JPS61223828A - Method for driving optical switching element - Google Patents

Abstract

PURPOSE:To prevent decline of optical switching performance and deterioration of element in the driving method of an optical switching element using a ferrodielectric substance, by using AC waveforms as much as possible in the driving waveform of the element. CONSTITUTION:Optical switching sections correspond to the intersecting points of signal electrodes S1-Sn and a common electrode C. When the optical switch of this invention is opened (selected condition), a voltage 2VO is applied across the common electrode C for a period TD only and no voltage is applied during another period TA. When the optical switch is closed (nonselected condition), no voltage is applied during a whole period TS. when such waveforms are impressed, the voltage VF which is applied across the common electrode C and signal electrode S1, namely, across a ferrodielectric substance becomes V51-VC and the time during which a DC voltage is applied can be shortened and an AC voltage can be applied during the remaining period. Accordingly, decline of the optical switching characteristic and DC deterioration of the liquid crystal material resulting from the charge up caused by continuous application of a DC voltage can be prevented.

Description

Detailed Description of the Invention (Field of Application of the Invention) The present invention relates to a method for driving an optical switch element, and in particular to a method for driving an optical switch element suitable for use in optical switch arrays for printers, displays, etc. using ferroelectric liquid crystals. Regarding the law.

[Background of the invention]

As an optical switch element using a ferroelectric substance, an optical switch element using transparent ceramics (PLZT) is well known (Japanese Unexamined Patent Publication No. 57-93316).

On the other hand, recently, optical switching devices using chiral smectic liquid crystals as ferroelectric materials have been attracting attention.
This switch element will be described below. First, in order to clarify the chiral smectic liquid crystal, Table 1 shows the chiral smectic C liquid crystal SmC'' (DO
The chemical structures and phase transition temperatures of BAMBC, OOBAMBCC) and chiral smectic H liquid crystal SmH'' (HOBACPC) are shown.

Next, Figure 4 shows these chiral smectic liquid crystal molecules (hereinafter abbreviated as liquid crystal molecules unless otherwise specified).
shows electric field response. As shown in Figure 4, liquid crystal molecules 2
is the helical axis 1 when no electric field is applied (E=O)
It has a "twisted structure" around it. In this way, when an electric field E greater than or equal to the critical electric field E0 determined by the physical properties of the liquid crystal (spontaneous polarization, twist viscosity) is applied to the liquid crystal molecules in a direction perpendicular to the helical axis 1, the liquid crystal molecules 2 move in the direction of spontaneous polarization 3. In order to align the liquid crystal molecules 2 in the direction of the electric field E, as shown in FIGS. ) to ensure uniform orientation. By utilizing the DC electric field responsiveness of such liquid crystal molecules 2, light can be transmitted,
An optical switch element that can be cut off can be obtained.

Next, FIG. 5 shows the structure and operating principle of a birefringence type optical switch element that transmits and blocks light by utilizing the birefringence of liquid crystal molecules 2. As shown in FIG. 5(a), in the birefringent type, the liquid crystal layer 4 is connected to a pair of transparent electrodes 6a,
6b are sandwiched in parallel between glass substrates 5a and 5b formed on their surfaces, and two polarizing plates 7a and 7b are placed on both sides of the substrates 5a and 5b.
are arranged so that their polarization axes are perpendicular to each other. At this time,
As shown in FIG. 5(b), the polarization axis 7ax of the polarizing plate 7a
If we set it at an angle θ with respect to the helical axis 1, we get
When a negative DC electric field IEI<IE (+1) is applied, as shown in FIG. 5(d), the alignment direction of the liquid crystal molecules 2 coincides with the polarization axis 7ax of the polarizing plate 7a, so that the light source 8 is connected to the optical switch element. The incident light 9 is not transmitted and is blocked.On the contrary, when a positive DC electric field (E>Eo) is applied, the alignment direction of the liquid crystal molecules 2 is aligned with the polarization axis, as shown in FIG. 5(c). 7ax, light is transmitted due to the birefringence effect.In this way, by reversing the polarity of the DC electric field E, an optical switch operation can be performed, and the response time is from several tens of microseconds to several tens of microseconds. Fertilizer and extremely fast.

Furthermore, if liquid crystal is used, the thickness of the liquid crystal layer 4 can be reduced to several micrometers, so that it can be driven at a low voltage of about 10 to 20V. On the other hand, transparent ceramics (PLZT
) requires several hundred V, which has the advantage of requiring an order of magnitude lower driving voltage. On the other hand, FIG. 6 shows the structure and operating principle of a guest-host type optical switch element in which a dichroic dye is mixed into the liquid crystal layer to control the amount of light transmission.

In guest-host type devices, dichroic dyes and
For example, if a black pigment is mixed in, only one polarizing plate is required. The polarization axis 7ax of the polarizing plate 7a is shown in FIG.
Arrange as shown in b). As shown in FIG. 6(d), when a negative DC voltage (IEI<IEcI) is applied, the dichroic dye molecules 10 are aligned in the same state as the liquid crystal molecules 2, so the absorption axis of the dichroic dye molecules 10 is (Long axis of the molecule) coincides with the polarization axis 7ax of the polarizing plate, and the light 9 incident on the liquid crystal layer 4 is absorbed and is not transmitted. Conversely, positive voltage (
When E>E) is applied, the direction in which the dichroic dye molecules are arranged by 1° is shifted from the polarization axis 7ax, so that the light is transmitted through the optical switch element without being absorbed.

In this way, the guest-host type optical switch element can also switch light by reversing the polarity of the DC electric field E, similar to the birefringence type optical switch element (FIG. 5). Like birefringent type elements, this type of element also responds quickly at low voltage.

The conventional driving method for optical switching devices using ferroelectric liquid crystals is to apply a positive or negative DC voltage as shown in Figure 7 based on the DC electric field response of ferroelectric liquid crystal molecules explained in Figures 3 to 5. Select time or.

The method was to always apply it during the non-selection time.

(Japanese Unexamined Patent Publication No. 59-138786) This method has the following problems as an optical switch element.

(1) It is necessary to constantly apply a DC voltage, which poses problems such as deterioration of the liquid crystal material and deterioration of switching characteristics due to charge-up.

(2) In the conventional driving method, the optical switch characteristic has two stages of light transmission and light blocking, and the amount of light transmission cannot be changed.

(3) As shown in (1), since DC voltage is always applied, the method for driving an optical switch array consisting of multiple optical switches is static drive, which requires the same number of drivers as optical switches. becomes.

[Purpose of the invention]

The purpose of the present invention is to prevent deterioration of optical switch performance and element deterioration and to freely change the amount of light transmission by changing the drive waveform to alternating current in a method for driving an optical switch element using a ferroelectric substance. [Summary of the Invention] Since ferroelectric liquid crystal molecules have spontaneous polarization, their response to an electric field is extremely fast compared to nematic liquid crystal molecules, and their response to applied voltage and the magnitude of spontaneous polarization is extremely fast. It depends, but if the applied voltage is 2
At 0V, it responds to voltage at a rate of 0.5 to 1 fertilization.

If this is converted into a frequency, it will respond to voltage pulses of IKHz to 2KHz. Conversely, I thought that if the frequency of the voltage was further increased, it would become impossible to follow voltage changes above a certain frequency.

FIG. 8 is a diagram showing the results of this experiment. The figures (a) and (c) show the selection voltage waveforms, and the figures (b) and (d) show the non-selection voltage waveforms. When the optical switch element is operated to select (switch open - light transmission) and non-select (switch closed - light cut off) at a constant period T8, first.

The DC voltage (
+vo(v) when selected, - when not selected
After applying V, (v)), an alternating current voltage is subsequently applied for a period T1. The period of the AC voltage waveform is set to T, and this frequency fK (1/T,) is increased. (a
)(b) shows the voltage waveform and light transmission characteristics when fl is relatively small.Since the liquid crystal molecules can somewhat follow voltage changes, the amount of light transmission changes in accordance with changes in the polarity of the AC voltage. On the other hand, when fl becomes very high, (c) (
As shown in d), it was found that the amount of light transmission was maintained in a fixed state by applying a DC voltage.

The reason why such a phenomenon occurs is considered to be that liquid crystal molecules are unable to follow voltage changes.

Therefore, the ferroelectric liquid crystal DABAMBC (shown in Table 1)
Chiral smectic C in the temperature range from 76℃ to 95℃
A device having the configuration shown in FIG. 5(a) was manufactured using the following phase (indicating phase), and the amount of light transmitted was measured by applying a voltage waveform as shown in FIG. 8 to the device. Note that the thickness of the liquid crystal layer is 5 μm. Regarding the changes in the amount of light transmission when applying the selection voltage waveforms as shown in Figure 8 (a) and (c), the amount of light transmission when DC voltage is applied is B6°, and the minimum amount of light transmission when AC voltage is applied is B6. As a technical measure, the light transmission amount retention rate was defined as Bx/B. 9th
The figure shows the relationship between the retention rate and the frequency of the AC voltage waveform.

As the frequency f1 increases, the retention rate increases,
When the liquid crystal temperature is 80℃,! , when the retention rate is 100% at 11KHz or higher and the temperature is 90℃, ! , is 15KHz
When it becomes more than that, the retention rate becomes 100%. The reason why it is necessary to increase fII as the temperature rises is because the viscosity of the liquid crystal decreases and molecules move more easily.

Similarly, when applying non-selective voltage waveforms as shown in Figures 8(b) and 8(d), a similar tendency was observed;
In this case, set fl to 11KHz or higher.

In the case of 90° C., it was found that when the frequency is set to 15 KHz or more, the amount of light transmission does not increase and the value of B is maintained.

As can be seen by comparing Figures 8(c) and 7(d) with Figure 7,
Although the voltage waveforms are different, the amount of light transmission is almost the same. In the case of Fig. 8(c) and (d), since the AC waveform is added during the TA period, the phenomenon of charge-up of the liquid crystal layer does not occur, and the DC voltage application time To is short, so the DC voltage deterioration of the material It also becomes less likely to occur.

[Embodiments of the invention]

FIG. 1 shows an embodiment in which the present invention is applied to an optical switch array. (a) shows the electrode arrangement, 5xtsz+
...Sl...S are signal electrodes, C is a common electrode, and the optical switch section corresponds to the intersection thereof.・(
b) shows the voltage waveform Vo applied to the common electrode C and the signal electrode S.
, is a voltage waveform V□ applied to . T is the opening/closing time of the optical switch; when the optical switch is open (selected state), a voltage of 2 V is applied for the period T, and 0 V is applied for the period T. Further, when the optical switch is closed (non-selected state), the voltage is set to zero V for the entire period of T6. When such a voltage waveform is applied, the voltage V applied between the common electrode C and the signal electrode S, that is, to one ferroelectric substance, is V□
-vc, and the voltage waveform is as shown in Figure 8 (C) when selected.
, when not selected, the waveform becomes as shown in FIG. 8(d), enabling the driving method of the present invention.

The present invention was applied to an optical switch array for an optical printer. The liquid crystal material used was DOBAMBC, and the temperature was controlled so that the temperature of the liquid crystal layer was 80°C.

To enable printing at a printing speed of 80 mm/sec with a resolution of 10 dots/nm, the processing time T for one line is 1.
It will be 25m5. Therefore, T1 was set to 0.5 ms.

T1 is 0.75m5, vl is 30V, V is 20
It was set to V. Further, the frequency of the AC voltage applied during the period T was set to 12 KHz. As a result, the same light transmission characteristics as when a DC voltage is constantly applied can be obtained, and the ratio of the amount of light transmission when the switch is open and when the switch is closed, that is, the contrast ratio Ba/Bo can be set to 20 to 30. Ta.

Further, since the opening/closing speed of the optical switch is determined by the DC voltage applied during the first TD period of each line processing time T1, there is no influence from the application of the AC voltage. In order to increase the switching speed, the DC voltage value v0 may be increased. Figure 2 shows the relationship between drive voltage v0 and response time. When v6 is 20V, it is 0.5ms.
becomes. .

It is preferable to ensure that the period T is enough for the liquid crystal molecules to completely unhelix and be uniformly aligned at the chilled angle θ by applying the voltage v0, that is, for the response time.

FIG. 1 shows the case of static drive. −
Of the line processing time T8, the period during which the voltage is applied to the signal electrode S is only T, so that it is possible to drive another line during the remaining period TA. In other words, time-division driving is also possible.

FIG. 3 is a diagram showing another embodiment. To control the print gradation of an optical printer, it is necessary to continuously change the amount of light transmitted through the optical switch. FIG. 3 shows a driving method for realizing this.

Voltage V applied to common electrode C. has a waveform as shown in FIG. 1(b), but the frequency of the AC voltage waveform is set to a relatively low frequency. In the voltage waveform V□ applied to the signal electrode S, the applied voltage during the period T in FIG. 1 was zero V, but in FIG. 3, a DC voltage is applied and this DC voltage value is changed. Accordingly, a DC bias is applied to the AC voltage applied to the liquid crystal layer during the period T1. As this DC bias value increases, the amount of light transmission increases. In FIG. 3, as the DC voltage value increases from Ov→v2→v3, the amount of light transmission increases from 5A4S to →So.

In FIG. 3, the amount of light transmission is adjusted by changing the bias voltage value, but it can also be done by changing the frequency of the AC voltage applied to the liquid crystal layer during the TA period (frequency modulation). This is obvious from the light transmission amount retention characteristic shown in FIG.

〔Effect of the invention〕

According to the present invention, as a method for driving an optical switch element using a ferroelectric substance, the DC voltage application time can be shortened, and
Since AC voltage is applied during the remaining period, it is possible to prevent deterioration of optical switching characteristics due to charge-up due to continuous application of DC voltage and DC deterioration of the liquid crystal material.

[Brief explanation of the drawing]

1 to 3 are diagrams showing one embodiment of the present invention, and FIG.
Figure 5 shows the electric field response of ferroelectric liquid crystal molecules, Figure 5 shows the structure and operating principle of an optical switch using birefringence, and Figure 6 shows the structure and operating principle of a guest-host type optical switch. Diagram of operation principle. FIG. 7 is a diagram showing the conventional driving method, and FIGS. 8 and 9 are diagrams showing the main points of the present invention. DESCRIPTION OF SYMBOLS 1... Helix axis, 2... Liquid crystal molecule, 3... Spontaneous polarization, 4... Liquid crystal layer, 5... Substrate, 6... Electrode, 7
... Polarizing plate, 8... Light source, 9... Incident light, 1o.
...Dichroic pigment.

Claims (1)

[Scope of Claims] 1. A method for driving an optical switching element that controls the amount of light transmitted through a pressed ferroelectric material by interposing a transparent ferroelectric material between a pair of electrodes and applying a voltage, the method comprising: An optical switching element characterized in that the selection voltage waveform and the non-selection voltage waveform each consist of a combination of a DC voltage waveform and an AC voltage waveform, and at least the polarity of the DC voltage during selection and non-selection is reversed. driving method. 2. The selected or unselected voltage waveform is a voltage waveform such that after the DC voltage is applied, the AC voltage is subsequently applied. Driving method for optical switch elements. 3. A patent characterized in that the amount of transmitted light of the optical switch is arbitrarily changed between the amount of transmitted light in a non-selected state and the amount of transmitted light in a selected state by changing the frequency of the AC voltage waveform. A method for driving an optical switch element according to claim 1. 4. The light according to claim 1, wherein the amount of light transmitted through the optical switch in a non-selected state or a non-selected state is changed by applying a DC bias to the AC voltage waveform. How to drive a switch element. 5. The method of driving an optical switch element according to claim 1, wherein the ferroelectric substance is a chiral smectic C liquid crystal or a chiral smectic H liquid crystal having ferroelectric properties.