CN101799590A - Digital Fresnel method and device for eliminating black-matrix effect of phase spatial light modulator (PSLM) - Google Patents
Digital Fresnel method and device for eliminating black-matrix effect of phase spatial light modulator (PSLM) Download PDFInfo
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Abstract
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技术领域technical field
本发明涉及相位型空间光调制器黑栅效应的消除方法,属于光信息处理技术。The invention relates to a method for eliminating the black grid effect of a phase-type spatial light modulator, which belongs to the optical information processing technology.
背景技术Background technique
空间光调制器(Spatial Light Modulator,SLM)是一种在信号源的控制下,能对光波的某个参量进行调制的器件,例如通过吸收调制振幅,通过折射率调制相位,通过偏振面的旋转调制偏振态等。它有效地利用了光的并行、互联和高速的处理能力,使之能够实时地在空间上调制光束,成为实时光学信息处理等系统的核心部件,在现代光学领域中具有越来越重要的地位和价值,它是光学、光电混合系统进行光互连、光学相关、光计算、模式识别、光学控制、光学检测、图像处理、显示技术等研究中的基本构件和关键器件。A spatial light modulator (Spatial Light Modulator, SLM) is a device that can modulate a certain parameter of a light wave under the control of a signal source, such as modulating the amplitude through absorption, modulating the phase through the refractive index, and through the rotation of the polarization plane. Modulate the polarization state, etc. It effectively utilizes the parallel, interconnected and high-speed processing capabilities of light, enabling it to spatially modulate light beams in real time, becoming the core component of real-time optical information processing systems, and playing an increasingly important role in the field of modern optics It is the basic component and key device in the research of optical and photoelectric hybrid systems for optical interconnection, optical correlation, optical computing, pattern recognition, optical control, optical detection, image processing, and display technology.
按SLM的调制方式,主要分成两类,一类是振幅调制型,一类是相位调制型。振幅调制型SLM是通过其透射率或反射率的变换调制入射光波的振幅,主要用于光开关、滤波和光学相关等应用。而SLM之所以在能够在光信息处理中愈来愈受到重视,应用越来越广,主要是因为它能够实现相位调制,它可以把图像的强度变化变成SLM的折射率变化,从而调制相位。这样无法直接用强度来表征的相位可以用一副强度图像的方式通过SLM实现相位调制的作用。因此,广泛的光信息处理系统,尤其是一些最新的技术,例如计算全息、衍射光学元件、光束整形、光镊等等都是使用相位型空间光调制。According to the modulation method of SLM, it is mainly divided into two types, one is the amplitude modulation type, and the other is the phase modulation type. Amplitude modulation SLM modulates the amplitude of the incident light wave through the transformation of its transmittance or reflectance, and is mainly used in optical switching, filtering and optical related applications. The reason why SLM has been paid more and more attention to in optical information processing and has been widely used is mainly because it can realize phase modulation. It can change the intensity change of the image into the refractive index change of SLM, thereby modulating the phase. . In this way, the phase that cannot be directly characterized by intensity can be used as an intensity image to achieve phase modulation through SLM. Therefore, a wide range of optical information processing systems, especially some of the latest technologies, such as computational holography, diffractive optical elements, beam shaping, optical tweezers, etc., use phase-type spatial light modulation.
空间光调制器通常是由单个分离像素组成,可以独立控制每个像素的变化,从而实现对光波面以像素为单位的调制。一副用于调制光波的数字图像写入SLM,这时SLM会显示这幅图像,入射光波经过SLM时,会受到空间调制。但由于每个像素周围均需电极包围,会在SLM上形成纵横交错的不透光区域,使得但该器件呈二维周期性栅格结构排列。光波经这种二维光栅式的结构发生衍射,会带来两个问题:中心亮斑和多级衍射像,尤其是中心亮斑,由于能量高度集中,其强度远高于其他衍射级。把这种现象称为空间光调制器的“黑栅效应”。A spatial light modulator is usually composed of a single discrete pixel, and the change of each pixel can be independently controlled, thereby realizing the modulation of the light wavefront in units of pixels. A digital image of the modulated light wave is written to the SLM, and the SLM displays the image. As the incident light wave passes through the SLM, it is spatially modulated. However, since each pixel needs to be surrounded by electrodes, criss-cross opaque regions will be formed on the SLM, so that the device is arranged in a two-dimensional periodic grid structure. Diffraction of light waves by this two-dimensional grating structure will cause two problems: the central bright spot and multi-level diffraction images, especially the central bright spot, whose intensity is much higher than other diffraction orders due to the high concentration of energy. This phenomenon is called the "black grid effect" of the spatial light modulator.
“黑栅效应”是SLM所固有的一种现象,即使在未向SLM写入任何图像时也会存在,而当写入调制图像时,则写入图像的衍射像的中心和中心亮斑重合,其高级衍射像的中心同样也会和空间光调制器的高级衍射亮斑重合,而使得在接收面上的图像的中心始终和空间光调制器固有的多级衍射亮斑耦合在一起无法分开,造成衍射像的质量严重下降,甚至无法使用。这种效应影响了空间光调制器的应用,为空间光调制器的应用带来极为不利的影响,使得空间光调制器在光信息处理中的应用受到了很大限制。The "black grid effect" is a phenomenon inherent in SLM, which exists even when no image is written to the SLM, and when a modulated image is written, the center of the diffraction image of the written image coincides with the central bright spot , the center of its advanced diffraction image will also coincide with the advanced diffraction bright spot of the spatial light modulator, so that the center of the image on the receiving surface is always coupled with the inherent multi-level diffraction bright spot of the spatial light modulator and cannot be separated , resulting in a serious decline in the quality of the diffraction image, or even unusable. This effect affects the application of the spatial light modulator, and brings extremely adverse effects to the application of the spatial light modulator, which greatly restricts the application of the spatial light modulator in optical information processing.
因此,寻求一种能够抑制黑栅效应的方法,对空间光调制器在光信息处理中的应用具有重要价值。Therefore, finding a method that can suppress the black grid effect is of great value for the application of spatial light modulators in optical information processing.
发明内容Contents of the invention
本发明的目的是提供一种能够消除相位型空间光调制器黑栅效应的方法,该方法提高了空间光调制器的相位调制性能。The purpose of the present invention is to provide a method capable of eliminating the black grid effect of a phase-type spatial light modulator, and the method improves the phase modulation performance of the spatial light modulator.
本发明是通过下述技术方案加以实现的:它通过在相位调制图上叠加数字菲涅耳透镜,利用菲涅耳透镜的聚集作用使得相位调制图的衍射像与黑栅的衍射像成像位置发生分离,以克服空间光调制器的黑栅效应,其特征在于包括以下过程:入射光波被起偏器,变换为线偏振光,该线偏振光入射到空间光调制器的受光面,空间光调制器工作在相位调制的状态下,空间光调制器后面放置一个检偏器,调整起偏器和检偏器的偏振轴的夹角角度,使空间光调制器工作在相位调制状态,且经检偏器透射出的光强不随相位调制度的大小而变化;在用于调制入射光波的相位调制图上叠加一个数字菲涅耳透镜,将叠加后的相位调制图写入空间光调制器,光波受菲涅耳透镜的折射,在菲涅耳透镜焦平面上能够接收到相位调制图所对应的衍射像,调节数字菲涅耳透镜的焦距以在不同位置接收观察到相位调制图衍射成像的结果;空间光调制器的二维周期性栅格结构并未受到数字运算的任何影响,在所加入的数字菲涅耳透镜的焦平面位置上,没有形成“黑栅效应”,黑栅衍射的光波传播到此,根据菲涅耳衍射的效应仅形成一个均匀的背景光,使得接收到的衍射像的背景较亮,从而使得相位调制图的衍射像与黑栅的衍射像在沿光轴的方向完全分开,以消除“黑栅效应”的影响,改善空间光调制器的调制性能。The present invention is achieved through the following technical scheme: it superimposes a digital Fresnel lens on the phase modulation diagram, and utilizes the converging effect of the Fresnel lens to make the imaging position of the diffraction image of the phase modulation diagram and the diffraction image of the black grid occur Separation, to overcome the black grid effect of the spatial light modulator, is characterized in that it includes the following process: the incident light wave is converted into linearly polarized light by the polarizer, and the linearly polarized light is incident on the light-receiving surface of the spatial light modulator, and the spatial light modulation When the optical modulator works in the state of phase modulation, an analyzer is placed behind the spatial light modulator, and the angle between the polarization axes of the polarizer and the analyzer is adjusted to make the spatial light modulator work in the state of phase modulation, and after the inspection The light intensity transmitted by the polarizer does not change with the degree of phase modulation; a digital Fresnel lens is superimposed on the phase modulation diagram used to modulate the incident light wave, and the superimposed phase modulation diagram is written into the spatial light modulator, and the light wave Refracted by the Fresnel lens, the diffraction image corresponding to the phase modulation pattern can be received on the focal plane of the Fresnel lens, and the focal length of the digital Fresnel lens can be adjusted to receive and observe the diffraction imaging results of the phase modulation pattern at different positions ; The two-dimensional periodic grid structure of the spatial light modulator is not affected by any digital operations, and at the focal plane position of the added digital Fresnel lens, no "black grid effect" is formed, and the light waves diffracted by the black grid Propagated here, according to the effect of Fresnel diffraction, only a uniform background light is formed, so that the background of the received diffraction image is brighter, so that the diffraction image of the phase modulation pattern and the diffraction image of the black grid are in the direction along the optical axis Completely separated to eliminate the influence of "black grid effect" and improve the modulation performance of the spatial light modulator.
在相位调制图写入SLM之前先在它上面叠加一个由计算机计算生成的数字菲涅耳透镜,在加入的菲涅耳透镜的焦平面上观察衍射成像的结果。由于菲涅耳透镜的会聚作用,相位调制图所对应的衍射像在其焦平面上成像,菲涅耳透镜对空间光调制器的“黑栅效应”没有影响,因此在其焦平面上“黑栅效应”,即中心亮斑和高级衍射像,尚未形成,黑栅带来的影响只是形成一个比较均匀的背景光,它仅会对相位图衍射像的比度产生影响,其带来的不利影响远小于中心亮斑。Before the phase modulation map is written into the SLM, a digital Fresnel lens generated by computer calculation is superimposed on it, and the result of diffraction imaging is observed on the focal plane of the added Fresnel lens. Due to the converging effect of the Fresnel lens, the diffraction image corresponding to the phase modulation pattern is imaged on its focal plane. Grid effect", that is, the central bright spot and high-level diffraction image have not yet formed. The effect of the black grid is only to form a relatively uniform background light, which will only affect the ratio of the diffraction image of the phase diagram. The disadvantages it brings The effect is much smaller than the central bright spot.
一种利用数字菲涅耳透镜消除相位型空间光调制器黑栅效应的装置,其特征在于:该装置由下列结构构成:光源(201)产生入射光波,入射光波经过第一透镜(202),第二透镜(203)后,入射光波被扩束成平行光束;平行光束经过起偏器(204)变换为线偏振光,该线偏振光经过透射式空间光调制器(205),该空间光调制器对该线偏振光执行相位调制,被调制的光束经过检偏器(206),入射到图像探测器(207),其成像被图像探测器(207)检测;中央处理器(208)接收输入的相位调制图并在相位调制图上叠加数字菲涅耳透镜,利用菲涅耳透镜的会聚作用使得相位调制图的衍射像与黑栅的衍射像成像位置分离,在被叠加了数字菲涅耳透镜的相位调制图的衍射像位置接收成像,黑栅衍射的光波传播在此位置,仅形成一个均匀的背景光,使得接收到的衍射像的背景较亮,从而消除“黑栅效应”的影响。A device for eliminating the black grid effect of a phase-type spatial light modulator by using a digital Fresnel lens, characterized in that the device is composed of the following structure: a light source (201) generates incident light waves, and the incident light waves pass through the first lens (202), After the second lens (203), the incident light wave is expanded into a parallel beam; the parallel beam is converted into linearly polarized light through a polarizer (204), and the linearly polarized light passes through a transmission type spatial light modulator (205), and the spatial light The modulator performs phase modulation on the linearly polarized light, and the modulated light beam passes through the analyzer (206) and is incident on the image detector (207), and its imaging is detected by the image detector (207); the central processing unit (208) receives The phase modulation image is input and a digital Fresnel lens is superimposed on the phase modulation image. Using the converging effect of the Fresnel lens, the diffraction image of the phase modulation image is separated from the imaging position of the diffraction image of the black grid, and the digital Fresnel lens is superimposed. The diffraction image position of the phase modulation pattern of the ear lens is received and imaged, and the light wave diffracted by the black grid propagates at this position, forming only a uniform background light, which makes the background of the received diffraction image brighter, thereby eliminating the "black grid effect" Influence.
而数字菲涅耳透镜对SLM的二维周期性的结构并没有任何影响,“黑栅效应”仍会存在,但“黑栅效应”是SLM固有二维周期结构的夫琅禾费衍射的结果,因此在菲涅耳透镜的焦平面上,“黑栅效应”还未形成,黑栅衍射的光波传播到在此位置仅仅会形成一个较为均匀的背景光,使得接收到的衍射像的背景比较亮,降低了图像对比度,但它所带来的不利影响远小于黑栅效应所带来的中心亮斑,因此这种方法很好的克服了空间光调制器的“黑栅效应”。The digital Fresnel lens has no effect on the two-dimensional periodic structure of the SLM, and the "black grid effect" will still exist, but the "black grid effect" is the result of Fraunhofer diffraction of the inherent two-dimensional periodic structure of the SLM , so on the focal plane of the Fresnel lens, the "black grid effect" has not yet formed, and the light waves diffracted by the black grid will only form a relatively uniform background light when it propagates to this position, making the background of the received diffraction image relatively Brightness reduces the image contrast, but its adverse effect is much smaller than the central bright spot caused by the black grid effect, so this method overcomes the "black grid effect" of the spatial light modulator very well.
上述的空间光调制器可以是透射式,也可以是反射式,如果是反射型SLM,可以通过一块分光棱镜,也使SLM工作在正入射的状态。正入射的工作状态不是必须条件,目的只是组成共轴的光学系统,便于调节。The above-mentioned spatial light modulator can be a transmission type or a reflection type. If it is a reflection type SLM, a beam splitting prism can be used to make the SLM work in the state of normal incidence. The working state of normal incidence is not a necessary condition, the purpose is only to form a coaxial optical system for easy adjustment.
上述的数字菲涅耳透镜由计算机产生,它可根据实际光学系统的设计需求很方便地改变焦距等光学参数,计算机生成后,直接叠加到相位调制图即可。The above-mentioned digital Fresnel lens is generated by a computer, which can easily change the optical parameters such as the focal length according to the design requirements of the actual optical system. After the computer is generated, it can be directly superimposed on the phase modulation diagram.
与已有的发明和技术相比,本发明具有以下优点:Compared with existing inventions and technologies, the present invention has the following advantages:
它通过在相位调制图上叠加数字菲涅耳透镜,利用菲涅耳透镜的聚集作用,使得相位调制图所对应的衍射成像与空间光调制器黑栅所对应的衍射像在光轴方向发生分立,从而消除了相位型空间光调制器的“黑栅效应”。该技术所用的数字菲涅耳透镜由计算机计算产生并叠加到相位调制图,此过程完全由计算机来自动完成,并且透镜的尺寸、焦距等光学参数都在数字域改变,通过设置由计算机完成,在消除了相位型空间光调制器的黑栅效应的同时,不用额外的傅里叶透镜或滤波器等装置,结构简单。It superposes a digital Fresnel lens on the phase modulation map, and utilizes the converging effect of the Fresnel lens to separate the diffraction image corresponding to the phase modulation map from the diffraction image corresponding to the black grid of the spatial light modulator in the direction of the optical axis. , thereby eliminating the "black grid effect" of the phase-type spatial light modulator. The digital Fresnel lens used in this technology is generated by computer calculation and superimposed on the phase modulation map. This process is completely automatically completed by the computer, and the optical parameters such as the size and focal length of the lens are changed in the digital domain, and are completed by the computer through setting. While eliminating the black grid effect of the phase-type spatial light modulator, no additional devices such as Fourier lenses or filters are needed, and the structure is simple.
附图说明Description of drawings
图1空间光调制器物理结构示意图。其中白色区域表示透光部分,黑色区域表示不透光部分。Fig. 1 Schematic diagram of the physical structure of the spatial light modulator. The white area represents the transparent part, and the black area represents the opaque part.
图2数字菲涅耳消除黑栅效应的原理示意图。Figure 2 is a schematic diagram of the principle of digital Fresnel eliminating black grid effect.
图3空间光调制器菲涅耳衍射和夫琅禾费衍射强度对比曲线。其中虚线为菲涅耳衍射强度曲线,实线为夫琅禾费衍射强度曲线。Fig. 3 Comparison curve of Fresnel diffraction and Fraunhofer diffraction intensity of spatial light modulator. The dotted line is the Fresnel diffraction intensity curve, and the solid line is the Fraunhofer diffraction intensity curve.
图4是透射式空间光调制器工作原理示意图。Fig. 4 is a schematic diagram of the working principle of the transmissive spatial light modulator.
图5是反射式空间光调制器工作原理示意图。Fig. 5 is a schematic diagram of the working principle of the reflective spatial light modulator.
图6是空间光调制器加入数字菲涅耳透镜前的衍射成像结果。Figure 6 is the diffraction imaging result before the spatial light modulator is added to the digital Fresnel lens.
图7是空间光调制器加入数字菲涅耳透镜后的衍射成像结果。Fig. 7 is the diffraction imaging result after the spatial light modulator is added with a digital Fresnel lens.
图8是空间光调制器在加入不同焦距的数字菲涅耳透镜后的实施例。Fig. 8 is an embodiment of a spatial light modulator after digital Fresnel lenses with different focal lengths are added.
图中101为初始的相位调制图,102为理想情况下所对应的衍射像,103为在空间光调制器黑栅效应影响下的衍射像,104为数字菲涅耳透镜,105为加入数字菲涅耳透镜后的衍射像。101 in the figure is the initial phase modulation diagram, 102 is the corresponding diffraction image under ideal conditions, 103 is the diffraction image under the influence of the black grid effect of the spatial light modulator, 104 is the digital Fresnel lens, and 105 is the digital Fresnel lens. Diffraction image behind a Neel lens.
图中201为光源,202为透镜一,203为透镜二,204为起偏器,205为透射式空间光调制器,206为检偏器,207为图像探测器,208为中央处理器,209为分光棱镜,210为反射式空间光调制器。In the figure, 201 is a light source, 202 is lens one, 203 is lens two, 204 is a polarizer, 205 is a transmission spatial light modulator, 206 is an analyzer, 207 is an image detector, 208 is a central processing unit, 209 is a beam splitting prism, and 210 is a reflective spatial light modulator.
具体实施方式Detailed ways
下面结合附图对技术方案的实施作进一步的详细描述。The implementation of the technical solution will be further described in detail below in conjunction with the accompanying drawings.
附图1是空间光调制器的物理结构示意图,白色区域表示透光部分,也代表了每个像素;黑色区域表示不透光部分,这种固有的物理结构使得SLM呈现一种二维栅格的结构。SLM的黑栅效应的影响主要在夫琅禾费衍射区,因此黑栅效应可以看作是二维栅格的夫琅禾费衍射,其在透镜焦距的傅里叶频谱面上的夫琅禾费衍射场可以看作是矩孔阵列的傅里叶变换。Attached Figure 1 is a schematic diagram of the physical structure of the spatial light modulator. The white area represents the light-transmitting part, which also represents each pixel; the black area represents the opaque part. This inherent physical structure makes the SLM present a two-dimensional grid Structure. The influence of the black grid effect of SLM is mainly in the Fraunhofer diffraction area, so the black grid effect can be regarded as the Fraunhofer diffraction of the two-dimensional grid, and its Fraunhofer diffraction on the Fourier spectrum plane of the focal length of the lens The Fei diffraction field can be regarded as the Fourier transform of the rectangular hole array.
定义SLM上的行方向坐标为fx,列方向坐标为fy,w为SLM的像素中心之间间距,w0为SLM的像素开口宽度,每个像素的透射率函数为整个M行N列的SLM的透射率函数为黑栅的透射率函数T(fx,fy):Define the row direction coordinates on the SLM as f x , the column direction coordinates as f y , w is the distance between the pixel centers of the SLM, w 0 is the pixel opening width of the SLM, and the transmittance function of each pixel is The transmittance function of the SLM with M rows and N columns is The transmittance function T(f x , f y ) of the black grid:
在平面波U0(fx,fy)照射下,平面波和黑栅透射率函数相乘的复振幅分布为:Under the irradiation of plane wave U 0 (f x , f y ), the complex amplitude distribution obtained by multiplying the plane wave and black grid transmittance function is:
在傅里叶频谱面上的夫琅禾费衍射图样为:The Fraunhofer diffraction pattern on the Fourier spectrum plane is:
其中,in,
通过公式(1)-(4)的理论推导,SLM上的二维栅格对入射平面波在傅里叶频谱面上的夫琅禾费衍射图样有卷积调制。这是由SLM物理结构决定,不论输入任何信息,这种效应都会存在。但既然黑栅效应是夫琅禾费衍射的结果,如果在近场观察或接收,则相位调制图和黑栅的衍射像多以菲涅耳衍射的形式传播了一段距离,此时如果在初始的相位调制图中叠加一个数字菲涅耳透镜,在其焦平面上会观察到相位调制图所对应的衍射像,而此时黑栅效应尚未形成。因此,一种利用数字菲涅耳透镜来消除SLM黑栅效应的办法其原理图如附图2所示。Through the theoretical derivation of formulas (1)-(4), the two-dimensional grid on the SLM has convolution modulation on the Fraunhofer diffraction pattern of the incident plane wave on the Fourier spectrum plane. This is determined by the physical structure of the SLM, regardless of any input information, this effect will exist. But since the black grid effect is the result of Fraunhofer diffraction, if it is observed or received in the near field, the phase modulation pattern and the diffraction image of the black grid will propagate for a certain distance in the form of Fresnel diffraction. A digital Fresnel lens is superimposed on the phase modulation diagram of the phase modulation diagram, and the diffraction image corresponding to the phase modulation diagram will be observed on its focal plane, and the black grid effect has not yet formed at this time. Therefore, a schematic diagram of a method for eliminating the SLM black grid effect by using a digital Fresnel lens is shown in Figure 2.
附图2是通过相位补偿来消除黑栅效应的原理示意图。相位调制图101在理想情况下所对应的衍射成像结果如102所示,当把相位调制图101写入空间光调制器205、210后,由于空间光调制器205、210的黑栅效应,此时衍射成像的结果为103所示,如图,黑栅效应造成了中心亮斑和高级衍射像与有效光信息耦合在一起无法分开。本发明在相位调制图101写入空间光调制器205、210之前,首先叠加一个数字菲涅耳透镜104,这时衍射像在透镜的后焦面上场分布为U′f (x,y)为:Figure 2 is a schematic diagram of the principle of eliminating the black grid effect through phase compensation. The diffraction imaging result corresponding to the phase modulation diagram 101 under ideal conditions is shown in 102. After the phase modulation diagram 101 is written into the spatial
其中U(x1,y1)为输入的相位调制图,(x1,y1)为输入面坐标,(x,y)为输出面坐标,λ为波长,f为焦距。则由(5)式可知,在焦平面的场分布是入射到菲涅耳透镜上的场的夫琅禾费衍射图样。此时黑栅在菲涅耳透镜的后焦面上场分布为Uf″(x,y):Where U(x 1 , y 1 ) is the input phase modulation map, (x 1 , y 1 ) is the input plane coordinates, (x, y) is the output plane coordinates, λ is the wavelength, and f is the focal length. It can be seen from formula (5) that the field distribution at the focal plane is the Fraunhofer diffraction pattern of the field incident on the Fresnel lens. At this time, the field distribution of the black grid on the back focal plane of the Fresnel lens is U f ″(x, y):
其中U′(x1,y1)为空间光调制器的函数,由(6)式可知,黑栅在菲涅耳透镜的焦平面的场分布为其菲涅耳衍射图样。如附图3所示,计算模拟的空间光调制器在菲涅耳透镜的焦平面上的菲涅耳衍射强度和其夫琅和费衍射强度分别表示为虚线和实线,显示在同一坐标系下。从附图3可以看到菲涅耳透镜的焦平面位置的衍射场,其强度远远小于夫琅禾费衍射的强度,并可近似的认为其光强分布是均匀的,为观察或接收面上的一个背景光强,此时的效果如附图2中105所示,接收到的图像背景较亮,但不会出现严重的中心亮斑问题。Where U′(x 1 , y 1 ) is a function of the spatial light modulator, and it can be known from formula (6) that the field distribution of the black grid at the focal plane of the Fresnel lens is its Fresnel diffraction pattern. As shown in Figure 3, the Fresnel diffraction intensity and its Fraunhofer diffraction intensity of the simulated spatial light modulator on the focal plane of the Fresnel lens are represented as dashed and solid lines, respectively, and are displayed in the same coordinate system Down. From the accompanying drawing 3, it can be seen that the intensity of the diffraction field at the focal plane of the Fresnel lens is much smaller than that of Fraunhofer diffraction, and it can be approximated that the light intensity distribution is uniform, which is the observation or receiving surface. A background light intensity on the above, the effect at this time is shown as 105 in the accompanying drawing 2, the background of the received image is brighter, but there will be no serious central bright spot problem.
所加入的数字菲涅耳透镜的相位因子为exp[jπ/λf(x1 2+y1 2)],把它作为相位补偿模板104,与原来的相位调制图101所对应的相位因子相乘,叠加后的结果105写入SLM。其中透镜焦距的取值范围取决于两个要素:一是焦距大小要确保空间光调制器的二维栅格的衍射满足菲涅耳衍射的近似条件,即不能因焦距太大而接近夫琅禾费远场衍射的距离;二是抽样对菲涅尔透镜最小焦距的限制,因为该相位因子的最大空间频率随焦距的减小而增大,焦距太小,会导致不满足抽样定理。The phase factor of the added digital Fresnel lens is exp[jπ/λf(x 1 2 +y 1 2 )], which is used as the
首先从传播距离考虑。当菲涅耳衍射成立时,f需满足:First consider the propagation distance. When Fresnel diffraction is established, f needs to satisfy:
夫琅和费衍射近似条件为:The approximate conditions for Fraunhofer diffraction are:
则焦距取值范围要在fn与fh之间。从满足抽样定理考虑,设空间光调制器抽样间隔(或像素大小)为d,像素数为N×N,则菲涅耳透镜函数在空间光调制器上的最大空间频率为:Then the value range of the focal length should be between f n and f h . From the consideration of satisfying the sampling theorem, if the sampling interval (or pixel size) of the spatial light modulator is d and the number of pixels is N×N, then the maximum spatial frequency of the Fresnel lens function on the spatial light modulator is:
根据抽样定理,抽样间隔需小于抽样函数的最大空间频率的倒数,即d≤1/εmax,由此得出对透镜焦距的限制条件为:According to the sampling theorem, the sampling interval must be smaller than the reciprocal of the maximum spatial frequency of the sampling function, that is, d≤1/ε max , thus the restriction on the focal length of the lens is obtained as follows:
因此,数字菲涅耳透镜焦距最大值要小于fh,最小值则要选取fn和fc中的较大者。Therefore, the maximum value of the focal length of the digital Fresnel lens should be less than f h , and the minimum value should be selected from the larger of f n and f c .
按照上述方法,实现相位型空间光调制器的黑栅效应抑制的系统主要包括光源、空间光调制器、起偏器和检偏器、空间滤波器、图像探测器和中央处理器。所说的光源可以是相干光源或部分相干光源;所说的空间光调制器可以是反射式,也可以是透射式;所说的起偏器和检偏器是一对偏振器件,主要作用是保证空间光调制器的相位调制状态;所说的图像探测器可以是CCD器件或CMOS器件;所说的中央处理器是数字信号处理器与可编程专用集成电路的组合,可以是计算机,也可以是嵌入式系统。其特征在于,中央处理器计算产生的相位调制图叠加数字菲涅耳透镜后写入空间光调制器,在入射光波的照射下发生衍射,在数字菲涅耳透镜所对应的焦平面上用图像探测器接收,再经中央处理器存储和显示成像结果。According to the above method, the system for realizing the suppression of the black grid effect of the phase-type spatial light modulator mainly includes a light source, a spatial light modulator, a polarizer and an analyzer, a spatial filter, an image detector and a central processing unit. Said light source can be a coherent light source or a partially coherent light source; said spatial light modulator can be reflective or transmissive; said polarizer and analyzer are a pair of polarizing devices, the main function of which is Ensure the phase modulation state of the spatial light modulator; the image detector can be a CCD device or a CMOS device; the central processing unit is a combination of a digital signal processor and a programmable application-specific integrated circuit, which can be a computer or a is an embedded system. It is characterized in that the phase modulation diagram generated by the calculation of the central processing unit is superimposed on the digital Fresnel lens and written into the spatial light modulator, and diffraction occurs under the irradiation of the incident light wave, and the image is displayed on the focal plane corresponding to the digital Fresnel lens The detector receives, and then the central processing unit stores and displays the imaging results.
下面结合附图对实施例作进一步的描述。The embodiment will be further described below in conjunction with the accompanying drawings.
附图4是透射式空间光调制器工作原理示意图。如图所示,光源201输出的光波经202透镜一和203透镜二组成的扩束准直系统扩束准直,通过起偏器204使得光波以线偏光入射到透射式空间光调制器205,光波经过透射式空间光调制205后,再经过一片检偏器206,起偏器204和检偏器206的作用是确保入射光波为线偏光,并且在线偏光入射的情况下透射式空间光调制器205工作在相位调制状态。中央处理器208计算产生或者把已存储的相位调制图叠加数字菲涅耳透镜后写入透射式空间光调制器205,并接收和显示由图像探测器207传输过来的衍射成像结果。Figure 4 is a schematic diagram of the working principle of the transmissive spatial light modulator. As shown in the figure, the light wave output by the
附图5是反射式空间光调制器工作原理示意图。如图所示,光源201输出的光波经202透镜一和203透镜二组成的扩束准直系统扩束准直,通过起偏器204使得光波以线偏光入射到分光棱镜209后被分成两束,其中透射光束经反射式空间光调制器210调制反射后,再经过分光棱镜209反射,入射到检偏器206。中央处理器208计算产生或者把已存储的相位调制图叠加数字菲涅耳透镜后写入反射式空间光调制器210,并接收和显示由图像探测器207传输过来的衍射成像结果。Figure 5 is a schematic diagram of the working principle of the reflective spatial light modulator. As shown in the figure, the light wave output by the
附图6是空间光调制器加入数字菲涅耳透镜前的衍射成像结果。其中(a)是原始图像,(b)是计算得到的(a)图相应的衍射光学元件相位图,(c)是(b)图直接写入SLM后,衍射成像的结果,由于黑栅效应的存在,很明显可看到中心亮斑。Figure 6 is the diffraction imaging result before the spatial light modulator is added to the digital Fresnel lens. Where (a) is the original image, (b) is the phase diagram of the diffractive optical element corresponding to the calculated image (a), (c) is the result of diffraction imaging after the image (b) is directly written into the SLM, due to the black grid effect The presence of a central bright spot is clearly visible.
附图7是空间光调制器加入数字菲涅耳透镜后的衍射成像结果。其中(a)是对附图6(b)叠加数字菲涅耳透镜后的图像,(b)是(a)图写入SLM后衍射成像的结果,很明显中心亮斑已经消除,黑栅的影响仅仅是使得图像的背景较亮,和附图6(c)相比,显著改善了空间光调制器的调制性能。Accompanying drawing 7 is the diffraction imaging result after the spatial light modulator is added with a digital Fresnel lens. Among them, (a) is the image after superimposing the digital Fresnel lens on the accompanying drawing 6 (b), (b) is the result of diffraction imaging after the image in (a) is written into the SLM, it is obvious that the bright spot in the center has been eliminated, and the black grid The effect is only to make the background of the image brighter, and compared with Fig. 6(c), the modulation performance of the spatial light modulator is significantly improved.
附图8是空间光调制器加入不同焦距的数字菲涅耳透镜后的结果。加入的数字菲涅耳透镜大小为512×512像素,像素间隔d为19μm,激光器的光波长为532nm,根据公式(7)~(10),计算得焦距f取值范围在0.2~45m之间,这是理论计算的适用范围,结合实际使用情况,典型的取值一般只要满足大于最小取值,并方便观察或成像即可,焦距太大,随着衍射距离的变大会使“黑栅效应”逐渐增强,影响成像质量。实施例中取焦距f分别为300mm、350mm、400mm、450mm、500mm,分别对应图(a)、(b)、(c)、(d)、(e)。在其焦平面上观察再现像,从图中看出,由于在本发明中不使用傅里叶透镜,所以黑栅现象只能在夫琅禾费远场衍射形成,对不同焦距的数字菲涅耳透镜,相位调制图所对应的成像结果均很清晰。Figure 8 is the result of adding digital Fresnel lenses with different focal lengths to the spatial light modulator. The size of the added digital Fresnel lens is 512×512 pixels, the pixel interval d is 19 μm, and the light wavelength of the laser is 532nm. According to formulas (7) to (10), the calculated focal length f ranges from 0.2 to 45m , this is the scope of application of theoretical calculations. Combined with actual use, the typical value generally needs to be greater than the minimum value, and it is convenient for observation or imaging. If the focal length is too large, the "black grid effect" will increase as the diffraction distance increases. "Gradually enhanced, affecting image quality. In the embodiment, the focal lengths f are taken as 300mm, 350mm, 400mm, 450mm, and 500mm respectively, corresponding to figures (a), (b), (c), (d), and (e) respectively. Observing the reproduced image on its focal plane, it can be seen from the figure that since the Fourier lens is not used in the present invention, the black grid phenomenon can only be formed by Fraunhofer far-field diffraction, for digital Fresnel with different focal lengths The imaging results corresponding to the ear lens and the phase modulation diagram are very clear.
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