JP2014139682A - Optical isolator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical isolator comprising fluoride single crystal enabling to enlarge size and having a large Verdet constant in wide wavelength regions and high transmittance.SOLUTION: An optical isolator comprises: a Faraday rotator consisting of fluoride single crystal. The fluoride single crystal is expressed by compositional formula: LMF, where L comprises Tb; M represents a group II element including at least one kind selected from a group consisting of Ca and Sr; x is larger than 0 and less than 1; and y is between -0.2 to 0.2).

Description

  The present invention relates to an optical isolator.

  In recent years, optical isolators have been used not only for optical communication but also for laser processing machines.

As a Faraday rotator material used for an optical isolator for a laser beam machine, a terbium gallium garnet single crystal (TGG: Tb ₃ Ga ₅ O ₁₂ ) has been developed and put into practical use (Non-patent Document 2).

Further, a terbium / aluminum / garnet single crystal (TAG: Tb ₃ Al ₅ O ₁₂ ) has been reported as one that exhibits a Faraday rotation angle larger than TGG (Patent Document 1 and Non-Patent Document 1).

Further, terbium / scandium / aluminum / garnet type single crystal (TSAG: Tb ₃ Sc ₂ Al ₃ O ₁₂ ) or terbium / scandium / lutetium partially substituted with lutetium is used to show a Faraday rotation angle larger than TGG. aluminum garnet-type single crystal _{(TSLAG: Tb 3 Sc 2-} x Lu x Al 3 O 12) , etc. are also known (Patent Document 2).

JP 2001-226196 A JP 2002-293893 A

Cryst. Res. Technol. 34 (1999) 5-6, p. 615-619 M.M. Y. A. Raja, D.D. Allien, W.H. Disk, Appl. Phys. Lett. 67 (1995) p. 2123

  However, although TAG has a larger Verde constant than TGG, it has an anharmonic melting composition (incongruent composition), so it is difficult to grow a large crystal by the Czochralski method or the like. Although it has been reported that TAG is grown by the micro-PD method, only a very thin rod-like crystal having a diameter of 2 mm is obtained at most. Thus, at present, it is impossible to industrially use TAG as a single crystal for an optical isolator.

  Furthermore, although TSAG and TSLAG have a larger Verde constant than TGG and can grow a large single crystal compared to TAG, cracks are more likely to occur than TGG, making it difficult to increase the size of the single crystal.

  Therefore, at present, only TGG is industrially used as a single crystal for an optical isolator.

  However, although the size of a single crystal can be increased, the yield of TGG is poor due to the rapid evaporation of gallium oxide as a raw material component during the growth of the single crystal. This is a factor that makes it difficult to reduce costs. In addition, TGG has a small Verde constant in a short wavelength region (for example, a wavelength region of 800 nm or less), so that a large crystal is required to rotate polarized light in the short wavelength region. Further, in TGG, the transmittance is drastically lowered particularly in a short wavelength region (for example, a wavelength region of 800 nm or less). For this reason, TGG is not appropriate for use as a Faraday rotator used in an optical isolator of a laser processing machine provided with a laser light source having an oscillation wavelength in a short wavelength region. Thus, TGG still has room for improvement in terms of crystal enlargement, Verde constant, and transmittance.

  The present invention has been made in view of the above circumstances, and provides an optical isolator including a fluoride single crystal that can be increased in size, has a large Verde constant over a wide wavelength range, and has a high transmittance. For the purpose.

  As a result of intensive studies focusing on fluoride single crystals used as optical parts such as optical lenses instead of garnet-type single crystals such as TGG, the present inventors have conducted group II elements including Ca and Sr. The present inventors have found that the above problems can be solved by introducing a rare earth element containing Tb that affects the Faraday rotation angle into a compound of benzene and fluorine, and the present invention has been completed.

That is, the present invention is an optical isolator having a Faraday rotator composed of a fluoride single crystal, wherein the fluoride single crystal has the following composition formula:
L _1-x M _x F _{3-x + y}
(In the above formula, L contains Tb. M represents a group II element containing at least one selected from the group consisting of Ca and Sr. x is greater than 0 and less than 1. y is -0.2. ~ 0.2.)
This is an optical isolator characterized by the following.

The Faraday rotator of this optical isolator is composed of a fluoride single crystal, and this fluoride single crystal has a large Verde constant in a wide wavelength range and high in a wide wavelength range as compared with TGG. It is possible to have transmittance. For this reason, the optical isolator of the present invention can be used as an optical isolator of a laser beam machine equipped with laser light sources having various wavelengths. Moreover, since the said fluoride single crystal can be enlarged, mass production of a Faraday rotator is possible. As a result, the price of the Faraday rotator can be reduced, and consequently the price of the optical isolator including the Faraday rotator can be reduced.

In the above composition formula, it is preferable that x satisfies the following formula.
0.1 <x ≦ 0.3

  In this case, the Faraday rotation angle can be remarkably increased as compared with the case where x exceeds 0.3, and the fluoride single crystal has a congruent composition, or compared with the case where x is 0.1 or less. Since it is closer to the composition, a larger fluoride single crystal can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the optical isolator provided with the fluoride single crystal which can be enlarged and has a large Verde constant in a wide wavelength range and has a high transmittance is provided.

It is the schematic which shows one Embodiment of the optical isolator which concerns on this invention. It is a figure which shows the process of growing the fluoride single crystal used for this invention using a crystal growth apparatus. 2 is a graph showing a transmission spectrum of the fluoride single crystal of Example 1.

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

  FIG. 1 is a schematic view showing an embodiment of an optical isolator according to the present invention. As shown in FIG. 1, the optical isolator 10 includes a polarizer 1, an analyzer 2, and a Faraday rotator 3 disposed between the polarizer 1 and the analyzer 2. Here, the polarizer 1 and the analyzer 2 are arranged so as to form an angle of 45 °, for example, so that their transmission axes are not parallel to each other.

  For example, the magnetic flux density B is applied to the Faraday rotator 3 along the direction from the polarizer 1 to the analyzer 2, that is, the incident direction of the light L. Is applied to rotate the plane of polarization of the light L that has passed through the polarizer 1 and pass through the transmission axis of the analyzer 2.

  Here, the Faraday rotator 3 will be described in detail.

The Faraday rotator 3 has the following composition formula:
L _1-x M _x F _{3-x + y}
(In the above formula, L includes Tb. M includes at least one selected from the group consisting of Ca and Sr. x is greater than 0 and less than 1. y is −0.2 to 0.2. is there.)
It is comprised with the fluoride single crystal represented by these. This fluoride single crystal has a hexagonal crystal structure and is a uniaxial crystal. Therefore, when light is incident in the [001] direction of the fluoride single crystal, the light can go straight without rotating its polarization plane.

  L represents a rare earth element containing Tb. Therefore, L may be a rare earth element containing only Tb, or a rare earth element mainly containing Tb and containing a small amount of trivalent and stable elements such as other rare earth elements.

  M represents a group II element containing at least one selected from the group consisting of Ca and Sr. Therefore, M may be a group II element including at least one selected from the group consisting of Ca and Sr, mainly including at least one selected from the group consisting of Ca and Sr, and 2 such as Mg and Ba. It may be a group II element containing a small amount of a stable and stable group II element.

  Here, x may be larger than 0 and smaller than 1. When x is 1, although the transmittance is high, the Verde constant is remarkably lowered. On the other hand, when x is 0, the fluoride single crystal does not become a uniaxial crystal but becomes a biaxial crystal, and the polarized light rotates without applying a magnetic flux density when light is incident. Therefore, it is not appropriate to use as the Faraday rotator 3. On the other hand, if y is out of the above range, defects in the fluoride single crystal increase, and the quality of the crystal decreases.

In the above composition formula, x preferably satisfies the following formula.
0.1 <x ≦ 0.3

  In this case, it becomes possible to make the Verde constant larger than when x exceeds 0.3, and compared with the case where x is 0.1 or less, the fluoride single crystal has a congruent composition or its composition. Therefore, a larger fluoride single crystal can be obtained.

In the above composition formula, it is more preferable that x satisfies the following formula.
0.1 ≦ x <0.5

  In this case, the growth of the fluoride single crystal is easier than when x is 0.5 or more, and the fluoride single crystal has a congruent composition or its composition as compared with the case where x is less than 0.1. Therefore, a larger fluoride single crystal can be obtained.

  In the above composition formula, y is preferably zero. In this case, the turbidity and cracks of the crystal due to defects are less likely to occur than when y is not zero.

In the above composition formula, M is Ca, Sr or a combination thereof. The group II element containing Ca or Sr was selected as M in particular, when M is a group II element not containing Ca or Sr, CaF ₂ and TbF ₃ do not form a congruent composition, and fluoride alone This is because it is difficult to increase the size of the crystal.

  The fluoride single crystal has a congruent composition or a composition close thereto. Therefore, there is almost no composition shift during the pulling of the crystal. As a result, the crystal can be enlarged. The fluoride single crystal can have a large Verde constant in a wide wavelength range as compared with TGG. Furthermore, the fluoride single crystal can have a high transmittance in a wide wavelength range as compared with TGG. In particular, the fluoride single crystal can have a high transmittance even in a short wavelength region of 800 nm or less.

  Therefore, the optical isolator 10 can be used as an optical isolator of a laser processing machine provided with laser light sources having various wavelengths. Moreover, since the said fluoride single crystal can be enlarged, the Faraday rotator 3 can be obtained in large quantities from one fluoride single crystal. For this reason, the price of the Faraday rotator 3 can be reduced, and as a result, the price of the optical isolator 10 including the Faraday rotator 3 can also be reduced.

  Next, a method for manufacturing the Faraday rotator 3 will be described.

  First, a crystal growth apparatus for growing a fluoride single crystal constituting the Faraday rotator 3 will be described with reference to FIG. FIG. 2 is a diagram showing a process of growing the fluoride single crystal according to the present invention using a crystal growing apparatus. As shown in FIG. 2, the crystal pulling furnace 20 includes an iridium crucible 21, a carbon inner heat insulating material 22A for housing the crucible 21, and an outer heat insulating material 22B provided so as to surround the inner heat insulating material 22A. The high-frequency coil 23 provided between the inner heat insulating material 22A and the outer heat insulating material 22B is mainly provided in the sealed housing 24. The high-frequency coil 23 is for generating an induced current in the crucible 21 and heating the crucible 21. Further, the crystal pulling furnace 20 includes a support portion 26 that supports the inner heat insulating material 22A and an elevating portion 25 that moves the support portion 26 up and down, and the crucible 21 is accommodated in the inner heat insulating material 22A. . In FIG. 2, reference numeral 29 indicates a seed crystal, reference numeral 30 indicates a grown crystal, arrow A indicates the rotation direction of the seed crystal 29, that is, the rotation direction of the grown crystal 30, and arrow C indicates the grown crystal. 30 pulling directions are shown.

  Next, a method for growing a fluoride single crystal using the crystal growth apparatus 20 will be described.

The fluoride single crystal can be grown using, for example, the Czochralski method. In this case, first, LF ₃ powder and MF ₂ powder are prepared.

When the composition of the fluoride single crystal to be grown, that is, x and y in the above composition formula is determined, the blending ratio of the LF ₃ powder and the MF ₂ powder is determined based on the composition. At this time, the blending ratio of the LF ₃ powder and the MF ₂ powder is as follows.

That, LF ₃ powder blending ratio of usually based on the total moles of LF ₃ powder and MF ₂ powder, and 70 to 90 mol%, preferably 75 to 85 mol%.

The blending ratio of the MF ₂ powder is usually 10 to 30 mol%, preferably 15 to 25 mol%, based on the total number of moles of the LF ₃ powder and the MF ₂ powder.

Then, to obtain a mixed powder by dry-mixing the LF ₃ powder and MF ₂ powder blending ratio determined as described above.

  Next, the mixed powder is packed in the crucible 21.

  Subsequently, a current is applied to the high frequency coil 23. Then, the crucible 21 is heated, the mixed powder is melted in the crucible 21, and the melt 28 is obtained. Subsequently, a rod-shaped seed crystal 29 is prepared, the tip of the seed crystal 29 is immersed in the melt 28, and then the seed crystal 29 is pulled up while rotating.

  At this time, as the seed crystal 29, for example, a crystal cut out from a solidified product obtained by solidifying the melt 28 can be used.

  The rotation speed of the seed crystal 29 is preferably 5 to 50 rpm, more preferably 8 to 15 rpm.

  The pulling speed of the seed crystal 29 is preferably 0.5 to 20 mm / h, more preferably 1 to 5 mm / h.

The pulling up of the seed crystal 29 is preferably performed in an atmosphere that does not contain oxygen or water vapor so as not to generate corrosive gas such as HF. Specifically, it is performed in an atmosphere of a fluorinated gas such as CF _4. preferable. In this case, excess oxygen or water vapor contained in the crucible 21 can react with the fluorinated gas to reduce the concentration of oxygen or water vapor, and the grown crystal 30 is corroded by a corrosive gas such as HF gas. Can be suppressed.

  When the seed crystal 29 is pulled up in this way, a bulk-shaped growth crystal 30 represented by the above composition formula can be obtained at the tip of the seed crystal 29.

  The present invention is not limited to the above embodiment. For example, in the above embodiment, an example in which a fluoride single crystal is used as a Faraday rotator of an optical isolator is shown, but the fluoride single crystal of the present invention uses a Faraday rotator to change the Faraday rotation angle. The present invention can also be applied to an optical magnetic field sensor that observes a change in magnetic field by measuring.

  Hereinafter, the content of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

Example 1
First, TbF ₃ powder and CaF ₂ powder were prepared, and these powders were dry-mixed to obtain a mixed powder. At this time, based on the total moles of TbF ₃ powder and CaF ₂ powder, each mixture ratio of TbF ₃ powder and CaF ₂ powder is 81 mol%, and 19 mol%.

  Subsequently, the mixed powder was packed in a cylindrical crucible 21 having a diameter of 60 mm and a depth of 100 mm.

  Next, a current was applied to the high-frequency coil 23 to heat the crucible 21 and melt the mixed powder to obtain a melt 28. Subsequently, a crystal cut into a square bar shape from the solidified product obtained by solidifying the melt 28 was prepared, and this was used as a seed crystal 29. At this time, the [001] direction of the seed crystal 29 was made to coincide with the longitudinal direction of the seed crystal 29.

Then, after the tip of the seed crystal 29 was immersed in the melt 28, the seed crystal 29 was pulled up at a pulling rate of 1 mm / hr while rotating at a rotation speed of 10 rpm. At this time, the inside of the crucible 21 was once evacuated and then filled with CF ₄ gas while being sealed to an atmospheric pressure. Thus, a large transparent crystal having a diameter of about 2.5 cm (about 1 inch) and a length of about 5 cm was obtained.

  When single crystal X-ray diffraction was performed on the thus obtained crystal, it was found that the obtained crystal had a hexagonal crystal structure and was a uniaxial crystal.

Furthermore, as a result of chemical analysis by ICP (inductively coupled plasma) and lanthanum alizarin complexone absorption method, a single crystal having a composition of Tb _0.81 Ca _0.19 F _2.80 was obtained. It was confirmed.

(Example 2)
Using SrF ₂ powder instead of the CaF ₂ powder, based on the total moles of TbF ₃ powder and SrF ₂ powder, to the respective mixture ratio of TbF ₃ powder and SrF ₂ powder, 76 mol%, and 24 mol% Except for the above, a fluoride single crystal was produced in the same manner as in Example 1. As a result, a large transparent single crystal having a diameter of about 2.5 cm (about 1 inch) and a length of about 5 cm was obtained.

  When single crystal X-ray diffraction was performed on the thus obtained crystal, it was found that the obtained crystal had a hexagonal crystal structure and was a uniaxial crystal.

Further, when the obtained crystal was subjected to chemical analysis by ICP and lanthanum alizarin complexone absorption method in the same manner as in Example 1, a single crystal having a composition of Tb _0.77 Sr _0.23 F _2.71 was obtained. It was confirmed that it was obtained.

(Comparative Example 1)
As a single crystal of Comparative Example 1, Tb ₃ Ga ₅ O ₁₂ (TGG) manufactured by Fujian Castech Crystals was used.

[Characteristic evaluation]
(Faraday rotation angle)
The Faraday rotation angle was measured for the crystals of Examples 1 and 2 and Comparative Example 1 obtained as described above.

  At this time, the Faraday rotation angle was measured as follows. That is, first, the analyzer was rotated to a quenching state without placing a single crystal between the polarizer and the analyzer. Next, the single crystals of Examples 1 and 2 and Comparative Example 1 were cut into a 3.5 mm × 3.5 mm × 16 mm square bar shape, which was placed between the polarizer and the analyzer, and the single crystal Light was incident along the longitudinal direction with a magnetic flux density of 0.42 T applied, and the analyzer was rotated again to be in a quenching state. Then, the difference between the rotation angle of the analyzer before sandwiching the single crystal between the polarizer and the analyzer and the rotation angle of the analyzer after sandwiching the single crystal is calculated, and this angular difference is calculated as the Faraday rotation angle. It was. At this time, the Faraday rotation angle was measured for the wavelength of the light source at 633 nm, 1064 nm, and 1303 nm, respectively. The results are shown in Table 1.

(Transmittance)
The crystals of Example 1 and Comparative Example 1 obtained as described above were formed into a square bar shape such that W [mm] × H [mm] × L [mm] = 3.5 mm × 3.5 mm × 12 mm. The transmittance in a wide wavelength region (200 to 1800 nm) was measured for the cut crystal. The results are shown in FIG. FIG. 3 is a graph showing the relationship between the transmittance and wavelength in the crystal of Example 1, that is, the transmission spectrum. In FIG. 3, the result of the transmission spectrum of TGG of Comparative Example 1 is also shown. In FIG. 3, the transmission spectrum of Example 1 is indicated by a solid line, and the transmission spectrum of Comparative Example 1 is indicated by a broken line. In addition, although the transmission spectrum of Example 2 is not illustrated, the result similar to Example 1 was shown.

  From the results shown in Table 1, it was found that the crystals of Examples 1 and 2 showed a large Faraday rotation angle in all wavelength regions of 633 nm, 1064 nm, and 1303 nm as compared with the crystal of Comparative Example 1. From this, it was found that the crystals of Examples 1 and 2 had a large Verde constant in a wide wavelength range as compared with the single crystal of Comparative Example 1.

  Further, the crystals of Examples 1 and 2 had higher transmittance than the crystal of Comparative Example 1 in a wide wavelength range of 200 to 1800 nm. In particular, the crystals of Examples 1 and 2 had significantly higher transmittance than the crystals of Comparative Example 1 in the short wavelength range of 200 to 800 nm.

  Furthermore, in Examples 1 and 2, a large single crystal having a diameter of about 2.5 cm and a length of about 5 cm could be easily obtained.

  From the above, it was confirmed that the fluoride single crystal used in the present invention can be increased in size, has a large Verde constant in a wide wavelength range, and has a high transmittance.

DESCRIPTION OF SYMBOLS 1 ... Polarizer 2 ... Analyzer 3 ... Faraday rotator 10 ... Optical isolator

Claims (2)

An optical isolator having a Faraday rotator composed of a fluoride single crystal,
The fluoride single crystal has the following composition formula:
L _1-x M _x F _{3−x + y}
(In the above formula, L contains Tb. M represents a group II element containing at least one selected from the group consisting of Ca and Sr. x is greater than 0 and less than 1. y is -0.2. ~ 0.2.)
An optical isolator characterized by the following: In the above composition formula, x is the following formula:
0.1 <x ≦ 0.3
The optical isolator according to claim 1, wherein:

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