JP2685107B2 - Laser radar device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a laser radar device,
In particular, it relates to a target distance and speed detection method.

[Prior Art] FIG. 3 shows, for example, OPTICAL ENGINEERING / January 1988 / V.
It is a block diagram of the conventional coherent laser radar apparatus as shown in ol.27 No.1 / p011 to p015. In the figure,
1 is a laser light source that oscillates continuously, 2 is a distributor that divides the output light of the laser light source 1 into two, 3 is a polarizer that transmits light in one polarization direction of the output light of the distributor 2, and 4 is a polarizer 3 Is a 1/4 wavelength plate that converts the polarized light of the output light into a circularly polarized light, 5 is an optical system that outputs the output light of the 1/4 wavelength plate 4 toward the target and receives the reflected light from the target, and 6 is a distribution A frequency shifter for converting the angular frequency of the other light of the output light of the device 2, 7 is a half-wave plate that rotates the polarization direction of the output light of the frequency shifter 6 by 90 °, and 8 is the output of the half-wave plate 7. A combiner for combining the light and the emitted light by the polarizer 3 and the reflected light from the target whose optical path has been separated, 9 is a light receiving element for converting the output light of the combiner 8 into electrical power, and 10 is a light receiving element. An amplifier for amplifying the output signal of 9 and a spectrum analyzer 11 are provided.

Next, the operation will be described. The angular frequency of the output light of the laser light source 1 is ω ₀ . One of the two light beams split by the distributor 2 passes through the polarizer 3, is circularly polarized by the quarter-wave plate 4, and is emitted from the optical system 5 toward the target. This light undergoes a Doppler shift when reflected by the target. The angular frequency ω ₁ of the light subjected to the Doppler shift is represented by the following equation.

Here, v is the target speed, and c is the speed of light.

This reflected light is received by the optical system 5 again, and the 1/4 wavelength plate 4 is received.
Is converted into linearly polarized light having a polarization direction orthogonal to that of the output light, and thus the light is input to the multiplexer 8 with the optical path different from that of the output light by the polarizer 3. On the other hand, the other output light to the distributor 2 undergoes frequency conversion by the frequency shifter 6 using Doppler shift by an acousto-optic device, for example. The angular frequency ω ₂ of the light subjected to the frequency conversion is expressed by the following equation.

ω ₂ = ω ₀ + ω _IF (2) where ω _IF is the Doppler shifter angular frequency due to the acousto-optic element. The output light of the frequency shifter 6 has its polarization direction converted by 90 ° by the half-wave plate 7 and has the same polarization direction as the reflected light from the target input to the multiplexer 8.
Is input to When the output light of the multiplexer 8 is electrically converted by the light receiving element 9, frequency mixing of the two lights input to the multiplexer 8 occurs, and the frequency mixing component in the output signal of the light receiving element 9 occurs.
I _mix is expressed by the following equation.

Where η is the sensitivity of the light receiving element 9, I ₁ and φ ₁ are the intensity and phase of the reflected light from the target on the surface of the light receiving element 9, respectively.
I ₂ and φ ₂ are the intensity and phase of the output light of the half-wave plate 7 on the surface of the light receiving element 9, respectively.

If this signal is amplified by the amplifier 10 and observed by the spectrum analyzer 19, ω _IF + ω _DP can be measured. Of these, ω
_{Since IF} is a known quantity, ω _DP is known, and the target speed v can be detected by the following equation.

[Problems to be Solved by the Invention] Since the conventional laser radar device is configured as described above, it is not possible to simultaneously measure the target distance and speed. To measure the distance, there is a method of emitting output light in pulses and measuring the pulse delay time. In this case,
Attempting to increase the range resolution lowers the signal energy that can be used for speed detection, and deteriorates the S / N of the speed detection signal. On the contrary, if the S / N ratio of the speed detection signal is increased, there is a problem that the distance resolution is deteriorated as in the conventional device.

The present invention has been made to solve the above-mentioned problems, and it is possible to simultaneously obtain a target distance and speed, high resolution, and S / S.
An object of the present invention is to obtain a laser radar device that can detect N well.

[Means for Solving the Problems] A laser radar device according to the present invention comprises a laser light source that continuously oscillates at an angular frequency ω ₀ and two laser light sources output light.
A splitter for splitting, a polarizer that transmits one of the splitter output light, and a polarization of the polarizer output light is converted into circularly polarized light.
The 1/4 wavelength plate, an optical system for emitting the 1/4 wavelength plate output light toward the target and receiving the reflected light from the target, and the other angular frequency of the distributor output light to ω ₀ + ω _IF A frequency shifter for conversion, a half-wave plate that rotates the polarization direction of the output light of the frequency shifter by 90 °, and an output light that is received by the optical system and converted into linearly polarized light by the quarter-wave plate and is output by the polarizer. And a multiplexer for multiplexing the reflected light from the target whose optical path is separated and the half-wave plate output light, a light-receiving element for optoelectrically converting the multiplexer output light, and the light-receiving element output In a laser radar device provided with an amplifier for amplifying a signal, the velocity B
( ^M / s) m sequence generator that generates 2 ^N -1 bit (N: integer) m sequence signal, and is placed on the optical path connecting the laser light source and the polarizer to generate the m sequence as an external signal. An optical modulator that modulates and outputs laser light output light from an m-sequence signal input from the m-sequence generator, and m from the m-sequence generator
A delay circuit for delaying a series signal according to an external signal, and an angular frequency ω _IF ± 5πB of the amplifier output signal
, A mixer for frequency mixing the output signal of the bandpass filter and the output signal of the delay circuit, and the power of the mixer output signal is measured, and an output signal corresponding to the power is generated. A power measuring device, a delay amount control circuit that outputs a signal to a delay circuit so as to maximize the output signal of the power measuring device, controls the delay amount, and outputs the delay amount at that time as a distance detection signal, and the mixer. A frequency discriminator for inputting an output signal, performing frequency discrimination, and outputting as a speed detection signal.

[Operation] In the laser radar device according to the present invention, the laser radar emission light is phase-modulated, for example, in accordance with the m-sequence signal, and the reflected light that has undergone the Doppler shift from the target is coherently detected. The detected electric signal can be synchronously detected by a signal obtained by delaying the m-sequence signal that modulates the laser radar output light, and the target distance can be detected from the delay amount that maximizes the detected output. At the same time, the target speed can be detected by discriminating the frequency of the synchronous detection signal.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and the same or corresponding parts as those of the conventional example of FIG. In the figure, 11 is the speed B (bit / se
In c), an m-sequence generator that generates a 2 ^N -1 bit (N: integer) m-sequence signal, 12 is placed on the optical path connecting the splitter 2 and the polarizer 3, and the m-sequence generator is used as an external signal. An optical modulator that modulates the output light of the laser light source 1 by the m-sequence signal input from 11 and outputs the modulated light, and 13 is a bit delay for the m-sequence signal from the m-sequence generator 11 according to an external signal. A delay circuit to give, 14 is an angular frequency ω _IF of the output signal of the amplifier 10.
Band-pass filter that passes signal components within ± 5πB, 15
Is the output signal of the bandpass filter 14 and the delay circuit 13
Mixer for frequency mixing the output signals of, 16 is this mixer
A power measuring instrument that measures the power of the output signal and generates an output signal according to the power. 17 outputs a signal to the delay circuit 13 so that the output signal of this power measuring instrument 16 becomes maximum, and the delay amount is adjusted. A delay amount control circuit for controlling and outputting a delay amount at that time as a distance detection signal, and a frequency discriminator 18 for inputting an output signal of the mixer 15 for frequency discrimination and outputting as a speed detection signal.

 Next, the operation will be described.

For example, considering a phase modulator as the optical modulator 12, the electric field strength E ₀ of the output light of the optical modulator 12 is expressed by the following equation.

E ₀ = ₀ exp (j (ω ₀ t + φ (t))) (5) where ₀ is the amplitude of the optical output field of the optical modulator 12, ω ₀ is the angular frequency of the output light of the laser light source 1, and φ (t) Is a phase, and takes a value of 0 or π for binary modulation.

When the target is irradiated with this light wave and the reflected light is received by the light receiving element 9, the electric field intensity E ₁ of the reflected light on the surface of the light receiving element 9 is given by the following equation.

E ₁ = ₁ exp {j (ω ₀ + ω _DP ) t + φ (t−τ)}
(6) where ₁ is the amplitude of the reflected light electric field on the surface of the light receiving element 9, and τ is the time at which the light represented by 2L / c (c: speed of light) travels back and forth to the target, where L is the distance to the target. Is. The power spectrum of this light is shown in FIG. In the figure, B is 2 ^N −
It is the transmission speed (bit / sec) of 1-bit m-sequence signal,
The carrier signal of ω ₀ + ω _DP is suppressed and becomes a spread spectrum signal.

On the other hand, the other light split by the distributor 2 has a carrier frequency of ω ₀ + ω _IF by the frequency shifter 6. The electric field intensity E ₂ of the output light of the half-wave plate 7 on the surface of the light receiving element 9 is given by the following equation.

E ₂ = ₂ exp {j (ω ₀ + ω _IF ) t + φ ₂ } (7) where ₂ is the amplitude of the 1/2 wavelength plate 7 output optical electric field, ω _IF
Is the frequency shift amount by the frequency shifter 6, and φ ₂ is the electric field phase. The power spectrum of this light is shown in FIG.

In the light receiving element 9, frequency mixing of the above two lights is performed, and the electrical signal I converted into light is expressed by the following equation.

I = η · (E ₁ + E ₂ ) ² = η · {| ₁ | ² + | ₂ | ² +2 | ₁ || ₂ | cos (j (ω _IF + ω _DP ) t + φ (t−τ))} ( 8) Here, η is the sensitivity of the light receiving element 9.

When this electric signal is amplified by the amplifier 10 and passed through the band-pass filter 14 that passes the signal having the central angular frequency ω _IF and the band width of 5πB or less, the spectrum of the electric signal becomes as shown in FIG. 2 (c).

On the other hand, of the output signals of the m-sequence generator 11, the signal V _D delayed by the delay circuit 13 is represented by the following equation.

V _D = V (t−τ ′) (9) Here, V (t) is the output signal of the m-sequence generator 11, and τ ′ is the delay time by the delay circuit 13. FIG. 2 (d) shows the power spectrum of this signal.

When the output signal of the band pass filter 14 and the output signal of the delay circuit 13 are mixed by the mixer 15, the average value of the output signal V _out is given by the following equation.

Here, R is the load resistance at the time of current-voltage conversion in the equation (8), and A is the gain of the amplifier 10. Mixer 15 when τ = τ ′
The power spectrum of the output signal is shown in FIG. The output signal of this mixer 15 is the frequency discriminator of the central angular frequency ω _IF 18
Ω _DP can be obtained by discriminating the frequency with
The target speed is known from the formula. Further, if the power of the output signal of the mixer 15 is measured by the power measuring device 16, and the delay amount control circuit 17 sets the delay amount of the delay circuit 13 so that this becomes maximum,
The distance L from the delay amount τ to the target is obtained by the following equation.

In practice, the delay circuit 13 gives a delay in bit units, so that if the delay bit amount is k (bit), the distance L to the target and the resolution ΔL are given by the following equations.

For example, to set the resolution to 1m, set the transmission speed Bbit / sec.
It should be 150 Mbit / sec.

That is, this laser radar device modulates light emitted to a target with an m-sequence signal, coherently detects reflected light from the target, and then synchronously detects with a signal obtained by delaying the m-sequence signal in bit units. It is a thing. Since the m-sequence signal is used as the modulation signal, the synchronous detection output becomes the correlation value of the two m-sequence signals, and the time required for the laser radar emission light to be reflected by the target and received again and the delay signal for the synchronous detection. Maximum output occurs when the delay times are matched. The target distance can be detected with high resolution as described above from the delay time of this delay signal, and the target speed can be detected from the frequency of this output. The output at this time is 2 ^N
This corresponds to the total energy of the -1 bit m-sequence pulse train, and the S / N is significantly improved as compared with the one using the energy of a single pulse.

In the above embodiment, the case where the phase modulator is used as the optical modulator 12 has been described, but an intensity modulator or a frequency modulator may be used, and the same effect as above can be obtained.

[Effects of the Invention] As described above, according to the present invention, light that is m-sequence modulated is used as the laser radar emission light, the reflected light from the target is coherently detected, and an m-sequence modulated signal that is further delayed is used. Since the synchronous detection is performed, it is possible to detect the target distance and speed from the delay amount and frequency at the same time with high resolution and to detect S / N well.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a laser radar device according to an embodiment of the present invention, FIGS. 2 (a) to 2 (e) are diagrams showing power spectra in respective portions of the embodiment, and FIG. 3 is a conventional laser radar device. It is a block diagram which shows. 1 is a laser light source, 2 is a distributor, 3 is a polarizer, 4 is a 1/4 wavelength plate, 5 is an optical system, 6 is a frequency shifter, 7 is a 1/2 wavelength plate, 8 is a multiplexer, and 9 is a light receiving device. Element, 10 is amplifier, 11 is m sequence generator, 12 is optical modulator, 13 is delay circuit, 14 is bandpass filter, 15 is mixer, 16 is power meter, 17 is delay amount control circuit, 18 is frequency Discriminator. In the drawings, the same reference numerals indicate the same or corresponding parts.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-57-168179 (JP, A) JP-A-62-249089 (JP, A) JP-A-57-86071 (JP, A)

Claims (1)

(57) [Claims] 1. A laser light source that continuously oscillates at an angular frequency ω ₀ , a distributor that divides the laser light source output light into two, a polarizer that transmits one of the distributor output light, and a polarizer output light. A 1/4 wavelength plate that converts polarized light into circularly polarized light, an optical system that outputs the 1/4 wavelength plate output light toward a target and receives reflected light from the target, and the other angle of the distributor output light A frequency shifter for converting the frequency to ω ₀ + ω _IF , a 1/2 wavelength plate for rotating the polarization direction of the output light of the frequency shifter by 90 °, and a linearly polarized light received by the optical system at the 1/4 wavelength plate A multiplexer that multiplexes the output light and the reflected light from the target whose optical path is separated by the polarizer and the half-wave plate output light, and a light receiving device that performs optoelectric conversion of the output light of the multiplexer. Element,
In a laser radar device including an amplifier for amplifying the output signal of the light receiving element, an m-sequence generator that generates an m-sequence signal of 2 ^N -1 bit (N: integer) at a speed B (bit / sec), An optical modulator that is placed on an optical path connecting a laser light source and a polarizer, modulates laser light output light from an m-sequence signal input from the m-sequence generator as an external signal, and outputs the modulated light. A delay circuit for delaying the m-sequence signal in accordance with an external signal, and the angular frequency ω _IF ± of the amplifier output signal.
A band-pass filter that passes a signal component within 5πB, a mixer that frequency-mixes the band-pass filter output signal and the delay circuit output signal, and the power of the mixer output signal is measured, and an output signal corresponding to the power is generated. And a delay amount control circuit that outputs a signal to a delay circuit so that the output signal of the power measuring device becomes maximum and controls the delay amount, and outputs the delay amount at that time as a distance detection signal, A laser radar device, comprising: a frequency discriminator that inputs a mixer output signal, performs frequency discrimination, and outputs a frequency detection signal.