CN101788671A - Multicycle modulation method applied to laser ranging device using chirp amplitude modulation based on heterodyne detection - Google Patents

Abstract

The invention discloses a multicycle modulation method applied to a laser ranging device using chirp amplitude modulation based on heterodyne detection. Continuous 4 to 10 cycles of chirp signals are used to modulate single laser pulse amplitude to realize laser ranging; and the method improves the signal frequency of the chirp difference frequency. In the invention, the laser ranging precision is improved due to the calculation method based on weighted average of spectrum power when in extraction of frequency information.

Description

Be applied to the multicycle modulator approach of heterodyne detection chirp amplitude laser ranging system

Technical field:

The present invention relates to the photoelectric instrument technology, specifically refer to a kind of multicycle modulator approach of the modulated laser distance measuring equipment that is applied to warble.

Background technology:

Laser ranging at present is many based on two kinds of systems, a kind of is to utilize the flight time of Laser Measurement pulse to realize range finding, the method is utilized the high power characteristic of pulsed laser, can realize telemeasurement, but distance accuracy is subjected to the restriction of laser pulse width, and the pumping source mission life of pulse laser is often limited; Another kind of then adopt the continuous wave laser or the long laser pulse of modulation, realize range finding by the relative variation of measuring modulation signal in the echo, these class methods can realize high-precision range observation, but because continuous laser power is limited, can't realize remote measurement.Modulation distance-measuring equipment based on direct detection often can only carry out the range observation of hundreds of rice, and need utilize cooperative target to increase reflection.In order to realize longer-distance high-acruracy survey, chirp amplitude distance measuring method based on heterodyne detection of laser is suggested, utilize the high sensitivity of heterodyne detection to improve the reach of modulation range finding, and heterodyne detection can draw the laser-Doppler frequency displacement, the realization velocity survey.

Chirp amplitude heterodyne detection range measurement system is utilized the long laser pulse of laser instrument emission, gets the local oscillator light signal of small part energy as heterodyne detection, after other parts process shift frequencies and the monocycle chirp signal amplitude modulation, is transmitted into target.The return laser beam of target reflection mixes with the local oscillator light signal, realizes mixing on detector, and is converted into the echo electric signal.

Comprise the chirp signal that is delayed in the echo electric signal, postpone to be τ, have a frequency difference f who is directly proportional with τ with the initial chirp signal _xDetect this frequency difference f _xJust can draw the delay of echo, and then get range-to-go.

The frequency f of chirp signal (t) is:

$f\left(t\right)=f_s+\frac{B}{T_C}t(-\frac{T_C}{2}\≤t\≤\frac{T_C}{2})---\left(1\right)$

f _sBe the centre frequency of chirp signal, B is the bandwidth of chirp signal, T _CLength for chirp signal.Then original chirp signal c (t):

$c\left(t\right)=\cos 2\mathrm{\π}[\∫f\left(t\right)\mathrm{dt}]=\cos [2\mathrm{\π}(f_{s}t+\frac{{\mathrm{Bt}}^2}{2T_C})+\mathrm{\φ}]---\left(2\right)$

Chirp signal c (t-τ) in the echo behind the delay τ:

$c(t-\mathrm{\τ})=\cos [2\mathrm{\π}(f_S(t-\mathrm{\τ})+\frac{{B(t-\mathrm{\τ})}^2}{2T_C})+\mathrm{\φ}]---\left(3\right)$

Behind the chirp signal and original chirp signal mixing that postpones, can obtain the frequency difference f of two chirp signals _x:

$f_x=\frac{\mathrm{B\τ}}{T_C}---\left(4\right)$

By analyzing mixing results, draw f _x, just can calculate echo delay:

τ＝f _xT _C/B (5)

Target range L then:

L＝Cf _xT _C/2B (6)

Wherein C is the light velocity.

For traditional chirp amplitude heterodyne detection distance-measuring equipment, laser pulse length T and chirp signal length T _CIdentical.Because

Longer as laser pulse length, when the chirp signal bandwidth B certain, this moment frequency difference f _xJust less.Frequency difference f then _xMeasurement just be subjected to the interference of system's low-frequency noise and laser phase noise easily, influence measuring accuracy.T is shorter as laser pulse length, and then the energy of laser pulse is low, and it is relatively poor to survey signal to noise ratio (S/N ratio), and frequency computation part time-frequency spectrum resolution is low, has reduced the resolution of measuring.Therefore there is certain contradiction in choosing of laser pulse length.

Summary of the invention:

The purpose of this invention is to provide a kind of modulation system that is used for heterodyne detection chirp amplitude laser ranging system, is to the improvement of modulation system of modulation range measurement system of warbling of traditional heterodyne detection, and has proposed the new method to the frequency difference frequence estimation of warbling.

Use 4 to 10 continuous cycle chirp signals that single laser pulse is modulated, shortened single chirp signal length, improved the difference frequency f that warbles _xFrequency, reduced laser phase noise and low frequency noises.Because laser pulse length is constant, do not influence the backward energy that receives simultaneously, and do not influence the spectral resolution when warbling the difference frequency analysis.

Distance measuring equipment uses the analog to digital converter collection difference frequency signal of warbling, and collection result is carried out fast Fourier transform (FFT), obtains the frequency spectrum of difference frequency signal.Choose 4 to 10 peak-peaks of frequency spectrum this moment, the frequency of these peak value correspondences is carried out weighted mean by the corresponding power of peak value, estimate to warble really the frequency difference frequency, can improve the precision of Frequency Estimation, promptly improve distance measuring precision.

Concrete technical scheme of the present invention is as follows:

Chirp amplitude heterodyne detection distance-measuring equipment comprises narrow linewidth laser, electrooptic modulator, acousto-optic frequency shifters, laser transmitting-receiving light path, optical-fiber laser mixed optical path, the balanced type photodetector, the chirp signal source, laser intermediate-freuqncy signal source, control circuit, radio frequency signal processing circuit (comprising amplifier and synchro detection circuit and wave filter), data acquisition system (DAS) and spectrum analysis software.

Under control circuit control, laser instrument emission Long Pulse LASER, length is T.While chirp signal source is sent a continuous N cycle (N=4～10) chirp signal and is used for pulse amplitude modulation, and each cycle chirp signal length is T _C=T/N.Target reflection laser echo signal and local oscillator light signal are finished mixing and are converted into electric signal after the light mixed optical path mixes on the balance detection device, the output signal of detector is the delay N cycle chirp signal that is modulated on the heterodyne intermediate frequency amplitude.

After amplify detector output earlier the radiofrequency signal processing section, with the mixing in the frequency mixer of warbling of initial N cycle chirp signal, obtain being modulated to N cycle on heterodyne intermediate frequency amplitude difference frequency signal of warbling, use the synchronous detection demodulation to obtain the difference frequency signal of warbling then.

The difference frequency signal frequency f of warbling that the use multicycle warbles and modulates _xFor:

$f_x=\frac{\mathrm{NB\τ}}{T}---\left(7\right)$

Target range L then:

L＝Cf _xT/2NB (8)

Wherein N is the chirp signal periodicity of modulating on the single laser pulse, and B is the chirp signal bandwidth, and T is a laser pulse length, and τ is an echo delay.

System uses analog to digital converter that demodulation result is converted into digital signal, carries out fast Fourier transform (FFT) then, obtains signal spectrum, and spectrum analysis software is analyzed it, draws the difference frequency f that warbles _xFrequency.Spectrum analysis software is at first sought the peak-peak of frequency spectrum, and its power is P ₀, corresponding signal frequency is f ₀Then around this peak value ± the 20KHz frequency range in, press the descending order of power, seek 3 to 9 other spectrum peaks again, peak value respective signal power is respectively P ₁, P ₂... P _k(k=3～9), frequency is respectively f ₁, f ₂... f _k(k=3～9).The spectrum peak quantity of utilizing is many more, and frequence estimation is accurate more, will be long more but calculate the time spent.The frequency of totally 4～10 peak values that will find is at last carried out the weighted mean by the corresponding power of peak value, estimates the difference frequency f that obtains warbling _xFrequency be:

$f_x=\frac{f_0*P_0+f_1*P_1+f_2*P_2+...+f_k*P_k}{P_0+P_1+P_2+...+P_k},(k=3~9)---\left(9\right)$

Principle of work: the length of laser instrument emission is that the laser pulse of T is sent N continuous (N=4～10) cycle chirp signal amplitude modulation by the chirp signal source, after send to target.Comprise the chirp signal of N continuous cycle from the echoed signal of target reflection through postponing.

Can obtain the N cycle difference frequency signal of warbling after N cycle chirp signal that postpones and the initial N cycle chirp signal mixing, its frequency f _xFor:

Wherein B is the chirp signal bandwidth, T _CBe chirp signal length, τ is an echo delay.Because system has used the chirp signal in N (N=4～10) cycle laser pulse to be carried out amplitude modulation, the length T of single chirp signal _C=T/N, T are laser pulse length.Therefore

Compare the modulating system of singly warbling, the frequency difference of warbling frequency has improved N doubly, has reduced the influence of system's low-frequency noise.

Owing to used the modulation of N (N=4～10) cycle chirp signal, the N cycle that the obtains difference frequency signal of warbling is similar to the warble convolution of difference frequency signal and N cycle impulse function of monocycle.The frequency spectrum of N cycle difference frequency signal equals the warble product of difference frequency signal frequency spectrum and N cycle impulse function frequency spectrum of monocycle.And the frequency spectrum of N cycle impulse function is approximately an impulse function sequence, therefore, in N cycle difference frequency signal spectrum, the monocycle phenomenon that the difference frequency signal frequency spectrum sampled by impulse function of warbling occurs.This moment as the peak-peak respective frequencies of only choosing Fourier transform results are as the difference frequency frequency, and then the frequence estimation precision is relatively poor.

By choosing warble 4 to 10 peak-peaks of difference frequency signal frequency spectrum of N cycle, carry out by the weighted mean of the peak power frequency difference frequency of estimating to warble, can from the frequency spectrum that is sampled, obtain the accurate position of spectrum peak more accurately, improve the precision of Frequency Estimation, improved distance accuracy then.

Advantage of the present invention is: convenience is implemented, but effectively raises the performance of heterodyne detection chirp amplitude range measurement system.

Description of drawings:

Fig. 1 is the structured flowchart of heterodyne detection chirp amplitude laser ranging system.

Fig. 2 is used to illustrate the range measurement principle of multicycle chirp amplitude, show among the figure for having modulated the chirp signal in 4 cycles continuously, with the fixedly difference frequency that postpones for the echo of τ be f _x

Fig. 3 illustrates and analyzes when warbling difference frequency, the phenomenon that the difference frequency spectrum information is sampled by impulse function because the multicycle that the multicycle modulation brings warbles.And illustrated when frequence estimation, the selection way of spectrum peak, 4 peak value has been selected in signal among the figure, and power is P ₀～P ₃, corresponding frequency is f ₀～f ₃

Embodiment:

Provide better embodiment of the present invention according to Fig. 1 below, so that further provide ins and outs of the present invention, enable feature of the present invention and functional characteristics are described better, but be not to be used for limiting scope of the present invention.

Multicycle chirp signal source is made of Direct Digital compositor DDS (Direct Digital Synthesizer), and output chirp signal initial frequency is 80MHz, signal frequency range be 80MHz to 280MHz, the single length of warbling is 1ms.The chirp signal frequency behind 280MHz, jumps back to 80MHz by the 80MHz linear growth at once, restarts one and warbles the cycle.Multicycle chirp signal source comprises the external trigger signal, sends the chirp signal in 4 cycles after triggering continuously, multicycle chirp signal total length 4ms.

Acousto-optic frequency shifters is the IPF-600-60-1550-2FP of U.S. BRIMOSE company, and the shift frequency frequency is 600MHz, and applicable wavelengths is 1550nm.Electrooptic modulator is the APE microwave simulation amplitude modulator of JDSU company, and the signal bandwidth that can modulate is 20GHz.

The narrow-linewidth laser wavelength that uses is 1550nm, and the individual pulse width is 4ms, and power is 1W, and live width is 5KHz, above the chirp signal in 4 cycles of modulation.The Laser emission laser beam divergence is 5 microradians, 10 centimetres of receiving telescope bores, 1 meter of focal length.

Steering logic is used for the chirp signal source, the sequential control of laser instrument and data acquisition.Under steering logic triggers, Laser emission, chirp signal source and data acquisition start.The data is used the PXI-5105 capture card of American National Instrument Instrument company, is the capture card of 8 passage 12bit, and is furnished with the external trigger input.The sampling rate of using is 10MHz, and a sampling time is 4ms.

Claims (2)

1. multicycle modulator approach that is applied to heterodyne detection chirp amplitude laser ranging system, it is characterized in that: system uses the chirp signal in 4 to 10 continuous cycles that single laser pulse is modulated, and choose 4 to 10 frequency spectrum peak-peaks of the difference frequency signal correspondence of warbling, by the frequency of these peak values is carried out calculating the frequency difference frequency of warbling by the weighted mean of each peak power.

2. a kind of multicycle modulator approach that is applied to heterodyne detection chirp amplitude laser ranging system according to claim 1 is characterized in that: described weighted average calculation by each peak power frequency difference frequency f of warbling _xFormula be:

$f_x=\frac{f_0*P_0+f_1*P_1+f_2*P_2+...+f_k*P_k}{P_0+P_1+P_2+...+P_k}---\left(1\right)$

Wherein: the k span is 3～9, f _k, P _kObtain by the following method: system uses analog to digital converter that the demodulation difference frequency signal that obtains warbling is converted into digital signal, carries out fast fourier transform then, obtains signal spectrum, and spectrum analysis software is analyzed frequency spectrum, draws the difference frequency f that warbles _xFrequency, spectrum analysis software is at first sought the peak-peak of frequency spectrum, its power is P ₀, corresponding signal frequency is f ₀Then around this peak value ± the 20KHz frequency range in, press the descending order of power, seek 3 to 9 other spectrum peaks again, peak value respective signal power is respectively P ₁, P ₂... P _k, frequency is respectively f ₁, f ₂... f _k

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