Device and method for improving range resolution of laser radar based on wavelength division multiplexing

Abstract

The invention discloses a laser radar range resolution improving device and method based on wavelength division multiplexing, and belongs to the field of laser radar system design and data processing.According to the wavelength division multiplexing principle, multi-wavelength laser signals are emitted, processed and output through a signal modulation module after wavelength division multiplexing, and then a plurality of linear frequency modulation signals are output; in a direct acquisition mode, heterodyne receiving is carried out on the demultiplexed signal, an imaging result of an equivalent large modulation bandwidth signal can be obtained through signal processing, and the method can be used for a laser radar system for high-precision distance measurement. Compared with a traditional single-wavelength laser radar structure, the limitation of the modulation bandwidth of an electro-optical modulator can be broken through, and the emission signal bandwidth is multiplied, so that the range resolution of the laser radar is greatly improved, and the imaging quality of a target object is better; meanwhile, the sampling rate requirement of each channel in a direct sampling mode can be reduced, and the problem that a high-bandwidth signal cannot be collected due to an AD rate bottleneck existing in a direct sampling condition is solved.

Description

Technical Field

[0001] The invention discloses a device and method for improving the range resolution of a laser radar based on wavelength division multiplexing, and belongs to the field of laser radar system design and data processing. Background Art

[0002] As a non-contact measurement system that can simultaneously obtain target distance and speed information, LiDAR has the advantages of resistance to ambient light interference, high precision, and high sensitivity. It is widely used in autonomous driving, target identification and tracking, precision manufacturing, etc. Range resolution is one of the important indicators for judging LiDAR systems. Range, also known as the slant range direction, refers to the distance direction along the light beam. The basis of range information, or range imaging, is the laser signal with a large time-width-bandwidth product emitted by the radar. After being reflected by the target object, the echo signal is received in the form of optical heterodyne detection, and a signal containing the Doppler frequency shift phase and target intensity is output. For heterodyne signals, pulse compression processing in the frequency domain can be achieved by means of demodulation and Fourier transform, or time domain pulse compression can be completed by matched filtering to obtain a high-resolution image in the range direction. The range resolution can be expressed as , where c is the speed of light and B is the bandwidth of the transmitted signal. Therefore, in order to improve the range resolution, the bandwidth of the transmitted signal needs to be increased as much as possible.

[0003] Based on the method of high-resolution range information and synthetic aperture, high-resolution range-azimuth imaging of the target can be achieved, namely the synthetic aperture laser radar system (Synthetic Aperture LADAR, SAL). Synthetic aperture technology originates from microwave imaging. Based on the relative motion between the radar platform and the target, the synthesis of virtual large aperture is achieved through algorithms. It is an effective method to improve spatial resolution. Synthetic aperture technology was first effectively verified in the microwave band and widely used in land exploration, terrain detection, military reconnaissance and other fields. The ultimate resolution of synthetic aperture radar is limited by the carrier wavelength and modulation bandwidth. In order to pursue higher resolution, laser is used as the carrier, the basic principle of synthetic aperture is followed, and some advantages of laser radar and traditional synthetic aperture microwave radar are combined to develop synthetic aperture laser radar for imaging fixed targets by moving radar system and inverse synthetic aperture laser radar (Inverse Synthetic Aperture LADAR, ISAL) for imaging moving targets by fixed radar system.

[0004] At present, many domestic and foreign scientific research institutions have conducted in-depth research on SAL / ISAL.

[0005] For example, the ground-based Firepond laser radar of the Lincoln Laboratory of MIT in the United States (Delaney WP, Ward W W. Radar Development at Lincoln Laboratory: An Overview of the First Fifty Years [J]. 2000.) is quite representative. The tracking experiment was carried out with the in-orbit ocean satellite (Seasat) at a distance of 800-1000km as the target, and the world's first range-Doppler image of an orbiting satellite was successfully obtained. In the subsequent sounding rocket "Firefly" experiment conducted by Lincoln Laboratory, rapid imaging and target capture were achieved, and sounding rockets and false targets were quickly identified. At this point, ISAL research has entered the strategically significant anti-missile field application.

[0006] For example, in 2002, the U.S. Naval Laboratory developed the first two-dimensional scanning synthetic aperture lidar verification system (Bashkansky M, Lucke RL, Funk E, et al. Two-dimensional synthetic aperture imaging in the optical domain [J]. Optics letters, 2002, 27 (22): 1983). The laser source uses an external cavity single-mode tunable laser, and the output laser signal is a linear frequency modulation signal. In the 1550nm infrared band, it achieves a two-dimensional imaging of a 1cm×1cm object with a resolution of 90μm×170μm, which is the minimum azimuth imaging resolution that can be achieved in all SAL / ISAL experiments at home and abroad.

[0007] For example, in 2021, the team led by Researcher Wei Kai from the Institute of Optoelectronics Technology of the Chinese Academy of Sciences successively announced the ISAL imaging results of 1 km and 6.9 km. A detailed analysis was conducted on the problem of laser signal amplitude distortion caused by the uneven electro-optical response of the I / Q modulator in the system, and for the first time, the self-heterodyne detection method was proposed to measure and compensate for the laser signal amplitude distortion. At the same time, motion compensation experiments were carried out using the platform, and a global joint motion error compensation algorithm based on the Nelder-Mead simplex method and particle swarm optimization was proposed. The parameter estimation error is mainly distributed within ±0.2%, and the parameter estimation accuracy and noise resistance performance are better than those of traditional ISAL imaging algorithms.

[0008] In addition, some scientific research institutes have also conducted preliminary research on SAL / ISAL indoor and outdoor system verification and airborne radar experiments, with significant results.

[0009] In the above studies, in order to obtain a large-bandwidth linear frequency modulation signal, the laser internal modulation and external modulation methods were mainly used, both of which have certain shortcomings. Since it is difficult for the seed source laser to output an ideal linear frequency modulation signal, the internal modulation method is often used to obtain it approximately, that is, to adjust the cavity length of the laser resonant cavity to output a wavelength linear modulation signal with a larger bandwidth. Obviously, since the wavelength is inversely proportional to the frequency, the frequency linearity of the signal obtained by this method is not ideal. Although the external modulation method (electro-optical modulation, acousto-optic modulation, etc.) can be controlled by RF signals to obtain a modulation effect with good frequency linearity, the modulation principle and device process greatly limit the signal bandwidth. In the existing ISAL imaging system based on electro-optical modulation, the bandwidth of the linear frequency modulation signal is mainly limited by the performance of the electro-optical modulator and the modulation control module (such as the arbitrary waveform generator AWG). If the arbitrary waveform generator is required to generate a linear frequency modulation signal with a larger bandwidth, the experimental cost will increase significantly, so it is difficult to achieve a doubling of the bandwidth in this mode.

[0010] In summary, in order to obtain high range resolution, it is necessary to improve the existing technology and effectively increase the equivalent bandwidth of the transmitted signal while ensuring the linearity of the frequency modulation.

[0011] Wavelength Division Multiplexing (WDM) technology, as an effective means to increase the amount of information transmitted, can provide a new idea for the laser radar system to expand the bandwidth of the transmitted signal. Wavelength Division Multiplexing is a technology that realizes the multiplexing and transmission of multiple wavelength optical signals in one optical fiber at the same time. Its basic principle is that in the transmission optical fiber, it is divided into several optical channels according to the wavelength or frequency. Each channel allows the optical carrier of a specific frequency (frequency band) to pass through. At the transmitting end, optical signals of different wavelengths are combined (called multiplexing) and coupled to the same optical fiber for transmission. At the receiving end, the optical signals of the combined wavelengths are separated (called demultiplexing), and the original signals are restored and sent to different terminals, thereby doubling the amount of information transmitted. Summary of the invention

[0012] The problem to be solved by the present invention is: how to achieve a significant increase in the equivalent bandwidth of the transmitted signal at a relatively low cost while ensuring the linearity of the frequency modulation in the SAL / ISAL imaging system, thereby improving the range resolution and further improving the imaging effect of the inverse laser radar.

[0013] The technical solution adopted by the present invention to solve its technical problems is: a device and method for improving the range resolution of a laser radar based on wavelength division multiplexing. Through wavelength division multiplexing, multi-wavelength signal light is used for laser radar range imaging. In direct acquisition mode, the target echo signal is received by coherent detection. By shifting and splicing the spectrum of the echo signal of multiple frequency bands, the total signal band after processing is expanded to nearly N times the modulation bandwidth B, thereby breaking through the limitation of the modulation bandwidth of the electro-optical modulation device. In theory, the imaging result of the equivalent multiple bandwidth modulation signal can be obtained, and the range resolution of the laser radar image can be improved.

[0014] In a first aspect, the present invention provides a method for improving the range resolution of a laser radar based on wavelength division multiplexing, the method comprising the following steps: Step 1: Generate N frequency modulation signal clusters with different center frequencies and the same modulation bandwidth according to the frequency modulation information, one part of which is wavelength division multiplexed and beam-combined to illuminate the target, and the other part is used as the local oscillator signal to participate in heterodyne detection; Step 2: Demultiplex and split the target echo signal according to the corresponding band of the center frequency of the linear frequency modulation signal, and perform homodyne detection processing together with the local oscillator signal of the corresponding frequency to obtain the corresponding N electrical signals; Step 3: Spectrum shifting and splicing of N electrical signals are performed. The total signal bandwidth after splicing is close to N times the modulation bandwidth. After matched filtering, the laser radar range imaging is completed.

[0015] Furthermore, the frequency bands of any two modulated signals in the linear frequency modulated signal cluster do not overlap.

[0016] Furthermore, the frequency modulation signal is a linear frequency modulation signal, a randomly modulated continuous wave signal or a phase coded signal.

[0017] Furthermore, the frequency modulated signal cluster is amplified and collimated before being transmitted to the transmission channel and irradiated on the target.

[0018] Furthermore, the target echo signal is demultiplexed and split to generate N sub-echo signals, and the N sub-echo signals and the local oscillator light of the corresponding band are subjected to optical coherent detection processing in a direct sampling mode to obtain corresponding N electrical signals.

[0019] Furthermore, the N electrical signals are respectively transmitted to corresponding phase modulators, and based on the prior conditions of the transmission signal frequency band, the phase modulator parameters are adjusted to achieve spectrum shifting and end-to-end splicing of the N electrical signals.

[0020] Furthermore, after being converted into digital signals, the N electrical signals are respectively transmitted to the digital domain for data processing and imaging, and spectrum shifting and end-to-end splicing of the N electrical signals are achieved based on frequency modulation.

[0021] On the other hand, the present invention also provides a wavelength division multiplexing-based laser radar distance resolution enhancement device, the device includes a laser signal generation module, a signal modulation module, a modulation control module, a laser amplification module, a signal transmission module, a signal receiving module, a signal detection module, a data acquisition module and a data processing and imaging module, wherein: The laser signal generating module is used to generate N single-frequency laser emission signals of different wavelengths, and couple the laser emission signals to the same optical fiber for transmission by wavelength division multiplexing. A part of the wavelength division multiplexing signal is transmitted to the signal modulation module, and the other part is transmitted to the signal detection module as a local oscillator signal for coherent detection of the target return light signal in the direct sampling mode. The signal modulation module is used to simultaneously modulate the N single-frequency laser emission signals with different wavelengths to generate N modulation signals with different center frequencies and the same modulation bandwidth, and send them to the laser amplification module; The modulation control module is used to generate specific modulation information to control the modulation process and modulation parameters of the modulation module; The laser amplification module is used to amplify the modulated signal cluster output by the signal modulation module to meet the detection power threshold requirement of the signal receiving module; The signal transmitting module is used to collimate the amplified modulated signal cluster and transmit it to the transmission channel to illuminate the target; The signal receiving module is used to receive the echo signal after being reflected by the target, and couple it into the optical fiber to transmit it to the signal detection module; The signal detection module includes a wavelength division multiplexer and N photoelectric detectors, which are used to demultiplex the echo signal according to the band setting to generate N sub-echo signals, which enter the corresponding N transmission optical fibers respectively; perform optical coherence detection processing on the N sub-echo signals and the local oscillator light of the corresponding band output by the laser generation module to obtain corresponding N electrical signals; shift the spectrum of the N electrical signals and splice them head to tail, and send the processed electrical signals to the data acquisition module; The data acquisition module is used to collect the processed echo electrical signal, convert the electrical signal into a digital signal, and send it to the data processing and imaging module; The data processing and imaging module is used to process the signal output by the data acquisition module to achieve an imaging result equivalent to an N-fold bandwidth modulation signal, thereby improving the range resolution of the laser radar image.

[0022] Furthermore, the signal detection module also has N phase modulators, and the obtained N electrical signals are respectively transmitted to the corresponding phase modulators. Based on the prior conditions of the transmission signal frequency band, the phase modulator parameters are adjusted to achieve spectrum shifting and end-to-end splicing of the N electrical signals.

[0023] Furthermore, after the N electrical signals are converted into digital signals, they are respectively transmitted to the digital domain of the data processing and imaging module to achieve spectrum shifting and end-to-end splicing of the N electrical signals.

[0024] Compared with the known single-wavelength laser radar system, the present invention has the following gain effects: 1. Based on the principle of wavelength division multiplexing, the present invention uses laser signals of multiple wavelengths for ISAL imaging. Under the premise of ensuring good coherence and frequency sweep linearity of the imaging system, the imaging result of equivalent large-bandwidth transmission signals is obtained by collecting and processing multi-wavelength target echo signals, which greatly improves the range resolution, while improving the frequency band utilization and the amount of information transmitted. The signal bandwidth and range resolution are multiplied at a relatively low cost, and the required AD sampling rate is reduced, which enhances the practicality of the technology.

[0025] 2. The ISAL system based on the wavelength division multiplexing principle proposed in the present invention can reduce the interference of noise on imaging and improve the signal-to-noise ratio. After the dense wavelength division multiplexing device demultiplexes the echo signal, it densely splits it into multiple narrow-band signals at specific wavelength intervals, which can be regarded as a narrow-band filter. The bandwidth of each branch signal output is narrow, which reduces the influence of noise, improves the efficiency of subsequent heterodyne detection, and improves the signal-to-noise ratio.

[0026] 3. The present invention has a simple design, compact structure, and is easy to integrate and load with other systems for use in conjunction. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0028] Figure 1 A general structural diagram of a wavelength division multiplexing-based inverse laser radar range resolution enhancement device provided in Example 1 of the present invention (direct sampling mode).

[0029] Figure 2 A schematic diagram of the optimized structure of a wavelength division multiplexing-based laser radar range resolution enhancement device provided in Example 1 of the present invention.

[0030] Figure 3 This is a schematic diagram of a wavelength division multiplexing output signal provided in Embodiment 1 of the present invention.

[0031] Figure 4This is a schematic diagram of a linear frequency modulation signal cluster provided in Embodiment 1 of the present invention.

[0032] Figure 5 This is a flow chart of a method for improving the range resolution of a synthetic aperture lidar based on wavelength division multiplexing provided in Example 2 of the present invention.

[0033] Figure 6 A schematic diagram of a signal spectrum after phase splicing provided in Embodiment 2 of the present invention.

[0034] Figure 7 This is the output signal after matched filtering provided in the second embodiment of the present invention.

[0035] Reference numerals: 1: laser signal generating module, 11: first single-frequency laser, 12: second single-frequency laser, 13: third single-frequency laser, 14: first dense wavelength division multiplexer, 2: signal modulation module, 3: modulation control module, 4: laser amplifying module, 5: signal transmitting module, 6: signal receiving module, 7: signal detection module, 71: first photodetector, 72: second photodetector, 73: third photodetector, 74: second dense wavelength division multiplexer, 75: first phase modulator, 76: second phase modulator, 77: third phase modulator, 8: data acquisition module, 9: data processing and imaging module. DETAILED DESCRIPTION

[0036] Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although certain embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as being limited to the embodiments described herein, which are instead provided for a more thorough and complete understanding of the present invention. It should be understood that the drawings and embodiments of the present invention are only for exemplary purposes and are not intended to limit the scope of protection of the present invention.

[0037] The terms "first", "second", "third", etc. (if any) in the description and claims of the embodiments of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0038] Embodiment 1: Figure 1 A general structural diagram of a wavelength division multiplexing-based laser radar range resolution enhancement device provided in Example 1 of the present invention (direct acquisition mode), including: a laser signal generating module 1, a signal modulation module 2, a modulation control module 3, a laser amplification module 4, a signal transmitting module 5, a signal receiving module 6, a signal detection module 7, a data acquisition module 8, and a data processing and imaging module 9.

[0039] Specifically, the laser signal generating module 1 is used to generate N single-frequency laser emission signals of different wavelengths, and couple the laser emission signals to the same optical fiber for transmission using wavelength division multiplexing. Part of the wavelength division multiplexing signal is transmitted to the signal modulation module 2, and part of it is transmitted to the signal detection module 7 as a local oscillator signal for coherent detection of the target return light signal in the direct acquisition mode.

[0040] Among them, laser heterodyne detection uses laser as the local oscillator source and performs optical mixing with the weak echo signal modulated by the object to be detected in the fiber coupler, which retains information such as the amplitude and phase of the measured light field, and has the advantages of noise suppression and high gain, which well meets the needs of high-precision distance imaging of lidar.

[0041] Preferably, in this embodiment, the laser signal generating module 1 can be a fiber laser, a solid laser, a semiconductor diode laser, a gas laser, a dye laser, a diode-pumped solid laser, a free electron laser or a Raman laser, etc., and the wavelength range can cover near-infrared light, visible light, and ultraviolet light.

[0042] Specifically, the signal modulation module 2 is used to simultaneously modulate N single-frequency laser emission signals with different wavelengths to generate N modulation signals with different center frequencies and the same modulation bandwidth, and send them to the laser amplification module 4.

[0043] In this embodiment, the signal modulation module 2 is an electro-optic modulator, an acousto-optic modulator or other components. The frequency bands of any two modulated signals in the signal cluster do not overlap, so as to ensure that the crosstalk during transmission is as small as possible.

[0044] Specifically, the modulation control module 3 is used to generate specific modulation information, such as a linear frequency modulation signal, or a randomly modulated continuous wave signal, or a phase coded signal, to control the modulation process and modulation parameters of the signal modulation module 2 .

[0045] In this embodiment, the modulation control module 3 is an arbitrary waveform generator, a radio frequency signal generator, a field programmable gate array (FPGA) or other components.

[0046] Specifically, the laser amplification module 4 is used to amplify the modulated signal cluster output by the signal modulation module 2 to meet the detection power threshold requirement of the signal receiving module 6 .

[0047] In this embodiment, the laser amplification module 4 can be an ytterbium-doped fiber amplifier YDFA, or an erbium-doped fiber amplifier EDFA, or other components. According to different central wavelengths of the laser signal generation module 1, a matching optical amplifier can be selected. For example, when the laser signal generation module 1 outputs a wavelength of 1 μm, the laser amplification module 4 uses YDFA; when the laser signal generation module 1 outputs a wavelength of 1550 nm, EDFA is selected; Specifically, the signal transmitting module 5 is used to collimate the amplified linear frequency modulation signal cluster and then transmit it to the transmission channel to illuminate the target.

[0048] In this embodiment, the signal transmitting module 5 is a laser beam combiner, a collimating lens or other components, which collimates the amplified linear frequency modulation signal cluster into the same optical path, transmits and illuminates the target surface to ensure that the transmission conditions of signals in each frequency band are as consistent as possible.

[0049] Specifically, the signal receiving module 6 includes a collimating lens or other components, which is used to receive the echo signal after being reflected by the target, and couple it into the optical fiber to transmit it to the signal detection module 7.

[0050] Specifically, the signal detection module 7 includes a wavelength division multiplexer, N phase modulators and N photodetectors, which can be a balanced amplified photodetector, a single photon counter, or other components, and is used to demultiplex the echo signal according to the band setting to generate N sub-echo signals, which enter the corresponding N transmission optical fibers respectively, and perform optical coherence detection processing on the N sub-echo signals and the local oscillator light of the corresponding band output by the laser generation module 1 to obtain the corresponding N electrical signals. The N electrical signals are respectively transmitted to the corresponding phase modulators, and the phase modulator parameters are adjusted based on the prior conditions such as the transmission signal frequency band to achieve the spectrum shifting and splicing of the N electrical signals. The processed electrical signals are sent to the data acquisition module 8; Specifically, the data acquisition module 8 may be a signal acquisition card, an oscilloscope, or other components, and is used to identify the different heterodyne echo electrical signals, and convert the electrical signals into digital signals, and send them to the data processing and imaging module 9; Specifically, the data processing and imaging module 9 is used to process the signal output by the data acquisition module 8 to achieve an imaging result equivalent to an N-fold bandwidth modulation signal, thereby improving the range resolution of the laser radar image.

[0051] Preferably, the spectrum shifting process of N echo signals is realized in the signal detection module 7 through corresponding N phase modulators. This process can also be completed in the digital domain by the data processing and imaging module 9, that is, the N phase modulators in the signal detection module 7 are removed, and the N electrical signals generated after zero-difference detection are directly transmitted to the data acquisition module 8.

[0052] Specifically, the device can solve the problem that high-bandwidth signals cannot be collected due to the AD rate bottleneck in direct sampling by splicing multiple channels. In scenarios where the target distance is unknown or the distance varies greatly, the advantage of the direct sampling mode is that, compared with the traditional de-linear frequency modulation (Dechrip) solution, it has low requirements for delay fiber length matching and a wider range of applications in actual scenarios; but it requires a high-speed, high-bandwidth AD sampling system to cooperate. The device and method samples and splices the corresponding sub-bands through N channels respectively, reducing the AD rate requirement of a single channel to 1 / N times the previous one.

[0053] Preferably, in order to describe this embodiment more specifically, Figure 1 On the basis of the general structural diagram provided, the device is further refined, and a natural number N=3 is taken in the laser signal generating module 1 and the signal detecting module 7, that is, the laser signal generating module 1 generates single-frequency laser emission signals of three different wavelengths, and the signal detecting module 7 includes three detectors, which perform heterodyne detection according to the corresponding bands.

[0054] Preferably, the laser signal generating module 1 can select three narrow-linewidth single-frequency lasers to generate laser signals of different wavelengths as local oscillator signals and transmission signals.

[0055] Figure 2 This is a schematic diagram of an optimized structure of a device for improving the range resolution of a laser radar based on wavelength division multiplexing provided in the first embodiment of the present invention. Figure 2 The experimental setup shown is Figure 1 On this basis, the device is further refined, including the following technical solutions: The laser signal generating module 1 is refined, including a first single-frequency laser 11, a second single-frequency laser 12, a third single-frequency laser 13, and a first dense wavelength division multiplexer 14; the signal detecting module 7 is refined, including: a first photodetector 71, a second photodetector 72, a third photodetector 73, a second dense wavelength division multiplexer 74, a first phase modulator 75, a second phase modulator 76, and a third phase modulator 77; the positions, connection methods, functions, and parameters of other components are the same as those of the present invention. Figure 1 remains unchanged.

[0056] Specifically, the first single-frequency laser 11, the second single-frequency laser 12, and the third single-frequency laser 13 are used to output a single-frequency narrow-linewidth laser signal respectively, a part of which is multiplexed and coupled by the first dense wavelength division multiplexer 14 and transmitted to the signal modulation module 2, and a part of which is transmitted to the signal detection module 7 as a local oscillator signal for heterodyne detection of the target return light signal.

[0057] Preferably, in this embodiment, for the consideration of frequency band utilization and eye safety, the center wavelengths of the first single-frequency laser 11, the second single-frequency laser 12, and the third single-frequency laser 13 are respectively set to 1550.0nm, 1550.6nm, and 1551.2nm, and the Lorentz line widths are all 1kHz.

[0058] Preferably, the adjacent wavelength interval of the first dense wavelength division multiplexer 14 is 0.3nm, and the signal crosstalk is not obvious during the wavelength division multiplexing, transmission, and demultiplexing of each central wavelength laser signal. The output signal schematic diagram of the first dense wavelength division multiplexer 14 is as shown in Figure 3 shown.

[0059] Preferably, for economic cost and equipment parameter considerations, the modulation bandwidth of the signal modulation module 2 is 4 GHz, corresponding to a wavelength span of 0.032 nm, to ensure that after modulation, the frequency bands of the linear frequency modulation signals do not overlap. Figure 4 shown.

[0060] Specifically, the second dense wavelength division multiplexer 74 is used to demultiplex the echo signal output by the signal receiving module 6 and output it in split beams.

[0061] Preferably, the second dense wavelength division multiplexer 74 outputs multiple beams of signals with adjacent wavelength intervals of 0.3nm. Optionally, corresponding to the first single-frequency laser 11, the second single-frequency laser 12, and the third single-frequency laser 13 in the laser signal transmitting module 1, the second dense wavelength division multiplexer 74 selects three sub-echo signals with central wavelengths of 1550.0nm, 1550.6nm, and 1551.2nm for output after demultiplexing. Since the sub-echo signals carry modulation information and target information, they have a certain spectral width, which is much smaller than the adjacent wavelength interval of 0.3nm. Therefore, each sub-echo signal can be demultiplexed without crosstalk, and the modulation information and target information are retained.

[0062] Specifically, the first photodetector 71 is used to mix the single-frequency local oscillator light output by the first single-frequency laser 11 with the sub-echo signal of its corresponding band and perform homodyne detection processing. The homodyne signal is output to the data acquisition module 8 in the form of an electrical signal through the first phase modulator 75.

[0063] Similarly, the second photodetector 72 is used to mix the single-frequency local oscillator light output by the second single-frequency laser 12 with the sub-echo signal of its corresponding band and perform homodyne detection processing. The homodyne signal is output to the data acquisition module 8 in the form of an electrical signal through the second phase modulator 76.

[0064] Similarly, the third photodetector 73 is used to mix the single-frequency local oscillator light output by the third single-frequency laser 13 with the sub-echo signal of its corresponding band and perform homodyne detection processing. The homodyne signal is output to the data acquisition module 8 in the form of an electrical signal through the third phase modulator 77.

[0065] Based on the prior conditions such as the frequency band of the transmitted signal, the parameters of the first photodetector 71, the second photodetector 72, and the third photodetector 73 of the phase modulator are adjusted to achieve the spectrum shifting and end-to-end splicing of the three electrical signals. The processed electrical signals are sent to the data acquisition module 8; different data processing methods can be used for different frequency modulation methods. If it is for linear frequency modulation, if the method of de-linear frequency modulation is used, then in theory, the signal can be directly spliced, and then Fourier transform is performed to achieve the end-to-end splicing of the electrical signal; if it is for linear frequency modulation or encoding, the matched filtering method is used, and in theory, the matched filtering should be completed first before the splicing of the spectrum.

[0066] Preferably, in order to reduce the complexity of the system, an optical frequency comb module or a multi-wavelength laser can be selected in the laser signal generating module 1 in combination with cost and actual needs to generate laser signals of different wavelengths, one part of which is directly used as a seed source signal and emitted after a modulation and amplification process, and the other part is demultiplexed by a dense wavelength division multiplexer, or processed by narrowband filtering to extract the response frequency band as a local oscillator signal. The implementation principle and effect are similar, and will not be repeated here one by one.

[0067] Embodiment 2: Figure 5 This is a flow chart of a method for improving the range resolution of synthetic aperture laser radar based on wavelength division multiplexing provided in the second embodiment of the present invention. Figure 5 As shown, the execution subject of the distance resolution improvement method provided in this embodiment is the wavelength division multiplexing-based laser radar distance resolution improvement device provided in Embodiment 1 of the present invention, and the method includes the following steps: Step 1: Generate multiple single-frequency laser emission signals with different wavelengths according to the linear frequency modulation information, perform wavelength division multiplexing and beam combining on one part, and use the other part as the local oscillator signal to participate in the heterodyne detection in step 4; in, Each local oscillator signal is

[0068]

[0069] …

[0070] in, is the signal amplitude, is the center frequency of the Nth local oscillator signal, is the random phase of the Nth local oscillator signal. Here, N=3 is taken as an example.

[0071] The signal after wavelength division multiplexing is:

[0072] Step 2: Modulate the wavelength division multiplexing signal to generate a linear frequency modulation signal cluster:

[0073] In the formula, , for full time, is the distance time, called fast time, It is the azimuth time, called slow time. is the time width of the signal distance direction, is the modulation frequency (the rate of change of the modulation frequency). The signal is amplified and collimated before being transmitted to the atmospheric channel; Step 3: After the target echo signal is received and converged, it is demultiplexed and split according to the frequency band corresponding to the center frequency of each local oscillator signal, and the specific frequency band information is output; The echo signal is

[0074] in, is the amplitude of the echo signal, and the time delay of the echo signal relative to the pulse signal , is the distance from the target to the radar, The speed of light.

[0075] Step 4: The demultiplexed signal is processed with the corresponding frequency local oscillator signal in step 1 to generate three homodyne signals:

[0076]

[0077]

[0078] Step 5: Based on the prior conditions such as the frequency band of the transmitted signal, phase modulate the three electrical signals and introduce a frequency shift phase to each of them:

[0079]

[0080]

[0081] The spectrum is shifted and spliced, and the frequency shift amount is close to the modulation bandwidth in step 2. It is adjusted according to the signal and imaging conditions in step 6, and it is optimized repeatedly to reduce the impact of the frequency band overlap area on imaging. The schematic diagram of the signal spectrum after phase splicing is shown in the figure. Figure 6 As shown, the bandwidth is nearly three times the frequency band of a single echo signal.

[0082] Step 6: Perform matching filtering and other processing on the processed electrical signal. In the traditional single-wavelength laser signal SAL imaging process, the matched filter function of the f1 frequency band is generally the complex conjugate of the input signal (taking the ideal case of a center frequency of 0 and no random phase as an example), that is,

[0083] The expression of matched filter output is:

[0084] like Figure 7 The pulse width of the output sinc function is the distance resolution after pulse compression.

[0085] because It is the modulation bandwidth of the linear frequency modulation signal. The resolution is inversely proportional to the bandwidth, so increasing the bandwidth can improve the range resolution. The phase-stitched signal obtained in step 5 can be equivalently regarded as the echo signal obtained after the N-times bandwidth modulation signal is reflected by the target. It is used for laser radar range imaging to achieve an imaging result equivalent to the N-times bandwidth modulation signal, so the range resolution can be improved to the 1 / N level.

[0086] The present invention is not limited to the above-mentioned specific implementation modes. Any modifications and changes to the present invention within the spirit and principle of the present invention should also fall within the protection scope of the claims of the present invention.

Claims

1. A method for improving the range resolution of a laser radar based on wavelength division multiplexing, characterized in that: The method comprises the following steps: Step 1: Generate N frequency modulation signal clusters with different center frequencies and the same modulation bandwidth according to the frequency modulation information, one part of which is wavelength division multiplexed and beam-combined to illuminate the target, and the other part is used as the local oscillator signal to participate in heterodyne detection; Step 2: Demultiplex and split the target echo signal according to the corresponding band of the center frequency of the linear frequency modulation signal, and perform homodyne detection processing together with the local oscillator signal of the corresponding frequency to obtain the corresponding N electrical signals; Step 3: Spectrum shifting and splicing of N electrical signals are performed. The total signal bandwidth after splicing is close to N times the modulation bandwidth. After matched filtering, the laser radar range imaging is completed.

2. According to the wavelength division multiplexing-based laser radar range resolution improvement method of claim 1, it is characterized in that: The frequency bands of any two modulated signals in the linear frequency modulated signal cluster do not overlap.

3. The method for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 1, characterized in that: The frequency modulation signal is a linear frequency modulation signal, a random modulation continuous wave signal or a phase coding signal.

4. The method for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 1, characterized in that: The FM signal cluster is amplified and collimated and then transmitted to the transmission channel, irradiating the target.

5. The method for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 1, characterized in that: The target echo signal is demultiplexed and split to generate N sub-echo signals, and the N sub-echo signals and the local oscillator light of the corresponding band are subjected to optical coherent detection processing in the direct sampling mode to obtain corresponding N electrical signals.

6. The method for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 5, characterized in that: The N electrical signals are respectively transmitted to the corresponding phase modulators, and based on the prior conditions of the transmission signal frequency band, the phase modulator parameters are adjusted to achieve spectrum shifting and end-to-end splicing of the N electrical signals.

7. The method for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 5, characterized in that: After being converted into digital signals, the N electrical signals are respectively transmitted to the digital domain for data processing and imaging, and the spectrum shifting and end-to-end splicing of the N electrical signals are realized based on the frequency modulation method.

8. A device for improving the range resolution of a laser radar based on wavelength division multiplexing, characterized in that: The device includes a laser signal generating module, a signal modulating module, a modulation control module, a laser amplifying module, a signal transmitting module, a signal receiving module, a signal detecting module, a data collecting module and a data processing and imaging module, wherein: The laser signal generating module is used to generate N single-frequency laser emission signals of different wavelengths, and couple the laser emission signals to the same optical fiber for transmission by wavelength division multiplexing. A part of the wavelength division multiplexing signal is transmitted to the signal modulation module, and the other part is transmitted to the signal detection module as a local oscillator signal for coherent detection of the target return light signal in the direct sampling mode. The signal modulation module is used to simultaneously modulate the N single-frequency laser emission signals with different wavelengths to generate N modulation signals with different center frequencies and the same modulation bandwidth, and send them to the laser amplification module; The modulation control module is used to generate specific modulation information to control the modulation process and modulation parameters of the modulation module; The laser amplification module is used to amplify the modulated signal cluster output by the signal modulation module to meet the detection power threshold requirement of the signal receiving module; The signal transmitting module is used to collimate the amplified modulated signal cluster and transmit it to the transmission channel to illuminate the target; The signal receiving module is used to receive the echo signal after being reflected by the target, and couple it into the optical fiber to transmit it to the signal detection module; The signal detection module includes a wavelength division multiplexer and N photoelectric detectors, which are used to demultiplex the echo signal according to the band setting to generate N sub-echo signals, which enter the corresponding N transmission optical fibers respectively; perform optical coherence detection processing on the N sub-echo signals and the local oscillator light of the corresponding band output by the laser generation module to obtain corresponding N electrical signals; shift the spectrum of the N electrical signals and splice them head to tail, and send the processed electrical signals to the data acquisition module; The data acquisition module is used to collect the processed echo electrical signal, convert the electrical signal into a digital signal, and send it to the data processing and imaging module; The data processing and imaging module is used to process the signal output by the data acquisition module to achieve an imaging result equivalent to an N-fold bandwidth modulation signal, thereby improving the range resolution of the laser radar image.

9. The device for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 8, characterized in that: The signal detection module also has N phase modulators. The obtained N electrical signals are transmitted to the corresponding phase modulators respectively. Based on the prior conditions of the transmission signal frequency band, the phase modulator parameters are adjusted to achieve spectrum shifting and end-to-end splicing of the N electrical signals.

10. The device for improving the range resolution of a laser radar based on wavelength division multiplexing according to claim 8, characterized in that: After the N electrical signals are converted into digital signals, they are transmitted to the digital domain of the data processing and imaging module respectively, so as to shift the spectrum of the N electrical signals and splice them end to end.

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