JP2670960B2 - Spatial light modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical address type spatial light modulator, and more particularly to an optical address type spatial light modulator which has a high diffraction efficiency and is suitable for a correlator and a hologram reproducing apparatus.

[0002]

2. Description of the Related Art Optical address type spatial light modulators (hereinafter referred to as
Examples of the "spatial light modulator") include an intensity modulation type spatial light modulator (TNLC-SLM) that uses a twisted nematic liquid crystal in a light modulation section, and a ferroelectric liquid crystal spatial light modulation that uses a ferroelectric liquid crystal. Devices (FLC-SLM) and the like have been conventionally proposed.

[0003]

[Problems to be Solved by the Invention] These conventional TNLCs
The -SLM and FLC-SLM have a problem that their diffraction efficiency is low. Therefore, when these are applied to a correlator, there is a problem that a strong correlation peak cannot be obtained and the S / N ratio becomes low. In particular, in the TNLC-SLM, since the readout light is always subjected to intensity modulation, there has been a problem that the efficiency of the correlator becomes very low. For the same reason, TNLC-SLM and FLC
Even when the SLM is applied to the hologram reproducing apparatus, there is a problem that the efficiency is low.

On the other hand, the present applicant has already filed Japanese Patent Application No. 4-20.
No. 3505 proposes an optical address type spatial light modulator that can obtain phase modulation with a wide dynamic range.

As shown in FIG. 7, in the spatial light modulator 1, a parallel-aligned nematic liquid crystal layer 16 as a light modulator is provided.
Are provided between the pair of alignment layers 15 and 17. A mirror 14 is provided on the side of the alignment layer 15 opposite to the liquid crystal layer 16,
Photoconductive layer 13 as an optical address portion and transparent conductive film 12
And the input side glass substrate 11 is provided in this order. A transparent conductive film 18 is provided on the side of the alignment layer 17 opposite to the nematic liquid crystal layer 16, and the read side glass substrate 1 is provided.
9 and are provided. Here, the rubbing directions of the alignment layers 15 and 17 are the same, and the nematic liquid crystal layer 1
As shown in FIG. 8A, the liquid crystal molecule 6 has a structure in which the liquid crystal molecules are aligned in parallel to the substrates 11 and 19 without being twisted (parallel alignment or homogeneous alignment). For comparison, FIG. 8B shows a twisted nematic alignment structure employed in the light modulation layer of the TNLC-SLM. The photoconductive layer 13 is made of, for example, amorphous silicon (a-S
i), CdS, Bi ₁₂ SiO ₂₀ , organic photoconductor (PV)
K) etc. The mirror layer 14 is made of, for example, a dielectric mirror. The alignment layers 15 and 17 may be produced by coating the mirror 14 and the transparent conductive film 18 with PVA or polyimide and then performing rubbing,
May be formed by oblique evaporation, formation of an LB film, or the like. An alternating drive voltage (amplitude V) is applied from the drive power supply 2 between the pair of conductive films 12 and 18 of the spatial light modulator 1.

The operation of the parallel alignment type nematic liquid crystal spatial light modulator 1 having such a configuration will be described below. When input light is incident on the photoconductive layer 13 via the input-side glass substrate 11, the resistance at that portion decreases according to the input light intensity, and a voltage according to the input light intensity is applied to the liquid crystal layer 16. The liquid crystal molecules move so as to rise with respect to the conductive films 12 and 18 to a degree corresponding to the voltage. As a result, the refractive index for light polarized in the length direction of the liquid crystal molecules (light having a polarization plane indicated by an arrow a in FIG. 8A) changes. However, the refractive index does not change at all for light polarized perpendicular to the length direction of the liquid crystal molecules (light having a polarization plane indicated by an arrow b in FIG. 8A). Therefore, when coherent read light polarized in the length direction of the liquid crystal molecules is incident on the liquid crystal layer 16 via the read side glass substrate 19, the light undergoes phase modulation according to the change in the refractive index. On the other hand, even if the reading light polarized perpendicular to the length direction of the liquid crystal molecules is incident, this is not affected at all.

As described above, since the spatial light modulator 1 can modulate only the phase of the read light polarized in the length direction of the liquid crystal molecules, the TNLC-SLM or the FLC.
It is considered that a higher diffraction efficiency can be obtained as compared with an SLM or the like.

Therefore, an object of the present invention is to use the phase modulation type spatial light modulator 1 to modulate read light with a high diffraction efficiency for various input light inputs. It is to provide a spatial light modulator.

[0009]

The inventors of the present invention have conducted extensive studies and studies on the spatial light modulator 1, and as a result, the spatial light modulator 1 has the characteristics shown in FIGS. 9 and 10 below. I found out what I was doing.

FIG. 9 shows a case where a striped input pattern having a constant spatial frequency is input to the spatial light modulator 1.
When the light intensity (light intensity average) of the input pattern is changed, how the phase change amount read out by the liquid crystal layer 16 changes with respect to the light is changed by a plurality of drive voltage values (more specifically, AC drive voltage). (Amplitude value of V). As is apparent from this figure, when the driving voltage is changed, the input pattern light intensity at which a certain desired phase change amount (△ Φ ₀ ) is obtained changes. That is, it can be seen that a constant phase change amount ΔΦ ₀ can always be obtained by controlling the drive voltage V or / and the input pattern light intensity.

Further, FIG. 10 shows the spatial light modulator 1 when a striped input pattern having a constant spatial frequency is input.
Shows how the diffraction efficiency of the first-order diffracted light changes when the amount of phase change (ΔΦ) given to the read light is changed. As is clear from this figure, the diffraction efficiency is maximum (about 33.9%) when the amount of phase change is 1.18π.

Therefore, it is clear from FIG. 10 that there is a phase change amount at which the maximum diffraction efficiency is obtained for an input pattern having a certain spatial frequency. Further, it is clear from FIG. 9 that an arbitrary amount of phase change can be obtained by controlling the drive voltage and / or the input light intensity. Therefore, it was found that it is possible to drive the spatial light modulator 1 so that the maximum diffraction efficiency can always be obtained with respect to an arbitrary input pattern by controlling the drive voltage and / or the input light intensity.

Based on the above findings, the present invention controls the input light intensity and / or the drive voltage of the spatial light modulator 1 so that the read light is always read with high diffraction efficiency for various input pattern lights. Can be modulated to achieve the above object.

In other words, in order to achieve the above-mentioned object, the spatial light modulator of the present invention has a pair of a light modulating portion made of a photoconductive layer and a light modulating portion made of a liquid crystal layer in which nematic liquid crystal molecules are aligned in parallel and without twist. An optical address type spatial light modulator laminated between transparent electrodes, drive voltage applying means for applying a drive voltage between the pair of transparent electrodes, and input light is applied to the optical address portion. In accordance with the output of the light intensity detecting means for detecting the intensity of the diffracted light formed by the read light applied to the light modulating portion in the state being modulated by the light modulating portion, hand,
The spatial light modulator is provided with a control means for controlling at least one of the intensity of the input light with which the optical address portion is irradiated and the driving voltage.

[0015]

In the spatial light modulator of the present invention having the above structure, the drive voltage applying means applies a drive voltage to the photo-address type spatial light modulator. The optical address section of the optical address type spatial light modulator is irradiated with input light, and the optical modulation section is irradiated with read light. The light modulator modulates the irradiated read light in accordance with the input light. The light intensity detecting means detects the intensity of the diffracted light formed by the modulated reading light. The control means controls at least one of the intensity of the input light and the drive voltage according to the output of the light intensity detection means.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

The present inventors manufactured the spatial light modulator 1 using the materials and methods described below. Glass substrate 1
1 was coated with an ITO (Indium-Tin-Oxide) film to form a transparent conductive film 12. Hydrogenated amorphous silicon (a-Si: H) was formed thereon to a thickness of 5 [μm] to form a photoconductive layer 13. On top of that, SiO ₂ and TiO ₂
Was formed as the mirror layer 14. On the mirror layer 14, SiO was obliquely vapor-deposited to form an alignment layer 15. On the other hand, the glass substrate 19 was also coated with the ITO film to form the transparent conductive film 18. An orientation layer 17 was formed thereon by obliquely depositing SiO. Here, the rubbing direction of the alignment layer 17 was formed in the same direction as the rubbing direction of the alignment layer 15. A space of 8 [μm] is provided between the alignment layers 15 and 17 thus formed.
A cell was formed so as to surround the periphery to a degree, and a nematic liquid crystal (here, “E44” manufactured by Merck) was injected. As a result, a liquid crystal layer 16 having a structure in which liquid crystal molecules were arranged in parallel to the glass substrates 11 and 19 without being twisted was formed.

The present inventors conducted the following experiment in order to investigate the characteristics of the spatial light modulator 1 thus manufactured. First, striped input pattern light was input to the hydrogenated amorphous silicon layer 13, and a refractive index grating was recorded on the liquid crystal layer 16. Uniform readout light was incident on the liquid crystal layer 16, and the light intensity of the + 1st-order diffracted light obtained by subjecting the light to phase modulation in the liquid crystal layer was measured to determine the diffraction efficiency.
Striped input pattern lights having various values of spatial frequency were repeatedly recorded, and each time, the light intensity of the obtained + 1st-order diffracted light was measured to determine the diffraction efficiency. FIG. 11 shows the measurement result.
As is clear from this figure, the spatial light modulator 1 has a maximum diffraction efficiency of 31%, which is close to the theoretical limit of 33.9%.

Next, while the striped input pattern light having a constant spatial frequency is incident on the photoconductive layer 13, the amplitude V of the AC drive voltage and / or the input pattern light intensity is changed to read the liquid crystal layer 16. The phase change amount (corresponding to the depth of the refractive index grating of the liquid crystal layer) given to the emitted light is changed to various values, and the obtained 0th order, + 1st order, and
The light intensity of the + 2nd-order and + 3rd-order diffracted light was measured, and the diffraction efficiency of each was calculated. FIG. 12 shows the measurement result.
The theoretical values calculated on the assumption of Raman-Nass diffraction are also shown. As is clear from this graph, the measurement results are in good agreement with the theoretical values. The measurement result corresponds to FIG. 10.

Next, while applying an AC drive voltage having a constant amplitude to the spatial light modulator 1, a striped input pattern having a constant spatial frequency is input to the spatial light modulator 1 and its light intensity (average light intensity) is calculated. Changed. In this case, it was measured how the amount of phase change given to the read light by the liquid crystal layer 16 changed. This measurement was performed while changing the drive voltage amplitude value V to various values. In addition, as the reading light,
In addition to light polarized in the length direction of liquid crystal molecules, light polarized perpendicular to the length direction of liquid crystal molecules was also used. FIG. 13 shows the measurement result. Note that this measurement result corresponds to FIG. 9. Also, from this figure, it is understood that the phase modulation is not performed on the reading light polarized perpendicularly to the length direction of the liquid crystal molecules.

As is clear from FIGS. 11 to 13, it has been found that the diffraction efficiency can be maximized by adjusting the drive voltage V and the input light intensity.

Further, in the state where an AC drive voltage having a constant amplitude is applied to the spatial light modulator 1, a striped input pattern having a constant light intensity (light intensity average) and a constant spatial frequency is inputted thereto. It was In this state, the light intensity of the read light was changed to various values, and the light intensity of the obtained + 1st-order diffracted light was measured to obtain the diffraction efficiency. For comparison, a spatial light modulator in which the liquid crystal layer 16 is twist-nematically aligned instead of parallel-aligned (that is, TNLC-SLM:
The same experiment was performed for FIG. 8 (B). FIG.
14A shows the + 1st-order diffracted light intensity and diffraction efficiency obtained for the spatial light modulator of the present invention, and FIG. 14B shows TNLC.
The + 1st order diffracted light intensity and the diffraction efficiency obtained for -SLM are shown. As is clear from this result, in the parallel alignment type spatial light modulator of the present invention, unlike the TNLC type, it was found that the diffraction efficiency hardly decreases even when the read light intensity is increased. In other words, the spatial light modulator 1 of the present invention
The read light passes through the mirror layer 14 and the photoconductive layer 13
It was found that there is an advantage that the influence on the characteristics is small even if the light is incident on the. The reflectance of the mirror layer 14 is usually 9
Although it is 9.9% or more, the amount of light cannot be ignored as the read light becomes stronger. Therefore, TNLC-SL
M, CdT between the photoconductive layer 13 and the mirror layer 14
A light shielding layer made of e or the like is provided. However, in the parallel orientation type spatial light modulator 1 of the present invention, as is apparent from the measurement result, the influence of the readout light is not so large, and thus there is no need to provide such a light shielding layer. Therefore, the spatial light modulator 1 of the present invention has advantages that the fabrication thereof is easy, the operation stability is good, and the resolution is not deteriorated. Although there is a type of FLC-SLM in which a light shielding layer is not provided, when driving the same, it is necessary to control the reading light so that the reading light does not enter during the writing operation. On the other hand, the spatial light modulator 1 of the present invention does not require such a complicated system.

From the experimental results detailed above, the parallel alignment type nematic liquid crystal spatial light modulator 1 shown in FIG.
It is considered to have the characteristics of FIGS. 9 and 10.

Therefore, in the present invention, the spatial light modulator 1 having the above-mentioned characteristics is utilized to construct the spatial light modulator 100 as shown in FIG. 1, which can always obtain a substantially maximum diffraction efficiency.

The spatial light modulator 100 of the present invention includes a spatial light modulator 1, a driving power supply 2, a diffracted light intensity detector 3, and a feedback controller 4. Here, the driving power source 2
Is a rectangular wave generator 21 for generating a rectangular wave voltage (AC voltage) having a frequency of 1 [kHz] and an amplitude of 5 [V], and the amplitude of the rectangular wave voltage from the rectangular wave generator 21 is attenuated. And an attenuator 22 for making The diffracted light intensity detector 3 is arranged to detect the light intensity of the diffracted light formed by the phase-modulated reading light in the liquid crystal layer 16. Specifically, the diffracted light intensity detector 3 detects the light intensity of the + 1st order or −1st order diffracted light formed by the phase-modulated read light (hereinafter, referred to as “first-order diffracted light intensity I”). Are arranged as follows. Such diffracted light intensity detector 3
Is, for example, a photodiode. The feedback controller 4 is connected to the diffracted light intensity detector 3 and the attenuator 22 of the driving power supply 2, and feedback-controls the attenuator 22 based on the detected diffracted light intensity value. More specifically, the feedback controller 4 feedback-adjusts the attenuation rate of the attenuator 22 based on the detected diffracted light intensity value to adjust the amplitude of the rectangular wave voltage applied to the spatial light modulator 1.

In the spatial light modulator 100 having such a configuration,
The feedback controller 4 feedback-controls the drive power source 2 by the so-called hill climbing method, and the first-order diffracted light intensity I
Oscillates around its maximum value. The operation of the spatial light modulator 100 will be described below with reference to FIGS.
This will be described in detail with reference to the flowchart of FIG. Input pattern light is incident on the photoconductive layer 13 of the spatial light modulator 1 through the input side glass substrate 11. Further, the reading light is incident on the liquid crystal layer 16 through the reading side glass substrate 19. The read light is coherent light in a linearly polarized state that is polarized in the length direction of the liquid crystal molecules of the liquid crystal layer 16. In this state, the feedback controller 4 first performs an initial setting, and sets a predetermined value V as an amplitude V ₁ of a rectangular wave of a first drive described later,
A predetermined value ΔV is set as the _first adjustment value ΔV _{1 which} will be described later (step S1). Next, the feedback controller 4 adjusts the attenuation rate of the attenuator 22 so that the rectangular wave voltage having the amplitude V ₁ (= V) is applied to the spatial light modulator 1. That is, the first drive of the spatial light modulator 1 is executed (step S2). As a result, the liquid crystal molecules of the liquid crystal layer 16 rise according to the intensity distribution of the input pattern, and the refractive index for light polarized in the length direction of the liquid crystal molecules changes. That is, the input pattern is written in the liquid crystal layer 16. The read light that has entered the liquid crystal layer 16 undergoes phase modulation according to the input pattern, is reflected by the mirror layer 14, and exits from the read-side glass substrate 19 again. The emitted read light forms a first-order diffracted light according to the input pattern. The diffracted light intensity detector 3 detects the light intensity I ₁ of the first-order diffracted light and outputs a signal indicating the detection result to the feedback controller 4. The feedback controller 4 stores the detected value I ₁ (step S3). Then, the feedback controller 4 re-adjusts the attenuation rate of the attenuator 22 so that the rectangular wave voltage having the amplitude of V ₂ = V ₁ + ΔV ₁ (= V + ΔV) is applied to the spatial light modulator 1. The second drive is executed (steps S4 and S5). Like the above, the resulting light intensity I ₂ of the first-order diffracted light is detected by the diffracted light intensity detector 3, the feedback controller 4 stores a detected value I ₂ (step S6). The feedback controller 4 compares the stored detection values I ₁ and I ₂ (step S7).
If I ₁ <I ₂ , the feedback controller 4 sets m = 2 in step S8 and sets ΔV ₂ = (− 1) ^m (ΔV ₁ ) = (− 1) as the _second adjustment value ΔV _2. ) ² (ΔV ₁ ) =
Set ΔV ₁ (= ΔV) (steps S10 and S1)
1). That is, ΔV ₂ is set equal to ΔV ₁ . After that, the feedback controller 4 readjusts the attenuation rate of the attenuator 22 so that the spatial light modulator 1 has V ₃ = V ₂ + ΔV ₂ (= V ₂ +
The rectangular wave voltage having the amplitude of ΔV ₁ = V ₂ + ΔV) is applied, and the third driving is executed (step S
5). On the other hand, if I ₁ > I ₂ , the feedback controller 4
Sets m = 1 in step S9 and sets ΔV ₂ = (− 1) ^m (ΔV ₁ ) = (− 1) ¹ as the _second adjustment value ΔV _2.
(ΔV ₁ ) = − ΔV ₁ = −ΔV is set (step S1)
0, 11). That is, ΔV ₂ is set so that the absolute value is equal to ΔV ₁ and the sign is opposite. Then, the feedback controller 4 resets the attenuation rate of the attenuator 22, and V ₃ = V ₂ + ΔV ₂ (= V ₂ − (ΔV ₁ ) = V ₂ − in the spatial light modulator 1.
A rectangular wave voltage with an amplitude of ΔV) is applied,
The third driving is executed (step S5).

In this way, the feedback controller 4
Is a I _i-1 obtained by the driving employing the rectangular wave amplitude _{V i-1 (i-1} ) -th amplitude _{V i (= V i-1} + Δ
I _i obtained by the i-th drive employing the rectangular wave of V _i−1 ).
(Step S7), and if I _i becomes larger than I _i-1 , the amplitude V _{i +1} (= V _i + ΔV _i ) to be adopted in the (i + 1) th driving is determined. ΔV _i is set to the same sign as ΔV _i−1 (steps S8, S10, S11), and I
who _i is the [Delta] V _i-1 and [Delta] V _i if became smaller than I _i-1 is set to the inverse code (step S9, S1
0, S11). When the spatial light modulator 1 is repeatedly driven while feedback controlling the amplitude of the rectangular wave by the hill climbing method, the first-order diffracted light intensity value I oscillates near its maximum value. Therefore, the spatial light modulator 1 can be feedback-controlled so that the diffraction efficiency thereof becomes substantially maximum.

The smaller the absolute value ΔV of the adjustment value ΔV _i , the better. This is because the first-order diffracted light intensity I can be brought closer to the maximum value. To approximate the further maximum value of the primary diffracted light intensity I, it is preferable to set the absolute value of the adjustment value [Delta] V _i as follows without a fixed value [Delta] V.
That is, in step S7, the absolute value | I _i −I _i−1 | of the difference between the detected values I _i and I _i−1 is obtained, and in step S11, the absolute value | I _i − of the adjustment value ΔV _i is calculated. It may be set to a value proportional to I _i-1 |. That is, ΔV _i = (− 1) ^m · α · |
It may be set so that I _i −I _i−1 |. (Note that α
Is a constant of proportionality. )

In the above embodiment, the diffracted light intensity detector 3 is arranged so as to detect the first-order diffracted light intensity.
You may arrange so that the 0th-order diffracted light intensity (henceforth "0th-order diffracted light intensity I '") may be detected. In this case,
The feedback controller 4 uses the hill-climbing method so that the 0th-order diffracted light intensity I ′ oscillates near its minimum value.
Should be controlled. The operation is shown in the flowchart of FIG. As is clear from FIG. 3, in this case, the feedback controller 4 adopts a rectangular wave having an amplitude V _i−1 (i
-1) I ′ _i−1 obtained by the first drive and amplitude V
_i (= V _i-1 + ΔV _i-1 ) is compared with I ′ _i obtained by the i-th drive using the square wave (step S7).
If I ′ _i becomes smaller than I ′ _i−1 , (i +
1) Amplitude V _{i + 1} (= V _i + ΔV _i ) adopted in the first driving
The [Delta] V _i to determine the [Delta] V _i-1 and the same reference numerals (step S
8, S10, S11), and if I ′ _i becomes larger than I ′ _i−1, ΔV _i may be set to the opposite sign to ΔV _i−1 (steps S9, S10, S11). By such adjustment, the spatial light modulator 1 can be controlled so that the diffraction efficiency thereof becomes substantially maximum.

In the spatial light modulator 100, the feedback controller 4 feedback-controls the drive voltage of the spatial light modulator 1. However, as shown by the dotted line in FIG. 1, the feedback controller 4 may be connected to the input pattern light source 5 to perform feedback control of the input pattern light intensity. Also in this case, the intensity of the input pattern light may be adjusted by the so-called hill climbing method shown in FIGS. Further, such feedback control of the input pattern light intensity may be performed together with the feedback control of the drive voltage.

As described above, the spatial light modulator 10 of the present invention
According to 0, it is possible to always maintain the diffraction efficiency of the spatial light modulator 1 at substantially the maximum for various input pattern lights.

An application example of the spatial light modulator 100 of the present invention will be described below.

FIG. 4 shows a joint transform correlator (JT) to which the spatial light modulator 100 is applied.
C) 200 is shown. The congruential conversion type correlator is for obtaining a correlation value between two input patterns. This joint conversion type correlator 200 employs the two spatial light modulators 100 of the present invention (hereinafter, referred to as "spatial light modulators 100A and 100B"). Here, the spatial light modulator 100A is of a type (type of FIG. 3) that detects the 0th-order diffracted light intensity I ′ and performs feedback control so as to minimize this, and the spatial light modulator 100B is the first-order diffracted light intensity. This is a type (type of FIG. 2) in which I is detected and feedback control is performed so as to maximize the value.

Regarding the congruential conversion type correlator 200,
The details will be described below. In the congruential conversion type correlator 200,
The spatial light modulator 1 of the spatial light modulator 100A (hereinafter referred to as "sky"
Inter-optical modulator 1A ”), the input side
Two input patterns P through the lath substrate 11₁・ P_TwoThe same
When the liquid crystal layer 16 has a pattern P₁And P_Two(Details
Or its intensity distribution). Liquid crystal molecules of liquid crystal layer 16
Read-out light in the linearly polarized state polarized in the length direction of
Space light through the half mirror 201.
The reading side glass substrate 19 of the modulator 1A is irradiated. liquid
Pattern P in crystal layer 16₁・ P_TwoSubject to phase modulation
The emitted reading light was transmitted through the half mirror 201.
Afterwards, the Fourier transform lens 202 spatially Fourier transforms
And the pattern P on the Fourier transform plane ₁・ P_TwoFu
Rie pattern (hereinafter referred to as the "first Fourier pattern"
U) P_fImage. Spatial light modulation on the Fourier transform plane
The spatial light modulator 1 of the device 100B (hereinafter, "spatial light modulator"
1B ”) is arranged. Therefore, the first
Fourier pattern P_fIs the input side of the spatial light modulator 1B
An image is formed on the photoconductive layer 13 through the substrate 11. This result
As a result, the first Fourier pattern P is formed on the liquid crystal layer 16._f(More details
Or its intensity distribution, that is, the pattern P₁, P_TwoPower
Spectrum) is written. Fourier transform lens 2
02 and the spatial light modulator 1B include a half mirror 203.
Is provided and the first Fourier pattern P_f0th light
The component is the diffracted light intensity detector 3 of the spatial light modulator 100A.
An image is formed on (hereinafter referred to as "diffracted light intensity detector 3A")
You. Feedback controller 4 (hereinafter referred to as "feedback"
Controller 4A ") is detected by the diffracted light intensity detector 3A.
First Fourier pattern P_fThe 0th order light component I'of
So that it is almost minimum, that is, the diffraction of the spatial light modulator 1A
Drive power supply 2 (hereinafter referred to as "drive power") is used to maximize efficiency.
Source 2A ").

The reading light (coherent light) in a linearly polarized state polarized in the length direction of the liquid crystal molecules of the liquid crystal layer 16 is applied to the reading side glass substrate 19 of the spatial light modulator 1B via the half mirror 204. To do. This read-out light undergoes phase modulation in the liquid crystal layer 16 according to the first Fourier pattern P _f , then exits, passes through the half mirror 204, and is spatially Fourier transformed by the Fourier transform lens 205. The first Fourier pattern P on the Fourier transform plane
_{An image} of a Fourier pattern of _f (hereinafter referred to as a second Fourier pattern) P _f 'is formed. Here, the second Fourier pattern P _f ′ is composed of 0th-order light corresponding to the intensities of the patterns P ₁ and P ₂ and + first-order light and −1st-order light arranged symmetrically with respect to the 0th-order light. The image formation positions of the + 1st-order light and the -1st-order light correspond to the relative positions of the patterns P ₁ and P ₂ , and the light intensity thereof corresponds to the correlation between the patterns P ₁ and P ₂ . A diffracted light intensity detector 3 (hereinafter referred to as “diffracted light intensity detector 3B”) of the spatial light modulator 100B is arranged at a position on the Fourier transform surface where the −first-order light is imaged. Feedback controller 4 (hereinafter referred to as “feedback controller 4
B ”) is such that the −1st-order light component I of the second Fourier pattern P _f ′ detected by the diffracted light intensity detector 3B becomes substantially maximum, that is, the diffraction efficiency of the spatial light modulator 1B is substantially the same. Drive power supply 2 (hereinafter, “drive power supply 2
B ”) is feedback-controlled. On the other hand, another diffracted light intensity detector 206 is arranged at a position on the Fourier transform surface where the + 1st order light is imaged. Detector 206
Is a photodiode, for example, and detects the + 1st-order light intensity and outputs a correlation signal indicating the correlation between the patterns P ₁ and P ₂ .

The congruential conversion type correlator 20 having the above configuration
According to 0, since the diffraction efficiency of the two employed spatial light modulators 1A and 1B is always controlled to the maximum, a strong correlation peak can be obtained. Therefore, the diffracted light intensity detector 206 can obtain a correlation signal with a high S / N ratio. Further, since the diffraction efficiency of the spatial light modulator can be always maximized for various patterns P ₁ and P ₂ , stable measurement with less noise can be performed.

In the spatial light modulators 100A and 100B of the above-mentioned congruential conversion type correlator 200, the drive voltage of the spatial light modulators 1A and 1B is feedback-controlled based on the detected diffracted light intensity. However, as shown by the dotted line in FIG. 4, the input patterns P ₁ and P ₂ are based on the diffracted light intensity.
The light intensity of may be feedback adjusted.
The present inventors construct a congruential conversion type correlator 200 using the spatial light modulator 1 manufactured as described above, and irradiate it with patterns P ₁ and P ₂ of various light intensities, At that time, the + 1st order light intensity (correlation signal) obtained by the diffracted light intensity detector 206 was detected. FIG. 5 shows the experimental results. (Note that
The vertical axis of the figure is the normalized + 1st order diffracted light intensity value with its maximum value being 1.0. From this figure, it can be seen that the magnitude of the + 1st order light intensity (correlation signal) changes according to the intensity of the input pattern to be irradiated. Therefore, it can be seen that it is extremely effective to adjust the light intensity of the input pattern by feedback.

FIG. 6 shows a spatial light modulator 100 of the present invention.
Hologram reproducing apparatus 30 as another application example of (FIG. 1)
0 is shown. This hologram reproducing device 300
Is a CGH (Comput
er Generated Hologram) A device for reproducing patterns. The CRT 302 is connected to the computer 301 and displays the CGH pattern. The lens 303 forms an image of the CGH pattern on the photoconductive layer 13 of the spatial light modulator 1 via the input side glass substrate 11. As a result, the CGH pattern is written into the liquid crystal layer 16 and the CGH pattern is written.
A phase hologram corresponding to the pattern is formed. The coherent reading light in the linearly polarized state, which is polarized in the length direction of the liquid crystal molecules of the liquid crystal layer 16, is reflected by the half mirror 304 and guided to the reading side glass substrate 19 of the spatial light modulator 1. This read light is CGH in the liquid crystal layer 16.
After undergoing phase modulation according to the pattern, the light is emitted, transmitted through the half mirror 304, and spatially Fourier transformed by the Fourier transform lens 305. As a result, the 0th order light, which is the Fourier pattern of the CGH pattern, and the hologram reproduction image are obtained on the Fourier transform surface. Spatial light modulator 10
The zero diffracted light intensity detector 3 is arranged at a position where the zero-order light forms an image. The feedback controller 4 performs the control shown in FIG. 3 based on the detection result of the detector 3 so that the diffraction efficiency of the spatial light modulator 1 becomes substantially maximum.

According to the hologram reproducing apparatus 300 having such a structure, the spatial light modulator 1 is controlled so that the diffraction efficiency thereof is always maximized, so that the hologram can be reproduced efficiently.

As shown in FIG. 6, the spatial light modulator 1
It is considered that the spatial frequency of the CGH pattern written in is constant. Therefore, the control corresponding to the experimental results of FIGS. 12 and 13 can be performed.

Further, in the spatial light modulator 100 of the hologram reproducing apparatus 300, the drive voltage of the spatial light modulator 1 is feedback-controlled based on the detected diffracted light intensity. However, as indicated by a dotted line in FIG. 6, the light intensity of the CGH pattern may be feedback-adjusted based on the diffracted light intensity.

The present invention is not limited to the spatial light modulator of the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. It should be noted that the spatial light modulator of the present invention is not limited to the spatial light modulator experimentally produced by the present inventors, but it goes without saying that a parallel alignment type nematic liquid crystal spatial light modulator can be generally applied. Is.

[0043]

As is clear from the above description, in the spatial light modulator of the present invention, the light address portion composed of the photoconductive layer and the light composed of the liquid crystal layer in which the nematic liquid crystal molecules are arranged in parallel and without twisting. The drive voltage applying means applies a drive voltage between the pair of transparent electrodes of the photo-addressable spatial light modulator in which the modulator is laminated between the pair of transparent electrodes. The optical address section of the optical address type spatial light modulator is irradiated with input light, and the optical modulation section is irradiated with read light. The light modulator modulates the irradiated read light in accordance with the input light. The light intensity detecting means detects the intensity of the diffracted light formed by the modulated reading light. The control means controls at least one of the intensity of the input light and the drive voltage according to the output of the light intensity detection means. Therefore, according to the spatial light modulator of the present invention, it is possible to constantly modulate the read light with high diffraction efficiency for various input lights.

[Brief description of the drawings]

FIG. 1 is a plan view of an optical system showing a spatial light modulator of the present invention using the phase modulation type spatial light modulator of FIG.

FIG. 2 is a flowchart showing a control operation in which a feedback controller 4 of the spatial light modulator of FIG. 1 performs feedback control so that the intensity of the first-order diffracted light becomes substantially maximum.

FIG. 3 is a flowchart showing a control operation in which a feedback controller 4 of the spatial light modulator of FIG. 1 performs feedback control so that a zero-order diffracted light intensity becomes substantially minimum.

4 is a plan view of an optical system showing a joint conversion type correlator to which the spatial light modulator of FIG. 1 is applied.

5 is a graph showing the relationship between the pattern light intensity and the correlation signal value in the congruential conversion type correlator shown in FIG. 4 produced by the present inventors.

6 is a plan view of an optical system showing a hologram reproducing device to which the spatial light modulator of FIG. 1 is applied.

FIG. 7 is a schematic cross-sectional view showing the structure of a phase modulation type spatial light modulator having a parallel alignment nematic liquid crystal layer as a light modulation unit, which is used in the spatial light modulator of the present invention.

FIG. 8A is an explanatory diagram showing a state of alignment of liquid crystals in the parallel alignment nematic liquid crystal layer, and FIG. 8B is an explanatory diagram showing a state of alignment of liquid crystals in the twisted nematic liquid crystal layer.

9 is an input / output characteristic of the phase modulation spatial light modulator of FIG.
That is, it is a graph showing the relationship between the input light intensity, the drive applied voltage, and the amount of phase change given to the read light by the optical modulator when the striped input pattern light having a constant spatial frequency is incident on the optical address portion. .

10 is a phase change amount given to the read light by the optical modulator when the striped input pattern light having a constant spatial frequency is incident on the optical address portion of the phase modulation spatial light modulator of FIG. It is a graph which shows the relationship with the obtained diffraction efficiency.

11 is a graph showing the spatial frequency dependence of the diffraction efficiency of the phase modulation type spatial light modulator of FIG. 7 produced by the present inventors.

12 is a graph showing the relationship between the amount of phase change given to the read light by the light modulator of the phase modulation type spatial light modulator of FIG. 7 manufactured by the present inventors and the diffraction efficiency.

13 is an input / output characteristic of the phase modulation type spatial light modulator of FIG. 7 manufactured by the present inventors, that is, input light when a striped input pattern light having a constant spatial frequency is incident on an optical address part. 7 is a graph showing the relationship between the intensity, the drive applied voltage, and the amount of phase change given to the light read out by the light modulator.

14A is a graph showing the diffraction efficiency and the + 1st-order diffracted light intensity with respect to the read light intensity of the phase modulation spatial light modulator of FIG. 7 manufactured by the present inventors, and FIG. 7 is a graph showing the diffraction efficiency and the + 1st order diffracted light intensity with respect to the read light intensity of the twisted nematic spatial light modulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Spatial light modulator 2 Drive power supply 3 Diffracted light intensity detector 4 Feedback controller 100 Spatial light modulator 100A, 100B Spatial light modulator 200 Congruent conversion type correlators 202, 205 Fourier transform lens 300 Hologram reproduction device 305 Fourier transform lens

Front Page Continuation (72) Inventor Terunari Hori 1 Hamamatsu Photonics Co., 1126 Nomachi, Hamamatsu City, Shizuoka Prefecture (72) Inventor Yuji Kobayashi 1 1126 1 Nono, Hamamatsu City, Shizuoka Prefecture Hamamatsu Photonics Co., Ltd. ( 72) Inventor Tsutomu Hara 1 1126 Ichinomachi, Hamamatsu City, Shizuoka Prefecture Hamamatsu Photonics Co., Ltd. (56) Reference JP-A-2-29631 (JP, A) JP-A-2-47072 (JP, A) JP 54-89759 (JP, A)

Claims (1)

(57) [Claims] 1. A photo-addressable spatial light modulator in which a photo-addressing part consisting of a photo-conductive layer and a photo-modulating part consisting of a liquid crystal layer in which nematic liquid crystal molecules are arranged in parallel and without twist are laminated between a pair of transparent electrodes. Drive voltage applying means for applying a drive voltage between the pair of transparent electrodes, and the read light radiated to the optical modulator while the input light is radiated to the optical address portion. Light intensity detection means for detecting the intensity of diffracted light formed by being modulated in the light modulation portion, and the light address portion of the spatial light modulator is irradiated with the light intensity detection means according to the output of the light intensity detection means. A spatial light modulator comprising: a control unit for controlling at least one of the intensity of input light and the driving voltage.