JP2008158227A - Optical fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber which is easily manufactured and has optical characteristic stable in the longitudinal direction and in which the generation of SBS is suppressed. <P>SOLUTION: The optical fiber includes: a core part in which speed of sound is reduced in a stepwise fashion or continuously from the center to the position of 85% of radius; and a clad part which is formed at the outer periphery of the core part and has a refractive index lower than that of the core part and in which the speed of sound is slower, wherein a several kinds of dopants including a dopant which increases the refractive index and reduces the speed of sound, and a dopant which increases both the refractive index and the speed of sound are added to the optical fiber. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical fiber that can suppress the occurrence of stimulated Brillouin scattering.

  In order to realize large-capacity optical communication, communication systems such as a wavelength division multiplexing (WDM) system and a time division multiplexing (TDM) system are employed. In such a communication system, when the optical power input to the optical fiber that is a transmission path increases, the occurrence of a nonlinear optical phenomenon in the optical fiber becomes remarkable. Stimulated Brillouin Scattering (SBS), which is one of nonlinear optical phenomena, is a phenomenon in which Brillouin scattered light of light input to an optical fiber causes stimulated scattering, and is an interaction between light propagating in the optical fiber and acoustic waves. appear. SBS is generated when the intensity of light input into an optical fiber becomes equal to or greater than a threshold (SBS threshold). When SBS occurs, the optical power that can be input into the optical fiber is limited. Therefore, non-linear optical phenomena such as optical fiber used for transmission lines, four-wave mixing (FWM), and self-phase modulation (SPM) are actively generated. A highly nonlinear optical fiber to be used is desired to have a high SBS threshold. In recent years, optical fiber lasers using optical fibers have been actively developed. However, the generation of SBS accompanying the increase in output of optical fiber lasers has become a problem, and an optical fiber having a high SBS threshold is desired. ing. Note that a gain resulting from stimulated scattering of Brillouin scattered light is referred to as a Brillouin gain.

  Conventionally, as a method of increasing the SBS threshold, a method of changing the characteristics of the optical fiber in the longitudinal direction by changing the amount of dopant added to the core in the longitudinal direction of the optical fiber or the core diameter has been proposed ( (See Patent Documents 1 and 2). According to these methods, since the shift amount of the Brillouin scattered light with respect to the input light on the frequency spectrum, that is, the Brillouin shift amount changes in the longitudinal direction of the optical fiber, SBS hardly occurs and the SBS threshold value becomes high. . On the other hand, as a method of realizing an optical fiber that raises the SBS threshold without changing the characteristics in the longitudinal direction, the intensity distribution of the fundamental propagation mode of light propagating in the optical fiber and the field distribution of the propagation mode of acoustic waves are the maximum values. There are disclosed a method of making different radii and a method of providing an annular acoustic core region in order to localize the propagation mode of acoustic waves (see Patent Documents 3 and 4).

Japanese Patent No. 2753426 Japanese Patent No. 3386948 JP 2006-154713 A JP 2006-184534 A

  However, the optical fibers whose characteristics are changed in the longitudinal direction described in Patent Documents 1 and 2 are difficult to manufacture even if the SBS threshold is increased and the generation of SBS can be suppressed, and the longitudinal direction of the optical fiber. However, there was a problem that the optical characteristics changed and stable characteristics could not be obtained. On the other hand, the optical fibers described in Patent Documents 3 and 4 can suppress the generation of SBS and the optical characteristics do not change in the longitudinal direction. However, for example, the effective core cross-sectional area is reduced to generate a nonlinear optical phenomenon. There is a problem that the degree of freedom in designing optical characteristics is not high, such as being difficult to do.

  The present invention has been made in view of the above, and an object thereof is to provide an optical fiber that is easy to manufacture, has stable optical characteristics in the longitudinal direction, and can suppress the occurrence of SBS.

  In order to solve the above-described problems and achieve the object, the optical fiber according to the present invention is doped with a plurality of dopants including a dopant that increases the refractive index and decreases the sound speed and a dopant that increases the refractive index and increases the sound speed. Then, the core part in which the sound speed is reduced stepwise or continuously from the center to the position of 85% of the radius, and the refractive index is lower and the sound speed is slower than the core part formed on the outer periphery of the core part. And a clad portion.

  The optical fiber according to the present invention is characterized in that, in the above-mentioned invention, the sound speed of the core portion is reduced stepwise or continuously from the center to a position of 90% of the radius.

  Further, in the optical fiber according to the present invention, in the above invention, the core portion includes a central core portion and an outer peripheral core portion that is formed on an outer periphery of the central core portion and has a lower sound velocity than the central core portion. Features.

  In the optical fiber according to the present invention as set forth in the invention described above, the sound velocity of the central core portion is 1% or more faster than the sound velocity of the outer peripheral core portion.

  Further, in the optical fiber according to the present invention, in the above invention, the clad part is formed on the outer periphery of the inner clad part and the inner clad part having a lower refractive index than the core part formed on the outer periphery of the core part. And an outer cladding portion having a refractive index lower than that of the core portion and higher than that of the inner cladding portion.

The optical fiber according to the present invention is characterized in that, in the above-mentioned invention, the core part is mainly composed of SiO ₂ glass, and the plurality of kinds of dopants include GeO ₂ and Al ₂ O ₃ .

Further, in the optical fiber according to the present invention, in the above invention, Al ₂ O ₃ is added so that the concentration of the core portion gradually or continuously decreases from the center to a position of 85% of the radius. It is characterized by.

Further, in the optical fiber according to the present invention, in the above invention, GeO ₂ is added so that the concentration of the core portion increases stepwise or continuously from the center to a position of 85% of the radius. Features.

  In the optical fiber according to the present invention, the absolute value of chromatic dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less.

The optical fiber according to the present invention is characterized in that, in the above-mentioned invention, an effective core area at a wavelength of 1550 nm is 18 μm ² or less.

  The optical fiber according to the present invention is characterized in that, in the above-mentioned invention, a relative refractive index difference between the core portion and the cladding portion at a wavelength of 1550 nm is 2% or more.

  According to the present invention, a plurality of types of dopants including a dopant that increases the refractive index and decreases the sound speed and a dopant that increases the refractive index and increases the sound speed are added and stepped from the center to the position of 85% of the radius. Or a core portion having a continuously reduced sound velocity, and a cladding portion having a lower refractive index and a slower sound velocity than the core portion formed on the outer periphery of the core portion, so that the light propagating in the optical fiber, The sum of spatial overlap with acoustic waves can be reduced. As a result, it is possible to realize an optical fiber that is easy to manufacture, has a stable optical characteristic in the longitudinal direction, and can suppress the occurrence of SBS.

  Embodiments of an optical fiber according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment 1)
FIG. 1 is a cross-sectional view schematically showing an optical fiber according to Embodiment 1 of the present invention. As shown in FIG. 1, the optical fiber 1 according to the first embodiment includes a core 11 part and a clad part 12 having a refractive index lower than that of the core part 11 formed on the outer periphery of the core part 11 and having a slower sound speed. And a single mode optical fiber mainly composed of SiO ₂ glass.

FIG. 2 is a diagram schematically showing a refractive index profile L1a and a sound velocity profile L1b in the radial direction of the optical fiber 1 according to the first embodiment. As shown in FIG. 2, the refractive index profile L1a has a step shape, and the refractive index values in the core portion 11 and the cladding portion 12 are constant at n11 and n12, respectively. Note that ns is the refractive index of pure SiO ₂ glass. The sound velocity profile L1b has a shape in which the sound velocity value is v11 at the center of the core portion 11, and the sound velocity continuously decreases from the center to the outer periphery of the core portion 11. The sound speed in the clad part 12 is constant at v12. Note that vs is the speed of sound in pure SiO ₂ glass.

The refractive index profile L1a and the sound velocity profile L1b shown in FIG. 2 include GeO ₂ that is a dopant that increases the refractive index of SiO ₂ glass and decreases the sound velocity, and Al ₂ O ₃ that is a dopant that increases the refractive index and increases the sound velocity. This is realized by adding fluorine (F), which is a dopant that lowers the refractive index and sound velocity of SiO ₂ glass, to the cladding portion 12 in addition to the core portion 11. FIG. 3 is a diagram schematically showing the concentration profile in the radial direction of GeO ₂ and Al ₂ O ₃ in the core portion 11 of the optical fiber 1. As shown in FIG. 3, the GeO ₂ concentration profile L1c has a shape in which the concentration continuously increases from the center to the outer periphery of the core portion 11, and the Al ₂ O ₃ concentration profile L1d The density continuously decreases from the center to the outer periphery. As a result, the refractive index profile L1a has a step shape, and the sound velocity profile L1b has a shape in which the sound velocity continuously decreases from the center to the outer periphery of the core portion 11. On the other hand, constant F is added to the cladding portion 12 in the radial direction. The core portion 11, by the addition of GeO _2, is realized a high refractive index remains low transmission loss.

  The characteristics of the propagation modes of light and acoustic waves propagating in the optical fiber depend on the refractive index profile and the sound velocity profile, respectively. The optical fiber 1 according to the first embodiment has a refractive index profile and a sound velocity profile as shown in FIG. 2, whereby light is localized in the core portion and in the vicinity of the core portion in the optical fiber 1. On the other hand, the acoustic wave does not propagate in the propagation mode localized in the core part and the vicinity of the core part, and the acoustic wave having a field distribution shape similar to the light intensity distribution form in the core part and the vicinity of the core part propagates. do not do. As a result, the optical fiber 1 has a small sum of spatial overlap between the light propagating in the optical fiber and the acoustic wave, can effectively suppress the occurrence of SBS, and has a high SBS threshold. Furthermore, the addition of dopants that have the same contribution to the increase / decrease in the refractive index but have different contributions to the increase / decrease in the sound speed to the core 11 increases the degree of freedom in designing the refractive index profile and the sound speed profile, thereby providing more flexible optics. Characteristic design is possible. In addition, since the sound velocity continuously decreases from the center of the core portion 11 to the outer periphery, the sound velocity does not change abruptly at the boundary surface between the core portion 11 and the clad portion 12, so that the acoustic wave is reflected at the boundary surface. The phenomenon in which the acoustic wave is confined in the core portion is suppressed, and the sum of spatial overlap between the light propagating in the optical fiber and the acoustic wave can be effectively reduced.

In addition, although the refractive index of the core part 11 is constant in the radial direction, the effect of the present invention can be obtained even if it increases or decreases in the radial direction. Further, the dopant to lower the raised and acoustic velocity a refractive index of the SiO ₂ glass is not limited to GeO ₂ dopant to raise up and the sound velocity a refractive index is not limited to Al ₂ O _3. 4, each dopant is a diagram showing the refractive index or the sound velocity of the increase or decrease in the SiO ₂ glass when added to SiO ₂ glass (CK Jen et al., " Role of guided acoustic wave properties in single-mode optical fiber design ", Electron. Lett., vol. 24, pp. 1419-1420, (1988)). Incidentally, if the upward arrow in the figure shows that the dopant by indicates that the refractive index or the acoustic velocity of SiO ₂ glass increases, decreases if downward. That is, by adding each dopant as shown in FIG. 4, the refractive index profile and the sound velocity profile can be controlled to a desired shape.

(Embodiment 2)
Next, a second embodiment of the present invention will be described. The optical fiber according to the second embodiment is different from the optical fiber according to the first embodiment in the configuration of the core portion.

FIG. 5 is a cross-sectional view schematically showing an optical fiber according to the second embodiment. As shown in FIG. 5, the optical fiber 2 according to the second embodiment includes a central core portion 211 and an outer peripheral core portion 212 having a lower sound speed than the central core portion 211 formed on the outer periphery of the central core portion 211. This is a single mode optical fiber including a core portion 21 and a cladding portion 22 having a lower refractive index and a lower sound velocity than the core portion 21 formed on the outer periphery of the core portion 21 and mainly composed of SiO ₂ glass.

  FIG. 6 is a diagram schematically showing a refractive index profile L2a and a sound velocity profile L2b in the radial direction of the optical fiber 2 according to the second embodiment. As shown in FIG. 6, in the refractive index profile L2a, the refractive index values in the central core portion 211, the outer peripheral core portion 212, and the cladding portion 22 are constant at n211, n212, and n22, respectively. In the sound velocity profile L2b, the sound velocity values at the central core portion 211, the outer core portion 212, and the cladding portion 22 are constant at v211, v212, and v22, and the sound velocity gradually increases from the center to the outer periphery of the core portion 21. It has a decreasing shape.

In the refractive index profile L1a and the sound velocity profile L1b shown in FIG. 6, GeO ₂ and Al ₂ O ₃ are added to the core portion 21 and F is added to the cladding portion 22 as in the optical fiber 1 according to the first embodiment. It is realized by doing. FIG. 7 is a diagram schematically showing the concentration profile of GeO ₂ and Al ₂ O ₃ in the core portion 21 of the optical fiber 2. As shown in FIG. 7, in the GeO ₂ concentration profile L2c, the outer core portion 212 has a higher GeO ₂ concentration than the central core portion 211, and the concentration gradually increases from the center of the core portion 21 to the outer periphery. Have a shape to On the other hand, the concentration profile L2d of Al ₂ O ₃ is, towards the outer circumferential core portion 212 than the center core portion 211 is the concentration of Al ₂ O ₃ low, stepwise concentration ranging from the center to the outer circumference of the core portion 21 is reduced It has a shape. Note that a certain F is added to the cladding portion 22 in the radial direction.

Since the optical fiber 2 has a refractive index profile and a sound velocity profile as shown in FIG. 6, like the optical fiber 1, the total sum of spatial overlap between the light propagating in the optical fiber and the acoustic wave is small. Thus, the occurrence of SBS can be effectively suppressed, and the SBS threshold is high. Further, each of the central core portion 211 and the outer peripheral core portion 212 is easy to manufacture because the concentrations of GeO ₂ and Al ₂ O ₃ are constant in the radial direction, and the refractive index profile and sound velocity profile as designed are even easier. Can be realized.

(Embodiment 3)
Next, a third embodiment of the present invention will be described. The optical fiber according to the third embodiment is different from the optical fiber according to the first embodiment in the configuration of the cladding portion.

FIG. 8 is a cross-sectional view of an optical fiber according to the third embodiment. As shown in FIG. 8, the optical fiber 3 according to the third embodiment includes a core part 31 and a clad part 32 having a refractive index lower than that of the core part 31 formed on the outer periphery of the core part 31 and having a slower sound speed. A single-mode optical fiber comprising SiO ₂ glass as a main component, and the clad part 32 further includes an inner clad part 321 formed on the outer circumference of the core part 31 and an inner clad formed on the outer circumference of the inner clad part 321 And an outer cladding portion 322 having a higher refractive index than the portion 321.

  FIG. 9 is a diagram schematically showing a refractive index profile L3a and a sound velocity profile L3b in the radial direction of the optical fiber 3 according to the third embodiment. As shown in FIG. 9, the refractive index profile L3a has a so-called W-shaped shape in which the refractive index values in the core portion 31, the inner cladding portion 321 and the outer cladding portion 322 are constant at n31, n321, and n322, respectively. . The sound velocity profile L3b has a shape in which the sound velocity value is v31 at the center of the core portion 31 and the sound velocity continuously decreases from the center to the outer periphery of the core portion 31. The values of sound velocity in the inner cladding portion 321 and the outer cladding portion 322 are constant at v321 and v322, respectively.

The refractive index profile L3a and the sound velocity profile L3b shown in FIG. 9 are similar to the optical fiber 1 according to the first embodiment, in which GeO ₂ and Al ₂ O ₃ are added to the core portion 31, and F is added to the inner cladding portion 321 and This is realized by adding to the outer cladding part 322. FIG. 10 is a diagram schematically illustrating the concentration profile of GeO ₂ and Al ₂ O ₃ in the core portion 31 of the optical fiber 3. As shown in FIG. 10, the GeO ₂ concentration profile L3c has a shape in which the concentration continuously increases from the center to the outer periphery of the core portion 31, and the Al ₂ O ₃ concentration profile L3d is the core portion 31. The density continuously decreases from the center to the outer periphery. As a result, the refractive index profile L3a has a step shape, and the sound velocity profile L3b has a shape in which the sound velocity continuously decreases from the center to the outer periphery of the core portion 31. The inner cladding portion 321 and the outer cladding portion 322 are each added with certain F in the radial direction.

Since the optical fiber 3 has a refractive index profile and a sound velocity profile as shown in FIG. 9, like the optical fiber 1, the total sum of spatial overlap between the light propagating in the optical fiber and the acoustic wave is small. Thus, the occurrence of SBS can be effectively suppressed, and the SBS threshold is high. Furthermore, since the refractive index profile L3a is W-type, the relative refractive index difference between the core portion 31 and the inner cladding portion 321 can be easily increased, the effective core area can be reduced, and the wavelength dispersion value can be reduced. It can be easily reduced and is suitable as a highly nonlinear optical fiber. For example, at a working wavelength of 1550 nm, the absolute value of chromatic dispersion is 10 ps / nm / km or less, the effective core area is 18 μm ² or less, and the relative refractive index difference between the core portion 31 and the inner cladding portion 321. Is 2% or more, a nonlinear optical phenomenon is effectively generated in the optical fiber, which is suitable as a highly nonlinear optical fiber.

In the third embodiment, the outer cladding portion 322 may be substantially made of pure SiO ₂ glass. Here, being substantially made of pure SiO ₂ glass means that a dopant for adjusting the refractive index is not included, and a Cl element that does not affect the refractive index may be included. In the second and third embodiments, the core portion or the clad portion has a two-layer structure, but may have a multilayer structure. Here, when the clad part has a multilayer structure, the layer having the lowest refractive index from the central core part in the radial direction is defined as the inner clad part, and the layer formed on the outer periphery is defined as the outer clad part. In the first to third embodiments, the sound velocity in the core portion decreases continuously or stepwise from the center to the outer periphery of the core portion, but is 85% of the radius from the center, preferably 90%. If the position reaches the position gradually or continuously, the effect of the present invention can be obtained even if the sound speed increases in a portion closer to the outer periphery.

  Next, the results of producing optical fibers according to examples and comparative examples of the present invention and measuring the Brillouin scattering spectrum will be described.

(Comparative Example 1)
First, an optical fiber according to Comparative Example 1 will be described. The optical fiber according to Comparative Example 1 includes a homogeneous core portion and a clad portion each composed mainly of SiO ₂ glass, the core diameter is 2 μm, the clad diameter is 60 μm, and the core portion has GeO ₂ as a dopant. 36.5 wt% is added, and the clad portion is substantially made of pure SiO ₂ glass.

FIG. 11 is a diagram showing a refractive index profile of an optical fiber according to Comparative Example 1, in which the horizontal axis indicates the radial distance, and the vertical axis indicates the relative refractive index difference based on the refractive index of pure SiO ₂ glass. . FIG. 12 is a diagram showing the sound velocity profile of the optical fiber according to Comparative Example 1, in which the horizontal axis represents the radial distance, and the vertical axis represents the specific sound speed difference based on the sound velocity of pure SiO ₂ glass. As shown in FIGS. 11 and 12, the optical fiber according to Comparative Example 1 has a step-type refractive index profile in which the relative refractive index difference with respect to the cladding portion of the core portion is 2.5% and the core portion with respect to the cladding portion. It has a step-type sound speed profile with a specific sound speed difference of -18%.

When the refractive index of pure SiO ₂ glass is ns and the sound speed is vs, the relative refractive index difference Δn and the specific sound speed difference Δv with respect to the pure SiO ₂ glass are expressed by equations (1) and (2), respectively.
Δn = [n−ns] / ns × 100 (%) (1)
Δv = [v−vs] / vs × 100 (%) (2)

The optical characteristics of the optical fiber according to Comparative Example 1 at a wavelength of 1550 nm are a chromatic dispersion value of −76 ps / nm / km, a dispersion slope of 0.072 ps / nm ² / km, and an effective core area of 12.9 μm ² . there were.

  FIG. 13 is a diagram showing a Brillouin scattering spectrum of the optical fiber according to Comparative Example 1. The horizontal axis of FIG. 13 is the frequency shift amount of the scattered light with respect to the incident light to the optical fiber, and the vertical axis is the relative intensity when the maximum peak value near the frequency shift amount of 9.3 GHz is 0 dB. Hereinafter, the optical fiber according to the example of the present invention and other comparative examples will be described with reference to the characteristics of the optical fiber according to the comparative example 1.

(Example 1)
Next, an optical fiber according to Embodiment 1 of the present invention will be described. The optical fiber according to Example 1 is similar to the optical fiber according to Embodiment 2 of the present invention, in which a core portion mainly composed of SiO ₂ glass, having a central core portion and an outer peripheral core portion, a cladding portion, The core diameter of the central core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.6 μm, and the cladding diameter is 60 μm. Further, as a dopant, 11.1 wt% of GeO ₂ in the central core portion, Al ₂ a O ₃ was added 7.9wt%, 18.8wt% of GeO ₂ in the outer peripheral core portion, the Al ₂ O ₃ 2. 9 wt% was added, and 3.7 wt% of F was added to the cladding.

  FIG. 14 is a diagram showing a refractive index profile of the optical fiber according to Example 1. The horizontal axis and the vertical axis are the same as those in FIG. FIG. 15 is a diagram showing a sound velocity profile of the optical fiber according to the first embodiment, and the horizontal axis and the vertical axis are the same as those in FIG. As shown in FIGS. 14 and 15, the optical fiber according to Example 1 has a step-type refractive index profile with a relative refractive index difference of 2.5% with respect to the cladding portion of the core portion, and the outer periphery of the core portion from the center. It has a sound speed profile in which the sound speed decreases step by step. In addition, the specific sound speed difference with respect to the outer periphery core part of a center core part was about 4.5%.

The optical characteristics of the optical fiber according to Example 1 at a wavelength of 1550 nm were a chromatic dispersion value of 0 ps / nm / km, a dispersion slope of 0.035 ps / nm ² / km, and an effective core area of 12.2 μm ^2. It was.

FIG. 16 is a diagram illustrating a Brillouin scattering spectrum of the optical fiber according to the first embodiment. The horizontal axis of FIG. 16 is the same as that of FIG. 13, but the vertical axis is the relative intensity when the maximum peak value of the spectrum shown in FIG. 13 is 0 dB. As shown in FIG. 16, the Brillouin scattering spectrum of the optical fiber according to Example 1 is significantly different from that shown in FIG. That is, there is no clear peak in the spectrum shown in FIG. 16, there is a broad peak in the vicinity of the frequency of 10.3 GHz to 11 GHz, the maximum peak value is about −7 dB, and the maximum of the spectrum shown in FIG. The peak value is greatly reduced from 0 dB. Therefore, in the optical fiber according to Example 1, the core part has a two-layer structure with different dopant concentrations, and the sound speed of the central core part is faster than that of the outer peripheral core part and the cladding part, from the center to the outer periphery of the core part. It was confirmed that SBS can be effectively suppressed by gradually reducing the sound speed. Furthermore, the optical fiber according to Example 1 is a suitable optical fiber as a highly nonlinear optical fiber because the chromatic dispersion value at a wavelength of 1550 nm is as small as 0 ps / nm / km and the effective core area is as small as 12.2 μm ^2. .

(Example 2)
Next, an optical fiber according to Embodiment 2 of the present invention will be described. The optical fiber according to the second embodiment, like the optical fiber according to the first embodiment, includes a core portion having a central core portion and an outer peripheral core portion, and a clad portion, the main component being SiO ₂ glass. In production, a layer made of pure SiO ₂ glass is formed on the outer periphery of the outer core portion, that is, on the outermost periphery of the core portion. This pure SiO ₂ glass layer was formed by volatilizing the dopant added to the core part glass base material in the surface part when the optical fiber was prepared using the core part glass base material prepared by the VAD method. It is considered a thing. In the optical fiber according to Example 2, the core diameter of the central core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.4 μm, and the thickness of the pure SiO ₂ glass layer is 0.1 μm. The outer diameter of the part is 3.6 μm. The clad diameter is 60 μm. Further, as a dopant, 11.1 wt% of GeO ₂ in the central core portion, Al ₂ a O ₃ was added 7.9wt%, 18.8wt% of GeO ₂ in the outer peripheral core portion, the Al ₂ O ₃ 2. 9 wt% was added, and 3.7 wt% of F was added to the cladding.

FIG. 17 is a diagram illustrating a refractive index profile of the optical fiber according to the second embodiment. FIG. 18 is a diagram illustrating a sound velocity profile of the optical fiber according to the second embodiment. As shown in FIGS. 17 and 18, the optical fiber according to Example 2 is a step type having a relative refractive index difference of 2.5% with respect to the cladding portion of the core portion, and has a pure SiO ₂ glass layer, and from the center. to the boundary position between the outer circumferential core portion of pure SiO ₂ glass layer, i.e. from the center up to 94% of the position of the core radius, stepwise sonic speed decreases, then the net specific acoustic velocity difference in SiO ₂ glass layer is 0% It has a sound speed profile that increases up to. The optical characteristics of the optical fiber according to Example 2 at a wavelength of 1550 nm are chromatic dispersion value of −3 ps / nm / km, dispersion slope of 0.033 ps / nm ² / km, and effective core area of 11.9 μm ² . there were.

FIG. 19 is a diagram illustrating a Brillouin scattering spectrum of the optical fiber according to the second embodiment. As shown in FIG. 19, the Brillouin scattering spectrum of the optical fiber according to Example 2 differs from that shown in FIG. 16 due to the influence of the pure SiO ₂ glass layer that exists at the outermost periphery of the core portion and has a high sound speed. Broad peaks exist in the vicinity of 10.5 GHz and 10.9 GHz, and the maximum peak value in the vicinity of 10.5 GHz was about −5 dB. Therefore, if the sound speed is gradually reduced from the center to the position of 94% of the radius in the core part, even if there is a part where the sound speed is fast such as a pure SiO ₂ glass layer on the outermost periphery of the core part, It was confirmed that SBS can be effectively suppressed.

(Example 3)
Next, an optical fiber according to Embodiment 3 of the present invention will be described. Similar to the optical fiber according to the second embodiment, the optical fiber according to the third embodiment has a layer made of pure SiO ₂ glass formed on the outermost periphery of the core portion at the time of manufacture. In the optical fiber according to Example 3, the core diameter of the central core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.2 μm, and the thickness of the pure SiO ₂ glass layer is 0.2 μm. The outer diameter is 3.6 μm. The clad diameter is 60 μm. Further, the kind and concentration of the dopant added to the central core portion, the outer peripheral core portion, and the clad portion are the same as those of the optical fiber according to the second embodiment.

FIG. 20 is a diagram illustrating a refractive index profile of the optical fiber according to the third embodiment. FIG. 21 is a diagram illustrating a sound velocity profile of the optical fiber according to the third embodiment. As shown in FIGS. 20 and 21, the optical fiber according to Example 3 is a step type having a relative refractive index difference of 2.5% with respect to the cladding part of the core part, and has a pure SiO ₂ glass layer, and from the center. to the boundary position between the outer circumferential core portion of pure SiO ₂ glass layer, i.e. from the center up to 88% of the position of the core radius, stepwise sonic speed decreases, then the net specific acoustic velocity difference in SiO ₂ glass layer is 0% It has a sound speed profile that increases up to. The optical characteristics of the optical fiber according to Example 3 at a wavelength of 1550 nm are as follows: the chromatic dispersion value is −6 ps / nm / km, the dispersion slope is 0.031 ps / nm ² / km, and the effective core area is 11.6 μm ² . there were.

  FIG. 22 is a diagram illustrating a Brillouin scattering spectrum of the optical fiber according to the third embodiment. As shown in FIG. 22, the Brillouin scattering spectrum of the optical fiber according to Example 3 has two broad peaks as shown in FIG. 19, and the maximum peak value near 10.6 GHz is about −3 dB. Met. Therefore, it was confirmed that SBS can be effectively suppressed if the sound speed is gradually reduced from the center to the position of 88% of the radius in the core portion.

(Comparative Example 2)
Next, an optical fiber according to Comparative Example 2 of the present invention will be described. Similar to the optical fibers according to Examples 2 and 3, the optical fiber according to Comparative Example 2 is formed by forming a layer made of pure SiO ₂ glass on the outermost periphery of the core portion at the time of manufacture. In the optical fiber according to Comparative Example 2, the core diameter of the central core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.0 μm, and the thickness of the pure SiO ₂ glass layer is 0.3 μm. The outer diameter is 3.6 μm. The clad diameter is 60 μm. Moreover, the kind and density | concentration of the dopant added to the center core part, the outer periphery core part, and the clad part are the same as those of the optical fibers according to Examples 2 and 3.

FIG. 23 is a diagram showing a refractive index profile of an optical fiber according to Comparative Example 2. FIG. 24 is a diagram showing a sound velocity profile of the optical fiber according to Comparative Example 2. As shown in FIGS. 23 and 24, the optical fiber according to Comparative Example 2 is a step type having a relative refractive index difference of 2.5% with respect to the cladding portion of the core portion, and has a pure SiO ₂ glass layer, and from the center. The sound speed gradually decreases until the boundary position between the outer peripheral core portion and the pure SiO ₂ glass layer, that is, from the center to the position of 83% of the radius, and then the specific sound speed difference reaches 0% in the pure SiO ₂ glass layer. It has an increasing sound speed profile. The optical characteristics of the optical fiber according to Comparative Example 2 at a wavelength of 1550 nm are chromatic dispersion value of −10 ps / nm / km, dispersion slope of 0.030 ps / nm ² / km, and effective core area of 11.4 μm ² . there were.

  FIG. 25 is a diagram showing a Brillouin scattering spectrum of an optical fiber according to Comparative Example 2. As shown in FIG. 25, the Brillouin scattering spectrum of the optical fiber according to Comparative Example 2 has two peaks, similar to that shown in FIG. 19, but the maximum peak value near 10.7 GHz is about −2 dB. Yes, the effect of suppressing SBS was small.

In addition, in the optical fibers according to Examples 2 and 3 and Comparative Example 2, the pure SiO ₂ glass layer formed on the outermost periphery of the core portion is not limited to the case where the glass preform for the core portion is manufactured by the VAD method. When a core glass preform is produced by a MCVD method using a pure SiO ₂ glass reaction tube, and then the reaction tube is cut and removed from the core glass preform before forming the cladding, one of It is also formed when the part remains. However, even if a part of the reaction tube remains and a pure SiO ₂ glass layer is formed in the core portion, the sound velocity from the center of the core portion to 85%, preferably 90% of the radius is reached. If is decreased stepwise or continuously, an optical fiber that exhibits the effects of the present invention is obtained.

Example 4
Next, an optical fiber according to Embodiment 4 of the present invention will be described. Similar to the optical fiber according to the first embodiment, the optical fiber according to the fourth embodiment includes a core portion including SiO ₂ glass as a main component and having a central core portion and an outer peripheral core portion, and a cladding portion. In the optical fiber according to Example 4, the core diameter of the central core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.6 μm, and the cladding diameter is 60 μm. Further, as a dopant, 14.1Wt% of GeO ₂ in the central core portion, Al ₂ a O ₃ was added 7.8wt%, 16.4wt% of GeO ₂ in the outer peripheral core portion, the Al ₂ O ₃ 6. 3 wt% was added, and 3.0 wt% of F was added to the cladding part.

  FIG. 26 is a diagram illustrating a refractive index profile of the optical fiber according to the fourth embodiment. FIG. 27 is a diagram illustrating a sound velocity profile of the optical fiber according to the fourth embodiment. As shown in FIGS. 26 and 27, the optical fiber according to Example 4 has a step-type refractive index profile with a relative refractive index difference of 2.5% with respect to the cladding portion of the core portion, and the outer periphery of the core portion from the center. The sound velocity profile is such that the sound velocity decreases step by step, and the specific sound velocity difference with respect to the outer core portion of the central core portion is about 1.4%, that is, the sound velocity of the central core portion is higher than the sound velocity of the outer core portion. It was about 1.4% faster. In addition, the optical characteristic in wavelength 1550nm of the optical fiber which concerns on Example 4 was the same as that of the optical fiber which concerns on Example 1. FIG.

  FIG. 28 is a diagram illustrating a Brillouin scattering spectrum of the optical fiber according to the fourth embodiment. As shown in FIG. 28, the Brillouin scattering spectrum of the optical fiber according to Example 4 has a narrower peak width and a higher value than that shown in FIG. 16, but the maximum peak value near 10.6 GHz is It was about -5 dB, and it was confirmed that SBS can be effectively suppressed.

(Example 5)
Next, an optical fiber according to Embodiment 5 of the present invention will be described. Similar to the optical fiber according to the fourth embodiment, the optical fiber according to the fifth embodiment includes a core portion that includes SiO ₂ glass as a main component, a central core portion and an outer peripheral core portion, and a cladding portion. The core diameter of the core portion is 2.1 μm, the core diameter of the outer peripheral core portion is 3.6 μm, and the cladding diameter is 60 μm. Further, as a dopant, the GeO ₂ in the central core portion was added 17.3 wt% and Al ₂ O ₃ 3.9 wt%, the 18.8Wt% and Al ₂ O ₃ and GeO ₂ on an outer peripheral core portion 2. 9 wt% was added, and 3.7 wt% of F was added to the cladding.

  FIG. 29 is a diagram illustrating a refractive index profile of the optical fiber according to the fifth embodiment. FIG. 30 is a diagram illustrating a sound velocity profile of the optical fiber according to the fifth embodiment. As shown in FIGS. 29 and 30, the optical fiber according to Example 5 had the same refractive index profile and sound velocity profile as those of the optical fiber according to Example 4, but the sound velocity of the central core portion was the outer core. It was about 1% faster than the sound speed of the club. The optical characteristics of the optical fiber according to Example 5 at the wavelength of 1550 nm were the same as those of the optical fiber according to Example 1.

  FIG. 31 is a diagram illustrating a Brillouin scattering spectrum of the optical fiber according to the fifth embodiment. As shown in FIG. 31, the Brillouin scattering spectrum of the optical fiber according to Example 5 has a narrower peak width and a higher value than that shown in FIG. 28, but the maximum peak value is about −3 dB. It was. That is, it was confirmed that SBS can be effectively suppressed if the sound speed of the central core portion is 1% or more faster than the sound speed of the outer core portion.

(Comparative Examples 3-5)
Next, optical fibers according to Comparative Examples 3 to 5 of the present invention will be described. First, as shown in FIGS. 32 and 33, the optical fiber according to Comparative Example 3 has the same refractive index profile and sound velocity profile as those of the optical fiber according to Comparative Example 1, but the optical characteristics at a wavelength of 1550 nm are as described in Examples. Similar to 1, the chromatic dispersion value is 0 ps / nm / km, the dispersion slope is 0.035 ps / nm ² / km, and the effective core area is 12.2 μm ² . As shown in FIG. 34, the optical fiber according to Comparative Example 3 has a maximum peak value in the Brillouin scattering spectrum of approximately 0 dB, which is as high as that of the optical fiber according to Comparative Example 1.

  Next, as shown in FIGS. 35 and 36, the optical fiber according to Comparative Example 4 has the same refractive index profile as that of the optical fiber according to Example 1, but the acoustic wave is localized in the core portion and in the vicinity of the core portion. The sound velocity profile of the clad portion is slower than the sound velocity of the core portion so as not to propagate through the optical fiber in the propagation mode. However, in the optical fiber according to the comparative example 4, although the acoustic wave is not localized in the core part and the vicinity of the core, the acoustic wave having a field distribution shape similar to the light intensity distribution shape propagates in the core part and the vicinity of the core part. Therefore, the total sum of spatial overlap between the light propagating in the optical fiber and the acoustic wave becomes large. As a result, as shown in FIG. 37, the maximum peak value in the Brillouin scattering spectrum was only slightly smaller than 0 dB.

  Next, as shown in FIGS. 38 and 39, the optical fiber according to Comparative Example 5 has the same refractive index profile as that of the optical fiber according to Comparative Example 1, but the intensity distribution shape of light in the core portion and in the vicinity of the core portion. The acoustic wave profile has a shape in which the sound speed increases stepwise from the center to the outer periphery of the core so that an acoustic wave having a field distribution shape similar to that does not propagate. However, since the optical fiber according to the comparative example 5 propagates through the optical fiber in the propagation mode in which the acoustic wave is localized in the core part and in the vicinity of the core part, the spatial relationship between the light propagating in the optical fiber and the acoustic wave The total sum of overlaps increases. As a result, as shown in FIG. 40, in the Brillouin scattering spectrum, there were peaks having substantially the same values at frequencies of 8, 7 GHz, and 9.4 GHz, and the peak value was −2 dB.

It is sectional drawing which showed typically the optical fiber which concerns on Embodiment 1 of this invention. 4 is a diagram schematically showing a refractive index profile and a sound velocity profile in the radial direction of the optical fiber according to Embodiment 1. FIG. FIG. 3 is a diagram schematically showing a concentration profile in the radial direction of GeO ₂ and Al ₂ O ₃ in the core portion of the optical fiber according to the first embodiment. Each dopant is a diagram showing the SiO ₂ refractive index or the sound velocity of the increase or decrease of the glass when added to SiO ₂ glass. It is sectional drawing which showed typically the optical fiber which concerns on Embodiment 2 of this invention. It is the figure which showed typically the refractive index profile and sound speed profile of the radial direction of the optical fiber which concerns on Embodiment 2. FIG. FIG. 6 is a diagram schematically showing a concentration profile of GeO ₂ and Al ₂ O ₃ in a core portion of an optical fiber according to a second embodiment. It is sectional drawing which showed typically the optical fiber which concerns on Embodiment 3 of this invention. It is the figure which showed typically the refractive index profile and sound velocity profile of the radial direction of the optical fiber which concerns on Embodiment 3. FIG. FIG. 6 is a diagram schematically showing a concentration profile of GeO ₂ and Al ₂ O ₃ in a core portion of an optical fiber according to a third embodiment. It is the figure which showed the refractive index profile of the optical fiber which concerns on the comparative example 1. It is the figure which showed the sound velocity profile of the optical fiber which concerns on the comparative example 1. FIG. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on the comparative example 1. 3 is a diagram illustrating a refractive index profile of an optical fiber according to Example 1. FIG. It is the figure which showed the sound velocity profile of the optical fiber which concerns on Example 1. FIG. 6 is a diagram showing a Brillouin scattering spectrum of the optical fiber according to Example 1. FIG. 6 is a diagram showing a refractive index profile of an optical fiber according to Example 2. FIG. It is the figure which showed the sound speed profile of the optical fiber which concerns on Example 2. FIG. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on Example 2. FIG. 6 is a diagram illustrating a refractive index profile of an optical fiber according to Example 3. FIG. It is the figure which showed the sound speed profile of the optical fiber which concerns on Example 3. FIG. 6 is a diagram showing a Brillouin scattering spectrum of an optical fiber according to Example 3. FIG. It is the figure which showed the refractive index profile of the optical fiber which concerns on the comparative example 2. It is the figure which showed the sound speed profile of the optical fiber which concerns on the comparative example 2. FIG. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on the comparative example 2. 6 is a diagram showing a refractive index profile of an optical fiber according to Example 4. FIG. It is the figure which showed the sound speed profile of the optical fiber which concerns on Example 4. FIG. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on Example 4. FIG. FIG. 10 is a diagram showing a refractive index profile of an optical fiber according to Example 5. It is the figure which showed the sound speed profile of the optical fiber which concerns on Example 5. FIG. 10 is a diagram showing a Brillouin scattering spectrum of an optical fiber according to Example 5. FIG. It is the figure which showed the refractive index profile of the optical fiber which concerns on the comparative example 3. It is the figure which showed the sound velocity profile of the optical fiber which concerns on the comparative example 3. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on the comparative example 3. It is the figure which showed the refractive index profile of the optical fiber which concerns on the comparative example 4. It is the figure which showed the sound speed profile of the optical fiber which concerns on the comparative example 4. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on the comparative example 4. It is the figure which showed the refractive index profile of the optical fiber which concerns on the comparative example 5. It is the figure which showed the sound speed profile of the optical fiber which concerns on the comparative example 5. FIG. It is the figure which showed the Brillouin scattering spectrum of the optical fiber which concerns on the comparative example 5.

Explanation of symbols

1-3 Optical fiber 11-31 Core part 12-32 Clad part 211 Central core part 212 Outer core part 321 Inner clad part 322 Outer clad part L1a-L4a Refractive index profile L1b-L4b Sound velocity profile L1c-L4c, L1d-L4d density Profile

Claims (11)

Add multiple types of dopants, including dopants that increase refractive index and decrease sound speed, and dopants that increase refractive index and increase sound speed, and gradually or continuously increase the sound speed from the center to the position of 85% of the radius. A reduced core, and
A clad part having a refractive index lower than that of the core part formed on the outer periphery of the core part and having a low sound velocity;
An optical fiber comprising:   2. The optical fiber according to claim 1, wherein the core portion reduces the speed of sound stepwise or continuously from a center to a position of 90% of the radius.   3. The optical fiber according to claim 1, wherein the core portion includes a central core portion and an outer peripheral core portion that is formed on an outer periphery of the central core portion and has a lower sound speed than the central core portion.   The optical fiber according to claim 3, wherein the sound speed of the central core portion is 1% or more faster than the sound speed of the outer peripheral core portion.   The clad part includes an inner clad part having a lower refractive index than the core part formed on the outer periphery of the core part, and a lower refractive index than the inner clad part formed on the outer periphery of the inner clad part. The optical fiber according to claim 1, further comprising an outer clad portion having a high refractive index. The core portion composed mainly of SiO ₂ glass, the plurality of kinds of dopants optical fiber according to any one of claims 1 to 5, characterized in that it comprises a GeO ₂ and Al ₂ O _3. The optical fiber according to claim 6, wherein Al ₂ O ₃ is added to the core portion so that the concentration decreases stepwise or continuously from the center to a position of 85% of the radius. 8. The optical fiber according to claim 6, wherein GeO ₂ is added to the core portion so that the concentration increases stepwise or continuously from the center to a position of 85% of the radius.   The optical fiber according to any one of claims 1 to 8, wherein an absolute value of chromatic dispersion at a wavelength of 1550 nm is 10 ps / nm / km or less. 10. The optical fiber according to claim 1, wherein an effective core area at a wavelength of 1550 nm is 18 μm ² or less.   The optical fiber according to claim 1, wherein a relative refractive index difference between the core portion and the clad portion at a wavelength of 1550 nm is 2% or more.

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