CN207019624U - A kind of dual wavelength superhet interferes real-time displacement measuring system - Google Patents

Abstract

The utility model discloses a dual-wavelength superheterodyne interference real-time displacement measurement system. The system consists of two lasers with a wavelength difference of Δλ, three polarization beamsplitters, four beamsplitters, two acousto-optic modulators, four quarter-wave plates, five plane mirrors, three polarizers, one It consists of an ultra-narrowband filter, two transimpedance photodetectors with large bandwidth, two high-sensitivity photodetectors with low bandwidth, a reference reflector and a measured reflector. The utility model utilizes the synthesized wavelength interference signal generated by dual wavelengths to increase the measuring range of the system, so that the measuring range of the system is much larger than that of the single-wavelength interference, and the phase of the synthesized wavelength can be directly measured to realize real-time measurement. Obtain a single-wavelength interference signal, and ensure the accuracy of single-wavelength interferometry while expanding the measurement range.

Description

A dual-wavelength superheterodyne interferometric real-time displacement measurement system

technical field

The utility model relates to a displacement measurement system, in particular to a dual-wavelength superheterodyne interference real-time displacement measurement system.

Background technique

Common single-wavelength laser interferometric displacement measurement systems have nanoscale displacement measurement accuracy, but since the single wavelength of laser is usually about 1 μm, the period of its interference signal is only hundreds of nanometers, which limits its measurement range. The displacement measurement system needs to increase the cycle count to accurately record the phase change. Once the displacement changes rapidly or encounters a large absolute displacement change such as steps, phase ambiguity will appear.

In order to solve the range problem of single-wavelength laser interference, Tilford et al. first proposed a method of using dual wavelengths to construct a synthetic wavelength to expand the range of the measurement system (CRTilford, Appl. Opt.16, 1857 (1977).). Two beams of laser light with wavelengths λ ₁ and λ ₂ can synthesize an interference signal with a wavelength of λ ₁ λ ₂ /(λ ₁ -λ ₂ ), when λ ₁ and λ ₂ are relatively close, the synthesized wavelength is much larger than the single wavelength, thus The measuring range of the interferometric displacement measurement system can be greatly expanded. However, with the expansion of the measurement range, dual-wavelength interferometry also faces two new problems: 1. The accuracy of interferometry based on the synthesized wavelength will decrease. It is difficult to realize real-time and fast measurement. In the following decades, the dual-wavelength interferometric displacement measurement has made great progress. For example, the patent No. 02112079. The measurement accuracy is high, but the stability of modulation methods such as temperature modulation laser wavelength is poor, and it is difficult to achieve theoretical accuracy; the superheterodyne interferometry proposed by Dandliker et al. Opt. Lett.13, 339 (1988)) adding different frequency shifts to two laser beams with different wavelengths can realize real-time measurement of the synthesized wavelength, but it still does not solve the problem of the decline in the measurement accuracy of the synthesized wavelength.

Existing technologies cannot solve the two problems caused by dual-wavelength interference at the same time, and meet the needs of large-scale and high-precision real-time interferometry.

Utility model content

Aiming at the above problems, the utility model proposes a dual-wavelength superheterodyne interference real-time displacement measurement system, which combines the advantages of synthetic wavelength interference, single-wavelength interference, and superheterodyne interference, and realizes a large range of high-precision real-time displacement measurement.

The utility model is realized through the following technical solutions.

It mainly consists of two lasers with a wavelength difference of Δλ, three polarization beamsplitters, four beamsplitters, two acousto-optic modulators, four quarter-wave plates, five plane mirrors, three polarizers, one Composed of ultra-narrowband filters, two large-bandwidth transimpedance photodetectors, two low-bandwidth high-sensitivity photodetectors, a reference reflector, a measured reflector, a signal processing circuit and a host computer;

The two lasers emit two beams of laser light with different wavelengths. Each laser beam passes through its own quarter-wave plate and then enters the polarization beam splitter. The modulator and the reflector are reflected, and the vertically polarized path is reflected after passing through the reflector. The reflected light of the two reflectors is incident on the splitter prism for beam combining output. The beam combining light contains two output lights with different frequencies and different polarization directions;

The two output lights are incident on the third beam-splitting prism for beam combining and splitting: a part of the beam-splitting light passes through the first polarizer and reaches the first photodetector as a reference signal for dual-wavelength superheterodyne interference; The other part of the light is incident on the third polarizing beamsplitter prism for reflection and transmission, and is divided into two paths of vertical polarization and horizontal polarization;

The vertically polarized light reflected by the third polarization beam splitter passes through the fourth quarter-wave plate and is reflected by the reference mirror, and then passes through the fourth quarter-wave plate to become horizontally polarized light and returns to the third polarization Dichroic prism; the horizontally polarized light transmitted by the third polarizing beam splitting prism passes through the third quarter-wave plate and is reflected by the mirror under test, and then passes through the third quarter-wave plate to become vertically polarized light and returns to To the third polarizing beam splitter, the measured reflector is fixed on the surface of the measured object;

The two laser beams returned to the third polarizing beam splitter are incident on the fourth beam splitting prism after the third polarizing beam splitting prism combines beams, and reflection and transmission beam splitting occurs; a part of the transmission passes through the second polarizer and is received by the second photodetector as The measurement signal of dual-wavelength superheterodyne interference; part of the reflection passes through the ultra-narrow-band filter and then is received by the third photodetector through the third polarizer, and the fourth photodetector is placed next to the third photodetector; the first, Both the second photodetector and the third and fourth photodetectors are connected to the host computer through a signal processing circuit.

The first photodetector and the second photodetector of the utility model are two large-bandwidth transimpedance photodetectors, the third photodetector and the fourth photodetector are two low-bandwidth high-sensitivity photodetectors, The sensitivity is greater than 0.1V/nW.

The signal processing circuit includes a self-mixer, a band-pass filter, a correlation phase detector, a follower and a differential amplifier circuit. The signal processing circuit specifically includes a first low noise amplifier, a second low noise amplifier, a first self-mixer, a second self-mixer, a first bandpass filter, a second bandpass filter, a first A correlation phase detector, an analog-to-digital conversion module, a differential amplifier and a second correlation phase detector; the first photodetector is sequentially connected to the first low noise amplifier, the first self-mixer and the first bandpass filter A correlation phase detector, the second photodetector is connected to the first correlation phase detector, the third photodetector and the fourth photodetector through the second low noise amplifier, the second self-mixer, and the second bandpass filter in turn The detectors are all connected to the second relative phase detector through the differential amplifier in turn, and both the first relative phase detector and the second relative phase detector are connected to the host computer through the analog-to-digital conversion module.

The two lasers emit laser light with different wavelengths, and the wavelength difference between the two laser beams is Δλ. The wavelength difference is Δλ less than 5nm and greater than 2nm.

The light emitted by the two lasers is linearly polarized light of different wavelengths, which becomes two beams of circularly polarized light after passing through a quarter-wave plate, and each beam of circularly polarized light is split into two beams of polarized light after passing through a polarization beam splitter prism. Linearly polarized light with directions perpendicular to each other.

The modulation frequencies of the two acousto-optic modulators are different.

In a specific implementation, the modulation frequencies f ₁ and f ₂ of the two acousto-optic modulators are 100 MHz and 100.01 MHz respectively, and the difference frequency between the two is 10 kHz.

The bandwidth of the first and second photodetectors is greater than the modulation frequency of the acousto-optic modulator.

The central wavelength of the ultra-narrow-band filter is the same as the smaller wavelength of the two beams of light emitted by the two lasers.

In a specific implementation, the light emitted by the two lasers is linearly polarized light with wavelengths of 632.8 nm and 635 nm. The central wavelength of the ultra-narrowband filter is 632.8nm, and the full width at half maximum is 1nm.

The third and fourth photodetectors are positioned close to each other and facing the same direction, so as to realize the difference from ambient light.

Compared with the prior art, the beneficial effects of the utility model are:

1. The utility model uses the synthetic wavelength to expand the measuring range of the measuring system, so that the measuring range of the measuring system is expanded to one-half of the synthetic wavelength, and the measuring range of the measuring system is kept within a reasonable range by rationally selecting dual wavelengths.

2. The utility model uses single-wavelength interference signals to ensure the measurement accuracy of the measurement system, so that the measurement system can ensure nanometer-level accuracy while expanding the measurement range.

Description of drawings

Fig. 1 is the system schematic diagram of the present utility model;

In the figure: laser 1, laser 2, first quarter wave plate 3, second quarter wave plate 4, first polarization beam splitter 5, second polarization beam splitter 6, first acousto-optic modulator 7 , the second acousto-optic modulator 8, the first reflector 9, the second reflector 10, the third reflector 11, the fourth reflector 12, the first dichroic prism 13, the second dichroic prism 14, the fifth reflector 15 , the third beam-splitting prism 16, the first polarizer 17, the first photodetector 18, the third polarization beam-splitting prism 19, the third quarter-wave plate 20, the measured mirror 21, the measured object 22, the fourth Quarter-wave plate 23, reference mirror 24, second polarizer 25, fourth dichroic prism 26, second photodetector 27, ultra-narrow band filter 28, third polarizer 29, third photodetector 30 , the fourth photodetector 31 .

detailed description

Further description will be given below in conjunction with specific examples.

The utility model provides a dual-wavelength superheterodyne interferometry system for realizing large-scale and high-precision real-time displacement measurement. The system includes a laser 1 with a wavelength of λ ₁ =632.8nm and a laser 2 with a wavelength of λ ₂ =635nm , the first quarter wave plate 3, the second quarter wave plate 4, the first polarizing beam splitting prism 5, the second polarizing beam splitting prism 6, the first acousto-optic modulator 7, the second acousto-optic modulator 8 , the first reflector 9, the second reflector 10, the third reflector 11, the fourth reflector 12, the first dichroic prism 13, the second dichroic prism 14, the fifth reflector 15, the third dichroic prism 16, the first dichroic prism A polarizer 17, a first photodetector 18, a third polarization splitter prism 19, a third quarter-wave plate 20, a measured mirror 21, an object to be measured 22, a fourth quarter-wave plate 23, Reference mirror 24 , second polarizer 25 , fourth dichroic prism 26 , second photodetector 27 , ultra-narrowband filter 28 , third polarizer 29 , third photodetector 30 , fourth photodetector 31 .

The concrete measurement principle of the utility model is described as follows:

As shown in Figure 1, laser 1 and laser 2 respectively emit linearly polarized light with wavelengths of λ ₁ =632.8nm and λ ₂ =635nm, wherein the linearly polarized light of wavelength λ ₁ is after passing through the first quarter-wave plate 3 Become the first circularly polarized light, the wavelength is that lambda ₂ becomes the second circularly polarized light after passing through the second quarter-wave plate 4, and the first and second circularly polarized light pass through the first polarizing beam splitter prism 5, the second polarized light respectively Polarizing beam splitting prism 6 splits the beam.

The first circularly polarized light is split into the first horizontally polarized light and the first vertically polarized light, and the first horizontally polarized light is introduced into a frequency shift of f ₁ =100MHz by the first acousto-optic modulator 7, and then passes through the first mirror 9 and The first vertically polarized light passing through the third reflector 11 is combined at the first dichroic prism 13. At this time, the frequency of the first horizontally polarized light is v ₁ +f ₁ , and the frequency of the first vertically polarized light is v ₁ , where v ₁ is the optical frequency corresponding to λ ₁ .

The second circularly polarized light is split into the second horizontally polarized light and the second vertically polarized light, and the second horizontally polarized light passes through the second acousto-optic modulator 8 and the second reflector 10, and passes through the second dichroic prism 14 and the fourth The second vertically polarized light reflected by the mirror 12 is combined, at this time, the frequency of the second horizontally polarized light is v ₂ +f ₂ , and the frequency of the second vertically polarized light is v ₂ , where f ₂ =100.01MHz, and v ₂ is λ ₂ corresponds to the optical frequency.

The first horizontally polarized light, the first vertically polarized light, the second horizontally polarized light and the second vertically polarized light four laser beams are combined and split at the third dichroic prism 16; where part of the light intensity passes through the first polarizer 17, the second _A polarizer 17 converts four laser beams with different polarizations into linearly polarized light with the same polarization state, wherein the _two laser beams with frequencies v1 and v1 ₊ f1 are coherently superimposed, and the laser beams with frequencies v2 and _{v2 + f2} The two laser beams are coherently superimposed to form a reference signal of dual-wavelength superheterodyne interference, which is received by the first photodetector 18 .

The other four laser beams after beam splitting are split again by the third polarization beam splitter prism 19, wherein the transmitted first horizontally polarized light and the second horizontally polarized light become the third circle respectively after passing through the third quarter-wave plate 20. The polarized light and the fourth circularly polarized light are vertically incident on the measured reflector 21 and reflected, and pass through the third quarter-wave plate 20 again to become the third vertically polarized light and the fourth vertically polarized light by the third polarization beam splitter prism 19 Reflection, wherein the measured mirror 21 is fixedly connected to the measured object 22; the reflected first vertically polarized light and the second vertically polarized light are reflected by the reference mirror 24 after passing through the fourth quarter-wave plate 23, and then passed through After the fourth quarter-wave plate 23, the third horizontally polarized light and the fourth horizontally polarized light pass through the third polarization splitter prism 19; wherein the third vertically polarized light, the fourth vertically polarized light, and the third horizontally polarized light and the frequencies of the fourth horizontally polarized light are respectively v ₁ +f ₁ , v ₂ +f ₂ , v ₁ , v ₂ , and the four laser beams are split into two parts by the fourth beam splitter 26; one part passes through the second polarizer 25 undergoes coherent superposition, and the measurement signal forming dual-wavelength superheterodyne interference is received by the second photodetector 27 . The reference signal and the measured signal of dual-wavelength superheterodyne interference are subsequently solved to obtain the phase change value containing displacement information. The phase change value is based on the synthetic wavelength λ _s , so it has a large range and relatively low precision.

The other part passes through the ultra-narrow-band filter 28 to leave only the third vertically polarized light and the fourth horizontally polarized light whose frequencies are v ₁ +f ₁ and v _1. The wavelengths of these two laser beams are all around λ ₁ , so they can pass through Narrowband filter 28, the central wavelength of the ultra-narrowband filter is 632.8nm, and the full width at half maximum is 1nm. The third vertically polarized light and the fourth horizontally polarized light passing through the ultra-narrowband filter 28 pass through the third polarizer 29 to undergo coherent superposition to form a single-wavelength interference signal that is received by the third photodetector 30 .

The fourth photodetector 31 receives the ambient light signal and uses it to make a difference from the single-wavelength interference signal to eliminate the influence of the ambient light. The single-wavelength interference signal and the environment-related signal are subsequently solved to calculate the phase change value containing displacement information. The phase change value is based _on the single wavelength λ1, so it has high precision.

The utility model utilizes the synthesized wavelength interference signal generated by dual wavelengths to increase the measuring range of the system, so that the measuring range of the system is far greater than the measuring range of single wavelength interference, and uses the superheterodyne interference method to demodulate and filter the output signal, which can directly measure the synthesized The phase of the wavelength realizes real-time measurement. At the same time, the ultra-narrow-band filter is used to collect single-wavelength interference signals, which ensures the accuracy of single-wavelength interferometry while expanding the measurement range.

The utility model has been described through the embodiments, and anyone familiar with the technology can modify or change the above embodiments without departing from the spirit and scope of the utility model. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed in the utility model should still be covered by the claims of the utility model.

Claims (5)

1. A dual-wavelength superheterodyne interference real-time displacement measurement system is characterized in that: two lasers (1, 2) send two beams of different wavelengths of laser light, and each beam of laser light is incident after passing through a quarter wave plate respectively The polarized beam splitter is divided into two paths: horizontal polarization and vertical polarization: one path of horizontal polarization is reflected by the acousto-optic modulator and mirror in turn, the other path of vertical polarization is reflected by the mirror, and the reflected light of the two mirrors Incident to the beam splitter prism combined output, the combined beam contains two output lights with different frequencies and different polarization directions; The two output lights are incident on the third dichroic prism (16) to be combined and split: a part of the beam-split light passes through the first polarizer (17) and reaches the first photodetector (18), and the other part of the beam-split light The light is incident on the third polarization beam splitter prism (19) for reflection and transmission, and is divided into two paths of vertical polarization and horizontal polarization; The one-way vertically polarized light reflected by the third polarization splitter prism (19) passes through the fourth quarter-wave plate (23) and is reflected by the reference reflector (24) again through the fourth quarter-wave plate (23) ) becomes horizontally polarized light and returns to the third polarization beam splitter prism (19); the horizontally polarized light transmitted through the third polarization beam splitter prism (19) is measured through the third quarter-wave plate (20) Reflecting mirror (21) becomes vertically polarized light again through the third quarter-wave plate (20) after reflection and returns to the third polarization beam splitter prism (19), and the measured reflecting mirror (21) is fixed on the measured object ( 22) surface; The two-way laser light that gets back to the third polarization beam splitter (19) is incident on the fourth beam splitter (26) after the third polarization beam splitter (19) combines beams, and reflection and transmission beam splitting occurs; a part of the transmission passes through the second polarization Plate (25) is received by the second photodetector (27), and a part of the reflection is received by the third photodetector (30) through the third polarizer (29) after the ultra-narrow-band filter (28), and the fourth photoelectric A detector (31) is placed next to the third photodetector (30). 2. A dual-wavelength superheterodyne interference real-time displacement measurement system according to claim 1, characterized in that: the two lasers emit lasers with different wavelengths, and the wavelength difference between the two laser beams is Δλ. 3. A kind of dual-wavelength superheterodyne interference real-time displacement measurement system according to claim 1, characterized in that: the light emitted by the two lasers is linearly polarized light of different wavelengths, and passes through a quarter wave plate Afterwards, it becomes two beams of circularly polarized light, and each beam of circularly polarized light is split into two beams of linearly polarized light whose polarization directions are perpendicular to each other after passing through a polarization beam splitter prism. 4. A kind of dual-wavelength superheterodyne interference real-time displacement measurement system according to claim 1, characterized in that: the central wavelength of the ultra-narrow-band filter and the smaller wavelength of the two beams of light emitted by the two lasers Likewise, the full width at half maximum is less than Δλ/2. 5. A dual-wavelength superheterodyne interference real-time displacement measurement system according to claim 1, characterized in that: said third and fourth photodetectors are placed next to each other and facing the same direction.