Single-mode optical fiber

Abstract

To provide an optical fiber with which a zero-dispersion slope and a wavelength dispersion value are reduced with low bending losses and low transmission losses by adjusting the refractive index distribution shape of a core region and an inside clad region.SOLUTION: Provided is a single-mode optical fiber composed of a core and a first and a second clad. When it is assumed that n0 denotes a central part refractive index, rcore denotes a position of n0×0.45, r2 denotes a position of rcore×2.2, n2 denotes a refractive index there, n3 denotes the minimum refractive index value of the first clad, r3 denotes a position of n3, r4 denotes the boundary position between the first and the second clads, and n4 denotes the average refractive index of the second clad. A relative refractive index difference Δn0 calculated from the core refractive index n0 and the second clad refractive index n4 is 0.3-0.5%. A relative refractive index difference Δn2 calculated from the refractive index n2 of the first clad and the refractive index n4 of the second clad is -0.12 to -0.05%. The Δn3 calculated from n3 and n4 is -0.20 to -0.12%, and the relative refractive index difference of the first clad decreases gently from rcore through to r3.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a single-mode optical fiber having characteristics of low bending loss and low transmission loss, and reduced zero-dispersion slope and wavelength dispersion value.

Background Art

[0002] Conventional single-mode optical fibers are characterized in that signal light propagates through the core portion of the optical fiber, and signals can be transmitted even when the optical fiber is slightly bent. Generally, in a single-mode optical fiber, as the bending radius decreases, the ratio of light that cannot be propagated and leaks from the core increases exponentially, resulting in an increase in transmission loss. This is the bending loss. In recent years, while optical fibers may be used in a bent state with a curvature radius of 15 mm or less to about 10 mm, optical fibers with lower loss are required. As a standard regarding the low bending loss of optical fibers, a standard defined in ITU-T G.657 is known. For example, in the G.657.A2 standard, it is specified that the bending loss at a wavelength of 1550 nm when bent with a radius of 10 mm is 0.10 dB / turn or less.

[0003] To reduce the bending loss, it is effective to increase the refractive index of the core to confine the light in the core. This is improved by reducing the mode field diameter (MFD). For this reason, optical fibers with an MFD of about 8.2 to 8.8 μm are often used. Also, by adding a down-dopant such as fluorine to the cladding layer on the outer periphery of the core layer and adopting a trench-type refractive index distribution with a lower refractive index, light can be confined in the core and the bending loss can be reduced.

[0004] In Patent Documents 1 to 3, a low-bending-loss fiber is achieved by appropriately controlling each structural parameter of the refractive index distribution, such as the width, depth, outer diameter of the trench layer, the refractive index difference of the core portion, and the core diameter. For the formation of the trench layer, a method of covering a fluorine-doped glass tube after forming the core and the inner cladding layer is generally used, but the manufacturing process of the base material increases and becomes complicated. Furthermore, impurities such as OH groups are likely to be mixed in the vicinity of the interface of the trench layer, and in addition, there is a composition difference (fluctuation of the interface) accompanying a rapid change in the refractive index at the interface, which may cause an increase in transmission loss.

[0005] To deepen the trench layer, it is necessary to increase the fluorine addition amount. On the other hand, when the fluorine addition amount is increased, the zero-dispersion wavelength decreases, and the zero-dispersion slope and the wavelength dispersion value decrease. In order to have characteristics of low bending loss and low transmission loss and for various optical characteristics to fall within a desired range, it is important to appropriately control the refractive index distribution shapes of the core region and the inner cladding region.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] Therefore, the present invention has been made in view of the above problems, and provides a single-mode optical fiber having characteristics of low bending loss and low transmission loss by adjusting the refractive index distribution shapes of the core region and the inner cladding region without forming a conventional complicated trench layer.

Means for Solving the Problems

[0008] The single-mode optical fiber of the present invention solves the above problems and is a single-mode optical fiber comprising a core layer, a first cladding layer adjacent to the outer periphery of the core layer, and a second cladding layer adjacent to the outer periphery of the first cladding layer. The refractive index at the center of the core is n0, and the radius position of the refractive index that becomes n0×0.45 with respect to the refractive index n0 is r コア is defined as, and in the first cladding layer, r コア ×2.2 is defined as the radius position r2, the refractive index at r2 is n2, the minimum value of the refractive index of the first cladding layer is n3, the radius position at which n3 is obtained is r3, the boundary radius position between the first cladding layer and the second cladding layer is r4, and the average refractive index of the second cladding layer is n4. When the relative refractive index difference Δn0 of the core layer calculated from the refractive index n0 of the core layer and the refractive index n4 of the second cladding layer is 0.3% or more and 0.5% or less, and the relative refractive index difference Δn2 calculated from the refractive index n2 of the first cladding layer and the refractive index n4 of the second cladding layer is -0.12% or more and -0.05% or less, and the relative refractive index difference Δn3 calculated from the refractive index n3 in the first cladding layer and the refractive index n4 of the second cladding layer is -0.20% or more and -0.12% or less, the non-refractive index difference of the first cladding layer is continuously and gently decreasing from the radius position r コア to r3.

[0009] In the core layer, when the radius position where r コア ×0.8 is defined as r1, it is desirable that for the differential value dΔ(r) / dr of the relative refractive index difference Δ(r) (%) with respect to the distance r (μm) from the center of the core layer, in the range where the radius position r is 0 to r1, dΔ(r) / dr≧-0.08% / μm is satisfied. Also, it is desirable that r コア is 3 to 6 μm and r4 is 14 to 20 μm.

[0010] The single-mode optical fiber of the present invention has a bending loss at a wavelength of 1550 nm when bent at a bending radius of 10 mm of 0.10 dB / turn or less.

[0011] The mode field diameter (MFD) at a wavelength of 1310 nm is 8.2 to 9.4 μm, the zero-dispersion wavelength (λ0) is in the range of 1300 to 1324 nm, and the dispersion slope at the zero-dispersion wavelength (zero-dispersion slope) is 0.091 ps / nm 2 / km or less, and the wavelength dispersion value at a wavelength of 1550 nm is 17.5 ps / nm / km or less. Regarding the cut-off wavelength, the value (λcc) measured with a fiber length of 22 m is 1260 nm or less. In the optical fiber of the present invention, germanium, fluorine, and chlorine are added to the core layer and the first cladding layer, and chlorine is added to the second cladding layer.

Advantages of the Invention

[0012] According to the present invention, excellent effects such as obtaining a single-mode optical fiber having characteristics of low bending loss and low transmission loss and reducing the zero-dispersion slope and the wavelength dispersion value can be achieved.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Modes for Carrying Out the Invention

[0014] The single-mode optical fiber of the present invention achieves an optical fiber with characteristics of low bending loss and low transmission loss by adjusting the refractive index distribution shapes of the core region and the inner cladding region, and reduces the zero-dispersion slope and the wavelength dispersion value. Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the single-mode optical fiber of the present invention is not limited to the examples of the following embodiments, and various modes are possible.

[0015] FIG. 1 shows an example of the refractive index distribution structure of the optical fiber according to this embodiment. The radius r of the core layer コア is the radius position at which the refractive index becomes n0 × 0.45 with respect to the refractive index n0 at the core center (r = 0). In the core layer, the radius position where r コア × 0.8 is r1, the refractive index at r1 is n1, in the first cladding layer, the radius position where r コア × 2.2 is r2, the refractive index at r2 is n2, the minimum value of the refractive index of the first cladding layer is n3, the radius position at which the refractive index is n3 is r3, the boundary radius position between the first cladding layer and the second cladding layer is r4, and the average refractive index of the second cladding layer is n4.

[0016] Also, the relative refractive index difference of each layer is defined as follows. The relative refractive index difference Δ(r) at the distance r from the center of the core layer is Δ(r) = 100 × (n(r) - n4) / n(r) with respect to the refractive index n4 of the second cladding layer. Therefore, the relative refractive index difference Δn0 of the core layer is calculated from the core center refractive index n0 and the refractive index n4 of the second cladding layer, the relative refractive index difference Δn2 of the first cladding layer is calculated from the refractive index n2 of the first cladding layer and the refractive index n4 of the second cladding layer, and the relative refractive index difference Δn3 of the first cladding layer is calculated from the refractive index n3 of the first cladding layer and the refractive index n4 of the second cladding layer. Note that the relative refractive index difference Δ(r) was measured and calculated at a pitch of 0.15 μm using an optical fiber with a diameter of 125 μm.

[0017] Here, the wavelength dispersion will be briefly described. Chromatic dispersion is a phenomenon in which light of different wavelengths propagates through an optical fiber at different speeds, resulting in differences in propagation time. Chromatic dispersion distorts the optical signal waveform and is one of the factors limiting transmission capacity. Chromatic dispersion is wavelength-dependent and is expressed as the sum of material dispersion and structural dispersion. Material dispersion is difficult to control because it is caused by the wavelength dependence of the refractive index of the glass material used in the optical fiber. On the other hand, structural dispersion is caused by the refractive index distribution shape, so it is important to properly control the refractive index distribution shape. The wavelength at which chromatic dispersion becomes zero is the zero-dispersion wavelength, and the zero-dispersion slope, which is also specified in the ITU-T standard, represents the inclination of chromatic dispersion at the zero-dispersion wavelength.

[0018] In the present invention, the reduction of the zero dispersion slope and chromatic dispersion at a wavelength of 1550 nm (hereinafter referred to as 1550 nm chromatic dispersion) is realized by adjusting the refractive index profile shape of the core region and the inner cladding region. The zero dispersion slope and 1550 nm chromatic dispersion generally have a negative correlation with the zero dispersion wavelength, and in order to reduce both the zero dispersion slope and the 1550 nm chromatic dispersion, it is effective to increase the zero dispersion wavelength.

[0019] Furthermore, in order to confirm which part of the refractive index profile is effective for the zero dispersion slope and 1550 nm wavelength dispersion, the optical characteristics were estimated while changing the shape of a specific part of the refractive index profile. To calculate the estimated values, a scalar wave equation was solved by the finite element method.

[0020] As a result of the estimated calculation, the core diameter r コア Whereas, r コア It was found that the relative refractive index difference Δn2 at the radial position r2 where Δn2 is 1×2.2 is one of the important factors. By increasing Δn2, it was possible to raise the zero-dispersion wavelength and reduce both the zero-dispersion slope and the 1550 nm wavelength dispersion. On the other hand, if Δn2 is made too large, the refractive index difference with the core layer becomes small, and the bending loss specified in ITU-T G.657.A2 cannot be obtained. It is desirable for the relative refractive index difference Δn2 in the first cladding layer to be -0.12 to -0.05%.

[0021] The relative refractive index difference Δn0 of the core layer is desirably 0.30% to 0.50%. When the relative refractive index difference is less than 0.3%, the refractive index difference from the cladding layer becomes small, and the bending loss defined in ITU-T G.657.A2 cannot be obtained. On the other hand, when it is 0.5% or more, the dopant concentration in the core portion becomes high, and there is a concern about the deterioration of transmission loss due to an increase in Rayleigh scattering.

[0022] The relative refractive index difference Δn3 in the first cladding layer is desirably -0.20 to -0.12%. When the relative refractive index difference is less than -0.20%, the negative dopant concentration in the cladding layer becomes high, and there is a concern about the deterioration of transmission loss due to an increase in Rayleigh scattering. On the other hand, when it is greater than -0.12%, the refractive index difference from the core layer becomes small, and the bending loss defined in ITU-T G.657.A2 cannot be obtained.

[0023] The core shape was also found to be one of the important factors for the zero-dispersion slope and the 1550 nm wavelength dispersion. A core shape in which the refractive index rapidly increases toward the center of the core can increase the zero-dispersion wavelength and lower the 1550 nm wavelength dispersion, but it was found to lead to an increase in the zero-dispersion slope. When the radius position where it becomes r_core × 0.8 in the core layer is r1, for dΔ(r) / dr, it is desirable that dΔ(r) / dr ≥ -0.08% / μm in the range where the distance r from the center of the core layer is 0 to r1.

[0024] The non-refractive index difference of the first cladding layer is desirably continuously and gently decreasing from the radius position r コア to r3 in order to suppress a rapid change in the refractive index, that is, a rapid change in the composition.

[0025] Next, a method for manufacturing a single-mode optical fiber according to the present invention will be described. First, a porous glass preform composed of a core layer and a first cladding layer is synthesized by the VAD method. At this time, burners for forming the core layer and burners for forming the first cladding layer are used. To the burner for forming the core layer, in addition to hydrogen, oxygen, and SiCl4 as a source gas, GeCl4 is supplied to dope germanium for increasing the refractive index. To the burner for forming the first cladding layer, hydrogen, oxygen, and SiCl4 are supplied as a source gas to deposit the first cladding layer and produce a soot preform. Note that the core shape can be controlled by adjusting the gas flow rate supplied to the burner for forming the core layer and the setting of the burner.

[0026] Next, the produced soot preform is sintered in the following procedure. First, as a dehydration treatment and a fluorine doping treatment of the soot preform, the entire length of the soot preform is heat-treated in a mixed gas atmosphere of Ar = 25 L / min, Cl2 = 1.0 L / min, and SiF4 = 0.25 L / min in the furnace at a sintering temperature of 1200°C and a lowering rate of 10 mm / min. Next, as a transparent vitrification treatment, the entire length of the soot preform is heat-treated in a gas atmosphere of He = 20 L / min in the furnace at a sintering temperature of 1500°C and a lowering rate of 5 mm / min.

[0027] Using the transparent core preform thus produced as a target, a second cladding layer is externally attached by the OVD method. The porous preform thus obtained is sintered and transparently vitrified to obtain an optical fiber preform. Thereafter, an optical fiber with a diameter of 125 μm can be obtained by heating the optical fiber preform to about 2100°C and spinning it.

Example

[0028] [Example 1] First, a porous glass base material composed of a core and a first cladding layer was synthesized by the VAD method. The core was doped with germanium to increase the refractive index. This porous glass base material was supplied with 1 liter per minute of chlorine gas, 0.25 liters per minute of silane tetrafluoride gas, and further 25 liters per minute of Ar gas, heated to about 1200 °C, and the porous glass base material was pulled down at 10 mm / min to perform dehydration and fluorine doping. Subsequently, 20 liters per minute of He gas was supplied, heated to about 1500 °C, and the porous glass base material was pulled down at 5 mm / min to obtain a transparent glass core base material. Note that methane tetrafluoride, ethane hexafluoride, or the like may be used instead of silane tetrafluoride gas.

[0029] Using the transparent core base material composed of the core and the first cladding layer thus produced as a target, a second cladding layer was externally attached by the OVD method. The obtained porous base material was sintered and vitrified to produce an optical fiber base material. The obtained base material was heated to about 2100 °C and spun to obtain an optical fiber with a diameter of 125 μm. The refractive index distribution of the obtained optical fiber is shown in Figure 2, and Figure 3 shows dΔ(r) / dr.

[0030] [Comparative Example 1-1] First, a porous glass base material composed of a core and a first cladding layer was synthesized by the VAD method. The core was doped with germanium to increase the refractive index. At this time, the gas flow rate supplied to the burner for forming the core layer and the setting of the burner were changed, and adjustments were made to obtain the core shape as shown in FIG. 4. This porous glass base material was supplied with 1 liter per minute of chlorine gas, 0.35 liter per minute of silane tetrafluoride gas, and further 25 liters per minute of Ar gas, heated to about 1230°C, and the porous glass base material was pulled down at 10 mm / min for dehydration and fluorine doping. Subsequently, 20 liters per minute of He gas was supplied, heated to about 1500°C, and the porous glass base material was pulled down at 5 mm / min to obtain a transparent glass core base material. Thereafter, a second cladding layer was externally attached in the same manner as in Example 1, vitrified to produce an optical fiber base material, and spun to obtain an optical fiber. The refractive index distribution of the obtained optical fiber is shown in FIG. 4, and FIG. 5 shows dΔ(r) / dr.

[0031] [Comparative Example 1-2] First, a porous glass base material composed of a core and a first cladding layer was synthesized by the VAD method. The core was doped with germanium to increase the refractive index. At this time, the gas flow rate supplied to the burner for forming the core layer and the setting of the burner were changed, and adjustments were made to obtain the core shape as shown in FIG. 6. This porous glass base material was supplied with 1 liter per minute of chlorine gas, 0.15 liter per minute of silane tetrafluoride gas, and further 25 liters per minute of Ar gas, heated to about 1180°C, and the porous glass base material was pulled down at 10 mm / min for dehydration and fluorine doping. Subsequently, 20 liters per minute of He gas was supplied, heated to about 1500°C, and the porous glass base material was pulled down at 5 mm / min to obtain a transparent glass core base material. Thereafter, a second cladding layer was externally attached in the same manner as in Example 1, vitrified to produce an optical fiber base material, and spun to obtain an optical fiber. The refractive index distribution of the obtained optical fiber is shown in FIG. 6, and FIG. 7 shows dΔ(r) / dr.

[0032] Table 1 shows various parameters of the optical fibers obtained in the examples and comparative examples.

[0033]

Table 1

[0034] In Example 1, the transmission loss at a wavelength of 1310 nm was 0.327 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.04 dB / turn, which met the bending loss value specified in ITU-T G.657.A2. Also, the zero-dispersion slope was 0.089 ps / nm 2 / km, and the wavelength dispersion at a wavelength of 1550 nm was 17.3 ps / nm / km, both of which were good values.

[0035] In Comparative Example 1-1, compared with Example 1, the fluorine doping advanced to the inside of the first cladding layer, and due to the decrease in Δn2 and Δn3, the bending loss decreased. The transmission loss at a wavelength of 1310 nm was 0.334 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.02 dB / turn, which met the bending loss value specified in ITU-T G.657.A2. However, on the other hand, due to the decrease in Δn2, the zero-dispersion slope was 0.093 ps / nm 2 / km, which deviated from the ITU-T standard. Also, the wavelength dispersion at a wavelength of 1550 nm increased to 17.9 ps / nm / km. Also, the refractive index of the core shape became steep from r1 to the center, and in the range of r = 0 to r1, the minimum value of dΔ(r) / dr was -0.09% / μm. Furthermore, due to the influence of the increase in the fluorine addition amount, the transmission loss at a wavelength of 1310 nm also increased compared with Example 1.

[0036] In Comparative Example 1-2, the transmission loss at a wavelength of 1310 nm was 0.329 dB / km, and the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm was 0.12 dB / turn, which deviated from the standard of ITU-T G.657.A2. The zero-dispersion slope was 0.087 ps / nm 2 / km, and the wavelength dispersion at a wavelength of 1550 nm was 17.1 ps / nm / km. In Comparative Examples 1-2, compared with Example 1, fluorine doping did not progress to the inside of the first cladding layer, and Δn2 and Δn3 became large, resulting in a large bending loss.

Claims

Claim 1 A single-mode optical fiber comprising a core layer, a first cladding layer adjacent to the outer periphery of the core layer, and a second cladding layer adjacent to the outer periphery of the first cladding layer, wherein the refractive index of the core center portion is n ０ , the refractive index n ０ , with respect to which n ０ ×0.45, the radius position of the refractive index is r コア , and in the first cladding layer, r コア ×2.2, the radius position is r ２ , the refractive index at the r ２ is n ２ , the minimum value of the refractive index of the first cladding layer is n ３ , the radius position where the n ３ is r ３ , the boundary radius position between the first cladding layer and the second cladding layer is r ４ , and the average refractive index of the second cladding layer is n ４ , when the refractive index n ０ of the core layer and the refractive index n ４ of the second cladding layer are used, the relative refractive index difference Δn ０ of the core layer is 0.3% or more and 0.5% or less, and the relative refractive index difference Δn ２ calculated from the refractive index n ４ of the first cladding layer and the refractive index n ２ of the second cladding layer is -0.12% or more and -0.05% or less, and the relative refractive index difference Δn ３ calculated from the refractive index n ４ in the first cladding layer and the refractive index n ３ of the second cladding layer is -0.20% or more and -0.12% or less, and the non-refractive index difference of the first cladding layer continuously and gently decreases from the radius position r コア to r ３ . A single-mode optical fiber characterized by this. Claim 2 In the core layer, when the radius position where r コア × 0.8 is r １ , with respect to the differential value dΔ(r) / dr of the relative refractive index difference Δ(r) (%) with respect to the distance r (μm) from the center of the core layer, when the radius position r is in the range of 0 to r １ , the single-mode optical fiber according to claim 1, satisfying dΔ(r) / dr ≥ -0.08% / μm. Claim 3 The aforesaid r コア is 3 to 6 μm, and the aforesaid r ４ is 14 to 20 μm. The single-mode optical fiber according to claim 1 or 2. Claim 4 The single-mode optical fiber according to claim 1 or 2, wherein the bending loss at a wavelength of 1550 nm when bent with a bending radius of 10 mm is 0.10 dB / turn or less. Claim 5 The single-mode optical fiber according to claim 1 or 2, wherein the mode field diameter at a wavelength of 1310 nm is 8.2 to 9.4 μm. Claim 6 The zero-dispersion wavelength is in the range of 1300 to 1324 nm, and the dispersion slope at the zero-dispersion wavelength is 0.091 ps / nm ２ / km or less, and the wavelength dispersion value at a wavelength of 1550 nm is 17.5 ps / nm / km or less. The single-mode optical fiber according to claim 1 or 2 Claim 7 The single-mode optical fiber according to claim 1 or 2, wherein the cut-off wavelength measured with a fiber length of 22 m is 1260 nm or less. Claim 8 The single-mode optical fiber according to claim 1 or 2, wherein germanium, fluorine, and chlorine are added to the core layer and the first cladding layer, and chlorine is added to the second cladding layer.

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