JPH09311221A - Light attenuatable optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen the influence of the clad mode which occurs at the connec tion. SOLUTION: This light attenuatable optical fiber consists of a core 1 and a clad which encloses this core 1 and has the refractive index lower than the refractive index of the core near the core 1. The clad has an inner clad 2 and a low-refractive index part 4 which is disposed on the outer side of this inner clad and has the refractive index smaller than the refractive index of the inner clad. The fiber contains a dopant to attenuate light signals in part or the whole of the core 1 and the inner clad 2.

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, there are known a method of inserting a filter into an optical fiber and a method of adding a light attenuating dopant to the optical fiber.

Among these methods, the method of adding a dopant for light attenuation has advantages such as ease of assembly, high reliability and low cost.

Such a technique is disclosed in, for example, Japanese Patent Laid-Open No. 54-54.
2754, Japanese Utility Model Laid-Open No. 63-96504, Japanese Utility Model Laid-Open No. 63-96506, Japanese Patent Laid-Open No. 6-303156 and the like.

[0006]

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

The conventional fixed attenuator is usually used by connecting an optical fiber to both ends or either one of the input and the output. At the time of connection, the cores of both optical fibers are deviated and the mode fields are different. This causes a slight component of the optical signal to be radiated into the clad (hereinafter referred to as the clad mode).

However, in the connection of a standard optical fiber used as a trunk line of public communication, etc., the intensity of the clad mode as described above is much weaker than the intensity of the optical signal in the core, and the optical fiber It can be considered that it almost disappears during propagation.

On the other hand, in the fixed attenuator having the above-mentioned structure, the inventor of the present invention uses the clad mode generated at the connection point of the input end because the optical fiber used has a short length and is fixed on a straight line. However, it was confirmed that was easily propagated to the output end of the fixed attenuator and was easily re-coupled to the core of the optical fiber connected at the connection point of the output end.

Although the measured value depends on the measurement conditions, in the measurement by the inventor, the propagation loss caused by the 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 the clad mode is not a problem in the ordinary connection of the optical fiber.

Similarly, even with a fixed attenuator, 10 dB
The following fixed attenuators are considered to have no practical problems.

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

Further, even in the case of a fixed attenuator having an attenuation amount of 10 dB or less, the propagation mode field shapes of the connected optical fiber and the optical fiber used for the fixed attenuator do not match,
When the shapes are different from each other, the relative intensity of the light radiated to the clad increases, so that it is obvious that the above-mentioned problem occurs.

Even when the mode field shapes are substantially the same, similar problems occur when the relative displacement of the connecting cores is large.

Further, as disclosed in Japanese Patent Laid-Open No. 6-109923, the problem of the cladding mode is not limited to the case of using the optical attenuating optical fiber, but is a problem widely occurring in general optical attenuators. is there.

As a method for solving this problem,
A method in which an attenuating fiber is used and the cladding of the attenuating fiber is made of an attenuating medium is disclosed in JP-A-5-2648.
No. 16 and Japanese Patent Application Laid-Open No. 6-109923.

However, in this method, since it is necessary to add an attenuating dopant to the entire cladding, there is a problem that the optical attenuating optical fiber becomes expensive.

In the case of a high attenuation product, it is necessary to add an amount of attenuating dopant corresponding to the core, so it becomes technically difficult to add a dopant of an equivalent concentration to the entire cladding, and an optical attenuating optical fiber is required. Will be expensive.

As described in JP-A-6-109923, when the amount of attenuation in the clad is larger than that of the core, it is necessary to further increase the concentration of the dopant.
The cost of the optical attenuating optical fiber is further increased.

It is very difficult to add a dopant having a concentration higher than that of the core to the entire cladding in a high attenuation product.

An object of the present invention is to provide a technique capable of reducing the influence of the clad mode generated at the time of connection.

Another object of the present invention is to provide an optical attenuator having excellent characteristic stability.

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

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

[0026]

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

(1) An optical attenuating optical fiber comprising 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, the clad being an inner clad and the inner clad. It has a low refractive index portion having a smaller refractive index than the inner cladding provided outside the cladding, and part or all of the core and the inner cladding contain a dopant that attenuates an optical signal.

According to the above-mentioned means, first, the optical signal leaked from the core to the clad is confined in the inner clad by forming the clad into a double structure of the inner clad and the low refractive index portion having a smaller refractive index than the inner clad. Will end up.

On the other hand, by adopting a structure in which an optical attenuating dopant is added to a part or all of the core and the inner cladding, an optical signal generated at the time of connection confined in the inner cladding can be attenuated, so that the influence of the cladding mode is reduced. be able to.

Since there is no change in the refractive index due to the addition of the light-attenuating dopant, an optical attenuator having excellent characteristic stability can be manufactured.

Since only a light-attenuating dopant needs to be locally added, a light-attenuating optical fiber having excellent characteristic stability can be manufactured at low cost.

That is, it is possible to reduce the influence of the clad mode generated at the time of connection and manufacture an optical attenuator having excellent characteristic stability at low cost.

Incidentally, Japanese Patent Application No. 8-4 filed by the same applicant
The optical attenuating optical fiber described in No. 1 has the following configuration.

(A) 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.

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

(C) 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.

Therefore, the optical attenuating optical fibers described in the present invention and Japanese Patent Application No. 8-4 are the same in that the cladding portion is composed of two or more claddings having different refractive indexes.

However, in the optical attenuation optical fiber described in Japanese Patent Application No. 8-4, the refractive index of the outer cladding is larger than the refractive index of the inner cladding, and the optical attenuation optical fiber of the present invention has a refractive index of the inner cladding. Since the indices are different in that they are larger than the refractive index of the outer cladding, the effects on the cladding modes are significantly different, as shown below.

In the optical attenuating optical fiber described in Japanese Patent Application No. 8-4, the optical signal propagating in the clad is absorbed by the outer clad, or is absorbed and attenuated.
Reduce the effects of cladding modes.

On the other hand, in the optical attenuation optical fiber of the present invention,
As described above, the influence of the cladding mode is reduced by confining the optical signal propagating in the cladding in the inner cladding and attenuating the core or the inner cladding with some or all of the doped dopant.

[0041]

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).

(Embodiment 1) FIG. 1 is a diagram showing a cross section and a refractive index distribution of an optical attenuating optical fiber according to Embodiment 1 of the present invention, in which 1 is a core for propagating light and 2 is an inner cladding. Reference numeral 3 is an outer cladding, and 4 is a low refractive index portion.

In FIG. 1, the core 1 has an inner cladding 2
In order to provide a difference in refractive index between the quartz glass and the glass, GeO ₂ is doped as a dopant.

The core diameter was 8 μm, the relative refractive index difference between the core 1 and the inner cladding 2 was 0.3%, and the cutoff wavelength at this time 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 mm and the optical attenuation amount at the wavelength of 1.55 μm is 30 dB.
The Co content is adjusted.

Co is an element suitable for optical attenuation in the wavelength band of 1.55 μm, as described in Japanese Patent Application No. 3-61547.

The inner clad 2 is a part for supplementing the clad mode, is substantially uniformly doped with Co as a dopant, and has an outer diameter of about 40 μm.

However, the Co concentration at this time was about half the concentration of the core 1, and there was almost no change in the refractive index due to the Co doping.
It was possible to reduce the amount of o-doping.

The outer cladding 3 is made of pure silica glass, and its outer diameter, that is, the outer diameter of the optical attenuating optical fiber of the first embodiment is the same as that of the standard optical fiber.
5 μm.

The low refractive index portion 4 is a layer in which F (fluorine) is uniformly doped outside the inner cladding 2, that is, inside the outer cladding 3 with a width of 15 μm, and its concentration has a relative refractive index of −0.15. It is set to be almost constant in%.

In the optical attenuating optical fiber of the first embodiment, the relative refractive index difference of the low refractive index portion 4 is such that the low refractive index portion 4 is separated from the core 1, and therefore the optical signal propagating through the core 1 is Had little effect on.

This is a design problem of the optical fiber,
As in the dispersion-shifted fiber, the structure of the low refractive index portion 4 is determined in advance in consideration of the influence on the optical signal, so that the problem of the fiber structure does not occur.

The relative refractive index difference of the low refractive index portion 4 given to the outer cladding 3 has almost no effect on the optical signal propagating through the core 1 because the outer cladding 3 is separated from the core 1.

FIG. 2 is a diagram showing a schematic structure when fixed in a ferrule as an example of the method of using the optical attenuating optical fiber according to the first embodiment. 5 is an optical attenuating optical fiber, and 6 is zirconia. It is a ferrule.

In FIG. 2, the optical attenuation optical fiber 5 is the produced optical attenuation optical fiber, and the zirconia ferrule 6 is used.
Is a member for holding a known optical fiber.

In the use example shown in FIG. 2, the optical attenuation optical fiber 5 is inserted into the ferrule 6 and fixed by adhesion, and then both end faces of the ferrule 6 are advanced PC polished, and the length thereof is 22 mm.

The length of the optical attenuation optical fiber 5 to be used is defined by the length of the ferrule 6, and the attenuation amount of the optical signal is determined by the concentration of the optical attenuation optical fiber 5 and Co that is a dopant.

Therefore, when the length of the optical attenuating optical fiber is not specified, for example, when a standard single mode optical fiber is used by fusion splicing on both sides or one side of the optical attenuating optical fiber 5, the optical attenuating property is set. By changing the length of the optical fiber 5, fixed attenuators with different attenuations can be manufactured from one type of optical attenuating optical fiber 5.

FIG. 3 is a diagram showing a schematic structure of an SC adapter type fixed attenuator using the ferrule containing the optical attenuating optical fiber shown in FIG. 2, wherein 7 is a sleeve and 8 is a housing.

In FIG. 3, the sleeve 7 is a member for aligning the central axes of the optical attenuating optical fibers 5 by applying a uniform force in the radial direction of the cylindrical ferrule 6, for example.

The housing 8 is a case of an SC adapter type fixed attenuator.

The inventor of the present application used the SC adapter type fixed attenuator shown in FIG. 3 and connected standard single mode optical fibers to both ends to measure the basic characteristics in the 1.55 μm band.

FIG. 4 is a diagram showing the measurement results of the wavelength dependence of the attenuation amount in the 1.55 μm band of the SC adapter type fixed attenuator shown in FIG.
It was found that the dB was very stable, and the beat of the attenuation value was not observed even when the wavelength was changed to some extent.

This is because there is no coupling between the optical signal and the cladding mode, and there are no obstructive factors for optical signal transmission, and 1.5
This shows that the allowable range of signal wavelength variation due to the light source and the like in the 5 μm band is wide.

Further, the attenuation value when the output of the fixed attenuator shown in FIG. 3 was directly received by the photodetector and measured, was almost unchanged from the value measured through the optical fiber and was stable. .

Further, as a condition that a clad mode is likely to occur, a core diffusion treatment is performed, and a fiber cord in which the mode field diameter at the tip of the connector is increased by 1.5 times is connected to both ends of the fixed attenuator in FIG. In case of 1.55
Even in the measurement of the wavelength dependence of the attenuation in the μm band, a change in the attenuation value due to an increase in splice loss was observed, but there was no change in the stability of the wavelength dependence of the attenuation and the cladding mode was effectively reduced. It was confirmed that it was done.

Further, when the optical attenuating optical fiber according to the first embodiment is manufactured, since the area of the inner cladding 2 is small,
The F doping technique for doping the low refractive index portion 4 is a widely used technique as well as the addition region of the light attenuating dopant can be small, and the entire optical fiber preform manufacturing process can be used as it is. It can be manufactured at a lower cost than the conventional optical attenuating optical fiber in which the optical attenuating dopant is added.

As described above, according to the optical attenuating optical fiber of the first embodiment, the low refractive index portion 4 outside the inner cladding 2 has a width of 15 μm and a relative refractive index of F (fluorine) of −.
By providing a layer that is uniformly doped so as to be substantially constant at a concentration of 0.15%, the core 1 to the inner cladding 2
Attenuating the propagating optical signal radiated into the inner clad 2 and at the same time doping Co into the core 1 and the inner clad 2 with Co as a dopant so that the propagating optical signal is attenuated by the doped Co. This can reduce the influence of the cladding mode.

That is, the cladding mode can be effectively reduced by locally doping the core 1 and the inner cladding 2 without doping the entire optical attenuating optical fiber with the dopant.

Further, since it is not necessary to dope the entire optical attenuating optical fiber with the dopant and it is not necessary to increase the concentration of the dopant, the optical attenuating optical fiber can be manufactured at a low cost.

Since the core 1 and the inner clad 2 hardly changed in refractive index due to Co doping, there is an advantage that connection characteristics such as reflection characteristics are not affected.

Since the method of changing the relative refractive index on the outer side of the inner clad 2, that is, the inner side of the outer clad 3 is a general technique, there is an advantage that the existing optical fiber manufacturing apparatus can be used. Further, since the region for changing the relative refractive index is narrow, the optical attenuating optical fiber can be manufactured at a lower cost.

In the first embodiment, only one kind of the light-attenuating dopant doped in the core 1 and its vicinity is used. However, the attenuating dopant doped in the core 1 and its vicinity is a light-attenuating basic material. It is needless to say that it is doped to control the characteristics, and thus is not limited to one kind.

Also, since it is not necessary to widen the doping region for the attenuating dopant with which the inner cladding 2 is doped,
It is needless to say that two or more kinds of dopants such as Cr (chromium) and V (vanadium) may be doped when the used wavelength region and fiber parameters are different.

Further, it goes without saying that the material, amount and doping method of the dopant in the optical attenuating optical fiber of the present invention are not limited to the material amount and doping method shown in the first embodiment.

Further, in the first embodiment, only the high attenuation optical attenuator is taken up, but the effect is clear even when applied to the low attenuation product. The effect is remarkable when different optical fibers are connected, or when a plurality of optical attenuators are continuously connected.

It is needless to say that the present embodiment has an effect when applied to a high attenuation product.

In the first embodiment, the operation and the effect when applied to the optical attenuator for the single mode optical fiber have been described. Needless to say, the same effect is obtained.

Needless to say, the present invention can be applied to optical fibers other than quartz glass, depending on the purpose of use.

(Embodiment 2) FIG. 5 is a diagram showing a cross section of a light attenuating optical fiber according to a second embodiment of the present invention and its refractive index distribution, and 9 is a light attenuating dopant region.

In FIG. 5, the light attenuating dopant region 9
Is, for example, a region doped with Co (cobalt),
This is the region of the inner cladding 2 that is in contact with the core 1.

However, also in the optical attenuating optical fiber of the second embodiment, as in the first embodiment, G
Co is doped (added) together with eO ₂ .

That is, in the optical attenuating optical fiber of the second embodiment, the doped region of the dopant of the optical attenuating optical fiber of the first embodiment is part of the inner cladding 2.

Further, as described in the first embodiment, in the region of the inner cladding 2, the Co-doped photo-attenuating dopant region 9 and the Co-undoped region are
Since there is almost no difference in the refractive index distribution, the refractive index distribution shown in FIG. 5 is obtained.

Even with such a structure, as shown in FIG. 5, the refractive index distribution is such that the core 1 has the largest refractive index and the inside including the light attenuating dopant region 9 outside the core 1. The cladding 2 is provided, and the low refractive index portion 4 is provided outside the inner cladding 2, and the refractive index distribution thereof is smaller than that of the inner cladding 2.

Therefore, similarly to the optical attenuating optical fiber of the first embodiment described above, the optical signal entering the inner cladding 2 contains the optical attenuating dopant region 9.
A light attenuating dopant region 9 confined within
Since the light-attenuating optical fiber according to the first embodiment has the same effect as the light-attenuating optical fiber according to the first embodiment, the doping region of the dopant can be further reduced, so that the optical-attenuating optical fiber according to the first embodiment can be manufactured at a lower cost. .

Although the core 1 is entirely doped with the dopant in the present embodiment, it is needless to say that a part thereof may be doped as in the inner cladding 2.

(Comparative Example) As a comparative sample of Embodiment 1 (Example 1), an optical attenuating optical fiber having the same core structure and a pure silica glass cladding, that is, not containing GeO ₂ was prepared. Then, a similar test was conducted as an SC adapter type optical attenuator using the same.

The optical attenuation is 30 dB at a length of 22 mm.
To achieve this, it was necessary to increase the concentration of Co doped (added) into the core 1 as compared with the optical attenuating optical fibers of the first and second embodiments.

FIG. 6 shows the result of measurement of the wavelength dependence of the attenuation amount of the conventional attenuator manufactured as described above, and particularly the wavelength dependence of the attenuation amount in the 1.55 μm band.

From this result, it is confirmed that a periodically changing beat reaching ± 5 dB is observed in the wavelength dependence of the attenuation amount of this comparative sample, and that the attenuation amount greatly changes due to a slight difference in wavelength. .

In addition, due to the coupling of the cladding modes, the optical signal may not propagate normally.

Therefore, it was clarified that the cladding mode can be effectively removed by using the optical attenuating optical fiber of the first embodiment shown in FIG.

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.

[0095]

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

(1) The influence of the cladding mode can be reduced.

(2) An optical attenuator having excellent characteristic stability can be manufactured.

(3) An optical attenuating optical fiber having excellent characteristic stability can be manufactured at low cost.

(4) It is possible to manufacture an optical attenuator that reduces the influence of the cladding mode and is excellent in characteristic stability at low cost.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a schematic configuration when fixed in a ferrule as an example of a method of using the optical attenuating optical fiber of the first embodiment.

FIG. 3 is a diagram showing a schematic configuration of an SC adapter type fixed attenuator using the ferrule containing the optical attenuating optical fiber shown in FIG.

[Fig. 4] 1.5 of the SC adapter type fixed attenuator shown in Fig. 3.
It is a figure which shows the measurement result of the attenuation amount wavelength dependence in a 5 micrometer band.

FIG. 5 is a diagram showing a cross section of a light attenuating optical fiber according to a second embodiment of the present invention and a refractive index distribution thereof.

FIG. 6 is a diagram showing a result of measuring the wavelength dependency of attenuation of a conventional attenuator.

[Explanation of symbols]

1 ... core, 2 ... inner cladding, 3 ... outer cladding, 4 ...
Low refractive index portion, 5 ... Optical attenuating optical fiber, 6 ... Zirconia ferrule, 7 ... Sleeve, 8 ... Housing, 9 ... Optical attenuating dopant region.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryo Nagase 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Inventor Naruyuki Mitachi 3--19-3 Nishishinjuku, Shinjuku-ku, Tokyo No. 2 within Nippon Telegraph and Telephone Corporation (72) Inventor Yuichi Morishita 2-1-1, Oda Sakae, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Showa Electric Cable Co., Ltd. (72) Inventor Asahi Kumagai Sakae Oda, Kawasaki-ku, Kawasaki-shi, Kanagawa 2-1-1 No. 1 Showa Cable Electric Co., Ltd. (72) Inventor Yumi Ariga 2-1-1 No. 1 Showa Cable Denki Co., Ltd. (72) Inventor Kenichi Muta Kawasaki City, Kanagawa Prefecture 2-1-1 Oda Sakae, Kawasaki-ku In Showa Electric Wire & Cable Co., Ltd. (72) Inventor Masashi Saijo 2-1-1 1-1 Odae, Kawasaki-ku, Kawasaki-shi, Kanagawa (72) Inventor Kazunari Sugi 2-1-1 1-1 Oda Sakae, Kawasaki-ku, Kawasaki City, Kanagawa Prefecture Showa Electric Cable Co., Ltd.

Claims (1)

[Claims] 1. An optical attenuating optical fiber comprising a core and a cladding surrounding the core and having a refractive index lower than that of the core at least in the vicinity of the core, wherein the cladding is an inner cladding and the inner cladding. An optical attenuator having a low refractive index portion provided outside the inner cladding and having a smaller refractive index than the inner cladding, and part or all of the core and the inner cladding contains a dopant that attenuates an optical signal. Optical fiber.