EP0603036A1 - Optical processing apparatus for electrical signals - Google Patents

Abstract

Optical processing device for electrical signals including: - an optical source (L) emitting a multiwavelength beam (B1); - a modulator (MOD) modulating this beam; - an optical fibre (F) receiving the modulated beam (B2) and variously delaying the components corresponding to the various wavelengths; - a dispersive grating (H) dispersing in different directions wavelengths contained in the modulated beam (B3); - a spatial light-modulator (SLM) controlling the optical intensity level of various directions of the dispersed beam (B4); - an optical detection system (PD) receiving the beam (B5) processed by the spatial light-modulator (SLM). Application: transverse filter - UHF signal correlator. <IMAGE>

Description

The invention relates to a device for optical processing of electrical signals and in particular a device applicable as a transverse filter or as a correlator of microwave signals.

More particularly, the invention relates to a set of fiber optic devices allowing the processing of very broadband microwave signals and in particular performing the functions of matched filter and correlator. These devices exploit the chromatic dispersion properties of optical fibers but also the possibility of inducing Bragg gratings therein permanently.

As is known in the art, a transverse filter performs the summation of samples of a signal, taken at different times, with a weighting law characteristic of the signal to be filtered. Using such a filter, one seeks to determine, for example, the date of appearance of a signal p (t), known a priori. This transient signal p (t) of finite duration T is mixed with a noise b (t) independent of p (t). It is therefore the signal x (t) = p (t) + b (t) that it is necessary to filter. Such a filter, if it maximizes the signal-to-noise ratio at time T, is said to be suitable. In the case of ideal white noise, the impulse response h (t) of the matched filter is h (t) = p (-t): when the noise is not white, this filter is no longer optimal but allows however, to determine the date of appearance of p (t) in most cases.

où |p_max| est la valeur maximale du module de p(t). A l'instant t₀, la somme y(t₀) des N+1 sorties pondérées vaut :

avec Nτ = T

avec t_k = t_o - kτThe weighting method described for example in the document J. MAX "Methods and techniques of signal processing and applications to physical measurements", Masson, 1987 is an exemplary embodiment of such a filter. As shown in Figure 1, the signal x (t) feeds a delay line consisting of N elements, each providing a delay T. There is also a sampling on N + 1 points of the signal p (t) : p (0), p (τ), ... p (Nτ). The signal from each element constituting the delay line is weighted by a coefficient λ _k such that: $${\text{λ}}_{\text{k}}{\text{= p ((Nk) τ) / | p}}_{\text{max}}\text{|}$$

where | p _max | is the maximum value of the modulus of p (t). At time t₀, the sum y (t₀) of the N + 1 weighted outputs is:

with Nτ = T

with t _k = t _o - kτ

This is indeed the result of the filtering adapted to the instant t₀-T. This function is now performed using digital electronic devices but is then limited in frequency and does not allow direct processing of signals at frequencies of the order of 20 GHz. Other solutions, analog this time, based on microwave guides or optical fibers as described in KP Jackson JJ Schaw "Fiber optics delay-line signal processors" in 'Optical Signal Processing "JL Horner Ed., Academic press allow '' consider reaching this frequency range but they come up against the difficulty of achieving a large number of coupling points.

The invention relates to a device making it possible to obtain a large number of samples on very high frequency signals, typically n ∼ 1024 from 0 to 20 GHz.

où
   R(t-t₀) est un signal de référence convenablement retardé
   S(t) est le signal à corréler
   T est le temps d'intégration
   b est la densité de puissance de bruit par Hz.In addition, it is often necessary in signal processing to calculate the correlation product: $${\text{C (t}}_{\text{o}}\text{) =}\frac{\text{1}}{\text{b}}{\text{∫}}_{\text{T}}{\text{R (t - t}}_{\text{o}}\text{) S (t) dt}$$

or
R (t-t₀) is a suitably delayed reference signal
S (t) is the signal to correlate
T is the integration time
b is the noise power density per Hz.

The object of this calculation is to determine the value of t₀ which ensures the maximum of the correlation function C (t₀). It is thus necessary to have a large number of samples of the reference signal delayed by different values of t₀ in order to ensure with precision the determination of t₀ which maximizes C (t₀). Such a function can be performed in electronics but it is limited to signals whose frequency and bandwidth do not exceed some 100 MHz. This limitation is due to the samples being too slow and the memory capacities too low.

Fiber-optic devices realizing the correlation of two optically transported signals have already been proposed (see for example French Patent Applications n ° 87 10120 and n ° 91 112040).

The correlator which is the subject of the invention has the advantage of not requiring a time reversal of one of the two signals and uses a photodetector with reduced bandwidth.

• une source optique émettant un faisceau optique multilongueur d'ondes ;
• au moins un modulateur électrooptique recevant le faisceau et le modulant à l'aide d'un signal électrique à traiter ;
• une fibre optique dispersive recevant le faisceau modulé et transmettant un faisceau dans lequel les composantes correspondant aux différentes longueurs d'ondes sont retardées les unes par rapport aux autres dans la fibre ;
• un réseau dispersif séparant les différentes longueurs d'ondes contenues dans le faisceau reçu de la fibre optique et fournissant un faisceau dispersé dans lequel chaque longueur d'onde est déviée selon une direction qui lui est caractéristique ;
• un modulateur spatial de lumière comportant une pluralité d'éléments de modulation recevant le faisceau dispersé et commandant le niveau d'intensité optique de différentes directions du faisceau dispersé ;
• un système de détection optique (PB) recevant le faisceau traité par le modulateur spatial de lumière.

The invention therefore relates to an optical processing device for electrical signals, characterized in that it comprises:

• an optical source emitting a multiwavelength optical beam;
• at least one electrooptical modulator receiving the beam and modulating it using an electrical signal to be processed;
• a dispersive optical fiber receiving the modulated beam and transmitting a beam in which the components corresponding to the different wavelengths are delayed with respect to each other in the fiber;
• a dispersive network separating the different wavelengths contained in the beam received from the optical fiber and providing a dispersed beam in which each wavelength is deflected in a direction which is characteristic thereof;
• a spatial light modulator comprising a plurality of modulation elements receiving the scattered beam and controlling the level of optical intensity from different directions of the scattered beam;
• an optical detection system (PB) receiving the beam processed by the spatial light modulator.

• la figure 1, un schéma théorique général d'un filtre transverse ;
• la figure 2, un exemple de réalisation d'un filtre transverse selon l'invention ;
• la figure 3, des courbes de dispersion chromatique de fibres optiques ;
• la figure 4, une variante de réalisation d'un filtre transverse selon l'invention ;
• la figure 5, un exemple de réalisation d'un corrélateur de signaux hyperfréquences selon l'invention ;
• la figure 6, un autre exemple de réalisation d'un corrélateur de signaux hyperfréquences de l'invention ;
• la figure 7, une variante de réalisation du corrélateur de la figure 6 ;
• la figure 8, une variante de réalisation applicable aux différents dispositifs des figures 2 à 7.

The various objects and characteristics of the invention will appear in the description which follows and in the appended figures which represent:

• Figure 1, a general theoretical diagram of a transverse filter;
• Figure 2, an embodiment of a transverse filter according to the invention;
• Figure 3, chromatic dispersion curves of optical fibers;
• Figure 4, an alternative embodiment of a transverse filter according to the invention;
• FIG. 5, an exemplary embodiment of a correlator of microwave signals according to the invention;
• FIG. 6, another exemplary embodiment of a correlator of microwave signals of the invention;
• Figure 7, an alternative embodiment of the correlator of Figure 6;
• FIG. 8, an alternative embodiment applicable to the various devices of FIGS. 2 to 7.

Referring to Figure 2, we will describe an embodiment of the device of the invention.

This device comprises in series a laser L, an electrooptical modulator MOD, an optical fiber F, a dispersive network H or dispersive wavelength device, a spatial light modulator SLM, a lens (LE), a photodetector PD.

The laser L provides a beam B1 multi-wavelength λ₁, ... λ _N. It is for example, a solid state laser pumped diode delivering a continuous broadband spectrum or a large set of longitudinal modes. This beam is coupled in the MOD modulator. This is for example, a modulator integrated on LiNbO₃ or on semiconductor. It has a passband extending between two frequencies F₁ and F₂ (example: F₁ = 0 F₂ = 20 GHz) and is excited by a signal x (t) to be processed.

There is thus available in the beam B2 an optical carrier multi-wavelengths of the signal to be processed. In fact, each wavelength λ₁ to λ _N can be considered as a carrier independent of the signal x (t).

The beam B2 from the MOD modulator is coupled into the single-mode optical fiber F, used in a spectral range where it is dispersive, that is to say where the refractive index n of the fiber depends on the wavelength. The beam B3 coming from the optical fiber F comprises the different wavelengths delivered by the source L all modulated by the modulator MOD, but these different wavelengths undergo different delays during the crossing of the fiber due to the different refractive index n for each wavelength.

The beam B3 then meets the dispersive network H, operating for example in transmission. The latter spatially separates the different wavelength components of the optical carrier. Each component then passes through an element of the SLM spatial light modulator. The transmission of each element of the modulator is variable according to the voltage which is applied to it and thus allows to apply to each component the desired weighting. An optical system LE then performs the summation of all the components, on a single photodetector PD.

où :

• C est la célébrité de la lumière
• S₀ et S₁ sont des valeurs d'intensités lumineuses telles que S₀ > S₁ |x_max|
• n_k est l'indice de réfraction de la fibre à la longueur d'onde λ_k.

At time t₀, the intensity of the optical carrier, on each channel, before crossing the SLM modulator is of the form: $${\text{I}}_{\text{k}}{\text{(t}}_{\text{o}}{\text{) = S}}_{\text{o}}{\text{+ S₁. x (t}}_{\text{o}}\text{- l}\frac{{\text{not}}_{\text{k}}}{\text{vs}}\text{)}$$

or :

• It's the celebrity of light
• S₀ and S₁ are values of light intensities such that S₀> S₁ | x _max |
• n _k is the refractive index of the fiber at the wavelength λ _k .

When crossing SLM, each channel is assigned a coefficient αk characteristic of a signal to be detected in x (t) and becomes: $${\text{I '}}_{\text{k}}{\text{(t}}_{\text{o}}{\text{) = S}}_{\text{o}}{\text{. α}}_{\text{k}}{\text{+ S₁. α}}_{\text{k}}{\text{. x (t}}_{\text{o}}\text{-}\frac{{\text{ln}}_{\text{k}}}{\text{vs}}\text{)}$$

The optical summation being inconsistent, the photodiode PD delivers a photocurrent proportional to the sum:

The first term Y₀ is a constant bias while the second Y₁ (t₀) is the result of the adapted filtering of x (t).

• Laser L :   laser état solide pompé diode émettant sur Δλ∼100nm entre 800 et 900nm, une puissance P₀∼20mW.
• Modulateur MOD :   modulateur optique intégré sur LiNbO₃
     large bande passante 0-->20 GHz
     profondeur de modulation 80 à 100 %
     pertes d'insertion : 6 dB
• Fibre :   monomode, en silice dont un exemple de courbes de dispersions est donné en figure 3. Il apparaît sur ces courbes qu'une fibre en silice pure est moins dispersive qu'une fibre de silice comportant un autre constituant. Ainsi il est possible d'adapter la dispersion de la fibre aux valeurs de retard désirées.
• Réseau dispersif H :   ce réseau autorise couramment une résolution de 0,1nm.
• Modulateur spatial de lumière SLM :   modulateur spatial à une dimension de 10³ pixels ; cellule à cristal liquide présentant une dynamique de 20 à 30 dB. Transmission ∼ 50 %.
• Détecteur optique PD :   photodiode rapide dont la puissance minimale détectable est typiquement de l'ordre de P₁ ≈ 10⁻¹³√B où B est sa bande passante de fonctionnement ; pour une bande passante Δf, l'incrément de retard T doit être au plus de : $$\text{τ = 1/2.ΔF}$$
  

We now give an example of the system and its performance:

• Laser L: solid state laser pumped diode emitting on Δλ∼100nm between 800 and 900nm, a power P₀∼20mW.
• MOD modulator: integrated optical modulator on LiNbO₃
  wide bandwidth 0 -> 20 GHz
  modulation depth 80 to 100%
  insertion loss: 6 dB
• Fiber: single-mode, made of silica, an example of which of the dispersion curves is given in FIG. 3. It appears on these curves that a pure silica fiber is less dispersive than a silica fiber comprising another constituent. Thus it is possible to adapt the dispersion of the fiber to the desired delay values.
• Dispersive network H: this network commonly allows a resolution of 0.1nm.
• SLM spatial light modulator: 10³ pixel spatial modulator; liquid crystal cell with a dynamic range of 20 to 30 dB. Transmission ∼ 50%.
• PD optical detector: fast photodiode whose minimum detectable power is typically of the order of P₁ ≈ 10⁻¹³√B where B is its operating bandwidth; for a bandwidth Δf, the delay increment T must be at most: $$\text{τ = 1 / 2.ΔF}$$
  

   d'où $$\text{l =}\frac{\text{c}}{\text{Δn}}\text{.}\frac{\text{N}}{\text{2. Δf}}$$

Thus, the length of fiber 1 enabling an N-channel device to be produced is determined by:

from where $$\text{l =}\frac{\text{vs}}{\text{Δn}}\text{.}\frac{\text{NOT}}{\text{2. Δf}}$$

For a silica fiber, used between 800 and 900nm we have Δn ∼ 2.10⁻³ from where if N = 10³, ΔF = 20 GHz
l = 3.8 km

Such a fiber length, at these wavelengths, introduces optical transmission losses of the order of 8dB (2dB / km).

où T est la transmission optique totale du dispositif et D la dynamique permise par SLM. Dans l'exemple donné : $$\text{ΔF/N ∼ 20 MHz}$$

d'où $\text{P₁ ∼ 10⁻¹³}\sqrt{\text{Δf / N}}{\text{= 4.10⁻¹⁰WetP}}_{\text{o}}\text{≧ 20mW}$

Furthermore, the passband of the photodiode must be of the order of ΔF / N. If P₁ is the minimum power detectable by this photodiode, it must satisfy: $${\text{P₁ ≦ P}}_{\text{o}}\text{. T.}\frac{\text{1}}{\text{NOT}}\text{.}\frac{\text{1}}{\text{D}}$$

where T is the total optical transmission of the device and D the dynamic range allowed by SLM. In the example given: $$\text{ΔF / N ∼ 20 MHz}$$

from where $\text{P₁ ∼ 10⁻¹³}\sqrt{\text{Δf / N}}{\text{= 4.10⁻¹⁰WetP}}_{\text{o}}\text{≧ 20mW}$

• Ce système permet de réaliser le filtrage adapté, sans transposition de fréquence, de signaux à très haute fréquence et à large bande passante. En effet, l'incrément de retard peut être aussi faible que désiré : il suffit pour cela d'utiliser la fibre optique sur un domaine spectral où sa dispersion chromatique est faible ou d'adapter la nature de la fibre à l'incrément désiré.
• Le contrôle de pondération α_k est assuré en parallèle au moyen d'un dispositif unique SLM. Celui-ci est commandé par des tensions faibles et assure à chaque instant la reconfigurabilité du système.
• Le contrôle indépendant sur chaque canal de la transmission du modulateur spatial SLM permet de compenser les non-uniformités du spectre émis par le laser ainsi que celles dues à la transmission de la fibre.
• Le volume du dispositif devrait être faible et ne pas excéder le litre. De plus sa consommation restera réduit, compte tenu des rendements des sources actuelles.

The device thus described finds a preferred application as a transverse filter and provides the following advantages:

• This system makes it possible to carry out the adapted filtering, without frequency transposition, of signals at very high frequency and with wide bandwidth. Indeed, the delay increment can be as small as desired: it suffices for this to use the optical fiber over a spectral range where its chromatic dispersion is low or to adapt the nature of the fiber to the desired increment.
• The weighting control α _k is carried out in parallel by means of a single SLM device. This is controlled by low voltages and ensures reconfigurability of the system at all times.
• The independent control on each channel of the transmission of the SLM spatial modulator makes it possible to compensate for the non-uniformities of the spectrum emitted by the laser as well as those due to the transmission of the fiber.
• The volume of the device should be low and not exceed one liter. In addition, its consumption will remain reduced, given the yields from current sources.

Referring to FIG. 4, a variant of the device in FIG. 2 will now be described.

In this variant, the optical fiber is no longer used as a dispersive medium. On the contrary, it is used at a wavelength for which the dispersion is minimal.

Bragg gratings, tuned to wavelengths λ₁, λ₂ ... λ _N , working in reflection are photoinduced in the fiber. Bragg's agreement at different wavelengths is obtained by varying the period of the photoinduced grating. The registration method is analogous to that described for example in the document G. Meltz, WW Morey, WH Glenn "Formation of Bragg gratings in optical fibers by a transverse holographic method" Opt. Lett., 14, 823 (1989) and uses a UV laser, ensuring the permanence of the networks.

The laser source L emits an extended spectrum Δλ, containing wavelengths λ₁ ... λ _N. In addition, the beam B1 which results therefrom is linearly polarized. It is then coupled in the MOD modulator identical to that previously described, excited by the hyper x (t) signal to be filtered. This multifrequency optical carrier is then coupled in the network fiber, where each component will undergo reflection at a different abscissa. This fiber is polarization maintaining in order to be able to easily separate the incident and reflected beams.

   où:

• n est l'indice de réfraction de la fibre ;
• l_k la position, dans la fibre, du réseau accordé à λ_k. De manière identique à ce qui précède, la sommation cohérente sur la photodiode fournit un photocourant qui rend compte du filtrage adapté de x(t). Afin de définir de façon précise l'échantillon temporel prélevé, il est nécessaire que l'épaisseur de chaque réseau soit petite devant la longueur d'onde du signal à traiter.

The quarter wave λ / 4 achromatic plate and the polarization splitter PBS (polarizer splitter cube) collect the light reflected by the fiber F. The dispersion-weighting-summation system remains identical to that previously described. Thus, the intensity of the optical carrier, on each channel, after crossing the SLM modulator, is of the form:

or:

• n is the refractive index of the fiber;
• l _k the position, in the fiber, of the network tuned to λ _k . Similarly to the above, the coherent summation on the photodiode provides a photocurrent which accounts for the adapted filtering of x (t). In order to precisely define the temporal sample taken, it is necessary for the thickness of each network to be small compared to the wavelength of the signal to be processed.

$${\text{2(l}}_{\text{k+1}}{\text{- l}}_{\text{k}}\text{) =}\frac{\text{c}}{\text{n}}\text{τ}$$

$$\text{l = N .}\frac{\text{c}}{\text{2n}}\text{.}\frac{\text{1}}{\text{2Δf}}$$

If Δf is the bandwidth to be treated and 1 the total fiber length: $$\text{τ = 1 / 2Δf}$$

$${\text{2 (l}}_{\text{k + 1}}{\text{- l}}_{\text{k}}\text{) =}\frac{\text{vs}}{\text{not}}\text{τ}$$

$$\text{l = N.}\frac{\text{vs}}{\text{2n}}\text{.}\frac{\text{1}}{\text{2Δf}}$$

So, for the application described above, we will have for example:
l _{k + 1} - l _k = 2.5 mm
l = 2.5 m
network coefficient = 250 µm
reflection thickness of each network = 10%

The previous dimensioning of the device remains valid since one substitutes for the losses by transmission in the fiber, the efficiency in reflection of the networks.

FIG. 8 represents another alternative embodiment in which, when the divergence of the multifrequency beam B4 is too large compared to the size of the pixels of the SLM modulator or when a very compact system is desired, it is advantageous to use the symmetrical system of figure 8.

L _c and L ' _c are symmetrical lenses, for example with the same focal length. In this case H and H 'are similar networks. All the wavelengths are thus recombined in a single direction before being summed by means of the output lens. The SLM pixels have the dimensions of the light lines formed by L _c .

According to another variant, the spherical output lens and the single detector of FIG. 8 are replaced respectively by a cylindrical lens, parallel to Lc, and by a strip of photodiodes. In addition SLM becomes a two-dimensional spatial light modulator (Nxp pixels). Each line of the SLM has q independently addressable pixels. Each pixel is associated with an element of the photodiodes array. The system thus makes it possible in parallel to carry out the filtering adapted to q different signals which may be contained in the signal x (t).

The device of the invention is also applicable to a correlator of electrical signals (microwave in particular).

• une source optique (laser) L
• un premier modulateur électrooptique MOD1
• une fibre optique dispersive F
• un deuxième modulateur électrooptique MOD2
• un réseau dispersif H
• un modulateur spatial de lumière SLM
• un dispositif de détection optique CCD.

FIG. 5 represents an example of such a correlator according to the invention. This correlator includes in series:

• an optical source (laser) L
• a first MOD1 electro-optical modulator
• a dispersive optical fiber F
• a second MOD2 electrooptical modulator
• a dispersive network H
• a SLM spatial light modulator
• a CCD optical detection device.

The various elements of this correlator have characteristics similar to those of the device described above. It is specified that the CCD optical detection device can comprise as many elementary detectors as there are image elements and that these detectors are coupled to a charge transfer device.

The role of this device is to correlate two electrical signals S (t) and R (t). The first electrooptical modulator MOD1 uses the signal S (t) to modulate the beam B1. The second electrooptical modulator MOD2 uses the signal R (t) to modulate the beam B3 coming from the fiber F.

The beam B3 is, as we have seen previously, made up of a plurality of elementary beams of different optical wavelengths and having undergone different delays in the optical fiber F. The modulator MOD2 therefore applies a modulation to each of these beams elementary. This therefore amounts to each of these elementary beams having an amplitude proportional to the product of the modulations S (t) and R (t), produced at different times for each of these elementary beams.

The dispersive network H spatially distributes the components of the beam B'3 each corresponding to a wavelength (or a narrow range of wavelengths). The different elementary beams of the beam B4 are modulated by the spatial light modulator SLM and then transmitted to the photodetectors CCD. The role of the SLM modulator is to correct the dispersions of the L source as well as of the transmission system (fibers in particular). However, according to an alternative embodiment, the SLM modulator may not exist and this correction can be made at the level of the detection on the CCD detector or at the level of the processing of the signal detected by the CCD.

où

• I_o,k est l'intensité du faiceau à λ_k reçue par l'élément photodétecteur en l'absence de modulation ;
• m₁ et m₂ les profondeurs de modulation du signal optique obtenues sur mod₁ et mod₂.

At time t, on each photodetector element of the CCD, the amplitude of the incident optical beam at the wavelength λ _k is proportional to:

or

• I _{o, k} is the intensity of the beam at λ _k received by the photodetector element in the absence of modulation;
• m₁ and m₂ the modulation depths of the optical signal obtained on mod₁ and mod₂.

Recall that the total bandwidth of the system is ΔF and that the number of samples of the correlation signal is N. In this case the bandwidth of each element of the CCD is of the order of Δf / N. Thus the integration time on each element of the CCD is worth: T = N / 2ΔF

   et rend bien compte, dans sa partie modulée, du produit de corrélation S(t)*R(t).Thus the photocurrent delivered by each element k of the CCD is proportional: $$\begin{gathered} {\text{i}}_{\text{k}}{\text{(t): <I}}_{\text{k}}\text{(t)> T} \\ {\text{= l}}_{\text{k, o}}{\text{× T + m₁. m₂l}}_{\text{k, o}}{\text{∫}}_{\text{T}}\text{S (t -}\frac{{\text{ln}}_{\text{k}}}{\text{vs}}\text{) R (t) dt} \end{gathered}$$

and gives a good account, in its modulated part, of the correlation product S (t) * R (t).

   où P₁ vaut ici typiquement 10⁻¹⁰ W (détectivité du CCD de l'ordre de 3.10⁻² pW/H2^1/2).
   D ∼ 40 dB
   et donc P_o >60 mWAs for the transverse filter, if ΔF = 20 GHz and N = 10³ we have:
1 ∼ 4km
and $${\text{P}}_{\text{o}}\text{≧ P₁}\frac{\text{1}}{\text{T}}\text{.ND}$$

where P₁ here is typically 10⁻¹⁰ W (CCD detectivity of the order of 3.10⁻² pW / H2 ^1/2 ).
D ∼ 40 dB
and therefore P _o > 60 mW

This device provides the same advantages as devices 2 and 4 and allows optically inconsistent detection on each element of the CCD.

FIG. 6 represents an alternative embodiment of the correlator of the invention.

The laser L, emitting on a broad spectrum Δλ, is coupled to two modulators MOD1 and MOD2 such as those described above (ΔF ∼ 20 GHz). They are respectively excited by the signals S (t) and R (t). The beams from these modulators are linearly polarized and pass through polarization splitters or cube polarization splitters PBS₁ and PBS₂. They are then coupled into two optical fibers F1, F2 with polarization maintenance of the same length 1 where networks have been photoinduced identical to those previously described. In the fiber F1, the networks are arranged so as to reflect successively λ₁ then λ₂, ... λ _N. The order is reversed in fiber F2. After reflection, the different components of the optical carriers S (t) and R (t) pass through the λ / 4 plates and are perfectly reflected by PBS₁ and PBS₂. The beam reflected by the fiber F1 undergoes a polarization rotation of 90 ° and passes through PBS₂. Thus, the carriers of the signals R (t) and S (t) are superimposed at the end of PBS₂ and their polarizations are crossed. This doubled beam then passes through a dispersive network H where the different wavelengths are spatially dispersed. Each of them passes through a first spatial light modulator SLM₁. The latter is, for example, a liquid crystal cell operating in electrically controlled birefringence. The polarization coincides for example with the optical axis of the liquid crystal molecules. Thus the refractive index seen by this polarization varies, depending on the voltage applied to the pixel, between n₀ and n _e (ordinary and extraordinary indices of the liquid crystal). On the contrary the polarization sees a constant refractive index n0. SLM₁ therefore makes it possible to control the relative phase shift φ of the carriers of S (t) and R (t). A polarizer P, oriented at 45 ° from the othogonal polarization directions allows the recombination of these two polarizations. A second spatial modulator SLM₂ attached to the first and counting the same number of pixels, makes it possible to control the weights ak assigned to each channel of wavelength component. At the output of this device, an optical system makes it possible to focus each channel on one of the elements of a multiple PDA photodetector, for example of the CCD type. Thus after integration, each pixel of the CCD delivers a signal proportional to the correlation product S (t) * R (t).

• à l'entrée de ces fibres les champs électriques associés aux deux ondes issues de mod₁ et mod₂ sont de la forme : $${\text{E}}_{\text{Z}}\text{(Z,t) = E₁₀}\sqrt{\text{S(t)}}\text{exp(jωt)}$$
  
  $${\text{E}}_{\text{Z}}\text{(Z,t) = E₂₀}\sqrt{\text{R(t)}}\text{exp(jωt)}$$
  
• sur l'élément l du photodétecteur multiple, les champs électriques incidents sont devenus :
  
  
     où :
     v : est la célérité de la lumière dans la fibre (Δλ est choisie au voisinage d'un minimum de dispersion de la fibre et donc v₁ = C/n_i = v = cst)
     L : la longueur totale des deux fibres
     I_F : la position du réseau réfléchissant λ_i dans la fibre 2
     w_i : la pulsation associée à la longueur λ_i
     φ_i : le déphasage relatif introduit par SLM₁ entre les deux composantes à λ_i qui interfèrent sur le photodétecteur i.

Indeed :

• at the entry of these fibers the electric fields associated with the two waves resulting from mod₁ and mod₂ are of the form: $${\text{E}}_{\text{Z}}\text{(Z, t) = E₁₀}\sqrt{\text{S (t)}}\text{exp (jωt)}$$
  
  $${\text{E}}_{\text{Z}}\text{(Z, t) = E₂₀}\sqrt{\text{R (t)}}\text{exp (jωt)}$$
  
• on element l of the multiple photodetector, the incident electric fields have become:
  
  
  or :
  v: is the speed of light in the fiber (Δλ is chosen in the vicinity of a minimum dispersion of the fiber and therefore v₁ = C / n _i = v = cst)
  L: the total length of the two fibers
  I _F : the position of the reflecting network λ _i in the fiber 2
  w _i : the pulsation associated with the length λ _i
  φ _i : the relative phase shift introduced by SLM₁ between the two components at λ _i which interfere on the photodetector i.

In this case, for an integration time T, the photocurrent delivered by the photodetector l is proportional to:

On each channel, the phase shift (φ _l is adjusted so that: $${\text{2ω}}_{\text{i}}\frac{\text{L - 2li}}{\text{v}}{\text{- φ}}_{\text{i}}\text{= 2Kπ (K ε N)}$$

Thus the optical path fluctuations are compensated by means of SLM₁. We thus find in the expression i₁ (t) two first terms which constitute a bias and a third term which accounts for the correlation product S (t) * R (t).

An example of the system and its expected dimensions is given below. The total bandwidth of the system is ΔF. The number of channels or samples of the correlation signal is N.

In this case, the integration time is worth at least: $$\text{T =}\frac{\text{1}}{\text{2Δf}}\text{. NOT}$$

$$\text{L = N.}\frac{\text{C}}{\text{2n}}\text{.}\frac{\text{1}}{\text{2Δf}}$$

As for the transverse filter: $${\text{l}}_{\text{i + 1}}{\text{- l}}_{\text{i}}\text{=}\frac{\text{VS}}{\text{2n}}\text{.}\frac{\text{T}}{\text{NOT}}$$

$$\text{L = N.}\frac{\text{VS}}{\text{2n}}\text{.}\frac{\text{1}}{\text{2Δf}}$$

So if
Δf = 20 GHz and N = 10³
T = 25 ns
l _{i + 1} -l _i = 2.5 mm
L = 2.5 m.

   où:

Tmod1 :

: perte d'insertion des modulateurs (∼ 6dB)

η_i

: coefficient de réflexion à λ_i du réseau photoinduit (∼ 10 %)

η_h

: efficacité de diffraction du réseau dispersif

T_slmk

: coefficient de transmission des modulateurs spatiaux (T_SLM1 ∼ 90 %, T_SLM2 ∼ 50 %)

If p _o is the optical power available at the output of the laser source, the maximum total optical power received on a channel is of the order: $${\text{P}}_{\text{o}}{\text{. T}}_{\text{mod}}{\text{. η}}_{\text{i}}{\text{. η}}_{\text{H}}{\text{. T}}_{\text{SLM1}}{\text{. T}}_{\text{SLM2}}\text{.}\frac{\text{1}}{\text{NOT}}$$

or:

Tmod1:

: loss of insertion of the modulators (∼ 6dB)

η _i

: reflection coefficient at λ _i of the photoinduced grating (∼ 10%)

η _h

: diffraction efficiency of the dispersive network

T _slmk

: transmission coefficient of space modulators (T _SLM1 ∼ 90%, T _SLM2 ∼ 50%)

(La durée de l'intégration n'est pas dans ce cas optimum puisque bien inférieure à la durée de la lecture de la barrette CCD (fréquence de lecture ∼ 20 MHz pour 10³ pixels)).In addition, a CCD pixel for an integration time of 1 ms, allows the detection of 1 pW, that is to say a detectivity of the order of 3.10W² pW / Hz ^1/2 For an integration time T the NEP ( noise equivalent power) which corresponds to the smallest detectable power, therefore becomes:

(The duration of the integration is not in this case optimum since it is much less than the duration of the reading of the CCD strip (reading frequency ∼ 20 MHz for 10³ pixels)).

For the dynamics of the system to be D it is then necessary to have: $${\text{P}}_{\text{o}}{\text{T}}_{\text{mod}}{\text{η}}_{\text{i}}{\text{η}}_{\text{H}}{\text{T}}_{\text{<figure-callout id="1" label="SLM1 T SLM2" filenames="imgf0001.png,imgf0003.png" state="{{state}}">SLM1 </figure-callout>}}{\text{T}}_{\text{SLM2}}{}_{}\frac{\text{1}}{\text{NOT}}\text{≧ D.NEP}$$

Hence here P₀> 140 mW (for D = 40dB) power compatible with current solid state laser sources pumped diode. It should however be noted that it is necessary, for each λ _i to have a coherence length greater than 2L in order to obtain the correlation product. Thus in the previously described case (L = 2.5 m) each W _i is defined better than 60 MHz. It therefore seems more realistic for this application to use a set of multidiodic laser sources.

• la corrélation ne nécessite aucune transposition de fréquences des signaux S(t), R(t) ;
• les contrôles de pondération des différentes composantes du faisceau B5 est reconfigurable à chaque instant ;
• la non-uniformité du spectre de la source L et de la transmission du système (de la ou des fibres notamment) peut être corrigée par le modulateur spatial SLM.

This correlator according to the invention has the same advantages as those indicated above for the filtering device. Indeed :

• the correlation does not require any transposition of frequencies of the signals S (t), R (t);
• the weighting checks of the various components of the beam B5 is reconfigurable at all times;
• the non-uniformity of the spectrum of the source L and of the transmission of the system (of the fiber or fibers in particular) can be corrected by the SLM spatial modulator.

FIG. 7 represents an alternative embodiment of FIG. 6. According to this alternative, the fiber F1 has chromatic dispersion over a domain of optical wavelength Δλ. In the same area, the F2 fiber is practically free from dispersion.

The PBS1 device located at the output of the fiber F1 is in fact a reflection device. The PBS2 device located at the outlet of fiber F2 makes it possible to combine the beams coming from fibers F1 and F2. By way of example in FIG. 7, the device SP located at the inputs of the fibers F1, F2 is a polarization splitter. However, the beams transmitted to the fibers F1, F2 could also be of same direction of polarization and the SP device could be a light splitter.

As before, the superimposed beams coming from the fibers F1, F2 are transmitted by the dispersive network H and the spatial light modulators SLM1 and SLM2 to the optical detection device CCD.

   c'est-à-dire

The product on each CCD detector thus has:

that is to say

We will now describe embodiments generally applicable to the various devices described above.

According to a first variant, the single laser source L is replaced after a set of p sources each emitting a spectrum Δλ / p. In this case we use a px1 coupler to combine the p sources in a single pigtail fiber connected to the modulator mod. It will thus be possible for example, for N = 1024 to use 64 semiconductor lasers of a few mW, each emitting 16 longitudinal modes distant by 0.1 nm.

Claims (13)

Optical processing device for electrical signals, characterized in that it comprises: - an optical source (L) emitting an optical beam (B1) multi-wavelength; - at least a first electrooptical modulator (MOD) receiving the optical beam (B1) and modulating it using a first electrical signal to be processed to provide a first modulated beam; - at least a first optical fiber (F) receiving the module beam (B2) and incorporating spatial separation means making it possible to transmit a beam (B3) in which the components corresponding to the different wavelengths are delayed relative to the others in fiber (F); - a dispersive network (H) separating the different wavelengths contained in the beam (B3) received from the optical fiber (F) and providing a dispersed beam (B4) in which each wavelength is deflected in a direction which is characteristic to him; - a reconfigurable spatial light modulator (SLM) comprising a plurality of modulation elements receiving the scattered beam (B4) and controlling the level of optical intensity from different directions of the scattered beam (B4); - an optical detection system (PD) receiving the beam (B5) processed by the spatial light modulator (SLM). Device according to claim 1, characterized in that it comprises a focusing device between the spatial light modulator (SLM) and the optical detection system (PD) for focusing the beam (B5) processed by the modulator (SLM), on the optical detection system (PD). Device according to claim 1, characterized in that the optical fiber (F) is a dispersive optical fiber. Device according to claim 1, characterized in that the spatial separation means comprise Bragg gratings inscribed in the optical fiber (F) each Bragg grating having a determined pitch so as to reflect the light of a determined wavelength ; and in that the device further comprises, between the modulator (MOD) and the optical fiber (F), a beam splitter (PBS) making it possible to transmit the light reflected by the Bragg gratings to the dispersive network (H). Device according to Claim 4, characterized in that the light transmitted by the modulator (MOD) is polarized in one direction and in that the device comprises, between the beam splitter (PBS) and the optical fiber (F), a blade quarter wave, the beam splitter then being a polarization splitter. Device according to Claim 1, characterized in that the optical detection system (PD) is an optical photodetector and in that the device comprises a focusing device situated between the spatial light modulator (SLM) and the optical detection system. Device according to claim 1, characterized in that it comprises a first cylindrical lens (L _c ) between the dispersive network (H) and the spatial light modulator (SLM) as well as a cylindrical lens (L ' _c ) symmetrical of the first cylindrical lens with respect to a space modulator (SLM) and a second dispersive network (H ') symmetrical with the first dispersive network with respect to the space modulator (SLM). Device according to Claim 7, characterized in that the optical detection system (PD) is a line of photodetectors and in that the device comprises a cylindrical lens comprised between the spatial light modulator and the optical detection system (PD). Device according to claim 1, characterized in that it comprises a second electrooptical modulator (MOD2) also receiving the multiwavelength optical beam and modulating it using a second electrical signal to be processed to provide a second modulated beam , this second modulated beam being superimposed on the first modulated beam before transmission to the dispersive network. Device according to claim 9, characterized in that the second electrooptical modulator (MOD2) is located between the optical fiber (F) and the dispersive network (H). Device according to claim 9, characterized in that the second electro-optical modulator (MOD2) receives in parallel with the first electro-optical modulator (MOD1) the multi-wavelength optical beam and that it retransmits it to a second optical fiber also comprising means allowing different delays of different wavelengths to be delayed; the beams from the two optical fibers being transmitted to a coupling system (FBS1, PBS2) which combines them and retransmits them to the dispersive network. Device according to claim 9, characterized in that the two optical fibers comprise Bragg gratings, each Bragg gratings having a not determined so as to reflect light of a determined wavelength; and in that the device further comprises modulators (MOD1, MOD2) and optical fibers (F1, F2), beam splitters (PBS1, PBS2) making it possible to transmit the light reflected by the Bragg gratings to the dispersive network . Device according to Claim 12, characterized in that the light transmitted by the modulators (MOD1, MOD2) is polarized in one direction and in that the device comprises a quarter-wave plate situated between the beam splitters (PBS1, PBS2) and the fibers (F1, F2), the beam splitters (PBS1, PBS2) then being polarization splitters.

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