JP2744467B2 - Optical isolator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical isolator used in the field of, for example, optical fiber transmission.

[Prior art] For example, in optical fiber transmission, high light transmission quality is achieved by stabilizing a light source such as a semiconductor laser and removing reflected return light reflected at a connection point and an input / output end of an optical fiber. An optical isolator using a magneto-optical element (Faraday rotator) is used to ensure this.

This optical isolator has a first polarizer and a second polarizer arranged at an incident position and an output position of a magneto-optical element, respectively.

Thereby, the polarization plane of the incident light converted into the polarized light by the first polarizer has a fixed angle (for example, in the magneto-optical element).
45 °) After being rotated, the light exits through the second polarizer.
On the other hand, a part of the emitted light is reflected by the end face of the optical fiber or the like, becomes reflected light, and enters the second polarizer.
This reflected light is polarized by the second polarizer,
After the plane of polarization is rotated by a fixed angle (45 °) in the magneto-optical element, the light enters the first polarizer. At this time, since the direction of rotation of the plane of polarization by the magneto-optical element is constant irrespective of the traveling direction of the normal light, the incident light and the reflected light incident on the first polarizer are the same as those of the magneto-optical element. The polarization planes are shifted from each other by an amount corresponding to the sum of the rotation angles (90 °). Therefore, the reflected light is blocked by the first polarizer.

[Problems to be Solved by the Invention] By the way, a conventional optical isolator of this type is usually
Adjustment and adjustment so that the specified characteristics can be exhibited under specific conditions (wavelength of laser beam used, environmental temperature, etc.)
The settings have been made. Therefore, when this optical isolator is used, it is necessary to set the use conditions of this optical isolator to the same conditions as those at the time of adjustment and setting.

That is, in general, the Faraday rotation angle of the magneto-optical element used in the optical isolator has wavelength dependency, and also has temperature dependency for the same wavelength. For this reason, when the wavelength of the laser beam used and the environmental temperature are different from the conditions at the time of the adjustment / setting, the predetermined characteristics cannot be exhibited. However, depending on the application, it is difficult to set the same temperature as the environmental temperature at the time of the adjustment / setting, or the wavelength of the laser beam to be used is set to the same wavelength as the wavelength at the time of the adjustment / setting. Not a few cases occur. Therefore, in such a case, there is a problem that the performance inherent in the optical isolator cannot be sufficiently exhibited.

The present invention has been made under the above-mentioned background,
It is an object of the present invention to provide an optical isolator that can exhibit the best characteristics under various conditions relatively easily.

[Means for Solving the Problems] The present invention solves the above-mentioned problems by adopting the following configuration.

A first polarizer that converts incident light into polarized light and emits the light; and a first polarizer that emits polarized light output from the first polarizer and rotates the plane of polarization by the Faraday effect to emit the light.
A second magneto-optical element that receives polarized light emitted from the first magneto-optical element, rotates the plane of polarization by the Faraday effect and emits the polarized light, and the first and second magnetic fields. A magnetic field generating means for generating a Faraday effect by forming a magnetic field in the optical element; and a second polarizer for inputting and emitting light emitted from the second magneto-optical element, wherein the magnetic field generating means The magnetic field generated in the first or second magneto-optical element can be changed.

[Operation] In the above-mentioned configuration, if the polarization light passing through the first and second magneto-optical elements is adjusted so that the polarization plane thereof is rotated by, for example, 45 °, the desired optical isolation characteristics can be obtained. It is possible to demonstrate.

In this case, the adjustment is performed by, for example, changing the positional relationship of the first or second magneto-optical element with respect to the magnetic field generating means, for example, by changing the magnetic field formed in the first or second magneto-optical element. It can be done by changing. Therefore, for example, even when conditions such as the wavelength of the laser beam to be used or the environmental temperature are different from the conditions at the time of the first adjustment, the intended adjustment is performed again by changing the magnetic field. It is possible to set to a state where the characteristics can be exhibited.

At this time, for example, as the first magneto-optical element, an element that can obtain most of the required rotation angle is used, and on the other hand, as the second magneto-optical element, a change in the magnetic field is prevented. By using a device whose rotation angle changes gradually and by changing the magnetic field formed in the second magneto-optical element, if the adjustment is performed, precise adjustment can be easily performed. It becomes possible.

FIG. 1 is a longitudinal sectional view of an optical isolator according to one embodiment of the present invention. Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG.

In FIG. 1, reference numeral 1 denotes a substantially cylindrical magneto-optical element,
Reference numeral 2 denotes a magnet member for forming a magnetic field in the magneto-optical element 1, reference numeral 3 denotes a second magneto-optical element, and reference numeral 4 denotes a magnet member for forming a magnetic field in the second magneto-optical element 3. Reference numeral 5 denotes a first polarizer, reference numeral 6 denotes a second polarizer, reference numeral 7 denotes a substantially cylindrical holder for holding the magneto-optical elements 1 and 3, and reference numeral 8 denotes the first polarizer. Reference numeral 9 denotes a rear side plate that holds the second polarizer 6, reference numeral 9 denotes a substantially bottomed cylindrical shape that holds the first and second magnet members 2 and 4, and the like. This is the outer case. The first and second magnet members 2 and 4 constitute a magnetic field generating means in the present invention.

The magneto-optical element 1 is, for example, one in which a single crystal (YIG) of yttrium iron garnet (Y ₃ Fe ₅ O ₁₂ ) is formed in a substantially columnar shape, and an anti-reflection film is formed on both ends thereof in parallel with each other. It is. Since the first magneto-optical element 1 is a ferromagnetic material, it is saturated at a relatively small magnetic field strength (about 1800 gauss), and the Faraday rotation angle rises rapidly with respect to the magnetic field strength. That is, a large Faraday rotation angle can be obtained with a relatively small magnetic field strength. On the other hand, it is not suitable for precisely adjusting the Faraday rotation angle by changing the magnetic field strength. The Faraday rotation angle θ is obtained by a well-known formula, θ = VlH (1: light path length of light propagating in a magneto-optical element, V: Verdet constant, H: magnetic field strength). In this embodiment, the first magneto-optical element 1 has a wavelength of 1.3 μm.
A Faraday rotation angle of 45 ° can be obtained for polarized light of 45 °, and 36 ° for polarized light of 1.55 μm wavelength.

The second magneto-optical element 3 is, for example, FR-6
Glass (trade name of HOYA CORPORATION) is formed in a cylindrical shape having a diameter larger than that of the first magneto-optical element 1, and an anti-reflection film is formed on both ends thereof in parallel with each other. Since the second magneto-optical element 3 is a paramagnetic substance, a large Faraday rotation angle cannot be obtained as compared with the first magneto-optical element 1, but the change in the Faraday rotation angle with respect to a change in magnetic field intensity is moderate. Therefore, fine adjustment of the Faraday rotation angle can be easily performed. In this embodiment, when the maximum magnetic field that can be formed on the second magneto-optical element 3 by the second magnet member 4 is applied,
A Faraday rotation angle of about 10 ° can be obtained for polarized light having a wavelength of 1.55 μm.

The first and second magneto-optical elements 1 and 3 are held inside a cylindrical holder 7 so that their central axes are common. That is, a small-diameter portion 7a and a large-diameter portion 7b are formed inside the holder 7, and the first magneto-optical element 1 and the second magneto-optical element 3 are respectively held therein. I have.

The first and second magnet members 2 and 4 are both Sm
-Forming a Co-based permanent magnet in a substantially cylindrical shape so that the inner diameter thereof is substantially the same as the outer diameter of the holder 7;
These are fitted coaxially to the holder 7 and the first
The second magnet member 4 surrounds the first magneto-optical element 1, and the second magnet member 4 surrounds the second magneto-optical element 3. And in this state,
The first and second magnet members 2 and 4 are fitted in the bottomed cylindrical outer case 10.

In this case, the first magnet member 2 is fixed to the holder 7, and the second magnet member 4 is formed so as to be movable along the outer peripheral surface of the holder 7 and the inner peripheral surface of the outer case 10. ing. The first and second magnet members 2
And 4, the north pole and the south pole are formed on both end surfaces of the cylindrical shape, respectively. Therefore, the first and second magneto-optical elements 1 and 3 are parallel to the axial direction. A strong magnetic field is formed. In this case, the first magnet member 2 and the second magnet member 4 are arranged so that the magnetic poles on the opposite end faces are the same.

A front side plate 8 is fixed to a left end surface of the outer case 10 in the drawing, and a rear side plate 9 is fixed to a right end surface of the case 10 in the drawing.

At approximately the center of the front side plate 8 and the rear side plate 9, storage portions 8a and 9a are provided, in which the first and second polarizers 5 and 6 are stored, respectively. And 9a, through-hole 8b for light passage
And 9b. In this case, the optical axes of the first polarizers 5 and 6 and the first and second magneto-optical elements 1
And 3 and the central axis of the through holes 8a and 9a are all formed to be common. The outer case 10 is provided with penetrating light 10a that extends through the through-hole 8b and communicates with the through-hole 8b to allow light to pass therethrough. Further, a bottomed circular hole 10b for fitting and supporting one end of the holder 7 is provided at the bottom of the outer case 10, and the other end of the holder 7 is fitted to the rear side plate 9. A circular hole 9c with a bottom that is supported together. Thus, the holder 7 is fixed to the outer case 10 in a predetermined positional relationship.

A bearing member 4a is fixed to the end face of the second magnet member 4 from the right side in the drawing, and a screw rod is attached to the bearing member 4a.
11 are fitted so that the distal end portion is rotatable and fixed in the horizontal direction in the figure. The threaded portion 11a of the threaded rod 11
Is screwed into a screw hole 9d formed in the rear side plate 9. A knob 12 is attached to the right end of the screw rod 11 in the drawing. Therefore, by operating the knob 12 and rotating the screw rod 11, the second magnet member 4 can be moved in the direction of the arrow p in the drawing.

In the above configuration, as indicated by an arrow A in the figure,
When the incident light is made to enter from the through hole 8b of the front side plate 8,
The incident light passes through the first polarizer 5, the first magneto-optical element 1, the second magneto-optical element 3, and the second polarizer 6, and is emitted to the outside.

In this case, the incident light is turned into polarized light by the first polarizer 5, and then the plane of polarization in the first magneto-optical element 1 is at a predetermined angle (for example, an angle close to 45 °). Note that the rotation angle differs depending on the wavelength.) After being rotated, the light enters the second magneto-optical element 3. Here, the relative position of the second magnet member 4 with respect to the second magneto-optical element 3 is changed by operating the knob 12 to change the magnetic field formed on the second magneto-optical element 3. Thus, the Faraday rotation angle by the second magneto-optical element 3 is finely adjusted, and the total rotation angle of the plane of polarization by the first and second magneto-optical elements 1 and 3 becomes 45 °. To do. Then, the second polarizer 6 is arranged such that its polarization plane has a relationship such that polarized light that has passed through the second magneto-optical element 3 can pass therethrough.

Thereby, a part of the light emitted from the second polarizer 6 is reflected on an end face of, for example, an optical fiber or the like, and becomes return light as shown by an arrow B in the drawing to become the second polarized light. Even when the light enters from the polarizer 6, the return light is transmitted to the first polarizer 5.
Can be blocked by That is, this return light is converted into polarized light by the second polarizer 6, and its polarization plane is changed to 45 by the second and first magneto-optical elements 3 and 1.
゜ After being rotated, the light enters the first polarizer 5. At this time, since the direction of rotation of the plane of polarization by the second and first magneto-optical elements 3 and 1 is constant irrespective of the traveling direction of the light, the incident light and the return light entering the first polarizer 5 are returned. In these, these are the first and second magneto-optical elements 1 and 3
The polarization planes are shifted from each other by the sum (90 °) of the rotation angles respectively rotated. Therefore, the reflected light is blocked by the first polarizer 5.

When light having a wavelength of 1.3 μm is incident as the incident light, the Faraday rotation angle in the first magneto-optical element 1 is 45 °. Therefore, when performing isolation with respect to the light having the wavelength of 1.3 μm, the second
By moving the magnet member 4 to the right end in the figure so that a magnetic field is not formed in the second magneto-optical element 3, the effect of isolation can be obtained.

Next, consider a case where light having a wavelength of 1.55 μm is made incident on this optical isolator. In this case, the rotation angle of the plane of polarization by the first magneto-optical element 1 is 36 °. Then, the knob 12 is operated to move the magnet member 4,
A magnetic field is formed in the second magneto-optical element 4 so that the rotation angle of the plane of polarization by the second magneto-optical element 3 is adjusted to 9 °. As a result, the total rotation angle of the first and second magneto-optical elements 1 and 3 becomes 45 °, and an effect of optical isolation can be obtained.

That is, according to the optical isolator of this embodiment, the effect of optical isolation can be obtained for any light having a wavelength of 1.3 to 1.55 μm. Therefore, it can be remarkably versatile.

In the above-described embodiment, the case where the wavelength of light to be applied is different is described as an example. However, this is the case where other conditions, for example, the environmental temperature during use is variously different, The present invention can be applied to the case where the isolation characteristic is to be obtained. That is, for example, for light of a specific wavelength, the Faraday rotation angle is θ ₁ (for example, an angle close to 45 °) at the temperature t ₁ as the first magneto-optical element, and at the temperature t ₂ When an element whose Faraday rotation angle is θ ₂ is selected, the Faraday rotation angle of the second magneto-optical element is set to θ ₁ + θ ₃ = at the temperature t ₁ .
45 ° becomes theta _3, at a temperature t _2, by setting the respective θ _{2 +} θ _{4 =} 45 ° becomes theta _4, it is possible at each of these temperatures to obtain the desired isolation characteristics.

Further, in the above-described embodiment, the example in which the second magnet member 4 is moved to change the magnetic field formed in the second magneto-optical element 3 has been described. The second magneto-optical element 3 may be moved while being fixed.

Further, the arrangement relationship between the first magneto-optical element and the second magneto-optical element may be reversed from that of the embodiment.

Also, in the above-described embodiment, an example is described in which the first and second magnet members (permanent magnets) are used as the magnetic field generating means.
This may use an electromagnet instead of a permanent magnet. When an electromagnet is used, there is no need to provide a mechanism for moving the second magnet member, as in the above-described embodiment. Can be changed.

Further, as the second magneto-optical element, instead of the FR-6 glass used in the above embodiment, another material having similar characteristics, for example, FR-5 glass (trade name of HOYA Corporation) Alternatively, TG or TGG (TbGa ₅ O ₁₂ ) or the like may be used. As the first magneto-optical element, instead of YIG used in the above-described embodiment, other materials having similar characteristics are used. material, for _{example, GIG, GIG (YbTbBi) 3} Fe 5 O 12, (HoTbB
i) ₃ Fe ₅ O ₁₂ or (GdBi) ₃ (FeAl) G may be used.

Further, a magnet member formed integrally with the first and second magnet members may be used so that either the first or second magneto-optical element can be moved.

[Effects of the Invention] As described above in detail, the present invention uses two magneto-optical elements, so that the magnetic field formed on one of the two magneto-optical elements can be fixed and the magnetic field formed on the other can be changed. When polarized light passes through these two magneto-optical elements, the total Faraday rotation angle by these two magneto-optical elements can be adjusted so as to be a predetermined value. It is possible to obtain an optical isolator that can exhibit the best characteristics easily and easily under various conditions.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an optical isolator according to one embodiment of the present invention. 1 ... first magneto-optical element, 2 ... first magnet member, 3 ... second magneto-optical element, 4 ... second magnet member, 5 ... first polarizer, 6 ... The second polarizer, 11 ... screw rod.

Claims (1)

(57) [Claims] 1. A first polarizer for converting incident light into polarized light and emitting the light; and a first polarizer for emitting polarized light emitted from the first polarizer and rotating the plane of polarization by the Faraday effect to emit the light. A second magneto-optical element that receives polarized light emitted from the first magneto-optical element, rotates the plane of polarization by the Faraday effect and emits the polarized light, and the first and second magnetic fields. A magnetic field generating means for generating a Faraday effect by forming a magnetic field in the optical element; and a second polarizer for inputting and emitting light emitted from the second magneto-optical element, wherein the magnetic field generating means An optical isolator which can change either one of the magnetic fields formed in the first or second magneto-optical element.