JPH04263205A - Double refractive diffraction grating type polarizer and optical isolator - Google Patents

Abstract

PURPOSE:To obtain the double refractive diffraction type polarizer which prevents the return of reflected light on the surface of the polarizer in the same route as the route of incident light and with which a high extinction ratio is obtainable. CONSTITUTION:The normal 4 of the crystal on the main plane of the crystal plate of the double refractive diffraction type polarizer is inclined with the x-axis or y-axis of the crystal in the range larger than 0 deg. and smaller than the 1st order diffraction angle when the grating vector is perpendicular to the optical axis. The optical axis of the crystal and the optical axis of the incident light can, therefore, be intersected orthogonally with each other and the deterioration in the extinction ratio is prevented even if the polarizer is inclined in order to escape the return light.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to birefringent polarizing plates used in various optical devices using semiconductor lasers, and particularly to grating-type polarizing plates and optical isolators whose diffraction efficiency varies depending on the polarization direction.

0002

[Prior Art] Polarizing elements, especially polarizing beam splitters,
This is an element that obtains specific polarized light by changing the propagation direction of light between orthogonal polarized lights. Such an element is
It is used as a component of optical isolators and optical circulators in light source modules for optical fiber communications and optical heads for optical disks.

[0003] Conventionally, polarizing beam splitters include those that separate optical paths by utilizing the difference in transmission or total reflection due to polarization on the light reflecting surface of a crystal with large birefringence, such as a Glan-Thompson prism or Rochon prism, or glass, etc. A total reflection prism made of an isotropic optical medium with a dielectric multilayer film on the reflective surface, and by utilizing the difference in refractive index due to polarization of this dielectric multilayer film, is often used to completely reflect or transmit light. has been done. However, these devices have drawbacks such as large size, low productivity, and high cost.

On the other hand, in recent years, a birefringent diffraction grating type polarizing plate described in JP-A-63-314502 has been known as a polarizing element characterized by its small size and high productivity. FIG. 4 is a perspective view showing the structure of the birefringence grating type polarizing plate described above, and FIGS. 5 to 7 are cross-sectional views. A birefringent diffraction grating type polarizing plate has an optical diffraction grating of periodic ion exchange regions 2 provided on the main surface of a lithium niobate substrate 1, and an ordinary light beam is formed between the regions subjected to ion exchange and the regions not subjected to ion exchange. It is equipped with a means for canceling out the phase change experienced by the polarized light, and separates the optical path by utilizing the difference in diffraction efficiency due to polarization. As a means for canceling the phase change experienced by the ordinary rays, as shown in the cross-sectional view of FIG. 5, the surface of the area where ion exchange has not been performed is shaved to a desired depth, or as shown in the cross-sectional view of FIG. 6. As shown in FIG. 7, the dielectric film 3 is formed on the ion-exchanged region 2, or as shown in the cross-sectional view of FIG. There are those in which a dielectric film 2 is formed. For example, when proton ion exchange is performed periodically on the main surface of lithium niobate, the refractive index for extraordinary rays with a wavelength of 1.3 μm increases by approximately 0.1 in the proton ion exchanged region, and the refractive index for ordinary rays increases by approximately 0.1. decreases by about 0.04. Therefore, the dielectric film thickness in the region subjected to proton ion exchange is made thicker than the dielectric film thickness in the region not subjected to proton ion exchange, and the refractive index for ordinary rays in the region subjected to proton ion exchange is decreased. By canceling out, the diffraction efficiency of the first or higher order of the ordinary ray and the zero-order diffraction efficiency of the extraordinary ray can both be made zero,
Become a polarizer.

[0005]

[Problem to be Solved by the Invention] Normally, in an element such as an optical isolator where it is necessary to sufficiently reduce the reflected return light from the element itself, the angle between the optical axis of the incident light and the element surface is slightly shifted from 90 degrees. , so that the reflected return light does not follow the same path as the incident light. However, in the birefringent grating type polarizing plate having the structure described above, if the angle between the optical axis of the crystal and the optical axis of the incident light deviates from 90 degrees, the extinction ratio deteriorates. In the case of an optical isolator in which two polarizing plates whose optical axes are tilted at 45 degrees to each other are pasted together across a Faraday rotator, 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, there is a problem that the extinction ratio of at least one of the polarizing plates deteriorates, and the isolation ratio of the optical isolator deteriorates. In addition, even when using a polarizer alone, since the polarizer can only be tilted around the optical axis, the returned light can only escape within the plane perpendicular to the optical axis, which increases the flexibility of the optical system. There is also the problem that it is small.

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

[0007]

[Means for Solving the Problems] The present invention provides an optical diffraction grating having an ion exchange region having a period on the main surface of a crystal plate having optical anisotropy, and an optical diffraction grating that passes through the ion exchange region. In a birefringent grating polarizer, the normal to the main surface of the crystal plate is more than 0 degrees with respect to the x-axis or y-axis of the crystal. This is a birefringent diffraction grating polarizer characterized by a grating vector tilted within a range smaller than the first-order diffraction angle when perpendicular to the optical axis. The present invention provides an optical isolator having a structure in which two birefringent diffraction grating type polarizers whose optical axes are inclined at 45 degrees to each other are pasted together with a Faraday rotator in between, in which one polarizer is connected to the normal to the main surface of the crystal. The x-axis or y-axis of the other polarizer is parallel, and the other polarizer has a grating whose rotation axis is an axis rotated 45 degrees from the z-axis within the An optical isolator, or one polarizer, is characterized by being tilted within a range smaller than the first-order diffraction angle when the vector is perpendicular to the optical axis. are parallel,
The other polarizer has its main surface normal to z within the yz plane of the crystal.
The optical isolator is characterized in that the rotation axis is rotated by 45 degrees from the axis and is tilted within a range greater than 0 degrees from the x-axis and smaller than the first-order diffraction angle when the grating vector is perpendicular to the optical axis. .

[0008]

[Operation] As mentioned above, the problem with conventional birefringent grating type polarizing plates is that the main plane of the polarizer is parallel to the optical axis of the crystal. The optical axis of the polarizer is tilted to the point where the extinction ratio of the polarizer deteriorates. Therefore, in the polarizer of the present invention, an angle is set in advance between the main surface of the polarizer and the optical axis of the crystal so that the optical axis of the crystal and the optical axis of the incident light are perpendicular to each other when the polarizer is arranged at an angle. be. By doing this, even if the polarizer is tilted, the optical axis of the crystal and the optical axis of the incident light can be made perpendicular to each other, allowing the reflected light on the polarizer surface to escape in any direction, and increasing the extinction ratio of the polarizer. Deterioration can be prevented.

When the grating vector of the diffraction grating is tilted by θ from the plane perpendicular to the optical axis, the angle formed by the incident light and the reflected light on the diffraction grating surface is 2θ, and the angle of the smaller first-order diffraction angle is 2θ. The angle φ is φ˜1/(1/φ0 +θ). However, φ0 is the first-order diffraction angle when the grating vector is perpendicular to the optical axis. Therefore, if θ is made 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, θ and φ are required to be equally large, so if θ=φ, then θ˜φ0/2. Therefore, the optimal angle between the principal plane of the polarizer and the optical axis of the crystal is half the first-order diffraction angle of light that is perpendicularly incident on the diffraction grating, and its upper limit is about the same as the first-order diffraction angle. be.

Furthermore, in an optical isolator having a structure in which a Faraday rotator is sandwiched between two birefringent diffraction grating type polarizers whose optical axes are tilted at 45 degrees, one polarizer is attached to the normal to the main surface. The x-axis or y-axis of the crystal is parallel,
The other polarizer is one in which the normal to the main surface is tilted by a certain angle from the y-axis or x-axis, with the rotation axis being an axis rotated 45 degrees from the z-axis in the xz plane or yz plane of the crystal. The optical axis of the other polarizer is orthogonal to the optical axis of the incident light. By tilting the entire optical isolator to this angle, the optical axes of the crystals of both polarizers can be made perpendicular to the optical axis of the incident light. Therefore, it is possible to cause the reflected return light on the surface of the optical isolator to escape from the optical axis of the incident light, and to obtain a high isolation ratio.

[0011]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of an embodiment of the birefringent grating type polarizer of the present invention. 1 is a crystal substrate having optical anisotropy, and the x-axis or y-axis of this crystal is inclined by θ from the normal 4 to the surface of the substrate 1. In the embodiment shown in 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 to the surface of the substrate 1. 2 is a proton ion exchange region, and this exchange region is formed periodically. Further, on this proton ion exchange region 2, a dielectric film 3 having a desired thickness is provided. Further, in the embodiment shown in FIG. 2, the x-axis of the crystal is inclined by θ from the normal vector 4 to the surface of the substrate 1, and other aspects are the same as the embodiment shown in FIG.

The zero-order diffracted light intensity of the diffraction grating shown in FIGS. 1 and 2 is COS2 {π[ΔnTp + (nd
−1) Td ]/λ}. However, λ is the wavelength of light, Δn is the refractive index difference between the proton ion exchange region 2 and the crystal substrate 1, Tp is the depth of the proton ion exchange region 2,
nd is the refractive index of the dielectric film 3, and Td is the thickness of the dielectric film 3. If the wavelength of light is 1.3 μm, the change in refractive index due to proton ion exchange is approximately +0.09 for extraordinary rays and approximately -0.04 for ordinary rays, so dielectric The film 3 is made of quartz (S) with a refractive index of 1.45.
If a (iO2) film is used, the depth of the proton ion exchange region 2 is about 5 μm, and the thickness of the quartz film is about 440 nm. The intensity of the diffracted light can be set to 1.

When 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 will be approximately 1.5 degrees. As mentioned earlier, the angle θ between the principal plane of the polarizer and the optical axis of the crystal is optimally half the first-order diffraction angle of the diffraction grating, so in the above case it is 0.
Approximately 75 degrees is optimal.

If the y-axis of the diffraction grating shown in FIG. 1 is arranged parallel to the optical axis of the incident light 5, the ordinary light component can be made zero for a specific linearly polarized light, so that the incident polarized light All light is diffracted. On the contrary, linearly polarized light orthogonal to the above polarized light has zero extraordinary light component and is not diffracted at all, allowing polarization separation. 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 from the surface of the polarizer from returning through the same path as the incident light 5. can. In the case of the diffraction grating shown in FIG. 2, exactly the same effect can be obtained if it is arranged so that the x-axis is parallel to the optical axis of the incident light 5.

Since the present polarizer can be formed in large quantities on thin lithium niobate crystal plates by a batch process, it is possible to obtain a thin and inexpensive polarizer.

FIG. 3 is a perspective view of an embodiment of the optical isolator of the present invention. The Faraday rotator 9 is aligned with the optical axis (Z
two birefringent diffraction grating type polarizers 7 whose axes) are tilted at 45 degrees;
8 are sandwiched and pasted together, and these are placed inside a cylindrical permanent magnet 10. One birefringent grating polarizer 8 has a main surface normal line parallel to the y-axis of the crystal, and the other polarizer 7 has a main surface normal line parallel to the crystal xz-plane from the z-z axis. It is tilted by θ from the y-axis using the axis rotated by 45 degrees as the rotation axis. Furthermore, the other birefringent grating polarizer 7 is rotated in a direction whose rotation axis is the optical axis (z axis) of the birefringent grating polarizer 8 whose main surface normal is parallel to the y axis of the crystal. If the entire optical isolator is tilted by 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 grating polarizers 7 and 8 are aligned with the optical axis of the incident light 5. It can be made perpendicular to the optical axis. Therefore, the reflected return light 6 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, the present invention provides a polarizing element that is thin, has a high extinction ratio, low insertion loss, and is less affected by reflected return light, and a thin polarizer that has a high isolation ratio and is less affected by reflected return light. A small optical isolator can be obtained.

[Brief explanation of the drawing]

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

FIG. 2 is a perspective view of another embodiment of the birefringent grating type 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 grating polarizer.

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

FIG. 6 is a cross-sectional view of an example of a birefringent grating type polarizer according to a conventional example.

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

[Explanation of symbols] 1 Lithium niobate crystal substrate 2. Proton ion exchange region 3 Dielectric film 4 Normal to the surface of substrate 1 5 Incident light 6 Reflected return light 7 Birefringence grating type polarizer 8 Birefringence grating type polarizer 9 Faraday rotator 10 Magnet 11 0th order diffracted light 12 ±1st order diffracted light

Claims (2)

[Claims] [Claim 1] On the main surface of a crystal plate having optical anisotropy,
In a birefringent diffraction grating type polarizer, which includes an optical diffraction grating of an ion-exchanged region having a period and means for compensating for a phase shift for light of a specific polarization component passing through the ion-exchanged region. , characterized in that the normal to the main surface of the crystal plate is tilted with respect to the x-axis or y-axis of the crystal within a range greater than 0 degrees and smaller than the first-order diffraction angle when the lattice vector is perpendicular to the optical axis. A birefringent diffraction grating type polarizer. [Claim 2] In an optical isolator having a structure in which two birefringent grating type polarizers whose optical axes are inclined at 45 degrees are laminated together with a Faraday rotator in between, one of the polarizers is aligned with the normal to the main surface. The x-axis or y-axis of the crystal is parallel, and the other polarizer has a rotation axis whose normal to the main surface is rotated by 45 degrees from the z-axis within the xz plane of the crystal, and whose rotation axis is greater than 0 degrees from the y-axis. An optical isolator characterized by a lattice vector tilted within a range smaller than the first-order diffraction angle when perpendicular to the optical axis, or a single polarizer is
The axes or y-axes are parallel, and the other polarizer has a lattice vector whose rotation axis is an axis rotated 45 degrees from the z-axis within the yz plane of the crystal, and whose lattice vector is larger than 0 degrees from the x-axis. An optical isolator characterized by being tilted within a range smaller than the first-order diffraction angle when perpendicular to the optical axis.