JP2000131645A - Optical isolator, optical circulator, and optical amplifier using the same - Google Patents

Abstract

(57) [Problem] To realize a function in which light of a certain wavelength propagates only in the forward direction and light of another certain wavelength propagates only in the reverse direction to the optical isolator itself without using a bypass. And an optical isolator for propagating light in a forward or backward direction depending on a wavelength. SOLUTION: Light of wavelength 1 propagating in the forward direction is cut into 0 ° polarized light by a polarizer 91 and passes through a 45 ° Faraday rotator 92 in the forward direction to become 45 ° polarized light. Next, the light of wavelength 1 remains 45 ° polarized light after passing through the birefringent medium 93, and is therefore almost completely transmitted through the 45 ° polarizer 94. On the other hand, the light of wavelength 2 propagating in the forward direction is cut into 0 ° polarization by the polarizer 91, and passes through the 45 ° Faraday rotator 92 in the forward direction to become 45 ° polarization. Further, the light of wavelength 2 passes through the birefringent medium 93 and becomes 135 ° polarized light.
It is almost completely blocked by the 45 ° polarizer 94.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator and an optical amplifier used as components in optical communication and the like.

[0002]

2. Description of the Related Art An optical isolator is an optical circuit that passes only light traveling in one direction and blocks light traveling in the other direction. The optical isolator includes, for example, a 45-degree Faraday rotator 11, a polarizer 12, and a polarizer 13, as shown in FIGS. FIG. 9 is a conceptual diagram illustrating a state of light propagating in a forward direction, and FIG. 10 is a conceptual diagram illustrating a state of light propagating in a backward direction.

A Faraday rotator is an optical non-reciprocal element. When light travels through the Faraday rotator, the polarization plane rotates in the traveling direction as the light travels, and always rotates in the same rotation direction. Therefore, since the polarization planes of light traveling in opposite directions rotate in opposite directions, the function of the optical isolator is achieved by combining the 45-degree Faraday rotator with the polarizer as shown in FIGS. 9 and 10. Obtainable.

In the configurations shown in FIGS. 9 and 10, there is a problem that the transmission loss significantly changes due to the polarization of the incident light. As a configuration for avoiding this, for example, the configurations shown in FIGS. 11 and 12 are used. Have been. Hereinafter, the fact that the configurations in FIGS. 11 and 12 have the function of an optical isolator will be described in detail.

In FIGS. 11 and 12, light input as a parallel beam from an input fiber 31 through a lens 32 is incident on a birefringent prism 33 and is divided into a 0 ° polarization component and a 90 ° polarization component. The 0 ° polarization component is converted to 45 ° polarization by passing through the polarization rotator 34, and further returned to 0 ° polarization by passing through the 45 ° Faraday rotator 35 in the forward direction. Back to a parallel beam.

After passing through the polarization rotator 34, the 90 ° polarization component becomes 135 ° polarization, and further passes through the 45 ° Faraday rotator 35 in a direction opposite to the direction of the magnetic field, thereby becoming 90 ° polarization. It is returned to the parallel beam by the birefringent prism 36. Lens 37 is arranged to couple the collimated beam to the output fiber, so that light input from input fiber 31 couples almost completely to output fiber 28. On the other hand, the light input from the output fiber 38 enters the birefringent prism 38 as a parallel beam through the lens 37, and is split into a 0 ° polarization component and a 90 ° polarization component.

[0007] The 0-degree polarized light component becomes 45-degree polarized light by passing through the 45-degree Faraday rotator 25 in the direction of the magnetic field.
Next, since the light passes through the polarization rotator 34 and becomes 90 ° polarized light, it does not return to a parallel beam even after passing through the birefringent prism 33.

The 90 ° polarized light component becomes 135 ° polarized light by passing through the 45 ° Faraday rotator 35 in the direction of the magnetic field, and then becomes 180 ° polarized light by passing through the polarization rotator 34. Even after passing through the refracting prism 33, it is not returned to a parallel beam. Since the lens 32 is arranged so that the parallel beam is coupled to the input fiber, almost all of the light input from the output fiber 38 is transmitted to the input fiber 3.
Not joined to 1. For the above reasons, the configurations of FIGS. 11 and 12 have a function of a polarization independent optical isolator.

Further, as another configuration example of the above-mentioned polarization independent optical isolator, there is a configuration shown in FIGS. 13 and 14, for example. In this configuration example, polarization beam splitters 51 and 52 are used as means for separating and combining light according to its polarization state.

Since the 45 ° Faraday rotator 53 and the polarization rotator 54 function in the same manner as the configuration shown in FIGS. 11 and 12, this configuration is similar to the configuration shown in FIGS. 11 and 12. In addition, it has the function of a polarization independent optical isolator. In addition, in this configuration, the counterpropagating light can be coupled to another port, so this configuration has the function of an optical circulator.

In the optical isolator described above, for example, in an optical fiber amplifier, an amplifier fiber is divided into two parts at an appropriate ratio, and the two parts are arranged in the middle, so that the parts constituting the amplifier or their connections are connected. Two or more reflection points, such as points, have been used to avoid multiple reflections or oscillations and degrade the amplification characteristics.

Further, in the same optical fiber amplifier, the noise figure can be reduced by blocking the backward Rayleigh scattered light propagating in the opposite direction to the signal light by the optical isolator, as described in P.B. Hansen et al.
1. , Electronics Letters, 3
2, pp. 2164-2165 (1996)).

[0013]

Among the above optical fiber amplifiers, in the fiber Raman amplifier, the transfer of the pump noise to the signal light is minimized by transmitting the pump light in the opposite direction to the signal light. (Publications P. B. Hansen et al.,
Ellectronics Letters, 32, p
p. 2164-2165 (1996)).

It is also known that crosstalk caused by mutual gain modulation when amplifying wavelength-division multiplexed signal light can be reduced by propagating the pump light in the opposite direction to the signal light. However, in this fiber Raman amplifier, if the optical isolator is arranged in the middle of the amplification medium in order to obtain the above-mentioned effect, there is a problem that the pump light propagating in the opposite direction is cut off.

In order to solve this problem, a configuration has been considered in which the wavelength division multiplexing filter 71 and the optical isolator 72 are configured as shown in FIG. 15, and the pump light propagating in the opposite direction bypasses the optical isolator. P.B.Hans
en et al. , E11electronics Le
ters, 32, pp. 2164-2165 (199
6)).

The present invention provides the optical isolator itself with a function of propagating light of a certain wavelength only in the forward direction and propagating light of another certain wavelength only in the reverse direction, as shown in FIG. It is an object of the present invention to provide an optical isolator that propagates light in a forward direction or a backward direction according to a wavelength by using a configuration in which no light is provided.

[0017]

According to a first aspect of the present invention, there is provided a polarizer, a birefringent prism, or a polarized beam, which blocks or determines the path of light at least in a preceding stage according to a polarization state. A first optical element of any of the splitters, a Faraday rotator, and a second stage of any of a polarizer, a birefringent prism, or a polarization beam splitter for selecting or combining light according to a polarization state in a subsequent stage. In an optical isolator including an optical element, n and m are arbitrary integers between the first optical element and the second optical element in an optical path of the optical isolator, A birefringent medium having the characteristics of rotating the polarized light by n × 360 ° and the polarized light by m × 360 ° + 90 ° with respect to the light of wavelength 2 is provided.

According to a second aspect of the present invention, at least a first birefringent prism that separates light in accordance with a polarization state in a preceding stage, a Faraday rotator, a polarization rotator, and a polarization state in a subsequent stage. An optical circulator comprising a second birefringent prism that couples light according to the first and second birefringent prisms in the optical path of the optical circulator. Is an arbitrary integer, and the polarization of the light of wavelength 1 is n ×
It is characterized in that a birefringent medium having a characteristic of rotating the polarization of light of 360 ° and wavelength 2 by m × 360 ° + 90 ° is disposed.

According to a third aspect of the present invention, there is provided an optical amplifier comprising a pumping light source and a gain fiber, wherein the optical isolator according to the first aspect is inserted in the middle of the gain fiber, so that the signal light and the signal light wavelength are increased. The noise light is propagated only in the forward direction, and the pump light is propagated only in the reverse direction.

According to a fourth aspect of the present invention, there is provided a fiber Raman amplifier comprising a pump light source and a gain fiber, wherein signal light propagates in the gain fiber in a forward direction and pump light propagates in the gain fiber in a reverse direction. By inserting the optical isolator described in the middle of the gain fiber, signal light and noise light of the signal light wavelength propagate only in the forward direction, and,
It is characterized in that the excitation light propagates only in the reverse direction.

According to a fifth aspect of the present invention, there is provided a fiber Raman amplifier comprising a pump light source and a gain fiber, wherein signal light propagates in the gain fiber in a forward direction and pump light propagates in the gain fiber in a reverse direction. By inserting the optical circulator described in the above in the middle of the gain fiber, the signal light and the noise light of the signal light wavelength propagate only in the forward direction, and the pump light propagates only in the reverse direction. Raman amplifier.

According to a sixth aspect of the present invention, the optical isolator is designed such that a minimum loss and a maximum isolation can be obtained with respect to the light having the wavelength 1, and the light having the wavelength 2 has a small amount. The optical isolator according to claim 1, wherein an increase in loss and a deterioration in isolation are allowed.

According to the present invention, there is provided a light path of an optical isolator, wherein a light is blocked or a path of light is determined at a preceding stage in accordance with a polarization state of a first one of a polarizer, a birefringent prism and a polarization beam splitter. N and m are arbitrary between a second optical element of a polarizer, a birefringent prism, or a polarization beam splitter that selects or combines light according to a polarization state in a subsequent stage. The polarization is n × 360 ° for light of wavelength 1 (light to be propagated only in the forward direction) and m is for light of wavelength 2 (light to be propagated only in the backward direction). × 360 + 90 °, and a birefringent medium having a rotating characteristic is arranged. By using the optical isolator configured as described above, it becomes possible to propagate light of wavelength 1 only in the forward direction and propagate light of wavelength 2 in the reverse direction.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIGS. 1 and 2 are conceptual diagrams of an optical isolator for explaining a first embodiment of the present invention. In the figure, the inclination of the axes in the circles indicated as the light of wavelength 1 and the light of wavelength 2 indicate the polarization angles of the light of wavelength 1 and the light of wavelength 2, respectively. Similar). In this embodiment, the optical isolator has a 45 ° Faraday rotator 92, a 0 ° polarizer 94 and a 45 ° polarizer 94, and a polarization of n × 360 ° for light of wavelength 1,
It is assumed that the birefringent medium 93 has a characteristic that the polarization of the light of wavelength 2 is n × 360 ° + 90 ° and the polarization is rotated.

First, the state of light having wavelengths 1 and 2 propagating in the forward direction will be described with reference to FIG. As shown in FIG.
The light of wavelength 1 propagating in the forward direction is changed to 0 by the polarizer 91.
It is cut into {polarized light and passes through the 45 ° Faraday rotator 92 in the forward direction, so that it becomes 45 ° polarized light. Next, the light of wavelength 1 remains 45 ° polarized light after passing through the birefringent medium 93, and is therefore almost completely transmitted through the 45 ° polarizer 94.

On the other hand, the light of the wavelength 2 propagating in the forward direction is cut into 0 ° polarized light by the polarizer 91 and passes through the 45 ° Faraday rotator 92 in the forward direction to become 45 ° polarized light. Further, the light of wavelength 2 passes through the birefringent medium 93 and becomes 135 ° polarized light, and becomes a 45 ° polarizer 94.
Almost completely shut off.

Next, the state of light of wavelengths 1 and 2 propagating in the backward direction will be described with reference to FIG. As shown in FIG. 2, the light of wavelength 1 propagating in the opposite direction is cut into 45 ° polarization by the polarizer 94, becomes 45 ° polarization after passing through the birefringent medium 93, and further becomes 45 ° Faraday. When the light passes through the rotator 92 in the reverse direction, it becomes 90 ° polarized light, and is almost completely cut off by the 45 ° 0 ° polarizer 91.

On the other hand, the light of wavelength 2 propagating in the opposite direction is cut into 45 ° polarized light by the luminaire 94 and passes through the birefringent medium 93 to become 135 ° polarized light.
When the light passes through the 45 ° Faraday rotator 92 in the opposite direction, it becomes 180 ° polarized light (= 0 ° polarized light), and the 0 ° polarizer 9
1 is transmitted almost completely.

Here, by making the birefringent medium have a predetermined thickness L, the polarization of the light of wavelength 1 is n × 360 °,
With respect to the light having the wavelength 2, the polarization can be obtained by rotating the polarization by m × 360 ° + 90 °, respectively. The reason will be described below. The thickness of the birefringent medium is L, the ordinary light refractive index of the medium is n0, and the extraordinary light refractive index of the medium is ne.

The birefringent medium is arranged so that, for example, the incident light of 0 ° linear polarization is an ordinary ray and the incident light of 90 ° linear polarization is an extraordinary ray. When light of λ is incident on the same medium, when the relationship of (ne−n0) 2πL / λ = 2πn (n is an integer) holds with respect to the thickness L of the medium, the polarization of the incident light does not rotate, and ne−n0) 2πL / λ = 2πn + π (n is an integer), the polarization of the incident light rotates by 90 °.

From this, the wavelength 1 shown in FIGS.
Λ1 and wavelength 2 as λ2, the thickness of the birefringent medium is such that (ne−n0) 2πL / λ1 = 2πn (n is an integer) (ne−n0) 2πL / λ2 = 2πm + π (m is an integer) By determining L, a birefringent medium 93 having the characteristics of rotating the light of wavelength 1 at n × 360 ° and the light of wavelength 2 at n × 360 ° + 90 °, respectively, is obtained. We can see that we can do it.

For the birefringent medium 93, for example, quartz can be used. Since ne of crystal is 1.55 and n0 is 1.54, for example, wavelength 1 is set to 1520n.
m, when the wavelength 2 is 1420 nm, the thickness is about 80 μm.
It can be understood that the crystal of No. can be used as a birefringent medium having the above characteristics. The birefringent medium 93 may be arranged at any position between the polarizer 91 and the polarizer 94, not necessarily at the place shown in FIGS.

(Second Embodiment) FIGS. 3 and 4 are conceptual diagrams of an optical isolator for explaining a second embodiment of the present invention. In this embodiment, the optical isolator comprises an input fiber 111, a lens 112, a birefringent prism 113, a 45 ° polarization rotator 114, and a light having a wavelength of n × 360 ° and a wavelength of 2 for light of wavelength 1. On the other hand, the polarization is n ×
360 ° + 90 °, birefringent medium 115 having a rotating characteristic, 45 ° Faraday rotator 116, birefringent prism 117, lens 118, and output fiber 1
19 is assumed.

First, the state of light of wavelengths 1 and 2 propagating in the forward direction will be described with reference to FIG. As shown in FIG. 3, the light of wavelength 1 input from the input fiber 111 is
The light enters the birefringent prism 113 through the lens 112 as a parallel beam, and is split into a 0 ° polarization component and a 90 ° polarization component.

The 0-degree polarization component is converted to a 45-degree polarization rotator 114.
, And becomes 45 ° polarized light, and then the birefringent medium 1
15, but remains at 45 ° polarization, and further passes through the 45 ° Faraday rotator 116 in the forward direction to obtain 0 °.
The light is returned to the polarized light and returned to the parallel beam by the birefringent prism 117.

The 90 ° polarization component is converted to the 45 ° polarization rotator 11.
4 then becomes 45 ° polarized light and then passes through the birefringent medium 115 but remains at 45 ° polarized light. In addition, 90
The {polarized light component is returned to 90 ° polarized light by passing through the 45 ° Faraday rotator 116 in a direction opposite to the direction of the magnetic field,
The beam is returned to a parallel beam by the birefringent prism 117.
The lens 118 outputs the parallel beam to the output fiber 1.
19. Therefore, the light of wavelength 1 input from the input fiber 111 is almost completely coupled to the output fiber 119.

On the other hand, the light of wavelength 2 input from the input fiber 111 passes through the lens 112 and is incident on the birefringent prism 113 as a parallel beam.
It is divided into 0 ° polarization components.

The 0 ° polarization component is converted to 45 ° polarization rotator 114.
, And becomes 135 ° by passing through the birefringent medium 115. Further, the 0 ° polarized light component becomes 90 ° polarized light by passing through the 45 ° Faraday rotator 116 in a direction opposite to the direction of the magnetic field.
It is not converted back to a parallel beam by the birefringent prism 117.

The 90 ° polarization component is converted to the 45 ° polarization rotator 11.
4 and 135 degree polarization, and 225 degree polarization by passing through the birefringent medium 115. further,
The 90-degree polarized light component passes through the 45-degree Faraday rotator 116 in the direction opposite to the direction of the magnetic field to produce 180-degree polarized light (= 0).
(゜ polarized light), and is not returned to a parallel beam by the refraction prism 117. Also, lens 118 is arranged to couple the parallel beam to the output fiber.
Therefore, the light of wavelength 2 input from the input fiber 111 is hardly coupled to the output fiber 99.

Next, the state of light of wavelengths 1 and 2 that propagate in the opposite direction will be described with reference to FIG. As shown in FIG. 4, the light of wavelength 1 input from the output fiber 119 is
The light enters the birefringent prism 117 as a parallel beam through the lens 118, and is split into a 0 ° polarization component and a 90 ° polarization component.

The 0 ° polarization component is converted into 45 ° polarization by passing through the 45 ° Faraday rotator 116 in the direction of the magnetic field. Then, the 0-degree polarized light component passes through the birefringent medium 115, but remains at the {45} -polarized light. Next, since the 0 ° polarization component becomes 90 ° polarization by passing through the 45 ° polarization rotator 114, it does not return to a parallel beam even after passing through the birefringent prism 113.

The 90 ° polarized light component becomes 135 ° polarized light by passing through the 45 ° Faraday rotator 116 in the direction of the magnetic field. The 90 ° polarization component then passes through the birefringent medium 115 but remains at 135 ° polarization. Next, the 90-degree polarization component passes through the 45-degree polarization rotator 114 so that
Since the light is 80 ° polarized light, it does not return to a parallel beam even after passing through the birefringent prism 113. Also, the lens 112
A parallel beam is arranged to couple to the input fiber. Accordingly, the light of wavelength 1 input from the output fiber 119 is hardly coupled to the input fiber 111.

On the other hand, the light of wavelength 2 input from the output fiber 119 passes through the lens 118 and is incident on the birefringent prism 117 as a parallel beam.
It is divided into 0 ° polarization components.

The 0 ° polarized light component becomes 45 ° polarized light by passing through the 45 ° Faraday rotator 116 in the direction of the magnetic field, and then becomes 135 ° polarized light by passing through the birefringent medium 115. Then, the 0 ° polarization component is converted to the 45 ° polarization rotator 11.
4, it becomes 180 ° polarized light (= 0 ° polarized light), and is returned to a parallel beam by the birefringent prism 114.

The 90 ° polarization component becomes 135 ° polarization by passing through the 45 ° Faraday rotator 96 in the direction of the magnetic field. Then, the 90 ° polarized light component becomes 225 ° polarized light by passing through the birefringent medium 115. Next, the 90 ° polarized light component passes through the 45 ° polarization rotator 114 to become 270 ° polarized light (= 90 ° polarized light).
13 returns to a parallel beam. Also, the lens 11
2 is arranged so that the parallel beam couples into the input fiber. Therefore, the light of wavelength 2 input from the output fiber 118 is almost completely coupled to the input fiber 111.

As in the first embodiment, when the wavelength 1 is 1520 nm and the wavelength 2 is 1420 nm, quartz having a thickness of about 80 μm can be used as the birefringent medium 115. The birefringent medium 115 is not necessarily at the place shown in FIGS.
13 and the birefringent prism 117.

(Third Embodiment) FIGS. 5 and 6 are conceptual diagrams of an optical circulator for explaining a third embodiment of the present invention. This embodiment is an example in which the present invention is applied to an optical circulator. In this embodiment, the optical circulator comprises a polarization beam splitter 131, 45 ° Faraday rotators 132 and 133, the polarization of which is n × 360 ° for the light of wavelength 1, and the polarization for the light of wavelength 2. Are n × 360 ° + 90 °, and birefringent media 134 and 135, 45 ° polarization rotator 13
6 and 137, and a polarization beam splitter 138.

First, the state of light incident from the port 1 will be described with reference to FIG. As shown in FIG.
The light of wavelength 1 input from the polarization beam splitter 1
According to 31, the light is divided into a 0 ° polarization component and a 90 ° polarization component.

The 90 ° polarized light component passes through the polarization beam splitter 131 and passes through the 45 ° Faraday rotator 132 in the direction of the magnetic field to become 135 ° polarized light. And
The 90 ° polarization component passes through the birefringent medium 134 but remains at 135 ° polarization. Next, the 90-degree polarization component passes through the 45-degree polarization rotator 136 to obtain 180-degree (= 0)
It becomes polarized light. The 90 ° polarized light component is reflected by the reflecting means H
After being reflected at 1, the light is reflected by the polarization beam splitter 138, so that the light is emitted to the port 2.

The 0 ° polarization component is converted to the polarization beam splitter 1
After being reflected at 31, and then reflected by the reflecting means H2, the light passes through the 45 ° Faraday rotator 133 in the direction of the magnetic field, and becomes 45 ° polarized light. Then, the 0 ° polarization component passes through the birefringent medium 135, but remains at 45 ° polarization. Next, the 0 ° polarization component becomes 90 ° polarization by passing through the 45 ° polarization rotator 137, and passes through the polarization beam splitter 138 and exits to the port 2. Therefore, port 1
The light of wavelength 1 input from the port is almost completely emitted from the port 2.

On the other hand, the light of wavelength 2 input from the port 1 is split by the polarization beam splitter 131 into a 0 ° polarization component and a 90 ° polarization component.

The 90 ° polarized light component passes through the polarization beam splitter 131 and passes through the 45 ° Faraday rotator 132 in the direction of the magnetic field to become 135 ° polarized light. And
The 90 ° polarization component passes through the birefringent medium 134 to
It becomes 5 ° polarized light. Next, the 90 ° polarization component passes through the 45 ° polarization rotator 136 to be 270 ° (= 90 °).
It becomes polarized light. Then, the 90 ° polarized light component is reflected by the reflection means H1.
After being reflected by the polarization beam splitter 138, the light is emitted to the port 4 for transmission.

The 0 ° polarization component is converted to the polarized beam splitter 1
After being reflected at 31, and then reflected by the reflecting means H2, the light passes through the 45 ° Faraday rotator 133 in the direction of the magnetic field, and becomes 45 ° polarized light. Then, the 0 ° polarization component becomes 135 ° polarization by passing through the birefringent medium 135. Next, the 0 ° polarization component is converted into 180 ° (= 0 °) polarization by passing through the 45 ° polarization rotator 137, and is reflected by the polarization beam splitter 138, and is output to the port 4. Therefore, the light of wavelength 2 input from port 1 exits port 4 almost completely.

Next, the state of light incident from the port 2 will be described with reference to FIG. As shown in FIG.
The light of wavelength 1 input from the polarization beam splitter 1
According to 38, the light is separated into a 0 ° polarization component and a 90 ° polarization component.

The 90 ° polarized light component passes through the polarization beam splitter 138 and passes through the 45 ° polarization rotator 137 to become 135 ° polarized light. The 90 ° polarization component then passes through the birefringent medium 135 but remains at 135 ° polarization. Next, the 90 ° polarized light component is returned to 90 ° polarized light by passing through the 45 ° Faraday rotator 133 in a direction opposite to the direction of the magnetic field. Then, the 90 ° polarized light component is reflected by the reflection means H2, and then passes through the polarization beam splitter 131, and thus is output to the port 3.

The 0-degree polarized light component is output from the polarization beam splitter 1.
After being reflected at 38, the light is reflected by the reflection means H1 and passes through the 45 ° polarization rotator 136 to be 45 ° polarized light. Then, the 0 ° polarization component passes through the birefringent medium 134,
゜ The polarization remains. Next, the 0 ° polarization component is returned to 0 ° polarization by passing through the 45 ° Faraday rotator 132 in a direction opposite to the direction of the magnetic field, and the polarization beam splitter 13
The light exits from port 3 because it is reflected at 1.

On the other hand, the light of wavelength 2 input from the port 2 is split by the polarization beam splitter 138 into a 0 ° polarization component and a 90 ° polarization component.

The 90 ° polarized light component passes through the polarization beam splitter 138 and passes through the 45 ° polarization rotator 137 to become 135 ° polarized light. Then, the 90 ° polarized light component becomes 225 ° polarized light by passing through the birefringent medium 135. Next, the 90 ° polarized light component passes through the 45 ° Faraday rotator 133 in a direction opposite to the direction of the magnetic field, and thereby becomes 180 °.
Returned to {(0}) polarization. Then, the 90 ° polarized light component is reflected by the reflection means H2 and then reflected by the polarization beam splitter 131, so that it is output to the port 1.

The 0 ° polarization component is converted to the polarization beam splitter 1
After being reflected at 38, the light is reflected by the reflection means H1 and passes through the 45 ° polarization rotator 136 to be 45 ° polarized light. Then, the 0 ° polarized light component becomes 135 ° polarized light by passing through the birefringent medium 134. Next, the 0 ° polarization component is returned to 90 ° polarization by passing through the 45 ° Faraday rotator 132 in a direction opposite to the direction of the magnetic field, and the polarization beam splitter 1
The light is emitted to port 1 for transmission through 31.

When the same is considered below, the optical circulator of the present configuration is arranged as shown in FIG. 8 in the direction of port 1 → port 2 → boat 3 → port 4 → port 1 for the light of wavelength 1. Has a circulator function, while the wavelength 2
It can be seen that the light has a circulator function in the direction of port 1 → port 4 → port 3 → port 2 → port 1.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment of the present invention.
It is a key map of an optical fiber amplifier explaining an embodiment. The fourth embodiment is an embodiment relating to an optical fiber Raman amplifier configured using the optical isolator according to claim 1 or 2 described in the first to third embodiments.

The optical fiber Raman amplifier of this embodiment is
As shown in FIG. 7, the amplification fiber 151, the excitation light source 1
52, an excitation light multiplexing means 153, and an optical isolator 154. Here, the optical circulator 15
4 is designed so that the signal light propagates only in the forward direction and the pump light propagates only in the reverse direction according to the wavelengths of the signal light and the pump light (that is, the wavelength 1 propagates only in the forward direction). Wavelength 2 propagates in the opposite direction).

In this optical fiber amplifier, as shown in FIG. 7, the signal light propagates only in the forward direction, and the pump light propagates only in the reverse direction. The reflected light of the signal light wavelength and backward Rayleigh scattered light propagating in the opposite direction are blocked by the optical isolator. Also, the reflected light of the excitation light wavelength and the backward Rayleigh scattered light propagating in the forward direction are blocked by the optical isolator. Optical circulator 154, in particular,
It may be designed so as to cause little loss to the pump light and some loss to the signal light.

As described above, one embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and a design change or the like may be made without departing from the gist of the present invention. The present invention is also included in the present invention.

[0065]

As described above, according to the present invention, by adding a polarization rotator having wavelength dependency to an optical isolator, light of wavelength 1 is emitted only in the forward direction and light of wavelength 2 is emitted in the reverse direction. Only in this case, there is an effect that it is possible to have a function of propagating. As a result, for example, in the above-described fiber Raman amplifier, the signal light is propagated only in the forward direction and the pump light is propagated only in the reverse direction according to the wavelengths of the signal light and the pumping light. Since the optical isolator can be inserted, the pump light propagating in the opposite direction can be effectively used without being blocked.

[Brief description of the drawings]

FIG. 1 shows that light of wavelength 1 is propagated only in the forward direction,
FIG. 2 is a diagram showing a configuration of a first embodiment of an optical isolator that propagates the light in the backward direction, and how the light propagates in the forward direction.

FIG. 2 shows that light of wavelength 1 is propagated only in the forward direction,
FIG. 2 is a diagram illustrating a configuration of a first embodiment of an optical isolator that propagates the light in the opposite direction and a state of light propagating in the opposite direction.

FIG. 3 shows that light of wavelength 1 is propagated only in the forward direction,
FIG. 9 is a diagram illustrating a configuration of an optical isolator according to a second embodiment for transmitting light in the backward direction and a state of light propagating in the forward direction.

FIG. 4 shows a case where light of wavelength 1 is propagated only in the forward direction,
FIG. 7 is a diagram illustrating a configuration of an optical isolator according to a second embodiment for transmitting light in the opposite direction and the state of light propagating in the opposite direction.

FIG. 5: Transmit light of wavelength 1 in the direction of ports 1 → 2 → 3 → 4 → 1 and transmit light of wavelength 2 to ports 1 → 4 → 3 → 2 → 1
FIG. 3 is a diagram illustrating a configuration example of an optical circulator that transmits light in the direction of FIG.

FIG. 6 transmits light of wavelength 1 in the direction of ports 1 → 2 → 3 → 4 → 1, and transmits light of wavelength 2 to ports 1 → 4 → 3 → 2 → 1.
FIG. 3 is a diagram illustrating a configuration example of an optical circulator that transmits light in the direction of FIG.

FIG. 7 shows that light of wavelength 1 is propagated only in the forward direction,
FIG. 2 is a diagram showing an example of the configuration of an optical fiber amplifier using an optical isolator that propagates the light of the opposite direction.

FIG. 8: Transmit light of wavelength 1 in the direction of port 1 → 2 → 3 → 4 → 1 and transmit light of wavelength 2 to port 1 → 4 → 3 → 2 → 1
FIG. 4 is a diagram illustrating an optical circulator that transmits light in the direction of FIG.

FIG. 9 is a conceptual diagram showing a configuration of an optical isolator and a state of light propagating therethrough in a forward direction.

FIG. 10 is a conceptual diagram showing a configuration of an optical isolator and a state of light propagating in the opposite direction.

FIG. 11 is a diagram illustrating a configuration of a polarization independent optical isolator and a state of light propagating in the forward direction.

FIG. 12 is a diagram showing a configuration of a polarization independent optical isolator and a state of light propagating in the opposite direction.

FIG. 13 is a diagram showing another configuration of the polarization independent optical isolator and a state of light propagating in the opposite direction.

FIG. 14 is a diagram illustrating another configuration of the polarization independent optical isolator and a state of light propagating in the opposite direction.

FIG. 15 is a diagram illustrating one method for propagating light of wavelength 1 only in the forward direction and propagating light of wavelength 2 in the reverse direction.

FIG. 16 is a diagram illustrating the concept of an optical isolator that propagates light of wavelength 1 only in the forward direction and propagates light of wavelength 2 in the reverse direction.

[Explanation of symbols]

 Reference Signs List 11 45 ° Faraday rotator 120 ° polarizer 13 45 ° polarizer 31 Input fiber 32 lens 33 Birefringent prism 34 Birefringent medium 35 45 ° Faraday rotator 36 Birefringent prism 37 Lens 38 Output fiber 51 Polarization beam splitter 52 Polarization beam splitter 53 45 ° Faraday rotator 54 45 ° polarization rotator 71 Wavelength division multiplexing filter 72 Optical isolator 91 0 ° polarizer 92 45 ° Faraday rotator 93 45 ° polarization rotator 94 45 ° polarizer 111 input Fiber 112 lens 113 birefringent prism 114 45 ° polarization rotator 115 birefringent medium 116 45 ° Faraday rotator 117 birefringent prism 118 lens 119 output fiber 131 polarization beam splitter 132 45 45 ° Faraday rotator 133 5 ° Faraday rotator 134 birefringent medium 135 birefringent medium 136 45 ° polarization rotator 137 45 ° polarization rotator 138 polarization beam splitter 151 amplifying fiber 152 the pumping light source 153 pumping light multiplexing means 154 optical isolator

Claims (6)

[Claims] At least a first optical element of a polarizer, a birefringent prism, or a polarization beam splitter that blocks light or determines a path of light in a preceding stage according to a polarization state, An optical isolator comprising: a rotator; and a second optical element of a polarizer, a birefringent prism, or a polarization beam splitter that selects or combines light in a subsequent stage according to a polarization state. Between the first optical element and the second optical element in the optical path of the isolator, where n and m are arbitrary integers, the polarization of the light of wavelength 1 is n × 360 ° and the wavelength of light of wavelength 2 is The polarization of the light is m
An optical isolator characterized by arranging a birefringent medium having a characteristic of rotating by 360 ° + 90 °. At least a first birefringent prism that separates light according to a polarization state at a previous stage, a Faraday rotator, a polarization rotator, and a light combiner according to a polarization state at a subsequent stage. In an optical circulator comprising a second birefringent prism, between the first birefringent prism and the second birefringent prism in the optical path of the optical circulator, n and m are arbitrary integers, The polarization of the light of wavelength 1 is n × 360 °, and the polarization of the light of wavelength 2 is m
An optical circulator characterized by disposing a birefringent medium having a characteristic of rotating by 360 ° + 90 °. 3. An optical amplifier comprising a pumping light source and a gain fiber, wherein the optical isolator according to claim 1 is inserted in the middle of the gain fiber to forward signal light and noise light of the signal light wavelength in the forward direction. An optical amplifier characterized in that the light propagates only in the opposite direction and the pump light propagates only in the opposite direction. 4. A fiber Raman amplifier comprising a pump light source and a gain fiber, wherein signal light propagates in the gain fiber in the forward direction and pump light propagates in the gain fiber in the reverse direction. A fiber Raman amplifier, which is inserted in the middle of a gain fiber so that signal light and noise light of a signal light wavelength propagate only in the forward direction and pump light propagates only in the reverse direction. 5. A fiber Raman amplifier comprising a pumping light source and a gain fiber, wherein signal light propagates in the gain fiber in the forward direction and pump light propagates in the gain fiber in the reverse direction, wherein the optical circulator according to claim 2 is provided. A fiber Raman amplifier, which is inserted in the middle of a gain fiber so that signal light and noise light of a signal light wavelength propagate only in the forward direction and pump light propagates only in the reverse direction. 6. The optical isolator is designed such that a minimum loss and a maximum isolation are obtained with respect to the wavelength 1 light, and the wavelength 2 light slightly increases loss and degrades isolation. The optical isolator according to claim 1, wherein