Patent Description:
In an optical fiber used in an optical communication system, in order to increase the possible transmission distance of an optical signal, it is required to improve an optical signal to noise ratio (OSNR). In order to improve the OSNR, it is important to reduce the transmission loss of an optical fiber.

As the optical fiber, a Ge-doped core fiber in which a core is doped with germanium dioxide is generally widely used. However, in a silica core fiber not including the additives, Rayleigh scattering can be further limited as compared to the Ge-doped core fiber, and thus a further reduction in transmission loss is expected. In the silica core fiber, it is important to reduce transmission loss caused by light absorption derived from binding defects of silicon dioxide (SiO<NUM>).

In general, in an optical fiber, transmission loss in an optical communication wavelength band increases due to light absorption of an OH group (hydroxy group) caused by binding defects of SiO<NUM>. In addition, in an optical fiber, light propagates mainly in a core. Therefore, in order to reduce transmission loss, it is necessary that the core is sufficiently dehydrated to remove an OH group. Therefore, Patent Document <NUM> discloses a method of dehydrating a core by causing halogen gas such as chlorine gas (Cl<NUM>) to flow and to react with hydrogen gas (H<NUM>) when a soot deposited using a vapor-phase axial deposition method (VAD method) is sintered. In this method, the dehydration is performed during the formation of a glass preform. Therefore, initial transmission loss of an optical fiber can be reduced.

As a result of a thorough investigation, the present inventors clarified that, even in an optical fiber in which a core is sufficiently dehydrated and initial transmission loss is low, when dehydration of a cladding layer is insufficient, optical transmission loss increases by being exposed to H<NUM> for a long period of time.

On the other hand, Patent Document <NUM> discloses a method of dehydrating a cladding layer using SiCl<NUM> after adding fluorine (F) to the cladding layer. However, in this method, a large amount of chlorine (Cl) is added to a cladding layer such that the refractive index of the cladding layer increases due to Cl. Therefore, in order to adjust a difference in refractive index between the core and the cladding, it is necessary to add an excess amount of F to the cladding layer to reduce the refractive index of the cladding layer. As compared to Cl, F tends to further increase Rayleigh scattering due to concentration fluctuation thereof, and the presence of an excess amount of F in the cladding layer in the vicinity of the core leads to an increase in the transmission loss of the optical fiber. In addition, for example, when an increase in raw material costs for adding an excess amount of F is taken into consideration, the method disclosed in Patent Document <NUM> also has room for improvement.

The present invention has been made in consideration of the above-described circumstances, and an object thereof is to provide an optical fiber in which a phenomenon of increasing transmission loss during exposure to hydrogen is limited.

According to a first aspect of the present invention for achieving the above-described object, there is provided an optical fiber according to claim <NUM>.

In addition, according to a second aspect of the present invention for achieving the above-described object, there is provided a method for manufacturing an optical fiber according to claim <NUM>.

In addition, according to a third aspect of the present invention for achieving the above-described object, there is provided an optical fiber preform for manufacturing an optical fiber according to claim <NUM>.

According to the aspects of the present invention, an optical fiber in which a phenomenon of increasing transmission loss during exposure to hydrogen is limited can be provided.

Hereinafter, an optical fiber, a method for manufacturing an optical fiber, and an optical fiber preform according to an embodiment of the present invention will be described.

The optical fiber according to the embodiment is an optical fiber <NUM> having, for example, a stepwise refractive index profile shown in <FIG> or a W-shaped refractive index profile shown in <FIG>. The refractive index profile of the optical fiber <NUM> is not limited to the example shown in <FIG>.

The optical fiber <NUM> includes a core <NUM> and a cladding layer <NUM> that is formed on an outer circumference of the core <NUM>.

The optical fiber <NUM> is a silica core fiber in which the core <NUM> is formed of silicon dioxide (SiO<NUM>) and additives such as germanium are not added. The optical fiber <NUM> shown in <FIG> includes the cladding layer <NUM> having a single-layer structure, and the optical fiber <NUM> shown in <FIG> includes the cladding layer <NUM> having a multiple-layer structure including an inner cladding layer 12a and an outer cladding layer 12b.

An optical fiber preform for manufacturing the optical fiber <NUM> according to the embodiment can be manufactured as follows.

First, a soot (hereinafter, referred to as "core soot") that forms the core <NUM> of SiO<NUM> is formed using a VAD method. At this time, in order to adjust the viscosity of a material that forms the core <NUM>, F may be added to the material using fluorine (F) -containing gas which includes fluorine, the F-containing gas such as SiF<NUM>, C<NUM>F<NUM>, SF<NUM>, or CF<NUM>. Alternatively, the material that forms the core <NUM> may include alkali metal such as sodium (Na) or potassium (K).

Next, Cl-containing gas which includes chlorine, the Cl-containing gas such as SOCl<NUM> or Cl<NUM> is diffused and is caused to react in the core soot such that the core soot is dehydrated. Here, F or the alkali metal for adjusting the viscosity may be added to the core soot.

Next, the dehydrated core soot is heated in a helium (He) atmosphere to be vitrified such that core glass is obtained. According to the invention, the dehydration is performed such that the core glass includes about <NUM> to <NUM> wt% of Cl. The shape of the refractive index profile of the core may become higher or lower from the outer circumference toward the inside, may be stepwise, or may be flat (uniform).

Next, a soot (hereinafter, referred to as "cladding soot") that forms the cladding layer <NUM> is formed on the outer circumference of the core glass using an outside vapor-deposition method. In the outside vapor-deposition method, "deposition" of depositing the cladding soot on the outer circumference of the core glass is performed. At this time, it is preferable that the deposition is performed in a state where the surface of the core glass is etched with etching gas such as SF<NUM> or C<NUM>F<NUM> to remove remaining water. During the formation of the cladding soot, the core glass and the cladding soot may be drawn in order to obtain an outer diameter in which the soot can be easily vapor-deposited. In addition, the cladding layer <NUM> may be vapor-deposited multiple times so as to obtain a desired outer diameter.

Next, Cl-containing gas such as SOCl<NUM> or Cl<NUM> and F-containing gas such as SiF<NUM> are mixed with each other, and this mixed gas is diffused and is caused to react in the cladding soot. As a result, the dehydration treatment and the F addition treatment of the cladding soot can be simultaneously performed.

Next, the cladding soot on which the dehydration treatment and the F addition treatment are performed is heated in a F-containing gas atmosphere to be vitrified such that the cladding layer <NUM> is formed. At this time, according to the invention, the cladding layer <NUM> is dehydrated such that an average Cl concentration distribution in a cross-section is <NUM> wt% or higher.

On the other hand, when the amount of Cl added to the cladding layer <NUM> is excessive, the transmission loss of the optical fiber <NUM> may increase due to Rayleigh scattering in the cladding layer <NUM>. In addition, when the amount of Cl added to the cladding layer <NUM> is excessive, it is difficult to adjust a difference in viscosity between the core <NUM> and the cladding layer <NUM> or the refractive index profile. Therefore, the cladding layer <NUM> is dehydrated such that an average Cl concentration profile in a cross-section is <NUM> wt% or lower.

According to the invention, the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%. The Cl concentration in the cladding layer <NUM> can be measured using a Fourier transform infrared spectrophotometer (FT-IR).

The shape of the refractive index profile and the Cl concentration distribution in the formed cladding layer <NUM> may become higher or lower from the outer circumference toward the inside, may be stepwise, or may be flat (uniform). In addition, a plurality of cladding layers <NUM> may be formed in order to obtain a desired shape of the refractive index profile and a desired Cl concentration distribution. However, in order to form a plurality of cladding layers <NUM>, the cladding formation using an outside vapor-deposition method is performed multiple times. Therefore, as long as optical characteristic suitable for the use can be obtained, the stepwise (<FIG>) or W-shaped (<FIG>) refractive index profile that can be realized with a small number of times of vapor-deposition is preferable.

By forming the cladding layer <NUM> as described above, an optical fiber preform can be obtained.

In addition, by melting and drawing the optical fiber preform obtained as described above, the optical fiber <NUM> can be manufactured.

Hereinafter, the embodiment will be described using specific Examples. The following Examples do not limit the present invention.

<FIG> is a graph showing a relationship between a Cl concentration in the cladding layer <NUM> and a transmission loss of the optical fiber <NUM> increased by exposure to hydrogen. The horizontal axis of <FIG> represents a Cl concentration (wt%) in the cladding layer <NUM>. The vertical axis of <FIG> represents a difference in transmission loss ΔLoss (=Δ2-Δ1). Here, Δ2 represents a value of transmission loss of the optical fiber <NUM> after exposure to hydrogen under conditions of H<NUM> concentration: <NUM>%, in room temperature, and for <NUM> hours. Δ1 represents a value of transmission loss of the optical fiber <NUM> before the exposure. A measurement wavelength of transmission loss was <NUM>, and the transmission loss was measured using an OTDR. That is, data of <FIG> was obtained by changing the Cl concentration in the cladding layer <NUM> in a state where the measurement wavelength of transmission loss was fixed.

It can be seen from <FIG> that, when the optical fiber <NUM> is exposed to hydrogen, the transmission loss at a wavelength of <NUM> of the optical fiber <NUM> increases. Further, it can be seen that, as the Cl concentration in the cladding layer <NUM> increases, ΔLoss at a wavelength of <NUM> decreases, and when the Cl concentration in the cladding layer <NUM> is <NUM> wt% or higher, ΔLoss is <NUM> dB/km or lower. That is, at a wavelength of <NUM>, Δ2-Δ1≤<NUM> dB/km is satisfied.

Here, in this example, the optical fiber <NUM> was exposed to high-concentration H<NUM> as an accelerated test. Therefore, the transmission loss of the optical fiber <NUM> significantly increased within a short period of time. On the other hand, H<NUM> is present even in a natural environment such as air or water. Therefore, it is considered that the same phenomenon occurs by exposing the optical fiber <NUM> in a natural environment for a long period of time. That is, when this phenomenon occurs, the transmission loss increases with the passage of time after providing the optical fiber <NUM> as a communication cable.

This way, the mechanism in which the transmission loss of the optical fiber <NUM> increases by exposing the optical fiber <NUM> to hydrogen will be discussed below.

<FIG> shows data of two kinds of the optical fibers <NUM> including the optical fiber <NUM> (Cl: <NUM> wt%) in which the cladding layer <NUM> was not sufficiently dehydrated and the optical fiber <NUM> (Cl: <NUM> wt%) in which the cladding layer <NUM> was sufficiently dehydrated. In <FIG>, the horizontal axis of the graph represents a measurement wavelength of transmission loss, and the vertical axis represents ΔLoss. That is, data of <FIG> was obtained by changing the measurement wavelength of transmission loss in a state where the Cl concentration in the cladding layer <NUM> was fixed in each of the two kinds of optical fibers <NUM>.

As shown in <FIG>, in the optical fiber <NUM> (Cl: <NUM> wt%) in which the cladding layer <NUM> was not sufficiently dehydrated, an increase in transmission loss having a peak in the vicinity of a wavelength range <NUM> to <NUM> was shown. The value of ΔLoss decreased as the measurement wavelength increased from the vicinity of a wavelength of <NUM>. However, the increase in transmission loss reached up to the vicinity of a wavelength of <NUM> (not shown). That is, an increase in transmission loss having a peak in the vicinity of a wavelength range <NUM> to <NUM> brings about an increase in transmission loss at a wavelength in the vicinity of a wavelength of <NUM> used for optical communication.

It is presumed that the increase in transmission loss having a peak in the vicinity of a wavelength range <NUM> to <NUM> occurs because the cladding layer <NUM> is not sufficiently dehydrated. When the cladding layer <NUM> is not dehydrated, due to the presence of a large number of OH groups in the cladding layer <NUM>, binding defects of SiO<NUM> having an action of absorbing light in the vicinity of a wavelength of <NUM> to <NUM> occurs at an interface between the core <NUM> and the cladding layer <NUM>. Due to the defects, the transmission loss in the vicinity of a measurement wavelength of <NUM> to <NUM> increased.

On the other hand, in the optical fiber <NUM> (Cl: <NUM> wt%) in which the cladding layer <NUM> was dehydrated, the value of ΔLoss was negative particularly in a measurement wavelength range of <NUM> or shorter. The value of ΔLoss being negative represents that the transmission loss was reduced by exposing the optical fiber <NUM> to hydrogen.

The reason why the transmission loss was reduced is presumed to be that an oxygen defective type defect present in an ultraviolet range, for example, E' center of SiO<NUM> react with hydrogen, -H is arranged at the terminal of the defect, and a Si-H structure having a low absorbance appears. That is, due to the appearance of the Si-H structure having a low absorbance, the light absorption amount was reduced, and the transmission loss was reduced as compared to that before the exposure to hydrogen.

As a result, an increase in transmission loss caused by exposure to hydrogen can be limited by dehydrating the cladding layer <NUM>.

Incidentally, when the dehydration treatment of the cladding layer <NUM> caused by Cl-containing gas is excessively performed, the refractive index of the cladding layer <NUM> increases due to Cl included in the cladding layer <NUM>, and it is necessary that the refractive index of the cladding layer <NUM> is lower than the refractive index of the core <NUM> by a predetermined amount. On the other hand, when F is added to reduce the refractive index of the cladding layer <NUM>, Rayleigh scattering increases, which causes deterioration in transmission loss. Accordingly, the Cl concentration in the cladding layer is according to the claimed range.

Hereinafter, the result of verifying the range of the Cl concentration in the cladding layer <NUM> and the like will be described using Table <NUM>.

The optical fibers <NUM> were prepared under a plurality of conditions (Comparative Examples <NUM> to <NUM> and Examples <NUM> to <NUM>), and a relationship between the Cl concentration in the cladding layer <NUM> and the transmission loss was verified. The results are shown in Table <NUM>.

"Cladding Cl Concentration" in Table <NUM> shows the results of measuring the average Cl concentration in the cladding layer <NUM> by an electron probe microanalyzer (EPMA).

"Cladding OH Group Concentration" of Table <NUM> shows the results of measuring the OH group concentration in the cladding layer <NUM> by an FT-IR. The lower detection limit of the OH group concentration by the FT-IR used herein was <NUM> ppm.

"Transmission Loss (Δ1) before Exposure to H<NUM>" of Table <NUM> shows the results of measuring the transmission loss of each of the optical fibers <NUM> by an OTDR before exposure to hydrogen, that is, in an initial state at a wavelength of <NUM>.

"Difference in Transmission loss (ΔLoss)" of Table <NUM> shows the value of transmission loss at a wavelength <NUM> increased after exposing each of the optical fibers <NUM> to hydrogen under conditions of H<NUM> concentration: <NUM>%, in room temperature (<NUM> to <NUM>), and for <NUM> hours.

Data shown in Table <NUM> are arranged in order from the lowest cladding Cl concentration to the highest. That is, data shown on the upper side of Table <NUM> was data of the optical fiber <NUM> in which the cladding layer <NUM> was weakly dehydrated, and data shown on the lower side of Table <NUM> was data of the optical fiber <NUM> in which the cladding layer <NUM> was strongly dehydrated.

Here, focusing on the values of "ΔLoss" in Table <NUM>, it can be seen that the difference in transmission loss (ΔLoss) of data (Examples <NUM> to <NUM>) in which the cladding Cl concentration was <NUM> wt% or higher was less than those of data (Comparative Examples <NUM> to <NUM>) in which the cladding Cl concentration was <NUM> to <NUM> wt%. That is, regarding the data in which the cladding concentration was <NUM> wt% or higher, ΔLoss was limited to be <NUM> dB/km or lower. However, regarding the data in which the cladding concentration was <NUM> wt% or lower, ΔLoss was relatively large at <NUM> dB/km or higher. It can be said from that above result that the cladding Cl concentration is <NUM> wt% or higher.

Next, focusing on the values of "Transmission Loss (Δ1) before Exposure to H<NUM>" of Table <NUM>, in the data (Examples <NUM> to <NUM>) in which the cladding Cl concentration was <NUM> wt% or lower, the transmission loss was limited to be <NUM> dB/km or lower. However, in the data (Comparative Example <NUM>) in which the cladding Cl concentration was <NUM> wt%, the transmission loss was <NUM> dB/km. In Comparative Example <NUM>, an excess amount of Cl was added to the cladding layer <NUM>. As a result, it was necessary to add F to reduce the refractive index of the cladding layer <NUM>, and the initial transmission loss (Δ1) was increased due to Rayleigh scattering caused by F. It can be said from that above result that the cladding Cl concentration is <NUM> wt% or lower.

In consideration of the above-described discussion, the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%.

By adjusting the Cl concentration in the cladding layer <NUM> to be <NUM> wt% or higher, an increase in the transmission loss of the optical fiber <NUM> caused by exposure to hydrogen can be limited. As a result, when the optical fiber <NUM> is exposed to hydrogen present in air or water for a long period of time after providing the optical fiber <NUM> as an optical fiber cable or the like, an increase in the transmission loss of the optical fiber <NUM> can be limited.

Further, by adjusting the Cl concentration in the cladding layer <NUM> to be <NUM> wt% or lower, the cladding layer <NUM> includes a large amount of Cl, and an increase in refractive index is limited. Therefore, it is not necessary that the cladding layer <NUM> includes a large amount of fluorine or the like.

That is, an increase in the transmission loss of the optical fiber <NUM> caused by exposure to hydrogen can be limited while limiting the initial transmission loss of the optical fiber <NUM> to be low.

In addition, when the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%, the difference in transmission loss (ΔLoss) between the value Δ2 of transmission loss after exposure to hydrogen at a wavelength of <NUM> and the value Δ1 of transmission loss before exposure to hydrogen at a wavelength of <NUM> is <NUM> dB/km or lower (refer to <FIG>). That is, at a wavelength of <NUM>, Δ2-Δ1≤<NUM> dB/km is satisfied. As a result, the optical fiber <NUM> can exhibit an excellent effect in that it has a wavelength band in which the transmission loss is reduced by exposure to hydrogen.

In addition, when the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%, at a wavelength of <NUM>, Δ2-Δ1≤<NUM> dB/km is satisfied (refer to <FIG>). As a result, the optical fiber <NUM> can be obtained in which an increase in transmission loss caused by exposure to hydrogen can be limited even in a wavelength band used for optical communication.

The optical fiber can be manufactured in which, when cladding layer <NUM> is dehydrated with Cl-containing gas such that the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%, Δ1 is <NUM> dB/km or lower at a wavelength of <NUM>, and Δ2-Δ1≤<NUM> dB/km. (Refer to <FIG> and Table <NUM>).

In addition, when the cladding layer <NUM> is dehydrated with mixed gas of Cl-containing gas and F-containing gas, the dehydration treatment and the F addition treatment of the cladding layer <NUM> can be simultaneously performed. Therefore, the optical fiber <NUM> can exhibit the above-described excellent effect while reducing the manufacturing time of the optical fiber <NUM>.

In addition, by melting and drawing the optical fiber preform in which the Cl concentration in the soot that forms the cladding layer <NUM> is <NUM> wt% to <NUM> wt%, the optical fiber <NUM> can be manufactured in which the Cl concentration in the cladding layer <NUM> is <NUM> wt% to <NUM> wt%.

Claim 1:
An optical fiber (<NUM>) comprising:
a core (<NUM>); and
a cladding layer (<NUM>) that is provided on an outer circumference of the core (<NUM>), characterized in that:
the core (<NUM>) is formed of silicon dioxide having no added germanium,
the core (<NUM>) includes <NUM> wt% to <NUM> wt% of Cl, and
an average Cl concentration distribution across a cross section of the cladding layer (<NUM>) is <NUM> wt% to <NUM> wt%.