JP2536510B2 - Mode conversion type optical isolator - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improvement of a mode conversion type optical isolator.

A non-reciprocal portion that performs irreversible mode conversion based on the Faraday effect by a magnetic field having a direction component parallel to the propagation direction of light, which is a part of a magnetic thin film provided on a substrate for guiding light, A reciprocal part that is a part of the magnetic thin film and performs reversible mode conversion of light that has passed through the non-reciprocal part by a magnetic field in a direction perpendicular to the light propagation direction, and the non-reciprocal part and the reciprocal part are formed according to the propagation direction. The forward-direction light that has sequentially passed is reflected outside the reciprocal portion, and when this reflected light passes through the reciprocal portion and the non-reciprocal portion in the opposite direction to the forward light, the reflected light passes through the non-reciprocal portion. A mode conversion type optical isolator of a type in which its deflection surface is in a mode perpendicular to the light in the forward direction is considered. In such a mode conversion type optical isolator, normally, forward light is one of the TM mode and TE mode in advance before passing through the non-reciprocal portion and the reciprocal portion by passing through the mode selecting portion, Since the mode of the reflected light is the opposite mode, the reflected light is blocked in the mode selection section.

Problems to be Solved by the Invention By the way, in the mode conversion type optical isolator of the above type, in order to reduce the outer shape, a non-reciprocal portion and a reciprocal portion having a length of, for example, several mm are mutually provided on a common substrate. Since they are provided adjacent to each other, even if an independent magnetic field forming device tries to form a magnetic field in a direction parallel to the light propagation direction and a perpendicular magnetic field in the non-reciprocal portion and the reciprocal portion, there is a disadvantage that they interfere with each other. On the other hand, if a common magnetic field forming device is used, it is efficient to form a magnetic field component parallel to the light propagation direction in the non-reciprocal portion and a magnetic field component perpendicular to the light propagation direction in the reciprocal portion. Could not be done.

Means for Solving the Problems The present invention has been made in view of the above circumstances, and the gist thereof is a part of a thin film made of a magneto-optical material provided on a substrate. The non-reciprocal part that performs irreversible mode conversion based on the Faraday effect by the magnetic field of the direction component parallel to the light propagation direction, and the magnetic field that is a part of the magnetic thin film and is perpendicular to the light propagation direction. A reciprocal portion that performs reversible mode conversion of light that has passed through the non-reciprocal portion, and forward light that has sequentially passed through the non-reciprocal portion and the reciprocal portion according to the propagation direction is reflected outside the reciprocal portion, and this reflection When the light again passes through the reciprocal portion and the non-reciprocal portion in the opposite direction to the light in the forward direction, the reflected light is in a mode in which the light in the forward direction before passing through the non-reciprocal portion is perpendicular to the deflection surface. In conversion type optical isolator ,
(A) a common magnetic field forming means for applying a magnetic field to each of the non-reciprocal portion and the reciprocal portion, and (b) the propagation direction of light passing through the non-reciprocal portion in the reciprocal portion is the propagation direction of light in the non-reciprocal portion. And a propagation direction conversion means for converting the propagation direction of light so as to be in the intersecting direction.

In this way, the propagation for converting the propagation direction of the light in the reciprocal part so that the propagation direction of the light passing through the non-reciprocal part intersects the propagation direction of the light in the non-reciprocal part. Since the direction changing means is provided, even a magnetic field formed by the common magnetic field forming means, a magnetic field component parallel to the traveling direction of light in the non-reciprocal portion and a magnetic field component perpendicular to the traveling direction of light in the reciprocal portion. Are efficiently formed.

Embodiment Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

In FIG. 1, on one surface of the transparent substrate 10 such as gadolinium / gallium / garnet (Gd ₃ G ₅ O ₁₂ ), sapphire, yttrium / iron / garnet (Y ₃ Fe ₅ O ₁₂ ),
Gadolinium / Iron / Garnet (Gd ₃ Fe ₅ O ₁₂ ), Bi _x Gd _3-x Fe ₅ O ₁₂ with some of them replaced with bismuth
-Dimensional optical waveguide consisting of magnetic thin film made of magneto-optical material such as
Twelve are provided. The three-dimensional optical waveguide 12 has an L-shape bent at about 90 degrees, and is provided in parallel with two adjacent sides of the substrate 10 and in a state of connecting between two sides facing each other. ing. The three-dimensional optical waveguide 12 is an etching method for selectively irradiating a magnetic thin film fixed to the substrate 10 with a laser beam in, for example, phosphoric acid by a well-known thin film forming method such as liquid phase epitaxy (LPE) or sputtering. Is applied to remove unnecessary portions. Then, metal clads 14 and 16 of aluminum or the like are provided on the input side end and the output side end of the three-dimensional optical waveguide 12, respectively. The respective portions of the three-dimensional optical waveguide 12 covered with the metal claddings 14 and 16 function as the first mode selecting section and the second mode selecting section, respectively.

As shown in FIG. 2, the magnetic field forming device 18 includes two iron plates fixed in an L shape and having a width equal to or larger than that of the substrate 10.
That is, the horizontal plate 20 and the vertical plate 22 is provided with a yoke 24 and permanent magnets 24 and 26 fixed to the horizontal plate 20 and the vertical plate 22, respectively, and the substrate 10 is parallel to the horizontal plate 20 and It is fixed at the same height as the permanent magnet 26. The horizontal plate
The 20 and the permanent magnet 26 are provided with notches for passing the laser light output from the semiconductor laser element 32, respectively. The arrow with J in the figure is from permanent magnet 26 to permanent magnet.
These are magnetic lines of force toward the 24, and the three-dimensional optical waveguide 12 on the substrate 10
Among them, the portion parallel to the magnetic force line J functions as the non-reciprocal portion 28, and the portion perpendicular to the magnetic force line J functions as the reciprocal portion 30. Therefore, a magnetic field is formed in the non-reciprocal portion 28 in parallel with the light propagation direction, and in the reciprocal portion 30, a predetermined angle θ (for example, 45 degrees) with respect to the vertical line of the substrate 10 surface in a plane perpendicular to the light propagation direction. A magnetic field is formed in the inclined direction.

 The operation and effect of this embodiment will be described below.

When the laser light emitted from the semiconductor laser device 32 is incident on the input side end of the three-dimensional optical waveguide 12, it is guided to the output side end while repeating total reflection by the boundary surface of the three-dimensional optical waveguide 12. Is emitted from. Usually, such a laser light is orthogonal to the z-axis in the propagation direction and the y-axis and the z-axis in the direction in which the refractive index changes (thickness direction of the three-dimensional optical waveguide 12), as shown in FIG. Further, assuming that the direction orthogonal to the y axis (the surface direction of the substrate 10) is the x axis, the TE mode wave having the field components (E _x , H _y , H _z ) and the field components (H _x , E _y , E _z). ), The TE mode wave has a polarization plane perpendicular to the TM mode wave.
Propagate in propagation mode.

In the propagation process of the laser light in the three-dimensional optical waveguide 12,
First, only the TM mode wave is attenuated during repeated reflection in the first mode selection section, which is the portion covered by the metal clad 14 of the three-dimensional optical waveguide 12, so as shown in FIG. Only TE mode waves having an electric field oscillating plane pass through the first mode selection unit. In the subsequent non-reciprocal part 28, the TE mode wave is converted to the TM mode wave by, for example, 50% based on the Faraday effect due to the magnetic field component parallel to the light propagation direction.
It is converted at a ratio of about. In this conversion, the propagation distance is set so that the mode conversion is 50%.
Not converted to 0% TM mode wave. Therefore, as shown in FIG. 5, the vibration plane of the electric field synthesized by the mixture of the TE mode wave component and the TM mode wave component is directed from the xz plane in the propagation direction (z direction) about the z axis. And turned 45 degrees counterclockwise.

The laser beam thus subjected to the mode conversion propagates along the three-dimensional optical waveguide 12 and thereby propagates in the direction perpendicular to the propagation direction up to that point, that is, in the direction orthogonal to the direction of the magnetic force line J, and therefore continues. In the reciprocal portion 30, the TM mode wave passing through the non-reciprocal portion 28 is converted into the TE mode wave by, for example, about 50% based on the Cotton-Mouton effect (effect similar to birefringence) due to the magnetic field component orthogonal to the light propagation direction. Converted in percentage. That is, the light passing through the reciprocal portion 30 is made into a substantially TE mode wave, and the vibration surface of the electric field is returned to the xz plane as shown in FIG. Then, this TE mode wave passes through the second mode selection unit (the portion of the three-dimensional optical waveguide 12 covered by the metal clad 16) and is emitted to the outside from the other end of the three-dimensional optical waveguide 12. In the second mode selection unit, only the TE mode wave is allowed to pass, like the first mode selection unit.

The polarized light emitted to the outside from the other end of the three-dimensional optical waveguide 12 is reflected as it is, and when this reflected light enters from the other end, it is allowed to pass through the second mode selection unit as it is. This reflected light is converted from the TE mode wave to the TM mode wave in the subsequent reciprocal portion 30 by the same effect as described above, but is converted at a rate of, for example, about 50% as described above. As described above, the oscillating plane of the electric field is changed from the xz plane to the z direction by combining the TE mode wave component and the TM mode wave component.
It is in a state in which it is turned 45 degrees clockwise around the axis in the propagation direction (-z direction). That is, in the vibrating surface of the electric field of the reflected light at this time, the light in the forward direction is not reciprocal.
The vibration surface is the same as when it passed through. In addition,
FIG. 7 and FIG. 8 which will be described later show a state of being viewed in opposition to the propagation direction of reflected light.

As described above, the reflected light in which the vibration surface of the electric field is rotated 45 degrees clockwise from the xz plane in the propagation direction about the z axis, has the Faraday effect as described above in the non-reciprocal portion 28. As a result of performing mode conversion based on this, the vibration surface of the electric field of the reflected light is further swung 45 degrees clockwise around the z axis in the propagation direction (−z direction),
As shown in FIG. 8, it becomes a yz plane and becomes a TM mode wave. The reflected light propagating in the TM mode in this way is attenuated in the subsequent first mode selection unit, and thus is not emitted from the input side end of the three-dimensional optical waveguide 12. Therefore, the reflected light of the laser light emitted from the semiconductor laser element 32 is prevented from entering the semiconductor laser element 32 again, and the operation of the semiconductor laser element 32 is stabilized.

As described above, according to the optical isolator of this embodiment,
Since the laser light is guided through the L-shaped three-dimensional optical waveguide 12, the propagation direction in the non-reciprocal portion 28 and the propagation direction in the reciprocal portion 30 intersect each other, so that the common magnetic field forming device 18 is provided. Even if is used, the magnetic field of the direction component parallel to the propagation direction in the non-reciprocal portion 28 and the magnetic field of the direction component perpendicular to the propagation direction in the reciprocal portion 30 can be efficiently obtained. Therefore, in this embodiment, the three-dimensional optical waveguide 12
Corresponds to the propagation direction changing means.

Next, another embodiment of the present invention will be described. In the following description, the same parts as those in the above-described embodiment will be designated by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 9, a cylindrical permanent magnet 40 may be used as the magnetic field forming means, and the substrate 10 may be arranged inside the permanent magnet 40 and inclined by a predetermined angle θ _e . In this case, since the direction of the magnetic field is parallel to the axis of the permanent magnet 40, if the magnitude of the magnetic field is H and the angle between the vertical line of the substrate 10 and the direction of the magnetic field is θ _e , non-reciprocal. The effective magnetic field magnitude in the portion 28 is H sin θ _e . Further, as shown in FIG. 10 as a magnetic field forming means for the non-reciprocal portion 28, a conductor pattern 44 including a large number of wirings 42 arranged in parallel across the three-dimensional optical waveguide 12 and having current flowing in the same direction. May be formed.

As shown in FIG. 11, the curved portion 46 of the three-dimensional optical waveguide 12 may be made of a material having no magneto-optical effect, for example, T _i O ₂ . By doing so, it is possible to prevent a complicated mode conversion due to the magneto-optical effect from occurring in the curved portion 46 by applying the magnetic field, and the characteristics of the optical isolator are further improved.

In the above-described embodiment, the laser light is guided through the three-dimensional optical waveguide 12 so that the propagation direction in the non-reciprocal portion 28 and the reciprocal portion 30
The propagation direction of the laser light is converted by approximately 90 degrees, but the propagation direction of the laser light may be converted by diffraction or reflection.

In FIG. 12, on the substrate 10, a two-dimensional optical waveguide 50 made of a magneto-optical material is formed on one surface by a well-known thin film forming means, and laser light is emitted on the two-dimensional optical waveguide 50. Metal clads 52 and 54 are fixed to the incident and outgoing portions, respectively. The portions of the two-dimensional optical waveguide 50 covered by the metal clads 52 and 54 function as a first mode selecting section and a second mode selecting section, respectively. A grating (diffraction grating) 56 is provided at one corner on the substrate 10. The grating 56 is, for example, a photoresist film as shown in FIG. 13, S _i O ₂ film multiple thin wire 5 parallel to each other due to
8 or a plurality of parallel grooves 6 as shown in FIG.
Consists of 0. Since the equivalent index of refraction in the two-dimensional optical waveguide 50 is periodically changed by such fine lines or fine grooves, it passes through the first mode selection section and the non-reciprocal section 28 and reaches the grating 56. The generated laser light is directed to the reciprocal portion 30 and the second mode selection portion by Bragg diffraction. Further, in one corner of the substrate 10 shown in FIG. 15, in place of the grating 56, a reflecting surface 62 formed obliquely for reflecting the laser light is formed. In this embodiment, the propagation direction of the laser light is changed by the reflection on the reflecting surface 62. Therefore, in the above embodiment, the grating 56 and the reflecting surface 62 correspond to the propagation direction changing means, respectively.

Although one embodiment of the present invention has been described above with reference to the drawings, the present invention is also applicable to other aspects.

For example, in the above-described embodiment, the propagation direction of the laser light in the non-reciprocal portion 28 and the propagation direction of the laser light in the reciprocal portion 30 are converted to a substantially right angle, but they need not be right angles.

The mode selection unit of the foregoing embodiment, the metal cladding on the magnetic thin film for guiding the laser beam is should be constituted by being fixed, the anisotropy material is secured, such as L _i N _b O ₃ It may be configured as follows.

Further, instead of the permanent magnet 40, an electromagnet having a wire wound in a cylindrical shape may be used, or the permanent magnets 24 and 26 may be used.
Instead of this, an electromagnet in which an electric wire is wound around a core may be used.

The above description is merely an example of the present invention, and the present invention can be variously modified without departing from the spirit thereof.

[Brief description of drawings]

FIG. 1 is a perspective view showing a substrate of the embodiment shown in FIG. FIG. 2 is a front view showing the configuration of an embodiment of the present invention. FIG. 3 is a diagram showing axes used in the description of the operation of the embodiment shown in FIG. 4, FIG. 5, FIG.
FIG. 7 and FIG. 8 are diagrams showing the light states of the respective parts in the embodiment of FIG. 9 and 10 are views for explaining the magnetic field forming means in another embodiment of the present invention. FIG. 11 is a diagram showing a three-dimensional optical waveguide according to another embodiment of the present invention. FIG. 12 shows another embodiment of the present invention and corresponds to FIG. Figures 13 and 14 are
FIG. 13 is a diagram showing in detail the gratings of the embodiment shown in FIG. 12; FIG. 15 is a diagram showing another embodiment of the present invention. 12: Three-dimensional optical waveguide (propagation direction changing means) 18: Magnetic field forming device (magnetic field forming means) 28: Non-reciprocal part 30: Reciprocal part 40: Permanent magnet (magnetic field forming means) 44: Conductor pattern (magnetic field forming means) 56 : Grating (propagation direction changing means) 62: Reflective surface (propagation direction changing means)

Claims (1)

(57) [Claims] 1. A non-reversible mode conversion based on the Faraday effect, which is a part of a thin film made of a magneto-optical material provided on a substrate and which has a magnetic field of a direction component parallel to the propagation direction of light in the thin film. A non-reciprocal portion according to the propagation direction, which includes a reciprocal portion and a reciprocal portion that is a part of the magnetic thin film and performs a reversible mode conversion of light passing through the non-reciprocal portion by a magnetic field in a direction perpendicular to the light propagation direction. Light in the forward direction that has sequentially passed through the reciprocal portion and the reciprocal portion is reflected outside the reciprocal portion, and when the reflected light passes through the reciprocal portion and the nonreciprocal portion in the opposite direction to the light in the forward direction, the reflected light is reflected. In the mode conversion type optical isolator of the type in which the light in the forward direction before passing through the non-reciprocal portion and the deflecting surface are in a vertical mode, common magnetic field forming means for applying a magnetic field to the non-reciprocal portion and the reciprocal portion, respectively, Smell of the reciprocal part And a propagation direction conversion unit that converts the propagation direction of light so that the propagation direction of the light passing through the non-reciprocal portion intersects with the propagation direction of the light in the non-reciprocal portion. Optical isolator.