CN110940992B - Signal detection method and system capable of improving detection distance and precision of laser radar - Google Patents

Abstract

The invention belongs to the technical field of laser ranging, in particular to a signal detection method and system which can improve the detection distance and accuracy of a laser radar. The method of the invention combines the photon counting technology and the weak signal detection technology, including aperiodic multi-pulse averaging and autocorrelation detection; the specific process is: after the reflected light signal reaches the laser radar, the stray light is filtered out by a narrow-band filter, Then, photon counting technology is used to perform photon counting amplification; non-periodic multi-pulse averaging is performed, that is, multi-pulse cumulative averaging is performed according to the pulse period; pulse autocorrelation detection method is used to measure the time interval between the start and end pulses. The system of the invention includes a pulse laser, a photon counter, a multiple averaging module, an autocorrelation detector, etc. The multiple averaging module is used for aperiodic multi-pulse averaging calculation, and the autocorrelation detector is used for autocorrelation operation to obtain a termination pulse signal. The invention can realize long-distance and high-precision target detection.

Description

Signal detection method and system for improving detection range and accuracy of lidar

technical field

The invention belongs to the technical field of laser ranging, and in particular relates to a signal detection method and system which can improve the detection distance and accuracy of a laser radar.

Background technique

Long-distance ranging for non-cooperative targets has a large application space for astronomy, military and other scientific research fields, especially for applications in the field of aerospace detection, usually the ranging distance is large (hundreds of kilometers to tens of thousands of kilometers), so Most of the targets to be detected are space satellites without cooperative targets. Due to the diffuse reflection characteristics of satellites, the received ranging pulses are very weak and difficult to detect directly. Therefore, the ranging of non-cooperative satellites has become a hot spot in modern space exploration.

For extended targets, the range equation for lidar is given by:

In the formula: Pr is the reflected laser power of the target received by the lidar; Pt is the laser transmission power; R is the distance from the target to the laser rangefinder; D is the effective optical receiving aperture; τo is the optical efficiency of the receiving optical path of the laser rangefinder ; τa is the two-way atmospheric transmittance.

The signal output by the amplifier is:

V _s =GR _v P _r

In the formula: G is the amplifier gain; Rv is the voltage responsivity of the detector.

so:

It can be seen from the above formula that the detection distance of the laser rangefinder is mainly related to the laser emission energy, the effective receiving optical aperture, the gain of the amplifier and the signal voltage. Since instruments are often limited by size, mass, and power consumption, the energy and optical aperture of the laser cannot be infinitely increased. This requires trying to increase the gain of the amplifier and minimize the output signal intensity under the condition of limited laser energy and optical aperture. Generally, when the detection rate is required to be greater than 95%, the signal-to-noise ratio (the ratio of the signal to the root mean square of the noise) cannot be lower than 10, so the weak signal detection technology should be used to make the noise unchanged (or change) when the amplifier gain increases. Small), the dynamic range of the signal becomes larger, thereby improving the sensitivity of the lidar.

There are many methods for weak signal detection, such as filtering, modulation amplification, lock-in amplification, correlation, accumulation and so on. Among them, modulation amplification and lock-in amplification technology are mainly used to detect the amplitude and phase of the sinusoidal signal submerged in the noise. For the weak laser pulse signal submerged in the noise, the lock-in amplifier is powerless. Because the fast rising and falling edges of the pulse waveform contain rich high-order harmonic components, the narrow-band filter of the lock-in amplifier output stage will filter out these high-frequency components, resulting in the distortion of the pulse waveform. The methods suitable for pulse signal processing include pulse accumulation method, autocorrelation detection method, correlated double sampling and so on.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a signal detection method which can improve the detection distance and precision of the laser radar, so as to realize long-distance and high-precision target detection.

The signal detection method that can improve the detection distance and accuracy of the laser radar proposed by the present invention is to combine the photon counting technology and the weak signal detection technology, including: aperiodic multi-pulse averaging and autocorrelation detection; the specific process is:

(1) When the reflected light signal reaches the lidar, the stray light is filtered out by a narrow-band filter, and then photon counting technology is used for photon counting amplification;

(2) Perform aperiodic multi-pulse averaging, that is, perform multi-pulse cumulative averaging according to the pulse period;

(3) The pulse autocorrelation detection method is used to measure the time interval of the start-stop pulse.

In the present invention, the multi-pulse cumulative average (refer to Figure 2) is performed, and the calculation formula is as follows:

Where: m is the average number of times; ΔT _j is the jth pulse interval.

The invention adopts aperiodic multi-pulse averaging, which can effectively reduce the noise signal and is beneficial to improve the signal-to-noise ratio.

In the present invention, the multi-pulse autocorrelation detection is to perform a correlation operation on the original pulse signal and the multi-pulse average signal (refer to Figure 2), and the calculation formula is as follows:

Among them: x(n) original pulse signal; y(n) is multi-pulse average signal; N is pulse width.

The invention adopts the multi-pulse autocorrelation detection method, which can effectively filter out the interference signal and accurately extract the rising edge signal.

In the present invention, the photon counting technology is adopted, and the specific content is as follows: the photon counting in the integration time is a discrete random variable, which obeys the Poisson distribution:

In the formula: Kn represents the average number of photons.

Since the photon detection of each integration interval is still a random process and is independent of each other, the detection process of summing N integration intervals still follows the Poisson distribution, the difference is that the mean and variance become NKn, and its signal probability density and The noise probability densities are:

The signal-to-noise ratio is:

倍。For photon counting systems, the signal-to-noise ratio can be improved by using the multi-pulse averaging method

times.

Corresponding to the above method, the present invention also relates to a signal detection system that can improve the detection distance and accuracy of the lidar, the structure of which is shown in FIG. 1 . The signal detection system includes: a pulsed laser 1, a first lens 2, a second lens 4, a narrow-band filter 5, a photon counter 6, an aperiodic multi-pulse averaging module (ie, a multiple averaging module) 7, and an autocorrelation detector 8 , beam splitter 9, sampling detector 10; wherein, pulse laser 1 emits aperiodic multi-pulse laser, after collimation by first lens 2, it is reflected by distant target 3; reflected signal is received by second lens 4, passes through narrow band Filter 5 filters, is received by photon counter 6 and carries out counting amplification; Then enter aperiodic multi-pulse averaging module 7, carry out aperiodic multi-pulse averaging calculation (by formula (1)), the calculation result and sampling detector 10 output The start pulse signal enters the correlation detector 8 together, and performs autocorrelation operation (by formula (1)) to obtain the end pulse signal.

In the present invention, the photon counter is composed of avalanche diode arrays operating in Geiger mode, and has the characteristics of high gain, high sensitivity, low bias voltage, insensitivity to magnetic fields, and compact structure.

In the present invention, the correlation detector can effectively filter out the interference signal and accurately extract the rising edge signal.

The beneficial effects of the present invention are:

Using the accumulation method, although the performance-to-noise ratio is improved, the accumulation of multiple pulses widens the width of the start pulse and the end pulse, and reduces the accuracy of ranging. In view of this situation, the pulse autocorrelation detection method is used to filter out the interference. Signal and atmospheric turbulence influence, measure the time interval of start-stop pulse, ranging sensitivity is proportional to the square root of the average number, ranging accuracy can be improved by 3-4 times.

Description of drawings

FIG. 1 is a schematic diagram of a signal detection system of the present invention (a block diagram of a laser radar signal processing).

FIG. 2 is a schematic diagram of the aperiodic multi-pulse averaging technique.

Figure 3 is a schematic diagram of the autocorrelation detection technology.

Labels in the figure: 1 is a pulsed laser, 2 is a first lens, 2 is a target, 4 is a second lens, 5 is a narrow-band filter, 6 is a photon counter, 7 is a multiple averaging module, and 8 is a correlation detector , 9 is the beam splitter, and 10 is the sampling detector.

Detailed ways

Below in conjunction with the accompanying drawings and specific embodiments, the present invention will be further clarified. 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. Modifications of equivalent forms all fall within the scope defined by the appended claims of this application.

Example:

The invention mainly utilizes the pulse flight time combined with the photon counting detection method to carry out the measurement experiment of the long-distance target laser ranging. The optical aperture of the experimental system is 100mm; the photon counter used is C14193-1325SA produced by Hamamatsu Corporation of Japan; the detection target distance is 1km; the narrowband filter 5 used has a bandwidth of 10nm and a transmittance of 80%.

As shown in Figure 1, the ranging laser 1 uses a semiconductor laser with an output wavelength of 905 nm, a pulse power of 75 W, and a pulse width of 20 ns. After a beam splitter, two laser beams are formed, and one laser beam is irradiated to the target as a ranging light source. Another beam of light enters the sampling detector 10, and after the laser light emitted to the target is reflected by the target, it is incident on the photon counter 6 through the receiving lens 4 and the narrow-band filter 5 to form a received signal. Use an oscilloscope (Agilent MSO7104B 1G bandwidth, 4G sampling rate) to observe and acquire data. Referring to Figure 2, repeat the measurement for many times, export the data collected by the oscilloscope, and use the computer for data processing.

The multi-pulse signal is integrated and accumulated and then averaged. The calculation formula is as follows:

Using the sampled pulse signal and the multi-pulse average signal to perform the correlation operation (see Figure 3), the calculation formula is as follows:

Among them: x(n) sampling pulse signal; y(n) is multi-pulse average signal; N is pulse width.

计算得到测距精度可以提高3-4倍。Experiments show that the minimum distinguishable signal amplitude is 0.5V without correlation detection. After using correlation detection, the minimum distinguishable signal amplitude can be increased by 10 times. According to the laser ranging formula

It is calculated that the ranging accuracy can be improved by 3-4 times.

This method combines the aperiodic cumulative average method with the correlation detection method, which not only improves the sensitivity of ranging, but also improves the accuracy of ranging. The accuracy can be improved by 3-4 times.

Claims (2)

1. A signal detection method capable of improving detection distance and precision of a laser radar is characterized in that a photon counting technology and a weak signal detection technology are combined, and the method comprises the following steps: non-periodic multi-pulse averaging and correlation detection; the specific process is as follows:

(1) when the reflected light signal reaches the laser radar, the stray light is filtered by a narrow-band filter, and then photon counting amplification is carried out by adopting a photon counting technology;

(2) non-periodic multi-pulse averaging is carried out, namely multi-pulse accumulation averaging is carried out according to the pulse period; the calculation formula is as follows:

wherein: m is the average number of times; delta T_jThe j-th pulse interval time;

(3) measuring the time interval of the start-stop pulse by adopting a multi-pulse correlation detection method; the multi-pulse correlation detection is to perform correlation operation on an original pulse signal and a multi-pulse average signal, and the calculation formula is as follows:

wherein: x (n) is the original pulse signal; y (n) is a multipulse average signal; and N is the pulse width.

2. A signal detection system capable of improving detection distance and accuracy of a laser radar is characterized by comprising: the device comprises a pulse laser, a first lens, a second lens, a narrow-band filter, a photon counter, a non-periodic multi-pulse averaging module, a relevant detector, a beam splitter and a sampling detector; the pulse laser emits non-periodic multi-pulse laser, and the non-periodic multi-pulse laser is collimated by the first lens and then reflected by a far target; the second lens receives the reflected signal, the reflected signal is filtered by the narrow-band filter, and the reflected signal is received by the photon counter for counting and amplifying; then entering a non-periodic multi-pulse averaging module to perform non-periodic multi-pulse averaging calculation, and entering a calculation result and an original pulse signal output by a sampling detector into a correlation detector together to perform correlation operation to obtain a termination pulse signal;

the photon counter consists of an avalanche diode array working in a Geiger mode;

the formula for the aperiodic multi-pulse averaging module to calculate the aperiodic multi-pulse average is as follows:

wherein: m is the average number of times; delta T_jThe j-th pulse interval time;

the formula for performing correlation operation in the correlation detector is as follows:

wherein: x (n) is the original pulse signal; y (n) is a multipulse average signal; and N is the pulse width.

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