WO2018185513A1

WO2018185513A1 - Optical filter, amplitude modulator, optical article and method for amplitude tuning an optical beam

Info

Publication number: WO2018185513A1
Application number: PCT/IB2017/000539
Authority: WO
Inventors: Eva OTON; Estelle NETTER
Original assignee: Nikon Corporation; Essilor International
Priority date: 2017-04-03
Filing date: 2017-04-03
Publication date: 2018-10-11

Abstract

The invention concerns an optical filter. According to the invention, the optical filter comprises : - a liquid crystal cell (1) filled with a liquid crystal mixture (3) having a Bragg state with a Bragg reflection wavelength within the 380-780 nm range in absence of applied voltage, the liquid crystal cell (1) comprising a plurality of adjacent pixels (41, 42, 43, 44) arranged in a matrix, and a controller (5) connected to the first electrode (111, 112, 113, 114) and to the second electrode (121, 122, 123, 124) of each pixel (41, 42, 43, 44) and configured for selecting a first portion of the plurality of pixels and/or a second portion of the plurality of pixels, and the controller (5) being configured for applying a first voltage (V1) to the first portion of the plurality of pixels so that the liquid crystal mixture in the first portion is in another state, which is an homeotropic state.

Description

OPTICAL FILTER, AMPLITUDE MODULATOR, OPTICAL ARTICLE AND METHOD FOR AMPLITUDE

TUNING AN OPTICAL BEAM

TECHNICAL FIELD OF THE INVENTION

The invention relates to a tunable amplitude optical filter.

More precisely the invention relates to a device and a method for adjusting or modulating the amplitude of an optical filter based on liquid crystals.

BACKGROUND INFORMATION AND PRIOR ART

Amplitude filtering or modulation of optical beams is used in many applications including metrology tools, such as microscopy apparatus and spectacle lenses.

Our eyes are exposed to natural light, which plays an important role in many aspects of our lives. Indeed, it was demonstrated that the blue radiation in the spectral range from 465 nm to 495 nm, or good blue range, has some positive effects for the human body related to the photo-synchronization of the circadian cycle, and thus to sleep, but also to mood, corporal temperature and psychomotricity. Ultraviolet (UV) and blue wavelengths correspond to the highest energy photons in the visible spectrum. Sunglasses protect from intense light radiation especially in the UV range. However, filtering glasses which absorb or reflect blue radiation affect the visual color perception of the environment.

Besides, we are more and more exposed to artificial light from electronic color display devices or LEDs. Some of these devices produce electromagnetic radiation in the visible range which is concentrated in particular in a narrow blue part of the spectrum. The eyes are thus more and more exposed to intense blue radiation. However, it is now recognized that the blue-violet light comprised between 415 nm and 455 nm, or bad blue range, induces cumulative damage to the retina of the human eye.

Thus there is a need for a wearer of ophthalmic lenses, to dispose of a therapeutic filter having a UV-only cut mode, for ensuring UV protection while providing an acceptable color perception. For efficient synchronization of circadian rhythms, exposure to the good blue range is required only for limited periods and at specific hours. Thus, there is a need for a therapeutic filter having a selective spectral bandwidth that can be modulated in amplitude especially in the blue range. Many active optical filter devices are based on liquid crystal technology. Different types of liquid crystals are used for manufacturing band-stop filters operating in reflection. In particular, cholesteric liquid crystals (CLC) comprise a liquid crystal host and a chiral dopant resulting in a CLC planar structure defined by a helix pitch and axis. Cholesteric liquid crystal devices can be tuned to reflect specific wavelengths for application to color filters.

The photonic band gap of a CLC device is generally controlled by adjusting the liquid crystal host and/or chiral parameters, or by adjusting external factors such as temperature, pressure, light irradiation or electric field.

However, an electrically driven CLC filter device generally operates as a switch between only two states (or switchable-only device), meaning that there are only two stable states, ON and OFF, respectively with 100% and 0% transmission coefficients for the wavelength range reflected, for circular polarized light. When switching between the On and Off states, CLC devices exhibit intermediate liquid crystal states. In these intermediate liquid crystal states, the CLC devices present a focal conic structure which leads to undesired scattering.

CLC devices are also available in polymer stabilized forms, such as helical polymer nano-dispersed liquid crystals or blue phase liquid crystals (BPLC). However, the polymer-based CLC versions require high threshold voltage and also exhibit undesired scattering.

Thus, for different applications, there is a need for a wavelength selective band-stop optical filter which is tunable in amplitude over a broad dynamic range and which is transparent in the rest of the visible range.

SUMMARY OF THE INVENTION

The above objects are achieved according to the invention by providing an optical filter.

According to the invention, the optical filter comprises a liquid crystal cell comprising a first plate having a first surface and a second plate having a second surface, the second surface being arranged opposite to the first surface, a liquid crystal mixture filling the liquid crystal cell between the first surface and the second surface, the liquid crystal mixture having a Bragg state displaying a Bragg reflection with a Bragg reflection wavelength within the 380-780 nm range in absence of applied voltage, the liquid crystal cell comprising a plurality of adjacent pixels arranged in a matrix, each pixel comprising a first electrode arranged on the first surface and a second electrode arranged on the second surface of the liquid crystal cell, and a controller connected to the first electrode and to the second electrode of each pixel and configured for selecting a first portion of the plurality of pixels and/or a second portion of the plurality of pixels, the first portion of the plurality of pixels and the second portion of the plurality of pixels forming the matrix of plurality of adjacent pixels and the controller being configured for applying a first voltage to the first portion of the plurality of pixels so that the liquid crystal mixture in the first portion of the plurality of pixels is in another state, which is an homeotropic state.

Thus, the first portion of pixels is in homeotropic state. In other words, the liquid crystal molecules of the first portion of pixels are aligned perpendicularly to the substrate. At the same time, the liquid crystal of the second portion of pixels is in cholesteric state.

As a result, the amplitude of Bragg reflection is reduced depending on the ratio of area of the first portion of pixels to the area of the second portion of pixels.

This optical filter device is tunable in amplitude and operates for a specific wavelength range. This specific wavelength range can be designed by selecting the liquid crystal mixture composition depending on the application.

Although the controller switches the applied voltage between only two tension values, the amplitude of reflection or transmission at the Bragg wavelength may be modulated within a dynamic range comprising a number of intermediate levels higher than two, the number of intermediate levels depending on the number of pixels in a cell.

According to a particular and advantageous aspect of the present disclosure, the controller is configured so that the first portion of the plurality of pixels and the second portion of the plurality of pixels are spatially selected to form a determined spatial distribution of pixels, so as to generate a predetermined amplitude of Bragg reflection or transmission at the Bragg reflection wavelength when the first voltage is applied.

In a particular operating condition, the controller is configured for applying a second voltage to the second portion of the plurality of pixels so that the liquid crystal mixture in the second portion is in the Bragg state.

According to a particular and advantageous aspect, the first portion of the plurality of pixels is selected randomly among the plurality of adjacent pixels so that pixels of the first portion are mixed with pixels of the second portion.

According to a particular embodiment, the plurality of first electrodes and/or the plurality of second electrodes comprise electrodes having uneven sizes and/or uneven shapes, and preferably random patterns or shapes.

Advantageously, the matrix of pixels has a spatially non-periodic structure.

Preferably, the liquid crystal mixture comprises a cholesteric liquid crystal, a blue-phase liquid crystal or a helical polymer liquid crystal layer.

According to an embodiment, the cholesteric liquid crystal mixture comprises a liquid crystal host material selected among nematic liquid crystal mixtures with a birefringence lower than 0,2 such as CB5, MLC-9100, MLC-9200, MLC-6241 , or MLC-6025, or birefringence higher than 0,2 such as MDA-98, MLC- 2140, MLC-2144, E7, E44, BL-003, BL-006, BL-038 or dual frequency nematic liquid crystals like W-1978C or MLC-2177.

Preferably, the cholesteric liquid crystal mixture comprises a chiral dopant selected among ISO(60BA)2, R-501 1 , S-501 1 , CB15, R-81 1 , S-81 1 , ZLI- 4571 , ZLI-4572, with a preference for R-501 1 , S-501 1 which have high twisting power.

According to a particular and advantageous aspect, the Bragg reflection is a selective Bragg reflection with a reflection bandwidth at half maximum smaller than 100 nm, and preferably smaller than 50nm.

Another object of the invention is to provide an amplitude modulator comprising a first optical filter according to any embodiment disclosed herein and a second optical filter according to any embodiment disclosed herein, the second optical filter having the same predetermined Bragg reflection wavelength as the first optical filter in absence of applied voltage, the first optical filter having a handedness and the second optical filter having an opposite handedness, the first optical filter and the second optical filter being arranged in a stack.

In an embodiment, the controller is configured to select the first portion of pixels and the second portion of pixels from the first optical filter and, respectively, another first portion of pixels and another second portion of pixels from the second optical filter, and wherein a ratio of an area of the first portion of pixels to an area of the second portion of pixels in the first optical filter is about the same as the ratio of an area of said another first portion of pixels to an area of said another second portion of pixels in the second optical filter.

A further object of the invention is to provide an optical article comprising an optical filter or, respectively, an amplitude modulator according to any embodiment disclosed herein, and further comprising a lens part, said optical filter, or, respectively, said optical modulator being configured for providing an optical phase modulation and/or an optical power modulation.

A further object of the invention is to provide a method for amplitude tuning an optical beam in reflection and/or in transmission, the method comprising the following steps:

- providing a liquid crystal cell comprising a liquid crystal mixture filling the liquid crystal cell between a first surface and a second surface, the liquid crystal cell comprising a plurality of adjacent pixels arranged in a matrix, each pixel comprising a first electrode arranged on the first surface and a second electrode arranged on the second surface of the liquid crystal cell, the liquid crystal mixture (3) having a Bragg state displaying a Bragg reflection with a Bragg reflection wavelength within the 380-780 nm range in absence of applied voltage,

- selecting a first portion of the plurality of pixels and/or a second portion of the plurality of pixels, the first portion of the plurality of pixels and the second portion of the plurality of pixels constituting the matrix of adjacent pixels of the liquid crystal cell and determining an area ratio of the first portion to the second portion of the plurality of pixels;

- applying a first voltage (V1 ) to the first portion of the plurality of pixels, the first portion of the plurality of pixels switching to an homeotropic state and becoming transmissive at the Bragg reflection wavelength, while applying no voltage to the second portion of the plurality of pixels, the second portion of the plurality of pixels displaying the Bragg reflection at the Bragg reflection wavelength.

According to a preferred aspect of the method, the first portion of the plurality of pixels and the second portion of the plurality of pixels are spatially selected to form a determined spatial distribution of pixels, depending on a predetermined amplitude of Bragg reflection or transmission at the Bragg reflection wavelength. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 schematically shows a cross-section of an optical device according to an embodiment;

Figure 2A, respectively 2B, schematically shows a top view of a first, respectively second, electrode patterned surface for a LC cell;

Figure 3 shows a microscope view of an exemplary device under crossed polarizers when all pixels are in the OFF state;

Figure 4 shows the reflectance R in arbitrary units (a.u.) of the device of Fig.1 measured as a function of wavelength using a spectrophotometer when all pixels are in the OFF state;

Figure 5 shows a microscope view of the same device when some pixels are in the OFF state and other pixels are in the ON state;

Figure 6 shows the reflectance of the device of Fig. 5 measured using a spectrophotometer;

Figure 7 shows a microscope view of the same device under crossed polarizers when all pixels are in the ON state;

Figure 8 shows the reflectance of the device of Fig. 7 measured using a spectrophotometer when all pixels are in the ON state.

DETAILED DESCRIPTION OF EXAMPLE(S)

The present disclosure concerns an optical filter adapted for modulating the amplitude in reflection and/or in transmission over a predetermined range of wavelengths.

The optical filter comprises at least one liquid crystal cell.

Figure 1 schematically shows an exemplary cross section of a liquid crystal cell.

The liquid crystal cell 1 comprises a first plate 1 1 having a first surface 1 1 0 and a second plate 1 2 having a second surface 1 20. The first plate 1 1 and the second plate 1 2 are made of a transparent material over the visible spectral range. A liquid crystal mixture 3 fills the liquid crystal cell between the first surface 1 1 0 and the second surface 1 20. In other words, the liquid crystal cell 1 comprises a layer of the liquid crystal mixture. Preferably, the first surface 1 1 0 and the second surface 1 20 are parallel. Micro sized spacers may be included in the LC cell so as to ensure a constant distance between the first surface 1 1 0 and the second surface 1 20 across the liquid crystal cell. In an example, the LC layer has a thickness of about 2 micrometers.

Figure 2A schematically shows a top view of the first plate 1 1 and figure 2B schematically shows a top view of the second plate 12.

The liquid crystal cell 1 comprises a matrix of adjacent pixels 41 , 42. A pixel 41 comprises a first electrode 1 1 1 arranged on the first surface 1 10 and a second electrode 121 arranged on the second surface 120 of the liquid crystal cell 1 . The first electrode 1 1 1 is placed opposite to the second electrode 121 of this pixel 41 . Similarly, an adjacent pixel 42 comprises a first electrode 1 12 arranged on the first surface 1 10 and a second electrode 122 arranged on the second surface 120 of the liquid crystal cell 1 . The first electrode 1 12 is placed opposite to the second electrode 122 of this pixel 12. The first electrode 1 1 1 , 1 12 and the second electrode 121 , 122 are preferably transparent conductive electrodes, consisted of an indium-tin-oxide (ITO) layer for example.

Pixel 41 comprises a portion of liquid crystal mixture 3 placed between the first electrode 1 1 1 and the corresponding second electrode 121 . Adjacent pixel 42 also comprises a portion of liquid crystal mixture 3 placed between the first electrode 1 12 and the corresponding second electrode 122.

In an embodiment, the liquid crystal cell 1 comprises no internal wall separating liquid crystal mixture of pixel 41 from the liquid crystal mixture of adjacent pixel 42.

In a variant, the liquid crystal cell comprises at least one internal wall separating the liquid crystal mixture of a pixel 41 from the liquid crystal mixture of another adjacent pixel 42.

A controller 5 is connected to the first electrodes 1 1 1 , 1 12 and/or to the second electrodes 121 , 122 of each pixel 41 , 42. Thus controller applies a voltage selected among two values between each pair of first electrode 1 1 1 and second electrode 121 . In this way, each pixel is electrically addressed to be either in the ON or OFF state.

Preferably, the LC layer comprises a cholesteric liquid crystal mixture 3. The cholesteric liquid crystal mixture 3 generally comprises a liquid crystal host material and a chiral dopant. The liquid crystal host material and the chiral dopant nature and amount are selected depending on the targeted spectral range for the optical filter.

Let us consider an OFF state, wherein a null voltage is applied by the controller 5 between the electrodes of every pixel of the LC cell. In this case, the LC layer for this pixel has a helical structure with a helix pitch P₀. The average refractive index <n> of the LC mixture depends on the LC host material. The helix pitch P₀ depends on the chiral dopant. Depending on the incidence condition, a light beam incident on the LC cell is at least partially reflected at a Bragg reflection wavelength. In other words, the pixels are in a Bragg state. The Bragg reflection wavelength is given by λ = <n>. P₀. The reflected spectral range is given by Δλ = Po (n_e - no) where n_e and n₀ are the extraordinary and ordinary refractive indices of the liquid crystal host material. Thus, the reflection bandwidth can be tuned by changing the pitch P₀. Generally, the spectral bandwidth of Bragg reflection for cholesteric liquid crystals ranges between at least 30-50nm and a few hundreds of nm at maximum.

In an example, the liquid crystal host is MLC9100 (from MERCK) and the chiral dopant is selected such that the pitch P₀ is 315 nm. In the OFF state, the optical filter exhibits a Bragg reflection at 480 nm under normal incidence with a bandwidth at half maximum of 27 nm. The light beam transmitted through the LC cell is attenuated in amplitude in the spectral bandwidth of about 27 nm around the Bragg wavelength of 480 nm. Incident light outside this spectral bandwidth is transmitted without attenuation and without scattering. When every pixel of a LC cell is in the OFF state or Bragg state, the LC cell forms an optical Bragg reflection filter. This Bragg reflection filter may have an application as a blue therapeutic filter for rejecting blue light in a determined spectral bandwidth.

Let us consider now another state, wherein the controller 5 applies another non-null voltage between the electrodes 1 1 1 , 1 12 of at least one pixel 41 of the LC cell. This pixel is now an ON state, wherein the LC layer exhibits locally a homeotropic state or orientation. In the ON state, the liquid crystal molecules of this pixel are aligned perpendicularly to the first and second surfaces 1 10, 120. Thus, in the ON state, this pixel does not produce a Bragg reflection. However, if the adjacent pixels are in the OFF state, the LC molecules in these adjacent pixels remain in the helical orientation.

The controller 5 is used to address each pixel individually. The controller may switch the applied voltage of each pixel between an OFF state, when applying a null voltage and an ON state, when applying a determined non-null voltage. The voltage is configured so as to avoid that the liquid crystal mixture exhibits an intermediate focal conic state when switching between the ON and OFF states and inversely.

The pixels are arranged in a matrix.

The controller addresses all the pixels of a LC cell. A first portion of the plurality of pixels of the LC cell are in the ON state, or homeotropic state and a second portion of the plurality of pixels of the LC cell are in the OFF state, or Bragg state. The amplitude of the Bragg reflection depends on the number of pixels in the first portion and the number of pixels in the second portion for the LC cell. More precisely, the amplitude of the Bragg reflection depends on the ratio of the area of the second portion of the plurality of pixels to the area of the first portion of the plurality of pixels. Consequently, a fraction of the incident light at the Bragg wavelength is transmitted.

When the controller 5 addresses the LC cell so that all the pixels are in the ON state or homeotropic state, the Bragg reflection disappears.

Thus, the optical filter enables to tune the amplitude of the reflected and transmitted incident light at the Bragg wavelength depending on the number of pixels that are in the ON state and in the OFF state respectively.

The number of tuning levels for the amplitude of Bragg reflection depends on the total number of pixels of the optical filter. The higher the number of pixels, the larger is the dynamic of the tuning range. As an example, the LC Cell comprises several hundreds and preferably thousands of pixels arranged in a matrix. Preferably, a patterned electrode grid forms electrodes having micrometric dimensions, thus delimiting pixels with micrometric sizes. Such a high number of micro-sized pixels enables to adjust precisely the exact level of reflectance at the Bragg wavelength according to the needs.

In this way, the optical filter operates without scattering as an amplitude modulator for a selected wavelength or wavelength range. Advantageously, the optical filter operates using a relatively low voltage range, generally less than a few volts, 5-50 V.

In an embodiment, the controller 5 is configured to address the pixels randomly. More precisely, the proportion of pixels in the ON state is determined while the spatial positions of the first portion of the plurality of pixels of the LC cell in the ON state is selected randomly by the controller 5.

Each pixel is defined by the overlap of the projections of the first electrode and the second electrode of said pixel. In one embodiment the first electrodes, respectively second electrodes, defining each pixel have identical sizes and shapes. For example, the pixels have square or rectangular shapes and are arranged in a periodic array. However, such a periodic structure with micro- sized pixels may produce undesired diffraction.

Alternatively, the LC cell comprises pixels formed by overlap of electrodes with different shapes and/or sizes. Such a configuration avoids forming a periodic structure. This configuration has the advantage of avoiding at the same time, diffraction due to switching on pixels and diffraction due to the ITO patterned layer itself. In one embodiment the electrodes form waved strips of un-periodic wavelength.

In another embodiment the first and second electrode may be positioned in a same plane, forming for example interspaced structures.

In another embodiment, one of the first or second electrode is controlled by a transistor so as to form an active grid; said electrode being for example a source or drain of said transistor. The shape of said electrode thus forms, by itself, a pixel of the optical filter.

By addressing the pixels that go ON or OFF in a randomized pattern and using electrodes with random shapes, light losses by diffraction are completely avoided.

An optical filter as disclosed herein forms a spectral band-stop filter whose attenuation in reflection over a determined spectral bandwidth can be modulated by switching the voltage applied to a matrix of pixels while the other wavelengths are unaltered both in transmission and reflection.

This optical filter finds several applications.

Such an optical filter can be used as a tunable amplitude beam splitter used for example in interferometric systems. For example, electrically-controlling a fine adjustment of the reflected and transmitted amplitude enables to finely adjust the contrast of the interference fringes by ensuring equal optical path without introducing losses.

Such an optical filter can also be used as a switchable dichroic mirror: By using only two amplitude levels, ON and OFF, the reflected wavelength can be switched to a variety of reflection levels, producing a device that behaves as a tunable amplitude mirror in a predetermined spectral range. Such an optical filter can also be used as a tunable amplitude modulation band stop therapeutic filter. The human eye can see wavelengths within a range of about 380 nm to about 780 nm. The optical filter is configured so as to reject the blue-violet light ranging between 415 nm and 495 nm in the OFF state. In the ON state, transmission of a portion of the blue light in the 465nm-495nm range is enabled for enhancing some non-visual functions for the human body.

A tunable amplitude modulation band stop therapeutic filter may be combined with ophthalmic lenses.

An exemplary device comprises two substrates with patterned ITO electrodes that form an 8x8 pixel grid. The substrate surfaces are conditioned using an alignment layer, for example a polyimide layer that is rubbed to induce an orientation of the LC molecules. The substrates are assembled using microsized spacers so as to form a cell of 2 μιη thickness.

The cell is infiltrated with a cholesteric LC mixture composed by a nematic liquid crystal and a chiral dopant. The amount of chiral dopant is fixed to obtain a determined pitch length for the Cholesteric LC that reflects a selected wavelength. For example, the liquid crystal mixture is CB5 (4-Cyano-4'- pentylbiphenyl) and the chiral dopant ISO-(6OBA)2.

Figure 3 shows a microscope view of this device under crossed polarizers. In Figures 3-4, no voltage is applied to any of the pixels. The pixels are hardly seen on figure 3. The LC layer looks blue in reflection under normal incidence across the whole surface of the device.

Figure 4 shows that the device reflects light in the wavelength range around 440nm, with a Full width at half maximum (FWHM) of about 30 nm.

In Figures 5-6, a voltage of about 25 to 30V is applied to 1 /3 of the pixels while no voltage is applied to 2/3 of the pixels. Thus, 1 /3 of the pixels are switched on, and, as the molecules switch in orientation perpendicular to the substrate (homeotropic orientation), these ON pixels look black under crossed polarizers. The reflection efficiency at the Bragg wavelength drops to approximately 2/3 of the total reflectance measured on fig. 4.

In Figures 7-8, a voltage of about 25 to 30V is applied to all the pixels of the LC cell so that all the pixels are in the ON state. When all pixels are switched ON, all LC molecules are in homeotropic orientation. All the pixels look black under crossed polarizers. No light is reflected, so the Bragg reflection peak disappears on Fig. 8.

In the above example, the liquid crystal mixture 3 comprises a cholesteric liquid crystal. The cholesteric liquid crystal mixture comprises a liquid crystal host material selected among nematic liquid crystal mixtures with intermediate or low birefringence (Δη<0.2) selected among CB5, MLC-9100, MLC-9200, MLC-6241 , or MLC-6025. Alternatively, the cholesteric liquid crystal mixture comprises a liquid crystal host material selected among nematic liquid crystal mixtures with high birefringence (Δη>0.2) selected among MDA-98, MLC-2140, MLC-2144, E7, E44, BL-003, BL-006, BL-038. Alternatively, the cholesteric LC mixture comprises a liquid crystal host selected among dual frequency liquid crystal mixtures like W- 1978C or MLC-2177.

An optical filter as described above comprising a single cholesteric liquid crystal layer may be either right-handed or left-handed depending on the handedness of the helix of the LC molecules in the OFF state. The Bragg reflection efficiency of such an optical filter with respect to un-polarized incident light is limited to 50%.

In another embodiment, an optical filter device comprises a left handed optical filter and a right handed optical filter arranged in a stack. Such a device enables to increase the Bragg reflection efficiency with respect to incident light up to 100%. This optical filter device is polarization-independent under normal incidence. Indeed, according to the invention, any of the left handed and the right handed optical filters is tunable in amplitude between 0% of reflection and 100% of reflection of light having both the Bragg reflection wavelength and the handedness corresponding to said left handed optical filter or right handed optical filter. Accordingly, while separately each of the left handed optical filter and the right handed optical filter can only reflect the ambient light at the Bragg wavelength up to 50%, when combined together in series, with each light ray going subsequently through both optical filters, each optical filter can filter, by reflection, from 0% to 50% of the light not reflected by the other filter for a total Bragg reflection controllable from 0% to 100% of the incident light having a wavelength corresponding to the Bragg reflection.

In an alternative embodiment, the liquid crystal mixture 3 comprises a blue-phase liquid crystal. These materials can be arranged to reflect a specific wavelength as well. However, blue-phase liquid crystal devices generally require a stabilization network, for example using a polymer. As a result, blue-phase liquid crystal devices operate under considerably higher applied voltages than cholesteric LC devices. Such high voltages are unsuitable for certain applications.

In still another example, the liquid crystal mixture 3 comprises a helical polymer liquid crystal layer. However, these material being organized in a network, the drawbacks are the same as using blue-phase liquid crystal devices.

In an embodiment the at least one cell comprises a liquid crystal mixture with dual frequency liquid crystals like W-1978C or MLC-2177 or W-1831 A and those dual frequency liquid crystals can be controlled to have at least two different values of Bragg reflection wavelength depending on a frequency of the voltage applied to the liquid crystal mixture. Accordingly, when the principle of the invention is applied to said cell, an amplitude tunable wavelength tunable optical filter can be produced.

Claims

1 . Optical filter, characterized in that it comprises :

- a liquid crystal cell (1 ) comprising a first plate (1 1 ) having a first surface (1 10) and a second plate (12) having a second surface (120), the second surface (120) being arranged opposite to the first surface (1 10),

- a liquid crystal mixture (3) filling the liquid crystal cell (1 ) between the first surface (1 10) and the second surface (120), the liquid crystal mixture (3) having a Bragg state displaying a Bragg reflection with a Bragg reflection wavelength within the 380-780 nm range in absence of applied voltage,

- the liquid crystal cell (1 ) comprising a plurality of adjacent pixels (41 , 42, 43, 44) arranged in a matrix, each pixel (41 , 42, 43, 44) comprising a first electrode (1 1 1 , 1 12, 1 13, 1 14) arranged on the first surface (1 10) and a second electrode (121 , 122, 123, 124) arranged on the second surface (120) of the liquid crystal cell (1 ), and

- a controller (5) connected to the first electrode (1 1 1 , 1 12, 1 13, 1 14) and to the second electrode (121 , 122, 123, 124) of each pixel (41 , 42, 43, 44) and configured for selecting a first portion of the plurality of pixels and/or a second portion of the plurality of pixels, the first portion of the plurality of pixels and the second portion of the plurality of pixels forming the matrix of plurality of adjacent pixels (41 , 42, 43, 44), and the controller (5) being configured for applying a first voltage (V1 ) to the first portion of the plurality of pixels so that the liquid crystal mixture in the first portion is in another state, which is an homeotropic state.

2. Optical filter according to claim 1 , wherein the controller (5) is configured so that the first portion of the plurality of pixels and the second portion of the plurality of pixels are spatially selected to form a determined spatial distribution of pixels, so as to generate a predetermined amplitude of Bragg reflection or transmission at the Bragg reflection wavelength when the first voltage is applied.

3. Optical filter according to any of claims 1 or 2, wherein the controller (5) is configured for applying a second voltage (V2) to the second portion of the plurality of pixels so that the liquid crystal mixture in the second portion is in the Bragg state.

4. Optical filter according to any of claims 1 to 3, wherein the first portion of the plurality of pixels is selected randomly among the plurality of adjacent pixels (41 , 42, 43, 44) so that pixels of the first portion are mixed with pixels of the second portion.

5. Optical filter according to any one of claims 1 to 4 wherein the plurality of first electrodes and/or the plurality of second electrodes comprise electrodes having uneven sizes and/or uneven shapes, and preferably random patterns or shapes.

6. Optical filter according to any one of claims 1 to 5 wherein the matrix of pixels has a spatially non-periodic structure.

7. Optical filter according to any one of claims 1 to 6 wherein the liquid crystal mixture (3) comprises a cholesteric liquid crystal, a blue-phase liquid crystal or a helical polymer liquid crystal layer.

8. Optical filter according to claim 7 wherein the cholesteric liquid crystal mixture (3) comprises a liquid crystal host material selected among nematic liquid crystal mixtures with a birefringence lower than 0,2 such as CB5, MLC-9100, MLC-9200, MLC-6241 , or MLC-6025, or birefringence higher than 0,2 such as MDA-98, MLC-2140, MLC-2144, E7, E44, BL-003, BL-006, BL-038 or dual frequency nematic liquid crystals like W-1978C or MLC-2177.

9. Optical filter according to claim 7 or 8 wherein the cholesteric liquid crystal mixture (3) comprises a chiral dopant selected among ISO(6OBA)2, R- 501 1 , S-501 1 , CB15, R-81 1 , S-81 1 , ZLI-4571 , ZLI-4572.

10. Optical filter according to any one of claim 1 to 9, wherein the Bragg reflection is a selective Bragg reflection with a reflection bandwidth at half maximum smaller than 100 nm, and preferably smaller than 50nm.

1 1 . Amplitude modulator comprising a first optical filter according to any one of claims 1 to 10 and a second optical filter according to any one of claims 1 to

10, the second optical filter having the same predetermined Bragg reflection wavelength as the first optical filter in absence of applied voltage, the first optical filter having a handedness and the second optical filter having an opposite handedness, the first optical filter and the second optical filter being arranged in a stack.

12. Amplitude modulator according to claim 1 1 , wherein the controller (5) is configured to select the first portion of pixels and the second portion of pixels from the first optical filter and, respectively, another first portion of pixels and another second portion of pixels from the second optical filter, and wherein a ratio of an area of the first portion of pixels to an area of the second portion of pixels in the first optical filter is about the same as the ratio of an area of said another first portion of pixels to an area of said another second portion of pixels in the second optical filter.

13. Optical article comprising an optical filter according to any one of claims 1 to 10 or, respectively, an amplitude modulator according to any one of claims 1 1 or 12, and further comprising a lens part, said optical filter, or, respectively, said optical modulator being configured for providing an optical phase modulation or an optical power modulation.

14. Method for amplitude tuning an optical beam in reflection and/or in transmission, the method comprising the following steps:

- providing a liquid crystal cell (1 ) comprising a liquid crystal mixture (3) filling the liquid crystal cell (1 ) between a first surface (1 1 0) and a second surface (120), the liquid crystal cell (1 ) comprising a plurality of adjacent pixels (41 , 42, 43, 44) arranged in a matrix, each pixel (41 , 42, 43, 44) comprising a first electrode (1 1 1 , 1 12, 1 13, 1 14) arranged on the first surface (1 10) and a second electrode (121 , 122, 123, 124) arranged on the second surface (120) of the liquid crystal cell (1 ), the liquid crystal mixture (3) having a Bragg state displaying a Bragg reflection with a Bragg reflection wavelength within the 380-780 nm range in absence of applied voltage,

15. Method according to claim 14, wherein the first portion of the plurality of pixels and the second portion of the plurality of pixels are spatially selected to form a determined spatial distribution of pixels, depending on a predetermined amplitude of Bragg reflection or transmission at the Bragg reflection wavelength.