CN102981194A - One-piece type optical element and design method of one-piece type diffraction optical element - Google Patents

Abstract

The invention discloses a single-chip optical element and a design method of the single-chip diffractive optical element. The monolithic optical element is used for color-separating and focusing the incident light containing multiple wavelengths, which includes an integrally formed grating for color-separating the incident light and for separating the incident light after color separation A combination of focusing lenses that focus light. The single-chip optical element of the present invention can achieve high-efficiency color separation focusing on the incident light by combining the lens and the grating into one body, avoiding the use of two independent optical elements, the entire optical system is too complicated, and the operation is inconvenient . Based on the focus lens and the blazed grating, the present invention designs a monolithic diffractive optical element with basically the same color separation and focusing function, breaking through the limitation of the thickness of conventional optical elements in the prior art, so that the designed monolithic diffractive optical element can Applied to more occasions, especially suitable for application in solar cells.

Description

Design method of monolithic optical element and monolithic diffractive optical element

technical field

The invention relates to the field of optics, and more specifically, the invention relates to a monolithic optical element and a design method of the monolithic diffractive optical element.

Background technique

Solar energy is a non-polluting, inexhaustible renewable energy source. An important way to utilize solar energy is to use solar cells to convert light energy into electrical energy. The main principle of solar cells, taking semiconductors as an example, is to use the photovoltaic effect of semiconductor materials to absorb the energy of sunlight and convert it into electrical energy. The two main factors restricting the widespread utilization of solar energy are low photoelectric conversion efficiency and high cost. Currently, cost reductions are mainly achieved by focusing sunlight to reduce the use of expensive solar cell materials. In actual use, due to the different bandgap structures of different semiconductor materials, light with energy lower than the bandgap cannot be absorbed and converted into electrical energy, while light with energy higher than the bandgap is absorbed, but the part beyond the bandgap Energy will be wasted in the form of heat, therefore, the conversion efficiency of solar cells using semiconductor materials with a single band gap is low. To this end, it is necessary to adopt a color separation scheme, that is, to use semiconductor materials with different band gaps to absorb and convert the energy of each band of sunlight, which is an important way to achieve high photoelectric conversion efficiency.

Based on the above ideas, color separation and focusing of sunlight is an important way to achieve high efficiency and low cost of solar energy. As far as color separation is concerned, there are currently two main types of research schemes in the world, namely series (also known as "cascade") and parallel (also known as "horizontal") methods. In the tandem structure, different semiconductor materials are grown sequentially from bottom to top along the vertical direction, and their band gap energy gradually increases. This method is usually called "tandem multi-junction battery" in the industry. At the same time, it is also necessary to provide a high-magnification focusing optical system to reduce costs. The disadvantage of this "serial" scheme is that lattice matching needs to be considered between different semiconductor layers, which not only reduces material selectivity, but also requires a tunnel junction between materials, which requires molecular beam epitaxy and other technologies for growth, which imposes high requirements on the process. In addition, due to the series connection between semiconductors with different band gaps, current matching is required in actual work, and the efficiency is also lost. Parallel structure can overcome the above disadvantages, so it has received more attention. The parallel structure refers to the use of an optical system to achieve color separation and focusing of sunlight at the same time, so that sunlight of different bands can be focused to different areas, and then semiconductor materials with the highest energy conversion efficiency for this band are placed on the corresponding areas, and each semiconductor material is independent Work.

At present, there are mainly two kinds of parallel structure realization schemes. The first one is to use a dichroic mirror to separate the sunlight, and divide the sunlight into long-wave and short-wave components. In order to obtain higher color separation efficiency, this kind of dichroic mirror usually needs to be coated with more than ten layers, or even dozens of layers, which is technically difficult. The second is the scheme of combining light splitting with lenses and prisms, which makes the optical device bulky. The disadvantage of the existing parallel structure is that the cost of the optical system will be very high.

The applicant disclosed in Chinese Invention Patent Application No. 201110351978.9 titled "A Diffractive Optical Element and Its Design Method and Application in Solar Cells" that can simultaneously separate the color of incident light containing multiple wavelengths. and a design method for a focused diffractive optical element, in which a so-called "thickness optimization algorithm" is used to improve the diffraction efficiency of the designed diffractive optical element. This application is also hereby incorporated by reference in its entirety. The design methodology includes:

Step 1: For each wavelength, calculate the modulation thickness for the wavelength at the current sampling point of the diffractive optical element; thereby correspondingly obtain multiple modulation thicknesses for multiple wavelengths;

Step 2: Obtain a series of mutually equivalent alternative modulation thicknesses for each modulation thickness;

Step 3: Select a modulation thickness from the alternative modulation thicknesses for each wavelength, and determine the designed modulation thickness of the current sampling point of the diffractive optical element according to the selected multiple modulation thicknesses corresponding to the multiple wavelengths.

The second and third steps are the "thickness optimization algorithm", which essentially expands the optional range of modulation thickness, and selects a design modulation that can better compromise multiple wavelengths within the expanded optional range thickness.

In this patent application, in the first step, methods such as the Yang-Gu algorithm need to be used to obtain the modulation thickness, which usually requires the design of complex computer programs and a large number of iterative processes, which is a very time-consuming process. Moreover, its calculation process and results may also be affected by the initial assignment of the iterative process.

Contents of the invention

In order to overcome at least one defect of the above-mentioned prior art, an object of the present invention is to provide a monolithic optical element. Another object of the present invention is to provide a method for designing a monolithic diffractive optical element based on the monolithic optical element. Another object of the present invention is to provide a method for designing a diffractive optical element based on conventional optical elements.

According to one aspect of the present invention, there is provided a monolithic optical element for color-separating and focusing incident light containing multiple wavelengths, which includes an integrally formed grating for color-separating the incident light Combination with a focusing lens for focusing the color-separated incident light.

Preferably, the grating is a transmission blazed grating, and the transmission blazed grating is configured to concentrate incident light of each wavelength on a predetermined single diffraction order.

The grating may be photolithographically formed on one side of the focusing lens.

According to another aspect of the present invention, a method for designing a monolithic diffractive optical element is provided, based on a grating used for color-separating incident light containing multiple wavelengths and for focusing the color-separated incident light The thicknesses at multiple sampling points of the focusing lens are used to obtain the designed modulation thicknesses at corresponding multiple sampling points of the monolithic diffractive optical element; the monolithic diffractive optical element has a The combination of lenses has basically the same optical function;

For each sampling point of the grating and the focusing lens, the design method includes:

Step 1: For each wavelength in the plurality of wavelengths, according to the thickness of the focusing lens at the current sampling point, obtain an equivalent modulation thickness equivalent in terms of phase modulation for the corresponding wavelength, and the equivalent modulation thickness is equivalent in diffraction Within the scale range of the optical element; thus, for the multiple wavelengths, multiple equivalent modulation thicknesses are correspondingly obtained;

Step 2: adding the plurality of equivalent modulation thicknesses to the thickness of the grating at the current sampling point respectively to obtain a plurality of initial modulation thicknesses at the current sampling point;

Step 3: Using a thickness optimization algorithm for the plurality of initial modulation thicknesses to determine the design modulation thicknesses at the corresponding sampling points of the monolithic diffractive optical element.

According to still another aspect of the present invention, a design method of a monolithic diffractive optical element is provided, based on a grating for color-separating incident light containing multiple wavelengths and for focusing the color-separated incident light The thicknesses at multiple sampling points of the focusing lens are used to obtain the designed modulation thicknesses at corresponding multiple sampling points of the monolithic diffractive optical element; the monolithic diffractive optical element has a The combination of lenses has basically the same optical function;

For each sampling point of the grating and the focusing lens, the design method includes:

Step 1: For each wavelength in the plurality of wavelengths, according to the thickness of the focusing lens at the current sampling point, obtain an equivalent modulation thickness equivalent in terms of phase modulation for the corresponding wavelength, and the equivalent modulation thickness is equivalent in diffraction Within the scale range of the optical element; thus, for the multiple wavelengths, multiple equivalent modulation thicknesses are correspondingly obtained;

Step 2: using a thickness optimization algorithm for the multiple equivalent modulation thicknesses to determine the lens design modulation thickness;

Step 3: adding the designed modulation thickness of the lens to the thickness of the grating at the current sampling point to obtain the designed modulation thickness at the corresponding sampling point of the monolithic diffractive optical element. In the above design method, the range of the modulation phase of the equivalent modulation thickness for the corresponding wavelength is [0, 2π).

In the above design method, there may be multiple focusing lenses, which are respectively used to focus incident light of corresponding wavelengths among the multiple wavelengths.

In one embodiment, the thickness optimization algorithm includes obtaining a corresponding series of candidate modulation thicknesses according to each of the initial modulation thicknesses; wherein, the candidate modulation thicknesses are limited within a predetermined thickness range.

In one embodiment, the thickness optimization algorithm includes obtaining a corresponding series of candidate modulation thicknesses according to each of the equivalent modulation thicknesses; wherein, the candidate modulation thicknesses are limited within a predetermined thickness range.

In the above design method, the predetermined thickness range may be determined according to the level of photolithography processing technology. The grating may be a transmission blazed grating, which concentrates the incident light of each wavelength on a predetermined single diffraction order.

According to another aspect of the present invention, a monolithic diffractive optical element designed by the above design method is provided. Preferably, the monolithic diffractive optical element is made by photolithography.

The present invention also provides a solar cell, comprising the above monolithic optical element or the above monolithic diffractive optical element.

The present invention also provides a method for designing a diffractive optical element based on a conventional optical element, which is used to obtain the design of the diffractive optical element at corresponding multiple sampling points according to the thickness of the conventional optical element at multiple sampling points Modulating thickness; said conventional optical element having an optical function of modulating incident light comprising a plurality of wavelengths to obtain outgoing light having a desired optical distribution, said diffractive optical element having substantially the same optical function as said conventional optical element ;

For each sampling point of the conventional optical element, the method includes:

Step 1: For each wavelength in the plurality of wavelengths, obtain an equivalent modulation thickness equivalent in terms of phase modulation for the corresponding wavelength according to the thickness at the current sampling point of the conventional optical element, the equivalent modulation thickness Within the scale range of the diffractive optical element; thus, for the plurality of wavelengths, correspondingly obtain a plurality of equivalent modulation thicknesses;

Step 2: Using a thickness optimization algorithm for the plurality of equivalent modulation thicknesses to determine the design modulation thickness at the corresponding sampling point of the diffractive optical element.

In one embodiment, the conventional optical element is a monolithic optical element composed of a grating for color-separating incident light containing multiple wavelengths and a focusing lens for focusing the color-separated incident light. The thickness of the conventional optical element at the current sampling point may be the thickness of the focusing lens at the current sampling point plus the thickness of the grating at the current sampling point.

Preferably, the range of the modulation phase of the equivalent modulation thickness for the corresponding wavelength is [0, 2π).

In the above method, the thickness optimization algorithm may include obtaining a series of candidate modulation thicknesses according to the equivalent modulation thickness; wherein, the candidate modulation thicknesses may be limited within a predetermined thickness range. The predetermined thickness range can be selected according to the level of photolithography processing technology.

The conventional optical element may include a focusing lens for focusing incident light of the plurality of wavelengths.

The grating may be a transmission blazed grating, which concentrates the incident light of each wavelength on a predetermined single diffraction order.

Embodiments of the present invention at least have the following technical effects:

1) The single-chip optical element of the present invention can achieve high-efficiency color separation and focusing on the incident light by combining the lens and the grating, avoiding the use of two independent optical elements to make the entire optical system too complicated, Inconvenient to operate.

2) According to the present invention, a monolithic diffractive optical element with basically the same color separation and focusing function can be designed based on the existing conventional focusing lens and blazed grating, which breaks through the limitation of the thickness of conventional optical elements in the prior art, making The designed diffractive optical element can be applied to more occasions, especially in solar cells. Moreover, according to the present invention, any conventional optical element that processes multiple wavelengths in the prior art can be thinned into a diffractive optical element, maintaining high diffraction efficiency on the basis of realizing the same optical function. Compared with the diffractive optical element design method proposed in Chinese Invention Patent Application No. 201110351978.9, this design method can make full use of the mature and reliable design achievements and results of existing conventional optical elements, greatly reducing the amount of calculation and workload. Furthermore, the monolithic diffractive optical element designed according to the present invention has high diffraction efficiency for multiple wavelengths respectively.

3) Since the thickness of the monolithic diffractive optical element designed according to the present invention has a greater selection range, it can be controlled arbitrarily according to actual needs, so that the thickness of the thinned monolithic diffractive optical element is controlled within a certain range , so that it is convenient to process the master plate through modern photolithography technology, and then apply imprint technology for mass production, so the cost is greatly reduced.

4) Since the monolithic diffractive optical element according to the present invention can greatly improve the optical efficiency, its application in solar cells has practical significance. Combined with the aforementioned ability to be mass-produced by modern photolithography, this provides an efficient and inexpensive route to harnessing solar energy.

Description of drawings

Figure 1 shows a schematic diagram of a focusing lens focusing parallel incident light to a focal point.

Figure 2 shows that the incident parallel light is coherent after being transmitted through the transmission blazed grating, and the main energy of a certain wavelength will be concentrated on a single diffraction order, so that different wavelengths will be separated into different directions.

Figure 3 shows that the focusing lens and the transmissive blazed grating are combined to form a single-piece optical component, which realizes color separation and focusing functions for the incident light at the same time.

Fig. 4 shows a schematic diagram of designing a focusing lens as a diffractive optical element according to the method of the present invention.

Figure 5(a) shows a schematic diagram of the combination of focusing lens and blazed grating.

Figure 5(b)-(d) respectively show the schematic diagrams of combining the converging lens with the blazed grating after being thinned for three wavelengths respectively.

Fig. 6(a) shows a cross-sectional view of a one-dimensional relief structure with 32 levels of quantization in the thickness of the monolithic diffractive optical element of the present invention.

Fig. 6(b) shows a partial three-dimensional relief structure diagram of the monolithic diffractive optical element in Fig. 6(a).

FIG. 7 shows the distribution diagram of light intensity on the exit panel after the three wavelengths pass through the monolithic diffractive optical element shown in FIG. 6 .

Fig. 8 shows the physical diagram of the monolithic diffractive optical element of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The inventors of the present application found that Chinese Invention Patent Application No. 201110351978.9 proposes a design method for diffractive optical elements purely based on diffraction theory, which has problems such as complex calculation, time-consuming, and may be affected by initial assignment. In fact, in the field of conventional optical element design, there are already some mature design methods that can obtain conventional optical elements with relatively satisfactory optical functions, such as conventional focusing lenses for focusing multi-wavelength incident light. The inventors have further found that, on the one hand, these existing conventional optical elements can be directly transformed in some cases; on the other hand, using the mature and reliable design achievements and As a result, it is also possible to realize a simpler and more reliable design method of diffractive optical elements.

Description of existing optics

In order to express the thickness distribution of the optical element, such a coordinate system can be established, that is, the direction of the incident light is set as the z direction, and the plane perpendicular to the incident light is set as the x-y plane.

Figure 1 shows the principle of lens focusing. The incident parallel light with wavelength _λα is focused from the input plane _P1 to a focal point on the output plane _P2 through the lens. The ideal focusing lens can be given according to the principle of equal optical path. Let its focal length be d and its aperture be 2L. For light with a wavelength of λ _α , its refractive index is n(λ _α ). Consider the thickness distribution of the focusing lens in one-dimensional case

${\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; \&PlusMinus;</span>\sqrt{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{\mathrm{<span\; class="notranslate">\; no</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

在某些特殊情形下，如焦距d>>L时，公式（1）可以简化成in

In some special cases, such as focal length d>>L, formula (1) can be simplified as

${\mathrm{<span\; class="notranslate">\; h</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; L</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

At this time, the shape of the focusing lens is a paraboloid.

For incident light with a wavelength of λ _α , its refractive index n can use its strict expression n(λ _α ) related to the wavelength λ _α . In this way, for different wavelengths λ _α , the thickness distribution h _c (x,λ _α ) of the focusing lens for the corresponding wavelength can be obtained.

When it is necessary to use a focusing lens to focus the incident light of multiple wavelengths λ _α (α=1, 2, 3,..., N _λ , N _λ represents the number of different wavelengths), for the usual focusing of glass materials The lens may also not consider the influence of dispersion, that is, the refractive index n may be set to a constant value independent of the wavelength λ _α , such as n=n(λ ₀ ). λ ₀ may be a representative wavelength, such as a central wavelength or an average wavelength of multiple wavelengths λ _α . In this way the thickness distribution h _c (x,λ ₀ ) of the focusing lens can be obtained.

It should be understood that other design methods for conventional focusing lenses described herein or existing in the prior art are well known to those skilled in the art. In conclusion, those skilled in the art already know how to obtain the thickness distribution h _c (x,λ _α ) of a focusing lens focusing on one wavelength λ _α , or the thickness distribution h _c ( x,λ ₀ ).

For the transmissive blazed grating shown in Fig. 2, it is composed of a series of small triangular prisms as repeating units, which is represented as a series of right triangles arranged vertically in Fig. 2 . The upper right corner of Figure 2 shows an enlarged right triangle, where the blaze angle is β, the dimension of the triangle thickness direction is a=λ/(n(λ)-1), and its grating period is a×cot(β), Here λ can be selected as any wavelength, or any one of the aforementioned multiple wavelengths λ _α , preferably the central wavelength or average wavelength λ ₀ of multiple wavelengths λ _α , that is, a=λ ₀ /(n(λ ₀ )-1). The transmissive blazed grating constructed in this way can separate the incident light of multiple wavelengths λ _α and concentrate them on a predetermined single diffraction order. As shown in Fig. 2, the incident light of three wavelengths λ ₁ , λ ₂ and λ ₃ propagates from the input plane P ₁ to the output plane P ₂ , and the main energy of the three wavelengths is respectively concentrated on a single diffraction order, so that the The outgoing light of the three wavelengths is split into different directions.

Considering the one-dimensional case, the thickness distribution h _s (x) of the transmission blazed grating can be given by

h _s (x)=Mod(－x×tan(β),λ ₀ /(n(λ ₀ ))－1)) (3)

Where n(λ ₀ ) represents the refractive index of light with wavelength λ ₀ in the medium. The effect of the Mod(x, a) function is (set a>0) to make x by adding an integer L of a to make (x+L×a) between 0 and a, where a=λ ₀ /(n( λ ₀ )-1).

It should be understood that other design methods for blazed gratings described here or existing in the prior art are well known to those skilled in the art.

Design a monolithic optical element with both dichroic focusing

The inventors of the present invention have found that, in applications such as the aforementioned solar cells with a parallel structure, it is best to concentrate as much energy as possible on one diffraction order when color-separating incident light of multiple wavelengths. The inventors found that the aforementioned blazed gratings can well fulfill this function. After the incident light of multiple wavelengths λα is separated by the blazed grating as shown in Figure 2, the light of multiple wavelengths _λα separated by the blazed grating is respectively focused through the conventional focusing lens shown in Figure 1, and then the Achieve very good color separation focusing effect. However, if in the field of solar cells, two independent optical elements are used to realize the function of color separation and focusing, the entire optical system will be too complicated and not very convenient to operate. Therefore, the inventors found that it would be very convenient to use if the focusing lens and the blazed grating are combined to form a single-piece optical element.

According to formulas (2) and (3), the thickness h(x) distribution of monolithic optical elements with dichroic focusing function can be given by

h(x)=h _s (x)+h _c (x) (4)

It should be noted that since the monolithic optical element separates and focuses the incident light of multiple wavelengths, the h _c (x) representing the thickness distribution of the focusing lens here should actually be the aforementioned without considering the dispersion The thickness distribution h _c (x,λ ₀ ) for the case.

The monolithic optical element determined by formula (4) can be used to separate and focus the incident light containing multiple wavelengths λ _α . The monolithic optical element essentially includes a combination of a transmissive blazed grating for color-separating incident light of multiple wavelengths _λα and a focusing lens for focusing the color-separated incident light. Fig. 3 shows a schematic diagram of simultaneous realization of color separation and focusing of incident light by a monolithic optical element obtained based on a combination of a lens and a transmission blazed grating according to the present invention. The incident light of three different wavelengths λ _α (α=1, 2, 3) is separated into different directions by the transmission blazed grating, and then converged to different positions of the focal plane by the focusing lens. The positions of λ ₁ , λ ₂ and λ ₃ in the figure respectively represent the focusing positions or areas of light of corresponding wavelengths on the output plane P ₂ . When processing the above-mentioned monolithic optical element, it can be processed according to the thickness of each point determined by formula (4). Alternatively, the grating is photolithographically formed on one side of the focusing lens. It is also possible to process the focusing lens on one side of the whole piece of optical glass, and process the grating on the other side, so as to obtain a single-piece optical element with an integrated structure.

Method of Designing Diffractive Optical Elements Based on Conventional Optical Elements

The thickness of the monolithic optical element shown in FIG. 3 is also relatively large due to the thickness of the conventional focusing lens. Not only the focusing lens, but other conventional optical elements (such as concave lenses, prisms, etc.) are usually relatively large in thickness. Therefore, the optical system using these conventional optical elements is also relatively bulky, which limits its application. If the thickness of these conventional optical elements can be thinned, basically reaching the scale of diffractive optical elements, and basically maintaining the same optical function, then modern photolithography technology can be used to process and batch replicate diffractive optical elements, and the cost of the optical system It will also be reduced a lot.

In addition, for the monolithic optical element shown in FIG. 3 , it has higher requirements on the processing technology, so the processing cost is also higher. If it can be thinned, it will also reduce the requirements for processing technology and reduce processing costs.

In the prior art, there are solutions for thinning conventional optical elements, such as Fresnel lenses, but they are designed for a single wavelength. This approach of thinning for a single wavelength is not available if the conventional optical element is used to process multiple wavelengths of incident light. On the one hand, this cannot maintain its original optical function for multiple wavelengths at the same time, for example, it cannot focus on multiple wavelengths separately; on the other hand, it will also reduce the focusing efficiency. Since it is common to use the same optical element to process multiple wavelengths in practical applications, it is hoped that the thinned optical element can minimize the optical loss for the incident light of multiple wavelengths and achieve basically the same optical function.

Fig. 4 shows a schematic diagram of an embodiment of designing a conventional focusing lens as a diffractive optical element according to the method of the present invention, and the conventional focusing lens is used for multiple wavelengths λ _α (α=1, 2, 3, ..., N _λ , N _λ represents the number of different wavelengths) of incident light are focused separately. As shown on the left side of FIG. 4, the conventional focusing lens is usually relatively thick. For example, in a more common situation, take d=800mm, 2L=21mm, λ ₀ =550nm, n=1.46, according to the formula (2), the maximum thickness of the lens can be obtained as 0.1498mm, which is not suitable for processing by micro-processing technology .

According to the principle of optics, for a sampling point whose thickness of the focusing lens is h _c , its phase modulation to the incident light with wavelength λ _α is △Φ _α =2π(n(λ _α )－1)h _c /λ _α , when When h _c increases or decreases by an integer number of △h _αc =λ _α /(n(λ _α )－1), the modulation phase correspondingly changes by an integer number of 2π, which is equivalent in terms of phase modulation and will not affect the focusing lens The focusing effect on that wavelength. In this way, in the present invention, the thickness h _c can be reduced by an integer number of Δh _αc until h _c is thinned to within the dimension of the diffraction element. The thinned thickness h _αc of the sampling point may be called an equivalent modulation thickness.

In one embodiment, the equivalent modulation thickness h _αc can be calculated according to formula (5)

h _αc =Mod(h _c ，△h _αc ) (5)

The range of the modulation phase Φ _α corresponding to the equivalent modulation thickness h _αc obtained by formula (5) is 0≤Φ _α <2π. And the maximum thickness of each sampling point of the diffractive optical element does not exceed λ _α /(n-1). In other embodiments, h _αc can also be selected such that the corresponding modulation phase Φ _α is in the range of several times of 2π.

In actual design, for the convenience of calculation, multiple representative sampling points can be set on the focusing lens. After the equivalent modulation thickness of each sampling point is obtained, the equivalent modulation thickness distribution h _αc (x) of the entire focusing lens after being thinned for the wavelength λ _α can be obtained. After the above thinning operation is performed on N _λ wavelengths, the equivalent modulated thickness distributions of N _λ thinned lenses respectively corresponding to N _λ wavelengths can be obtained, as shown in the middle part of Fig. 4 .

At this time, for each sampling point of the focusing lens, N _λ equivalent modulation thicknesses h _αc (α=1~N _λ ) respectively corresponding to N _λ wavelengths are obtained. Finally, for the N _λ equivalent modulation thicknesses h _αc , a thickness optimization algorithm is used to determine the design modulation thickness h _D at the corresponding sampling point of the required diffractive optical element. After covering all the sampling points, the entire thickness distribution h _D (x) of the diffractive optical element can be obtained, as shown on the far right side of FIG. 4 . As for the thickness optimization algorithm, it has been described in detail in Chinese Invention Patent Application No. 201110351978.9, and will not be repeated here.

Although the focusing lens is used as an example above to give the embodiment of designing diffractive optical elements based on conventional optical elements, those skilled in the art can easily understand that any incident light with multiple wavelengths is modulated to obtain the desired optical distribution Conventional optical elements with optical functions of outgoing light can be obtained through the design method of the present invention to obtain diffractive optical elements with substantially the same optical functions, not limited to focusing lenses. In the present invention, conventional optical elements may include thicker optical elements such as focusing lenses, refracting lenses, and prisms, whose thickness is at least on the order of millimeters, which is far greater than that of diffractive optical elements. For the monolithic optical element composed of focusing lens and grating in the present invention, its thickness is mainly determined by the thickness of the focusing lens. In the present invention, such monolithic optical element is also treated as a conventional optical element.

Therefore, it is also possible to design a monolithic diffractive optical element based on the monolithic optical element with basically the same optical function as the above-mentioned method of designing a diffractive optical element based on a conventional optical element. The "conventional optical element" here is a monolithic optical element composed of a grating for color-separating incident light containing multiple wavelengths and a focusing lens for focusing the color-separated incident light. Specifically, for each sampling point of the grating and focusing lens that constitute the monolithic optical element, the thickness of the focusing lens at the current sampling point is added to the thickness of the grating at the current sampling point to obtain the initial modulation thickness h at the current sampling point (x)=h _s (x)+h _c (x); for each wavelength in the N _λ wavelengths, according to the initial modulation thickness h(x) at the current sampling point, the phase modulation equivalent to the corresponding wavelength is obtained The equivalent modulation thickness h _α (x) (α=1~N _λ ), the equivalent modulation thickness h _α (x) is within the scale range of the diffractive optical element. Correspondingly, for N _λ wavelengths, N _λ equivalent modulation thicknesses h _α (x) are correspondingly obtained. Then, for the N _λ equivalent modulation thicknesses h _α (x), a thickness optimization algorithm is used to determine the design modulation thickness at the corresponding sampling point of the monolithic diffractive optical element. After covering all the sampling points, the entire thickness distribution of the monolithic diffractive optical element can be obtained.

Design a monolithic diffractive optical element with both color separation and focusing functions

According to the above-mentioned method of designing diffractive optical elements based on conventional optical elements, it is basically the same idea, and the single-piece optical element composed of blazed gratings and conventional focusing lenses shown in Figure 3 can also be thinned and designed to obtain both color separation and Monolithic diffractive optical element for focusing function.

As shown in Figure 5(a), the monolithic optical element with dichroic focusing function (shown on the right side of the "=" sign) consists of a conventional focusing lens (left side of the "+" sign) and a blazed grating ( to the right of the "+" sign). The thickness distribution of the monolithic optical element is shown in formula (4) as h(x)=h _s (x)+h _c (x), where h _s (x) and h _c (x) are the blazed grating and the thickness distribution of a conventional focusing lens. As mentioned above, both h _s (x) and h _c (x) can be obtained by design according to existing methods.

Combining with FIG. 5(a), FIG. 5(b)-(d) presents a schematic flowchart of a first embodiment of a monolithic diffractive optical element. The left side of the "+" sign in Figure 5(b)-(d) schematically represents the thinned conventional focusing lens in Figure 5(a) for the exemplary three wavelengths λ ₁ , λ ₂ and λ ₃ respectively As a result, the equivalent modulation thickness distributions h _1c (x), h _2c (x) and h _3c (x) of the conventional focusing lens for the three wavelengths are respectively obtained. Then, these three equivalent modulation thickness distributions h _1c (x), h _2c (x) and h _3c (x) are added to the thickness distribution h _s (x) of the blazed grating respectively, so as to obtain the The initial modulation thickness distributions h ₁ (x), h ₂ (x) and h ₃ (x) of . For general N _λ different wavelengths λ _α , the initial modulation thickness distributions of N _λ different wavelengths λ _α are obtained respectively:

h _α (x)=h _αc (x)+h _s (x),α=1~N _λ (6)

For the N _λ initial modulation thicknesses h _α of each sampling point, the thickness optimization algorithm can be performed to obtain the final required design modulation thickness of the corresponding sampling point of the diffractive optical element. After going through all the sampling points, the designed modulation thickness distribution h _DOE1 (x) of the monolithic diffractive optical element can be obtained. This diffractive optical element has basically the same optical function as the monolithic optical element shown in Fig. 5(a), and basically maintains a high diffraction efficiency.

In the second embodiment, the design thickness distribution h _D (x) of the corresponding thinned diffractive optical element can be obtained directly based on the thickness distribution h _c (x) of the conventional focusing lens according to the method described above. Then, combine the thickness distribution h _D (x) with the thickness distribution h _s (x) of the blazed grating to obtain the desired thickness distribution h _DOE2 (x)=h _s (x )+h _D (x). This can also thin the monolithic optical element shown in FIG. 3 or FIG. 5(a) to obtain a corresponding monolithic diffractive optical element.

In the above first and second embodiments, both of which involve a single conventional focusing lens that simultaneously focuses incident light of multiple wavelengths, this actually does not consider the dispersion of incident light of different wavelengths by the conventional focusing lens. Although in most applications, the influence of the dispersion is very small, a design method of a monolithic diffractive optical element with separation and focusing function is also proposed here when the dispersion is considered. This method is similar to the design method of the first and second embodiments, except that the influence of dispersion is considered when selecting the thickness distribution of the conventional focusing lens, _and the corresponding focusing lens thickness distribution h _c (x, λ _α ), and when the focusing lens is thinned for the corresponding wavelength, such as shown on the left side of the "+" sign in Fig. 5(b)-(d), it is based on the corresponding focusing lens thickness distribution h _c (x ,λ _α ) to carry out.

In a specific embodiment, the monolithic diffractive optical element of the present invention is designed for incident light with three wavelengths λ ₁ =450nm, λ ₂ =550nm, and λ ₃ =650nm according to the method of the aforementioned first embodiment, wherein The central wavelength is λ ₀ =550nm, and the refractive index is n=n(λ ₀ )=1.46. When performing the thickness optimization algorithm, it is necessary to obtain a corresponding series of alternative modulation thicknesses according to each equivalent modulation thickness, and the alternative modulation thicknesses can be limited within a predetermined thickness range according to the level of photolithography processing technology. In this embodiment, the maximum modulation phase of the monolithic diffractive optical element to the central wavelength _λ0 can be limited to 12π, that is, the maximum possible thickness of the monolithic diffractive optical element is limited, and the monolithic diffractive optical element thus obtained The maximum thickness of the diffractive optical element is about 7 μm. At this time, the theoretical diffraction efficiencies of the single-chip diffractive optical element for the three wavelengths are 69.96%, 80.80%, and 80.50%, respectively. In the actual production process, since multiple overlays are required for processing, it is necessary to quantify the design modulation thickness h. In one embodiment, 32 levels of quantization can be used, and the quantization thickness h _q (x)=Int[h(x)/step]×step, Int[y] means taking the integer part of the real number y, and step means each quantization level second thickness, here step=(Max(h(x))-Min(h(x)))/31, where Max(h(x)) and Min(h(x)) represent h(x) respectively maximum and minimum values of . Thus, a monolithic diffractive optical element with a one-dimensional relief structure with a thickness of 32 steps can be obtained, and its morphology is shown in Figures 6(a) and 6(b). FIG. 7 shows the light intensity distribution diagram of the incident light of these three wavelengths on the exit panel after passing through the monolithic diffractive optical element. The incident lights of three wavelengths are respectively focused at three different positions, which shows that the monolithic diffractive optical element has a good effect of color separation and focusing. Fig. 8 shows the physical picture of the monolithic diffractive optical element obtained by using the micromachining photolithography technology and using 5 times of overlay processing. It can be seen from FIG. 8 that the effective area of the single-chip diffractive optical element is about 2 cm×1 cm, which can be very conveniently applied to various occasions that require color separation and focusing.

Applications in solar cells

When the monolithic optical element and the monolithic diffractive optical element designed according to the method of the present invention perform color separation and focusing, theoretical analysis shows that in the visible light band, the average diffraction focusing efficiency can reach 85% and 77%, respectively. This therefore makes such optical elements promising for use in systems such as high-efficiency solar cells.

In the solar cell of the present invention, the monolithic optical element or the monolithic diffractive optical element is used to separate the incident sunlight according to selected multiple wavelengths (λ _α , α=1-N _λ ) and focus them on the same Different positions of the output plane. A variety of semiconductor materials are placed at corresponding focusing positions on the output plane, and these semiconductor materials are respectively used to absorb sunlight of corresponding wavelength bands.

Since the monolithic optical element or monolithic diffractive optical element designed according to the present invention greatly improves the diffraction efficiency, so that more solar energy can be utilized, so that the monolithic optical element and monolithic optical element of color separation focus The type diffractive optical element has practical application value in solar cells.

The above are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall fall within the protection scope of the present invention.

Claims (14)

1. the method for designing of an one chip diffraction optical element is carried out the grating of color separation and the thickness of a plurality of sample point of the condenser lens that is used for the incident light after the color separation is focused on obtains the design modulation thickness of corresponding a plurality of sample point of described one chip diffraction optical element based on being used for to the incident light that comprises a plurality of wavelength; Described one chip diffraction optical element has and the essentially identical optical function of the combination of described grating and described condenser lens;

For each sampled point of described grating and described condenser lens, described method for designing comprises:

Step 1: for each wavelength in described a plurality of wavelength, the thickness of stating condenser lens according to the current sampling point place obtains for the equivalence modulation thickness of corresponding wavelength in equivalence aspect the phase-modulation, and described equivalence modulation thickness is in the diffraction optical element range scale; Thus, for described a plurality of wavelength, obtain accordingly a plurality of equivalence modulation thickness;

Step 2: described a plurality of equivalence modulation thickness are added that respectively the current sampling point place states the thickness of grating, to obtain accordingly a plurality of initial modulation thickness at current sampling point place;

Step 3: adopt the thickness optimization algorithm to determine the design modulation thickness of the corresponding sample point of described one chip diffraction optical element to described a plurality of initial modulation thickness.

2. the method for designing of an one chip diffraction optical element is carried out the grating of color separation and the thickness of a plurality of sample point of the condenser lens that is used for the incident light after the color separation is focused on obtains the design modulation thickness of corresponding a plurality of sample point of described one chip diffraction optical element based on being used for to the incident light that comprises a plurality of wavelength; Described one chip diffraction optical element has and the essentially identical optical function of the combination of described grating and described condenser lens;

For each sampled point of described grating and described condenser lens, described method for designing comprises:

Step 1: for each wavelength in described a plurality of wavelength, the thickness of stating condenser lens according to the current sampling point place obtains for the equivalence modulation thickness of corresponding wavelength in equivalence aspect the phase-modulation, and described equivalence modulation thickness is in the diffraction optical element range scale; Thus, for described a plurality of wavelength, obtain accordingly a plurality of equivalence modulation thickness;

Step 2: adopt the thickness optimization algorithm to determine Lens Design modulation thickness to described a plurality of equivalence modulation thickness;

Step 3: described Lens Design modulation thickness added the thickness of stating grating in the current sampling point place is with the design modulation thickness of the corresponding sample point that obtains described one chip diffraction optical element.

3. method for designing according to claim 1 and 2 is characterized in that, described equivalence modulation thickness for the scope of the phase modulation of corresponding wavelength be [0,2 π).

4. method for designing according to claim 1 is characterized in that, described thickness optimization algorithm comprises a series of alternative modulation thickness that obtains correspondence according to each described initial modulation thickness; Wherein, in the thickness range that described alternative modulation thickness is limited in being scheduled to.

5. method for designing according to claim 2 is characterized in that, described thickness optimization algorithm comprises a series of alternative modulation thickness that obtains correspondence according to each described equivalence modulation thickness; Wherein, in the thickness range that described alternative modulation thickness is limited in being scheduled to.

6. according to claim 4 or 5 described methods for designing, it is characterized in that described predetermined thickness range is determined according to the lithography process technological level.

7. each described method for designing is characterized in that according to claim 1-6, and described grating is the transmission-type blazed grating, and its incident light with each wavelength concentrates on respectively on the predetermined single order of diffraction.

8. method for designing according to claim 1 and 2 is characterized in that, described condenser lens is a plurality of, is respectively applied to the incident light of corresponding wavelength in described a plurality of wavelength is focused on.

9. one chip diffraction optical element according to each described method for designing design among the claim 1-8.

10. one chip diffraction optical element according to claim 9 is characterized in that, described one chip diffraction optical element is made with photoetching method.

11. an one chip optical element is used for the incident light that comprises a plurality of wavelength is carried out color separation and focusing, it comprises the combination of the condenser lens that integrated grating and being used for for described incident light being carried out color separation focuses on the incident light after the color separation.

12. one chip optical element according to claim 11 is characterized in that, described grating is the transmission-type blazed grating, and described transmission-type blazed grating is configured to the incident light of each wavelength is concentrated on respectively on the predetermined single order of diffraction.

13. according to claim 11 or 12 described one chip optical elements, it is characterized in that described grating is formed in a side of described lens in the mode of photoetching.

14. a solar cell comprises such as claim 9 or 10 described one chip diffraction optical elements or such as each described one chip optical element among the claim 11-13.

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