System for detecting and tracking a coherent light source among thermal and / or chaotic sources

Abstract

The present invention concerns a system for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources in a given field of view, comprising: an optical imaging system (11, 21) designed to collect incoming light from the given field of view; a detector assembly (13, 14, 22) designed to receive the incoming light from the optical imaging system (11, 21) and to detect intensity of the light impinging on the detector assembly (13, 14, 22); and a processor (15, 23) configured to compute a light intensity image based on the intensities of the light detected by the detector assembly (13, 14, 22) and an image related to the second order correlation function of the light by determining the second order correlation function of the light based on the intensities of the light detected by the detector assembly (13, 14, 22), and to detect the light signal emitted by the coherent light source based on the light intensity image and the image related to the second order correlation function of the light.

Description

[0001] SYSTEM FOR DETECTING AND TRACKING A COHERENT LIGHT SOURCE AMONG THERMAL AND / OR CHAOTIC SOURCES

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to a system for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources .

[0004] STATE OF THE ART

[0005] As is known, in the context of space imaging and deep space missions, optical links and pointing of optical instruments are becoming more and more demanding. This is quite natural for modern instruments which intrinsically provide resolution at prad and sub-prad level . For the same intrinsic rationale, an optical solution represents the most adequate approach.

[0006] When it is necessary to establish an optical link between two instruments and they do not know the exact position of the counterpart, one of the most used solutions is to emit, from at least one of the terminals, an optical signal which is acquired by the other, remote terminal for subsequent accurate pointing. When this signal is extremely low, because of power limitations or overwhelming distances (and, hence, due to diffraction losses) , it can be hardly detectable or distinguished from other spurious sources, apparently similar, in the field of view.

[0007] The Applicant has investigated the known solutions for establishing an highly accurate link (pointing error in prad range) in situations where the counterpart cannot be seen and it can only be recognized by an extremely weak coherent signal, whichever is the reason for its weakness, from other spurious natural sources (e. g. , stars in space applications) which are present in the field of view and even brighter than the signal itself . The weakness of the signal could derive from power limitations of the source (by design or due to external causes) , or from the diffraction losses resulting from propagation across hundreds of millions or even billions of km, as it is the case for communication links in planetary exploration.

[0008] Three main approaches are known to establish links in these conditions :

[0009] • source power - use of a sufficiently powerful beacon, combined with highly sensitive detectors (e . g. , cryogenic single-photon detectors) , which allows to achieve an adequate Signal-to-Noise Ratio (SNR) and, hence, to identify the position of the source;

[0010] • indirect pointing 1 - if the target position can be assumed almost identical to some visible object, this last can be targeted assuming the real target is very close; e . g. , when a communication terminal is on Earth surface, the Earth disk can be used as a beacon from planetary distance;

[0011] • indirect pointing 2 - by using ephemerides and accurate star trackers, the platform points the target where it is expected to be .

[0012] The above three approaches have the following drawbacks :

[0013] • source power - on ground technology can provide large amount of optical power; however, for Mars distance and beyond it doesn' t look feasible : in case of a Ground Station this would pose relevant problems of safety for shooting very intense beams; in case of a flying terminal, technological constraints could pose severe limits to the availability of electrical power necessary to feed the beacon / source; if the reason for weak conditions is directly related to the application or to the source itself, the solution is excluded by definition.

[0014] • indirect pointing 1 - being indirect, if the size of the target is larger than the requested pointing accuracy, the target centroid could provide insufficient accuracy; moreover, it can be strongly affected by external disturbances : in the example of the Earth disk, the Earth albedo derived from Sun illumination could alter the apparent centroid as a function of the relative Sun-Earth position; once acquired the reference, the target still needs to be found; it cannot be used for targets orbiting far from Earth (e. g. , a communication hub orbiting in a Lagrangian point) or, in general, isolated;

[0015] • indirect pointing 2 - pointing is blind and it can be confirmed only after having received and analysed the data (to recognize the coded message) ; uncertainty, error propagation, thermoelastic deformation between the Guidance, Navigation and Control (GNC) sensors and the optical terminal can be sufficient to move the target across the acquisition sensor far from the expected position and far from the link sensor; the large field of view is not exploited and a timeconsuming scan procedure is required.

[0016] OBJECT AND SUMMARY OF THE INVENTION

[0017] An object of the present invention is that of providing a technical solution such that to alleviate, at least in part, the above technical drawbacks .

[0018] This and other objects are achieved by the present invention in that it relates to a system for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources in a given field of view, as defined in the appended claims .

[0019] In particular, the system according to the present invention comprises :

[0020] • an optical imaging system designed to collect incoming light from the given field of view;

[0021] • a detector assembly designed to receive the incoming light from the optical imaging system and to detect intensity of the light impinging on said detector assembly; and

[0022] • a processor configured to

[0023] - compute a light intensity image based on the intensities of the light detected by the detector assembly,

[0024] - compute an image related to the second order correlation function of the light by determining the second order correlation function of the light based on the intensities of the light detected by the detector assembly, and

[0025] - detect the light signal emitted by the coherent light source based on the light intensity image and the image related to the second order correlation function of the light .

[0026] Preferably, the processor is configured to detect the light signal by comparing the light intensity image and the image related to the second order correlation function of the light .

[0027] Conveniently :

[0028] • the detector assembly includes two time-synchronized detector arrays;

[0029] • the system further comprises an optical splitter configured to split up the incoming light from the optical imaging system into two beams reaching, each, a respective time-synchronized detector array;

[0030] • the light intensity image is computed by averaging over time the sum of the intensities of corresponding pixels on the two time-synchronized detector arrays; and

[0031] • the image related to the second order correlation function of the light is computed by averaging over time the product of the intensities of the corresponding pixels on the two time-synchronized detector arrays .

[0032] Preferably, the two time-synchronized detector arrays are time-synchronized through time-to-digital converting electronics .

[0033] Moreover, the two time-synchronized detector arrays can be two distinct detector arrays or two disjoint parts of one and the same detector array device.

[0034] Alternatively :

[0035] • the detector assembly includes a single detector array;

[0036] • the light intensity image is computed based on an intensity autocorrelation computation for each pixel on the single detector array or by averaging over time the sum of the intensities of neighboring, time-synchronized pixels on the single detector array; and

[0037] • the image related to the second order correlation function of the light is computed by averaging over time the product of the intensity of each pixel by itself or by a neighboring, time-synchronized pixel .

[0038] The optical imaging system can include a telescope .

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] For a better understanding of the present invention, preferred embodiments, which are intended purely by way of non-limiting, non-binding examples, will now be described with reference to the attached drawings (all not to scale) , wherein :

[0041] • Figures 1 and 2 schematically illustrate two systems according to two alternative embodiments of the present invention;

[0042] • Figures 3 and 4 show examples of results computed with the systems in Figures 1 and 2, respectively;

[0043] • Figure 5 shows a comparison between two different images computed according to the present invention for two overlapping light sources; and

[0044] • Figure 6 shows an example of a specific typology of image computed according to the present invention for a series of sources .

[0045] DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0046] The following description is presented to enable a person skilled in the art to comprehend, make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the present invention as claimed. Thence, the present invention is not intended to be limited to the embodiments shown and described but is to be accorded the widest scope of protection consistent with the features defined in the appended claims .

[0047] Instead of simply collecting photons to build up a sufficient Signal-to-Noise Ratio (SNR) , the present invention is based on the ability of using statistical properties of the light to distinguish coherent light sources (e .g. , laser or single-photon beams) , from thermal or chaotic ones (e . g. , stars and other celestial objects) , while performing imaging of an extended Field of View (FoV) . This should allow the identification and tracking of the light sources still using a direct method, but extending the capability to extreme conditions (down to the single-photon level) , when an intensity-based solution is not efficient, replacing it with the measurement of the spatial-temporal correlations of second order or higher.

[0048] In general terms, a system according to the present invention for detecting and tracking light sources comprises :

[0049] • an optical imaging system for acquiring the FoV;

[0050] • a detector assembly for acquiring photons, conveniently with proper time synchronization; and

[0051] • a processor for computing image statistics .

[0052] More specifically, the present invention concerns a system for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources in a given FoV, comprising:

[0053] • an optical imaging system designed to collect incoming light from the given FoV;

[0054] • a detector assembly designed to receive the incoming light from the optical imaging system and to detect intensity of the light impinging on the detector assembly; and

[0055] • a processor configured to

[0056] - compute a light intensity image based on the intensities of the light detected by the detector assembly,

[0057] - compute an image related to the second order correlation function of the light by determining / estimating / computing / measuring the second order correlation function of the light based on the intensities of the light detected by the detector assembly, and

[0058] - detect the light signal emitted by the coherent light source based on the light intensity image and the image related to the second order correlation function of the light .

[0059] Preferably, the processor is configured to detect the light signal by comparing the light intensity image and the image related to the second order correlation function of the light .

[0060] Conveniently :

[0061] • the detector assembly includes two time-synchronized detector arrays (that are conveniently time-synchronized through time-to-digital converting electronics) ;

[0062] • the system further comprises an optical splitter configured to split up the incoming light from the optical imaging system into two (symmetric) beams reaching, each, a respective time-synchronized detector array;

[0063] • the light intensity image is computed by averaging over time the sum of the intensities of corresponding pixels on the two time-synchronized detector arrays; and

[0064] • the image related to the second order correlation function of the light is computed by averaging over time the product of the intensities of the corresponding pixels on the two time-synchronized detector arrays .

[0065] The two time-synchronized detector arrays can be two distinct detector arrays or two disjoint parts of one and the same detector array device .

[0066] According to the present invention, two detector arrays are considered synchronized in time if their measurements can be tagged with timestamps that follow a shared absolute time flow. Therefore, relative time-of-arrival measurements are always possible between the two detectors .

[0067] Alternatively : the detector assembly includes a single detector array;

[0068] • the light intensity image is computed based on an intensity autocorrelation computation for each pixel on the single detector array or by averaging over time the sum of the intensities of neighboring, time-synchronized pixels on the single detector array; and

[0069] • the image related to the second order correlation function of the light is computed by averaging over time the product of the intensity of each pixel by itself or by a neighboring, time-synchronized pixel .

[0070] The optical imaging system can include a telescope .

[0071] The proposed solution is based on the measurement of statistical properties of the light while performing imaging of a large field of view. The objective is to allow the identification and discrimination of coherent light (generated by the target to be acquired) from other thermal / chaotic sources present in the same image. It defines a unique capability in identifying and acquiring artificial sources in extreme conditions in contrast to classical direct light detection (intensity) but nevertheless with a direct method.

[0072] In mathematical terms, the solution provides spatial mapping of the light correlation function.

[0073] The proposed solution aims at providing a direct light detection method for faint light coherent sources through the exploitation of detector arrays where each pixel can detect intensity fluctuations or even, in photon counting regime, grant photon detection with associated time-tagging and reduced arrival jitter .

[0074] The combination of the matrix spatial resolution with the recording of intensity fluctuations or the single photon time-tagging feature of the detector, grants the spatial identification of the second order correlation function for each light source entering the detector field of view, even very faint (single photon level) ones . The invention is based on the measurement of the second order correlation function of the light at two space-time coordinates with i = 1,2, which in the quantum formalism can be written as : where are the negative / positive frequency parts of the quantized electric field operator at detector i = 1,2, namely: where k is the polarization vector, e is the unit vector perpendicular to k, ωkis the frequency of the mode, V is the volume, the creation operator, and μ. identifies two orthogonal polarization states .

[0075] For the device applications, a classical source of light is considered and thus a switch is performed from the quantum to the classical formalism, replacing the creation and annihilation operators in the field with the classical field amplitudes . This enables rewriting the second order correlation function as the correlation between the intensities It measured at the two coordinates i = 1,2 through a detector (pixel) :

[0076] In the above expression, one position coordinate can be dropped as it is assumed that the two positions (pixels) are within the transverse coherence area of the detected light, obtaining :

[0077] Examples of classical light are so-called coherent light (like light from a laser) or thermal and chaotic light (like light from a star) .

[0078] The average denotes a statistical (ensemble) average. Whenever the signal statistics is stationary, the average can be performed across time and the function depends only on the delay

[0079] From an experimental point of view, the ensemble averaging can be performed by taking consecutive pictures of the light intensity. For coherent light, like that one produced by a laser, the intensity readings at time t are uncorrelated to the intensity at time t + 1 , hence :

[0080] While for coherent light the two detection events are uncorrelated, for light produced by other kinds of classical sources, like chaotic light, it is possible to have correlations between intensity fluctuations . In these cases, one talks of bunched light . This means that, given a detection event at t, there is a higher probability of another detection event at times close to t. These kinds of sources therefore satisfy

[0081] Since we are interested in discerning coherent to chaotic light, the proposed device is set to measure the correlation function at T = 0 and it is possible to write : Given these premises, the solution is based on the use of a classical optical imaging system (e . g. , a telescope) to image a portion of the field of view containing the light source of interest . The sources present in the field of view will be imaged on a detector array or, in an alternative approach, split and reimaged on either two disjoint parts of the same detector array or on two synchronized detector arrays .

[0082] In this respect, Figure 1 schematically illustrates a synchronized two-detector array configuration, wherein light is focused on the detectors through an optical imaging system and split up into two perfectly symmetric paths . The two detectors are synchronized in time through a link. A processor computes the intensity and ^®(0) images through a time average as it will be described hereinafter. The coherent sources of light are detected through a comparison of the intensity and g^2\0) images .

[0083] More in detail, in use, the light is inputted in a camera optics 11, then an optical splitter 12 splits up the light along two symmetric paths to reach two detector arrays that are time-synchronized through time-to-digital converting electronics 13, 14, and a processor 15 computes intensity image and g^ffi) image (block 16) based on pixel readings from the time-synchronized detector arrays 13, 14.

[0084] Figure 2 schematically illustrates a single-detector array configuration, wherein a processor computes the intensity and g^2\0) images and through a comparison of these images detects the coherent sources of light .

[0085] More in detail, in use, the light is inputted in a camera optics 21, the light reaches a detector array 22, and a processor 23 computes intensity image and image (block 24) based on pixel readings from the detector array 22.

[0086] In the following, operation of the systems based on the two-detector arrays configuration and on the single-detector array configuration will be described in detail . In case of two synchronized detectors, each corresponding pixel of the two detectors, i . e . two pixels which are both conjugate to the same point in object space, will be time-synchronized by a coincidence counter in order to build the correlation function. Detector (pixel) 1 measures intensity / 1, while detector (pixel) 2 measures / 2, both splitted by the original source through a beam splitter, and can be computed on each pixel . The ensemble averaging can be evaluated as a time average over different acquired pictures, with an overall acquisition speed granted by the shutter frequency or the detector dead time .

[0087] The resulting output of the device will be two pictures :

[0088] - a picture of intensities, given by the average of the sum of the two pixel arrays readings,

[0089] - a picture of the computed for each pixel .

[0090] Each pixel on detector 1 measures the same focused wavefront as its time-synchronized pixel on detector 2, hence rt= r2= r, as specified in Eq. 4.

[0091] An example for the intensity and results obtained by the system based on the synchronized two-detector arrays configuration is shown in Figure 3 for four different light sources .

[0092] In the example shown in Figure 3, the single readings of detectors 1 and 2 are called (a) and (b) , respectively. These readings are sampled randomly in time and are time- synchronized. The operations made with this data are shown in the picture . The intensity is obtained by averaging over time the sum of the two detectors readings, while the by averaging over time the intensity readings product . The resulting result highlights that the central signal is a coherent source, since its intensity is nonzero while its

[0093] For clarity, the of the background is set equal to zero in these examples . This is an ideal result when the background intensity reaching the detector is also zero. In space, the background is filled by the cosmic radiation, which can be compared to a thermal source . Therefore, depending on the acquisition method, the of the background can span between different non-zero values . This result does not affect the working principle of the device, which is based on the comparison between the intensity and the image .

[0094] In case of a single detector array, the autocorrelation can be computed on the single pixel when possible (detector dead time lower than the correlation time of the light emission) . When this is not possible, it can be computed between neighbouring pixels that are time-synchronized. This latter configuration gives useful results only if the coherence area of the imaged source is larger than the pixel size, therefore :

[0095] An example of results obtained by the single-detector configuration is shown in Figure 4 , wherein a+1identifies a neighbouring pixel or the same pixel a, depending on the detector performance .

[0096] These two devices' examples can be built through different detectors families and different data processing approaches . In case of intensity statistics, the detectors provide intensity fluctuations representative of photons stream fluctuations (high efficiency detectors) . The processor adequately samples the signals and performs a correlation product . For a single detector, the shutter speed is smaller than the correlation time of chaotic / thermal sources to obtain the desired results . For the 2x detectors configuration also the time-synchronization jitter is smaller than the correlation time .

[0097] In case of photon statistics, the detectors provides photon counting capability including time tagging. The processor builds up a statistics of photon events . The timesynchronization jitter of the detectors is smaller than the correlation time of chaotic / thermal sources to obtain correct results . Example of overlapping sources

[0098] The resulting plot can discern between the coherent and thermal / chaotic source even when these sources overlap on the detector image . Practical examples of this situation are signals sent by a satellite along the same line of sight of a star or other thermal / chaotic sources .

[0099] The result can be computed analytically through the classical equations at τ = 0. The intensity read on the pixel will be a sum of the two intensities lcand IT, respectively for the coherent and thermal sources .

[0100] The plot is expected to show a result like the one reported in Figure 5 that shows a comparison between the intensity distribution and the plots for two overlapping sources, namely a large chaotic source and a smaller coherent one . The higher the intensity lcwith respect to IT, the higher the contrast on the plot .

[0101] Single photon source detection

[0102] Under certain conditions, the device is also able to detect and discern single photon sources . When dealing with single photon Fock states, a single photon counting detector is required. Moreover, a quantum treatment of the equations is used.

[0103] The classical predictions of may not hold when dealing with purely quantum light states, such as Fock (number) states that yield a value in the range Using the quantum formalism, a coherent source like a laser described classically by a travelling wave as per can be written in terms of photon number (Fock state) of the coherent state :

[0104] When projected on the Fock state \ri) , the statistical distribution of emitting a certain number of photons is obtained : which is a Poissonian distribution. The key feature of the coherent state is that the variance is equal to the statistical average Δn2= (n) . This feature can be measured through a photon counting detector. The variance defines the so-colled shot noise of a laser source .

[0105] The photon number statistics can be sampled through the measurement of the In the quantum formalism, the function can be written using the normal ordering and the number operator N :

[0106] Experimentally, a single photon source can be detected with a single photon counting detector and using the configuration described in Figure 1. Each synchronized couple of pixels on the two detectors define an Hanbury Brown-Twiss interferometer, from which the for single photons sources can be extracted.

[0107] Figure 6 shows an example of image for a series of sources : a coherent source (center) , two chaotic sources (top) , and a quantum single-photon source (not visible) .

[0108] The invention can be exploited whenever the establishment of very accurate links is required and the signal is extremely weak or with unfavourable SNR. The reason could be in the weakness of the source (for short range links) or in the huge distance covered by the signal and consequent diffraction losses (e .g. , long range links, deep space links, etc . ) .

[0109] Applicable domains, products and solutions for the present invention include :

[0110] • interception by direct method of coherent signals (SIGINT) , also in absence of encoded information;

[0111] • Pointing-Acquisition-Tracking subsystem for Quantum Free Space Optical (FSO) Communications using very weak beacons (down to Photon Counting regime) ;

[0112] • Large Field-of-View Single Photon Source sensors for quantum communication and cryptography, quantum metrology, quantum sensing;

[0113] • Deep Space Optical Communications (Mars distance and beyond) ;

[0114] • acquisition of large constellations (millions of km) .

[0115] It is worth noting that recent advances in the field of Deep Space Optical Communications (DSOC) are based on the development of superconducting cryogenically cooled single photon detectors able to collect very faint signals (down to photon counting regime) and extract the information, if any, encoded in some kind of modulation. This is a different approach to signal detection. The approach according to the present invention is based on the nature of light used for the transmission and not on the encoded information.

[0116] Further industrial applications in different domains (e . g. , microscopy, security) can be realistically envisaged.

[0117] The innovative features and technical advantages of the present invention are immediately clear from the foregoing.

[0118] In particular, it is worth noting the following features of the present invention:

[0119] • an optical imaging system to acquire the Field of View (FoV) ; • according to an embodiment of the invention, an optical splitter can be conveniently used to create two instances of the same image on two detector arrays that are time- synchronized through time-to-digital converting electronics;

[0120] • a single or two detector arrays designed to acquire photons, conveniently with proper time synchronization; and

[0121] • a processor configured to compute image statistics; according to the operative mode, it can compute statistics of photon counts or perform correlation analysis of intensity fluctuations .

[0122] In all cases, the solution according to invention possesses the capability to perform spatial discrimination of the statistical properties of the sources simultaneously present in a single large-FoV image, identifying the position in space of a specific signature.

[0123] In general, the present invention concerns :

[0124] • a device for spatial discrimination of a faint coherent source from other thermal and chaotic sources found in space, like stars or other radiating bodies, simultaneously present in a single large-FoV image, analysing the statistics of intensity fluctuations;

[0125] • the above device wherein the statistics is analysed on photon counts;

[0126] • the above device wherein the image is split up into two identical and synchronized images to exploit the full resolution

[0127] • a device for imaging faint artificial light sources with a specific statistical signature, providing instantaneously a spatial mapping of their distribution;

[0128] • the above device with the extension to very long distances, up to the boundaries of the Solar System;

[0129] • a device for the identification of a quantum communication channel (based on single photon transmission) over a large FOV, and with respect to other classical sources like lasers and chaotic lights present in the same FOV.

[0130] In conclusion, it is clear that numerous modifications and variants can be made to the present invention, all falling within the scope of the invention, as defined in the appended claims .

Claims

CLAIMS1. System for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources in a given field of view, comprising:• an optical imaging system (11, 21) designed to collect incoming light from the given field of view; and• a detector assembly (13, 14, 22) designed to receive the incoming light from the optical imaging system (11, 21 ) and to detect intensity of the light impinging on the detector assembly ( 13, 14, 22) ; characterized by further comprising a processor ( 15, 23) configured to:• compute a light intensity image based on the intensities of the light detected by the detector assembly (13, 14, 22) ;• compute an image related to the second order correlation function of the light by determining the second order correlation function of the light based on the intensities of the light detected by the detector assembly (13, 14, 22) ; and• detect the light signal emitted by the coherent light source based on the light intensity image and the image related to the second order correlation function of the light .

2. The system of claim 1, wherein the processor (15, 23) is configured to detect the light signal by comparing the light intensity image and the image related to the second order correlation function of the light .

3. The system according to claim 1 or 2, wherein:• the detector assembly includes two time-synchronized detector arrays ( 13, 14 ) ;• the system further comprises an optical splitter (12) configured to split up the incoming light from the optical imaging system ( 11, 21) into two beams reaching, each, a respective time-synchronized detector array (13, 14 ) ;• the light intensity image is computed by averaging over time the sum of the intensities of corresponding pixels on the two time-synchronized detector arrays (13, 14) ; and• the image related to the second order correlation function of the light is computed by averaging over time the product of the intensities of the corresponding pixels on the two time-synchronized detector arrays (13, 14) .

4. The system of claim 3, wherein the two time- synchronized detector arrays (13, 14 ) are time-synchronized through time-to-digital converting electronics .

5. The system according to claim 3 or 4, wherein the two time-synchronized detector arrays (13, 14 ) are two distinct detector arrays or two disjoint parts of one and the same detector array device .

6. The system according to claim 1 or 2, wherein:• the detector assembly includes a single detector array (22) ;• the light intensity image is computed based on an intensity autocorrelation computation for each pixel on the single detector array (22 ) or by averaging over time the sum of the intensities of neighboring, time-synchronized pixels on the single detector array (22) ; and• the image related to the second order correlation function of the light is computed by averaging over time the product of the intensity of each pixel by itself or by a neighboring, time-synchronized pixel .

7. The system according to any claim 1-6, wherein the optical imaging system (11, 21) includes a telescope .

8. Electronic processing unit (15, 23) for detecting and tracking a light signal from a coherent light source among thermal and / or chaotic sources in a given field of view, designed to be coupled to :• an optical imaging system (11, 21) designed to collect incoming light from the given field of view; and• a detector assembly ( 13, 14, 22) designed to receivethe incoming light from the optical imaging system (11, 21) and to detect intensity of the light impinging on the detector assembly (13, 14, 22) ; characterized by being configured to:• compute a light intensity image based on the intensities of the light detected by the detector assembly (13, 14, 22) ;• compute an image related to the second order correlation function of the light by determining the second order correlation function of the light based on the intensities of the light detected by the detector assembly (13, 14, 22) ; and• detect the light signal emitted by the coherent light source based on the light intensity image and the image related to the second order correlation function of the light .

9. The electronic processing unit of claim 8, configured to detect the light signal by comparing the light intensity image and the image related to the second order correlation function of the light .

10. The electronic processing unit according to claim 8 or 9, designed to be coupled to:• a detector assembly that includes two time- synchronized detector arrays (13, 14) ; and• an optical splitter (12) configured to split up the incoming light from the optical imaging system (11, 21) into two beams reaching, each, a respective time-synchronized detector array (13, 14 ) ; the electronic processing unit (15) being configured to compute :• the light intensity image by averaging over time the sum of the intensities of corresponding pixels on the two time-synchronized detector arrays (13, 14 ) ; and• the image related to the second order correlation function of the light by averaging over time the product ofthe intensities of the corresponding pixels on the two time- synchronized detector arrays (13, 14) .

11. The electronic processing unit according to claim 8 or 9, designed to be coupled to:• a detector assembly that includes a single detector array (22 ) ; the electronic processing unit (23) being configured to compute :• the light intensity image based on an intensity autocorrelation computation for each pixel on the single detector array (22 ) or by averaging over time the sum of the intensities of neighboring, time-synchronized pixels on the single detector array (22) ; and• the image related to the second order correlation function of the light by averaging over time the product of the intensity of each pixel by itself or by a neighboring, time-synchronized pixel .

12. Computer program product comprising one or more software code portions loadable on, and executable by, a processor, and such that to cause, when loaded, the processor to become configured as the electronic processing unit as claimed in any claim 8-11.

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