Reflective fabry-perot grating interferometric displacement measurement method and system

Abstract

The application provides a reflective Fabry-Perot grating interference displacement measurement method and system, relates to the field of optical measurement, and the technical points are that: the light beam emitted by a frequency stabilization light source is split by a splitting device to output a first light beam and a second light beam; the first light beam enters an F-P cavity composed of a first cavity mirror and a grating after passing through a first loop device; a first interference light beam is received by a first photodetector after sequentially passing through the first cavity mirror and the first loop device; the second light beam enters the F-P cavity composed of a second cavity mirror and the grating after passing through a second loop device; a second interference light beam is received by a second photodetector after sequentially passing through the second cavity mirror and the second loop device; two interference signals are calculated by a signal processing module to obtain two multi-beam interference phases, and decoupling is performed to obtain two degrees of freedom displacement information of the grating in-plane and out-of-plane. Compared with the prior art, the complexity of the optical path is reduced, the range of the grating out-of-plane displacement is improved, and the working distance limit is broken.

Description

Technical Field

[0001] This application relates to the field of optical measurement technology, and in particular to a method and system for measuring displacement by reflection-type Fabry-Perot grating interferometry. Background Technology

[0002] Grating interferometers are an effective means of displacement measurement. Their measurement principle is based on the phase changes in the interference signal caused by the Doppler frequency shifts generated by in-plane and out-of-plane motion of the grating, thus revealing displacement information for a single degree of freedom in the grating plane or two degrees of freedom in-plane and out-of-plane motion. Examples include the commonly used two-beam interferometry or transmission-type multi-beam interferometry. However, existing two-beam grating interferometers typically suffer from drawbacks such as complex structure, high manufacturing costs, difficult assembly and adjustment, and limitations in miniaturization design of the sensing head, hindering the miniaturization requirements of high-end equipment. While multi-beam Fabry-Perot grating interferometers simplify the probe structure to some extent, the use of transmission signals from multi-beam grating interference limits the measurement range of out-of-plane displacement and the working distance (i.e., the distance between the sensing head and the grating), still presenting certain structural shortcomings. Summary of the Invention

[0003] This application provides a reflective Fabry-Perot grating interferometric displacement measurement method and system, which has the advantages of simple structure, convenient assembly and adjustment, and flexible working distance, and can measure two degrees of freedom motion in and out of the grating plane.

[0004] To achieve the above objectives, the first aspect of this application provides a reflective Fabry-Perot grating interferometric displacement measurement method, wherein a beam of light is emitted from a frequency-stabilized light source and sequentially split by a beam splitter.

[0005] The first beam output by the beam splitter passes through the first annular device and generates the first incident beam, which enters the FP cavity composed of the first cavity mirror and the grating. The generated first interference beam passes through the first cavity mirror and the first annular device in sequence and is received by the first photodetector.

[0006] The second beam output from the beam splitter passes through the second annular device and generates a second incident beam, which enters another FP cavity composed of a second cavity mirror and a grating. The generated second interference beam passes through the second cavity mirror and the second annular device in sequence and is received by the second photodetector.

[0007] The first interference signal received by the first photodetector and the second interference signal received by the second photodetector are transmitted to the signal processing module to obtain the first multi-beam interference phase and the second multi-beam interference phase, and the grating two-degree-of-freedom displacement information is decoupled to obtain the grating.

[0008] In some embodiments of the first aspect, the first cavity mirror and the second cavity mirror are reflective beam splitters or diffractive beam splitters.

[0009] In some embodiments of the first aspect, the optical axes of the first incident light and the second incident light form a +m or -m Littrow angle with the grating normal, where m is a positive integer greater than or equal to 1; or,

[0010] The optical axes of the first and second incident beams are parallel to the grating normal; or,

[0011] The optical axes of the first incident light and the second incident light are neither parallel to the grating normal nor do they form any order Littrow angle.

[0012] In some embodiments of the first aspect, a light deflection device is placed between the first annular device, the second annular device, and the grating such that the optical axes of the first incident light and the second incident light form a +m or -m Littrow angle with the grating normal, where m is a positive integer greater than or equal to 1.

[0013] In some embodiments of the first aspect, the light deflecting device includes a first light deflecting device and a second light deflecting device.

[0014] The second aspect of this application provides a reflective Fabry-Perot grating interferometric displacement measurement system, comprising a frequency-stabilized light source, a beam splitter, a first circulator, a second circulator, a first cavity mirror, a second cavity mirror, a grating, a first photoelectric detection module, a second photoelectric detection module, and a signal processing module:

[0015] The first cavity mirror and the grating, and the second cavity mirror and the grating, respectively constitute an FP cavity;

[0016] A beam of light emitted from a frequency-stabilized light source is split by a beam splitter.

[0017] The first beam output by the beam splitter passes through the first annular device and generates the first incident beam, which enters the FP cavity composed of the first cavity mirror and the grating. The generated first interference beam passes through the first cavity mirror and the first annular device in sequence and is received by the first photodetector.

[0018] The second beam output from the beam splitter passes through the second circulator and generates a second incident beam, which enters the FP cavity composed of the second cavity mirror and the grating. The generated second interference beam passes through the second cavity mirror and the second circulator in sequence and is received by the second photodetector.

[0019] The first interference signal received by the first photodetector and the second interference signal received by the second photodetector are transmitted to the signal processing module to obtain the first multi-beam interference phase and the second multi-beam interference phase, and decouple to obtain the single-degree-of-freedom displacement information in the grating plane, or the synchronous displacement information of two degrees of freedom in the plane and out of the plane.

[0020] In some embodiments of the second aspect, the first incident light output from the first annular device enters the first FP cavity after passing through the first collimating device to form a spatial transmission optical path; and / or,

[0021] The second incident light output from the second ring device enters the second FP cavity after passing through the second collimating device to form a spatial transmission optical path.

[0022] In some embodiments of the second aspect, the first cavity mirror is integrated onto the light-emitting surface of the first collimating device by means of bonding, coating, mechanical scribing, or photolithography; and / or,

[0023] The second cavity mirror is integrated into the light-emitting surface of the second collimator by means of bonding, coating, mechanical scribing or photolithography.

[0024] In some embodiments of the second aspect, the optical axis of the spatial transmission optical path entering the first FP cavity is perpendicular to the first cavity mirror and forms a Littrow angle of +m or -m with the grating normal, where m is a positive integer greater than or equal to 1; and / or,

[0025] The optical axis of the spatial optical path entering the second FP cavity is perpendicular to the second cavity mirror and forms a Littrow angle of +m or -m with the grating normal, where m is a positive integer greater than or equal to 1.

[0026] In some embodiments of the second aspect, the light beam emitted by the light source passes through an optical isolator before entering the beam splitter; and / or,

[0027] There is at least one light deflection device between the first cavity mirror, the second cavity mirror, and the grating.

[0028] This application has the following beneficial effects:

[0029] Compared with the existing two-beam grating interferometry scheme, it effectively reduces the complexity of the optical path, the difficulty of miniaturization design, and the difficulty and cost of processing and assembly. Compared with the existing transmission Fabry-Perot grating interferometry scheme, it effectively improves the range of out-of-plane displacement of the grating, and breaks through the limitation of the working distance by the image plane size of the photodetector. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1This is a schematic diagram of the connection structure of each module in a reflective Fabry-Perot grating interferometric displacement measurement system according to an embodiment of this application;

[0032] Figure 2 This is a schematic diagram of the connection between the collimating device and the endoscope in Embodiment 1 of this application;

[0033] Figure 3 This is a schematic diagram of the connection between the collimating device and the endoscope in Embodiment 2 of this application;

[0034] Figure 4 This is a schematic diagram of the connection between the collimating device and the cavity mirror in Embodiment 3 of this application.

[0035] Figure label:

[0036] 1. Frequency-stabilized light source; 2. Transmission optical path; 21. Beam splitter; 22a. First ring device; 22b. Second ring device; 23a. First collimator; 23b. Second collimator; 3. Fabry-Perot cavity; 31a. First cavity mirror; 31b. Second cavity mirror; 32. Grating; 4a. First photodetector; 4b. Second photodetector; 5. Signal processing module; 6a. First light deflector; 6b. Second light deflector. Detailed Implementation

[0037] The embodiments of this application will be described in further detail below with reference to the accompanying drawings and examples. The terminology used in the embodiments section of this application is only used to explain the specific embodiments of this application and is not intended to limit this application.

[0038] In existing technologies, most grating interferometric displacement measurement methods are based on the principle of two-beam interference. For example, publication CN102353327A discloses a dual-frequency laser grating interferometric measurement method and system for measuring single-degree-of-freedom motion within the grating surface. Its two-beam interference principle involves obtaining the grating's single-degree-of-freedom displacement information through phase decoupling operations between the reference interference beam and the measurement interference beam. Another example is publication CN109579694B, which discloses a high-tolerance two-degree-of-freedom heterodyne grating interferometric measurement method and system. Its two-beam interference principle involves using a reference interference beam, a first measurement beam, and a second measurement beam, and obtaining the grating's two-degree-of-freedom displacement information through phase decoupling operations between the three interference beams. However, the two-beam grating interferometric scheme is structurally complex, costly to manufacture, and difficult to assemble and adjust, which is not conducive to the miniaturization design of the sensing head and cannot yet meet the miniaturized, embedded grating interferometric measurement requirements of high-end equipment.

[0039] Another approach utilizes the principle of transmission-based multi-beam interferometry. For example, publication CN112444194B discloses a Fabry-Perot grating interferometer for two-degree-of-freedom displacement measurement, its measurement method, and a six-degree-of-freedom interferometer. The multi-beam interference is achieved by incident light from a laser source at a Littoral angle onto the grating, forming a self-collimating optical path structure and diffracting on the grating surface. A Fabry-Perot cavity (FP cavity) is formed between a portion of the transmission surface and the grating. The phase of the transmitted signal from the multi-beam grating interference formed by the two FP cavities is detected, further decoupling to obtain the two-degree-of-freedom displacement information. While the transmission-based multi-beam grating interferometry simplifies the probe structure to some extent, the use of the transmitted signal from the multi-beam grating interference limits the measurement range of out-of-plane displacement and also limits the working distance (i.e., the distance between the sensing head and the grating), thus still having certain structural limitations.

[0040] like Figure 1 As shown, in order to solve the above-mentioned technical problems in the prior art, this application provides a reflective Fabry-Perot grating interferometric displacement measurement system, including a frequency-stabilized light source 1, a transmission optical path 2, a Fabry-Perot cavity 3, a photodetector, and a signal processing module 5;

[0041] The transmission optical path 2 includes a beam splitter 31, a first ring device 22a, and a second ring device 22b. The beam splitter 31 is connected to the first ring device 22a and the second ring device 22b respectively.

[0042] The Fabry-Perot cavity 3 includes a first cavity mirror 31a, a second cavity mirror 31b, and a grating 32, with the grating 32 connected to the first cavity mirror 31a and the second cavity mirror 31b respectively.

[0043] The photodetector includes a first photodetector 4a and a second photodetector 4b;

[0044] The frequency-stabilized light source 1 is connected to the beam splitter 21;

[0045] The first photodetector 4a is connected to the signal processing module 5 and the first ring device 22a respectively; the second photodetector 4b is connected to the signal processing module 5 and the second ring device 22b respectively.

[0046] The first cavity mirror 31a is connected to the first annular device 22a, and the second cavity mirror 31b is connected to the second annular device 22b.

[0047] It should be understood that the “connection” in this embodiment can be either an indirect connection or a direct connection, and an indirect connection includes an optical path connection.

[0048] The laser emitted by the frequency-stabilized light source 1 is transmitted to the beam splitter 21 to generate a first beam and a second beam. The first beam and the second beam pass through the first annular device 22a and the second annular device 22b, respectively, to form a first incident beam and a second incident beam. The first incident beam and the second incident beam enter the two Fabry-Perot cavities 3 formed by the first cavity mirror 31a and the grating 32, and the second cavity mirror 31b and the grating 32, respectively, resulting in multi-beam interference. The transmitted signals from the two Fabry-Perot cavities 3 pass through the first cavity mirror 31a and the second cavity mirror 31b again, and are oriented by the first annular device 22a and the second annular device 22b, respectively, before reaching the first photodetector 4a and the second photodetector 4b. The electrical signals obtained from the photodetectors are input to the signal processing module 5, which calculates the in-plane single-degree-of-freedom displacement or the in-plane and out-of-plane two-degree-of-freedom displacement of the grating 32.

[0049] like Figure 2 As shown, in Embodiment 1 of this application, the beam splitter 21 is an optical fiber beam splitter, the first circulator 22a and the second circulator 22b are optical fiber circulators, the first cavity mirror 31a and the second cavity mirror 31b are semi-reflective mirrors, and the reflective Fabry-Perot grating interferometric displacement measurement system also includes a first collimator 23a and a second collimator 23b. The first cavity mirror 31a and the second cavity mirror 31b are respectively integrated into the light-emitting surfaces of the first collimator 23a and the second collimator 23b by coating. The beam output by the collimator forms a Litterrow angle with the grating 32.

[0050] like Figure 3 As shown in Embodiment 2 of this application, the first collimating device 23a and the second collimating device 23b are placed perpendicular to the grating 32. The first cavity mirror 31a and the second cavity mirror 31b are semi-reflective lenses, which are integrated into the light-emitting surfaces of the first collimating device 23a and the second collimating device 23b respectively by coating. The reflective Fabry-Perot grating interferometric displacement measurement system also includes a first optical path deflection element 6a and a second optical path deflection element 6b. The first optical path deflection element 6a and the second optical path deflection element 6b are added to the Fabry-Perot cavity 4 so that the light beam output by the collimating device forms a Litterrow angle with the grating 32.

[0051] like Figure 4 As shown, in Embodiment 3 of this application, the first collimating device 23a and the second collimating device 23b are placed perpendicular to the grating 32. The first cavity mirror 31a and the second cavity mirror 31b are transmission gratings, which are directly etched on the light-emitting surfaces of the first collimating device 23a and the second collimating device 23b, so that the light beam output by the collimating device forms a Litterrow angle with the grating 32.

[0052] The working principle of the reflective Fabry-Perot grating interferometric displacement measurement system provided in this application embodiment is as follows: Let the in-plane displacement of the grating 32 be x, and the out-of-plane displacement be z; then the displacement corresponding to the phase of the reflective multi-beam interference signal obtained by the first photodetector 4a can be expressed as x + kz (the coefficient k is a constant determined by the Littorau angle of the optical path); the displacement corresponding to the phase of the reflective multi-beam interference signal obtained by the second photodetector 4b is x - kz, so that the displacement information in both directions can be calculated by the signal processing module 5, that is, to realize the function of single-degree-of-freedom displacement measurement in the grating plane or the function of synchronous measurement of two-degree-of-freedom displacement in and out of the plane.

[0053] Combination Figures 1 to 4 As shown, this application embodiment also provides a reflective Fabry-Perot grating interferometric displacement measurement method, in which a beam is emitted from a frequency-stabilized light source 1 and sequentially split by a beam splitter 21;

[0054] The first beam output by the beam splitter 21 passes through the first annular device 22a and generates the first incident light, which enters the FP cavity (first FP cavity) composed of the first cavity mirror 31a and the grating 32. The generated first interference beam passes through the first cavity mirror 31a and the first annular device 22a in sequence and is received by the first photodetector 4a.

[0055] The second beam output from the beam splitter 21 passes through the second annular device 22b and generates a second incident beam, which enters another FP cavity (second FP cavity) composed of the second cavity mirror 31b and the grating 32. The generated second interference beam passes through the second cavity mirror 31b and the second annular device 22b in sequence and is received by the second photodetector 4b.

[0056] The first interference signal received by the first photodetector 4a and the second interference signal received by the second photodetector 4b are transmitted to the signal processing module 5. The signal processing module 5 demodulates the first multi-beam interference phase and the second multi-beam interference phase, and decouples the in-plane and out-of-plane two-degree-of-freedom displacement information of the grating 32.

[0057] In some embodiments of this example, the first cavity mirror 31a and the second cavity mirror 31b are reflective beam splitters or diffractive beam splitters.

[0058] In some embodiments of this example, the optical axes of the first and second incident lights form a +m or -m Littrow angle with the normal of the grating 32, where m is a positive integer greater than or equal to 1; or,

[0059] The optical axes of the first and second incident lights are parallel to the normal of grating 32; or,

[0060] The optical axes of the first incident light and the second incident light are neither parallel to the normal of grating 32 nor do they form any order Littrow angle.

[0061] In some embodiments of this example, a light deflection device is placed between the first annular device 22a, the second annular device 22b and the grating 32 so that the optical axes of the first incident light and the second incident light form a +m or -m Littrow angle with the normal of the grating 32, where m is a positive integer greater than or equal to 1.

[0062] In some embodiments of this example, the light deflection device includes a first light deflection device 6a and a second light deflection device 6b.

[0063] The in-plane and out-of-plane two-degree-of-freedom displacement synchronous measurement method and system provided in this application have the advantages of simple structure, convenient assembly and adjustment, and flexible working distance, and the probe does not impose any limitations on the range of the two measured degrees of freedom. Compared with the existing dual-beam grating interferometry technology, it effectively reduces the complexity of the optical path, reduces the difficulty of miniaturization design, and reduces the difficulty and cost of processing and assembly and adjustment; compared with the existing multi-beam grating interferometry technology, it effectively improves the range of out-of-plane displacement of the grating, and at the same time breaks through the limitation of the working distance by the image plane size of the photodetector.

[0064] In the description of the embodiments of this application, it should be noted that "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship. Terms such as "including" and / or "having" can be interpreted as indicating a specific characteristic, number, operation, constituent element, component, or combination thereof, but should not be interpreted as excluding the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0065] In the description of the embodiments of this application, it should be noted that the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Features defined with "first" or "second" may explicitly or implicitly include one or more of that feature. Unless otherwise stated, "multiple" means two or more.

[0066] The above embodiments are merely explanations of this application and are not intended to limit it. After reading this specification, those skilled in the art can make modifications to the implementation methods of this application without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A reflective Fabry-Perot grating interferometer displacement measurement method, characterized by, The light beam emitted by the frequency stabilized light source (1) is split by the light splitting device (21); The first light beam output by the light splitting device (21) passes through the first loop device (22a) to generate first incident light, which enters the F-P cavity composed of the first cavity mirror (31a) and the grating (32), and the generated first interference light beam passes through the first cavity mirror (31a) and the first loop device (22a) in turn and is received by the first photodetector (4a); The second light beam output by the light splitting device (21) passes through the second loop device (22b) to generate second incident light, which enters another F-P cavity composed of the second cavity mirror (31b) and the grating (32), and the generated second interference light beam passes through the second cavity mirror (31b) and the second loop device (22b) in turn and is received by the second photodetector (4b); The first interference signal received by the first photodetector (4a) and the second interference signal received by the second photodetector (4b) are transmitted to the signal processing module (5), and the first multi-beam interference phase and the second multi-beam interference phase are obtained after processing by the signal processing module (5), and the in-plane and out-of-plane two-degree-of-freedom displacement information of the grating (32) is decoupled; A light deflection device is placed between the first loop device (22a), the second loop device (22b) and the grating (32) to make the optical axis of the first incident light and the second incident light form a +m order or -m order Littrow angle with the normal line of the grating (32), wherein m is a positive integer greater than or equal to 1.

2. The reflective Fabry-Perot grating interferometer displacement measurement method according to claim 1, characterized in that, The first cavity mirror (31a) and the second cavity mirror (31b) are reflective light splitting devices or diffractive light splitting devices.

3. The reflective Fabry-Perot grating interferometer displacement measurement method of claim 1, wherein, The optical axis of the first incident light and the second incident light forms a +m order or -m order Littrow angle with the normal line of the grating (32), wherein m is a positive integer greater than or equal to 1; or, The optical axis of the first incident light and the second incident light is parallel to the normal line of the grating (32); or, The optical axis of the first incident light and the second incident light is neither parallel to the normal line of the grating (32) nor forms any order Littrow angle.

4. The reflective Fabry-Perot grating interferometer displacement measurement method of claim 1, wherein, It comprises at least one light deflection device.

5. A reflective Fabry-Perot grating interferometer displacement measurement system characterized by, It comprises a frequency stabilized light source (1), a light splitting device (21), a first loop device (22a), a second loop device (22b), a first cavity mirror (31a), a second cavity mirror (31b), a grating (32), a first photodetector (4a), a second photodetector (4b) and a signal processing module (5): The first cavity mirror (31a) and the grating (32), and the second cavity mirror (31b) and the grating (32) respectively form F-P cavities; The light beam emitted by the frequency stabilized light source (1) is split by the light splitting device (21); The first light beam output by the light splitting device (21) passes through the first loop device (22a) to generate first incident light, which enters the F-P cavity composed of the first cavity mirror (31a) and the grating (32), and the generated first interference light beam passes through the first cavity mirror (31a) and the first loop device (22a) in turn and is received by the first photodetector (4a); The second light beam output by the light splitting device (21) generates second incident light after passing through the second ring-shaped device (22b) and entering the F-P cavity composed of the second cavity mirror (31b) and the grating (32), and the generated second interference light beam passes through the second cavity mirror (31b) and the second ring-shaped device (22b) in turn and is received by the second photodetector (4b); The first interference signal received by the first photodetector (4a) and the second interference signal received by the second photodetector (4b) are transmitted to the signal processing module (5), the first multi-beam interference phase and the second multi-beam interference phase are obtained after the signal processing module (5) processes the first interference signal and the second interference signal, and the in-plane single-degree-of-freedom displacement information of the grating (32) or the in-plane and out-of-plane two-degree-of-freedom synchronous displacement information of the grating (32) is obtained after decoupling; The light beam emitted by the frequency-stabilized light source (1) passes through an optical isolator before entering the light splitting device (21); and / or, there is at least one light deflection device (6) between the first cavity mirror (31a), the second cavity mirror (31b) and the grating (32).

6. The reflective Fabry-Perot grating interferometer displacement measurement system of claim 5, wherein, The first incident light output by the first ring-shaped device (22a) enters the first F-P cavity after passing through the first collimating device (23a) to form a spatial transmission light path; and / or, The second incident light output by the second ring-shaped device (22b) enters the second F-P cavity after passing through the second collimating device (23b) to form a spatial transmission light path.

7. The reflective Fabry-Perot grating interferometer displacement measurement system of claim 5, wherein, The first cavity mirror (31a) is integrated on the light-emitting surface of the first collimating device (23a) by means of bonding, coating, mechanical scribing or photolithography; and / or, The second cavity mirror (31b) is integrated on the light-emitting surface of the second collimating device (23b) by means of bonding, coating, mechanical scribing or photolithography.

8. The reflective Fabry-Perot grating interferometer displacement measurement system of claim 5, wherein, The optical axis of the spatial transmission light path entering the first F-P cavity is perpendicular to the first cavity mirror (31a) and forms a +m or -m order Littrow angle with the normal line of the grating (32), where m is a positive integer greater than or equal to 1; and / or, The optical axis of the spatial transmission light path entering the second F-P cavity is perpendicular to the second cavity mirror (31b) and forms a +m or -m order Littrow angle with the normal line of the grating (32), where m is a positive integer greater than or equal to 1.

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