Rotary Doppler effect coherent detection method based on multimode receiving

Abstract

The invention relates to the technical field of photoelectric detection and signal processing, and particularly discloses a rotating Doppler effect coherent detection method based on multi-mode receiving, which comprises the following steps: irradiating a rotating object by using a fundamental mode Gaussian beam at a transmitting end, and carrying out amplitude and spiral phase modulation on the Gaussian detection beam through the rotating object, forming a scattered light field; a vortex light beam is used as reference light, frequency beating is carried out on fundamental mode components in scattered light of the reference light and all OAM modes in received echo signal light of a rotating object, the different OAM modes in the echo signal light are mapped to a rotating Doppler frequency spectrum, and multi-mode receiving of the echo signal light is achieved. According to the method, the energy utilization rate of the echo signal during rotation Doppler effect measurement is improved, a single peak value of a frequency domain during traditional rotation Doppler effect measurement is changed into a plurality of peak values, cross validation and adaptive extraction of a detection result are facilitated, and rotation Doppler effect remote sensing practical application is promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of optoelectronic detection and signal processing, and particularly relates to a coherent detection method for rotational Doppler effect based on multimode reception. Background Art

[0002] The linear Doppler effect is the frequency change of a wave caused by the relative linear motion between a wave source and an observer. This effect is caused by linear motion in a Cartesian coordinate system and is related to the linear momentum of the wave. It is widely used in fields such as atmospheric measurement, object recognition, and motion monitoring. When there is a relative rotational motion between the wave source and the observer, there is also a Doppler effect - the rotational Doppler effect. This effect is the counterpart of the linear Doppler effect on a rotating object. It is caused by rotational motion in a polar coordinate system and is related to the angular momentum of the wave. Just as the linear Doppler effect requires a change in the linear momentum of the wave, the rotational Doppler effect requires a change in its angular momentum. In classical rotational Doppler effect measurements, a vortex beam is usually used as the detection beam. It is a special beam carrying orbital angular momentum, with a helical wavefront and a central phase singularity. The characteristic that the direction of its own Poynting vector has an angle with the beam propagation direction is used to detect the rotational Doppler frequency shift signal. However, the vortex light source increases the complexity of the entire system. At the same time, to ensure detection sensitivity, a vortex beam with a large topological charge number is often required as the detection beam, and the topological charge number is positively correlated with the far-field divergence angle of the vortex beam, which makes the detection sensitivity and detection distance restrict each other.

[0003] Currently, for the rotational Doppler effect measurement method based on receiver modulation, regardless of what kind of detection beam is emitted, a single-point detector at the receiver only receives a single specific orbital angular momentum mode in the scattered light field of the rotating object. Although using a planar array detector can separate different orbital angular momentum modes to different positions on the detector, limited by the frame rate of the planar array detector, it cannot detect high-frequency information and is not suitable for remote sensing applications of the rotational Doppler effect. According to the mode decomposition theory, the scattered light field can be decomposed into multiple orbital angular momentum modes. Receiving only a single mode will discard other modes, which is essentially a serious waste of echo energy and offsets to a certain extent the advantage of higher echo energy brought by using a Gaussian light source in lidar compared to a vortex light source with a smaller divergence angle. In remote sensing applications of the rotational Doppler effect, the detection distance will be greatly reduced. At the same time, in actual measurements, due to atmospheric turbulence disturbances, the proportion of each orbital angular momentum mode is unstable, further reducing the signal-to-noise ratio. Therefore, in actual rotational Doppler effect remote sensing, how to achieve multimode reception of orbital angular momentum modes during receiver modulation has become an urgent problem to be solved. Summary of the Invention

[0004] To achieve the object of the present invention, the present application provides a coherent detection method for the rotational Doppler effect based on multimode reception, including: Step S1: At the transmitting end, a fundamental-mode Gaussian beam is used to irradiate a rotating object, and the Gaussian detection beam is amplitude- and helical-phase-modulated by the rotating object to form a scattered light field; Step S2: At the receiving end, a vortex beam is used as a reference light. The fundamental-mode component in the scattered reference light is beat with each OAM mode in the received echo signal light from the rotating object, and different OAM modes in the echo signal light are mapped onto the rotational Doppler spectrum to achieve multimode reception of the echo signal light.

[0005] In some specific embodiments, Step S1 further includes: Using a fundamental-mode Gaussian beam as a detection beam to irradiate a rotating object, and the scattered signal light modulated by the rotating object includes orbital angular momentum modes of different orders.

[0006] In some specific embodiments, Step S2 includes: At the receiving end, a vortex beam is used as a reference light, and different orbital angular momentum modes in the scattered signal light are mapped onto the rotational Doppler spectrum by using the principle that there is coherence between the scattered signal light and the fundamental-mode component of the scattered reference light.

[0007] In some specific embodiments, Step S1 further includes: When using a fundamental-mode Gaussian beam for detection, the detection beam is determined according to the following formula: ; where, represents the three coordinates in cylindrical coordinates, represents time, represents the initial light intensity of the Gaussian beam, represents the wave vector, represents the angular frequency.

[0008] In some specific embodiments, the scattered light field formed after the detection beam irradiates the rough surface of the rotating object is determined according to the light field function of the detection beam and the object modulation function: ; where, represents the reflectivity coefficient of a point on the rough surface in the direction pointing to the receiving end at that point, represents the order of the helical mode on the object surface, is the complex amplitude of the -order helical mode and has where

[0009] The beneficial effects of the above technical solutions: The technical solution provided by this application has strong compatibility and a wide range of applicable scenarios. It can be made compatible with the measurement of the rotational Doppler effect by simply modifying the receiving end of the existing lidar using the linear Doppler principle. Moreover, this method is still applicable under extreme conditions such as high-speed rotation. Using laser as the detection medium, it has a long transmission distance, a fast response speed, and is less affected by the environment, with broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the technical solutions in the embodiments of this application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of this application. For those skilled in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0011] Figure 1 Schematic flow chart of a rotational Doppler effect coherent detection method based on multimode reception provided by an embodiment of the present invention; Figure 2 Schematic structural diagram of a rotational Doppler effect coherent detection system based on multimode reception provided by an embodiment of the present invention; Figure 3 Schematic detection optical path diagram of a rotational Doppler effect coherent detection system based on multimode reception provided by an embodiment of the present invention; Figure 4 Schematic experimental result diagram of a rotational Doppler effect coherent detection system based on multimode reception provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0012] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, rather than all, of the embodiments of the present invention.

[0013] Examples of the embodiments are shown in the drawings, where the same or similar reference numerals indicate the same or similar elements or elements with the same or similar functions throughout. The embodiments described below with reference to the drawings are exemplary and are intended to explain the present invention, and should not be construed as limiting the present invention.

[0014] Embodiment 1 An embodiment of the present invention provides a rotational Doppler effect coherent detection method based on multimode reception. Referring to Figure 1 as shown, it includes: Step S1: Irradiate a rotating object with a fundamental mode Gaussian beam at the transmitting end, and perform amplitude and spiral phase modulation on the Gaussian detection beam through the rotating object to form a scattered light field.

[0015] In a specific embodiment of the present invention, step S1 further includes: Using a fundamental mode Gaussian beam as a probe beam to irradiate a rotating object, the scattered signal light modulated by the rotating object includes orbital angular momentum modes of different orders.

[0016] In a specific embodiment of the present invention, step S1 further includes: When using a fundamental mode Gaussian beam for detection, the probe beam is determined according to the following formula: ; Wherein, represents three coordinates in cylindrical coordinates, represents time, represents the initial light intensity of the Gaussian beam, represents the wave vector, represents the angular frequency.

[0017] In a specific embodiment of the present invention, the scattered light field formed after the probe beam irradiates the rough surface of the rotating object is determined according to the probe beam light field function and the object modulation function: ; Wherein, represents the reflectivity coefficient of a point on the rough surface in the direction pointing to the receiving end at that point, represents the order of the spiral mode on the object surface, is the complex amplitude of the order spiral mode and has is the angular velocity of the object.

[0018] Specifically, the linearly polarized Gaussian beam emitted by the laser is divided into two paths. One path is used as a probe beam to irradiate the rotating object, and the signal light field reflected and modulated by the rotating object enters the 50:50 beam splitter prism again. The other path passes through a quarter-wave plate, a vortex wave plate and a polarizer to generate a linearly polarized single-state vortex light. Only the fundamental mode is selected after the reference light interacts with the target. The scattered signal light and the fundamental mode component of the vortex reference light are coherently mixed in the 50:50 beam splitter prism, and after being focused by a lens and filtered by a small hole, they are transmitted to the photodetector. After Fourier transform, the multimode rotational Doppler signal is extracted in the first Fourier spectrum.

[0019] Step S2: Using the vortex beam as the reference light at the receiving end, the fundamental mode component in the scattered light of the reference light is beat with each OAM mode in the received echo signal light of the rotating object, and different OAM modes in the echo signal light are mapped to the rotational Doppler spectrum, realizing the multimode reception of the echo signal light.

[0020] In a specific embodiment of the present invention, step S2 includes: At the receiving end, a vortex beam is used as the local oscillator light, and different orbital angular momentum modes in the scattered signal light are mapped onto the rotational Doppler spectrum by utilizing the principle that there is coherence between the scattered signal light and the fundamental mode component of the reference light scattered light.

[0021] In terms of the signal energy utilization rate, by using a vortex beam as the reference light at the receiving end, different orbital angular momentum modes in the scattered signal light are mapped onto the rotational Doppler spectrum, greatly improving the utilization rate of the echo signal.

[0022] Specifically, when a fundamental mode Gaussian beam is used for detection, the detection beam can be expressed as: (1) where, represents the three coordinates in the cylindrical coordinates of the detection beam, represents time, represents the initial light intensity of the Gaussian beam, represents the wave vector, represents the angular frequency.

[0023] In actual detection, the rotating object often consists of a rough surface, which modulates both the amplitude and phase of the detection beam. The modulation function of the rough surface on the beam can be expressed as: (2) where, represents the reflectivity coefficient of a certain point on the rough surface in the direction pointing to the receiving end at that point, represents the order of the orbital angular momentum mode of the object surface, is the complex amplitude of the th order orbital angular momentum mode and has (3) When the object rotates at an angular velocity , the phase modulation function will add an angular variable that changes with time, expressed as: (4) The scattered light field formed after the detection beam irradiates the rough surface of the rotating object can be expressed as the product of the light field function of the detection beam and the object modulation function: (5) It can be seen that the scattered light field modulated by the rotating object contains numerous OAM modes of different orders, carrying different rotational Doppler frequency shifts. As the eigenmode of OAM, the LG beam is a set of orthogonal and complete basis vectors in the Hilbert space, forming a complete infinite-dimensional basis. With the increase of the topological charge number, the difficulty of receiving high-order modes gradually increases. Considering the influence of the aperture of the receiving system, only a finite number of OAM modes in the scattered light field can be received by the receiving system. To ensure the receiving efficiency of the receiving system, the probing beam should be irradiated onto the rotating object in a collimated state, so that each OAM mode in the scattered light field has a small far-field divergence angle to ensure the number of OAM modes entering the receiving system. The OAM modes received by the receiving system can be expressed as: (6) It can be seen that there are modes passing through the receiving system. At this time, we use a vortex beam as the reference beam, which can be expressed as: (7) Among them, is the initial light intensity of the vortex beam, is the topological charge number of the vortex beam.

[0024] Similarly, the scattered light field formed after the reference beam irradiates the rough surface of the rotating object can be expressed as the product of the reference beam light field function and the object modulation function: (8) If the reference beam is irradiated onto the rotating object in a focused state with a small spot size and a large far-field divergence angle, then each OAM mode in the scattered light field modulated by the rotating object has a large far-field divergence angle, and the high-order modes will be dissipated. Only the fundamental mode beam, that is, the component with OAM = 0, can return to the receiving system, which can be expressed as: (9) The probing light and the reference light are both reflected after being modulated by the rotating object and are coherent on the detector. The photocurrent generated can be expressed as: (10) Among them, represents the responsivity of the detector. It can be seen that the photocurrent generated by the signal contains three parts, namely the DC term, the beat signal between different modes of the signal light, and the beat signal between the signal light and the local oscillator light. Since the intensity of the local oscillator light is much greater than that of the signal light, the beat signal between different modes of the signal light can be ignored. The intermediate-frequency signal of the photocurrent can be approximately expressed as: (11) It can be seen that if Without changing the symbols, each OAM mode corresponds one-to-one with the beat frequency shift. In this way, the present application successfully realizes the multi-mode reception of the RDE signal in the scattered light field of the rotating object.

[0025] Specifically, a vortex beam is used as the reference light. The fundamental mode component in the scattered light of the reference light is beat with each OAM mode in the received echo signal light of the rotating object, and different OAM modes in the echo signal light are mapped onto the rotational Doppler spectrum, realizing the multi-mode reception of the echo signal light.

[0026] Embodiment 2 An embodiment of the present invention provides a rotational Doppler effect coherent detection system based on multi-mode reception, including: Beam emission module: used to irradiate a rotating object with a fundamental mode Gaussian beam, and the Gaussian detection beam is amplitude- and helical phase-modulated by the rotating object to form a scattered light field; Beam reception module: uses a vortex beam as the reference light. The fundamental mode component in the scattered light of the reference light is beat with each OAM mode in the received echo signal light of the rotating object, and different OAM modes in the echo signal light are mapped onto the rotational Doppler spectrum to achieve multi-mode reception of the echo signal light. In a specific embodiment of the present invention, the beam processing module is further used for: Using a fundamental mode Gaussian beam as the detection beam to irradiate a rotating object, and the scattered signal light modulated by the rotating object includes orbital angular momentum modes of different orders.

[0027] In a specific embodiment of the present invention, the beam scattering module is used for: At the receiving end, a vortex beam is used as the local oscillator light, and different orbital angular momentum modes in the scattered signal light are mapped onto the rotational Doppler spectrum by using the principle that there is coherence between the scattered signal light and the fundamental mode component of the scattered light of the reference light.

[0028] In a specific embodiment of the present invention, when a fundamental mode Gaussian beam is used for detection, the detection beam is determined according to the following formula: ; Wherein, represents the three coordinates in cylindrical coordinates, represents time, represents the initial light intensity of the Gaussian beam, represents the wave vector, represents the angular frequency.

[0029] In a specific embodiment of the present invention, the scattered light field formed after the detection beam irradiates the rough surface of the rotating object is determined according to the light field function of the detection beam and the object modulation function: ; Among them, represents the reflectivity coefficient of a certain point on the rough surface in the direction pointing to the receiving end at that point, represents the order of the spiral pattern on the object surface, is the complex amplitude of the n-th order spiral pattern and has , is the angular velocity of the object.

[0030] The present invention uses a fundamental mode Gaussian beam as the detection beam and a single-state vortex beam as the reference beam, and the implementation object is a spatially rotating object, specifically as follows: The optical path required for detection is as Figure 3 shown. First, the laser 1 generates a narrow-linewidth linearly polarized Gaussian beam with a wavelength of 532 nm. After passing through the 10:90 beam splitter prism 2, it is divided into two beams. One beam, as the detection beam, passes through the 50:50 beam splitter prism 9 and then irradiates onto the rotating object 8 simulated by the DMD. The signal light field modulated and reflected by the rotating object enters the 50:50 beam splitter prism 9 again. The other beam, as the reference beam, is reflected by the mirror 3 onto the quarter-wave plate 4. By adjusting the fast axis orientation of the quarter-wave plate 4, the polarization state of the beam incident on the vortex half-wave plate 5 can be adjusted. Thus, the sign and polarization state of the single-state vortex beam can be selected and prepared. Then, the linear polarizer 6 is used to adjust the circular polarization state of the reference beam to the same linear polarization state as the detection beam. Finally, it is reflected by the mirror 7 and enters the 50:50 beam splitter prism 9. In the 50:50 beam splitter prism 9, the fundamental mode components of the scattered signal light and the reference light are coherently mixed. After being focused by the lens 10 and filtered by the small hole 11, it is transmitted to the photodetector 12. After Fourier transform, the multi-mode rotational Doppler signal is extracted in the first Fourier spectrum. In this application, the DMD is used in the experiment to simulate the rotating object, and the polarization-preserving characteristic of the DMD is utilized to prevent the scattered light field of the rotating object from depolarizing. By loading different simulated rotational phase holograms on the DMD, the OAM components and ratios in the reflected light field can be quantitatively adjusted. In the experiment, this application loads to a total of 10-mode spiral phase holograms on the object.

[0031] For example, using the Figure 3 shown optical path, using a fundamental mode Gaussian beam as the detection beam, and using a vortex beam with a topological charge number as the local oscillator beam at the receiving end, detecting a rotating object with a rotational speed of 100. By adjusting the presence or absence of the local oscillator light, the effects before and after using this method are compared. The received scattered light signal is processed using the Figure 2 shown analysis process, and the experimental results are as Figure 4As shown, (a) and (b) are respectively the scattered light signals and Fourier spectra without the vortex local oscillator light. (c) and (d) are respectively the scattered light signals and Fourier spectra without the vortex local oscillator light. It can be seen that without the local oscillator vortex light, the orbital angular momentum component in the scattered light field cannot be received, and no rotational Doppler frequency shift can be observed. Only the fundamental frequency flicker signal of the light illuminating the object itself and its higher harmonics can be observed. After adding the vortex local oscillator light, the mode of the vortex local oscillator light and the to modes carried by the object's scattered light field have a topological charge sum of 17 to 26, and the frequency shift caused in the Fourier spectrum is a cluster of peaks with an interval of 100 Hz in the range from 1700 Hz to 2600 Hz. Ten different orbital angular momentum modes in the scattered light field correspond to different rotational Doppler frequency shifts, and the difference between the rotational Doppler frequency shifts of adjacent modes is exactly the product of the mode difference and the rotational speed of the object. Thus, the present application realizes the multi-mode reception of rotational Doppler signals and simultaneously utilizes all the orbital angular momentum modes carried in the scattered light field.

[0032] The above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, and all should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

[0033] Each embodiment in this specification is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other. The embodiments of the present invention are described with reference to the flowcharts and / or block diagrams of the methods, terminal devices (systems), and computer program products according to the embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, and the combination of the processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing terminal devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing terminal devices generate a device for realizing the functions specified in Figure 1 one process or multiple processes and / or blocks Figure 1 one block or multiple blocks. These computer program instructions can also be stored in a computer-readable memory that can guide the computer or other programmable data processing terminal devices to work in a specific manner, so that the instructions stored in the computer-readable memory generate a manufactured product including an instruction device, and the instruction device realizes the functions in the process Figure 1One process or multiple processes and / or boxes Figure 1 The functions specified in one box or multiple boxes. These computer program instructions can also be loaded onto a computer or other programmable data processing terminal device, so that a series of operation steps are executed on the computer or other programmable terminal device to generate a computer-implemented process. Thus, the instructions executed on the computer or other programmable terminal device provide steps for implementing the functions specified in Figure 1 One process or multiple processes and / or boxes Figure 1 The functions specified in one box or multiple boxes. Although the preferred embodiments of the embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the appended claims are intended to be construed as including the preferred embodiments and all changes and modifications that fall within the scope of the embodiments of the present invention. Finally, it should also be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or terminal device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or terminal device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the presence of additional identical elements in the process, method, article or terminal device comprising the element.

[0034] The method and device provided by the present invention have been introduced in detail above. Specific examples are used in this article to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

[0035] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", "one specific embodiment" or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A coherent detection method for rotational Doppler effect based on multimode reception, characterized in that, Including: Step S1: Irradiate a rotating object with a fundamental mode Gaussian beam at the transmitting end, and modulate the amplitude and helical phase of the Gaussian beam through the rotating object to form a scattered light field. Step S2: Use a vortex beam as a reference light at the receiving end. The fundamental mode component in the scattered reference light beats with each OAM mode in the received echo signal light of the rotating object, and different OAM modes in the echo signal light are mapped onto the rotational Doppler spectrum to achieve multimode reception of the echo signal light.

2. The coherent detection method of rotational Doppler effect based on multimode reception according to claim 1, characterized in that, Step S1 further includes: Use a fundamental mode Gaussian beam as a probing beam to irradiate the rotating object, so that the scattered signal light modulated by the rotating object includes orbital angular momentum modes of different orders.

3. The coherent detection method of rotational Doppler effect based on multimode reception according to claim 1, wherein Step S2 includes: Use a vortex beam as a reference light at the receiving end, and map different orbital angular momentum modes in the scattered signal light onto the rotational Doppler spectrum by utilizing the principle that there is coherence between the scattered signal light and the fundamental mode component of the scattered reference light.

4. The coherent detection method of rotational Doppler effect based on multimode reception according to claim 2, wherein Step S1 further includes: When using a fundamental mode Gaussian beam for probing, the probing beam is determined according to the following formula: ； Among them, represent the three coordinates in cylindrical coordinates, represents time, represents the initial light intensity of the Gaussian beam, represents the wave vector, represents the angular frequency.

5. The coherent detection method of rotational Doppler effect based on multimode reception according to claim 3, characterized in that, The scattered light field is determined according to the light field function of the probing beam and the object modulation function as: ； Among them, represents the reflectivity coefficient of a certain point on the rough surface in the direction pointing to the receiving end at that point, represents the order of the spiral pattern on the object surface, is the complex amplitude of the -order spiral pattern and has is the angular velocity of the object.

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