FR3098931A1 - FREQUENCY CONVERSION ACTIVE IMAGING SYSTEM - Google Patents

Abstract

The invention relates to an imaging system (10) comprising: an illumination device (DI) configured to generate an illumination beam (Fi) comprising an illumination wavelength (λi) and to illuminate a portion of space (1) with said illumination beam, a source (Sp) called a pump configured to generate a pump beam (Fp) comprising a pump wavelength (λp), an optical device (DO) configured to collect a backscattered beam (Fd) by at least one element (2) present in the portion of space, to form a signal beam (Fs), a frequency conversion device (CF) configured to generate a converted beam (Fc) from the signal beam (Fs) and the pump beam (Fp) by a non-linear frequency summation method, the converted beam comprising a converted wavelength (λc), the optical device being further configured to form an image of at least a portion of said at least one element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye (OE) or by a detection device (Det). Figure to be published: Fig. 5

Description

Frequency conversion active imaging system

The present invention relates to active imaging systems preferably intended to operate in the infrared (IR) and portable.

The active imaging technique consists of associating an imaging sensor with its own light source to illuminate the scene at the time of observation. This technique can for example be applied to vision in the infrared, and more particularly in the near IR band. We define the NIR band (for "near" IR in English) between 0.75 μm and 1.4 μm, and the SWIR band (for "Short Wave" IR in English between 1.4 μm and 3 μm.

Active imaging systems allow, for example, 2D or 3D, high-resolution and small-field imaging functions for identification and vision through scattering media such as fog, day and night.

One of the main problems of this type of active imagers is the detection of the signals to be observed generally located in the SWIR band, which requires a dedicated detection matrix and a means of visualization. Active imaging systems are currently very complex, sensitive cameras of SWIR technologies are expensive and bulky. Until now, all active imaging systems marketed are produced in the NIR band, and based on light intensifiers (IDL) or detector arrays. Active imaging systems are being developed in the SWIR band and use cameras based on InGaAs or based on HgCdTe to image the scene, and screens or displays in the eyepiece(s) for the restitution of the image. on the operator's eye after treatments.

However, compact and portable systems are difficult to achieve.

On the other hand, one of the great advantages of active imaging systems is to be able to perform 3-dimensional imaging using pulsed lasers with temporal aliasing of detection (or "range gating"), whose principle is recalled in Figures 1a, 1b, 1c and 1d. Figure 1a illustrates the starting situation. It is sought to visualize the content of a predetermined slice of space 13 comprising an object 14. Instead of illuminating the scene continuously, it is illuminated with brief laser pulses IL (of durations typically between a few ns and a few µs) emitted by an illumination laser L. During their journey they encounter a certain number of elements, disturbances of the suspended particle type, etc. which will generate backscattered light 12. The imager 11 (for example a camera ) is closed, as illustrated by the STOP shutter, and is therefore not dazzled by the backscattered light 12. In a second phase illustrated in FIG. 1b, the pulse IL is reflected/backscattered by the object 14 to form the pulse 15 which returns to the lighting system. Imager 11 is always closed. A synchronization device 16 determines the time between the emission of the pulse IL and the return of the pulse 15 taking into account the round trip distance traveled to correspond to the distance of the space slice 13. In a third phase illustrated in FIG. 1c, at the moment when the light wave 15 reflected by the object 14 returns from the slice of space 13 (determined by the synchronization device 16) the imager 11 is open, generally for an equivalent duration to that of the laser pulse. In a fourth phase illustrated in figure 1d the imager is closed again, once the pulse 15 has been detected. Thus this temporal aliasing, associated with a pulsed illumination laser, selects a slice of space to observe, by suppressing the parasitic signals (backscattered) coming from other parts of the space.

To be compatible with the temporal aliasing technique, the detectors used must have very short exposure times, which is difficult to achieve with SWIR infrared cameras.

Thus there are currently no existing solutions to these problems: in general in the SWIR band, active imaging is not carried out and optical binoculars, IDL binoculars, or passive IR binoculars are used.

An object of the present invention is to remedy the aforementioned drawbacks by proposing a compact and portable infrared active imaging system. In addition, the imaging system according to the invention is compatible with direct use with the eye of a human observer as the final sensor.

DESCRIPTION OF THE INVENTION

• un dispositif d’illumination configuré pour générer un faisceau d’illumination comprenant une longueur d’onde d’illumination et une fréquence d’illumination, et pour illuminer une portion d’espace avec ledit faisceau d’illumination,
• une source dénommée pompe configurée pour générer un faisceau pompe comprenant une longueur d’onde de pompe et une fréquence pompe,
• un dispositif optique configuré pour collecter un faisceau rétrodiffusé par au moins un élément présent dans la portion d’espace, pour former un faisceau signal,
• un dispositif de conversion de fréquence configuré pour générer un faisceau converti à partir du faisceau signal et du faisceau pompe par un procédé non linéaire de sommation de fréquence, le faisceau converti comprenant une longueur d’onde convertie et une fréquence convertie, la fréquence convertie étant égale à la somme de la fréquence d’illumination et de la fréquence pompe,

le dispositif optique étant en outre configuré pour former une image d’au moins une partie dudit élément, ladite image, dénommée image convertie, étant formée avec le faisceau converti et destinée à être détectée par un œil d’observateur ou par un dispositif de détection.The subject of the present invention is an imaging system comprising:

• an illumination device configured to generate an illumination beam comprising an illumination wavelength and an illumination frequency, and to illuminate a portion of space with said illumination beam,
• a source called pump configured to generate a pump beam comprising a pump wavelength and a pump frequency,
• an optical device configured to collect a beam backscattered by at least one element present in the portion of space, to form a signal beam,
• a frequency conversion device configured to generate a converted beam from the signal beam and the pump beam by a nonlinear method of frequency summing, the converted beam comprising a converted wavelength and a converted frequency, the converted frequency being equal to the sum of the illumination frequency and the pump frequency,

the optical device being further configured to form an image of at least part of said element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye or by a detection device .

According to a variant, the pump beam propagates in the frequency conversion device in a direction opposite to that of the signal beam.

According to a first variant, the pump beam and the illumination beam are pulsed, and the imaging system further comprises a synchronization device configured to act on the illumination device and the pump so that a signal pulse and a pulse pump are superimposed in the frequency conversion device.

Preferably, the synchronization device comprises a delay device coupled to the illumination device and to the pump so that the pump emits a pump pulse with a time delay relative to the emission of the determined illumination pulse, a time delay value making it possible to image an associated slice of space included in said portion of space.

According to one embodiment, the delay device is configured so that the time delay is variable.

According to one embodiment, the time delay is manually adjustable.

According to another embodiment, the time delay follows a predetermined law of variation.

According to a variant, the imaging system further comprises a telemetry device configured to determine a distance from said element, said time delay then being adjusted so as to image a slice of space located at a distance D from the imaging system.

According to a second variant, the illumination beam and the pump beam are continuous, and the imaging system further comprises a laser cavity adapted for the pump beam and comprising a laser crystal, and in which the frequency conversion device is arranged .

According to one embodiment, the optical system is further configured to form an image in the visible spectrum called the visible image, which is superimposed on the converted image.

• un cristal dénommé cristal retourné pour réaliser la conversion de fréquence, le cristal comprenant un matériau dont le signe du coefficient non linéaire d’ordre 2 est inversé périodiquement,
• des cristaux massifs du même matériau mais ne comprenant pas d’inversion du signe du coefficient non linéaire d’ordre 2, disposés autour du cristal retourné.

According to one embodiment, the frequency conversion device comprises:

• a crystal called a flipped crystal to carry out the frequency conversion, the crystal comprising a material whose sign of the nonlinear coefficient of order 2 is periodically inverted,
• massive crystals of the same material but not comprising any inversion of the sign of the nonlinear coefficient of order 2, arranged around the inverted crystal.

According to one embodiment, the imaging system further comprises at least one removable mirror arranged on an optical path of the pump beam and of the illumination beam, and configured to redirect said pump and illumination beams so that the beam coming of the pump becomes the illumination beam and the beam coming from the illumination device becomes the pump beam, the initial pump beam transformed into an illumination beam being configured to perform a target designation function.

According to one embodiment, the illumination wavelength and the pump wavelength are chosen so that the converted wavelength is included in the visible spectral band. Typically the illumination wavelength is within the spectral band [1.4 – 3 µm] and the pump wavelength is within the spectral band [0.75 - 1.4 µm].

According to one embodiment, the imaging system is of the telescope type, the image being detected by an eye of the observer.

According to another embodiment, the imaging system is of the binocular type and comprises an optical device and an additional frequency conversion device, each optical device/frequency conversion device pair forming a channel of the binocular, each image being detected by an eye of the observer, the pump being common to the two channels.

• A générer un faisceau d’illumination comprenant une longueur d’onde d’illumination et une fréquence d’illumination, et illuminer une portion d’espace (1) avec ledit faisceau d’illumination,
• B collecter un faisceau rétrodiffusé par au moins un élément présent dans la portion d’espace, afin de former un faisceau signal,
• C générer un faisceau pompe comprenant une longueur d’onde de pompe et une fréquence pompe,
• D générer un faisceau converti à partir du faisceau signal et du faisceau pompe par un procédé non linéaire de sommation de fréquence, le faisceau converti comprenant une longueur d’onde convertie et une fréquence convertie, la fréquence convertie étant égale à la somme de la fréquence d’illumination et de la fréquence pompe,
• E former une image d’au moins une partie dudit au moins un élément, ladite image, dénommée image convertie, étant formée avec le faisceau converti et destinée à être détectée par un œil d’observateur ou par un dispositif de détection (Det).

According to another aspect, the invention relates to an active imaging method comprising the steps consisting in:

• Generating an illuminating beam comprising an illuminating wavelength and an illuminating frequency, and illuminating a portion of space (1) with said illuminating beam,
• B collect a beam backscattered by at least one element present in the portion of space, in order to form a signal beam,
• C generate a pump beam comprising a pump wavelength and a pump frequency,
• D generating a converted beam from the signal beam and the pump beam by a non-linear frequency summation method, the converted beam comprising a converted wavelength and a converted frequency, the converted frequency being equal to the sum of the frequency illumination and pump frequency,
• E forming an image of at least a part of said at least one element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye or by a detection device (Det).

The following description presents several embodiments of the device of the invention: these examples do not limit the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics related to the embodiments considered.

The invention will be better understood and other characteristics, objects and advantages thereof will appear during the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which:

FIG. 1a already cited illustrates the starting situation of the principle of active imaging by temporal aliasing: the laser pulse is emitted;

FIG. 1b already cited illustrates the continuation of the principle of active imaging by temporal aliasing: the object reflects the laser pulse;

FIG. 1c already cited illustrates the continuation of the principle of active imaging by temporal aliasing: the imager is open for the time of detection;

FIG. 1d already cited illustrates the last step of the principle of active imaging by temporal aliasing: the imager is closed just after the passage of the pulse.

FIG. 2 recalls the principle of conversion by frequency summation.

Figure 3 illustrates the principle of frequency conversion with quasi-phase-matching (QAP) material.

FIG. 4 illustrates the concept of the invention for the Infrared to visible case.

Figure 5 illustrates the imaging system according to the invention.

FIG. 6a illustrates the preferred variant of the invention for which the final detector is the eye of an observer, in an embodiment of the telescope/identification type.

FIG. 6b illustrates the preferred variant of the invention for which the final detector is the eye of an observer, in a binocular type embodiment.

FIG. 7 illustrates a variant of the optical device of the imaging system which is configured to form an image in the visible spectrum which is superimposed on the converted image.

FIG. 8 describes a variant of the invention in which the illumination beam generated by the illumination device is pulsed and which comprises a synchronization device DS configured to act on the illumination device and on the pump so that a signal pulse and a pump pulse are superimposed in the frequency conversion device.

Figure 9 illustrates an embodiment of the invention wherein the timing device includes a delay device coupled to the illumination device and the pump, such that the pump outputs a pump pulse with a time delay relative to upon emission of the determined illumination pulse.

FIG. 10 illustrates an embodiment according to the invention in which the imaging system comprises a telemetry device.

FIG. 11a illustrates, for the case of the telescope, an embodiment according to the invention in which the system further comprises a laser cavity comprising a laser crystal adapted for the pump beam.

FIG. 11b illustrates, for the case of the binoculars, an embodiment according to the invention in which the system also comprises a laser cavity comprising a laser crystal adapted for the pump beam.

FIG. 12a illustrates the imaging system according to the invention compatible with operation according to two modes between which one switches with at least a removable mirror, in a standard mode.

FIG. 12b illustrates the imaging system according to the invention compatible with operation according to two modes between which one switches with at least a removable mirror, in a so-called inverted mode in which the pump and illumination beams are redirected so that the beam from the pump becomes the illumination beam and the beam from the illumination device becomes the pump beam.

The imaging system according to the invention is an active system which combines an IR illumination device and detection comprising a wavelength conversion part, so as to perform detection in the converted band, preferably the visible spectral band. .

The wavelength conversion of IR, for example from a wavelength around 1.5 μm (eye safety) to the visible is carried out according to the invention by a frequency conversion device configured to generate a beam converted Fc from a signal beam Fs (to be detected) and a pump beam Fp by a non-linear method of frequency summing.

The principle of conversion by frequency summation is recalled in figure 2. The pump beam Fp has a pump wavelength λp and a pump (optical) frequency fp equal to c/λp, the signal beam Fs has a signal wavelength λs and a signal (optical) frequency fs equal to c/λs, and the converted beam Fc has a converted wavelength λc and a converted (optical) frequency fc equal to c/λc. The frequency conversion device comprises a non-linear optical crystal 20 having a non-linear susceptibility of order 2 or χ2 allowing the generation, from the signal beam and the pump beam, of a converted beam having a length of diminished wave (ie an increased frequency hence the term up conversion) so that: fs+fp = fc. After the conversion to the crystal output, in addition to the converted beam, there remains a residual signal beam Fs(r) and the unused pump Fp. It is generally considered that the pump beam, which is much more powerful than the signal beam, is only very slightly reduced by the conversion.

For the conversion process to be effective, this mechanism requires phase matching between the high intensity pump beam, the signal beam to be detected and therefore to be converted, and the converted beam during their propagation in the nonlinear medium, which is expressed by the relationship below between the respective wave vectors of the 3 beams:

This condition can be satisfied with certain birefringent nonlinear crystals by playing on the crystallographic orientation and the polarization of the waves involved.

Another approach consists in using materials 30 with quasi-phase matching (QAP) whose sign of the nonlinear coefficient of order 2 χ ⁽²⁾ is periodically alternated (period Λ), in order to restore the correct phase relationship, as shown in Figure 3. The phase matching relationship becomes:

This last method is generally preferable because it makes it possible to best exploit the non-linear properties of the material and minimizes the pump power necessary to obtain a good conversion efficiency.

Among the usual QAP materials, we can distinguish ferroelectric materials such as PPLN (Periodically Poled Lithium Niobate) which cover the spectral range from the visible to about 4 µm, and semiconductor materials such as OP-GaAs (Orientation Patterned Gallium Arsenide) which make it possible to work further into the infrared, for example up to 16 µm.

For example, PPLN can be used to convert a signal at 1.5 µm using a pump laser emitting towards 1 µm or 2 µm. In this case, a signal converted around 0.60 µm or 0.86 µm is obtained, allowing the use of a silicon detector or a human eye (for 0.6 µm). Typically a 1.06 µm pump (YAG laser) allows conversion from 1.5 µm to 0.63 µm.

The conversion characteristics are dependent on the characteristics of the pump laser, so a dedicated pump laser allows selection of the detection characteristics. To obtain good conversion efficiencies, peak pump powers of the order of several kW are generally necessary. To simply obtain these peak powers, pulsed pump lasers are preferred.

IR to visible frequency conversion applied to an elementary (passive) imaging system has been demonstrated in the publication “Near infrared to visible upconversion imaging using a broadband pump laser” by Romain Demur et al, Optics Express Vol.26, No .10, 2018. The phase matching condition limits the number of spatial modes, or pixels, in the converted image. To increase it significantly, it is shown in the publication that an enlargement of the pump spectrum to a few nm makes it possible to obtain sufficiently resolved images for DRI (Detection, Identification, Recognition) applications. The conversion takes place over a field of view (“Field of View” or FOV) of about a hundred mrad.

The innovative concept of the invention, illustrated in figure 4 for the IR to visible case, is to integrate a frequency conversion into an active imaging system.

The imaging system 10 according to the invention is illustrated in FIG. 5. It comprises an illumination device DI configured to generate an illumination beam Fi comprising an illumination wavelength λi and an illumination frequency fi, and to illuminate a portion of space 1 with the illumination beam Fi. The illumination device comprises an illumination laser source Li. According to one option, the divergence of the source is adapted to illuminate the desired portion of space 1. According to another option, the device DI also comprises illumination optics making it possible, from a very weakly divergent illumination laser source, to illuminate the portion of space 1. The incident beam Fi on an element 2 present in the space 1 is reflected/backscattered by it then forming the backscattered beam Fd.

The imaging system 10 according to the invention also comprises an optical device DO configured to collect the backscattered beam Fd by at least one element 2 present in the portion of space 1. The fraction of the beam Fd collected by DO is called the signal beam fs.

- The imaging system 10 also comprises a source Sp, preferably a laser source, called pump, configured to generate a pump beam Fp of pump wavelength λp and pump optical frequency fp and a frequency conversion device CF configured to generate a converted beam Fc from the signal beam Fs and the pump beam Fp by a nonlinear method of frequency summing. The converted beam Fc has a converted wavelength λc and a converted optical frequency fc. The converted frequency fc is equal to the sum of the illumination frequency fi and the pump frequency fp: fc = fi + fp.

The optical device DO is further configured to form an image of at least part of the element 2, the image called converted image Ic being formed with the converted beam and intended to be detected by an eye OE of the observer or by a detection device Det. The optical information to be detected/imaged is initially carried by the signal beam Fs which is converted into a beam Fc of lower wavelength to form the image Ic. Preferably, the device CF comprises a nonlinear crystal 20 or 30 as described above. The frequency conversion device CF is arranged inside the optical imaging device DO.

According to a preferred variant, the illumination wavelength λi and the pump wavelength λp are chosen so that the converted wavelength λc is included in the visible spectral band. According to a preferred option, the visible converted image is detected by a human observer's eye.

In a preferred embodiment of this preferred variant, the illumination wavelength λi is included in the spectral band [1.4 – 3 µm] and the pump wavelength λp is included in the spectral band [0.75-1.4 µm]. An example of a wavelength triplet is:

λi = 1.5 µm (ocular safety, discretion, good atmospheric transmission); λp = 1.06 µm (YAG laser); which leads to λc = 0.63 µm in the red.

According to another embodiment, a visible detector is integrated into the system 10, typically a low-cost CMOS or CCD detector. The system can also be adapted to the eye of an observer and include a removable mirror placed after the conversion to direct the image Ic towards a visible detector/camera, in order to share the information.

The concept can also be transposed with a wavelength converted no longer in the visible but in the infrared, the system 10 then integrating an ad-hoc detector. It is also possible to perform illumination at 2 μm or in MWIR (3-5 μm) or LWIR (8-12 μm) band.

It can be seen that with the preferred variant of system 10, the infrared signal to be observed is converted to the visible, allowing the human eye to be used directly as a detector. We therefore allow a human to “see” the infrared in active imaging. The scene is seen directly to the eye without the need for an intermediate sensor. With System 10, the detection barrier is lifted by providing a solution that is both very sensitive and multifunctional (see below) which directly integrates the eye into the viewing function. The conversion takes place via a nonlinear χ2 crystal and a pump laser. The system according to the invention is portable and constitutes a portable active imager system which did not exist so far in the SWIR range because of the problem of detection.

In order to avoid dangerous light for the human eye, according to one embodiment, the pump beam Fp propagates in the frequency conversion device CF in a direction opposite to that of the signal beam Fs (Fp and Fs counter-propagating). According to another embodiment, a device is inserted to protect the eye against too high a luminous flux which allows low light powers to pass and which absorbs when the flux becomes too high, based on a 2-photon absorber or a saturable absorber reverse. According to yet another embodiment, the system 10 comprises a dichroic mirror at the output of CF to reflect the pump signal outside the optics after passing through CF.

Active imagers are very small field, typically sub-degree. The wavelength conversion solution using an optical crystal, whose angular acceptance is limited by the phase matching condition, is well suited to this application.

According to one embodiment of the preferred variant, the system according to the invention is of the sight/identification telescope type, as illustrated in FIG. 6a.

According to another embodiment of the preferred variant, the imaging system according to the invention is of the twin type as illustrated in FIG. 6b. It comprises an additional optical device DO' and an additional frequency conversion device CF', each optical device/frequency conversion device pair (DO, CF) and (DO', DF') forms a channel of the twin, and each image Ic, Ic' is respectively detected by an eye OE, OE' of the observer. Preferably, the pump is common to both channels. Preferably, the pump beam successively passes through the two devices CF, CF', so as to benefit, for each conversion, from the full power of the pump. According to one embodiment, an existing binocular is adapted by inserting the CF device. As explained above, there is currently no active imaging binocular due to the problem of the detector. There are only optical, light intensifier or passive IR binoculars. The binocular according to the invention integrates active imaging into a portable system for human vision.

According to a variant, the optical device DO of the imaging system according to the invention is further configured to form an image in the visible spectrum (Δλv band) called visible image Iv, which is superimposed on the converted image Ic, such that illustrated in figure 7. A large field passive visible channel is superimposed on the small field active imaging system (typically >1°). This allows the operator, for example, to easily locate the small field area of the active imaging through the large field visible passive imaging.

The anti-reflective treatments of the different optics are suitable for this configuration. It is the same for the components of the optical device before and after the crystal, to correct the aberrations, in particular chromatic, and to correctly superimpose the converted image Ic on the visible passive image Iv.

The nonlinear quasi-phase-matching crystals used, called flipped crystals (because they have a periodic inversion of the χ2) have a parallelepipedic matchstick shape (typically 1 mm x 1 mm, by 20 mm in length). These are bad optical systems, with a small aperture. To increase the aperture for passive imaging, in one embodiment, bulk crystals of the same composition as the flipped crystal but not including a sign reversal of the 2nd order nonlinear coefficient are placed around the flipped crystal to form a sandwich of crystals having a large opening.

The characteristics of the pump laser used to perform the conversion directly determine the properties of the detection. According to a preferred variant, the pump beam Fp is pulsed, which makes it easy to obtain the narrow temporal aliasing function necessary for the good depth resolution of these systems. Indeed as long as there is no pump power there is no converted image. The impulsive character of the pump acts as a shutter of the system: the pump pulses naturally achieve a narrow temporal aliasing of the detection, very useful for the applications targeted in active imaging (decamouflage, vision through clouds, etc.).

Preferably, the illumination beam generated by the illumination device is pulsed. The system 10 also comprises a synchronization device DS configured to act on the illumination device DI and on the pump Sp so that a signal pulse Is and a pump pulse Ip are superimposed in the frequency conversion device CF, such as as shown in figure 8.

Preferably, the synchronization device DS comprises a delay device 12 coupled to the illumination device and to the pump, so that the pump emits a pump pulse with a time delay Del (also called delay) with respect to the emission of the determined illumination pulse, as illustrated in FIG. 9. A determined time delay value makes it possible to superimpose Ip with a pulse Is coming from an associated slice of space TE, included in said portion of space 1. image therefore only this portion TE according to the principle of temporal aliasing. Thanks to the synchronization device DS, the system correctly superimposes in time the retroreflected signal pulse Is and the pump pulse Ip. Preferably the delay device 12 is configured so that the time delay between the two pulses is variable, in other words adjustable. By varying the time delay, we can image a continuum or a plurality of slices of space.

According to one embodiment, the time delay is manually adjustable. For example, a wheel located on the case of a binocular allows you to manually adjust the delay.

According to another embodiment, the time delay follows a predetermined law of variation to automatically scan a set of depths in the observed space.

According to another embodiment illustrated in Figure 10, the imaging system further comprises a telemetry device TM configured to determine a distance D from an element 2 (in association with the illumination laser source). For this, a typically removable mirror 5, located before the CF device, makes it possible to deviate the IR beam to direct it towards a point detector typically made of InGaAs used for telemetry. The time delay is then automatically adjusted so as to image a slice of space located at distance D from the imaging system. We thus pass from the telemetry function to the active imager function.

The telemetry function, associated for example with a binocular, thus makes it possible to determine the distance from the scene to be observed and therefore the delay between the two pulses which is then automatically adjusted. It is also possible to use a wheel as described above to manually adjust the delay to fine-tune the setting or modify the observed depth.

Thus the different time delay setting modes can be combined or successively used.

According to an embodiment illustrated in Figure 10, but also compatible with the system of Figure 9, the device DS comprises an external triggering device 11 of the illumination pulse, coupled to the illumination device DI, the device 12 being then coupled to this device 11.

According to a variant, the illumination beam and the pump beam are continuous. To have sufficient pump power, according to an embodiment illustrated in FIGS. 11a and 11b, the system 10 further comprises a laser cavity 20 comprising an LC laser crystal (typically an Nd:YAG crystal) suitable for the pump beam and in which the frequency conversion device CF is arranged. Figure 11a illustrates the case of the telescope and Figure 11b the case of the binoculars. In the latter case, the laser cavity 20 and the laser crystal LC are shared by the two channels, each channel comprising an associated frequency conversion device, CF, CF′ both arranged in the laser cavity.

According to an embodiment illustrated in Figures 12a and 12b, the imaging system 10 further comprising at least one removable mirror MA disposed on an optical path of the pump beam and of the illumination beam, and configured to redirect said pump and d beams. illumination so that the beam from the pump becomes the illumination beam and the beam from the illumination device DI becomes the pump beam. The initial pump beam transformed into an illumination beam is further configured to perform a target designation function. FIG. 12a recalls the so-called initial operation of the imaging system 10 according to the invention as described above. Figure 12b illustrates the reverse operation achieved by positioning the mirror MA at the crossroads of the Fp and Fi beams (it is also possible to use two removable mirrors instead of one, a mirror positioned on the path of Fi and a mirror positioned on the path of Fp). The MA mirror sends the pump beam towards the observed area which is configured not to designate a target. The use of a laser at λp = 1.06 µm is well suited to this function. The detection is carried out with an Fs beam at λp, which pumped at λi (for example λi=1.5 µm) is converted into λc. The switch between the initial mode (standard) and the inverted mode is made by positioning or not the removable mirror(s) on the path of the illumination and pump beams.

• A générer un faisceau d’illumination (Fi) comprenant une longueur d’onde d’illumination (λi) et une fréquence d’illumination (fi), et illuminer une portion d’espace (1) avec ledit faisceau d’illumination,
• B collecter un faisceau rétrodiffusé (Fd) par au moins un élément (2) présent dans la portion d’espace, afin de former un faisceau signal (Fs),
• C générer un faisceau pompe (Fp) comprenant une longueur d’onde de pompe (λp) et une fréquence pompe (fp),
• D générer un faisceau converti (Fc) à partir du faisceau signal (Fs) et du faisceau pompe (Fp) par un procédé non linéaire de sommation de fréquence, le faisceau converti comprenant une longueur d’onde convertie (λc) et une fréquence convertie (fc), la fréquence convertie (fc) étant égale à la somme de la fréquence d’illumination (fi) et de la fréquence pompe (fp),
• E former une image d’au moins une partie dudit au moins un élément, ladite image, dénommée image convertie, étant formée avec le faisceau converti et destinée à être détectée par un œil (OE) d’observateur ou par un dispositif de détection (Det).

According to another aspect, the invention relates to an active imaging method comprising the steps consisting in:

• To generate an illumination beam (Fi) comprising an illumination wavelength (λi) and an illumination frequency (fi), and to illuminate a portion of space (1) with said illumination beam,
• B collecting a beam backscattered (Fd) by at least one element (2) present in the portion of space, in order to form a signal beam (Fs),
• C generate a pump beam (Fp) comprising a pump wavelength (λp) and a pump frequency (fp),
• D generating a converted beam (Fc) from the signal beam (Fs) and the pump beam (Fp) by a non-linear method of frequency summing, the converted beam comprising a converted wavelength (λc) and a converted frequency (fc), the converted frequency (fc) being equal to the sum of the illumination frequency (fi) and the pump frequency (fp),
• E forming an image of at least a part of said at least one element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye (OE) or by a detection device ( Det).

Claims (17)

Imaging system (10) comprising:

• an illumination device (DI) configured to generate an illumination beam (Fi) comprising an illumination wavelength (λi) and an illumination frequency (fi), and to illuminate a portion of space ( 1) with said illumination beam,
• a source (Sp) called a pump configured to generate a pump beam (Fp) comprising a pump wavelength (λp) and a pump frequency (fp),
• an optical device (DO) configured to collect a beam backscattered (Fd) by at least one element (2) present in the portion of space, to form a signal beam (Fs),
• a frequency conversion device (CF) configured to generate a converted beam (Fc) from the signal beam (Fs) and the pump beam (Fp) by a nonlinear method of frequency summing, the converted beam comprising a length d converted wave (λc) and a converted frequency (fc), the converted frequency (fc) being equal to the sum of the illumination frequency (fi) and the pump frequency (fp),

the optical device being further configured to form an image of at least part of said element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye (OE) or by a detection device (Det). Imaging system according to Claim 1, in which the pump beam propagates in the frequency conversion device in a direction opposite to that of the signal beam. Imaging system in which the pump beam and the illumination beam are pulsed, and further comprising a synchronization device (DS) configured to act on the illumination device (DI) and the pump (Sp) so that a signal pulse (Is) and a pump pulse (Ip) are superimposed in the frequency conversion device (CF). An imaging system according to claim 3 wherein the synchronization device (DS) comprises a delay device (12) coupled to the illumination device and to the pump such that the pump emits a pump pulse with a time delay ( Del) with respect to the emission of the determined illumination pulse, a time delay value making it possible to image an associated slice of space (TE) comprised in said portion of space. An imaging system according to claim 4 wherein the delay device is configured such that the time delay is variable. An imaging system according to claim 5 wherein the time delay is manually adjustable. Imaging system according to Claim 5, in which the time delay follows a predetermined law of variation. Imaging system according to claim 5 further comprising a telemetry device (TM) configured to determine a distance (D) of said element, said time delay then being adjusted so as to image a slice of space located at a distance D from the imaging system. An imaging system according to claim 1 in which the illumination beam and the pump beam are continuous, and further comprising a laser cavity (20), adapted for the pump beam and comprising a laser crystal (LC) and in which the frequency conversion device is arranged. [Claim 10] Imaging system according to one of the preceding claims, in which the optical system is further configured to form an image in the visible spectrum, referred to as the visible image, which is superimposed on the converted image. Imaging system according to claim 9 wherein the frequency conversion device (CF) comprises:

• a crystal called a flipped crystal to carry out the frequency conversion, the crystal comprising a material whose sign of the nonlinear coefficient of order 2 is periodically inverted,
• massive crystals of the same material but not comprising any inversion of the sign of the nonlinear coefficient of order 2, arranged around the inverted crystal.

Imaging system according to one of the preceding claims further comprising at least one removable mirror (MA) disposed on an optical path of the pump beam and of the illumination beam, and configured to redirect said pump and illumination beams so that the beam from the pump becomes the illumination beam and the beam from the illumination device becomes the pump beam, the initial pump beam transformed into an illumination beam being configured to perform a target designation function. Imaging system according to one of the preceding claims, in which the illumination wavelength (λi) and the pump wavelength (λp) are chosen so that the converted wavelength is included in the visible spectral band. Imaging system according to Claim 13, in which the illumination wavelength (λi) is included in the spectral band [1.4–3 µm] and the pump wavelength (λp) is included in the spectral band [0.75-1.4µm]. Imaging system according to one of Claims 13 or 14 of the telescope type in which the image is detected by an eye of the observer. Imaging system according to one of Claims 13 or 14 of the twin type comprising an additional optical device (DO') and a frequency conversion device (CF'), each optical device/frequency conversion device pair forming a of the binocular, each image being detected by an eye (OE, OE') of the observer, the pump being common to the two channels. Active imaging method comprising the steps of:

• To generate an illumination beam (Fi) comprising an illumination wavelength (λi) and an illumination frequency (fi), and to illuminate a portion of space (1) with said illumination beam,
• B collecting a beam backscattered (Fd) by at least one element (2) present in the portion of space, in order to form a signal beam (Fs),
• C generate a pump beam (Fp) comprising a pump wavelength (λp) and a pump frequency (fp),
• D generating a converted beam (Fc) from the signal beam (Fs) and the pump beam (Fp) by a nonlinear method of frequency summing, the converted beam comprising a converted wavelength (λc) and a converted frequency (fc), the converted frequency (fc) being equal to the sum of the illumination frequency (fi) and the pump frequency (fp),
• E forming an image of at least a part of said at least one element, said image, called converted image, being formed with the converted beam and intended to be detected by an observer's eye (OE) or by a detection device ( Det).