Laser radar ranging and imaging system and method

Abstract

The invention discloses a laser radar ranging and imaging system and method. The system comprises a dual-wavelength laser emission module, a common-path optical transceiver module and a light-splitting-free detection module. The common-optical-path optical transceiver module is used for combining the purple laser pulse and the near-infrared laser pulse into a coaxial laser beam, guiding the coaxial laser beam to a target, receiving an echo optical signal which is returned from the target and comprises the purple laser pulse and the near-infrared laser pulse, and integrally exporting the echo optical signal; the light-splitting-free detection module comprises a gallium nitride photoelectric detector, a silicon photomultiplier detector and a signal processor. The underwater echo and the water surface echo are directly separated from each other on the physical level by using the dual-function characteristics of purple light absorption and infrared transmission of the gallium nitride detector. According to the design, sounding errors caused by signal aliasing of a traditional single-wavelength system are fundamentally eliminated, and the measurement precision is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser ranging and imaging technology, in particular to a laser radar ranging and imaging system and method. BACKGROUND

[0002] The fields of laser radar technology and semiconductor optoelectronic devices have developed rapidly in recent years and have shown important value in multiple industries. Laser radar has a wide range of applications in high-precision measurement, environmental perception, and other aspects, especially in water depth measurement, which provides an effective means for obtaining water surface and water bottom distance information. With the continuous progress of related technologies, people have higher requirements for the precision, efficiency, and cost of laser radar in water depth measurement, expecting it to better adapt to complex measurement environments and provide more accurate data support for related fields of research and application.

[0003] In the laser radar technology for water depth measurement, traditional methods have their own characteristics to solve measurement problems. Single-wavelength lasers such as green light are commonly used to obtain water bottom echo signals by utilizing the penetration ability of laser in water. Meanwhile, to address the shortcomings of single-wavelength systems, the industry has proposed a dual-wavelength detection scheme. In addition, traditional silicon-based detectors are often used for signal detection.

[0004] However, the existing technology has obvious defects. Single-wavelength technology cannot effectively distinguish between reflection signals from the water surface and reflection signals from the water bottom. When the water body is turbid or there is wave disturbance on the water surface, the two signals are easily mixed and overlapped, leading to measurement errors. Traditional silicon-based detectors have low quantum efficiency in the blue-green light band and are easily disturbed by ambient light, resulting in insufficient sensitivity for detecting weak echo signals after penetrating the water body. The receiving end of the existing dual-wavelength detection scheme usually needs to use optical elements such as beam splitters to separate signals of different wavelengths, making the optical path complex and causing significant loss of optical energy, which reduces the overall detection efficiency of the system. SUMMARY

[0005] To solve the technical problems in the existing technology, the present application provides a laser radar ranging and imaging system and method.

[0006] The laser radar ranging and imaging system and method provided by the present application adopt the following technical solutions: A laser radar ranging and imaging system, comprising: a dual-wavelength laser emission module for emitting a purple light laser pulse in a first wavelength range and a near-infrared laser pulse in a second wavelength range; a common-path optical transceiver module for combining the purple light laser pulse and the near-infrared laser pulse into a coaxial laser beam and directing it towards a target, and receiving a return echo light signal containing the purple light laser pulse and the near-infrared laser pulse from the target and guiding the entire return echo light signal out. The non-splitting detection module includes a gallium nitride photodetector, a silicon photomultiplier detector, and a signal processor. The gallium nitride photodetector is arranged on the outgoing path of the echo light signal and used for absorbing and detecting the violet light part in the echo light signal. The silicon photomultiplier detector is arranged on the side of the gallium nitride photodetector away from the echo light signal and used for receiving and detecting the near-infrared part in the echo light signal penetrating the gallium nitride photodetector. The signal processor is in communication connection with the gallium nitride photodetector and the silicon photomultiplier detector and used for calculating distance or three-dimensional shape information according to the received signals.

[0007] In some embodiments, the dual-wavelength laser emission module includes a violet laser, a near-infrared laser, a first collimating mirror, and a second collimating mirror. The violet laser is used for generating violet laser pulses. The near-infrared laser is used for generating near-infrared laser pulses. The first collimating mirror is arranged on a first emission light path of the violet laser. The second collimating mirror is arranged on a second emission light path of the near-infrared laser. The first emission light path and the second emission light path are coaxial, and the emission end of the violet laser and the emission end of the near-infrared laser are arranged opposite to each other. The common-path optical transceiver module includes a beam combining mirror having a first reflecting surface and a second reflecting surface perpendicular to each other. The first reflecting surface is arranged on the first emission light path and forms a 45° angle with the first emission light path. The second reflecting surface is arranged on the second emission light path and forms a 45° angle with the second emission light path. The violet laser pulses and the near-infrared laser pulses generated by the violet laser and the near-infrared laser, respectively, are first collimated by the first collimating mirror and the second collimating mirror, respectively, and then reflected by the first reflecting surface and the second reflecting surface, respectively, and combined into the coaxial laser beam.

[0008] In some embodiments, the common-path optical transceiver module further includes an aperture mirror and a scanning galvanometer assembly. A central hole is formed in the center of the aperture mirror, and the central hole is located on the path of the coaxial laser beam. The scanning galvanometer assembly is arranged on the side of the aperture mirror away from the beam combining mirror. The coaxial laser beam passes through the central hole and is guided to the target by the scanning galvanometer assembly. The echo light signal returned from the target is guided to the mirror surface of the aperture mirror by the scanning galvanometer assembly and is reflected to the receiving end of the gallium nitride photodetector.

[0009] In some embodiments, the scanning galvanometer assembly comprises a first rotatable galvanometer, a second rotatable galvanometer, and a first galvanometer driver and a second galvanometer driver respectively driving the rotation of the two galvanometers, wherein, in the transmitting path, the first rotatable galvanometer deflects the coaxial laser beam for the first time, and the second rotatable galvanometer deflects the light beam reflected by the first rotatable galvanometer for the second time to guide it to the target; in the receiving path, the echo light signal is collected and guided by the second rotatable galvanometer and the first rotatable galvanometer in reverse order.

[0010] In some embodiments, the non-splitting detection module further comprises a filter arranged on the light path between the gallium nitride photodetector and the silicon photomultiplier detector, for filtering out stray light and allowing only near-infrared light in a preset wavelength range to pass through.

[0011] In some embodiments, the gallium nitride photodetector comprises a sapphire substrate, an n-type gallium nitride semiconductor layer formed on the sapphire substrate, a p-type gallium nitride semiconductor layer formed on the n-type gallium nitride semiconductor layer and constituting a p-n junction with the n-type gallium nitride semiconductor layer, and a p-type electrode and an n-type electrode electrically connected to the p-type gallium nitride semiconductor layer and the n-type gallium nitride semiconductor layer, respectively. The selective response of the gallium nitride photodetector to violet light and near-infrared light is based on the band gap energy of the gallium nitride material constituting the n-type gallium nitride semiconductor layer and the p-type gallium nitride semiconductor layer: the energy of the violet light photons is higher than the band gap energy, so they are absorbed by the p-n junction to generate photocurrent; the energy of the near-infrared light photons is lower than the band gap energy, so they are transmitted through the gallium nitride photodetector.

[0012] In some embodiments, the distance between the gallium nitride photodetector and the silicon photomultiplier detector along the light signal incident path is less than 1 mm, and they constitute a confocal plane integrated package.

[0013] In some embodiments, the first wavelength ranges from 380 nm to 420 nm, and the second wavelength ranges from 895 nm to 915 nm.

[0014] The application also provides a laser radar ranging and imaging method, which is suitable for the laser radar ranging and imaging system and comprises the following steps: The dual-wavelength laser emitting module and the common-path optical transceiver module are used to synchronously and coaxially emit violet light laser pulses and near-infrared laser pulses to the water body; The common-path optical transceiver module combines the violet laser pulses and the near-infrared laser pulses into a coaxial laser beam and directs the coaxial laser beam to the water body, and receives and leads out the echo optical signal containing the violet laser pulses and the near-infrared laser pulses returned from the water body, wherein the violet laser pulses penetrate the water body and are reflected by the water bottom to carry the water bottom distance information, and the near-infrared laser pulses are reflected by the water surface to carry the water surface distance information; The gallium nitride photodetector absorbs and detects the violet laser pulse part in the echo optical signal to generate a first electric signal, and the silicon photomultiplier tube detector receives and detects the near-infrared laser pulse part in the echo optical signal penetrating the gallium nitride photodetector to generate a second electric signal; The time difference between the first electric signal and the second electric signal is calculated by a signal processor, and the water depth of the water body is calculated according to the time difference.

[0015] In some embodiments, the water depth of the water body is calculated by the following formula:

[0016] wherein, Depth is the water depth of the water body, Δt is the time difference between the first electric signal and the second electric signal, c is the speed of light in vacuum, n is the refractive index of the water body.

[0017] In summary, the present application includes at least one of the following beneficial technical effects: 1. The dual-function characteristics of the gallium nitride (GaN) detector, i.e. absorption of violet light and transmission of infrared light, directly separate the water bottom echo (violet light) and the water surface echo (infrared light) at the physical level. This design fundamentally eliminates the depth measurement error caused by signal aliasing in traditional single-wavelength systems, significantly improves the measurement accuracy, and the innovative GaN+SiPM confocal plane integrated detection module eliminates the need for dichroic mirrors and other light splitting elements required by traditional dual-wavelength systems, greatly simplifying the optical system structure. This non-splitting design avoids serious light energy loss, and revolutionarily improves the light energy utilization rate of the echo signal from about 45% in the traditional scheme to more than 95%.

[0018] 2. Based on the physical properties of gallium nitride material, the detector has natural cutoff and suppression ability to most of the visible light (>450nm) in sunlight, which can effectively suppress more than 90% of environmental light interference, reduce the requirement for subsequent light filtering system, and enhance the robustness of the system under various lighting conditions. The silicon photomultiplier tube detector has a built-in narrowband filter to resist solar background noise; and has the potential for all-weather operation.

[0019] 3. By adopting the open-hole mirror, the transmitting light path and the receiving light path are ingeniously integrated on the same optical axis, realizing the high-efficiency common light path design, which utilizes the diameter difference between the transmitting light beam and the returning light beam. Most of the returned signal photons are effectively collected by the mirror surface and reflected to the detector, which almost does not affect the receiving efficiency, ensuring extremely high echo signal receiving efficiency. Only a very small part of the echo energy coinciding with the transmitting path will pass through the center hole and be lost, which is negligible for the entire echo signal. This ensures that the detector can receive as much effective signal energy as possible, minimizing signal loss. 4. Since the expensive and precisely calibrated light splitting element is omitted, and the highly integrated light splitting-free detection module is adopted, the overall manufacturing cost, volume and debugging difficulty of the system are significantly reduced, laying the foundation for providing a commercially viable, high-precision hydrographic surveying and mapping innovation solution that can be mounted on a mobile platform. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a structural schematic diagram of a laser radar ranging and imaging system provided by an embodiment of the present application; Figure 2 is a structural schematic diagram of a gallium nitride photodetector in Figure 1 Figure 3 is a pulse timing diagram of a laser radar ranging and imaging system; Mark 1, dual-wavelength laser emitting module; 11, violet laser; 12, near-infrared laser; 13, first collimating mirror; 14, second collimating mirror; 2, common light path optical transceiver module; 21, beam combiner; 22, open-hole mirror; 23, scanning galvanometer assembly; 231, first rotatable galvanometer; 232, second rotatable galvanometer; 24, aperture stop; 3, light splitting-free detection module; 31, gallium nitride photodetector; 311, sapphire substrate; 312, n-type gallium nitride semiconductor layer; 313, p-type gallium nitride semiconductor layer; 314, p-type electrode; 315, n-type electrode; 32, silicon photomultiplier detector; 33, optical filter; 34, signal processor; 35, third collimating mirror; 4, target. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be further described in detail below with reference to the drawings. The described embodiments are only possible technical implementations of the present application, but are not limited thereto, and those skilled in the art can obtain other embodiments without creative labor under the circumstances of the embodiments of the present application.

[0022] Embodiment 1: Laser radar ranging and imaging system; Please refer to Figure 1 ​The laser radar ranging and imaging system provided by the embodiment of the application comprises a dual-wavelength laser emitting module 1, a common-path optical transmitting and receiving module 2 and a non-splitting detection module 3. The dual-wavelength laser emitting module 1 is used for emitting a violet laser pulse in a first wavelength range and a near-infrared laser pulse in a second wavelength range. The common-path optical transmitting and receiving module 2 is used for combining the violet laser pulse and the near-infrared laser pulse into a coaxial laser beam and guiding the coaxial laser beam to a target 4, and receiving a return light signal returned from the target 4 and containing the violet laser pulse and the near-infrared laser pulse and guiding the return light signal as a whole out. The non-splitting detection module 3 comprises a gallium nitride photodetector 31, a silicon photomultiplier detector 32 and a signal processor 34. The gallium nitride photodetector 31 is arranged on a guiding-out path of the return light signal and is used for absorbing and detecting a violet part in the return light signal. The silicon photomultiplier detector 32 is arranged on a side of the gallium nitride photodetector 31 opposite to the guiding-out of the return light signal and is used for receiving and detecting a near-infrared part in the return light signal penetrating through the gallium nitride photodetector 31. The signal processor 34 is in communication connection with the gallium nitride photodetector 31 and the silicon photomultiplier detector 32 and is used for calculating distance or three-dimensional shape information according to the received signals. The effect of high-precision and high-efficiency dual-wavelength laser radar ranging and imaging is achieved. The reason is that the system realizes dual-wavelength detection and physical separation of different wavelength signals, avoids signal aliasing, improves light energy utilization and enhances the detection capability of weak signals.

[0023] The specific description of each module is as follows: Please refer to Figure 1 The dual-wavelength laser emitting module 1 is responsible for generating and preliminarily processing laser pulses of two wavelengths. In a preferred scheme, it comprises a violet laser 11, a near-infrared laser 12, a first collimating mirror 13 and a second collimating mirror 14. The violet laser 11 can be a violet LED or other type of laser, which is used to generate a violet laser pulse. The near-infrared laser 12 can be an infrared laser diode or the like, which is used to generate a near-infrared laser pulse. The first collimating mirror 13 is arranged on a first emitting light path of the violet laser 11 and can collimate the light emitted by the violet laser 11 into a parallel light beam. A lens group or the like can also be used to replace the first collimating mirror 13. The second collimating mirror 14 is arranged on a second emitting light path of the near-infrared laser 12 and also plays a role of collimating the near-infrared light. Other collimating structures can also be used to replace the second collimating mirror 14. The first emitting light path and the second emitting light path are coaxial, and the emitting end of the violet laser 11 is arranged opposite to the emitting end of the near-infrared laser 12.

[0024] In the embodiment, the violet laser 11 can be a violet LED with a center wavelength of 405 nm, and the near-infrared laser 12 can be an infrared laser diode with a center wavelength of 905 nm. The emitting ends of the two lasers are arranged opposite to each other, and the light paths (the first emitting light path and the second emitting light path) of the two lasers are on the same straight line.

[0025] The common-path optical transceiver module 2 is the optical path hub of the system, responsible for beam combining, scanning, emission and reception. It includes a combining mirror 21, a hole mirror 22, a scanning galvanometer assembly 23 and an aperture stop 24, wherein: The combining mirror 21 has a first reflecting surface and a second reflecting surface perpendicular to each other, the first reflecting surface is arranged on the first emission light path and forms a 45° angle with the first emission light path, and the second reflecting surface is arranged on the second emission light path and forms a 45° angle with the second emission light path. In this way, the violet laser pulses and near-infrared laser pulses generated by the violet laser 11 and the near-infrared laser 12 respectively first pass through the first collimating mirror 13 and the second collimating mirror 14 for collimation, and then are reflected by the first reflecting surface and the second reflecting surface respectively, and are combined into a coaxial laser beam. The combining mirror 21 here can also be replaced by other optical structures with combining function.

[0026] The hole mirror 22 has a central hole in the center, which is located on the path of the coaxial laser beam, for allowing the emitted coaxial laser beam to pass through unobstructed.

[0027] The aperture stop 24 is arranged on the light path after the hole mirror 22, for regulating the shape of the coaxial laser beam and controlling the quality of the coaxial laser beam.

[0028] The scanning galvanometer assembly 23 is arranged on the side of the hole mirror 22 away from the combining mirror 21. After passing through the central hole, the coaxial laser beam is guided to the target 4 by the scanning galvanometer assembly 23, and the return light signal from the target 4 is guided to the mirror surface of the hole mirror 22 by the scanning galvanometer assembly 23 and is reflected to the receiving end of the gallium nitride photodetector 31. The scanning galvanometer assembly 23 can accurately control the direction of the laser beam to realize scanning of the target 4 area.

[0029] Specifically, please refer to Figure 1 The scanning galvanometer assembly 23 includes a first rotatable galvanometer 231, a second rotatable galvanometer 232, and a first galvanometer driver and a second galvanometer driver for driving the two to rotate, respectively. On the emission path, the first rotatable galvanometer 231 performs the first deflection on the coaxial laser beam, and the second rotatable galvanometer 232 performs the second deflection on the light beam reflected by the first rotatable galvanometer 231 to guide it to the target 4; on the receiving path, the return light signal is collected and guided in reverse order by the second rotatable galvanometer 232 and the first rotatable galvanometer 231, respectively. The first rotatable galvanometer 231 and the second rotatable galvanometer 232 can be a lens structure driven by a galvanometer motor, and the first galvanometer driver and the second galvanometer driver can be a step motor driver, of course, other driving modes that can realize the rotation control of the galvanometer can also be used.

[0030] The non-splitting detection module 3 is responsible for separating and detecting the echo signals of two wavelengths without splitting. It includes a gallium nitride photodetector 31, a silicon photomultiplier detector 32, a filter 33, and a signal processor 34.

[0031] Please refer to Figure 1 and Figure 2 The gallium nitride photodetector 31, as the first-level detector, has a specific multilayer structure, including a sapphire substrate 311, an n-type gallium nitride semiconductor layer 312 formed on the sapphire substrate 311, a p-type gallium nitride semiconductor layer 313 formed on the n-type gallium nitride semiconductor layer 312 and constituting a p-n junction with it, and a p-type electrode 314 and an n-type electrode 315 electrically connected to the p-type gallium nitride semiconductor layer 313 and the n-type gallium nitride semiconductor layer 312, respectively. The selective response of the gallium nitride photodetector 31 to violet light and near-infrared light is based on the band gap energy of the gallium nitride material constituting the n-type gallium nitride semiconductor layer 312 and the p-type gallium nitride semiconductor layer 313: the energy of the violet light photon is higher than the band gap energy, so it is absorbed by the p-n junction to generate photocurrent; the energy of the near-infrared light photon is lower than the band gap energy, so it transmits through the gallium nitride photodetector 31. The sapphire substrate 311 here can also be replaced by other substrate materials with similar support and optical properties.

[0032] The core principle of the technical solution is to cleverly use the band gap energy characteristics of gallium nitride (GaN) semiconductor material. Semiconductor materials can only absorb photons with energy higher than their band gap energy, and are transparent to photons with energy lower than their band gap energy. This scheme accurately selects two laser wavelengths, so that their photon energies are just on both sides of the band gap energy of the GaN material: Violet light (about 405 nm): its photon energy is higher than the band gap energy of the gallium nitride semiconductor material, so it can be efficiently absorbed by the gallium nitride detector to produce a photoelectric effect.

[0033] Near-infrared light (about 905 nm): its photon energy is lower than the band gap energy of the gallium nitride semiconductor material, so it cannot be effectively absorbed and can penetrate the gallium nitride detector at a high proportion.

[0034] Through this physical property, the gallium nitride detector itself plays the role of a "dichroic mirror" in the traditional optical path, directly realizing the separation of the two wavelengths of signals at the physical level, thereby constructing a "non-splitting" detection module.

[0035] The silicon photomultiplier detector 32 (SiPM) as the second-level detector is arranged closely behind the gallium nitride photodetector 31. The distance between them is less than 1 mm, constituting a confocal plane integrated package to maximize light energy utilization and reduce system size.

[0036] The filter 33 is arranged on the light path between the gallium nitride detector and the silicon photomultiplier detector 32, and is usually a narrowband filter 33 allowing only a specific wavelength range (e.g. 905nm ± 10nm) of near-infrared light to pass, for filtering out ambient stray light and improving the signal-to-noise ratio.

[0037] The signal processor 34 is in communication connection with the gallium nitride detector and the silicon photomultiplier detector 32, and is responsible for receiving the electrical signals and performing subsequent calculations.

[0038] Preferably, the reflected echo light signal is focused by a third collimating mirror 35 before being irradiated onto the gallium nitride photodetector 31, to ensure that the size of the echo light spot matches the effective area of the gallium nitride photodetector 31, thereby optimizing the receiving efficiency.

[0039] The system working process is as follows: Signal transmission: the system sends a trigger signal, and the violet laser 11 and the near-infrared laser 12 in the dual-wavelength laser emission module 1 synchronously emit violet laser pulses with a central wavelength of 405nm and near-infrared laser pulses with a central wavelength of 905nm, respectively. After collimation by the first collimating mirror 13 and the second collimating mirror 14, the two beams of light are accurately combined into a coaxial dual-wavelength laser beam by the beam combiner 21.

[0040] Scanning and emission: the coaxial laser beam passes through the central hole of the aperture mirror 22 and reaches the scanning galvanometer assembly 23. The first rotatable galvanometer 231 and the second rotatable galvanometer 232 are controlled by the driver to perform accurate deflection, thereby modulating the path of the laser beam in two dimensions and making it scan towards the water body (target 4).

[0041] Signal reception: after the light beam reaches the water body, the two wavelengths undergo different physical interactions: the near-infrared light with an energy of 905nm is highly reflective at the water-air interface, and most of its energy is directly reflected by the water surface; while the violet light with an energy of 405nm penetrates the water body and is reflected by the underwater target 4 due to its excellent penetration in water. Therefore, two echo light signals carrying information about the water surface and the water bottom, respectively, are generated. These two echo light signals return along approximately the same path, are collected by the first rotatable galvanometer 231 and the second rotatable galvanometer 232 of the scanning galvanometer assembly 23 in reverse order, and are directed to the mirror surface of the aperture mirror 22.

[0042] Detection and separation: The open aperture mirror 22 reflects the echo light beams carrying water surface and water bottom information to the non-split detection module 3. The light beams reflected by the open aperture mirror 22 are first irradiated to the gallium nitride photodetector 31. The p-n junction inside it will absorb the 405 nm violet echo (water bottom signal) and generate a photocurrent, completing the signal detection of the far layer target 4 such as the water bottom. The 905 nm near-infrared echo (water surface signal) will directly penetrate the gallium nitride photodetector 31 with an infrared transmittance of about 60%, and after further purification by the optical filter 33, it is received by the rear silicon photomultiplier detector 32, completing the signal detection of the near layer target 4 such as the water surface.

[0043] Solving and imaging: The electrical signals generated by the two detectors are sent to the signal processor 34. The module calculates the time difference between the water surface signal detected by the silicon photomultiplier detector 32 and the water bottom signal detected by the gallium nitride detector to solve the water depth, and finally generates the water depth information or seabed three-dimensional image of the target 4. The whole process realizes high-precision water depth measurement, high light energy utilization, and is suitable for hydrographic surveying and mapping, environmental monitoring, underwater security and other fields.

[0044] It should be pointed out that the central hole will not significantly affect the reflection of the echo signal, and the principle is as follows: the system emits a collimated laser beam with a very small diameter. The spot diameter is precisely controlled to be smaller than the size of the central hole, so it can pass through without hindrance. When the laser irradiates the target 4 (such as the water surface or the water bottom) far away, it will be diffusely reflected. The returning echo signal is no longer a narrow beam, but a much larger diameter divergent light cone. When this wide light cone returns to the open aperture mirror 22, its coverage area is much larger than the area of the central hole. Therefore, most of the echo photons are irradiated on the mirror surface around the central hole and are effectively collected and reflected to the detector. Only a very small part (coinciding with the emission path) of the energy will pass through the small hole and be lost, which is negligible for the whole echo signal. This design cleverly takes advantage of the divergence of the light beam after long-distance transmission and diffuse reflection, and realizes the coaxial integration of the emission and reception paths without affecting the reception efficiency of the echo signal.

[0045] The laser radar ranging and imaging system of the embodiment emits laser pulses of different wavelengths through the dual-wavelength laser emission module 1, combines and directs them to the target 4 using the common-path optical transceiver module 2, and then receives the return light signal. The beamsplitting detection module 3 uses a gallium nitride photodetector 31 and a silicon photomultiplier tube detector 32 to physically separate and detect signals of different wavelengths, avoiding the problems of signal aliasing and light energy loss in traditional technologies. The signal processing module calculates the distance or three-dimensional shape information according to the detected signals, realizing high-precision and high-efficiency dual-wavelength laser radar ranging and imaging, with higher precision, lower cost and stronger anti-interference ability compared with existing technologies.

[0046] Embodiment 2: Laser radar ranging and imaging method The laser radar ranging and imaging method provided by the embodiment of the application comprises the following steps: The dual-wavelength laser emission module 1 and the common-path optical transceiver module 2 synchronously and coaxially emit violet light laser pulses and near-infrared laser pulses to the water body; The common-path optical transceiver module 2 combines the violet light laser pulses and the near-infrared laser pulses into a coaxial laser beam and directs it to the target 4 water body, and receives the return light signal containing the violet light laser pulses and the near-infrared laser pulses from the water body and exports the return light signal as a whole, wherein the violet light laser pulses penetrate the water body and are reflected by the water bottom, carrying the water bottom distance information, and the near-infrared laser pulses are reflected by the water surface, carrying the water surface distance information; The gallium nitride photodetector 31 absorbs and detects the violet light laser pulse part in the return light signal to generate a first electric signal, and the silicon photomultiplier tube detector 32 receives and detects the near-infrared laser pulse part in the return light signal that has penetrated the gallium nitride photodetector 31 to generate a second electric signal; The signal processor 34 calculates the time difference between the first electric signal and the second electric signal, and calculates the water depth of the water body according to the time difference.

[0047] Specifically, the water depth of the water body is calculated by the following formula:

[0048] wherein, Depth is the water depth of the water body, Δt is the time difference between the first electric signal and the second electric signal, c is the speed of light in vacuum, n is the refractive index of the water body.

[0049] Figure 3is a pulse timing diagram, which explains the Time-of-Flight (ToF) principle of the system for distance measurement. In which, t0 (trigger): represents the trigger time of laser emission, which is the starting point of timing.

[0050] tNIR1, tNIR2: represents the first returned near-infrared (NIR) signal. Since NIR is reflected by the nearest surface (such as the water surface), it reaches the detector first.

[0051] tv(n), tv(n+1)...: represents the later returned violet light (Violet) signal. Since the violet light penetrates the first layer of surface and is reflected by the more distant target (such as the water bottom), it needs a longer flight time. n represents the nth measurement point in the scanning process.

[0052] tof(tv(n)): represents the total flight time of the violet light signal, and its calculation formula is tof = tv(n) - t0. The system calculates the accurate distance to the distant target by accurately measuring this tof value, multiplying it by the speed of light and dividing it by 2. At the same time, by calculating the time difference between tv(n) and tNIR, the thickness or distance between the two layers of surfaces (such as water depth) can be obtained.

[0053] The implementation principle of the embodiment is: based on the violet light absorption / infrared transmission dual-function characteristics of the gallium nitride photodetector 31, the physical separation of violet light (water bottom) and infrared (water surface) signals is realized, the depth measurement error caused by signal aliasing in traditional single-wavelength systems is eliminated, the confocal plane integrated module (GaN+SiPM) eliminates dichroic mirror, optical filter 33 and other optical elements, and the light energy utilization rate is improved from 45% of the traditional dual-wavelength system to more than 95%; the natural ultraviolet cutoff characteristics of gallium nitride material suppress 90% of visible light interference, the built-in narrowband optical filter 33 (905nm±10nm) of the silicon photomultiplier detector 32 resists solar background noise, realizes all-weather detection, and provides a high-precision, low-cost and strong robustness innovative solution for hydrographic surveying and mapping.

[0054] The installation and preparation method of the system is as follows: Step 1, obtain or prepare a gallium nitride detector. Specifically, the gallium nitride detector can be obtained by any one of photolithography process, ICP (Inductively Coupled Plasma) dry etching or electron beam evaporation.

[0055] Step 2, according to the size of the gallium nitride detector, match a suitable SiPM detector, the distance between the gallium nitride detector and the SiPM detector is 0.5mm, fixed by transparent epoxy resin, and the top is covered with a cutoff optical filter 33.

[0056] Step 3, install a receiving optical receiving system on the integrated probe system.

[0057] The beneficial effects of the technical solutions provided by the present application include: 1. The dual-function characteristics of the gallium nitride (GaN) probe, i.e., the absorption of violet light and the transmission of infrared light, directly separate the water bottom echo (violet light) and the water surface echo (infrared light) at the physical level. This design fundamentally eliminates the depth measurement error caused by signal aliasing in traditional single-wavelength systems, significantly improves the measurement accuracy, and the innovative GaN+SiPM confocal plane integrated detection module eliminates the need for dichroic mirrors and other light splitting elements required by traditional dual-wavelength systems, greatly simplifying the optical system structure. This non-splitting design avoids severe light energy loss and revolutionarily improves the light energy utilization rate of the echo signal from about 45% in the traditional scheme to more than 95%.

[0058] 2. Based on the physical properties of gallium nitride material, the probe has natural cutoff and suppression capability for most visible light (>450nm) in sunlight, which can effectively suppress more than 90% of environmental light interference, reduce the requirements for subsequent light filtering systems, and enhance the robustness of the system under various lighting conditions. The silicon photomultiplier detector 32 has a built-in narrowband filter 33 to resist solar background noise; and has the potential for all-weather operation.

[0059] 3. By using an open hole mirror 22, the emission light path and the receiving light path are ingeniously integrated on the same optical axis, realizing an efficient common light path design. This design takes advantage of the diameter difference between the emitted light beam and the returned light beam. Most of the returned signal photons are effectively collected and reflected by the mirror to the detector, hardly affecting the receiving efficiency, ensuring extremely high echo signal receiving efficiency. Only a very small part of the echo energy coinciding with the emission path will pass through the center hole and be lost, which is negligible for the entire echo signal. This ensures that the detector can receive as much effective signal energy as possible, minimizing signal loss. 4. Since the expensive and precisely calibrated light splitting elements are eliminated, and the highly integrated non-splitting detection module 3 is adopted, the overall manufacturing cost, volume and debugging difficulty of the system are significantly reduced, laying a foundation for providing a commercially viable, mobile platform-mounted high-precision hydrographic surveying and mapping innovation solution.

[0060] The specific embodiments of the present application described above do not constitute a limitation on the scope of protection of the present application. Any various other corresponding changes and modifications made in accordance with the technical concept of the present application should be included within the scope of protection of the present application.

Claims

1. A ladar ranging and imaging system, comprising: The application relates to a dual-wavelength laser emission module (1) for emitting violet laser pulses in a first wavelength range and near-infrared laser pulses in a second wavelength range; a common-path optical transceiver module (2) for combining the violet laser pulses and the near-infrared laser pulses into a coaxial laser beam and directing the coaxial laser beam towards a target (4), and receiving a return light signal containing the violet laser pulses and the near-infrared laser pulses returned from the target (4) and guiding the return light signal as a whole outwards; and a non-splitting detection module (3) comprising a gallium nitride photodetector (31), a silicon photomultiplier detector (32) and a signal processor (34), wherein the gallium nitride photodetector (31) is arranged on a guiding path of the return light signal and used for absorbing and detecting a violet light part in the return light signal, the silicon photomultiplier detector (32) is arranged on a side of the gallium nitride photodetector (31) opposite to the return light signal and used for receiving and detecting a near-infrared part in the return light signal penetrating through the gallium nitride photodetector (31), and the signal processor (34) is in communication connection with the gallium nitride photodetector (31) and the silicon photomultiplier detector (32) and used for calculating distance or three-dimensional shape information according to received signals. The dual-wavelength laser emission module (1) comprises a violet laser (11), a near-infrared laser (12), a first collimating mirror (13) and a second collimating mirror (14), the violet laser (11) is used for generating violet laser pulses, the near-infrared laser (12) is used for generating near-infrared laser pulses, the first collimating mirror (13) is arranged on a first emission light path of the violet laser (11), the second collimating mirror (14) is arranged on a second emission light path of the near-infrared laser (12), the first emission light path and the second emission light path are coaxial, and an emission end of the violet laser (11) is arranged opposite to an emission end of the near-infrared laser (12); The common-path optical transceiver module (2) comprises a beam combining mirror (21), the beam combining mirror (21) has a first reflecting surface and a second reflecting surface perpendicular to each other, the first reflecting surface is arranged on the first emission light path and forms a 45-degree angle with the first emission light path, the second reflecting surface is arranged on the second emission light path and forms a 45-degree angle with the second emission light path, the violet laser pulses and the near-infrared laser pulses generated by the violet laser (11) and the near-infrared laser (12) respectively are collimated by the first collimating mirror (13) and the second collimating mirror (14) respectively, then are reflected by the first reflecting surface and the second reflecting surface respectively, and are combined into the coaxial laser beam. ​ 2. The lidar ranging and imaging system of claim 1, wherein, ​ ​ 3. The ladar ranging and imaging system of claim 2, wherein, The common-path optical transceiver module further comprises an aperture mirror (22) and a scanning galvanometer assembly (23), the aperture mirror (22) has a central aperture in the center, the central aperture is located on the path of the coaxial laser beam, the scanning galvanometer assembly (23) is arranged on the side of the aperture mirror (22) away from the beam combiner (21), the coaxial laser beam passes through the central aperture and is guided to the target (4) by the scanning galvanometer assembly (23), and the echo light signal returned from the target (4) is guided to the mirror surface of the aperture mirror (22) by the scanning galvanometer assembly (23) and is reflected to the receiving end of the gallium nitride photodetector (31).

4. The ladar ranging and imaging system of claim 3, wherein, The scanning galvanometer assembly (23) comprises a first rotatable galvanometer (231), a second rotatable galvanometer (232), and a first galvanometer driver and a second galvanometer driver for driving the two rotatable galvanometers, respectively, wherein, in the emission path, the first rotatable galvanometer (231) performs the first deflection on the coaxial laser beam, and the second rotatable galvanometer (232) performs the second deflection on the light beam reflected by the first rotatable galvanometer (231) to guide it to the target (4); in the receiving path, the echo light signal is collected and guided by the second rotatable galvanometer (232) and the first rotatable galvanometer (231) in reverse order.

5. The ladar ranging and imaging system of claim 1, wherein, The non-splitting detection module (3) further comprises a filter (33) arranged on the optical path between the gallium nitride photodetector (31) and the silicon photomultiplier detector (32), for filtering out stray light and allowing only near-infrared light in a predetermined wavelength range to pass through.

6. The ladar ranging and imaging system of claim 1, wherein, The gallium nitride photodetector (31) comprises a sapphire substrate (311), an n-type gallium nitride semiconductor layer (312) formed on the sapphire substrate (311), a p-type gallium nitride semiconductor layer (313) formed on the n-type gallium nitride semiconductor layer (312) and constituting a p-n junction with the n-type gallium nitride semiconductor layer (312), and a p-type electrode (314) and an n-type electrode (315) electrically connected to the p-type gallium nitride semiconductor layer (313) and the n-type gallium nitride semiconductor layer (312), respectively. The selective response of the gallium nitride photodetector (31) to violet light and near-infrared light is based on the band gap energy of the gallium nitride material constituting the n-type gallium nitride semiconductor layer (312) and the p-type gallium nitride semiconductor layer (313): the energy of the violet light photon is higher than the band gap energy, so it is absorbed by the p-n junction to generate photocurrent; the energy of the near-infrared light photon is lower than the band gap energy, so it transmits through the gallium nitride photodetector (31).

7. The ladar ranging and imaging system of claim 1, wherein, The distance between the gallium nitride photodetector (31) and the silicon photomultiplier detector (32) along the light signal incident path is less than 1 mm, and they constitute a confocal plane integrated package.

8. The ladar ranging and imaging system of claim 1, wherein, The first wavelength ranges from 380 nm to 420 nm, and the second wavelength ranges from 895 nm to 915 nm.

9. A method for laser radar ranging and imaging, suitable for the laser radar ranging and imaging system as claimed in any one of claims 1-8, and comprising the steps of: synchronously and coaxially emitting violet light laser pulses and near-infrared laser pulses to a water body through the dual-wavelength laser emission module (1) and the common-path optical transceiver module (2); the common-path optical transceiver module (2) combines the violet light laser pulses and the near-infrared laser pulses into a coaxial laser beam and directs it to the water body, and receives and leads out the echo optical signals containing the violet light laser pulses and the near-infrared laser pulses returned from the water body, wherein the violet light laser pulses penetrate the water body and are reflected by the water bottom, carrying the water bottom distance information, and the near-infrared laser pulses are reflected by the water surface, carrying the water surface distance information; a gallium nitride photodetector (31) absorbs and detects the violet light laser pulse part in the echo optical signals to generate a first electrical signal, and a silicon photomultiplier detector (32) receives and detects the near-infrared laser pulse part in the echo optical signals that has penetrated the gallium nitride photodetector (31) to generate a second electrical signal; calculating the time difference between the first electrical signal and the second electrical signal by a signal processor (34), and calculating the water depth of the water body according to the time difference.

10. The ladar ranging and imaging method of claim 9, wherein, The water depth of the water body is calculated by the following formula: ， wherein, Depth is a water depth of the water body, Δt is a time difference between the first and second electrical signals, c is a speed of light in a vacuum, n is a refractive index of the water body.

Images