CN117805854B - Laser SAL wide-field imaging device and method based on MIMO - Google Patents

Abstract

The present invention provides a MIMO-based laser SAL wide-field imaging device and method, which belongs to the field of synthetic aperture laser radar wide-field imaging, draws on microwave multi-input multi-output synthetic aperture radar SAR technology, solves the laser wide field of view and long-distance detection problems, proposes a multi-input multi-output array synthetic aperture laser radar, constructs an optical microlens array, and realizes a large field of view in azimuth and distance. On this basis, the light field energy superposition in the field of view is realized through array transmission, and different receiving and transmitting channel combinations are used to form equivalent phase center signals at different positions, so as to realize high-resolution wide-surveying-band synthetic aperture laser radar imaging. The present invention adopts MIMO array synthetic aperture laser radar technology, and multiple channels simultaneously transmit and receive mutually separable transmission waveforms to complete synthetic aperture laser radar imaging.

Description

Laser SAL wide-field imaging device and method based on MIMO

Technical Field

The present invention belongs to the field of synthetic aperture laser radar (Synthetic Aperture Ladar, SAL) wide field of view imaging, and specifically relates to a laser SAL wide field of view imaging device and method based on multiple-input multiple-output (Multiple-Input-Multiple-output, MIMO).

Background technique

Since 2000, with the maturity of laser technology and synthetic aperture technology, there has been a boom in synthetic aperture lidar imaging research at home and abroad. At present, the research on laser SAL is mainly based on the single-transmitter single-receiver SAL system. As an ideal way to observe long-distance high resolution, laser SAL technology is developing towards higher resolution, longer range, and larger mapping width.

In the traditional single-transmitter single-receiver SAL system, the laser divergence angle commonly used for long-distance detection is small, and the contradiction between the range mapping bandwidth and the azimuth resolution is inevitable, which limits the application of laser SAL technology. There are two main reasons for the narrow range mapping bandwidth:

First, the laser wavelength is short and the beam is narrow. The distance mapping bandwidth of the laser SAL is affected by the distance beam width of the system. Increasing the beam width will increase the mapping swath width, but will also reduce the detection distance. That is, there is a contradiction between the divergence angle and the detection distance, making it difficult to achieve a long-distance wide mapping swath.

Second, there is a contradiction between the range mapping bandwidth and the azimuth resolution, which is caused by their different requirements for the system pulse repeat frequency (PRF). When the single-transmitter and single-receiver laser SAL system performs high-resolution observations in azimuth, its range mapping bandwidth will be limited to a certain range.

In order to solve the first problem mentioned above, a multi-fiber array is used as the light source in the transmitting and receiving antenna to increase the number of array elements in the range direction. For example, Tang Yu's research group at Xidian University uses multi-transmit and multi-receive in the range direction to achieve a wide range mapping zone. For the second reason, Tang Yu's research group uses multi-transmit and multi-receive in azimuth to increase the divergence angle in azimuth and achieve high azimuth resolution imaging of synthetic aperture lidar. The above two solutions theoretically solve the problems of high resolution in azimuth and width in range. However, considering that the core diameter of the optical fiber of the commonly used light source is 10μm, the cladding diameter is 127μm, the minimum spacing of the optical fiber array is 127μm, and the filling factor between array elements is 10/127. The filling factor is defined as the ratio of the diameter of the array element to the spacing between the array elements. For the above multi-transmit SAL in the range direction, the array light source is imaged as multiple separated imaging areas in the far field, making it difficult to achieve continuous expansion of the mapping zone. For the above multi-transmit and multi-receive synthetic aperture lidar in azimuth, the azimuth array light source is imaged as multiple separated imaging areas in the far field, making it difficult to continuously increase the azimuth beam width.

Summary of the invention

In view of the above problems and considering the technical difficulties of discontinuous imaging in reality, the present invention provides a MIMO-based laser SAL wide-field imaging device and method, draws on microwave MIMO SAR technology to solve the contradiction between divergence angle and detection distance, proposes a multi-transmitter and multi-receiver array laser SAL, constructs an optical small-aperture microlens array, and realizes a wide field of view through a small aperture. On the basis of the wide field of view, a microlens array is used to increase the light field energy in the field of view and improve the detection distance. Different combinations of transmit and receive channels are used to form equivalent phase center signals at different positions, thereby meeting the application requirements of laser SAL imaging with high resolution and wide mapping band.

In order to achieve the above object, the present invention adopts the following technical scheme:

A MIMO-based laser SAL wide-field imaging device comprises 16 array transceiver devices with 4 columns in azimuth and 4 rows in distance, wherein a MIMO mode is formed between the array transceiver devices, and the array transceiver device comprises a transmitting system and a receiving system; the transmitting system comprises a laser source, a modulation module, an optical amplification module, a transmitting optical fiber array and a microlens array; the receiving system comprises an array detection unit and an array acquisition imaging processing unit; the transmitting optical fiber array and the microlens array constitute a transmitting array, and the array detection unit constitutes a receiving array; each transmitting optical fiber array is used for emitting a laser beam, and the emitted laser beam is collimated and irradiated to a far field through a microlens array to obtain a larger surveying and mapping bandwidth; an echo signal enters the array detection unit, and then enters the array acquisition imaging processing unit for array acquisition imaging processing.

The present invention also provides a MIMO-based laser SAL wide-field imaging method, comprising the following steps:

Step 1) Design the azimuth and range divergence angle of the emitted beam according to the resolution and width requirements in the laser SAL application;

Step 2) According to the detection distance requirement and based on the radar equation, determine the number of multiple-transmit and multiple-receive arrays in azimuth and range directions, design a microlens array corresponding to the transmitting optical fiber array, and form a multiple-transmit and multiple-receive array;

Step 3) Assume that the multi-transmitter multi-receiver array with a microlens array is a two-dimensional array, in which each transmitting optical fiber and the corresponding microlens of the transmitting system are a transmitting array element, and the near-field beam amplitude is obtained by synthesizing the microlens array;

Step 4) The far-field light intensity distribution is regarded as the Fourier transform of the output light field of the microlens array, thereby obtaining the field distribution formed by the output light of the multi-transmitter and multi-receiver array in the far field;

Step 5) When calculating different arrangements, the optimal arrangement of the transmitting position and receiving position of the multi-transmitter multi-receiver array is obtained according to the field distribution formed by the output light of the multi-transmitter multi-receiver array in the far field and the energy distribution of the far-field light spot.

Beneficial effects:

(1) The present invention utilizes the small aperture distribution of the azimuth and distance arrays to achieve a large imaging field of view.

(2) Based on a small-aperture lens, the array transmitting unit is combined to improve the energy distribution in the field of view and achieve long-distance and wide-band detection.

(3) The present invention utilizes multiple arrays to transmit and receive separable waveforms, simulates and designs an array transmission system based on the existing microlens array arrangement technology, and generates an imaging transmission and receiving device with high main lobe energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a MIMO-based laser SAL wide-field imaging device according to an embodiment of the present invention;

FIG2 is a schematic diagram of the arrangement of the transmitting optical fiber and the corresponding microlenses;

Figure 3 is a 2D 4×4 laser SAL emission array and far-field spot distribution diagram;

FIG4a is a far-field beam distribution diagram of an equally spaced multiple-transmitter and multiple-receiver array;

FIG4b is a diagram showing the intensity distribution of the far-field beam of an equally spaced multi-transmitter and multi-receiver array;

FIG5a is a far-field spot distribution diagram of the optimized multi-transmit multi-receive MIMO sparse array;

FIG5b is a diagram showing the intensity distribution of the far-field spot of the optimized multi-transmit multi-receive MIMO sparse array.

Detailed ways

The present invention uses a two-dimensional array multi-transmit and multi-receive method to achieve laser SAL high-resolution wide-field imaging, and uses an optimization algorithm to design the multi-transmit and multi-receive array element arrangement to achieve high main lobe energy and low side lobe energy; each receiving detector receives echo data at the same time, and adopts a corresponding waveform separation method according to different transmission waveforms (such as using a matched filtering method when transmitting orthogonal waveforms of the same frequency), separates the echoes of different transmission waveforms, obtains echoes of different transmission and reception combinations, and then performs coherent imaging processing on the echoes of all transmission and reception combinations. At the same time, the present invention uses a theoretical simulation method for verification, which verifies the effectiveness of the device of the present invention.

The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention, and do not limit the present invention.

According to an embodiment of the present invention, as shown in FIG1 , a MIMO-based laser SAL wide-field imaging device is provided, including a transmitting system and a receiving system, wherein the transmitting system includes a laser source, a modulation module, an optical amplification module, a transmitting optical fiber array, and a microlens array, etc. The echo signal enters the receiving system and couples with the intrinsic light, and the receiving system includes an array detection unit and an array acquisition imaging processing unit, etc. The transmitting optical fiber array and the microlens array constitute a transmitting array, and the receiving array includes an array detection unit. The transmitting optical fiber array is collimated and irradiated to the far field through the microlens array. Each transmitting optical fiber array is used to emit a laser beam, and the emitted laser beam is collimated and irradiated to the far field through the microlens array to obtain a large mapping bandwidth; the echo signal enters the array detection unit, and the detection output signal is processed by array acquisition imaging.

According to the requirements of high resolution and width in azimuth of laser SAL, the azimuth and distance divergence angles of the transmitting array are designed, and the number of multi-transmitting and multi-receiving arrays in azimuth and distance is determined. The MIMO-based laser SAL wide-field imaging method of the present invention comprises the following steps:

Step 1) Based on the current requirements of laser SAL for bandwidth, the bandwidth is set to 56.7m at 3km, and the divergence angle of the system emission is calculated to be 0.0189rad. According to the fiber parameters of commonly used lasers, the core diameter of a single output light source is selected to be 10μm, and the fiber numerical aperture NA=0.12. The transmitting fiber array is used to output the laser beam to form a multi-transmit and multi-receive MIMO transmitting array.

Step 2) According to the requirement of 3km detection distance, based on the radar equation and the above array transmission beam divergence angle, determine the multi-transmit and multi-receive array arrangement in azimuth (route direction) and range (slant range direction): For the above-mentioned transmitting optical fiber array, a microlens array corresponding to the transmitting array is designed, and the diameter of the microlens is 127μm, the focal length is 529μm, and the spacing is 254μm. As shown in FIG2 , the transmitting optical fiber array is placed one by one at the focal plane position of the microlens array and fixed. The transmitting optical fiber and the corresponding microlens in front of it are a transmitting array element. After passing through the microlens array, the divergence angle θ of a _single transmitting array element is 0.0189rad, where/> , f is the focal length of the microlens, and d is the aperture diameter of the transmitting array element. Using the MIMO array-to-array beam combining mode, the divergence angle of the multi-transmitter multi-receiver array beam combining laser SAL is/> .

On the basis of a single aperture with large divergence angle, the beam combining mode between arrays can improve the energy in the field of view, increase the detection distance and mapping width, and the resolution of the array laser SAL in azimuth In order to avoid Doppler ambiguity in the azimuth direction, the pulse repetition frequency PRF of the single-transmit and single-receive laser SAL satisfies the inequality/> ,/> It is the speed of the imaging device in the azimuth direction. In the imaging process, in order to prevent the range ambiguity, the echo signal in the surveying band needs to be received by the antenna within the same pulse repetition period, that is, the range surveying band is wide/> , c is the speed of light.

Step 3) As shown in Figure 3, assume that the transmitting system with the microlens array is a two-dimensional array with the number of array elements being , M is the number of arrays in azimuth, N is the number of arrays in range, the coordinates of the (m,n)th array element are (x _m ,y _n ), and the initial phase of each array element is/> , the axial amplitude of the array element is/> , the coordinates of the far-field plane are expressed by the beam propagation direction angle/> To express, each array element is approximately a Gaussian beam, and the near-field beam amplitude U(x, y) synthesized by the microlens array is:

(1)

Where i is an imaginary number, ω=63.5μm is the spot radius of each emitting element after passing through the microlens array, exp() is an exponential operation, and (x, y) represents the coordinates of the emitting element.

Step 4) According to the multi-transmitter multi-receiver theory, the far-field light intensity distribution can be regarded as the Fourier transform of the output light field of the microlens array. The field distribution formed by the output light of the multi-transmitter multi-receiver array in the far field is Expressed as:

(2)

Far field light intensity distribution It can be expressed as:

(3)

The above formula is simplified to formula (4):

(4)

in, Refers to proportion, /> , /> , /> , where/> represents the initial phase component of the (m,n)th transmitting array element in the x direction,/> Represents the initial phase component of the (m,n)th transmitting array element in the y direction.

The far-field spot energy distribution is calculated according to the above formula (4).

Step 5) Calculate the far-field light spot energy distribution for different arrangements, the far-field light field total energy distribution _Itotal , the main lobe energy distribution _Imain , and the side lobe energy distribution _Iside .

In order to maximize the energy concentration of the far-field main lobe after beam combining and suppress the grating lobe, the evaluation function is set ;

The main lobe energy concentration of the array beam is , where/> is the light field intensity in the polar coordinate system, (r, z, θ) is the corresponding coordinate representation in the polar coordinate system, and ω is the spot radius.

Contrast of grating lobe characteristics ; Where I _max is the ratio of the energy of the far-field main lobe to the total energy, k ₁ is the weighted value of the main lobe characteristic; C _max is the contrast between the main lobe and the maximum grating lobe, that is, the ratio of the energy of the far-field main lobe minus the energy of the maximum grating lobe to the main lobe energy, and k ₂ is the weighted value of grating lobe suppression. Based on the evaluation function k ₁ =0, k ₂ =1, the array element spacing is optimized with grating lobe suppression as the adaptation function, and the position arrangement method of the array transmitting device is obtained.

In a uniform linear array, the spacing between adjacent array elements is the same, and the same position difference will produce the same phase difference, resulting in grating lobes in the array arrangement with equal spacing. Based on this, a sparse MIMO system is proposed to obtain the maximum radar effective aperture. The following is the process and results of the simulation verification of the transmit array.

According to the current application requirements, the laser SAL is designed to have a millimeter-level azimuth resolution and a width of 56.7m when detecting images at a distance of 3km. Considering the divergence angle of a single array is 0.0189rad, the spot diameter after microlens collimation is 127μm, and the array element spacing is 254μm. Based on these parameters, according to the azimuth resolution calculation formula , the azimuth resolution of a single array element is ρ=0.041mm, according to the PRF calculation formula/> ,/> is the running speed. At this time, PRF = 1219.5KHz, and the survey width at a detection distance of 3km is 56.7m.

Without increasing the pulse repetition frequency (PRF), in the case of array MIMO beam combining, 4 array units are set in the azimuth direction; 4 array units are set in the range direction, and the light field energy in the imaging area is increased while keeping the azimuth divergence angle at 18.9 mrad and the range imaging width at 56.7 m unchanged. According to this requirement, the parameters in Table 1 are used for simulation.

Table 1

,

Then, the far-field spot distribution of the multi-transmitter multi-receiver array was simulated, with the parameters of 4 array elements in azimuth, 4 array elements in range, wavelength λ=1.550μm, and array spot radius of 63.5μm after collimation. The simulation results are shown in Figures 4a and 4b. Figure 4a shows the two-dimensional light field energy distribution in the far field, and Figure 4b is the light intensity distribution in one dimension. Then, the optimized sparse array was used, and the position interval of the two-dimensional array elements was (127μm, 254μm, 190μm). The simulated two-dimensional light field and light intensity distribution are shown in Figures 5a and 5b below. The simulation results show that after optimization, the energy concentration of the central main lobe is enhanced, and the peak energy of the grating lobe is suppressed.

The above-mentioned MIMO simultaneously transmits mutually separable light beams. After the transmitted light beams hit the target, an array reception method is adopted to receive the echo signals of multiple channels separately, and signal energy superposition and imaging processing are performed. Different combinations of receiving and transmitting channels will form equivalent phase centers at different positions to complete synthetic aperture lidar imaging.

It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (2)

1. A MIMO-based laser SAL wide-field imaging method, characterized in that it comprises the following steps: Step 1) Design the azimuth and range divergence angle of the emitted beam according to the resolution and width requirements in the laser SAL application; Step 2) According to the SAL detection distance requirement and based on the radar equation, determine the number of multi-transmit and multi-receive arrays in azimuth and range, design a microlens array corresponding to the transmitting optical fiber array, and form a multi-transmit and multi-receive array; Assuming that the number of multi-transmit and multi-receive arrays in azimuth is M, and the number of multi-transmit and multi-receive arrays in range is N, the number of multi-transmit and multi-receive MIMO arrays is M. N, the spot diameter of the emitting optical fiber is 10μm, the numerical aperture of the optical fiber is NA=0.12; the diameter of the microlens is 127μm, the focal length is 529μm, and the arrangement of the microlens array is M/> N array, M/> N transmitting optical fibers are placed one by one in M/> The focal plane positions of N microlenses are fixed, and the values of M and N are calculated according to the radar equation; Step 3) Assume that the multi-transmitter multi-receiver array with a microlens array is a two-dimensional array, in which each transmitting optical fiber and the corresponding microlens of the transmitting system are a transmitting array element, and the near-field beam amplitude is obtained by synthesizing the microlens array; The number of transmitting array elements is M N, the coordinates of the (m,n)th transmitting element are (x _m ,y _n ), and the initial phase of the (m,n)th transmitting element is/> , the axial amplitude of the transmitting array element is A _mn , and the coordinates of the far-field plane are the beam propagation direction angle/> To indicate that each emitting array element is approximately a Gaussian beam, the amplitude of the near-field beam synthesized by the microlens array is recorded as U(x,y), which can be expressed as: (1) Wherein, (m,n) is the coordinate of the mth array element in the x-axis direction and the nth array element in the y-axis direction, i is an imaginary number, ω=63.5μm is the spot radius of each emitting array element after passing through the microlens array, exp() is an exponential operation, and (x, y) represents the coordinate of the emitting array element; Step 4) The far-field light intensity distribution is regarded as the Fourier transform of the output light field of the microlens array, thereby obtaining the field distribution formed by the output light of the multi-transmitter and multi-receiver array in the far field; The field distribution formed by the outgoing light of the multi-transmitter and multi-receiver array in the far field is recorded as ,Expressed as: (2) Among them, (x, y, z) is the coordinate of the coordinate system of the initial plane, (x', y', z') is the coordinate of the coordinate system of the far-field observation plane, and/> is the spatial frequency; k = 2π/λ is the wave vector, λ is the laser wavelength, dx, dy represent the integral of the initial plane; _θx represents the tangential distribution in the x-axis direction, and _θy represents the tangential distribution in the y-axis direction; The far-field light intensity distribution I(θ _x ,θ _y ) is expressed as: (3) in, represents the initial phase of the (m,n)th array element, the above formula is simplified to formula (4): (4) Among them, ∝ refers to proportionality, ,/> ,/> , where/> represents the initial phase component of the (m,n)th transmitting array element in the x direction,/> Represents the initial phase component of the (m,n)th transmitting array element in the y direction; Step 5) When calculating different arrangements, the optimal arrangement of the transmitting position and receiving position of the multi-transmitter multi-receiver array is obtained according to the field distribution formed by the output light of the multi-transmitter multi-receiver array in the far field and the energy distribution of the far-field light spot. 2. A MIMO-based laser SAL wide-field imaging method according to claim 1, characterized in that, in the step 5), the far-field main lobe energy of the equally spaced MIMO array mode and the sparse MIMO array mode are compared to obtain the optimal arrangement of the transmitting position and the receiving position of the multi-transmitter and multi-receiver array.