EP0505235A1 - Wide-band intercorrelation method and apparatus - Google Patents

Abstract

Two-dimensional optical architecture allowing intercorrelation of two instantaneously-wide-band time signals. <??>The beam originating from a single-mode laser makes it possible to obtain two carriers (W1, W2) for signals R(t) and S(t) by means, for example, of integrated optical modulators (mod1, mod2). These two carriers have orthogonal polarisations and are distributed in a two-dimensional structure comprising spatial light modulators as well as polarisation separator elements. PxP independent channels (C1 to Cn) are thus produced. Their detection on a matrix of photodetectors (mod1 to modn) makes it possible to obtain, on each of them, the intercorrelation signal, for various delays of the signals R(t) and S(t). <??>Applications: correlation of wide-band electrical signals. <IMAGE>

Description

The invention relates to a broadband intercorrelation method and to a device implementing this method.

It is applicable in particular in any system such as a system allowing the correlation of wideband electrical signals.

It is known that to perform such a correlation, signals R (t) and S (t) to be correlated are applied to the two ends of a delay line comprising N coupling points (typically N = 128) regularly spaced (FIG. 1 ). The spacing between two adjacent points corresponds to a time τ of propagation of the signal S (t). Thus at each of these points, of index p, we can take a fraction of the signal S (t-pτ) + R [t- (N-p) τ]. Each coupling point is followed by a diode which realizes the squared of the signal. A bandpass filter then makes it possible to isolate, on each output, the product S (t-pτ) R [t- (N-p) τ] which is then integrated. The signals S (t) and R (t) can have an instantaneous band which can cover a band of 20 GHz. Such a band requires a sampling such that τ is worth approximately 12.5 ps. Currently the number of increments limited to N = 128 only allows an intercorrelation over 1.6 ns, to the detriment of a good determination of the frequencies contained in the signal.

The method and the device of the invention make it possible to operate with a fineness of increment authorizing 20 GHz of bandwidth, the number of these increments being able to be brought to N = 1,024 or 2,048. This embodiment uses an optical architecture based on two-dimensional space light modulators

The invention therefore relates to a method for correlating electrical signals characterized in that a first and a second light wave are modulated respectively by a first electrical signal and by a second electrical signal, the two waves being polarized differently and made collinear in a single light beam; then the light beam is separated into at least two channels and in each channel a delay is introduced on one polarization with respect to the other; finally, the channels which provide the cross-correlation function are detected and the channel which provides the maximum of the cross-correlation function makes it possible to detect the delay existing between the first and the second electrical signal.

• un premier et un deuxième modulateur électrooptique (mod1, mod2) recevant respectivement un premier et un deuxième signal électrique de commande ainsi qu'une onde lumineuse chacun, chaque onde lumineuse étant modulée par un signal électrique et étant polarisée selon une direction de polarisation qui lui est propre ;
• un dispositif de couplage (PBS) superposant les deux ondes modulées en un seul faisceau lumineux ;
• un dispositif de séparation de faisceau séparant le faisceau lumineux en au moins deux canaux ;
• un dispositif de rotation de polarisation commutable (M1, Mk) associé à chaque canal ;
• un dispositif de séparation de polarisation (B1 à Bk) associé à chaque dispositif de rotation de polarisation et transmettant une polarisation sur un premier trajet et l'autre polarisation sur un deuxième trajet ;
• un dispositif de recombinaison des premier et deuxième trajets vers des photodétecteurs.

The invention also relates to a device for correlating electrical signals, characterized in that it comprises at least:

• a first and a second electrooptical modulator (mod1, mod2) receiving respectively a first and a second electrical control signal as well as a light wave each, each light wave being modulated by an electrical signal and being polarized according to a direction of polarization which is clean ;
• a coupling device (PBS) superimposing the two modulated waves in a single light beam;
• a beam splitting device separating the light beam into at least two channels;
• a switchable polarization rotation device (M1, Mk) associated with each channel;
• a polarization separation device (B1 to Bk) associated with each polarization rotation device and transmitting one polarization on a first path and the other polarization on a second path;
• a device for recombining the first and second paths to photodetectors.

• la figure 1, un dispositif à ligne à retard selon la technique connue ;
• la figure 2, un exemple de réalisation général d'un dispositif de corrélation selon l'invention ;
• la figure 3, un exemple de réalisation détaillé du dispositif de corrélation selon l'invention ;
• la figure 4, un exemple de réalisation plus détaillé du dispositif de l'invention ;
• les figures 5a, 5b, des exemples de réalisation des circuits réalisant les retards ;
• la figure 6, un diagramme de fonctionnement du dispositif de la figure 4 ;
• la figure 7, un diagramme de fonctionnement d'un miroir à conjugaison de phase de la figure 4 ;
• les figures 8 et 9 des variantes de réalisation permettant d'introduire des retards importants ;
• les figures 10 et 11, une variante de réalisation incorporant un translateur de fréquence ;
• la figure 12, une variante de réalisation dans laquelle la conjugaison de phase se fait dans un dispositif à mélange quatre ondes ;
• la figure 13, une variante de réalisation ne prévoyant qu'un seul photodétecteur ;
• la figure 14, une autre variante de création de retards ;
• la figure 15, une autre variante du dispositif de la figure 14.

The various objects and characteristics of the invention will appear more clearly in the description which follows made by way of example and in the appended figures which represent:

• Figure 1, a delay line device according to the known technique;
• Figure 2, a general embodiment of a correlation device according to the invention;
• Figure 3, a detailed embodiment of the correlation device according to the invention;
• Figure 4, a more detailed embodiment of the device of the invention;
• FIGS. 5a, 5b, exemplary embodiments of the circuits producing the delays;
• Figure 6, an operating diagram of the device of Figure 4;
• FIG. 7, an operating diagram of a phase conjugation mirror of FIG. 4;
• Figures 8 and 9 alternative embodiments for introducing significant delays;
• Figures 10 and 11, an alternative embodiment incorporating a frequency translator;
• FIG. 12, an alternative embodiment in which the phase conjugation takes place in a four-wave mixing device;
• Figure 13, an alternative embodiment providing only one photodetector;
• FIG. 14, another variant for creating delays;
• FIG. 15, another variant of the device of FIG. 14.

Referring to Figure 2, we will first describe a simplified embodiment of the invention. This device comprises two electro-optical modulators mod1 and mod2 each receiving an optical wave W1 and W2 and each controlled by a electrical modulation signal R (t) for the mod1 modulator and S (t) for the mod2 modulator. The electrical signals S (t) and R (t) are the electrical signals that we want to compare by correlation.

The two modulated optical waves are polarized in different directions (perpendicular for example) and are made collinear in the form of a beam W3.

The beam W3 is separated into several channels C1 to Cn, each channel therefore comprising polarized light ↑ (that modulated by R (t)) and polarized light ⊙ (that modulated by S (t)).

The different channels are coupled to a delay creation circuit CR which introduces on each channel different lengths of paths for the two polarizations of the channel. The channels are then coupled to photodetectors mpd1 to mpdn. The photodetector which detects the maximum intensity corresponds to the channel which introduces a delay making it possible to compensate for the delay existing between the electrical signals R (t) and S (t).

It can therefore be seen that the method of the invention consists in modulating two light waves, which are preferably of the same wavelength, using the two electrical signals which it is desired to correlate.

The two waves being polarized differently (perpendicularly for example) either before modulation or after modulation, they are made collinear then the beam obtained is separated into several channels (at least 2). Then we introduce into each channel a determined and known delay which affects a polarization with respect to the other. Finally, all the channels which provide the cross-correlation function are detected and the channel which provides the maximum of the cross-correlation function makes it possible to identify the delay existing between the input signals R (t) and S (t).

Figure 3 shows an embodiment of the device of the invention. It includes modulators mod1 and mod2. A CF coupler combining the two modulated and perpendicularly polarized waves. A beam splitter S1 separates the beam obtained into several channels. Switchable polarization rotation devices R1.1 to R1.n rotates the polarization directions contained in the different channels at 90 °. A first polarizer b1 reflects a determined polarization of each channel and transmits the other. Then the reflected polarization is made collinear by ml, m2 mirrors and a coupler, with the transmitted polarization. One polarization is therefore delayed with respect to the other in each channel. A second set of polarization rotation devices R2.1 to R2.n of a polarizer G2 and of mirrors m3, m4 perform a similar function. Then the different channels are coupled to photodetectors mpd1 to mpdn.

As can be seen in FIG. 3, if we consider the polarizations of the channel C1, one of the polarizations is delayed by the first then by the second delay circuit. If each circuit brings a delay of value t, the two polarizations are liable to be delayed by 2t when arriving at the photodetector MPD1, assuming that they were synchronous at the start, that is to say that the signals R ( t) and S (t) were in phase.

For channel C2, one of the polarizations is delayed in the first delay circuit while it is the other polarization which is delayed in the second delay circuit. The phase relationship between the two polarizations is therefore preserved

It can be seen that by increasing the number of delay circuits, the number of possible delays between the polarizations is increased. In this way, the possibilities of delay compensations to be detected are increased.

Referring to Figure 4, we will now describe a detailed embodiment of the invention.

The polarized beam from a laser L₁ passes through an isolator I and is then separated into two by means of a beam splitter, for example, a CF fiber coupler. A part passes through a modulator of intensity mod1 excited by the signal R (t). There is thus at the output of this modulator an optical carrier whose intensity is modulated by R (t). Similarly, the other part of the beam is modulated in mod2 by S (t). Mod1 and mod2 are for example optical modulators integrated on LiNbO₃ or semiconductor. The performance of such modulators indeed allows bandwidths extending from 0 to 20 GHz and a dynamic range compatible with broadband signals to be processed. The polarizations of the two beams thus modulated are made orthogonal (Figure 4). However, the polarization of the beams can also be done before modulation. Then the two beams are superimposed by means of a coupler such as a polarization splitter cube PBS and then extended by an afocal system BE so as to cover the surface of a spatial light modulator M₁. This modulator is for example a liquid crystal cell comprising P (P ∼ 1024) pixels. Each pixel of the modulator M₁ determines a light channel. The spatial modulator M₁ is arranged so that it cuts P parallel paths in the extended laser beam. On each pixel, the polarization of the incident light is rotated from 0 to 90 ° depending on the applied voltage. It can be noted that only two states (0 and 90 °) are necessary and that therefore, ferroelectric liquid crystal cells are well suited.

A set of polarization splitter cubes and total reflection prisms is placed at the output of the modulator. As can be seen in FIG. 5a, the choice of the state of polarization on the modulator M₁ makes it possible to choose, for each channel (pixel) the path followed by each of the two polarizations.

The position of the prism P₁ (FIG. 5a) is adjusted so that the difference in optical path between the two orthogonal polarizations of a channel corresponds to a delay of value τ. The assembly constituted by the spatial modulator M₁, the polarization splitter cube B1 and the reflection devices P1 therefore constitute a delay creation system. Several delay creation systems thus congruent are arranged in series. As an example in the second delay creation system, the position of P₂ is chosen to provide a delay 2τ; that of P _i , for a delay 2 ^i-1 τ. With the delay creation system, we thus have 2 ^k possible delay values for the two polarizations (O, τ, 2τ, ..., 2 ^k τ).

The different modulators M₁ to Mk are identical and are aligned so that the different pixels of the modulators are aligned on the optical paths of the different channels.

If the signal carrier R (t) is polarized ↑ at the input of the delay structure. That of S (t) is then polarized ⊙. Thus if we are interested in a particular channel, we see that the paths followed by the two polarizations are necessarily complementary to each other from the delay point of view (Figure 5). Indeed on each spatial modulator M _i , the two orthogonal polarizations ↑ and ⊙, passing through the same pixel, undergo the same polarization rotation value (⊙ remains ⊙ when ↑ remains ↑. On the other hand ⊙ becomes ↑ when ↑ is changed in ⊙). We thus see in the example of FIG. 6 of a structure providing a maximum delay of 15 τ (for example) that the carrier R (t) is delayed 11 τ while that of S (t) is delayed of 4 τ. More generally, for a device providing a maximum delay Nτ, when on a channel the carrier of R (t) undergoes a delay pτ then on the same channel, the carrier S (t) undergoes the complementary delay (Np) τ.

When the two carriers of R (t) and S (t), divided into P parallel channels have crossed the delay structure, they pass through a MA spatial modulator controlling the intensity transmitted a _p on each of the channels. This two-dimensional modulator can for example be a liquid crystal cell placed between polarizers.

• dans la région B ; le faisceau incident 1 donne naissance au faisceau 2, du fait de diffusion sur des microdéfauts du cristal.
• le faisceau 2, deux fois réfléchis par le dièdre formé par les deux faces du cristal, fournit 3'.
• par ailleurs, dans la région A, le faisceau 1 donne naissance à 2', également par diffusion. 2' suit le trajet inverse de 3' et après réflexion sur le dièdre fournit le faisceau 3.
• l'interaction 1, 2, 3 dans la région B, ainsi que celle de 1 2' et 3' dans la région A, donne naissance au faisceau 4 par mélange à quatre ondes. Ce faisceau est en outre la réplique, conjuguée en phase de 1.

An optical system L then ensures the focusing of all these parallel channels on a set of two phase conjugate mirrors MCP₁ and MCP₂. These two mirrors are preceded by a cube to separate CS2 from polarization. MCP₁ is illuminated by polarization ⊙ while MCP reçoit receives only polarized beams ↑. MCP1 and MCP2 can be, for example, photorefractive crystals of barium titanate BaTiO₃. Each phase conjugation mirror is said to be "pumped self" because it is the incident waves alone which create the photoinduced networks from which the conjugate waves will be created. In the simple case of a single incident wave, the creation of a conjugate wave is shown diagrammatically in FIG. 7 and is described in the document by J. FEINBERG, Optics Letters, 2, 486, 1982:

• in region B; the incident beam 1 gives rise to the beam 2, due to scattering on microdefects of the crystal.
• the beam 2, twice reflected by the dihedral formed by the two faces of the crystal, provides 3 '.
• moreover, in region A, the beam 1 gives rise to 2 ′, also by diffusion. 2 'follows the reverse path of 3' and after reflection on the dihedral provides the beam 3.
• the interaction 1, 2, 3 in region B, as well as that of 1 2 'and 3' in region A, gives rise to beam 4 by mixing with four waves. This beam is also the replica, conjugated in phase 1.

For each polarization, the set of channels coming from the MA modulator is thus conjugated in phase. The use of BaTiO₃ makes it necessary to prepare the polarizations on MCP₁ and MCP₂, because this crystal is only sensitive to polarization.

After conjugation, the two polarizations are superimposed again by the separator cube CS2. The beam thus reconstituted, having the same characteristics as the incident beam but propagating in the opposite direction, again crosses the various delay creation systems. A part, extracted by the semi-reflecting plate (LS) is directed towards a phase modulator MP comprising the same number of pixels as there are channels and that there are pixels in the modulators M1 to Mk. passage in MP, the P channels cut by the modulators are detected by an MPD matrix of P photodetectors. In addition, a polarizer P has been placed in front of MPD, oriented at 45 ° from the ⊙ and ↑ directions and which ensures its recombination on a single polarization direction. On each channel and therefore on each PDM detector (FIG. 4), it is thus possible to detect the carriers of the signals R (t) and S (t) in a coherent manner.

où :
   . λ longueur d'onde de L₁
   Kp ε

   0 < r_p < 1Thanks to the afocal BE system, it is an almost flat wave which crosses the delay structure towards MCP₁ and MCP₂. Phase conjugation provides exact compensation for all phase faults encountered by this wave. It is therefore still a plane wave which emerges from the structure after double passage and is reflected on (LS). This conjugation, however, does not compensate for delays which are of another order of magnitude. Indeed, for each channel we can write: $${\text{pcτ = Kp .λ + r}}_{\text{p}}\text{λ}$$

or :
. λ wavelength of L₁
Kp ε

0 <r _p <1

où cette relation rend compte d'une moyenne sur le temps de réponse du détecteur. Dès lors que la longueur de cohérence de L₁ est plus importante que la plus importante différence de marche entre les porteuses R(t) et S(t) on a :

It is the fraction r _p λ which will be compensated by phase conjugation. Furthermore, taking into account the respective values of λ and t, we have Kp >> r _p . The photocurrent provided by the detector corresponding to pτ is thus:

where this relation gives an average over the response time of the detector. As soon as the coherence length of L₁ is greater than the largest path difference between the carriers R (t) and S (t) we have:

• le modulateur d'amplitude de MA en contrôlant l'amplitude permet de compenser les différences de coefficient de transmission sur les différents canaux (le nombre de dioptre rencontré n'est pas le même quelle que soit la valeur du retard) ainsi que les différences de coefficient de réflexion des MCP₁ et MCP₂(en fonction de l'incidence). Ce modulateur peut être une cellule à cristal liquide réalisée à l'aide d'un cristal liquide nématique twisté entre polariseurs.
• MP agit sur les phases respectives des deux polarisations recombinées sur P. On agit sur la phase de ⊙, par exemple, en fonction de la tension appliquée sur le pixel, sans affecter ↑.

The double pass doubles the value of the delays fixed by the position of the prisms P _i (τ → 2τ). Strictly speaking, we do not consider S₁ (t-2pτ) but S₁ (t-2K _p λ / C). However, we can neglect the influence of phase conjugation on the value of the delays assigned to R (t) and S (t). In addition, MCP₁ and MCP₂, even if they compensate for phase differences greater than 2π (r _p > 1) these do not exceed a few units and do not affect the value p.τ. The spatial modulators of amplitude MA and of phase MP make it possible to obtain, in the absence of modulations R (t) and S (t), a level of signal detected uniform over the whole of MPD:

• the AM amplitude modulator by controlling the amplitude makes it possible to compensate for the differences in transmission coefficient on the different channels (the number of diopter encountered is not the same whatever the value of the delay) as well as the differences in coefficient of reflection of MCP₁ and MCP₂ (depending on the incidence). This modulator can be a liquid crystal cell produced using a nematic liquid crystal twisted between polarizers.
• MP acts on the respective phases of the two recombined polarizations on P. One acts on the phase of ⊙, for example, according to the tension applied to the pixel, without affecting ↑.

This phase modulator therefore makes it possible to control only the amplitude of the product R (t) and S (t).

Each MPD photodetector is followed by an integrator. The integration time T is necessarily greater than the maximum delay 2Nτ.

After integration, each channel provides a signal C _p (T) such that:

qui, à une différence d'origine près est bien le signal de corrélation recherché. On réalise donc bien la fonction d'intercorrélation désirée sur P canaux, en tirant parti du parallélisme de l'architecture optique à deux dimensions. La détermination du signal de plus forte amplitude fournit le centre de la fonction d'intercorrélation qui détermine la valeur du retard existant entre les deux signaux électriques R(t) et S(t).The term R (T) + S (T) is common to all channels. These differ only in

which, with a difference of origin, is the desired correlation signal. The desired cross-correlation function on P channels is therefore well realized, taking advantage of the parallelism of the two-dimensional optical architecture. The determination of the signal of greatest amplitude provides the center of the intercorrelation function which determines the value of the delay existing between the two electrical signals R (t) and S (t).

• L₁ : laser état solide, pompé diode monomode longitudinal, quelques 100 mW, λ = 1,3 µm - 1,5 µm
• mod₁, mod₂ :
  • . modulateurs optiques intégrés sur LiNbO₃
  • . large bande 0 → 20 GHz
  • . profondeur de modulation : 80 à 100 %
  • . pertes d'insertion : ∼ 6 dB
• M_i :
  • . cellules à cristal liquide nématique twisté ou ferroélectrique 40 x 40 mm²
  • . 32 x 32 pixels commandés individuellement
  • . taux d'extinction entre polariseurs croisés : 1 : 1000 (compatible avec la dynamique requise pour la corrélation)
• MA :
  • . cellule identique aux cellules M_i précédentes mais placée entre polariseurs croisés
• MP :
  • . cellule à cristal liquide nématique parallèle. Les axes de la cellule coïncident avec les polarisations ⊙ et ↑
• MPD :
  • . matrice de photodiodes rapides + intégrateur
  • . suivant les temps d'intégration nécessaire, l'ensemble photodiodes + intégrateur peut être remplacé par un détecteur CCD
• Valeur des retards :
  • . pour une bande passante de 20 GHz → 2π = 25 ps.
  • . 1024 incréments sont nécessaires à une corrélation de dynamique 30 dB. L'architecture comprend donc 10 modulateurs M_i.
  • . pour les incréments les plus faibles (25 ps → 1 mm), la figure 5b donne un exemple de réalisation : une lame Lp d'épaisseur e fournit la différence de marche entre les deux polarisations (τ = en/c)
  • . ainsi lorsque τ = 12,5 ps, e = 2,5 mm. De même pour 2τ, 4τ et 8τ. Les épaisseurs respectives sont 5 mm, 10 mm et 20 mm. A partir de 16 τ = 200 ps et jusqu'à 128 τ = 1600 ps le schéma de la figure 5a reste utili sable. Pour cette dernière valeur, la distance entre P₇ et B₇ est de 0,4 m. Pour les deux valeurs de retard les plus importantes, 256 τ et 512 τ, il faut réaliser le schéma de la figure 8 qui permet un repliement des trajets optiques. Pour une matrice de 32 x 32 mm, L ∼ 300 mm.
    256 τ D = 5 cm
    512 τ D = 10 cm
    La longueur totale du dispositif est alors de l'ordre de 1,2 m (700 mm pour les 8 premiers étages, 300 mm pour les deux derniers), sa largeur de 0,3mm. Son épaisseur peut ne pas excéder 40 mm, autorisant ainsi un repliement de l'ensemble sur deux couches superposées (volume) total ∼ 12 litres). On peut noter que pour un nombre de prises de 128 (au lieu de 1024) les dimensions sont de 400 x 600 x 40 mm³.

As an example of an embodiment, the device of FIG. 4 is produced with components which may have the following characteristics:

• L₁: solid state laser, pumped longitudinal single mode diode, some 100 mW, λ = 1.3 µm - 1.5 µm
• mod₁, mod₂:
  • . optical modulators integrated on LiNbO₃
  • . broadband 0 → 20 GHz
  • . modulation depth: 80 to 100%
  • . insertion losses: ∼ 6 dB
• M _i :
  • . twisted or ferroelectric nematic liquid crystal cells 40 x 40 mm²
  • . 32 x 32 pixels individually controlled
  • . extinction ratio between crossed polarizers: 1: 1000 (compatible with the dynamics required for correlation)
• MY :
  • . cell identical to the previous M _i cells but placed between crossed polarizers
• MP:
  • . parallel nematic liquid crystal cell. The axes of the cell coincide with the polarizations ⊙ and ↑
• PDM:
  • . matrix of fast photodiodes + integrator
  • . depending on the integration times required, the photodiodes + integrator assembly can be replaced by a CCD detector
• Value of delays:
  • . for a bandwidth of 20 GHz → 2π = 25 ps.
  • . 1024 increments are necessary for a 30 dB dynamic correlation. The architecture therefore includes 10 modulators M _i .
  • . for the smallest increments (25 ps → 1 mm), Figure 5b gives an example of an embodiment: a blade Lp of thickness e provides the path difference between the two polarizations (τ = en / c)
  • . so when τ = 12.5 ps, e = 2.5 mm. Likewise for 2τ, 4τ and 8τ. The respective thicknesses are 5 mm, 10 mm and 20 mm. From 16 τ = 200 ps and up to 128 τ = 1600 ps the diagram in Figure 5a remains useful. For this latter value, the distance between P₇ and B₇ is 0.4 m. For the two most important delay values, 256 τ and 512 τ, it is necessary to carry out the diagram of figure 8 which allows a folding of the optical paths. For a 32 x 32 mm matrix, L ∼ 300 mm.
    256 τ D = 5 cm
    512 τ D = 10 cm
    The total length of the device is then of the order of 1.2 m (700 mm for the first 8 stages, 300 mm for the last two), its width of 0.3 mm. Its thickness may not exceed 40 mm, thus allowing folding of the assembly on two superimposed layers (volume) total ∼ 12 liters). It can be noted that for a number of sockets of 128 (instead of 1024) the dimensions are 400 x 600 x 40 mm³.

• Ce dispositif permet la corrélation de signaux large bande.
• Les signaux de corrélations sont obtenus en parallèle sur P voies.
• Un contrôle de l'amplitude de chaque voie permet la compensation de la dispersion des niveaux. Il permet en outre d'adapter la dynamique des signaux reçus à celle des photodétecteurs.
• L'architecture proposée est entièrement, et à chaque instant, totalement reconfigurable. La valeur de l'incrément de retard peut ainsi être, en permance, adaptée à la bande de signal reçu.
• Un nombre de voies supérieur à celui des retards autorise la défaillance de certains pixels sans affecter les perfor mances du système (de même pour les photodiodes).
• Les miroirs à conjugaison de phase permettent de compenser toutes les distorsions de phase introduites sur la porteuse optique, sans pour autant affecter la précision avec la quelle sont déterminés les retards hyperfréquence. Cette conjugaison permet en outre de se passer de systèmes optiques imageant les modulateurs spatiaux M_i entre eux et qui autrement seraient rendus nécessaires par la diffraction due aux pixels.

Such a device has the following advantages:

• This device allows the correlation of broadband signals.
• The correlation signals are obtained in parallel on P channels.
• A control of the amplitude of each channel allows compensation for the dispersion of the levels. It also makes it possible to adapt the dynamics of the signals received to that of the photodetectors.
• The architecture proposed is entirely, and at all times, completely reconfigurable. The value of the delay increment can thus be, permanently, adapted to the received signal band.
• A number of channels greater than that of the delays authorizes the failure of certain pixels without affecting the performance of the system (likewise for photodiodes).
• The phase conjugation mirrors make it possible to compensate for all the phase distortions introduced on the optical carrier, without however affecting the precision with which the microwave delays are determined. This conjugation also makes it possible to dispense with optical systems imagining the space modulators M _i between them and which otherwise would be made necessary by the diffraction due to the pixels.

FIG. 9 represents an alternative embodiment of the device of the invention.

In the case of a system where a minimum delay is already provided, the storage of this delay is ensured by a length of fiber L _M at the output of a modulator, mod₁ for example (FIG. 9). The rest of the architecture remains the same. The fiber L _m thus makes it possible to "center" the values of delays provided by the architecture around a value corresponding to the average range of the system.

Figure 10 shows another alternative embodiment of the device of the invention.

In this variant, the proposed architecture operates in a single pass, without phase conjugation, thanks to a frequency offset between the carriers of the signals R (t) and S (t).

où φ_p est contrôlé par M_p (ω′ = ω + 2πf)

A Trans frequency translator modifies the frequency of the laser beam passing through the modulator mod₂ (ω becomes ω + 2πf). This translation is at fixed frequency and constant level. The two carriers of R (t) and S (t) remain orthogonally polarized. After recombining, they pass through the same delay device as that described in FIG. 4. All of the channels thus cut through pass through a phase modulator MP whose operation has been described previously. It is followed by a polarizer P at 45 ° from ⊙ and ↑, then by a matrix of fast photodiodes. Each photodiode will therefore provide a photocurrent of the form:

where φ _p is controlled by M _p (ω ′ = ω + 2πf)

The phase Φ _P depends on the position of the prisms but its value cannot be fixed a priori since the precision required on their positioning is of the order of magnitude of λ, wavelength of L₁.

A bandpass filter F (Figures 10 and 11) centered on f allows the term product to be isolated. This requires that f> 3B / 2 where B is the spectral range of R (t) and S (t). One thus avoids the influence of the constant term appearing during integration.

On each channel, during a first calibration phase, we choose the tels _p such that:

This solution however has the drawback of remaining sensitive to vibrations, unlike that incorporating phase conjugation. The combination of frequency translation and phase conjugation as achieved in Figure 4 overcomes this problem.

FIG. 12 represents another alternative embodiment in which the self-pumped phase conjugation mirrors MPC1 and MPC2 are replaced by phase conjugation mirrors resulting from a four-wave interaction as shown in FIG. 12. The photorefractive material can stay the same (BaTiO₃). The increased complexity of the assembly is compensated by a gain in reflectivity of the mirror. It can indeed be greater than 1, the amplification of the conjugate wave being provided by the pump beams (also from L₁).

FIG. 13 represents another alternative embodiment in which the architecture of the device is identical to that proposed in FIG. 4, except as regards the detection matrix. In addition, on the modulator mod₁ is applied not the signal R (t) but the signal R (-t) (after storage of the signal over the observation time).

Ce dernier terme peut s'écrire encore : $${\text{A(t) =Σ}}_{\text{i}}{\text{S₁(t}}_{\text{i}}{\text{-t) S₂(t}}_{\text{i}}{\text{-2Nτ+t) cos φ}}_{\text{p}}$$

After double passage in the delay device, modulation by MP and recombination of the polarizations by P, all the channels are summed by means of an optic (L₂) on a single photodetector PD. This photodiode then delivers a photocurrent of the form:

This last term can still be written: $${\text{A (t) = Σ}}_{\text{i}}{\text{S₁ (t}}_{\text{i}}{\text{-t) S₂ (t}}_{\text{i}}{\text{-2Nτ + t) cos φ}}_{\text{p}}$$

There is thus at each instant t the correlation product of the signals R (t) and S (t). In addition, each term of the sum can be assigned a weight of -1 to 1.

For signals R (t) and S (t) varying little in the interval of the observation time, C (t) varies practically as A (t), correlation product.

One can also choose, as in the variant represented in FIG. 10, to shift in frequency one of the carriers so as to be able to filter A (t).

FIG. 14 shows an alternative embodiment in which, in order to reduce the bulk, the most important delay values can be produced by means of bundles of optical fibers FB. In certain embodiments these fibers will have the same length for the different channels.

According to the alternative embodiment of FIG. 14, advantage is taken of the phase conjugation to best achieve the summation of the different channels. The MP phase modulator is deleted. The different weights necessary for the summation are assigned to the channels thanks to MA. The afocal BE system provides the summation, through the polarizer P, of all the channels on the photodiode PD.

It is obvious that the above description has been given only by way of nonlimiting example. Other variants can be envisaged without departing from the scope of the invention. The types of optical elements, such as the types of liquid crystal cells, the types of polarizers, the types of polarization splitters have been indicated only to illustrate the description.

Claims (18)

Method for intercorrelating electrical signals characterized in that a first and a second light wave are modulated respectively by a first electrical signal and by a second electrical signal, the two waves being polarized differently and made collinear in a single light beam; then the light beam is separated into at least two channels and in each channel a delay is introduced on one polarization with respect to the other; finally, the channels which provide the cross-correlation function are detected and the channel which provides the maximum of the cross-correlation function makes it possible to detect the delay existing between the first and the second electrical signal. Method according to claim 1, characterized in that the two waves are polarized perpendicularly. Method according to claim 2, characterized in that the different paths of the two waves of each channel are obtained by polarization separation and transmission of the two polarizations to two paths of different lengths, the polarization separation being further preceded or not by a rotation 90 ° of polarizations. Method according to claim 1, characterized in that the first and the second light waves have the same wavelength. Device for intercorrelating electrical signals according to the method of claim 1, characterized in that it comprises at least: a first and a second electrooptical modulators (mod1, mod2) respectively receiving a first and a second electrical control signal as well as a light wave each, each light wave being modulated by an electrical signal and being polarized in its own polarization direction; - a coupling device (PBS) superimposing the two modulated waves in a single light beam; - a beam splitting device separating the light beam into at least two channels; - a switchable polarization rotation device (M1, Mk) associated with each channel; - a polarization separation device (B1 to Bk) associated with each polarization rotation device and transmitting one polarization on a first path and the other polarization on a second path; - a device for recombining the first and second paths to photodetectors. Device according to claim 5, characterized in that the beam separation device and the polarization rotation device are the same separation and rotation device (M1 to Mk). Device according to claim 6, characterized in that the separation and rotation device (M1 to Mk) is a liquid crystal cell. Device according to claim 5, characterized in that the polarization separation device (B1 to Bk) comprises: - a first polarization separation prism transmitting a first polarization and reflecting a second polarization; - a reflection device (P1 to Pk) receiving the second reflected polarization and returning it to a second polarization separation prism placed on the path of the second polarization so that the second polarization is reflected and brought back collinearly with the first polarization thus being delayed with respect to the first polarization. Device according to claim 8, characterized in that a beam separation device, a polarization rotation device and a polarization separation device form a delay creation assembly and that several delay creation assemblies are placed in series . Device according to claim 9, characterized in that it comprises, in series with the delay creation assemblies, at least one phase conjugation mirror (MPC1, MPC2) reflecting the different polarizations according to their direction of incidence; a semi-reflecting device being located between the first set of delay creation and the modulators (mod1, mod2). Device according to claim 5, characterized in that one of said first or second paths comprises transmission devices (P1, ..., Pn) so that the polarization transmitted along this path traverses a determined path and is then brought back collinearly with the other polarization. Device according to claim 5, characterized in that one of said first or second paths comprises one or more optical fibers (FB). Device according to claim 5, characterized in that it comprises a delay circuit (Lm) placed in series with one of the modulators (mod1, mod2). Device according to claim 13, characterized in that the delay circuit (Lm) is an optical fiber. Device according to claim 5, characterized in that it comprises a frequency translator (Trans) placed in series with one of the modulators (mod1, mod2) as well as a filter (F) placed at the output of the photodetectors and filtering the information provided by the photodetectors. Device according to claim 10, characterized in that that it includes a polarization separator / recombiner device, as well as a first phase conjugation mirror (MCP1) with photorefractive crystal reflecting a first polarization and a second phase conjugation mirror (MCP2) with photorefractive crystal reflecting the second polarization . Device according to claim 10, characterized in that the phase conjugation mirror is a four-wave mixing device. Device according to claim 5, characterized in that one of the signals R (t) is inverted in time R (-t) and that it comprises a single photodetector (PD), means being provided for concentrating the light having crossed the different sets of delays to this photodetector (PD).

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