WO2012103559A1

WO2012103559A1 - Method and device for controlling of switching and phase modulation of a liquid crystal cell

Info

Publication number: WO2012103559A1
Application number: PCT/AM2011/000002
Authority: WO
Inventors: Hakob MARGARYAN; Vladimir Aroutiounian; Nune HAKOBYAN; Valeri ABRAHAMYAN; David HOVHANNISYAN
Original assignee: "YEREVAN STATE UNIVERSITY" STATE NOT COMMERCIAL ORGANIZATION
Current assignee: "YEREVAN STATE UNIVERSITY" STATE NOT COMMERCIAL ORGANIZATION
Priority date: 2011-01-31
Filing date: 2011-06-20
Publication date: 2012-08-09
Anticipated expiration: 2013-07-31

Abstract

The invention refers to photonics, in particular to the development of optoelectronic components of photonics. Possible fields of the invention application are: adaptive optics, optical correlation, image processing, holographic record, as well as displays. In order to control the birefringence of planar oriented nematic liquid crystal simultaneously and synchronously with the voltage applied between two substrates of nematic liquid crystal cell an additional voltage - bipolar electric pulse, is applied along to one of substraits of the liquid crystal cell. The controlling of the bievergence of nematic liquid crystal with proposed method create an opportunity to reduce switching time and improve the modulation characteristics of the nematic liquid crystal cell. To realize the method a nematic liquid crystal cell is proposed in which one contact area is formed on the transparent conducting layer of one substrate, and two contact areas are formed on the transparent conducting layer of the other substrate to provide control voltages application.

Description

Method and device for controlling of switching and phase modulation of a liquid crystal cell.

This invention refers to photonics, in particular to the development of optoelectronic components of photonics. Possible fields of the invention application are: adaptive optics, optical correlation, image processing and analysis, holographic record of information, as well as displays.

The electro-optical method of controlling of birefringence of liquid crystals (LC) is known [1] according to which a layer of the nematic liquid crystal (NLC) with uniform distribution of director is reoriented in external electric field (Freedericksz electro-optical effect). This effect is commonly used in such LC-based devices as phase retarders, light valves, spatial light modulators and LC displays. The basic technical characteristics of these devices are: the depth of phase modulation of the light and switching time. In order to achieve improved modulation and time characteristics of LC-based devices either a new type of LC is used [2] or the thickness of LC layer is reduced, and the latter in conjunction with reduced switching time introduces reduction in the depth of phase modulation.

The electro-optical method of controlling of LC birefringence is chosen for the closest prototype, according to which an electric field is applied between the LC cell substrates in consequence the birefringence of NLC with uniform distribution of director changes [3].

It is known, that LC cells have the following two operation modes: as an electro-optically controlled light valve and as a phase retarder.

The LC cell, as a light valve in the prototype, works as follows: planar oriented NLC cell is placed between the crossed polarizers so that the incident light beam to be oriented at 45° angle with respect to the of NLC director, and the control voltage is applied between the two substrates of the NLC cell. A change in the value of the applied voltage effects a different orientation of the NLC director and thereby different magnitudes of intensity of the light beam passing through the cell. Thus the intensity of monochromatic light passing through the system is controlled by varying the amplitude of applied voltage in a proper way.

The switching time, the basic characteristics of such a system, is conditional upon the speed of both the reorientation and relaxation processes in NLC. In the prototype, the minimum on/off transition time is achieved by changing the induced birefringence (by applying corresponding voltage) so that the phase shift to be π radian. In the prototype quick reorientation is achieved by applying a voltage of around the value of 1.3 times higher than the threshold voltage of Freedericksz transition in order to circumvent random and undesirable hydrodynamic flows [3-5]. To reduce the relaxation time the method of double-frequency control is used [2, 6, 7], which, however, is applicable to the special type of double-frequency LCs only.

As phase retarder the LC cell operates in the following way: a voltage is applied between the substrates of NLC cell, and due to electro-optical effect the birefringence of NLC is changed. Therefore, the light passing through the NLC layer is subjected to a phase-shift in accordance with the applied voltage. Thus, the phase of the light beam can be controlled by changing the amplitude of external voltage.

A basic characteristic of such a device is the dependence of phase shift Δφ on the variation of the voltageAF , i.e. the transfer coefficient δφ/δ¥ . In the initial linear region on phase shift vs. applied voltage curve, A (AV) a slight variation of AV i_eads to the significant change of phase shift Δφ = k-AV , where & = δφ/δΓ is the transfer coefficient. With farther increase in control voltage the Δφ(ΔΚ) curve reaches its saturation region, with further decrease in transfer coefficient. In this region, as the voltage changes by AV the phase shift changes slightly in close proximity of π . On one hand it is preferable the device to operate in this range of control voltage with higher value of k and on the other hand in case of greater values of k the noise component of external voltage will cause random and uncontrolled changes in the phase. Besides, from the point of view of increasing the speed it is advantageous to run the device in the range of higher values of control voltage, where transfer coefficient k decreases significantly (saturation mode). Though in this case maximum speed and noise resistance is achieved, the transfer coefficient δφ/δΡ and thereby the modulation depth is significantly low. From aforesaid it could be concluded that there is an inconsistency between high speed and deep modulation.

The objective of this invention is the improvement of time and modulation characteristics of LC-based devices with a method enabling to choose optimum operation mode of LC cells evading inconsistency between high speed and deep modulation.

What is claimed is:

In the known method birefringence of a liquid crystal is controlled by applying external voltage (hereafter- transverse voltage, V_± ) between two substrates of a LC cell, in this invention the birefringence of NLC is controlled by applying two control voltages: conventional V_L and additional voltage along one of substrates of planar oriented NLC cell (hereafter - longitudinal voltage, V_n). The method is put into action with a device comprising two glass substrates, inner surfaces of which are covered with transparent conducting layers; a spacer between the two substrates providing fixed distance between the substrates; nematic liquid crystal filling the space between the substrates; alignment layer providing planar orientation of nematic liquid crystal; one electric contact formed on each transparent conducting layer. In this invention on transparent conducting layer of one of substrates a second electric contact is used to apply the additional control voltage.

In Fig.1 a device embodying claimed method is schematically given. It comprises of the LC cell with two glass substrates (1) inner surfaces of which are covered with light-transparent conducting layers (Indium-Tin Oxide, ITO) (2), spacers providing a fixed gap between the substrates (3), nematic liquid crystal layer (4) sandwiched between the substrates, a polymer alignment layer (5) providing planar orientation, and three metal contact areas (6) designed to apply control voltage: two on transparent conducting layer on one of substrates, and one on transparent conducting layer on the other substrate.

In Fig. 2 the schematic diagram of setup of embodying claimed method is given. It comprises a two-channel generator run by NI 6025 DAQ card (7), a computer (8), a voltage amplifier (9), the LC cell (12) arranged between two crossed polarizers (10, 11), a light source (632.8 nm He:Ne laser) (13), and a photo-detector (14).

In Fig.3 the switching dynamics of the LC electro-optically controlled light valve and the control signal shape are given in case of controlling with known method.

In Fig.4 the switching dynamics of LC electro-optically controlled light valve and the control signal shape are given in case of controlling with claimed method.

In Fig.5 the phase shift of LC phase retarder vs. control voltage is given in case of known method.

In Fig.6 the dependence of phase shift vs. longitudinal voltage of a phase retarder is given for fixed values of transverse voltage.

In Fig.7 the phase shift vs. transverse voltage is given for fixed values of longitudinal voltage in case of claimed method.

The claimed method is realized for the LC cell given in Fig.1.

The claimed method is realized with an setup schematically given in Fig.2. It comprises of a two-channel generator run by a NI 6025 DAQ card (7) and a voltage amplifier (9). A software package developed in LabView environment enables one to shape both bipolar rectangular electrical pulses of different amplitudes, frequency and duration, and control signals of other shape, for example TNE (Trancient Nematic Effect). The transverse control voltage is applied between NLC substrates from the first channel, and the longitudinal control voltage from the second channel. Both control signals are generated either synchronously or with fixed phase shift. Incident light (13), passing through NLC cell (12) arranged in between two crossed polarizers (10, 11), then is registered by the photo-detector (14). The signal read by NI 6025 card (7) is inputted to the computer (8) in the form of digitalized data.

Example 1:

In the known method the NLC light valve is controlled with 1 kHz transverse voltage V_L of TNE shape. The amplitudes of the TNE signal fractions being chosen so to provide complete "on" and "off states and on/of switching in the range of phase shift at close proximity of π . According to switching dynamics given in Fig.3, the switch-on time is 15.17 ms, and the switch-off time is 11.16 ms.

In the present invention a NLC cell operates as a light valve in the following way: TNE shape transverse voltage V_± provides the valve running. Synchronously with the "on" front of TNE signal the longitudinal control voltage V_n is applied in a shape of a single bipolar pulse. In this case, as it is seen from the transient dynamics (Fig.4) the switch-on time is 700 με, with switch-off time of 1 1.06 ms. Hence the claimed method provides an opportunity to decrease the switch-on time of a NLC electro-optical gate 20 times and the overall switching time-2.5 times.

Example 2:

In the known method as a phase retarder the NCL cell operates by applying V transverse voltage of 1 kHz and from the phase shift vs. control voltage curve, φ(ν_χ) (Fig.5) the voltage value corresponding to necessary phase shift is chosen.

In this invention the NLC cell is controlled by synchronously and simultaneously applied transverse and longitudinal electric pulses. In the claimed method the phase shift of light is dependant on both transverse V_± and longitudinal V_u control electric pulses. At a fixed value of V by changing the amplitude of the longitudinal voltage by Δ^, a deviation in phase shift of is obtained, where V_Lth is the threshold value of transverse voltage, at which

Frederiksz transition occurs (Fig.6). And vice versa, having fixed V_HQ value and changing the amplitude of transverse voltage by AV_± a deviation in phase shift AV_± is obtained

(Fig.7). In the claimed method electro-optically induced phase shift is presented as

The availability of two independent control voltages provides an opportunity to control the value of transfer coefficient δφ/δΓ working in the range of maximum speed and noise-resistance, chosing proper amplitudes of V_± and V_n . In other words, the possibility of additional control of phase shift is created.

In the case of controlling NLC phase retarder by the known method (Fig.5), the variation of control voltage V_± from 26V to 36V (in the range of maximum operational speed) the phase shift is changed from 2.05 π to 2.08 π and the transfer coefficient is 0.003 .

In the case of controlling NLC phase retarder by the claimed method as it is seen Fig.6, when controlling with the fixed amplitude of transverse voltage V (in given case the value of 26V is chosen, which corresponds to the maximum operating speed of device) variation of the amplitude 26.14V to 27.07V a phase shift alteration of 0.33 π is possible to reach. The transfer coefficient δφ/δΓ , in this case is 0.35 π .

Thus, the claimed method gives an opportunity to increase transfer coefficient δφ/δΚ in two orders.

Information sources:

1. L.M. Blinov, Electro-Optical and Magneto-Optical Effects in Liquid Crystals, Wiley, Chichester, 1984.

2. A.B. Golovin, S.V. Shiyanovskij, O.D. Lavrentovich. Fast Switching Dual-Frequency Liquid Crystal Cells with High Pretilt Angle, Driven by an Amplitude and Frequency Modulated Voltage. U.S. Provisional Patent Application. Serial No. 60/350,747.

3. L.M.Blinov, V.G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials, Springer, 1993 - closest prototype.

4. V.G. Chigrinov and V.V. Belyaev, Kristallografiya 22, 603 (1977). 5. V.V. Belyaev, L.M. Blinov, and V.G. Rumyantsev, Mikroelektronika 5, 552 (1976).

6. V.V. Belyaev and V.G. Chigrinov, Kristallografiya 23, 811 (1978).

7. G. Baur, A. Stieb, and G. Meier, Appl. Phys. 2, 349 (1973); 6, 309 (1975).

8. H.K. Bucher, R.T. Klingbiel, and J.P. van Meter, Appl. Phys. Lett. 25, 186 (1974).

Claims

1. A method of controlling of switching and phase modulation of liquid crystal cell, according to which an electric field is applied between the substrates of a planar-oriented nemtic liquid crystal cell, changing thus the birefringence of nematic liquid crystal with uniform director distribution characterized by: in order to contol the birefringence of the liquid crystal, simultaneuosly and synchronously with the basic electric field an additional voltage - a bipolar electric pulse is applied along one of substrates of planar oriented liquid crystal cell.

2. A device for controlling of switching and phase modulation of liquid crystal cell comprising: two glass substrates, inner surfaces of which are covered with transparent conducting layer; a spacer between these substrates; nematic liquid crystall filling the space between the substrates; an alingment layer providing the planar orientataion of the nematic liquid crystal; one electric contact on transparent conducting layer of each substrates,

characterized by: one contact area is formed on the transparent conducting layer of one substrate, and two contact areas are formed on the transparent conducting layer of the other substrate to provide control voltages application.