Patent Publication Number: US-6700704-B2

Title: System comprising a device necessitating the reception of linearly polarized beams and corresponding method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to any field of application using processing that necessitates the reception of linearly polarized beams. In particular, the invention can be used in array antenna beam-shaping and aiming systems that use delay generation with polarization switching. 
     Unlike the generation of phase shifts between the elements of an array antenna, the generation of true delays gives an aiming direction that does not depend on the frequency. 
     Microwave delays can be created by optical means. The use of optics to convey microwave signals gives devices that have low dependency on the electrical frequency conveyed. These properties are especially valuable in electronic scanning antennas that have to work in a wide frequency band. Furthermore, devices using optics have reduced mass and space requirements. 
     2. Description of the Prior Art 
     Multiple devices for the creation of microwave delays by optical means are known. 
     The patent Thomson-CSF FR 2659754 proposes a device of this kind using polarization switches. FIG. 1 gives a schematic view of an example of this multiple device  5  for the creation of delays (hereinafter called a multiple delay-correction device) using polarization switches CP i  (1≦i≦n). Delays in several parallel optical paths can be commanded by means of polarization switches CP i  consisting for example of matrices of pixels such as spatial light modulators, liquid crystal matrices, etc. Since the optical carriers are modulated by microwave signals, the delays will also be applied to these signals. 
     The association of several switches CP i  consisting of pixel arrays, polarization splitter/recombiner elements SP i  and reflector elements R i  achieve the quantified control of the delay of each optical channel. Indeed, by controlling each pixel of a given polarization switch Cp i , the delay that will be undergone by each path at the crossing of the assembly [Sp i , R i ] (direct or delayed route) is determined in binary fashion. 
     This concept has the advantage of providing for a multiple processing of the different spatially separated optical channels. 
     FIG. 2 shows a possible implementation of a multiple delay-creation device  5  using prior art polarization switches CP. This exemplary application is the supply of an antenna array working in transmission (with beam-shaping at transmission). 
     A modulated optical source  1  gives a beam to the coupler  2 . The coupler  2  maintains the polarization and distributes the entering beams to the polarization-maintaining fibers  3   M . These beams are transmitted by the fibers  3   M  though the array of lenses  4   M  to the multiple delay-creation device  5  using polarization switches CP. The processed beams (delayed or not) at output of the multiple delay-creation device  5  are transmitted to the photodetectors  6  through the array of lenses  4   V  and the optical fibers  3   V  for which the maintaining of the polarization is not necessary. The array of lenses  4   M  and  4   V  provide for accurate coupling between the fibers  3   M  and  3   V  respectively and the multiple delay-creation device  5 . Each photodetector  6  is connected to an antenna element or sub-array  7 . 
     In order that the selection of a delay by polarization switching may be efficient, the polarization switch must receive a linearly polarized beam. This is why the implementation of the multiple delay-creation device  5  with polarization switches CP i  described in the patent FR 2659754 requires polarization-maintaining elements  2  and  3   M  upline from the multiple delay-creation device  5 . This constraint is not negligible because these polarization-maintaining elements, couplers  2  for example, of the fibers  3   M  are more costly and more difficult to implement than elements that do not maintain polarization. 
     The present invention is used to overcome or at least reduce these drawbacks by proposing an alternative solution. 
     SUMMARY THE INVENTION 
     It proposes a system comprising a processing device that necessitates the reception of linearly polarized beams at input wherein it furthermore comprises at least one element for polarization splitting in open space, placed upline from said device. 
     This system may comprise for example: 
     a polarization switch downline from said device, and 
     an element for superposing polarization in open space downline from the polarization switch. 
     The invention furthermore proposes a method comprising a step for the processing of linearly polarized beams, the method comprising at least the splitting of polarizations in open space achieved prior to said processing step. 
     This method for example comprises the following steps: 
     an additional polarization switching achieved after said processing step, and 
     the superposing of polarization in open space achieved after said additional switching step. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention shall appear more clearly from the following description given by way of an example and the appended figures, of which: 
     FIG. 1 shows an example of a multiple delay-creation device  5  using prior art polarization switches CP i , 
     FIG. 2 exemplifies the implementation of a multiple delay-creation device  5  using prior art polarization switches CP i , 
     FIG. 3 shows a system implementing a processing method  5   G  necessitating the reception of linearly polarized beams according to the invention, 
     FIGS.  4 ( a ) and  4 ( b ) show the system of FIG. 3 implementing a multiple delay-creation device  5   G  comprising respectively two identical multiple delay-creation devices such as the one of FIG. 1 and a multiple delay-creation device common to both routes, according to the invention. 
    
    
     MORE DETAILED DESCRIPTION 
     FIG. 3 proposes an example according to the invention of a system implementing a processing device  5   G  that necessitates upline maintaining of polarization. 
     In the present example, the initial optical beams are transmitted by means of optical fibers  3   M , that do not necessitate polarization maintaining, and an array of lenses  4   M  to an element for the splitting polarization in open space  5   M . This splitter element  5   M , comprising for example a polarization splitter  51   M  and a mirror  52   M , is placed upline with respect to the device  5   G , to which it delivers a group of polarization beams ↑ and a group of orthogonal polarization beams ⊙ (the symbols ↑ and ⊙ indicate the orientation of the polarization). The two groups of beams entering the device  5   G  are therefore linearly polarized as required by the processing operations contained in this device  5   G . The device  5   G  may for example be a multiple delay-creation device  5  with polarization switches CP i  such as the one shown in FIG.  1 . The splitter element  5   M  generates the doubling of the number of pixels of the multiple delay-creation device  5  as compared with the solution using polarization maintaining elements (fibers, etc.). 
     The two polarization states coming from a given initial beam undergo the same processing by the device  5   G . For example, the multiple delay-creation device  5  applies identical delays to the two orthogonal polarization beams coming from an initial beam. Each group of beams processed at output of the device  5   G  goes through a polarization switch CP +  used to determine the polarization of each of the groups at its output. 
     The additional switch CP +  may for example have either 2N pixels or two switches CP 1+  and CP 2+  with N pixels (not shown in the figures), one for each of the two groups of beams. 
     The role of the polarization switch CP +  is twofold and differs from the role of the polarization switches CP i  of the multiple delay-creation device  5  of FIG. 1 responsible for selecting the direct or delayed route that must be followed by a given initial beam. Indeed, the switch CP +  ensures that, 
     1) all the beams of a group whose polarization was respectively ↑ and ⊙ before processing by the device  5   G  are in a same polarization at output of CP + , and that 
     2) the polarization of the first group of beams is orthogonal to the polarization of the second group of beams. 
     This makes it possible, first of all, to reorganize the polarization of the beams of the two groups which are modified by the use of the different polarization switches CP i  of the multiple delay-creation device  5   G . Furthermore, this enables the polarizing of the two groups of beams according to the position of the polarization superimposing element  51   V  with respect to the polarization switch CP+ such that the two groups of beams at output of the device  5   G  are recombined by the polarization superposing element  51   V . 
     The initial beams may then be reconstituted by means of the open space superposing element  5   V . This open space superposing element  5   V  placed after the polarization switch CP +  superposes signals coming from the previously split polarizations, for example by means of a device using a mirror  52   V  and a polarization splitter mirror  51   V  (herein playing the role of a recombiner). The post-processing superposition of the two polarization states coming from a given initial beam therefore mitigates the fluctuations in levels that may exist between either of the polarization states. The beams thus processed are sent to a user circuit, for example through the network of lenses  4   V  and the fibers  3   V  not requiring polarization maintenance. 
     In one possible variant of the system the fibers  3   M  and  3   V  and the lenses  4   M  and  4   V  are replaced by system for the propagation of the beams in open space. 
     Furthermore, the polarization splitters  51   M  and  51   V  may for example be polarization splitting cubes but it is also possible to consider using other elements such as, for example, spatial splitters using birefringent materials and prompting a beam deflection depending on the polarization. 
     Inasmuch as the doubling of the number of pixels remains compatible with the technology for making the multiple device for the creation of delays in open space, the solution proposed here above simplifies the implementation of the system by eliminating the need to maintain the polarization before the feeding of the multiple delay-creation system  5 . 
     It may be noted that, because of its structure, the proposed system comprising a multiple delay-creation system S with polarization switches CP i  is reciprocal (i.e. a two-way system). It therefore enables the creation of microwave delays by optical means for beam shaping and aiming in array antennas working in transmission or reception especially in the case of electronic scanning antennas that have to work in a wide band. 
     As specified here above, the device  5   G  is not necessarily a multiple delay-creation device  5  with polarization switching but may be any type of device requiring linearly polarized beams at input. 
     FIGS.  4 ( a ) and  4 ( b ) show two versions of the system according to FIG. 3 when the processing device  5   G  is a multiple delay-creation device. 
     The first version of the system shown in FIG.  4 ( b ) is such that the multiple delay-creation device  5   G  comprises, for each group of orthogonal or almost orthogonal polarization beams, a multiple delay-creation device such as the multiple delay-creation device  5  of FIG.  1 . It has for example: 
     n polarization splitters/recombiners SP i  (1≦i≦n), each polarization splitter/recombiner SP i  separating and then recombining the two beams switched by the upline polarization switch CP i  with the other non-switched beams, 
     n delay devices R i  (1≦i≦n) each delay device R i  delaying the two beams split by the polarization splitter/recombiner SP i  by a same delay τ i  (1≦i≦n) before their recombination by the polarization splitter/recombiner SP i . 
     The second version of the system presented by FIG.  4 ( a ) is such that the multiple delay-creation device  5   G  has a structure similar to that of the multiple delay-creation device  5  of FIG.  1 . It comprises at least, on each of the two paths (j=1,2): 
     n polarization switches CP i   j  (1≦i≦n and j=1,2), the polarization switch) CP i   j  of the first path and the polarization switch CP i   2  of the second path each, in a similar way, switching a beam corresponding to one of the two polarization states of an initial beam entering said system, 
     n polarization splitters/recombiners SP i   j  (1≦i≦n and j=1,2), the polarization splitter/recombiner SP i   j  separating and then recombining the beam switched by the upline polarization switch CP i   j  with the other beams, 
     n delay devices R i   j  (1≦i≦n and j=1,2), the delay device R i   1  and the delay device R i   2  respectively delaying the beam separated by the polarization splitter/recombiner SP i   1  and the beam separated by the polarization splitter/recombiner SP i   2  by a same delay τ i  (1≦i≦n) before their recombination by, respectively, the polarization splitter/recombiner SP i   1  and the polarization splitter/recombiner SP i   2 . 
     The splitters/recombiners SP 1 , SP 2 , . . . , SP n  are represented in FIG.  4 ( b ) in proportions such that they facilitate the reading of FIG.  4 ( b ) without being exhaustive. The proportions of the splitters/recombiners SP 1 , SP 2 , . . . , SP n  of FIG.  4 ( b ) may for example be similar to those of the splitters/recombiners SP 1   1 , SP 1   2  . . . SP 1   n  and SP 2   1 , SP 2   2  . . . SP 2   n  of FIG.  4 ( a ). 
     The multiple delay-creation device  5   G  is capable of delaying 2N beams, namely twice the number of the beams delayed by the multiple delay-creation device  5  of FIG.  1 . For this purpose, the matrices of the polarization switches CP 1 , CP 2 , . . . , CP n  used by the multiple delay-creation device  5   G  have 2N pixels. The delays induced by the multiple delay-creation device  5   G  for the two groups of beams are such that two groups of orthogonal polarization beams coming from an initial beam entering the system shown in FIGS.  4 ( a ) and  4 ( b ) undergo the same delay τ 1 , τ 2  . . . τ n  created by the delay devices of the first path R 1   1 , R 2   1  . . . R n   1  and the second path R 1   2 , R 2   2  . . . R n   2  in the case of FIG.  4 ( a ) or the delay devices R 1 , R 2 , . . . , R n  in the case of FIG.  4 ( b ). 
     The matrices of the polarization switches of the first path CP 1   1 , CP 2   1  . . . CP n   1  and of the second path CP 1   2 , CP 2   2  . . . CP n   2  may for example be identical (CP 1   1 =CP 1   2 , CP 2   1 =CP 2   2  . . . CP n   1 =CP n   2 ), as in FIG.  4 ( a ). Or again the N first pixels CP 1   1 , CP 2   1  . . . CP n   1  of the matrices of the polarization switches CP 1 , CP 2 , . . . , CP n  may for example be identical to the last N pixel CP 1   2 , CP 2   2  . . . CP n   2  of the matrices of the polarization switches CP 1 , CP 2 , . . . , CP n  (CP 1   1 =CP 1   2 , CP 2   1 =CP 2   2  . . . CP n   1 =CP n   2 ) as in the case of FIG.  4 ( b ). 
     Let us follow an initial beam F which has to be delayed, for example by a duration τ 2 +τ 5 . This beam F is separated by the polarization splitter  51   M , according to two orthogonal or almost orthogonal polarization states, into two beams F 1  and F 2 . The polarization switches CP 2   1  and CP 2   2  of FIG.  4 ( a ) or the polarization switches CP 2  of FIG.  4 ( b ) change the polarization state of the beams F 1  and F 2  such that respectively the polarization splitters SP 2   1  and SP 2   2  of FIG.  4 ( a ) or the polarization splitter SP 2  of FIG.  4 ( b ) modify the route of these two beams F 1  and F 2  with respect to all the beams. The beams F 1  and F 2  are then delayed by a duration τ 2  either by the delay devices R 2   1  and R 2   2  of FIG.  4 ( a ) or the delay device R 2  of FIG.  4 ( b ). The polarization splitters SP 2   1  and SP 2   2  of FIG.  4 ( a ) or the polarization splitter SP 2  of FIG.  4 ( b ) then recombine the delayed beams F 1  and F 2  with all the beams that have followed a direct route between the input of the polarization switches CP 2   1  and CP 2   2  of FIG.  4 ( a ) or of the polarization switch CP 2  of FIG.  4 ( b ) and the output of the polarization splitters SP 2   1  and SP 2   2  of FIG.  4 ( a ) or of the polarization splitter SP 2  of FIG.  4 ( b ). The delay τ 5  is applied in the same way. The polarization switch CP +  places the beam F 1  in a given polarization state identical for all the beams having followed the first route and the beam F 2  in a given polarization state identical for the all the beams having followed the second route. The polarization state of the beam F 2  is orthogonal or almost orthogonal to the state of the beam F 1 . One of the beams F 1  or F 2  directly reaches one of the inputs of the polarization superposing element  51   V , and the other beam F 2  or F 1  is redirected by a mirror  52   V  to the second input of the superposing element  51   V  which then superposes the two delayed beams F 1  and F 2  so as to obtain the delayed beam F. 
     The multiple delay-creation device serving as a basis for the creation of the multiple delay-creation device  5   G  may also be any multiple delay-creation device other than the one presented in FIG. 1 such as for example those presented in the patent FR 2659754. 
     One variant of the system comprising a patent device requiring the reception of linearly polarized beams at input such as for example a multiple delay-creation device  5   G  may comprise only the element  51   M  upline from said collective delay-creation device  5   G . A system of this kind doubles the number of delays.