Target rotating speed detection device based on self-feedback rotating Doppler effect

Abstract

The present application relates to a rotational speed detection device based on self-feedback rotating Doppler effect. Its main components include microchip laser, lens, light splitting prism, photodetector, acousto-optic frequency shifter, polarizer, quarter wave plate, vortex plate. The laser generated by the microchip laser is expanded by the lens and then split into two beams by the light splitting prism. The reflected beam irradiates on the photodetector, and the transmitted beam is frequency shifted twice by the acousto-optic frequency shifter to generate a frequency shift Δf smaller than the relaxation oscillation frequency of the microchip laser. Then, the polarization state of the beam is controlled by the polarizer and the quarter wave plate, and then transmitted through the vortex plate to become vortex light. After passing through another linear polarizer, the polarization state of the vortex beam can be further modulated. Finally, the vortex light irradiates on the surface of the rotating object to generate a rotating Doppler shift f mod , and the scattered light returns to the microchip laser cavity again. After amplification in the laser cavity, the photodetector can detect the size of the rotating Doppler shift, thereby realizing the measurement of the rotational motion information of the target. The device has the outstanding advantages of compact structure and high sensitivity.

Description

TECHNICAL FIELD

[0001] The present application mainly relates to the field of optics, physics, laser, photoelectric conversion, and particularly to the technical methods of light field regulation of light beams and beat frequency detection. TECHNICAL BACKGROUND

[0002] The characteristics and performance of a laser can be seriously affected or modulated by the feedback light. The electric field re-entering the laser cavity is highly coherent under the reflection condition, but it is only partially coherent or weakly coherent if the light beam is scattered back by a scattering surface or a scattering body. Since the discovery of laser, the laser feedback phenomenon has become the source of many laser problems, such as increasing the noise and instability of the laser. On the other hand, controlled laser feedback has great practical use, such as playing an important role in narrowing the linewidth of the laser and improving the frequency stability.

[0003] In recent years, with the in-depth study of the principle of laser feedback interference, a measurement system based on laser feedback interference has been invented. The intensity of the laser is modulated by the feedback light from the external object, which is closely related to the reflectivity of the target surface, the distance, and the motion parameters. The first scheme for measuring the distance and velocity based on laser feedback interference was proposed by King and Steward in 1963. In the following years, phase-sensitive techniques were also used to measure the direction of target motion, measure the axial mode number of the laser, measure the velocity, measure the frequency stability of the laser, and metrology applications.

[0004] In the frequency conversion feedback, the microchip laser exhibits very high feedback sensitivity (10^6). This characteristic has been widely used in various physical measurements, such as Doppler velocimeter based on feedback laser, optical feedback vibration meter, optical feedback tomography technology, and optical feedback displacement measurement. Due to its high sensitivity, microchip laser feedback measurement has unique advantages in detecting weak signals. According to the rate equation model, the feedback light intensity of the microchip laser under the frequency conversion condition can be represented as,

[0005]

[0006] where ΔI represents the intensity modulation signal of the laser, I s is the output light intensity of the laser in the stable state (without feedback light and quantum limit noise), I s can be obtained by solving the rate equation of the laser; κ is the light intensity feedback factor determined by the external cavity, φ is the external cavity phase caused by the external cavity length, φ s is a fixed phase; G(x) is the feedback light amplification factor, which is related to the frequency. When the feedback light frequency shift 2Ω is close to the relaxation oscillation frequency ω rThis feedback amplification factor has a maximum value at the time. For microchip lasers, this amplification factor can reach 10 6 .

[0007] Because of the laser self-feedback effect, the frequency shift of the weak feedback light can be detected effectively, so it has a wide application prospect in metrology. Doppler effect is a common phenomenon in nature. According to Einstein's relativity principle and the principle of invariance of light speed, the linear Doppler effect in classical optics can be expressed as:

[0008]

[0009] Where Δf1 represents the frequency shift of the light beam irradiating on the moving object surface, f c represents the frequency of the probe light beam itself, v represents the relative motion speed between the object and the wave source, and c represents the light speed.

[0010] In 1992, Allen et al. first proposed the concept that the structured light beam with a helical phase factor may carry orbital angular momentum (OAM), which triggered a research boom on the structured light beam with a helical phase. The light beam with a helical phase is called vortex light, which has a ring-shaped light intensity distribution. The wavefront of the vortex light beam is no longer a plane, but a helical phase surface, resulting in that the Poynting vector of the light beam is no longer parallel to the direction of the light beam propagation, but has an angle. Because of the existence of the angle, we can decompose the Poynting vector of the vortex light beam into two directions along the light beam propagation and perpendicular to the light beam cross section, according to this characteristic, when the vortex light beam interacts with a moving object, it will simultaneously respond to the motion along the light beam propagation and perpendicular to the light beam propagation.

[0011] In 2013, Padgett et al. of Glasgow University in the UK systematically summarized and proposed a scheme for detecting the rotation speed of a rotating object by using superposition state vortex light, which first realized the measurement of the rotation speed of a rotating object by using vortex light beam, and revealed the great application potential of vortex light detection. The related results were published in Science. The angle between the Poynting vector of the vortex light and the optical axis can be expressed as sinβ = lλ / 2πr, where l is the topological charge number of the vortex light, λ represents the wavelength of the light beam, and r is the distance between any position in the light field and the optical axis. When a vortex light beam is aligned with the rotation axis to detect a rotating object, for each micro scattering body, there is an angle between the Poynting vector of the light beam and the optical axis. By substituting the linear Doppler frequency shift formula under the inclination condition, we can get:

[0012]

[0013] Wherein, Δf2 represents the rotational Doppler shift generated by the vortex light irradiating on the surface of the rotating object. SUMMARY

[0014] The technical problem solved by the present application is:

[0015] For the wide existence of rotational motion in nature, non-cooperative, high-precision, non-contact measurement of target rotating speed has broad application scenarios, and the existing measurement means is difficult to realize high-precision real-time measurement when measuring the rotating target with no obvious features on the surface, in addition, it is also difficult to realize effective extraction of the rotating signal when the object surface scattering is weak. The present application innovatively combines the rotational Doppler effect of the structured light beam with the laser self-feedback coupling detection technology, which can realize the rotating speed information acquisition of the non-cooperative and weak scattering rotating object. The device has simple structure, small size and is easy to integrate, and can be developed into a portable rotating speed measurement equipment.

[0016] The technical solution of the present application is: the present application relates to a target rotating speed detection device based on self-feedback rotational Doppler effect, as shown in Figure 1 , the main components include: microchip laser (1), convex lens (2), beam splitter prism (3), photodetector (4), acousto-optic frequency shifter 1 (5), acousto-optic frequency shifter 2 (6), polarizer 1 (7), quarter wave plate (8), vortex plate (9), polarizer 2 (10). First, the microchip laser (1) generates a laser with a frequency of f0, which is subjected to the joint action of two acousto-optic frequency shifters to generate a frequency shift of Δf0. At this time, the beam frequency is f0+Δf0.

[0017] Secondly, the light beam after frequency shift by the acousto-optic frequency shifter becomes vortex light with topological charge number m after passing through the vortex plate (9) with order m again. Then it irradiates on the surface of the rotating object with rotating speed Ω. Due to the influence of rotational Doppler effect, the scattered light from the object surface will have a frequency shift of f mod .

[0018] Next, the scattered light returns through the original light path, and the frequency becomes f0+2Δf0+f mod after passing through the two acousto-optic frequency shifters again. And it returns to the laser resonant cavity, and after amplification in the laser resonant cavity, it forms beat frequency coupling with the original light beam with frequency f0.

[0019] Finally, the light beam emitted from the microchip laser irradiates on the photodetector (4) after being reflected by the beam splitter prism (3), and the feedback light frequency shift of 2Δf0+f mod can be detected. At this time, the fixed frequency shift introduced by the acousto-optic frequency shifter is subtracted, and the rotating speed of the rotating object can be calculated according to the formula .

[0020] The principle of the present application is:

[0021] (1) Laser self-feedback effect

[0022] Since the discovery of laser, laser feedback phenomenon has been the source of many laser problems, such as increasing the noise and instability of the laser. On the other hand, controlled laser feedback has great practical use, such as the phenomenon of narrowing the laser linewidth and improving the frequency stability. One of the potential applications is the laser feedback interferometry system, the intensity of the laser is modulated by the external surface feedback light. This feedback phenomenon will be affected by the reflectivity, distance and motion of the target surface. The first laser feedback interferometry distance and velocity measurement scheme was proposed by King and Steward in 1963. In the following years, phase-sensitive techniques were also used to measure the direction of target motion, measure the axial mode number of the laser, measure the velocity, measure the frequency stability of the laser, and metrology applications.

[0023] According to the L-K rate formula and the laser self-feedback effect, the laser output photon rate can be represented as,

[0024]

[0025] Under the condition of weak light feedback, the gain of DBR fiber laser can be represented as,

[0026]

[0027] where η represents the normalized pump coefficient, which depends on the ratio of laser output to laser threshold; Ω r = 2πf r represents the laser relaxation oscillation frequency, <p out > and φ s represent the stable laser output photon rate and the fixed additional phase, respectively. In addition, γ c = 1 / τ c , γ1= 1 / τ1represent the cavity damping rate and the population inversion damping rate.

[0028] From the above formula, when 2Ω = Ω r , the gain rate G(2Ω) has a maximum value, which will cause a very large self-feedback modulation enhancement effect. The Doppler frequency shift angle frequency generated by the target is ω D = 2πf D , and the frequency shift generated by the frequency shift device (such as an acousto-optic frequency shifter) is Ω = 2πf. Because the feedback light enters the laser cavity twice through the frequency shift device, the change of the frequency shift of the feedback laser relative to the laser generated in the cavity is ω D + 2Ω, and the modulation frequency of the beat frequency intensity formed by the feedback light and the laser in the cavity is also ω D + 2Ω.

[0029] According to the device diagram shown in the last section, the light beam emitted from the microchip laser can be represented as,

[0030]

[0031] After passing through the acousto-optic frequency shifter (AOM1 and AOM2), the frequency of the light beam becomes,

[0032]

[0033] where Δω1 and Δω2 represent the frequency shift of the two acousto-optic frequency shifters, respectively. This is the final frequency of the laser emitted from the laser. In the implementation of the feedback optical path, after the probe light beam passes through the object, it will pass through the acousto-optic frequency shifter again in the return optical path, at which time the reflected light becomes a light beam that has undergone frequency shift. After the feedback passes through the acousto-optic frequency shifter, the light beam becomes,

[0034]

[0035] where Δf represents the frequency shift amount generated after the scattered light interacts with the moving object.

[0036] In order to make the frequency shift generated by the feedback light near the relaxation oscillation frequency of the microchip laser, the frequency shift directions of the two acousto-optic frequency shifters should be opposite to each other. Therefore, the final signal frequency detected by the probe through this feedback interference system is,

[0037] Δf s = 2(Δω1+Δω2)+Δf (9)

[0038] where Δω1 and Δω2 are known quantities set in the experimental system, so that after feedback amplification, the light beam frequency shift Δf caused by the moving target can be detected.

[0039] (2) Based on the principle of rotational Doppler effect of vector vortex light

[0040] The distribution of the liquid crystal principal axis of the vortex wafer in its cross-section in the polar coordinate system can be represented as:

[0041]

[0042] where m is the order of the vortex wafer, which is an integer multiple of 0.5. α0 is the initial principal axis direction at .

[0043] The Jones matrix of the vortex wafer can be represented as,

[0044]

[0045] where a is determined by equation (10). The polarization state of any beam can be obtained by multiplying the Jones vector of the incident light with the Jones matrix of the optical element.

[0046] The electric field distribution and its polarization state of the laser with wavelength of 1064 nm emitted from the microchip laser (Nd-YVO) can be expressed in cylindrical coordinates as,

[0047]

[0048] where is the angular coordinate, and k is the wave vector of the plane wave. After passing through a QWP placed at 45°, the linearly polarized light will become left-handed circularly polarized light. Equation (12) becomes,

[0049]

[0050] If the above beam is transmitted through a vortex plate with topological charge m, a right-handed vortex beam with the same polarization state will be generated. This process can be expressed as,

[0051]

[0052] It can be seen that the polarization state of the beam is reversed in the above process.

[0053] Here, the conversion from a plane wave to a vortex wave in a self-feedback interference system is completed. Next, the interaction of the beam with an object is considered.

[0054] According to the spiral spectrum decomposition theory, the surface phase of any object can also be represented by a series of spiral spectra. If a pure phase object is considered to rotate at an angular velocity Ω, its surface phase can be represented as,

[0055]

[0056] In this experiment, a spatial light modulator is used to simulate a rotating pure phase object. The spatial light modulator only responds to horizontally polarized light, so the circularly polarized vortex light is adjusted to be horizontally polarized vortex light by passing through a QWP before illuminating the SLM. After the circularly polarized vortex light of equation (14) passes through the QWP and is coaxially illuminated on the rotating object surface, the light field of the scattered light can be represented by the product of the incident light and the phase of the scattered light, that is,

[0057]

[0058] where n represents the spiral order of the pure phase-rotating target added on the SLM, which is usually unknown, but the order of this spiral can be determined by the difference between the topological charge of the scattered light and the incident light. After knowing the order of the received scattered light, the order of the spiral on the object surface can be expressed as n = 1 s - m. At this time, it can be seen that when only the base film (l = 0) order component of the scattered light is received, the frequency shift Δf generated by the scattered light is determined by the size of the topological charge of the incident light, that is, Δf = -mΩ.

[0059] Due to the addition of the blazed grating on the SLM, the light beam reflected by the simulated object will no longer return along the original light path, but will have a certain angle with the return path. In order to verify the effect of vortex light self-feedback measurement as much as possible, the above light beam is adjusted to the original light path through a mirror. At this time, the scattered light beam is still a horizontally linearly polarized light beam, which becomes a right-handed circularly polarized light beam after being reversely transmitted through the QWP. The liquid crystal principal axis of the vortex sheet is observed in reverse again, which is exactly chiral symmetric with the phase angle shown in formula (10). At this time, combined with formula (10), the transmitted light can be expressed as,

[0060]

[0061] where the beam phase Φ = kz- ωt+ α0, α0 represents the initial azimuth angle of the vortex sheet principal axis, and when α is zero, this phase term is the same as the phase of the outgoing light. After being filtered by the vertical polarizer again, the feedback light will enter the feedback cavity and interfere with the local light to generate beat frequency.

[0062] The feedback light is obviously different from the outgoing light in that the spiral phase factor is added. After passing through the vortex sheet twice, the phase factor becomes and the rotation frequency shift term inΩt, which depends on the rotation speed and the topological charge of the rotating phase object. Since the base film can only resonate with the local light after the feedback light enters the laser cavity, the frequency detected finally is only the frequency shift of the base film component in formula (17), and the condition for the base film component in the above formula is 2m+n = 0, that is, the order of the spiral on the object surface is twice the order of the detected vortex light. At this time, the size of the generated frequency shift is,

[0063] Δf = 2mΩ (18)

[0064] It is worth noting that, unlike the traditional rotating Doppler effect expression, although a single-state vortex light is used as the probe light here, the rotating Doppler frequency shift generated is twice that of the ordinary single-state vortex light probe.

[0065] (3) Coherent measurement of light beam

[0066] As can be seen from the patent device figure, the signal finally collected by the photoelectric detector is the mixed frequency laser signal emitted from the laser. The frequency shift is generated because the scattered light from the object surface of the laser external cavity is reflected back into the laser, amplified by the resonant cavity and coherently frequency-mixed with the laser in the cavity, and the basic principle is as follows:

[0067] The expressions of the feedback light and the local laser light can be written as And Where A0 represents the light intensity, w represents the angular frequency of the light wave, t represents time, (k is the wave number, is the distance of the light beam along the propagation direction, is the initial phase) represents the initial phase and the phase change caused by the propagation distance. After the experimental light beam is reflected by the object and superimposed with the detection light, the light intensity at the photoelectric detector can be represented as:

[0068]

[0069] The frequency of the laser is generally 10 14 orders of magnitude, far exceeding the response frequency of the photoelectric detector, so the double frequency term in formula (19) will become a direct current component, and the difference frequency term can generally fall within the response range of the photoelectric detector according to its size range, so the period size change of the difference frequency term is reflected in the signal of the photoelectric detector.

[0070] In the present patent, the laser light source generates a linearly polarized light beam with the same frequency, which is still linearly polarized after being modulated by the object outside the cavity and entering the laser resonant cavity, so that the frequency-mixed interference with the laser in the cavity can be realized. The frequency shift of the external feedback light is often directly related to the movement of the object, so that the movement information of the object can be obtained. This detection method using beam coherence, removing the common mode part and retaining the difference mode part, is widely used in high-frequency electrical wave detection.

[0071] The main advantages of the present invention are:

[0072] (1) The device is novel in technology, simple to operate, convenient to use and easy to control.

[0073] (2) The device has high sensitivity when detecting targets with weak surface scattered light. According to the design principle, it can be seen that the device has a self-feedback amplification effect of laser, which can amplify the small light intensity through resonant cavity, and has high sensitivity.

[0074] (3) The device has high detection precision. Since the movement of external objects will change in nature, and the displacement can be measured by laser interference with a detection precision of laser wavelength level, the detection precision can be realized at the nanometer level.

[0075] (4) The device uses light wave as detection medium, has long transmission distance, fast measurement speed, is not affected by temperature, environment and other factors, has rapid response and wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0076] Figure 1 A schematic diagram of the detection device is shown in the figure.

[0077] Figure 2 Single-state left / right circularly polarized vortex beam;

[0078] Figure 3 Superposition-state linearly polarized vector vortex beam;

[0079] Figure 4 Detection results at rotational speeds of 1700 deg / s and 1600 deg / s, respectively;

[0080] Figure 5 Direction identification detection results DETAILED DESCRIPTION

[0081] The present application detects based on laser self-feedback effect and vector vortex light regulation technology, and the specific implementation steps are as follows:

[0082] First, as shown in the basic device principle diagram, the microchip laser (1) generates laser with a frequency of f0, and after the joint action of two acousto-optic frequency shifters (5) and (6), a frequency shift of Δf0 is generated, and at this time the beam frequency is f0+Δf0. Figure 1 Second, the light beam after frequency shift by the acousto-optic frequency shifter again transmits through the vortex sheet (9) with order m and becomes vortex light with topological charge m. According to different angles of the 1 / 4 sheet (8) and the linear polarizer (10) in the device, the outgoing vortex light can present left circular polarization, right circular polarization (as shown in

[0083] ), radial vector polarization and linear polarization (as shown in Figure 2 ) several states. Then it irradiates the surface of a rotating object with a rotational speed of Ω, and due to the influence of the rotational Doppler effect, the scattered light from the surface of the object will have a frequency shift of f mod . Figure 3 Next, the scattered light returns through the original light path, and again transmits through the polarizer (10), the vortex sheet (9), the 1 / 4 sheet (8), the two acousto-optic frequency shifters (5) and (6), and then the frequency becomes f0+2Δf0+f mod , and returns to the laser resonant cavity (1). After amplification in the laser resonant cavity, beat coupling is formed with the original light beam with a frequency of f0.

[0084]

[0085] ​Finally, the light beam from the microchip laser (1) is reflected by the beam splitter prism (3) and irradiates the photoelectric detector (4) to detect the feedback light frequency shift of 2Δf0+f mod When the fixed frequency shift introduced by the acousto-optic frequency shifter is subtracted, the rotating object's rotational speed can be calculated according to the formula For example, when the target setting rotational speed is 5 rps, the feedback light frequency shift produced under the condition that the light beam modulation is superposition state vortex light is shown in Table 1; if the polarization state of the vortex light is changed and the vortex light beam modulation is single state vortex light, the frequency shift produced under the condition that the target rotates clockwise and counterclockwise will be distributed on different sides of the reference signal light, and the corresponding detection results are shown in Table 2. Figure 4 Figure 5

[0086] The contents not described in detail in the present application belong to the prior art known to those skilled in the art.​​

Claims

1. A target rotational speed detection device based on self-feedback rotational Doppler effect, the components of which include: Microchip laser (1), convex lens (2), light splitting prism (3), photodetector (4), acousto-optic frequency shifter 1 (5), acousto-optic frequency shifter 2 (6), polarizer 1 (7), quarter wave plate (8), vortex plate (9), polarizer 2 (10), each component is installed as follows: The convex lens (2) is arranged behind the microchip laser (1) in the light emitting direction, and is used for expanding and collimating the emitted laser; The light splitting prism (3) is arranged behind the convex lens (2), and the emitted laser is split into two beams; The photodetector (4) is arranged on the reflected light path of the light splitting prism (3), and is used for detecting the intensity change of the emitted laser; The acousto-optic frequency shifter 1 (5) is arranged on the projected light path of the light splitting prism (3), and is used for frequency shifting the detected light; The acousto-optic frequency shifter 2 (6) is arranged behind the acousto-optic frequency shifter 1 (5), and is used for frequency shifting the light beam again, so that the frequency shifting frequency of the light beam is near the relaxation oscillation frequency of the light source; The polarizer 1 (7) is arranged behind the acousto-optic frequency shifter 2 (6), and is used for modulating the frequency of the detected light into linear polarization; The quarter wave plate (8) is arranged behind the polarizer 1 (7), and by adjusting the fast axis direction of the quarter wave plate, left-handed, right-handed or linearly polarized light can be outputted; The vortex plate (9) is arranged behind the quarter wave plate (8), and is used for modulating the laser to generate vortex light; The polarizer 2 (10) is arranged behind the vortex plate (9), and is used for further modulating the polarization state of the vortex light, and generating detected vortex light with a specified polarization state.

2. The target rotational speed detecting device based on self-feedback rotational Doppler effect according to claim 1, characterized in that, The microchip laser (1) generates laser with frequency f0, which is acted on by two acousto-optic frequency shifters to generate a frequency shift of Δf0, at this time the frequency of the light beam is f0+Δf0, and after passing through the vortex sheet (9) with order m, the light beam becomes vortex light with topological charge m, and is irradiated on the surface of a rotating object with rotation speed Ω. Due to the influence of the rotating Doppler effect, the scattered light from the surface of the object will have a frequency shift of f mod , and the scattered light returns to the original light path, and after passing through the two acousto-optic frequency shifters again, the frequency becomes f0+2Δf0+f mod , and returns to the laser resonant cavity. After amplification in the laser resonant cavity, beat coupling is formed with the original light beam with frequency f0, so that the photodetector (4) can detect the feedback light frequency shift of 2Δf0+f mod . At this time, the fixed frequency shift introduced by the acousto-optic frequency shifter is subtracted, and the rotation speed of the rotating object can be calculated according to the formula .

3. The target rotational speed detecting device based on self-feedback rotational Doppler effect according to claim 2, characterized in that, The polarizer 1 (7) and the quarter wave plate (8) form a light beam polarizer, when the angle between the quarter wave plate (8) and the fast axis of the polarizer 1 (7) is +45°, the laser passing through the quarter wave plate is right-handed circularly polarized light, after passing through the vortex plate (9) with order m, the generated vortex light is single state m order left-handed circularly polarized vortex light, when the angle between the quarter wave plate (8) and the fast axis of the polarizer 1 (7) is -45°, the laser passing through the quarter wave plate is left-handed circularly polarized light, after passing through the vortex plate (9) with order m, the generated vortex light is single state m order right-handed circularly polarized vortex light, when the angle between the quarter wave plate (8) and the fast axis of the polarizer 1 (7) is 0°, after passing through the vortex plate with order m, the generated vortex light is superposition state ±m order radial polarization vortex beam.

4. The target rotational speed detecting device based on self-feedback rotational Doppler effect according to claim 2, characterized in that, The device has rotation target rotation direction identification capability, when the detection light beam is +m order single state circular polarization vortex light, if the rotation target rotation speed direction is clockwise, a rotation Doppler frequency shift f is generated mod The final photoelectric detector detects the feedback light frequency shift size 2Δf0+f mod If the rotation target rotation speed direction is counterclockwise, a rotation Doppler frequency shift-f is generated mod The final photoelectric detector detects the feedback light frequency shift size 2Δf0-f mod According to the frequency shift size relationship obtained by detection, the rotation direction information of the target to be detected can be judged.

5. The self-feedback rotation Doppler effect based target rotational speed detection device according to claim 1, wherein The detected light and the feedback light are completely in the same light path, and the transmitting and receiving are coaxial, the light beam scattered by the object surface and entering the laser cavity and the original laser in the cavity have a certain coherence.

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