CN1367398A - Continuous vari-focus Fresnel lens - Google Patents

Abstract

The invention belongs to the field of optical devices and relates to a continuous zoom Fresnel lens. It is characterized in that it is composed of upper and lower glass substrates and a liquid crystal interlayer, and the ITO conductive film on the inner surface of the substrate is etched into a banded electrode structure. The refractive index of the liquid crystal layer is controlled by an electric field, so that the phase difference between the two light beams passing through adjacent bands to the main focus is 2π, so the light-gathering ability is stronger than that of the conventional Fresnel zone plate. In addition, through liquid crystal alignment technology, no polarizer is needed, and the light utilization rate is high. And because the refractive index of the liquid crystal is continuously adjustable, the main focal length of the lens can be continuously changed in the range of 0.5f to 2f.

Description

Continuous Zoom Fresnel Lens

1. Technical field

The invention belongs to the field of optical devices, and relates to a Fresnel lens whose focal length is continuously adjustable by utilizing the characteristic of continuously changing refractive index of liquid crystal under the action of an electric field. The liquid crystal layer is used to make up the half-wave phase to minimize the loss of light energy, and at the same time, the lens is continuously zoomed by electronically controlling the effective refractive index of the liquid crystal.

2. Background technology

Lenses are fundamental optics that can be found everywhere in optical instruments and equipment. With the development of optical technology, the requirements for lenses are getting higher and higher. One is a continuously variable focal length of the lens, and the other is a lens with a large focal length. Either way, meeting the requirements will make the optical system bulky and complex.

Microlens is also an important optical device. At present, the traditional methods of making microlens mainly include ion exchange method, compression molding method, photosensitive glass thermoforming method and photoresist thermoforming method. Due to the limitations of materials used and traditional techniques, the microlenses made by these methods have a common feature, that is, the lens has only one focal length. To change the focal length of the lens, a group of lenses is required, which can be realized by mechanically adjusting the distance between the lenses. Using a lens group not only increases the cost of the device and increases the volume of the device, but also makes it difficult to effectively achieve the required focal length. Therefore, it is desired to develop a variable focus lens with simple structure, light weight and low cost.

In recent years, it has been proposed to use liquid crystal technology to make variable-focus microlenses. The liquid crystal microlens adopts light transmission mode, which has the advantages of simple control, strong reliability and low driving voltage. This device has huge potential applications. At present, several lens devices have been designed with liquid crystal technology, such as the linear separation electrode structure, see U.S. Journal of Applied Physics, Vol.24, No.8, 1985pp.L626-628), porous electrode structure (APPLIED OPTICS/Vol.36, No.20/10 July1997, pp4772-4778). The lenses with the above structures are all refraction liquid crystal lenses, some of which have complex manufacturing techniques. We have used polymer dispersed liquid crystal technology to prepare Fresnel band diffraction devices (CN2348405Y, 1999, 11, 10). By designing the structure of ITO (indium tin oxide) electrodes on the substrate, we can achieve odd half-band or even Electric-field tunable Fresnel liquid crystal waveband devices with half-waveband action.

3. Contents of the invention

According to the conventional Fresnel wave zone structure, the present invention utilizes the property that the refractive index of liquid crystal changes under the action of changing electric field, and aims to provide a Fresnel lens with large focal length or miniature continuous zoom.

The structure of a conventional Fresnel zone plate is shown in Figure 1. It uses the method of covering the odd half-wave band or the even half-wave band, so that the light waves with the same phase or a phase difference of 2π integer times are diffracted through the zone plate. And through interference, the light waves are concentrated on the axis of the wave band, thus having the function of a lens. However, due to the selective shielding of light, this zone plate reduces the utilization rate of light by half, and its main focal length cannot be adjusted. It can be seen from Figure 1 that if the odd half-wave band (or even half-wave band) is not shielded, but the light wave passing through the odd half-wave band (or even half-wave band) is regenerated into bits of π or odd multiples of π If there is a phase difference, this zone plate is a phase zone plate, and this purpose can be achieved by using liquid crystal technology.

Figure 2 shows the phase difference caused by the same thickness and different refractive index, n ₁ represents the refractive index of the upper half of the liquid crystal in the liquid crystal cell, n ₂ represents the refractive index of the lower half of the liquid crystal, Δn is |n ₁ -n ₂ |, d is the thickness of the liquid crystal cell. If light passes through the two parts, the optical path difference , λ is the light wavelength, and these two parts are regarded as two adjacent wave bands, then the phase type liquid crystal zone plate can be designed.

According to the above principles and analysis, the present invention designs the band structure as shown in FIG. 3 . Fig. 3 is the ITO electrode etching pattern and the common electrode part of the upper and lower glass substrates.

，r₁为最内圆的半径，m为正整数是波带环的序数。可以看出，如果给上下基板施加电压，则只有镜相对称的环状电极部分之间存在电场，即电场的形状为波带状。用波带状的电场驱动液晶，则液晶层亦呈波带状，因此，液晶和菲涅耳波带结合可用来制作液晶透镜。According to the structure designed in Figure 3, the ITO electrode on one glass substrate has a ring-shaped band structure, and the other glass substrate also has a ring-shaped electrode, and the ring-shaped bands on each substrate are connected together by a common straight-band electrode , the ring-shaped bands of ITO on the upper and lower substrates are exactly the same. The two ring-shaped electrode surfaces are used as inner surfaces, the electrode patterns of the upper and lower substrates are mirror images, and the common electrodes do not overlap, and a liquid crystal layer is sandwiched between the two substrates. The liquid crystal alignment layer polyimide (PI) is coated on the two annular band electrodes. The liquid crystal layer between the two glass substrates is twisted nematic phase liquid crystal material doped with chiral agent. The gap d between the two substrates, that is, the thickness of the liquid crystal layer can be selected between 4-6 μm. In this device, the radius of each band circle is expressed as

, r ₁ is the radius of the innermost circle, m is a positive integer and is the ordinal number of the band ring. It can be seen that if a voltage is applied to the upper and lower substrates, only the electric field exists between the mirror-symmetrical ring-shaped electrode parts, that is, the shape of the electric field is banded. When the liquid crystal is driven by a zonal electric field, the liquid crystal layer is also zonal. Therefore, the combination of liquid crystal and Fresnel band can be used to make a liquid crystal lens.

In order to facilitate the understanding of the present invention, further description is given with FIG. 4 . Figure 4 shows the alignment structure of liquid crystal molecules in the Fresnel zone. Figure 4(a) shows the alignment of liquid crystals when no electric field is applied. Figure 4(b) shows the alignment of liquid crystals when the Fresnel zone electrodes are applied with an electric field. Condition. Wherein, na _ave is the average refractive index of the liquid crystal when no electric field is applied, and n _eff is the effective refractive index of the alignment of the liquid crystal when the electric field is applied. In Fig. 4, 1—glass substrate, 2—is the ITO electrode, 3—is the alignment layer, 4—the liquid crystal layer.

According to the optical anisotropy characteristics of liquid crystals, in order to make monochromatic natural light pass through the liquid crystal layer to produce the same phase delay in any light wave vibration direction, the liquid crystals should be arranged in a 180° twist. Figure 4(a) shows the light transmission process of two adjacent wavebands without voltage applied, and the transmission conditions are the same because of the same refractive index. (b) in Fig. 4 is the case where there is an applied electric field in adjacent bands. When the liquid crystal is driven, the liquid crystal in the driven part has the same effective refractive index n _eff in any direction, while the liquid crystal not driven has the average refractive index na _ave . The refractive index of two adjacent bands is different, but the optical path difference generated by the transmitted light is the same in any direction. By adjusting the effective refractive index, the liquid crystal phase zone plate can be controlled, which not only significantly improves the light utilization rate of the device, but also changes the focal length of the device.

In order to obtain the relationship between the structure of this band device and the focal length, Figure 5 shows the structure of the liquid crystal band device. When light enters from the left side of the device and focuses on the right side, the phase difference of adjacent wavebands should satisfy: $2\mathrm{\π}=\frac{2\mathrm{\π}}{\mathrm{\λ}}[{\mathrm{no}}_{\mathrm{ave}}-{\mathrm{no}}_{\mathrm{eff}}\left(V\right)]d+\frac{2\mathrm{\π}}{\mathrm{\λ}}b,---\left(1\right)$ in

为光通过液晶层时产生的位相差，b为光由相邻波带到聚焦点处产生的光程差，O为焦点。

is the phase difference generated when the light passes through the liquid crystal layer, b is the optical path difference generated by the light from the adjacent band to the focal point, and O is the focus.

It can be deduced from formula (1)

λ=[n _ave -n _eff (V)]d+b (2)

According to the principal focal length formula of the traditional zone plate:

From the above two formulas: $f_{\mathrm{LCFZ}}={{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2/\mathrm{\&lambda;}=\frac{{{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2}{[{\mathrm{no}}_{\mathrm{ave}}-{\mathrm{no}}_{\mathrm{eff}}\left(V\right)]d+b}---\left(4\right)$ It can be seen from formula (4) that the focal length _fLCFZ of the Fresnel liquid crystal zone plate has the ability to continuously adjust the electric field.

Figure 7 shows the light-gathering characteristics of this liquid crystal zone plate measured by CCD when different voltages are applied. The distance between the liquid crystal device and the CCD remains unchanged, and the light source used is a He-Ne laser. Fig. 7(a) is the focusing situation of the present invention when no voltage is applied, and (b) and (c) are the focusing situations of the present invention when the voltage is 3V and 4V respectively. It can be seen that when no voltage is applied, the He-Ne laser beam passes through the liquid crystal cell without being focused, and when 3V and 4V are applied, the transmitted beam is obviously narrowed and the light intensity is significantly increased. The light intensity obtained under the two conditions of 3V and 4V is different, indicating that the focal length of the device is different. Therefore, the focal length of this device has electric field tunability. The light intensity of the transmitted light was detected by a polarizing microscope, and it was found that the light intensity was not polarized.

According to the expression of formula (3), if the zone plate is regarded as an amplitude zone plate, its focal length is calculated to be about 40mm, considering the first wave zone and the second wave zone, the second wave zone and the third wave zone Due to the inconsistency of b changes in the wave bands, the light focusing ability is reduced. According to the geometric structure in Figure 5, it can be calculated by the Pythagorean theorem. When the optical path difference between adjacent wave bands does not exceed λ/20, it can be considered that there is still light focusing ability . So we can get: $\sqrt{{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="A0112512800091.png,A0112512800101.png"\; state="\{\{state\}\}">f</figure-callout>}}^2+{2{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2}-\sqrt{{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="A0112512800091.png,A0112512800101.png"\; state="\{\{state\}\}">f</figure-callout>}}^2+{{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2}=\sqrt{{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="A0112512800091.png,A0112512800101.png"\; state="\{\{state\}\}">f</figure-callout>}}^2+3{{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2}-\sqrt{f^2-2{{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="A0112512800101.png"\; state="\{\{state\}\}">r</figure-callout>}}_1}^2}\&PlusMinus;\frac{\mathrm{\&lambda;}}{20}-----\left(5\right)$ From this formula, it can be estimated that the adjustable range of the focal length of this device is 1 cm to 100 cm, so we believe that the adjustable range of the effective focal length is about 0.5 to 2 times the original intrinsic focal length.

Theoretically, if the intensity of the laser is I _o , the vibration amplitude generated by the laser passing through each wave band at the main focus is a (I _o = a ² ), then the total vibration amplitude is 2×16a, and the light intensity is I=(2×16a) ² , that is, the light intensity will increase by 1024 times.

4. Description of drawings

Figure 1 is a structural diagram of a conventional Fresnel zone plate.

Fig. 2 shows the optical path difference caused by light transmitted through adjacent two-phase liquid crystal bands in the present invention, where n ₁ and n ₂ are the refractive indices of the liquid crystal.

Fig. 3 is a structural schematic diagram of the present invention.

Fig. 4 is the change of the refractive index of adjacent wave bands under the action of electric field in the present invention.

Fig. 5 is a schematic diagram of focusing in the present invention.

Fig. 6 is a band structure observed under a polarizing microscope according to an embodiment of the present invention.

FIG. 7 shows the light-gathering characteristics measured by the CCD under the condition of applying different voltages according to the present invention.

5. Specific implementation

The ITO electrodes on the two glass substrates are etched into the same band structure by photolithography. The innermost radius of the band is 0.5mm. There are 16 odd bands in total, and the respective bands are connected by common electrodes. Coat the polyimide alignment layer on the surface of the ITO electrode and rub it in one direction. The glass plates are placed symmetrically to form a liquid crystal cell, the rubbing directions are antiparallel, and the common electrodes do not overlap. A moderate voltage is applied to the common electrode, and the band structure observed under a polarizing microscope is shown in Figure 6 . In the photo, the odd-numbered bands are the part of the liquid crystal alignment under the action of the electric field, and the even-numbered bands are the part of the initial alignment of the liquid crystal.

Claims (4)

1. A continuous zoom Fresnel lens, composed of a glass substrate with a transparent electrode, a liquid crystal layer and another glass substrate with a transparent electrode, characterized in that the transparent electrode on the glass substrate is etched into a Fresnel annular wave zone Electrode structure, the electrode patterns of the upper and lower substrates are mirror-symmetrical, and these band electrodes are connected by a common straight-band electrode, and the common electrodes of the upper and lower substrates do not overlap; the electrode surface is used as the inner surface of the substrate, and the two ring-shaped band electrodes are coated with liquid crystal Alignment layer; the liquid crystal layer between the upper and lower substrates is a nematic liquid crystal material doped with a chiral agent. 2. The continuous zoom Fresnel lens according to claim 1, characterized in that the liquid crystals are twisted and rotated at 180°, and the helical axis is along the normal direction of the substrate surface. 3. The continuous zoom Fresnel lens according to claim 2, characterized in that the refractive index change difference Δn of the liquid crystal is in the range of 0.2-0.3, and the equal-thickness gap between the two substrates is 4-6 μm. 4. The continuous zoom Fresnel lens according to claim 1, characterized in that the refractive index of the liquid crystal is modulated by an electric field, so that the focal length of the lens changes continuously within the range of 0.5 to 2 times the original focal length.

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