CN2419594Y - Optical Measuring Instrument for Object Vibration Amplitude - Google Patents

Abstract

An optical measuring instrument for the vibration amplitude of an object includes a first lens, a second lens, a beam splitter, a polarization beam splitter, and a polarization converter arranged in sequence along the forward direction of the light beam emitted by a light source to the object to be measured. The light beam with speckle noise reflected by the object to be measured for the first time is reflected by the polarization beam splitter and then converged to the photorefractive crystal by the fourth lens. The phase conjugate light generated by the photorefractive crystal is reflected to the object to be measured by the polarization beam splitter, and the phase conjugate light reflected by the object to be measured no longer has speckle noise. Therefore, regardless of whether the surface of the object to be measured is smooth or rough, its vibration amplitude can be detected with sub-nanometer resolution.

Description

Optical Measuring Instrument for Object Vibration Amplitude

The utility model relates to a measuring instrument for the vibration amplitude of an object, in particular, the measuring instrument is not only suitable for the measured object with a smooth surface, but also suitable for the measured object with a rough surface.

In the prior art, as an effective non-contact precision measuring instrument, laser interferometer has been widely used. When using a traditional homodyne or high-precision heterodyne interferometer for measurement, in order to obtain an interference signal with a high signal-to-noise ratio, it is necessary for the surface of the measured object to be a smooth surface close to a mirror. However, the surface of most measured objects is rough. Light reflected from such surfaces contains a large amount of speckle noise, which makes high-precision measurements difficult. For example, Mr. Suzuki Xiaochang (Takamasa Suzuki) of Niigata University in Japan proposed an interferometer for measuring vibrations, (see prior art: Takamasa Suzuki, Takao Okada, OsamiSasaki, and Takeo Maruyama, "Real-time vibration measurement using a feedback type of laser diode interferometer with an optical fiber,"Opt.Eng.,1997,36(9),2496-2502.) If the object to be measured is an object with a rough surface, the high quality of the vibration amplitude cannot be achieved Measurement. In addition, the vibration amplitude measurement accuracy of this interferometer is only 40 nanometers (4×1O ^-8 meters) in the best case.

The purpose of this utility model is to overcome the deficiencies in the above-mentioned prior art and provide an optical measuring instrument for measuring the vibration amplitude of an object. It will use the phase compensation characteristics of phase conjugate light and combine the structure of sinusoidal phase modulation to realize sub-nanometer (10 ^-10 meters to 10 ^-12 meters) to accurately measure the vibration amplitude of the object, and the surface of the measured object can be measured whether it is smooth or rough.

The optical measuring instrument of the vibration amplitude of the object of the present utility model has a structure as shown in Fig. 1 . It includes a light source 1, a first lens 2, a second lens 3 and a beam splitter 5 are sequentially arranged along the advancing direction of the linearly polarized light beam _G0 emitted by the light source 1 on the same optical axis OO as the light source 1. The light beam G ₀ is split into the transmitted light beam Gt ₁ and the reflected light beam _{Gf 1} by the beam splitter 5 , and a polarization beam splitter 8 and a polarization converter 11 are sequentially placed on the optical path of the transmitted light beam Gt 1 to irradiate the measured object 12 . A reference attenuating reflector 4 is placed on the optical path of the reflected light beam Gf ₁ on the reflective surface of the beam splitter 5 opposite to the light source 1 . A third lens 6 and a photoelectric receiving element 7 are arranged along the optical path of the reflected light beam Gf ₂ on the reflective surface of the beam splitter 5 facing the object 12 to be measured. The electrical signal output by the photoelectric receiving element 7 is input into the computer 15 through the amplifier 13 and the data acquisition card 14 . A fourth lens 9 is arranged in a direction perpendicular to the optical axis OO of the light source 1 where the reflection direction of the polarization beam splitter 8 relative to the reflection surface of the measured object 12 is perpendicular. A photorefractive crystal 10 is disposed at the focal point of the fourth lens 9 . That is to say, the polarization direction of the polarized light beam Gp ₂ passing through the polarization converter 11 after being reflected by the measured object 12 is perpendicular to the polarization direction of the light beam Gp ₁ before passing through the polarization converter 11 for the first time. The polarized light beam Gp ₂ is reflected by the polarizing beam splitter 8 and converged onto the photorefractive crystal 10 through the fourth lens 9 . The phase conjugate light generated by the photorefractive crystal 10 is collimated by the fourth lens 9, reflected by the polarization beam splitter 8, and then irradiated on the measured object 12 for the second time after passing through the polarization converter 11. After the phase conjugate light reflected by 12 passes through the polarization converter 11 , the polarization direction is the same as that of the light beam Gp ₁ before incident on the measured object 12 for the first time. The light beam passes through the polarizing beam splitter 8 and then returns to the beam splitter 5 , and then is reflected by the beam splitter 5 and converged onto the photoelectric receiving element 7 through the third lens 6 .

The light source 1 mentioned above refers to a laser light source with a certain wavelength that can make the photorefractive crystal 10 generate phase conjugate light in the measuring device of the present utility model, which is a gas laser, or a semiconductor laser, or a solid-state lasers, etc.

Said reference attenuation reflector 4 is a light beam reflection element, and its reflectivity satisfies that after cooperating with beam splitter 5 during measurement, the light intensity ratio of the object light received by photoelectric receiving element 7 and the reference light is close to 1:1. Therefore, the reference attenuating reflector is composed of a parallel flat plate 401 and an attenuating plate 404 coated with an optical analysis film and an anti-reflection film on both sides, as shown in FIG. 2 . Alternatively, it is composed of a parallel plate 401 coated with an optical analysis film and an anti-reflection film on both sides and two polarizers 402 and 403, as shown in FIG. 1 . Or it is only composed of a parallel flat plate coated with a light analysis film and an anti-reflection film on both sides.

Said beam splitter 5 refers to an element capable of splitting incident light into two beams of light according to a certain beam splitting ratio.

The photoelectric receiving element 7 is a photodiode, or a photoelectric conversion device such as a photovoltaic cell.

Said polarizing beam splitter 8 is capable of separating two beams of light whose polarization directions are perpendicular to each other. Let another beam of light with polarization perpendicular to A be reflected. It is a polarizing beamsplitter prism, or a polarizing parallel plate, etc.

The so-called photorefractive crystal 10 refers to a photorefractive crystal that can generate self-pumped phase conjugate light after the light beam emitted by the light source 1 is irradiated on the photorefractive crystal 10 by the above-mentioned optical elements, which is barium titanate (BaTiO ₃ ) crystal, or potassium sodium strontium barium niobate (KNSBN) crystal, or strontium barium niobate crystal, etc.

The so-called polarization converter 11 refers to an optical element that passes a beam of polarized light through it, is reflected by the measured object 12, and returns to the optical element that changes the polarization direction by 90 degrees after passing through it again. It is a Faraday rotator or a quarter-wave plate wait.

With the structure shown in Figure 1 and Figure 2, when the linearly polarized light G ₀ emitted by the light source 1 with the polarization direction A is expanded by the first lens 2 and the second lens 3, the light beam Gt ₁ passing through the beam splitter 5, It is irradiated onto the measured object 12 through the polarization beam splitter 8 and the polarization converter 11 . After the light beam reflected by the measured object 12 and containing a lot of speckle noise passes through the polarization converter 11 , the polarization direction of the polarized light beam Gp ₂ whose polarization direction changes to B is perpendicular to the polarization direction A. The polarized beam Gp ₂ is reflected by the polarizing beam splitter 8 and converged to the photorefractive crystal 10 by the fourth lens 9 . The phase conjugate light generated by the photorefractive crystal 10 is collimated by the fourth lens 9, reflected by the polarization beam splitter 8, and then irradiates the measured object 12 after passing through the polarization converter 11. Since the phase conjugate light has a phase Compensation characteristics, so the phase conjugate light reflected by the measured object 12 for the second time no longer contains speckle noise. At the same time, after the phase conjugate light passes through the polarization converter 11, the polarization direction becomes A again. After passing through the polarization beam splitter 8, the light beam is reflected by the beam splitter 5, and then converged to the photoelectric receiving element 7 by the third lens 6. Above, this beam of light is called object light. The reflected beam Gf 1 after the linearly polarized beam G ₀ emitted by the light source 1 is reflected by the beam splitter ₅ is irradiated onto the reference attenuating reflector 4 . The reflected light beam from the reference attenuating reflector 4 passes through the beam splitter 5 and then converges to the photoelectric receiving element 7 by the third lens 6. The light beam reflected by the reference attenuating reflector 4 is called reference light. The reference light interferes with the object light. After the electrical signal received by the photoreceiving element 7 is amplified by the amplifier 13, the data acquisition card 14 converts the analog electrical signal output by the amplifier 13 into a digital signal and stores it in the computer 15 for data processing.

The interference signal detected by the photoelectric receiving element 7

I(t)=I ₀ (t)+S ₀ (t)cos[zcos(ω _c t+θ)+α ₀ +α(t)], (1) Among them, I ₀ (t) and S ₀ ( t) are the amplitudes of the DC component and the AC component of the interference signal, respectively, and α ₀ is the phase of the interference signal when the measured object 12 is stationary. α(t) is the phase change introduced by the vibration of the measured object 12 . ω _c is the frequency of the sinusoidal phase modulation, t is the time, and θ is the initial phase of the modulated signal. The amplitude of the phase modulation of the interference signal z=4πa/λ ₀ , where a is the vibration amplitude to be measured, and λ ₀ is the center wavelength of the light source 1 . Perform Fourier transform on formula (1) to obtain z value. The vibration amplitude of the measured object 12

a=λ ₀ z/4π. (2) It is relatively easy to realize that the measurement accuracy of z reaches 0.01rad. If an argon ion laser with a wavelength of 514nm is used, the measurement accuracy of the amplitude is 0.4nm. If the measurement accuracy of z is increased to 0.001rad, the measurement accuracy of the amplitude is increased to 0.04nm (4×10 ^-11 meters).

The utility model has the advantages of:

1. Since the measuring instrument of the present invention contains the photorefractive crystal 10 , it generates phase conjugate light with phase compensation characteristics, which can eliminate the speckle noise brought by the measured object 12 . Therefore, the measuring instrument of the utility model is applicable to the measured object 12 no matter the surface is smooth or rough, and greatly expands the scope of the measuring object. The surface of the measured object 12 is expanded from a smooth surface close to a mirror surface to a generally rough surface such as stainless steel or aluminum plate.

2. Since the measuring instrument of the present invention contains the photorefractive crystal 10 , it generates phase conjugate light with phase compensation characteristics, which can eliminate speckle noise caused by the measured object 12 . Even if the surface to be measured is a rough surface, the resolution of the measurement can still reach sub-nanometer. Therefore, the measurement accuracy is high. Even if the surface to be measured is a rough surface, the measurement accuracy can still reach the sub-nanometer level (10 ^-10 meters to 10 ^-12 meters).

3. The measuring instrument of the utility model has a simple, compact and reasonable structure.

Description of drawings

Fig. 1 is the schematic diagram of the optical measuring instrument of the vibration amplitude of the object of the present invention, wherein the reference attenuation reflector 4 is made of a parallel plate 401 and two polarizers 402 and 403 coated with an optical analysis film and an anti-reflection film on both sides respectively.

Fig. 2 is a schematic structural view of the measuring instrument of the present invention, wherein the reference attenuation reflector 4 is composed of a parallel flat plate 401 and an attenuation plate 404 coated with a light-splitting film and an anti-reflection film on both sides.

Example:

The structure shown in Figure 2. Wherein the light source 1 is an argon ion gas laser with a wavelength of 514nm, and the beam splitter 5 is a parallel plate coated with a light analysis film on one side and an anti-reflection film on the other side. The photoreceiving element 7 is a photodiode. The polarizing beam splitter 8 is a polarizing beam splitting prism. The reference attenuating reflector 4 is composed of a parallel flat plate 401 and an attenuating plate 404 coated with a light-splitting film and an anti-reflective film on both sides respectively. The polarization converter 11 is a Faraday rotator. The photorefractive crystal 10 is a barium titanate (BaTiO ₃ ) crystal. The measured object 12 is an aluminum plate with a rough surface. The measured vibration amplitude of the aluminum plate is 130nm, the measurement accuracy is 0.5nm, and the resolution is less than 5×10 ⁻¹⁰ meters.

Claims (6)

1. the optical measuring instrument of an object vibration amplitude comprises: light source (1), and along the linearly polarized light beam (G of light source (1) emission ₀) on the working direction, with the same optical axis of light source (1) (O-O) be equipped with first lens (2), second lens (3) and beam splitter (5) successively; Beam splitter (5) transmitted light beam (Gt ₁) light path on be equipped with polarisation transformer (11) and testee (12); Folded light beam (Gf on the reflecting surface of the relative light source (1) of beam splitter (5) ₁) light path on be equipped with reference to reflection attenuation device (4); Folded light beam (Gf on the reflecting surface of the relative testee (12) of beam splitter (5) ₂) light path on be equipped with the 3rd lens (6) and photoelectric apparatus (7); The electric signal of photoelectric apparatus (7) output is by amplifier (13) and data collecting card (14) input computing machine (15); It is characterized in that between beam splitter (5) and the polarisation transformer (11), be equipped with polarization beam apparatus (8) (O-O) with beam splitter (5) and the same optical axis of polarisation transformer (11); Be equipped with the 4th lens (9) on polarization beam apparatus (8) direction that relatively reflection direction of testee (12) reflecting surface is vertical with light source (1) optical axis (O-O); The focus place of the 4th lens (9) is equipped with photorefractive crystal (10).

2. the optical measuring instrument of object vibration amplitude according to claim 1 is characterized in that said light source (1) is a gas laser, or semiconductor laser, or solid state laser.

3. the optical measuring instrument of object vibration amplitude according to claim 1 is characterized in that said photorefractive crystal (10) is a barium titanate crystal, or potassium sodium strontium barium niobate crystal, or the strontium barium niobate crystal.

4. the optical measuring instrument of object vibration amplitude according to claim 1, it is characterized in that said is to plate the parallel flat (401) of analysing light film and anti-reflection film respectively by the two sides to plate parallel flat (401) and two polarizers (402 of analysing light film and anti-reflection film respectively with attenuator (404) formation or by the two sides with reference to reflection attenuation device (4), 403) constitute, or plate the parallel flat of analysing light film and anti-reflection film respectively by a two sides and constitute.

5. the optical measuring instrument of object vibration amplitude according to claim 1 is characterized in that said polarization beam apparatus 8 is polarization splitting prisms, or the polarization parallel flat board.

6. the optical measuring instrument of object vibration amplitude according to claim 1 is characterized in that said polarisation transformer 11 is Faraday rotators, or quarter-wave plate.

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