RU2071179C1 - Method for transmission and receiving information through multiple-mode fiber-optical waveguide - Google Patents

Abstract

FIELD: devices for communication, confidential transmission of information through multiple-mode fiber-optical waveguide. SUBSTANCE: method involves manipulation by some characteristic of light beam according to information signal to be transmitted, feeding manipulated light beam to multiple-mode waveguide, detection of light beam at receiving part of device and decoding information signal. Width of spectral line of light beam is manipulated at input. Optimal decoding spectral structures at output end of multiple-mode fiber-optical waveguide according to their contrast is achieved by means of matrix of photodetectors. EFFECT: increased functional capabilities. 1 dwg

Description

 The invention relates to the field of fiber optic communication and can be used for covert information transmission over a multimode fiber waveguide (MVS).

 Known methods for transmitting digital information via the MIF [1] are that on the transmitting side one of the parameters of the optical field (amplitude, frequency, phase or intensity) is manipulated in accordance with the transmitted information, the manipulated optical radiation is introduced into the MIF, and on the receiving side detect optical radiation and extract the information signal.

Closest to the technical nature of the present invention is a method of transmitting digital information via the MIF by manipulating the intensity, which consists in the fact that to increase the secrecy of the transmission of information is manipulated only 1% of the intensity of optical radiation. At the same time, there is an increase in stealth with respect to the reconnaissance receiver, which is a single photodetector [2]
The disadvantage of this method is the low secrecy of the transmission of information in relation to the reconnaissance receiver, identical to the receiver used at the output end of the MVS.

 At present, various methods have been developed for removing radiation from the MIF through the side surface [3, 4, 5]. These methods allow unauthorized output of 3–7% of the total radiation energy. In this case, optical radiation can be supplied to the input of the reconnaissance receiver, the power of which is comparable in magnitude with the power of the radiation supplied to the input of the optical receiver of the fiber optic transmission system (FOTS). In this case, if a receiver identical to that used in the FOTS is used as a reconnaissance receiver, their noise immunity will be approximately equal. The secrecy of the transmission of information in this case is determined by noise immunity, which in turn is comparable to the noise immunity of the VOSP receiver. Given the high noise immunity of the VOSP, its secrecy with respect to such a reconnaissance receiver is low. The reason for this is that, at the point of reconnaissance connection, the reception conditions are potentially better than at the output end of the MVS due to the fact that the distance from the input end of the MVS to the connection point of the reconnaissance receiver is less than the length of the MVS.

 The aim of this invention is to increase the secrecy of the transmission of information through a multimode fiber.

 This goal is achieved by the fact that the width of the spectral line of the radiation is manipulated, and on the receiving side, the speckle structures at the output end of the MVS are optimally differentiated by their contrast using the photodetector array. On the transmitting side, by manipulating the width of the spectral line of the radiation in binary transmission, two equally-energetic optical signals of the same frequency with different coherence times are alternately generated. In MVS, both signals form speckle structures at a certain distance from its input end. Further, as it propagates along the MSS, the speckle structure formed by radiation with a shorter coherence time (with a wider spectral line) begins to collapse, which manifests itself in a decrease in its contrast. In this case, the speckle structure formed by radiation with a long time of specificity retains its contrast. With the complete disappearance of the contrast of the speckle structure formed by radiation with a shorter coherence time and the preservation of a high-contrast speckle structure formed by radiation with a long coherence time, the best distinguishability of these signals is achieved. It is in this place that it is necessary to install the receiving module of the fiber optic information transmission system. At the same time, a reconnaissance receiver connected at a shorter distance from the optical transmitter will have low noise immunity due to the poor distinguishability of speckle structures with approximately equal contrasts.

где σ дисперсия МВС,
t_к время когерентности источника.This invention meets the requirement of industrial applicability, since sources with an adjustable width of the radiation spectrum are used to estimate dispersion in the MVS [6]. An expression is obtained for the speckle structure contrast:

where σ is the dispersion of the MVS,
t _to the source coherence time.

где n_o показатель преломления сердцевины МВС;

n₁ показатель преломления оболочки МВС; С скорость света в вакууме; L длина МВС.For MVS, intermode dispersion is of decisive importance, which for a gradient MVS is equal to [7]

where n _o the refractive index of the core MVS;

n ₁ the refractive index of the shell MVS; With the speed of light in a vacuum; L is the length of the MVS.

где l длина волны излучения. Подставляя выражение (2) для градиентного МВС и (3) в формулу (1) получим

(4)
Рассмотрим МВС с n_o 1,46, n₁ 1,45, входной торец которого облучается излучением с длиной волны 1,55 мкм. Пусть при передаче информационного символа "1" это излучение имеет ширину спектральной линии Δλ₁= 10^-4 нм, а при передаче символа "О" Δλ₀= 2•10^-3нм.. Пусть длина связи составляет 10 км. На таком расстоянии контраст спекл-структуры при передаче "1" равен 0,85, а при передаче "О" 0,014. В этом случае наблюдается большое различие в контрастах спекл-структур. Пусть разведприемник подключается на расстоянии 5 км от оптического передатчика. На его входе будут наблюдаться слаборазличимые спекл-структуры, контрасты которых равны 0,999 и 0,8547. Кроме того, применение для различения данных сигналов невозможно, так как разница их времени когерентности (ширин спектральных линий) меньше точности измерения, обеспечиваемой данными методами [8]
На приемной стороне осуществляется оптимальное различение спекл-структур на выходном торце МВС по их контрасту с помощью матрицы фотодетекторов. Отношение правдоподобия для данных сигналов имеет вид:

где α количество пространственных степеней свободы (спеклов);
b_l количество временных степеней свободы при передаче информационных символов l(0,I);
K количество фотодетекторов в матрице;
n_i количество фотоэлектронов, зафиксированных 1-м фотодетектором матрицы;

(a)_b символ Похгаммера.The coherence time with a rectangular envelope of the signal as follows depends on the width of the spectral line of radiation Δλ:

where l is the radiation wavelength. Substituting expression (2) for the gradient MBC and (3) in the formula (1) we obtain

(4)
Consider a MVS with n _o 1.46, n ₁ 1.45, the input end of which is irradiated with radiation with a wavelength of 1.55 μm. Suppose that when transmitting the information symbol "1" this radiation has a spectral line width Δλ ₁ = 10 ^-4 nm, and when transmitting the symbol "O" Δλ ₀ = 2 • 10 ^-3 nm .. Let the communication length be 10 km. At such a distance, the speckle structure contrast for transmission “1” is 0.85, and for transmission “O” 0.014. In this case, there is a large difference in the contrasts of speckle structures. Let the reconnaissance receiver connect at a distance of 5 km from the optical transmitter. At its entrance, subtle speckle structures with contrasts of 0.999 and 0.8547 will be observed. In addition, it is impossible to distinguish between these signals, since the difference in their coherence time (spectral line widths) is less than the measurement accuracy provided by these methods [8]
On the receiving side, the speckle structures at the output end of the MVS are optimally distinguished by their contrast using the photodetector array. The likelihood ratio for these signals is:

where α is the number of spatial degrees of freedom (speckles);
b _{l the} number of temporary degrees of freedom in the transmission of information symbols l (0, I);
K is the number of photodetectors in the matrix;
n _{i is the} number of photoelectrons recorded by the 1st photodetector of the matrix;

(a) _{b is} the Pohhammer symbol.

To achieve optimal reception, the current at the output of each photodetector is measured, the corresponding number of photoelectrons, n _i , i 1, K, is calculated, with the help of which the likelihood ratio is calculated according to formula (5). Then, the obtained likelihood ratio is compared with a threshold value, which, if the a priori probabilities of information symbols are equal and the damage from receiving different symbols is the same, is equal to 1. For Λ> 1, a decision is made in favor of the information symbol "T", otherwise, a decision is made to accept the information symbol "ABOUT".

 The drawing shows a structural diagram of a fiber optic information transmission system that implements this method. The system contains a source of binary information sequence 1, a manipulator 2, source 3 with an adjustable spectral line width, MVS 4, a photodetector matrix 5. Block 6 calculates the likelihood ratio, threshold block 7, receiver 8 of the binary information sequence. Source 1 generate a sequence of information signals, which is fed to the input of the manipulator 2, which controls the source 3, manipulating it in accordance with the transmitted message. The manipulated message is fed to the input of the MVS 4, from the output of which to the photodetector array 5. Each output of the photodetector array 5 is connected to the corresponding input of the unit 6. Each photodetector of the matrix 5 registers a certain amount of radiation incident on it. At the output of each photodetector, a current is proportional to the number of photodetectors. Block 6 operates according to expression (5) and generates a voltage at its output, the value of which is equal to the calculated likelihood ratio. This voltage is applied to the input of the threshold block 7, which compares with the threshold value of the likelihood ratio. When the threshold is increased, the information symbol "I" is generated, otherwise the information symbol "O". The received signal is fed to the input of the recipient 8 binary information.

Claims (1)

 1. A method of transmitting and receiving digital information through a multimode fiber waveguide, namely, that on the transmitting side, one of the parameters of the optical radiation is manipulated in accordance with the transmitted information signal, the manipulated optical radiation is introduced into the multimode optical fiber, and the optical side is detected radiation and the selection of the information signal, characterized in that the width of the spectral line of radiation is manipulated, and on the receiving side estvlyaetsya optimal distinction speckle patterns at the output end of the multimode optical fiber for their contrast with the help of the matrix of photodetectors.