AU2020101889A4

AU2020101889A4 - A three-channel optical receiver for airborne dual-frequency lidar

Info

Publication number: AU2020101889A4
Application number: AU2020101889A
Authority: AU
Inventors: Jiandong Wei; Guoqing Zhou
Original assignee: Guilin University of Technology
Current assignee: Guilin University of Technology
Priority date: 2020-08-19
Filing date: 2020-08-19
Publication date: 2020-09-24
Anticipated expiration: 2028-08-19

Abstract

The invention discloses a three-channel optical receiver for airborne dual-frequency lidar, which is mainly aimed at the reception of 532nm and 1064nm detection echoes in the shallow sea area of the airborne dual-frequency lidar. The device is equipped with three echo receiving channels, the sea surface, the shallow sea area, and the deep water area, adopting the method of first splitting the echo and then splitting the field of view. The device uses a large-aperture receiving objective lens to increase the receiving field of view (FOV). When echos reach the receiving objective lens,the device first reflects the 1064nm echo to the sea surface channel by the dichroic beam splitter. The 532nm echo passed through the central field of view of the dichroic mirror is reflected to the shallow sea channel by the small reflector, and the remaining echoes passing the hollow lens enter the deep sea channel. The invention effectively reduces the optical crosstalk between the optical element and the channel, which realizes the energy collection of channels in different water depths, dynamically compresses the photoelectric signal, improves the signal-to-noise ratio, and effectively improve the detection accuracy of dual-frequency lidar. 4/4 T----- ------ Phoheiti Figure 4

Description

4/4

T----- ------ Phoheiti

Figure 4

A three-channel optical receiver for airborne dual-frequency lidar

TECHNICAL FIELD

[01] The invention relates to a dual-wavelength optical receiving device, which is mainly aimed at receiving 532nm and 1064nm detection echoes of airborne dual frequency lidar. By the method of first splitting the echo and then splitting the field of view, effectively reduces the optical crosstalk between components, solves the problem of low detection of weak light in near-coast aerial detection under large dynamic range.

BACKGROUND

[02] There are many intertidal zones, islands and reefs in coastal areas, and traditional shipborne acoustic measurement methods are extremely inefficient. Therefore, three-dimensional comprehensive measurement in coastal areas has always been a difficult in the field of remote sensing. Because aquamarine green light windows have good transparency, the laser point cloud data can quickly and accurately distinguish the characteristics of shallow sea floor terrain. At present, the most effective coastal area detection method in the world is airborne dual-frequency lidar detection technology, which has the characteristic of fast measurement speed and wide coverage. The laser outputs 1064nm and 532nm dual-wavelength lasers. The 1064nm laser forms echoes on the sea surface, and the 532nm laser penetrates seawater to form shallow and deep sea echoes. However, in the process of seawater propagation, the amount of photon scattering increases with the increase of water depth, which will cause the attenuation of the echo signal. Because the cross-section of the light propagating in seawater is close to half of the water depth, and the large light spot is more severely affected by the scattering of the water body, the echo energy received by the sensor is reduced, which will greatly limit the maximum sounding accuracy of the laser. As a result, the detection of weak light in a large dynamic range is not enough, which is a difficult for near-shore aerial detection.

[03] The dual-frequency lidar optical receiving system is an important part of lidar ocean depth sounding system, and a series of researches have been carried out at home and abroad. At present, the representative lidar sounding systems in the world include the EAARL system developed by NASA, which uses only 532nm monochromatic green wave laser pulses for water depth detection. It's laser pulse energy is low and pulse width is short, which can seamlessly measure underwater and coastal terrain. However, its laser echo receiving field of view (FOV) is narrow. Although it can effectively weaken the influence of sunlight reflection and water scattering on the laser echo waveform, the received echo energy is low, which affects its detection accuracy. The most prominent domestic dual-frequency lidar is the airborne dual-frequency lidar prototype with independent intellectual property rights developed by the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. Although the optical receiving system also adopts the method of sub-field of view, each receiving channel needs to be installed a filter, which greatly increases the axial distance of the entire optical receiving system. To ensure the safety of the human eye is a common problem in the current airborne lidar receiving system. So, the laser energy per unit area must be limited, and the method of expanding the spot area is usually used. However, the large light spot is more severely affected by the scattering of water body. As a result, the reduced echo energy received by the sensor will greatly limit the maximum measurement depth of laser detection. Based on the existing problems of most current laser radar optical receiving structures, the present invention designs a dual-frequency lidar three-channel optical receiving device to increase the receiving field of view, and then divide the enlarged receiving field of view. The innovative method of first splitting the echo and then splitting the field of view reasonably allocates the wavelength and detection depth to the receiving field of view angle, effectively reducing the optical crosstalk between optical components and channels, realizing energy collection of channels in different water depth. The structure dynamically compresses the photoelectric signal, improves the signal-to-noise ratio, and effectively improves the detection accuracy of the dual-frequency lidar.

SUMMARY

[04] The present invention is a three-channel dual-wavelength laser receiving device. By expanding the receiving field of view and further dividing the field of view, the three-channel optical receiver improve the signal-to-noise ratio and significantly enhance the echo signal energy of each channel.

[05] In order to achieve the purpose of the present invention, the following technical solutions are adopted to achieve the following objects: the receiving optical system is designed with three echo receiving channels: sea surface, shallow water and deep water. Each receiving channel corresponds to a different detector module respectively. The echo signal from the sea surface and bottom is received by the Cassegrain telescope system, adopting a large-aperture receiving objective lens to increase the field of view (FOV) of the receiving system. There are three types of echoes entering the receiving system, including 1064nm sea surface echo, 532nm shallow sea echo and 532nm deep water echo. The three-channel optical receiver adopts the method of first splitting the echo and then splitting the field of view. The 1064nm echo is reflected into the sea channel by the dichroic beam splitter. The 1064nm echo is collimated in the channel and incident on the photosensitive surface of the APD. Because the cross-section of light propagating in seawater is close to half of the water depth, the sub-fields of 532nm echo of dichroic beam splitter are used to collect echoes of different depths respectively. For a small central field of view of 5mrad, a mirror of 450 is embedded in the center of first mirror, which is in the collimating structure of the large field of view on the horizontal light path 532nm echo passed through the dichroic mirror. The532nm echo of small field of view is reflected into the shallow sea channel and collimated into the single quadrant PMT. The remaining 5-50mrad edge large field of view is used as a deep water channel, and the 532nm echo of large field of view is collimated into the high-gain four-quadrant PMT.

[06] The beneficial effects of the present invention is as below.

[07] Three-channel receiving optical system of the dual-channel lidar effectively reduces the optical crosstalk between optical elements and channels, and realizes the energy collection of channels in different water depth. This structure dynamically compresses the photoelectric signal and improves the signal-to-noise ratio. By reducing the use of filters, the axial distance of each channel is reduced, and the size and weight of the system are reduced, the detection accuracy of airborne dual-frequency lidar is effectively improved.

BRIEF DESCRIPTION OF THE FIGURES

[08] Figure 1 is an overall schematic diagram of the three-channel optical receiving device of the present invention;

[09] Figure 2 is a schematic diagram of the bracket structure of the three-channel optical receiving device of the present invention;

[010] Figure. 3 is a schematic diagram of the hollow lens structure of the three channel optical receiving device of the present invention with a mirror embedded;

[011] Figure 4 is a schematic diagram of the working principle of the three channel optical receiving device of the present invention;

[012] [Description of Reference Signs]

[013] 1 Receiving window,

[014] 2 Reflector,

[015] 3 Dichroic beam splitter,

[016] 4 Collimating lens

[017] 5 Focuslens

[018] 6 Fixing bracket

[019] 7 Hollow lens with 450 small reflector,

[020] 8 Fixed base of device

DESCRIPTION OF THE INVENTION

[021] In order to clearly illustrate the technical characteristics of the present solution, the present invention will be described in detail below by specific implementations and accompanying drawings.

[022] As shown in Figure 1, a dual-frequency optical receiving device is mainly for detecting laser echoes in shallow waters of 1064nm and 532nm. The dual-frequency laser echo is incident from the receiving window 1, and then the light path is changed by the 450 reflector at position 2. There are three main types of received echoes: 1064 sea surface echo, 532 deep water echo and 532 shallow water echo. Three kinds of echoes are received at the same time. Using the dichroism spectrometer at position 3, the sea echo at 1064 nm is reflected into channel 1. Then in channel 1, the double lens collimation structure at position 4 is used for collimating and focusing, and the focus is transmitted to the photodetector. The 532nm echo passing through the beam splitter, including deep water echo and shallow water echo, reaches position 5 along the channel to the right to. A new lens structure was innovatively designed at position 5. As shown in Figure 2, a reflector is embedded in the middle of the hollow lens and extends out in a cylindrical shape. Then installing a reflector on the 45° inclined plane to achieve the purpose of dividing the field of view. Because the cross-section of light propagating in seawater is close to half of the water depth,,the amount of photon scattering increases as the water depth increases. Therefore, the deep water echo is received with a large field of view, and the shallow water echo is received with a small field of view. In order to prevent the small field of view from blocking the large field of view, extending the center of the hollow lens to embed a 45° reflector. The 532nm shallow sea echo is reflected into channel 2, and is focused onto the photodetector by the collimating structure at position 6. Excluding the reflection in the central part, the ring part at position 5 emits the 532 nm echo of a large field of view. The collimation is also performed at the 7 position and then incident on the photodetector.

[023] The biggest innovation of the present invention is to divide the field of view according to the detection target and depth. Different echoes are received by different channels respectively, and further collimated in each channel. The energy of each type echo reaches the maximum on corresponding photodetector. By this method of channel division, the optical crosstalk between optical elements and channels is effectively reduced, and the energy collection of channels in different water depth is realized. The structure dynamically compresses photoelectric signal and improves the signal-to-noise ratio. By the method of first splitting the echo and then splitting the field of view, filters of each channel are avoided, the axial distance of each channel is reduced, and the size and weight of the device are reduced, the detection accuracy of the airborne dual frequency lidar is effectively improved.

[024] Anything not described in detail in the present invention is well known to those skilled in the art.

[025] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms, in keeping with the broad principles and the spirit of the invention described herein.

[026] The present invention and the described embodiments specifically include the best method known to the applicant of performing the invention. The present invention and the described preferred embodiments specifically include at least one feature that is industrially applicable

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A dual-wavelength optical receiving device, characterized in that: the receiving optical system is designed with three echo receiving channels: sea surface, shallow water and deep water; each receiving channel corresponds to a different detector module; echoes from the sea surface and the seabed are received by the Cassegrain telescope system adopting a large-aperture receiving objective lens; the device adopts the method of first splitting the echo and then splitting the field of view, the 1064nm echo is reflected into the sea surface channel by the dichroic beam splitter, collimated in the channel and incident on the photosensitive surface of the APD; Because the cross-section of light propagating in seawater is close to half of the water depth, the sub-field of view formed by the 532nm echo passing through the dichroic beam splitter is used to collect echoes of different depth respectively on the horizontal optical path the 532nm echo passed through the dichroic mirror; for a small central field of view of 5mrad, a 450 mirror is embedded in the center of the first mirror in the collimating structure of the large field of view; the 532nm echo of the small field of view is reflected into the shallow sea channel and collimated to the single-quadrant PMT; the remaining 5-50mrad edge large field of view serves as a deep water channel, the 532nm echo of a large field of view is collimated into the high-gain four-quadrant PMT.

2. A dual-wavelength optical receiving device according to claim 1, characterized in that a large-aperture receiving objective lens is adopted, and the method of first splitting the echo and then splitting the field of view is adopted.

3. A dual-wavelength optical receiving device according to claim 1, where in the characteristic dichroic beam splitter, 1064nm sea surface echo is reflected into the channel one, and then focused in the channel by the double lens collimation structure.

4. A dual-wavelength optical receiving device according to claim 1, characterized in that a reflector is embedded in the middle of the hollow lens extending out cylindrically; and a reflector is installed on the inclined plane of 45° to achieve the purpose of dividing the field of view.