JP7496100B2 - Rare earth doped optical fiber - Google Patents

Description

The present invention relates to rare earth element-doped optical fibers.

Optical fibers generally have a core and a cladding that covers the core. In this type of optical fiber, the refractive index of the core is made larger than that of the cladding, and light is totally reflected at the boundary between the core and cladding, confining the light within the core and propagating it over long distances as an optical signal.

Optical fibers are also used as optical amplifiers. As optical fibers for optical amplifiers, rare-earth-doped optical fibers in which rare-earth elements are added to the core are known. Among optical fiber amplifiers using rare-earth-doped optical fibers, Er-doped optical fibers in which Er (erbium) is added are mainly used for amplifying the 1550 nm band, which is the main wavelength band in terrestrial optical fiber communication networks and space optical communication. In space optical communication, high-speed optical communication over long distances where relay amplification is basically not possible is required, so high output is required, and the pumping power may be insufficient in core-pumped optical fiber amplifiers using single-mode laser diodes. For this reason, clad-pumped optical fiber amplifiers using double-clad fibers to which high-output pumping multimode laser diodes in the 980 nm band can be applied are used. In this case, in Er-doped optical fibers in which only Er is added to the core, both the absorption band and the absorption amount in the core of the 980 nm band pumping light propagating through the clad portion are likely to be insufficient. In order to improve the absorption band and amount of absorption in the 980 nm band, optical fiber amplifiers using Er/Yb-doped optical fiber, in which Er and Yb (ytterbium) are co-doped, are being considered.

In rare-earth-doped optical fibers used as optical amplifiers, it is effective to increase the rare-earth element content in the core in order to improve the gain. However, if the rare-earth element (particularly Er) content is too high, clustering (gathering together) of the rare-earth elements occurs, causing concentration quenching and actually decreasing the gain. To prevent this clustering of the rare-earth elements, rare-earth-doped optical fibers are co-doped with P (phosphorus) and Al (aluminum) in the core to disperse the rare-earth elements.

Patent document 1 discloses that a rare-earth-doped optical fiber has a core doped with rare-earth elements and at least P, and a cladding provided around the core, and that the minimum transmission loss of the core is 3 dB/km to 100 dB/km in the wavelength band from 1000 nm to 1700 nm, the core NA is 0.05 to 0.25, and the concentration of the rare-earth element doped in the core is 2 wt% to 10 wt%. The examples of this patent document 1 disclose a rare-earth-doped optical fiber in which the core contains 3.3 wt% Yb, 0.28 wt% Er, 6.9 wt% P, 0.1 wt% Ge, and 0.35 wt% F.

JP 2014-143287 A

In rare earth element doped optical fiber doped with Er and Yb, the excitation band and absorption amount in the 980 nm band are significantly improved by the ground level absorption of Yb ions from the ² F _7/2 level to the ² F _5/2 level. Then, Yb ions that absorb the 980 nm band excitation light and transition to the ² F _5/2 level indirectly excite Er ions to the ⁴ I _11/2 level through ion-ion interaction, making it possible to use the energy absorption of Yb ions for the light amplification action of Er ions. However, the conversion efficiency from the pump light to the signal light may be reduced due to energy losses caused by the reverse transition from ^the _4I11 / ₂ level of the Er ion to the ^2F5 _{/ 2} level of the ^Yb ion, the excited state absorption (ESA) from the ^4I11 _/2 ^level of the Er ion to the ^4F7 /2 level, and the emission between the levels of the Yb ion (the transition from the 2F5/2 level to the _2F7/2 level).

The present invention was made in consideration of these circumstances, and aims to provide a rare-earth element-doped optical fiber containing Er and Yb that has low energy loss and high conversion efficiency from pump light to signal light.

In order to solve the above problems, the rare earth element-doped optical fiber of the present invention has a core portion and a cladding portion surrounding the core portion, the core portion containing Yb, Er, and P, the ratio P/(Yb+Er) of the content of P to the total content of Yb and Er is within the range of 15 to 25.1 in atomic concentration ratio, the ratio Yb/Er of Yb to Er is within the range of 2 to 50 in mass ratio, and the ratio P/Er of P to Er is within the range of 13 to 360 in mass ratio.

In the rare earth element-doped optical fiber of the present invention, the core portion may contain Ge, and the Ge content may be 0.5 mass% or more and 12.0 mass% or less.

In addition, in the rare earth element-doped optical fiber of the present invention, the Er content may be 0.05 mass% or more.

The present invention makes it possible to provide a rare-earth element-doped optical fiber containing Er and Yb that has low energy loss and high conversion efficiency from pump light to signal light.

1 is a cross-sectional view of a rare earth-doped optical fiber according to an embodiment of the present invention;

Below, an embodiment of the rare earth element doped optical fiber according to the present invention will be described with reference to the attached drawings. Note that the drawings used in the following description may show enlarged characteristic parts for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may differ from the actual ones.

FIG. 1 is a cross-sectional view of a rare earth doped optical fiber according to one embodiment of the present invention.
1, the rare-earth-element-doped optical fiber 1 has a core 2 and a cladding 3 that covers the periphery of the core 2. The cladding 3 has a double-cladding structure including a first cladding layer 3a formed around the core 2 and a second cladding layer 3b formed on the outside of the first cladding layer 3a. The outside of the second cladding layer 3b is covered with a coating layer 4.

The core portion 2 contains Yb (ytterbium), Er (erbium), and P (phosphorus).
The Er contained in the core portion 2 has the function of being excited by light (excitation light) having a wavelength of 980 nm or 1480 nm, and generating light (signal light) having a wavelength of 1550 nm.

Yb is excited by light with a wide wavelength range of 800 nm to 1100 nm, and has the effect of exciting Er through energy transfer from the excited Yb to Er. This action can increase the amount of light generated with a wavelength of 1550 nm. Yb also has the effect of suppressing the occurrence of Er clustering by coordinating around Er and increasing the distance between Er atoms. This action can suppress concentration quenching of Er even when the Er content is high, which can also increase the amount of light generated with a wavelength of 1550 nm.

P has the effect of suppressing the occurrence of Er clustering by coordinating around Er and lengthening the distance between Er. This effect can suppress the concentration quenching of Er even if the Er content is high, thereby increasing the amount of light generated at a wavelength of 1550 nm. P also has the effect of suppressing the reverse transition from the ⁴ I _11/2 level of Er ions to the ² F _5/2 level of Yb ions and the excited state absorption (ESA) from the ⁴ I _11/2 level of Er ions to the ⁴ F _7/2 level.

The contents of Yb, Er and P in the core 2 are set so that the ratio P/(Yb+Er) of the content of P to the total content of Yb and Er is within the range of 15 to 50 in atomic concentration ratio. By setting the atomic concentration ratio of P/(Yb+Er) within this range, the transition from the ^2F5 _/2 ^level of Yb ions to the _4I11/2 level of Er ions due to the ionic interaction between Yb ions and Er ions is likely to occur, so that the energy loss of Yb ions transitioned to the ^2F5 _/2 level is reduced. Therefore, the rare earth element doped optical fiber 1 of this embodiment has a high conversion efficiency from pumping light to signal light.

If the atomic concentration ratio of P/(Yb+Er) becomes too low (the P content becomes too low, or the total content of Yb and Er becomes too high), clustering of the rare earth elements may occur, causing the clouding of the core portion 2. In addition, if the P content becomes too low, reverse transition from the ⁴ I _11/2 level of Er ions to the ² F _5/2 level of Yb ions or excited state absorption from the ⁴ I _11/2 level of Er ions to the ⁴ F _7/2 level may easily occur, resulting in increased energy loss of Er ions. On the other hand, if the atomic concentration ratio of P/(Yb+Er) becomes too high (the content of P becomes too high, or the total content of Yb and Er becomes too low), the inter-ionic interaction between Yb ions and Er ions makes it difficult to excite Er ions to the ⁴ I _11/2 level, and the Yb ions excited to ² F _5/2 return to the ground level by emitting light between the Yb levels, so that the conversion efficiency from excitation light to signal light may decrease. The atomic concentration ratio of P/(Yb+Er) is preferably 18.0 or more, and more preferably 20.0 or more. The atomic concentration ratio of P/(Yb+Er) is preferably 35.0 or less, and more preferably 30.0 or less.

The Er content in the core 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, there is a risk of concentration quenching due to the occurrence of clustering, so it is necessary to co-do Yb and P within the ranges shown in the following sections.

The Yb content of the core portion 2, or the ratio Yb/Er of Yb to Er, is preferably in the range of 1 to 50 in mass ratio, more preferably in the range of 2 to 50 in mass ratio, even more preferably in the range of 4 to 50 in mass ratio, particularly preferably in the range of 4 to 40 in mass ratio, and particularly preferably in the range of 4 to 20 in mass ratio. By keeping the Yb content within this range, it is possible to more reliably increase the generation of light with a wavelength of 1550 nm.

The P content of the core portion 2, that is, the ratio P/Er of P to Er, is preferably in the range of 5 to 360 in mass ratio, more preferably in the range of 13 to 360, and particularly preferably in the range of 18 to 360. When the P content is within this range, the occurrence of Er clustering, the reverse transition from the ^4I11 _/2 level of Er ions to the ^2F5 _/2 level of Yb ions, and the excited state absorption from the ^4I11 _/2 level of Er ions to the ^4F7 _/2 level can be more reliably suppressed.

The core 2 of the rare earth element-doped optical fiber 1 may contain Ge (germanium). Ge has the effect of suppressing the generation of defect absorption caused by radiation irradiation due to the P contained in the core 2, thereby improving the radiation resistance of the rare earth element-doped optical fiber 1.

The Ge content of the core portion 2 is preferably in the range of 0.5% by mass to 12.0% by mass. The lower limit of the Ge content is more preferably 1.0% by mass or more, even 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, even more preferably 9.0% by mass or less, particularly preferably 8.6% by mass or less, even more preferably 6.0% by mass or less, and most preferably 5.0% by mass or less. If the Ge content is too low, it may be 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 saturate, and the NA (numerical aperture) may become too large.

The first cladding layer 3a of the cladding section 3 functions as a cladding for the core section 2, and is also called a pumping guide, functioning as a multimode waveguide for the excitation light. There are no particular limitations on the material of the first cladding layer 3a, so long as it has a lower refractive index than the core section 2. For example, silica glass (quartz glass) can be used as the material of the first cladding layer 3a.

The second cladding layer 3b functions as a cladding for the pumping guide of the first cladding layer 3a. There are no particular limitations on the material of the second cladding layer 3b, so long as it has a lower refractive index than the first cladding layer 3a. The second cladding layer 3b may be a resin cladding type that uses a silicone or acrylic low refractive index resin, or an air hole type in which air holes (voids) are arranged in the cladding.

The coating layer 4 is formed from a resin material. As the resin material, a curable resin material such as a thermosetting resin or an ultraviolet curable resin can be used.

The diameter of the core portion 2 of the rare earth element-doped optical fiber 1 is not particularly limited, but may be in the range of 3 μm to 110 μm. The thickness of the first cladding layer 3a is not particularly limited, but may be in the range of 25 μm to 350 μm. The thickness of the second cladding layer 3b is not particularly limited, but may be in the range of 5 μm to 100 μm.

Next, the method for manufacturing the rare earth element-doped optical fiber of this embodiment will be described taking as an example a case in which the second cladding layer is a resin cladding type.
The rare-earth-doped optical fiber of this embodiment can be manufactured by a method including, for example, a base material preparation step of preparing a base material (preform) for manufacturing the rare-earth-doped optical fiber, a drawing step of drawing the obtained base material to manufacture a single-clad type rare-earth-doped optical fiber having a core portion covered with a first cladding layer, a second cladding layer formation step of forming a second cladding layer on the obtained single-clad type rare-earth-doped optical fiber, and a coating layer formation step of coating the periphery of the second cladding layer with a protective resin.

(Base material production process)
The base material prepared in the base material preparation step has a core forming portion and a first cladding forming portion that covers the periphery of the core forming portion. The core forming portion is made of a silica composition containing Er, Yb, P, and optionally Ge, and the first cladding forming portion is made of silica or a silica composition having a lower refractive index than the core forming portion.

The base material can be obtained by preparing a silica composite having a core-forming portion and a portion of the first cladding portion, and then adding a shortage of the first cladding portion around the silica composite, so that the outer diameter of the core-forming portion and the first cladding portion can be set to a desired ratio.
The silica composite can be produced by the MCVD (Modified Chemical Vapor Deposition Method). Specifically, the quartz tube is heated by an oxyhydrogen burner while a raw material gas containing a silica source (e.g., SiCl ₄ ), an Er source (e.g., Er(DPM) ₃ ), a Yb source (e.g., Yb(DPM) ₃ ), a P source (e.g., POCl ₃ ), and, if necessary, a Ge source (e.g., GeCl ₄ ), and oxygen is supplied to the quartz tube. At this time, the amount of Er, Yb, P, and Ge contained in the core forming portion can be adjusted by adjusting the type and composition of the raw material gas. In this way, soot consisting of glass composition particles containing Er, Yb, P, and Ge is deposited on the quartz tube. Next, the quartz tube is heated by an oxyhydrogen burner to make 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 collapse the hollow portion. This results in a silica composite having a core forming portion made of a silica composition containing Er, Yb, P, and Ge, and a first cladding forming portion covering the periphery of the core forming portion. In addition to the above-mentioned MCVD method, various methods that are generally used as a method for producing a base material for optical fiber manufacturing, such as the OVD method (Outside Vapor Deposition Method) and the VAD method (Vapor phase Axial Deposition Method), can be used as a method for producing the silica composite.

The jacket method (also called the rod-in-tube method) can be used as a method for adding the missing first cladding portion around the silica composite. Specifically, the base material obtained by the MCVD method is inserted into a quartz tube having the same or similar composition as the quartz tube used in the MCVD method, and the quartz tube is heated by an oxyhydrogen burner to integrate the base material and the quartz tube. In this way, a base material for manufacturing rare earth element-doped optical fiber can be obtained with the desired ratio of outer diameters of the core forming portion and the first cladding forming portion.

(Wire drawing process)
In the drawing process, the preform for manufacturing the rare earth element-doped optical fiber produced in the preform production process is heated and melted, and the melted preform is stretched into a filament and cooled. This makes it possible to obtain a single-clad type rare earth element-doped optical fiber. As a method for stretching the preform into a filament, various methods commonly used in the manufacture of optical fibers can be used.

(Second cladding layer forming step)
In the second cladding layer forming step, the single cladding type rare earth element doped optical fiber obtained in the above drawing step is covered with a low refractive index resin having a refractive index lower than that of the first cladding layer to form the second cladding layer. Specifically, the single cladding type rare earth element doped optical fiber is covered with a curable low refractive index resin, and the curable low refractive index resin is cured to form the second cladding layer. In this way, a double cladding type rare earth element doped optical fiber can be obtained.

(Covering layer forming process)
The rare-earth-doped optical fiber of the present embodiment is obtained by coating the second cladding layer of the obtained double-clad rare-earth-doped optical fiber with a protective resin to form a coating layer. As a method for coating with a protective resin, for example, a method of applying a curable resin and curing it can be used.

The drawing process, second cladding layer formation process, and coating layer formation process are generally performed continuously as a series of processes. This is because the bare optical fiber glass wire has low strength against bending and pulling, and is prone to breakage when wound around a bobbin in its bare state. By forming the second cladding layer and coating layer continuously as a series of processes, the strength can be increased.

Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and substitutions can be made within the scope of the present invention. For example, in the above embodiment, the rare earth element doped optical fiber was described as a double clad optical fiber (DCF), but the rare earth element doped optical fiber of the present invention is not limited to this. The rare earth element doped optical fiber of the present invention may be a single clad optical fiber, and can be in various forms used as optical fibers, such as multimode optical fiber (MMF), single mode optical fiber (SMF), and polarization maintaining optical fiber.

The effects of the present invention are explained below using examples.

[Example 1]
(Preparation of preform for manufacturing rare earth element doped optical fiber)
A glass composite (diameter: 13 mm) was obtained by the MCVD method, in which the core forming portion was made of a silica composition having a Yb content of 0.84 mass%, an Er content of 0.17 mass%, a P content of 7.31 mass%, and a Ge content of 6.71 mass%, and a part of the first cladding forming portion was made of silica glass. Next, a first cladding was added around the obtained glass composite by a jacket method using a quartz tube having the same composition as the first cladding forming portion, to produce a base material (diameter: 30 mm) for manufacturing a rare earth element-doped optical fiber.

(Preparation of rare earth element doped optical fiber)
The above-mentioned base material was drawn to obtain a single-clad rare-earth-doped optical fiber having a core diameter of 8.1 μm and a diameter of the first cladding layer of 125 μm. Next, the periphery of the obtained single-clad rare-earth-doped optical fiber was coated with an ultraviolet-curable acrylic low-refractive index resin, and the acrylic low-refractive index resin was cured to obtain a double-clad rare-earth-doped optical fiber having a second cladding layer of acrylic low-refractive index resin with a thickness of 35 μm. Then, the periphery of the obtained double-clad rare-earth-doped optical fiber was coated with a protective resin to form a coating layer, thereby producing a double-clad (2LP-DCF) rare-earth-doped optical fiber propagating in 2LP mode. The drawing process, the second cladding layer forming process, and the coating layer forming process were performed continuously as a series of processes. The obtained 2LP-DCF rare-earth-doped optical fiber had a diameter of 250 μm including the coating layer.

[Examples 2 to 13]
A preform for manufacturing a rare earth element-doped optical fiber was prepared in the same manner as in Example 1, except that the composition of the core forming portion was set to the composition shown in the following Table 1. Then, the obtained preform was drawn to have a core diameter of the value shown in the following Table 1, in the same manner as in Example 1, to prepare a 2LP-DCF type rare earth element-doped optical fiber.

[evaluation]
The maximum light-to-light conversion efficiency, spectral intensity, and gamma ray resistance characteristics of the 2LP-DCF type rare earth element doped optical fibers obtained in Examples 1 to 13 were measured by the following methods. The results are shown in Table 2 below.

(Maximum light-to-light conversion efficiency)
Using light with a wavelength of 975 nm as pump light, the ratio of the output power of the signal light (wavelength 1550 nm) to the input power of the pump light was calculated as the maximum light-to-light conversion efficiency. Note that in Examples 1 to 5, forward pumping evaluation and backward pumping evaluation were performed, in Example 6, only forward pumping evaluation was performed, and in Examples 7 to 13, only backward pumping evaluation was performed. The forward pumping evaluation means an evaluation when the pump light is incident in the same direction as the transmission direction of the signal light, and the backward pumping evaluation means an evaluation when the pump light is incident in the opposite direction to the transmission direction of the signal light.

(Spectral Intensity)
When a signal light of 1550 nm wavelength was amplified using light of 975 nm wavelength as pump light, the intensity of light of 548 nm wavelength emitted from the side of a rare earth element-doped optical fiber was measured using a spectrophotometer. The light of 548 nm wavelength is emitted when ^{the 4} S _3/2 level of Er ions transitions to the ⁴ I _15/2 level. When the excited state absorption of Er ions from the ⁴ I _11/2 level to the ⁴ F _7/2 level occurs, the transition occurs in the order of the ⁴ F _7/2 level to the ² H _11/2 level and ^{the 4} S _3/2 level. Therefore, a low spectral intensity means that the excited state absorption of Er ions from the ⁴ I _11/2 level to the ⁴ F _7/2 level is low. However, since excited state absorption is a phenomenon accompanying energy transfer from Yb ions to Er ions, if the spectral intensity at 548 nm is too small, it also indicates that energy transfer from Yb ions to Er ions is not efficient. Note that in Examples 1 to 5, forward pumping evaluation and backward pumping evaluation were performed, in Example 6, only forward pumping evaluation was performed, and in Examples 7 to 13, only backward pumping evaluation was performed. The forward pumping evaluation means an evaluation when the pumping light is incident in the same direction as the transmission direction of the signal light, and the backward pumping evaluation means an evaluation when the pumping light is incident in the opposite direction to the transmission direction of the signal light.

(γ-ray resistance)
The optical loss spectrum, with the horizontal axis representing the wavelength of light and the vertical axis representing the optical absorption loss, was measured by the following method.
<Method for measuring optical loss spectrum>
The loss spectrum was measured by the cutback method. White light was incident on the fiber under test with a length of L ₀ [m], and the transmitted light spectrum was measured using an optical spectrum analyzer. The optical power at this time was set to P ₀ [dBm]. Next, the length of the fiber under test was cut back to L ₁ [m], and the transmitted light spectrum of the fiber under test after the cutback was measured. The optical power at this time was set to P ₁ [dBm]. L ₀ was set to about 10 m, and L ₁ was set to about 2 m. The absorption loss [dB/m] was calculated by (P ₁ - _{P 0} )/(L ₀ - _{L 1} ).

Next, the rare earth doped optical fiber was irradiated with 1000 Gy of gamma rays (corresponding to 10 years of stationary placement in geostationary orbit) using a cobalt 60 gamma ray irradiation device. The optical loss spectrum of the rare earth doped optical fiber after gamma ray irradiation was measured again. The optical loss spectrum of the rare earth doped optical fiber after gamma ray irradiation was subtracted from the optical loss spectrum before gamma ray irradiation to calculate the increase in absorption loss. The wavelength (nm) of the optical loss spectrum was converted to photon energy (eV), and a graph was created in which the horizontal axis was the photon energy (eV) and the vertical axis was plotted with the increase in absorption loss. Gaussian fitting was performed using the plotted data to obtain an approximate curve. Gaussian fitting was performed using parameters described in the following literature. Table 2 shows the increase in absorption loss of light with a wavelength of 1570 nm.
"D.L. Griscom, E.J. Friebele, K.J. Long and J.W. 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."

It is seen that the rare earth element-doped optical fibers of Examples 1 to 13, in which the atomic concentration ratio of P/(Yb+Er) is within the range of the present invention, exhibit a high maximum light-light conversion efficiency. In particular, it is seen that the rare earth element-doped optical fibers of Examples 2 to 11 and 13, in which the atomic concentration ratio of P/(Yb+Er) is 40.0 or less, Yb/Er is 2 or more, and P/Er is 13 or more, exhibit a high maximum light-light conversion efficiency. It is also seen that the rare earth element-doped optical fibers of Examples 1 to 4 and 6 to 13, in which the atomic concentration ratio of P/(Yb+Er) is 18.0 or more, exhibit low spectral intensity, and the excited state absorption from the ⁴ I _11/2 level to the ⁴ F _7/2 level of Er ions is reduced.

In addition, the rare earth element-doped optical fibers of Examples 1 to 5 and 7 to 13, in which the core portion contains Ge, show a low increase in light absorption loss after exposure to gamma rays, and show improved gamma ray resistance.

The rare earth element-doped optical fiber of the present invention can be suitably used as an optical amplifier component for optical communication, or as a component for various sensors that use optical fiber, such as an optical fiber gyroscope. In particular, a rare earth element-doped optical fiber containing Ge in the core can be suitably used in applications that require radiation resistance, such as optical communication devices and optical fiber gyros mounted on spacecraft (e.g., rockets, artificial satellites, spacecraft), optical amplifiers in nuclear reactors, and optical communication devices mounted on aircraft.

1...Rare earth element doped optical fiber, 2...Core portion, 3...Clad portion, 3a...First clad layer, 3b...Second clad layer, 4...Coating layer

Claims (3)

a rare-earth-element-doped optical fiber having a core portion and a cladding portion surrounding the core portion, the core portion containing Yb, Er, and P, a ratio P/(Yb+Er) of a content of P to a total content of Yb and Er being within a range of 15 to 25.1 in terms of atomic concentration ratio, a ratio Yb/Er of Yb to Er being within a range of 2 to 50 in terms of mass ratio, and a ratio P/Er of P to Er being within a range of 13 to 360 in terms of mass ratio. The rare earth element-doped optical fiber according to claim 1, wherein the core portion contains Ge and the Ge content is 0.5 mass% or more and 12.0 mass% or less. The rare earth element-doped optical fiber according to claim 1 or 2, in which the Er content is 0.05 mass% or more.

Images