JPH10206608A - Refractive index distributed lens and optical fiber connector using this - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide refractive index distributed cylindrical lenses which can improve a transmission band to transmit a light signal using multimode step index type (SI type) optical fibers and are used to increase a transmittable information quantity by providing the specific refractive index distribution. SOLUTION: An optical fiber connector 23 is arranged in a coaxial shape between SI type optical fibers 21 and 22. The connector 23 is composed of two refractive index distributed cylindrical lenses 3 and 4 and a single refractive index distributed cylindrical lens 5 arranged between these. In both the cylindrical lenses 3 to 5, a refractive index is a function of a distance (r) from the central axis. The refractive index distribution of the cylindrical lenses 3 and 4 takes a shape of a monotone reduction function by which the refractive index is highest in a position of the central axis and the refractive index gradually becomes small as a radius becomes large. The refractive index distribution of the cylindrical lens 5 becomes a convex curve besides taking a maximum value in a specific value (rm ).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal transmission technique using an optical fiber, and more particularly, to a multimode step index type optical fiber (hereinafter simply referred to as "SI type optical fiber"). The present invention relates to a mode switching optical fiber connector used to increase the amount of information that can be transmitted in the used optical signal transmission.

[0002]

2. Description of the Related Art SI type optical fibers are widely used as a medium for transmitting optical signals. There are many modes in light propagating through an SI type optical fiber. Therefore, in the optical signal transmission using the SI type optical fiber, the light of the higher order mode having a large incident angle and the output angle with respect to the end face of the optical fiber is more light than the light of the low order mode having the small incident angle and the output angle with the end face of the optical fiber. Since the propagation speed in the longitudinal direction of the fiber becomes slow, there is a mode dispersion phenomenon that the propagation speed in the longitudinal direction of the optical fiber differs depending on the mode. As a result, the pulse shape of the signal pulse incident from the incident end is deteriorated at the output end by combining the low-order mode light and the high-order mode light. For this reason, there is a problem that a signal of a particularly high frequency cannot be transmitted.

[0003] The frequency range of a signal that can be transmitted is called a transmission band, and is generally indicated by an upper limit frequency value at which transmission is possible. The fact that the transmission band is wide means that the amount of information that can be transmitted in a given time can be increased.

In the optical signal transmission using the SI type optical fiber, as described above, the transmission band is limited due to the signal degradation due to the mode dispersion. Low at 10 MHz.

Accordingly, an object of the present invention is to provide an optical element used to greatly improve the transmission band of optical signal transmission using an SI type optical fiber and increase the amount of information that can be transmitted. Things.

[0006]

According to the present invention, a refractive index distribution n (r) [0 ≦ r ≦ r expressed as a function of a distance r from a central axis is achieved to achieve the above object. ₀ ], and the refractive index distribution n (r) is such that r exceeds 0 and r
Is a convex function above having maximum values at a particular value r _m is less than _0, the refractive index distribution type cylindrical lens, is provided.

In one aspect of the invention, r _m = r ₀ /
2.

A preferred embodiment of the refractive index distribution n (r) is, for example, in _{r m = r 0/2 n} (r m -r) = n (r m +
a convex function on satisfying r), in particular can be approximated by such n (r) = a _{r m -a (r-r m} ) 2 wherein in the case where the refractive index at r _m and n _m Function.

According to the present invention, a first optical fiber on the light incident side and a light emitting side on the light incident side when transmitting an optical signal using a step index type optical fiber are provided. An optical fiber connector interposed between the optical fiber connector and the second optical fiber, wherein the optical fiber connector has an optical axis common to the output end of the first optical fiber and the input end of the second optical fiber. The refractive index distribution is a function of the distance R from the center axis, and is a smooth monotonous convex on the first end face and the second end face of the refractive index distribution type cylindrical lens 5. A first gradient index cylindrical lens 3 and a second gradient index cylindrical lens 4 each represented by a decreasing function N (R) are provided, and the first gradient index cylindrical lens 3 is Output from the output end of the first optical fiber Having the function of guiding the incident light to the first end face of the refractive index distribution type cylindrical lens 5, and the second refractive index distribution type cylindrical lens 4 is emitted from the second end face of the refractive index distribution type cylindrical lens 5. Has a function of converging and guiding the light to be incident on the incident end of the second optical fiber, and the higher-order mode light emitted from the first optical fiber with a relatively large inclination with respect to the optical axis is the Low-order mode light is incident on the second optical fiber with a relatively small inclination with respect to the optical axis as low-order mode light, and is emitted from the first optical fiber with a relatively small inclination with respect to the optical axis. An optical fiber having a mode switching function, wherein the mode light is incident on the second optical fiber as higher-order mode light with a relatively large inclination with respect to the optical axis. Connector, but It is subjected.

According to the present invention, in order to achieve the above object, at least one convex lens having a similar function in the optical fiber connector in place of the first gradient index cylindrical lens 3 is provided. And place
An optical fiber connector having a mode switching function, in which at least one convex lens having a similar function is arranged instead of the second gradient index cylindrical lens 4.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

First, an optical signal transmission method using an optical fiber connector having a mode switching function according to the present invention will be described. FIG. 1 is a schematic sectional view showing one embodiment of this optical signal transmission method, and FIG. 2 is a schematic view showing the entire transmission line.

In the following, for convenience of explanation, light near the optical axis is representatively illustrated and described as low-order mode light, and light near the outer peripheral portion (peripheral portion) farthest from the optical axis is described as high-order mode light. A representative illustration will be given. However, the low-order mode light and the high-order mode light referred to in the present invention are, of course, not limited to these illustrated ones.

1 and 2, reference numeral 21 denotes a first SI.
Reference numeral 22 denotes a second SI type optical fiber, and reference numeral 23 denotes an optical fiber connector for connecting these two optical fibers 21 and 22. The optical fiber 21 includes a core 21a and a clad 21b, and the optical fiber 22 includes a core 22a and a clad 22b.

As shown in FIG. 2, an optical signal is input from one end of the optical fiber 21 (IN), is output from the other end of the optical fiber 21, and is output from one end of the optical fiber connector 23. Optical fiber connector 2
The light exits from the other end of the optical fiber 22, enters one end of the optical fiber 22, and exits from the other end of the optical fiber 22 (OUT).

This will be described in more detail with reference to FIG. The low-order mode light (light having a small incident angle) A and the high-order mode light (light having a large incident angle) B simultaneously incident on one end face of the optical fiber 21 propagate through the optical fiber 21, respectively. The light exits from the other end face of the optical fiber 21 as A ′ and B ′. At the time of this emission, the higher-order mode light B ′ has a time delay Δt with respect to the lower-order mode light A ′. These lights A ′ and B ′ enter one end face of the optical fiber 22 via the optical fiber connector 23.

In this embodiment, the optical fiber connector 23
At the time of incidence, the light A ', which was in the low-order mode at the time of incidence, is converted into high-order mode light and emitted, and the light B', which was at the high-order mode at the time of incidence, is converted into low-order mode light and emitted. Thus, the mode is switched in the optical fiber connector 23, and the light A ′ enters the one end face of the optical fiber 22 as the higher-order mode and the light B ′ enters the lower-order mode.

Accordingly, the low-order mode light B ′ and the high-order mode light A ′ that have entered the one end face of the optical fiber 22 with a time shift Δt propagate through the optical fiber 22 and the other end of the optical fiber 22. When the light is emitted from the end face as B ″ and A ″, the time shift Δt at the time of incidence on the optical fiber 22 is eliminated, and the low-order mode light B ″ and the high-order mode light A ″ are emitted almost simultaneously. This eliminates mode dispersion,
The transmission bandwidth is improved.

In principle, the length of the optical fiber 21 and the length of the optical fiber 22 should be equal. In this case, the effect of improving the transmission band is maximized. Even if the length is slightly different, a sufficient effect is recognized. For example, even if the length of the optical fiber 21 differs from the length of the optical fiber 22 by about 20%, the decrease in the transmission band is slight. Further, even if the difference between the length of the optical fiber 21 and the length of the optical fiber 22 is increased to about 50%, a transmission band about twice as large as when the transmission method of the present invention is not used can be obtained, and a clear effect is obtained. Is recognized.

FIG. 3 is a schematic sectional view showing a first embodiment of the optical fiber connector 23 of the present invention used in the above-described optical transmission method, and FIG. 4 is a schematic perspective view thereof.

In this embodiment, the exit end of the SI optical fiber 21 and the entrance end of the SI optical fiber 22 are coaxially opposed to each other. Between these, the optical fiber connector 23 is coaxially arranged. The optical fiber connector 23 includes two first type gradient index cylindrical lenses 3,
4 and a second type of graded index cylindrical lens 5 disposed between them.

FIGS. 5 and 6 are schematic diagrams for explaining the characteristics and functions of the cylindrical lenses 3, 4, and 5 described above. As shown in FIG. 5, cylindrical lenses 3, 4, 5
Are the functions of the refractive indices r and R from the central axis Z. Assuming that the refractive index distributions of the cylindrical lenses 3 and 4 are N (R) and the refractive index distribution of the cylindrical lens 5 is n (r), these refractive index distributions are as shown in FIG. That is, examples of the cylindrical lenses 3 and 4 include, for example, those used in an array as an equal-magnification imaging element. The refractive index distribution N (R) is determined by the position of the central axis Z (arranged so as to match the optical axes of the optical fibers 21 and 22) (that is, R =
0) has the highest refractive index, and takes the form of a monotonically decreasing function in which the refractive index gradually decreases as the radius R increases.
This curve is approximated by, for example, a quadratic curve or a quartic curve. In contrast, the refractive index of the cylindrical lens 5 according to the present invention the distribution n (r), as shown in solid line in FIG. 6, certain values r _m
The curve becomes an upwardly convex curve having a maximum value at. For the whole cross section of the cylindrical lens 5 to function effectively, it is desirable to equalize the r _m to r _0/2. r _m is r _0/2 smaller than the radius no longer utilized outer circumference exceeding 2r _m, r _m
There r _0/2 greater than a radius (2r _m -r ₀₎ no longer less than the peripheral portion is utilized, resulting in waste.

As shown in FIG.
The light emitted from the emission end of the lens passes through the cylindrical lens 3 and enters one end surface of the cylindrical lens 5. At that time, the optical fiber 2
1 is converged to a position corresponding to the exit angle, the higher-order mode light enters the periphery of the cylindrical lens 5 as shown by the broken line, while the lower-order mode light As shown by the solid line, the light enters the center of the cylindrical lens 5. By appropriately setting the length, the refractive index, and the distribution of the cylindrical lens 5, the higher-order mode light that has entered the peripheral portion of the one end surface of the cylindrical lens 5 can be emitted from the central portion of the other end surface, The low-order mode light that has entered the center of one end of the lens 5 can be emitted from the periphery of the other end. These outgoing lights enter the cylindrical lens 4, are converted into light having an angle corresponding to the incident position, and then enter the incident end of the optical fiber 22. That is, the cylindrical lens 4
Is to make the light from the outer peripheral portion of the output end face of the cylindrical lens 5 incident on the second optical fiber 22 as higher-order mode light, and to transmit the light from the central portion of the output end face of the cylindrical lens 5 to the second optical fiber 22. At a low-order mode.

Thus, when the higher-order mode light that has entered the peripheral portion of the cylindrical lens 5 from the optical fiber 21 through the cylindrical lens 3 enters the optical fiber 22 through the cylindrical lens 4, Since the angle between them is small, the light is introduced into the optical fiber 22 as low-order mode light, while
From the optical fiber 21 through the cylindrical lens 3 to the cylindrical lens 5
When entering the optical fiber 22 through the cylindrical lens 4, the low-order mode light that has entered the center of the optical fiber has a large angle with the optical axis, so that it enters the optical fiber 22 as higher-order mode light. Is introduced. Thus, the low-order mode light and the high-order mode light are converted (replaced) by the optical fiber connector 23.

As a method for producing the cylindrical lenses 3 and 4, for example, three or more uncured mixtures composed of an organic polymer and a monomer are concentrically multilayered and spun from the center to the outer periphery. Forming a fiber-like uncured product laminate whose refractive index decreases gradually toward the other, while interdiffusing components between adjacent layers so that the refractive index distribution between each layer of the laminate changes continuously or After mutual diffusion, the laminate is cured to obtain a refractive index distribution type optical fiber (see JP-A-3-174105), and the optical fiber is cut into appropriate lengths. Further, the refractive index distribution can be adjusted by stretching the fiber to reduce the diameter.

Similarly, the cylindrical lens 5 of the present invention can be manufactured. In this case, at the time of multilayer spinning, a fiber-like uncured product laminate is formed in which the refractive index is sequentially increased from the central portion to the outer peripheral portion, and then the refractive index is sequentially decreased. The laminate may be cured while inter-diffusion of components between adjacent layers is performed or after inter-diffusion is performed so that the refractive index distribution between the layers of the laminate continuously changes.

FIG. 7 shows a second embodiment of the optical fiber connector according to the present invention.
FIG. 8 is a schematic cross-sectional view showing the embodiment, and FIG. 8 is a schematic perspective view thereof. In these drawings, members having the same functions as those in FIGS. 1 to 6 are denoted by the same reference numerals.

In this embodiment, the optical fiber connector 23
Is composed of a cylindrical lens 5, two convex lenses 9, 10 arranged on the light incident side to the cylindrical lens 5, and two convex lenses 11, 12 arranged on the light emitting side of the cylindrical lens 5.
The convex lens 9 is attached to the exit end of the optical fiber 21, and the convex lens 11 is attached to the entrance end of the optical fiber 22, and these convex lenses 9 and 11 are equivalent. The convex lens 10 is attached to the entrance end of the cylindrical lens 5, and the convex lens 12 is attached to the exit end of the cylindrical lens 5, and these convex lenses 10 and 12 are equivalent.

In this embodiment, the function of the cylindrical lens 3 in the embodiment of FIGS. 3 and 4 is borne by the convex lenses 9 and 10, and the function of the cylindrical lens 4 in the embodiment of FIGS. , 12. Convex lens 9,1
0 may be integrated, and the convex lenses 11 and 12 may be integrated.

This embodiment has the same operation and effects as those of the embodiment shown in FIGS.

[0031]

As described above, the mode dispersion can be suppressed by interposing the optical fiber connector having the mode switching function using the special gradient index lens according to the present invention in the middle of the optical fiber transmission line. Thus, the transmission band can be improved, and the amount of information that can be transmitted can be increased.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an embodiment of an optical signal transmission method.

FIG. 2 is a schematic diagram showing the entire transmission path of the optical signal transmission method of FIG.

FIG. 3 is a schematic sectional view showing an embodiment of the optical fiber connector of the present invention.

FIG. 4 is a schematic perspective view of the optical fiber connector of FIG.

FIG. 5 is a schematic diagram for explaining characteristics and functions of a gradient index cylindrical lens used in the optical fiber connector of the present invention.

FIG. 6 is a schematic diagram for explaining characteristics and functions of a gradient index cylindrical lens used in the optical fiber connector of the present invention.

FIG. 7 is a schematic sectional view showing an embodiment of the optical fiber connector of the present invention.

FIG. 8 is a schematic perspective view of the optical fiber connector of FIG. 7;

[Explanation of symbols]

 3, 4, 5 Refractive index distribution type cylindrical lens 9, 10, 11, 12 Convex lens 21, 22 Optical fiber 23 Optical fiber connector

Claims (4)

[Claims] 1. A refractive index distribution n (r) [0 ≦ r ≦ r ₀ ] represented by a function of a distance r from a central axis, wherein the refractive index distribution n (r) is larger than 0 and a function of convex above having maximum values at a particular value r _m is less than r _0, the refractive index distribution type cylindrical lens. Wherein a r _m = r _0/2, a refractive index distribution type cylindrical lens according to claim 1. 3. An optical fiber connector interposed between a first optical fiber on the light incident side and a second optical fiber on the light emitting side when transmitting an optical signal using a step index type optical fiber. 3. The optical fiber connector according to claim 1, wherein the optical fiber connector is disposed so as to have a common optical axis with an output end of the first optical fiber and an input end of the second optical fiber. 4. The refractive index distribution is a function of the distance R from the center axis, and is a smooth monotonically decreasing function N (R) convex upward on the first end face and the second end face of the refractive index distribution type cylindrical lens (5). A first gradient index cylindrical lens (3) and a second gradient index cylindrical lens (4)
And the first gradient index cylindrical lens (3) is configured to convert the light emitted from the emission end of the first optical fiber to the first end face of the gradient index cylindrical lens (5). And the second gradient index cylindrical lens (4) converts the light emitted from the second end face of the gradient index cylindrical lens (5) into the second optical fiber. A higher-order mode light emitted from the first optical fiber with a relatively large inclination with respect to the optical axis to the second optical fiber with respect to the optical axis. The low-order mode light which is incident as a low-order mode light with a relatively small inclination and is emitted from the first optical fiber with a relatively small inclination with respect to the optical axis is incident on the second optical fiber. High with relatively large inclination to the optical axis An optical fiber connector having a mode switching function, wherein the optical fiber connector is configured to be incident as next mode light. 4. The method according to claim 3, wherein at least one convex lens having a similar function is disposed in place of the first gradient index cylindrical lens (3), and the second gradient index cylindrical lens ( An optical fiber connector in which at least one convex lens having a similar function is arranged instead of 4).