〉〉 Communications Solutions

Hollow Core Fiber Cable
Kazunori Mukasa*

Hollow core fibers (HCF) are innovative optical fibers having the potential tobreak the limits of conventional optical fibers. Examples of innovation are ultralow loss potential, ultra-low nonlinearity, and long interaction lengths with gases confined to thecore. In addition, ultra-low latency characteristics have attracted much attention in recent years,
particularly amongst financial trading communication systems. For the first time in the world, we
have succeeded in developing a high-performance HCF with practical single mode (SM) properties achieved by a resonant couple phenomenon, to implement the HCF in the low latency transmission. Furthermore, we have successfully developed HCF connectors and HCF cables toachieve the actual low latency transmission. These development results are reported.

ABSTRACTINTRODUCTION
1.1.INTRODUCTION
The confinement of light in HCF is different from the conventional mechanism of total internal reflection (TIR)
which relies on a high index core surrounded by a low
index cladding. On the other hand, since HCF can propagate 99% or more light in the hollow core with a refractive
index of approximately 1.0 by the photonic bandgap(PBG) or the anti-resonant principle, then the effective
group index of light becomes about 1.002~1.003. This issignificantly lower than the 1.47 obtained with conventional glass fibers, and other various characteristics thatare difficult to achieve with the conventional glass corefiber are possible to achieve.1)-3) Confining and guiding
light in an air core which has a low refractive index
enabled novel properties that are difficult to achieve inconventional optical fibers, such as low latency, low nonlinearity while maintaining low bending loss characteristics as summarized in Figure 1.
Among these, the ultra-low latency characteristics are
attracting attention for applications such as in the highspeed trading in the financial services, the high-speed
computing, Beyond 5G, various remote controls, and wiring in data centers.4), 5) Even with the glass fiber, for example, by using a pure silica core and a depressed typecladding a certain improvement is possible, but its
improvement rate is small about few percent at best.
Whereas, by changing the core from glass to the hollow
core, 30% or more of significant improvement is possible
in the latency characteristics. The improvement becomes
more remarkable as the link length becomes longer.
Figure 2 shows the improvement rate, with length, of thelatency in HCF. The improvement on the order of a microsecond is obtained for link lengths exceeding 1 km. It can*

R & D Division, Telecommunications & Energy Laboratories

Unique principal of the hollow
core fiber

Innovative characteristics
Ultra-low latency characteristics
Ultra-low loss potential

Light is confined not in the glass
core but in the air core

Ultra-low nonlinearity
Super high reliability /Resistance feature to severe
environment
Interaction with a gas in thecore

Light confinement is based on aprinciple different from theconventional total internalreflection mechanism

Ultimate low bending loss
characteristics
Unique dispersion / polarization
characteristics

Figure 1 Examples of innovative characteristics achieved by
HCF.
Low latency transmission
Input

Output0 1 0 1 0 1 0 1

Hollow core 1 0 1

0 1 0 1 0 1 0 1

Glass 1
core0 1
Diﬀerential delay

Link length

Glass corelatency time

Hollow corelatency time

Improvementof latency

300 m

1.47 µs

1.00 µs

0.46 µs

500 m2.44 µs

1.67 µs

0.77 µs

1,000 m4.89 µs

3.34 µs

1.54 µs

10,000 m

48.9 µs33.4 µs15.4 µs

Figure 2 Improvement rate of the latency characteristics in
HCF.

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DEVELOPMENTNEWHOLLOWHOLLOW
2.2.DEVELOPMENT
OFOF
AA
NEW
COREFIBERFIBERANDHAVINGAN EFFECTIVE
CORE
AN EFFECTIVE
SMSM-PROPERTY(PRISM-PBGF)
PBGF(PRISM-PBGF)
As described, in PBGF, achieving both low loss and SM
operation are a key issue for its application in the actual
transmission. To achieve these desirable properties, weapplied a new structure that is different from the conventional structure. With the new structure, it became possible to suppress the propagation of HOMs while maintaining the core of 19-cell. 9) -11) A Scanning Electron
Microscopy (SEM) of the structure is shown in Figure 3.
The most notable feature of this fiber is that it uses a
19-cell type core which can achieve a low transmission
loss, but has a special structure called Perturbed
Resonance for Increased Single Modedness (PRISM)
structure, which has two cores next to the center core
(Shunt cores) corresponding to 7 cells. With this structure, the HOMs in the center core are selectively coupled
to the shunt cores to suppress its propagation, and thisachieves an effective SM transmission. By matching theeffective group index of the HOMs with the effective
group index of the fundamental mode of the cores, theHOM of the light signal in the center core is coupled withthe shunt cores at high efficiency. Since the shunt cores
are located near the edges of the cladding of the light, thecoupled light leaks out efficiently without propagating
through the shunt cores. By this newly developed structure that applies this unique principle, preferable properties of low loss and effective SM that were not possible
with the conventional PBGFs can be achieved. This concept is important and novel that would get rid of the
major obstacles in the practical application of HCF.
A very advanced technology is required for the fabrication of the PRISM type PBGF.

Eﬀective group index
of modesbe said that the transmission line with HCF has potentially extremely high needs, since it is considered to be an
important characteristics not only in niche fields such as
financial transactions, but also in fields such as autonomous-driving, remote medical services and factory operations where transmission latency can be fatal in the
future 6), 7).
However, not only the innovations but also many issues
which are necessary to be solved exist in HCF for its
practical applications. For example, though theoreticalpredictions indicate ultra-low loss potential, the loss ofthe current HCF is higher compared to that of the conventional silica fiber. Further investigation is necessary to
reduce the loss of HCF by optimizing the processes. In
addition, it is a significant problem that multi modes (MM)propagetes in HCFs, which is highly undesirable, as SMtransmissions are expected to become the standard inmany low-latency transmission routes. There are several
types of HCF such as the photonic bandgap fiber (PBGF)
and the anti-resonant fiber (ARF). PBGF has been investigated for the longest time, and we also have been studying with PBGF type. PBGF has a regular array structure
formed in its cladding in order to prohibit wavelength
groups and angle groups of light from propagating within
the cladding by the photonic bandgap (PBG) phenomenon. By forming an irregular structure (defects in structure) in the central part, launched light into the hollow
core is prohibited from propagating in the cladding due to
PBG phenomenon and a mode is formed in the core andpropagates. The dominant factor of the loss is scattering
loss which is induced at the rough interface (frozen-in
capillary wave) between air and glass created by heat
during the fabrication process.2) PBGF is classified into
3-cell type, 7-cell type, 19-cell type, etc. depending on
how many times sizes of the core are larger compared tothe size of regular structures in the cladding. The larger
core has a smaller intensity of light that overlaps with theglass surface, and the transmission loss tends todecrease. On the other hand, the larger the core, themore the High Order Modes (HOMs) are confined, and it
allows the propagation of more modes.8) This is a major
obstacle in applying PBGF to the low latency transmissions that use existing transmitters and receivers. For theactual use of HCF in low latency transmission, the establishment of peripheral technologies, such as connection
and cable fabrication, is also necessary, but very little
investigation has been made on those technologies. As
described above, though the potential of HCF is very
high, there are many issues that need to be overcome forthe practical use of HCF in low latency transmission.

Fundamental modein the center core

Coupling
leak
HOM
Fundamental
in the center coremode in the
shunt core

Figure 3 SEM image of the newly developed PBGF.

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Hollow Core Fiber Cable

Figure 4 shows an example of a fabrication process. A
large number of thin glass capillaries are put (stacked)
into a silica jacket tube and a fiber is drawn while maintaining a regular arrangement. At first, a regular hexagonal close packed structure is assembled in a jacket tube
without the distortions of the structure to obtain the basic
structure. This is an important process for obtaining therequired properties in PBGF. Not only the regularity of thecladding but also the appropriate position and structureof the center core and the shunt cores are necessary, anda very advanced processing technology is required. The
drawing process is also a key process in order to achieve
PBGF with good characteristics. The assembled capillaries and tubes need to be drawn to obtain the optical fiber,
while collapsing the interstitial holes between the capillaries and the tubes without any significant distortion to theoverall structure.
We have established the advanced process technologies and successfully achieved a PBGF with very regularstructures and desirable optical characteristics over along length of fiber. Figure 5 shows an example of thecross-sectional structure and transmission loss.
As described above, the most significant feature of the
HCF presented here is that the loss of the HOM is suppressed by the PRISM structure. Figure 6 shows the field
distribution of a typical 19-cell HCF and the PRISM fibers

50

Loss / dB/km

40
30
20
10
0
1475
Figure 4 Concept diagram of the newly developed PBGF
processes.
Cross sectional images

1500

1600

Figure 5 Structure and loss spectrum of the newly developed
PBGF.
Example of a HOM characteristics spectrum and a ﬁeld distribution
600

λ= 1500.001 nm

DGD / ps

500

Conventional
type
19-cell PBGF

1525
1550
1575
Wavelength / nm

400
300
200
100
0
1500

1510 1520 1530
wavelength / nm

1000

λ= 1580 nm

PRISM
structure
19-cell PBGF

DGD / ps

800
600
400
200
0
1560 1570 1580 1590 1600
wavelength / nm

Figure 6 Examples of traditional and the newly developed PBGF mode profiles and spectrograms.

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Hollow Core Fiber Cable

along with the spectrogram11) of the PRISM fibers. The
blue darker areas correspond to the area where the propagation of HOM is suppressed and the red and green
areas correspond to an area with HOM components. As
Figure 6 shows, effective SM operation, which was difficult to achieve with the conventional 19-cell PBGF, canbe achieved by effectively coupling HOM to the shuntcores in a PRISM fiber. Field distributions of the PRISM
fiber also represent a more Gaussian-like field distribution, typically associated with a fundamental mode of an
SMF. Thus, it was confirmed that the newly developed
PRISM type PBGF has characteristics more suitable for apractical use.
In order to suppress further higher order modes,
PBGFs with 6 shunt cores were developed, as shown in
Figure 7.12) The concept is the same, but effective SM
performance can be ensured more reliably, as the problem of a coupling efficiency reduction caused by a bending direction is suppressed by shunt cores that couple
HOM at six places. As shown in Figure 7, by increasing
number of shunt cores from 2 to 6, no deterioration in thetransmission loss of PBGF was observed.
Figure 8 shows HOM characteristics of 2-shunt coretype and 6-shunt core type fibers (5 m in length) wound
on a 15 cm of coil. As is clear from Figure 8, even with the
2-shunt core type, the effective SM characteristics havebeen obtained, but by adopting the 6-shunt core type,
HOM component in the transmission band is further suppressed. So, it can be seen that achievement of lowtransmission loss characteristics and further improvementof SM performance, which are important for the actual
transmission, are achieved.
When installing HCF cable in the actual field, the HCF
has to be robust and withstand environmental conditions
such as sunlight, hot and cold temperatures and mechanical vibration. The 6-shunt core type PBGF with further
stable mode characteristics was used for the trial cablefabrication described later. The characteristics of the two
PBGFs used in the trial cable fabrication are shown in
Figure 9. Not only in the spool state but also after thecable fabrication, the loss increase was small and stable
low loss characteristics were obtained. Also, the dispersion characteristics of all the fibers used in the trial cablefabrication were controlled to low values at 1550 nm,
which were preferable characteristics for low-latencytransmission.

min loss = 2.7±0.3 dB/km at 1545.8 nm

30

Loss / dB/km

25
20
15
10
5
0

1500

1520

1540

1560

1580

1600

1620

Wavelength / nm
Figure 7 Stacked preform and SEM of the fiber for 6-shunt core type PBGF (left) and its transmission loss (right).
-10

1000

1000

800

800

DGD / ps

1200

DGD / ps

1200

600

-40
400

200

200
1540

1560
1580
Wavelength / nm

1600

-30

600

400

0

-20

0

-50

1540

1560
1580
Wavelength / nm

1600

-60

Figure 8 Mode spectrograms of 2-shunt core (left) and 6-shunt core (right) PBGFs.

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Hollow Core Fiber Cable

1000
500

25

Cabled C

20

Spooled ﬁber C

15

Cabled D

CD / ps/nm

Loss / dB/km

30

Spooled ﬁber D

10

0
-500

HCF cable C
HCF cable D
HCF cable A
HCF cable B
total CD (ps/nm)

-1000
-1500

5
0
1538

1543

1548

1553

1558

-2000
1543 1545 1547 1549 1551 1553 1555 1557 1559

1563

Wavelength / nm

Wavelength / nm

Figure 9 Examples of structure and loss of 6-shunt core PBGF and wavelength dispersion characteristics.

DEVELOPMENTTHE
RELATEDTECH3.3.DEVELOPMENT
OFOF
THE
RELATEDTECHNOLOGIESFOR PRACTICALNOLOGIES
FOR PRACTICALUSE USE
3.1 Development of PBGF Connectors
For the practical application of HCF, connection technology is an important factor. Fusion splicing and a connector
can be considered as important connection technologies.
For the fusion splicing, various investigations have been
carried out13)-15), but there was a problem that the fusion
splicing loss was high because of the significant mode
field mismatch between HCF and the conventional SMF
and deformation of the PBGF structure by splicing. Also,
reflection loss tended to be high. With the cable using
PBGF, we have since lowered splice losses to the order of
1 dB. The PBGF connector is a key component for easeof installation into existing transmission systems, but therealization has been difficult. We have succeeded in thedevelopment of PBGF connector for the first time in theworld16). For the connector, we used PRISM type PBGF
which is our original structure. Since the 19-cell core
structure is used for the center core, in spite of low transmission loss of 3.9 ±1.2 dB/km (1571 nm), the effective
SM performance is achieved. An effective core area (Aeff)of this PBGF is 200 μm2, which is much larger comparedto the 80 μm2 of conventional fibers. Therefore, a coupling loss is expected to be large when connecting thisconnector with SMF due to the difference in Aeff. Then,the coupling loss caused by a physical bonding between
PBGF and solid fibers having different Aeff values prepared by flat cleave, was investigated. The results areshown on the left side of Figure 10, and it can be seenthat when Aeff of the solid fiber is about 165 μm2, thecoupling loss is minimized to about 0.25 dB. This is close
to the theoretical limit, considering the coupling of a solid
fiber with a circular Gaussian mode and PBGF with amore hexagonal field. Based on this result, we fabricatedan FC/PC connector using a solid fiber with Aeff of
165 μm2. In order to suppress the reflection loss, AntiReflection (AR) coating was applied for this time. The
right side of Figure 10 shows the reflection loss of the flat
cleaved fiber and the AR coated connector, and it isfound that the reflection loss at 1585 nm can be reduced
to –42.5 dB by the AR coating.

-10

0.9

-15

0.8

-20
Return loss / dB

Coupling loss / dB

1.0

0.7
0.6
0.5

ﬂat cleaved ﬁber
AR-coated connector

-25
-30
-35

0.4
0.3

-40

0.2
80

100

120
140
160
Launch Aeﬀ / µm2

180

200

-45

1500

1520

1540
1560
Wavelength / nm

1580

1600

Figure 10 Coupling loss and connector reflection loss characteristics by physical coupling between PBGF and solid fiber.

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Hollow Core Fiber Cable

Regarding the HCF connector, HCF was inserted into
the central void part of the FC/PC connector as shown in
Figure 11, the protruding part was flat cleaved, and thefiber end surface was pulled back to the surface of theconnector and fixed with epoxy resin. The fabricated HCF
connector was joined with the aforementioned AR coatedenlarged-Aeff solid fiber connector. The right side of
Figure 11 shows the obtained optical properties. The
insertion loss of a jumper cable which consists of twoconnectors was 1.4 dB at 1571 nm, which was only
0.6 dB higher than the loss of the fiber itself. The difference in the insertion loss from the incident direction was
hardly observed. And, the reflection loss at 1571 nm wasvery low at –31.3 dB by using the AR coating, but further
improvement is possible by adjusting the transmission
window of the PBGF to match with the effective wavelength of the AR coating.
The coupling loss from the AR coated Aeff enlarged
fiber connector to the HCF connector was approximately
0.3 dB, which is slightly higher than the 0.25 dB for physical contact bonding, but the loss obtained by the fusion
splice optimization was approximately 0.75 dB, andwhich shows a clear advantage. It is also a dramatic
improvement compared to the 1.2 dB11), which is a recordof angle-cleaved physical contact losses between 7-cell

PBGF and SMF. Therefore, the developed technology on
the HCF connector would be a key for the practical application of HCF.
3.2 Development of PBGF Cables
It is well known that the optical properties of an opticalfiber can be changed such as due to tensile strain during
the cable fabrication process, or an environment effectafter cable fabrication. Similar with conventional fibers, itis necessary for HCF to evaluate characteristics such as
loss, dispersion and polarization mode dispersion (PMD)in the conditions in which the cable is actually used.
Using the PBGF with 6-shunt cores, as described in section 2, the world’s first loose tube type PBGF cable wasfabricated and evaluated.17) Each cable was terminatedwith an LC/UPC connectors that were fusion spliced to
SMFs. The increase in loss after cabling was less than
0.7 dB/km at wavelengths between 1544.9 and 1558.6 nm,
and no significant transmission penalty has been
observed in the severe environmental tests at – 40°C to
80°C of temperature. PMD characteristics are important
optical characteristics in PBGF the same as with the conventional fibers. Figure 12 shows the PMD of PBGF
drawn without twisting (left) and the PMD of the PBGF
drawn with twisting (right). The fiber drawn without twisting was measured in a spool state (orange) and in a cable

Transmission and return loss / dB

ﬁber transmission
fully connectorized, 200m length of HCF
return loss
HCF butcoupled to uncoated ULA ﬁber
return loss
fully connectorized, 200m length of HCF
0
-10
-20
-30
-40
1500

1520

1540 1560 1580 1600
Wavelength / nm

Figure 11 Images (left) and characteristics (right) of PBGF connectors.
100

100
Spooled ﬁber

Cabled80
DGD / ps

DGD / ps

80
60
40

HCF cable D

60

HCF cable A
HCF cable B

40
20

20
0
1545

HCF cable C

1547

1549
1551
1553
Wavelength / nm

1555

1557

0
1543 1545 1547 1549 1551 1553 1555 1557 1559
Wavelength / nm

Figure 12 PMD characteristics of the fiber without twisting (left) and the fiber with twisting in cables (right).

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Hollow Core Fiber Cable

BER

state (black). In a spool state, the PMD reduction effectwas observed due to the coupling of polarized lightcaused by overlapping of the fibers, but after being
cabled the effect was reduced and appeared the PMD
that the fibers had intrinsically. As shown in the right sideof Figure 12, we successfully reduced PMD even after
cabling, by the effect of polarization coupling which is
caused by continuous twisting when drawing at a pitch
longer than the beat length of the fiber. The typical PMD
in 1 km of length was less than 10 ps.
In order to evaluate the performance of the fabricated
HCF cable, 10Gb/s non-return-to-zero (NRZ) signal WDM
transmission experiment was carried out. 35 signal lightchannels with wavelengths from 1544.92 nm to 1558.58
nm were multiplexed at 50 GHz spacing by separating
the odd-numbered channels and even-numbered channels, and incident on HCF. A booster amplifier wasapplied at the input side of the transmission link which
consisted of two 0.59 km length HCF cables and two
1 km length HCF cables, approximately 3.1 km in total,each fiber is terminated with LC/UPC connectors fusion
spliced with conventional SMFs.
This time, since the configuration includes fusion splices, the total link loss at 1550 nm was 30 dB, but further
reduction would be possible by using the abovedescribed PBGF / large Aeff fiber connector. The transmitted signal was amplified, demultiplexed, and sent to
the receiver, where the Bit Error Rate (BER) was measured.

1.0E-06
1.0E-07
1.0E-08
1.0E-09
1.0E-101.0E-11
1.0E-121.0E-13
1.0E-14
1.0E-15
1.0E-16
1.0E-1710

B2B
1545.32 nm
1555.75 nm
1558.58 nm

12

14

16

1544.92 nm
1550.12 nm
1558.17 nm

18

20

22

OSNR / dB/0.1 nm
1E-09
1E-10
1E-11

BER

1E-12
1E-13
1E-14
1E-15

1544.92 nm

The obtained characteristics are shown in Figure 13. The
upper side of Figure 13 shows the relationship between
the Optical Signal to Noise Ratio (OSNR) and BER for thesignals at 2 wavelengths near the center of 35 channels in
the wavelength range from 1544.92 nm to 1558.58 nm,
and the signals at 4 wavelengths near the edges. BER ofless than 10 –15 were obtained in 33 channels in between
1545.32 to 1558.17 nm. Most of the channels have OSNR
penalty of 4 to 5 dB at BER of 10 –15, but the channels
near the edge have penalty of approximately 6 dB. On thelower side of Figure 13, the BER and several eye diagrams
are shown when the received OSNR is 20 dB / 0.1 nm.
Error-free transmission (BER<10 –15 ) was confirmed for 33
channels, and the system margin of 4.5 dB / 0.1 nm wasconfirmed on average. From the eye diagrams, it wasobserved that short wavelength penalties are mainly
caused by dispersion, and long wavelength penalties aremainly caused by PMD. As a result, very good transmission performance was confirmed with the actual HCF
cable. We believe this achievement is very important
ensuring the high performances when the HCF cables will
be used in many real practical applications, such as low
latency transmission or supercomputing, in the future.

CONCLUSION
4.4.CONCLUSION
Remarkable progress has been made in the research on
HCF cables, which are expected to expand into applications such as financial trading transmission. By optimizing the unique and novel PRISM structure and the fabrication process, we succeeded in achieving both a low
loss and a practical SM of HCF. In addition, the HCF connector and the cable connected with LC / MC connector
were successfully developed, making great strides
towards its application to real low-latency transmission.
The effect on PMD by cabling was investigated, and bysolving the problem in the fiber drawing process, the HCF
was successfully cabled with no significant degradationin the optical properties. By these optimizations, BER freetransmission (BER < 10 –15 ) in 33 channels over 3.2 km ofcabled HCF has been demonstrated. We believe thatsuch comprehensive research results for practical use willlead to the actual use of HCF in low-latency transmission
lines.

1558.58 nm

20 ps/div

20 ps/div
1551.72 nm

20 ps/div

1E-16
1E-17
1544 1546 1548 1550 1552 1554 1556 1558
Wavelength / nm

ACKNOWLEDGEMENT

This review paper was mostly supported by investigations
by Brian J. Mangan, Jeffrey W. Nicholson, Zhu Benyuan,
Tristan Kremp, Merari Cortes, Matt Corrado, Gabe Puc,Yuriy Dulashko, Vitaly Mikhailov, Robert S. Windeler, Eric
M. Monberg, Kyle Dube, Ken Marceau, Brian Violette,
Donald Cartsounis, Ralph Lago, Brian Savran, Tahar Irid,
Robert J. Blakley, Ji Lie, Daryl Inniss, David J. DiGiovanniand other members in OFS.

Figure 13 Transmission characteristics of HCF.

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REFERENCES
1) P. J. Roberts et al.: “Ultimate low loss of hollow-core photonic
crystal fibres”, Opt. Express, Vol. 13, No.1, pp.236-244, (2005).
2) B. J. Mangan et al.: “Low loss (1.7dB/km) hollow core photonic
bandgap fiber”, OFC 2004, paper PD24, (2004).
3) T. D. Bradley et al.: “Antiresonant hollow core fiber with 0.65dB/
km attenuation across C and L telecommunication bands”, ECOC
2019 PD.3.1, (2019).
4) B. J. Mangan et al.: “First demonstration of hollow-core fiber for
intra data center low-latency data transmission”, OFC 2015, paper
M3D.4, (2015).
5) Y. Chen et al.: “Demonstration of an 11 km hollow core photonic
bandgap fiber for broadband low-latency data transmission”,
OFC2015, paper Th5A.1, (2015).
6) K. Mizushima et al.: “Evaluation of influence of delay of image
information on steering maneuver in remotely controllable automated driving system”, Transactions of the Society of Automotive
Engineers of Japan, Inc., Vol. 50, No. 3, pp.970-976, (2019). (inJapanese)
7) I. Maida et al.: “Influence of visual delay on minute pointing andpush down by upper limb assuming telesurger y application”,
Transaction of the Virtual Reality Society of Japan, Vol.22 No. 1,
pp.61-69, (2017). (in Japanese)8) M.N. Petrovich et al.: “Robust single mode hollow core photonic

bandgap fiber”, Opt. Express 16, pp.4337-4346, (2008).
9) J. M. Fini: “Aircore microstructured fibers with suppressed higher-order modes”, Opt. Express 14, pp.11354-11361, (2006).
10) J. M. Fini et al.: “Low-loss hollow-core fibers with improved single-modedness”, Opt. Express, 21(5), pp.6233-6242, (2013).
11) J. W. Nicholson et al.: “Measuring higher-order modes in a lowloss, hollow-core, photonic-bandgap fiber”, Opt. Express 20,
pp.20494-20505, (2012).
12) B. J. Mangan et al.: “Single mode, polarization maintaining hollow
core fibre with significantly improved higher order mode reduction”, ECOC 2015, Mo.3.1.2, (2015).
13) F. Couny et al.: “Reduction of Fresnel back-reflection at spliceinterface between hollow core PCF and single-mode fiber”, IEEE
Phot. Tech. Letter 19, pp1020-1022, (2007).
14) L. Xiao et al.: “Fusion splicing photonic crystal fibers and conventional single-mode fibers: Microhole Collapse Ef fect”, J.
Lightwave Technology 25, pp3563-3574, (2007).
15) K. Z. Aghaie et al.: “Optimization of splice loss between photonicbandgap fibers and conventional single-mode fibers”, Opt. Lett.
35, pp.1938-1940, (2010).
16) J. W. Nicholson et al.: “Low-loss low return-loss coupling between
SMF and single-mode, hollow-core fibers using connectors”,
CLEO Applications and Technology, paper JTu4A, (2014).
17) B. Zhu et al.: “First demonstration of hollow-core-fiber cable forlow latency data transmission”, OFC 2020, paper Th4B.3, (2020).

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