Gas refractive index measuring device and method based on Michelson interferometer

Abstract

The invention provides a gas refractive index measuring device and method based on a Michelson interferometer, based on the Michelson interference principle, one interference arm of the Michelson interferometer is closed to serve as a gas chamber, and a plane mirror at the tail end is replaced by a grating; when the refractive index of gas in the cavity changes, the diffraction emergence angle of the grating and the propagation optical path of light in the cavity can change, an interferogram is changed, the refractive index can be calculated through inversion of the fringe number of interference fringes or the change of interference phases, and the structure is simple, convenient and rapid.

Description

Technical Field

[0001] The present invention belongs to the technical field related to optical measurement, and in particular relates to a gas refractive index measuring device and method based on a Michelson interferometer. Background Art

[0002] The gas refractive index is an important inherent property of gas media and one of the important measurement objects in the field of optical precision measurement. The accurate measurement of the gas refractive index is of great significance in the fields of quantum vacuum metrology, atmospheric monitoring and photogrammetry. Laser interferometry is an important technical means commonly used for refractive index measurement. Its basic principle is that the change of the refractive index of the gas medium causes the change of the optical path, and the change of the optical path difference will cause the change of the interference image or signal. The refractive index of the gas medium can be obtained by inverting the change of the optical path compared to the vacuum environment. Interference methods such as Twyman-Green interferometer, Fabry-Perot cavity and Michelson interferometer are often used to measure the gas refractive index.

[0003] Michelson interferometer is the most common optical interferometer. Its basic optical structure consists of a beam splitter and two plane mirrors. It uses the amplitude division method to generate double-beam interference. By adjusting, it can generate both equal-thickness interference and equal-inclination interference fringes. The principle and structure of Michelson interferometer are simple, and the optical path difference measurement accuracy is high. It has a very wide range of applications in the field of optical metrology. However, in order to achieve optical path difference scanning, Michelson interferometer requires high-precision moving parts to move one of the mirrors, which increases the complexity of the structure and puts higher requirements on the design and control of the structure. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a gas refractive index measurement device and method based on Michelson interferometer, which has a simple structure and has the ability to measure the gas refractive index in real time and conveniently.

[0005] A gas refractive index measuring device based on Michelson interferometer, comprising a laser light source, a front collimator lens assembly, an interferometer body, an imaging lens assembly, a detector, and a computer; wherein the interferometer body comprises a beam splitter, a plane reflector, and a grating, and the passage between the beam splitter and the grating is set as a sealed cavity for containing the gas to be measured;

[0006] The outgoing light of the laser light source enters the interferometer body after being collimated by the front collimator assembly, and is divided into two interference arms by the beam splitter. One interference arm returns along the original path after being reflected by the plane reflector, and the other interference arm also returns after being diffracted by the grating. The angle between the direction of the light returning after diffraction by the grating and the incident direction of the interference arm incident on the grating is related to the refractive index of the gas to be measured.

[0007] The light rays returned by the two interference arms meet in space to form interference fringes, which are imaged by the imaging mirror assembly to the detector; the detector converts the received interference fringes into electrical signals and inputs them into the computer, which obtains the refractive index of the gas to be tested based on the number of fringes in the interference fringes.

[0008] Furthermore, the grating is tilted and fixed at a Littrow angle.

[0009] Furthermore, the Littrow angle of the grating is 10.9434°.

[0010] Furthermore, the line density of the grating is 100 lp / mm, and the wavelength of the output light of the laser light source is 633.5 nm.

[0011] A method for measuring the gas refractive index based on a gas refractive index measuring device based on a Michelson interferometer comprises the following steps:

[0012] Assuming that the direction of the wavefront of the light reflected by the plane mirror is the X-axis, the direction of the light reflected by the plane mirror is the Y-axis, and the foot point of the wavefront corresponding to the light reflected by the plane mirror is the origin, then the intersection of the wavefronts corresponding to the two light rays returned by the two interference arms is X(x,0), and x is the horizontal coordinate of the intersection X;

[0013] The mapping relationship between the light intensity I(x) of the interference fringes at the intersection point X and the refractive index n of the gas to be measured is constructed as follows:

[0014]

[0015] Where I0 is the intensity of the emitted light from the laser light source, σ is the wave number of the emitted light from the laser light source, and σ L is the Littrow wave number of the grating, θ L is the Littrow angle of the grating;

[0016] Perform Fourier transform on the light intensity I(x) to obtain the spectrum, then extract the single-sided spectrum and perform inverse Fourier transform to obtain the complex interference pattern;

[0017] The original phase of the emitted light of the laser light source is obtained according to the real and imaginary parts of the complex interference pattern. The continuous phase distribution is obtained after the original phase is expanded. The phase interval corresponding to each interference fringe period is 2π. The phase change in the entire sampling interval corresponding to the continuous phase distribution is divided by 2π to obtain the interference fringe number N. fringe ;

[0018] According to the spatial frequency f of the interference fringes x and the number of interference fringes N fringe The mapping relationship between fringe =f x·Δx inverts the spatial frequency f of the interference fringes x , where Δx is a known fixed parameter of the system determined by the detector size, imaging magnification, and grating effective area;

[0019] According to the spatial frequency f x The mapping relationship between the refractive index n The refractive index n of the gas to be measured is inverted.

[0020] Beneficial effects:

[0021] 1. The present invention provides a gas refractive index measuring device based on a Michelson interferometer. Based on the Michelson interference principle, one interference arm of the Michelson interferometer is sealed as a gas chamber, and the plane reflector at the end is replaced by a grating. When the refractive index of the gas in the chamber changes, the diffraction exit angle of the grating and the propagation path of the light in the cavity will change, changing the interference pattern. The refractive index can be calculated by inverting the change in the number of interference fringes or the interference phase. The structure is simple and convenient.

[0022] 2. The present invention provides a gas refractive index measurement method based on a Michelson interferometer. Based on the changing relationship between the refractive index and the number of fringes and the spatial frequency, the light intensity of the interference fringes is Fourier transformed to obtain the number of fringes, and then the spatial frequency of the fringes is obtained according to the number of fringes. Finally, the refractive index is calculated from the spatial frequency, thereby realizing real-time and convenient measurement of the refractive index of the gas in the cavity. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The schematic diagram of the basic device for measuring gas refractive index using a Michelson interferometer based on a single grating and single reflector structure;

[0024] Figure 2 is the diffraction effect when the Littrow wave number and the general wave number are incident on the grating surface respectively;

[0025] Figure 3 Schematic diagram of optical path difference analysis for measuring interferometer image plane;

[0026] Figure 4 The comparison diagram of interference fringes when the gas refractive index is 1.0000 and 1.0003 respectively;

[0027] Figure 5 Schematic diagram of the change of Littrow wavelength of the grating when the refractive index changes;

[0028] Figure 6 It is a schematic diagram of the change of the number of interference fringes in the sampling interval when the refractive index changes;

[0029] Figure 7It is a schematic diagram of the change of phase at different pixel positions when the refractive index changes;

[0030] Figure 8 Schematic diagram of the phase inversion process of interferometric data. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0032] like Figure 1 As shown, a gas refractive index measuring device based on Michelson interferometer includes a laser light source, a front collimator lens assembly, an interferometer body, an imaging lens assembly, a detector, and a computer; wherein the interferometer body includes a beam splitter, a plane reflector, and a grating, and the passage between the beam splitter and the grating is set as a sealed cavity for containing the gas to be measured;

[0033] When the gas to be measured is prepared, the outgoing light of the laser light source enters the interferometer body after being collimated by the front collimator assembly, and is divided into two interference arms by the beam splitter. One interference arm returns along the original path after being reflected by the plane reflector, and the other interference arm also returns after being diffracted by the grating. The angle between the direction of the light returning after being diffracted by the grating and the incident direction of the interference arm incident on the grating is related to the refractive index of the gas to be measured.

[0034] The light rays returned by the two interference arms meet in space to form interference fringes, which are imaged by the imaging mirror assembly to the detector; the detector converts the received interference fringes into electrical signals and inputs them into the computer, which obtains the refractive index of the gas to be tested based on the number of fringes in the interference fringes.

[0035] It should be noted that the interferometer body of the present invention is modified based on the Michelson interferometer structure. The plane reflector at the end of one arm of the interferometer is changed to a grating fixed at a Littrow angle, and the space between the beam splitter and the grating of this arm is closed to form a gas chamber, which can be pumped and de-gassed, and the device measures the refractive index of the gas in the chamber.

[0036] That is to say, the core of the measuring device in the present invention is the interferometer part. The interference principle of the interferometer is similar to that of the spatial heterodyne interferometer. The spatial modulation of the interference optical path difference is achieved through the diffraction of the grating to form Fizeau interference fringes. The incident light enters the interferometer through a collimator and is divided into two optical arms by a beam splitter. In one optical arm, the light is incident on the plane reflector along the optical axis and returns to the beam splitter along the original path; in the other optical arm, according to the diffraction effect of the grating, when the incident light is incident along the optical axis, only light of a specific wavelength can return along the original path. This wavelength can be called the Littrow wavelength. Light of other wavelengths has an angle with the optical axis when it returns. The size of the angle is determined by the wavelength difference between the incident wavelength and the Littrow wavelength, such as Figure 2 shown.

[0037] According to the basic working principle of the interference device, as shown in the attached Figure 3 As shown in the figure, for any point X(x,0) on the interference image plane, wavefront 1 and wavefront 2 are two outgoing wavefronts from two optical arms passing through the point, respectively. The propagation paths of the light rays of the two optical arms are ACO and ABDE, respectively. The difference in the lengths of the two propagation paths is the optical path difference at point X. When the inclination angle of the outgoing wavefront is γ, the optical path difference at point X can be expressed as:

[0038] OPD=x·sinγ (1)

[0039] Among them, OPD is the abbreviation of optical path difference, which means optical path difference;

[0040] For the interferometer, the inclination angle of the outgoing wavefront can be obtained from the diffraction relationship of the grating surface. According to the grating equation, when the Littrow wavelength and other wavelengths of light are incident on the grating, they are:

[0041]

[0042] where d grating is the grating constant of the grating, θ L is the Littrow angle of the grating, m is the diffraction order, σ L is the Littrow wave number of the grating, and σ is the wave number of the incident light. Comparing the two equations in (2), we can get:

[0043] σsinγ=2(σ-σ L )tanθ L (3)

[0044] Then the optical path difference on the interference image plane can be expressed as:

[0045]

[0046] Then the theoretical expression of the intensity distribution of Fizeau interference fringes on the image plane can be expressed as:

[0047]

[0048] In the above analysis, the refractive index of the environment during the diffraction of the grating is considered to be 1. When the influence of the refractive index n of the grating environment is considered, the diffraction angle of the light is related to the refractive index, and the distribution of the optical path difference of the interference avoidance also changes accordingly. Considering the refractive index n, the diffraction relationship in equation (4) can be written as:

[0049]

[0050] Among them, σ n,L is the Littrow wave number corresponding to the refractive index n. Then, by reanalyzing the optical path difference according to the diffraction relationship, we can obtain the theoretical expression of the interference fringes including the influence of the refractive index:

[0051]

[0052] It can be seen from the expression that in the interferometer measurement device proposed in the present invention, the influence of the refractive index is mainly to cause the drift of the grating Littrow wave number, and the changed Littrow wave number is σ L / n.

[0053] In this way, use the attached Figure 1 In the device shown, when the refractive index of the gas in the cavity changes, the Littrow wave number of the grating changes. According to equation (7), the spatial frequency of the interference fringes changes. In this way, under the condition that the wavelength of the incident light is known, the measured interference pattern can be inverted, and the refractive index of the gas in the cavity can be measured according to the number of fringes or the change of the interference phase of the interference pattern. The number of fringes and the phase of the interference pattern can be obtained by the inversion method based on Fourier transform. The advantages of this measurement method are the same as those of the spatial heterodyne interferometer. It has a simple structure and has the ability to measure multiple spectral lines simultaneously, and can be conveniently calibrated in real time.

[0054] When the refractive index of the gas in the interferometer arm cavity is changed, according to the grating equation (6), the diffraction exit angle of the light changes, and the inclination angle of the wavefront on the image plane changes the optical path difference distribution, resulting in a change in the spatial frequency of the interference fringes. The present invention shows an example, the Littrow angle of the grating used in the interferometer is 10.9434°, the diffraction order used is -6, the grating line density is 100lp / mm, and the wavelength of the incident light is 633.5nm. When the refractive index of the gas in the cavity is 1.0000 and 1.0003 respectively, the interference fringes are compared. Figure 4 As shown, the spatial frequency of the visible interference fringes changes significantly.

[0055] Furthermore, by continuously changing the refractive index of the gas in the cavity from 1.0000 to 1.0003, the change of the interferometer Littrow wavelength can be observed as follows: Figure 5 As shown, it can be observed that the number of fringes within the sampling range changes as follows Figure 6 As shown, the phase changes of the five pixel positions of 1, 256, 512, 768, and 1024 are as follows Figure 7 As shown in the figure, it can be seen that the Littrow wavelength, the number of interference fringes, the interference phase, etc. all show a linear change trend with the refractive index. The refractive index of the gas to be measured can be inversely solved by using the change in the number of fringes or the phase.

[0056] The number of fringes and phase of the interference pattern can be obtained by Fourier inversion. The basic steps are as follows: Figure 8 As shown, the following steps are included:

[0057] Assuming that the direction of the wavefront of the light reflected by the plane mirror is the X-axis, the direction of the light reflected by the plane mirror is the Y-axis, and the foot point of the wavefront corresponding to the light reflected by the plane mirror is the origin, then the intersection of the wavefronts corresponding to the two light rays returned by the two interference arms is X(x,0), and x is the horizontal coordinate of the intersection X;

[0058] The mapping relationship between the light intensity I(x) of the interference fringes at the intersection point X and the refractive index n of the gas to be measured is constructed as follows:

[0059]

[0060] Where I0 is the intensity of the emitted light from the laser light source, σ is the wave number of the emitted light from the laser light source, and σ L is the Littrow wave number of the grating, θ L is the Littrow angle of the grating;

[0061] Perform Fourier transform on the light intensity I(x) to obtain the spectrum, then extract the single-sided spectrum and perform inverse Fourier transform to obtain the complex interference pattern;

[0062] The original phase of the emitted light of the laser light source is obtained according to the real and imaginary parts of the complex interference pattern. The continuous phase distribution is obtained after the original phase is expanded. The phase interval corresponding to each interference fringe period is 2π. The phase change in the entire sampling interval corresponding to the continuous phase distribution is divided by 2π to obtain the interference fringe number N. fringe ;

[0063] According to the spatial frequency f of the interference fringes x and the number of interference fringes N fringe The mapping relationship between fringe =f x ·Δx inverts the spatial frequency f of the interference fringes x, where Δx is a known fixed parameter of the system determined by the detector size, imaging magnification, and grating effective area;

[0064] According to the spatial frequency f x The mapping relationship between the refractive index n The refractive index n of the gas to be measured is inverted.

[0065] That is to say, the number of interference fringes N is calculated according to the Fourier inversion method. fringe After that, the spatial frequency f of the fringes can be calculated x , f x In the expression of the light source wave number σ, the interferometer Littrow wave number σ L As well as the grating Littrow angle, they are all known system parameters, so after obtaining the spatial frequency, the refractive index n can be directly calculated from them.

[0066] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may certainly make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. A gas refractive index measuring device based on Michelson interferometer, characterized in that: It includes a laser light source, a front collimator lens assembly, an interferometer body, an imaging lens assembly, a detector, and a computer; wherein the interferometer body includes a beam splitter, a plane reflector, and a grating, and the passage between the beam splitter and the grating is set as a sealed cavity for containing the gas to be measured; The outgoing light of the laser light source enters the interferometer body after being collimated by the front collimator assembly, and is divided into two interference arms by the beam splitter. One interference arm returns along the original path after being reflected by the plane reflector, and the other interference arm also returns after being diffracted by the grating. The angle between the direction of the light returning after diffraction by the grating and the incident direction of the interference arm incident on the grating is related to the refractive index of the gas to be measured. The light rays returned by the two interference arms meet in space to form interference fringes, which are imaged by the imaging mirror assembly to the detector; the detector converts the received interference fringes into electrical signals and inputs them into the computer, which obtains the refractive index of the gas to be tested based on the number of fringes in the interference fringes.

2. A gas refractive index measuring device based on Michelson interferometer as claimed in claim 1, characterized in that: The grating is tilted and fixed at the Littrow angle.

3. A gas refractive index measuring device based on Michelson interferometer as claimed in claim 2, characterized in that: The Littrow angle of the grating is 10.9434°.

4. A gas refractive index measuring device based on Michelson interferometer as claimed in claim 1, characterized in that: The line density of the grating is 100 lp / mm, and the wavelength of the output light of the laser light source is 633.5 nm.

5. A method for measuring gas refractive index based on the gas refractive index measuring device based on Michelson interferometer according to claim 1, characterized in that: The following steps are involved: Assuming that the direction of the wavefront of the light reflected by the plane mirror is the X-axis, the direction of the light reflected by the plane mirror is the Y-axis, and the foot point of the wavefront corresponding to the light reflected by the plane mirror is the origin, then the intersection of the wavefronts corresponding to the two light rays returned by the two interference arms is X(x,0), and x is the horizontal coordinate of the intersection X; The mapping relationship between the light intensity I(x) of the interference fringes at the intersection point X and the refractive index n of the gas to be measured is constructed as follows: Where I0 is the intensity of the emitted light from the laser light source, σ is the wave number of the emitted light from the laser light source, and σ L is the Littrow wave number of the grating, θ L is the Littrow angle of the grating; Perform Fourier transform on the light intensity I(x) to obtain the spectrum, then extract the single-sided spectrum and perform inverse Fourier transform to obtain the complex interference pattern; The original phase of the emitted light of the laser light source is obtained according to the real and imaginary parts of the complex interference pattern. The continuous phase distribution is obtained after the original phase is expanded. The phase interval corresponding to each interference fringe period is 2π. The phase change in the entire sampling interval corresponding to the continuous phase distribution is divided by 2π to obtain the interference fringe number N. fringe ; According to the spatial frequency f of the interference fringes x and the number of interference fringes N fringe The mapping relationship between fringe =f x ·Δx inverts the spatial frequency f of the interference fringes x , where Δx is a known fixed parameter of the system determined by the detector size, imaging magnification, and grating effective area; According to the spatial frequency f x The mapping relationship between the refractive index n The refractive index n of the gas to be measured is inverted.

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