JPH09184919A - Optically attenuating optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical attenuator which has superior characteristic stability at a low cost by reducing the influence of a clad mode that is generated at the time of connection. SOLUTION: This optically attenuating optical fiber consists of a core 1 and clads 2 and 3 which surround the core 1 and are lower in refractive index at least nearby the core 1 than the core 1, and contains a dopant for attenuating a light signal in the core or clad part 2 nearby the core, and the high refractive index clad part 3 which has a refractive index larger than the refractive index nearby the core is installed partially in the clads 2 and 3 not nearby the core. Further, a part or the whole high refractivel index clad part 3 contains the dopant which attenuates the light signal. This optically attenuating optical fiber contains the dopant which attenuates the light signal partially in the clads 2 and 3 not nearby the core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical attenuating optical fiber, and more particularly to an optical attenuator used for attenuating the intensity of an optical signal at a constant rate in the fields of optical communication, optical measurement, CATV system and the like. And an optical attenuating optical fiber used therein.

[0002]

2. Description of the Related Art Conventionally, in an optical attenuator using an optical fiber, some methods for attenuating an optical signal by a certain amount have been proposed. However, a high attenuating product requires further stability to withstand a high input power. Become. As attenuating means capable of coping with such high attenuation products, a method of inserting a filter into an optical fiber and a method of adding a light attenuating dopant to the optical fiber are known. Among them, the method of adding the dopant for light attenuation is easy to assemble, highly reliable,
It has excellent features such as low price. Such a technique is disclosed in, for example, Japanese Patent Laid-Open No. 54-2754 and Japanese Utility Model Laid-Open No. 63-96.
504, Japanese Utility Model Laid-Open No. 63-96506, JP-A-6-3031.
56 and the like.

[0003]

SUMMARY OF THE INVENTION As a result of studying the above prior art, the present inventor has found the following problems.

The fixed attenuator is usually used by connecting an optical fiber to both ends thereof or one of input and output ends. However, at the time of connection, an optical signal is generated due to a deviation of cores of both optical fibers and a difference in mode field. Will be radiated into the cladding. In a standard optical fiber connection, the intensity of such a cladding mode is much weaker than that of the optical signal in the core, and it can be considered that the cladding mode almost disappears during the propagation of the optical fiber.

However, in an optical fixed attenuator having such a structure, we have used a short optical fiber and fixed it on a straight line, so that the cladding generated at the connection point of the input end is used. It was confirmed that the mode easily propagates to the output end of the fixed attenuator and is easily re-coupled to the core of the optical fiber connected at the connection point of the output end.

The measured value depends on the measurement conditions, but in our measurement, the propagation loss of this cladding mode was about 40 dB including the coupling loss at the time of recombination. If the optical fiber is long and has a curvature, this value is further reduced, so that the propagation of this clad mode does not pose a problem in the connection of ordinary optical fibers. Further, even if the optical fixed attenuator is a fixed attenuator of 10 dB or less, it is considered that there is no practical problem.

However, in a fixed attenuator having a large amount of attenuation, the intensity of the optical signal propagating in the attenuated core and the intensity of the cladding mode are relatively close to each other. Therefore, due to the mode coupling state, a beat is generated depending on the wavelength of the attenuation value. However, there is a problem that the attenuation value becomes unstable and the optical signal does not propagate normally.

Here, it is assumed that the propagation mode field shapes of the optical fiber to be connected and the optical fiber used for the fixed attenuator are substantially the same, but when they are different from each other, the relative intensity of the light emitted to the cladding is Further increase,
It is clear that the above problem occurs even with a fixed attenuator having an attenuation amount of 10 dB or less. Even when the mode field shapes are substantially the same, the same problem occurs when the relative displacement of the connected cores is large.

Further, here, the influence of the cladding mode has been described in the case of the optical attenuator using the optical attenuating optical fiber. However, as described in JP-A-6-109923, the problem of the cladding mode is the optical attenuation. This is a problem that occurs widely in general optical attenuators, not limited to the case of using a flexible optical fiber.

A method for overcoming this problem is proposed in JP-A-5-264816, JP-A-6-109923 and the like. That is, this is a method of using an attenuating fiber and forming the clad with an attenuating medium. However, this method has a problem that the attenuating optical fiber becomes expensive because it is necessary to add the attenuating dopant to the entire cladding. Since it is necessary to add an amount of attenuating dopant equivalent to the core in high attenuation products, it is technically difficult to add the same concentration of dopant to the entire cladding, and optical attenuating optical fibers are expensive. Becomes

As described in JP-A-6-109923, when the amount of attenuation in the clad is larger than that in the core,
Further, it becomes necessary to increase the dopant concentration, which further increases the cost of the optical attenuating optical fiber. It is very difficult to add a dopant with a concentration higher than that of the core to the entire cladding in a high attenuation product.

From the viewpoint of characteristics, the wavelength dependence of the attenuation amount greatly depends on the property of the attenuating dopant in the method of manufacturing the entire cladding with the attenuating medium. Usually, the absorption spectrum of this kind of dopant has wavelength dependence, so that the optical attenuation characteristic depends on the wavelength of the optical signal.

Therefore, this structure has a problem that it is more difficult than ever to manufacture an optical attenuator which has a small wavelength dependency of the attenuation amount and can be used in two or more multi-wavelengths. As a solution to this problem, it is conceivable to offset the wavelength dependence of the attenuation amount of the dopant by doping the cladding with two or more kinds of attenuating substances. It is technically difficult to dope the cladding so that the optical properties are offset, and the cost of the optical attenuating optical fiber increases.

A common problem in the method of adding the attenuating dopant to the entire cladding is an increase in the refractive index due to the addition of the attenuating dopant. As the refractive index of the clad increases, the refractive index of the core also needs to increase correspondingly, and the dopant for increasing the refractive index of the core needs to be increased to adjust the relative refractive index difference. Furthermore, if the refractive index of the core and the clad is increased more than necessary, the refractive index will be different from that of a normal optical fiber, and the optical signal at the connection point when the optical fiber is connected to the optical attenuator However, there is a problem in that the reflection of light increases and the basic characteristics of the optical attenuator are impaired.

An object of the present invention is to provide a technique capable of reducing an influence of a cladding mode generated at the time of connection and obtaining an optical attenuator excellent in characteristic stability at a low cost.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0017]

The following is a brief description of an outline of a typical invention among the inventions disclosed by the present application.

(1) A dopant which comprises a core and a clad which surrounds the core and has a refractive index lower than that of the core at least in the vicinity of the core, and which attenuates an optical signal in the core or the clad portion near the core. An optical attenuating optical fiber containing the above, wherein a high-refractive-index clad portion having a refractive index higher than the refractive index in the vicinity of the core is provided in a part of the clad other than in the vicinity of the core.

(2) In the optical attenuating optical fiber of (1), a part or all of the high refractive index clad portion contains a dopant that attenuates an optical signal.

(3) A dopant which comprises a core and a clad which surrounds the core and has a refractive index lower than that of the core at least in the vicinity of the core, and which attenuates an optical signal in the core or the clad portion near the core. And a dopant for attenuating an optical signal is contained in a part of the clad other than near the core.

The above-mentioned means is provided as an optical attenuating optical fiber structure in which an absorption portion for the optical signal propagated through the clad is provided in the clad sufficiently separated from the core so that the optical signal propagated through the clad is not recombined with the core. That is the most important feature.

In the conventional fixed attenuator, the clad has no attenuation or the entire attenuator, and the absorption of the optical signal propagated in the clad is sufficiently separated from the core. There was no parted structure. The inventor has confirmed that the cladding mode which is a problem in the optical attenuator is basically not a mode trapped in the vicinity of the core but a mode reaching a region sufficiently distant from the core of the cladding. It was an opportunity to do so. In addition, it was confirmed that it is sufficient to provide a portion that absorbs the cladding mode in a cladding region that is sufficiently distant from the core to reduce the cladding mode.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings together with its embodiments (examples).

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
FIG. 3 is a diagram showing a cross section of the optical attenuating optical fiber of (Example 1) and its refractive index distribution, in which 1 is a core for propagating light, 2 is an inner cladding, and 3 is an outer cladding, which is an absorption portion of a cladding mode. . The core 1 is formed by doping silica glass with GeO _{2 in} order to provide a refractive index difference with the cladding. The core diameter was 8 μm, and the relative refractive index difference between the core and the clad was 0.3%. The cutoff wavelength was about 1.1 μm. The core 1 is further doped with Co (cobalt) so that the intensity of the optical signal is attenuated.

In this case, the optical fiber length is 22.4 mm
Then, the Co content was adjusted so that the optical attenuation amount at a wavelength of 1.31 μm was 30 dB. The inner clad 2 was made of pure quartz glass and had an outer diameter of about 40 μm. The outer cladding is doped with GeO ₂ in the same way as the core, and the concentration of GeO ₂ is gradually increased from the inner side so that the concentration near the outer periphery is almost constant with a relative refractive index difference of 0.15%. did. Outer diameter is the same as standard optical fiber, 125
μm. The relative refractive index difference of the outer clad is about half that of the core, but since it is sufficiently away from the core, it did not affect the optical signal propagating through the core.

FIG. 2 shows an example of a structure in which the optical attenuating optical fiber is fixed in a ferrule as an example of how to use the optical attenuating optical fiber. Reference numeral 4 is the manufactured optical attenuating optical fiber, 5 is a zirconia ferrule, and the length thereof is 22.4 mm. The length of the optical attenuating optical fiber 4 used is defined by the length of the zirconia ferrule 5, and the attenuation value of the optical signal is determined by the length of the optical attenuating optical fiber 4 and the Co concentration.

Therefore, when the standard single-mode optical fiber is used by fusion splicing on both sides or one side of the optical attenuating optical fiber 4, if the length of the optical attenuating optical fiber 4 is not specified, the optical attenuating optical fiber 4 is attenuated. By changing the length of the optical attenuating optical fiber 4, it is possible to manufacture a solid-state attenuator with a different attenuation amount from one type of optical attenuating optical fiber 4.

FIG. 3 shows an example of a SC adapter type fixed attenuator using a ferrule containing the optical attenuating optical fiber 4. 6 is a sleeve, and 7 is a housing.

Using this SC adapter type fixed attenuator, standard single mode optical fiber is connected to both ends, and 1.3 μm
The basic characteristics of the band were measured. Figure 4 shows the measurement results of the wavelength dependence of the attenuation in the 1.3 μm band. The attenuation value of 30 dB was very stable, and even if the wavelength was slightly changed, the beat of the attenuation value was not observed and was stable. This is because there is no coupling between the optical signal and the cladding mode, and there is no obstruction factor for optical signal transmission, indicating that the allowable range of signal wavelength fluctuation due to the light source in the 1.3 μm band is wide.

When the output of the fixed attenuator was directly received by the photodetector, the attenuation value was stable with almost no change from the value measured through the optical fiber. Furthermore, as a condition that the clad mode is likely to occur, core diffusion treatment was performed, and a similar test was conducted by connecting an optical fiber cord with the mode field diameter at the tip of the connector enlarged to about 1.5 times to both ends, but It was confirmed that the stability of the value did not change and the cladding mode was effectively reduced.

Regarding the price, GeO is added to a part of the clad.
The optical attenuating optical fiber 4 when doped with ₂ is Ge
It is obvious that the clad doping of O ₂ can be manufactured at a lower cost than adding an attenuating dopant to the entire clad because the ordinary optical fiber preform manufacturing process can be used as it is.

(Comparative Example 1) As a comparative sample of the present embodiment 1 (Example 1), the core structure is the same and Ge to the clad is used.
An optical attenuating optical fiber containing no O ₂ was prepared, and the same test was performed using the optical attenuating optical fiber. FIG. 5 shows the measurement results of the attenuation wavelength dependency in the 1.3 μm band of the comparative sample in this Comparative Example 1. In the attenuation wavelength dependency, a periodically changing beat reaching ± 5 dB was observed, and it was confirmed that the attenuation value significantly changes due to a slight difference in wavelength. The difference between these results clearly shows the cladding mode removing effect of the optical attenuating optical fiber according to the present invention.

(Second Embodiment) FIG. 6 shows a second embodiment of the present invention.
FIG. 3 is a diagram showing a cross section and a refractive index distribution of the optical attenuating optical fiber of (Example 2), in which 1 is a core for propagating light, 2 is an inner cladding, 8 is an intermediate cladding which is an absorption portion of a cladding mode, 3 is an outer cladding. The core 1 is formed by doping silica glass with GeO _{2 in} order to provide a refractive index difference with the cladding. The relative refractive index difference between the core 1 and the cladding is 0.
3%, and the diameter of the core 1 was 8 μm. Cutoff wavelength is about 1.1
μm. The core 1 contains V (vanadium) so that the intensity of the optical signal is attenuated.

In this case, the optical fiber length is 22.4 mm
Then, the content of V was adjusted so that the light attenuation amount at a wavelength of 1.31 μm was 20 dB. The inner clad 2 and the outer clad 3 were made of pure silica glass, the outer diameter of the inner clad 2 was about 75 μm, and the outer diameter of the outer clad 3 was 125 μm, which is the same as a standard optical fiber. The intermediate cladding 8 is doped with GeO ₂ as in the core 1, and has a relative refractive index difference of 0.1.
It was set to be almost constant at 3%. The outer diameter of the intermediate cladding 8 was 100 μm. Using this optical attenuating optical fiber, an SC adapter type fixed attenuator was manufactured in the same manner as in the first embodiment (Example 1).

Using this SC adapter type fixed attenuator, standard single-mode optical fibers are connected at both ends to 1.31 μm.
The basic characteristics of the band were measured. The attenuation value was very stable, and even if the wavelength was changed a little, the beat of the attenuation value was not observed and was stable.

(Third Embodiment) FIG. 7 shows a third embodiment of the present invention.
It is a figure which shows the cross section of the optical attenuating optical fiber of (Example 3), and its refractive index distribution, 1 is a core which propagates light, 9 is a center line part of the core which has an optical attenuating property, 2 is an inner clad. ,
An outer cladding 3 is an absorption / attenuation portion of the cladding mode. The core 1 is formed by doping silica glass with GeO _{2 in} order to provide a refractive index difference with the cladding. The relative refractive index difference between the core 1 and the clad was 0.75%, and the mode field diameter was about 8 μm. The cutoff wavelength was about 1.2 μm. The center line portion 9 of the core 1 contains Co so that the intensity of the optical signal is attenuated.

In this case, the optical fiber length is 22.4 mm
Then, the Co doping amount and the doping region (range of the core central portion) were adjusted so that the optical attenuation amounts at the wavelengths of 1.31 μm and 1.55 μm were both 25 dB. The inner clad 2 was made of pure quartz glass and had an outer diameter of about 85 μm. As in the core 1, the outer cladding 3 was doped with GeO ₂ so that the relative refractive index difference was kept constant at 0.1%. The outer cladding 3 was further doped with Co substantially uniformly. The Co addition concentration was set to about 1/5 of the Co addition concentration of the core 1. The outer diameter was 125 μm, which is the same as that of a standard optical fiber. Using this optical attenuating optical fiber, an SC adapter type fixed attenuator was manufactured in the same manner as in the first embodiment (Example 1).

Using this SC adapter type fixed attenuator, standard single-mode optical fibers were connected at both ends, and the basic characteristics in the 1.3 μm band were confirmed by connecting dispersion-shifted optical fibers to 1.
The basic characteristics in the 55 μm band were measured. The attenuation value was very stable, and even if the wavelength was changed a little, the beat of the attenuation value was not observed and was stable. This is 1.3 μm band and 1.55
This shows that the allowable range of signal wavelength variation due to the light source in the μm band is wide. There is no effect on the optical signal. Also, the attenuation value when the output of this fixed attenuator is directly received by the photodetector is almost the same as the value measured through the optical fiber,
It was stable in both wavelength bands of 1.3 μm band and 1.55 μm band.

When measuring the characteristics in the 1.3 μm band,
Standard single mode optical fiber was connected to both ends of the attenuator,
The mode field diameter of the standard single-mode optical fiber is larger than the mode field diameter of the prototyped optical attenuating optical fiber, and the stability of the attenuation value changes despite the large emission of light to the cladding at the connection point. However, it shows that the cladding mode is effectively captured and reduced. As described above, although the cladding of the attenuating dopant was added only to the outermost shell and the concentration thereof was smaller than that of the core 1, the reduction of the cladding mode was effective.

The same effect can be expected when GeO ₂ is not clad-doped and the outer clad is doped with only an attenuating dopant. In that case, if the amount of the attenuating dopant is increased more than in the third embodiment (Example 3), the influence of the cladding mode can be more effectively reduced.

Regarding the price, since a part of the cladding is doped with an attenuating substance, the price increase as an optical attenuating optical fiber is higher than that of the first and second embodiments (Examples 1 and 2). However, it is clear that it can be manufactured at a lower cost than adding an attenuating dopant to the entire cladding.

(Fourth Embodiment) FIG. 8 shows a fourth embodiment of the present invention.
It is a figure which shows the cross section of the optical attenuating optical fiber of (Example 4), and its refractive index distribution, 1 is a core which propagates light, 9 is a center line part of the core which has an optical attenuating property, 2 is an inner cladding. ,
Reference numeral 8 is an intermediate cladding that is a cladding mode attenuating portion, and 3 is an outer cladding that is a cladding mode absorbing portion. The core 1 has a cladding and a refractive index.
It is doped with eO ₂ . The relative refractive index difference between the core 1 and the clad was 0.75%, and the mode field diameter was about 8 μm. The cutoff wavelength was about 1.2 μm. Co is contained in the central portion 9 of the core 1 so that the intensity of the optical signal is attenuated.

In this case, the optical fiber length is 22.4 mm
Then, the doping amount of Co and the doping region (range of the core center line portion 9) were adjusted so that the optical attenuation amounts at the wavelengths of 1.31 μm and 1.55 μm were both 30 dB. The inner clad 2 was made of pure quartz glass and had an outer diameter of about 80 μm.
Co was added to the intermediate clad 8 almost uniformly. The Co addition concentration was about 1/2 of the Co addition concentration of the core. The outer diameter of the intermediate cladding 8 was 100 μm. The outer cladding 3 was doped with GeO2 as in the core 1, and the relative refractive index difference was kept constant at 0.2%. The outer diameter was 125 μm, which is the same as that of a standard optical fiber. Using this optical attenuating optical fiber, a SC adapter type fixed attenuator was manufactured in the same manner as in the first embodiment (Example 1).

Using this SC adapter type fixed attenuator, the same test as in Embodiment 3 (Example 3) was conducted. 1.
The attenuation values in the 3 μm band and 1.55 μm band were very stable, and even if the wavelength was slightly changed, the beat of the attenuation value was not observed and was stable.

(Comparative Example 2) As a comparative sample of Embodiment 4 (Example 4), the core structure was the same, and an attempt was made to add Co to the entire clad, but an amount similar to that of the core 1 was added to the entire clad. Doping requires many steps, resulting in a very expensive optical attenuating optical fiber, which is not practical,
An optical attenuating optical fiber was produced in which only the clad portion near the core with an outer diameter of 50 μm was doped with Co in an amount similar to that of the core, and the same test was conducted using the optical attenuating optical fiber. In the wavelength dependence, a beat whose attenuation value periodically changes by about 1.5 dB was observed, and the cladding mode removing effect was not sufficient.

As for the average attenuation amount, the attenuation amount in the 1.3 μm band was about 29 dB, but the attenuation amount in the 1.55 μm band was about 35 dB, and the attenuation amount wavelength dependency was observed. In the case where Co is doped only in the core 1, the Co-doped region hardly depends on the doping amount, and is substantially constant to cancel the attenuation wavelength dependency. However, when Co is also doped in the clad near the core, it is necessary to consider the attenuation of the core propagation mode oozing into the clad, and it depends largely on the distribution state of Co in the clad, so Control factors increase. Actually, even if the Co-doped region in the core 1 is readjusted, it becomes difficult to cancel the wavelength dependence, which causes a problem that the yield of the optical attenuating optical fiber decreases. Furthermore,
In order to keep the optical attenuation value constant, the Co-doped region at the center of the core 1 needs to be reduced by the amount corresponding to the reduction of the Co-doped region, which makes it difficult to manufacture a high-attenuation product. Have.

Embodiments 1 to 4 described above (Examples 1 to 4)
In the above, the case where there is only one type of attenuating dopant doped in the core 1 and the vicinity of the core 1 has been described, but the attenuating dopant doped in the vicinity of the core 1 and the core 1 controls the optical attenuation basic characteristics. However, it is not limited to one type. Further, it is not necessary to widen the doping region of the attenuating dopant to be doped in the clad, and therefore, two or more kinds of dopants may be doped in some cases. The point is to select the most effective doping method.

In addition, the above-described first to fourth embodiments (Examples 1 to 4)
In 4), only the high-attenuation optical attenuator was taken up, but the effect is clear even for low-attenuation products, especially when multiple optical attenuators are used by connecting optical fibers with different mode field diameters. The effect is remarkable when the vessels are continuously connected and used.

Further, the above-mentioned first to fourth embodiments (Examples 1 to 1)
In 4), an optical attenuator for a single mode optical fiber is taken up, but it goes without saying that the same effect can be expected with an optical attenuator for a multimode optical fiber. Further, depending on the purpose of use, the same effect can be expected with optical fibers other than silica glass.

As described above, the present invention is implemented in the above-mentioned embodiment (example).
Although the present invention has been specifically described based on the above, the present invention is not limited to the above-described embodiments (examples), and it goes without saying that various modifications can be made without departing from the scope of the invention.

[0051]

The effects obtained by typical aspects of the invention disclosed in the present application will be briefly described as follows.

(1) The cladding mode can be effectively reduced without affecting the attenuation characteristic of the optical attenuator.

(2) Since the clad mode absorption portion is provided in a region sufficiently distant from the core, the mode field shape of the fundamental mode propagating through the core is not changed even if the refractive index of the absorption portion is changed. The addition of an attenuating dopant to the absorbing portion does not change the basic characteristics such as the attenuation wavelength characteristic of the optical attenuator, so the optical attenuation characteristic can be controlled only by the structure near the core.

(3) Since the refractive index of the clad near the core is not changed, it does not affect the connection characteristics such as reflection characteristics.

(4) Since it is not necessary to make the absorption part of the cladding mode the entire cladding, and only the relative refractive index needs to be changed, the cladding can be manufactured only by an ordinary optical fiber manufacturing apparatus, and an attenuating dopant is added. In that case, since it is not necessary to increase the region and the concentration, it is possible to suppress the cost increase of the attenuation fiber.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross section of a light attenuating optical fiber according to a first embodiment (Example 1) of the present invention and a refractive index distribution thereof.

FIG. 2 is a diagram showing a structure of a zirconia ferrule for a fixed optical attenuator of the first embodiment (Example 1).

FIG. 3 is a diagram showing a structure of an SC adapter type optical attenuator of the first embodiment (Example 1).

FIG. 4 is a diagram showing measurement results of attenuation wavelength dependency in the 1.3 μm band according to the first embodiment (Example 1).

FIG. 5 shows a comparison example of the first embodiment (Example 1) .1.
It is a figure which shows the measurement result of the attenuation amount wavelength dependence in a 3 micrometer band.

FIG. 6 is a diagram showing a cross section of a light attenuating optical fiber according to a second embodiment (Example 2) of the present invention and a refractive index distribution thereof.

FIG. 7 is a diagram showing a cross section of a light attenuating optical fiber according to a third embodiment (Example 3) of the present invention and a refractive index distribution thereof.

FIG. 8 is a diagram showing a cross section of a light attenuating optical fiber according to a fourth embodiment (Example 4) of the present invention and a refractive index distribution thereof.

[Explanation of symbols]

1 ... core, 2 ... inner cladding, 3 ... outer cladding, 4 ...
Optical attenuating optical fiber, 5 ... Zirconia ferrule, 6 ...
Sleeve, 7 ... Casing, 8 ... Intermediate cladding, 9 ... Core center line portion.

Front page continuation (72) Inventor Ryo Nagase 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (72) Inventor Naruyuki Mitachi 3-19-3 Nishishinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Yuichi Morishita, Yu 1-1 Morita, Kawasaki-ku, Kanagawa Prefecture 2-1-1 Oda Sakae, Showa Electric Cable Co., Ltd. (72) Inventor, Asahi Kumagaya 2-chome, Odasakae, Kawasaki-ku, Kanagawa Prefecture No. 1 in Showa Cable Denki Co., Ltd. (72) Inventor Kenichi Muta 2-1-1, Oda Sakae, Kawasaki-ku, Kawasaki-ku, Kanagawa Prefecture (72) Masashi Saijo, Inventor Masashi Saijo Oda, Kawasaki-ku, Kawasaki-shi, Kanagawa Sakae 2-1-1 No. 1 Showa Cable Electric Co., Ltd. (72) Inventor Kazunari Sugi 2-1-1 1-1 Oda Sakae, Kawasaki-ku, Kawasaki City, Kanagawa Prefecture Showa Cable Co., Ltd.

Claims (3)

[Claims] 1. A dopant, which comprises a core and a cladding that surrounds the core and has a refractive index lower than that of the core at least in the vicinity of the core, and a dopant that attenuates an optical signal in the core or the cladding portion near the core. A light-attenuating optical fiber containing, characterized in that, in a part other than the core vicinity of the clad, a high refractive index clad portion having a refractive index larger than the refractive index of the core vicinity is installed. Attenuating optical fiber. 2. The optical attenuating optical fiber according to claim 1, wherein a part or all of the high refractive index clad portion contains a dopant that attenuates an optical signal. 3. A core and a clad that surrounds the core and has a refractive index lower than that of the core at least in the vicinity of the core, and a dopant that attenuates an optical signal is added to the core or the clad portion near the core. An optical attenuating optical fiber containing the optical attenuating optical fiber, wherein a part of the clad other than near the core contains a dopant for attenuating an optical signal.