CN104897270A - Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarizing beamsplitting - Google Patents

Abstract

The Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting belongs to the field of laser interferometry; the polarization beam splitter PBS is used to split the light to form the reference arm and the measurement arm, and the incident beam on the reference arm is diffracted by the acousto-optic modulator. The frequency-shifted first-order diffracted light is adjusted by the beam deflection element, so that the beam propagation direction is parallel to the incident beam; by adjusting the reference corner cube, the reflected light is located above the AOM, so that the reference light only passes through the AOM once. , adjust the measurement corner cube to make the reference light and the measurement light beams overlap when they pass through the polarization beam splitter PBS again, and interference occurs; the invention uses fewer optical elements to achieve heterodyne laser interferometry, and the adjustment of the optical path is simple, which can effectively solve the current problem. There are problems such as complex optical path adjustment in some technical solutions, and it has significant technical advantages in the field of ultra-precision vibration measurement.

Description

Michelson Heterodyne Laser Vibrometer Based on Single Acousto-optic Modulation and Polarization Spectroscopy

technical field

The invention belongs to the field of laser interferometry, and mainly relates to a Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting.

Background technique

As an ultra-precision measuring instrument that can trace the vibration value to the laser wavelength, the laser vibrometer is widely used in the fields of dynamic displacement measurement, vibration measurement and monitoring, ultra-precision equipment and system integration, scientific research and experiments. Laser vibrometers based on laser interferometry can be divided into two categories, homodyne and heterodyne, according to the principle. The two have significant differences in working principle, optical path structure and technical characteristics. The homodyne measurement method uses a single-frequency laser as the light source, based on the classic or improved Michelson laser interference principle, and directly measures the displacement by measuring the phase change of the interference fringe; while the heterodyne measurement method generally uses a dual-frequency laser as the light source, based on the Doppler The Le effect, by measuring the Doppler frequency difference to measure the motion speed of the tested part and indirectly measure the displacement. The signal processing of the homodyne laser vibrometer is essentially to measure the light intensity. The system itself is a DC system, which has the advantages of simple structure, high measurement accuracy, wide dynamic range, and easy compensation for nonlinearity. The disadvantage is poor anti-interference ability and measurement The results are greatly affected by changes in light intensity. The signal processing of the heterodyne laser vibrometer (commonly known as Doppler laser vibrometer) is essentially frequency measurement. The system itself is a dynamic AC system, which has strong anti-interference ability, high measurement accuracy, and the measurement results are affected by light intensity changes. Therefore, the heterodyne measurement method has always been concerned by researchers and has become a research hotspot in this field.

Heterodyne interferometry requires a certain frequency difference between the two interference arms of the vibrometer. The method of producing frequency difference mainly utilizes Zeeman effect and acousto-optic modulation. The Zeeman effect is affected by the frequency difference blocking phenomenon, and the resulting dual-frequency frequency difference is generally small, and usually the maximum frequency difference does not exceed 4MHz. The frequency difference obtained by the acousto-optic modulation method is usually large, and the frequency difference can reach more than 20MHz, and the frequency stability is very good, which can meet the needs of high-speed and high-precision measurement.

Existing heterodyne laser vibrometers based on acousto-optic modulation to generate frequency difference (1. Martin B, et.al. "High-precision laservibrometers based on digital Doppler-signal processing", Proceedings of SPIE, Vol.4827: 50- 61, 2002; 2. Stefan Franz.et.al. "Heterodyne interferometer having an optical modulator". US Patent: US7688451B2;) is based on the Mach-Zehnder interferometer, using a frequency-stabilized laser as a light source, through a polarization beam splitter PBS Light splitting forms two interference arms, as the measurement arm and the reference arm, using an acousto-optic modulator on at least one interference arm to achieve frequency shift, so that a frequency difference is formed between the two interference arms; And the depolarizing beam splitter NBS combines the light beams. The frequency difference of the dual-frequency laser produced by the single acousto-optic frequency shifting technology scheme is large, which requires high speed of the system hardware. In order to solve this problem, the dual-acousto-optic modulation technology scheme can be used to generate two beams with relatively small frequency difference, respectively as Although the dual acousto-optic modulation scheme for reference light and measurement light can reduce the frequency difference, it increases the complexity of the optical system. In summary, the disadvantages of the prior art solutions are: 1) the prior art solutions are all based on the Mach-Zehnder interferometer, the interference arm and the measurement arm are rectangular as a whole, and the light is split at an apex of the rectangle by a polarization beam splitter, Then through the additional polarization beam splitter PBS, depolarization beam splitter NBS and reflectors to combine light at the opposite angle of the splitting point, this technical solution has multiple polarization beam splitters PBS, there is polarization leakage phenomenon, resulting in significant nonlinear error of the optical path; 2) According to the working principle of the acousto-optic modulator, only the first-order diffracted light will shift in frequency, but there is a certain angle with the incident beam, which makes it difficult to combine light in the optical path, and it is difficult for the reference light and the measurement light to interfere.

Due to factors such as the first-order diffracted light of the acousto-optic modulator is not parallel to the incident light, and the optical components are not ideal, especially the polarization leakage of the polarization beam splitter PBS, it is difficult to adjust the technical solution of the existing heterodyne laser vibrometer, and the optical path Due to the limitations of the structure, principle and the characteristics of the optical device itself, there are nonlinear errors that are difficult to overcome, and it is difficult to meet the requirements of sub-nanometer or even picometer-level precision vibration measurement. Therefore, it is of great significance to provide a heterodyne laser vibration measurement technology solution with simple optical path adjustment or small nonlinear error through the innovation of optical path structure and principle.

Contents of the invention

The purpose of the present invention is to provide a Michelson sensor based on single acousto-optic modulation and polarization splitting for the existing Mach-Zehnder heterodyne laser vibrometer technology solutions in terms of optical path structure and principle. The heterodyne laser vibrometer, through the innovation of the optical path structure and principle, is based on the Michelson interferometer, and uses fewer optical components to achieve heterodyne laser interferometry. The adjustment is simple and convenient, which can effectively solve the existing technical solutions. Polarization leakage and polarization aliasing are significant, and optical path adjustment is difficult.

Technical solution of the present invention is:

A Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting, consisting of a laser, a half-wave plate, a polarizing beam splitter, a quarter-wave plate, a measuring cube, and an acousto-optic modulator , beam deflection element, reference corner cube, analyzer, and high-speed photodetector. The laser emits linearly polarized light, the polarization direction is adjusted by a half-wave plate, and then split by a polarizing beam splitter. The reflected light The first beam is formed as the measurement light, and the transmitted light forms the second beam as the reference light; the first beam becomes circularly polarized light after passing through the quarter-wave plate, and then passes through the quarter-wave plate again after being reflected by the measuring mirror to become The linearly polarized light is then transmitted through the polarizing beam splitter to form the third beam; the second beam passes through the acousto-optic modulator to generate a diffracted beam, and after the diffracted beam is adjusted by the beam deflection element, the beam propagation direction is parallel to the second beam, and passes through the reference angle cone After being reflected by the prism, it is reflected by the polarizing beam splitter to form the fourth beam; the optical paths of the third beam and the fourth beam overlap, and the polarization directions are orthogonal; the third beam and the fourth beam interfere after passing through the analyzer, and are detected by high-speed photoelectric detection Receiver.

The beam deflection element is a plane reflector or a wedge block.

The first-order diffracted light of the acousto-optic modulator is orthogonal to the polarization of the incident light.

The laser is a frequency-stabilized laser.

The technical innovation of the present invention and the good effect that produce are:

(1) The present invention proposes a Michelson heterodyne laser vibration measurement technology scheme based on acousto-optic modulation. The technical scheme is based on the Michelson interferometer, the optical path is simple, the components are less, and the link of nonlinear error is reduced; the frequency shift of the first-order diffracted light of the acousto-optic modulator is used to make the difference between the two interference arms The frequency difference is formed; the corner cube prism is used as the measuring mirror and the reference mirror, so that the return light of the interference arm is located above the acousto-optic modulator, and only passes through the acousto-optic modulator once, thereby realizing heterodyne laser interferometry. Through the above technological innovations, the problems of many optical elements in the optical path and many sources of nonlinear errors in the prior art solutions are effectively solved.

(2) The present invention only needs to adjust the beam deflection element on the interference arm, so that the propagating direction of the first-order diffracted beam is parallel to the incident beam or the zero-order beam, and the position of the measuring corner cube and the reference corner cube can be adjusted to make The reference light and measurement light interfere, and the light path adjustment is very convenient. The problem of inconvenient adjustment of the optical path in the prior art solution is solved.

Description of drawings

Fig. 1 is the three-dimensional diagram of an embodiment of the Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting of the present invention;

Fig. 2 is the schematic diagram of the optical path of an embodiment of the Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting in the present invention;

Fig. 3 is a three-dimensional diagram of another embodiment of the Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting of the present invention;

Fig. 4 is the optical path schematic diagram of another embodiment of the Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting in the present invention;

Fig. 5 is the operating principle of the acousto-optic modulator of the present invention;

Fig. 6 is a signal processing schematic diagram of the present invention;

In the figure, 1 laser, 2 polarizing beam splitter, 3 quarter-wave plate, 4 measuring corner cube, 5 acousto-optic modulator, 6 beam deflection element, 7 reference corner cube, 8 high-speed photodetector, 9 First beam, 10 second beam, 11 diffracted beam, 12 third beam, 13 fourth beam, 14 half-wave plate, 15 analyzer.

Detailed ways

The specific implementation manner of the present invention will be described in detail below with reference to the accompanying drawings, and examples will be given.

A Michelson heterodyne laser vibrometer based on single acousto-optic modulation and polarization splitting, consisting of a laser 1, a half-wave plate 14, a polarization beam splitter 2, a quarter-wave plate 3, and a measuring corner cube prism 4 , an acousto-optic modulator 5, a beam deflection element 6, a reference corner cube 7, a polarizer 15, and a high-speed photodetector 8. The laser 1 emits linearly polarized light, and the polarization is adjusted by a half-wave plate 14 direction, and then split the light through the polarizing beam splitter 2, the reflected light forms the first beam 9 as the measuring light, and the transmitted light forms the second beam 10 as the reference light; the first beam 9 becomes circularly polarized after passing through the quarter-wave plate 3 The light, after being reflected by the measuring mirror 4, passes through the quarter-wave plate 3 again to become linearly polarized light, and then transmits through the polarizing beam splitter 2 to form a third beam 12; the second beam 10 passes through the acousto-optic modulator 5 to generate a diffracted beam 11 , after the diffracted beam 11 is adjusted by the beam deflection element 6, the beam propagation direction is parallel to the second beam 10, after being reflected by the reference corner cube 7, and then reflected by the polarizing beam splitter 2 to form the fourth beam 13; the third beam 12, The optical paths of the fourth light beam 13 overlap and the polarization directions are orthogonal; the third light beam 12 and the fourth light beam 13 interfere after passing through the analyzer 15 and are received by the high-speed photodetector 8 .

The beam deflection element 6 is a plane reflector or a wedge.

The first-order diffracted light of the acousto-optic modulator 5 is perpendicular to the polarization of the incident light.

The laser 1 is a frequency-stabilized laser.

An embodiment of the present invention is given in conjunction with Fig. 1 and Fig. 2 . In this embodiment, the beam deflecting element 6 is a plane mirror. Laser 1 uses a frequency-stabilized He-Ne laser with a wavelength of 632.8nm, noise <0.05% rms, output power of 1mW, polarization of 1000:1, and frequency of ω ₀ . In the space coordinate system xyz, laser 1 emits Linearly polarized light, the polarization direction is the x-axis, that is, P light. The half-wave plate 14 is placed in the xy plane, and when the fast axis of the half-wave plate 14 forms an angle of 22.5° with the x-axis, the P light becomes 45° linearly polarized light, which is split by the polarization beam splitter 2, The reflected light of the S component forms a first light beam 9 as measurement light, and the transmitted light of the P component forms a second light beam 10 as reference light. On the measuring arm, the quarter-wave plate 3 is placed in the yz plane, and when the fast axis of the quarter-wave plate 3 forms an angle of 45° with the z-axis, the first beam 9 of the S component first passes through a quarter The wave plate becomes circularly polarized light, which is reflected by the measuring corner cube prism 4 and then turned into P light by the quarter wave plate 3 , and then transmitted by the polarizing beam splitter 2 to form the third light beam 12 . The movement of the measurement corner cube produces a Doppler frequency shift ±ω _d , so the frequency of the returned light from the measurement arm is ω ₀ ±ω _d . On the reference arm, the second light beam 10 of the P component passes through the acousto-optic modulator 5, the driving frequency of the acousto-optic modulator 5 is ω _c , the outgoing diffracted beam 11 is S light, the frequency is ω ₀ +ω _c , and the propagation direction There is a certain angle with the incident light. After the diffracted light beam 11 is adjusted by the plane reflector, the propagation direction is parallel to the second light beam 10, reflected by the reference corner cube 7, the return light of the reference arm is located above the acousto-optic modulator 5, and then reflected by the polarizing beam splitter 2 to form a fourth Beam 13. The third beam 12 of the measuring light and the fourth beam 13 of the reference light overlap, and the polarization directions are orthogonal, and interference occurs after passing through the analyzer 15. When the polarization direction of the analyzer 15 forms an included angle of 45° with the z-axis, the intensity of the interference signal is the largest . The interference signal with frequency ω _c ±ω _d is received by the high-speed photodetector 8 . In this embodiment, the high-speed photodetector 8 is a Si PIN photodetector with a photosensitive area diameter of 0.8 mm and a bandwidth of 200 MHz.

3 and 4 show another embodiment of the present invention. In this embodiment, the beam deflection element 6 is a wedge-shaped block, which changes the propagation direction of the beam according to the law of refraction.

Fig. 5 is a schematic diagram of the working principle of the AOM of the present invention. In this embodiment, the acousto-optic crystal of the acousto-optic modulator 5 is a Bragg cell. Due to the inherent characteristics of Bragg acousto-optic diffraction, the frequency of the ultrasonic wave cannot be too small, generally not less than 20MHz, and the frequency difference of the dual-frequency laser that can be obtained is equal to or Greater than 20MHz. When the incident light is obliquely incident on the acousto-optic modulator 5, the outgoing light only has the diffracted zero-order light and first-order light, the frequency and direction of the diffracted zero-order light are the same as the incident light, and the frequency of the diffracted first-order light is shifted, and the propagation direction is the same as that of the incident light. Zero order light or incident light has a deflection angle. The polarization state of the diffracted zero-order light is consistent with that of the incident light, and the polarization of the first-order light is orthogonal to the incident light.

Fig. 6 is a schematic diagram of the signal processing principle of the present invention. The measurement signal cos(ω _c ±ω _d )t received by the high-speed photodetector 8 is respectively multiplied by the reference signal cos ω _c t of the acousto-optic modulator 5 and the sinω _c t of the reference signal phase-shifted by 90°, and respectively passed through the low After filtering and A/D conversion, the quadrature signal is obtained, and then the phase of the moving object is obtained by arctangent calculation

Claims (4)

1. the Michelson heterodyne laser vialog based on monophone optical modulation and polarization spectro, by laser instrument (1), / 2nd wave plates (14), polarization spectroscope (2), quarter-wave plate (3), measured angular cone prism (4), acousto-optic modulator (5), light beam is turned back element (6), reference angle cone prism (7), analyzer (15), high-speed photodetector (8) forms, it is characterized in that: described laser instrument (1) sends linearly polarized light, through 1/2nd wave plates (14) adjustment polarization direction, then light splitting is carried out through polarization spectroscope (2), reflected light forms the first light beam (9) as measuring light, transmitted light forms the second light beam (10) as reference light, first light beam (9) becomes circularly polarized light after quarter-wave plate (3), after measuring mirror (4) reflection, again become linearly polarized light through quarter-wave plate (3), then form the 3rd light beam (12) through polarization spectroscope (2) transmission, second light beam (10) produces diffracted beam (11) through acousto-optic modulator (5), diffracted beam (11) is turned back after element (6) adjustment through light beam, direction of beam propagation is parallel to the second light beam (10), after reference angle cone prism (7) reflection, then reflect to form the 4th light beam (13) through polarization spectroscope (2), 3rd light beam (12), the 4th light beam (13) light path overlap and polarization direction is orthogonal, 3rd light beam (12), the 4th light beam (13) interfere after analyzer (15), are received by high-speed photodetector (8).

2. the Michelson heterodyne laser vialog based on monophone optical modulation and polarization spectro according to claim 1, is characterized in that: described light beam turns back element (6) for plane mirror or wedge.

3. the Michelson heterodyne laser vialog based on monophone optical modulation and polarization spectro according to claim 1, is characterized in that: the first-order diffraction light of described acousto-optic modulator (5) is orthogonal with incident light polarization.

4. the Michelson heterodyne laser vialog based on monophone optical modulation and polarization spectro according to claim 1, is characterized in that: described laser instrument (1) is frequency stabilized laser.