LIGHT SHUTTER ARRAY ELEMENT

Abstract: An optoelectronic shutter array element applicable as a writing head for an optical printer and using a transparent ce­ ramic (1) having the electrooptical effect, which element re­ duces abrupt change in the electric field strength so as to im­ prove the stability of permeability of the optoelectronic shut­ ter, the element being provided on the main surface of the transparent ceramic (1) with opposite grooves (7) for forming therein the electrodes (2, 3) so that the electrodes are formed across the inner wall surfaces of the grooves and the surfaces in the vicinity of the grooves on the main surface of the trans­ parent ceramic.

Description: The present invention relates to a solid-state light shutter array element usable for a writing head of an optical printer. Recently, computers have progressed to promote automation in every technical field, and the field of information processing technique has been remarkably noticed. A printer as the information output apparatus is one of those playing a very important role. Various types of printers have actively been developed, among which an optical printer, a kind of non-impact printer, has been the most noticeable one. A device comprising a light shutter formed in array using electro-optical effect is applicable as the writing device for the optical printer. At present, a light shutter array element which employs an electro-optical material is well known. The element comprises a transparent substrate of lanthanum modified lead zirconate titanate (PLZT) having a composition of La/Zr/Ti = 9/65/35 showing very large Kerr effect, a plurality of opposing electrodes formed on the substrate, and polarizing plates disposed at both sides of the substrate. The principle of operation of the light shutter is that an electric voltage is applied between the opposite electrodes formed on PLZT substrate to cause birefringence thereon, thereby varying polarization condition of an incident light. The light shutter using the PLZT substrate is characterized in that the response speed is fast, which has hitherto been fully studied and also examined as the light shutter array. The light shutter array using PLZT substrate was reported in "PLZT LINEAR ARRAY PAGE COMPOSER" by George R. Laguna, Ferro-electrics Vol. 10 (1976), pp. 35-38, and Elektronic Industrie Vol. 6 (1980). p. 26. Such PLZT light shutter array, however, has a large problem that the permeability will vary after a long time use of the shutter array. The reason for this is that the electrostriction phenomenon of the PLZT material causes mechanical stress on the PLZT substrate. Especially, the stress is caused by variation in the electrostriction caused by abrupt variation in the electric field distribution at the end portion of each electrode. An improvement for solving the above problem has been disclosed in Japanese Laid-Open Patent Application No. 50-130464. This improvement, however, is very difficult to realize and not applicable to the light shutter array having many light shutter portions. SUMMARY OF THE INVENTION An object of the present invention is to provide a light shutter array element less in variation in the permeability caused by being driven for a long time and high in reliability. In order to attain this object, the present invention provides a light shutter array element comprising: a transparent ceramic substrate having two flat mutually parallel surfaces and exhibiting the Kerr effect; a plurality of paced electrodes provided on one of the surfaces of the substrate, the areas of the substrate between each electrode and a contiguous one defining individually addressable light shutters; and first and second polarizing plates provided on both sides of the said transparent ceramic substrate and having axes of polarization perpendicular to each other, characterised in that the surface of the substrate having the electrodes is provided with a plurality of grooves, each one of said plurality electrodes partly covering the inner surface of one of the grooves and partly covering an area of the flat substrate surface in the vicinity of said one of the grooves. An electric field applied to the transparent substrate in its thicknesswise direction increases, and the concentration of the electric field at the ends of the electrodes is reduced. Furthermore, an insulating resin film may be provided to cover the electrodes and a part of the main surface of the transparent ceramic substrate in the vicinity of the electrodes, thereby enabling the above effect to be further improved. The above and other objects and features of the invention will more fully appear from the following description taken in connection with the accompanying drawings, in which:- Fig. 1 is a structural view of a first embodiment of light shutter array element of the invention, Fig. 2 is a sectional view taken on the line (A) - (B) in Fig. 1, Fig. 3 is a graph showing variations in permeability with the lapse of time of the light shutter array element in Fig. 1 and that of the light shutter array element of the conventional planer type opposite electrodes, when driven for a long time, Fig. 4 is a structural view of a second embodiment of a light shutter array element of the invention. Fig. 5 is a sectional view taken on the line (A) - (B) in Fig. 4, and Fig. 6 is a graph showing variation in permeability with the lapse of time of the second embodiment when driven for a long time. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1 showing a first embodiment of light shutter array element of the invention, reference numeral 1 designates a PLZT substrate, 2 designates a common electrode provided on a main surface of the PLZT substrate 2, and 3 designates a group of electrodes for applying a voltage opposing the common electrode 2. The electrodes 2 and 3 are formed on inner wall surfaces of electrode forming grooves 7 and a part of the main surface of the PLZT substrate in the vicinity of the grooves. Numeral 4 designates an insulating resin film formed on the electrodes and a part of the surface of the light shutter portion 8 in the vicinity of the electrodes, and 5 and 6 designate a light polarizer and a light analyser respectively, which are adapted to have polarizing axes of +45° and -45° respectively with respect to the direction of the electric field generated when the voltage is applied between the common electrode 2 and the voltage applying electrode group 3. Fig. 2 shows a sectional view taken along a line (A) - (B) in Fig. 1. In Fig. 2, the polarizers are not shown. Next, explanation will be given on operation of the light shutter array element constructed as above. When a light is irradiated onto the polarizer 5 from the rear thereof, the light is linearly polarized by the polarizer 5, and the linearly polarized light is incident onto the PLZT substrate 1. When no voltage is applied between the common electrode 2 and the voltage applying electrodes 3, the incident light, while keeping its polarizing direction, is transmitted through the PLZT substrate 1 and intercepted by the light analyser 6. On the other hand, when a voltage is applied between the common electrode 2 and the voltage applying electrode 3, the PLZT substrate 1 generates birefringence so that the linearly polarized light is changed in the polarizing condition by the PLZT substrate 1 to be an elliptically polarized light. Hence, a light in the polarizing direction identical with the polarization axis of the light analyser 6 is transmitted therethrough. Now, it is known that the electrostriction phenomenon exists in the PLZT substrate, which generates a strain proportional to a square of the intensity of the electric field and the value of the strain is very large. Hence, when the voltage is applied between the common electrode 2 and the voltage applying electrodes 3 as abovementioned, the strain corresponding to the intensity of the electric field as well as the birefringence is generated. The strain is not generated at a portion of the PLZT substrate 1 under the electrodes, but a large discontinuity of the electric field is caused at the border between the light shutter portion 8 and the electrode formed portion of the PLZT substrate 1. In the first embodiment, the electrodes, as seen from Fig. 2, are formed on the inner wall surfaces of the grooves and a part of the main surface of PLZT substrate 1 in the vicinity of the grooves. Therefore the electric field distribution in the thicknesswise direction of the PLZT substrate 1 is increased to reduce the necessary electric field at each portion, and simultaneously to deteriorate concentration of electric field at the end portion of each electrode (the portion a ○ in Fig. 2) on the surface of PLZT substrate 1. Also, the portion b ○ in Fig. 2 which would be structurally easy to be subjected to the concentration of stress, is covered with the electrodes to make zero the electric field so as not to cause strain. Furthermore, the portion a ○ at the end of each electrode is formed of a resin film to reduce a difference of strain between the electrode and the shutter portion 8, thereby reducing stress caused by the strain created at the portions a ○ and b ○ . In the first embodiment, the PLZT substrate 1 employs a (Pb 0.91, La 0.09) (Zr 0.65, Ti 0.35)O₃ substrate of 300 µm thick, sliced and polished. The grooves on the main surface of the PLZT substrate are formed by a chemical etching method using the photolithography technique. A liquid of a series of HF + HNO₃ + H₂O is used for etching the PLZT substrate. A depth of the etching is about 5 µm. Then, Cr-Au is vapor-deposited on the entire surface of the PLZT substrate and the photolithography technique is again used to form a plurality of electrodes. The Cr-Au electrodes are about 1 µm in thickness. Next, polymide resin is used to form the resin film pattern by the photolithography technique. In Fig. 1, each electrode of the voltage applying electrode group 3 is of 50 µm wide and an interval between the electrodes is 70 µm. In Fig. 2, an interval between the opposite grooves for the opposing electrodes on the light shutter portion is 60 µm, that between the opposing electrodes is 50 µm, and that between the opposing resin film patterns is 40 µm. The light shutter array element constructed as above showed its maximum permeability when applied with a voltage of 100 V. Furthermore, we have examined variation with time in permeability of the light shutter array element of the invention and of conventional one of the planer construction, when applied with rectangular waveform pulse of 100 V and 1 KHz for a long time. Fig. 3 shows the results of the above examination, in which A designates the conventional light shutter array element and B the light shutter array element of the invention. As seen from Fig. 3, the element of conventional construction largely changes in permeability when the number of times of pulse applications becomes more than 10⁹, while, that of the invention shows small variation in permeability. As a result, in the light shutter array element of the first embodiment, it can be said very effective for improving the stability for a long time drive. In other words the difference in intensity of electric field at the boundary between the shutter portion 8 and the electrode and the difference in strain are remarkably reduced. Referring to Figs 4 and 5 showing a second embodiment of the invention, the operating mechanism and basic construction thereof are same as those of the first embodiment. In the second embodiment, as seen from the sectional view in Fig. 5, electrodes 2, 3 each comprises a first electrode 2a or 3a formed in each electrode disposing grooves 7 provided on the PLZT substrate 1 and a second electrode 2b or 3b provided to cover the first electrode and a part of the main surface of the PLZT substrate 1 in the vicinity of the first electrodes. Cr-Au is vapor-deposited on the substrate 1, and then the first electrodes are formed by the lift-off method. Then, Cr-Au are again vapor-deposited on the first electrodes and the PLZT substrate, and the second electrodes are formed by use of the photolithography technique. The first electrodes are about 2 µm thick, and the second electrodes are about 5000 Å thick. Other design conditions are the same as those of the first embodiment. Fig. 6 shows the result of variation in permeability with time in the same examination condition as that of the first embodiment. As seen from Fig. 6 the variation of the permeability is very small even after the long time drive as same as the first embodiment. The above described embodiments are merely exemplary of the invention and may be modified variously within the scope of the invention defined solely by the appended claims.

Claims: A light shutter array element comprising: a transparent ceramic substrate (1) having two flat mutually parallel surfaces and exhibiting the Kerr effect; a plurality of spaced electrodes (2,3) provided on one of the surfaces of the substrate, the areas (8) of the substrate between each electrode and a contiguous one defining individually addressable light shutters; and first and second polarizing plates (5,6) provided on both sides of said transparent ceramic substrate and having axes of polarization perpendicular to each other, characterised in that the surface of the substrate having the electrodes is provided with a plurality of grooves (7), each one of said plurality electrodes partly covering the inner surface of one of the grooves and partly covering an area of the flat substrate surface in the vicinity of said one of the grooves. A light shutter array element according to claim 1, further comprising an insulating resin film (4) formed to cover said electrodes (2,3) and a part of the flat substrate surface in the vicinity of the ends of said electrodes. A light shutter array element according to claim 1 or 2, wherein each electrode (3,2) consists of a first electrode part (2a, 3a) covering the inner surface of said of the grooves and of a second electrode part (2b,3b) covering said first electrode part (2a, 3a) and an area of the flat substrate surface in the vicinity of said one of the grooves. A light shutter array element according to claim 3, further comprising an insulating resin film (4) formed to cover said second electrode part (2b,3b) and a part of the flat substrate surface in the vicinity of the ends of said second electrode part (2b,3b). A light shutter array element according to any one of claims 1 to 4, wherein said transparent ceramic substrate (1) is a PLZT substrate.

Inventors: HORIBE, YASUTAKA, TAKUBO, YONEHARU, YAMANISHI, NOBUE
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1985
Main classification: G02F1
IPC codes: G02F1