Optical cable

In the optical cable in accordance with the present invention, the reversal angle .phi. from one reverse portion to the next reverse portion in an S-Z type helical groove is at least 180 degrees; and, letting W, T, and n be the width and thickness of each optical fiber ribbon and the number of stacked sheets of optical fiber ribbons, respectively, and a and b be the width and depth of the helical groove, respectively, at least each reverse portion of the helical groove has a cross-sectional form satisfying: EQU nT<a.ltoreq.W.sup.2 +L +(nT+L ).sup.2 +L (1) EQU W<b (2) PA1 whereas the remaining portion of the helical groove has a cross-sectional form satisfying: EQU W.sup.2 +L +(nT+L ).sup.2 +L <min(a, b) (3).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical cable having an S-Z type
 helical groove for containing stacked optical fiber ribbons.
 2. Related Background Art
 As a technique in such a field, Japanese Patent Application Laid-Open No.
 HEI 8-211264has been known. This publication discloses a technique in
 which the width and depth of an S-Z type helical groove provided in a
 chamber element are made greater than the diagonal of a stack of optical
 fiber ribbons.
 SUMMARY OF THE INVENTION
 The above-mentioned conventional optical cable has the following problems.
 Namely, while it is preferred that the optical fiber ribbons be contained
 within the S-Z type helical groove from the viewpoint of securing a
 long-term reliability, it is necessary to keep the optical fiber ribbons
 from collapsing within the groove from the viewpoint of preventing
 transmission loss from increasing due to cabling. In particular, it has
 been known that the stack of optical fiber ribbons generally tends to move
 toward the opening of the groove in the vicinity of reverse portions of
 the groove in order to reduce the curvature of the optical fiber ribbons.
 Therefore, if the groove width is broader than necessary in the
 conventional optical cable, then the stack may rotate within the groove,
 whereby the form of the stack may collapse, which may cause transmission
 loss to increase.
 In order to overcome the above-mentioned problems, it is an object of the
 present invention to provide, in particular, an optical cable which
 reliably inhibits, over the whole length of a stack of optical fiber
 ribbons contained in a helical groove of a chamber element, the stack from
 collapsing.
 For overcoming the above-mentioned problems, the optical cable in
 accordance with the present invention is an optical cable comprising a
 tension member disposed at a center thereof, an elongated chamber element
 having at least one S-Z type helical groove whose direction of strand on
 an outer periphery reverses periodically, and a plurality of optical fiber
 ribbons stacked within the helical groove of the chamber element; wherein
 a reversal angle from one reverse portion to a next reverse portion in the
 helical groove is at least 180 degrees; and wherein, letting W, T, and n
 be the width and thickness of each optical fiber ribbon and the number of
 stacked optical fiber ribbons, respectively, and a and b be the width and
 depth of the helical groove, respectively, at least the reverse portions
 of the helical groove have a cross-sectional form satisfying:
EQU nT&lt;a.ltoreq.W.sup.2 +L +(nT+L ).sup.2 +L (1)
EQU W&lt;b (2)
 whereas the remaining portion of the helical groove has a cross-sectional
 form satisfying:
EQU W.sup.2 +L +(nT+L ).sup.2 +L &lt;min(a,b) (3)
 In this optical cable, the twist of the stack itself is peaked in the
 reverse portions of the helical groove, so that the form of the stack is
 most likely to collapse there. Hence, the width of the helical groove is
 made smaller than the length of the diagonal of the stack at least in the
 reverse portions, whereby the stack is reliably prevented from collapsing
 when moving from the groove bottom toward the groove opening. In the
 portion where the stack is hard to collapse, on the other hand, the width
 and depth of the helical groove are made greater than the length of the
 diagonal of the stack, so that the stack is contained within the helical
 groove with a margin. Namely, in this portion, the relative movement of
 the helical groove with respect to the stack is made smooth. Therefore,
 the stack can be contained in a stable state over the whole length of the
 helical groove, whereby transmission loss is appropriately inhibited from
 increasing due to the collapsing of the stack.
 Alternatively, the optical cable in accordance with the present invention
 is an optical cable comprising a tension member disposed at a center
 thereof, an elongated chamber element having at least one S-Z type helical
 groove whose direction of strand on an outer periphery reverses
 periodically, and a plurality of optical fiber ribbons stacked within the
 helical groove of the chamber element; wherein a reversal angle from one
 reverse portion to a next reverse portion in the helical groove is at
 least 180 degrees; and wherein, letting W, T, and n be the width and
 thickness of each optical fiber ribbon and the number of stacked optical
 fiber ribbons, respectively, and a and b be the width and depth of the
 helical groove, respectively, the helical groove has a cross-sectional
 form satisfying the above-mentioned expressions (1) and (2) in an area
 where a rotational angle with reference to a transit center portion
 located between neighboring reverse portions is at least 90 degrees,
 whereas the remaining portion of the helical groove has a cross-sectional
 form satisfying the above-mentioned expression (3).
 In this optical cable, the twist of the stack itself is peaked in the
 reverse portions of the helical groove, so that the form of the stack is
 most likely to collapse there. Hence, the width of the helical groove is
 made smaller than the length of the diagonal of the stack in an area where
 the rotational angle with reference to a transit center portion located
 between neighboring reverse portions is at least 90 degrees, i.e., in the
 area where the stack is likely to collapse within the helical groove,
 whereby the stack is reliably prevented from collapsing when moving from
 the groove bottom toward the groove opening. In the portion where the
 stack is hard to collapse, on the other hand, the width and depth of the
 helical groove are made greater than the length of the diagonal of the
 stack, so that the stack is contained within the helical groove with a
 margin. Namely, in this portion, the relative movement of the helical
 groove with respect to the stack is made smooth. Therefore, the stack can
 be contained in a stable state over the whole length of the helical
 groove, whereby transmission loss is appropriately inhibited from
 increasing due to the collapsing of the stack.
 Here, it is preferred that the cross-sectional form of the helical groove
 of the chamber element have a bottom portion which is substantially shaped
 like an arc. As a consequence, it becomes easier for the stack within the
 helical groove to shift to a state having the least twist.
 Letting r be the radius of the arc of the bottom portion of this helical
 groove, it is preferred that
 ##EQU1##
 be satisfied.
 Preferably, a stack constituted by a plurality of the optical fiber ribbons
 is contained in the helical groove with a ribbon plane thereof facing the
 bottom face of the helical groove in the vicinity of the transit center
 portion located between neighboring reverse portions, whereas the stack is
 contained in the helical groove with a ribbon side face thereof facing the
 bottom face of the helical groove in the vicinity of the reverse portions.
 When the stack is contained within the helical groove in such a state, the
 transmission loss of coated optical fibers can be suppressed as much as
 possible.
 The present invention will be more fully understood from the detailed
 description given hereinbelow and the accompanying drawings, which are
 given by way of illustration only and are not to be considered as limiting
 the present invention.
 Further scope of applicability of the present invention will become
 apparent from the detailed description given hereinafter. However, it
 should be understood that the detailed description and specific examples,
 while indicating preferred embodiments of the invention, are given by way
 of illustration only, since various changes and modifications within the
 spirit and scope of the invention will be apparent to those skilled in the
 art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the following, preferred embodiments of the present invention will be
 explained in detail with reference to the accompanying drawings. To
 facilitate the comprehension of the explanation, the same reference
 numerals denote the same parts, where possible, throughout the drawings,
 and a repeated explanation will be omitted.
 FIG. 1 is a sectional view showing the optical cable in accordance with the
 present invention. The optical cable 1 shown in this drawing has a
 cylindrical elongated chamber element 2 extending over the whole length
 thereof, whereas a tension member 3 made of a steel wire, FRP, kevlar.TM.,
 or the like is embedded in the chamber element 2 at the center thereof.
 Further, the peripheral face of the chamber element 2 is formed with five
 S-Z type helical grooves 4 each extending in the longitudinal direction
 and having a rectangular cross section. Within each helical groove 4, a
 stack 7 constituted by five stacked optical fiber ribbons 6 is contained.
 A pressing wrap 8 made of nylon threads and nonwoven tapes is employed in
 the optical cable 1 while in a state where the stack 7 is contained within
 the helical groove 4, and a sheath 9 made of polyethylene envelops the
 pressing wrap 8.
 As shown in FIGS. 2, 3A, and 3B, the S-Z type helical groove 4 is formed
 with a predetermined period in the chamber element 2, such that a transit
 portion is formed between a reverse portion S1 and a reverse portion S2,
 with a transit center portion S0 being located in the middle thereof.
 FIGS. 3A and 3B schematically show these portions. Here, symbol .phi.
 indicates the reversal angle of the helical groove 4 from the reverse
 portion S1 to the next reverse portion S2. In these drawings, the reversal
 angle .phi. is 280 degrees.
 As shown in FIGS. 2, 3A, 3B, and 4, in the transit center portion S0, the
 stack 7 is contained within the helical groove 4 while in a state where a
 ribbon plane 7a of the stack 7 faces the bottom face 4a of the helical
 groove 4. On the other hand, as shown in FIG. 5, the stack 7 is contained
 within the helical groove 4 in the reverse portions S1 and S2 while in a
 state where a ribbon side face 7b faces the bottom face 4a of the helical
 groove 4. It is necessary that such contained states be maintained over
 the whole length of the cable 1.
 These states will be explained with reference to FIGS. 3A, 3B, and 6A to
 6G. Here, sectional views at positions indicated by (a) to (g) in FIG. 3B
 correspond to FIGS. 6A to 6G, respectively. Namely, in the area extending
 from the position (a) (see FIG. 6A) corresponding to the transit center
 portion S0 to a predetermined position (d) (see FIG. 6D) rotated by 90
 degrees therefrom in the helical groove 4, the stack 7 is contained within
 the helical groove 4 in an untwisted state. Namely, in this area, the
 stack 7 is hard to collapse within the helical groove 4, and the helical
 groove 4 relatively rotates with respect to the stack 7. On the other
 hand, in the area where the rotational angle with reference to the transit
 center portion S0 exceeds 90 degrees until it reaches the reverse portion
 S2, i.e., from the predetermined position (d) to a predetermined position
 (g) (see FIGS. 6D to 6G) in the helical groove 4, the stack 7 is likely to
 collapse within the helical groove 4, and distortion stress is likely to
 occur in each optical fiber ribbon 6 upon forcible bending.
 Therefore, for coping with such discrepancies, it is preferred that, while
 the state where the ribbon plane 7a of the stack 7 abuts against the side
 wall face 4b located on the upper side of the helical groove 4 is being
 maintained as shown in FIG. 6E, the stack 7 itself be moved toward the
 opening 4c of the helical groove 4 as shown in FIG. 6G.
 For realizing this, the size of the helical groove 4 in the area extending
 from the position (a) (see FIG. 6A) corresponding to the transit center
 portion S0 to the predetermined position (d) (see FIG. 6D) rotated by 90
 degrees therefrom is needed to be such that the stack 7 is allowed to
 rotate smoothly with respect to the helical groove 4, since the stack 7
 rotates relative to the helical groove 4 in this area. Therefore, letting
 W, T, and n be the width and thickness of each optical fiber ribbon 6 and
 the number of stacked optical fiber ribbons 6, respectively, and a and b
 be the width and depth of the helical groove 4, respectively, both of the
 width a and depth b of the helical groove 4 in this portion are needed to
 be greater than the length W.sup.2 +L +(nt).sup.2 +L of the diagonal L of
 the stack 7 when the latter is assumed to be a rectangle. Here, as shown
 in FIG. 8, P is a circumscribed circle of the stack 7, which touches each
 apex of the stack 7 when the latter is assumed to be a rectangle
 (indicated by the dash-single-dot line) for the sake of convenience,
 whereas L is the diagonal thereof.
 In the area extending from the position (d) (see FIG. 6D) to the position
 (g) (see FIG. 6G) corresponding to the reverse portion S2, on the other
 hand, it is hardly necessary for the stack 7 to be rotated relative to the
 helical groove 4. Therefore, it will be sufficient if the width a of the
 helical groove 4 is not smaller than the height nT of the stack 7 and if
 the depth b of the helical groove 4 is not smaller than the width W of the
 optical fiber ribbon 6. Preferably, the width a and depth b of the helical
 groove 4 have clearances which take account of tolerances in the
 manufacture of the chamber element 2 and optical fiber ribbons 6.
 Further, from the position (d) to position (g) (see FIGS. 6D to 6G), the
 stack 7 is contained in the helical groove 4 so as to be shifted toward
 the opening 4c thereof. Here, if the width a of the helical groove 4 is
 not smaller than the above-mentioned diagonal L, then the stack 7 rotates
 in excess with respect to the helical groove 4, thereby causing the stack
 7 to collapse as shown in FIGS. 7F and 7G. It is because of the fact that
 the individual optical fiber ribbons 6 tend to take the shortest path
 within the helical groove 4 in the process of making the optical cable. In
 particular, the optical fiber ribbons 6 closer to the bottom face 4a of
 the helical groove 4 tend to keep their current positions without
 following the relative rotation of the helical groove 4.
 For appropriately preventing the stack 7 from collapsing due to such a
 phenomenon, the width a of the helical groove 4 is made smaller than the
 length of the diagonal L of the stack 7 in the area where the rotational
 angle with reference to the transit center portion S0 becomes 90 degrees
 or greater, i.e., from the position (d) to position (g) (see FIGS. 6D to
 6G). Namely, employed is a groove form in which the circumscribed circle P
 projects out of the side wall face 4b as shown in FIG. 5, so as to prevent
 the stack 7 from rotating in excess.
 For preventing the stack 7 from collapsing as such, it is not necessary for
 the above-mentioned setting to be made in all the area where the
 rotational angle with reference to the transit center portion S0 becomes
 90 degrees or greater. The aimed object can be achieved if the
 above-mentioned conditions are satisfied at least in the reverse portions
 S1, S2.
 Here, tests for verifying various characteristics in the above-mentioned
 embodiment were carried out. As shown in FIG. 1, the optical cable 1 in
 this case has five helical grooves 4, whereas five sheets of optical fiber
 ribbons 6 each including four optical fibers are contained in each helical
 groove 4. In the area from the position where the rotational angle is 90
 degrees to the reverse portion S2, the width a of the helical groove 4 is
 reduced in proportion to the length of the chamber element 2.
 The optical cable 1 was subjected to a transmission loss test under the
 conditions shown in Table 1 for all the one hundred optical fibers by
 means of OTDR (having a wavelength of 1.55 .mu.m). The results are shown
 in Table 2.
 TABLE 1
 Outside
 diameter Re-
 of versal
 chamber angle Diagonal L Groove Groove
 element .phi. Pitch of stack Portion Width a Depth b
 10.0 mm 280.degree. 300 mm 1.9 mm Transit 2.0 mm 2.0 mm
 (W = 1.1 mm, Center
 T = 0.32 mm, 90.degree. 2.0 mm
 n = 5) Reverse 1.7 mm
 TABLE 2
 Optical fiber After
 itself After contained Sheathing
 Max. 0.21 dB/km 0.22 dB/km 0.22 dB/km
 Ave. 0.19 dB/km 0.20 dB/km 0.20 dB/km
 As shown in Table 2, the difference between the transmission loss of the
 optical fiber ribbons 6 themselves and the transmission loss in the case
 where the optical fiber ribbons 6 are contained in the chamber element 2
 is 0.01 dB/km at the maximum, and is also 0.01 dB/km on average. As a
 consequence, it has been proved that the stack 7 is appropriately
 prevented from collapsing within the helical groove 4 in the manufacturing
 process, whereby transmission loss is appropriately inhibited from
 increasing.
 For corroborating the effect of the above-mentioned embodiment, a
 comparative example shown in FIGS. 9 and 10 will now be set forth, so as
 to verify its characteristics. In this comparative example, the width a
 and the depth b of a helical groove 100 are made uniform over the whole
 length thereof. FIG. 9 is a sectional view showing the transit center
 portion of the helical groove 100.
 The optical cable having the configuration of the comparative example was
 subjected to a transmission loss test under the conditions shown in Table
 3 for all the one hundred optical fibers by means of OTDR (having a
 wavelength of 1.55 .mu.m). The results are shown in Table 4.
 TABLE 3
 Outside
 diameter Re-
 of versal
 chamber angle Diagonal L Groove Groove
 element .phi. Pitch of stack Portion Width a Depth b
 10.0 mm 280.degree. 300 mm 1.9 mm Transit 2.0 mm 2.0 mm
 (W = 1.1 mm, Center
 T = 0.32 mm, 90.degree. 2.0 mm
 n = 5) Reverse 2.0 mm
 TABLE 4
 Optical fiber After
 itself After contained Sheathing
 Max. 0.21 dB/km 0.25 dB/km 0.26 dB/km
 Ave. 0.19 dB/km 0.21 dB/km 0.21 dB/km
 As shown in Table 4, the difference between the transmission loss of the
 optical fiber ribbons 6 themselves and the transmission loss in the case
 where the optical fiber ribbons 6 are contained in the chamber element 2
 is 0.04 to 0.05 dB/km at the maximum, and is 0.02 dB/km on average. As a
 consequence, it is seen that a large transmission loss is generated in a
 part of the optical fiber ribbons 6, and that the collapse of the stack 7
 is generated in the reverse portions S1, S2 as shown in FIGS. 7A to 7G and
 10 in the manufacturing process.
 The groove is not limited to rectangular grooves, but may be a U-shaped
 groove such as the one shown in FIGS. 11 and 12. FIGS. 11 and 12
 correspond to FIGS. 4 and 5 in the case of the above-mentioned rectangular
 groove 4, respectively.
 In the area from the position (a) corresponding to the transit center
 portion S0 to the predetermined position (d) rotated by 90 degrees
 therefrom in FIG. 3B, whose sectional views are shown in FIGS. 6A to 6D,
 it is necessary that the stack 7 be rotated relative to the U-shaped
 helical groove 14. To this aim, it is necessary for the helical groove 14
 to have such a form that a circle having a diameter equal to the diagonal
 L of the stack 7, when the latter is assumed to be a rectangle, is
 completely contained within the cross section of the helical groove 14.
 Namely, as in the rectangular helical groove 4 shown in FIG. 4, both of
 the width a and depth b are needed to be greater than the length W.sup.2
 +L +(nt).sup.2 +L of the diagonal L. Also, letting r be the diameter of
 the arc of the bottom face 14, the stack 7 can be rotated smoothly within
 the helical groove 14 if r.gtoreq.L/2.
 On the other hand, from the position (d) to the position (g) corresponding
 to the reverse portion S2 in FIG. 3B, whose sectional views are shown in
 FIGS. 6D to 6G, it is hardly necessary for the stack 7 to be rotated
 relative to the helical groove 14. Hence, as in the rectangular helical
 groove 4 shown in FIG. 5, it will be sufficient if the width a of the
 helical groove 14 is not smaller than the height nT of the stack 7 and if
 the depth b of the helical groove 14 is not smaller than the width W of
 the optical fiber ribbon 6. Preferably, the width a and depth b of the
 helical groove 14 have clearances which take account of tolerances in the
 manufacture of the chamber element 2 and optical fiber ribbons 6.
 Without being restricted to the above-mentioned embodiments, the present
 invention may be provided with a LAP sheath, HS sheath, or water-absorbing
 tape wrap, for example, and it may be a self-supporting type cable as a
 matter of course. Also, the chamber element may be a linear one crawling
 in the longitudinal direction along the peripheral face of a body having a
 cylindrical form.
 From the invention thus described, it will be obvious that the invention
 may be varied in many ways. Such variations are not to be regarded as a
 departure from the spirit and scope of the invention, and all such
 modifications as would be obvious to one skilled in the art are intended
 for inclusion within the scope of the following claims.