JPS63284518A - Optical isolator - Google Patents

Abstract

PURPOSE:To improve the matching to an optical fiber for maintaining the plane of polarization and to prevent the degradation of isolation due to the influence of a background light by reciprocating a forward transmitted light in a 45 deg. Faraday rotator by reflection mirror so as to make the deflection plane of the exit light perpendicular to the reference plane of the incident light. CONSTITUTION:When a linearly deflected light which has the deflection plane on the incidence face of a deflection separating film 26 of a first deflection separating prism 20 of an optical isolator is made incident on the direction of an arrow IN, the incident light passes the prism 20, a 45 deg. Faraday rotator 28, and a second deflection separating prism 30 and is reflected on a reflection mirror 38. This light is transmitted through the prism 30 and the rotator 28 and is reflected on the separating film 26 of the prism 20 and is emitted in the direction of an arrow OUT. The deflection plane of this emitting face is perpendicular to that of the incidence face to facilitate mechanical matching of the deflection plane of the optical fiber maintaining the plane of polarization. Since the reflected return light is orthogonal to the incident light, the background light due to aberrations of a lens or the like is prevented from going in the direction of incidence.

Description

Detailed Description of the Invention Overview With the above configuration, the incident light that has passed through the first polarization separation prism travels back and forth within the 45° Faraday rotator and the second polarization separation prism, and then passes through the polarization separation film of the first polarization separation prism. It is reflected and emitted.

Therefore, since the emitted light is perpendicular to the incident light, deterioration of isolation due to background light is prevented. Further, since the polarization plane of the emitted light and the polarization plane of the transmitted light of the first polarization splitting prism are perpendicular to each other, connection becomes easy, especially when a polarization maintaining fiber is used as a transmission path.

INDUSTRIAL APPLICATION FIELD The present invention relates to an optical isolator used on the light emitting source side in optical communication systems and the like.

In optical communication systems that use optical fiber as the transmission path,
Light that enters an optical fiber from a light source may be reflected at a joint between the optical fibers, and a portion of the light may return to the light source. When such reflected feedback light occurs, the intensity noise of the semiconductor laser as a light source increases, making it difficult to achieve a required transmission distance, especially in a gigabit (Gb/s) class high-speed system. For this reason, an optical isolator that can pass light only in the forward direction is required. Conventional technology Figure 5 is a plan view of a conventional optical isolator using a polarization splitting prism. The diagram is for explaining the polarization state at each point on the optical path shown in FIG. 5. For convenience, an orthogonal three-dimensional coordinate system (x, y, z) is set, which has a Z-axis in the direction of the incident optical axis and a y-axis that is perpendicular to the plane of FIG. 5 and penetrates from the back side to the front side.

Reference numeral 2 designates a first polarization separation prism in which a polarization separation film 2C is interposed between the triangular prisms 2a and 2b.
is located on the plane F, represented by Z=X. 1st
The second polarization separation prism 4 is formed by moving the polarization separation prism 2 in parallel in one direction and rotating it clockwise by 45 degrees toward the Z direction.
It is constructed by interposing. A 45° Faraday rotator 6 is provided between the first polarization splitting prism 2 and the second polarization splitting prism 4, and rotates the polarization plane of transmitted light by 45° clockwise in two directions.

The light incident on the first polarization splitting prism 2 is separated into a p-wave component having a polarization plane on the plane of incidence on the polarization splitting film 2C and an S-wave component having a polarization plane perpendicular to this.
The wave component is optically rotated by a 45° Faraday rotator 6 to become a p' wave having a polarization plane making an angle of 45° with the polarization plane of the p wave. , p' waves are transmitted through the second polarization separation prism 4, emitted, and coupled to an optical fiber (not shown).

The reflected return light that is reflected from the end face of the optical fiber, etc.
Normally, the polarization state is not preserved and an S' wave component having a polarization plane perpendicular to the polarization plane of the P' wave is generated, but this reflected feedback light is separated in the second polarization separation prism 4. Then, only the p' wave component is incident on the 45° Faraday rotator 6.

Then, this p' wave is optically rotated counterclockwise toward the -2 direction and emitted as an S wave, and is separated from the forward incident optical axis in the first polarization splitting prism 2 and emitted. In this way, reflected feedback light is prevented from returning to a light source (not shown).

Problems to be Solved by the Invention Incidentally, the emitted light of a semiconductor laser serving as a light source is generally linearly polarized light. X parallel to the paper
It is important to match the Z plane (hereinafter referred to as the reference plane).
This is advantageous in terms of insertion loss of the optical isolator. At this time, the light emitted from the second polarization separation prism 4 forms an angle of 456 with respect to the reference plane, as described above.

Therefore, when the optical isolator is applied to a coherent optical communication system using a polarization-maintaining fiber as a transmission path, one of the two polarization-maintaining planes of the polarization-maintaining fiber, which are orthogonal to each other, is It is necessary to connect the connectors at an angle of 45° with respect to the reference plane, which may lead to complicated work such as alignment of the connectors during mechanical connection.

In addition, in coherent optical communication systems, the spectral purity of the signal light is extremely high, so it is easily affected by reflected feedback light, and for this reason, the optical isolator is required to have high isolation capability. Since the directions of the incident light and the outgoing light of the optical isolator are the same, there is a risk that the background light of the reflected feedback light may return to the light source, which is a problem.

The present invention was created in view of these circumstances, and its purpose is to provide an optical isolator that does not suffer from the above-mentioned problems.

Means for Solving the Problems In order to solve the problems of the prior art, the present invention provides a method in which the polarization plane of the emitted light is perpendicular to the reference plane, and the incident light and the emitted light are orthogonal to each other. This provides an optical isolator with excellent performance.

Figure 1 is a basic configuration diagram of this optical isolator, (a
) is given forward transmitted light, and part (b) is given reflected feedback light in the reverse direction.

10 is a first polarization separation pulley having a polarization separation film 8.
- It is a rhythm.

12 is a 45° Faraday rotator that rotates transmitted light by 45°.

16 is a second polarization separation prism having a polarization valve 1111 film 14, and the polarization separation film 14 rotates the polarization separation film 8 by 45 degrees.
The Faraday rotator 12 is rotated 456 times in the direction of optical rotation.

18 is a reflecting mirror that reflects incident light in that direction.

And each of these optical elements 10.12.16°18 is
They are arranged in this order on the optical axis that is incident in the direction of the arrow IN.

Action Figure 2 shows each point (A) in the forward direction and reverse direction in Figure 1.
~J) shows the polarization state. Uppercase letters indicating each point in Figure 1 and lowercase letters attached to each graph in Figure 2 are as follows:
Each corresponds to the other. The polarization plane of light incident in the direction of the arrow IN from a semiconductor laser (not shown) is made to coincide with the XZ plane so that it becomes a P wave that can pass through the polarization separation film 8 (a>.P). An S wave with a plane of polarization perpendicular to that of the wave is actually 20 times smaller than a P wave.
Although it has a small intensity of about dB, this S wave component is
The light is reflected and separated by the polarization separation film 8 (b). The P wave (C) that has passed through the first polarizing prism 10 is optically rotated by 45° clockwise in the Z direction by the 45° Faraday rotator 12 . For convenience, this is called a P' wave. The second polarization splitting prism 16 has a diameter of 45mm with respect to the first polarization splitting prism 10.
Since the P′ wave is tilted, the second polarization splitting prism 1
6, is reflected by a reflecting mirror 18, and then passes through the second polarization separation prism 16 again. Then, the light is rotated by 45° counterclockwise in one or two directions by the 45° Faraday rotator 12 to become an S wave (e). This S wave enters the first polarization separation prism 10 and is reflected by the polarization separation film 8, and is
It is emitted in the UT direction (f). In this way, the reference plane and the polarization plane of the emitted light are perpendicular to each other, making it easy to connect polarization-maintaining fibers and the like.

On the other hand, when the transmission path is a polarization-maintaining fiber, almost no P-wave component is generated in the reflected return light, but even if some P-wave component is generated (g), this P-wave component is
The light passes through the polarization separation film 8 and is separated (h). The S wave (1) reflected by the polarization separation film 8 and emitted from the first polarization separation prism 10 is optically rotated by 45 degrees clockwise in the Z direction by the 45 degree Faraday rotator 12 and then emitted. For convenience, this is called an S' wave (j). Since this S' wave has a polarization plane perpendicular to the polarization plane of the P' wave at the point, it is separated by the polarized separation film 14 of the second FA light separation prism 16 and cannot reach the reflecting mirror 18. do not have. In this way, reflected feedback light is prevented from returning to the light source. At this time, since the direction of the reflected return light beam and the direction of the incident light beam from the light source are orthogonal to each other, deterioration of isolation due to iW of background light is prevented.

Example Embodiments of the present invention will be described below based on the drawings.

FIGS. 3(a) and 3(b) are perspective views of an optical isolator constructed by applying the present invention, in which the optical axes of forward transmitted light (a) and backward reflected feedback light (b) are respectively Granted. This optical isolator includes a first polarization separation prism 2
0.45° Faraday rotator 28, second polarization separation prism 30. and a reflecting mirror 38 are integrated. 1st
The polarization separation prism 20 is a triangular prism 22 having the same shape.
A polarization separation film 26 is formed by coating one of the slopes of the triangular prisms 22 and 24 with, for example, a dielectric multilayer film, and the slopes of the triangular prisms 22 and 24 are adhered to each other using an optical adhesive. It can be obtained by The 45° Faraday rotator 28 is a normal one made of a magneto-optical crystal such as a YIG single crystal, and rotates the transmitted light by 45° according to a saturation magnetic field applied from a permanent magnet (not shown).
The length is set to allow optical rotation. The second polarization separation prism 30 has a polarization separation film 36 at a position obtained by rotating the polarization separation film 26 of the first polarization separation prism 20 by 45° clockwise toward the incident optical axis direction. 3
6 is obtained by interposing a dielectric multilayer film between the glass blocks 32 and 34 similarly to the first polarization separation prism 20, and then cutting both glass blocks 32 and 34 into a predetermined shape.

The reflecting mirror 38 can be obtained by forming a total reflection film on the polished surface of the second polarization separation prism 30 by means such as metal vapor deposition, but it may also be an independent member attached with an optical adhesive. .

Note that the arrangement of each optical element 20, 28, 30 with respect to the incident optical axis and the output optical axis and the positional relationship between each element are substantially the same as the basic configuration shown in FIG. Omitted.

Now, suppose that linearly polarized light having a plane of polarization on the plane of incidence on the polarization separation film 26 is incident from a semiconductor laser (not shown) in the direction of the arrow IN in the figure. It passes through the Faraday rotator 28 and the second polarization splitting prism 30, is reflected by the reflecting mirror 38, and further passes through the second polarization splitting prism 30 and the 45° Faraday rotator 28, and then is polarized by the first polarization splitting prism 20. The light is reflected by the film 26 and emitted in the same OUT direction.

The polarization plane of the output light is perpendicular to the polarization plane of the input light, so compared to the conventional case where it is tilted at 45 degrees, the polarization plane of the polarization preserving fiber can be mechanically aligned. becomes easier. Furthermore, since the direction of the reflected return ray shown in FIG. 3(b) is perpendicular to the direction of the incident ray shown in FIG. It is prevented from proceeding in the direction of the light beam,
There is no risk of deterioration of isolation.

The reflected feedback light incident on the first polarization separation prism 20 is
4 after passing through the polarization separation film 26 or reflecting it.
The light passes through the 5° Faraday rotator 28, is reflected by the polarization separation film 36, and is emitted. Therefore, the reflected feedback light does not reach the reflecting mirror 38, and is therefore prevented from returning to the incident side.

FIG. 4 is for explaining part of the manufacturing process of the optical isolator shown in FIG. 3. In this embodiment, each optical component is integrated using an optical adhesive or the like, so the manufacturing process can be simplified by integrally forming all or part of these components in advance. That is, for example, a prism base material 20' consisting of triangular prisms 22', 24' and a polarization separation film 26' and a magneto-optic crystal base material 28' are integrated, and this is shown by a dashed line in FIG. By cutting at the plane, the first
Polarization separation prism 20 and 45° Faraday rotator 28
It is possible to obtain a plurality of integrated products consisting of: At this time, each surface of the base material can be polished, which improves productivity.

Effects of the Invention As detailed above, according to the present invention, the forward transmitted light is reciprocated within the 45° Faraday rotator by the reflecting mirror, so the polarization plane of the output light is the same as the polarization plane of the incident light ( Reference plane)
This has the effect of improving the matching with the polarization maintaining fiber. Furthermore, since the direction of the incident light beam and the direction of the output light beam are perpendicular to each other, there is also the effect that there is no possibility that the isolation will deteriorate due to the influence of background light. Furthermore, when each optical element is integrated as in the embodiment, operations such as polishing can be performed simultaneously, which has the effect of simplifying the manufacturing process.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle of the present invention, and is a plan view of an optical isolator (a: forward transmitted light, b: backward reflected feedback light). FIG. 2 is a diagram showing the principle of the present invention, and FIG. Figure 3 is a diagram for explaining the polarization state at each point A to J on the optical axis shown in the figure. Figure 3 is an embodiment of the present invention. Light, b; Reverse reflected feedback light)
, FIG. 4 is a perspective view of a part of the base material of the optical isolator shown in FIG. 3, FIG. 5 is a plan view of a conventional optical isolator, and FIG. 6 is a diagram showing an embodiment of the present invention. 5 is a diagram for explaining the polarization state at each point af on the optical path shown in FIG. 5. FIG. 2.10.20...first polarization separation prism, 4.16
．． 30...Second polarization separation prism, 6.12.28.
...45° Faraday rotator, 2C, 4G, 8.14°26.36... Polarization separation film, 18.38... Reflector. (a) (b) (C) Water bucket 5 Sun/Month 2nd (d) (e) (f) Kyubara map Ita 5 Next A row Figure 6 y y y (8, Shō's polarization 4 fire, state ) figure

Claims (1)

[Claims] A first polarization separation prism (10) having a polarization separation film (8) in the direction of the incident optical axis.
), a 45° Faraday rotator (12), and a second polarization separation prism (16) having a polarization separation film (14) formed by rotating the polarization separation film (8) by 45° toward the optical axis direction. , a reflecting mirror (18), arranged in this order.