JPH06214194A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain an optical isolator without the dispersion of polarization by composing plural polarizers of a set of two mediums with equal thickness and having birefringet property and arranging the polarizers so that an ordinary ray in a first polarizer and an extraordinary ray in a second polarizer are generated when a polarization light is made incident on the first and the second polarizers from the forward direction. CONSTITUTION:A plural number of polarizers 1, 3, 4, 6 are composed of a set of two mediums with equal thickness and having bi-refringent property. A light incident from an optical fiber 7 passes through a first polarizer 1, a first Faraday rotator 2, a second polarizer 3, a third polarizer 4, a second Faraday rotator 5 and a fourth polarizer 6 and is made incident on an optical fiber 8 in the order of forward travel. Respective polarizers 1, 3, 4, 6 are composed of a birefringent crystal such as rutile, their optical axis is inclined by 45 degrees to the traveling direction of light and the polarizers function so as to make an ordinary ray pass as it is and an extraordinary ray shift in parallel. Respective Faraday rotators 2, 5 also act so as to rotate the polarization direction by 45 degrees.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an optical isolator.

[0002]

2. Description of the Related Art In recent years, with the new development of optical fiber communication, various forms of optical fiber networks have been constructed.
Various optical components have been used in those networks.

Therefore, in order to maintain the quality of communication,
It has become more important than ever before to eliminate the reflected return light from the end faces of each optical component.

Light that has propagated through an ordinary optical fiber generally takes an arbitrary polarization state, and the polarization state changes due to ambient temperature changes, bending or twisting of the fiber, or the like.

Therefore, the optical isolator used in the optical fiber communication network is required to have no polarization dependence not only in the backward direction but also in the forward direction incident light.

FIG. 3 is a block diagram showing the configuration of an example of a conventional optical isolator. In the optical isolator of this example, the light incident from the optical fiber 14 has the first order in the forward order.
Polarizer 9, first Faraday rotator 10, second polarizer 11, second Faraday rotator 12, third polarizer 1
It is emitted to the optical fiber 15 through the optical fiber 3.

Each of the polarizers 9, 11, and 13 is made of a birefringent crystal such as rutile, and its optical axis is inclined by 45 degrees with respect to the traveling direction of light, allowing ordinary rays to pass therethrough and extraordinary rays. Function to shift in parallel.

Further, each Faraday rotator 10, 12 functions to rotate the polarization direction by 45 degrees.

FIG. 4 is a diagram showing changes in the polarization direction and the center point of light at points between the elements in the light traveling direction in FIG.

First, from the optical fiber 14 to the optical fiber 15
The light traveling to will be described with reference to FIGS. 3 and 4A.

Point Y ₀ shows the state of the light immediately after it emerges from the optical fiber 14, and polarizations F ₅ and F ₆ orthogonal to each other are shown.
The center points O ₅ and O ₆ of the two coincide with each other.

[0012] Next, since the abnormal light F ₆ by passing through the polarizer 9 is displaced, abnormal light as shown in Y ₁ point F ₆
The center point O ₆ of the normal light F ₅ moves to a position different from the center point O ₅ of the ordinary light F ₅ .

Further, when passing through the Faraday rotator 10, the polarized light F ₅ and the polarized light F ₆ are rotated 45 degrees counterclockwise as viewed from the side of the optical fiber 14 as indicated by the point Y ₂ , and then passed through the polarizer 11. Then, the center point O ₆ of the extraordinary light F ₆ moves as shown by the point Y ₃ .

Then, after passing through the Faraday rotator 12, the polarized light F ₅ and polarized light F ₆ are reflected by the optical fiber 1 as indicated by the point Y _4.
It rotates 45 degrees counterclockwise when viewed from the 4 side, and finally, when it passes through the polarizer 13, the center point O ₆ of the extraordinary ray F ₆ moves as shown by the point Y ₅ and moves to the center point O of the ordinary ray F _5. Match ₅

That is, the thickness ratio of the three polarizers is set to 1: root 2: 1 and the shift direction is set horizontally so that the sum of the vectors of the shifts of the extraordinary ray in the forward direction by the three polarizers becomes zero. When the direction is 0 degree, they are 0 degree, -135 degree, and 90 degree. Therefore, in this case, both polarized lights F ₅ and F ₆ are incident on the optical fiber 15 without loss.

Next, from the optical fiber 15 to the optical fiber 14
The light traveling to will be described with reference to FIGS. 1 and 4B.

Point Y ₅ shows the state of the light immediately after it emerges from the optical fiber 15, and the polarized lights F ₇ and F ₈ are orthogonal to each other.
The center points O ₇ and O ₈ of the two coincide with each other.

Next, since the extraordinary ray F ₈ is displaced by passing through the polarizer 13, the extraordinary ray F ₈ is displaced as indicated by a point Y _4.
Center point O _{8 8} moves to different positions with the center point O ₇ normal light F _7.

Further, when passing through the Faraday rotator 12, the polarized light F ₇ and the polarized light F ₈ are rotated 45 degrees counterclockwise as viewed from the side of the optical fiber 14 as indicated by the point Y ₃ , and then passed through the polarizer 11. Then, the center point O ₇ of the extraordinary light F ₇ moves as shown by the point Y ₂ .

[0020] Then, the polarization F ₇ passes through the Faraday rotator 10, polarization F ₈ is the optical fiber 1 as shown in _a point Y
When it rotates 45 degrees counterclockwise when viewed from the 4 side and finally passes through the polarizer 9, the center point O ₈ of the extraordinary light F ₈ moves as shown by the point Y ₀ .

Here, as can be seen by comparing the points Y ₀ in FIGS. 4B and 4A, the central points O ₇ of the polarizations F ₇ and F ₈ are
O ₈ is at a position different from the central points O ₅ and O _{6 of} the polarized light F ₅ and the polarized light F ₆ . Therefore, in this case, the polarized light F ₇ and the polarized light F ₈ do not enter the optical fiber 14.

An optical isolator having such a structure is described in detail in Japanese Patent Laid-Open No. 60-49297.

[0023]

In recent years, the development of optical fiber amplifiers and the like has led to the realization of an optical fiber communication system that is longer and more repeatable than before. In such a system, polarization dispersion of light is one of the factors that limit the transmission distance and transmission band.

In the above-mentioned conventional polarization-independent optical isolator, since the light passing through in the forward direction is an ordinary ray or an extraordinary ray in all polarizers, the relationship between the polarization and the crystal optical axis is the same. And an extraordinary ray cause a difference in optical path length, which causes polarization dispersion.

[0025]

An optical isolator according to the present invention is an optical isolator including an even number of polarizers, wherein the even number of polarizers are a set of two and have the same thickness and birefringence. It is characterized by being composed of a medium having properties.

Further, when polarized light is incident on the first and second polarizers forming a set of the two sheets from the forward direction, the first
The second polarizer is arranged to be an ordinary ray and the second polarizer to be an extraordinary ray.

The polarizer is made of a rutile crystal.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an optical isolator of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the optical isolator of the present invention.

As shown in FIG. 1, the optical isolator of the present embodiment has a first polarizer 1, a first Faraday rotator 2 and a second polarization in the order in which the light incident from the optical fiber 7 advances in the forward direction. The light is emitted to the optical fiber 8 through the child 3, the third polarizer 4, the second Faraday rotator 5, and the fourth polarizer 6.

Each of the polarizers 1, 3, 4, and 6 is made of a birefringent crystal such as rutile, and its optical axis is inclined by 45 degrees with respect to the traveling direction of light, and ordinary rays are transmitted as it is. It works by shifting extraordinary rays in parallel.

Further, the Faraday rotators 2 and 5 function to rotate the polarization direction by 45 degrees.

First, the light traveling from the optical fiber 7 to the optical fiber 8 will be described with reference to FIGS. 1 and 2A.

FIG. 2 is a diagram showing changes in the polarization direction and the center point of light at points between the respective elements in the light traveling direction in FIG.

Point Z ₀ shows the state of the light immediately after it emerges from the optical fiber 7, and the central points O ₁ and O ₂ of the polarized lights F ₁ and F _{2 which} are orthogonal to each other coincide with each other.

Next, since the abnormal light F ₂ passes through the polarizer 1 is displaced, abnormal light as shown in _one point Z F ₂
The central point O ₂ of the normal light F ₁ moves to a position different from the central point O ₁ of the ordinary light F ₁ .

Further, when passing through the Faraday rotator 2, the polarized light F ₁ and the polarized light F ₂ are reflected by the optical fiber 7 as shown by the point Z _2.
When viewed from the side, it rotates 45 degrees counterclockwise, and when it passes through the polarizer 3, the center point O ₁ of the extraordinary light F ₁ moves as shown by the point Z ₃ .

Next, when passing through the polarizer 4, the central point O ₄ of the extraordinary light F ₂ moves as shown by the point Z ₄ .

After passing through the Faraday rotator 5, the polarized light F ₁ and the polarized light F ₂ are rotated 45 degrees counterclockwise as viewed from the side of the optical fiber 7 as indicated by the point Z ₅ , and finally, the polarizer 6 is rotated.
After passing through, the center point O ₁ of the extraordinary light F ₁ moves as shown at the point Z ₆ and coincides with the center point O ₂ of the ordinary light F ₂ .

That is, in the polarizer 1 and the polarizer 3, the polarization F
₂ becomes extraordinary light, and the polarized light F becomes
_{Since 1} becomes extraordinary light, the ratio of the four polarizers 1, 2, 3, 4 is 1: route 2: route 2: 1, and the shift direction is 0 ° when the horizontal direction is 0 °, −45 °, -135 degrees, -1
The angle becomes 80 degrees, and the central points O ₁ and O ₂ of the polarized lights F ₁ and F ₂ coincide with each other.

Therefore, in this case, both polarized lights F ₁ and F ₂ are incident on the optical fiber 8 without loss. At this time, the polarized light F ₁ , F ₂
Have the same optical path length, the effect of making the polarization dispersion zero can be obtained.

Next, the light traveling from the optical fiber 8 to the optical fiber 7 will be described with reference to FIGS. 1 and 2B.

Point Z ₆ shows the state of the light immediately after it emerges from the optical fiber 8, and the central points O ₃ and O ₄ of the polarized lights F ₃ and F _{4 which} are orthogonal to each other coincide with each other.

Next, since the extraordinary light F ₃ passes through the polarizer 6 is displaced, abnormal light F ₃ as shown in Z ₅ points
The central point O ₃ of the normal light F ₄ moves to a position different from the central point O ₄ of the ordinary light F ₄ .

Further, when passing through the Faraday rotator 5, the polarized light F ₃ and the polarized light F ₄ are converted to the optical fiber 7 as indicated by the point Z _4.
When viewed from the side, it rotates 45 degrees counterclockwise, and when it passes through the polarizer 4, the center point O ₃ of the extraordinary light F ₃ moves as shown by the point Z ₃ .

Next, when passing through the polarizer 3, the central point O ₄ of the extraordinary light F ₄ moves as shown by the point Z ₂ .

After passing through the Faraday rotator 2, the polarized lights F ₃ and F ₄ rotate 45 degrees counterclockwise as viewed from the side of the optical fiber 7 as indicated by the point Z ₁ , and finally the polarizer 1 is rotated. When passing, the center point O ₄ of the extraordinary light F ₄ moves as shown at the point Z ₀ .

Here, as can be seen by comparing the Z ₀ points of FIGS. 2B and 2A, the center points O ₃ and O ₄ of the polarized lights F ₃ and F ₄ are the polarized lights F ₁ and Center point O _{1 of} F ₂ ,
It is in a different position than O ₂ . Therefore, in this case both polarizations F
₃ , F ₄ will not enter the optical fiber 7.

Further, when the thickness of the polarizer is the same as that of the conventional optical isolator, the shift amount of the light beam in the opposite direction is doubled by the route, and the isolation is improved.

The thickness of the polarizer 1 and the polarizer 6 is 0.500 mm.
Of rutile, the thickness of the polarizer 3 and the modifier 4 is 0.70
When an optical isolator with the structure shown in FIG. 1 was manufactured using 7 mm rutile, the amount of shift of the light beam in the opposite direction was about 10
The isolation is 0 μm and the isolation is 55 dB, and the isolation is improved by about 15 dB as compared with the conventional optical isolator having the configuration of FIG. 3 using the same thickness of rutile.

In this embodiment, an example using four polarizers has been described, but it is also possible to use 2n (n = 3, 4, ...) Polarizers.

[0051]

As described above, in the optical isolator of the present invention, even-numbered polarizers are made up of a pair of mediums having the same thickness and birefringence. When polarized light is incident on the child from the forward direction, the first polarizer becomes an ordinary ray and the second polarizer becomes an extraordinary ray, so that there is no polarization dispersion and the isolation is improved. Has the effect.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical isolator of the present invention.

FIG. 2 is a diagram showing how the polarization direction and the center point of light change at points between elements in the light traveling direction in FIG.

FIG. 3 is a block diagram showing a configuration of an example of a conventional optical isolator.

FIG. 4 is a diagram showing how the polarization direction and the center point of light change at points between the elements in the light traveling direction in FIG. 3;

[Explanation of symbols]

1, 3, 4, 6, 9, 11, 13 Polarizer ₂ , ₅ , ₁₀ , 12 Faraday rotator 7, 8, 14, 15 Optical fiber F ₁ , F ₂ , F ₃ , F ₄ Polarized O ₁ , O Center point of ₂ , O ₃ , and O ₄ polarization

Claims (3)

[Claims] 1. An optical isolator including an even number of polarizers, wherein the even number of polarizers is a set of two and is made of a medium having an equal thickness and birefringence. Optical isolator. 2. When polarized light is incident on the first and second polarizers forming a set of the two sheets from the forward direction, the first polarizer becomes an ordinary ray and the second polarizer is abnormal. The optical isolator according to claim 1, wherein the optical isolator is arranged so as to be a light beam. 3. The optical isolator according to claim 1, wherein the polarizer is made of a rutile crystal.