A long-distance ghost imaging method

Abstract

The present invention belongs to the technical field of wading optics and underwater security in the on-site security technology system, and particularly relates to a method for realizing long-distance ghost imaging by using a Bessel beam modulated by a random intensity distribution. The method includes the following steps: The light emitted by a light source passes through a Bessel beam generating device to generate a Bessel beam, and the Bessel beam is then modulated by a light field intensity modulation device. The modulated Bessel beam will have the characteristics of a random speckle light field with intensity modulation, and this beam is used to perform ghost imaging on the target. By utilizing the non-diffracting characteristics of the Bessel beam and the characteristics of the random speckle light field with intensity modulation, it is applied to the research of long-distance underwater ghost imaging, and long-distance ghost imaging in such a high-scattering environment as underwater is realized through a new idea.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater optical and underwater security in the on-site security technology system, and particularly relates to an underwater long-distance ghost imaging method and system based on intensity-modulated Bessel beams. Background Art

[0002] Underwater ghost imaging technology is one of the important technologies in underwater optics and underwater security in the on-site security technology system. Ghost imaging is a new imaging technology that non-locally obtains target object information based on the correlation characteristics of quantum entanglement or classical optical field fluctuations, through the intensity correlation between the reference optical field and the target detection optical field. The concept of ghost imaging was first realized by generating spatially entangled photon pairs through parametric down-conversion for imaging. For traditional ghost imaging, after the light emitted by the light source is modulated, a beam splitter is used to divide the light into two beams. One beam of light irradiates on the target object and is called the signal optical path, and an optical detector without spatial resolution (bucket detector) is placed behind to collect the total light intensity reflected or transmitted after irradiating on the target object. The other beam directly irradiates on the camera and is called the reference optical path. The camera has spatial resolution and scans the cross-section of the light beam in this optical path to obtain spatial distribution information. After multiple measurements, the light intensity of the target object multiple times and the corresponding spatial distribution information multiple times are correlated and calculated to restore the image of the target object. Subsequently, studies have shown that the light source used in ghost imaging can be an entangled light source, a thermal light source, or a pseudo-thermal light source, that is, quantum ghost imaging and classical ghost imaging, which provides the possibility for developing new ghost imaging methods. In 2008, Shapiro proposed the computational ghost imaging theory that can omit the reference optical path. Usually, a spatial light modulator or a digital micromirror device is used for phase or amplitude modulation to generate a prefabricated modulated optical field. There is no need for a CCD to detect, and a bucket detector is directly used to receive the light intensity signal of the detection target. The intensity second-order correlation algorithm is adopted for weighted averaging, and finally the target image is inversely reconstructed.

[0003] Underwater ghost imaging is one of the most basic topics in computational imaging research. When imaging in underwater and similar high-scattering environments, the transmitted modulated optical field is very easy to be distorted by effects such as scattering and diffraction, and the optical field distribution is easily damaged. And the imaging distance and imaging quality of computational ghost imaging highly depend on the transmitted modulated optical field. To ensure that the modulated optical field is not distorted, the Bessel beam with non-diffracting characteristics is one of the choices. In 2012, Ronald E. Meyers et al. (Appl. Phys. Lett. 100, 061126 (2012)) first reported using a continuously displaced Bessel optical field for ghost imaging. After that, there are few reports on using Bessel beams for ghost imaging. The key problem is that simply using Bessel beams and performing ghost imaging through displacement transformation will generate strong background noise in the reconstructed image and reduce the imaging quality.

[0004] It can be seen that in underwater and similar highly scattering environments, how to utilize the non-diffracting property of Bessel beams for long-distance ghost imaging is a key problem to be urgently solved. Summary of the Invention

[0005] To solve the problems that in underwater and similar highly scattering environments, the computational ghost imaging based on the traditional intensity-modulated light field has a short imaging distance, and the computational ghost imaging based on the displacement-varying Bessel beam has a low imaging quality, the present invention proposes an underwater long-distance ghost imaging method based on intensity-modulated Bessel beams. By combining the non-diffracting property of Bessel beams and the characteristics of the intensity-modulated random speckle light field, the intensity-modulated random speckle Bessel beam is used in underwater long-distance ghost imaging. In underwater and similar highly scattering environments, compared with using only intensity-modulated random speckles, the method has the advantage of a long imaging distance; compared with using only the Bessel beam with position offset, the method has the advantage of high imaging quality. This method uses the Bessel light field as a carrier to carry random intensity-modulated speckles, realizing long-distance and high-quality ghost imaging in underwater and similar highly scattering environments, and will expand the application scope of ghost imaging technology.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] An underwater long-distance ghost imaging method and system based on intensity-modulated Bessel beams disclosed by the present invention. The underwater long-distance ghost imaging method based on intensity-modulated Bessel beams includes the following steps:

[0008] Step 1: A light source (1) generates a Gaussian beam or a plane beam with a certain divergence angle, and an intensity-modulated Bessel beam generating device (2) generates a randomly intensity-modulated Bessel speckle light field from the Gaussian beam or the plane beam; wherein, the intensity-modulated Bessel beam generating device (2) includes a Bessel beam generator and a light field intensity modulator, the Bessel beam generator generates a randomly intensity-modulated Bessel beam; the light field intensity modulator modulates the randomly intensity-modulated Bessel beam to generate a randomly intensity-modulated Bessel speckle light field;

[0009] Step 2: According to the randomly intensity-modulated Bessel speckle light field generated in Step 1, it is sequentially projected onto a target (3), and a photodetector (3) that does not require spatial resolution ability measures the total light intensity signal reflected or transmitted by the target, and the total light intensity signal is used as the inner product between the target image and the light projection pattern; then, the total light intensity signal is transmitted to the data synchronization and processing module of a computer (4);

[0010] Step 3: In the data synchronization and processing module, calculate and reconstruct the total light intensity signal obtained in Step 2 and the randomly intensity modulated Bessel speckle light field of the corresponding projection to obtain the image of the target, realizing underwater long-distance ghost imaging.

[0011] The calculation and reconstruction in Step 3 is a method for underwater long-distance ghost imaging based on intensity modulated Bessel beam, and the following correlation calculation formula is adopted:

[0012]

[0013] Wherein, G(x, y) is the image of the target to be measured (3), < 〉 represents taking the arithmetic mean of N measurements, I(x, y) is the intensity distribution of the nth random pattern not loaded into the intensity modulated Bessel beam generating device, and B is the total light intensity value corresponding to I(x, y) detected by the photodetector.

[0014] In one embodiment, the Bessel beam generator uses a cone lens or a spatial light modulator, and the light field intensity modulator uses a digital micromirror device.

[0015] In one embodiment, the wavefront distribution of the Gaussian beam or plane beam is changed by the cone lens to generate a Bessel beam; or, the Gaussian beam or plane beam is phase modulated by the spatial light modulator to directly generate a Bessel beam.

[0016] In one embodiment, the light field intensity modulator adopts a digital micromirror device to generate a light field with random spatial intensity fluctuations on the surface of the digital micromirror device. After the Bessel beam passes through the digital micromirror device, a randomly intensity modulated Bessel speckle light field is generated.

[0017] An underwater long-distance ghost imaging system disclosed by the present invention includes a light source, an intensity modulated Bessel beam generating device, the intensity modulated Bessel beam generating device includes a cone lens or a spatial light modulator for generating a Bessel beam, a digital micromirror device for modulating the Bessel beam, a photodetector and a computer.

[0018] In one embodiment, the light source is a Gaussian beam or a plane beam with a certain divergence angle, and an LED or a laser is adopted.

[0019] In one embodiment, the intensity modulated Bessel beam generating device includes a Bessel beam generator and a light field intensity modulator, wherein the Bessel beam generator uses a cone lens or a spatial light modulator, and the light field intensity modulator uses a digital micromirror device.

[0020] In one embodiment, the photodetector adopts a photomultiplier tube, a photodiode, a photoconductive device or a charge coupled device.

[0021] In one embodiment, the computer includes a main control circuit, a two-dimensional modulated light field generation module, and a data synchronization and processing module.

[0022] The beneficial effects of the present invention are as follows: By taking advantage of the characteristics of non-diffracting Bessel beams, which can travel long distances and achieve long imaging distances in underwater and similar highly scattering environments, the method uses the Bessel light field as a carrier to carry randomly intensity-modulated speckles, enabling high-quality long-distance ghost imaging in underwater and similar highly scattering environments, thus greatly expanding the application scope of ghost imaging technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic structural diagram of the system of the present invention;

[0024] The reference numerals in the figure are: 1 - light source, 2 - intensity-modulated Bessel beam generation device, 3 - bucket detector, 4 - computer.

[0025] Figure 2 It is a schematic structural diagram of Embodiment 1 of the present invention;

[0026] The reference numerals in the figure are: 21 - light source, 22 - intensity-modulated Bessel beam generation device (including: conical lens and digital micromirror device), 23 - bucket detector, 24 - computer, 25 - displacement platform.

[0027] Figure 3 It is a schematic structural diagram of Embodiment 2 of the present invention;

[0028] The reference numerals in the figure are: 31 - light source, 32 - intensity-modulated Bessel beam generation device (including: spatial light modulator and digital micromirror device), 33 - bucket detector, 34 - computer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will further clarify the present invention with reference to the accompanying drawings in the embodiments of the present invention. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. After reading the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0030] The present invention proposes an underwater long-distance ghost imaging method. Refer to Figure 1 , Figure 1 It is a schematic structural diagram of the system of the present invention, 1 - light source, 2 - intensity-modulated Bessel beam generation device, 3 - bucket detector, 4 - computer.

[0031] Embodiment 1

[0032] As Figure 2 shown, when the Bessel beam generator uses a conical lens, the system includes a light source (21), an intensity modulation Bessel beam generating device (22), a bucket detector (23), a computer (24), and a displacement platform (25). The intensity modulation Bessel beam generating device (22) includes a conical lens and a digital micromirror device. The light source emits light, and a Bessel beam is generated by the conical lens. The light source and the conical lens need to be fixed on the electric displacement platform, and the displacement platform randomly moves around the starting point as the center point in the y-z plane, thereby generating a displacement Bessel light field with a randomly offset center, and the displacement range does not exceed the light passing aperture of the digital micromirror device. Then, the displacement Bessel beam is aimed at the digital micromirror device for random intensity modulation to obtain a Bessel distribution light field with spatially fluctuating randomness. Using this beam to aim at the target, after long-distance transmission, the transmission medium can be an environment such as underwater, air, fog, smoke, etc. After the beam passes through the target or is reflected by the target, the intensity signal is received by the bucket detector (23), completing the underwater long-distance ghost imaging method based on intensity modulation Bessel beam.

[0033] The modulation method is that after a Gaussian beam or a plane beam passes through a conical lens, the wavefront distribution of the beam will be changed to produce a Bessel light field. The light field intensity modulator uses a digital micromirror device, and a light field with spatially fluctuating intensity is generated on the surface of the digital micromirror device. After the Bessel beam passes through the digital micromirror device, a Bessel speckle light field with random intensity modulation is generated.

[0034] Here, after a Gaussian beam or a plane beam passes through a conical lens, its distribution can be considered to be approximately the same as that of an ideal Bessel beam, because the distribution of an ideal Bessel beam has no limit and extends infinitely in the radial direction, which does not exist in practical applications. For this beam approximately distributed as an ideal Bessel distribution, its light intensity expression uses the first kind of Bessel function:

[0035]

[0036] where n is the order of the Bessel function; ρ is the distance from a point on the plane to the origin, in the xoy plane, m is the number of summation terms; Γ(x) is the gamma function, defined as:

[0037] After the Bessel beam passes through the digital micromirror device, a random intensity distribution Ir(x, y) is introduced, and the total incident intensity field is:

[0038] I(x, y) = J n (x, y) + I r (x, y)

[0039] The computer (24) described above includes a main control circuit, a speckle generation module, and a data synchronization and processing module. The main control circuit includes connections to a digital micromirror device and a bucket detector (23). The main control circuit is used to input random forced speckles to the digital micromirror device and collect target intensity signals from the bucket detector (23). The speckle generation module is used to generate randomly intensity-modulated speckles. The data synchronization and processing module is used to process the target intensity signals collected from the bucket detector (23), perform correlation calculations with the matching randomly intensity-modulated speckles, and obtain a reconstructed image.

[0040] Embodiment 2

[0041] As Figure 3 As shown, when the Bessel beam generator uses a spatial light modulator (32), the system includes a light source (31), an intensity-modulated Bessel beam generation device (32), a bucket detector (33), and a computer (34). The computer (34) includes a main control circuit, a Bessel light field generation module, a speckle generation module, and a data synchronization and processing module. The intensity-modulated Bessel beam generation device (32) includes a spatial light modulator and a digital micromirror device. The light source emits light, which generates a displaced Bessel beam after being reflected or projected by the spatial light modulator, and then a Bessel distribution light field with random spatial fluctuations is reflected and output by the digital micromirror device. Using this beam to aim at the target, after long-distance transmission, the transmission medium can be an underwater, air, fog, smoke, or other environment. After the beam passes through the target or is reflected by the target, the intensity signal is received by the bucket detector (33), completing an underwater long-distance ghost imaging method based on intensity-modulated Bessel beams.

[0042] The modulation method is that the spatial light modulator performs phase modulation on a Gaussian beam or a plane beam to directly generate a Bessel light field. The light field intensity modulator uses a digital micromirror device, and a light field with random spatial intensity fluctuations is generated on the surface of the digital micromirror device. After the Bessel beam passes through the digital micromirror device, a randomly intensity-modulated Bessel speckle light field is generated.

[0043] The spatial light modulator is used to generate a displaced Bessel image. The main control circuit is used to input random forced speckles to the digital micromirror device and collect target intensity signals from the bucket detector (33). The speckle generation module is used to generate randomly intensity-modulated speckles with random spatial fluctuations. The data synchronization and processing module is used to process the target intensity signals collected from the bucket detector (33), perform correlation calculations with the matching randomly intensity-modulated speckles with random spatial fluctuations, and obtain a reconstructed image.

[0044] The above embodiments are the preferred embodiments of this patent, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent substitution methods and are all included in the protection scope of the present invention.

Claims

1. An underwater long-distance ghost imaging method, comprising the following steps: Step 1: A light source (1) generates a Gaussian beam or a plane beam with a certain divergence angle, and an intensity modulation Bessel beam generating device (2) generates a randomly intensity-modulated Bessel speckle light field from the Gaussian beam or the plane beam; wherein, the intensity modulation Bessel beam generating device (2) includes a Bessel beam generator and a light field intensity modulator, and the Bessel beam generator generates a randomly intensity-modulated Bessel beam; the light field intensity modulator modulates the randomly intensity-modulated Bessel beam to generate a randomly intensity-modulated Bessel speckle light field; Step 2: According to the randomly intensity-modulated Bessel speckle light field generated in Step 1, it is sequentially projected onto a target (3), and a photodetector (3) that does not require spatial resolution ability measures the total light intensity signal reflected or transmitted by the target, and the total light intensity signal is used as the inner product between the target image and the light projection pattern; then, the total light intensity signal is transmitted to the data synchronization and processing module of a computer (4); Step 3: In the data synchronization and processing module, the total light intensity signal obtained in Step 2 is calculated and reconstructed with the corresponding projected randomly intensity-modulated Bessel speckle light field to obtain an image of the target, realizing underwater long-distance ghost imaging.

2. The method according to claim 1, characterized in that: wherein the Bessel beam generator uses a conical lens or a spatial light modulator, and the light field intensity modulator uses a digital micromirror device.

3. The method according to claim 2, characterized in that: The wavefront distribution of the Gaussian beam or the plane beam is changed by the conical lens to produce a Bessel beam; or, the Gaussian beam or the plane beam is phase-modulated by the spatial light modulator to directly generate a Bessel beam.

4. The method according to claim 1, characterized in that: The light field intensity modulator uses a digital micro mirror device to generate a light field with random spatial intensity fluctuations on the surface of the digital micromirror device. After the Bessel beam passes through the digital micromirror device, a randomly intensity-modulated Bessel speckle light field is generated.

5. An underwater long-distance ghost imaging system configured to execute the method according to any one of claims 1 to 4, wherein the underwater long-distance ghost imaging system includes a light source (1), an intensity modulation Bessel beam generating device (2), the intensity modulation Bessel beam generating device (2) includes a Bessel beam generator for generating a Bessel beam and a light field intensity modulator for modulating the Bessel beam, a photodetector (3) and a computer (4).

6. The system according to claim 5, characterized in that: The light source (1) is a Gaussian beam or a plane beam with a certain divergence angle, and an LED or a laser is adopted.

7. The system according to claim 5, characterized in that: wherein the Bessel beam generator uses a conical lens or a spatial light modulator, and the light field intensity modulator uses a digital micromirror device.

8. The system according to claim 5, characterized in that: The photodetector (3) adopts a photoelectric A photomultiplier tube, a photodiode, a photoconductive device, or a charge-coupled device.

9. The system according to claim 5, wherein: the computer (4) includes a main control circuit, a two-dimensional modulated light field generation module, and a data synchronization and processing module.

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