Summary of the invention
The purpose of this patent is to be the linear frequency modulation continuous wave radar in the coherent detection laser radar technique, refers in particular to the linear tuning radar that can realize super large bandwidth (THz level) signal, proposes a kind of its echo channel phase error that can make and is able to adaptive equalization; Realize the frequency spectrum compression of signal; Also be the system of pulse compression, thereby realize the high resolving power truly of corresponding bandwidth, the linear tuning pulse of adopting semiconductor laser directly to launch; Need not to carry out other modulation; Based on separating the pulse compression technique that line is transferred frequently, utilize phase error adaptive equalization program, in conjunction with the information of reference channel data the echo channel data are carried out phase error compensation; Frequency domain compression excess of export burst pulse in the echo channel data; The position of pulse is corresponding one by one with distance, realizes the range observation of high score rate, also just makes the radar of this form really to use.And this system also inherited the advantage of laser radar, and range resolution does not increase with distance and reduces.
The linear in time tuning pulse signal of wavelength after emission, echoed signal with exist between the echo local oscillation signal of fine delay one with the corresponding time-delay of distance, owing to the wavelength tuning chirp signal; Also can be similar to and regard linear FM signal as; Frequency is linear change in time, and therefore through after the relevant mixing, fixing time-delay converts fixing frequency difference into; Through detecting frequency difference; That is to say the corresponding frequency location of peak value in the frequency spectrum, thereby just can obtain the echo time-delay, and then obtain target range." frequency-time " expression formula of chirp signal is (sawtooth wave, promptly frequency is only linear in time in the monocycle descends):
(0≤t≤T)(1)
f
cBe the initial frequency of chirp signal, B is the bandwidth of chirp signal, and T is the cycle of chirp signal change of frequency, K
rBe the linear frequency modulation rate.Then original chirp signal is:
Through a segment distance, the echoed signal behind the time-delay Δ ts is:
Through the less specific echo local oscillation signal that prolongs Δ tl be:
Wherein, Time-delay is all corresponding with distance, i.e. signal lag
local oscillator time-delay
Obtain by formula (3) and (4), be balanced the detector heterodyne reception after echoed signal and the mixing of echo local oscillation signal, can obtain the difference frequency of two signals:
S
IF(t)=E
IF_S?cos[2πK
r(Δts-Δtl)(t-Δtl)+2πf
c(Δts-Δtl)-πK
r(Δts-Δtl)
2](5)
The echoed signal frequency values is:
So, theoretic range resolution:
The two-part phase constant item in back does not influence frequency in the formula (5), does not therefore influence the measurement of distance yet.
Visible by formula (5), can obtain the range information of the single frequency value of corresponding (6) in theory.But because the laser tuning pulse of adopting is that wavelength linear is tuning; Frequency
is inversely proportional to wavelength; And above-mentioned analysis is that the supposition frequency is a linear change, and this certainly exists the error of nonlinear frequency modulation; And because this through regulating the semiconductor laser that chamber length is adjusted the oscillation light wavelength, in fact regulate wavelength and will exist non-linearly, also can bring a little nonlinear frequency modulation error.These two factors have caused having comprised in the final formula (5) phase error of various orders (secondary and more than the secondary).
Because the existence of these phase errors; Make the phase place of the middle simple signal of formula (5) become non-linear; Therefore not single burst pulse just in frequency field; Can't obtain the frequency burst pulse peak value of correspondence
; But be the frequency range that non-constant width is contained at the center with the theoretical peak; So the theoretical burst pulse on the frequency spectrum just by broadening a lot of a lot, cause the non-constant of frequency resolution, the resolution characteristic of respective has also deteriorated into obsolete stage.
Therefore this patent has proposed a kind of system that can really bring into play the advantage of the big bandwidth of wavelength tuning semiconductor laser---the linear tuning laser range measurement system of submillimeter level; And rely on this system; Realized a kind of effective signal processing algorithm program; Its core concept is a differential concept, adopts the reference channel with fixed delay, nonlinear frequency modulation phase error that this passage comprises and echoed signal passage closely similar; So compensate the phase error that the echoed signal passage is comprised with it; And the delay length of reference channel do not need the echoed signal passage measured apart from priori, belong to a kind of self-adapting compensation method, also can subtractive (5) in the two-part phase constant item in back; Compensation effect is very good; Can eliminate the phase error of echoed signal passage basically, near the high resolving power of theoretical value
, this patent system can realize submillimeter level resolution at present basically in realization.The reference channel design is of formula (8):
The reference delay signal that postpones Δ tr through particular fixed is:
The reference local oscillator signal is the same with the echo local oscillator, all is the signal (less specific delays Δ tl) that separates through a 50:50 coupling mechanism:
Wherein, Time-delay is all corresponding with distance, promptly with reference to time-delay
local oscillator time-delay
Obtain by formula (8) and (9), be balanced the detector heterodyne reception behind reference delay signal and the reference local oscillator signal mixing, can obtain the difference frequency of two signals:
R
IF(t)=E
IF_R·cos[2πK
r(Δtr-Δtl)(t-Δtl)+2πf
c(Δtr-Δtl)-πK
r(Δtr-Δtl)
2](10)
With reference to the time delayed signal frequency values be:
The core concept of self-adapting compensation method is based on the difference theory, and the said signal of formula (10) is gathered, and obtains reference channel signal data (being made as B); The said signal of formula (5) is gathered; Obtain echo channel signal data (being made as A); The phase error that the two was comprising very similar (size for linear relationship) distributes, and just can utilize the phase error of reference channel data to compensate the phase error that the echo channel data are comprised.Main process is following:
1. earlier the data of echo channel signal data A and reference channel signal data B are added the Hamming window w (t) isometric with data length, obtain the echo channel data after the windowing:
A
1=A×w(t)(12)
With the reference channel data:
B
1=B×w(t)(13)
When 2. utilizing-the echo channel data A of frequency analysis after the windowing
1In extract phase place distribution in time
Reference channel data B after the windowing again
1In extract phase place distribution in time
3. with data B
1Phase place
Deduct the PHASE DISTRIBUTION of desired reference passage and obtain reference channel
Wherein: the PHASE DISTRIBUTION of desired reference passage
is:
In the formula: K
rBeing the linear frequency modulation coefficient, is a constant for particular system,
T is the time width of tuning pulse signal, and B is a signal bandwidth, B=c/ λ-c/ λ 2, λ
1Be the initial wavelength of laser tuning, λ
2Be the termination wavelength of laser tuning, c is the light velocity; R
rFor with reference to the light path in the time delay optical fiber, equal with reference to the length of time delay optical fiber and the product of optical fibre refractivity 1.5.
4. phase error is distributed
Multiply by a change of scale factor ξ, ξ is certain number between 0~1, and the phase error that obtains after the conversion distributes
Take data A
1PHASE DISTRIBUTION
The phase error that deducts after the conversion distributes
Echo channel data A after promptly being compensated
1PHASE DISTRIBUTION:
5. the echo channel data A after the compensation
1PHASE DISTRIBUTION
With original echo channel data A
1Amplitude distribution A
Abs(t), be combined into echo channel data C after the compensation:
6. the echo channel data C after the compensation is carried out Fourier transform, and calculates sharpening function S (ξ) value:
Wherein
is the Fourier transform to
, || be the delivery value;
7. change change of scale factor ξ, 4.-6. calculated the value of sharpening function again by step, circulation is found out and is made pairing ξ when sharpening function has maximal value
Max, and utilize this ξ
Max, obtain the echo after the final compensation
8. again the echo channel data D after the final compensation is carried out Fourier transform, obtain spectrogram, the spectrogram of this moment has the peak value corresponding with target range; And each peak value is very narrow; Very high frequency resolution is arranged,, calculates the distance R of target by the frequency location f at peak value place in the spectrogram:
In theory, the change of scale factor at sharpening function peak value place should equal the ξ value car following-theory value that the signal handler in
this patent calculates and coincide.
In view of the above, this patent proposes the linear tuning laser range measurement system of a kind of submillimeter level, and like Fig. 1, system's ingredient and function are following:
1. emission and local oscillator light path: by linear tunable semiconductor lasers 1,90:10 fiber coupler 2, emission collimating mirror 3, polarize beam splitter 4, λ/4 wave plate 5, receive collimating mirror 6 and form.The laser beam S of laser instrument 1 output
0Be divided into two parts through 90:10 fiber coupler 2, a part is designated as S as bias light
1, another part is designated as S as emission light
2, S
2Through collimating mirror 3 emissions, during through polarize beam splitter 4, part reflection, another most of transmission, S
2Transmissive part laser is through behind λ/4 wave plates 5, and the laser light polarization direction converts circular polarization into by linear polarization, shines on the target 16 through the free space path; Bias light S
1I7 is divided into local oscillator light S through the 50:50 fiber coupler
3With reference light S
4Two parts, local oscillator light S
3II8 is divided into echo local oscillator light S through the 50:50 fiber coupler
3-1With reference local oscillator light S
3-2
2. echo receiving light path: 16 couples of S of target
2The reflection of transmissive part laser signal, the flashlight that reflects is designated as echoed signal R
0Process, it changes linearly polarized light into after through λ/4 wave plates 5, and the polarization direction is vertical through the polarization direction before λ/4 wave plates 5 during with emission, during through polarize beam splitter 4, echoed signal R
0Most of light is reflected, and be coupled in the optical fiber through receiving collimating mirror 6 reflecting part, is designated as echo received signal
3. coherent detection and balance detection light path: comprise two passages altogether.The one,, echo received signal R
1With echo local oscillator light S
3-1Be coupled after getting into 50:50 fiber coupler III9, mixing takes place, surveyed reception by echo channel balance detection device 11, light signal converts electric signal into, is designated as echo channel signal data A; The 2nd,, reference light S
4Through one section with reference to time delay optical fiber 13 back and reference local oscillator light S
3-2Get into 50:50 fiber coupler IV10 again, be coupled, mixing takes place, surveyed reception by reference channel balance detection device 12, light signal converts electric signal into, is designated as reference channel signal data B.
4. data acquisition module 14: adopt the PXI data collecting card, two-way balance detection device output signal Synchronization is gathered.
5. the signal handler module 15: according to systematic parameter; Utilize reference channel data B that echo channel data A is carried out nonlinear frequency modulation phase error compensation adaptively; Echo data after will compensating again carries out Fourier transform, obtains frequency spectrum, calculates the distance of target according to spectrometer.
The concrete workflow of the linear tuning laser range measurement system of submillimeter level is following:
The laser beam S of laser instrument 1 output
0Be divided into two parts through 90:10 fiber coupler 2, a part is designated as S as bias light
1, another part is designated as S as emission light
2, S
2Through collimating mirror 3 emissions, during through polarize beam splitter 4, part reflection, another part transmission, S
2Transmissive part laser is through behind λ/4 wave plates 5, and the laser light polarization direction converts circular polarization into by linear polarization, is transmitted on the target 16 through the free space path; Bias light S
1I7 is divided into local oscillator light S through the 50:50 fiber coupler
3With reference light S
4Two parts, local oscillator light S
3II8 is divided into echo local oscillator light S through the 50:50 fiber coupler
3-1With reference local oscillator light S
3-2
16 couples of S of target
2The reflection of transmissive part laser signal, the flashlight that reflects is designated as echoed signal R
0Process, it changes linearly polarized light into after through λ/4 wave plates 5, and the polarization direction is vertical through the polarization direction before λ/4 wave plates 5 during with emission, during through polarize beam splitter 4, echoed signal R
0Most of light is reflected, and be coupled in the optical fiber through receiving collimating mirror 6 reflecting part, is designated as echo received signal R
1
Echo received signal R
1With echo local oscillator light S
3-1Be coupled after getting into 50:50 fiber coupler III9, mixing takes place, surveyed reception by echo channel balance detection device 11, light signal converts electric signal into, is designated as echo channel data A; Reference light S
4Through one section with reference to time delay optical fiber 13 back and reference local oscillator light S
3-2Get into 50:50 fiber coupler IV10 again, be coupled, mixing takes place, surveyed reception by reference channel balance detection device 12, light signal converts electric signal into, is designated as reference channel data B;
The data of two passages are admitted to data acquisition module 14, gather the electric signal of two balance detection device outputs simultaneously; Entering signal processing module 15, this module is handled echo channel data A and reference channel data B, finally obtains the range data of target, and concrete steps are following:
1. the data to echo channel data A and reference channel data B add the Hamming window w (t) isometric with data length, obtain the echo channel data after the windowing:
A
1=A×w(t)(21)
With the reference channel data:
B
1=B×w(t)(22)
When 2. utilizing-the echo channel data A of frequency analysis after the windowing
1In extract phase place distribution in time
Reference channel data B after the windowing again
1In extract phase place distribution in time
3. with data B
1Phase place
Deduct the PHASE DISTRIBUTION of desired reference passage and obtain reference channel data B
1Phase error distribute:
Wherein: the PHASE DISTRIBUTION of desired reference passage
is:
In the formula: K
rBeing the linear frequency modulation coefficient, is a constant for particular system,
T is the time width of tuning pulse signal, and B is a signal bandwidth, B=c/ λ
1-c/ λ
2, λ
1Be the initial wavelength of laser tuning, λ
2Be the termination wavelength of laser tuning, c is the light velocity; R
rFor with reference to the light path in the time delay optical fiber, equal with reference to the length of time delay optical fiber and the product of optical fibre refractivity 1.5.
4. phase error is distributed
Multiply by a change of scale factor ξ, ξ is certain number between 0~1, and the phase error that obtains after the conversion distributes
Take data A
1PHASE DISTRIBUTION
The phase error that deducts after the conversion distributes
Echo channel data A after promptly being compensated
1PHASE DISTRIBUTION:
5. the echo channel data A after the compensation
1PHASE DISTRIBUTION
With original echo channel data A
1Amplitude distribution A
Abs(t), be combined into echo channel data C after the compensation:
6. the echo channel data C after the compensation is carried out Fourier transform, and calculates sharpening function S (ξ) value:
Wherein
is the Fourier transform to
, || be the delivery value;
7. change change of scale factor ξ, 4.-6. calculated the value of sharpening function again by step, circulation is found out and is made pairing ξ when sharpening function has maximal value
Max, and utilize this ξ
Max, obtain the echo channel data after the final compensation, be designated as the echo channel data D after the final compensation:
8. again the echo channel data D after the final compensation is carried out Fourier transform, obtain spectrogram, the spectrogram of this moment has the peak value corresponding with target range, by the frequency location f at peak value place in the spectrogram, calculates the distance of target:
The advantage of native system is:
1) system stability is good, and most of module of system all is optical fibre devices, and is better than Free Space Optics device anti-interference.
2) detection sensitivity is high, has adopted coherent detection and balance detection mode that light signal is carried out opto-electronic conversion, with respect to direct detection, has suppressed relative intensity noise, has bigger conversion gain, has improved detection sensitivity, low energy detection 10 in this experiment
-10The echo power of w.
3) the signal processing module robustness is good, and range resolution is high, though have the big bandwidth of THz to be subject to big nonlinear frequency modulation error can not well realize pulse compression with conventional method drawback even if fundamentally solved linear tuned laser; In native system, realized the resolution of submillimeter level.
4) signal processing module has adaptivity, and the range information of undesired signal passage priori can carry out the Adaptive matching compensation according to specific reference channel, has bigger practical value.
5) distance accuracy is high, repeatedly measure, to survey the distance value variance very little.
Embodiment:
The linear tuning laser range measurement system of submillimeter level is divided into following a few step to the distance measurement process of target:
1) setting laser device frequency tuning range and tuning speed are opened laser instrument 1, and laser instrument adopts NewFocus company semiconductor laser, wavelength 1550~1630nm; Peak power output 50mW1590nm, live width 30kHz (when 120 μ s postpone) sets initial wavelength 1550nm, stops wavelength 1560nm; Tuning speed 20nm/s triggers laser scans, and wide during laser tuning pulse (approximate chirped pulse) is 0.5s; Wavelength change is following, and preceding 0.5s is that linear wavelength rises, and ensuing 0.5s is 1560nm; Thereafter the linear wavelength of 0.5s descends, and thereafter 0.5s is that 1550nm is linear decline, circulation in this way again.Laser instrument is output as the fixing laser in polarization direction.The laser beam S of laser instrument 1 output
0Be divided into two parts through 90:10 fiber coupler 2, fraction is designated as S as bias light
1, another major part is designated as S as emission light
2, S
2Through collimating mirror 3 emissions, during through polarize beam splitter 4, part reflection, another part transmission, S
2Transmissive part laser is through behind λ/4 wave plates 5; The laser light polarization direction converts circular polarization into by linear polarization; Be transmitted on the target 16 through the free space path, put emission collimating mirror 3, polarize beam splitter 4, λ/4 wave plate 5, receive collimating mirror 6 and fixing by Fig. 2 relative position;
2) target 16 stationkeeping;
3) the adjusting light path gets in the reception collimating mirror 6 light of target reflection; To transmit and receive all to adjust and aim at the mark; All very straight on the level of assurance light path and the vertical direction, last available horsepower meter measurement target behind collimating mirror reflects, again the luminous power of coupled into optical fibres; When luminous power is maximum, can think that light path reaches optimum;
4) after receiving collimating mirror 6, insert the fibre delay line of one section 500m, be used for the simulated target distance;
5) by Fig. 1 connect 50:50 fiber coupler I7,50:50 fiber coupler II8,50:50 fiber coupler III9,50:50 fiber coupler IV10, echo channel balance detection device 11, reference channel balance detection device 12, with reference to time delay optical fiber 13, connect the detector power supply;
6) open computing machine, detector output is inserted the PXI data collecting card input interface on the industrial computer, carry out the data acquisition of two passages;
7) utilize the data after 15 pairs of collections of signal handler module to handle computed range.
At chirp bandwidth B is 1.240694789THz, and wide when warbling is 0.5s, apart from 1.86m, is 120 microns according to formula (7) theory of computation range resolution.Certain experimental result picture that native system is implemented to differentiate to 3 distance objectives is shown in Fig. 2~6; As a comparison, certain experimental result picture of 2 distance objectives being implemented to differentiate is shown in Fig. 7~8.
Fig. 2 is the frequency spectrum of echo channel data A, the distance of the corresponding 1.5m of peak width, and resolution is very poor, almost can not be applied to distance and differentiate.Fig. 3 is the frequency spectrum of reference channel data B; Fig. 4 is exactly according to the resulting scale factor functional arrangement of formula (12), and the corresponding horizontal ordinate 0.801 of peak value is exactly best scale factor here; Fig. 5 is according to the best scale factor 0.801; With the frequency spectrum of the echoed signal passage behind the phase error compensation that reference channel comprised, and horizontal ordinate (distance) converts according to formula (6) among Fig. 2 and Fig. 5, and horizontal ordinate among Fig. 3 (distance) converts according to formula (11).Fig. 6 is the figure after Fig. 5 is amplified along horizontal ordinate; By finding out that the horizontal ordinate position of three peak values is respectively 1.596m, 1.598m among the figure; 1.6m locate; Minimum amplitude also exceeds more than the background 10dB, and obviously find out the width of each peak value corresponding apart from width all less than 1mm, about 0.2mm.Explain that resolution characteristic has reached the submillimeter magnitude, be in close proximity to the theoretical resolution that calculates according to formula (7).Therefore with the method for this patent, can be so that tuning super large bandwidth (THz) system of wavelength linear be able to be applied to laser ranging.
Fig. 7 as a comparison and Fig. 8 are the design sketchs that two targets are differentiated, and for easy, only show signalling channel figure and enlarged drawing thereof after the compensation here, as can be seen from Figure 8 1.596m, and two peak values at 1.598m place, spike width is also less than 1mm (submillimeter).