Laser detection background noise suppression system and method based on vortex light characteristics

Abstract

The invention relates to a laser detection background noise suppression system and method based on vortex light characteristics, and belongs to the technical field of laser detection and optical filtering. The response difference of laser (coherent light) and sunlight (incoherent light) under vortex phase modulation is utilized: signal light is subjected to phase modulation to form a hollow annular light ring of which the central intensity is zero and the main energy is expanded outwards on a focal plane; background light is not modulated due to phase incoherence, and is focused to form a solid round spot with highly concentrated central energy. Incoherent light spots located on the central axis are blocked in the physical space through the circular shielding piece, so that vortex signal light located in an annular area passes through in a lossless mode, and therefore the signal-to-noise ratio is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of laser detection and optical filtering technology, and relates to a laser detection background noise suppression system and method based on the characteristics of vortex light. Specifically, it relates to a device and method for separating signal and background noise by utilizing the difference in characteristics between laser (signal) and sunlight (background noise). Background Technology

[0002] Among numerous laser detection technologies, lidar plays a crucial role in atmospheric meteorology and environmental fields due to its three-dimensional imaging and high-precision ranging capabilities. Currently, there is an urgent need for high spatiotemporal resolution atmospheric observation data in research on weather forecasting, meteorological services, climate change, and the atmospheric environment. Therefore, research on all-weather lidar technology and methods is imperative. Background radiation and multiple scattering light reduce the signal-to-noise ratio of signals detected by lidar (optical detection and ranging) and optical communication systems. During daytime lidar detection, solar background light is the most significant source of interference noise. Noise reduction techniques for strong solar background light during the day have always been a key technology for achieving all-weather, high-precision lidar detection, and remain a major challenge in all-weather lidar detection.

[0003] Currently, denoising of solar background light can be broadly categorized into hardware denoising and software denoising. Existing hardware denoising methods, such as using narrowband or ultra-narrowband interference filters, can suppress solar background light to some extent, but are limited by bandwidth and can only be used for lidar detection at night and during dawn / dusk. FP interferometer filters have a relatively stable structure and are not limited by wavelength, but their manufacturing process is complex and costly. Using a small field-of-view telescope receiving system can reduce solar background noise and improve lidar detection capabilities. However, due to the spectral overlap between atmospheric echo signals and solar background light, as well as the nonlinear and non-stationary characteristics of the echo signals, hardware filtering methods cannot effectively filter out solar background light. In recent years, many scholars have also conducted research on software denoising. For example, the moving average method can improve the signal-to-noise ratio of lidar echo signals, but it loses effective data points, reducing accuracy. Wavelet denoising has good time-frequency localization characteristics and can be used for low signal-to-noise ratio signal processing. Empirical mode decomposition (EMD) has the characteristic of requiring no prior information and decomposing entirely based on the signal itself, making it suitable for high signal-to-noise ratio situations, but it suffers from problems such as mode aliasing. Ensemble empirical mode decomposition has also proven applicable to noise reduction of atmospheric lidar signals. Therefore, it is of great significance to find novel optical filters that take into account the effects of atmospheric turbulence, are usable all day long, and effectively increase the signal-to-noise ratio.

[0004] To find a new method to improve the signal-to-noise ratio (SNR) of lidar signals, the combination of vortex beams and lidar has attracted researchers' attention. Some progress has been made in the field of vortex beam and lidar noise reduction, such as using a Laguerre-Gaussian laser beam carrying orbital angular momentum as the emission source and employing a photon sieve as a diffraction filter at the receiver to physically separate the lidar return signal from solar background noise; and using a spatial light modulator (SLM) to modulate the laser beam into a vortex beam, utilizing the different response behaviors of the light field coherence differences in spatial mode evolution to achieve physical separation. However, this method does not address wavefront distortion caused by turbulence in the real atmosphere, and the simulation using a point source to model sunlight has certain limitations. It also does not consider the possibility of multiple scattering in reality, which could disrupt the spiral wavefront. Based on the above analysis, designing a spiral filter that significantly improves the SNR while fully considering the solar angular spread (angle) and atmospheric transmission effects is of great significance.

[0005] This invention aims to solve the technical challenge of a sharp decrease in the signal-to-noise ratio of laser detection systems under strong background radiation (sunlight environment) and considering the solar divergence angle. Specifically, it addresses how to utilize simple and efficient phase modulation elements to construct a laser detection filtering system that can completely separate laser light and sunlight of the same wavelength band from each other in terms of physical spatial distribution, while simultaneously achieving high light energy utilization and low engineering implementation difficulty. Summary of the Invention

[0006] This invention relates to a laser detection background noise suppression system based on the characteristics of vortex light, comprising a modulation end, a transmission end and a filtering end, employing a cascaded architecture consisting of a dual vortex phase modulation element and a telescope system, transforming the traditional solid Gaussian distribution into a hollow ring-shaped distribution with a large-diameter dark core by performing step-like phase superposition of the laser.

[0007] In one embodiment, a first vortex wave plate is placed at the receiver entrance to perform helical phase modulation on the laser, and at this time, lenses l0 and l1 are used to expand and balance the laser beam to reach the incident surface.

[0008] In one embodiment, the first shielding plate has a radius of 24 μm, and the second shielding plate at the center of the circle has a radius of 3 mm.

[0009] In one embodiment, the aperture diameter D is 20 mm - 40 mm, and the Gaussian beam waist is... The first vortex waveplate has an order of [10, 20], the second vortex waveplate has an order of [20, 40], the lens l1 has a focal length of 100mm-300mm, the lens l2 has a focal length of 40mm-60mm, and the lens l3 has a focal length of 40mm-60mm.

[0010] This invention relates to a method for suppressing background noise in laser detection based on the characteristics of vortex light, comprising the following steps: S1. Vortex Light Generation: The laser emitted by the lidar is incident on a vortex waveplate, converting it into an optical field distribution that satisfies... A vortex beam, where r is the radial coordinate, i.e., the distance from a point on the beam cross-section to the center. These are azimuth coordinates, i.e., the angular position of a point on the beam cross-section. Radial light field distribution, Here, i is the spiral phase factor, i is the imaginary unit, and l is the topological charge number. Indicates phase as a function of azimuth angle It exhibits a linear change; S2. Filtering out the center of sunlight: Place a circular first shielding plate with radius r behind the glass plate. At this time, the laser has become a vortex light with a central dark spot and the radius of the dark spot is greater than r, thus filtering out the center of sunlight. S3. Increase laser topological charge: Adopt a double vortex waveplate structure. By cascading the first and second vortex waveplates, the radius of the dark nucleus on the focal plane is increased, leaving tolerance space for placing physical shielding plates. S4. Filtering out sunlight: After the above steps, the laser light is distributed in a ring shape on the back focal plane of lens l2, and the sunlight is a dark spot; place a second shielding plate on the incident surface of lens l3 to filter out the sunlight and obtain a high signal-to-noise ratio laser. S5. LiDAR Integration: The first vortex wave plate and lens l0 are integrated into the front end of the transmitting module, and the second vortex wave plate, the first circular central blocking plate, the second circular central blocking plate, and lenses l1l2l3 are integrated into the receiving module.

[0011] In one embodiment, the circular center radius of the first shielding plate is r = 24 μm.

[0012] In one embodiment, the cascading of the first vortex waveplate l=16 and the second vortex waveplate l=32 brings the total topological charge to 48.

[0013] In one embodiment, the circular center radius of the second shielding plate is r = 3 mm.

[0014] The technical effects achieved by this invention are as follows: Extremely high background noise suppression capability and higher accuracy: Utilizing the "zero-intensity dark core" characteristic formed by laser modulation with higher-order topological charges, this scheme achieves spatial separation of signal and noise on the focal plane. Simulation data shows that under the conditions of topological charge number l=32 and circular shield radius r=24um, the incoherent sunlight intensity in the central region is almost completely blocked. The scheme achieves a signal-to-noise ratio as high as 96.34 in simulation calculations under specific configurations, effectively solving the "blind zone" problem of all-weather lidar under strong sunlight backgrounds, and achieving an order-of-magnitude leap in detection signal-to-noise ratio.

[0015] Extremely high light energy utilization and detection sensitivity, with lower energy consumption: Existing technologies use photon sieves that are essentially diffractive optical elements. A large amount of incident light is lost due to diffraction orders, resulting in extremely low energy utilization of the echo signal (typically only 10%-20%). This invention uses a combination of vortex waveplates (VR), which are phase modulation elements and theoretically have a near 100% conversion efficiency for transmitted light energy. This not only significantly reduces the attenuation of the echo signal but also significantly improves the system's detection accuracy and range for distant, weak echo targets, with particularly obvious advantages in all-weather detection.

[0016] Lower maintenance costs and a simpler, less complex system: This solution uses conventional refractive lens groups and spatial shielding plates. The shielding plate has a simple structure and stable properties, and does not involve complex micro / nano structure fabrication; at the same time, the spatial shielding design reduces the sensitivity of optical path alignment and enhances the system's robustness in harsh environments such as outdoor vibration and temperature differences. In contrast, photon sieves contain millions of tiny light-transmitting holes, requiring sub-micron level fabrication precision, making them extremely expensive to manufacture and susceptible to dust. Furthermore, existing technologies often use "perforated mirrors" for separation, which makes collimation extremely difficult. Attached Figure Description

[0017] The invention will now be described in more detail with reference to embodiments and the accompanying drawings. Figure 1 A schematic diagram of the system structure of the present invention is shown; Figure 2 A flowchart of the present invention is shown; Figure 3 The intensity and phase distribution of the laser image at the incident surface of lens l1; Figure 4 The intensity and phase distribution of the laser image at the focal plane of lens l1; Figure 5 The intensity and phase distribution of the laser image at the focal plane of lens l2; Figure 6 The intensity and phase distribution of the laser image at the back focal plane of lens l3; Figure 7Intensity distribution of sunlight imaged on the back focal plane of lens l1; Intensity distribution of sunlight imaged on the back focal plane of lens; Figure 8 This represents the intensity distribution of sunlight imaged on the back focal plane of lens l3; Figure 9 A comparison of the intensity distribution of laser light and sunlight on the back focal plane of lens l2 along the center line; Figure 10 A comparison of the intensity distribution of laser light and sunlight on the back focal plane of lens l3 along the center line. Detailed Implementation

[0018] The invention will now be further described with reference to the accompanying drawings.

[0019] This invention relates to a laser detection background noise suppression system based on vortex phase modulation and spatial filtering. The core structure of the system is as follows: Figure 1 As shown, the system includes a modulation end, a transmission end, and a filtering end. It employs a cascaded architecture consisting of a dual-vortex phase modulation element and a telescope system. By performing step-wise phase superposition of the laser beam, the traditional solid Gaussian distribution is transformed into a hollow ring-shaped distribution with a large-diameter dark core. Compared to single modulation, this architecture can more precisely control the focal plane light field, achieving complete decoupling of the laser and sunlight in physical space modes. The aperture diameter D is 30mm, the Gaussian beam waist ω0 is 2mm, the first vortex waveplate (VR) is of order 16, the second vortex waveplate (VR) is of order 32, lens l1 has a focal length of 200mm, lens l2 has a focal length of 50mm, and lens l3 has a focal length of 50mm. At the receiving end, the first vortex waveplate is placed at the entrance to perform helical phase modulation of the laser beam. Simultaneously, the lenses are used to expand and balance the laser beam until it reaches the glass plane, which is the incident surface. Sunlight enters directly through the glass plate. It first undergoes a first filtering process through the first shielding plate. Then, it passes through a telescope system consisting of two lenses, l1 and l2, with a second vortex wave plate placed at the central focal plane for phase remodulation, causing the laser to be superimposed to a specific higher-order topological charge. Finally, a circular shield of a specific radius is placed coaxially in front of the incident lens l3 for a second filtering process. Finally, a high signal-to-noise ratio laser is obtained at the rear focal plane of lens l3.

[0020] like Figure 2 As shown): Utilizing the difference in response between laser (coherent light) and sunlight (incoherent light) under vortex phase modulation: 1. The signal light is phase-modulated to form a hollow ring-shaped light halo with zero central intensity and outward expansion of main energy at the focal plane; 2. Due to phase incoherence, the background light is not modulated and, after focusing, forms a solid circular spot with highly concentrated central energy.

[0021] By physically blocking the incoherent light spot on the central axis with a circular shield, the vortex signal light in the annular region can pass through without loss, thereby achieving a significant improvement in the signal-to-noise ratio.

[0022] Unlike traditional spectral filtering (filters), this invention utilizes spatial distribution differences caused by coherence for noise reduction: taking advantage of the characteristics of incoherent sunlight forming a narrow, focused peak at the center of the focal plane, while coherent laser light forms a strong, ring-shaped peak at the center, a physical shield is added before photoelectric detection. This "physical stripping" method can effectively filter out strong background interference of the same wavelength and frequency as the signal.

[0023] This invention specifically optimizes the modulation order, using a topological charge configuration of l=32 as the core or cascaded up to 48, and a first circular central blocking plate with a radius of 24µm and a second circular central blocking plate with a radius of 3mm. Simulation verification shows that these specific parameters can generate a sufficiently large dark core radius, avoiding the obstruction area of ​​the circular blocking plates while ensuring a high concentration of signal energy within the annular region. This is a key quantitative characteristic for achieving a high signal-to-noise ratio.

[0024] This invention relates to a laser detection background noise suppression method based on vortex light characteristics, comprising the following steps: S1. Vortex Light Generation: The laser light (fundamental mode Gaussian beam) emitted by the lidar is incident onto a vortex waveplate, converting it into an optical field distribution that satisfies... A vortex beam, where r is the radial coordinate, i.e., the distance from a point on the beam cross-section to the center. These are azimuth coordinates, i.e., the angular position of a point on the beam cross-section. Radial light field distribution, Here, i is the spiral phase factor, i is the imaginary unit, and l is the topological charge number. Indicates phase as a function of azimuth angle It exhibits a linear change; S2. Filtering out the center of sunlight: Place a circular first shielding plate with radius r behind the glass plate. At this time, the laser has become a vortex light with a central dark spot and the radius of the dark spot is greater than r, thus filtering out the center of sunlight. S3. Increasing the Laser Topological Charge: A dual-vortex waveplate structure was adopted. Simulation data shows that for lasers, the dark nucleus radius generated by a single low-order vortex waveplate is small, which is not conducive to spatial separation. By cascading the first vortex waveplate (l=16) and the second vortex waveplate (l=32), the total topological charge reaches 48, significantly increasing the dark nucleus radius on the focal plane and leaving tolerance space for placing physical blocking plates. Sunlight is incoherent light and is not affected by the vortex waveplates; it is only scaled by the lens, thus providing spatial support for subsequent separation. S4. Filtering out sunlight: After the above steps, the laser light is distributed in a ring shape on the back focal plane of lens l2, and the sunlight is a dark spot; place a second circular shield with a radius r=3mm at the center of the incident surface of lens l3 to filter out the sunlight and obtain a high signal-to-noise ratio laser. S5. LiDAR Integration: The first vortex wave plate and lens l0 are integrated into the front end of the transmitting module, and the second vortex wave plate, the first circular central blocking plate, the second circular central blocking plate, and lenses l1l2l3 are integrated into the receiving module.

[0025] In one embodiment, the present invention is based on a laser detection background noise suppression system using vortex phase modulation and spatial filtering. The core structure of the system is as follows: Figure 1 As shown, the system includes a modulation end, a transmission end, and a filtering end. It employs a cascaded architecture consisting of a dual-vortex phase modulation element and a telescope system. By performing step-wise phase superposition of the laser beam, the traditional solid Gaussian distribution is transformed into a hollow ring-shaped distribution with a large-diameter dark core. Compared to single modulation, this architecture can more precisely control the focal plane light field, achieving complete decoupling of the laser and sunlight in physical space modes. The aperture diameter D is 20 mm - 40 mm, the Gaussian beam waist ω0 is 1 mm - 3 mm, the first vortex waveplate (VR) has an order of 10-20, the second vortex waveplate (VR) has an order of 20-40, lens l1 has a focal length of 100 mm - 300 mm, lens l2 has a focal length of 40 mm - 60 mm, and lens l3 has a focal length of 40 mm - 60 mm. At the receiving end, the first vortex waveplate is placed at the entrance to perform helical phase modulation of the laser beam. Simultaneously, the lenses are used to expand and balance the laser beam until it reaches the glass plane, which is the incident surface. Sunlight enters directly through the glass plate. It first undergoes a first filtering process through the first shielding plate. Then, it passes through a telescope system consisting of two lenses, l1 and l2, with a second vortex wave plate placed at the central focal plane for phase remodulation, causing the laser to be superimposed to a specific higher-order topological charge. Finally, a circular shield of a specific radius is placed coaxially in front of the incident lens l3 for a second filtering process. Finally, a high signal-to-noise ratio laser is obtained at the rear focal plane of lens l3.

[0026] In one embodiment, the laser detection background noise suppression method based on vortex light characteristics of the present invention includes the following steps: S1. Vortex Light Generation: The laser light (fundamental mode Gaussian beam) emitted by the lidar is incident onto a vortex waveplate, converting it into an optical field distribution that satisfies... A vortex beam, where r is the radial coordinate, i.e., the distance from a point on the beam cross-section to the center. These are azimuth coordinates, i.e., the angular position of a point on the beam cross-section. Radial light field distribution, Here, i is the spiral phase factor, i is the imaginary unit, and l is the topological charge number. Indicates phase as a function of azimuth angle It exhibits a linear change; S2. Filter out the center of sunlight: Place a circular first shield with a radius r = 20um-30um behind the glass plate. At this time, the laser has become a vortex light with a central dark spot and the radius of the dark spot is greater than r, thus filtering out the center of sunlight. S3. Increasing the Laser Topological Charge: A dual-vortex waveplate structure is adopted. Simulation data shows that for lasers, the dark nucleus radius generated by a single low-order vortex waveplate is small, which is not conducive to spatial separation. By cascading the first vortex waveplate (l=10-20) and the second vortex waveplate (l=20-40), the total topological charge reaches 30-60, significantly increasing the dark nucleus radius on the focal plane and leaving tolerance space for placing physical blocking plates. Sunlight is incoherent light and is not affected by the vortex waveplates; it is only scaled by the lens, thus providing spatial support for subsequent separation. S4. Filtering out sunlight: After the above steps, the laser at the back focal plane of lens l2 is distributed in a ring shape, and the sunlight is a dark spot; place a circular second shield with a radius r = 1mm-5mm at the center of lens l3 to filter out the sunlight and obtain a high signal-to-noise ratio laser. S5. LiDAR Integration: The first vortex wave plate and lens l0 are integrated into the front end of the transmitting module, and the second vortex wave plate, the first circular central blocking plate, the second circular central blocking plate, and lenses l1l2l3 are integrated into the receiving module.

[0027] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A laser detection background noise suppression system based on vortex light characteristics, characterized in that, It includes a modulation end, a transmission end and a filtering end, and adopts a cascaded architecture consisting of a dual vortex phase modulation element and a telescope system. By performing step-like phase superposition on the laser, the traditional solid Gaussian distribution is transformed into a hollow ring distribution with a large-diameter dark core.

2. The system according to claim 1, characterized in that, The receiver has a first vortex plate placed at the incident port to perform helical phase modulation on the laser, and at this time, lenses l0 and l1 are used to expand and balance the laser beam to reach the incident surface.

3. The system according to claim 1, characterized in that, The first shielding plate has a radius of 24µm, and the second shielding plate at the center of the circle has a radius of 3mm.

4. The system according to claim 1, characterized in that, The aperture diameter D is 20 mm - 40 mm, and the Gaussian beam waist is... The first vortex waveplate has an order of [10, 20], the second vortex waveplate has an order of [20, 40], the lens l1 has a focal length of 100mm-300mm, the lens l2 has a focal length of 40mm-60mm, and the lens l3 has a focal length of 40mm-60mm.

5. A method for suppressing background noise in laser detection based on the characteristics of vortex light, characterized in that, Includes the following steps: S1. Vortex Light Generation: The laser emitted by the lidar is incident on a vortex waveplate, converting it into an optical field distribution that satisfies... A vortex beam, where r is the radial coordinate, i.e., the distance from a point on the beam cross-section to the center. These are azimuth coordinates, i.e., the angular position of a point on the beam cross-section. Radial light field distribution, Here, i is the spiral phase factor, i is the imaginary unit, and l is the topological charge number. Indicates phase as a function of azimuth angle It exhibits a linear change; S2. Filtering out the center of sunlight: Place a circular first shielding plate with radius r behind the glass plate. At this time, the laser has become a vortex light with a central dark spot and the radius of the dark spot is greater than r, thus filtering out the center of sunlight. S3. Increase laser topological charge: Adopt a double vortex waveplate structure. By cascading the first and second vortex waveplates, the radius of the dark nucleus on the focal plane is increased, leaving tolerance space for placing physical shielding plates. S4. Filtering out sunlight: After the above steps, the laser light is distributed in a ring shape on the back focal plane of lens l2, and the sunlight is a dark spot; place a second shielding plate on the incident surface of lens l3 to filter out the sunlight and obtain a high signal-to-noise ratio laser. S5. LiDAR Integration: The first vortex wave plate and lens l0 are integrated into the front end of the transmitting module, and the second vortex wave plate, the first circular central blocking plate, the second circular central blocking plate, and lenses l1l2l3 are integrated into the receiving module.

6. The method according to claim 5, characterized in that, The circular center radius of the first shielding plate is r = 24 μm.

7. The method according to claim 5, characterized in that, The cascading of the first vortex waveplate l=16 and the second vortex waveplate l=32 brings the total topological charge to 48.

8. The method according to claim 5, characterized in that, The circular center radius of the second shielding plate is r=3mm.

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