Terahertz free-form surface off-axis three-mirror collimation system

Abstract

The present invention discloses a terahertz free-form surface off-axis three-mirror collimating system. The system adopts a self-developed free-form surface design method, and adopts an XY polynomial surface designed by the free-form surface design technology, so that the system has no central obstruction, no chromatic aberration, and a long working band, and achieves a large field of view and low distortion imaging effect, and better realizes the correction and balance of the aberration of the system, effectively eliminates the aberration, and improves the sensitivity and imaging quality of the system. The off-axis three-mirror optical design scheme is adopted to reduce the volume and weight of the system and increase the effective aperture. It is suitable for focal plane array detectors and can improve the sensitivity and imaging quality of terahertz imaging systems. The present invention has broad application prospects and can be used for non-destructive testing in the fields of aerospace, electronic products, food safety, biomedicine, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz imaging, and particularly relates to a terahertz free-form off-axis three-reflection quasi-optical system. Background Art

[0002] Terahertz wave (THz) is an electromagnetic wave between microwave and infrared light, with a frequency range of 0.1 THz to 10 THz and a wavelength of 3 mm to 30 μm. Terahertz wave has unique advantages such as strong penetrability, non-contact detection, and wide spectral lines, and has broad application prospects in the field of non-destructive testing. In recent years, terahertz imaging technology has been widely applied in the fields of aerospace, electronic products, food safety, biomedicine, etc.

[0003] Currently, the focal plane array detector (FPA) is one of the main detectors used in terahertz imaging technology. In order to improve the sensitivity and imaging quality of the FPA, a quasi-optical system is usually required to focus the terahertz wave. There are mainly two types of quasi-optical systems used in existing focal plane array detectors: (1) Refractive lens quasi-optical system: The refractive lens quasi-optical system usually consists of 2 or 3 refractive lenses and can effectively focus the terahertz wave. However, the transmission loss of the refractive lens material and the interference between different interfaces will reduce the sensitivity and imaging quality of the system. (2) Reflector quasi-optical system: The reflector quasi-optical system usually consists of 2 or 3 reflectors and can avoid material transmission loss. However, it is difficult for traditional spherical or aspherical reflectors to eliminate aberration, resulting in poor imaging quality.

[0004] The information disclosed in this background art section is only intended to enhance the overall understanding of the present invention and should not be regarded as an admission or any form of suggestion that this information constitutes prior art already known to those of ordinary skill in the art. Summary of the Invention

[0005] The purpose of the present invention is to provide a terahertz free-form off-axis three-reflection quasi-optical system, which uses an independently developed free-form design method to effectively eliminate aberration and improve the sensitivity and imaging quality of the system.

[0006] To achieve the above purpose, a specific embodiment of the present invention provides a terahertz free-form off-axis three-reflection quasi-optical system, including:

[0007] A first reflector, arranged on the outgoing light path of the light, for reflecting the light to form a first reflected light beam;

[0008] A second reflector, arranged on the reflection light path of the first reflector, for secondarily reflecting the first reflected light beam to form a second reflected light beam;

[0009] The third reflector is disposed on the reflection optical path of the second reflector and is configured to reflect the second reflected light beam again to form a third reflected light beam;

[0010] The detector is located on the reflection optical path of the third reflector and at the converging light spot of the third reflected light beam, and is configured to receive the third reflected light beam;

[0011] The reflecting surfaces of the first reflector, the second reflector and the third reflector all adopt an xy polynomial surface type, and the XY polynomial is an even polynomial of X, and the highest degree of X is 6;

[0012] A global three-dimensional rectangular coordinate system (X, Y, Z) is established, and the centers of the first reflector, the second reflector and the third reflector are respectively located at points O 1 , point O 2 and point O 3 ; the distances between point O 1 , point O 2 and point O 3 satisfy the following relationship: d 23 >= d 12 >= d 13 ;

[0013] wherein, d 12 is the distance between point O 1 and point O 2 , d 23 is the distance between point O 2 and point O 3 , d 13 is the distance between point O 1 and point O 3 ;

[0014] The clear aperture of the terahertz free-form off-axis three-reflector quasi-optical system is 60 nm - 160 mm, the F number is 1 - 1.3, and the field of view angle range is (±4.2°×±4.2°)~(±4.6°×±4.6°).

[0015] In one or more embodiments of the present invention, the terahertz free-form off-axis three-reflector quasi-optical system further includes an aperture stop, and the aperture stop is disposed on the second reflector; or,

[0016] The second reflector is an aperture stop.

[0017] In one or more embodiments of the present invention, the reflecting surfaces of the first reflector, the second reflector and the third reflector are all 6th-order XY polynomial free-form surfaces containing even terms of X, and the XY polynomial free-form surface equation is:

[0018]

[0019] Among them, the radius of curvature of the first reflector is 2646.08299075151 mm, the conic coefficient is 109.049764178204, the X half-width is 160 mm, the Y half-width is 160 mm, and the surface parameters of the XY polynomial are:

[0020]

[0021] The radius of curvature of the second reflector is -646.297743530395 mm, the conic coefficient is 10.670946693987, the X half-width is 80 mm, the Y half-width is 80 mm, and the surface parameters of the XY polynomial are:

[0022]

[0023] The radius of curvature of the third reflector is -436.76242195341 mm, the conic coefficient is -0.00275362836076844, the radius of curvature is 180 mm, and the surface parameters of the XY polynomial are:

[0024]

[0025]

[0026] In one or more embodiments of the present invention, the center point O of the first reflector 1 and the center point O of the second reflector 2 The distance d 12 is 400.008 mm; the center point O of the second reflector 2 and the center point O of the third reflector 3 The distance d 23 is 400.071 mm; the center point O of the third reflector 3 and the distance from the plane center of the detector is 399.935 mm.

[0027] In one or more embodiments of the present invention, the distance between the lowermost light ray of the light incident on the first reflector and the upper edge of the second reflector is greater than 10 mm; the distance between the third reflected beam of the third reflector and the lower edge of the second reflector is at least greater than 10 mm.

[0028] In one or more embodiments of the present invention, the focal length of the terahertz free-form off-axis three-mirror quasi-optical system is 160 mm, the clear aperture is 120 mm - 160 mm, and the F number is 1 - 1.3;

[0029] The detector is a focal plane array detector. The array scale number of the focal plane array detector is 32×32 or 64×64. The size of each array unit is 0.4 mm, and the array scale of the focal plane array detector is 25.6 mm×25.6 mm;

[0030] The field of view angle range of the terahertz free-form off-axis three-reflection quasi-optical system is ±4.6°×±4.6°.

[0031] In one or more embodiments of the present invention, the root mean square radius within the field of view of the terahertz free-form off-axis three-reflection quasi-optical system is 5.424 μm - 25.947 μm, and the diffraction limit at a wavelength of 0.882 μm is 0.27 lp / mm.

[0032] In one or more embodiments of the present invention, the reflecting surfaces of the first mirror, the second mirror, and the third mirror are all 6th-order XY polynomial free-form surfaces containing X even terms. The XY polynomial free-form surface equation is:

[0033]

[0034] Among them, the radius of curvature of the first mirror is 1323.06433748304 mm, the conic coefficient is 109.049764178204, the X half-width is 80 mm, the Y half-width is 80 mm, and the surface type parameters of the XY polynomial are:

[0035]

[0036]

[0037] The radius of curvature of the second mirror is -323.154450880609 mm, the conic coefficient is 10.670946693987, the X half-width is 40 mm, the Y half-width is 40 mm, and the surface type parameters of the XY polynomial are:

[0038]

[0039]

[0040] The radius of curvature of the third mirror is -218.384981294618 mm, the conic coefficient is -0.00275362836076844, the X half-width is 95 mm, the Y half-width is 110 mm, and the surface type parameters of the XY polynomial are:

[0041]

[0042] In one or more embodiments of the present invention, the center point O of the first mirror1 and the distance d 2 from the center point O of the second mirror 12 is 200.008 mm; the center point O of the second mirror 2 and the distance d 3 from the center point O of the third mirror 23 is 200.039 mm; the distance from the center point O of the third mirror 3 to the plane center of the detector is 199.971 mm.

[0043] In one or more embodiments of the present invention, the distance between the lowermost ray of the light incident on the first mirror and the upper edge of the second mirror is greater than 10 mm; the distance between the third reflected beam of the third mirror and the lower edge of the second mirror is at least greater than 10 mm.

[0044] In one or more embodiments of the present invention, the focal length of the terahertz free-form off-axis three-reflection quasi-optical system is 80 mm, the clear aperture is 60 mm - 80 mm, and the F number is 1 - 1.2;

[0045] The detector is a focal plane array detector. The array scale number of the focal plane array detector is 32×32 or 64×64. The size of each array unit is 0.4 mm, and the array scale of the focal plane array detector is 12.8 mm×12.8 mm;

[0046] The field of view angle range of the terahertz free-form off-axis three-reflection quasi-optical system is ±4.2°×±4.2°.

[0047] In one or more embodiments of the present invention, the root mean square radius within the field of view of the terahertz free-form off-axis three-reflection quasi-optical system is 2.785 μm - 7.689 μm, and the diffraction limit is 0.33 lp / mm at a wavelength of 0.882 μm.

[0048] In one or more embodiments of the present invention, the terahertz wave is an electromagnetic wave with a frequency range of 0.1 THz to 10 THz.

[0049] Compared with the prior art, the terahertz free-form off-axis three-reflection quasi-optical system of the present invention adopts the XY polynomial surface type designed by the free-form design technology, so that the system has no central obstruction, no chromatic aberration, a long working wavelength band, realizes the imaging effect of a large field of view and low distortion, better realizes the correction and balance of the system's aberrations, effectively eliminates aberrations, and improves the imaging quality.

[0050] The off-axis three-reflection quasi-optical system with a terahertz freeform surface of the present invention uses a freeform XY polynomial surface for the first mirror, the second mirror, and the third mirror to correct off-axis aberrations. At the same time, the structure of each mirror surface has eccentricity and tilt in the plane. The freeform surface used is symmetric about the sagittal plane, and only the even-term parameters of X are changed, ensuring that the entire system is symmetric about the sagittal plane, greatly reducing the production and processing difficulty and increasing the system field of view.

[0051] For the off-axis three-reflection quasi-optical system with a terahertz freeform surface of the present invention, the operand is used to constrain that the distance between the lowermost incident light ray and the upper edge of the second mirror is greater than 10 mm, and the distance between the third reflected light beam reflected by the third mirror and the lower edge of the second mirror is greater than 10 mm, ensuring that all light rays are unobstructed during the optimization process, preventing the reflected light rays from intersecting on different mirror surfaces, resulting in serious stray light and affecting the imaging quality.

[0052] The off-axis three-reflection quasi-optical system with a terahertz freeform surface of the present invention adopts an off-axis three-reflection optical design scheme, reducing the system volume and weight and increasing the effective aperture.

[0053] The off-axis three-reflection quasi-optical system with a terahertz freeform surface of the present invention is applicable to a focal plane array detector and can improve the sensitivity and imaging quality of a terahertz imaging system.

[0054] The off-axis three-reflection quasi-optical system with a terahertz freeform surface of the present invention has broad application prospects and can be used for non-destructive testing in fields such as aerospace, electronic products, food safety, and biomedicine. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0056] Figure 1 It is a schematic structural diagram of the off-axis three-reflection quasi-optical system with a terahertz freeform surface in Embodiment 1 of the present invention;

[0057] Figure 2 It is the transfer curve of the off-axis three-reflection quasi-optical system with a terahertz freeform surface in Embodiment 1 of the present invention in the terahertz band (0.882 mm);

[0058] Figure 3 It is the spot diagram of the off-axis three-reflection quasi-optical system with a terahertz freeform surface in Embodiment 1 of the present invention;

[0059] Figure 4It is the ray trace diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 1 of the present invention;

[0060] Figures 5a - 5d It is the field curvature / F-Tan(Theta) distortion diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 1 of the present invention;

[0061] Figure 6 It is the geometric image analysis diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 1 of the present invention.

[0062] Figure 7 It is the structural schematic diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention;

[0063] Figure 8 It is the transfer curve of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention at the terahertz band (0.882 mm);

[0064] Figure 9 It is the spot diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention;

[0065] Figure 10 It is the ray trace diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention;

[0066] Figures 11a - 11d It is the field curvature / F-Tan(Theta) distortion diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention;

[0067] Figure 12 It is the geometric image analysis diagram of the terahertz freeform off-axis three-reflection quasi-optical system in Embodiment 2 of the present invention. Detailed implementation manners

[0068] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0069] As described in the background art, the focal plane array detector (FPA) is one of the main detectors used in terahertz imaging technology. In order to improve the sensitivity and imaging quality of the FPA, a quasi-optical system is usually required to focus terahertz waves. There are mainly two types of quasi-optical systems used in existing focal plane array detectors: (1) Refractive lens quasi-optical system: The refractive lens quasi-optical system usually consists of two or three refractive lenses and can effectively focus terahertz waves. However, the transmission loss of the refractive lens material and the interference between different interfaces will reduce the sensitivity and imaging quality of the system. (2) Mirror quasi-optical system: The mirror quasi-optical system usually consists of two or three mirrors and can avoid material transmission loss. However, it is difficult for traditional spherical or aspherical mirrors to eliminate aberrations, resulting in poor imaging quality.

[0070] Aiming at the limitations of the existing terahertz quasi-optical system for detectors, the transmission loss of terahertz refractive lenses, and the problems that the interference between different interfaces will reduce the sensitivity and imaging quality of the system, etc., the present invention proposes a novel terahertz free-form off-axis three-mirror quasi-optical system. This system adopts an independently developed free-form design method. Through the XY polynomial surface type designed by using free-form design technology, the system has no central obstruction, no chromatic aberration, a long working wavelength band, realizes an imaging effect with a large field of view and low distortion, better realizes the correction and balance of the system's aberrations, effectively eliminates aberrations, and improves the sensitivity and imaging quality of the system. Adopting an off-axis three-mirror optical design scheme reduces the volume and weight of the system and increases the effective aperture. It is applicable to focal plane array detectors and can improve the sensitivity and imaging quality of terahertz imaging systems. The present invention has broad application prospects and can be used for non-destructive testing in fields such as aerospace, electronic products, food safety, and biomedicine.

[0071] As Figure 1 and Figure 7 As shown, the terahertz free-form off-axis three-mirror quasi-optical system of the present invention includes a first mirror 10, a second mirror 20, a third mirror 30, and a detector 40.

[0072] The first mirror 10 is arranged on the outgoing light path of the light to reflect the light and form a first reflected light beam; the second mirror 20 is arranged on the reflected light path of the first mirror to reflect the first reflected light beam twice and form a second reflected light beam; the third mirror 30 is arranged on the reflected light path of the second mirror to reflect the second reflected light beam again and form a third reflected light beam; the detector 40 is located on the reflected light path of the third mirror and at the converging spot of the third reflected light beam to receive the third reflected light beam. The incident light is reflected successively by the first mirror 10, the second mirror 20, and the third mirror 30 and finally forms an image on the detector 40.

[0073] Among them, in order to correct off-axis aberrations and improve imaging quality, the reflecting surfaces of the first mirror 10, the second mirror 20, and the third mirror 30 all adopt an XY polynomial surface type, and the XY polynomial is an even polynomial of X, and the highest degree of X is 6. An aperture stop is provided on the reflecting surface of the second mirror 20, or the second mirror 20 is an aperture stop. The detector 40 is a focal plane array detector. Terahertz waves are electromagnetic waves in the range of 0.1 THz to 10 THz.

[0074] In the terahertz free-form off-axis three-reflection quasi-optical system of the present invention, the optical path during operation is as follows: The incident terahertz wave on the object to be measured is reflected on the first mirror 10 to form a first reflected beam; the first reflected beam irradiates on the second mirror 20 and is reflected to form a second reflected beam; the second reflected beam irradiates on the third mirror 30 and is reflected to form a third reflected beam, and finally an image is formed on the detector 40.

[0075] For ease of description, a global three-dimensional rectangular coordinate system (X, Y, Z) is established, and the centers of the first mirror 10, the second mirror 20, and the third mirror 30 are respectively located at points O 1 、point O 2 and point O 3 ; among them, the distances between point O 1 、point O 2 and point O 3 satisfy the following relationship: d 23 > = d 12 > = d 13 ; where d 12 is the distance between point O 1 and point O 2 , d 23 is the distance between point O 2 and point O 3 , d 13 is the distance between point O 1 and point O 3 . The above layout is designed to optimize the optical path and reduce the system volume.

[0076] In the present invention, the clear aperture of the terahertz free-form off-axis three-reflection quasi-optical system is 60 nm - 160 mm, the F number is 1 - 1.3, and the field of view angle range is (±4.2°×±4.2°) - (±4.6°×±4.6°).

[0077] The technical solutions of the present application are elaborated in detail below through two specific embodiments in conjunction with the accompanying drawings.

[0078] Embodiment 1:

[0079] The radius of curvature of the first mirror 10 is set to 2646.08299075151 mm, the conic coefficient is 109.049764178204, and the X half-width and Y half-width are both 160 mm. The XY polynomial surface parameters of this first mirror are listed in detail to ensure effective correction of light rays.

[0080] The reflecting surface of the first mirror 10 is an x 1 y 1 polynomial free-form surface, and the equation of this x 1 y 1 polynomial free-form surface can be expressed as:

[0081]

[0082] where z is the surface sag, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the j-th term in the polynomial. Since the terahertz free-form off-axis three-mirror quasi-optical system is symmetric about the y 1 z 1 plane, therefore, only the even terms of x 1 can be retained. Preferably, the reflecting surface of the first mirror 10 is an x 1 y 1 polynomial free-form surface, and the x 1 y 1 polynomial is an even polynomial of x 1 with the highest degree of x 1 being 6. The equation of this x 1 y 1 polynomial free-form surface can be expressed as

[0083]

[0084] The surface parameters are:

[0085]

[0086] The second mirror 20 also serves as the aperture stop. Its radius of curvature is -646.297743530395 mm, the conic coefficient is 10.670946693987, and the X half-width and Y half-width are both 80 mm.

[0087] The reflecting surface of the second mirror 20 is an x 2 y 2 polynomial free-form surface, and the equation of this x 2 y 2 polynomial free-form surface can be expressed as:

[0088]

[0089] Among them, z is the sag of the surface, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the j-th term in the polynomial. Since the terahertz freeform off-axis three-reflection quasi-optical system is symmetric about the y 2 z 2 plane, therefore, only the even terms of x 2 can be retained. Preferably, the reflecting surface of the second mirror 20 is an x 2 y 2 polynomial freeform surface, and the x 2 y 2 polynomial is an even polynomial of x 2 The highest degree of x 2 is 6, and the equation of this x 2 y 2 polynomial freeform surface can be expressed as

[0090]

[0091] The surface parameters are as follows:

[0092]

[0093]

[0094] The radius of curvature of the third mirror 30 is -436.76242195341 mm, and the conic coefficient is -0.00275362836076844. The surface shape is a circle with a radius of 180 mm. The XY polynomial surface parameters of each mirror are accurately calculated to achieve the best imaging effect.

[0095] The reflecting surface of the third mirror 30 is an x 3 y 3 polynomial freeform surface, and the equation of this x 3 y 3 polynomial freeform surface can be expressed as:

[0096]

[0097] Among them, z is the sag of the surface, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the j-th term in the polynomial. Since the terahertz freeform off-axis three-reflection quasi-optical system is symmetric about the y 3 z 3 plane, therefore, only the even terms of x 3 can be retained. Preferably, the reflecting surface of the third mirror 30 is an x 3 y 3 polynomial freeform surface, and the x 3 y 3 polynomial is an even polynomial of x 3 The highest degree of x 3The highest degree is 6, and this x 3 y 3 The equation of the polynomial free-form surface can be expressed as

[0098]

[0099] The surface parameters are as follows:

[0100]

[0101] The center point O of the first mirror 10 1 and the center point O of the second mirror 20 2 The distance d 12 is 400.008 mm; the center point O of the second mirror 20 2 and the center point O of the third mirror 30 3 The distance d 23 is 400.071 mm; the center point O of the third mirror 30 3 The distance to the plane center of the focal plane array detector 40 is 399.935 mm. These precise distance settings ensure the focusing performance of the system.

[0102] The system design takes into account the incident angle of the light, ensuring that the distance between the lowermost light of the incident light and the upper edge of the second mirror 20 is greater than 10 mm, and at the same time, the distance between the reflected light of the third mirror 30 and the lower edge of the second mirror 20 is greater than 10 mm, avoiding the problem of light occlusion.

[0103] The clear aperture of the terahertz free-form off-axis three-mirror quasi-optical system in this Embodiment 1 is designed to be 120 mm - 160 mm, the focal length is 160 mm, the F-number range is 1 - 1.3, the array scale of the focal plane array detector is 25.6 mm × 25.6 mm, and the field of view angle range of the terahertz free-form off-axis three-mirror quasi-optical system is ±4.6° × ±4.6°. The root mean square radius within the field of view of the system is 5.424 μm - 25.947 μm. The diffraction limit at a wavelength of 0.882 μm is 0.27 lp / mm, and the transfer function is better than 0.68 at 0.2 lp / mm.

[0104] Figure 2The MTF graph shown presents the MTF values from 0 to 0.2 line pairs per millimeter (lp / mm), which reflect the system's ability to respond to different detail sizes. The higher the spatial frequency, the smaller the details the system can resolve. From the MTF graph, it can be observed that for the system of this embodiment, at a spatial frequency of 0.16 lp / mm, the MTF value is 0.76, and at a spatial frequency of 0.2 lp / mm, the MTF value is 0.65. Since an MTF value greater than 0.5 indicates that the system has the corresponding resolution ability, it is thus proven that Embodiment 1 has the corresponding resolution ability. The MTF graph usually presents the MTF values in the meridional and sagittal directions respectively. In an ideal situation, the MTF values in both directions should be close, indicating that the system has consistent imaging performance in all directions. The diffraction limits in the meridional and sagittal directions are also marked in the graph, which is the upper limit of the system performance determined by the diffraction characteristics of light. The degree of closeness between the MTF values at different field angles and the diffraction limits indicates the performance potential of the system in these directions. The MTF value changes with the increase of the field angle, and this change reflects the imaging consistency of the system at different field positions. In Embodiment 1, the field positions from 0° to ±4.6° are all close to the imaging quality of the diffraction limit.

[0105] By analyzing the spot diagram of the terahertz freeform off-axis three-reflection quasi-optical system of Embodiment 1, as Figure 3 shown. At the central field (0°, 0°), the RMS radius of the spot is 5.42 μm, and the spot is concentrated and uniform, indicating that the system has high imaging quality at the central field. At the positions of (3.25°, 0°) and (-3.25°, 0°), the RMS spot radii are both 9.28 μm, and the spot distribution has no diffusion, and the overall imaging quality is good. At the positions of (0°, 4.6°) and (0°, -4.6°), the RMS spot radii are 15.48 μm and 20.575 μm respectively, showing obvious distortion, and the spot size increases significantly, but the distribution is uniform and still within an acceptable range. The overall range of the RMS spot radius is between 5.424 μm - 25.947 μm, reflecting the uniform imaging quality of the system.

[0106] The ray trace diagram of the terahertz freeform off-axis three-reflection quasi-optical system of Embodiment 1 clearly shows the propagation of light rays in the optical system, as Figure 4 shown. By Figure 4It can be seen that the light rays incident at different field angles follow the expected propagation paths after passing through each optical element, without generating stray light. In this ray trace diagram, the markers of different colors represent the positions of different field angles, specifically including 0°, 3.25°, 4.6°, etc. By observing the focal points of the light rays on the image plane, the imaging quality of the system can be evaluated. The ray trace diagram shows that the focal points of the light rays on the image plane are evenly distributed. Even at the edge of the field of view, the light rays can be well focused, which is one of the advantages of using a freeform surface design in this Embodiment 1. This ray trace diagram provides the minimum and maximum positions of the light rays at different field angles in the X and Y directions, which are -12.84 mm to 12.84 mm and -13.07 mm to 13.09 mm respectively. This is crucial for evaluating the field of view range and the size of the imaging area of the system.

[0107] By analyzing the field curvature and distortion of the terahertz freeform off-axis three-mirror quasi-optical system in Embodiment 1 in different directions, the analysis results are shown in Figure 5, covering the X positive direction ( Figure 5a )), X negative direction ( Figure 5b ), Y positive direction ( Figure 5c ), and Y negative direction ( Figure 5d ) within the field angle range of 0° to 4.6°. For the X positive direction, the meridional field curvature is 0.0314 mm, the sagittal field curvature is 0.0659 mm, and the maximum distortion is 0.0115%, which indicates that the distortion in the X positive direction is effectively controlled. Similarly, the field curvature in the X negative direction is basically the same as that in the X positive direction, the meridional field curvature is also 0.0313 mm, and the maximum distortion in the X negative direction is also 0.0115%, showing the symmetry of the system. In the Y positive direction, the meridional field curvature increases to 0.1239 mm, and the sagittal field curvature is 0.0668 mm. Compared with the X direction, the field curvature in the Y positive direction is larger, which means that the optical performance in the Y direction decreases slightly. The maximum distortion in the Y positive direction is 0.0495%, which is higher than that in the X direction, indicating that the distortion control in the Y positive direction is not as good as that in the X direction. This is mainly because the system is mainly off-axis in the Y direction. For the Y negative direction, the meridional field curvature is significantly higher than other directions, reaching 0.1383 mm, and the sagittal field curvature is 0.0344 mm. The maximum distortion in the Y negative direction is 0.1555%, which is the largest distortion value among the four directions. Based on the above analysis, this freeform off-axis three-mirror optical system shows good symmetry and low distortion in the x positive direction and x negative direction of the image plane, and the field curvature is well controlled.

[0108] Geometric image analysis is a method of pure geometric ray tracing used to evaluate the imaging performance of an optical system and is known as the "gold standard" for simulated images. By performing geometric image analysis on the terahertz freeform off-axis three-mirror quasi-optical system in Embodiment 1, the results are as Figure 6As shown. The analysis results show that the imaging resolution of the system is extremely high, and the clarity and detail distinguishability of the images have reached a satisfactory level. In addition, there is no obvious distortion in the images, which further confirms that the system has extremely low distortion.

[0109] Figures 2 to 6 It shows that the terahertz free-form off-axis three-reflection quasi-optical system in Embodiment 1 has high imaging quality and resolution.

[0110] It can be seen from Embodiment 1 that applying the free form to the off-axis reflection system simultaneously solves the problems of large field of view, high image quality, high sensitivity, and high signal-to-noise ratio. It can be used for non-destructive testing of terahertz focal plane array detectors, achieving an imaging effect of large field of view and low distortion, better realizing the correction and balance of the system's aberrations, and having important practical significance and application prospects.

[0111] The terahertz free-form off-axis three-reflection quasi-optical system of the present invention effectively corrects off-axis aberrations by adopting the free-form XY polynomial surface type. At the same time, due to the eccentric and tilted design of the mirror structure and the symmetry of the sagittal plane, the production and processing difficulty is reduced and the system field of view is increased. The operand constraints in the system design ensure that there is no light occlusion during the optimization process, ensuring high imaging quality and system performance. In addition, the system has significant advantages in achieving large field of view, high image quality, high sensitivity, and high signal-to-noise ratio, is suitable for non-destructive testing of terahertz focal plane array detectors, and has important practical significance and broad application prospects.

[0112] Embodiment 2

[0113] The radius of curvature of the first mirror 10 is set to 1323.06433748304 mm, the conic coefficient is 109.049764178204, the X half-width is 80 mm, and the Y half-width is 80 mm. The XY polynomial surface type parameters of this first mirror are listed in detail to ensure the effective correction of light rays.

[0114] The reflecting surface of the first mirror 10 is the x 1 y 1 polynomial free form, and the equation of this x 1 y 1 polynomial free form can be expressed as:

[0115]

[0116] Among them, z is the surface sag, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the jth term in the polynomial. Since the terahertz free-form off-axis three-reflection quasi-optical system is symmetric about the y 1 z 1 plane, therefore, only the x 1Even-order terms. Preferably, the reflecting surface of the first mirror 10 is an x 1 y 1 polynomial free-form surface, where x 1 y 1 the polynomial is an even-order polynomial of x 1 with the highest degree of 6, and this x 1 y 1 y 1 The equation of the polynomial free-form surface can be expressed as

[0117]

[0118] The surface parameters are:

[0119]

[0120] The second mirror 20 also serves as the aperture stop, with a radius of curvature of -323.154450880609 mm, a conic coefficient of 10.670946693987, and both the X half-width and the Y half-width are 40 mm.

[0121] The reflecting surface of the second mirror 20 is an x 2 y 2 polynomial free-form surface, and this x 2 y 2 The equation of the polynomial free-form surface can be expressed as:

[0122]

[0123] where z is the surface sag, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the j-th term in the polynomial. Since the terahertz free-form off-axis three-mirror catadioptric system is symmetric about the y 2 z 2 plane, therefore, only the even-order terms of x 2 can be retained. Preferably, the reflecting surface of the second mirror 20 is an x 2 y 2 polynomial free-form surface, where x 2 y 2 the polynomial is an even-order polynomial of x 2 with the highest degree of 6, and this x 2 y 2 y 2 The equation of the polynomial free-form surface can be expressed as

[0124]

[0125] The surface parameters are:

[0126]

[0127]

[0128] The radius of curvature of the third mirror 30 is -218.384981294618 mm, the conic coefficient is -0.00275362836076844, the X half-width is 95 mm, and the Y half-width is 110 mm. The XY polynomial surface parameters of each mirror are accurately calculated to achieve the best imaging effect.

[0129] The reflecting surface of the third mirror 30 is an 3 y 3 x polynomial free-form surface, and the 3 y 3 equation of the x polynomial free-form surface can be expressed as:

[0130]

[0131] where z is the surface sag, c is the surface curvature, k is the conic coefficient, and Cj is the coefficient of the jth term in the polynomial. Since the terahertz free-form off-axis three-mirror quasi-optical system is symmetric about the 3 z 3 y plane, therefore, only the even terms of 3 x can be retained. Preferably, the reflecting surface of the third mirror 30 is an 3 y 3 x polynomial free-form surface, and the 3 y 3 polynomial is an even polynomial of 3 x, the highest degree of 3 x is 6, and the equation of the 3 y 3 x polynomial free-form surface can be expressed as

[0132]

[0133] The surface parameters are:

[0134]

[0135] The distance d 1 between the center point O of the first mirror 10 2 and the center point O of the second mirror 20 12 is 200.008 mm; the distance d 2 between the center point O of the second mirror 20 3 and the center point O of the third mirror 30 23 is 200.039 mm; the distance d 3The distance from the plane center of the focal plane array detector 40 is 199.971 mm. These precise distance settings ensure the focusing performance of the system.

[0136] The system design takes into account the incident angle of light, ensuring that the distance between the bottommost light of the incident light and the upper edge of the second mirror 20 is greater than 10 mm, and at the same time, the distance between the reflected light of the third mirror 30 and the lower edge of the second mirror 20 is greater than 10 mm, avoiding the problem of light occlusion.

[0137] In this embodiment 2, the clear aperture of the terahertz freeform off-axis three-mirror anastigmat system is designed to be 60 mm - 80 mm, the focal length is 80 mm, the F-number range is 1 - 1.2, the array scale of the focal plane array detector is 12.8 mm × 12.8 mm; the field of view angle range of the system is ±4.2° × ±4.2°. The root mean square radius within the field of view of the system is 2.785 μm - 7.689 μm. The diffraction limit at a wavelength of 0.882 μm is 0.33 lp / mm, and the transfer function is better than 0.70 at 0.2 lp / mm.

[0138] Figure 8 The shown MTF graph shows the MTF values from 0 to 0.2 line pairs per millimeter (lp / mm), and these values reflect the system's response ability to different detail sizes. The higher the spatial frequency, the smaller the details that the system can resolve. The goal of this embodiment is to achieve an imaging resolution of 3 mm, which corresponds to a spatial frequency of 0.16 lp / mm. According to the MTF graph, it can be observed that at a spatial frequency of 0.16 lp / mm, the MTF value is 0.7, and at a spatial frequency of 0.2 lp / mm, the MTF value is 0.6. Since an MTF value greater than 0.5 can be considered that the system has the corresponding resolution ability, it is proved that this embodiment 2 has the corresponding resolution ability. The MTF graph usually shows the MTF values in the meridional and sagittal directions respectively. In an ideal situation, the MTF values in the two directions should be close, indicating that the system has consistent imaging performance in all directions. The diffraction limits in the meridional and sagittal directions are also marked in the graph, which is the upper limit of the system performance determined by the diffraction characteristics of light. The degree of closeness between the MTF values of different field of view angles and the diffraction limit indicates the performance potential of the system in these directions. The MTF value changes with the increase of the field of view angle, and this change reflects the imaging consistency of the system at different field of view positions. In this embodiment 2, the field of view positions from 0° to ±4.6° are all close to the imaging quality of the diffraction limit.

[0139] By analyzing the spot diagram of the terahertz freeform off-axis three-mirror anastigmat system of this embodiment 2, as Figure 9As shown. At the central field of view (0°, 0°), the RMS radius of the light spot is 2.78 μm, and the light spot is concentrated and uniform, indicating that the system has high imaging quality at the central field of view. At the positions of (3.25°, 0°) and (-3.25°, 0°), the RMS radius of the light spot is 4.71 μm for both, and the distribution of the light spot has no diffusion, and the overall imaging quality is good. At the positions of (0°, 4.6°) and (0°, -4.6°), the RMS radius of the light spot is 7.68 μm for both, showing obvious distortion, the size of the light spot increases significantly, but the distribution is uniform and still within an acceptable range. The overall range of the RMS radius of the light spot is between 2.785 μm and 7.689 μm, reflecting the uniform imaging quality of the system.

[0140] The ray trace diagram of the terahertz freeform off-axis three-mirror quasi-optical system in this Embodiment 2 clearly shows the propagation of light rays in the optical system, as Figure 10 shown. From Figure 10 it can be seen that after the light rays incident at different field angles pass through each optical element, their propagation paths meet the expectations and there is no stray light. In this ray trace diagram, the marks of different colors represent the positions of different field angles, specifically including 0°, 3.25°, 4.6°, etc. By observing the focal points of the light rays on the image plane, the imaging quality of the system can be evaluated. The ray trace diagram shows that the distribution of the focal points of the light rays on the image plane is uniform. Even at the edge of the field of view, the light rays can be well focused, which is one of the advantages of using the freeform design in this Embodiment 2. This ray trace diagram provides the minimum and maximum positions of the light rays at different field angles in the X and Y directions, which are -6.41 mm to 6.41 mm and -6.53 mm to 6.55 mm respectively, and this is crucial for evaluating the field of view range and the size of the imaging area of the system.

[0141] By analyzing the field curvature and distortion of the terahertz free-form off-axis three-mirror anastigmat (TMA) quasi-optical system in Example 2 in different directions, the analysis results are shown in Figure 11, covering the positive X direction (Figure 11.a), negative X direction (Figure 11.b), positive Y direction (Figure 11.c), and negative Y direction (Figure 11.d) within the field of view angle range of 0° to 4.6°. For the positive X direction, the meridional field curvature is 0.0156 mm, the sagittal field curvature is 0.0330 mm, and the maximum distortion is 0.0114%, indicating that the distortion in the positive X direction is effectively controlled. Similarly, the field curvature in the negative X direction is the same as that in the positive X direction, with the meridional field curvature also being 0.0156 mm, and the maximum distortion in the negative X direction is also 0.0114%, showing the symmetry of the system. In the positive Y direction, the meridional field curvature increases to 0.0618 mm, and the sagittal field curvature is 0.0334 mm. Compared with the X direction, the field curvature in the positive Y direction is larger, indicating a slight decline in the optical performance in the Y direction. The maximum distortion in the positive Y direction is 0.0507%, higher than that in the X direction, indicating that the distortion control in the positive Y direction is not as good as that in the X direction, mainly because the system is mainly off-axis in the Y direction. For the negative Y direction, the meridional field curvature is significantly higher than other directions, reaching 0.0696 mm, and the sagittal field curvature is 0.0172 mm. The maximum distortion in the negative Y direction is 0.1537%, which is the largest distortion value among the four directions. Based on the above analysis, the free-form off-axis three-mirror optical system shows good symmetry and low distortion in the positive and negative x directions of the image plane, and the field curvature is well controlled.

[0142] Geometric image analysis is a method of pure geometric ray tracing used to evaluate the imaging performance of an optical system and is known as the "gold standard" for simulating images. By performing geometric image analysis on the terahertz free-form off-axis three-mirror anastigmat (TMA) quasi-optical system in Example 2, the results are as Figure 12 shown. The analysis results show that the imaging resolution of this system is extremely high, and the clarity and detail distinguishability of the letters in the image have reached a satisfactory level. In addition, there is no obvious deformation of the letters in the image, which further confirms that the distortion of the system is extremely small.

[0143] Figures 8 to 12 It shows that the terahertz free-form off-axis three-mirror anastigmat (TMA) quasi-optical system in Example 2 has high imaging quality and resolution.

[0144] It can be seen from Example 2 that applying the free form to the off-axis reflection system simultaneously solves the problems of large field of view, high image quality, high sensitivity, and high signal-to-noise ratio. It can be used for non-destructive testing of terahertz focal plane array detectors, achieving an imaging effect of large field of view and low distortion, and better realizing the correction and balance of the system's aberrations, which has important practical significance and application prospects.

[0145] Compared with the prior art, in the terahertz free-form off-axis three-reflection quasi-optical system of the present invention, by adopting the XY polynomial surface type designed by the free-form surface design technology, the system has no central obstruction, no chromatic aberration, a long working wavelength band, realizes an imaging effect with a large field of view and low distortion, better realizes the correction and balance of the system's aberrations, effectively eliminates aberrations, and improves the imaging quality.

[0146] In the terahertz free-form off-axis three-reflection quasi-optical system of the present invention, the first mirror, the second mirror, and the third mirror are selected with the free-form XY polynomial surface type to correct the off-axis aberration. At the same time, the structures of each mirror surface are eccentric and tilted in the plane. The free-form surface used is symmetric about the sagittal plane, and only the even-term parameters of X are changed, ensuring that the entire system is symmetric about the sagittal plane, greatly reducing the production and processing difficulty, and at the same time increasing the system's field of view.

[0147] In the terahertz free-form off-axis three-reflection quasi-optical system of the present invention, the operand is used to constrain that the distance between the lowest incident light ray and the upper edge of the second mirror is greater than 10 mm, and the distance between the third reflected light beam reflected by the third mirror and the lower edge of the second mirror is greater than 10 mm, ensuring that during the optimization process, all light rays are not blocked, preventing the reflected light rays from intersecting on different mirror surfaces, resulting in serious stray light and affecting the imaging quality.

[0148] In the terahertz free-form off-axis three-reflection quasi-optical system of the present invention, an off-axis three-reflection optical design scheme is adopted to reduce the volume and weight of the system and increase the effective aperture.

[0149] The terahertz free-form off-axis three-reflection quasi-optical system of the present invention is applicable to a focal plane array detector and can improve the sensitivity and imaging quality of the terahertz imaging system.

[0150] The terahertz free-form off-axis three-reflection quasi-optical system of the present invention has broad application prospects and can be used for non-destructive testing in the fields of aerospace, electronic products, food safety, biomedicine, etc.

[0151] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed rights.

[0152] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A terahertz free-form surface off-axis three-mirror collimation system, characterized in that: include: A first reflector, disposed on an outgoing light path of the light, for reflecting the light to form a first reflected light beam; A second reflector, disposed on a reflection light path of the first reflector, and configured to reflect the first reflected light beam twice to form a second reflected light beam; A third reflector, disposed on the reflected light path of the second reflector, and used to reflect the second reflected light beam again to form a third reflected light beam; a detector, located on the reflected light path of the third reflector and at a converging spot of the third reflected light beam, and configured to receive the third reflected light beam; The reflection surfaces of the first reflector, the second reflector and the third reflector all adopt an XY polynomial surface type, and the XY polynomial is an even-order polynomial of X, and the highest order of X is 6; A global three-dimensional rectangular coordinate system (X, Y, Z) is established, wherein the centers of the first reflector, the second reflector, and the third reflector are respectively located at point O1, point O2, and point O3 in the coordinate system (X, Y, Z), and the distances between point O1, point O2, and point O3 satisfy the following relationship: d 23 >=d 12 >=d 13 ; Among them, d 12 is the distance between point O1 and point O2, d 23 is the distance between point O2 and point O3, d 13 is the distance between point O1 and point O3; The terahertz free-form surface off-axis three-mirror collimating light system has a light aperture of 60nm-160mm, an F number of 1-1.3, and a field of view angle range of (±4.2°×±4.2°) to (±4.6°×±4.6°).

2. The terahertz free-form surface off-axis three-reflection collimation system according to claim 1, characterized in that: It also includes an aperture stop, and the aperture stop is arranged on the second reflecting mirror; or, The second reflector is an aperture stop.

3. The terahertz free-form surface off-axis three-mirror collimation system according to claim 1, characterized in that: The XY polynomial surface equation is: Among them, the radius of curvature of the first reflector is 2646.08299075151mm, the quadratic surface coefficient is 109.049764178204, the X half width is 160mm, the Y half width is 160mm, and the surface parameters of the XY polynomial are: The radius of curvature of the second reflector is -646.297743530395 mm, the quadratic surface coefficient is 10.670946693987, the X half width is 80 mm, the Y half width is 80 mm, and the surface parameters of the XY polynomial are: The radius of curvature of the third reflector is -436.76242195341mm, the quadratic surface coefficient is -0.00275362836076844, the radius of curvature is 180mm, and the surface parameters of the XY polynomial are:

4. The terahertz free-form surface off-axis three-reflection collimation system according to claim 3, characterized in that: The distance d between the center point O1 of the first reflector and the center point O2 of the second reflector 12 is 400.008 mm; the distance d between the center point O2 of the second reflector and the center point O3 of the third reflector 23 is 400.071 mm; the distance between the center point O3 of the third reflector and the plane center of the detector is 399.935 mm.

5. The terahertz free-form surface off-axis three-mirror collimation system according to claim 3, characterized in that: The distance between the lowest light beam incident on the first reflector and the upper edge of the second reflector is greater than 10 mm; the distance between the third reflected light beam of the third reflector and the lower edge of the second reflector is at least greater than 10 mm.

6. The terahertz free-form surface off-axis three-reflection collimation system according to claim 3, characterized in that: The terahertz free-form surface off-axis three-mirror collimation system has a focal length of 160 mm, a clear aperture of 120 mm-160 mm, and an F number of 1-1.3; The detector is a focal plane array detector, the array size of the focal plane array detector is 32×32 or 64×64, the size of each array unit is 0.4 mm, and the array size of the focal plane array detector is 25.6 mm×25.6 mm; The field of view angle range of the terahertz free-form surface off-axis three-reflection collimation light system is ±4.6°×±4.6°.

7. The terahertz free-form surface off-axis three-reflection collimation system according to claim 3, characterized in that: The root mean square radius within the field of view of the terahertz free-form surface off-axis three-mirror collimation system is 5.424 μm-25.947 μm, and the diffraction limit is 0.27 lp / mm at a wavelength of 0.882 μm.

8. The terahertz free-form surface off-axis three-reflection collimation system according to claim 1, characterized in that: The XY polynomial surface equation is: Among them, the radius of curvature of the first reflector is 1323.06433748304mm, the quadratic surface coefficient is 109.049764178204, the X half width is 80mm, the Y half width is 80mm, and the surface parameters of the XY polynomial are: The radius of curvature of the second reflector is -323.154450880609mm, the quadratic surface coefficient is 10.670946693987, the X half width is 40mm, the Y half width is 40mm, and the surface parameters of the XY polynomial are: The radius of curvature of the third reflector is -218.384981294618 mm, the quadratic surface coefficient is -0.00275362836076844, the X half width is 95 mm, the Y half width is 110 mm, and the surface parameters of the XY polynomial are:

9. The terahertz free-form surface off-axis three-reflection collimation system according to claim 8, characterized in that: The distance d between the center point O1 of the first reflector and the center point O2 of the second reflector 12 is 200.008 mm; the distance d between the center point O2 of the second reflector and the center point O3 of the third reflector 23 is 200.039 mm; the distance between the center point O3 of the third reflector and the plane center of the detector is 199.971 mm.

10. The terahertz free-form surface off-axis three-reflection collimation system according to claim 8, characterized in that: The distance between the lowest light beam incident on the first reflector and the upper edge of the second reflector is greater than 10 mm; the distance between the third reflected light beam of the third reflector and the lower edge of the second reflector is at least greater than 10 mm.

11. The terahertz free-form surface off-axis three-reflection collimation system according to claim 8, characterized in that: The terahertz free-form surface off-axis three-mirror collimation system has a focal length of 80 mm, a clear aperture of 60 mm-80 mm, and an F number of 1-1.2; The detector is a focal plane array detector, the array size of the focal plane array detector is 32×32 or 64×64, the size of each array unit is 0.4 mm, and the array size of the focal plane array detector is 12.8 mm×12.8 mm; The field of view angle range of the terahertz free-form surface off-axis three-reflection collimation system is ±4.2°×±4.2°.

12. The terahertz free-form surface off-axis three-mirror collimation system according to claim 8, characterized in that: The root mean square radius within the field of view of the terahertz free-form surface off-axis three-reflection collimation system is 2.785 μm-7.689 μm, and the diffraction limit is 0.33 lp / mm when the wavelength is 0.882 μm.

Images