JP2505760B2 - Driving method of optical modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for driving an optical modulation element, and more particularly to a method for driving a ferroelectric liquid crystal element having at least two stable states.

[Description of Prior Art]

2. Description of the Related Art Conventionally, a liquid crystal display element in which a scanning electrode group and a signal electrode group are configured in a matrix, a liquid crystal compound is filled between the electrodes to form a large number of pixels, and an image or information is displayed is well known. . As a driving method of this display element, there is a time-division drive in which an address signal is sequentially and selectively applied to the scanning electrode group and a predetermined information signal is selectively applied to the signal electrode group in parallel in synchronization with the address signal. Has been adopted.
Most of these were put to practical use, for example, "Applied Physics Letters".
Letters ") 1971, 18 (4), pages 127-128," Voltage Day Pendant Optical Activity Off "co-authored by M. Schadt and W. Helfrich.・ A twisted nematic / liquid crystal (“Voltage Dependent
Optical Activity of a Twisted Nematic Liquid Cryst
It was a TN (twisted nematic) type liquid crystal shown in al ").

In recent years, as an improved type of the conventional liquid crystal element, the use of a liquid crystal element having bistability has been disclosed in both Clark and Lagerwall by JP-A-56-107216.
No. 4,367,924 and the like. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC *) or an H phase (SmH *) is generally used. Has the property of taking one of the first optical stable state and the second optical stable state, and maintaining the state when an electric field is not applied, that is, stability.
Further, it is expected to be widely used in fields such as a high-speed and storage type display device, which has a quick response to a change in an electric field.

[Problems to be solved by the invention]

However, a problem arises when the number of display pixels is extremely large and high-speed driving is required. That is, the threshold voltage for providing the first stable state in the ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -V.
_th1 city, + V threshold voltage to give the second stable state
_{If th2} is set, the display state (for example, white state) written in the pixel is different from the other display state (for example, black state) when the voltage is continuously applied for a long time without _exceeding these threshold voltages. May be reversed. FIG. 1 shows the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 1 shows DOBAMBC (12 in the figure) as a ferroelectric liquid crystal.
And HOBACPC (11 in the figure) are plots of the application time dependence of the threshold voltage (V _th ) required for switching.

As is clear from FIG. 1, it is understood that the threshold V _th has an application time dependency, and the shorter the application time, the steeper the gradient. From this, when applied to an element that has a large number of scanning lines and is driven at high speed, for example, even if a certain pixel is switched to a bright state at the time of scanning, an information signal of V _th or less is always obtained after the next scanning. It can be understood that if the voltage is continuously applied, the pixel may be inverted to the dark state in the middle of the scanning of one screen. In addition, when driven for a long time, the DC component becomes large as a whole, and the same problem as described above may occur.

It is an object of the present invention to provide a novel method of driving an optical modulation element, which solves the problems in the conventional liquid crystal display element or liquid crystal light shutter as described above.

Another object of the present invention is to provide a method of driving an optical modulator suitable for expressing gradation. Still another object of the present invention is to provide a method of driving an optical modulator that eliminates the flicker that appears on the display screen.

[Means for Solving Problems] and [Operation] The present invention relates to a method of driving an optical modulation element in which an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group. A scanning selection signal and a scanning non-selection signal, which have different voltages in the first to third periods in the selection period, are selectively and sequentially supplied to the group, and the signal electrode group is responsive to the information to be displayed. By supplying an information signal, a voltage waveform having a zero DC component in the selection period is applied to the pixel on the selected scan electrode to determine the display state of the pixel, and the step includes maximizing the pixel. When one of the dark (black) or the brightest (white) state is set, the same potential as the scanning non-selection signal or a first potential based on the potential is used as the reference during the first period (first phase). A first voltage having a polarity of 1 and not exceeding the inversion threshold is applied to the pixel. In addition, in the second period (second phase) after the first period, a second voltage having a second polarity with respect to the potential and exceeding the inversion threshold is applied to the pixel. The pixel is erased to one of the states by applying the same potential to the pixel during the third period (third phase) after the second period or the same potential as the scanning non-selection signal. A third voltage having a first polarity and not exceeding an inversion threshold is applied, and further, the process includes the first voltage if the pixel is in the other state of darkest or brightest. During the period, a fourth voltage, which has the same potential as the scanning non-selection signal or a voltage of the first polarity based on the potential and which does not exceed the inversion threshold value, is applied to the pixel, and during the second period. Applying a fifth voltage, which is a voltage of the second polarity based on the electric potential and exceeds the inversion threshold value, to the pixel to erase the pixel in the one state, During, by applying a voltage exceeding the inversion threshold in polarity of the first to the pixel, and applying a sixth voltage to change the display state of the pixel. The effect of the present invention is that the voltage applied before erasing does not exceed the threshold value, and therefore the pixel does not reverse to the white or black state opposite to the erasing state before erasing. Moreover, the effect of the present invention is that the DC component is zero, so that unnecessary inversion can be prevented.

〔Example〕

The optical modulation substance used in the driving method of the present invention has at least two stable states, and particularly takes one of the first optical stable state and the second optical stable state depending on the applied electric field. That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property is preferable.

As a liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and among them, a chiral smectic C phase (SmC *) or an H phase (SmH *). ) Liquid crystal is suitable. This ferroelectric liquid crystal is described in "Le Journal de
"Fujitsu Letter"("Le Journal de Physics lett
er ") Volume 36 (L-69), 1975" Ferroelectric Liquid Crystals "(" Ferroelectric Liquid
Crystals ");" Applied Physics Letters "36 (11) 1980
Year of "Submicron Second By Stable Electro-Optical Switching In-Liquid
Crystal ”(“ Submicro Second Bistable Electroop
tic Switching in Liquid Crystals ”);“ Solid physics ”1
6 (141) 1981 “Liquid crystal” and the like, and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include decyloxybenzylidene-p'-
Amino-2-methylbutyl cinnamate (DOBAMBC),
Hexyloxybenzylidene-p'-amino-2-chloropropylcinnamate (HOBACPC) and 4-o-
(2-methyl) -butyl resorcylidene-4'-octylaniline (MBRA8) and the like can be mentioned.

When using these materials to form an element, the liquid crystal compound is kept in a temperature state where it becomes the SmC * phase or SmH * phase. Can be supported.

Further, in the present invention, in addition to SmC * and SmH * described above, it is also possible to use a ferroelectric liquid crystal which appears in a chiral smectic F phase, I phase, J phase, G phase or K phase.

FIG. 2 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 21a and 21b are substrates (glass plates) coated with transparent electrodes such as In ₂ O ₃ , SnO ₂ and ITO (Indium-Thein-Oxide), between which the liquid crystal molecular layer 22 is perpendicular to the glass surface. The liquid crystal of SmC * phase that is oriented like this is enclosed. A thick line 23 represents a liquid crystal molecule,
The liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 21a and 21b, the liquid crystal molecules 23
The orientation of the liquid crystal molecules 23 can be changed so that the helical structure is unwound and all the dipole moments (P⊥) 24 are oriented in the electric field direction. The liquid crystal molecules 23 have an elongated shape, and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicols position above and below a glass surface are placed. It is easy to understand that the liquid crystal optical modulator has optical characteristics that change depending on the polarity of voltage application. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μ), the helical structure of the liquid crystal molecules is unraveled and the dipole moment Pa or Pb is The state is either upward (34a) or downward (34b). In such a cell, as shown in FIG. 3, electric fields Ea of different polarities and above a certain threshold are provided.
Or, if Eb is applied for a certain period of time, the dipole moment becomes an electric field.
The direction is changed upward 34a or downward 34b with respect to the electric field vector of Ea or Eb, and the liquid crystal molecules are oriented to either the first stable state 33a or the second stable state 33b accordingly.

There are two advantages of using such a ferroelectric liquid crystal as the optical modulation element. First, the response speed is extremely fast, and the alignment of the second liquid crystal molecules has a bistable state. The second point will be explained with reference to FIG. 2, for example.
When the electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a, but this state is stable even when the electric field is cut off. or,
When the electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented in the second stable state 33b to change the orientation of the molecules, but they remain in this state even when the electric field is turned off. Electric field
As long as Ea does not exceed a certain threshold, each alignment state is also maintained. With such a fast response speed,
In order to effectively realize the bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 2.0 μ, particularly 1 μ to 5 μ is suitable.

A preferred specific example of the driving method of the present invention is shown by the following figures.

FIG. 4 is a schematic diagram of a typical ferroelectric liquid crystal cell 41 having a matrix pixel structure in which a bistable ferroelectric liquid crystal is sandwiched between a scan electrode group 42 and a signal electrode group 43. Although the present invention can be applied to multi-value or analog gradation display, in order to simplify the description, a case of displaying three values of white and one intermediate level and black will be described as an example.
In FIG. 4, the pixel A indicated by cross-hatching corresponds to "black", the pixel B indicated by single-hatching corresponds to the intermediate level, and the other pixels C correspond to "white".

FIG. 5 shows an example of specific drive waveforms when image erasing and writing are sequentially performed for each line, and the image after writing corresponds to FIG.

FIG. 5 (a) shows the voltage waveforms applied to the scan electrodes S _S and S _NS and the signal electrodes I _S , I _HS , and I _NS and the pixel liquid crystal sandwiched between the scan electrodes and the signal electrodes. Voltage. Here, the horizontal axis represents time and the vertical axis represents voltage.

Here, S _S is a line in which image information is written, that is, a drive waveform applied to the selected scan electrode, and S _NS is a line in which no image information is written at that time, that is, a drive waveform applied to a non-selected scan electrode. Is. I _S is a driving waveform for writing “black” between the intersection with the selected line, I _HS is an intermediate level, and I _NS is a driving waveform for writing “white”. .

At this time, the voltages respectively applied to the liquid crystals forming the pixels are I _S −S _S , I _HS −S _S , I _NS −S _S , I _S −S _NS , I _HS −
S _NS , I _NS −S _NS .

When the inversion threshold of the bistable ferroelectric liquid crystal used here is Vth, | ± 2Vo | <between the inversion threshold _Vth and the drive voltage Vo
The drive voltage Vo is selected so that | V _th , | <1 ± 3 Vo |. Here, the inversion threshold value V _th may be slightly different between the sides due to the alignment process or the like applied to a normal liquid crystal cell, but in this case, the drive potentials on the side and the side are slightly different in each drive waveform. It is assumed that a correction or the like is taken, and here, for convenience of description, | + V _th | (side inversion threshold) = | −V
_{Let th} | (side inversion threshold).

In the above case, when the absolute value of the voltage applied to each pixel is 2Vo or less, inversion of the liquid crystal molecules does not occur, and when it is 3Vo or more, the inversion occurs and the absolute value is As it gets bigger, the reversal becomes stronger.

 Here, each waveform will be described.

The scan selection signal S _s applied to the selected scan electrode has four phases within the writing period of one line, and the line erase is applied in the second phase and the line erase is applied to the signal electrode in the third phase. Pixel writing according to the signal is performed. Therefore, 2Vo for the second phase and 3V for the third phase, respectively.
2Vo pulse waveform is applied. Further, a potential of substantially zero (reference potential) is auxiliary applied to the first phase and the fourth phase.

On the other hand, the scanning non-selection signal S _NS applied to the unselected scanning electrodes is fixed to the reference potential (here, 0 V).

Next, the potential waveform applied to the signal electrode is applied with the erasing signal 2Vo in the second phase thereof in synchronism with the respective phases of the scanning selection signal, and in this phase, the respective signal electrodes and the selected scanning electrode S will be the voltage of 4Vo is applied between the _s, to cross the liquid crystal inversion threshold V _tH,
Invert all this lines to the erase side (white). next,
In the third phase, a voltage according to the gray scale is applied to each of the signal electrodes that intersect the selected scan electrode. At this time, 2Vo is set as the potential for making the pixel "black", Vo as the potential for setting the intermediate level (gray), and the potential to be kept as "white" is the same level as the potential level of the scanning non-selection signal, that is, zero (reference Potential). By doing this, in the third phase, the voltages applied to the pixels on this line are 4Vo, 3Vo and 2Vo, respectively, and write "black", "grey" (intermediate level) and "white" to the pixels respectively. become. Next, the first phase and the fourth phase that are auxiliary applied will be described. First, in the fourth phase, the voltage applied to the pixel on the selected scanning electrode is set to zero (reference potential) which is the same level as the voltage level of the scanning non-selection signal. Further, as the first phase, a voltage according to the write signal in the pixel of the third phase is applied. That is, the voltage applied to the pixel in the first phase is
The voltage is set to the same level as the scanning non-selection signal, or the scanning non-selection signal has the same polarity as the voltage applied to the pixel in the third phase with reference to the voltage level of the scanning non-selection signal. Is set to a voltage level between the voltage level of 1 and the threshold voltage level of the ferroelectric liquid crystal.
Further, at this time, in the present invention, in order to eliminate the flicker on the display screen, the sum of the voltage values applied to the pixels in the first phase and the third phase is constant over the pixels on the selected scanning electrodes. Is desirable.

The mechanism of switching by the electric field of the ferroelectric liquid crystal in the bistable state is not necessarily microscopically clear, but generally after switching to a predetermined stable state with a strong electric field for a predetermined time, no electric field is generated. When left unapplied, it is possible to maintain that state almost semi-permanently, but a weak electric field that does not cause switching within a predetermined time (in the example explained above, a voltage below V _th Even if it is "correspondence", when an electric field of opposite polarity is applied for a long time, the orientation state is inverted again to the opposite stable state, and as a result, correct information display and modulation cannot be achieved. Can occur. The inventors of the present invention have found that the ease of occurrence of the transition reversal phenomenon (a kind of crosstalk) of the alignment state due to the application of such a weak electric field for a long time is
We have recognized that it is affected by roughness and liquid crystal material, but we have not yet grasped it quantitatively. However, it was confirmed that uniaxial substrate treatment for aligning liquid crystal molecules such as rubbing or oblique vapor deposition of SiO 2 tends to increase the inversion phenomenon. In particular, it was confirmed that the tendency was stronger when the temperature was high than when the temperature was low.

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

In the present invention, the voltages of the same polarity have two or more phases,
Subsequently, the above problem was solved by not applying the voltage.

As can be seen from FIG. 5, the voltage applied to the scan electrode group and the signal electrode group is in each phase, and the range of voltage selection is zero voltage and alternating voltage. Since the voltages of opposite polarities exist, the voltages of the same polarity are not continuously applied to the pixels.

In addition, the voltage applied to the pixel in the fourth phase is kept constant at almost zero, so that the voltage in the fourth phase crosses the voltage in the previous phase, that is, the voltage in the third phase that determines the pixel state. Since no talk is given, good and stable gradation pixel expression can be performed. It is also possible to apply the voltage of the fourth phase here in the first phase. Of course, by selecting only the binary values corresponding to "white" and "black" as the voltage level, it is naturally possible to express only the binary values.

Further, in the above description, a three-valued image has been described, but in FIG. 5, the potential in the third phase of the drive waveform applied to the signal electrode is, for example, from −2Vo to zero, or correspondingly. An image with multiple gradations or analog gradations can be obtained by dividing the potential of the auxiliary signal in the first phase into multiple values from zero to −2Vo or by selecting values in an analog manner.

〔The invention's effect〕

According to the present invention described above, it is possible to stably perform gradation expression even in a matrix optical modulator having high density pixels.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a change in threshold voltage of a ferroelectric liquid crystal with respect to a voltage application time. 2 and 3 are schematic perspective views of the ferroelectric liquid crystal device used in the present invention. FIG. 4 is a plan view of the matrix electrode structure used in the present invention. FIG. 5 (a) is a drive voltage waveform diagram, and FIG. 5 (b) is its time-series waveform diagram.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Masahiko Enari 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Mitsutoshi Kuno 3-30-2 Shimomaruko, Ota-ku, Tokyo Ki Within Canon Inc. (56) References JP 61-52630 (JP, A) JP 60-173591 (JP, A)

Claims (8)

(57) [Claims] 1. A method of driving an optical modulation element, wherein an optical modulation substance is arranged between a pair of substrates having a scanning electrode group and a signal electrode group, wherein the scanning electrode group is provided with a first to a third group during a selection period. By selectively and sequentially supplying a scanning selection signal and a scanning non-selection signal having different periods of voltage, and supplying an information signal corresponding to the information to be displayed to the signal electrode group, the DC component of the selection period Is applied to the pixel on the selected scan electrode to determine the display state of the pixel, and the step includes the step of bringing the pixel into one of the darkest state and the brightest state. During the first period, a first voltage that is the same potential as the scanning non-selection signal or a voltage of a first polarity that is based on the potential and that does not exceed the inversion threshold is applied to the pixel, In the second period after the first period, the pixel is referenced to the potential with the first period. A second voltage, which is a bipolar voltage and exceeds the inversion threshold, is applied to erase the pixel in the one state, and the pixel is scanned in the third period after the second period. A third voltage having the same potential as the non-selection signal or the voltage of the first polarity and not exceeding the inversion threshold is applied, and the step is performed by setting the pixel to the other one of the darkest state and the brightest state. In that case, during the first period, a fourth voltage that is the same potential as the scanning non-selection signal or a voltage of the first polarity that is based on the potential and that does not exceed the inversion threshold is applied to the pixel. Then, during the second period, a fifth voltage that is a voltage of the second polarity based on the potential and that exceeds the inversion threshold is applied to the pixel,
Erasing the pixel to the one state, applying a voltage exceeding the inversion threshold with the first polarity to the pixel during the third period, and applying a sixth voltage that changes the display state of the pixel. A method of driving an optical modulator, which is characterized by: 2. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a chiral smectic liquid crystal. 3. The method according to claim 1, wherein the step further includes, after the third period, a fourth period in which a voltage having the same potential as the scanning non-selection signal is applied to the pixel. Driving method of optical modulator. 4. The optical device according to claim 1, wherein the step further comprises a period before the first period, in which a voltage having the same potential as the scanning non-selection signal is applied to the pixel. Driving method of modulator. 5. The method according to claim 5, wherein the sum of voltage values applied to the pixel in the first period and the third period is constant over all pixels on the selected scan electrode. Claim 1st
A method for driving the optical modulator according to the item 1. 6. The method of driving an optical modulation element according to claim 1, wherein the information to be displayed is binary information. 7. The information to be displayed is multivalued information corresponding to gradation information, and the third voltage changes according to the gradation information. A method for driving the described optical modulator. 8. The method for driving an optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal.