JP2967257B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator that blocks reflected light that is reflected on the surface of an optical element in an optical communication path and returns toward a light source, thereby preventing noise.

[0002]

2. Description of the Related Art In an optical communication system or an optical measuring instrument, transmission light oscillated from a laser light source is reflected on an incident surface of various optical elements in an optical communication path, and the reflected light reaches the laser light source. is there. The reflected light disturbs the light emitting action of the light source and often causes noise. The optical isolator is provided in such an optical transmission path, and transmits only light traveling in a predetermined transmission direction, and selectively blocks reflected light traveling in a light source direction.

Japanese Patent Publication No. 58-28561 discloses an optical isolator using a birefringent plate. In the optical isolator, as shown in FIG. 1, a first birefringent plate 2, a gyromagnetic element 3, a retardation plate 4, and a second birefringent plate 5 are sequentially arranged. The first birefringent plate 2 and the second birefringent plate 5 are provided so that the optical axis directions of the crystals are parallel. The magnetic rotating optical element 3 has an effect of rotating the polarization plane of the transmitted light by 45 degrees by the magnetic field of the surrounding magnet 10. The phase difference plate 4 has the function of rotating the polarization plane of the transmitted light by 45 degrees in the right-handed screw direction.

[0004] In the optical isolator shown in FIG. 1, the transmission light ₇₁ of circular polarization from the signal transmission optical fiber 11 ₁ is connected to the light source side is incident lens 1 in the first birefringence plate 2 is transmitted through , divided into an ordinary ray ₈₁ in the birefringent plate 2 and the extraordinary ray 9 _2. At this time the ordinary ray ₈₁ is transmitted through the magneto-rotating optical element 3,
The plane of polarization is rotated clockwise by the action of the magnetic field. Next, when the light passes through the retardation plate 4, the polarization plane is further changed in the same direction.
Rotate 45 degrees. Therefore ordinary ray _81, the optical axis of the crystal relative to the second birefringent plate 5 parallel to the first birefringence plate 2, the plane of polarization is incident become rotated and extraordinary rays 8 ₂ 90 degrees Refracted and emitted. Birefringent plate 2 with extraordinary rays 9 ₂ which is divided between the ordinary ray ₈₁ also enters the magnetic rotating optical element 3 is rotated by 45 degrees the polarization plane to the right screw direction and then rotates another 45 degrees, even the phase difference plate 4 . Therefore, the extraordinary ray 9 ₂ for the second birefringent plate 5 is incident becomes an ordinary ray 9 _1. Ordinary ray 9 ₁ and extraordinary ray 8 ₂
Transmitted through the lens 6 and the optical path is met, it becomes the transmitted light ₇₁ of the circularly polarized light emitted from the second birefringent plate 5 and the optical fiber 11 ₂
Head for.

[0005] Most of the transmitted light ₇₁ incident on the optical fiber 11 in _2, a portion is reflected at such an end face of the optical fiber 11 _2. Therefore resulting reflected light 7 _2, as shown in FIG. 2, is transmitted through the lens 6 is incident on the second birefringent plate 5, divided again extraordinary ray ₈₂ an ordinary ray 9 ₁ and. Extraordinary ray 8 ₂ phase difference plate 4
And the polarization plane is rotated 45 degrees in the right-handed direction, and the magnetic rotating optical element 3 is rotated 45 degrees in the left-handed direction by the magnetic field. Therefore extraordinary ray ₈₂ is refracted incident on the first birefringence plate 2 remains the same polarization plane angle as when exiting the second birefringent plate 5, is emitted. Second ordinary ray divided and extraordinary ray ₈₂ in the birefringent plate 5 9 ₁ in the right screw direction of the polarization plane by the phase difference plate 4
The optical element 3 rotates 45 degrees in the opposite left-handed screw direction. Ordinary ray 9 ₁ enters the first birefringent plate 2 remains polarization plane angles of the ordinary ray 9 _1. Therefore, the extraordinary ray ₈₂ which is divided by the second birefringent plate 5 is increased more and more a distance from each other in the first birefringence plate 2. Ordinary ray 9 ₁ and extraordinary ray 8 ₂
Also the amount of light reaching from the not focused on the end face of the optical fiber 11 ₁ to the light source is small and results in the generation of noise can be prevented by the lens 1 and.

Conventionally, quartz is used as the material of the phase difference plate 4. In the case of a quartz plate, a contrivance is adopted in which the same plate is bonded in the direction in which the optical axes of the crystals intersect so that the function is not disturbed by the temperature or the wavelength of the transmitted light. Nevertheless, the quenching performance of the quartz plate rotating the plane of polarization of transmitted light is 15
There is a problem that the conventional optical isolator using this is still insufficient to prevent communication trouble.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an object of the present invention is to provide an optical isolator to which a birefringent plate is applied, which has an extremely high ability to cut off reflected light.

[0008]

An optical isolator made to achieve the above object is shown in FIG.
It will be described according to.

In the optical isolator of the present invention, the magneto-rotational optical element 3 and the phase difference plate 4 are provided between the first birefringent plate 2 and the second birefringent plate 5 in which the optical axis of the crystal is parallel. Placed,
An optical isolator in which lenses 1 and 6 are provided between each of a first birefringent plate 2 and a second birefringent plate 5 and a light source or a light receiving system, wherein the phase difference plate 4 is a glass plate. Metal particles are oriented on the surface.

[0010] The thickness of the glass used as the phase difference plate 4 is 0.2 to 2 m.
m is good. The metal particles are, for example, silver particles, the particle size of which is 50 to 1200 nm in vertical length, 10 to 150 nm in width,
Use an optical isolator that has a large extinction ratio at a wavelength where the optical isolator is used, such as an aspect ratio of 2: 1 to 15: 1.

The magneto-rotating optical element 3 has the formula (GdBi) ₃ (FeGa) ₅ O
Crystals obtained by growing the garnet crystal represented by ₁₂ by a liquid crystal epitaxial method are preferable. When the optical path length is set to about 0.3 mm, it has an effect of rotating the polarization plane of 1550 nm light used for communication by 45 degrees in a magnetic field. The positions of the magneto-rotational optical element 3 and the phase difference plate 4 do not matter before or after each other.

It is preferable that both the first birefringent plate 2 and the second birefringent plate 5 use rutile crystal (TiO ₂ ).

[0013]

[Action] transmission light ₇₁ from the optical fiber 11 ₁ side of the light source side when incident on the first birefringence plate 2 divided into ordinary ray ₈₁ and extraordinary ray 9 _2. These polarization plane is rotated 90 degrees, with respect to the second birefringent plate 5 is incident in the form of each transposed into extraordinary rays 8 ₂ an ordinary ray 9 ₁ and. Therefore when emitting the second birefringent plate 5 matches the optical path of each other, toward the optical fiber 11 ₂ becomes the transmitted light ₇₁ of circular polarization.

[0014] reflected light generated by such an end face of the optical fiber 11 ₂
7 ₂ enters from the lens 6 in the second birefringent plate 5, divided therefrom even the extraordinary ray ₈₂ in the ordinary ray 9 _1. These lights midway its polarization plane to the right screw direction are rotated 45 degrees at a time alternately in the left-hand screw direction, the first extraordinary ray ₈₂ with respect to the birefringent plate 2, incident ordinary ray 9 _1, refracted , Further apart from each other. The separated extraordinary ray 8 ₂ and ordinary ray 9 ₁ are separated from the lens 1 by the optical fiber 11 _1.
Cannot be formed on the end surface of the light source, and as a result, the amount reaching the light source is extremely reduced.

The phase difference plate 4 made of glass can easily form a wide light receiving surface. This glass contains oriented metal particles, has small variations, and can be manufactured at low cost. When metal particles such as silver are oriented on a glass plate, the quenching performance of rotating the plane of polarization increases. Its function hardly changes even if the temperature or the wavelength of the transmitted light changes. Further, the phase difference plate made of glass is more suitable for downsizing because the thickness can be made thinner than that made of quartz.

[0016]

Embodiments of the present invention will be described below.

FIG. 1 shows an embodiment of the optical isolator according to the present invention.

The first collimator lens 1 and the first compound lens are arranged in the direction from the light source side to the light receiving side on the optical path of the signal transmission optical fiber 11 (11 ₁ , 11 ₂ ) which is partially cut away. A refracting plate 2, a magnetic rotating optical element 3, a retardation plate 4, a second birefringent plate 5, and a second collimator lens 6 are arranged in this order. First collimator lens 1 is focused on the end of the signal transmission optical fiber 11 ₁ in the light source side. The second collimator lens 6 is a signal transmission optical fiber 11 ₂ on the light receiving system side.
Focus on the end. Around the magnetic rotating optical element 3 so as to form a magnetic field that rotates the plane of polarized light in the right screw direction with respect to light traveling from the signal transmission optical fiber 11 ₁ side, is disposed Sm-Co permanent magnets 10 ing.

The phase difference plate 4 has a thickness in which silver particles are oriented on the surface so as to rotate the plane of polarization of the traveling light in the right-handed screw direction.
It is formed of a 0.5mm glass plate. The transmission loss of the phase difference plate 4 is 0.04 dB, and the extinction performance is 38 dB.

A first birefringent plate 2, a gyroscopic optical element 3,
Each of the retardation plate 4 and the second birefringent plate 5 has a light receiving surface of 3 mm in length and width and an antireflection film on the surface. Both the first birefringent plate 2 and the second birefringent plate 5 have an optical path length of 10 mm. 0.2 dB transmission loss, 8 ₁ ordinary ray
Resolution divided into the extraordinary ray 9 ₂ two beams is about 1 mm. The transmission loss of the magneto-rotating optical element 3 is 0.04 dB, and the optical path length is
0.3 mm.

The transmission light ₇₁ is emitted from the optical fiber 11 ₁ in the light source side, turns into parallel light refracted incident on the first collimator lens 1 and incident on the first birefringence plate 2. Transmission light
7 ₁ is divided into the ordinary ray ₈₁ and the extraordinary ray 9 ₂ in the first birefringence plate 2. FIG. 1 mainly shows only the optical axis rays. The separate ordinary ray ₈₁ a magnetic rotating optical element 3, the plane of polarization rotated 90 degrees and as it passes through the retardation plate 4, for the second birefringent plate 5 extraordinary ray ₈₂ of the polarized light The light enters at a plane angle, is refracted, and exits. Also the first birefringence plate 2 at ordinary ray ₈₁ and separate extraordinary ray 9 ₂ the polarization plane rotated by 90 degrees, with respect to the second birefringent plate 5 is incident in the polarization plane angles of the ordinary ray 9 ₁ . Therefore, the ordinary ray
9 ₁ of the outgoing optical path matches the emission optical path of the refracted becoming abnormal ray _82, passes through the second collimating lens 6 becomes a transmission light ₇₁ is an ordinary ray 9 ₁ and extraordinary ray _82, light It focused on the fiber 11 _2. When focused light is reflected like the end face of the optical fiber 11 _2, the reflected light 7 ₂ becomes parallel passes through the second collimating lens 6, the second extraordinary ray incident on the birefringent plate 5 8 ₂
And divided into ordinary ray 9 _1. Extraordinary ray 8 ₂ polarization plane is rotated 45 degrees to the right screw direction and incident on the phase difference plate 4 and then rotated 45 degrees to the left screw direction opposite to the magnetic rotating optical element 3. Therefore extraordinary ray ₈₂ is incident remains polarization angle of the extraordinary ray 8 ₂ to the first birefringent plate 2, emits while being refracted. Second ordinary ray divided extraordinary ray ₈₂ and a birefringent plate 5 9 ₁ also retardation plate 4
In is rotated 45 degrees the polarization plane to the right screw direction is rotated 45 degrees to the left screw direction opposite the magnetic rotating optical element 3 is incident on the first birefringence plate 2 by the polarization angle of the ordinary ray 9 _1. Therefore the extraordinary ray ₈₂ and the ordinary ray 9 ₁ further separation between in the first birefringence plate 2, not focused on the end face of the optical fiber 11 ₂ is also transmitted through the lens 1. Therefore, the amount of light reaching the light source is extremely small.

A performance test was performed on such an optical isolator as follows. That is, oscillated laser light having a wavelength of 1550nm to the signal transmission optical fiber 11 _{in two} directions of the light receiving system from the signal transmission optical fiber 11 ₁ in communication with the light source side (transmission light). An optical loss of 0.75 dB was recorded.
Oscillated laser light having a wavelength of 1550nm (the reflected light) to the signal transmission optical fiber 11 _{in one} direction of the light source side from the signal transmission optical fiber 11 _{and second} light-receiving system. An optical loss of 36.5dB was recorded. From this, it was confirmed that the optical isolator of this example had high extinction performance against reflected light.

In order to compare with the effect of the phase difference plate made of a glass plate, a conventional type of phase difference plate having an anti-reflection film is formed by superposing 3 mm square quartz plates with their optical axes orthogonal to each other, and transmitting the same. The loss and extinction performance were examined. The transmission loss of the laser light having a wavelength of 1550 nm was 0.06 dB, and the extinction performance was 17.5 dB.

For comparison with the effects of the above-described embodiment, an optical isolator having the same configuration as that of the above-described embodiment was formed using the above-mentioned retardation plate in which quartz plates were overlapped, and the optical loss thereof was examined. Light loss direction directed from the signal transmission optical fiber 11 ₁ in the light source side to the light receiving system signal transmission optical fiber 11 ₂ was 0.78DB.
Conversely, the light loss in the direction from the signal transmission optical fiber 11 _{and second} light-receiving systems to the signal transmission optical fiber 11 ₁ in the light source side
It was 17.3dB. It was found that the extinction performance for reflected light was weak.

[0025]

As described in detail above, the optical isolator of the present invention uses an inexpensive glass plate instead of a quartz plate as a phase difference plate, and thus can reduce the cost industrially. Since the metal fine particles are oriented, the function of rotating the polarization plane of light is hardly affected by the difference in temperature or the wavelength of transmitted light, and effectively blocks the progress of reflected light.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an optical path traveling in a transmission direction of an optical isolator to which the present invention is applied.

FIG. 2 is a configuration diagram illustrating a backward optical path of an optical isolator to which the present invention is applied.

[Explanation of symbols]

1 is a lens, 2 is a first birefringent plate, 3 is a magnetic rotating optical element, 4 is a retardation plate, 5 is a second birefringent plate, 6 is a lens,
7 ₁ is transmitted light, 7 ₂ is reflected light, 8 ₁・ 9 ₁ is ordinary light, 8 ₂・ 9 ₂ is extraordinary light, 10 is permanent magnet, 11 (11 ₁ , 11 ₂ ) is optical fiber for signal transmission is there.

Claims (1)

(57) [Claims] A first birefringent plate, wherein a magneto-rotational optical element and a phase difference plate are disposed between a first birefringent plate and a second birefringent plate whose optical axis directions of the crystals are parallel to each other; And an optical isolator provided with a lens between each of the second birefringent plate and the light source or the light receiving system, wherein the phase difference plate is formed by orienting metal particles on a surface of a glass plate. An optical isolator characterized in that: