Improvements in classical and quantum free-space communication by separating the reference beam and signal beam using adaptive optics and time delays for light sources moving relative to the detector.

Abstract

Disclosed is a method for improving information transmission speed, comprising the steps of reducing atmospheric distortion by emitting a reference light source for adaptive optics correction and a signal light source for optical communication at the same or nearly the same wavelength and by adjusting the time delay between the reference light source and the signal light source and / or the delay time in the adaptive optics control and / or the apparent angular velocity of the reference light source and the signal light source relative to the detection module and / or the physical separation between the reference light source and the signal light source, juxtaposing detection of the reference light source beam and detection of the signal light source beam, and simultaneously correcting the distortion of the signal light source using adaptive optics to correct the waveform distortion of the reference light source. The reference light source is brighter than the signal light source, the reference light source (pulsed or continuous) is emitted earlier than the signal light source (pulsed or continuous), and the optical paths of the reference light source beam and the signal light source beam have nearly the same wavefront distortion.

Description

[Technical Field]

[0001] Cross-reference of related applications In accordance with applicable patent laws and / or the rules governing the Paris Convention, this application timely asserts the priority and interests of International Patent Application PCT / CN2021 / 096100, filed on 26 May 2018. For all purposes, the entire disclosure of International Patent Application PCT / CN2021 / 096100 is incorporated by reference as part of the disclosure of this application.

[0002] Disclosed are a system for improving information transmission speed, an information transmission system, a method for improving information transmission speed, and related methods. [Background technology]

[0003] The reflectivity of air changes slightly with variations in physical parameters such as density, pressure, and temperature. As a result, atmospheric turbulence dynamically distorts the wavefront of light rays, causing blur and drift in transmitted images. Adaptive optics (AO) is a technique that corrects such image distortion. Its basic idea is to correct wavefront distortion through feedback control. The most commonly used method is to dynamically adjust the deformable optical elements of the imaging system. AO technology is widely used in fields such as astronomy, optical communications, and microscopy.

[0004] To correct distorted images as quickly and accurately as possible, a sufficiently bright reference light source is necessary. In typical classical optical communication applications, the signal source itself is sufficiently bright and also functions as the reference light source. Many implementations have been proposed and developed. One example is to divide the received image into multiple sub-apertures. By dynamically adjusting the phase shifter of each sub-aperture, the instantaneous output signal-to-noise ratio of the entire received signal can be maximized.

[0005] Not all classical optical communications, including optical arrays (AO), use a signal source. One example uses sunlight reflected from a mirror on a satellite as a reference light source. AO techniques are used to correct for wavefront distortion between this reference light source and the optical signal source emitted from a nearby satellite. However, this method has three problems. First, sunlight is not always available. Second, the two satellites are at different altitudes, making them too far apart to apply AO correction techniques effectively. Furthermore, very bright reflected sunlight causes significant tube currents in the air inside the telescope used to detect and correct the reference sunlight. This degrades the performance of AO correction.

[0006] In astronomical applications, to observe faint celestial objects, astronomers use a nearby bright star or an artificial guide star as a reference light source (in terms of apparent angular separation observed from the telescope). In either case, the reference light source and the observed celestial object pass through the same telescope's optical system. The idea is that because the two light sources are angularly close, they should experience some degree of the same wavefront distortion. Therefore, if wavefront correction of the reference light source using AO (Automatic Angle) techniques is successful, image correction of faint celestial objects should also be successful.

[0007] Signal correction can become more difficult when the reference light source and signal source move relative to the detector. The movement of the signal source suggests a reduction in the effective spatial and temporal scales of atmospheric turbulence. To achieve a similar level of AO correction, the faster the relative movement, the faster the AO control must be. This is true whether the atmospheric turbulence is spatially heterogeneous or temporally static. [Overview of the project]

[0008] The following provides a simplified overview of the invention to offer a basic understanding of some aspects of the invention. This overview is not a comprehensive overview of the invention. It is not intended to identify any key or important elements of the invention, nor to clarify its scope. Rather, the sole purpose of this overview is to present some of the concepts of the invention in a simplified form as a preliminary step to the more detailed description provided below.

[0009] Information transmission speed can be severely affected by atmospheric distortion in free-space communication, particularly in the visible spectrum, when the light source is moving relative to the detector. This document describes a method for correcting the effects of atmospheric distortion using a reference beam and adaptive optics, while simultaneously transmitting classical or quantum information via a nearby delayed signal beam. This technique is effective when a wavefront detection module, which detects and corrects atmospheric distortion in the reference beam, and a signal detection module, which detects the actual optical communication signal, are located in close proximity to each other at the receiving end.

[0010] Disclosed herein is a method for improving the rate of information transmission. The method includes the steps of reducing atmospheric distortion by emitting a reference light source for adaptive optics correction and a signal light source for optical communication at the same or substantially the same wavelength, and by adjusting the time delay between the reference light source and the signal light source, and / or the delay time in adaptive optics control, and / or the apparent angular velocity of the reference light source and the signal light source with respect to the detection module, and / or the physical separation between the reference light source and the signal light source; the steps of simultaneously detecting the reference light source beam and the signal light source beam; and the steps of simultaneously correcting the distortion of the signal light source by using adaptive optics to correct the distortion of the waveform of the reference light source. The reference light source is brighter than the signal light source, the reference light source and the signal light source move relative to the detection module, the (pulsed or continuous) reference light source is emitted earlier than the (pulsed or continuous) signal light source, and the optical paths of the reference light source beam and the signal light source beam have substantially the same wavefront distortion.

[0011] Further disclosed is a system for improving the information transmission speed using a wavefront detection module that detects and corrects atmospheric distortion of a reference beam, and a signal detection module that is positioned close to each other at the receiving end of an information transmission system and detects the actual optical communication signal. The wavefront detection module and the signal detection module are positioned close to each other such that the center of the image of the reference beam overlaps at least with the center of the photosensitive surface of the wavefront detection module.

[0012] Further disclosed is an information transmission system comprising: a first emitter for generating a signal light source for optical communication; a second emitter for generating a reference light source with the same or substantially the same wavelength as the signal light source; a first detector; a second detector; and an adaptive optics system. The reference light source is brighter than the signal light source. The optical paths of the reference light source beam and the signal light source beam have substantially the same wavefront distortion. The first detector detects the signal light source beam. The second detector detects the reference light source beam. The first and second detectors are arranged adjacent to each other. The adaptive optics system corrects the waveform distortion of the light source and simultaneously corrects the distortion of the signal light source.

[0013] To achieve the aforementioned and related objectives, the present invention includes features fully described below, particularly those shown in the claims. The following description and accompanying drawings illustrate in detail specific exemplary embodiments and examples of the present invention. However, these represent only a fraction of the various methods in which the principles of the present invention may be employed. Other objectives, advantages, and novel features of the present invention will become apparent from the following detailed description of the invention, when considered in conjunction with the drawings. [Brief explanation of the drawing]

[0014] [Figure 1] The arrangement of ground-to-ground communication according to one embodiment is shown. [Figure 2] This shows a satellite-to-ground communication configuration according to another embodiment. [Figure 3] Further, an arrangement of communication from a flying object to the ground, according to another embodiment, is shown. [Figure 4]Furthermore, a communication configuration for use with a telescope, according to yet another embodiment, is shown. [Figure 5] This shows the spatial communication arrangement of an embodiment of the operational mode of AO technology. [Figure 6] This shows the temporal communication configuration of an embodiment of the operational mode of AO technology. The light source moves around the detector from time t=0 to t=Tr. By carefully adjusting Tr, the value of θ1 can be minimized, thereby allowing the initial reference beam and the delayed signal beam to travel along somewhat the same optical path. [Figure 7] This shows the simulation setup at the receiving end. [Figure 8] This shows a schematic representation of a communication channel. [Figure 9] This diagram schematically shows both beams heading towards the receiver, with the distance between the beams changing with z. [Figure 10] This schematically represents the coherent efficiency γ versus separation distance L at different zenith angles ζ, depending on the presence or absence of AO. [Figure 11] This schematically represents the coherent efficiency γ versus the zenith angle ζ for various separation distances L in meters between the reference beam and the signal beam. [Figure 12] This diagram schematically represents the coherent efficiency γ versus zenith angle ζ for spatial separation systems and WDM systems. [Figure 13] This shows the Greenwood frequency versus zenith angle. [Modes for carrying out the invention]

[0015] Recently, AO technology has been applied to ground-based free-space secure quantum communication over a distance of 19.2 km, where two photon sources with different but close wavelengths (one for AO correction and the other for secret key generation) are used via frequency multiplexing. This implementation has three problems. First, it cannot be scaled when the communication distance increases due to frequency dispersion. Second, the separation of the two frequency signals is not effective, resulting in a very low signal transmission speed. Third, it is ineffective when the signal source moves relative to the detector. In fact, the effectiveness decreases as the relative speed increases.

[0016] Here, the problems of free-space secure quantum communication are solved by using two artificial light sources that emit at the same or approximately the same wavelength (a bright reference light source for effective AO correction and a weak signal light source for actual optical quantum communication). Referring to Figures 1 to 4, these two light sources are physically located close together. Similarly, a wavefront detection module that detects the reference light source beam and a signal detection module that detects the signal light source beam are located next to each other. The timing of the (pulsed or continuous) reference beam, the timing of the (pulsed or continuous) signal beam, and the response time of the AO control are carefully, and in some cases dynamically and compensatoryly, adjusted. In this way, the optical paths of the two sets of light sources of the same or approximately the same wavelength experience some degree of the same wavefront distortion. Thus, the correction of the waveform distortion of the reference light source by AO can simultaneously correct the distortion of the weaker signal light source. Indeed, since the two sets of light sources are located far enough apart, the influence of diffraction and scattering of the reference light source on the signal light source is negligibly small, and vice versa. Consequently, a favorable feature of this method is that the signal transmission speed does not depend on the reference light source.

[0017] Referring to Figures 5 and 6, the exemplary embodiment illustrates the optical relationship between two artificial light sources emitting a signal beam and a reference beam, an adaptive optics system, a reference detection module, a signal detection module, and feedback control that changes the adaptive optics system based on the readings of the reference detection module. That is, the relative angular velocity (θ² / T) between the two beams and the two detection modules.r ) and the angle (θ) between the reference beam and the signal beam observed from the detection module. S Using the information of ), the optical path of the reference beam emitted at time t=0 and the time T characterized by the angle (θ1) r The difference between the path of the signal beam emitted and the optical path it travels can be minimized. This time difference T r However, if the timescale is about an order of magnitude shorter than the atmospheric turbulence fluctuations and longer than the response time of the AO system, the waveform distortion correction implemented by the controller in the adaptive optics system can minimize atmospheric distortion and thereby improve the communication speed in free-space communication of the signal beam. Indeed, T r The value of can be adjusted dynamically and compensatoryly. Furthermore, by changing the angular velocity of these two light source beams relative to the detector module and / or the physical separation between the two light source beams, the difference between the optical paths through which the leading reference beam and the delayed signal beam travels can also be minimized; however, these methods are technically difficult and may not be economical using current technology.

[0018] In this specification, "approximately the same wavelength" means that the two wavelengths are within a range of 50 nm from each other, "the reference beam and signal beam moving relative to the detection module" means that the relative angular velocity between the two beams and the two detection modules is greater than the mean solar angular velocity, i.e., about 360° per day, and "wavefront detection module" means an apparatus or technique for directly or indirectly measuring and / or reconstructing a wavefront. In other embodiments, "approximately the same wavelength" means that the two wavelengths are within a range of 25 nm from each other. In yet another embodiment, "approximately the same wavelength" means that the two wavelengths are within a range of 10 nm from each other.

[0019] In this respect, the methods described herein are similar to standard artificial guide star techniques used in astronomy. However, there are at least a few significant differences. First, all light sources used herein are artificial. Second, the reference light sources described herein are positioned in close proximity to (one or more) signal light sources (not merely in close proximity in terms of apparent angular separation). Third, there is no need to use advanced reference light sources or to adjust the response time of the AO system.

[0020] It should be noted that this method is not solely for secure quantum communication. The method described here can be directly applied to classical optical communication in free space, provided that the signal source moves relative to the detector. In this case, the signal source intensity does not need to be low. Furthermore, this method is applicable to ground-based communications, air-to-ground communications, and satellite-to-ground communications.

[0021] In other words, this specification describes the following implementation: 1. Ground-to-ground communication between two moving points, and 2. Low Earth orbit (LEO) satellites that travel in a circular orbit at an altitude of 550 km above the ground for ground-to-earth communications.

[0022] Although not explicitly stated, it can be easily seen that the technologies described herein are also applicable to drone-to-ground and aircraft-to-ground optical communications. To further illustrate these implementations, two specific cases of telescope setups in which the detection module is positioned at the focal plane are described below. The first specific case is based on a commercially available telescope, and the second is based on an actual satellite-to-ground experiment. These are a 356mm diameter telescope with a closed diameter of 114mm and an effective focal length of 3910mm, and a 1m diameter reflecting telescope with a focal length of 10m.

[0023] Design of reference beam and signal beam The intuition of the exemplary embodiment is that two light beams of similar frequency and physically close together will pass through the same air column to some extent, and therefore their wavefronts, arriving at the detector end almost simultaneously, should be distorted in approximately the same way. As a result, a single wavefront correction method should be able to correct both light beams simultaneously with high fidelity. One might wonder why the invention does not place the two beams together. Time multiplexing techniques should also work if the time interval between beam switching is much larger than the change in wavefront distortion in the atmosphere. The answer is that, as demonstrated in recent experiments, pure wavelength division multiplexing works, but this technique can achieve a better secret key rate for moving light sources. By placing a reference beam ahead of the signal beam along the direction of movement of the light source relative to the receiver, the method can better correct wavefront distortion. More importantly, by carefully adjusting the apparent angular distance between the two beams and the delay time used in the AO feedback loop, the two beams can be made to travel through approximately the same optical path. As a result, if the time scale of atmospheric turbulence fluctuations is sufficiently short, the level of AO correction should be comparable to that of a stationary light source, although it will not be as effective as with a moving light source.

[0024] Returning to Figure 5, the two light sources are physically located close together. Similarly, the wavefront detection module that detects the reference light source beam and the signal detection module that detects the signal light source beam are located next to each other. To reduce photon loss in long-distance communication, each beam source is positioned at the focal point of the telescope on the satellite, and the light beams emitted near the light sources can be well approximated by traveling plane waves. In this way, the optical paths of two sets of light sources having the same or nearly the same wavelength experience some degree of the same wavefront distortion. The reference detection module estimates the distortion due to the atmosphere and generates a feedback signal to the control system. The control system then drives the actuators of the deformable mirrors or spatial light modulators of the AO system. Thus, the waveform distortion correction by AO for the reference light source also corrects the distortion of the weaker signal light source. Of course, the two sets of light sources must be placed far enough apart so that the effects of diffraction and scattering of the reference light source on the signal light source and vice versa can be ignored. The advantage of this method is that the signal transmission speed does not depend on the reference light source.

[0025] This method is similar to standard artificial guide star techniques used in observational astronomy. This invention demonstrates that this method is not only effective for quantum communications. It can also be directly applied to classical optical communications in free space. In this case, there is no need to reduce the signal source intensity. Furthermore, this method can be applied to ground-based communications, air-to-ground communications, satellite-to-ground communications, detection modules, and stationary and moving light sources relative to the detection modules. However, it should be noted that there are two major differences from standard artificial guide star methods. First, all light sources used in this invention are artificial. Second, the reference light source is positioned in close proximity to (one or more) signal sources (not merely in close proximity in terms of apparent angular separation).

[0026] Phase screen simulation To verify the effectiveness of this method, the present invention simulates the spatial profiles of a reference beam and a signal beam. To simplify the problem, the present invention ignores the effects of haze and clouds. Furthermore, since the angular velocity of the LEO satellite is high, the present invention ignores the time dependence of reflectance fluctuations. In other words, the results are obtained by AO correction for a random sample of spatially non-uniform reflectances in the atmosphere. (The present invention will discuss the effects of the time dependence of atmospheric turbulence later.) The present invention simulates the propagation of light in this medium using the PROPER library written in Matlab. The present invention models atmospheric phase disturbances by a set of phase screens used to change the phase of the light wave. These phase screens are generated using FFT on random complex numbers with a distribution according to Kolmogorov's turbulence theory. Here, the present invention presents the details of the formulas and parameters in phase screen generation. For near-field and far-field light propagation, the present invention uses modified von Kalman phase noise power spectral density (PSD), spectral algorithms, and Fresnel-approximation Fourier algorithms. The present invention also provides routines for simulating telescopes and deformable mirrors. Diffraction effects of the telescope are included in the simulation to obtain more accurate results. Figure 7 shows the setup of the receiving end telescope and AO system used in the simulation.

[0027] In the free-space channel, the present invention divides the atmosphere into two layers. The upper layer has one phase screen, and the lower layer has ten phase screens. The satellite altitude, layer division altitude, and receiver altitude are 400 km, 20 km, and 0 km, respectively. The size of the phase screen is 1024 × 1024, and the present invention repeats the simulation 1000 times for each scenario. The parameters used in the simulation are shown in Table I. The telescope specifications are based on the actual telescope at the Shikabayashi Astronomical Observatory. Also, for simplicity, the present invention ignores the diffraction effect of supported spider vanes in the simulation. The present invention uses a 780 nm wavelength photon source because this wavelength is superior in terms of spatial filtering strategy, geometric coupling, and focal spot size.

[0028] Here, the present invention considers a situation in which quantum signal beam detection is triggered by a reference beam. The reference light source transmits a relatively strong coherent pulse, which slightly precedes the quantum signal pulse. This setup automatically compensates for zero-order distortion due to turbulence. More importantly, phase information of the reference beam is extracted as a feedback signal. This phase is compared to an ideal optical beam propagating through a perfect vacuum channel. The difference in profiles is used to compensate for the phase error between the reference beam and the spatially separated signal beam by applying a deformable mirror (DM).

[0029] AO System Parameters Signal wavelength 780nm DM actuator array size: 64 x 64 Initial beam diameter 0.05m Primary mirror diameter: 1.03m Secondary mirror diameter: 0.36m Focal length 8m

[0030] Table I. AO system parameters used in simulations based on the actual Cassegrain telescope at the Shikabayashi Astronomical Observatory.

[0031] This invention models atmospheric phase disturbances using a set of phase screens used to change the phase of light waves. These phase screens are generated using FFT on random complex numbers with a distribution according to Kolmogorov's turbulence theory. Here, the invention presents the details of the formulas and parameters for phase screen generation. This invention uses modified von Kalman phase noise power spectral density (PSD). TIFF0007910779000001.tif17170 Here, κ0 = 2π / L0, κ m = 5.92 / l0, where κ is the spatial frequency (rad / m). r0 in meters is the coherence diameter of the atmosphere, also known as the Fried parameter. Here, L0 in meters is the average size of the largest eddy (Eddy), also called the outer scale of turbulence, and l0 in meters is the average size of the smallest eddy, also called the inner scale of turbulence. This invention assumes that L0 follows the Coulman-Vernin profile. TIFF0007910779000002.tif18170 Here, h is the altitude expressed in meters. The value of r0 changes along with the altitude and zenith angle according to the following formula. TIFF0007910779000003.tif14170 Here, ζ is the zenith angle and k is the wavenumber of light. TIFF0007910779000004.tif6150 is a refractive index structural parameter. In this invention, The Hufnagel-Valley model will be used for the simulation of TIFF0007910779000005.tif6150. That is, TIFF0007910779000006.tif25170 Here, v = 21 m / s is the wind speed.

[0032] The Fourier transform method using subharmonics is used to generate the phase screen. The phase screen for the Fourier transform method can be written as follows: TIFF0007910779000007.tif12170 Here, fx and f y are the spatial frequencies along the x - direction and y - direction, respectively. Further, c n,m is a random complex coefficient and has a circular complex Gaussian distribution with the variance given by the following equation. TIFF0007910779000008.tif15170

[0033] The present invention generates a low - frequency phase screen using the sub - harmonic method proposed by Lane et al. More precisely, a low - frequency phase screen is generated using sub - harmonics and added to the FT phase screen. The screen φ LF (x, y) is calculated by summing the NP phase screens. TIFF0007910779000009.tif11170

[0034] The p used by the present invention th The frequency interval of the screen is Δf p = 1 / (3 p L).

[0035] FIG. 8 is a schematic diagram of a communication channel. The gray - scale plate here is a randomly generated (time - independent but spatially non - uniform) phase screen. The gray - scale of each pixel represents the phase change when light passes through that region.

[0036] To simulate the spatial correlation between the reference beam and the signal beam, in the present invention, both (spatially separated) beams pass through the same set of phase screens. As shown in FIG. 8, the region where the two beams overlap on the phase screen increases as the beams propagate. This means that as the transmission distance increases, the reference beam contains more turbulence information of the signal beam. When h is high, there is little turbulence, so the distortion of the wavefront by the first phase screen is almost zero. As a result, even if the two beams do not overlap on the first phase screen, the performance of the system is not affected.

[0037] It should be noted that as the individual beam sizes or beam transmission distances increase, the overlap area of ​​the two beams also increases. As shown in Figure 9, the beams are tilted at a small angle toward the receiver. This invention assumes that the centers of the beams reach the receiving end at the same position. Therefore, when passing through the phase screen, one beam travels a certain distance Δx = L(z max -z / z max It shifts by that much. Here, L, z max L< <z max Therefore, the beams travel the same distance, and the relative inclination angle can be ignored.

[0038] The signal aberration caused by turbulence is quantified by the coherent efficiency γ. TIFF0007910779000010.tif21170 In the above formula, E ideal E is the electric field in the ideal state when a beam passes through a vacuum channel. received This is the distorted or corrected electric field. Furthermore, the integral is performed over the receiver surface. Clearly, 0 ≤ γ ≤ 1, and γ = 1 is E ideal and E received This means they are a perfect match.

[0039] Figure 10 shows the simulation results of the coherent efficiency γ versus separation distance L graph. Without AO correction, γ is approximately 0.3 at zenith angle ζ=0° and approximately 0.05 at ζ=75°. As expected, γ increases after AO is used. For example, with L=2m, the system can correct distortion up to γ=0.958 at ζ=0° and γ=0.566 at ζ=75°. It should be noted that as L increases, the overlapping area of ​​the beams decreases, so γ decreases. The phase distortion of the reference beam is not very relevant to the signal beam. For each ζ, the coherent efficiency drops sharply as L increases. Moreover, the distance L at which this sharp drop occurs decreases with ζ. This drop is related to the equifront separation of the turbulence. When the angle between two light sources is smaller than the equifront separation, their distortions can be considered almost the same. Therefore, as L increases and the angular separation between the two light sources becomes greater than the equi-wavefront separation, the effect of AO correction decreases sharply. Lastly, and importantly, when L is constant, as the zenith angle ζ increases, the value of γ decreases for two reasons: the light beam needs to travel through a longer optical path, and the Fried parameter r0 in equation (3) becomes smaller.

[0040] It must be noted that, due to the limited number of actuators in the DM, the system cannot completely restore the signal even when L=0. Therefore, it is not possible to completely compensate for higher-order turbulence. The contribution of higher-order strain increases as ζ increases. Consequently, γ decreases instead of remaining at 1, as shown in Figure 11.

[0041] Spatial dependence of turbulence Comparison of wavelength division multiplexing and this method This invention compares the coherent efficiency of this method with that of a system that couples a signal beam and a reference beam using wavelength division multiplexing (WDM). Phase deviation is inversely proportional to wavelength. In the simulation, this invention adjusts the phase screen according to the ratio of the wavelengths of the signal beam and the reference beam. This invention sets the reference wavelength to 808 nm, the standard optical communication wavelength. The results for the WDM system are compared with a system in which the signal beam and reference beam are separated by 2 m. The comparison is shown in Figure 12. The coherent efficiency of the spatial separation method is at least 10% higher than that of the AO system coupled by WDM. When ζ≦30°, the coherent efficiency of L=2m is approximately 0.96, which is about 13% higher than the WDM method. It should be noted that chromatic aberration of the equipment is not included in the simulation. The actual performance of the WDM method will likely be lower. Figure 12 shows the coherent efficiency γ versus zenith angle ζ for the spatial separation method and the WDM method. The upper line is the line calculated using L=2m. The lower line is the line calculated using a reference wavelength of 808 nm.

[0042] Maximum path difference between the leading reference beam and the delayed signal beam It is clear that optical optics (AO) techniques will work if the optical paths of two light sources always experience some degree of the same optical distortion. This requirement is met when their angular separation over the atmospheric optical path is less than the equifront separation angle θ0. A typical value for this can be estimated using the Hufnagel-Valley model. In special case 2, where the satellite is approximately 550 km above the ground, an AO system can perform well if the physical separation of the two sets of light sources is approximately 3.5 m. It should be noted that satellite-to-ground optical communication is most effective when the satellite is close to the zenith. Furthermore, the distance between the satellite and the ground station changes slowly as the satellite moves slightly around the zenith position. As the zenith angle increases, the value of θ0 decreases. However, even when the angular separation of the light sources is greater than θ0, the AO system can still provide some improvement to the signal. As long as the angular separation between the light sources is on the same order as θ0, the overlapping region of the beam paths is large enough for the system to extract turbulent information from the signal beam.

[0043] Maximum physical distance between the reference light source and the signal light source If the optical paths of two light sources always experience some degree of the same optical distortion, then AO (Optical Angle) technology is clearly effective. This requirement is met when the separation distance between the light sources is z max This condition is satisfied when θ is less than θ0. TIFF0007910779000011.tif13170 Here, θ0 is the equiwavefront separation angle, h max This is the altitude of the light source. This fact has also been verified by our simulation results. The simulation results show that when the beam separation is too large compared to the equiwavefront separation, the coherent efficiency decreases significantly.

[0044] The maximum delay time between the preceding reference beam and the delayed signal beam, and the maximum response time of the AO system. Standard AO correction techniques can be used to correct image drift (by dynamically adjusting the tilt of the optical elements) and blur (by dynamically adjusting the shape of the optical elements). Effective AO correction as used herein means that the AO system operates with a response time at least about an order of magnitude shorter than the dynamic timescale of the optical distortion of either optical path. Furthermore, this response time must be less than or equal to the delay time between the preceding reference beam and the delayed signal beam. In special cases 1 and 2, the methods described herein work when the response time of the entire AO system, including the electronics, control unit and mechanical parts, and the delay time between the reference beam and the signal beam are ≤ ≈ t0, where t0 is the dynamic timescale of the wavefront distortion. Typically, t0 is at least 10 ms.

[0045] Indeed, the two light sources and detection modules must be properly synchronized. Furthermore, the two light sources must be precisely aligned with respect to each other. Fortunately, the present invention requires this to be done only once. The present invention also requires the dynamic alignment of the light source and detector optics with very precise tracking.

[0046] Minimum size of the photosensitive surface of the wavefront detection module The size of the photosensitive surface of the wavefront detection module must be large enough for effective AO correction. Here, frequency ν, wavelength λ, and maximum electric field strength E R Consider a point light source having E. (More precisely, E R This should be considered the maximum electric field intensity of the photon beam just before it enters the detection optics. Essentially, this is the actual E of the light source after subtracting absorption and scattering by the atmosphere. R ) If this light source is located at a distance R from a circular aperture of diameter D (in other words, in the case of a refracting telescope), then the electric field strength at an angle θ with respect to the circular aperture due to diffraction in the case of a far field of view is: This is equivalent to TIFF0007910779000012.tif16119, where J1 is the Bessel function of the first kind. More generally, for a circular aperture with a central circular occlusion of diameter bD (i.e., a catadioptric telescope with a Cassegrain focus), E is given by the following equation: TIFF0007910779000013.tif17170

[0047] The same calculation can be performed for Newtonian reflectors, but it becomes more complex due to the presence of mechanical supports that obstruct part of the optical path.

[0048] After AO correction, the electric field strength of the image received by the photosensitive surface of the wavefront detection module follows either equation (10) or (11), depending on the optical design of the detection telescope. The image correction method works best when at least two diffraction rings are recorded by this detection module. For a telescope with an effective focal length f, this is the size l of the photosensitive surface of the wavefront detection module. w However, this means that for all b less than or equal to 1, the following inequality must be satisfied. TIFF0007910779000014.tif15125 If the wavelength of the light source is the same as that of known satellite-to-ground communication experiments, λ=405nm, then in either telescope setup (i.) or (ii.), l w ≥ 14 nm. w This value is easily achievable with current technology.

[0049] Minimum physical distance between the reference light source and the signal light source The minimum possible distance between the reference light source and the signal light source is determined by both the resolution of the optical system and the "interference" between the two light sources. It should be noted that, if AO correction is successful, the center of the image of the reference beam should be near the center of the photosensitive surface of the wavefront detection module. The linear size of the photosensitive surface of the signal detection module is l s Furthermore, the separation between the photosensitive surface of the wavefront detection module and the signal detection module is d sepAssume that the light intensity of the reference beam at a distance x from the center is This is equal to TIFF0007910779000015.tif13170, where f is the effective focal length of the telescope, and b = 0.36 / 1.03 is the ratio of the diameters of the secondary mirror to the primary mirror of the Cassegrain telescope used. R (0) ≈ 2ε0E R 2 π 2 (D / 2) 4 / R 2 Therefore, the total light energy flux of the reference beam irradiated onto the photosensitive surface of the signal detection module is ∫∫sI R (x)dA, and this integral is greater than or equal to the area of ​​the field aperture of the signal detection module. For example, when L=2m, ∫∫sI R (x)dA = 4.36 × 10 -15 I(0) is the minimum distance. The minimum distance must be set according to the required attenuation from the beam center. Otherwise, stray reference beam photons will have a serious impact on the signal detection statistics. The integration is performed over the photosensitive surface of the signal detection module S. This energy flux is at least, for example, 10 times, greater than the energy flux of the signal beam irradiating the photosensitive surface of the signal detection module. -4 ~10 -3 It must be at least twice as weak. Otherwise, the stray reference beam photons will have a serious impact on the signal detection statistics. This is due to D, f, l s d sep This can be easily achieved by adjusting the following: Because when x is large, |J1(x)|~x -1 / 2 Therefore

[0050] Time dependence of turbulence In the above discussion, the present invention considered only the spatial correlation of the beam. In reality, the system responds in a short time. For a stationary ground-based observer, the apparent angular velocity of a LEO satellite is much faster than the angular velocity of the celestial body, thus imposing more stringent requirements on AO systems in satellite communications.

[0051] To compare the differences between a stationary light source and a moving light source, the present invention uses the Greenwood frequency f G Use the Greenwood frequency f. G This is an effective method for approximately quantifying the rate of change of turbulence [7,22]. Please recall TIFF0007910779000016.tif13170. Here, v(h)=v wind (h)+v app (h) is the sum of the natural wind speed and the apparent wind speed due to the satellite's movement. The LEO satellite is v app >>v wind Since the movement is at a large angular velocity, this assumption of simply adding the two speeds as a scalar is justified. The present invention further assumes that the natural wind speed follows an altitude-dependent Bufton wind profile. TIFF0007910779000017.tif14170 Here, v g Let's assume the natural wind speed near the ground is 5 m / s. Apparent wind speed v app (h=ω s Adding h, the total wind speed is It can be written as TIFF0007910779000018.tif13170, where ωs is the angular rotation velocity of the satellite. For simplicity, this invention assumes that the satellite is moving in a circular orbit. Therefore, the angular rotation velocity is equal to the following equation: TIFF0007910779000019.tif16170 Here, G is the gravitational constant, M + and R + These are the mass and radius of the Earth, respectively. app >>v wind Therefore, the Greenwood frequency for LEO satellite tracking can be much higher than the natural frequency of atmospheric turbulence. As shown in Figure 13, when the zenith angle is 0°, the channel-specific f G While the frequency is approximately 64Hz, the frequency including rotation is f GThis is approximately 380 Hz. Figure 13 shows the Greenwood frequency versus the zenith angle. The dashed line curve is calculated under conditions of rotation and no spatial separation. The dotted line curve is the channel-specific Greenwood frequency. The solid and dashed lines are calculated under conditions of spatial separation of 2.5 m and response times of 1 ms and 0.5 ms, respectively.

[0052] The idea proposed by this invention can reduce the apparent wind speed when the reference beam is positioned in front of the signal beam. The system response time is T r Let's assume that when the system receives the reference signal at t=0, t=T r The signal is corrected. Figure 6 shows t=0 and t=T r This shows the satellite's position and beam path. Here, θ1 is the angle between the leading reference beam and the delayed signal beam (solid and dashed lines), and θ2 is the angle between t=0 and t=T r This is the angle between the signal beam paths (dotted and dashed lines). Figure 6 shows that when both beams are in the same position, the angle between the two timestamps is larger than when the two beams are spatially separated. Therefore, the apparent wind speed is smaller by θ1 / θ2. The equivalent angular rotation speed is, TIFF0007910779000020.tif16170, where θ s =L / z max θ is the angular distance between the reference beam and the signal beam. Combining equations (14) and (16), we get θ s / T r =ω s Therefore, it can be seen that the influence of apparent wind speed can be completely eliminated, and the optimal performance of the AO system can be achieved. In fact, this was observed by the present invention in Figure 13.

[0053] θ s / T r <ω s In this case, it should be noted that the setup performance will be worse than that of a stationary light source because the response time of the AO system is not fast enough for the pulse signal and the reference beam to travel through approximately the same optical path. More interesting is θ s / T r >ωs is the case. In this case, f G The performance degradation reflected in the value of is due to the system response time T r being too fast. Certainly, by artificially increasing T r , for example, by appropriately increasing the delay of AO feedback control, the present invention can reduce f G to the optimal case.

[0054] Finally, in FIG. 13, when the zenith angle is not large, the Greenwood frequency curve calculated at L = 2.5 m is lower than the curve calculated under the condition of no spatial separation. As the zenith angle increases, ω s and θ s / T r both decrease, so the curve decreases and approaches the natural frequency curve. Also, ω s decreases more rapidly than θ s / T r according to ζ, so the curve with spatial separation intersects the curve without turning. This means that at that point θ s / T r = ω s . When the zenith angle is larger than this point, θ s / T r > ω s , and it is necessary to shorten the response time to maintain near the natural frequency.

[0055] Scattering noise by strong beam Scattering by a strong reference beam affects the final key rate. A part of the photons from the reference light source enters the signal receiving module and may cause an error. In this section, the present invention estimates the scattering by a strong laser in a clear sky scenario. Here, the present invention uses the approach of sky scattering noise to obtain an estimate of the laser scattering noise. The formula for calculating the number of sky noise photons incident on the system is given by TIFF0007910779000021.tif14170. Here, Hb (Wm -2 sr μm) is the sky radiance, Ω FOV = πΔθ2 1 / 4 is the solid angle field of view by the field stop, D R is the diameter of the receiver primary optical system, Δλ is the bandpass of the spectroscopic filter in μm units, and Δt is the photon integration time of the receiver. Here, Δθ is calculated by D FS / f, and D FS is the diameter of the field stop. Since both beams use the same wavelength or approximately the same wavelength, the present invention assumes Δλ = 1, and the spectroscopic filter cannot block photons from the reference beam.

[0056] In astrophotography, a bright star close to the target can be used as a reference for searching for channels. Therefore, the luminance of the reference laser must be similar to that of a bright star. The glow of the sky caused by the laser can be estimated by the glow of the sky by the star. In the situation of a clear night without the moon, the typical sky radiance luminance is 1.5×10 -5 Wm -2 sr μm. Using the above parameters and setting Δt = 1 ns, the probability of receiving reference photons is on the order of 10 -8 and is sufficient for practical use.

[0057] The present disclosure proposes a new method of applying AO technology to an optical communication system. The main idea of this method is to spatially separate the reference beam and the signal beam. Since both beams use the same or approximately the same frequency, the signal distortion information collected from the reference beam may be more accurate than that of a system using WDM. The present invention analyzes this by using phase screen simulation. The results show that for the case of LEO satellites, the performance of this method is superior to that of the WDM method. Furthermore, for a signal source moving at high speed, this design can reduce the apparent wind speed caused by the movement of the object. Thereby, the Greenwood frequency of the turbulence can be lowered. The present invention analytically verifies this by using the Bufton wind profile. Finally, the present invention estimates the crosstalk caused by the diffraction and scattering of the reference beam. Since there is an FS in the reference receiving module and the power of the reference beam is not high, the crosstalk caused by the reference beam can be ignored.

[0058] This disclosure improves classical and quantum communication speeds in free space in situations where atmospheric wavefront distortion exists for one or more light sources moving relative to a detector. More specifically, this disclosure uses adaptive optics techniques in which an artificial reference beam light source is placed near one or more potentially much weaker signal sources, and the wavefront sensing module and the receiving signal sensing module are placed close to each other. Furthermore, the delay time between the emission of the reference beam and the emission of the signal beam, and the response time of the AO system, are possibly dynamically and compensatoryly adjusted so that the reference beam and the delayed signal beam travel through the same optical path to some extent.

[0059] Unless otherwise indicated in the examples, specification, or claims, all parts and percentages are by weight, all temperatures are in degrees Celsius, and pressures are atmospheric pressure or near atmospheric pressure.

[0060] A numerical range may be generated by combining a range of numerical values ​​or parameters with another range of numerical values ​​or parameters relating to the same characteristic, with respect to any numerical value or range of numerical values ​​concerning a given characteristic.

[0061] Except in the examples of operation, or where otherwise suggested, all numerical values, values, and / or expressions used herein and in the claims to refer to the quantities of ingredients, reaction conditions, etc., should be understood in all cases to be modified by the term “approximately.”

[0062] Although the present invention is described in relation to specific embodiments, it should be understood that various modifications thereof will be apparent to those skilled in the art by reading the specification. Therefore, it should be understood that the invention disclosed herein is intended to cover modifications that fall within the scope of the appended claims.

Claims

1. A method for improving the speed of information transmission, A step of emitting a reference light source for adaptive optics correction and a signal light source for optical communication at the same wavelength or approximately the same wavelength, Steps include: detecting a reference light source beam and a signal light source beam side by side, dynamically and / or compensatoryly adjusting the time delay between the reference light source and the signal light source, and / or the delay time in adaptive optics control, and / or the apparent angular velocity of the reference light source and the signal light source with respect to the detection module, and / or the physical separation between the reference light source and the signal light source; The step includes simultaneously correcting the distortion of the signal light source by using adaptive optics to correct the distortion of the waveform of the reference light source, The reference light source is brighter than the signal light source. The reference light source and the signal light source move relative to the detection module. The reference light source emits light earlier than the signal light source, The optical path of the reference light source beam and the optical path of the signal light source beam have substantially the same wavefront distortion. method.

2. The method according to claim 1, wherein time-multiplexing and / or spatial model-multiplexing are used in the reference light source beam and / or the signal light source beam.

3. The method according to claim 1, wherein the reference light source is adjacent to the signal light source.

4. The method according to claim 1, wherein the information transmission speed is within the range of an optical communication method.

5. The method according to claim 1, wherein the information transmission is within the scope of classical communication methods, quantum communication methods, or a combination of classical communication methods and quantum communication methods.

6. The method includes one or more of the following: a ground-based, celestial surface-based, flying object-based, satellite-based, and / or space probe-based reference light source beam and a signal light source beam. This includes detecting one or more reference light source beams and signal light source beams on the ground, on the surface of a celestial body, on an flying object, on a satellite, and / or on a space probe. The method according to claim 1.

7. The method according to claim 1, wherein the reference light source beam and the signal light source beam partially or completely pass through the telescope.

8. The method according to claim 1, wherein the reference light source beam and the signal light source beam partially or completely pass through interplanetary space, the atmosphere of a celestial body, the fluids of the Earth, and / or the fluids of a celestial body.

9. The method according to claim 1, wherein information transmission is performed by a classical network, a quantum network, or a combination of a classical network and a quantum network.

10. A system for improving the speed of information transmission, The system comprises one or more pairs of wavefront detection modules and signal detection modules, wherein the wavefront detection module receives and detects a reference beam from a reference light source, and the signal detection module receives and detects an actual optical communication signal from a signal light source, and the wavefront detection module and the signal detection module are configured to dynamically and / or compensatory adjust the time delay between the reference light source and the signal light source, and / or the delay time in adaptive optics control, and / or the apparent angular velocity of the reference light source and the signal light source with respect to the detection module, and / or the physical separation between the reference light source and the signal light source, wherein the reference beam and the actual optical communication signal have the same or substantially the same wavelength, the reference beam is emitted earlier than the actual optical communication signal, and the optical path of the reference beam and the optical path of the actual optical communication signal have substantially the same wavefront distortion. Each wavefront detection module uses adaptive optics to directly or indirectly detect and correct atmospheric distortion of the corresponding reference beam, and simultaneously detect and correct distortion of the actual optical communication signal. Each pair of wavefront detection modules and signal detection modules is positioned in the information transmission system near the receiving end of the corresponding reference beam, and is positioned close to each other such that the center of the corresponding image of the reference beam overlaps at least with the center of the corresponding photosensitive surface of the wavefront detection module. system.

11. An information transmission system, A pair of one or more emitters, A pair of one or more detectors, The first emitter of each pair of the aforementioned pair of one or more emitters generates a signal light source for optical communication. The second emitter of each pair of the one or more emitter pairs generates a reference light source with the same or substantially the same wavelength as the signal light source. The reference light source is brighter than the signal light source, The optical paths of the reference light source beam and the signal light source beam have approximately the same wavefront distortion. The first detector of each pair of the one or more detectors detects the signal light source beam. The second detector of each pair of the one or more detectors detects the reference light source beam. The first detector and the second detector are arranged side by side. The time delay between the reference light source and the signal light source, and / or the delay time in adaptive optics control, and / or the apparent angular velocity of the reference light source and the signal light source with respect to the detection module, and / or the adjustment of the physical separation between the reference light source and the signal light source are performed dynamically and / or compensatoryly. The information transmission system further comprises a compensating optical system for correcting the waveform distortion of the reference light source and simultaneously correcting the distortion of the signal light source. Information transmission system.

12. The information transmission system according to claim 11, wherein the first emitter and the second emitter of each pair are composed of a terrestrial structure, an object on the surface of a celestial body, a flying object, a satellite, or a space probe.

13. The information transmission system according to claim 11, wherein the signal light source beam and the reference light source beam pass through a telescope.

14. The information transmission system according to claim 11, wherein the compensating optical system is replaced by other real-time signal processing and / or signal post-processing techniques.

15. The information transmission system according to claim 11, wherein the distance or angle between the optical paths of the preceding reference beam and the delayed signal beam of the moving light source relative to the detector module is smaller than the corresponding distance or angle between the optical paths of the two beams when the light source is stationary relative to the detector module.

16. The imaging system according to any one of claims 11 to 15.

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