WO2004038474A1

WO2004038474A1 - Optical drop cable

Info

Publication number: WO2004038474A1
Application number: PCT/JP2003/013637
Authority: WO
Inventors: Takeshi Honjyou; Kazunaga Kobayashi; Masahiro Kusakari; Osamu Koyasu; Satoru Shiobara; Ken Osato; Masashi Hara; Shimei Tanaka; Keiji Ohashi
Original assignee: Fujikura Ltd.
Priority date: 2002-10-25
Filing date: 2003-10-24
Publication date: 2004-05-06
Also published as: AU2003274752A1

Abstract

Optical fibers 3-1 to 3-8 are aligned in a row, and buffer tapes 13 are longitudinally attached to a top surface and an undersurface so as to sandwich the optical fibers 3-1 to 3-8. The optical fibers 3-1 to 3-8 and the buffer tapes 13 are housed in a hollow portion 5 provided in a cable sheath 9. Substantially V-shaped notches 11 for indicating a splitting position are provided on planes of the cable sheath 9 parallel to the longitudinal direction of the hollow portion 5. Strength members 7 for absorbing tension in a tensile direction of the cable are provided in the vicinities of both ends of the hollow portion 5 of the cable sheath 9. Since the optical fibers 3-1 to 3-8 can only move one-dimensionally, it is possible to reduce transmission losses.

Description

DESCRIPTION OPTICAL DROP CABLE

TECHNICAL FIELD The present invention relates to an optical drop cable to be drawn from a closure connected to a distribution line system of an optical fiber network into the premises of a housing complex, for example. More specifically, the present invention relates to an optical drop cable including a hollow in a cable sheath thereof.

BACKGROUND ART Fiber to the Home (FTTH) has been proposed in recent years in an effort to promote achievement of an advanced information communication society. The FTTH is a network designed to offer integrated communication including telephones, computer communication, CATV (i.e. a method of wiring cable television and an optical network) , and the like by means of drawing an optical fiber into each household. The FTTH is typically achieved by locating a remote terminal on a tail end at a central telephone office, then laying an optical cable for a distribution line system starting from the remote terminal, and laying an optical drop cable from a closure provided on the distribution line to each household or housing complex.

A conventional optical drop cable is either a single-fiber or two-fiber cable drawn out of the closure connected to a feeder line system into an optical termination box provided at a housing complex, for example, and is further wired from the optical termination box to a wall outlet provided in each household in the housing complex, for example. Such a conventional optical drop cable is disclosed in Japanese Unexamined Patent Publication No. 2000-171673, for example. In this optical drop cable 101, as shown in Fig. 1, either a single fiber 111 or two fibers are housed in a hollow portion 103 provided in a cable sheath 107 with a rectangular cross-section. Meanwhile, notches 109 are formed on the laterals of the cable sheath 107, and strength members 105 are provided in the vicinity of the hollow portion 103 so as to withstand the tensile force in a longitudinal direction.

DISCLOSURE OF INVENTION

A hollow exists in the hollow portion of the conventional optical drop cable shown in Fig. 1 because the optical drop cable adopts a loose tube structure. Accordingly, when a plurality of optical fibers are housed in the hollow portion, for example, the optical fibers may move two-dimensionally (in a radial direction) inside the hollow portion. For this reason, depending on manufacturing conditions, both an optical fiber of a normal length and any other optical fiber of a relatively longer length manufactured in a meandering manner inside the cable tend to coexist in the same hollow portion.

If there is a difference in fiber lengths between the housed optical fibers, the longer optical fiber may be more susceptive and deformed by lateral pressure due to deformation of the cable sheath exposed to a low temperature and the cable sheath is caused to contract, for example. In this way, the transmission loss of optical fibers is increased. Moreover, when the plurality of optical fibers are housed in the hollow portion, the optical fibers exist inside the hollow portion may tangle each other inside the hollow portion of the optical drop cable. Therefore, it is difficult to select or identify a desired optical fiber from the other optical fibers.

The present invention has been made in consideration of the foregoing problems. According to the present invention. it is possible to provide an optical drop cable which can stabilize the transmission loss characteristics and facilitate fiber identification inside the optical drop cable. According to a first technical aspect of the present invention, an optical drop cable includes a plurality of optical fibers, a cable sheath with a rectangular in cross-section configured to house the plurality of optical fibers while providing a gap around the plurality of optical fibers, and at least one buffer tape to be housed in the gap. According to a second technical aspect of the present invention, the plurality of the optical fibers of the optical drop cable are aligned in one row like an array.

According to a third technical aspect of the present invention, in addition to the first technical aspect, the plurality of optical fibers of the optical drop cable are aligned in a plurality of rows , and the buffer tapes are housed in the gaps around the respective rows of the aligned optical fibers .

According to a fourth technical aspect of the present invention, in addition to the first technical aspect, the buffer tape of the optical drop cable possesses a water-absorbing property.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a cross-sectional view showing a structure of a conventional optical drop cable.

Fig. 2 is a cross-sectional view showing a structure of an optical drop cable according to a first embodiment of the present invention. Fig. 3 is a table showing results of measurement of redundancy ratios among optical fibers which are housed in the optical drop cable according to the present invention. Fig. 4 is a table showing results of measurement of redundancy ratios among optical fibers which are housed in the conventional optical drop cable.

Fig. 5 is a cross-sectional view showing a structure of an optical drop cable according to a second embodiment of the present invention.

Fig. 6 is a cross-sectional view showing a structure of an optical drop cable according to a third embodiment of the present invention. Fig. 7 is a view showing watertightness test set-up applying a method of testing watertightness of an intermediate between a cable core and a sheath.

Fig. 8 is a view showing watertightness test set-up applying a method of testing water permeability of an entire cross-sectional structure.

Fig.9 is a schematic drawing showing an aspect of laying the optical drop cable according to any of the first to fourth embodiments .

DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will be described with reference to the accompanying drawings . First Embodiment

Fig. 2 is a cross-sectional view showing a structure of an optical drop cable according to a first embodiment of the present invention. An optical drop cable 1 shown in Fig. 2 is composed of optical fibers 3-1 to 3-8, a hollow portion 5, strength members 7, a cable sheath 9, notches 11, and buffer tapes 13 (13-1, 13-2). Each of the optical fibers 3 (3-1 to 3-8) is a single-mode optical fiber having a fiber coat diameter of 0.25 mm. The optical fibers 3 are adjacently aligned parallel in the cross section thereof and linearly disposed in a hollow provided as the hollow portion 5 in the cable sheath 9.

The cable sheath 9 has a rectangular shape with a section size of 4.0 mm wide and 2.0 mm high, and is made of flame-retardant low density polyethylene. The cable sheath 9 houses the optical fibers 3-1 to 3-8 in the hollow portion 5 to jacket the optical fibers 3-1 to 3-8. The cable sheath 9 includes V-shaped notches 11 in two positions perpendicular to a direction of row of the optical fibers , so as to indicate a splitting position for ripping the hollow portion 5 in the middle .

In addition, the cable sheath 9 jackets two respective strength members 7 disposed in the vicinities of both ends of a longitudinal cross section of the hollow portion 5 , which are made of steel wires with an outer diameter of 0.4 mm that have been subjected to corrosion prevention such as galvanization, and which are designed to withstand the tensile force in a longitudinal direction. Glass fiber reinforced plastic (glass FRP) or aramid fibers such as Kevlar (trademark) can be used instead of the steel wires with the outer diameter of 0.4 mm.

The buffer tapes 13 are applied longitudinally to a top surface and an undersurface of the eight optical fibers 3-1 to 3-8 aligned in one row, and are laid in the hollow portion 5 of the cable sheath 9 parallel to the aligning direction (in an X-Z plane) of the optical fibers 3-1 to 3-8. That is, the eight optical fibers 3 are laid parallel to constitute one row within a plane intersecting a longitudinal direction (Z direction) of the optical drop cable (an X-Y plane in the drawing) , and the buffer tapes 13 are housed in the hollow at the top surface and the undersurface (as represented by positive and negative directions along the Y axis) of the optical fibers 3 so as to be parallel to the respective surfaces . As a result, a multilayer structure including a first buffer tape 13-1 layer, a layer of the optical fibers 3, and a second buffer tape 13-2 layer is formed inside the cable sheath 9 as shown in Fig. 2.

The buffer tape 13 is made of paper, fabrics, nonwoven fabrics, or thermoplastic resin (such as polyethylene terephthalate (PET), polypropylene (PP), or polyethylene (PE)). However, the material of the buffer tape 13 is not limited to those cited above. It is possible to use any other materials as long as any optical fiber and the buffer tape 13 do not fuse each other by heat for extruding the sheath 9 over the optical fibers. The buffer tapes 13 are laid in the hollow portion 5 and gaps among the optical fibers 3-1 to 3-8, and thereby restrain a degree of freedom in a direction intersecting the aligning direction (X direction) of the optical fibers. In this way, the buffer tapes 13 restrain relative movements of the optical fibers within the plane intersecting the longitudinal direction (Z direction) of the optical drop cable 1.

A method of manufacturing the optical drop cable 1 will be described with reference to Fig.2. First, as shown in Fig. 2, the optical fibers 3-1 to 3-8 composed of the plurality of optical fibers are prepared. Subsequently, the buffer tapes 13 are applied longitudinally and parallel to the top and bottom faces of the theoretical plane which is defined by the optical fibers 3-1 to 3-8, and then the buffer tapes 13 and the optical fibers 3-1 to 3-8 are laid in the hollow portion 5 provided in the cable sheath 9.

The V-shaped notches 11 for indicating the splitting position are formed on the laterals of this cable sheath 9 which are parallel to the longitudinal direction of the hollow portion 5, and the strength members 7 are provided in the vicinities of both ends of the cross section of the hollow portion 5 of the cable sheath 9 so as to withstand the tensile force in the longitudinal direction of the cable.

In this embodiment, the optical fibers 3-1 to 3-8 aligned in one row are laid in the hollow portion 5 along with the buffer tapes 13 longitudinally attached thereto. The V-shaped notches 11, which indicate the splitting position for ripping the hollow portion 5 in the middle, are formed in the direction intersecting the aligning direction (X direction) of the optical fibers. Therefore, it is possible to easily rip the cable sheath 9 in two by use of the notches 11 as a start and to take out the optical fibers 3-1 to 3-8. The eight optical fibers 3-1 to 3-8 adjacently aligned parallel are laid between the buffer tapes 13, and the optical fibers 3-1 to 3-8 and the buffer tapes 13 are integrally housed in the hollow portion 5. Accordingly, it is possible to restrain movements of the optical fibers in a Y direction along the cross section of the cable, and to limit the freedom of movements to be one-dimensional. As a result, no difference in line lengths occurs among the housed optical fibers . Therefore, it is possible to prevent the optical fibers from deformation by lateral pressure attributable to contraction of the cable sheath 9. It is possible to suppress an increase in the transmission loss in the optical fibers. Moreover, it is possible to reduce external forces applied to the optical fibers 3-1 to 3-8 by use of the buffer tapes 13.

In this embodiment, the cable sheath jacketing the optical fibers 3-1 to 3-8 still retain the strength member 7 even when the optical drop cable sheath 9 is split in two. Therefore, the cable can withstand the tensile force in the longitudinal direction.

In addition, in this embodiment, the plurality of optical fibers 3-1 to 3-8 are housed in the hollow portion 5 of the cable sheath 9. Accordingly, it is possible to distribute the optical fibers to a plurality of users by use of a single cable. The plurality of optical fibers 3-1 to 3-8 are aligned in one row and housed in the hollow portion 5. Therefore, it is possible to reduce the thickness of the cable and to facilitate wiring in a limited space. The plurality of optical fibers 3-1 to 3-8 are aligned in one row inside the hollow portion 5. Accordingly, when colorings or markings are applied to the optical fibers 3-1 to 3-8 for identification, it is possible to use the same colors or marks for different optical fibers repeatedly or periodically as long as at least two outmost optical fibers in the row are identified by different colors. As a result, it is possible to reduce the number of colors or marks used therein while facilitating identification among the fibers .

In the above-described case where the optical drop cable 1 includes the eight optical fibers 3-1 to 3-8, it is possible to reduce the number of colors for identification down to five colors by means of coating the optical fibers 3-1 to 3-8 with colorings in the order of "blue, yellow, green, red, purple, yellow, green, red", for example. Results of measurement of redundancy ratios and transmission losses of the optical fibers housed in the optical drop cable of the present invention will be described with reference to Fig.3. On the other hand, results of measurement of redundancy ratios and transmission losses of optical fibers housed in a conventional optical drop cable will be described with reference to Fig.4 for the purpose of comparison between the present invention and the related art . In the measurement described above, a redundancy ratio (ΔL/L) is calculated by measuring a ratio of an extra length ΔL of a fiber to a length L of an optical drop cable . Meanwhile, a transmission loss is obtained by measuring a transmission loss when transmitting light in a wavelength region of 1.55 μm in a condition at a temperature of -40 C° . The redundancy ratios and the transmission losses were measured for all the eight fibers housed in the optical drop cable.

As shown in Fig. 3, the maximum redundancy ratio among fibers 1 to 8 housed in the optical drop cable of the present invention was +0.03%, and the minimum redundancy ratio was 0%. Two out of these fibers showed no redundancy. As for the transmission losses, all the fibers showed 0.2 dB/km.

On the contrary, the maximum redundancy ratio among fibers 1 to 8 housed in the conventional optical drop cable was +0.10%, and the minimum redundancy ratio was 0%. Only three out of these fibers showed values within 0.03%, and other five fibers' values exceeded the range from -0.05% to +0.05%. Meanwhile, the transmission losses thereof ranged from 0.2 to 0.3 dB/km.

It is apparent from the above measurement results that the fibers in a row housed in the optical drop cable of the present invention showed smaller redundancy ratios as compared to the fibers in a bundle in the conventional optical drop cable. The redundancy ratios were uniform, and no extremely high value was observed. In addition, the fibers housed in the optical drop cable of the present invention showed transmission losses which were smaller than and therefore superior to the fibers in the conventional optical drop cable. The eight optical fibers housed in the optical drop cable of the present invention are aligned in one row and are interposed between the buffer tapes disposed on the top surface and the undersurface. Accordingly, the optical fibers can only move one-dimensionally in terms of the cross section of the cable. A difference in redundancy is reduced for this reason. Since no extreme extra fiber length exists in the hollow portion in the optical drop cable of the present invention, it is possible to drastically reduce deformation caused by fiber entanglement or lateral pressure being applied to meandering fibers. In this way, the transmission losses can be reduced. In other words, according to this embodiment, it is possible to reduce a difference in line lengths of the optical fibers and thereby to stabilize loss characteristics. Since the plurality of the optical fibers are aligned in one row and disposed in the hollow portion while being sandwiched by the buffer tapes, the same colors or marks can be used repeatedly and thereby the number of colors or marks for identification can be reduced.

Second Embodiment Fig. 5 is a cross-sectional view showing a structure of an optical drop cable 21 according to a second embodiment of the present invention. In comparison with the optical drop cable 1 as shown in Fig. 2, the optical drop cable 21 further provides a self-supporting wire 15 as another strength member through a web portion 17 as shown in Fig.5. A steel wire having a diameter of 1.2 mm is applied to as the self-supporting wire 15.

In the optical drop cable 21 of this embodiment, the self-supporting wire 15 for supporting optical fibers 3-1 to 3-8 for aerial application are integrally formed on one side face of the cable sheath 9 through the web portion 17, that is, on an extension line of two strength members 7 in the cross section of the optical drop cable 21.

A method of manufacturing the optical drop cable 21 will be described with reference to Fig. 5. As shown in Fig. 5, the optical fibers 3-1 to 3-8 composed of a plurality of optical fibers are prepared.

Subsequently, buffer tapes 13 are applied to the optical fibers 3-1 to 3-8, and then the buffer tapes 13 and the optical fibers 3-1 to 3-8 are disposed in a hollow portion 5 provided in the cable sheath 9. Two V-shaped notches 11 for indicating a splitting position are formed on laterals of this cable sheath 9 which are substantially parallel to the longitudinal direction (X direction) of the hollow portion 5. The strength members 7 are provided in the vicinities of both ends of the cross section of the hollow portion 5 of the cable sheath 9 so as to withstand the tensile force in the longitudinal direction of the cable. In addition, the self-supporting wire 15 is laid on one side of the cable sheath 9 through the web portion 17.

According to this embodiment, in addition to the characteristics of the first embodiment, the plurality of optical fibers 3-1 to 3-8 are aligned in order and housed in the hollow portion 5 and unnecessary gaps are thereby reduced. Therefore, the thickness of the cable can be reduced and wiring in a limited space is facilitated. According to this embodiment, the plurality of optical fibers 3-1 to 3-8 always maintain the same arrangement in one row in a longitudinal direction inside the hollow portion 5. Therefore, in a case where colorings or markings are applied to the optical fibers 3-1 to 3-8 for the purpose of identification, the same colors or marks can be used for different optical fibers repeatedly or periodically as long as at least two outmost optical fibers in the same row are identified by different colors. As a result, the number of colors or marks can be reduced while distinction among the fibers is facilitated.

Third Embodiment

Fig. 6 is a cross-sectional view showing a structure of an optical drop cable 31 according to a third embodiment of the present invention. In the optical drop cable 31 shown in Fig. 6, four optical fibers being adjacently aligned parallel feature one fiber row, and three fiber rows are laminated vertically in parallel. Buffer tapes 13 are applied to and interposed between top surfaces and undersurfaces of all the fiber rows .

Specifically, in the optical drop cable 31 of this embodiment, a first fiber row defined by paralleling four optical fibers 3a-1 to 3a-4, a second fiber row defined by paralleling four optical fibers 3b-l to 3b-4, and a third fiber row defined by paralleling four optical fibers 3c-1 to 3c-4 are laminated in parallel while applying to and interposing the buffer tapes 13 on both sides of the respective fiber rows and then housed in a hollow portion 5. In other words, as similar to the first embodiment, the optical drop cable 31 has a multilayer structure in which layers of the buffer tapes 13 and layers of the optical fibers 3 are alternately laminated inside the cable sheath 9. And, when two strength members 7, which are disposed orthogonal to the laminating direction of the optical fibers so as to sandwich the hollow portion 5 , are integrally formed with the cable sheath 9 , substantially V-shaped notches 11 for indicating a splitting position are provided on the cable sheath 9 in the laminating direction of the optical fibers. A self-supporting wire 15 as a messenger wire for supporting the optical fibers through the web portion 17 are integrally formed on one side face of the cable sheath 9 , that is , on an extension line of the two strength members 7.

A method of manufacturing the optical drop cable 31 will be described. As shown in Fig.6, the first fiber row including the four optical fibers 3a-1 to 3a-4, the second fiber row including the optical fibers 3b-1 to 3b-4, and the third fiber row including the optical fibers 3c-1 to 3c-4 are prepared.

Subsequently, when laminating these fiber rows in parallel, the buffer tapes 13 are inserted between the respective fiber rows and are housed inside the hollow portion 5 provided in the cable sheath 9.

Substantially V-shaped notches 11 for indicating a splitting position are formed in two positions on this cable sheath 9 in the laminating direction (Y direction) of the fiber rows . The strength members 7 are provided in the vicinities of both ends of the cross section of the hollow portion 5 of the cable sheath 9 so as to withstand the tensile forth in the longitudinal direction of the cable. Then, the self-supporting wire 15 as the messenger wire is added to the cable through the web portion 17 the cable sheath 9 is formed.

According to the optical drop cable 31 of this embodiment, in a housing complex, the cable sheath 9 can be ripped and split in two by applying tearing forces to the notches 11 as a center. In this way, it is possible to easily take out the optical fibers 3a-l to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4. Since the optical fibers 3a-l to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4, and the buffer tapes 13 are integrally housed in the hollow portion 5, external forces induced or generated in the optical fibers 3a-l to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4 can be reduced.

According to this embodiment, since the plurality of optical fibers 3a-l to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4 are respectively aligned in a row in a cross section thereof and are housed in three rows in the hollow portion 5 , gaps around the optical fibers can be reduced. Accordingly, it is possible to reduce a dimension in a direction (X direction) normal to the aligned direction of optical fibers 3. In this connection a wind pressure load can be also reduced.

In this embodiment , each fiber row includes four optical fibers which are adjacently aligned. It is noted that the number of optical fibers in each row is not limited thereto and may be three fibers, five fibers, six fibers, or the like. Similarly, the number of lamination is not limited to three layers and may be four layers , five layers , or the like.

In addition, according to this embodiment, the optical fibers 3a-l to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4 are partitioned by the buffer tapes and are aligned in three rows in the hollow portion 5. The plurality of optical fibers 3 are arranged in the plurality of layers and the buffer tapes 13 are at least inserted between the respective layers. For this reason, the optical fibers 3 are laid parallel in the direction intersecting the respective layer planes (Y direction in the drawing) and are thereby restrained to move along the respective layers. Therefore, in a case where colorings or markings are applied to the optical fibers 3a-1 to 3a-4, 3b-l to 3b-4, and 3c-l to 3c-4 for the purpose identification, the same colors or marks can be used for the different optical fibers repeatedly or periodically as long as at least two outmost optical fibers in the same row are identified by different colors. As a result, the number of colors or marks used therein can be reduced while facilitating identification among the fibers.

In other words, the number of colors for identification can be reduced to three colors by means of jacketing the optical fibers 3a-1 to 3a-4 as shown in Fig. 6 with colorings in the order of "blue, yellow, blue, red", jacketing the optical fibers 3b-1 to 3b-4 with colorings in the order of "yellow, red, yellow, blue", and jacketing the optical fibers 3c-l to 3c-4 with colorings in the order of "red, blue, red, yellow", for example.

Fourth Embodiment

When running the optical fiber of an optical drop cable from an aerial closure into a subscribers house and the like by use of the conventional optical drop cable shown in Fig. 1, the hollow portion which houses the optical fibers has a loose tube structure and therefore includes a space inside the hollow portion. When it rains, water may penetrate the aerial closure or an optical termination box and reach this hollow. In a case where water reaches a line terminating equipment of a system through this hollow in the cable, there is a risk of damaging an optical transmission system and the like. Unless water penetration in a cable joint is properly handled, a damage scale of the optical transmission system expands further.

As it is generally known, lattice defects in atomic structure in the optical fiber shortens the life of the fiber when the lattice defects chemically react to water. Such lattice defects, due to OH radicals originated from absorbed water via hydrogen generation deteriorates transmission characteristics .

Such a reduction in the life time of the fiber due to penetration of water and an increase in the transmission loss in the fiber due to hydrogen generation are applicable not only to aerial closure housing optical fibers but also to installed optical drop cable with a crack on a lateral thereof.

Therefore, an optical drop cable according to this embodiment is the optical drop cable of the first embodiment as shown in Fig. 2, in which the buffer tapes 13 further possesses a water-absorbing property. Other structures of the optical drop cable of this embodiment are similar to those in the first embodiment .

Eight optical fibers 3-1 to 3-8 with longitudinal application of water-absorbent tapes 13 on a top surface and an undersurface thereof are disposed in a hollow portion 5 located in the center of a cross section of a cable sheath 9. The water-absorbent tape 13 is fabricated firstly by forming a tape with a material such as absorbent cotton, paper (pulp), a fabric, a nonwoven fabric, or thermoplastic resin. Then the water-absorbent tape 13 is processed to enhance the water-absorbing property by means of any of coating and adhering a water-absorbent polymer such as starch, cellulose, polyvinyl alcohol or a polyacrylate as a water absorbent, wrapping the water absorbent with the tape, or kneading the tape with the water absorbent . The thermoplastic resin includes polyethylene terephthalate (PET) , polypropylene (PP) , or polyethylene (PE) or the like.

According to this embodiment, the water-absorbent tapes 13 sandwiching the eight optical fibers 3-1 to 3-8 aligned in a row in the cross section thereof are integrally housed in the hollow portion 5. Therefore, in addition to the characteristics of the first embodiment, the optical drop fiber cable of this embodiment forms a watertight dam when water penetrates a cross section of the cable, by means of a reaction between the water-absorbent and the water in the vicinity of a location where the water penetrated. In this way, the optical drop fiber cable can prevent water penetration. As a result, it is possible to prevent hydrogen generation inside the cable and thereby to suppress an increase in the transmission loss in the optical fiber.

Evaluation of watertightness

Watertightness of the optical drop cable 1 of the fourth embodiment was evaluated in accordance with the following methods. Figs. 7 and 8 are views showing watertightness testing equipment based on IEC 60794-1-2.

Two methods (F5A and F5B) are described as methods of watertightness testing. The method shown in Fig. 7 (F5A) is designed to test watertightness of an intermediate between a cable core and a sheath. On the contrary, the method F5B shown in Fig. 8 is designed to test water permeability of an entire cross-sectional structure provided with watertightness.

In the test set-up of Fig. 7, the sheath of an optical drop cable are removed by a width of 25 mm along the circumference at a position 3 meters away from one end. Then a pipe 41 to be filled with water is put on to the circumference the sheath so that water pressure at a height of 1 m (a water head hwl) can be applied thereto. An end cap 43 is fitted to one end of the optical drop cable.

In the test set-up of Fig. 8, an optical drop cable 1 having a length longer by 1 m than a test length (not more than 3 m) is prepared. A transparent acrylic pipe 45 having the inner diameter of 18 mm, which is provided with a rubber plug fitted around an end thereof, is inserted into one end of an L-shaped polyvinyl chloride resin pipe 47. Meanwhile, the other end of the optical drop cable 1 wrapped with a Selbon (trademark) tape is inserted into the other end of the polyvinyl chloride resin pipe 47 to ensure water-tightness. In that event, one end 48 of the optical drop cable 1 is sealed to be watertight so that water pressure at a height of 1 m (a water head hw2) can be applied thereto. Subsequently, the optical drop cable 1 is held horizontal and the transparent acrylic pipe 45 is held vertical, and water is poured into the transparent acrylic pipe 45 so as to apply water pressure at 0.1 atm to an open end face of the optical drop cable 1. A length of water penetration along the cable length is measured 24 hours later.

As a consequence of executing these waterproof tests , no water penetration was detected from the open end face of the optical drop cable. The optical transmission characteristics thereof was confirmed to be equivalent to that of the conventional optical drop cable.

As a result , even if a crack occurs in the cable sheath 9 of the optical drop cable 1 and water penetrate the crack, the water absorbent absorbs the water resulting in expansion of the water absorbent's volume and forms the watertight dam inside the cable sheath 9. In this way, it is possible to suppress rapid water penetration inside the hollow portion 5 and to maintain the transmission characteristics normal until the damaged cable is replaced by a new cable . In other words , according to this embodiment, it is possible to stabilize the optical transmission characteristics by preventing penetration of water into the hollow portion which houses the optical fibers .

It is easily understood that the water-absorbent buffer tape of this embodiment can be also applied to the optical drop cables according to the second and third embodiments. In this case as well, in addition to the characteristics of each of the embodiments, it is possible to prevent penetration of water to the inside when water penetrates the cross section of the cable, because the water-absorbent material of the buffer tape reacts with the water to extend the water absorbent ' s volume and form the watertight dam inside the cable sheath 9. Laying the optical drop cable

Fig. 9 is a view showing an aspect of installing the optical drop cable according to any of the first to fourth embodiments. As shown in Fig. 9, an end of an optical fiber cable 53 distributed from a central telephone office (not shown) is connected to an aerial closure 55 in the vicinity of a telephone pole 51. Of an optical drop cable to be disposed between this aerial closure 55 and a building or a housing complex 50, one end is connected to the aerial closure 55 and the other end is connected to an optical termination box 59 installed in an administrative room of the housing complex.

To connect the aerial closure 55 to the optical termination box 59 , the web portion 17 of the optical drop cable is partially ripped to detach the cable body housing the optical fibers from the self-supporting wire 15. One end 19A of the detached self-supporting wire 15 is fixed to a drop cable clamp 57 of the telephone pole 51. Meanwhile, the other end 19B of the self-supporting wire 15 is fixed to another one (not shown) near the administrative room. One end 15A of the cable body is connected to the aerial closure 55 in the vicinity of the telephone pole 51, and the other end 15B thereof is connected to the main distribution frame (MDF) 59 in the administrative room.

Subsequently, the optical drop cable 1 extended to the administrative room through the main distribution frame (MDF) 59 is connected to a router and an optical hub installed in the administrative room. Then, the optical drop cable 1 according to the first embodiment is connected to one of output ends of multiple optical transmission paths branched off by the optical hub. The other end of the optical drop cable 1 is extended to each housing unit along a duct inside the housing complex. The optical drop cable extended to the each housing unit is connected to a fiber distribution frame (FD) 61 installed in the housing unit . In that event , it is easily possible to take out the housed optical fibers 3-1 to 3-8 only by ripping the cable sheath 9 in two by use of the notches 11 as a start.

Claims

1. An optical drop cable comprising: a plurality of optical fibers; a long-shaped cable sheath configured to house the plurality of optical fibers while providing a gap around the plurality of optical fibers; and at least one buffer tape housed in the gap.

2. The optical drop cable according to claim 1, wherein the plurality of optical fibers are aligned in one row on a plane intersecting a longitudinal direction of the optical drop cable.

3. The optical drop cable according to claim 1, wherein the plurality of optical fibers are aligned in a plurality of rows on a plane intersecting a longitudinal direction of the optical drop cable, and the buffer tapes are housed in gaps around the respective rows of the aligned optical fibers.

4. The optical drop cable according to claim 1 , wherein the plurality of optical fibers are arranged in a plurality of layers , and the buffer tapes are disposed at least between the respective layers .

5. The optical drop cable according to any one of claims 1 to 3, wherein the buffer tape possesses a water-absorbing property.

6. The optical drop cable according to claim 1 , wherein the plurality of optical fibers constitutes a tape core wire.

7. The optical drop cable according to any one of claims 1 to 3, further comprising: a substantially V-shaped notch formed on a side of the cable sheath; and at least one strength member provided in the vicinity of the plurality of optical fibers .

8. The optical drop cable according to any one of claims 1 to 3 , further comprising a support line connected to a side of the cable sheath and configured to support the optical fibers .