CN114217454B - Design and implementation method of spatial frequency spectrum modulation device based on diffraction optical element - Google Patents
Design and implementation method of spatial frequency spectrum modulation device based on diffraction optical element Download PDFInfo
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Abstract
The invention discloses a design and implementation method of a spatial frequency spectrum modulation device based on a diffractive optical element, and belongs to the field of optical devices. The method comprises the following steps: processing a light-suppressing region on a substrate device of a diffraction optical element to obtain a spatial filter; determining the phase distribution of the diffraction optical element, and converting the phase distribution into the structural parameters of each diffraction unit; and processing the spatial filter according to the structural parameters of each diffraction unit to obtain the spatial frequency spectrum modulation device. The invention uses the diffraction optical element with mature processing technology as the main optical field modulation device; before the diffraction optical element is processed, a region with the distribution characteristics of light-tight property, light-gradual change, light transmission property and the like with specific design parameters is processed in the center of a substrate device through a coating process, so that the space spectrum modulation device provided by the invention has the capability of carrying out complex modulation on a frequency domain light field, and simultaneously has the advantages of customization and zero power consumption.
Description
Technical Field
The invention belongs to the field of optical devices, and particularly relates to a design and implementation method of a spatial frequency spectrum modulation device based on a diffractive optical element.
Background
The spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum of light, can conveniently and accurately analyze and modulate the property of an optical system by modulating the spatial frequency spectrum, and is widely applied to the fields of imaging, optical analog calculation and the like.
The existing spatial spectrum modulation device mainly comprises a spatial light modulator, a traditional optical device, a super-surface and other emerging integrated optical elements.
Spatial light modulators are most commonly liquid crystal spatial light modulators and digital micromirror arrays, which can generally control the distribution and properties of modulation unit arrays on the devices through a computer, so as to modulate the phase or amplitude of a frequency domain light field, and can realize flexible modulation effect in a very convenient and efficient manner.
The traditional optical device is mainly a fixed device, the main problem is that the device is difficult to flexibly adjust and change according to the requirement, and the structure and the working characteristics of the traditional optical device cause the volume of a system using the device to be huge.
The super surface is a novel emerging planar optical device with ultrahigh integration level in recent years, and compared with a spatial light modulator and a traditional optical device, the super surface is extremely high in design flexibility and higher in modulation freedom degree, but the super surface is higher in design difficulty, large in processing technology difficulty and high in cost at present, and large-scale practical application is difficult to realize.
Meanwhile, because the optical field has amplitude and phase characteristics, the existing spatial spectrum modulation devices often modulate single amplitude or phase, and are difficult to simultaneously perform complex modulation on the optical field (simultaneously modulate amplitude and phase), although complex modulation functions can be realized to a certain extent by combining various modulation devices through optical design, the complexity of the whole optical system can be improved, and the final efficiency is greatly reduced.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a design and implementation method of a space spectrum modulation device based on a diffractive optical element, and provides the space spectrum modulation device and an edge detection device by taking the implementation of optical edge detection as an implementation case, aiming at solving the problems that the existing space spectrum modulation device is difficult to give consideration to flexibility, high efficiency, low energy consumption and even zero energy consumption, and the novel optical element represented by a super surface is difficult to design and high in processing cost.
In order to achieve the above object, in a first aspect, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including: processing a light-suppressing region on a substrate device of a diffraction optical element to obtain a spatial filter; determining the phase distribution of the diffraction optical element, and converting the phase distribution into the structural parameters of each diffraction unit; and processing the spatial filter according to the structural parameters of each diffraction unit to obtain the spatial frequency spectrum modulation device.
Furthermore, the light suppression area is an opaque area or an area with light transmittance gradually increased from the center to the periphery in the radial direction.
Further, the determining the phase distribution of the diffractive optical element comprises: the phase distribution of orbital angular momentum with a topological charge value of ± 1 is taken as the phase distribution of the diffractive optical element.
Further, the determining the phase distribution of the diffractive optical element comprises: the phase distribution of orbital angular momentum with a topological charge value of +/-1 is superposed with the phase distribution with a focusing function, and the superposed phase distribution is used as the phase distribution of the diffraction optical element.
In a second aspect, the present invention provides a spatial spectrum modulation device based on a diffractive optical element, obtained by using the method according to the first aspect.
In a third aspect, the present invention provides an edge detection apparatus based on a diffractive optical element, comprising: the spatial spectrum modulation device is positioned on a back focal plane of the first lens and a front focal plane of the second lens; the first lens is used for transforming the light field to be processed on the front focal plane of the first lens into a frequency domain space and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the first lens; the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution; the second lens is used for converting the modulated spatial frequency spectrum distribution into a spatial domain and obtaining the light field distribution after edge detection on a back focal plane of the second lens.
In a fourth aspect, the present invention provides an edge detection apparatus based on a diffractive optical element, comprising: the spatial spectrum modulation device is positioned on a back focal plane of the lens; the lens is used for transforming the light field to be processed on the front focal plane of the lens into a frequency domain space, and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the lens; the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution and obtaining the light field distribution after edge detection on the focal plane of the spatial frequency spectrum modulation device.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) compared with the existing spatial frequency spectrum modulation device, the invention uses the diffractive optical element with mature processing technology as the main optical field modulation device; before the diffractive optical element is processed, an opaque or gradually-changing transparent area and other areas with specific design distribution are processed on a substrate device through a film coating process, and the areas and the phase distribution provided by the diffractive optical element act together, so that the final spatial frequency spectrum modulation device has the capability of carrying out complex modulation on an optical field, and the characteristic enables the frequency spectrum modulation device realized based on the scheme to have higher theoretical upper limit in the aspect of optical field modulation;
(2) Compared with the traditional method for carrying out complex modulation on the optical field, the method has smaller limitation on the specific design and implementation, can theoretically realize the amplitude and phase distribution of various designs, and simultaneously has higher integration level of the final device, thereby avoiding the problems of large volume, low efficiency and the like caused by the cascade connection of a plurality of optical elements for carrying out complex modulation to a certain extent;
(3) compared with the existing spatial frequency spectrum modulation device, the spatial frequency spectrum modulation device provided by the invention has the characteristics of flexibility of an electric control active spatial light modulator such as a liquid crystal spatial light modulator and the like and low power consumption and even zero power consumption of traditional optical elements such as a spiral phase plate and the like;
(4) the device and the system for optical edge detection, which are provided by the invention, utilize the characteristic of large phase degree of freedom of the diffractive optical element, and superpose phase distribution with a specific focal length and focusing capacity on the basis of modulating a generated spiral phase optical field. The focusing phase is equivalent to an ideal thin lens without any loss of amplitude, and is effectively completely equivalent to the second fourier lens of the optical 4f system, functioning to perform a two-dimensional fourier transform on the optical field. Therefore, the spatial frequency spectrum modulation device provided by the invention only needs to work under an optical 3f system, and the defect of poor integration level of the optical 4f system is improved to a certain extent;
(5) Compared with the scheme of processing a lighttight area in the center of a substrate device of a diffractive optical element, the invention also provides the scheme of processing an area with radially and gradually increased light transmittance from the center to the periphery in the center of the substrate device of the diffractive optical element, so that the amplitude distribution and the phase distribution of the obtained spatial frequency spectrum modulation device meet the distribution required by the differential property of standard Fourier transform and are theoretically equivalent to a standard first-order optical differentiator, and the spatial frequency spectrum modulation device has the capability of realizing the theoretical optimal effect obtained by differential calculation in the aspect of edge detection;
(6) compared with the scheme of using the super surface as a modulation device, the edge detection device based on the diffractive optical element has the advantages that the design difficulty and the processing cost are far lower than those of the super surface, and the edge detection device based on the diffractive optical element is more advantageous for practical application. And the efficiency of the edge detection device based on the diffractive optical element can reach more than 90% generally, which is obviously higher than other optical edge detection schemes such as surface plasma polaritons, spin hall effect and the like.
Drawings
Fig. 1 is a schematic flowchart of a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a spiral phase field distribution with a topological charge value of 1 according to an embodiment of the present invention.
Fig. 3 is a schematic flow chart of a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element according to a second embodiment of the present invention.
Fig. 4 is a schematic flow chart of a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element according to a third embodiment of the present invention.
Fig. 5 is a schematic diagram of the phase field distribution after superimposing the focusing phase on the spiral phase field with a topological charge value of 1 according to the third embodiment of the present invention.
Fig. 6 is an amplitude distribution diagram of a high-pass spiral phase device according to a third embodiment of the present invention.
Fig. 7 is a schematic phase distribution diagram of a high-pass spiral phase device according to a third embodiment of the present invention.
Fig. 8 is a schematic flow chart of a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element according to a fourth embodiment of the present invention.
Fig. 9 is an amplitude distribution diagram of a first-order optical differentiator according to a fourth embodiment of the present invention.
Fig. 10 is a schematic diagram of a phase distribution of a first-order optical differentiator according to a fourth embodiment of the present invention.
Fig. 11 is a schematic flowchart of a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element according to a fifth embodiment of the present invention.
Fig. 12 is a schematic structural diagram of an edge detection apparatus according to a sixth embodiment of the present invention.
Fig. 13 is a schematic structural diagram of an edge detection apparatus according to a seventh embodiment of the present invention.
Fig. 14 is an input image provided by the eighth embodiment of the present invention.
Fig. 15 is a schematic diagram of a simulation result according to an eighth embodiment of the present invention.
Fig. 16 is an image to be detected according to an eighth embodiment of the present invention.
Fig. 17 is a schematic diagram of an edge detection result according to an eighth embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
In the present application, the terms "first," "second," and the like (if any) in the description and the drawings are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order.
Diffraction Optical Elements (DOE) are a kind of ultra-light and ultra-thin Optical devices designed based on Optical diffraction effect, and can realize high-precision Optical field modulation. Compared with a spiral phase plate and a spatial light modulator, the spiral phase plate and the spatial light modulator are high in design freedom degree, zero in power consumption and small in integrated volume, can be very conveniently inserted into an optical system, compared with a super surface, the design and processing difficulty of a diffraction optical element is far lower than that of the super surface, and compared with the super surface, the spiral phase plate and the spatial light modulator have very obvious advantages in large-scale application, and most of devices realized based on the diffraction optical element are polarization-independent devices.
The first embodiment is as follows:
as shown in fig. 1, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including:
s101, processing a light suppression region on a substrate device of a diffractive optical element to obtain a spatial filter;
in this embodiment, the substrate device may be a common silicon dioxide material substrate, an ITO conductive glass substrate, and other conventional transparent substrates. The light suppression region can be an opaque region or a gradual change light transmission region, when the opaque region or the gradual change light transmission region is processed on the substrate through a coating process, compared with the original transparent substrate, due to the influence of material action, the light field can be limited to a certain extent when passing through the regions, namely, the light transmittance of the processed part can be suppressed (the final suppression effect depends on the design target), so that the light suppression region is called, and the whole device plays a role of a spatial filter in terms of the effect of the device.
It should be noted that, according to the design objective, the light-suppressing region may be any shape, such as a circle, a square, a hexagon, etc., wherein, the circle as the most basic isotropic shape easily satisfies the modulation requirement of the isotropic device; and can be processed at any location on the device substrate. Preferably, in this embodiment, the circular opaque region is disposed at a central position of the device substrate.
Illustratively, when the spatial frequency spectrum modulation device is applied to the field of edge detection, the amplitude distribution of the spatial frequency spectrum modulation device requires that an opaque region or a region with gradually increased light transmittance from the center to the periphery is arranged at the center of a device substrate, the region can be processed by using a coating process by using any opaque material theoretically, the size of the region depends on the actual size of the whole optical system in an application scene, and in principle, the larger the size of a dark region is, the better the edge detection effect is, but the device efficiency is reduced.
After the step is completed, the obtained device may be referred to as a spatial filter according to its characteristic features, and further, since the central light-suppressing region plays a role of suppressing low-frequency components in the frequency domain, high-frequency components may normally pass through, and thus may be specifically referred to as a high-pass filter.
S102, determining the phase distribution of the diffraction optical element, and converting the phase distribution into the structural parameters of each diffraction unit;
in this embodiment, the phase distribution of the diffractive optical element is determined according to actual design requirements, for example, when the spatial spectrum modulation device is applied to the edge detection field, taking this embodiment as an example, the diffraction unit distribution of the diffractive optical element needs to be designed to generate a spiral phase optical field; when the spatial spectrum modulation device is applied to off-axis imaging, the distribution of diffraction units of the diffractive optical element needs to be designed to generate a phase light field with off-axis focusing capability; when the spatial spectrum modulation device is applied to multi-focus imaging, the distribution of diffraction units of the diffractive optical element needs to be designed into a phase light field with a plurality of focus focusing capacities; it should be noted that, according to the design features proposed in the present invention, the phase distribution of the diffractive optical element should be determined in combination with the transmittance distribution of the spatial filter, taking into account the complex amplitude modulation capability brought by the combination.
After the phase distribution of the diffractive optical element is determined, the phase distribution of the diffractive optical element is converted into the structural parameters of each diffraction unit according to the process requirements of actual processing.
Illustratively, taking as an example that the diffraction cell distribution of the diffractive optical element is designed to generate a spiral phase optical field, a phase distribution of orbital angular momentum with a topological charge value of ± 1 may be taken as the phase distribution of the diffractive optical element.
The phase distribution form of the spiral phase comes from the optical first-order radial Hilbert transform theory, and when the frequency domain modulation function meets the distribution required by the theory, the phase distribution form of the spatial spectrum modulation device is just the form of the spiral phase with a topological charge value of +/-1. The diffractive optical element with helical phase further modulates the optical field such that any 2-point symmetric about radial direction in the frequency domain will have the same phase after modulation
A helical phase distribution of this nature exactly coincides with an orbital angular momentum beam with a topological charge value of ± 1), which causes the final transformation back into the optical field of the spatial domain, the non-edge portions of which cause destructive interference, the edge portions of which, due to the presence of amplitude or phase gradients, impose a phase difference that is not critical for the spectral modulator
This results in final destructive interference imperfection, which ultimately has the effect of emphasizing the edges. A spiral phase field profile with a topological charge value of + -1 is shown in FIG. 2.
And S103, processing the spatial filter according to the structural parameters of the diffraction units to obtain a spatial frequency spectrum modulation device.
In this embodiment, the diffractive optical element is directly processed by a gray scale exposure technique. The technology can process the diffraction unit structure array only by the gray level images with specific distribution, and compared with the steps of etching and the like in the traditional photoetching process, the gray level exposure technology has higher flexibility and efficiency.
The second embodiment:
as shown in fig. 3, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including:
s201, processing a light-tight area on a substrate device of the diffraction optical element to obtain a spatial filter;
s202, taking the phase distribution of orbital angular momentum with the topological charge value of +/-1 as the phase distribution of the diffraction optical element; converting the phase distribution of the diffraction optical element into the structural parameters of each diffraction unit;
and S203, processing the spatial filter according to the structural parameters of the diffraction units to obtain the spatial frequency spectrum modulation device.
Based on this embodiment, the obtained spatial spectrum modulator may be referred to as a high-pass spiral spatial spectrum modulator according to its structural features and properties.
Example three:
as shown in fig. 4, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including:
s301, processing a light-tight area on a substrate device of the diffractive optical element to obtain a spatial filter;
s302, superposing the phase distribution of orbital angular momentum with the topological charge value of +/-1 with the phase distribution with the focusing function, and taking the superposed phase distribution as the phase distribution of the diffraction optical element; converting the phase distribution of the diffraction optical element into the structural parameters of each diffraction unit;
and S303, processing the spatial filter according to the structural parameters of each diffraction unit to obtain a spatial frequency spectrum modulation device.
The difference between the present embodiment and the second embodiment is that, in the present embodiment, the device phase distribution of the diffractive optical element is superimposed with a phase distribution with a specific focal length and focusing capability on the basis of generating the spiral phase optical field by modulation. This focusing phase is equivalent to an ideal thin lens without any loss of amplitude, and is effectively completely equivalent to the second fourier lens of the optical 4f system, functioning to perform a two-dimensional fourier transform on the optical field. The phase field distribution after superimposing the focusing phase on the spiral phase field with a topological charge value of + -1 is shown in FIG. 5.
The equivalence and effectiveness of the superimposed focusing phases are specifically described below with reference to the calculation formulas. According to an ideal lens imaging formula, the design method is equivalent to placing an ideal thin lens close to a space spectrum modulation device, and recording the distribution of a light field which is just modulated by the space spectrum modulation device as
Then, on the focal plane of the lens, an output field distribution is obtained as
In whichx、yRepresenting the coordinates of the position in space,Rrepresents the radius value of the clear aperture of the system,
the circular function is expressed, the transmittance of the round hole with the radius R on the infinite opaque screen is described, the transmittance of the round hole with the radius R is taken as 1, and the circular opaque screen has no light intensity loss,findicating the frequency of the optical wave,
which represents the wavelength of the light wave,ithe number of the units of the imaginary number is expressed,kwhich is a representation of the spatial wave vector,
andvrespectively, two-dimensional spatial frequency components of the light field, and x and y respectively, two-dimensional spatial position coordinates of the light field. The output field distribution obtained in the above case for a standard optical 4f system is as follows
It can be seen that the two distributions have only one phase term difference, so that the intensity distribution characteristics of the final light field are not affected.
Based on this embodiment, after the step is completed, the obtained final spatial spectrum modulation device may be referred to as a high-pass spiral spatial spectrum modulation device with focusing capability according to the structural features and properties thereof, and the schematic diagrams of the amplitude distribution and the phase distribution thereof are shown in fig. 6 and fig. 7. It should be noted that the scale data in fig. 6 and 7 are provided only for the sake of example, and do not represent the actual size of the device proposed in the present invention in real cases, and the actual size may be designed as needed.
Example four:
as shown in fig. 8, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including:
s401, processing a gradual change light transmission area on a substrate device of a diffraction optical element to obtain a spatial filter;
in this embodiment, the substrate device may be a common silicon dioxide material substrate, an ITO conductive glass substrate, and other conventional transparent substrates. The gradual change light transmission region may be a radial gradual change (the light transmittance increases or decreases along the radial direction from the center to the periphery), a linear gradual change (the linear gradual change light transmittance in any direction), and the specific distribution needs to be determined by the practical application target.
The reason for selecting the radial gradient increasing distribution of the transmittance is explained below by combining a specific calculation formula, which can be known from the differential property of the first-order fourier transform formula, 
，iThe number of the units of the imaginary number is expressed,
representing the frequency. Function of spatial domain
Is equivalent to a differential fourier transform
The Fourier transform of (A) is modulated in the frequency domain with the frequency domain
The frequency domain modulation function is the modulation function that can be provided by the device to be constructed in this embodiment. Because the calculation target is to realize isotropic differential calculation in a two-dimensional space, a first-order differential modulation function is applied to each diameter along the radial direction of a frequency spectrum plane by referring to the idea of optical radial Hilbert transform, which can be regarded as rotation and superposition of infinite first-order differential modulation functions, and the obtained two-dimensional amplitude and the finally obtained amplitude are obtainedAnd the phase distributions are shown in fig. 9 and 10, respectively.
S402, taking the phase distribution of orbital angular momentum with the topological charge value of +/-1 as the phase distribution of the diffraction optical element; converting the phase distribution of the diffraction optical element into the structural parameters of each diffraction unit;
and S403, processing the spatial filter according to the structural parameters of each diffraction unit to obtain the spatial frequency spectrum modulation device.
The difference between the present embodiment and the second embodiment is that in the present embodiment, a region in which the transmittance gradually increases radially from the center to the periphery is processed in the center of the substrate device of the diffractive optical element, and compared to the second embodiment, the device obtained in the present embodiment theoretically satisfies the amplitude distribution required by the differential property of the standard fourier transform, and in the application of edge detection, the optimal edge detection effect can be theoretically obtained.
Example five:
as shown in fig. 11, the present invention provides a method for designing and implementing a spatial spectrum modulation device based on a diffractive optical element, including:
s501, processing a gradual change light-transmitting area on a substrate device of the diffraction optical element to obtain a spatial filter;
in this embodiment, the gradual light transmission region is preferably a region in which the light transmittance gradually increases radially from the center to the periphery.
S502, superposing the phase distribution of orbital angular momentum with the topological charge value of +/-1 with the phase distribution with the focusing function, and taking the superposed phase distribution as the phase distribution of the diffraction optical element; converting the phase distribution of the diffraction optical element into the structural parameters of each diffraction unit;
and S503, processing the spatial filter according to the structural parameters of each diffraction unit to obtain the spatial frequency spectrum modulation device.
The difference between this embodiment and the fourth embodiment is that the phase distribution of the devices of the diffractive optical element in this embodiment is superimposed with a phase distribution with a specific focal length and focusing power on the basis of the spiral phase optical field generated by modulation. This focusing phase is equivalent to an ideal thin lens without any loss of amplitude, and is effectively completely equivalent to the second fourier lens of the optical 4f system, functioning to perform a two-dimensional fourier transform on the optical field.
Based on this embodiment, the amplitude distribution and phase distribution of the obtained spatial spectrum modulation device, which is also called a first-order optical differentiator, are schematically shown in fig. 9 and 10.
Example six:
as shown in fig. 12, the present invention provides an edge detection apparatus based on a diffractive optical element, including: the spatial spectrum modulation device is positioned on a back focal plane of the first lens and a front focal plane of the second lens; the first lens is used for transforming the light field to be processed on the front focal plane of the first lens into a frequency domain space and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the first lens; the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution; the second lens is used for converting the modulated spatial frequency spectrum distribution into a spatial domain and obtaining the light field distribution after edge detection on a back focal plane of the second lens.
In this embodiment, the light field with the target feature distribution enters from the input surface, then sequentially passes through the first lens, the spatial frequency spectrum modulation device and the second lens, and finally the light field distribution after edge detection is presented on the observation surface. The focusing phase of the spatial frequency spectrum modulation device can be designed into different focal lengths according to actual needs, and the relationship between the focal length of the focusing phase and the focal length of the lens can influence the size of the final actual imaging.
It should be noted that the spatial spectrum modulation device in this embodiment is obtained by using the method of the second embodiment or the fourth embodiment.
Example seven:
as shown in fig. 13, the present invention provides an edge detection apparatus based on a diffractive optical element, including: the spatial frequency spectrum modulation device is positioned on a back focal plane of the lens; the lens is used for transforming the light field to be processed on the front focal plane of the lens into a frequency domain space, and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the lens; the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution and obtaining the light field distribution after edge detection on the focal plane of the spatial frequency spectrum modulation device.
In this embodiment, the light field with the target feature distribution enters from the input surface, then sequentially passes through the lens and the spatial spectrum modulation device, and finally, the light field distribution after edge detection is presented on the observation surface. The focusing phase of the spatial frequency spectrum modulation device can be designed into different focal lengths according to actual needs, and the relationship between the focal length of the focusing phase and the focal length of the lens can influence the size of the final actual imaging.
It should be noted that the spatial spectrum modulation device in this embodiment is obtained by using the method in the third embodiment or the fifth embodiment. In the embodiment, on the basis of generating the spiral phase optical field by modulating the device phase distribution of the optical filter, a phase distribution with a specific focal length and focusing capability is superposed to perform a two-dimensional Fourier transform function on the optical field, so that the optical field distribution after edge detection can be directly obtained on a focal plane of the spatial frequency spectrum modulator without a second lens.
Example eight:
in the embodiment, through simulation, a first-order optical differentiator is taken as a reference, and a more appropriate size value of a central dark area of the high-pass spiral phase device is selected. The simulation parameters are as follows: the working wavelength is 532 nanometers, the size of a calculation matrix is 5000 multiplied by 5000, the size of a single square pixel is 2 micrometers, the focal length of a Fourier lens is 20000 micrometers, the focal length provided by a focusing phase superposed on a spatial frequency spectrum modulation device is 40000 micrometers, the aperture of the whole circular clear light of the system is 5000 micrometers, and the diffraction propagation algorithm is an angular spectrum method.
Firstly, a high-pass spiral spatial spectrum modulation device of the amplitude and phase distribution shown in fig. 6 and 7 is constructed, the device principle is based on the high-pass filtering theory and the spiral phase matching technology, and the details are described in the first, second and third embodiments.
It is noted that the device superimposes a phase with focusing power on the basis of the modulation phase.
The first order optical differentiators for the amplitude and phase distributions shown in fig. 9 and 10 were subsequently constructed, the device principle being derived from the differential nature of the fourier transform and the idea of the radial hilbert transform, the device being constructed in amplitude in such a way that the transmission increases radially and gradually from the center to the periphery, with the same phase as the phase distribution of the orbital angular momentum with a topological charge value of ± 1 (only the distribution characteristics being the same).
It should be noted that the device superimposes a phase with focusing power on the basis of the modulation phase.
It should be noted that the distribution on an infinite plane is required in the theory of fourier transform, but in the specific implementation process, the distribution is limited by factors such as the aperture of a device, and compared with the theoretical distribution, the specific distribution can only be approximate distribution, but the essential characteristics and the capability of the distribution are not affected, and the final modulation effect can still approach the theoretically optimal effect.
It should be noted that the phase distributions of these two types of devices are modulation phase and superposition focusing phase, and the final operation result of the device is increased by a coefficient on a phase term compared with the standard optical 4f system, but the coefficient does not affect the final light intensity distribution, and the final operation result can still be considered as a differential operation result.
It should be noted that, in the simulation, the geometric dimensions of the high-pass spiral spatial frequency spectrum modulator and the first-order optical differentiator are completely consistent with the equivalent focal length value of the focusing phase, and only the dark area dimension of the high-pass spiral phase device is taken as a variable value.
The image shown in fig. 14 is then used as a simulation input to calculate a sharpness indicator for the simulation results, for example by calculating its central transverse cross-section and calculating the inverse of the full width at half maximum of the central position. For the high-pass spiral phase device, different central dark area size values are used for simulation, and all simulation results are drawn on a curve shown in fig. 15. For the first-order optical differentiator, when the geometric dimension is fixed, all the parameters are fixed, so only one set of simulation parameters is plotted in the form of a dotted line in fig. 15, and it can be seen that under the current simulation parameters, when the radius of the central dark area of the high-pass helical phase device is about 160um, the sharpness index is equivalent to that of the first-order optical differentiator, but the realization difficulty is far lower than that of the first-order optical differentiator.
Fig. 16 is a picture with H-shaped, and the edge detection result obtained after the modulation by the high-pass spiral phase device according to the present invention is shown in fig. 17.
The invention has the characteristics of simplicity, high efficiency, less limitation, mature processing technology and low cost, so the invention can be used as a front-end processing component of a vision system for large-scale practical application.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (7)
1. A design and realization method of a spatial spectrum modulation device based on a diffractive optical element is characterized by comprising the following steps:
processing a light-suppressing region on a substrate device of a diffraction optical element to obtain a spatial filter with amplitude modulation capability;
determining the phase distribution of the diffraction optical element, and converting the phase distribution into the structural parameters of each diffraction unit;
and processing the spatial filter according to the structural parameters of each diffraction unit to obtain the spatial frequency spectrum modulation device with complex modulation capability.
2. The method as claimed in claim 1, wherein the light-suppressing region is an opaque region or a region with gradually increasing light transmittance radially from the center to the periphery.
3. The method for designing and implementing a diffractive optical element based spatial spectrum modulation device according to claim 2, wherein said determining a phase distribution of a diffractive optical element comprises:
the phase distribution of orbital angular momentum with a topological charge value of ± 1 was taken as the phase distribution of the diffractive optical element.
4. The method for designing and implementing a diffractive optical element based spatial spectrum modulation device according to claim 2, wherein said determining the phase distribution of the diffractive optical element comprises:
the phase distribution of orbital angular momentum with a topological charge value of +/-1 is superposed with the phase distribution with a focusing function, and the superposed phase distribution is used as the phase distribution of the diffraction optical element.
5. Spatial spectrum modulation device based on diffractive optical elements, characterized in that it is obtained with the method according to any one of claims 1 to 4.
6. An edge detection device based on a diffractive optical element, comprising: a first lens, a spatial spectrum modulation device obtained by the method according to claim 3 and a second lens, wherein the spatial spectrum modulation device is positioned in a back focal plane of the first lens and a front focal plane of the second lens;
The first lens is used for transforming the light field to be processed on the front focal plane of the first lens into a frequency domain space and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the first lens;
the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution;
the second lens is used for converting the modulated spatial frequency spectrum distribution into a spatial domain and obtaining the light field distribution after edge detection on a back focal plane of the second lens.
7. An edge detection device based on a diffractive optical element, comprising: a lens and a spatial spectrum modulation device obtained by the method of claim 4, wherein the spatial spectrum modulation device is positioned on a back focal plane of the lens;
the lens is used for transforming the light field to be processed on the front focal plane of the lens into a frequency domain space, and obtaining the spatial frequency spectrum distribution of the light field to be processed on the back focal plane of the lens;
the spatial frequency spectrum modulation device is used for modulating the spatial frequency spectrum distribution and obtaining the light field distribution after edge detection on the focal plane of the spatial frequency spectrum modulation device.
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