CN111289994B - Frequency modulation continuous wave laser radar ranging method based on double heterodyne mixing - Google Patents
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Abstract
The invention discloses a frequency modulation continuous wave laser radar distance measurement method based on double heterodyne frequency mixing, which does not need complicated laser feedback control, effectively reduces the implementation cost, offsets the phase noise of a laser in a beat frequency mode, has good expansibility, and can multiply improve the sweep frequency range by increasing the number of cascaded phase modulators, thereby expanding the sweep frequency range of the laser and improving the linearity of the sweep frequency. The problems of complex process, high cost and unsatisfactory effect of the conventional method for improving the ranging precision of the frequency modulation continuous wave laser radar are solved.
Description
Technical Field
The invention relates to the technical field of microwave photonics, in particular to a frequency modulation continuous wave laser radar ranging method based on double heterodyne mixing.
Background
At present, because of the use of optical signals with shorter wavelength, the laser radar has the advantages of good directionality, high spatial resolution, strong anti-interference capability, small volume, light weight and the like, and is widely applied to the field of high-precision distance measurement. The lidar may be classified into a pulse lidar and a continuous wave lidar, and the continuous wave lidar may be classified into a phase-type lidar and a frequency-modulated continuous wave lidar. Because the distance measurement mode used by the phase type laser radar has fuzzy distance, the measurement precision is difficult to ensure, and the method is not widely applied. The frequency modulation continuous wave laser radar adopts an absolute ranging mode, has high ranging precision, large measuring range and high measuring speed, and can be used for measuring a non-cooperative target. Compared with the traditional laser pulse distance measurement method, the distance measurement mode of the frequency modulation continuous wave laser radar has wider tuning bandwidth, and can achieve higher distance measurement precision and distance measurement resolution, so that the method has wider application prospect in the field of industrial large-size precision measurement.
The conventional frequency modulation continuous wave laser radar is mainly limited by the influence of frequency sweep range, linearity and phase noise, so that the ranging precision of the frequency modulation continuous wave laser radar cannot be further improved. For the influence of the sweep frequency range and the linearity, a mature predistortion algorithm is generally adopted at present, the linear tuning of the output frequency of the laser is realized by controlling the cavity length of the laser in a feedback mode, or a frequency-pulling type sweep frequency mode is generated by modulating an optical frequency comb by using a sweep frequency electric signal, and the output of a main laser is locked on a reference sweep frequency optical comb by using an optical phase-locked loop. For the influence of phase noise of frequency modulation continuous wave laser radar, the phase noise output by a laser is reduced mainly by an optical phase-locked loop technology and feedback phase locking at present. However, the existing measures for reducing the above three aspects of influences require a complex predistortion algorithm and a cumbersome circuit design in the implementation process, so that it is difficult to reduce the design cost, and the achieved effect is not ideal.
Therefore, how to provide a frequency modulation continuous wave laser radar ranging method with lower cost and higher ranging accuracy is a problem that needs to be solved urgently by those skilled in the art.
Disclosure of Invention
In view of the above, the invention provides a frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing, which obtains an optical frequency comb through electro-optical modulation, eliminates phase noise of a laser according to a coherent relation among frequency components of the optical frequency comb, can expand a frequency sweep range of the laser, improves linearity of frequency sweep, and solves the problems of complex process, high cost and unsatisfactory effect of the existing method for improving the ranging precision of the frequency modulation continuous wave laser radar.
In order to achieve the purpose, the invention adopts the following technical scheme:
a frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing comprises the following steps:
s1, modulating the linear sweep frequency radio frequency signal to a laser signal output by a laser through a phase modulator to generate an optical frequency comb, and dividing the optical frequency comb into detection light and reference light after passing through a coupler;
s2, the detection light is emitted into a space to be detected through the circulator and the collimator, and echo signals reflected by a target object in the space to be detected are received by the collimator and return to the circulator;
s3, dividing the reference light into two paths after frequency shifting by an acousto-optic modulator driven by a fixed frequency, and enabling the two paths of light to enter different optical filters to respectively filter two light wave signals with different frequencies;
s4, dividing the echo signal into two beams by a coupler, and respectively performing beat frequency on the two filtered light wave signals with different frequencies to obtain two beat frequency signals;
and S5, performing frequency beating again on the two frequency beating signals to obtain a final frequency beating signal, and calculating the actual distance of the measurement target object according to the finally obtained frequency beating signal.
Further, in step S1, the ratio of the probe light to the reference light is 90: 10.
Further, in step S1, a plurality of phase modulators are provided, and the plurality of phase modulators are cascaded to the output end of the laser in sequence. In the practical implementation process, one phase modulator can be arranged, or a plurality of phase modulators can be arranged, so that the number of the cascade phase modulators is increased, the modulation depth can be further improved, the spectrum range of the optical frequency comb is widened, and the measurement precision is increased.
Further, in step S4, two balanced receivers are used to receive the beat signals respectively, and since the balanced receivers have limited bandwidths, the beat signals output by the balanced receivers are only the beat frequencies of the filtered frequency light and the probe light with the corresponding frequency, and taking the light with the frequency of 0 order and 1 order as an example, the two beat signals are:
wherein the content of the first and second substances,
representing the phase noise of the laser, fARepresents the frequency shift frequency of the acousto-optic modulator, gamma represents the sweep speed, tau represents the delay difference between the reflected light and the reference light path, R represents the reflectivity of the reflection point, A represents the light intensity, wherein 01, 02 correspond to the intensity before and after the frequency shift of the 0-order comb teeth, 11 and 12 correspond to the intensity before and after the frequency shift of the 1-order comb teeth.
Further, in step S5, the final beat signal is:
wherein f isτγ τ represents the frequency of the beat signal, the frequency information includes information of time delay, the actual distance R of the measurement target can be calculated from the time delay and the speed of light, and a represents the reflectivity of the reflection point01、A02、A11、A12Both represent light intensity.
Phase noise term, as seen by the final beat signal formula
Has been eliminated.
According to the technical scheme, compared with the prior art, the frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing is provided, complex laser feedback control is not needed, implementation cost is effectively reduced, phase noise of a laser is offset in a beat frequency mode, good expansibility is achieved, the frequency sweep range can be doubled by increasing the number of cascaded phase modulators, accordingly, the frequency sweep range of the laser is expanded, the linearity of frequency sweep is improved, the ranging result obtained through the method is high in precision and strong in practicability.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 is a schematic flow chart of a frequency modulated continuous wave lidar ranging method based on double heterodyne frequency mixing provided by the present invention;
fig. 2 is a schematic structural diagram of an implementation system of an optical carrier frequency modulation continuous wave radar based on electro-optical modulation in an embodiment of the present invention;
fig. 3 is a schematic diagram of the power spectral density of the ranging signal before and after phase noise is removed according to the embodiment of the invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to the attached figure 1, the embodiment of the invention discloses a frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing, which comprises the following steps:
s1, modulating the linear sweep frequency radio frequency signal to a laser signal output by a laser through a phase modulator to generate an optical frequency comb, and dividing the optical frequency comb into detection light and reference light after passing through a coupler;
s2, transmitting the detection light to a space to be detected through the circulator and the collimator, and receiving an echo signal reflected by a target object in the space to be detected through the collimator and returning the echo signal to the circulator;
s3, dividing the reference light into two paths after frequency shifting by an acousto-optic modulator driven by a fixed frequency, and enabling the two paths of light to enter different optical filters to respectively filter two light wave signals with different frequencies;
s4, dividing the echo signal into two beams by a coupler, and respectively performing beat frequency with the two filtered light wave signals with different frequencies to obtain two beat frequency signals;
and S5, performing frequency beating again on the two frequency beating signals to obtain a final frequency beating signal, and calculating the actual distance of the measurement target object according to the finally obtained frequency beating signal.
The above method is specifically described below by specific application examples.
Referring to fig. 2, the present embodiment discloses an implementation system of an optical frequency modulation continuous wave radar based on electro-optical modulation, which includes a laser, a phase modulator, a fractional-n phase-locked loop, an acousto-optic modulator, an optical filter (including an optical filter 1 and an optical filter 2), a balanced receiver (including a balanced receiver 1 and a balanced receiver 2), a circulator and an optical fiber coupler, wherein an output end of the laser is connected with an input end of the phase modulator, and an output end of the phase modulator forms two output ends after passing through the optical fiber coupler 1; the first output end enters the circulator and is output to the collimator to be transmitted to the measurement space, the second output is divided into two paths through the optical fiber coupler 2 after frequency shift of the acousto-optic modulator, and the two paths of output respectively pass through different wave band filters and then enter the balance receiver together with echo signals received by the circulator.
The method of the present invention is specifically described below based on the system for implementing the electro-optically modulated optical carrier frequency modulated continuous wave radar.
A frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing comprises the following steps:
step 1: inputting a linear frequency sweeping video signal to a phase modulator, modulating a linear frequency sweeping radio frequency signal to a laser signal output by a laser by using the phase modulator to generate a frequency sweeping optical frequency comb, and dividing an optical frequency comb into detection light and reference light after passing through a 90:10 coupler;
step 2: the detection light is transmitted to a space to be detected through the circulator and the collimator, the echo signal is received by the collimator and returned to the circulator, and the echo signal is received at the 3 port of the circulator; the light paths through which the light with different frequencies in the detection light passes are consistent, and the influence of dispersion can be ignored when the wavelength difference is small, so that the phase noise of the light with different wavelengths is the same.
And step 3: the reference light is divided into two paths after frequency shift of an acousto-optic modulator driven by fixed frequency, the two paths of light enter different optical filters 1 and 2, and two optical comb teeth with different frequencies are respectively filtered;
and 4, step 4: meanwhile, the echo signal is divided into two beams by the optical fiber coupler 3, and beat frequency is carried out on the two beams and the filtered light waves with different frequencies. The beat frequency signals are received by the balanced receiver 1 and the balanced receiver 2 respectively, and the beat frequency signals of the two balanced receivers are, for example, light with 0-order and 1-order frequencies:
wherein the content of the first and second substances,
representing the phase noise of the laser, fARepresenting the frequency shift frequency of the acousto-optic modulator, gamma representing the sweep frequency speed, tau representing the delay time difference of reflected light and a reference light path, R representing the reflectivity of a reflection point, and A representing the light intensity, wherein 01 and 02 respectively correspond to the intensity before and after the frequency shift of 0-order comb teeth, and 11 and 12 respectively correspond to the intensity before and after the frequency shift of 1-order comb teeth.
And 5: the beat frequency signals received by the two balanced receivers are subjected to electrical beat frequency again, so that the phase noise of the laser can be counteracted, and more accurate distance information can be obtained. The beat signal can now be expressed as:
wherein f isτγ τ represents the frequency of the beat signal, the frequency information includes information of time delay, the actual distance R of the measurement target can be calculated from the time delay and the speed of light, and a represents the reflectivity of the reflection point01、A02、A11、A12All representLight intensity.
Phase noise term, as seen by the final beat signal formula
Has been eliminated.
Referring to fig. 3, comparing the power spectral densities of the ranging signals before and after eliminating the phase noise, it can be seen that the measurement result is very poor when the phase noise exists, and the accuracy is significantly increased after the processing by the method disclosed in this embodiment.
In some embodiments, the ranging accuracy can be improved by using a fractional-n pll to generate a swept rf signal with low phase noise.
Preferably, the optical comb with the larger order difference is selected by setting the range of the filter and increasing the modulation depth, so that the secondary measurement precision can be improved.
Preferably, the number of the cascade phase modulators is increased, so that the modulation depth can be further improved, the spectrum range of the optical frequency comb is widened, and the measurement precision is increased.
Specifically, the parameters of the two balanced receivers are the same in this embodiment.
In this embodiment, the electro-optical modulator may use an intensity modulator in addition to the phase modulator.
In some embodiments, phase information of the measuring light waves can be obtained through a Fourier transform spectrum of the echo signals, when the distance of the target object changes slightly, the frequency measurement result is unchanged, but the phase of the frequency point changes, and the phase information of the measuring light waves can be used for sensing the small displacement of the target object, so that the ranging precision can be further improved.
In summary, compared with the prior art, the frequency modulation continuous wave lidar ranging method based on double heterodyne frequency mixing provided by the embodiment of the invention has the following advantages:
1. complicated laser feedback control is not needed, the cost can be reduced, and the measurement precision can be improved.
2. The phase-locked loop circuit board is mature in technology at present, low in cost and excellent in phase-locking effect, and avoids complex control over the laser.
3. The beat frequency can offset the phase noise of the laser, and even if the laser with poor phase noise and wide line width is used, accurate distance measurement can be realized.
4. The device has good expansibility, and the sweep frequency range can be doubled by increasing the number of the cascaded electro-optical modulators, so that the measurement precision is improved.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed by the embodiment corresponds to the method disclosed by the embodiment, so that the description is simple, and the relevant points can be referred to the method part for description.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (3)
1. A frequency modulation continuous wave laser radar ranging method based on double heterodyne frequency mixing is characterized by comprising the following steps:
s1, modulating the linear sweep frequency radio frequency signal to a laser signal output by a laser through a phase modulator to generate an optical frequency comb, and dividing the optical frequency comb into detection light and reference light after passing through a coupler;
s2, the detection light is emitted into a space to be detected through the circulator and the collimator, and echo signals reflected by a target object in the space to be detected are received by the collimator and return to the circulator;
s3, dividing the reference light into two paths after frequency shifting by an acousto-optic modulator driven by a fixed frequency, and enabling the two paths of light to enter different optical filters to respectively filter two light wave signals with different frequencies;
s4, dividing the echo signal into two beams by a coupler, and respectively performing beat frequency on the two filtered light wave signals with different frequencies to obtain two beat frequency signals;
s5, performing beat frequency on the two beat frequency signals again to obtain a final beat frequency signal, and calculating the actual distance of the measured target object according to the finally obtained beat frequency signal;
in step S4, the two beat signals are:
wherein the content of the first and second substances,
representing the phase noise of the laser, fARepresenting the frequency shift of the acousto-optic modulator, gamma representing the sweep speed, tau representing the delay difference between the reflected light and the reference light path, R representing the reflectivity of the reflection point, A01、A02Respectively corresponding to the intensity before and after the frequency shift of the 0-order comb teeth; a. the11、A12Respectively corresponding to the intensity before and after the frequency shift of 1-order comb teeth;
in step S5, the final beat signal is:
wherein f isτγ τ denotes the frequency of the beat signal, R denotes the reflectivity of the reflection point, a01、A02、A11、A12Both represent light intensity.
2. The frequency-modulated continuous wave lidar ranging method based on double-heterodyne mixing as claimed in claim 1, wherein in step S1, a ratio of the probe light to the reference light is 90: 10.
3. The frequency modulation continuous wave lidar distance measurement method based on double-heterodyne frequency mixing of claim 1, wherein in step S1, the number of the phase modulators is multiple, and the multiple phase modulators are cascaded to the output end of the laser in sequence.
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