JP2001091738A - Birefringent plate and optical isolator using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a birefringent plate and an optical isolator using a crystal which is completely different from a crystal conventionally used as a birefringent plate and which is extremely large in birefringent and can be easily produced. SOLUTION: The birefringent plate 1, 2 consists of a single crystal of PbMoO4 (lead molybdate), K5(BixNd1-x)(MoO4)4 (wherein 0<x<1) or BaLiNb5O15. The optical isolator S is obtained by disposing the birefringent plates 1, 2 on the entrance side and exit side of light of a Faraday rotator 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent plate such as a prism used for a polarizing plate or a polarizing beam splitter, etc.
And an optical isolator using the same.

[0002]

2. Description of the Related Art Conventionally, when unpolarized light is incident on an anisotropic crystal such as calcite, quartz, rutile (TiO ₂ ), two refracted light rays are observed. It is well known. This phenomenon is generally called birefringence, and the two refracted rays are called an ordinary ray that obeys Snell's law and an extraordinary ray that does not obey Snell's law.

Here, the larger the birefringence, which is the difference between the refractive index of ordinary light (ordinary refractive index) and the refractive index of extraordinary light (extraordinary refractive index), the more the direction that is not parallel to the optical axis of the crystal. Since the light incident on the lens has larger birefringence, the separation angle between the ordinary ray and the extraordinary ray becomes larger.

Therefore, if the separation interval between the ordinary ray and the extraordinary ray is constant, the element length can be made smaller by using a crystal having a large birefringence. ) Are expected to be significantly reduced in size, for example, in an optical reading unit or an optical isolator of an optical recording apparatus.

However, in the case of, for example, calcite, the birefringence is about 0.172, which is very large as compared with other crystal materials. However, since it is a material produced only naturally, inexpensive and high quality crystals can be obtained. There is a problem that it is difficult to supply it stably.

In the case of quartz, a crystal can be synthesized by a hydrothermal synthesis method in addition to those naturally produced, but the birefringence is very small at about 0.009, so that the element becomes very large. As a result, there is a problem that the entire device such as an optical isolator using this element becomes large.

In the case of rutile single crystal, the birefringence is as large as about 0.282, and it is used as a polarizer for an optical isolator. However, it can be grown only by a method such as Bernoulli method or floating zone method. In addition, since it is difficult to produce a material having good crystallinity, there is a problem that the yield is reduced due to the presence of grain boundaries and internal strain, and it is difficult to obtain a large crystal.

[0008] From the above, since a small birefringent plate has not been easily obtained in the past, it is possible to reduce the size of a device using the plate (ie, an optical reading unit or an optical isolator of an optical recording apparatus). There was no.

Accordingly, the present invention has been proposed to solve the above-mentioned conventional problems, and is completely different from a crystal which has been conventionally used as a birefringent plate, and has a very large birefringence. It is another object of the present invention to provide a birefringent plate and an optical isolator using a crystal that can be easily manufactured.

[0010]

To achieve the above object, according to the Invention The birefringent plate of the present invention, PbMoO ₄ (lead _{molybdate), K 5 (Bi x Nd} 1-x) (MoO 4) 4 ( provided that , 0 <x <1) or a single crystal of BaLiNb ₅ O ₁₅ .

Further, the optical isolator of the present invention comprises the birefringent plates provided on each of the light incident side and the light emitting side of the Faraday rotator, and is particularly provided on the light incident side of the Faraday rotator. The light exit surface of the birefringent plate and the light incident surface of the birefringent plate disposed on the light exit side of the Faraday rotator are surfaces parallel to the c-axis. Further, a light exit surface of a birefringent plate disposed on the light incident side of the Faraday rotator,
The light incident surface of the birefringent plate disposed on the light exit side of the Faraday rotator may be a surface parallel to the a-axis.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a birefringent plate and an optical isolator according to the present invention will be described below in detail with reference to the drawings.

[0013] _{PbMoO 4, K 5 (Bi x} Nd 1-x)
(MoO ₄ ) ₄ (where 0 <x <1) or BaL
iNb single crystal body consisting of ₅ O ₁₅ is capable of obtaining a high-quality crystal easily large by the Czochralski method. That is, after the raw material is put into a crucible made of platinum or the like in a resistance heating furnace or a high-frequency heating furnace and melted, a growing rod having a seed crystal attached to the tip is immersed in the melt in the crucible, and a predetermined number of rotations with respect to the crucible is performed. By pulling up the growing crystal while rotating the growing rod, the outer diameter is 80 mm or more and the length is 100 mm
It is possible to easily obtain a single crystal having the above-mentioned uniformity and excellent crystallinity.

Then, the grown single crystal is diced and cut into wafers of a predetermined size.
A plate having a substantially rectangular parallelepiped shape as shown in FIG. Further, the surface of this plate body through which light passes is precisely polished by a polishing device for mechanochemical to form a birefringent plate.

Here, the vertical direction in FIG.
Then, the angle θ1 with respect to the vertical direction E of the inclined surface K through which light passes is set to 3 to 5 °. The reason for setting this angle is to make the emission angle of the return light sufficiently large and to realize a sufficient transmittance. One surface M is parallel to the vertical direction E, that is, a surface substantially perpendicular to the lower surface U (substantially perpendicular to the light incident direction and the light emitting direction), and c
The plane is parallel to the axis or the a-axis. In particular, as shown in FIG. 1B, the c-axis (or a-axis) with respect to one side of the birefringent plate
Is preferably about 22.5 °.

Next, when the ordinary refractive index and the extraordinary refractive index of the birefringent plate were measured by a refractive index measuring device equipped with a prism coupler and the like, the birefringent plate had a very large refractive index and a birefringence of 0.1. 1 to 0.2. As described above, it was found that birefringence was much larger than that of quartz.

Therefore, as shown in FIGS. 2A and 2B, for example, the light incident side and the light emitting side of the Faraday rotator 3 made of a YIG (yttrium iron garnet) crystal or the like are respectively provided on the light incident side and the light emitting side as shown in FIG. An optical isolator S in which the refraction plates 1 and 2 are disposed so that the vertical plane M faces the Faraday rotator 3 and both the ordinary ray and the extraordinary ray are shifted by 45 °.
According to the method, the size of the birefringent plates 1 and 2 occupying a large area of the component parts can be reduced, so that the optical isolator S can be significantly reduced in size compared to the related art.

In particular, the light exit surface of the birefringent plate disposed on the light incident side of the Faraday rotator and the light incident surface of the birefringent plate disposed on the light exit side of the Faraday rotator are respectively denoted by c.
When the plane is parallel to the axis, the refractive index difference (birefringence) can be maximized. As a result, it is possible to minimize the elements that obtain the same characteristics.

A commonly used Faraday rotator rotates the plane of polarization by 45 °. However, when the birefringent plate is formed in a substantially rectangular parallelepiped shape, as shown in FIG. Is about 22.5 °, the same birefringent plate is simply arranged as shown in FIG.
Can be configured. The optical isolator S
Affixes two of the birefringent plates to each of the light entrance and exit surfaces of the Faraday rotator 3 with an adhesive such as an ultraviolet curing resin,
It is configured by being arranged at a predetermined interval position.

The light exit surface of the birefringent plate disposed on the light incident side of the Faraday rotator and the light incident surface of the birefringent plate disposed on the light exit side of the Faraday rotator are respectively denoted by a
If the plane is parallel to the axis, it is easy to grow a single crystal having good crystallinity in the case of pulling up the a-axis, particularly in the structure of the PbMoO ₄ single crystal. For this reason, if the light incidence surface of the birefringent plate is set to the a-plane, it is possible to produce a birefringent plate having excellent crystallinity by a wafer process using an a-plane wafer easily obtained from an ingot of an a-axis pulled crystal. it can. Also,
In this case, when the birefringent plate is formed into a substantially rectangular parallelepiped shape in the same manner as described above, as shown in FIGS. 1A and 1B, if the angle θ2 of the a axis with respect to one side is about 22.5 °. An optical isolator can be configured by simply arranging the same birefringent plate as shown in FIG.

Furthermore, the PbMoO ₄ single crystal is (001)
Alternatively, (010) striae (distribution of the refractive index) often occurs, and when the incident surface is not the a-plane, the striae obliquely intersect with the traveling direction of the light. , The insertion loss increases. From these facts, it can be said that the advantage is greatest in terms of manufacturing cost, characteristics, and yield when the incident surface is the a-plane.

Next, an optical isolator according to the present invention will be described. As shown in FIG. 2A, in the optical isolator S utilizing the birefringence phenomenon, two wedge-shaped birefringent plates 1 and 2 are arranged on the light incidence side and the light emission side of the Faraday rotator 3, respectively. Things. When the Faraday rotator 3 does not have spontaneous magnetization, a magnet for generating a magnetic field may be provided around the Faraday rotator 3.
The magnetic field generating means is not shown for simplicity.

The light L0 emitted from the optical fiber 4 is converged by the lens 5 and is incident on the optical isolator S. After that, the ordinary ray L1 and the extraordinary ray L2 separated by the birefringent plate 1 are separated by a Faraday rotator 3 at a predetermined angle. Rotated.
Then, the ordinary ray L1 and the extraordinary ray L2 are emitted from the birefringent plate 2 to the lens 6 as parallel rays, and are focused on the optical fiber 7 from the lens 6.

On the other hand, as shown in FIG. 2B, the light L3 in the opposite direction is emitted from the optical fiber 7 through the lens 6, and
After being split into an ordinary ray L5 and an extraordinary ray L4 by the birefringent plate 2, the ordinary ray L5 and the extraordinary ray L4 which are rotated by the Faraday rotator 3 at a predetermined angle and spread by the birefringent plate 1 are emitted. 4 is not focused.

Thus, a compact and high-performance birefringent plate and optical isolator can be realized. In the present embodiment, an optical isolator has been described as an example of a device using a birefringent plate, but any device using such a birefringent plate can be applied, and is used for, for example, a polarizing beam splitter. The invention can be applied to various known polarizers such as a prism and an optical reading unit of an optical recording apparatus.

[0026]

EXAMPLE PbMoO ₄ (lead molybdate), K ₅ (BiNd) (MoO ₄ ) ₄ , and BaLiNb were placed in a platinum crucible placed on a support placed in a single crystal growing furnace.
The raw material of ₅ O ₁₅ is put, and the raw material is melted to form a melt, and a growing rod provided with a seed crystal at the tip is immersed in the melt,
A single crystal 24 having an outer diameter of about 80 mm and a length of about 100 mm was grown at a predetermined pulling speed (1 mm / h) while rotating the growth rod at 5 to 10 rpm.

Next, the outer diameter is about 75 m by dicing.
A wafer having a thickness of about 1 mm and a thickness of about 1 mm was cut out to prepare a plate constituting a prism.

The cut surface of such a plate was polished by a polishing device for mechanochemical and the refractive index was measured. As a result, the average ordinary refractive index (no) and extraordinary light were measured at a measurement wavelength of 632.8 nm. The refractive index (ne) of lead molybdate is no = 1.79 and ne = 1.6, respectively.
2, no = 2 in K ₅ (Bi, Nd) (MoO ₄ ) ₄ .
41, ne = 2.28, no in BaLiNb ₅ O ₁₅
2.32, ne = 2.22. Here, the measurement of the refractive index was performed as follows.

That is, the ordinary light refractive index and the extraordinary light refractive index were measured by a refractive index measuring device (Metrion model 2010) equipped with a prism coupler and the like. As shown in FIG. 3, light is made incident on 14 points (points indicated by the crosses P: on a line extending from the center to the outer periphery) which are measurement points of the wafer W cut out from the upper and lower portions of the grown single crystal. Measure
The average of the measured values was calculated and determined.

The thus obtained ordinary light refractive index (n
o) and extraordinary refractive index (ne), birefringence = no-ne
Is 0.13 lead molybdate, K ₅ (Bi, Nd)
0.17 for (MoO ₄ ) ₄ and 0.1 for BaLiNb ₅ O ₁₅ .
1, which was found to have a very large value.

Further, when the presence or absence of grain boundaries and strains was examined by X-ray topography and crossed nicols, no abnormalities were observed at all, and it was found that the crystals were very good and homogeneous.

[0032]

As described above, according to the present invention,
Birefringence can be made larger than conventionally used quartz, and it is possible to grow by a very simple method than calcite or rutile, so that a small and high-performance birefringent plate can be easily provided, As a result, miniaturization and high performance of an apparatus using such a birefringent plate, in particular, an optical reading section and an optical isolator of an optical recording apparatus are realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a birefringent plate according to the present invention,
(A) is a front view, (b) is a side view.

FIGS. 2A and 2B are schematic diagrams illustrating a schematic configuration and operation of an optical isolator, respectively.

FIG. 3 is a plan view showing a measurement position of a refractive index on a wafer.

[Explanation of symbols]

 1: 2: birefringent plate 3: Faraday rotator S: optical isolator

Claims (4)

[Claims] 1. A _{PbMoO 4, K 5 (Bi x} Nd 1-x)
(MoO ₄ ) ₄ (where 0 <x <1) or BaLi
Birefringent plate made of a single crystal of Nb ₅ O _15. 2. An optical isolator comprising the birefringent plate according to claim 1 disposed on each of a light incident side and a light emitting side of a Faraday rotator. 3. A light emitting surface of the birefringent plate disposed on a light incident side of the Faraday rotator and a light incident surface of the birefringent plate disposed on a light emitting side of the Faraday rotator. 3. The optical isolator according to claim 2, wherein each of the surfaces is a plane parallel to the c-axis. 4. A light emitting surface of the birefringent plate provided on a light incident side of the Faraday rotator and a light emitting surface of the birefringent plate provided on a light emitting side of the Faraday rotator. 3. The optical isolator according to claim 2, wherein each of the surfaces is parallel to the a-axis.