RU2350018C2 - Method of detection of radiation coupling from lateral surface of optical fibre - Google Patents

Abstract

FIELD: information technologies.

SUBSTANCE: invention can be used for detection of infringement of working capacity of the protected fiber-optical systems of information transfer (FOTS) of the limited access and as a way of the constant control of a fiber-optical line of transfer (FOTL) from leak on the optical channel. In the way, consisting in reception, detecting of optical signals from the fiber-optical communication line, strengthening, integration and analogue-digital transformation of the received analogue electric signals which in the digital form are compared to control value in such a manner that in case of decrease in amplitude of a signal below control value on the set size the alarm signal is formed. The new thing is that through each quantity of readout N, making a cycle set by the timer, average selective value, for example, in j cycle

(1) which is remembered and used in the subsequent cycles for calculation of a control signal under the formula

(2) is calculated, and each i size of a control signal is compared to the threshold value in advance set and written down in memory calculated under formula Φ_n=lg Λ_n+0.5Q₂N, (3) where Λ_n - the set threshold relation of credibility; Q - the set relation of amplitude of the signal called by loss of radiation in a fiber-optical line of transfer, to average quadratic value of noise, thus, if /Φ_i/≥Φ_n the ALARM signal is formed, otherwise after the termination of each j cycles the received size Φ_j is remembered and subtracted from the value received in previous j-1 the cycle, the received difference is compared to in advance set threshold of detection Φ_o and in case /Φ_j-Φ_j-1/≤Φ_o, (4) that for size of average selective value in j+1 a cycle is accepted the size calculated in j cycle, the size used in j-1 a cycle otherwise is accepted.

EFFECT: increase of probability of detection of reserved attempt of unloading an optical signal from a lateral surface of an optical fibre and decrease in probability of false operations at operation.

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Description

The invention relates to methods for detecting a malfunction of protected fiber-optic transmission systems (FOTS) of limited access information and can be used as a method of continuous monitoring of a fiber-optic transmission line (FOCL) from leakage through an optical channel.

A known method of controlling FOCLs from the patent "Device that controls the operation of fiber-optic communication, and a receiver that includes the said device" (French patent No. 2568431 from 1988 on application No. 8411970 from 1984).

The method consists in the fact that the optical signals received from the fiber optic line are detected and amplified to the required value. After that, to eliminate impulse noise and reduce the influence of noise, the signal is integrated and converted from analog form to digital form using analog-to-digital conversion. Analog-to-digital conversion is performed periodically with a given frequency. The resulting value is compared with one or more previous values. If the difference exceeds the set value, an alarm is triggered. The comparison reveals a violation in the operation of the communication system, since it is believed that sudden changes indicate a deliberate violation, and slow changes are caused by external and internal factors.

The control device that implements the method operates as follows. Optical signals received from a fiber optic line are detected by a photodetector and amplified by a photocurrent amplifier. Next, the signals are fed to the output driver and integrator. Thus, at the output of the shaper output information signals are generated. From the output of the integrator, the sequence of signals is converted in such a way that a low-frequency control signal is formed at the output of the integrator, the amplitude of which is proportional to the attenuation of the optical signal in the fiber-optic communication line. Next, the control signal is converted into digital form using an analog-to-digital converter (ADC), which is clocked by a generator. The signals from the output of the ADC are fed to random access memory (RAM) and to the input of a digital comparator. The comparator compares the value of the received signal with one or more previous values retrieved from RAM. If the amplitude of the received signal is less than that of one or more of the previous ones by a predetermined value, an ALARM signal is generated.

The disadvantages of this method of controlling FOCL include the following facts:

1. Low noise immunity due to the fact that the control uses a once-measured signal value, which can be distorted by impulse noise or noise emission.

2. The ability to skip access to the fiber optic line if the exposure time exceeds the maximum storage time of the samples.

3. High probabilities of signal skipping and false alarm due to a small signal processing base (product of the signal frequency band and its processing time).

The well-known "Method of detecting a slow output of optical radiation through the lateral surface of a fiber-optic communication line" (Russian patent No. 2251810 dated January 20, 2003 according to the application No. 2003101467/09), which is closest to the claimed method and therefore adopted as a prototype.

The method consists in the fact that the optical signals received from the fiber optic line are detected and amplified to the required value. After that, the signal is integrated and converted from analog form to digital form using the ADC. After that, the sum of the deviations of the current signal values from the control value is calculated taking into account the sign during the time specified by the timer. After that, the obtained current values of the sums are compared with a predetermined reference signal, and an alarm signal is generated if the difference in the current value of the sum exceeds the reference signal by the value of the specified threshold value. Moreover, the threshold value is periodically adjusted.

A device that implements the method operates as follows. Optical signals from a fiber optic line are detected by a photodetector, amplified by a photocurrent amplifier, and fed to the output through an output signal shaper. The signals from the output of the photocurrent amplifier go to the input of the integrator, from the output of which - to the input of the ADC, where they are converted from analog form to digital form. The ADC is clocked by a generator, the signals from which are fed to the timer input. The timer counts the time during which the total is calculated. From the output of the ADC, the signals are sent to an arithmetic logic unit (ALU), which performs the summation (taking into account the sign) of the obtained value with the previously received sum. The amount is stored in RAM. The digital comparator compares the current values of the signal received from the output of the ADC with the previous (or several previous), as well as the received amount with a given threshold value. If the current value is lower than the previous (previous) by a limit value or the amount reaches a predetermined value, the alarm device generates an alarm. The timer counts down the period of time after which the zero amount is set in RAM. Thus, slow changes in attenuation that occur during a time longer than the storage time of the current values will be detected, since they will lead to a constant increase in the sum (with a negative sign) and, ultimately, to achieve the response value of the alarm device.

The disadvantages of this detection method include:

1. The method meets the criterion of optimal signal isolation against a background of noise (the most possible signal-to-noise ratio when making a decision) only for a constant amplitude pick-up signal. This leads to the fact that for pickup signals that vary during the calculation of the sum, the extremely low values of the probabilities of skipping the signal and false alarm are not achieved.

2. The maximum duration of the detected signal is limited by the period of time through which the zero amount is set in RAM.

The technical task to be solved is an increase in the probability of detecting a covert attempt to pick up an optical signal from the side surface of an optical fiber and a decrease in the likelihood of false positives during operation.

The technical result is to increase the signal-to-noise ratio when making decisions for pick-up signals of any shape, amplitude and duration.

This technical result is achieved by the method of detecting the output of radiation through the side surface of the optical fiber, which consists in receiving, detecting optical signals from a fiber optic communication line, amplifying, integrating and analog-to-digital conversion of the obtained analog electrical signals, which are digitally compared with a control value such so that in the case of a decrease in the signal amplitude below the control value by a predetermined value, an alarm is generated.

What is new is that after each number of samples N constituting the cycle specified by the timer, the average sample value is calculated, for example, in the jth cycle

which is remembered and used in subsequent cycles to calculate the control signal according to the formula

each i-th value of the control signal is compared with a predetermined and stored in the memory threshold value calculated by the formula

where Λ _n is a given threshold likelihood ratio;

Q is the predetermined ratio of the signal amplitude caused by the loss of radiation in the fiber optic transmission line to the root mean square noise value,

in this case, if | Ф _i | ≥Ф _n , then an ALARM signal is generated, otherwise after the end of each j-th cycle, the obtained value of Ф _{j is} stored and subtracted from the value obtained in the previous j-1 cycle, the resulting difference is compared with in advance a given detection threshold f _o and if

then the value calculated in the j-th cycle is taken as the average sample value in the j + 1 cycle; otherwise, the value used in the j-1 cycle is taken.

The set of essential features of the proposed method allows to increase the signal-to-noise ratio when making decisions for signals of any shape, amplitude and duration, since in the calculations for the most probable signal amplitude the absolute value of the implementation at a given time is taken. In addition, a detection threshold has been introduced, upon exceeding which a pickup signal of any duration is detected.

The figure 1 presents a functional diagram of a photodetector that implements the inventive method: 1 - optical fiber (S); 2 - receiving optoelectronic module (PROM); 3 - amplifier-limiter (UO); 4 - linear amplifier (LU); 5 - amplitude detector (HELL); 6 - integrator (INT); 7 - ADC; 8 - clock generator (TG); 9 - device for calculating the average (UVS); 10 - average RAM; 11 - device for calculating the squared deviation (UVCO); 12 - adder (SUM); 13 - digital comparator (COM); 14 - RAM alarm threshold; 15 - RAM amount; 16 - subtraction device (SUB); 17 - digital comparator (COM); 18 - RAM detection threshold.

The figure 2 presents diagrams of three characteristic cycles of operation of the device: 1 - the cycle of calculating the average sample (no signal), 2 - the detection cycle of the pick-up signal (formation of prohibition of overwriting), 3 - the cycle of generating the alarm.

A device that implements the inventive method works as follows. A permanently received optical information signal with an OB (1) is fed to the input pole of the PROM (2), where it is converted into an electrical signal. From the PROM output, the signal goes to the input of the UO (3), which forms a digital information signal. The same signal is fed to the input of the amplifier (4), which amplifies the amplitude of the signal to the desired level. After that, HELL (5) converts the signal amplitude to a constant control level (KU), which is fed to the input of INT (6). INT limits the frequency band of the control signal and accordingly reduces noise.

After that, the analog signal is converted into a digital Kotelnikov series using the ADC (7), which is clocked by the TG (8) (the clock period is Δt). A sample of the input implementation Yi arrives at the UVS (9), which calculates the average sample value for a given period of time T (the number of samples N = T / Δt). The obtained average sample, in the absence of a ban on overwriting, is recorded in the secondary RAM (10), from which it enters the first input of the UVCO (11), and the second input of which receives a sample of the input implementation Yi. UVCO calculates the square of the deviations ΔYi ² received at the input of the SMS (12), which calculates the sum of the squares of the deviations Фi.

The sum of the squared deviations is constantly compared by KOM (13) with the alarm threshold Ф _n previously recorded in RAM (14). If the threshold is exceeded, an ALARM signal is generated. After the end of each cycle (N - samples), the signal Ф _{j is} supplied to the CONTRACT (16), which calculates the modulus of the difference of two adjacent values Ф _{j -} Ф _j-1 , the last of which is stored in RAM (15) and is overwritten after the end of each cycle. The obtained value | ΔФ _j | is compared using KOM (17) with the detection threshold Ф _o , which is previously recorded in RAM (18).

If the modulus of the difference of the compared values is greater than zero, a prohibition signal for overwriting the average sample value in RAM is generated (10).

In the prototypes of the converters FOBOS-100M and FOBOS-100S that implement the inventive method, the following elements and devices were used: receiving optoelectronic modules PROM-364-155 and HFBR-2316T; amplifier limiter K500LP116; linear amplifier K500LP116; amplitude detector 2 BC523 VM; Integrator OPA177GS; ADC; clock generator; average calculator; RAM average; deviation square calculator; adder; digital comparator; RAM alarm threshold; RAM amount; digital comparator; The detection threshold RAM is implemented in the PIC16C717 microcontroller with specially developed DEMOS software.

Tests of the samples showed the following results: detection threshold 0.01 dB, alarm threshold 0.05 dB, detection of signals of any duration, uptime of at least 270 hours.

Claims (1)

A method for detecting radiation output through the side surface of an optical fiber, which comprises receiving, detecting optical signals from a fiber optic line, amplifying, integrating and analog-to-digital converting the obtained analog electrical signals, which are digitally compared with a control value in such a way that in the case of reducing the amplitude of the signal below the control value by a predetermined amount an alarm is generated, characterized in that through each quantity set by the timer counts of N that make up the cycle, the average sample value is calculated, for example, in the j-th cycle

which is remembered and used in subsequent cycles to calculate the control signal according to the formula

moreover, each i-th value of the control signal is compared with a predetermined and stored in the memory threshold value calculated by the formula

where Λ _n is a given threshold likelihood ratio;
Q is the predetermined ratio of the signal amplitude caused by the loss of radiation in the fiber optic transmission line to the root mean square noise value, while if / Ф _i / ≥Ф _n , an alarm is generated, otherwise after the end of each j-th cycle the obtained value of Ф is stored and subtracted from the value obtained in the previous j-1 cycle, the obtained difference is compared with a predetermined detection threshold of Ф _о and if

then the value calculated in the j-th cycle is taken as the average sample value in the j + 1 cycle; otherwise, the value used in the j-1 cycle is taken.

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