JPS59104609A - Waveguide type optical isolator - Google Patents

Abstract

PURPOSE:To improve adhesive strength with a magneto-optical crystal thin film, to adjust magnitude of a double refraction extending over a wide range, and also to select freely a main axis direction of the double refraction by using a structural double refraction medium as a constituting element of a waveguide type optical isolator. CONSTITUTION:A medium 8 is vapor-deposited to suitable thickness on the surface of a substrate 10, subsequently, the medium 8 is scraped like a lattice by means of plasma etching, etc., and next, a medium 9 is vapor-deposited, by which a structural double refraction medium is obtained. A main axis direction of a double refraction can be selected to an optional direction by changing a direction of etching. Also, magnitude of the double refraction can be changed extending over a wide range by changing a ratio of thickness of the medium 8 and 9. Also, adhesive strength of this double refraction medium and a magneto- optical crystal thin film is good. A waveguide type isolator having said feature is obtained by laminating an isotropic medium, the magneto-optical crystal thin film and a structural anisotropic medium in this order.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for preventing reflected light in an optical circuit.

As shown in FIG. 1, only a conventional device of this type called a one-bulk type has been put into practical use.

A bulk type optical isolator consists of two polarizers 1 and 11 whose axes are tilted by 45 degrees to each other, and a magneto-optic crystal 2 sandwiched between these polarizers 1 and 11.
It is made up of. A magnetic field H is applied to the magneto-optic crystal 2, and the magneto-optic crystal 2 functions as a so-called optical isolator that allows only one-way passage of the light beam 3 by using the non-reciprocity of the magneto-optic crystal 2. In order to operate this type of optical isolator, a large magnetic field of about i,ooo (06) or more is required. - Magneto-optic crystals are very expensive.
The optical isolator itself has drawbacks such as being expensive and having a large overall size, for example, 9 mm x 9 mm x 20 ttm rods, because it allows the light beam to pass through. In order to improve some of these drawbacks, it has been proposed to make the magneto-optic crystal 2 into a film with a thickness of about 100 μm, so that the required magnetic field is about 100 (Oe) and the overall dimension is 5 ms. It has been confirmed that the size can be reduced to about 5 mm x 6 mm. However, even with such improvements, it remains essentially a bulk type.

In recent years, research and development of optical transmission systems has made remarkable progress, and the need for waveguide optical isolators that are smaller and can be incorporated as so-called optical I (3) has increased.

As shown in FIG. 2, such a waveguide optical isolator includes an isotropic crystal 4, a magneto-optic crystal thin film 5, and a birefringent crystal 6.
Warner proposed in 1972 that it could be constructed using metal thin films 7, 7' as mode filters. In such a configuration, the required magnetic field is 10 (
oe ) or less ・Dimensions are L as X L a
m x 5 mm or less, single mode operation is possible, and efficient coupling with optical elements such as lasers and single mode optical fibers is possible. However, since it is extremely difficult to bring the birefringent crystal 6 and the magneto-optic crystal thin film 5 into close contact with each other, the operation of the optical isolator with the configuration shown in FIG. 2 has not been confirmed even experimentally.

In order to fundamentally solve these drawbacks, the present invention is devised to use a structured birefringent medium that is amorphous but has birefringence instead of the birefringent crystal 6.

The present invention will be explained in detail below with reference to the drawings.

FIG. 8 is a diagram showing the structure and principle of the structured birefringent medium 6, which is an important component of the present invention.

The structured birefringent medium is obtained by regularly arranging two types of media 8°9 having different refractive indexes in a plate shape with thicknesses t□t'cg that are sufficiently smaller than the light wavelength λ. At this time, for electromagnetic waves whose plane of polarization lies in the xy plane, the electric field E becomes continuous at the interface between medium 8 and medium 9, whereas for electromagnetic waves whose plane of polarization lies in the biaxial direction, medium 8 Anisotropy occurs because the electric flux density becomes continuous at the interface between the medium 9 and the medium 9. In other words, the effective permittivity ε11 in the xy plane is εI+ −(t□ε, +t2ε,)/(1□+t2)
In contrast to (1), the effective permittivity ε± in the Z-axis direction is ε soil - (t□+t2)ε, ε2/(t□ε2+t2ε,
)' (2), and between ε11 and ε earth there is ε do ε on −1112(ε, −ε2 n /(1□ε2+
The difference given by t2ε□)(1□+t,)(3) arises. Here ε0. ε2 is the dielectric constant of medium 8 and medium 9 (
the square of the refractive index).

That is, if the refractive index of medium 8 and medium 9 is different, -
It behaves in the same way as a ring-negative crystal (εworks ε11).

There are many substances in nature that have orientational properties in their refractive index,
Most of this is due to the crystal structure, and the size of the anisotropy cannot be changed arbitrarily.0 Also, when constructing an optical isolator, 1 magneto-optic crystal thin film 5 and birefringent medium 6 are used.
Although it is absolutely necessary that the two surfaces are optically in close contact with each other, it is unavoidable that a non-uniform gap of about 0.1 mm is created using normal optical polishing techniques. This gap has a fatal effect on the operation of the optical isolator. The only way to achieve such close contact without any gaps is to grow the entire birefringent crystal directly on the magneto-optic crystal thin film 5, but this was impossible because these two crystal structures are completely different. The present invention solves all these problems. That is (3)
As shown in the formula, the magnitude of birefringence (ε11−εk)
1. , 1. By changing the ratio of 0 to (ε
, -ε, )/g(ε, 162). Further, since the mediums 8 and 9 are each an amorphous dielectric material, they can be brought into close contact with any crystal by, for example, a vacuum evaporation method.

Very high birefringence can be obtained using very common media. For example, quartz (5i
n2) and titanium dioxide (Ti0g), ε
□−(i−′)” +ε, −(2,5)2. t, −
As t2, ε1 and εk are 0.94. This is 0.4 of LiIQ)08, a typical birefringent crystal in nature.
It is sufficiently larger than 0.51 for 0, L, 1Io8 and 0.62 for calcite.

Furthermore, the magnitude of birefringence can be controlled by appropriately selecting the ratio between t□ and t2.

A structural birefringent medium in which the principal axis direction of birefringence is perpendicular to the substrate surface is S.
It can be easily obtained using vacuum deposition techniques such as electron beam evaporation or sputtering methods. That is, the fourth
As shown in the figure, media 8 and 9 may be deposited alternately on a suitable substrate IO.

The thicknesses g t, , t of the media 8 and 9 need to be sufficiently smaller than the wavelength; however, the rate of vacuum evaporation should be kept at 0.0000000000000000000000000000000 per minute.
It is possible to select the thickness to be about 0.01 μm or less. To operate a waveguide optical isolator, the main axis direction of birefringence (the third
2 axes in the figure) must be within the plane of the substrate.

FIG. 5 is a diagram showing the manufacturing procedure of a structured birefringent medium having a principal axis within the plane of the substrate. FIG. 5 (aJ indicates the substrate 10, and as shown in FIG. 5(b), the medium 8 is evaporated to an appropriate thickness on the surface of the substrate 100. Next, as shown in FIG. 8 in a grid pattern. For this purpose, interference exposure of the photoresist and plasma etching can be applied. Next, by vapor depositing the medium 9, as shown in FIG. 5 ((1)
) is obtained. The direction of the principal axis of birefringence can be arbitrarily selected by changing the direction of etching.

In the above description, the operation has been explained using a slab type optical isolator as an example, but the structurally anisotropic medium can be used in the same manner in a rectangular cross-section optical isolator.

As explained above, since the waveguide optical isolator of the present invention uses a structured birefringent medium as its component, good adhesion between the birefringent medium and the magneto-optic crystal thin film can be obtained, and the birefringence is large. The angle can be adjusted over a wide range, and the direction of the principal axis of birefringence can be selected in any direction within the plane of the substrate.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing the structure of a conventional bulk type optical isolator, Fig. 2 is a perspective view showing the structure of a waveguide type optical isolator, and Fig. 3 is a structure of a structurally anisotropic medium which is a component of the present invention. Fig. 4 is a perspective view showing an example of creating a structurally anisotropic medium, Fig. 5(a), ■> + Cc
No+ (The crests are diagrams showing the procedure for creating a structurally anisotropic medium in which the anisotropy axis lies within the plane of the substrate. 1.1'...Polarizer, 2...Magneto-optical crystal, 3...
- Light beam, 4... Isotropic substrate, 5... Magneto-optic crystal thin film, 6... Refractive index anisotropic medium, 7,7'... Metal thin film, 8... Amorphous medium , 9... amorphous medium, 10...
··substrate.

Claims (1)

[Claims] 1. An isotropic medium, a magneto-optic crystal thin film, and a refractive index anisotropic medium are laminated in this order, and a metal thin film is provided as a cladding at a light input end and a light output end on the magneto-optic crystal thin film. 1. A waveguide optical isolator characterized in that a structural anisotropic medium is used as the refractive index anisotropic medium.