JPH0584487B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field The present invention relates to a light modulation device suitable for various devices such as optical display, optical recording, optical coupling, optical communication, and optical calculation.

(2) Conventional technology Conventionally, typical optical modulation devices include
There were devices using electro-optic crystals and liquid crystals such as PLZT and BSO.

A device using an electro-optic crystal includes intersecting comb-shaped electrodes on sliced electro-optic crystal surfaces, a polarizer and an analyzer in front and behind the electro-optic crystal, and an electric field applied to the comb-shaped electrodes. There is a device that controls the light flux that passes through a device consisting of a polarizer, an electro-optic crystal, and an analyzer by changing the birefringence within the crystal. Although this device has relatively excellent response characteristics and a high contrast ratio for monochromatic light,
Usually, the driving voltage is very high, ranging from 100V to several kV.
Moreover, it has the disadvantage that it is difficult to increase the area.

In addition, in devices using liquid crystals, liquid crystals are filled between transparent electrodes that are aligned in directions perpendicular to each other, and the liquid crystals are oriented in a spiral shape.In a static state, the light beam passes through polarizing plates that are orthogonal to each other. However, when an electric field is applied, there is a device in which the liquid crystal is oriented in the direction of the electric field and the light beam is blocked by a polarizing plate installed on the output side, making it impossible to transmit the light. Although devices using such liquid crystals have low driving voltage and are made of relatively inexpensive materials, they have problems with switching response speed, temperature stability, etc., and are not satisfactory in terms of contrast ratio, light utilization efficiency, etc.

Not only the typical light modulation device mentioned above, but also most conventional light modulation devices have random polarization because they use light with specific polarization characteristics as incident light, which is usually linearly polarized light. A polarizing plate had to be used for incident light, and when the incident light was transmitted through the polarizing plate, the light utilization efficiency was greatly reduced.

(3) Summary of the Invention An object of the present invention is to eliminate the conventional drawbacks and provide a light modulation device that has high light utilization efficiency for light having arbitrary polarization characteristics.

In order to achieve the above object, a light modulation device according to the present invention has first and second grating layers arranged in the traveling direction of incident light, and the first and second grating layers are transparent. It has an optical member and a variable refractive index medium, and the direction of the optical axis of the medium is different between the first grating layer and the second grating layer, and by controlling the optical axis of the medium, arbitrary polarization characteristics can be obtained. An optical modulation device that modulates light, comprising means for spatially separating the variable refractive index medium of the first grating layer and the variable refractive index medium of the second grating layer. It is said that Note that the state in which multiple gratings are lined up in the direction in which the incident light travels indicates that each grating always exists in the direction in which the incident light travels, and the incident angle does not necessarily vary with respect to this device. It is not necessarily vertical.

The substance having the predetermined optical axis is a so-called refractive index variable medium, and the refractive index of the refractive index variable medium is changed by an electric field, a magnetic field, pressure, heat, etc. to impart predetermined characteristics to the grating.

Examples of the variable refractive index medium include liquid crystal,
PLZT, LiNbO ₃ , LiTaO ₃ , TiO ₂ , PMMA,
CCl ₄ , KDP, ADP, ZnO, BaTiO ₃ , Bi ₁₂ SiO ₂₀ ,
Ba ₂ NaNb ₅ O ₁₅ , MnBi, EuO, CS ₂ , Gd ₂
( _M0O4 ) ₃ , _{Bi4Ti3O12 , CuCl ,} CaAs, ZnTe,
As ₂ Se ₃ , Se, AsGeSeS, DKDP, MNA,
Examples include mNA, UREA, photoresist, etc. In particular, liquid crystals such as positive dielectric nematic liquid crystals and ferroelectric liquid crystals are suitable because they are inexpensive, have a large refractive index difference Δn (difference between extraordinary refractive index and ordinary refractive index), and have a simple control method. In addition, the method for creating the grating includes a method combining photolithography and dry etching, a replica method using a thermosetting resin or an ultraviolet curable resin, a cutting method using a ruling engine, an embossing method, etc. Various methods can be mentioned.

(4) Embodiment Figures 1, 2, and 3 show configuration examples of a light modulation device according to the present invention, in which 1 and 1' are variable refractive index media whose optical axes are in different directions, and 2 is a transparent medium. The optical members include a transparent electrode 3, a transparent spacer 4, and a transparent heater 5.

FIG. 1 shows an example of the configuration of this electric field control type optical modulation device, in which two gratings are formed in one device. The apparatus shown in FIG. 1a is a transparent optical member 2.
has a triangular grating, and variable index media 1 and 1' are arranged above and below with a flat transparent spacer 4 in between. Further, the transparent electrode 3 is formed on the transparent optical member 2. In the apparatus shown in FIG. 1b, one grating is formed on one transparent optical member 2, the grating formed on a transparent spacer 4 is placed facing the other transparent optical member, and the refraction is reflected through the transparent spacer 4. Variable rate media 1 and 1'
are arranged above and below. Further, a transparent electrode 3 is provided on each transparent optical member 2. In the apparatus shown in FIG. 1c, transparent optical members 2 having transparent electrodes 3 are placed facing each other, a transparent spacer 4 having two gratings is placed between them, and refractive index variable media 1 and 1' are placed in the gap. It is located in

FIG. 2 shows an example of the configuration of this thermally controlled optical modulation device, and the basic structure is almost the same as that in FIG. 1. However, in FIG. 2a, a transparent heater 5 is arranged in place of a spacer, and the variable refractive index media 1 and 1' are divided into upper and lower parts. Furthermore, in FIG. 2b, a transparent heater 5 is used in place of the transparent electrode 3 in FIG. 1b.

Figure 3 shows two optical modulation devices each consisting of one grating arranged side by side, and the refractive index variable media 1 and 1' used in each element have their optical axes in different directions in the initial state. It's on. Each element has a grating formed on a transparent optical member 2, and a transparent electrode 3 provided on the transparent optical member 2 applies an electric field to the refractive index variable medium 1 or 1'.
It controls the refractive index of.

Hereinafter, the modulation principle of the present optical modulation device will be explained in detail using the drawings. Note that light having arbitrary polarization characteristics can be considered as being divided into two predetermined orthogonal polarization components.

FIG. 4 is an explanatory diagram of the modulation principle of this optical modulation device, and the basic structure of the device is the same as the device shown in FIG. 1a. Here, 6 is the incident light, 7 and 7' are mutually orthogonal polarization components of the incident light 6, 8 and 8' are the directions of the optical axes of the variable refractive index media 1 and 1', and the first
Components similar to those in the figures are given the same numbers.

In FIG. 4, the optical axis of the first layer variable refractive index medium 1 is directed toward the grating groove direction, and the optical axis of the second layer variable refractive index medium 1' is directed toward the grating arrangement direction. . It is also assumed that the refractive index of the variable refractive index media 1 and 1' is controlled by an electric field.

In a static state with no electric field applied, in the first layer, the polarized light component 7' of the incident light 6 senses the extraordinary refractive index n _e of the variable index medium 1; We feel that the ordinary refractive index of n ₀ . In the second layer, the polarized light component 7' senses the ordinary refractive index _n'0 of the variable index medium 1', and the polarized light component 7 senses the extraordinary refractive index _n'e of the variable index medium 1'. Here, the refractive index of the transparent optical member 2 forming the first layer grating is n _g , the refractive index n' _g of the transparent optical member 2 forming the second layer grating, the wavelength of the incident light is λ, 1
If the thicknesses of the gratings in the first and second layers are T and T', respectively, the diffraction efficiencies η' ₀ and η' ₀ of the zero-order transmitted diffracted light in the gratings in each layer are as follows (1)
It can be expressed as equation (1)′.

η ₀ = sinc ² (π△n・T/λ) …(1) η′ ₀ = sinc ² (π△n′・T′/λ) …(1)′ From the above equation, △n=0, or △ When n′=0, η ₀ =1 or η′ ₀ =1, and △nT=mλ or △n′T′=mλ
When the conditions (m=1, 2, 3,...) are satisfied, η ₀
It can be seen that =0 or η′ ₀ =0.

If n ₀ =n _g or ne = n _g is satisfied in the first layer, one of the polarization components 7 and 7' will pass through, and the other will be modulated according to equation (1). 2
Even in the layer, if n' ₀ = n' _g or n' _e = n' _g is satisfied, one of polarized light components 7 and 7' will pass through, and the other will pass through equation (1)'. Modulated according to

Next, when an electric field is applied to the variable index media 1 and 1', the direction of the optical axis of the variable index media 1 and 1' changes, and the refractive index felt by the polarized light components 7 and 7' changes accordingly. Therefore, modulation is performed in accordance with equations (1) and (1)' in the first and second layers, respectively.

For example, if the same liquid crystal is used for the variable refractive index media 1 and 1', ne = n'e, n ₀ = n' ₀ , and the initial conditions are n _g = n' _g = n ₀ , T = T ′＋｜n _e −n _g
If |·T=mλ is set, the equations expressing the diffraction efficiency of the zero-order transmitted diffracted light in the first layer and the second layer both become equation (1).

It is assumed that the refractive index of the spacer 4 is approximately equal to n _g . At this time, in a static state, the polarized light component 7' of the incident light 6 passes through the first layer, and the polarized light component 7' becomes η ₀ = 0 from equation (1), so no zero-order transmitted light is emitted and all the higher-order diffracted light becomes. Further, in the second layer, the polarization component 7 has η ₀ =0 according to equation (1), so no zero-order transmitted light is emitted and all become high-order diffracted light. Note that the polarized light component 7' passes through the second layer as high-order diffracted light. Therefore, there is no light emitted in the zero-order direction. Next, when a predetermined electric field is applied and the optical axes (orientation directions) of the liquid crystals 1 and 1' are directed perpendicular to the grating surface, that is, in the direction of incidence of the light flux, the polarized light components 7 and 7' are In the second layer, all the light senses the ordinary refractive index n ₀ of the liquid crystal, and passes through it as zero-order transmitted light. Therefore, by applying an electric field, it is possible to control the transmittance of zero-order transmitted diffracted light of light having arbitrary polarization characteristics. In the above explanation, zero-order diffracted light was considered as modulated light, but it goes without saying that higher-order diffracted light can also be used.

In addition, in reality, the directions of the optical axes of the variable refractive index media 1 and 1' do not necessarily have to be orthogonal as in this embodiment, and the directions of the optical axes of the variable index media 1 and 1' are not necessarily perpendicular to each other as in this embodiment. It would be good if the directions were different. Therefore, how much difference should be made in the direction of the optical axis?
It varies depending on constraints from the medium, polarization characteristics of the incident light used, manufacturing conditions, and specifications of the device.
Furthermore, in the embodiments shown in FIGS. 1 to 4, the direction of grating in each layer is the same;
This light modulation device does not depend on the arrangement direction of the gratings, and the plurality of gratings may be oriented in any direction as long as the light modulation effect cannot be prevented.

FIG. 5 shows an example of the shape of a grating formed by the present optical modulation device, and all numbers in the figure indicate the same members as in FIG. 1.

In FIG. 5a, two rectangular gratings are formed by transparent optical members 2 with spacers 4 interposed therebetween. Also, in FIG. 5b, two sinusoidal gratings are similarly formed.

The present optical modulation device has a light modulation function regardless of the shape of the grating, but if the shape of the grating differs, a difference will appear in the equation of diffraction efficiency shown in equation (1) above. For example, in the case of a rectangular grating, the following equation (2) is obtained.

η ₀ = 1/2 {1 + cos (2π△n・T/λ)} ...(2) Note that it does not matter if multiple gratings have different shapes, and the shape of the gratings can be determined based on the shape of the grating that is easy to manufacture. The decision should be made taking into consideration gender, etc. Further, the triangular grating used in the above embodiment has an excellent diffraction effect not only for monochromatic light but also for white light.

Below, we will describe the manufacturing process and performance evaluation results of this optical modulator having the basic configuration shown in FIG. 1a.

FIG. 6 shows the manufacturing process of this optical modulation device, in which 9 is a liquid crystal and other numbers refer to the same members as in FIG. 1.

Transparent PBMA resin substrate 2 (50×25×1.5mm ² , n _g =
Both sides of 1.56) were made transparent planes, and a triangular grating with a pitch of 3 μm and a depth of 2.6 μm was formed by embossing on a predetermined portion (10×10 mm ² ) of one side as shown in FIG. 6a. Next, as shown in FIG. 6b, the substrate 2 is
An ITO film 3 was formed thereon to a thickness of 1000 Å in the form of a strip including the grating region. Prepare another transparent PBMA substrate 2 created by the same method as above,
5μm thick with orientation treatment in the direction that the front and back are perpendicular to each other
Place the transparent Teflon spacer 4 between the two transparent sheets above.
A positive dielectric liquid crystal is sandwiched between the PBMA substrates 2 and in the gap between the upper and lower grating layers and the spacer 4.
Filled with MBBA (n ₀ =1.56, n _e =1.786), a device as shown in FIG. 6c was prepared.

D line (λ = 589.3 nm) of Na light source with random polarization component in the optical modulator shown in Figure 6c
In a static state with no voltage applied, almost no zero-order transmitted diffraction light was emitted, and the transmittance of the incident light was 1% or less. Also, frequency 1kHz,
When a rectangular wave voltage with an effective voltage of 10 V was applied, almost all incident light passed through, and the transmittance was over 80%. Furthermore, the rise time for voltage application is 1 msec,
The fall time was 5 msec.

As described above, it was confirmed that the device can operate effectively for incident light having arbitrary polarization characteristics. Next, other embodiments will be described below.

A transparent PMMA resin film was rolled using a rectangular wave hot rolling roller with a pitch of 3 μm and a depth as shown in Figure 7.
2.1 μm rectangular wave gratings were formed on both sides. However, the directions of the grating grooves on the front and back sides are orthogonal. Next, two BK7 boards (50 x 25 x 1 mm ² ,
ng=1.490) were made transparent planes, and an ITO film with a thickness of 1000 Å was formed on a predetermined portion (10×40 mm ² ) of each side. Next, the film 4 having the grating described above was sandwiched between the ITO films of these two BK7 substrates as a spacer, and a positive dielectric liquid crystal ZLI-2141-000 (manufactured by Merck, no = 1.49, n _e =
1.64) was filled. At this time, the liquid crystal display is the above film 4.
The upper and lower liquid crystals are oriented by the grating and separated by the film 4, and their alignment directions are perpendicular to each other. Using the optical modulator of this example, a He-Ne laser beam with random polarization components (λ=
632.8 nm) was incident, and the same measurements as in the above example were performed, and almost the same results as in the above example could be obtained.

(5) Effects of the Invention As explained above, the light modulation device according to the present invention is a device that does not require a polarizing plate and has a high light utilization effect for incident light having any polarization component.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, and FIG. 3 are diagrams showing an example of the configuration of a light modulation device according to the present invention. FIG. 4 is an explanatory diagram of the modulation principle of the present optical modulation device. FIG. 5 is a diagram showing an example of the shape of a grating used in the present optical modulation device. FIG. 6 is a diagram showing the manufacturing process of the present optical modulation device. FIG. 7 is a diagram showing a grating used in another embodiment of the present optical modulation device. DESCRIPTION OF SYMBOLS 1, 1'...Refractive index variable medium with different optical axis directions, 2...Transparent optical member, 3...Transparent electrode, 4...Transparent spacer, 5...Transparent heater, 6...Incoming light,
7, 7'... Polarization components perpendicular to each other, 8, 8'... Direction of the optical axis of the variable refractive index medium, 9... Liquid crystal.

Claims (1)

[Scope of Claims] 1. First and second grating layers arranged in the traveling direction of incident light, the first and second grating layers having a transparent optical member and a variable refractive index medium. and the direction of the optical axis of the medium is different between the first grating layer and the second grating layer,
In the light modulation device that modulates light having arbitrary polarization characteristics by controlling the optical axis of the medium, the variable refractive index medium of the first grating layer and the variable refractive index medium of the second grating layer are provided. 1. A light modulation device characterized by having means for spatially separating the light modulator and the light modulator. 2. The light modulation device according to claim 1, wherein the separating means also serves as at least one of the transparent optical members.