JP2000284225A - Optical isolator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical isolator which uses a polarizer requiring no polishing process and which is more inexpensive without increasing the insertion loss or the loss in the opposite direction although the dispersion of polarized waves is zero. SOLUTION: The incident light in the forward direction is divided according to the components of polarized light by transmission or reflection by a first reflection type polarizer 1. The reflected light is converted by a first reflection mirror 3 into light in the parallel direction to the propagation direction of the incident light in the forward direction to enter a 45 deg. Faraday rotator 5 and to enter a second polarizer 2. The polarized light component transmitting through the first polarizer 1 is transmitted through the 45 deg. Faraday rotator 5 and then reflected by a second reflection mirror 4 to enter the second polarizer 2. When the light is reflected by the second polarizer, the light is mixed with the light which is reflected by the first polarizer 1 and guided to the second polarizer and transmitted to transmit so that the optical path length of these optical paths is made equal to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for optical communication equipment, optical information processing equipment, etc., and transmits light in one direction only.
The present invention relates to an optical isolator that is an element that blocks light in the reverse direction.

[0002]

2. Description of the Related Art Conventionally, practically used polarization independent optical isolators include a combination of a parallel plate birefringent crystal and a 45 ° Faraday rotator, and a wedge type birefringent single crystal. There is a combination of a 45 ° Faraday rotator and the like. For example, a configuration in which four parallel plate birefringent crystals are combined with a 45 ° Faraday rotator is disclosed in
-185419 (hereinafter referred to as prior art 1).

FIG. 3 is a diagram showing a schematic configuration of an optical isolator according to prior art 1. Referring to FIG. 3, a first parallel plate birefringent crystal 51 and a fifth parallel plate birefringent crystal 5
5, the second parallel plate birefringent crystal 52 and the third parallel plate birefringent crystal 54 have the same thickness, and the first parallel plate birefringent crystal 51 and the second parallel plate birefringent crystal 52
The ratio of the separation distance between the extraordinary light and the ordinary light is 1: tan (2
2.5 ゜).

A polarization-independent isolator is constituted by these two sets of parallel plate birefringent crystals and one 45 ° Faraday rotator. This optical isolator has an equal optical path length for two orthogonally polarized lights, and has zero polarization dispersion in principle.

[0005]

However, these polarizers are expensive in material and require high precision in processing steps such as cutting and polishing, and it is difficult to reduce the manufacturing cost. As a result, a polarization independent optical isolator was expensive.

Therefore, a technical problem of the present invention is to use a polarizer that does not require polishing, have the same insertion loss and reverse loss as a conventional optical isolator, and have zero polarization dispersion. Regardless, it is an object of the present invention to provide an optical isolator at a lower price than before.

[0007]

According to the present invention, a first and a second reflective polarizer, a 45 ° Faraday rotator, a first and a second polarizer, which reflect one of two orthogonally polarized lights and transmit the other. A polarization independent optical isolator comprising a second reflection mirror, wherein the first reflection polarizer separates forward incident light by transmitting or reflecting the same in accordance with a polarization component. The reflected light separated by the first reflective polarizer is converted by the first reflecting mirror into a direction parallel to the traveling direction of the forward incident light, and after being incident on the 45 ° Faraday rotator, While being incident on the second polarizer, the polarized light component transmitted through the first polarizer is transmitted through the 45 ° Faraday rotator, and then reflected by the second reflection mirror, and Led to the second polarizer When the light is reflected by the second polarizer, the light is combined with the light guided by the second polarizer and transmitted after being reflected by the first polarizer, An optical isolator characterized in that the two paths have the same optical path length is obtained.

Further, according to the present invention, in the optical isolator, the first and second reflection-type polarizers and the first and second reflection mirrors are respectively formed on both surfaces of the same parallel plane substrate. Thus, an optical isolator characterized in that:

According to the present invention, there is provided an optical isolator according to any one of the optical isolators, wherein the first and second reflective polarizers are made of a photonic crystal.

According to the present invention, in any one of the optical isolators, of the first and second reflection mirrors, in the forward optical path, the reflection mirror is located closer to the front than the 45 ° Faraday rotator. An optical isolator characterized by replacing the reflection mirror with a third reflection type polarizer is obtained.

According to the present invention, there is provided the optical isolator, wherein the first and third reflective polarizers are formed on both sides of a parallel plane substrate.

Further, according to the present invention, in any one of the above optical isolators, the first, second, and third reflective polarizers are made of a photonic crystal. .

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the embodiments of the present invention, the principle of the present invention will be described.

First, among the reflective polarizers used in the optical isolator of the present invention, a polarizer made of a photonic crystal and having excellent optical characteristics will be described.

In recent years, the density of states of photons in an artificial periodic structure composed of a medium having a high refractive index and a medium having a low refractive index has been studied. In two linearly polarized light beams orthogonal to each other, each has an independent relationship between frequency and wave vector. The band gap, that is, the frequency band in which the density of states of photons becomes zero is also unique to each polarized light. In a certain frequency band, the state density for one polarized light may be zero and the state density for the other polarized light may not be zero. In this frequency band, it can function as a polarizer. That is, the periodic structure reflects one polarized light and transmits the other polarized light while preserving the wave vector.

In particular, among photonic crystal polarizers, a photonic crystal composed of a multilayer film of a high-refractive-index medium and a low-refractive-index medium deposited while preserving the concavo-convex shape formed on the substrate surface is an optical element. It has excellent characteristics as a polarizer for isolators.

The structure, characteristics, and fabrication method of a polarizer made of a photonic crystal are described in detail in Japanese Patent Application No. 10-2574.
No. 26 (inventor: Shojiro Kawakami, Yasuo Odera, Takayuki Kawashima, applicant: Shojiro Kawakami, title of invention "Polarizer and its manufacturing method").

The optical isolator of the present invention includes a reflection type polarizer made of a photonic crystal or the like as a component, and has a structure for utilizing the features of the polarizer.

Next, an embodiment of the present invention will be described. Here, the used polarizer can be used as a polarization splitting element to transmit one of two orthogonally polarized lights and reflect the other.

Further, two polarized lights transmitted through the non-reciprocal element can be combined. As a result, the operation of the polarization independent optical isolator is enabled.

Next, an embodiment of the present invention will be described.

FIG. 1 is a diagram showing a configuration of an isolator according to an embodiment of the present invention. FIG. 2 (a) and (b)
FIG. 2 is a diagram showing a transmission polarization direction and a reflection polarization direction in the first and second reflective polarizers 1 and 2 of FIG. 1.

In FIG. 1, a first reflection type polarizer 1 and a second reflection type polarizer 2 (hereinafter, simply referred to as first and second polarizers) are made of a photonic crystal. Acts as a separating element.

Referring to FIGS. 1 and 2A and 2B, the first polarizer 1 is set at an angle θ with respect to an incident light beam.

The second polarizer 2, the first total reflection mirror 3, and the second total reflection mirror 4 are set in parallel with the first polarizer 1. In the following description,
The first and second total reflection mirrors 3, 4 are simply referred to as the first
And a second mirror.

The 45 ° Faraday rotator 5 reflects the light transmitted through the first polarizer 1 and the light reflected by the first polarizer 1,
The light reflected by the mirror 3 is transmitted through different regions.

After being reflected by the first mirror 3, the second polarizer 2 transmits the light whose polarization plane has been rotated by the 45 ° Faraday rotator 5 and transmits the first polarizer 1. Then, the polarization plane is rotated by the 45 ° Faraday rotator 5, and the light whose traveling direction is changed by the mirror 4 is reflected and multiplexed on the same optical path 7.

Next, the change in the polarization direction of the forward light will be described.

As shown in FIG. 2A, the first polarizer 1
The direction of the transmitted polarized light 12 is rotated by 45 ° from the plane defined by the incident light beam and the perpendicular to the light transmitting surface of the first polarizer 1 in the direction of the left-handed screw.

On the other hand, the direction of the reflected polarized light 11 is rotated by 45 ° from the plane defined by the incident light beam and the perpendicular to the light transmitting surface of the first polarizer 1 in the direction of the right-hand screw.

Since the first polarizer 1 is set at an angle θ with respect to the incident light beam, the direction of the transmitted polarized light 12 and the reflected polarized light of the first polarizer 1 viewed from the direction of the incident light beam. The angle φ formed by the directions of 11 is 180-2 tan
^-1 (cos θ) is greater than 90 °. However, the loss caused by the difference is, when θ = 10 °,
It is about 0.001 dB and can be ignored.

Further, the light reflected by the first polarizer 1 is further reflected by the first mirror 3 and travels parallel to the optical path 6 of the incident light. Next, the polarization plane is rotated by the 45 ° Faraday rotator 5. Here, as shown in FIG. 2B, the direction of the transmitted polarized light 13 of the second polarizer 2 is a plane defined by the perpendicular direction of the light transmission surface of the second polarizer 2 and the incident light direction. , The light is transmitted.

On the other hand, the light transmitted through the first polarizer 1 is 45
後 After the polarization plane is rotated by the Faraday rotator 5, the light is reflected by the second mirror 4 and enters the second polarizer 2. Here, as shown in FIG. 2B, the polarization direction of the light coincides with the direction of the reflected polarized light 14 of the second polarizer 2, so that the light is reflected.

Incidentally, the two mirrors of the first and second mirrors 3 and 4 and the two polarizers of the first and second polarizers 1 and 2 are set in parallel with each other. The lights separated by one polarizer 1 are multiplexed by a second polarizer 2.

On the other hand, since these two optical path lengths are equal,
In principle, the polarization dispersion is zero.

Next, the change in the polarization direction of light in the opposite direction will be described.

The fact that the direction of rotation of the polarization plane by the 45 ° Faraday rotator depends only on the direction of magnetization, and that FIG.
According to the relationship between the directions of the transmission polarizations 12 and 13 and the directions of the reflection polarizations 11 and 14 of the first and second polarizers 1 and 2 shown in FIGS. Polarizer 1
The light reflected by and transmitted through the second polarizer 2 transmits through the first polarizer 1. As a result, no light is coupled into the optical path of the forward incident light. That is, the operation of the polarization-independent optical isolator is possible.

By replacing the first mirror 3 with a reflective polarizer made of a photonic crystal, which is the same as the first polarizer 1 and the second polarizer 2, and adjusting the direction of the reflected polarization. , The isolation can be improved. The reason is as follows.

When the temperature and the wavelength deviate from the center value, the rotation angle of the Faraday rotator 5 deviates from 45 °. At this time, light transmitted through the second polarizer 2 from the opposite direction is reflected without transmitting through the first polarizer 1, and a polarized light component that is coupled to the optical path 6 of the incident light is generated. This is because the reflection type polarizer provided by changing the polarization component to the first mirror 3 can be removed.

Further, in the embodiment of the present invention,
If the reflection type first polarizer 1 and the first mirror 3 made of a photonic crystal are formed on both sides of one parallel flat substrate, a mechanism for adjusting them in parallel becomes unnecessary.

The same applies to the reflection type second polarizer 2 and the second mirror 4 made of a photonic crystal. The reflection mirror 3 is replaced with a reflection type polarizer made of a photonic crystal. In this case, it is preferable to form a reflective polarizer made of a photonic crystal on both surfaces of the parallel plane substrate.

The reflection type polarizer using a photonic crystal has an insertion loss of 0.2 dB or less and an extinction ratio of 45 dB.
It has excellent polarization characteristics of dB or more. Further, even when the reflection type polarizer using the photonic crystal used in the above embodiment is replaced with a reflection type polarizer using a composite of a metal and a dielectric, the polarization independent optical isolator which operates in the same manner. Is obtained.

[0043]

As described above, in general, a reflection type polarizer using a photonic crystal or the like is generally a thin film type and can have a large area. By using it as an element, a low-cost polarization-independent optical isolator with few components can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical isolator according to an embodiment of the present invention.

FIGS. 2A and 2B are diagrams respectively showing the transmission polarization direction and the reflection polarization direction of the first and second polarizers of FIG. 1;

FIG. 3 is a diagram illustrating a configuration of a polarization-independent optical isolator according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st polarizer 2 2nd polarizer 3 1st mirror 4 2nd mirror 5 45 degree Faraday rotator 6 Optical path of the incident light of a forward direction 7 Optical path of the output light of a forward direction 10, 50 Optical isolator 51 First parallel plate birefringent crystal 52 Second parallel plate birefringent crystal 53 45 ° farate rotator 54 Third parallel plate birefringent crystal 55 Fourth parallel plate birefringent crystal

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Honma 6-7-1, Koriyama, Taishira-ku, Sendai, Miyagi Prefecture Tokinnai Co., Ltd. (72) Inventor Haruhiko Tsuchiya 6-7-1, Koriyama, Taishiro-ku, Sendai, Miyagi No. Tokin Co., Ltd. (72) Inventor Takashi Sato 25-6 Akiramaki Shinmeicho, Aoba-ku, Sendai, Miyagi Prefecture No. 202 Corpora Shinmei 202 (72) Inventor Shojiro Kawakami 236 Toi, Wakabayashi-ku, Sendai, Miyagi Atagobashi Mansion Pharaoh C-09 F term (reference) 2H099 AA01 BA02 CA01

Claims (6)

[Claims] 1. A polarization-independent light comprising a first and a second reflection type polarizer, a 45 ° Faraday rotator, and a first and a second reflection mirror for reflecting one of two orthogonally polarized lights and transmitting the other. An optical isolator, wherein the first reflective polarizer separates forward incident light by transmitting or reflecting the incident light according to a polarization component, and is separated by the first reflective polarizer. The reflected light is converted by the first reflecting mirror into a direction parallel to the traveling direction of the forward incident light, and after being incident on the 45 ° Faraday rotator, is incident on the second polarizer. The polarized light component transmitted through the first polarizer is transmitted through the 45 ° Faraday rotator, reflected by the second reflection mirror, guided to the second polarizer, and then transmitted to the second polarizer. With two polarizers, When reflecting this light, the light is combined with light guided by the second polarizer and transmitted after being reflected by the first polarizer,
An optical isolator characterized in that the optical path lengths of these two paths are equal. 2. The optical isolator according to claim 1, wherein said first and second reflection-type polarizers and said first and second reflection mirrors are formed on both surfaces of the same parallel plane substrate. An optical isolator characterized in that: 3. The optical isolator according to claim 1, wherein said first and second reflective polarizers are made of a photonic crystal. 4. The optical isolator according to claim 1, wherein, of the first and second reflection mirrors, in a forward optical path, the 45 ° Faraday rotator is more than the Faraday rotator.
An optical isolator characterized in that a reflection mirror located in front is replaced by a third reflection type polarizer. 5. The optical isolator according to claim 4, wherein said first and third reflective polarizers are formed on both sides of a parallel plane substrate. 6. The optical isolator according to claim 4, wherein said first, second, and third reflective polarizers are made of a photonic crystal.