A three-dimensional topography measurement system for antennas based on an optical fiber image bundle

Abstract

A three-dimensional topography measurement system for antennas based on an optical fiber image bundle of the present invention uses two optical frequency combs with time and phase simultaneously locked to a microwave frequency reference as a reference light source and a test light source respectively, and encodes and images the test light by using the optical fiber image bundle. The test light respectively returned by the reference mirror and the surface of the antenna to be measured interferes with the reference light to form an interference signal, and this interference signal is responded to by a photodetector, and the detection signal is recorded by an oscilloscope. The test optical signal is spatially decoded according to the image calibration of the optical fiber image bundle. The time delay between the femtosecond optical pulse returned by the reference mirror and the femtosecond optical pulse returned by the surface of the antenna to be measured is used for the restoration of the three-dimensional topography of the antenna to be measured. By encoding and imaging the test light of the optical frequency comb by using the optical fiber image bundle, the three-dimensional topography of the antenna to be measured is obtained in real time, overcoming the problems of small dynamic range and low update rate of the existing antenna surface shape detection methods, realizing fast topography measurement with a wide dynamic range, and the system structure is simple.

Description

Technical Field

[0001] The invention belongs to the field of measurement technology, and in particular relates to an antenna three-dimensional shape measurement system based on an optical fiber image transmission bundle and applicable to a large-aperture terahertz antenna. Background Art

[0002] With the continuous development of satellite payloads towards large-scale, high reliability and batch production, as well as the in-depth research on terahertz technology, antennas, as an indispensable component of communication, remote sensing and detection systems, play an important role in the development of terahertz technology. At present, due to the high operating frequency, small relative wavelength and large spatial transmission loss of the terahertz band, terahertz antennas are developing in the direction of high gain, high reliability, high pointing accuracy and large aperture. The accuracy of the antenna reflector is a key performance indicator of the antenna, which directly affects the far-field electrical performance of the antenna and determines the transmission quality of the antenna. Especially for large-aperture reflector antennas, the inaccuracy of the reflector surface shape and the deformation of the surface shape have a more significant impact on the electrical performance of the antenna.

[0003] Usually, the surface accuracy of the antenna should be 1 / 16 to 1 / 32 of the antenna's working wavelength, and the measurement accuracy should be better than the surface accuracy by one order of magnitude or more. The shorter the working wavelength, the more stringent the measurement requirements. For millimeter wave / submillimeter wave antennas, the classic theodolite measurement method has many shortcomings, such as low measurement accuracy, strict temperature requirements before and during measurement, etc. Digital close-range industrial photogrammetry (referred to as industrial photogrammetry) technology uses professional camera photography and computer image processing to measure and image physical quantities such as the geometric size of an object and its position and posture in space. This technology has the characteristics of non-contact measurement, relatively high measurement accuracy, fast measurement speed, and low requirements for environmental conditions. It is often used for rapid detection of antenna surfaces. The general detection accuracy of this method is in the order of 10-100 microns. For higher-precision surface detection, optical means are required, especially interference methods based on stable frequency light fields. The detection accuracy of this technology can reach the nm level. However, due to the limitation of optical wavelength, optical interference detection technology has the dilemma of small range and cannot achieve both large range and high precision, that is, the measurement dynamic range is limited.

[0004] In summary, although there are many traditional surface shape detection methods, they all have technical deficiencies and defects, and it is difficult to meet the high-precision detection requirements of large-aperture terahertz antenna surfaces. Summary of the invention

[0005] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to propose an antenna three-dimensional shape measurement system based on an optical fiber image transmission bundle, to realize rapid three-dimensional shape detection of the terahertz antenna surface shape, to be fully applied in the field of three-dimensional shape measurement, and to realize full-field real-time measurement of objects with larger calibers.

[0006] The technical solution of the present invention is as follows:

[0007] A three-dimensional shape measurement system for an antenna based on an optical fiber image bundle, comprising: a first optical frequency comb, a second optical frequency comb, a first beam expanding system, a semi-transmissive and semi-reflective mirror, a reference mirror, a beam reducing system, a first lens, an optical fiber image bundle, a second lens, a second beam expanding system, a reflecting mirror, a beam combining mirror, a third lens, a photodetector, a data acquisition module, and a computer;

[0008] The output light of the first optical frequency comb is expanded and collimated by the first beam expanding system;

[0009] The test light passes through the semi-transmissive and semi-reflective mirror and the reference mirror and is incident on the surface of the antenna to be measured; the test lights reflected by the reference mirror and the front surface of the antenna to be measured both return along the original path, forming a reference mirror test light and a surface to be measured test light. There is a corresponding time delay between these two test lights due to the surface topography of the antenna to be measured;

[0010] The reference mirror test light and the surface to be measured test light pass through the beam reducing system and the first lens successively and are incident on the input end face of the optical fiber image bundle, and are encoded and imaged by N single-fiber optical fibers; N encoded test lights are emitted from the output end face of the optical fiber image bundle;

[0011] The N encoded test lights are collimated by the second lens, and then combined with the output light of the second optical frequency comb that is expanded and collimated by the second beam expanding system and reflected by the reflecting mirror by the beam combining mirror. The combined light is focused on the surface of the photodetector by the third lens for beat frequency detection to obtain a detection light signal; the detection light signal is collected by the data acquisition module and input into the computer; first, the distances from different spatial positions on the surface of the antenna to be measured to the reference mirror are calculated through the detection light signal, and then the three-dimensional shape of the antenna to be measured is restored according to the calibrated encoding format.

[0012] Preferably, the diameter of the spot corresponding to the test light after being expanded and collimated by the first beam expanding system is 10 - 20 times the diameter of the spot of the output light of the first optical frequency comb.

[0013] Preferably, the reference mirror is a reference plane mirror.

[0014] Preferably, the surface shape accuracy of the reference mirror is better than 1 / 10 wavelength.

[0015] Preferably, the aperture of the reference mirror is not less than the size of the test light spot after being expanded by the first beam expanding system.

[0016] Preferably, the N is a positive integer, and N≥3000.

[0017] Preferably, the difference Δfr between the repetition frequencies of the second optical frequency comb and the first optical frequency comb is at least three orders of magnitude lower than fr1; fr1 is the repetition frequency of the first optical frequency comb.

[0018] Preferably, the method for calculating the distance s between different spatial positions on the surface of the antenna under test and the reference mirror is specifically as follows:

[0019] s = 1 / 2 * (t - t0) * c,

[0020] where c is the speed of light in vacuum, t is the time of the femtosecond pulse returned from the surface of the antenna under test, and t0 is the time of the femtosecond pulse reflected by the reference mirror.

[0021] Preferably, the first beam expander system is used to increase the diameter of the light spot corresponding to the test light.

[0022] Preferably, the calibrated coding format represents the position correspondence between each single fiber in the fiber optic image bundle and the surface of the antenna under test.

[0023] The advantages of the present invention compared with the prior art are as follows:

[0024] Based on the optical frequency comb technology, the present invention proposes an antenna three-dimensional topography measurement system based on a fiber optic image bundle, which breaks through the problems of narrow dynamic range and slow update frequency in optical interference measurement. The surface of the antenna under test is optically imaged and position calibrated by using the fiber optic image bundle, realizing high-precision and rapid surface shape detection of the terahertz antenna, and providing key technical support for the design and implementation of high-gain spaceborne terahertz antennas. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of an antenna three-dimensional topography measurement system based on a fiber optic image bundle.

[0026] Figure 2 Schematic diagram of time delay. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] An antenna three-dimensional topography measurement system based on a fiber optic image bundle according to the present invention includes: a first optical frequency comb 1-1, a second optical frequency comb 1-2, a first beam expander system 3-1, a beam splitter 4, a reference mirror 5, a beam reducer system 7, a first lens 8-1, a fiber optic image bundle 9, a second lens 8-2, a second beam expander system 3-2, a reflector 11, a beam combiner 10, a third lens 8-3, a photodetector 12, a data acquisition module 13, and a computer 14.

[0028] The emitted light of the first optical frequency comb 1-1 is expanded, collimated, and focused by the first beam expander system 3-1. The test light passes through the semi-transparent and semi-reflective mirror 4 and the reference mirror 5, and is incident on the surface of the antenna under test 6; the test lights respectively reflected by the reference mirror 5 and the front surface of the antenna under test 6 return along the original path, forming a reference mirror test light and a surface under test test light. There is a corresponding time delay between these two test lights due to the surface topography of the antenna under test 6.

[0029] The reference mirror test light and the surface under test test light successively pass through the beam reduction system 7 and the first lens 8-1, and are incident on the input end face of the fiber optic image bundle 9, and are encoded and imaged with a certain resolution by N single-fiber optical fibers; N encoded test lights are emitted from the output end face of the fiber optic image bundle. After the N encoded test lights are collimated by the second lens 8-2, they are combined with the emitted light of the second optical frequency comb 1-2 that is expanded, collimated by the second beam expander system 3-2, and reflected by the mirror 11 using the beam combiner 10. The combined light is then focused by the third lens 8-3 onto the surface of the photodetector 12 for beat frequency detection to obtain a detected optical signal; the detected optical signal is collected by the data acquisition module 13 composed of an oscilloscope or a data acquisition card and input into the computer 14; first, the distances between different spatial positions on the surface of the antenna under test and the reference mirror are calculated from the detected optical signal, and then the three-dimensional topography of the antenna under test is restored according to the calibrated coding format.

[0030] The diameter of the spot corresponding to the test light after being expanded, collimated, and focused by the first beam expander system 3-1 is 10-20 times the diameter of the spot of the emitted light of the first optical frequency comb 1-1; when the aperture of the antenna under test is larger than the diameter of the spot of the expanded test light, a scanning and stitching test is required.

[0031] The reference mirror 5 is a standard plane mirror. The surface form accuracy of the reference mirror 5 is better than 1 / 10 wavelength. The aperture of the reference mirror 5 is not less than the size of the test light spot after being expanded by the first beam expander system 3-1. Encoding and imaging are performed by N single-fiber optical fibers, where N refers to the number of single-fiber optical fibers in the fiber optic image bundle, and N≥3000.

[0032] The so-called certain resolution means that the light reflected from different positions on the surface of the antenna under test passes through the beam reduction system and a lens and is focused on the end face of the fiber optic image bundle. Since the fiber optic image bundle is composed of multiple single-fiber optical fibers, the focused light spots are respectively converged into different single-fiber optical fibers, which is the measurement resolution of the three-dimensional topography of the surface of the antenna under test.

[0033] The so-called according to the calibrated coding format means that the system needs to be calibrated before measuring the antenna under test. The position correspondence between each single-fiber optical fiber in the fiber optic image bundle and the surface of the antenna under test is calibrated using a resolution target.

[0034] The repetition frequency of the first optical frequency comb is f1 = f0 + nfr1, and the repetition frequency of the second optical frequency comb is f2 = f0 ’ + n(fr1 + Δfr), where f0 and f0 ’ are the carrier-envelope phase drift frequencies of the first and second optical frequency combs respectively, Δfr is the difference between the repetition frequencies of the second and the first optical frequency combs, and the difference is at least three orders of magnitude lower than fr1; n is the number of teeth of the 1-1 comb of the first optical frequency comb; the number of teeth of the 1-1 comb of the first optical frequency comb is equal to that of the 1-2 comb of the second optical frequency comb, and fr1 is the repetition frequency of the 1-1 comb of the first optical frequency comb. The repetition frequencies and the carrier-envelope phase drift frequencies of these two optical frequency combs are locked to the frequency reference of the rubidium clock 2.

[0035] The time delay mentioned refers to the time delay of the femtosecond pulse emitted by the optical frequency comb. Taking the time of the femtosecond pulse reflected by the standard mirror as the reference time t0, then the time of the femtosecond pulse returning from the surface of the antenna under test through the standard mirror is t, and 1 / 2*(t - t0)*c (where c is the speed of light in vacuum) is the distance from the surface of the antenna under test 6 to the standard mirror 5. After being encoded and imaged through the fiber optic image bundle, N test lights with different time delays will be emitted at the rear end of the fiber optic image bundle. These N test lights correspond to the distances from N different positions on the surface of the antenna under test to the standard mirror according to the calibrated encoding format. First, determine the three-dimensional spatial positions of the N distances obtained by processing according to the encoding format, and then perform coordinate transformation to obtain the three-dimensional topography of the antenna under test.

[0036] The method for calculating the distance s between different spatial positions on the surface of the antenna under test 6 and the standard mirror 5 is specifically as follows:

[0037] s = 1 / 2*(t - t0)*c,

[0038] where c is the speed of light in vacuum, t is the time of the femtosecond pulse returning from the surface of the antenna under test 6, and t0 is the time of the femtosecond pulse reflected by the standard mirror 5.

[0039] The first beam expander system 3-1 is used to increase the diameter of the spot corresponding to the test light.

[0040] The features of the present invention and other related features will be further described in detail below with reference to the accompanying drawings through embodiments for easy understanding:

[0041] See Figure 1, the principle of the three-dimensional topography measurement method of the antenna based on the fiber optic image bundle is as follows: First, it is necessary to use a resolution target to calibrate the imaging of the fiber optic image bundle to determine the spatial position relationship between the N single-fiber optical fibers of the fiber optic image bundle and the surface of the antenna to be measured. The pixel of the fiber optic image bundle determines the resolution of the surface shape detection of the antenna to be measured. Secondly, use a beam expander system to expand the femtosecond pulsed light emitted by the optical frequency comb. After passing through the beam splitter, it returns along the original path from the standard mirror and the surface of the antenna to be measured respectively. In the test light output by each single-fiber optical fiber in the fiber optic image bundle, there are paired test light signals that do not return simultaneously from the standard mirror and the surface of the antenna to be measured. When the test light signals output by the N single-fiber optical fibers in the fiber optic image bundle interfere and beat with the reference light, and then after the photoelectric conversion and data acquisition by the photodetector, the time delays Δt1, Δt2,..., Δt corresponding to different positions on the surface of the antenna to be measured are obtained. N , see Figure 2 . Calculate 1 / 2*c*Δt1, 1 / 2*c*Δt2,..., 1 / 2*c*Δt N respectively, and the distances between different spatial positions on the surface of the antenna to be measured and the standard mirror can be obtained. Finally, according to the imaging calibration result of the fiber optic image bundle, coordinate transformation can be performed to obtain the unit topography method of the surface of the antenna to be measured.

[0042] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modification, equivalent change and decoration made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0043] The content not detailedly described in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A three-dimensional topography measurement system for an antenna based on an optical fiber image bundle, characterized in that Including: A first optical frequency comb (1-1), a second optical frequency comb (1-2), a first beam expander system (3-1), a beam splitter (4), a reference mirror (5), a beam reducer system (7), a first lens (8-1), a fiber optic image bundle (9), a second lens (8-2), a second beam expander system (3-2), a reflector (11), a beam combiner (10), a third lens (8-3), a photodetector (12), a data acquisition module (13), and a computer (14); The output light of the first optical frequency comb (1-1) is expanded and collimated by the first beam expander system (3-1); The test light passes through the beam splitter (4) and the reference mirror (5) and is incident on the surface of the antenna under test (6); The test lights reflected by the reference mirror (5) and the front surface of the antenna under test (6) respectively return along the original path, forming a reference mirror test light and an antenna surface under test light. There is a corresponding time delay between these two test lights due to the surface topography of the antenna under test (6); The reference mirror test light and the antenna surface under test light pass through the beam reducer system (7) and the first lens (8-1) successively and are incident on the input end face of the fiber optic image bundle (9), and are encoded and imaged by N single-fiber optical fibers; N encoded test lights are emitted from the output end face of the fiber optic image bundle; The N encoded test lights are collimated by the second lens (8-2) and then combined with the output light of the second optical frequency comb (1-2) that is expanded and collimated by the second beam expander system (3-2) and reflected by the reflector (11) using the beam combiner (10). The combined light is then focused on the surface of the photodetector (12) by the third lens (8-3) for beat frequency detection to obtain a detection light signal; The detection light signal is collected by the data acquisition module (13) and input into the computer (14); The computer (14) first calculates the distance between different spatial positions on the surface of the antenna under test and the reference mirror through the detection light signal, and then restores the three-dimensional topography of the antenna under test according to the calibrated coding format; The calibrated coding format represents the position correspondence between each single-fiber optical fiber in the fiber optic image bundle (9) and the surface of the antenna under test (6).

2. The three-dimensional shape measurement system of an antenna based on an optical fiber image bundle according to claim 1, characterized in that: The diameter of the spot corresponding to the test light after being expanded and collimated by the first beam expander system (3-1) is 10 - 20 times the diameter of the spot of the output light of the first optical frequency comb (1-1).

3. The three-dimensional topography measurement system of an antenna based on an optical fiber image bundle according to claim 1, characterized in that: The reference mirror (5) is a reference plane mirror.

4. The three-dimensional topography measurement system of an antenna based on an optical fiber image bundle according to claim 3, characterized in that: The surface form accuracy of the reference mirror (5) is better than 1 / 10 wavelength.

5. The three-dimensional topography measurement system of an antenna based on an optical fiber image bundle according to claim 4, characterized in that: The aperture of the reference mirror (5) is not less than the size of the test light spot after being expanded by the first beam expander system (3-1).

6. A three-dimensional shape measurement system for an antenna based on an optical fiber image bundle according to any one of claims 1 to 5, characterized in that: The N is a positive integer, and N≥3000.

7. The three-dimensional shape measurement system of an antenna based on an optical fiber image bundle according to claim 6, characterized in that: The difference Δfr between the repetition frequencies of the second optical frequency comb and the first optical frequency comb is at least 3 orders of magnitude lower than fr1; fr1 is the repetition frequency of the first optical frequency comb (1-1).

8. The three-dimensional shape measurement system of an antenna based on an optical fiber image bundle according to claim 7, wherein: The method for calculating the distance s between different spatial positions on the surface of the antenna under test (6) and the reference mirror (5) is specifically: s = 1 / 2*(t - t0)*c, where c is the speed of light in vacuum, t is the time of the femtosecond pulse returned from the surface of the antenna under test (6), and t0 is the time of the femtosecond pulse reflected by the reference mirror (5).

9. The three-dimensional topography measurement system of an antenna based on an optical fiber image bundle according to claim 8, wherein: The first beam expander system (3-1) is used to increase the diameter of the spot corresponding to the test light.

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