JP3038942B2 - Birefringent diffraction grating polarizer and optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent polarizing plate for use in various optical devices utilizing a semiconductor laser, and more particularly to a grating-type polarizing plate and an optical isolator having different diffraction efficiencies depending on the polarization direction.

[0002]

2. Description of the Related Art Polarizing elements, especially polarizing beam splitters,
An element that obtains a specific polarization by making the propagation direction of light different between orthogonal polarizations. Such an element
It is used as a component of an optical isolator and an optical circulator in a light source module for optical fiber communication, an optical head for an optical disk, and the like.

Conventionally, a polarizing beam splitter, such as a Glan-Thompson prism or a lotion prism, which separates an optical path by utilizing the difference in transmission or total reflection by polarized light on a light reflecting surface of a crystal having a large birefringence, or glass, etc. A dielectric multilayer film is provided on the reflecting surface of a total reflection prism made of an isotropic optical medium, and a material that totally reflects or transmits light by utilizing the difference in refractive index due to polarization of the dielectric multilayer film is often used. Have been. However, these devices have disadvantages such as large size, low productivity, and high price.

On the other hand, a birefringent diffraction grating type polarizing plate described in Japanese Patent Application Laid-Open No. 63-314502 is known as a polarizing element characterized by small size and high productivity in recent years. FIG. 4 is a perspective view showing the configuration of the birefringent diffraction grating polarizer described above, and FIGS. 5 to 7 are cross-sectional views. The birefringent diffraction grating polarizer is provided with an optical diffraction grating of a periodic ion exchange region 2 on the main surface of a lithium niobate substrate 1 and has an ordinary ray between an ion-exchanged region and a non-ion-exchanged region. This is provided with means for canceling the phase change received by the light source, and separates the optical path by utilizing the difference in diffraction efficiency due to polarization. As means for canceling the phase change received by the ordinary ray, the surface of the region not subjected to ion exchange is cut to a desired depth as shown in the sectional view of FIG. 5, and the sectional view of FIG. The dielectric film 3 is formed on the ion-exchanged region 2 as described above, or is thick on the ion-exchanged region 2 and thin on the non-ion-exchanged region as shown in the sectional view of FIG. For example, a dielectric film 2 is formed. For example, when proton ion exchange is periodically performed on the main surface of lithium niobate, the refractive index for an extraordinary ray having a wavelength of 1.3 μm increases by about 0.1 in the area where the proton ion exchange is performed, and the refractive index for ordinary rays. Is reduced by about 0.04. Therefore, the thickness of the dielectric film in the region where the proton ion exchange is performed is made thicker than the dielectric film thickness in the region where the proton ion exchange is not performed, and the refractive index of the region where the proton ion exchange is performed with respect to ordinary light is reduced. By canceling out, both the first-order and higher-order diffraction efficiencies of the ordinary rays and the zero-order diffraction efficiencies of the extraordinary rays can be made zero, and the polarizer becomes a polarizer.

[0005]

Normally, in an element such as an optical isolator, which is required to sufficiently reduce reflected return light from the element itself, the angle between the optical axis of incident light and the element surface is slightly shifted from 90 degrees. , So that the reflected return light does not pass through the same path as the incident light. However, in the birefringent diffraction grating polarizer having the structure described above, the extinction ratio deteriorates when the angle between the optical axis of the crystal and the optical axis of the incident light deviates from 90 degrees. In the case of an optical isolator stuck with a Faraday rotator sandwiched between two polarizing plates whose optical axes are inclined at 45 degrees to each other, the optical axes of both polarizing plates remain at 90 degrees with respect to the optical axis of the incident light. The angle between the surface of the polarizing plate and the optical axis of the incident light cannot be shifted from 90 degrees. Therefore, when the optical isolator is tilted from the optical axis of the incident light, the extinction ratio of at least one of the polarizing plates deteriorates,
There is a problem that the isolation ratio of the optical isolator deteriorates. Also, when the polarizer is used alone, the polarizer can only be tilted about the optical axis as the center of rotation, so the return light can only escape to a plane perpendicular to the optical axis, and the degree of freedom of the optical system can be reduced. Is also small.

It is an object of the present invention to provide a polarizing element having a high extinction ratio and a low insertion loss and little influence of reflected return light, and an optical isolator having a high isolation ratio and little influence of reflected return light.

[0007]

SUMMARY OF THE INVENTION The present invention provides an optical diffraction grating having a periodic ion-exchange region on a principal surface of a crystal plate having optical anisotropy and a specific grating passing through the ion-exchanged region. In a birefringent diffraction grating polarizer formed with means for compensating for a phase shift with respect to the light of the polarized light component, the normal of the main surface of the crystal plate is a lattice base with respect to the x-axis or y-axis of the crystal.
Angle that is about half of the first-order diffraction angle when the vector is perpendicular to the optical axis
This is a birefringent diffraction grating polarizer characterized by being inclined at an angle. The present invention relates to an optical isolator having a structure in which two birefringent diffraction grating polarizers whose optical axes are inclined at 45 degrees to each other with a Faraday rotator interposed therebetween, wherein one polarizer has a normal to a principal surface and a crystal. rating of a parallel x-axis or y-axis, and the other one polarizer and an axis normal to the principal plane is rotated 45 degrees from the z axis in the xz plane of a crystal from the y-axis as a rotation axis
Approximately half the first-order diffraction angle when the child vector is perpendicular to the optical axis
Optical isolator characterized by being inclined at an angle of
Alternatively, one polarizer has a normal to the principal surface and the x-axis or y of the crystal.
The axes are parallel, and the other polarizer has a principal plane normal when the lattice vector is perpendicular to the optical axis from the x axis with the axis rotated 45 degrees from the z axis in the yz plane of the crystal as the rotation axis . 1st round
An optical isolator characterized by being inclined at about half the angle of the bend.

[0008]

As described above, the problem with the conventional birefringent diffraction grating polarizer is that the main surface of the polarizer is parallel to the optical axis of the crystal. In that the extinction ratio of the polarizer is degraded. Therefore, in the polarizer of the present invention, an angle is previously provided between the main surface of the polarizer and the optical axis of the crystal such that the optical axis of the crystal and the optical axis of the incident light are orthogonal to each other when the polarizer is arranged at an angle. is there. By doing so, even if the polarizer is tilted, the optical axis of the crystal can be made orthogonal to the optical axis of the incident light, the return light reflected on the polarizer surface can escape in any direction, and the extinction ratio of the polarizer can be reduced. Deterioration can be prevented.

When the grating vector of the diffraction grating is inclined by θ from the plane perpendicular to the optical axis, the angle between the incident light and the reflected light on the surface of the diffraction grating is 2θ, and the angle of the first-order diffraction angle is smaller. The angle φ is φ〜1 / (1 / φ ₀ + θ). Here, φ ₀ is the first-order diffraction angle when the lattice vector is perpendicular to the optical axis. Therefore, if θ is too large, φ becomes small, and the degree of separation between the 0th-order diffracted light and the 1st-order diffracted light, that is, the degree of polarization separation deteriorates. Normally, it is required that the theta and phi to increase in the same way, by placing the theta = phi, the θ~φ _0/2. Therefore, the angle between the principal plane of the polarizer and the optical axis of the crystal is optimally half the primary diffraction angle of the light perpendicularly incident on the diffraction grating, and the upper limit is the same as the primary diffraction angle. is there.

In an optical isolator having a structure in which a Faraday rotator is sandwiched between two birefringent diffraction grating polarizers whose optical axes are inclined at 45 degrees with respect to each other, one polarizer has a normal to the principal surface and The x or y axis of the crystal is parallel,
The other polarizer is such that the normal to the principal surface is inclined at an angle from the y-axis or x-axis, respectively, with the axis rotated 45 degrees from the z-axis in the xz plane or the yz plane of the crystal as the rotation axis. The normal axis of the principal surface is used as a rotation axis with the optical axis of the polarizer parallel to the x-axis or y-axis of the crystal, and the optical axis of the crystal of the other polarizer is orthogonal to the optical axis of the incident light. By tilting the entire optical isolator up to the desired angle, the optical axes of the crystals of both polarizers can be made orthogonal to the optical axis of the incident light. Therefore, the return light reflected on the surface of the optical isolator can escape from the optical axis of the incident light, and a high isolation ratio can be obtained.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view of an embodiment of the birefringent diffraction grating polarizer of the present invention. Reference numeral 1 denotes a crystal substrate having optical anisotropy, and the x-axis or the y-axis of the crystal is inclined by θ from the normal 4 of the surface of the substrate 1. In the embodiment of FIG. 1, lithium niobate is used for the crystal substrate 1, and the y-axis of the crystal is inclined by θ from the normal vector 4 on the surface of the substrate 1.
Reference numeral 2 denotes a proton ion exchange region, which is formed periodically. Further, a dielectric film 3 having a desired thickness is provided on the proton ion exchange region 2. Further, in the embodiment of FIG. 2, the x-axis of the crystal is inclined by θ from the normal vector 4 on the surface of the substrate 1, and the rest is the same as the embodiment of FIG.

The intensity of the 0th-order diffracted light of the diffraction grating shown in FIGS. 1 and 2 is COS ² ｛π [ΔnT _p + (n _d −
1) It is given by T _d ] / λ}. Where λ is the wavelength of light,
Δn is the refractive index difference between the proton ion exchange region 2 and the crystal substrate 1, T _p is the depth of the proton ion exchange region 2, n _d is the refractive index of the dielectric film 3, and T _d is the thickness of the dielectric film 3. is there. Assuming that the wavelength of light is 1.3 μm, the change in refractive index due to proton ion exchange is about +0.09 for an extraordinary ray and about −0.04 for an ordinary ray. If a quartz (SiO ₂ ) film having a refractive index of 1.45 is used as the film 3, the depth of the proton ion exchange region 2 is about 5 μm, and the thickness of the quartz film is about 440 nm. Can be set to 0, and the 0th-order diffracted light intensity of the ordinary ray can be set to 1.

If the wavelength of light is λ and the pitch of the diffraction grating is Λ, the first-order diffraction angle of the diffraction grating is given by λ / Λ.
Therefore, if the wavelength is 1.3 μm and the pitch is 50 μm, 1
The next diffraction angle is about 1.5 degrees. As described above, since the angle θ between the main surface of the polarizer and the optical axis of the crystal is optimally a half of the first-order diffraction angle of the diffraction grating, the angle θ in the above case is equal to 0.
Approximately 75 degrees is optimal.

If the y-axis of the diffraction grating shown in FIG. 1 is arranged so as to be parallel to the optical axis of the incident light 5, the ordinary light component can be made zero for a specific linearly polarized light. All light is diffracted. Conversely, the linearly polarized light orthogonal to the above-mentioned polarized light has no extraordinary light component and is not diffracted at all. Moreover, since the normal 4 to the surface of the polarizer is inclined by θ from the optical axis 5 of the incident light, it is possible to prevent the return light 6 reflected on the surface of the polarizer from returning along the same path as the incident light 5. it can. In the case of the diffraction grating shown in FIG. 2, the same effect can be obtained if the x-axis is arranged so as to be parallel to the optical axis of the incident light 5.

Since the present polarizer can be formed in a large amount on a thin lithium niobate crystal plate by a batch process, a thin and inexpensive polarizer can be obtained.

FIG. 3 is a perspective view of an embodiment of the optical isolator according to the present invention. The Faraday rotator 9 is connected to the optical axis (Z
Two axes of the birefringent diffraction grating polarizer 7, whose axes are inclined by 45 degrees,
8 and these are put in a cylindrical permanent magnet 10. One birefringent diffraction grating polarizer 8 has a normal to the principal surface parallel to the y-axis of the crystal, and another polarizer 7 has a principal normal to the x-plane of the crystal from the z-axis. The axis rotated by 45 degrees is inclined by θ from the y axis with the axis of rotation being the rotation axis. Furthermore, the direction of the rotation of the optical axis (z axis) of the birefringent diffraction grating polarizer 8 whose normal line to the main surface is parallel to the y axis of the crystal is set to the direction of the other birefringent diffraction grating polarizer 7. When the entire optical isolator is arranged at an angle of θ so that the y-axis of the crystal is parallel to the optical axis of the incident light 5, the optical axes of the crystals of both birefringent diffraction grating polarizers 7 and 8 are adjusted to the incident light 5. It can be orthogonal to the optical axis. Therefore, the return light 6 reflected on the surface of the optical isolator can escape from the optical axis of the incident light 5 and a high isolation ratio can be obtained.

[0017]

As described above, according to the present invention, a polarizing element which is thin, has a high extinction ratio and low insertion loss and is small in the influence of reflected return light, and is thin and has a high isolation ratio and is affected by the reflected return light. An optical isolator having a small size can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view of a birefringent diffraction grating polarizer according to an embodiment of the present invention.

FIG. 2 is a perspective view of another embodiment of the birefringent diffraction grating polarizer of the present invention.

FIG. 3 is a perspective view of an embodiment of the optical isolator of the present invention.

FIG. 4 is a perspective view of an example of a conventional birefringent diffraction grating polarizer.

FIG. 5 is a sectional view of an example of a conventional birefringent diffraction grating polarizer.

FIG. 6 is a sectional view of an example of a conventional birefringent diffraction grating polarizer.

FIG. 7 is a cross-sectional view of an example of a conventional birefringent diffraction grating polarizer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Lithium niobate crystal substrate 2 Proton ion exchange area 3 Dielectric film 4 Normal line of substrate 1 surface 5 Incident light 6 Reflection return light 7 Birefringent diffraction grating polarizer 8 Birefringent diffraction grating polarizer 9 Faraday rotator 10 Magnet 11 0th order diffracted light 12 ± 1st order diffracted light

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 5/30 G02B 5/18 G02B 27/28 G02B 27/42

Claims (2)

(57) [Claims] 1. An optical diffraction grating having a periodic ion-exchange region on a principal surface of a crystal plate having optical anisotropy, and a light having a specific polarization component passing through the ion-exchanged region. In a birefringent diffraction grating polarizer provided with a means for compensating for a phase shift, the normal to the main surface of the crystal plate is x
When the lattice vector is perpendicular to the optical axis with respect to the axis or y-axis
A birefringent diffraction grating polarizer, which is tilted at about half the primary diffraction angle . 2. An optical isolator having a structure in which two birefringent diffraction grating polarizers whose optical axes are inclined at 45 degrees to each other with a Faraday rotator interposed therebetween are bonded to each other. The x-axis or y-axis of the crystal is parallel, and the other polarizer has a lattice vector from the y-axis with the normal to the principal surface rotated 45 degrees from the z-axis in the xz plane of the crystal as the rotation axis.
An optical isolator characterized in that the light is tilted at about half the angle of the first diffraction order when the light is perpendicular to the optical axis , or
In one polarizer, the normal of the principal surface is parallel to the x-axis or y-axis of the crystal, and in the other polarizer, the normal of the principal surface is rotated by 45 degrees from the z-axis in the yz plane of the crystal. Is the primary diffraction angle when the lattice vector is perpendicular to the optical axis from the x-axis with
An optical isolator characterized by being inclined at a half angle .