10 · Ultra-High-Fiber-Count and High-Density Slotted Core Cable with Pliable 12-Fiber Ribbons

INFOCOMMUNICATIONS

1. Introduction

Reflecting the recent advancement of cloud computing and other big-data processing technologies, a growing number of large-scale data centers are currently being constructed. As future increase in data transmission capacity between these facilities is expected, the demand for high-count, high-density optical cables is growing. Optical cables that connect these data centers are usually installed in cable ducts located outdoors, which requires technology that allows high-density installation of these cables in a limited conduit space. To meet this demand, we have developed a series of high-fiber-count, high-density optical cables that are flexible in all directions. In partic-ular, the new optical cables use pliable 12-fiber ribbons to ensure high flexibility and facilitate mass-fusion splicing.

In addition, the core of these cables is slotted and a strength member*1 is passed through the center of the core. This paper describes the designs and characteristics of 1728- and 3456-fiber-count slotted core cables shown in Table 1. These two cables are representative models of the new cable series that has been developed to increase the fiber density in 1.5-inch and 2.0-inch cable ducts, which are in wide use today.

2. Design and Evaluation of Pliable 12-Fiber Ribbon

The pliable ribbon that we used for the new cables is schematically illustrated in Fig. 1. The flexibility of the ribbon and the alignment of its fibers for mass-fusion splicing can be controlled by adjusting the lengths and the ratio of the adhesive part to the non-adhesive part. This study compared the mass-fusion splicing workability of a single-fiber type pliable ribbon shown in Fig. 1 (a) and with that of a two-fiber type pliable ribbon shown in Fig. 1 (b).

For the comparison, we measured the total time required to complete the following work procedures:

(1) Setting test ribbon on fiber holder of fusion splicer(2) Removing coating resin with heating type remover(3) Cutting off fiber end with fiber cutter(4) Setting test ribbon on fusion splicer to splice fibers

togetherIn particular, we compared the fusion splicing time

between the pliable ribbons (a) and (b) shown in Fig. 1 by varying the adhesive pitch “P” with the ratio between the adhesive part “a” and non-adhesive part “c” kept constant.

The results of fusion splicing time measurement are shown in Fig. 2. The single-fiber type pliable ribbon required relatively long time even for a shorter adhesive pitch range. The main reason for this is that the single-fiber type pliable

Ultra-High-Fiber-Count and High-Density Slotted Core Cable with Pliable 12-Fiber Ribbons

Fumiaki SATO*, Keisuke OKADA, Takao HIRAMA, Kentaro TAKEDA, Ryoei OKA and Ken TAKAHASHI

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------This paper describes the configuration of a new ultra-high-count and high-density slotted optical fiber cable with 12-fiber pliable ribbons. We combined the pliable ribbon technology with a slotted core cable design to create the cable that doubles the fiber count while the diameter remains the same as conventional cables. The pliable ribbon consists of fiber adhesive parts and non-adhesive parts coming in turns in a longitudinal and transverse direction, enabling high fiber density within a limited duct space and mass fusion splicing. A slotted core cable is designed with a non-preferential bending axis, allowing for easy installation in space-constrained areas. Its best characteristics, such as easy handling, good identification, and mass fusion splicing, are retained in our new cable. This paper describes the design of the 12-fiber ribbon and the test results of mass fusion splicing. It also refers to the designs and characteristics of 3456-fiber-count slotted core cables, which are the highest fiber-count optical cables in commercial use, and 1728-fiber-count slotted core cables.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: ultra-high-fiber-count, high-density, mass-fusion splicing, pliable 12-fiber ribbon, slotted core cable

Table 1. Structures of newly developed ultra-high-count optical cables

Conventional opticalcable

Structures of newlydeveloped optical cable

1.5-inchcable duct

2.0-inchcable duct

864-fiber-count 1728-fiber-count

1728-fiber-count 3456-fiber-count

SEI TECHNICAL REVIEW · NUMBER 83 · OCTOBER 2016 · 11

ribbon was easily dislocated when set on the holder of the fusion splicer, and required the time for realignment. In contrast, the fibers of the two-fiber type pliable ribbon remained in position after being placed on the holder. We confirmed that, as long as an appropriate adhesive pitch is selected, the two-fiber type pliable ribbon is spliced no later than splicing time for conventional 12-fiber ribbons, which is shown by the dotted box in Fig. 2. In this compar-ative study, we used 1.5 times the adhesive pitch to conventional pliable ribbon. At this adhesive pitch, the 12-fiber ribbon exhibited high flexibility while maintaining its original alignment.

Figure 3 compares the distribution of mass fusion splice loss between the pliable 12-fiber ribbon used in the newly developed optical cables and a conventional 12-fiber ribbon. The results confirm that the fusion splice loss of the new 12-fiber ribbon is comparable to that of the conven-tional 12-fiber ribbon.

In addition to the above test, we carried out a heat cycle test of the new 12-fiber ribbon with its fusion-spliced joint covered with a protective sleeve. The objective of this test was to check the functional reliability of the fusion-spliced joint. In the test, we connected 10 fusion-spliced joints in series as shown in Fig. 4, and exposed them to 20 heating cycles at a temperature range from -40°C to 70°C. The test specimen did not exhibit any increase in fusion splice loss even after exposure to the heating cycle, thereby verifying the functional reliability of the fusion-spliced joints on the new pliable 12-fiber ribbons.

3. Ultra-High-Fiber-Count and High-Density Slotted Core Optical Cable

3-1 Design of the optical cableThe newly developed optical cable comprises a

stranded steel wire passing through the axial center and a slotted core that makes the cable significantly flexible. As typical examples of the optical cable, the design of a 1728-fiber-count slotted core optical cable and the cross

Fig. 2. Comparison of mass-fusion splicing time between single-fiber type and two-fiber type pliable ribbons

(a) Single-fiber type pliable ribbon

(b) Two-fiber type pliable ribbon

Adhesive pitch P

Adhesive pitch P

Adhesivepart length

a

Non-adhesivepart length

c

Adhesivepart length

a

Non-adhesivepart length

c

Fig. 1. Schematic illustration of pliable 12-fiber ribbons

Fig. 3. Fusion splice loss of 12-fiber ribbons

Fig. 4. Heat cycle test results for fusion spliced joints (10 joints) of pliable 12-fiber ribbon

12 · Ultra-High-Fiber-Count and High-Density Slotted Core Cable with Pliable 12-Fiber Ribbons

section of a 3456-fiber-count slotted core optical cable are schematically illustrated in Figs. 5 and 6, respectively. The optical fibers used in these cables are single-mode fibers (ITU-T G.657A1) with enhanced bending property. Combined use of these bendable optical fibers and pliable ribbons has significantly contributed to the increased fiber density in the cable core, thereby reducing the cable size and weight compared with conventional optical cables.

In a high-fiber-count optical cable, each fiber ribbon needs to be identified. For the new optical cable, we printed a series of bars on each fiber ribbon as shown in Fig. 7 so that they can be identified according to the number of the bars. The use of bars in place of conventional numerical figures offers better legibility and easy recognition.

Each groove of a 3456-fiber-count slotted core optical cable housed 36 pliable 12-fiber ribbons. Thirty six pattern markings allow the identification of each ribbon in the groove. The grooves in the optical cable are distinguished from each other by identification marks printed on the surface of the slot ribs.

3-2 Study of water-sealing mechanismUnlike slotted core optical cables with conventional

fiber ribbons, the newly developed ultra-high-fiber-count, high-density optical cable consists of grooves packed with fiber ribbons at a high density. At the development stage of the new optical cable, the conventional water-sealing system was unlikely to be able to deliver the effect of the water absorbing powder to the bottom of the grooves. To clear the above problem, we investigated the effects of the fiber density in the slotted optical fiber grooves on the water absorption performance of the optical cable. The relation (relative value) between fiber density in the groove and water penetration after 240 hours is shown in Fig. 8. Water penetration increases as fiber density increases. For the new optical cable, we determined the optical fiber packaging density so that it would not significantly degrade the water penetration length.

3-3 Study of cable installation workabilityIn contrast to conventional optical cables containing

strength members on both sides of the outer jackets, the new slotted optical cable is characterized by its omnidirectional flexural property owing to a strength member passed through the longitudinal center of the core. In addition, the flexural

Fig. 8. Relationship between optical fiber density in groove and water penetration length

SheathRipcordWater-blocking tapePliable 12-fiber ribbonSlotted coreStranded steel strength member Outer diameter: 26 mm

Fig. 5. Schematic illustration of 1728-fiber-count optical cable with pliable 12-fiber ribbon

Outer diameter: 34mm

Pliable 12-fiber ribbon

Identification mark

Slotted core

Stranded steel strength member

Water-blocking tape

Sheath

Ripcord

Fig. 6. Cross sectional view of 3456-fiber-count optical cable with pliable 12-fiber ribbon

Identification of pliable fiber ribbons according tothe number of bars printed on them

Fig. 7. Schematic illustration of bars printed on pliable 12-fiber ribbon

SEI TECHNICAL REVIEW · NUMBER 83 · OCTOBER 2016 · 13

property of the new cable can be enhanced by optimally selecting the type of the strength member and the structure of the outer jacket. To check the difference of flexural prop-erty between the newly developed 1728-fiber-count slotted core optical cable and a conventional central-tube type optical cable, we carried out a flexural rigidity test.

The flexural rigidity test system is shown in Fig. 9, and the test results are shown in Table 2. As the table shows, the flexural rigidity of the new cable was nearly one-half that of the conventional cable. This result suggests that the new cable is advantageous in that it is easier to be installed in a hand-drilled duct or other restricted spaces.

3-4 Evaluation of transmission and mechanical propertiesWe evaluated the transmission and mechanical prop-

erties of the newly developed 1728-fiber-count and 3456-fiber-count slotted core cables. The heat cycle test was conducted at a temperature range from -40°C to +70°C for a prototyped 3456-fiber-count cable wound around a drum. As shown in Fig. 10, the 3456-fiber-count optical cable is stable in attenuation even after it is exposed to the heat cycle. Table 3 summarizes the results of the transmis-sion and mechanical property evaluation for the 1728-fiber-count and 3456-fiber-count slotted core cables, confirming that both of them have sufficient properties.

3-5 Comparison of fiber countFigure 11 compares the outer diameter and fiber count

of the new optical cables with those of conventional ribbon

IEC60794 Stiffness (Method E17A): flexural rigidity B = x3/48•(F/y)

Load cell

Test cableTest load F

Displacement y

Distance x

Fig. 9. Flexural rigidity test system for optical cable

Fig. 10. Attenuation change of 3456-fiber-count slotted core optical cable

Table 2. Flexural rigidity comparison test results for two types of optical cables

Table 3. Transmission and mechanical properties of 1728-fiber- count and 3456-fiber-count slotted core optical cables

Conventional optical cable Newly developed optical cable

* The flexural rigidity of the conventional optical cable was measured in the diagonal direction to strength member (the direction in which the cable could be bent).

864-fiber count(outer diameter: 25 mm)

Flexural rigidity: 11.4 N･m2

1728-fiber count(outer diameter: 26 mm)

Flexural rigidity: 5.5 N･m2

Item Test method Evaluation result

Attenuation Coefficient IEC60793-1-40 < 0.25 dB/km

(1550 nm)

Temperature Cycling

FOTP-3-40°C/+70°C × 2 cyc. < 0.10 dB/km

Compressive Loading

FOTP-41220 N/cm, 1 minute followed by 110 N/cm, 10 minutes

< 0.1 dB No defect in cable's external appearance

Impact Test

FOTP-25Impact Energy: 4.4 N-m 2 impacts, 3 locationsλ = 1550 nm

Cyclic Flexing

FOTP-104 I and IVSheave diam. ≤ 20 × cable diameter 25 cycles at 30 cyc./minλ = 1550 nm

Cable Twist Test

FOTP-85Sample Length ≤ 2 m10 cycles ± 180°λ = 1550 nm

Long Tensile Loading and

Fiber Strain Test

FOTP-33a) 600 Ib (rated)b) 180 Ib (residual)

Fiber strain (Rated) ≤ 60% fiber proof strain Fiber strain (Residual) ≤ 20% fiber proof strain < 0.1 dB

Fig. 1 1. Comparison of fiber count between conventional and newly developed optical cable

14 · Ultra-High-Fiber-Count and High-Density Slotted Core Cable with Pliable 12-Fiber Ribbons

loose tube optical cables.*2 The fiber density of 1728-fiber-count and 3456-fiber-count slotted core optical cables is twice that of a conventional optical cable of the same outer diameter.

4. Conclusion

We have developed a series of ultra-high-fiber-count, high-density slotted core optical cables with pliable 12-fiber ribbons. The ribbon is so constructed as to enhance its fiber density without deteriorating mass fusion splicing workability. The newly developed cable comprises a slotted core and a strength member passed through the center of the core. Out of the new series of optical cables, the 1728-fiber-count cable has an outer diameter of 26 mm or less while the 3456-fiber-count cable has an outer diameter of 34 mm or less. These optical cables are expected to enhance optical transmission density and contribute to the effective use of restricted spaces.

Technical Terms＊1 Strength member: a tensile strength member that is

used to relieve the tension that is loaded to the optical fibers of an optical cable when it is installed.

＊2 Loose tube optical cable: an optical cable that is made by twisting optical fibers after installing them in a thin plastic tube.

References(1) Y. Yamada et al, “Ultra-High-Density Optical Fiber Cable with Rollable

Optical Fiber Ribbons,” The Institute of Electronics, Information and Communication Engineers (2008), p.292

(2) Y. Yamada et al, “High-Fiber-Count and Ultra-High-Density Optical Fiber Cable with Rollable Optical Fiber Ribbons,” The Institute of Electronics, Information and Communication Engineers (2009), p.503

(3) K. Okada et al, “Enhanced Peelable Ribbon and Its Application to Access Network Cables,” Proc. of 53rd IWCS (2005), p.55-60

(4) F. Sato et al, “Low Polarization Mode Dispersion and Thin Ribbon Type Optical Cable with Peelable Ribbon “EZbranch®”,” Proc. of 55th IWCS (2007), p.55-60

(5) M. Takami et al, “Design of Ultra-High-Density Optical Fiber Cable with Rollable 4-Fiber Ribbons for Aerial Deployment,” Proc. of 61st. IWCS (2012), p. 433-436

(6) F. Sato et al, “Flame Retardant Optical Fiber Cords with Pliable Ribbons for Easy MPO terminations,” Proc. of 63rd. IWCS (2014), p.742-746

(7) F. Sato et al, “Design of Ultra-High-Density 2000-Optical Fiber Cable with Pliable 8-fiber Ribbons for Underground Deployment,” IWCS (2015), p.659

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

F. SATO*• Assistant General Manager, Optical Fiber & Cable

Division

K. OKADA• Assistant General Manager, Optical Fiber & Cable

Division

T. HIRAMA• Assistant Manager, Optical Fiber & Cable Division

K. TAKEDA• Assistant General Manager, Optical Fiber & Cable

Division

R. OKA• Assistant General Manager, Optical Fiber & Cable

Division

K. TAKAHASHI• Group Manager, Optical Fiber & Cable Division