A single-photon laser ranging method based on laser pulse frequency modulation deblurring

Abstract

The application discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring, and belongs to the field of laser ranging. In the ranging process, two different pulse frequency references are set to correspond to two encoding code elements, n-bit random codes are generated according to n times non-blurring distance expansion requirements, and the n-bit random codes are ensured to have no shift symmetry, then a laser source repeatedly emits corresponding pulse signals according to a sequence represented by the random codes, then a sequence of echo signals is received, a single-photon cumulative histogram is obtained, and a cross-correlation operation is performed on the single-photon cumulative histogram and the pulse sequence represented by the known random codes to obtain an operation result, one having the maximum amplitude in the operation result is determined, and a histogram time position corresponding to the operation result is calculated, that is, a ranging result without blurring can be calculated. The application can overcome the distance blurring problem of pulse radar ranging, and the time sliding sequence matching method proposed in the cross-correlation operation has the characteristics of high efficiency and flexibility, and effectively reduces the time complexity of the cross-correlation operation.

Description

Technical Field

[0001] This invention relates to a single-photon laser ranging method based on laser pulse frequency modulation deblurring, belonging to the field of laser ranging. Background Technology

[0002] Laser ranging systems based on the time-of-flight (TOF) method of photons are widely used in distance measurement, determining distance information by utilizing the arrival time of laser pulses. For single-photon laser ranging systems, photon accumulation and counting can be achieved using a high repetition rate (PRR) laser source with low single-pulse energy and a highly sensitive single-photon detector, thereby extracting the distance. Generally, the higher the pulse repetition frequency, the more accumulations per unit time, resulting in a higher signal-to-noise ratio (SNR) for the ranging result. However, at higher PPR frequencies, the interval between adjacent emitted pulses is shorter than the pulse flight time, leading to ranging ambiguity. For example, when the maximum distance measurement reaches 1.5 km, the highest pulse repetition frequency without ranging ambiguity is 100 kHz. When the maximum distance requirement reaches tens or hundreds of kilometers, the repetition frequency needs to be reduced to the 1 kHz level or even lower to avoid ranging ambiguity, which severely limits the speed of single-photon detection.

[0003] In the field of pulse detection radar, there are two commonly used methods to resolve range ambiguity:

[0004] 1. The staggered repetition frequency method: This method requires transmitting two pulse sequences with different repetition frequencies, and uses the least common multiple of the periods of the two transmitted pulses to extend the maximum unambiguous distance range. However, this method cannot eliminate distance ambiguity; it can only increase the maximum unambiguous ranging range to a limited extent. Further increasing the unambiguous ranging range requires setting more pulses with different repetition frequencies, which increases the complexity of the equipment.

[0005] 2. The pulse discarding method: This method discards one pulse from every M transmitted pulses, using it as an additional marker in the transmitted pulse train. Assuming the discarded pulse is A2 from A1-Am, starting from the moment corresponding to A2 of the transmitted pulse, the number of transmitted pulses is accumulated until the echo pulse is "lost." The recorded number is the number of ambiguity periods, m. This method increases the maximum unambiguous range by a factor of M. However, this method cannot distinguish between two targets that meet a specific echo interval. That is, if the pulseless gap of a distant target coincides with the return pulse of a nearby target and is received by the radar, the pulse discarding method cannot be used because the gap of the discarded pulse does not exist.

[0006] For single-photon laser ranging systems, photon accumulation and counting can be achieved using a high repetition rate, low single-pulse energy laser source and a highly sensitive single-photon detector, thereby extracting the distance. However, at higher repetition rates, the interval between adjacent emitted pulses is shorter than the pulse flight time, leading to ranging ambiguity. This creates a contradiction between the maximum detection rate and the maximum ambiguity-free distance of single-photon detection.

[0007] Based on the above analysis, it is extremely necessary to propose a suitable method to solve the ranging ambiguity problem that occurs when conducting long-distance tests. Summary of the Invention

[0008] To address the problem that existing technologies cannot completely eliminate ambiguity during ranging and fail under certain conditions, this invention provides a single-photon laser ranging method based on laser pulse frequency modulation for deambiguation, which has the advantages of high ranging accuracy and high ranging efficiency.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] This invention discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring, comprising the following steps:

[0011] Step 1: Set two pulse frequency references with unit amplitude, with repetition frequencies of 2f and 3f respectively. It is necessary to ensure that 2f and 3f are within the highest repetition frequency range supported by the laser source, and that the interval between adjacent pulses is higher than the dead time of the single-photon detector; set two coding symbols 0 and 1 to represent repetition frequencies 2f and 3f respectively.

[0012] Step 2: Encode and arrange the two types of code symbols set in Step 1 to obtain a pulse sequence, called a random code; generate an n-bit random code according to the requirement of expanding the maximum ranging distance by n times, and ensure that the random code does not have shift symmetry;

[0013] Step 3: The laser source repeatedly transmits pulse signals of the modulation frequency according to the sequence represented by the random code. Taking the start time of the sequence transmission as the start time of counting, the signal is received according to the time-correlated single-photon counting method to obtain the single-photon cumulative histogram of the time span corresponding to the coded code length, which is represented in the form of a sequence.

[0014] Step 4: Cross-correlate the single-photon cumulative histogram obtained in Step 3 with the pulse sequence represented by the random code. The correlation result will show several peaks with different amplitudes. Find the one with the largest amplitude in the cross-correlation result, record the time coordinate, and subtract the zero point to obtain the time difference. The time difference is the time interval between the echo represented by the single-photon cumulative histogram and the transmitted pulse sequence represented by the random code, which is the true target echo flight time for resolving ranging ambiguity. Calculate the ranging distance based on the true target echo flight time, that is, realize single-photon laser ranging based on laser pulse frequency modulation to resolve range ambiguity.

[0015] Step 4 involves cross-correlation between the single-photon accumulation histogram obtained in step 3 and the pulse sequence represented by the random code. The specific implementation steps are as follows:

[0016] Step 4-1: Set a threshold to distinguish between signal and noise echoes based on the single-photon cumulative histogram. This threshold should be between the highest noise count and the lowest signal count level, and is called the signal threshold.

[0017] Step 4-2: Normalize the count values ​​of the single-photon accumulation histogram. The maximum count value of the pulse echo is normalized to the unit amplitude, which is consistent with the pulse unit amplitude in Step 1.

[0018] Step 4-3: Align the time zeros of the normalized single-photon accumulation histogram and the sequence represented by the random code, and slide the single-photon accumulation histogram along the time axis to match the sequence represented by the random code. The initial sliding step size is the time resolution of the single-photon counter.

[0019] Step 4-4: When the sliding matching correlation result shows a peak value, and the peak value is higher than the signal threshold described in step 4-1, the sliding step size is changed to the period corresponding to the least common multiple frequency of the two symbol modulation frequencies; continue sliding until all sequences have completed the cross-correlation operation and a cross-correlation result with different amplitudes is obtained.

[0020] Furthermore, in step 4-1, the signal threshold is determined based on the signal and noise count levels in the single-photon accumulation histogram. The signal is processed using a double-sliding-window energy detection method to divide the single-photon accumulation histogram into signal and noise time periods. The signal threshold should be greater than the maximum value of the noise time period and not exceed the minimum value of the signal time period.

[0021] Furthermore, in step 4-4, the histogram cumulative pulse has a predetermined width in time. When the correlation calculation yields a result higher than the signal threshold, the two sequence pulses may be in a state of local overlap. In order to avoid introducing ranging errors, it is necessary to keep the sliding step size set to the time resolution of the single photon counter until the two sequence pulses reach the maximum overlap width, which is then determined to be a peak in the sliding matching correlation result.

[0022] Beneficial effects:

[0023] 1. This invention discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring. During the ranging process, a laser source cyclically emits a sequence of laser pulses modulated by frequency encoding. Correlation calculations are performed between the received laser echo signal sequence and known modulated pulses to identify potential targets. The maximum correlation coefficient among all potential targets is calculated, and the maximum value of the maximum correlation coefficient is determined. The potential target corresponding to the maximum value is identified as the true target, and the histogram time position corresponding to the maximum value is calculated, thus enabling the calculation of the ranging distance. This invention has the advantages of low computational complexity and high ranging efficiency.

[0024] 2. The present invention discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring. The time-sliding matching method in sequence matching has the characteristics of high efficiency and flexibility. The sliding step size can be adjusted according to the cross-correlation calculation results. When the pulses are not completely matched and the correlation is very low, the counter time resolution is used as the step size. When the calculation results of individual pulses are matched, the period corresponding to the least common multiple frequency of the two symbol modulation frequencies is used as the sliding step size, thereby reducing the time complexity of correlation calculation.

[0025] 3. The single-photon laser ranging method based on laser pulse frequency modulation deblurring disclosed in this invention, when the result obtained by relevant calculation is higher than the signal threshold, the two sequence pulses may be in a state of local overlap. At this time, it is necessary to keep the sliding step size set to the time resolution of the single-photon counter to avoid introducing ranging error and ensure the accuracy of laser ranging. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a single-photon laser ranging method based on laser pulse frequency modulation deblurring disclosed in this invention;

[0027] Figure 2 This is an example diagram illustrating the setting of the signal threshold in step 4-1 of the method of the present invention. Detailed Implementation

[0028] To better illustrate the purpose and advantages of the present invention, the invention will be further described below in conjunction with the accompanying drawings and examples.

[0029] Example 1:

[0030] This embodiment discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring. By emitting a laser pulse sequence modulated by frequency coding, a suspected target is determined by performing correlation calculations between the received laser echo signal sequence and a known modulated pulse. The actual target is determined based on the maximum correlation coefficient, and the time difference between the two sets of sequences is obtained. The time coordinate of the maximum correlation coefficient among all matching results is calculated to obtain the target with the highest matching degree with the laser frequency modulated pulse. This solves the problem that existing technologies cannot completely eliminate ambiguity during ranging and fail under certain conditions.

[0031] This embodiment discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring, and the specific implementation steps are as follows:

[0032] Step 1: According to the required maximum ranging distance, select a nanosecond or sub-nanosecond pulsed laser source with appropriate single-pulse energy, and record the pulse amplitude as the unit amplitude. Determine two pulse repetition frequencies 2f and 3f such that 2f and 3f are within the highest repetition frequency range supported by the laser source, and the adjacent pulse interval time is higher than the dead time of the single-photon detector; set two coding symbols 0 and 1, representing the repetition frequencies 2f and 3f respectively;

[0033] Step 2: Generate an n-bit random code according to the requirement of expanding the maximum ranging distance by n times. The code elements are respectively recorded as "0" and "1", and are distinguished by the pulse frequencies 2f and 3f respectively. It can be set that the first bit of the random code is "0", corresponding to the frequency 2f. For example, when the maximum ranging distance is expanded by 5 times, a 5-bit random sequence "01100" is generated. This random code is denoted as S(n), and it does not have shift symmetry. The so-called shift symmetry is specifically described as follows:

[0034] If S' after S is displaced by k (k < n) positions forms a completely coincident or mirror-symmetric relationship with the original form, then S is said to have shift symmetry. Mathematically, it can be expressed as:

[0035] For left shift: S(i + k) = S'(i), where the value range of i is within [0, n - k - 1], and S'(i) is the string obtained after S is displaced k positions to the left. For right shift: S(i - k) = S'(i), where the value range of i is within [k, n - 1], and S'(i) is the string obtained after S is displaced k positions to the right.

[0036] Step 3: During ranging, the laser source emits a repeated laser pulse sequence with a code length of n as the period and frequency modulation. Taking the start moment of the sequence emission as the counting start moment, receive the signal according to the time-correlated single-photon counting method, and obtain the single-photon cumulative histogram within the time span of the coding symbol, which includes noise counts that follow a uniform distribution within the entire counting period and signal echo pulses that follow a Poisson distribution within the pulse width time.

[0037] Step 4: Use the obtained single-photon cumulative histogram to extract and match the cumulative pulse echo target. The extraction and matching method is to perform a correlation operation on the histogram statistical value and the known frequency-modulated pulse sequence to obtain several result sequences with different amplitudes.

[0038] Step 5: Find the result with the largest amplitude, and perform cross-correlation to obtain the result sequence with the largest amplitude within the coding length range. The difference between its corresponding time coordinate and the zero moment is the actual target photon flight time, and the actual distance is calculated.

[0039] Correlation calculation can be performed using conventional methods. The computational load is determined by the temporal resolution of the single-photon counter and the total cumulative duration of the entire single-photon cumulative histogram. To reduce the computational load without affecting detection performance, the following correlation calculation method is adopted:

[0040] After obtaining the long-term single-photon cumulative histogram of the encoded code, the start time of the known encoded frequency pulse is aligned with the start time of the cumulative histogram G(t), denoted as time zero. Then, the encoded pulse F(t) is slid backwards with a certain step size Δτ, and the two sequences are cross-correlated until the end time of the known encoded frequency pulse F(t) aligns with the end time of the cumulative histogram G(t). Let the cross-correlation result be H(t), then we have...

[0041] Specifically, the time sliding step size Δτ depends on the result of the current convolution operation. Since the returned accumulated pulse has a certain width, a threshold comparison can be used to determine the operation result. If the amplitude of the sequence obtained by convolution is very small, below the threshold, it is determined that the current histogram sliding position does not coincide with any pulse of the modulation sequence, and the sliding step size Δτ is set to the time resolution of the single-photon counter. If the amplitude of the sequence obtained by convolution has a maximum value higher than the threshold, it is determined that the current histogram sliding position coincides with at least one pulse in the known sequence, and the sliding step size is set to the period corresponding to the least common multiple frequency of the two symbol modulation frequencies. Under the condition that the modulation reference frequencies of the two symbols are 2f and 3f respectively, Δτ = 1 / 6f.

[0042] The decision threshold is determined based on the cumulative count level of the histogram, such as... Figure 2 As shown, the curve represents the correlation between a typical return peak and the impact signal. The example threshold given in the figure should be higher than the maximum cross-correlation value between the impact signal and noise.

[0043] Furthermore, the histogram cumulative pulse has a certain width in time. When the correlation calculation yields a result higher than the threshold, the two sequence pulses may be in a state of local overlap. In order to avoid introducing ranging errors, it is necessary to keep the sliding step size set to the time resolution of the single photon counter until the two sequence pulses reach the maximum overlap width, which is then determined to be a peak in the sliding matching correlation result.

[0044] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A single-photon laser ranging method based on laser pulse frequency modulation deblurring, characterized in that, Comprising the following steps: Step 1, set two pulse frequency references with unit amplitude, the repetition frequencies are 2f and 3f respectively, need to ensure that 2f, 3f are in the highest repetition frequency range supported by the laser source, and the adjacent pulse interval time is higher than the dead time of the single photon detector; Set two encoding code elements 0 and 1, which represent the repetition frequencies 2f and 3f respectively; Step 2, encode the two encoding code elements set in step 1 to get the pulse sequence, called random code; According to the requirement of expanding the maximum ranging distance by n times, generate n-bit random code, and ensure that the random code does not have shift symmetry; Step 3, the laser source repeatedly emits pulse signals of modulation frequency according to the sequence represented by the random code, and the starting time of sequence emission is taken as the counting starting time; According to the time correlation single photon counting method, the signal is received to obtain the single photon cumulative histogram corresponding to the time span of the code length, which is represented in sequence form; Step 4, cross-correlate the single photon cumulative histogram obtained in step 3 with the pulse sequence represented by the random code, and a number of peaks with different amplitudes will appear in the correlation result, find one with the maximum amplitude in the cross-correlation result, record the time coordinate, and subtract the zero time to get the time difference; The time difference is the time interval between the echo represented by the single photon cumulative histogram and the emission pulse sequence represented by the random code, which is the true target echo flight time of the range ambiguity; According to the true target echo flight time, the ranging distance is calculated, that is, the single photon laser ranging is realized based on laser pulse frequency modulation to remove distance ambiguity.

2. A single-photon laser ranging method based on laser pulse frequency modulation deblurring according to claim 1, characterized in that, In step 4, the single photon cumulative histogram obtained in step 3 is cross-correlated with the pulse sequence represented by the random code, and the specific implementation steps are as follows: Step 4-1, set a threshold to distinguish signal and noise echoes according to the single photon cumulative histogram, which should be between the highest noise count and the lowest signal count, called signal threshold; Step 4-2, normalize the count value of the single photon cumulative histogram, and normalize the maximum count value of the pulse echo to unit amplitude, which is consistent with the pulse unit amplitude in step 1; Step 4-3, align the time zero points of the normalized single photon cumulative histogram and the sequence represented by the random code, and slide the single photon cumulative histogram along the time axis to match the sequence represented by the random code, and the initial sliding step is the time resolution of the single photon counter; Step 4-4, when the sliding matching correlation result appears a peak value, and the peak value is higher than the signal threshold in step 4-1, the sliding step is changed to the least common multiple frequency corresponding to the period of the two code element modulation frequencies; Continue to slide until all sequences complete cross-correlation operation to get a segment of cross-correlation results with different amplitudes.

3. A single-photon laser ranging method based on laser pulse frequency modulation deblurring according to claim 2, characterized in that, In step 4-1, the signal threshold is determined according to the signal and noise count levels in the single photon cumulative histogram; The signal is detected by the energy detection method of double sliding window, so as to divide the signal time period and the noise time period in the single photon cumulative histogram, and the signal threshold should be greater than the maximum value of the noise time period and not more than the minimum value of the signal time period.

4. A single-photon laser ranging method based on laser pulse frequency modulation deblurring according to claim 2, characterized in that, The histogram accumulated pulse in step 4-4 has a predetermined width in time, when the correlation calculation results above the signal threshold are obtained, the two sequence pulses can be in the state of local coincidence, in order to avoid introducing ranging error, at this time, the sliding step needs to be kept to be set as the time resolution of the single photon counter until the two sequence pulses reach the maximum coincidence width, and it is determined that the sliding matching correlation result appears the peak value.

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