Liquid crystal polarization grating filter assembly

Abstract

The present application relates to the technical field of optical components, and discloses a liquid crystal polarization grating filter assembly, which realizes 0-order filtering based on the principle of diffraction and ensures high transmittance under a relatively narrow half-width. The assembly comprises N (N>=2) pieces of liquid crystal polarization gratings with the same period connected in cascade, and each liquid crystal polarization grating is provided with an aperture stop at the exit end; the aperture size of each aperture stop is equal to the size of the incident light spot, and the distance between any aperture stop and the previous stage liquid crystal polarization grating is greater than or equal to the product of the diameter of the incident light spot and the tangent value of the diffraction angle corresponding to the minimum wavelength in the estimated applicable wavelength range; the thickness of the liquid crystal layer of the next stage polarization grating is twice the thickness of the corresponding liquid crystal layer of the previous stage polarization grating; and the thickness d of the liquid crystal layer of the first stage liquid crystal polarization grating needs to meet a specific condition.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of optical components, in particular to a liquid crystal polarization grating filter assembly. BACKGROUND

[0002] A band-pass filter is a filter with a transmission band on both sides of which there are adjacent cutoff bands. The band-pass filter is roughly divided into wide-band filters and narrow-band filters according to the spectral characteristics, and the two kinds of filters are usually combined. Most of the existing products are band-pass filters prepared by applying the principle of light wave interference. Based on the constructive interference condition of the Fabry-Perot cavity, the light within a small range on both sides of the central wavelength is effectively transmitted, and the destructive interference can prevent the transmission of light outside the passband.

[0003] Among them, the filter based on the principle of polarized light interference uses the superposition of wave peaks and wave troughs in the interference spectrum and the cooperation of the polarizer to filter. Since the polarizer (usually a traditional non-liquid crystal linear polarizer used to filter linearly polarized light in a specific direction) is used, the final transmission peak power is low, and the peak transmittance of the final transmission peak is difficult to reach more than 50%.

[0004] The band-pass filter has been widely used in various fields, but there is no passive band-pass filter prepared based on the diffraction principle of liquid crystal polymers on the market at present. SUMMARY

[0005] The application aims to disclose a liquid crystal polarization grating filter assembly to realize 0-order filtering based on the diffraction principle and ensure high transmittance under a relatively narrow half-width.

[0006] To achieve the above purpose, the application discloses a liquid crystal polarization grating filter assembly, which comprises:

[0007] N pieces of liquid crystal polarization gratings with the same period are connected in cascade; N is greater than or equal to 2;

[0008] Each of the liquid crystal polarization gratings is provided with an aperture stop at the exit end; the aperture size of each aperture stop is equal to the size of the incident light spot, and the distance between any aperture stop and the previous stage liquid crystal polarization grating is greater than or equal to the product of the diameter of the incident light spot and the tangent value of the diffraction angle corresponding to the smallest wavelength in the transmission range.

[0009] Each stage of the liquid crystal polarization grating is respectively provided with a glass substrate, an alignment layer and a liquid crystal layer, wherein the thickness of the liquid crystal layer of the next stage polarization grating is twice the thickness of the corresponding liquid crystal layer of the previous stage polarization grating; and the thickness d of the liquid crystal layer of the first stage liquid crystal polarization grating is the minimum thickness satisfying the following conditions:

[0010]

[0011]

[0012] d·Δn3=m3·λ3;

[0013] Wherein, Δn1, Δn2, Δn3 are the birefringence of the liquid crystal material corresponding to the wavelength of λ1, λ2, λ3; when N=2, d needs to satisfy the above three equations, λ1, λ2 are the wavelengths at the trough of the transmission spectrum of the liquid crystal polarization grating filter assembly, λ3 is the central wavelength of the transmission peak, λ1, λ2, λ3 satisfy the relationship λ1<λ3<λ2; when N>2, d only needs to consider the third equation above to satisfy the full wave condition of λ3; m1, m2, m3 are positive integers greater than 1, and m1, m2, m3 are not equal.

[0014] Preferably, the spacing between each level of polarization grating is equal, and the spacing value is greater than or equal to 0.3mm and less than 150mm.

[0015] Preferably, in each level of polarization grating, the orientation layer is spin-coated on the glass substrate, the liquid crystal layer is spin-coated on the orientation layer, and the aperture diaphragm is arranged on the liquid crystal layer.

[0016] The present application has the following beneficial effects:

[0017] Without using traditional polarizing plates, in the cascaded multi-level liquid crystal polarization grating structure, due to the multiplication of the thickness of the liquid crystal layer, the change period of the peak of the transmission spectrum of the liquid crystal polarization grating will be reduced, and the bandwidth of the peak will be narrowed, and finally only one peak exists in the transmission spectrum after the superposition of each level of liquid crystal polarization grating, and the central wavelength of the transmission peak of the assembly can be considered to be determined by the thickness d of the liquid crystal layer of the first level of liquid crystal polarization grating, and the bandwidth of the final transmission peak can be further reduced by cascading more liquid crystal polarization gratings, and the transmittance of the transmission peak is not lost. Therefore, the present application realizes 0-level filtering through the unique cascaded structure and specific parameter design, and still effectively improves the peak power of the transmission peak in the case of narrow half-width.

[0018] The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0019] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application, and are incorporated in and constitute a part of this application. The embodiments of the present application illustrated in the drawings and their descriptions are used to explain the present application and are not intended to limit the present application. In the drawings:

[0020] Figure 1 is a structural schematic diagram of the liquid crystal polarization grating filter assembly disclosed by the embodiment of the present application.

[0021] Figure 2 is Figure 1 is the 0-level transmission spectrum schematic diagram corresponding to the liquid crystal polarization gratings 1, 2, 3, 4 in the above table respectively.

[0022] Figure 3 is Figure 2 Fig. 2 is a schematic diagram of the overall transmission spectrum after superposition of the transmission spectra of the four liquid crystal polarization gratings. DETAILED DESCRIPTION

[0023] The embodiments of the present application will be described in detail below with reference to the accompanying drawings, but the present application can be implemented in various different ways as defined and covered by the claims.

[0024] Embodiment 1

[0025] This embodiment discloses a liquid crystal polarization grating filter assembly, as shown in Fig. 1, wherein 1, 2, 3, and 4 are respectively a first liquid crystal polarization grating, a second liquid crystal polarization grating, a third liquid crystal polarization grating, and a fourth liquid crystal polarization grating, each of which includes a glass substrate, an orientation layer, and a liquid crystal layer; generally, the orientation layer is spin-coated on the glass substrate, and the liquid crystal layer is spin-coated on the orientation layer. Figure 1

[0026] The liquid crystal polarization grating is a grating whose diffraction characteristics are related to the polarization state of the incident light, and in the case of linearly polarized light incidence, 0th order, ±1st order will be generated; in the case of circularly polarized light incidence, only 1st order or -1st order diffracted light will be generated, depending on the handedness of the circular polarization. For details, reference can be made to the many patents related to polarization gratings previously applied by the applicant, which will not be described here.

[0027] In this embodiment, 5 is incident linearly polarized light, 6 is 0th order diffracted light emitted after the incident linearly polarized light 5 passes through the first liquid crystal polarization grating 1, 7 is +1st order diffracted light, 8 is -1st order diffracted light, and 9 is an aperture stop. The working principle of the liquid crystal polarization grating filter assembly is as follows:

[0028] When the incident linearly polarized light passes through the liquid crystal polarization grating 1, due to the diffraction effect of the liquid crystal polarization grating on the incident light, 0th order and ±1st order diffracted light will be emitted. The 0th order diffracted light is consistent with the polarization state and transmission direction of the incident light, and the transmission direction of the ±1st order diffracted light will be deflected relative to the transmission direction of the incident light, and the polarization state will change, wherein the +1st order diffracted light is left-handed circularly polarized light, and the -1st order diffracted light is right-handed circularly polarized light.

[0029] ​The 0th order and ±1st order diffraction light generated by the liquid crystal polarization grating 1 is transmitted to the liquid crystal polarization grating 2. Due to the aperture diaphragm 9, the ±1st order diffraction light generated by the liquid crystal polarization grating 1 is blocked, and only the 0th order diffraction light generated by the liquid crystal polarization grating 1 is in the light transmission aperture of the aperture diaphragm. When the 0th order diffraction light 6 is transmitted through the liquid crystal polarization grating 2, the situation is the same as when the incident linearly polarized light 5 is transmitted through the liquid crystal polarization grating 1. The subsequent analysis of the incident linearly polarized light transmitted through the liquid crystal polarization gratings 3 and 4 is the same. Finally, the transmission spectrum of the liquid crystal polarization grating filter is equivalent to the superposition of the 0th order transmission spectra of the liquid crystal polarization gratings, that is, the multiplication of the 0th order transmission spectra.

[0030] The 0th order diffraction efficiency η of the liquid crystal polarization grating is calculated by the following formula:

[0031]

[0032] Where Γ is the phase retardation, which is the product of the birefringence difference of the liquid crystal material and the thickness of the liquid crystal layer.

[0033] The thickness d of the liquid crystal layer of the liquid crystal polarization grating 1 is determined by the following formula:

[0034]

[0035]

[0036] d·Δn3=m3·510;

[0037] Alternatively, the same material is used in several cascaded polarization gratings. Due to the dispersion characteristics, the birefringence difference of the material at different wavelengths is not the same. In this embodiment, the birefringence difference corresponding to the 445 nm wavelength is 0.17; the birefringence difference corresponding to the 510 nm wavelength is 0.15; and the birefringence difference corresponding to the 635 nm wavelength is 0.14. Correspondingly, in a specific component product, m1=5, m2=3, m3=2, and d=6.66 μm. The thicknesses of the liquid crystal layers of the liquid crystal polarization gratings 2, 3, and 4 are 2d, 4d, and 8d, respectively. The 0th order transmission spectra of the liquid crystal polarization gratings 1, 2, 3, and 4 and the transmission spectra of the second-level cascaded liquid crystal polarization grating filter and the fourth-level cascaded liquid crystal polarization grating filter formed by the combination thereof are shown in Figure 2 、 Figure 3 Due to the multiplication of the thickness of the liquid crystal layer, the change period of the wave peak in the transmission spectrum of the liquid crystal polarization grating will decrease, and the bandwidth of the wave peak will narrow. Finally, there is only one wave peak in the transmission spectrum after the superposition of the liquid crystal polarization gratings 1, 2, 3, and 4, and the bandwidth of the final transmission peak can be further reduced by superimposing more liquid crystal polarization gratings without losing the transmittance of the transmission peak. Figure 3It can be seen that the transmittance of the transmission peak of the second cascade liquid crystal polarization grating filter is the same as that of the fourth cascade liquid crystal polarization grating filter, but the half-height width of the transmission peak of the fourth cascade liquid crystal polarization grating filter is smaller than that of the second cascade liquid crystal polarization grating filter, that is, the half-height width of the transmission peak of the fourth cascade liquid crystal polarization grating filter is only 10 nm, compared with the half-height width of 80 nm of the transmission peak of the second cascade liquid crystal polarization grating filter. In practical applications, the selection of the cascade number of the cascade liquid crystal polarization grating filter is determined by the required half-height width of the transmission peak and the cost.

[0038] In the embodiment, the aperture size of each aperture diaphragm is equal to the size of the incident light spot, and the product of the distance between any aperture diaphragm and the front-stage liquid crystal polarization grating and the tangent value of the diffraction angle corresponding to the minimum wavelength in the transmission range is greater than or equal to the diameter of the incident light spot. The center wavelength of the transmission peak corresponding to the applicable wave range is 510 nm.

[0039] Preferably, the period of each polarization grating in the embodiment is less than 5 microns, the distance between the polarization gratings is equal, the distance value is greater than or equal to 0.3 mm and less than 150 mm, the aperture (diameter) size of the aperture diaphragm is selected within 10-15 mm, and the transmission range of the applicable wave band can be selected between 400 nm-2000 nm.

[0040] In summary, the liquid crystal polarization grating filter assembly disclosed in the embodiment of the application has at least the following beneficial effects:

[0041] Without using a traditional polarizer, in the cascade multi-stage liquid crystal polarization grating structure, due to the multiplication of the thickness of the liquid crystal layer, the change period of the wave peak in the transmission spectrum of the liquid crystal polarization grating is reduced, and the bandwidth of the wave peak is narrowed. Finally, only one wave peak exists in the transmission spectrum after the superposition of the liquid crystal polarization gratings of each stage. The center wavelength of the transmission peak of the assembly can be considered to be determined by the thickness d of the liquid crystal layer of the first-stage liquid crystal polarization grating, and the bandwidth of the final transmission peak can be further reduced by cascading more liquid crystal polarization gratings without losing the transmittance of the transmission peak. Therefore, the application realizes 0-level filtering through the unique cascade structure and specific parameter design, and still effectively improves the peak power of the transmission peak in the case of a narrow half-height width.

[0042] The above only describes the preferred embodiments of the application and is not used to limit the application. For those skilled in the art, the application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A liquid crystal polarizing grating filter assembly, characterized in that, The application relates to a liquid crystal polarization grating filter assembly, which comprises the following steps: A plurality of liquid crystal polarization gratings are cascaded, and the periods of the liquid crystal polarization gratings are the same; N>=2; An aperture diaphragm is arranged at the exit end of each liquid crystal polarization grating; the aperture size of each aperture diaphragm is equal to the size of an incident light spot, and the distance between any aperture diaphragm and the previous liquid crystal polarization grating is greater than or equal to the product of the minimum wavelength in the transmission range and the tangent value of the +1 or -1 diffraction angle corresponding to the minimum wavelength. Each liquid crystal polarization grating is provided with a glass substrate, an orientation layer and a liquid crystal layer, wherein the thickness of the liquid crystal layer of the next liquid crystal polarization grating is twice the thickness of the liquid crystal layer of the previous liquid crystal polarization grating; and the thickness d of the liquid crystal layer of the first liquid crystal polarization grating is the minimum thickness that satisfies the following conditions: d*Delta n3=m3*lambda3; Wherein, Delta n1, Delta n2 and Delta n3 are the birefringence differences of the liquid crystal material corresponding to wavelengths lambda1, lambda2 and lambda3; when N=2, d needs to satisfy the above three equations, lambda1 and lambda2 are the wavelengths at the troughs of the transmission spectrum of the liquid crystal polarization grating filter assembly, and lambda3 is the central wavelength of the transmission peak; lambda1, lambda2 and lambda3 satisfy the relationship lambda1<lambda3<lambda2; when N>2, d only needs to satisfy the third equation to meet the full-wave condition of lambda3; m1, m2 and m3 are positive integers greater than 1, and m1, m2 and m3 are all different.

2. The liquid crystal polarization grating filter assembly of claim 1, wherein, The distances between the liquid crystal polarization gratings are equal, and the distance value is greater than or equal to 0.3 mm and less than 150 mm.

3. The liquid crystal polarization grating filter assembly of claim 1 or 2, wherein, In each liquid crystal polarization grating, the orientation layer is spin-coated on the glass substrate, the liquid crystal layer is spin-coated on the orientation layer, and the aperture diaphragm is arranged on the liquid crystal layer.

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