CN104035101B - Based on the Synthetic Aperture Laser Radar system of intensity coding - Google Patents

Abstract

The invention discloses a kind of Synthetic Aperture Laser Radar system based on intensity coding, it is applied to synthetic aperture laser radar imaging.System of the present invention is by symbol signal generation unit, 90:10 fiber coupler, transmitting-receiving camera lens, 50:50 fiber coupler, 2 balanced detector, data acquisition module and signal handler module compositions.Intensity coding Synthetic Aperture Laser Radar system of the present invention is based on multistage intensity coding modulated laser pulse signal, adopt heterodyne coherent detection and balance detection technology to obtain echoed signal, distance to orientation to compress process after obtain the two dimensional image of target.System advantage is that signal madulation speed is exceedingly fast, and under the prerequisite meeting airborne platform flying speed, modulation band-width can reach more than 5GHz, system adopts polarization maintaining optical fibre structure, and range stability is high, is easy to adjust light path, detection sensitivity is high, and range resolution reaches centimetres.

Description

Based on the Synthetic Aperture Laser Radar system of intensity coding

Technical field

The present invention relates to Synthetic Aperture Laser Radar field, especially, relate to a kind of Synthetic Aperture Laser Radar system based on intensity coding modulation signal.

Background technology

Present laser radar system can be divided into direct detection laser radar and coherent detection laser radar by the detection mode of echo.Direct detection structure is simple, is most widely used, and this system is that the time delay (Δ t) by measuring transmitting and receiving light signal calculates distance c is the light velocity, but will obtain high resolving power must use ultrashort pulse, high-speed sampling, and the very high avalanche photodide of sensitivity is as detector, higher to requirement on devices, affected by noise larger.Coherent detection utilizes the mixing on the detector of continuous print local oscillator light signal and echo, detection sensitivity is high, and can show that echo frequency and phase place change, detection range compares direct detection obviously to be increased, and is applied in large-scale coherent laser radar and laser Doppler radar.

From laser radar theory, during power limited, the distance detected is far away, wide larger when requiring pulse; And the range resolution that will realize is higher, then the bandwidth needed is larger, simple pulse time wide bandwidth product close to l, in time, widely restricts mutually with bandwidth, can not increase simultaneously.Therefore the simple laser pulse to this constant wavelength, maximum operating range and range resolution can not be taken into account simultaneously.Best solution is exactly, and adopts the sophisticated signal form with large Timed automata as transmitting.

Signal theory is pointed out for a long time, and time large, wide transmitting with bandwidth product, must use complicated modulation waveform.Conventional has three kinds of forms: linear frequency modulation, nonlinear frequency modulation and phase encoding are modulated.Due to linear FM signal (LFM signal, i.e. chirp signal) be easy to produce, be convenient to process, to characteristics such as Doppler shift are insensitive, early stage laboratory demo system all adopts this modulation system.But wideband frequency modulation speed is low and to cause repetition frequency not high, wideband frequency modulation poor linearity, range resolution is required to the high speed motion platforms such as higher airborne platform cannot use which, limit the application of Synthetic Aperture Laser Radar; The processing of signal sources of nonlinear frequency modulation laser is comparatively complicated, seldom has research; Random code phase-modulation laser radar can realize signal madulation fast, but modulating mechanism is complicated, needs accurately to control, Project Realization is subject to many limitations to the modulation of phase place.

Based on more than, at present the biggest obstacle of restriction airbome synthetic aperture laser radar engineering is, (sub-millisecond) modulation cannot obtain large Timed automata radar signal in very short time.

Summary of the invention

The object of this invention is to provide a kind of Synthetic Aperture Laser Radar based on intensity coding, realize the large Timed automata radar signal of extremely short modulating time (sub-millisecond).

The present invention provides a kind of new fast modulation mode, meets airbornely to wait rapid movement platform, realize distance to orientation to high-resolution imaging, modulating equipment is simple and easy to use, is beneficial to realization.The intensity coding modulation system that the present invention proposes adopts N rank random code, and symbol width is T, and he number is L, is modulated in transponder pulse light by photoelectricity intensity modulator.Benefit from the modulating speed of existing electrooptic modulator quickly (lithium niobate intensity modulator modulating speed can reach 40Gb/s, and this is that microwave regime does not possess), very high bandwidth can be obtained under the prerequisite ensureing high repetition frequency.Below advantage of the present invention is derived in detail.

Choose the continuous laser of λ=1550nm wavelength in Synthetic Aperture Laser Radar reality as light source, emission signal frequency is designated as ω ₀, then laser emitting signal can be expressed as:

s(t)＝E _sexp(-j(ω ₀t))(1)

E _srepresent signal amplitude, for the purpose of simple, be set to identical.

Signal, through copped wave, obtains the pulse signal that width is T*L, repetition frequency f _raccording to orientation to Doppler frequency f _dmore than 2 times arrange,

$f_d=\frac{2V}{\mathrm{\λ}}\sin \left(\mathrm{\θ}\right)---\left(2\right)$

Wherein V be carrier aircraft in orientation to movement velocity, θ is the angle of carrier aircraft current location and target location.

Code symbols signal is expressed as A (t), then have

$A\left(t\right)=\underset{n=1}{\overset{L}{\mathrm{\Σ}}}p\left(n\right)\mathrm{rect}(t-(n-1)T)---\left(3\right)$

Wherein p (n) is 1 random number sequence arriving N random value, and number is L.

Light signal after modulation is

$\begin{array}{c}{s}_m\left(t\right)=A\left(t\right)E_s\exp (-j\left({\mathrm{\ω}}_{0}t\right))\\ =\underset{n=1}{\overset{L}{\mathrm{\Σ}}}p\left(n\right)\mathrm{rect}(t-(n-1)T)*E_s\exp (-j\left({\mathrm{\ω}}_{0}t\right))\end{array}---\left(4\right)$

After target reflection, get back to receiving system, if the time delay of process is τ, then echo is expressed as

$s_s\left(t\right)=\underset{n=1}{\overset{L}{\mathrm{\Σ}}}p\left(n\right)\mathrm{rect}(t-\mathrm{\τ}-(n-1)T)*E_s\exp (-j\left({\mathrm{\ω}}_0(t-\mathrm{\τ})\right))---\left(5\right)$

Carry out relevant balance detection with local oscillator, the symbol signal finally obtained is

$A_s\left(t\right)=\underset{n=1}{\overset{L}{\mathrm{\Σ}}}p\left(n\right)\mathrm{rect}(t-\mathrm{\τ}-(n-1)T)---\left(6\right)$

To A _st () carries out matched filtering, can obtain the position of the corresponding spike of target, complete distance to data compression.Compress accordingly to Doppler frequency spectrum in orientation, finally obtain high resolution 2 d imaging, orientation to compression be classical method due to what adopt, do not repeat them here.

Theoretical according to matched filtering and Radar Signal Processing, radar distance to resolution to be directly proportional to modulation band-width ρ _r=c/2B, c are the lighies velocity, and B is modulation band-width.Specific in the present invention, the bandwidth that B is corresponding is 1/T.Consider that signal generator produces the restriction of chip rate, the present invention selects B=2GHz, and corresponding range resolution can reach 7.5cm, and along with the raising of chip rate, resolution can also promote further.In order to alleviate the pressure that rear end AD samples, have employed I/Q quadrature demodulation, sampling rate is reduced half.

Accordingly, the present invention propose based on intensity coding modulation Synthetic Aperture Laser Radar system, as Fig. 1, system element and function as follows:

1, system optic fibre light path.By narrow linewidth continuous wave laser 1,90/10 fiber optic splitter 2, image intensifer 4, optical fiber circulator 5, transmitting-receiving camera lens 6,90 ° of phase shifter 7,50/50 fiber optic splitters 10, three-dB coupler 15, three-dB coupler 16, chopper 17 forms.Narrow linewidth continuous wave laser 1 Output of laser power at more than 100mW, below live width 100KHz.Chopper 17 repetition frequency is at more than 1KHz.The output power of image intensifer 4 is determined according to operating distance, general selection 5W to 20W.

2, radar signal modulating part.Be made up of with AWG (Arbitrary Waveform Generator) 9 lithium niobate intensity modulator 3, wideband radio frequency amplifier 8.Lithium niobate intensity modulator 3 modulation band-width will reach more than 1Gb/s, and wideband radio frequency amplifier 8 bandwidth of operation needs more than 5GHz, gain 20dB; AWG (Arbitrary Waveform Generator) 9 more than output bandwidth 1GHz, and can symbol width be adjusted.In very short time (sub-millisecond), the modulation of large Timed automata radar signal can be realized.

3, echo signal processing part.By No. 1 balanced detector-13, No. 2 balanced detector-14, broadband high-speed analog to digital converter-11, data processing unit-12 forms.Wavelength 1550nm centered by No. 1 balanced detector-13 and No. 2 balanced detector-14, the pin detectors such as bandwidth 500M-3GHz; Broadband high-speed analog to digital converter-11 about bandwidth 500M-3GHz, sampling rate 1-3GS/s, significance bit is greater than 7.

System specific works flow process is as follows:

Narrow linewidth continuous wave laser 1 produces the continuous laser of centre wavelength 1550nm, sends into 90/10 fiber optic splitter 2, and wherein 90% energy sends into lithium niobate intensity modulator 3,10% as local oscillation signal s2 as the s1 that transmits;

Produce random N exponent unit by AWG (Arbitrary Waveform Generator) 9, N chooses 6 to 16.Be amplified to applicable voltage amplitude through wideband radio frequency amplifier 8, be input to the prevention at radio-frequency port of lithium niobate intensity modulator 3, amplitude code modulation is carried out to signal s1, export modulation signal s3;

S3, through chopper 17, launches exited signal s4 via optical fiber circulator 5 and transmitting-receiving camera lens 6;

Exited signal s4 is reflected, and obtains echo r1 at transmitting-receiving camera lens 6, exports, energy is on average given three-dB coupler I 15 and three-dB coupler II 16 from No. 2 mouths of optical fiber circulator 5;

Local oscillation signal s2 is divided into equal signal L1 and L2 of two-way energy through 50/50 fiber optic splitter 10, and wherein L2 is through phase shifter 7 time delay 90 ° of phase places;

L1 and L2 is sent into respectively three-dB coupler I 15 and three-dB coupler II 16, after mixing, sampled by broadband high-speed analog to digital converter 11 after balanced detector I 13 and balanced detector II 14 opto-electronic conversion respectively, be stored in data processing unit 12 and carry out data processing, through compression coding and matched filtering, obtain view data.

In system, random N exponent unit, its amplitude is according to 1 to N stochastic distribution, and transmit s1 after intensity modulated, amplitude envelops changes according to coded magnitude.

The invention has the advantages that: system signal modulating speed is exceedingly fast, under the prerequisite meeting airborne platform flying speed, modulation band-width can reach more than 5GHz, system adopts polarization maintaining optical fibre structure, and range stability is high, is easy to adjust light path, detection sensitivity is high, and range resolution reaches centimetres.

Accompanying drawing explanation

Accompanying drawing is used to provide a further understanding of the present invention, and forms a part for instructions, is used from explanation the present invention, but is not construed as limiting the invention with embodiment one below.

Fig. 1 is random strength modulated symbol schematic diagram, and Baud Length is 6, represents 315783;

Fig. 2 is length is 500, is highly the autocorrelogram of 1 to 16 random arrangement code elements;

Fig. 3 is that system realizes schematic block diagram, and wherein each several part is respectively:

1. narrow linewidth continuous wave laser, 2.90/10 fiber optic splitter, 3. lithium niobate intensity modulator, 4. image intensifer, 5. optical fiber circulator, 6. receive and dispatch camera lens, 7.90 ° of phase shifter, 8. wideband radio frequency amplifier, 9. AWG (Arbitrary Waveform Generator), 10.50/50 fiber optic splitter, 11. broadband high-speed analog to digital converters, 12. data processing units, 13. balanced detector I, 14. balanced detector II, 15.3dB coupling mechanisms I, 16.3dB coupling mechanism II, 17. chopper.

Embodiment

Below in conjunction with accompanying drawing, the specific embodiment of the present invention is described in detail.Should be understood that, embodiment described herein, only for instruction and explanation of the present invention, is not limited to the present invention.

The two-dimensional imaging process of intensity coding modulation Synthetic Aperture Laser Radar system to target is divided into the following steps:

1, arrange symbol width and the code element exponent number of coding, choose random code sequence, this step sets the modulation intelligence transmitted.AWG (Arbitrary Waveform Generator) 9 selects the AWG5102C of Imtech, sets symbol width according to range resolution, selects 5ns symbol width.Output level peak-to-peak value is set to 300mV, and according to 8 rank intensity, namely code element output amplitude is 300mV/8*n, n=1, and 2,3 ... 8.Baud Length is set to 1000.Consider that output level scope is smaller, and general intensity modulator half-wave voltage is upper and lower at 5V, centre adds wideband radio frequency amplifier 8 (select the SHF806e of German SHF company, gain amplifier is 26dB) and amplifies code element.

2, the light regulating light path that target is reflected enters in transmitting-receiving camera lens 6.Transmitting-receiving camera lens 6 designs according to angle of divergence 3mrad, bore 30mm.Transmitting-receiving camera lens 6 adjustment is aimed at the mark, ensures on the level of light path and vertical direction all very straight, reflect by power meter measurement target after collimating mirror, again the luminous power of coupled into optical fibres, when luminous power is maximum, can think that light path reaches optimum;

3, laser instrument exports energy adjustment.Because native system modulates intensity, while meeting narrow linewidth and low phase noise, the output intensity of laser instrument is needed to stablize.Laser instrument selects the ETH-180-1550.12-2-PZ10B-SL130-TT-PM of OrbitsLightwave company, start after one hour energy basicly stable, up-down error, at about nW, meets systematic survey demand substantially;

4, all in connection layout 1 optical fibre devices, optical fibre device all adopts polarization maintaining optical fibre, comprises 2, and 10,7,15,16,13,14,5 etc.;

5, adjust I/Q quadrature demodulation fiber lengths, the phase unbalance degree of quadrature sampling and amplitude degree of unbalancedness are calibrated;

6, transmitting-receiving camera lens 6 is fixed on displacement platform, command displacement platform according to 500um/s speed linear uniform motion (for airborne situation, system is placed on stable platform), open laser instrument simultaneously, system starts, balanced detector I 13 and balanced detector II 14 adopt the 1617-FC of NewFocus, and bandwidth is 800MHz, and conversion gain is 1V/A; Broadband high-speed analog to digital converter 11 adopts the U1064A capture card of Agilent company, sampling rate 1GS/s, significance bit 8bit.

7, after the synthetic aperture time terminates, the sampled data of storage is carried out distance to compression (code element convolution algorithm), then do Data in Azimuth Direction compression (matched filtering).

Below the preferred embodiment of the present invention is described in detail by reference to the accompanying drawings; but; the present invention is not limited to the detail in above-mentioned embodiment; within the scope of technical conceive of the present invention; can carry out multiple simple variant to technical scheme of the present invention, these simple variant all belong to protection scope of the present invention.Such as, random code element can be changed into certain code element specific, optic fibre light path be changed into free space light path etc.

It should be noted that in addition, each concrete technical characteristic described in above-mentioned embodiment, in reconcilable situation, can be combined by any suitable mode, such as this system be used for Doppler ranging and test the speed, instead of Synthetic Aperture Laser Radar.In order to avoid unnecessary repetition, the present invention illustrates no longer separately to various possible array mode.

In addition, also can carry out combination in any between various different embodiment of the present invention, as long as it is without prejudice to thought of the present invention, it should be considered as content disclosed in this invention equally.

Claims (2)

1. the Synthetic Aperture Laser Radar system based on intensity coding, it comprises narrow linewidth continuous wave laser (1), 90/10 fiber optic splitter (2), lithium niobate intensity modulator (3), image intensifer (4), optical fiber circulator (5), transmitting-receiving camera lens (6), 90 ° of phase shifters (7), wideband radio frequency amplifier (8), AWG (Arbitrary Waveform Generator) (9), 50/50 fiber optic splitter (10), broadband high-speed analog to digital converter (11), data processing unit (12), balanced detector I (13), balanced detector II (14), three-dB coupler I (15), three-dB coupler II (16), chopper (17), it is characterized in that:

Narrow linewidth continuous wave laser (1) produces continuous laser, sends into 90/10 fiber optic splitter (2), and wherein 90% energy sends into lithium niobate intensity modulator (3) as the s1 that transmits, and 10% as local oscillation signal s2;

Random N exponent unit is produced by AWG (Arbitrary Waveform Generator) (9), N chooses 6 to 16, applicable voltage amplitude is amplified to through wideband radio frequency amplifier (8), be input to the prevention at radio-frequency port of lithium niobate intensity modulator (3), amplitude code modulation is carried out to signal s1, exports modulation signal s3;

S3, through chopper (17), launches exited signal s4 via optical fiber circulator (5) and transmitting-receiving camera lens (6);

Exited signal s4 is reflected, and obtains echo r1 in transmitting-receiving camera lens (6), exports, energy is on average given three-dB coupler I (15) and three-dB coupler II (16) from No. 2 mouths of optical fiber circulator (5);

Local oscillation signal s2 is divided into equal signal L1 and L2 of two-way energy through 50/50 fiber optic splitter (10), and wherein L2 is through phase shifter (7) time delay 90 ° of phase places;

L1 and L2 is sent into respectively three-dB coupler I (15) and three-dB coupler II (16), after mixing, sampled by broadband high-speed analog to digital converter (11) after balanced detector I (13) and balanced detector II (14) opto-electronic conversion respectively, be stored in data processing unit (12) and carry out data processing, through compression coding and matched filtering, obtain view data.

2. a kind of Synthetic Aperture Laser Radar system based on intensity coding according to claim 1, it is characterized in that: described random N exponent unit, its amplitude is according to 1 to N stochastic distribution, and transmit s1 after intensity modulated, amplitude envelops changes according to coded magnitude.