JPS61100721A - Driving method of optical switch - Google Patents

Abstract

PURPOSE:To obtain an optical switch driving method characterized by high contrast and high reliability by making the waveform of a driving voltage appropriate. CONSTITUTION:A voltage to be impressed to a liquid crystal element selects a waveform (b) at the transmission of light (ON) or a waveform (c) at the interruption of lgiht (OFF), the frequency fL2 (=1/TL2), fL1 (=1/TL1) of respective high frequency voltages are set up 10kHz and the value of V1 is set up to zero. A positive high frequency pulse voltage of 10kHz is always applied to a common electrode and a positive high frequency pulse voltage of 10kHz shifted at its phase by TLx2 at the transmission of light and to be turned to zero potential at the interruption of light is impressed to a signal electrode. Consequently, a prescribed voltage can be impressed between both the electrodes and proper change of light transmission can be obt.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for driving an optical switch using a ferroelectric substance such as a ferroelectric liquid crystal, and in particular to an optical switch array used in a printing head for an optical printer. The present invention relates to a method for driving an optical switch that is suitable as a method for driving an optical switch.

[Background of the invention]

Ferroelectric liquid crystals, unlike conventional nematic liquid crystals, respond to direct current voltage and have an extremely fast response speed of 1 ms or less. Therefore, the development of a print head for an optical printer using an optical switch that takes advantage of this high-speed response is expected.

Figure 3 shows the configuration of a printing head consisting of an optical switch array 3 having a micro optical switch section 7, a light source 6, and a SELFOC lens 2 that focuses light on the photoreceptor drum 1.
5 is a rod lens that focuses the light from the light source onto the optical switch array 3.

As shown in FIG. 4, the optical switch array is composed of minute optical switch sections 4bt+, 4b, .
w iz*... is opened and closed to create a light pulse train for printing one line. 4. indicates a common m-pole. In order to obtain a light pattern as shown in FIG. 5 using such an optical switch, the optical switch parts 4, 1 . ..., fully open 46, 2nd and 3rd line T,...
Fully open at T, fully open at 4th line T4, 5th line T
In S, it is fully closed.

Next, the electric field responsiveness of ferroelectric liquid crystal molecules will be explained using FIG. Take structure. A characteristic of this type of liquid crystal is that the liquid crystal molecules 8 have spontaneous polarization 9 in a direction perpendicular to the molecular axis 7, and an electric field E larger than the threshold electric field E is applied from a direction perpendicular to the helical axis 10. Then, the liquid crystal molecules 8 are oriented so that the direction of the spontaneous polarization 9 is aligned with the direction of the electric field. As a result, by reversing the direction of the electric field E,
The liquid crystal molecules 8 are centered around the helical axis 10.

They are arranged at an angle (tilt angle) θ with respect to the helical axis 10 on the left and right. In this way, ferroelectric liquid crystal molecules respond to DC voltage (electric field).

FIG. 7 is a diagram showing the structure and operating principle of an optical switch element that utilizes the birefringence of liquid crystal molecules. Two substrates 12a each having a liquid crystal fill and transparent electrodes formed thereon.

The substrates 12b are held in parallel, and two polarizing plates 13a and 13b are placed on the outside of the substrates so that their polarization axes are perpendicular to each other. At this time, the angle formed between one polarization axis 13a and the helical axis 10 is made equal to the tilt angle θ of the liquid crystal molecules 8.

When an electric field larger than the operating threshold electric field E of the liquid crystal molecules 8 is applied from the back side to the front side of the paper (negative voltage applied) as shown in FIG. 7(d), the liquid crystal molecules 8 are Since the arrangement direction of 8 coincides with the polarization axis of the polarizing plate 13a,
The light 14 that enters the liquid crystal element from the light source 6 cannot pass through the element, and the light is blocked. 6 Conversely, when an electric field is applied from the front side of the paper toward the back side (positive voltage applied), the result shown in Fig. 6 (a) is Since the alignment direction of the liquid crystal molecules 8 does not coincide with either of the polarizing plates 13a and 13b, the light 14 incident on the liquid crystal screen is transmitted through the element due to the birefringence of the liquid crystal molecules. The method described above uses two polarizing plates, but there is also a guest-host type optical switch element in which a dichroic dye is mixed into the liquid crystal layer 11 and one polarizing plate is used. Detailed explanation will be omitted.

In either method, the ferroelectric liquid crystal element functions as an optical switch element by reversing the polarity of the DC voltage.The only characteristics required for optical switch arrays for printers and optical switches for optical waveguides are high-speed response. However, it has high contrast characteristics, and in the case of a printer, a contrast of at least 10 or more is required, and in the case of an optical waveguide, a contrast of 20 or more is required.

As described above, higher contrast is required compared to display elements.

FIG. 8 shows the voltage applied between each signal electrode 4.1 to 46 and the common electrode 4 necessary to obtain the optical pattern array shown in FIG. This figure shows the voltage waveform.

(2) is a voltage sufficiently higher than the operating threshold voltage.

In this case, the liquid crystal molecules are as shown in Figure 7(c) when a voltage of +V is applied, and as shown in Figure 7(d) when a voltage of -V is applied.
), so the contrast B, /Bfh is 2
It becomes a very large value of 0 or more.

Although high contrast can be obtained with this method,
If the state of light transmission or light blocking continues for a long time, a DC voltage of the same polarity will be continuously applied. Continuously applying DC voltage to liquid crystal materials is undesirable because the liquid crystal deteriorates due to electrochemical reactions.Therefore, liquid crystal elements used in calculators, watches, graphic displays, etc. that use nematic liquid crystals use AC voltage. , the driving waveform is such that the average voltage is zero. Special request 1975
In the device shown in Japanese Patent No. 8-216469, the drive voltage waveform is selected so that the average voltage becomes zero within one line printing time T, as shown in FIG. 9(a). That is, V, -T1=Vt -T.

satisfy. However, in this case, if the light source 6 is a continuously lit light source, light leakage 15 occurs during the period when light should not be transmitted (OFF period).
A flashing light 6' that flashes at a constant cycle as shown by the broken line in C)
By adopting.

This eliminates the 15" light leakage that was a problem when using a steady light source. As a result, the xenon lamp However, although the contrast can be increased with the proposed method, a method is proposed in which the steady light source is converted into flashing light by using a light source or by providing another optical switch element between the steady light source 6 and the liquid crystal optical switch array 3. In order to make the average voltage zero during a certain period T of the line application time T, it is necessary to apply a voltage with the opposite polarity to the voltage applied to the liquid crystal layer during period T1. , when the light transmission state is selected, the period during which light actually passes through one line printing time T is as short as T1.Therefore, 1
The light energy that can be applied to the photosensitive drum 1 within the line printing time is T1/T. If v+, is 20V and V, is 50V, the value of Tz /T is 5/7 (=0.7
＞, and the light utilization rate decreases. There is no problem when the light from the light source is sufficiently large or when printing is performed at low speed, but when performing high-speed printing using a fluorescent lamp as the light source, the amount of light becomes insufficient. When using a 20-30W fluorescent lamp,
The maximum printing speed is 18 with a resolution of 10 dots/I1m.
The limit is 00 lines/minute. If one period is completed, the time T
1000 lines/minute is fully possible if it can be used.

Furthermore, when a fluorescent lamp is used, a flashing light conversion element is required, which increases the number of parts and increases costs.

Further, a method for driving a display element using a ferroelectric liquid crystal material is described in Japanese Patent Laid-Open No. 179890/1983. The basic idea is to always make the average voltage zero, and a waveform as shown in FIG. 9(a) is used. When a steady light source is used, Figure 9 (b
) As shown in Figure 9(a), light leaks when the display is off, but since the display is intended for human eyes, the contrast ratio is only an apparent value; The combination with the drive voltage waveform shown in can be used.

However, in optical switch arrays used for printers and optical switches for optical waveguides, contrast has become more important, and it is necessary to minimize light leakage, especially when the switch is off.

[Purpose of the invention]

The purpose of the present invention is to: (1) optimize the drive voltage waveform in an optical switch element that transmits or blocks light by applying a voltage to a ferroelectric material, particularly a ferroelectric liquid crystal material;
An object of the present invention is to provide a method for driving an optical switch that has high contrast and is excellent in reliability.

[Summary of the invention]

In order to achieve the above object, the present invention provides an optical switching element that transmits and blocks light by interposing a ferroelectric substance between a pair of electrodes and applying a driving voltage between the electrodes. The voltage is a high frequency voltage in which unipolar voltage pulses with a peak value larger than the operating threshold voltage of the ferroelectric material and voltage pulses of opposite polarity with a peak value smaller than the operating threshold voltage are alternately and periodically repeated. This drive voltage is applied with opposite polarity when transmitting light and when blocking light.

[Embodiments of the invention]

Next, a method for driving an optical switch according to the present invention will be explained based on the drawings.

First, before explaining a specific example, a fundamental explanation of the driving method will be described.

In the polarizing plate arrangement shown in FIG. 7, a conventional DC voltage has been applied in order to obtain a light transmitting state (ON state).

On the other hand, a positive pulse voltage value V with a period T1 as shown in FIG. 10(a). , a negative pulse voltage value - ■1 frequency f where an AC voltage waveform of 1 was applied to the element and the light transmission characteristics were investigated.
In the case of low frequencies where L (=1/TL) is relatively low,
As shown in FIG. 10(b), when the applied voltage becomes -V1, the amount of light transmission decreases to B (<100%), and the total amount of light transmission within the -period TL period becomes small.

Therefore, Figure 10 (C) shows the change in the amount of light transmission when the frequency f1 is increased to the first order.

As shown in (d), the change in the amount of light transmission becomes small.

FIG. 11 shows the experimental results. The strong HM of the device, the p-segment of the liquid crystal layer is 4 μm, and the operating threshold voltage value V is 3V. Curve A is the frequency characteristic of the light transmission amount B when the positive pulse voltage value V is 20V and the negative pulse voltage value -■ is -2■. As the frequency becomes higher, B becomes larger, and at IKHz, B becomes 90% of the saturation value. On the other hand, the curve 11IB shows the frequency characteristics of the light transmission amount B when the positive pulse voltage value is +20V and the negative pulse voltage value is -20V. I hope to see you soon.

The frequency starts to increase rapidly around IKHz.

At around 10 KHz, B becomes 90% of the saturation value.In the case of 0 mB, as B increases, the frequency increases, but the average voltage value becomes zero, which is preferable from the viewpoint of the lifespan of the liquid crystal. The values change slightly depending on the type of material and the thickness of the liquid crystal layer, but the trend remains the same.

Figure 12 shows a negative pulse voltage -v, contrary to Figure 10(c).
The absolute value of o is the operating threshold voltage V of the liquid crystal. This figure shows the light transmission characteristics when the positive pulse voltage v1 is set to be larger than V and smaller than V.

When the applied voltage becomes +V, the amount of light transmission B increases:
When obtaining a light-blocking state, the smaller the value of B, the better.

FIG. 13 shows the frequency characteristics of the amount of light transmission (leakage light t) B when -V is -20V and Vl is 2V. As will be seen, as the frequency increases, the leakage light amount B decreases, and when the frequency exceeds IKHz, the leakage light amount becomes as small as 3%, assuming that the maximum transmission softness in the light transmission state is 100%.

From the following, when obtaining a light transmission state, Fig. 10 (C)
In order to obtain a light blocking state, the voltage waveform of
By using the voltage waveform a) and appropriately selecting the frequency, the ratio of the amount of light transmitted in the light transmitting state to the amount of leaking light in the light blocking state, that is, the contrast ratio, can be made extremely high at 30 to 40.

FIG. 14 shows the driving voltage waveform of a printing head for a printer using ferroelectric liquid crystal based on the above experimental data. Here, T is one line printing time. The drive voltage waveform in the light transmitting state (ON state) is shown in (a) in Figure 14.
Alternatively, the voltage waveform in FIG. 14(c) is used as the waveform in FIG. 14(b) and the drive voltage waveform in the light cutoff state (OFF state). In Figure 14, the frequency of the voltage waveforms in (a) and (C) is Cb
) is shown lower than the frequency of the voltage waveform, but it goes without saying that the same effect can be obtained with the same frequency.
The waveform in FIG. 14(b) is an AC voltage waveform with an average voltage of zero, so there is no problem of DC deterioration of the liquid crystal material.Furthermore, in the case of waveforms as shown in FIGS. 14(a) and (c), Although the average voltage is not zero, the time it is actually applied to the liquid crystal layer is at most 1/2 of the one-line impression time, so
This has the advantage that deterioration of the liquid crystal is less likely to occur.

In general, electrophotographic printers that print by irradiating light onto a photoreceptor drum have two types of development methods: reversal development, in which toner is deposited on areas that are irradiated with light, and regular development, in which toner is deposited on areas that are not irradiated with light. There is.

Normally, on computer terminals and word processors, there is more blank space for white characters than the area on which characters and patterns are printed.6 Therefore, if the regular development method is used as the development method, light irradiation As the time becomes longer, the waveform of FIG. 14(b) in which the average voltage is completely zero can be used, and the deterioration of the liquid crystal gradually decreases.

Next, the present invention will be explained in more detail with reference to FIG.

A signal ff1ll+substrate 11b having a group of signal electrodes 4b and a common electrode substrate 11a having a common electrode 4a are held in parallel, and a ferroelectric liquid crystal DOBAMBC (76° C. to 9° C.
We prototyped a liquid crystal optical switch array in which a liquid crystal optical switch (which exhibits a chiral smectic C phase at 5°C) was sealed.

Reference numeral 16 denotes a liquid crystal alignment film made of a polyimide rubbing film which also serves as an insulating film. The liquid crystal layer thickness was 4 μm.

The printing head was targeted to have a resolution of 10 dots/m and a printing speed of 80 m/sec (approximately 1000 lines/min).

1 line printing time T is 1.25m5 (800Hz)
becomes.

The 14th waveform of the voltage applied to the liquid crystal element when light is transmitted (ON)
In Figure (b), the waveform in Figure 14 (c) is selected as the applied voltage waveform during optical cutoff (OFF), and the frequency of each high-frequency voltage is set! , , (=1/TL2) and f Lm (=1
/TLl) was set to 10KHz. Further, the value of vl was set to zero.

Figure 2 shows the voltage waveform v0 applied to the common electrode, the voltage waveform v19 applied to the signal electrode, and the voltage v19 applied between both electrodes.
, -v, and the light transmission waveform. A positive high frequency pulse voltage of 10 KHz is always applied to the common electrode, and the signal electrode has a voltage of TL2 for v8 when light is transmitted.
10KHz positive high frequency pulse voltage waveform with a phase shift of /2.

When light is cut off, the potential is zero. Thereby, a predetermined voltage can be applied between both electrodes, and a favorable change in the amount of light transmission can be obtained.

As a result of experiments, it was found that the ratio of the amount of transmitted light B when light is transmitted and the amount of leaked light B0 when light is blocked, that is, the contrast, is about 30, and a high contrast can be obtained. Furthermore, it was found that the response time from the ON state to the OFF state and from the OFF state to the ON state was 0.2 to 0.3 ms, which enabled a high-speed response.

FIG. 15 is a diagram showing another embodiment, in which light transmission 1
The first voltage waveform applied to the liquid crystal element when (ON)
Figure 4 (a) shows the voltage waveform V applied to the common electrode when the waveform of Figure 14 (c) is adopted as the applied voltage waveform during light cutoff (OFF), (Figure (a)) and the voltage waveform applied to the signal electrode. The voltage waveform V, lb)) and the voltage v applied between both electrodes
, -y, and the light transmission waveform. Frequency of high-frequency pulse f Lm (=' 1 / T 1.-
) was set to 10KHz. The differences from Figure 1 are ■, and vt
are in the same phase, and the applied voltage of (2) is 2vn. Contrast and response characteristics similar to those shown in FIG. 1 can be obtained.

In each of the above embodiments, T5 and T, , 2 are the same, but T -2 > T L 1 or T L 2 <
T ,, may be 1 0 Frequency-wise, Figs. 11 and 13
The value of vl is set to zero as long as it satisfies the conditions in the figure, but this value can be freely selected as long as it is smaller than V6.Furthermore, the pulse voltage value can be set to duty50.
%(T,=TL/2), but there is no need to fix it to this, and T,<T,/2 or T,>TL/2 may be used.

As shown in FIG. 7, the direction of the polarization axis of the polarizing plate 13a was made to match the alignment direction of the ferroelectric liquid crystal molecules when a negative DC voltage was applied; It may be made to match the alignment direction of the dielectric liquid crystal molecules, and in this case, the ON and OFF states in each of the above-mentioned embodiments will be reversed.

The driving method described above has a first-order effect.

(1) Since the DC time applied to the liquid crystal layer can be reduced to at least 1/2 or less of the high frequency pulse voltage compared to conventional methods, DC deterioration of the liquid crystal material can be suppressed.

(2) It is not necessary to use a flashing light as a light source, a single point dimming conversion element is not required, and a low-cost fluorescent lamp can be used as is, which is effective in terms of economy and fill simplification.

(3) Since the light irradiation time can be made equal to the one-line printing time T, there is an effect of improving the utilization efficiency of light energy.

〔Effect of the invention〕

As described above, according to the present invention, since light is transmitted and blocked by a drive voltage with an appropriate voltage waveform, it is possible to transmit and block light with little direct current deterioration and high contrast.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view of an optical switch to which the driving method according to the present invention is applied, Fig. 2 is a waveform diagram showing the relationship between the driving voltage waveform and the amount of light transmission according to the present invention, and Fig. 3 is a general print head. FIG. 4 is a perspective view of a general optical switch array, FIG. 5 is an explanatory diagram showing the light transmission and blocking patterns of the optical switch array, and FIG. 6 is an exploded perspective view showing the structure of ferroelectric liquid crystal molecules. Figure 7 is an explanatory diagram showing the operating principle of liquid crystal molecules, Figure 8 is a waveform diagram showing the conventional voltage waveform necessary to obtain the light pattern of Figure 5, and Figure 9 is an explanatory diagram showing the electric field response of . 10 is a waveform diagram showing an example of a conventional driving voltage waveform, FIG. 10 is a waveform diagram showing the relationship between the driving voltage waveform and the amount of transmitted light to explain the driving method according to the present invention, and FIG. 11 is a frequency characteristic showing experimental results. 12 is a waveform diagram showing the relationship between the drive voltage waveform and the amount of light transmission for obtaining a light blocking state in the present invention, and FIG.
Fig. 3 is a frequency characteristic diagram when driven with the drive voltage shown in Fig. 12, Fig. 14 is a waveform diagram showing a preferred drive voltage waveform in the present invention, and Fig. 15 is a drive voltage waveform and light transmission amount in another embodiment. FIG. 1... Photosensitive drum, 2... Selfoc lens,
3... Optical switch array, 4... Electrode, 5... Rod lens, 6... Light source, 7... Minute optical switch section, 8... Liquid crystal molecule, 9... Spontaneous polarization, 10... Spiral axis, 11... Liquid crystal layer, 12... Substrate, 13...
- Polarizing plate, 14... Incident light from the light source, 15... Light leakage, 16... Alignment film, v3... Liquid crystal operating threshold voltage, V, , V,... Pulse voltage value .

Claims (1)

[Claims] 1. In an optical switch element that transmits and blocks light by interposing a ferroelectric substance between a pair of electrodes and applying a driving voltage between the electrodes, the driving voltage is applied to the operation of the ferroelectric substance. A high-frequency voltage is used in which a unipolar voltage pulse with a peak value larger than the threshold voltage and a voltage pulse of opposite polarity with a peak value smaller than the operating threshold voltage are alternately and periodically repeated, and this driving voltage is set as a driving voltage when light is transmitted. A method for driving an optical switch, characterized in that voltage is applied with opposite polarity at the time of and cutoff, respectively.