CN102654575B - Terahertz (THz) level large bandwidth laser synthetic aperture radar imaging system - Google Patents

Abstract

The invention discloses a terahertz (THz) level large bandwidth laser synthetic aperture radar imaging system. The system comprises a linear tuning semiconductor laser, a 99:1 optical fiber coupler, a 90:10 optical fiber coupler, a transmission collimation lens, a polarized polarization beam splitter, a lambda/4 wave plate, a target displacement platform, a receiving collimation lens, four 50:50 optical fiber couplers, two balanced detectors, a reference delaying optical fiber, a molecular wavelength reference device, an optical power meter, a data acquisition module and a data processing program module. The THz level large bandwidth laser synthetic aperture radar imaging system disclosed by the invention is based on a wide range of linear tuning laser pulse signals, adopts a homodyne coherent detection technology, a balance detection technology and a synthetic aperture technology, and uses a special data processing program to calculate a two-dimensional image of a target; the THz level large bandwidth laser synthetic aperture radar imaging system has the advantages that the laser signal wavelength is short, the bandwidth is large, the imaging resolution is high, and the characteristic of all-weather application is realized; in addition, most part of the system is based on optical fiber components, so that the stability is high, the structure is simple, the detection sensitivity is high, and the operating distance is long.

Description

The large bandwidth laser synthetic aperture radar image-forming of THz level system

Technical field

This patent relates to the laser infrared radar imaging technology, specifically refers to the large bandwidth laser imaging system of a kind of level of the THz for imaging and data processing method thereof.

Background technology

Synthetic-aperture radar (Synthetic Aperture Radar is called for short SAR) is a kind of active imaging radar, is operated in microwave region, but round-the-clock, round-the-clock, and SAR has obtained ripe application at present.In the SAR development, laser instrument also develops rapidly, and the spatial resolution of real aperture laser radar is subject to the restriction of the diffraction limit of transmitting aperture.Thereby microwave section synthetic aperture technique is transplanted to optical region, to obtain more high-resolution image, laser synthetic aperture radar (Synthetic Aperture Ladar is called for short SAL) just arises at the historic moment.The same SAR of principle, be the linear FM signal of wide-bandwidth product when distance is upwards large by emission, to target echo carry out pulse compression obtain distance to high resolving power, range resolution ρ in theory _r=c/2B(c is the light velocity, and B is the linear FM signal bandwidth); Orientation is upwards based on the synthetic aperture principle, utilize the relative motion between platform and target, the simulation array of apertures, synthesize a large aperture, a synthetic aperture, in the time, collection is recorded platform diverse location place and is carried the target echo signal of phase information, and the method for processing by data realizes that orientation is to focusing, obtain orientation to high resolving power, azimuth resolution ρ in theory _a=D _T/ 2(D _TFor the system length of real aperture).

According to the radar detection system, just comprising/oblique side-looking strips S AL of laser synthetic aperture radar, scanning SAL, bunching type SAL etc., and scan-type and the bunching type direction of visual lines of wave beam in imaging process needs to adjust, implement difficulty larger, so native system adopts strip-type.Positive side-looking band pattern laser synthetic aperture laser radar refers to that radar makes uniform motion along straight line, and in motion, the line of sight (LOS) of emission light beam is vertical with radar track, to the laser synthetic aperture radar of the banded zone imaging that is parallel to radar direction of motion.

Signal theory points out, the transmitting of wide and bandwidth when large, must be used complicated modulation waveform.Commonly used have three kinds of forms: linear frequency modulation, nonlinear frequency modulation and phase encoding modulation.Because linear FM signal (LFM signal, i.e. chirp signal) is easy to produce, be convenient to processing, insensitive to Doppler shift, so it is most widely used.Microwave radar has had successful application, in like manner, uses for reference microwave radar, and some New Systems have also appearred in laser radar, comprise linear frequency modulation continuous wave (LFMCW) laser radar, pseudo-random code phase modulation laser radar and chirp signal amplitude modulation laser radar etc.The methods such as the generation acousto-optic modulation of LFM signal, electrooptical modulation, wavelength linear tuning (hereinafter to be referred as Linear Tuning).Acousto-optic and electrooptical modulation are due to system and device cause, and its modulation band-width generally, in hundreds of MHz effect, is difficult to realize the modulation of larger bandwidth; And the wavelength linear regulative mode that adopts the Linear Tuning semiconductor laser can be realized the modulation of tens nm easily, corresponding bandwidth is the THz magnitude, and it is even less that theoretical resolution can reach submillimeter level.Therefore the first-selected Linear Tuning semiconductor laser of SAL system is as transmiting signal source.Because this laser instrument is to realize the frequency modulation rate by adjusting wavelength, the linear change of wavelength, frequency

(c is the light velocity), frequency is inevitable is not linear change, therefore there is larger nonlinear frequency modulation error in the Laser emission pulse, only copying the system of traditional microwave SAR and adopting the resolution that conventional impulse compression method is obtained is meter level, differ greatly with theoretical value, can not realize the high-resolution imaging under this system.Therefore must utilize specific system, in conjunction with effective data processing method, just can make the SAL of this system realize high-resolution imaging.

No matter system adopts the carrier wave of which kind of form, no matter adopt which kind of modulation system, the signal that obtains large bandwidth is final target, the bandwidth maximum that adopts the Linear Tuning semiconductor laser to obtain, tuning range can reach tens ~ to hundreds of nm, this patent has only selected (corresponding bandwidth is 1.24THz) between 1550 ~ 1560nm to carry out Linear Tuning, and purpose is the sampling rate pressure of reduction system.This type of tunable laser of the existing Multiple Type of existing market, tuned speed has 20nm/s, 100nm/s, 2000nm/s does not wait, and adopts the tuned speed of 20nm/s in the system of this patent.Therefore in the situation that increase sampling rate to meet the image-forming range requirement, the bandwidth of this system also can continue to increase, and respective distances resolution can continue to increase.This patent has been to build special imaging system, rely on specific data processing method, solved the nonlinear frequency modulation phase error existed in the destination channel of this Linear Tuning mode, completed distance to orientation to two-dimension focusing, make the large bandwidth laser pulse of this THz level to be achieved the high-resolution laser synthetic aperture radar image-forming.

Summary of the invention

The objective of the invention is for the laser synthetic aperture radar in coherent detection laser imaging radar technology, refer in particular to the laser synthetic aperture radar with the pulse of the large bandwidth Emission Lasers of THz level, on system, compare common coherent detection passage, except destination channel, increased again two extra passages, the one, intrinsic tool is determined the reference channel of time delay, the nonlinear phase error be used in the Compensation Objectives passage; Another one is synchronizing channel, is used for the reference position of each pulse in the Accurate align imaging process.On data processing method, utilize synchronizing channel to carry out synchronously the data of each pulse, proposed a kind ofly can compensate adaptively the nonlinear frequency modulation phase error comprised in destination channel, realize that the distance of each pulse is to focusing, then pass through again matched filtering azimuth focus and Gradient Phase Autofocus processing, finally obtain system and the data processing algorithm of high-resolution two dimensional image, thereby realize the high-resolution laser synthetic aperture radar image-forming of corresponding bandwidth.The Linear Tuning pulse that adopts the Linear Tuning semiconductor laser directly launch, without the external modulation of carrying out other, and this system also inherited the advantage of SAR, and range resolution and azimuthal resolution are with reducing apart from increase.

Wavelength in time Linear Tuning laser pulse signal through the emission after, between the target local oscillation signal of echo signal and same fine delay, exist one with time delay apart from corresponding, because can being similar to, this signal regards linear FM signal as, frequency is linear change in time, therefore by after relevant mixing, fixing time delay is converted to fixing frequency difference, by detecting frequency difference, that is to say frequency location corresponding to peak value in frequency spectrum, just can realize that the distance of individual pulse is to focusing, this is the basic premise of synthetic aperture imaging." frequency-time " expression formula of chirp signal is (sawtooth wave, i.e. only linear decline in time of frequency within the monocycle):

$f\left(t\right)=f_c-\frac{B}{T}t=f_c-K_r\·t,(0\≤t\≤T)---\left(1\right)$

F _cFor the initial frequency of chirp signal, the bandwidth that B is chirp signal, the cycle that T is the chirp signal frequency change, K _rFor the linear frequency modulation rate.Original chirp signal is:

$p\left(t\right)=E_s\mathrm{rect}\left(\frac{t}{\mathrm{\&tau;}}\right)\exp \{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}[\&Integral;f\left(t\right)\mathrm{dt}\left]\right\}=E_s\mathrm{rect}\left(\frac{t}{\mathrm{\&tau;}}\right)\exp [-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}(f_{c}t-{\mathrm{<figure-callout\; id="2"\; label="K\; r\; t"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">K</figure-callout>}}_r\frac{t^{}}{}\frac{{}^2}{2})]---\left(2\right)$

Through a segment distance, the echo signal after time delay Δ ts is:

$s\left(t\right)=E_s\mathrm{rect}\left(\frac{t-\mathrm{\&Delta;ts}}{\mathrm{\&tau;}}\right)\exp \{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}[f_c(t-\mathrm{\&Delta;ts})-K_r\frac{{(t-\mathrm{\&Delta;ts})}^2}{2}\left]\right\}---\left(3\right)$

Target local oscillation signal through less specific delays Δ tl is:

$L\left(t\right)=E_s\mathrm{rect}\left(\frac{t-\mathrm{\&Delta;tl}}{\mathrm{\&tau;}}\right)\exp \{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}[f_c(t-\mathrm{\&Delta;tl})-K_r\frac{{(t-\mathrm{\&Delta;tl})}^2}{2}\left]\right\}---\left(4\right)$

Wherein, time delay is all corresponding with distance, i.e. target time delay

The local oscillator time delay

Obtained by formula (3) and (4), after echo signal and the mixing of target local oscillation signal, be balanced the detector heterodyne reception, 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 echo signal frequency values is:

$f_s=K_r(\mathrm{\&Delta;ts}-\mathrm{\&Delta;tl})=\frac{{2K}_r}{c}(R_s-R_l)---\left(6\right)$

Therefore, theoretic range resolution:

${\mathrm{\&rho;}}_r=\mathrm{\&delta;}\left(\mathrm{\&Delta;R}\right)=\frac{c}{2K_r}\&CenterDot;\mathrm{\&delta;}\left(f_s\right)=\frac{c}{{2K}_r}\&CenterDot;\frac{1}{T}=\frac{c}{2B}---\left(7\right)$

In formula (5) second of the phase bit position, the 3rd be the phase constant item, does not affect frequency, so does not also affect the measurement of distance.

From formula (5), can obtain in theory the range information of the single frequency value of corresponding (6).But because the laser tuning pulse adopted is that wavelength linear is tuning, frequency

With wavelength, be inversely proportional to, and above-mentioned analysis is that the supposition frequency is linear change, this certainly exists the error of nonlinear frequency modulation; And, due to this semiconductor laser of adjusting the oscillation light wavelength by regulating chamber length, in fact adjusting wavelength will exist non-linearly, also can bring a little nonlinear frequency modulation error.These two factors have caused in final formula (5) having comprised the phase error of various orders (secondary and more than secondary).

Due to the existence of these phase errors, make the phase place of simple signal in formula (5) become non-linear, in frequency field, not just therefore single burst pulse, can't obtain correspondence

Frequency burst pulse peak value, but contain the frequency range of non-constant width centered by theoretical peak, the theoretical burst pulse on frequency spectrum, just by serious broadening, causes the frequency resolution variation like this, corresponding distance is to not completing high-resolution focusing.This is also the place different to processing from the distance of SAR.

Therefore the present invention proposes a kind of system that can really bring into play the advantage of the large bandwidth of Linear Tuning Laser Diode Pulse--the large bandwidth laser synthetic aperture of THz level imaging system, and rely on this system, realized a kind of effective data processing method program, its core concept is to adopt the reference channel with fixed delay, the nonlinear frequency modulation phase error that this passage comprises and destination channel closely similar, therefore the phase error of coming the Compensation Objectives passage to comprise with it, and the delay length of reference channel do not need destination channel measured apart from priori, belong to a kind of self-adapting compensation method, compensation effect is good, can basically eliminate the phase error of destination channel, realize substantially approaching theoretical value

high resolving power, at present system of the present invention can realize distance to submillimeter level differentiate.Reference channel design is as described in formula (8):

Reference delay signal through specific fixed delay Δ tr is:

$R\left(t\right)=E_r\mathrm{rect}\left(\frac{t-\mathrm{\&Delta;tr}}{\mathrm{\&tau;}}\right)\exp \{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}[f_c(t-\mathrm{\&Delta;tr})-K_r\frac{{(t-\mathrm{\&Delta;tr})}^2}{2}\left]\right\}---\left(8\right)$

The reference local oscillator signal is the same with the target local oscillator, is all the signal (less specific delays Δ tl) separated through a 50:50 coupling mechanism:

$L\left(t\right)=E_s\mathrm{rect}\left(\frac{t-\mathrm{\&Delta;tl}}{\mathrm{\&tau;}}\right)\exp \{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}[f_c(t-\mathrm{\&Delta;tl})-K_r\frac{{(t-\mathrm{\&Delta;tl})}^2}{2}\left]\right\}---\left(9\right)$

Wherein, time delay is all corresponding with distance, with reference to time delay The local oscillator time delay

Obtained by formula (8) and (9), after reference delay signal and the mixing of reference local oscillator signal, be balanced the detector heterodyne reception, 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:

$f_r=K_r(\mathrm{\&Delta;tr}-\mathrm{\&Delta;tl})=\frac{{2K}_r}{c}(R_r-R_l)---\left(11\right)$

The distance to nonlinear frequency modulation phase error compensation method be, the described signal of formula (10) is gathered, obtain reference channel data (being made as B); The described signal of formula (5) is gathered, obtain destination channel data (being made as A), the two phase error that is comprising very similar (size is linear relationship) distributes, and just can utilize the phase error of reference channel data to compensate the phase error that the destination channel data comprise.Process is as shown in step 2.

It is as follows that data are processed main process:

Step 1, the triple channel data are synchronous: laser pulse of every emission, three passages can be gathered once simultaneously, and acquisition rate is consistent with length, is designated as respectively destination channel data A, reference channel data B and synchronizing channel data C.Find out the position of first absorption peak from synchronizing channel data C, according to this position, intercept out isometric destination channel data A from destination channel data A, reference channel data B respectively ₀, reference channel data B ₀, consistent with the initial phase between pulse to guarantee pulse.

Step 2, distance is to compression: each pulse data is carried out to distance to the compression processing, is mainly nonlinear frequency modulation phase error compensation method, and the phase error compensation that utilizes the reference channel packet to contain falls the phase error in the destination channel data, obtain Range Focusing image I, operate as follows:

1. to destination channel data A ₀With reference channel data B ₀Data added with data length isometric

A ₁＝A×w(t) （12）

With reference channel data B ₁:

B ₁＝B×w(t) （13）

While 2. utilizing-the destination channel data A of frequency analysis from windowing ₁In extract phase place distribution in time

Reference channel data B from windowing again ₁In extract phase place distribution in time

3. by 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

For:

In formula: K _rBeing the linear frequency modulation coefficient, is a constant for particular system,

The time width that T is tuning pulse signal, B is signal bandwidth, B=c/ λ ₁-c/ λ ₂, λ ₁For the initial wavelength of laser tuning, λ ₂For the termination wavelength of laser tuning, c is the light velocity; R _rFor the light path in the reference time delay optical fiber, equal with reference to the length of time delay optical fiber and the product of optical fibre refractivity 1.44;

4. phase error is distributed

Be multiplied by a change of scale factor ξ, ξ is certain number between 0 ~ 1, and the phase error after being converted distributes

Take data A ₁PHASE DISTRIBUTION

The phase error deducted after conversion distributes

The destination channel data A after being compensated ₁PHASE DISTRIBUTION:

5. the destination channel data A after compensation ₁PHASE DISTRIBUTION

With original destination channel data A ₁Amplitude distribution A _abs(t), be combined into the destination channel data C after compensation:

6. the destination channel data C after compensation is carried out to Fourier transform, and calculates sharpening function S (ξ) value:

Wherein

For right

Fourier transform, || be the delivery value;

7. change change of scale factor ξ, again by step, 4.-6. calculate the value of sharpening function, corresponding ξ while making sharpening function that maximal value be arranged is found out in circulation _max, and utilize this ξ _max, obtain the destination channel data after final compensation, be designated as the destination channel data D after final compensation:

8. again the destination channel data D after final compensation is carried out to Fourier transform, obtain frequency spectrum data, this frequency spectrum data has and target range R _TCorresponding peak value, target range R _TWith the relation of frequency spectrum medium frequency coordinate f, as formula (9), show:

$R_T=\frac{f\&CenterDot;c}{2K_r}---\left(20\right)$

Complete the distance of individual pulse data to compression.Data after processing are lined up according to precedence, and individual pulse data occupy row of array, form the two-dimensional array data, obtain Range Focusing image I.

In theory, the change of scale factor at sharpening function peak value place should equal

According to systematic parameter, can only calculate the coarse value of ξ, carry out phase error compensation though can not directly apply it, can be used as the comparison of program calculated result value, with the determining program result, whether serious deviation appears.The ξ value car following-theory value that data processor in the present invention calculates is coincide on the order of magnitude.

Step 3, orientation is to compression: with the matched filtering algorithm, Range Focusing image I (complex image) is processed to (being the line direction of array) along orientation, all " OK " all is disposed and completes orientation to compression, obtain the azimuth focus image II, operate as follows:

1. along the equal corresponding same distance of the data of line direction, be designated as range unit data E, E carried out to Fourier transform, obtain data F:

F＝F{E} （21）

2. according to the Doppler FM slope

V is the target translational speed, sets reference function to be:

h(t)＝exp(jπ·K _d·t _a ²) （22）

T wherein _aFor orientation to the time.Again to reference function h (t) a synthetic aperture time T _SaInside carry out Fourier transform, obtain:

H _a＝F{h(t)} （23）

The synthetic aperture time is R wherein ₀For transmitting aperture range-to-go, D _TTransmitting aperture size (diameter) for system.

3. by data F and H _aConjugate multiplication, then carry out Fourier inversion, obtain orientation to the data G compressed, wherein conj is for getting conjugation:

G＝IFFT{F·conj(H _a)} （24）

4. all capable data are carried out to orientation after the compression processing, obtain the azimuth focus image II.

Step 4, the Gradient Phase self-focusing: with the Gradient Phase autofocus algorithm, orientation focusedimage II (complex image) is processed, based on data compensation orientation to the high-order phase error, further sharpening image, iteration obtains the final image III for several times, operates as follows:

1. reconnaissance: on the azimuth focus image II, along line direction, travel through each range unit u _n, select to meet the normalization amplitude variance that Steinberg proposes

Range unit as the aobvious point of a spy range unit u _Spec,

As formula (25) shows:

${\mathrm{\&sigma;}}_{\mathrm{<figure-callout\; id="2"\; label="un"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">un</figure-callout>}}^2=\stackrel{\&OverBar;}{{(u_n-\stackrel{\&OverBar;}{u_n})}^2}/\stackrel{\&OverBar;}{{\mathrm{<figure-callout\; id="2"\; label="u\; n"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">u</figure-callout>}}_n^{}}=1-{\stackrel{\&OverBar;}{u_n}}^2/\stackrel{\&OverBar;}{{\mathrm{<figure-callout\; id="2"\; label="u\; n"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">u</figure-callout>}}_n^{}}---\left(25\right)$

Wherein

The average of n range unit target echo sequence amplitude,

It is its mean square value.Choose maximal value as the aobvious point of spy from each the special aobvious some range unit found.

2. center displacement: adopt the circular shifting method, the aobvious point of spy (be in each special aobvious some range unit strong scattering point) is moved to centre position, obtain u _CsAs formula (26), to eliminate because of the different Doppler shifts of introducing in scattering point position.

u _cs＝circshift{u _spec} （26）

3. windowing: the suitable window function according to the picture contrast Feature Selection, as Hamming window, by the Data in Azimuth Direction windowing, remove the strong scattering point impact of scattering point on phase error estimation and phase error on every side, only retain the fuzzy region that scattering point causes because of phase error.If window function length is excessive, in window, the data signal to noise ratio (S/N ratio) reduces, and not only causes iterations to increase, and affects the phase estimation precision; If window function length is too small, will cause the phase error estimation and phase error distortion, even cause and estimate to disperse.In addition, windowing has vital role to the robustness that improves the PGA algorithm.In operating process, the length l en of window function will stipulate in advance, and changes along with iteration! As formula (27):

u _w＝hamming(len)·u _cs （27）

4. FFT: to the data after windowing along orientation to carrying out Fourier transform, by data transformation to the distance-Doppler territory, as formula (28):

I _w(n,X)＝F{u _w}＝F{hamming(len)·u _cs} （28）

Wherein, I _w(n, X) be orientation after the image windowing to Fourier transform, the sequence number that n is the distance row.

5. phase gradient is estimated: according to optimal estimation criterion (as linear unbiased minimum variance estimation, maximal possibility estimation etc.), utilize phase error adjust the distance to redundancy properties phase gradient is estimated.Here adopt maximal possibility estimation, along orientation to, estimate phase gradient As follows:

$\mathrm{\&Delta;}\widehat{\mathrm{\&Phi;}}\left(X\right)=\arg \{\underset{n=1}{\overset{M}{\mathrm{\&Sigma;}}}{I}_w^{*}(n,X-1)\&CenterDot;I_w(n,X)\}---\left(29\right)$

Wherein, arg is the function of asking phasing degree, the number that M is special aobvious some range unit, and X is that orientation is to sequence number.

6. ask phase error: to estimated phase gradient, along orientation to X integration summation, show that phase error distributes, as formula (30):

$\widehat{\mathrm{\&Phi;}}\left(X\right)=\underset{l=1}{\overset{X}{\mathrm{\&Sigma;}}}\mathrm{\&Delta;}\widehat{\mathrm{\&Phi;}}\left(l\right)---\left(30\right)$

7. phase compensation: by the data u of original azimuth focus image II _nCarry out Fourier transform, obtain F{u _n, then the phase error distributed compensation of trying to achieve to F{u _nUpper, along orientation to the X conjugate multiplication, and then convert back image area with IFFT, obtain the image I II that this focuses on _i.

${\mathrm{III}}_i=\mathrm{IFFT}\left\{F\right\{u_n\}\&CenterDot;\exp [-j\widehat{\mathrm{\&Phi;}}\left(X\right)\left]\right\}---\left(31\right)$

Wherein, subscript n refers to range units all in image II is all carried out to this operation.

8. will dwindle the length l en of window, the image I II tried to achieve with this _i(complex image) replaces image II, repeats 1.-7., obtain image I II _I+1(complex image), proceed iteration, makes image further focus on, until meet the number of times requirement, obtains the final image III.

Accordingly, this patent proposes the large bandwidth laser synthetic aperture radar image-forming of a kind of THz level system, and as Fig. 1, system ingredient and function are as follows:

1. launch and the local oscillator light path: by Linear Tuning semiconductor laser 1,99:1 fiber coupler 2,90:10 fiber coupler 3, emission collimating mirror 4, polarize beam splitter 5, λ/4 wave plates 6, reception collimating mirror 7, formed.The laser beam S of laser instrument 1 output ₀Be divided into two parts through 99:1 fiber coupler 2, a part is designated as S ₁, another part is designated as S ₂, S ₂Be further divided into two parts through 90:10 fiber coupler 3, a part is designated as S as utilizing emitted light ₃, another part is designated as S as bias light ₄, S ₃Through 4 emissions of emission collimating mirror, during through polarize beam splitter 5, part reflection, another most of transmission, S ₃Transmissive portion laser is by after λ/4 wave plates 6, and the laser light polarization direction is converted to circular polarization by linear polarization, through the free space path, shines on target 17; Bias light S ₄Through the 50:50 fiber coupler, I8 is divided into local oscillator light S ₅With reference light S ₆Two parts, local oscillator light

2. target echo receiving light path: 17 couples of S of displacement of targets platform ₃The reflection of transmissive portion laser signal, the flashlight reflected, be designated as target echo signal R ₀, it changes linearly polarized light into after by λ/4 wave plates 6, and polarization direction is vertical through the polarization direction before λ/4 wave plates 6 during with emission, during through polarize beam splitter 5, target echo signal R ₀Most of light is reflected, and be coupled in optical fiber through receiving collimating mirror 7 reflecting part, is designated as target echo and receives signal R ₁

3. coherent detection and balance detection light path: comprise altogether two passages.The one,, target echo receives signal R ₁With target local oscillator light S _5-1Be coupled after entering 50:50 fiber coupler III10, mixing occurs, by destination channel balance detection device 11, surveyed and receive, light signal is converted to electric signal, is designated as destination channel data A; The 2nd,, reference light S ₆Through one section with reference to after time delay optical fiber 14 with reference local oscillator light S _5-2Enter 50:50 fiber coupler IV12 again, be coupled, mixing occurs, referenced channel balance detector 13 is surveyed and is received, and light signal is converted to electric signal, is designated as reference channel data B;

4. synchronizing signal light path: it is S that the signal separated by fiber coupler 2 absorbs postscripts through molecule wavelength reference device 15 _1-1, S _1-1By light power meter 16, received, light signal is converted to electric signal, is designated as synchronizing channel data C;

5. displacement of targets platform: target is placed on the miter angle inclined-plane of displacement of targets platform 17, the movement of the moving belt moving-target of displacement platform;

6. data acquisition module 18: adopt the PXI data collecting card, two-way balance detection device output signal and light power meter output signal are synchronously gathered;

7. the data processor module 19: according to systematic parameter, utilize synchronizing channel data C to carry out synchronously each pulse, at each burst length internal reference channel data B, destination channel data A is carried out to nonlinear frequency modulation phase error compensation adaptively, again the target echo data after compensation are carried out to pulse compression, obtain Range Profile, data after pulse compression by autofocus algorithm carry out orientation to focusing, obtain two-dimentional laser synthetic aperture image.

The large bandwidth laser synthetic aperture radar image-forming of THz level system specific works flow process is as follows:

The laser beam S of laser instrument 1 output ₀Be divided into two parts through 99:1 fiber coupler 2, a part is designated as S ₁, absorbing postscript through molecule wavelength reference device 15 is S _1-1A part is designated as S in addition ₂, being further divided into two parts through 90:10 fiber coupler 3, a part is designated as S as utilizing emitted light ₃, another part is designated as S as bias light ₄, S ₃Through collimating mirror 4 emissions, during through polarize beam splitter 5, part reflection, another part transmission, S ₃Transmissive portion laser is by after λ/4 wave plates 6, and laser polarization direction is converted to circular polarization by linear polarization, through the free space path, is transmitted on displacement of targets platform 17; Bias light S ₄Through the 50:50 fiber coupler, I8 is divided into local oscillator light S ₅With reference light S ₆Two parts, local oscillator light S ₅Through the 50:50 fiber coupler, II9 is divided into target local oscillator light S _5-1With reference local oscillator light S _5-2.

Target is to S ₃The reflection of transmissive portion laser signal, the flashlight reflected, be designated as target echo signal R ₀Change linearly polarized light into after λ/4 wave plates 6, polarization direction and when emission the polarization direction before through λ/4 wave plates 6 vertical, during through polarize beam splitter 5, target echo signal R ₀Most of light is reflected, and be coupled in optical fiber through receiving collimating mirror 7 reflecting part, is designated as target echo and receives signal R ₁

Target echo receives signal R ₁With target local oscillator light S _5-1Be coupled after entering 50:50 fiber coupler III10, mixing occurs, by destination channel balance detection device 11, surveyed and receive, light signal is converted to electric signal, is designated as destination channel data A; Reference light S ₆Through one section with reference to after time delay optical fiber 14 with reference local oscillator light S _5-2Enter 50:50 fiber coupler IV12 again, be coupled, mixing occurs, referenced channel balance detector 12 is surveyed and is received, and light signal is converted to electric signal, is designated as reference channel data B; Signal is through light power meter 16, and light signal is converted to electric signal, is designated as synchronizing channel data C.

The data of three passages are admitted to data acquisition module 18, gather together the electric signal of two balance detection devices and light power meter output; Enter data processing module 19, this module is processed destination channel data A, reference channel data B and synchronizing channel data C, finally obtains the two dimensional image of target, and concrete steps are as follows:

Step 1, the triple channel data are synchronous: laser pulse of every emission, three passages can be gathered once simultaneously, and acquisition rate is consistent with length, is designated as respectively destination channel data A, reference channel data B and synchronizing channel data C.Find out the position of first absorption peak from synchronizing channel data C, according to this position, intercept out isometric destination channel data A from destination channel data A, reference channel data B respectively ₀, reference channel data B ₀, consistent with the initial phase between pulse to guarantee pulse.

Step 2, distance is to compression: each pulse data is carried out to distance to the compression processing, is mainly nonlinear frequency modulation phase error compensation method, and the phase error compensation that utilizes the reference channel packet to contain falls the phase error in the destination channel data, obtain Range Focusing image I, operate as follows:

1. to destination channel data A ₀With reference channel data B ₀Data added the Hamming window w (t) isometric with data length, obtain the destination channel data A after windowing ₁:

A ₁＝A×w(t) （32）

With reference channel data B ₁:

B ₁＝B×w(t) （33）

While 2. utilizing-the destination channel data A of frequency analysis from windowing ₁In extract phase place distribution in time

Reference channel data B from windowing again ₁In extract phase place distribution in time

3. by 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

For:

In formula: K _rBeing the linear frequency modulation coefficient, is a constant for particular system,

The time width that T is tuning pulse signal, B is signal bandwidth, B=c/ λ ₁-c/ λ ₂, λ ₁For the initial wavelength of laser tuning, λ ₂For the termination wavelength of laser tuning, c is the light velocity; R _rFor the light path in the reference time delay optical fiber, equal with reference to the length of time delay optical fiber and the product of optical fibre refractivity 1.44;

4. phase error is distributed

Be multiplied by a change of scale factor ξ, ξ is certain number between 0 ~ 1, and the phase error after being converted distributes

Take data A ₁PHASE DISTRIBUTION

The phase error deducted after conversion distributes

The destination channel data A after being compensated ₁PHASE DISTRIBUTION:

5. the destination channel data A after compensation ₁PHASE DISTRIBUTION

With original destination channel data A ₁Amplitude distribution A _abs(t) be combined into the destination channel data C after compensation:

6. the destination channel data C after compensation is carried out to Fourier transform, and calculates sharpening function S (ξ) value:

Wherein

For right Fourier transform, || be the delivery value;

7. change change of scale factor ξ, again by step, 4.-6. calculate the value of sharpening function, corresponding ξ while making sharpening function that maximal value be arranged is found out in circulation _max, and utilize this ξ _max, obtain the destination channel data after final compensation, be designated as the destination channel data D after final compensation:

8. again the destination channel data D after final compensation is carried out to Fourier transform, obtain frequency spectrum data, this frequency spectrum data has and target range R _TCorresponding peak value, target range R _TWith the relation of frequency spectrum medium frequency coordinate f, as formula (9), show:

$R_T=\frac{f\&CenterDot;c}{2K_r}---\left(40\right)$

Complete the distance of individual pulse data to compression.Data after processing are lined up according to precedence, and individual pulse data occupy row of array, form the two-dimensional array data, obtain Range Focusing image I.

Step 3, orientation is to compression: with the matched filtering algorithm, Range Focusing image I (complex image) is processed to (being the line direction of array) along orientation, all " OK " all is disposed and completes orientation to compression, obtain the azimuth focus image II, operate as follows:

1. along the equal corresponding same distance of the data of line direction, be designated as range unit data E, E carried out to Fourier transform, obtain data F:

F＝F{E} （41）

2. according to the Doppler FM slope

V is the target translational speed, sets reference function to be:

h(t)＝exp(jπ·K _d·t _a ²) （42）

T wherein _aFor orientation to the time.Again to reference function h (t) a synthetic aperture time T _SaInside carry out Fourier transform, obtain:

H _a＝F{h(t)} （43）

The synthetic aperture time is

R wherein ₀For transmitting aperture range-to-go, D _TTransmitting aperture size (diameter) for system.

3. by data F and H _aConjugate multiplication, then carry out Fourier inversion, obtain orientation to the data G compressed, wherein conj is for getting conjugation:

G＝IFFT{F·conj(H _a)} （44）

4. all capable data are carried out to orientation after the compression processing, obtain the azimuth focus image II.

Step 4, the Gradient Phase self-focusing: with the Gradient Phase autofocus algorithm, orientation focusedimage II (complex image) is processed, based on data compensation orientation to the high-order phase error, further sharpening image, iteration obtains the final image III for several times, operates as follows:

1. reconnaissance: on the azimuth focus image II, along line direction, travel through each range unit u _n, select to meet the normalization amplitude variance that Steinberg proposes

Range unit as the aobvious point of a spy range unit u _Spec,

As formula (45) shows:

${\mathrm{\&sigma;}}_{\mathrm{<figure-callout\; id="2"\; label="un"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">un</figure-callout>}}^2=\stackrel{\&OverBar;}{{(u_n-\stackrel{\&OverBar;}{u_n})}^2}/\stackrel{\&OverBar;}{{\mathrm{<figure-callout\; id="2"\; label="u\; n"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">u</figure-callout>}}_n^{}}=1-{\stackrel{\&OverBar;}{u_n}}^2/\stackrel{\&OverBar;}{{\mathrm{<figure-callout\; id="2"\; label="u\; n"\; filenames="HDA0000147887730000011.png"\; state="\{\{state\}\}">u</figure-callout>}}_n^{}}---\left(45\right)$

Wherein

The average of n range unit target echo sequence amplitude,

It is its mean square value.Choose maximal value as the aobvious point of spy from each the special aobvious some range unit found.

2. center displacement: adopt the circular shifting method, the aobvious point of spy (be in each special aobvious some range unit strong scattering point) is moved to centre position, obtain u _CsAs formula (46), to eliminate because of the different Doppler shifts of introducing in scattering point position.

u _cs＝circshift{u _spec} （46）

3. windowing: the suitable window function according to the picture contrast Feature Selection, as Hamming window, by the Data in Azimuth Direction windowing, remove the strong scattering point impact of scattering point on phase error estimation and phase error on every side, only retain the fuzzy region that scattering point causes because of phase error.If window function length is excessive, in window, the data signal to noise ratio (S/N ratio) reduces, and not only causes iterations to increase, and affects the phase estimation precision; If window function length is too small, will cause the phase error estimation and phase error distortion, even cause and estimate to disperse.In addition, windowing has vital role to the robustness that improves the PGA algorithm.In operating process, the length l en of window function will stipulate in advance, and changes along with iteration! As formula (47):

u _w＝hamming(len)·u _cs （47）

4. FFT: to the data after windowing along orientation to carrying out Fourier transform, by data transformation to the distance-Doppler territory, as formula (48):

I _w(n,X)＝F{u _w}＝F{hamming(len)·u _cs} （48）

Wherein, I _w(n, X) be orientation after the image windowing to Fourier transform, the sequence number that n is the distance row.

5. phase gradient is estimated: according to the optimal estimation criterion (as linear unbiased minimum variance estimation, maximum seemingly

$\mathrm{\&Delta;}\widehat{\mathrm{\&Phi;}}\left(X\right)=\arg \{\underset{n=1}{\overset{M}{\mathrm{\&Sigma;}}}{I}_w^{*}(n,X-1)\&CenterDot;I_w(n,X)\}---\left(49\right)$

Wherein, arg is the function of asking phasing degree, the number that M is special aobvious some range unit, and X is that orientation is to sequence number.

6. ask phase error: to estimated phase gradient, along orientation to X integration summation, show that phase error distributes, as formula (50):

$\widehat{\mathrm{\&Phi;}}\left(X\right)=\underset{l=1}{\overset{X}{\mathrm{\&Sigma;}}}\mathrm{\&Delta;}\widehat{\mathrm{\&Phi;}}\left(l\right)---\left(50\right)$

7. phase compensation: by the data u of original azimuth focus image II _nCarry out Fourier transform, obtain F{u _n, then the phase error distributed compensation of trying to achieve to F{u _nUpper, along orientation to the X conjugate multiplication, and then convert back image area with IFFT, obtain the image I II that this focuses on _i.

${\mathrm{III}}_i=\mathrm{IFFT}\left\{F\right\{u_n\}\&CenterDot;\exp [-j\widehat{\mathrm{\&Phi;}}\left(X\right)\left]\right\}---\left(51\right)$

Wherein, subscript n refers to range units all in image II is all carried out to this operation.

8. will dwindle the length l en of window, the image I II tried to achieve with this _i(complex image) replaces image II, repeats 1.-7., obtain image I II _I+1(complex image), proceed iteration, makes image further focus on, until meet the number of times requirement, obtains the final image III.

The advantage of native system is:

1) system stability is good, and most of module of system is all optical fibre devices, than Free Space Optics device strong interference immunity;

2) laser wave length, the device in system and element size will be more much smaller than corresponding size of electronic devices, so laser synthetic aperture radar is more conducive to the miniaturization of system and integrated realization;

3) detection sensitivity is high, has adopted coherent detection and balance detection mode to carry out opto-electronic conversion to light signal, with respect to direct detection, has suppressed relative intensity noise, has larger conversion gain, has improved spy

4) modulation band-width of Linear Tuning signal is large, and range resolution is high;

5) although the Linear Tuning laser instrument has the modulation band-width of THz level, the existence of nonlinear frequency modulation error cause adopting conventional method not arrive high range resolution.In native system, by reference channel is set, compensating non-linear frequency modulation error, successfully realize the range resolution of submillimeter level;

6) absorption spectra of molecule wavelength reference device is 1530-1560nm, and spectral resolution is high, and the wavelength uncertainty, at 0.04-0.24pm, can be good at synchronous each pulse signal;

7) data processing module has adaptivity, does not need the range information of destination channel priori, can carry out the Adaptive matching compensation according to specific reference channel, has larger practical value.

The accompanying drawing explanation

The block diagram that Fig. 1 is the large bandwidth laser synthetic aperture radar image-forming of THz level system, each several part is:

1. Linear Tuning semiconductor laser;

2.99:1 fiber coupler;

3.90:10 fiber coupler;

4. emission collimating mirror;

5. polarize beam splitter;

6. λ/4 wave plates;

7. reception collimating mirror;

8.50:50 fiber coupler I;

9.50:50 fiber coupler II;

10.50:50 fiber coupler III;

11. destination channel balance detection device;

12.50:50 fiber coupler IV;

13. reference channel balance detection device;

14. with reference to time delay optical fiber;

15. molecule wavelength reference device;

16. light power meter;

17. displacement of targets platform;

18. data acquisition module;

19. data processor module.

Fig. 2 is that the experiment test target is arranged schematic diagram.

Fig. 3 ~ 5 are the large bandwidth laser synthetic aperture radar image-forming of THz level system experimentation figure as a result.

The data of the synchronizing channel that Fig. 6 is certain subpulse.

Embodiment:

The large bandwidth laser synthetic aperture radar image-forming of THz level system is divided into the following steps to the imaging process of target:

1) setting laser device frequency tuning range and tuning speed, open laser instrument 1, laser instrument adopts NewFocus company semiconductor laser, wavelength 1550 ~ 1630nm, peak power output 50mW@1590nm, when live width 30kHz(120 μ s postpones), set initial wavelength 1550nm, stop wavelength 1560nm, tuning speed 20nm/s, trigger laser scans, wide during laser tuning pulse (approximate chirped pulse) is 0.5s, wavelength variations is as follows, front 0.5s is that linear wavelength rises, ensuing 0.5s is 1560nm, thereafter the linear wavelength of 0.5s descends, thereafter 0.5s is that 1550nm is linear decline again, circulation in this way.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 99:1 fiber coupler 2, sub-fraction is designated as S ₁, another major part is designated as S ₂, S ₂Be further divided into two parts through 90:10 fiber coupler 2, fraction is designated as S as bias light ₄, another major part is designated as S as utilizing emitted light ₃, S ₃Through collimating mirror 4 emissions, during through polarize beam splitter 5, part reflection, another part transmission, S ₃Transmissive portion laser is by after λ/4 wave plates 6, the laser light polarization direction is converted to circular polarization by linear polarization, be transmitted on target 17 through the free space path, by relative position in figure, put emission collimating mirror 4, polarize beam splitter 5, λ/4 wave plates 6, receive collimating mirror 7 fixing;

2) target is placed on the miter angle inclined-plane of displacement of targets platform 17;

3) regulating light path enters in reception collimating mirror 7 light that target reflects, to transmit and receive all and adjust and aim at the mark, all very straight on the level of assurance light path and vertical direction, last available horsepower meter measurement target after collimating mirror reflects, the luminous power of coupled into optical fibres again, when luminous power is maximum, can think that light path reaches optimum;

5) by Fig. 1 connect 50:50 fiber coupler I8,50:50 fiber coupler II9,50:50 fiber coupler III10, destination channel balance detection device 11,50:50 fiber coupler IV12, reference channel balance detection device 13, with reference to time delay optical fiber 14, molecule wavelength reference device 15, light power meter 16, connect balance detection device and light power meter power supply;

6) open displacement of targets platform 17, the displacement of targets platform is with the speed stepping of 50um/s, and the displacement of each stepping is 50um;

7) open computing machine, by the PXI data collecting card input interface on detector and light power meter output access industrial computer, a secondary data is adopted in the every stepping of displacement of targets platform once, and each sampling time was 1 second, altogether gathers 512 times (gathering 512 pulses);

8) utilize the data after 19 pairs of collections of data processor module to be processed, draw the two dimensional image of target.

At chirp bandwidth B, be 1.240694789THz, wide while warbling is 0.5s, and apart from 1.86m, calculating theoretical range resolution according to formula (7) is 120 microns.Native system is chosen 5 targets and is inclined on the displacement of targets platform, the upper interval 1mm of adjacent target distance, and orientation upwards dislocation place, in the zone that 5 target distribution are 11mm * 5mm in size, as shown in Figure 2.Certain experimental result picture that system is imaged to this is as shown in Fig. 3 ~ 5.As shown in Figure 6, it is the data that reference point intercepts destination channel and reference channel that native system adopts the minimum value of first absorption peak to the data of the synchronizing channel of certain subpulse, with this, comes the start-up phase site of synchronous each pulse.

Because sample frequency is 200KHz, the distance of calculating the respective frequencies coordinate according to formula should be 0 ~ 6.045m to coordinate, and target covers distance is to yardstick 5 millimeter only, for the sharpening image shows, Fig. 3 ~ 5 provide be distance to enlarged drawing (indication range 1.546 ~ 1.555m), orientation to yardstick do not have convergent-divergent.

By all can obviously finding out on Fig. 3 ~ 5 that distance, to telling 5 targets, is spaced apart 1mm, illustrate that resolution is better than 1mm.Fig. 3 is the Range Focusing image I obtained after the nonlinear frequency modulation phase error is crossed in over-compensation, and distance is to focusing on, but single target is wider to comparing the target actual width on (laterally) in orientation, illustrate that orientation is to also failing focusing well.

Fig. 4 is the azimuth focus image II after matched filtering through Fig. 3, orientation has obtained compression to image, five targets obviously narrow down in the horizontal, trend towards point target, although seem that image is discontinuous, effect is not as Fig. 3 seemingly, but this just because of image after azimuth focus, different range units is because the focal position difference that the noise cause causes causes.Fig. 5 final image III that to be Fig. 4 cross through the Gradient Phase Autofocus processing, its image Fig. 4 that compares only has a little to improve on the azimuth focus degree, and this is to the focus level of bearing images very high cause due to matched filtering.

The direction corresponding relation of target and image is: transversely, be orientation to, with respect to the moving direction of transmitting aperture as shown by the arrows in Figure 2, hot spot moves to the right in the left side of target the displacement of targets platform from Fig. 2, and the left side of the image therefore become is all corresponding to the left side of target; Vertically upper, distance to, the distance in Fig. 3 ~ 5 increases progressively to distance value from top to bottom, therefore the below of image is corresponding with the upside of real goal.

Can obviously find out that by Fig. 3 the adjacent target spacing all is less than 1mm, in the 0.2mm left and right.Illustrate that resolution characteristic is close to the theoretical resolution calculated according to formula (7); And orientation is approaching to also having realized

Resolution.Therefore use method of the present invention, can realize meeting the high-resolution laser synthetic aperture radar image-forming of the large bandwidth of THz level.

Claims (1)

1. the large bandwidth laser synthetic aperture radar image-forming of a THz level system, it comprises Linear Tuning semiconductor laser (1), 99:1 fiber coupler (2), 90:10 fiber coupler (3), emission collimating mirror (4), polarize beam splitter (5), λ/4 wave plates (6), receive collimating mirror (7), 50:50 fiber coupler I(8), 50:50 fiber coupler II(9), 50:50 fiber coupler III(10), destination channel balance detection device (11), 50:50 fiber coupler IV(12), reference channel balance detection device (13), with reference to time delay optical fiber (14), molecule wavelength reference device (15), light power meter (16), displacement of targets platform (17), data acquisition module (18) and data processor module (19), it is characterized in that:

The laser beam S of Linear Tuning semiconductor laser (1) output ₀Be divided into two parts through 99:1 fiber coupler (2), a part is designated as S ₁, absorbing postscript through molecule wavelength reference device (15) is S _1-1, another part is as being designated as S ₂, S ₂Be further divided into utilizing emitted light S through 90:10 fiber coupler (3) ₃With bias light S ₄, S ₃Through emission collimating mirror (4) emission, during through polarize beam splitter (5), part reflection, another part transmission, S ₃Transmissive portion laser is by after λ/4 wave plates (6), and the laser light polarization direction is converted to circular polarization by linear polarization, through the free space path, is transmitted on displacement of targets platform (17); Bias light S ₄Process 50:50 fiber coupler I(8) be divided into local oscillator light S ₅With reference light S ₆Two parts, local oscillator light S ₅Process 50:50 fiber coupler II(9) be divided into target local oscillator light S _5-1With reference local oscillator light S _5-2

Displacement of targets platform (17) is to S ₃The reflection of transmissive portion laser signal, the flashlight reflected, be designated as target echo signal R ₀, it changes linearly polarized light by λ/4 wave plates into after (6), and in polarization direction and when emission,, the polarization direction front through λ/4 wave plates (6) was vertical, during through polarize beam splitter (5), target echo signal R ₀Most of light is reflected, and be coupled in optical fiber through receiving collimating mirror (7) reflecting part, is designated as target echo and receives signal R ₁

Target echo receives signal R ₁With target local oscillator light S _5-1Enter 50:50 fiber coupler III(10) after be coupled, mixing occurs, surveyed and receive by destination channel balance detection device (11), light signal is converted to electric signal, is designated as destination channel data A; Reference light S ₆Through one section with reference to after time delay optical fiber (14) with reference local oscillator light S _5-2Enter again 50:50 fiber coupler IV(12), be coupled, mixing occurs, referenced channel balance detector (13) is surveyed and is received, and light signal is converted to electric signal, is designated as reference channel data B; S ₁Signal S after absorbing _1-1Be converted to electric signal through light power meter (16), be designated as synchronizing channel data C;

The electric signal that data acquisition module (18) gathers two balance detection devices and light power meter output obtains destination channel data A, reference channel data B and synchronizing channel data C, then utilize data processor module (19) to destination channel data A, reference channel data B and synchronizing channel data C process the two dimensional image that obtains target.

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