fig. 2: Bend loss as a function effective area for design (b).
However microbending loss becomes a limiting factor for large effective area [45]. If we use the fiber-coating system
currently used for standard single mode fiber (G.652), it has been reported that the effective area can only be increased
to approximately 120 µm2 due to microbending loss increase [46]. Microbending is an attenuation increase caused by
high frequency longitudinal perturbations to the waveguide. These perturbations couple power from the guided
fundamental mode in the core to higher-order cladding modes that are lost to the outer medium [47]. A
phenomenological model introduced by Olshansky captures the importance of treating the glass and coating as a
composite system by predicting that the microbending losses scale with [48]
a4 3/ 2
γ micro ∝ 6 3 E
b∆
(5)
where a is the core radius, b is the cladding radius, ∆ is the relative refractive index of the core and E is the elastic
modulus of the primary coating layer that surrounds the glass. As it is mentioned earlier that core delta and core radius
are determined by the effective area and the cutoff wavelength, these variables are not completely independent and
together do not offer a significant lever for reducing microbending sensitivity. This leaves the inner primary modulus as
a key factor for addressing the increased microbending sensitivity in large effective fibers. Making the inner primary
coating softer will help cushion the glass from external perturbations and hence improve the microbending performance.
The role of the primary modulus in mitigation of microbending was demonstrated experimentally. In the experiment,
fibers of different effective areas were made with two coatings which have inner primary moduli of approximately 0.43
and 0.13 (Coating A and B, respectively). Fig. 3 shows measured fiber attenuation. For effective area between 110 and
115 µm2, the attenuation of fibers with the two coatings are the same. This shows that intrinsic attenuation can be
realized with effective area less than 115 µm2. For the effective area larger than 120 µm2, the microbending loss starts to
increase with Coating A, while Coating B keeps the microbending loss to nearly zero level for effective area up to 135
µm2. It is evident that to derive the benefits of ultra-low attenuation, a fiber with very large effective area requires a
coating with an optimized inner primary modulus to protect against microbending. It is estimated that effective area of
about 150 µm2 is possible with further optimization of primary coating.
Fig. 3: Attenuation of fibers with different effective areas and different coatings.
The fiber effective area and attenuation affect optical signal to noise ratio (OSNR) of a transmission system. The
main factors that determine the OSNR for a given system link at a given distance are the channel launch power into each
span, the noise figure of the optical amplifiers, the loss per span, and the total number of spans in the link. Of these, the
factors that are directly related to optical fiber parameters are channel launch power and span loss. The channel launch
power is limited by optical nonlinear impairments in the fiber, which scale with the ratio of the fiber effective area A eff
over n 2 , the nonlinear index of refraction. The span loss is directly related to the fiber attenuation coefficient α. To
quantify benefits of large effective area and low loss on OSNR, a fiber figure of merit (FOM) is defined [49]:
 L
 Aeff ⋅ n2, ref 
 − (α − α ref )⋅ L − 10 log eff
Fiber FOM(dB) = 10 log
L

A
 eff , ref
 eff , ref ⋅ n2 




(6)
where A eff , α, n 2 , and L eff are the effective area, attenuation coefficient, nonlinear refractive index and effective length of
the fiber under consideration, and A eff,ref , α ref , n 2,ref , and L eff,ref are the effective area, attenuation coefficient, nonlinear
refractive index and effective length of the reference fiber, and L is the span length.
-20
-20
GeO -doped silica core, n = 2.3x10
2
Silica core, n = 2.1x10
2
2
140
130
120
110
100
0.16
0.162
0.164
0.166
0.17
0.168
0.172
0.174
0.176
0.178
0.18
0.182
0.184
0.186
0.188
0.19
0.192
0.194
0.196
0.2
0.198
90
Effective area (sq. um)
150
80
5-5.5
4.5-5
4-4.5
3.5-4
3-3.5
2.5-3
2-2.5
1.5-2
1-1.5
0.5-1
0-0.5
Fiber attenuation (dB/km)
Fig. 4: Fiber FOM contour plot as a function of attenuation and effective area for 75
km span length. Red circle represents parameters of reference fiber [49].
Fig. 4 illustrates the fiber FOM as a function of effective area A eff and attenuation α for a system with 75 km spans
[49]. The reference fiber parameters used here are for a typical standard single mode fiber with 0.2 dB/km attenuation
and 80 μm2 effective area. The discontinuity observed in the contour plot in Fig.4 between 0.179 dB/km and 0.178
dB/km is the result of an assumption made here that for attenuation values ≤ 0.178 dB/km, the fiber will have to be a
silica core fiber rather than a conventional Germanium-doped fiber. Fig. 4 shows clearly that increasing the fiber FOM
can be attained by either increasing the fiber effective area or decreasing the fiber attenuation. In fact, for this example of
75 km span length, >5 dB increase in FOM can be obtained by moving to an ultra-low loss fiber with attenuation of
0.162 dB/km and an effective area of at least 145 μm2. For longer spans or unrepeated long links, the ultra-low loss
becomes even more important. An analysis shows that an attenuation decrease of -0.035 dB/km is equivalent to
increasing the effective area by 34, 55, and 83 μm2 above the reference fiber’s 80 μm2 effective area for 50, 75, and 100
km spans, respectively.
Table 1. Transmission experiments using low loss and large effective area fibers
Fiber
System
Modulation
No of
format
channels
α
(dB/km)
A eff
(µm2)
Span
(km)
Amplifier
Data rate
(Gb/s)
0.163
0.162
0.165
76-128
134
85,134
365
100
200
EDFA/Raman
EDFA
EDFA/Raman
112
112
112
PM-QPSK
PM-QPSK
PM-QPSK
0.161
0.160
50
60.6
EDFA
EDFA
112
120.5
0.17
112
112,
146
80
125
EDFA
0.162
112
100
Raman
112
42.8
112
PDM-QPSK
PDM-8QAMOFDM
PDM-QPSK
CRZ-DQPSK
PM-QPSK
40
16
8
32
80
115
1
4
40
Channel
spacing
(GHz)
50
50
50
Total
capacity
(Tb/s)
4
1.6
0.8
3.2
8
11.5
Ref.
50
25
Reach
length
(km)
365
7200
5400
6000
9000
10181
50
3000
0.26
55
50
10200
4
49
50
51
52
53
54
The benefits of optical fibers with ultra-low loss and large effective area as quantified by the fiber FOM have been
experimentally demonstrated by several recent 100 Gb/s transmission experiments [49-55]. Table 1 summarizes seven
experiments. In these experiments, optical fibers used have attenuation of 0.16-0.17 dB/km, and effective area of 80 to
146 µm2. The transmission system investigated in Ref. 49 is an unrepeatered system with span length 365 km. In the
other experiments, the span length ranges from 50 to 200 km. Effective area management approach is used in Ref. 50
and 52 to reduce nonlinearity using a larger effective area fiber at beginning of each span and to increase Raman
efficiency using a smaller effective area at the end of each span. Reach lengths from 3000 km to 10200 km are
demonstrated, which can cover system link lengths for terrestrial and submarine applications. These experiments and
results highlight the long reach lengths and good system performance enabled by the fiber attributes.
III. New Multimode fibers for short reach systems
The fast growing internet traffic demands for high speed transmission and storage of huge amount of information by
datacenters, super computers and consumer electronics. These applications have short reach distances from a few meters
to a few hundred meters, where multimode fiber (MMF) is the fiber of choice for low cost system solutions. The
increased data flows demand for MMF with higher and higher bandwidth. At same time, the systems require lower bend
loss multimode fibers for improving power operating margins, space savings, cooling efficiency and overall connection
and cable management. In this section, we review recent new developments in MMF and discus new trends for high data
rate MMF applications [40].
III-1. High bandwidth MMF at 850 nm
MMF is designed with an alpha profile to minimize the modal group delays to achieve high bandwidth:
  r α 
∆ = ∆ 0 1 −   
  r0  
where r 0 is the core radius, and ∆ 0 is maximum relative refractive index change in the core:
∆0 =
n02 − n12
2n02
(7)
(8)
where n 0 is the refractive index in the center of the core, and n 1 is the refractive index of the cladding.
When the α value is properly chosen, the modal bandwidth of the MMF can be optimized at a specified wavelength
[56]. Fig. 5 shows a modeled bandwidth of a 1% delta 50 µm MMF at 850 nm. For this fiber, the theoretical peak
bandwidth is over 13 GHz.km. However, the bandwidth is very sensitive to the α value as shown in Fig. 5. Various
imperfections in the manufactured core profile limit the actual bandwidth.
Fig.5: Bandwidth vs. alpha of 1% delta 50 µm MMF
Fig. 6: Relative bandwidth versus core delta of MMF.
Fiber
Core ∆ 0
(%)
OM1
OM2
OM3
OM4
2
1
1
1
Table 1. Bandwidth and link distance of different types of MMF
Core diameter OFL
BW EMB
BW Link distance (m) (850 nm)
(MHz.km)
(MHz.km)
(µm)
850
1310
850 nm
1G
10G
40 G 100 G
nm
nm
62.5
200
500
N/A
275 33
N/A N/A
50
500
500
N/A
550 82
N/A N/A
50
1500
500
2000
N/A 300
100
100
50
3500
500
4700
N/A 550
150
150
As fiber manufacturing processes and designs have improved, MMF bandwidth has improved tremendously to meet new
bandwidth demands. Table 1 shows different types of standard MMF. The 62.5 µm MMF has a higher NA and a larger
core which enables more light coupled to the fiber from a LED source, which could support 2 km transmission at 10
Mb/s and even faster data rates, up to 100 Mb/s, in shorter distance applications like the “Fast Ethernet” standard of the
early 1990’s. In the mid-1990s, developments of a 1 Gb/s optical Ethernet standard and the low cost VCSELs at 850 nm
favored the use of 50 µm fiber with its lower modal dispersion and higher bandwidth due to its lower refractive index
difference. The coupling to 50 µm fibers was no longer an issue due to the smaller spot size and lower NA of VCSELs.
Therefore, 50 µm fibers have become the fiber of choice for 1 Gb/s and 10 Gb/s Ethernet applications. 50 µm MMF has
evolved from OM2 (500 MHz.km) to OM3 (2000MHz.km) and now to OM4 (4700MHz.km). Further improvement in
bandwidth is theoretically possible through tighter profile control but at the higher cost of fiber manufacturing process
control.
The core ∆ 0 also affects the maximum bandwidth that can be achieved because the bandwidth scales with 1/∆2 as
shown in Fig. 6. The bandwidth is doubled when the core ∆ 0 is reduced from 1% to 0.75%. However, reducing the core
∆ will increase the bend loss. This problem can be mitigated by reducing the core diameter and using a low index trench
in the cladding as discussed in the section below.
For transmission systems using multimode VCSELs operating at 850 nm, further increasing the bandwidth of MMF
beyond OM4 offers very small benefits as the system is limited by chromatic dispersion. For multimode VCSELs, some
chromatic dispersion effects can be compensated by designing a MMF with slightly left tilt in differential mode delay
(DMD) [57] or using a short MMF compensation jumper that has the desired DMD tilt [58]. The latter offers flexibility
to adapt VCSELs with different dispersion characteristics. To take full advantages of bandwidth higher than OM4, we
can use single mode 850 nm VCSELs [59] or move to the system to longer wavelengths where the chromatic dispersion
is lower.
III-2. Bend-insensitive MMF
For data center applications, bend-insensitive MMF is attractive because it offers improved power operating
margins, enables smaller cable, hardware and equipment designs that can deliver space savings, easier handling for
frequent changes and better cooling efficiency, and better overall connection and cable management.
Fig. 7: Schematic of a bend-insensitive MMF refractive index profile
Fig. 8: Bend loss at 850 nm of standard and bend-insensitive MMF.
Fig. 7 shows a bend-insensitive MMF refractive index profile design [60]. It has an alpha graded index core with a low
index trench in the cladding. The trench reduces the optical power of guided modes in the cladding region, thus
improving their bend performance. The core delta and trench volume are carefully selected to balance the bend
performance and the compatibility with standard MMF. The trench’s location is also important to achieve high
bandwidth by correcting delays of outer mode groups. By properly designing the core and the trench, high bandwidth
OM4 MMF with low bend loss can be achieved. Fig. 8 shows measured bend loss at 850 nm for a bend-insensitive fiber
compared to a standard MMF. The bend loss of the bend-insensitive MMF is more than 10 times lower than the standard
MMF without the low index trench.
III-3. MMF for long wavelengths
As bandwidth reaches OM4 levels for 850 nm MMF, the chromatic dispersion becomes a limiting factor for high
data rate and long reach links because of large transceiver linewidths employed with VCSELs. Single-mode fibers or
restricted launches into the fundamental mode of an 850 nm MMF at a long wavelength using a single mode laser [61]
have been suggested for increasing the data rate or distance; however the high alignment precision required results in
high packaging costs for laser to fiber coupling, leading to higher optical transceiver costs.
One attractive solution is to use a MMF optimized for high bandwidth at a longer wavelength, for example 980
nm/1060 nm, or 1310 nm in conjunction with long wavelength sources such as long wavelength VCSELs and integrated
silicon-photonics (SiPh) transceivers. The long wavelength MMF system retains the advantage of low loss coupling and
passive alignment of conventional 850 nm MMF systems. At the same time, the chromatic dispersion and attenuation of
the fiber are much lower at a longer wavelength. This can be seen in Fig. 9, where the chromatic dispersion and loss are
plotted as a function of wavelength. At 1060 nm, both the chromatic dispersion and loss are reduced by more than 2
times, and at 1310 nm, the chromatic dispersion is nearly zero and the loss is only one fifth of that at 850 nm. The low
loss and low chromatic dispersion together with the high modal bandwidth enables higher data rate and longer length
transmissions. Recently, 25 Gb/s transmissions have been demonstrated through 820 nm MMF with a 1310 nm SiPh
transceiver [62], and through 500 m of MMF with a 1060 nm VCSEL transceiver [63], which clearly show the
advantages of long wavelength MMF transmissions.
Fig. 9: Chromatic dispersion and loss of MMF.
III-4. MMF for Consumer Applications and very short distance networks
With the increasing deployment of Fiber to the Home (FTTH), it is becoming apparent that optical communication
may find new opportunities in short-reach consumer electronics interconnects or home-networking. For fiber designs,
consumer interconnects represent a new application space that has different requirements from traditional applications
such as in data centers. In this application space, in addition to the bandwidth requirement, the fiber must be designed to
minimize the total link loss in the presence of misalignments, which allow the use of inexpensive optical components
and low cost assembly processes. The deployment conditions require the fiber for even lower bend loss, and high
mechanical reliability under small-radius bend conditions.
Fig. 10. Optical link loss in dB under misaligned conditions.
To increase tolerance to misalignments, it is beneficial to increase the core diameter and NA of the fiber. A
comprehensive analysis was performed for a high-speed link operating at 10 Gb/s [64] using an 850 nm VCSEL. The
result is shown in Fig. 10. A marked improvement of link loss occurs when the NA increases up to ~0.3 and the core
diameter increases up to ~80 µm. Further increase in the core diameter or NA provide only a marginal advantage
because the coupling improvements at the transmitter and in-line connectors become marginally better, while at the same
time the coupling degradation when focusing the light onto the photodiode increases. The total link loss is approximately
6.2 dB for a fiber with 80 µm core diameter and NA=0.3, compared to 11.5 dB for a standard MMF with 50 µm core
diameter and NA=0.2.
To make a fiber to withstand consumer handling, particularly in transient short term tight bend conditions of about 3
mm diameter, a fiber design with smaller glass diameter reduces the applied stress in bend and can increase the lifetime
by several orders of magnitude. Studies show that at 3 mm bend diameter, the lifetime is increased by approximately 4
orders of magnitude for a 100 µm glass fiber compared a fiber with 125 µm glass diameter.
The need for high data transfer rates requires the fiber to have a graded index core, since a step index core cannot
achieve sufficiently high bandwidth and support >10 Gb/s over relatively long distances. The addition of a low index
trench in the cladding is beneficial for achieving both high bandwidth and low bend performance as discussed in Sec.
III-2. By using a core delta of 2% with a low-index trench, bend loss on the order of 1 dB in a 3 mm bend diameter was
achieved. Transmissions at 10 Gb/s on a 50 m link with less than 1 dB power penalty were demonstrated [64].
IV. Fibers for space division multiplexing systems
Space division multiplexing is a promising approach for future capacity growth. However, the conventional single
mode or multimode fibers are not suitable for this application. New fibers such as multicore fiber and the other is fewmode fiber need to be developed. In this section, we discuss the two types of fiber and review recent progress in space
division multiplexing using these fibers.
IV-1. Few mode fibers
Mode-division multiplexing (MDM) in a multimode fiber is not a new concept. The potential of using different
modes to carry independent channels was recognized at early days of fiber optics. MDM was first demonstrated over a
short distance of 10-m using a conventional multimode fiber, in which the distance was limited by mode coupling [65].
The success of multiple-input-multiple-output (MIMO) systems in wireless communications has motivated interesting
investigations of MIMO in optical fiber communications [66]. MIMO technology can deal with the mode coupling issue,
increasing the transmission distance and capacity but requires more complex decoding techniques and digital signal
processing power. Conventional MMF with 1% core delta and 50 μm core diameter has more than 100 modes, which
requires very high complex DSP for long haul transmission. So far, the research efforts have been focused on few mode
fiber (FMF) transmission systems. For FMF transmission, it is desirable to minimize differential mode group delay
(DMGD) in order to reduce decoding complexity. It is also advantageous to have large effective areas to reduce the
nonlinear effects.
Fig.11 shows profile designs of FMF. Fig. 11(a) is a step index design and Fig. 11(b) is a graded index design. Both
designs can be described by the α profile function described by Eq. (7). For a step index profile, the α value approaches
infinity. The step index design is simple. The number of modes is determined by the core ∆ 0 and the core radius r 0 .
However, it is impossible to design a fiber with low DMGD over a transmission band like C and L bands. The graded
index profile has shape factor α, which adds one more degree of freedom. By controlling the α value, low DMGD can be
realized [67]. To improve the fiber bending performance, a low index trench can be added in the cladding as shown in
Fig. 11(c). The low index trench does not only reduce the fiber bending loss, but also reduces the mode delay of the
outer mode group if the location of trench is optimized, similarly to the bend insensitive MMF described in Ref. [60]
(a)
(c)
(b)
∆0
∆0
∆0
r0
r0
r0
∆1
w
r1
Fig. 11: FMF profile designs: (a) step index, (b) graded index, (c) graded index with trench.
With graded index profile designs, very low DMGD can be obtained theoretically. Fig. 12 shows the root mean
square (RMS) DMGD for optimum alpha profiles designs with different delta values. It can be seen that DMGD is less
than 10 ps/km in the whole C+L WDM window. As an example, fiber properties of a low DMGD profile design (Fiber 0)
are listed in in Table 1. In this example, the DMGD is lower than 1 ps/km for a wavelength window between 1500 and
1600 nm. DMGDs as low as 50 ps/km have been achieved for 3-LP mode fibers [69, 70]. Fibers with 6-LP modes and
DMGD less than 85 ps/km have been reported [71, 72].
Fig. 12: RMS DMGD for optimum alpha profile designs.
In addition to the very low delay, Table 1 shows also that the effective areas of the LP 01 and LP 11 are both very
large. The Effective area of LP 01 mod is 177 µm2 and the effective area of LP 11 mode is 238 µm2. Although the effective
areas are much larger than single mode fibers, the bending performance is good enough for practical cable applications
because the cutoff of the LP 11 mode is around 4 µm. The large effective areas are suitable for reducing nonlinear effects
for long haul transmission.
Positive DMGD
Negative DMGD
Fig. 13. DMGD managed fiber link.
Another way to realize low DMGD in a FMF link is to use a DMGD managed approach as shown in Fig. 13, where
FMF segments with positive and negative DMGD are concatenated to form a span with an ultra-low cumulative DMGD
[68, 73]. Positive and negative DMGD can be designed by changing the α value in a graded index profile design.
Fig. 14 shows differential mode group delays as a function of wavelength for two fiber designs. Table 2 shows
optical properties of the two fibers (Fibers 1 and 2). The two fibers have opposite delays and delay slopes. By choosing
the fiber length ratio of 1.2:1 for the two fibers, a fiber link with nearly zero delay can be constructed. For example, the
net delay obtained by combining 552 km of Fiber 1 with 460 km of Fiber 2 is plotted in Fig. 3. The net delay of the
~1000 km fiber link is less than 0.5 ps/km over the entire wavelength range between 1.5 to 1.6 µm.
Fig. 14. Fiber design examples with potive and negative DMGD.
Table 2. Optical properties of fiber design examples
Fiber
LP 01 MFD at 1550 nm (µm)
LP 01 Effective Area at 1550 nm (µm2)
LP 01 Dispersion (ps/nm/km)
LP 11 Cutoff (µm)
LP 11 Effective Area 1550 (µm2)
LP 11 Dispersion (ps/nm/km)
DMGD at 1500 (ps/km)
DMGD at 1550 (ps/km)
DMGD at 1600 (ps/km)
DMGD slope at 1550 nm (ps/nm/km)
0
15.0
177
21.0
4.085
238
21.1
−0.76
0.39
0.68
0.0191
1
15.3
186
21.2
4.178
242
21.4
0.1012
0.1057
0.1094
0.06123
2
14.7
168
20.9
3.976
234
20.8
−0.1243
−0.1269
−0.1304
−0.04363
DMGD compensation has been demonstrated experimentally [68]. Fig. 15 shows the DMGD measured for four
spools of fibers (A to D) with different refractive index designs. The lengths of the four fibers are 10, 18, 22 and 50 km,
respectively. Fibers A and B have positive DMGD with negative DMGD slope. Fiber C has negative DMGD and
positive DMGD slope. Fiber D was targeted to have near zero DMGD. The measure DMGD for this fiber was between
about -20 to 0 ps/km, which are slight larger than the design target. To demonstrated DMGD compensation, we spliced
the 18, 10 and 22-km spools together, and connected them to the 50-km spool with a connector to construct a 100-km
link and measured the total DMGD. In Fig. 16, it is observed that average DMGD varies from −6 to +5 ps/km across the
C-band from 1530 nm to 1565 nm. The measured attenuation for LP 01 and LP 11 in the link was approximately the same
0.25 dB/km.
Fig. 15. Measured DMGD of four experimental fibers.
20
15
Av
era
ge
D
M
G
D
10
5
0
(ps/
km)
-5
-10
-15
-20
1530
1535
1540
1545
1550
1555
1560
Wavelength (nm)
Fig. 16. DMGD compensation in 100-km fiber link.
Mode division multiplexing transmission over few mode fibers has been demonstrated in several system
experiments. Table 3 summarizes a few most recent experiments that have been reported, which represent the state of the
art in this area. The number of modes used in the transmission experiments is 3 or 6 that require 6x6 or 12x12 MIMO
processing. A larger number of modes is possible but will increase significantly the complexity of MIMO and processing
time. Few mode EDFAs using specially designed dopant profiles to equalize modal gain have demonstrated, but further
optimization is required to improve further the FM EDFA performance and to extend to a larger number of modes. For
the mode mux/demux devices, both devices using free space optics with phase plates and 3D-waveguides are
demonstrated. Among them, the 3D-waveguide approach is more promising to extend to a large number of modes and to
realize compact and integrated devices.
Table 3: Mode division multiplexing transmission experiments using few mode fibers
Fiber
No of
DMGD
modes (ps/km)
Span
(km)
Mode
Mux/Demux
3
25
50
Phase pate
6
183
59
3
5.4
3
50
84,
35
30
3Dwaveguide
Phase pate
Phase pate
Amplifier
FM
EDFA
SM
EDFA
FM
EDFA
SM
EDFA
Data rate
(Gb/s)
76
System
No of
channels
128
146
16
32
128
96
80
1
Channel
spacing
(GHz)
25
Total
capacity
(Tb/s)
66.57
7.30
26.4
Spectral
efficiency
(b/Hz)
18.2
74
25
Reach
length
(km)
500
100
177
Ref.
32.0
75
50
119
57.6
12
76
1200
0.19
77
IV-2 Multicore fibers
Multicore fiber is another potential fiber for space division multiplexing. Different multicore fiber structures have
been proposed. Fig. 17 shows schematics of some MCF structure designs. Fig. 17(a) is hexagonal design. This design
has the highest packing density. However, the crosstalk in the central core is the worst because it has 6 neighboring cores.
To avoid this problem, one can use one ring design as shown in Fig. 17 (b). Fig. 17(c) is a linear array design. A linear
array can have nxm cores, which can be designed to match semiconductor transceiver arrays. The designs of Fig. 17(a)(c) have a round fiber cladding. One limitation of round fiber cladding is that the number of cores in a fiber is limited by
the cladding diameter because if the cladding diameter is too large, the fiber loses it flexibility. To overcome this limit,
we can use a ribbon MCF design as shown Fig. 17(d). A ribbon structure offers the advantage of scalability for number
of cores in one dimension while keeping the other dimension small enough to maintain fiber flexibility in that dimension.
(a)
(b)
(c)
(d)
Fig. 17: Multicore fiber designs
One most important aspect for MCF designs is the crosstalk among the cores. The crosstalk depends on core
refractive index profile designs and the distance between two neighboring cores. One way to model the crosstalk is to
use the coupled mode theory [78]. Let’s consider two perfect identical cores separated by a distance D. From the coupled
mode theory, if we launch light into core 1, the powers P 1 and P 2 transmitted in two cores will change sinusoidally. The
power crosstalk can be calculated using the following equation:
P 


4κ 2

X = 10 log 2  = 10 log 2
2 
2
 P1 
 4 g ctan (gz ) + (∆β ) 
(9)
where z is the propagation distance, κ is the coupling coefficient, ∆β is the mismatch in propagation constant between
the modes in two cores when they are isolated, and g is a parameter depending on κ and ∆β,
 ∆β 
g2 = κ 2 + 

 2 
2
(10)
The crosstalk depends on the coupling coefficient κ that depends on the core design and distance between the two cores,
and ∆β that depends on the difference in refractive index profile between the two cores. This simple two-core model can
provide good guidelines for designing MCFs although a more complicated model is needed to determine the crosstalk
more accurately among all the cores in a MCF. According to Eq. (9), the most important factor to reduce the crosstalk is
to reduce the coupling coefficient. The coupling coefficient depends on the overlap integral of electrical fields of the
fundamental modes guided in two neighboring cores. Increasing the space between the two cores reduces the coupling
coefficient but results in lower packing density. A low index trench around the core in the cladding is effective to
confine the field closer to the core to suppress the crosstalk. Another factor is the mismatch in propagation constant
between the two cores. A small mismatch reduces effectively the maximum power that can be transferred from one core
to the other core. Therefore a heterogeneous core design can have lower crosstalk than homogeneous core design.
Eq. (9) shows that the power conversion efficiency oscillates sinusoidally with a coupling length of L=2π/g. If the
fiber length is much less than the half of the coupling length, Eq. (9) can be used to calculate the crosstalk. However, for
fiber length much longer than the coupling length, it was reported that the measured crosstalk of fabricated
homogeneous MCFs did not oscillate as shown Eq. (9), but accumulated linearly along the fiber length [79]. In addition,
the measured crosstalk of fabricated heterogeneous MCFs was reported to be ~40 dB larger than the power conversion
efficiency [80]. Such discrepancies may be due to inhomogeneity of cores along the fiber and fiber bending effects.
Taking into account fiber bending and statistical nature of crosstalk, a simple formula is derived for the average crosstalk
[81]:
X =2
κ2 R
L
β D
(11)
where κ is the coupling coefficient, β is the propagation constant, R is bending radius, D is the distance between two
cores, and L is the fiber length. Eq. (11) shows that the average crosstalk scales with fiber length, which explains the
linear scaling of crosstalk observed in experiments.
To design a MCF with low crosstalk, it is important to reduce the overlap between the electrical fields of the two
modes propagating in two neighboring cores. Step and trench assisted MFCs have been studied in detail in order to
maximize the core density [82]. Fig. 18 plots the crosstalk as a function of core spacing for step and trench assisted
profiles designs. The crosstalk was calculated using Eq. (11) with coupling coefficients taken approximately from Ref.
[82]. The bending radius used in the calculation was 50 cm, and the fiber length was 100 km. It can be seen that, for the
effective area of 80 µm2 with a step index profile design, the core spacing needs to be greater than 45 µm to ensure a
crosstalk of less than -30 dB after100 km propagation. With a trench profile design, the core spacing can be reduced to
about 37 µm for the same effective area. Even for the effective area of 100 µm2, the core spacing is smaller than the step
index with 80 µm2.
To accommodate more cores in a MCF, the cladding diameter can be increased. However, the fiber diameter is
limited by the mechanical reliability requirements. It has been reported that the maximum fiber diameter should be
smaller than 230 µm to ensure an acceptable failure rate under 50 mm bend radius [83]. With this fiber diameter, the
largest number of cores can be put into a multicore fiber is about 19 assuming the worst 100 km crosstalk.
Fig. 18 Crosstalk as function of core spacing for step and trench assisted profiles designs.
Low loss and low crosstalk MCFs have been demonstrated and used in transmission experiments using space
division multiplexing. Table 4 summarizes most recent experiments on space division multiplexing transmission using
MCFs, which represents the state of the art in MCF transmission. Total transmission capacity of over 1 Pb/s has been
demonstrated with spectral efficiency around 100 b/Hz [84, 85]. Long distance transmission over 6000 km has been
achieved using multicore EDFA [86]. Also, the number of core of 19 has been fabricated and used for a transmission
experiment [87].
Table 4: Space division multiplexing transmission experiments using multicore fibers
No of
cores
Core
spacing
Fiber
Fiber
diameter
Effective
area
Span
(km)
Data rate
(Gb/s)
No of
channels
System
Channel
Reach
spacing
length
Total
capacity
Spectral
efficiency
Ref.
12
12 SM
2 FM
7
19
(µm2)
(µm)
37
45
(µm)
225
216
81
80
52
3
56
35
200
200
99
72
55
10.1
456
92, 103
213
128
86
222
385
(GHz)
50
25
(km)
52
3
(Tb/s)
1010
1050
(b/Hz)
91.4
109
84
85
40
100
50
100
6160
10.1
28.8
305
14.4
30.5
86
87
MCFs are also attractive for high density short reach parallel optical data links. The use of VCSEL array and
multicore fiber to realize multichannel transmission was proposed in Ref. [88], where a transmission over a 2x2 MCF
using direct coupling with a linear VCSEL array at 1-Gb/s was demonstrated. Recently, a transmission over a hexagonal
7 core multimode MCF using tapered multicore connectors and 850-nm VCSELs was reported [89]. Also, MCFs with
linear geometry arrangements have been proposed for used with Silicon Photonics linear array transceivers [90,91].
MCFs with 1x4 and a 2x4 linear array core arrangements have been made. It has been shown that a step index profile
design with core separation of 47 µm resulted in crosstalk between two neighboring cores below -45 dB for fiber length
of 200 m, which is suitable for short reach applications. In a recent report, a ribbon MCF with rectangular fiber crosssection has been demonstrated with low crosstalk [92].
IV-3 Challenges for practical space division multiplexing systems
Although significant progress has been made in both FMFs and MCFs and SDM transmission systems, there are still
significant challenges for both the fibers and the components before they can be used in practical networks. From the
fiber side, fiber manufacturing companies need to understand fiber design space and identify optimum fiber designs for
both FMFs and MCFs in terms of crosstalk, mode coupling, multipath interference and attenuation. Most importantly,
there is a need to develop practical low cost processes for large scale production FMFs and MCFs.
From the component side, component makers need to develop optical components for SDM systems including
transceiver arrays, low cost multiplex and de-multiplex components, optical amplifiers for simultaneously amplifying
multimodes or multicores, and precision coupling components and connectors. These components are more challenging
to develop than FMFs and MCFs. In addition to developing SDM components, it is important to integrate these
components together with transceivers and WDM components to form subsystems. Without subsystem integration, it
will be hard to realize the cost benefits of SDM.
The current belief is that it will take many years to overcome the challenges mentioned above so there is still a long
way to go before the SDM technologies can be used in real communications systems. However, multicore fibers may be
used first in short reach applications because some of the components such as multicore amplifiers, mux/demux that are
required for long haul transmissions are not needed for short reach links. In addition, MCFs can adapt transceiver linear
arrays made with low cost silicon photonics, which has the potential to provide higher bandwidth-distance capability in
meeting the power and density needs of data centers and high performance computer interconnects.
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