JP3355575B2 - Single mode optical fiber and method for expanding core of single mode optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to optical communication, optical measurement,
The present invention relates to a single mode optical fiber used for transmitting an optical signal in a field such as a CATV system, and more particularly to a single mode optical fiber having excellent connectivity.

[0002]

2. Description of the Related Art Conventionally, since a single mode optical fiber has a very small core diameter of about 10 μm or less, connection loss of an optical signal is apt to occur due to a slight displacement of a core at the time of connection.
There has been a problem that joining with an accuracy of the order of sub-μm is required. In addition, the connection between different kinds of fibers has a drawback that a connection loss occurs due to a difference in a mode field shape of an optical signal propagating through the optical fibers.
Also, when a groove is formed vertically in a part of the optical fiber and various functions are introduced by inserting a filter, a wavelength plate, etc., the propagation loss of light increases rapidly as the gap between the cores increases. However, there is a disadvantage that the gap width is greatly limited and the function is limited.

As a method for compensating for these drawbacks, there has been proposed a core-expanded fiber in which the core at the end of the optical fiber is enlarged by heating the connection portion of the optical fiber.

An ordinary single mode optical fiber is made of synthetic silica glass for the purpose of ensuring low transmission loss characteristics, etc., and has a cladding made of pure silica glass and a core made of pure silica glass doped with Ge. Most of the single-mode optical fibers practically used have this configuration.

[0005] Another single-mode optical fiber configuration is to add F to the cladding, which is suitable for relatively low-temperature processing.
There are single-mode optical fibers with a core made of pure synthetic quartz and single-mode optical fibers with multi-component glass.
These are different from the standard single-mode optical fiber described above in that the core has a different refractive index, so that there is a drawback that a sufficient amount of return loss of an optical signal at the time of connection cannot be obtained, and thus they are not practically used. In addition, since these single mode optical fibers have a different melting point from standard single mode optical fibers, there is a disadvantage that low loss fusion splicing with standard single mode optical fibers is difficult. However, a low-loss connection effect due to the core expansion could not be expected.

[0006] Some silica glass single mode optical fibers are called W-type single mode optical fibers in which the cladding near the core is doped with F to increase the relative refractive index difference from the Ge-doped core. Although there is a fiber, it is expensive and is hardly commercially available at present.In addition, since the refractive index of the cladding is not constant in the radial direction, the cut-off wavelength is greatly changed by the core expansion processing, and the core expansion is not possible. Not suitable.

For these reasons, the study of a practical core-expanded fiber has been carried out exclusively with a standard quartz glass-based single mode optical fiber doped with Ge near the core. Therefore, in order to diffuse Ge into the cladding made of pure synthetic quartz glass, processing at a high temperature of 1200 ° C. or more is necessary for the core expansion processing of the single mode optical fiber. High temperature treatment at a temperature of at least ℃ was required.

[0008]

The basic method of expanding the core of an optical fiber is to expand the core additive material to the cladding by heating a part of the optical fiber to a temperature near the softening point.

As a heating means for the optical fiber, an electric furnace,
Although there are micro burners and electric discharge machining, in order to accelerate the diffusion by any means, it is necessary to soften the silica glass which is the cladding material, and therefore the disadvantage that the outer shape of the optical fiber tends to change during the diffusion process there were.

When a micro burner is used, it is technically difficult to control the diameter of the enlarged core, and there is a disadvantage that the yield is poor for low-loss connection. Further, in order to increase the diameter of the enlarged core to a certain degree or more, it is necessary to heat the micro burner while repeatedly sweeping it in the longitudinal direction of the optical fiber, and there is a disadvantage that the yield is further reduced.

In the case of using an electric furnace, since the variation in the outer shape is suppressed to some extent, it is possible to carry out the treatment at a reduced heating temperature, and the controllability of the enlarged core diameter is better than that of the micro burner, but the heat treatment time is greatly increased. Had the disadvantage of doing so. Also,
Even if the heating temperature is suppressed, a decrease in the viscosity of the clad is unavoidable, so that it was not possible to suppress a change in the outer shape on the order of μm during a long-time heat treatment. Furthermore, batch processing is possible in the heating process using an electric furnace, but it is necessary to cut the optical fiber to be put into the furnace and remove the entire coating, and in actual use, recoat or fuse the shortened optical fiber. Since a connection or the like is required, there is a disadvantage that it is not practical.

In addition, when electric discharge machining is used, the heat treatment time is short, but the optical fiber is locally heated to a considerably high temperature, so that variations in the outer shape cannot be avoided and the expansion range is limited. And its scope was limited (Electronics Letters, Volume 27, Number 17, Page1
597-1599, 'Simple fusion splicing technique for re
reducing splicing loss between standard singlemode f
ibres and erbium-dopedfibre 'by HYTAM).

As another core expansion method, S. Ishika
As in wa et al., OFC'93, TuB4, a method has been proposed to release the stress strain left during the production of optical fibers by heat treatment.However, in order to leave stress in the core, the softening temperature of the core must be reduced. Since the temperature must be higher than the softening temperature of the cladding and a fiber of pure silica core / F-doped cladding is used, the same problem as described above occurs. Also, in this case,
There is a drawback that the mode field diameter to be enlarged is limited to about twice. Furthermore, since the V value changes in the portion where the stress remains, there is a possibility that a problem may occur in the transmission characteristics of the optical fiber, and the residual stress strain itself may cause an increase in loss or deterioration of the polarization characteristics. There was a risk of causing this. For this reason, this method is only a proposal and has not become a widely spread technology.

As an application example of the core enlargement technology, SC
Although application to optical fiber connectors has been proposed, in the conventional method of enlarging the core, even if the heating temperature is suppressed by using an electric furnace or the like, it is inevitable that the viscosity of the clad will decrease. could not. Therefore, it has not been possible to enlarge the core of the optical fiber so that it can be stably inserted into an optical fiber insertion hole such as an optical connector ferrule in which the inner diameter of the optical fiber insertion hole differs from the outer diameter of the optical fiber by only about 1-2 μm. Therefore, an optical connector using such a core-expanded optical fiber is only a proposal, and has not been a technology used for practical use.

An object of the present invention is to provide an inexpensive single mode optical fiber and a method for expanding the core of a single mode optical fiber, which can stably expand the core without deforming the outer shape. .

[0016]

In order to solve the above-mentioned problems, according to the present invention, a cladding of a single mode optical fiber is constituted by an inner cladding near a core and an outer cladding covering the inner cladding, and the core is made of GeO _2. And add
On the side cladding, GeO ₂ with a concentration of about half of the core and G
Adds an amount of fluorine to offset the increase in refractive index due to eO ₂
By doing so, it is possible to realize an increase in the diffusion rate of the dopant in the portion where the core is to be expanded, thereby reducing the temperature required for the core expansion process, shortening the processing time, and facilitating the core expansion process. The greatest feature is that the deformation of the clad outer shape that occurs during the core enlargement processing is eliminated, and the controllability of the cross-sectional shape of the enlarged core is greatly improved as compared with the related art. In addition, since the core enlargement process can be performed at a lower heating temperature than before, the present invention has a feature that the range of selection of the heating means is expanded.

[0017]

Next, the present invention will be described in detail with reference to the drawings. However, the embodiments disclosed below are merely exemplifications, and do not limit the scope of the present invention.

FIG. 1 shows an embodiment of a single mode optical fiber according to the present invention, and shows a cross section of the optical fiber and its refractive index distribution. In the figure, 1 is a core for guiding and propagating light,
Numeral 2 denotes a clad comprising an inner cladding 2a near the core 1 and an outer cladding 2b covering the inner cladding 2a.

The core 1 is doped with GeO ₂ in order to give a refractive index difference to the cladding 2. Also,
The core diameter was about 8 μm, and the relative refractive index difference between the core 1 and the clad 2 was 0.3%. The cutoff wavelength was 1.12 μm.

The inner cladding 2a was doped with GeO ₂ at a concentration about half that of the core 1, and at this time, an increase in the refractive index was suppressed by doping an amount of F that offset the increase in the refractive index. The outer diameter of the inner cladding 2a is about 24
μm. The outer cladding 2b was made of pure silica glass, and had an outer diameter of 125 μm, similar to a standard optical fiber.

The above-mentioned optical fiber is disclosed in Japanese Patent Application Laid-Open No. H5-2734.
Heating was performed using a micro heater disclosed in Japanese Patent Publication No. 29, and a core enlargement process was performed.

FIG. 2 shows the change over time in the mode field diameter at the center of heating when heated at 1150 ° C. with a wavelength of 1.3.
It is a result measured in μm. The mode field diameter increased almost linearly from about 10 μm before the treatment, reached about 31 μm in about 20 minutes, and was thereafter substantially constant and stable. This is presumably because Ge diffuses almost uniformly throughout the inner cladding 2a and the outer cladding 2b is made of pure silica glass, so that Ge diffusion hardly occurs at 1150 ° C.

It can be seen that in this fiber, the allowable range of the heating time for obtaining a mode field diameter of about 31 μm is sufficiently wide.

FIG. 3 shows an enlarged mode field diameter distribution of the optical fiber which has been heated at 1150 ° C. for 20 minutes. From this figure, it can be seen that the reproducibility of the enlarged mode field diameter is very good.

The reason why Ge diffuses into the inner cladding 2a at 1150 ° C. is considered to be that the addition of F lowers the melting point of the inner cladding 2 and increases the Ge diffusion coefficient. Can be Also, the heat treatment at this temperature did not cause any deformation of the outer cladding 2b.

FIG. 4 shows a change in connection loss when a heat treatment is performed at 1150 ° C. for 20 minutes, two optical fibers cut at the center of the heat are butted in parallel to each other, and the relative positions of the cores are laterally offset. 3 shows the result of comparing the sample with the sample before the core enlargement treatment. From this figure, it can be seen that the range where the excess loss is within 0.5 dB has been expanded three times or more.

[0027] Twenty core-enlarged optical fibers were manufactured and inserted into the connector ferrule to assemble the SC optical connector, but all could be inserted without any problem. The connection loss of these connectors was very small at 0.05 dB on average, and the return loss was stable at 50 dB or more. When a conventional general optical fiber is used, a temperature of 1400 ° C. or more is required to perform a practical core enlargement process, and a change in outer shape is inevitable. Therefore, the production yield of an optical fiber that can be inserted into a ferrule Was very bad and poor in practicality.

As described above, when the enlarged end of the core of the optical fiber is applied to an optical connector or the like, the allowable range of the displacement of the core at the time of connection is greatly expanded, and the positional accuracy is greatly relaxed. The processing accuracy of the optical connector parts can be greatly reduced. Furthermore, stable low-loss connection is possible, connection reliability is greatly improved, and cost is greatly reduced. In addition, if the present invention is applied to connection of optical fibers having different mode field diameters, low-loss connection between them is also possible.

FIG. 5 shows a heat treatment at 1150 ° C. for 20 minutes, two optical fibers cut at the center of the heat were butted in parallel to each other, and the distance between the cores (fibers) was changed while keeping the axes of the cores aligned. 6 shows the result of measuring the change in connection loss at the time. From this figure, it can be seen that the range where the loss is within 0.5 dB has been expanded five times or more. This is because the NA sharply decreases as the core expands. Thus, when a groove is formed in a part of an optical fiber and an optical element is inserted, the limitation on the thickness of the optical element is greatly relaxed, and insertion can be performed with low loss.

[0030]

As described above, according to the present invention,
The cladding of the single-mode optical fiber has a structure including an inner cladding near the core and an outer cladding covering the inner cladding , GeO ₂ is added to the core, and the inner cladding has a core.
GeO ₂ with a concentration of about half that of a and the refractive index due to GeO ₂
By adding an amount of fluorine to offset the increase of the required diffusion coefficient of the refractive index control agent in the inner cladding by was low comb than the diffusion coefficient of the refractive index control agent in the outer cladding, the core enlargement process Since it is possible to reduce the temperature and the processing time, it is possible to provide an inexpensive core-expanded fiber that has excellent characteristics and reproducibility and does not largely depend on the heating means.

Also, since there is no deformation of the outer shape that occurs at the time of the core enlargement processing, the insertion into the connector ferrule can be stably performed, and the application to the optical connector becomes practical. Also,
Even when optical fibers having different mode field diameters are connected to each other, a mode field diameter determined in advance by the structure of the optical fiber can be stably obtained, so that connection with good reproducibility and low loss can be performed. Furthermore, if the advantage that there is no deformation of the outer shape and the diameter of the enlarged core is stable is used, the application range is greatly expanded as compared with the prior art.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an embodiment of a single mode optical fiber of the present invention.

FIG. 2 is a diagram showing a result of measuring a time change of a mode field diameter at a heating center when heated at 1150 ° C. at a wavelength of 1.3 μm.

FIG. 3 is a diagram showing an enlarged mode field diameter distribution of an optical fiber heated at 1150 ° C. for 20 minutes.

FIG. 4 is a diagram showing a change in connection loss when optical fibers according to the present invention are butted in parallel to each other and the relative position of the core is laterally offset.

FIG. 5 is a diagram showing a change in connection loss when the optical fibers according to the present invention are butted against each other and the distance between the fibers is changed.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... Clad, 2a ... Inner clad, 2b ...
Outer cladding.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-224060 (JP, A) JP-A-8-190030 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6/00 G02B 6/10 G02B 6/16-6/255 G02B 6/36-6/40 G02B 6/44

Claims (3)

(57) [Claims] 1. A single-mode optical fiber covering the core and the core is guided through the optical and refractive index than the core is made of slightly lower cladding, the inner cladding and the inner side of the core near the clad GeO ₂ is added to the core, and the inner cladding has a G concentration of about half that of the core.
The amount of eO ₂ and the amount of flux that offset the increase in the refractive index due to GeO ₂
A single mode optical fiber characterized by adding nitrogen . 2. The single mode optical fiber according to claim 1,
The edge was locally heated, and the Ge added to the core was
Diffusion into the inner cladding, core only at the end of the optical fiber
Of single mode optical fiber characterized by increasing diameter
Core expansion method. 3. The local heating is performed by a micro heater.
The single-mode optical fiber according to claim 2, wherein
How to enlarge the core of the ba.