A method for accurate measurement of subaperture baseline in optical synthetic aperture imaging system

Abstract

The application discloses a kind of optical synthetic aperture imaging system sub-aperture baseline accurate measurement methods, the method specifically is: first, two femtosecond laser ranging probes are installed on respective miniature adjusting frame in opposite direction, ensure that optical axis is parallel, opposite direction is regarded as sub-aperture baseline measuring instrument, it is placed in the reference mirror of known interval and carries out two probe interval calibration, then it is respectively moved to each sub-aperture primary mirror and the conical lens installed on system optical axis, the distance of each sub-aperture primary mirror and fixed conical lens is measured respectively, then according to the structure parameters of sub-aperture and conical lens, the entrance pupil baseline is calculated.The method solves the problem that the sub-aperture entrance pupil baseline in the optical synthetic aperture imaging system with variable baseline and invariable baseline is difficult to accurately and simply measure, provides technical support for the measurement and control of imaging system pupil mapping error, and further provides technical support for the observation field of view and observation efficiency of imaging system.

Description

Technical Field

[0001] The present invention relates to the technical field of astronomical observation and precise distance measurement, and in particular to a method for precisely measuring a subaperture baseline of an optical synthetic aperture imaging system. Background Art

[0002] According to the diffraction limit formula, the spatial resolution of a telescope is limited by the wavelength of light and the aperture of the optical system. As astronomical observations place increasing demands on the spatial resolution of telescope systems, telescope systems operating within a given wavelength band need to have their apertures continuously increased to meet the ever-increasing spatial resolution demands of astronomy. However, in practice, increasing the aperture of a single-aperture system is extremely difficult due to various factors, such as manufacturing materials, manufacturing technology, mechanical structure, and rocket payload volume and weight. Spatially distributed synthetic aperture technology provides a new approach to improving the spatial resolution of optical imaging systems.

[0003] The 500-meter Aperture Spherical Radio Telescope built in Guizhou, my country, uses a method of splicing mirrors to increase the aperture of a single telescope. The James Webb Space Telescope launched by NASA also uses a method of splicing mirrors to increase the aperture of a single telescope. The former is a ground-based radio telescope, and a single-aperture optical telescope cannot reach a few hundred meters. The latter is an optical space telescope, but its aperture is 6.5 meters. NASA spent more than 20 years and tens of billions of dollars to achieve its successful launch. Compared with a single-aperture telescope, an optical synthetic aperture uses a telescope array to achieve the equivalent spatial resolution of a single large-aperture telescope, breaking through the diffraction limit of a single-aperture telescope due to the limitations of aperture size and splicing difficulty. Its equivalent aperture can reach hundreds of meters or even larger.

[0004] An ideal Fizeau-type synthetic aperture imaging system requires that the entrance pupil baselines of the subaperture array (the distance between the optical axes of the subaperture entrance pupils; in this example, four subapertures comprise six baselines) and the entrance pupil baselines of the combined telescope (the distance between the optical axes of the combined imaging beams; in this example, four combined imaging beams comprise six baselines) be strictly similar, with the similarity ratio being equal to the angular magnification of the subapertures. Therefore, precise measurement and control of the entrance pupil baselines of the subaperture array and the combined telescope are crucial. While a Michelson-type synthetic aperture imaging system does not require this principle, precise measurement and control of the entrance pupil baselines of the subaperture array and the combined telescope are equally important. Precise measurement and control of the baselines ensures that the actual positions of the subapertures after baseline movement are consistent with the theoretically analyzed UV coverage points. Regardless of the system type, accurate subaperture baselines are required to control the system baseline mapping error, and thus the system pupil mapping error, to maintain the system field of view and observation efficiency.

[0005] Existing methods and means:

[0006] 1. NASA established the Wide-Field Imaging Interferometry (WIIT) to validate wide-field interferometric imaging algorithms for space interferometer projects such as SPIRIT (infrared), SPECS (submillimeter wave), and TPF-I. The system model was established and verified. Two subtelescopes are mounted at either end of a linear air-bearing stage. Each subaperture is 25 mm, and the baselines of the two subtelescopes are variable between 25 mm and 250 mm. UV coverage is achieved through baseline adjustment and synchronized rotation of the target and detector. Subaperture position information is obtained using feedback from a linear absolute encoder mounted on an air-bearing guide rail. This encoder only determines the position of the guide sleeve on the rail and cannot determine the absolute distance between the subaperture entrance and exit pupil baselines. The paper mentions the need for well-calibrated absolute baseline metrology in the future. However, no introduction or experimental verification of absolute baseline metrology was found in subsequent papers on WIIT.

[0007] 2. The National Astronomical Observatories of the Chinese Academy of Sciences designed and built a Fizeau Imaging Interferometer Test (FIIT). This test setup primarily consists of a light source module for simulating an infinite object, three 100mm aperture sub-telescopes arranged in a Golay-3 pattern (equilateral triangle), three yaw / pitch correction modules and an optical path delay module, three detector telescopes for phase imaging, and a beam combining telescope. The setup achieved broadband white-light (400-700nm) phase imaging in the laboratory with a field of view of 2 arc minutes. The baseline lengths of the three sub-telescopes and the sub-beam baseline length are fixed at 200mm and 40mm, respectively.

[0008] The system entrance pupil baseline is determined by the aperture placed in front of the sub-telescope. The aperture has three entrance pupils that have been precisely machined and tested, and are aligned with the three sub-apertures respectively. The baseline length is strictly limited and can be used as the system entrance pupil baseline reference.

[0009] The exit pupil baseline is determined using a calibration plate, lens, and camera. A precisely machined and tested calibration glass plate is placed in the path of the three sub-beams. The calibration glass plate is engraved with three circular lines arranged in an equilateral triangle. Each circular line has a cross mark passing through the center of the circle. The center distance of the circular lines has been precisely tested and can be used as a reference for exit pupil baseline testing. After the sub-beams pass through the glass plate and lens, six circular images are displayed on the camera: images of the three sub-beams and images of the three circular lines on the calibration plate. After identifying the coordinates of the centers of the six images, the baseline of the sub-beams is calculated using the center of the image on the calibration plate as the reference, thereby obtaining the system exit pupil baseline. Combining the system entrance pupil baseline and the system exit pupil baseline yields the system baseline mapping error.

[0010] 3. The China Resources Satellite Application Center analyzed and studied intersatellite high-precision baseline measurement and processing methods and workflows based on the operating characteristics of distributed interferometric synthetic aperture radar (InSAR) satellite systems, including GNSS and SAR payloads. The distributed InSAR satellite system first obtains a GNSS measurement baseline through GNSS dual-frequency carrier phase differential measurement and high-precision ground-based orbit calculation. Then, through position correction, the baseline reference point is shifted from the satellite centroid to the SAR antenna phase center, resulting in a spatial domain baseline. Experimental results demonstrate that the intercomparison of InSAR baselines obtained through orbit determination has a statistical accuracy of approximately 1 mm.

[0011] 4. For dual-antenna InsAR systems with flexible long baselines, motion errors caused by flexible baseline jitter severely impact radar imaging quality and interferometric measurement accuracy. Researchers from the Remote Sensing Satellite General Administration and Xi'an University of Science and Technology have proposed a combined baseline measurement system using a single camera and femtosecond laser ranging. Based on this system, a new joint dynamic baseline solution method was proposed. This method, based on the collinearity equations of the unit quaternary elements, fuses the camera measurement data with the femtosecond laser data. This method effectively improves the solution accuracy of the exterior orientation elements.

[0012] The solution primarily consists of five components: a measurement sensor mounting platform, a target tracker (including a high-precision CCD camera and a femtosecond laser rangefinder), a platform position and attitude measurement system (star sensor + GPS), and a target (LED and cone angle). The measurement platform is connected to a single antenna via an antenna support arm, which is up to 60 meters long. The phase centers of the two antennas form a flexible long baseline. The target tracker is rigidly connected to the platform's position and attitude measurement system. The target tracker obtains the position and attitude of the single antenna relative to the measurement platform. The star sensor and GPS systems measure the platform's absolute position and attitude.

[0013] 5. The Beijing Great Wall Metrology and Testing Technology Institute of the Aviation Industry Corporation of China has invented a femtosecond laser length measurement device and method based on the principle of variable frequency phase measurement. This invention includes a femtosecond laser, a filter, a beamsplitter, a first plane reflector, a second plane reflector, a third plane reflector, a first angle-coupled reflector prism, a second angle-coupled reflector prism, a first concave reflector, a second concave reflector, a focusing lens, a first photodetector, a second photodetector, etc. This invention achieves distance measurement without requiring adjustment of the measuring arm length or the stringent requirement that the number of repetition cycles must be changed by only one. However, the distance measurement method involved requires a large number of instruments and components, which takes up considerable space and costs a lot.

[0014] 6. Tsinghua University Shenzhen Graduate School has invented a femtosecond laser ranging device. This device comprises a first reference optical path, a second reference optical path, and a measurement optical path. The first and second reference optical paths have a fixed optical path difference and are each equipped with a shutter. The shutters are used to switch between reference optical paths during ranging, allowing the corresponding reference light to be used for measurement. This device effectively addresses the dead zone issue inherent in femtosecond laser ranging, but the distance measurement method involved requires a large number of instruments and components, which occupies a large space and is costly.

[0015] Disadvantages of existing technology:

[0016] (1) Currently, most existing optical synthetic aperture imaging systems at home and abroad use a static method in which the subaperture baseline cannot be changed. The baseline measurement method using a reference aperture and a calibration plate is only applicable to systems with an immutable baseline and cannot measure the subaperture baseline of a system with a variable baseline.

[0017] (2) Currently, the baseline measurement of the existing variable baseline optical synthetic aperture imaging system at home and abroad mostly adopts the method of installing a grating ruler on the guide rail or using a motor encoder to feedback the sub-aperture position. This method can obtain the guide sleeve position information. However, due to the position deviation between the guide sleeve and the sub-aperture and sub-beam, the sub-aperture baseline length information cannot be directly obtained.

[0018] (3) The existing femtosecond laser baseline measurement methods at home and abroad need to be used in conjunction with CCD cameras, targets, corner conic mirrors, etc., and data fusion algorithms need to be developed. Many instruments and mechanisms are required, the system is relatively complex, and it is difficult to implement.

[0019] The information disclosed in this background technology section is only intended to deepen the understanding of the overall background technology of the present invention and should not be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art. Summary of the Invention

[0020] In view of the problems existing in the prior art, the purpose of the present invention is to provide a method for accurately measuring the sub-aperture baseline of an optical synthetic aperture imaging system, so as to solve the difficult problem of accurate and simple measurement of the sub-aperture entrance pupil baseline in optical synthetic aperture imaging systems with variable baseline and fixed baseline.

[0021] In order to achieve the above object, the present invention adopts the following technical solutions:

[0022] A method for accurately measuring the subaperture baseline of an optical synthetic aperture imaging system, the method specifically comprising:

[0023] First, two femtosecond laser ranging probes are mounted facing each other on their respective micro-adjustment frames, ensuring that the optical axes are parallel and in opposite directions. They are regarded as sub-aperture baseline measuring instruments and placed in a reference mirror with a known spacing to calibrate the spacing between the two probes. Then, they are moved respectively between each sub-aperture primary mirror and the aconic lens mounted on the optical axis of the system. The distance between each sub-aperture primary mirror and the fixed aconic lens is measured respectively, and then its entrance pupil baseline is calculated based on the sub-aperture and aconic lens structural parameters.

[0024] Furthermore, the sub-aperture baseline measuring instrument needs to accurately measure the distance between the two femtosecond laser ranging probes before being integrated between the sub-apertures of the optical synthetic aperture imaging system. The specific method is as follows:

[0025] The principle of using a femtosecond laser ranging probe to measure the absolute distance between itself and the target to be measured is used. The sub-aperture baseline measuring instrument is placed inside the reference mirror. The distance between the two target surfaces (18) and (19) of the reference mirror has been measured by a high-precision three-coordinate measuring instrument and is recorded as L0. The postures of the two femtosecond laser ranging probes are adjusted to align them with the surfaces (18) and (19). The distance between the probe (15) and the surface (18) is measured and recorded as L1. The distance between the probe (16) and the surface (19) is measured and recorded as L2. The distance between the probes (15) and (16) is L 仪 = L0-(L1+ L2), L 仪 As the basic parameter of the femtosecond laser rangefinder, the accuracy reaches the micron level.

[0026] Furthermore, the specific steps for accurate measurement of sub-aperture baseline are as follows:

[0027] 1) Measure the distance between the two target surfaces of the reference mirror. Install the sub-aperture baseline measuring instrument in the reference mirror. Adjust the postures of the two femtosecond laser ranging probes so that they are aligned with the two target surfaces. Measure the distance between the two probes and their respective target surfaces and calculate L. 仪 Specific values;

[0028] 2) Install the subaperture baseline measuring instrument on the side of a subaperture primary mirror, adjust the posture of the two femtosecond laser ranging probes so that they are aligned with the small facet on the side of the subaperture primary mirror and one side of the aconic lens, and measure the distances from the two probes to their respective target surfaces;

[0029] 3) The distance between the sub-aperture entrance pupil and the optical axis of the optical synthetic aperture imaging system is calculated based on the distance between the measured plane on the side of the sub-aperture primary mirror and the center of the primary mirror's entrance pupil and the distance between the side of the axicon and its axis;

[0030] 4) Repeat step 3) to obtain the distance between the entrance pupil of each sub-aperture and the optical axis of the system;

[0031] 5) The spacing of each sub-aperture entrance pupil to the optical axis of the system is obtained to obtain the sub-aperture entrance pupil baseline data;

[0032] 6) If the optical synthetic aperture imaging system is not variable baseline, the sub-aperture baseline measurement of the system is completed;

[0033] 7) If the optical synthetic aperture imaging system is variable baseline, the steps 1) to 5) are repeated after the system is variable baseline.

[0034] Further, the specific process of the sub-aperture baseline measurement in step 6) is as follows:

[0035] The femtosecond laser range finder is installed on the side of a sub-aperture primary mirror, the poses of two femtosecond laser ranging probes are adjusted to align with the facet on the side of the sub-aperture primary mirror and a side of the conical lens, respectively, and the distances from the two probes to the measured target surfaces are measured, respectively, and are recorded as L 主 and L 锥 ;

[0036] If the sub-aperture of the optical synthetic aperture imaging system is not variable baseline, according to the structural parameters of the sub-aperture, the spacing between the measured plane on the side of the primary mirror and the entrance pupil center of the primary mirror is L 主-入 , the spacing of the sub-aperture entrance pupil to the optical axis of the system is L 入-轴1 =L 仪 +L 主 +L 锥 +L 主-入 +d 锥 , wherein d 锥 is the distance from the side of the conical lens to the axis thereof; the spacing L 入-轴2 , L 入-轴3 , and L 入-轴4 of each sub-aperture entrance pupil to the optical axis of the system is measured by using the above principle, and the sub-aperture entrance pupil baseline is obtained.

[0037] By using the technical scheme, the present application has the following beneficial effects:

[0038] 1) The technology involved is relatively mature and easy to implement. The machining and detection precision of the domestic mature femtosecond laser ranging probe and conical lens can reach the micron level, meeting the precision requirement of the method;

[0039] 2) The method is widely applicable. The method is generally applicable to optical synthetic aperture imaging systems with variable baseline and non-variable baseline, and is generally applicable to optical synthetic aperture imaging systems with different numbers of sub-apertures, and is applicable to both symmetrically arranged sub-aperture baseline measurement and non-symmetrically arranged sub-aperture baseline measurement, and only the configuration of the side of the conical lens needs to be changed;

[0040] 3) The method is widely applicable. The method provides a train of thought and scheme for measuring the absolute distance between multiple components;

[0041] 4) High cost performance. The femtosecond laser ranging probe, cone lens, adjusting frame and other components used in the method are small in size, mature in technology and low in price, and the overall structure is simple, small in space occupation and high in cost performance;

[0042] 5) Absolute measurement. Unlike the traditional relative measurement which can only measure the change of the baseline, the method can realize the absolute distance measurement between multiple components;

[0043] 6) High measurement accuracy. Through accurate calibration and installation of the baseline measuring instrument, micron-level absolute distance measurement can be realized;

[0044] 7) Small influence on the system. The components involved in the method are small in size and light in weight, and are installed in an area that does not affect the sub-aperture, so the influence on the position and attitude of the sub-aperture is small. BRIEF DESCRIPTION OF DRAWINGS

[0045] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the description of the specific embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0046] Figure 1 The structure diagram of the device for the optical synthetic aperture imaging system sub-aperture baseline accurate measurement method of the present application.

[0047] Figure 2 The principle diagram of the sub-aperture baseline accurate measurement device of the present application.

[0048] Figure 3 The femtosecond laser ranging probe spacing calibration device schematic diagram of the present application.

[0049] Figure 4 The sub-aperture entrance pupil baseline schematic diagram of the present application.

[0050] Figure 5 The time domain and frequency domain characteristics of the laser frequency comb of the present application.

[0051] Figure 6 The double optical frequency comb interference method measurement system schematic diagram of the present application.

[0052] Figure 7 The off-axis two-mirror sub-aperture structure schematic diagram of the present application. DETAILED DESCRIPTION

[0053] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0054] The following describes the specific embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0055] This invention proposes a method for accurately measuring the subaperture baseline of an optical synthetic aperture imaging system. This method first installs two femtosecond laser ranging probes facing each other on their respective micro-adjustment mounts, ensuring that their optical axes are parallel and in opposite directions. These probes are then placed within a reference mirror with a known spacing to calibrate the distance between the two probes. The probes are then moved between each subaperture primary mirror and an axicon mounted on the system's optical axis. The distance between each subaperture primary mirror and the fixed axicon is measured, and the entrance pupil baseline is calculated based on the subaperture and axicon structural parameters.

[0056] This method solves the problem of accurate and simple measurement of the sub-aperture entrance pupil baseline in optical synthetic aperture imaging systems with variable and fixed baselines, provides technical support for the measurement and control of pupil mapping errors in imaging systems, and further provides technical guarantee for the observation field of view and observation efficiency of imaging systems.

[0057] Combine Figure 1-4 As shown, the method for accurately measuring the subaperture baseline of an optical synthetic aperture imaging system proposed in the present invention is used to measure the baseline of the subaperture of an optical synthetic aperture imaging system. The required structures include a collimator 1, subapertures 2-5, subaperture support structures 6-9, subaperture variable baseline guide rails 10-11, an axicon 12, an axicon mounting and adjustment structure 13, and a subaperture baseline measuring instrument 14. The subaperture baseline measuring instrument 14 is composed of a femtosecond laser ranging probe A 15, a femtosecond laser ranging probe B 16, and a femtosecond laser ranging probe connector 17. Furthermore, a femtosecond laser ranging probe spacing calibration device consists of the subaperture baseline measuring instrument 14 and a reference mirror 20 with a known spacing between surfaces 18 and 19. The side profile of the axicon 12 is consistent with the system's subaperture layout. In this example, there are four subapertures arranged in a rectangular shape. Therefore, the axicon also has four side surfaces, which are parallel to the side surfaces (facets) of the four subaperture primary mirrors.

[0058] In the present application, a collimator 1 capable of emitting parallel light is installed at the front end of the system as an observation target simulator of an optical synthetic aperture imaging system.

[0059] There are four subapertures 2-5, arranged behind collimator 1 and mounted on subaperture support structures 6-9. Subaperture variable baseline guide 10 supports subapertures 2 and 4 via subaperture support structures 6 and 8, with the optical axis connecting subapertures 2 and 4 forming baseline 22. Subaperture variable baseline guide 11 supports subapertures 3 and 5 via subaperture support structures 7 and 9, with the optical axis connecting subapertures 3 and 5 forming baseline 21.

[0060] Subaperture variable-baseline guides 10 and 11 are positioned at angles of -30° and +30° relative to the horizontal plane, respectively. Driven by these guides, subapertures 2-5 can all undergo variable-baseline motion. Under any baseline condition, the optical axes of subapertures 2-5 are at the same distance from the collimator. The optical axes of subapertures 2-5 form a rectangle in the vertical plane. The length of baseline 21 is equal to that of baseline 22, and baselines 21 and 22 intersect at the optical axis of collimator 1.

[0061] The light beams emitted from sub-apertures 2-5 enter the subsequent subsystems.

[0062] The axicon 12 is mounted in the axicon mounting and adjustment structure 13, which can be adjusted in five dimensions. The optical axis of the axicon coincides with the center of the rectangle formed by the optical axis of the collimator and the optical axes of the sub-apertures 2-5, and the four side surfaces of the axicon 12 are parallel to the primary mirror side reference planes of the sub-apertures 2-5.

[0063] The sub-aperture baseline measuring instrument 14 is installed on the side of the primary mirror side reference surface of one of the sub-apertures 2-5, and moves as the sub-aperture 2-5 changes its baseline. The femtosecond laser ranging probe A15 is aligned with the primary mirror side reference surface of one of the sub-apertures 2-5, and the relative distance remains unchanged. The femtosecond laser ranging probe B16 is aligned with the side surface of one of the conical lenses, and the relative distance changes as the sub-aperture 2-5 changes its baseline.

[0064] Before the sub-aperture baseline measuring instrument 14 is installed in the system, the relative distance between the femtosecond laser ranging probe A15 and the femtosecond laser ranging probe B16 needs to be calibrated offline. The sub-aperture baseline measuring instrument 14 is placed in the reference mirror 20, and the femtosecond laser ranging probe A15 is aligned with the surface 18, and the femtosecond laser ranging probe B16 is aligned with the surface 19.

[0065] Here's how this application works:

[0066] This method uses a sub-aperture baseline measuring instrument mainly composed of two femtosecond laser ranging probes and a reference mirror to perform offline calibration of the probe spacing. The calibrated sub-aperture baseline measuring instrument is then used to precisely measure the relative position of the sub-aperture of an optical synthetic aperture imaging system with a variable or fixed baseline. The entrance pupil baseline is then calculated based on the sub-aperture and aconic lens structure parameters.

[0067] Femtosecond laser absolute distance measurement process:

[0068] Combine Figure 5-6 As shown in Figure 1, femtosecond laser absolute distance measurement is an absolute distance measurement method based on an optical frequency comb. A femtosecond laser frequency comb is a broadband spectrum output by a passively mode-locked femtosecond laser. It consists of tens of thousands of equally spaced coherent spectral lines that are locked to the mode during the passive mode-locking process. The frequency of any spectral line can be expressed in the following concise form:

[0069] f n =f ceo +n·f rep ;

[0070] Among them, n is the order of the spectrum; f rep is the pulse repetition frequency of the femtosecond laser, corresponding to the spectral line spacing; f ceo It is called the carrier-envelope offset frequency and is determined by the phase velocity v of the femtosecond pulse carrier. p and the group velocity v of the outer envelope g The difference causes the frequency comb to have an overall frequency shift.

[0071] Mode locking refers to the phase locking of the longitudinal modes within a laser, creating a beat pattern that causes the laser to produce periodic, short pulses in the time domain. The wider the gain spectrum of the laser's working medium, the greater the number of longitudinal modes that can be locked within the laser cavity, enabling narrower laser pulses and higher peak power.

[0072] By locking the carrier frequency and carrier envelope phase of a femtosecond laser frequency comb and applying Fourier transforms, a series of spectral lines, uniformly distributed and stably positioned in the frequency domain, can be generated. This is called a laser frequency comb. In the time domain, a laser frequency comb appears as ultrashort pulses with femtosecond widths, while in the frequency domain, it manifests as a comb-like spectrum of regularly arranged frequency lines with wide spectral coverage.

[0073] Dual optical frequency comb interferometry: Dual optical frequency comb ranging uses two frequency comb light sources with a small repetition frequency difference. The repetition frequency deviation Δf is much smaller than the repetition frequency f r After passing through the interferometer, the two pulses are periodically superimposed with a period of τ = 1 / Δf. Analyzing the superimposed interference signal in both the time and frequency domains enables fast, high-precision absolute distance measurement.

[0074] Figure 6 In FIG, OFC1 represents optical frequency comb signal 1; OPC2 represents optical frequency comb signal 2; M1 represents a reference mirror; M2 represents a measurement mirror; and APD represents an avalanche photodetector.

[0075] The first optical frequency comb is used as a measurement optical frequency comb, and the measurement light generates a repetition frequency of f r1(i.e. the repetition time is T r1 ) passes through the Michelson interferometer and is reflected by the measuring mirror and the reference mirror to produce separated sub-pulses; the second optical frequency comb is used as the local oscillator optical frequency comb, and the repetition frequency of the local oscillator light is f r2 (i.e. the repetition time is T r2 ) interferes with the signal light, and the repetition frequency difference between the two optical frequency combs is: Δf = f r1 -f r2 The interference light passes through an optical bandpass filter with a bandwidth of Δv and is then received by a photodetector. Explained in the time domain, two sequences of measurement light pulses with a time delay of Δτ are linearly sampled by the local oscillator light, and the sampling step is as follows:

[0076] ;

[0077] The period of the interference signal is T in =m⋅T r1 =1 / Δf r , where m = f r1 / Δf r , which is the number of measured light pulses in one cycle. At this time, the measured time delay Δτ is magnified to Δt=m⋅Δτ, and the measured distance L can be further obtained. double , as shown below:

[0078] ;

[0079] Explained in the frequency domain, the dual optical frequency comb ranging system produces two RF combs with different phase spectra. Fundamentally, the main source of heterodyne interference is the experimental adjustment method using single-correlated optical sampling, which directly receives the lower-frequency interference signal in adjacent optical frequency comb pulses. In this case, the heterodyne is specifically expressed as Δf r The absolute distance is accurately measured by spectrum phase analysis. The distance to be measured L is calculated. ω As shown in the following formula:

[0080] ;

[0081] Where b is the parameter of delay information, v g is the group velocity of the central wavelength, m is the number of integer phase cycles, φ0 is the initial phase, and λ is the central wavelength of the optical frequency comb.

[0082] The sub-aperture baseline measuring instrument calibration process is as follows:

[0083] The calibration of the sub-aperture baseline measuring instrument refers to the precise measurement of the distance between its two femtosecond laser ranging probes before integrating it between the sub-apertures of the optical synthetic aperture imaging system, so that the sub-aperture baseline can be measured using the distance value as a known quantity.

[0084] Using the principle that the femtosecond laser ranging probe measures the absolute distance between itself and the target, the sub-aperture baseline measuring instrument is placed inside the reference mirror. The distance between the two target surfaces 18 and 19 of the reference mirror has been measured by a high-precision three-dimensional coordinate measuring instrument and is recorded as L0. The postures of the two femtosecond laser ranging probes are adjusted to align with surfaces 18 and 19 respectively, and the distance between probe 15 and surface 18 is measured, recorded as L1, and the distance between probe 16 and surface 19 is measured, recorded as L2. The distance between probes 15 and 16 is L 仪 = L0-(L1+L2), L 仪 As a basic parameter of femtosecond laser rangefinder, the accuracy can reach the micron level.

[0085] The sub-aperture baseline measurement process is as follows:

[0086] The femtosecond laser rangefinder is installed on the side of a sub-aperture primary mirror. The postures of the two femtosecond laser ranging probes are adjusted to align with the small plane on the side of the sub-aperture primary mirror and one side of the aconic lens. The distances from the two probes to their respective target surfaces are measured, which are recorded as L. 主 and L 锥 .

[0087] If the sub-aperture of the optical synthetic aperture imaging system has an invariable baseline, then according to the structural parameters of the sub-aperture, the distance from the measured plane on the side of the main mirror to the center of the entrance pupil of the main mirror is L 主-入 , the distance L between the sub-aperture entrance pupil and the system optical axis 入-轴1 =L 仪 +L 主 +L 锥 +L 主-入 +d 锥 , where d 锥 is the distance from the side of the axon to its axis. Using the above principle, the distance L between each sub-aperture entrance pupil and the system optical axis is measured separately. 入-轴2 , L 入-轴3 , L 入-轴4 , the sub-aperture entrance pupil baseline can be obtained.

[0088] like Figure 7 As shown, if the subaperture of an optical synthetic aperture imaging system has a variable baseline, the subaperture will produce motion errors when the variable baseline moves, including three angular errors and three positional errors. If the subaperture motion error is large, the subaperture baseline value measured using this method will also have a large cosine error. In this example, the subaperture variable baseline motion error was measured and corrected. The angular and positional errors were on the order of microradians and micrometers, which are small and their impact on the subaperture baseline measurement can be ignored.

[0089] The working process of this application is as follows:

[0090] 1) Measure the distance between the two target surfaces of the reference mirror. Install the sub-aperture baseline measuring instrument in the reference mirror. Adjust the postures of the two femtosecond laser ranging probes so that they are aligned with the two target surfaces. Measure the distance between the two probes and their respective target surfaces and calculate L. 仪 Specific values;

[0091] 2) Install the subaperture baseline measuring instrument on the side of a subaperture primary mirror, adjust the posture of the two femtosecond laser ranging probes so that they are aligned with the small facet on the side of the subaperture primary mirror and one side of the aconic lens, and measure the distances from the two probes to their respective target surfaces;

[0092] 3) The distance between the sub-aperture entrance pupil and the optical axis of the optical synthetic aperture imaging system is calculated based on the distance between the measured plane on the side of the sub-aperture primary mirror and the center of the primary mirror's entrance pupil and the distance between the side of the axicon and its axis;

[0093] 4) Repeat step 3) to obtain the distance between the entrance pupil of each sub-aperture and the optical axis of the system;

[0094] 5) The sub-aperture entrance pupil baseline data can be obtained by superimposing the distances between each sub-aperture entrance pupil and the system optical axis;

[0095] 6) If the optical synthetic aperture imaging system has an immutable baseline, then complete the sub-aperture baseline measurement;

[0096] 7) If the optical synthetic aperture imaging system has a variable baseline, then repeat steps 1) to 5) after the system changes its baseline.

[0097] In summary, compared with the prior art, the innovations of this application include:

[0098] 1) This method uses a subaperture baseline measuring instrument connected to the subaperture and an aconic lens placed on the optical axis of the system to perform absolute distance measurement on the position of the subaperture and the baseline between the subapertures.

[0099] 2) Since this method uses absolute ranging technology rather than relative ranging technology, it can perform baseline measurement for both variable-baseline and fixed-baseline optical synthetic aperture imaging systems;

[0100] 3) The subaperture baseline measuring instrument involved in this method can be connected to the subaperture or installed at a position between each subaperture, ensuring that the probe of the subaperture baseline measuring instrument is aligned with the side of the subaperture;

[0101] 4) The aconic lens involved in this method can also be eliminated, and the two probes of the aperture baseline measuring instrument can be aligned with two sub-apertures on the same line. However, this method is only applicable to the layout scheme where the sub-apertures are on the same line.

[0102] 5) This method is generally applicable to optical synthetic aperture imaging systems with different numbers of sub-apertures. It is suitable for both symmetrically arranged sub-aperture baseline measurements and asymmetrically arranged sub-aperture baseline measurements. It only requires changing the side configuration of the axicon.

[0103] 6) This method uses a reference mirror to calibrate the two probes of the subaperture baseline measuring instrument, which is the key to absolute subaperture baseline measurement;

[0104] 7) This method has certain requirements for the sub-aperture variable baseline motion accuracy of the variable baseline optical synthetic aperture imaging system, and is suitable for the case of low sub-aperture motion error. If the sub-aperture motion error is large, it should be corrected first or the number of probes should be increased for calibration.

[0105] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for accurately measuring the subaperture baseline of an optical synthetic aperture imaging system, characterized in that: The method is specifically as follows: First, two femtosecond laser ranging probes are mounted facing each other on their respective micro-adjustment mounts, ensuring that their optical axes are parallel and in opposite directions. These probes are considered subaperture baseline measuring instruments and are placed inside a reference mirror with a known spacing to calibrate the spacing between the two probes. The probes are then moved between each subaperture primary mirror and an axicon mounted on the system's optical axis. The distance between each subaperture primary mirror and the fixed axicon is measured, and the entrance pupil baseline is calculated based on the subaperture and axicon structural parameters. The distance between the sub-aperture entrance pupil and the optical axis of the optical synthetic aperture imaging system is calculated based on the distance between the measured plane on the side of the sub-aperture primary mirror and the center of the entrance pupil of the primary mirror, as well as the distance between the side of the aconic lens and its axis. The above process is repeated to obtain the distance between each sub-aperture entrance pupil and the optical axis of the system. The sub-aperture entrance pupil baseline data can be obtained by superimposing the distances between the entrance pupils of each sub-aperture and the optical axis of the system.

2. The method for accurately measuring subaperture baselines of an optical synthetic aperture imaging system according to claim 1, wherein: Before the sub-aperture baseline measuring instrument is integrated between the sub-apertures of the optical synthetic aperture imaging system, it is necessary to first accurately measure the distance between the two femtosecond laser ranging probes. The specific method is as follows: The principle of using a femtosecond laser ranging probe to measure the absolute distance between itself and the target to be measured is used. The sub-aperture baseline measuring instrument is placed inside the reference mirror. The distance between the two target surfaces (18) and (19) of the reference mirror has been measured by a high-precision three-coordinate measuring instrument and is recorded as L0. The postures of the two femtosecond laser ranging probes are adjusted to align them with the surfaces (18) and (19). The distance between the probe (15) and the surface (18) is measured and recorded as L1. The distance between the probe (16) and the surface (19) is measured and recorded as L2. The distance between the probes (15) and (16) is L 仪 = L0-(L1+ L2), L 仪 As the basic parameter of the femtosecond laser rangefinder, the accuracy reaches the micron level.

3. The method for accurately measuring subaperture baselines of an optical synthetic aperture imaging system according to claim 1, wherein: The specific steps for accurate measurement of sub-aperture baseline are as follows: 1) Measure the distance between the two target surfaces of the reference mirror. Install the sub-aperture baseline measuring instrument in the reference mirror. Adjust the postures of the two femtosecond laser ranging probes so that they are aligned with the two target surfaces. Measure the distance between the two probes and their respective target surfaces and calculate L. 仪 Specific values; 2) Install the subaperture baseline measuring instrument on the side of a subaperture primary mirror, adjust the posture of the two femtosecond laser ranging probes so that they are aligned with the small facet on the side of the subaperture primary mirror and one side of the aconic lens, and measure the distances from the two probes to their respective target surfaces; 3) The distance between the sub-aperture entrance pupil and the optical axis of the optical synthetic aperture imaging system is calculated based on the distance between the measured plane on the side of the sub-aperture primary mirror and the center of the primary mirror's entrance pupil and the distance between the side of the axicon and its axis; 4) Repeat step 3) to obtain the distance between the entrance pupil of each sub-aperture and the optical axis of the system; 5) Superimpose the distances between each sub-aperture entrance pupil and the system optical axis to obtain the sub-aperture entrance pupil baseline data; 6) If the optical synthetic aperture imaging system has an immutable baseline, then complete the sub-aperture baseline measurement; 7) If the optical synthetic aperture imaging system has a variable baseline, then repeat steps 1) to 5) after the system changes its baseline.

4. The method for accurately measuring subaperture baselines of an optical synthetic aperture imaging system according to claim 3, wherein: Step 6) The specific process of neutron aperture baseline measurement is as follows: The femtosecond laser rangefinder is installed on the side of a sub-aperture primary mirror. The postures of the two femtosecond laser ranging probes are adjusted to align with the small plane on the side of the sub-aperture primary mirror and one side of the aconic lens. The distances from the two probes to their respective target surfaces are measured, which are recorded as L. 主 and L 锥 ; If the sub-aperture of the optical synthetic aperture imaging system has an invariable baseline, then according to the structural parameters of the sub-aperture, the distance from the measured plane on the side of the main mirror to the center of the entrance pupil of the main mirror is L 主-入 , the distance L between the sub-aperture entrance pupil and the system optical axis 入-轴1 =L 仪 +L 主 +L 锥 +L 主-入 +d 锥 , where d 锥 is the distance from the side of the axon to its axis; using the above principle, the distance L from each sub-aperture entrance pupil to the system optical axis is measured respectively. 入-轴2 , L 入-轴3 , L 入-轴4 , the sub-aperture entrance pupil baseline can be obtained.

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