JPH06202057A - Optical waveguide method and its execution device - Google Patents

Abstract

(57) [Summary] [Objective] To obtain an inexpensive optical waveguide method and optical isolator suitable for optical integration. [Structure] An optical waveguide method and a guided optical isolator according to the present invention.
Is composed of a non-reciprocal optical waveguide, at least a part of which is formed of a magneto-optical material, and a phase constant due to a difference in traveling direction of light guided through the optical waveguide due to a magnetization vector in the magneto-optical material. Of the TM mode to the TE mode due to optical anisotropy and the forward wave of the TM mode is not coupled to the TE higher mode due to phase mismatch. , And the backward wave of the TM mode is coupled with the TE higher mode by phase matching. Further, the waveguide type optical isolator comprises an optical waveguide having a non-reciprocity, at least a part of which is formed of a magneto-optical material, and the waveguide is formed into a channel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide method used for optical communication, optical measurement and the like, and an apparatus for implementing the same, and in particular, it utilizes a non-reciprocal coupling between a TM mode and a TE high-order mode. The present invention relates to a waveguide type optical isolator.

[0002]

2. Description of the Related Art FIG. 13 shows a conventional bulk type optical isolator.
FIG. 51 is a schematic diagram for explaining the operating principle of the
2 is a port, 53 and 54 are polarization beam splitters that transmit only S-polarized light, 55 is a Faraday rotator such as YIG (yttrium iron garnet), and 56 is a magnetic field passing through the Faraday rotator 55. .

In a conventional optical isolator, as shown in FIG. 13, S-polarized light (vertical polarized light) incident from a port 51 is entered.
Light having a wavelength is transmitted through the polarizer 53, and the Faraday rotator 55
Polarization is rotated by 45 ° by passing through
After passing through the tilted analyzer 54, the light is emitted from the port 52. S-polarized light incident from port 52 (4 from vertical
The polarized light having a tilt of 5 ° passes through the polarization beam splitter 54 and passes through the Faraday rotator 55 so that the polarized light becomes 45
The light beam is rotated to become horizontal, becomes P-polarized light (horizontal polarized light) to the polarization beam splitter 53, and cannot be transmitted.

However, in this bulk type optical isolator, the Faraday rotator 55 is expensive, and the Faraday rotator 55 and the polarization beam splitter 54 are arranged so that the coincidence of the optical axes and the plane of polarization form a predetermined angle. Had to be adjusted with high precision, resulting in poor reliability and extremely high cost. Further, there is a problem that integration is difficult. Therefore, realization of a waveguide type optical isolator suitable for integration has been awaited.

FIG. 14 shows the structure of a conventionally proposed waveguide type optical isolator (see Reference 1: IEICE, Opto-Quantum Electronics Research Group, OQE89-29 (1989)).

In FIG. 14, reference numeral 61 is a substrate, 62 is a waveguiding layer of a magneto-optical film, 63 is a magnetization vector in the magneto-optical film 62, and 64 is a mode selector using a metal loading film.

In this waveguide type optical isolator, a forward traveling light wave incident in the TE basic mode is partially converted into the TM basic mode due to phase mismatch with the TM basic mode. However, at the output end, only the TE basic mode is output as it is. On the other hand, the light wave in the opposite direction that is incident in the TE basic mode is
The TM basic mode is in a phase matching state with the TM basic mode that has undergone a non-reciprocal phase shift change due to the magnetization component perpendicular to the propagation direction.
Converted to mode, absorbed by the mode selector 64, and not output.

[0008]

However, in this structure, in order to obtain a large amount of non-reciprocal phase shift, a region in which the TM mode is close to the cutoff is required. The phase constant of the mode becomes larger than that of the TM basic mode, and phase matching becomes impossible. Therefore, in the above-mentioned Document 1, it is proposed to assume phase matching by assuming birefringence, but a large birefringence is required for isolator operation, and it is difficult to control its birefringence. Therefore, in reality, the device has not been realized yet.

That is, in the case of a slab waveguide, T
The phase constant of the E mode is larger than the phase constant of the TM mode, and is particularly remarkable near the cutoff (called structural anisotropy). The birefringence originally means that the TE mode and the TM mode have different refractive indexes. By using a birefringent material exhibiting a birefringence in which the TM mode has a higher refractive index, the phase difference due to the structural anisotropy is caused. Catch the match. However, it is difficult and not feasible to find an amount of material that can catch the phase mismatch (see the right column of p27 in Ref. 1).

The present invention has been made to solve the above problems, and an object of the present invention is to provide an inexpensive optical waveguide method and optical isolator suitable for optical integration.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[0012]

In order to achieve the above object, an optical waveguide method and a waveguide type optical isolator according to the present invention are provided.
At least a part of the optical waveguide is formed of a magneto-optical material and exhibits non-reciprocity, and the magnetization vector in the magneto-optical material causes the phase constants to be mutually different due to a difference in traveling direction of light guided through the optical waveguide. In contrast, due to the existence of the coupling coefficient from the TM mode to the TE mode due to the optical anisotropy, the light wave in the forward direction of the TM mode does not combine with the TE higher-order mode due to the phase mismatch. The main feature is that the light waves are guided as they are and the light waves in the opposite direction of the TM mode are coupled with the TE higher-order mode by phase matching.

The waveguide type optical isolator is characterized in that at least a part of the waveguide type optical waveguide is formed of a magneto-optical material and exhibits non-reciprocity, and the waveguide is formed into a channel.

That is, it is a waveguide type optical isolator utilizing non-reciprocal coupling between the TM mode and the TE higher-order mode, and the operation principle and structure are different from those of the prior art.

[0015]

According to the above-mentioned means, the phase constants are different from each other due to the difference in the traveling direction of the light propagating in the optical waveguide due to the magnetization vector in the magneto-optical material, and the optical anisotropy changes the TM mode to the TE mode. Due to the existence of the coupling coefficient to the TE mode, the light wave in the forward direction of the TM mode is TE due to the phase mismatch.
It is guided as it is without being coupled to the higher-order mode, and the light wave in the opposite direction of the TM mode is coupled to the TE higher-order mode by phase matching. Therefore, it is inexpensive and highly suitable for optical integration. A waveguide type optical isolator can be obtained.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

In all the drawings for explaining the embodiments, parts having the same function are designated by the same reference numerals and their repeated description will be omitted.

(Embodiment 1) FIG. 1 is a perspective view showing a schematic structure of Embodiment 1 in which the present invention is applied to a waveguide type optical isolator, wherein 1 is a rib conductor formed of a magneto-optical film. Waveguide, 2
Is a magneto-optical film, 3 is a substrate, 4 is a magnetization vector M, 5 in the magneto-optical thin film induced by an applied external magnetic field, and coordinates are coordinates.

In FIG. 1, light is guided through a rib waveguide 1 having a high refractive index. Non-reciprocal phase shift effect due to the magnetization vector component in the y-axis direction in the magneto-optic thin film (Reference 2: IEICE Transactions '72 / 10 Vol.55-C No.10 pp550-708, Reference 3: Electronic Information Communication) Academic journal '88 / 5 Vol.j71-C No.5 pp
702-708), and the phase constant of the TM mode light is different from the light wave in the forward direction (hereinafter referred to as forward wave) and the light wave in the reverse direction (hereinafter referred to as backward wave).

The operation principle of the optical isolator operating in the TM (like) mode of the first embodiment will be described with reference to the phase constant shown in FIG. The lower side shows the phase constant β (TM) of TM (like) mode, and the upper side shows the phase constant β (TE) of TE (like) mode. β ^x _11F and β ^x _11B are
TM basic E ^x ₁₁ mode (M
arcartili notation: Bell.Syst.Tech.J.48,2071-2102 (1
969))) and shows the forward and backward wave phase constants,
β ^y ₁₁ and β ^y ₁₂ are the TE basic E ^y ₁₁ mode and T, respectively.
E shows the phase constant of the higher-order E ^y ₁₂ mode. In Figure 2, β ^x _11F
<Beta is a ^x _11B, by reversing the direction of magnetization vector M changes the direction of the externally applied magnetic field, as shown in FIG. 3 beta ^x
_The effect is the same even if _11F > β ^x _11B . TM basic E ^x ₁₁
Mode backward wave and TE higher-order E ^y ₁₂ mode are phase-matched (β ^x
_11B = β ^y ₁₂ ) state. K represents the coupling coefficient between the TM mode and the TE mode, and is given by the magneto-optical effect, optical anisotropy, etc. caused by the magnetization vector component in the z-axis direction. Furthermore, the following relations should be satisfied.

[0021]

[Equation 1]

[0022]

KL = π / 2 + nπ where L is the waveguide length and Δ = | β ^x _11F −β ^x _11B | / 2
m and n are 0 or natural numbers. If this condition is satisfied, the TM fundamental E ^x ₁₁ mode of the forward wave will not be coupled to the TE higher-order E ^y ₁₂ mode due to phase mismatch (β ^x _11F ≠ β ^y ₁₂ ), and will be used as it is in the waveguide. The TM fundamental E ^x ₁₁ mode of the backward wave is completely coupled with the TE higher-order E ^y ₁₂ mode by phase matching (β ^x _11B = β ^y ₁₂ ) and is emitted from the waveguide. m =
Optical power from TM mode to TE mode when n = 0
The migration rate of is shown in FIG. As described above, it is understood that the waveguide type optical isolator operating in the TM mode can be realized.

The phase matching condition of FIG. 2 is achieved as follows. The non-reciprocal phase shift effect is an asymmetric structure in which the refractive index of the substrate 3 is different from that of air, and since it is necessary to set the film thickness under the condition that the TM mode light is close to the cutoff, the waveguide has a slab structure. As shown in FIG. 5, the phase mismatch (difference in phase constant) becomes large near the cutoff of the TM mode, and the TM mode and the TE mode cannot be phase matched in any order.

However, by adopting the channel structure as shown in FIG. 1, the phase matching as shown in FIG. 2 is realized.
In order to show that, FIG. 6 shows changes in the respective phase constants when the waveguide width W is changed. The film thickness of the waveguide is formed so that the non-reciprocal phase shift amount becomes large. As can be seen from FIG. 6, at the point A, the TM basic E ^x ₁₁ mode and the TE high-order E ^y ₁₂ mode have phase matching (β ^x _11B = β ^y ₁₂ ). Also at the point B, the TM basic E ^x ₁₁ mode and TE higher order E ^y
Phase ₁₃ (β ^x _11B = β ^y ₁₃ ) occurs in _{13 mode} ,
The same effect is obtained. The relationship between the phase constants at point B in FIG. 6 is as shown in FIG. 7 (β ^x _11F <β ^y ₁₃ ) or FIG. 8 (β ^x _11F >
β ^y ₁₃ ). By further widening the waveguide width, it is possible to achieve phase matching with a higher-order TE mode, and the same effect can be obtained.

The coupling coefficient K is caused by the optical anisotropy of the film or the substrate (non-diagonal components ε _xy and ε _{yz of the} dielectric constant tensor), and the light induced by the film stress when forming the magneto-optical film on the substrate. It is given by the elastic effect, the magneto-optical effect due to the z-axis or x-axis component of the magnetization vector in the magneto-optical film, and the like. In the latter case, the magnetization vector is like a magnet, so it can be easily controlled by applying a magnetic field from the outside. Thereby, the coupling coefficient K can be easily induced. In FIG. 1, the direction M of the magnetization vector is inclined by θ from the y-axis to the z-axis to give the magnetization vector of the z-direction component.

FIG. 9 shows a waveguide type optical isolator of the first embodiment.
6A to 6C are cross-sectional views in each step for explaining the manufacturing method of the magnetic tape.

In the waveguide type optical isolator of the first embodiment, first, as shown in (a), GGG (Gd ₃ Ga ₅ O ₁₂ ),
A magneto-optical thin film 2 of Ce-substituted YIG or the like is formed by RF sputtering on a garnet substrate 3 of cation-doped GGG, NGG (Nd ₃ Ga ₅ O ₁₂ ), or the like. In order to increase the forward and reverse isolation, it is necessary to increase | β ^x _11F −β ^x _11B |, and for that reason, a Ce-substituted YIG film having a large magneto-optical effect was used (M.Gomi et al. Jpn.JA
ppl.Phy.27, L1536 (1988), Shintaku et al .: 50th Autumn Bibliographic Proceedings, 28a-x-10).

Next, as shown in (b), a photoresist is formed on the magneto-optical film 2 by using a photolithography technique.
A mask material 6 such as Cr or Ti is formed.

Next, as shown in (c), reactive ion etching with a chlorine-based gas such as BCl ₃ or the like, ion milling with Ar ions or the like, or chemical etching with hot phosphoric acid or the like is performed to form a waveguide. .

Finally, as shown in (d), the mask material 6 is removed and the magneto-optical waveguide 1 is formed.

(Second Embodiment) FIG. 10 shows a second embodiment of the present invention.
3 is a cross-sectional view showing a schematic configuration of the waveguide type optical isolator of FIG. As shown in FIG. 10, the waveguide type optical isolator according to the second embodiment has a structure in which the magneto-optical film 7 is formed in a square shape and embedded with a cover layer 8 such as a SiO ₂ film.

(Third Embodiment) FIG. 11 shows a third embodiment of the present invention.
3 is a cross-sectional view showing a schematic configuration of the waveguide type optical isolator of FIG. In the waveguide type optical isolator of the third embodiment, as shown in FIG. 11, a high refractive index film 9 was formed on the magneto-optical film 2 and processed into a rib type waveguide. With such a structure, the amount of non-reciprocal change in the propagation constant increases.

(Fourth Embodiment) FIG. 12 shows a fourth embodiment of the present invention.
3 is a cross-sectional view showing a schematic configuration of the waveguide type optical isolator of FIG. As shown in FIG. 12, the waveguide type optical isolator according to the fourth embodiment is obtained by forming the high refractive index film 9 in the third embodiment into a strip shape 10 and can obtain the same effect as that of the third embodiment. it can.

In the first, second, third, and fourth embodiments described above,
Although the present invention has been described as an example in which it is applied to a waveguide type optical isolator, it goes without saying that the present invention can be applied to an optical circulator and the like. For example, an optical circulator can be constructed by changing the polarizer / analyzer of the optical isolator to one whose input / output positions are largely different depending on the polarization component of the optical beam.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various modifications can be made without departing from the scope of the invention. Absent.

[0036]

As described above, according to the present invention, it is possible to obtain an inexpensive and highly reliable waveguide type optical isolator suitable for optical integration.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a first embodiment in which the present invention is applied to a waveguide type optical isolator,

FIG. 2 is a diagram for explaining the phase constants of TE mode and TM mode of light for explaining the operation principle of the waveguide type optical isolator of the first embodiment.

FIG. 3 is a diagram for explaining the TE and TM mode phase constants of light for explaining the operation principle of the waveguide type optical isolator of the first embodiment;

FIG. 4 is a graph showing the power transfer rate from TM mode to TE mode for explaining the operation of the waveguide type optical isolator according to the first embodiment.

FIG. 5 is a diagram showing that the phase mismatch becomes large in the vicinity of the cutoff in each order of the TE mode and the TM mode of the waveguide type optical isolator,

FIG. 6 is a diagram showing the relationship between the phase constant of the waveguide type optical isolator of the first embodiment and the width of the magneto-optical waveguide;

FIG. 7 is a diagram for explaining the relationship of the phase constants of the waveguide type optical isolator of the first embodiment,

FIG. 8 is a diagram for explaining the relationship between the phase constants of the waveguide type optical isolator of the first embodiment,

FIG. 9 is a sectional view in each step for explaining the manufacturing steps of the waveguide type optical isolator according to the first embodiment,

FIG. 10 is a sectional view showing a schematic configuration of a waveguide type optical isolator according to a second embodiment of the present invention,

FIG. 11 is a sectional view showing a schematic configuration of a waveguide type optical isolator according to a third embodiment of the present invention,

FIG. 12 is a sectional view showing a schematic configuration of a waveguide type optical isolator according to a fourth embodiment of the present invention,

FIG. 13 is a schematic configuration diagram of a conventional bulk type optical isolator,

FIG. 14 is a schematic configuration diagram of a conventionally-guided optical isolator.

[Explanation of symbols]

1 ... Magneto-optical rib waveguide, 2 ... Magneto-optical film, 3 ... Substrate,
4 ... Magnetization vector M, 5 ... Coordinates, 6 ... Mask material, 7 ... Rectangular magneto-optical waveguide, 8 ... Cover layer, 9, 10 ... High refractive index layer, 51, 52 ... Port, 53, 54 ... Polarization beam splitter, 55 ... Faraday rotator, 56 ... Magnetic field passing through Faraday rotator 55, 61 ... Substrate, 62 ... Magneto-optical film, 63 ... Magnetization vector, 64 ... Metal mode selector.

Claims (2)

[Claims] 1. A non-reciprocal optical waveguide at least a part of which is formed of a magneto-optical material, and the magnetization vector in the magneto-optical material causes a difference in traveling direction of light guided through the optical waveguide. Since the phase constants are different from each other and there is a coupling coefficient from the TM mode to the TE mode due to optical anisotropy, the light wave in the forward direction of the TM mode is TE high-order mode due to phase mismatch. The optical wave in the opposite direction of the TM mode is guided by the waveguide without being coupled to the T mode, and the optical wave in the opposite direction of the TM mode is T
E An optical waveguide method characterized by coupling with a higher-order mode. 2. A waveguide type optical isolator, at least a part of which is formed of a non-reciprocal optical waveguide formed of a magneto-optical material, and the waveguide is formed into a channel.
Ta.