JP3662669B2 - Optical fiber - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical fiber for communication, and more particularly to an improvement in the amount of transmission information in a multimode optical fiber.
[0002]
[Prior art]
There are various types of optical fibers used for communication, including a single mode type and a multimode type in terms of modes, and a step index type and a gradient index type in terms of the refractive index profile. . In particular, the multi-mode step index type can have a core diameter of, for example, about several hundred μm to 1 mm, and is easy to handle in connection with a light source or a light receiver or between a fiber and a fiber. Because of its low cost compared to a gradient index type capable of a large core diameter, it is frequently used as a relatively short-distance communication and data transmission means in equipment.
[0003]
However, multimode step index type optical fiber has a large effect of different propagation speed depending on the mode of propagating light beam, so-called mode dispersion is large, so that the time width of the incident light pulse is increased as the propagation distance is increased. There is a phenomenon that the shape tends to collapse. For this reason, multimode step index type optical fibers have a narrow transmission bandwidth of about one hundredth over the same transmission distance compared to single mode step index type and gradient index type, and the amount of information that can be transmitted per unit time. There is much less. However, even in a multimode step index type optical fiber, it is possible to widen the transmission band by reducing the difference in refractive index between the core and the clad. However, this is not practical because the numerical aperture decreases and the transmission efficiency deteriorates.
[0004]
However, if the transmission distance is short, the decrease in the transmission band due to mode dispersion is relatively small, and the transmission band required so far in fields such as near field communication where multi-mode step index type is frequently used is too large. For example, the mode dispersion as described above was not so much a problem. However, recent information processing devices tend to have higher processing speeds, and as a result, a wider transmission band is required even in short-distance communication, and the limitation of the transmission band in the multi-mode step index type is a problem. It is becoming.
[0005]
By the way, the gradient index type optical fiber has a transmission band several hundred times that of the multimode step index type optical fiber as described above, and can have a large core diameter that facilitates connection and the like. Therefore, by using this gradient index type optical fiber, it is possible to cope with the increase in speed as described above. However, the gradient index shape gives a refractive index distribution of a parabolic distribution by diffusing a material for refractive index distribution, as is known, for example, in Japanese Patent Laid-Open No. 4-97302 and Japanese Patent Publication No. 5-808488. The manufacturing process, etc. is significantly more complicated than the step index type, and there is a problem that the cost is increased. Therefore, it is not always appropriate to use a gradient index type optical fiber for the short-range communication system.
[0006]
[Problems to be solved by the invention]
This is the background to the present invention, which has the same ease of manufacturing and handling as the conventional multimode step index optical fiber, and the conventional multimode. The purpose is to provide a new optical fiber that enables the transmission band required by the speeding up of the information processing device beyond the limit of the transmission band in the step index type.
[0007]
[Means for Solving the Problems]
For the above purpose, in the present invention, the core in the optical fiber is provided in contact with the clad and has an outer core having a circular cross section having a refractive index larger than that of the clad, and the outer core provided in the outer core. In principle, the inner core is formed with an inner core having a different refractive index, the inner core has a circular cross-sectional shape, and the inner core is non-concentric with the outer core, that is, the inner core center is the outer core. Shifting with respect to the center of the core, multimode transmission is performed between the outer core and the inner core, and a random speed change is caused in the skew ray .
[0008]
In the case of an optical fiber having a circular multilayer core structure in which such a core is composed of a circular inner core as well as a circular outer core, the propagation speed is uniform by the following mechanism for light beams having different incident conditions on the optical fiber end face. The transmission band can be widened. For example, in the case of a structure in which the inner core is a single layer and the entire core is a two-layer structure, the transmission speed is approximately 2.5 times that of a conventional step index optical fiber with the same numerical aperture. In the case of a structure in which the inner core has two layers and the entire core has three layers, the transmission band can be widened to about four times.
[0009]
Light rays propagating through the optical fiber can be regarded as substantially all skew rays, that is, light rays that deviate from the meridional plane including the central axis of the optical fiber. In an optical fiber having a circular multilayer core structure, the core has a structure in which a circular inner core is provided non-concentrically inside a circular outer core. Most of them, except those that are confined in a core having a relatively high refractive index and propagate only through the core, most of them propagate through the core while alternately passing through the outer core and the inner core having different refractive indexes. At the same time, the pattern of incidence and reflection at the interface between the outer core and the clad and the interface between the outer core and the inner core is changed aperiodically, that is, randomly. As a result, the skew ray during the propagation has a period of one period (one period is from a certain reflection at the interface between the outer core and the clad to the next reflection). The frequency is changed randomly every period), and a speed change is randomly generated. By the averaging action by the random speed change, the propagation speed is effectively uniformed.
[0010]
Since the numerical aperture of this form of optical fiber is determined by the highest refractive index in the core, the refractive index of the portion having a low refractive index in the core is approximately equal to the refractive index of the portion having a high refractive index in the core and the refractive index of the cladding. By setting the intermediate value, the transmission band of the two kinds of propagation light components becomes extremely larger than the transmission band of the step index fiber having the same numerical aperture. Also, the average propagation speed of both components can be made equal by adjusting the shape of the inner core and the refractive index, so that the transmission band resulting from the addition of both components is as large as both components. .
[0011]
The optical fiber having a circular multi-layer core structure according to the present invention enables a wide transmission band as described above, but at the same time has ease of manufacture. That is, the optical fiber according to the present invention can basically be manufactured by the same manufacturing method as that generally used in the conventional step index type optical fiber, and thus the core has a multilayer structure. By making the degree of multilayering of the core within an appropriate range, it is possible to manufacture under the same conditions as in the conventional step index type optical fiber.
[0012]
In this way, the circular multi-layer core structure that can make the propagation speed uniform can also increase the degree of uniformity as the inner core is made multi-layered and the number of layers is increased. However, increasing the number of inner core layers increases the difficulty of manufacturing. Therefore, the optimum number of inner core layers is determined by the balance between required performance and cost. However, it is most effective to use 1 to 3 layers (2 to 4 layers for the entire core), and manufacturing is easy. A significant 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.
[0013]
In order to make the core have a three-layer structure, the inner core is formed of a first inner core having a circular cross-sectional shape and a second inner core having the same circular cross-sectional shape and provided in the first inner core. In addition, both the second inner core and the first inner core are provided non-concentrically with respect to the outer core, and the second inner core is provided non-concentrically with respect to the first inner core.
[0014]
Embodiment
The first embodiment of the present invention relates to a core having a three-layer structure. As shown in FIG. 1, the core 1 of the optical fiber according to the present embodiment is an outer layer provided in contact with the clad C as a circular cross-sectional shape. Three layers comprising a core 2, a first inner core 3 having a circular cross-sectional shape provided inside the outer core 2, and a second inner core 4 having the same circular cross-sectional shape provided inside the first inner core 3 Structure.
[0015]
Both inner cores 3 and 4 are provided non-concentrically with respect to the outer core 2 by shifting the respective centers with respect to the center of the outer core 2, and the second inner core 4 is also the first inner core. 3, the center is shifted from the center of the first inner core 3 so as to be non-concentric. Further, the inner cores 3 and 4 are provided so that the distance between the outer peripheral surface of each of the inner cores 3 and 4 and the outer peripheral surface of the outer core 2 is sufficiently narrow as long as there is no problem in manufacturing. By doing so, the inner cores 3 and 4 can be overlapped so as to cover the skew ray propagation path propagating through the optical fiber without leakage, and the random speed in the skew ray described above can be achieved. The averaging effect due to the change can be obtained more effectively.
[0016]
An example of the numerical values of the outer core and inner core sizes and the respective refractive indexes in this embodiment is as follows. The refractive index of the second inner core = 1.505, the refractive index of the first inner core = 1.500, the refractive index of the outer core = 1.495, the refractive index of the cladding = 1.490, the radius of the outer core = 0.50 mm, the first inner core Radius = 0.40mm, second inner core radius = 0.285mm. The transmission band improvement effect under these numerical conditions is more than four times when calculated from a calculation based on a comparison with a conventional step index type optical fiber having the same numerical aperture.
[0017]
The second embodiment of the present invention relates to a two-layer structure. As shown in FIG. 2, the optical fiber according to this embodiment has an inner core 13 whose core 11 has a circular cross-sectional shape similar to the outer core 12 having a circular cross-sectional shape. A two-layer structure consisting of 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.
[0018]
A numerical example in this embodiment is as follows. The refractive index of the inner core = 1.505, the refractive index of the outer core = 1.500, the refractive index of the cladding = 1.490, the radius of the outer core = 0.50mm, and the radius of the inner core = 0.25mm. The transmission bandwidth improvement effect under this numerical condition is about 2.5 times when calculated from a calculation based on a comparison with a conventional step index type optical fiber.
[0019]
Here, in each of the above embodiments, the distribution is a forward distribution in which the refractive index decreases from the inside toward the outside, but for example, a distribution in which the refractive index increases from the inside to the outside, or other distributions may be used. Is possible. Such a thing often shows a higher transmission band improvement effect than the forward distribution type, but the transmission loss may be slightly increased depending on the form.
[0020]
【The invention's effect】
As described above, according to the present invention, the optical fiber can be manufactured under substantially the same conditions as a conventional step index optical fiber, and the transmission band can be greatly widened as compared with the conventional step index optical fiber. The optical fiber which can be provided can be provided, and it can contribute greatly to the function improvement of the communication system using an optical fiber.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an optical fiber according to a first embodiment.
FIG. 2 is a cross-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 ... Cladding

Claims (2)

Light in which a core is formed by an outer core having a circular cross-sectional shape provided in contact with the clad and giving a refractive index larger than that of the clad, and an inner core provided in the outer core and given a refractive index different from that of the outer core A fiber,
The cross-sectional shape of the inner core is circular, and the inner core is provided non-concentrically with respect to the outer core, and multimode transmission is performed between the outer core and the inner core, causing a random speed change in the skew ray. an optical fiber which is characterized in that so as to. The inner core is formed of a first inner core having a circular cross-sectional shape and a second inner core having the same circular cross-sectional shape and provided in the first inner core, and the second inner core and the second inner core The optical fiber according to claim 1, wherein both of the first inner cores are provided non-concentrically with respect to the outer core, and the second inner core is provided non-concentrically with respect to the first inner core.

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