EP0708491A1 - Optical control system for an antenna with electronic scanning - Google Patents

Abstract

The system includes two light sources (T1,T2) which transmit orthogonal polarisation light beams (F1,F2) through a delay mechanism (DCR). A delay (DCRn) is created for each of the phase shifter elements. A mirror separator (MD1-MDn) separates out the two light polarisations. One set is detected (PDR1,1-n) and the phase shift operates the transmitter (ED1-EDn). The other polarisation delay set passes to a receiver detection circuit (FFC) which operates the receiver phase shifters.

Description

The invention relates to an optical scanning antenna optical control system and in particular a system ensuring both the formation of a beam on transmission from the antenna and the reception by the antenna of a beam reflected by a target. .

The operation over an instantaneous wide frequency band of electronically scanned radars requires their control, both at transmission and at reception, in time delays. This necessity results from the dispersivity of the phased networks.

Numerous architectures for optical scanning antenna controls have been proposed, mono or two-dimensional, in order to control the radiation pattern on emission and their operating principle has generally been validated. They make it possible to envisage, taking into account the performance of current optoelectronic components, the solution, in the short or medium term, on operational equipment, to the problem of the depointing of network antennas with frequency. An example of such an architecture is found in French Patent No. 2,659,754.

However, the problem of beam formation in reception, using time delays has not been resolved to date. The main obstacle to the use of an optical architecture in this area remains the very important dynamic of the signals to be processed.

The invention provides a solution to this problem.

• le premier faisceau est polarisé selon une première direction déterminée,
• les circuits optiques de création de retards (DCR) reçoivent également un deuxième faisceau lumineux modulé par un signal électrique et polarisé selon une deuxième direction orthogonale à la première direction, chaque circuit à retards induisant des retards complémentaires par rapport à une valeur de temps déterminée sur les lumières des premier et deuxième faisceaux qu'il reçoit ;
• il comporte un séparateur de faisceau couplé à chaque sortie des circuits à retards transmettant la lumière du premier faisceau au premier photodétecteur et transmettant la lumière du deuxième faisceau à un deuxième photodétecteur ; ainsi qu'un dispositif de formation du faisceau (FFC) par corrélation des signaux électriques fournis par les deuxièmes photodétecteurs et des signaux électriques détectés par les éléments de réception d'antenne.

The invention therefore relates to a scanning antenna optical control system comprising phase-shifting elements to be controlled. This system comprising a set of optical circuits for creating delays each receiving a first light beam modulated by an electrical signal and each supplying on an output this first beam affected by an appropriate delay, each phase-shifting element of the antenna being coupled to a output of a delay circuit by a first photodetector, characterized in that:

• the first beam is polarized in a first determined direction,
• the optical delay creation circuits (DCR) also receive a second light beam modulated by an electrical signal and polarized in a second direction orthogonal to the first direction, each delay circuit inducing additional delays with respect to a time value determined on the lights of the first and second beams which it receives;
• it comprises a beam splitter coupled to each output of the delay circuits transmitting light from the first beam to the first photodetector and transmitting light from the second beam to a second photodetector; as well as a beam forming device (FFC) by correlation of the electrical signals supplied by the second photodetectors and the electrical signals detected by the antenna reception elements.

• la figure 1, un exemple général de réalisation du système de l'invention ;
• la figure 2, un exemple de réalisation plus détaillé du système de l'invention ;
• la figure 3, un schéma permettant l'explication du fonctionnement de l'antenne ;
• les figures 4a à 4c, une variante de réalisation du système de l'invention.

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

• Figure 1, a general embodiment of the system of the invention;
• Figure 2, a more detailed embodiment of the system of the invention;
• Figure 3, a diagram for the explanation of the operation of the antenna;
• Figures 4a to 4c, an alternative embodiment of the system of the invention.

Referring to Figure 1 we will therefore describe a general embodiment of the system of the invention.

The system comprises a set DCR of optical circuits for creating delays DCR1 to DCRn. To simplify the explanation, as many circuits DCR1 to DCRn are provided as there are antenna phase shift elements ED1 to EDn to be controlled. Light sources T1, T2 provide light beams F1, F2 modulated either in frequency or in amplitude. These two beams are linearly polarized in orthogonal directions. They are preferably of different wavelengths λ1 for the beam F1 and λ2 for the beam F2.

In addition, although this is not compulsory, the frequencies of the modulation signals of these beams are different.

The light beams F1 and F2 are superimposed and transmitted on the inputs of the different delay circuits each delay circuit causes the appropriate delay of the light it transmits. This is symbolized in FIG. 1 by circuits DCR1 to DCRn having different lengths (transmission lengths).

Each circuit DCR1 to DCRn delays the light coming from one of the beams, F1 for example, by an adjustable time ti. It then delays by a time T -ti the light coming from the other beam (F2 according to the example taken). The time T is a fixed fixed time which is the same for all the delay circuits. Preferably, this time T is equal to or greater than the maximum delay provided by each circuit.

At the output of each circuit is provided a beam splitter MD1 to MDn. They direct the light coming from the beam F1 towards photodetectors PDR1,1 to PDRn, 1 which control the radiating elements ED1 to EDn of an antenna. These separators are preferably wavelength separators or dichroic separators in the case where the beams are of different wavelengths and are adapted to these wavelengths.

The photodetectors supply the phase shift elements ED1 to EDn with currents delayed with respect to one another according to the delays brought by the delay circuits DCR1 to DCRn. Under these conditions, the control of the delay circuits makes it possible to act on the direction of emission of the antenna.

The beam splitters MD1 to MDn direct the light coming from the beam F2 towards photodetectors PDR1,2 to PDRn, 2 which supply photoconduction currents to an FFC detection circuit. It also receives, phase shift elements ED1 to EDn, detection currents corresponding to a reception beam received by the antenna. The FFC circuit establishes the correlation of the signals supplied by the photodetectors PDR1,2 to PDRn, 2 and by the phase shift elements ED1 to EDn.

In the case where a beam emitted by the antenna is reflected by a target, the FFC circuit identifies it by correlation with the signals supplied by the photodetectors PDR1,2 to PDRn, 2.

Referring to Figure 2, we will now describe a more detailed embodiment of the system of the invention.

In this system the light from the beams F1, F2 is frequency modulated. A first source L1 emits a single-frequency light beam of wavelength λ1 (pulsation ω1). A frequency translator T1 receives this light and transmits light at ω1 and light at ω1 + 2πf _e modulated using a signal of frequency f _e .

A second light source emits another single-frequency light beam of wavelength λ2 (pulsation ω2). A frequency translator T2 receives this light and transmits light at ω2 and light at ω2 + 2π (f _e + f _o ) modulated by a signal of frequency f _e + f _o .

In an application to the control of an electronically scanned antenna, the frequency f _e is situated in the microwave range and corresponds to the emission frequency of the antenna. The frequency f _o takes the place of the local oscillator frequency for the reception mode of the antenna in the following description.

The light emitted by the translator T1 is polarized in a determined direction. That emitted by the translator T2 is polarized in a direction perpendicular to that emitted by T1.

An optical mixing system ME superimposes the light coming from the translator T1 to that coming from the translator T2. The resulting beam therefore comprises light polarized in two orthogonal directions, as is symbolized in FIG. 2, and at the different frequencies from the translators T1 and T2.

The resulting beam is extended by a beam splitter SE so as to be distributed over the different inputs of a set of DCR delay circuits.

This set of DCR delay circuits can be produced as described in French patent application No. 92 34 467.

Each delay circuit delays the light from the source L1 and the light from the source L2 differently. More precisely, according to a preferred embodiment, if T is the maximum delay induced by a delay circuit, the light coming from the source L1 is delayed by a time ti and the light coming from the source L2 is delayed by a time T-ti complementary to time T.

Also preferably, the times T of the different delay circuits are equal.

For example, the DCR delay circuits comprise a set of spatial light modulators comprising pxp pixels (same number of pixels as radiating antenna elements) and making it possible to control the phase shift and the delay assigned to each of the channel pxp thus cut. The DCR delay circuits provide delays in geometric progression so that N spatial modulators are sufficient to obtain 2 ^N distinct delay values on each of the pxp channels of the architecture. The switching of the delays is based on the controlled rotation, thanks to the spatial light modulators, of the polarization of the beams. In order to obtain a local oscillator adapted in direction, the property of the DCR is exploited, which is to generate, on each channel, additional delays for crossed polarization states at the input. Indeed, if the beam from L₁ undergoes a delay on channel i, then the beam from L₂ undergoes a delay Tt _i (T is the crossing time of the DCR).

Each output Sd of a delay circuit provides light at the wavelength λ1 modulated at the frequency f _e and light at the wavelength λ2 modulated at the frequency f _e + f _o .

Detection circuits PDRi and PDRn are connected to the outputs Sd for example by optical fibers. These circuits are produced as shown at the bottom right of FIG. 2. Each circuit comprises a chromatic separator MD separating the light at the wavelength λ1 from the light at the wavelength λ2.

Light at the wavelength λ1 is transmitted to a photodetector PDRi, 1 which emits a photocurrent of frequency f _e towards a radiating element ED1.

This photocurrent results from the beat between the light at ω1 and the light at ω1 + 2πf _e .

The transmitted photocurrent is amplified by an amplifier so as to be compatible with the radiated power necessary for the emission of the radiating element of the radar.

By providing appropriate delays in the various delay circuits, the radiation pattern of the antenna is checked. The antenna orientation of the antenna is thus optically controlled.

Furthermore, the light at the wavelength λ2 is transmitted to another photodetector PDRi, 2 by the chromatic separator. This emits a photocurrent resulting from the beat between the light at ω2 and ω2 + 2π (f _e + f _o ). This photocurrent is applied to a microwave mixer Mk which also receives a signal received by an antenna element. It should be noted that a directional coupler DC makes it possible to couple, on the one hand, the photocurrent of PDRi, 1 to an antenna element in the emission direction and to couple, on the other hand, a detection current of an antenna element (in the receiving direction) to the frequency mixer Mk.

The set of signals from PDRI photodetectors, 2 actually constitutes a local oscillator (homodyne or heterodyne) adapted to the direction of emission of the antenna.

Thus, the signal received by an antenna element EDk is amplified and is applied together with the signal from PDR _k2 , on a microwave mixer M _K. Indeed, if the signal emitted by the antenna element EDk is of the form S (t-τ _k ), the same element receives a signal R (t '+ τ _k ) which must therefore be mixed with a local oscillator S '(t' + T + τ _k ).

• numérisation au niveau de chaque élément d'antenne et sommation de l'ensemble de ces signaux dans un processeur numérique classique de formation fine de faisceau par le calcul. Ce processeur peut en outre être déporté par rapport à l'antenne de réception au moyen d'un nombre réduit de liaisons numériques à fibres optiques multiplexées en longueur d'onde.
• excitation des pxp pixels d'un modulateur spatial de lumière bidimensionnel au moyen de ces pxp signaux basse fréquence afin de mettre en oeuvre un traitement optique cohérent de la voie retour.

Low frequency signals from the mixers are processed in two ways:

• digitization at the level of each antenna element and summation of all of these signals in a conventional digital processor for fine beam formation by calculation. This processor can also be remote from the receiving antenna by means of a reduced number of digital optical fiber links multiplexed in wavelength.
• excitation of the pxp pixels of a two-dimensional spatial light modulator by means of these pxp low frequency signals in order to implement a coherent optical processing of the return path.

Referring to Figure 3 we will now describe an example of sizing of the system of the invention.

FIG. 3 represents two radiating elements ED1, EDN external of an antenna.

• pour l'élément ED1 :
  • un signal émis :   S(t)
  • un signal reçu :   R(t')
  • un signal d'oscillateur local :   S(t-T)
• pour l'élément EDN au même instant :
  • un signal émis :   S(t+τ)
  • un signal reçu :   R(t'-τ)
  • un signal d'oscillateur local :   S(t-T-τ)

If the antenna transmits in a direction θ such that the delay between the extreme elements of the antenna ED1 and EDN, is τ we have:

• for element ED1:
  • an emitted signal: S (t)
  • a signal received: R (t ')
  • a local oscillator signal: S (tT)
• for the EDN element at the same time:
  • an emitted signal: S (t + τ)
  • a received signal: R (t'-τ)
  • a local oscillator signal: S (tT-τ)

• pour l'élément ED1 : $${\text{2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) (t -T) - 2π f}}_{\text{r}}\text{t'}$$
  
  $${\text{[2π(f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) t - 2π f}}_{\text{r}}{\text{t'] - 2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}\text{) T}$$
  

If we assume the sinusoidal signals with the frequencies f _e (emission) f _e + f _o (local oscillator) and f _r (received signal) the phases of the low frequency signals from the mixers Mk are then:

• for element ED1: $${\text{2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) (t -T) - 2π f}}_{\text{r}}\text{you}$$
  
  $${\text{[2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) t - 2π f}}_{\text{r}}{\text{t '] - 2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}\text{) T}$$
  

In this relation (t'-t) represents the antenna-target distance.

$${\text{[2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) t - 2 π f}}_{\text{r}}{\text{t'] - 2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) T + 2π (f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{- f}}_{\text{o}}\text{) τ}$$

For the EDN element: $${\text{2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) (t - T -τ) - 2π f}}_{\text{r}}\text{(t '- τ)}$$

$${\text{[2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) t - 2 π f}}_{\text{r}}{\text{t '] - 2π (f}}_{\text{e}}{\text{+ f}}_{\text{o}}{\text{) T + 2π (f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{- f}}_{\text{o}}\text{) τ}$$

   c'est-à-dire $${\text{f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{- f}}_{\text{o}}\text{<<}\frac{\text{1}}{\text{τ}}$$

To be able to sum the N channels directly, either in analog or in digital, the additional phase term 2π (f _r - f _e - f _o ) τ must be negligible. If it is for the element of rank N it is for the elements of lower rank). $${\text{2π (f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{- f}}_{\text{o}}\text{) τ << 1}$$

that is to say $${\text{f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{- f}}_{\text{o}}\text{<<}\frac{\text{1}}{\text{τ}}$$

• τ est donné par les dimensions de l'antenne : par exemple, pour une antenne de 5 mètres de côté, ayant un angle de balayage maximum de 45°, le retard τ est sensiblement de 10 ns
     d'où f_r - f_e - f_o << 100 MHz

The frequency difference f _r - f _e corresponds substantially to the dopler shift due to the movement of the target which is of the order of magnitude 10 to 100 kHz.

• τ is given by the dimensions of the antenna: for example, for an antenna with a side length of 5 meters, having a maximum scanning angle of 45 °, the delay τ is substantially 10 ns
  hence f _r - f _e - f _o << 100 MHz

   par exemple : 100 kHz << f_o << 100MHzIn order to keep the entire doppler around the intermediate frequency f _o we can choose: $${\text{f}}_{\text{r}}{\text{- f}}_{\text{e}}{\text{<< f}}_{\text{o}}\text{<< 100 MHz =}\frac{\text{1}}{\text{τ}}$$

for example: 100 kHz << f _o << 100MHz

Under these conditions, one can for example choose f _o in a frequency range from 1 to 10 MHz

We find the same constraint linking the size of the antenna to the signal band used in the case of a phase-controlled antenna.

Referring to Figures 4a to 4c, we will now describe an alternative embodiment of the system of Figure 2 to obtain not only delays but also phase shifts of microwave signals.

FIG. 4a represents the chain of optical circuits up to the mixing system ME and the delay creation circuits DCR.

According to this alternative embodiment, the light emitted by the frequency translators respectively at ω1 and ω2 is polarized in a direction of polarization. The light emitted respectively ω1 + 2πf _e and ω2 + 2π (f _e + f _o ) is polarized in a polarization direction perpendicular to the previous one.

A single mode laser beam supplied by the source L1 is processed by a frequency translator which supplies two superimposed beams offset in frequency, one at ω1, the other at ω1 + 2πfe. These two beams are linearly polarized in orthogonal directions. The embodiment of this translator can be as shown in Figure 4b.

The transverse and longitudinal monomode laser beam (ω₂ / 2π) supplied by L1 is focused in an acoustooptic Bragg cell BC operating in an anisotropic regime. This cell is excited by a continuous microwave signal fe. The transmitted beam ((ω₁ / 2π) and the diffracted beam ((ω / 2π + f _e ) are orthogonally polarized. These two components are superimposed by means of a PBS polarization separator cube for example. This superposition is done with a low path difference so as not to degrade the spectral purity of the microwave signal which will be transmitted This dual-frequency beam crosses a half-wave λ / 2 plate which rotates the two orthogonal polarizations by 45 °.

The beam supplied by the λ / 2 plate is extended (FIG. 4a) by means of an afocal BE system (lenses LE1.1 and L2.1) and passes through a spatial modulator M01. This modulator is for example a nematic liquid crystal cell, comprising p x p pixels and which is used in controlled birefringence (molecules parallel to each other and to the walls). As can be seen in Figure 4c. This modulator allows analog control over each pixel of the phase of the microwave signal because it allows control of the relative phase shift between the two components of the dual-frequency beam.

   où λ est la longueur d'onde du laser, e l'épaisseur de cristal liquide de M_o, Δn(Vk) = n(V_k) - n_o et i_o = √i_ω i_ω+2πf avec i_ω (resp. i_ω+2πfe) photocourant délivré par une photodiode détectant le faisceau à la fréquence ω/2π seul (resp. ω/2π+f) et τ_k le retard affecté à ce canal.The polarization of the beam component at the frequency ω₁ / 2π coincides with the long axis of the liquid crystal molecules. Thus, according to the voltage V _k applied to each pixel, the refractive index n (V _k ) seen by this polarization varies continuously between n _o and n _e , respectively ordinary and extraordinary indices of the liquid crystal. On the contrary, the component ω₁ / 2π + f _e constantly sees the refractive index n _o . These two polarizations are then recombined by means, for example, of a polarization separator cube PBS1 which provides beams polarized in a direction. These beams undergo delays in the DCR device and are transmitted to the photodetectors PDRi, 1 (see FIG. 2). Each photodetector delivers an amplitude microwave beat signal:

where λ is the laser wavelength, e the liquid crystal thickness of M _o , Δn (Vk) = n (V _k ) - n _o and i _o = √i _ω i _{ω + 2πf} with i _ω ( resp. i _{ω + 2πfe} ) photocurrent delivered by a photodiode detecting the beam at the frequency ω / 2π alone (resp. ω / 2π + f) and τ _k the delay assigned to this channel.

The dual-frequency beam (ω1, ω1 + 2πfe) having only one direction of polarization then crosses a set of DCR delay circuits allowing the choice of delay values assigned to each radiating element of the antenna.

According to the example of the invention of FIG. 4a, if there are p x p radiating antenna elements, the modulator MO has p x p image elements and the assembly DCR has p x p delay creation circuits. The various devices of the system are aligned so that a portion of the light beam processed by an image element of the MO modulator is received by a delay creation circuit (DCRi) which transmits this portion of the beam suitably delayed to a photodetector (PDRi, 1) assigned to an antenna radiating element.

We have described Figure 4a in its operation in transmission.

• le translateur de fréquence T2 est excité par un signal de fréquence f_e +f_o et fournit de la lumière à une fréquence correspondant à ω2 et de la lumière à ω2 + 2π(f_e + f_o), ces deux lumières étant polarisées orthogonalement ;
• après modulation par le modulateur MO₂ le cube séparateur de polarisations PBS2 fournit de la lumière entièrement polarisée selon une direction orthogonale à celle fournie par le cube PBS1. Cette lumière est transmise au cube PBS1 (cela pourrait être l'inverse) et celui-ci fournit sur chaque entrée de l'ensemble de circuits DCR :
• de la lumière polarisée selon une première direction à une fréquence correspondant ω1 + 2πf_e éventuellement déphasée par rapport à la précédente ;
• de la lumière polarisée selon une deuxième direction, à ω2 et de la lumière polarisée selon cette deuxième direction à ω2 + 2π(f_e + f_o) éventuellement déphasée par rapport à la précédente.

For reception, use is made of the light at the wavelength λ2 supplied by the source L2. The circuits for processing the beam supplied by the source L2 are similar to those described above for processing the beam supplied by the source L1, taking into account the following differences:

• the frequency translator T2 is excited by a signal of frequency f _e + f _o and supplies light at a frequency corresponding to ω2 and light at ω2 + 2π (f _e + f _o ), these two lights being orthogonally polarized ;
• after modulation by the MO₂ modulator, the polarization separator cube PBS2 provides light entirely polarized in a direction orthogonal to that provided by the cube PBS1. This light is transmitted to the cube PBS1 (it could be the reverse) and it provides on each input of the DCR circuit set:
• light polarized in a first direction at a frequency corresponding ω1 + 2πf _e possibly out of phase with the previous one;
• light polarized in a second direction, at ω2 and light polarized in this second direction at ω2 + 2π (f _e + f _o ) possibly out of phase with the previous one.

The architecture of FIG. 4a therefore makes it possible, on pxp independent channels, to obtain 2 ^N delay values and continuous control of the phase of the signal between 0 and 2π. In order to supply the transmitting antenna, each photodetector is then connected to a microwave amplifier A _j and to a radiating element E _j .

Claims (7)

Scanning antenna optical control system comprising radiating elements to be controlled (ED1) this system comprising a set (DCR) of optical circuits for creating delays each receiving a first light beam (F1) modulated by an electrical signal and each providing on an output, this first beam affected by an appropriate delay, each phase-shifting element of the antenna being coupled to an output of a delay circuit by a first photodetector (PDRi, 1), characterized in that: the first beam (F1) is polarized in a first determined direction, the optical delay creation circuits (DCR) also receive a second light beam (F2) modulated by an electrical signal and polarized in a second direction orthogonal to the first direction, each delay circuit inducing additional delays with respect to a value determined time (T) on the lights of the first and second beams it receives; - it comprises a beam splitter (MD1, ... MDn) coupled to each output of the delay circuits transmitting the light of the first beam to the first photodetector (PDRi.1) and transmitting the light of the second beam to a second photodetector (PDRi , 2); as well as a detection circuit (FFC) by correlation of the electrical signals supplied by the second photodetectors and of the electrical signals detected by the antenna reception elements. System according to claim 1, characterized in that it comprises: - a first light source (L1) emitting the first light beam (F1) at a first wavelength (λ1); - a first electrooptical modulator (T1) modulating the first light beam with a first electrical signal (S1) and supplying this first modulated beam, polarized in the first direction to the delay creation circuits; - a second light source (L2) emitting the second light beam (F2) at a second wavelength (λ2); - A second electrooptical modulator (T2) modulating the second light beam with a second electrical signal (S2) and supplying this second modulated beam, polarized in the second direction to the circuits for creating delays. System according to claim 1, characterized in that said determined time value (T) is the same for all the circuits of the set of circuits for creating delays and equal to the maximum value of the delay which can be induced by each circuit to delay; System according to one of claims 1 or 2, characterized in that the modulated beams are modulated in amplitude or in frequency. System according to claim 4, characterized in that the beams are frequency modulated using signals of different frequencies. System according to claim 2, characterized in that the beam splitter (MD) is a chromatic separator separating the light at the first wavelength to transmit it to the first photodetectors, from the light at the second wavelength for the transmit to the second photodetectors. System according to claim 2, characterized in that each modulator (T1, T2) comprises in series: - a frequency translator (BC1, BC2) receiving light at a wavelength (λ1, λ2) transmitting light at this wavelength polarized in a third direction and light at a wavelength frequency translated and polarized in a fourth direction perpendicular to the third direction; - a polarization rotation device; - a spatial light modulator (M01, M02) acting differently on the two directions of light polarization; - a polarization separator (PBS1, PBS2), the separator (PBS1) of the first modulator (T1) retaining only light polarized in the first direction and transmitting it to the set of delay creation circuits (DCR) while the separating device (PBS2) of the second modulator (T2) retaining only the light polarized in the second direction and transmitting it to the set of delay creation circuits (DCR).

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