Single-photon laser ranging method based on laser pulse frequency modulation deblurring

Abstract

The invention 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 firstly set to correspond to two coding code elements respectively, n-bit random codes are generated according to n-time non-fuzzy distance expansion requirements, it is guaranteed that the n-bit random codes do not have displacement symmetry, then a laser source is made to repeatedly emit corresponding pulse signals according to a sequence represented by the random codes, then echo signal sequences are received, and the pulse frequency references correspond to the two coding code elements respectively. And performing cross-correlation operation on the single photon accumulation histogram and a pulse sequence represented by a known random code to obtain an operation result, determining one of the single photon accumulation histograms with the maximum amplitude, and calculating a histogram time position corresponding to the result, namely, calculating a distance measurement result of de-blurring. According to the method, the problem of range ambiguity of pulse radar ranging can be solved, the time sliding sequence matching method proposed in cross-correlation operation has the advantages of being efficient and flexible, and the time complexity of cross-correlation operation is effectively reduced.

Description

Technical Field

[0001] The invention relates to a single-photon laser ranging method based on laser pulse frequency modulation deblurring, and belongs to the field of laser ranging. Background Art

[0002] Laser ranging systems based on the photon time-of-flight method are widely used in distance measurement. They use the arrival time of laser pulses to determine distance information. For single-photon laser ranging systems, photon accumulation counting can be achieved through a laser source with a high repetition rate and low single pulse energy and a single-photon detector with extremely high sensitivity, thereby extracting distance. Generally, the higher the pulse repetition frequency, the more accumulation times per unit time, and the higher the signal-to-noise ratio of the ranging result. However, at a higher repetition rate, the interval between adjacent transmitted pulses is shorter than the pulse flight time, and ranging ambiguity will occur. For example, when the farthest distance measurement reaches 1.5km, the maximum pulse repetition frequency without ranging ambiguity is 100kHz. When the farthest distance measurement is required to reach tens or hundreds of kilometers, in order to avoid ranging ambiguity, the repetition frequency needs to be reduced to the order of 1kHz or even lower, which seriously restricts the speed of single-photon detection.

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

[0004] 1. The repetition frequency difference method, this method requires the emission of two pulse sequences with different repetition frequencies, and uses the least common multiple of the two emission pulse periods to expand the maximum unambiguous distance range. However, this method cannot eliminate distance ambiguity, but can only increase the maximum unambiguous ranging range to a limited extent. To further increase the unambiguous ranging range, more pulses with different repetition frequencies need to be set, which increases the complexity of the equipment.

[0005] 2. Pulse discarding method: This method discards one pulse in every M pulses transmitted as an additional mark of the transmitted pulse train. Assuming that the A2 pulse in A1-Am is discarded, just start from the corresponding moment of A2 of the transmitted pulse, accumulate the number of transmitted pulses, until the echo pulse is "lost", and the number recorded is the number of ambiguous cycles m. This type of method expands the maximum unambiguous distance by M times, but this method cannot distinguish between two targets that meet a specific echo interval, that is, the pulse-free gap of the distant target coincides with the return pulse of the near target and is received by the radar. At this time, since there is no gap for the discarded pulse, the pulse discarding method cannot be used.

[0006] For single-photon laser ranging systems, photon accumulation counting can be achieved through a high repetition rate, low single pulse energy laser source and a highly sensitive single-photon detector, thereby extracting distance. However, at a higher repetition rate, the interval between adjacent emitted pulses is shorter than the pulse flight time, and ranging ambiguity will occur. This creates a contradiction between the maximum detection rate and the maximum unambiguous 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 when implementing long-distance testing. Summary of the invention

[0008] In order to solve the problem that the blur cannot be completely eliminated during distance measurement in the prior art and fails under specific conditions, the present invention provides a single-photon laser ranging method based on laser pulse frequency modulation deblurring, which has the advantages of high distance measurement accuracy and high distance measurement efficiency.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] The present 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 maximum repetition frequency range supported by the laser source, and the interval between adjacent pulses is higher than the dead time of the single-photon detector; set two encoding code elements 0 and 1, representing the repetition frequencies 2f and 3f respectively;

[0012] Step 2: Encode and arrange the two encoding code elements set in step 1 to obtain a pulse sequence, which is called a random code; generate an n-bit random code according to the requirement of extending 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 emits a pulse signal of a modulated frequency according to a sequence represented by a random code, takes the start time of the sequence emission as the counting start time, receives the signal according to the time-correlated single photon counting method, and obtains a single photon accumulation histogram of the time span corresponding to the encoding code length, which is expressed in a sequence form;

[0014] Step 4, use the single-photon cumulative histogram obtained in step 3 to perform cross-correlation with the pulse sequence represented by the random code. The correlation operation result will have several peaks of different amplitudes. Find the one with the largest amplitude in the cross-correlation result, record the time coordinate, and subtract the zero time to obtain 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, that is, the real target echo flight time for resolving ranging ambiguity; the ranging distance is calculated based on the real target echo flight time, that is, single-photon laser ranging is realized based on laser pulse frequency modulation to remove distance ambiguity.

[0015] In step 4, the single photon accumulation 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:

[0016] Step 4-1, according to the single photon accumulation histogram, a threshold for distinguishing between signal and noise echo is set. The threshold should be between the highest noise count and the lowest signal count level, which is called the signal threshold.

[0017] Step 4-2, normalize the count value of the single photon accumulation histogram, and normalize the maximum count value of the pulse echo to a unit amplitude, which is consistent with the pulse unit amplitude in step 1.

[0018] Step 4-3, aligning the time zero points of the normalized single-photon accumulation histogram and the sequence represented by the random code, allowing the single-photon accumulation histogram to slide along the time axis with the sequence represented by the random code, and the initial sliding step length is the time resolution of the single-photon counter;

[0019] Step 4-4, when a peak appears in the sliding matching correlation result and the peak is higher than the signal threshold described in step 4-1, the sliding step is changed to the period corresponding to the least common multiple frequency of the two code element modulation frequencies; continue sliding until all sequences complete the cross-correlation operation and obtain a cross-correlation result with different amplitudes.

[0020] Furthermore, the signal threshold in step 4-1 is determined according to the signal and noise count levels in the single photon accumulation histogram. The signal is detected by the energy detection method of the double sliding window, thereby dividing the signal time period and the noise time period in the single photon accumulation histogram. 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, the histogram accumulated pulses in step 4-4 have a predetermined width in time. When the correlation calculation obtains a result higher than the signal threshold, the two sequence pulses may be in a state of partial 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 and it is determined that a peak appears in the sliding matching correlation result.

[0022] Beneficial effects:

[0023] 1. The present invention discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring. In the ranging process, the laser source is made to cyclically emit a laser pulse sequence modulated by frequency coding. The suspected target is determined based on the received laser echo signal sequence and the known modulated pulse for correlation operation, and the maximum correlation coefficient among all suspected targets is calculated. The maximum value of the maximum correlation coefficient among all suspected targets is determined, and the suspected target corresponding to the maximum value is determined to be the real target. The time position of the histogram corresponding to the maximum value is calculated, and the ranging distance can be calculated. The present invention has the advantages of small calculation amount 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 is efficient and flexible. The sliding step can be adjusted according to the cross-correlation calculation result. When the pulses are not completely matched and the correlation is very low, the counter time resolution is used as the step. When the calculation result of individual pulses has been matched, the period corresponding to the least common multiple of the two code element modulation frequencies is used as the sliding step, thereby reducing the time complexity of the correlation operation.

[0025] 3. The present invention discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring. When the correlation calculation obtains a result higher than the signal threshold, the two sequence pulses may be in a state of partial overlap. At this time, it is necessary to keep the sliding step size set to the time resolution of the single-photon counter, which can avoid introducing ranging errors and ensure the accuracy of laser ranging. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0028] In order to better illustrate the purpose and advantages of the present invention, the invention is further described below with reference to the accompanying drawings and examples.

[0029] Embodiment 1:

[0030] The present embodiment discloses a single-photon laser ranging method based on laser pulse frequency modulation deblurring, which emits a laser pulse sequence that has been frequency coded modulated, determines a suspected target based on a correlation operation between a received laser echo signal sequence and a known modulated pulse, determines an actual target based on the maximum value of the correlation coefficient, obtains the time difference between the two groups of sequences, calculates the time coordinate of the maximum correlation coefficient in all matching results, and obtains the target with the highest matching degree with the laser frequency modulation pulse, so as to solve the problem that the prior art cannot completely eliminate blur during ranging and fails under specific 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 code elements 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, if 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 coincides completely or is mirror-symmetric 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: When 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 time of the sequence emission as the counting start time, 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 code element, which includes noise counts uniformly distributed over the entire counting period and signal echo pulses following the 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 time is the actual target photon flight time, and the actual distance is calculated.

[0039] Among them, the correlation calculation can be performed according to the conventional method. The amount of calculation is determined by the time resolution of the single-photon counter and the total cumulative duration of the entire single-photon accumulation histogram. In order to reduce the amount of calculation without affecting the detection performance, the following correlation calculation method is used:

[0040] After obtaining the single photon accumulation histogram of the coding code for a long time, align the starting time of the known coding frequency pulse with the starting time of the cumulative histogram G(t), record it as time zero, and then slide the coding pulse F(t) backward according to a certain step length Δτ to make the two sequences cross-correlated until the end time of the known coding frequency pulse F(t) is aligned with the end time of the cumulative histogram G(t). The cross-correlation result is recorded as H(t), then

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

[0042] Among them, the decision threshold is determined according to the histogram cumulative count level, such as Figure 2 The curve is the correlation result between a typical return peak and an impulse signal. The figure gives an example of setting the threshold, which should be higher than the maximum value of the cross-correlation between the impulse signal and the noise.

[0043] Furthermore, the histogram accumulated pulses have a certain width in time. When the correlation calculation obtains a result higher than the threshold, the two sequence pulses may be in a state of partial 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 and it is determined that a peak appears in the sliding matching correlation result.

[0044] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above 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 in 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: The steps include: 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 maximum repetition frequency range supported by the laser source, and the interval between adjacent pulses is higher than the dead time of the single-photon detector; set two encoding code elements 0 and 1, representing the repetition frequencies 2f and 3f respectively; Step 2: Encode and arrange the two encoding code elements set in step 1 to obtain a pulse sequence, which is called a random code; generate an n-bit random code according to the requirement of extending the maximum ranging distance by n times, and ensure that the random code does not have shift symmetry; Step 3, the laser source repeatedly emits a pulse signal of a modulated frequency according to a sequence represented by a random code, takes the start time of the sequence emission as the counting start time, receives the signal according to the time-correlated single photon counting method, and obtains a single photon accumulation histogram of the time span corresponding to the encoding code length, which is expressed in a sequence form; Step 4, use the single-photon cumulative histogram obtained in step 3 to perform cross-correlation with the pulse sequence represented by the random code. The correlation operation result will have several peaks of different amplitudes. Find the one with the largest amplitude in the cross-correlation result, record the time coordinate, and subtract the zero time to obtain 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, that is, the real target echo flight time for resolving ranging ambiguity; the ranging distance is calculated based on the real target echo flight time, that is, 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 as claimed in claim 1, characterized in that: In step 4, the single photon accumulation 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, setting a threshold for distinguishing signal and noise echoes according to the single photon accumulation histogram. The threshold should be between the highest noise count and the lowest signal count level, which is called the signal threshold; Step 4-2, normalizing the count value of the single photon accumulation histogram, and normalizing the maximum count value of the pulse echo to a unit amplitude, which is consistent with the pulse unit amplitude in step 1; Step 4-3, aligning the time zero points of the normalized single-photon accumulation histogram and the sequence represented by the random code, allowing the single-photon accumulation histogram to slide along the time axis with the sequence represented by the random code, and the initial sliding step length is the time resolution of the single-photon counter; Step 4-4, when a peak appears in the sliding matching correlation result and the peak is higher than the signal threshold described in step 4-1, the sliding step is changed to the period corresponding to the least common multiple frequency of the two code element modulation frequencies; continue sliding until all sequences complete the cross-correlation operation and obtain a cross-correlation result with different amplitudes.

3. A single-photon laser ranging method based on laser pulse frequency modulation deblurring as claimed in 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 double sliding window energy detection method to divide the signal time period and the noise time period in the single-photon cumulative histogram. 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.

4. A single-photon laser ranging method based on laser pulse frequency modulation deblurring as claimed in claim 2, characterized in that: In step 4-4, the histogram accumulated pulse has a predetermined width in time. When the correlation calculation obtains a result higher than the signal threshold, the two sequence pulses may be in a state of partial 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, and it is determined that a peak appears in the sliding matching correlation result.

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