A common means for deploying fiber optic cable
is by jetting (sometimes referred to as blowing) the
cable into installed ducts. To support these
deployments, cable designs must be optimized
for installation. For example, a general rule of
thumb is the cable outer diameter (OD) should be
less than 80% of the duct inner diameter (ID).
Therefore, in order to maximize the number of
fibers that can be installed in a given duct, the
fiber density—the ratio of optical fibers to the
cable cross-sectional area—must be maximized.
However, the cable design must be sufficient to
protect the fibers during installation and operation.
This article will introduce an innovative, ultra-high density micro-duct cable having a 10.4 mm OD and containing 288 fibers. The cable is designed to withstand an installation tensile load exceeding 300 pounds and for operational temperatures between -40°C and 70°C. Test results of various fiber types will be presented. Additionally the article will discuss results of a field trial where the cable was installed in a micro-duct with a 13 mm ID.
Fiber optic cables are designed to protect optical fibers from external forces during installation and throughout the life of the cable. Historically, Outside Plant (OSP) Cable—cable intended for use in an outdoor environment—has been designed to meet the stringent mechanical and environmental requirements of Telcordia Technology’s GR-20- CORE, Generic requirements for Optical Fiber and Optical Fiber Cable and the Insulated Cable Engineer Association’s ANSI/ICEA S-87-640, Standard for Optical Fiber Outside Plant Communications Cables.
OSP cables are most commonly installed aerially or buried underground. Cables intended for aerial installation are typically constructed of alldielectric components while cables intended for underground installation routinely contain a metallic armor under the outer sheath to offer additional protection to the fibers. Regardless, both types of construction are subjected to the same mechanical and environmental requirements during product qualification. While some special handling is required during installation, these constructions are quite robust and limit the forces that are applied directly to the optical fibers.
As fiber networks continue to grow and expand, particularly in urban locations and rough terrain, OSP cables are often installed in conduit or ducts. In particular, micro-ducts—factory bundled ducts with multiple pathways as shown in Figure 1—are advantageous because they facilitate future expansion since only the cable currently needed is installed. Additionally, the micro-ducts provide additional protection to the fibers, further insulating them from external forces.
GR-20-CORE recognizes the micro-duct cable type and indicates that the small size of the micro-duct cables results in generally lower tensile strength, crush resistance and the like . However, the robust construction described in this article is designed to prevent damage that could occur during installation and operation, while still providing ultra-high fiber density.
TRADITIONAL OSP CABLES
While OSP cable constructions may vary, two
common types include loose tube (LT) and central
tube cables. In both constructions, fibers are
enclosed within a buffer tube. Both constructions
use rigid and/or non-rigid strength members
to provide tensile strength and an outer sheath
provides additional protection to the buffered
OSP Loose Tube Cables
Loose tube cables typically contain 12 or 24 fibers per buffer tube and multiple tubes are stranded together, over a rigid strength member, with a reverse oscillating lay (ROL). In addition to mechanical protection, the buffer tubes serve to separate groups of fibers for easy identification in splice boxes. The ROL allows for simple mid- span access to the fibers and creates a fiber strain free window so that strain on the fibers is much less than the cable elongation at the rated installation load.
OSP Central Tube Cables
Central tube cables contain a single buffer tube and often include more than 24 fibers per tube. Fiber groups are commonly identified with colored threads or ring marks. Alternatively, ribbonized fibers may be used in central tube constructions. Strength members are applied over the tube and/or embedded in the cable sheath. Mid-span access to the fibers can be achieved, but is more difficult than with loose tube cables. Further, since the cable has little or no fiber strain free window, additional strength members are needed or the cable tensile rating may be reduced.
OSP MICRO-DUCT CABLES
Traditional Micro-Duct Cables
The use of cable ducts and eventually micro-ducts began expanding in the 1980s. The common approach was to reduce the size of components by reducing free space and wall thickness and eliminating the metallic armor. As shown in Figure 2, these changes resulted in a diameter reduction of 56% for 72 fiber constructions and an increase in fiber density from 0.5 f/mm2 to 2.4 f/mm2.
Conversely, the tensile rating for the cable was reduced from 2,670 N to 667 N, the compression resistance was reduced from 440 N/cm to 100 N/ cm and the minimum operating temperature was limited to -30°C.
Micro-Duct Fiber Units
￼ Thin-walled, reduced-diameter buffer tubes allow a significant reduction in cable diameter but they can be prone to kinking when expressed and coiled in enclosures. To overcome this issue, alternate cable constructions were developed with micro-modules—thin-walled sub-units made of lower modulus, thermoplastic elastomers that are less likely to kink when coiled. The small diameter and thin walled micro-modules allow for increased fiber density over traditional micro-duct cables. However micro-modules and micro-duct fiber units offer minimal protection to the optical fibers. Moreover, the tensile rating and compression resistance of these cables may be reduced even more and operating temperature may be further limited .
ROBUST MICRO-DUCT CABLES WITH EXTREME PERFORMANCE
In many applications, the most likely opportunity
for cable damage occurs during installation.
Cables are pulled off of the reel, flexed around
bends, twisted and even occasionally stepped on.
Since these same opportunities for damage
exist during the installation of micro-duct cables,
robust micro-duct cables have been designed to
withstand these forces . Similar to traditional
micro-duct cables, robust micro-duct cables
contain reduced-diameter buffer tubes; however,
the tubes are designed and produced to be more
Ultra-High Fiber Density Constructions
In some instances, duct space is limited, but high fiber counts are needed. To create the ultra-high fiber density construction, a cable has been designed by combining the ROL strand of a loose tube cable with the high fiber density of a central tube construction, thus reducing the size of the cable components.
The ultra-high density construction utilizes a buffer tube containing 48, 250 μm fibers. As shown in Figure 3, the fiber density of this tube is 6.4 f/mm2 where the fiber density of four micro- modules, each containing 12 fibers, is 4.0 f/mm2. Additionally, Figure 4 shows the compression resistance of different buffer tubes, relative to traditional OSP LT buffer tubes.
Fiber identification is achieved by color coding as described in ANSI/TIA-598-D, where groups of 12 fibers are contained within color-coded threads. Alternatively, fibers may be ring marked. Additional development work is underway with SpiderWeb Ribbon (SWR). Unlike traditional ribbon, SWR contains intermittently bonded fibers that can be rolled or formed into a compact bundle to allow for tight packing  and . To construct the 288 fiber construction, six of the ultra-high fiber density buffer tubes are stranded over a central strength member (CSM) with a ROL. This, along with the robustness of the tubes, allows for the extended tensile rating, along with the ability to access buffer tubes and fibers in mid-span applications. An outer jacket is then applied over the core, providing a final cable OD of 10.4 mm.
Ultra-High Fiber Density Cable Testing
Cable qualification testing was performed on 48 live fibers per GR-20-CORE and IEC 60794-5-10. Table 1 below describes the four different fiber types that were evaluated.
Figure 5 shows the tensile performance of the test cable. The measured fiber strain is below the measured cable strain, confirming the fiber strain free window. All of the fibers exhibited ≤0.2 dB power loss at the 600 lbs load and no measurable power loss was recorded in any fiber when the load was reduced to 180 lbs.
Table 2 shows the results of crush testing using a 990 N load, the same load used to evaluate OSP LT cables. Note that IEC 60794-5-10 requires a 500 N load. Results include the maximum and average power loss by fiber type while the cable is under the crush load and after the crush load has been released.
Similarly, cable impact testing was performed using 4.4J and two impacts at three locations, the same energy and impacts used to evaluate OSP LT cables. Note that IEC 60794-5-10 requires 1J and one impact at three locations. The residual power loss was ≤0.1 dB for all four fiber types. As shown in Figure 6, the buffer tube kink diameter for the ultra-high fiber density tube is slightly higher than the traditional OSP LT buffer tube, but much less than 100 mm.
Results of the cable temperature cycling are presented in Figure 7 and results of the cable temperature cycling after heat aging at 85°C for 168 hours are presented in Figure 8. To evaluate mid-span buffer tube performance, 4.9 m of buffer tubes were expressed from the ultra-high fiber density cable and stored in a LightGuard® 350XL Splice Closure. As shown in Figure 9, this commercially available closure is designed for large count fiber splicing .
The cable and stored buffer tubes were conditioned per FOTP-244 and the results from the second cold cycle are shown in Figure 10. Notice that fiber types A and B only meet the optical requirements at -20°C, but the bend- insensitive fiber types, fibers C and D, meet the optical requirements of GR-20-CORE at -40°C. As commonly seen with ITU-T G652.D fibers, the power loss in type A and B fibers is directly related to the MAC value.
After the mid-span buffer tube performance test, the buffer tubes were inspected for damage. No signs of breaks, cracks, blisters, kinking or excessive discoloration were observed.
Installation Performance Testing
The installation performance test was conducted in a 7-Way FuturePathTM duct. This duct consists of seven 16 mm (OD) x 13 mm (ID) micro-ducts. Note that the ratio of the ultra-high fiber density cable OD (10.4 mm) to the micro-duct ID (13 mm) is 80%. As shown in Figure 11, the 284 m length of duct was installed and the elevation change between the ends of the duct was 7.2 m. Four micro-ducts were connected in series so that the total path length was 1136 m and the loops where the micro-ducts were connected were 1.5 m in diameter.
Prior to performing the installation performance test, the micro-duct route verification test was completed. As described in IEC 60794-1-21, Method E23, a plastic sphere with the same OD as the test cable was successfully blown through the path, confirming the path was free from obstructions. Subsequently, a sponge coated with lubricant was blown through the path. The jetting apparatus selected for the installation performance test was a Plumett Minijet®. Figure 12 shows the pneumatic jetting device that is intended for cables with OD ≤16 mm with a recommended maximum install speed of 100 m/min .
The Minijet® was connected to the 13 mm ID micro-duct and the installation test was started. During the test, the outside temperature was 28°C with 65% relative humidity. The cable was jetted through the 1136 m path in 21 minutes with a maximum air pressure of 1.0 MPa, meeting installation performance expectations.
Fiber Access with Ultra-High Density Buffer Tube
Similar to traditional OSP and Micro-duct cable, fiber access is achieved by first removing the outer sheath and string binders and unwrapping the buffer tubes as shown in Figure 13. Note that the appropriate access length is documented in the instructions for the selected pedestal or closure.
Using a MS-6 Mid-Span Slitter made by Jonard Tools, score the tube and separate into two halves as shown in Figure 14. Ring cut the tube near the split and remove the buffer tube. Separate the four bundled groups of 12 fibers; each group is contained within two counter helical color-coded threads as shown in Figure 15. Next, anchor the buffer tube in the splicing tray, clean and coil the non-accessed binder groups and remove the string binders from the group to be accessed as shown in Figure 16. Finally, clean and prepare the fibers for splicing.
Using a ring cutting tool, score and then remove the buffer tube, exposing the fiber bundles. Separate the four bundled groups of 12 fibers; each group is contained within two counter helical color-coded threads. Load the buffer tubes into splitters and route the bundles of fiber into furcation tubes as shown in Figure 17. Finally, clean and prepare the fibers for splicing.
Additional Fiber Density Increase with 200μm Fibers
By applying the same design criteria used for the ultra-high fiber density cable, with 200 μm fibers, the buffer tube OD is reduced to 2.4 mm and the OD for a 288 fiber cable is reduced to 8.1 mm. This construction is suitable for installation into 10 mm ID micro-ducts.
The ultra-high fiber density cable that was
presented is capable of withstanding tensile
loads in excess of 300 pounds and operating at
temperatures from -40°C to 70°C. Utilizing a
robust, ultra-high fiber density buffer tube, the
construction minimizes the risk of fiber damage
during installation and throughout the life of the
cable. With bend-insensitive fiber, the cable is
suitable for mid-span buffer tube access.
Furthermore, the installation performance
testing indicates excellent jetting properties.
Considering all of the data presented, the 288
fiber ultra-high density cable demonstrates
extreme operating performance and meets or
exceeds industry standards for micro-duct cables.
Special thanks to David Reeve, Lonnie Phillips, Kim Smith, Bu Kijsamnong and Chad Mossberg for supporting the cable testing. Also, special thanks to Rick Dvorak and Al Crandall for supporting the installation performance testing.
1. Telcordia Technologies Generic Requirements, “Generic Requirements for Optical Fiber and Optical Fiber Cable,” GR-20-CORE, Issue 4, (July 2013) p 6-62.
2. IEC 60794 International Standard, “Optical fibre cables – Part 5-20: Family specification- Outdoor micro-duct fibre units, micro-ducts and protected micro-ducts for installation by blowing,” IEC, Ed. 1.0 (2014-02).
3. Justin Quinn, “The Evolution of Micro-Cables,” Proceedings of the 63rd International Wire & Cable and Connectivity Symposium, (2014) pp 649-652.
4. Mizuki Isaji, et al, “Ultra-High Density Wrapping Tube Optical Fiber Cable with 12-Fiber SpiderWeb® Ribbon,” Proceedings of the 62nd International Wire & Cable Symposium, (2013) pp 605-609.
5. Daiki Takeda, et al, “Development of Wrapping Tube Cable with SpiderWeb® Ribbon Using 200 μm Coated Fiber,” Proceedings of the 63rd International Wire & Cable and Connectivity Symposium, (2014) pp 757-761.
6. http://www.aflglobal.com/Products/Fiber-Outside-Plant/LightGuard-Sealed-Fiber-Optic-Splice-Closures/LightGuard-Sealed-Fiber-Optic- Splice-Closures/LightGuard-350XL-Sealed-Fiber-Optic-Splice-Closure.aspx
Justin Quinn is a Product Development Engineer
for Cable Systems at AFL. He earned a Bachelor
of Science in Mechanical Engineering from
Clemson University in 2000 and began his career
in fiber optics that year. In his current role, Justin
is responsible for development of micro-duct, OSP
loose tube, ADSS and premise cables. Prior to
joining AFL in 2012, Justin held various process
and development engineering roles with Alcatel
and Draka Communications. Justin has been
granted eight U.S. patents and has authored
multiple IWCS papers.
Olaf Storaasli is the Eastern Enterprise Sales Manager at Dura-Line. He holds a Bachelor of Science in Mechanical Engineering from Virginia Tech, a Master of Science in Aerospace Engineering from Old Dominion University and a MBA in Global Business from Lenoir-Rhyne University. Olaf has 15+ years of experience in fiber optics from various roles in R&D, Application Engineering, Product Management, Marketing and Sales. Olaf has been granted 10 U.S. patents and has authored numerous IWCS and industry papers, including many on micro cables.