Edge enhancement imaging method and system based on nonlinear angle filtering

Abstract

The invention discloses an edge enhancement imaging method based on nonlinear angle filtering, and the method comprises the following steps: S1, employing mid-infrared coherent light as signal light to vertically irradiate a to-be-imaged object from one side of the to-be-imaged object, enabling the signal light to pass through the to-be-imaged object, and carrying out the diffraction, and obtaining the angular spectrum component of the signal light; s2, a nonlinear crystal is placed on the side, away from the signal light, of the object to be imaged, and the angular spectrum component, obtained in the step S1, of the signal light is transmitted to the nonlinear crystal and collected by the nonlinear crystal; s3, pump light is loaded in the nonlinear crystal, frequency conversion is carried out on the angular spectrum meeting specific conditions, nonlinear angle filtering is achieved, and a conversion light signal is obtained; and S4, the converted light signals obtained in the step S3 are collected by an imaging system and are imaged, and edge detection of the image of the object to be imaged is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of object edge detection, and in particular to an edge enhancement imaging method and system based on nonlinear angle filtering. Background Art

[0002] Edge detection technology is a means of obtaining information about image intensity mutations, which helps to complete tasks such as object recognition and feature detection. Object intensity information can be collected by a camera and then processed numerically by a computer.

[0003] In the field of edge detection technology, the method based on all-optical image processing has the advantages of low energy consumption, fast processing speed, and parallel processing of large amounts of data compared to image edge detection based on numerical calculation. Common all-optical edge detection technologies, such as dark field imaging and vortex phase contrast imaging, are based on the 4f framework, which cuts off the low-frequency components of the image on the system Fourier plane, applies a phase difference between the low frequency and the high frequency, and thus obtains the edge information of the image. Several important indicators of all-optical edge detection technology are high sensitivity, integration, wide working bandwidth, and high tunability. In addition, some recently developed micro-nanostructure devices use the transmission and reflection characteristics of materials to light as an angle-sensitive filter to directly realize image Fourier information processing on the object plane, thus realizing edge detection with integration and high tunability.

[0004] However, current all-optical edge detection technology is still concentrated in the visible light and near-infrared bands, and it is difficult to expand the working bandwidth to bands such as the mid-infrared.

[0005] The mid-infrared band has the advantages of long range and good anti-interference, and is widely used in fields such as biological tissue imaging, night vision imaging and military monitoring. However, due to the limitations of current mid-infrared detectors, it is a technical challenge to extend the optical edge enhancement technology to the mid-infrared band.

[0006] Traditional detection and imaging devices used in mid-wave infrared are usually based on materials such as mercury cadmium telluride and indium antimonide. They have large dark noise and require low-temperature refrigeration devices when used. In terms of core performance indicators such as the number of pixels, pixel size and operating frame rate, they are far inferior to silicon-based detectors in the visible light band.

[0007] In addition, the detection efficiency of emerging micro-nano materials in the mid-infrared band is low, and their preparation technology is still in the development stage. Therefore, the performance of existing micro-nano devices and their preparation scale limit their performance in the detection direction of the outer edge of the mid-infrared band. It is urgent to develop a highly sensitive, low-noise, integrated and tunable mid-infrared edge enhancement imaging technology. Summary of the invention

[0008] The purpose of the present invention is to provide an edge enhancement imaging method and system based on nonlinear angle filtering to solve the technical problem that the all-optical edge detection technology is difficult to expand the working bandwidth to the mid-infrared band.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention discloses an edge enhancement imaging method based on nonlinear angle filtering, comprising the following steps:

[0011] S1. Using mid-infrared coherent light as signal light to illuminate the object to be imaged vertically from one side of the object to be imaged, the signal light passes through the object to be imaged and diffracts, and angular spectrum components of the signal light are obtained;

[0012] S2, placing a nonlinear crystal on a side of the object to be imaged away from the signal light, and the angular spectrum components of the signal light obtained in step S1 propagate to the nonlinear crystal and are collected by the nonlinear crystal;

[0013] S3, loading pump light into the nonlinear crystal, performing frequency conversion on the angular spectrum that meets specific conditions, realizing nonlinear angle filtering, and obtaining a converted optical signal;

[0014] S4. The converted light signal obtained in step S3 is collected and imaged by an imaging system to achieve edge detection of the image of the object to be imaged.

[0015] The working principle of this scheme is: this scheme illuminates the object to be imaged with mid-infrared coherent light, and obtains angular spectrum components diverging at various angles behind the object to be imaged. Subsequently, these angular spectrum components enter the crystal and interact with the pump light. The angular spectrum components and the pump light follow the wave vector synthesis law caused by phase matching for up-conversion. By utilizing the characteristic that the up-conversion process is sensitive to the angle of incident light, only plane waves at specific angles meet the phase matching characteristics of the up-conversion crystal. The angular spectrum components incident perpendicular to the nonlinear crystal do not meet the phase matching conditions and cannot be efficiently converted, thereby realizing edge detection of the image. Finally, the image information after angle filtering under the crystal phase matching conditions is collected by the imaging system and recorded by a visible light band detector.

[0016] Preferably, in step S1, the mid-infrared coherent light adopts a continuous light source or a pulse light source, which can emit continuous light or pulse light.

[0017] Preferably, in step S1, the angular spectrum components of the signal light refer to that, by illuminating the corresponding imaging object with mid-infrared coherent light, the object information can be decomposed into a large number of plane waves, and each plane wave has a propagation direction, and each plane wave propagating along each propagation direction is transmitted to the crystal and collected by the crystal.

[0018] Preferably, in step S2, the crystal is a nonlinear crystal; a nonlinear crystal refers to a nonlinear material that is transparent in the mid-infrared to visible light band, including but not limited to lithium niobate, lithium tantalate and potassium titanyl phosphate nonlinear materials; the nonlinear crystal has a phase matching characteristic, and can directly perform selective angle filtering on the angular spectrum of the object in the object plane, and convert the mid-infrared information into the visible light / near-infrared band.

[0019] Preferably, in step S3, the propagation direction of the pump light is input from the signal light output end of the nonlinear crystal, and propagates in the nonlinear crystal toward the signal light input end of the nonlinear crystal. At the same time, the end of the nonlinear crystal facing the signal light input is coated with a reflective film for reflecting the pump light propagating thereto, so as to change the propagation direction of the pump light.

[0020] Preferably, in step S3, the specific condition refers to regulating the transfer function of the system by changing the phase matching condition of the crystal, so that each plane wave incident on the nonlinear crystal at an inclined angle is frequency converted, while the plane wave incident perpendicular to the nonlinear crystal cannot be efficiently converted.

[0021] The preferred method for regulating the transfer function of the system is to use a wavelength tunable signal light source, or to use broadband light source illumination combined with narrowband filter devices, or to use a narrowband signal light source combined with crystal temperature regulation.

[0022] Preferably, the imaging system uses a single lens to image the light signal after nonlinear angle filtering, or uses a combined microlens to image the light signal after nonlinear angle filtering.

[0023] Preferably, the converted optical signal in step S3 and step S4 is up-converted light, and the up-converted light refers to a light wave whose frequency is converted from an original light wave to a higher frequency.

[0024] In a second aspect, the present invention further discloses an edge enhancement imaging system based on nonlinear angle filtering, which is used to implement the edge enhancement imaging method based on nonlinear angle filtering as described above, comprising a signal light source for emitting signal light, wherein a light emitting end of the signal light source is sequentially provided with a first quarter wave plate and a first half wave plate; the first quarter wave plate is used to convert the signal light into linearly polarized light; the first half wave plate is used to adjust the linearly polarized light into vertically polarized light;

[0025] An object to be imaged is placed at the light emitting end of the first half-wave plate; a nonlinear crystal is arranged on the side of the object to be imaged away from the first half-wave plate; a reflective film is plated on the end of the nonlinear crystal facing the signal light source; a temperature-controlled furnace is placed at the bottom of the nonlinear crystal for adjusting the temperature of the nonlinear crystal;

[0026] The imaging system is also provided with a pump light source for generating pump light; a light emitting end of the pump light source is provided with a second quarter wave and a second half wave plate in sequence; the second quarter wave is used to convert the pump light into linearly polarized light; the second half wave plate is used to adjust the linearly polarized light into vertically polarized light;

[0027] An isolator is provided at the light output end of the second half-wave plate to prevent the pump light from being reflected back to the pump light source; a reflective silver mirror and a dichroic mirror are provided in sequence at the light output end of the isolator; the reflective silver mirror is used to change the direction of the pump light, and the dichroic mirror is used to transmit the up-converted visible light and reflect the pump light to the signal light output end of the nonlinear crystal, so that the pump light propagates from the signal light output end of the nonlinear crystal to the reflective film, and is vertically reflected at the reflective film, and then propagates in the same direction as the signal light;

[0028] The light emitting end of the dichroic mirror is provided with a convex lens, a filter system and a visible light detector in sequence; the convex lens is an imaging lens for collecting and imaging the up-converted light, the filter system is used to filter out noise, and the visible light detector is used to collect and record the up-converted visible light information.

[0029] The present invention has the following beneficial effects: the present invention uses nonlinear crystal as a Fourier image information processing device, has the characteristics of reconfigurable angle filtering, can convert mid-infrared information into visible / near-infrared bands with the help of nonlinear processes, and then make full use of the high-performance visible light detector to achieve high-quality mid-infrared imaging, and then can achieve high-sensitivity, low-noise mid-infrared edge detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to make the purpose, technical solution and advantages of the invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0031] Figure 1 The figure is a schematic diagram of the composition of an edge enhancement imaging system based on nonlinear angle filtering according to an embodiment of the present invention.

[0032] Figure 2 It is a schematic diagram of the imaging effect when the signal light of the present invention is incident on the nonlinear crystal at an inclined angle.

[0033] Figure 3It is a schematic diagram of the imaging effect when the signal light of the present invention is incident on the nonlinear crystal perpendicularly to the crystal.

[0034] Figure 4 It is a schematic diagram of the principle of the edge enhancement imaging method based on nonlinear angle filtering of the present invention.

[0035] Figure 5 This is a wavelength parameter simulation diagram of the edge enhancement imaging method based on nonlinear angle filtering according to an embodiment of the present invention.

[0036] Figure 6 This is a temperature parameter simulation diagram of the edge enhancement imaging method based on nonlinear angle filtering according to an embodiment of the present invention.

[0037] Figure 7 It is an original image of the object to be imaged according to an embodiment of the present invention.

[0038] Figure 8 This is a simulation diagram of a bright field image of an object to be imaged according to an embodiment of the present invention.

[0039] Fig. 9 This is a simulated dark field image of an object to be imaged obtained by changing the wavelength of a signal light source according to an embodiment of the present invention.

[0040] Fig.10 This is a simulated dark field image of an object to be imaged obtained by changing the temperature of a nonlinear crystal according to an embodiment of the present invention.

[0041] Explanation of the accompanying drawings: 1. signal light source; 2. first quarter wave plate; 3. first half wave plate; 4. object to be imaged; 5. periodically poled lithium niobate crystal; 6. temperature-controlled furnace; 7. pump light source; 8. second quarter wave plate; 9. second half wave plate; 10. isolator; 11. reflective silver mirror; 12. dichroic mirror; 13. convex lens; 14. filter system; 15. visible light detector. DETAILED DESCRIPTION

[0042] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0043] It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings. In the description of the present invention, it should be noted that the orientation or position relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inside", "outside", etc. is based on the orientation or position relationship shown in the drawings, or the orientation or position relationship in which the invention product is usually placed when used, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance. In addition, the terms "horizontal", "vertical", etc. do not mean that the components are absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted. In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0044] The present invention can be applied to the field of all-optical edge detection using mid-infrared, solving the technical problem that all-optical edge detection technology is difficult to expand the working bandwidth to the mid-infrared band.

[0045] In a first aspect, based on the technical problem solved above, the present invention discloses an edge enhancement imaging method based on nonlinear angle filtering, comprising the following steps:

[0046] S1. Using mid-infrared coherent light as signal light to illuminate the object to be imaged vertically from one side of the object to be imaged, the signal light passes through the object to be imaged and diffracts, and angular spectrum components of the signal light are obtained;

[0047] S2, placing a nonlinear crystal on a side of the object to be imaged away from the signal light, and the angular spectrum components of the signal light obtained in step S1 propagate to the nonlinear crystal and are collected by the nonlinear crystal;

[0048] S3, loading pump light into the nonlinear crystal, performing frequency conversion on the angular spectrum that meets specific conditions, realizing nonlinear angle filtering, and obtaining a converted optical signal;

[0049] S4. The converted light signal obtained in step S3 is collected and imaged by an imaging system to achieve edge detection of the image of the object to be imaged.

[0050] The working principle of this scheme is: this scheme illuminates the object to be imaged with mid-infrared coherent light, and obtains angular spectrum components diverging at various angles behind the object to be imaged. Subsequently, these angular spectrum components enter the crystal and interact with the pump light. The angular spectrum components and the pump light follow the wave vector synthesis law caused by phase matching for up-conversion. By utilizing the characteristic that the up-conversion process is sensitive to the angle of incident light, only plane waves at specific angles meet the phase matching characteristics of the up-conversion crystal. The angular spectrum components incident perpendicular to the nonlinear crystal do not meet the phase matching conditions and cannot be efficiently converted, thereby realizing edge detection of the image. Finally, the image information after angle filtering under the crystal phase matching conditions is collected by the imaging system and recorded by a visible light band detector.

[0051] At present, the existing edge enhancement technology based on nonlinear frequency conversion mainly uses the 4f system to apply a vortex phase to the pump light, place the crystal in the Fourier plane, and perform frequency conversion and Fourier image processing at the same time to obtain an edge-enhanced image. In the existing scheme, the complex amplitude distribution of the pump light determines the transfer function of the system. It is necessary to obtain a vortex beam with a specific complex amplitude distribution at the crystal position to achieve ideal edge detection. The existing scheme system is relatively complicated, the transfer function regulation speed is slow, and it does not meet the requirements of integration and high tunability. This type of method has application difficulties in scenarios where it is necessary to obtain the bright and dark field information of the object in real time.

[0052] This scheme uses nonlinear crystals as a Fourier image information processing device with the characteristics of reconfigurable angle filtering. This scheme can convert mid-infrared information to visible / near-infrared bands with the help of nonlinear processes, and then make full use of the high-performance visible light detector to achieve high-quality mid-infrared imaging. It can then achieve high-sensitivity, low-noise mid-infrared edge detection. The mid-infrared edge detection system implemented based on the method of the present invention performs Fourier information processing on the object plane, has the advantages of compact structure and easy adjustment of bright and dark field imaging. It can be used in the fields of biological tissue imaging, non-destructive material testing, and object feature recognition.

[0053] Preferably, in step S1, the mid-infrared coherent light adopts a continuous light source or a pulse light source, which can emit continuous light or pulse light.

[0054] Preferably, in step S1, the angular spectrum components of the signal light refer to that, by illuminating the corresponding imaging object with mid-infrared coherent light, the object information can be decomposed into a large number of plane waves, and each plane wave has a propagation direction, and each plane wave propagating along each propagation direction is transmitted to the crystal and collected by the crystal.

[0055] Preferably, in step S2, the crystal is a nonlinear crystal; a nonlinear crystal refers to a nonlinear material that is transparent in the mid-infrared to visible light band, including but not limited to lithium niobate, lithium tantalate and potassium titanyl phosphate nonlinear materials; the nonlinear crystal has a phase matching characteristic, and can directly perform selective angle filtering on the angular spectrum of the object in the object plane, and convert the mid-infrared information into the visible light / near-infrared band.

[0056] Preferably, in step S2, the crystal is arranged behind the object to be imaged and close to the object to be imaged to increase the receiving angle of the system and reduce the size of the system.

[0057] Preferably, in step S3, the propagation direction of the pump light is input from the signal light output end of the nonlinear crystal, and propagates in the nonlinear crystal toward the signal light input end of the nonlinear crystal. At the same time, the end of the nonlinear crystal facing the signal light input is coated with a reflective film for reflecting the pump light propagating thereto, so as to change the propagation direction of the pump light.

[0058] Preferably, in step S3, the specific condition refers to regulating the transfer function of the system by changing the phase matching condition of the crystal, so that each plane wave incident on the nonlinear crystal at an inclined angle is frequency converted, while the plane wave incident perpendicular to the nonlinear crystal cannot be efficiently converted.

[0059] See also Figure 2 and Figure 3 , demonstrating the different imaging effects of the nonlinear crystal when the signal light is incident at an oblique angle or perpendicular to the crystal.

[0060] Specifically, in step S3, the pump light adopts a pulse or high-intensity continuous light source.

[0061] Specifically, in step S3, the phase matching condition of the crystal is changed by adjusting parameters such as temperature or incident wavelength of the signal light.

[0062] One of the preferred methods for regulating the transfer function of the system is to use a signal light source with tunable wavelength.

[0063] A second preferred method for regulating the transfer function of the system is to use broadband light source illumination combined with narrowband filter devices.

[0064] Specifically, the narrowband filter adopts a narrowband filter or an acousto-optic tunable filter.

[0065] A third preferred method for regulating the transfer function of the system is to use a narrowband signal light source in combination with crystal temperature regulation.

[0066] As one implementation of the imaging system, the imaging system uses a single lens to image the light signal after nonlinear angle filtering.

[0067] As one implementation of the imaging system, the imaging system uses a combined microlens to image the optical signal after nonlinear angle filtering.

[0068] Specifically, in step S4, a silicon-based camera is used to detect the edge of the image, and the silicon-based camera can also record the edge information of the detected image.

[0069] Preferably, the converted optical signal in step S3 and step S4 is up-converted light, and the up-converted light refers to a light wave whose frequency is converted from an original light wave to a higher frequency.

[0070] The working principle of the present invention is as follows: the present invention illuminates the object to be imaged by mid-infrared coherent light to obtain angular spectrum components diverging in various angles, that is, the information of the object can be expressed as plane waves diverging in various directions; then, these plane waves enter the crystal and interact with the pump light, and by utilizing the characteristic that the up-conversion process is sensitive to the angle of the incident light, only the plane waves at a specific angle meet the phase matching characteristics of the up-conversion crystal, and the angular spectrum components incident perpendicular to the nonlinear crystal do not meet the phase matching conditions and cannot be efficiently converted, thereby realizing edge detection of the image; in addition, by adjusting the temperature and the wavelength of the incident light, the transfer function of the corresponding system can be adjusted, and the bright and dark field information of the image can be obtained through the imaging system; in this process, the mid-infrared coherent light is converted to the visible / near-infrared band by the frequency up-conversion method, and the required Fourier spectrum information is obtained by adjusting the phase matching angle; finally, the image information filtered by the crystal phase matching condition angle is collected by the imaging system and recorded by a visible light camera.

[0071] The following content is an introduction to the principle of the edge enhancement imaging method based on nonlinear angle filtering of the present invention:

[0072] Specifically, the amplitude of each plane wave can be obtained by Fourier transform. In spherical coordinates, the propagation angle corresponds to the spatial frequency, and the expression is:

[0073]

[0074] In formulas (1) and (2), k x ,k y are the spectral components in the horizontal and vertical directions, θ, are the polarization angle and radial angle respectively, k0 is the wave number, k0=2π / λ, λ represents the wavelength, the angular spectrum with smaller angles corresponds to the information of the object's flat intensity area, and the angular spectrum with larger angles corresponds to the information of the object's sudden intensity change.

[0075] For an object with a complex amplitude distribution of O(x,y) on the object plane, its Fourier spectrum is expressed as Under the action of the filter, the spatial distribution of the complex amplitude output by the system is:

[0076]

[0077] In formula (3), H(k x ,k y ) is the transfer function of the system, which is related to the angle. Various materials that are sensitive to the angle of incident light can be used to design the transfer function.

[0078] Here we use periodically poled lithium niobate crystal as the angle-sensitive device, considering Figure 2 and Figure 3 The scene shown. A nonlinear crystal is placed close to the object and, based on the angle sensitivity of the crystal upconversion process, each spatial frequency component is selectively upconverted.

[0079] The principle of angle filtering through upconversion is as follows: the upconversion process adds the energy of signal light photons and pump light photons and converts them into visible light photons. This process needs to meet the conditions of energy conservation and momentum conservation. Among them, the energy conservation condition requires where ω u,s,p are the angular frequencies of the upconverted light, pump light and signal light, respectively. is Planck's constant. The momentum conservation condition requires that the signal light, pump light wave vector and up-conversion light wave vector satisfy the vector synthesis condition, such as Figure 4 shown.

[0080] In the systems currently considered, the pupil is the most restrictive component of the crystal / lens.

[0081] The pump light is incident from the rear end of the crystal through a reflector and is reflected by the high-reflection film on the front end of the crystal, making the propagation direction consistent with the signal light. The pump light can be a pulse or a high-power continuous light source. At this time, the pump light is incident on the crystal along the axial direction perpendicular to the crystal, and the angle of the signal light relative to the center of the crystal is recorded as θ s , the phase mismatch expression of the up-conversion process along the propagation direction is as follows:

[0082]

[0083] In formula (4), n u,s,p Represent the refractive index of upconverted light, signal light and pump light in the crystal, λ u,s,p is the corresponding wavelength, and Λ is the polarization period of the periodically poled crystal.

[0084] Here we use an approximate condition where θ uis the angle of the upconverted light. This approximate condition comes from the phase matching in the vertical propagation direction. In the case of the small signal approximation, the normalized intensity of the upconverted light intensity incident at a certain angle is:

[0085] I∝sinc 2 [Δk(T,λ s ,λ p ,θ s )L / 2] (5)

[0086] In formula (5), T is the crystal temperature and L represents the crystal length.

[0087] It can be seen that the up-conversion process is a process that is sensitive to the angle of the incident light. Only when the phase mismatch term is a relatively small amount can the wave vector synthesis condition be met, thereby efficiently obtaining the converted light. By selectively designing the phase matching condition, the selective conversion of the corresponding angular spectrum components can be achieved. Under the small angle approximation, switching to the frequency domain expression, the system transfer function caused by the crystal in the up-conversion process is:

[0088]

[0089] Equation (6) shows that the transfer function of the system is related to temperature and signal wavelength.

[0090] Next, the image intensity distribution of the upconverted light is calculated. The coupled wave equation that describes the upconversion process is as follows:

[0091]

[0092] In formula (7), E u,s,p represent the complex amplitudes of upconverted light, signal light, and pump light, respectively; z is the propagation distance in the crystal; ε0 is the vacuum magnetic permeability; c is the propagation speed of light in vacuum; d eff is the effective nonlinear coefficient of the crystal.

[0093] Since the crystal has a certain length, the upconversion process may occur at every position of the crystal. In theoretical analysis, the distribution of the light field at each spatial position is valuable.

[0094] The entire imaging process can be divided into two parts: one is spatial propagation, and the other is frequency conversion.

[0095] First, the diffracted light field of the object propagates freely from z1 to the front end of the crystal, and the propagation of light is calculated using the Fresnel diffraction integral formula. The complex amplitude distribution of the light field at z is E(x, y, z), and the complex amplitude distribution of the light field after propagating along the air for d is E(x', y', z+d). The Fresnel diffraction integral formula is expressed as follows:

[0096]

[0097] Calculation of the light field in the crystal, taking into account the truncation of the transfer function and the frequency conversion process:

[0098]

[0099] In formula (8) and formula (9), F cry (z) = sign[cos(2πz / Λ)], which is the spatial modulation function of the periodically poled crystal, represents the lateral Laplace operator, dz is the spatial propagation step length;

[0100] In addition, the second term on the right side of the equal sign in equation (9) describes the lateral diffusion term in the process of light field propagation; the third term on the right side of equation (9) describes the calculation of the intensity of spectrum truncation and up-conversion, indicating that by truncating the angular spectrum of vertical incidence and only converting the angular spectrum of oblique incidence, the edge information detection of the object image can be achieved.

[0101] Specifically, in step S4, the up-converted light field emitted from the end of the crystal continues to propagate z2 to the lens, and finally converges to form an image at a position z3 behind the lens, and is detected by a silicon-based detector.

[0102] The edge enhancement imaging method based on nonlinear angle filtering disclosed in the present invention has the following technical effects:

[0103] 1. In view of the difficulties in mid-infrared edge detection imaging, the image edge enhancement method based on nonlinear angle filtering of the present invention utilizes nonlinear crystals with periodic polarization structures to change the phase matching angle by adjusting parameters such as the wavelength and temperature of the incident light, thereby performing angle-selective up-conversion on the angular spectrum emitted by the object and converting the mid-infrared image to the visible light band.

[0104] 2. The present invention utilizes the frequency conversion characteristics of nonlinear crystals as an angle filter to directly perform Fourier information processing on the object plane, and can simultaneously perform mid-infrared frequency conversion and image edge detection. The system structure is simple and compact.

[0105] 3. The present invention utilizes the frequency conversion characteristics of nonlinear crystals to perform Fourier information processing. The crystal has the effect of reconfigurable optical angle filtering. By adjusting the phase matching of the nonlinear crystal, such as changing parameters such as temperature and incident wavelength, the transfer function of the system can be switched.

[0106] 4. The present invention utilizes the difference in response of nonlinear crystals to different wavelengths, and can obtain the bright and dark field information of the object at the same time through illumination by a broadband light source.

[0107] Second, see Figure 1The present invention also discloses an edge enhancement imaging system based on nonlinear angle filtering, which is used to implement the edge enhancement imaging method based on nonlinear angle filtering as described above, including a signal light source 1. Specifically, the signal light source 1 in this embodiment adopts a coherent light source, and the coherent light source in this embodiment takes mid-infrared 3070nm as an example; in addition, the coherent light source can also select a broadband light source, and the coherent light source is a pulse light or a continuous light source. In this embodiment, Figure 5 As shown, the angle-dependent, intensity-normalized upconverted light intensity at different wavelengths is shown.

[0108] The light emitting end of the signal light source 1 is provided with a first quarter wave plate 2 and a first half wave plate 3 in sequence;

[0109] Specifically, the first quarter wave plate 2, here considering that the signal light is initially circularly polarized light, becomes linearly polarized light after passing through the quarter wave plate; the first half wave plate 3 adjusts the linearly polarized light after passing through the first quarter wave plate 2 into vertically polarized light;

[0110] An object 4 to be imaged is placed at the light emitting end of the second half-wave plate 9;

[0111] Specifically, the object to be imaged 4 in this embodiment is an amplitude type or phase type object.

[0112] A nonlinear crystal 5 is arranged on the side of the object to be imaged 4 away from the first half-wave plate 3. In this embodiment, the nonlinear crystal 5 is a periodically poled lithium niobate crystal.

[0113] Specifically, the nonlinear crystal 5 has a length of 5 mm, a lateral size of 2*2 mm, and a polarization period of 20.4 μm. As a nonlinear frequency conversion medium, it converts the signal light into the visible light band. At the same time, as an angle filter, it only converts the signal light at a certain angle. The wave vector synthesis principle of angle selection is as follows: Figure 4 What is shown;

[0114] One end of the nonlinear crystal 5 facing the signal light source is coated with a 1030nm reflective film, so that the pump light and the signal light in the nonlinear crystal 5 propagate in the same direction.

[0115] A temperature control furnace 6 is placed at the bottom of the nonlinear crystal 5 to adjust the temperature of the nonlinear crystal 5;

[0116] Specifically, the temperature variation leads to an angle-dependent intensity normalized transfer function, such as Figure 6 shown.

[0117] The imaging system is further provided with a pump light source 7 for generating pump light;

[0118] Specifically, in this embodiment, the pump light source 7 can generate relatively high intensity pump light with a wavelength of 1030 nm, and a continuous light source or a pulse light source is selected.

[0119] The light emitting end of the pump light source 7 is provided with a second quarter wave 8 and a second half-wave plate 9 in sequence; the second quarter wave 8, here considering that the pump light is initially circularly polarized light, becomes linearly polarized light after passing through the quarter wave plate; the second half-wave plate 9 adjusts the linearly polarized light after passing through the quarter wave plate 8 into vertically polarized light.

[0120] An isolator 10 is provided at the light emitting end of the second half-wave plate 9 to prevent the pump light from reflecting and damaging the laser;

[0121] A reflective silver mirror 11 and a dichroic mirror 12 are sequentially arranged at the light output end of the isolator 10; the reflective silver mirror 11 is used to change the direction of the pump light, and the dichroic mirror 12 is used to transmit the up-converted light in the visible light band, and at the same time reflect the 1030nm pump light to the signal light output end of the nonlinear crystal 5, so that the pump light propagates from the signal light output end of the nonlinear crystal 5 to the reflective film, and is vertically reflected at the reflective film, and then propagates in the same direction as the signal light;

[0122] The light emitting end of the dichroic mirror 12 is provided with a convex lens 13, a filter system 14 and a visible light detector 15 in sequence; the convex lens 13 is an imaging lens, which is used to collect and image the up-converted light information. In this embodiment, the focal length of the convex lens 13 is 50 mm, the filter system 14 is used to filter out the signal light, pump light and noise caused by parameter conversion, and the visible light detector 15 is used to collect and record the up-converted visible light information.

[0123] The following is the specific implementation process:

[0124] First, the signal light source 1 emits 3070nm circularly polarized light, which becomes vertically linearly polarized light after passing through the first quarter wave plate 2 and the first half wave plate 3. After illuminating the object 4 to be imaged, it is diffracted into plane waves propagating in all directions. The original image of the object 4 to be imaged is shown in the attached figure. Figure 7 As shown, it is an amplitude type object with a size of 2*2mm;

[0125] Then, these plane waves enter the nonlinear crystal 5 located 1 mm behind the object. The temperature control furnace 6 is used to change the crystal temperature. The corresponding angle-dependent normalized upconversion intensity changes with temperature as shown in Figure 6 As shown;

[0126] Afterwards, the pump light source 7 generates light with a wavelength of 1030nm, which becomes vertically polarized light after passing through the quarter wave plate 8 and the second half wave plate 9, passes through the isolator 10, is reflected to the dichroic mirror 12 by the reflective silver mirror 11, and then is reflected to the inside of the crystal 5, and finally is reflected by the 1030nm high-reflection film plated on the front end of the nonlinear crystal 5; the crystal temperature is set to 180°C by the temperature control furnace 6;

[0127] Afterwards, a series of plane wave components generated by the pump light and the object selectively undergo a frequency conversion process according to the direction of the wave vector. Plane waves in a small angle range perpendicular to the nonlinear crystal incident on the nonlinear crystal can be efficiently converted, while plane waves greater than the phase matching angle cannot be converted, that is, angle filtering and frequency conversion are performed simultaneously to generate 771nm upconversion light, which is collected by a convex lens 13 located 54mm behind the crystal. After other noise signals except the upconversion light are filtered out by a filter system 14, an image is formed at a detector 15 located 300mm behind the lens, as shown in the attached figure. Figure 8 Bright field image shown;

[0128] If only the wavelength of the signal light source 1 is changed to 3090 nm, the plane wave incident perpendicular to the nonlinear crystal cannot be converted. Fig. 9 The dark field image shown in the figure; Similarly, if the crystal temperature is only changed to 160°C, the image shown in the figure will appear Fig.10 Dark field image shown.

[0129] In addition, in a specific implementation, if broadband wavelength illumination is used, since the system transfer functions at different wavelengths are inconsistent, both bright field and dark field information can be obtained at the same time.

[0130] Compared with the traditional up-conversion edge detection imaging method and system, the present invention uses nonlinear crystals as a reconfigurable optical angle filter, and uses the angle selection characteristics of up-conversion to directly perform image Fourier information processing on the object plane, making the system structure more compact. At the same time, it can be illuminated by broadband signal light to achieve bright field imaging and dark field imaging effects at different wavelengths. An edge enhancement imaging method based on nonlinear angle filtering implemented by the method of the present invention has a compact structure and tunability, and can achieve the effects of bright and dark field imaging at the same time.

[0131] It is understood that the present invention is described by some embodiments, and those skilled in the art are aware that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. Under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. The embodiments described in the present invention are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein may be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all other embodiments obtained by ordinary technicians in the art without creative work based on the embodiments in the present invention belong to the scope of protection of the present invention.

Claims

1. An edge enhancement imaging method based on nonlinear angle filtering, characterized in that: The following steps are involved: S1. Using mid-infrared coherent light as signal light to illuminate the object to be imaged vertically from one side of the object to be imaged, the signal light passes through the object to be imaged and diffracts, and angular spectrum components of the signal light are obtained; S2, placing a nonlinear crystal on a side of the object to be imaged away from the signal light, and the angular spectrum components of the signal light obtained in step S1 propagate to the nonlinear crystal and are collected by the nonlinear crystal; S3, loading pump light into the nonlinear crystal, performing frequency conversion on the angular spectrum that meets specific conditions, realizing nonlinear angle filtering, and obtaining a converted optical signal; S4. The converted light signal obtained in step S3 is collected and imaged by an imaging system to achieve edge detection of the image of the object to be imaged.

2. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S1, the mid-infrared coherent light adopts a continuous light source or a pulse light source, which can emit continuous light or pulse light.

3. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S1, the angular spectrum components of the signal light refer to that, by illuminating the corresponding imaging object with mid-infrared coherent light, the object information can be decomposed into a large number of plane waves, and each plane wave has a propagation direction. Each plane wave propagating along each propagation direction is transmitted to the crystal and collected by the crystal.

4. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S2, the crystal adopts a nonlinear crystal; a nonlinear crystal refers to a nonlinear material that is transparent in the mid-infrared to visible light band, including but not limited to nonlinear materials such as lithium niobate, lithium tantalate and potassium titanyl phosphate; the nonlinear crystal has a phase matching characteristic, and can directly perform selective angle filtering on the angular spectrum of the object in the object plane, and at the same time convert the mid-infrared information into the visible light / near-infrared band.

5. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S3, the propagation direction of the pump light is input from the signal light output end of the nonlinear crystal, and propagates in the nonlinear crystal toward the signal light input end of the nonlinear crystal. At the same time, a reflective film is coated on one end of the nonlinear crystal facing the signal light input, which is used to reflect the pump light propagating thereto, so as to change the propagation direction of the pump light.

6. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S3, the specific condition refers to regulating the transfer function of the system by changing the phase matching condition of the crystal, so that each plane wave incident on the nonlinear crystal at an inclined angle is frequency converted.

7. The edge enhancement imaging method based on nonlinear angle filtering according to claim 6, characterized in that: The method for regulating the transfer function of the system is to use a wavelength tunable signal light source, or to use a broadband light source combined with a narrowband filter device, or to use a narrowband signal light source combined with crystal temperature regulation.

8. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: In step S4, the imaging system uses a single lens to image the optical signal after nonlinear angle filtering, or uses a combined microlens to image the optical signal after nonlinear angle filtering.

9. The edge enhancement imaging method based on nonlinear angle filtering according to claim 1, characterized in that: The converted optical signal in step S3 and step S4 is up-converted light, and the up-converted light refers to a light wave whose frequency is converted from an original light wave to a higher frequency light wave.

10. An edge enhancement imaging system based on nonlinear angle filtering, used to implement the edge enhancement imaging method based on nonlinear angle filtering as claimed in any one of claims 1 to 9, characterized in that: It includes a signal light source for emitting signal light, wherein a first quarter wave plate and a first half wave plate are sequentially arranged at a light emitting end of the signal light source; the first quarter wave plate is used to convert the signal light into linearly polarized light; and the first half wave plate is used to adjust the linearly polarized light into vertically polarized light; An object to be imaged is placed at the light emitting end of the first half-wave plate; a nonlinear crystal is arranged on the side of the object to be imaged away from the first half-wave plate; a reflective film is plated on the end of the nonlinear crystal facing the signal light source; a temperature-controlled furnace is placed at the bottom of the nonlinear crystal for adjusting the temperature of the nonlinear crystal; The imaging system is also provided with a pump light source for generating pump light; a light emitting end of the pump light source is provided with a second quarter wave and a second half wave plate in sequence; the second quarter wave is used to convert the pump light into linearly polarized light; the second half wave plate is used to adjust the linearly polarized light into vertically polarized light; An isolator is provided at the light output end of the second half-wave plate to prevent the pump light from being reflected back to the pump light source; a reflective silver mirror and a dichroic mirror are provided in sequence at the light output end of the isolator; the reflective silver mirror is used to change the direction of the pump light, and the dichroic mirror is used to transmit the up-converted visible light and reflect the pump light to the signal light output end of the nonlinear crystal, so that the pump light propagates from the signal light output end of the nonlinear crystal to the reflective film, and is vertically reflected at the reflective film, and then propagates in the same direction as the signal light; The light emitting end of the dichroic mirror is provided with a convex lens, a filter system and a visible light detector in sequence; the convex lens is an imaging lens for collecting and imaging the up-converted light, the filter system is used to filter out noise, and the visible light detector is used to collect and record the up-converted visible light information.

Images