FR2667470A1 - Electrooptical device with electronic addressing - Google Patents

Abstract

The invention relates to an electrooptical device comprising an electrooptical element (1), the connecting element (10) having a first arrangement of electrodes (8) spaced from one another and means for integrating the electrooptical device and the connection element (10). The electrooptical device comprises, on a first surface, a second arrangement of electrodes (3) corresponding to the said first arrangement (8) as well as a deformable, interposed interconnection element (2) arranged between the connection element (10) and the electrooptical element (1) in such a way as to interconnect the first (8) and the second (3) arrangement.

Description

ELF.CTRO-OPTICAL DEVICE WITH ELECTRONIC ADDRESSING.

 The present invention relates to an electro-optical device comprising an electro-optical element, a connection element having a first arrangement of electrodes spaced from each other and means for securing the electro-optical device and the connection element .

 Such a device is known from US patent 438 0373 (Robert A. SPRAGUE-XEROX cor.), More particularly with a LiNbO3 crystal, for modulating the polarization of a light beam in relation to the data signals applied to electrodes of line of an image, distributed over the width of the crystal in electrical contact with it and corresponding to a line of an image. The electrodes are part of a resilient support, in particular an integrated circuit, which is applied against the electro-optical element by means of an elastomeric pad, so as to allow the electrodes to conform to the surface of the electrooptical element. This device has a single modulation line, and works by total reflection.

 This arrangement has the drawbacks on the one hand of being difficult to produce and on the other hand of applying stresses to the electro-optical element which are added to the other stresses normally present such as those linked to thermal expansion.

 The subject of the invention is an electro-optical device of the type mentioned above but making it possible to avoid the aforementioned drawback.

 The basic idea of the invention consists in mechanically decoupling the electro-optical device and the support by means of an interconnecting interconnection element.

 An electro-optical device according to the invention is thus characterized in that it comprises on a first surface a second arrangement of electrodes corresponding to said first arrangement and in that it comprises a deformable interconnection interlayer element, disposed between the support and the electro-optical element so as to interconnect the first and the second arrangement.

 According to a first embodiment1 the intermediate element comprises an insulating film having metallized holes arranged so as to make said interconnection.

 According to a second embodiment, the intermediate element comprises balls of malleable conductive material, for example comprising indium.

 The connection element can be an integrated circuit, advantageously of dimensions substantially equal to those of the electro-optical element, and having an electronic switching circuit intended to charge and discharge sequentially selected electrodes of the second arrangement.

 A matrix addressing corresponding for example to pixels of an image can be obtained with arrangements of two-dimensional electrodes. The connection element can then have a line decoder, a column decoder and a matrix of switches corresponding to said pixels.

According to a particularly advantageous embodiment, the row and column decoders comprise r switches in series respectively for each row and for each column and, to control said switches, q addressing signals are provided for each decoder, the number 1 of row, and the number c of column being both less than or equal to cr.

q
According to a preferred embodiment, the connection element is mechanically integral with a first block of material having a coefficient of thermal expansion close to that of the optical element, (in particular a single crystal of DKDP with a block in fluorite) which element can then be secured to a second block of material having a coefficient of thermal expansion close to its own (in particular also fluorite in the above case). In this way, the stresses exerted on the electro-optical element are minimized.

 To homogenize the temperature of the assembly, it is advantageous to interpose at least one thermally conductive wedge between the first and the second block.

 The invention will be better understood on reading the description which follows, given by way of nonlimited example, in connection with the drawings which represent - FIGS. 1 and 2 respectively a first and a second embodiment of the Figure 3 is a two-dimensional embodiment of the electro-optical element.

- Figure 4 a two-dimensional addressing mode.

- Figures 5 and 6 an embodiment of a row or column decoder, and the corresponding time diagrams.

- Figure 7 a matrix of switches controlled by line and column decoders.

- And Figure 8 a preferred method of assembling an electro-optical device according to the invention.

 According to FIG. 1, an electro-optical element 1 has electrodes 3 distributed on its upper face and at least one transparent counter-electrode 2 on its lower face. The electro-optical element 1 can be in particular a monocrystal of deuterated potassium acid di-phosphate (DRDP), implementing the Pockels effect as for example in TITUS tubes. In such tubes, an addressing is obtained using an electron beam which sequentially scans all the points of the crystal (see Revue Acta Electronica Vol.18 n02 and 3 - 1975).

 It is thus possible to modulate images at video rate. The high spatial resolution of the electron gun allows imaging applications, in particular video projection. On the other hand, the control by electron beam leads to a significant congestion, and the image frequency is limited by the value of the beam current imposed by the geometric resolution and by the complexity of the video electronics. Finally, the phase modulation of the light is limited to a firing phase shift / 2 (variation of optical path of A / 4, A being the wavelength of the radiation used).

 To perform direct electronic addressing, using electrodes, a connection element 10, for example a printed circuit, a hybrid circuit or an integrated circuit, has electrodes 8 arranged opposite the electrodes 3 and a circuit of interconnection 12, here constituted by an insulating film 5 (such as that known under the brand KAPTON) having holes 6 metallized on its edges at 7. Such an insulating film 5 has a certain flexibility which means that there is no need to exert constraints on the element 1 or on the support 10 to ensure good interconnection. In the aforementioned prior art US 4 380 0773, significant constraints had to be exerted by using the resilience of the integrated circuit to apply its electrodes to a surface devoid of electrode of the optoelectronic element. To build the interconnection circuit mentioned above is intrinsically deformable, that is to say, in the sense of demand, much more resilient than the connection element 10.

 For an operation by reflection, other than by total reflection, an insulating and reflecting layer It is provided under the electrodes 3, and the electrode 2 is transparent. Alternatively, the electrodes 3 can be reflective and separated by an insulating and reflective layer.

 In FIG. 2, the interconnection element 12 consists of indium balls with a diameter of around 30 microns, preferably shaped by heating during assembly so as to increase the contact surface. An advantageous variant consists in using cn eutectic indium-bismuth beads, the advantage of which is furthermore to present a lower solidification temperature than that of indium. As in the above case, the balls are much more resilient than the support 10.

 Figure 3 illustrates the case of two-dimensional video scanning. The electro-optical element 1 has electrodes 3, one per image point (pixel), while the counter-electrode 2 covers the entire surface of the electro-optical element I.

 To carry out the addressing, a line decoder and a column decoder are sufficient to control two switches in series per image point. When an image point is selected by the decoders, the two corresponding switches are closed and the video signal is applied to the electrode of the selected image point. If N is the number of rows and columns of the matrix (assumed to be square), then 4N2 switches are required.

 FIG. 4 represents a configuration which requires a lower number of switches, here MOS field effect transistors operating in switching mode. Let 1 be the number of rows and c the number of columns. Each image point (represented by a capacitor Cnp) has a switching transistor in series Tnp, n indicating the line number, and p the column number, the transistors of the same index p being all controlled by the same output of the line decoder. 20 receiving the LS line signal. For each line, a transistor Tn is controlled by the nth output of the line decoder 20. The transistor Tn is on for the video signal VI when the nth line is selected by the line signal LS and is connected to the line electrode 2 corresponding to the nth row. The column signal TS applied to the input of the column decoder 30 makes it possible to scan the p transistors of the nth row selected.

 This circuit takes account of the following imperatives - between two successive addresses of the same pixel, conservation of the electric charge of the pixel capacity Cnp - during the addressing of the pixel (n, p): charge or discharge of the capacity Cnp of the pixel according to the sign of the difference between the signals of the image being written and the previous image.

According to FIGS. 5 and 6, the total number of switching transistors of the decoders and of the column selection transistors (Tp in FIGS. 3 and 4) is reduced by using a combinatorial decoding according to which several switches are placed in series on the lines and on the columns. If q is the number of addressing signals and r the number of switches placed on each line and on each column, it is possible to address a number of lines and a number of columns up to Cq with cr = q! q -
For example, with three switches (r = 3) per row and per column, it is possible to address a square matrix of 120 x 120 pixels with q = 10 signals or 286 x 286 pixels with q = 13 signals.

 Of course, we can choose different values of q and r for the rows and columns, for non-square matrices.

 Addressing can be carried out with a small number of connections, namely those of the video signal and of the q addressing signals, which can be produced by electronics external to the component.

In Figures 5 and 6, an example is given of a diagram of such a line addressing circuit in the simplified case of two switches per line (r = 2) and four
2 addressing signals (q = 4) for controlling C4 lines, i.e. six lines.

For each line two transistors operating in switching mode have their main current path in series, namely (Q1, Q'1) (Q2, Q 2) (Q3 Q'3) (Q41 Q'4) (Q, Q'5 ) and (Q6, Q'6), and connected between a video signal supply terminal VI and the respective image point lines. The line selection signal LS comprises four logic signals S1, S2,
S3 and S4.

Signal S1 is applied to the control electrode of transistors Q1, Q2 and Q3r Signal S2 is applied to the control electrode of transistors Q'1, Q4 and Qs, signal S3 is applied to the electrode for controlling the Q'2 transistors,
Q'4 and Q6 and the signal S4 is applied to the control electrode of the transistors Q'3, Q's and Q'6. During a cycle of field T (Figure 6b), presenting six sequences t1, t2, t3, t4, t5 and t6, the signal S1 is at the high level during t1, t2 and t3, the signal S2 is at the high level during t1 , t4 and t5, signal S3 is high for t2, t4 and t6 and the signal
S4 is at the high level during t3, t5 and t6.

 During the instant t1, S1 and S2 are at the high level, from where the transistors Q1 and Q'1 are put into the on state and selection of the first line. During time t2, S1 and S3 are at the high level, from where selection of the second line, and so on, S1 and S4 at the high level selecting the third line, S2 and S3 at the high level selecting the fourth line, S2 and S4 at the high level selecting the fifth line and S3 and S4 at the high level selecting the sixth line. A similar reasoning could apply to the column decoder 30 with column selection signals On S'2, S'3 and S'4. For the column decoder 30, a DC voltage signal V (for example 12 Volts) can be used, this voltage being used to control the switching transistors Tnp arranged at the level of each pixel.

FIG. 7 shows such a configuration, the main current path of the transistors (tn1,
Tn2 ... Tnc) of the same line n being in series between on the one hand the series transistors (Qnr Q'n ^ ...) corresponding to the selection of this same line n by the decoder line 20, and on the other hand the respective capacities (Cn11 Cn2... Cnc) materializing the image points. As regards the column of rank p, the switching transistors (Pp, P'p ...) are in series with on the one hand the supply terminal at the DC voltage V and on the other hand the electrode for controlling the transistors (T1p, T2p ... Tlp) of the matrix 40 for controlling the electro-optical element.

 According to FIG. 8, the problem of differential expansion between the material constituting the connection element 10 and that constituting the electro-optical element î is resolved by joining, in particular by gluing, the connection element 10 with a solid wall 52 whose coefficient of thermal expansion is close (or equal) to that of the electro-optical element 1. Indeed, by way of example, the differential expansion existing between a silicon support (integrated circuit) and a DKDP crystal between -500C and 1500C (difference of 220 C) corresponds to a relative displacement of 25u for a pellet of 7.5 mm in diameter, i.e. a half image point (or pixel). In this case, one can use for the wall 52 a block of fluorite (CaF2), or any other material having the same coefficient of thermal expansion as that of the electro-optical element 1.

 According to a preferred embodiment, the electro-optical element 1 is also secured to a transparent solid wall 51 having a coefficient of thermal expansion close to that of element 1, for example fluorine (CaF2) in the example above, and is interposed between the walls 51 and 52 a heat conducting wedge 53 so as to obtain better thermal homogeneity.

 In the case where the interconnection element 12 has balls 4, it is advantageous to use an indium-bismuth eutectic whose melting point (720C) is lower than that of indium.

 On the wall 52 can be mounted a Peltier cooling circuit 56 coupled by a heat conducting block 55. The Peltier circuit 56 is supported by a support 60.

Claims (15)

 1. Electro-optical device comprising an electro-optical element, a connection element having a first arrangement of electrodes spaced from each other and means for securing the electro-optical device and the connection element characterized in that the electro-optical device comprises on a first surface a second electrode arrangement (3) corresponding to said first arrangement (8) and in that it comprises an intermediate deformable interconnection element (2) disposed between the support (10) and the electro-optical element (1) so as to interconnect the first (8) and the second (3) arrangement. 2. Electro-optical device according to claim 1 characterized in that the intermediate element (12) comprises an insulating film (5) having metallized holes arranged so as to make said interconnection. 3. Electro-optical device according to claim I characterized in that the intermediate element (12) comprises balls (4) made of malleable conductive material. 4. Device according to claim 3 characterized in that said material comprises indium. 5. Device according to one of the preceding claims characterized in that the support (10) is an integrated circuit having an electronic switching circuit intended to charge and discharge selected electrodes of the second arrangement (3). 6. Device according to claim 5 characterized in that the integrated circuit (10) has dimensions substantially equal to those of the electro-optical element (1). 7. Device according to one of the preceding claims, characterized in that the electrode arrangements (3, 8) are two-dimensional. 8. Device according to claim 7 characterized in that the second arrangement of electrodes (3) corresponds to pixels of an image. 9. Device according to claim 8 characterized in that the electro-optical element has, on a second surface opposite to the first r a transparent electrode (2). 10. Device according to claim 9 characterized in that the support comprises a line decoder (20), a column decoder (30) and a matrix of switches (T Tic, T21 --- Tl1 ... TlC) corresponding to said pixels. 11. Device according to claim 10 characterized in that the row and column decoders (20, 30) comprise q and q 'switches in series respectively for each row and for each column and for controlling the switches r and r' signals d addresses for the line and column decoders respectively and in that 1 (Cq; c with C r q '!  q 'r' (q'-r ') l  1 = number of rows and c = number of columns. 12. Device according to one of the preceding claims, characterized in that the connection element (10) is mechanically integral with a first block (52) of material having a coefficient of thermal expansion close to that of the electro-optical element ( 1). 13. Device according to claim 12 characterized in that the electro-optical element (1) is mechanically integral with a second block of material (51) having a coefficient of thermal expansion close to its own. 14. Device according to claim 13 characterized in that at least one thermally conductive wedge (53) is interposed between the first (52) and the second (51) block. 15. Device according to one of claims 12 to 14 characterized in that the electro-optical element (1) is a single crystal of deuterated potassium acid di-phosphate (DKDP), and in that at least one block (51, 52) is fluorite (CaF2).