Method for accurately measuring sub-aperture base line of optical synthetic aperture imaging system

Abstract

The invention discloses an optical synthetic aperture imaging system sub-aperture baseline accurate measurement method, which specifically comprises the following steps of: oppositely mounting two femtosecond laser ranging probes on respective micro adjusting frames, ensuring that optical axes are parallel and the directions are opposite, and taking the two femtosecond laser ranging probes as a sub-aperture baseline measurement instrument; the method comprises the following steps of: placing a plurality of sub-aperture primary mirrors in a reference mirror with a known distance to calibrate the distance between the two probes, respectively moving the sub-aperture primary mirrors between the sub-aperture primary mirrors and a conical lens arranged on an optical axis of a system, respectively measuring the distance between each sub-aperture primary mirror and the fixed conical lens, and calculating an entrance pupil base line according to structural parameters of the sub-apertures and the conical lens. According to the method, the problem that the sub-aperture entrance pupil base line in the optical synthetic aperture imaging system with the variable base line and the invariable base line is difficult to accurately and simply measure is solved, technical support is provided for measurement and control of the pupil mapping error of the imaging system, and technical guarantee is provided for the observation view field and the observation efficiency of the imaging system.

Description

Technical Field

[0001] The invention relates to the technical field of astronomical observation and precision distance measurement, and in particular to a method for accurately measuring a sub-aperture 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, it is necessary to continuously increase the aperture of telescope systems operating in a certain wavelength band to meet the increasing demand for spatial resolution in astronomy. However, in practical applications, due to various factors, such as manufacturing materials, manufacturing technology, mechanical structure, rocket carrying volume and weight, it becomes extremely difficult to increase the aperture of a single aperture system. Spatially distributed synthetic aperture technology provides a new path to improve 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 several hundred meters. The latter is an optical space telescope, but its aperture is 6.5 meters. It took NASA more than 20 years and cost tens of billions of dollars to successfully launch it. Compared with a single-aperture telescope, the optical synthetic aperture uses a telescope array method 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 several hundred meters or even larger. ‌ The ideal Fizeau type optical synthetic aperture imaging system needs to follow the strict similarity of the entrance pupil baseline of the sub-aperture array (the distance between the optical axes of the entrance pupils of the sub-apertures, in this example, the four sub-apertures contain six baselines) and the entrance pupil baseline of the light-combining telescope (the distance between the optical axes of the light-combining imaging beams, in this example, the four light-combining imaging beams contain six baselines), and the similarity ratio is the angular magnification of the sub-aperture, so the precise measurement and control of the entrance pupil baseline of the sub-aperture array and the entrance pupil baseline of the light-combining telescope become crucial. Although the Michelson type optical synthetic aperture imaging system does not need to follow this principle, the precise measurement and control of the entrance pupil baseline of the sub-aperture array and the entrance pupil baseline of the light-combining telescope are equally significant. Through the precise measurement and control of the baseline, the actual position of the sub-aperture after the variable baseline moves can be consistent with the UV coverage point position analyzed in its theoretical analysis. Regardless of the type of system, the baseline of its sub-aperture needs to be accurately determined in order to control the system baseline mapping error, thereby controlling the system pupil mapping error value to ensure the system field of view angle and the system observation efficiency.

[0004] Existing methods and means: 1. The Wide Field Imaging Interferometer (WIIT) established by NASA verifies the wide field interferometric imaging algorithm for space interferometer projects such as SPIRIT (infrared), SPECS (submillimeter wave), and TPF-I. The system model is established and verified. Two sub-telescopes are installed at both ends of a linear air-floating stage. Each sub-aperture is 25mm. The baseline of the two sub-telescopes is variable between 25mm and 250mm. UV coverage is achieved through baseline adjustment and synchronous rotation of the target and detector. The position information of the sub-aperture is obtained by feedback from the linear absolute encoder installed on the air-floating guide rail. The encoder can only obtain the position information of the guide sleeve on the guide rail, and cannot obtain the absolute distance of the sub-aperture entrance pupil and exit pupil baseline. At the end of the article, it is mentioned that well-calibrated absolute baseline measurement is needed in the future. No introduction and experimental verification of absolute baseline measurement was found in the subsequent papers on WIIT.

[0005] 2. The National Astronomical Observatory of the Chinese Academy of Sciences designed and built a Fizeau Imaging Interferometer Test Device (FIIT), which mainly consists of a light source module for simulating infinite objects, three sub-telescopes with an aperture of 100mm in a Golay-3 distribution (equilateral triangle), three yaw / pitch correction modules and an optical path delay module, as well as three detection telescopes for phase imaging and a beam synthesis telescope. Broadband white light (400-700nm) phase imaging with a field of view of 2 arc minutes was achieved in the laboratory. The baseline length of the three sub-telescopes in FIIT is 200mm, and the baseline length of the sub-beam is 40mm, both of which cannot be changed.

[0006] 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, which 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.

[0007] The exit pupil baseline is determined by the calibration plate, lens and camera. A precisely processed 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 distributed in an equilateral triangle. Each circular line has a cross line passing through the center of the circle. The center distance of the circular lines has been precisely tested and can be used as the exit pupil baseline test benchmark. After the sub-beam passes through the glass plate and the lens, there are 6 circular images on the camera, which are the images of the 3 sub-beams and the images of the circular lines of the 3 calibration plates. After identifying the coordinates of the center of the 6 images, the baseline of the sub-beam is calculated based on the image center of the circular lines of the calibration plate, thereby obtaining the system exit pupil baseline. The system baseline mapping error can be obtained by combining the system entrance pupil baseline and the system exit pupil baseline.

[0008] 3. China Resources Satellite Application Center analyzed and studied the intersatellite high-precision baseline measurement and processing methods and baseline processing procedures based on the working characteristics of the distributed interferometric synthetic aperture radar satellite system GNSS, SAR and other payloads. The distributed InSAR satellite system first obtains the GNSS measurement baseline through GNSS dual-frequency carrier phase differential measurement and ground high-precision orbit solution; then, through position correction, the reference point of the baseline is converted from the satellite centroid to the SAR antenna phase center to obtain the spatial domain baseline. The experimental results show that the statistical accuracy of the InSAR baseline intercomparison results obtained by orbit determination is about 1 mm.

[0009] 4. For the dual-antenna InsAR system with flexible long baseline configuration, the motion error caused by the jitter of the flexible baseline seriously affects the radar imaging quality and interferometric measurement accuracy. The Remote Sensing Satellite General Department and Xi'an University of Science and Technology and other units proposed a combined baseline measurement system using a single camera and femtosecond laser ranging. Based on this system, a new baseline joint dynamic solution method is proposed. This method is based on the collinear equation of the unit four elements, and the camera measurement data and femtosecond laser data are fused and processed. This method can effectively improve the solution accuracy of the exterior orientation elements.

[0010] The solution mainly includes the following components: measurement sensor installation platform, target tracker (including 1 high-precision CCD camera and 1 femtosecond laser rangefinder), platform position and attitude measurement system (star sensor + GPS), target (LED and cone). The measurement platform is connected to the single-sided antenna through an antenna support arm, which is up to 60m long. The phase centers of the two antennas form a flexible long baseline. The target tracker is rigidly connected to the platform posture measurement system. The target tracker can obtain the position and attitude information of the single-sided antenna relative to the measurement platform. The star sensor and GPS system are used to measure the absolute position and attitude of the platform.

[0011] 5. The Beijing Great Wall Metrology and Testing Technology Research Institute of China Aviation Industry Corporation has invented a femtosecond laser length measurement device and method based on the frequency conversion phase measurement principle. The invention includes a femtosecond laser, a filter, a spectroscope, 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. The invention can achieve distance measurement without adjusting the measuring arm length or the harsh condition that the number of repeated cycles can only be changed by one, but the distance measurement method involved requires more instruments and parts, which takes up more space and costs more.

[0012] 6. Tsinghua University Shenzhen Graduate School invented a femtosecond laser distance measurement device, including a first reference optical path, a second reference optical path and a measurement optical path. The first reference optical path and the second reference optical path have a fixed optical path difference and are respectively provided with shutters, which are used to switch the reference optical path during the distance measurement process to use the corresponding reference light for measurement. The device can effectively solve the dead zone problem existing in femtosecond laser distance measurement, but the distance measurement method involved requires more instruments and parts, occupies more space and has a larger cost.

[0013] Disadvantages of existing technology: (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.

[0014] (2) Currently, the baseline measurement of the existing variable baseline optical synthetic aperture imaging systems at home and abroad is mostly carried out by 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. 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.

[0015] (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. A large number of instruments and mechanisms are required, the system is relatively complex, and it is difficult to implement.

[0016] 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 acknowledging or suggesting in any form that the information constitutes the prior art already known to those skilled in the art. Summary of the invention

[0017] 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 problem that the sub-aperture entrance pupil baseline is difficult to accurately and simply measure in optical synthetic aperture imaging systems with variable baselines and fixed baselines.

[0018] In order to achieve the above object, the present invention adopts the following technical solutions: A method for accurately measuring subaperture baseline of an optical synthetic aperture imaging system, the method specifically comprising: First, two femtosecond laser ranging probes are installed facing each other on their respective micro-adjustment frames to ensure 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 conical lens installed on the optical axis of the system, and the distance between each sub-aperture primary mirror and the fixed conical lens is measured respectively. Finally, its entrance pupil baseline is calculated based on the sub-aperture and conical lens structural parameters.

[0019] 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: According to the principle that a femtosecond laser ranging probe measures the absolute distance between itself and a target, a sub-aperture baseline measuring instrument is placed inside a reference mirror. The distance between 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 the surfaces (18) and (19), respectively, and 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 L1. 仪 = L0-(L1+ L2), L 仪 As a basic parameter of a femtosecond laser rangefinder, its accuracy reaches the micron level.

[0020] Furthermore, the specific steps for accurate measurement of sub-aperture baselines 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 to align them with the two target surfaces, measure the distances between the two probes and their respective target surfaces, and calculate L 仪 Specific value; 2) Install the sub-aperture baseline measuring instrument on the side of a sub-aperture primary mirror, adjust the postures of the two femtosecond laser ranging probes to align them with the small plane on the side of the sub-aperture primary mirror and one side of the conical 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 entrance pupil of the primary mirror and the distance between the side of the aconic lens 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 steps 1) to 5) are repeated after the system changes its baseline.

[0021] Furthermore, the specific process of step 6) 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 rangefinder probes are adjusted to align with the small plane on the side of the sub-aperture primary mirror and one side of the conical 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, it can be known that the distance between the measured plane on the side of the primary mirror and the center of the entrance pupil of the primary 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 axle to its axis; using the above principle, measure the distance L from each sub-aperture entrance pupil to the system optical axis 入-轴2 , L 入-轴3 , L 入-轴4 , the sub-aperture entrance pupil baseline can be obtained.

[0022] By adopting the above technical solution, the present invention has the following beneficial effects: 1) The technology involved is relatively mature and easy to implement. The processing and detection accuracy of mature femtosecond laser ranging probes and conical lenses in China can reach the micron level, which meets the accuracy requirements of this method; 2) Strong versatility. This method is generally applicable to optical synthetic aperture imaging systems with variable baselines and fixed baselines. At the same time, this method is generally applicable to optical synthetic aperture imaging systems with different numbers of sub-apertures. It is applicable to both symmetrically arranged sub-aperture baseline measurements and asymmetrically arranged sub-aperture baseline measurements. It only requires changing the side configuration of the aconic lens; 3) Strong scalability. This method provides ideas and solutions for measuring the absolute distance between multiple components; 4) High cost performance. The femtosecond laser ranging probe, conical lens, adjustment frame and other components used in this method are small in size, relatively mature and low in price. The overall structure is relatively simple, occupies little space and has high cost performance; 5) Absolute measurement. This method is different from the traditional relative measurement that can only measure the change of the baseline. It can measure the absolute distance between multiple parts; 6) High measurement accuracy. Micron-level absolute distance measurement can be achieved through accurate calibration and installation of the baseline measuring instrument; 7) Small impact on the system. The parts involved in this method are small in size and light in weight, and are installed in an area that does not affect the sub-aperture, with little impact on the position and posture of the sub-aperture. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 The present invention is a structural diagram of a device used in the method for accurately measuring the sub-aperture baseline of an optical synthetic aperture imaging system.

[0025] Figure 2 This is a schematic diagram of the sub-aperture baseline precise measurement device of the present invention.

[0026] Figure 3 The figure is a schematic diagram of the femtosecond laser ranging probe spacing calibration device of the present invention.

[0027] Figure 4 It is a schematic diagram of the sub-aperture entrance pupil baseline of the present invention.

[0028] Figure 5 Schematic diagram of the time domain characteristics and frequency domain characteristics of the laser frequency comb of the present invention.

[0029] Figure 6 Schematic diagram of the dual optical frequency comb interferometry measurement system of the present invention.

[0030] Figure 7 It is a schematic diagram of the off-axis two-trans sub-aperture structure of the present invention. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings. It should be understood that the specific implementation described here is only used to illustrate and explain the present invention, and is not used to limit the present invention.

[0033] The present invention proposes a method for accurately measuring the subaperture baseline of an optical synthetic aperture imaging system. The method first installs two femtosecond laser ranging probes facing each other on their respective micro-adjustment frames to ensure that the optical axes are parallel and in opposite directions, and regards them as subaperture baseline measuring instruments. The probes are placed in a reference mirror with a known spacing to calibrate the spacing between the two probes, and then the probes are moved respectively between each subaperture primary mirror and an aconic lens installed on the optical axis of the system, and the distance between each subaperture primary mirror and the fixed aconic lens is measured respectively, and then the entrance pupil baseline is calculated according to the subaperture and aconic lens structural parameters.

[0034] 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 baseline and fixed baseline, provides technical support for the measurement and control of pupil mapping errors of imaging systems, and further provides technical guarantee for the observation field and observation efficiency of imaging systems.

[0035] Combination Figure 1-4 As shown, the optical synthetic aperture imaging system subaperture baseline accurate measurement method proposed by the present invention is used to measure the baseline of the optical synthetic aperture imaging system subaperture, and the required specific structures include collimator 1, subaperture 2-5, subaperture support structure 6-9, subaperture variable baseline guide rail 10-11, aconic lens 12, aconic lens installation adjustment structure 13, subaperture baseline measuring instrument 14, etc. The subaperture baseline measuring instrument 14 is composed of femtosecond laser ranging probe A15, femtosecond laser ranging probe B16, femtosecond laser ranging probe connector 17, etc. In addition, the femtosecond laser ranging probe spacing calibration device is composed of subaperture baseline measuring instrument 14 and reference mirror 20 with known spacing between surface 18 and surface 19. The side configuration of the aconic lens 12 is consistent with the subaperture layout of the system. In this example, the number of subapertures is 4, arranged in a rectangular shape, and the aconic lens also has 4 side surfaces, which are parallel to the side surfaces (small planes) of the 4 subaperture primary mirrors.

[0036] 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.

[0037] The number of subapertures 2-5 is 4, which are arranged behind the collimator 1 and mounted on the subaperture support structures 6-9. The subaperture variable baseline guide 10 carries subapertures 2 and 4 through subaperture support structures 6 and 8, and the optical axis connection line of subapertures 2 and 4 is the baseline 22. The subaperture variable baseline guide 11 carries subapertures 3 and 5 through subaperture support structures 7 and 9, and the optical axis connection line of subapertures 3 and 5 is the baseline 21.

[0038] The angles of the sub-aperture variable baseline guides 10 and 11 with the horizontal plane are -30° and +30° respectively, and the sub-aperture 2-5 can perform variable baseline movement under the drive of the sub-aperture variable baseline guides 10 and 11. Under any baseline condition, the optical axis of the sub-aperture 2-5 is at the same distance as the optical axis of the collimator, the optical axes of the sub-aperture 2-5 form a rectangle in the vertical plane, the length of the baseline 21 is equal to the length of the baseline 22, and the baselines 21 and 22 intersect at the optical axis of the collimator 1.

[0039] The light beam emitted from sub-aperture 2-5 enters the subsequent subsystem.

[0040] The conical lens 12 is installed in the conical lens installation and adjustment structure 13, and the conical lens installation and adjustment structure 13 can be adjusted in five dimensions. The optical axis of the conical lens coincides with the center of the rectangle formed by the optical axis of the parallel light tube and the optical axis of the sub-aperture 2-5, and the four side surfaces of the conical lens 12 are respectively parallel to the primary mirror side reference plane of the sub-aperture 2-5.

[0041] 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 with the variable baseline movement of the sub-aperture 2-5. 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 with the variable baseline movement of the sub-aperture 2-5.

[0042] 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, 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.

[0043] Here’s how this application works: The 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, and then uses the calibrated sub-aperture baseline measuring instrument to precisely measure the relative position of the sub-aperture of an optical synthetic aperture imaging system with a variable baseline or a fixed baseline, and then calculates its entrance pupil baseline based on the sub-aperture and conical lens structure parameters.

[0044] Femtosecond laser absolute distance measurement process: Combination Figure 5-6 As shown in the figure, femtosecond laser absolute distance measurement is an absolute distance measurement method based on optical frequency comb. Femtosecond laser frequency comb refers to the broadband spectrum output by passively mode-locked femtosecond laser, which is composed of tens of thousands of equally spaced coherent spectral lines that are mode-locked in the passive mode-locking process. The frequency of any spectral line is expressed in the following concise form: f n =fceo +n·f rep ; 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.

[0045] Mode locking refers to the phase locking of the longitudinal modes in the laser, thus forming a beat, which makes the laser produce periodic short pulse output in the time domain. The wider the gain spectrum of the laser working medium, the more longitudinal modes can be locked in the laser cavity, thus supporting narrower laser pulse output and higher peak power.

[0046] By locking the carrier frequency and carrier envelope phase of the femtosecond laser frequency comb and using Fourier transform, a series of spectral line comb teeth that are evenly distributed and stably positioned in the frequency domain can be obtained, which is called a laser frequency comb. In the time domain, the laser frequency comb appears as ultrashort pulses with a width of femtoseconds, and in the frequency domain, it appears as comb-shaped frequency lines with regular arrangement and wide spectral coverage.

[0047] 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 , the two pulses are periodically superimposed after passing through the interferometer, with a period of τ=1 / Δf. By analyzing the superimposed interference signal in both the time domain and the frequency domain, fast and high-precision absolute distance measurement can be achieved.

[0048] Figure 6 , 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.

[0049] The first optical frequency comb is used as a measurement optical frequency comb, and the measurement light is generated with 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 local oscillator light repetition frequency 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 r2The interference light passes through an optical bandpass filter with a bandwidth of Δv and is received by a photodetector. From the perspective of 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 shown in the following formula: ; The period of the interference signal is T in =m⋅T r1 =1 / Δf r , where m = f r1 / Δf r , that is, the number of measured light pulses in one cycle. At this time, the measured time delay Δτ is amplified to Δt=m⋅Δτ, and the measured distance L can be further obtained. double , as shown below: ; Explained in the frequency domain, the dual optical frequency comb ranging system produces two RF combs with different phase spectra. Fundamentally speaking, the main source of heterodyne interference is the experimental adjustment method using a single correlated optical sampling, which presents a lower frequency interference signal that can be directly received in adjacent optical frequency comb pulses. At this time, 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 below: ; 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.

[0050] The sub-aperture baseline measuring instrument calibration process is as follows: The calibration of the subaperture baseline measuring instrument refers to the precise measurement of the spacing between its two femtosecond laser ranging probes before integrating it between the subapertures of the optical synthetic aperture imaging system, so that the spacing value can be used as a known quantity to measure the subaperture baseline.

[0051] 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, 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 a femtosecond laser rangefinder, its accuracy can reach the micron level.

[0052] The sub-aperture baseline measurement process 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 rangefinder probes are adjusted to align with the small plane on the side of the sub-aperture primary mirror and one side of the conical lens. The distances from the two probes to their respective target surfaces are measured, which are recorded as L. 主 and L 锥 .

[0053] 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, it can be known that the distance between the measured plane on the side of the primary mirror and the center of the entrance pupil of the primary 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 axle lens 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.

[0054] like Figure 7 As shown in the figure, if the subaperture of the optical synthetic aperture imaging system has a variable baseline, the subaperture will produce motion errors when the variable baseline moves, including 3 angle errors and 3 position errors. If the subaperture motion error is large, the cosine error of the subaperture baseline value measured by this method is also large. In this scheme example, the subaperture variable baseline motion error is measured and corrected. The angle and position errors are in the order of micro-radians and micrometers, and the error values ​​are small, so its influence on the subaperture baseline measurement can be ignored.

[0055] The working process of this application is 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 to align them with the two target surfaces, measure the distances between the two probes and their respective target surfaces, and calculate L 仪 Specific value; 2) Install the sub-aperture baseline measuring instrument on the side of a sub-aperture primary mirror, adjust the postures of the two femtosecond laser ranging probes to align them with the small plane on the side of the sub-aperture primary mirror and one side of the conical 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 entrance pupil of the primary mirror and the distance between the side of the aconic lens 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) Superimposing the distances between each sub-aperture entrance pupil and the system optical axis can 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 steps 1) to 5) are repeated after the system changes its baseline.

[0056] In summary, compared with the prior art, the innovations of this application include: 1) This method uses a subaperture baseline measuring instrument connected to the subaperture and a conical 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.

[0057] 2) Since this method uses absolute ranging technology instead of relative ranging technology, it can perform baseline measurement for both variable baseline and fixed baseline optical synthetic aperture imaging systems; 3) The sub-aperture baseline measuring instrument involved in this method can be connected to the sub-aperture or installed at a certain position between each sub-aperture, and it is sufficient to ensure that the probe of the sub-aperture baseline measuring instrument is aligned with the side of the sub-aperture; 4) The conical lens involved in this method can also be eliminated, and the two probes of the aperture baseline measuring instrument can be respectively aligned with the two sub-apertures on the same straight line. However, this method is only used for the layout scheme where the sub-apertures are on the same straight line. 5) This method is generally applicable to optical synthetic aperture imaging systems with different numbers of sub-apertures. It is applicable to both symmetrically arranged sub-aperture baseline measurements and asymmetrically arranged sub-aperture baseline measurements. It only requires changing the side configuration of the axicon. 6) This method uses a reference mirror to calibrate the two probes of the sub-aperture baseline measuring instrument, which is the key to the absolute measurement of the sub-aperture baseline; 7) This method has certain requirements on 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 to calibrate it.

[0058] 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned 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 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 installed facing each other on their respective micro-adjustment frames to ensure 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 conical lens installed on the optical axis of the system, and the distance between each sub-aperture primary mirror and the fixed conical lens is measured respectively. Finally, its entrance pupil baseline is calculated based on the sub-aperture and conical lens structural parameters.

2. The method for accurately measuring subaperture baseline of an optical synthetic aperture imaging system according to claim 1, characterized in that: 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: According to the principle that a femtosecond laser ranging probe measures the absolute distance between itself and a target, a sub-aperture baseline measuring instrument is placed inside a reference mirror. The distance between 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 the surfaces (18) and (19), respectively, and 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 L1. 仪 = L0-(L1+ L2), L 仪 As a basic parameter of a femtosecond laser rangefinder, its accuracy reaches the micron level.

3. The method for accurately measuring subaperture baseline of an optical synthetic aperture imaging system according to claim 1, characterized in that: 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 to align them with the two target surfaces, measure the distances between the two probes and their respective target surfaces, and calculate L 仪 Specific value; 2) Install the sub-aperture baseline measuring instrument on the side of a sub-aperture primary mirror, adjust the postures of the two femtosecond laser ranging probes to align them with the small plane on the side of the sub-aperture primary mirror and one side of the conical 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 entrance pupil of the primary mirror and the distance between the side of the aconic lens 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 the system changes its baseline and then repeats steps 1) to 5).

4. The method for accurately measuring subaperture baseline of an optical synthetic aperture imaging system according to claim 3, characterized in that: 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 rangefinder probes are adjusted to align with the small plane on the side of the sub-aperture primary mirror and one side of the conical 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, it can be known that the distance between the measured plane on the side of the primary mirror and the center of the entrance pupil of the primary 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 axle to its axis; using the above principle, measure the distance L from each sub-aperture entrance pupil to the system optical axis 入-轴2 , L 入-轴3 , L 入-轴4 , the sub-aperture entrance pupil baseline can be obtained.

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