JPH04217224A - Light isolator - Google Patents

Abstract

PURPOSE:To provide a light isolator which can prevent isolation decrease with no dependency on a wavelength of incident light. CONSTITUTION:Since wavelength dependency is negated each other with a Faraday rotor 3 in which a direction of rotation of a polarization direction is determined by not an incident direction of incident light but only an applied magnetic field by providing a polarization rotor 2 of reversing the direction of rotation of the polarization direction in accordance with the incident direction of the incident light, the wavelength dependency as a light isolator can be eliminated, and isolation decrease as the light isolator can be prevented.

Description

Description: FIELD OF THE INVENTION The present invention relates to an optical isolator that prevents degradation of isolation due to wavelength dependence. [0002] Optical isolators are used to prevent light reflected from the end faces of optical fibers, lens systems, and connectors in optical communications that use semiconductor lasers and the like as light sources, writable video disks, and the like. Conventionally, Fig. 9
The configuration shown in is common. That is, as shown in FIG. 9, a polarizer 101 and an analyzer 10 whose polarization directions differ by 45 degrees from each other
4, a magneto-optic crystal having a thickness that allows a Faraday rotation angle of 45 degrees is interposed as a Faraday rotator 103,
A permanent magnet 105 is arranged around this Faraday rotator 103 for applying an external saturation magnetic field. When a magnetic field 109 shown in FIG. 10 is applied to the Faraday rotator 103 by the permanent magnet 105, due to the Faraday effect,
The polarization angle of the passing light is rotated 45 degrees clockwise in the figure. In addition, these polarizers 101 and Faraday rotators 10
3. The analyzer 104 and the permanent magnet 105 are housed in a case 106, and this case 106 is provided with a light input section 107 and a light output section 108. Therefore, as shown in FIG. 10, when light 110a enters the optical isolator in the forward direction, a specific linearly polarized light 1 of this light 102a
Only light 10b passes through polarizer 101. This linearly polarized light 1
The light beam 10b enters the Faraday rotator 103, and its plane of polarization rotates 45 degrees clockwise in the figure, becoming linearly polarized light 110c. This linearly polarized light 110c passes through an analyzer 104 whose polarization direction is tilted 45 degrees clockwise in the figure with respect to the polarizer 101, and is emitted. On the other hand, when light 111a is incident on the optical isolator in the opposite direction as shown in FIG.
Among them, only a specific linearly polarized light 111b passes through the analyzer 104. This linearly polarized light 111b is transmitted through the Faraday rotator 1
03, the plane of polarization rotates 45 degrees clockwise in the figure, and becomes linearly polarized light 111c. Here, Faraday rotator 1
Due to its non-reciprocity, 03 does not depend on the direction of incidence of light,
Only the direction of the magnetic field 109 causes Faraday rotation. Therefore, the linearly polarized light 111c that has passed through the Faraday rotator 103 in the opposite direction is perpendicular to the polarization direction of the polarizer 101 and cannot pass through the polarizer 101. [0004] However, in the above-mentioned prior art, the Faraday rotation angle of the Faraday rotator 103 changes depending on the wavelength, so there is a drawback that the isolation of the optical isolator deteriorates. . for example,
As shown in the wavelength characteristics of FIG. 6, the Faraday rotation coefficient of YIG or Ga-YIG used in a 1 μm band isolator changes greatly with respect to wavelength. Therefore, when the wavelength of the light incident on the optical isolator changes, the polarization rotation angle by the Faraday rotator 103 deviates from 45 degrees, and as shown in FIG. The polarization direction no longer makes an angle of 90 degrees with the polarization direction of the polarizer 101. For this reason, in the optical isolator, the isolation that prevents light from passing in the opposite direction is degraded, and linearly polarized light may leak out from the polarizer 101. The present invention has been made in view of the above-mentioned prior art, and it is an object of the present invention to provide an optical isolator that does not depend on the wavelength of incident light. Means for Solving the Problems The configuration of the present invention that achieves the above object includes a polarizer that transmits a specific linearly polarized light, a polarizer that can rotate the polarization direction of the passing light, and The polarization rotation element reverses the direction of polarization rotation depending on the incident direction of the light, and the polarization direction of the passing light is set to 4 at the reference wavelength.
A Faraday rotator that can be rotated by 5 degrees and whose polarization direction is determined only by the applied magnetic field; a magnet that applies a magnetic field to the Faraday rotator; and a specific linearly polarized light that can be transmitted; The polarization rotation angle at the reference wavelength by the polarization rotator with respect to the polarizer is 45
It is characterized by having an analyzer whose polarization direction is tilted by an angle of 100 degrees. [Embodiment] FIG. 1 shows a first embodiment of the present invention. As shown in the figure, the optical isolator of this embodiment has a polarizer 1 and an analyzer 4 that allow only light in a specific polarization direction to pass through.
A polarization rotator 2 and a Faraday rotator 3 are arranged, and a permanent magnet 5 is arranged around the Faraday rotator 3. These polarizer 1, polarization rotator 2, Faraday rotator 3, analyzer 4, and magnet 5 are housed in a case 6, and this case 6 is provided with a light input section 7 and a light output section 8. As the polarization rotator 2, an optically active medium such as crystal is used, and the polarization direction of the light incident on the polarization rotator 2 is rotated by α degrees and output. However, the rotation direction of the polarization rotator 2 differs depending on the direction of incidence of the incident light. When light enters from the forward direction shown in Figure 2, the polarization direction rotates clockwise in the figure.
As shown in FIG. 3, when light enters from the opposite direction, the polarization direction rotates counterclockwise in the figure. Faraday rotator 3 is
Faraday rotation is caused only by the direction of the magnetic field 9 generated by the permanent magnet 5, regardless of the direction of incidence of the light. When the magnetic field 9 acts in the directions shown in FIGS. 2 and 3, the polarization angle of light incident from any direction is rotated by 45 degrees clockwise in the drawings. The polarization direction of the analyzer 4 is tilted clockwise by (45+α) degrees in FIGS. 2 and 3 with respect to the polarization direction of the analyzer 1. Therefore, as shown in FIG. 2, when light 10a is incident on the optical isolator in the forward direction, only a specific linearly polarized light 10b of the incident light 10a passes through the polarizer 1. When this linearly polarized light 10b passes through the polarization rotator 2, the polarization direction is rotated by α degrees, and becomes linearly polarized light 10c tilted clockwise by α degrees with respect to the polarizer 1 in the figure. Furthermore, this linearly polarized light 10
When c passes through the Faraday rotator 3, the polarization direction becomes 45
degree (45+) clockwise in the figure with respect to polarizer 1.
α) It becomes linearly polarized light 10d tilted by degrees. Here, analyzer 4
As mentioned above, the polarization direction of is tilted clockwise by (45+α) degrees in FIG. Output. On the other hand, when the light 11a is incident on the optical isolator in the opposite direction as shown in FIG.
Only the specific linearly polarized light 11b of the light a passes through the analyzer 4. This linearly polarized light 11b is tilted clockwise (45 degrees + α) degrees in FIG. 3 with respect to the polarizer 1, but when passing through the Faraday rotator 3, the polarization direction is rotated 45 degrees clockwise in the figure. As a result, linearly polarized light 11c whose polarization direction is tilted clockwise in the figure with respect to the polarizer 1 by (90 degrees + α) degrees is obtained. Furthermore,
When this linearly polarized light 11c passes through the polarization rotator 2, the polarization direction is rotated -α degree clockwise in the figure, and becomes linearly polarized light 11d tilted 90 degrees clockwise in the figure with respect to the polarizer 1. . The polarization direction of the linearly polarized light 11d is the same as the polarization direction of the polarizer 1.
Since the angle is 0 degrees, this linearly polarized light 11d is polarized by polarizer 1.
cannot pass through. Generally, the optically active substance used as the polarization rotator 2 has the property that when light that has passed through a medium is reflected and passed through the medium again, the plane of polarization returns to its original state. In other words, if the rotation angle when light passes through the polarization rotator 2 in the forward direction is α degrees as in this embodiment, the rotation angle when the light passes through the medium in the reverse direction is −α degrees. It is. Although wavelength dependence was not considered in FIGS. 2 and 3, in reality, the rotation angles of the polarization rotator 2 and Faraday rotator 3 change depending on the wavelength. As shown in FIG. 7, the wavelength characteristics of the crystal used as the polarization rotator 2 are similar to the wavelength characteristics of the Faraday rotator shown in FIG.
The rotation coefficient tends to decrease as the wavelength increases. . Incidentally, the polarization rotation angle per unit length of the crystal is expressed by the following formula. ρ=7.2484×106/λ2 + 1.27636
×108/λ8-0.2048 (deg/mm) Here, λ is the wavelength (nm) of light. Therefore, by appropriately adjusting the direction of the magnetic field applied to the Faraday rotator 3 and the length of the polarization rotator, wavelength dependence can be mutually canceled out when light is passed in opposite directions as shown in FIG. Therefore, wavelength dependence as an optical isolator can be eliminated. That is, FIGS. 4 and 5 show cases in which incident light having a wavelength different from that in FIGS. 2 and 3 is incident from the forward direction and the reverse direction. As shown in FIG. 4, when light 12a having a wavelength different from that shown in FIG. This linearly polarized light 12b is transmitted to the polarization rotator 2
When passing through, the polarization direction is rotated by (α+δ) degrees, and becomes linearly polarized light 12c tilted clockwise by (α+δ) degrees with respect to the polarizer 1 in the figure. Here, the δ degree is caused by the wavelength dependence of the polarization rotator 2. Furthermore, this linearly polarized light 12
When c passes through the Faraday rotator 3, the polarization direction becomes (4
The linearly polarized light 12d is rotated by 5+δ) degrees and tilted clockwise by (45+α+2δ) degrees with respect to the polarizer 1 in the figure. Here, the δ degree depends on the wavelength dependence of the Faraday rotator 3, and was adjusted to be the same as that of the polarization rotator 2. Furthermore, as described above, the polarization direction of the analyzer 4 is tilted clockwise by (45+α) degrees in FIG. At this time, the output is reduced to a cos22δ times the intensity and output as linearly polarized light 12e. However, the loss of analyzer 4 was ignored. On the other hand, as shown in FIG. 5, when light 13a having a wavelength different from that shown in FIG. . This linearly polarized light 13b is tilted clockwise by (45 degrees + α) degrees in FIG. The light rotates to become linearly polarized light 13c whose polarization direction is tilted clockwise in the drawing with respect to the polarizer 1 by (90 degrees + α + δ) degrees. Furthermore, when this linearly polarized light 11c passes through the polarization rotator 2, the polarization direction is rotated by -(α+δ) degrees clockwise in the figure, resulting in a straight line tilted 90 degrees clockwise in the figure with respect to the polarizer 1. The polarized light becomes 13d. Linear polarized light 1
Since the polarization direction of light 3d is at 90 degrees with the polarization direction of polarizer 1, this linearly polarized light 13d cannot pass through polarizer 1. In this embodiment, as shown in FIG. 5, when light with a wavelength different from the reference wavelength is incident from the opposite direction, the wavelength dependence of the Faraday rotator 3 and the polarization rotator 2 have completely opposite characteristics. Therefore, they cancel each other out,
Wavelength dependence as an optical isolator is eliminated. Therefore, even if the incident light has a wavelength different from the reference wavelength, the isolation of the optical isolator will not deteriorate. Note that, as shown in FIG. 4, when light of a wavelength different from the reference wavelength is incident from the forward direction, the intensity of the output light decreases slightly, but this is hardly a problem in practice. Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, the arrangement of the polarization rotator 2 and the Faraday rotator 3 is reversed from that of the previous embodiment. That is, in the above embodiment, the analyzer 1, the polarization rotator 2
, Faraday rotator 3, and analyzer 4, but in this embodiment, the analyzer 1, Faraday rotator 3, polarization rotator 2, and analyzer 4 are arranged in this order. The other configurations are the same as those of the embodiment described above, and the same effects can be achieved. [Effects of the Invention] As explained above in detail based on the embodiments, the present invention adds a polarization rotator in which the rotation direction of the polarization direction is reversed depending on the direction of incidence of the incident light. Therefore, the wavelength dependence can be mutually canceled out between the Faraday rotator and the Faraday rotator whose polarization rotation direction is determined only by the direction of the applied magnetic field regardless of the direction of incidence of the incident light. Therefore, in the optical isolator of the present invention, deterioration in isolation can be prevented.

[Brief explanation of the drawing]

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

FIG. 2 is an explanatory diagram showing a case where light of a reference wavelength enters the optical isolator shown in FIG. 1 from the forward direction.

FIG. 3 is an explanatory diagram showing a case where light of a reference wavelength enters the optical isolator shown in FIG. 1 from the opposite direction.

FIG. 4 is an explanatory diagram showing a case where light having a different wavelength from a reference wavelength enters the optical isolator shown in FIG. 1 from the forward direction.

FIG. 5 is an explanatory diagram showing a case where light having a different wavelength from a reference wavelength enters the optical isolator shown in FIG. 1 from the opposite direction.

FIG. 6 is a graph showing wavelength characteristics of a Faraday rotator.

FIG. 7 is a graph showing wavelength characteristics of a polarization rotator.

FIG. 8 is a configuration diagram showing an optical isolator according to a second embodiment of the present invention.

FIG. 9 is a configuration diagram showing a conventional optical isolator.

FIG. 10 is an explanatory diagram showing a case where light enters a conventional optical isolator from the forward direction.

FIG. 11 is an explanatory diagram showing a case where light enters a conventional optical isolator from the opposite direction.

[Explanation of symbols]

1 Polarizer 2 Polarization rotator 3 Faraday rotator 4 Analyzer 5 Permanent magnet 6 Case 7 Optical input section 8. Light output section 9 Magnetic field 101 Polarizer 103 Faraday rotator 104 Analyzer 105 Permanent magnet 106 case 107 Optical input section 108 Light output section 109 Magnetic field

Claims (1)

[Claims] 1. A polarizer that transmits a specific linearly polarized light, and a polarization rotation element that can rotate the polarization direction of the light that passes therethrough and that reverses the direction of polarization rotation depending on the direction of incidence of the incident light. , a Faraday rotator that can rotate the polarization direction of passing light by 45 degrees at a reference wavelength, and the polarization direction is determined only by the applied magnetic field; a magnet that applies a magnetic field to the Faraday rotator; It is characterized by comprising an analyzer capable of transmitting linearly polarized light and tilting the polarization direction with respect to the polarizer by an angle equal to 45 degrees added to the polarization rotation angle at the reference wavelength by the polarization rotator. optical isolator.