CLAIMS

1. Device (14) for detecting optical pulses, advantageously laser pulses, each optical pulse having a pulse width, the device (14) comprising a sensor (20) comprising a plurality of pixels, each pixel comprising:

- a receiver (30) configured to receive optical pulses and to generate an electrical signal having variations representative of the received optical pulses, each variation of the electrical signal having a frequency in the frequency domain dependent on the pulse width of the pulse corresponding to said variation,

- an event detection unit (32) comprising at least one frequency filter, the or at least one frequency filter having at least one adjustable cutoff frequency (f 1 , f2), the cutoff frequency (f 1 , f2) of each filter defining a passband for the event detection unit (32), the passband having a lower bound and an upper bound, the adjustable cutoff frequency (f1, f2) of the or each filter being set so that the upper limit is greater than or equal to 1 Megahertz, the event detection unit (32) being configured to detect variations over time in the electrical signal generated by the receiver (30) only when the frequency in the frequency domain of said variations is within the bandwidth of the event detection unit (32), and

- a chronometry unit (34) configured to date each variation of the electrical signal detected by the event detection unit (32).

2. Device (14) according to claim 1, in which the or each adjustable cut-off frequency is adjusted as a function of an adjustment voltage applied to the corresponding frequency filter, the adjustment being advantageously carried out during the design of the detection device ( 14).

3. Device (14) according to claim 2, in which each event detection unit (32) comprises at least one low-pass frequency filter at a first adjustable cut-off frequency (f1) and a high-pass frequency filter at a second adjustable cutoff frequency (f2), the adjustment voltages applied to each filter being such that the first cutoff frequency (f1) is the upper limit of the bandwidth of the event detection unit (32) and the

second cut-off frequency (f2) is the lower limit of the bandwidth of the event detection unit (32).

4. Device (14) according to any one of claims 1 to 3, wherein, for each pixel, the adjustable cut-off frequency (f1, f2) of the or each filter of the event detection unit (32) is set so that the lower limit of the bandwidth is greater than or equal to 100 kilohertz, preferably greater than or equal to 1 Megahertz.

5. Device (14) according to any one of claims 1 to 4, wherein the receiver (30) is configured to generate an electrical signal only when the wavelength of the optical pulses received is between 380 nanometers and 1, 7 micrometers.

6. Device (14) according to any one of claims 1 to 5, wherein the device (14) comprises at least one spectral filter (40) upstream of the sensor (20), the spectral filter being centered on a length d wave of interest, advantageously the spectral filter (40) having a transmission window of width greater than or equal to 30 nanometers or greater than or equal to 100 nanometers.

7. Device (14) according to any one of claims 1 to 6, in which each optical pulse is received on the sensor (20) in the form of a task (T), the device (14) comprising a unit of control of the size and shape of the spot (T) so that the spot (T) extends over at least two adjacent pixels of the sensor (20), advantageously over at least four adjacent pixels of the sensor (20), each pixel comprising a measurement unit (35) of the variations of the electric signal detected by the corresponding event detection unit (32), the device (14) further comprising a computer (24) configured for, the case applicable,comparing the measurement performed by the measurement unit (35) of each pixel over which the task (T) extends to a predetermined threshold and determining whether or not the received optical pulse is measurement noise according to the result of the comparison.

8. Device (14) according to any one of claims 1 to 7, in which each optical pulse is received on the sensor (20) in the form of a task (T), the device (14) comprising a unit of control of the size and shape of the spot (T) so that the spot (T) extends over at least two adjacent pixels of the sensor (20), advantageously over at least four adjacent pixels of the sensor (20), each pixel comprising a unit for measuring (35) variations in the electrical signal detected by the corresponding event detection unit (32), the pixels of the sensor (20) being grouped together, each set comprising at least a first pixel (P1) and a second pixel (P2), adjacent to the first pixel (P1),each first pixel (P1) comprising an interference filter capable of transmitting only a first spectral band (B1), the first spectral band (B1) being a spectral band of interest centered on a wavelength of interest, each second pixel (P2) comprising a filter capable of transmitting only a second spectral band (B2), the second spectral band (B2) being different from the first spectral band (B1), the second spectral band (B2) being a chosen reference spectral band in the group consisting of:the second spectral band (B2) being different from the first spectral band (B1), the second spectral band (B2) being a reference spectral band chosen from the group consisting of:the second spectral band (B2) being different from the first spectral band (B1), the second spectral band (B2) being a reference spectral band chosen from the group consisting of:

- a spectral band centered on a wavelength different from the or each wavelength of interest,

- a spectral band disjoint from the or each spectral band of interest, and

- a spectral band in which at least one spectral band of interest is strictly included,

the device (14) comprising a computer capable of comparing, where appropriate, the measurements made by the measurement unit (35) of each of the first and of the second pixel (P1, P2) and of classifying the optical pulse received into depending on the result of the comparison.

9. Device (14) according to claim 6 or 8, wherein at least one wavelength of interest is between 1.05 micrometers and 1.07 micrometers or between 1.50 micrometers and 1.70 micrometers, of preferably between 1.55 micrometers and 1.65 micrometers.

10. Optronic system comprising a detection device (14) according to any one of claims 1 to 9.

DESCRIPTION
TITLE: Optical Pulse Detection Device

The present invention relates to an optical pulse detection device. The invention also relates to an optronic system comprising such a detection device.

Optronic systems are conventionally equipped with functions for detecting optical pulses, in particular laser pulses.

The detection of laser pulses by means of such optronic systems is subject to numerous problems. In particular, one of the problems consists in reducing the rate of false alarms (caused, for example, by parasitic solar fluxes) while allowing precise angular location of the emission source.

An existing solution consists in using four-quadrant detectors which have a large passband, which makes it possible, with suitable filtering, to detect pulses of very short duration, typically laser pulses, the slower events being rejected. Nevertheless, such four-quadrant detectors have a limited sensitivity linked to daytime noise (especially for large fields) and have an imprecise location (outside the center of the detector).

Another solution consists in using imaging matrix detectors which have good localization (position of the pixel in the matrix). However, due to their long integration times (a few milliseconds), these detectors cannot distinguish very short events (a few nanoseconds) due to laser pulses from longer events, in particular due to solar reflections, which generates many false alarms.

There is therefore a need for an optical pulse detection device making it possible to reliably detect pulses of short duration, typically of the order of ten nanoseconds, while allowing precise angular localization of the emission source. .

To this end, the subject of the present description is a device for detecting optical pulses, advantageously laser pulses, each optical pulse having a pulse width, the device comprising a sensor comprising a plurality of pixels, each pixel comprising:

- a receiver configured to receive optical pulses and to generate an electrical signal having variations representative of the optical pulses received,

each variation of the electrical signal having a frequency in the frequency domain depending on the pulse width of the pulse corresponding to said variation,

- an event detection unit comprising at least one frequency filter, the or at least one frequency filter having at least one adjustable cutoff frequency, the cutoff frequency of each filter defining a bandwidth for the event detection unit events, the bandwidth having a lower limit and an upper limit, the adjustable cut-off frequency of the or each filter being set so that the upper limit is greater than or equal to 1 Megahertz, the event detection unit being configured to detect variations over time in the electrical signal generated by the receiver only when the frequency in the frequency domain of said variations is included in the bandwidth of the event detection unit, and

- a chronometry unit configured to date each variation of the electrical signal detected by the event detection unit.

According to other advantageous aspects, the detection device comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

- the or each adjustable cut-off frequency is adjusted as a function of an adjustment voltage applied to the corresponding frequency filter, the adjustment being advantageously carried out during the design of the detection device;

- each event detection unit comprises at least one low-pass frequency filter at a first adjustable cut-off frequency and one high-pass frequency filter at a second adjustable cut-off frequency, the adjustment voltages applied to each filter being such that the first cutoff frequency is the upper limit of the passband of the event detection unit and the second cutoff frequency is the lower limit of the passband of the event detection unit;

- for each pixel, the adjustable cut-off frequency of the or each filter of the event detection unit is set so that the lower limit of the bandwidth is greater than or equal to 100 kilohertz, preferably greater than or equal to 1 Megahertz;

- The receiver is configured to generate an electrical signal only when the wavelength of the optical pulses received is between 380 nanometers and 1.7 micrometers;

- the device comprises at least one spectral filter upstream of the sensor, the spectral filter being centered on a wavelength of interest, advantageously the spectral filter having a transmission window of width greater than or equal to 30 nanometers or greater than or equal at 100 nanometers;

- each optical pulse is received on the sensor in the form of a spot, the device comprising a unit for controlling the size and shape of the spot so that the spot extends over at least two adjacent pixels of the sensor , advantageously on at least four adjacent pixels of the sensor, each pixel comprising a unit for measuring the variations of the electrical signal detected by the corresponding event detection unit, the device further comprising a computer configured for, if necessary , comparing the measurement performed by the measurement unit of each pixel over which the task extends with a predetermined threshold and determining whether or not the received optical pulse is measurement noise as a function of the result of the comparison;

- each optical pulse is received on the sensor in the form of a spot, the device comprising a unit for controlling the size and shape of the spot so that the spot extends over at least two adjacent pixels of the sensor , advantageously over at least four adjacent pixels of the sensor, each pixel comprising a unit for measuring variations in the electrical signal detected by the corresponding event detection unit, the pixels of the sensor being grouped together, each set comprising at least one first pixel and a second pixel, adjacent to the first pixel, each first pixel comprising an interference filter capable of transmitting only a first spectral band, the first spectral band being a spectral band of interest centered on a wavelength of interest,each second pixel comprising a filter capable of transmitting only a second spectral band, the second spectral band being different from the first spectral band, the second spectral band being a reference spectral band chosen from the group consisting of:

- a spectral band centered on a wavelength different from the or each wavelength of interest,

- a spectral band disjoint from the or each spectral band of interest, and

- a spectral band in which at least one spectral band of interest is strictly included,

the device comprising a computer capable of comparing, where applicable, the measurements made by the measurement unit of each of the first and of the second pixel and of classifying the optical pulse received as a function of the result of the comparison;

- at least one wavelength of interest is between 1.05 micrometers and 1.07 micrometers or between 1.50 micrometers and 1.70 micrometers, preferably between 1.55 micrometers and 1.65 micrometers.

The invention also relates to an optronic system comprising a detection device as described above.

Other characteristics and advantages of the invention will appear on reading the following description of embodiments of the invention, given by way of example only and with reference to the drawings which are:

- [Fig 1], Figure 1, a schematic representation of an example of a light source and an optronic system comprising a detection device,

- [Fig 2], figure 2, a schematic representation of an example of elements of a detection device, namely: a sensor, a unit for controlling the size and shape of a spot on the sensor and a spectral filter,

- [Fig 3], figure 3, an example of a functional representation of the elements of a pixel of a sensor of a detection device,

- [Fig 4], figure 4, an example of a Bode diagram of the frequency filters of figure 3 after application of adjustment voltages to the filters,

- [Fig 5], figure 5, a schematic representation of an example of elements of a detection device, namely: a sensor and a unit for controlling the size and shape of a spot on the sensor , and

- [Fig 6], Figure 6, a schematic representation of an example of a sensor of a detection device receiving an optical pulse in the form of a task. Figure 1 illustrates a light source 12 and an optronic system 13 comprising a detection device 14.

The light source 12 is capable of emitting optical pulses in particular in the direction of the optronic system 13. The optical pulses emitted by the light source 12 have an emission spectral band B1. Each optical pulse has a pulse width (also called pulse duration). In the rest of the description, the term “optical pulse” designates pulses in the broad sense, that is to say both in the visible, infrared and ultraviolet range.

The light source 12 is, for example, a laser transmitter or a broadband source, such as the sun.

The optronic system 13 is configured to detect either directly or indirectly the optical pulses emitted in its direction.

The optronic system 13 is, for example, a laser range finder, a pointer, a laser designator, a laser spot detector, a laser pointer detector, a laser warning detector or a missile guidance system (in English "beamriders ").

Advantageously, the optronic system 13 and the light source 12 evolve in an external environment on the same scene. A scene designates a theater of operations, that is to say the place where an action takes place. The stage is therefore an extended space with sufficient dimensions to allow an action to take place.

The optronic system 13 is, for example, intended to be integrated into a platform, such as the platform of an aircraft or a land vehicle.

In the example illustrated by FIG. 1, the detection device 14 comprises a sensor 20, a control unit 22 and a computer 24.

The sensor 20 is capable of receiving optical pulses, in particular optical pulses emitted by the light source 12. As visible in FIG. 2, the optical pulse is received on the sensor 20 in the form of a task T.

The sensor 20 is a matrix sensor, that is to say a sensor formed from a matrix of pixels. The pixel matrix is ​​configured so that each signal is received on a number of pixels of the pixel matrix strictly less than the total number of pixels of the pixel matrix. Typically, each pixel in the matrix is ​​associated with a direction. Thus, this allows the direction of each signal received by the pixel array to be determined.

Advantageously, the pixels of sensor 20 are independent of each other. By the term “independent”, it is understood that each pixel is configured to operate autonomously, without taking into account the optical pulses received by the other pixels.

Advantageously, the sensor 20 is an event detection sensor. Such a sensor, conventionally used for the compression of video streams, has been adapted for the detection of optical pulses as described below.

As illustrated by FIG. 3, at least one pixel of the sensor 20 comprises a receiver 30, an event detection unit 32 and a chronometry unit 34. Preferably, each pixel of the sensor 20 comprises such a receiver 30, such a event detection unit 32 and such a chronometry unit 34, as illustrated in FIG. 3. Advantageously, each pixel of sensor 20 further comprises a measurement unit 35 illustrated in FIG. 3.

The receiver 30 is configured to receive optical pulses and generate an electrical signal depending on the received optical pulses. The electrical signal notably exhibits variations representative of the optical pulses received. In particular, each variation of the electrical signal has a frequency in the frequency domain depending on the pulse width of the pulse corresponding to said variation. The term "frequency domain" is to be understood as opposed to "time domain", the transition from a signal in the time domain to the corresponding signal in the frequency domain being done by applying a Fourier transform to the signal in the time domain . Thus, a signal in the frequency domain has a frequency or distribution of frequencies depending on the width of the corresponding signal in the frequency domain.

time domain. It will be understood that in the present description, the term “frequency in the frequency domain” is to be understood in the broad sense and can also designate a distribution of frequencies. Moreover, it will also be understood that the considered variations of the electrical signal are the variations corresponding to optical pulses.

The receiver 30 is, for example, a photodiode.

Preferably, the receiver 30 is configured to generate an electrical signal only when the wavelength of the optical pulses received is between 1 micrometer and 1.7 micrometers. The receiver 30 is then adapted to detect such wavelengths. Indeed, for certain detection applications, the wavelengths of interest are:

- between 1.05 μm and 1.07 μm, advantageously equal to 1.064 μm, and/or

- Between 1.50 μm and 1.70 μm, preferably between 1.55 μm and 1.65 μm.

The event detection unit 32 is configured to detect a variation in the electrical signal generated by the receiver 30. A variation means that the corresponding pixel is illuminated at the present time (time of reception) in a different way from the previous moment. Such a variation can in particular be explained by the reception on the sensor 20 of an optical pulse originating from a laser.

The variation is detectable only when the frequency in the frequency domain of said variations is included in the passband of the event detection unit 32. The passband of the detection unit 32 comprises a lower limit (in frequency) and an upper limit (in frequency), the lower limit being strictly lower than the upper limit.

Event detection unit 32 includes at least one frequency filter. The term “frequency filter” is understood to mean a filter configured to filter a signal as a function of the frequency of said signal in the frequency domain.

Each frequency filter is preferably an analog filter.

At least one frequency filter has at least one adjustable cutoff frequency. By the term “adjustable”, it is understood that the cut-off frequency can be modified, the adjustment being made either during the design of the detection device 14 (in this case, once the adjustment has been made, it is no longer modifiable), or when using the detection device 14 (the adjustment can then be made several times over time). The adjustable cut-off frequency is, for example, fixed as a function of an adjustment voltage applied to the filter. The cutoff frequency of each filter defines the bandwidth of the event detection unit 32.

Advantageously, the adjustment voltage, also called bias voltage, applied to each filter is such that the upper limit of the bandwidth of the detection unit 32 is greater than or equal to 1 Megahertz (MHz), preferably greater than or equal to at 10 MHz (while remaining below the upper limit). The detection unit 32 is then adapted to detect variations in the electrical signal corresponding to short optical pulses, typically of the order of ten nanoseconds.

Preferably, the adjustment voltage applied to each filter is such that the lower limit of the bandwidth of the detection unit 32 is greater than or equal to 100 kilohertz (kHz), preferably greater than or equal to 1 MHz. The detection unit 32 is then adapted not to detect variations in the electrical signal corresponding to relatively long pulses, typically greater than a microsecond.

Preferably, the adjustment voltage applied to each filter is such that the bandwidth of the event detection unit 32 extends over at most 10 MHz.

Thus, the event detection unit 32 is configured to detect variations in the electrical signal corresponding to short pulses, typically of the order of ten nanoseconds, without taking account of pulses of longer duration, typically greater than the microsecond.

In the example illustrated by FIG. 4, each event detection unit 32 comprises at least one low-pass frequency filter at a first adjustable cut-off frequency f1 and a high-pass frequency filter at a second adjustable cut-off frequency f2 .

In a conventional operation of the event detection unit 32 for a video stream compression application, the adjustment voltages applied to each filter are such that the first cutoff frequency f1 is strictly greater than the second cutoff frequency f2 . The first cut-off frequency f1 is typically of the order of ten kilohertz. The second cutoff frequency f2 is of the order of a few Hz, typically 0 Hz. Such a detection unit 32 does not make it possible to detect variations in the electrical signal corresponding to short pulses, typically of the order of ten nanoseconds, without taking into account pulses of longer duration, typically less than a microsecond.

In an operation adapted for the detection of optical pulses, in particular optical pulses of the order of ten nanoseconds, the adjustment voltages applied to each filter are such that the cut-off frequencies have been adapted for the detection of optical pulses. In particular, the first cut-off frequency f1 is strictly greater than the second cut-off frequency f2, which is illustrated by FIG. 4. The low-pass and high-pass frequency filters then form a band-pass frequency filter. The first cutoff frequency f1 is then the upper limit of the bandwidth of the event detection unit 32 and the second cutoff frequency f2 is the lower limit of the bandwidth of the event detection unit 32 In accordance with what has been explained previously, the first cutoff frequency f1 is typically greater than or equal to 1 MHz, preferably greater than or equal to 10 MHz. The second cutoff frequency f2 is typically greater than or equal to 100 kHz, preferably greater than or equal to 1 MHz.

The chronometry unit 34 is configured to date each variation of the electrical signal detected by the event detection unit 32. The chronometry unit therefore assigns to each variation detected the date of reception of the corresponding optical pulse. This makes it possible, in the case of periodic optical pulses, to go back to the repetition frequency of the optical pulses, and thus to deduce therefrom the characteristics of the emission source.

The measurement unit 35 is configured to measure the electrical signal generated by the receiver 30 when a variation has been detected by the event detection unit 32. Such a measurement is, for example, an illumination measurement.

The unit 22 for controlling the size and shape of the spot T is configured to control the size and shape of the spot T forming on the sensor 20 from the optical pulse emitted by the light source 12.

Advantageously, the control unit 22 is configured so that the task T extends over at least two adjacent pixels of the sensor 20, advantageously over at least four adjacent pixels of the sensor 20.

The control unit 22 is, for example, an optical device configured to defocus the light flux received by the sensor 20. The control unit 22 comprises, for example, an optical lens (as illustrated by FIG. 2) or a optical diffuser upstream of the sensor 20.

Optionally, as illustrated by FIG. 2, a spectral filter 40 is arranged upstream of the pixels of the sensor 20. The spectral filter 40 is, for example, arranged downstream of the control unit 20.

The spectral filter 40 is centered on a wavelength of interest, that is to say a wavelength that the detection device 14 is configured to detect. The wavelength of interest is typically the central wavelength of the spectral band of the laser which it is desired to detect.

The spectral filter 40 has, for example, a transmission window with a width greater than or equal to 30 nanometers (typically for a laser centered on 1.06 μm) or greater than or equal to 100 nanometers (typically for a laser centered on 1.50 pm). the

computer 24 is, for example, a processor. The computer 24 comprises, for example, a data processing unit, memories, an information carrier reader and a man/machine interface.

In the example illustrated by FIG. 1, the computer 24 is carried by the optronic system 12. Alternatively, the computer 24 is remote from the optronic system 12 and is installed in an entity which is, for example, on the ground. This makes it possible to deport the processing carried out by the computer 24 outside the optronic system 12.

For example, the computer 24 interacts with a computer program product which comprises an information carrier. The information medium is a medium readable by the computer 24, usually by the data processing unit of the computer 24. The readable information medium is a medium suitable for storing electronic instructions and capable of being coupled to a computer system bus. By way of example, the readable information medium is a diskette or floppy disk (from the English name floppy disk), an optical disk, a CD-ROM, a magneto-optical disk, a ROM memory, a RAM memory, an EPROM memory, an EEPROM memory, a magnetic card or an optical card. On the information carrier is stored the computer program product comprising program instructions.

The computer program can be loaded onto the data processing unit and is suitable for causing the implementation of steps aimed at processing the measurements made by the pixels of the sensor 20 when the computer program is implemented on the computer processing unit 24.

In another example, the computer 24 is made in the form of one or more programmable logic components, such as FPGAs (Field Programmable Gate Array), or even in the form of one or more dedicated integrated circuits , such as ASICs (Application Specifies Integrated Circuit). The computer 24 is in this case configured to implement the steps aimed at processing the measurements made by the pixels of the sensor 20 as will be described below.

The operation of the detection device 14 will now be described. Advantageously, such operation takes place at any time, that is to say in real time.

Sensor 20 receives optical pulses over time. Each optical pulse is received in the form of a spot T on the sensor 20. Advantageously, the spot T is spread over at least two adjacent pixels, preferably over at least four adjacent pixels of the sensor 20 as illustrated in FIG. 2.

The receiver 30 of each pixel receiving the optical pulse generates an electrical signal which is a function of the optical pulse received. In particular, the electrical signal exhibits a variation representative of the received optical pulse, the variation of the electrical signal having a frequency in the frequency domain depending on the pulse width of the pulse corresponding to said variation.

The event detection unit 32 of each pixel receiving the optical pulse detects any variation in the electrical signal generated by the receiver 30 with respect to the previous time when the frequency domain frequency of the electrical signal is in the band bandwidth of the event detection unit 32.

When a variation has been detected by the event detection unit 32, the corresponding chronometry unit 34 dates each variation of the electrical signal detected by the event detection unit 32.

Optionally, the measurement unit 35 measures the detected variation of the signal.

Advantageously, the computer 24 receives the measurements made by each pixel and compares them with a predetermined threshold. Depending on the result of the comparisons, the computer 24 classifies the optical pulse as being measurement noise or not. For example, if the value of the measurement is lower than the predetermined threshold for a number of adjacent pixels greater than or equal to a predetermined number (for example four), the optical pulse is considered to be measurement noise. Otherwise, the optical pulse is considered to be coming from a light source. Typically, it is considered that an optical pulse, different from a simple measurement noise, has been received on the sensor 20 when several adjacent pixels (for example 4) have exceeded the predetermined threshold simultaneously.

Furthermore, depending on the pixels of the sensor 20 having received the optical pulse, the computer 24 locates the emission source.

Thus, the detection device 14 described makes it possible to detect and locate laser pulses. In particular, the detection device 14 is particularly suitable for detecting pulses of short duration (a few tens of ns) and with precise wavelengths (for example 1.06 μm or 1.55 μm). This is made possible by the high bandwidth of the event detection unit. Matched filtering also makes it possible to focus detection on the wavelengths of interest.

The detection device 14 makes it possible in particular to detect laser pulses with precise dating making it possible to determine the frequency of repetition of the pulses (including at very high rate) or a time code. The detection device 14 also makes it possible to precisely locate the source of such laser pulses. The detection device 14 also makes it possible to reject pulses that do not correspond to laser pulses, typically pulses of long duration.

(greater than a microsecond). Finally, the matrix sensor 20 of the detection device 14 is compatible with the generation of an image of the scene in which the location of the detected optical pulses would be embedded.

In particular, the detection unit 32 of each pixel (basically designed for the compression of video streams) has been configured to detect extremely short events (a few tens of nanoseconds) by modifying the values ​​of the frequency filters of the unit detection 32 (in particular high-pass and low-pass).

The unit for controlling the size and shape of the task T makes it possible to retain only the events for which several contiguous pixels (typically 4) have exceeded the predetermined threshold simultaneously in a constrained time window. This makes it possible to further reduce false events and discriminate between false events generated by the noise of the detector (intrinsically isolated spatially and temporally).

Thus, the detection device 14 makes it possible to reliably detect pulses of short duration, in particular lasers, and to locate them precisely while rejecting the sources of potential false alarm (such as solar reflections for example) on time criteria (in particular when the durations of the pulses are different from the durations sought).

A second embodiment of the detection device 14, complementary to the embodiment described previously (first embodiment), is described with reference to FIGS. 5 and 6. The elements identical to the detection device 14 according to the first embodiment do not are not repeated. Only the differences are highlighted.

The pixels of sensor 20 are grouped into sets. Each set preferably comprises the same number of pixels.

Each set includes at least a first pixel P1 and a second pixel P2. The second pixel P2 is adjacent to the first pixel P1.

Each first pixel P1 comprises an interference filter capable of transmitting only a first spectral band B1. An interference filter (also called a dichroic filter) is a filter whose transmission and reflection properties depend on the wavelength. The interference filter is, for example, of the Fabry Pérot type.

The first spectral band B1 is a spectral band of interest centered on a wavelength of interest. The wavelength of interest is typically the central wavelength of the spectral band of the laser which it is desired to detect.

For example, the wavelength of interest belongs to the short infrared, that is to say to the range of wavelengths comprised between 0.9 micrometers (pm) and 1.7 pm. More specifically, the wavelength of interest is between 1.05 μm and 1.07 μm and is advantageously equal to 1.064 μm. In another example, the wavelength of interest is between 1.50 gm and 1.70 gm, preferably between 1.55 gm and 1.65 gm.

Each second pixel P2 includes a filter capable of transmitting only a second spectral band B2. The filter of the second pixel P2 is, for example, a high-pass filter.

The second spectral band B2 is different from the first spectral band B1. The second spectral band B2 is a reference spectral band. A reference spectral band makes it possible to define an illumination reference locally or step by step for each set of pixels. The reference spectral band is chosen from the group consisting of:

- (i): a spectral band centered on a wavelength different from the or each wavelength of interest,

- (ii): a spectral band disjoint from the or each spectral band of interest, and

- (iii): a spectral band in which at least one spectral band of interest is strictly included.

Preferably, when the reference spectral band is of type (iii), the reference spectral band is a wide spectral band. A wide spectral band is defined as being a spectral band with a width greater than or equal to 100 nm. When the reference spectral band is of type (i) or (ii), the reference spectral band is a wide spectral band or a narrow spectral band. A narrow spectral band is defined as a spectral band with a width of less than 100 nanometers (nm).

Advantageously, each set comprises at least one third pixel P3 among the plurality of pixels of the sensor 20. The third pixel P3 is adjacent to at least one of the first and the second pixel P2 of the set.

Each third pixel P3 includes a filter capable of transmitting only a third spectral band B3.

The third spectral band B3 is different from the first spectral band B1 and from the second spectral band B2. The third spectral band B3 is a spectral band of interest centered on a wavelength of interest.

As a variant, the third spectral band B3 is a different reference spectral band from the second spectral band B2.

Advantageously, when the third spectral band B3 is a spectral band of interest, the filter of the third pixel P3 is an interference filter.

Advantageously, each set comprises a plurality of pixels, such as several first pixels P1 and/or several second pixels P2 and/or several third pixels P3 and/or pixels different from the first, second and third pixels P1, P2, P3.

In this case, each pixel comprises a filter matched to the spectral band corresponding to the pixel. When each set comprises pixels different from the first, second and third pixels P1, P2, P3, the spectral bands of said pixels P1, P2, P3 are spectral bands of interest or reference spectral bands.

The arrangement of pixels in each set is predefined. Advantageously, the arrangement of the pixels of each set is identical from one set to another. For example, the position of the different types of pixels on the sensor 20 is chosen so as to form a periodic pattern. Advantageously, the different types of pixels are arranged relative to each other according to an interlaced tiling.

For example, the positions of the first pixels on the sensor 20 are chosen so as to form a predefined pattern (for example, a staggered pattern) and the position of the other pixels, in particular of the second pixels, are the positions not occupied by the first pixels (in the example, the voids of the staggered pattern).

An example of arrangement of the first, second and third pixels P1, P2, P3 is illustrated by FIG. 6. In this example, the second pixels P2 (reference pixels) are arranged according to a periodic pattern on the sensor 20 and the first pixels P1 (pixel of interest) and third pixels P3 (pixels of interest or reference) are arranged periodically in the spaces not occupied by the first pixels.

The computer 24 is able to compare, if necessary, the measurements made by the pixels of each set and to classify the optical pulse according to the result of the comparison. For example, the results obtained at the end of the comparison are compared with a database of results obtained for known light sources, which allows the classification of the light source to be identified.

The comparisons are, for example, made detection by detection, that is to say each time a flow is received on the sensor 20.

As a variant, the comparisons are made after several detections, for example after a duration greater than the duration of a laser shot (of the order of 1 second at most). This facilitates the classification of the light source 11.

The operation of the detection device 14 according to the second embodiment will now be described. In what follows only the operating differences of the detection device 14 with respect to the first embodiment are highlighted.

The sensor 20 receives an optical pulse in the form of a spot T spread over at least one set of pixels of the sensor 20.

In response, the measurement unit 35 of each pixel of the set receiving the optical pulse generates a measurement (assuming that a variation has been detected by the detection unit 32 of each pixel).

The computer 24 then compares the measurements to a predetermined threshold similarly to the first mode of operation.

The measurements judged to be valid (that is to say as corresponding to optical pulses different from a simple noise) are then compared between the pixels of the same set. For this, the computer 24 calculates the ratios between the measurements of each set and then compares the ratios obtained with at least one predetermined value to classify the light source 11 for emission.

In the example considered, the computer 24 compares the measurements of the first, second and third pixels P1, P2, P3 of the set of illuminated pixels.

For example, the computer 24 calculates the ratios between the measurements obtained for the first, second and third pixels P1, P2, P3, which amounts to calculating the ratios between the first, second and third spectral bands B1, B2, B3. For example, when the first and the third spectral band B1 , B3 are spectral bands of interest and the second spectral band B2 is a reference spectral band, the calculated ratios are, on the one hand, the ratio B1 /B2 between the first spectral band B1 (of interest) and the second spectral band B2 (reference) and, on the other hand, the ratio B3/B2 between the third spectral band B3 (of interest) and the second spectral band B2 ( reference).

These ratios are then compared with predetermined values, which makes it possible to determine whether the emission source is a laser source centered on the wavelength of interest or a broadband source. Advantageously, the estimation of the equivalent temperature of the light source 11 makes it possible to classify the source more precisely. For example, equivalent temperatures of the order of 5800 Kelvins (K) will make it possible to reject a solar reflection. Temperatures below 2000 K will make it possible to classify muzzle fires or propulsion of rockets or missiles.

In a variant implementation, the measuring unit 35 does not allow flux variations to be measured. This is particularly the case when the pulse received is very short. In this case, in response to the reception of an optical pulse, the knowledge of the pixels having detected the optical pulse makes it possible to classify the emission source. For example, when each of the pixels of interest (P1, P3) and reference (P2) of a set has detected an optical pulse, the emission is considered to be coming from a solar reflection. When only the first pixel of interest P1 and the reference pixel P2 of a set have detected an optical pulse, the emission is considered to be coming from a laser source centered on a wavelength included in the first band spectral

B1. When only the third pixel of interest P3 and the reference pixel P2 of a set have detected an optical pulse, the emission is considered to be coming from a laser source centered on a wavelength included in the third band spectral B3.

Thus, in addition to the advantages of the first embodiment, the detection device 14 according to the second embodiment makes it possible, by comparing ratios, or more simply by comparing the pixels of a set having carried out a detection, to distinguish emissions lasers (particularly in the short infrared) of solar reflections, which makes it possible to reduce the rate of false alarms. In particular, in the case of a matrix laser warning detector, the false alarm rate is reduced for the detection of laser flux emitted, for example, by multi-pulse range finders, laser designators, imager illuminators active or otherwise.

Since the sensor 20 of the detection device 14 is a matrix sensor, precise angular location of the emission source is also possible. Furthermore, such a matrix sensor makes it possible to perform two functions: a detection function (allowing the emission source to be classified) and an imaging function.

In addition, the pixels corresponding to the reference spectral bands make it possible to define a local illumination reference of the scene. In particular, during the day, the average solar flux reflected by the scene is taken into account.

Thus, the detection device 14 makes it possible to reliably detect pulses of short duration, in particular lasers, and to locate them precisely while rejecting the sources of potential false alarm (such as solar reflections for example) on time criteria (in particular when the durations of the pulses are different from the durations sought).

Those skilled in the art will understand that the embodiments described above can be combined to form new embodiments provided that they are technically compatible.