JPH02201420A - Faraday rotation element and optical switch using it - Google Patents

Abstract

PURPOSE:To obtain high reliability and enable low-electric-power driving by arranging the yoke of a semihard magnet which has passing holes for light so that a Faraday rotator is included in the center and a coil is also included, and adjusting a magnetic field applied to the yoke by supplying a current to the coil. CONSTITUTION:The passing holes 4a and 4b for light are formed penetrating the hollow yoke 1 which is made of a semihard magnet partially or wholly and the Faraday rotator 2 is arranged in the yoke 1 where the optical axis of the light passes to form a magnetic circuit close to a closed magnetic path by the Faraday rotator 1 and yoke 1. Further, the coil 3 for changing the magnetism state of the yoke 1 is put in the space surrounded with the yoke 1 and Faraday rotator 2. This constitution is much less in magnetic resistance than a structure of conventional technique, the magnetism of the Faraday rotator 2 can be inverted with a small current, and the current need not be supplied continuously to the coil 3 because of the saturation of the Faraday rotator. Consequently, no large electric power is required to invert the magnetism and the practical element is realized.

Description

Detailed Description of the Invention "Industrial Application Field" This invention relates to a Faraday rotation element that changes the polarization plane of light using an electric current, and an optical switch that uses this Faraday rotation element to switch the propagation direction of light between two directions. It is related to.

4. Prior Art For example, when trying to propagate light generated by a semiconductor laser as a light source alternately to two optical fibers as necessary, conventionally the prism 13 was mechanically moved as shown in FIG. An optical switch is used which has a mechanism for changing the optical path of the incident light 5 to a direction. Here, 12a, 1
2b is a total reflection prism. However, while this method has the advantage of obtaining a high extinction ratio, it does not require mechanical movement. tn, when used for a long period of time, deterioration such as optical axis misalignment occurs due to wear of the movable parts, resulting in reliability problems. One way to solve this problem is to
As shown in Figure 3, optical switches using the magneto-optical effect were produced in Utility Model Publication No. 63-144614 and Utility Model Publication No. 63-14.
It is disclosed in Japanese Patent No. 4615.

This utilizes the principle of an optical isolator. When a magnetic field 7a is generated by passing a current through the coil 3 in one direction, and the Faraday rotator 2 is thereby magnetized,
After the incident light 5 passes through the first polarizer 6a, the plane of polarization is rotated by 45 degrees by the Faraday rotator 2, and the incident light 5 passes through the second polarizer 6b whose transmission direction is aligned with the plane of polarization.

In this state, if the magnetization direction of the Faraday rotator 2 is reversed by reversing the direction of the current and reversing the magnetic field direction to 7b, the incident light 5 will be perpendicular to the transmission direction of the second polarizer 6b, so that The light cannot pass through this and is bent in the right angle direction as shown in the direction shown in the figure. Therefore, by connecting an optical waveguide such as an optical fiber in this direction, light can be propagated in another direction.

That is, by reversing the magnetization direction of the Faraday rotator 2, the incident light 5 can realize a so-called electromagnetically controllable optical switch in which the propagation direction of the light can be switched from direction a to direction b.

[Problems to be Solved by the Invention] However, in an optical switch using the function of a conventional optical isolator as shown in FIG. 3 above, an extremely large 1'I! There was a question of power or necessity.

That is, in the magnetic circuit shown in FIG. 3, the magnetization of the Faraday rotator 2 could not be reversed unless a large current was passed through the wire 3 wound around the holder 15.

One way to solve this problem is to use a Faraday rotator 2 made of a YIG single crystal film formed on a substrate 9 shown in Fig. 4. , 513-14. In the optical switch shown in Fig. 4, the magnetic yoke l has a ring shape with a portion missing, and a thin film type Faraday rotator 2 is placed in the missing portion, creating a magnetic circuit that is substantially close to a closed magnetic path. .

Further, the wire ring 3 is wound around the ring-shaped magnetic yoke l.

This method uses a thin film Faraday rotator 2, so the demagnetizing field is extremely small, and has the advantage that the magnetization state of the Faraday rotator 2 can be reversed with a small magnetic field, that is, with a small current.

However, this method cannot pass light with a large beam diameter, which naturally limits its range of application.

As described above, the conventional optical switch structure using the magneto-optic effect requires a large amount of electric power to reverse the magnetization, and there are various problems in realizing a practical device.

An object of the present invention is to provide a Faraday rotation element using a magneto-optic effect having a magnetic circuit with a new structure to solve this problem, and an optical switch using the same.

[Means for Solving the Problems] A light passage hole is provided so as to penetrate through a hollow yoke that is partially or entirely made of a semi-hard magnet, and a Faraday is provided inside the yoke at a position through which the optical axis of the light passes. Arrange the rotor,
A magnetic circuit close to a closed magnetic path is formed by the Faraday rotator and the yoke, and a wire ring for changing the magnetization state of the yoke is included in a space surrounded by the yoke and the Faraday rotator. This is a Faraday rotation element.

In the present invention, by magnetizing the yoke of the semi-hard magnet, the Farabout rotator can be magnetized by the magnetic field generated by the yoke, and the plane of polarization of incident light can be rotated. By using a semi-hard magnet, it is possible to maintain a predetermined rotation angle of the plane of polarization without constantly passing current through the coil.

The Faraday rotator element configured in this manner can be used as an optical switch by instantaneously passing a large current in the opposite direction to reverse the magnetization state of the semi-hard magnet, that is, the magnetization state of the Faraday rotator.

Furthermore, it is preferable to arrange polarizers on both sides of the Faraday rotator to align the planes of polarization incident on the Faraday rotator, since the extinction ratio can be improved.

FIG. 1 is an explanatory diagram showing the principle structure of the optical switch of the present invention. Two polarizers 6a on both sides of the Faraday rotator 2
, 8b are disposed, and a wire ring 3 is wound around the Faraday rotator 2 in order to reverse the rotation direction of the plane of polarization of light passing through the Faraday rotator 2.

Furthermore, a yoke 1 made of a semi-hard magnet having light passage holes 4a and 4b is disposed such that it encloses a Faraday rotator 2 in the center and also encloses a wire ring 3. The yoke 1 and the Faraday rotator 2 constitute a nearly closed magnetic circuit. The lead wires 14a and 14b of the eight wheels 3 are taken out of the yoke l, and by passing a current through them, the magnetic field applied to the yoke l is adjusted. Therefore, when this current is reversed, the magnetization direction of the yoke 1 is reversed, and the Faraday rotor magnetization direction is also reversed.

Due to this structure, the structure of the present invention shown in FIG. 1 has much lower magnetic resistance than the prior art structure shown in FIG. Therefore, it is no longer necessary to keep current flowing through the coil 3.

In this case, when the magnetic field points in the solid line direction 7a, the incident light 5 propagates in the a direction, but when the magnetic field points in the dotted line direction 7b, the incident light 5 propagates in the b direction.

Furthermore, there is a very close relationship between the diameter of the Faraday rotator and the diameter of the light passage hole in the yoke in terms of performance, and these are important parameters for the strength of the magnetic field applied to the Faraday rotator. The coil current required for the Faraday rotator to saturate becomes very small when the diameter of the Faraday rotator is larger than the diameter of the light passage hole in the yoke.

Further, in the Faraday rotator element described above, the structure of only the yoke, Faraday rotator, and wire ring is shown, but a bobbin for holding the wire ring may be added, or a yoke, Faraday rotator, wire ring, etc. may be added. Of course, a support member may be inserted to fix the position of the ring.

When constructing an optical switch, a polarizing beam splitter or other birefringent prisms such as a Glan-Thompson prism or a wedge prism can be used as the polarizer.

"Example" Examples of the present invention will be described in detail below.

(Example 1) An optical switch having the structure shown in FIG. 1 was created. Here, 6a and 6b are polarizers made of polarizing beam splitters, 2 is a Faraday rotator made of cylindrical YIG through which the incident light 5 passes, 3 is a wire ring that applies a magnetic field to the yoke l, and l
is F in which the wire ring 3 and the Faraday rotator 2 are mounted in the hollow part.
Yokes 4a and 4b made of e-Cr-Co magnets are light passing holes provided in the yoke l. Note that the rotation angle of the Faraday rotator 2 was 45 degrees at saturation.

Lead wires 14a and 14b of the wire ring 3 are taken out of the yoke and connected to the circuit shown in FIG. That is, in the initial state, the switches SWI, SW2 . SW3. SW4 is o
It is in the state r; F, and electric charge is accumulated in the capacitor C (20 μF) from the electric current JE via the resistor R.

When capacitor C is sufficiently charged, SWl,
When SW2 is turned on, a discharge current with a high peak value flows in the forward direction (solid line direction) of the coil 3. When the discharge is almost completed, SWI and SW2 are turned off again. In this state, the yoke 9 made of a Fe--Cr--Co magnet is magnetized in one direction, and thereby the Faraday rotator 2 is also stably magnetized in one direction. At this time, the light is propagated in the direction a in FIG.

If you want to switch the light propagation direction from direction a to direction b, use the circuit shown in FIG. 5 and switch SW3. This can be achieved by turning on SW4 almost simultaneously. At this time, the discharge current flows in the opposite direction (
Since the light flows in the direction of the dotted line), the yoke 1 made of a semi-hard magnet and the Faraday rotator 2 are magnetized in opposite directions, and the light propagates in the direction b in FIG. FIG. 6 shows the relationship between ON/OFF of each switch shown in FIG. 5 and current.

This embodiment is characterized in that a stable magnetization state of the Faraday rotor 1 can be achieved without continuing to supply current to the coil 3.

Comparing the structure of the present invention shown in FIG. 1 with the structure of the prior art shown in FIG. Magnetization can be reversed.

As a result of experiments, the conventional structure requires a continuous current of approximately 1 OA, but in the embodiment of the present invention, the peak value is 5 A or less, the applied voltage is 50 V, and the pulse width of the current discharged from capacitor C is 1 m5 ec. The following was sufficient.

Moreover, it is not necessary to keep current flowing in order to maintain a stable state.

(Example 2) A Faraday rotation element shown in FIG. 7 was created.

The second Faraday rotation element is the yoke 1 shown in Example 1 above.
, a Faraday rotator element consisting of a Faraday rotator 2 and a wire ring 3, in which a divided portion 8 is provided in the yoke l.

Using this Faraday rotation element, the light passing holes 4a and 4b are
The current value of the coil 3 necessary for saturation of the Faraday rotator 2 was measured by changing the ratio of the diameter DO of the Faraday rotor 2 to the diameter DY of the Faraday rotator 2. The results are shown in FIG.

As shown in Figure 8, when DO is larger than DY, the saturation current is quite large at 5 A or more, but on the other hand, when Do is larger than DY
When it is smaller, the saturation current will be much smaller than 5A.

In this example, the shape of the Faraday rotator is cylindrical, but any engineer related to this technology will know that the effect is the same whether the cross-sectional shape of the Faraday rotor is rectangular or polygonal. It will be easy to understand. Furthermore, various shapes can be considered for the light passage hole at the center of the yoke.

In these cases, the size relationship of the dimensions obtained in FIG. 8 can be considered by converting DO into the minimum-one method of the passage hole and DY into the maximum dimension of the cross-sectional shape of the Faraday rotator.

(Example 3) The optical switch shown in Example 1 was manufactured with the relationship Do=0.8DY, and the wavelength of the incident light 5, extinction ratio, and insertion loss were measured. Figure 9 shows the results. In FIG. 9, the solid line indicates the case where the current is in the forward direction, that is, the internal magnetic field direction is 78 in FIG. This is the case when Further, the wavelength of the incident light was set around 1.55 μI assuming a semiconductor laser.

As shown in Figure 9, the insertion loss is less than 1 dB in both directions, and the amount of attenuation of leaked light in the undesirable propagation direction is 30 dB.
dB or more is ensured, indicating that an excellent optical switch can be obtained.

(Embodiment 4) FIG. 10 shows another embodiment of the present invention. When the beam diameter of the incident light is relatively large, the diameter of the Faraday rotator 2 becomes large, so the demagnetizing field coefficient becomes large. , it is necessary to pass a large current. In this case, as shown in FIG. 10, a protrusion 10 is provided on the inner surface of the light passage hole, and
It is effective to increase the volume of the ring.

(Example 5) FIG. 11 shows another example of the present invention. As shown in the figure, a notch 11 is provided in the peripheral portion of the hollow yoke 1. By doing so, the temperature increase in the wire ring 3 caused by passing a current through the wire ring 3 can be suppressed to some extent by the cooling effect of air convection. Furthermore, the magnetic resistance in the peripheral portion of the yoke 1 and the magnetic resistance in the vicinity of the light passage holes 4a and 4b can be designed to be approximately the same, and the magnetic charge appearing on the surface of the yoke 1 can be minimized.

Further, in the embodiment of the present invention, the entire yoke 1 is made of a semi-hard magnet, but even if the yoke 1 is divided and some portions are made of semi-hard magnets, the effects of the present invention will not change. Especially the 10th
It is effective to make only the portion of the protrusion 10 on the inner surface of the light passage hole shown in the figure a semi-hard magnet.

In the embodiments of the present invention, the Faraday rotator is made of YIG single crystal, but other materials such as Bi-substituted L P E
(Liquid Phase Epitaxial)
Membranes etc. can also be used. However, since the LPE film is too thin, it may not be possible to secure enough space for the excitation coil, or even if it is possible, the distance between the center of the yoke and the LPE film may be too far, increasing magnetic resistance. .

"Effects of the Invention" As described above, the Faraday rotation element of the present invention has high reliability and can be driven with low power. Furthermore, since the coercive force of a yoke made of a semi-hard magnet is used, it is particularly effective as a low-speed optical switch.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of the configuration of a Faraday rotation element according to the present invention, FIGS. 2, 3, and 4 are diagrams showing the structure of a conventional optical switch, and FIG. 5 is a diagram showing the structure of a conventional optical switch. 6 is an explanatory diagram of the discharge current in the power supply circuit of FIG. 5, FIG. 7 is a diagram showing the relationship between the light passage hole and the Faraday rotator, and FIG. 9 is a diagram showing the relationship between instantaneous current and Faraday rotation angle, FIG. 9 is a diagram showing an example of the extinction ratio and insertion loss of the Faraday rotation element of the present invention, and FIG.
Figures 1 and 11 are diagrams showing other embodiments of the present invention. l; Yoke, 2; Faraday rotator, 3; wire ring, 4 a, 4 b; light passage hole, 5; incident light 6
a, 6b; polarizer; 7 a, 7 b; magnetic field direction. 10; Protrusion, 11; Notch Fig. 1 Fig. 3 Fig. 2 Fig. 4 Fig. 4

Claims (7)

[Claims] (1) A light passage hole is provided so as to penetrate through a hollow yoke partially or entirely made of a semi-hard magnet, and a Faraday rotator is arranged at a position where the optical axis of the light passes through the hollow part of the yoke. The Faraday rotator and the yoke form a magnetic circuit close to a closed magnetic path, and a space surrounded by the yoke and the Faraday rotator includes a wire ring for changing the magnetization state of the yoke. Characteristic Faraday rotation element. (2) The minimum dimension of the light passage hole provided in the yoke according to claim 1 is smaller than the maximum dimension in the direction perpendicular to the optical axis of the Faraday rotator. Faraday rotary element as described. (3) The Faraday rotation element according to claim 1 or 2, wherein the yoke according to claim 1 or 2 can be divided into two or more parts. (4) A Faraday rotation element, wherein a part of the yoke according to any one of claims 1 to 3 is removed. (5) In the Faraday rotator according to claims 1 to 4, the Faraday rotator is YIG (Y_3Fe_5O
A Faraday rotation element characterized by using a garnet type ferrite single crystal containing _1_2) as a main component. (6) An optical switch characterized in that polarizers are disposed on both sides of the Faraday rotator according to any one of claims 1 to 5, and the optical path is changed by reversing the magnetization direction of the Faraday rotator. (7) Claim 6 characterized in that one stable state of the Faraday rotor is obtained by passing current through the wire for a certain period of time in the forward or reverse direction and magnetizing the yoke.
Optical switch described in.