JPS60172029A - Driving method of optical modulation element - Google Patents

Abstract

PURPOSE:To cause no problem in a liquid crystal display element or a liquid crystal-an optical shutter, and to obtain the titled element having high-speed responsiveness by providing a scanning electrode group and a signal electrode group for forming a picture element at an intersection part, and placing a bistable optical modulation substance on an electric field between said electrode groups. CONSTITUTION:The titled element has a scanning electrode group 32 selected successively and periodically based on a scanning signal, a signal electrode group 33 selected, based on a prescribed information signal, and a bistable optical modulation substance on an electric field, which is held between both the electrodes. Also, it has a phase T<0> having a voltage for giving an electric field of one direction to orient the optical modulation substance to the first stable state in spite of any electric signal, and a phase T for writing an information signal having a voltage for helping to orient said substance again to the second stable state in accordance with an electric signal of the signal electrode. The phase T consists of an information signal phase T<1> and an auxiliary signal phase T<2>, and they apply an electric signal having an opposite voltage polarity, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an optical modulation element, and more particularly to a method for time-divisional driving of an optical modulation element such as a display element or an optical shutter array.

Conventionally, liquid crystal display elements are well known in which a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display images or information. .

The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. However, this display element and its driving method had major and fatal drawbacks as described below.

That is, it is difficult to increase the pixel density or enlarge the screen. Among conventional liquid crystals, most of them are practically used as display elements because they have a relatively high response speed and low power consumption, for example, M, 5ch.
adt and W, He1frich @Applied P
hysics Letters' Vo 18 v
& 4 (1971, 2, 15), P127-128 “Voltage-Dependent Optical
Activity of a Twisted Ne
This type of liquid crystal uses a T N (twisted nematic) type liquid crystal shown in ``MaticLiquid Crystal.'' In this type of liquid crystal, molecules of nematic liquid crystal with positive dielectric anisotropy are twisted in the thickness direction of the liquid crystal layer in the absence of an electric field. On the other hand, when an electric field is applied, positive dielectric anisotropy liquid crystal is used to form a matrix electrode structure in which liquid crystal molecules are arranged in parallel on both electrode surfaces. When the display element is configured by the structure,
A voltage higher than the threshold required to align liquid crystal molecules perpendicularly to the electrode surface is applied to the region where both the scanning electrode and the signal electrode are selected (selection point), and the region where both the scanning electrode and the signal electrode are not selected (the non-selection point) is applied. No voltage is applied to the selected point), so the liquid crystal molecules maintain a stable alignment parallel to the electrode plane. By arranging linear polarizers above and below such a liquid crystal cell in a cross nicol relationship with each other, light does not pass through selected points, but light passes through non-selected points, making it possible to use it as an image element. . However, when a matrix electrode structure is configured, scan electrodes are selected and signal electrodes are not selected in areas or scan electrodes are not selected.
A finite electric field is also generated in the region where the signal electrode is selected (so-called "half selection point"). If the difference between the voltage applied to the selected point and the voltage applied to the half-selected point is sufficiently large, and the voltage threshold required to align liquid crystal molecules perpendicular to the electric field is set to a voltage value in between, the display element will function normally. However, if the number of scanning lines is increased, the effective value for one selection point decreases while scanning the entire screen (one frame). For this reason, when repeated scanning is performed, the effective voltage difference between selected points and non-selected points becomes smaller as the number of scanning lines increases, and as a result, reduction in image contrast and crosstalk can be avoided. This is a serious drawback. This phenomenon is caused by liquid crystals that do not have bistability (the stable state is when the liquid crystal molecules are aligned horizontally with respect to the electrode surface, and they are aligned vertically only while an electric field is effectively applied). ) is essentially an unavoidable problem that arises when driving using the temporal accumulation effect (ie, repeated scanning).

In order to improve this point, voltage averaging method, dual frequency drive method, multiple matrix method, etc. have already been proposed, but all of these methods are insufficient, and it is difficult to increase the screen size and density of display elements. Currently, the number of scanning lines has reached a plateau due to the inability to increase the number of scanning lines sufficiently.

On the other hand, looking at the field of printers, laser beam printers provide electrical image signals in the form of light to electrophotographic photoreceptors in terms of both pixel density and speed, as a means of obtaining hard copies using electrical signals as input. (LBP) is currently the best. However, LBP has the following problems: 1. The device used as a printer becomes large.

2. There are disadvantages such as high-speed driving parts such as polygon scanners, which generate noise and require strict mechanical precision. In order to overcome these drawbacks, a liquid crystal shutter array has been proposed as an element that converts electrical signals into optical signals. However, when providing pixel signals using a liquid crystal shutter array, for example, in order to write pixel signals at a rate of 10 dat/mm within a length of 200 mm, it is necessary to have 2000 signal generating units, each of which In order to provide independent signals to each, it was originally necessary to wire lead wires to send signals to all of the signal generating parts, which was difficult to manufacture.

For this reason, an attempt has been made to provide pixel signals for I LINE (line) in a time-division manner for each row using a signal generating section that is divided into several rows.

This is because by doing so, it is possible to use a common electrode for applying a signal to a plurality of signal generating sections, and the actual number of wiring lines can be significantly reduced. However, in this case, if the number of lines is increased using a liquid crystal that does not have bistability, as is usually done, the signal ON time becomes essentially 1, and the amount of time that can be obtained on the photoreceptor increases. There are disadvantages in that the amount of light decreases and crosstalk problems occur.

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel method for driving an optical modulation element that solves all the problems of conventional liquid crystal display elements or liquid crystal female shutters as described above.

Another object of the present invention is to provide a method for driving an optical modulation element with high-speed response.

Another object of the present invention is to provide a method for driving an optical modulation element having a high density of pixels.

Furthermore, another object of the present invention is to provide a method for driving an optical modulation element that does not generate crosstalk.

Furthermore, another object of the present invention is to provide a novel method for driving a liquid crystal device using a liquid crystal that is bistable with respect to an electric field, particularly a chiral smect C-phase or H-phase liquid crystal that has ferroelectricity. There is a particular thing.

Furthermore, another object of the present invention is to provide a high-speed driving method suitable for an optical modulation element having a high density of pixels and a large screen area.

That is, an object of the present invention is to have a scanning electrode group and a signal electrode group that form pixels at their respective intersections, and an optical modulation material that is bistable with respect to an electric field between the scanning electrode group and the signal electrode group. In a driving method of an optical modulation element having a structure in which a bistable optical modulation material corresponding to a pixel on an Nth scanning electrode is arranged in one stable state, A second phase in which a write signal is applied to the signal electrode group in synchronization with the scanning signal applied to the N
This is achieved by a method of driving an optical modulation element having a third phase, which arranges the bistable optical modulation material corresponding to the pixel on the +1st scanning electrode in one stable state.

In a preferred embodiment of the present invention, a scanning electrode group is sequentially and periodically selected based on a scanning signal, a signal electrode group facing the scanning electrode group and selected based on a predetermined information signal, and a gap between the two electrodes. selected scanning electrodes of an optical modulation element having at least an optical modulation material that is held at a constant temperature and that is bistable with respect to an electric field, the optical modulation material is placed in a first stable state regardless of the electrical signal of the signal electrode. a phase TO having a voltage that provides a unidirectional electric field to be oriented, and a phase in which an information signal is written having a voltage that assists in reorienting the optically modulating material to a second stable state in response to the electrical signal of the signal electrode. T
More preferably, the phase T consists of an information signal phase T1 and an auxiliary signal phase T2, and the phase T
In No. 2, the signal electrode group can be driven by applying an electric signal having a voltage polarity opposite to that of the electric signal applied at the phase TI based on predetermined information.

The bistable optical modulation material used in the driving method of the present invention has a first optically stable state and a second optically stable state with respect to an electric field, and has a different bistable state with respect to an electric field. On the other hand, a liquid crystal having bistability as described above is used.

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic C phase (Smc") or H phase (SmH') liquid crystal having ferroelectric properties is suitable. Regarding the LCD, please refer to “LB JOU”
RNAL DB PHY8IQUELETTER8”
36 (L-69) 1975, “Ferroel
etricLiquid CrystalsJ;
”Applied Physics Letters
”36 (11) 1980 7 Submicro 5e
cond B15tableElectrooptic
Switching in Liquid Crys
talsJ; 1 Solid State Physics” 16 (141) 1981
ferroelectric liquid crystals disclosed in these documents can be used in the present invention.

Specific examples of ferroelectric liquid crystal compounds include decyloxybenzylidene-E'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-E'-amino-2-chlorobutylcinnamate) (
HOBACPC), 4-o-(2-methyl)-butylresorsilitin-4'-octylaniline (MBRA8
).

FIG. 1 schematically depicts an example of a ferroelectric liquid crystal cell. 11 and 1r are In, 03.5n02 and I
A substrate (glass plate) coated with a transparent electrode such as TO (Indium-Tin Oxide), between which a layer 12 is oriented perpendicularly to the glass surface.
C" phase or SmH" phase liquid crystal is sealed. A thick line 13 represents a liquid crystal molecule, and this liquid crystal molecule 13 has a dipole moment in the direction perpendicular to the molecule) 1
4 (Pyo). When a voltage equal to or higher than a certain threshold is applied between the electrodes on the substrate 11 and 1r, the helical structure of the liquid crystal molecules 13 is unraveled, and the liquid crystal molecules 13 can change their alignment direction so that all the dipole moments 14 are oriented in the direction of the electric field. can. The liquid crystal molecules 13 have an elongated shape,
It exhibits refractive index anisotropy in the long axis direction and short axis direction, and therefore, for example, if cross-niffle polarizers are placed above and below the glass surface, it becomes a liquid crystal modulation element whose optical characteristics change depending on the polarity of voltage application. easily understood. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ),
As shown in Figure 2, even when no electric field is applied, the helical structure of the liquid crystal molecules unwinds and becomes a non-helical structure, and the dipole moment P or P' is either upward (24) or downward (24'). take a state. In such a cell, as shown in Figure 2, an electric field E of different polarity above a certain threshold value is applied.
or E', the dipole moment changes its direction upward 24 or downward 24' in response to the electric field E or the electric field vector of E', and accordingly, the liquid crystal molecules are in the first stable state 23 or Orientation to one of the second stable states 23'.

There are two advantages to using such a ferroelectric liquid crystal as a light modulation element. First, the response speed is extremely fast.
Second, the alignment of liquid crystal molecules has bistability.

To explain the second point with reference to FIG. 2, for example, the electric field E
When the voltage is applied, the liquid crystal molecules are aligned in a first stable state 23, and this state remains stable even when the electric field is turned off. Furthermore, when an opposite electric field E' is applied, the liquid crystal molecules enter the second stable state 2.
3' and changes the direction of the molecule, but it remains in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively achieve such fast response speed and bistability, it is preferable for the cell to be as thin as possible, generally 0.5μ to 20μ, especially 1μ.
~5μ is suitable. Using this kind of ferroelectric liquid crystal,
Liquid crystal electro-optical devices having a matrix electrode structure are disclosed, for example, by Clark and Ragabal in US Pat. No. 4,367,924.
It is proposed in the publication.

A preferred embodiment of the driving method of the present invention is shown in FIG.

FIG. 3 is a schematic diagram of a cell 31 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 32 is a scanning electrode group, and 33 is a signal electrode group. For the sake of simplicity, an example will be shown in which a binary signal of white and black is displayed.

In FIG. 3, it is assumed that the pixels indicated by diagonal lines correspond to "black" and the other pixels correspond to "white". Figure 4(a)
and (b) respectively show the electrical signals given to the selected scanning electrode and the electrical signals given to the other scanning electrodes (unselected scanning electrodes), and FIGS. 4(c) and (d)
are the electric signals given to the selected (black) signal electrodes.

Of these, FIG. 4(C) shows the result when the previous signal was black, and FIG. 4(d) shows the result when the previous signal was white. Further, FIGS. 4(e) and 4(f) are electrical signals applied to the unselected (white) signal electrodes, respectively.

Of these, FIG. 4(e) is when the previous signal was black, and FIG. 4(f) is when the previous signal was white. In this figure, To is the phase for aligning all pixels on one scanning electrode to white, and T is the phase for writing information signals. In this example, an example of To: T = Δt is shown. FIG. 5 shows drive waveforms when the display shown in FIG. 3 is performed using these signals. 81 in Figure 5
~S, is the signal applied to the scanning electrode, I, and ■3 are,
The signals A and C applied to the signal electrodes 11 and I3 are voltage waveforms applied to the pixels C and C in FIG. 3, respectively. Now, the threshold voltage at the application time Δt to give the first stable state (this is white) of the liquid crystal cell having bistability is -Vth2, and the second stable state (this is black) Let vth be the threshold voltage at application time Δt to give
Then, the VoLv value is vo<vthl<
2V, , -2Vo<-vth2<-v.

It is set so that

As is clear from FIG. 5, all pixels on one scanning electrode are rewritten to "white". Subsequently, "black" or "white" is specified based on the information, and the pixel corresponding to "black" causes an inversion from "white" to "black", and information is written, but the information is written on this scanning electrode. At the phase (time) in which information is written, all pixels on the next scanning electrode are simultaneously rewritten to white. Therefore, it is possible to write the entire screen at high speed by scanning one frame.

6 and 7 represent another embodiment of the driving method of the invention.

FIGS. 6(a) and 6(b) respectively show electrical signals applied to selected scanning electrodes and electrical signals applied to unselected scanning electrodes. Figures 6(C) to (f) are
It represents the information signal applied to the signal electrode group, and the figure (
C) and (e) make the previous signal black, and (d) and (f)
) makes the previous signal white, the same figure (e) and (
In d), Vo corresponding to black is applied as an information signal,
In (e) and (f) of the same figure, the information signal corresponds to white. is applied.

FIG. 7 shows the driving waveform when obtaining the display shown in FIG. 3. 81 to S5 in FIG. 7 are signals applied to the scanning electrodes, 11 and I3 in FIG. 7 are signals applied to the signal electrodes I, and A and C in FIG. 3 represents the voltage waveform applied to pixels A and C shown in FIG.

Now, the mechanism by which a ferroelectric liquid crystal changes due to an electric field in a bistable state is not necessarily clear from a microscopic perspective, but in general, a strong electric field for a predetermined period of time is applied to a predetermined (first) stable state. If the state is left in a state where no electric field is applied after switching, it is possible to maintain that state almost semi-permanently; Ba,
If an electric field of opposite polarity is applied for a long time even if the voltage is higher than vth, the orientation state will transition again to the opposite (second) stable state, and as a result, Situations may arise in which displaying or modulating superior information is not achievable.

The inventors of the present invention acknowledge that the ease with which the transition of the orientation state (a type of crosstalk) occurs due to the application of such a weak electric field for a long time is affected by the material of the substrate surface, the grain quality, the liquid crystal material, etc. However, we have not yet fully understood the deterministic f.

However, it has been confirmed that when uniaxial substrate processing is performed to orient liquid crystal molecules, such as rubbing or oblique evaporation of SiQ 9, the tendency for the above-mentioned transition to occur tends to increase. It was also confirmed that this tendency was stronger at higher temperatures.

In any case, in order to achieve correct information display and modulation, it is preferable that an electric field in a certain direction be applied over a long distance.

Therefore, the auxiliary signal phase T2 in the more preferred driving method of the present invention is a phase for preventing a weak electric field from continuing to be applied in a certain direction, and a specific example thereof is shown in the eighth example.
As shown in Fig. and Fig. 9.

FIGS. 8(a) and 8(tb) show electrical signals applied to selected scanning electrodes and electrical signals applied to unselected scanning electrodes, respectively. The signal electrode group is shown in Fig. 8fc.
) to (f), the information signal applied at phase 17 -
Signals with different polarities (C) and (d) in the figure correspond to black, and signals with different polarities (e) and (fl correspond to white) in the figure are applied at phase T2.For example, when trying to display the pattern shown in Figure 3, In this case, if a driving method without phase T2 is used, when the scanning electrode S1 is scanned, the pixel A becomes black, but after S2, the electric signal applied to the signal electrode 1 continues to be -■. Since that voltage is applied to the pixel as it is, there is a risk that the pixel will eventually turn white, but by providing the auxiliary signal phase T', it becomes clear from the time series signal shown in Figure 8. As such, there is no risk of crosstalk.

Also, in Figures 8 (C) and (e), the previous signal was black, and the 8th
Figures (d) and (f) represent the case where the previous signal was white.

FIG. 9 shows the driving waveform when obtaining the display shown in FIG. 3. S and -S in FIG. 9 are signals applied to the scanning electrodes, and 1. and ■, are signal electrodes ■1, I, V
Signals A and C in FIG. 9 represent voltage waveforms applied to pixels A and C shown in FIG. 3, respectively.

10 and 11 depict another embodiment of the invention. In this example, between the wing and vth or vth2
(Vth, (3Vo, -3■o(-Vth, (-V
.

The relationship is set as follows. Figure 10 (al and (bl,
It shows electrical signals given to selected scan electrodes and electrical signals given to unselected scan electrodes, respectively.

Now, the optimal time interval of the auxiliary signal phase T2 depends on the magnitude of the voltage applied to the signal electrode in this phase, and is a voltage of opposite polarity to the voltage applied in the information signal phase T1. Generally speaking, if the voltage is large, v
The time interval c is short, and if the voltage is small, it is preferable to make the time interval long, but if the time interval is long, it will take a long time to scan the entire screen.

Therefore, it is preferable to set T2≦TI.

Figure 1O [C) to ridges) represent the information signals applied to the signal electrode group, and the same figure (C1 and (e) are the previous signals in black, the same figure (d) ridges) are the previous signals. When the traffic light turns white,
In the same figure (C) and (d) at phase T1, V corresponding to black is used as an information signal. is applied, and IN (e) and (f) represent a woodcutter in which m-, which corresponds to white, is applied as an information signal.

FIG. 11 shows the driving waveform when obtaining the display ¥i- shown in FIG. 3. 11 in FIG. 11, signals applied to the scanning electrodes, 1. in FIG. Toko work, signal electrode work,
Signals A and C in FIG. 11 represent the voltage waveforms applied to pixels A and C shown in FIG. 3, respectively.

Example 1 About 3001 polyimide films were formed using a spin cord on a set of glass plates in which transparent conductive films (ITO) were patterned to form a 500 x 5000 matrix. Each substrate was subjected to a rubbing treatment using a roller whose surface was wrapped with a terrene cloth, and the substrates were bonded together so that the rapping/greasing directions matched to form a cell. The cell spacing at this time was approximately 1.2 μ. DOBAMBC, which is a ferroelectric liquid crystal, was injected into this cell, heated to a molten state, and then slowly cooled to obtain a uniform monodomain state in the SmC state. The cell temperature was controlled at 70°C, and the voltage was set to 10V based on the driving method shown in Figure 10.
, T0='r'=T'-Δt=50 μsec, and line sequential scanning was performed, and an extremely good image was obtained.

The method of the present invention can be widely applied to optical shanks or display fields such as liquid crystal optical shutters and liquid crystal televisions.

[Brief explanation of the drawing]

FIGS. 1 and 2 are perspective views showing a liquid crystal crystal used in the present invention. FIG. 3 is a plan view of the pole structure used in the driving method of the present invention. FIGS. 4(a) to 4(f) are explanatory diagrams showing waveforms of electrical signals applied to the electrodes. FIG. 5 is an explanatory diagram showing voltage waveforms when voltages are applied in time series. FIGS. 6(a) to 6) are explanatory diagrams showing other waveforms of electrical signals applied to the electrodes. FIG. 7 is an explanatory diagram showing another voltage waveform when voltage is applied in time series. 8th
FIGS. 8(a) to 8(a) are explanatory diagrams showing other waveforms of electrical signals applied to one electrode. FIG. FIG. 9 is an explanatory diagram showing another voltage waveform when voltage is applied in time series. FIGS. 10(a) to 10(f) are explanatory diagrams showing other waveforms of electrical signals applied to the electrodes. FIG. 11 is an explanatory diagram showing another voltage waveform when voltage is applied in time series. Figure 1 1 Fear 2 Figure 1 Figure 4 (b) (e) (C) (F) D00C00-12WestL-1. □ 5th drawing 3 o Go L work m - Drawing 6 rb) (e) (C) (f) (b) (e) (C) (f) Procedural amendment stamp plow January 21, 1985 Manabu Shiga, Commissioner of the Japan Patent Office2, Title of the invention: Driving method for an optical modulation element3, Relationship with the case of the person making the amendment Patent applicant address: 3-30-2 Shimomaruko, Ota-ku, Tokyo Name (100)
) Canon Co., Ltd. Representative: Ryu Kaku, Department 4, Agent address: 3-30-25 Shimomaruko, Ota-ku, Tokyo 146, Drawing 6 subject to amendment, Contents of amendment (1) Figure 3 of the drawing is attached as an attachment. Correct as shown in the drawing.

Claims (8)

[Claims] (1) It has a structure including a scanning electrode group and a signal electrode group that form pixels at each intersection, and an optical modulation material that is bistable with respect to an electric field is arranged between the scanning electrode group and the signal electrode group. In a method for driving an optical modulation element, a first phase for arranging bistable optical modulation substances corresponding to pixels on the Nth scanning electrode in one stable state, a scanning signal applied to the Nth scanning electrode, and A second phase in which a write signal is applied to the signal electrode group in synchronization, and a third phase in which a bistable optical modulation material corresponding to a pixel on the N+1 scanning electrode is arranged in one stable state. Features: Driving method of optical modulation element. (2) The method for driving an optical modulation element according to claim 1, wherein the second phase and the third phase are simultaneous. (3) The method for driving an optical modulation element according to claim 1, wherein the write signal applied at the second phase has an information signal phase and an auxiliary signal phase. (4) The method for driving an optical modulation element according to claim 3, wherein the information signal and the auxiliary signal have different voltage polarities. (5) When the time interval of the information signal phase is TI, and the time interval of the auxiliary signal phase is T2, there is a relationship of T, ≧T2 between T1 and T2. Driving method of the optical modulation element described in 2. (6) The method for driving an optical modulation element according to claim 1, wherein the bistable optical modulation substance is a ferroelectric liquid crystal. (7) The method for driving an optical modulation element according to claim 6, wherein the ferroelectric liquid crystal is a liquid crystal having a chiral smectic C phase or H phase. (8) The method for driving an optical modulation element according to claim 7, wherein the liquid crystal having the chiral smectic C phase or H phase is a liquid crystal phase forming a non-helical structure.