Coaxial confocal laser vibration measurement system

Abstract

The invention discloses a coaxial confocal laser vibration measurement system. A signal light laser emits signal laser to a beam splitter; the beam splitter splits the received signal laser according to a preset proportion, one beam serves as signal light to be transmitted to the circulator, and the other beam serves as local oscillation light to be transmitted to the frequency mixer; the circulator transmits signal light to the beam combiner, receives a light return signal of the signal light transmitted by the beam combiner, and feeds back the light return signal to the frequency mixer; the frequency mixer is coupled with the local oscillation light emitted by the beam splitter and a return light signal of the signal light output by the circulator, so that the return light signals of the local oscillation light and the signal light are interfered; the beam combiner receives the indication light emitted by the indication light laser, transmits the signal light and the indication light to the telescope system, and receives a light return signal of the signal light transmitted by the telescope system; the telescope system is used for transmitting signal light and indication light and receiving light return signals of the signal light. According to the invention, optical fiber beam combination coaxial indication light is designed, so that light spots can be observed at a long distance, and the efficiency of operating an instrument by people is improved.

Description

Technical Field

[0001] The invention relates to the technical field of Doppler vibrometers, and in particular to a coaxial confocal laser vibrometer system. Background Art

[0002] Laser Doppler vibrometer is a detection instrument using laser as the medium. The laser hits the object to be measured, and the laser return signal and the local oscillator light signal of the object to be measured are interfered to measure the object's vibration frequency, relative displacement and other parameters. Existing laser vibrometers mostly use helium-neon laser light sources in the visible light band of the human eye (400nm-700nm) and 1550nm near-infrared laser light sources. Helium-neon laser light sources are mostly used for close-range testing, and near-infrared laser light sources are suitable for vibration measurement of rough surfaces and low-reflectivity surfaces, long-distance vibration measurement, and high-frequency vibration measurement.

[0003] Existing visible light helium-neon lasers are limited by the low power of the laser itself, which makes the test distance of the vibrometer relatively short. If long-distance testing is required, a 1550nm near-infrared laser with greater energy is required. However, the 1550nm infrared laser is invisible to the human eye, and it is impossible to effectively determine whether the laser is hitting the object being tested. Summary of the invention

[0004] Technical purpose: In view of the defects in the prior art, the present invention discloses a coaxial confocal laser vibrometer system. By designing a solution of optical fiber beam combining and coaxial indicator light, the position where the indicator light is emitted is the position where the signal light is located, so that the light spot can be well observed at a long distance, thereby improving the efficiency of people operating the instrument. It creatively solves the problem of difficulty in aligning the artificial light spot and low alignment efficiency in long-distance testing of the laser Doppler vibrometer.

[0005] Technical solution: In order to achieve the above technical objectives, the present invention adopts the following technical solution.

[0006] A coaxial confocal laser vibrometer system, characterized in that it comprises: a signal light laser, a beam splitter, a circulator, a beam combiner, a telescope system, an indicator light laser, and a mixer;

[0007] The signal light laser is connected to the beam splitter and is used to emit the signal laser to the beam splitter;

[0008] The beam splitter is connected to the circulator and the mixer, and is used to split the received signal laser according to a preset ratio, one beam is transmitted to the circulator as signal light, and the other beam is transmitted to the mixer as local oscillator light;

[0009] The circulator is connected to the beam combiner and the mixer, and is used to transmit the signal light to the beam combiner, receive the return light signal of the signal light transmitted by the beam combiner, and feed it back to the mixer;

[0010] The mixer is used to couple the local oscillator light emitted by the beam splitter and the return light signal of the signal light output by the circulator, so that the return light signals of the local oscillator light and the signal light interfere with each other;

[0011] The beam combiner is connected to the telescope system and the indicator light laser, and is used to receive the indicator light emitted by the indicator light laser, transmit the signal light and the indicator light to the telescope system, and receive the return light signal of the signal light transmitted by the telescope system;

[0012] The telescope system is used to transmit signal light and indicator light, and receive the return light signal of the signal light.

[0013] Preferably, the beam combiner is a fiber beam combiner, which utilizes total reflection of optical fiber to transmit light, thereby achieving coaxial transmission of signal light and indicator light.

[0014] Preferably, the signal light laser is a 1550nm narrow-linewidth polarization-maintaining laser; and the indicator light laser is a 650nm laser.

[0015] Preferably, the telescope system comprises a focus-adjustable structure and a lens, and the lens is engaged with the focus-adjustable structure.

[0016] Preferably, the lens is a double-cemented lens, which is formed by bonding concave and convex lenses.

[0017] Preferably, in the double-cemented lens, the refractive index of the convex lens is less than 1.62, and the Abbe number is less than 28; the refractive index of the concave lens is greater than 1.8, and the Abbe number is greater than 60.

[0018] Preferably, the adjustable focus structure includes a rotary adjustment member and interfaces and slots located at both ends of the rotary adjustment member; the interface is a fiber optic head socket connected to the combiner; the slot is used to engage the lens to achieve coaxiality between the lens and the adjustable focus structure, and the rotary adjustment member 52 realizes forward and backward rotation focusing.

[0019] Preferably, the vibration measurement system also includes a target to be measured, with adjustable frequency, amplitude and distance.

[0020] Beneficial effects:

[0021] (1) The present invention designs a solution for coaxial indicator light of optical fiber beam combination. The position of the indicator light is the same as the position of the signal light. The light spot can be well observed at a long distance, which improves the efficiency of people operating the instrument. It creatively solves the problem that the alignment of the artificial light spot is difficult and the alignment efficiency is low in the long-distance test of the laser Doppler vibrometer.

[0022] (2) The present invention designs an achromatic telescope system to eliminate the chromatic aberration caused by the wavelength difference between visible light and near-infrared light, so that the spot sizes of visible light and near-infrared light are consistent at different distances;

[0023] (3) The present invention uses lens focusing to adapt to long-distance and short-distance testing. The 650nm visible light and 1550nm near-infrared light cannot be focused due to the large wavelength difference. The laser vibrometer uses continuous zoom to achieve a detection range from near to far. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the system structure of an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of the lens structure of a telescope system according to an embodiment of the present invention;

[0026] Figure 3 A schematic diagram of a simulation point array according to an embodiment of the present invention;

[0027] Figure 4 is a schematic structural diagram of a lens structure according to an embodiment of the present invention;

[0028] Figure 5 Schematic diagram of signal-to-noise ratio results according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] 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. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0030] A coaxial confocal laser vibrometer system of the present invention comprises: a signal light laser 1, a beam splitter 2, a circulator 3, a beam combiner 4, a telescope system 5, an indicator light laser 6, and a mixer 8;

[0031] The signal light laser 1 is connected to the beam splitter 2 and is used to emit a signal laser to the beam splitter 2. In some embodiments of the present invention, the signal light laser is a 1550nm band narrow line width (line width less than 500khz) polarization-maintaining laser.

[0032] The beam splitter 2 is connected to the circulator 3 and the mixer 8, and is used to split the received signal laser according to a preset ratio, one beam is transmitted to the circulator 3 as signal light, and the other beam is transmitted to the mixer 8 as local oscillator light; in some embodiments of the present invention, the beam splitter 2 uses a 1550nm polarization-maintaining beam splitter with a splitting ratio of 0.1:99.9, wherein the 0.1 splitting ratio is used to transmit the local oscillator light, and the 99.9 ratio is used to transmit the signal light;

[0033] The circulator 3 is connected to the beam combiner 4 and the mixer 8, and is used to transmit the signal light to the beam combiner 4, receive the return light signal of the signal light transmitted by the beam combiner 4, and feed it back to the mixer 8; in some embodiments of the present invention, the circulator 3 uses a 1550nm polarization-maintaining circulator, and the crosstalk index is greater than 60dB;

[0034] The mixer 8 is used to couple the local oscillator light emitted by the beam splitter 2 and the return light signal of the signal light output by the circulator 3, so that the return light signals of the local oscillator light and the signal light interfere with each other, and output two optical signals with a phase difference of 90°. The optical signals are sent to the detector to complete the photoelectric conversion; the phase shift deviation of the return light signal of the signal light and the local oscillator light of the mixer 8 is 90°±4° when the beat frequency is generated, and the output optical power difference is less than 0.8dB and enters the detector.

[0035] The beam combiner 4 is connected to the telescope system 5 and the indicator light laser 6, and is used to receive the indicator light emitted by the indicator light laser 6, transmit the signal light and the indicator light to the telescope system 5, and receive the return light signal of the signal light transmitted by the telescope system 5; the beam combiner 4 is a fiber beam combiner, which uses total reflection of the optical fiber to transmit light, and realizes the coaxial transmission of the signal light and the indicator light; in some embodiments of the present invention, the beam combiner 4 must satisfy the requirement that both visible light and signal light can pass, that is, it is compatible with the 650nm band and the 1550nm band; the indicator light laser 6 uses a 650nm laser that emits light visible to the human eye;

[0036] The telescope system 5 is used to transmit signal light and indicator light, and receive the return light signal of the signal light; the telescope system 5 includes a focusable structure and a lens, and the lens is clamped on the focusable structure; the lens in the telescope system 5 adopts a double-cemented lens, and the double-cemented lens is formed by bonding concave and convex lenses, the refractive index of the convex lens is less than 1.62, and the Abbe number is less than 28; the refractive index of the concave lens is greater than 1.8, and the Abbe number is greater than 60.

[0037] Focusable structural parts such as Figure 4 As shown, the focus-adjustable structure includes a rotary adjustment member 52 and interfaces 51 and a slot 53 located at both ends of the rotary adjustment member 52; the interface 51 is a fiber head socket, connected to the beam combiner 4, and is used to insert the optical fiber and fix the optical fiber at the center of the focus-adjustable structure; the slot 53 is used to clamp the lens to achieve coaxiality between the lens and the focus-adjustable structure. The lens is a glued lens and is a double convex surface. The slot 53 is a circular slot. The rotary adjustment member 52 can be rotated forward and backward to adjust the focus. The distance between the interface 51 and the slot 53 is adjusted by rotating the rotary adjustment member 52 forward and backward to achieve focusing and ensure concentricity with 1′ accuracy.

[0038] The system disclosed in the present invention also includes a target to be measured 7, which is connected to a signal generator for drive control. The vibration frequency and vibration amplitude of the target to be measured can be changed by adjusting the frequency and voltage amplitude of the driving voltage signal. The distance of the target to be measured can be artificially pulled to achieve testing at different distances.

[0039] Example

[0040] A coaxial confocal laser vibrometer system in this embodiment has the following design principles:

[0041] The near-infrared laser Doppler vibrometer uses near-infrared laser as signal light to hit the object to detect the vibration characteristics of the object. The visible light laser coaxial with the signal light is used as the indicator light to facilitate people to observe whether the laser hits the object to be measured. The telescope system has the function of sending and receiving signal light while converging the indicator light on the object to be measured, and the focal length is adjusted to ensure that the light spots of the signal light and the indicator light are small enough.

[0042] A coaxial confocal laser vibrometer system designed in this embodiment is a coaxial system that allows the signal light and the indicator light to have good coaxiality. This system must not only ensure the energy transmission efficiency of the signal light, minimize the signal light loss, but also ensure that the indicator light has good transmittance, so that the human eye can observe the indicator light spot at a long distance. The design of the telescope system needs to be compatible with the difference in wavelengths of 650nm and 1550nm light, so that the two wavelengths are confocal and the chromatic aberration caused by the wavelength difference is eliminated, so that the two wavelengths can always keep the same spot size during the focusing process. When designing the lens, the spot should be small enough at 10 meters, with a diameter less than 10mm. According to the radar equation:

[0043]

[0044] Among them, P r is the return signal of the signal light. The larger the return signal of the signal light, the higher the signal-to-noise ratio of the system, and the more beneficial it is to the entire system. τ, η are constants, and ρ t is the reflectivity of the object being measured, P t is the total power emitted by the signal light laser, A t is the receiving area of ​​the telescope system 5 on the target 7, A r is the aperture area of ​​the telescope system 5, A l is the spot area on the target 7, θ is the angle between the spot and the plane projection of the target; R is the distance between the target 7 and the system. r and spot area A l Inversely proportional, A l If it is too large, the energy scattering will be more serious and the return signal will be reduced, which is not conducive to the system receiving the return signal of the signal light, so the spot area is required to be as small as possible.

[0045] like Figure 1 As shown, in this embodiment, a coaxial confocal laser vibrometer system, a signal light laser 1 is connected to a beam splitter 2, and is used to emit a signal laser to the beam splitter 2, and the signal laser is 1550nm; the beam splitter 2 is connected to a circulator 3 and a mixer 8, and is used to split the received signal laser according to a preset ratio, one beam is transmitted to the circulator 3 as a signal light, and the other beam is transmitted to the mixer 8 as a local oscillator light; the circulator 3 is connected to a beam combiner 4 and a mixer 8, and is used to transmit the signal light to the beam combiner 4, receive the return light signal of the signal light transmitted by the beam combiner 4, and feed it back to the mixer 8; the mixer 8 is used to couple the local oscillator light emitted by the beam splitter 2 and the circulator 3 The return light signal of the output signal light causes the return light signal of the local oscillator light and the signal light to interfere; the beam combiner 4 is connected to the telescope system 5 and the indicator light laser 6, and is used to receive the indicator light emitted by the indicator light laser 6, transmit the signal light and the indicator light to the telescope system 5, and receive the return light signal of the signal light transmitted by the telescope system 5; the beam combiner 4 is a fiber beam combiner, and the fiber beam combiner uses the total reflection of the optical fiber to transmit light; the indicator light emitted by the indicator light laser 6 is a 650nm red indicator light; the telescope system 5 is used to emit the signal light and the indicator light, and receive the return light signal of the signal light; the target 7 to be measured has adjustable frequency, amplitude, and distance;

[0046] In this embodiment, the working process of the system is:

[0047] After laser 1 emits 1550nm signal light, it enters combiner 4 after being transmitted by beam splitter 2 and circulator 3. Combiner 4 uses a fiber optic combiner, which uses total reflection of the optical fiber to transmit light. Laser 6 outputs 650nm red indicator light. After beam combiner 4, the indicator light and signal light are combined in the optical fiber and the coaxiality of the two beams is very high. After ensuring the coaxiality of the two beams, we need to consider the large wavelength difference between the indicator light (650nm) and the signal light (1550nm), so we need to design an achromatic telescope system to eliminate the chromatic aberration caused by the wavelength difference.

[0048] Telescope system 5 uses double-cemented lenses, which are made of concave and convex lenses glued together. In order to effectively eliminate the chromatic aberration caused by the wavelength difference between 1550nm and 650nm, and to make the two light spots small enough, the refractive index of the convex lens is less than 1.62, and the Abbe number is less than 28; the refractive index of the concave lens is greater than 1.8, and the Abbe number is greater than 60. The designed lens can eliminate the chromatic aberration of the signal light and the indicator light and ensure that the two light spots are small enough. In order to verify, the lens is designed using zemax software, and the focal length parameter of the lens is set to f=32mm, the central thickness of the lens is greater than 1mm and less than 20mm, the edge thickness of the lens is greater than 1.5mm, the numerical aperture parameter of the lens is 0.12, the lens material parameters, etc. The point diagram of the entire system is optimized, and it is verified that the refractive index of the convex lens is less than 1.62, and the Abbe number is less than 28; the refractive index of the concave lens is greater than 1.8, and the Abbe number is greater than 60. The designed lens can eliminate the chromatic aberration of the signal light and the indicator light and ensure that the two light spots are small enough. After simulation and actual measurement verification, the diameters of the signal light and the indicator light are both about 4mm at 10m. The adjustable focus structure in the telescope system 5 uses the hole-axis combination method to ensure coaxiality and realize lens focal length adjustment.

[0049] Specifically, the lens of the telescope system 5 is as follows: Figure 2 As shown, it is made of two concave and convex lenses made of glass with different optical properties. Convex lenses are often made of glass with a small refractive index (refractive index less than 1.62) and a large dispersion ability (Abbe number less than 28), and concave lenses are made of glass with a large refractive index (refractive index greater than 1.8) and a small dispersion ability (Abbe number greater than 60). The aberration lens can generally only overlap the images and points of the two main colors of light, thereby eliminating the chromatic aberration of the two colors of light, and the difficulty of installation and adjustment is relatively small. The lens is designed according to the fiber diameter and fiber port position of the combiner 4. The numerical aperture of the fiber is fixed at 0.12, so the numerical aperture of the lens is designed to be 0.12. The fiber is inserted into the structure, and the Gaussian imaging formula is used to obtain:

[0050]

[0051] Wherein, f is the focal length of the lens in the telescope system 5, u is the image distance, and v is the object distance;

[0052] The designed lens focal length f = 32mm, the focal length is an empirical value obtained from the optical fiber outlet to the lens installation position, the image distance u is set to 10cm to 10m, and the object distance v needs to be adjusted in the range of 36.3mm to 29.1mm. According to the simulation and actual measurement, Figure 3 As shown, the diameter of the light spot is less than 10 mm at 10 m.

[0053] Vibration test was performed based on the vibration measurement system of this embodiment, using red light as the indicator light. The red light indicator was turned on throughout the process to determine whether the signal light hit the target. Then, the signal light return intensity from 10 cm to 3 m under different reflectivity materials was tested. The experimental results are shown in Table 1:

[0054] Table 1 Signal light return intensity test table

[0055]

[0056]

[0057] Reflectivity of the object being measured ρ t (10%~70%), the total power of the signal light laser emission P t The receiving surface area A of the telescope system on the target 7 is 5mW. t 3.14cm 2 、Aperture area A of telescope system 5 r 3.14cm 2 , the spot area A on the target being measured l 0.125cm 2 , the distance R between the target and the vibration measurement system is 10cm to 3m. Substituting the above parameters into the radar equation and comparing them with the measured return light energy diagram of the system, it is found that the data has a good linear correlation; the signal-to-noise ratio diagram is as follows Figure 5 As shown, the ordinate is the signal-to-noise ratio (SNR) value at different distances, the abscissa is the sampling number, the ordinate is the signal-to-noise ratio value, and different colors represent the signal-to-noise ratio curves at different distances. As the sampling number of the abscissa increases, it is observed that when the return light conditions are good, the signal-to-noise ratio is high and the data is stable. When the test distance increases, the system return light becomes worse, and the signal-to-noise ratio begins to fluctuate. This shows that after the laser vibration measurement system is added with red light indication, self-developed achromatic lens and supporting structural parts, through focal length rotation, it can not only meet the needs of finding the object to be measured through signal light more quickly and accurately at a long distance, but also ensure the detection capability of the detection system.

[0058] The invention solves the problem that the artificial light spot alignment of the laser Doppler vibrometer is difficult and the alignment efficiency is low in the long-distance test. By designing a solution of optical fiber beam combining coaxial indicator light and an achromatic telescope system, the spot sizes of visible light and near-infrared light are consistent at different distances. The detection range of the laser vibrometer from near to far is realized by continuous zooming. The position where the indicator light is hit is the position where the signal light is located. Figure 3 From simulation calculation, it can be found that when the optimization mode of the evaluation function of Zemax software is set to the spot diagram, the system will automatically optimize the spot diagram to the minimum, so that the light spot can be observed well at a long distance, thus improving the efficiency of people operating the instrument.

[0059] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A coaxial confocal laser vibrometer system, characterized in that: include: Signal light laser, beam splitter, circulator, beam combiner, telescope system, indicator light laser, mixer; The signal light laser is connected to the beam splitter and is used to emit the signal laser to the beam splitter; The beam splitter is connected to the circulator and the mixer, and is used to split the received signal laser according to a preset ratio, one beam is transmitted to the circulator as signal light, and the other beam is transmitted to the mixer as local oscillator light; The circulator is connected to the beam combiner and the mixer, and is used to transmit the signal light to the beam combiner, receive the return light signal of the signal light transmitted by the beam combiner, and feed it back to the mixer; The mixer is used to couple the local oscillator light emitted by the beam splitter and the return light signal of the signal light output by the circulator, so that the return light signals of the local oscillator light and the signal light interfere with each other; The beam combiner is connected to the telescope system and the indicator light laser, and is used to receive the indicator light emitted by the indicator light laser, transmit the signal light and the indicator light to the telescope system, and receive the return light signal of the signal light transmitted by the telescope system; The telescope system is used to transmit signal light and indicator light, and receive the return light signal of the signal light.

2. The coaxial confocal laser vibrometer system according to claim 1, characterized in that: The beam combiner is a fiber beam combiner, which utilizes total reflection of optical fibers to transmit light, thereby achieving coaxial transmission of signal light and indicator light.

3. The coaxial confocal laser vibrometer system according to claim 1, characterized in that: The signal light laser is a 1550nm band narrow line width polarization maintaining laser; the indicator light laser is a 650nm laser.

4. The coaxial confocal laser vibrometer system according to claim 1, characterized in that: The telescope system comprises a focus-adjustable structure and a lens, wherein the lens is engaged with the focus-adjustable structure.

5. The coaxial confocal laser vibrometer system according to claim 4, characterized in that: The lens is a double-cemented lens, which is formed by bonding concave and convex lenses.

6. The coaxial confocal laser vibrometer system according to claim 5, characterized in that: In the double-cemented lens, the refractive index of the convex lens is less than 1.62, and the Abbe number is less than 28; the refractive index of the concave lens is greater than 1.8, and the Abbe number is greater than 60.

7. The coaxial confocal laser vibrometer system according to claim 4, characterized in that: The focus-adjustable structure includes a rotary member and interfaces and slots at both ends of the rotary member; the interface is a fiber head socket connected to the beam combiner; the slot is used to engage the lens to achieve coaxiality between the lens and the focus-adjustable structure, and the rotary member 52 realizes forward and backward rotation focusing.

8. The coaxial confocal laser vibrometer system according to claim 1, characterized in that: It also includes the target to be measured, with adjustable frequency, amplitude and distance.

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