KR100928242B1 - All-optical pulsed fiber laser module - Google Patents

Abstract

The present invention relates to an all-optical pulsed optical fiber laser module for obtaining a high quality laser pulse in a simple manner, having an optical fiber for laser oscillation between a first FBG and a second FBG, wherein the reflectance of the first FBG is the second; A resonant cavity greater than the reflectance of the FBG; A first pumping light source for supplying a first pumping light for exciting the laser oscillation optical fiber to obtain an oscillation laser; And an all-optical variable type optical attenuator for generating a pulsed laser by inducing periodic Q-switching of the oscillating laser or the first pumping light.

Fiber optic, laser, pulse

Description

An all-fiber pulsed fiber laser module

 The present invention relates to a fiber laser, and more particularly to an all-optical pulsed fiber laser module for obtaining a high quality laser pulse in a convenient manner.

In recent years, due to better beam quality, quantum efficiency and system miniaturization, fiber lasers are more attractive than conventional solid state lasers in many respects. High power or pulsed fiber lasers are often an excellent alternative for a variety of practical applications.

Q-switching is a well known technique for generating pulsed laser output and many methods have been proposed. In order to achieve Q-switching in fiber lasers, acoustic optical modulators and various saturable absorbers have been widely used.

However, when using the AOM, the laser power is not large due to loss of connection to the modulator because it is not all-optical. Therefore, the final laser product is not economical because the laser power at the modulator input must be increased. In addition, since the modulator is used, the pulse period is limited to tens of ns.

On the other hand, bulk Q-switching devices must use fiber collimating elements to couple into the fiber laser cavity, which increases system complexity and decreases laser output stability.

Therefore, there is an urgent need to construct an all-optical pulsed fiber laser to obtain high quality laser pulses in a simple manner.

In order to solve the problems of the prior art as described above, it is an object of the present invention to provide an all-optical pulsed fiber laser module for obtaining a high quality laser pulse in a simple manner.

A configuration according to the present invention for achieving the above object is an all-optical pulsed optical fiber laser module, having a laser oscillation optical fiber between the first FBG and the second FBG, the reflectance of the first FBG is the reflectance of the second FBG Larger resonant cavities; A first pumping light source for supplying a first pumping light to excite the laser oscillation optical fiber to obtain an oscillation laser; And an all-optical variable light attenuator for inducing periodic Q-switching of the oscillating laser or the first pumped light to generate a pulsed laser.

Preferably, the all-optical variable type optical attenuator includes: a nonlinear optical fiber in which a long period grating pair is formed in a predetermined pattern; And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements.

Preferably, the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe.

Preferably, the metal fine particles are selected from the group consisting of Au, Ag, Cu.

Preferably, the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm.

Preferably, the all-optical variable type optical attenuator includes: a pair of optical fibers in which a long period grating is formed in each of a predetermined pattern to form a long period grating pair as a whole; A nonlinear optical fiber fused between one end of each of the pair of optical fibers; And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements.

Preferably, the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe.

Preferably, the metal fine particles are selected from the group consisting of Au, Ag, Cu and Si.

Preferably, the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm.

Preferably, the all-optical variable type optical attenuator includes: an optical fiber having a third FBG formed along a predetermined pattern; A nonlinear optical fiber fused to one end of the optical fiber; And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements.

Preferably, the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe.

Preferably, the metal fine particles are selected from the group consisting of Au, Ag, Cu and Si.

Preferably, the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm.

Preferably, the all-optical variable type optical attenuator includes: a nonlinear optical fiber having a third FBG formed in a predetermined pattern; And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements.

Preferably, the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe.

Preferably, the metal fine particles are selected from the group consisting of Au, Ag, Cu and Si.

Preferably, the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm.

According to the present invention, it is possible to obtain an all-optical pulsed fiber laser module that provides a high quality laser pulse in a simple manner.

According to the present invention, variable by varying the dopant in the core of the non-linear optical fibers constituting the second optical attenuator 10 ^-3 s, microseconds, 10 ^-6 s pre, 10 ^-9 s nanosecond, picosecond 10 ^-12 s, femtosecond It is possible to obtain an all-optical pulsed fiber laser module having various switching times such as 10 ^-15 s.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 schematically shows the overall configuration of an all-optical pulsed fiber laser module according to the present invention.

The all-optical pulsed optical fiber laser module of the preferred embodiment according to the present invention comprises a laser oscillation optical fiber 13 between the first FBG 11 and the second FBG 12 (the reflectance of the first FBG> the reflectance of the second FBG). And a first pumping light source 15 for supplying a first pumping light to excite the laser oscillating optical fiber 13 to obtain an oscillation laser, and the periodic Q-switching of the oscillating laser or the first pumping light. It consists of an all-optical variable type optical attenuator 14 which induces a pulsed laser.

The laser oscillation optical fiber 13 means an optical fiber having an active medium in the core layer. As the active medium, any one or more selected from the group consisting of rare earth elements and metal fine particles or semiconductor fine particles can be used.

Examples of the rare earth element and the metal fine particles are Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Sc, Ti, V, Cr, Mn, Fe , Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os, Ir, Pt, Au, Tl , Pb, Bi, and mixtures thereof. Examples of the semiconductor fine particles include any one or more selected from the group consisting of PbTe, PbSe, PbS, SnTe, CuCl, CdS, CdSe, Si, and the like.

Then, the first FBG 11 is arranged at the input end side of the laser oscillation optical fiber 13 and the second FBG 12 is arranged at the output end side, and the first FBG 11 and the second FBG 12 are arranged. Serves as a mirror to form a resonant cavity. The first FBG 11 and the second FBG 12 having the same center wavelength as the oscillation wavelength of the laser oscillation optical fiber 13 are formed to form a laser beam having the oscillation wavelength of the first FBG 11 and the second FBG. The laser oscillation optical fiber 13 in 12 continuously moves repeatedly to increase the laser power. At this time, the reflectance of the first FBG 11 at the laser oscillation wavelength is set higher than the reflectance of the second FBG 12 so that the laser can be emitted through the output terminal. The reflectance of the first FBG 11 and the second FBG 12 serving as the resonator of the fiber laser can be selected in the range of 1 to 45 dB according to the final laser oscillation performance, and the first FBG 11 and the second FBG ( 12) length can be selected in the range of 5 to 30mm depending on the laser beam characteristics.

Then, a first pumping light source 15 for energizing the laser oscillation optical fiber 13 and supplying energy into the resonance cavity for obtaining an oscillation laser is connected, and several LD pumps are connected to an optical fiber combiner (if necessary). Fiber Combiner) may be connected to the first FBG 11 constituting the input end of the resonant cavity. The wavelength of the first pumping light source 15 may be configured as 808, 915, 980, or the like depending on the absorption wavelength of the laser oscillation optical fiber 13.

Optionally, fiber isolators 17 and 18 may be included in the resonant cavity, where the fiber isolators 17 and 18 run the pumping source and the fiber laser of a particular wavelength in only one direction to block the reflected laser. It serves to prevent the loss of the pumping source.

Then, between the laser oscillation optical fiber 13 and the second FBG 12, in order to generate a pulsed laser by inducing periodic Q-switching through the oscillation laser or the variable light attenuation of the first pumping light, An all-optical variable type optical attenuator 14 serving as a pulse generator is fusion-spliced or fusion-spliced between the fiber isolator 17 and the first FBG 11, or the power meter of the second FBG 12 and the output stage ( It is also possible to form a fusion spliced connection between 19).

As shown in Fig. 2, the first configuration of the all-optical variable type optical attenuator includes a pair of optical fibers 21, 22, in which a long period grating is formed in each of a predetermined pattern to form a long period grating pair 21 ', 22' as a whole. ); A nonlinear optical fiber 23 fused between ends of each of the pair of optical fibers 21 and 22; And a second pumping light source 24 for supplying a second pumping light to the nonlinear optical fiber 23, wherein the core layer of the nonlinear optical fiber 23 includes nanometer-sized semiconductor particles and nanometer-sized metal particles. Or a rare earth element. The second pumping light source 24 and the OSA 25 may be connected to one end of the optical fiber 21, 22 by the WDMs 26, 27, respectively. In addition, a fiber isolator 28 may be arranged between the second pumping light source 24 and the WDM 26.

As shown in Fig. 3, the second configuration of the all-optical variable type optical attenuator includes a nonlinear optical fiber 33 in which long period grating pairs 31 'and 32' are formed in a predetermined pattern; And a second pumping light source 34 for supplying a second pumping light to the nonlinear optical fiber 33, wherein the core layer of the nonlinear optical fiber 33 includes nanometer-sized semiconductor particles and nanometer-sized metal particles. Or a rare earth element. The second pumping light source 34 and the OSA 35 may be connected to both ends of the nonlinear optical fiber 33 by the WDMs 36 and 37, respectively. In addition, a fiber isolator 38 may be arranged between the second pumping light source 34 and the WDM 36.

The reason why the above-described first and second configurations are taken is that when the second pump light is incident on the second pumping light sources 24 and 34, the nonlinear optical fibers 23 and 33 are different from the second pump light. As a result, the refractive index is changed (ie, the length of the optical signal path is long), so that the optical interference pattern generated by the long period grating is shifted toward the longer wavelength. Therefore, when the intensity of the second pump light is changed, the change of the refractive index is also changed, so that the light transmittance at a specific wavelength is changed. The second pumping light sources 24 and 34 may be configured to 808, 915, 980, or the like, depending on the absorption wavelength of the nonlinear optical fibers 23 and 33 constituting the all-optical variable optical attenuator.

In the long period grating, a part of the optical signal passing through it exits the cladding part, and the light transmittance is rapidly reduced at a certain wavelength. In the present invention, when one pair is added and the pair is added, the cladding mode proceeds through one long period grating (core). The light transmittance changes in the band erasing mode as it is coupled out of the mode, and then the cladding mode is recoupled to the core mode through another long-period lattice, which is divided into several areas in a narrow area due to the interference of the two modes. Loss of light represents an interference pattern. Since the band erasing occurs in a narrow wavelength region, even if the light interference pattern is shifted slightly, the light transmittance is large, so that the desired light attenuation can be easily obtained even if the intensity of the second pump light is reduced. That is, according to the configurations 1 and 2, the width of the light attenuation can be widened even at a low second pump light intensity.

As shown in Fig. 4, the third configuration of the all-optical variable type optical attenuator includes: an optical fiber 41 in which a short period grating 41 'is formed along a predetermined pattern; A nonlinear optical fiber 42 fused to one end of the optical fiber 41; And a second pumping light source 43 for supplying a second pumping light to the nonlinear optical fiber 42, wherein the core layer of the nonlinear optical fiber 42 includes nanometer-sized semiconductor particles and nanometer-sized metal particles. Or a rare earth element. The second pumping light source 43 and the OSA 44 may be connected to one end of the optical fiber 41 and the nonlinear optical fiber 42 by WDMs 45 and 46, respectively. In addition, a fiber isolator 48 may be arranged between the second pumping light source 43 and the WDM 45.

As shown in Fig. 5, the fourth configuration of the all-optical variable type optical attenuator includes: a nonlinear optical fiber 52 in which the short period grating 51 'is formed in a predetermined pattern; And a second pumping light source 53 for supplying a second pumping light to the nonlinear optical fiber 52, wherein the core layer of the nonlinear optical fiber 52 includes nanometer-sized semiconductor particles and nanometer-sized metal particles. Or a rare earth element. The second pumping light source 53 and the OSA 54 may be connected to both ends of the nonlinear optical fiber 52 by the WDMs 55 and 56, respectively. In addition, a fiber isolator 58 may be arranged between the second pumping light source 53 and the WDM 55.

In the third and fourth configurations of the all-optical variable type optical attenuator, a short period grating is used instead of a long period grating pair, and the short period grating is called fiber bragg grating (FBG). This name is more general. The third FBGs 41 'and 51' are grids having a period of 0.3 to 0.5 um which is much smaller than the long period lattice having a lattice period of 0.3 to 0.5 mm (about 1/1000). 41 ', 51') is a change in light transmittance caused by the reflection of the optical signal passing through the core (not exiting the cladding). In this case, you don't need to use a pair, only one. The advantage of this is that the reflectance can be up to 99.9% or more, ie the light intensity can be greatly reduced by reflection at a particular wavelength. Therefore, the change of light transmittance can be easily made to about 45 dB. In addition, when the second pump light intensity of the second pump light sources 43 and 53 is changed, the light transmission spectrum (changed by reflection) can be shifted, and the change in the light transmittance causes the VOA function. The second pumping light source may be configured as 808, 915, 980, or the like according to the absorption wavelength of the nonlinear optical fiber constituting the all-optical variable type optical attenuator.

As described above, the third FBGs 41 'and 51' in the third and fourth configurations are advantageous over the long period gratings in the first and second configurations of the all-optical variable type optical attenuator. The long period grating has a disadvantage of increasing the grid formation time to vary up to about 45dB. In the case of the third FBGs 41 'and 51', it is easy to increase the light attenuation range up to 45dB, and the line width of the light transmission spectrum can be easily adjusted.

The pulse period of the all-optical pulsed optical fiber laser is determined according to the switching speed of the variable optical attenuator, and the switching speed of the variable optical attenuator originates from the characteristics of the nonlinear optical fibers 23, 33, 42, and 52 constituting the variable optical attenuator. do. Therefore, the frequency of the pulsed laser is determined by the switching time at a specific wavelength of the all-optical variable type optical attenuator, so that the characteristics of the nonlinear optical fibers 23, 33, 42, 52 constituting the variable optical attenuator, for example, the nonlinear optical fiber ( 23, 33, 42, and 10 ^-3 s jeongwangsik seconds, microseconds, 10 ^-6 s pre, 10 ^-9 s nanosecond, picosecond 10 ^-12 s, femtosecond 10 ^-15 s by varying the dopant in the core 52) of the The all-optical pulsed fiber laser module having various pulse types can be configured.

As described above, the switching time of the all-optical variable optical attenuator is determined by an additive material, i.e., a dopant, present in the core region of the nonlinear optical fibers 23, 33, 42 and 52 constituting the all-optical variable optical attenuator. If an all-optical variable optical attenuator using a special optical fiber containing a metal additive (Au, Ag, Cu and Si, etc.) or a semiconductor additive (PbTe, PbS, PbSe, SnTe, CuCl, CdS, etc.) has a switching time of about 10 ^-11 s And all-optical variable light attenuators using special optical fibers containing rare earth additives (Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm, etc.) with a switching time of about 10 ^-3 s A pulsed all-optical optical fiber laser is obtained.

And finally, the power meter 19 measures the power of the laser coming out through the all-optical pulsed fiber laser. This can be measured by placing an oscilloscope and an optical spectrum analyzer, or OSA, in addition to the power meter 19 at the final output stage to analyze the laser characteristics.

Embodiments 1, 2, 3 and 4 of the present invention to be described later are distinguished according to the type of the all-optical variable type optical attenuator used, except for the first FBG, the second FBG, and the laser oscillation optical fiber (Yb ³⁺ / Al ^3+). -doped optical fiber), and the remaining components, such as the first pumping light source and the second pumping light source, are identical.

Example 1

Referring to FIG. 2, the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module according to Embodiment 1 is fused between a pair of long period gratings (LPG), Yb-doped fiber, which is a kind of nonlinear optical fiber. Take the connected configuration. Where L = 30,5 cm L ₁ = 25.5 cm and L ₂ = 2.5 cm and d = 0.5-3 cm.

Example 2

Referring to FIG. 3, the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module according to Embodiment 2 is configured to directly form a long period grating pair by exposing ultraviolet rays to a core of a nonlinear optical fiber including Ge. Where L, L ₁ , L ₂ and d are the same as in Example 1.

Example 3

Referring to FIG. 4, the all-optical variable type optical attenuator used in the all-optical pulsed optical fiber laser module according to the third embodiment of the optical fiber in which the Yb-doped fiber, which is a kind of nonlinear optical fiber, is engraved with a short period lattice (FBG) The configuration is fused and connected to one end. Here, the L = 28cm, L ₁ = 25.5cm and L ₂ = 2.5cm and, d = 0.5 ~ 3cm.

Example 4

Referring to FIG. 5, the all-optical variable type optical attenuator used in the all-optical pulsed optical fiber laser module according to the fourth embodiment is configured to directly form a short-period grating pair by exposing ultraviolet rays to a core of a nonlinear optical fiber including Ge. . Here, L, L ₁ , L ₂ and d were the same as in Example 3.

The all-optical pulsed optical fiber laser modules using the all-optical variable optical attenuators of Embodiments 1 to 4 output 100% of the output power at the laser oscillation wavelength by the special optical fiber for laser oscillation through modulation of the second pump light source. And 0% output (OFF) periodically to generate Q-switching by the periodic change of the second pumping light to generate an all-optical pulsed fiber laser.

FIG. 6 shows the result of implementing CW laser as a pulse laser through modulation simultaneously with the first pump LD pumping for laser oscillation without using a variable optical attenuator. Pulse characteristics in FIG. 6 were measured through an oscilloscope using PD. As can be seen from the result of FIG. 6, the square wave shape is not shown through the modulation of the first pump LD.

However, FIG. 7 is a diagram illustrating pulse characteristics of the all-optical pulsed fiber laser module according to the present invention using a variable optical attenuator using PD. As can be seen from the measurement result, it showed a square wave shape of 1V and had a pulse width of about 800us.

1 schematically shows the overall configuration of an all-optical pulsed fiber laser module according to the present invention.

Fig. 2 shows a first configuration of the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module of the present invention.

3 shows a second configuration of the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module of the present invention.

Fig. 4 shows a third configuration of the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module of the present invention.

Fig. 5 shows a fourth configuration of the all-optical variable type optical attenuator used in the all-optical pulsed fiber laser module of the present invention.

FIG. 6 shows the result of implementing CW laser as a pulse laser through modulation simultaneously with the first pump LD pumping for laser oscillation without using a variable optical attenuator.

FIG. 7 is a diagram illustrating pulse characteristics of the all-optical pulsed fiber laser module according to the present invention using an all-optical variable type optical attenuator using PD.

Claims (17)

All-optical pulsed fiber laser module, A resonant cavity having a laser oscillation optical fiber between the first FBG and the second FBG, wherein the reflectance of the first FBG is greater than the reflectance of the second FBG; A first pumping light source for supplying a first pumping light for exciting the laser oscillation optical fiber to obtain an oscillation laser; And And an all-optical variable type optical attenuator for generating a pulsed laser by inducing periodic Q-switching of the oscillating laser or the first pumping light. According to claim 1, The all-optical variable type optical attenuator, A nonlinear optical fiber in which a long period grating pair is formed in a predetermined pattern; And And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements. All-optical pulsed fiber laser module. 3. The all-optical pulsed optical fiber laser module according to claim 2, wherein the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe. 3. The all-optical pulsed optical fiber laser module according to claim 2, wherein the metal fine particles are selected from the group consisting of Au, Ag, Cu, and Si. 3. The all-optical pulsed optical fiber laser module according to claim 2, wherein the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm. According to claim 1, The all-optical variable type optical attenuator, A pair of optical fibers in which a long period grating is formed in each of a predetermined pattern to form a long period grating pair as a whole; A nonlinear optical fiber fused between one end of each of the pair of optical fibers; And And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements. All-optical pulsed fiber laser module. 7. The all-optical pulsed optical fiber laser module according to claim 6, wherein the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS and CdSe. The all-optical pulsed optical fiber laser module according to claim 6, wherein the metal fine particles are selected from the group consisting of Au, Ag, Cu, and Si. 7. The all-optical pulsed optical fiber laser module according to claim 6, wherein the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm. According to claim 1, The all-optical variable type optical attenuator, An optical fiber having a third FBG formed along a predetermined pattern; A nonlinear optical fiber fused to one end of the optical fiber; and And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements. All-optical pulsed fiber laser module. The all-optical pulsed optical fiber laser module according to claim 10, wherein the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS, and CdSe. 11. The all-optical pulsed optical fiber laser module according to claim 10, wherein the metal fine particles are selected from the group consisting of Au, Ag, Cu, and Si. 11. The all-optical pulsed optical fiber laser module according to claim 10, wherein the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm. According to claim 1, The all-optical variable type optical attenuator, A nonlinear optical fiber having a third FBG formed in a predetermined pattern; And And a second pumping light source for supplying a second pumping light to the nonlinear optical fiber, wherein the core layer of the nonlinear optical fiber contains nanometer-sized semiconductor particles, nanometer-sized metal particles, or rare earth elements. All-optical pulsed fiber laser module. 15. The all-optical pulsed optical fiber laser module according to claim 14, wherein the semiconductor fine particles are selected from the group consisting of PbTe, PbS, PbSe, SnTe, CuCl, CdS, and CdSe. 15. The all-optical pulsed optical fiber laser module according to claim 14, wherein the metal fine particles are selected from the group consisting of Au, Ag, Cu, and Si. 15. The all-optical pulsed optical fiber laser module according to claim 14, wherein the rare earth element is selected from the group consisting of Er, Nd, Yb, Tb, Pr, Eu, Dy, Tm, Ho and Sm.

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