SU607169A1 - Kerr electrooptical modulator - Google Patents

Description

(54) ELECTROOPTICAL KERRA MODULATOR

The invention relates to electro-optical light modulators and can be used in photo-telegraph equipment, in recording equipment and in other devices that require the conversion of electrical signals into visible elements of the image and recording is performed on the photosensitive layers by modulating the luminous flux. The electro-optic light module operating on the Kerr effect is known. These modulators are low-inertia, but they have a very small optical input opening, which almost completely excludes the possibility of their use in telegraph equipment (photo) and sound recording. In order not to increase the distance between the control electrodes, an almost parallel beam of light with a very small cross section (about) is fed to the input of the known modulator. Increasing this distance leads to the need for a proportional increase in the control voltage. Known liquid modulators based on the Kerr effect, which also have some use in the technique of high frequencies in the range finder (radio geodesy). Such modulators are low-inertia; in a constructive sense, they are very simple and exon cally justified, for example, when using them in a range finder, the cost of such a modulator was about 50 rubles, while crystalline modulators have a cost from 1000 to 2500 rubles. Known modulators provide operation at frequencies of hundreds of megahertz, but to reduce the control voltage and the power supplied to the modulator, the modulation depth is somewhat limited (up to 50%). Kpcwe addition, in all these devices they have such a constructive implementation, which provides the ability to work only when applied to the input of the luminous flux in the form of a parallel beam of rays (most of the beam diameter from 1 to 3 mm). The closest in technical essence to the described invention is an electro-optical modulator containing an optical focusing element (lens), a polarizer, a modulator cell made in the form of a cell filled with nitrobenzene, in which two electrodes are installed parallel to each other, and an analyzer Z. Although the light beam entering the cell is focused in the modulator, it does not ensure the luminous efficiency of the modulator and the parameters of its element are not defined, i.e. It has a low luminous efficiency. In order to increase the luminous efficiency in the proposed modulator, the optical focusing element is made in such a way that it forms a convergent — a beam of light with an aperture angle of more than 10, on the path of which K is successively located, the modulator cell is selected from egciod- M)) -2 V in y where K is the nearest integer; m is the required modulation depth as a percentage, the fraction of the luminous flux received at the exit of a single cell set to darkness due to the reflection of light from the internal surfaces of electrodes on which a layer of a substance with a light refractive index less than that of nitrobenzene is applied, For example, light kroner glass or fused quartz, D on the electrodes in the direction of light propagation is determined by the condition. M tg-c. where N is the number of reflections from the inner surfaces of the electrodes for the extreme rays of a converging light incident on the modulator (N 100 (3 is the distance between the electrodes, d., & angle between the inner surface of the electrodes and the direction of incidence of the extreme beam on it, the width of the electrodes is chosen within the transverse size of the light beam incident on the modulator. In order to simplify the design in the proposed modulator, the electrodes of the modulator cells together with the analyzers can be enclosed in one cell with the previous analyzer It serves as a polarizer for the subsequent ki. When a converging luminous flux with an aperture angle of 20-30 ° is applied, the amount of light that can be passed through such a modulator can increase 500-2000 times, which significantly: increases its luminous efficiency Partial depolarization of light, which occurs when reflected from the internal surfaces of the electrodes, can lead to a decrease in the modulation depth of the light flux by up to 80%, depending on the length of the electrodes and the distance between the electrodes. In cases where this is unacceptable, the modulation depth can be increased to the desired value by re-passing light through the same modulator (only without a polarizer). FIG. 1 shows the scheme of the proposed modulator; in fig. 2 is a diagram of the passage of a light beam in a modulator cell; in fig. 3 and 4 are diagrams using cylindrical and corresponding spherical lenses as optical elements for obtaining a convergent light flux; FIG. 5 is a diagram of a modulator with the passage of light through two successively located modulator cells; in fig. b - implementation of a modulator, when two series-connected modulator cells are made in the form of one common construction; in fig. 7 - modulation characteristics for operation of one modulator cell and two series-installed cells, respectively; in fig. 8 and 9 are diagrams of the relative positions of the transmission planes of the polarizer and both analyzers for two modes of operation, with the passage of light through two modulator cells. The proposed modulator contains a modulator cell made in the form of a glass cell 1 filled with a liquid (e.g., nitrobenzene), which becomes anisotropic under the action of an electric field. Two electrodes 2 are placed inside the glass cuvette 1. A converging beam of light 4 is fed to the entrance slit formed between these electrodes by means of the optical element 3. A polarizer 5 is placed in front of the modulator cell, followed by an analyzer B. Control length K ) electrodes 2 in the direction of light propagation t at a given value of the aperture angle ip and the distance between the electrodes c (. is set such that for most of the light rays they are reflected from the inner surfaces of the control electrodes. reflections should be small, since each reflection reduces the modulation depth of a single-cell modulator by approximately 1% (with optimal ratios of refractive indices) N 100-M, where LL is the modulation depth; N is the number of reflections; between, the inner surface of the electrodes 2 and the direction of incidence of the beam in the environment of nitrobenzene.

FIG. 2 shows the path of the rays inside the modulator cell, while the PD is the refractive index of the medium from which the light beam 1 enters the cell; however, for the material from which the cuvette is made; the same for nitrobenzene; P, - the same for the material with which the inner surface of the mounting electrode 2 is covered; f is the angle of incidence of the light beam at the entrance of the cell; 1dol refraction in the thickness of the glass; & - the angle formed by the padak beam 7 with the surface of the control electrode,

At the point of incidence of the beam 7, total internal reflection takes place, and this beam is deflected at the same angle

The modulator operates as follows.

When a convergent beam of light is supplied to the modulator input, two options are possible.

In the first embodiment, a light cone is created of a converging light beam 4 (Fig. 3), the axis of this cone is located normally to the optical axis of the anisotropic medium, determined by the direction of the electric field between the control electrodes 2 of the modulator. Such formation of a cone of rays is obtained, for example, when a collecting spherical lens is placed in a parallel beam of light, a converging beam of light 4 is collected at the entrance slit of the modulator 8 in the form of a small spot light, In a variant (FIG. 4) in the path of the parallel beam of light a collecting cylindrical lens is placed, with a ps of which a converging light beam 4 is created, compressed only in one plane, and in another arc (mutually perpendicular to the plane) the rays remain parallel to each other.

A narrow strip of light is created at the cell entrance. In the first version, the planes of incidence of all the light rays are different and they form different angles with the planes of the main section of the optical system, and in the second version, the planes of incidence of all light rays are parallel to each other and they coincide with the planes of the main section of the optical system, therefore Effects in the system of heterogeneity in illumination are significantly reduced. Each of these options for shaping the luminous flux has its advantages and disadvantages, and the final choice of them is determined in each individual case by the conditions of operation of the device.

The first option provides a smaller interelectrode capacitance and, consequently, less inertia, i.e. it is possible to pass a wider frequency band within many hundreds of thousand Hertz, and with an increase in the control power, the megahertz,.

. The second variant of the formation of a convergent beam of light leads to the need to increase the size of the control electrodes in the direction perpendicular to the propagation of light and, consequently, their electrical capacitance. This, in turn, increases the inertia of the system compared with the first option. However, the quality of the light of the image at the output of the modulator

10 is obtained in this case more uniform, therefore, for the frequency band within hundreds of thousands of hertz, the second variant should be recommended for the formation of a convergent light flux, and for

5 high speeds of work it is better to take the first variant of the formation of a converging lumen.

- In order to reduce the degree of depolarization of light in reflections in the modulator, such materials are used so that the difference between the refractive indices of the optically denser medium and the optically less dense would be insignificant. So for example for a corner

25 drops of light rays. At the modulator input, the optimal value of this difference is 0.03-0.04. In this case, the plane-polarized beam at each reflection, although it becomes eleptic polarized, but the phase difference between the components of the reflected wave lying in the plane of incidence and perpendicular to the J plane of incidence, does not exceed (2.5–3). Calculation of this shift

35 sU is produced according to the following formula

tc, .- l-V ((2) 2 lij einV

where Il2 is the refractive index of the optically more dense medium;

llj is the refractive index of the optically less dense medium;

C - the angle of inclination of the rays incident HI interface of the media in relation to the standards (Fig. 2)

. ": V" j- "

Q is the angle formed by the padak tsim

beam with the surface of the control electrode ..

In the modulator cell, the main medium is nitrobenzene, the refractive index of which is in the range of lengths

waves from L-0.5 microns to L 0.645 microns matter. from P 1,5685 to P2 1,5472. The inner surface of the control electrodes is covered, for example, with light crown glass, which has

The specified wavelength range of the refractive index is from 521 to Hj 1.51. This difference between the refractive indices of nitrobenzene and glass is a slight kroner for the conditions used, which is close to the optimum. Fused quartz, having a value of the index of refraction, is also suitable for the visible range of the rays from 1.4560 to 1.4697. However, since the difference in the value of the convergence index in quartz and in nitrobenzene is greater than that of lightweight crowns and vibrobenzene, this leads to a somewhat greater depolarization of light, which is less expensive. With the correct and optimal choice of structural elements, the described design allows a significant increase in the amount of light passing through the modulator (more than 500 times) with extremely insignificant light losses due to reflection (without increasing the control voltage on the electrodes). In the same cases when, according to the operating conditions of the device, the control voltage is required to be reduced significantly, the length of the electrodes has to be increased (in the direction of light propagation) and the distance d between the electrodes is reduced, since The definition of C from the expression where B is the Kerr constant; id | 2 is the half-wave voltage (i.e., the voltage at which the polarization plane is rotated by an angle. As follows from (1), this is a T1 reduction path U d leads to an increase in the number of reflections N and, consequently, to an increase in the delol of the part of the world. For example, with mm, (, 5 mm, f 20 °, the number of reflections, N according to expression (1) is .... N “- | -tgre | - | tgre, (5) where angle & is determined for benzene nitrate as follows. If the aperture angle of the tf 20 light rays incident on the modulator and the refractive index for nitrobenzene, 563 (dl, 52 mm), you can write the following relations (Fig. 2) Pr 61P 4 h in Hjj sine-, (b since , (for air),, 563, then From where, according to expression (1)., 224-18 When mm,, 5 mm and If 20 N 9. 9. With such a relatively large number of reflections (for the most extreme converging light rays) The depolarized part of the world in one case is LO 17%, and in the other case 8.5%, which leads to a corresponding decrease in the modulation depth in one case to almost 80%, and in another see case-up to 90% (see option no. 1 and 2 in the table). In cases where a modulation depth of 80% and 90% is acceptable according to the operating conditions of the device, this mode can be accepted, However, if this is not acceptable, then the following path can be used (to reduce the effect of the depolarizing part of the world on the modulation depth: the luminous flux after passing through the first modulator cell is directed to the same second modulator cell (Fig. five). At the output of the second modulator cell, due to depolarization, there will again be a luminous flux equal to 17% of the input flux for the first variant and 8.5% for the second variant. Thus, for the first variant after the second cell, the depolarizing part of the light is 0.17 x O, l7, 029, i.e. 2.9%, and for the second variant - 0.085 X 0.085 0.0072, i.e. 0.72%, which increases the modulation depth in one case to 97%, and in the other to 99% (the mode of the modulator set to darkness is considered). If it is required to further suppress the depolarizing part of the world, then the light flux can be transmitted through the third modulator cell, which leads to a further increase in the modulation depth. The number of required modulator cells K to provide the required modulation depth can be determined from the following expression egcioo-MV2: j.4 (the nearest integer and Va number) where M is the required modulation depth,%; f, .... - the proportion of the light flux at the output of one modulator cell, which is set to the darkness of the flux at the input (due to depolarization). In cases where, for constructive reasons, it is undesirable to include several modulator cells in series, it is possible to use a cell with a smaller number of reflections. For example, in item 8, the table with mm, 30.5 mm, the number of reflections of the light flux from internal surfaces, the modulation depth in one cell is 98%, and with two cells it reaches almost 100%. With successive passage of light through a series of modulator cells, the analyzer installed in the previous modulator cell simultaneously performs the functions of a polarizer for the subsequent modulator cell. The modulation characteristic for two series-connected modulator cells (FIG. 7, second curve) has a greater steepness than for one single cell (FIG. 7, first curve). If a constant bias of about 0.75id / 2 is brought to such a system, then, when operating in black and white mode to control the modulator, a signal is required, only O, 25 id / 2 are equal. When operating in halftone mode, this characteristic needs to be modified by incorporating non-linear elements to ensure correct reproduction of optical densities. Instead of passing light through a series of consecutively connected modulator cells (Fig. 5), it is possible, in order to simplify and reduce the cost of construction, to install a three-way modular cell (Fig. 6) consisting of a glass cell filled with nitrobenzene, two or three pairs of control electrodes 2 and analyzer 6 (one or two, depending on the number of pairs of electrodes), clamped closely between both pairs of electrodes adjacent to it. On the input side of this device, there was a polarizer 5 and an optical system 3, which provided for obtaining convergent light, and on the output side, the analyzer was also an optical system. Due to this method of solving the problem, as compared with the method of sequential light transmission over individual (one cell) modulators (Fig. 5). there is no need to install optical elements between modulators; the overall dimensions of the device and the total loss of light are reduced. In the practical implementation of a two cell modulator (in one case, the entire luminous flux that emerges from the slot of the first cell must fall into the entrance slot of the second cell. To do this, the distance between the electrodes in the second cell is taken slightly higher than the distance between In the first cell, the magnitude of this difference (d - d,) is determined by the thickness of the analyzer clamped between the cells. When used to modulate two separate modulator cells (Fig. 1) or one two cell modulator (Fig. 6), optimal identification of the transmission planes of the polarizer and both analyzers. A large number of options can be proposed to solve this problem. For each of these options, a definite correlation is required between the values of the voltages applied to both pairs of control electrodes. as in liquid electro-optical moduli of light, quadratic dependence of the phase difference on the modulating voltage takes place, and the intensity of the light at the output of the jitter is proportional to the square of the sine t of the phase difference, the initial portion of the growth of the light flux from the applied voltage is very flat, i.e. practically ineffective (Fig. 7), the relative growth of the luminous flux is plotted along the ordinate axis, and the relative value of the voltage applied to the electrodes is plotted along the abscis axis. Curve 1 (Fig. 7) for one modulator cell, and curve 2 for two series-connected modulator cells, and for curve 1 in the voltage variation region from zero to (0.3 - 0.4) id, 2 light flux there is very little change (by 3-5%), so it is better to apply a constant voltage equal to d, 2 to such a modulator, and then reduce it to (0.60, 7) from Uj (2, .B In this case, the changing part of the voltage on the electrodes should be only (60-70%) of the maximum, which facilitates the circuit realization of the device. The mutual position of the floor transmission plane when working according to the scheme shown in Fig. 5 or Fig. 6 ,, / will be as follows (see diagram in Fig. 8) D analyzer 6 is located crosswise with analyzer B. In the absence of a signal from the channel In this connection, the polarization planes in both modulators are rotated by 90. The rush through the system passes the maximum. At the moment of arrival of the control signal from the control signal, the voltage U and U begin to decrease, and the polarization planes rotate in the opposite direction aspire to your starting position. In this case, in the first In the second cell, the illuminated light is delayed by the analyzer ClCl 6. The polarized light, i.e. That part, in which the oscillation plane coincides with the direction of transmission of the analyzer b, passes through this analyzer, but at the same time this remainder of the luminous flux becomes flat polarized again. Having passed through the cell of the second modulator, this residue is again significantly reduced and practically can not be further taken into account. In those cases when in the initial position (in the absence of a signal from the communication channel) the light through the modulator should not pass, the mutual position of the polarization transmission planes and both analyzers should be as follows (Fig. 9) :: analyzers 6 and 6 They are parallel to the polarizer 5. In the initial position, voltage is applied to both control electrodes, 2 and .cBeT in this case does not pass through the system. With the arrival of the control signal from the communication channel

eleven

the voltages U and Uj simultaneously decrease, and the light through the modulator system increases, reaching its full value when U and Ug fall to the set value.

For phototelegraphic equipment, working in a telephone channel, you need a modulator with not very high voltage for control (points 2 and 3 of the table). It is possible to make a comparative assessment of the luminous efficiency of the modulator under item 2 of the table (And 20 mm, with | 0.5 V) when a convergent light flux with an aperture angle of 20 is applied to it with a modulator, also made on the basis of the Kerr effect with the same distance between the electrodes, «0.5 mm, but when a parallel beam of light d Ю Ю, 5 O4 is applied to its input. For the formation of a gathering of the dashes Where d C N a and „, and -d M 12

Light flux can be used (for example, the first lens installed in the Neva photo-telegraphic apparatus provides a converging light flux onto the aperture with an aperture angle of about 20. The diameter of this lens is 12 mm, 2F 17,4 17.4 mm, the lens area is 110 mm. If roughly assume that the illumination of the light beam in the cross sections is the same as the illumination in the cross section of a parallel light beam, 5 mm, incident on the input of the modulator with a Kerr cell, then the ratio between the light fluxes for the first and second variants will be equal to Hf ciW, i.e., the luminous efficiency of the proposed modulator is 560 times higher than that of liquid electro-optical modulators of light (with the same value: control voltage on the electrodes).

Claims (3)

The table is the length of the electrodes in the direction of light propagation; the distance between the electrodes; electrical, interelectrode capacitance (if the size of the electrodes in the direction perpendicular to the light is 12 mm); the number of reflections from the internal surfaces of the electrodes; half wave voltage; the proportion of the residual light flux at the outlet of one modulator cell when it is completely closed; On the other hand, at the output of two Sequentially switched on modulator cells, t is the span of the control voltage when operating in the semitone mode; control voltage range when operating in the black-bel- sion mode, modulation depth,%. Claim 1. Electro-Kerr modulator containing an optical focusing element, a polarizer, a modulator cell made in the form of a cell filled with nitrobenzene, in which two electrodes are mounted in parallel to each other, and an analyzer, and so that, in order to increase the luminous efficiency, the optical focusing element is emitted | 1 yen. so that it forms a convergent beam of light with an aperture angle of more than 10, on the path of which K moduli cells are successively located, with K chosen from the condition ggdoo-MVz e, (. where K is the nearest integer; M is the required depth modulus percent, the luminous flux receiving at the exit of a single cell that is set to darkness due to depolarization of light while reflecting it from the inner surfaces of electrodes on which a layer of a substance with a light refractive index less than that of nitrobenzene, such as glass, is applied or fused / quartz, the length of the electrodes in the direction J 4 of the light propagation is determined by the condition where N is the number of reflections from the inner surfaces of the electrodes for the extreme rays of the converging light incident on the modulator (N 100-M) f d is the distance between the electrodes; & - the angle between the inner surface of the electrodes and the direction of incidence of the extreme beam on it, the width of the electrodes is chosen within the transverse size of the light beam incident on the modulator., 2, Modulator according to n, 1, and so that in order to simplify the construction In addition, the electrodes of the modulator cells: together with the analyzers are enclosed in a single cuvette, while the analyzer of the previous cell serves as a polarizer of the subsequent cell. Sources of information taken into account in the examination: 1.Orlovsky E.L. Theoretical foundations of phototelegraphy. M., Holy Hour, 1957, p. 299. . 2.Mustel E.R., Parygin V.N. Methods of modulation and scanning of light, Moscow, Science, 1970 p. 143. 3. USSR author's certificate number 127840, cl. (502 F 1/07, 1959.