CN102857294B - Method and device for suppressing signal fading in laser communication from ground to GEO (geosynchronous) satellite - Google Patents
Method and device for suppressing signal fading in laser communication from ground to GEO (geosynchronous) satellite Download PDFInfo
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Abstract
Disclosed are a method and a device for suppressing signal fading in laser communication from ground to a GEO (geosynchronous) satellite. The method includes: measuring intensity fluctuation waveform of downlink beacon light emitted from the GEO satellite by a single-mode fiber on a ground station, and transmitting uplink laser beam along a direction opposite to the downlink beacon light via the same single-mode fiber, and then emitting the uplink laser beam from an end face of the single-mode fiber; modulating to-be-transmitted uplink data waveform according to the reciprocal of the intensity fluctuation waveform of the downlink beacon light measured by the single-mode fiber, driving a laser to emit uplink laser, and transmitting the uplink laser beam to the GEO satellite. Accordingly, atmospheric turbulence and signal fading caused by alignment error of a ground transmitter can be eliminated automatically, and power of laser received by the satellite is stable. In addition, average transmission power of the uplink laser beam can be reduced on the condition of the rated fading probability.
Description
Technical field
The present invention relates to satellite laser communications technical field, specifically a kind of in the up laser communication of ground to geostationary orbit (GEO) satellite, the signal fadeout that reduction atmospheric turbulance and alignment error cause, thus a kind of method reducing average emitted luminous power.
Background technology
Satellite laser communications has the advantages such as message capacity is large, volume is little, antijamming capability is strong, security performance is good, is communication technology of satellite important development direction.The impact of earth atmosphere turbosphere will inevitably be subject to the laser communication on ground and ground to the laser communication of satellite at satellite.Atmospheric turbulance can make the wavefront of laser beam produce distortion, and make laser signal intensity fluctuation (flicker), produce decline, the time scale of intensity fluctuation is about 10 milliseconds of magnitudes, and this can have a strong impact on the performance of satellite laser communications system.
Atmospheric turbulance is different on satellite to the laser communication on ground and ground to the impact of the laser communication of satellite.For the descending laser communication of satellite to ground, first laser beam passes through remote free space transmission, then the atmospheric turbosphere passing about 20 kilometers arrives the optical receiver of ground station.Turbulent flow only works in the relatively short segment distance near receiver.For the laser communication system on the satellite-ground of employing intensity modulated/direct detection, by increasing terrestrial receiving antenna diameter, utilizing wide-aperture aperture smoothing effect or adopting the methods such as the array received of multiple aperture significantly can reduce the impact of turbulence intensity flicker.
For the up laser communication of ground to satellite, the scintillation that atmospheric turbulance causes is more serious.Uplink laser beam first through the atmospheric turbosphere of about 20 kilometers, and then through the free space transmission of at least hundreds of kilometer.Due to receiver aperture size on satellite limited (being generally less than 25 centimetres), be difficult to adopt large aperture or array received antenna, therefore laser arrives after on satellite, and scintillation is very large, can produce deep fading.For the up laser transmission of ground to satellite, multiple beam lift-off technology is a kind of turbulence intensity flicker suppression technology the most frequently used at present, has good Project Realization.As adopted
nindividual mutual incoherent beamlet is launched, then total scintillation will reduce to 1/ of single beam intensity flicker
n.But this method requires each beamlet all to have independently catches, follows the tracks of and aims at (ATP) subsystem, and has independently optical antenna.Therefore the total cost of multiple beam emission system is by for single beam emission system
ndoubly.And cause laser communication terminal volume to increase, be not easy to mobile.
Suppressing the signal strength signal intensity of wireless channel to rise and fall and also have a kind of method be in daily use, is exactly automated power control technology.The carrier signal that transmitter sends is after wireless channel transmission, the intensity fluctuation of Received signal strength is measured by receiver, then by reverse link, this intensity fluctuation is sent to transmitting terminal, again according to the transmitting power that this intensity fluctuation controls transmitter, thus the signal strength signal intensity fluctuating quantity making receiver receive weakens, received power is kept to stablize.
This transmitting terminal automatic power control method is widely applied in wireless mobile communications, also similar method (Liu Zili can be adopted in space optical communication, Tu Cuixia, the control method of free-space optical communication system and transmitted power thereof, Chinese patent, application number: 200410052212.0), but this method is only applicable to transmitter, receiver at a distance of comparatively near, and under the lower application conditions of signal strength signal intensity fluctuating frequency.For the communication between ground and satellite, due to apart from far, the passback time delay of fading signal waveform is very large, and the duration of the signal fadeout that turbulent flow causes only has 10 Milliseconds, now the decline of channel is uncorrelated already, and the automated power control technology of this form therefore cannot be adopted to decline to the fast signal suppressing turbulent flow to cause.For ease of ground station launch laser beam accurately to satelloid, satellite needs ground station to launch a branch of descending beacon beam, the imageing sensor of ground station can restraint descending beacon beam according to this, obtain the azimuth information of target satellite, then drive quick deflecting mirror to make uplink laser beam accurately point to satellite, thus eliminate the impact of various alignment error.
The early-stage Study of applicant finds, the two-way laser adopted between the ground of duplexer system and satellite is transmitted, the spatial amplitude of the uplink laser beam of launching as made terrestrial optical antenna place and PHASE DISTRIBUTION, with without the spatial amplitude of beacon beam descending during turbulent flow and PHASE DISTRIBUTION consistent, when then there is turbulent flow, the intensity fluctuation of descending beacon beam at ground receiver optical antenna focus place and the uplink laser beam intensity fluctuation on satellite plane optical axis will be consistent, strong correlation.Utilize this characteristic, by descending beacon beam, ground station not only can obtain the azimuth information of target satellite, also can obtain the light intensity fluctuation waveform of uplink laser beam on satellite, and do not need to measure the intensity fluctuation waveform of uplink laser beam on satellite and pass back to ground station, thus can continue to adopt automated power control technology automatically to control the transmitting power of uplink laser beam, reach and reduce the decline of up laser signal, improve the object of laser communication system performance.
For ground to GEO(geostationary orbit) the up laser communication of satellite, in order to overcome the signal fadeout of the uplink laser beam that atmospheric turbulance and ground transmitter alignment error cause.The intensity fluctuation of descending beacon beam at ground receiver optical antenna back focus place and uplink laser beam is utilized to be this consistent features in the intensity fluctuation rule at GEO satellite place, a monomode fiber is adopted to measure the intensity fluctuation waveform of descending beacon beam in real time, then this intensity fluctuation waveform is utilized to carry out adaptive control to the transmitting power of uplink laser beam, the signal power that GEO satellite reception is arrived is steady, thus suppress the signal fadeout of uplink laser beam, because the light intensity of uplink laser beam on satellite is very little in the angle that the intensity fluctuation at terrestrial optical antenna back focus place is relevant to descending beacon beam, therefore this turbulence intensity flicker restraint method can only be applied to ground station (can high-speed motion) in the up laser communication of GEO satellite.
Summary of the invention
Technical problem to be solved by this invention: for prior art because the light intensity of uplink laser beam on satellite is very little in the angle that the intensity fluctuation at terrestrial optical antenna back focus place is relevant to descending beacon beam, turbulence intensity flicker restraint method can only be applied to ground station to GEO(geostationary orbit) satellite up laser communication in.
The technical scheme that the present invention solves the problems of the technologies described above is: the suppressing method proposing signal fadeout in a kind of ground to GEO satellite laser communications.The method comprises, the descending beacon laser sent from GEO satellite is by being received by optical antenna (1-1) after atmospheric turbulance, after quick deflecting mirror (1-2) and spectroscope (1-3), by lens (1-5), descending beacon beam is coupled in the monomode fiber (1-6) of back focal plane, after wavelength division multiplexer (1-7), use photodetector (1-8) that beacon beam is converted to the signal of telecommunication, signal processing module (1-9) amplifies this signal of telecommunication and after filtering process, can obtain the intensity fluctuation waveform of descending beacon beam.Because descending beacon beam intensity fluctuation is identical with uplink laser beam intensity fluctuation rule, therefore this intensity fluctuation waveform can reflect the intensity fluctuation of the uplink laser beam transmitted along same hydraulic flow path.This intensity fluctuation waveform is sent into the judgement of signal transacting center (1-10) settling signal power, power controlled automatically by sending control module (1-11) and completes the modulation of data to be transferred again, wherein the method that automatically controls of power by input signal opto-electronic conversion after power compare with the power needed for launching, by Feedback of Power circuit, power is compensated, make it reach required performance number; The modulated process of data to be transferred is then utilize the waveform reciprocal of descending beacon beam intensity fluctuation to go to modulate upstream data waveform to be passed.
Finally utilize power demand to obtain the drive current mated, drive laser (1-12) sends laser, and this laser power just has the power fluctuation of descending beacon beam intensity fluctuation waveform form reciprocal.The laser that laser (1-12) sends is coupled in monomode fiber (1-6) by wavelength division multiplexer (1-7), along transmitting with the descending antipodal direction of beacon beam signal.Launched by optical antenna (1-1) after lens (1-5), spectroscope (1-3), fast deflecting mirror (1-3), form the uplink laser beam of collimation, finally arrive GEO satellite.
Due to the existence of alignment error and atmospheric turbulance inclination of wave front, for ensure descending beacon beam correct be coupled in monomode fiber (1-6), ensure that uplink laser beam accurately points to GEO satellite, also need an automatic tracking system to eliminate the inclination of wave front of alignment error and atmospheric turbulance.
Automatic tracking system separates a part of beacon light beam by spectroscope (1-3), by imaging on imageing sensor (1-14) after diaphragm (1-15), lens (1-13).The incident orientation of descending beacon beam can be obtained by the upper light spot image of imageing sensor (1-14), by drive control module (1-16), quick deflecting mirror (1-2) is deflected, eliminate the inclination of wave front of alignment error and atmospheric turbulance.
Because descending beacon beam and uplink laser beam are by same monomode fiber, its transmission path is identical, just direction is contrary, therefore automatically tracing process can complete descending beacon beam to the coupling of monomode fiber (1-6) and uplink laser beam aligning GEO satellite simultaneously, diaphragm (1-15) is used for limiting beacon light beam diameter, makes the inclination of wave front caused by turbulent flow of being measured by imageing sensor (1-14) substantially can reflect the turbulent wave top rake of uplink laser beam.
In the present invention, monomode fiber (1-6) is adopted to obtain the intensity fluctuation waveform of descending beacon beam, and simultaneously as the output channel of uplink laser beam.The spatial amplitude being realized the uplink laser beam that terrestrial optical antenna (1-1) is launched by this root monomode fiber (1-6) distributes, and distributes mate with the spatial amplitude without beacon beam descending during turbulent flow.The spatial amplitude that the uplink laser beam exported from back focal plane monomode fiber (1-6) is transferred to optical antenna (1-1) is distributed as Gaussian Profile, and its Wave-front phase is a plane.
And be also a plane without the Wave-front phase of beacon beam descending during turbulent flow at terrestrial optical antenna (1-1) place, but its spatial amplitude is distributed as and is uniformly distributed.This uniform plane wave continues be transferred to back focal plane monomode fiber (1-6) and be coupled in optical fiber.Theoretical according to optical coupling, the amplitude of this root monomode fiber to descending beacon beam plays space filtering effect, equivalence is in optical antenna (1-1) plane, and after filtering, the actual distribution of amplitudes worked is by identical with the gaussian-shape distribution of amplitudes of uplink laser beam.Like this, under the condition that there is turbulent flow, the intensity fluctuation of the descending beacon beam that monomode fiber (1-6) receives can reflect the intensity fluctuation of the uplink laser beam that satellite receives completely.
In addition, monomode fiber (1-6) also can ensure that the transmission path of descending beacon beam and the ground-launched uplink laser beam received is completely the same, but direction is contrary, realizes more high-precision beam alignment.And in this case, the intensity fluctuation of the uplink laser beam that the satellite caused by the residual alignment error of automatic tracking system receives, the intensity fluctuation of the descending beacon beam received with monomode fiber, its Changing Pattern is also identical.Therefore this method can suppress atmospheric turbulance and residual alignment error to cause earth signal to decline simultaneously without distinction.
The invention has the beneficial effects as follows:
(1) the intensity fluctuation waveform of the laser signal arrival receiver end that transmitter sends can be obtained in real time at transmitter terminal, avoid at receiver end measured intensity fluctuating waveform, and intensity fluctuation waveform is sent back to the passback process of transmitter from receiver, make in the laser communication between remote ground to GEO satellite, still can carry out adaptive control to the transmitting power of transmitter and cause fast signal to decline to suppress turbulent flow.
(2) only adopt a uplink laser beam just can atmospheric turbulence intensity be suppressed greatly to glimmer, reduce the dafing depth of up laser signal.
(3) signal fadeout of the present invention's uplink laser beam that ground transmitter alignment error also can be suppressed to cause.
(4) technical scheme is simple, launches relative to single beam, and cost increases little, but significantly can reduce turbulence intensity flicker.Compared with multiple beam lift-off technology, can greatly reduce costs.
Accompanying drawing explanation
Fig. 1 is structural representation of the present invention.
Fig. 2 adopts the intensity fluctuation waveform of forward signal prediction to obtain and process structural representation.
Fig. 3 is the beneficial outcomes figure of the embodiment of the present invention according to Multiple-Phase-Screen numerical simulation.
Embodiment
Utilize the correlation of the intensity fluctuation of descending beacon beam and uplink laser beam intensity fluctuation to suppress an embodiment of uplink laser beam intensity fluctuation as shown in Figure 1.From GEO(geostationary orbit) the descending beacon laser that sends of satellite by after atmospheric turbulance by terrestrial optical antenna (1-1) (diameter
about 5 ~ 20 centimetres) receive, optical antenna (1-1) is a confocal optical system, and descending beacon beam is still collimated light beam after optical antenna (1-1), but beam diameter
compressed.And then after quick deflecting mirror (1-2), spectroscope (1-3) and lens (1-5), lens (1-5) back focal plane is coupled in monomode fiber (1-6), after wavelength division multiplexer (1-7), use photodetector (1-8) that beacon beam is converted to the signal of telecommunication, signal processing module (1-9) amplifies this signal of telecommunication and after filtering process, can obtain descending beacon beam intensity fluctuation waveform.This intensity fluctuation waveform is sent into the judgement of signal transacting center (1-10) settling signal power, power controlled automatically by sending control module (1-11) and completes the modulation of data to be transferred again, wherein the method that automatically controls of power by input signal opto-electronic conversion after power compare with the power needed for launching, by Feedback of Power circuit, power is compensated, make it reach required performance number.The modulated process of data to be transferred is then utilize the waveform reciprocal of descending beacon beam intensity fluctuation to go to modulate upstream data waveform to be passed.
Finally utilize power demand to obtain the drive current mated, drive laser (1-12) sends laser, and this laser power just has the power fluctuation of descending beacon beam intensity fluctuation waveform form reciprocal.The laser that laser (1-12) sends is coupled in monomode fiber (1-6) by wavelength division multiplexer (1-7), and the right focus place being transferred to lens (1-5) along the direction contrary with descending beacon beam signal exports.After spectroscope (1-3), fast deflecting mirror (1-2), arrive optical antenna (1-1) surface again, form the uplink laser beam of collimation.The distribution of amplitudes of uplink laser beam is Gaussian Profile herein, and Wave-front phase is approximately a plane.
For eliminating the alignment error of ground transmitter and the inclination of wave front of atmospheric turbulance, descending beacon beam efficiently being entered in monomode fiber (1-6), and makes uplink laser beam accurately point to GEO satellite, also needing an automatic tracking system.A part of beacon beam is separated, by imaging on imageing sensor (1-14) after diaphragm (1-15), lens (1-13) by spectroscope (1-3).The incident orientation of descending beacon beam can be obtained by the upper light spot image of imageing sensor (1-14), by drive control module (1-16), quick deflecting mirror (1-3) is deflected, eliminate alignment error and atmospheric turbulance inclination of wave front.This automatic tracing process can complete descending beacon beam simultaneously and aim at GEO satellite to the coupling of monomode fiber (1-6) and uplink laser beam.
May there is relative position between monomode fiber (1-6) and imageing sensor (1-14) to drift about, the reference position of automatic tracking system is changed, its result makes the optical axis deviation GEO satellite position of uplink laser beam.Common solution is increase by electronic shutter (1-17) and prism of corner cube (1-4), make the actual exit direction of imageing sensor (1-14) also detectable uplink laser beam, thus the tracking benchmark of calibration automatic tracking system.
Short wavelength (0.85
) spot size of monomode fiber
general about 2 ~ 3 microns, for making the beacon beam after compression more can enter in monomode fiber (1-6) by efficient coupling, need meet following condition:
, in formula
for the focal length of lens,
for optical maser wavelength, lens (1-5) diameter is
,
for the spot size of monomode fiber.Under usual conditions
, under this condition, beacon beam is 81.45% to the maximum coupling efficiency of monomode fiber, and as considered the loss that other factors causes again, the coupling efficiency of 50% can be accomplished.When reaching lens (1-5) from the Gaussian beam of monomode fiber end face outgoing, the beam waist radius of Gaussian beam is
.
Diaphragm (1-15) is used for being limited in the descending beacon light beam diameter that imaging gone up by imageing sensor (1-14), the inclination of wave front caused by turbulent flow that imageing sensor (1-14) is measured can reflect the turbulent wave top rake of uplink laser beam substantially, then eliminates by the deflection of quick deflecting mirror (1-2) inclination of wave front and alignment error that turbulent flow causes.The aperture radius of general diaphragm (1-15)
gaussian beam beam waist radius up with lens (1-13) place
quite, namely
.
As descending beacon beam and uplink laser beam adopt different wavelength, then need optical antenna (1-1) and lens (1-5) achromatism, while ensureing that descending beacon beam can be focused at monomode fiber (1-6) end after lens (1-5), ensure that the uplink laser beam sent from monomode fiber end face is collimation after optical antenna (1-1) outgoing.Optical antenna (1-1) can adopt transmission-type antenna, also can adopt mirror antenna, and mirror antenna does not exist aberration problem.
The intensity fluctuation waveform of descending beacon beam is obtained from monomode fiber (1-6), and go with this intensity fluctuation waveform the transmitting power controlling uplink laser beam, need certain signal processing time, within this period of time, due to the impact of wind speed, atmospheric turbulance can change, and make the intensity fluctuation of uplink laser beam not exclusively the same with the intensity fluctuation possibility of descending beacon beam, mechanical oscillation also can make the position of monomode fiber change in addition.The scintillation of general atmosphere turbulent flow and the frequency of mechanical oscillation are at below 100Hz, and for reducing the error that time delay causes, signal processing time is sufficiently short, and being less than 1 millisecond is needs, preferably can reach less than 0.1 millisecond.
In the acquisition to intensity fluctuation waveform and in the processing time, atmospheric turbulance and monomode fiber position may change.Better signal processing mode is the correlation that using degree rises and falls, according to former intensity fluctuation value prediction time delay
after intensity fluctuation value.
As shown in Figure 2, the intensity fluctuation waveform of the descending beacon beam measured from monomode fiber (1-6), become digital signal through A/D converter (2-1), then by forward signal prediction module (2-2) according to the digital signal of a front n sampling instant
,
...,
, the prediction algorithm prediction time delays such as self adaptation least mean-square error forward signal prediction algorithm can be adopted
after value
analog signal is converted to again by D/A converter (2-3), this analog signal is sent into the judgement that signal transacting center (1-10) completes its signal power, power controlled automatically by sending control module (1-11) and completes the modulation of data to be transferred again, then utilize power demand to obtain the drive current mated, last drive laser (1-12) sends laser.
Suppose the signal sampling time interval
with signal processing time
equal (
), forward signal prediction is namely according to signal
the sampled value in moment in the past
, obtain
estimated value,
represent the number for the given data predicted.Following self adaptation least mean-square error forward signal prediction algorithm can be adopted:
(1) by the sampled signal in former moment
,
,
represent sample data number when calculating auto-correlation,
represent the integer ordinal number of sampled data,
require enough large to obtain signal autocorrelation value more accurately, calculate signal
autocorrelation value
,
,
represent that two adopt signal time difference.
(2) basis
write out signal
with
autocorrelation matrix
,
,
,
represent the integer ordinal number of sampled data,
with
represent respectively
individual and
the signal sampling value in individual moment.Then ask
inverse matrix
.
and
be all
matrix.
(3) basis
write out signal
with
autocorrelation matrix
,
.
for
matrix.
(4) signal estimation coefficient is calculated
.
(5) last according to formula:
,
, obtain signal
estimated value.
In the implementation case, the intensity fluctuation source of the descending beacon beam measured from monomode fiber (1-6) mainly contains following several respects: the intensity noise of laser on (1) GEO satellite, (2) descending laser beam causes intensity fluctuation to the alignment error of ground station, (3) intensity fluctuation that causes of atmospheric turbulance, (4) lens (1-5) back focal plane beacon hot spot can not be aimed at completely with monomode fiber and cause intensity fluctuation, the measure error that (5) photodetection circuit noise causes.
And the intensity fluctuation source of the uplink laser beam that GEO satellite receives comprises: (1) ground intensity noise of planar laser, (2) atmospheric turbulence intensity rises and falls, (3) intensity fluctuation that causes of ground transmitter alignment error, the intensity fluctuation of (4) descending beacon beam intensity fluctuation form reciprocal.If do not considered the impact that the time delay of signal transacting and descending beacon beam and uplink laser beam wavelength are inconsistent, the intensity fluctuation of the GEO satellite optical signal then caused to the alignment error of GEO satellite by atmospheric turbulance and uplink laser beam, it is on all four for can not aiming at monomode fiber the intensity fluctuation rule causing light signal in monomode fiber with atmospheric turbulance and lens (1-5) back focal plane beacon hot spot completely, after adopting the solution of the present invention, the impact of these two kinds of factors can be offset.
Therefore the intensity fluctuation that the light signal last GEO satellite received will comprise following several factor and causes:
(1) intensity noise of laser on GEO satellite, (2) intensity noise of planar laser, (3) descending laser beam causes intensity fluctuation to the alignment error of ground station, the measure error that (4) photodetection circuit noise causes.
The intensity noise of general laser is smaller, substantially can ignore, and the power averaging reducing amount that descending laser beam causes to the alignment error of ground station and intensity fluctuation variance are
In formula
for uniaxial alignment error to standard deviation
with descending beacon light beam width
(
power points overall with) ratio,
for without the largest light intensity on optical axis during alignment error,
for the light intensity that terrestrial optical antenna when having alignment error receives.Requirement in common engineering
, corresponding intensity fluctuation variance
, power averaging reducing amount
(average intensity is 86% of largest light intensity value on optical axis).Increase descending beacon light beam width
intensity fluctuation variance can be reduced and make average intensity
increase.The inverse of the intensity fluctuation waveform caused to the alignment error of ground station with descending laser beam (namely
) go to modulate upstream data to be passed after, signal fadeout on GEO satellite can not be caused, only signal can be made to strengthen, but correspondingly need the uplink laser beam average power of launching can increase by 1.16 times (about 0.6dB), Power penalty not quite.
The intensity fluctuation waveform how measuring descending beacon beam in monomode fiber is accurately the problem needing carefully process, photodetection circuit noise can make intensity fluctuation waveform measurement inaccurate, finally can be reflected in the light intensity of GEO satellite reception, low noise detection circuit measuring descending beacon beam intensity fluctuation waveform need be adopted, the fluctuating frequency of atmospheric turbulance and alignment error, generally at below 100Hz, therefore needs the interference paying special attention to suppress various low-frequency noise.Intensity fluctuation waveform signal to noise ratio need be greater than 20dB ~ more than 30dB.
Be specifically described from following 2, one is descending beacon beam has strong correlation at the intensity fluctuation waveform of terrestrial optical antenna back focus and uplink laser beam at the intensity fluctuation waveform at satellite place.Two is that monomode fiber can carry out space filtering to the amplitude of the descending beacon beam of mean amplitude of tide approaches uniformity distribution on optical antenna, makes the actual descending beacon beam distribution of amplitudes worked identical with the gaussian-shape distribution of amplitudes of uplink laser beam.
For first problem, adopt the method for Multiple-Phase-Screen, turbulent flow is divided into many sublayers
(
for integer ordinal number, represent the
layer), the distance between each layer is respectively
, in each turbulent flow sublayer, turbulent flow produces additional phase perturbation to the light wave of transmission and is respectively
, propagation between layers adopts Fresel diffraction principle.Whole process is similar to the Multiple-Phase-Screen numerical simulation of laser transmission.
For uplink laser beam, if the initial multiplexed optical wave distribution of amplitudes at transmitting optics aerial position place is
, then complex amplitude when laser is upwards transferred to each sublayer
,
...,
for:
(1)
(2)
…
(3)
In formula
for laser free space wave number,
for optical maser wavelength,
represent the two-dimensional coordinate on each sublayer,
represent imaginary part unit.
Top layer air is from the distance of satellite
very large, after light wave reaches turbulent flow top layer, the complex amplitude of satellite place light wave can be calculated according to Fraunhofer diffraction
.
(4)
The coefficient irrelevant with complex amplitude spatial distribution is not write out in formula (1) ~ (4)
.Formula (1) ~ (3) are substituted into formula (4),
(5)
(suppose identical with uplink laser beam wavelength for descending beacon beam, differ very little in practical application, its difference can be ignored), first pass through remote free-space propagation, during arrival atmospheric turbulance top, beam diameter is very large, can think uniform plane wave, suppose that amplitude is 1, then propagate into terrestrial optical antenna place from top to bottom, be transferred to complex amplitude during each sublayer
be respectively
(6)
…
(7)
(8)
On ground, the spot diameter of descending beacon beam is very large, generally reach more than 10 meters, and it is smaller for the optical antenna diameter of beam-capturing, aligning and tracking, be generally less than 0.2 ~ 0.3 meter, like this in beacon beam receiver aperture, without thinking during turbulent flow that the space COMPLEX AMPLITUDE of beacon beam is uniform.A space complex amplitude filter function is applied to this equally distributed complex amplitude
, then at the complex amplitude receiving optical antenna back focus place light wave be
(9)
As before, the coefficient irrelevant with complex amplitude spatial distribution is not write out in formula (6) ~ (9) yet
.Formula (6) ~ (8) are substituted into formula (9),
(10)
From formula (5) and formula (10), as the space complex amplitude filter function of descending beacon beam
with the initial complex amplitude of uplink laser beam
spatial distribution identical, namely
=
, (A is invariant), then light wave complex amplitude on satellite
with terrestrial optical aerial focus place multiplexed optical wave amplitude
changing Pattern by identical.Namely the beacon beam intensity fluctuation waveform at terrestrial optical aerial focus place is identical with the intensity fluctuation waveform of the up beacon beam signal that satellite receives.
The following describes and adopt same monomode fiber to launch uplink laser beam and to receive descending beacon beam, can make
.
If terrestrial optical antenna radius is
, focal length is
.Spot size is
monomode fiber be placed on focus place, then the Gaussian beam of unit power is after monomode fiber outgoing, arrives the COMPLEX AMPLITUDE at optical antenna place to be:
(11)
Wherein
for the up Gaussian beam beam waist radius in terrestrial optical antenna place.
Be coupled into the descending beacon beam complex amplitude of monomode fiber
can calculate in monomode fiber end, also can calculate at optical antenna plane place, two kinds of methods are (Wallner O of equal value, Winzer P J, Leeb W R. Alignment tolerances for plane wave to single-mode fiber coupling and their mitigation by use of pigtailed collimators. Appl.Opt, 2002,41 (4): 637-643).As calculated at antenna plane place, be equivalent to the descending beacon beam complex amplitude at terrestrial optical antenna place
the complex amplitude of optical antenna is arrived with the Gaussian beam from monomode fiber outgoing
carry out related operation, namely
(12)
Compare with formula (9), have
, namely utilize the space filtering effect of monomode fiber, the actual descending beacon beam COMPLEX AMPLITUDE worked can be made identical with initial uplink laser beam COMPLEX AMPLITUDE.Even if when monomode fiber location error and optical antenna edge produce truncation effect to Gaussian beam, they are also identical.As monomode fiber location error (random fluctuation), then site error will produce an inclination of wave front amount in formula (11), this inclination of wave front makes uplink laser beam produce alignment error, make uplink laser beam optical axis deviation satellite position, satellite received signal produces intensity fluctuation.On the other hand, the site error of monomode fiber also can make the descending beacon beam be coupled in monomode fiber produce intensity fluctuation.Both fluctuating rules are identical.Therefore the uplink laser beam intensity fluctuation adopting same monomode fiber also can suppress ground transmitter alignment error to cause.
There is the stronger region of a turbulent flow apart from ground 10km At The Height, have larger contribution to uplink laser beam scintillation, and this region wind speed is comparatively large, can reach 30 meter per seconds.
Descending beacon beam propagates into ground from this region through 10km hydraulic flow path, is then turned back to the time in this region by ground-launched uplink laser beam, at least needs 0.07 millisecond, and as comparatively large in zenith angle, and this time can be longer.Therefore the acquisition of descending beacon beam intensity fluctuation waveform and the time according to its waveform modulated laser generation uplink laser beam reciprocal, can refer to the design of this time, desirable 0.1 ~ 1 millisecond, shorter signal processing time has little significance.
After adopting method of the present invention, ground can reduce greatly to the scintillation value of the uplink laser beam of GEO satellite.Figure 3 shows that the beneficial outcomes figure (only considered the impact of atmospheric turbulance) after according to the invention process of Multiple-Phase-Screen numerical simulation.Simulated conditions: adopt Hufnagel-Valley 5/7 atmospheric turbulance Refractive Index Model, Bufton wind model (Chen Jingyuan, atmospheric turbulance intermittence and the impact on light wave propagation thereof, China Engineering Physics Research Institute Ph.D. Dissertation, 2005.1), zenith angle
, uplink laser beam is Gaussian beam, and waist diameter is 7.5 centimetres, wavelength 0.85 micron, descending beacon wavelength 0.83 micron.When not adopting of the present invention (uplink laser beam Initial Trans is constant), turbulent wave top rake is followed the tracks of the rear scintillation index of uplink laser beam near optical axis and is about 0.21.Application the present invention and signal processing time be when being 0.1 millisecond, turbulent wave top rake follow the tracks of after the scintillation index of uplink laser beam near optical axis be about 0.0045, scintillation index reduces more than 40 times.Application the present invention and signal processing time are when being 1 millisecond, and after tilting to follow the tracks of, the scintillation index of uplink laser beam near optical axis is about 0.036, and scintillation reduces nearly six times.
The angular range relevant to descending beacon beam intensity fluctuation due to uplink laser beam is very little, only has a few microradian.Preferably the angular deviation between uplink laser beam and descending beacon beam is controlled in actual applications, below 1 microradian, at this moment just to have obvious beneficial effect.As the angular deviation between uplink laser beam and descending beacon beam is comparatively large, then the intensity fluctuation of light signal satellite received can increase on the contrary.
In addition the present invention can only be used in the face of GEO satellite up laser communication in, because if the motion of satellite relative ground station, then uplink laser beam needs lead, and this lead angle generally can reach tens microradians, considerably beyond the related angle of uplink laser beam and descending beacon beam intensity fluctuation.But ground station might not require static, it can be high-speed motion.As ground station's high-speed motion, then require shorter signal processing time, the requirement of signal processing time and ground station's movement velocity are inversely proportional to.
Claims (6)
1. the restraining device of signal fadeout in a ground to GEO satellite laser communications, it is characterized in that: the descending beacon beam that the GEO satellite of reception sends is coupled into monomode fiber by terrestrial optical antenna, the uplink laser beam that acquisition atmospheric turbulance and ground transmitter alignment error cause is at the intensity fluctuation waveform of GEO satellite optical signal, measure the intensity fluctuation waveform of beacon beam in monomode fiber, power judgement is carried out to this intensity fluctuation waveform in signal transacting center, send control module automatically control power and complete the modulation of data to be transferred, drive laser sends up laser, this up laser is coupled in monomode fiber by wavelength division multiplexer, the end face being transferred to monomode fiber along the direction contrary with descending beacon beam signal exports, again through lens, spectroscope, after quick deflecting mirror, the uplink laser beam of collimation is formed from optical antenna outgoing, the described power that carries out judges to be specially, and the intensity fluctuation waveform of descending beacon beam, by the digital signal of forward signal prediction module according to a front n sampling instant after A/D converter
,
...,
, prediction time delay
after value
, be converted to analog signal by D/A converter, send into the judgement that signal transacting center completes its signal power, describedly carry out automatically controlling to be specially to power: the power after the conversion of input signal photoelectricity compares with the power needed for launching, and Feedback of Power circuit compensates power, makes it reach required performance number, the modulation of data to be transferred is specially: utilize the waveform reciprocal of descending beacon beam intensity fluctuation to go to modulate upstream data waveform to be passed.
2. restraining device according to claim 1, it is characterized in that: for eliminating the alignment error of ground transmitter and the inclination of wave front of atmospheric turbulance, a part of beacon beam is separated by spectroscope, by imaging on the image sensor after diaphragm, lens, the incident orientation of descending beacon beam is obtained by the light spot image that imageing sensor is formed, by drive control module, quick deflecting mirror is deflected, eliminate alignment error and atmospheric turbulance inclination of wave front.
3. restraining device according to claim 1, is characterized in that: for making beacon beam efficient coupling enter in monomode fiber, need meet following condition:
, in formula,
for monomode fiber spot size,
for the focal length of lens,
for optical maser wavelength,
for lens diameter.
4. the suppressing method of signal fadeout in a ground to GEO satellite laser communications, it is characterized in that: the descending beacon beam that the GEO satellite of reception sends is coupled into monomode fiber by terrestrial optical antenna, the uplink laser beam that acquisition atmospheric turbulance and ground transmitter alignment error cause is at the intensity fluctuation waveform of GEO satellite optical signal, measure the intensity fluctuation waveform of beacon beam in monomode fiber, power judgement is carried out to this intensity fluctuation waveform in signal transacting center, send control module automatically control power and complete the modulation of data to be transferred, drive laser sends up laser, this up laser is coupled in monomode fiber by wavelength division multiplexer, the end face being transferred to monomode fiber along the direction contrary with descending beacon beam signal exports, again through lens, spectroscope, after quick deflecting mirror, the uplink laser beam of collimation is formed from optical antenna outgoing, the described power that carries out judges to be specially, and the intensity fluctuation waveform of descending beacon beam, by the digital signal of forward signal prediction module according to a front n sampling instant after A/D converter
,
...,
, prediction time delay
after value
, be converted to analog signal by D/A converter, send into the judgement that signal transacting center completes its signal power, the method that described power controls automatically is specially: the power after the conversion of input signal photoelectricity compares with the power needed for launching, and Feedback of Power circuit compensates power, makes it reach required performance number, the modulation of data to be transferred is specially: utilize the waveform reciprocal of descending beacon beam intensity fluctuation to go to modulate upstream data waveform to be passed.
5. suppressing method according to claim 4, it is characterized in that: for eliminating the alignment error of ground transmitter and the inclination of wave front of atmospheric turbulance, a part of beacon beam is separated by spectroscope, by imaging on the image sensor after diaphragm, lens, the incident orientation of descending beacon beam is obtained by the light spot image that imageing sensor is formed, by drive control module, quick deflecting mirror is deflected, eliminate alignment error and atmospheric turbulance inclination of wave front.
6. suppressing method according to claim 4, is characterized in that: for making beacon beam efficient coupling enter in monomode fiber, need meet following condition:
, in formula,
for the focal length of lens,
for optical maser wavelength,
for lens diameter,
for monomode fiber spot size.
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