CN114942486A - Liquid crystal holographic film and preparation method thereof, detection light path and detection method - Google Patents

Abstract

The invention belongs to the technical field of optics, and discloses a liquid crystal hologram, a preparation method, a detection optical path and a detection method; the liquid crystal hologram comprises a substrate, an alignment layer and a liquid crystal layer; The control is the same orientation as the computational hologram corresponding to the optical element to be detected; the photo-controlled orientation method is used to control the different orientations of adjacent stripes in the alignment layer, and the stripes with different orientations of liquid crystal molecules induced by the PB phase principle will respond to the same incident direction. The incident light in the polarization direction will have different refractive indices. After the incident light passes through the liquid crystal material with a fixed thickness and different refractive index, a fixed phase difference will be formed; thus the substrate, the alignment layer and the liquid crystal layer form a fixed phase difference that compensates for the incident light. Functional liquid crystal hologram; adjusting the thickness of the liquid crystal material or changing the liquid crystal material can change the refractive index of the liquid crystal material, thereby realizing the controllable design of the retardation.

Description

A liquid crystal holographic sheet and preparation method, detection optical path and detection method

technical field

The invention belongs to the technical field of optics, and relates to a liquid crystal holographic sheet, a preparation method and an aspherical surface detection optical path.

Background technique

At present, there are many detection methods for aspheric surface, which can be divided into two categories: contact measurement method and non-contact measurement method. Among many non-contact aspheric surface measurement methods, computational holography method has great advantages. It can measure very complex surface shapes without the use of expensive reference entities and zero-position lenses. It is a high-precision interferometric detection method. It can generate the ideal wavefront required for any aspheric surface to be measured and analyze it. Phase compensation and, therefore, computational holography are widely used in aspheric detection.

1) Detection principle based on computational holography

The light wave emitted from the interferometer is a standard spherical wave. After the calculated hologram corresponding to the aspheric surface to be measured is designed, the standard spherical wave is phase-compensated by the hologram, and all the light rays are incident vertically along the normal direction of the aspheric surface to be measured. The aspheric surface, and the light wave reflected by the aspheric surface to be measured carries its surface shape information. After the reflected light wave is compensated by the hologram again, it becomes a spherical wave and enters the interferometer, forming interference with the reference wave in the interferometer, resulting in The interference fringe pattern can be obtained by analyzing the fringe pattern to obtain the surface deviation of the aspheric surface.

2) Problems existing in the detection of existing computational holograms

Computational holograms are similar in pattern to circular gratings and are usually fabricated on glass or fused silica optical substrates. Computational holograms can be divided into two categories due to different ways of recording information: one is amplitude-type holograms, and the other is phase-type holograms. The amplitude-type hologram can be obtained by the traditional photolithography process, the process is simple, and the processing accuracy is high, but its diffraction efficiency is lower than that of the phase-type hologram. The phase hologram is realized by multi-step lithography. Theoretically, the diffraction efficiency of 2 steps is 42%, and the diffraction efficiency of 4 steps is 78%. However, multi-step lithography and etching transfer have the problems of difficult processing and large transfer error.

In the process of pattern etching and transfer of traditional computational holograms on the substrate, if the etching depth is too large, the etching time will be long, and the grooves will be easily deformed by heat or mask deformation, which will cause the etching stripes. The position is shifted, resulting in a decrease in accuracy, affecting the phase distribution of the light wave after passing through the hologram, and thus affecting the detection accuracy of the aspheric system.

SUMMARY OF THE INVENTION

In order to solve the problems pointed out in the background art, the proposed technical solutions are as follows.

A liquid crystal hologram, used for optical element detection, comprising a substrate, an alignment layer provided on the substrate, and a liquid crystal layer provided on the alignment layer; the alignment layer contains azo dye molecules, and the azo dye molecules are oriented by light control The technology is manipulated to be in the same orientation as the computational hologram corresponding to the optical element to be tested; the liquid crystal layer is made of liquid crystal polymer material and cured by UV cross-linking.

The azo dye molecules in the alignment layer in the liquid crystal hologram are arranged to detect the same orientation of the computational hologram corresponding to the optical element to be tested. The liquid crystal molecules in the liquid crystal layer are induced by the azo dye molecules in the alignment layer to calculate The hologram is arranged and oriented, and the calculated hologram arrangement and orientation of the liquid crystal molecules are cured by ultraviolet cross-linking curing.

The light-controlled alignment method is used to control the different orientations of adjacent stripes in the alignment layer. The stripes with different orientations of liquid crystal molecules induced by the PB phase principle will produce different refractive indices for incident light in the same incident direction and polarization direction. After the liquid crystal material with different thicknesses of refractive index, a fixed phase difference will be formed; thus the substrate, the alignment layer and the liquid crystal layer constitute a liquid crystal hologram with the function of compensating the fixed phase difference of the incident light; adjusting the thickness of the liquid crystal material or changing the liquid crystal material can change the liquid crystal The refractive index of the material, and then the controllable design of the retardation is realized. Liquid crystal can use the optical anisotropy of liquid crystal molecules to generate polarization state and phase modulation to form a "liquid crystal phase grating" to realize the function of a computational hologram. That is, by precisely adjusting the orientation distribution of the liquid crystal molecules, the phase distribution of the outgoing light wave is modulated.

In order to achieve the maximum refractive index difference between adjacent stripes, the orientations of adjacent liquid crystal molecular regions are perpendicular to each other; according to the compensated phase distribution, the calculated hologram distribution includes: grating distribution and annular band distribution.

Preferably, based on the above-mentioned liquid crystal holographic sheet, the present invention also discloses a liquid crystal holographic sheet for detecting aspherical surfaces, comprising a lens barrel, and a polarizer, a quarter wave and a film, liquid crystal hologram. When in use, the incident detection light emitted by the laser light source is firstly modulated by the polarizer to modulate the incident detection light into linearly polarized light, and then undergo polarization modulation by a quarter-wave plate to become circularly polarized light. It will make the incident light wave get phase compensation and polarization state inversion, and then become an ideal detection light wave to detect aspheric surface shape.

A method for preparing a liquid crystal hologram, which uses an azo dye as a material for an alignment layer and coats a substrate; a photo-controlled alignment method is adopted, so that the azo dye molecule is manipulated into a computational hologram corresponding to an optical element to be detected. The same orientation forms an alignment layer; the surface of the alignment layer is coated with a liquid crystal layer, and the liquid crystal molecules in the liquid crystal layer are induced by the azo dye molecules in the alignment layer to form a computational hologram arrangement orientation using the PB phase principle. The UV cross-linking curing cures the CGH alignment of the liquid crystal molecules.

Compared with traditional holograms, liquid crystal holograms have many advantages. In terms of process production, liquid crystal holograms have more steps in self-smoothing than traditional holograms. Therefore, the diffraction efficiency value of liquid crystal-based phase gratings is theoretically higher than that of traditional holograms. It is also easy to meet the detection accuracy requirements. Secondly, the liquid crystal hologram does not need to be etched and transferred, and there will be no problems with precision or stripe deformation. In the preparation process, the preparation of traditional holograms requires photolithography and etching transfer, while liquid crystal holograms are relatively easy to prepare by polarized exposure to alignment agents, spin coating of liquid crystal polymers and UV curing.

The liquid crystal polymer is used as the aspheric detection hologram. The liquid crystal polymer forms a stable optical layer through UV cross-linking, which has the advantages of high reliability, high transmittance and flexibility.

A detection optical path is used to detect the surface shape error of optical components, including a laser, a beam splitter, and a liquid crystal hologram; the incident light emitted by the laser is divided into two beams by the beam splitter, one of which is used as a reference beam, and the other beam is used as a reference beam. The light is modulated by the liquid crystal hologram into a standard detection light wave that matches the surface of the optical element to be tested. The standard detection light wave is reflected by the optical element to be tested to form a detection light that carries the surface shape error of the optical element to be tested. The detection light interferes with the reference light. form interference fringes. By interpreting the interference fringes, the surface shape error of the optical element to be tested can be known.

Preferably, a CCD is also arranged in the detection optical path, and the controller is used for receiving the interference fringe image and transmitting the image to the controller, and the controller is used for interpreting the interference fringe and outputting the surface shape error distribution of the optical element to be tested. picture.

Description of drawings

1 is a schematic diagram of a liquid crystal hologram;

2 is a schematic diagram of a liquid crystal hologram compensator for detecting optical elements;

3 is a schematic diagram of light wave detection and modulation of a liquid crystal hologram compensator;

4 is a flow chart of a method for preparing a liquid crystal hologram;

5 is a schematic diagram of the distribution of grating-type liquid crystal molecules;

6 is a schematic diagram of the distribution of rotationally symmetric annular liquid crystal molecules;

FIG. 7 is a schematic diagram of a detection optical path.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be described in further detail below with reference to the accompanying drawings.

Example 1

A liquid crystal hologram, used for optical element detection, comprises a substrate 1, an alignment layer 2 provided on the substrate, and a liquid crystal layer 3 provided on the alignment layer as shown in FIG. 1; the alignment layer contains azo dye molecules, The azo dye molecules are manipulated into the same orientation as the computational hologram corresponding to the optical element to be detected by the light-controlled orientation technology; the liquid crystal layer is made of liquid crystal polymer material and cured by ultraviolet cross-linking.

The effect of polarization modulation of the liquid crystal hologram will change the polarization state of the incident light, which will affect the detection light path. Therefore, a polarizer and a quarter-wave plate are installed in front of the liquid crystal hologram; polarizer and quarter-wave plate , liquid crystal holographic sheet together to form a compensator for detecting optical components liquid crystal holographic sheet compensator.

In at least one application example, as shown in FIG. 2 , the polarizer 5 , the quarter-wave plate 6 , and the liquid crystal hologram 7 are assembled in the lens barrel 4 to form a liquid crystal hologram compensator for detecting optical elements. As shown in Figure 3, a polarizer and a quarter-wave plate are added in front of the hologram to modulate the incident light wave into circularly polarized light, and then through the phase compensation effect of the liquid crystal hologram, so that the outgoing light wave has an ideal aspheric surface. Light waves for surface information.

Embodiment 2

A preparation method of a liquid crystal hologram, comprising the following steps:

(a). The azo dye is used as the material of the alignment layer, and the substrate is coated with 0.5% azo dye, and the coating speed is 3000 RPM for 30 s and heated at 100 degrees for 5 minutes;

(b). Using a photo-controlled orientation method, the azo dye molecules are manipulated into the same orientation as the computational hologram corresponding to the optical element to be detected,

(c) Coating the liquid crystal layer, irradiating the UV lamp at 80 degrees for 5 minutes, the liquid crystal molecules in the liquid crystal layer are induced by the azo dye molecules in the alignment layer to form a computational hologram arrangement orientation by using the PB phase principle, and cross-linking by UV The curing solidifies the computational hologram arrangement orientation of the liquid crystal molecules.

In a specific example, a method for preparing a liquid crystal hologram, as shown in Figure 4, the compensation phase of the hologram is a grating distribution, the first linearly polarized light in step (b). ; Then a mask is added to change the polarization direction of the incident light to be perpendicular to the first time to form the second orientation; In the adjacent areas where the liquid crystal molecules are oriented vertically, if the laser with the same polarization state is incident, due to the anisotropy of the liquid crystal molecules, the corresponding refractive indices in the adjacent areas are different, resulting in a phase grating effect.

In another specific example, for a liquid crystal hologram, if the compensation phase of the hologram is a rotationally symmetrical distribution, the liquid crystal polarization hologram is a rotationally symmetrical central ring. The arrangement of liquid crystal molecules of the holographic sheet is shown in FIG. 6 , in which each ring represents a region with the same orientation of liquid crystal molecules, and the orientations of liquid crystal molecules of adjacent rings are perpendicular to each other.

Embodiment 3

A detection optical path is used to detect the surface shape error of an optical element. The detection optical path is a Fizeau-type detection optical path. As shown in Figure 7, it includes: a laser 8, a beam splitter 9, a polarizer 5, and a half mirror 10. The first condensing lens 11, the quarter wave plate 6, the liquid crystal hologram 7, the second condensing lens 13, the CCD14;

The light wave emitted by the laser passes through the beam splitter, first passes through the polarizer, and then passes through the half mirror. The light returned from the front surface of the half mirror is the reference light, and the reference light is reflected by the back surface of the beam splitter and passes through the second half mirror. The condensing lens enters the CCD; the transmitted light through the half mirror passes through the first lens, quarter-wave plate, and liquid crystal hologram in sequence, and is modulated into a standard detection light wave that is incident along the normal direction of the optical element 12 to be measured. , and then reflected by the optical element to be tested; the detection light carrying the surface deviation information of the optical element to be tested returns according to the original path, and is reflected by the rear surface of the beam splitter and enters the CCD through the second condensing lens. The reference light interferes, and the CCD receives the interference fringes.

Fizeau type detection optical path The detection light and the reference light share the same optical path, which reduces the source of error and solves the problem of unstable interference fringes caused by different effects of temperature and vibration in the environment; the polarizer in the detection optical path is placed in the optical path. Between the beam splitter and the half mirror, the output beam of the laser is modulated into linearly polarized light, and after the beam splitting effect of the half mirror, both the reference light and the detection light become linearly polarized light;

The polarizer and the quarter-wave plate, the polarizer and the quarter-wave plate modulate the polarization state of the light wave emitted by the laser, so that the light wave incident on the liquid crystal hologram is circularly polarized light, and then the hologram works. In the detection optical path, the deviation of the aspheric surface shape is detected; the interference between the reference light and the detection light is polarized light interference. In the optical path, there will be no change of polarization state and influence of detection, and neither the detection light nor the reference light is polarized light. In the detection light path of the liquid crystal hologram, both the detection light and the reference light are modulated by the polarizer, and the interference of the two light waves is polarized light interference;

Polarizer: change the polarization state of incident light to obtain linearly polarized light;

Quarter-wave plate: Make the optical axis of the quarter-wave plate and the vibration direction of the polarized light at a 45-degree angle, so that the incident ray polarized light is converted into circularly polarized light;

Liquid crystal hologram: The modulation effect of the polarization grating on the polarization state of the incident light is the same as that of the polarization grating. When the circularly polarized light is incident, the outgoing circularly polarized light with opposite rotation is used as the detection light wave and then acts on the aspheric surface to be measured to detect the surface shape;

The second condensing lens: The fringe pattern after the interference of the two linearly polarized lights is imaged on the CCD.

On the basis of this embodiment, a detection method is used to detect the surface shape error of aspheric optical elements, and a two-dimensional grating type liquid crystal hologram is produced according to the preparation method of liquid crystal hologram; the diameter of the quartz substrate is 52*52*8mm , the transmitted wavefront of the quartz substrate is required to be less than 0.2 times the wavelength;

Then, the liquid crystal hologram is processed in the order of azo dye coating, primary exposure, cover mask, secondary exposure, coating liquid crystal material, and UV lamp curing, and the grating period is 2 μm to 20 μm. Here, a hologram with a grating period of 2 microns is taken as an example, and the actual hologram produced is used for the aspheric detection optical path.

The conditions for using the hologram are met, and the wavelength of the laser used in the experiment is 632.8 nm. A rotationally symmetric aspheric surface can be modeled, resulting in rotationally symmetric residual wavefronts.

The parameters of the measured aspherical surface are: the clear aperture is 50.8mm, and the curvature radius is 155.04mm. The true surface shape error is 0.16 times the wavelength, and the wavefront error value is 0.084 times the wavelength.

The calculated hologram based on liquid crystal polarization uses the optical path of laser interferometry to detect the surface shape error data value of aspheric optical elements to 0.17 times the wavelength, and the wavefront error value is 0.080 times the wavelength. Compared with the actual aspheric surface shape error and wave surface error value, the liquid crystal hologram has a higher detection accuracy for aspheric surface shape, indicating that the liquid crystal polarization-based computational hologram is feasible and effective for aspheric surface shape detection.

Claims (10)

1. a liquid crystal holographic sheet, used for optical element detection, is characterized in that: comprise the alignment layer provided on substrate, substrate, the liquid crystal layer provided on alignment layer; comprise azo dye molecule in the described alignment layer, azo The dye molecules are manipulated into the same orientation as the computational hologram corresponding to the optical element to be detected by the light-controlled orientation technology; the liquid crystal layer is made of liquid crystal polymer material and cured by UV cross-linking. 2 . The liquid crystal hologram according to claim 1 , wherein the orientations of adjacent liquid crystal molecular regions in the liquid crystal layer are perpendicular to each other. 3 . 3 . The liquid crystal hologram according to claim 1 , wherein the computational hologram corresponding to the optical element to be measured comprises: grating distribution and annular band distribution. 4 . 4 . The liquid crystal hologram according to claim 1 , wherein the liquid crystal molecules in the liquid crystal layer are induced to be aligned by the azo dye molecules in the alignment layer by using the PB phase principle. 5 . 5 . The liquid crystal hologram according to claim 1 , wherein the different orientations of adjacent stripes in the alignment layer are controlled by a light-controlled alignment method. 6 . 6. A liquid crystal holographic sheet for detecting aspheric surfaces, characterized in that it comprises a lens barrel, and a polarizer, a quarter-wave plate, and claims 1 to 5 that are sequentially assembled along the center axis of the lens barrel in the lens barrel. One of the liquid crystal holograms. 7 . The liquid crystal hologram according to claim 6 , wherein the optical axis of the quarter-wave plate and the vibration direction of the polarized light form an angle of 45 degrees. 8 . 8. A method for preparing a liquid crystal holographic sheet, characterized in that: using azo dyes as a material of an alignment layer, coating on a substrate; adopting a light-controlled alignment method, so that azo dye molecules are manipulated to detect the optical The alignment layer is formed by the same orientation of the computational hologram corresponding to the element; the liquid crystal layer is coated on the surface of the alignment layer, and the liquid crystal molecules in the liquid crystal layer are induced by the azo dye molecules in the alignment layer to form the alignment of the computational hologram using the PB phase principle. Using UV lamp irradiation, the CGH alignment of liquid crystal molecules is cured by UV cross-linking curing. 9. A detection optical path for detecting the surface shape error of an optical element, characterized in that: comprising a laser, a beam splitter, and the liquid crystal hologram described in one of claims 1 to 5; the incident light emitted by the laser is passed through the beam splitter It is divided into two beams, one of which is used as a reference light, and the other beam is modulated into a standard detection light wave that matches the surface of the optical element to be measured through the liquid crystal hologram. The detection light of the surface shape error of the optical element, the detection light interferes with the reference light to form interference fringes, and the surface shape error of the optical element to be tested can be known by interpreting the interference fringes. 10. A detection optical path according to claim 9, characterized in that: a CCD is also provided, a controller; the CCD is used to receive the interference fringe image, and transmit the image to the controller, and the controller is used to interpret the interference fringe And output the surface shape error distribution map of the optical element to be tested.

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