JP2011225400A - Single crystal for magnetooptical element, and device using the single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crystal body for a magnetooptical element having a Verdet constant higher than that of a TGG single crystal, and to provide a method for applying a crystal body other than the isometric system to a magnetooptical element.SOLUTION: The crystal body is a single crystal of a multiple oxide for a magnetooptical element, which has a chemical composition of MVO(wherein, M is at least one kind of element selected from Sc, Y and lanthanoids). The multiple oxide is made into a single crystal by a crystal growth method including growing a crystal from a melt, such as a floating zone method (FZ method). Further, a magnetooptical device for laser, having such a constitution that the c-axis in the crystal orientation of a single crystal is allowed to coincide with the propagation direction of the laser, is obtained by using the single crystal.

Description

The present invention relates to a complex oxide single crystal for a magneto-optical element. The present invention also relates to a magneto-optical device using the single crystal as a magneto-optical element.

An optical isolator using the magneto-optical effect is a magneto-optical element used in a laser system. The optical isolator includes a polarizer, a Faraday rotator, an analyzer, and a magnet. The phenomenon that the plane of polarized light rotates when polarized light passes through a material placed in a magnetic field is known as the Faraday effect. The rotation angle Θ depends on the strength H of the magnetic field and the length L of the substance ( 1) Formula Θ = VHL (1)
It is represented by The proportional coefficient V is called the Verde constant, which is a characteristic value depending on the material. When a material having a large V is used for the Faraday rotator, the same isolation performance can be obtained even if the Faraday rotator and the permanent magnet are small, so that the element can be miniaturized. Since the optical isolator has the function of preventing the reflected light from returning to the laser and operating the laser stably, a laser for semiconductor microfabrication, a semiconductor laser for optical fiber communication, a laser for cutting and heat treatment of steel and ceramics, It is incorporated in medical laser scalpels and is used in a wide range of industrial fields.

In addition to the Verde constant, the performance required for an optical isolator requires a high extinction ratio and a low insertion loss. When the extinction ratio is low, the controllability of the polarization of the laser light is poor, and as a result, the separation ability (isolation performance) of the return light is impaired. If the insertion loss is high, loss of laser output and endurance of the optical isolator due to heat absorption are reduced.

A garnet crystal mainly composed of iron is used for an optical isolator of a laser for optical communication of 1.3 μm to 1.5 μm. However, garnet crystals containing iron cannot be used for lasers in this wavelength range because of absorption in the visible to near-infrared wavelength range. For this reason, crystals and glasses containing rare earth elements have been proposed as Faraday rotators for optical isolators in the visible to near-infrared wavelength region. The rare earth element here is Sc of atomic number 21, Y of atomic number 39, lanthanoids of atomic number 57 to 71 (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and is represented by the symbol M below.

For crystals containing rare earth elements, garnets mainly composed of trivalent terbium ions (Tb ³⁺ ) have been put into practical use as Faraday rotators. For example, a TGG (terbium gallium garnet, chemical formula Tb ₃ Ga ₅ O ₁₂ ) single crystal is used for an optical isolator for a fiber laser of 1.06 μm. However, since the Berde constant of TGG is as small as 40 rad / (T · m), a crystal having a length of 2 cm to 3 cm is usually required for the Faraday rotator. Furthermore, in order to shorten the Faraday rotator length, a single crystal material having a large Verde constant is desired.

Currently, Faraday rotators of optical isolators that are in practical use are limited to crystals belonging to the equiaxed system such as glass and garnet crystals. The reason for this is that single crystals belonging to crystal systems other than equiaxed crystals, such as tetragonal and hexagonal, do not quench due to birefringence, and do not function as an isolator even if the Verde constant is large. It was because it was thought.
JP2002-293693 Journal of Applied Physics, Volume 35, Number 8, p2338

The problem to be solved by the present invention is to provide a crystal for a magneto-optical element having a larger Verde constant than a conventional TGG single crystal. It is another object of the present invention to provide a method of applying a crystal belonging to a crystal system other than the equiaxed crystal system to the magneto-optical element.

The present invention
(1) A single crystal characterized in that it is a complex oxide for a magneto-optical element, and its chemical composition is MVO ₄ (M is one or more elements selected from Sc, Y, and lanthanoid) (2) Magnetism TbVO ₄ single crystal for optical element (3) A magneto-optical element using the above-mentioned single crystal, characterized in that the c-axis of the crystal orientation of the single crystal coincides with the light propagation direction. Magneto-optical device (4) The magneto-optical device (5) is characterized in that the accuracy of coincidence between the c-axis of the crystal orientation of the single crystal and the light propagation direction is within ± 0.5 degrees. The present invention relates to a method for producing a single crystal for a magneto-optical element, wherein the single crystal is formed by a crystal growth method from a melt.

According to the present invention (1) and (2), a single crystal for a magneto-optical element having a Verde constant larger than TGG can be obtained. This single crystal can provide a large Faraday rotation angle even with a size smaller than TGG.
According to the present invention (3) and (4), a smaller and higher performance magneto-optical device is possible. Small magneto-optical devices are suitable for mounting on fiber lasers.
According to the present invention (5), it is possible to industrially produce a single crystal for a magneto-optical element with necessary and sufficient dimensions.

A method for manufacturing a crystal will be described. The following describes a method for producing a rare earth vanadate (MVO ₄ ) crystal having a 1: 1 stoichiometric ratio of rare earth elements to vanadium elements. Rare earth vanadate belongs to the tetragonal system and has a zircon crystal structure. Rare earth vanadate is known to have a solid solution region in the vicinity of a stoichiometric ratio of 1: 1, but even a crystal having a deviation from such a stoichiometric ratio is a tetragonal zircon type. A similar manufacturing method can be applied to a crystal body having a crystal structure.

The raw material can be used by mixing a rare earth element oxide and a vanadium element oxide. The purity of the raw material is preferably 99.9% or more, and more preferably 99.99% or more. In addition to oxides, compounds that decompose into volatile substances and oxides before being melted by heating, such as ammonium salts, carbonates and hydroxides, such as carbonates, ammonium salts and hydroxides are used as raw materials. Also good. The elemental ratio of rare earth to vanadium may be a stoichiometric ratio of 1: 1. Since vanadium evaporates from the melt, a larger amount of vanadium may be added in consideration of evaporation.

The weighed raw materials are sufficiently mixed using existing equipment such as a ball mill mixer, a V-type mixer, a shaker, and an agate mortar. The mixed powder is formed into a compact by cold isostatic pressing (CIP).

There are a plurality of crystal growth methods for producing rare earth vanadate single crystals from the melt, such as the Czochralski method (CZ method), the floating zone method (FZ method), the Bridgman method, and the EFG method. In either method, raw materials other than the seed crystal are heated and melted, and then the melt is gradually solidified so as to take over the orientation of the seed crystal to grow a large single crystal. The heating method may be selected from resistance heating, induction heating, condensing heating and the like.

The atmosphere for growing a single crystal from the melt is preferably air or an atmosphere having a lower partial pressure of oxygen in order to prevent Tb ions from being oxidized from trivalent to tetravalent.

The crystal growth rate may be selected in the range of 0.1 mm / h to 10 mm / h in consideration of crystal quality and productivity according to the crystal growth method. When the single crystal grows to a predetermined size, the growth is stopped and the solution is slowly cooled to room temperature.

The grown crystal can be heat-treated as necessary. The purpose of the heat treatment is to reduce the thermal strain of the crystal to prevent cracking during processing, to improve the extinction ratio due to strain birefringence, and to reduce the absorption in the wavelength range used due to crystal defects.

Next, a procedure for producing a magneto-optical element from a single crystal is shown. The illustrated element is an optical isolator.

From the grown single crystal, a square or circular element having a cross section of several millimeters, and a prismatic or cylindrical element having a length of several millimeters to several centimeters in the direction of propagation of laser light is cut. The size of the cross section depends on the laser optical system used. The length depends on the magnetic field strength of the magnet and the design of the optical isolator.

When processing the columnar element, processing is performed so that the central axis of the prism or cylinder coincides with the c-axis of the crystal orientation of the rare earth vanadate single crystal. The crystal orientation can be determined using X-rays. For cutting, an outer peripheral blade cutting machine, an inner peripheral blade cutting machine, a multi-wire saw, an ultrasonic core drill, or the like is used.

Both end surfaces of the element are mirror-polished. The flatness of the polishing is desirably λ / 8 or less with respect to the wavelength λ of the laser to be used. The parallelism of the polished surface should be within 1 minute. Further, an antireflective coating may be applied to the polished surface.

The configuration of the optical isolator may be the same as when a normal garnet crystal or glass is used as the Faraday rotation element. For example, in the configuration of a polarization-dependent optical isolator, a rare earth vanadate single crystal of a Faraday rotation material is surrounded by a magnet, and a polarizer and an analyzer are placed before and after the crystal.

The magneto-optical characteristics of the produced crystal can be evaluated by an optical system having the same element arrangement as that of the optical isolator. The Verde constant can be calculated from the equation (1) by measuring the rotation angle Θ of the polarization plane when a magnetic field of strength H is applied to a crystal of optical path length L. The insertion loss is obtained from the energy lost when the light emitted from the laser device passes through the optical isolator toward the irradiation target. The extinction ratio measures how much the return light from the object side is attenuated by the optical isolator. The laser used for the measurement has the same wavelength as the optical isolator. For example, in the case of an optical isolator for a 1.06 μm fiber laser, characteristics are measured using a commercially available laser having an oscillation wavelength of 1.06 μm as a light source. When the extinction ratio is 20 dB or less, the return light is not sufficiently attenuated, and it is not practical as an optical isolator.

Examples of the present invention will be described below.

(Example 1) 34 g of Tb ₄ O ₇ powder having a purity of 99.99% and 17 g of V ₂ O ₅ powder having a purity of 99.99% were weighed so as to have a molar ratio of 1: 1. Each powder and 50 cc of ethyl alcohol were put into an alumina mortar and mixed using a pestle for 2 to 3 hours until the ethyl alcohol evaporated and the mixture was not liquid. The mixed powder was further air-dried.

This powder was packed in a rubber tube, shaped into a rod shape having a diameter of 5 mm and a length of 100 mm, and then molded using CIP at a pressure of 100 MPa. The molded bar was sintered at 1300 ° C. to 1600 ° C. to obtain a dense sintered bar of TbVO ₄ .

The single crystal was produced by the FZ method using an image furnace by condensing heating. A conceptual diagram of the furnace used is shown in FIG. The lamp 2 is placed at one focal point of the spheroid mirror 1 as a heat source, and the material rod 3 at the other focal point is condensed and heated. The raw material rod 3 is fixed to the upper shaft 7, and the seed crystal 4 is fixed to the lower shaft 8. The lower end of the raw material rod 3 is heated and melted, and then the upper and lower shafts are brought close to each other to join the seed crystal 4 to form a melt zone 5 having an appropriate length. Both shafts can be rotated in opposite directions to agitate and homogenize the melt zone. When the melt zone is slowly moved to the raw material rod side, the single crystal 6 grows between the seed crystal and the melt zone. The melt zone is isolated from the atmosphere by a transparent quartz tube 9 so that the crystal growth atmosphere can be freely controlled.

The formed raw material rod and seed crystal were attached to a furnace, the output of the lamp was adjusted to melt the lower end of the raw material rod, and then joined to the seed crystal. The melt zone was moved at 1-10 mm / hr to grow a TbVO ₄ single crystal. The obtained single crystal was yellow to brown.

A square pillar Faraday rotator was processed from the produced single crystal. The coincidence accuracy between the central axis of the square pillar and the c-axis of the crystal orientation of the TbVO ₄ single crystal was within ± 0.05 degrees. The length of the central axis was 2.8 mm, and the shape of both end faces was a 3 mm × 3 mm square. Both end surfaces were mirror-polished.

The transmission spectrum of the polished Faraday rotator was measured. The wavelength range from 1400 nm to 400 nm was measured using a Hitachi spectrophotometer U4100. A sharp absorption band due to trivalent terbium was observed around 488 nm. At 800 nm or less, the absorption gradually increased and the transmittance decreased, but there was almost no absorption in the wavelength region of 1400 nm to 1000 nm, indicating a high transmittance. It was shown to be suitable as an optical material for lasers in the near-infrared wavelength region, including fiber lasers having a wavelength of 1.06 μm.

Next, the magneto-optical characteristics were measured. The Faraday rotator was adjusted and installed so that the laser propagation direction coincided with the c-axis of the TbVO ₄ single crystal with an accuracy within ± 0.05 degrees. The element was placed in a 0.5 T magnetic field. FIG. 2 shows the result of measuring the Verde constant according to the extinction angle by the analyzer by rotating the polarizer in a 1.06 μm laser sandwiched between the polarizer and analyzer by the Glan-Thompson prism. The Verde coefficient was 62 rad / (T · m). The extinction ratio at this time was 35 dB.

An optical isolator using a TbVO ₄ single crystal as a Faraday rotator was produced. The configuration of the optical isolator is shown in FIG. A TbVO ₄ single crystal 12 in which the c-axis of the crystal is aligned in parallel with the laser propagation direction 10 is arranged at the center. A magnet 13 surrounds the crystal. Polarizers 11 are arranged before and after the crystal. The crystal, magnet, and polarizer are housed in the housing 14. Since the TbVO ₄ single crystal has a large Verde constant, an optical isolator using the TbVO ₄ single crystal can be reduced in size.

(Example 2)
A procedure similar to that in Example 1 was followed, and a single crystal was produced by the FZ method. When processing into a square column Faraday rotator, the center axis of the square column and the c-axis of the single crystal were shifted by 0.2 degrees and 0.3 degrees. The length of the central axis was 2.8 mm. The shape of both end surfaces was a 3 mm × 3 mm square, and mirror polishing was performed. The laser beam was propagated to the central axis of the square pillar. The extinction ratio was 28 dB when the central axis of the square pillar and the c-axis of the single crystal were shifted by 0.2 degrees, and 25 dB when shifted by 0.3 degrees.

(Example 3)
228.1 g of Tb ₂ O ₃ powder having a purity of 99.99% and 113.4 g of V ₂ O ₅ powder having a purity of 99.99% were weighed. Two types of powders were mixed with a shaker for 2 hours, and then pressure-molded using CIP.

The raw material for molding was filled in an iridium crucible, and the raw material was melted by high frequency induction heating. A seed crystal was put into the melt and a single crystal was grown by a general Czochralski method. The cultivation was performed in a low oxygen atmosphere. After growing to a predetermined length, the crystal was separated from the melt and slowly cooled.

A square columnar Faraday rotator was produced from the grown single crystal in the same procedure as in Example 1. The central axis of the square pillar and the c axis of the single crystal were matched with an accuracy of 0.1 degree. Magneto-optical properties were measured. The Verde coefficient was 61 rad / (T · m). The extinction ratio was 34 dB.

(Comparative Example 1)
A procedure similar to that in Example 1 was followed, and a single crystal was produced by the FZ method. When processing into a square pillar Faraday rotator, the central axis of the square pillar and the c axis of the single crystal were shifted from -0.8 degrees to +1.2 degrees. The length of the central axis was 2.8 mm. The shape of both end surfaces was a 3 mm × 3 mm square, and mirror polishing was performed. The laser beam was propagated to the central axis of the square pillar. The extinction ratio was only as low as 14 dB when the deviation angle was +1.2 degrees and 17.5 dB when the deviation angle was -0.6 degrees. FIG. 4 shows the influence of the light propagation direction and the c-axis deviation angle on the extinction ratio. It has been found that when the deviation angle exceeds ± 0.5 degrees, the extinction ratio becomes smaller than 20 dB, and sufficient performance as an optical isolator cannot be obtained.

As shown in the above test results, it can be seen that a crystal body having a large Verde constant and extinction ratio and suitable for a magneto-optical element having high transmittance is obtained. Furthermore, a high-performance optical isolator can be realized by using this crystal as a Faraday rotation material.

Schematic diagram of the floating zone method Verde constant measurement result 1 Schematic diagram of the fabricated optical isolator Light propagation direction, c-axis deviation angle and extinction ratio

DESCRIPTION OF SYMBOLS 1 Rotating elliptical mirror 2 Lamp 3 Raw material rod 4 Seed crystal 5 Melt zone 6 Single crystal 7 Upper shaft 8 Lower shaft 9 Quartz tube 10 Laser propagation direction 11 Polarizer 12 Rare earth vanadate crystal 13 Magnet 14 Housing

Claims (5)

Single crystal characterized in that it is a complex oxide for a magneto-optical element, and its chemical composition is MVO ₄ (M is one or more elements selected from Sc, Y, and lanthanoid) TbVO ₄ single crystal for magneto-optic element 3. A magneto-optical device using the single crystal according to claim 1, wherein the c-axis of the crystal orientation of the single crystal coincides with the light propagation direction. 4. The magneto-optical device according to claim 3, wherein the coincidence accuracy between the c-axis of the crystal orientation of the single crystal and the light propagation direction is within ± 0.5 degrees. A method for producing a single crystal for a magneto-optical element, wherein the single crystal according to claim 1 or 2 is single-crystallized by a crystal growth method from a melt.