JP2981116B2 - Optical fiber unit and optical fiber cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber cable, and more particularly to an optical fiber unit.

[0002]

2. Description of the Related Art With an increase in demand for optical fibers, a demand for a high-density multi-core optical cable has been increasing. As a multi-core optical cable currently in practical use, a slot type cable is the most common. As shown schematically in FIG. 3, the slot type cable has a structure in which the optical fiber tape 10 is stacked and housed in a spiral groove 22 of a slot 20, and a holding roll 24 and a sheath 26 are provided. Note that 12
Indicates an individual optical fiber.

However, this slot type cable has a disadvantage that the optical fiber tape 10 cannot be stored at a high density. Therefore, a U-groove type cable has recently been proposed as a cable that can be stored at a higher density. This is, as schematically shown in FIG.
Are stacked and stored in a U-groove 28 to form a U-groove unit 30, which is twisted and assembled around a central tension member 32, and is provided with a presser winding 34 and a sheath 36 (1993, Electronic Information and Communication). (See Spring Meeting of the Society, Proceedings, B-873).

However, even in the case of this U-groove type cable, when a large number of optical fiber tapes 10 are laminated in a groove of one U-groove 28 for high density, a transmission loss easily occurs in the optical fiber at the bottom of the groove. There is.

[0005]

SUMMARY OF THE INVENTION In order to solve these problems and to enable high-density mounting,
We have previously proposed an optical cable in which the following H-groove units are assembled. FIG. 5A is a side view of the H-groove 42, and FIG. The H-groove 42 has an elongated cord-like body 44 (generally circular cross section)
In addition, two back-to-back grooves 46, 46 are provided in a spiral shape, and a cross section perpendicular to the longitudinal direction is substantially H-shaped. The optical fiber tape 10 is stacked and stored in the groove 46 of the H groove 42 to form an H groove unit 40 (c).

[0007] As shown in FIG. 5 D, the H-groove unit 40 is twisted around the center tension member 49 to form an optical fiber cable.

The structure of the H-groove 42 is determined by the parameters shown in FIG. 6, a radius, a radius, a groove width, gw, a groove depth, gh, and a groove bottom thickness, gbt. These parameters are designed only from the viewpoint of the groove space, such as the width and the number of optical fiber tapes 10 to be mounted.

Since the groove for accommodating the H-groove 42 is divided into two grooves, even if the optical fiber tape is mounted at a high density, transmission loss is less likely to occur in the optical fiber at the bottom of the groove. However, there is a problem that the bending rigidity has directionality due to its cross-sectional shape. That is, the bending stiffness is greatly different between the a direction and the b direction in FIG. 7, and the structure is easy to bend in the a direction and hard to bend in the b direction.

For this reason, when the bending diameter is small when the H groove 42 is bent, the above-described directionality of the bending rigidity causes
An unnatural way of bending is obtained as shown in FIG. That is, groove 4
Since H is spiral, the H-grooves 42 alternately face the direction a or the direction b with respect to the bending direction. At A (portion in the a direction), the body is extremely bent and the radius of curvature is locally reduced. On the other hand, B (b-direction portion) does not bend much (FIG. 8 is exaggerated). When the H-groove 42 becomes like this, a transmission loss occurs in the optical fiber tape 10 housed and assembled in the groove 46.

One way to improve this is to increase the number of groove grooves, for example, as shown in FIG.
It is conceivable that the number 6 is changed to 3 (a) or 4 (b). However, this method has a drawback that the structure approximates to a conventional slot rod, so that the mounting density of the optical fiber decreases and the outer diameter of the cable increases.

Therefore, as an effective method for improving the directionality of bending rigidity, the following is considered (Japanese Patent Application No. 5-222).
No. 271). That is, as shown in FIG. 1A, a notch 48 is provided on a side surface of the main body 44 of the H groove 42 in the direction b. In addition to ur, gw, gh, and gbt in FIG. 6 described above, the direction of bending rigidity is designed in consideration of the radius sr of the notch 48 and the distance sd from the center of the notch 48 to the side surface of the main body 44. .

The bending stiffness in the b direction is the product of [Second moment of area with respect to axis aoa] and [Young's modulus]. The value of the second moment of area with respect to axis aoa is: Area] and [distance from axis aoa 2]
To the power]. The same applies to the bending stiffness in the a-direction. When the H groove 42 is made of a homogeneous material, the direction of the bending rigidity is determined from the cross-sectional shape. That is, a, b
The difference in the second moment of area in the direction produces the directionality of the bending.

As described above, if the difference in the bending stiffness in the a and b directions, that is, the difference in the second moment of area, is greater than a certain level, the H-groove 42 is bent as shown in FIG. Transmission loss occurs in the optical fiber tape 10 housed in the groove 46. This transmission loss is a, b
The larger the difference in the second moment of area in the direction is, the larger the difference is. Therefore, generally, an H-groove is designed and used so that the difference is as small as possible.

However, as a result of further studies, it has been found that the transmission loss occurring in the tape 10 may exceed the permissible limit even in the case where the difference in the second moment of area is designed to be small. Was.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a ratio of a second moment of area of the H-groove 42 in the b-axis direction to the a-axis direction of 5 or less. The spiral pitch p of the groove 46 is set to 700 mm or less;
Solved the problem.

[0016]

[Operation] When the ratio of the second moment of area in the b-axis direction and the a-axis direction of the H groove is reduced, the bending directionality is reduced, so that transmission loss due to bending is less likely to occur. However, even if the ratio of the second moment of area is small, the degree of local bending is larger in the case of the spiral pitch p (a) than in the case of the small pitch p (b), as shown in FIG. Is large (the radius of curvature is small). As a result, the transmission loss of the optical fiber tape 10 housed in the groove 46 increases.

Therefore, it is necessary to design not only the ratio of the second moment of area in the b-axis direction and the a-axis direction of the H groove but also the spiral pitch of the groove in consideration of the balance. The balance point between the two obtained from the experiment is that the ratio of the second moment of area in the b-axis direction and the a-axis direction of the H groove is 5 or less, and the spiral pitch of the groove is 700 mm or less. By satisfying this condition, the transmission loss occurring in the optical fiber tape 10 can be reduced to 0.25 dB / km or less as shown in Table 1 below.

[0018]

FIG. 1A is a sectional view of an H groove 42,
(B) is a cross-sectional view of the H groove unit 40, (c) is H
It is a side view of the groove 42. The material of the H groove 42 is PBT (polybutylene terephthalate). ur =
7 mm, gw = 3.6 mm, gh = 2.0 mm, and gbt = 1.2 mm. By changing sr and sd, the ratios of the moments of inertia in the directions b and a of 6, 5.5, 5 and 3.5 were produced. Also, b,
For the ratios of the moments of inertia in the a-direction above, the helical pitches p are 800 mm, 750 mm and 700 m, respectively.
Made m and 300mm. In the groove 46 of each H groove 42, five 12-fiber optical fiber tapes 10 each having a width of 2.5 mm and a thickness of 0.25 mm were accommodated. The transmission loss at a wavelength of 1.55 μm of the optical fiber tape when these were wound around a 600 mmφ drum was measured. Table 1 shows the measurement results.

[0019]

[Table 1] From this table, if the ratio of the second moment of area in the b-axis direction and the a-axis direction of the H groove is set to 5 or less and the spiral pitch of the groove is set to 700 mm or less, the transmission loss generated in the optical fiber tape 10 is 0.25 dB. / Km or less.

In the above case, the H groove 42
Is 7.0 mm (ur = 3.5 mm), but if the thickness of the H groove 42 changes, the characteristics may change even with the same helical pitch. In consideration of the influence of the thickness of the H groove, a more accurate expression can be obtained by taking the [ratio of the helical pitch p and the diameter 2 ur] instead of the [helical pitch p]. Therefore, considering the groove thickness, the transmission loss is 0.25 dB / km.
(Wavelength 1.55 μm) or less, assuming that the ratio of the second moment of area in the b-axis direction and the a-axis direction of the H groove is 5 or less,
In addition, the ratio between the spiral pitch and the diameter of the groove is 100 or less.

[0021]

As described above, in the H-shaped groove according to the present invention, the ratio of the second moment of area in the b-axis direction to the a-axis direction is as small as 5 or less, so that the bending directionality is reduced. And the spiral pitch of the groove is 700mm
Since it is smaller than that described below, even when it is wound around a drum or the like, a bent portion having a small bending radius does not occur locally. For this reason, even when the optical fiber tape is mounted at a high density, transmission loss is less likely to occur, and a high-density multicore cable having good transmission characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an operation in the present invention.

FIG. 3 is an explanatory view of a slot type optical cable.

FIG. 4 is an explanatory view of a U-groove type optical cable.

FIG. 5 is an explanatory diagram of an H-groove unit and an optical cable using the same.

FIG. 6 is an explanatory diagram of parameters for determining a cross-sectional shape of a slot.

FIG. 7 is an explanatory view showing that a slot has a bending direction.

FIG. 8 is a schematic explanatory view showing a state where a directional slot is bent.

FIG. 9 is an explanatory diagram of a directionality improvement measure of a slot.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Optical fiber tape 12 Optical fiber 20 Slot 22 Slot groove 24, 34 Holding roll 26, 36 Sheath 28 U groove 30 U groove unit 32, 49 Center tension member 40 H groove unit 42 H groove 44 H groove main body 46 Spiral groove 48 Notch

──────────────────────────────────────────────────続 き Continued on the front page (56) References JP-A-6-337335 (JP, A) JP-A-60-169607 (JP, U) JP-A-1-103812 (JP, U) JP-A-2-103 119314 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) G02B 6/44

Claims (3)

(57) [Claims] 1. An optical fiber unit comprising an elongate rod-shaped main body and spirally provided two back-to-back grooves in an H-groove, wherein an optical fiber tape is stacked and housed in said groove, wherein said H-groove is in the b-axis direction. And the ratio of the moment of inertia in the a-axis direction to 5 or less, and the spiral pitch of the groove is 700 m
fiber optic unit that is less than m. 2. The optical fiber according to claim 1, wherein the ratio of the second moment of area in the b-axis direction to the a-axis direction of the H-groove is 5 or less, and the ratio between the helical pitch and the diameter of the groove is 100 or less. unit. 3. An optical fiber cable comprising the optical fiber units according to claim 1 or 2.