JPH0688876B2 - Magneto-optical crystal - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of characteristics of a magneto-optical crystal used for optical communication, optical measurement, optical recording and the like.

2. Description of the Related Art An optical isolator using a garnet crystal is often used to suppress reflected return light that causes noise in a semiconductor laser (hereinafter referred to as LD) used as a light source in the fields of optical communication and optical measurement. In addition, Bi-substituted garnet using the liquid phase epitaxial method has been actively researched recently because of its high crystal productivity.

Generally, garnet containing iron is a ferromagnetic material, and its magnetization changes when temperature changes, so that the Faraday rotation angle determined by the magnetization also changes. The Faraday rotation angle per unit length is called the Faraday rotation coefficient θ _F degcm ^-1 . At this time, the Faraday rotation angle θ deg of the light transmitted through the crystal with the thickness tcm is θ deg.
Is θ = θ _F t. For optical isolators, it is necessary to adjust the crystal thickness so that the absolute value of the Faraday rotation angle | θ | becomes | θ | = 45deg.

However, since θ _F of a garnet crystal changes with temperature, even if the crystal thickness t is adjusted so that | θ | becomes 45 deg at a temperature of 20 ° C., if the temperature changes, | θ |
It changes from g and the isolation ratio deteriorates. 45d of θ
If the temperature change from eg is defined as β deg · ° C ^-1 , the deterioration of the isolation ratio (calculated value) due to the temperature change of θ is as shown in Fig. 2. Ordinary yttrium / iron / garnet (YIG) is | β | 0.04deg ・ ° C ^-1 , and Bi containing garnet is usually | β | 0.06-0.11deg ・ ° C ^-1 .

In recent years, various attempts have been made to reduce this | β | in Bi substitution type garnet.

For example, the 10th Annual Meeting of the Japan Society for Applied Magnetics 4pE-5, Honda et al. Realized that (TbBi) ₃ Fe ₅ O ₁₂ ββ 0.04 deg ・ ° C ^-1 and also 4aE-1, Asahara et al. By (GdBi) ₃ (Fe
It has been attempted to set β0.00deg · ° C ^-1 by superimposing two kinds of crystals of AlGa) ₅ O ₁₂ and (TbYbBi) ₃ Fe ₅ O ₁₂ .

Problems to be Solved by the Invention In the above (TbBi) ₃ Fe ₅ O ₁₂ , Tb atoms play a role of reducing the temperature change of θ, but as the concentration of Tb increases, the concentration of Bi decreases and therefore the Faraday rotation coefficient. Becomes smaller, and the thickness of the crystal becomes thicker in order to obtain | θ | = 45 deg required for the optical isolator. Also in this system β ≤ 0.03
No crystals have been obtained, and β cannot be adjusted.
Further, when the concentration of Tb was increased, there was a problem that a substrate having an appropriate lattice constant was not commercially available when growing a crystal by epitaxial growth.

Next, in the method in which (GdBi) ₃ (FeAlGa) ₅ O ₁₂ and (TbYbBi) ₃ Fe ₅ O ₁₂ are superposed, (GdBi) ₃ (FeAlGa) ₅ O ₁₂ becomes (TbYbBi) ₃ Fe.
It is used as a temperature compensating crystal that reduces the temperature change of the Faraday rotation angle of ₅ O ₁₂ . However, since the temperature compensation effect is small, absorption of light increases as the thickness of the crystal increases, and it takes time to grow each crystal. Also, two crystals are required. Therefore, when actually used as an optical isolator, there is a drawback that the number of steps is increased and the cost is increased because polishing of the crystal and non-reflection coating for preventing light reflection on the crystal surface are performed on the two crystals.

Means for Solving the Problems In order to solve the above problems, the present invention provides (Bi _x Gd _3-x ) (Fe _5-y
In the garnet crystal of Ga _y ) O ₁₂ , 0.7 ≦ x ≦ 1.3,
Further, it is a magneto-optical crystal in which a rare earth garnet containing Bi is epitaxially grown on a crystal having a composition of 1.0 ≦ y ≦ 1.7.

Effect The present invention shows that a crystal can be provided with good productivity and at low cost, in which the temperature change of the Faraday rotation angle of the optical isolator is zero in the operating temperature range, or | θ | increases with increasing temperature.

FIG. 3 shows a principle diagram for improving the temperature characteristic of the Faraday rotation angle θ. The iron atom of the iron site is replaced by another non-magnetic ion (M
Which is slightly substituted with, or is not substituted at all, is negative near θ at room temperature of Bi-substituted garnet,
It approaches zero as the temperature rises. At this time, the temperature change of the Faraday rotation angle becomes positive (β is positive). Next, in Bi-substituted rare earth garnets (rare earth elements Re = Gd, Tb, Dy, etc.) in which the iron sites are largely replaced by nonmagnetic ions M, θ becomes positive and approaches zero (β is negative) as the temperature rises. Therefore, using (BiR) ₃ (FeM) ₅ O ₁₂ with a large negative β in which iron sites are replaced by a large amount of nonmagnetic ions as a temperature compensation crystal, the temperature of θ of Bi-substituted garnet with a positive β is It is possible to adjust for changes.

If the Faraday rotation coefficient of the temperature compensation crystal is θ _F1 , the thickness of the crystal is t ₁ , the Faraday rotation coefficient of the Bi-substituted rare earth garnet is θ ₂ , and its thickness is t ₂ , then | θ | = | θ _{F1 in the} optical isolator _{· t 1 + θ F2 · t} 2 | = 45deg By adjusting the film thicknesses t ₁ and t _{2 so} that the crystal can be obtained, the Faraday rotation angle hardly changes with temperature near the operating temperature. Further, in the case of a module in which the optical isolator and the LD are integrated, the central wavelength of the LD shifts to the long wavelength side as the temperature rises, and | θ | becomes smaller than 45 deg due to the wavelength dependence of θ of the garnet. In this case, θ =
If −45 deg, β <0.00 deg · ° C. ⁻¹ , | θ | becomes larger than 45 deg as the temperature rises. Therefore, it is possible to adjust the decrease of the Faraday rotation angle due to the shift of the LD central wavelength.

Next, in (Bi _x Gd _3-x ) (Fe _5-y Ga _y ) O ₁₂ , 0.7 ≦ x ≦ 1.3, 1.
It is shown that the temperature compensation effect is high when 0 ≦ y ≦ 1.7, and therefore a crystal with a small Faraday rotation angle temperature change and small light absorption can be epitaxially grown with a thin crystal.

In order to be effective as a temperature compensating crystal, the Faraday rotation angle must be positive and the rate of temperature change must be large (has a large negative β) in the operating temperature range (-20 ° to 60 °). is necessary. Since the temperature change of the Faraday rotation angle becomes large near the Curie temperature T _c , it is necessary to set T _c slightly higher than the operating temperature range (about 80 to 200 ° C).

It is also necessary that there is no compensation temperature T _{comp at} which the sign of the Faraday rotation angle is reversed in the operating temperature range of the optical isolator.

Fig. 4 shows that for (Bi ₁ Gd ₂ ) (Fe _5-y Ga _y ) O ₁₂ , the Ga substitution amount y
And the relationship between T _c and T _comp . y = 1.0 to 1.7 and T _c = 190 to 8
It becomes 0 ℃, and the operating temperature range of the optical isolator is -20 to 6
You can see that there is no T _{comp at} 0 ° C.

Since T _c is as low as 80 to 190 ° C., the temperature compensating effect of this crystal is large, and therefore temperature compensation can be performed with a thin crystal, and light absorption can be reduced because the crystal is thin.

In carnet crystals, in order to epitaxially grow a film without cracks or defects, a lattice mismatch with the substrate must be used.
It is necessary to set Δa | ≦ 0.02Å. Currently available substrates have a lattice constant of a = 12.490-12.500Å Ca-Mg-Zr
Substitute GGG substrate or NdGG substrate with a = 12.509Å.

In FIG. 5 _{(Bi x Gd 3-x)} (Fe 5-y Ga y) of the lattice constant of the O ₁₂ x, indicating a y-dependent. 12.477 ≦ when 0.7 ≦ x ≦ 1.3 and 1.0 ≦ y ≦ 1.7
a ≦ 12.517Å, the lattice mismatch with the substrate is 0.013 at maximum.
However, since epitaxial growth is possible without cracks and defects up to a lattice constant difference of about | Δa | <0.02Å with the substrate, an epitaxial film can be obtained as a temperature-compensated crystal without problems.

Furthermore, if a Bi-substitution garnet crystal is epitaxially grown on this temperature-compensating crystal, a single substrate can be grown as compared with the case where the temperature-compensation crystal and the Bi-substitution garnet crystal are separately grown on the substrate. A non-reflective coating of the cut chip and mirror polishing are half of the case of combining two crystals, and it is possible to reduce the number of man-hours when creating an optical isolator and realize a low-cost optical isolator crystal.

Examples Example 1 As a first example of the present invention, (Bi ₁ Gd ₂ ) (Fe _3.5 Ga _1.5 ) O ₁₂
The case of epitaxially growing (Bi _1.2 Yb _0.6 Gd _1.2 ) Fe ₅ O ₁₂ will be described above. Using a PbO-B ₂ O ₃ -Bi ₂ O _{3 based} flux, on a substrate with Ca-Ma-Zr substitution type GGG and lattice constant a = 12.497 Å, (Bi ₁ Gd ₂ ) (Fe _3.5 Ga _1.5 ). O ₁₂ was grown to 10 μm.
Separately, (Bi _1.2 Yb _0.6 Gd _1.2 ) Fe ₅ O ₁₂ was grown to 10 μm,
Each property was measured.

・ (Bi ₁ Gd ₂ ) (Fe _3.5 Ga _1.5 ) O ₁₂ ・ (Bi _1.2 Yb _0.6 Gd _.12 ) Fe ₅ O ₁₂ From this, the film thickness t ₁ of the temperature compensating crystal (Bi ₁ Gd ₂ ) (Fe _3.5 Ga _1.5 ) O ₁₂ is t ₁ = 167 μm, and (Bi _1.2 Yb _0.6 Gd _1.2 ) Fe ₅ O ₁₂ is t ₂ t ₂ = 303 μm, the rotation angle: θ = θ _F1 · t ₁ + θ _F2 · t ₂ = −45 deg Becomes Also, these lattice constants are the same, and | Δa | <
Since the condition of 0.02Å is satisfied, epitaxial growth is possible. Therefore, on top of Ca-Mg-Zr-substituted GGG, (Bi ₁ Gd ₂ ) (Fe
_3.5 Ga _1.5 ) O ₁₂ was grown to 170 μm, and (Bi _1.2 Yb _0.6 Gd
Fig. 1 shows the results of measuring the temperature change of the Faraday rotation angle after growing _1.2 ) Fe ₅ O ₁₂ 305 μm. (Bi _1.2 Yb _0.6 G
d _1.2 ) Fe ₅ O ₁₂ has β = 0.06 deg ・ ° C ^-1 , while β0.00 when grown as a temperature compensating crystal (T = -20 to 60 ° C).
A crystal with a deg. ° C. ^-1 was obtained.

Example 2 (Bi ₁ Gd ₂ ) (Fe _3.5 Ga _1.5 ) O ₁₂ 230 μm and (Bi _1.2 Yb _0.6 Gd) on a Ca-Mg-Zr-substituted GGG substrate from the flux under the same conditions as in Example 1
_1.2 ) As a result of growing Fe ₅ O ₁₂ by 325 μm, rotation angle: θ = −45 deg (T = 20 ° C.) Got At this time, | θ | was 45.8 ° at 60 ° C, and a crystal was obtained in which the absolute value of the Faraday rotation angle increased with increasing temperature.

As a second example of the third embodiment the present _{invention, (Bi 1.1 Gd 1.9) (} Fe 3.7 Ga 1.3) O 12 on a _{(Bi 0.9 Gd 2.1) Fe 5} O 12
The case of growing is described. The characteristics of each crystal are: (Bi _1.2 Gd _1.8 ) (Fe _3.7 Ga _1.3 ) O ₁₂ ・ (Bi _0.9 Gd _2.1 ) Fe ₅ O ₁₂ Met. Therefore, using NdGG as a substrate, (Bi _1.2 Gd _1.8 ) (Fe _3.7 Ga _1.3 ) O ₁₂ was grown 215 μm and (Bi _0.9 Gd _2.1 ) Fe ₅ O ₁₂ was grown 430 μm.
θ = -45deg, β = 0.00deg ・ ° C ^-1 in temperature range -20 to 60 ° C
Realized about.

As an example, as a Bi substitution type garnet (Bi _1.2
Yb _0.6 Gd _1.2 ) Fe ₅ O ₁₂ , (Bi _0.9 Gd _2.1 ) Fe ₅ O ₁₂ is shown, but garnet containing Bi may have other composition, for example, (BiTb) ₃ Fe ₅ O ₁₂ ,. (BiGdLu) ₃ Fe ₅ O ₁₂ and (BiGdYb) ₃ (Fe _4.8 Ga _0.2 ) O ₁₂ containing a small amount of Ga may be used.

EFFECTS OF THE INVENTION As described above, according to the present invention, with one magneto-optical crystal, there is no temperature change in the Faraday rotation angle, or a crystal in which the absolute value of the Faraday rotation angle increases as the temperature rises. It can be provided with high productivity and at low cost, and its industrial value is high.

[Brief description of drawings]

FIG. 1 is a temperature characteristic diagram of the Faraday rotation angle θ of the magneto-optical crystal according to the present invention, FIG. 2 is a characteristic diagram showing deterioration of the isolation ratio due to temperature change of θ, and FIG. 3 is for eliminating temperature change of θ. 4 is a characteristic diagram for explaining the principle of the above, FIG. 4 is a characteristic diagram showing the Ga substitution amount dependency of Curie temperature and compensation temperature, and FIG. 5 is a characteristic diagram showing the relationship between Bi substitution amount, Ga substitution amount and lattice constant. Is.

Claims (1)

[Claims] 1. A _{(Bi x Gd 3-x)} (Fe 5-y Ga y) O 12 having a composition formula, a 0.7 ≦ x ≦ 1.3 and 1.0 ≦ y ≦ 1.7 becomes first garnet crystal, including Bi A second garnet crystal is formed by epitaxially growing a rare earth garnet crystal, and the relationship between the Faraday rotation coefficient and the thickness of the first garnet crystal and the second garnet crystal is | θ _F1 · t ₁ + θ _F2 · t ₂ | = 45deg (Where, θ _F1 and θ _F2 are Faraday rotation coefficients of the first and second garnet crystals, and t ₁ and t ₂ are thicknesses of the first and second garnet crystals). .