A laser echo ranging method and system based on spread spectrum system

Abstract

The application discloses a kind of laser echo ranging method and system based on spread spectrum system, to realize low frequency, low peak power laser detection.The application designs correlation sequence in receiving end, improves echo signal peak discernibility, realizes long-distance, high-precision ranging.FPGA controller generates a piece of pseudo-random code, semiconductor laser emits light source of specific wavelength, is received by SPAD after target reflection, SPAD output electric pulse is handled by signal shaping circuit to obtain single-photon signal, which is read by FPGA controller, correlation operation is carried out on received echo photon and correlation sequence based on pseudo-random code using the correlation characteristics of pseudo-random code, and the distance of target is obtained.The laser echo signal processing method based on spread spectrum system proposed in the application has good performance in single-photon ranging accuracy, and the correlation sequence designed overcomes the influence of pulse width, improves echo signal peak discernibility, and realizes long-distance, high-precision ranging.

Description

Technical Field

[0001] This invention belongs to the field of laser signal processing technology, specifically relating to a laser echo ranging method and system based on spread spectrum. Background Technology

[0002] Single-photon lidar is a new type of lidar technology based on weak light detection, which can realize the detection and counting of single photons, thereby enabling long-distance measurement.

[0003] Because photon detectors have high sensitivity, laser ranging using photon counting methods requires a low background noise environment to count the echo photons. Therefore, research was conducted on time-correlated photon counting methods to fully utilize the correlation characteristics of target photons and effectively extract target echo photons mixed with noise. When the ranging system acquires the echo signal of a high-frequency pulse, due to the long measurement distance and weak echo signal, a single pulse train can only receive approximately one target echo photon. The acquisition results of multiple pulse trains are then summed to obtain a photon count distribution histogram. Furthermore, both target photons and noise photons can cause avalanche effects in the detector. The random triggering of avalanche effects by noise photons results in weak time correlation and a uniform distribution within the detection time interval. In contrast, target echo photons exhibit strong time correlation; therefore, appropriate signal processing methods can effectively avoid noise interference.

[0004] As the operating distance increases, single-photon detectors need to accumulate more photons to achieve a high signal-to-noise ratio. Under the same signal-to-noise ratio conditions, ranging distant targets requires more sampling time, i.e., a longer photon accumulation time, which to some extent limits the maximum operating distance of single-photon detectors.

[0005] Increasing the repetition rate of the emitted laser can accumulate more echo photons within a certain time, thereby improving the signal-to-noise ratio and enhancing distance detection capabilities. On the one hand, high-repetition-rate lasers are expensive, and increasing the repetition rate reduces the pulse period, leading to distance ambiguity when ranging from distant targets. On the other hand, when the pulse period is less than the dead time, once the detector is excited and trapped in the dead time, it cannot detect the returning photons, rendering the higher repetition rate ineffective. Therefore, it is necessary to explore single-photon ranging under low repetition rate and low power conditions.

[0006] Since the maximum unambiguous distance is finite at a single ranging frequency, applying coding demodulation theory to ranging technology can solve the problem of distance ambiguity. A pseudo-random sequence is used as a pulse compression signal to modulate the transmitted signal, and at the receiving end, the measured signal is correlated with a reference signal. Currently, single-photon counting systems using pseudo-random code modulation obtain the time of flight by distinguishing different symbol return information. Their ranging accuracy is highly dependent on the repetition frequency of the transmitted pulse. However, neither the threshold method nor the peak compensation method used can obtain a more accurate round-trip time for the ultrasonic signal. There is no in-depth study and analysis of the characteristics of the target's returned information within a detection period, resulting in a large error between the characteristics of the returned information and the theoretical value.

[0007] On the other hand, methods for processing directly detected echo signals include peak methods and centroid methods. These methods utilize the weak time correlation of noise photons and the strong time correlation of target echo photons to obtain echo information, but they ignore the influence of pulse width on error. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a laser echo ranging method and system based on spread spectrum, so as to solve the defects in laser array signal processing in the prior art and thus improve the performance of single-photon ranging.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A laser echo ranging method based on spread spectrum technology includes the following steps:

[0011] S1, the FPGA controller generates a pseudo-random code, which modulates the semiconductor laser to emit laser light. Based on the pulse width information of the laser, the pseudo-random code is expanded to obtain an expanded pseudo-random code.

[0012] S2, after the laser is collimated, it is reflected by the target and received by the single-photon detector; the single-photon detector sends a signal to the FPGA based on the received laser echo; the FPGA controller obtains the laser return time and then outputs the time interval between the laser emission and return times, and the multiple time intervals obtained repeatedly are used to form an echo sequence; the cumulative photon histogram is obtained by statistics.

[0013] S3. When the photon accumulation histogram and the extended pseudo-random code are synchronized and the time delay is stable, perform correlation calculation on the extended pseudo-random code and the echo sequence to obtain the correlation value. Based on the correlation value, obtain the detection distance.

[0014] A further improvement of the present invention is that:

[0015] Preferably, in S1, the formula for expanding the pseudo-random code using the laser-based pulse width signal is as follows:

[0016]

[0017] Among them, t bin τ is the minimum time resolution of the FPGA controller. width Let PR be the full width at half maximum (FWHM) of the laser pulse waveform, and PR be a randomly generated 1×N... PR The pseudo-random code, N PR Let PR be the length of the pseudo-random code PR, and ExPR be the extended 1×N. PR N i The pseudo-random code, N i This represents the number of time intervals occupied by a single pulse cycle on the FPGA controller side.

[0018] Preferably, in S2, the specific process by which the FPGA controller obtains the laser return time is as follows:

[0019] The FPGA records the photon return time index within the entire pseudo-random code period in time intervals. The simulation obtains the photogenerated electron value in each time interval and determines whether the photogenerated electron value exceeds the detection threshold of the single-photon detector. If the single-photon detector is excited, the FPGA obtains the laser return time.

[0020] Preferably, the formula for calculating the number of photogenerated electrons in each time interval is:

[0021]

[0022] Where j is the position of the target time interval, η is the quantum efficiency of the detector, and p(t) is the normalized laser pulse.

[0023] Preferably, the formula for calculating the laser pulse is:

[0024]

[0025] Where n is a parameter, the formula for normalizing the laser pulse is:

[0026] make but

[0027] Preferably, in S2, the return time of the laser includes the time jitter when the single-photon detector is excited.

[0028] Preferably, in S2, the photons in the laser light received by the single-photon detector include reflected signal photons, background noise photons, and dark current noise photons.

[0029] Preferably, in S3, the formula for correlation calculation is:

[0030]

[0031] Where N is the number of cycles, ExPR is the extended pseudo-random code, I is the cumulative histogram of reflected photons, and τ is the time delay.

[0032] Preferably, the formula for calculating the detection distance is:

[0033]

[0034] Where c is the speed of light and γ is the correlation value.

[0035] A laser echo ranging system based on spread spectrum technology includes:

[0036] The pseudo-random code module is used by the FPGA controller to generate a pseudo-random code. The pseudo-random code modulates the laser emitted by the semiconductor laser. Based on the pulse width information of the laser, the pseudo-random code is expanded to obtain an expanded pseudo-random code.

[0037] The photon accumulation module is used to reflect the laser after collimation to the target and receive it through a single-photon detector. The single-photon detector sends a signal to the FPGA based on the received laser echo. The FPGA controller obtains the laser return time and outputs the time interval between the laser emission and return times. The multiple time intervals obtained repeatedly are used to form an echo sequence. The photon accumulation histogram is obtained by statistics.

[0038] The detection range module is used to perform correlation calculations on the extended pseudo-random code and the echo sequence when the photon accumulation histogram and the extended pseudo-random code are synchronized and the time delay is stable, to obtain the correlation value, and to obtain the detection range based on the correlation value.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] This invention discloses a laser echo ranging method based on spread spectrum to achieve low-frequency, low-peak-power laser detection. At the receiving end, this invention utilizes the orthogonality of the spreading code, considers the laser pulse width factor, and designs a correlation sequence to improve the peak resolution of the echo signal, achieving long-distance, high-precision ranging. An FPGA controller generates an n-order M-sequence pseudo-random code. A semiconductor laser emits a light source of a specific wavelength, which is reflected by the target and received by a SPAD. The SPAD output electrical pulse is processed by a signal shaping circuit to obtain a single-photon signal, which is read by the FPGA controller. Using the correlation characteristics of the pseudo-random code, correlation calculations are performed on the received echo photons and the correlation sequence based on the pseudo-random code to obtain the target distance. The effective signal returned by the target received by the laser detector lens inevitably contains noise such as spatial noise. The effective signal and various noises together form the total signal. After processing the total signal using a photon counting method, the photon signal is extracted and analyzed using correlation detection technology to obtain the laser beam flight time, thus calculating the measured distance.

[0041] Data obtained through experimental simulations demonstrate that the laser echo signal processing method based on spread spectrum proposed in this invention achieves excellent performance in single-photon ranging accuracy. Simulations of errors under different repetition frequencies and pulse widths show the potential of spread spectrum in single-photon ranging. Compared to previous research in this field, where ranging accuracy is highly dependent on the transmission repetition frequency, this invention focuses on high-precision detection at low frequencies and low peak power, while also considering the impact of pulse width on the results. By utilizing the orthogonality of extended pseudo-random codes to extract photon information, and designing correlation sequences to overcome the influence of pulse width, the peak resolution of the echo signal is improved, achieving long-distance, high-precision ranging. Attached Figure Description

[0042] Figure 1 This is a block diagram of a pseudo-random code modulated lidar ranging system.

[0043] Figure 2 The time-domain waveform of the pseudo-random code;

[0044] Figure 3 To detect the cumulative histogram of events;

[0045] Figure 4 Autocorrelation curves for extended pseudo-random codes;

[0046] Figure 5 For cross-correlation histograms;

[0047] Figure 6 Error plots at different repetition frequencies;

[0048] Figure 7 Error graphs for different pulse widths;

[0049] Figure 8 This is a flowchart of a laser echo ranging method based on spread spectrum technology according to the present invention. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0051] In order to improve the accuracy of long-distance detection in single-photon ranging systems, pseudo-random code modulation laser ranging systems use laser pulse transmitters with high repetition rates. The time of flight is obtained by distinguishing the returned information of different symbols. Its ranging accuracy is highly dependent on the repetition rate of the transmitted pulse. High repetition rate lasers are not only expensive, but also require higher detection frequencies from the single-photon detector at the receiving end.

[0052] See Figure 1 The laser echo signal processing system of the present invention includes a main control unit, a transmitting unit, and a receiving unit. The main control unit includes an FPGA controller, the transmitting unit includes a laser and a laser collimation module, and the receiving unit includes a single-photon detector (SPAD) receiver. The FPGA controller sends a signal to the laser, and the laser emitted by the laser passes through the laser collimation module and is sent to the target object. The target object reflects the laser back to the single-photon detector (SPAD) receiver.

[0053] To address the aforementioned problems, this invention discloses a laser echo signal processing method based on a spread spectrum mechanism, see [link to relevant documentation]. Figure 8 The method includes the following steps:

[0054] Step 1: Set up the system. The FPGA controller records the start time of photons. First, it repeatedly generates an n-order M-sequence pseudo-random code as the start signal. Based on the start signal, the pseudo-random code modulates a semiconductor laser to emit a 1550nm wavelength light source. Then, based on the laser emitted by the semiconductor laser, using the laser pulse width information, the pseudo-random code is expanded into a correlation sequence without changing the correlation of the pseudo-random sequence. Specifically:

[0055] Laser pulses are not constant, and the emission waveforms of various lasers are also different. To adapt to various lasers and ranging systems, this invention proposes a correlation method to solve Time-of-Flight (TOF) information. Compared to the peak method, the correlation method can better utilize the characteristics of the laser emission waveform and improve ranging accuracy. Therefore, this invention utilizes laser pulse width information to expand the pseudo-random code as the correlation sequence without changing the correlation of the pseudo-random sequence. The process is as follows:

[0056]

[0057] Among them, t bin τ is the minimum time resolution of the FPGA controller. widthGiven the full width at half maximum (FWHM) of the laser pulse waveform, randomly generate 1×N... PR Pseudo-random codes PR, N PR The length of the pseudo-random code PR is expanded to 1×N. PR N i The pseudo-random code ExPR,N i The extended correlation sequence is the number of time intervals occupied by a single pulse cycle on the FPGA controller side. It can not only process multi-pulse laser return signals, but also overcome the influence of pulse width and reduce ranging error.

[0058] A pseudo-random code sequence modulated laser emitter emits a laser with energy E T A laser pulse, after being reflected by a smooth, diffusely reflective target (Lambertian target) at a distance R and a reflectivity of ρ, returns an average of S photons. P-total , can be represented as:

[0059]

[0060] in, P T It is the output laser power, τ width λ is the laser pulse width, h is Planck's constant, λ is the laser wavelength, c is the speed of light, FOV is the field of view of the receiving system, and θ is the laser pulse width. T It is the divergence angle of the emitted laser, θ target It is the angle between the laser beam and the normal direction of the target surface, A R It is the size of the receiving aperture of the receiving system, η T It is the efficiency of the launch system, η R It is the receiving efficiency of the receiving system, η A It is the atmospheric transport rate.

[0061] Step 2: A 1550nm wavelength laser is emitted by a semiconductor laser. After passing through a collimation module and being reflected by the target, the laser is received by a single-photon detector (SPAD). The SPAD outputs an electrical pulse signal as a termination signal. The FPGA controller records the interval between the start and end times to obtain the return time. After statistical analysis of multiple return times, a cumulative photon histogram is obtained.

[0062] Specifically, the SPAD detector receives the excited echo photon and transmits a signal to the FPGA controller. The return time index of the photon is recorded in time intervals bin. During simulation, the number of photogenerated electrons in each interval is calculated by R(i) to determine whether it exceeds the detection threshold that can excite the detector. If not, the detector does not enter the dead time and continues to detect. If the detector is excited, since there is time jitter every time the detector is excited, the FPGA records the SPAD detector pulse data, which includes not only the actual return time of the signal photon, but also the time jitter of the synchronization pulse, the detector, and the FPGA.

[0063] In the above process, the SPAD detector is excited by echo photons within a certain time interval, including signal photons reflected from the target, background noise photons, dark current noise photons, etc. Single-photon detectors lack photon number resolution; within a detection interval, regardless of whether the detector receives target-reflected photons, noise photons, or multiple photons simultaneously, it can only output one counting pulse to the FPGA controller, and its distribution can approximate a Poisson distribution. Due to time jitter and noise interference, the actual output pulse time has a certain error compared to the detector excitation time, necessitating the determination of the final target time based on the system's detection characteristics and parameters.

[0064] During the entire laser transmission process, the system noise mainly includes background stray light and internal dark current. Therefore, the total noise photoelectron count of the system can be expressed as:

[0065] N = N b +N d =ηK b +N d (3)

[0066] Where η is the quantum efficiency of the detector, and K b N represents the number of photons generated by background stray light. d This represents the dark count generated by the detector's own dark current. Typically, the background photon count and the dark count approximate a Poisson distribution.

[0067] The laser pulse model is shown below:

[0068]

[0069] Here, the parameter n determines the laser model; for a typical Q-switched laser, n=1 is suitable. For n=1, the relationship between the laser's half-width and τ is P FWHM =3.5τ, normalize the laser pulse:

[0070] make but

[0071] Assuming background noise and internal dark current noise are constant, define N as the total average excitation noise intensity, composed of background noise and dark current noise; S i Let t be the laser pulse reflection intensity in time interval i. bin Let be the minimum time resolution of the FPGA controller. In the absence of an echo signal, the generation of the primary electron is only related to background noise, and the probability of primary electron generation within time interval j is:

[0072] p j =exp[-N(j-1)τ bin ][1-exp(-Nτ bin (5)

[0073] Wherein, N is defined as the total average excitation noise intensity, which consists of background noise and dark current noise; S i Let t be the laser pulse reflection intensity in time interval i. bin This represents the minimum time resolution of the FPGA controller.

[0074] In the presence of an echo signal, the probability of primary electron generation in the SPAD detector is related to both the photons reflected from the target and the background noise. The probability of primary electron generation within time interval j is:

[0075] p j =exp[-N(j-1)τ bin ][1-exp(-(S i +N)τ bin (6)

[0076] Define N as the total average excitation noise intensity, composed of background noise and dark current noise; S i Let t be the laser pulse reflection intensity in time interval i. bin This represents the minimum time resolution of the FPGA controller.

[0077] Based on the probability p of the original electron production j And the original electron excitation probability p m A random number generator was constructed to handle two random processes: the generation and excitation of the primary electron, thereby determining whether the detector in the j-time interval is excited or not.

[0078] In a laser pulse, the number of laser photons in the i-th time interval is:

[0079]

[0080] Where, N L This represents the total number of laser photons in the laser pulse echo signal.

[0081] Assuming background noise and internal dark current noise are constant, and defining N as the total average excitation noise frequency, composed of background noise and dark current noise, then the number of echo photons in the i-th time interval during the entire detection process is:

[0082]

[0083] Where j represents the position of the target time interval, and η represents the quantum efficiency of the detector. Calculate the number of electrons generated in each time interval.

[0084] The generation of primary electrons is determined sequentially for each time interval according to the time axis: a Poisson distribution generator with the number of echo photoelectrons R(i) in that time interval as a parameter is used to generate random numbers. If the generated random number is greater than 0, it is considered that primary electrons have been generated in that time interval. Then, based on the time jitter characteristics of the detector, the time offset value of the detector output signal is randomly generated using the Poisson distribution. The photon flight time is obtained from the time interval index of the primary electron generation and the time offset value, thereby determining the target position.

[0085] Step 3: At the FPGA receiver, utilizing the orthogonality of the generated extended pseudo-random code, and considering the laser pulse width—the largest factor causing ranging errors—the peak resolution of the echo signal is improved, achieving long-distance, high-precision ranging. The strong correlation between the pseudo-random code pulse and the returned photons emitted by this unique laser source is utilized; the cross-correlation is as follows:

[0086]

[0087] Where N is the number of cycles, ExPR is the extended pseudo-random code, I is the cumulative histogram data of the detected return photon signal, and τ is the time delay.

[0088] When the histogram of returned photons detected by the FPGA changes synchronously with the extended pseudo-random code, and the time delay caused by timing jitter between the synchronization pulse, detector, and FPGA is constant, a correlation calculation can be performed on the extended pseudo-random code sequence and the received echo photon number sequence to observe the strong correlation peak max(γ) in the measurement. The detection distance is:

[0089]

[0090] Example 1

[0091] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0092] like Figure 5 As shown, the laser echo signal processing method using spread spectrum requires five steps: parameter setting, original electron number generation, time axis bin-based calculation of echo signal photon number, SPAD detector excitation, and data processing. Figure 2 The time-domain waveform of the pseudo-random code is shown. Table 2 shows the system simulation parameter settings. After simulating laser pulse generation, primary electron number generation, and time-axis bin-based calculation of echo signal photon count, the SPAD detector is excited. The interval between the SPAD detector's excitation time and the pulse emission start time is recorded to obtain the photon flight time. The cumulative histogram of detection events is then calculated, as shown below. Figure 3 As shown. Based on the designed pseudo-random code, the augmented correlation sequence is obtained, and its good autocorrelation is shown in the figure. Figure 4 As shown, the time-off histogram after correlation processing is as follows: Figure 5 As shown, the peak value is taken as the detected flight time. Figure 6 The error graphs at different repetition frequencies are shown after 20 experiments. Figure 7 Error graphs from 20 experiments with different pulse widths are shown.

[0093] Table 1 System Simulation Parameter Settings

[0094]

[0095]

[0096] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A laser echo ranging method based on spread spectrum, characterized in that, Includes the following steps: S1, the FPGA controller generates a pseudo-random code, which modulates the semiconductor laser to emit laser light. Based on the pulse width information of the laser, the pseudo-random code is expanded to obtain an expanded pseudo-random code. In S1, the formula for expanding the pseudo-random code based on the laser pulse width signal is as follows: (1) in, This represents the minimum time resolution of the FPGA controller. The full width at half maximum (FWHM) of the laser pulse waveform. For random generation pseudo-random code, Pseudo-random code Length, For extension pseudo-random code, This represents the number of time intervals occupied by a single pulse cycle on the FPGA controller side. S2, after the laser is collimated, it is reflected by the target and received by the single-photon detector; the single-photon detector sends a signal to the FPGA based on the received laser echo; the FPGA controller obtains the laser return time and then outputs the time interval between the laser emission and return times, and the multiple time intervals obtained repeatedly are used to form an echo sequence; the cumulative photon histogram is obtained by statistics. The specific process by which the FPGA controller obtains the laser return time is as follows: The FPGA records the photon return time index within the entire pseudo-random code period in time intervals. The simulation obtains the photogenerated electron value in each time interval and determines whether the photogenerated electron value exceeds the detection threshold of the single-photon detector. If the single-photon detector is excited, the FPGA obtains the laser return time. The formula for calculating the number of photogenerated electrons in each time interval is: (8) in, j This refers to the location of the target time interval. The quantum efficiency of the detector. Normalized laser pulse; S3. When the photon accumulation histogram and the extended pseudo-random code are synchronized and the time delay is stable, perform correlation calculation on the extended pseudo-random code and the echo sequence to obtain the correlation value. Based on the correlation value, obtain the detection distance.

2. The laser echo ranging method based on spread spectrum as described in claim 1, characterized in that, The formula for calculating laser pulses is: (4) in, The formula for normalizing the laser pulse, where is the parameter, is: make ,but .

3. The laser echo ranging method based on spread spectrum as described in claim 1, characterized in that, In S2, the laser return time includes the time jitter when the single-photon detector is excited.

4. The laser echo ranging method based on spread spectrum according to claim 1, characterized in that, In S2, the photons in the laser light received by the single-photon detector include reflected signal photons, background noise photons, and dark current noise photons.

5. The laser echo ranging method based on spread spectrum according to claim 1, characterized in that, In S3, the formula for correlation calculation is: (9) Where N is the number of periods, It is an extended pseudo-random code. It is the cumulative histogram of reflected light. It's a time delay.

6. The laser echo ranging method based on spread spectrum according to claim 1, characterized in that, The formula for calculating the detection distance is: (10) in, It's the speed of light. This is a relevant value.

7. A laser echo ranging system based on a spread spectrum architecture for implementing the laser echo ranging method of claim 1, characterized in that, include: The pseudo-random code module is used by the FPGA controller to generate a pseudo-random code. The pseudo-random code modulates the laser emitted by the semiconductor laser. Based on the pulse width information of the laser, the pseudo-random code is expanded to obtain an expanded pseudo-random code. The photon accumulation module is used to reflect the laser after collimation to the target and receive it through a single-photon detector. The single-photon detector sends a signal to the FPGA based on the received laser echo. The FPGA controller obtains the laser return time and outputs the time interval between the laser emission and return times. Multiple time intervals obtained repeatedly are used to form an echo sequence. The photon accumulation histogram is obtained by statistics. The detection range module is used to perform correlation calculations on the extended pseudo-random code and the echo sequence when the photon accumulation histogram and the extended pseudo-random code are synchronized and the time delay is stable, to obtain the correlation value, and to obtain the detection range based on the correlation value.

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