A dual-loop laser doppler velocimetry method and system

Abstract

The application discloses a kind of double-loop laser doppler velocimetry method and system, the method includes: first, the incident laser beam is split into signal light and reference light;Signal light is again split into first signal light and second signal light parallel in emission direction, so that first signal light and second signal light irradiate to the surface of the measured moving object, and the two-way scattered light of original road returns is combined into one way;While modulating reference light, reference light is split into two ways and irradiated on the photosensitive surface of first detector and second detector together with scattered light, generating two-way beat frequency signals;Finally, the positive and negative input terminals of differential amplifier are connected respectively by first detector and second detector, and high signal-to-noise ratio signal is obtained, and then the speed and direction of moving object are measured simultaneously by signal processing solution.The application is applied to the field of optical measurement technology, and can effectively improve the utilization of light energy, solve the problem that the existing laser velocimeter cannot distinguish direction and the utilization of light energy is low.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical measurement, in particular to a dual-loop laser Doppler velocimetry method and system. BACKGROUND

[0002] Laser Doppler velocimetry technology is based on Doppler effect, which uses the Doppler shift of moving particles to obtain velocity information. As a new type of velocity sensor, it has the advantages of non-contact measurement, high precision, high spatial resolution, fast dynamic response, etc., and has been widely used in aerospace, medical monitoring, industrial measurement and other fields.

[0003] However, because the Doppler shift generated by the moving particles of equal size and opposite direction is the same, the traditional laser velocimeter cannot distinguish the direction of the object. In order to solve this problem, optical frequency shifting technology is often used to add frequency shifting devices in the optical path to shift the frequency and distinguish the direction of the velocity. However, the optical path with frequency shifting devices increases the number of beam splitters, which causes a large amount of measurement light to be split, resulting in low utilization of light energy. For objects with weak scattered light, the signal-to-noise ratio of the velocity measurement system is greatly reduced. SUMMARY

[0004] In view of the above problems in the prior art, the present application provides a dual-loop laser Doppler velocimetry method and system, which can effectively improve the utilization of light energy and solve the problems of the existing laser velocimeter that cannot distinguish the direction and low utilization of light energy.

[0005] To achieve the above purpose, the present application provides a dual-loop laser Doppler velocimetry method, comprising the following steps:

[0006] Based on the first beam splitting structure, the incident laser is split into signal light and reference light;

[0007] Based on the second beam splitting structure and the reflection structure, the signal light is split into first signal light and second signal light with parallel outgoing directions;

[0008] The first signal light and the second signal light are irradiated to the surface of the measured moving object, and based on the second beam splitting structure, the two paths of scattered light returned along the original path are combined into one path, and based on the third beam splitting structure, the scattered light combined into one path is split into two paths and irradiated on the light-sensitive surfaces of the first detector and the second detector, respectively;

[0009] Based on the acousto-optic frequency shifter, the reference light is modulated, and based on the third beam splitting structure, the reference light is split into two paths and irradiated on the light-sensitive surfaces of the first detector and the second detector, respectively, so that the light-sensitive surfaces of the first detector and the second detector generate beat signals;

[0010] The first detector and the second detector are connected to positive and negative input terminals of a differential amplifier respectively, so that a signal with high signal-to-noise ratio is obtained, and the speed and direction of the moving object are measured simultaneously through signal processing.

[0011] In one of the embodiments, the first detector and the second detector are symmetrically arranged on two sides of the third light splitting structure, and the first detector and the second detector receive beat signals with a phase difference of π.

[0012] In one of the embodiments, when the reference light is modulated based on the acousto-optic frequency shifter, the reference light is incident on the surface of the acousto-optic frequency shifter at a Bragg angle.

[0013] In one of the embodiments, after the reference light is modulated based on the acousto-optic frequency shifter, the reference light is irradiated on the third light splitting structure after passing through an aperture, so as to select the required first-order diffracted light.

[0014] In one of the embodiments, the light splitting ratio of the first light splitting structure is 95:5-70:30.

[0015] To achieve the above-mentioned purpose, the application further provides a dual-loop laser Doppler velocimetry system, which adopts the above-mentioned method to measure the speed and direction of the moving object simultaneously.

[0016] The laser Doppler velocimetry system comprises:

[0017] a laser for generating incident laser;

[0018] a first light splitting structure located on the optical path of the incident laser, for splitting the incident laser into signal light and reference light;

[0019] a second light splitting structure located on the optical path of the signal light, for splitting the signal light into transmitted light and reflected light, and combining the two paths of scattered light returned along the original path into one path, wherein the transmitted light is the first signal light;

[0020] a reflecting structure located on the optical path of the reflected light, for reflecting the reflected light to be parallel to the direction of the transmitted light, wherein the reflected light is the second signal light;

[0021] an acousto-optic frequency shifter located on the optical path of the reference light, for modulating the reference light;

[0022] a third light splitting structure located on the output optical path of the acousto-optic frequency shifter and on the optical path of the combined scattered light, for splitting the reference light and the combined scattered light into two paths and irradiating them on the light-sensitive surfaces of the first detector and the second detector respectively;

[0023] A first detector is located in the transmission direction of the third light splitting structure, and is used to receive one path of reference light and scattered light and generate beat frequency signals.

[0024] A second detector is located in the reflection direction of the third light splitting structure, and is used to receive another path of reference light and scattered light and generate beat frequency signals.

[0025] A differential amplifier has a positive input end and a negative input end, the positive input end is connected to the first detector, and the negative input end is connected to the second detector, which is used to obtain signals with high signal-to-noise ratio, and the speed and direction of the measured moving object are calculated through signal processing.

[0026] In one embodiment, the laser Doppler velocimeter system further comprises a diaphragm, which is located between the acousto-optic frequency shifter and the third light splitting structure.

[0027] In one embodiment, the reflection structure comprises a first plane full mirror, a second plane full mirror and a third plane full mirror.

[0028] The first plane full mirror, the second plane full mirror, the third plane full mirror and the second light splitting structure are arranged in a rectangular shape.

[0029] In one embodiment, the laser Doppler velocimeter system further comprises a fourth plane full mirror, which is located between the first light splitting structure and the acousto-optic frequency shifter.

[0030] Compared with the prior art, the present application has the following beneficial technical effects:

[0031] 1. The present application adopts a Michelson interferometer structure, is based on optical frequency shifting technology, uses an acousto-optic frequency shifter for frequency modulation, and realizes the measurement of the speed and direction of a moving object according to the change of the frequency of an output signal relative to a bias frequency.

[0032] 2. The present application changes the propagation direction of original useless reflected light into measurement light by adding a reflection structure, which can effectively improve the utilization rate of light energy.

[0033] 3. The present application uses balanced detection method for signal measurement, which can effectively enhance the amplitude of the signal and improve the measurement accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or the prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description only show some embodiments of the present application, and all other drawings obtained by those of ordinary skill in the art without creative effort based on these drawings also belong to the protection scope of the present application.

[0035] Figure 1 Figure 1 is a structural schematic diagram of a double-loop laser Doppler velocimeter system in the embodiment of the present application.

[0036] Figure 1 is a structural schematic diagram of a double-loop laser Doppler velocimeter system in the embodiment of the present application.

[0037] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the accompanying drawings. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0039] In addition, the technical solutions in each embodiment of the present application can be combined with each other, but it must be based on the fact that those of ordinary skill in the art can realize it. When the combination of technical solutions appears contradictory or unachievable, it should be considered that the combination of technical solutions does not exist and is not within the protection scope of the present application.

[0040] As Figure 1 Figure 1 is a structural schematic diagram of a double-loop laser Doppler velocimeter system in the embodiment of the present application.

[0041] Specifically, the single-frequency solid laser 1, the first light splitting structure 2, the second light splitting structure 3 and the surface of the measured moving object 11 are sequentially and spacedly arranged along a first direction, and the fourth plane full mirror 4, the acousto-optic frequency shifter 5, the diaphragm 6 and the third light splitting structure 7 are sequentially and spacedly arranged along a second direction. The first direction is parallel to the second direction, and the fourth plane full mirror 4 is located in the reflection direction of the first light splitting structure 2. The first light splitting structure 2, the second light splitting structure 3, the third light splitting structure 7 and the fourth plane full mirror 4 are in a rectangular structure distribution, and the first plane full mirror 8, the second plane full mirror 9, the third plane full mirror 10 and the second light splitting structure 3 are in a rectangular structure distribution. The first detector 12 is located in the transmission direction of the third light splitting structure 7, the second detector 13 is located in the reflection direction of the third light splitting structure 7, the differential amplifier 14 has a positive input end and a negative input end, the positive input end of the differential amplifier 14 is electrically connected with the first detector 12, and the negative input end of the differential amplifier 14 is electrically connected with the second detector 13.

[0042] In the specific implementation process, the first light splitting structure 2, the second light splitting structure 3 and the third light splitting structure 7 can all use a light splitting mirror, or other light splitting devices.

[0043] The specific working process of the double-loop laser Doppler velocimeter system in the embodiment is as follows:

[0044] The single-frequency solid laser 1 emits a light beam with a frequency of f0, which is split into two beams by the first light splitting structure 2. One of the beams is used as reference light and is incident on the acousto-optic frequency shifter 5 through the fourth plane full mirror 4 for modulation, and the modulation frequency is f s Under the Bragg diffraction condition, after the reference light passes through the acousto-optic frequency shifter 5, the high-order diffraction lights cancel each other out, and only the zero-order and first-order diffraction lights appear, so that a higher diffraction efficiency is obtained. Then, the diaphragm 6 is used to select the first-order diffraction light, i.e., the light with a frequency of f0+f sThe light beam is incident on the first detector 12 and the second detector 13 after passing through the third light splitting structure 7, and the light beam is reference light. Another light beam is incident on the second light splitting structure 3 after passing through the first light splitting structure 2, and the light beam is signal light. The signal light is split into two light beams, i.e., a transmitted light beam and a reflected light beam, wherein the transmitted light beam is directly incident on the measured moving object 11, and the reflected light beam is also incident on the measured moving object 11 after passing through the first plane full mirror 8, the second plane full mirror 9 and the third plane full mirror 10 in sequence. The two light beams are scattered by the measured moving object 11, and scattered light is scattered in each direction. Two light beams that return along the original direction are combined into one light beam after passing through the second light splitting structure 3. The light beam is incident on the first detector 12 and the second detector 13 after passing through the third light splitting structure 7, and the light beam is signal light. The signal light and the reference light are heterodyne interfered on the light-sensitive surface of the first detector 12 to obtain a photocurrent il, and the signal light and the reference light are heterodyne interfered on the light-sensitive surface of the second detector 13 to obtain a photocurrent i2. The photocurrent il and the photocurrent i2 are connected to the positive input terminal and the negative input terminal of the differential amplifier 14, respectively. The output of the differential amplifier 14 is a high signal-to-noise ratio signal, i.e., a differential current signal of the photocurrent. The differential current signal is processed by a subsequent signal processing circuit to calculate a corresponding frequency value, so that the size and direction of the speed of the measured moving object 11 are calculated.

[0045] In the embodiment, the relationship between the Doppler frequency and the speed of the laser Doppler velocimeter system is as follows:

[0046]

[0047] wherein V represents the speed of the measured moving object 11, θ represents the included angle between the signal light and the moving direction of the surface of the measured moving object 11, and λ represents the wavelength of the laser.

[0048] Suppose that the electric field expressions of the reference light and the signal light incident on the first detector 12 and the second detector 13 are as follows:

[0049]

[0050]

[0051] wherein E1 and E2 represent the amplitudes of the reference light and the signal light, respectively, f s represents the modulation frequency of the acousto-optic frequency shifter 5, f D represents the Doppler frequency, and represent the initial phases of the reference light and the signal light, respectively. The output current i(t) of the first detector 12 and the second detector 13 is proportional to the square of the amplitude of the light field, and high-frequency terms are ignored. Thus, the output current i(t) is as follows:

[0052]

[0053] Where B is a constant related to the detector itself.

[0054] According to the signal frequency f s -f D Relative to the modulation frequency f s If it is increased or decreased, the direction of the measured moving object 11 can be determined, that is, if it is defined that (f S -f D )-f S > 0, the speed direction of the measured object 11 is positive, then (f S -f D )-f S < 0, the speed direction of the measured object 11 is negative. Then by extracting the Doppler frequency f D in the signal, the speed of the measured moving object 11 can be obtained.

[0055] In order to realize the function of direction discrimination in this embodiment, two beam splitters are added compared with the traditional laser speed meter, forming a Mach-Zehnder ring structure, and only part of the light beam is used during detection, and the light energy is not fully utilized. In order to solve this problem, the first plane total reflection mirror 8, the second plane total reflection mirror 9 and the third plane total reflection mirror 10 are added on the reflected light path of the second beam splitting structure 3, forming a second loop, so that the originally wasted reflected light also becomes detection light.

[0056] Suppose the average scattering coefficient of the measured moving object 11 is β, the reflectivity of the second beam splitting structure 3 is r3, the transmittance is t3, and the absorption is zero, then r3+t3=1. Similarly, suppose the reflectivity of the third beam splitting structure 7 is r7, the transmittance is t7, and r7+t7=1. Then the scattering light signal P S1 received by the first detector 12 is:

[0057] P S1 = (P0r3βt3+P0t3βr3)r7=2P0βr3t3r7

[0058] Where P0 is the light power transmitted from the first beam splitting structure 2 to the second beam splitting structure 3. When r3=t3, P S1 takes the maximum value of

[0059] Similarly, the scattering light signal received by the second detector 13 is P S2 =2P0βr3t3t7, the maximum value is

[0060] The maximum scattered light signal that the first and second detectors 12 and 13 can measure is or

[0061] Meanwhile, according to the relationship between the signal-to-noise ratio and the optical power:

[0062]

[0063] wherein η is the quantum efficiency of the detector, e is the electronic charge, h is the Planck constant, v is the central frequency of the laser, and Δ is the bandwidth of the detector.

[0064] According to the above formula, the signal-to-noise ratio is doubled after the reflected light of the second light splitting structure 3 is utilized in the embodiment.

[0065] Meanwhile, in the traditional optical path, only one detector is used for heterodyne coherent detection, and only the partially transmitted light of the signal light and the partially reflected light of the reference light (or the partially reflected light of the signal light and the partially transmitted light of the reference light) are received, and the optical energy is not fully utilized. Therefore, two identical photoelectric detectors (i.e., the first detector 12 and the second detector 13) are symmetrically placed on both sides of the third light splitting structure 7 in the embodiment, and balanced detection is used for detection, so that the energy of the two beams of light is fully utilized.

[0066] According to the balanced detection and heterodyne coherent detection principles, the output current i1 of the first detector 12 is:

[0067]

[0068] The output current i2 of the second detector 13 is:

[0069]

[0070] wherein A1 and B1 are the amplitude of the direct current component and the alternating current component of the output current i1, respectively, and A2 and B2 are the amplitude of the direct current component and the alternating current component of the output current i2, respectively.

[0071] If the splitting ratio of the third light splitting structure 7 is 50:50, A1=A2 and B1=B2. After the differential amplifier 14, the output current i is:

[0072]

[0073] As can be seen from the above formula, the detection method can suppress the DC in the signal and enhance the amplitude of the AC component, making it twice the original amplitude. In combination with the use of the reflected light in the previous step, the signal strength measured by the laser Doppler velocimetry system of the double-loop in this embodiment is enhanced to four times the original.

[0074] The above only describes the preferred embodiments of the present application, and does not limit the patent scope of the present application. Any equivalent structural transformation made under the inventive concept of the present application, using the content of the present application specification and drawings, or direct / indirect application in other related technical fields is included in the patent protection scope of the present application.

Claims

1. A dual-loop laser Doppler velocimetry method, characterized by, The method comprises the following steps: The incident laser is split into signal light and reference light based on a first light splitting structure; The signal light is split into first signal light and second signal light with parallel exit directions based on a second light splitting structure and a reflection structure; The first signal light and the second signal light are irradiated to the surface of the moving object under test, and the two paths of scattered light returning along the original path are combined into one path based on the second light splitting structure, and then the scattered light combined into one path is split into two paths based on a third light splitting structure and irradiated on the light sensitive surfaces of a first detector and a second detector respectively; The reference light is modulated based on an acousto-optic frequency shifter, and then the reference light is split into two paths based on the third light splitting structure and irradiated on the light sensitive surfaces of the first detector and the second detector respectively, so that the light sensitive surfaces of the first detector and the second detector both generate beat signals; The first detector and the second detector are respectively connected to the positive and negative input terminals of a differential amplifier to obtain signals with high signal-to-noise ratio, and the signals are processed to realize the simultaneous measurement of the speed, size and direction of the moving object.

2. The dual-loop laser Doppler velocimetry method of claim 1, wherein, The first detector and the second detector are symmetrically arranged on the two sides of the third light splitting structure, and the beat signals received by the first detector and the second detector are out of phase by π.

3. The dual-loop laser Doppler velocimetry method of claim 1, wherein, When the reference light is modulated based on the acousto-optic frequency shifter, the reference light is incident on the surface of the acousto-optic frequency shifter at a Bragg angle.

4. The dual-loop laser Doppler velocimetry method according to claim 1 or 2 or 3, characterized in that, After the reference light is modulated based on the acousto-optic frequency shifter, the reference light passes through an aperture and is then irradiated on the third light splitting structure to select the required first-order diffracted light.

5. The dual-loop laser Doppler velocimetry method according to claim 1 or 2 or 3, characterized in that, The light splitting ratio of the first light splitting structure is 95:5-70:

30.

6. A dual-loop laser Doppler velocimetry system, characterized by, The method of any one of claims 1-5 is used to realize the simultaneous measurement of the speed, size and direction of the moving object. The laser Doppler velocimeter system comprises: a laser for generating incident laser; a first light splitting structure located on the optical path of the incident laser for splitting the incident laser into signal light and reference light; a second light splitting structure located on the optical path of the signal light for splitting the signal light into transmitted light and reflected light, and combining the two paths of scattered light returning along the original path into one path, wherein the transmitted light is the first signal light; a reflection structure located on the optical path of the reflected light for reflecting the reflected light to be parallel to the exit direction of the transmitted light, wherein the reflected light is the second signal light; an acousto-optic frequency shifter located on the optical path of the reference light for modulating the reference light; a third light splitting structure located on the output optical path of the acousto-optic frequency shifter and on the optical path of the scattered light combined into one path, for splitting the reference light and the scattered light combined into one path into two paths and irradiating them on the light sensitive surfaces of a first detector and a second detector respectively; the first detector located in the transmission direction of the third light splitting structure for receiving one path of reference light and scattered light and generating beat signals; the second detector located in the reflection direction of the third light splitting structure for receiving the other path of reference light and scattered light and generating beat signals; The differential amplifier has a positive input end and a negative input end, the positive input end is connected with the first detector, the negative input end is connected with the second detector, is used for obtaining a signal with high signal-to-noise ratio, and is used for obtaining the speed and direction of the measured moving object through signal processing.

7. The dual-loop laser Doppler velocimetry system of claim 6, wherein, Further comprising a light barrier between the acousto-optic frequency shifter and the third light splitting structure.

8. The dual-loop laser Doppler velocimetry system of claim 6, wherein, The reflection structure comprises a first plane total reflector, a second plane total reflector and a third plane total reflector. The first plane total reflector, the second plane total reflector, the third plane total reflector and the second light splitting structure are distributed in a rectangular shape.

9. The dual-loop laser Doppler velocimetry system of claim 6, wherein, Further comprising a fourth plane total reflector between the first light splitting structure and the acousto-optic frequency shifter.

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