JP2003315737A - Optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive optical isolator which has simpler constitution than before by using reflection type polarization. <P>SOLUTION: The polarization dependent type optical isolator can be realized by combining a reflection type polarizer which has different polarization dependency for each place, a Faraday rotating element with a rotational angle of 22.5°, and a total reflecting mirror. In respective different-characteristic parts of the reflection type polarizer, transmission polarization direction is deviated by 45°. One polarizer is needed although two polarizers are necessary before and the Faraday rotating element may be a half as thick as before. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used for optical communication equipment, optical information processing equipment and the like and transmitting light of a specific polarization only in a specific direction.

[0002]

2. Description of the Related Art Generally, a polarization-dependent optical isolator is constructed by combining two polarizers and a Faraday rotator whose polarization direction is rotated by applying a magnetic field.

In practically used optical isolators, a birefringent optical crystal, a glass polarizer containing metal particles, a composite multilayer film made of a dielectric and a metal is used as a material of a polarizer. These all function as an optical isolator by transmitting a specific polarized light, and two Faraday rotators are required to be sandwiched between them.

Further, in principle, the rotation angle of the Faraday rotator is required to be 45 degrees. Conventionally, a thick film of garnet crystal having a magneto-optical effect is used, but a thickness of 500 μm
It requires a certain degree of crystal growth, requires a long manufacturing time, and is expensive, and even costs about half of the total cost of the parts used for the optical isolator. If an optical isolator having a configuration in which the necessary rotation angle of the Faraday rotator is operated at an angle smaller than 45 degrees can be realized, the Faraday rotator can be made thinner, and the cost of the entire optical isolator can be significantly reduced. It becomes possible and an inexpensive optical isolator is realized.

[0005]

SUMMARY OF THE INVENTION Therefore, the present invention uses a reflective polarizer that reflects a specific polarized light.
The purpose of the present invention is to provide an optical isolator that operates by providing only one polarizer on one side of the Faraday rotator, and that the thickness of the Faraday rotator is half that of conventional optical isolators.

[0006]

The basic structure of an optical isolator according to the present invention is shown in FIG. It is composed of one reflection polarizer shown by reference numeral 4, a Faraday rotation element shown by reference numeral 2 and a total reflection mirror shown by reference numeral 3. The polarizer used transmits specific polarized light,
Polarized light in a direction perpendicular to it is a reflective polarizer that reflects light. As shown in FIG. 2, the polarizer portion is divided into two regions, and the polarization directions of the regions 5 and 6 are different from each other by 45 degrees.
The thickness of the Faraday rotator has a rotation angle of 22.
It is adjusted to 5 degrees, and the light travels back and forth through it to realize rotation of the polarization direction of 45 degrees.

With this structure, a specific polarized light incident from the polarizer side is transmitted, and an arbitrary polarized light incident from the Faraday rotator element side is blocked to function as an isolator.

As the reflection type polarizer, one using a polymer multilayer film and one using a photonic crystal (Patent Publication No. 2000-56133) can be considered. When a polymer multilayer film is used, it is necessary to stick two polarizers in a mutually shifted form in order to realize different polarization characteristics depending on the location. On the other hand, in the case of a polarizer using a photonic crystal, it is possible to fabricate a polarizer having polarization characteristics in an arbitrary direction on a single substrate at an arbitrary location by patterning the substrate. Such a polarizer can be easily realized on the substrate.

[0009]

Example 1 A photonic crystal polarizer will be described in detail. A photonic crystal that has a two-dimensional or three-dimensional periodicity of the refractive index and whose period is in the wavelength order is called a photonic crystal. Its optical characteristics are the refractive index of the material used, the periodicity of the structure, and the periodic array and its direction. Dependent. To give an example of the optical characteristics to be realized, multiple reflections of light in each cycle cause Bragg cutoff, and a phenomenon occurs in which light is cut off in a specific wavelength band. Further, such optical characteristics can be provided with structural anisotropy.

A major feature of the photonic crystal is that it is an artificial structure, so that the optical characteristics can be designed by designing the structure. Therefore, it becomes possible to realize a specific optical characteristic at a specific place.

Consider, for example, a structure in which two two-dimensional periodic structures as shown in FIG. 3 are combined. In such an artificial periodic structure consisting of a high refractive index medium and a low refractive index medium,
The two polarization components orthogonal to each other have independent dispersion relations (relationship between frequency and wave vector). In FIG. 3, the band gap, that is, the wavelength range in which light is blocked, differs between the polarization component parallel to the column and the polarization component perpendicular to the column. That is, in a certain wavelength range, one polarization mode may be blocked and the other polarization mode may propagate. That is, in this wavelength range, this periodic structure can operate as a polarizer that reflects or diffracts one polarized light and transmits the other polarized light. In addition, a theoretically high extinction ratio can be obtained (Tetsuko Hamano, Masayuki Izutsu,
Hideki Hirayama, "Possibility of Polarizers Using Two-Dimensional Photonic Crystals", Proceedings of the 58th Annual Response Period Cycle, paper 2a
-W-7, 1997, Akira Sato, Masahiro Takebe, "Optically anisotropic multilayer film by structural birefringence", Optics Japan
n'97, Proceedings, paper 30pDO1, 1
997). In reference numeral 8, the reference numeral 7 is obtained by producing a structure obtained by rotating the structure of reference numeral 7 by 45 degrees in the xy plane.
It is possible to realize a characteristic that polarized light in a direction shifted by 5 degrees is reflected and polarized light in a direction perpendicular to it is transmitted. Code 7 and code 8
Has a structure in which the optical axis is rotated by 45 degrees.

Depending on the required function, the transmission and reflection characteristics can be changed by design, and there may be a plurality of regions with different angles.

As a method for realizing a photonic crystal, for example, a self-cloning method can be mentioned (Patent publication number:
JP-A-10-335758). This reflects the pattern of the substrate by depositing an alternate multilayer film on the substrate on which the concavo-convex pattern is formed by using a film formation method that uses diffused incidence of deposited particles represented by bias sputtering and sputter etching. This is a method of stacking while retaining the uneven shape. This mechanism has the following three effects, (1) the effect of slowing the deposition rate of the recessed portion that becomes a shadow due to the diffused incidence of deposited particles, and (2) the tilt angle of about 50 due to sputter etching
The effect of maximizing the etching rate on the surface from 60 degrees to 60 degrees, and the effect of re-adhering the particles scraped by sputter etching on the (3) surface to another place on the substrate, at an appropriate ratio. Explain (Shojiro Kawakami, Takashi Sato, Takayuki Kawashima, “3D made by bias sputtering method”
Formation Mechanism of Periodic Nanostructure ", IEICE Journal C-
1, vol. J81-C-1, no. 2, pp. 108
-109, February 1998).

In the self-cloning method, the concavo-convex pattern on the substrate is formed by lithography and etching, so it is possible to form an arbitrary pattern depending on the location, and the photonic crystal formed thereon also has the pattern. Reflecting this, a photonic crystal that varies from place to place is realized.

In the two-dimensional periodic structure produced by the self-cloning method, a polarizer having high performance has been realized (Patent Publication No. 2000-5613).
3).

By preparing a substrate as shown in FIG. 4 and depositing a multi-layered film on it by a self-cloning method, a structure as shown in FIG. 5 can be realized. In such a structure, it is possible to give different polarization dependences between the portions 12 and 13 as in FIG.

A method for producing a photonic crystal polarizer using the self-cloning method used in the present invention will be described in detail with reference to the part indicated by reference numeral 12 in FIG. In FIG. 5, reference numeral 14 is a layer of amorphous SiO ₂ (SiO ₂ layer),
Reference numeral 15 is an amorphous Si layer (Si layer). x
The period Lx in the axial direction is 0.5 μm, and the period Lz in the z-axis direction is 0.57 μm. The SiO ₂ layer 14 and the Si layer 15 have a shape which is periodically bent along the x-axis direction while slightly changing the thickness. Reference numeral 13 is reference numeral 1
2 has a structure in which the z axis is rotated by 45 degrees about the z axis.

First, a periodic resist pattern is formed on a substrate by an electron beam lithography technique. The groove has a width of 0.25 μm, a depth of 0.2 μm, and a lateral period of 0.5 μm. The schematic diagram is shown in FIG. Reference numeral 9 is a substrate, reference numeral 10 is an antireflection coating layer, and reference numeral 11 is a periodic groove portion. Generally, depending on the selection of the size of the periodic structure, the reference numerals 10 and 11 are selected from a material different from that of the substrate, but it is also possible to form the groove on the same material as that of the substrate. This time, a SiO ₂ layer and a Si layer were alternately laminated on the quartz substrate by a bias sputtering method using targets of SiO ₂ and Si. At that time, it is important to perform film formation while preserving the shape of the periodic unevenness of each layer in the x-axis direction. The conditions were as follows: Ar gas pressure 2 Pa, target RF power 800 W, substrate RF power 20 W; S for the deposition of SiO _2.
For the film formation of i, the Ar gas pressure was 0.15 Pa and the target high frequency power was 400 W. 1 layer of SiO ₂ and Si
0 layers were laminated. The laminated thickness is about 6 μm.

In addition, in order to prevent reflection caused between the periodic grooves on the substrate and the multilayer film and between the multilayer film and the air due to a difference in refractive index between the multilayer film, films each having a thickness of 10 layers above and below the multilayer film are adjusted. By inserting, the multi-layer film is matched with the substrate or air, and reflection is reduced. This time, air is used as the top of the multilayer film, but another substance can be used.

FIG. 6 shows the results of measuring the transmittance for each polarized wave when light is vertically incident on the portion 12 of the manufactured structure while changing the wavelength. Here, the polarized wave parallel to the groove is referred to as TE wave, and the polarized wave perpendicular to the groove is referred to as TM wave. Code 1
The TM polarized wave is transmitted and the TE polarized wave is blocked near the wavelength of 1.5 μm shown by 7. The blocked TE polarized light is reflected as reflected light. In addition, as a result of introducing the non-reflective layer into the stacking start part and the end part, the transmittance of TM polarization shows a high value in the vicinity of the wavelength of 1.5 μm, and between the multilayer film and the substrate and between the interfaces of the multilayer film and the air. Due to the effect of multiple reflections that occur, flat characteristics are obtained without changing the transmittance with changes in wavelength.

[0021]

Second Embodiment The operation of the optical isolator according to the present invention will be described with reference to the optical path diagram. Here, the direction from the polarizer to the Faraday rotator is the forward direction and the opposite direction is the reverse direction, and optical path diagrams in each case are shown in FIGS. The polarizer is formed on a quartz substrate having a refractive index of 1.5, and the total reflection mirror is formed on the surface of the Faraday rotation element by vapor deposition or the like.

First, the operation in the forward direction will be described with reference to FIG. Assuming that the magnetic charges of the Faraday element are aligned by an appropriate external or internal magnetic field, when polarized light in the x direction enters from the reference numeral 23, the polarizer of the reference numeral 21 transmits. Then, in the Faraday rotator, as viewed in the traveling direction, it rotates 22.5 degrees clockwise. Bi-based garnet thick film is used for the Faraday rotation element, and the thickness is 225μ.
m, the refractive index is 2.5. The light rotated by 22.5 degrees is reflected by the total reflection mirror installed on the back surface of the Faraday rotator, and travels inside the Faraday rotator again.
At this time, due to the non-reciprocity of the Faraday rotator, the rotation of the polarization direction rotates counterclockwise with respect to the traveling direction. That is, in FIG. 7, the direction of rotation is the same when going right through the garnet and when going left. Therefore, the light emitted from the Faraday rotator has its polarization direction rotated by 45 degrees as compared with the time of incidence. Here, by obliquely entering the light into the Faraday rotator, it is easy to shift the light from the position where it enters the Faraday rotator and let it exit the polarizer of the portion 22. For example, when the light enters the Faraday rotator from the polarizer side at an angle θ shown in the figure of 11 °, the light travels in the garnet at an angle of 7 ° due to refraction. In that case, it is displaced by 55 μm. The polarizer used operates with sufficient characteristics even at an incident angle of 11 degrees.

In the portion 22, the polarization direction of the incident light is deviated by 90 degrees from the polarization direction passing through this polarizer, so that it is reflected, enters the Faraday element again, and is emitted from the back surface of the Faraday element. The rotation of the polarization here does not make sense.

It is important in terms of preventing the generation of extra stray light that the deviation of the positions of incidence on the portions 21 and 22 is sufficiently large. Therefore, it is possible to obtain the distance by placing a polarizer on the right side of the substrate as shown in FIG. 9. For example, as shown in FIG. 10, one transparent plate is provided between the Faraday rotator and the total reflection mirror. It is possible to get a sufficient distance by sandwiching. For example, by using a transparent plate having a thickness of 1 mm and a refractive index of 1.5, it is possible to provide a distance of 500 μm between the portion that enters the polarizer and the position that is returned by the total reflection mirror.

In this case, the size of the Faraday rotator is further adjusted as shown in FIG. 11, and the Faraday rotator is adhered only to a necessary portion, so that the number of times of traveling in the Faraday rotator is reduced, and the Faraday rotator has Since the absorption loss can be reduced, it is possible to reduce the insertion loss.

Further, by adjusting the size of the Faraday rotator, it is possible to extract the light reflected at the portion 22 without passing through the Faraday rotator.

In the forward direction, the optical isolator operates only in a specific polarization direction. However, for example, in an optical isolator installed at the emitting end of the laser for stabilizing the laser, only the specific polarization is emitted from the laser. It only has to work in the direction. Since light of any polarization state is incident in the opposite direction, it is necessary to block light of all polarization states.

Next, the light traveling from the opposite direction will be described with reference to FIG. The light incident from the reference numeral 30 first has its polarization direction rotated in the Faraday rotator.
The rotation here does not make sense because in the opposite direction any polarization has to be considered. When incident on the polarizer 29, the light is transmitted or reflected depending on the polarization direction at that time. The polarized light in the direction rotated 45 degrees counterclockwise from the y-axis as viewed in the -z direction is transmitted and the light is emitted as it is. Since this position is displaced from the incident position in the forward direction, the light does not couple to the reference numeral 31. On the other hand, the polarization direction of the reflected light is a direction rotated by 45 degrees clockwise from the y-axis as viewed in the -z direction. The reflected light passes through the Faraday rotator, is reflected by the 22.5 ° rotator and the total reflection mirror, and returns by 22.5 ° by returning inside the Faraday rotator. The rotation direction is the same as the case where light travels in the forward direction due to the non-reciprocity of the Faraday rotator. As a result, polarized light in the y direction, which is deviated by 90 degrees from the direction of transmitting the polarizer of the code 28, enters the code 28. There, the light is reflected again, passes through the Faraday rotation element, and is emitted to the outside. Reflection exists when the light is emitted from the Faraday rotator, and the reflected light is 31
It is not connected to the reference numeral 31 to return to the dislocated position.

The polarized light transmitted through the portion of the reference numeral 29 is reflected on the back surface of the substrate of the polarizer, which is reflected again on the polarizer,
There is a possibility of returning to the code 31, but it is possible to shift the position by adjusting the thickness of the polarizer substrate, or by using a wedge-shaped substrate instead of a flat plate for the back surface. It is possible to let the reflected light at 11 escape in a direction completely different from the reference numeral 31.

In the self-cloning method, various substances can be used for the substrate, and for example, a Faraday rotation element can be used for the substrate of the polarizer. In this case, an optical isolator can be realized with the configuration shown in FIG. 12, and its thickness is substantially only the thickness of the Faraday rotator. The incident position on the part of reference numeral 51 and the reference numeral 52
If the distance from the incident position on the
By inserting a transparent plate between the total reflection mirror and the Faraday rotator, it is possible to increase the extinction ratio by increasing the distance.

It is also possible to further reduce the thickness of the Faraday rotator by reflecting the light inside the Faraday rotator multiple times. (N + 1) regions where adjacent regions of the polarizer are different from each other by 45 / n degrees (n is an integer) are made, and n is generated by multiple reflection in the Faraday rotator.
By reciprocating once, a rotation angle of 45 degrees can be obtained. In this case, the thickness of the Faraday rotation element can be set to 2 / n as shown in FIG.

[0032]

An optical isolator can be realized by combining reflection type polarizers having different polarization directions to be transmitted and arranging it on only one side of a Faraday rotator having a thickness half that conventionally required. Furthermore, by using a polarizer using a photonic crystal as the reflective polarizer, it is easy to fabricate a polarizer having different directivity on a single substrate at one time. This photonic crystal polarizer does not require processing such as polishing, is excellent in mass productivity, and has performance comparable to existing high-performance polarizers. In addition, half the cost is required for the polarizer, which previously required two sheets. Therefore, it is possible to realize a device having characteristics similar to those of the existing optical isolator at a cost of 1/2 or less.

[Brief description of drawings]

FIG. 1 is a basic structure of an optical isolator according to a first embodiment of the present invention.

FIG. 2 shows a polarizer having different polarization dependence depending on the location used in the present invention.

FIG. 3 is a conceptual diagram of a photonic crystal having a polarization dependence on location.

FIG. 4 is a substrate necessary for realizing the photonic crystal corresponding to FIG. 2 by the self-cloning method.

FIG. 5 is a diagram showing a structure in which the photonic crystal corresponding to FIG. 2 is realized by a self-cloning method.

FIG. 6 is a diagram showing a relationship between wavelength and transmittance of a photonic crystal polarizer used in the present invention.

FIG. 7: Optical path diagram in the forward direction

FIG. 8: Optical path diagram in the opposite direction

9 is a diagram showing a structure in which a reflective polarizer is installed on the side opposite to the Faraday rotator in the basic structure shown in FIG.

FIG. 10 is a diagram showing a structure in which a transparent plate is sandwiched between the Faraday rotator and a polarizer in the basic structure shown in FIG.

11 is a diagram showing a structure in which the size of the Faraday rotator is adjusted in the structure shown in FIG. 10 to reduce the number of times light travels in the Faraday rotator.

FIG. 12 is a diagram showing a structure in which a photonic crystal polarizer is directly formed on the surface of a Faraday rotator in the basic structure shown in FIG.

[Explanation of symbols]

1 Polarizer Substrate 2 Faraday Rotating Element 3 Total Reflection Mirror 4 Polarizer with Different Polarization Dependence Depending on Location 5 Polarized light in the direction rotated 45 degrees clockwise from the y axis when viewed in the z direction is transmitted, and polarized light perpendicular to it A two-dimensional photonic crystal 8 having a uniform structure in the x direction. A two-dimensional photonic crystal having a uniform structure in the x direction. Two-dimensional photonic crystal 9 having a similar structure 9 Substrate 10 Anti-reflection coating layer 11 Periodic grooves 12 Two-dimensional photonic crystal having a uniform structure in the y direction produced by the self-cloning method 13 Produced by the self-cloning method acts as a polarizer that transmits the two-dimensional photonic crystal 14 SiO ₂ layer 15 Si layer 16 substrate molded layer 17 TM wave having a uniform structure in the direction rotated 45 degrees from been y-direction One of the wavelength bands 18 Substrate 19 Faraday rotator 20 Total reflection mirror 21 Polarizer 22 that transmits polarized light in the x direction and reflects polarized light in the y direction 22 The characteristic of the polarizer 21 is 45 degrees with the z axis as the rotation axis. Polarizer 23 having rotated characteristics 23 Incident position in forward direction 24 Ejection position in forward direction 25 Substrate 26 Faraday rotator 27 Total reflection mirror 28 Polarizer 29 that transmits polarized light in the x direction and reflects polarized light in the y direction 29 Polarizer 30 has a characteristic that the characteristic of the polarizer is rotated by 45 degrees about the z-axis as a rotation axis 30 Ejection position in the forward direction 31 Incident position in the forward direction 32 Substrate 33 Faraday rotator 34 Total reflection mirror 35 Transmits polarized light in the x direction Polarizer 36 that reflects polarized light in the y-direction 36 Polarizer 37 having the property that the property of the polarizer 35 is rotated by 45 degrees about the z-axis as the rotation axis 37 Substrate 38 Transparent plate 39 Faraday rotator 40 Total reflection mirror 41 Polarizer 42 that transmits polarized light in the x-direction and reflects polarized light in the y-direction Polarized light having the characteristic of a polarizer rotated by 45 degrees about the z-axis as the rotation axis Child 43 Substrate 44 Transparent plate 45 Faraday rotator 46 Total reflection mirror 47 Polarizer 48 that transmits polarized light in the x direction and reflects polarized light in the y direction The characteristic of the polarizer 41 is rotated by 45 degrees about the z axis as the rotation axis. Polarizer with characteristics 49 Faraday rotator 50 Total reflection mirror 51 Polarizer 52 that transmits polarized light in the x direction and reflects polarized light in the y direction The characteristic of the polarizer denoted by reference numeral 45 is that rotated by 45 degrees about the z axis as the rotation axis. With a polarizer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Osamu Ishikawa             2-21-21 Shiomidainan, Shichigahama-cho, Miyagi-gun, Miyagi Prefecture             No. 8 (72) Inventor Nao Sato             1-2-5 Tomizawaminami, Taihaku-ku, Sendai City, Miyagi Prefecture               Bonard Tomizawa No. 302 F-term (reference) 2H049 BA02 BA08 BA46 BB03 BC06                       BC25                 2H099 AA01 BA02 CA01 CA17 DA05

Claims (6)

[Claims] 1. At least one reflective polarizing element having a plurality of regions having polarization characteristics different from each other by 45 degrees and a rotation angle of 2.
Polarization dependent optical isolator consisting of at least one 2.5 degree Faraday element and one total reflection mirror 2. A polarization-dependent optical isolator characterized by using the reflection-type polarization element according to claim 1 formed on a single substrate by using a photonic crystal. 3. A polarization-dependent optical isolator, wherein the total reflection mirror according to claim 1 or 2 is a reflection type polarizer. 4. A polarization-dependent optical isolator, characterized in that a photonic crystal is used as a reflection-type polarizer used in place of the total reflection mirror according to claim 3. 5. A polarization-dependent optical isolator characterized in that a reflective polarization element using the photonic crystal according to claim 2 is directly formed on a Faraday element. 6. The polarization-dependent optical isolator according to claim 1, wherein at least one transparent plate is inserted.