JP7149515B2 - Rare-earth-doped optical fiber and method for improving radiation resistance of rare-earth-doped optical fiber - Google Patents

Description

The present invention relates to a rare-earth-doped optical fiber and a method for improving the radiation resistance of the rare-earth-doped optical fiber.

An optical fiber generally has a core portion and a clad portion covering the core portion. In this type of optical fiber, the refractive index of the core is made higher than that of the clad, and the light is totally reflected at the boundary between the core and the clad. Propagate far away as a signal.

Optical fibers are also used as amplifiers. Rare-earth-element-doped optical fibers having a core portion doped with a rare-earth element are known as optical fibers for amplifiers. An Er-doped optical fiber doped with erbium (Er) as a rare earth element is used as an amplifier in the 1.55 μm band.
In a rare earth element-doped optical fiber used as an amplifier, it is effective to increase the content of the rare earth element in the core portion in order to improve the gain. However, if the content of the rare earth elements (especially Er) is too high, clustering (aggregation) of the rare earth elements occurs, concentration quenching occurs, and the gain may rather decrease. In order to prevent this clustering between rare earth elements, in a rare earth element-doped optical fiber, phosphorus (P) or aluminum (Al) is co-doped in the core portion to disperse the rare earth elements. .

2. Description of the Related Art With recent progress in optical communication technology, optical fibers have come to be used in various environments. For example, optical fibers are also used in environments where radiation exists, such as outer space and nuclear power plants. However, an optical fiber having a core portion doped with a metal element has a problem that it is easily deteriorated by radiation (especially electromagnetic radiation such as X-rays and γ-rays).

For example, it is known that an optical fiber doped with Ge to improve the refractive index of the core portion of the optical fiber is inferior to an optical fiber having a pure silica core in radiation resistance. Therefore, increasing the refractive index of the core portion without using Ge is being studied. In Patent Document 1, as a method for increasing the refractive index of the core portion without using Ge, a quartz glass clad portion containing a high concentration of fluorine is used as the clad portion, and the clad portion has a lower refractive index than the core portion. A method of relatively increasing the refractive index of the core portion is disclosed.

In addition, when a rare earth element-doped optical fiber in which P or Al is added to the core portion to prevent clustering of the rare earth element from occurring, defect absorption of light is generated and the gain is lowered when irradiated with radiation. It has been known. This defect absorption of light is said to be caused by P and Al added to the core portion.

US Pat. No. 6,200,000 discloses a rare earth element-doped optical fiber that does not require the addition of any other radiation sensitive dopants. A doped optical fiber is disclosed. In the rare-earth-element-doped optical fiber disclosed in Patent Document 2, a silica-based matrix that does not contain phosphorus or aluminum is used as the core matrix.

Patent Document 3 discloses that the core portion is co-doped with cerium (Ce) in order to suppress reduction in gain due to radiation caused by phosphorus. In Patent Document 3, the core portion has an erbium concentration of 100 to 1000 ppm, an ytterbium concentration of 500 to 10000 ppm, a phosphorus concentration of 2 to 10 atomic %, and a cerium concentration of 500 to 10000 ppm. An optical fiber is described.

JP-A-60-257408 JP 2010-135801 A U.S. Patent Application Publication No. 2013/0101261

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel rare earth element-doped optical fiber having excellent radiation resistance. That is, the present invention provides a novel rare-earth element-doped optical fiber that has high radiation resistance while containing a rare-earth element and phosphorus in the core portion, and does not easily reduce gain even when used in an environment where radiation exists. With the goal. Another object of the present invention is to provide a novel method for improving the radiation resistance of a rare earth element-doped optical fiber containing a rare earth element and phosphorus in the core portion.

In order to solve the above problems, the rare earth element-doped optical fiber of the present invention has a core portion and a clad portion surrounding the core portion, and the core portion contains erbium, ytterbium, phosphorus and germanium. The erbium content is 0.05% by mass or more, the phosphorus content is 13.5 or more and 92.6 or less in mass ratio P/Er between the phosphorus and the erbium, and the germanium is 0.5% by mass or more and 12.0% by mass or less, and the content of the ytterbium is 3.9 or more and 11.6 or less in terms of the mass ratio Yb/Er between the ytterbium and the erbium. is characterized by

In the rare earth element-doped optical fiber of the present invention, the germanium content may be 0.5% by mass or more and 8.6% by mass or less .

Further, in the rare earth element-doped optical fiber of the present invention, the phosphorus content may be 0.9% by mass or more and 18% by mass or less.
Further, in the rare earth element-doped optical fiber of the present invention, the ytterbium content may be 0.2% by mass or more and 2.5% by mass or less.

Further, in the rare earth element-doped optical fiber of the present invention, the core portion may contain cerium, and the cerium content may be 0.12% by mass or less.

Further, in the rare earth element-doped optical fiber of the present invention, the core portion may contain aluminum, and the content of the aluminum may be less than 1.0 as a mass ratio Al/P between the aluminum and the phosphorus. .

Further, in the rare earth element-doped optical fiber of the present invention, the core portion may contain boron, and the boron content may be 0.6% by mass or less.

A method for improving radiation resistance of a rare earth element-doped optical fiber according to the present invention has a core portion and a clad portion surrounding the core portion, and the core portion contains a rare earth element containing erbium, ytterbium and phosphorus. A method for improving the radiation resistance of a doped optical fiber, wherein the core portion contains 0.05% by mass or more of erbium, and the mass ratio P/Er of the phosphorus and the erbium is 13.5 to 92.6. Hereinafter, the germanium is added at 0.5% by mass or more and 12.0% by mass or less, and the ytterbium is added at a mass ratio Yb/Er of 3.9 or more and 11.6 or less between the ytterbium and the erbium .
In the method for improving the radiation resistance of a rare earth element-doped optical fiber according to the present invention, the content of germanium can be 0.5% by mass or more and 8.6% by mass or less.
In the method for improving the radiation resistance of the rare earth element-doped optical fiber according to the present invention, the phosphorus content can be 0.9% by mass or more and 18% by mass or less.
In the method for improving the radiation resistance of a rare earth element-doped optical fiber according to the present invention, the content of ytterbium can be 0.2% by mass or more and 2.5% by mass or less.
In the method for improving the radiation resistance of the rare earth element-doped optical fiber according to the present invention, the core portion may contain cerium, and the cerium content may be 0.12% by mass or less.
In the method for improving the radiation resistance of the rare earth element-doped optical fiber according to the present invention, the core portion contains aluminum, and the content of the aluminum is 1.0 in terms of the mass ratio Al/P of the aluminum and the phosphorus. can be less than
In the method for improving the radiation resistance of the rare earth element-doped optical fiber according to the present invention, the core portion may contain boron, and the boron content may be 0.6% by mass or less.

According to the present invention, even though the core portion contains erbium, germanium, ytterbium, and phosphorus , the mass ratio P/Er between phosphorus and erbium is 13.5 or more and 92.6 or less, and germanium is 0.5% by mass or more. Ytterbium is added at a mass ratio Yb/Er of ytterbium to erbium of 12.0% by mass or less, from 3.9 to 11.6. It is possible to provide a novel rare-earth element-doped optical fiber that is difficult to reduce.
The above-mentioned erbium addition makes it possible to obtain the action of generating signal light by excitation light in the core portion, and the above-mentioned germanium addition suppresses the generation of defect absorption due to radiation irradiation caused by phosphorus addition, improving radiation resistance. can. Furthermore, by adjusting the mass ratio of ytterbium and erbium as described above, even if the erbium content is high, erbium concentration quenching can be suppressed and the amount of signal light generated can be increased. Furthermore, by adjusting the mass ratio of erbium and phosphorus as described above, it is possible to reliably suppress the occurrence of erbium clustering.
Further, according to the present invention, by selecting the above-described erbium content and germanium content, and by selecting the above-described mass ratio P/Er and mass ratio Yb/Er , erbium, germanium, ytterbium , and phosphorus are formed in the core portion. It is possible to provide a novel method for improving the radiation resistance of a rare earth element-doped optical fiber containing

4 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 1. FIG. 5 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 2. FIG. 7 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 3. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 4. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 5. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 6. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 7. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 8. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 9. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 10. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 11. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 12. FIG. 13 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 13. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 14. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 15. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 16. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 17. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 18. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 19. FIG. 10 is a graph showing the concentration distribution of the additive element in the rare earth element-doped optical fiber preform produced in Example 20. FIG. 5 is a graph showing the amount of light absorption coefficient increase after X-ray irradiation of the rare earth element-doped optical fiber preforms produced in Examples 1 to 10. FIG. 10 is a graph showing the amount of increase in light absorption coefficient after X-ray irradiation of the rare earth element-doped optical fiber preforms produced in Examples 11 to 20. FIG. 5 is a graph showing the increase in light absorption loss of the rare earth element-doped optical fibers produced in Examples 1, 6, 8, 10 and Comparative Example 1 after γ-ray irradiation. 5 is a graph showing the amount of increase in light absorption loss after γ-ray irradiation of the rare earth element-doped optical fibers produced in Examples 11, 12, 13, 18, 19, 21, 22, and 23 and Comparative Example 1. FIG. 4 is a graph showing the relationship between the Ge doping concentration in the core portion of the rare-earth-element-doped optical fibers produced in Examples 1 to 23 and Comparative Example 1, X-ray resistance, and γ-ray resistance.

Embodiments of the rare earth element-doped optical fiber and the method for improving the radiation resistance of the rare earth element-doped optical fiber according to the present invention will be described below.

The rare-earth element-doped optical fiber of this embodiment has a core portion and a clad portion surrounding the core portion. The core is made of a silica composition containing erbium (Er), ytterbium (Yb), phosphorus (P) and germanium (Ge). The core portion may further contain cerium (Ce), aluminum (Al), and boron (B) as necessary.

The rare-earth-element-doped optical fiber of this embodiment is an Er-doped optical fiber whose core contains Er. The rare earth element-doped optical fiber of this embodiment is used as an optical fiber such as a multimode optical fiber (MMF), a single mode optical fiber (SMF), a double clad optical fiber (DCF), and a polarization maintaining optical fiber. It can be in various forms. The rare earth element-doped optical fiber of this embodiment can be used, for example, as an amplifier for light in the 1.55 μm band. The rare-earth-element-doped optical fiber of this embodiment can be used for optical communication equipment and optical fiber gyros mounted on spacecraft (for example, rockets, artificial satellites, spacecraft), optical fiber amplifiers in nuclear reactors, and optical communication equipment for aircraft. It can be suitably used in applications that require radiation resistance such as.

Er contained in the core portion has the effect of generating light (signal light) with a wavelength of 1.55 μm when excited by light (pumping light) with a wavelength of 0.98 μm or 1.48 μm.
The Er content of the core portion is preferably 0.05% by mass or more, and particularly preferably 0.09% by mass or more. On the other hand, if the Er content is too high, concentration quenching may occur due to clustering, so it is necessary to co-dope Yb and P within the range shown in the following section.

Yb is excited by light with a wide wavelength range of 0.8 to 1.1 μm, and has the action of exciting Er by energy transition from the excited Yb to Er. This action can increase the amount of light with a wavelength of 1.55 μm. Yb also has the effect of suppressing the occurrence of clustering of Er by coordinating around Er and increasing the distance between Er. Due to this action, concentration quenching of Er can be suppressed even if the Er content is high, and this also makes it possible to increase the amount of light with a wavelength of 1.55 μm. The Yb content in the core portion is preferably in the range of 4 to 50, more preferably in the range of 4 to 40, and particularly preferably in the range of 4 to 20 in terms of the mass ratio Yb/Er of Yb and Er. . By setting the Yb content within the above range, it is possible to reliably increase the generation of light with a wavelength of 1.55 μm.

P has the effect of suppressing the occurrence of clustering of Er by coordinating around Er and lengthening the distance between Er. Due to this action, the concentration quenching of Er can be suppressed even if the Er content is high, so that the amount of light with a wavelength of 1.55 μm can be increased. P also has the effect of suppressing excited state absorption (ESA) of Er and back transfer of energy from excited Er to Yb.
The P content in the core portion is preferably in the range of 18 or more and 360 or less in mass ratio P/Er of P and Er. By setting the P content within the above range, it is possible to reliably suppress the occurrence of Er clustering.

Ge has the effect of suppressing the generation of defect absorption due to P-induced radiation irradiation and improving the radiation resistance, as will be shown in the examples below.
The inventor of the present invention added various elements to the core portion in order to improve the radiation resistance of the rare earth element-doped optical fiber containing P. As a result, unexpectedly, the addition of Ge, whose use was conventionally avoided from the viewpoint of radiation resistance, suppresses the generation of defect absorption due to irradiation of radiation caused by P, and the rare earth element optical fiber is improved. It was found that radiation resistance is improved.

The Ge content of the core portion is preferably in the range of 0.5% by mass or more and 12.0% by mass or less. The lower limit of the Ge content is more preferably 1.0% by mass or more, still more preferably 2.3% by mass or more, and particularly preferably 3.2% by mass or more. The upper limit of the Ge content is more preferably 10% by mass or less, more preferably 9.0% by mass or less, particularly preferably 8.6% by mass or less, still more preferably 6.0% by mass or less, and most preferably It is 5.0% by mass or less. If the Ge content is too low, it may become difficult to sufficiently improve the radiation resistance. On the other hand, if the Ge content is too high, the effect of improving the radiation resistance may be saturated and the NA (numerical aperture) may become too large.

Ce has the effect of improving the radiation resistance together with Ge described above.
The Ce content in the core portion is preferably 0.12% by mass or less, and more preferably in the range of 0.08% by mass to 0.12% by mass. By setting the Ce content within the above range, the radiation resistance can be improved more reliably.

Al, together with P described above, coordinates around Er to lengthen the distance between Er, thereby suppressing the occurrence of clustering of Er. In addition, Al has the effect of leveling the amplification gain and the effect of reducing the NA of the core portion by being co-doped with P.
The Al content in the core portion is preferably less than 1.0 as the mass ratio Al/P between Al and P. If the content of Al is too high, the absorption of defects caused by Al is likely to occur due to radiation irradiation, and the radiation resistance may decrease. While suppressing the generation of defect absorption due to the irradiation of , the occurrence of clustering of Er can be further suppressed, and the NA of the core portion can be reduced.

B has the effect of reducing the NA of the core without degrading the radiation resistance. From the viewpoint of reducing the NA of the core, since B can substitute for Al, the addition of B can reduce the amount of Al added.
The content of B in the core portion is preferably 0.6 mass % or less, particularly preferably 0.1 mass % or more and 0.6 mass % or less. If the content of B is too high, it is difficult to add both B and a rare earth element at a high concentration when manufacturing a rare earth element-doped optical fiber preform (preform) by the MCVD method. A decrease in the content of the element may reduce the effect of adding the rare earth element. Also, if the B content is too high, the difference in linear expansion coefficient between the core portion and the clad portion of the rare earth element-doped optical fiber preform increases during the production of the rare earth element-doped optical fiber, and the preform cracks. It may get lost. If the B content is less than 0.1% by mass, the addition of B may reduce the effect of reducing the NA of the core.

In the rare-earth element-doped optical fiber of the present embodiment, the material of the cladding is not particularly limited as long as it has a lower refractive index than the core. Silica (quartz glass), for example, can be used as the material of the clad.

Although the optical fiber of this embodiment is not particularly limited, the diameter of the cladding portion may be in the range of 50 μm or more and 400 μm or less. The diameter of the core portion is not particularly limited, but may be in the range of 3 μm or more and 110 μm or less.

Next, a method for manufacturing the rare earth element-doped optical fiber of this embodiment will be described.
The rare-earth element-doped optical fiber of the present embodiment is produced by, for example, a preform manufacturing step of manufacturing a rare-earth element-doped optical fiber preform having a core portion and a clad portion surrounding the core portion; and a drawing step of drawing the element-doped optical fiber preform to manufacture the rare-earth element-doped optical fiber.

(Base material manufacturing process)
In the base material preparation step, a core forming portion comprising a silica composition containing Er, Yb, P and Ge, and optionally Ce, Al and B, and a clad forming portion surrounding the core forming portion. A rare earth element-doped optical fiber preform having The MCVD method (Modified Chemical Vapor Deposition Method) can be used as a method for producing the rare earth element-doped optical fiber preform. Specifically, a silica source (eg, SiCl ₄ ), an Er source (eg, Er(DPM) ₃ ), a Yb source (eg, Yb(DPM) ₃ ), and a P source (eg, POCl ₃ ) are placed in a quartz tube. , a Ge source (e.g., GeCl ₄ ), a Ce source (e.g., Ce(DPM) ₄ ), an Al source (e.g., AlCl ₃ ), a B source (e.g., BCl ₃ ), and a source gas containing oxygen, A quartz tube is heated by an oxyhydrogen burner. At this time, the amounts of Er, Yb, P, Ce, Al, and B contained in the core portion can be adjusted by adjusting the type and composition of the raw material gas. Thus, soot composed of glass composition particles containing Er, Yb, P, Ce, Al and B is deposited on the quartz tube. The quartz tube is then heated by an oxyhydrogen burner to turn the soot into a transparent glass layer. Finally, the supply of the raw material gas is stopped, and the quartz tube is heated by an oxyhydrogen burner to soften the quartz tube and crush the hollow portion. As a result, a rare-earth element-doped optical fiber preform having a core-forming portion comprising a silica composition containing Er, Yb, P, Ge, Ce, Al, and B and a clad-forming portion surrounding the core-forming portion is produced. can get.

In addition to the above-described MCVD method, various methods generally used as a method for manufacturing an optical fiber preform, such as the OVD method (Outside Vapor Deposition Method) and the VAD method, can be used as the method for manufacturing the rare earth element-doped optical fiber preform. (Vapor phase Axial Deposition Method) or the like can be used.

(drawing process)
In the drawing step, the rare earth element-doped optical fiber preform prepared in the preform preparation step is heated and melted, and the melted rare earth element-doped optical fiber preform is drawn into a filament and cooled. As a method for stretching the rare-earth-element-doped optical fiber preform into a filamentous shape, various methods commonly used in the manufacture of optical fibers can be used.

The obtained rare earth element-doped optical fiber can be used by coating the outer circumference of the clad portion with a protective resin. As a method of coating the outer periphery of the clad portion with the protective resin, various methods commonly used in the manufacture of optical fibers can be used.

According to the rare-earth element-doped optical fiber of the present embodiment configured as described above, since the core portion contains Er, Yb, P, and Ge, even if the Er content is increased, clustering of Er occurs. is less likely to occur, and has high radiation resistance, and the gain is less likely to decrease even when used in an environment where radiation exists.

A method for improving the radiation resistance of a rare earth element-doped optical fiber according to the present embodiment has a core portion and a clad portion surrounding the core portion, and the core portion comprises a rare earth element containing Er, Yb and P. This is a method for improving the radiation resistance of doped optical fibers. In this embodiment, Ge is added to this core portion. Ce, Al, and B may be added to the core portion, if necessary.

Specifically, the method for improving the radiation resistance of the rare-earth-doped optical fiber of the present embodiment is to incorporate Ge into the core-forming portion when fabricating the base material of the rare-earth-doped optical fiber. The amounts of Er, Yb, P, Ce, Al, and B contained in the core portion of the optical fiber obtained by using the method for improving the radiation resistance of the rare earth element-doped optical fiber of this embodiment are as described above. be.

According to the method for improving the radiation resistance of the rare earth element-doped optical fiber of the present embodiment, the core portion of the optical fiber containing Er, Yb, and P is doped with Ge, so that radiation caused by P is reduced. The generation of defect absorption due to irradiation can be suppressed, thereby improving the radiation resistance of the rare earth element-doped optical fiber.

Below, the effects of the present invention will be described with reference to examples.

[Example 1]
(Fabrication of rare earth element-doped optical fiber preform)
Silica composition in which the core portion has an Er content of 0.12% by mass, a Yb content of 0.69% by mass, a P content of 9.95% by mass, and a Ge content of 4.0% by mass by the MCVD method A rare earth element-doped optical fiber preform (diameter: 13 mm) having a cladding made of silica glass was produced.

(Fabrication of optical fiber)
A rare earth element-doped optical fiber was produced by drawing the above rare earth element-doped optical fiber preform. The produced rare earth element-doped optical fiber was a multimode optical fiber (MMF) having a cladding diameter of 125 μm and a core diameter of 35 μm.

[Examples 2 to 23, Comparative Example 1]
(Fabrication of rare earth element-doped optical fiber preform)
A rare earth element-doped optical fiber preform was produced in the same manner as in Example 1, except that the composition of the core portion was set as shown in Tables 1 and 2 below. Tables 1 and 2 show the composition of the core portion as Er content %, Yb content, P content, Ge content, Ce content, Al content, B content, Yb/Er mass ratio, P/Er mass ratio and Al/Er mass ratio are shown.

(Fabrication of rare earth element-doped optical fiber preform)
In Examples 6, 8, 10, 11, 12, 13, 18, 19, 21, 22, and 23 and Comparative Example 1, the obtained rare earth element-doped optical fiber preforms were used, and the following Tables 1 and 2 A rare-earth element-doped optical fiber having the structure shown was manufactured. In Examples 6, 21 and 22, and in Examples 10 and 23, the same rare earth element-doped optical fiber preform was used to manufacture rare earth element-doped optical fibers. In Tables 1 and 2, MMF is multimode optical fiber, SMF is single mode optical fiber, DCF is double clad optical fiber, and 2LP-DCF is 2LP of LP01 mode and LP11 mode. Mode-propagating double-clad optical fiber.
In SMF, DCF, and 2LP-DCF, silica glass (cladding glass) is added to the outer periphery of the rare earth element-doped optical fiber preform by a jacket method (also called a rod-in-tube method), and then the core diameter is as shown in Tables 1 and 2. and the diameter of the clad portion was 125 μm.

The obtained rare earth element-doped optical fiber was coated with a first resin layer, and then the first resin layer was coated with a second resin layer. The second resin layer was coated so as to have an outer diameter of 250 μm. In MMF and SMF, a resin having a higher refractive index than silica glass (cladding) was used as the material of the first resin layer on the rare earth element-doped optical fiber side. In DCF and 2LP-DCF, a resin having a refractive index lower than that of silica glass was used as the material for the first resin layer, and a resin having a refractive index higher than that of silica glass (cladding) was used as the material for the second resin layer. .

[evaluation]
Using the rare earth element-doped optical fiber preform, the concentration distribution of the added element and the X-ray resistance characteristics were measured. Also, the γ-ray resistance was measured using the rare-earth element-doped optical fiber. The measurement method is shown below.

(Concentration distribution of additive element in rare earth element-doped optical fiber preform)
A rare earth element-doped optical fiber preform was cut radially. The concentration of the additive element was linearly analyzed using an EPMA (electron probe microanalyzer) along the center line of the cut surface of the obtained cut piece. EPMA measurement conditions are shown below.
<EPMA measurement conditions>
Accelerating voltage: 15.0 kV
Irradiation current: 1.17×10 ⁻⁷ A
Beam shape: circle
Probe diameter: 50 μm
Number of measurement points: 200
Scan width: 1cm
Measurement time: 2 sec

The results obtained for the rare earth element-doped optical fiber preforms produced in Examples 1 to 20 are shown in FIGS. 1 to 20 in order. 1 to 20, the horizontal axis indicates the measurement position from the base material center, and "0" indicates the center of the core portion of the base material. The vertical axis represents the concentration of each additive element contained in the base material. It was confirmed that the average concentration of each additive element in Examples 1 to 20 calculated from FIGS. .

(X-ray resistance)
A rare earth element-doped optical fiber preform was cut radially. With respect to the obtained cut piece, the light absorption coefficient of the core portion of the cut surface was measured under the following conditions.
<Conditions for measurement of light absorption coefficient>
Wavelength range: 200-1600nm
Resolution: 0.5nm

Next, the core portion of the cut surface of the cut piece was irradiated with X-rays having a wavelength of 0.71 Å (Mo) at an output of 17.5 keV for 100 minutes. The light absorption coefficient of the core portion of the cut surface of the cut piece after the X-ray irradiation was measured again. Then, a value obtained by subtracting the light absorption coefficient of the core portion before X-ray irradiation from the light absorption coefficient of the core portion after X-ray irradiation was calculated as the absorption coefficient increase amount. In the rare earth element-doped optical fiber preform produced in Comparative Example 1, the core portion was colored reddish purple after X-ray irradiation, and it was clear that the light absorption coefficient was significantly increased. No re-measurement of coefficients was performed.

The results obtained for the rare earth element-doped optical fiber preforms produced in Examples 1 to 10 are shown in FIG. 21, and the results obtained for the rare earth element-doped optical fiber preforms produced in Examples 1 to 10 are shown in FIG. . Absorption of light with wavelengths of 240 nm, 400 nm, 510 nm and 570 nm is induced defect absorption by P-OHC (phosphorus oxygen hole center). From the results of FIGS. 21 and 22, it can be seen that the core portion of the rare earth element-doped optical fiber after X-ray irradiation has increased light absorption by P-OHC. Tables 1 and 2 show the amount of increase in absorption coefficient for light with a wavelength of 550 nm as X-ray resistance.

(γ-ray resistance)
Regarding the rare earth element-doped optical fibers produced in Examples 1, 6, 8, 10, 11, 12, 13, 18, 19, 21, 22, and 23 and Comparative Example 1, the horizontal axis is the wavelength of light, and the vertical axis is A light loss spectrum as light absorption loss was measured by the following method.
<Method for measuring optical loss spectrum>
Loss spectra were measured by the cutback method. White light was incident on the fiber under test having a length L ₀ [m], and the transmitted light spectrum was measured using an optical spectrum analyzer. The optical power at this time was P ₀ [dBm]. Next, the length of the fiber to be measured was cut back to L ₁ [m], and the transmitted light spectrum of the fiber to be measured after cutback was measured. The optical power at this time was P ₁ [dBm]. _L0 was about 10m and L1 was _about 2m. Absorption loss [dB/m] was determined by (P ₁ -P ₀ )/(L ₀ -L ₁ ).

Next, the rare-earth element-doped optical fiber was irradiated with 1000 Gy of γ-rays (equivalent to the case of standing in a geostationary orbit for 10 years) using a cobalt-60 γ-ray irradiator. The optical loss spectrum of the rare-earth element-doped optical fiber after gamma-ray irradiation was measured again. Then, a value obtained by subtracting the light loss spectrum before γ-ray irradiation from the light loss spectrum of the rare earth element-doped optical fiber after γ-ray irradiation was calculated as the amount of increase in absorption loss. The wavelength (nm) of the loss spectrum of light is converted to the amount of photon energy (eV), the horizontal axis is the amount of photon energy (eV), and the vertical axis is the amount of increase in absorption loss. Using the obtained data, Gaussian fitting was performed to obtain an approximate curve. Gaussian fitting was performed using the parameters described in the following literature.
“DL Griscom, EJ Friebele, KJ Long and JW Fleming, “Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus-doped silica glass and optical fibers,” Journal of Applied Physics, vol. 54, no. 7, pp. 3743-3762, 1983.”

FIG. 23 shows the results obtained for the rare-earth element-doped optical fibers fabricated in Examples 1, 6, 8, 10 and Comparative Example 1. FIG. 24 shows the results obtained for the rare earth element-doped optical fiber produced in Example 1. FIG. Absorption of light with a wavelength of 1570 nm is induced defect absorption by phosphorus (P1). From the results of FIGS. 23 and 24, it can be seen that the optical fiber doped with a rare earth element after gamma-ray irradiation has increased light absorption by P1. Tables 1 and 2 show the increase in absorption loss of light with a wavelength of 1570 nm as γ-ray resistance.

In Comparative Example 1 in which Ge was not added to the core portion, the light absorption coefficient after X-ray irradiation and the light absorption loss after γ-ray irradiation were increased. It is considered that this is because the absorption of induced defects by POHC and P1 increased due to irradiation with radiation.

On the other hand, in Examples 1 to 23 in which Ge was added to the core portion, the light absorption coefficient after X-ray irradiation and the light absorption loss after γ-ray irradiation were small. That is, from the results of Examples 1 to 23, it was confirmed that the radiation resistance of the rare earth element-doped optical fiber was improved by the addition of Ge. Further, from the results of Examples 7 and 8, it was confirmed that the radiation resistance was further improved by adding Ce together with Ge. Furthermore, from the results of Examples 9, 10, 11 to 20, and 23, it was confirmed that Ge is also effective for rare earth element-doped optical fibers containing P and Al.

FIG. 25 shows the relationship between the Ge doping concentration in the core portion of the rare-earth element-doped optical fiber, the X-ray resistance, and the γ-ray resistance. The horizontal axis of FIG. 25 indicates the Ge doping concentration in the core portion, and the RIA of P-OHC (X-ray) (550 nm) on the left vertical axis indicates the increase in the absorption coefficient of light with a wavelength of 550 nm after X-ray irradiation, RIA of P1 (Gannima-ray) (1570 nm) on the right vertical axis indicates the increase in absorption loss of light with a wavelength of 1570 nm after γ-ray irradiation. The white and black circles of P-Ge are the measurement results of the rare earth element-doped optical fiber preform and optical fiber containing Yb, Er, P and Ge, and the white and black triangles of P-Ge-Ce are the Yb and The measurement results of the rare earth element-doped optical fiber preform containing Er, P, Ge, and Ce and the optical fiber are shown. The measurement results of the doped optical fiber preform and the optical fiber, the white rhombus and the black rhombus P-Ge-Al-B are the rare earth element doped optical fiber preform and light containing Yb, Er, P, Ge, Al and B 4 shows fiber measurement results. White circles, white triangles, white squares, and white rhombuses indicate the amount of increase in absorption coefficient of light with a wavelength of 550 nm after X-ray irradiation, and black circles, black triangles, black squares, and black rhombuses indicate a wavelength of 1570 nm after γ-ray irradiation. shows the amount of increase in optical absorption loss of .

The graph in FIG. 25 reveals the following.
(1) From the measurement results of the rare earth element-doped optical fiber preform containing Yb, Er, P, and Ge and the optical fiber (white circles and black circles), X-ray resistance and γ-ray resistance are proportional to the doping concentration of Ge. and improve.
(2) The effect of (1) tends to saturate as the Ge concentration increases.
(3) From the measurement results of the rare-earth element-doped optical fiber preform containing Yb, Er, P, Ge, and Ce and the optical fiber (white triangle and black triangle), it was found that even if the doping concentration of Ge was the same, Ce was coexistent. The addition improves X-ray resistance and γ-ray resistance.
(4) From the measurement results of the rare earth element-doped optical fiber preform containing Yb, Er, P, Ge, and Al, and the optical fiber (white squares and black squares), Al is added within a range where Al/P is less than 1.0. The co-doped rare-earth element-doped optical fiber exhibits X-ray resistance and gamma-ray resistance equivalent to those of the rare-earth element-doped optical fiber containing Yb, Er, P, and Ge (white circles and black circles).
(5) From the measurement results of the rare-earth element-doped optical fiber preform containing Yb, Er, P, Ge, Al, and B and the optical fiber (white rhombus and black rhombus), B was co-doped to lower the Al concentration. The rare-earth element-doped optical fiber exhibits X-ray resistance and gamma-ray resistance equivalent to those of the rare-earth-element-doped optical fiber preform containing Yb, Er, P, and Ge and the optical fibers (white circles and black circles).

From the above results, according to the present invention, a novel rare-earth-element-doped light that has high radiation resistance while containing a rare-earth element and P in the core portion and that is difficult to reduce the gain even when used in an environment where radiation exists. It was confirmed that the fiber can be provided. Further, it has been confirmed that the present invention can provide a novel method for improving the radiation resistance of a rare earth element-doped optical fiber containing a rare earth element and P in the core portion.

Claims (14)

having a core portion and a clad portion surrounding the core portion, the core portion containing erbium, ytterbium, phosphorus and germanium;
The erbium content is 0.05% by mass or more,
The phosphorus content is 13.5 or more and 92.6 or less in mass ratio P/Er of the phosphorus and the erbium and
The germanium content is 0.5% by mass or more and 12.0% by mass or less,
The content of the ytterbium is 3.9 or more and 11.6 or less as a mass ratio Yb/Er of the ytterbium and the erbium. Rare earth element doped optical fiber. The germanium content is 0.5% by mass or more and 8.6% by mass or less, The rare earth element-doped optical fiber according to claim 1. The phosphorus content is 0.9% by mass or more and 18% by mass or less, 3. The rare earth element-doped optical fiber according to claim 1 or 2. The ytterbium content according to any one of claims 1 to 3, wherein the content of ytterbium is 0.2% by mass or more and 2.5% by mass or less. Rare earth element doped optical fiber. The rare earth element-doped optical fiber according to any one of claims 1 to 4, wherein the core contains cerium, and the cerium content is 0.12% by mass or less. The rare earth element according to any one of claims 1 to 5 , wherein the core portion contains aluminum, and the content of the aluminum is less than 1.0 in terms of the mass ratio Al/P of the aluminum and the phosphorus. Doped optical fiber. The rare earth element-doped optical fiber according to any one of claims 1 to 6 , wherein said core portion contains boron, and the content of said boron is 0.6% by mass or less. A method for improving radiation resistance of a rare-earth-element-doped optical fiber having a core and a cladding surrounding the core, wherein the core contains erbium, ytterbium, and phosphorus, comprising:
The core portion contains 0.05% by mass or more of erbium, a mass ratio P/Er between the phosphorus and the erbium of 13.5 to 92.6, and 0.5% by mass or more of germanium to 12.0% by mass. % or less of the ytterbium, the mass ratio Yb/Er of the ytterbium and the erbium is 3.9 or more and 11.6 or less , a method for improving the radiation resistance of a rare earth element-doped optical fiber. 9. The method for improving radiation resistance of a rare earth-doped optical fiber according to claim 8, wherein the germanium content is 0.5% by mass or more and 8.6% by mass or less. 10. The method for improving the radiation resistance of a rare earth-doped optical fiber according to claim 8, wherein the phosphorus content is 0.9% by mass or more and 18% by mass or less. The method for improving the radiation resistance of a rare earth element-doped optical fiber according to any one of claims 8 to 10, wherein the ytterbium content is 0.2% by mass or more and 2.5% by mass or less. The method for improving radiation resistance of a rare earth element-doped optical fiber according to any one of claims 8 to 11, wherein the core portion contains cerium, and the cerium content is 0.12% by mass or less. . The rare earth element according to any one of claims 8 to 12, wherein the core portion contains aluminum, and the aluminum content is less than 1.0 in terms of the mass ratio Al/P of the aluminum and the phosphorus. A method for improving the radiation resistance of a doped optical fiber. The method for improving radiation resistance of a rare earth element-doped optical fiber according to any one of claims 8 to 13, wherein the core portion contains boron and the boron content is 0.6% by mass or less. .

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