OPTICAL TARGET DETECTION SYSTEM USING FREQUENCY SUM SIGNAL CONVERSION

Abstract

OPTICAL TARGET DETECTION SYSTEM USING FREQUENCY SUM SIGNAL CONVERSION S A system (10) is proposed configured to illuminate a target (20) with an illumination pulse (Iill), receive a reflected pulse (Ir) resulting from the reflection of the illumination pulse on the target, comprising: - a single laser source (110) configured to generate an initial signal (Sp-110) of pump frequency (), - a first unit (120) adapted to be traversed by the initial signal and produce a resulting signal (Sp-120), and - a second unit (130) adapted to be traversed by the resulting signal and to produce by frequency conversion the illumination pulse, the illumination and reflected pulses being of probe frequency () distinct from the pump frequency.The first unit is configured to generate a converted pulse (Ic) of converted frequency ( ), via a nonlinear pump and probe frequency summation operation, the system further comprising a detector (140) of the converted pulse. Figure for the abstract: [Fig.1].

Description

Title of the invention: OPTICAL TARGET DETECTION SYSTEM USING CONVERSION SIGNAL BY FREQUENCY SUM technical field

[0001] The present invention relates generally to the field of target detection, and in particular to an optical system for detecting a target using optical signal conversion by frequency summation, as well as an optical method for detecting a target.

[0002] Optical target detection systems are commonly used in scene (or area) observation systems under active illumination for telemetry, LIDAR, active imaging or spectroscopy applications.

[0003] To probe and detect a target, existing optical systems conventionally exploit the optical properties of the atmosphere, which presents transparent optical windows for optical signals in the general infrared (IR) range, particularly the near-infrared (1.5 to 2 µm) and infrared (2 to 12 µm). These wavelength ranges are compatible with eye safety requirements.

[0004] Moreover, in the field of optics, many advances have been made to provide a wide variety of IR laser sources in terms of power and accessible wavelengths, which makes it possible to use a wide range of illumination techniques to actively probe an area that may be very far from the optical system.

[0005] However, the detection and analysis of reflected optical signals, known as 'probe optical signals', on a target remain limited due to the sensitivity of the receiving channel of optical systems.

[0006] In particular, in existing optical systems with IR probe optical signals, a known method for detecting reflected optical signals involves using IR detectors and / or cameras with very limited performance in terms of sensitivity and spectral bandwidth, or in terms of implementation constraints. For example, an implementation constraint might be significant cooling of the detector, which could be, for example, a detector based on HgCdTe (or mercury-cadmium telluride) requiring cooling to 77 K.

[0007] In existing optical systems with IR probe optical signals, another known detection method consists of converting reflected signals to frequencies in order to use detectors and / or cameras with lower spectral ranges (i.e., visible and near-infrared) to improve detection sensitivity. Such Frequency conversion of reflected signals is commonly performed using a crystal with a second-order nonlinear susceptibility and a pump laser to perform frequency summation.

[0008] However, some known solutions based on this frequency conversion have the drawback of requiring several specific optical sources integrated into the system to generate, on the one hand, the probe signal to be propagated to the target and, on the other hand, the pump signal for the conversion. Separating the sources makes it possible to obtain sufficient power for effective illumination of the source and to reach a performance threshold essential for efficient conversion. Such solutions have been described, for example, in the article “Room-temperature mid-infrared single-photon spectral imaging” by JS Dam et al., Nature Photonics, 2012, 6, pp. 788-793, or in the article “Long-wavelength-pumped upconversion single-photon detector at 1550 nm: performance and noise analysis” by JS Pelc, Optics Express, 2011, 19, pp. 21445-21456.However, solutions using multiple optical sources have high manufacturing costs and result in a large system volume and weight.

[0009] Other known solutions based on frequency conversion use a single laser source to generate the probe signal and the pump signal enabling conversion upon reception of the reflected signal. For example, a first solution based on this approach was described in the article “Upconversion imaging using an all-fiber supercontinuum source” by L. Huot, Optics Letters, 2016, 41, pp. 2466-2469. However, in this first solution, the same initial pulse from a single laser source is split into two distinct pulses (or “subpulses”) sharing the energy of the initial pulse. The first subpulse is used as the probe signal, and a delay is applied to the second subpulse, which is then used in a frequency summation operation upon reception of the reflected signal. Such a solution is therefore limited, particularly in terms of object (i.e., target) detection at very short distances.This is particularly the case if the delay applied to postpone the second sub-impulse aims to be realistic.

[0010] Furthermore, a second solution based on the conversion approach using a single laser source was described in patent EP3679395 Bl. However, according to this second solution, the single laser source emits two pulses per emission cycle, a first pulse being used as the probe signal and a second pulse being used in the frequency summation signal conversion. For most distances to the objects to be detected (i.e., up to about 10 km), the energy stored in the single laser source does not have time to be completely renewed between the two pulses. This results in a sharing of the extractable energy. between the two pulses of the same emission cycle, which reduces both the power of the laser beam of the probe signal and the conversion efficiency at reception.

[0011] Thus, the solutions proposed in the prior art remain limited in terms of target detection distance, system complexity, energy consumption, and / or conversion efficiency because illuminating a target to be detected requires short probe pulses with high peak power, while the peak pump power required to obtain good conversion efficiency is also very high (i.e., several hundred watts). Known solutions using a single optical source are therefore not sufficiently efficient.

[0012] There is therefore a need for an optimized optical system for detecting a target, as well as an associated optical process. Summary of the invention

[0013] To this end, an optical system is proposed that is configured to illuminate a target with an illumination pulse, the optical system being capable of receiving a reflected pulse corresponding to a reflection of the illumination pulse on the target. The optical system comprises:

[0014] - a single laser source configured to generate an initial pump signal formed by a continuous or quasi-continuous pulse train comprising at least a first pump pulse followed by a second pump pulse, the first and second pump pulses having the same frequency, called the pump signal frequency,

[0015] - a first conversion unit adapted to be traversed by the pump signal initial and produce a resulting pump signal comprising the first pump pulse, and

[0016] - a second conversion unit adapted to allow the signal to pass through pump resulting and produce the illumination pulse by applying a frequency conversion from the first pump pulse, the illumination pulse and the reflected pulse having the same frequency, called the probe signal frequency, distinct from the pump signal frequency.

[0017] The first conversion unit is further configured to generate a converted pulse by applying a nonlinear frequency summation operation to the second pump pulse and the reflected pulse, the converted pulse having a frequency, referred to as the converted signal frequency, equal to the sum of the pump signal frequency and the probe signal frequency. The optical system further includes a detection unit capable of detecting the converted pulse.

[0018] In embodiments, the first conversion unit may comprise a nonlinear optical crystal having a nonlinear susceptibility of order 2.

[0019] Advantageously, the second conversion unit may include a conversion element configured to apply a second-order nonlinear frequency conversion operation, the conversion element being inserted into a laser cavity Optical Parametric Oscillator.

[0020] The second conversion unit may be a Raman laser comprising a conversion element configured to apply a 3rd order nonlinear frequency conversion operation, the conversion element being inserted in a resonant cavity.

[0021] Alternatively, the second conversion unit may be a gain-switching laser comprising an optical amplifier-type conversion element inserted in a resonant cavity.

[0022] The single laser source can be a laser selected from an actively triggered, gain-solid-state laser, and a MOPA-type fiber optic laser.

[0023] Advantageously, the single laser source can be configured to generate the initial pump signal with a repetition rate, the second pump pulse being temporally separated from the first pump pulse by a separation delay defined from the repetition rate, the target being at a distance from the optical system. The optical system can be configured to determine the distance between the target and the optical system as a function of the repetition rate of the single laser source.

[0024] The optical system can be configured to vary the repetition rate according to a narrow temporal aliasing to determine the distance between the target and the optical system.

[0025] In embodiments, the optical system may include a servo unit configured to apply a servo control of the repetition rate of the laser source, if the distance to the target varies.

[0026] The servo unit can be configured to detect an optical signal having a probe signal frequency, the servo unit comprising at least one polarizing mirror adapted to separate a reflected signal comprising the reflected pulse received by the optical system, into two signals comprising:

[0027] - a first signal defined according to a first polarization, the first signal being intended to be propagated to the first conversion unit, and

[0028] - a second signal defined according to a second polarization, the second signal being intended to be detected by the servo unit.

[0029] According to certain embodiments, the optical system can further be configured to illuminate the target with a complementary pulse having a frequency, called the idler signal frequency, resulting from the frequency conversion applied to form the illumination pulse, the idler signal frequency being distinct from the probe signal frequency and the pump signal frequency, the optical system being configured to receive a reflected complementary pulse in response to a reflection of the complementary pulse off the target. The servo unit can be configured to detect the reflected complementary pulse.

[0030] Another object of the invention is an optical method, implemented in an optical system, comprising the steps of:

[0031] - generate, using a single laser source, an initial pump signal having a frequency, called pump signal frequency, the initial pump signal comprising a first pump pulse, then a second pump pulse,

[0032] - passing the initial pump signal through a first conversion unit, to produce a resulting pump signal comprising at least the first pulse,

[0033] - passing the resulting pump signal through a second conversion unit to produce at least one illumination pulse obtained from the first pulse by applying a frequency conversion,

[0034] - illuminate a target with the illumination pulse, and

[0035] - receive a reflected impulse in response to a reflection of the impulse Illumination on the target, the illumination pulse and the reflected pulse having the same frequency, called the probe signal frequency, the probe signal frequency being distinct from the pump signal frequency,

[0036] The process further comprises the steps of:

[0037] - to pass the reflected impulse through the first conversion unit, for to produce a converted pulse from the reflected pulse and the second pump pulse by applying a nonlinear frequency summation operation, the converted pulse having a frequency, called the converted frequency, equal to the sum of the probe signal frequency and the pump signal frequency, and

[0038] - detect the converted pulse.

[0039] Embodiments of the invention thus provide an optimized method and optical system for detecting a target using optical signal conversion by frequency summation.

[0040] They thus enable the detection of objects at short and long ranges, using a single laser source to generate the probe signal emission and to convert the reflected signal received from the reflection of the probe signal off the target into a frequency. Furthermore, the simplified overall architecture of the system optimizes the use of the energy produced by the single laser source. This results in optimized costs, energy consumption, manufacturing, and assembly of the optical system.

[0041] Embodiments of the invention allow in particular optimized use of the optical system for telemetry and / or active imaging applications. DESCRIPTION OF THE FIGURES

[0042] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0043] [Fig.1] [Fig.1] is a diagram representing an optical system, according to the embodiments.

[0044] [Fig.2] [Fig.2] is a time diagram of the position of optical pulses in the optical system and on the target, according to embodiments.

[0045] [Fig.3] [Fig.3] shows diagrams 3(a) and 3(b) representing an element of the converter operating by frequency summation, according to embodiments.

[0046] [Fig.4] [Fig.4] is a diagram representing an optical system, according to embodiments.

[0047] [Fig.5] [Fig.5] is a diagram representing an optical system comprising a signal frequency detection servo unit, according to the embodiments.

[0048] [Fig.6] [Fig.6] is a diagram representing an optical system comprising a frequency-sensing idler servo unit, according to embodiments.

[0049] [Fig.7] [Fig.7] is a diagram representing an optical system with an external telemetry unit, according to embodiments.

[0050] [Fig.8] [Fig.8] is a flowchart representing the optical process, according to embodiments.

[0051] [Fig.9] [Fig.9] is a flowchart representing sub-steps of an optical process, according to embodiments of the invention.

[0052] [Fig. 10] [Fig. 10] shows the flowcharts 10(a) and 10(b) representing sub-steps of servo control of an optical process, according to the embodiments of the invention.

[0053] Identical reference numerals are used in the figures to designate identical or analogous elements. For clarity, the elements shown are not to scale. DETAILED DESCRIPTION OF THE INVENTION

[0054] Fig. 1 schematically represents an optical system 10 comprising a single pulsed laser source 110, a first conversion unit 120, a second conversion unit 130 and a detection unit 140, according to embodiments of the invention.

[0055] The optical system 10 is configured to illuminate a target 20 with a probe optical signal Ss_i0, generated from a single pulsed laser source 110. The target 20 is positioned at a distance d from the optical system 10. The optical system 10 is further configured to recover a reflected optical signal Sr_io resulting from a reflection or backscattering of the probe optical signal Ss_i0 on the target 20, as shown in [Fig. 1].

[0056] The optical system 10 can be implemented, for example and without limitation, in the form of an optronic rangefinding system, a LIDAR system, and / or an active imaging system. Furthermore, the optical system 10 can be implemented on an optical platform comprising complementary imaging systems, such as, for example and without limitation, a visible imaging system, a passive near-infrared imaging system, and / or a thermal imaging system.

[0057] As used here, an "optical signal" refers to a wave (or wave train) resulting from one or more pulses of light originating from a pulsed optical source, such as a laser beam. The electromagnetic wave (or beam) carrying the optical signal is characterized, in particular, by a given wavelength A, relative to a signal frequency ω specific to the wave. The electromagnetic wave can also be characterized by a given polarization, a given phase associated with a given wave vector Σ. A signal comprising a plurality of pulses can also be characterized by a repetition rate (or frequency) p of the different pulses of the optical wave train.

[0058] Unless otherwise indicated, the term "signal" used herein shall refer to an optical signal.

[0059] The single pulsed laser source 110 (also more simply called the 'laser source') is configured to generate an initial pump signal Sp 4 i 0 at a frequency, called the pump signal frequency (or 'pump frequency') denoted ^p. The probe signal Ss_i0 is obtained from the initial pump signal Sp ii 0.

[0060] The probe signal Ss_i0 comprises at least one illumination pulse Im. The reflected signal Sr.i0 comprises at least one reflected pulse Ir resulting from the reflection of the illumination pulse U on the target 20. The illumination pulse U and the reflected pulse Ir have the same frequency, called the probe signal frequency (or 'probe frequency') and denoted ^s. The probe signal frequency is distinct from the pump signal frequency ^p.

[0061] The wave train of the initial pump signal Sp ii 0 is a continuous or quasi-continuous pulse train, comprising at least a first pump pulse Ip (n) followed by a second pump pulse Ip (n+1). The second pump pulse Ip (n+1) is temporally separated from the first pump pulse Ip (n) by a time (or separation delay, duration) due defined from the repetition rate p of the laser source 110.

[0062] The optical system 10 is configured to pass the initial pump signal Sp iio into the first conversion unit 120 to form a resulting pump signal Sp -i 2 o (the initial pump signal Sp ii 0 thus passes through the first conversion unit 120 which provides at output a resulting pump signal Sp i 2 o)- The optical system 10 is configured to then pass the resulting pump signal Sp 12 o into the second conversion unit 130 (also called 'transmission unit') to generate the probe signal Ss_i0 (the resulting pump signal Sp 12 o thus passes through the second conversion unit 130 which produces at output the probe signal Ss.i0).

[0063] The resulting pump signal Sp i20 from the first conversion unit 120 includes at least the first pulse Ip (n), which is unaltered and unattenuated after passing through the first conversion unit 120. The first pulse Ip (n) thus retains all its pulse energy between the generation of the pulse at the output of the laser source 110 and its passage through the first conversion unit 120. The illumination pulse Im of the probe signal Ss.i0, from the second conversion unit 130, can be obtained from the first pulse Ip (n) by a nonlinear frequency conversion process or by pumping a laser by gain switching.

[0064] The optical system 10 is further configured to pass the reflected signal Sr_i0 (i.e. the reflected pulse L) into the first conversion unit 120 to generate a converted signal Sc 12 0 to be detected by the detection unit 140 (the reflected signal Sr.i0 thus passes through the first conversion unit 120 to produce at the output the converted signal Sc 4 2 0). The converted signal Sc_i 2 0 from the first conversion unit 120 comprises at least one converted pulse L which can be obtained by a nonlinear frequency summation process, from the reflected pulse Ir and the second pump pulse Ip (n+i) of the initial pump signal Sp ii 0. The converted pulse Ic has a signal frequency, hereafter called the 'converted signal frequency' (or 'converted frequency') and designated by the notation ^c. The converted signal frequency is distinct from the probe signal frequency and the pump signal frequency ^p.In particular, the converted signal frequency is equal to the sum of the probe signal frequency and the pump signal frequency ^p. .

[0065] To facilitate understanding of the embodiments of the invention, Figure 2 represents a simplified time chronogram of the position of the different pulses (Ip (n), Ip (n+1), Im, Ir and Ic) on an optical path O corresponding to the optical path of the different signals in the optical system 10, as well as towards the target 20 and from the target 20.

[0066] As shown in [Fig.2], at a time tb the first pump pulse Ip (n) generated by the laser source 110 passes into the first conversion unit 120. At a time t2 (close to ti), the first pump pulse Ip (n) (not altered by passing through the first conversion unit 120) passes into the second conversion unit 130 to generate the illumination pulse Im which illuminates the target 20 at a time t3. At time t4, the reflected pulse Ir (resulting from the reflection of the illumination pulse Im on the target 20) is received by the optical system 10 and passes into the first conversion unit 120. At the same time t4, the second pump pulse Ip (n+1) generated by the laser source 110 passes into the first conversion unit 120. The converted pulse Ic is then obtained from the spatial and temporal coincidence of the reflected pulse Ir and the second pump pulse Ip (n+i) in the first conversion unit 120.At a time t5 (close to L), the converted pulse L is detected by the detection unit 140.

[0067] As used here, the expression "spatial and temporal coincidence" refers to a spatial and temporal superposition (or overlap) of the reflected pulse Ir and the second pump pulse Ip (n+1) in the conversion unit 120, i.e. the pulses overlap spatially and simultaneously.

[0068] The repetition frequency p of the laser source 110, relative to the separation time At between the first pump pulse Ip(n) and the second pump pulse Ip(n+1) (and therefore between times ti and t4 on the timing diagram in Figure 2), is adapted so that the second pump pulse Ip(n+1) and the reflected pulse Ir pass through the first conversion unit 120 substantially simultaneously (i.e., substantially at the same time). Since the various distances between units in the optical system 10 are significantly smaller than the distance d between the target 20 and the optical system 10, the separation time At can be defined, in a simplified manner, as a function of this distance d, according to the following equation (01):

[0069] At = (2xd) / c(0D

[0070] In equation (01), c corresponds to the speed of light.

[0071] The time between two converted pulses L, formed at the output of the first conversion unit 120, can be the separation time between the pulses At.

[0072] For example, taking the value of c to be 3.108 m / s, if the target 20 is located at a distance d from the optical system 10 equal to 10 km, the separation time At between the two pulses Ip(n) and Ip(n+i) (and therefore the detection time between two converted pulses Ic) can be equal to 667 T25, for a repetition rate λ of the laser source 110 of 1.5 kHz. Similarly, if the target 20 is located at a distance d equal to 1 km, for example, the separation time At can be equal to approximately 6.7 L1S, for a repetition rate p of 150 kHz.

[0073] Advantageously, the repetition rate y of the laser source 110 can be adjusted and / or set according to the observation distance d of the target by the optical system 10. The repetition rate p of the laser source 110 can, for example, be continuously variable or incrementally variable. Alternatively, the repetition rate y of the laser source 110 can be fixed according to a measurement of the distance d.

[0074] The arrangement of the different units, and in particular the conversion units, forming a very specific optical path O in the optical system 10, allows the use of laser pulses from the same wave train to generate the illumination pulse Im and to convert the reflected pulse Ir by frequency summation. Such an optical path û allows for ideal optimization of the use of the available energy of each optical pulse at the output of the laser source 110. Indeed, the initial propagation of the pump pulses (Ip(n), Ip(n+i)) of the wave train in the first conversion unit 120 (before propagation in the second conversion unit 130) does not alter these pulses and thus allows all the available energy of each of them to be used to generate the illumination pulses Im.It should also be noted that a pump pulse is said to be "unaltered and unattenuated," which in reality means that a pump pulse is "virtually unattenuated or virtually unmodified" during the conversion in the first conversion unit 120 of the reflected pulse Ir into the converted pulse Ic, because the return signal (comprising reflected pulses Ir) is very weak compared to the pump signal. However, the frequency summation process cannot consume more photons from the pump pulses than 'signal photons' in the reflected pulses. In the presence of a spatial and temporal coincidence of the reflected pulse Ir and the second pump pulse Ip (n+1) in the first conversion unit 120, forming a converted pulse Ic, the detection unit 140 can detect a converted signal Sc_i 2 0.

[0075] In some embodiments, the laser source 110 can be a high-energy pulse pump laser. The laser source 110 can be, for example and without limitation, an actively triggered solid-state laser with gain, or a MOPA (Master Oscillator Power Amplifier) ​​fiber optic laser, corresponding to a fiber optic laser injected into a fiber amplifier.

[0076] According to certain embodiments, the first conversion unit 120, which performs the conversion of the reflected signal Sr_i0 into a converted signal Sc_i2o, detectable by the detection unit 140, may include a conversion element 122 adapted to convert a signal of probe signal frequency into a signal of converted signal frequency ^c. The converted signal frequency then corresponds to a higher frequency (and therefore at a shorter wavelength). In particular, the conversion element 122 of the first conversion unit 120 can be adapted to implement a nonlinear frequency summation process from a pump signal frequency ^p, according to the following equation (02):

[0077] û7s+wp= ûL(02)

[0078] Figure 3 shows diagrams 3(a) and 3(b) representing examples of implementation of the conversion element 122 capable of performing signal conversion by frequency summation or SFG (acronym for the English expression 'Sum Frequency Generation') according to embodiments of the invention. The conversion element 122 of the first conversion unit 120 can be, for example, a nonlinear optical crystal having a second-order nonlinear susceptibility.

[0079] The conversion method implemented by the conversion element 122 of the first conversion unit 120 uses a spatial and temporal superposition between a high-intensity pump beam (i.e., the second pump pulse Ip (n+i) of pump signal frequency ^P), and a signal beam to be detected and therefore converted (i.e., the reflected pulse Ir of probe signal frequency ws). The conversion method implements a phase match between the high-intensity pump beam, the signal beam to be converted, and the converted beam (i.e., the converted pulse Ic of converted signal frequency wc, resulting from the propagation of the pulses Ip (n+i) and Ir propagating in the nonlinear medium).

[0080] Phase matching is a vector relationship between the wave vectors of the pump beam, the signal beam and the converted beam.

[0081] In some embodiments, the phase agreement between the wave vector of the pump beam, signal beam wave vector and wave vector of the The converted beam can be defined by a first vector relation (corresponding to a first phase matching condition to be respected), according to the following equation (03):

[0082] Tk=Q = ^c -Ts -Tp <03)

[0083]

[0084] The phase-matching condition according to equation (03) can be satisfied by using certain birefringent nonlinear crystals and adjusting the crystallographic orientation and polarization of the waves involved. In particular, the diagram in [Fig. 3](a) schematically represents a summation of frequencies in a nonlinear crystal whose crystal orientation and wave polarization are chosen to satisfy the phase-matching condition of equation (03). Alternatively, the phase alignment between the wave vector of the pump beam, the wave vector of the signal beam, and the wave vector of the converted beam can be defined by a second vector relation (corresponding to a second phase agreement condition to be met), according to the following equation (04): [°° 85 i Âi=o=xr s -ï;-(^)2< 04 '

[0086] The second phase-matching condition according to equation (04) can be satisfied by using quasi-phase-matched (or QAP) materials whose orientation is periodically alternated (A) in order to restore the correct phase relationship. In particular, the diagram in Figure 3(b) schematically represents a summation of frequencies in a nonlinear quasi-phase-matched crystal whose orientation periodicity A is chosen to compensate for the phase mismatch. Such a method makes optimal use of the nonlinear properties of the material and minimizes the pump power required to obtain good conversion efficiency.

[0087] The conversion element 122 of the first conversion unit 120 may, for example, be a PPLN type crystal (acronym for 'Periodically Poled Lithium Niobate') enabling the conversion of a signal beam whose spectral range covers visible wavelengths and IR wavelengths up to approximately 4 µm. The conversion element 122 may also be made of a semiconductor material such as OP-GaAs (acronym for 'Orientation-Patterned Gallium Arsenide'), enabling the conversion of a signal beam whose spectral range covers IR wavelengths up to approximately 16 Z²™, or OP-GaP (acronym for 'Orientation-Patterned Gallium Phosphide').

[0088] For example, and without limitation, a PPLN-type crystal can be used to convert a probe signal beam with a frequency corresponding to a wavelength of 2 μm using a pump beam with a frequency corresponding to a wavelength of 1 or 1.5 μm, to obtain a converted beam with a frequency corresponding to a wavelength of 600 or 860 nm. Furthermore, an OP-GaAs-type crystal can be used to convert a probe signal beam with a frequency corresponding to a wavelength of 8 to 12 μm using a pump beam with a frequency corresponding to a wavelength of 1.9 μm, to obtain a converted beam with a frequency corresponding to a wavelength of 1.53 or 1.64 μm.Similarly, an OP-GaP type crystal can be used to convert a probe signal beam frequency corresponding to a wavelength of 3 to 1011111, using a pump beam frequency ^p corresponding to a wavelength of 1 to obtain a beam. converted signal frequency wc corresponding to a wavelength in the near-infrared of 0.75 to 0.91

[0089] In embodiments, the optical system 10 (for example the first conversion unit 120) may include one or more optical elements, such as a dichroic mirror M21 (reflecting at the wavelength of the pump signal frequency ^p for example) schematically illustrated in Figures 4, 5 and 6. Such optical elements may be configured to inject the pump beam from the laser source 110 into the conversion element 122.

[0090] According to some embodiments, the second conversion unit 130 may include a conversion element 132 configured to convert a high-intensity pump beam (i.e., the resulting pump signal Sp i2o comprising the first pump pulse Ip (n) of pump signal frequency ^p) into a probe signal Ss_i0 (i.e., comprising the illumination pulse Im of probe signal frequency ws) suitable for illuminating the target 20.

[0091] In a first embodiment, the conversion element 132 can be adapted to apply a second-order nonlinear frequency conversion operation and can be inserted into an OPO laser cavity (acronym for 'Optical Parametric Oscillator'). Such a conversion induces the formation of a complementary signal (also called 'idleP' in French and English) comprising a complementary pulse h of signal frequency, called the idler (or intermediate) signal frequency (or 'idler frequency'). The idler signal frequency is related to the frequencies and by the following equation (05):

[0092] ü)p=(Ps+œi(Q5)

[0093] A laser cavity OPO can be formed for example by mirrors, M31 and M32, reflecting at the wavelength of the pump signal frequency ^p, to generate the illumination pulse U, as schematically illustrated in figures 4, 5 and 6.

[0094] For example, and without limitation, the second conversion unit 130 may be capable of generating a probe signal Ss.i0 with a probe signal frequency ws corresponding to a wavelength of 2037 nm, from a pump beam with a pump signal frequency ^p corresponding to a wavelength of 1030 nm, while inducing a complementary signal with an idler signal frequency corresponding to a wavelength of 2083 nm. The wavelength of 2037 nm corresponds to a wavelength within the atmospheric transmission band.

[0095] In some embodiments, the optical system 10 may comprise one or more optical elements (implemented, for example, in the first unit conversion unit 120 and / or the second conversion unit 130), such as a dichroic mirror M22 and a dichroic mirror M33 reflecting at the wavelength of the pump signal frequency ^p, suitable for injecting the resulting pump signal from the first conversion unit 120 into the OPO laser cavity, as illustrated by Figures 4, 5 and 6.

[0096] In embodiments, the optical system 10 may further include one or more other optical elements, implemented for example in the second conversion unit 130, such as for example a dichroic mirror M34 reflecting at the wavelength of the pump signal frequency ^p, capable of deflecting or ejecting from the optical path O the residues of the pump signal, corresponding to the conversion losses, at the output of the laser cavity OPO, as illustrated by Figures 4, 5 and 6.

[0097] In embodiments, the optical system 10 may include one or more other optical elements (implemented for example in the second conversion unit 130), such as a dichroic mirror M35 reflecting at the wavelength of the idler signal frequency ^i, capable of deflecting or ejecting from the optical path O the complementary signal at the output of the laser cavity OPO, as illustrated by Figures 4 and 5.

[0098] In a second embodiment, the conversion element 132 of the second conversion unit 130 can be adapted to apply a third-order nonlinear frequency conversion operation, of the Raman type, to convert a high-intensity pump beam Sp_i2o into a probe signal Ss_i0. The conversion element 132 can thus be inserted into a resonant cavity (or oscillator, also called a 'Raman laser') so as to translate the pump signal frequency ^p into a probe signal frequency ws

[0099] In a third embodiment, the conversion element 132 can be of the optical amplifier type inserted in a resonant cavity. The resulting second conversion unit 130 then forms a laser unit pumped by the high-intensity pump beam Sp i2o (to emit a probe signal Ss_i0 having a longer wavelength). This type of pumped laser can be, for example, a gain-switched laser.

[0100] In certain aspects, the optical system 10 may include one or more output optical elements 151, such as, for example, a lens L51, capable of transmitting the probe signal Ss.i0 to the target 20 (for example, by directing or propagating it), as illustrated by Figures 4, 5 and 6.

[0101] In one embodiment, the optical system 10 may include one or more input optical elements 152, such as, for example, a lens L52, suitable for receive the reflected signal Sr_i0 resulting from the reflection of the probe signal S, |() on the target 20, as illustrated by figures 4, 5 and 6.

[0102] According to some embodiments, the optical system 10 can be a so-called all-optical fibre device, such that in particular the optical paths between the different units of the system are fibre-connected.

[0103] The detection unit 140 can be any detection device, such as a detector or a detection camera, adapted to detect optical signals defined in the visible range. For example, and without limitation, the detection unit 140 can be a silicon detector adapted to detect a converted beam with a converted signal frequency corresponding to a wavelength of 600 to 900 nm. Advantageously, detection cameras defined in the visible range exhibit extremely low dark noise, allowing for long exposure times and thus increased resolution through signal integration.

[0104] Alternatively, the detection unit 140 may be any detection device, such as a detector or a detection camera, adapted to detect defined optical signals in the infrared range and in particular the near-infrared range. For example, and without limitation, the detection unit 140 may be an InGaAs (i.e., indium gallium arsenide) detector adapted to detect a converted beam with a converted signal frequency corresponding to a wavelength of 0.9 to 1.65 Ω.

[0105] In certain aspects, the optical system 10 may include one or more optical elements (implemented for example in the first conversion unit 120 and / or in the detection unit 140), such as for example an optical filter F23, configured to filter wavelengths other than those relating to the signal converted to the detection unit 140, as schematically illustrated in Figures 4, 5 and 6.

[0106] In embodiments, the optical system 10 can be configured to perform a telemetry function consisting of performing a telemetry measurement to determine the distance d of the target 20 relative to the optical system 10. The telemetry function can be implemented by performing a variation (or a sweep) of the repetition rate p of the laser source 110 until a converted signal Sc i2o is obtained and detected by the detection unit 140. The distance d of the target 20 can then be determined by implementing signal processing using equation (01) and the value of the repetition rate p associated with the detection.

[0107] For example, and without limitation, the execution of the telemetry function may include a first phase of scanning the repetition rate p of the laser source 110 over a large possible frequency range, and then, in response to the detection of a converted signal Sc i 2 o, a second phase of scanning the repetition rate p over a more restricted frequency range, in order to improve the detection of the converted signal Sc 4 2 o- A detection unit 140 suitable for performing such a telemetry function can be a sensitive and fast sensor configured to detect low-intensity converted pulses Ic. The detection unit 140 can also use signal integration, at each repetition rate p experienced according to a short integration time compared to the possible frequency variation f of the repetition rate p of the laser source 110, during the execution of the telemetry function.Since the duration of the pulses propagating along the optical path O of the optical system 10, towards and from the target 20, is very short, the execution of the rangefinding function can include a narrow temporal aliasing relative to the frequency of variation f of the repetition rate p of the laser source 110 (for example during a second scanning phase implemented), which makes it possible to effectively filter the backscattered signal before and after the target 20, particularly for a target 20 located in a highly diffusive medium, such as for example in a medium where the target 20 is immersed in fog or a medium where the target is surrounded by smoke or camouflage smoke.

[0108] In embodiments, a detection unit 140 adapted to perform the telemetry function can be a point sensor, such as an adapted photodetector.

[0109] According to certain embodiments, the optical system 10 can be further configured to apply an active imaging function to the target 20. The detection unit 140 associated with such an active imaging function can thus be, for example, a camera comprising a plurality of sensors. The active imaging function can be implemented to image at least a portion of the target 20 by implementing suitable image processing and forming an image comprising a plurality of pixels.

[0110] In embodiments, the optical system 10 can be configured to apply a telemetry function and an active imaging function of the target 20 with a suitable detection unit 140, as shown in [Fig.4].

[0111] In embodiments, such a suitable detection unit 140 may be a sensitive and fast (i.e. time-resolved) camera, or SPAD arrays (acronym for the Anglo-Saxon expression 'Single-Photon Avalanche Diode' meaning single-photon avalanche detectors).

[0112] Alternatively, a suitable detection unit 140 can be a standard camera, known as a "slow" camera. Such a "slow" camera can be characterized by an image refresh rate of between 100 Hz and a few kHz at full resolution (i.e., when acquiring images according to the maximum number of pixels of the camera) and this with a relatively long exposure time.

[0113] In embodiments, the optical system 10 can be configured to implement a frequency scan (i.e., variation of the repetition rate p of the laser source 110) allowing an image to appear (or to be generated) on the camera (fast or slow) when the time At (relative to the repetition rate y) corresponds to the round-trip time on the target of the probe pulses.

[0114] In the case of using a high-speed camera, such a frequency scan can be an ultra-high-speed scan, corresponding to the generation of an image on the camera per scanned frequency. These embodiments can, for example, be implemented for the detection of stationary targets or targets moving at high or low speeds.

[0115] When using a slow camera, frequency scanning can be implemented using a camera "windowing" method, i.e., by using only a limited number of the camera's pixels, which has the effect of proportionally increasing the image acquisition speed. These embodiments are particularly suitable for stationary targets or targets moving at low speeds.

[0116] In embodiments where the detection unit 140 used is characterized by a long integration time compared to the variation frequency f, the target 20 can be positioned at a known distance d. The distance d can then be defined from a prior rangefinding measurement, and the repetition rate y of the laser source 110 can be fixed during the implementation of the imaging function.

[0117] In certain aspects, the optical system 10 may further comprise a servo unit (160 or 170) configured to apply servo control to the repetition rate p of the laser source 110 in certain cases, such as when the distance d to the target 20 varies (slightly but sufficiently) during the measurement by the detection unit 140 in active imaging mode. Such a servo unit (160 or 170) may correspond to a rangefinder integrated into the optical system 10. These embodiments may, for example, be implemented for the detection of stationary targets or targets moving at high or low speeds.

[0118] In some embodiments, such a servo unit 160 can be adapted to detect a portion of the reflected signal Si0 obtained in response to the reflection of the probe signal Si0 on the target 20, and to detect a signal associated with a probe signal frequency ws. This is the case regardless of the nature of the second conversion unit 130 (OPO laser cavity, a Raman laser, a gain-switched laser, etc.). In such embodiments, the probe signal frequency-detection servo unit 160 can include a signal detection and analysis unit 161 configured to detect an optical signal at a probe signal frequency, to perform a telemetry function relative to this signal, and to transmit an electrical servo signal of the repetition rate y to the laser source 110, as illustrated in Figure 5. To acquire the optical signal to be analyzed, the unit The 160 signal frequency detection servo system may include one or more signal separation optical elements, such as a polarizing mirror P62 schematically illustrated in [Fig. 5], configured to separate the reflected signal Sr_i0 received by the optical system 10 along two distinct polarization axes, forming two distinct signals comprising: - a first signal defined, according to a first polarization, the first signal being intended to be propagated towards the first conversion unit 120, and - a second signal, called leakage signal, defined according to a second polarization, the second signal being intended to be propagated towards the detection and analysis unit 161, and optionally to be focused on the detection and analysis unit 161.

[0119] Although the probe signal Ss_io at the output of the optical system 10 is polarized, for example with linear polarization, reflection from the target 20 to form the reflected signal Sr_i0 induces at least a partial depolarization of the electromagnetic wave. Furthermore, the conversion of the reflected signal S, io is generally applied with only one polarization in the first conversion unit 120. This is particularly the case in a PPLN-type crystal where phase matching occurs through birefringence. Thus, the formation of the converted signal Sc_i2o, detected by the detection unit 140, uses only a polarized portion of the reflected signal Sr_i0, with a polarization inferred by the conversion element 122 (for example, vertical polarization). A portion (called the 'depolarized portion') of the reflected signal Sr_io can then be naturally filtered during the conversion process by the conversion element 122.This filtered part is not used subsequently. The frequency-sensing servo configuration of the probe signal therefore does not decrease the conversion rate of the reflected signal Sr.i0 into the converted signal Sc 4 2 o- .

[0120] In embodiments where the conversion unit 130 is an OPO laser cavity, the servo unit 170 can be adapted to detect a portion of the reflected signal from the reflection of the complementary signal on the target 20, and thus to detect a signal associated with an idler signal frequency ^i. In this case, the complementary signal at idler signal frequency resulting from the conversion of the pump signal is not optically separated from the probe signal Ss_io at the output of the conversion unit 130. The servo unit 170 can thus include a signal detection and analysis unit 171 configured to detect an optical signal that is at an idler signal frequency ^i, to perform a telemetry function relative to this signal, and to transmit an electrical servo signal of the repetition rate p to the laser source 110, as illustrated in [Fig. 6].To acquire the optical signal to be analyzed, the servo unit 170 can include one or . several optical separation elements, such as for example a dichroic mirror M72, configured to separate the reflected signal, from the reflection of the complementary signal on target 20, and the reflected signal Sr.i0, from the reflection of the probe signal Ss.io on target 20. The reflected signal Sr.io is propagated to the first conversion unit 120, and the reflected signal, from the reflection of the complementary signal on target 20, is propagated to the detection and analysis unit 171.

[0121] In some embodiments, the optical system 10 can use a rangefinder 30, as shown in Figure 7. The rangefinder 30 can be internal or external to the optical system 10. The rangefinder 30 is configured to perform a rangefinding measurement of the target 20 relative to the optical system 10 and to use this rangefinding measurement to apply a servo control of the repetition rate p of the laser source 110. The rangefinder 30 can be any suitable ad-hoc rangefinder.

[0122] The [Fig.8] is a flowchart representing the optical process implemented by the optical system 10, according to embodiments of the invention.

[0123] In step 1010, the initial pump signal Sp.no, comprising the first pump pulse Ip (n), and the second pump pulse Ip (n+i), both defined at the pump signal frequency ^p, is generated from the laser source 110. The first pump pulse Ip (n) is temporally separated from the second pump pulse Ip (n+i) by a separation delay At defined as a function of the repetition rate p applied to the laser source 110.

[0124] At step 1020, the initial pump signal Sp.no passes through the first conversion unit 120 of the optical system 10, which produces the resulting pump signal Sp i2o comprising the first pump pulse Ip (n) having the pump signal frequency ^p. The first pump pulse Ip (n) after passing through the first conversion unit 120 is unaltered and unattenuated.

[0125] At step 1030, the resulting pump signal Sp i2o passes through the second conversion unit 130 of the optical system 10 to produce the probe signal Ss.i0, comprising the illumination pulse Im of frequency corresponding to the probe signal frequency. For example, and without limitation, the second conversion unit 130 can be an OPO laser cavity using a second-order nonlinear frequency conversion method applied to the first pump pulse Ip(n), the probe signal frequency being equal to the difference between the pump signal frequency ^p and the idler signal frequency (^p~^f). Alternatively, the second conversion unit 130 can be, for example, a Raman laser, or a gain-switched laser to produce the probe signal Ss_i0.

[0126] At step 1040, the target 20, located at a distance d from the optical system 10, is illuminated by the probe signal Ss.i0, which is then reflected back to the optical system 10, providing a reflected signal Sr_i0.

[0127] At step 1050, the reflected signal Sr_i0, comprising the reflected pulse Ir of probe signal frequency ws, is received by the optical system 10.

[0128] At step 1060, the reflected signal Sr_i0 passes through the first conversion unit 120 of the optical system 10 to produce the converted signal Sc_i2o comprising the converted pulse Ic having the frequency of the converted signal using a nonlinear frequency summation process applied to the second pump pulse Ip(n+i) and to the reflected pulse Ir, the converted signal frequency then being equal to the sum of the probe signal frequency and the pump signal frequency ^p. In particular, the second pump pulse Ip(n+i) also passes through the first conversion unit 120 at step 1060.

[0129] At step 1070, the converted signal Sc_no is detected by the detection unit 140 of the optical system 10.

[0130] The [Fig.9] is a flowchart representing sub-steps of the optical process, for example a telemetry or active imaging process, implemented by the optical system 10, from the signal detected in step 1070, according to embodiments of the invention.

[0131] At step 1080, different values ​​of the repetition rate p of the laser source 110 can be applied.

[0132] At step 1090, the value of the distance d of the target 20 with respect to the optical system 10 can be determined as a function of the value of the repetition rate p applied from the laser source 110.

[0133] In embodiments, the value of the distance d can be determined in response to the detection of a converted signal Sc_120 (by the detection unit 140), at step 1070.

[0134] Advantageously, the variation of the repetition rate p can in particular be applied so as to maximize the power of the converted signal Sc_120, the value of the distance d can then be determined in response to the maximization of the power of the converted signal Sc.120 detected during the modification of the values ​​of the repetition rate p in step 1080.

[0135] Fig. 10 shows diagrams 10(a) and 10(b) representing respectively two variants of a servo method, implemented by the optical system 10, from the signal detected at step 1070, according to embodiments of the invention.

[0136] At step 1100, the reflected signal Sr_i0 can be separated into two distinct polarization signals comprising a first signal defined according to a first The first signal, defined according to the first polarization, can be propagated to the first conversion unit 120. The second signal, defined according to the second polarization, can be propagated to a detection and analysis unit 161 within a frequency-detection servo unit 160. The servo unit 160 can then apply a telemetry function related to this detection and transmit an electrical signal for controlling the repetition rate u to the laser source 110.

[0137] In some embodiments, at step 1110, the complementary frequency signal of the idler signal from the conversion of the resulting pump signal Sp.i2o, can be generated at the output of the conversion unit 130. The target 20 can then be illuminated by the complementary signal, which is then reflected back to the optical system 10, which provides a reflected complementary signal.

[0138] At step 1120, the complementary reflected signal having the frequency of the idler signal can be received by the optical system 10 and detected by a servo unit 170 which can then apply a telemetry function related to this detection and transmit an electrical servo signal of the repetition rate p to the laser source 110.

[0139] A person skilled in the art will also understand that certain steps or substeps of the process can be carried out simultaneously, sequentially, successively, independently or not, and / or in an order defined by the implementation of the optical system 10, according to the embodiments of the invention.

[0140] A person skilled in the art will also readily understand that the various elements of the optical system, according to the embodiments of the invention, can be implemented in various ways by hardware, or by a combination of hardware and software, for example in the form of program code that can be distributed as a program product or in various forms, in particular to control the hardware part of this combination.

[0141] The invention is not limited to the embodiments described above by way of non-limiting examples. It encompasses all the alternative embodiments that can be considered by those skilled in the art. In particular, those skilled in the art will readily understand that the invention is not limited to the different units of the optical system, described by way of non-limiting examples.

Claims

Demands

1. An optical system (10) configured to illuminate a target (20) with an illumination pulse (Im), the optical system being capable of receiving a reflected pulse (Ir) corresponding to a reflection of said illumination pulse (Im) on the target (20), said optical system (10) comprising: - a single laser source (110) configured to generate an initial pump signal (Sp_n0) formed by a continuous or quasi-continuous pulse train comprising at least a first pump pulse (Ip(n)) followed by a second pump pulse (Ip(n+1)), said first and second pump pulses (Ip(n) and Ip(n+1)) having the same frequency, referred to as the pump signal frequency (P), - a first conversion unit (120) adapted to be traversed by said initial pump signal (Sp_n0) and to produce a resulting pump signal (Sp_n20) comprising said first pump pulse (Ip (n)),and - a second conversion unit (130) adapted to be traversed by said resulting pump signal (Sp.i2o) and to produce said illumination pulse (Im) by applying a frequency conversion from said first pump pulse (Ip (n)), said illumination pulse (Im) and said reflected pulse (Ir) having the same frequency, said probe signal frequency (ws), distinct from the pump signal frequency (^P), said first conversion unit (120) being further configured to generate a converted pulse (Ic) by applying a nonlinear frequency summation operation from said second pump pulse (Ip (n+i)) and said reflected pulse (Ir), said converted pulse (Ic) having a frequency, said converted signal frequency (wc), equal to the sum of the pump signal frequency (WP) and the probe signal frequency (ws),said optical system (10) further comprising a detection unit (140) capable of detecting said converted pulse (Ic).

2. Optical system (10) according to claim 1, wherein the first conversion unit (120) comprises a nonlinear optical crystal (122) having a nonlinear susceptibility of order 2.

3. Optical system (10) according to any one of claims 1 to 2, wherein the second conversion unit (130) comprises a conversion element (132) configured to apply a second-order nonlinear frequency conversion operation, said conversion element (132) being inserted into a laser cavity Optical Parametric Oscillator.

4. Optical system (10) according to any one of claims 1 to 2, wherein the second conversion unit (130) is a Raman laser comprising a conversion element (132) configured to apply a 3rd order nonlinear frequency conversion operation, said conversion element (132) being inserted in a resonant cavity.

5. Optical system (10) according to any one of claims 1 to 2, wherein the second conversion unit (130) is a gain-switching laser comprising an optical amplifier-type conversion element (132) inserted in a resonant cavity.

6. Optical system (10) according to any one of claims 1 to 5, wherein said single laser source (110) is a laser selected from an actively triggered, gain-solid-state laser, and a MOPA-type optical fiber laser.

7. Optical system (10) according to any one of claims 1 to 6, wherein said single laser source (110) is configured to generate said initial pump signal (Sp.no) with a repetition rate (p), said second pump pulse (Ip (n+i)) being temporally separated from said first pump pulse (Ip (n)) by a separation delay (At) defined from said repetition rate (p), said target being at a distance (d) from said optical system (10), and wherein the optical system (10) is configured to determine said distance (d) between the target (20) and the optical system (10) as a function of said repetition rate (y) of said single laser source (110).

8. Optical system (10) according to claim 7, wherein said optical system (10) is configured to vary the repetition rate (y) according to a narrow temporal aliasing to determine the distance (d) between said target (20) and the optical system (10).

9. Optical system (10) according to claim 7, wherein said optical system (10) comprises a servo unit (160 or 170) configured to apply a rate servo control of repetition (y) of the laser source (110), if the distance (d) of said target varies.

10. Optical system (10) according to claim 9, wherein said servo unit (160) is configured to detect an optical signal having a probe signal frequency (ws), said servo unit (160) comprising at least one polarizing mirror (P62) adapted to separate a reflected signal (Sr.i0) comprising said reflected pulse (Ir) received by said optical system (10), into two signals comprising: - a first signal defined according to a first polarization, the first signal being intended to be propagated to the first conversion unit (120), and - a second signal defined according to a second polarization, the second signal being intended to be detected by said servo unit (160).

11. Optical system (10) according to claims 3 and 9, wherein said optical system (10) is further configured to illuminate said target (20) with a complementary pulse (f dier) having a frequency, said idler signal frequency (^i), resulting from said frequency conversion applied to form said illumination pulse (Im), said idler signal frequency (^i) being distinct from the probe signal frequency (ws) and the pump signal frequency (^p), said optical system (10) being configured to receive a complementary reflected pulse in response to a reflection of said complementary pulse (h dier) on said target, and wherein said servo unit (170) is configured to detect said complementary reflected pulse.

12. An optical method, implemented in an optical system (10), comprising the steps of: - generate (1010), using a single laser source, an initial pump signal (Sp.no) having a frequency, called pump signal frequency (^P), said initial pump signal (Sp no) comprising a first pump pulse (Ip (n)), then a second pump pulse (Ip (n+l)), - to pass (1020) said initial pump signal (Sp no) through a first conversion unit (120), to produce a resulting pump signal (Sp_i20) comprising at least said first pulse (Ip (n)), - to pass (1030) said resulting pump signal (Sp i2o) through a second conversion unit (130) to produce at least one illumination pulse (Im) obtained from said first pulse (Ip (n)) by applying a frequency conversion, - illuminate (1040) a target (20) with said illumination pulse (Im), and - receive (1050) a reflected pulse (Ir) in response to a reflection of said illumination pulse (Im) on the target (20), said illumination pulse (Im) and said reflected pulse (Ir) having the same frequency, said probe signal frequency (Ws), the probe signal frequency (ws) being distinct from said pump signal frequency (^P), and in that the process includes the steps of: - to pass (1060) said reflected pulse (Ir) through said first conversion unit (120), to produce a converted pulse (Ic) from said reflected pulse (Ir) and said second pump pulse (IP <n+ i )) en appliquant une opération non linéaire de sommation de fréquences, ladite impulsion convertie (Ic) présentant une fréquence, dite fréquence convertie (wc), égale à la somme de la fréquence de signal sonde (ws) et de la fréquence de signal de pompe (^P), et - detect (1070) the converted pulse (Ic).

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