JP2001166172A - Optical fiber, optical signal receiver and optical signal transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-speed and high-speed interchangeable optical fiber which allows the transmission of both of a low-speed signal and a high-speed signal by using the same one optical transmitter. SOLUTION: The optical fiber 2 includes a first core 4, first clad 8, third clad 12, second core 6 and second clad 10 successively from the central part toward the outer peripheral part. The optical fiber has the two of the first core 4 and the second core 6 as an optical transmission part. The first core 4 has a GI type refractive index and the second core 6 an SI type refractive index. When the refractive indices of the center of the first core 4 and the first clad 8, the second core 6, the second clad 10 and the third clad 12 are respectively defined as n1, n2, n3 and n4, respectively the relations n1>n2>n4 and n3>n4 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber, an optical receiver for detecting an optical signal transmitted through the optical fiber,
And an optical transmission device for inputting and outputting an optical signal to and from an optical fiber.

[0002]

2. Description of the Related Art As a plastic optical fiber for transmitting an optical signal, a step index type plastic optical fiber and a graded index type plastic optical fiber are known.

[0003] Japanese Patent Application Laid-Open No. Hei 9-101423 discloses a step index type plastic optical fiber (hereinafter referred to as an SI type plastic optical fiber) in which two layers of cladding having different refractive indexes are laminated on a core in order to reduce bending loss. Is described. However, the transmission band of such a two-layer clad SI optical fiber is the same as that of a normal single-layer clad SI optical fiber and is narrow. For example, when transmitting 50 m, the transmission speed is 250 Mbps or less.

Japanese Patent Application Laid-Open No. H11-95045 discloses that a plurality of plastic materials having different refractive indexes are concentrically laminated so that the refractive index at the center is the largest and the refractive index gradually decreases toward the outside. A multilayer optical fiber is described. This multilayer optical fiber is designed to obtain a transmission band substantially equal to that of a graded index plastic optical fiber (hereinafter referred to as a GI plastic optical fiber) whose refractive index continuously decreases from the center. Called layer plastic optical fiber. With such a multilayer optical fiber, 50
With a transmission distance of m, high-speed transmission of 500 Mbps or more can be performed.

However, in order to perform such high-speed transmission, it is necessary to use a photodetector having a light receiving element having a small light receiving area in order to increase the response speed. Therefore, in order to adapt to this small light receiving area, the core diameter of the multilayer type or GI type optical fiber must be reduced. More specifically, the SI for low-speed transmission described above is used.
In general, the type plastic optical fiber has a core diameter of 980 μm and a numerical aperture of 0.3, whereas the multilayer or GI type optical fiber has a core diameter of 5 μm.
Generally, those having a numerical aperture of about 0.15 to 0.2 and a numerical aperture of about 0.005 to 750 μm. When the refractive index of the _core is n _core and the refractive index of the cladding is n _clad , the numerical aperture NA is (n ²
_core - n ² _clad) is represented by ^1/2, when the maximum value of the light load angle of the optical fiber at the transmission side and the alpha, NA = s
There is a relationship of inα.

[0006]

As described above, there is a difference in the core diameter and the numerical aperture between the low-speed SI plastic optical fiber and the high-speed GI plastic optical fiber. For a low-speed optical fiber and a high-speed optical fiber having different core diameters and numerical apertures, it is necessary to use a low-speed optical transmission device and a high-speed optical transmission device having different light receiving element areas and the like.

Japanese Patent Application Laid-Open No. Hei 10-239566 describes an optical transmission device that receives light within a predetermined angle range from an output optical axis, converts the light into an electric signal, and shapes the waveform. The conventional optical transmission device cannot adapt to both the high-speed transmission and the low-speed transmission.
If the I-type plastic optical fiber and the high-speed GI-type plastic optical fiber are used in common, there is a problem that the signal light amount is reduced and the maximum transmission distance is significantly reduced.

It is an object of the present invention to provide a low-speed / high-speed compatible optical fiber which can transmit both a low-speed signal and a high-speed signal using one and the same optical transmission device. Another object of the present invention is to provide an optical receiver suitable for separating and receiving both a low-speed signal and a high-speed signal from the low-speed / high-speed compatible optical fiber.

[0009]

The above object is achieved by the following means. That is, an optical fiber comprising: a first core having a first refractive index; and a cylindrical second core having a second refractive index and surrounding the first core. is there.

[0010] According to the above configuration, since the first core is located at the center of the optical fiber, the angle of incidence of light on the optical fiber on the transmission side is small, so that the transmission band is high and a high-speed optical signal can be transmitted. . This optical fiber can transmit one or both of a high-speed optical signal and a low-speed optical signal, and it is not necessary to separately lay optical fibers for low-speed and high-speed as in the related art.

The first refractive index of the first core has a value decreasing from the center toward the periphery, and the second refractive index of the second core is
Is larger than the minimum value of the first refractive index of the first core. By providing the first core with a cross-sectional distribution of the refractive index, the transmission band in the first core can be increased. In addition, since the refractive index of the first core and the refractive index of the second core are different from each other at the interface, light traveling from one core to the other core is reflected, and crosstalk can be prevented.

A first clad is formed between the first core and the second core, a second clad is formed on an outer surface of the second core, and a second refractive index of the second core is formed. Preferably, the difference between the refractive index of the first cladding and the refractive index of the second cladding is larger than the difference between the minimum value of the first refractive index of the first core and the refractive index of the first cladding. By doing so, the light emitted from each core to the outside is reflected by the cladding, so that the light can be confined in each core. Light that has passed through the first cladding from the first core can also be reflected by the second cladding and returned to the first core.

The object of the present invention can also be solved by the following means. That is, a plurality of first cores each covered with a first clad, and a plurality of second cores each covered with a second clad are provided.
Are disposed in a central region, the plurality of second cores are disposed in a peripheral region, and a gap between the cores in the central region and the peripheral region is filled with a third clad. Is smaller than the refractive indexes of the first and second claddings.

In the above-mentioned optical fiber, since the plurality of first cores are arranged in the central region, the angle of incidence of light from the light source on each of the first cores is small, so that the transmission band is high and the high-speed signal is high. Can be transmitted on multiple channels. Further, the plurality of second cores arranged in the peripheral area can transmit a low-speed signal through a plurality of channels.

The following means can be used in place of the above means. That is, a plurality of first cores each covered with a first clad, and a plurality of second cores each covered with a second core are provided, and the plurality of first cores are arranged in a central region. And disposing the plurality of second cores in a peripheral region;
The gap between the cores in the central region is filled with a third clad, and the gap between the cores in the peripheral region is filled with a fourth clad, and the refractive index of the fourth clad is the third clad. An optical fiber having a smaller refractive index.

In the optical fiber, since the incident angle is large, the light transmitted through the first core and traveling from the central region to the peripheral region can be reflected at the interface between the third clad and the fourth clad. This makes it possible to reduce the distance between the light source and the fiber on the transmission side.

Preferably, the diameter of the first core is 330 to 820 μm, and the diameter of the second core is 930 to 1030 μm. By doing so, it is possible to ensure dimensional compatibility with the conventional GI plastic optical fiber and the conventional SI plastic optical fiber.

Preferably, the numerical aperture of the first core is 0.1 to 0.25, and the numerical aperture of the second core is 0.2 to 0.3. With such a value, the conventional GI
Compatibility with respect to the numerical aperture of the type plastic optical fiber and the conventional SI type plastic optical fiber can be secured.

Another object of the present invention is solved by the following means. That is, the first light emitted from the center of the end of the optical fiber
And a second optical signal, which separates and guides the second optical signal emitted from the annular periphery of the end of the optical fiber to the first and second photodetectors, respectively. The light receiving portion of the detector is annular, and the first photodetector is disposed inside the annular light receiving portion of the second photodetector.

By using the above-mentioned optical receiver, a high-speed optical signal and a low-speed optical signal, which are spatially separated and propagated through the optical fiber of the present invention, can be separated and received with a simple configuration.

[0021] Amplifying means connected to the output of the first photodetector, and switching means connected to the output of the second photodetector, wherein the switching means is such that the second photodetector is significant. Preferably, the output of the second photodetector is connected to the amplifying means when a proper optical signal is detected, and the sum of the first optical signal and the second optical signal is amplified. By doing so, the first
Noise can be suppressed from being mixed into the signal of the photodetector.

The light separating means has a first optical waveguide and a second optical waveguide, the second optical waveguide is cylindrical, and the first optical waveguide is formed of the second cylindrical optical waveguide. It preferably extends coaxially within the optical waveguide. According to this structure, the responsiveness of the photodetector can be improved by reducing the diameter of the exit side of the first and second optical waveguides to narrow the light and reducing the area of the light receiving element.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a first embodiment of the optical fiber of the present invention will be described. FIG. 1 is a diagram showing a cross-sectional view and a refractive index cross-sectional distribution of the optical fiber according to the first embodiment. The optical fiber 2 has a first core 4, a first clad 8, a third clad 12, a second
It is configured to include the core 6 and the second clad 10. It has two optical transmission parts, a first core 4 and a second core 6. The first core 4 has a GI-type refractive index distribution, and the second core 6 has an SI-type refractive index. Having. Third clad 12
Has a lower refractive index than the first cladding 8, and the second cladding 10 and the third cladding 12 are made of the same material and have the same refractive index. Center of the first core 4, a first clad 8, the second core 6, when the refractive index of the second clad 10 and the third cladding 12 and n _1, n _2, n _3, n _4, respectively, n _{1
> N 2> n 4 n 3} > relation n ₄ is present.

In the optical fiber 2, a transparent plastic is used for the core and the clad. The core and cladding materials are not limited to specific materials, and can be appropriately selected as long as the materials satisfy the above relational expression. For example, acrylic, methacrylic, carbonate-based, amorphous olefin-based, sulfone-based, silicone-based, vinyl-based plastics or fluorine compounds such as poly (4-methylpentene-1), Polytetrafluoroethylene, poly (1,1-dihydroperfluorohexyl acrylate), poly (1,1-dihydroperfluorobutyl acrylate), polychlorotrifluoroethylene, polytrifluoroisopropyl methacrylate, polytriethoxysilicole methacrylate, Polybutyl acetate, polyethyl acrylate,
Polyvinyl acetate, polyvinyl butyral, polymethyl acrylate, polyisopropyl methacrylate, polyisobutyl methacrylate, polymonofluorothiel methacrylate, poly (n-hexyl methacrylate), polyethyl methacrylate, poly (n-butyl methacrylate, poly (β-ethoxy methacrylate) ), Poly (n-propyl methacrylate), poly (3,3,5-trimethylcyclohexyl methacrylate), polymethyl methacrylate, poly (2-nitro-
2-methylpropyl methacrylate), polytriethylcarbyl methacrylate, poly (α-methylaryl methacrylate), poly (3-methylcyclohexyl methacrylate), poly (4-methylcyclohexyl methacrylate), polyisobutylene, polyboronyl methacrylate, polycyclohexyl Methacrylate, poly (β-
(Methacrylyl methacrylate), polytetrahydrofurfuryl methacrylate, poly (1-methylcyclohexyl methacrylate), polyethylene glycol monomethacrylate, polyvinyl chloroacetate, polyvinyl methacrylate, polyethylene chlorohydrin methacrylate, polymethyl-α-chloroacrylate, poly (2-chloro) Cyclohexyl methacrylate), polyallyl methacrylate, polyacrylonitrile, polymethacrylonitrile, polycyclohexyl-cyclohexyl methacrylate, poly (1,3-dichloropropyl 1-2-methacrylate), polycyclohexyl-α-chloroacrylate, poly (β-chloro Ethyl chloroacrylate), polybutyl mercaptyl methacrylate, sec
-Butyl α-bromoacrylate, cyclohexyl α-
Bromoacrylate, poly (β-bromoethylmethacrylate), polyethylsulfide methacrylate, polycyclohexylbromoacrylate, poly (α-phenylethylmethacrylate), poly (p-methoxybenzylmethacrylate), polyvinylfuran, poly (p-isopropylstyrene) ), Polyethylene glycol benzoate methacrylate (α-phenylallyl methacrylate), poly (p-cyclohexylphenyl methacrylate), poly (β-phenylethyl methacrylate),
Poly-α- (o-chlorophenylethyl methacrylate), poly (1-phenylcyclohexyl methacrylate), polymethyl α-bromoacrylate, polybenzyl methacrylate, poly (β-phenylsulfonethyl methacrylate), poly (m-cresyl methacrylate), Acrylonitrile-styrene copolymer, polydiallyl cinnamate, poly (o-methoxyphenyl methacrylate), polyphenyl methacrylate, poly (2,3
-Dibromopropyl methacrylate, poly (2-β-diphenylethyl methacrylate), poly (o-chlorobenzyl methacrylate), poly (m-nitrobenzyl methacrylate), polycarbonate, polystyrene, poly (o-methoxystyrene), polycinnamyl methacrylate , Polybenzylidyl methacrylate, poly (p-
Bromophenyl methacrylate), poly (p-methoxystyrene), poly (o-chlorobenzylidyl methacrylate), 3,3 ′, 5,5′-tetrachlorodiphenyl-phosgene copolymer, polypentachlorophenyl methacrylate, poly (o -Chlorostyrene), polyphenyl α-bromoacrylate, poly (p-divinylbenzene), polydichlorostyrene, poly (2,6-dichlorostyrene), polyvinylphenyl sulfide, and the like.

Further, methyl methacrylate, benzyl methacrylate, phenoxyethyl methacrylate, vinyl benzoate, phenyl methacrylate, styrene,
Copolymers of monomers such as alkyl fluoride (meth) acrylates, alkyl fluoride-α-fluoroacrylates, and pentafluorophenyl methacrylate can also be used. When a copolymer is used, the refractive index can be continuously changed depending on its composition. For example, a polymer having a refractive index of 1.37 to 1.59 can be obtained by using the above-mentioned monomer.

The formation of the GI-type plastic optical fiber of the first core 4 is not limited to any particular method, but may be performed by a known technique, for example, a method in which two types of monomers having different refractive indices are homopolymerized. It can be formed by an interfacial gel method of polymerizing and stretching inside, or a method of heat-treating after preparing a multilayer fiber using a plurality of polymer mixtures having different refractive indexes. Alternatively, it may be constituted by a multilayer plastic optical fiber described in the related art.

The numerical aperture NA of the optical fiber is as follows, where n _{core is} the refractive index of the _core and n _clad is the refractive index of the cladding.

[0028]

(Equation 1)

## EQU1 ## Further, there is a relation of NA = sin α between the numerical aperture NA and the maximum light taking-in angle α of the optical fiber.

As a conventional low-speed SI plastic optical fiber, for example, an ATM-LAN having a core diameter of 930 to 1030 μm and a numerical aperture of 0.28 to 0.32 is used. To ensure compatibility with the conventional SI plastic optical fiber, the core diameter of the second core 6 is set to 930 to 1030 μm and the numerical aperture is set to 0.28 to
Desirably, it is 0.32.

The numerical aperture of the first core 4 having a GI-type refractive index distribution is desirably 0.10 to 0.25. Conventionally, a GI plastic optical fiber having a numerical aperture of about 0.15 has been used. This is because if the numerical aperture is less than 0.1, the amount of light incident on the optical fiber is less than half that of the conventional GI plastic optical fiber and the transmission distance is short, and the numerical aperture is 0.25.
This is because it is difficult to precisely control the refractive index distribution and the transmission band is reduced. Therefore, when the diameter of the second core 6 is 980 μm, which is the center value of the conventional SI plastic optical fiber, and the numerical aperture of the light incident on the second core 6 is 0.3, the incident light on the first core 4 is The diameter of the first core 4 is set to 330 μ
m and the numerical aperture of the light incident on the first core 4 is 0.25
In order to reduce the diameter to below, the diameter of the first core 4 needs to be 820 μm or less. From the above, the diameter of the first core 4 is 33
Desirably, it is in the range of 0 μm to 820 μm.

In the above embodiment, the light incident on the GI-type first core 4 is mainly confined in the fiber by the refractive index distribution of the first core 4.
Light transmitted through the cladding 8 can also be confined by being totally reflected at the interface between the first cladding 8 and the third cladding 12. As shown in FIG. 2, when an angle between the light source 18 and the peripheral portion of the first clad 8 is θ,

[0033]

(Equation 2)

If the refractive indices n ₂ and n ₄ are selected so as to satisfy, all the light incident on the first core 4 is confined in the first core 4. Therefore, the numerical aperture defined by the refractive index of the first cladding 8 and the third cladding 12 is desirably large, and preferably 0.2 to 0.25. The core diameter of the SI type plastic optical fiber is generally 980 μm,
The numerical aperture of the incident light (the sine of the angle formed between the light source and the peripheral edge of the core) is set in the range of 0.2 to 0.3. If the distance l from the light source 18 to the optical fiber 2 is in the range of 1.56 to 2.4 mm, the numerical aperture of the incident light falls within the above range.

When the distance 1 is 1.56 mm, if the diameter r of the first core is 0.64 mm or more, the numerical aperture of the light incident on the first core 4 becomes 0.2 or more. If the diameter of the core 4 is 0.80 mm or less, the numerical aperture of light incident on the first core 4 is 0.25 or less. Therefore, it is desirable to set the diameter r of the first core 4 in the range of 0.64 to 0.8 mm.

When the numerical aperture of the light incident on the first core 4 is 0.2 and the numerical aperture of the first clad 8 and the third clad 12 is in the range of 0.2 to 0.25, the light source 18 Is closer to the optical fiber 2, the first clad 8 and the third clad 1
The light can be confined by the reflection at the interface with the second.

In the above embodiment, n ₁ = 1.505, n ₂
= 1.492, n ₃ = 1.501, and n ₄ = 1.471. Therefore, the numerical aperture between each layer is as follows.

[0038]

(Equation 3)

In the above embodiment, since the light incident on each core is confined in each core, there is no increase in the optical transmission loss that occurs in a normal multilayer structure, and the optical transmission is almost the same as that of a single-core optical fiber. Loss can be reduced. Further, since the low-speed and high-speed optical signals are confined in the respective cores and transmitted respectively, there is no possibility that the optical signal propagating in the second core 6 is mixed into the first core 4 to degrade the band of the first core 4. . For example, even if light transmitted through the third clad 12 is generated due to scattering caused by bending or a defect of the fiber, the third clad 12 and the second clad 1
Since the refractive index of 0 is the same, this light
And is absorbed by a jacket provided outside the second clad 10 (not shown).

The refractive index of the third cladding 12 is smaller than the refractive index of the first cladding 8. Thus, light having an incident numerical aperture (incident angle) greater than the numerical aperture determined from the refractive index distribution of the first core can be confined at the interface with the first cladding 8. In addition, there is an effect that bending loss in the first core 4 is reduced. As described above, since it is assumed that the light beam passes through the first cladding 8 and is reflected at the interface with the third cladding 12, it is desirable that the transmission loss of the first cladding 8 is small, and the transmission loss of the first core 4 is small. Is preferably twice or less.

A second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a diagram showing a cross section and a cross section distribution of a refractive index of the optical fiber of the second embodiment. Optical fiber 2
Includes a first core 4, a first clad 8, a second core 6, and a second clad 10 from the center toward the outer periphery. The first core 4 has a GI-type refractive index distribution, and the second core 6
It has an I-type refractive index. Center of the first core 4, a first clad 8, the second core 6, the refractive index of the second cladding 10, respectively, when expressed as _{n 1, n 2, n 3} , n 4, n 1> n 2 n 3 > satisfy the relationship of n ₂ n _2> n _4.

The light incident on the first core 4 is mainly
However, the light transmitted through the first clad 8 without being confined in the first core 4 is also confined by the refractive index distribution of the second core 6.
After transmission, the light is totally reflected at the interface between the second core 6 and the second clad 10, so that the light can be confined. That is, the first core 4
To

[0043]

(Equation 4)

The light incident at an incident angle を 満 た す that satisfies the above conditions is transmitted through the first clad 8 and confined at the interface between the second core 6 and the second clad 10 even if the light is not confined in the first core 4 of the GI type. , Second core 6, first clad 8, first core 4
Are sequentially transmitted through the optical fiber 2. Also, the second core 6

[0045]

(Equation 5)

The ray incident at an incident angle を 満 た す that satisfies
The transmission is confined in the second core 6 by the interface between the cladding 8 and the second cladding 10. Furthermore, the second core 6

[0047]

(Equation 6)

The ray incident at an incident angle を 満 た す that satisfies
The light is totally reflected by the interface between the core 6 and the second clad 10, and is transmitted through the second core 6, the first clad 8, and the first core 4.

It is desirable that the light incident on each core be confined in the incident core and transmitted because the increase in transmission loss is small. Therefore, it is desirable that the incident angle on the first core 4 is small, and preferably, the numerical aperture of the incident light beam is 0.1 to
0.2. The numerical aperture at the interface between the first cladding 8 and the second core 6 is preferably large,
It is preferably 0.3. When the distance l from the light source 18 to the optical fiber 2 is 1.56 mm, the first core 4
Is set to 0.64 mm or less, the numerical aperture of the light beam incident on the first core 4 can be set to 0.2 or less. When the distance l from the light source 18 to the optical fiber 2 is set to 2 mm, the numerical aperture of the light ray incident on the first core 4 is set to 0.2 or less by setting the diameter of the first core 4 to 0.82 mm or less. can do. Further, when the distance l from the light source 18 to the optical fiber 2 is 2.4 mm, if the diameter of the first core 4 is 0.48 mm or more, the numerical aperture of the light ray incident on the first core 4 becomes 0.1 or more. Become. From the above, the diameter of the first core 4 is desirably 0.48 to 0.82 mm, preferably 0.48 to 0.64 mm. In order to reduce the angle of incidence on the first core 4, it is preferable that the light source 18 and the optical fiber 4 are provided separately, and it is preferable that the light source 18 and the optical fiber 4 be separated by 2 mm or more. However, as described above, even if light is incident on the first core 4 from the light source at a large incident angle, the light is confined by the interface between the second core 6 and the second clad 10, so that an increase in transmission loss is small. In this case, the light is
Therefore, the loss of the first cladding 8 is desirably small, and is preferably not more than twice the loss of the first core 4.

In the second embodiment, n ₁ = 1.500,
_{n 2 = 1.492, n 3 =} 1.509, n 4 = 1.479
And the numerical aperture between the layers is as follows.

[0051]

(Equation 7)

In the second embodiment, the number of cladding layers is smaller and the configuration of the optical fiber 2 is simpler than in the first embodiment, and the production of the optical fiber 2 is easy. Further, when the light source 18 can be installed at a distance from the optical fiber 2, the light incident on each core can be confined in each incident core, so that the transmission loss of the optical fiber 2 is almost the same as that of a single-core optical fiber. Since the incident light is confined in each core and transmitted, the first core 4
This prevents the optical signal of the second core 6 from being mixed into the first core 4 and deteriorating the transmission band of the first core 4.

A third embodiment of the present invention will be described with reference to FIG. FIG. 4 is a diagram illustrating a cross section and a cross section distribution of a refractive index of the optical fiber according to the third embodiment. Optical fiber 2
Includes a first core 4, a second core 6, a second clad 10, and a third clad 12 from the center toward the outer periphery. First
The core 4 has a GI type refractive index distribution, and the second core 6
Has a refractive index of SI type. The center of the first core 4, the second
Core 6, a second cladding 10, the refractive index of the third cladding, respectively, when expressed as _{n 1, n 2, n 3} , n 4, satisfy the relationship of _{n 1> n 2> n 3} > n 4.

In the third embodiment, the light incident on the first core 4 is mainly confined in the optical fiber 2 by the refractive index distribution of the first core 4, but is not confined in the GI type first core 4. The light incident on the second core 6 also passes through the second core 6 and is totally reflected at the interface between the second core 6 and the second clad 10,
Can be confined. That is, the incident angle ψ

[0055]

(Equation 8)

Even if the incident light is not confined in the GI-type first core 4, it is totally reflected at the interface between the second core 6 and the second clad 10, passes through the second core 6, and passes through the first core 4. Transmitted within. Also, the light incident on the second core 6 is totally reflected by the second core 6 and the second clad 10, passes through the second core 6, and is transmitted in the first core 4.

In the third embodiment, n ₁ = 1.5
05, n ₂ = 1.492, n ₃ = 1.462, n ₄ = 1.
406, and the numerical aperture between the layers is as follows.

[0058]

(Equation 9)

In the third embodiment, the optical fiber 2
Is bent, and even if light leaks from the second clad, the proportion of light reflected at the interface between the second clad 10 and the third clad 12 is large, and light can be returned to the second core 6 again. As described above, the third cladding 12 is provided to reduce bending loss, and may be provided in the optical fibers of the first and second embodiments in the same manner. The configuration of the optical fiber according to the third embodiment is further simplified as compared with the second embodiment, and the fabrication of the optical fiber is easy.

In each of the embodiments described above, the core having the SI type refractive index is provided. However, the core may have a multilayer structure. In this case, the numerical aperture increases toward the outside. Is desirable.

A fourth embodiment of the present invention will be described with reference to FIG. FIG. 5 is a diagram showing a cross section and a refractive index cross section distribution of the optical fiber of the fourth embodiment. The optical fiber 2 according to the fourth embodiment is a multi-core fiber in which a plurality of cores having an SI type refractive index are bundled. The optical fiber 2
Has a plurality of optical fiber cores 14 having a low numerical aperture on the inner side and a plurality of optical fiber cores 16 having a high numerical aperture on the outer side. The first cladding 8 is formed on each core. Further, the outside of the first cladding 8 of each core is filled with the third cladding 12. The core and the clad are made of a transparent plastic. Low numerical aperture optical fiber core 14, high numerical aperture optical fiber core 16, first cladding 8, and third cladding 1
When the refractive indices of 2 are represented as n ₁ , n ₂ , n ₃ , and n ₄ , respectively, the relationship of n ₂ > n ₁ > n ₃ > n ₄ is satisfied.

If the numerical aperture is reduced, the transmission band can be increased even for the SI type fiber, so that high-speed communication can be performed using the low numerical aperture optical fiber core 14 arranged at the center. If the diameter of each core is reduced, bending loss can be suppressed even if the numerical aperture is reduced. In addition, by arranging the optical fiber core 16 having a high numerical aperture on the periphery having a large incident numerical aperture, a light ray having a large incident angle can be taken in, and a large amount of light can be taken in the optical fiber 2.

In the fourth embodiment, for example, 500
It is desirable that the numerical aperture of the optical fiber core 14 be 0.2 or less in order to enable 50 m transmission at Mbps or more, while the numerical aperture is 0.1 in order to reduce bending loss.
It is desirable to make the above. The optical fiber core 16
Is preferably 0.2 to 0.35.
The diameter of the region constituted by the optical fiber core 14 is 490 to 490.
830 μm is desirable. When the diameter of this region is 700 μm, the numerical aperture of the incident light becomes 0.14 or more when the distance to the light source 18 is set to 2.4 mm or less, and the numerical aperture of the incident light becomes 1.7 when the distance is 1.71 mm or more. Can be set to 0.2 or less. From the above, it is desirable that the distance from the light source 18 to the optical fiber 2 be 1.71 to 2.4 mm.

Further, in the fourth embodiment, the space between the cores is filled with the third clad 12, and the third clad 12 is made of a material having a small refractive index. The bending loss of the low numerical aperture optical fiber core 14 is small. Since the bending loss decreases as the numerical aperture increases, the numerical aperture determined by the optical fiber core 14 and the third cladding 12 is preferably 0.3 to 0.65.

In the fourth embodiment, n ₁ = 1.4
92, n ₂ = 1.509, n ₃ = 1.479, n ₄ = 1.
424, and the numerical aperture between the layers is as follows.

[0066]

(Equation 10)

In the fourth embodiment, since all the cores have the SI type refractive index, it is not necessary to form a refractive index distribution, and the optical fiber 2 can be easily manufactured. Also, the cladding material is only two types, the first cladding 8 and the third cladding 12, and the number of components of the optical fiber can be reduced. Moreover, the light leaked from each optical core reaches the optical fiber jacket (not shown) without being confined to another core and is absorbed there, so that an optical signal is prevented from being mixed into another core.

A fifth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a diagram illustrating a cross section and a cross section distribution of a refractive index of the optical fiber according to the fifth embodiment. The fifth embodiment is also a multi-core fiber in which a plurality of optical fibers having an SI type refractive index are bundled. The optical fiber 2 has a low numerical aperture optical fiber core 14 at the center and a high numerical aperture optical fiber core 16 at the outer periphery. The optical fiber core 14 has a first
A clad 8 is provided, and the space between the cores is filled with a third clad. The optical fiber core 16 is provided with the second clad 10, and the space therebetween is filled with the fourth clad 13. Each core and cladding are made of transparent plastic, and the same material is used for the optical fiber core 14 and the optical fiber core 16. The optical fiber core 14, the optical fiber core 16, first clad 8, the second cladding 10, the refractive index each of the third cladding 12, and the fourth cladding _{13, n 1, n 2,} n 3, n 4, n 5 Then, the relationship of n ₁ > n ₂ > n ₄ n ₁ > n ₃ > n ₅ is satisfied.

Also in the fifth embodiment, 500 Mbp
In order to enable transmission of 50 m at s or more, it is desirable to set the numerical aperture of the optical fiber core 14 to 0.2 or less,
On the other hand, in order to reduce bending loss, it is desirable that the numerical aperture be 0.1 or more. The numerical aperture of the optical fiber core 16 is desirably 0.2 to 0.35. The diameter of the third cladding 12 is desirably 490 to 830 μm. When the diameter of the third cladding 12 is 700 μm, the numerical aperture is 0.1 if the distance to the light source is 2.4 mm or less.
When the distance is 1.71 mm or more, the numerical aperture of incident light can be 0.2 or less. From the above,
The distance to the light source is desirably 1.71 to 2.4 mm.

In the fifth embodiment, the space between the fibers is filled with the third clad 12 or the fourth clad 13, and the third clad 12 and the fourth clad 1 are filled.
Since a material having a small refractive index is used for 3, the bending loss of the optical fiber core 14 having a low numerical aperture is small. The numerical aperture between the optical fiber core 14 and the third cladding 12 and between the optical fiber core 16 and the third cladding 13 are desirably 0.3 to 0.65 in order to reduce bending loss.

In the fifth embodiment, n ₁ = 1.5
09, n ₂ = 1.492, n ₃ = 1.479, n ₄ = 1.
424, and the n ₅ = 1.424, the numerical aperture between each layer is as follows.

[0072]

[Equation 11]

In the fifth embodiment, the optical fiber core 14
The optical fiber 2 having a small transmission loss can be formed by using the same material having a small loss for the optical fiber core 16 and the optical fiber core 16. Further, even when light having a large numerical aperture enters the optical fiber core 14 having a low numerical aperture, the light is reflected at the interface between the third clad 12 and the fourth clad 13, so that the distance between the light source and the optical fiber can be reduced. it can.

Hereinafter, the conventional optical transmission device (for 500 Mbps and 250 Mbps) is added to the optical fiber of each embodiment of the present invention described above, the conventional SI plastic optical fiber and the conventional GI plastic optical fiber.
) Are compared and the results of signal transmission are compared.

A conventional SI plastic optical fiber having a core diameter of 980 μm, a numerical aperture of 0.3, and a transmission loss of 7.5 dB / 50 m at a wavelength of 650 nm was used. A conventional GI plastic optical fiber having a core diameter of 700 μm, a numerical aperture of 0.2, and a transmission loss of 8 dB / 50 m at a wavelength of 650 nm was used. The 500 Mbps optical transmission device used is for a GI type plastic optical fiber, and the light source has a wavelength of 65 nm.
A semiconductor laser of 0 nm is used, and the numerical aperture of the emitted light beam is 0.1 mm.
It was 15. Also, the optical transmission device for 250 Mbps is
This was for an SI-type plastic optical fiber, and an LED having a center wavelength of 650 nm was used as a light source, and the numerical aperture of emitted light was 0.3. In the above combination, distance 5
An experiment was conducted to determine whether 0 m transmission was possible. Table 1 shows the results.

[0076]

[Table 1]

The same results were obtained for all of the first to fourth embodiments of the present invention. Note that “Embodiment” in Table 1 represents the optical fiber of each embodiment of the present invention described above, and “Comparative Example”
Represents a conventional optical fiber.

As is apparent from Table 1, in the conventional SI optical fiber, the transmission side and the reception side optical transmission devices are 500
When it is for Mbps, signal transmission is not possible,
On the other hand, in the conventional GI type optical fiber, transmission cannot be performed when the optical transmission devices on the transmission side and the reception side are for 250 Mbps. On the other hand, in the optical fibers according to the first to fourth embodiments of the present invention, signal transmission is possible regardless of whether the optical transmission device on the transmission side and the reception side is for 500 Mbps or for 250 Mbps. It can be seen that the low-speed transmission system and the high-speed transmission system are compatible.

According to the present invention, a core having a high bandwidth is arranged at the center and a core having a large numerical aperture is arranged at the periphery.
It is characterized by having both high band characteristics and a large amount of light taken in.
It is not limited to the above embodiment. In the above embodiment, the plastic optical fiber has been described. However, the present invention is not limited to this, and can be similarly applied to a glass fiber or an optical fiber combining glass and plastic.

Next, an embodiment of the optical transmission device of the present invention will be described. FIG. 7 is a diagram showing a longitudinal section of the receiving section of the first embodiment of the optical transmission device of the present invention, and FIG. 8 is a perspective view thereof. In the present embodiment, the optical signal emitted from the optical fiber 2 is separated using the optical waveguide 34. The optical waveguide 34 has a first optical waveguide core 36 for separating and extracting high-speed optical signals, and a second optical waveguide core 38 for separating and extracting mainly low-speed optical signals. An optical waveguide cladding 40 having a gradually increasing thickness is provided between the first optical waveguide core 36 and the second optical waveguide core 38, so that the two cores are in contact with each other on the incident side, but the output cores are in contact with each other. Side away. First optical waveguide core 3 on the incident side of optical waveguide 34
6 and the diameter of the second optical waveguide core 38 are substantially equal to the diameters of the first core 4 and the second core 6 of the corresponding optical fiber 2, respectively. On the output side, the optical signals from the first optical waveguide core 36 and the second optical waveguide core 38
The light is received by the light detector 20 and the light detector 22 and detected.
Since the diameters of the first optical waveguide core 36 and the second optical waveguide core 38 gradually decrease from the incident side to the emission side, the numerical aperture of the optical waveguide 34 is made larger than the numerical aperture of the second core of the optical fiber 2, and the diameter is increased. It is desirable that no light loss occurs even if the size is reduced. For this purpose, the numerical aperture of the optical waveguide 34 is set to 0.
Preferably, the numerical aperture is 3 to 0.65, which is 1.5 times or more larger than the numerical aperture of the optical fiber 2 by the second core. In the above embodiment, since the light from the optical fiber 2 is narrowed down and coupled to the photodetector, a high-speed photodetector having a small light receiving area can be used.

FIG. 9 is a configuration diagram of a photodetector and a detection circuit of the optical transmission device according to the first embodiment of the present invention. The photodetector is composed of a first photodetector 20 for detecting an optical signal propagating in the central portion of the optical fiber 2 and a second photodetector 22 for detecting an optical signal propagating in the peripheral portion of the optical fiber 2. The first photodetector 20 detects a high-speed optical signal, and the second photodetector 22 detects a low-speed optical signal. The signal detected by the first photodetector 20 is amplified by the preamplifier 50, delayed by the delay circuit 54, and amplified by the amplifier 56. When a signal is detected by the second photodetector 22, the signal is amplified by the preamplifier 52, and is switched by the switching circuit 60 so that the signal flows to the amplifier 56.
The signal is amplified together with the signal from the delay circuit 54. Control circuit 5
8 controls the switching circuit 60 and cuts off the connection between the preamplifier 52 and the amplifier 56 when the signal from the second photodetector does not have a level higher than a predetermined value.

As described above, the switching circuit 6 is controlled by the control circuit 58.
By controlling 0, even if an insignificant minute optical signal is incident on the second photodetector 22 during high-speed communication through the central portion of the optical fiber 2, this signal is transmitted to the first optical detector 22. Since it does not mix with the signal from the detector 20, deterioration of the detection signal is prevented. When the high-speed optical signal and the low-speed optical signal are sufficiently separated in the optical fiber and the crosstalk between the photodetectors is sufficiently small, the control circuit 58 and the switching circuit 60 can be omitted.

The pulse light signal propagating through the peripheral low-speed transmission core has a wider pulse width than the pulse light signal propagating through the central high-speed transmission core. Occurs. If the signals are delayed and added by the delay circuit 54 by the delay of the optical signal peak, the transmission band can be made higher than that of the conventional SI plastic optical fiber having the same numerical aperture as the low-speed transmission core. . In this case, it is desirable to control the delay time according to the length of the optical fiber, but if such control is not performed, the delay time that occurs at half the maximum possible fiber length In consideration of the above, it is desirable to estimate the delay time of the delay circuit 54.

In the optical transmission device according to the first embodiment, by providing two photodetectors coaxially, compared with the case where the optical transmission device is constituted by a single photodetector, the individual photodetectors can be used. Since the light receiving area can be reduced, the photodetector cutoff frequency can be increased, and higher speed optical signals can be transmitted and received.

In the above embodiment, the high-speed optical signal and the low-speed optical signal are spatially separated and transmitted in the optical fiber. Therefore, on the output side, a simple and compact optical waveguide or the like as described above is used. These signals can be easily separated by using the optical system described above.

Next, a second embodiment of the optical transmission device of the present invention will be described with reference to FIG. FIG. 10 is a longitudinal sectional view of a light detection optical system of the optical transmission device according to the second embodiment. In the second embodiment, a hologram lens 24 is used to separate and condense optical signals. Optical signals from the first core 4 and the second core 6 are incident on the first photodetector 20 and the second photodetector 22, respectively, by the first hologram lens 26 and the second hologram lens 28 constituting the hologram lens 24. . The second hologram lens 28 condenses the light into a cylindrical shape and condenses the light on the ring-shaped second photodetector 22. Since the light is narrowed down by the hologram lens 24, a high-speed photodetector having a small light receiving diameter can be used as in the first embodiment.

Also in the second embodiment, since the high-speed optical signal and the low-speed optical signal are spatially separated and transmitted in the optical fiber, the hologram lens 24 does not need to be separated from the optical fiber 2 and the optical system is Can be small. In addition, there is a certain degree of freedom in setting the focusing positions of the first hologram lens 26 and the second hologram lens 28, and the second hologram lens 28 can be focused in a cylindrical shape. Diffraction efficiency can be increased by blazing the hologram lens 24.

Next, a third embodiment of the optical transmission device of the present invention will be described. FIG. 11 is a longitudinal sectional view of a light detection optical system of the optical transmission device according to the third embodiment. In the third embodiment, an optical signal is separated by using a separation prism 30 and condensed by a lens 32. The optical signal from the first core 4 passes through the separation prism 30, forms an image on the lens 32, and is detected by the first photodetector 20. The optical signal from the second core 6 is refracted by the inclined surface of the separation prism 30, forms an image by the lens 32, and is detected by the second photodetector 22.

In the transmitting section of the optical transmission device, it is desirable that the numerical aperture of the incident light is small,
Is preferred. In FIG. 2, the light source 18 and the optical fiber 2
The numerical aperture is reduced by increasing the distance l to the distance l, and the distance l is desirably 1.56 to 2.4 mm, and particularly preferably 2.0 to 2.4 mm. It is desirable to use a semiconductor laser as the light source 18. When the divergence angle of the light emitted from the laser is small, as in the case of a surface-emitting type semiconductor laser, the light source 18 and the optical fiber 2 can be placed close to each other.

Hereinafter, the optical fiber of each embodiment of the present invention, the conventional SI plastic optical fiber, and the conventional The result of signal transmission on an optical transmission line combining various types of GI plastic optical fibers will be described. Conventional SI plastic optical fibers have a core diameter of 980 μm, a numerical aperture of 0.3, and a wavelength of 65.
The transmission loss at 0 nm was 7.5 dB / 50 m. A conventional GI plastic optical fiber has a core diameter of 700 μm, a numerical aperture of 0.2, and a wavelength of 650.
The transmission loss in nm was 8 dB / 50 m. Conventional optical transmission devices for 500 Mbps and 250 Mbps were used. The optical transmission device for 500 Mbps was for a GI plastic optical fiber, and used a semiconductor laser as a light source. The wavelength of the emitted light was 650 nm and the numerical aperture of the emitted light was 0.15. In addition, 250Mb
The optical transmission device for ps is for an SI type plastic optical fiber, uses an LED as a light source, and has a center wavelength of 650 n.
At m, the numerical aperture of the emitted light beam was 0.3. In the above combinations, it was examined whether transmission at a distance of 50 m was possible. Table 2 shows the comparison results.

[0091]

[Table 2]

The same results were obtained with the configuration of the optical fiber according to each embodiment of the present invention and the configuration of the optical transmission device according to each embodiment of the present invention. In Table 2, “Embodiment” represents the optical fiber or the optical transmission device of each embodiment of the present invention, and “50”.
0 ”and“ 250 ”are 2 for the conventional 500 Mbps, respectively.
It represents an optical transmission device for 50 Mbps.

When a conventional optical transmission device for 500 Mbps is used on the transmission side and the reception side, the conventional S
Even if an I-type fiber is used, low-speed communication at 250 Mbps cannot be performed. Also, when a conventional GI fiber is used, at least one of the transmitting side and the receiving side has a conventional 250-degree fiber.
If an optical transmission module for Mbps is used, communication cannot be performed.

On the other hand, if the optical transmission device of the present invention is used on both or one of the transmission side and the reception side, communication at 250 Mbps can be performed using the SI type fiber. When the optical fiber of the embodiment of the present invention is used, when the optical transmission device of the embodiment of the present invention is used on both sides, 500
Both Mbps and 250 Mbps communication are possible. If the optical transmission device of the embodiment of the present invention is used on one side and the conventional 250 Mbps optical transmission device is used on one side, communication can be performed at 250 Mbps.

From the above, it is possible to connect the optical transmission device according to the embodiment of the present invention to a conventional SI plastic optical fiber, and it is also possible to combine the optical transmission device with a conventional SI plastic optical fiber optical transmission module. It is possible. Thus, the optical transmission device of the present invention is
Compatible with optical transmission equipment for plastic optical fibers. Furthermore, if the optical transmission device of the present invention is used on both the transmitting side and the receiving side, high-speed communication can be performed using the conventional GI plastic optical fiber or using the optical fiber of the present invention. .

[0096]

According to the present invention, a low-speed / high-speed compatible optical fiber capable of transmitting both a low-speed optical signal and a high-speed optical signal using one and the same optical transmission device is obtained. Further, an optical receiving device capable of separately receiving both a low-speed signal and a high-speed signal from the low-speed / high-speed compatible optical fiber can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a first embodiment of the optical fiber of the present invention and a diagram showing a cross-sectional distribution of a refractive index.

FIG. 2 is a diagram illustrating a relationship between a light incident angle between the optical fiber of FIG. 1 and a light source.

FIG. 3 is a cross-sectional view of a second embodiment of the optical fiber of the present invention and a diagram showing a cross-sectional distribution of a refractive index.

FIG. 4 is a cross-sectional view of a third embodiment of the optical fiber of the present invention and a diagram showing a cross-sectional distribution of a refractive index.

FIG. 5 is a cross-sectional view of a fourth embodiment of the optical fiber of the present invention and a diagram showing a cross-sectional distribution of a refractive index.

FIG. 6 is a cross-sectional view of a fifth embodiment of the optical fiber of the present invention and a diagram showing a cross-sectional distribution of a refractive index.

FIG. 7 is a longitudinal sectional view of a light detection optical system according to the first embodiment of the optical transmission device of the present invention.

8 is a perspective view of the light detection optical system of FIG.

9 is a configuration diagram of a detection circuit of the optical transmission device of FIG.

FIG. 10 is a longitudinal sectional view of a light detection optical system according to a second embodiment of the optical transmission device of the present invention.

FIG. 11 is a longitudinal sectional view of a third embodiment of the optical transmission device according to the present invention and another photodetection optical system.

[Explanation of symbols]

 2 Optical fiber 4 First core 6 Second core 8 First clad 10 Second clad 12 Third clad 13 Fourth clad 14 Low numerical aperture fiber core 16 High numerical aperture fiber core 18 Light source 20 First photodetector 22 Second Photodetector 24 Hologram lens 26 First hologram lens 28 Second hologram lens 30 Separating prism 32 Lens 34 Optical waveguide 36 First optical waveguide core 38 Second optical waveguide core 40 Optical waveguide clad 50, 52 Preamplifier 54 Delay circuit 56 Amplifier 58 Control circuit 60 Switching circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/13 H04B 9/00 Y 10/12 H04J 14/00 14/04 14/06 H04B 10/28 10 / 26 10/04 10/06

Claims (12)

[Claims] 1. A semiconductor device comprising: a first core having a first refractive index; and a cylindrical second core having a second refractive index and surrounding the first core. Optical fiber. 2. The first core according to claim 1, wherein the first refractive index of the first core has a value decreasing from the center to the periphery, and the second refractive index of the second core is the first refractive index. An optical fiber having a refractive index greater than a minimum value of the optical fiber. 3. The method according to claim 2, wherein a first clad is formed between the first core and the second core, and a second clad is formed on an outer surface of the second core. The difference between the second refractive index of the second core and the refractive index of the second cladding is the minimum of the first refractive index of the first core and the refractive index of the first cladding. An optical fiber characterized by being larger than the difference between: 4. A plurality of first cores each covered with a first clad, and a plurality of second cores each covered with a second clad, wherein the plurality of first cores are centered. A plurality of second cores disposed in a peripheral region, a gap between the central region and the cores in the peripheral region is filled with a third cladding, and a refractive index of the third cladding. Is an optical fiber having a refractive index smaller than that of the first and second claddings. 5. A semiconductor device comprising: a plurality of first cores each coated with a first clad; and a plurality of second cores each coated with a second core, wherein the plurality of first cores are centered. Locating the plurality of second cores in a peripheral region,
A gap between the cores in the center region is filled with a third clad, and a gap between the cores in the peripheral region is filled with a fourth clad, and the refractive index of the third clad is the fourth clad. An optical fiber characterized by having a refractive index larger than the refractive index of the cladding. 6. An optical fiber according to claim 1, wherein said first and second cores are formed of a plastic material. 7. The optical fiber according to claim 1, wherein the diameter of the first core is 330 to 820 μm, and the diameter of the second core is 930 to 1030 μm. 8. The method according to claim 1, wherein the numerical aperture of the first core is 0.1 to 0.25, and the numerical aperture of the second core is 0.2 to 0.3. Characteristic optical fiber. 9. The first light emitted from the center of the end of the optical fiber.
And a second optical signal, which separates and guides the second optical signal emitted from the annular periphery of the end of the optical fiber to the first and second photodetectors, respectively. An optical receiving device, wherein the light receiving portion of the detector is annular, and the first photodetector is disposed inside the annular light receiving portion of the second photodetector. 10. The switching means according to claim 9, further comprising: amplification means connected to an output of said first photodetector; and switching means connected to an output of said second photodetector. Connects the output of the second photodetector to the amplifying means when the second photodetector detects a significant optical signal, and calculates the sum of the first optical signal and the second optical signal. An optical receiver characterized by amplifying. 11. The light separating means according to claim 9, wherein said light separating means has a first optical waveguide and a second optical waveguide, wherein said second optical waveguide is cylindrical. The optical receiving device, wherein the optical path extends coaxially in the second optical waveguide. 12. An optical receiving device according to claim 9, further comprising: an optical transmitting device, wherein a distance between a light source of the optical transmitting device and the optical fiber is 1.56 mm to 2.0 mm.
An optical transmission device characterized by being in the range of mm.