GB2306828A - Covert 3-dimensional imaging lidar - Google Patents

Abstract

The tomoscopic laser detection device according to the invention comprises means (1, 2) to illuminate a target (3) at a wavelength that is unusual for a laser source, means (4) to obtain a temporal gate on the wave that is back-scattered or partially reflected by the target (3) following the laser illumination and means (5) to convert the unusual wavelength into a wavelength appropriate for an image sensor (6), enabling discretion of detection to be ensured. Application to reconnaissance, "de-camouflage", 3D navigation.

Description

TOMOSCOPIC LASER DETECTION DEVICE The present invention relates to a tomoscopic laser detection device, it can be applied chiefly to active laser imaging.

Imaging systems generally perform the functions of navigation, detection and identification.

For each of these functions, it is necessary to takes measures to counter the non-cooperative nature of the target and of the medium of propagation (which is a naturally or artificial scattering medium).

Tomoscopy enables a considerable improvement in the functional quality of the viewing means implemented in standard imaging systems.

With regard to the navigation function, tomoscopy provides the possibility of obtaining a pseudo-3D image particularly relevant in an application to the piloting of helicopters.

For the detection function, tomoscopy provides the possibility of increasing the detection capacity of the viewing device by obtaining a decorrelation, through temporal gating, between, on the one hand, the target and, on the other hand, the effects related to the propagation medium (smoke, fumigenic phenomena, fog etc.) the optronic countermeasure means and backgrounds with low contrast (through an increase in the contrast).

For the identification function, tomoscopy offers the possibility of obtaining an image of a target with an exposure time (for integration) that is extremely small without in any way causing an unacceptable deterioration in the signal-to-noise ratio. The quality of the image is thereby improved by freezing the effects of atmospheric distortion and of stabilisation of the line of sight.

There has been much work on imaging by tomoscopy in the near infrared range with the use of a monochromatic source.

In the military field, a demonstrator has been made with a 1.06 pm laser and a light intensifier tube.

Systems developed from silicon-based and tube-based photoconductors are being profitably used in detection.

In the medical field, the renewal of interest is related to the development of femtosecond sources which can be used to obtain imaging on small-sized objects immersed in highly scattering environments. The heightening of the contrast and the use of the gate detection technique making use of optical nonlinearities (X(2) and X(3)) have been extensively demonstrated, especially in an article by R. MAHON, M.D. DUNCAN, L.L. TANKERSLEY, J. REINTJES, "Time-gated imaging through dense scatterers with a Raman amplifier" in Appl. Opt. 32 (36), 20th December 1993, p. 7425.

Furthermore, the making of parametric amplifiers working in the visible range on a sub-picosecond time scale has also enabled the performance of imaging in these same environments. Amplifiers of this kind are notably described in an article by J. WATSON, T.

LEPINE, P. GEORGES, A. BLUN, "Femtosecond parametric generation and amplification in the visible spectrum", in OSA Proc. On Adv. Solid-State Lasers, 1994, Vol. 20, p. 425.

However, all these systems have the drawback of having the laser emission wavelength included in the spectral band of the imagers used especially in battlefield conditions. This drawback may vitiate the discretion of the device used to detect these systems.

The present invention is aimed at overcoming the above-mentioned drawback. To this end, an object of the invention is a tomoscopic laser detection device characterised in that it comprises means to illuminate a target at a wavelength that is unusual for a laser source and in that it comprises means to obtain a temporal gate on the wave that is back-scattered or partially reflected by the target following the laser illumination and means to convert the unusual wavelength into a wavelength appropriate for an image sensor, enabling discretion of detection to be ensured.

The invention has the advantage of transferring the laser flux from a spectral band unusual for a laser to a spectral band appropriate for an image sensor, enabling discretion with respect to the laser flux emitted, 3D imaging and "de-camouflage" type detection.

Other advantages and characteristics of the present invention shall appear more clearly from the following description and from the appended figures, of which: - Figure 1 is a block diagram of a discrete laser tomoscopy device in which the invention is embodied, and - Figure 2 shows a device in which the invention is embodied.

The combination of tomoscopy with frequency transposition enables the making of a temporal gate and the conversion of an infrared wave, constituting the "flash" laser, in an usual spectral band into a different spectral domain. In a configuration of this kind, the spectral domain of the flash illumination is not included in any of the spectral bands of the battlefield sensors. This property provides for discretion and enables the detection system to be hardened.

Figure 1 shows a block diagram of a discrete tomoscopy laser device. It comprises a first source also called a primary source that is tuneable and covers, for example, band II in the infrared range. This source may consist of an Nd: YAG type pulsed laser pump 1, working for example at a wavelength Apl = 1.064 Wm that excites an optical parametric oscillator (OPO) type of assembly 2. At the output of this assembly, a wavelength-agile emission is obtained with a wavelength ranging from 1.4 to 4.43 pm.

This is obtained by using a non-linear medium such as an LiNbO3 crystal.

In the non-linear interaction, the pump wave generated by the pump laser 1 at the pulse op generates two waves, a signal wave xs and a complementary wave These two waves meet the conditions of conservation of energy and phase-matching known as the "conservation of momentum": op = Xs + ic (1) k= ks + k, (2) where kss kS and kc correspond respectively to the wave vectors associated with op, os and xc Only the complementary wave xc is used and serves to illuminate the target 3.During its propagation in the atmosphere, and depending on the arrangement of the targets along the path, a part of the light is either back-scattered or partly reflected.

The return signal consists of continuous background with an exponential decrease resulting from the backscattering mechanism on which there are superimposed the echoes associated with the different image planes linked to the objects or structures present. The fact of working in band II considerably minimises the losses by Rayleigh scattering proper to the medium of propagation. By using the fact that the time of propagation of a laser pulse corresponds to a distance, it is possible to analyse a plane image by using an optical gate for the sampling.

The device in which the invention is embodiea furthermore comprises means to obtain the difference in frequency between the wave emitted, back-scattered or partly reflected by the target 3 and an auxiliary pump wave p2 These means can be subdivided into first means 5 used to obtain a temporal gate on the wave that is emitted and back-scattered or partly reflected by the target 3, using for example an optical gate controlled by the auxiliary pump wave, and second means 5 for the frequency transposition, within the gate, of the wave emitted and back-scattered or partially reflected by the target 3 into a spectral domain for which efficient detection systems such as image sensors 6 are available.In particular, by performing the following operation: - ~ Xc = Zs (3) we arrive at the creation of a signal wave os. Taking again the values of the previous example, the wavelength of this signal wave is in the range of As = 1.5 pm, in mixing the wave that is back-scattered or partly reflected by the target 3 with the auxiliary pump wave 2. This signal wave oS exists only when the other two waves are present simultaneously. Thus, by the use of a pulsed auxiliary pump wave op2 as a control signal and through checks on the time of application of this pulse with respect to the wave that is back-scattered or partially reflected by the target 3, an optical gate function is obtained.

For this purpose, the first means comprise a second laser source 4, called an auxiliary pump source, that generates the control signal at a specified instant and at a specified wavelength, for example a wavelength of 1.064 urn to stay within the framework of the previous example. The spatial resolution is of course linked to the temporal width of the pulse coming from the auxiliary pump 4.

The mixing of the wave that is back-scattered or partially reflected by the target 3 with the auxiliary pump wave op2 is done by the second means 5 consisting of a non-linear medium, for example an LiNbO3 crystal.

The non-linear interaction brought into play enables the transposition of the beam, interacting with the objects located in the propagation medium, into the spectral domain used by the image sensors 6.

Furthermore, the efficiency of transfer depends on the peak power delivered by this pump wave.

The fact of using a tuneable primary source 1 provides knowledge of the phase-matching conditions that must be met if the interaction leading to the frequency transposition is to be efficient. This phase-matching condition is naturally represented by the following relationship: k"- 4= k, Figure 2 illustrates a device in which the invention is embodied. A first laser source 7 sends out a pulsed laser beam F1 at the laser wavelength of x1 = 1.06 pm for example. The beam F1 goes through a first non-linear medium 8, for example an LiNbO3 crystal, enabling a wavelength-agile emission by non-linear interaction.A first laser beam and a second laser beam F2 and F3 are generated respectively in the vicinity of the wavelengths x2 = 3.44 urn and x3 = 1.54 urn. A first dichroic plate 9 transmits the second beam F3 and reflects the first beam F2 to a first focusing lens 10 focusing the first beam F2 on a delaying element 11. This delaying element 11 is, for example, an optic fiber whose length determines a temporal delay T = T. At the output of the fiber 11, the first beam F2 is collimated by a second lens 12.

After reflection on a mirror 13 and a second dichroic plate 14, the two beams F2 and F3, again joined at the output of the dichroic plate 14, are collimated by a third lens 15 oriented towards a specified target 16.

The scattered fluxes, RF2 and RF3, respectively at the wavelengths A2 = 3.44 pm and x3 = 1.54 pm, are collected by a fourth lens 17. A third dichroic plate 18 separates the fluxes RF2 and RF3. The flux RF3 at the wavelength A3 = 1.54 pm is focused by a fifth lens 19 on a detector 20, for example a GaInAs avalanche detector symbolised by a diode, and enables a detection of the incident flux RF3. A first electronic processing device 21 measures the instant to of arrival of the flux RF3.A second management electronic device 22 activates a laser beam F4 generated by a second laser source 23 also called an auxiliary pump, emitting at the wavelength A4 = 1.06 pm at an instant to + T corresponding also to the instant of arrival of the flux RF2 on the third dichroic plate 18. A first filter 24, positioned between the dichroic plate 18 and a second non-linear medium 25, has the role of transmitting the wavelength 2 = 3.44 pm and A4 = 1.064 pm, corresponding respectively to the wavelengths of the fluxes RF2 and F4, and of cutting off the wavelength x3 = 1.54 pm corresponding to the flux RF3.

After a mixing of the two laser fluxes F4 and RF2 in the second non-linear medium 25, which is an LiNbO3 crystal for example, a non-linear operation to obtain the difference in frequency generates a flux F5 at the wavelength X5 = 1.54 pm.

This flux goes through a second filter 26 cutting off the wavelength s2 = 3.44 urn and x4 = 1.064 pm. The flux F5 is focused by a sixth lens 27 on a matrix of detectors 27, symbolised by a diode. This matrix of detectors 27 is sensitive to the wavelength A5 = 1.54 urn and is used to obtain an image of the target 16 corresponding to a "distance" section equal to the duration of the laser pulse of the auxiliary pump 23, divided by the speed of light.

A third electronic device 29 shapes the detected signal for a processing operation, if any, especially in order to obtain a display on a video screen 30.

As variants and without departing from the framework of the present invention, it is possible to refrain from using the optical filter and to obtain the temporal gate by an external command. It is also possible to use a laser working at a fixed rate, for example at 5 kHz, and to use a filter that does not cut off the 1.54 pm wavelength. An image at 50 Hz of the scene will then be obtained in the infrared band I in the vicinity of 1.54 pm comprising 100 "distant" planes. This operation gives the operator an impression of a 3D or near-3D display.

Claims (11)

1. A tomoscopic laser detection device comprising means to illuminate a target at a wavelength that is unusual for a laser source and means to obtain a temporal gate on the wave that is back-scattered or partially reflected by the target following the laser illumination and means to convert the unusual wavelength into a wavelength appropriate for an image sensor, enabling discretion of detection to be ensured.

2. A tomoscopic device according to claim 1, wherein the means for the laser illumination of the target are obtained by means of a pulsed laser source called a pump laser that excites an optical parametric oscillator generating a wavelength-agile laser wave.

3. A tomoscopic device according to claim 2, wherein the pump laser is an Nd:YAG laser.

4. A tomoscopic device according to claim 2, wherein the optical parametric oscillator comprises an LiNbO3 non-linear crystal.

5. A tomoscopic device according to any one of the claims 1 to 7, wherein the means used to obtain the temporal gate comprise a laser source called an auxiliary pump that generates a laser wave called a pump wave, applied simultaneously with the wave that is backscattered, or partly reflected by the target at the input of the conversion means, and for a specified duration.

6. A tomoscopic device according to claim 5, wherein the auxiliary pump is a pulsed laser generating a pulse whose width defines the duration of the conversion.

7. A tomoscopic device according to claim 5, wherein the auxiliary pump is a laser working at a specified fixed rate.

8. A tomoscopic device according to any of the claims 1 to 7, wherein the conversion means comprise a non-linear medium mixing the wave that is back-scattered or partially reflected by the target with the pump wave generated by the auxiliary wave, enabling the frequency transposition of the wave that is back-scattered, or partially reflected by the target.

9. A tomoscopic device according to claim 8, wherein the non-linear medium is a LiNbO3 crystal.

10. A tomoscopic device according to any one of claims 1 to 9, wherein it works in the band II.

11. A tomoscopic device substantially as described hereinbefore with reference to the accompanying drawings and as shown in Figure 1 or Figure 2 of those drawings.