JP2701110B2 - Lidar equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention realizes high-sensitivity measurement by capturing a signal without waste in a lidar apparatus using a high-speed repetition laser as a light source, and densely measures a spatial distribution of a high-speed varying phenomenon. The present invention relates to a lidar device which enables measurement close to an actual state.

[0002]

2. Description of the Related Art Generally, when a spatial distribution of a target object is measured by a lidar apparatus (laser radar), a time width narrow enough to obtain a required spatial resolution (between the space width and the time width, A pulse laser beam having a relationship of “space width” = “time width” × (c / 2) (c: light speed)) is transmitted into a medium, and an echo signal from a scatterer is received by a light receiving telescope. At the same time, this is converted into an electric signal to perform signal processing.

Next, the processed signal is transferred to a personal computer to perform integration, calculation, display, and recording. The repetition cycle of the pulse laser used at that time is often 10 to 100 Hz. The reason for this is that the repetition cycle of a high-output pulse laser (for example, Nd: YAG laser) is often in the above range, and the time required to transfer data converted by the signal processing device to the personal computer side is as follows. Conventionally, the limit is about 10 to 100 Hz.

Recently, attention has been paid to lasers that do not use flash lamps, such as solid-state lasers excited by semiconductor lasers (LD). In the case of a semiconductor laser pump pulsed solid-state laser, when a cw-operated semiconductor laser is used, a repetition rate higher than 1 KHz is possible because the Q switch is performed in an electronic process.

[0005] In particular, when a Q-switch pulse operation is performed using a photoacoustic (AO) element, the operation is unstable and the output is small at a repetition frequency of several hundred Hz or less. . On the other hand, when the repetition frequency increases, the average power increases and approaches a constant value. Therefore, a high-speed repetition pulse operation of usually 5 to 10 KHz is performed.

A semiconductor laser pumped solid-state laser is suitable as a lidar light source because of its small pumping efficiency, reliability, and small shape. However, the problem of data sampling remains to acquire data at high speed. Was. Other examples of laser light sources that have substantially the same energy per pulse and that can be repeated at high speed include AlGaAs-based semiconductor lasers whose maximum output is limited by the intensity of the end face, and resonance of cw operation solid-state lasers. This corresponds to the case where a modulation element is inserted inside the device and the Q switch is turned on.

[0007] Next, what the transfer speed will be in a conventional lidar device will be considered. FIG. 5 is a configuration diagram illustrating an example of a conventional rider device. As shown in FIG. 5, first, the signal light scattered by the scatterer 9 to be measured is collected by the light receiving telescope 10. The collected optical signal is converted into an electric signal by the photodetector 27, and if necessary, is amplified by the preamplifier 13 according to the signal intensity.

The rider signal amplified to the required level by the preamplifier 13 is supplied to a transient recorder (high-speed multi-channel AD converter) 28 for digitizing an electric signal. Then, after being amplified to a predetermined level by the amplifier 17 in the transient recorder 28, it is converted into a digital signal by the AD converter 18. The AD conversion operation is controlled by a time controller 19 and is performed at time intervals of a clock signal generated by a clock generator 20.

The digitized lidar signal is temporarily stored in the buffer register 24, transferred to the personal computer 15 through the interface board 21, and processed. In this case, the integration, processing, recording, etc. of the data are all performed by the personal computer 15 in real time. The repetition setting unit 25 sends a start signal to the time controller 19 under the control of the personal computer 15 and controls the light emission operation of the laser light transmission unit 26.

The data acquisition speed depends on the clock frequency of the transient recorder 28 and the sampling memo.
The number is controlled by the number of clocks, the number of clocks required for integration in the personal computer 15, and the like. For example, at present I
Considering the speed of C, if the clock frequency of the transient recorder 28 is 30 MHz and the number of sampling memories is 2 kilowords (2048), one cycle is 68.3 μs.
Becomes

Next, consider a case where the data is sequentially transferred to the personal computer 15 and added together with the sampling memory at the beginning of the data. It is assumed that the number of quantization bits of the AD converter 18 is 6 or 8 bits, and the data is transmitted in parallel on a bus line. Since the transfer rate at this time depends on the clock frequency of the personal computer 15, for example, if the frequency is set to 20 MHz, the sampling rate of 2048 per cycle is used.
Assuming that the number of transfer clocks required for the integration on the personal computer 15 side to transfer the sampling memory is 10 clocks, the number of sampling memories × 10 times the number of clocks is required, so that 1024 μs is required.

[0012]

This means that the maximum transfer rate is 0.975 KHz, even if the processing steps required for other transfers are negligible. Actually, the transfer speed is further reduced due to the exchange of commands between the transient recorder 28 and the personal computer 15, and the like.
The limit is about z. Therefore, there has been a limit in increasing the speed at which data is taken in the past.

In the conventional method, for example, after the transient recorder 28, a signal averager (S
A: A signal integrator) can be added to the PC 15
Although it is possible to perform the integration before transferring the data to the storage device, it is difficult to employ the pipeline system in such a case, and it is difficult to increase the improvement of the integration speed.
The present invention has been made in view of the above-described problems, and has as its object to increase the speed of capturing data in a lidar device using a high-speed repetitive laser as a light source.

[0014]

According to the present invention, there is provided a lidar device comprising:
In order to achieve the above object, a light transmitting unit that transmits high-speed repetitive pulsed laser light into a medium, a light receiving optical system that receives backscattered light from the medium, and light that is received by the light receiving optical system A photoelectric conversion unit that converts the lidar signal light into an electric signal, an amplifier that amplifies the lidar signal converted into the electric signal, and a lidar signal output from the amplifier, which is converted into digital data at high speed, A signal processing device that integrates the sampling time in synchronization with the laser pulse and integrates the data with an adder, and computes and calculates digital data sent from the signal processing device.
It is characterized by comprising a calculation / display / recording unit for displaying / recording. According to another feature of the present invention, the apparatus further includes a light transmission timing detection unit that detects a pulse laser beam transmitted toward the medium and determines an operation timing of the signal processing device. .

[0015]

Since the present invention comprises the above technical means, it is assumed that all systematic noise is removed from the noise entering the lidar device, and that only random noise remains, the signal / noise ratio (hereinafter "SN") The ratio is called as follows:

[0016]

(Equation 1)

Here, N is the number of times of integration, I _S (R) is a current due to a signal from the distance R, i _S (R) is a shot noise due to Is (R), i _B is a shot noise due to the background photocurrent, i _D is APD (avalanche photodiode)
, _InA represents amplifier noise, and _ifn represents thermal noise due to feedback resistance in the amplifier.

In the present invention, as in the embodiment, a case where an avalanche photodiode APD is used as a detector from the viewpoint of all-solid-state devices will be discussed. In the case where another element is used, it can be handled in exactly the same way.

First, consider the SN ratio when the data acquisition time (integrated time) is fixed. The sampling time is Δt
In order to measure at a spatial resolution of ΔR = c · Δt / 2, the laser pulse width needs to be smaller than the sampling time Δt. The shot noise I _S (R) in Expression 1 is a value sampled at the sampling time Δt, and is proportional to the energy E ₀ per pulse.

If the energy E ₀ per pulse is constant irrespective of the repetition frequency f, the integration number N becomes f
Therefore, the SN ratio increases in proportion to f ^1/2 . When the average power is constant irrespective of the repetition frequency f, the energy E ₀ per pulse, ie, Is (R), is proportional to 1 / f. When the signal shot noise limit (the signal noise i _S (R) ² is the largest in the denominator noise term of Equation 1 and the others are negligible), i _S (R) ² is Is
Since it is proportional to (R), the S / N ratio of Equation 1 is independent of the repetition frequency f, and there is no advantage of this method.

In the denominator of the equation 1, i is larger than that of other noises.
_{When S} (R) ² is negligible (noise is dominated by noise other than signal shot noise), the S / N ratio deteriorates in proportion to 1 / f ^1/2. There is no. Furthermore, when noise is dominated by noise other than signal shot noise, the SNR increases with repetition frequency f because the decrease in energy E ₀ per pulse is 1 / f
^This is a case where the time is slower than ^1/2 (when f ^1/2 · E ₀ increases with f).

In the present invention, the transient recorder 28 in FIG. 5 is replaced by the high-speed data processing device 14 in FIG. 1, and the addition operation is performed in the processor in synchronization with the clock of the high-speed data processing device 14. Thus, it is possible to “acquire data at high speed”, which is the gist of the present invention. The transfer is practically omitted as being performed after the addition of data. When data is acquired with a sampling memory number of 2K words, a clock operation of 30 MHz is normally possible, so that data can be acquired at a high speed of up to 15 KHz. did.

In the present invention, the input signal is AD-converted at 30 MHz by the ADC 18 in the figure, and the first channel of the register is added the required number of times in accordance with the start of the laser, and the clock frequency of the high-speed data processor 14 is increased. To get data at high speed. Then, a pipeline system is adopted in order to efficiently perform addition within a time.

For example, when operating at 5 KHz, 1 second integration adds 5,000 times and then 1
The measurement is stopped for the period (68.3 μs at 30 MHz). Then, after the start of the next measurement, the data is transferred to the personal computer 15 (the transfer time is 10 to 100 ms), and the process proceeds to the next measurement. Therefore, the time (10 to 100 ms) during which data transfer is prevented by transfer to the personal computer 15 is almost negligible. If the same data is transferred by the conventional method, the transfer to the personal computer 15 takes 5,000 times longer.

Further, if the clock frequency is 30 MHz (clock time 33.3 ns), clock time Δt
2 (a) and 2 (b) depending on the size of the product of the sampling memory number and 68.3 μs and the laser oscillation interval _TL . In FIG. 2, Δt is a clock time, and N is
The number of sampling memories , _TL , indicates the laser interval and 1 / f
It is.

FIG. 2A shows a case where data writing to the register is completed before the next laser pulse arrives, and there is a time that is not used for measurement until the next data is added. In addition, in FIG. 2B, contrary to FIG. 2A, the case where the next pulse comes before the addition is completed, the first sample is newly added by the subsequent pulses.
From the memory . 63.8 μs in this case
2 (a) or FIG. 2 at the reciprocal of 14.6 KHz of FIG.
The operation is divided into (b). In the operation at a higher speed (for example, 20 KHz), only a part of the entire sampling memory is used.

Therefore, if the sampling memory is filled before the next pulse arrives at the signal processor, nothing is done until the next signal arrives, and when the next pulse arrives before reaching the last sampling memory. Is the back sampling
It is desirable to have a function of returning to the first sampling memory while leaving the memory blank. In addition to setting the timing in accordance with the clock, a light transmission timing detection unit 8 is provided to monitor the laser oscillation light so that the light transmission timing has a timing function for starting digital sampling and integration. It may be.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a schematic configuration of the lidar device of the present embodiment. In the lidar device of the present embodiment, the light source of the light transmitting unit 29 is an Nd: YAG laser 1 (250 mW at CW oscillation) excited by a 1 W class LD.
AO modulation element 2 is inserted into the resonator, and a Q-switch is applied by the external oscillator 3 to set
A pulse width of 40 ns is obtained.

In this case, the modulation frequency becomes the laser pulse repetition frequency as it is, and the operation is performed at 1 KHz. The energy per pulse is 70 μJ. The lidar device in the embodiment is intended to measure suspended particles such as aerosols in the atmosphere, and FIG. 3 shows a measurement example of a reflected echo from a building 300 m away.

The laser signal is emitted from the LD-pumped YAG laser 1 and reaches the beam expander 6 via the output mirror 4 and the beam splitter 5. After the beam spread is narrowed by the beam expander 6, the beam direction is adjusted by the direction adjuster 7 composed of two wedges, and the beam is emitted to the atmosphere.

The beam emitted into the atmosphere is reflected by the scatterer 9 and returns as scattered light. The scattered light is collected by the light receiving telescope 10, converted into an electric signal by the avalanche photodiode APD 12, and amplified by the amplifier 13.

The lidar signal S1 amplified by the amplifier 13
Is then supplied to the high-speed data processing device 14, and the 6-bit A provided in the high-speed data processing device 14
It is digitized by the D converter 18. Then, it is given to the adder 22, and is repeatedly and continuously added at a timing based on the laser light in the sampling memory 2048, and the addition result is stored in the register 23. And a personal computer (CPU) every required time
15 to be calculated and analyzed, and displayed on the display device 16.

Further, in this embodiment, a light transmission timing detecting section 8 is provided to monitor the laser oscillation light to generate a start signal S2 and have a timing function as a starting point of digital sampling and integration. . further,
The light emitting and receiving unit of the laser beam is installed on a sweeping pedestal 11 and is swept over a predetermined range. In the measurement of this example, 5,000 times of integration (5 seconds) were performed.

As a comparative example, the Q of a Nd: YAG laser at a laser pulse repetition frequency of 10 Hz according to a conventional method was used.
Activate the switch and further increase the pulse energy to 1 KH
FIG. 4 shows the result of comparing the SN ratio by dimming the light to 70 μJ, the same value as in the z operation, and performing integration for 5 seconds each in the present invention and the conventional method. As is clear from FIG. 4, in the case of the lidar apparatus of the present invention in which the repetition period is 1 KHz, the SN ratio is 10 (100) compared to 10 Hz in the conventional method.
^1/2 ) times better.

[0035]

As described above, the present invention captures a lidar echo signal in synchronization with a laser pulse, converts the captured signal into digital data at a high speed, and accumulates the position in synchronization with the laser pulse. Therefore, high-precision measurement can be performed by increasing the SN ratio, and high-speed processing of data can be performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a lidar device of the present invention.

FIG. 2 is a diagram showing a relationship between a laser pulse interval and the number of sampling memories .

FIG. 3 is an intensity characteristic diagram showing an experimental example of detecting an echo from a building.

FIG. 4 is a characteristic diagram showing a comparison of the SN ratio between the rider device of the present invention and the conventional rider device.

FIG. 5 is a configuration diagram illustrating an example of a conventional lidar signal processing unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 LD excitation YAG laser 2 AO modulator 3 Oscillator 4 Output mirror 5 Beam splitter 8 Detector 9 Scatterer 10 Light receiving telescope 11 Sweep mount 13 Preamplifier 14 High-speed signal processing device 15 CPU (PC) 16 Display device 17 Amplifier 18 A / D converter 19 Time controller 20 Clock generator 21 Interface board 22 Adder 23 Register

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-76484 (JP, A) JP-A-53-147570 (JP, A) JP-A-53-34549 (JP, A) 252586 (JP, A)

Claims (2)

(57) [Claims] A light transmitting section for transmitting a high-speed repetitive pulsed laser beam into a medium; a light receiving optical system for receiving backscattered light from the medium; and a lidar signal light received by the light receiving optical system. A photoelectric conversion unit that converts the electric signal into an electric signal; an amplifier that amplifies the lidar signal converted into the electric signal; a lidar signal that is output from the amplifier is taken in; converted into digital data at high speed; and synchronized with a laser pulse. A signal processing device for integrating sampling times and adding the signals by an adder; and a calculation, display, and recording unit for calculating, displaying, and recording digital data sent from the signal processing device. Lidar device. 2. A light transmitting section for transmitting high-speed repetitive pulsed laser light into a medium, a light receiving optical system for receiving backscattered light from the medium, and a lidar signal light received by the light receiving optical system. A photoelectric conversion unit that converts the electric signal into an electric signal, an amplifier that amplifies the lidar signal converted into the electric signal, a lidar signal output from the amplifier is taken and converted into digital data at a high speed, and the laser signal is converted into a laser pulse. A signal processing device for synchronizing the sampling time and integrating by an adder; a calculation / display / recording unit for calculating / displaying / recording the digital data sent from the signal processing device; A light transmission timing detecting unit for detecting the pulsed laser light transmitted by the light source and determining the operation timing of the signal processing device. Device.