An underwater detection device for angular momentum light beams based on volume holographic gratings

Abstract

The application discloses an underwater detection device for angular momentum light beams based on volume holographic gratings, which comprises the following steps: laser output light is modulated to become an angular momentum light beam, and the angular momentum light beam is reflected by a two-dimensional scanning mirror after passing through a hole mirror, so that the two-dimensional scanning of the angular momentum light beam on an underwater target is realized; the signal light reflected by the underwater target and the scattering light of water bodies re-enter the system through the two-dimensional scanning mirror, and the aperture of the light beam is reduced by a beam-reducing device after being reflected by the hole mirror; the transmission direction of the light beam is selected by a volume holographic grating, and the scattering light of the water bodies is filtered out; then, the light beam is spatially coherent filtered by a spiral phase plate; finally, the light beam is focused by a focusing lens, a signal is received by a detector, and the detection of the underwater target is realized. The application fully utilizes the anti-water scattering interference characteristics of the angular momentum light beam and the angle selection characteristics of the volume holographic grating, and has the advantages of long underwater detection distance, high detection signal-to-noise ratio and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of underwater laser radar detection, and particularly relates to an angular momentum beam underwater detection device based on a volume holographic grating. BACKGROUND

[0002] The underwater sonar detection technology has problems such as being susceptible to interference, large ranging error, and difficulty in identifying small targets. The laser radar is used for detecting, imaging and identifying underwater targets, and can realize high-anti-interference high-resolution imaging and high-precision ranging, and has an important application prospect in the fields of underwater robot operation, underwater target monitoring and underwater environment monitoring.

[0003] However, due to the strong scattering and absorption effect of water and impurities therein on laser, the image signal-to-noise ratio obtained by the underwater laser radar is low, and the effective detection distance is also small. In order to reduce the limitation of the strong scattering and attenuation effect of the water medium on the system performance, the distance-gated imaging technology is usually used to suppress the backscattering light, but the distance-gated method needs to know the distance between the detection system and the target, which is limited in actual application. Single-photon detection technology is also used to extend the underwater detection distance, although the sensitivity is very high, but the single-photon detection technology still cannot effectively distinguish the water scattering light and the target light, and the system detection performance is limited.

[0004] The vortex beam has the characteristics of orbital angular momentum and phase singularity, and by adding a spiral phase plate in the receiving light path, the scattering light and the target reflected light can be distinguished from the spatial coherence, and has a spatial filtering effect. However, as the underwater detection distance increases, the coherence of the target reflected light will gradually decrease, resulting in that the signal light distributed on the vortex ring may be completely submerged in the scattering light and cannot be detected. SUMMARY

[0005] The technical problem to be solved by the present application is that the underwater laser radar has a limited detection distance due to the influence of water scattering light. The present application provides an angular momentum beam underwater detection device based on a volume holographic grating. The device uses the strong light direction angle selection characteristic of the volume holographic grating for single-wavelength laser, and by adding a volume holographic grating in the receiving light path, the target signal light propagating along the near optical axis direction is passed, while the water scattering light deviating from the optical axis direction is blocked, thereby realizing effective suppression of the scattering light. At the same time, the transmission rate of the vortex beam after passing through the water scattering medium is higher than that of the Gaussian beam, and the vortex beam is also used at the transmitting end of the detection system, further improving the underwater detection distance.

[0006] The technical solution adopted by the present invention to solve its technical problem is: an underwater detection device based on a volume holographic grating with angular momentum beam, including a laser 1, an angular momentum beam generating device 2, a perforated mirror 3, a two-dimensional scanning mirror 4, a beam shrinking device 5, a volume holographic grating 6, a spiral phase plate 7, a focusing lens 8, and a detector 9.

[0007] The output light from laser 1 is modulated by angular momentum beam generator 2 to become an angular momentum beam. After passing through the perforated mirror 3, it is reflected by the two-dimensional scanning mirror 4 to achieve two-dimensional scanning of underwater targets. The signal light reflected by the underwater target and the scattered light from the water body re-enter the system through the two-dimensional scanning mirror 4. After being reflected by the perforated mirror 3, the beam aperture is reduced by the beam shrinking device 5. Then, the beam transmission direction is angle-selected by the volume holographic grating 6 to filter out the scattered light from the water body. Next, the beam is spatially coherently filtered by the spiral phase plate 7. Finally, the beam after angle selection and spatial coherence filtering is focused by the focusing lens 8, and the signal is received by the detector 9. After subsequent data processing, long-distance detection of underwater targets can be achieved.

[0008] The laser 1 is a blue-green pulsed laser, suitable for underwater transmission and target distance detection;

[0009] The angular momentum beam generating device 2 can be a spatial light modulator or a phase plate with a fixed vortex phase distribution, used to generate an angular momentum beam that can travel a longer distance underwater than a Gaussian beam.

[0010] The main purpose of the beam-shrinking device 5 is to reduce the requirement for the aperture size of the volume holographic grating and reduce the processing difficulty of the volume holographic grating.

[0011] The volume holographic grating 6 has high diffraction efficiency for underwater target echo signal light that meets the Bragg condition, but very low diffraction efficiency for water body scattered light that does not meet the Bragg condition, thereby filtering out background stray light and improving the detection signal-to-noise ratio.

[0012] The angle selection characteristic of the volume holographic grating 6 can be adjusted according to the laser wavelength, and parameters such as the thickness, refractive index modulation, and period of the volume holographic grating can be adjusted to obtain the optimal angle selection characteristic.

[0013] The spiral phase plate 7 is used to modulate the target reflected light with good spatial coherence into a ring-shaped vortex light, while not modulating the water body scattered light with poor spatial coherence into a vortex light, thereby achieving spatial separation of the target reflected light and the water body scattered light.

[0014] The detector 9 can be an array detector for simultaneously recording the signal light in annular distribution and the scattered light in uniform distribution, or a plurality of single-point detectors for recording the signal light at a point on the vortex ring and the scattered light at the center of the annular vortex field, respectively, the single-point detector having higher sensitivity and being more conducive to detecting the underwater target at a long distance.

[0015] Compared with the prior art, the present application has the following advantages:

[0016] The present application combines the advantages of the anti-water scattering interference ability of the angular momentum light beam and the light direction selection characteristic of the volume holographic grating, on the one hand, enhances the transmission distance of the light beam in the water body, and on the other hand, filters out most of the water scattering light while retaining the echo signal light of the detection target, effectively improving the detection signal-to-noise ratio and detection distance of the system. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a schematic diagram of an underwater detection device based on an angular momentum light beam and a volume holographic grating.

[0018] Figure 2 is a curve of the diffraction efficiency of the volume holographic grating varying with the Bragg angle offset.

[0019] The reference signs are explained as follows: 1 is a laser, 2 is an angular momentum light beam generating device, 3 is a mirror with a hole, 4 is a two-dimensional scanning mirror, 5 is a beam shrinking device, 6 is a volume holographic grating, 7 is a spiral phase plate, 8 is a focusing lens, and 9 is a detector. DETAILED DESCRIPTION

[0020] The present application will be described in detail below in combination with the drawings and specific embodiments. However, the following embodiments are only for explaining the present application, and the protection scope of the present application should include the entire content of the claims, and the skilled in the art can also realize the entire content of the claims of the present application through the following embodiments.

[0021] Embodiment:

[0022] The embodiment of the present application is an underwater detection device using a volume holographic grating with an angle selection range of 1° and a Bessel-Gauss light beam.

[0023] Figure 1is a schematic diagram of an underwater detection device of an angular momentum beam based on volume holographic grating in the embodiment of the present application, the laser 1 is a pulsed laser with a wavelength of 532 nm, the angular momentum beam generating device 2 is a spatial light modulator, the laser output by the laser 1 is 2 mm, becomes a Bessel-Gauss beam after being modulated by the spatial light modulator, and passes through the hole of the hole mirror 3 and is incident on the surface of the two-dimensional scanning mirror 4. Among them, the aperture of the hole mirror 3 and the two-dimensional scanning mirror is 100 mm. The servo mechanism is used to drive the two-dimensional scanning mirror to deflect in the x direction and the y direction, and the deflection angle is 10°, so that the scanning detection of the Bessel-Gauss beam on the underwater target can be realized.

[0024] Because the propagation speed of light is very fast, the signal light reflected by the underwater target and the scattering light of the water body are collected by the two-dimensional scanning mirror 4 into the system at the same time, and after being reflected by the reflection surface of the hole mirror 3, the aperture of the light beam is reduced to 25 mm by the 4 times beam shrinking device 5; the transmission direction of the light beam is selected by the volume holographic grating 6, and the scattering light of the water body is filtered out. Among them, the working wavelength of the volume holographic grating 6 is 532 nm, the refractive index is 1.52, the refractive index modulation degree is 0.005, the thickness is 50 μm, the period is 1000 lines / mm, the grating vector direction is perpendicular to the surface of the volume holographic grating element, and the diffraction efficiency curve changes with the Bragg angle offset as shown in Figure 2 Obviously, the half width of the volume holographic grating diffraction efficiency curve is about 1°, so that the angle selection within 1° around the Bragg angle can be realized, and the scattering light in the range larger than 1° is suppressed. Then, the spatial coherence filtering of the light beam is carried out by the spiral phase plate 7, the signal light can form a vortex ring light due to good coherence, and the residual scattering light which is not filtered out by the volume holographic grating cannot form a vortex ring light due to poor coherence, so that the separation of the scattering light and the signal light is realized; finally, the light beam after the angle selection and spatial coherence filtering is focused by the focusing lens 8, and the signal is received by the detector 9; in subsequent data processing, the average light intensity of the vortex ring light is subtracted from the average value of the scattering light intensity in the middle of the ring beam, so that the signal light with higher signal-to-noise ratio can be obtained, and the long-distance detection of the underwater target is realized.

[0025] The part not described in detail in the present application belongs to the known technology of those skilled in the art.

Claims

1. An underwater detection device of angular momentum beam based on volume holographic grating, comprising a laser (1), an angular momentum beam generating device (2), a perforated mirror (3), a two-dimensional scanning mirror (4), a beam shrinking device (5), a volume holographic grating (6), a spiral phase plate (7), a focusing lens (8), and a detector (9), characterized in that: The laser (1) outputs light which is modulated by the angular momentum beam generating device (2) to become an angular momentum beam, is reflected by the two-dimensional scanning mirror (4) after being reflected by the aperture mirror (3), and realizes two-dimensional scanning of the angular momentum beam on the underwater target; the signal light reflected by the underwater target and the scattering light of the water body re-enter the system through the two-dimensional scanning mirror (4), are reflected by the aperture mirror (3), and the aperture of the light beam is reduced by the beam-reducing device (5); then, the transmission direction of the light beam is angle-selected by the volume holographic grating (6), and the scattering light of the water body is filtered out; then, the light beam after the angle selection and the spatial coherence filtering is focused by the focusing lens (8), the signal is received by the detector (9), and remote detection of the underwater target is realized through data processing; The laser (1) is a blue-green pulsed laser, which is used for underwater transmission and detection of the target distance; The angular momentum beam generating device (2) is a spatial light modulator or a phase plate with a fixed vortex phase distribution, which is used for generating an angular momentum beam; The beam-reducing device (5) is used for reducing the aperture of the reflected signal light beam; The volume holographic grating (6) has high diffraction efficiency for the underwater target echo signal light satisfying the Bragg condition, and low diffraction efficiency for the water body scattering light not satisfying the Bragg condition, so as to realize filtering of the background stray light; The volume holographic grating (6) adjusts the thickness, refractive index modulation degree and period of the volume holographic grating according to the laser wavelength, so as to obtain the best angle selection characteristic; The spiral phase plate (7) is used for modulating the target reflected light with good spatial coherence into annularly distributed vortex light, and not modulating the water body scattering light with poor spatial coherence into vortex light, so as to realize spatial separation of the target reflected light and the water body scattering light; The detector (9) is an array detector, which is used for recording the annularly distributed signal light and the uniformly distributed scattering light at the same time; or a plurality of single-point detectors, which respectively record the signal light at a point on the vortex annulus and the scattering light at the center of the annular vortex field; the data processing includes subtracting the average value of the scattering light intensity in the middle of the annular light beam from the average light intensity of the vortex annular light to obtain signal light with high signal-to-noise ratio; The laser output by the laser (1) is modulated by the spatial light modulator to become a Bessel-Gauss light beam, and passes through the aperture of the aperture mirror (3) to be incident on the surface of the two-dimensional scanning mirror (4); the two-dimensional scanning mirror is driven by a servo mechanism to deflect in the x direction and the y direction, so as to realize scanning detection of the Bessel-Gauss light beam on the underwater target.

2. The device according to claim 1, wherein, The aperture of the aperture mirror (3) and the two-dimensional scanning mirror is 100 mm; the deflection angles of the two-dimensional scanning mirror in the x direction and the y direction are both 10°.

Images