JPH0926517A - Dispersion compensation optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the induced Brillouin backward scattering of a dispersion compensation optical fiber and to make the transmission of high-power light possible by changing at least either of a core diameter and a difference in the specific refractive index between the core and a clad in the longitudinal direction of the optical fiber. SOLUTION: This dispersion compensation optical fiber is so designed as to have the negative wavelength dispersion value of the absolute value larger than the absolute value of a quartz single mode optical fiber at a use wavelength by making the core diameter smaller than the core diameter of the quartz single mode optical fiber having a zero dispersion wavelength at a 1.3μm band and making the difference in the specific refractive index between the core and the clad larger. The dispersion compensation optical fiber is so formed that the fiber outside diameter and the core diameter change periodically in the longitudinal direction of the optical fiber. The smaller wavelength dispersion value of the dispersion compensation optical fiber is more preferable and the wavelength dispersion value is preferably set at <=-50ps/km/nm. At least either of the core diameter and the difference in the specific refractive index between the core and the clad are changed in the longitudinal direction of the optical fiber in such a manner.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber capable of compensating for chromatic dispersion, and more particularly to a dispersion compensating optical fiber capable of suppressing generation of stimulated Brillouin backscattered light which is a problem in an optical communication system using the optical fiber. Regarding

[0002]

2. Description of the Related Art In recent years, an extremely low loss wavelength band of a silica-based optical fiber is in the 1.5 μm band, so that optical communication using this wavelength band has become popular. By the way, chromatic dispersion becomes a problem in a long distance and large capacity optical communication system. Most installed optical communication systems use a single mode optical fiber having a zero-dispersion wavelength in a normal 1.3 μm band.
The chromatic dispersion of this normal single mode optical fiber is 1.55
Approximately 20 ps / km / nm in μm (positive dispersion value)
However, if this is used to perform optical communication in the 1.5 μm band, a large chromatic dispersion will occur. Such a large chromatic dispersion value poses a serious problem especially during high-speed communication, and the extremely low loss characteristics of the silica-based optical fiber cannot be fully utilized in the installed optical communication system.

Therefore, a dispersion compensating optical fiber having a relatively large absolute value of chromatic dispersion (negative dispersion value) in a wavelength band of 1.5 μm and capable of canceling chromatic dispersion generated in a normal single mode optical fiber. Was developed. Since such a dispersion compensating optical fiber has a negative chromatic dispersion value and a large absolute value, it has a smaller core diameter than a single mode optical fiber having a zero dispersion wavelength in a normal 1.3 μm band. In addition, the core-clad relative refractive index difference is increased. When the dispersion compensating optical fiber is used by inserting it into an optical communication system using a normal single-mode optical fiber, the amount of chromatic dispersion in the entire system becomes almost zero even if optical communication is performed in the wavelength 1.5 μm band. It is possible. Therefore, an optical communication system with a wavelength of 1.3 μm, which has already been laid using a normal single mode optical fiber,
It can be reconstructed into an optical communication system with a wavelength of 1.5 μm band. Also, with the progress of optical amplification technology, for example, 1.5μ
By inserting an m-band erbium-doped optical fiber amplifier before, after, or in the middle of the system,
It has become possible to increase the intensity of transmitted light and carry out further long-distance transmission.

[0004]

However, when the intensity of the light incident on the optical fiber exceeds a certain threshold value, stimulated Brillouin backscattering occurs and the incident light power is saturated. The stimulated Brillouin backscattering is such that the intensity of light incident on the optical fiber is strong, the length of the optical fiber is long, the core-clad relative refractive index difference of the optical fiber is large, and the concentration of the light intensity on the core is high. (The smaller the core diameter is, the larger it becomes.) As a result, most of the increased energy of incident light is changed to stimulated Brillouin backscattered light and reflected toward the incident end, and the optical transmission distance cannot be extended. .

In particular, a dispersion compensating optical fiber has a smaller core diameter and a larger relative refractive index difference than a normal single-mode optical fiber, so that the threshold value of stimulated Brillouin backscattering is lowered and effective incident light is reduced. The power is limited to a small amount. Therefore, when constructing a long-distance and large-capacity optical communication system in the wavelength 1.5 μm band using a dispersion-compensating optical fiber having a large absolute value of chromatic dispersion, a high-power light is incident to increase the transmission distance. It was difficult to plan. The present invention has been made in view of the above circumstances, and an object thereof is to suppress the occurrence of stimulated Brillouin backscattering in a dispersion compensating optical fiber and to enable transmission of high-power light.

[0006]

In order to solve the above-mentioned problems, the dispersion compensating optical fiber of the present invention has a smaller core diameter than a silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band, and A dispersion-compensating optical fiber having a large core-clad relative refractive index difference, wherein at least one of a core diameter and a core-clad relative refractive index difference is changed in the length direction of the optical fiber. To do. Especially, the chromatic dispersion value at the wavelength used is -50 ps.
/ Km / nm or less, and the amount of change in the core diameter in the length direction of the optical fiber is preferably ± 5% or more. Or the chromatic dispersion value at the used wavelength is -50 ps / km / n
It is preferable that it is m or less, and the amount of change in the core-clad relative refractive index difference in the optical fiber length direction is ± 5% or more.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
Part of the light propagating in the optical fiber is absorbed as kinetic energy of the constituent molecules of the glass. The absorbed energy is released as acoustic phonons when it exceeds a certain value. Brillouin scattering is a phenomenon in which Stokes' light shifted by the frequency of the acoustic phonon is generated due to mutual interference between the emitted acoustic phonon and the transmitted light. Due to the law of conservation of momentum between transmitted light and acoustic phonons, Stokes light is observed only as backscattered light. When light having the same wavelength as Stokes light is present in the vicinity of a molecule in the excited state by absorbing energy, stimulated emission occurs, and as a result, acoustic phonons are easily generated. This is stimulated Brillouin scattering. If the optical fiber has a homogeneous structure,
Induction phenomenon due to the same Stokes light wavelength occurs over the entire length of the optical fiber, and the threshold value is greatly reduced by the self-induction gain effect. The Brillouin shift frequency is determined by the propagation of excitation light and acoustic waves in the optical fiber, and the core diameter of the optical fiber, the refractive index of the core, the optical fiber diameter, the glass composition (Young's modulus, density, Poisson's ratio, etc.) It changes depending on structural parameters such as residual strain. The Brillouin shift frequency of a normal single mode optical fiber is about 10 GHz. Based on such a principle, the present invention changes the Brillouin shift frequency in each part in the length direction by making the structural parameters of the optical fiber non-uniform in the length direction. As a result, the center frequency of the stimulated Brillouin backscattered light in each part in the length direction of the optical fiber changes, the threshold value of the stimulated Brillouin backscattering is improved, and the incident power to the optical fiber can be increased.

Here, the dispersion compensating optical fiber in the present invention has a core diameter smaller than that of a silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band, and has a core.
It is designed to have a negative chromatic dispersion value whose absolute value is larger than that of a silica-based single mode optical fiber at a used wavelength by increasing the difference in cladding relative refractive index. A silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band usually has a core diameter of about 9 μm and a core
The clad relative refractive index difference is designed to be about 0.35%.

FIG. 1 shows a first dispersion compensation optical fiber of the present invention.
7 shows a change in the fiber outer diameter in the embodiment of FIG. The dispersion compensating optical fiber of this embodiment is formed so that the fiber outer diameter and the core diameter periodically change in the optical fiber length direction as shown in FIG. The wavelength used in the dispersion compensating optical fiber is preferably set in the range of 1.525 to 1.575 μm, and the core diameter range and the core diameter are set so that a desired chromatic dispersion value, cutoff wavelength and bending loss can be obtained in this wavelength range. -Clad relative refractive index difference (hereinafter, simply referred to as relative refractive index difference) is suitably set.
The smaller the chromatic dispersion value of the dispersion compensating optical fiber, the more preferable, but it is preferably -50 ps / km / nm or less for the reason described later. The cutoff wavelength is preferably set to 1.5 μm or less so that single mode transmission can be performed in the above wavelength range. The bending loss is preferably small, and is preferably set to, for example, 1.0 dB / m (bending diameter 20 mm) or less. And the refractive index distribution is
In order to increase the dispersion value of the optical fiber, it is preferable that the refractive index difference at the interface between the core and the clad is large. For example, a step type refractive index distribution or a single peak type is preferably formed.

FIG. 2 shows how the chromatic dispersion changes when the relative refractive index difference and the core diameter are changed, taking a case where a wavelength of 1.55 μm is used in a single mode optical fiber having a step type refractive index distribution. It shows the result of the calculation of the change to. In FIG. 2, A indicates a boundary where the bending loss at a bending diameter of 20 mm is 1.0 dB, and the bending loss is usually too large in the lower left region, which is not preferable. Further, B indicates a boundary where the cutoff wavelength is 1.5 μm, and the cutoff wavelength increases in the upper right region, which is not preferable.

C is a boundary where the relative refractive index difference is 3.5%, and if the relative refractive index difference is larger than this, cracking due to thermal strain occurs during the preparation of the base material, which makes it extremely difficult to produce. It is not preferable. In FIG. 2, the larger the relative refractive index difference and the smaller the core diameter, the smaller the wavelength dispersion value (negative) (the larger the absolute value). D is wavelength 1.5
It shows a boundary where the wavelength dispersion value at 5 μm is −50 ps / km / nm, and the region above this is the region where the wavelength dispersion value is −50 ps / km / nm or less. Therefore, when the used wavelength is 1.55 μm, by setting the core diameter and the relative refractive index difference so as to be included in the shaded regions in FIG.
A dispersion compensating optical fiber satisfying the preferable conditions of the cutoff wavelength can be obtained. Note that FIG. 2 shows an example of a method of calculating the usable range of the core diameter and the relative refractive index difference under given conditions such as the wavelength used, the wavelength dispersion value, and the bending loss. It is needless to say that the range of the core diameter and the relative refractive index difference can be calculated similarly under the above conditions.

In the dispersion compensating optical fiber of this embodiment, the core diameter is formed so as to periodically change in the length direction. The core diameter can be changed within the preferable range shown in FIG. If the amount of change in the core diameter is too small, the effect of suppressing stimulated Brillouin backscattering cannot be obtained, and in the case of a dispersion compensating optical fiber with a chromatic dispersion value of -50 ps / km / nm or less, the experimental result is ± 5% or more. It was confirmed that the scattering suppression effect can be obtained by changing it. The larger the amount of change in core diameter, the greater the effect of suppressing scattering. In this embodiment, the core diameter changes in a triangular wave shape as shown in FIG. 1 in proportion to the outer diameter of the fiber. However, the shape is not limited to this waveform, and the core diameter may be non-uniform in the length direction. Is. Therefore, the shape of the core diameter change may be sinusoidal or the like, and the period can be set appropriately. In addition, the core diameter may be uniformly increased or decreased from one end of the optical fiber to the other end. The more the core diameter varies in the optical fiber length direction, the greater the scattering suppressing effect. However, a fiber in which the core diameter changes abruptly, such as one in which optical fibers having different refractive index distributions are connected in multiple stages or one in which the core diameter changes in a rectangular wave shape, is not preferable because it causes an increase in transmission loss. The degree of change in the core diameter is such that the transmission loss at the wavelength used is 0.5 dB /
It is preferable that it is gentle to the extent that it is not more than km.

Here, the chromatic dispersion value of the dispersion compensating optical fiber is -50 ps / km / nm or less, and the transmission loss is 0.5.
The reason why it is preferably set to be dB / km or less is as follows. That is, the normal 1.3 μm that has already been laid
Most single band optical fiber communication systems have a length of 5
The wavelength dispersion in the 1.5 μm band is approximately 20 ps / km / nm. Therefore,
The total chromatic dispersion amount in the 1.5 μm band is 1000 to 200
It is about 0 ps / nm. On the other hand, the transmission line loss is about 0.4 dB / km at the wavelength of 1.3 μm and the wavelength of 1.5
It is about 0.2 dB / km in the μm band. At this time, the total loss amount is 20 to 40 dB in the wavelength band of 1.3 μm and 10 to 20 dB in the wavelength band of 1.5 μm.

Therefore, if the following conditions (1) and (2) are satisfied, the optical communication system can be reconstructed into an optical communication system in the wavelength band of 1.5 μm. (1) Total chromatic dispersion in 1.5 μm band = total chromatic dispersion in single mode optical fiber + total chromatic dispersion in dispersion compensating optical fiber ≈ 0 (2) total loss in 1.5 μm band = 1.5 μm band Total loss of single mode optical fiber in + + total loss of dispersion compensating optical fiber in 1.5 μm band ≦ total loss in 1.3 μm band = total loss of single mode optical fiber in 1.3 μm band where: dispersion compensating optical fiber Loss of 0.5 dB / k
If m or less, the length of the dispersion compensating optical fiber is required to be 20 to 40 km according to (2). Therefore, it can be seen from (1) that the chromatic dispersion value of the dispersion compensating optical fiber is preferably -50 ps / km / nm or less.

The dispersion compensating optical fiber of this embodiment can be manufactured, for example, as follows. First, a base material composed of a core and a clad is formed to have a refractive index distribution that satisfies the conditions shown in FIG. The outer diameter and the refractive index distribution of the base material are made uniform in the length direction. When the base material is drawn by using a heating furnace, the drawing temperature, the drawing speed, and the sending speed of the base material are appropriately controlled so that the outer diameter of the optical fiber becomes a triangular wave as shown in FIG. Spinning so that it changes in the vertical direction. Thus, by periodically changing the outer diameter of the optical fiber during spinning, a dispersion-compensating optical fiber in which the core diameter also changes in the lengthwise direction in a triangular wave in proportion to the outer diameter of the fiber can be obtained. In the obtained dispersion compensating optical fiber, since the core diameter changes in the length direction, the Brillouin shift frequency in each part in the length direction changes, and as a result, the amount of stimulated Brillouin backscattering in the entire length of the optical fiber is suppressed. be able to.

Another method of manufacturing a dispersion compensating optical fiber whose core diameter changes in the length direction is a method of changing the core diameter at the stage of the base material. That is, when forming the base material, first, a glass rod to be a core portion is produced, and the outer peripheral surface of the glass rod is externally cut, or after the external cutting, stretching is performed to reduce the glass rod diameter to, for example, a triangle. The glass layer is changed into a wavy shape in the length direction, and a glass layer to be a clad portion is formed on the circumference thereof. If such a base material is spun so as to have a uniform outer diameter to form an optical fiber, it is possible to obtain a dispersion compensating optical fiber in which the core diameter changes in the length direction.

Next, a second embodiment of the dispersion compensating optical fiber of the present invention will be described. The dispersion-compensating optical fiber of this embodiment is different from the first embodiment in that the fiber outer diameter and core diameter are uniform in the length direction, and the relative refractive index difference changes in the length direction. Is. Also in this embodiment, the core diameter and the relative refractive index difference are set so as to simultaneously satisfy the conditions of the dispersion value, the cutoff wavelength, and the bending loss. For example, when the wavelength used is 1.55 μm, the relative refractive index difference can be changed so as to fall within the preferable range shown in FIG. If the amount of change in the relative refractive index difference is too small, the effect of suppressing stimulated Brillouin backscattering cannot be obtained, and in the case of a dispersion compensating optical fiber with a chromatic dispersion value of -50 ps / km / nm or less, the experimental result shows that Refractive index difference ±
It was confirmed that the scattering suppressing effect can be obtained by changing the amount by 5% or more. The larger the amount of change in the relative refractive index difference, the greater the scattering suppressing effect.

The relative refractive index difference may be non-uniform in the length direction, and may be changed periodically, for example, in a triangular wave shape, a sine wave shape, or the like, and the cycle can be set appropriately. Alternatively, the relative refractive index difference may be uniformly increased from one end of the optical fiber to the other end. The more the relative refractive index difference varies in the optical fiber length direction, the greater the scattering suppressing effect. However, those in which optical fibers having different refractive index distributions are connected in multiple stages, or those in which the relative refractive index difference changes abruptly, such as those in which the relative refractive index difference changes in a rectangular wave, are not preferable because they cause an increase in transmission loss. The degree of change in the relative refractive index difference is preferably moderate so that the transmission loss at the used wavelength is 0.5 dB / km or less.

In the dispersion compensating optical fiber in which the relative refractive index difference changes in the lengthwise direction, for example, when a preform having a uniform relative refractive index difference in the longitudinal direction is prepared and the preform is drawn. By changing the stress on the core, the residual strain of the core can be changed, and as a result, the refractive index of the core can be changed. That is, first, a base material composed of a core and a clad is formed to have a refractive index distribution that satisfies the conditions shown in FIG. At this time, when the base material is in a molten state in the subsequent drawing step, the type and amount of dopant are set so that the viscosity of the glass forming the core is higher than the viscosity of the glass forming the clad. . Further, the outer diameter and the refractive index distribution of the base material are made uniform in the longitudinal direction.

By thus providing a difference in viscosity between the core and the clad during melting, the core is first solidified when the base material is melt-drawn and solidified, and the clad is completely completed with a slight delay. To be solidified. The core of the single-mode optical fiber formed by drawing has a very small cross-sectional area of about 0.5 to 1.0% of the clad, so that the core is solidified and the clad is not solidified. Tension for drawing the optical fiber is selectively applied to the core. Therefore, the core is solidified in a tensioned state, and thereafter the clad on the periphery thereof is solidified, so that in the obtained optical fiber, tensile strain due to the linear tension force remains in the core. Then, the drawing speed and the heating temperature during drawing are changed continuously or periodically to change the drawing force, and thereby the strain remaining in the core of the optical fiber is changed in the length direction of the optical fiber. be able to. Such a change in strain remaining in the core causes a change in the refractive index of the core due to the photoelastic effect, and an optical fiber having a changed relative refractive index difference can be obtained.

Alternatively, when the base material is formed, the amount of the dopant added to the core or the clad is changed in the length direction of the optical fiber to change the relative refractive index difference, and the base material is drawn by a usual drawing method. Also by spinning, a dispersion compensating optical fiber whose relative refractive index changes in the length direction can be obtained.

In the first embodiment, the core diameter is changed in the lengthwise direction of the optical fiber, and in the second embodiment, the relative refractive index difference is changed in the lengthwise direction of the optical fiber. In the above, at least one of the core diameter and the relative refractive index difference may be changed, and both may be changed.

[0023]

【Example】

Example 1 A dispersion compensating optical fiber having a core diameter varying in the length direction was manufactured as follows. An optical fiber preform having a unimodal refractive index distribution was manufactured by the VAD method. The relative refractive index difference in the central portion of the core was 2.2%, and the thickness of the clad was clad diameter / core diameter = about 54. The drawing temperature, the drawing speed, and the feeding speed of the base material during the drawing of the base material are periodically changed so that the outer diameter of the optical fiber is periodically changed into a triangular wave shape as shown in FIG. did. The average value of the outer diameter of the optical fiber is 125 μm, and the variation of the outer diameter is ±
5%. The change cycle is every 500m, and the total length is 5km.
Was obtained. Wavelength of the obtained optical fiber 1.5
The wavelength dispersion value at 5 μm is −50.7 ps / km / n
m.

With respect to the obtained dispersion-compensating optical fiber, in order to evaluate the degree of change in the Brillouin shift frequency, the gain was measured by Brillouin amplification using the measurement system shown in FIG. This measurement system oscillates a light having a wavelength of 1.55 μm from a semiconductor laser 1, amplifies this laser light by an optical amplifier 2, and then makes it enter one side of a measuring optical fiber 3 and a white light from a white light source 4. Is incident on the measuring optical fiber 3 from the other side. Then, the Stokes light is induced by the wavelength component of the white light and deviates from the wavelength of the laser light (1.55 μm) by the Brillouin shift frequency, and the Stokes light is generated. It is amplified (Brillouin amplification) and emitted from one of the measuring optical fibers 3. This back scattered light is measured by the light receiving element 5,
If analyzed by the optical spectrum analyzer 6, a gain peak due to Brillouin amplification can be obtained. The result measured in this way is shown by a solid line in FIG. In FIG. 4, the horizontal axis represents the Brillouin shift frequency, and the vertical axis represents the gain with respect to the incident laser light. The wider the gain peak, the greater the change in the Brillouin shift frequency and the lower the gain peak height. This indicates that stimulated Brillouin backscattering is suppressed.

With respect to the dispersion compensating optical fiber obtained in this example, the threshold value of stimulated Brillouin scattering was measured using the measuring system shown in FIG. In FIG. 5, a single mode CW semiconductor laser with an external resonator was used as a light source, and a narrow line width pulse of 500 kHz was confirmed at a wavelength of 1.55 μm. The LD output is amplified by the Er-doped optical fiber amplifier 11, and after passing through the optical isolator 12, the amplified signal is branched by the optical fiber coupler 13 at about 8: 2. The 20% branched signal light is guided to the first power meter 14 and the incident light intensity is monitored. The 80% branched signal light is directly guided to the test fiber 15. On the other hand, the transmitted light intensity is measured from the output end of the test fiber 15 to the optical isolator 16
After passing through, it is detected by the second power meter 17. Further, the backscattered light generated in the test fiber passes through the optical fiber coupler 13 again, and is then detected by the third power meter 18.

FIG. 6 shows changes in the forward scattered light and the back scattered light when the intensity of the incident light is changed.
The backscattered light exhibits a non-linear behavior that sharply rises from a certain point when the incident light intensity increases. This is a change corresponding to the occurrence of stimulated Brillouin scattering. On the other hand, when the incident light intensity is low, the backscattered light shows a linear change. This linear change was used as the background in the backscattered light and was shown by the broken straight line. The deviation from this straight line is taken as the contribution due to stimulated Brillouin scattering, and the intensity of the incident light when it becomes 3 dB higher than the background was taken as the threshold value for stimulated Brillouin scattering. As indicated by the solid line in the figure, the threshold value of the stimulated Brillouin scattering of the dispersion compensating optical fiber of this example was about 12 dBm.

(Comparative Example 1) A base material similar to that of the above-mentioned Example 1 was prepared, and the drawing temperature, drawing speed, and
An optical fiber having a total length of 5 km was obtained by spinning so that the outer diameter of the optical fiber was constant without changing the feed rate of the base material.
The wavelength dispersion value of the obtained optical fiber at a wavelength of 1.55 μm was −55.0 ps / km / nm. Further, the gain was measured by Brillouin amplification in the same manner as in Example 1 above. The measurement result is shown by a broken line in FIG. Further, when the threshold value of stimulated Brillouin scattering was measured in the same manner as in Example 1, it was about 8 dBm as shown by the broken line in FIG.

From the result of FIG. 4, the Brillouin gain width of the dispersion compensating optical fiber obtained in Example 1 is widened from 30 MHz to 50 MHz as compared with that obtained in Comparative Example 1, and the Brillouin gain is also reduced. It was recognized that
This is because the core diameter of the optical fiber is non-uniform in the length direction, so the Brillouin shift frequency in each part in the length direction becomes non-uniform, which causes the center frequency of the stimulated Brillouin backscattering in each part in the length direction of the optical fiber. Shows that stimulated Brillouin backscattering over the entire length of the optical fiber is suppressed. Moreover, from the result of FIG.
It was confirmed that the dispersion-compensating optical fiber obtained in Example 1 had a higher threshold value than that obtained in Comparative Example 1.

Example 2 A dispersion compensating optical fiber having a relative refractive index difference changing in the length direction was manufactured as follows. An optical fiber preform having a unimodal refractive index distribution was manufactured by the VAD method. The soot deposition conditions were controlled so that the amount of the dopant added to the core during the production of the base material changed periodically in the length direction. The average value of the relative refractive index difference in the central portion of the core was 2.3%, and the change amount of the relative refractive index difference was ± 5%. The change cycle was 500 m, and the clad thickness was clad diameter / core diameter = about 54. This base material was spun so that the outer diameter of the optical fiber was constant, and an optical fiber having a total length of 5 km was obtained. Wavelength of the obtained optical fiber 1.55 μm
The chromatic dispersion value in was 50.9 ps / km / nm. The threshold value of the stimulated Brillouin scattering of the obtained dispersion-compensating optical fiber was measured in the same manner as in Example 1 and found to be about 12 dBm, which is higher than that obtained in Comparative Example 1 above. It was found to be increasing.

Example 3 A dispersion compensating optical fiber in which the core diameter and the relative refractive index difference were changed in the lengthwise direction was manufactured as follows. An optical fiber preform having a unimodal refractive index distribution was manufactured by the VAD method. The soot deposition conditions were controlled so that the amount of the dopant added to the core during the production of the base material changed periodically in the length direction. The average value of the relative refractive index difference in the central portion of the core was 2.3%, and the change amount of the relative refractive index difference was ± 5%. The change cycle was 500 m, and the clad thickness was clad diameter / core diameter = about 54. The drawing temperature, the drawing speed, and the base material feeding speed at the time of drawing the base material were periodically changed, and spinning was performed so that the outer diameter of the optical fiber was changed periodically. The average value of the optical fiber outer diameter is 12
At 5 μm, the change in outer diameter was ± 5%. The cycle of change was set to every 500 m, and an optical fiber having a total length of 5 km was obtained. The wavelength dispersion value of the obtained optical fiber at a wavelength of 1.55 μm was -50.3 ps / km / nm. The threshold value of the stimulated Brillouin scattering of the obtained dispersion compensating optical fiber was measured in the same manner as in Example 1 and found to be about 14d.
It was Bm, and it was confirmed that the threshold value was increased as compared with that obtained in Comparative Example 1.

[0031]

The wavelength used in the dispersion compensating optical fiber of the present invention has a smaller core diameter than that of a silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band, and has a core-clad relative refractive index difference. A dispersion-compensating optical fiber formed to be large in size, characterized in that at least one of a core diameter and a core-clad relative refractive index difference is changed in the length direction of the optical fiber. According to the present invention, even in a dispersion compensating optical fiber having a large absolute value of chromatic dispersion value, the amount of stimulated Brillouin backscattering can be suppressed, the threshold value of scattering can be improved, and the incident power can be increased. Can be increased. Therefore, by inserting and using the dispersion compensating optical fiber of the present invention in an optical communication system which has already been constructed with a normal silica optical fiber, the existing system can be used with an optical amplifier of 1.5 μm.
It became possible to reconstruct an m-band optical communication system,
Moreover, since the incident light power can be increased, it is possible to further increase the transmission distance and increase the transmission capacity.

[Brief description of drawings]

FIG. 1 is a graph showing changes in core diameter in an embodiment of the present invention.

FIG. 2 is a graph showing a preferable range of the core diameter and the relative refractive index difference in the embodiment of the present invention.

FIG. 3 is a schematic configuration diagram showing a Brillouin gain measurement system in an example of the present invention.

FIG. 4 is a graph showing Brillouin gain measurement results in an example of the present invention.

FIG. 5 is a schematic configuration diagram showing a Brillouin scattered light measurement system in an example of the present invention.

FIG. 6 is a graph showing incident light power dependence of forward and backward scattered light due to Brillouin scattering in the example of the present invention.

[Explanation of symbols]

 1 ... Light source, 3, 15 ... Dispersion compensation optical fiber.

Claims (3)

[Claims] 1. A dispersion compensating optical fiber having a smaller core diameter and a larger core-clad relative refractive index difference than a silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band. A dispersion compensating optical fiber, wherein at least one of the core diameter and the core-clad relative refractive index difference is changed in the length direction of the optical fiber. 2. A chromatic dispersion value at a used wavelength is -50p.
2. The dispersion compensating optical fiber according to claim 1, wherein the dispersion compensating optical fiber is s / km / nm or less, and the variation of the core diameter in the length direction of the optical fiber is ± 5% or more. 3. The chromatic dispersion value at the used wavelength is -50p.
2. The dispersion compensating optical fiber according to claim 1, wherein the dispersion compensating optical fiber is s / km / nm or less, and the variation of the core-cladding relative refractive index difference in the optical fiber length direction is ± 5% or more.