Pulse modulation type laser frequency modulated continuous wave based super-resolution ranging method and system

Abstract

The application discloses a super-resolution ranging method and system based on pulse modulation type laser frequency modulation continuous wave, which comprises a light source generating device, a processing device, an adjustable optical delay line, an interference device, a signal acquisition device and a data processing device; the light source generating device is used for outputting a continuous light source; the processing device is used for processing the continuous light source; the adjustable optical delay line is used for scanning and finding a peak value of a pulse autocorrelation curve; the interference device is used for dividing the continuous light source processed by the processing device into two paths to generate interference and output; the signal acquisition device is used for signal acquisition, analysis and storage; and the data processing device is used for calculating a ranging result. Compared with a conventional light frequency modulation continuous wave ranging system, the application can break through the limitation of Fourier transform spectral resolution, realize super-resolution ranging, and meanwhile, the characteristics of low detection bandwidth of the light frequency modulation continuous wave ranging system are maintained.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of laser ranging, and particularly relates to an ultra-resolution ranging method and system based on a pulse modulation type laser frequency modulated continuous wave. BACKGROUND

[0002] Traditional laser ranging technology directly detects the flight time of a laser pulse. In order to achieve high-precision measurement, a high-speed receiver is often needed. Optical frequency modulated continuous wave (OFMCW) ranging technology uses a frequency modulated continuous wave laser as a light source, and uses a photodetector to receive a beat signal generated by a non-balanced Mach-Zehnder interference structure to demodulate distance information to be measured. The OFMCW ranging technology has the characteristics of low power and low receiving bandwidth while achieving high-resolution measurement.

[0003] Compared with traditional microwave ranging technology, the carrier frequency of laser ranging technology is much higher than the microwave frequency, the carrier cycle is smaller, and more information can be carried. In addition, laser ranging technology has the characteristics of good directivity, good monochromaticity, and strong anti-interference ability, and can achieve higher resolution and accuracy of target distance measurement.

[0004] In the OFMCW ranging technology, the output signal of the photodetector can be expressed as

[0005] i b ~cos{2π[F s τ d t / T s +ν0τ d -f s τ d 2 / (2T s )]},

[0006] wherein τ d is the delay difference of the two arms of the non-balanced Mach-Zehnder interference structure, F s , ν0, T s are the sweep range, initial frequency, and sweep duration of the linear sweep light source, and t is time. From the above formula, it can be seen that the frequency of the beat signal is:

[0007] f b =F s τ d / T s ,

[0008] When the parameters of the sweep light source are fixed, the frequency of the beat signal is proportional to the delay τ d , so the position information can be demodulated from the beat signal according to the known system parameters.

[0009] In order to extract beat frequency information in the coherent waveform, after the output signal of the photoelectric detector is collected by the oscilloscope, the fast Fourier transform (FFT) is usually used to analyze the beat frequency signal, and the frequency spectrum resolution Δf obtained is inversely proportional to the signal length T s The spatial resolution of the OFMCW system ranging can be represented as

[0010]

[0011] Wherein, c is the speed of light.

[0012] It can be seen that the larger the sweep range F s of the OFMCW system, the finer the spatial resolution Δx, and the higher the measurement accuracy. However, due to the limitation of the bandwidth of the photoelectric device and other factors, it is very difficult to greatly improve the sweep range while maintaining the quality of the sweep laser (such as the line width, sweep linearity, etc.), which also limits the improvement of the ranging accuracy of the OFMCW system. SUMMARY

[0013] The present application provides a kind of based on pulse modulation type laser frequency modulated continuous wave (PT-OFMCW) super-resolution ranging method and system, the beat frequency signal power pulse autocorrelation peak is found by periodically pulse type modulation to OFMCW and scanning the delay of reference road, to calculate the accurate distance of the object to be measured, realize super-resolution ranging.

[0014] To achieve the above object, the present application provides the following scheme:

[0015] The super-resolution ranging system based on pulse modulation type laser frequency modulated continuous wave includes: light source generating device, processing device, adjustable optical delay line, interference device, signal acquisition device and data processing device;

[0016] The light source generating device is connected with the processing device, and the light source generating device is used to output continuous light source;

[0017] The processing device is also connected with the interference device, and the processing device is used to process the continuous light source;

[0018] The adjustable optical delay line is connected with the interference device, and the adjustable optical delay line is used to scan and find the peak value of pulse autocorrelation curve;

[0019] The interference device is also connected with the signal acquisition device, and the interference device is used to divide the continuous light source into two paths based on the continuous light source processed by the processing device, and the interference device is used to generate interference and output.

[0020] The signal collection device is used for signal collection, analysis and storage.

[0021] The data processing device is connected with the signal collection device, and the data processing device is used for calculating a ranging result.

[0022] Preferably, the light source generating device comprises a linear sweep light source.

[0023] The linear sweep light source is used for outputting a continuous light source with a linearly changing light frequency over time.

[0024] Preferably, the processing device comprises a pulse generator and an intensity modulator.

[0025] The pulse generator is connected with the intensity modulator, and the pulse generator is used for generating a pulse sequence.

[0026] The intensity modulator is used for cutting the continuous light source based on the pulse sequence to generate a linear sweep light pulse sequence.

[0027] Preferably, a sweep frequency range of the linear sweep light source is equal to a repetition frequency of the pulse generated by the pulse generator.

[0028] Preferably, the interference device comprises a coupler and an unbalanced Mach-Zehnder interference structure.

[0029] The coupler comprises a first coupler and a second coupler; the first coupler is connected with the intensity modulator and the unbalanced Mach-Zehnder interference structure, and the first coupler is used for dividing the linear sweep light pulse sequence into two paths to obtain a reference path linear sweep light pulse sequence and a signal path linear sweep light pulse sequence.

[0030] The unbalanced Mach-Zehnder interference structure is also connected with the second coupler, and the unbalanced Mach-Zehnder interference structure comprises a reference path and a signal path; the signal path comprises a circulator and a collimating mirror.

[0031] The circulator is used for controlling the signal path linear sweep light pulse sequence to be transmitted in a fixed direction.

[0032] The collimating mirror is connected with the circulator, and the collimating mirror is used for emitting and receiving the signal path linear sweep light pulse sequence.

[0033] The second coupler is used for the reference path linear sweep light pulse sequence and the signal path linear sweep light pulse sequence to interfere and be averagely divided into two paths for output.

[0034] Preferably, the signal collection device comprises a balanced detector and an oscilloscope.

[0035] The balance detector is connected with the oscilloscope, and the balance detector is used for converting the optical beat signal into an electrical signal and transmitting the electrical signal to the oscilloscope.

[0036] The oscilloscope is used for collecting, analyzing and storing the electrical signal.

[0037] Preferably, the first coupler adopts an unbalanced coupler.

[0038] Preferably, the second coupler adopts a 50:50 coupler.

[0039] The application also provides an ultrahigh-resolution ranging method based on a pulse-modulated laser frequency-modulated continuous wave, comprising the following steps:

[0040] S1, generating a continuous light, wherein the continuous light is a continuous light with a linear change in optical frequency period over time;

[0041] S2, cutting the continuous light to obtain a linear sweep light pulse sequence, and dividing the linear sweep light pulse sequence into two paths to obtain a reference linear sweep light pulse sequence and a signal linear sweep light pulse sequence;

[0042] S3, interfering the reference linear sweep light pulse sequence and the signal linear sweep light pulse sequence, and dividing the interference signal into two paths for output;

[0043] S4, collecting, storing and analyzing the two interference signals to obtain a beat frequency and a power;

[0044] S5, drawing a power-delay curve based on the power, judging whether the power-delay curve has covered a peak value of a pulse autocorrelation curve, if not, recycling from S1 by adjusting a delay amount of a variable optical delay line, and if yes, entering S6;

[0045] S6, fitting the power-delay curve to obtain a ranging result.

[0046] The application has the following beneficial effects:

[0047] Compared with a common OFMCW ranging system, the application uses a pulse-modulated light frequency-modulated continuous wave to replace the original OFMCW as a detection light source, and further adds a variable optical delay line (VODL) in a reference arm, thereby breaking the limitation of Fourier transform spectral resolution and realizing ultrahigh-resolution ranging due to the narrow full width at half maximum of the pulse, while maintaining the low detection bandwidth characteristic of the OFMCW ranging system. BRIEF DESCRIPTION OF DRAWINGS

[0048] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a schematic diagram of the super-resolution ranging system based on pulse-modulated laser frequency-modulated continuous wave of the present invention;

[0050] Figure 2 This is a schematic diagram of the super-resolution ranging method based on pulse-modulated laser frequency-modulated continuous wave of the present invention;

[0051] Figure 3 This is a schematic diagram illustrating the principle of the present invention;

[0052] Figure (a) is a schematic diagram of frequency scanning; Figure (b) is a schematic diagram of the power waveform of the frequency sweep pulse; Figure (c) is a schematic diagram of the change of beat frequency power with delay; Figure (d) is a schematic diagram of the super-resolution ranging principle.

[0053] Figure 4 Example image of super-resolution ranging experiment results;

[0054] Figure 5 This is a schematic diagram comparing the measurement values ​​of the pulse-modulated laser frequency-modulated continuous wave super-resolution ranging system and the optical frequency-modulated continuous wave ranging system of the present invention. Detailed Implementation

[0055] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0056] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0057] Example 1

[0058] like Figure 1 As shown, the super-resolution ranging system based on pulse-modulated laser frequency-modulated continuous wave of the present invention includes: a light source generating device, a processing device, an adjustable optical delay line, an interferometer, a signal acquisition device, and a data processing device.

[0059] The light source generating device is connected with the processing device, and the light source generating device comprises a linear sweep light source which periodically outputs continuous light with a linearly changing light frequency over time; the processing device comprises a pulse generator and an intensity modulator, the intensity modulator being connected with the pulse generator and the linear sweep light source; the pulse generator inputs a pulse sequence generated by the pulse generator into the intensity modulator, cuts the continuous light output by the linear sweep light source, and generates a linear sweep light pulse sequence. The sweep frequency range of the linear sweep light source is equal to the pulse repetition frequency.

[0060] The processing device is further connected with an interference device, and the interference device comprises a coupler and an unbalanced Mach-Zehnder interference structure; the coupler comprises a first coupler and a second coupler; the first coupler is connected with the intensity modulator and the unbalanced Mach-Zehnder interference structure, and the first coupler is used for dividing the linear sweep light pulse sequence into two paths to obtain a reference linear sweep light pulse sequence and a signal linear sweep light pulse sequence; the second coupler is connected with the unbalanced Mach-Zehnder interference structure and a signal acquisition device, and the second coupler is used for interference between the reference linear sweep light pulse sequence and the signal linear sweep light pulse sequence and average distribution of the two sequences into two paths for output.

[0061] The unbalanced Mach-Zehnder interference structure comprises a reference path and a signal path, the reference path is connected with an adjustable optical delay line; the signal path comprises a circulator and a collimator; the circulator is used for controlling transmission of the signal linear sweep light pulse sequence in a fixed direction; the collimator is connected with the circulator, and the collimator is used for emitting and receiving the signal linear sweep light pulse sequence.

[0062] In view of the fact that spatial light emission and target reflection in the signal path will cause large loss, the first coupler is an unbalanced coupler, and the optical power allocated to the signal path is appropriately increased to obtain a stronger beat signal; the second coupler is a 50:50 coupler. The linear sweep light pulse sequence is input into the first coupler through the intensity modulator, the first coupler divides the linear sweep light pulse sequence into two paths and sends the two paths into two interference arms of the unbalanced Mach-Zehnder structure; the reference linear sweep light pulse sequence passes through the adjustable optical delay line and enters the second coupler; the signal linear sweep light pulse sequence passes through the circulator and the collimator in sequence, is reflected by the surface of a to-be-measured object, and then enters the second coupler through the collimator and the circulator. The reference linear sweep light pulse sequence and the signal linear sweep light pulse sequence interfere with each other in the second coupler, and then are evenly distributed into two paths.

[0063] The interference device is further connected with a signal acquisition device, and the signal acquisition device comprises a balanced detector and an oscilloscope; the balanced detector is connected with the second coupler and the oscilloscope; the two ports of the balanced detector receive two paths of light respectively, and convert the light beat signal into an electrical signal which is transmitted to the oscilloscope for storage and analysis processing.

[0064] The data processing device is connected with the signal collecting device, and the data processing device is used for calculating the ranging result.

[0065] Wherein, according to actual needs, the adjustable optical delay line or its accessories need to have the function of recording the delay change amount, and the oscilloscope can be replaced by other signal collecting and processing instruments.

[0066] Embodiment two

[0067] As Figure 2 shown, the application also provides a kind of super-resolution ranging method based on pulse modulation type laser frequency modulation continuous wave, comprising the following steps:

[0068] S1, generates a continuous light, and the continuous light is continuous light with linearly time-varying optical frequency periodicity, as shown in Figure 3 (a);

[0069] In this embodiment, the sweep range of linear sweep light source is F s =1GHz, and the repetition period is T s =2ms, at this time, FFT spectrum resolution Δf=500Hz.

[0070] S2, the continuous light is cut to obtain a linear sweep light pulse sequence, as shown in Figure 3 (b);

[0071] The repetition period of pulse sequence is set to T p =1 / F s , in this embodiment, T p =1ns, at this time, the frequency difference of adjacent pulses of pulse sequence is Δf p =Δf=500Hz.

[0072] The linear sweep light pulse sequence is divided into two paths to obtain reference linear sweep light pulse sequence and signal linear sweep light pulse sequence;

[0073] Reference linear sweep light pulse sequence and signal linear sweep light pulse sequence are represented as:

[0074] s ref (t)=A T (t)cos{2π[ν0+F s t / (2T s )]t},

[0075] s sig (t)=A T (t-τ d )cos{2π[ν0+F s (t-τ d ) / (2Ts )](t-τ d )}.

[0076] wherein A T (t) =∑ k A p (t-kT p ) is the modulation of the amplitude of the continuous wave light by the pulse sequence, where A p (t) is the amplitude modulation function of a single pulse period, k is an integer; v0 is the initial frequency of the linearly swept light source; τ d represents the delay difference between the signal path and the reference path; t represents time.

[0077] S3, interfering the linearly swept light pulse sequence of the reference path and the linearly swept light pulse sequence of the signal path, and dividing the interfered signal into two paths;

[0078] S4, collecting, storing and analyzing the two interfered signals to obtain the beat frequency and power;

[0079] The output of the balanced detector can be expressed as:

[0080]

[0081] wherein f b represents the beat frequency; meanwhile is the optical path difference between adjacent pulses at the second coupler, and n is a positive integer;

[0082] The power of the beat signal can be expressed as:

[0083] P b (τ d ) ~ [∫A T (t-τ d )A T (t)dt] 2

[0084] From the above formula, when τ d changes, the beat power changes in the form of the square of the autocorrelation function of A T (t). Meanwhile, due to the periodicity of the pulse sequence, when the delay τ d changes, the autocorrelation peak will appear repeatedly with a period of T p , as shown in Figure 3 (c). And since Δf p = Δf, when the beat frequency is an integer multiple of Δf, the beat power is exactly the peak value of the autocorrelation curve, that is, when the beat power reaches the peak value, the beat frequency read from the FFT spectrum is accurate and is not limited by the resolution of the Fourier transform;

[0085] S5, forFigure 3 The distance to be measured τ in (c) d = τ0, based on the power, a power-delay curve is plotted as Figure 3 (d) shown, the horizontal axis is the change amount Δτ of the adjustable optical delay line delay VODL , and the vertical axis is the beat signal power obtained from the FFT spectrum; it is determined whether the power-delay curve has covered A T (t) the autocorrelation curve peak value; if not, the delay amount of the adjustable optical delay line is adjusted, and the cycle is repeated from S1; if yes, S6 is entered;

[0086] S6, the power-delay curve plotted is fitted to obtain the ranging result.

[0087] When the beat power reaches the autocorrelation curve peak value, the frequency of the peak on the FFT spectrum is f b =nΔf, wherein n is a positive integer, representing the nth grid of the single sideband FFT spectrum;

[0088] Further, the delay at the object to be measured is obtained as:

[0089]

[0090] , wherein is the size of Δτ b when P VODL reaches the peak value;

[0091] Further, the distance of the object to be measured is obtained as:

[0092] d=cτ0 / 2

[0093] , wherein c represents the speed of light.

[0094] The present application introduces pulse modulation in the OFMCW ranging system, so that the spatial resolution can be represented as:

[0095] Δx b =cτ b / 2

[0096] , wherein τ b is the full width at half maximum (FWHM) of a single peak of P b (τ d ), and its value is proportional to the FWHM (τ p ) of the single pulse power envelope in A T (t) on the premise that the pulse shape is unchanged. The ratio of the spatial resolution of the present application to that of the conventional OFMCW ranging is r=Δx b / Δx=τ b F s, which is the resolution-enhanced coefficient. It should be noted that when the method is used for measurement, the step size of the adjustable optical delay line should be less than half of τ b to avoid loss of spatial resolution.

[0097] In the case of a linear swept source with a sweep range F s = 1 GHz and a sweep period T s = 2 ms, the spatial resolution of the conventional OFMCW ranging corresponds to Δx = 15 cm; let the pulse repetition period T p = 1 ns and the input clock frequency of the pulse generator be 11 GHz. The object to be measured is a retroreflector. By adjusting the delay difference between the reference path and the measurement path and recording the power value of the FFT spectrum frequency peak, a relationship curve can be plotted as shown in Figure 4 , where f b = 21.0045 MHz when it reaches the highest point, and n = 42009 is calculated. The data is fitted using a quadratic function, and from the graph, it can be seen that τ0 = 6301.396 m is obtained. In addition, the spatial resolution of the present embodiment is calculated to be Δx b = 1.46 cm, which is about 1 / 10 of the spatial resolution of the OFMCW system with the same sweep range, and r = Δx b / Δx = 0.0973 is obtained.

[0098] As shown in Figure 5 , the object to be measured is moved by 1.5 cm as a step size, and 29 different positions are measured using the method, and the measurement root mean square error is 0.198 cm, which is about 1 / 75 of the spatial resolution of the OFMCW system with the same sweep range, and the ranging accuracy is obviously improved.

[0099] The above-described embodiments are only descriptions of the preferred modes of the present application and do not limit the scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application as defined by the claims.

Claims

1. A super-resolution ranging system based on pulse-modulated laser frequency-modulated continuous wave, characterized in that, The application relates to a linear sweep frequency light source and a method for generating the linear sweep frequency light source. The application comprises a light source generating device, a processing device, an adjustable light delay line, an interference device, a signal collecting device and a data processing device. The light source generating device is connected with the processing device, and the light source generating device is used for outputting continuous light sources. The processing device is further connected with the interference device, and the processing device is used for processing the continuous light sources; the processing device comprises a pulse generator and an intensity modulator. The pulse generator is connected with the intensity modulator, and the pulse generator is used for generating a pulse sequence. The intensity modulator is used for cutting the continuous light sources based on the pulse sequence to generate a linear sweep frequency light pulse sequence. The tunable optical delay line is connected to the interference device, and the tunable optical delay line is used to scan and find the peak value of the pulse autocorrelation curve; when the delay difference τ between the signal path and the reference path... d When changes occur, the beat frequency power is expressed in A T (t) The form of the square of the autocorrelation function changes, where A T (t) represents the modulation function of the pulse signal on the amplitude of the frequency-modulated continuous light; changing the delay difference τ d At that time, the autocorrelation peak is T p For periodic repetition, the beat frequency power reaches the peak of the autocorrelation curve when the beat frequency is an integer multiple of the FFT spectral resolution Δf; where the autocorrelation curve is a power-delay curve, and the horizontal axis represents the change in delay Δτ of the tunable optical delay line. VODL The vertical axis represents the beat frequency signal power obtained from the FFT spectrum; The interference device is further connected with the signal collecting device, and the interference device is used for dividing the continuous light sources processed by the processing device into two paths to generate interference and output. The signal collecting device is used for signal collection, analysis and storage. The data processing device is connected with the signal collecting device, and the data processing device is used for calculating a ranging result.

2. The super-resolution range finding system based on pulse modulation type laser frequency modulated continuous wave of claim 1, wherein, The light source generating device comprises a linear sweep frequency light source. The linear sweep frequency light source is used for outputting continuous light sources with linearly changed light frequencies over time.

3. The super-resolution range finding system based on pulse modulation type laser frequency modulated continuous wave of claim 1, wherein, The sweep frequency range of the linear sweep frequency light source is equal to the repetition frequency of the pulse generated by the pulse generator.

4. The super-resolution range finding system based on pulse modulation type laser frequency modulated continuous wave of claim 1, wherein, The interference device comprises a coupler and an unbalanced Mach-Zehnder interference structure. The coupler comprises a first coupler and a second coupler; the first coupler is connected with the intensity modulator and the unbalanced Mach-Zehnder interference structure, and the first coupler is used for dividing the linear sweep frequency light pulse sequence into two paths to obtain a reference linear sweep frequency light pulse sequence and a signal linear sweep frequency light pulse sequence. The unbalanced Mach-Zehnder interference structure is further connected with the second coupler, and the unbalanced Mach-Zehnder interference structure comprises a reference path and a signal path; the signal path comprises a circulator and a collimating mirror. The circulator is used for controlling the signal linear sweep frequency light pulse sequence to be transmitted along a fixed direction. The collimating mirror is connected with the circulator, and the collimating mirror is used for emitting and receiving the signal linear sweep frequency light pulse sequence. The second coupler is used for the reference linear sweep frequency light pulse sequence and the signal linear sweep frequency light pulse sequence to generate interference and be averagely divided into two paths to be output.

5. The super-resolution range finding system based on pulse modulation type laser frequency modulated continuous wave of claim 1, wherein, The signal collecting device comprises a balanced detector and an oscilloscope. The balanced detector is connected with the oscilloscope, and the balanced detector is used for converting an optical beat signal into an electric signal and transmitting the electric signal to the oscilloscope. The oscilloscope is used for collecting, analyzing and storing the electric signal.

6. The super resolution range finding system based on pulse modulation type laser frequency modulated continuous wave according to claim 4, characterized in that, The first coupler adopts an unbalanced coupler.

7. The super resolution range finding system based on pulse modulation type laser frequency modulated continuous wave (FMCW) of claim 4, wherein, The second coupler adopts a 50:50 coupler.

8. The method for super-resolution ranging based on pulse modulation type laser frequency modulated continuous wave, characterized in that, The application further discloses a method for generating the linear sweep frequency light source. S1, a continuous light is generated, and the continuous light is continuous light with periodically linearly changed light frequencies over time; S2, the continuous light is cut to obtain a linear sweep frequency light pulse sequence, and the linear sweep frequency light pulse sequence is divided into two paths to obtain a reference linear sweep frequency light pulse sequence and a signal linear sweep frequency light pulse sequence. S3, interfering the reference linearly swept optical pulse sequence and the signal linearly swept optical pulse sequence, and dividing the interference signal into two paths; S4, collecting, storing and analyzing the two interference signals to obtain beat frequency and power; S5, drawing a power-delay curve based on the power, and determining whether the power-delay curve has covered the peak value of the pulse autocorrelation curve; if not, adjusting the delay amount of the adjustable optical delay line and recycling from S1; if yes, entering S6; S6, fitting the power-delay curve to obtain the ranging result.

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