JP3386948B2 - Optical fiber - Google Patents

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 injecting and transmitting a high level of light while suppressing the occurrence of stimulated Brillouin scattering.

[0002]

2. Description of the Related Art In recent years, a large output light can be obtained by an optical amplifier such as a high output optical booster amplifier. Therefore, a long span of non-repeatered transmission without using a repeater is studied by using this. ing.

[0003]

However, in an optical transmission system using such an optical amplifier, there is a problem that stimulated Brillouin scattering is likely to occur because the intensity of the signal light incident on the optical fiber becomes large. there were. FIG. 10 shows the forward transmitted light intensity emitted from the other end (indicated by a circle in the figure) and the backscattered light emitted from the incident end when the intensity of the incident light incident from one end of the optical fiber is increased. The change in strength (indicated by ● in the figure) is shown.
As shown in this figure, the incident light intensity is 7 dBm (5 m
At W) and above, the backscattered light intensity increases dramatically and the forward transmitted light intensity decreases. That is, even if high-intensity light is incident on the optical fiber, if the incident light intensity exceeds a certain value, most of the incident light returns to the incident end as stimulated Brillouin scattered light. Therefore, there is a disadvantage that the high intensity light obtained by the optical amplifier cannot be effectively transmitted through the optical fiber and the non-repeatered transmission distance of the optical signal cannot be extended.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical fiber capable of suppressing the occurrence of stimulated Brillouin scattering.

[0005]

In order to solve the above-mentioned problems, the optical fiber according to claim 1 of the present invention is a silica single mode optical fiber, wherein the core raises at least one kind of refractive index. SiO ₂ containing a first dopant that slows down the longitudinal acoustic wave velocity and at least one second dopant that lowers the refractive index and slows down the longitudinal acoustic wave velocity.
The first dopant and the second dopant are made of glass.
-The amount of pant added is constant along the length of the optical fiber,
In addition, the core diameter is changed in the length direction of the optical fiber. Further, in the optical fiber according to claim 2, the first dopant is GeO ₂ , and the _second dopant is GeO 2.
Is a dopant of F.
The optical fiber according to claim 3 is the optical fiber according to claim 1 or 2.
The optical fiber according to any one of the above, wherein the cladding is a dopper.
It is characterized in that it is made of SiO ₂ to which
Of. The optical fiber according to claim 4 is the optical fiber according to claim 3.
Of the optical fiber of the
The punt is from the group consisting of GeO ₂ , F, and B ₂ O _3.
It is characterized by being at least one selected.
The optical fiber according to claim 5 is the optical fiber according to claim 3 , wherein the cladding is made of SiO ₂ glass containing at least one third dopant for increasing the refractive index and increasing the longitudinal acoustic wave velocity. It is characterized by becoming.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
A quartz single mode optical fiber is used as the optical fiber of the present invention. The single-mode optical fiber here is a fiber that is capable of substantially single-mode transmission, and even if the secondary mode can theoretically be transmitted, it is a relatively short distance, for example, a distance of 1 km or less. Those that are attenuated and may be regarded as substantially single mode transmission are also included.

In the optical fiber of the present invention, the core is made of SiO ₂ glass containing at least one first dopant and at least one second dopant.
The first dopant contained in the core has the functions of increasing the refractive index of SiO ₂ glass and slowing the velocity of longitudinal acoustic waves propagating through the core. Examples of such a dopant include GeO ₂ and TiO _2. , Na ₂ O, K ₂ O, Cs ₂
There are O, CaO, SrO, BaO, PbO and the like. Any one of these dopants may be added to the core,
Further, plural kinds may be added to the core. The second dopant contained in the core has a function of lowering the refractive index of SiO ₂ glass and slowing the velocity of longitudinal acoustic waves propagating in the core. Examples of such a dopant include F and B ₂ There are O ₃ etc. One of these dopants
Seeds may be added to the core, or multiple types may be added to the core.

Since the shape of the refractive index distribution of the core and the refractive index distribution of the optical fiber is changed by the addition of the first dopant and the second dopant, a preferable core-clad specific refractive index according to the characteristics of the optical fiber to be obtained. The addition amounts of these dopants are set so that a difference in index (hereinafter, simply referred to as a relative refractive index difference) and a preferable refractive index distribution can be obtained. Then, the larger the amount of the second dopant added to the core and the larger the amount of the first dopant added, the greater the effect of suppressing the stimulated Brillouin scattering, and the higher the threshold value of the stimulated Brillouin scattering. be able to. However, as the concentration of the dopant in the core increases, the Rayleigh scattering coefficient increases, which increases the transmission loss.Therefore, in order to increase the increase in the stimulated Brillouin scattering threshold per transmission loss as much as possible for each dopant. It is preferable to set the addition amount. For example in the case of using F as a second dopant, the absolute value of the relative refractive index difference between pure SiO ₂ glass upon addition of F only pure SiO ₂ glass does not exceed 0.7% amount (relative refractive The rate difference is −
An amount greater than 0.7, hereinafter simply referred to as greater than -0.7% with respect to pure SiO ₂ , etc.), preferably pure S 2.
It is preferably set to -0.3% or more with respect to iO ₂ .

In the optical fiber of the present invention, the clad may be pure SiO ₂ glass or SiO ₂ with a dopant added. It is preferable to add a dopant to the clad because the viscosity difference between the core and the clad during spinning can be reduced. This is because if the difference in viscosity between the core and the clad is large, strain remains at the interface between the core and the clad after spinning, which causes a large transmission loss. Aim to reduce the viscosity difference between core and clad
The dopant added to the clad is longitudinal acoustic
It may not have the effect of increasing the speed of the wave, for example,
For example, it is possible to add appropriate materials such as GeO ₂ , F and B ₂ O _3.
it can. However, in the present invention, it is added to the clad.
Dopant has no action to increase the velocity of longitudinal acoustic waves.
If not, transmission loss due to reduction of viscosity difference between core and clad
Loss reduction effect, while the addition amount of the dopant is
If too much, the effect of suppressing stimulated Brillouin scattering is impaired, so
The increase in the stimulated Brillouin scattering threshold per transmission loss is
The amount of the dopant added should be as large as possible.
It is preferable to set. Further, as a dopant added to the clad, those having an action of increasing the refractive index of Al ₂ O ₃ or Li ₂ O and increasing the velocity of longitudinal acoustic waves (third
The use of dopants), a core - not only reduce the viscosity difference between the cladding, it is possible to increase the stimulated Brillouin scattering suppression effect as described later favored
Good

In the optical fiber of the present invention, the core diameter changes in the optical fiber length direction. This change in core diameter is
It suffices that the core diameter is non-uniform in the length direction, and may be unidirectionally changed from the start end to the end of the optical fiber, or may be changed periodically. When the optical fiber is long, it is preferable to change it periodically. The range of the core diameter changes the characteristics such as the wavelength dispersion characteristics of the optical fiber, so that it is preferably set according to the characteristics of the optical fiber to be obtained.

The operation of the present invention will be described below.
Stimulated Brillouin scattering is a phenomenon in which scattered light whose frequency is shifted is generated due to mutual interference between longitudinal acoustic waves in glass constituting an optical fiber and transmitted light. by the way,
When the velocity of the acoustic wave in the core material is slower than the velocity of the acoustic wave in the clad material, the acoustic wave in the optical fiber having the core-clad structure has a mode structure that propagates in the core as in the case of light. , The velocity of the longitudinal acoustic wave propagating in the core changes depending on the core diameter. Further, it is known that when the velocity of the longitudinal acoustic wave changes, the Brillouin shift frequency changes in proportion to this, and these relationships are expressed by the following mathematical formula (I) (Reference 1; C. Jen, A. Safaai, and GWFarne
ll, ”Leaky modes in weakly guiding fiber acoustic
waveguides ", IEEE Trans, Ultrason, Ferroelectr, F
req. Countr. UFFC-33,634, 1986,).

[0012]

[Equation 1]

FIG. 8 is described in the above-mentioned document 1 and shows how the velocity of the longitudinal acoustic wave and the Brillouin shift frequency in the optical fiber change with the core diameter. As shown in this figure, the velocities of the guided modes (L ₀₁ , L ₁₁ , ...) Of the longitudinal acoustic wave are the acoustic wave velocity in the core material (V _core = 5736 m / sec) and the velocity in the clad material. It changes between the acoustic wave velocity (V _clad = 5933 m / sec) as the core diameter changes. Therefore,
When the core diameter is changed in the length direction of the optical fiber, the velocity of the longitudinal acoustic wave changes in the length direction of the optical fiber, so that the Brillouin shift frequency can be changed in the length direction of the optical fiber. The present inventors have already proposed an optical fiber capable of suppressing stimulated Brillouin scattering by changing the Brillouin shift frequency in the longitudinal direction by changing the core diameter (Japanese Patent Application No. 166403).

Furthermore in FIG. 8, if it is possible to lower the value of V _core, the difference between V _core and V _cl a _d increases, the slope of the curve of each waveguide mode increases. Then, the change in the velocity of the guided mode becomes large when the core diameter changes. According to the present invention, the greater the difference between the velocity of the acoustic wave in the core material and the velocity of the acoustic wave in the cladding material, the greater the degree of change in the velocity of the longitudinal acoustic wave due to the change in the core diameter. This property is used to obtain a large effect of suppressing the stimulated Brillouin scattering.

That is, the value of the relative refractive index difference of the optical fiber is defined by the characteristics to be obtained,
Usually, the relative refractive index difference is controlled by adding a dopant for increasing the refractive index to the core of the optical fiber and / or adding a dopant for decreasing the refractive index to the cladding. Therefore, the amount of the dopant added to the core of the optical fiber is determined by the value of the relative refractive index difference. In the optical fiber of the present invention, a first dopant for increasing the refractive index and a second dopant for decreasing the refractive index are added to the core. Therefore, as compared with the case where only the first dopant that raises the refractive index of the core is added to obtain a predetermined relative refractive index difference, the total amount of the dopant added is equal to that of the second dopant, and Can be increased by the amount of the first dopant increased. Since the first dopant and the second dopant having the function of slowing down the velocity of the longitudinal acoustic wave are used, the velocity _Vcore of the acoustic wave in the core material is greatly reduced.

As a result, the velocity V of the acoustic wave in the core material V
The difference between the _core and the velocity V _clad of the acoustic wave in the clad material becomes large. Therefore, the change in velocity of the longitudinal acoustic wave in the core caused by changing the core diameter in the length direction of the optical fiber becomes large, and the change in the Brillouin shift frequency in the length direction of the optical fiber becomes larger. From this, compared to the case of an ordinary optical fiber in which the core contains only a dopant for increasing the refractive index, the threshold value of the stimulated Brillouin scattering can be increased even if the amount of change in the core diameter is the same. A large effect of suppressing stimulated Brillouin scattering is obtained. Moreover, since the change in the refractive index due to the newly added second dopant can be offset by increasing the addition amount of the first dopant, suppression of the stimulated Brillouin scattering can be achieved without changing the optical characteristics. The effect can be enhanced.

If a first dopant and a second dopant as described above are added to the core and a third dopant for increasing the refractive index is added to the clad, a predetermined relative refractive index difference can be obtained. The amount of addition of the first dopant for increasing the refractive index of the core can be further increased by an amount corresponding to the third dopant. This slows down the velocity _Vcore of the acoustic wave in the core material. Since the third dopant has an action of increasing the velocity of the longitudinal acoustic wave, the difference between the velocity V _core of the acoustic wave in the core material and the velocity V _clad of the acoustic wave in the clad material is In comparison with the case where the third dopant is not added, the effect of suppressing stimulated Brillouin scattering is further enhanced.

FIG. 9 shows the addition amount of each dopant and the acoustic wave velocity when GeO ₂ as the first dopant and F as the second dopant are co-doped in the core (core diameter is constant) of the optical fiber. It shows the relationship. still,
In this graph, the amount of each dopant added is pure Si.
Pure S when only the dopant is added to O ₂ glass
It is represented by the absolute value (%) of the relative refractive index difference from iO ₂ glass. As shown in this figure, as the dopant concentration increases, the acoustic wave velocity decreases linearly with it. Therefore, by increasing the concentration of these dopants, the acoustic wave velocity V in the cladding material made of pure SiO ₂ (GeO ₂ = 0%, F = 0%) is obtained.
The difference between _clad and the acoustic wave velocity V _core in the core material can be increased.

FIG. 1 schematically shows a first embodiment of the optical fiber of the present invention, which is an example of constructing a low dispersion optical fiber in which the chromatic dispersion at the used wavelength is close to zero. is there. In the optical fiber 1 of the present embodiment, the core diameter of the core 2 is uniformly reduced from the starting end 1A to the terminating end 1B, and the optical fiber diameter is constant from the starting end 1A to the terminating end 1B. The length of the optical fiber 1 is not particularly limited, but is usually several kilometers to several tens of kilometers.

In the optical fiber 1 of this embodiment, the core 2 is Ge.
It is made of SiO ₂ glass containing O ₂ and F, and the cladding 3 is made of pure SiO ₂ glass. The refractive index distribution shape of the optical fiber 1 is a step index type, a dual core type having a staircase core portion, or the like, and the shape at the start end 1A and the shape at the end end 1B are substantially similar. The refractive index distribution of the optical fiber 1 can be controlled by changing the amount of at least one dopant contained in the core 2 in the radial direction, but the addition amount of F is kept constant in the radial direction and GeO ₂ A desired refractive index distribution can be preferably obtained by changing the addition amount of the compound in the radial direction.

For example, in the case of a low-dispersion optical fiber whose wavelength dispersion in the 1.55 μm band is close to zero, a low-dispersion optical fiber in which stimulated Brillouin scattering is suppressed by changing the core diameter in the longitudinal direction, A larger effect of suppressing the stimulated Brillouin scattering can be expected as the core-clad relative refractive index difference increases. Further, as described above, the larger the amount of GeO ₂ and F added to the core, the greater the effect of suppressing stimulated Brillouin scattering, but the transmission loss increases. By the way, a typical 1.55 μm band low-dispersion optical fiber has a refractive index distribution of, for example, a unimodal central core portion having a peak value of a relative refractive index difference of about 0.8% and a relative refractive index difference outside thereof. Is a dual core type having a flat staircase core portion of 0.2%. Therefore, in the case of configuring the low dispersion optical fiber in which the stimulated Brillouin scattering according to the present invention is configured, the transmission loss is increased by increasing the relative refractive index difference between the central core portion and the step core portion within a designable range. In order to maximize the increase of the stimulated Brillouin scattering threshold per hit,
It is preferable to set the refractive index distribution. For example, the relative refractive index difference of the central core portion is preferably set to about 0.8 to 1.5%, and the relative refractive index difference of the staircase core portion is preferably set to about 0.2 to 0.3%.

Further, in this embodiment, in order to obtain a chromatic dispersion close to 0, the range of the core diameter of the optical fiber 1 is negative and the portion of the core diameter where the chromatic dispersion that changes with the change of the core diameter becomes positive. It is set so as to straddle the part of the core diameter that becomes. Specifically the core diameter r _A in starting 1A, a core diameter r _B at the end 1B, are defined as follows.

FIG. 2 shows a certain transmission wavelength (for example, 1.55 μm) corresponding to a change in core diameter of a single mode optical fiber.
It is a graph which shows the example of a change of chromatic dispersion in m) typically. As shown in this graph, when the core diameter of the single-mode optical fiber changes, the chromatic dispersion also changes in response to the change, and exhibits a change represented by a curve such as curve (a) or curve (d). Four kinds of curves (a) shown in the graph
(D) is for a single-mode optical fiber with different structural parameters, and the relationship between the core diameter and chromatic dispersion is generally expressed by a quadratic curve. In the curve (d), the minimum point is the core diameter. Is in a small region, and the curve (C) has a minimum point in a region where the core diameter is large.

Then, by appropriately determining the structural parameters of the single mode optical fiber, the portion of the core diameter where the value of chromatic dispersion becomes positive as shown by the curves (a) and (c) shown in the graph of FIG. There is a continuous core diameter range in which the negative core diameter portion continues. And
In the range of the core diameter satisfying such conditions, the optical fiber 1
The core diameter r _A at the start end and the core diameter r _B at the end are determined.

For example, regarding the curve (a), two regions (N,
The two core diameters r _A and r _B are determined so that the area of the region (L) having a negative value and the sum of the areas of M) are equal or nearly equal. Similarly, regarding the curve (c), the area of the region where the chromatic dispersion value takes a positive value and the area of the region where the chromatic dispersion value takes a negative value are equal or almost equal to each other.
To determine the core diameters r _A and r _B. By doing so, it is possible to suppress chromatic dispersion to such an extent that there is no practical problem.

When the variance value is further lowered,
The following circumstances need to be considered. That is, when a base material processed into a taper shape is spun so as to have a constant outer diameter as described below, a portion having a large outer diameter of the base material is spun longer than a portion having a smaller outer diameter. Therefore, the average value of chromatic dispersion over the entire length of the optical fiber is
That is, the weight of the chromatic dispersion on the side of r _{B in} FIG. If the core diameters at both ends of the optical fiber are determined in consideration of the above circumstances, it is possible to suppress the chromatic dispersion to a lower value.

The two core diameters r thus determined
_A and r _B are the start end 1A and the end end 1B of the optical fiber 1.
Of the core diameter. As a result, the starting end 1A of the optical fiber 1
The positive chromatic dispersion and the negative chromatic dispersion generated while the light having the transmission wavelength incident from the terminal 1B reaches the terminal 1B are canceled, and as a result, the light emitted from the terminal 1B has zero wavelength dispersion. The chromatic dispersion is ± 3 ps /
It is possible to suppress it within km / nm.

[0028] In principle, the core diameter r _B of the core diameter r _A and the end 1B of the starting end 1A of the optical fiber 1 is defined as above, in fact, between the core diameter r _A and r _B , The chromatic dispersion at any core diameter is within ± 5 ps / km / nm, the length of the core diameter where the cutoff wavelength exceeds the transmission wavelength does not become too long, and the bending loss and Rayleigh loss do not become excessive. Thus, the core diameter and other structural parameters need to be defined. From these points, it is preferable that the optical fiber 1 is a dual core type having a refractive index distribution shape having a core composed of a central core portion and a staircase core portion. On the other hand, generally, when a low dispersion optical fiber is designed as a single-peak type refractive index distribution shape, bending loss becomes large, so that a sufficient core diameter variation range cannot be obtained.

Next, a method of manufacturing the low dispersion optical fiber of this embodiment will be described. First, the core part is made of SiO ₂ glass to which GeO ₂ and F are added, and the clad part is made of pure S.
A glass base material made of i0 ₂ and having a constant outer diameter and a constant core diameter in the length direction is prepared, and the refractive index distribution of the glass base material is measured. Predict various characteristics of optical fibers obtained by spinning this glass base material with various core diameters from this measurement result,
calculate. From these characteristics, the transmission wavelength, for example 1.55μ
It is possible to obtain how the chromatic dispersion at m changes in accordance with the change in core diameter, that is, the curve in the graph of FIG. When it is assumed, for example, the curve as the curve (i) is obtained, and the core diameter r _A in the starting end 1A of the optical fiber as described above, the core diameter r _B at the end 1B
Determine.

Next, the core diameter r _B of the core diameter r _A and the end 1B of the outer diameter and the starting end 1A of the optical fiber 1 that is set in advance, defining an outer diameter in the longitudinal direction of the optical fiber preform. (still,
Here, the spinning is performed from the end of the base material, and the end of the base material is the starting end of the optical fiber 1. For that purpose, first, the diameter ratio (core-cladding ratio) of the core portion and the cladding portion at the starting end of the optical fiber preform matches the core-cladding ratio at the terminating end 1B of the optical fiber 1 after spinning. The core-clad ratio at the terminal end of the optical fiber preform must match the core-clad ratio at the starting end 1A of the optical fiber 1 after spinning. Since the optical fiber preform has a constant outer diameter and core diameter, in order to satisfy the above conditions, the outer diameter of the optical fiber preform is cut into a tapered shape,
The outer diameter at the start end may be made small, and the outer diameter at the end may be made large.

At this time, depending on the initial size of the optical fiber preform, the thickness of the clad portion may be insufficient and the core-clad ratio at the terminal end may not be satisfied. In this case,
A new additional clad portion may be formed by an external attachment method to satisfy the above core-clad ratio, and then ground into a tapered shape. Next, the taper-shaped optical fiber preform thus obtained is spun to have a constant outer diameter, whereby the low dispersion optical fiber shown in FIG. 1 can be obtained. The outer diameter of this spinning can be made constant by gradually changing the take-up speed. Alternatively, the outer diameter can be made constant by gradually changing the descending speed of the base material when the base material is lowered and introduced into the heating furnace.

In such a low dispersion optical fiber 1, the core 2 contains the first dopant and the second dopant for slowing the velocity of the longitudinal acoustic wave, and the core diameter is the same as that of the optical fiber 1. Due to the change in the length direction, generation of stimulated Brillouin scattering is suppressed. Further, as described above, since the average value of the chromatic dispersion over the entire length of the optical fiber 1 is zero or approaches zero, the chromatic dispersion in the entire optical fiber 1 can be suppressed within ± 3 ps / km / nm. Therefore, this low-dispersion optical fiber has a chromatic dispersion of zero or close to zero, and is capable of inputting and transmitting high-output signal light.

In the above embodiment, the core diameter of the optical fiber 1 varies uniformly from the starting end 1A to the terminating end 1B, but the present invention is not limited to this. In the length direction of the optical fiber of
The uniform change in the core diameter may be repeated twice or more, and the chromatic dispersion is averaged to zero in each range of the uniform change in the core, so that the chromatic dispersion is zero as a whole. Or it will be close to zero. Therefore, even if multiple optical fibers whose core diameter changes uniformly from the start end to the end, such as fusion splicing in the direction in which the core diameters of the end portions are equal, are connected, the total length of the connected optical fibers is the same. The chromatic dispersion can be maintained at zero or a value close to zero. Such a configuration is suitable when the optical fiber is long.

Next, as a second embodiment of the optical fiber of the present invention, an example in which a dispersion compensating optical fiber is constructed will be described. The dispersion compensating optical fiber of the present embodiment is used by making the core diameter smaller and making the core-clad relative refractive index difference larger than the silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band. At wavelength 1.3
It is designed to have a negative chromatic dispersion value having an absolute value larger than that of the silica single mode optical fiber of μm band. A silica single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band is usually designed to have a core diameter of about 9 μm and a core-clad relative refractive index difference of about 0.35%.

FIG. 3 shows changes in the outer diameter of the fiber in the dispersion compensating optical fiber of this embodiment. In the dispersion compensating optical fiber of this embodiment, the core is made of SiO ₂ glass containing GeO ₂ and F, and the clad is made of pure SiO ₂ glass. Further, the fiber outer diameter and the core diameter are formed so as to periodically change in the optical fiber length direction as shown in FIG. In this embodiment, the wavelength used in the dispersion compensating optical fiber is 1.525 to 1.575 μm.
It is preferably set in the range of m, and the range of the core diameter and the relative refractive index difference are preferably set so that a desired wavelength dispersion value, cutoff wavelength and bending loss can be obtained in this wavelength range.

The smaller the chromatic dispersion value of the dispersion compensating optical fiber is, the more preferable it is, but -50 ps / km for the reason described later.
/ Nm or less is preferable. 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. Also, the bending loss should be small, for example 1.0 dB / m (bending diameter 20
mm) or less. The refractive index distribution preferably has a large difference in refractive index between the core and the clad in order to increase the dispersion value of the optical fiber, and is preferably formed in, for example, a step type refractive index distribution or a unimodal type. The refractive index distribution can be controlled by appropriately setting the amount of at least one dopant contained in the core. When the refractive index of the core changes in the radial direction, a desired refractive index distribution can be preferably obtained by, for example, making the addition amount of F constant in the radial direction and changing the addition amount of GeO _{2 in} the radial direction. . Alternatively, a constant amount of GeO ₂ in the radial direction, may change the amount of F in the radial direction, or may be varied the amount of GeO ₂ and F in the radial direction.

FIG. 4 shows a case where a wavelength of 1.55 μm is used in a single mode optical fiber having a step type refractive index distribution, and when the relative refractive index difference and the core diameter are changed, bending loss and cut It shows the result of calculation of how the off-wavelength and the chromatic dispersion value change. In this figure, A indicates a boundary at which 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, cracks and the like due to thermal strain occur when the base material is manufactured, making it extremely difficult to manufacture. It is not preferable. In this figure, the larger the relative refractive index difference and the smaller the core diameter, the smaller the chromatic dispersion value (negative) (the larger the absolute value). D has a wavelength of 1.
Wavelength dispersion value at 55 μm is -50 ps / km / nm
Is shown, and the region above this is a region where the chromatic dispersion value is −50 ps / km / nm or less. Therefore, when the wavelength used is 1.55 μm, the chromatic dispersion value, the bending loss, and the cutoff are set by setting the core diameter and the relative refractive index difference so as to fall within the shaded regions in FIG. A dispersion compensating optical fiber satisfying the preferable wavelength is obtained. It should be noted that FIG. 4 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 used wavelength, the chromatic 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 scattering cannot be obtained, and as the amount of change in the core diameter is large, the effect of suppressing stimulated Brillouin scattering is greater. For example, the core has a unimodal refractive index distribution,
For a dispersion-compensating optical fiber having a relative refractive index difference of about 2.3% between the central part of the core and the clad and a wavelength dispersion value of −50 ps / km / nm or less, stimulated Brillouin scattering is performed by changing the change amount of the core diameter. The amount of increase in the threshold value was measured, and when the amount of change in core diameter was set to ± 5%, the threshold value was 4d.
On the other hand, when the amount of change in core diameter was set to ± 3%, the threshold value increased by only 1 dB, while the value increased by about B.

In the present embodiment, the core diameter changes in a triangular wave shape as shown in FIG. 3 in proportion to the outer diameter of the fiber. However, it is not limited to this waveform, and the core diameter may be non-uniform in the longitudinal direction. It ’s good. 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, causes an increase in transmission loss and is not preferable. The degree of change in core diameter is such that the transmission loss at the wavelength used is 0.5d.
It is preferable that the degree of graduality is B / km or less.

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 the length is m or less, the length of the dispersion compensating optical fiber is allowed to be 20 to 40 km according to (2). Therefore
From (1), the chromatic dispersion value of the dispersion compensating optical fiber is -50 ps.
It is understood that it is preferable to set it to be / km / nm or less.

The dispersion compensating optical fiber of this embodiment can be manufactured, for example, as follows. First, a base material having a core portion made of SiO ₂ glass to which GeO ₂ and F are added and a cladding portion made of pure SiO ₂ is formed to have a refractive index distribution satisfying 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, the core contains the first dopant and the second dopant for slowing the velocity of the longitudinal acoustic wave, and the core diameter changes in the length direction. Therefore, the Brillouin shift frequency in each part in the length direction of the optical fiber changes, and the amount of stimulated Brillouin scattering generated over the entire length of the optical fiber can be suppressed. Therefore, this dispersion compensating optical fiber is capable of compensating for chromatic dispersion and being able to enter and transmit high-power signal light.

Another method of manufacturing a dispersion compensating optical fiber in which the core diameter changes in the length direction is to change the core diameter at the stage of the base material. That is, when forming the base material, first, a glass rod to be the core portion is produced, and the outer peripheral surface of the glass rod is externally cut, or after the external cutting, the glass rod diameter is, for example, triangular. 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.

[0046]

EXAMPLES Specific examples are shown below, but the invention is not limited thereto. (Example 1) A low-dispersion optical fiber whose chromatic dispersion in the 1.55 µm band is close to zero was manufactured as follows. GeO
A fiber preform for a low-dispersion optical fiber comprising a core made of SiO ₂ glass doped with ₂ and F and a clad made of pure SiO ₂ glass was prepared. The amount of F added to the core of the fiber base material was uniform in the radial direction and was -0.4% with respect to pure SiO ₂ . Further, the addition amount of GeO ₂ was changed in the radial direction so that the refractive index distribution of the base material became a dual core type as shown in FIG. GeO ₂ was added so that the peak position at the center of the core would be + 1.6% with respect to pure SiO ₂ . The outer diameter of the fiber base material is 30 mm,
The diameter of the core portion was 3.6 mm, the core clad ratio was 8.3, and this dimension was constant in the length direction.

As a result of the analysis of the refractive index distribution of the fiber base material, it was calculated that the chromatic dispersion at the transmission wavelength of 1.55 μm changes as the core diameter changes, as shown by the curve (e) in FIG. Was done. From this curve (e), when the core diameter at the starting end and the core diameter at the terminating end of the fiber were determined so that the areas of both the positive and negative chromatic dispersion areas were substantially equal, r _A was 25. Values of 0 μm and r _B 12.5 μm were obtained.

If the diameter of the fiber 1 after spinning is constant at 125 μm, the core-clad ratio at the starting end 1A of the fiber 1 after spinning is 125/25 = 5, and the core-cladding ratio at the terminal end 1B is 125/12. 5 = 10. On the other hand, the above-mentioned prepared fiber preform has a uniform core-clad ratio of 8.3, so that the glass having the same refractive index as that of the clad portion is adjusted so that the core-clad ratio on the terminal side of the fiber preform becomes 10 or more. Was attached externally, and its outer diameter was set to 36 mm. Then, as shown in FIG. 7, the fiber base material 1 having an outer diameter of 36 mm
1 on its end side 11B to an outer diameter of 36 mm, and its start end side 1
It was ground in a taper shape with an outer diameter of 18 mm at 1A. Note that reference numeral 12 in FIG. 7 indicates a core portion.

The tapered fiber preform 11 was spun with a constant outer diameter of 125 μm to obtain an optical fiber having a total length of 14 km. When the characteristics of the obtained fiber were measured, they were as follows. Starting Edge 1A Starting Edge 1B Cladding Outer Diameter (μm) 125 125 Core Diameter (μm) 25 12.5 1.55 μm Mode Field Diameter (μm) 6.43 8.63 1.55 μm Wavelength Dispersion over Full Length +0. 43 (ps / km / nm) Further, the threshold value of the incident light amount that causes stimulated Brillouin scattering of this fiber was about 13 dBm.

(Comparative Example 1) In Example 1 above, F was not added to the core, and the amount of GeO ₂ added was adjusted to be + 1.2% with respect to pure SiO ₂ at the peak position in the center of the core. A low dispersion optical fiber was manufactured in the same manner except for the above. When the characteristics of the obtained fiber were measured, it was 1.55 μm.
Dispersion over the entire length at +0.43 ps / km
/ Nm, which was the same as that of Example 1 above, but the threshold value for stimulated Brillouin scattering was a low value of about 11 dBm.

(Comparative Example 2) A fiber preform having GeO ₂ and F added to the core was prepared in the same manner as in Example 1 above, and the fiber preform was not trimmed, but with a constant outer diameter of 125 μm. Spun The core diameter of the obtained fiber was 15 μm and was constant over the entire length. The threshold value of stimulated Brillouin scattering of this fiber was about 7 dBm.

From the results of Example 1 and Comparative Example 1 above,
It was found that in a low-dispersion optical fiber with a changed core diameter, by adding F in addition to GeO ₂ to the core, the effect of suppressing the induced Brillouin backward dispersion can be enhanced without changing the chromatic dispersion characteristics. . From the results of Example 1 and Comparative Example 2 described above, it was confirmed that the effect of suppressing stimulated Brillouin scattering can be obtained by adding GeO ₂ and F to the core and changing the core diameter.

(Example 2) As described below, 1.55
A dispersion compensating optical fiber having a large negative chromatic dispersion in the μm band was manufactured. A fiber base material for a dispersion compensating optical fiber was prepared, which was composed of a core made of SiO ₂ glass to which GeO ₂ and F were added and a clad made of pure SiO ₂ glass. The amount of F added to the core of the fiber base material was uniform in the radial direction and was -0.4% with respect to pure SiO ₂ . Further, the addition amount of GeO ₂ was changed in the radial direction so that the refractive index distribution of the base material became a monomodal type. GeO ₂ was added so that the peak position at the center of the core would be + 2.7% with respect to pure SiO ₂ . The thickness of the clad was clad diameter / core diameter = about 54, and the dimensions of the core and the clad were constant in the length direction of the fiber preform.

When the base material is drawn, the drawing temperature, the drawing speed, and the feed rate of the base material are periodically changed, and the outer diameter of the optical fiber is periodically changed in a triangular wave shape as shown in FIG. Was spun. Average optical fiber outer diameter is 125 μm
The change in outer diameter was ± 5%. Change cycle is 500
An optical fiber having a total length of 5 km was obtained every m. The wavelength dispersion value of the obtained optical fiber at a wavelength of 1.55 μm is −5.
It was 0.7 ps / km / nm. In addition, the threshold value of the amount of incident light that causes stimulated Brillouin scattering in this fiber was about 11 dBm.

Comparative Example 3 In Example 2, F was not added to the core, and the amount of GeO ₂ added was adjusted to + 2.3% with respect to pure SiO ₂ at the peak position in the center of the core. A dispersion compensating optical fiber was manufactured in the same manner except the above.
The characteristics of the obtained fiber were measured and found to be 1.55μ.
chromatic dispersion over the entire length in m is -50.7 ps / k
m / nm was the same as that of Example 2 above, but the threshold value for stimulated Brillouin scattering was a low value of about 10 dBm.

(Comparative Example 4) A fiber preform with GeO ₂ and F added to the core was prepared in the same manner as in Example 2, and the drawing temperature, the drawing speed, the preform when drawing the preform 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 delivery speed. The chromatic dispersion value at a wavelength of 1.55 μm of the obtained optical fiber is −
It was 55.0 ps / km / nm. The threshold value for stimulated Brillouin scattering was about 8 dBm.

From the results of Example 2 and Comparative Example 3 above,
It was found that in the dispersion-compensating optical fiber with a changed core diameter, by adding F in addition to GeO ₂ to the core, the induced Brillouin backward dispersion suppressing effect can be enhanced without changing the chromatic dispersion characteristics. . From the results of Example 2 and Comparative Example 4 described above, GeO ₂ was added to the core.
It was confirmed that the effect of suppressing the stimulated Brillouin scattering can be obtained by adding and F and changing the core diameter.

Example 3 A low-dispersion optical fiber whose chromatic dispersion in the 1.55 μm band is close to zero was manufactured in the same manner as in Example 1 except that Al ₂ O ₃ was added to the clad. That is, a fiber preform for a low-dispersion optical fiber comprising a core made of SiO ₂ glass added with GeO ₂ and F and a clad made of SiO ₂ glass added with Al ₂ O ₃ was prepared. The amount of F added in the core of this fiber base material was uniform in the radial direction, and was -0.4 with respect to pure SiO ₂ .
%. Further, the addition amount of GeO ₂ was changed in the radial direction so that the refractive index distribution of the base material became a dual core type as shown in FIG. GeO ₂ was added so that the peak position at the center of the core would be + 2.0% with respect to pure SiO ₂ . The addition amount of Al ₂ O ₃ in the clad was uniform in the radial direction and was + 0.4% with respect to pure SiO ₂ . The outer diameter of the fiber base material was 30 mm, the diameter of the core portion was 3.6 mm, and the core-clad ratio was 8.3, and this dimension was constant in the length direction.

This fiber base material was ground into a taper shape in the same manner as in Example 1 above, and then spun with a constant outer diameter of 125 μm.
An optical fiber having a total length of 14 km was obtained. When the characteristics of the obtained fiber were measured, the outer diameter of the cladding (μm), the core diameter (μm), and the mode field diameter (μ) at 1.55 μm were measured.
m), and the chromatic dispersion over the entire length at 1.55 μm was the same as in Example 1. In addition, the threshold value of the amount of incident light that causes stimulated Brillouin scattering in this fiber is about 1
It was 4 dBm, and a larger effect of suppressing stimulated Brillouin scattering was obtained as compared with the optical fiber of Example 1.

[0060]

As described above, the optical fiber according to claim 1 of the present invention is a silica-based single mode optical fiber, in which the core raises the refractive index of at least one type and increases the velocity of the longitudinal acoustic wave. It is made of SiO ₂ glass containing a first dopant for slowing down and at least one second dopant for lowering the refractive index and slowing down the longitudinal acoustic wave velocity, and the core diameter changes in the optical fiber length direction. It is characterized by being present.

According to the optical fiber of the present invention, the core has the first
By adding the second dopant in addition to the above dopant, the effect of suppressing the stimulated Brillouin scattering obtained by the change in the core diameter can be made larger than that of the optical fiber in which the core contains only the first dopant. Therefore, the threshold value for the occurrence of scattering can be increased, so that an optical fiber capable of entering and transmitting a high level of light can be obtained. Further, according to the present invention, the effect of suppressing stimulated Brillouin scattering can be enhanced without changing the optical characteristics of the optical fiber. Therefore, the low dispersion optical fiber that suppresses stimulated Brillouin scattering is configured by changing the core diameter. It is suitable for improving the scattering threshold of dispersion compensation optical fibers and the like. Therefore, if a low-dispersion optical fiber or a dispersion-compensating optical fiber is configured by the optical fiber of the present invention and an optical transmission system is constructed using this, not only excellent chromatic dispersion characteristics but also high-power light is incident. Thus, a system capable of transmission can be obtained, and the transmission distance can be further increased and the transmission capacity can be increased.

Further, the optical fiber according to claim 3 is characterized in that the clad is made of SiO ₂ glass containing at least one third dopant for increasing the refractive index and increasing the longitudinal acoustic wave velocity. Is. Thus, by adding the first dopant and the second dopant to the core and adding the third dopant to the cladding, the amount of the first dopant added to the core can be increased, The effect of suppressing the stimulated Brillouin scattering obtained by changing the core diameter can be further increased.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram schematically showing an example of a low dispersion optical fiber according to the present invention.

FIG. 2 is a graph showing the relationship between core diameter and chromatic dispersion according to the present invention.

FIG. 3 is a graph showing changes in core diameter in an example of the dispersion compensating optical fiber according to the present invention.

FIG. 4 is a graph showing a preferable range of a core diameter and a relative refractive index difference in an example of the dispersion compensating optical fiber according to the present invention.

FIG. 5 is a diagram showing a refractive index distribution of a fiber preform used in Examples.

FIG. 6 is a graph showing the relationship between core diameter and chromatic dispersion in an example.

FIG. 7 is a schematic cross-sectional view of a fiber preform that has been ground in an example.

FIG. 8 is a graph showing the dependence of longitudinal acoustic wave velocity and Brillouin shift frequency on core diameter.

FIG. 9 is a graph showing the relationship between the amount of dopant and the acoustic wave velocity.

FIG. 10 is a graph showing incident light power dependence of forward transmitted light and back scattered light by stimulated Brillouin scattering.

[Explanation of symbols]

1 ... Low dispersion optical fiber, 2 ... Core, 1A ... Start end, 1B ...
Termination, 11 ... Fiber preform.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Akira Wada 1440 Rokuzaki, Sakura-shi, Chiba Fujikura Co., Ltd. Sakura factory (72) Ryozo Yamauchi 1440 Rokuzaki, Sakura-shi, Chiba Fujikura Co., Ltd. 72) Inventor Shoji Ohashi 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Kazuhide Nakajima 3-192-3, Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation In-house (56) Reference JP-A-5-249329 (JP, A) JP-A-4-367539 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 6/10

Claims (5)

(57) [Claims] 1. A silica-based single-mode optical fiber, wherein the core comprises at least one first dopant for increasing the refractive index and slowing the longitudinal acoustic wave velocity, and at least one for lowering the refractive index. made of SiO ₂ glass containing a second dopant to slow the velocity of longitudinal acoustic waves, said first de
-The amount of the dopant and the second dopant is an optical fiber
An optical fiber which is constant in the length direction and whose core diameter changes in the length direction of the optical fiber. 2. The optical fiber according to claim 1, wherein the first dopant is GeO ₂ and the second dopant is F. 3. The cladding is S doped with a dopant.
iO ₂ 3. The method according to claim 1 or 2, characterized in that
The optical fiber according to the above. 4. A dopant added to the cladding.
But GeO ₂ , F, and B ₂ O ₃ Selected from the group consisting of
4. The optical fiber according to claim 3, wherein the optical fiber is one or more types.
Viva. 5. The optical fiber according to claim 3 , wherein the cladding is made of SiO ₂ glass containing at least one third dopant for increasing the refractive index and increasing the velocity of longitudinal acoustic waves.