Waveplate lenses and methods for their fabrication

Abstract

The invention provides for lenses fabricated as planar thin film coatings with continuous structure. The lensing action is due to optical axis orientation modulation in the plane of the lens. The lenses of the current invention are fabricated using photoalignment of a liquid crystal polymer wherein the polarization pattern of radiation used for photoalignment is obtained by propagating the light through an optical system comprising a shape-variant nonlinear spatial light polarization modulators.

Description

CLAIM OF PRIORITY

This application claims priority to Provisional Application No. 61/801,251 filed Mar. 15, 2013, the contents of which are relied upon and incorporated herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No. W911QY-12-C-0016.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

CROSS REFERENCES

U.S. Patent Documents

14,193,027	February 2014	Tabirian et al.
61/757,259	January 2013	Tabirian et al.

FIELD OF THE INVENTION

This invention relates to optical lenses, aspheric and apodizing components, and the method of their fabrication. The field of applications of such components includes imaging systems, astronomy, displays, polarizers, optical communication and other areas of laser and photonics technology.

BACKGROUND OF THE INVENTION

Lenses are commonly made by shaping an optical material such as glass. The weight of such lenses increases strongly with diameter making them very expensive and prohibitively heavy for applications requiring large area. Also the quality of a lens typically decreases with increasing size. Diffractive lenses such as Fresnel lenses are relatively thin, however, the structural discontinuity adds to aberrations. Uses of holographic lenses are limited by the compromise of efficiency and dispersion.

In the present invention, such components are obtained on the basis of diffractive waveplates. An exemplary structure of one of the optical components of interest is schematically shown in FIG. 1. Essentially, it is an optically anisotropic film 100 with the optical axis orientation 101 rotating in the plane of the film, the x,y plane in FIG. 1. The thickness L of the film is defined by half-wave phase retardation condition L=λ/(n_∥−n_⊥), where n_∥ and n_⊥ are the principal values of the refractive indices of the material; and λ is the radiation wavelength. The required half-wave phase retaradation condition can be met for as low as a few micrometer thick films, particularly, for liquid crystalline materials. Such a structure imposes a phase shift Φ=±2 α(x,y) on circular polarized beams propagating through it with the sign depending on the handedness of polarization. In simplest realization, the rotation angle α of the optical axis orientation is a linear function of a single coordinate, α=2πx/Λ with Λ characterizing the period of the pattern. With account of α=2πx/Λ=qx, where q=2π/Λ, an unpolarized beam is thus diffracted by the diffractive waveplate into +/−1st diffraction orders with the magnitude of the diffraction angle equal to λ/Λ. The phase Φ in the equation above, known as geometrical or Pancharatnam phase, does not depend on wavelength, hence the broadband nature of the diffraction. Due to its half-wave plate nature, there are well developed techniques for making the component essentially achromatic in a wide range of wavelengths. In case of quadratic variation pattern of the optical axis orientation, α˜x² or, in two dimensional case, α˜x²+y², the parabolic phase modulation profile produces cyclindrical or spherical lens action, correspondingly.

Thus, there is a need and an opportunity provided by the current invention for fabricating lenses and other nonlinear phase modulating components that could be obtained in the form of thin film structurally continuous coatings on a variety of substrates.

BRIEF SUMMARY OF THE INVENTION

The objective of the present invention is providing structurally continuous thin film lenses, positive or negative.

The second objective of the present invention is providing a polarizing lens.

The third objective of the present invention is providing a lens with polarization dependent power sign.

The fourth objective of the present invention is providing a lens wherein the power can be varied (increased) without varying (increasing) thickness.

The fifth objective of the present invention is providing means for fabricating aspherical optical components in the form of thin film coatings.

The sixth objective of the present invention is providing thin film variable lens.

The seventh objective of the present invention is providing micro lenses and their arrays as thin film coatings.

The eight objective of the present invention is providing broadband/achromatic thin film lenses for different spectral range.

The ninth objective of the present invention is providing a method for fabricating and replicating waveplate lenses.

Still another objective of the present invention is providing electrically controlled waveplate lenses.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A shows spatial distribution of optical axis orientation in a vortex waveplate lens.

FIG. 1B shows spatial distribution of optical axis orientation in a spherical waveplate lens.

FIG. 1C shows spatial distribution of optical axis orientation in a cylindrical waveplate lens.

FIG. 2 schematically shows a shape-variant polarization converter comprising a concave and a convex lens sandwiching a planar aligned nematic liquid crystal in-between. A cylindrical system is shown for drawing simplicity.

FIG. 3A schematically shows an optical system for recording nonlinear variation patterns for liquid crystal orientation on a substrate coated with a photoaligning material film.

FIG. 3B schematically shows an optical system with a biriefringent wedge acting as spatial light polarization modulator, and twist oriented liquid crystal cells as polarization converter components.

FIG. 4 shows photos of a laser beam received at the output of the optical recording system observed with and without a polarizer for both a cylindrical and a spherical birefringent lens.

FIG. 5 shows photos of liquid crystal polymer films photoaligned with a cylindrical and spherical lens. Photos are obtained between crossed polarizers.

FIG. 6A shows photos of a laser beam focused with a cylindrical waveplate-lens for different polarization states: linear, right-hand circular, and left-hand circular.

FIG. 6B shows far field photos of a laser beam focused with a cylindrical waveplate-lens for different polarization states: linear, right-hand circular, and left-hand circular.

FIG. 6C shows photos of projections of a triangular aperture with a spherical waveplate-lens for different polarization states: linear, right-hand circular, and left-hand circular. Photos are obtained at planes before, at, and far from the focus of the lens.

FIG. 6D shows switching the focusing state to defocusing state by switching the handedness of a circular polarized beam.

FIG. 7 shows photos of a cylindrical and spherical waveplate-microlenses.

FIG. 8 schematically shows an array of spherical microlenses.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not limitation.

Patterns of optical axis orientation demonstrating examples of waveplate lenses of current invention are shown in FIG. 1. An example of a key component of spatially nonlinear polarization converting system of the present invention shown in FIG. 2 comprises a cylindrical plano-convex lens 202 and a cylindrical plano-concave lens 201 sandwiching a planar aligned nematic liquid crystal (NLC) layer 203. Cylindrical or spherical lenses are used for fabricating cylindrical or spherical waveplate lenses, correspondingly. The focal length of plano-convex and plano-concave lenses can be chosen to create a cell gap between the lenses with depth as small as 1 μm or as large as 1 mm. For example, a lense of focal length F1=150 mm with curvature radii ?? can be combined with a lens of focal length F2=−100 mm with curvature radii ?? for obtaining a gap of 340 μm.

To align the NLC layer within the lenses, the surfaces of the lenses that are in touch with the NLC can be spin-coated with 0.5 wt. % solution of PVA in distilled water at 3000 rpm for 30 s. Then, they can be dried during 20 min at 100° C. and rubbed with a soft cloth in one direction. NLC E48 (Merck) can be used to fill in the cells, as an example.

This shape-variant birefringent film 203 provides spatially varying phase retardation acting as a spatial light polarization modulator (SLPM). The polarization control system may further incorporate additional polarizing optics that ensures equality of the electric field strength of ordinary and extraordinary wave components generated in the SLPM film. For example, when using a linear polarized laser beam 301 in FIG. 3A, a quarter-waveplate 311 can be used for creating a circular polarized light beam incident on the SLPM film 320 as shown in FIG. 3A. Alternatively, a twisted liquid crystal polarization rotator 313 can be used to rotate the polarization of the incident light beam 301 to arrange it at 45 degrees with respect to the anisotropy axis of the SLPM film 320 as show in FIG. 3B. The spatial modulation of polarization obtained at the output of the film is further transformed by a second quarter-waveplate 312 in the schematic shown in FIG. 3A or a twisted nematic film 313 in the example shown in FIG. 3B. Thus, cyclodal distribution of light polarization can be obtained in a particular case of a SLPM film in the form of a birefringent wedge 321 in FIG. 3B. The twist angle, when using NLC cells for controlling the beam polarization at the input and output of the SLPM film may be 45 degrees as depicted schematically in FIG. 3B. The mutual alignment of the axes of the quarter-waveplates in FIG. 3A or the twist NLC cells in FIG. 3B shall be such as the transmitted beam is linear polarized at the absence of the SLPM film.

An expanded beam of an Argon ion laser operating at 488 nm wavelength providing a power density 12 mW/cm² can be used for photoalignment of the photoaligning layer 330 deposited on a support substrate 340. The beam propagates through two quarter waveplates and the SLPM film 320 between them in FIG. 3A. In the example shown in FIG. 3B, the beam propagates through the system of two polarization rotators and the SLPM film 321 between them.

The pattern of polarization distribution of the beam at the output of the system can be verified with a linear polarizer on a screen. Cylindrical and spherical cycloidal distribution of polarization is shown in FIG. 4. The item 401 in FIG. 4 corresponds to the pattern observed without polarizers. It shows homogeneous distribution of light intensity since the only parameter being modulated is polarization. In contrast, polarization modulation is revealed between polarizers as parabolic fringes in case of cylindrical lens 402 and concentric modulation pattern in case of a spherical lens 403.

The polarization modulation patterns can be recorded, as an example, on PAAD series photoalignment material layers (available at beamco.com). The PAAD layer is created on a substrate, glass, for example, by spin-coating a solution of PAAD-72(1%)/DMF at 3000 rpm during 30 s. PAAD layer can be pre-exposed with linear polarized LED light, 459 nm wavelength, for example, before recording the lens; the pre-exposure time is approximately 10 min at power density 10 mW/cm². The pre-aligned PAAD layer is exposed then to the Argon ion laser beam during 60 s.

Having thus created the required alignment conditions, the PAAD coated substrate can be coated with layers of liquid crystal monomer solution, for example, RMS-03-001C (Merck, Ltd.), followed by photopolymerization with unpolarized UV light at 365 nm wavelength during 5 min. The first layer of the RMS-03-001C can be spin-coated on PAAD-72 layer at a speed 3000 rpm during 1 min. A second layer of RMS-03-001C can be spin-coated on the first layer at a 2000 rpm during 1 min followed by photopolymerization as indicated above to create half-wave phase retardation condition at, for example, 633 nm wavelength.

Alternatively, photoaligned substrates can be used for making electrically or optically controlled liquid crystal cells resulting in electrically or optically controlled waveplate lenses. Details?

Cylindrical and spherical LC polymer lens structures are shown in FIG. 5, items 501 and 502, correspondingly, between crossed polarizers. Focusing and defocusing patterns of a red laser beam on above structures are shown in FIG. 6 at different conditions. The Focal length of the cylindrical waveplate-lenses used in FIG. 6 is 84 cm and 7 cm: photos 601-603 (F=84 cm) are obtained in its focal plane. Photos 607-609 (F=7 cm) correspond to far field zone. Photos 601, and 607 correspond to linear polarized incident light. Photos 602, and 608 correspond to left-hand circular polarized incident beam. The photos 603, and 609 correspond to right-hand circular polarized incident beam.

Projection of a mask with a triangular opening with the aid of a spherical cycloidal lens is shown in FIG. 6. Photos 611, 612, and 613 were taken behind the lens before focus. Photos 621, 622, and 623 were taken at the focus of the lens (F=190 mm), and the Photos 631, 632, and 633 were taken far from the focus. The photos 611, 621 and 631 correspond to linear polarized or unpolarized incident beam. The photos 612, 622 and 632 correspond to left-hand circular polarized incident beam, and the photos 613, 623, and 633 correspond to right-hand circular polarized beam, correspondingly. The focusing conditions can be controlled by using electrically or optically controlled phase retardation plates to modulate the polarization state and distribution in the input light.

Lenses of different focal length can be recorded by simply changing the size of the polarization modulation pattern projected onto the photoalignment layer. FIG. 7 shows examples of lenses of less than a mm diameter. The projection can be simply made by a focusing lens placed after the output quarter-wave plate. Lens size of 0.6 mm, for example, corresponded to a focal length of 3 mm while a lens size of 6 mm exhibited a focal length of 475 mm. Arrays of such microlenses can be printed as schematically shown in FIG. 8.

Although the present invention has been described above by way of a preferred embodiment, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.

Claims

1. An optical film comprising a birefringent material, the optical axis orientation angle of said material being a nonlinear function of coordinates in the plane of the film.

2. The optical film as in claim 1 wherein said nonlinear function comprises at least a section of parabolic dependence along a Cartesian coordinate axis in the plane of the film.

3. The optical film as in claim 1 wherein said nonlinear function comprises at least a section of parabolic dependence on the radius of a polar coordinate system in the plane of the film.

4. The optical film as in claim 1 wherein said nonlinear function comprises at least a section of linear dependence of the optical axis orientation angle on the radius of a polar coordinate system in the plane of the film.

5. The optical film as in claims 1-4 wherein said birefringent film is a liquid crystal polymer.

6. The optical film as in claim 5 wherein the optical and mechanical characteristics of said liquid crystal polymer can be controlled by stimuli such as temperature, mechanical stress, and radiation.

7. The optical film as in claims 1-4 wherein said birefringent film is a liquid crystal.

8. The optical film as in claim 7 wherein the optical characteristics of said liquid crystal can be controlled by stimuli such as temperature, radiation, and electric fields.

9. The optical film as in claim 7 further comprising electrodes for application of electric fields across the layer of the film, controlling said orientation pattern.

10. A method for fabricating diffractive waveplate optics of nonlinear phase modulation profile comprising:

(a) providing a substrate;

(b) dispensing a photoaligning material layer over said substrate;

(c) providing a light source with wavelength in the absorption band of said photoalignment material;

(d) propagating said beam through polarization control optics comprising a polarization converter outputting a spatially nonlinear distribution of linear polarization;

(e) further propagating said beam through an optical projection system;

(f) exposing said photoaligning material layer to said light beam at the output of said optical projection system.

(g) dispensing a polymerizable liquid crystal precursor over the photoaligned material layer;

(h) in-situ reacting said polymerizable liquid crystal to form a polymer film.

11. The method for fabricating diffractive waveplate optics as in claim 10 further comprising deposition of the second or more liquid crystal polymer layers, generally, possessing with different material characteristics, including chirality sign.

12. The method as in claim 10 wherein said spatially nonlinear polarization converter comprises an assembly of a first and a second substrates with at least one of them having non-planar surface topology, said substrates treated for inducing alignment of liquid crystalline materials, and a liquid crystalline material being sandwiched in-between.

13. The method as in claim 12 wherein said substrates are plano-convex and plano-concave lenses.

14. The method as in claim 10 wherein said polarization control optics further comprises an input and an output quarter waveplates with their axes aligned parallel or perpendicular to each other.

15. The method as in claim 10 wherein said polarization control optics further comprises an input and an output liquid crystal cells, each possessing a twist, oriented such as the resultant polarization is linear at the absence of the nonlinear polarization converter.

16. The method as in claim 10 wherein said nonlinear polarization converter comprises a diffractive waveplate-lens meeting half-wave phase retardation condition for the wavelength of the recording radiation.

17. The method as in claim 10 wherein said nonlinear polarization converter comprises material with spatially varying optical anisotropy.

18. An imaging system comprising at least one waveplate-lens film.

19. The system as in claim 18 further comprising at least one variable phase retardation plate.

20. An optical film made of a liquid crystalline material between substrates inducing different orientation states, including a combination of planar alignment and a spatially nonlinear alignment pattern.