CN111289994B - A Ranging Method for Frequency Modulated Continuous Wave Lidar Based on Double Heterodyne Mixing - Google Patents

Abstract

The invention discloses a frequency modulation continuous wave laser radar ranging method based on double heterodyne mixing. The method does not need complex laser feedback control, effectively reduces the implementation cost, cancels the phase noise of the laser by means of beat frequency, and has the advantages of Good scalability. By increasing the number of cascaded phase modulators, the frequency sweep range can be doubled, thereby expanding the frequency sweep range of the laser and improving the linearity of the frequency sweep. The accuracy of the ranging results obtained by this method is higher. , more practical. The method solves the problems of complicated process, high cost and unsatisfactory effect of the existing method for improving the ranging accuracy of the frequency-modulated continuous wave laser radar.

Description

A Ranging Method for Frequency Modulated Continuous Wave Lidar Based on Double Heterodyne Mixing

technical field

The invention relates to the technical field of microwave photonics, and more particularly to a ranging method for frequency-modulated continuous wave laser radar based on double heterodyne mixing.

Background technique

At present, LiDAR is widely used in the field of high-precision ranging because it uses optical signals with shorter wavelengths, has the advantages of good directionality, high spatial resolution, strong anti-interference ability, small size, and light weight. Lidar can be divided into pulse Lidar and continuous wave Lidar, among which continuous wave Lidar can be further divided into phase Lidar and Frequency Modulated Continuous Wave Lidar. Due to the ambiguous distance in the ranging method used by the phased lidar, the measurement accuracy is difficult to guarantee, and it has not been widely used. The FM continuous wave lidar adopts the absolute ranging method, which has high ranging accuracy, large measuring range and fast measuring speed, and can be used for the measurement of non-cooperative targets. Compared with the traditional laser pulse ranging method, the ranging method of FM continuous wave lidar has a wider tuning bandwidth, which can achieve higher ranging accuracy and ranging resolution, so it has more advantages in the field of industrial large-scale precision measurement. Broad application prospects.

The existing frequency-modulated continuous wave lidar is mainly limited by the three aspects of sweep frequency range, linearity and phase noise, so that the ranging accuracy of the frequency-modulated continuous wave lidar cannot be further improved. For the influence of sweep frequency range and linearity, mature pre-distortion algorithms are generally used at present. Linear tuning of laser output frequency is realized by feedback control of laser cavity length, or frequency sweep electrical signal is used to modulate optical frequency comb to generate frequency pulling sweep. The main laser output is locked on the reference frequency swept optical comb with an optical phase-locked loop. For the phase noise effect of frequency-modulated continuous wave lidar, the phase noise of the laser output is mainly reduced by the optical phase-locked loop technology and feedback phase-locking. However, the existing measures to reduce the influence of the above three aspects require complex pre-distortion algorithms and cumbersome circuit design in the implementation process, which makes it difficult to reduce the design cost and achieves unsatisfactory effects.

Therefore, how to provide a FM continuous wave laser radar ranging method with lower cost and higher ranging accuracy is an urgent problem to be solved by those skilled in the art.

SUMMARY OF THE INVENTION

In view of this, the present invention provides a frequency-modulated continuous wave laser radar ranging method based on double heterodyne mixing, obtaining an optical frequency comb through electro-optic modulation, and eliminating lasers according to the coherent relationship between the frequency components of the optical frequency comb At the same time, it can expand the frequency sweep range of the laser, improve the linearity of the frequency sweep, and solve the problems of cumbersome process, high cost and unsatisfactory effect of the existing method of improving the ranging accuracy of FM continuous wave laser radar.

In order to achieve the above object, the present invention adopts the following technical solutions:

A FM continuous wave lidar ranging method based on double heterodyne mixing, the method comprising:

S1, modulate the linear frequency swept radio frequency signal to the laser signal output by the laser through the phase modulator, to generate an optical frequency comb, and the optical frequency comb is divided into probe light and reference light after passing through the coupler;

S2. The detection light passes through the circulator and is emitted into the space to be measured through the collimator, and the echo signal reflected by the target in the space to be measured is received by the collimator and returned to the circulator;

S3, the reference light is divided into two paths after frequency shifting by the acousto-optic modulator driven by a fixed frequency, and the two paths of light enter different optical filters to filter out two light wave signals of different frequencies respectively;

S4. Divide the echo signal into two beams with a coupler, and perform beat frequency respectively with the two filtered light wave signals of different frequencies to obtain two beat frequency signals;

S5 , beat the two beat signals again to obtain a final beat signal, and calculate the actual distance of the measurement target according to the finally obtained beat signal.

Further, in step S1, the ratio of the probe light to the reference light is 90:10.

Further, in step S1, there are multiple phase modulators, and the multiple phase modulators are cascade-connected to the output end of the laser in sequence. In the actual implementation process, one phase modulator can be set, or multiple phase modulators can be set, increasing the number of cascaded phase modulators can further improve the modulation depth and broaden the spectral range of the optical frequency comb, thereby increasing the measurement accuracy.

Further, in step S4, two balanced receivers are used to receive the beat signal respectively. Since the bandwidth of the balanced receiver is limited, the beat signal output by the balanced receiver is only the beat frequency of the filtered frequency light and the probe light of the corresponding frequency. , taking the light of the 0th and 1st order frequencies as an example, the two beat signals are:

代表激光器的相位噪声，f_A代表声光调制器的移频频率，γ代表扫频速度，τ代表反射光和参考光路径的延时差，R代表反射点的反射率，A代表光强度，其中01、02分别对应0阶梳齿移频前和移频后的强度，11、12分别对应1阶梳齿移频前和移频后的强度。in,

represents the phase noise of the laser, f _A represents the frequency-shift frequency of the acousto-optic modulator, γ represents the frequency sweep speed, τ represents the delay difference between the reflected light and the reference optical path, R represents the reflectivity of the reflection point, A represents the light intensity, Among them, 01 and 02 correspond to the intensity of the 0-order comb tooth before and after frequency-shifting, respectively, and 11 and 12 correspond to the intensity of the first-order comb tooth before and after frequency-shifting, respectively.

Further, in step S5, the final beat frequency signal is:

Among them, f _τ =γτ represents the frequency of the beat signal, and the frequency information includes the information of time delay. The actual distance of the measurement target can be calculated from the time delay and the speed of light. R represents the reflectivity of the reflection point, A ₀₁ , A ₀₂ , A ₁₁ , and A ₁₂ all represent light intensity.

已经被消除。As can be seen from the final beat signal formula, the phase noise term

has been eliminated.

It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a frequency-modulated continuous wave laser radar ranging method based on double heterodyne mixing, which does not require complex laser feedback control and effectively reduces the need for implementation. cost, the phase noise of the laser is offset by the beat frequency, and it also has good scalability. By increasing the number of cascaded phase modulators, the sweep frequency range can be doubled, thereby expanding the sweep frequency range of the laser and improving the sweep frequency. The linearity of the distance measurement results obtained by this method is more accurate and more practical.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to the provided drawings without creative work.

Fig. 1 accompanying drawing is a kind of schematic flow chart of the method for ranging FM continuous wave laser radar based on double heterodyne mixing provided by the present invention;

2 is a schematic diagram of the structure 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 eliminating phase noise in an embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Referring to FIG. 1, an embodiment of the present invention discloses a method for ranging FM continuous wave laser radar based on double heterodyne mixing, the method includes:

S1. Modulate the linearly swept radio frequency signal to the laser signal output by the laser through the phase modulator to generate an optical frequency comb, which is divided into probe light and reference light after passing through the coupler;

S2. The probe light passes through the circulator and is emitted into the space to be measured through the collimator, and the echo signal reflected by the target in the space to be measured is received by the collimator and returned to the circulator;

S3. The reference light is divided into two paths after frequency shifting by the acousto-optic modulator driven by a fixed frequency, and the two paths of light enter different optical filters to filter out two light wave signals of different frequencies respectively;

S4. Divide the echo signal into two beams with a coupler, and perform beat frequency respectively with the two filtered light wave signals of different frequencies to obtain two beat frequency signals;

S5 , beat the two beat signals again to obtain a final beat signal, and calculate the actual distance of the measurement target according to the finally obtained beat signal.

The above method will be described in detail below through specific application examples.

Referring to FIG. 2, this embodiment discloses an implementation system of an optical carrier frequency modulation continuous wave radar based on electro-optic modulation, including a laser, a phase modulator, a fractional frequency division phase-locked loop, an acousto-optic modulator, an optical filter (including an optical filter 1 and optical filter 2), balanced receiver (including balanced receiver 1 and balanced receiver 2), circulator and fiber coupler, wherein the output end of the laser is connected with the input end of the phase modulator, and the phase modulation The output end of the device passes through the fiber coupler 1 to form two output ends; the first output end enters the circulator and is output to the collimator to be emitted into the measurement space, and the second output is frequency shifted by the acousto-optic modulator and then passed through The optical fiber coupler 2 is divided into two channels, which respectively enter the balanced receiver together with the echo signal received by the circulator after passing through different band filters.

The method of the present invention will be specifically described below based on the implementation system of the above-mentioned electro-optical modulation optical carrier frequency modulation continuous wave radar.

A frequency-modulated continuous wave lidar ranging method based on double heterodyne mixing, comprising:

Step 1: Input the linearly swept frequency video signal to the phase modulator, and use the phase modulator to modulate the linearly swept frequency radio frequency signal to the laser signal output by the laser to generate a swept frequency optical frequency comb, which is coupled by 90:10. After the detector, it is divided into probe light and reference light;

Step 2: The probe light passes through the circulator and is emitted into the space to be measured through the collimator, and the echo signal is received and returned to the circulator through the collimator, and the echo signal is received at the 3 ports of the circulator; different frequencies in the probe light The optical path of the light passing through is the same, and the wavelength difference is very small, and the influence of dispersion can be ignored, so the phase noise of light of different wavelengths is the same.

Step 3: The reference light is divided into two paths after frequency shifting by the acousto-optic modulator driven by a fixed frequency, and the two paths of light enter different optical filter 1 and optical filter 2, respectively, to filter out two optical combs with different frequencies;

Step 4: At the same time, the echo signal is divided into two bundles by the fiber coupler 3, and the beat frequency is performed with the filtered light waves of different frequencies. Due to the limited bandwidth of the balanced receiver, the beat frequency signal output by the balanced receiver is only the filtered frequency. The beat frequency of the light and the probe light of the corresponding frequency. Balanced receiver 1 and balanced receiver 2 are used to receive the beat signal respectively. Taking the light of the 0th and 1st order frequencies as an example, the beat signals of the two balanced receivers are:

代表激光器的相位噪声，f_A代表声光调制器的移频频率，γ代表扫频速度，τ代表反射光和参考光路径的延时差，R代表反射点的反射率，A代表光强度，其中01、02分别对应0阶梳齿移频前和移频后的强度，11、12分别对应1阶梳齿移频前和移频后的强度。in,

represents the phase noise of the laser, f _A represents the frequency-shift frequency of the acousto-optic modulator, γ represents the frequency sweep speed, τ represents the delay difference between the reflected light and the reference optical path, R represents the reflectivity of the reflection point, A represents the light intensity, Among them, 01 and 02 correspond to the intensity of the 0-order comb tooth before and after frequency-shifting, respectively, and 11 and 12 correspond to the intensity of the first-order comb tooth before and after frequency-shifting, respectively.

Step 5: The beat frequency signal received by the two balanced receivers is subjected to electrical beat frequency again, which can cancel the phase noise of the laser itself and obtain more accurate distance information. At this time, the beat frequency signal can be expressed as:

Among them, f _τ =γτ represents the frequency of the beat signal, and the frequency information includes the information of time delay. The actual distance of the measurement target can be calculated from the time delay and the speed of light. R represents the reflectivity of the reflection point, A ₀₁ , A ₀₂ , A ₁₁ , and A ₁₂ all represent light intensity.

已经被消除。As can be seen from the final beat signal formula, the phase noise term

has been eliminated.

Referring to FIG. 3 , comparing the power spectral density of the ranging signal before and after eliminating phase noise, it can be seen that the measurement result is very poor when there is phase noise, and the accuracy is significantly improved after processing by the method disclosed in this embodiment.

In some embodiments, a fractional frequency division phase-locked loop is used to generate a swept-frequency radio frequency signal with low phase noise, which can improve the ranging accuracy.

Preferably, by selecting an optical comb with a larger order difference by means of the filter setting range and increasing the modulation depth, the secondary measurement accuracy can be improved.

Preferably, increasing the number of cascaded phase modulators can further increase the modulation depth, broaden the spectral range of the optical frequency comb, and thus increase the measurement accuracy.

Specifically, the parameters of the two balanced receivers in this embodiment are the same.

In this embodiment, the electro-optical modulator may use an intensity modulator in addition to the phase modulator.

In some embodiments, the phase information of the measured light wave can be obtained through the Fourier transform spectrum of the echo signal. When the distance of the target object changes slightly, since the frequency measurement result does not change, but the phase of the frequency point will change accordingly. By measuring the phase information of the light wave, the tiny displacement of the target can be sensed, so that the ranging accuracy can be further improved.

To sum up, compared with the prior art, the FM continuous wave laser radar ranging method based on double heterodyne mixing provided by the embodiment of the present invention has the following advantages:

1. No complex laser feedback control is required, which can reduce costs and improve measurement accuracy.

2. The current technology of the phase-locked loop circuit board is mature, the cost is low, and the phase-locking effect is excellent, which avoids the complicated control of the laser itself.

3. The phase noise of the laser can be offset by the beat frequency, and accurate distance measurement can be achieved even if the laser with poor phase noise and wide line width is used.

4. It has good scalability. By increasing the number of cascaded electro-optic modulators, the frequency sweep range can be doubled, thereby improving the measurement accuracy.

The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other. As for the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant part can be referred to the description of the method.

The above description of the disclosed embodiments enables 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 implemented in 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,

representing the phase noise of the laser, f_ARepresenting 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 is_τγ 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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