JP3662670B2 - 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 inscribed in the clad and provided with a circular cross-sectional outer core having a higher refractive index than the clad, and the outer core provided in the outer core. Basically, it is formed with inner cores with different refractive indexes. In addition to this, a plurality of inner cores are provided, and each inner core is provided discretely, that is, separated so as not to overlap each other. Te performs multimode transmission at these outer core and the inner core, and the so that to cause random speed change in the skew rays.
[0008]
In an optical fiber having a discrete composite core structure in which such a core is composed of an outer core and a plurality of discrete inner cores, light beams having different incident conditions on the optical fiber end face are propagated by the following mechanism. The time can be made uniform and the transmission band can be widened. The effect of making the propagation time uniform is that the transmission band can be expanded to about five times that of a conventional step index type optical fiber having the same numerical aperture.
[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 the case of an optical fiber having a discrete composite core structure, since the core has a structure in which a plurality of inner cores are discretely provided inside a circular outer core, most of the skew ray is an outer core. Or either of the inner core, either the light confined in the core having a relatively high refractive index and propagating only through the core, or the light propagating while alternately passing through both the outer core and the inner core These both change the pattern of incidence / reflection at the interface between the outer core and the clad and the interface between the outer core and the inner core in a non-periodic manner, that is, randomly. As a result, the skew ray during propagation has a period of one cycle (one period is a period from one reflection to the next reflection at the interface between the outer core and the clad) in a region having different propagation speeds due to different refractive indexes. ) At random, causing a speed change at random, and an averaging effect due to this random speed change results in an effective uniformization of the propagation time.
[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 discrete composite 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 be basically manufactured by a manufacturing method similar to that generally used in the conventional step index type optical fiber, and thus the core is complex. However, by setting the composite degree of the core within an appropriate range, for example, by setting the number of inner cores to two, it is possible to manufacture under the same conditions as in the conventional step index type optical fiber.
[0012]
The cross-sectional shape of each inner core in the optical fiber having a discrete composite core structure as described above may be circular, but is more preferably non-in the configuration where the refractive index of the inner core is higher than the refractive index of the outer core. A circle. That is, by making the cross-sectional shape of the inner core non-circular, it is possible to eliminate a component having a particularly low propagation speed in the light beam confined in the inner core and propagating it to the outer core. The averaging action due to the change can be made to function more effectively. Various forms are possible for non-circular. However, if a discontinuous bent portion is included in the interface between the outer core and the inner core, transmission loss is increased. Therefore, it is desirable that the outer surface shape of the inner core is composed of only a continuous curved surface. Moreover, it can be said that a continuous curved surface is easier to manufacture if it does not include irregularities. An elliptical shape is most suitable as a non-circular shape that satisfies such conditions. Therefore, each inner core preferably has an elliptical cross-sectional shape.
[0013]
Further, for the optical fiber having the above-described discrete composite core structure, it is more preferable that the refractive index of each inner core is made different. By doing in this way, the frequency | count which passes through the area | region where refractive indexes differ alternately increases, said average effect | action can be heightened, and uniformization of propagation time can be made more effective.
[0014]
Embodiment
One embodiment of the present invention relates to a structure having two inner cores. As shown in FIG. 1, an optical fiber core 1 according to the present embodiment has an outer core 2 provided inscribed in a clad C as a circular cross-sectional shape. The outer core 2 includes first and second inner cores 3 and 4 which are provided so as not to overlap each other. Both the inner cores 3 and 4 are formed in an elliptical cross-sectional shape, and are provided so as to be symmetric with respect to the center of the outer core 2. 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 omission, 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.
[0015]
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. Refractive index of the first inner core = 1.505, Refractive index of the second inner core = 1.500, Refractive index of the outer core = 1.495, Refractive index of the cladding = 1.490, Radius of the outer core = 0.50mm, Short of each inner core Radius = 0.2425mm, long radius of each inner core = 0.33mm. The transmission band improvement effect under these numerical conditions is about a little less than five times when calculated from a calculation based on a comparison with a conventional step index type optical fiber having the same numerical aperture.
[0016]
Here, in the above-described embodiment, the refractive index of the inner core is made larger than the refractive index of the outer core, but this can be reversed. This type has a slightly higher transmission band improvement effect than the type of the above embodiment, but tends to increase transmission loss.
[0017]
【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.
[Explanation of symbols]
1 ... Core 2 ... Outer core 3 ... First inner core 4 ... Second inner core C ... Cladding

Claims (3)

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,
A plurality of the inner cores are provided, and the inner cores are separately provided so as not to overlap each other , and multimode transmission is performed between the outer cores and the inner cores. An optical fiber characterized in that it can cause various speed changes . The optical fiber according to claim 1, wherein the cross-sectional shape of each inner core is an ellipse. The optical fiber according to claim 1 or 2, wherein each inner core has a different refractive index.

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