Abstract:
A multispectral imaging device for satellite observation utilizing “push broom” scanning over an observed area centered on one or more wavelengths which can be electrically controlled to produce a filtering function wavelength band, thus obviating the need for conventional stacking.

Description:
PRIORITY CLAIM 
     This application claims priority to PCT Patent Application Number PCT/EP2008/005424, entitled Multispectral Imaging Device With MOEMS Type Filter For Satellite Observation, filed Jul. 3, 2008. 
     FIELD OF THE INVENTION 
     The invention relates to a multispectral observation device used notably for the acquisition of satellite observation images of the ground by “push-broom” scanning from strips of detectors of the charge-coupled type (CCD) for example, scrolling facing the observed area. 
     BACKGROUND OF THE INVENTION 
     The push-broom principle is illustrated in  FIG. 1  diagrammatically in the case of a strip of detectors  1 . This strip on board the satellite carries out the successive observation, as the satellite moves, of rows L 1 , L 2 , . . . , L N  perpendicular to the direction of displacement D. A wide field optic  2  forms the image of the ground on a row of detectors located in the focal plane. The row scan is obtained by reading the sensitive elements of the detection row. The scan of the landscape in the perpendicular direction results from the movement of the satellite in its orbit. It is also possible to use a spectral splitter that makes it possible in addition to conduct this observation in different spectral windows and thus produce the multispectral imaging. 
     In a known manner when wanting to produce a polychromatic image, strips of individual detectors are used that are coupled to interference filters as illustrated in  FIG. 2  that represent the example of four filters having respective spectral bands B 0 , B 1 , B 2 , B 3  physically separated by a distance L i-j . Notably, it is known to use filters of small thickness called “match” filters. To reconstruct the various spectral components, a detector coupled to four filters of very small thickness can conventionally be used. These filters are difficult to manufacture because they are made up of stacks of thin layers on the surface of a substrate.  FIG. 3  shows an example of layer stacking, typically around 20 layers distributed over both faces of a substrate may be necessary to form a filter in a given wavelength range. This type of filter notably has two types of drawbacks. The first is associated with the edge effects in an area z i  that appear because of the stacking of all of these layers with a thickness of the order of λ/4 and that, given a large number of layers, embrittle the filters. The second drawback is associated with the fact that the various filters are produced in a connected way on one and the same substrate as illustrated in  FIG. 3  that diagrammatically represents the production of two types of stacking that make it possible to provide filtering functions in wavelength bands B i  and B j . The layer stacking technologies entail imposing minimum separation distances between two filters, of the order of a few millimeters, which amounts to taking images of scenes on the ground that are several kilometers away. 
     SUMMARY OF THE INVENTION 
     In order notably to resolve this problem of excessive distance, the present invention proposes a multispectral imaging device comprising a unique structure that can be controlled electrically so as to produce a filtering function in a chosen wavelength band, and thus no longer using the conventional stacks of layers. The benefit of the invention also lies in the possibility of varying the spectral filtering function in wavelength and spectral width. 
     More specifically, the subject of the invention is a multispectral imaging device for satellite observation by “push-broom” scanning over an observed area, operating in N wavelength bands, respectively centered on a first wavelength (λ 1 ), . . . , an nth wavelength (λ N ) comprising:
         a source emitting a light beam in a set of the N wavelength bands;   a wide-field optic;   a set of N rows of detectors making it possible to acquire images of said observed area;   optical filtering means,
 
characterized in that it also comprises:
   a first dispersion element (R 1 ,R) making it possible to disperse the light beam toward the filtering means;   optical filtering means comprising at least one micro-opto-electro-mechanical system (MOEMS) capable of carrying out N filtering functions for the N spectral bands, wavelength-tunable;   control means for said micro-opto-electro-mechanical system making it possible to select the filtering function;   a second dispersion element (R 2 ) making it possible to recombine all the filtered beams at the output of the filtering means.       

     According to a variant of the invention, the first dispersive element is an array. 
     According to a variant of the invention, the first dispersive element is a prism, or an array, or a component incorporating the array and prism functions. 
     According to a variant of the invention, the control means for the micro-opto-electro-mechanical system include means for varying the filtering function with the same period as the acquisition time for an image corresponding to the displacement time equivalent to a scrolling row of detectors facing the observed area. 
     According to a variant of the invention, the device also comprises a second dispersive element that makes it possible to recombine all the filtered beams at the output of the filtering means. 
     According to a variant of the invention, the second dispersive element is an array, or an array, or a component incorporating the array and prism functions. 
     According to a variant of the invention, the first and second dispersive elements are one and the same component. 
     According to a variant of the invention, the micro-opto-electro-mechanical component comprises a micro-mirror structure suspended relative to a substrate, of which the distance or angle with said substrate can be controlled electrically. 
     According to a variant of the invention, the micro-mirror structure comprises unitary elements having lengths of a few tens of microns and widths of a few microns. 
     According to a variant of the invention, the unitary elements are separated by a pitch of a few microns. 
     According to a variant of the invention, the control means can be programmed from the ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other benefits will become apparent from reading the description that follows, given by way of nonlimiting example and from the appended figures in which: 
         FIG. 1  diagrammatically represents an exemplary device according to the invention; 
         FIG. 2  illustrates an exemplary multispectral imaging device according to the state of the art using a “match” type filter; 
         FIG. 3  illustrates a detailed view of all the layers needed to produce a “match” type filter; 
         FIG. 4  illustrates a first exemplary imaging device according to the invention; 
         FIG. 5  illustrates a second exemplary imaging device according to the invention in which the optical combinations are highlighted; 
         FIG. 6  illustrates a third exemplary imaging device according to the invention in which the optical combinations are highlighted; 
         FIG. 7  illustrates in more detail the behavior of the MOEMS-based filter used in the inventive device; 
         FIGS. 8   a  and  8   b  illustrate perspective and cross-sectional views of an MOEMS component with no voltage applied; 
         FIGS. 9   a  and  9   b  illustrate perspective and cross-sectional views of an MOEMS component with voltage applied; 
         FIGS. 10   a  and  10   b  illustrate the addressing of the filtering functions coupled to the displacement of the strips of detectors in an inventive device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally, the inventive device comprises wavelength filtering means to produce different colored beams so as to carry out multispectral imaging comprising an MOEMS (micro-opto-electro-mechanical system) type component, simultaneously offering mechanical, electrical and optical functions. This type of component, when judiciously associated with a dispersive element in a multispectral imaging device, makes it possible to produce a number of filtering functions in different wavelength bands using an electrical control. The spectral adjustment can be ultrafast thanks to the high speed of the MOEMS components as will be explained hereinbelow. 
     In a first exemplary embodiment of the invention, the device can comprise a single diffraction array on the incident path at the level of the MOEMS component and of the reflected path. Thus, according to one example of this type of configuration, illustrated in  FIG. 4 , the device comprises a wide field optic O 1  directing a beam F of polychromatic light emitting in a wide spectral band. The device also comprises a diffracting array R making it possible to diffract a beam ΣF R  consisting of unitary beams F Ri  by dispersing in different directions through a lens L, said unitary beams comprising wavelengths belonging to the spectral band. These different beams are sent to different portions of the component C MOEMS  that provides a wavelength-controllable filtering function, by reflecting only some wavelengths in a beam ΣF R ′ through the lens L toward the same array R. Thus, all the wavelengths are recombined after filtering into a beam F′ which, via an optic O 2 , is sent toward the area to be observed (not represented). 
       FIG. 5  illustrates in more detail a second exemplary device according to the invention in which two arrays R 1  and R 2  are used. More specifically, an incident, light beam is sent through a slot F e  toward a collimation lens L col  and a first diffraction array R 1 . The latter diffracts, in different directions, a beam ΣF R  onto the component C MOEMS  through lenses L, then is once again sent, via a mirror Mr, toward a second diffraction array R 2 . The beam ΣF R ′ is then refocused using a focusing lens L f  toward the focal plane P r  of the detection lines. 
       FIG. 6  illustrates a third exemplary device according to the invention in which the dispersing element is a prism P r . According to this example, the beam ΣF R  is sent toward the component C MOEMS  through a collimation lens L col  toward the prism which disperses it in a beam ΣF R ′ sent to a focusing lens L f  toward the focal plane P f  of the detection lines. 
     The diagram of  FIG. 7  illustrates in more detail the diffracted beams, in this case four represented F R0 , F R1 , F R2  and F R3 , sent toward different sectors of locally-controllable components so as to be able to locally reflect or not reflect a determined wavelength. 
     There now follows a more detailed description of the behavior of this type of component. It is a microstructure that can provide a mirror function with regard to a multispectral light beam as illustrated in  FIG. 8   a .  FIG. 8   b  relates to a cross-sectional view of the structure represented in  FIG. 8   a . Under the effect of an applied electrical field, and by electrostatic force, certain unitary elements M oi  may be brought closer to the substrate so creating an array structure as illustrated in  FIGS. 9   a  and  9   b . Typically, the elements M oi  can have lengths of around a few tens of microns for widths of around a few microns. Arrays of micro-mirrors are thus produced that are capable of reflecting or not reflecting the light beam F i  and that can be electrically driven. 
     According to the invention, when beams F R0 , F R1 , F R2 , F R3 , respectively centered on the wavelengths λ 0 , λ 1 , λ 2 , λ 3  with spectral bandwidths Δλ 0 , Δλ 1 , Δλ 2 , Δλ 3 , arrive on a component C MOEMS , as illustrated in  FIG. 7 , certain wavelengths may be switched off or dispersed so as to restore, in reflection and in a given direction, beams having spectral bands that are partially filtered compared to the beams F R0 , F R1 , F R2 , F R3 . By rerouting these partially filtered beams toward the array R, these different beams are recombined by virtue of the reverse principle of light. 
     Thus, more specifically, the MOEMS component can be driven successively so that it reflects, for example, in succession:
         the beams F′ R1 , F′ R2 , F′ R3  corresponding to an emission band called B 0 ;   the beams F′ R0 , F′ R2 , F′ R3  corresponding to an emission band called B 1 ;   the beams F R0 , F′ R1 , F′ R3  corresponding to an emission band called B 2 ;   the beams F R0 , F R1 , F′ R2  corresponding to an emission band called B 3 .       

     Since the integration time by a row of strips of detectors is T in , advantageously and according to the invention the filtering functions are swopped concomitantly, also every T in , so that a set of four strips of diodes can integrate all of four “colored” images. In practice, as illustrated in  FIG. 8   a , firstly the four strips of detectors D 0 , D 1 , D 2 , D 3  integrate filtered images with the band B 0 , then, when the satellite has moved by a unit equivalent to a distance equal to the pitch of a detector strip (typically this pitch can be of the order of around 10 microns corresponding to a distance on the ground of a few tens of meters, unlike the few kilometers in observation obtained with the filters of the prior art), the four strips integrate filtered images with the band B 1 , and so on so that, after a time equal to 4T in  as diagrammatically illustrated in  FIG. 8   b , each of the strips has integrated all of the four colored images with the four filtering functions. It is thus possible to control, typically after a time T in  of the order of a microsecond, the change of filter needed to acquire a color image. The invention described here consists (for example in the case of four spectral bands) in using a matrix detector with four rows of detectors. However, generally, the number of rows of detectors and the number of spectral bands can advantageously be set by having N rows for N spectral bands. 
     Thus, according to the invention, the spectral function (spectral band) is varied cyclically (B 0 -&gt;B 1 -&gt;B 2 -&gt;B 3 -&gt;B 0  . . . ) with the same period as the integration time (displacement equivalent to one row). A scene can then be observed successively in the four spectral bands. The benefits of this solution lie in the absence of match filters and the possibility of easily increasing the number of bands. The spectral profile can also be easily programmed from the ground.