Non-line-of-sight imaging device and method

Abstract

The present disclosure provides a non-line-of-sight imaging method, comprising: S1, projecting a frequency-modulated continuous wave laser onto an intermediate wall so that the intermediate wall reflects the frequency-modulated continuous wave laser onto a non-line-of-sight object and attenuates another frequency-modulated continuous wave laser; S2, detecting the laser signal reflected from the non-line-of-sight object and the attenuated other laser signal, mixing the laser signal with the other laser signal to form a mixed signal, and converting the mixed signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; S4, reconstructing the photon number histogram to form an image of the non-line-of-sight object. The non-line-of-sight imaging method disclosed in the present invention innovatively introduces frequency-modulated continuous wave imaging into the field of non-line-of-sight imaging. Compared with traditional time-domain non-line-of-sight imaging systems, this system can achieve non-line-of-sight imaging accuracy that exceeds that of time-domain systems by utilizing a frequency-modulated laser with a larger frequency modulation range, achieving non-line-of-sight imaging accuracy at the millimeter level or even the micrometer level.

Description

Technical Field

[0001] The present disclosure relates to the field of non-line-of-sight imaging, and in particular to a non-line-of-sight imaging device and method. Background Art

[0002] Optical non-line-of-sight imaging (NLOS) is a novel imaging concept. Because light propagates in straight lines through a homogeneous medium, optical imaging generally captures scenes within the human eye's line of sight or the imaging device's field of view. This limited field of view prevents the capture of a vast amount of visual information in nature that lies beyond the field of view. However, NLOS imaging technology, leveraging diffuse reflection from intervening surfaces, can capture objects hidden from view, enabling a "through-the-wall" approach. This technology transcends the limitations of traditional optical imaging and expands human imaging capabilities. It is foreseeable that NLOS imaging technology will play a vital role in numerous fields, including counterterrorism warning, intelligent driving, emergency rescue, and healthcare, in the near future. Figure 1 (a) to (c) schematically illustrate the application of non-line-of-sight imaging technology in the fields of intelligent driving, emergency rescue, medical health, etc. Figure 1 As shown in (a) to (c), in the field of intelligent driving, non-line-of-sight imaging technology can effectively detect road conditions at intersections; in the field of emergency rescue, this technology can provide relevant information about trapped people and the surrounding environment; and in the field of healthcare, this technology can be used to examine the inner walls of human organs. (Source [arXiv 1910, 05613(2019)])

[0003] This technology originated in the early 2000s. In 2009, the MIT Media Lab first proposed the basic concept of optical non-line-of-sight imaging at the International Conference on Computational Vision and demonstrated non-line-of-sight detection of multiple flat panels in a hidden space. In 2012, the same research team proposed a back-projection algorithm to solve the non-line-of-sight problem and, using a femtosecond pulsed laser and a streak camera, demonstrated the first experimental non-line-of-sight imaging of a hidden three-dimensional object. Since then, researchers from around the world have joined the research community in non-line-of-sight imaging, achieving a series of significant results.

[0004] In particular, since 2016, the Defense Advanced Research Projects Agency (DARPA) has launched the REVEAL (Revolutionary Enhancement of Visibility by Exploiting Active Light-fields) research program, with a funding exceeding $27 million. This program has prioritized non-line-of-sight imaging (NLOS) technology, significantly advancing its development. Supported by this program, several landmark NLOS imaging technologies have been proposed. In 2018, Stanford University published an academic paper in Nature proposing a NLOS imaging technique based on confocal scanning and a light-cone transform algorithm. Compared to previous techniques, this technique offers lower temporal and spatial complexity and higher reconstruction accuracy. In 2019, Carnegie Mellon University proposed a NLOS imaging technique based on Fermat paths, analyzing the continuity of NLOS echo signals. This technique can accurately recover the fine three-dimensional structure of hidden targets. This work won the Best Paper Award at CVPR-2019. In the same year, the University of Wisconsin-Madison published an academic paper in the journal Nature, proposing a non-line-of-sight imaging technology based on virtual wave phase field. This technology can effectively restore complex non-line-of-sight scenes by considering more than three diffuse reflection processes.

[0005] At present, mainstream non-line-of-sight imaging technologies, including the above-mentioned landmark works, all adopt the time-domain imaging system, that is, directly measuring the flight time of laser pulses (Pulsed time-of-flight or Pulsed ToF). Specifically, this time-domain imaging system emits a laser pulse signal with a narrow pulse width (picosecond or femtosecond level) and uses a high-sensitivity, high-time-precision single-photon detector to directly record the flight time information of the echo signal photons after multiple diffuse reflections. However, due to the limitations of the time accuracy that electronic devices can achieve (about tens of picoseconds), the imaging accuracy of this type of system is limited to the centimeter level. For example, the non-line-of-sight imaging system constructed based on the above-mentioned technologies generally has an overall system time jitter of no less than 60ps, corresponding to an imaging accuracy of about 1em. How to achieve higher-precision non-line-of-sight imaging and thus achieve detailed perception of complex non-line-of-sight scenes is the core issue currently being addressed in the field of non-line-of-sight imaging. Summary of the Invention

[0006] To address the above-mentioned deficiencies in the prior art, the present invention provides a non-line-of-sight imaging device and method, which innovatively introduces a frequency domain imaging system into the field of non-line-of-sight imaging. The imaging accuracy of traditional time-domain non-line-of-sight imaging systems is generally on the order of centimeters; however, the frequency domain imaging system of this application utilizes a frequency-modulated laser with a large frequency modulation range, achieving non-line-of-sight imaging accuracy exceeding that of the time-domain system. Currently, the frequency modulation range of various frequency-modulated lasers can basically reach several THz, enabling non-line-of-sight imaging accuracy at the millimeter level or even the micrometer level.

[0007] The present invention provides a non-line-of-sight imaging method, comprising: S1, projecting a frequency-modulated continuous laser onto an intermediate wall so that the intermediate wall reflects the frequency-modulated continuous laser onto a non-line-of-sight object; S2, detecting a laser signal reflected from the non-line-of-sight object and converting the laser signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; and S4, reconstructing the photon number histogram to form an image of the non-line-of-sight object.

[0008] A non-line-of-sight imaging method includes: S1, projecting a frequency-modulated continuous laser onto an intermediate wall so that the intermediate wall reflects the frequency-modulated continuous laser onto a non-line-of-sight object and attenuates another frequency-modulated continuous laser; S2, detecting a laser signal reflected from the non-line-of-sight object and another attenuated laser signal, mixing the laser signal with the other laser signal to form a mixed signal, and converting the mixed signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; and S4, reconstructing the photon number histogram to form an image of the non-line-of-sight object.

[0009] Optionally, the mixed signal is a beat signal formed by interference between a laser signal and another laser signal.

[0010] Optionally, S2 includes: outputting a first electrical signal when the non-visual object meets a first condition; and outputting a second electrical signal when the non-visual object meets a second condition.

[0011] Optionally, if the non-visual-area object satisfies the first condition, the non-visual-area object is in a stationary state; if the non-visual-area object satisfies the second condition, the non-visual-area object is in a moving state.

[0012] Optionally, the first electrical signal is I(t), where

[0013]

[0014] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); E0 is half of the amplitude of the light field of the frequency modulated continuous laser; n x ×n y is the number of discrete pixels of non-viewing objects; α jis the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object; τ j is the time delay corresponding to any pixel point on the non-viewing object; γ is the slope of the frequency modulation of the laser signal; f0 is the starting frequency of the frequency modulated continuous laser.

[0015] Optionally, the second electrical signal is I′(t), where

[0016]

[0017]

[0018]

[0019] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); E0 is half of the amplitude of the light field of the frequency modulated continuous laser; n x ×n y is the number of discrete pixels of non-viewing objects; α j is the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object; τ j is the time delay corresponding to any pixel point on the non-viewing object; γ is the slope of the frequency modulation of the laser signal; f0 is the starting frequency of the frequency modulated continuous laser, and v0 is the speed of the non-viewing object.

[0020] Optionally, the frequency modulated continuous laser is collimated before passing through the intermediate wall and before being detected.

[0021] The present invention also provides a non-line-of-sight imaging device, comprising: a frequency-modulated laser for emitting a frequency-modulated continuous laser and another frequency-modulated continuous laser; an attenuator for attenuating the other frequency-modulated continuous laser; a detector for capturing a laser signal reflected from a non-line-of-sight object and the attenuated other laser signal, mixing the laser signal with the other laser signal to form a mixed signal, and then converting the mixed signal into an electrical signal; a circulator for adjusting the frequency-modulated continuous laser so that it is incident on an intermediate wall through a collimator or adjusting a laser signal reflected from a non-line-of-sight object received by the collimator so that it enters the detector; a collimator for collimating the emitted frequency-modulated continuous laser so that it is incident on the intermediate wall in a specified direction; or for receiving a laser signal reflected from a non-line-of-sight object.

[0022] Optionally, the system further includes: a second beam splitter, configured to receive the laser signal that has passed through the circulator and another laser signal that has been attenuated, and interfere the two to form a beat frequency signal.

[0023] The non-line-of-sight imaging method disclosed in this invention innovatively introduces a frequency-domain imaging system into the field of non-line-of-sight imaging. In traditional time-domain systems, the temporal resolution of non-line-of-sight systems is limited by the bandwidth of the detector and electronics, typically tens of picoseconds, corresponding to centimeter-level imaging accuracy. In the frequency-domain imaging system of this application, imaging accuracy depends on the bandwidth of the frequency-modulated laser, i.e., the frequency modulation range. Currently, the frequency modulation range of various frequency-modulated lasers can reach several terahertz, enabling non-line-of-sight imaging accuracy at the millimeter or even micrometer level.

[0024] The present invention discloses a non-line-of-sight imaging method that can simultaneously perform multi-factor detection, including non-line-of-sight imaging and velocity measurement. Existing time-domain non-line-of-sight imaging technologies can achieve imaging by measuring time of flight, but are unable to measure the speed of dynamic targets in real time. The present invention utilizes the Doppler effect to obtain the speed of non-line-of-sight targets.

[0025] The present invention discloses a non-line-of-sight imaging method that employs two laser paths: a signal path and a reference path, while the mainstream setup only includes a single signal laser path. The local reference path laser signal can be used to enhance the laser signal reflected from non-line-of-sight objects, thereby improving the detection sensitivity of the entire setup.

[0026] The non-line-of-sight imaging method disclosed in the present invention adopts a coherent detection method, while the mainstream setting is incoherent detection. Coherent detection has a higher tolerance to environmental noise than incoherent detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 (a) to (c) schematically illustrate the application of non-line-of-sight imaging technology in intelligent driving, emergency rescue, medical health and other fields;

[0028] Figure 2 The flowchart of the non-line-of-sight imaging method according to an embodiment of the present disclosure is schematically shown;

[0029] Figure 3 Schematically shows a histogram of an electrical signal converted from a laser signal collected by the non-line-of-sight imaging method according to an embodiment of the present disclosure;

[0030] Figure 4 Schematically shows the Figure 3 Photon number histogram after Fourier transformation;

[0031] Figure 5 A schematic diagram of a non-line-of-sight imaging method according to an embodiment of the present disclosure is schematically shown;

[0032] Figure 6 The following schematically shows a structural diagram of a non-line-of-sight imaging device according to an embodiment of the present disclosure;

[0033] Figure 7 (a) to (c) schematically illustrate an original front view, a top view, and a side view of a non-viewing area object according to an embodiment of the present disclosure;

[0034] Figure 8 (a) to (c) schematically illustrate the front view, top view, and side view of the non-viewing object reconstructed by a 60ps time-domain system in the prior art;

[0035] Figure 9 (a) to (c) schematically illustrate the front view, top view, and side view of the non-line-of-sight object reconstructed by the 1 THz frequency modulation system according to an embodiment of the present disclosure;

[0036] In the figure, non-line-of-sight imaging device-1, intermediate wall-2, obstruction-3, non-line-of-sight object-4, frequency-modulated laser-5, first beam splitter-6, circulator-7, collimator-8, attenuator-9, signal path-10, reference path-11, second beam splitter-12, first photodetector-13, second photodetector-14, and balance detection device-15. DETAILED DESCRIPTION

[0037] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0038] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise," "include," etc. used herein indicate the presence of the features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0039] Figure 2 The flowchart of the non-line-of-sight imaging method according to an embodiment of the present disclosure is schematically shown.

[0040] Figure 3 The histogram schematically shows an electrical signal converted from a laser signal collected by the non-line-of-sight imaging method according to an embodiment of the present disclosure.

[0041] Figure 4 Schematically shows the Figure 2 Photon count histogram after Fourier transformation.

[0042] The embodiment of the present disclosure provides a non-line-of-sight imaging method, such as Figure 2 As shown, it includes: S1, projecting a frequency modulated continuous laser onto the intermediate wall 2, so that the intermediate wall 2 reflects the frequency modulated continuous laser onto the non-viewing object 4; S2, detecting the laser signal reflected from the non-viewing object 4 and converting the laser signal into an electrical signal, such as Figure 3 As shown; S3, Fourier transform electrical signal, forming a photon number histogram, as shown Figure 4 As shown; S4, reconstruct the photon number histogram to form an image of the non-viewing object.

[0043] A non-line-of-sight imaging method disclosed in the present invention innovatively introduces a frequency domain imaging system into the field of non-line-of-sight imaging. Compared with the traditional time domain imaging system, it only needs to measure the electrical signal converted by the detected laser signal, and then obtain a photon number histogram by Fourier transforming the electrical signal. Since the time resolution of the time domain system is limited by electronics, the electronics accuracy is generally tens of ps; in the frequency domain imaging system of the present application, the electrical signal is positively correlated with the frequency modulation range of the frequency modulated continuous laser. At present, the frequency modulation range of various types of frequency modulated lasers 5 can basically reach several THz, thereby achieving non-line-of-sight imaging accuracy at the millimeter level or even the micron level.

[0044] An embodiment of the present disclosure also provides a non-line-of-sight imaging method, including: S1, projecting a frequency-modulated continuous laser onto an intermediate wall 2, so that the intermediate wall 2 reflects the frequency-modulated continuous laser onto a non-line-of-sight object 4, and attenuates another frequency-modulated continuous laser; S2, detecting a laser signal reflected from the non-line-of-sight object 4 and another attenuated laser signal, mixing the laser signal with the other laser signal to form a mixed signal, and converting the mixed signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; S4, reconstructing the photon number histogram to form an image of the non-line-of-sight object.

[0045] The present invention discloses a non-line-of-sight imaging method that employs two frequency-modulated continuous laser beams, while the mainstream setup only includes one frequency-modulated continuous laser beam. The attenuated laser signal can be used to enhance the laser signal reflected from the non-line-of-sight object 4, thereby improving the detection sensitivity of the entire setup.

[0046] In some embodiments, the mixed signal is a beat signal formed by interference between a laser signal and another laser signal.

[0047] The non-line-of-sight imaging method disclosed in the present invention adopts a coherent detection method, while the mainstream setting is incoherent detection. Coherent detection has a higher tolerance to environmental noise than incoherent detection.

[0048] In some embodiments, S2 includes: outputting a first electrical signal when the non-visual object 4 meets a first condition; and outputting a second electrical signal when the non-visual object 4 meets a second condition.

[0049] In some embodiments, the non-visual-area object 4 satisfies the first condition that the non-visual-area object 4 is in a stationary state; and the non-visual-area object 4 satisfies the second condition that the non-visual-area object 4 is in a moving state.

[0050] In some embodiments, the non-viewing object 4 is stationary, and the corresponding first electrical signal is I(t), where:

[0051]

[0052] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); E0 is half of the amplitude of the light field of the frequency modulated continuous laser; n x ×n y is the number of discrete pixels of the non-viewing object; α i is the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object 4; τ j is the time delay corresponding to any pixel point on the non-viewing object 4; γ is the slope of the frequency modulation of the laser signal; f0 is the starting frequency of the frequency modulated continuous laser.

[0053] Figure 5 A schematic diagram of a non-line-of-sight imaging method according to an embodiment of the present disclosure is schematically shown.

[0054] The derivation process of this formula will be described in detail below: For the sake of simplicity, the following description adopts a coaxial non-line-of-sight imaging setting (that is, the laser irradiation point and the detection receiving point on the wall of the intermediate wall 2 coincide with each other), and takes an arbitrary measurement point O on the intermediate wall 2 as the starting point of the model description.

[0055] According to the non-line-of-sight imaging process, such as Figure 5 As shown, the laser will be diffusely reflected at O, scattered to various points on the non-visual object 4, and then diffusely reflected back to O from each point. First, arbitrarily select three points T1, T2, and T3 on the non-visual object 4, and examine the photoelectric detection signals I1, I2, and I3 corresponding to the three points respectively; the frequency modulated laser output light field can be uniformly expressed as in, is the overall phase of the light field, which is a time-dependent function, i is an imaginary unit, and linear frequency modulation is considered here, that is, thereby, Where f0 is the starting frequency of the frequency modulation, and γ is the slope of the linear frequency modulation. Note that the following derivation is based on linear frequency modulation. Other modulation forms can be derived from the corresponding imaging models through similar derivation. The basic model of frequency modulated continuous wave and coherent detection can be used to obtain the photoelectric detection signals I1, I2, and I3 corresponding to the three points T1, T2, and T3:

[0056]

[0057]

[0058]

[0059] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); is the energy attenuation of the reference path light field; α1, α2, and α3 are the attenuation of the complex amplitude of the signal path light field at points T1, T2, and T3 on the non-viewing object 4; Describes the reflectivity of each of T1, T2, and T3 on the non-viewing object 4; τ 1，2，3 =2r 1，2，3 / c describes the time delay corresponding to each of the three points (c is the speed of light, r 1，2，3 are the lengths of OT1, OT2, and OT3 respectively); E0 is half of the amplitude of the FM continuous laser light field; f0 is the starting frequency of FM, and γ is the slope of the linear FM modulation.

[0060] Therefore, if the non-viewing object 4 is discretized into n x ×n y pixels, considering τ j =2r j / c is the time delay corresponding to any pixel point on the non-viewing object 4, r j is the distance from any pixel point on the non-viewing object 4 to the measurement point O on the intermediate wall 2, α j is the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object 4, corresponding to is the reflectivity at any pixel point on the non-viewing object 4. Therefore, the response of the detection end is the linear superposition of the responses of each pixel point:

[0061]

[0062] Furthermore, combined with the balanced detection principle, the specific expression of the non-line-of-sight imaging model based on FMCW can be obtained:

[0063]

[0064] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); E0 is half of the amplitude of the light field of the frequency modulated continuous laser; n x ×n y is the number of discrete pixels of the non-viewing object; α j is the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object 4; τ jis the time delay corresponding to any pixel point on the non-viewing object 4; γ is the slope of the frequency modulation of the laser signal; f0 is the starting frequency of the frequency modulated continuous laser. The above is derived to obtain I(t) corresponding to the stationary non-viewing object 4.

[0065] Performing Fourier transform on this result yields:

[0066]

[0067] H(τ) is the equivalent time-domain photon number-flight time histogram, where δ(*) is the Dirac function, which takes the value of 1 at δ(0) and 0 at other locations. The histogram can be understood as a vector, where different positions on the vector represent time, and the specific value of each position is the number of photons. Therefore, H(τ) is the value of the time at τ. j The value is α j Subsequently, reconstruction can be achieved by applying various standard non-line-of-sight imaging reconstruction algorithms to H(τ).

[0068] In some embodiments, the non-viewing object moves, and the corresponding second electrical signal is I′(t), where:

[0069]

[0070]

[0071]

[0072] Among them, α f is the attenuation of the complex amplitude of the light field of another frequency modulated continuous laser (reference path); E0 is half of the amplitude of the light field of the frequency modulated continuous laser; n x ×n y is the number of discrete pixels of the non-viewing object; α j is the attenuation of the complex amplitude of the signal path light field by any pixel point on the non-viewing object 4; τ j is the time delay corresponding to any pixel point on the non-viewing object 4; γ is the slope of the frequency modulation of the laser signal; f0 is the starting frequency of the frequency modulated continuous laser, and v0 is the speed of the non-viewing object 4.

[0073] The derivation process of this formula will be described in detail below: Consider a moving non-visual object 4. For simplicity, consider the object to move in a uniform straight line with a velocity v0. Then the time delay corresponding to any pixel point needs to be rewritten as Replace τ in the stationary object expression I(t) j Replaced by τ′ j , we can get the expression corresponding to the moving object:

[0074]

[0075]

[0076]

[0077]

[0078] Performing Fourier transform on this result yields:

[0079]

[0080] Further considering the triangular wave case in linear modulation, in the part where the modulation slope is γ, we have:

[0081]

[0082] in, thereby:

[0083]

[0084] Similarly, by using various standard non-line-of-sight imaging reconstruction algorithms on H(τ), the reconstruction result can be obtained, and at the same time the speed of the dynamic object can be obtained:

[0085]

[0086] Optionally, the frequency modulated continuous laser light is collimated before passing through the intermediate wall 2 and before being detected.

[0087] Optionally, various standard non-line-of-sight imaging reconstruction algorithms, such as LCT, Phasor field, FK, etc., may be used when reconstructing the photon number histogram.

[0088] Figure 6 The schematic diagram of the structure of the non-line-of-sight imaging device 1 according to an embodiment of the present disclosure is shown schematically.

[0089] The present invention also provides a non-line-of-sight imaging system, such as Figure 6 As shown, it includes: a non-line-of-sight imaging device 1; an intermediate wall 2, which is used to reflect the frequency-modulated continuous laser to the non-line-of-sight object, and reflect the light signal reflected from the non-line-of-sight object to the detector; and an obstruction 3, which prevents the non-line-of-sight object 4 from being directly observed by the non-line-of-sight imaging device 1.

[0090] Among them, the non-line-of-sight imaging device 1 includes a frequency-modulated laser 5, which is used to emit a frequency-modulated continuous laser (signal path) and another frequency-modulated continuous laser (reference path); an attenuator 9, which is used to attenuate the other frequency-modulated continuous laser; a detector, which is used to capture the laser signal reflected from the non-line-of-sight object and the other laser signal after attenuation, and mix the laser signal with the other laser signal to form a mixed signal, and then convert the mixed signal into an electrical signal; a circulator 7, which is used to adjust the frequency-modulated continuous laser so that it is incident on the intermediate wall 2 through the collimator 8 or adjust the laser signal reflected from the non-line-of-sight object 4 received by the collimator 8 so that it enters the detector; the collimator 8, which is used to collimate the emitted frequency-modulated continuous laser so that it is incident on the intermediate wall 2 in a specified direction; or to receive the laser signal reflected from the non-line-of-sight object 4.

[0091] Optionally, a first beam splitter 6 is included for splitting the laser signal emitted by the frequency modulated laser 5 into two beams of frequency modulated continuous laser light; a second beam splitter 12 is included for receiving the laser signal passing through the circulator 7 and another attenuated laser signal, and interfering the two to form a beat signal.

[0092] Alternatively, a frequency-modulated continuous laser beam passing through circulator 7 and collimator 8 forms signal path 10, while another frequency-modulated continuous laser beam passing through attenuator 9 forms reference path 11. In the present invention, two frequency-modulated continuous laser beams (signal path 10 and reference path 11) are provided, while the mainstream non-line-of-sight configuration only includes one laser beam (signal path 10). This configuration can utilize local laser signals to enhance laser signals reflected from non-line-of-sight objects, thereby improving the detection sensitivity of the entire configuration.

[0093] Optionally, the first beam splitter 6 is a one-to-two beam splitter, and the second beam splitter 12 is a two-to-two beam splitter. The laser signals of the signal path 10 and the reference path 11 are mixed in the second beam splitter 12 to form an interference beat signal. are detected by the first photodetector 13 and the second photodetector 14 respectively. + (t) is detected by the first photodetector 13 via the upper part, I - (t) is detected by the second photodetector 14 through the lower half, and then passes through the balanced detection device 15 to form I(t).

[0094] Optionally, the frequency modulated laser 5 can be a tunable laser of brands such as New Focus (TLB-8800-H-CL), Luna (Phoenix 1400), and Santec (TSL770), or can be self-developed as needed.

[0095] Optionally, the beam splitter, attenuator 9, circulator 7, etc. all use single-mode optical fiber devices, among which the coaxial optical path design of "circulator 7 + collimator head" can be replaced with a dual-axis optical path design of "dual collimator head" as needed.

[0096] Alternatively, the detector can be a Thorlabs PDB series balanced detector or a high-sensitivity single-photon detector. When using a balanced detector, the reference optical power should be as high as possible, typically in the milliwatt range. When using a single-photon detector, the reference optical power should be close to the signal optical power, typically in the femtowatt range or below.

[0097] Regarding the time accuracy of the frequency domain system: γτ1-γτ2=1 / T, therefore, the accuracy of H(τ) is 1 / γT=1 / B, where B is the frequency modulation range and T is the frequency modulation time.

[0098] In the frequency domain imaging system, Δt = 1 / B, where Δt is the time accuracy of the system. For example, for a system with B = 1 THz, its Δt can reach 1 ps, so the imaging accuracy = Δt*C / 2 = 150 μm, where C is the speed of light.

[0099] At present, the frequency modulation range of various frequency modulated lasers 5 can basically reach several THz, thereby achieving non-line-of-sight imaging accuracy at the millimeter level or even the micrometer level.

[0100] Figure 7 (a) to (c) schematically illustrate an original front view, a top view, and a side view of a non-viewing-zone object according to an embodiment of the present disclosure.

[0101] Figure 8 (a) to (c) schematically illustrate the front view, top view and side view of the non-viewing object reconstructed by the 60ps time domain system in the prior art.

[0102] Figure 9 (a) to (c) schematically illustrate the front view, top view, and side view of the non-line-of-sight object reconstructed by the 1 THz frequency modulation system according to an embodiment of the present disclosure.

[0103] like Figures 7 to 9 As shown in the figure, in this comparison, the non-line-of-sight target is a resolution plate (with four sizes of intervals: 9.4mm, 12.5mm, 15.6mm, and 18.8mm). It can be seen that the 60ps time-domain non-line-of-sight system can only roughly locate the position of the resolution plate, while the 1THz frequency modulation system can accurately reconstruct the specific shape of the resolution plate.

[0104] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A non-line-of-sight imaging method, characterized in that: include: S1, projecting a frequency-modulated continuous laser onto an intermediate wall (2), so that the intermediate wall (2) diffusely reflects the frequency-modulated continuous laser onto a non-viewing object (4); S2, detecting the laser signal reflected from the non-viewing object (4) by using a coherent detection method, and converting the laser signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; S4, reconstructs the photon number histogram to form an image of the non-viewing object (4), Wherein, the S2 includes: when the non-visual object (4) meets the first condition, outputting a first electrical signal; when the non-visual object (4) meets the second condition, outputting a second electrical signal, The non-visual object (4) satisfies the first condition that the non-visual object (4) is in a stationary state; and the non-visual object (4) satisfies the second condition that the non-visual object (4) is in a moving state.

2. A non-line-of-sight imaging method, characterized in that: include: S1, projecting a frequency-modulated continuous laser onto an intermediate wall (2), so that the intermediate wall (2) diffusely reflects the frequency-modulated continuous laser onto a non-viewing object (4), thereby attenuating another frequency-modulated continuous laser; S2, using a coherent detection method to detect the laser signal reflected from the non-viewing object (4) and another laser signal after attenuation, interfering the laser signal with the other laser signal to form a mixed signal, and converting the mixed signal into an electrical signal; S3, Fourier transforming the electrical signal to form a photon number histogram; S4, reconstructs the photon number histogram to form an image of the non-viewing object (4), Wherein, the S2 includes: when the non-visual object (4) meets the first condition, outputting a first electrical signal; when the non-visual object (4) meets the second condition, outputting a second electrical signal, The non-visual object (4) satisfies the first condition that the non-visual object (4) is in a stationary state; and the non-visual object (4) satisfies the second condition that the non-visual object (4) is in a moving state.

3. The non-line-of-sight imaging method according to claim 2, wherein: The mixed signal is a beat signal formed by interference between the laser signal and the other laser signal.

4. The non-line-of-sight imaging method according to claim 2, wherein: The first electrical signal is I(t), in, is the attenuation of the complex amplitude of the light field of the other frequency-modulated continuous laser (reference path); It is half of the amplitude of the frequency modulated continuous laser light field; is the number of discrete pixels of the non-viewing object (4); is the attenuation of the complex amplitude of the signal path light field at any pixel point on the non-viewing object (4); is the time delay corresponding to any pixel point on the non-viewing object (4); is the slope of the frequency modulation of the laser signal; is the starting frequency of the frequency modulated continuous laser.

5. The non-line-of-sight imaging method according to claim 2, wherein: The two electrical signals are ,in, in, is the attenuation of the complex amplitude of the light field of the other frequency-modulated continuous laser (reference path); It is half of the amplitude of the frequency modulated continuous laser light field; is the number of discrete pixels of the non-viewing object (4); is the attenuation of the complex amplitude of the signal path light field at any pixel point on the non-viewing object (4); is the time delay corresponding to any pixel point on the non-viewing object (4); is the slope of the frequency modulation of the laser signal; is the starting frequency of the frequency modulated continuous laser, is the speed of the non-viewing object (4), and c is the speed of light.

6. The non-line-of-sight imaging method according to claim 1 or 2, characterized in that: The frequency modulated continuous laser first passes through a collimating device before passing through the intermediate wall (2) and before detection.

7. A non-line-of-sight imaging device (1), used to implement the method according to any one of claims 1 to 6, characterized in that: include: A frequency modulated laser (5) for emitting a frequency modulated continuous laser and another frequency modulated continuous laser; an attenuator (9), used for attenuating the other frequency-modulated continuous laser light; A detector, comprising a second beam splitter (12), a first photodetector (13), a second photodetector (14), and a balanced detection device (15), for capturing a laser signal reflected from a non-viewing object and another laser signal after attenuation, mixing the laser signal with the other laser signal to form a mixed signal, and then converting the mixed signal into an electrical signal; A circulator (7) is used to adjust the frequency modulated continuous laser light so that it enters the intermediate wall (2) via the collimator (8) or to adjust the laser signal reflected from the non-viewing object (4) received by the collimator (8) so that it enters the detector; The collimator (8) is used to collimate the emitted frequency-modulated continuous laser light so that it is incident on the intermediate wall (2) in a specified direction; or to receive the laser signal reflected from the non-viewing object (4).

8. The non-line-of-sight imaging device (1) according to claim 7, characterized in that: The detector also includes: The second beam splitter (12) is used to receive the laser signal that has passed through the circulator (7) and another attenuated laser signal, and interfere the two to form a beat frequency signal.

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