JPH09311241A - Optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate the manufacture and handling and to expand the transmission band by making the sectional shapes of an inner core elliptic and providing the inner cores concentrically with an outer core. SOLUTION: The core 1 of the optical fiber is in three-layered structure consisting of the outer core 2 which is sectioned circularly and inscribed with a clad C, the 1st inner core 3 which is elliptically sectioned and provided in the outer core 2, and the 2nd inner core 4 which is elliptically sectioned and provided in the 1st inner core 3. Both the inner cores 3 and 4 are concentric with the outer core 2 and provided so that the intervals between the outer peripheral surface of the outer core 2 and the outer peripheral surface of the 1st inner core 3 and the interval between the outer peripheral surface of the 1st inner core 3 and the outer peripheral surface of the 2nd inner core are made narrow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber for communication, and more particularly to an improvement in the amount of information transmitted in a multimode optical fiber.

[0002]

2. Description of the Related Art Optical fibers used for communication include single-mode and multi-mode optical fibers.
There are various types of refractive index distribution, such as a step index type and a gradient index type, and each type has its own characteristics. Among them, the multimode step index type is, for example, several hundred μm to 1 mm.
It is possible to make the core diameter to some extent, it is easy to handle in connection with light source and light receiver, fiber-to-fiber connection, etc. Also, it is lower cost than the gradient index type which can also have large core diameter. For this reason, it is often used as a relatively short-distance communication or a data transmission means in a device.

However, in a multimode step index type optical fiber, there is a large effect that the propagation speeds of propagating light rays differ depending on the mode, that is, so-called mode dispersion, whereby the time width of an incident light pulse is increased according to the increase of the propagation distance. There is a phenomenon that the pulse shape is easily spread and the pulse shape is easily broken.
For this reason, the multimode step index type optical fiber has a narrow transmission band of several hundredths at the same transmission distance compared to the single mode step index type or gradient index type, and the amount of information that can be transmitted in a unit time. Is significantly less. Even in a multimode step index type optical fiber, however, it is possible to widen the transmission band by reducing the refractive index difference between the core and the cladding. But doing this
This is not practical because the numerical aperture decreases and the transmission efficiency deteriorates.

However, if the transmission distance is short, the reduction of the transmission band due to mode dispersion is relatively small, and the transmission band required up to now in the field of near field communication in which the multimode step index type is frequently used. The mode dispersion as described above has not been so much a problem because, for example, is not so large. However, the processing speed of information processing devices in recent years tends to increase more and more, and accordingly, a wider transmission band is required even in short-distance communication, and the limit of the transmission band in the multimode step index type becomes a problem. It is becoming.

By the way, the gradient index type optical fiber has a transmission band several hundred times as large as that of the multimode step index type optical fiber as described above, and a large core diameter which facilitates handling such as connection is possible. Is. Therefore, by using this gradient index type optical fiber, it is possible to cope with the above-mentioned high speed. However, the gradient index type gives a refractive index distribution of a parabolic distribution by diffusing a substance for refractive index distribution, as known from, for example, Japanese Patent Application Laid-Open No. 4-97302 and Japanese Patent Application Laid-Open No. 5-808488. However, there is a problem in that the manufacturing process is much more complicated than in the step index type and the cost is increased. However, it is not always appropriate to use the gradient index type optical fiber for the short-distance communication system because the performance becomes excessive.

[0006]

The present invention has been made against such a background, and the present invention is similar to the conventional multimode / step index type optical fiber in terms of easiness of manufacture and easy handling of connection. It is an object of the present invention to provide a new optical fiber which has a high transmission property, and which exceeds the limit of the transmission band in the conventional multimode step index type and enables the transmission band required by the speeding up of information processing equipment.

[0007]

To achieve the above object, according to the present invention, an outer core having a circular cross section is provided in which an optical fiber core is inscribed in a clad and has a refractive index larger than that of the clad, and the outer core. It is basically formed with an inner core that is provided inside the core and has a refractive index different from that of the outer core.In addition to this, the cross-sectional shape of the inner core is elliptical, and this inner core is concentric with the outer core. That is, the center of the inner core is made to coincide with the center of the outer core.

In an optical fiber having an elliptic multilayer core structure having such a core consisting of a circular outer core and an elliptical inner core, light rays having different incident conditions on the end face of the optical fiber are propagated by the following mechanism. The speed can be made uniform, and the transmission band can be widened. For example, in the case of a structure in which the inner core has a single layer and the entire core has two layers, the effect of making the propagation velocity uniform is about three times as high as that of the conventional step index type optical fiber with the same numerical aperture. The transmission band can be widened, and the inner core is made up of 2 layers and the entire core is 3 layers.
In the case of a layered structure, the transmission band can be widened to about 5 times.

Light rays propagating through an optical fiber can be regarded as rays which are substantially all skew rays, that is, rays which deviate from the meridional plane including the central axis of the optical fiber. In an optical fiber with an elliptic multilayer core structure, the core has a structure in which an elliptical inner core is provided inside a circular outer core, so most of the skew rays are of the inner core or outer core. In either case, it is either light propagating while being confined in a core having a relatively high refractive index, or light propagating while alternately passing through both the inner core and the outer core. The incident / reflection patterns at the interface between the outer core and the clad and the interface between the outer core and the inner core are changed aperiodically, that is, randomly. As a result, in particular, in the latter skew ray, the passage distance in the region where the propagation velocity is different due to the different refractive index is one cycle (one cycle is a cycle from one reflection at the interface between the outer core and the clad to the next reflection). ), The velocity is changed randomly, and the averaging effect of this random velocity change effectively equalizes the propagation velocity.

Since the numerical aperture of the optical fiber of this form is determined by the highest refractive index in the core, the refractive index of the portion with the lower refractive index in the core is compared with the refractive index of the portion with the higher refractive index in the core and the refractive index of the clad. By setting the value approximately in between, the transmission band of the above two kinds of propagating light components becomes extremely larger than the transmission band of the step index fiber having the same numerical aperture. Also, the average propagation velocity of these two components can be made equal by adjusting the shape of the inner core and the refractive index, and by doing so, the transmission band resulting from the addition of both components will be about the same size as both components. .

The optical fiber having a multi-layer core structure according to the present invention enables a wide transmission band as described above, but at the same time has an easy manufacture. That is, the optical fiber according to the present invention can be basically manufactured by the same manufacturing method as that generally used in the conventional step index type optical fiber, and therefore the core has a multilayer structure. By setting the degree of multi-layering of the core within an appropriate range, it is possible to manufacture under the same conditions as those in the conventional step index type optical fiber.

In the elliptic multi-layer core structure capable of making the propagation velocity uniform in this way, the inner core can also be multi-layered, and the degree of homogenization can be increased by increasing the number of layers. However, increasing the number of layers of the inner core increases the difficulty of manufacturing. Therefore, the optimum number of inner core layers is determined by the balance between the required performance and cost, but it is most effective to use 1 to 3 layers (2 to 4 layers for the entire core), and it is easy to manufacture. A large band improvement effect can be obtained. In particular, a structure in which the inner core has two layers and the entire core has three layers is preferable.

To make the core a three-layer structure, the inner core is formed of a first inner core and a second inner core each having an elliptical cross-sectional shape, and the second inner core is the first inner core. It is arranged to be concentric with the first inner core in the core.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention relates to a core having a three-layer structure. As shown in FIG. The outer core 2 provided so as to be in contact with the outer core 2, the first inner core 3 having an elliptical cross section provided inside the outer core 2, and the second inner core 3 having the same elliptical cross section provided inside the first inner core 3. It has a three-layer structure including the inner core 4. Both of the inner cores 3 and 4 are concentric with the outer core 2, and the distance between the outer peripheral surface of the outer core 2 and the outer peripheral surface of the first inner core 3 in the major axis direction of the ellipse, and the first The distance between the outer peripheral surface of the inner core 3 and the outer peripheral surface of the second inner core 4 is sufficiently narrowed within a range that does not hinder manufacturing. By doing so, both the first and second inner cores can be overlapped so as to cover the propagation path of the skew ray propagating in the optical fiber without leakage,
It is possible to more effectively obtain the averaging action due to the random speed change in the skew ray. In addition, since such a structure has mirror symmetry with respect to two orthogonal mirror surfaces, expansion due to temperature change or moisture absorption,
The risk of deformation such as warp or twist of the fiber due to the amount of shrinkage depending on the material can be reduced.

A numerical example of the sizes of the outer core and the inner core and the respective refractive indexes in this embodiment is as follows. Refractive index of second inner core = 1.505, refractive index of first inner core = 1.500
, Outer core refractive index = 1.495, cladding refractive index = 1.490, outer core radius = 0.50 mm, first inner core minor radius = 0.35 mm, first inner core major radius = 0.49 mm, second Inner core minor radius = 0.18mm, 2nd
Inner core long radius = 0.48 mm. The effect of improving the transmission band under these numerical conditions is about five times or less when calculated from the calculation based on the comparison with the conventional step index type optical fiber having the same numerical aperture.

A second embodiment of the present invention relates to a core having a two-layer structure, and an optical fiber according to this embodiment is
As shown in FIG. 2, the core 11 has a two-layer structure including an outer core 12 having a circular cross section and an inner core 13 having an elliptical cross section. The inner core 13 is provided on the outer core 12 in the same manner as the first inner core 3 in the first embodiment.

An example of one numerical value in this embodiment is as follows. Inner core refractive index = 1.505, outer core refractive index = 1.498, clad refractive index = 1.49
0, outer core radius = 0.50 mm, inner core short radius = 0.20 mm, inner core long radius = 0.49 mm. The effect of improving the transmission band under these numerical conditions is about three times or more when calculated from the comparison with the conventional step index type optical fiber as a model.

In each of the above embodiments, the forward distribution is such that the refractive index decreases from the inner side to the outer side. However, for example, the distribution is such that the refractive index increases from the inner side to the outer side, and other distributions. It is also possible. Such a device often shows a higher transmission band improvement effect than the forward distribution type, but may slightly increase transmission loss depending on the form.

[0019]

As described above, according to the present invention, it is possible to manufacture under substantially the same conditions as the conventional step index type optical fiber, and the transmission band is significantly larger than that of the conventional step index type optical fiber. It is possible to provide an optical fiber that can widen
It can greatly contribute to the improvement of the function of the communication system using the optical fiber.

[Brief description of drawings]

FIG. 1 is a sectional view of an optical fiber according to a first embodiment.

FIG. 2 is a sectional view of an optical fiber according to a second embodiment.

[Explanation of symbols]

 1 …… Core 2,12 …… Outer core 3 …… First inner core 4 …… Second inner core 13 …… Inner core C …… Clad

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Noriji Oishi 2-12-8 Tsujido Shinmachi, Fujisawa City, Kanagawa Prefecture Izawa 103

Claims (2)

[Claims] 1. An outer core having a circular cross-section, which is inscribed in the clad and has a larger refractive index than the clad, and an inner core which is provided in the outer core and has a different refractive index from the outer core. An optical fiber having a core, wherein the inner core has an elliptical cross section and the inner core is provided concentrically with the outer core. 2. An inner core is formed of a first inner core and a second inner core each having an elliptical cross-sectional shape, and the second inner core is contained in the first inner core. The optical fiber according to claim 1, which is provided concentrically with the core.