JPH04142009A - Magnetic garnet crystal - Google Patents

Abstract

PURPOSE:To eliminate various kinds of problems associated with Bi substitution and obtain a collectively excellent magneto-optical garnet crystal by forming the garnet crystal to have a chemical composition expressed by Lu3-(x+y+z) YxTbyBizFe5O12. CONSTITUTION:This magnetic garnet crystal has a chemical composition expressed by Lu3-(x+y+z)YxTbyBizFe5O12 (where, the x, y, and z respectively represents number of 0.1-1.0, 0.1-1.8, and 0.5-2.0). When the garnet crystal is formed in such way, addition of Y becomes effective without giving any adverse influence to the magneto-optical characteristic of the crystal by using Bi as a substitutable element in a large amount. Therefore, large-amount substitution of Bi, improvement in temperature characteristic of the Farady rotation coefficient, reduction in spontaneous nuclear defects, etc., can be achieved collectively.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides an optical isolator using the Faraday effect;
This invention relates to bismuth (Bi)-substituted magnetic garnet crystals used in magneto-optical elements such as optical circulators and optical switches.

[Prior Art] In an optical transmission circuit that uses a semiconductor laser (LD) as a light source, if reflected return light from optical components such as connectors and switches, or light receiving elements enters the LD, laser oscillation becomes unstable and transmission quality deteriorates. known to deteriorate. As a countermeasure to this problem, an optical isolator that blocks reflected light returning to the LD has been proposed, and is currently being put into practical use.

Optical isolators make good use of the non-reciprocity of the Faraday effect, which is a magneto-optical effect that rotates the plane of linear polarization of transmitted light.

A Faraday rotator material that exhibits the Faraday effect and is used in the near-infrared region with a wavelength of 1.3 to 1.55 μm is a magneto-optic garnet crystal. Among these, a yttrium-iron-garnet (YIG) crystal was first used as a Faraday rotator in an optical isolator.

In recent years, the miniaturization and cost reduction of optical isolators have rapidly progressed, and along with this, there has been a strong desire for miniaturization of elements and reduction in manufacturing costs.

In order to downsize optical isolators, it is essential to downsize the Faraday rotator, which is the main component.

By the way, when used as a Faraday rotator, it needs to be long enough to rotate the incident polarization plane by 45 degrees.
This length is proportional to the magnitude of the Faraday rotation coefficient. Therefore, in order to further downsize the Faraday rotator, it is necessary to select a material with a large Faraday rotation coefficient.

Garnet containing Bi as a solid solution is generally known as a material with a large Faraday rotation coefficient.

This material can be manufactured by a CVD method, a sputtering method, a flux method, a liquid phase epitaxial (LPE) method, or the like. However, the CVD method and the sputtering method mainly have problems with crystallinity, and it is still difficult to put them into practical use.

In the flux method, the garnet components yttrium oxide and iron oxide as well as the flux components lead oxide and boron oxide are melted, the melt is maintained above the liquidus temperature to make it uniform, and then the melt is slowly cooled. This allows garnet crystals to grow by natural nucleation. In this method, some drawbacks were observed, such as concentration variations due to the segregation of garnet components, and cracks in the crystals as the flux solidified, etc., or these were improved by the improved flux method.

However, since natural nuclei are grown, crystal growth requires a long time, precision processing is required to form devices, and mass productivity is poor.

Under these circumstances, the LPE method, which was developed as a method for growing garnet thin films for magnetic field devices, has attracted attention, and various studies have been conducted to develop Bi-substituted magnetic garnet thick films using this method.

Well, Th1n 5olid Films, 114 (1
As shown in 984)69, the Faraday rotation coefficient increases in proportion to the amount of Bi substitution. However, since the solid solubility of Bi in garnet crystals is extremely small,
It is also known that replacement is very difficult.

On the other hand, Materials Science Journal Vol.123. No3 (1987) 15
As stated in 1, YIG's rare earth site is
1. Gd substituted with Gd element etc. 2. lB io, 9F
e4.6A10. lGa0.3012 (G B I G
) has a large temperature coefficient of Faraday rotation coefficient. For example, the temperature coefficient of YIG is about 0.04deg/'C, but the temperature coefficient of the above material is 0.10deg/'C, which is about 2
That's more than double that.

When this material is used for a Faraday rotator of an optical isolator, the isolation (Es) of the optical isolator deteriorates. Es is expressed by the following formula.

Es=-10・log-sin2(Δθ) where Δθ(
deg) is the deviation of θ from 45 degrees. If changes in the oscillation wavelength of the semiconductor laser are not considered, YIG can obtain an Es of 36 dB in the temperature range of 0 to 50°C, but
The above material drops to 29dB. Optical isolator Es
is desired to be 30 dB or more, but in order to ensure this Es, a temperature coefficient of 0.07 deg/''C or less is required.

As mentioned above, miniaturization of the Faraday rotator is essential for miniaturization of optical isolators. In order to achieve this, it is important to replace a large amount of Bi to dramatically increase the Faraday rotation coefficient, but on the other hand, it also has the drawback of increasing the temperature coefficient of the Faraday rotation coefficient as described above.

Various studies have been made to resolve these two contradictory propositions.

[Problems to be Solved by the Invention] In order to replace a large amount of Bi, it is necessary to increase the solid solubility of B1. In order to achieve this, (a) searching for an optimal component system, (b) optimizing LPE conditions, etc. have been proposed. (B) is (GdBi)3(
FeAIGa) 5012 is a typical example, but it has the drawback of deteriorating the temperature characteristics of the Faraday rotation coefficient. (B) mainly involves increasing the degree of supercooling (T: saturation temperature - film growth temperature) and lowering the film growth temperature (TG). However, eight T
As the number increases, natural nuclei are more likely to be generated. This natural nucleus serves as a seed crystal in the flux method, but in the LPE method, it is an undesirable product that contributes to membrane defects. Also,
As ΔT increases and the temperature of TG decreases, Pb, which is a flux component in the melt, and Pt, which is a material of the crucible and substrate fixing jig, become easily mixed into the film.

When pb and pt are mixed into the film, Journal of the Japan Society of Applied Magnetics, 10, 2. (1986) 161, the light absorption increases. Also, Pt is J, Magn, S
oc, J pn, 11. As is known from SI (1987) 347, it contributes to film defects, and defects are known to cause deterioration of optical properties, as stated in Journal of the Japan Society of Applied Magnetics, 10.2 (1986) 147. ing.

Therefore, 8T is preferably about 30°C or lower, and TG is preferably about 730°C or higher.

By the way, it is known that if the lattice constants of the film and substrate are mismatched, it can cause film cracking or substrate cracking.
When Bi ions with a large ionic radius are replaced, the lattice constant of the film increases. Therefore, in order to match the lattice constant of the film, it is necessary to select a substrate with a larger lattice constant, but the types of garnet substrates that can be used industrially are limited. Therefore, rare earth sites or Fe
It is common practice to match the lattice constant of the film to the substrate by substituting sites with various elements and strictly adjusting the amount of substitution, but as mentioned above, the adverse effects of substitution pose problems. was.

The present invention solves various problems associated with Bi substitution and provides an overall excellent magneto-optic garnet crystal.

[Means for solving problems] In order to solve the above problems, the present invention provides the following general formula: %formula% (In the formula, X is 0.1 to 1.0, and y is 0.1 to 1.

8, Z is 0. The present invention provides a magnetic garnet crystal characterized by having a composition represented by a number of 5 to 2.0.

The purpose of the present invention is to comprehensively achieve (a) a large amount of Bi substitution, (b) an improvement in the temperature characteristics of the Faraday rotation coefficient, and (c) a reduction in natural nuclear defects.

The basic idea of the present invention is shown below. As mentioned above, Gd is known as a host rare earth element that allows a large amount of Bi to be substituted, but it has the drawback of deteriorating the temperature characteristics of the Faraday rotation coefficient. Therefore, as a result of various studies, the present inventors determined that B1 can be used in large amounts without adversely affecting the magneto-optical properties.
It has been found that addition of Y is effective as an element that can replace .

If the value is less than 0.1, no effect will be shown, and if it is greater than 1°0, the amount of substitution of other elements to be substituted into the rare earth site will be reduced, which is not preferable. Preferably,
It should be between 0.2 and 0.9.

The main cause of the deterioration of the temperature characteristics of the Faraday rotation coefficient due to B1 substitution is said to be a change in the sign of the Faraday rotation coefficient. That is, although the Faraday rotation coefficient increases dramatically by B1 substitution, at the same time its sign becomes negative and its value decreases as the temperature rises. Therefore, in order to improve the temperature characteristics of the Faraday rotation coefficient, methods have been proposed such as forming a solid solution of crystals with positive and negative Faraday rotation coefficients, bonding two plates together, or growing two layers. It was done. However, with methods other than solid solution,
It has been pointed out that this method suffers from the disadvantage that the insertion loss of incident light becomes large and is therefore impractical.

In the present invention, we focused on the solid solution method and investigated various elements, and as a result, we found that Tb, which has a Faraday rotation coefficient with a positive sign, is promising.

If the value is less than O,1, no improvement effect is seen;
If it is larger than 8, the amount of B1 substitution decreases, which is not preferable. Desirably, it is in the range of 0.2 to 1.5. B1 is not preferable because if it is less than 0.5, a sufficient Faraday rotation coefficient cannot be obtained and the required film thickness becomes large.

On the other hand, if it is larger than 2.0, ΔT becomes 30° C. or more, which is not preferable because the precipitation of natural nuclei becomes intense and film defects increase. Desirably, it should be between 0.8 and 1.6.

By the way, there are only a few types of industrially usable garnet substrates for LPE, and they are usually Gd3Ga5O12 (GGG
) and Ca-Mg-Zr substituted GGG substrates dominate the market, with Sm3Ga5O12 and Nd3Ga5012
The board is only used in some parts. therefore,
It is common practice to control the component composition of the film to match the lattice constant of the substrate.

As mentioned above, the lattice constant of the film increases as the amount of Bi substitution increases, so reducing the lattice constant of the base material in which Bi is to be substituted is the basis for replacing a large amount of Bi.

Th1n 5olid Films, 114 (1984
) 33, the lattice constant of Lu3Fe5012 is the smallest among rare earth iron garnets. Therefore, in the present invention, Lu is positioned to play a role in balancing the rare earth sites, and the amount of Lu added is determined according to the amount of Bi substitution. Therefore, we define its definition as Lu3− (x y
10z).

[Examples] The present invention will be described in detail below using Examples. Note that the mismatch between the lattice constants of the substrate and the film was measured using an X-ray diffractometer.

Example <Example 1> Using the melt composition shown in Example 1 in Table 1, Ca-M
Growth temperature 76 on g-Zr substituted Gd3Ga5O12 substrate
Lu0.3Tbo grown by LPE at 2℃, ΔT 21℃
, 7Y0.7Bi1.3Fe5012 composition has a Faraday rotation coefficient of 1480 deg/cm at the working wavelength of 1.55 μm and a temperature coefficient of 0.
05 de g/''C. Furthermore, no natural nuclei were generated during the growth, and no membrane defects due to the influence of natural nuclei were observed.

The Faraday rotation coefficient and temperature characteristics of GBIG are 11020 de/cm and 0.10 deg/”C, respectively.
Therefore, the garnet film of the present invention is superior in all properties.

Note that the Faraday rotation coefficient was determined by dividing the angular difference between the incident polarized light and the output polarized light by the film thickness. Temperature characteristics range from 0"C to 50
The deviation of the Faraday rotation angle θ between zero and It was confirmed using

(Hereinafter, blank space) Table 1 Unit: Mol% Component Example 1 Example 2 Example 3 Example 4F e
aoj 12.55 15.25 13.
22 9.58Y20. 0.12 0.04
0.08 0.08L u 203 0.04
0.03 0.06 0.05T b40?
0.07 0.16 0.04 0.02Bi2
0s 27.19 25.85 31.45 3
3.32B, 0. 6.90 6.42 6.
16 6.99PbO53,1352,2548,99
49,96 <Example 2> Using the melt composition shown in Example 2 in Table 1, Ca-M
LuO grown by LPE on a g-'1rflt converted Gd3Ga5O12 substrate at a growth temperature of 790°C and ΔTl1°C,
3T bl, 6Y0.2B i 0.9F e5012
The composition of the garnet film has a Faraday rotation coefficient of 11040 de/cm at a wavelength of 1.55 μm.

It showed a temperature coefficient of 0.04 deg/°C from 0 to 50°C. Furthermore, no natural nuclei were generated during growth, and no membrane defects due to the influence of natural nuclei were observed.

<Example 3> Using the melt composition shown in Example 3 in Table 1, Ca-M
Growth temperature 74 on g-Zr substituted Gd3Ga5O12 substrate
0°C, ΔT28. Luo grown by LPE at 5°C
, 4T bo, 5Y0.5B i 1.6F e501
The 2-composition garnet film has a Faraday rotation coefficient of 1880 deg/am and 0 to 50 at a wavelength of 1.55 μm.
It showed a temperature coefficient of 0.07 deg/'C. Furthermore, no natural nuclei were generated during growth, and no membrane defects due to the influence of natural nuclei were observed.

<Example 4> Using the melt composition shown in Example 4 in Table 1, Ca-M
Growth temperature 71 on g-Zr substituted Gd3Ga5O12 substrate
LPE grown at 8℃, ΔT 45℃, 3T
bo, I Yo, 5B i 2. The garnet film having the I Fe5012 composition exhibited a Faraday rotation coefficient of 2470 deg/cm at the used wavelength of 1.55 μm. However, natural nuclei precipitated in the melt, and many membrane defects occurred with these natural nuclei as nuclei. As a result, crystallinity deteriorated, making it impossible to measure optical properties.

[Effects of the Invention and Idea] Since the garnet crystal having the composition of the present invention has a large Faraday rotation coefficient, it is possible to miniaturize the Faraday rotator. Furthermore, since the temperature coefficient of the Faraday rotation angle is small, the isolation of the optical isolator is less likely to deteriorate. Furthermore,
Growth yield is greatly improved by reducing natural nuclear defects.

From the above, the practical value of the present invention is extremely large.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the amount of Bi and the amount of Y in the film.
The figure shows the relationship between the amount of Tb in the film and the temperature coefficient of the Faraday rotation angle. Figure 1 Y content x 10-'atoms/f, u Figure 2 Tb Content 0 'atoms/f, u

Claims (1)

[Claims] General formula; Lu_3_-_(_X_+_Y_+_Z_)Y_xTb
_YBi_zFe_5O_1_2 (in the formula, x is 0.1 to
1.0, y is a number from 0.1 to 1.8, and z is a number from 0.5 to 2.0).