CROSS-REFERENCE

This application is the U.S. National Stage of International Application No. PCT/ES2019/070599 filed on Sep. 11, 2019, which claims the benefit of priority from Spanish Patent application No. P201830891 filed on Sep. 14, 2018, the contents of which are both herein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a configurable (or reconfigurable) optical device capable of introducing a radial variation in the phase profile of arbitrary incident radiation.

The device is based on one or more liquid crystal cells in series, provided with electrodes structured so that all the electrodes are accessible from the periphery of the cell, and consequently are able to be directed independently. The specific design of the electrodes, which is part of the invention, allows reconfigurable lenses and wavefront correctors to be manufactured.

The present invention also relates to a method for manufacturing the aforementioned configurable optical device.

The device object of the present invention has application in the field of design and commercialisation of optical devices for different types of industries, such as the aerospace, health, security, transportation, communications industries, etc.

Description of Related Art

Lenses that are commonly used are based on light refraction. Lenses based on light diffraction are also well known, also known as Fresnel lenses, which are diffractive lenses with a fixed focus, without a topological charge (or zero topological charge).

Certain types of vortex lenses are known in the state of the art, such as spiral diffractive lenses. A spiral diffractive lens is characterised by the profile thereof, which is described by the topological charge thereof, and in the case of a focusing lens, by the focal length thereof.

Optical vortex beams, or vortices, are less common and not well-known by the general public. A vortex is an optical beam wherein the phase of the light varies spatially in an azimuthal manner around the centre of the beam. Typically, the phase varies by a number which is an integer multiple of 2π, in a revolution around the centre of the beam. The number, an integer multiple of 2π, is known as the topological charge of the vortex.

The elements defined in this patent are all elements which change the topological charge of an incident beam.

Likewise, a type of contact lens is known, such as the one disclosed in document U.S. Pat. No. 5,408,281 A, which has a spiral geometry and the capacity to focus.

Likewise, document US 2008226844 A1 is known, relating to configurable spiral phase plates, but based on non-uniform alignment and which does not use lenses in series.

Likewise, document WO 2012/080532 is known, which defines a reconfigurable beam reflectarray for frequencies in the terahertz and millimetre-wave ranges, based on reconfigurable resonant dipole structures in liquid crystal cells.

To date, no optical device is known that uses one same passive electrode structure to create spiral diffractive lenses with different focal lengths, depending only on the field applied to the different electrodes and without aliasing present in the pixelated devices.

To date, no optical device is known which combines two vortex-creating structures in series. The combination of such structures in series of the present invention makes it possible to emulate, by means of a single device, practically all types of optical lenses, as well as configure the optical features thereof according to what is needed.

A device of this type can compete with spatial light modulators (SLM), with the added advantages of having a higher fill factor, not having aliasing and lacking electronic components in the active part thereof, therefore transmitting more light. It is also structurally and electronically simpler and therefore cheaper.

The present invention takes advantage of some of the basic properties of liquid crystals (LCs), specifically the dielectric and optical molecular anisotropy thereof. The fluid nature of LCs combined with the intrinsic dielectric anisotropy thereof implies that, on a macroscopic scale, the material tends to orient itself offering the highest dielectric constant thereof to the applied field.

In the most common LCs, nematic calamitic liquid crystals, the molecules have an elongated shape; the longest axis of the molecule corresponds approximately to the axis of the highest dielectric constant and the optical axis of anisotropy, also called the indicatrix. Thus, the highest refractive index and highest dielectric constant are parallel to the longest axis of the molecule. These materials are called positive LCs.

There are LCs with negative dielectric anisotropy, wherein the molecular axis coincides with the highest refractive index (the indicatrix) and the lowest dielectric constant. In this description, uniaxial anisotropies are assumed, an approximation which is valid for all relevant liquid crystal (LC) cases in this invention.

In other LC materials, such as blue phase liquid crystals, an isotropic material can transform into an anisotropic material by applying an electric field. In such cases, the index tends to increase in the direction of the field, and decrease in the perpendicular direction.

Additionally, LCs are ordered fluids. They have an intrinsic macroscopic order generated by intermolecular viscoelastic forces which extends to hundreds of μm, which allows them to be aligned, in the absence of external fields, with relatively weak surface interactions.

Thus, it is possible to induce a preferential alignment of the LC by means of the appropriate surface treatment, which is transmitted to the rest of the material, provided that the volume of the LC is confined in a cell with parallel faces located at a short distance. Said alignment can be eventually altered by applying an external electric field. Commutation between one state and another is determined by the plane defined by the alignment direction and the direction of the electric field.

As a consequence, the refractive index or indices perceived by the light passing through an LC can be modified by means of applied electric fields, causing the optical path length (OPL) or the state of polarisation (SOP) of the incident light.

The modification of the SOP is the basis on which liquid crystal displays (LCDs) are built. By controlling the SOP, the amount of light passing through the system can be controlled, with the help of one polariser or two polarisers. An LCD is usually made up of an LC cell that is backlit or provided with a rear mirror, whereon the LC cell is positioned between polarisers. The cell has an electrical excitation system formed by electrodes in the form of segments (alphanumeric displays) or a rectangular matrix of pixels such as the kind carried by mobile phones, projectors and TVs.

OPL variations are the basis for LC lenses, and more generic LC photonic devices such as spatial light modulators (SLM). These devices make it possible to manipulate at least one of the polarisations of the incident wavefront.

Transparent LC lenses are usually based on concentric electrodes to which different voltages are applied. Connecting concentric elements is complex if multilayer assemblies of the electrodes are not chosen, which in turn have their own problems. Consequently, complex directing schemes with very limited tunability ranges must be used, or intertwined electrodes with spiral shapes.

SLMs are based on technologies derived from the commercial production of LCDs, although they must be prepared to achieve maximum phase lags of 2π, instead of the lag of π usually required for LCD displays. High-performance SLMs are based on very high-density rectangular arrays of pixels, which allow almost arbitrary adjustments of the wavefronts. However, most displays are direct vision displays, with a pixel size of tens of μm (less than the visual acuity of the human eye), or transmissive or reflective projection devices such as LCoS (liquid crystal on silicon).

Direct vision devices generally use thin-film transistors (TFTs) in an active matrix, which reduces the number of electrodes required to control the large number (millions) of individual pixels that a display carries. A significant portion of the active area of the display is occupied by the TFTs and the circuitry thereof, reducing the so-called fill factor (light transmission) of the display. The problem is exacerbated in transmissive projection devices, because the small size of the pixels thereof reduces the fill factor to the point that the resolution of such devices is limited in practice. In reflective LCoS devices, the transistors and microelectronic circuitry are installed behind the display and do not affect the fill factor.

In either case, the control electronics are above the pixels or below the active area, which is undesirable in certain applications, specifically those wherein the LC cell is exposed to intense electromagnetic radiation or ionising radiation, which can deteriorate the micro- and nanoelectronic elements of the circuits.

The present invention is presented within this context. It is a transparent device formed by electrodes with various shapes, capable of creating LC lenses of any profile with an arbitrary number of pixels, which are directed from the perimeter of the active area. The device has several advantages over current devices:It is transparent, unlike most high-resolution SLMs.It lacks electronic elements in the active areaIt can be directed with an arbitrary number of individual pixels, with no electrode interconnection or overlap in any region.It has a remarkably high fill factor, limited only by the gap between electrodesIt intrinsically lacks aliasing

Apart from liquid crystals, there are other materials with an electro-optical effect wherein the optical anisotropy of the material changes when an electric field is applied. In these materials, the refractive index tends to increase in the direction of the field and decrease in the perpendicular direction, or vice versa. In the present invention, LCs can be replaced by materials with an electro-optical effect such as lithium niobate or by liquids that exhibit the Pockels or Kerr effect.

BRIEF SUMMARY OF THE INVENTION

In view of the prior art, the following configurable (and/or tunable) optical device, object of the present invention, is presented.

The configurable optical device object of the present invention comprises a configurable optical element or various configurable optical elements arranged in series. Each optical element comprises an active region made up of an entry surface (or transparent substrate) and an exit surface (or transparent substrate) for light beams.

Each optical element includes at least one first transparent electrode and at least one transparent counter electrode (or second electrode).

The electrical connections for the electrodes are located in the perimeter region of each optical element, so that there are no connecting tracks penetrating the active region.

According to a possible embodiment, the first electrode of at least one of the optical elements is divided into sections, so that the device is configured to generate electric fields with different intensities between each section of the first electrode and the second electrode.

The device is configured such that, upon application of a potential difference between different sections of the electrodes of each optical element, different electric fields that alter the degree of commutation in different areas of the active region of each optical element are generated, thus creating a varying OPL profile in each of the optical elements of the device. This configures a certain topological charge and a certain focus of the incident light beam, according to the pattern of electric fields applied to the sections of the first electrode.

According to a possible embodiment, the optical elements are liquid crystal cells (or polymerisable liquid crystal cells) with the corresponding electrodes thereof. As an alternative to liquid crystal, the optical elements could be made up of some other electro-optical material (with the ability to alter the refractive index thereof in the presence of an electric field) and the corresponding electrodes thereof.

Thus, there are partial commutations (depending on the number and arrangement of sections of the electrodes), which allow the refractive index of different regions of each optical element to be locally altered.

According to a possible embodiment, the counter electrode is also divided into sections. In this case, both electrodes are configured both to have different electrical potentials in each section and to have equal electrical potentials in each section. The section distribution of the first electrode can be different from the section distribution of the counter electrode, thus allowing the device to be reconfigured (tuned) in order to create two independent sets of optical path profiles, which changes the topological charge and the focus of an incident light beam in different ways.

Optical elements wherein the counter electrode is not divided into sections, or optical elements wherein the counter electrode is divided into sections, or a series of optical elements wherein one has the counter electrode divided into sections and the other has the counter electrode not divided into sections, can be arranged in series.

According to a possible embodiment, an optical element acts as a variable spiral diffractive lens. Depending on the applied pattern, an optical path is produced with a certain focal length and topological charge. Thus, upon application of a first suitable pattern of electric fields, the optical path profile of the optical element corresponds to that of a spiral diffractive lens with a first focal length and a first topological charge, while upon application of a second suitable pattern of electric fields, the optical path profile of the optical element corresponds to that of a spiral diffractive lens with a second focal length and a second topological charge.

According to another possible embodiment, a pair of optical elements acts together, emulating a variable diffractive lens.

According to another possible embodiment, a pair of optical elements acts together, emulating a variable axicon.

According to another possible embodiment, a pair of optical elements acts together, emulating a variable spiral diffractive lens.

According to another possible embodiment, a pair of optical elements acts together, emulating the combination of at least two of the following systems: a lens, an axicon or a spiral phase plate.

Thus, upon application of the corresponding electric fields, the optical path profile of at least one of the adjacent optical elements placed in series can correspond to that of a spiral diffractive lens, while the optical path profile of at least one of the remaining adjacent optical elements placed in series may correspond to that of a spiral phase plate.

According to a particular embodiment, the first electrode of two adjacent optical elements placed in series is divided into sections, the distribution of the sections of the first electrode of an optical element being a mirror image of the distribution of the sections of the first electrode of the adjacent optical element.

Thus, upon application of the corresponding electric fields, the commutation profile of the two adjacent optical elements placed in series corresponds to that of two spiral diffractive lenses with inverse topology, and the combination of elements emulates a diffractive lens.

According to another particular embodiment, upon application of the corresponding electric fields, the commutation profile of two adjacent optical elements placed in series corresponds to that of two spiral diffractive lenses. Preferably, both spiral diffractive lenses are positioned in series, with a certain gap between them, so that the focal lengths of both lenses coincide, which allows them to function as a tunable telescope or beam expander, altering the topological charge and focus of the lenses, applying the suitable patterns of electric fields to the sections of the electrodes of the optical elements.

According to another particular embodiment, a third element is added to the previous embodiment so that the combination of elements results in a device that does not affect the topological charge of the light.

According to a possible embodiment, at least one of the electrodes of at least one optical element includes structures with electromagnetic resonance frequencies which are variable depending on the degree of commutation of the adjacent liquid crystal.

According to one possible embodiment, the electrodes take the form of poles having electromagnetic resonance frequencies for frequencies in the terahertz and millimetre-wave and micrometre-wave ranges. Upon application of a polarisation field to the dipoles, a phase profile can be created for these frequencies, as well as the phase profile created for the embodiments for light beams.

The present invention in turn envisages a method for manufacturing a configurable optical device.

The manufacturing method comprises:arranging an optical element or various optical elements placed in series, wherein each optical element comprises an active region consisting of an entry surface and an exit surface for light beams, wherein each optical element comprises at least one first transparent electrode and at least one second transparent electrode, wherein each optical element has electrical connections for the electrodes, said electrical connections being located in the perimeter region of each optical element outside the active region, wherein each of the optical elements of the device changes the topological charge of an incident light beam.applying a potential difference between the electrodes of each optical element, thus producing an electric field that alters the refractive index of the optical element, thus creating a selective commutation profile in each of the optical elements of the device, thus altering the optical path profile of each optical element, which changes the topological charge of an incident light beam.

Preferably, the method envisages that the optical elements are liquid crystal lenses, each lens being made up of a liquid crystal cell located between at least one first electrode and at least one second electrode.

Also, preferably, the method envisages that the liquid crystal of the lenses is polymerisable, also known as reactive mesogen (RM). The method then comprises the possibility of curing (by means of UV radiation or another curing method) the polymerisable liquid crystal, obtaining a diffractive device with a variation profile having a fixed or partially tunable phase.

According to a possible embodiment, the method comprises applying an electric field between the electrodes of two adjacent optical elements placed in series creating a commutation profile of the two adjacent optical elements corresponding to that of two spiral diffractive lenses. Likewise, according to a possible embodiment, the method comprises positioning both spiral diffractive lenses in series, with a certain gap between them, so that the focal lengths of both lenses coincide, which allows them to function as a tunable telescope or beam expander, altering the topological charge of the lenses.

