JPH10273397A - Faraday element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a faraday element having high isolation characteristics by combining a Bi-substituted rare earth iron garnet single crystal film not containing Co which is formed into a film by a liquid phase epitacial method with a rare earth iron garnet single crystal film containing Co which is formed into a film by a liquid phase epitaxial method under specific composition conditions. SOLUTION: This faraday element is obtained by combining the following films A and B which are formed into films by liquid phase epitaxial method. [A]: a film of magnetic garnet single crystal of formula I [R is a rare earth element containing Y; Ma is a trivalent positive element not containing Co; 0.6<=x<=1.9; 0<=y<=0.5]. [B]: a film of magnetic garnet single crystal of formula II [R' is a rare earth element containing Y; Mb is a trivalent positive element; Mc is a tetravalent positive element; 0<=k<=0.3; 0<=1<=0.5; 0<=m<=0.23; 0.02<=n<=0.28]. The films A and B are superposed so as to provide such thickness as to have 45 deg. synthetic faraday rotation angle and the films A is combined with the film B so that dependence of each Faraday rotation angle of the films A and B to wavelength counteracts to each other and minimum value Kmin of isolation is 1550 nm wavelength band is >=35 dB.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Faraday element formed of a magnetic garnet single crystal formed by a liquid phase epitaxial (hereinafter abbreviated as "LPE") method and used in a 1550 nm band. The A film composed of Bi (bismuth) -substituted rare earth iron garnet single crystal not containing Co (cobalt) and the B film composed of rare earth iron garnet single crystal containing Co are overlapped, and the wavelength dependence of the Faraday rotation coefficient of both films is superposed. The present invention relates to a Faraday element having a wide band using a difference in gender.

[0002]

2. Description of the Related Art In recent years, the wavelength used in optical fiber communication has become mainstream in the 1550 nm band with the practical use of erbium-doped fiber amplifiers (EDFA). As a magnetic garnet single crystal used in this wavelength band, Bi-substituted rare earth iron garnet, for example, Tb _1.85 Bi _1.15 Fe _4.75 A
An LPE film having a composition of l _0.25 O ₁₂ is known. Here, the reason why the LPE film is adopted is that the LPE method is excellent in mass productivity. One of the criteria for evaluating the characteristics of such a magnetic garnet single crystal is a Faraday rotation coefficient θ.
_F (deg / cm). The larger the absolute value of the Faraday rotation coefficient is, the more the film thickness for obtaining the required Faraday rotation angle can be reduced, so that the Faraday rotation coefficient is preferable because it is easy to manufacture. Bi substitution is performed because the Faraday rotation coefficient can be increased thereby.

Recently, a wavelength multiplexing transmission system has been expected to realize large-capacity optical communication in the 1550 nm band. To perform wavelength multiplex transmission, a broadband optical isolator for a wavelength of 1550 nm is required. A characteristic required as a Faraday element for this broadband optical isolator is that the rate of change in wavelength of the Faraday rotation coefficient is small. Further, it is desired that the magnetic garnet single crystal can be formed by the LPE method from the viewpoint of mass productivity as described above.

[0004]

Generally, the Faraday rotation angle of a magnetic garnet single crystal film changes depending on the wavelength. Thus, for a particular wavelength of the light source, the polarization plane is just 4
Even if the Faraday element is set to rotate by 5 degrees, when the wavelength of the light source changes, the rotation angle of the polarization plane deviates from 45 degrees. As a result, the degree of erasure of the light incident in the opposite direction decreases, so that the isolation of the optical isolator deteriorates.

[0005] In recent wavelength division multiplexing transmission, which is expected for large-capacity optical communication in the 1550 nm band as described above, experiments are performed in a wavelength range of 40 nm or less centered on a wavelength of 1550 nm. This is because the wavelength range that can be amplified by the erbium-doped fiber amplifier is about 1530 nm to 1570 nm. Therefore, an optical isolator used in such a wavelength division multiplex transmission system needs to exhibit good isolation in this wide wavelength band. However, for example, the magnetic garnet single crystal having the above-mentioned composition has a problem that the wavelength dependence of the Faraday rotation coefficient is considerably large, and is not suitable for such a broadband optical isolator.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Faraday element having a small wavelength dependence of a Faraday rotation angle in a wide band of a wavelength of 1550 nm and exhibiting high isolation characteristics.

[0007]

Means for Solving the Problems The present invention includes a A film composition formula was deposited is made of a magnetic garnet single crystal represented by _{R 3-x Bi x Fe 5} -y Ma y O 12 by LPE method, also composition formula was formed by LPE method R _'3-k Bi k Fe
The _{5-lmn Mb l Mc m Co} n O 12 B film of a magnetic garnet single crystal represented by the superimposed thicknesses their synthesis Faraday rotation angle is 45 degrees, and the wavelength of the Faraday rotation angle A film depends This is a Faraday element used in the 1550 nm band, which is combined such that the minimum value K _min of the isolation in the 1550 nm band is 35 dB or more by canceling out the wavelength dependency of the Faraday rotation angle of the B film with the property. Here, R and R 'are rare earth elements containing yttrium, Ma is a trivalent positive element not containing cobalt, Mb is a trivalent positive element, Mc is a tetravalent positive element, and 0.6 ≦ x ≦ 1.9. 0 ≦ y ≦ 0.5 0 ≦ k ≦ 0.3 0 ≦ l ≦ 0.50 ≦ m ≦ 0.23 0.02 ≦ n ≦ 0.28

The present inventor has made trial productions of LPE magnetic garnet single crystal films of various compositions and has repeated basic experiments for measuring various properties thereof. As a result, Bi containing no Co
It has been found that the wavelength dependence of the Faraday rotation coefficient at a wavelength of 1550 nm is reversed between the substituted rare earth iron garnet single crystal and the rare earth iron garnet single crystal containing Co. As an index for evaluating the wavelength dependence, a wavelength change rate FWC (% / nm) of the Faraday rotation coefficient at a wavelength of 1550 nm defined by the following equation was used. FWC = (θF _(1570nm) -θF _(1550nm) ) / | θ
_{F (1550 nm)} | / 20 × 100 where θ _{F (1550 nm)} and θ _{F (1570 nm)} are each 155
Faraday rotation coefficients at 0 nm and 1570 nm.
An example of the results of the above basic experiment is shown in Table 1 and the Bi substitution amount and the wavelength change rate FW of the Faraday rotation coefficient for each sample.
The relationship with C is shown in FIG.

[0009]

[Table 1]

As can be seen from Table 1, the wavelength change rate FWC of the Faraday rotation coefficients of Co and Samples 11 and 12 is shown.
Is a minus sign, but the wavelength change rate FW of the Faraday rotation coefficient of the sample that does not contain other Co.
C has all positive signs and 0.12 to 0.15%
/ Nm is almost constant irrespective of the type of rare earth element and the Bi substitution amount (see FIG. 1). From these results, the Bi-substituted rare-earth iron garnet single crystal not containing Co and the rare-earth iron garnet single crystal containing Co have the opposite sign of the wavelength dependence of the Faraday rotation coefficient. By combining them, there is a possibility that the wavelength dependence of the Faraday rotation coefficient can be reduced as compared with the case where each magnetic garnet single crystal is used alone.

As an example, Bi in which Co is not substituted
The Faraday rotation coefficient θ of the A film of the substituted rare earth iron garnet single crystal (film number A-4 in the example described later) and the B film of the Co substituted rare earth iron garnet single crystal (film number B-4 in the example described later) FIG. 2 shows the wavelength dependence of _F. Wavelength change rate FW of Faraday rotation coefficient of A film not substituted with Co
C has a plus sign. On the other hand, the Co-substituted B film has a thickness of 1 due to the Faraday rotation caused by Co.
550nm wavelength change rate of Faraday rotation coefficient FWC
Has a minus sign. This is a phenomenon found only in rare earth iron garnet containing Co.

By the way, the isolation in the optical isolator is the ratio of the insertion loss in the forward direction to the insertion loss in the reverse direction.
In the present invention, in order to evaluate the performance of the Faraday element,
The following evaluation method was adopted. This is because a Faraday element having a synthetic Faraday rotation angle of 45 degrees at a wavelength of 1550 nm is manufactured by combining an A film and a B film having different Faraday rotation coefficient wavelength change rates FWC, and an optical isolator is manufactured using the Faraday element. 50 dB for polarizer and analyzer
A rutile single crystal having the above extinction ratio is used. Then, the isolation in the wavelength region of 1530 nm to 1570 nm is measured, and the minimum value K of the isolation at this time is measured.
_min (dB) is determined and a performance comparison is performed. "
This is the evaluation method.

The Bi-substituted rare earth iron garnet single crystal is
As the i-substitution amount increases, the absolute value of the Faraday rotation coefficient increases (however, the sign is minus), and the film thickness required for obtaining a 45-degree Faraday rotation angle can be reduced. And
As described above, the Bi-substituted rare earth iron garnet single crystal having no Co can be used regardless of the type of the rare earth element and the amount of Bi.
The wavelength change rate FWC of the Faraday rotation coefficient near 50 nm has a plus sign and takes a substantially constant value (however, the sign of the Faraday rotation coefficient is minus). On the other hand, the wavelength change rate FWC of the Faraday rotation coefficient near 1550 nm of the Co-substituted rare earth iron garnet single crystal has a minus sign,
The value changes depending on the valence (divalent or trivalent) of Co and the amount of substitution (however, the sign of the Faraday rotation coefficient is plus). Therefore, 1550 is adjusted by adjusting the thickness of both films.
By setting the synthetic Faraday rotation angle in nm to 45 degrees and combining films having the respective properties, the wavelength change rate FWC of the Faraday rotation coefficient near 1550 nm can be offset.
In other words, in the present invention, the Faraday rotation angle is gained by the Bi-substituted A film, and the wavelength change rate FWC of the Faraday rotation coefficient is adjusted by the B film containing Co. The Faraday rotation angle of the film A is-(45 + α).
Degree (actually about -50 degrees), and the thickness of the B film is such that the Faraday rotation angle is + α degrees (actually about +5 degrees).
Set so that If the α degree is too small, the film thickness will be too thin to make it difficult to produce, or the effect of canceling the wavelength change rate of the Faraday rotation coefficient will be poor. Since it becomes difficult, it is preferable to set the angle to about 5 degrees as described above. Then, the wavelength dependence of the Faraday rotation angle is canceled as much as possible between the film A and the film B. Suitable conditions can be determined by adjusting the thickness ratio and selecting the composition of the film.

In the present invention, a Co-containing Bi-substituted rare earth iron garnet single crystal (A film) and a Co-containing rare earth iron garnet single crystal (B film) are each separately LP-coated.
It is preferable to stack them after they are formed by E growth.
This is because the separate LPE growth has less restrictions on the substrate material and the film composition. In this case, for example, the A film is not necessarily a single film, and a plurality of films may be stacked. Alternatively, a configuration may be employed in which either the A film or the B film is first grown on the nonmagnetic garnet substrate by LPE, and the other film remaining thereon is grown by LPE. Usually, the non-magnetic garnet substrate used when forming the LPE film is removed by polishing in order to reduce the insertion loss in the forward direction. Others
The configuration may be such that the A film is LPE-grown on one surface of the non-magnetic garnet substrate and the B film is LPE-grown on the other surface of the substrate. However, this configuration cannot remove the substrate,
This is not preferable because the thickness increases and the insertion loss increases.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As R in the A film, one or more rare earth elements selected from Y (yttrium), Tb (terbium), Gd (gadolinium) and La (lanthanum) are preferably used. . Bi amount x in A film
Is set to 0.6 ≦ x ≦ 1.9. This is because the Faraday rotation coefficient increases with an increase in the Bi substitution amount x, and if it is too small, the film thickness becomes extremely large in order to obtain a desired Faraday rotation angle. In particular, it is generally difficult to grow the film to a thickness of 600 μm or more by the LPE method.
Considering that it can be composed of a single film, the Bi amount x is
It is preferable that 1.15 ≦ x. Conversely, if the Bi amount x is too large, cracks occur and growth is impossible. Although Ma may not be contained in the A film, a part of the Fe site may be changed to Al (aluminum), In (indium), or Ga.
Substitution with one or more members selected from (gallium) is preferable because the saturation magnetization of the LPE film becomes small and the magnet can be made small when forming an optical isolator. However, if the substitution amount y is too large, the Faraday rotation coefficient decreases, which is not preferable.

R 'in the B film is preferably one or more selected from Y, Tb and Gd, and a very small portion thereof may be substituted with Bi. Mb and Mc may not be contained in the B film, but Mb is, for example, one or two selected from Al, In, and Ga.
Or more, as the Mc, for example, Ge (germanium), Z
It is preferable to use one or more selected from r (zirconium), Sn (tin), and Si (silicon). Co
The reason why the content n is set to 0.02 ≦ n ≦ 0.28 is that the effect of Co does not appear unless a certain amount or more is contained, and if the content n is too large, the minimum isolation in the 1550 nm band is minimized. This is because the value K _min does not increase. Note that M
Substitution with b is mainly performed to match the lattice constant with the substrate at the time of film formation by the LPE method, and substitution with Mc is performed to control the valence of Co. The respective contents are preferably 0 ≦ l ≦ 0.5 and 0 ≦ m ≦ 0.23.

[0017]

EXAMPLE A Bi-substituted rare earth iron garnet single crystal film (A film) was grown on a nonmagnetic garnet substrate by the LPE method. The flux was used _{Bi 2 O 3 -B 2 O 3} -PbO. Grown samples (four kinds of film numbers A-1 to A-4)
Are shown in the upper half of Table 2. The sample with film number A-5 was cracked and could not be grown.

As shown in Table 2, as the Bi content increases, the absolute value of the Faraday rotation coefficient increases. However,
If the Bi content x is too large (x> 1.9), cracks will occur due to the difference in thermal expansion with the substrate, making it impossible to grow a single crystal. In an optical isolator using these films, K _min is about 33 dB at best. The film composition of A-1 is Bi
Since the amount is small, the Faraday rotation coefficient is small, and accordingly, it is necessary to increase the film thickness or to configure a plurality of films.
However, since it is difficult to increase the film thickness by the LPE method, it is desirable to set the Bi amount x to 1.15 or more if possible.

Similarly, on a non-magnetic garnet substrate,
A rare earth iron garnet single crystal (B film) containing Co was grown by the LPE method. The flux is B ₂ O ₃ -PbO
(However, only the flux of Bi- ₂ O ₃ −
With B ₂ O ₃ -PbO). The lower half of Table 2 shows the compositions, magneto-optical properties, and substrates of the grown samples (13 types from B-1 to B-13). B-1 to B-6 are Co ³⁺
The substituted films, B-7 to B-11, are considered to be Co ²⁺ substituted films, and B-12 and B-13 are considered to be Co ³⁺ and Co ²⁺ simultaneous substituted films.

As shown in Table 2, the Co-substituted rare earth iron garnet film has a positive Faraday rotation coefficient within the range of the Co substitution amount shown in Table 2. K _min is at best 24d
B, which is much worse than the Bi-substituted film. The sign of the wavelength change rate FWC of the Faraday rotation coefficient is minus.
The wavelength change rate FWC of the Faraday rotation coefficient is -0.14% / nm in B-7 where the amount of Co substitution is 0.02 / fu, and its absolute value is almost the same as that of the A film. Exceeds 0.05 / fu, the absolute value of the wavelength change rate FWC of the Faraday rotation coefficient increases, and is significantly deteriorated as compared with that of the conventional Bi-substituted rare-earth iron garnet that is not Co-substituted.

[0021]

[Table 2]

Therefore, a 45 ° Faraday element is manufactured by combining the various A films and B films grown as described above, and the minimum value K _min of the isolation when an optical isolator is assembled using the Faraday device is measured. did. Table 3 shows an example of the experimental results.

[0023]

[Table 3]

In the experimental example, the A-1 film has a two-layer structure because a single film has a small Faraday rotation angle. In each of the experimental examples, the same film thickness is used for A-1 to A-4, and the synthetic Faraday rotation angle is adjusted to 45 degrees by changing the film thickness of B-1 to B-13. I have. Therefore, each combination of the A film and the B film does not always have the optimum thickness ratio. In the combinations marked with *, the minimum isolation K _min is 35 dB
In particular, the combination marked with **
_{A min} of 42 to 44 dB was obtained, which was improved by about 10 dB or more compared to the case of using the A film alone.

The reason is that 4 is currently used at 1550 nm.
The fifth-degree Faraday element is composed of Tb _1.85 Bi _1.15 Fe _4.75 Al
It is an LPE film of _0.25 O ₁₂ , and the minimum value K _min of the isolation of the optical isolator using the LPE film is 33 dB.

[0026]

As described above, the present invention provides an A film made of a Bi-substituted rare earth iron garnet single crystal containing no Co and a Co film.
Is a Faraday element composed of a rare earth iron garnet single crystal containing B film so that the synthetic Faraday rotation angle becomes 45 degrees and the wavelength dependence of the Faraday rotation angle of both films is canceled as much as possible. The minimum value K _min of the isolation in the band can be made 35 dB or more, and it is possible to realize a wideband optical isolator that can support wavelength multiplex transmission in the 1550 nm band.

[Brief description of the drawings]

FIG. 1 shows the wavelength change rate of Bi substitution amount and Faraday rotation coefficient.
The figure which shows the relationship with FWC.

FIG. 2 shows a Faraday rotation coefficient θ _{F of the} A-4 film and the B-4 film.
FIG. 4 is a diagram showing the wavelength dependence of the wavelength.

Claims (4)

[Claims] 1. A composition formula _{R 3-x Bi x Fe 5} -y Ma y O
₁₂ where R is a rare earth element containing yttrium, Ma is a trivalent positive element not containing cobalt, and a liquid phase epitaxial magnetic garnet single crystal represented by 0.6 ≦ x ≦ 1.90 ≦ y ≦ 0.5. and A film made, composition formula _{R '3-k Bi k Fe} 5-lmn Mb l Mc m Co
_n O ₁₂ where R 'is a rare earth element containing yttrium and Mb is 3
Liquid positive epitaxial element, Mc is a tetravalent positive element, liquid phase epitaxial magnetism represented by 0 ≦ k ≦ 0.30 ≦ l ≦ 0.50 ≦ m ≦ 0.23 0.02 ≦ n ≦ 0.28 The B films made of garnet single crystals are superimposed on each other so that their combined Faraday rotation angles become 45 degrees, and the wavelength dependence of the Faraday rotation angle of the A film and the wavelength dependence of the Faraday rotation angle of the B film cancel each other out. A Faraday element used in the 1550 nm band, which is combined so that the minimum value K _min of the isolation in the 1550 nm band is 35 dB or more. 2. The Bi amount x in the A film is 1.15 ≦ x
The Faraday element according to claim 1, wherein ≤ 1.9. 3. R in the A film is Y, Tb, Gd, La.
One or more kinds selected from the group consisting of Al, In, G
a or two or more selected from a, and R ′ in the B film is one or two or more selected from Y, Tb and Gd;
The Faraday element according to claim 1, wherein Mb is one or more selected from Al, In, and Ga, and Mc is one or two or more selected from Ge, Zr, Sn, and Si. 4. The Faraday element according to claim 1, wherein the Faraday element is used in a wavelength band of 1550 nm, which is a wavelength of 1530 nm to 1570 nm.