JPS58190904A - Magneto-optical thin film optical waveguide - Google Patents

Abstract

PURPOSE:To accord the transmission phase constant of TE waves with that of TM waves, by installing a buffer film and a loss-causing film on the optical waveguide of a magneto-optical crystal layer, and controlling thickness of these films. CONSTITUTION:A crystal film of Gd0.44Y0.26Fe5O12 or the like higher in refractive index and lattice constant than a magneto-optical crystal plate 1 of GGG crystals or the like is grown on this plate 1 to form an optical waveguide layer 2. On this upper surface a buffer film 3 and a metallic film 4 are formed, and the transmission phase constants of the TE waves and TM waves can be accorded with each other by controlling thickness of these films.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a guiding waveguide in which the propagation phase constants of optical waveguide modes in which oscillating electric field components are orthogonal to each other are matched.

Optical communication has entered the practical stage due to the rapid progress in research and development of its basic components, especially optical fibers, and higher performance and higher reliability of communication systems are desired than ever before, and along with this, the simplification of optical circuit elements is required. , miniaturization and integration have progressed, and optical waveguides have been created by providing a waveguide layer with a high refractive index on a dielectric or semiconductor substrate, instead of a configuration that combines conventional optical components such as lenses and prisms, and various types of light can be transmitted within the waveguide. The trend is toward constructing circuit elements.

TE waves and TM waves exist independently within the optical waveguide.

When their propagation phase constants are matched, a TM wave can be converted into a TE wave or vice versa, and a TFi wave can be converted into a TM wave using an electro-optic effect or a magneto-optic effect. By utilizing this mode conversion, applications to various optical circuit elements such as optical modulators, optical switches, and optical isolators are being considered.

Among these, the optical isolator is important as an element that blocks light returning from the fiber to the light source.Efficient mode conversion of TB and TM waves is essential to form the element in the form of a waveguide and obtain large isolation. However, TE and TM propagating through an optical waveguide with a high refractive index waveguide layer on the substrate
Since there is a difference between the wave propagation phase constants, efficient mode conversion does not occur.

Therefore, it is necessary to implement efficient mode conversion using some kind of method, and methods such as Hiro's are known. One of them is gadolinium, gallium, carnet) (
GGG)% Magnetic garnet (Ys Ga,
, Seo,, Fe1-10+t) thin film is grown epitaxially, and an electric circuit of the conductive thin film is provided on this garnet substrate, and mode conversion is performed using the magneto-optic effect. (For details, see Applied Fix Letters, Vol. 21, No. 8, p. 394).

This method involves folding an electric circuit so that the direction of the magnetic field is alternately reversed according to the distance at which conversion from TE waves to TM waves occurs, and converting TM waves to T waves over the entire distance that light travels.
The configuration is such that mode conversion to E waves is added. However, this configuration has the disadvantage that a complicated electric circuit must be set with high precision. In addition, to perform mode conversion using the same magneto-optic effect, a waveguide is formed by providing a crystal layer of yttrium, iron, and garnet on a GGG crystal substrate, and an iodine layer, which is an optically anisotropic crystal, is placed on the waveguide. There is one in which lithium oxide (LiI'Os) is adhered (
For details, see the literature I Triple E Transactions, MT
T-23, page 70) In this case, light leaks from the waveguide to the anisotropic crystal, and the birefringence in this anisotropic crystal,
The Faraday effect of the magnetic film is used to impart directionality to the coupling between polarized lights and at the same time match the propagation phase constant. However, the adhesion between the waveguide layer and the anisotropic crystal has disadvantages in terms of stability and reliability, as it has to rely on techniques with poor yields, such as adhesion. Furthermore, a simple method to achieve phase matching is to utilize the birefringence that occurs in a thin film when a crystal with a lattice constant different from that of the substrate is grown on a crystal substrate (for details, see Patent Application No. 36988 of 1982). There is. In this structure, a magnetic crystal film having a higher refractive index and a larger lattice constant than a crystal substrate is provided on a garnet substrate such as GGG by a liquid phase or vapor phase growth method. At this time, a tensile force is generated in the plane of the waveguide layer due to the lattice constant mismatch between the substrate and the waveguide layer, and furthermore, due to the photoelastic effect, the in-plane refractive index CT
The refractive index n, or n, ) specific to the waveguide N for E waves decreases, and as a result, becomes smaller than the refractive index in the thickness direction (the refractive index n, or n, specific to the waveguide layer for TM waves). When the thickness of the waveguide layer is small, the isolateral refractive index (the value obtained by dividing the propagation phase constant β by the propagation phase constant of free space) of the TE and 'I'M waves propagating in the waveguide layer is determined by the refraction of the substrate. If the isolateral refractive index of the TE wave is thicker than that of the TM wave and the thickness of the waveguide layer is sufficiently large, the isolateral refractive index of the TE and TM waves is slightly larger than that of the TE wave. The value is approximately equal to the refractive index specific to each corresponding waveguide layer, and the isolateral refractive index for the TM wave is larger than the isolateral refractive index for the TE wave. Therefore, at a certain thickness of the waveguide layer in the middle, the equilateral refractive indexes of the TE wave and the TM wave become equal (1), then phase matching is established. however,
The thickness t of the waveguide layer and the amount of negative birefringence (Δn = n, −n
There is a drawback that x) must be set strictly. −
When used as an optical isolator with an in-resicon performance of about 30 dβ, it is necessary to keep the equivalent refractive index difference between TE and TM waves within lXl0', but to do so, the thickness and birefringence of the waveguide layer must be 4±0. 05μm, -3
, it must be set with an accuracy of 2 years old Xl0-4, which is difficult in actual manufacturing.

Therefore, in any of the above cases, in order to realize phase matching, [H
1. It requires a complicated structure and strict manufacturing conditions, and it also poses problems in terms of stability and reliability.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned difficulties and provide a phase-matched leading waveguide that is easy to manufacture.

The magneto-optic thin film guiding waveguide of the present invention is an optical waveguide in which an optical waveguide of a magneto-optic crystal layer having a higher refractive index than the substrate and a larger lattice constant than the substrate is formed on a crystal substrate. A film and a lossy film are provided, and the size of the propagation phase constant for a leading wave mode (TE wave) having an oscillating electric field component in the horizontal direction and for an optical waveguide mode (TM wave) having an oscillating electric field component in the vertical direction is provided on the substrate. The thickness of the lossy film is determined so as to match the magnitude of the propagation phase constant, and there are nine characteristics.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention. III.For example, gadolinium gallium garnet single crystal.

A crystal film having a higher refractive index and a larger lattice constant than the crystal substrate by a liquid phase or vapor phase growth method on the crystal substrate,
For example, Gd6.44 Y6. , @Fe, OK.

The waveguide layer 2 is provided with a crystalline film such as the one shown in FIG.

Then, a buffer film 3 is formed on the upper surface of the waveguide layer, and a metal film 4 is further formed on the buffer film 3. A metal film, such as aluminum, can be formed by sputtering or vacuum evaporation, but if it is formed directly on the top surface of the waveguide layer 2, the birefringence of the waveguide layer will change due to the distortion caused by the addition of the metal film. A buffer film 3 is provided between them to absorb the elastic strain caused by the addition of the metal film.

Further, when forming the buffer film 3, it is necessary to avoid distortion due to its addition, and the buffer film can be formed by spin coating of an organic material such as polyimide or siloxane, or by a plasma CVD method of Sin. At this time, in order not to lose the buffering effect, the thickness of the buffer film ti
loooA is necessary, and if it is too large, the optical effect due to the addition of the metal film will be lost, so it needs to be 3000A or less.

As mentioned above, if the lattice constant of the waveguide layer is made larger than that of the substrate, and the refractive index n of the waveguide layer for TM waves is set to be larger than the refractive index n of the waveguide layer for TE waves, the result shown in Fig. 2 is obtained. The thickness t of a given waveguide layer is shown by the solid line. The equilateral refractive index nTEt nTM of the TE wave and the TM wave match, and phase matching is established. However, when used as an optical isolator with an isolation performance of about -30 dβ, the equivalent refractive index difference △neff between the #'iTE wave and the TM wave needs to be within 10-3, so the thickness of the Vcti waveguide layer When is set to 4 μm, the thickness of the waveguide layer and the amount of birefringence (
Δn=narnx) respectively 4±0.05μm,
It must be set with an accuracy of -3.2±0.1×10−4, which is difficult to manufacture. Therefore, the actually fabricated waveguide layer is set to deviate from the 9 phase matching conditions, which is different from the design value (('''i'o).A new waveguide layer is added or the waveguide layer is removed by etching. Although it is possible to compensate for manufacturing errors by increasing or decreasing the thickness, it is not appropriate because it is expected to change the refractive index due to mechanical strain on the waveguide layer or increase scattering loss due to roughness of the waveguide surface.

Therefore, in the present invention, a buffer film and a metal film are added to the upper surface of the waveguide layer in order to compensate for the setting error of the birefringence Δn or the film thickness t of the waveguide layer without modifying the waveguide layer. By adding the buffer film and the metal film, the equivalent refractive index difference of the TE%TM wave is reduced, and as a result, phase matching can be realized at t=t' as shown by the broken line in FIG. As is well known, when an isotropic dielectric film is added to the top surface of the waveguide layer, TB waves and T
Since the equivalent refractive index difference △"ett Fi of M waves decreases, the amount of birefringence of the waveguide layer 〇 or the film thickness t can be made small △n
Compensation for Δn5ff is possible only when eo>O. By increasing the birefringence or film thickness of the waveguide layer, Δn @
It is necessary to use a lossy film such as a metal or a semiconductor whose addition of Ku to achieve phase matching even when f(<0) increases the equivalent refractive index difference.

A metal film is suitable for ease of use in the study, and the thickness of the metal film is 1. The equivalent refractive index difference can be changed by adjusting. FIG. 3 shows the relationship between the metal thickness t and the isolateral refractive index, and the equivalent refractive index difference reaches its maximum when tl is approximately 0.08 μm, and remains constant at thicknesses greater than that.

A buffer film (polyimide) and gold R (aluminum) are each 0.15 μm thick on the top surface of the 4 μm thick waveguide layer.

If approximately 0.08 μm is provided, the equivalent refractive index difference △n5ff =
``TEnTM increases by 1.2X10''4, and when only the buffer film is added (f+=0) [ti3X10-' decreases.These changes in Δneff are caused by variations in the waveguide layer thickness t-
〇, 6 nm, +0.1 μm. Therefore, the film thickness of the waveguide layer, the manufacturing error of birefringence, △n, are -0, 1<, respectively.
t@<, +0.6μm, -1,2x 10-'<Δn
Even if the thickness of the metal film is 1. 0
．． Phase matching can be achieved by appropriately setting the distance between 0.8 μm and 0.08 μm.

When only a buffer film is added without using metal, Kt, Δna
FiO, 15am<: t, <o, O<△ne<+
4.times.10@-s, and by using a metal film, the range in which manufacturing errors in the thickness t of the waveguide layer and the birefringence .DELTA.n can be compensated for can be greatly expanded.

The above describes a method for realizing phase matching by adjusting the thickness of the metal film.The same effect can be obtained even if the thickness of the metal film is kept constant and the thickness of the buffer film is adjusted.

Conventionally, in order to achieve phase matching, it was necessary to strictly set the birefringence and film thickness of the waveguide layer, but in this waveguide with a buffer film and a lossy film added to the top surface of the waveguide layer, This is advantageous in terms of manufacturing because phase matching can be achieved by correcting errors that occur during the manufacturing of the waveguide layer.

As described above, according to the present invention, it is possible to obtain an optical waveguide that is easy to manufacture and in which the phase constants of the TE wave and the TM wave match.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, in which 1 is a garnet substrate, 2 is a magneto-optic garnet crystal waveguide layer having a higher refractive index than the substrate, 3 is a buffer film, and 4 is a metal film. FIG. 2 is a schematic diagram of the relationship between the thickness of the waveguide layer and the equivalent refractive index, and FIG. 3 is a schematic diagram of the relationship between the thickness of the metal film and the equivalent refractive index.

Claims (1)

[Claims] In a guiding waveguide in which an optical waveguide of a magneto-optic crystal layer having a higher refractive index and a larger lattice constant than that of the substrate is formed on a crystal substrate, a buffer film and a lossy film are provided on the upper surface of the waveguide. The size of the propagation phase constant for an optical waveguide mode (TE wave) having an oscillating electric field component in the horizontal direction and the optical waveguide mode (TE wave) having an oscillating electric field component in the vertical direction are provided on the substrate.
1. A magneto-optic thin film guided waveguide, characterized in that the thickness of the lossy film is determined so as to match the magnitude of a propagation phase constant for a TM wave.