阅读说明：本技术 光纤和光传输系统 (Optical fiber and optical transmission system ) 是由 寒河江悠途 中岛和秀 松井隆 于 2018-07-03 设计创作，主要内容包括：本发明的光纤具备：芯体；设置于芯体的外周、与芯体相比为低折射率的第一包层；以及设置于第一包层的外周、与第一包层相比为低折射率的第二包层。关于本发明的光纤,波长1.55μm时的模场直径为11.5μm以上,截止波长为1.53μm以下,弯曲半径30mm和波长1.625μm时的弯曲损耗为2dB/100turns以下,在波长1.55μm下每单位长度的传播光的延迟时间为4.876μs/km以下。(The optical fiber of the present invention has a mode field diameter of 11.5 [ mu ] m or more at a wavelength of 1.55 [ mu ] m, a cutoff wavelength of 1.53 [ mu ] m or less, a bending loss of 2dB/100turns or less at a bending radius of 30mm and a wavelength of 1.625 [ mu ] m, and a delay time per unit length of propagation light of 4.876 [ mu ] s/km or less at a wavelength of 1.55 [ mu ] m.)

An optical fiber of kinds, comprising:

a core body;

th clad layer having a lower refractive index than the core and provided on the outer peripheral portion of the core, and

a second clad layer provided at an outer peripheral portion of the th clad layer and having a lower refractive index than the th clad layer,

wherein the mode field diameter at a wavelength of 1.55 μm is 11.5 μm or more,

a cutoff wavelength of 1.53 μm or less,

a bending loss at a bending radius of 30mm and a wavelength of 1.625 μm of 2dB/100turns or less,

the delay time per unit length of light propagating at a wavelength of 1.55 μm is 4.876 μ s/km or less.

2. The optical fiber of claim 1,

the radius of the core is 1.0 [ mu ] m or more and 4.3 [ mu ] m or less,

the radius of the th cladding satisfies the following equations (1) and (2),

[ mathematical formula 1]

[ mathematical formula 2]

In addition, in the formulae (1) and (2), a₁Is the radius [ mu ] m of the core]，a₂Radius [ mu ] m of the cladding of the th layer]，Δ₁Is the relative refractive index difference [% of the th cladding layer relative to the core body]。

3. The optical fiber of claim 2,

the relative refractive index difference of the second cladding with respect to the th cladding satisfies the following formula (3),

[ mathematical formula 3]

。

4. The optical fiber of any of claims 1-3,

in the cross-sectional view of the device,

a plurality of low-retardation cores having the core as the th core and the th clad provided on the outer periphery of the th core are arranged on a concentric circle at the center of the second clad.

5. The optical fiber of claim 4,

the core as a second core is disposed in the center of the second clad.

6. The optical fiber of any of claims 1-3,

in the cross-sectional view of the device,

a low-retardation core having the core as an th core and the th clad provided on the outer periphery of the th core is arranged at the center of the second clad,

the core as a third core is disposed concentrically around the low-delay core.

7. The optical fiber of any of claims 1-3,

in the cross-sectional view of the device,

a low-retardation core having the core as an th core and the th clad provided on the outer periphery of the th core is arranged at the center of the second clad,

the core as a fourth core is filled in a closest-packed manner with the low-delay core as a center.

An optical transmission system of types, comprising:

the optical fiber of any of claims 1-7;

a transmitter connected to ends of the optical fiber, and

a receiver connected to the other end of the optical fiber.

Technical Field

The present application is based on Japanese application No. 2017-130725 filed on 7/3.2017, the contents of which are incorporated herein by reference.

Background

In recent years, with diversification of use of communication networks, reduction of transmission delay is demanded. For example, in communication between computers that are frequently used in financial transactions performed on an international scale, a reduction in transmission delay of 1ms has a great influence on the transmission performance of communication, financial transaction services, and the profit or loss of customers. In the future, it is expected that the demand for increasing the reduction amount of the transmission delay will be accelerated.

In long-distance communication networks, such as submarine optical cable networks spanning the pacific, the communication lines run over lengths of thousands of km. In order to improve the transmission performance of a long-distance communication network, it becomes important to reduce the delay generated in the transmission line. With respect to the submarine fiber cable network, studies have been made to reduce the delay of communication lines by optimizing the laying path of submarine fiber cables across continents. Through this study, it is reported that the transmission delay of a long-distance communication network is reduced by about several ms.

The delay of the communication network includes a delay generated inside a transmission device or the like and a delay generated in a transmission path. In a long-distance communication network, the delay time occurring in the transmission path occupies most of the delay time occurring in the entire network, and is so large as to be non-negligible.

The delay time per unit length of an optical fiber constituting a transmission path of a communication network is mainly determined by the refractive index of an optical fiber medium. In order to reduce the transmission delay of a communication network, it is effective to use a medium with a low refractive index as an optical fiber medium. The conventional cutoff-displacement optical fiber used in the submarine optical cable network has a core formed of high-purity silica glass. Thus, the conventional submarine optical fiber cable network can perform optical transmission with a delay time of about 4.876 μ s/km in a wavelength range including 1.55 μm. Non-patent document 1 reports that the delay time of a photonic bandgap fiber is reduced to about 3.448 μ s/km. The photonic band gap fiber has a hollow core in which the refractive index of the medium is lowered to the limit.

Disclosure of Invention

Problems to be solved by the invention

As described above, in the long-distance communication network, the delay time is improved by optimizing the routing path. However, when optimizing a paving route in practice, there are restrictions on geographical conditions and paving cost, and this restriction causes a problem that the reduction amount of delay time caused by optimizing the paving route is reduced.

In addition, a hollow core having a low refractive index is formed in the photonic band gap fiber. However, the propagation loss of the photonic band gap fiber reaches about several dB/km, and thus the photonic band gap fiber is not suitable for a transmission path of a long-distance communication network.

The present invention has been made in view of the above problems, and provides an optical fiber including: the fiber can be applied to a long-distance communication network, has a Mode field diameter (Mode field diameter: MFD) and a bending loss equivalent to those of a conventional cut-off displacement fiber, and has a delay time shorter than that of the cut-off displacement fiber.

In addition, the present invention provides kinds of optical transmission systems having the excellent characteristics of the optical fiber.

Means for solving the problems

As an optical fiber for solving the above problems, the present inventors newly found design conditions and structures of kinds of optical fibers having all five features of (1) a core, a -th clad adjacent to the outer periphery of the core, and a second clad adjacent to the outer periphery of the -th clad, (2) a core radius of 4 μm or less, (3) a relative refractive index difference of the -th clad with respect to the core of 0.0% or less, (4) a Mode Field Diameter (MFD) at a wavelength of 1.55 μm of 11.5 μm or more, and (5) a bending loss at a bending radius of 30mm and a wavelength of 1.625 μm of 2.0dB/100turns or less.

The optical fiber of the present invention includes a core, an th clad provided on an outer peripheral portion of the core and having a refractive index lower than that of the core, and a second clad provided on an outer peripheral portion of the th clad and having a refractive index lower than that of the th clad, wherein a mode field diameter of the optical fiber of the present invention at a wavelength of 1.55 μm is 11.5 μm or more, a cutoff wavelength of the optical fiber of the present invention is 1.53 μm or less, a bending loss of the optical fiber of the present invention at a bending radius of 30mm and a wavelength of 1.625 μm is 2dB/100 tus or less, and a delay time per unit length of the core of light propagating through the core of the optical fiber of the present invention at a wavelength of 1.55 μm is 4.876 μ s/km or less.

In the optical fiber of the present invention, the radius of the core may be 1.0 μm or more and 4.3 μm or less, and the radius of the th cladding may satisfy the following expressions (1) and (2).

[ mathematical formula 1]

[ mathematical formula 2]

In addition, in the formulae (1) and (2), a₁Is the radius [ mu ] m of the core]。a₂Radius [ mu ] m of the cladding of the th layer]。Δ₁Is the relative refractive index difference [% of the th cladding layer relative to the core body]。

In the optical fiber of the present invention, the relative refractive index difference between the second cladding and the -th cladding may satisfy the following expression (3).

[ mathematical formula 3]

In the optical fiber of the present invention, a plurality of low-retardation cores may be arranged on a concentric circle at the center of the second clad in a cross-sectional view, the low-retardation core having the core as an th core and the th clad provided at an outer peripheral portion of the th core.

In the optical fiber according to the present invention, the core may be disposed as a second core at the center of the second clad.

In the optical fiber of the present invention, a low-retardation core may be disposed at the center of the second clad in a cross-sectional view, the low-retardation core may include the core as an th core and the th clad provided on the outer peripheral portion of the th core, and the core as a third core may be disposed concentrically around the low-retardation core.

In the optical fiber of the present invention, a low-retardation core having the core of the th core and the th clad provided on the outer peripheral portion of the th core may be disposed at the center of the second clad in a cross-sectional view, and the core may be filled with the closest packing with the low-retardation core as the center to form a fourth core.

The optical transmission system of the present invention includes the optical fiber described above, a transmitter connected to ends of the optical fiber, and a receiver connected to the other end of the optical fiber.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, an optical fiber having an MFD and a bending loss equivalent to those of conventional cutoff-displacement optical fibers and having a delay time smaller than that of the cutoff-displacement optical fiber can be obtained. According to the present invention, the affinity of the optical fiber to the existing submarine optical cable network can be obtained, and therefore the delay time of the optical fiber is reduced. According to the present invention, since the transmission path is constituted by the optical fiber of the present invention, the delay time occurring in the transmission path of the optical transmission system is reduced, and the delay time occurring in the entire optical transmission system is also reduced.

Drawings

Fig. 1 is a graph showing the relationship between the optical characteristics and the delay time of a conventional silica core optical fiber.

Fig. 2 is a diagram showing a refractive index profile of a Single Mode Fiber (SMF) according to the present invention.

FIG. 3 shows the radius a of an optical fiber of the present invention which satisfies the conditions of 1.53 μm of cutoff wavelength, 11.5 μm or more of MFD, 2dB/100turns or less of bending loss, and delay time of the cutoff-shift optical fiber or less of delay time when the radius of the core is 1.0. mu.m₂And relative refractive index difference Δ₁A graph of the conditions of (a).

FIG. 4 shows the radius a of an optical fiber of the present invention which satisfies the conditions of 1.53 μm of cutoff wavelength, 11.5 μm or more of MFD, 2dB/100turns or less of bending loss, and delay time of the cutoff-shift optical fiber or less of delay time when the radius of the core is 1.5 μm₂And relative refractive index difference Δ₁A graph of the conditions of (a).

FIG. 5 shows the radius a of an optical fiber of the present invention which satisfies the conditions of 1.53 μm of cutoff wavelength, 11.5 μm or more of MFD, 2dB/100turns or less of bending loss, and delay time of the cutoff-shift optical fiber or less of delay time when the radius of the core is 2.0 μm₂And relative refractive index difference Δ₁A graph of the conditions of (a).

Fig. 6 is a graph showing the relationship between the fitting coefficient of the delay time of the SMF of the present invention and the radius of the core, which satisfies the prescribed requirements.

Fig. 7 is a graph showing the fit coefficient of the MFD of the SMF of the present invention versus the radius of the core, which satisfies the specified requirements.

Fig. 8 is a graph showing the relationship of the fitting coefficient of the bending loss of the SMF of the present invention to the radius of the core, which satisfies the prescribed requirements.

FIG. 9 shows a radius a of an optical fiber of the present invention which satisfies a plurality of conditions that a cutoff wavelength is 1.53 μm, MFD is 11.5 μm or more and 12.5 μm or less, a bending loss is 0.1dB/100turns or less, and a delay time is not more than a delay time of a cutoff displacement fiber when a radius of a core is 1.0 μm₂And relative refractive index difference Δ₁A graph of the conditions of (a).

FIG. 10 shows a radius a of an optical fiber of the present invention which satisfies a plurality of conditions that a cutoff wavelength is 1.53 μm, MFD is 11.5 μm or more and 12.5 μm or less, a bending loss is 0.1dB/100turns or less, and a delay time is not more than a delay time of a cutoff displacement fiber when a core radius is 1.5 μm₂And relative refractive index difference Δ₁A graph of the conditions of (a).

FIG. 11 shows a radius a of an optical fiber of the present invention which satisfies a plurality of conditions that a cutoff wavelength is 1.53 μm, MFD is 11.5 μm or more and 12.5 μm or less, a bending loss is 0.1dB/100turns or less, and a delay time is not more than a delay time of a cutoff displacement fiber when a core radius is 2.0 μm₂And relative refractive index difference Δ₁A graph of the conditions of (a).

Fig. 12 is a graph showing the relationship between the fitting coefficient of the bending loss and the radius of the core of the SMF of the present invention that satisfies the predetermined requirement.

FIG. 13 shows the radius a of the SMF of the present invention having a cutoff wavelength of 1.53 μm or less₂Relative refractive index difference Delta₁A graph of the relationship of (a).

FIG. 14 is a fitting coefficient κ showing the cutoff wavelength of the SMF of the present invention₉、κ₁₀Radius a of the core₁And relative refractive index difference Δ of the cladding with respect to the core region₁A graph of the relationship of (a).

FIG. 15 is a fitting coefficient κ showing the cutoff wavelength of the SMF of the present invention₁₁、κ₁₂Radius a of the core₁A graph of the relationship of (a).

FIG. 16 is a fitting coefficient κ showing the cutoff wavelength of the SMF of the present invention₁₃Radius a of the core₁A graph of the relationship of (a).

Fig. 17 is a graph showing the refractive index distribution of the SMF of the present invention having a low refractive index layer in the second cladding layer.

Fig. 18 shows examples of optical transmission systems including the SMF of the present invention.

Fig. 19A is a cross-sectional view of th example of the optical fiber of the present invention having a plurality of cores.

Fig. 19B is a cross-sectional view of a second example of the optical fiber of the present invention having a plurality of cores.

Fig. 19C is a cross-sectional view of a third example of the optical fiber of the present invention having cores.

Fig. 19D is a cross-sectional view of a fourth example of the optical fiber of the present invention having cores.

Fig. 20 shows examples of optical transmission systems using the optical fiber of the present invention.

FIG. 21 is a graph showing the relationship between the Rayleigh scattering loss and the radius of the core, which is received by the core and the th cladding, in the electric field distribution having a mode field diameter of 11.5 μm in the optical fiber of the present invention.

FIG. 22 is a graph showing Rayleigh scattering loss and radius a of an electric field distribution having a mode field diameter of 15.0 μm in an optical fiber of the present invention due to a core and an th cladding₁A graph of the relationship of (a).

FIG. 23 is a graph showing Rayleigh scattering loss α in an optical fiber in accordance with the present invention of recommendation G.654.D_RMFD, group delay time and radius a of light propagating from the core per unit length₁A graph of the relationship of (a).

FIG. 24 is a graph showing Rayleigh scattering loss α in an optical fiber in accordance with the present invention of recommendation G.654.E_RMFD, group delay time and radius a of light propagating from the core per unit length₁A graph of the relationship of (a).

Fig. 25 is a view showing a refractive index distribution of a trial-produced optical fiber of the present invention.

Fig. 26 is a graph showing measurement/evaluation results of optical characteristics of the trial-produced optical fiber of the present invention and the common SMF and CSF (various optical fibers).

Fig. 27 is a graph showing measurement results of the wavelength dependence of MFD of various optical fibers.

Fig. 28 is a graph showing measurement results of wavelength dependence (loss wavelength spectrum) of propagation loss of various optical fibers.

Fig. 29 is a graph showing-4 power plots of the wavelengths of the loss wavelength spectra of various optical fibers and a fitted straight line of these plots.

Fig. 30 is a graph showing the incident light power dependence measurement result of the phase shift amount by the CW-SPM method in the nonlinear coefficient evaluation of various optical fibers and a fitted straight line of each measurement result.

Fig. 31 is a graph showing the measurement results of the group delay time by the impulse response method using various optical fibers.

Fig. 32 is a graph showing the measurement results of the wavelength dependence of the group delay time by the impulse response method using various optical fibers.

Detailed Description

The embodiments of the present invention will be described below with reference to the drawings, the embodiments described below are examples of the implementation of the present invention, and the present invention is not limited to the embodiments described below.

The conventional silica core optical fiber has a high purity Silica (SiO) of 99.8 wt% or more₂) The core (sometimes referred to as a core region) is formed to have a general step-index-type refractive index distribution. As is well known, in the step-index-type refractive index distribution, the refractive index of the core that transmits light and the refractive index of the cladding (sometimes referred to as a cladding region) are uniform. Fig. 1 shows the relationship between the optical characteristics (the radius a (μm) of the core made of quartz and the relative refractive index difference Δ (%) of the clad with respect to the core) and the delay time (GD) of a conventional quartz core optical fiber. The refractive index of quartz at a wavelength of 1.55 μm was 1.444377.

Optical characteristics of a cut-off displacement optical fiber mainly used for an undersea optical cable are specified as recommendation g.654.d of ITU-T (international Telecommunication Union-Telecommunication Standardization Sector: international Telecommunication Union Telecommunication Standardization Sector ). in recommendation g.654.d, the MFD of the cut-off displacement optical fiber at a wavelength of 1.55 μm is specified to be 11.5 μm or more and 15.0 μm or less, and in recommendation g.654.d, the bending loss of the cut-off displacement optical fiber is specified to be 2.0dB/100turns or less and the cut-off wavelength of the cut-off displacement optical fiber is specified to be 1.53 μm or less at a wavelength of 1.625 μm and a bending radius of 30 mm.

The optical characteristics of optical fibers used in terrestrial core networks for long-range communications are specified as recommendation g.654.e by ITU-T. In recommendation g.654.e, the MFD of the optical fiber for terrestrial core network at a wavelength of 1.55 μm is specified to be 11.5 μm or more and 12.5 μm or less. In recommendation G.654.E, it is specified that the bending loss of the optical fiber for terrestrial core network is 0.1dB/100turns or less and the cutoff wavelength of the optical fiber for terrestrial core network is 1.53 μm or less at a wavelength of 1.625 μm and a bending radius of 30 mm.

The solid line (A) of FIG. 1 shows the relationship between the radius a (μm) of the core and the relative refractive index difference Δ (%) of the cladding with respect to the core of a silica core optical fiber having an MFD of 11.5 μm as defined in recommendation G.654.D, the solid line (B) of FIG. 1 shows the bending loss α_bThe relationship between the radius a and the relative refractive index difference Δ of a silica core optical fiber of 2.0dB/100 turns. The solid line (C) of FIG. 1 shows the cutoff wavelength λ_cThe relationship between the radius a of the core of a silica core optical fiber having a thickness of 1.53 μm and the relative refractive index difference Δ.

As shown by the arrows in FIG. 1, the MFD is 11.5 μm or more and the bending loss α satisfying the predetermined conditions in compliance with the recommendation G.654.D_bIs 2.0dB/100turn or less and has a cut-off wavelength lambda_cThe region in which each of the conditions of 1.53 μm or more is a region in which the side where the relative refractive index difference Δ is low as compared with the solid line (a) (i.e., the side where the relative refractive index difference Δ is close to 0, the upper side of the graph of fig. 1), the side where the radius a is large as compared with the solid line (B), and the side where the radius a is small as compared with the solid line (C) overlap each other.

The broken lines in FIG. 1 show the relationship between the radius a and the relative refractive index difference Δ of the silica core fiber having a group Delay time (Gloup Delay: GD, sometimes simply referred to as Delay time) of each of values 4.861 μ s/km, 4.87 μ s/km, 4.873 μ s/km, 4.876 μ s/km and 4.879 μ s/km. furthermore, the silica core fiber satisfying the specification of recommendation G.654.E is required to have a lower bending loss than the silica core fiber satisfying the specification of recommendation G.654. D. the region satisfying the specification of recommendation G.654.E is changed to the side where the radius a becomes larger than the hatched portion shown in FIG. 1.

As understood from the relative position to the diagonal portion shown in fig. 1, the minimum group delay time that can be achieved by the cut-off displacement fiber is 4.876 μ s/km. In addition, it is understood that the smaller the radius a, the smaller the group delay time is reached. When the radius a is 4.4 μm or less, the bending loss increases, and it becomes difficult to reduce the group delay time. The group delay time is difficult to be reduced because the electric field distribution of the propagating light is almost confined in the core, and the speed of the propagating light is determined dominantly by the refractive index of the high-purity quartz which is the material of the core. In a silica core fiber having a refractive index distribution other than a step-index type, which has been developed to achieve both expansion of the core and reduction of loss, the electric field distribution is well confined in the core, and therefore the delay time is equal to or lower than that of the step-index type silica core fiber. As the refractive index profile other than the step index type, for example, a W-type refractive index profile is cited.

FIG. 2 shows the distribution of the refractive index n of a Single Mode optical fiber (SMF) according to the present invention. In the present invention, the relative refractive index difference Δ is not absolute, but substantially negative. The optical fiber of the present invention has a core (r.ltoreq.a) from the center of a cross section in the longitudinal direction in a direction overlapping a radius r₁) th cladding (a)₁<r≤a₂) And a second cladding layer (r)>a₂) That is, the th clad layer is provided on the outer peripheral portion of the core, and the second clad layer is provided on the outer peripheral portion of the th clad layer₁And high purityRefractive index of the silica glass (refractive index n at a wavelength of 1.55 μm)_SiO2=1.444377) refractive index of silica glass of equal to or less than refractive index of high purity refractive index n of clad layer₂Lower than the refractive index n₁. Refractive index n of the second cladding₃Lower than the refractive index n₂。

At a radius a₁In particular, confinement of light of higher-order mode, which has a characteristic such that the electric field distribution of light is compared with , has little influence on light propagation in the core₁And the relative refractive index difference delta of the second cladding with respect to the th cladding₂To optimize the cut-off wavelength of the SMF of the present invention, in another aspect, the fundamental mode is influenced by the refractive index profile near the center of the fiber cross-section₁、a₂And is set appropriately.

(embodiment )

FIG. 3 shows the relationship between the optical characteristics and the delay time of the SMF of the th embodiment of the present invention and the radius a of the SMF of the st embodiment₁Is set to 1.0 μm. Taking into account the relative refractive index difference Δ₂(%) and radius a₂The cutoff wavelength of the SMF of embodiment is set to 1.53 μm or less, and the solid line (D) in FIG. 3 shows the radius a of the SMF of embodiment having an MFD of 11.5 μm or more₂Relative refractive index difference Delta₁The solid line (E) in FIG. 3 shows the radius a of the SMF of the th embodiment with bending loss of 2.0dB/100turns or less₂Relative refractive index difference Delta₁The relationship (2) of (c).

As can be understood by comparing FIG. 1 with FIG. 3, the relative refractive index difference Δ of the SMF of embodiment having a delay time (4.876 μ s/km) equivalent to that of the conventional cut-off displacement fiber is shown₁With radius a₂Increase in relative refractive index difference Δ₁Toward the large side (i.e., relative refractive index difference Δ)₁ side away from 0, lower side of the graph of fig. 3) with fiber displacement from cutoffRadius a of SMF in th embodiment of equivalent delay time₂By relative refractive index difference Δ₁And radius a₁Function of (k)₀(a₁)、κ₁(a₁)、κ₂(a₁) And is represented by the formula (4). In the present specification, the function represents a fitting function, and the argument may be omitted.

[ mathematical formula 4]

As described above, the radius a of the SMF according to embodiment ₁At 1.0 μm, the function κ₀(a₁) Has a value of 2.00, function k₁(a₁) Has a value of-1.42, function κ₂(a₁) The value of (A) is 0.50. As shown in FIG. 3, the relative refractive index difference Δ at the boundary (solid line (D) in FIG. 3) of SMF having an MFD of 11.5 μm or more₁Radius a in the case of 0.0%₂It was 5.56 μm.

With radius a₂As the result, the th cladding has less effect on MFD, and the refractive index profile of SMF is closer to the step index profile₁Refractive index n₁Etc.) are determined. Thus, with the radius a₂Increase in relative refractive index difference Δ₁Convergence to be dependent on radius a₁Thus, the MFD of the SMF of the th embodiment is a radius a of 11.5 μm₂By relative refractive index difference Δ₁Radius a₁Function of (k)₃(a₁) And due to the radius a₂Increase in relative refractive index difference Δ₁Converged function k₄(a₁) And is represented by the formula (5).

[ math figure 5]

Radius a of SMF in embodiment ₁At 1.0 μm, the function κ₃(a₁) Has a value of-3.94, function k₄(a₁) Has a value of-0.88. As shown in FIG. 3, the relative refractive index difference Δ is measured at the boundary (solid line (E) in FIG. 3) of the SMF having a bending loss of 2.0dB/100turns or less₁At 0.0%, the radius a₂It was 7.68 μm.

When following the radius a₂When the refractive index distribution of the SMF is increased to approach the simple step index type as described above, the bending loss is also determined only by the structure of the core. Due to the following radius a₂Increase in relative refractive index difference Δ₁Converge to a radius a₁Determined value, so that the bending loss of SMF is 2.0dB/100turns of radius a₂By relative refractive index difference Δ₁And radius a₁Function of (k)₅(a₁)、κ₆(a₁) And is represented by the formula (6).

[ mathematical formula 6]

Radius a of SMF in embodiment ₁At 1.0 μm, the function κ₅(a₁) Has a value of-1.08, function κ₆(a₁) The value of (D) is-0.82.

The shaded portion in fig. 3 indicates a design region of the SMF according to embodiment , which satisfies the specification of recommendation g.654.d and can realize a delay time (4.876 μ s/km or less) equal to or less than the cutoff-displacement fiber.

As described above, the relative refractive index difference Δ is limited by the conditions of the delay time, the MFD, and the bending loss₁Radius a₂By adopting the structure corresponding to the hatched portion in fig. 3, the SMF according to embodiment realizes the same optical characteristics as the conventional cut-off displacement fiber and the delay time equal to or less than the delay time of the cut-off displacement fiber, the more the relative refractive index difference Δ is made to be₁The delay time of the SMF of the th embodiment is reduced as it is lower.

FIG. 4 shows radius a₁Radius a of the SMF of the th embodiment of 1.5 μm₂Relative refractive index difference Delta₁The relationship (2) of (c). Solid lines (D) and (E) in the graphs of fig. 4 and 5 and fig. 9 to 11 shown later represent the same contents as those of the solid lines (D) and (E) in fig. 3. Delay time is a relative refractive index difference Δ of SMF below the delay time of the cut-off displacement fiber₁Radius a₂The boundary line of the selection range of (4) is expressed by the following expression. A difference Δ between the relative refractive index of SMF having an MFD of 11.5 μm or more and the relative refractive index₁Radius a₂The boundary line of the selection range of (2) is represented by the formula (5). Relative refractive index difference Delta of SMF with bending loss below 2dB/100turns₁Radius a₂The boundary line of the selection range of (2) is represented by equation (6). At a radius a₁At 1.5 μm, the function κ₀(a₁) Has a value of 2.00, function k₁(a₁) Has a value of-0.86, function κ₂(a₁) Has a value of 0.43 and a function k₃(a₁) Has a value of-3.94, function k₄(a₁) Is-0.50. the shaded portion of fig. 4 shows the design region of the SMF of embodiment that meets the specifications of recommendation g.654. d.

FIG. 5 shows the radius a of the core region₁Radius a of th cladding of SMF 2.0 μm₂Relative refractive index difference Delta₁The relationship (2) of (c). The boundary line of the structure having the delay time equal to or shorter than the cut-off displacement fiber is expressed by the above-mentioned expression (4). The boundary line of the structure having an MFD of 11.5 μm or more is represented by the above-mentioned expression (5). The boundary line of the structure having a bending loss of 2dB/100turns or less is expressed by the above expression (6). Radius a in the core region₁At 2.0 μm, the function κ₀(a₁) Has a value of 2.00, function k₁(a₁) Has a value of-0.50, function κ₂(a₁) Has a value of 0.38, function k₃(a₁) Has a value of-3.94, function k₄(a₁) Has a value of-0.36. The shaded portion of fig. 5 shows a design region of the SMF satisfying the specification of ITU-T recommendation g.654.d with the single-mode optical fiber of the present invention.

As shown in fig. 3 to 5, the relative refractive index difference Δ₁And radius a₂Is dependent on the radius a₁But may vary. By expressing the coefficient of the function representing the boundary lines surrounding the hatched parts shown in fig. 3 to 5 as the radius a₁, in the present invention, the condition that the optical fiber can be easily manufactured is considered, provided that the radius a₁Is 1.0 μm or more.

The function κ is a boundary line (i.e., a boundary line expressed by equation (1)) of the structure of the SMF that realizes a delay time equal to or less than the delay time of the cutoff-shift fiber₀(a₁) Is independent of the radius a of the core region₁And is 2.00. FIG. 6 shows the function κ₁(a₁)、κ₂(a₁) Respective value and radius a₁The relationship (2) of (c). As shown in FIG. 6, the function κ₁(a₁) Expressed by the formula (7), function κ₂(a₁) Is represented by the formula (8).

[ math figure 7]

[ mathematical formula 8]

By passing through a radius a₁Relative refractive index difference Δ₁And radius a₂The boundary line of the structure having a delay time of 4.876 μ s/km is shown, and thus the design region of the SMF of embodiment (i.e., the region showing the optical characteristics) is shown as expression (1).

[ mathematical formula 9]

The function κ is a boundary (boundary represented by formula (5)) of a structure having an MFD of 11.5 μm or more₃(a₁) Is independent of the radius a₁And is-3.94. FIG. 7 shows the function κ₄(a₁) Value of (a) and radius a₁The relationship (2) of (c). As shown in FIG. 7, the function κ₄(a₁) Represented by the formula (10).

[ mathematical formula 10]

By passing through a radius a₁Relative refractive index difference Δ₁And radius a₂The boundary line of the structure having the MFD of 11.5 μm is shown, and the design region of the SMF of the -th embodiment is shown as equation (11).

[ mathematical formula 11]

FIG. 8 shows a function κ with respect to a boundary (a boundary represented by formula (6)) of a structure having a bending loss of 2.0dB/100turns or less₅(a₁)、κ₆(a₁) Respective value and radius a₁The relationship (2) of (c). As shown in FIG. 8, the function κ₅(a₁) Expressed by the formula (12), the function κ₆(a₁) Is represented by the formula (13).

[ mathematical formula 12]

[ mathematical formula 13]

By passing through a radius a₁Relative refractive index difference Δ₁And radius a₂The boundary line of the structure having the bending loss of 2.0dB/100turns is shown, and the design region of the SMF of the -th embodiment is shown as the following expression (9).

[ mathematical formula 14]

Based on the above, the SMF of the th embodiment has a radius a₁A core of 1.0 μm to 4.3 μm in radius a₂Relative refractive index difference Δ₁Satisfies the above-mentioned expression (1) and the following expression (2).

[ mathematical formula 15]

By relative to radius a₁Relative refractive index difference Δ₁Radius a₂By satisfying the above conditions, the SMF according to embodiment achieves the same optical characteristics as the cut-off displacement fiber and achieves a delay time equal to or shorter than the delay time of the cut-off displacement fiber.

Table 1 shows examples of the design parameters of the SMF of the th embodiment, in the design shown in table 1, the optical characteristics equivalent to those of a cutoff-displacement optical fiber are realized, and the reduction of the delay time of 0.05 μ s/km is realized, by using the SMF thus designed, the reduction of the delay time of about 1ms is realized in a long-distance network having a network length of about 10000km as in the case of the pacific ocean bottom optical cable, the design parameters shown in table 1 are examples satisfying the above conditions, and the same effects as those of the SMF having the design parameters shown in table 1 can be obtained by the SMF having a structure satisfying the above conditions.

[ Table 1]

(second embodiment)

FIG. 9 shows that the radius a is satisfied₁Radius a in the case of 1.0 μm₂Relative refractive index difference Delta₁And the design area of the SMF of the second embodiment of the invention of g.654.e. Based on relative refractive index difference Δ₂And radius a₂The cut-off wavelength of the SMF of the second embodiment is set toThe thickness of the optical fiber is 1.53 μm or less, the solid line (D) in fig. 9 shows the boundary line of the structure having an MFD of 11.5 μm as in the th embodiment, the solid line (E) in fig. 9 shows the boundary line of the structure having a bending loss of 0.1dB/100turns or less as in the th embodiment, and the shaded portion in fig. 9 shows a design region in which the above-described conditions relating to the cutoff wavelength, the MFD, and the bending loss are completely satisfied and the delay time (4.876 μ s/km or less) of the cutoff-shifted optical fiber can be realized.

At a radius a₁When the thickness is 1.0 μm, the boundary line of the delay time equal to or less than the delay time of the cut-off displacement fiber is expressed by the equation (4) as in the case of embodiment ₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are compared with the function k explained in the embodiment ₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are the same. The boundary line having an MFD of 11.5 μm or more is represented by the formula (5). Function kappa₃(a₁)、κ₄(a₁) The respective values are compared with the function k explained in the embodiment ₃(a₁)、κ₄(a₁) The respective values are the same.

The boundary line of the structure with the bending loss of less than 0.1dB/100turns is in the relative refractive index difference delta₁At 0.0%, the radius a₂It was 6.88 μm. With radius a₂The larger the th cladding, the less the bending loss is affected, which is determined only by the structure of the core region₂Increase in relative refractive index difference Δ₁Converging to a radius a of the core region₁The determined value. Thus, the SMF of the present invention has a bending loss of 0.1dB/100turns radius a₂By relative refractive index difference Δ₁And radius a₁Function of (k)₇(a₁)、κ₈(a₁) And is expressed as equation (14).

[ mathematical formula 16]

At a radius a₁At 1.0 μm, the function κ₇(a₁) Has a value of-3.74 and a function k₈(a₁) Has a value of-1.36. Relative refractive index difference Δ₁The high design area is limited due to the increase in delay time. MFD and radius a₂The design range of (2) is limited by the required bending loss condition. As shown in FIG. 9, the relative refractive index difference Δ₁The lower the delay time is.

FIG. 10 shows that the radius a is satisfied₁Radius a at 1.5 μm₂Relative refractive index difference Delta₁And the design area of the SMF of the second embodiment of g.654.e. At a radius a₁In the case of 1.5 μm, the boundary line of the delay time equal to or less than the delay time of the cut-off displacement fiber is also expressed by the expression (4). Function kappa₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are compared with the function k explained in the embodiment ₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are the same. The boundary line having an MFD of 11.5 μm or more is represented by the formula (5). Function kappa₃(a₁)、κ₄(a₁) The respective values are compared with the function k explained in the embodiment ₃(a₁)、κ₄(a₁) The respective values are the same.

The boundary line of the structure in which the bending loss is 0.1dB/100turns or less is expressed by the expression (14). Due to the radius a₁Is 1.5 μm, so the function κ₇(a₁) Has a value of-5.60, function k₈(a₁) Has a value of-0.87. The shaded portion in fig. 10 represents a design region in which the above-described conditions relating to the cutoff wavelength, the MFD, and the bending loss are completely satisfied and the delay time equal to or less than the delay time of the cutoff-shifted fiber can be realized.

FIG. 11 shows that the radius a is satisfied₁Radius a in the case of 2.0 μm₂Relative refractive index difference Delta₁And the design area of the SMF of the second embodiment of g.654.e. At a radius a₁Is 2.0 muIn the case of m, the boundary line of the delay time equal to or less than the delay time of the cut-off displacement fiber is also expressed by expression (4). Function kappa₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are compared with the function k explained in the embodiment ₀(a₁)、κ₁(a₁)、κ₂(a₁) The respective values are the same. The boundary line having an MFD of 11.5 μm or more is represented by the formula (5). Function kappa₃(a₁)、κ₄(a₁) The respective values are compared with the function k explained in the embodiment ₃(a₁)、κ₄(a₁) The respective values are the same.

The boundary line of the structure in which the bending loss is 0.1dB/100turns or less is expressed by the above expression (14). Due to the radius a₁Is 2.0 μm, so the function κ₇(a₁) Has a value of-6.25, function κ₈(a₁) Has a value of-0.61. The shaded portion in fig. 11 represents a design region in which the above-described requirements regarding the cutoff wavelength, the MFD, and the bending loss are completely satisfied and the delay time equal to or less than the delay time of the cutoff-shifted fiber can be realized.

As shown in fig. 9 to 11, the relative refractive index difference Δ₁And radius a₂Is dependent on the radius a₁. The boundary lines shown in fig. 9 to 11 at which the MFD is 11.5 μm or more are the same as those shown in fig. 3 to 5. The structure of SMF having an MFD of 11.5 μm or more is represented by formula (9). The boundary lines of the delay times equal to or less than the delay time of the cutoff-displacement fiber shown in fig. 9 to 11 are the same as those shown in fig. 3 to 5. The structure of the SMF that realizes a delay time equal to or less than the delay time of the cutoff-shift fiber is expressed by expressions (9) and (11).

The boundary line of the structure having a bending loss of 0.1dB/100turns or less is expressed by the expression (14). FIG. 12 shows the relationship with the radius a₁Function of (k)₇(left axis of FIG. 12) and function κ₈(the right axis of fig. 12). As shown in FIG. 12, the function κ₇(a₁) Expressed by the formula (15), function κ₈(a₁) Through (1)6) The formula is shown.

[ mathematical formula 17]

[ mathematical formula 18]

As described above, the radius a of the SMF of the second embodiment having the bending loss of 0.1dB/100turns or less₂The design region of (2) is represented by the formula (17).

[ math figure 19]

In light of the foregoing, the SMF of the second embodiment has a radius a₁A core body of 1.0 μm or more and 4.3 μm or less, and having a radius a₂Relative refractive index difference Δ₁Satisfies the design region of the expressions (1) and (2).

[ mathematical formula 20]

[ mathematical formula 21]

By relative to radius a₁Relative refractive index difference Δ₁Radius a₂By satisfying the above conditions, the SMF according to the second embodiment achieves the same optical characteristics as those of the cut-off displacement fiber and achieves a delay time equal to or shorter than that of the cut-off displacement fiber.

In FIG. 13, the radius a in the core region₁In each of the case of 1.0 μm and the case of 2.0 μm, regarding the relative refractive index difference Δ₁Radius a when set to-0.1% and when set to-0.8%₂And relative refractive index difference Δ₂A boundary line with a cutoff wavelength of 1.53 μm or less is shown. Relative refractive index difference Δ in comparison to these boundary lines₂Lower side (i.e., relative refractive index difference Δ)₂ side near 0, upper side of the graph of fig. 13) satisfying the aforementioned requirements regarding cutoff wavelength, MFD, and bending loss as shown in fig. 9, the boundary line with cutoff wavelength of 1.53 μm or less is dependent on the core radius a₁And relative refractive index difference Δ₁But may vary. Thus, the relative refractive index difference Δ having a cutoff wavelength of 1.53 μm₂Passing through radius a₁Relative refractive index difference Δ₁And radius a₂To indicate.

When radius a₂The difference in the boundary-to-relative refractive index Δ of 1.53 μm at the cutoff wavelength increases₂Lower side change when radius a₂When the refractive index is increased, the influence of the core structure on the cut-off wavelength is reduced, and thus the relative refractive index difference Δ₂Independent of radius a₁And relative refractive index difference Δ₁And converges. Relative refractive index difference Δ₂The convergence value of (A) was-0.033%. When using the relative refractive index difference Δ₂Convergence value and radius a₁And relative refractive index difference Δ₁Function k set as variable₉(a₁，Δ₁)、κ₁₀(a₁，Δ₁) When the relative refractive index difference Δ is 1.53 μm at the cutoff wavelength₂Expressed as formula (20).

[ mathematical formula 22]

FIG. 14 shows a radius a₁Relative refractive index difference Δ of 1.0 μm, 2.0 μm, 4.3 μm₁Function of (k)₉(left axis of FIG. 14) and function κ₁₀(axis on right side of fig. 14). As described above, the relative refractive index difference Δ₁In the case of 0.0%, the relative refractive index difference Δ₁And function k₉、κ₁₀Using the relationship of (a)₁Function k set as variable₁₁(a₁)、κ₁₂(a₁)、κ₁₃(a₁) And are expressed as the expressions (21) and (22).

[ mathematical formula 23]

[ mathematical formula 24]

At a radius a₁4.3 μm, relative refractive index difference Δ₁When the ratio is-0.531% or less, the ratio to the radius a₂And relative refractive index difference Δ₂Independently, the cutoff wavelength is 1.53 μm or more. Thus, at radius a₁In the case of 4.3 μm, the relative refractive index difference Δ₁The formulae (21) and (22) are applied to the structure of an SMF of-0.531% or more.

FIG. 15 shows the relationship with the radius a₁Function of (k)₁₁(left axis of FIG. 15) and function κ₁₂(the right axis of fig. 15). At a radius a₁At 0.0 μm, the function κ₁₁、κ₁₂Loss of relative refractive index difference Δ₁Of the function, thus the function κ₁₁、κ₁₂Becomes 0 (zero). The curve shown by the solid line in fig. 15 is represented by expression (23). The curve shown by the broken line in fig. 15 is represented by the formula (24).

[ mathematical formula 25]

[ mathematical formula 26]

FIG. 16 shows the relationship with the radius a₁Function of (k)₁₃A change in (c). At a radius a₁At 0.0 μm, the function κ₁₃Loss of relative refractive index difference Δ₁Of the function, thus the function κ₁₃Becomes 0 (zero). The curve shown by the solid line in fig. 16 is expressed by expression (25).

[ mathematical formula 27]

Based on the above, the boundary line of the structure of the SMF according to the second embodiment having the cutoff wavelength of 1.53 μm or less passes through the radius a₁Relative refractive index difference Δ₁And radius a₂Thereby expressing the relative refractive index difference Delta₂Expressed as formula (3).

[ mathematical formula 28]

The SMF of the second embodiment described above is designed to have a radius a₁、a₂And relative refractive index difference Δ₁、Δ₂The above-described preferred conditions are satisfied.

In the present invention, by designing as the radius a₁In the range of 1.0 μm or more and 4.3 μm or less, the radius a represented by the formulae (1) and (2) is determined in the SMF conforming to the recommendation G.654.D₂And relative refractive index difference Δ₁The preferred design area of (c). Similarly, in the SMF complying with recommendation g.654.e, the radius a represented by expressions (18) and (19) is determined₂And relative refractive index difference Δ₁The preferred design area of (c). In the preferred region described above, the delay time of the SMF that can be achieved is determined by the relative refractive index difference Δ₁Roughly determine the relative refractive index difference Δ₁The lower the delay time is. By suitably selecting the radius a in the above-mentioned preferred region₁、a₂And relative refractive index difference Δ₁Whereby the cutoff wavelength satisfies the condition required when applying SMF to a long-distance communication network, and the phase is determined by the expression (3)Difference of p-refractive index Δ₂。

The second cladding of the SMF according to the second embodiment is not limited to the second cladding having the refractive index profile shown in fig. 2. FIG. 17 shows the secondary radius a of the second cladding shown in FIG. 2₃To radius a₄A region having a refractive index n₄And a relative refractive index difference with respect to the second cladding layer is Δ₃That is, radius a. examples of the refractive index profile of the low refractive index region₃Greater than radius a₂Refractive index n₄Lower than the refractive index n₃. It is known that the low refractive index region of the second cladding layer alleviates the trade-off resulting from the optical characteristics of MFD, bending loss, and the like. It is expected that the design area is preferably enlarged by the SMF of the second embodiment. By forming a void in the second cladding layer instead of the low refractive index region, the same effect as that of the SMF having a low refractive index region in the second cladding layer can be obtained.

(third embodiment)

Fig. 18 shows the structure of an optical transmission system 100 of the present invention. The optical transmission system 100 includes a transmitter 102, an optical fiber (SMF)104 according to the present invention, and a receiver 106. The transmitter 102 and the optical fiber 104 are connected to each other by a connector. The optical fiber 104 and the receiver 106 are connected to each other by a connector. The optical transmission system 100 is provided with the optical fiber 104, thereby reducing the transmission delay of the optical transmission system 100. Thus, the optical transmission system 100 can meet a demand for reducing the delay between the transmitter 102 and the receiver 106.

(fourth embodiment)

Fig. 19A, 19B, 19C, and 19D show the structures of optical fibers 51, 52, 53, and 54 in which a plurality of cores are arranged as a single core optical fiber when viewed in a cross section with respect to the longitudinal direction, wherein or more cores of the optical fibers 51, 52, 53, and 54 are low-delay cores that satisfy the conditions described in embodiment or the second embodiment and can reduce the delay time of the optical fibers.

Fig. 19A shows a cross section of an optical fiber in which only four low-retardance cores 60A are arranged in a second cladding 66 having a diameter of 125 μm (i.e., a second cladding in the th embodiment and the second embodiment), each low-retardance core 60A has a core (a th core) 62 at the center and a th cladding 64 provided at the outer peripheral portion of the core 62 in a cross-sectional view, fig. 19B shows a cross section of an optical fiber in which a core (a second core) 60B is arranged at the center of the second cladding 66 having an outer diameter of 125 μm and four low-retardance cores 60A are arranged concentrically with respect to the center of the core 60B, fig. 19C shows a cross section of an optical fiber in which a second cladding 66 having a diameter of 175 μm and a cladding 70 having a diameter of 250 μm is provided at the outer peripheral portion of the second cladding 66, in an SMF shown in fig. 19C, a low-retardance core 60A is arranged at the center of the second cladding 66 and a core 60A core (a third cladding 60C) is arranged concentrically with a core 60D with a center and a maximum reliability of the cladding 60D of the cladding 60A is considered to be arranged with a maximum reliability of the outer peripheral diameter of the cladding of the low-retardance cores arranged at the center of the second cladding 60A, i.e., the highest reliability of the cladding 60A cladding arranged with the highest reliability of the cladding of the filled with.

According to the optical transmission system including the optical fibers 51, 52, 53, and 54 illustrated in fig. 19A to 19D, conventional optical communication and optical communication with reduced time delay can be simultaneously and preferably realized.

(fifth embodiment)

Fig. 20 shows an optical transmission system 200 including an optical fiber according to the present invention, the optical transmission system 200 includes an SMF (optical fiber) 150, a plurality of transmitters 172, and a plurality of receivers 174, the SMF 150 may be any of the SMFs of the th to third embodiments and the optical fibers 51, 52, 53, 54 of the fourth embodiment, at least or more of the plurality of transmitters 172 are coupled to ends of the SMF 150 via a Fan-in device (Fan-in device), at least or more of the plurality of receivers 174 are coupled to another end of the SMF 150 via a Fan-in device, and according to the optical transmission system 200, it is possible to simultaneously achieve conventional optical communication and optical communication in which a transmission delay between transmission devices is reduced, whereby the optical transmission system 200 can flexibly cope with a demand for reducing a delay of a transmission path.

(more preferred design conditions for optical fiber)

FIG. 21 relates to Rayleigh scattering loss α due to the effect of cladding on the electric field distribution of light propagating through the core of the SMF at MFD of 11.5 μm_RAnd shows that the radius a is obtained by numerical calculation₁Relative refractive index difference Delta₁FIG. 22 relates to Rayleigh scattering loss α due to the effect of cladding on the electric field distribution of light propagating through the core of the SMF at MFD of 15.0 μm_RAnd shows that the radius a is obtained by numerical calculation₁Relative refractive index difference Delta₁The result of the relationship of (1). It is understood that the radius a is reduced as shown in fig. 21 and 22₁And the relative refractive index difference Delta₁Reduce, and thus the Rayleigh scattering loss α of SMF_RAnd is increased.

Fig. 23 shows rayleigh scattering loss α in SMF complying with recommendation g.654.d based on the numerical calculation results shown in fig. 21 and 22_RMFD, group delay time and radius a of light propagating through the core per unit length₁Fig. 24 shows rayleigh scattering loss α in SMF complying with recommendation g.654.e based on the numerical calculation results shown in fig. 21 and 22_RMFD, group delay time and radius a of light propagating through the core per unit length₁The relationship (2) of (c).

Rayleigh scattering loss α for SMF considered general_RAbout 0.17 dB/km. When the group delay time of the cut-off displacement fiber (CSF) is considered to be about 4.877 μ s/km, the radius a corresponding to the hatched portion in FIGS. 23 and 24 is set₁And relative refractive index difference Δ₁Thereby enabling simultaneous achievement of optical characteristics and low loss/low delay in compliance with recommendation g.654.d and recommendation g.654.e, respectively, providing a better design region of the optical fiber of the present invention the sloped line portions of fig. 23 and 24 show a boundary line with MFD of 11.5 μm or more, rayleigh scattering loss α in a manner satisfying each recommendation_RThe difference in relative refractive index between each boundary line of the boundary lines of 0.17dB/km was Δ₁Lower side (i.e., relative refractive index difference Δ)₁ side near 0, upper side of the graph of fig. 13) overlap each other, that is, in each of the above embodiments, rayleigh is included by step Free scattering loss α_RUnder such conditions of 0.17dB/km or less, thereby enabling both optical characteristics and low loss/low retardation in compliance with the recommendation g.654.d and recommendation g.654.e, respectively, and providing a better design area.

(examples)

Based on the relative relationship between the design region and the parameters and the preferred conditions described in the above embodiments, the SMF (optical fiber) of the present invention was produced in a trial manner. Fig. 25 shows a refractive index distribution of a trial SMF. The characteristics of the SMF of the present invention prepared in a test were evaluated, and the characteristics of the SMF and CSF that are commonly used were evaluated.

FIG. 26 shows MFD at a wavelength of 1.55 μm and an effective cross-sectional area (A) at a wavelength of 1.55 μm for a trial-produced optical fiber of the present invention (hereinafter, may be referred to as a trial-produced optical fiber)_eff) For comparison, the same measurement as that of the optical fiber of the present invention (the "trial fiber" in fig. 26) that was manufactured in a trial was performed with respect to the common SMF and the cut-off displacement fiber (CSF) according to the recommendation g.654.d for the purpose of comparison, as for the optical fiber of the present invention, the measurement was performed with respect to the cut-off wavelength, the bending loss per turns at a bending radius of 15mm at the wavelength of 1.625 μm, the propagation loss at the wavelength of 1.55 μm, the rayleigh scattering loss at the wavelength of 1.55 μm, the wavelength dispersion at the wavelength of 1.55 μm, the wavelength set as a reference in the measurement of the nonlinear coefficient at the wavelength of 1.55 μm, the measurement results, and the measurement.

Fig. 25 shows a structure of an optical fiber which was trial-manufactured in a design region conforming to the structural parameters of recommendation g.654.d which specifies the characteristics of a cutoff-shifted optical fiber, which was determined based on the numerical calculation results, the trial-manufactured optical fiber has a core formed of pure silica glass, the radius of the core is 1.0 μm, the radius of the th cladding is 6.4 μm, and further, the relative refractive index difference between the core and the th cladding is-0.38%, and the relative refractive index difference between the th cladding and the second cladding is-0.24%.

As shown in FIG. 26, the trial fiber had MFD and A equivalent to those of CSF as designed_effThe propagation loss of the trial fiber was as low as that of the general-purpose SMF, and the wavelength dispersion and nonlinear coefficient of the trial fiber were equal to those of CSF at step .

Fig. 27 shows the results of measurement of the wavelength characteristics of MFD of various optical fibers, for the trial-manufactured optical fiber, MFD is expanded to the same extent as CSF in the entire region of C-band + L-band, and the tendency that MFD is expanded is very close to as a result of numerical calculation shown by a solid line.

Fig. 28 shows the measurement results of the loss wavelength spectrum of each optical fiber. The shape of the loss wavelength spectrum of the trial fiber is the same as the shape of the common SMF and CSF. The propagation loss of the trial fiber was comparable to that of the general-purpose SMF over the entire region of the measurement wavelength.

Fig. 29 shows the result obtained by plotting the loss wavelength spectrum shown in fig. 28 to the-4 th power of the wavelength λ. When based on λ^-4Is from 0.52 μm^-4To 0.80 μm^-4When the Rayleigh scattering loss at a wavelength of 1.55 μm was analyzed by the slope of the fitted straight line, it was 0.161dB/km for the trial fiber, 0.166dB/km for the general SMF, and 0.146dB/km for the CSF. From the results, it was confirmed that the rayleigh scattering loss of the trial fiber was about the same as that of the general-purpose SMF.

Fig. 30 shows the measurement results of the incident light power dependence of the phase shift amount obtained by the CW-SPM method when evaluating the nonlinear coefficient of each optical fiber. Regarding the CW-SPM method, the nonlinear coefficient (n) can be analyzed by using the slope of incident light power dependence according to the amount of phase shift in the formula (26)₂/A_eff)。

[ mathematical formula 29]

In the formula (26), the compound represented by the formula (I),is the amount of phase shift, λ is the wavelength, L_effIs the effective length, P, of the various optical fibers_inIs the power of the incident light for the various fibers. The nonlinear coefficient analyzed based on the slope of the fitted straight line shown by the dotted line was 1.79 × 10 in the trial fiber, the general SMF, and the CSF, respectively^-10/W、2.95×10^-10/W、1.90×10^-10and/W. According to the aboveAs a result, it was confirmed that the trial fiber had low nonlinearity to the same extent as CSF.

FIG. 31 shows the measurement results of group delay time obtained by the impulse response method using various optical fibers, the abscissa of the graph of FIG. 31 shows the measured group delay time in terms of the group delay time per unit length, and for the measurement, the pulse width of the pulse emitted from the pulsed light source is modulated to 100ps, and the measured fiber length is set to 350m, and for CSF, the group delay time is reduced by 0.018. mu.s/km. compared with the general-purpose SMF, and for this, it is confirmed that for the trial fiber, the group delay time is reduced by 0.016. mu.s/km in steps with respect to CSF .

Fig. 32 shows the results of measuring the wavelength dependence in the C band + L band with respect to the measurement results of the group delay time shown in fig. 31. It was confirmed that the group delay time of the trial-produced optical fiber was reduced by about 0.016. mu.s/km compared to the CSF over the entire region of the C-band + L-band.

The general SMF has a three-layer structure including a core, an th clad layer, and a second clad layer, and the following parameters were used in the above numerical calculation for the general SMF.

Radius of core. 3.5 μm

Radius of th cladding 6.5. mu.m

The relative refractive index difference of the core to the second cladding 0.38%

Radius of the second cladding 62.5. mu.m

relative refractive index difference of 0.05% relative to the second cladding

The refractive index (wavelength 1.55 μm) · · 1.444377 of the second cladding.

The CSF has a three-layer structure including a core, an th clad layer, and a second clad layer, and the following parameters were used in the above numerical calculation of the CSF.

Radius of core. 6 μm

Refractive index of core (wavelength 1.55 μm). 1.444377

Radius of th cladding 25 μm

th relative refractive index difference of cladding to core-0.35%

Radius of the second cladding 62.5. mu.m

The relative refractive index difference of the second cladding layer with respect to the core is-0.25%.

Industrial applicability

The present invention can be applied to widely to optical fibers in application fields where reduction of transmission delay is required, such as optical fibers for long-distance communication networks.

Description of the symbols

60A low retardation core, 60B core (second core), 60C core (third core), 60D core (fourth core), 62 core ( th core), 150 SMF (optical fiber), 172 transmitter, 174 receiver, a1, a2, a3 radius, Delta₁、Δ₂、Δ₃: relative refractive index difference.