RU2538449C1 - Method of increasing range of high-speed open optical links with underwater facilities - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method employs a laser beam consisting of pulses with duration of not less than 1 ns, which are generated from a plurality of waves by phase synchronisation and interference. Laser pulses with duration of 1 ns or less are generated by synchronising mode components and coherent interaction thereof on a propagation path. Pulse-periodic optical pulses are generated from the plurality of continuous mode components, wherein the repetition of said pulses is determined by the spectral composition of the laser radiation. The pulse sequence is detected by a photodetector as presence (or absence) at given moments in time of a radiation pulse by chirping the spectral frequency of the laser radiation. A binary code is generated to transmit information.

EFFECT: longer range of optical communication owing to improved transmission of signals through attenuating portions of a path.

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Description

The present invention relates to the field of optical communication, and in particular to a technique for transmitting and receiving information using laser communication.

Despite the widespread use of radio communications, the Navy constantly uses an optical system for transmitting information using laser communications or spotlights [A.A. Katanovich, Yu.L. Nikolashin. Ship optical communication systems. Shipbuilding, St. Petersburg, 2009, 239 pp.].

Airborne laser communication systems can be installed on all land-based and deck-based anti-submarine defense aircraft, relay aircraft and air command posts. To control submarines operating in a rapidly changing tactical environment, real-time data transmission to their address is required. This transmission is carried out using a narrow laser beam from an airplane to a submarine.

However, the reliability of the laser communication line is affected by cloudiness, fog, precipitation and some other factors.

Known "Optical system of open communication." Katanovich A.A. and others. Patent of the Russian Federation on PM No. 62316. Bull. No. 9, 2007. The optical system consists of an optical transmitter (laser), a nonlinear optical element, a separator, a differential amplifier and an optical receiver (photodiode).

The system provides the ability to transmit information in both digital and analog form in open optical communication lines. However, cloudiness, fog, drizzle, etc., affect the reliability of optical communications.

The Known Method of optical signaling in fog. USSR copyright certificate No. 527729, 1976. According to this method, in order to increase the communication range, a low-diverging infrared beam is formed coaxially with the light beam. The disadvantage of this method is the insufficient brightness of the luminous zone.

The aim of the invention is to increase the range of optical communication due to the better passage of signals through the weakening sections of the route.

This goal is achieved in that a method for increasing the range of high-speed open optical communication channels, which consists in using a laser beam to organize optical communication, the laser beam consisting of pulses of at least 1 ns duration, which are formed from multiple waves by phase synchronization and interference, as well as in a nonlinear medium due to the dispersion of wave velocities of various frequencies, and the phenomena caused by intermode beats lead to the formation of pulsed radiation, while laser pulses of 1 ns duration or less are formed due to the synchronization of mode components and their coherent interaction on the propagation path, and pulse-periodic optical pulses are formed from the set of continuous mode components, the repetition period of which is determined by the spectral composition of laser radiation, and the form of the optical pulses generated with a frequency f _o = 5.63762 · 10 ¹⁴ Hz with an inter-mode interval Δf = 5 MHz, with a repetition rate of 5 MHz and a half-width Δf = 20 ns, using frequency modulation (chirping), pulses are formed with a single double repetition period, while the pulse train is perceived by the photodetector as the presence (or absence) of a radiation pulse at given times, due to the chirping of the frequency of the laser radiation spectrum, a binary code is generated for transmitting information.

It should be noted that in the proposed method for the formation of nanosecond and shorter pulses, there is a fundamental difference associated with the use of intermode beats. These beats represent the interaction (interference effect) of the spectral components of the radiation mixed on the intermode interval.

, а скорость перемещения интерференционных полос $$v=\Delta \omega {\left|к\right|}^{-2}$$

, к=к₁к₂ - величина векторная, где к₁ и к₂ - волновые векторы интерферирующих лучей. При этом максимумы интерференционной картины будут перемещаться с групповой скоростью.Consider the transmission of an information signal using an interference stream. The interference flow of two flows of electromagnetic waves oscillates in the direction of their propagation. In an interference field formed in a weakening medium, the energy loss of optical radiation occurs only in the region of bright bands. The interference pattern step is $$a=2\pi {\left|\mathrm{to}\right|}^{-2}$$

, and the speed of movement of interference fringes $$v=\Delta \omega {\left|\mathrm{to}\right|}^{-2}$$

, k = k ₁ to ₂ is a vector quantity, where k ₁ and k ₂ are wave vectors of interfering rays. In this case, the maxima of the interference pattern will move with a group velocity.

The wave vectors of the individual spectral components are parallel. Therefore, for the spectral components (necessary for the formation of short pulses) with an intermode interval of 10 MHz, the group velocity in air far from the absorption line coincides with the phase velocity (speed of light). For two interfering waves, the envelope of the total wave is described by a sinusoid, and as the number of components increases, the wave energy is localized in a narrow region of the maximum of the interference band.

- добротность резонатора; r₁, r₂ - коэффициенты отражения зеркал резонатора; λ - длина волны излучения). Углы распространения поперечных мод составляют $${\theta }_m=\sqrt{m^{\lambda /L}}$$

, где m=1,2,3,… - номер модовой составляющей. Поперечные моды, распространяющиеся под углом к оптической оси лазера, пройдя некоторое расстояние по открытым трассам, уходят из пучка. Для L=1,0 м и длины волны излучения 0,8 мкм это расстояние менее 10 м, что подтверждает справедливость вывода о совпадении групповой скорости (скорости перемещения интерференционных полос) со скоростью света.The beat of radiation fluxes at different frequencies can be observed both in the laser cavity and on the propagation path. The intermode interval of the resonator can be determined from the relation Δf = c (2L) ^-1 , where L is the length of the resonator; c is the speed of light. Theoretical half-width of the laser generation line in the single-frequency mode (line width of one mode component) Δv = 2πhνp ^-1 (Δν _p ) ² , where p is the laser power; Δv _p = v / Q is the half-width of the passband of the resonator $$\left(Q=\frac{2\pi L}{\lambda \left(\mathrm{one}-r_{\mathrm{one}}{r}_2\right)}\right)$$

- quality factor of the resonator; r ₁ , r ₂ are the reflection coefficients of the mirrors of the resonator; λ is the radiation wavelength). The angles of propagation of the transverse modes are $${\theta }_m=\sqrt{m^{\lambda /L}}$$

, where m = 1,2,3, ... is the mode component number. The transverse modes propagating at an angle to the optical axis of the laser, after passing some distance along open paths, leave the beam. For L = 1.0 m and a radiation wavelength of 0.8 μm, this distance is less than 10 m, which confirms the validity of the conclusion that the group velocity (the velocity of movement of the interference fringes) coincides with the speed of light.

Let's make a calculation. The electrical component of the light wave (as well as the magnetic component) can be represented as

$$E^i=E^i{}_o\cos \left({\omega }_{i}l-k_{i}z+{\varphi }_o^i\right),\left(\mathrm{one}\right)$$

where l is the coordinate in the direction of radiation propagation; ω _i = 2πf _i .

When the laser is simultaneously synchronized to generate N spectral components (mode components) simultaneously, the amplitude value at different points of the path can be determined by adding the amplitudes of the individual components (taking into account the signs of the spectral components and the spectral width of each mode):

$$E={\displaystyle \sum _{l=\mathrm{one}}^n{E}^i}={\displaystyle \sum _{i=\mathrm{one}}^n{E}_o^i\cos \left({\omega }_{i}t-k_{i}z+{\varphi }_o^i\right)}.\left(2\right)$$

As is known, the power of electromagnetic radiation along the propagation path in a weakening medium decreases exponentially (Bouguer’s law): I = I _o exp (-αz), where α is the attenuation coefficient.

The radiation intensity I is proportional to the square of the amplitude of the electric component of the electromagnetic wave.

$$I~{\left({\displaystyle \sum _{i=\mathrm{one}}^n{E}^i}\right)}^2={\left({\displaystyle \sum _{i=\mathrm{one}}^n{E}_o^i\cos ({\omega }_{i}t-k_{i}z}\right)}^2\left(3\right)$$

When studying the possibility of signal transmission through the attenuating portions of the optical communication channels using the interference flow, transmission coefficients were calculated for various relative positions of the attenuating portion and phase of short (10 ^-9 s) periodically radiation pulses (5 MHz). To form periodic laser pulses by coherent summation of several components, synchronize the phases of the individual spectral components. For simplicity of calculations, the initial phases of the waves in formula (2) were taken equal to zero.

The calculations showed that when the interference of 150 mode components, the amplitude of the signal passing through a 66 m long section with an attenuation coefficient of 0.17 m ^-1 is 98% of the initial value. The intensity was calculated for time moments spaced from each other by 10 ⁻¹³ s. The attenuating section was located symmetrically with respect to the pulses in the region of predominantly tunnel transmission of radiation energy.

The calculations are as follows. At the starting point (with zero coordinate), the sum of the amplitude values of all harmonic components was found. For the remaining calculated points of the optical beam path, the signal intensity was determined by adding harmonic signals taking into account their signs (cosine value) - vector addition. To determine the radiation intensity, the sum was squared and multiplied by a coefficient c / 8π, since the light intensity is related to the complex amplitude of the wave

$$I=\frac{c}{8\pi }{\left|E^{\mathrm{one}}\right|}^2$$

The attenuation was calculated in two ways. In the first case (scalar incoherent addition of waves) to determine the total initial intensity, all spectral components were added in intensity (squares of amplitudes). The resulting value was multiplied by a factor of s / 8π. According to Bouguer's law, the intensity of the total wave after the attenuating layer was determined. Without a weakening section, the radiation intensity of the incoherent beam remained constant along the entire length of its propagation path.

For a beam of coherent components at each point of the path, vector summation of waves at different wavelengths was performed. For the attenuating section, according to Bouguer’s law, the change in the effective intensity value of each harmonic component between two calculated points was determined taking into account the ratio of the total intensity values at the current point to the intensity of the incoherent beam. In the next calculation section, the amplitudes of the electric field intensities of individual waves were replaced by their reduced values at the end of the previous section, determined from the expression

$$E_o^{\mathrm{at}sx}\left(i\right)=E_o^{\mathrm{at}x}\sqrt{\mathrm{one}-\delta I_o\left[\mathrm{one}-\exp \left(-\alpha \Delta l\right)\right]}$$

where α is the attenuation coefficient; Δl is the length of the calculated section; δI _o is a coefficient that determines the magnitude of the change in the intensity of coherent radiation relative to incoherent in the calculation area due to the interference flow.

As can be seen from figure 1, between the two maxima there are slight declining bursts of intensity. The region of minimum values of additional pulses is located in the middle between two pulses. Since the coefficient of transmission through the attenuating portion of the path, which is located symmetrically relative to the middle between the pulses, was calculated, which provides the best conditions for the passage of the optical radiation flux (minimal attenuation).

The dependence of the transmittance of the attenuating portion on the spectral composition for various values of optical density is shown in figure 2. It is seen that with an increase in the number of spectral components, regardless of the optical density of the attenuating region, an increase in the transmittance of the laser radiation flux is observed. With a shift of 5 MHz components, the beat period between different spectral components of the radiation with a central frequency f _o = 5.637362 · 10 ¹⁴ Hz is 60.81 m. The coefficient of laser radiation transmission through the attenuating layer is calculated for a section 53.38 m long, located symmetrically between interference highs.

For comparison with the obtained results, the transmission coefficient of the incoherent radiation flux having the same spectral composition as the coherent beam was calculated. The dependence of the transmission coefficient of the laser and incoherent radiation through the attenuating section on its length is shown in Fig.3. Moreover, the transmittance for the site with a coefficient of attenuation of 0.05 m ^-1 is 0.056; for a site with 0.15 m ^-1 - 1.78 · 10 ^-4 ; for an attenuating layer with 0.3 m ^-1 or more, it is practically zero (less than 10 ^-7 ). Thus, a coherent beam (solid lines in the figures) passes through a weakening layer with much lower losses than an incoherent one (dashed lines).

Figure 4 shows graphs of the dependences of the transmission coefficient through the attenuating layer of laser (solid lines) and incoherent (dashed lines) radiation from the spectral composition. Calculations show that a coherent beam propagates along attenuating paths with much smaller losses than an incoherent beam. Changes in the spectral composition (shear frequency of adjacent components Δf) are not significantly affected. The data were calculated for an attenuating layer located symmetrically and occupying 96% of the distance between the maximums of interference with a change in the magnitude of the spectral shift between the components.

The data shown in figures 3 and 4 are calculated for N = 120 components. The incoherent beam intensity decreases in the attenuating layer according to the exponential law (Δf = 5 MHz). At small values of the ratio of the length of the attenuating section to the distance between the maximums of interference (Δl = 55 m), the influence of the attenuating layer is insignificant. When the region of the maximum of interference is combined with the attenuating layer, the radiation loss increases.

A decrease in the transmittance of the layer with a decrease in the shift of the spectral components occurs due to an increase in the distance between the intensity maxima. At the same time, the length of the attenuating layer taken into account in the calculations increases simultaneously. The graphs in Fig. 4 make it possible to estimate the ratio of the coefficients of coherent and incoherent beams of different spectral composition with the limiting length of the attenuating layer for the coherent beam.

Thus, the above calculations indicate the existence of a fundamental possibility of transmitting a coherent optical radiation flux through attenuating sections (rain, fog, cloud cover, etc.) of open atmospheric paths using an interference flux. In this case, it becomes possible to transmit optical radiation over long distances through open atmospheric communication channels.

Claims (1)

A method for increasing the range of high-speed open optical communication channels with underwater objects, which consists in using a laser beam to organize optical communication, characterized in that the laser beam consists of pulses of at least 1 ns duration, which are formed from many waves by phase synchronization and interference , as well as in a nonlinear medium due to the dispersion of wave velocities of different frequencies, while the phenomena caused by intermode beats lead to the formation of pulsed radiation, and laser pulses of 1 ns duration or less are formed due to the synchronization of mode components and their coherent interaction on the propagation path, and pulse-periodic optical pulses are formed from a variety of continuous mode components, the repetition period of which is determined by the spectral composition of laser radiation, and the form of the optical pulses generated frequency f _o = 5.63762 · 10 ¹⁴ Hz with an inter-mode interval Δf = 5 MHz, with a repetition rate of 5 MHz and a half-width Δf = 20 ns, using frequency modulation ( pulping), generate pulses with a single double repetition period, while the pulse train is perceived by the photodetector as the presence (or absence) of a radiation pulse at given times due to chirping of the frequency of the laser radiation spectrum, form a binary code for transmitting information.

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