JPS63293519A - Optical isolator - Google Patents

Abstract

PURPOSE:To miniaturize structure, to increase coupling efficiency and to reduce cost by means of the small number of parts by setting up the optical axis of a polarizing prism for penetrating light radiated from a Faraday rotor to a direction different from a light transmitting direction. CONSTITUTION:Light from a light source 18 is rotationally polarized by 45 deg. through the Faraday rotor 20, made incident upon the polarizing prism 22, and since the optical axis 22a is set up to a direction different from incident light, separated into a normal light component 101 and an abnormal light component 102. When the component 101 is coupled with an SMF, its reflected feedback light is rotationally polarized by the rotor 20 and returned to a light source 18 as straight polarized light. The abnormal light component from the prism is rotationally polarized through the rotor 20 and projected to a direction different from the light source. Thereby, the structure can be miniaturized and the coupling efficiency can be increased by means of the small number of parts, and since only the small quantity of an expensive optical crystal material is used, cost can be reduced.

Description

Detailed Description of the Invention Overview An optical isolator is composed of a 45° Farade rotator and a polarizing prism made of an axle crystal that transmits the output light, and the propagation direction of the light passing through the polarizing prism is determined by the optical direction of the polarizing prism. Make sure the direction is different from the axis. As a result, the polarization component of the reflected feedback light that was not removed by the optical isolator is perpendicular to the polarization plane of the forward incident light, so if the light source is a semiconductor laser that emits linearly polarized light, the polarization component of the reflected feedback light that is not removed by the optical isolator is
g) The influence of the feedback light on the light source is almost eliminated.

INDUSTRIAL APPLICATION FIELD The present invention relates to an optical isolator for eliminating the influence of reflected feedback light in a light emitting section of an optical communication device, a light emitting section of a laser f-isk device, and the like.

For example, in an optical communication system using an optical fiber as a transmission path, a portion of light that enters the optical fiber from a light source may be reflected at a joint between the optical fibers and return to the light source. When such reflected feedback light occurs, especially when the light source is a semiconductor laser, its operation becomes unstable and deterioration of transmission quality is unavoidable. For this reason, there is a need for an optical isolator that can eliminate the influence of reflected feedback light, and a simplification of its configuration is desired.

2. Description of the Related Art A known optical isolator of this type is one in which a Faraday rotator 6 is interposed between a pair of polarizing prisms 2 and 4, as shown in FIG. 4, for example. The polarizing prism 2.4 is made of an axle crystal having birefringence, and the crystal axes 2a and 4a are inclined at 45° with respect to each other. A saturation magnetic field is applied to the Faraday rotator 6 from a permanent magnet (not shown), and the incident light is rotated by 45 degrees.

For example, in the light source module shown in the figure, this optical isolator 8 connects the emitted light of a semiconductor laser (hereinafter referred to as LD) 10 to a single mode optical fiber (hereinafter referred to as S).
lens 12.14 for coupling to 16 (referred to as MF)
The signal light in the forward direction (from the left to the right in the figure) is transmitted in a direction parallel to the incident direction, and the reflected return light in the reverse direction (from the cloth to the left in the figure) is polarized. Regardless of the direction, it functions to separate and emit in a direction opposite to LDlo. Therefore, the reflected feedback light is
Therefore, the influence of reflected feedback light can be eliminated.

Problems to be Solved by the Invention In the above-mentioned optical isolator 8, the three optical crystals constituting it are arranged in series, so the length depends on the material of the optical crystal. The reality is that the number of shoes is ~8. By the way, in the light source module as shown in FIG.
are located at the focal point of each lens 12.14,
Moreover, it is said that the coupling loss is minimized when the separation distance between the lenses 12 and 14 is set equal to the sum of the two focal lengths. However, lens 12.
The focal length of lens 12 and 14 is small due to the relationship between the numerical apertures of LDIO and SMF 16, so if optical isolator 8 has the above size, the distance between lenses 12 and 14 should be set at the confocal position. There is a problem in that the coupling efficiency decreases because the distance has to be made larger than the original distance.

Furthermore, since there are three optical crystals that make up the optical isolator 8, there are six interfaces between these and the medium, and losses due to reflection at these interfaces cannot be ignored. is the reality.

The present invention was created in view of these circumstances, and an object of the present invention is to provide an optical isolator with a small loss of forward transmitted light.

In general, the emitted light of the LD is substantially linearly polarized light, and the polarized light component having a polarization plane perpendicular to this polarized light plane is about 30 dB smaller in intensity than the polarized light component of the emitted light. It is known that. In addition, if the reflected feedback light returning to the LD is linearly polarized light having a polarization plane perpendicular to the polarization plane of the emitted light, that is, if the reflected feedback light does not have the polarization component of the emitted light, the reflected feedback light returns to the LD. It is known that there is almost no effect of light.

The present invention has been made with attention to these facts, and by configuring the reflected feedback light to exclude only the polarization component having the same polarization plane as the polarization plane of the light emitted from the light source, the light This is intended to simplify the configuration of the isolator.

That is, the optical isolator of the present invention has a basic configuration as shown in FIG. The polarizing prism 22 is made of a central axis crystal that transmits the emitted light. The optical axis 22a of the polarizing prism 22 is set in a direction different from the propagation direction of light transmitted through the polarizing prism 22, for example, in a direction perpendicular to the propagation direction.

For convenience of explanation, it is assumed that the polarizing prism 22 is made of a positive uniaxial crystal, and its refractive index for ordinary rays is n. , let n be the maximum value of the refractive index for extraordinary rays (no<n.

). Then, it is assumed that the light is propagating in the direction of the arrow S at the origin 0 of the orthogonal three-dimensional coordinate axis with the optical axis 22a of the polarizing prism 22 as the Z axis, and the projection of the arrow S onto the xy plane coincides with the y axis. (Figure 2). At this time, the refractive index ellipsoid is expressed as -I-y/n+z''/nX/no2o282-1...(1).

The refractive index n0 for ordinary rays is always constant, and the origin O up to the point P where the circle A cut by the xy plane of the refractive index ellipsoid intersects the ellipse B cut by the plane orthogonal to the propagation direction S at the origin O It is expressed as the distance mop from . In addition, the refractive index n, l for the extraordinary ray changes depending on the angle θ formed by the propagation direction S and 2@, and is expressed as the distance mOQ from the origin 0 to the point Q where the ellipse B and the yz plane intersect. be done. That is, the refractive index n' for the extraordinary ray varies from n to n depending on the propagation direction S of the light within the polarizing prism 4. It changes continuously until The relationship between θ, n, and J at this time is 1, 2 2 2 ne iy +z...(2)z-
n ′ stn θ
-(3), so no' = n n /(ne2CO32θ+n
sin θ) (4).

Here, when θ-〇, it becomes n'-n O. Although the apparent anisotropy of the uniaxial crystal forming the upper width optical prism 22 is lost, in the optical isolator of the present invention, Since the optical axis 22a is set in a direction different from the propagation direction of the transmitted light, n'≠n always holds, and the refractive index differs depending on the polarization state of the transmitted light, so the influence of reflected feedback light is is prevented.

In the optical isolator shown in FIG. 1, if the light source 18 emits linearly polarized light with a plane of polarization parallel to the plane of the paper, this emitted light is transmitted by the Faraday rotator 20 in the forward direction (from left to right in the figure). direction) (clockwise) 4
The light is rotated by 5 degrees and enters the polarizing prism 22. Since the optical axis 22a of the polarizing prism 22 is set in a direction different from the propagation direction of the incident light, for example, perpendicularly,
The light transmitted through the polarizing prism 22 is separated into an ordinary ray component 101 and an extraordinary ray segment 102 and then emitted. Therefore, the one having a relatively higher intensity among these separated light beams can be coupled to an SMF (not shown). At this time, the intensity ratio is
For example, in FIG. 2, when the polarization plane of the propagating light includes the line segment OP, the output light is determined to be only the ordinary ray component. and the polarization plane of the propagating light is the line segment O
When Q is included, the emitted light can be made up of only the extraordinary ray component. By doing so, only one of the light beams 101 and 102 will be emitted, and a decrease in coupling efficiency can be prevented.

FIG. 1(b) shows the ordinary ray component 101 of the polarizing prism 22.
This is for explaining the optical path of the reflected feedback light when it is coupled to an SMF (not shown).

Generally, the reflected feedback light is not linearly polarized light, but produces a polarized light component having a polarization plane perpendicular to the polarization plane of the forward emitted light. Of this random polarized light, the one that returns along the same optical path as the forward direction is the ordinary ray component of the polarizing prism 22, and this polarized light component returns to the Faraday rotator 20.
The light is rotated counterclockwise by 45 degrees in the opposite direction, and the light source 1 is converted into linearly polarized light with a polarization component perpendicular to the plane of the paper.
Return to 8. On the other hand, since the extraordinary ray component of the polarizing prism 22 has a polarization plane perpendicular to the polarization plane of the ordinary ray component, it is rotated by 45 degrees in the same direction by the Faraday rotator 20, and has a polarization plane parallel to the plane of the paper. As linearly polarized light with
The light is separated and emitted in a different direction from the light source 18.

In this way, all the polarization components of the reflected feedback light having the same polarization plane as the polarization plane of the emitted light from the light source 18 are separated in a direction different from that of the light source 18, so when the light source 18 is an LD, the influence is to be prevented.

The separation angle at this time increases or decreases according to the difference e between the refractive index n of the ordinary ray and the refractive index n' of the extraordinary ray in FIG. 2, so that the angle θ between the optical axis and the propagation direction is approximately 90 ' It is desirable to make the separation angle large. This is to prevent the separated light from returning to the light source 18 as background light.

Large-U Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

FIG. 3 is a block diagram of an LD module including an optical isolator constructed based on the basic configuration shown in FIG. 1. In this LD module, an LD 24 is used as a light source, and the emitted light is sent to a Faraday rotator 28 via a lens 26. l) 1) I try to shake it. Furthermore, the light emitted from the polarizing prism 30 is coupled to the SMF 34 via a lens 32. The Faraday rotator 28 is a normal one made of YIG single crystal, and its length and applied magnetic field are set so that the amount of optical rotation is 45°. The material of the polarizing prism 30 is rutile crystal, and its wedge angle α is set to 30''.The optical axis 30a of the polarizing prism 30 is approximately perpendicular to the propagation direction of light transmitted through the polarizing prism 30. , and its orientation is set so that the transmitted light becomes an ordinary ray of the polarizing prism 30.

Assume now that the incident angle φ of the incident light from the LD 24 to the Faraday rotator 28 is 40°. Faraday rotator 28
The reason why the light is not incident perpendicularly to is to prevent reflection on the surface of the Faraday rotator 28. At this time, the angle β between the light emitted from the polarizing prism 30 and the direction of incidence on the Faraday rotator 28 is 49.2°.

Under these conditions, the separation angle γ of the separated light 103 having the same polarization plane as the polarization plane of L024 among the reflected feedback lights is
is 10.6° as shown in FIG. 3(b), and a sufficient extinction ratio is obtained. Further, since the polarization plane of the feedback light 104 returning to the LD 24 is perpendicular to the polarization plane of the output light from the LD 24, this feedback light 104 has no effect on the LD 24.

In this way, the Faraday rotator 28 and the polarizing prism 3
Since the optical isolator was constructed from only the lens 26.0, its length could be 4 m+ or less.
32 can be brought closer to the confocal position, and a decrease in coupling efficiency can be prevented. Furthermore, since the number of components of the optical isolator is reduced from 3 to 2, the number of reflective surfaces is also reduced from 6 to 4, increasing coupling efficiency. LD associated with these effects
The improvement in the coupling efficiency of the module is 0.5-1°0 dB.

Effects of the Invention As detailed above, according to the present invention, it is possible to configure an optical isolator to be built into an LD module with a reduced number of parts, thereby achieving miniaturization of the device and increase in coupling efficiency. This effect is achieved. Moreover, since it is possible to reduce the use ω of optical crystal materials, which are generally expensive, there is also the effect that it becomes possible to provide optical isolators at low cost.

Further, as in the embodiment, by making the optical axis of the polarizing prism and the propagation direction of transmitted light approximately orthogonal, it is possible to further increase the coupling efficiency.

In addition to these, as in the embodiment, by making the transmitted light become either the ordinary ray or the extraordinary ray of the polarizing prism, there is also the effect that the coupling efficiency can be further increased. .

[Brief explanation of the drawing]

Fig. 1 is a basic configuration diagram of the optical isolator of the present invention (a: forward transmitted light, b: reverse reflected return light), and Fig. 2 is a diagram of the principle of operation of the present invention, and an explanation of the refractive index ellipsoid. Fig. 3 shows an embodiment of the present invention, and Fig. 4 shows a configuration diagram of an LD module (a: forward transmitted light, b: reverse reflected feedback light), and Fig. 4 shows an embodiment of the present invention. FIG. 2 is a configuration diagram of a light source module (a: forward transmitted light, b: backward reflected feedback light). 2.4, 22.30...Polarizing prism, 6.20.2
8... Faraday rotator, 10.24... LD,
16.34...SMF. 2゜z (b) Basic configuration diagram of the present invention Fig. 1

Claims (3)

[Claims] (1) Consists of a Faraday rotator (20) that rotates incident light by 45 degrees and outputs it, and a polarizing prism (22) made of a uniaxial crystal that transmits the light emitted from the Faraday rotator (20), and polarizes the light. The optical axis of the prism (22) is the polarizing prism (22).
) An optical isolator characterized in that the optical isolator is set in a direction different from the propagation direction of light passing through the interior. (2) Optical axis of polarizing prism (22) and polarizing prism (
22) The optical isolator according to claim 1, wherein the propagation direction of light passing through the optical isolator is approximately perpendicular to the propagation direction. (3) Claim 1 or 2, characterized in that the light that passes through the polarizing prism (22) is either an ordinary ray or an extraordinary ray of the polarizing prism (22). optical isolator.