US 12,618,950 B2 · Photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR

Abstract

A photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR is designed, through a photomultiplier tube gating technology, based on dual optical receiving channels, main control module with STM32 single chip microcomputer and, high-speed AD sampling module. The system includes: calculating laser echo receiving power ratios of different optical receiving channels, respectively; acquiring, by AD sampling module, laser echo signal, and performing peak determination on acquired data, and transmitting peak information to the main control module; and collecting, by the main control module, echo signal intensity information, performing photomultiplier tube gating control according to the received echo signal intensity and the calculated echo receiving efficiency ratios of different optical receiving channels, and stopping the photomultiplier tube through photomultiplier tube gating control if saturated echo signal occurs, and adjusting external laser device power, thus achieving multiple protection of the photomultiplier tube.

A method for operating a photomultiplier tube protection system with dual optical receiving channels for a bathymetry LiDAR, the method comprising the following operations:

step 1:
calculating laser echo receiving power ratios of different optical receiving channels respectively;
wherein, an output current of a photomultiplier tube is calculated using Formula (1):
I
I
=
P
p
*
S
p
*
G
s
(
1
)
wherein II denotes the output current of the photomultiplier tube, Sp denotes sensitivity of a photocathode surface of the photomultiplier tube, Pp denotes received optical power of the photomultiplier tube after light attenuated in environment, and Gs denotes a bias voltage of the photomultiplier tube;
a received echo power of the optical receiving channel is expressed by Formula (2);
P
p
=
P
b
(
h
)
+
P
s
(
h
)
(
2
)
wherein Pb(h) is water-bottom echo power, and Ps(h) is water-body backscattering power;
backscattered laser echo signal receiving powers of water bottom and water body are calculated by Formula (3) and Formula (4), respectively:
P
b
(
h
)
=
P
i
⁢
ρ
b
π
⁢
∑
η
⁢
cos
2
⁢
θ
w
(
H
+
h
)
2
⁢
e
[
-
2
⁢
(
a
+
b
b
)
cos
⁢
θ
w
]
⁢
F
⁡
(
h
)
(
3
)
P
s
(
h
)
=
P
i
⁢
c
⁢
τ
p
2
⁢
n
⁢
β
⁢
∑
η
⁢
cos
2
⁢
θ
w
(
H
+
h
)
2
⁢
e
[
-
2
⁢
(
a
+
b
b
)
c
⁢
o
⁢
s
⁢
θ
w
]
⁢
F
⁡
(
h
)
(
4
)
wherein Pb(h) is seabed echo signal power, h is a water depth, Pi is laser peak power, ρb is water-bottom reflectivity, Σ is an aperture area of a receiving field, η is receiving efficiency of the optical receiving channel, θw is an included angle between a propagation direction and a vertical direction after laser enters seawater, H is an equivalent flight altitude, a is an absorption coefficient of the water body, bb is a backscattering coefficient of the water body, F(h) is a field loss factor, τp is a laser pulse width, n is a refractive index of the water body, βπ is a value when a volume scattering function β(θ) is 180°;
according to a design of the optical receiving channel, LiDAR echo signal receiving efficiency ratios of different optical receiving channels are calculated in combination with Formula (1), Formula (2), Formula (3) and Formula (4);
step 2:
transmitting an echo signal received by the optical receiving channel to a high-speed AD acquisition module after passing through the photomultiplier tube and a back-end processing circuit, performing peak determining on acquired data by an AD sampling module designing a peak module, and transmitting peak information to a main control module;
wherein, an attenuator in the back-end circuit is configured to attenuate a signal acquired by the photomultiplier tube to a threshold range acquired by the AD sampling module, and voltage conversion of the attenuator is expressed by Formula (5);
20
⁢
lg
⁡
(
U
i
U
o
)
=
B
(
5
)
wherein Ui denotes an input voltage, Uo denotes an output voltage, and B is an attenuation amount;
an acquisition voltage of the AD sampling module ranges from −0.85 v to +0.85 v, a number of conversion bits is 14, and a corresponding analog-to-digital conversion formula is expressed by Formula (6):
V
i
V
o
=
D
o
2
16
-
1
(
6
)
wherein Vi is the acquisition voltage, Vo is the range of the acquisition voltage, Do is a converted value after acquisition;
a peak determination module is designed in the high-speed AD acquisition module to match a laser pulse period, to record the converted value after acquisition in each clock cycle of the high-speed AD acquisition module, and compare the converted value with a value in a previous clock cycle to record a maximum value;
when all values in a complete laser pulse period are compared, a resultant maximum value is transmitted to the main control module of the photomultiplier tube protection system through a serial port as echo signal intensity information of the protection system;
and
step 3:
after receiving the echo signal intensity information, according to the calculated echo receiving efficiency ratios of different optical receiving channels, by the main control module, performing a precalculation on an echo signal intensity acquired by a second photomultiplier tube and a second back-end processing circuit, connected to the strong light channel, in a current operating state;
when it is determined that the photomultiplier tube is able to operate in a normal operating state, inputting a gating start signal to the second photomultiplier tube to make the second photomultiplier tube operate as normal;
when it is determined that the photomultiplier tube is in danger of damage, prohibiting the gating start signal from being input, and turning down laser energy;
wherein, during operation, current echo signal intensities acquired by a first photomultiplier tube and a first back-end processing circuit as well as the second photomultiplier tube and the second back-end processing circuit are detected in real time, and when a saturated echo signal occurs, the input of the gating start signal is stopped, and an external laser intensity is adjusted to achieve multiple protection of the photomultiplier tube.

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Photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR

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Photomultiplier tube protection system with dual optical receiving channels for bathymetry LiDAR

Life of this patent

Abstract

Claims1