WO2010084200A1

WO2010084200A1 - Device for amplitude modulation

Info

Publication number: WO2010084200A1
Application number: PCT/EP2010/050837
Authority: WO
Inventors: Stephan Reichelt; Steffen Buschbeck
Original assignee: Seereal Technologies S.A.
Priority date: 2009-01-26
Filing date: 2010-01-26
Publication date: 2010-07-29

Abstract

Devices for phase modulation of light bundles by means of electrowetting cells (EW cells) comprising a flat plate are described in applications of the applicant. Such devices should be modified for an amplitude modulation or complex modulation of coherent light according to the principle of the Fabry-Perot interferometer. Each EW cell of the device comprises a chamber having three liquids, which form two bounding surfaces as a flat plate. The bounding surfaces can be controlled by control means (CM) via electrodes (U1, U2, D3, U4). The flat plate formed can be variably tilted for phase modulation. For amplitude modulation, the covering means (AM) of the chamber have a semitransparent mirrored surface (V), which reflects incoming light bundles within the chamber multiple times. The tilting of the plane-parallel plate can be controlled with an optical path length that lies within half a wavelength of the light used. A corresponding amplitude value results from each phase change in said range. Other types of modulation are possible by modifying the device. The field of application is, for example, a holographic display.

Description

Device for amplitude modulation

The invention relates to a device for the spatial modulation of the amplitude of

Wave fronts containing electrowetting cells using the principle of a Fabry-Perot interferometer. The electrowetting cells are arranged in a matrix. An electrowetting cell (EW ZeIIe) each has a chamber with transparent covering means, which include three transparent fluids in the chamber and two controllable interfaces formed as a plane plate, wherein with a control means and the EW ZeIIe connected electrodes control tilting of the plane-parallel plate.

Fields of application of the invention are modulation devices in which a rapid modulation of the amplitude of incident interfering light is necessary. In combination with a series-connected phase-modulating EW cell, the invention can be used for complex modulation. Such modulation means are used for coherent light e.g. needed in a holographic display to display 3D objects.

The electrowetting technology allows the position and / or shape of the interface between two immiscible fluids within a microvial to be changed. The fluids have a different refractive index and one of the fluids is electrically conductive and thus controllable by electrodes.

By a targeted via external electrodes and the conductive fluid adjustable

Voltage difference, the interface can be changed. The microcontainer with the fluids is known as the electrowetting cell (EW cell).

A fluid may be a liquid, a gel or a gaseous medium.

In the present document liquids are preferably used as fluids.

The liquids may e.g. Oil and water or any other combinations. For three fluids, all three may be different or two equal fluids may include a third. A liquid can also be replaced by another fluid.

EW cells can form a matrix-like arrangement in any number and as

Array designed to realize a display for information presentation. Among other

Advantages are their fast switching times in the kHz range for modulation of phase and amplitude of light to use. With appropriate formation of the EW cells, phase and amplitude can be modulated together as complex values.

It is known to change the optical path of a light beam by using a plane-parallel plate in the optical path of an optical system. If a light bundle passes through this plane-parallel plate (plane plate) of fluid material, the optical path of the light bundle can be increased by tilting the plane plate by a predetermined angle. As a result, the light beam receives a phase difference with respect to a non-tilted ground state. This fact can be used in a three-lattice EE cell to produce different changes in the pathlength within the PEM through different tilt angles of the plano-plate for the light beam, thereby modulating the light beam in phase.

This is described in the not yet published patent application of the applicant (GB 0720484). In an EW cell with three different fluids, the central fluid forms a plane plate when actuated. The two interfaces between the different liquids which define the plane plate are tilted parallel to one another by the predetermined control values of an electrical voltage provided by the control. Tilting the plane plate by setting equal tilt angles at the two interfaces increases the optical path of the light beam in the EW cell. The interference-capable light bundle passes through the EW cell as a plane wavefront. In this way, a phase modulator is realized which alters the phase of the light as a function of the tilt of the two interfaces or the plane plate.

Furthermore, the use of a plane-parallel plate in a Fabry-Perot interferometer is known. This interferometer usually consists of two glass or quartz plates, between which a plane-parallel air plate is formed. The mutually facing surfaces of the plates are permeable mirrored and aligned exactly parallel to each other. A light beam entering the first plate is reflected several times between the plates before leaving the second plate. The emerging light beam has interference patterns. Similar interference phenomena as in the case of the air plate are provided by a transparent mirrored glass or quartz plate on both sides. Absolute determinations of wavelengths can not be made with this plate. However, the accurate determination of small wavelength differences is possible. This property of the Fabry-Perot interferometer for determining small wavelength differences should be used according to the invention for the modulation of light in an EW cell.

The object of the invention is to provide a modulation device based on electrowetting cells, which allows using a plane-parallel plate, the modulation of the phase or amplitude or a complex modulation of planar wavefronts without much effort.

The solution according to the invention is based on a device for light modulation based on a Fabry-Perot interferometer.

The device has, for the amplitude modulation of light, matrix-arranged electrowetting cells in each of which an electrowetting cell (EW cell) contains a chamber with transparent covering means and is connected to a control means and controllable electrodes. The transparent cover means include three transparent fluids in the chamber and two controllable interfaces formed as a plano plate, the cover means having a partially transparent coating which multiply reflects an incident light beam within the chamber, the driven electrodes for the plane-parallel plate setting an angle of inclination realized for the reflected light beam depending on the driving a path length difference.

The multiple reflections create multi-beam interferences within the chamber.

In an embodiment of the invention, the light bundle incident into the chamber strikes a plane plate which can be tilted under control with a variably adjustable voltage difference and sets an optical path length for the multiply reflected light bundle which is within half the wavelength of the light used. This implements the principle of the Fabry-Perot interferometer in an EW cell. On this basis, it is possible to set by the control means arbitrary phase values for the light beam passing through the chamber, which then supply a corresponding amplitude value for amplitude modulation. These phase values produce a phase offset of the amplitude-modulated light beam, but this does not affect the amplitude modulation.

In order to realize a complex modulation with the device according to the invention, the amplitude modulating EW cell in the light path of the light beam is serially combined with an EW cell which has a chamber with transparent covering means and modulates the phase of the light beam.

The relative phase offset occurring in the amplitude-modulating EW cell, which interferes with the complex modulation, is compensated by the phase-modulating EW cell. The compensation takes place by way of example in that the control means integrates the relative phase offset into the voltage difference of the driving electrodes to be set.

In order to obtain a multicolor modulation of incident light bundles of different colors, preferably the colors red (R), green (G) and blue (B), the monochrome light beams are modulated in succession for each color in a complex manner.

Advantageously, the fluids which enclose the plane plate are identical in their optical properties.

An EW cell according to the invention can also be used as an optically variable wavelength-selective filter. Furthermore, the EW cell may optionally be designed to be transmissive or reflective.

A display device for displaying two-dimensional stereo images can be realized in a further embodiment of the invention with a device according to at least one of the subclaims.

Further, here not explicitly stated training and uses of an EW cell with the integrated function of a Fabry-Perot interferometer, the Those skilled in the art can realize without inventive step, are also covered by the invention.

The advantages of the invention are as follows: When the EW cells according to the invention are formed as an array, it is possible to realize a fast switching amplitude modulator. In combination with an array of identical three-phase phase-change EW cells, a sandwich arrangement can be created to modulate complex values. The identical EW cells are advantageous for the manufacturing process.

However, other phase-modulating arrangements may be combined with the modulation means of the invention. The designed as an amplitude modulator device can also be used as a normal 2D display display. Compared to a LC display, the display of the invention can switch much faster. In conjunction with a strongly scattering film on the display surface, the angular range of the emitted light can be increased.

Another significant advantage over known amplitude modulators for 2D displays is the independence of the polarization. It can be dispensed polarized light and completely on polarizing films, which have only a transmission efficiency of 80%.

A further advantageous embodiment of the EW ZeIIe invention is that it can also be used as a single element in the form of an optically variable filter.

The device according to the invention is described below with reference to drawings. In the drawings show schematically as a sectional view in side view

Fig. 1 an inventive electrowetting cell (EW ZeIIe) with three

Liquids which generate a plane plate by controlled electrodes,

Fig. 2 shows the initial position of the plane plate of EW-ZeIIe of FIG. 1 with an incident light beam, and Fig. 3 is a tilt of the plane plate of FIG. 2 and the resulting course of the light beam.

The invention is based on the principle of the optical effect of a plane plate: by tilting the plane plate, an interference-capable light bundle passing through the EW element obtains a phase shift of Δφ (see FIG. 3) as a function of the predetermined thickness of the plane plate and the tilt angle v. The tilt can be performed variably with different tilt angles Y, whereby variable phase shifts Δφ of incident light beams can be realized. Tilting is performed by changing the contact angles, which are normally 90 ° between the interface of two immiscible liquids and the sidewalls of the chamber of the EW cell. In the case of three liquids, both contact angles for both boundary surfaces must be set equal in terms of magnitude, since the surfaces of the plane plate must remain parallel even when tilted.

It is known to keep the interface between two liquids flat by creating voltage differences between the opposing electrodes. This is achieved by setting contact angles on opposite sidewalls of the EW row so that they add up to a total of 180 °. The voltage values required for this condition, which build up an electric field between the respective side electrodes and the conductive liquid, can be determined approximately from the so-called electrowetting equation or by means of a calibration. The electrowetting equation or calibration indicates the macroscopic contact or wetting angle that forms for a given material and given thickness of the dielectric when a voltage is applied between the conductive liquid and the dielectrically isolated electrode.

The chamber of the EW cell here preferably has a quadrangular cross-section. In the figures, the arrow to the left of the EW row indicates the light incident direction.

Fig. 1 shows an EW ZeIIe invention, the chamber contains three liquids, two of which are identical and have the refractive index ni and a Surround liquid with the refractive index ri ₂ like a sandwich. In the example shown, an electrically non-conductive liquid is enclosed by two electrically conductive liquids, eg oil between two aqueous salt solutions. Control means CM set the voltage of the electrodes Ui, U ₂ , U ₃ and U ₄ so that the boundary surfaces between each two adjacent liquids are tilted parallel to each other and a plane plate is formed. The EW cell has transparent covering means AM at the light entry and light exit sides. These are provided on the inside of the EW ZeIIe with a partially transparent coating V and aligned parallel to each other. When driving the four electrodes Ui, U ₂ , U ₃ and U ₄ for adjusting the voltages, it is advantageous to control each electrode individually. The electrodes can also be controlled in pairs by cross. The latter can lead to inaccuracies, however, as a result of tolerances or material inequalities, the predetermined voltage difference may be subject to errors.

The mirroring V of the covering means AM makes it possible to realize a Fabry-Perot interferometer in the EW cell.

One part of the incident light beam passes directly through the EW cell and another part is reflected several times at the cover means AM and leads to multiple interferences within the EW cell. This can be seen in FIGS. 2 and 3. The part of the light beam running directly through the EW cell is drawn as a continuous arrow line, while the reflected part is shown in dashed lines.

In Fig. 2, the plane plate can be seen in its initial position, wherein the boundary surfaces extend parallel to the fixed cover means. The means for controlling the EW cell are not needed here for the sake of understanding and are therefore not shown.

FIG. 3 shows a tilting of the plane plate shown in FIG. 2 and the curve resulting from the tilting of a light bundle entering the EW cell. The flat plate is tilted relative to its initial position in Fig. 2 by the angle 90 ° - Y. This fact can be used to make different changes in the optical spectrum for the light beam due to different tilt angles Y of the plane plate To generate path length within the EW-ZeIIe and thus to modulate the light beam in the phase φ (Fig. 2).

The parallelism of the interfaces is maintained by the tilt. However, the optical path length changes both in the directly passing and in the multiply reflected part of the continuous light beam. The path length of the light beam in the EW cell is greater than in the starting position according to FIG. 2. The light beam passes through the tilting of the plane plate with a relative phase shift by Δφ to a light beam that is currently passing through the EW cell. This light beam is shown by a dash-dotted line.

In the initial position of the plane plate, a phase value is set as a reference value at which the change in the optical path length of the multiply reflected light beam for amplitude modulation is exactly half a wavelength. At this value, a destructive interference of the light beams results, which leads to the total extinction of the amplitude on exit from the EW cell. If you change the phase of the light beam that passes through the EW cell by slightly tilting the plane plate, you get a nonzero amplitude value. If the phase of the light bundle passing through the EW cell is then changed by a corresponding voltage difference at the electrodes, which are controlled by the control means CM, an amplitude value correspondingly assigned to this phase and a phase shift are obtained. The resulting phase shift does not interfere with the amplitude modulation. If a plurality of amplitude-modulating EW cells are combined to form an array, a spatial amplitude modulator can be realized. The phase shift follows in a three-liquid containing EW ZeIIe in which the formed plane-parallel plate has a tilt angle 90 ° - Y, no linear course, but is approximately square (see Fig. 1 (a) and (b) in the appendix). This fact leads to a widening of the central peak of the intensity and amplitude curves in the light modulation with the Fabry-Perot amplitude modulator according to the invention (compare Figures 2 (a) and (b)) and thus reduces the edge steepness. In order to realize complex values for a hologram coding, for example, in addition to the amplitude, the phase of the transmitted light is also modulated. This can be achieved, for example, by serially assigning a phase modulator described in the prior art to the amplitude modulator. Both modulators are identical except for the covering means and the structure of the amplitude modulator then corresponds to the inventive EW cell device, while in the phase modulator the covering means are transparent.

In the case of complex modulation, the phase shift resulting from the amplitude modulation, which is known from its size, must be compensated again. For this purpose, the control means CM integrates the value of the phase shift in the voltage difference to be set of the electrodes of the EW cell to be controlled.

If a multicolor modulation of incident light bundles of different colors, preferably the colors red (R), yellow (G) and blue (B) is to be generated with the device according to the invention, each color must be modulated successively in a complex manner. In order to compensate for a lateral offset .DELTA.l (see FIG. 3) of the light bundles after passing through the EW cells lying serially in the light path, the last EW cell is to be provided with an aperture at the light exit side.

An EW cell may optionally be embodied as transmissive or reflective and in each case form a transmissive or reflective EW cell array with a plurality of EW cells.

Further embodiments of the invention provide that tailored, designed for a given application of the light modulation cell geometries of the EW cells are used. For example, the chamber may have a variable cross-section, for example, such that the operation curve / flank of the light modulator becomes more linear. Or in the EW cells instead a solid, partially absorbing layer is introduced. The designed as an amplitude modulator device can also be used as a normal 2D display display. Compared to a LC display, the display of EW cells according to the invention can switch much faster. In conjunction with a strongly scattering film on the surface of the display, the angular range of the emitted light can be further increased.

Another significant advantage over known amplitude modulators for 2D displays is the independence of the polarization of the light. It can therefore be dispensed with polarized light and completely on polarizing films, which have only a transmission efficiency of 80%.

The application is not limited to just one amplitude modulator. The device can also be implemented as a variable / tunable filter.

Investment:

^■ underlying principle: multi-beam interferences on plane-parallel surfaces ¹

^■ The statements made below are based in part on findings obtained through simulations (see appendix)

^■ Assessment of technical feasibility:

- Phase (liquid tilt angle) must be set accurately (the same or even higher requirements apply to the phase modulators)

- parallelism of the partially mirrored surfaces must be ensured (this requirement is higher than for pure phase modulators, but theoretically a wedge error of the end surfaces could be compensated by a small wedge of the inner liquid → it is only important that the mirror surfaces are always hit vertically)

- 6 electrodes are required per cell (4 for plane-parallel tilting of the inner fluid and 2 for ensuring the 90 ° angle on the other two side walls)

- Mirroring / partial mirroring of the cover glasses is state of the art

- Can be used in combination with all phase modulators as a sandwich solution for complex-valued modulation

- Even an amplitude modulator with limited

Modulation width / contrast can be used in combination with a phase modulator

Almost identical production technology to an EW-3Liquid phase modulator possible,

¹ The transmission of flat plate resonators is described by the Airy function. Lateral beam offset is lower than the EW phase modulator, but must still be considered (aperture solution is possible)

Not only amplitude modulator (array) but also variable filter

Limitation of the parameter range, further embodiments: transmissive + reflective; also 'tailor-made' cell geometries ('variable-section' cylinders, for example, such that the operation curve / flank becomes more linear; or introducing a solid, partially-absorbing layer); Application is not limited to amplitude SLM, in principle it is a variable / tunable filter, which can also be implemented as a single element.

Appendix - Simulations:

Based on the phase shift within a 3-liquid EW cell (composite plane plate), the intensities and amplitudes of a Fabry-Perot etalon (interferometer) were calculated

γ = tilt angle of the liquids φ = phase shift

Fig. A1: Typical phase profile in an EW cell (3-liquid) with a composite flat plate

The phase shift in a 3-liquid EW cell with tiltable inner liquid does not follow a linear course, but is approximately square (FIG. A1 (a)). This fact is favorable for the proposed Fabry-Perot amplitude modulus, since this leads to a widening of the central peak of the intensity and amplitude profiles (see diagrams below) and thus the edge steepness is reduced.

FIG. A2: Graphical representation of the different phase progression between a standard Fabry-Perot SLM and an SLM according to the invention with EW cells with a composite plan plate according to Fabry-Perot (influence on half-width and gradient of the central maximum of the amplitude)

dependencies:

Influence of finesse ¹ F (adjustment by reflectance / transmittance of the end mirror) on the exit amplitude or intensity

The so-called finesse F is used to characterize the resonator. It is defined as the ratio between the free spectral range and the half-width of a single peak:

* Δλ

The finesse assumes high values at high reflectivities R of the mirrors or at low attenuation in the resonator:

Fig. A3: Graphic representation of a finesse compromise: A flat edge causes a reduced contrast

As the finesse F increases, the half-width of the peak becomes narrower according to FIG. A4a, and the amplitude dynamics increase according to FIG. A4b, meaning that no amplitudes close to zero can be realized with small finesse (compare the graph with the finesse F = 5 in FIG Fig. A3).

Influence of the refractive index differences of the liquid water and oil water = 1 -34; % first liquid noi = 1.6 ... 1.7; % second liquid water = 1 -34; % third liquid

Fig. A4: Peak broadening with smaller index contrast

The larger the index contrast, the smaller the required angle range of the liquid menisci (tilting of the middle liquid).

Claims

claims

Anspruch [en] A device for amplitude modulation with matrix-arranged electrowetting cells, in which in each case an electrowetting cell (EW cell) is connected to a control means and controllable electrodes and has a chamber with transparent covering means, wherein in the chamber three transparent fluids and two trapped by the fluids controllable Form a plane-parallel plate, wherein the transparent cover means have a partially transparent reflection, which reflect an incident light beam within the chamber multiply, wherein the driven electrodes for the plane-parallel plate set an inclination angle, for the multiply reflected light beam, a change in the optical path length within a half wavelength of the light used.

2. Device according to claim 1, wherein a set by the control means phase value for the light beam passing through the chamber provides a corresponding amplitude value for amplitude modulation.

3. Device according to claim 2, wherein in each case an amplitude-modulating EW cell in the light path of the light beam in series with a phase-modulating EW

ZeIIe is combined to perform a complex modulation.

4. Device according to claim 3, wherein the amplitude-modulating EW ZeIIe partially transparent mirrored covering means and the phase-modulating EW ZeIIe transparent covering means for complex modulation.

5. Device according to claim 4, wherein the phase-modulating EW cell compensates for a relative phase offset occurring in the complex modulation in the amplitude-modulating EW cell.

6. Device according to claim 3, wherein for generating a multicolor modulation incident light bundles of different colors, preferably RGB, are modulated in sequence for each color complex.

7. Device according to claim 1, wherein the fluids, which include the plane-parallel plate, are identical in their optical properties.

8. Device according to claim 1, which is used as an optically variable wavelength-selective filter.

9. Display device for displaying two-dimensional stereo images, comprising a device according to one of the preceding claims.