No. 96 · APRIL 2023

SUMITOMO ELECTRIC TECHNICAL REVIEWFEATURED TOPIC

Fiber Bundle Fan-in/Fan-out for Four-Core
Multi-Core Fiber
Takahiro KIKUCHI, Osamu SHIMAKAWA, and Hidehisa TAZAWA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Next-generation transmission systems using multi-core fiber (MCF) for data center applications require fan-in/fan-out (FIFO)
devices with low insertion loss (IL) and high return loss (RL). A fiber bundle FIFO can achieve low IL and high RL by high-precision
core eccentricity control of the fiber bundle and physical contact connection between the MCF and the fiber bundle. Although ourFIFO is designed to allow only rotational alignment*1 between the MCF and the bundle, the optical characteristics of the fabricated
FIFO devices achieved IL of less than 0.3 dB and RL of more than 50 dB.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: space division multiplexing transmission technology, multi-core fiber, fan-in/fan-out

1. Introduction
In recent years, data traffic has rapidly increased withthe spread of high-definition video streaming services.
Correspondingly, the transmission capacity of single-corefiber (SCF) has been expanded by dense wavelength division multiplexing technology, but it is reaching the fiber
input power limit.(1) Since the transmission capacity isexpected to reach the limit in the near future, space division multiplexing transmission technology is being developed to increase transmission capacity by spatially dividing
transmission paths. Multi-core fiber (MCF), which has
multiple cores (light transmission paths) within a single
optical fiber, is one promising technology and can increasetransmission capacity in proportion to the number of coreswithout increasing the installation area.(2)It is assumed that the transmission systems using
MCF will start with a configuration that MCF is connected
to transceivers with an SCF interface using fan-in/fan-out(FIFO) devices. Since the use of FIFOs in the transmission
path increases the number of connection points comparedto the current system, FIFOs are required to have lowinsertion loss (IL) as well as high return loss (RL) above 55
dB to prevent transmission errors due to multi-path interference*2 between connection points.(3),(4)We have developed a fiber bundle type FIFO (bundleFIFO) to achieve low IL by core eccentricity control andhigh RL by physical contact (PC) connection. This FIFO issuitable for uncoupled 4-core MCFs that are expected to beadopted for data center applications.(5) This paper introduces the basic design and the characteristics of the FIFO.

2. Design of Bundle FIFO
The bundle FIFO consists of an uncoupled 4-core
MCF with a 40 µm square lattice configuration and an SCF
bundle. Figure 1 shows the specifications of the 4-core
MCF and the bundle. The MCF is a single mode (SM) witha mode field diameter (MFD) of 8.6 μm at the wavelengthof 1.31 μm and a cladding diameter of 125 μm. Bendinsensitive SCFs with the same MFD as the MCF wereEtched SCFs Bundle

Four-Core MCF

4040MCF

Etched SCF

CoreSM/UncoupledCoreSM/Trench

MFD

8.6

MFD

8.6

Cladding

CladdingCore pitch

Fig. 1. Specifications of 4-core MCF and bundle

etched from 125 μm to 40 μm in diameter. Four etched
SCFs were inserted into the hole of a zirconia ferrule, andthen terminated. The hole diameter is designed so that thecores of the SCFs are arranged in a square lattice configuration. The core position of the bundle can be easily
matched to that of the MCF.
One feature of this FIFO is the PC connection
between the MCF and the bundle. PC connection is aconnection technique used in optical fiber connectors.
When a pressing force is applied between the ferrules with
their tips butted against each other, the fiber ends are elastically deformed, and adhere tightly without gaps, resulting
in a high RL at the PC connection.
Figure 2 shows how to assemble a FI or FO device(FO device). The MCF and the bundle are rotated andaligned in a zirconia split sleeve, and then placed in
housing to complete the PC connection. For PC connection, the housing design is based on LC connector*3 parts.
The housing size is 63 mm × 5.6 mm × 5.6 mm, including
the boot components. Photo 1 shows a picture of a FIFO
pair with FO devices connected to both ends of the MCF.
Fiber Bundle Fan-in/Fan-out for Four-Core Multi-Core Fiber55（1）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 96 · APRIL 2023

Fig. 3. Eccentricity overview

Fig. 2. FO device assembly and PC connection

Photo 1. Picture of FIFO pairThe optical characteristic targets for the FO devices are set
to IL <0.5 dB, RL >55 dB, and crosstalk (XT) <-50 dB.

a square of 40 μm per side placed at the center of theferrule outer circumference.
3-2 Bundle Core Eccentricity Control
We performed simulations to determine the target forthe outer diameter variation of etched fibers, which is themain cause of bundle core eccentricity. 40 μm is the ideal
outer diameter of the etched fiber to be inserted into theferrule hole without gaps to match the MCF core pitch of40 μm. In this case, the ferrule hole diameter would beapproximately 96.6 μm. In actual manufacturing, however,slightly larger hole or thinner fibers must be used because
clearance is required to insert the fiber into the ferrule hole.
Therefore, in the simulation, IL was calculated assuming afiber with an outer diameter of 39-40 μm for a ferrule holediameter of Φ97 μm. Based on the Monte Carlo method,
cores were randomly positioned so that the distance
between cores in the ferrule was larger than the fiber diameter, and the core eccentricity d was calculated. IL can becalculated from the core eccentricity d using Eq. 1.(6) The
IL value of the bundle was defined as the maximum IL of
the four cores calculated from the core eccentricities.
................................... (1)3. Core Eccentricity Control
3-1 Core Eccentricity
The IL of the FIFO is degraded mainly by the core
position mismatch between the MCF and the bundle, whichis caused by core eccentricity of the bundle due to manufacturing variations. Figure 3 shows an overview of thebundle core eccentricity. In this bundle FIFO design, theferrules of the MCF and bundle are inserted into a split
sleeve to fix the XY axis for ease of assembly, and onlyrotational alignment can be performed. Since ferrules,
which have been developed in the field of optical connectors, have high coaxiality of inner and outer circumference
and high accuracy of inner and outer diameter, rotationalalignment in SCF connection is easily achieved by aligning
the outer circumference of the ferrule. However, in the caseof bundles, eccentricity control is more difficult due to (1)eccentricity of the ferrule hole relative to the ferrule outercircumference, (2) misalignment of the fiber due to clearance in the ferrule hole, and (3) variation in the outer diameter of the etched fiber. The core eccentricity of bundles
and MCFs is defined here as the deviation of the actual
core position from the designed core position at the apex of

The cumulative probability of IL obtained as a resultof 10,000 calculations is shown in Fig. 4. The result showsthat a yield rate of 90% for a target IL <0.5 dB is achievable
for a fiber outer diameter of 39.5 μm or larger. Based on thisresult, we set the fiber diameter control target to ±0.25 μm.

Fig. 4. IL yield rate by fiber diameter
Fiber Bundle Fan-in/Fan-out for Four-Core Multi-Core Fiber56（2）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 96 · APRIL 2023

We fabricated eight bundles by preparing etched fibers
based on the results of the previous simulation. Figure 5
shows the core eccentricity of the eight bundles measured
with an image-processing measuring instrument. The coreeccentricity of the bundles was 0.42 μm on average. In addition, the distribution of IL estimates calculated from the coreeccentricity with respect to the ideal MCF is shown in
Fig. 6. The average IL was 0.06 dB and the maximum IL
was 0.30 dB. The core eccentricities were suppressed byusing etched fibers with an appropriate outer diameter tothe ferrule hole diameter and a small diameter variation.
Fig. 7. Schematic diagram of a rotational alignment

Fig. 5. Core eccentricity of the fabricated bundles measurement

maximum of 1.02 μm. Figure 8 shows an example of IL
variation by rotational angle when a bundle with an averagecore eccentricity of 0.85 μm and an MCF with an averagecore eccentricity of 0.79 μm are rotationally aligned. Since
the four cores are arranged in a square configuration, there
is an angle at which the total IL value of the four cores
becomes small every 90 degrees. Among them, the IL of
the four cores is smallest near one direction where theferrule hole eccentricity direction coincides.
The results of IL estimation simulating rotational alignment with 8 MCFs are shown in Fig. 9. Since the rotationalalignment compensated for each other’s core eccentricity,Fig. 6. IL estimates calculated from bundle core eccentricity

Fig. 8. IL variation by rotation angle in rotational alignment

3-3 Rotational Alignment
In FIFO assembly, the core eccentricity of the bundle
is compensated by rotational alignment with MCFs that
also have core eccentricity. A schematic diagram of rotational alignment is shown in Fig. 7. The cores of the MCF
and bundle rotate with the center of the ferrule outercircumference in the split sleeve. Since both ferrule hole
centers have eccentricity to the outer circumference center,when only rotational alignment is performed, there arepositions where the distance between the cores of the MCFand bundle is the largest and the smallest.
The core eccentricity of eight MCFs was alsomeasured to simulate the rotational alignment. The coreeccentricity of the MCF was 0.60 μm on average and aFig. 9. IL estimates considering rotational alignment
Fiber Bundle Fan-in/Fan-out for Four-Core Multi-Core Fiber57（3）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 96 · APRIL 2023

the maximum IL value was estimated to be 0.1 dB. Theseresults suggest that even a simple assembly with only rotational alignment can be sufficient for low IL FIFO production if eccentricity-controlled bundles are used.

4. Optical Characteristics of FIFO
Four FIFO pairs were fabricated by MCFs andbundles whose core eccentricity was measured, and their
optical properties were evaluated. Measurements weremade at wavelengths of 1.31 µm and 1.55 µm, respectively.
Table 1 summarizes the results of the measurements of IL,
RL, and XT, which are the main characteristics.

Fig. 11. RL measurement result

Table 1. Measurement results of optical characteristics(1.31/1.55 μm)[Unit: dB]
ILRL

XTN

16

32

48

Ave.

0.29/0.33

57.5/58.3-68.0/-61.6

Min.

0.16/0.12

51.7/54.5-74.9/-68.4

Max.

0.51/0.57

61.1/61.5-63.7/-55.8

SD.

0.10/0.11

2.2/1.9

2.4/3.1

Figure 10 shows the distribution of IL for a FIFO pair.
Simply multiplying the result of measurements by 0.5 toconvert the IL per FO device unit, the maximum value was
0.255 dB at the wavelength of 1.31 μm. The measured ILs
were higher than the value of less than 0.1 dB obtained in
the rotational alignment simulation. The reasons for thisdeviation may be the inclusion of LC connector connection
loss in this measurement system and fiber bending loss inthe bundle ferrule.

Figure 12 shows the XT results for the FIFO pair. XTproduced in the nth core with light in the mth core is definedas Eq. 2. XT achieved the target value at both wavelengths.
.... (2)The FIFO pair was placed in a thermostatic oven toevaluate the IL temperature dependence. The evaluation
conditions were -40°C to 85°C, referring to the “Category
OP+ HD ” conditions of IEC61753-1. Figure 13 shows the IL
variation at the wavelength of 1.31 μm for one sample. In

N = 48 patterns

Fig. 12. XT measurement results

Fig. 10. IL measurement result

Figure 11 shows the measurement results of the RL of
the FIFO devices. Note that the RL was measured using theOTDR*4 system and was measured for each FO device.
Although some cores failed to the target of 55 dB, theaverage was 57.5/58.3 dB (wavelength: 1.31/1.55 µm).

Fig. 13. Temperature-dependent loss
Fiber Bundle Fan-in/Fan-out for Four-Core Multi-Core Fiber58（4）SUMITOMO ELECTRIC TECHNICAL REVIEW

all cores, the IL followed the temperature change, but thefluctuation range was within ±0.1 dB.

5. Conclusion
We introduced the design and optical characteristics
of a bundle FIFO suitable for data center transmission
systems using 4-core MCFs. Low IL by high-precision coreeccentricity control and high RL by PC connection wereachieved.
The core eccentricity data of the fabricated bundles
showed that the core eccentricity can be suppressed bycontrolling the etched fiber diameter. The core eccentricitiesof the bundle and MCF are compensated by the rotationalalignment, and an IL less than 0.1 dB due to the coreeccentricity is possible.
The bundle FIFO achieved the target in IL and XT,
and RL missed the target by 55 dB in some cases but
achieved the target in the average value.
In the future, we would like to contribute to the spreadof MCF transmission systems by further improving FIFO
characteristics.

No. 96 · APRIL 2023

References(1)	T. Morioka, “New Generation Optical Infrastructure Technologies:"EXAT Initiative” Towards 2020 and Beyond,” in Proc. Beyond,” inProc. OECC2009, FT4(2)	G. Rademacher et al, “10.66 Peta-Bit/s Transmission over a 38-CoreThree-Mode Fiber,” in Optical Fiber Communication Conference(OFC) 2020, paper Th3H.1(3)	Y. J. Wen et al, “Mitigation of optical multipath interference impact for
directly detected PAMn systems,” Opt. Express 28, 38317-38333(2020)(4)	“IEEE Standard for Ethernet Amendment 10: Media Access Control
Parameters, Physical Layers, and Management Parameters for 200 Gb/s
and 400 Gb/s Operation.”(5)	O. Shimakawa, M. Shiozaki, T. Sano and A. Inoue, “Pluggable fan-out
realizing physical-contact and low coupling loss for multi-core fiber,”
2013 Optical Fiber Communication Conference and Exposition and theNational Fiber Optic Engineers Conference (OFC/NFOEC)(6)	D. Marcuse, “Loss analysis of single-mode fiber splices,” The Bell
System Technical Journal 56 (1977): 703-718

Contributors The lead author is indicated by an asterisk (*).

T. KIKUCHI*• ‌Assistant Manager, Optical Communications
Laboratory

Technical Terms＊1	Alignment: To align the optical axis in the assemblyof an optical device. Adjustments are made in thevertical, horizontal, forward/backward, rotational,and tilt directions by usually monitoring opticalpower.
＊2	Multi-path interference: Interference between multiple
modes and reflected light of an optical signal
propagating in an optical fiber, leads to degradation oftransmission quality due to fluctuations of opticalpower in time.
＊3	LC connector: A compact PC connector used inplaces where high-density wiring is required, such as
rack-to-rack wiring in servers.
＊4	OTDR: Optical Time Domain Reflectometer (OTDR)
is a measuring instrument used to evaluate loss andreflection measurements of optical fiber transmission
paths.

O. SHIMAKAWA• ‌Senior Assistant Manager, Optical Communications
Laboratory

H. TAZAWA• ‌Ph.D.
Group Manager, Optical Communications
Laboratory

Fiber Bundle Fan-in/Fan-out for Four-Core Multi-Core Fiber

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