FR3071929A1 - LASER RADAR FOR THE DETECTION OF SPATIAL DEBRIS - Google Patents

Abstract

Radar (1) comprising: - a transmission channel (10) for generating a burst of pulses, the transmission channel comprising an ultra-short pulse laser driven by an HF oscillator and a device for temporally stretching pulses and pulses; amplifying; - a return channel (20) for receiving the echo of the pulse burst, comprising an optical compressor (21) for reducing the time duration of the pulses and a scanning camera (22) synchronized to the HF oscillator.

Description

LASER RADAR FOR DETECTION OF SPATIAL DEBRIS

The present invention relates to laser radars suitable for detecting space debris and determining their orbits.

Space congestion with space debris is a growing concern as this debris poses a risk to satellites and people sent into space. This debris is currently detected using radars operating in the centimeter bands from Earth, which does not allow the detection of the smallest debris. However, we admit that a piece of debris is dangerous from a size on the order of a millimeter, given its speed. The uncertainty which reigns over the knowledge of the orbits of space debris forces satellites and other spacecraft to perform anticipatory avoidance maneuvers, most of which would be useless if one could know the orbit with greater precision. waste.

Various solutions have already been proposed for detecting space debris, mainly based on rather complex image processing methods.

Thus, US Pat. No. 8,923,561 describes a method for simulating the trajectories of virtual space debris and for processing images using a motion vector constructed from the simulations.

Application CN 101929859 discloses a method of detecting space debris using cameras in full format scan mode.

CN 101916439 discloses a method of detecting space debris based on an image processing method using the HilbertHuang transformation.

EP 2 894 842 discloses a detection method which is based on image processing with pixel offset correction.

The article "Long range, high resolution laser radar" (Optics Communications 105 (1994) -63-66) by A. Braun et al, describes a radar system in which a time-stretched pulse is sent on a reflective object having two surfaces reflective spatially shifted and in which the corresponding echoes are compressed and then directed towards an autocorrelation device making it possible to know the delay between the two echoes. Such a device is not intended to measure the speed of a moving object such as space debris.

The article "Demonstration designs for the remediation of space debris firom the International Space Station" (Acta Astronautica 112 (2015) 102-113) describes a system for desorbing space debris. The system includes an EUSO (Extreme Universe Space Observatory) telescope with a detector sensitive to photons reflected by debris and a CAN (Coherent Amplification Network) laser system with a single-mode fiber amplifier network. Such a system makes it possible to locate debris but is not suitable for providing the speed of the latter with great precision.

There is therefore a need to be able to detect space debris and determine their orbits with precision, with a sensitivity suitable for small debris, and the invention meets this need thanks to a radar comprising:

- an emission channel for generating a burst of pulses, the emission channel comprising an ultra-short pulse laser controlled by an HF oscillator and a device for temporally stretching the pulses and amplifying them,

- a return channel to receive the echo of the pulse burst, comprising an optical compressor to reduce the temporal duration of the pulses and a synchronized scanning camera on the HF oscillator.

The invention provides a particularly precise means for knowing the velocity vector of a distant object and thus makes it possible to determine with much greater precision than the methods which have been proposed so far the orbits of the debris and thus to estimate more precisely the potential danger that they represent vis-à-vis a satellite or any other spacecraft evolving in orbit.

The invention thus makes it possible to estimate the potential for collision of small debris, in particular from 1 mm to 10 cm. These estimates have so far been unreliable due to uncertainty about their speeds.

The use of a scanning camera synchronized with the laser oscillator makes it possible to quickly and precisely determine the displacement of the object as well as, if necessary, its shape and its own kinematic components such as rotation on itself 'object. It is thus possible not only to determine the orbit of the space debris but also to identify it.

Sending temporally stretched pulses instead of compressed pulses reduces the self-focusing and phase-modulating effects. Thus it is possible to obtain a long range and high resolution laser radar.

Preferably, the radar includes a telescope on the return path of the light. This can for example be an EUSO telescope as described in the article "Demonstration designs for the remediation of space debris from the international Space Station" cited above.

The ultra-short pulses are produced by the oscillator. Preferably, the duration of ultra-short pulses of the laser is less than or equal to 1 ps, better than 100 fs. This increases the detection accuracy, for example less than 1mm, over a long distance. Preferably, the pulse spectrum remains constant after the oscillator, throughout the time stretch, the energy boost and the time compression.

Preferably, the burst of pulses comprises a series of pulses (also called “pulses”) emitted at a frequency greater than or equal to 100 Hz, better at 500 Hz, even better at 1 kHz, for example between 500 Hz and 10 KHz . Such a repetition rate makes it possible to measure the speed vector of the debris with good precision.

Each pulse preferably has an energy level greater than or equal to 0.5J, better than 1 J.

In particular, the burst of pulses can be obtained using the ultrashort draw amplification technique known as CPA (Chirped draw amplification). This technique is for example described in the article "Compression of Amplified Chirped OpticalPuises" (Opt. Commun, 56, 219-221, December 1985).

The emission channel preferably includes a fiber amplifying medium, for example a network of single-mode fibers known as such, encountered in particular in CAN lasers, for example such as that described in the article "The future is fiber accelerators" (Nat.Photonics 7 (2013) 258-261, 2013).

The fiber amplifying medium can be configured to allow at least partial ionization of a space debris on which the laser is focused.

The frequency of the laser oscillations is preferably stabilized by an optical clock. This technique makes it possible to synchronize the camera to the burst of pulses with an accuracy lower than the ps.

The radar may include a calculator to calculate, on the basis of a series of echoes received from a satellite object, its orbit and / or its shape.

The subject of the invention is also a method for determining the orbit of an object, in particular a space debris, in orbit around the earth, comprising the steps consisting in:

- send towards the object a burst of pulses emitted by a radar according to the invention as defined above, placed in orbit,

- calculate from the received echo the speed of the object and its orbit.

The method may include calculating, from the received echo, the shape of the object.

The method may include sending a pulse to the object having sufficient power to ionize the object on the surface with the formation of a plasma and analysis of the spectrum of the radiation emitted by the plasma to determine the composition of the object.

A laser having a suitable power is for example described in the article "A spaceborne, pulsed UV laser System for re-entering or nudging LEO debris, and reorbiting GEO debris" (Acta Astronautica 118 (2016) 224-236).

The method may include calculating, from the orbit of the previously determined object, a probability of collision with another listed satellite object.

The method may include the implementation of an avoidance strategy in the event of a collision risk greater than a predefined threshold.

The invention will be better understood on reading the detailed description which follows, of an example of non-limiting implementation thereof, and on examining the appended drawing, in which FIG. 1 shows in a way schematic and partial an example of a radar according to the invention.

The laser radar 1 according to the invention, represented in FIG. 1, comprises an emission channel 10 and a reception channel 20 for processing the echo of the light emitted on a space debris D, therefore the largest dimension can be less than or equal to 5 mm, or even 1mm. Of course, the invention is not limited to a particular size of object, and also covers the determination of the speed of objects having for example a dimension of the order of 10 cm.

The laser radar 1 is arranged to generate a burst of light pulses 11, which are sent to the debris D through a telescope 30 which makes it possible to reduce the divergence of the beam. The debris is for example located at a distance from the laser of between 100 and 1000 km.

The light echoes which are reflected on the debris D are picked up by the telescope 30 and directed towards the reception channel 20.

The laser comprises a laser cavity 13, for example of the mode lock type, making it possible to generate ultra-short light pulses 14, for example of the order of ps, in particular of duration between 0.03 and 100 ps, at a rate for example between 1 and 1000 MHz, for example between 10 and 200 MHz, in particular of the order of 100 MHz. These pulses 14 are synchronized with an oscillator which produces a reference signal 15, preferably sinusoidal as illustrated. The energy of each ultrashort pulse 14 before amplification by CPA is for example between the pJ. and the pJ.

The pulses 14 are filtered by a device 17 which comprises an electronically controlled shutter, in order to reduce the frequency of the pulses by letting pass only one pulse every n pulses, with n integer chosen to obtain for example a burst 18 where the frequency pulse repetition is of the order of 10 kHz, against 100 MHz upstream of the device 17.

The ultra-short pulses of the burst 18 are stretched in time for their amplification in a known manner by the CPA technique (“Chirped pulsed Amplification”), using a device 16 (“stretcher”), for example a pair of diffraction grids, to obtain a burst 28 of pulses. This burst 28 is amplified in an amplifier 19 to produce the pulses 11 which are directed through the telescope 30 to the debris D whose speed and shape are sought to be known. The time stretch of the ultra-short pulses reduces the non-linear effects during amplification. The time stretch factor is for example between 1000 and 100,000. For example, after stretching, the duration of a pulse is extended from the order of ps to the order of ns. The gain in amplification will preferably be between 1000 and 10 ^1θ .

The amplifier 19 is preferably a fiber amplifier so as to facilitate the cooling of the amplifying medium. Such an amplifier, for example used in a CAN laser as described in the article “The future is fiber accelerators” cited above, may comprise a network of single-mode fibers, for example of a number between 1000 and 10000. L the amplifier receives as input a seed signal, for example having an energy level of the order of pJ. After amplification in the fiber the signal becomes of the order of mJ. The same operation is performed on all fibers. The signal is amplified by each fiber in the network and is combined in phase and at the output. Thus, the amplifier makes it possible to produce at the output a signal having an energy level of the order of IJ, while having a relatively high wall-plug efficiency, for example of the order 30%. The signal amplified by this phase network has other useful properties. In particular, as the amplifier is formed by a network of amplifying fibers, the amplified pulses, all coming from a single pulse, have between them a phase imposed by electro-optical or acousto-optical elements of the network. Thus, the wavefront can be changed very quickly, for example in lms, or the time form of the pulse can be changed at will. For example, if the phase difference is zero between the fibers, a pulse with a plane wave front will be emitted by the network. This wavefront can be changed at will by changing the phase difference between the fibers. This can make more effective the vaporization / partial ionization of the debris necessary for their elemental analysis.

In order to determine the composition of a debris, the telescope 30 can focus the amplified laser beam on the debris until ionization occurs with the formation of a plasma. The spectrum of the radiation emitted by the plasma is characteristic of the elements that constitute the debris. This method of analyzing the elementary chemical composition of a sample is known as LIBS (/ 'Laser Induced Breakdown Spectroscopÿf

The radiation emitted by the plasma can be collected using an optical fiber connected to a spectrometer coupled to a detector. For example, the telescope 30 includes an optical fiber suitable for collecting the radiation emitted. The scanning camera 22 of the invention can be used as a detector. Alternatively, another camera, for example intensified charge transfer (ICCD) can be used.

Preferably, the laser oscillation in the cavity 13 is controlled by an optical clock according to a known method which consists in separating using a spectral comb different modes of oscillations of the cavity and in extracting one of the modes using a frequency doubler to generate a harmonic. The two modes are combined to produce a signal, which is used to control all of the modes of the laser pulse. The principle of an optical clock is for example described in the article "Ab solute optical frequency measurable of the cesium Dl line with a mode-locked laser" (Phys.Rev.Lett. 82,3568 (1999)). The stability df / f of the laser oscillation frequency is extreme and can be better than 10 ¹⁸ .

The reception channel 20 comprises a spectral compressor 21, for example a pair of diffraction gratings, which is in phase conjugation with the stretching device 16 to compress each pulse reflected on the space debris D, which reduces the duration d impulse and increases its power. The reception channel 20 provides at the output of the compressor 21 a train of pulses 22 where each pulse has a shorter duration, for example of the same order as the ultrashort pulses 14, and a higher power. The compressor 21 can be relatively inexpensive because the power of the pulses to be compressed is low, being an echo of light.

The pulses 22 are sent to the slot of a synchronized scanning camera 40. This slot cuts a fine section of the return signal. The longitudinal direction of the slot is perpendicular to the direction in which the sweep is carried out. Such a camera typically includes an optical input 41, a photoelectric conversion element 42, for example a photocathode, which transforms the incident photons into electrons which can be accelerated in an accelerating cavity 43 by passing between scanning electrodes 44, subjected to a scanning potential which is synchronized with the reference signal 15 supplied by the laser oscillator 13. The slit is therefore illuminated by an optical signal, which varies along it, as is the case for an echo coming from a non-uniform relief surface with peaks and valleys of a few mm. The scan of the slit provides the temporal profile of the spatial variations of the target surface. This property can be used to identify debris, as described below. An electronic circuit 45 makes it possible to generate the high-voltage scanning voltage applied to the scanning electrodes 44 from the reference signal 15. Thus, the electrons generated by the photons echoed on the space debris D are deflected in a way which is dependent on the time difference existing between the moment when the well was emitted by the laser cavity 13 and that when the electrons pass between the scanning electrodes 44. The camera can allow a precision better than the ps, with an efficiency of more of the order of 10% compared to the incoming light flux.

A detector 46 makes it possible to detect the incident electrons and to materialize the angle of deflection of these electrons, which is a function of the phase of the signal applied to the scanning electrodes 44. It is thus possible to know with great precision the time taken by the light to reach the debris and reach, after reflection on it, the detector 46. The repetition frequency of the echo pulses received, of the order of kHz for example as indicated above, makes it possible to know the speed vector with precision, so easy.

As described above, when a photon front is emitted in the same plane, these photons can be scattered with a time phase shift which is linked to the shape of the object. This time shift produces on the detector 46 a signal representative of the shape of the surface on which the light emitted by the laser is reflected. The signal supplied by the detector 46 can then be processed so as to calculate the speed of the debris relative to the laser as well as its own shape and kinematic characteristics, for example its speed of rotation on itself. Sub-millimeter accuracy can be obtained.

Knowing the speed of the space debris makes it possible to precisely calculate its orbit and thus to implement avoidance strategies.

Of course, the invention is not limited to the example which has just been described.

When the desired precision is less, the laser may not include an optical clock for locking its frequency. A conventional oscillator with a stability of 10 ⁹ provides an accuracy of the order of 1mm at a distance of 1000km.

Speed detection can also be associated with detection of its chemical composition by sending laser pulses powerful enough to cause local ablation on the surface of the space debris, and analyze the light emitted using a spectrometer to recognize depending on this the presence of certain chemical elements.

Claims (15)

1. Radar (1) comprising: - an emission channel (10) for generating a burst of pulses, the emission channel comprising an ultra-short pulse laser controlled by an HF oscillator and a device for temporally stretching the pulses and amplifying them, - a return channel (20) for receiving the echo of the pulse burst, comprising an optical compressor (21) to reduce the temporal duration of the pulses and a camera (22) with synchronized scanning on the HF oscillator. 2. Radar according to claim 1, comprising a telescope (30) on the return path of the light. 3. Radar according to one of claims 1 and 2, the duration of the ultrashort pulses (18) of the laser being less than or equal to 1 ps. 4. Radar according to any one of claims 1 to 3, the ultra-short pulses (18) comprising a series of pulses at a frequency greater than or equal to 100 Hz, better at 500 Hz, even better at 1 kHz. 5. Radar according to any one of the preceding claims, the emission channel (10) comprising a fiber amplifying medium (19). 6. Radar according to claim 5, the fiber amplifying medium (19) comprising a network of single-mode fibers. 7. Radar according to claim 5 or 6, the fiber amplifying medium (19) being configured to allow at least partial ionization of a space debris on which the laser is focused. 8. Radar according to any one of claims 1 to 7, the frequency of the laser oscillations being stabilized by an optical clock. 9. Radar according to any one of the preceding claims, comprising a calculator for calculating, on the basis of a series of echoes received from a satellite object, its orbit. 10. Radar according to any one of the preceding claims, comprising a calculator for calculating, on the basis of a series of echoes received from a satellite object, its shape. 11. Method for determining the orbit of an object, in particular a space debris, in orbit around the earth, comprising the steps consisting in: - send towards the object (D) a burst of pulses emitted by a radar (1) according to any one of the preceding claims, placed in orbit, 5 - calculate from the received echo the speed of the object and its orbit. 12. The method of claim 11, wherein the shape of the object is calculated from the echo received. 13. Method according to one of claims 11 and 12, wherein one sends on the object a pulse having a sufficient power to ionize on the surface the object with the 10 formation of a plasma and your analysis of the spectrum of the radiation emitted by the plasma to determine the composition of the object. 14. Method according to any one of claims 11 to 13, in which a probability of collision with another listed satellite object is calculated from the orbit of the object thus determined. 15. The method as claimed in claim 14, in which an avoidance strategy is implemented in the event of collision risk greater than a predefined threshold.