JPH0690374B2 - Optical modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical modulator, and more particularly to a ferroelectric liquid crystal device having at least two stable states.

[Description of Prior Art]

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 an image 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 practical applications were, for example, “Applied Physics Letters”.
tters ") 1971, 18 (4), pages 126-128, co-authored by M. Schadt and W. Helfrich," Voltage Dependant Optical. "
Activity of a Twisted Nematic Liquid Crystal ”(Voltage Dependent Opti
cal Activity of a Twisted Nematic Liquid Crystal "
The TN (twisted-nematic) liquid crystal shown in FIG.

In recent years, as an improved version of the conventional liquid crystal device, the use of a liquid crystal device having bistability has been disclosed by both Clark and Lagerwall in JP-A-56-107216 and US Pat. No. 4,367,924. Is proposed in the book.
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, a first liquid crystal in response to an applied electric field is used. It has either the 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, has stability, and has a quick response to a change in the electric field, Wide application is expected in the field of high-speed and memory type display devices.

[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 giving the first stable state in the ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -V _th1.
And the threshold voltage for giving the second stable state is + V _th2
Then, even if these threshold voltages are not exceeded, when the voltage is continuously applied for a long time, the display state (for example, white state) written in the pixel is inverted to another display state (for example, black state). I have something to do. FIG. 1 shows the threshold characteristic of a bistable ferroelectric liquid crystal cell.

Figure 1 shows H of DOBAMBC (12 in the figure) as a ferroelectric liquid crystal.
It is a plot of the application time dependence of the threshold voltage (V _th ) required for switching when OBACPC (11 in the figure) is used.

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 it is applied to an element that has extremely many scanning lines and is driven at high speed,
For example, even if a pixel is switched to the bright state during scanning, if the information signal of V _th or less is continuously applied after the next scanning, that pixel is inverted to the dark state during the scanning of one screen. It turns out that there is a risk of being lost.

[Means for Solving Problems] and [Operation] An object of the present invention is to provide a novel optical modulator which solves the problems in the conventional liquid crystal display element or liquid crystal optical shutter as described above. .

Another object of the present invention is to provide an optical modulator having a fast response.

Another object of the present invention is to provide an optical modulator having high density pixels.

The present invention includes: a. A matrix electrode formed of a scan electrode group formed of a plurality of scan electrodes and a signal electrode group formed of a plurality of signal electrodes, and different alignment states depending on voltage application of different polarities exceeding a threshold voltage. An optical modulation element provided with a ferroelectric liquid crystal that causes a.b. means for simultaneously applying a polarity voltage exceeding the threshold voltage of the ferroelectric liquid crystal at the intersection of the scanning electrode group and the signal electrode group, c. The scan electrodes are sequentially scanned, and the scan electrodes selected for scanning are
Based on the voltage applied to the unselected scan electrodes,
A scan selection signal having a bipolar pulse composed of one polarity pulse and the other polarity pulse, and a voltage average value of the bipolar pulse is set to 0 based on the voltage applied to the unselected scan electrode is applied. Means, and d. Applying one information signal having a bipolar pulse having the same phase as the bipolar pulse to the selected signal electrodes of the signal electrode group in synchronization with the bipolar pulse of the scanning selection signal. As a result, a voltage that does not exceed the threshold voltage of the ferroelectric liquid crystal is applied to the intersection of the scan electrode to which the scan selection signal is applied and the selected signal electrode, and the remaining signal electrodes are connected to the bipolar electrodes. Bipolar signal in the information signal is applied to the intersection of the scan electrode to which the scan selection signal is applied and the remaining signal electrode by applying the other information signal having the bipolar pulse having the opposite phase to the sex pulse. During the second half pulse of the applying is characterized in an optical modulator having a means for applying the other polarity voltage exceeding the threshold voltage of the ferroelectric liquid crystal.

〔Example〕

The optical modulator used in the device of the present invention has at least two stable states, and in particular, 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 used.

As a liquid crystal having bistability that can be used in the device of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and a chiral smectic C phase (SmC *) or H phase (SmH *) liquid crystal is suitable. ing. About this ferroelectric liquid crystal, "Le Journal de Physioue l"
etter ") Volume 36 (L-69), 1975" Ferroelectric Liquid Crystals "(" Ferroelectric L "
iquid Crystals ”);“ Applied Physics Letters ”36 volumes (11
No.) 1980 "Submicron Second Bistable Electro-Optic Switching In Liquid Crystal"("Submicro Second Bistable")
Electrooptic Switching in Liquid Crystal
s) ”;“ Solid State Physics ”16 (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 device of the present invention include desiloxybenzylidene-P '.
-Amino-2-methylbutyl cinnamate (DOBAMB
C), hexyloxybenzylidene-P'-amino-2
-Chloropropyl cinnamate (HOBACPC) and 4-o
Examples include-(2-methyl) -butylresorcylidene-4'-octylaniline (MBRA8).

When using these materials to form an element, the liquid crystal compound maintains a temperature state where it becomes the SmC * phase or SmH * phase. Therefore, if necessary, use an element such as a copper block with a heater embedded in it. Can be supported.

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

FIG. 2 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 21a and 21b are In ₂ O ₃ , SnO ₂ and ITO (Indium-
It is a substrate (glass plate) coated with a transparent electrode such as thein-oxide), and SmC * phase liquid crystal oriented so that the liquid crystal molecular layer 22 is perpendicular to the glass surface is enclosed between the substrates. A thick line 23 represents a liquid crystal molecule, and 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 helical structure of the liquid crystal molecules 23 is unraveled, and all the dipole moments (P⊥) 23 are oriented in the electric field direction. Can be changed. 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 easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. 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 unwound and the dipole moment Pa or Pb thereof is released even when no electric field is applied as shown in FIG. Takes a state of either upward (34a) or downward (34b). When an electric field Ea or Eb having a polarity different from a certain threshold value is applied to such a cell for a predetermined time as shown in FIG. 3, the dipole moment becomes upward 34a or downward 34b with respect to the electric field vector of the electric field Ea or Eb. The orientation is changed, and the liquid crystal molecules are aligned in one of the first stable state 33a and the second stable state 33b accordingly.

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast, and the alignment of the second liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 2, for example, when an 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. 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 33b and changes its molecular orientation, but it remains in this state even when the electric field is turned off. Also, the applied electric field
As long as Ea does not exceed a certain threshold, they are also maintained in their respective orientations. 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 20 μ, particularly 1 μ to 5 μ is suitable.

A preferred embodiment of the device of the present invention is shown in the following figures.

FIG. 4 shows a cell 41 having a matrix pixel structure in which a bistable ferroelectric liquid crystal is sandwiched between a scan electrode group and a signal electrode group.
FIG. 42 is a scanning electrode group, and 43 is a signal electrode group. Now, in order to simplify the description, a case where a black and white binary signal is displayed will be described as an example. In FIG. 4, the shaded pixels correspond to "black" and the other pixels correspond to "white".

FIG. 5 shows a full-clear step T for applying a signal (referred to as full-clear signal) for aligning the screen to "white" prior to writing. That is, FIG. 5 (a)
Represents a voltage waveform 2V _o applied to all or predetermined portions of the scanning electrode group 42 at one time or as a scanning signal. FIG. 5B shows a voltage waveform −V _o applied to all or a predetermined part of the signal electrode group in synchronization with the signal applied to the scan electrode group 42. Further, FIG. 5 (c) shows a voltage waveform −3V _o when the pixel is applied. Since the above-mentioned full-clear signal -3V _o is applied to all or a predetermined part of the pixels at a voltage _exceeding the threshold voltage -V _th1 of the ferroelectric liquid crystal, the ferroelectric liquid crystal in one of the stable states ( The pixels are oriented in the first stable state), and the display state of the pixels is aligned with, for example, a "white" display state. That is, in this step T, the entire screen is arranged in the state of “white” at 1 o'clock or sequentially.

FIGS. 6 (a) and 6 (b) respectively show an electric signal given to the selected scan electrode and an electric signal given to the other scan electrodes (scan electrodes not selected), and FIGS. d) is not selected as an electric signal applied to the selected signal electrode (shown as black) and is not selected (shown as white)
Represents an electrical signal applied to the signal electrode. Figure 6 (a)
(D) The horizontal axis represents time and the vertical axis represents voltage, respectively. t ₂ and
t ₁ represents the phase to which the information signal (and the scanning signal) is applied and the phase to which the auxiliary signal is applied, respectively. In this example, t ₁ =
An example of t ₂ = Δt is shown.

Sequential scanning signals are selected for the scanning electrode group 42. Now, the application time Δ for giving the first stable state (display state based on the first stable state is white) of the liquid crystal cell having bistability
The threshold voltage at t is set to −V _th1, and the threshold voltage at the application time Δt for giving the second stable state (the display state based on the second stable state is black) is set to V _th2 . electrical signal applied to the scanning electrodes a sixth as phase (time) is shown in Figure (a) t ₁ At 2V _o, phase (time) in t ₂ -
The voltage is 2V _o . The other scanning electrodes are grounded as shown in FIG. 6 (b), and the electric signal is 0. On the other hand, a selected electrical signal applied to the signal electrodes sixth diagram to a phase t ₁ as shown in (c) Te at -V _o, a phase t ₂ V _o, also applied to the signal electrodes are not selected The electric signal to be transmitted has a phase as shown in Fig. 6 (d).
V _o at t ₁ and −V _o at phase t ₂ . In the above,
The voltage value V _o is set to a desired value that satisfies V _o <V _th2 <3V _o and −V _o > −V _th1 > −3V _o . FIG. 7 shows the waveform of the voltage applied to each pixel when such an electric signal is applied.

In FIG. 7, (a) and (b) are on the selected scanning line, and "black" and "white" are at the pixels to be displayed, and (c) and (d) are respectively. It is a voltage waveform applied to the pixel on the scanning line which is not selected.

In the pixel on the selected scanning electrode, the pixel on the selected signal electrode, that is, the pixel to be displayed as "black", is applied to the scanning line at the phase t _{1 as} shown in FIG. 7 (a). voltage the absolute value of (Figure 6 (a)) | 2V _o | absolute value of the signal line to the voltage applied (Figure 6 (c)) | V _o | added value of | 3V _o
|, And the voltage of the polarity on the side that gives the first stable state −
3V _o is applied. The pixel to which −3V _o is applied at the phase t ₁ is in the first stable state in advance by the full-surface clear signal, and thus the “white” state formed by the above-mentioned full-clear step is held. Further, the pixels on the signal electrodes are not selected, -V _o in phase t ₁ as shown in FIG. 7 (b)
However, since the voltage −V _o is set to be equal to or lower than the threshold voltage, the display state is not changed for the pixel which is already in the white state in the entire clear step.

In the pixel on the selected scanning electrode and the pixel on the selected signal electrode in the phase t ₂ , 3V _o is applied as shown in FIG. 7 (a). Therefore, in the pixel selected at this phase t ₂ , the second stable state threshold voltage V _th2 of the ferroelectric liquid crystal is
A voltage of 3 V _{o exceeding} the threshold voltage is applied to cause a transition to the display state based on the second stable state, that is, the black state.
Further, as shown in FIG. 7 (b), the voltage + V _o is applied to the pixels on the signal electrode which are not selected in the phase t ₂ ,
Since this voltage + V _o is set below the threshold voltage,
The display state at the phase t ₁ is maintained as it is. Therefore, the phase t ₂ means a phase (display state determining phase) that determines the writing state of the pixel on the scan electrode line. Further, at the phase t ₁ described above, since a voltage exceeding the threshold voltage is not applied to the pixels on the scan electrode line, it is possible to use an auxiliary phase that does not change the display state in the above-mentioned full clear step T. The signal applied to the signal electrode group at this time can be used as an auxiliary signal.

FIG. 8 shows the drive signals described above in time series. S _{1 to} S ₅ are electric signals applied to the scanning electrodes, I ₁ and I ₃ are electric signals applied to the signal electrodes, and I ₁ -S ₁ and I ₃ -S ₃ are shown in FIG. 4, respectively. It is a voltage waveform applied to pixels A and C.

The mechanism of switching due to the electric field of the ferroelectric liquid crystal in the bistable state is not always clear microscopically, but generally, after switching to a predetermined stable state for a predetermined time with a strong electric field, there is no electric field at all. It is possible to maintain the state of almost semi-permanently when it is left unapplied, but a weak electric field that does not switch for a predetermined time (in the example explained above, V _th or less (Corresponding to the voltage), if an electric field of opposite polarity is applied for a long time, the orientation state will be inverted again to the opposite stable state, and as a result, correct information display and modulation will be possible. Phenomena that 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 not affected by the material, roughness, liquid crystal material, etc. of the substrate surface. I recognized it, but I 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.

Therefore, the first phase t _{1 in} the device of the present invention is a phase capable of preventing a weak electric field in a constant direction from being continuously applied,
As a preferable example thereof, as shown in FIGS. 6 (c) and 6 (d), the information signal ((c) corresponds to black, (d) corresponds to white) and the polarity applied to the signal electrode group at the phase t _1. Different signals are applied at phase t ₂ . For example, when the pattern shown in FIG. 4 is to be displayed, if the driving method without the phase t ₁ is performed, the pixel A becomes black when the scan electrode S ₁ is scanned, but the signal electrode is generated after S _2. The electric signal applied to I ₁ has a continuous −V _o , and the voltage is the same as that of the pixel A.
Therefore, the pixel A is likely to be inverted to white in due course. According to the present invention, as described above, all the pixels on the screen are previously set to “white” and the phase is set when writing “black”.
While the voltage of the display -3 V _o is applied at t _1, this does not mean to determine the display state in the phase t _1, the voltage written "black" in the subsequent phase t ₂ 3V _o is applied.

If the writing time at this time is Δt,
The application time at t ₁ and t ₂ is Δt. Further, when "white" is held, the voltage is | ± V _o | and the application time is Δt. Furthermore, the voltage applied to each pixel is at maximum | ± V _o |
And, no matter how these states last, | ± V _o | will not last longer than time 2Δt except during the writing period.
Crosstalk does not occur at all, and once the scanning of the entire screen is completed, the displayed information is as semi-permanently retained as in a display element using a normal TN liquid crystal that does not have bistability. , No refresh process is needed.

Figure 9 is another embodiment of a full clear signal, Figure 9 (a) is a voltage waveform applied to the scan line, 2V _o at phase P _1, the phase P ₂ 2V _o, 9 Figure (b) is a waveform of the voltage applied to the signal line, and -V _o in phase P ₁ V _o, the phase P _2. 9th
In FIG. (C) is a voltage applied to the pixel, V _o becomes at P _1, -3 V _o becomes "white" state is written once to become but the phase P ₂ following "black". By doing so, the voltage applied to all the pixels can always have an average value of 0, and therefore the possibility of causing the above-mentioned crosstalk is completely eliminated.

〔The invention's effect〕

According to the present invention, even when a display panel using a ferroelectric liquid crystal element is driven at high speed, the maximum pulse width of the voltage waveform continuously applied to the pixels on the scan electrode line to which the scan non-selection signal is applied is Since it is twice the pulse Δt at the time of writing, it is possible to effectively prevent the phenomenon that the display state is inverted to another display state during the writing scanning of one screen.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing threshold characteristics of a ferroelectric liquid crystal. 2 and 3 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 4 is a plan view of the matrix pixel structure used in the present invention. Fig. 5 (a)
(C) is an explanatory view showing a voltage waveform at the time of the full clear step. FIGS. 6A to 6D are explanatory diagrams showing voltage waveforms of signals applied to the electrodes. FIGS. 7A to 7D are explanatory diagrams showing voltage waveforms of signals applied to pixels, respectively. FIG. 8 is an explanatory diagram of voltage waveforms representing the above signals in time series. FIG. 9 (a)-
(C) is an explanatory view showing another voltage waveform at the time of full clear step.

Claims (2)

[Claims] 1. A matrix electrode formed of a scanning electrode group formed of a plurality of scanning electrodes and a signal electrode group formed of a plurality of signal electrodes, and different orientations according to voltage application of different polarities exceeding a threshold voltage. An optical modulation element having a ferroelectric liquid crystal that causes a state, b. Means for simultaneously applying a polarity voltage exceeding the threshold voltage of the ferroelectric liquid crystal at the intersection of the scanning electrode group and the signal electrode group, c. .The scan electrodes are sequentially scanned, and the scan electrodes selected for scanning are
Based on the voltage applied to the unselected scan electrodes,
A scan selection signal having a bipolar pulse composed of one polarity pulse and the other polarity pulse, and a voltage average value of the bipolar pulse is set to 0 based on the voltage applied to the unselected scan electrode is applied. Means, and d. Applying one information signal having a bipolar pulse having the same phase as the bipolar pulse to the selected signal electrodes of the signal electrode group in synchronization with the bipolar pulse of the scanning selection signal. As a result, a voltage that does not exceed the threshold voltage of the ferroelectric liquid crystal is applied to the intersection of the scan electrode to which the scan selection signal is applied and the selected signal electrode, and the remaining signal electrodes are connected to the bipolar electrodes. Bipolar signal in the information signal is applied to the intersection of the scan electrode to which the scan selection signal is applied and the remaining signal electrode by applying the other information signal having the bipolar pulse having the opposite phase to the sex pulse. During the second half pulse of the applying means for applying the other polarity voltage exceeding the threshold voltage of the ferroelectric liquid crystal, an optical modulation device having a. 2. A voltage average value of bipolar pulses of the one and the other information signals is 0 with reference to a voltage applied to an unselected scan electrode.
An optical modulator according to the item.