JPH1096828A - Stimulated Brillouin scattering suppression optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stimulated Brillouin scattering-suppressed optical fiber which is easy to manufacture, has almost zero chromatic dispersion throughout the longitudinal direction of the optical fiber, and has low optical loss. SOLUTION: The relationship that the chromatic dispersion with respect to a designated wavelength of an optical communication signal becomes zero can be obtained by changing the relative refractive index difference of the optical fiber and the core diameter in the same direction. Refractive index difference and core diameter R
Is changed in the longitudinal direction of the optical fiber, and the core diameter R is reduced as the relative refractive index difference of the core decreases, and the core diameter R is increased as the refractive index difference of the core increases. Is formed substantially equal to zero over the entire area in the longitudinal direction of the optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stimulated Brillouin scattering suppression optical fiber mainly used for optical communication.

[0002]

2. Description of the Related Art In optical communication using an optical fiber, an attempt has been made to increase the intensity of signal light input to the optical fiber in order to increase the relay interval between a light transmitting section and a receiving section. In recent years, the development of erbium-doped fiber amplifiers (EDFAs) has enabled high-intensity light to enter optical fibers.

By the way, when the development of the EDFA allows high-intensity light to enter the optical fiber, a nonlinear phenomenon occurs in the optical fiber, and it becomes necessary to consider this nonlinear phenomenon in optical communication. It became so. Non-linear phenomena include self-phase modulation, cross-phase modulation,
Although there are characteristics such as four-wave mixing (FWM), FWM is an effective characteristic that can be used as a light source for wavelength division multiplexing transmission (WDM). It has been confirmed that the nonlinear effect is generated more efficiently as the incident light intensity on the optical fiber increases.

However, when signal light is input to an optical fiber and transmitted, stimulated Brillouin scattering (SBS)
Therefore, there is a problem that even if a very strong input signal light is input to the optical fiber, the transmitted light power cannot be increased. The stimulated Brillouin scattering in the optical fiber is one of nonlinear phenomena, and is caused by inelastic scattering between incident light and acoustic phonons in the fiber. The stimulated Brillouin scattering causes the light signal to be scattered backward, and the degree increases sharply when the threshold is exceeded.
Due to the increase in stimulated Brillouin scattering, the transmitted light power hardly changes even if the incident light power is increased beyond the threshold, so that stimulated Brillouin scattering obtains a nonlinear effect like FWM in optical communication. This is a major obstacle when trying to do so.

It is known that the stimulated Brillouin scattering is more likely to occur as the fiber structure such as the refractive index of the optical fiber becomes more uniform. Therefore, in order to suppress the stimulated Brillouin scattering, it is necessary to reduce the stimulated Brillouin scattering in the longitudinal direction of the optical fiber. What is necessary is to give a change to make the structure of the optical fiber non-uniform in the longitudinal direction.

Accordingly, for example, Japanese Patent Application Laid-Open No. 5-249329
As disclosed in Japanese Patent Application Laid-Open Publication No. H10-163, in an optical fiber having a core containing GeO ₂ (germanium oxide) -doped quartz as a main component and a clad containing pure quartz as a main component, these cores and the cladding have F (fluorine). ), And the F dopant concentration is continuously changed in the longitudinal direction of the core and the clad, so that the refractive index of the core and the refractive index of the clad are continuously changed in the longitudinal direction of the core and the clad, respectively. An optical fiber of the Brillouin scattering suppression type is obtained.

Since the optical fiber is required to have uniform chromatic dispersion in the longitudinal direction, the refractive index distribution of the cross section of the optical fiber is determined by the maximum refractive index of the core. The refractive index distribution obtained by standardizing with the index is the longitudinal direction of the optical fiber (axial direction)
(The relative refractive index of the cladding with respect to the maximum refractive index of the core is formed to be equal over the entire length of the core and the cladding), whereby the chromatic dispersion characteristic of the optical fiber in the longitudinal direction is reduced. We are aiming for uniformity.

[0008]

However, if both the core and the clad are doped with F, as in the optical fiber of this proposal, the diffusion of GeO ₂ doped in the core occurs due to the influence of the doping of F, As a result, there has been a problem that the transmission loss of the optical fiber increases.

[0009] Further, when producing the optical fiber of this proposal, when producing an optical fiber preform for forming the optical fiber, an optical cladding portion (from the core) which affects the dispersion characteristics of the core and the optical fiber. It is necessary to simultaneously synthesize the core (the part where light seeps into the cladding side) and the core.
(Vapor-phase Axial Deposition) method.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to make it easy to manufacture and to make the chromatic dispersion in the longitudinal direction of an optical fiber almost zero in a used wavelength band. It is an object of the present invention to provide a stimulated Brillouin scattering suppression optical fiber with a possible low optical loss.

[0011]

Means for Solving the Problems To achieve the above object, the present invention provides means for solving the problems by the following constitution. That is, the first invention provides an optical fiber having a characteristic that the relationship that the chromatic dispersion with respect to a designated wavelength of an optical communication signal becomes zero is obtained by changing the relative refractive index difference and the core diameter of the optical fiber in the same direction. The relative refractive index difference and the core diameter of at least the core of the optical fiber formed by forming the cladding on the outer peripheral side of the core are changed in the longitudinal direction of the optical fiber, and the relative refractive index difference of the core is monotonically small. As the core diameter monotonically decreases while maintaining the characteristic that the chromatic dispersion in the specified wavelength band becomes substantially zero, the chromatic dispersion in the specified wavelength band increases as the relative refractive index difference of the core increases monotonically. It is characterized in that it is formed monotonically large while maintaining the characteristic of becoming substantially zero, and the chromatic dispersion in the designated wavelength band is made substantially zero over the entire area in the longitudinal direction of the optical fiber.

In the second invention, the relationship that the chromatic dispersion with respect to the designated wavelength of the optical communication signal becomes zero has a characteristic obtained by changing the relative refractive index difference of the optical fiber and the core diameter in different directions. The optical fiber having a clad formed on the outer peripheral side of the core has a relative refractive index difference and a core diameter of at least the core that change in the longitudinal direction of the optical fiber, and the relative refractive index difference of the core is As the diameter decreases monotonically, the core diameter increases monotonically while maintaining the characteristic that the chromatic dispersion in the specified wavelength band becomes almost zero, and as the relative refractive index difference of the core increases monotonically, the core diameter increases in the specified wavelength band. The chromatic dispersion is monotonically reduced while maintaining the characteristic that the chromatic dispersion is substantially zero, and the chromatic dispersion in the designated wavelength band is formed to be substantially zero over the entire length of the optical fiber in the longitudinal direction.

Further, according to a third aspect of the present invention, a clad having a smaller refractive index than the core is disposed outside the core so as to surround the core, and the core surrounds the center core at the center and the center core. An optical fiber having a refractive index smaller than that of the center core and a side core having a larger refractive index than the cladding, and having a step-shaped refractive index distribution, wherein the outer diameter of the side core is substantially constant in the longitudinal direction of the optical fiber. The ratio between the relative refractive index difference of the center core and the relative refractive index difference of the side cores maintains the characteristic of making the chromatic dispersion of the optical communication signal in the specified wavelength band substantially zero over the entire length of the optical fiber. It is characterized in that it is varied in the longitudinal direction of the fiber.

Further, in one of the first to third aspects of the present invention, one of the features is that the designated wavelength of the optical communication signal is 1.55 μm.

In the present invention having the above structure, the relative refractive index difference and the core diameter are formed in correspondence with the relationship between the zero dispersion wavelength of the optical fiber, the relative refractive index difference, and the core diameter. 1
In the invention, the core diameter is monotonically reduced as the relative refractive index difference of the core monotonically decreases, and the core diameter is formed monotonically large as the relative refractive index difference of the core monotonically increases, thereby using Since the chromatic dispersion in the wavelength band (specified wavelength band) is formed to be substantially zero over the entire length of the optical fiber in the longitudinal direction, the increase in the chromatic dispersion of the optical communication signal can be suppressed. In the second invention,
As the relative refractive index difference of the core monotonically decreases, the core diameter is monotonically increased, and as the relative refractive index difference of the core monotonically increases, the core diameter is formed monotonically small. Since the dispersion is formed to be substantially zero over the entire area in the longitudinal direction of the optical fiber, the increase in the chromatic dispersion of the optical communication signal can be similarly suppressed.

As described above, since the relative refractive index difference and the core diameter of the core of the optical fiber change in the longitudinal direction of the optical fiber, the optical fibers of the first and second inventions both Stimulated Brillouin scattering is suppressed by making the structure of the optical fiber non-uniform in the longitudinal direction.

Further, also in the third invention, since the characteristic that the chromatic dispersion is substantially zero is maintained over the entire region in the longitudinal direction of the optical fiber, it is possible to suppress an increase in the chromatic dispersion of the optical communication signal. Moreover, since the ratio of the relative refractive index difference between the center core and the side core is changed in the longitudinal direction of the optical fiber, the effect of suppressing stimulated Brillouin scattering is obtained in the same manner as in the first and second inventions. Can be

Furthermore, unlike the conventional optical fiber in which stimulated Brillouin scattering is suppressed by changing the F doping concentration in the longitudinal direction of the optical fiber, the optical fiber of the present invention causes transmission loss due to GeO ₂ diffusion accompanying F doping. In addition, since it is not necessary to simultaneously synthesize the core portion and the optical clad portion of the optical fiber preform at the time of producing the optical fiber, the production of the optical fiber preform becomes easy. Therefore, the optical fiber of the present invention is easy to produce, and is an optical fiber that suppresses stimulated Brillouin scattering with low optical loss, and has an excellent effect that chromatic dispersion in the longitudinal direction of the optical fiber can be made almost zero. is there.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the first and second embodiments of the stimulated Brillouin scattering suppression optical fiber according to the present invention. FIGS. 2 and 3 show the first and second embodiments.
The refractive index distribution structures of the stimulated Brillouin scattering suppression optical fiber of the embodiment are shown respectively. As shown in these figures, the stimulated Brillouin scattering suppression optical fiber of the first and second embodiments is an optical fiber formed by forming a clad 9 on the outer peripheral side of a core 8 and has a length of 20 km. It is. In the drawing, R indicates the diameter (diameter) of the core 8, R _IN indicates the core diameter at the optical fiber input end, R _EX indicates the core diameter at the optical fiber output end, and Δ + indicates the core 8.
3 shows a relative refractive index difference with respect to the cladding 9.

As shown in FIGS. 2A and 2B,
The refractive index distribution structure (profile) of the stimulated Brillouin scattering suppression optical fiber of the first embodiment is a single-peak type, while the stimulated Brillouin scattering suppression optical fiber of the second embodiment is the same as that of FIG. As shown in FIGS. 2 and 3, the core 8 includes a center core 8a and a side core 8b surrounding the outer periphery of the center core 8a.
Is the relative refractive index difference Δc of the center core 8a.
+ (Δc + = Δ +), and has a stepped profile. In FIGS. 2 and 3, Δ + IN is the relative refractive index difference of the core 8 at the input end of the optical fiber, and Δ + EX is the core 8 at the output end of the optical fiber.
Are shown, respectively.

As shown in FIG. 1 to FIG.
In the stimulated Brillouin scattering suppressing optical fiber of the embodiment, the diameter R of the core 8 and the relative refractive index difference Δ + of the core 8 change in the longitudinal direction of the optical fiber. As the relative refractive index difference Δ + of the core 8 decreases monotonically, the diameter R of the core 8 decreases monotonically, and as the relative refractive index difference Δ + of the core 8 increases monotonically, the diameter R of the core 8 increases monotonically. , Thereby using wavelength band (1.55μm band)
Is formed to be substantially zero over the entire length of the optical fiber in the longitudinal direction. Each of the optical fibers of the first and second embodiments has the characteristics shown in the area a in FIG. That is, the relationship that the chromatic dispersion of the optical communication signal with respect to the specified wavelength (1.55 μm) becomes zero over the entire length of the fiber is the relative refractive index difference Δ of the optical fiber.
This is obtained by changing the relative refractive index difference Δ + of the core and the radius of the core 8 in the same direction.

By the way, in the case of the step index optical fiber having a stepped refractive index profile like the optical fiber of this embodiment, for example, the value of the zero dispersion wavelength λ ₀ of the optical fiber is 1.55 μm and the stepped optical fiber has a stepped type. The relationship between the relative refractive index difference Δ + of the core and the radius a of the core in the optical fiber having the profile becomes a relationship as shown in FIG. 4 when calculated by a known method. The slope is reversed. The optical fiber having the relationship shown in FIG. 4, when the ratio of the .DELTA.c + and Delta] s + and R _d, the core diameter ratio of the center core and the side cores and R _a, R
_This is an optical fiber in which _d is 0.2 and Ra is 0.4.

Here, in this optical fiber, the characteristics necessary to keep the zero dispersion wavelength λ ₀ at 1.55 μm while actually changing the relative refractive index difference Δ are the characteristic line in region a in FIG. The characteristic line has one of the characteristic lines. The present applicant has determined that the relationship of the characteristic line A shown in FIG.
Relationship and the cutoff wavelength lambda _c and, the relationship between the characteristic line B, that focuses on the relationship between the mode field diameter of the relative refractive index difference Δ and the optical fiber. When the characteristic line of FIG. 4 corresponds to the characteristic line A of FIG. 5, the region a in FIG.
Corresponds to the characteristic line A in the region b ', and conversely,
The characteristic line of the area b in FIG. 4 is the area a 'in FIG.
Corresponds to the characteristic line A.

On the other hand, the characteristic line B in FIG. 5 shows the relationship between the relative refractive index difference and the mode field diameter. The mode field diameter of the optical fiber becomes smaller as the radius a of the core becomes larger, and vice versa. In consideration of the fact that the core radius a increases as the radius a decreases, the characteristic line B and the characteristic line in FIG. 4 correspond to each other, and the characteristic line of the region a in FIG. Corresponding to the line B, the characteristic line of the region b in FIG. 4 corresponds to the characteristic line B of the region a ″ in FIG.

Therefore, when the characteristic line in the region b in FIG. 4 is selected, it can be seen from FIG. 5 that the cutoff wavelength λ _c is 500 nm.
(0.5 μm) and the mode field diameter is about 14 μm, which makes it impossible to satisfy the propagation condition of the optical signal in the wavelength band of 1.5 μm. Therefore, the relative refractive index difference Δ
A practically useful optical fiber capable of maintaining the zero dispersion wavelength λ ₀ at 1.55 μm while changing the optical characteristics has characteristics within the region a in FIG. The optical fiber according to the present embodiment has the characteristics in the region a in FIG. 4, that is, the specified wavelength 1.55 μm over the entire region in the longitudinal direction of the fiber length.
Is maintained by changing the relative refractive index difference Δ of the optical fiber and the radius of the core 8 in the same direction, which is obtained by the present inventors. Has been confirmed by

It is to be noted that a relationship similar to that shown in FIG. 4 can be obtained for an optical fiber having a single-peak type refractive index distribution profile, and the same application as an optical fiber having a step-shaped profile is possible.

Table 1 shows the results obtained by summarizing the parameters such as the refractive index distribution structure for the stimulated Brillouin scattering suppression optical fiber of the first and second embodiments. Table 1 shows that the diameter R of the core 8 and the relative refractive index difference Δ + do not change in the longitudinal direction of the optical fiber as Comparative Example A, and the relative refractive index of the core 8 as Comparative Example B. The difference Δ + does not change in the longitudinal direction of the optical fiber, and the results similarly summarized for an optical fiber in which the diameter R of the core 8 changes in the longitudinal direction of the optical fiber are also shown.

[0028]

[Table 1]

The optical fibers of the first and second embodiments and the optical fibers of Comparative Examples A and B both form a core 8 (a center core 8a and a side core 8b in the case of a stepped profile). Ge-doped quartz soot is formed using the VAD method, and a cladding 9
After pure silica soot was formed and formed, the glass was vitrified to produce an optical fiber preform, and thereafter, an optical fiber was drawn.

In the optical fibers of the first and second embodiments and the optical fiber of Comparative Example A, the center core 8a or the Ge of the burner for forming the core 8 is synthesized during VAD synthesis.
By changing the concentration, the center core 8a or the core 8
The relative refractive index difference Δ + (Δ + = Δc +) was changed in the longitudinal direction of the optical fiber. When the diameter of the core 8 is changed in the longitudinal direction of the optical fiber as in the first and second embodiments and the comparative example B, after the Ge-doped quartz soot is synthesized, the core 8 is cut by external machining. The diameter R was changed in the longitudinal direction, and thereafter pure silica soot was deposited.

As is apparent from Table 1, in the stimulated Brillouin scattering suppression optical fiber of the first and second embodiments, the Δ change rate (% /
km) are 0.025 and 0.019, respectively. In the optical fibers of Comparative Examples A and B, the value of the rate of change is 0.
And

Table 2 shows the chromatic dispersion characteristics, the stimulated Brillouin scattering (SBS) generation threshold, and the presence or absence of FWM light for the optical fibers of the first and second embodiments and the optical fibers of Comparative Examples A and B. The results of an actual examination by the present applicant are shown.

[0033]

[Table 2]

In Table 2, the short dispersion IN indicates the chromatic dispersion measurement result at 1.3 m of the optical fiber incident end side, and the short dispersion EX indicates the chromatic dispersion measurement result at 1.3 m of the optical fiber exit end side. , And the average dispersion shows the results of chromatic dispersion measurement over the entire length of the optical fiber.

As is clear from Table 2, the SBS occurrence threshold values of the first and second embodiments are 15.0 dB, respectively.
m, 13.0 dBm, and it was confirmed that the SBS suppression effect was high, and high intensity FWM light could be obtained.

According to the present embodiment, the relationship that the chromatic dispersion of the optical communication signal with respect to the designated wavelength becomes zero has a characteristic obtained by changing the relative refractive index difference of the optical fiber and the core diameter in the same direction. The relative refractive index difference Δ + of the core 8 and the diameter R of the core 8 change in the longitudinal direction of the optical fiber, and as the relative refractive index difference Δ + of the core 8 decreases monotonically, the diameter of the core 8 decreases. Since R is monotonically reduced and the diameter R of the core 8 is monotonically increased as the relative refractive index difference Δ + of the core 8 monotonically increases, the 1.55 μm band which is the used wavelength band (specified wavelength band) is used. Can be formed substantially equal to zero over the entire length of the optical fiber in the longitudinal direction.

The optical fiber of this embodiment has a structural change in the longitudinal direction due to a change in the relative refractive index difference Δ + of the core 8 and the diameter of the core 8 in the longitudinal direction. Can be effectively suppressed.

Furthermore, unlike the proposed optical fiber in which the F-doping concentration is changed in the longitudinal direction of the optical fiber, the optical fiber of the present embodiment does not need to combine the optical cladding with the core 8 at the same time. Since only the core portion for forming the core 8 can be easily formed by using the VAD method and then the cladding portion can be deposited and formed on the outer peripheral side, the production of the optical fiber preform becomes easy and the optical fiber can be easily formed. Ge in the core with F doping
An optical fiber with low optical loss can be obtained without increasing the transmission loss of the optical fiber by O ₂ diffusion.

The present invention is not limited to the above-described embodiment, but can adopt various embodiments. For example,
The parameters such as the profile and length of the optical fiber, the relative refractive index difference Δ + of the core 8 and the diameter R of the core 8 are not necessarily shown in Table 1.
The relationship that the chromatic dispersion for the specified wavelength of the optical communication signal becomes zero is not limited to the value as shown in the above, and the characteristic obtained by changing the relative refractive index difference of the optical fiber and the core diameter in the same direction is large and small. It is sufficient if the optical fiber has a specific refractive index difference between the core and the core diameter in the longitudinal direction of the optical fiber, and the core diameter decreases as the relative refractive index difference of the core decreases. By increasing the core diameter as the value increases, it becomes possible to form an optical fiber whose chromatic dispersion in the used wavelength band is almost equal to zero over the entire length of the optical fiber in the longitudinal direction.

For example, contrary to the above embodiment, the relative refractive index difference Δ + IN of the core at the optical fiber input end may be smaller than the relative refractive index difference Δ + EX of the core at the optical fiber output end. Has a relationship of core diameter IN <core diameter EX.

The relationship that the chromatic dispersion with respect to the designated wavelength of the optical communication signal becomes zero is an optical fiber having characteristics obtained by changing the relative refractive index difference and the core diameter of the optical fiber in different directions. At this time (for example, when the characteristic line has the characteristic line in the region b in FIG. 4), the relative refractive index difference and the core diameter of the core are changed in the longitudinal direction of the optical fiber, and
As the relative refractive index difference of the core decreases, the core diameter increases, and as the relative refractive index difference of the core increases, the core diameter decreases, and the chromatic dispersion in the used wavelength band is made uniform over the entire length of the optical fiber in the longitudinal direction. do it. However,
In this case, the cutoff wavelength is around 0.5 μm as described above.

Further, in the above-described embodiment, the relative refractive index difference of the core 8 and the diameter R of the core 8 are both changed in the longitudinal direction of the optical fiber. However, as in the second embodiment, In an optical fiber having a stepped profile having a center core 8a and a side core 8b, the ratio of the relative refractive index difference between the center core 8 and the side core 8b is changed without changing the diameter R of the core 8 in the longitudinal direction of the optical fiber. A stimulated Brillouin scattering suppression optical fiber can also be formed.

For example, the present applicant has actually determined that the relative refractive index difference (core Δ + IN) of the core 8 on the optical fiber incident end side is 1.23%,
The relative refractive index difference (core Δ + E
X) is 0.93%, and the relative refractive index difference Δs + of the side core 8b is 0.
08%, and an optical fiber in which the diameter R of the core 8 is 4.01 μm over the entire region in the longitudinal direction of the optical fiber.
%, And the rate of change in core Δ is 0.015% / km). The dispersion, loss and the like shown in Table 2 were measured for this optical fiber, and the short dispersion IN was +0.12 ps.
/ Nm / km, short dispersion EX is +0.08 ps / nm / km, average dispersion is +0.10 ps / nm / km, loss is 0.36 dB / km, and chromatic dispersion is almost zero over the entire area in the longitudinal direction of the optical fiber. And the optical transmission loss can be reduced, and the SBS occurrence threshold value can be reduced.
It was as high as 12.0 dBm, and generation of high intensity FWM light was also confirmed.

[0044]

According to the first and second aspects of the present invention, the structure in the longitudinal direction of the optical fiber is changed by changing the relative refractive index difference and the core diameter of the core of the optical fiber in the longitudinal direction of the optical fiber. In addition to being able to effectively suppress the generation of stimulated Brillouin scattering by giving a change, the relationship that the chromatic dispersion with respect to the specified wavelength of the optical communication signal becomes zero depends on the relative refractive index difference of the optical fiber and the core diameter in the same direction or By changing the relative refractive index difference of the core and the core diameter in accordance with the characteristics obtained by changing the size in different directions, the chromatic dispersion of the used wavelength band can be reliably ensured over the entire longitudinal direction of the optical fiber. It can be formed equally.

In addition, when the structural change in the longitudinal direction of the optical fiber is given, for example, the conventional proposed stimulated Brillouin scattering suppressing optical fiber in which the F-doping concentration is changed in the longitudinal direction of the optical fiber to cause a change in the refractive index. No,
An optical fiber preform for forming an optical fiber can be easily produced, and transmission loss of the optical fiber does not increase due to GeO ₂ diffusion in the core accompanying F doping, and the production is easy, and An optical fiber with low optical loss can be obtained.

Further, in the invention according to the third aspect, the refractive index distribution is stepped and the outer diameter of the side core is made substantially constant in the longitudinal direction of the optical fiber. , The increase in chromatic dispersion of the optical communication signal can be suppressed, and the ratio of the relative refractive index difference between the center core and the side cores in the longitudinal direction of the optical fiber. Since it has been changed, the effect of suppressing stimulated Brillouin scattering can be obtained in the same manner as in the first and second aspects of the present invention.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a stimulated Brillouin scattering suppression optical fiber according to the present invention.

FIG. 2 is an explanatory diagram showing a refractive index distribution structure of a first embodiment of the stimulated Brillouin scattering suppression optical fiber according to the present invention.

FIG. 3 is an explanatory diagram showing a refractive index distribution structure of a second embodiment of the stimulated Brillouin scattering suppression optical fiber according to the present invention.

FIG. 4 is a graph showing an example of the relationship between the relative refractive index difference and the radius of the core so that the zero dispersion wavelength of the step index optical fiber becomes 1.55 μm.

FIG. 5 is a graph showing an example of the relationship between the relative refractive index difference of the step index optical fiber, the cutoff wavelength, and the mode field diameter.

[Explanation of symbols]

 8 core 8a center core 8b side core 9 clad

Claims (4)

[Claims] An optical fiber having a characteristic that the chromatic dispersion of an optical communication signal with respect to a designated wavelength becomes zero is obtained by changing a relative refractive index difference and a core diameter of the optical fiber in the same direction. The relative refractive index difference and the core diameter of at least the core of the optical fiber having the clad formed on the outer peripheral side of the core are changed in the longitudinal direction of the optical fiber, and as the relative refractive index difference of the core decreases monotonically, the core becomes smaller. The diameter is monotonically reduced while maintaining the characteristic that the chromatic dispersion in the specified wavelength band is substantially zero, and the chromatic dispersion in the specified wavelength band is substantially zero as the relative refractive index difference of the core monotonically increases. An stimulated Brillouin scattering suppression optical fiber, characterized in that the optical fiber is formed monotonically large while maintaining the following characteristics, and the chromatic dispersion in a specified wavelength band is formed to be substantially zero over the entire longitudinal direction of the optical fiber. 2. An optical fiber having characteristics obtained by changing the relative refractive index difference and the core diameter of an optical fiber in directions different from each other in such a manner that the chromatic dispersion of the optical communication signal with respect to a designated wavelength becomes zero, The relative refractive index difference and the core diameter of at least the core of the optical fiber having the clad formed on the outer peripheral side of the core are changed in the longitudinal direction of the optical fiber, and as the relative refractive index difference of the core decreases monotonically, the core becomes smaller. The diameter is monotonically increased while maintaining the characteristic that the chromatic dispersion in the specified wavelength band is substantially zero, and the chromatic dispersion in the specified wavelength band is substantially zero as the relative refractive index difference of the core monotonically increases. An stimulated Brillouin scattering suppression optical fiber, characterized in that the chromatic dispersion in a specified wavelength band is formed to be substantially zero over the entire longitudinal direction of the optical fiber, while being monotonically reduced while maintaining the following characteristics. 3. A cladding surrounding the core and having a lower refractive index than the core is disposed outside the core, the core comprising:
An optical fiber comprising a center core at a central portion and a side core surrounding the center core and having a smaller refractive index than the center core and a larger refractive index than the clad, and having a step-shaped refractive index distribution. The outer diameter of the optical fiber is substantially constant in the longitudinal direction of the optical fiber, and the ratio of the relative refractive index difference of the center core to the relative refractive index difference of the side core indicates the chromatic dispersion in the specified wavelength band of the optical communication signal. An stimulated Brillouin scattering suppression optical fiber characterized in that the optical fiber is changed in the longitudinal direction of the optical fiber while maintaining a characteristic of being substantially zero in all directions. 4. The stimulated Brillouin scattering suppression optical fiber according to claim 1, wherein the designated wavelength of the optical communication signal is 1.55 μm.