JPH0311304A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain an optical isolator which has a short element length and a small insertion loss and has a simple structure and is easily produced to have a high productivity by using materials having the magnetooptical effect for a substrate or an upper layer part also to approximate the refractive index of the substrate or the upper layer part to that of a waveguide layer and approximating the thickness of the waveguide layer film to that of a cut-off film. CONSTITUTION:A magnetic body having a first-order magnetooptic factor whose sign is opposite to that of the magnetic body which is used for a waveguide layer 14 and has a magnetooptical effect is used for a substrate 13 or the upper layer part, and the refractive index of the substrate 13 or the upper layer part is approximated to that of the waveguide layer 14, and the thickness of a phase matching film is approximated to that of the cut-off film. The direction of magnetization of magnetic bodies used for the waveguide layer 14 and the substrate 13 or the upper layer part is inclined from the film surface in a plane perpendicular to the light propagation direction. The mode conversion due to the first-order magnetooptic effect in the waveguide layer 14 and that in the substrate 13 are added to realize the optical isolator having a short element length and a small insertion loss, and this optical isolator has a simple structure and a high productivity.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention prevents reflected light from various optical elements from returning to a semiconductor laser used as a light source for optical communication or optical disks, etc. This relates to an isolator, which is a unidirectional waveguide used to stabilize oscillation.

[Prior Art] Conventionally, an optical isolator of this type composed of a leading waveguide with sufficient characteristics has not been realized. FIG. 6 shows an example of the configuration of a conventional three-area optical isolator, in which a substrate 61 such as GGG (Gd3 GabO+2)
YIG created by liquid phase epitaxy (LPE) method etc.
(Y3 F es O+ 2 ) or Bi-substituted Y
IG (B i xY3-xF e5 o.

2, O<x<3. Hereinafter, it will be written as Bi:YIG.

) etc. magnetic thin film 62 and A7 etc. metal cladding 63°6
It consists of 4. Mode selectors 65 and 66 using metal clads 63 and 64 greatly attenuate the 7M mode and pass only the TE mode. The mode converter 67 consists of a non-reciprocal mode converter 68 and a reciprocal mode converter 69,
The magnetizations are parallel to the light propagation direction, perpendicular to the light propagation direction, and tilted by θ from the direction perpendicular to the film surface. In the non-reciprocal mode converter 68 and the reciprocal mode converter 69, TE-TM mode conversion occurs by 50% each due to the Faraday effect and the Kotton-Mouton effect, which cancel each other in the forward direction and add together in the reverse direction. That is, for the light incident from the left end, only the TE mode component is transmitted by the mode selector 65, and the light is converted to the 7M mode by the non-reciprocal mode converter 68.
% conversion, but since the mode conversion in the non-reciprocal mode converter 68 is canceled in the reciprocal mode converter 69,
7M mode is converted to TE mode and becomes only TE mode again. Therefore, it passes through the mode selector 66 and is emitted from the right end. Conversely, the light incident from the right end becomes only the TE mode component at the mode selector 66, and the light enters the reciprocal mode converter 6.
50% conversion to 7M mode in one case. Furthermore, the mode conversion by the mesh mode converter 68 is added, and all remaining TE modes are also converted to the 7M mode. This 7
Since the M mode is attenuated by the mode selector 65, it is not emitted from the left end.

Other examples of conventional optical isolators are plants.

Miyazaki, Institute of Electronics and Communication Engineers Technical Research Report MW86-124 (
Single area type optical isolators are known as shown in Taki, Miyazaki, IEICE technical research report MW86-126 (1986).

[Problem to be solved by the invention] However, in the three-area optical isolator, two adjacent
It is necessary to orient the magnetization of the two regions in different directions, but this is difficult in practice because the direction of magnetization changes in a complicated manner near the boundary between the nonreciprocal mode converter and the reciprocal mode converter. However, since the magnitude of mode conversion also changes, desired characteristics cannot be obtained. Furthermore, conventional single region type optical isolators have the disadvantage that the element length becomes long and insertion loss increases.

The present invention has been made to solve the above-mentioned problems, and uses a material having a magneto-optic effect not only for the waveguide layer but also for the substrate or upper layer, and also by changing the refractive index of the substrate or upper layer. By approaching the waveguide layer and making the waveguide layer thickness close to the cut-off thickness, we have created an optical isolator that has a short element length and low insertion loss, and has a simple structure that is actually easy to create and has high productivity. is intended to provide.

[Means for Solving the Problems] In order to achieve this object, the optical isolator of the present invention has a primary magnetism of a magnetic material having a magneto-optic effect used in the waveguide layer on the substrate or upper layer. A magnetic material having a first-order magneto-optic factor with the opposite sign to the optical factor is used, and its refractive index is made close to that of the waveguide layer, so that the phase matching film thickness is made close to the cutoff film thickness. ing. Furthermore,
The magnetization of the magnetic material used in the waveguide layer and the substrate or upper layer is tilted from the film surface in a plane perpendicular to the light propagation direction.

[Operation] In the present invention having the above configuration, a magnetic material having a magneto-optic effect having a first-order magneto-optic effect with a sign opposite to that of the waveguide layer is used in the substrate or upper layer of the mode converter. , non-reciprocal mode conversion due to the first-order magneto-optic effect contributed by the propagation direction component of the electric field generated in the waveguide layer and the substrate or upper layer are added together. In addition, by setting the refractive index of the substrate or upper layer close to the refractive index of the waveguide layer and making the phase matching film thickness close to the cutoff film thickness, the propagation direction component of the electric field increases, and the first-order magneto-optic effect The number of mode conversions due to this will also increase. Furthermore, by tilting the magnetization of the magnetic material from the film surface in a plane perpendicular to the light propagation direction, reciprocal mode conversion and 1 The following mode conversion due to magneto-optic effect is made equal. As a result, in the forward direction, the mode conversions due to the second-order and first-order magneto-optic effects cancel each other out, and the TE mode is transmitted as it is and is emitted through the mode selector. In the reverse direction, these mode conversions add up, and the TE mode is completely converted to the 7M mode, which is attenuated by the mode selector and is therefore not emitted. This allows light to be transmitted only in the forward direction.

[Example] Hereinafter, an example embodying the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of an optical isolator of the present invention. The optical isolator consists of a mode converter 10 and mode selectors 11, 12 provided at both ends thereof. The mode converter 10 includes a substrate 13. Waveguide layer 14. The upper layer part 15 is composed of three layers, the substrate 13 and the waveguide layer 14 are magnetic materials having a magneto-optic effect, and the upper layer part 15 is made of a sinusoidal material.
.

It is a dielectric material such as BK7 glass or ZnO. The signs of the first-order magneto-optic effects of the magnetic materials of the waveguide layer 14 and the substrate 13 are opposite. Bi, which has a negative first-order magneto-optic effect, is used as the waveguide layer 14.
:YIG, G with positive first-order magneto-optic effect as substrate 15
a Massive substitution (BiGd)3 (FeGa)b O+
2 can be used.

Mode selectors 11 and 12 are the same, AA? The metal cladding 16, 17, etc. gives a large attenuation only to the 7M mode and functions to pass only the TE mode.

The dielectric constant tensor of the magnetic garnet used for the waveguide layer 14 and the substrate 13 is expressed as 11. If ε2, then ε1. ε2 is given by (1). Here, when the magnetization of the waveguide layer 14 and the substrate 13 is tilted by θ from the film surface in a plane perpendicular to the light propagation direction (Z direction), each component is e, m(n, +Δ
n-)”+2 (faaMrilXXl
1.5intθ ε, -n,"+2 (f14M")-cosθ1
y y L
Lε ・n ・2 IZZ 1 ε−= (f4tM”), sin 2θ
(2) txy tε
・“J (f+M), cos θ
IZX lε, -j
(f? M) -5in θ1yZ
l However, n and Δn represent the refractive index and birefringence of the magnetic garnet, and f, M, f, and □M2 represent the first-order and second-order magneto-optic effects. Among these, ε, and ε, contribute to mode conversion. Therefore, due to ε, the Ey acid component of the TE mode is coupled with the E component of the TMxy mode, resulting in mode conversion. This mode conversion is reciprocal mode conversion due to the second-order magneto-optic effect as shown in equation (2).

On the other hand, due to ε, the E component of the TE mode and Ttyz
E in 7M mode
A combination with the components occurs. This mode conversion is non-reciprocal mode conversion due to the first-order magneto-optic effect, as shown by equation (2). The magnitude of the mode conversion caused by this ε, and ε, is expressed by the coupling coefficients K and K in the following equations.

xy yz K −ε2 f'E EXdx+xY
x)''')o2>' 2Xε+
J'5x E+ E+' dx (3)xy
Y x K = e2
fOE2 E2 dx+yz
yz 噌 yzE+ f7E+ E
+ Kyodx (4)yz y
Figure 2 shows the electric field distribution in the mode converter.

From the distribution of E, the product numerator 6E2 E2 dx and 2
Be yzfτEl ,E
The signs of l-dx are opposite to each other.

Therefore, in order to increase K, ε2 and yz
yzε,
That is, the sign of the first-order magneto-optic effect of the substrate 13 and the waveguide layer 14 must be opposite. Thereby, not only the waveguide layer 14 but also the magneto-optic effect of the substrate 13 can be effectively utilized, and as a result, the element length can be shortened. Further, the refractive index of the substrate 13 is brought close to the refractive index of the waveguide layer,
By bringing the phase matching film thickness closer to the cutoff film thickness, E, which is a component in the propagation direction of the electric field, increases. As a result, ε, which was ignored in conventional two-region optical isolators where the phase matching film thickness is sufficiently larger than the cutoff film thickness,
mode conversion becomes large. Furthermore, it is possible to make K and K equal by appropriately selecting the elevation angle θ of magnetization using an external magnet (not shown).
yz can be done. Therefore, in the forward direction, ε,
The mode conversion due to and ε, cancels lx y
1 y Z matching mode conversion does not occur, and in the opposite direction, mode conversion due to ε, and ε are added LX5/i
yz, a complete mode conversion occurs. That is, in the forward direction, the TE mode that enters from the left end through the mode selector 11 is not converted into mode by the mode converter 10, passes through the mode selector 12 as the TE mode, and is output. On the other hand, in the opposite direction, the light incident from the right end is
The mode selector 12 selects only the TE mode, and the mode converter 10 converts it completely to the 7M mode. This 7M mode is attenuated by the mode selector 11 and is not output. In this way, a destination isolator that transmits light in only one direction can be constructed.

As a specific design example, at a wavelength of 1.15 m, the waveguide layer 1
4, refractive index n1-2.18° birefringence Δrll = 2.5xlO-', first order magneto-optic effect (f? M)+ = -2,79X10-', second-order magneto-optic effect (f44 ”), s+w-5,5
8Xio-4, (BiY)z(Fe
A-A')so+□, GGG (Gd3G
refractive index n2=2.137. Birefringence Δn2-2゜5
X 10-3.1st order magneto-optic effect (f7M) 2”3.
85X10-', second-order magneto-optic effect (ft□M2)2
--7.70X10', film thickness 2.5m or more (B 1G
d) By using BK7 glass with a refractive index of n3-1.50 as the upper layer of the 3 (FeGa)s O+ 2 cladding layer, an optical isolator can be created with an element length of the mode converter 10 of 4.68 mm. . The isolation characteristics at this time are shown in FIG. Elevation angle of magnetization θ-79
It can be seen that a forward/reverse ratio of 30 dB or more can be obtained at ~80''.

The present invention is not limited to the embodiments described above,
Various changes can be made without departing from the spirit of the invention.

For example, instead of a magneto-optical substrate, a sufficiently thick cladding layer 42 of magnetic garnet may be used on a dielectric 41 such as GGG as shown in FIG. 4. If the cladding layer 42 is thick enough, the dielectric 41 The effect is negligible, and the cladding layer 42
can be treated as a substrate. Alternatively, as shown in FIG. 5, a waveguide layer 52 and an upper layer portion 53 having a magneto-optic effect may be formed on an isotropic substrate 51 such as GGG. At this time,
The refractive index of the upper layer portion 53 is greater than that of the substrate 51, and the signs of the first-order magneto-optic effects of the upper layer portion 53 and the waveguide layer 52 may be opposite to each other.

Furthermore, all three layers, ie, the substrate, the waveguide layer, and the upper layer, may be made of materials having a magneto-optic effect. At this time, the sign of the first-order magneto-optic factor of the substrate and the upper layer having a refractive index close to that of the waveguide layer may be reversed to that of the waveguide layer.

[Effects of the Invention] As is clear from the detailed description above, according to the present invention, a material having a magneto-optic effect is used not only for the waveguide layer but also for the substrate or upper layer, and the primary Since the sign of the magneto-optic effect is opposite to that of the waveguide layer, mode conversion due to the first-order magneto-optic effect in the waveguide layer and substrate is combined, realizing an optical isolator with short element length and low insertion loss. can do. Furthermore, by bringing the refractive index of the substrate or upper layer closer to the refractive index of the waveguide layer and the thickness of the waveguide layer closer to the cutoff film thickness, a mode converter can be realized with a single magnetization direction, resulting in a structure with a simple structure. A simple and highly productive optical isolator can be provided.

[Brief explanation of the drawing]

1 to 5 show embodiments embodying the present invention. FIG. 1 is a configuration diagram of an optical isolator to which this embodiment is applied, and FIG. 2 is a mode conversion diagram of the optical isolator. FIG. 3 is a diagram showing the electric field distribution in the device, and FIG. 4 is a diagram showing specific isolation run characteristics in this example.
FIG. 5 is a sectional view showing the configuration of a modified example of this embodiment, and FIG.
The figure is a configuration diagram of a conventional optical isolator. In the figure, 10 is a mode converter, 11 and 12 are mode selectors, 13 is a substrate, 14 is a waveguide layer, and 15 is an upper layer. Fig. 1 11 0 2 Jin 1 peep distance layer

Claims (1)

[Claims] 1. A mode converter consisting of a substrate, a waveguide layer, and an upper layer, a mode selector provided at both ends of the mode converter, and a magnetic field perpendicular to the light propagation direction and tilted from the film surface in the mode converter. An optical isolator comprising: a means for applying . 2. In the optical isolator according to claim 1, at least one of the substrate or the upper layer of the mode converter and the waveguide layer are made of a material having a magneto-optic effect, and the first-order magneto-optic factor of the waveguide layer is , an optical isolator characterized in that the magneto-optic factors of one of the substrate and the upper layer have opposite signs. 3. The optical isolator according to claim 1, wherein the substrate of the mode converter is a cladding layer formed on a dielectric material and having a sufficiently thick magneto-optic effect. 4. In the optical isolator according to claim 1, by bringing the refractive index of the substrate or upper layer of the mode converter close to the refractive index of the waveguide layer, the thickness of the waveguide layer can be brought close to the cutoff thickness. Features of optical isolators.