JP3059979B2 - Phase modulation method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for modulating the phase of light. The phase modulation method and apparatus can be used for an optical information processing apparatus, electronic holography, an optical coupling element in an optical computer, and the like.

[0002]

2. Description of the Related Art In optical information processing, electronic holography, and optical coupling devices in optical computers, it is necessary to modulate the phase of light. As one of techniques for performing phase modulation of light, it is known to use a twisted nematic liquid crystal panel in which the major axis direction of liquid crystal molecules is twisted by 90 °. When performing phase modulation of light, it is desirable that the size of the pixel of the liquid crystal panel be small. The number of pixels in currently marketed liquid crystal panels is about 400 × 480, and the size of one pixel is about 50 μm. In a pixel having such an ultra-fine pixel, the thickness of the liquid crystal is small, for example, less than 5 μm.
On the other hand, the amount of phase modulation of a liquid crystal panel is represented by "birefringence x thickness of liquid crystal layer", so a small thickness of the liquid crystal layer results in a small phase modulation capability, and a large amount of phase modulation can be obtained. Can not.

[0003]

In view of the above, it has been considered to transmit light to be phase-modulated twice through the liquid crystal panel in order to obtain phase modulation using a thin liquid crystal panel. Conventionally, as a technique of passing light through a liquid crystal panel twice, a mirror is attached to the back surface of a liquid crystal layer, and light to be phase-modulated is incident from one side of the liquid crystal panel and transmitted through the liquid crystal panel. This is a method in which the light is reflected by a mirror on the back surface of the panel and transmitted through the liquid crystal panel again. But,
Since the light actually passes through the liquid crystal layer while changing its state, it was later revealed that the amount of phase modulation does not double even though the light passes through the liquid crystal layer twice.

The present invention has been made in view of the above circumstances, and provides a phase modulation method and apparatus capable of reliably doubling the amount of phase modulation by passing a liquid crystal layer having a small thickness twice. It is intended to provide.

[0005]

In response to this object, a phase modulation method according to the present invention provides a light to be phase-modulated by a first polarization.
The long axis direction of the liquid crystal molecules is twisted by 90 ° through the facing plate, the light is incident from one surface of the twisted nematic type liquid crystal panel, is transmitted therethrough, and is emitted from the other surface through the second deflecting plate , The emitted light is reflected, incident on the other surface through the same optical path, and transmitted. Further, the phase modulation device of the present invention includes a twisted nematic type liquid crystal panel in which the major axis direction of liquid crystal molecules is twisted by 90 °, and light to be phase-modulated incident on one surface of the liquid crystal panel through the first deflecting plate. And a light reflecting device for reflecting light emitted from the other surface of the liquid crystal panel and passing through the second deflecting plate and incident on the other surface through the same optical path. I have.

[0006]

The light to be phase-modulated enters from one surface of the liquid crystal panel, passes through the liquid crystal panel, and exits from the other surface.
The emitted light is reflected by the reflecting device, passes through the same optical path, and enters from the emission point on the other surface. Light incident from the other surface passes through the liquid crystal layer and exits from one surface. The emitted light has received twice the amount of phase modulation. The reflecting device functions to re-emit the light emitted from the other surface to the emission point of the liquid crystal layer again through the same optical path.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the drawings showing one embodiment. 1 and 2, reference numeral 1 denotes a phase modulation device. The phase modulation device 1 includes a liquid crystal panel 2, a light incident device 4 located on one surface 3 side of the liquid crystal panel 2, and a light reflection device 6 arranged on the other surface 5 side of the liquid crystal panel 2. . The light incidence device 4 includes a laser light source 7, an objective lens 8, a pinhole 11, and a lens 12, and is disposed on one surface 3 of the liquid crystal panel 2 via a half mirror 13. The light reflecting device 6 includes a lens 14, a spatial filter 15, a lens 16 and a mirror 17, the lens 14 and the liquid crystal panel 2 are located apart from each other by a focal length f of the lens 14, and the spatial filter 15 is Is located at a focal length f. Lens 16 and spatial filter 15
Is located away from the focal length f 'of the lens 16, and the mirror 17 is located at the focal length f' of the lens 16. A polarizing plate 18 is arranged facing one surface 3 of the liquid crystal panel 2, and a polarizing plate 21 is arranged facing the other surface 5 of the liquid crystal panel 2.

[0008] The phase modulation operation of the phase modulation device 1 configured as described above is as follows. The light 22 to be subjected to phase modulation emitted from the laser light source 7 is made parallel by the objective lens 8, the pinhole 11, and the lens 12, reflected by a half mirror, and made incident on one surface 3 of the liquid crystal panel 2. . At this time, the incident light is polarized by the polarizing plate 18. The light incident on the liquid crystal panel 2 is
, And undergoes phase modulation and exits from the other surface 5.
At this time, the emitted light is polarized by the polarizing plate 21. The light polarized by the polarizing plate 21 passes through the lens 14, the pinhole 15,
The light is filtered by the lens 16, reflected by the mirror 17, and reversely follows the same path as the light incident on the mirror 17 to enter the liquid crystal panel 2. The reflecting device 6 is adjusted so that the light emitted from the liquid crystal panel follows exactly the same optical path in reverse, and the light enters the same position on the liquid crystal panel.
Light that has entered the liquid crystal panel from the other surface 5 is again phase-modulated by the liquid crystal panel, passes through the polarizing plate 18, passes through the half mirror 13, and is extracted. The light transmitted through the liquid crystal panel 2 twice from the front and back has a phase modulation amount twice as large as that of the light transmitted once, as shown in the following equation. That is, the Jones matrix J of the liquid crystal panel is given by the following equation (1).

[0009]

(Equation 1)

Here, c: loss due to absorption and surface reflection d: liquid crystal layer thickness λ: wavelength ne: extraordinary ray refractive index no: ordinary ray refractive index θ: liquid crystal molecule tilt angle

The Jones matrix P of the polarizer is expressed by the following equation (2).

[0012]

(Equation 2)

The Jones vector of the input light is expressed by the following equation (3).

(Equation 3) The Jones vector of the output light is

(Equation 4)

Calculating the amount of phase modulation by the liquid crystal panel using the Jones vector as follows:・ In the case of a one-way optical path (following equation 5)

[0015]

(Equation 5)

## EQU1 ## At this time, the phase modulation amount is δ excluding the constant part.

In the case of a reciprocating optical path In a liquid crystal panel in which the director of molecules is twisted by 90 °, the Jones matrix of the panel for light transmitted in the opposite direction is the transposed inverse matrix of the original matrix, so that the light wave in the reciprocating optical path is The calculation of the amount of phase modulation by the Jones vector is as follows (formula 6).

[0018]

(Equation 6)

At this time, the amount of phase modulation is 2δ, which is twice that in the case of a one-way optical path. Therefore, double phase modulation is possible.

(Experimental Example) In order to confirm the amount of phase modulation by the above calculation by actual measurement, the first polarizing plate was set at 28 °, the angle of the second polarizing plate was set at 75 °, and light was transmitted once through the liquid crystal panel. Actually measured and compared two times from before and after. The results are as shown in FIG. 3. The amount of phase modulation when the liquid crystal panel is passed twice is twice as large as that when the light is transmitted only once, and the measured value agrees well with the theoretical value. I understand.

Next, in order to examine the relationship between the angle of the polarizing plate and the phase difference, the angle of the polarizing plate 18 was fixed at 30 °, and the phase difference when the angle of the polarizing plate 21 was changed was measured and measured by computer simulation. confirmed. As a result, as shown in FIG. 4, when the angle of the polarizing plate 21 is 30 ° and the angle of the polarizing plate 21 is approximately 70 ° to 90 °, the phase modulation capability may be larger than π radian. confirmed.

[0022]

As described above, according to the present invention, by passing the light to be phase-modulated twice through the liquid crystal layer, the amount of phase modulation can be reliably doubled as compared with the case of passing the light once. A phase modulation method that can obtain a large amount of phase modulation even when a liquid crystal panel having a small thickness is used can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of a phase modulation amount adjusting device.

FIG. 2 is a schematic diagram of an optical system of the phase modulation device.

FIG. 3 shows a phase modulation amount for an input video signal.

FIG. 4 is a graph showing the relationship between the amount of phase modulation and the angle of an analyzer.

[Explanation of symbols]

 Reference Signs List 1 phase modulation device 2 liquid crystal panel 3 one surface 4 light incidence device 5 other surface 6 light reflection device 7 laser light source 8 objective lens 11 pinhole 12 lens 13 half mirror 14 lens 15 spatial filter 16 lens 17 mirror 18 polarizing plate 21 Polarizer 22 light

Claims (2)

(57) [Claims] 1. A light beam to be phase-modulated is passed through a first polarizing plate. A long axis direction of liquid crystal molecules is twisted by 90 °.
The nematic type liquid crystal panel is made to enter and transmit from one surface and emit from the other surface through the second deflecting plate , reflect the emitted light, and enter and transmit from the other surface through the same optical path. Characteristic phase modulation method. 2. A twisted nematic liquid crystal panel in which the major axis direction of liquid crystal molecules is twisted by 90.degree., And a light incidence device for causing light to be phase-modulated to enter one surface of the liquid crystal panel through a first deflecting plate. And a light reflection device that reflects light emitted from the other surface of the liquid crystal panel and passed through the second deflecting plate to be incident on the other surface through the same optical path. apparatus.