JPH0627320A - Double refractive diffraction grating type polarizer - Google Patents

Abstract

PURPOSE:To obtain an extinction ratio sufficient for practicable use over a wide wavelength range of incident light. CONSTITUTION:The diffraction gratings of proton exchange regions 18 and phase compensation films 19 are formed on a surface 16 of a lithium niobate substrate 15. The diffraction gratings of proton exchange regions 20 and phase compensation films 21 are formed on the surface 17. The phase difference between the line parts and base parts of the diffraction gratings of the surface 16 is 0 with an ordinary light component and pi with an extraordinary light component in the case of a wavelength lambda1 of incident light. The phase difference between the line parts and base parts of the diffraction gratings of the surface 17 is 0 with the ordinary light component and pi with the extraordinary light component in the case of a wavelength lambda2 (lambda1<lambda2) of the incident light. The high extinction ratio is obtd. by the diffraction gratings formed on the surface 16 if the wavelength of the incident light is smaller than lambda1. The high extinction ratio is obtd. by the diffraction gratings formed on the surface 17 if the wavelength of the incident light is larger than lambda2. The necessary extinction ratio is assured by adequately determining the values of the ' lambda1, lambda2 even if the wavelength of the incident light exists between the lambda1 and the lambda2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent diffraction grating type polarizer used in an optical communication device or an optical information processing device.

[0002]

2. Description of the Related Art A polarizer is an element for extracting only a specific polarization component of incident light, and is used in an optical system such as a light source module for optical communication, an optical disk head, and a projection display. As the type of polarizer, one that utilizes the difference in the refractive index due to polarization in birefringent crystals such as Glan-Thompson prisms, or a dielectric multilayer film is coated on the surface of glass The one that uses the difference between the transmittance and the reflectance is often used. On the other hand, in recent years, a birefringent diffraction grating type polarizer as disclosed in Japanese Patent Laid-Open No. 63-314502 has been proposed, which is characterized by its small size and low price.

FIG. 3 shows the structure of a conventional birefringent diffraction grating type polarizer. A bilayer diffraction grating including a proton exchange region 2 and a phase compensation film 3 is formed on a lithium niobate substrate 1 having birefringence. As the phase compensation film 3, Nb ₂ O ₃ is used, for example. In addition, the line portion and the space portion of the diffraction grating have the same width. Here, if the symbols are defined as follows, equations (1) to (3) are established.

Η ₀ : Diffraction efficiency (transmittance) of 0th order light η ₁ : Sum of diffraction efficiency of ± 1st order light λ: Wavelength of incident light T _p : Depth of proton exchange region T _d : Phase compensation film the thickness [Delta] n: refractive index difference of the proton exchange region and the non-exchange region n _d: refractive index ^{_{η 0 = cos 2 γ ... (}} 1) of the phase compensation film _{η 1 = 2 (2 / π} ) 2 sin 2 γ ... (2 ) Γ = (π / λ) [T _p Δn + T _d (n _d -1)] (3) FIG. 4 shows the phase change of incident light in this birefringent diffraction grating type polarizer. (A) corresponds to the polarization component (ordinary component) perpendicular to the optical axis, and the sign of the phase change amount 4 in the proton exchange region and the sign of the phase change amount 5 in the phase compensation film are opposite. (B) corresponds to the polarization component (extraordinary light component) parallel to the optical axis, and the signs of the phase change amount 6 in the proton exchange region and the phase change amount 7 in the phase compensation film are the same. As shown in the figure, by appropriately setting T _p and T _d , the phase difference 2γ between the line portion and the space portion of the diffraction grating is set to 0 for the ordinary light component and π for the extraordinary light component. be able to. Therefore, according to the equations (1) and (2), the ordinary light component is η ₀ = 1 and η ₁ = 0, so that all are transmitted without being diffracted, and the extraordinary light component is η ₀ = 0, η ₁ = 0.8
Since it is 1, all light is diffracted without being transmitted.

[0005]

In the conventional birefringent diffraction grating type polarizer shown in FIG. 3, when the wavelength of the incident light deviates from the design wavelength of the polarizer, the phase change of the incident light also changes as shown in FIG. It deviates from the state shown. Here, the design wavelength of the polarizer is set to λ ₀ , and since the wavelength dependence of Δn and n _d is small, it is ignored. For the ordinary light component, either of the phase change amount 4 in the proton exchange region and the phase change amount 5 in the phase compensation film shown in FIG. On the other hand, for the extraordinary light component,
Since the deviation between the phase change amount 6 in the proton exchange region and the phase change amount 7 in the phase compensation film shown in (b) is added, γ = (πλ ₀ / 2λ).

An important performance of the polarizer is the extinction ratio given by the ratio of the transmittance of the ordinary light component and the transmittance of the extraordinary light component. If the extinction ratio is ε ₁ , then equation (4) holds.

Ε ₁ = −10 log [cos ² (πλ ₀ / 2λ)] (4) When λ ₀ = 520 nm in FIG. 5, the wavelength λ and the extinction of incident light in this birefringent diffraction grating polarizer are set. The result of calculating the relationship of the ratio ε ₁ is shown. It can be seen that the extinction ratio decreases as the wavelength of the incident light deviates from the design wavelength, and only an extinction ratio of about 6 dB can be obtained at the lower limit wavelength of 390 nm and the upper limit wavelength of 780 nm in the visible light region.

In order to suppress such a decrease in the extinction ratio, a method has been proposed in which the diffraction grating is duplicated. Figure 6
The structure of the birefringent diffraction grating type polarizer doubled is shown in FIG. On the first surface 9 of the lithium niobate substrate 8, a two-layer diffraction grating composed of the proton exchange region 11 and the phase compensation film 12 is formed. Furthermore, on the second surface 10 of the lithium niobate substrate 8, a two-layer diffraction grating including the proton exchange region 13 and the phase compensation film 14 is formed. The depths of the proton exchange regions 11 and 13 are both T ₀ , and the thicknesses of the phase compensation films 12 and 14 are both T _d . Also, 2
The directions of the grooves of the two diffraction gratings are orthogonal to each other so that the diffracted light does not overlap the transmitted light. If the extinction ratio of this doubled polarizer is ε ₂ , then equation (5) holds.

Ε ₂ = −20 log [cos ² (πλ ₀ / 2λ)] (5) When λ ₀ = 520 nm in FIG. 7, the wavelength λ and the extinction of incident light in this birefringent diffraction grating polarizer are set. The result of calculating the relationship of the ratio ε ₂ is shown. The amount of decrease in the extinction ratio due to the wavelength shift of the incident light is suppressed, and an extinction ratio of about 12 dB is obtained at the lower limit wavelength of 390 nm and the upper limit wavelength of 780 nm in the visible light region, but it is still sufficient. It can not be said.

An object of the present invention is to provide a birefringent diffraction grating type polarizer which can obtain a sufficient extinction ratio over a wide wavelength range of incident light.

[0011]

A birefringent diffraction grating type polarizer of the present invention comprises: a first surface of a crystal substrate having birefringence;
A diffraction grating composed of a periodic ion exchange region and a dielectric film provided on the ion exchange region is formed, and the periodic ion exchange region and the second face opposite to the first face are formed. In the birefringence diffraction grating type polarizer in which the diffraction grating formed of the dielectric film provided on the ion exchange region is formed such that the direction of the groove is orthogonal to the diffraction grating formed on the first surface, In the diffraction grating formed on the first surface, the phase difference between the portion of the incident light of the first wavelength that passes through the ion exchange region and the dielectric film and the portion that does not pass through is 0 for the ordinary light component, and the extraordinary light. For the component, it has a depth of the ion exchange region and a thickness of the dielectric film such that it becomes π,
And, in the diffraction grating formed on the second surface, the phase difference between the portion that passes through the ion exchange region and the dielectric film and the portion that does not pass through is the ordinary light of the incident light of the second wavelength different from the first wavelength. 0 for the component and π for the extraordinary light component
And a thickness of the dielectric film.

[0012]

In the birefringent diffraction grating type polarizer of the present invention,
Design wavelengths are different in the diffraction grating formed on the second surface of the diffraction grating formed on the first surface of the crystal substrate. The design wavelength for each of the diffraction grating lambda _1, and lambda _2, lambda ₁
It is assumed that <λ ₀ <λ ₂ . When the wavelength of the incident light is smaller than λ ₁ , the extinction ratio higher than that of the conventional example can be obtained mainly due to the contribution of the diffraction grating formed on the first surface. On the other hand, when the wavelength of the incident light is larger than λ ₂ , the extinction ratio higher than that of the conventional example can be obtained mainly due to the contribution of the diffraction grating formed on the second surface. Further, when the wavelength of the incident light is between λ ₁ and λ ₂ , there is a region where the extinction ratio is lower than that of the conventional example, but it is necessary to set the values of λ ₁ and λ ₂ appropriately. It is possible to secure an extinction ratio. As a result, a practically sufficient high extinction ratio can be obtained over a wider wavelength range than the conventional example.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing an embodiment of the birefringent diffraction grating type polarizer of the present invention.

First surface 16 of lithium niobate substrate 15
A two-layer diffraction grating composed of the proton exchange region 18 and the phase compensation film 19 is formed on the top. Further, on the second surface 17 of the lithium niobate substrate 15, a two-layer diffraction grating including the proton exchange region 20 and the phase compensation film 21 is formed.

The depths of the proton exchange regions 18 and 20 are T _p1 and T _p2 , respectively, and the phase compensation films 19 and 21.
_Have thicknesses of T _d1 and T _d2 , respectively. The directions of the grooves of the two diffraction gratings are orthogonal to each other so that the light diffracted twice does not overlap the transmitted light. The values of T _p1 and T _d1 are 0 for the ordinary light component and π for the extraordinary light component when the incident light wavelength is λ ₁ and the phase difference between the line portion and the space portion of the diffraction grating is 0. Has been defined. The values of T _p2 and T _d2 are such that the phase difference between the line portion and the space portion of the diffraction grating is 0 for the ordinary light component and π for the extraordinary light component when the wavelength of the incident light is λ _2. Is set to be. If the extinction ratio of this polarizer is ε, then equation (6) holds.

Ε = −10 log [cos ² (πλ ₁ / 2λ)] −10log [cos ² (πλ ₂ / 2λ)] (6) When λ ₁ = 420 nm and λ ₂ = 690 nm in FIG. 2, The results of calculating the relationship between the wavelength λ of incident light and the extinction ratio ε in this birefringent diffraction grating type polarizer are shown. A high extinction ratio of 16 dB or more has been obtained over a wide wavelength range from 380 nm to 810 nm that completely includes the visible light region, and the wavelength range in which the necessary extinction ratio can be secured is expanded compared to the conventional example. I understand.

[0017]

As described above, according to the present invention, it is possible to obtain a birefringent diffractive polarizer capable of realizing a practically sufficient high extinction ratio over a wide wavelength range of incident light.

[Brief description of drawings]

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

FIG. 2 is a correlation diagram showing the relationship of the extinction ratio of the wavelength of incident light in this example.

FIG. 3 is a side view showing an example of a conventional birefringent diffraction grating type polarizer.

FIG. 4 is a diagram showing a phase change of incident light in a conventional example.

FIG. 5 is a correlation diagram showing the relationship between the wavelength of incident light and the extinction ratio in the conventional example.

FIG. 6 is a perspective view showing a doubled birefringent diffraction grating type polarizer of a conventional example.

FIG. 7 is a correlation diagram showing the relationship between the wavelength of incident light and the extinction ratio in the doubled birefringent diffraction grating type polarizer of the conventional example.

[Explanation of symbols]

 1 Lithium niobate substrate 2 Proton exchange region 3 Phase compensation film 4 Phase change amount in proton exchange region 5 Phase change amount in phase compensation film 6 Phase change amount in proton exchange region 7 Phase change amount in phase compensation film 8 Lithium niobate substrate 9 First surface 10 Second surface 11 Proton exchange region 12 Phase compensation film 13 Proton exchange region 14 Phase compensation film 15 Lithium niobate substrate 16 First surface 17 Second surface 18 Proton exchange region 19 Phase compensation film 20 Proton exchange region 21 Phase compensation film

Claims (1)

[Claims] 1. A diffraction grating composed of a periodic ion exchange region and a dielectric film provided on the ion exchange region is formed on a first surface of a crystal substrate having birefringence,
Further, on the second surface facing the first surface, a diffraction grating composed of the periodic ion exchange region and a dielectric film provided on the ion exchange region is formed on the first face. In a birefringent diffraction grating type polarizer in which the directions of the diffraction grating and the groove are orthogonal to each other, in the diffraction grating formed on the first surface, in the incident light of the first wavelength, the ion exchange region and the dielectric The depth of the ion exchange region and the thickness of the dielectric film are such that the phase difference between the part that passes through the film and the part that does not pass becomes 0 for the ordinary light component and π for the extraordinary light component, and In the diffraction grating formed on the second surface, the phase difference between the portion of the incident light of the second wavelength different from the first wavelength that passes through the ion exchange region and the dielectric film and the portion that does not pass is the ordinary light component. Is 0 for, and π is for the extraordinary light component. A birefringent diffraction grating type polarizer having a depth of an exchange region and a thickness of a dielectric film.