JP3037474B2 - Faraday rotator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Faraday rotator having a small change in Faraday rotation angle with temperature and suitable for use in optical isolators, optical circulators, optical switches and the like.

[0002]

2. Description of the Prior Art 1.3 μm band, which is the wavelength of optical communication,
As a Faraday rotator material used for an optical isolator for the 1.55 μm band, a magnetic garnet material is known. In particular, Bi-substituted rare-earth iron garnet materials have great potential for Faraday rotation and are considered promising, but have the disadvantage that the temperature change of the Faraday rotation angle is large. It has recently been proposed to adjust the composition of garnet to improve this temperature characteristic. For example, Japanese Patent Application Laid-Open No. Sho 62-10
No. 5931. Tb _2.6 Bi _{0. 4} Fe ₅ O in the present invention
It is shown that No. ₁₂ has an excellent composition that makes the temperature change rate of the rotation angle almost zero. The coefficient (rotational power) is small. To date, there is no known material composition in which the Bi substitution amount is sufficiently large and the temperature change is almost zero.

As an improvement method other than composition adjustment, Japanese Utility Model Publication No. Sho 61-9376 discloses a temperature compensated optical isolator. This is a method of compensating for the temperature change of the Faraday rotation angle due to the temperature change of the saturation magnetic field of the Faraday rotator by changing the applied magnetic field, is an effective method for improving the temperature characteristics, and the present invention also utilizes this principle. However, the disadvantage of Japanese Utility Model Publication No. 61-9376 is that it has a complicated structure in which a magnetic shunt steel is arranged in addition to the permanent magnet, and is intended for the 0.8 μm band.

The near-infrared wavelength (1.3 μm, 1.5
Magnetic garnet materials practically used as Faraday rotators of 5 μm) are usually used in a saturated magnetic field,
The Faraday rotation angle is excellent in that the Faraday rotation angle does not change even if the external magnetic field changes to some extent. However, as described above, if Bi is large, the temperature characteristic is deteriorated.

In general, when a magnetic field H is applied to a rare earth iron garnet, the Faraday rotation angle typically changes as shown in FIG. Above the saturation magnetic field H _S , the Faraday rotation angle becomes the saturation value θ _fs and does not change. When this garnet is used as a Faraday rotator for an optical isolator, θ _fs is often simply referred to as a Faraday rotation angle because it is usually used in a saturation magnetic field (H> H _S ).

Rare earth iron garnets with a large amount of Bi substitution have a large Faraday rotation capability (θ _fs / thickness), but have the disadvantage of a large temperature change of θ _fs . For example, in Bi _x R _3-x Fe ₅ O ₁₂ (R: rare earth), when x> 0.5, the temperature coefficient of θ _fs is usually −0.15% / ° C.
It is about.

[0007]

A Faraday rotator having a composition with a small rotation angle temperature change rate as disclosed in Japanese Patent Application Laid-Open No. Sho 62-105931 has a small Faraday rotation coefficient (rotation ability) due to a small amount of Bi substitution. On the other hand, a device having a temperature compensating device as disclosed in Japanese Utility Model Publication No. 61-9376 requires a complicated structure and complicated adjustment. Accordingly, an object of the present invention is to provide a Faraday rotator having a small temperature dependency and a large Faraday rotation angle.

[0008]

[Means for Solving the Problems]

The present invention, the composition _{Bi x P y Q 3-Xy} Fe 5-w M w O 12 ( where P: Y, La, Sm, Eu, Tm, Yb, Lu
At least one of the following: Q: at least one of Gd, Tb, Dy, Ho, and Er; M: at least one of elements that can be substituted for Fe ;
y and w are 0.7 ≦ x ≦ 2.0, 0.5 ≦ y ≦ 2.3, and 0 ≦ 3-x
The garnet material expressed by satisfying -y ≦ 1,0.5 ≦ w ≦ 1.0) , formed by combining a magnet for applying a smaller magnetic field than the saturation field of the material, the file
Rotation angle at a magnetic field smaller than the saturation magnetic field of the Raday rotator
Has a positive temperature coefficient, and the magnet changes the temperature coefficient
A Faraday rotator set to have a negative temperature coefficient to cancel out .

The present inventors have previously disclosed in Japanese Patent Application No. 2-13698.
No. 8 and Japanese Patent Application No. Hei 3-94851, the composition Bi _x
Py Q _3-xy Fe _5-w M _w O ₁₂ (where P: Y, La, Sm, Eu, Tm, Yb, Lu
Q: at least one or more of Gd, Tb, Dy, Ho, and Er; M: at least one of elements that can be substituted for Fe; 0.7 ≦ x ≦ 2.0, 0.5 ≦ y ≦ 2.3, 0 ≦ 3-x
-Y ≦ 1, w <0.5) and a Faraday rotator composed of a magnet for applying a magnetic field smaller than a magnetic field at which the Faraday rotation angle is saturated (saturation magnetic field) to the material. did.

The difference between the prior invention and the present invention is that
Is only the element M to be substituted. w was less than 0.5. This has a feature that the garnet material is used immediately below the saturation magnetization and has an automatic temperature compensation function, and has a large Faraday rotation ability due to a large amount of Bi, but has a saturation magnetization of about 1.6 kG. Since it is large, the magnet for magnetizing the magnetization slightly lower than that of about 1.5 kG becomes large, and there is a disadvantage that the overall size of the Faraday rotator becomes large. The present invention reduces the saturation magnetization of the garnet material by making the amount of the element M greater than that of the prior application, thereby reducing the size of the Faraday rotator. The size of the magnet is increased because the magnet is used without saturating the garnet material, so that it is necessary to increase the length of the magnet sufficiently in order to make the magnetic field uniform and to avoid the influence of the shape of the magnet as much as possible.

The garnet material used in the present invention has a function of self-adjustment of a large Faraday rotation angle and temperature characteristics similarly to the above-mentioned prior application, and further has a low saturation magnetization. Can be configured.

The principle of the temperature compensation according to the present invention will be described with reference to FIG. Figure shows the relationship between the Faraday rotation angle theta _f and the magnetic field H at two temperatures T ₁ <T _2. In the present invention, the applied magnetic field H _M is set smaller than the saturation magnetic field H _S , so the Faraday rotation angle θ _fM when a magnetic field H _M equal to or less than the saturation magnetic field is applied is a problem, not the Faraday rotation angle θ _fs at the time of saturation. Becomes Magnets decrease in magnetic field strength with increasing temperature (H _M1 > H _M2 , subscripts ₁ and ₂ correspond to temperatures T ₁ and T ₂ , respectively), whereas materials of the composition of the present invention become unsaturated with increasing temperature. Θ _fs1 / H _S1 <θ
Since at _fs2 / H _S2, little Faraday rotator temperature changes is obtained since the Faraday rotation angle as shown in FIG even decreases the magnetic field up temperature increases. In addition,
Since the temperature coefficient of the unsaturated part of the garnet element of the composition of the present invention is much smaller than the temperature coefficient of the saturated part, a much more stable rotation angle can be obtained than before, regardless of the rate of decrease of the magnetic flux of the magnet due to the temperature. . In other words, better results can be obtained than with conventional magnets even when a magnet with a zero temperature coefficient is used.However, since the temperature coefficient of a magnet is generally negative, for example, a magnet having a small temperature coefficient such as a rare-earth cobalt-based magnet is used. The use of the compound gives a further improved effect. For example in a conventional rare earth cobalt magnet is used for the optical isolator, the temperature coefficient of the H _M is -0.03~-0.045% / ℃, also the temperature coefficient in general to other magnet negative. After all, when the temperature change of H _{M and} the temperature change of V = θ _fs / H _s cancel each other, the temperature change of the rotation angle becomes the smallest.

In the present invention, since the amount of M satisfies 0.5 ≦ w ≦ 1, the saturation magnetization of the garnet material (Faraday rotator) of the prior application is about 1.5 kG to about 1.1 G, for example. (Corresponding to Hs ₁ and Hs _{2 in} FIG. 2).
Therefore, the size of the magnet constituting the Faraday rotator can be reduced.

Next, the garnet material used in the present invention will be described. The temperature coefficient of V = θ _fs / H _s depends on the composition of the rare earth iron garnet. _{Bi x P y Q 3-xy} Fe
_{In 5-w} M _w O ₁₂ , P (Y, La, Sm, Eu, T
m, Yb, Lu) are known to make the temperature change of V positive, and cancel out the temperature change of the magnet. On the other hand, Q (G
d, Tb, Dy, Ho, Er) are known to make the temperature change of V negative, and by adjusting this, the temperature coefficient of the garnet material can be adjusted so as to offset the temperature change of the magnetic field of the magnet. . When 0.7 ≦ x, the Faraday rotation ability decreases when the Bi substitution amount is small, and when x ≦ 2, the LPE growth becomes difficult when the Bi substitution amount is too large.
≤y≤2.3 has the effect of making the temperature change of V positive,
This is offset by the effect of making the temperature change of V negative by 0 ≦ 3-xy ≦ 1 of Q, and the amount of M is 0.5
When ≦ w ≦ 1, the saturation magnetization decreases.

The saturation magnetization can be reduced by substituting Fe with a nonmagnetic element M. That is, Fe
It has been found that the element M which can be substituted for the d site, that is, the element D can lower the saturation magnetization. On the other hand, a
The element M which can be substituted at the site, that is, the element A has an adverse effect, but it may be present in a small amount in relation to the element at the d site. Therefore, particularly in the composition of the garnet material of the present invention, M
_{When w} is A _a D _d (w = a + d), 0 ≦ a <0.
3. It is preferable that 0.5 ≦ d ≦ 1.0 is satisfied. The element A constituting the nonmagnetic ion which can be substituted for the a-site of garnet includes Sc and In, and the element D which can be substituted for the d-site of garnet includes Ga and Al.

A specific example of the composition is (YLaHo) _1.6 B
i _1.4 Fe _4.5 (Ga, Al) _0.5 O ₁₂ (however, Ho /
It is preferable that Y = 0.25 and La / (Y + La) = 0.1). For example, those containing no Al in this composition have the following characteristics. Saturation magnetization = 0.9 kG Faraday rotation angle θ _f = 1900 deg / cm (130
(0 nm) Temperature characteristic of Verdet constant ~ + 0.03% / ° C Lattice constant = 12.496 In contrast, the one without a nonmagnetic substitution ion at the d site had a saturation magnetization of about 1700G.

If a magnetic field is applied in the direction of the optical axis and the magnetization is less than the saturation magnetization, small magnetic domains having a magnetization opposite to the direction of the magnetic field remain. In order that the Faraday rotation angle does not change depending on the position where the light beam enters the material, the beam diameter must always include approximately the same number of these small magnetic domains. That is, the light beam diameter needs to be sufficiently larger than the size of the magnetic domain. If the direction of magnetization is uniform in a direction parallel to the magnetic field, it is acceptable to have a magnetic domain in which the magnetization is reversed in a direction perpendicular to the magnetic field.

Next, a description will be given of the diffraction loss that occurs when light is incident on a magnetic garnet material to which a magnetic field less than the saturation magnetic field is applied. When linearly polarized light parallel to the magnetization direction is incident, the polarization plane rotates due to the Faraday effect, and the polarization plane of the light is + in a magnetic domain in which the magnetization is in the same direction as the light traveling direction.
Rotate, while the opposite direction of magnetization-rotate. As described above, since the rotation angle of the polarization plane differs depending on the location of the magnetic domain, diffraction occurs. Such a multi-domain structure remains even when a magnetic field slightly smaller than the saturation magnetization is applied, and when light enters the multi-domain structure, the multi-domain structure acts as a diffraction grating to diffract a part of the incident light. I do. As a result, the detection light causes diffraction loss. This problem can be solved by using a plurality of garnet plates magnetized perpendicular to the optical axis direction, separated from each other, or by using a material whose magnetization has an in-plane component, thereby eliminating the temperature change of the Faraday rotation angle and diffracting. Can reduce the loss of the detection light or prevent the diffraction from occurring. These are described in the aforementioned Japanese Patent Application No. 3-94851.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The problem of the present invention is the non-uniform distribution of the Faraday rotation angle depending on the location. Conventionally, the Faraday rotation angle distribution is good even if the magnetic field distribution is bad if there is a magnetic field higher than the saturation magnetic field because the magnetic field is saturated. However, in the present invention, since the magnetic field distribution is not saturated, the magnetic field distribution directly reflects the Faraday rotation angle distribution. Japanese Patent Application No. Hei 2-136988
This point is considered in this issue, and when the magnet is formed in a disk shape having an inner diameter Di, an outer diameter Do, and a length L, and a disk-shaped Faraday rotating element made of garnet material is disposed at the center thereof, the most uniform condition: magnetic field is represented by _{L / D i = (0.4 ~1.0} ) +0.5 D o / D i. Incidentally, D _o is not more than 5 times the D _i so compact if the outer diameter exceeds 5 times the inner diameter becomes difficult.

Preferred specific dimensions are as follows.
Material thickness: 0.5 mm as 45 degree rotator for wavelength 1.3 μm or 1.55 μm due to large Bi substitution
It is as follows. Material dimensions: usually 1.5-5 mm. However, the larger the diameter of the light beam used, the larger it needs to be. Inner diameter of magnet: The size where the material can be placed. Magnet length and outer diameter: Determined by the above formula. Magnetic field strength: The material of the present invention has a saturation magnetization of about 900 G, which is smaller than about 900 Oe to saturate it.

FIG. 3 shows an example in which the Faraday rotator according to the present invention is applied to an isolator. The rare earth iron garnet crystal element 1 is arranged in a uniform magnetic field by the permanent magnet 2.
The magnetic field is generated by magnets of strength, shape and size adjusted to be used in a region where the element 1 does not saturate and at a rotation angle as large as possible. The Faraday rotator of the present invention refers to a combination of the garnet element 1 and the magnet 2. Reference numeral 3 denotes a polarizer for incident light, and 4 denotes a polarizer on the output light side. These polarizers transmit the incident light through a Faraday rotator and completely block light reflected from an optical fiber or the like. It is arranged to be.

Example Material (YLaHo) _1.6 Bi _1.4 Fe by LPE method
_4.5 (Ga, Al) _0.5 O ₁₂ (Ho / Y = 0.2
5, La / (Y + La) = 0.1). The saturation magnetic field is about 900 Oe, and the rotating power of 1.3 μm wavelength is 1900 de.
g / cm, temperature coefficient of θ _FS is −0.13% / ° C., V = θ _fs /
The temperature coefficient of H _S is + 0.03% / ° C. Met. Inner diameter 3
In the center of a rare-earth cobalt magnet (temperature coefficient -0.03% / ° C, central magnetic field 850 Oe) of a hollow cylinder having a diameter of 4.5 mm, an outer diameter of 4.5 mm and a length of 4.2 mm, When the Faraday rotator was arranged, the Faraday rotation angle was 45 ° and the temperature coefficient of the rotation angle was almost 0.

Conventional Example When the above-mentioned material having a size of 2 mm and a thickness of 0.237 mm was placed at the center of a rare earth cobalt magnet (central magnetic field of 1400 Oe) having an inner diameter of 3 mm, an outer diameter of 5.0 mm and a length of 3 mm, the Faraday rotation angle was at room temperature. 45 °, the temperature coefficient was as bad as -0.13% / ° C. Comparative Example According to Japanese Patent Application No. 2-136988 (YLaHo) _1.6 B
i _1.4 Fe ₅ O ₁₂ (however, Ho / Y = 0.25, La /
(Y + La) = 0.1, the saturation magnetization is about 1700 G
In order to obtain the same result as the present invention, the dimensions of the rare earth cobalt magnet are 3 mm in inner diameter, 6.0 mm in outer diameter, and 5.
7mm hollow cylinder rare earth cobalt magnet (temperature coefficient-
0.03% / ° C, and a central magnetic field of 1800 Oe).

[0024]

The effects of the present invention are summarized as follows. (1) The temperature change of the Faraday rotation angle is small. (2) Since a material having a large amount of Bi substituted is used, the Faraday rotation ability of the material is large and the thickness of the material can be reduced, so that the material can be easily manufactured by the LPE method. (3) The saturation magnetization is small, and therefore the magnetic field for magnetizing the garnet may be small, and the size of the magnet can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing characteristics of a Faraday rotator.

FIG. 2 is a graph showing the principle of the present invention.

FIG. 3 shows a diagram in which the present invention is applied to an isolator.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-297083 (JP, A) JP-A-4-324817 (JP, A) JP-A-4-31821 (JP, A) JP-A-4-324 304418 (JP, A) Jikku Sho 61-9376 (JP, Y2) US Patent 4,981,341 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/09-1/095 G02F 1/29-1/313 G02B 27/28 CA (STN) REGISTRY (STN) WPI (DIALOG)

Claims (2)

(57) [Claims] 1. A composition _{Bi x P y Q 3-Xy} Fe 5-w M w
O ₁₂ (where P: Y, La, Sm, Eu, Tm, Yb, Lu
At least one of the following: Q: at least one of Gd, Tb, Dy, Ho, and Er; M: at least one of the elements that can replace Fe, and x, y, and w are 0. 7 ≦ x ≦ 2.0, 0.5 ≦ y ≦ 2.3, 0 ≦ 3-x
The garnet material expressed by satisfying -y ≦ 1,0.5 ≦ w ≦ 1.0) , formed by combining a magnet for applying a smaller magnetic field than the saturation field of the material, the file
Rotation angle at a magnetic field smaller than the saturation magnetic field of the Raday rotator
Has a positive temperature coefficient, and the magnet changes the temperature coefficient
A Faraday rotator set to have a negative temperature coefficient to cancel out . 2. A profile D _o, an inner diameter Di, the length L is satisfy the following relationship _{L / D i = (0.4~1.0)} +0.5 D o / D i D o / D i <5 And a magnet whose magnetization direction is the length direction and a Faraday rotator material made of a ferromagnetic material having a saturation magnetic field larger than the magnetic field applied by the magnet are arranged near the hollow center of the magnet. Faraday rotator according to the item.