JP2772759B2 - Polarization plane switch and optical switch using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical polarization plane switch which switches the polarization direction of light by reversing the direction of a magnetic field applied to a Faraday rotator, and an optical switch using the same. It is. More specifically, according to the present invention, when a magnetic field is applied, a part of the Faraday rotator is in a state of non-saturation, and the remaining part is in a state of magnetization saturation. The present invention relates to a polarization plane switch in which a light beam passes in a direction substantially perpendicular to the polarization plane.

[0002]

2. Description of the Related Art In an optical communication system or an optical measuring device, an optical switch for spatially switching the traveling direction of light is required. For this, a 1 × 1 type optical switch (optical shutter), a 2 × 2 type optical switch, or a combination thereof is used. Various optical switch structures have been proposed, such as a mechanical type, a type using an electro-optical effect or an acousto-optical effect, or a magneto-optical type. Among them, the magneto-optical type is a type in which the polarization plane of the light beam is magnetically switched by a magneto-optical type polarization plane switch installed inside, and can be operated at a higher speed and smaller than a mechanical type. It has the advantage of having excellent long-term reliability.

A typical magneto-optical polarization plane switch includes a yoke made of a magnetic material, a winding provided on the yoke, and a thin Faraday rotator inserted into a gap of the yoke. As the Faraday rotator, a bismuth-substituted iron garnet single crystal film by the LPE method is being used frequently. The reason is that bismuth-substituted iron garnet has a large Faraday rotation coefficient, so that a relatively thin film structure can be obtained, and the LPE method has an advantage of high productivity.

[0004] The Faraday rotator is inserted into a gap portion provided in the yoke perpendicularly to the direction of the gap length. In that case, the Faraday rotator has such a size that a part thereof protrudes outward from the gap part, and the light beam passes through the protruding part. Here, in the prior art, the above-mentioned protruding portion of the Faraday rotator is made as small as possible, and a magnetic field is applied so that the magnetization of the garnet single crystal film is saturated over the entire surface by the magnetic field of the yoke.

[0005]

The switching time in the conventional magneto-optical switch is several tens to 250 .mu.sec (typically about 80 .mu.sec in comparison with the prior art by the present inventors).
It is. By the way, 1 × 1 type optical switch (optical shutter)
One of the uses is an excessive light blocking device proposed by the present inventors. This is because when the optical amplifier causes abnormal oscillation,
Light of excessive intensity exceeding the specified level propagates,
Optical components such as the light-receiving element and electronic components downstream of the light-receiving element may be destroyed.Therefore, an optical power detector, a delay fiber, and an optical switch are arranged in series in the optical path. This device protects optical components and electronic components by turning off the switch and blocking excessive light passing through the delay fiber.

The cutoff time required in the above optical switch is determined by the delay time of the fiber. For example, when the fiber length is 200 m, it is about 1 μs, and when the fiber length is 2000 m, it is about 10 μs. The fiber length directly affects the size of the excessive light blocking device, and it is needless to say that the shorter the fiber length, the better. Practically, an optical switch that can shut off light at a high speed of 10 μsec or less and has high reliability is required.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magneto-optical polarization plane switch and an optical switch that switch at high speed.

[0008]

Means for Solving the Problems The present inventors have studied the relationship between the switching speed and the applied magnetic field distribution for a Faraday rotator made of an iron-containing garnet single crystal film. As a result, the magnetic field in which the magnetization of the iron-containing garnet single crystal film is more spatially distributed than when a magnetic field that saturates the entire surface of the iron-containing garnet single crystal film is applied, and the magnetization of a part of the iron-containing garnet single crystal film becomes unsaturated It was found that the switching speed was improved when multiplied by. The present invention has been made based on such knowledge.

The present invention is a polarization plane switch comprising a Faraday rotator made of an iron-containing garnet single crystal film, and magnetic field reversing magnetic field applying means for applying a magnetic field to the Faraday rotator. In the present invention, when the magnetic field is applied by the magnetic field applying means, the magnetization of one part of the Faraday rotator is in an unsaturated state, and the remaining part is in the state of saturation of magnetization, and the part where the magnetization is saturated is formed on the film surface. It has a structure in which a light beam passes in a direction substantially perpendicular to that, which is characteristic.

Specifically, for example, a substantially C-shaped plate-like yoke made of a semi-hard magnetic material, a winding provided on the yoke, and a gap provided in the yoke are provided with respect to the direction of the gap length. A Faraday rotator made of an iron-containing garnet single crystal film inserted vertically, and the Faraday rotator has a size such that most of the Faraday rotator protrudes outward from the gap portion, and the tip of the protruding portion is magnetized. The light beam passes through the saturated region of magnetization in the protruding portion in a direction substantially perpendicular to the film surface. Alternatively, a magnetic shield material may be attached to the tip of the protruding portion of the Faraday rotator to form a magnetization unsaturated region. There is also a configuration in which a main yoke and an auxiliary yoke are used, and a magnetic field generated by the main yoke and the auxiliary yoke is applied to the Faraday rotator in opposite directions. Further, the gap portion of the yoke may be configured such that a portion having a short gap length is provided at a plurality of intervals, and a portion having a long gap length becomes an unsaturated region of magnetization.

The Faraday rotator is made of a bismuth-substituted iron garnet single crystal film grown by a liquid phase epitaxy method and having a compensation temperature, at a temperature of 1120 ° C. or more and 1180 ° C. or less and within 7 hours. The one that has been heat-treated under the conditions is optimal.

According to the present invention, for example, two wedge-shaped birefringent single crystals arranged to face each other, the above-described polarization plane switch having a Faraday rotator arranged between them, and an input and output lens An optical switch is constituted by the body optical fiber. Alternatively, two polarization beam splitters disposed opposite to each other, a polarization plane switch as described above in which a Faraday rotator is disposed therebetween, and 1/1 positioned between the Faraday rotator and one of the polarization beam splitters. An optical switch can also be configured with a two-wavelength plate.

[0013]

According to the present invention, the Faraday rotator made of the iron-containing garnet single crystal film is inserted into the gap of the yoke, and the vicinity of the gap is magnetically saturated. is there. With such a magnetized state, the switching speed is improved. The reason is considered as follows. That is, the conventional magnetization reversal process of the iron-containing garnet single crystal film when the entire surface is magnetically saturated,
First, a part of the magnetic moment is directed in the opposite direction, and the nucleus serves as a nucleus to cause domain wall displacement, which eventually leads to all magnetization reversals. In the present invention, since the magnetization is in an unsaturated state in a part of the iron-containing garnet single crystal film, a nucleus for magnetization reversal already exists in the first stage of the switching operation. Therefore, in the present invention, since the nucleation step can be omitted, the magnetization reversal occurs earlier and the switching time is shortened.

In the present invention, since the light beam passes through the magnetically saturated portion near the gap, the isolation and insertion loss are not different from those of the conventional product.

Further, when an appropriate heat treatment is applied to the iron-containing garnet single crystal film, the area S surrounded by the hysteresis related to the magnetic field dependence of the Faraday rotation angle, the saturation magnetization, and the magnetic field required for saturation does not change. Decreased,
Further, under the optimum condition, the area S becomes substantially zero. The area surrounded by the hysteresis has a correlation with the switching time, and the use of the Faraday rotator greatly speeds up the switching of the optical switch.

[0016]

FIG. 1A shows an embodiment of the optical switch according to the present invention. This is an example of a 1 × 1 type optical switch (optical shutter). The polarization plane switch includes a substantially C-shaped flat yoke 10 made of a semi-hard magnetic material, and a winding 12
And a gap portion 14 provided in the yoke 10
And a plate-like Faraday rotator 16 inserted perpendicular to the direction of the gap length. Here, the Faraday rotator 16 is made of a bismuth-substituted iron garnet single crystal film grown by an LPE method (liquid phase epitaxial method) and having a compensation temperature. The Faraday rotator 16 has a size such that most of the Faraday rotator protrudes outward from the gap portion 14, and as shown in FIG. 1B, the tip of the protruding portion is an unsaturated region 16a of magnetization. The light beam passes through the magnetization saturation region 16b of the protruding portion in a direction substantially perpendicular to the film surface. On both sides of the Faraday rotator 16, a wedge-shaped birefringent single crystal 18,
20 and a lens-integrated optical fiber 2 for input.
2 and an output lens-integrated optical fiber 24 are provided.

That is, in this embodiment, the Faraday rotator 16 is enlarged so that most of the Faraday rotator protrudes from the gap portion 14, and the magnetization of the Faraday rotator 16 is changed by the magnetic field applying means using the yoke 10 and the winding 12. It is set so as not to be saturated over the entire film. Conversely, the leakage magnetic field generated by the magnetic field applying means is set to a magnitude such that the magnetization of the Faraday rotator 16 is not saturated in the entire film.

The Faraday rotator 16 is formed by processing a single crystal film of (GdBi) ₃ (FeAlGa) ₅ O ₁₂ formed by the LPE method to 2 × 7 × 0.50 mm, and processing a 2 × 7 mm surface to a mirror surface. It was done. The composition of the single crystal film is Gd _2.02 B
i _0.98 Fe _4.43 Al _0.44 Ga _0.13 O ₁₂ , which is −
It has a compensation temperature at 5 ° C. When magnetized perpendicular to the film surface, the saturation magnetization at room temperature is 80 G, and the magnetic field Hs required for saturation is
Is 80 Oe, and the Faraday rotation angle at a wavelength of 1.55 μm is 45 degrees. As the birefringent single crystals 18 and 20 serving as a polarizer and an analyzer, wedge-shaped rutile single crystals were used. Single mode fibers were used for the lens-integrated optical fibers 22 and 24.

FIG. 2 is an explanatory diagram of the operation. When viewed in the direction of the input light, the wedge-shaped birefringent single crystal 18 serving as a polarizer and the wedge-shaped birefringent single crystal 20 serving as an analyzer have different thicknesses on the left and right, and a thick portion and a thin portion face each other. Are combined. And a wedge-shaped birefringent single crystal 18 serving as a polarizer
Has an angle α (= 2) clockwise with respect to the vertical direction.
The optical axis of the wedge-shaped birefringent single crystal 20 serving as an analyzer is inclined counterclockwise at an angle α (= 22.5 degrees) with respect to the vertical direction. The arrow H indicates the direction of the magnetic field applied to the Faraday rotator 16 in the forward direction. In this state, the input light is applied to both wedge-shaped birefringent single crystals 18, 2
0 is transmitted and emitted. When the drive voltage to the winding is switched and a magnetic field in the opposite direction is applied, the Faraday rotation direction differs by 90 degrees, and the input light beam cannot pass through the wedge-type birefringent single crystal 20 serving as an analyzer and is blocked. In these switching operations, the light beam is applied to the Faraday rotator 16.
The yoke 10 passes through the magnetization saturation region of
The tip portion moving away from 4 is always an unsaturated region. Therefore, since there is always a nucleus for magnetization reversal, transmission and cutoff of input light can be controlled at high speed by supplying a pulse current to the winding. According to the results of the trial production, the switching time of the optical switch having the above configuration was 25 μsec. The insertion loss was 0.5 dB and the isolation was 40 dB.

FIG. 3 shows another embodiment of the optical switch according to the present invention. Since the basic configuration may be the same as that of the example of FIG. 1, corresponding portions are denoted by the same reference numerals for simplification of description. Also in this embodiment, the Faraday rotator 1
6 has a size that most of the size protrudes outward from the gap portion 14, and the tip of the protruding portion is covered with a U-shaped magnetic shield material. A magnetic field is applied to the Faraday rotator 16 by a magnetic field applying means including the yoke 10 and the winding 12. As a result, the portion not covered with the magnetic shield material 28 becomes a saturation region of magnetization, but the portion covered with the magnetic shield material 28 remains unsaturated. Although this configuration increases the number of components, it has the advantage that the size of the Faraday rotator can be made smaller than in the configuration of FIG.

FIG. 4A shows a further embodiment of the optical switch according to the present invention. In addition to a substantially C-shaped plate-shaped main yoke 30 made of a semi-hard magnetic material and a winding 32 provided on the main yoke 30, a substantially C-shaped plate-shaped auxiliary yoke 31 made of a semi-hard magnetic material, and an auxiliary yoke 31 Winding 33 applied to
These are arranged so that the gap portion 34 provided on the main yoke 30 and the gap portion 35 provided on the auxiliary yoke 31 are on the same plane. The Faraday rotator 36 made of an iron-containing garnet single crystal film is inserted in the gap portions 34 and 35 in a direction perpendicular to the direction of the gap length. The DC pulse current is supplied so that the magnetic field of the main yoke 30 and the magnetic field of the auxiliary yoke 31 are applied to the Faraday rotator 36 in opposite directions, and the magnetic field generated by the main yoke 30 is To be larger than the magnetic field generated by Accordingly, for example, as shown in FIG. 4B, the Faraday rotator 36 is magnetized downward by the auxiliary yoke 31 while the major portion 36 a is magnetized downward by the main yoke 30. Will be. The light beam passes through the portion 36a where the magnetization is saturated by the main yoke of the Faraday rotator 36 in a direction substantially perpendicular to the film surface. On both sides of the Faraday rotator 36, wedge-shaped birefringent single crystals 38 and 40 are arranged, respectively.

The switching of the magnetic field by the two magnetic field applying means may be performed at the same timing. However, in this type of optical switch, the switching is usually not frequently performed. The switching of the optical switch is performed by supplying a pulse current to the auxiliary yoke 31. Thereafter, a pulse current is supplied to the auxiliary yoke 31 at an appropriately delayed timing to locally magnetize in the opposite direction so as to prepare for the next switching. Is good. When the magnetization is switched by the main yoke 30, a nucleus for the magnetization in the opposite direction is generated by the auxiliary yoke 31, so that the switching speed is extremely high as in the above-described embodiments.

FIG. 5A shows another embodiment of the optical switch according to the present invention. A substantially C-shaped flat yoke 50 made of a semi-hard magnetic material, and a winding 52 applied to the yoke 50
And a gap 54 provided in the yoke 50
And a Faraday rotator 56 made of an iron-containing garnet single crystal film inserted perpendicular to the direction of the gap length. Here, the gap portion of the yoke 50 has a structure in which two protrusions 51 are provided opposite to each other on both sides of the opposing end surface, thereby forming portions having a short gap length at two intervals. . As shown in FIG. 5B, since the magnetic field concentrates on the protrusion 51, the portion of the Faraday rotator corresponding to the portion where the gap length is short is a region where the magnetization is saturated, while the other portion is the region where the magnetization is unsaturated. So that Then, the light beam passes through the two locations where the magnetization is saturated in a direction substantially perpendicular to the film surface.

A polarizing beam splitter 58 is provided on one side of the Faraday rotator 56, and a half-wave plate 59 and a polarizing beam splitter 60 are provided on the other side. The polarization beam splitters 58 and 60 have a structure in which a polarization splitting film is sandwiched between a parallelogram prism and a right-angled triangle prism.
A total reflection film is formed on a surface of the parallelogram prism parallel to the polarization separation film. In the ON state of the optical switch, the input light is separated into P-polarized light and S-polarized light by the polarization separation film, the polarization plane is rotated by the 45-degree Faraday rotator 56, and the half-wave plate 5 is rotated.
9, the P-polarized light is converted into S-polarized light, and the S-polarized light is converted into P-polarized light, and combined by the polarization beam splitter 60 to become output light. In the off state, since the rotation of the polarization plane by the 45-degree Faraday rotator 56 is in the opposite direction, the output light from the polarization beam splitter 60 has a direction different from the direction of the output light by 90 degrees, and is emitted to the regular output port. do not do. Thus, it operates as a 1 × 1 type optical switch.

The Faraday rotator used in the present invention is a bismuth-substituted iron garnet single crystal film grown by the LPE method and having a compensation temperature, at a temperature of 1120 ° C. or more and 1180 ° C. or less and within 7 hours. Heat-treated under the above conditions. As a specific composition of the single crystal film, for example, Gd _2.02 Bi _0.98 F
e There is _4.43 Al _0.44 Ga _0.13 O ₁₂ . The above heat treatment conditions were derived from various experiments. The experimental heat treatment conditions are shown in FIG. The heat treatment was performed in an air atmosphere by placing the element on a platinum plate and placing the element in an electric furnace. The heating rate and the cooling rate were each set to 120 ° C./hour, and the maximum temperature (top temperature) and the holding time were variously changed. 3 at 1150 ° C
FIG. 7 shows the measurement results of the magnetic field dependence of the Faraday rotation of the element (product of the present invention) heat-treated for a time. The area S surrounding the hysteresis is 0.1 kdeg / Oe, and the extinction ratio is 30 dB or more.

In FIG. 6, the hatched area (including the boundary line) is the range of the preferable top conditions of the heat treatment.
Considering the application as an optical switch, the characteristics required for the Faraday rotator include a magnetic field Hs required for saturation of 100 Oe or less, a Faraday rotation angle of 45 degrees, and a coercive force H
c must be 5 Oe or less, and the area S surrounding the hysteresis relating to the magnetic field dependence of the Faraday rotation angle must satisfy the condition of 1 kdeg / Oe or less. Since the heat treatment goes through a process of raising the temperature, maintaining the top condition, and lowering the temperature in the furnace, the top condition may be a case where the holding time is zero (that is, a process of immediately lowering the temperature after raising the temperature to a predetermined temperature). Including. If the heat treatment temperature is lower than 1120 ° C., the coercive force Hc increases, and the area S surrounded by the hysteresis does not decrease. If it exceeds 1180 ° C., the Faraday rotation angle decreases, so that the film thickness must be increased and the surface becomes rough. This is considered to be due to the scattering of bismuth from the crystal, and the composition is shifted. If the heat treatment time becomes longer as it exceeds 7 hours, the magnetic field Hs required for saturation increases. It is considered that this is because the perpendicular magnetic anisotropy decreases and approaches the in-plane magnetization film. As a result of these various experiments, in particular, a temperature of 1150 ° C.
The heat treatment for 7 hours was optimal. When such a Faraday rotator is used, for example, in the configuration of FIG. 1, the switching time of the optical switch is increased by one digit or more as compared with the conventional product, such as 6 μs.

It is to be noted that the polarization plane switching device according to the present invention can be used not only for the 1 × 1 type optical switch as described above, but also for a 2 × 2 type optical switch and other magneto-optical devices. Not even.

[0028]

As described above, according to the present invention, the magnetization of the Faraday rotator is not saturated over the entire surface of the film, and is partially in an unsaturated region (or the magnetization is reversed even if saturated). Region), there is always a nucleus of magnetization reversal at the time of switching, so that high-speed switching is possible. In particular, in the case of heat treatment under appropriate conditions, the hysteresis relating to the magnetic field dependence of the Faraday rotation angle becomes very small, and together with the fact that the unsaturated region is provided as described above, it is 10 μsec or less. The switching speed is significantly improved. As a result, when configuring an excessive light blocking device or the like, the size of the device can be reduced, and a highly reliable system can be configured.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing one embodiment of an optical switch according to the present invention.

FIG. 2 is an explanatory diagram of the operation.

FIG. 3 is an explanatory view showing another embodiment of the optical switch according to the present invention.

FIG. 4 is an explanatory view showing still another embodiment of the optical switch according to the present invention.

FIG. 5 is an explanatory view showing another embodiment of the optical switch according to the present invention.

FIG. 6 is an explanatory diagram showing heat treatment conditions for a Faraday rotator.

FIG. 7 is a graph showing the magnetic field dependence of the Faraday rotation angle and the extinction ratio of a Faraday rotator heat-treated under optimal conditions.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Yoke 12 Winding 14 Gap 16 Faraday rotator 18, 20 Wedge type birefringent single crystal 22, 24 Lens integrated fiber

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Rikukawa 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Tsugio Tokumas 5-36-11 Shimbashi, Minato-ku, Tokyo No. Fuji Electric Chemical Co., Ltd. (56) References JP-A-7-104225 (JP, A) JP-A-6-34894 (JP, A) JP-A-62-138832 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) G02F 1/09-1/095 G02F 1/29-1/31 G01R 15/24 G01R 33/032

Claims (7)

(57) [Claims] 1. A polarization plane switching device comprising: a Faraday rotator made of an iron-containing garnet single crystal film; and a magnetic field inverting means for applying a magnetic field to the Faraday rotator, wherein a magnetic field is applied by the magnetic field applying means. Occasionally, the magnetization of one part of the Faraday rotator is in an unsaturated state, and the remaining part is in a saturated state, and the light beam passes through the saturated part in a direction almost perpendicular to the film surface. Polarization plane switch. 2. A yoke made of a magnetic material, a coil wound on the yoke, and a single crystal iron-containing garnet film inserted into a gap provided in the yoke perpendicularly to a gap length direction. The Faraday rotator has a size that most of the Faraday rotator protrudes outward from the gap portion, and the tip of the protruding portion is an unsaturated region of magnetization. A polarization plane switch through which a light beam passes in a direction substantially perpendicular to the film plane through the saturation region of magnetization. 3. A yoke made of a magnetic material, a winding provided on the yoke, and an iron-containing garnet single crystal film inserted into a gap provided in the yoke perpendicularly to the direction of the gap length. The Faraday rotator has a size that most of the Faraday rotator protrudes outward from the gap portion, and a magnetic shield material is attached to the tip of the protruding portion to form an unsaturated region of magnetization. A polarization plane switch in which a light beam passes through a magnetization saturation region of the protruding portion in a direction substantially perpendicular to the film surface. 4. A yoke made of a magnetic material, a winding applied to the yoke, and an iron-containing garnet single crystal film inserted into a gap provided in the yoke perpendicular to the direction of the gap length. A gap portion of the yoke, a portion having a short gap length is provided at a plurality of intervals, and a portion of the Faraday rotator corresponding to the portion having a short gap length is a saturation region of magnetization. A polarization plane switch in which the other portion is a magnetization non-saturation region and the light beam passes through the magnetization saturation region in a direction substantially perpendicular to the film surface. 5. The Faraday rotator is made of a bismuth-substituted iron garnet single crystal film grown by a liquid phase epitaxy method and having a compensation temperature.
At 80 ° C. below the temperature, and the top condition within 7 hours are those that have been heat-treated in the claims 1 to 4 polarization plane switcher according. 6. A wedge-shaped birefringent single crystal arranged oppositely and a Faraday rotator arranged between them.
4. An optical switch comprising: the polarization plane switching device according to any one of 3 to 3 ; input / output optical fibers; and a condenser lens. 7. The polarization beam splitter according to claim 1, wherein two polarization beam splitters are disposed opposite to each other, and a Faraday rotator is disposed therebetween.
An optical switch consisting of 4 and the polarization plane switcher according, the half-wave plate positioned between the Faraday rotator and one of the polarizing beam splitter.