RU2778048C1 - Method for receiving pulsed optical signals - Google Patents

Abstract

FIELD: optical technology.

SUBSTANCE: invention relates to the reception of optical signals, in particular to the technique of receiving signals using avalanche photodiodes. A method for receiving pulsed optical signals using an avalanche photodiode, including threshold signal processing using a threshold device and the formation of output pulses, provides for pre-determining the non-multipliable component

of the square of the noise current brought to the output of the photodiode, set the threshold $$$ brought to the output of the photodiode, within

where

is the square of the non-multipliable noise current brought to the output of the photodiode, setting the offset voltage of the photodiode at the level at which the avalanche multiplication coefficient $$$ where α is the noise coefficient of the photodiode,

the square of the multiplied component of the primary noise current brought to the output of the photodiode; e is the electron charge; I₁ is the primary multiplied dark current of the photodiode; Δf is the bandwidth of the receiving path, after that, the offset voltage is reduced until the frequency f_nM of noise emissions exceeding the threshold drops to the maximum permissible level, while the offset voltage is fixed and the threshold is reduced to a level at which the frequency of noise emissions exceeding the threshold does not reach the nominal intermediate value $$$ , then the threshold is increased $$$ times, where f₀ is the frequency of zero-level noise crossing, and fm is the maximum permissible frequency of false alarms in operating mode, after which the threshold is fixed and signals are received.

EFFECT: at maximum speed, sensitivity close to the maximum achievable is provided in all modes, including in the presence of microplasma breakdowns.

4 cl, 3 dwg

I_{пор0 –} I_thr0

М_{опт –} M_opt.

f_{шМ –} f_nM

_m

Description

The present invention relates to the reception of optical signals, in particular to the technique of receiving signals using avalanche photodiodes, and can be used in location, communications and other photoelectronic systems.

A known method of receiving optical signals using avalanche photodiodes [1]. There are also known methods for stabilizing the avalanche mode of the photodiode, for example, by thermal compensation of the operating point of the bias voltage [2].

Closest to the proposed technical solution is a method for receiving pulsed optical signals using an avalanche photodiode, the bias voltage of which is maintained by stabilizing the frequency of noise pulses that occur in the photodiode during avalanche multiplication [3].

The disadvantage of this method is the possibility of introducing a photodiode into the microplasma breakdown mode [4]. Microplasma current pulses (microplasmas) 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 [5]. In this mode, the noise of the avalanche photodiode consists of two independent components - normal noise [6] and "telegraph" 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 [3] is unpredictable. Under certain temperature conditions, adjusting the avalanche mode according to the frequency of photodiode noise emissions, including microplasmas, can lead to the system reaching a non-optimal avalanche multiplication mode, i.e. to the deterioration of the threshold sensitivity of the photodetector or to the unacceptable probability of false alarms caused by microplasmas.

The objective of the invention is to provide high sensitivity in all operating conditions with a minimum time to reach the operating mode.

квадрата шумового тока, приведенного к выходу фотодиода, устанавливают порог I_пор0, приведенный к выходу фотодиода, в пределах

где

- квадрат неумножаемого шумового тока, приведенного к выходу фотодиода, устанавливают напряжение смещения фотодиода на уровне, при котором коэффициент лавинного умножения

где α - коэффициент шума фотодиода,

- квадрат умножаемой составляющей первичного шумового тока, приведенного к выходу фотодиода; е - заряд электрона; I₁ - первичный умножаемый темновой ток фотодиода; Δf - полоса пропускания приемного тракта, после этого уменьшают напряжение смещения до тех пор, пока частота f_шM превышений порога шумовыми выбросами не упадет до предельно допустимого уровня, при этом фиксируют напряжение смещения и начинают уменьшать порог до уровня, при котором частота шумовых превышений порога не достигнет номинального промежуточного значения f_п >> f_шM, затем увеличивают порог в

раз, где f₀ - частота пересечения шумом нулевого уровня, a f_p - предельно допустимая частота ложных срабатываний в рабочем режиме, после чего фиксируют порог и приступают к приему сигналов.This problem is solved due to the fact that in the known method for receiving pulsed optical signals using an avalanche photodiode, including threshold processing of signals using a threshold device and the formation of output pulses, the non-multipliable component is preliminarily determined

squared noise current, reduced to the output of the photodiode, set the threshold I _por0 , reduced to the output of the photodiode, within

where

- the square of the non-multipliable noise current, reduced to the output of the photodiode, set the bias voltage of the photodiode at a level at which the avalanche multiplication factor

where α is the photodiode noise figure,

- the square of the multiplied component of the primary noise current, reduced to the output of the photodiode; e is the electron charge; I ₁ - primary multiplied dark current of the photodiode; Δf is the bandwidth of the receiving path, after that, the bias voltage is reduced until the frequency f _wM of threshold exceedance by noise emissions falls to the maximum permissible level, while the bias voltage is fixed and the threshold is reduced to a level at which the frequency of noise threshold exceedance does not reaches the nominal intermediate value f _p >> f _wM , then increase the threshold in

times, where f ₀ is the frequency of noise crossing the zero level, af _p is the maximum allowable frequency of false positives in the operating mode, after which the threshold is fixed and signals are received.

The frequencies f _w , f _p and f _p are determined in the corresponding modes as the ratio of the number of noise emissions for the control time interval T.

The maximum allowable level of the frequency of microplasmas _fshM can be taken as the number of microplasmas no more than one per time T.

The frequency f _p is chosen from the condition f ₀ >> f _p >> f _p .

In FIG. 1 shows a diagram of a photodetector that implements this method. In FIG. 2a), b) - examples of the implementation of noise at the input of the threshold device. In FIG. 3 shows plots of η(M) for germanium (Fig. 3a) and silicon (Fig. 3b) avalanche photodiodes.

The photodetector contains an avalanche photodiode 1, an amplifier 2 and a threshold device 3 connected in series. The bias voltage is applied to the photodiode 1 from a power source 4 connected in series and a thermal compensation circuit 5. The threshold device is covered by a feedback circuit in the form of a noise automatic threshold control circuit (SHARP) 6 connected between the output of the threshold device and its control input. The output of the threshold device is connected to the input of the reduction circuit 7, and the output of the latter is connected to the thermal compensation circuit 5 and the threshold device 3. The reduction circuit 7 is also connected to the control input of the SHARP circuit 6. Mode synchronization is carried out by the control unit 8 associated with blocks 6 and 7.

The method is carried out as follows.

Preliminarily, at the design stage, the temperature dependence of the avalanche multiplication factor on the bias voltage of the photodiode is determined. At the same time, the temperature course of the optimal avalanche is determined, that is, such an avalanche multiplication factor M _opt (T) at which the signal-to-noise ratio at the output of the photodiode takes on a maximum value at each temperature.

где

- квадрат неумножаемого шумового тока на выходе лавинного фотодиода.In accordance with this, at the manufacturing stage, the thermal compensation circuit 5 is adjusted so that, over the entire temperature range, the avalanche multiplication factor M=M(T) ~ M _opt (T). At the same time, the SHARP 6 circuit is adjusted so that the response threshold of the threshold device reduced to the output of the photodiode is within

where

is the square of the non-multipliable noise current at the output of the avalanche photodiode.

раз и начинают прием оптических сигналов.At the initial moment of time, set by a command from the control unit 8, the input of the threshold device opens, and pulses appear at its output, corresponding to microplasma breakdowns of the photodiode. These pulses are applied to the step down circuit, forcing it to lower the bias voltage of the photodiode and, thereby, reduce the avalanche multiplication factor and the microplasma frequency, defined as the number of microplasmas during the control time interval T (see Fig. 2). When this number during the next cycle T decreases to 1 or 0, the step-down circuit 7 stops the change in the bias voltage and simultaneously turns on the SHARP circuit 6, which begins to lower the response threshold until normal noise appears due to the factor I _w ² \u003d I ₀ ² + Im ² , where Im ² is the square of the multiplied noise current. At the frequency of operation of the threshold device of the level f=f _p , the SHARP circuit fixes the operation threshold at this level, and the preparatory stage ends, after which the operating threshold is set by increasing the threshold by

times and start receiving optical signals.

The described method provides the maximum signal-to-noise ratio in the presence of microplasma breakdowns, which are usually not taken into account, which leads to significant losses in the real sensitivity of the receiving devices.

The optimal value of the avalanche multiplication factor M can be determined as follows. The equivalent square of the noise current acts at the output of the avalanche photodiode

- квадрат неумножаемого шумового тока;

is the square of the non-multipliable noise current;

e is the electron charge;

I ₁ - primary reverse current of the photodiode;

Δf is the bandwidth of the linear 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 [6].

Noise/Signal Ratio W Squared

Zero Derivative Condition

Or

where

When assessing the probability p of a false event (generation of microplasma) by calculating the relative frequency of false events [10, p. 226] as the ratio w of the number m of false events and the total volume k of tests, there is a confidence limit p _in the probability estimate p for m=1.

where w=1/k;

t - confidence factor (reliability of the estimate); for example, a confidence level of 0.95 corresponds to a reliability of t=1.96 [10].

From (7) follows the required amount of tests k=K at m=1 to ensure a given probability p with reliability t

Statistical volume K is required for each step ΔU _cm of bias voltage regulation.

Duration T ₁ control cycle at each stage ΔU _cm

where Δt is the duration of the cycle of checking the presence of microplasma, corresponding to the discreteness of measuring the delay of the received pulse.

Example 1

p _in =0.001; t=2. The criterion for entering the working avalanche mode m≤1 (Fig. 2). In accordance with (8) K≥3⋅10 ³ .

The resolution Δt is usually in the range of 3.33-33.3 ns.

Example 2

K= ¹⁰⁴ ; Δt=10 ^-8 s.

In accordance with (9) the duration of one cycle T ₁ =K Δt=10 ^-4 s.

Number Q steps of bias voltage regulation

where U _cmM - the initial level of the bias voltage;

U _cm0 - final level of bias voltage;

ΔU _cm is a significant change in the bias voltage, at which the observed change in the frequency of microplasmas occurs.

Example 3

U _cmM - U _cm0 =20 V: ΔU _cm =1 V.

Q=20/1=20.

The maximum time to reach the operating mode of displacement

Example 4

Under the conditions of examples 1-3

Т=20⋅10 ^-4 =2⋅10 ^-3 s.

Reducing the avalanche multiplication factor to a level slightly below the optimal level does not lead to a significant deterioration in the signal-to-noise ratio

Example 5 (Fig. 3a).

отличается от максимального значения, обеспечиваемого при М=М_опт=3, не более, чем на 2%.germanium photodiode. I ₁ \u003d 10 ^-7 A. I _m ² \u003d 3.2⋅10 ^-19 A ² . α=1. The area of microplasmas starts from M=4. The operating point of the photodiode is maintained at M=1.8...3.5. In this case, the maximum signal-to-noise ratio provided by the method, that is, the value

differs from the maximum value provided at M=M _opt =3, no more than 2%.

Example 6 (Fig. 3b).

отличается от максимального значения, обеспечиваемого при М=М_опт=30, не более, чем на 2%.Silicon photodiode .I ₁ \u003d 10 ^-9 A. I _m ² \u003d 3.2⋅10 ^-21 A ² . α=0.5. The area of microplasmas starts from M=25. The working point of the photodiode is maintained at M=20...25. In this case, the maximum signal-to-noise ratio provided by the method, that is, the value

differs from the maximum value provided at M=M _opt =30, no more than 2%.

где f₀ - частота пересечения шумом нулевого уровня. Такой процесс стабилизации режима порогового устройства также занимает время порядка 10^-3 с [9].If the process of stabilization of the threshold/noise ratio occurs at a frequency f _p of noise exceeding the threshold exceeding the frequency f _p of such events in the operating mode, then before switching to signal reception, the threshold is increased by

where f ₀ is the frequency of noise crossing the zero level. Such a process of stabilization of the threshold device mode also takes a time of the order of 10 ^-3 s [9].

Thus, at maximum speed, a sensitivity close to the maximum achievable sensitivity is provided in all modes, including in the presence of microplasma breakdowns.

Sources of information

1 Ross M. Laser receivers. - "Mir", M., 1969 - 520 p.

2 RF Patent No. 2248670. The device for switching on the avalanche photodiode in the receiver of optical radiation. 2005

3 US Pat. 4,077,718. Receiver for optical radar. 1978. - prototype.

4 Filachev A.M., Taubkin I.I., Trishenkov M.A. Solid state photoelectronics. Physical bases. Moscow, Fizmatgiz. 2007, - S. 345.

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

6 Vilner V.G., Leichenko Yu.A., Motenko B.N. Analysis of the input circuit of a photodetector with an avalanche photodiode and anti-noise correction. Optical-mechanical industry, 1981, No. 9, - S. 59.

7 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.

8 Vilner V.G. Design of threshold devices with noise threshold stabilization. - Optical-mechanical industry, 1984, No. 5, p. 39-41.

9 RF Patent No. 2718856. A method for automatically stabilizing the frequency of crossing the threshold level by noise process emissions, 2020

10 Gmurman V.E. Theory of Probability and Mathematical Statistics. Moscow, Higher School, 1977, - 480 p.

Claims (4)

1. A method for receiving pulsed optical signals using an avalanche photodiode, including threshold processing of signals using a threshold device and the formation of output pulses, characterized in that the non-multipliable component is preliminarily determined

squared noise current, reduced to the output of the photodiode, set the threshold I _por0 , reduced to the output of the photodiode, within

where

- the square of the non-multipliable noise current, reduced to the output of the photodiode, set the bias voltage of the photodiode at a level at which the avalanche multiplication factor

where α is the photodiode noise figure,

- the square of the multiplied component of the primary noise current, reduced to the output of the photodiode; e is the electron charge; I ₁ - primary multiplied dark current of the photodiode; Δf is the bandwidth of the receiving path, after that, the bias voltage is reduced until the frequency f _wM of exceeding the threshold by noise emissions drops to the maximum allowable level, while fixing the bias voltage and starting to reduce the threshold to a level at which the frequency of noise exceeding the threshold does not reaches the nominal intermediate value f _p >> f _wM , then increase the threshold in

times, where f ₀ is the frequency of noise crossing the zero level, af _p is the maximum allowable frequency of false positives in the operating mode, after which the threshold is fixed and signals are received. 2. The method for receiving pulsed optical signals according to claim 1, characterized in that the frequencies f _w , f _p and f _p are determined in the appropriate modes as the ratio of the number of noise emissions over the control time interval T. 3. The method of receiving pulsed optical signals according to claim 1, characterized in that the maximum allowable level of microplasma frequency f _wM is taken as the number of microplasmas no more than one per time T. 4. The method of receiving pulsed optical signals according to claim 1, characterized in that the frequency f _n is selected from the condition f ₀ >> f _p >> f _p .

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