JP2960567B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical isolator used for optical communication and optical measurement.

[0002]

2. Description of the Related Art A conventional example of an optical isolator is shown in FIG. 11 (written by Sato, "Light and Magnetism", Asakura Shoten, p. 165 (198)).
8)). In the figure, 51 and 52 are ports, 53 and 54
Is a polarization beam splitter that transmits only S-polarized light, 55 is a Faraday rotator such as YIG (5F ₂ O ₃ .3Y ₂ O ₃ ),
Reference numeral 56 denotes a magnetic field passing through the Faraday rotator 55.
The light having the S-polarized light (vertical polarized light) incident from the port 51 passes through the polarization beam splitter 53 and then passes through the Faraday rotator 55, whereby the polarization is rotated by 45 degrees and tilted by 45 degrees in the same direction. After passing through the polarization beam splitter 54 of the analyzer, the light exits from the port 52. The S-polarized light (polarized light inclined at 45 degrees from the vertical) transmitted from the port 52 passes through the polarizing beam splitter 54, passes through the Faraday rotator 55, rotates the polarized light by 45 degrees, and becomes horizontal. P
Because it becomes polarized light (horizontal polarization), it cannot be transmitted.
An optical isolator is configured.

[0003]

The optical isolator shown in the above prior art uses an expensive Faraday rotator,
Since it is necessary to adjust the optical axis and the polarization angle in the Faraday rotator and the polarization beam splitter with high accuracy,
There are drawbacks of poor reliability and very high cost. Further, the conventional optical isolator has a disadvantage that it is difficult to integrate an optical circuit.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive optical isolator suitable for optical integration without having to adjust expensive components such as a Faraday rotator with high precision.

[0005]

An object of the present invention is to provide a non-reciprocal optical waveguide formed at least in part by a magneto-optical material, and to guide the optical waveguide by a magnetization vector in the magneto-optical material. The phase constants are different from each other depending on the direction of light propagation.
The cutoff of the E mode is larger than the cutoff of the TM mode, and the cutoff of the TE mode is between the phase constant of the forward wave and the phase constant of the backward wave. , The forward wave of the TM mode is guided as it is without being coupled to the TE mode due to phase mismatch, and the backward wave of the TM mode is coupled to the radiated TE mode and Achieved by dissipating optical power from the core.

[0006]

The present invention comprises a waveguide using a magneto-optical material, and TM mode light guided through the waveguide has a phase constant between a forward wave and a backward wave due to non-reciprocal phase shift based on the magneto-optical effect. Although different, the cutoff of the TE mode is set between the respective phase constants of the forward wave and the backward wave, so that the light in the forward direction is guided in the waveguide according to the difference in the traveling direction of the light. Utilizing the fact that the wave and the light in the opposite direction are radiated and not guided, it is possible to obtain an optical isolator completely different in configuration, forming method and operation principle from the conventional technology.

[0007]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of a first embodiment of an optical isolator according to the present invention, FIG. 2 is a diagram showing phase constants of light TE mode and TM mode for explaining the operation principle of the present invention, and FIG. FIG. 4 is a diagram showing the relationship between the phase constant of the layered slab waveguide and the film thickness of the magneto-optical film, FIG. 4 is a diagram showing the manufacturing process of the first embodiment, and FIG. (A) shows a pattern when a magnetic field is applied at 10 Oe and corresponds to a forward wave, and (b) shows a pattern when a magnetic field is applied at -10 Oe. FIG. 6 is a diagram corresponding to a backward wave, FIG. 6 is a diagram illustrating a measurement system for measuring the isolation of the optical isolator of the present invention, FIG. 7 is a diagram illustrating isolation characteristics,
8 is a sectional view showing a second embodiment of the present invention, FIG. 9 is a sectional view showing a third embodiment of the present invention, and FIG. 10 is a sectional view showing a fourth embodiment of the present invention.

First Embodiment In FIG. 1 showing a first embodiment of the present invention, 1 is a rib waveguide formed of a magneto-optical film, 2 is a magneto-optical film, 3 is a substrate, and 4 is an applied external magnetic field. The magnetization vectors M and 5 in the magneto-optical film induced by the above indicate coordinates, and light is guided through the rib waveguide 1 having a high refractive index.

The operation principle of the optical isolator operating in the TM (like) mode of this embodiment will be described with reference to the phase constant shown in FIG. In order to realize this isolator, the following conditions must be satisfied.

Using the structural anisotropy of the waveguide, the TE mode cutoff β _{TEC is changed} to the TM mode cutoff β _TMC
Larger than.

By applying the Y-direction magnetization vector My to the magneto-optical film, a non-reciprocal phase shift occurs in the phase constant of the TM mode, and the phase constant β _TMF of the forward wave is changed to the phase constant β of the backward wave.
_Make it larger than _TMC .

The waveguide structure is designed so that β _TMB <β _TEC <β _TMF .

By giving optical anisotropy (photoelastic effect, electro-optic, magneto-optic, etc.) to a film or a substrate, TE
A coupling coefficient K for coupling from the mode to the TM mode is given.

Because of the above conditions, FIG.
As shown in the _figure , the forward wave of the TM mode having the phase constant β _TMF does not couple with the TE guided mode and the TE radiation mode due to phase mismatch (β _TEC <β _TMF , β _TMF ≠ β _TE ). , As it is in the waveguide. On the other hand, the phase constant β
It becomes a backward wave of the TM guided mode having the _TMB, and can be phase-matched with the phase constant β _TER of the TE radiation mode (β _TMB =
β _TER ), while traveling through the waveguide, it becomes a TE radiation mode and is radiated.

Next, a design method for satisfying the above conditions using the equivalent refractive index method will be described.
FIG. 3 shows the relationship between the phase constant of the waveguide mode and the thickness of the waveguide in the three-layer slab waveguide. Generally, as shown in the figure, the phase constant of the slab waveguide is larger in the TE mode than in the TM mode. When the thickness t ₂ of the magneto-optical film in the slab portion (outside the rib portion) is set so that the slab waveguide mode exists, the slab waveguide mode becomes a radiation mode in the rib waveguide. Therefore, the cutoff of the TE mode is β _TEC shown in FIG. 3 and the cutoff of the TM mode is β _TMC , which satisfies the above conditions. Next, the above condition is based on the magnetic vector component in the y-axis direction My
Is generated in the magneto-optical film, and a non-reciprocal phase shift is caused by the magneto-optical effect (Transactions of the Institute of Electronics, Information and Communication Engineers '88 / 5, Vol.
C, No. 5, pp 702-708). This effect requires an asymmetric structure in which the refractive indices of the substrate and air are different. Next, the TM mode phase constant of the rib waveguide can take a value between the phase constant β _TM1 of the slab waveguide for the film thickness t ₁ and the phase constant β _TMC for the film thickness t ₂ by adjusting the pattern width W. Therefore, the conditions described above can be satisfied by adjusting the pattern width W. TE mode phase constant β of rib waveguide
_TE is determined in the same manner, but is larger than the phase constant of the TM mode. The above condition is caused by the optical anisotropy (off-diagonal components ε _xy , ε _{yz of the} dielectric constant tensor) of the film or the substrate, and the light induced by the film stress when the magneto-optical film is formed on the substrate. It is given by the elastic effect, the magneto-optical effect by the magnetization vector of the z-axis or x-axis component of the magneto-optical film, and the latter magnetization vector can be easily controlled, so that the coupling coefficient K can be easily induced. is there. In FIG. 1, the direction M of the magnetization vector is inclined by θ from the y axis to the direction of the z axis to give a magnetization vector of the z direction component.

Next, the manufacturing method will be described. FIG.
FIG. 4 is a diagram showing a manufacturing process of the optical isolator of the embodiment. _{GGG (Gd 3 Ga 5 O 12} ) , as shown in FIG. 4 (a),
Ce on a garnet substrate 3 such as NGG (Nd ₃ Ga ₅ O ₁₂ )
A magneto-optical thin film 2 such as a substituted YIG is formed by RF sputtering. In order to increase the forward and reverse isolation, it is necessary to increase β _TMF −β _TMB ,
For this purpose, a Ce-substituted YIG film having a large magneto-optical effect was used (M. Gomi et al .: Japanese Journal.
Of Applied Physics (Jpn. J. Appl. Ph.
ys. ) 27, L1536 (1988), Shintaku et al .: No. 50
Proceedings of the Fall of Applied Physics, 28a-x-10). Next, as shown in FIG. 4B, a mask material 6 such as a photoresist, Cr, or Ti is formed on the magneto-optical film 2 using a photolithography technique. Next, as shown in FIG. 4C, reactive ion etching with a chlorine-based gas such as BCl ₃ ,
A waveguide portion is formed by performing ion milling with Ar ions or the like or chemical etching with hot phosphoric acid or the like. Finally, as shown in FIG. 4D, the mask material 6 is removed to form the magneto-optical waveguide 1. When an external magnetic field is applied substantially in the y-axis direction, the coupling between the TE guided mode and the TM radiation mode is induced by the film stress or the like during the formation of the magneto-optical thin film even without the presence of the magnetic field component in the z-direction component. This was achieved by optical anisotropy.

FIG. 5 shows a near-field pattern at the emission end when light having a wavelength of 1.55 μm is guided.
(A) is a pattern when a magnetic field is applied at 10 Oe and corresponds to a forward wave. It turns out that it has become a beautiful guided mode. (B) is a pattern when a magnetic field of −10 Oe is applied, and corresponds to a backward wave. It can be seen that the radiation mode is guided to the magneto-optical thin film outside the core. FIG. 6 shows a measurement system for measuring the isolation of the optical isolator of the present invention. Light 20 having a wavelength of 1.55 μm is
, And the light is condensed by the lens 22 and is incident on the end face of the sample 24. The light emitted from the sample 24 is collimated by the lens 25, passes through the slit 27, and is received by the Ge photodiode (Ge-PD) 27. The magnetic field 23 is applied in a direction substantially perpendicular to the plane of the paper (y-axis direction), and the slit 27 eliminates a radiation mode. FIG. 7 shows the measurement results obtained by the above measurement system. A magnetic field of -10 to 10 Oe is applied. The positive magnetic field corresponds to the forward wave and the negative magnetic field corresponds to the backward wave, and about 8 dB of isolation is obtained from the difference in light intensity between positive and negative.

Second Embodiment FIG. 8 is a sectional view showing a second embodiment of the present invention. Substrate 3
A high-refractive film 31 was formed on the magneto-optical film 2 formed on the surface of the substrate, and the high-refractive film 31 was processed into a rib waveguide. With the above structure, the amount of non-reciprocal change of the propagation constant becomes large (Transactions of the Institute of Electronics, Information and Communication Engineers '88 / 5,
Vol. J71-C, No. 5, pp 702-708).

Third Embodiment FIG. 9 is a sectional view showing a third embodiment of the present invention, in which the high refractive index film of the second embodiment is formed in a strip shape 32, and is the same as the second embodiment. Has an effect.

Fourth Embodiment FIG. 10 is a sectional view showing a fourth embodiment of the present invention. In the first embodiment, an absorber 33 is provided at a position that does not affect the light guided through the core. Absorber 33
Metals such as Co, Al, and Ti are usually used. The light in the forward direction is confined in the core on which the ribs are formed, and is guided without loss because it is not affected by the absorber 33.
3 causes a large loss. Therefore, a large isolation can be obtained.

[0021]

As described above, the optical isolator according to the present invention comprises a non-reciprocal optical waveguide formed at least in part by a magneto-optical material. The optical isolator is formed by a magnetization vector in the magneto-optical material. The phase constants are different from each other depending on the traveling direction of the guided light, and the cutoff of the TE mode is larger than the cutoff of the TM mode due to the structural anisotropy of the waveguide. Since the coupling coefficient from the TM mode to the TE mode due to optical anisotropy exists between the phase constant and the phase constant of the backward wave, the forward wave of the TM mode is coupled with the TE mode due to phase mismatch. Instead, the backward wave of the TM mode couples with the radiated TE mode and dissipates the optical power from the core of the waveguide, thereby eliminating expensive components. It is possible to obtain an optical isolator without using it and without requiring high-precision adjustment, and since the optical isolator can be integrally formed on a substrate, an inexpensive optical isolator suitable for optical integration can be obtained. Can be

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a first embodiment of an optical isolator according to the present invention.

FIG. 2 is a diagram illustrating phase constants of light TE mode and TM mode for explaining the operation principle of the present invention.

FIG. 3 is a diagram showing the relationship between the phase constant of a three-layer slab waveguide and the thickness of a magneto-optical film.

4 (a) to 4 (d) show the manufacturing steps of the first embodiment, respectively.

FIG. 5 is a near-field pattern at an emission end when light is guided through the waveguide of the optical isolator according to the present invention, wherein (a) shows a pattern when a magnetic field of 10 Oe is applied and corresponds to a forward wave; (B) is a diagram showing a pattern when a magnetic field of −10 Oe is applied and corresponding to a backward wave.

FIG. 6 is a diagram showing a measurement system for measuring the isolation of the optical isolator of the present invention.

FIG. 7 is a diagram illustrating isolation characteristics.

FIG. 8 is a sectional view showing a second embodiment of the present invention.

FIG. 9 is a sectional view showing a third embodiment of the present invention.

FIG. 10 is a sectional view showing a fourth embodiment of the present invention.

FIG. 11 is a diagram showing a configuration of a conventional optical isolator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Magneto-optical rib waveguide 2 Magneto-optical film 4 Magnetization vector M

Claims (2)

(57) [Claims] 1. A non-reciprocal optical waveguide at least a part of which is formed of a magneto-optical material, and a phase difference is caused by a difference in a traveling direction of light guided through the optical waveguide due to a magnetization vector in the magneto-optical material. The constants are different from each other, and the cutoff of the TE mode becomes larger than the cutoff of the TM mode due to the structural anisotropy of the waveguide, and the cutoff of the TE mode is between the phase constant of the forward wave and the phase constant of the backward wave. Because of the existence of the coupling coefficient from the TM mode to the TE mode due to optical anisotropy, the forward wave of the TM mode is guided without being coupled to the TE mode due to phase mismatch. The backward wave is the radiation TE
An optical isolator that couples with a mode and dissipates optical power from the waveguide core. 2. The optical isolator according to claim 1, wherein said optical waveguide is provided with a light absorber at a position other than a core portion to absorb said radiation mode.