JPS61140924A - Driving method of optical modulation element - Google Patents

Abstract

PURPOSE:To realize bistability and high speed responsiveness by making the polarity of an electric signal supplied to a scanning line and a data line by an erasing step opposite in polarity to the polarity of a scan selecting signal at write stepping, respectively. CONSTITUTION:In order to put a picture plane in order in one direction, a signal of a prescribed voltage pulse is applied to all scan electrode groups, and a signal of a prescribed pulse is supplied to all signal electrode groups. That is to say, by an erasing step, an electric signal of the opposite polarity to a scan selecting signal at write stepping, and an electric signal of the opposite polarity to an information selecting signal at write stepping are applied synchronously to the scan electrode group and the signal electrode group, respectively. When an electric field E or E' each different in polarity and exceeding a prescribed threshold value is supplied, a dipole moment changes its direction in accordance with an electric field spectrum of the electric field E or E', and in accordance with the result, a liquid crystal molecule is oriented in one direction. An advantage for using such a ferroelectric liquid crystal as an optical modulation element is first a fact that a response speed is extremely high, and secondly a fact that an orientation of the liquid crystal molecule has bistability.

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.

Traditionally, scanning lines and data lines are configured in a matrix,
2. Description of the Related Art Liquid crystal display elements are well known in which a liquid crystal compound is filled between electrodes to form a large number of pixels to display images or information. As a driving method for this display element, time-division driving is adopted, in which address signals are selectively and periodically applied to the scanning lines, and predetermined information signals are selectively applied in parallel to the data lines in synchronization with the address signals. However, this display element and its driving method had major and fatal drawbacks as described below.

In other words, it is important to increase the pixel density or increase the screen size. Among conventional liquid crystals, most of them are practically used as single display elements because of their relatively high response speed and low power consumption, for example, M, 8ch.
dt and W, written by He1frich” Applied P
hysics Letters ” vo isl /
l64 (1971, 2, 15), P127-128' Voltage-Dependen tOpti
cal Activity of a Tvriste
d Namatic Liquid Crystal”
K shown fi T N (twistednemaNc
) type of liquid crystal, and this liquid crystal has a positive dielectric anisotropy in the absence of an electric field.The liquid crystal molecules form a structure (helical structure) twisted in the thickness direction of the liquid crystal layer.
The molecules of this liquid crystal form a structure arranged in parallel on both poles. On the other hand, when an electric field is applied, nematic liquid crystals with positive dielectric anisotropy are aligned in the direction of the electric field, resulting in light modulation. When a display element is constructed with a matrix electrode structure using this type of liquid crystal, the area (selection point) K where both the scanning line and the data line are selected is equal to or larger than the threshold required to align the liquid crystal molecules perpendicularly to the electrode surface. A voltage is applied to the area where neither the scanning line nor the data line is selected (unselected point), and therefore 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 create a high-picture element. Become. However, when a matrix electrode structure is configured, there is a finite area in which scanning lines are selected and data lines are not selected, or in areas where scanning lines are not selected and data lines are selected (so-called "half-selected points"). An electric field of If so, one display element will operate normally, but
When the number of scanning lines (N) is increased to 9, the time during which an effective electric field is applied to the selected point while scanning the entire screen (one frame) (duty ratio) decreases at the rate of N. I'm done. Due to this rounding, the effective voltage difference between selected points and non-selected points when scanning is repeated becomes smaller as the number of scanning lines increases, resulting in a decrease in contrast and crosstalk. This is a drawback that is difficult to avoid. 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 often arises when driving using the temporal accumulation effect (ie, repeated scanning). In order to improve this point, methods such as the button, voltage averaging method, two-frequency drive method, and multiple -f+7x method have already been proposed, but all of these methods are insufficient and strange, and it is difficult to increase the screen size of the display element. At present, the trend toward higher density has reached a plateau unless the number of scanning lines can be sufficiently increased.

On the other hand, looking at the field of printers, it is used as a means to obtain hard copies using electrical signals as input.

Laser beam printers (LBPs), which apply electrophotographic signals in the form of light to an electrophotographic photoreceptor, are currently superior in terms of both pixel density and speed. However, LBP has the following problems: 1. The device used as a printer becomes large.

2. It has the following drawbacks: It has high-speed moving parts such as a polygon scanner, which generates noise and requires 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 IQ dat/mm within a length of 2QQmm, it is necessary to have 2000 signal generating sections. In order to provide independent signals to each of the K, it was originally necessary to wire lead wires to send signals to each signal generating section, which was difficult to manufacture.

Therefore, an attempt has been made to provide xLINE (line) worth of pixel signals to each row in a time-divided manner using a signal generating section divided into several rows.

This is because it is possible to use a common signal generator for a plurality of signal generating portions of the electrode that provides a signal, and the actual number of wiring lines can be significantly reduced. However, in this case, if the number of lines (N) is increased using a liquid crystal that does not have bistability, as is usually done, the signal ON time becomes essentially N, and the amount of time that can be obtained on the photoreceptor increases. There is a problem in that a cyclotalk problem occurs in which the amount of light is reduced.

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

Another object of the present invention is to provide a method for driving a liquid crystal element having high-speed response.

Another object of the present invention is to provide a method for driving a liquid crystal device having high density pixels.

That is, an object of the present invention is to drive an optical modulation element having a structure in which a scanning line and a data line are arranged, and an optical modulation material having bistability with respect to an electric field is disposed between the scanning line and the data line. The method includes an erasing step of applying a voltage signal between the scanning line and the data line to align the bistable optical modulating material to a first stable state, and a scanning selection signal is applied to the scanning line. a writing step of applying an information selection signal to the data line in synchronization with the scanning selection signal to orient the optical modulation substance in a second stable state according to the information; and the erasing step This is achieved by a method of driving the optical modulation element in which the polarities of the electrical signals applied to the scanning line and the data line are respectively opposite to the polarity of the scanning selection signal and information selection signal during the writing step.

The optical modulation material used in the driving method of the present invention has a bistable state consisting of a first optically stable state and a second optically stable state with respect to the electric field.

In particular, a liquid crystal having bistability as described above with respect to the Kfi field is used.

As the liquid crystal having bistable properties that can be used in the driving method of the present invention, a chiral smectate C phase (SmC) or H phase (8 mH") liquid crystal having ferroelectricity is suitable. Regarding dielectric liquid crystals, see “I,E JO
URNAL DB PHYSIQUELETTER8″
36 (L-69) 1975. [Ferroelect
ricLiquid Crystals J: ”A
pplied Physics Letters”36
(11) 1980 “8ubmicro 5eco
nd B15tableElectrooptic S
witching in Liquid Crystal
als':0 Solid State Physics "16 (141) 1981r Liquid Crystals" and the like. In the present invention, the ferroelectric liquid crystals disclosed therein can be used.

Furthermore, in the present invention, in addition to the above-mentioned 8mC" and 8mH", chiral smectic I phase (8mI) and J phase (Sm
J), G phase (8mG"), F phase (SmF), and carbon phase (8+nK") can be used.

Figure 1 schematically depicts an example of a ferroelectric liquid crystal cell.
-t' exists. 11 and 11' are Ir1203e 5n0
A transparent substrate (glass plate) coated with 2 or ITO (Indium-Tin Oxide), between which a liquid crystal of 8mC "phase" or 8mH" phase with layer 12 oriented perpendicular to the glass surface. It is enclosed.

A thick line 13 represents a liquid crystal molecule, and this liquid crystal molecule 13 has a dipole moment 14 (P) in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 11 and 11, the liquid crystal molecules 1
The orientation direction of the liquid crystal molecules 13 can be changed so that the helical structure of the liquid crystal molecules 3 is unraveled and all of the dipole moments 14 are oriented in the direction of the electric field. The liquid crystal molecules 13 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the It is easily understood that this results in a liquid crystal modulation element whose optical properties change. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), K
As shown in Figure 2, even when no electric field is applied, the helical structure of the liquid crystal molecules unravels (non-helical structure), and its dipole moment) P or P' points upward (24) or downward (24'). take either of the following states. In such a cell, as shown in Fig. 2, a different electric field E of the other pole above a certain threshold value is applied.
or E' is given, 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 the liquid crystal molecules change to the first stable state 23 or to the downward direction 24' accordingly. 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. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. To explain the second point with reference to FIG. 2, for example, when an electric field E is applied, the liquid crystal molecules enter the first stable state 2.
3Vc orientation, and this state is stable even when the electric field is turned off. When an electric field E' in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 23' and change their orientation, but they remain 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.

It is preferable for cells such as K, which can effectively achieve high response speed and bistability, to be as thin as possible, and generally 0.5μ to 20μ, particularly 0.5μ to 5μ is suitable. ing. A liquid crystal electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal has been proposed, for example, in US Pat. No. 4,367,924 by Clark and Ragabal.

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

FIG. 3A is a schematic diagram of a cell 31 having a matrix pixel structure in which a bistable optical modulation material is sandwiched between scanning lines (scanning electrode group) and data lines (signal electrode group). 32
is a scanning electrode group, and 33 is a signal electrode group. now,
To simplify the explanation, an example will be shown in which black and white binary signals are displayed. In FIG. 3(A), it is assumed that the hatched pixels in K correspond to "black"J&C1 and the other pixels are "white". First, to align the screen to "white" K (this step is referred to as an erase step), K may align the bistable optically modulating material to a first stable state. For this purpose, a predetermined voltage pulse (for example +
2Vo, time width Δt), and apply a predetermined pulse signal (for example, voltage -Vo, Δt) to all signal electrode groups. That is, in this erasing step, an electric signal of opposite polarity to the scanning selection signal during the writing step to be completed is applied to the scanning electrode group, and an electric signal of opposite polarity to the information selection signal (IliF writing signal) during the writing step is applied to the signal electrode group. The air signals are applied in synchronization with each other.

Fig. 3 (B)-(a) and (B)-(b) are electrical signals given to selected scanning electrodes (scanning selection signals), respectively.
Figures 3 (B)-(C) and (
B)-(d) are each selected (these are black)
It represents the electric signal (information selection signal) applied to the signal '1 pole and the electric signal applied to the unselected (white) signal electrode.

Figure 3 (B)-(a) to (d) The horizontal axis represents time, respectively.
The vertical axis represents voltage. T1 and T2 are each an information signal (
and the scanning signal) are applied and the auxiliary signal is applied. In this example, an example of T□=T2=Δt is shown.

The scanning electrode groups 32 are sequentially selected. Now, the threshold voltage at the application time Δt to give the first stable state (white) of the liquid crystal cell having bistability is -vth2, and the threshold voltage at the application time Δt to give the second stable state (black) to the bistable liquid crystal cell is -vth2. The threshold voltage of v
thl, the hypochondriacal signal given to the selected scanning electrode has a phase (time) as shown in Fig. 3(B)-(a)K.
The voltage is -2vo at T1 and 0 at phase (time) T2. In addition, for other scans and one pole, the third
As shown in Figures (B)-(b), it is in a grounded state and the electrical signal is O. On the other hand, the electric signal applied to the selected signal electrode is vo at phase t1 and -vo at phase t2VC, as shown in FIGS. 3(B)-(C), and is applied to the unselected signal electrode. The electrical signal is -v at phase T□ as shown in Figure 3 CB)-(ψ
o and +vo at phase Tz. In the above,
The voltage value vo is vo<■th□<3v0 and -vo>
Vthz > -3Vo is satisfied and set to the desired value.

FIG. 3(C) shows the voltage waveform applied to each pixel when such an electric signal is applied.

Figure 3 (C) In K, (a) and (b) are on the selected scanning line, respectively, and the pixels to be displayed "black" and "white" are 1 or (C) and (d ) are voltage waveforms applied to pixels on each unselected scanning line.

In the first phase T1, the information selection signal 0vo is applied to a pixel on the scanning line to which the scanning selection signal -2Vo is applied and should be displayed as "black", and this rounding exceeds the threshold voltage vth1 for this pixel.・One pressure 3vo is applied, the bistable liquid crystal is oriented to the second optically stable state, and the pixel is "
``black'' (writing step). or,
For pixels that are on the same scanning line and should be displayed as "white", the applied voltage at the first phase Tll1c is the threshold voltage vthl.
Since the voltage vo does not exceed , it remains in the first optically stable state, ie white.

On the other hand, on unselected scanning lines, the voltages applied to all pixels are ±V or O, neither of which exceeds the threshold voltage. Therefore, the liquid crystal molecules maintain the orientation corresponding to the signal state when scanned without changing the orientation state. That is, when the scanning electrode is selected in the optically stable state of "white" on one side, writing of signals for - lines is performed in the first phase T1, and the frame is completed. Even after that, the signal state can be maintained. FIG. 4 shows the drive signals described above in chronological order. 81~Ss are electrical signals applied to the scanning electrodes, I! and I3 are electrical signals applied to the signal electrodes, and A and C are voltage waveforms applied to pixels A and CK, respectively, shown in FIG. 3(A).

Now, the mechanism of electric field-induced switching of ferroelectric liquid crystals in a bistable state is not necessarily clear from a microscopic perspective, but generally, after switching to a predetermined stable state with a strong electric field for a predetermined time, the electric field disappears completely. If it is left unapplied, it is possible to maintain that state semi-permanently; Even if an electric field of opposite polarity is applied for a long time, the orientation state will be reversed again to the opposite stable state, and as a result, correct information display or modulation cannot be achieved. phenomenon may occur. The present inventors have demonstrated that the ease with which the orientation state transition reversal phenomenon (-crosstalk) occurs due to the long-term application of such a weak electric field is affected by the substrate surface material, roughness, liquid crystal material, etc. I have recognized this, but I have not yet understood it quantitatively. However, it has been confirmed that when a uniaxial substrate treatment for aligning liquid crystal molecules is performed, such as rubbing or oblique evaporation of SiO, etc., the tendency for the above-mentioned inversion phenomenon to occur tends to increase. In particular, it was confirmed that this tendency appears more strongly at high temperatures than at low temperatures.

In any case, it is preferable to avoid applying an electric field in a fixed direction for a long time in order to achieve correct information display or modulation.

Therefore, the second phase T2 in the driving method of the present invention is a phase for preventing the continued application of a weak electric field in a certain direction, and a preferable example thereof is shown in FIG. 3(B)-(
As shown in C) and (d), a signal with a different polarity from the information signal applied to the signal electrode group at phase T1 ((C) corresponds to black, (d) corresponds to white) is applied at phase T2. It is applied. For example, when trying to display the pattern shown in FIG. 3(A), if a driving method without phase T2 is used,
When scan electrode S1 is scanned, #J amateur becomes black, but s
After S2, the signal is applied to the pole work 1.The 1-ki signal is.

-V is continuous, and since that voltage is directly applied to the pixel, there is a high possibility that the pixel will eventually turn white.

In this embodiment, an erasing step is performed in which the entire screen becomes "white", and a writing step and K are performed in which corresponding pixels are written as "black" in accordance with the information in the first phase T1. In the first phase T1, the voltage for writing "black" is 3vo, and the application time is Δt. Also, the voltage applied to each pixel during periods other than scanning is a maximum of 1±Vo.
1, and the longest time it is continuously applied is 2Δt at 40 points shown in FIG. '' corresponds to the scanning time, which is the most severe condition, but this is still 4Δt (41) and the application time is short.
There is no crosstalk at all, and once the entire screen has been scanned, the displayed information is retained semi-permanently, so it is not refreshed, as is the case with display elements using ordinary TN liquid crystals, which do not have bistability. Uni Chengsha is not necessary at all.

Now, the optimal time interval for the second phase T2 also depends on the magnitude K of the voltage applied to the signal electrode in this phase, and the voltage applied as an information signal in the first phase TlK and When applying a voltage of opposite polarity, it is generally preferable to use a short time interval when the voltage is large, and a long time interval when the voltage is small. This means that it takes a long time to scan. Therefore, it is preferable to set T2≦T□.

FIGS. 5 and 6 show nine examples of other drive modes based on the present invention in chronological order. In these driving forms, the value of vo is set so that the threshold for orientation transition is between IVol and 21 Vol with respect to the pulse width Δt.

In Fig. 5, after the signal for erasing the image is applied as an electrical signal of +V to the scanning electrode group and -vo to the information signal electrode group in the erasing step, the next writing step starts from S□. A scanning signal -vo is sequentially applied, and in synchronization with this scanning signal, an information selection signal -vo is applied to the signal electrodes to perform writing.

The examples shown in FIGS. 5 and 6 are cases in which there is no auxiliary signal, and the example in FIG. 7 is a case in which there is an auxiliary signal. The voltage value of each drive pulse is shown in the figure, but in the example of FIG. 7, the erase signals applied to the scanning electrode and the signal electrode during the erase step have polarities different from those during information writing. It is inverted and the absolute value is smaller than that (Tv
o), and is an electrical signal with a large pulse width (2Δt). How to erase like this.

The threshold magnetic pressure of liquid crystal has pulse width dependence, and the width is 2Δ
The threshold mvth"'t for t is v t h ""≦4
■.

It is possible if you are satisfied.

Examples of ferroelectric liquid crystal compounds include hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (H
OBACPC), 4-O-(2-methyl)-butyl resol cylidene-4'-octylaniline (MBRA8)
can be used. In addition to the above-mentioned 8mC'' and 8mH'', other ferroelectric liquid crystals such as chiral smect I phase, F phase, G phase, and J phase can be used as the ferroelectric liquid crystal.

When constructing an element using these materials, the element may be supported by a copper block with a heater embedded in it, if necessary, in order to maintain the temperature at which the liquid crystal compound enters the chiral smectate phase. can.

The method of the present invention can be widely applied to the field of optical shutters and displays such as liquid crystal-optical shutters and liquid crystal televisions.

Hereinafter, specific examples of the present invention will be shown.

Example 1 The transparent conductive films (ITO) are 500xsoo each other.

Approximately 300λ was applied to one of a set of glass plates patterned to form an i-trix by spinning
A polyimide film was formed. The substrate is subjected to a rubbing process using a roller with a terrane cloth wrapped around the surface.
A cell was formed by bonding it to the other substrate not coated with the polyimide film. The cell spacing at this time is approximately 1.6μ. In this cell, a ferroelectric liquid crystal, desyloxybenzylidene-P'-amino-2-methylbutylcynamate (
By injecting DOBAMBC) and gradually cooling it from the heated molten state17), a uniform monodomain state was obtained at 8rrC state. Controlling the cell temperature at 70°C, based on the driving method shown in Figure 3, VO = 10V, T1 = T
When line sequential scanning was performed with the setting of 2=Δt=80ttsec, an extremely good image was obtained.

[Brief explanation of the drawing]

FIGS. 1 and 2 are perspective views of a liquid crystal element used in the driving method of the present invention. FIG. 3(A) is a plan view of the electrode structure used in the driving method of the present invention. ig3 diagram (B) (a)
-(d) are explanatory diagrams showing waveforms of electrical signals applied to electrodes. FIG. 3(C)(a) to (d) are explanatory diagrams showing voltage waveforms applied to pixels. Figures 4 and 5
6 and 7 are explanatory diagrams showing voltage waveforms when magnetic pressure is applied in time series. Patent applicant: Canon Co., Ltd. No. 4 (21-11Va-Otsu procedure amendment blind type) April 7, 1985 Manabu Shiga, Commissioner of the Patent Office, Indication of case Patent application No. 263662 of 1988, Title of the invention Driving method for optical modulation element 3, relationship to the case of the person making the correction Patent applicant address 3-30-2 Shimomaruko, Ota-ku, Tokyo Name (100
) Canon Co., Ltd. Representative Noh Kaku 3-4, Agent Address: 3-30-25 Shimomaruko, Ota-ku, Tokyo 1413
Date of amendment order: March 26, 1985 (shipment date) 6. Specification subject to amendment 7. Contents of amendment (1) “LEJO” on page 9, lines 13 to 17 of the specification
URNAL・-Liquid Crystals J
; J as “LE JOURNAL DE PHYSIQ”
UE LETTERS”) 36 (L-69) 1975.
r Ferroelectric Fist Liquid Crystals J
(r FerroelectricLiquid C
r7stals J) ; “Applied e7
Izzyx Letters (“Applied Ph
ysicsLetters”) 36 (11) 1980
r Submicro 5econd Electro-optic Switching in Liquid Crystals J
B15tableElectrooptic Swi
tching in LiquidCrystal
sJ); Correct it as J.

Claims (5)

[Claims] (1) In a method for driving an optical modulation element having a structure in which a scanning line and a data line are arranged, and an optical modulation material having bistability with respect to an electric field is arranged between the scanning line and the data line, an erasing step of applying a voltage signal between the scanning line and the data line to align the bistable optical modulation material to a first stable state; and sequentially applying a scan selection signal to the scanning line to erase the data. a writing step of applying an information selection signal to the line in synchronization with the scanning selection signal to orient the optical modulation material in a second stable state according to the information;
A method for driving an optical modulation element, characterized in that the polarities of the electric signals applied to the scanning line and the data line in the erasing step are opposite to the polarities of the scanning selection signal and the information selection signal in the writing step, respectively. (2) The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal. (3) The method for driving an optical modulation element according to claim 2, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (4) The chiral smectate liquid crystal has a C phase, an H phase,
4. The method of driving an optical modulation element according to claim 3, wherein the optical modulation element is in G phase, I phase, J phase, F phase, or K phase. (5) The method for driving an optical modulation element according to claim 3, wherein the chiral smect liquid crystal has a non-helical structure.