CN107462165A - High optical fine dual-frequency grating interferometer based on bigrating structures - Google Patents

Abstract

The invention discloses a high optical subdivision dual-frequency grating interferometer based on a double-grating structure. The grating interferometer includes a dual-frequency orthogonal laser, a non-polarizing beam splitter, a polarizing beam splitter, a quarter-wave plate, Reflectors, scale gratings, index gratings, polarizers, photodetectors, data acquisition and processing units and other components. According to the Doppler effect, when the laser beam is non-perpendicularly incident on the moving measuring grating, the diffracted light will carry the Doppler frequency shift item. Doubling the frequency-shifted component, so the grating interferometer has a high optical subdivision structure. In order to improve the signal strength of the measurement beam and reduce the influence of environmental noise, both the scale grating and the indicator grating are ‑1 order high diffraction efficiency reflective gratings. The invention has great value in measurement fields such as improving the resolution and stability of the grating interferometer.

Description

High Optical Subdivision Dual Frequency Grating Interferometer Based on Dual Grating Structure

technical field

The invention belongs to the field of high-precision displacement measurement, and is characterized by a high optical subdivision double-frequency grating interferometer based on a double-grating structure.

Background technique

With the development of high-precision manufacturing in the direction of miniaturization and portability, the demand for high-precision positioning and measurement technology at the micro-nano level is constantly increasing, especially in processing and measurement systems. At present, there are mainly two types of systems used to measure displacement with micronano precision, one is laser interferometer, and the other is grating interferometer. Laser interferometers are widely used in various measurement systems. The laser interferometer has the advantages of high resolution, high precision, and large range, but its production cost is high and the device is complicated, and because the laser interferometer uses the laser wavelength as the measurement reference, it is easily affected by environmental factors, resulting in large random measurement errors , so the use of laser interferometers is limited. The grating interferometer better makes up for this defect. Since the measurement basis of the grating interferometer is the grating period, the grating interferometer has strong resistance to environmental interference factors, so the grating interferometer is widely used.

Whether it is a laser interferometer or a grating interferometer, increasing the resolution can improve the electronic subdivision and optical subdivision. Electronic subdivision is the identification of signals based on optical subdivision. Therefore, it is very important to improve the optical subdivision by improving the optical path to improve the resolution. The grating interferometers such as Heidenhain in Germany and Renishaw in Britain, which are widely used in the market now, generally use the first-order diffraction double optical subdivision, or the second-order diffraction four-fold optical subdivision. Such as the patent US5574558 of Heidenhain, the patent US5442172 of IBM Corporation of the United States. There are patented inventions in my country that use high-magnification optical subdivision, such as CN104729411A, CN104729402B, and CN104613865A. Compared with the above inventions, the present invention adopts a more compact self-collimation structure of double gratings with different periods, simple manufacturing process, low cost and small installation tolerance. The present invention uses a double grating structure to improve optical subdivision and reduce installation errors, and finally improve the resolution and accuracy of the grating interferometer. If the grating is well-made, the negative first-order diffraction efficiency is extremely high, reaching 90% and above. Theoretically, the optical subdivision can be increased to thirty-two subdivisions and above.

Contents of the invention

The invention is a dual-grating-based high optical subdivision dual-frequency grating interferometer, which aims to solve the problem of low optical subdivision multiples of the grating interferometer widely used in the market. The invention adopts a double grating structure, one is a scale grating and the other is an indicating grating, both of which are minus-order high diffraction efficiency gratings. The measurement beam is incident on the scale grating at an approximate Littrow angle, and the negative first-order beam is emitted to the index grating. A suitable period of the index grating is selected so that the negative first-stage beam enters the scale grating at an approximate Littrow angle again, and high-efficiency negative first order diffraction. The measuring beam is diffracted back and forth in the double grating structure, and finally enters the indicating grating at the Littrow angle, and the beam returns along the original path. The measuring beam is diffracted on the moving scale grating multiple times, so the optical subdivision of the grating interferometer is improved.

Technical solution of the present invention:

A highly optically subdivided dual-frequency grating interferometer based on a dual-grating structure, which is characterized in that it includes a dual-frequency orthogonal linearly polarized laser source, a non-polarizing beam splitter, a polarizing beam splitter, and a first quarter-wave plate , the second quarter-wave plate, the first reflector, the second reflector, the scale grating, the index grating, and the first analyzer with a 45° angle to the dual-frequency orthogonally polarized light and the corresponding first polarizer A photodetector, a second analyzer that is 45° to the dual-frequency orthogonally polarized light, and a demodulation phase unit that is jointly formed by the corresponding second photodetector and the data acquisition and processing control unit;

The dual-frequency orthogonally polarized light emitted by the laser light source is divided into reflected light and transmitted light through a non-polarizing beam splitter. The reflected light enters the first analyzer at 45° to the dual-frequency orthogonally polarized light as a reference quantity. device, the first photodetector detects the interference signal as the reference light of the dual-frequency heterodyne interferometry, and transmits it to the data acquisition and processing control unit;

The transmitted light as the measurement signal is divided into two beams by a polarization beam splitter, one beam is P light whose polarization state is parallel to the incident plane, and the other beam is S light whose polarization state is perpendicular to the incident plane, and the S light passes through the first quarter A wave plate becomes left-handed circularly polarized light, which enters the scale grating after being reflected by the first mirror, and the -1 order diffracted light passing through the scale grating enters the indicating grating, and the -1 order diffracted light passing through the indicating grating enters the scale grating again, and turns left-handed circularly The polarized light diffracts back and forth between the scale grating and the index grating, and finally enters the index grating at the Littrow angle, and returns according to the original path, passes through the first reflector and enters the first quarter-wave plate again to become P light; The P light emitted by the polarizing beam splitter becomes right-handed circularly polarized light through the second quarter-wave plate, and enters the scale grating after being mirror-reflected by the second mirror, and the -1 order diffracted light through the scale grating enters the indicating grating, and passes through the The -1st-order diffracted light of the index grating enters the scale grating again, and the right-handed circularly polarized light diffracts back and forth between the scale grating and the index grating in turn, and finally enters the index grating at the Littrow angle, and the right-handed circularly polarized light returns through the second The reflector enters the second quarter-wave plate again to become S light; the P light returned by the original path passes through the polarizing beam splitter and the S light returned by the original path is reflected by the polarizing beam splitter and overlapped at On the same optical path, they are jointly incident on the second analyzer to form an interference signal, which is received by the second detector and transmitted to the data acquisition, processing and control unit as a measurement signal of dual-frequency heterodyne interferometry.

Compared with prior art, technical effect of the present invention:

A compact self-collimating structure of double gratings with different periods is adopted, so that the measurement beam continuously undergoes negative first-order diffraction back and forth at the approximate Littrow angle in the double grating structure. Due to the Doppler effect, each time the measurement beam undergoes negative order diffraction on the measurement grating, it will carry a Doppler frequency shift component. Finally, the measurement beam enters the grating at the Littrow angle and returns along the original path. At this time, the frequency shift component is doubled, and the two measurement beams interfere with the polarization beam splitter.

In order to enhance the environmental resistance of the grating interference system, the present invention adopts a symmetrical structure. If a single measuring beam performs N=3 times of negative first-order diffraction on the measuring grating, the structure performs 12 optical subdivisions.

Description of drawings

Fig. 1 is the structural representation of the high optical subdivision dual-frequency grating interferometer based on the double grating structure of the present invention

Figure 2 is a schematic diagram of double optical subdivision double grating on one side

Figure 3 is a schematic diagram of a four-fold optical subdivision double grating on one side

Figure 4 is a schematic diagram of a single-side six-fold optical subdivision double grating

detailed description

The present invention will be further described below in conjunction with the embodiments and accompanying drawings, but the protection scope of the present invention should not be limited thereby.

Fig. 1 is a structural schematic diagram of a high optical subdivision dual-frequency grating interferometer based on a dual-grating structure in the present invention. As shown in the figure, a dual-frequency orthogonal linearly polarized laser light source 9 emits an orthogonal dual-frequency polarized beam through a non-polarizing beam splitter 8 is divided into two beams, one beam is injected into the first analyzer 10 to form an interference signal, and after being received by the first detector 12, it is transmitted to the data acquisition, processing and control unit 14 as a reference signal of dual-frequency heterodyne interferometry, and the other One beam is split into transmitted P light and reflected S light by a polarizing beam splitter 7 . The S light coming out of the polarizing beam splitter 7 is transformed into left-handed circularly polarized light through the first quarter-wave plate 5, and enters the high-density scale grating 1 through the first reflector 3, and the diffracted by the scale grating 1- The 1st-order diffracted light hits the index grating 2, undergoes -1-order diffraction through the index grating 2, and then enters the high-density scale grating 1, and then the left-handed circularly polarized light diffracts back and forth between the scale grating 1 and the index grating 2, and finally forms a The Littrow angle is incident on the index grating 2, and the left-handed circularly polarized light returns along the original optical path through the first mirror 3, and becomes P light through the first quarter-wave plate 5. The P light coming out of the polarizing beam splitter 7 is transformed into right-handed circularly polarized light through the second quarter-wave plate 6, enters the high-density scale grating 1 through the second reflector 4, and is diffracted by the scale grating 1 The -1 order diffracted light hits the index grating 2, and the -1 diffracted light of the index grating 2 enters the high-density scale grating 1 again, and accordingly the right-handed circularly polarized light diffracts back and forth between the scale grating 1 and the index grating 2, and Finally, the index grating 2 is incident at the Littrow angle, and the right-handed circularly polarized light returns along the original optical path through the second mirror 4, and becomes S light through the second quarter-wave plate. The S light returning from the original path passes through the polarizing beam splitter and the P light returning from the original path is reflected by the polarizing beam splitter 7 and overlapped on the same optical path, and they are incident on the second analyzer 11 to form an interference signal, which is detected by the second detector 13 After receiving, it is transmitted to the data acquisition and processing and control unit 14 as a measurement signal of dual-frequency heterodyne interferometry. The displacement of the scale grating 1 can be obtained by analyzing the reference signal and the measurement signal.

Since the two beams of measuring light are symmetrically incident on the double-grating structure, the single-side structure is analyzed.

As shown in Figure 2, after the light beam emerges from the polarizing beam splitter 7, it passes through the quarter-wave plate 6, enters the second mirror (4), and is reflected into the double grating structure. The angle between the second mirror 4 and the scale grating 1 is:

η is the angle between the second reflector 4 and the scale grating 1, and in _is the incident angle of the light incident on the scale grating for the first time. The light enters the scale grating and the index grating from the second reflector 4, and the incident angles entering the scale grating are i _n , i _n-1 , i _n-2 ... i ₃ , i ₂ , i ₁ , and the diffraction angles on the scale grating θ _n , θ _n-1 , θ _n-2 ... θ ₃ , θ ₂ , θ ₁ , the light is diffracted by the scale grating and then enters the index grating, and the incident angles are i' _n , i' _n-1 , i ' _n-2 ...i' ₃ , i' ₂ , i ₁ '. Indicates that the diffraction angles of the grating are α _n , α _n-1 , α _n-2 ... α ₃ , α ₂ , α ₁ .

On the measurement grating, the incident angle i _n and the diffraction angle θ _n satisfy the grating equation:

d ₁ *(sin i _n +sinθ _n )＝mλ (1)

In the formula, d ₁ is the period of the scale grating, λ is the laser wavelength, and m is the diffraction order. In this patent, m=-1

On the scale grating, the incident angle i' _n and the diffraction angle α _n also satisfy the grating equation:

d ₂ *sin(i' _n +α _n )＝mλ (2)

In the formula, d ₂ is the measurement grating period, λ is the laser wavelength, m is the diffraction order, and m=-1 in this patent

And satisfy the following angle relationship:

θ _n =i' _n (3)

α _n =i _n-1 (4)

i' ₁ =α ₁ (5)

When determining d ₁ , d ₂ and the diffraction number n of the measuring beam, the optical path on the scale grating and indicating grating and the installation angle of the second reflector 4 can be determined through the above five equations.

In the example, as shown in Figure 2, n=1, d ₁ =1000nm, d ₂ =1500nm, λ=632.8nm, the incident angle on the scale grating is i ₁ =24.9525°, and the diffraction angle is θ ₁ =12.1771° , indicating that the incident angle on the grating is i′ ₁ =12.1771°, and the diffraction angle is α ₁ =12.1771°. The included angle between the reflector and the scale grating is 79.9788°, assuming that the negative first-order diffraction efficiency of the grating is 95%, the return light intensity of the original path is 85.7375% of the incident light intensity, that is, the detection light power of the incident scale grating structure is 20mw, and the original path The optical power returned to the polarization beam splitter is 19mw, which can reach the threshold power of the photodetector.

As shown in Figure 3, n=2, d ₁ =1000nm, d ₂ =1300nm, λ=632.8nm, the incident angle on the scale grating is i ₁ =22.9181°, i ₂ =32.3741°, and the diffraction angle is θ ₁ =14.0864°, θ ₂ =5.5868°, indicating that the incident angles on the grating are i′ ₁ =14.0864°, i′ ₂ =5.5868°, and the diffraction angles are α ₁ =14.0864°, α ₂ =22.9181°. The included angle between the reflector and the scale grating is 83.6871°, assuming that the negative first-order diffraction efficiency of the grating is 95%, the return light intensity of the original path is 69.8337% of the incident light intensity, that is, the detection light power of the incident scale grating structure is 20mw, and the original path The light power returned to the polarization beam splitter is 13.96674mw, which can reach the threshold power of the photodetector.

As shown in Figure 4, n=3, d ₁ =1000nm, d ₂ =1100nm, λ=632.8nm, and the incident angles on the scale grating are i ₁ =20.1918°, i ₂ =23.7645°, i ₃ =27.4012° , the diffraction angles are θ ₁ =16.7165°, θ ₂ =13.3035°, θ ₃ =9.9380° in turn. The incident angles on the indicating grating are i′ ₁ =16.7165°, i’ ₂ =13.3035°, i’ ₃ =9.9380°, and the diffraction angles are, α ₁ =16.7165°, α ₂ =20.1918°, α ₃ =23.7465 °. The included angle between the mirror and the scale grating is 81.2006°. Assuming that the grating’s negative first-order diffraction efficiency is 95%, the original return light intensity is 56.88% of the incident light intensity, that is, the detection light power of the incident scale grating structure is 20mw, and the original return light power reaching the polarization beam splitter is 11.376mw Achievable photodetector threshold power. The three diffraction situations are shown in Table 1.

Table 1

Claims (1)

1. a kind of high optical subdivision double frequency grating interferometer based on double grating structure, it is characterized in that, comprise double frequency orthogonal linearly polarized laser source (9), non-polarization beam splitter (8), polarization beam splitter ( 7), the first quarter-wave plate (5), the second quarter-wave plate (6), the first mirror (3), the second mirror (4), the scale grating (1), the indicator The grating (2), as well as the first analyzer (10) and the corresponding first photodetector (12) at 45° to the dual-frequency orthogonally polarized light, are formed at 45° to the dual-frequency orthogonally polarized light The second polarizer (11) and the demodulation phase unit (15) jointly constituted by the corresponding second photodetector (13) and data acquisition and processing control unit (14); The dual-frequency orthogonally polarized light emitted by the laser light source (9) passes through the non-polarizing beam splitter (8) and is divided into two parts: reflected light and transmitted light. The first polarizer (10) of °, the first photodetector (12) detects the interference signal as the reference light of double-frequency heterodyne interferometry, and transmits to the data acquisition and processing control unit (14); The transmitted light is divided into two beams through the polarization beam splitter (7) as the measurement signal, one beam is P light whose polarization state is parallel to the incident plane, and the other beam is S light whose polarization state is perpendicular to the incident plane, and the S light passes through the first The quarter-wave plate (5) becomes left-handed circularly polarized light, which enters the scale grating (1) after being reflected by the first reflector (3), and the -1 order diffracted light from the scale grating (1) enters the indicator grating (2) , the -1st-order diffracted light of the index grating (2) enters the scale grating (1) again, and the left-handed circularly polarized light diffracts back and forth between the scale grating (1) and the index grating (2) in turn, and finally enters the index at the Littrow angle grating (2), and return according to the original path through the first reflector (3) and enter the first quarter-wave plate (5) again to become P light; the P light emitted by the polarizing beam splitter (7) After passing through the second quarter-wave plate (6), it becomes right-handed circularly polarized light, and enters the scale grating (1) after being reflected by the second mirror (4), and the -1 order diffracted light of the scale grating (1) enters The index grating (2), the -1 order diffracted light of the index grating (2) enters the scale grating (1) again, and the right-handed circularly polarized light is diffracted back and forth between the scale grating (1) and the index grating (2) in turn, and finally the Littrow angle incident index grating (2), right-handed circularly polarized light returns according to the original path through the second reflector (4) and enters the second quarter-wave plate (6) again to become S light; the original path The returned P light passes through the polarizing beam splitter (7) and the S light returned from the original path is reflected on the same optical path through the polarizing beam splitter (7), and is jointly incident on the second analyzer ( 11) Forming an interference signal, which is received by the second detector (13) and transmitted to the data acquisition, processing and control unit (14) as a measurement signal of dual-frequency heterodyne interferometry.