JPH0718991B2 - Driving method of optical modulator - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for driving an optical modulation element, particularly an element using a ferroelectric liquid crystal such as a chiral smectic liquid crystal.

[Conventional technology]

2. Description of the Related Art Conventionally, a liquid crystal display element in which scanning electrodes and signal electrodes are formed in a matrix and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display pixels or information 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”.
Lette rs ") 1971, 18 (4), pages 127-128, co-authored by M. Schadt and W. Helfrich," Voltage Day Pendant Optical Activity. " Of 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 version of conventional liquid crystal devices, the use of bistability liquid crystal devices has been disclosed by both Clark and Lagerwall in Japanese Patent Laid-Open No. 56-107216 and U.S. Pat. No. 4,367,924. It is proposed in the specification etc. As the bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or an H phase (SmH ^* ) is generally used, and in these states, the ferroelectric liquid crystal responds to an applied electric field. It has either the first optical stable state or the second optical stable state, and has the property of maintaining that state when an electric field is not applied, that is, stability, and has a quick response to changes in the electric field. Therefore, it is expected to be widely used in the field of high-speed and memory type display devices and the like.

[Problems to be solved by the invention]

As a method of driving the above-mentioned ferroelectric liquid crystal element, for example, a positive signal for orienting the ferroelectric liquid crystal in a first stable state corresponding to a white display state and a second signal corresponding to a black display state are provided.
The method of selectively applying a negative polarity signal for orienting the ferroelectric liquid crystal to the stable state is adopted. Generally, it is necessary to output a voltage signal of both positive and negative polarities as a multi-valued voltage of three values or more, which makes the output waveform complicated. Therefore, the driving circuit is complicated as compared with the case of the conventional TN crystal element, which causes the cost to increase accordingly.

[Means for Solving Problems] and [Operation] Accordingly, an object of the present invention is to provide a driving method which solves the above-mentioned drawbacks, has a simple driving waveform, and does not require a multi-value output driving circuit. Especially.

That is, in a pixel including a pair of electrodes facing each other, an insulating film, and a film of an optical modulation material such as a ferroelectric liquid crystal, the optical modulation material film has a relatively low resistivity. The applied voltage becomes a differential waveform of the voltage applied between the electrodes, and the above-mentioned object of the present invention is achieved by using such a differential waveform. Specifically, the present invention is
In a method of driving an optical modulation element having a plurality of pixels configured by arranging an optical modulation substance exhibiting first and second stable states based on an applied electric field at an intersection of a scanning electrode group and a signal electrode group, The scan electrodes are sequentially selected to supply a single-polarity pulse to the scan electrodes, and the information electrodes are supplied with a single-polarity pulse to provide a selection period for selecting a stable state of the pixel. In the selection period, after applying a differential waveform of one polarity having a peak value exceeding a threshold value to the optical modulation substance, applying a differential waveform of the other polarity having a peak value exceeding a threshold value to the optical modulation substance, A first step of changing the pixel to the first stable state and then to the second stable state; and a differential waveform of one polarity having a peak value exceeding a threshold value in the optical modulation substance during the selection period. After applying the voltage, the threshold is exceeded. That the other polarity of the differential waveform having a peak value so as not applied to the optical modulation substance, and carrying out the second step of the pixel in the first stable state, the selectively.

〔Example〕

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

FIG. 1 schematically shows an example of a ferroelectric liquid crystal cell. 11a and 11b are In ₂ O ₃ , SnO ₂ and ITO (Indium-
It is a substrate (glass plate) coated with a transparent electrode such as theine-oxide), and a liquid crystal of SmC ^* phase (chiral smectic C phase) in which the liquid crystal molecular layer 12 is 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 (P⊥) 14 in a direction orthogonal to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 11a and 11b, the helical structure of the liquid crystal molecules 13 is unraveled, and all the dipole moments (P⊥) 14 are oriented in the electric field direction.
The orientation direction of the liquid crystal molecules 13 can be changed. Liquid crystal molecule
13 has an elongated shape, and exhibits refractive index anisotropy in the major axis direction and the minor axis direction thereof, and 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 applied polarity. Further, 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 becomes a non-helical structure even when no electric field is applied, as shown in FIG. Moment Pa or Pb is upward (24a) or downward (2a
Take either of the states of 4b). When electric fields Ea or Eb having different polarities above a certain threshold value are applied to such a cell, the dipole moment electric field Ea or Eb is directed upward 24a or downward 24b in accordance with the electric field vector. And the liquid crystal molecules are aligned in either the first stable state 23a or the second stable state 23b accordingly.

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. 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 23a, but this state is stable even when the electric field is cut off. Moreover, when the electric field Eb in the opposite direction is applied, the liquid crystal molecules are in the second stable state.
It is oriented to 23b and changes its molecular orientation, but it remains in this state even when the electric field is turned off. Also, the applied electric field Ea
As long as the values do not exceed a certain threshold, they are still maintained in their respective orientations. In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 20 μ, particularly 1 μ to 5 μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using this type of ferroelectric liquid crystal,
For example, Clark and Ragabal, U.S. Pat.
No. specification.

FIG. 3 is a sectional view showing a pixel portion of the optical modulator used in the present invention, and FIG. 4 is an equivalent circuit diagram thereof. In the pixel shown in FIG. 3, substrates 31a and 31b are provided with supported electrodes 32a and 32b, respectively, which are supported by each other.
Insulator films 33a and 33b are overlaid on and 32b to prevent shorting. The insulating pair films 33a and 33b are subjected to a uniaxial alignment process such as a rubbing process for controlling the alignment of the ferroelectric liquid crystal film 34. Further, although not shown, an orientation control film may be separately provided on the insulator films 33a and 33b. Further, a color filter layer (not shown) may be provided on the upper and lower sides of either one of the insulator films. At this time, in the color filter, a blue dye filter (B), a green dye filter (G) and a red dye filter (R) are arranged for each pixel, and these B, G and R constitute one color pixel. You can In this liquid crystal element, the periphery of substrates 31a and 31b is sealed with a sealant 36 such as an epoxy adhesive, and a pair of crossed Nichols polarizers 36a and 36b for detecting optical modulation are provided on both sides of the cell.
Are arranged.

The insulating material used in the insulator films 33a and 33b of the present invention is not particularly limited, but silicon nitride,
Hydrogen-containing silicon nitride, silicon carbide, hydrogen-containing silicon nitride, silicon oxide, boron nitride, hydrogen-containing boron nitride, cerium oxide, aluminum oxide, zirconium oxide, titanium oxide Or inorganic insulating materials such as magnesium fluoride, polyvinyl alcohol, polyimide, polyamide imide, polyester imide, polyparaxylylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyvinyl acetate, polyamide, polystyrene, cellulose resin, melamine An organic insulating material such as resin, urea resin, acrylic resin or photoresist resin is used as the insulating film. The film thickness of these insulating films is not more than 5000 Å, preferably 100 Å to 5000 Å, particularly 500 Å to 3000 Å.

Further, in the case of the capacitance formed by these insulator films 33a and 33b, by setting the capacitance to be 5.5 × 10 ³ PF / cm ² or more, the above-mentioned inversion phenomenon can be more effectively prevented. . Its preferable capacity is 5.5 × 10 ³ PF / cm ² ~ 3.0 × 1
In the range of 0 ⁵ PF / cm ² , it is necessary to maintain particularly good insulation 9.
0 × 10 ³ PF / cm ^{2 to} 5.5 × 10 ⁴ PF / cm ² are suitable.

The ferroelectric liquid crystal 34 used in the present invention is most preferably a chiral smectic liquid crystal, among which the chiral smectic C phase (SmC ^* ), H phase (SmH ^* ), I phase (SmI ^* ), J
Phase (SmJ ^* ), K phase (SmK ^* ), G phase (SmG ^* ) and F phase (Sm
F ^* ) LCD is suitable.

More specifically, the ferroelectric liquid crystal 34 includes p-decyloxybenzylidene-p'-amino-2-methylbutyl cinnamate (DOBAMBC), p-hexyloxybenzylidene-p'-amino-2-chloropropyl. Cinnamate (HOBACPC), p-decyloxybenzylidene-p'-amino-2-methylbutyl-α-cyanocinnamate (DO
BAMBCC), p-tetradecyloxybenzylidene-p'-
Amino-2-methybutyl-α-cyanocinnamate (TD
OBAMBCC), p-octyloxybenzylidene-p'-
Amino-2-methylbutyl-α-chlorocinnamate (OOBAMBCC), p-octyloxybenzylidene-p ′
-Amino-2-methylbutyl-α-methylcinnamate, 4,4'-asoxycinnamamide-bis (2
-Methylbutyl) ester 4-0- (2-methyl) -butylresorcylidene-4'-octylaniline (MBRA
8), 4- (2'-methylbutyl) phenyl-4'-octyloxybiphenyl-4-carboxylate, 4-
Hexyloxyphenyl-4- (2 "-methylbutyl)
Biphenyl-4'-carboxylate, 4-octyloxyphenyl-4- (2 "-methylbutyl) biphenyl-4'-carboxylate, 4-heptylphenyl-4
-(4 "-methylhexyl) biphenyl-4'-carboxylate, 4- (2" -methylbutyl) phenyl-4
-(4 "-methylhexyl) buphenyl-4'-carboxylate and the like can be used.

These ferroelectric liquid crystal compounds may be used alone or in combination of two or more, and may be mixed with other non-dielectric liquid crystals such as nematic liquid crystals, cholesteric liquid crystals (chiral nematic liquid crystals) and smectic liquid crystals.
The ferroelectric liquid crystal 34 described above may have the spiral structure shown in FIG. 1 or may have the non-helical structure shown in FIG. In particular, in the case of the spiral structure shown in FIG. 1, a ferroelectric liquid crystal having a negative dielectric anisotropy is used, and by applying an AC bias between both electrodes, a non-helical structure is obtained. It is preferable to apply a driving method that imparts bistability as a structure. At this time, it is also possible to apply the above-mentioned driving method of applying an AC bias to the liquid crystal element which forms the non-helical structure by making the cell thickness of the liquid crystal layer sufficiently small.

In the equivalent circuit shown in FIG. 4, C ₁ is a capacitance including the insulator films 33a and 33b and other capacitance components. C ₂ is the capacitance of the ferroelectric liquid crystal film 34, and R is its resistance. At this time, the resistances of the insulator films 33a and 33b are usually sufficiently large and can be ignored. Further, V is a voltage applied between the electrodes facing each other, and V _LC is a voltage applied to the ferroelectric liquid crystal.

Therefore, if a rectangular wave voltage pulse with a pulse width ΔT shown in FIG. 5 is applied between the electrodes facing each other, a differential waveform pulse that attenuates with a time constant of τ = (C ₁ + C ₂ ) R is obtained, and τ is smaller than ΔT. For example, this differential waveform pulse has the waveform shown in FIG. That is, the positive and negative voltages are applied to the ferroelectric liquid crystal film at the rising edge and the falling edge of the externally applied voltage, respectively. In the present invention, by utilizing the positive voltage and the negative voltage, writing can be performed by a unipolar applied voltage. To reduce the time constant τ, for example, the specific resistance of the ferroelectric liquid crystal is 10 ⁶ Ω ・ cm to 10 ⁹ Ω ・
It is desirable to use cm (measured with an LCR meter). As a method of setting the specific resistance of the liquid crystal film within the above range, for example, a method of adding a surfactant to the liquid crystal can be used. Is formed with an extremely thin film thickness of 2 μm or less. Therefore, since there is a problem that shorts are generated between the upper electrode and the lower electrode through the fine particles mixed in the element, the electrode provided in the element is An insulator film for preventing shorts is provided.

As described above, since the insulating film is formed between the facing electrodes, the rectangular wave pulse source V ₀ (writing pulse of pulse width ΔT) is generated in the liquid crystal layer from which the ferroelectric liquid crystal is completely inverted. As shown in FIG. 6, V ₀ at the time of applying a pulse has a time constant τ of
= R (C ₁ + C ₂ ), a voltage drop of ΔV ₀ occurs. This is because the electric field in the opposite direction (-Δ
V ₀ ) is the cause.

FIG. 7 shows an example of applied waveforms when the above principle is applied to matrix driving. FIG. 7 (a) is a waveform applied to the scanning electrodes, FIG. 7 (b) is a waveform applied to the information electrodes, and FIG. 7 (c) is a waveform applied to the scanning electrodes and the information electrodes described above. The difference between the waveforms, that is, the voltage waveform applied between the electrodes facing each other, and FIG. 7D shows the voltage waveform applied to the ferroelectric liquid crystal film in time series.

At time t ₁ , when the negative polarity pulse −V _S of the scan electrode falls, the peak value appears on the liquid crystal film. Positive differential waveform voltage is applied, and by appropriately selecting V _S so that this voltage waveform becomes equal to or higher than the inversion threshold value, all pixels on this scan electrode are, for example, based on the first alignment state. The display state of "white" is written. Then the time
At the same time as the scan electrode pulse rises at t ₂ , a positive pulse V ₁ (<V _S ) is applied to the information electrode of the non-selected pixel (the pixel that does not invert to the “black” display state based on the second alignment state). It is applied and left at 0 volts for selected pixels (pixels to write "black"). At this time, the peak value is displayed on the liquid crystal film of the unselected pixel. The differential waveform voltage of the page is applied, but by appropriately selecting V _I , this voltage waveform can be made equal to or higher than the inversion threshold value, so that the pixel is not inverted and remains in the “white” display state. On the other hand, the liquid crystal film of the selected pixel has a peak value The negative differential waveform voltage of is applied and "black" is written.

Next, at time t ₃ , the information electrode pulse falls, and the peak value of the differential voltage waveform applied to the liquid crystal film at this time is Then, by properly selecting V _I , this voltage waveform can be set below the threshold value, and the pixel is kept in the “white” display state.

In the above embodiment, the peak value of the differential waveform is It is necessary to select V _S and V _I so that (Vth; threshold voltage of ferroelectric liquid crystal), and from the viewpoint of voltage margin (voltage margin), V _S = 2V _I is most preferable.

Further, the voltage waveform applied to both electrodes of the pixel does not necessarily have to be the rectangular wave as in the above-described embodiment, and may be, for example, a sawtooth voltage. However, it is generally desirable that the voltage applied to the liquid crystal film has a peak value that is as high as possible and that the time until it decays to 0 volt is as long as possible, and a rectangular wave pulse is most suitable for this purpose.

Therefore, in the present invention, when the time (t ₂ −t ₁ ) is set to the phase T ₁ , the pixels on the row are aligned to the white display state at 1 o'clock at this phase T ₁ , and the time (t ₃ −t ₂ ) is set to the phase T _1. When T ₂ is set, only the selected pixel on the row is inverted to the black display state and writing of one line is performed. By sequentially performing this writing row by row, one screen is written.

〔The invention's effect〕

According to the present invention, the drive waveform can be simplified, and in particular, the output of the drive circuit need only have one of positive and negative polarities, and the drive circuit output level can be binary.

Further, by making the drive waveform a rectangular wave, the peak value of the voltage applied to the liquid crystal film can be increased and the width can be increased, thereby shortening the writing time.

[Brief description of drawings]

1 and 2 are perspective views of a ferroelectric liquid crystal device used in the present invention. FIG. 3 is a sectional view of a ferroelectric liquid crystal device used in the present invention.
FIG. 4 is an explanatory diagram showing an equivalent circuit of the pixel. FIG. 5 is an explanatory diagram showing a write pulse waveform applied between a pair of electrodes facing each other, and FIG. 6 is an explanatory diagram showing a voltage waveform applied to the liquid crystal film when the pulse is applied to the liquid crystal film. FIG. 7 is an explanatory diagram showing the method of the present invention in time series.

Front page continuation (72) Inventor Akihiro Mori 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP-A-56-107216 (JP, A) JP-A-50-38494 ( JP, A) Actually developed 50-2351 (JP, U)

Claims (2)

[Claims] 1. An optical modulation element having a plurality of pixels, which are formed by arranging optical modulation substances exhibiting first and second stable states based on an applied electric field at intersections of scanning electrode groups and signal electrode groups. In order to select a stable state of the pixel, the scanning electrodes are sequentially selected to supply a single-polarity pulse to the scanning electrode and the information electrode is supplied to a single-polarity pulse. And applying a differential waveform of one polarity having a peak value exceeding the threshold value to the optical modulation substance in the selection period, and then applying a differential waveform of the other polarity having a peak value exceeding the threshold value to the optical modulation substance. To the second stable state after the pixel is set to the first stable state by applying to the first stable state, and a differential waveform of one polarity having a peak value exceeding a threshold value in the selection period. Applied to the optical modulator After that, a second step of bringing the pixel into the first stable state by selectively preventing the differential waveform of the other polarity having a peak value exceeding the threshold value from being applied to the optical modulation substance, is selectively performed. Driving method of optical modulator. 2. The method for driving an optical modulation element according to claim 1, wherein the optical modulation material is a chiral smectic liquid crystal.