RU2791151C1 - Method for incoherent accumulation of pulsed light-location signals - Google Patents

Abstract

FIELD: optical signals reception.

SUBSTANCE: invention relates to the reception of optical signals, in particular to the technique of receiving signals by means of avalanche photodiodes. The substance of the claimed method for incoherent accumulation of pulsed light-location signals is as follows. In each K-th probing cycle, the reflected signal is received in N range channels, in which the received signal is compared with one or more analog threshold levels, and the sum of excesses of these levels is accumulated, taking into account the level weighting factor. The presence of a signal in a particular channel is detected by comparing the sum of excesses with a threshold number. Then, reflected signals are received by means of an avalanche photodiode, in the preparatory mode, in the absence of probing laser pulses, the optimal avalanche multiplication factor M_opt of the photodiode is set for the operating mode, at which the signal-to-noise ratio is maximum. Next, in the M_opt mode, the presence of microplasma pulses is detected and the minimum amplitude of microplasma pulses U_Mmin is determined, an additional threshold level U_M is set, after which they switch to the signal accumulation operating mode, in which weighted sums Σ_j of operating threshold excesses are accumulated in each j-th range channel. In this case, in parallel, in each j-th range channel, the sums Σ_Mj of exceeding the U_M level are accumulated. At the end of a series of probing cycles in each range channel, the difference values ΔΣ_j=Σ_j-E_Mj are calculated. In the event that ΔΣ_j exceeds the set threshold number, then a decision is made about the presence of a signal in a given range channel and the distance to the target is judged by the number of this range channel.

EFFECT: ensuring the maximum probability of detecting light-location signals by the accumulation method, regardless of the microplasmas arising during the avalanche multiplication of the signal in the photodiode.

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Description

The invention relates to laser technology, namely to laser ranging.

There is a method of incoherent accumulation of pulses when they are repeated many times, for example, to detect received signals during laser or radar sounding of remote objects [1-5]. This method consists in performing a series of probing cycles, in each probing cycle, the received signal is compared with an analog threshold (binary quantization is performed), the number of analog threshold exceedances is counted, and a decision is made on the presence of a signal if this number exceeds a predetermined number. This method does not allow realizing the potential probability of detecting signals due to the loss of information during binary quantization of the received signal.

The closest in technical essence to the proposed method is a method of incoherent accumulation of signals, including a series of probing cycles, in each probing cycle, receiving a reflected signal and comparing the received signal with one or more analog threshold levels, accumulating the sum of exceedances of analog threshold levels, according to which, after the completion of the series the presence of a signal is judged by comparing the sum of exceedances with a threshold number [6].

The advantages of this method are realized to the maximum if the reflected signal is received using an avalanche photodiode, which has the best threshold sensitivity compared to other receivers [7]. However, in the optimum sensitivity mode of avalanche multiplication, the formation of explosive (“telegraph”) noises caused by microplasma breakdowns (microplasmas) in the semiconductor junction of the photodiode is possible [8]. Microplasma current pulses have a rectangular shape and a constant amplitude, which increases as the reverse voltage increases. An increase in the amplitude is accompanied by an increase in the duration of the pulses and a decrease in the duty cycle [9]. In this mode, the noise of an avalanche photodiode consists of two independent components - normal noise [7] and explosive noise of microplasmas. The microplasma component of the photodiode noise is not statistically comparable with the normal component, and its participation in the process of controlling the photodiode bias is unpredictable [10].

The objective of the invention is to ensure the maximum probability of detecting light-location signals by the accumulation method in the presence of microplasmas arising from the avalanche multiplication of the signal in the photodiode.

This problem is solved due to the fact that in the known method of incoherent accumulation of pulsed light-location signals, including K cycles of probing the target with laser pulses, in each probing cycle, the reception of the reflected signal in N range channels, where N=R _max /ΔR, R _max is the range of measured distances, ΔR=s τ/2, s is the speed of light, τ is the clock period of the reflected signal delay meter, in each range channel, comparison of the received signal with one or more analog operating threshold levels, accumulation of the sum of excesses of the analog threshold levels, taking into account the weighting coefficient of the level , according to which the presence of a signal in this channel is judged by comparing the sum of excesses with a threshold number, the reflected signals are received using an avalanche photodiode, in the preparatory mode, in the absence of probing laser pulses, the optimal avalanche multiplication coefficient M _opt of the photodiode is set for the operating mode, at which the signal ratio /noise maximum , in M _opt mode, the presence of microplasma pulses is detected and the minimum amplitude of microplasma pulses U _Mmin is determined, an additional threshold level U _M is set within U _cmax <U _M <U _Mmin , where U _cmax is the maximum expected signal amplitude, after which they switch to the operating mode signal accumulation, in each j-th range channel accumulate weighted sums Σ _j of exceedances of operating thresholds, in parallel in each j-th range channel accumulate sums Σ _Mj of level exceedances U _M , at the end of a series of probing cycles in each range channel calculate the difference values ΔΣ _j =Σ _j -Σ _Mj and, if ΔΣ _j exceeds a predetermined threshold number, then a decision is made about the presence of a signal in a given range channel and the distance to the target is judged by the number of this range channel.

раз, где α - параметр шум-фактора лавинного умножения F=М^α, порог

фиксируют и с помощью напряжения смещения фотодиода устанавливают такой коэффициент лавинного умножения М=М_опт, при котором частота f_M шумовых превышений порога U_M становится равной частоте f в безлавинном режиме М=1 при пороге U, по достижении частоты f_M=f фиксируют достигнутый коэффициент лавинного умножения М=М_опт и включают рабочий режим приема сигналов.The optimal mode M _opt is set, including the avalanche-free bias mode of the photodiode M=1, then the response threshold U is set at a level corresponding to the frequency of noise responses of the threshold device f<<f ₀ , where f ₀ is the frequency of the noise crossing the zero level, then the threshold is increased in

times, where α is the noise factor parameter of the avalanche multiplication F=M ^α , threshold

fix and using the bias voltage of the photodiode set such an avalanche multiplication factor M=M _opt at which the frequency f _M noise exceeding the threshold U _M becomes equal to the frequency f in the avalanche-free mode M=1 at the threshold U, when the frequency f _M =f is reached, the achieved avalanche multiplication factor M=M _opt and include the operating mode of signal reception.

If the amplitude of the signal in the operating mode exceeds the threshold level U _M then reduce the avalanche multiplication factor to the level 1≤M≤M _opt at which the nonlinear distortion of the signal is minimal.

If a signal is recorded in any of the range channels in two accumulation cycles in a row, then a decision is made about the presence of a target in this channel and accumulation is stopped, while the probability of the appearance of two microplasmas in a row in any range channel should not exceed the value W _2N =W ₂ ⋅ N<(1-D), where W ₂ is the probability of two microplasmas in a row in one range channel; N is the number of range channels; D is the given probability of correct detection of the signal.

In FIG. 1 shows the cyclogram of the method. In FIG. 2 - the nature of the mixture of signal and fluctuation noise at the input of the two-threshold amplitude analyzer. In FIG. 3 shows a diagram of a possible hardware implementation.

, где

- частота пересечения шумом нулевого порога; R''(0) - вторая производная корреляционной функции шума на входе порогового устройства R(t) при задержке t=0; σ² - дисперсия шума I₀ ² в размерности порога U.At the first stage of the preparatory mode (Fig. 1) the bias voltage of the photodiode corresponds to the avalanche multiplication factor M=1. In this case, the fluctuation noise of the preamplifier with the mean square noise current I ₀ ² dominates. From the frequency f of exceeding the available threshold levels by noise emissions, one can judge the root mean square value of the noise. For the threshold U, we know the relation

, Where

- frequency of noise crossing the zero threshold; R''(0) - the second derivative of the correlation function of the noise at the input of the threshold device R(t) at a delay of t=0; σ ² - noise variance I ₀ ² in the dimension of the threshold U.

раз и установлении лавинного режима, при котором увеличенному порогу соответствует та же частота f шумовых превышений, коэффициент лавинного умножения М=М_опт, обеспечивает максимальное отношение сигнал/шум [14]. В первой стадии подготовительного режима в течение времени Т_подг1 устанавливают порог U (фиг. 1) Во второй стадии подготовительного режима устанавливают порог

и увеличивают напряжение смещения фотодиода до тех пор, пока частота шумовых превышений снова не станет равна f. Тем самым, устанавливают оптимальный коэффициент лавинного умножения М_опт, после чего включают рабочий режим приема светолокационных сигналов.It follows from this that if in the avalanche-free mode M=1 the threshold U corresponds to the frequency f, then with an increase in the threshold in

times and the establishment of an avalanche mode, in which the increased threshold corresponds to the same frequency f of noise excesses, the avalanche multiplication factor M=M _opt , provides the maximum signal-to-noise ratio [14]. In the first stage of the preparatory mode during the time T _preg1, the threshold U is set (Fig. 1) In the second stage of the preparatory mode, the threshold is set

and increase the bias voltage of the photodiode until the frequency of noise excesses again becomes equal to f. Thus, the optimal avalanche multiplication factor M _opt is set, after which the operating mode for receiving light signals is switched on.

In the receive mode, each range channel accumulates weighted sums _{Σj of} threshold exceedances with weight coefficients corresponding to exceeded threshold levels [11]. Simultaneously with this process, a similar multi-channel accumulation of excesses of the high threshold U _M corresponding to large-amplitude emissions, identified as microplasmas, is carried out. In each range channel, the sums of these excesses Σ _Mj are accumulated. The difference between these amounts ΔΣ _j =Σ _j -Σ _Mj characterizes the level of the useful signal, and when the value of ΔΣ _j exceeds the threshold value, a decision is made about the presence of a light-location signal in the j-th range channel.

In accordance with the proposed solution, false alarms from microplasmas are excluded from the signal + noise mixture. Features of further signal extraction were studied in [11]. An essential criterion is the accumulation efficiency E, which is an improvement in the signal-to-noise ratio at the input and output of the storage:

where M(k) - the average value of the accumulated amount;

σ _K is the standard deviation of the accumulated amount after K accumulation cycles;

A is the signal amplitude at the accumulator input;

σ is the rms value of the input noise.

The dependence of the accumulation efficiency on the relative value of the threshold levels Δ _u /σ at their symmetrical position from the zero level was studied [11] (Fig. 2). At the optimal position of the thresholds, the two-level and four-level accumulation modes with a symmetrical placement of the threshold levels approach the theoretical limit in terms of efficiency

There is an optimal value of the avalanche multiplication factor M, which, in the absence of microplasmas, can be determined as follows. At the output of the avalanche photodiode, the equivalent square of the noise current acts:

where I ₀ ² is the square of the non-multipliable equivalent noise current;

e is the electron charge;

I ₁ - primary reverse current of the photodiode;

Δf - bandwidth of the receiving path to the input of the threshold device;

M - coefficient of avalanche multiplication;

M ^α - noise factor of avalanche multiplication;

α - coefficient determined by the material of the photodiode.

Noise/signal ratio W squared:

where J _M ² =2eI ₁ Δƒ.

Zero derivative condition:

or

where

The task of the present invention is solved due to the hardware interpretation of microplasmas not as false alarms, but as signal skipping factors. Due to this technique, it is possible to allow a higher probability of microplasmas and, thereby, to maintain the avalanche multiplication factor closer to the optimal level (6).

In FIG. 3 shows an example of a multi-threshold (eg, two-threshold) structure for implementing the method.

This location structure contains a transmitting channel 1, a photoreceiving channel 2 with a photodiode 3 avalanche mode stabilization circuit, a working multi-threshold device 4 and an additional threshold device 5 connected to multi-channel storage devices 6 and 7. The output data of the storage devices is fed to the decision device 8. At the output of the photoreceiving channel the noise analyzer 9 is switched on, the output of which is connected through the control circuit 10 to the control input of the photodiode 3 avalanche mode stabilization circuit.

раз и повышает напряжение смещения фотодиода до тех пор, пока не восстановится частота f, при которой коэффициент лавинного умножения М соответствует уровню М_опт. После этого переходят в рабочий режим. По команде от блока управления 10 передающий канал 1 излучает на цель серию K зондирующих импульсов. Одновременно блок управления запускает синхронизацию многоканальных накопителей 6 и 7, переключая их ячейки накопления с периодом τ (фиг. 2). Отраженные целью сигналы принимаются фотоприемным каналом 2, на выходе которого образуется смесь отраженного сигнала, флуктуационного шума и импульсов микроплазм. С выходов порогового устройства 4 сигналы поступают поочередно в ячейки накопителя 6, соответствующие текущему каналу дальности, включаемому тактовым сигналом блока управления 10. Сигналы в накопителе 6 суммируются с весом, соответствующим превышенному порогу. Например, при двухуровневом преобразовании с симметричным положением порогов относительно нуля при пересечении шумовым выбросом верхнего порога вверх в накопитель заносится «+1», а при пересечении нижнего порога вниз заносится «-1». При таком построении аппаратуры обеспечивается выигрыш Е в улучшении отношения сигнал/шум, близкий к теоретическому пределу

где K - количество циклов (объем) накопления [2]. В накопителе 7 содержится информация о количестве микроплазм в каждом канале дальности. Решающее устройство 8 вычисляет разность накопленных сумм ΔΣ_j=Σ_j-Σ_Mj, компенсируя наличие микроплазм.Before receiving signals, the photoreceiving channel is switched on and brought to the optimal mode. To do this, the noise analyzer 9 is switched on. The noise analyzer is a threshold device with a low threshold U, such that the frequency of its exceedance by noise emissions f>>f _M *, where f _M * is the expected frequency of microplasmas. Under this condition, the microplasmas do not affect the mode of the photodiode. Upon reaching the steady value of f, circuit 9 increases the threshold by

times and increases the bias voltage of the photodiode until the frequency f is restored, at which the avalanche multiplication factor M corresponds to the level M _opt . After that, they go into working mode. On command from the control unit 10, the transmitting channel 1 emits a series of K probing pulses to the target. Simultaneously, the control unit starts synchronization of multichannel storage devices 6 and 7, switching their accumulation cells with a period τ (Fig. 2). The signals reflected by the target are received by the photodetector channel 2, at the output of which a mixture of the reflected signal, fluctuation noise, and microplasma pulses is formed. From the outputs of the threshold device 4, the signals are fed in turn to the cells of the drive 6 corresponding to the current range channel, switched on by the clock signal of the control unit 10. The signals in the drive 6 are summed with a weight corresponding to the exceeded threshold. For example, in a two-level transformation with a symmetrical position of the thresholds relative to zero, when the noise emission crosses the upper threshold upwards, “+1” is entered into the accumulator, and when the lower threshold is crossed downwards, “-1” is entered. With such a construction of equipment, a gain E in improving the signal-to-noise ratio is provided, close to the theoretical limit

where K is the number of cycles (volume) of accumulation [2]. The drive 7 contains information about the number of microplasmas in each range channel. The solver 8 calculates the difference between the accumulated sums ΔΣ _j =Σ _j -Σ _Mj , compensating for the presence of microplasmas.

The signal skip due to noise masking, characterized by the probability Q _w , and the signal skip due to microplasma blocking, characterized by the probability Q _M , are mutually independent events [3], therefore, the given probability of signal skip Q=1-D in one measurement can be represented as a sum Q=Q _w +Q _M , where D is the probability of correct signal detection.

When choosing conditions:

And

it is possible to almost completely eliminate the influence of microplasmas on the detection characteristics.

Condition (8) is equivalent to the relation:

where m is the allowable number of microplasmas in one range channel during the accumulation time;

K _then - the threshold value of the accumulated sum in one channel, at which a decision is made about the presence of a signal;

обеспечивающий условие (7).

providing condition (7).

.The accumulated sum K is a random variable with the mathematical expectation corresponding to the level of the received signal and at the threshold value of this value K=K _then with the standard deviation

.

The critical amount of microplasmas in one accumulation channel m should not significantly affect the statistics of the accumulated amount due to the signal, which is indicated by expression (9).

Example 1

; количество каналов накопления N=10⁴; ширина канала τ=10^-8 с; коэффициент

Accumulation volume K=200; the average value of the accumulated amount in the absence of the signal K _cp (0)=0; set threshold

; the number of accumulation channels N=10 ⁴ ; channel width τ=10 ^-8 s; coefficient

Under these conditions, the average number of microplasmas per channel during the accumulation time is:

Average number of microplasmas per range channel per cycle:

The duration of the accumulation cycle Т=Nτ=10 ⁴ ⋅10 ^-8 =10 ^-4 s.

Permissible frequency of microplasmas

The probability of the appearance of microplasma in one channel in two cycles in a row - W ₂ =m ₁ ² .

Under these constraints, such an event means that a high-amplitude signal is likely to occur. Therefore, a decision is made to receive a signal in a given range channel and the accumulation process is stopped.

Example 2

Under the conditions of the previous example, the probability W ₂ =m ₁ ² =4⋅10 ^-6 . In this case, the probability of two microplasmas in a row in any range channel is W _2N =W ₂ ⋅N=4⋅10 ^-2 . In current practice, the signal skipping with such a probability is considered acceptable.

Thus, the task of the invention is ensured - the achievement of theoretically limiting sensitivity in all operating conditions, regardless of microplasma breakdowns following with a frequency of up to 200 kHz.

The proposed method of incoherent signal accumulation provides the maximum probability of detecting signals with a minimum amount of equipment and can be implemented in portable laser rangefinders.

Information sources

1 Ya.D. Shirman, V.N. Golikov "Fundamentals of the theory of detection of radar signals and measurement of their parameters." Ed. "Soviet radio", M, 1963, p. 179.

2 Ya.D. Shirman, V.N. Manzhos "Theory and technique of processing radar information against the background of interference." Ed. "Radio and communication", M., 1981, p. 81-83.

3 V.E. Gmurman, Probability Theory and Mathematical Statistics. Ed. "Higher School", M., 1977, 479 p.

4 V.G. Vilner Design of threshold devices with noise threshold stabilization. Optical-mechanical industry, 1984, No. 5.

5 Patent WO 2005/006016 Al "Laser rangefinder and method thereof.

6 Patent of the Russian Federation No. 2359226 according to No. 2007137271 dated 10.10.2007. "Method of incoherent accumulation of light-location signals". - Prototype.

7 Tikhonov V. I. Emissions of random processes. Ed. "Science", M, 1970, 392 p.

8 Filachev A.M., Taubkin I.I., Trishenkov M.A. Solid state photoelectronics. Physical bases. Moscow, Fizmatgiz. 2007.

9 Vishnevsky A.I., Rudenko V.S., Platonov A.P. Power ion and semiconductor devices. Textbook for universities. Ed. V.S. Rudenko. Moscow, Higher School, 1975.

10 Shashkina A.S. Avalanche breakdown of a p-n-junction in radio engineering problems. - Scientific and technical bulletin of information technologies, mechanics and optics, 2016, volume 16, no. 5, p. 864-871.

11 Vilner VG et al. Evaluation of the capabilities of a light-location pulse range meter with accumulation. Photonics, 2007, No. 6, p. 22-27.

12 RF Patent No. 2390724. "Method for light-location determination of range by the method of incoherent accumulation".

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14 RF Patent No. 2755603. "Method for Threshold Detection of Optical Signals".

Claims (4)

1. The method of incoherent accumulation of pulsed light-location signals, including K cycles of probing the target with laser pulses, in each probing cycle receiving the reflected signal in N range channels, where N=R _max /ΔR, R _max is the range of measured ranges, ΔR=с τ/2 , c is the speed of light, τ is the clock period of the reflected signal delay meter, in each range channel, comparison of the received signal with one or more analog threshold levels, accumulation of the sum of exceedances of the analog threshold levels, taking into account the weighting factor of the level, which is used to judge the presence of a signal in this channel by comparing the sum of excesses with a threshold number, characterized in that the reflected signals are received using an avalanche photodiode, in the preparatory mode, in the absence of probing laser pulses, the optimal avalanche multiplication coefficient M _opt of the photodiode is set for the operating mode, at which the signal-to-noise ratio is maximum, in mode M _opt detect the presence of a pulse microplasmas and determine the minimum amplitude of microplasma pulses U _Mmin , set an additional threshold level U _M within U _cmax <U _M <U _Mmin , where U _cmax is the maximum expected signal amplitude, after which they switch to the signal accumulation operating mode, in each j- m range channel accumulate weighted sums Σ _j of exceedances of operating thresholds, in parallel in each j-th range channel accumulate sums Σ _Mj of level exceedances U _M , at the end of a series of probing cycles in each range channel calculate the difference values ΔΣ _j =Σ _j -Σ _Mj , and if ΔΣ _j exceeds a predetermined threshold number, then a decision is made on the presence of a signal in a given range channel, and the distance to the target is judged by the number of this range channel. 2. The method according to claim 1, characterized in that the optimal mode M _opt is set, including the avalanche-free bias mode of the photodiode M=1, then the operating threshold U is set at a level corresponding to the frequency of noise operations of the threshold device f<<f ₀ , where f ₀ - the frequency of the noise crossing the zero level, then increase the threshold in

times, where α is the noise factor parameter of the avalanche multiplication F=M ^α , threshold

fix and using the bias voltage of the photodiode set such an avalanche multiplication factor M=M _opt at which the frequency f _M noise exceeding the threshold U _M becomes equal to the frequency f in the avalanche-free mode M=1 at the threshold U, when the frequency f _M =f is reached, the achieved avalanche multiplication factor M=M _opt and include the operating mode of signal reception. 3. The method according to claim 1, characterized in that if the signal amplitude in the operating mode exceeds the maximum of the threshold levels U _max , then the avalanche multiplication factor is reduced to the level 1≤M<M _opt , at which the nonlinear distortion of the signal is minimal. 4. The method according to claim 1, characterized in that if a signal is recorded in any of the range channels in two accumulation cycles in a row, then a decision is made about the presence of a target in this channel and the accumulation is stopped, while the probability of the appearance of two consecutive microplasmas in any the range channel should not exceed the value of W _2N =W ₂ ⋅N<(1-D), where W ₂ is the probability of two microplasmas in a row in one range channel; N is the number of range channels; D is the given probability of correct detection of the signal.

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