Abstract:
Embodiments of the present invention are directed to a nuller that is used to significantly reduce or eliminate a monochromatic radiation signal within a polychromatic object field. In one embodiment, a method of nulling a coherent light from a light beam having the coherent light and an incoherent light comprises collimating the light beam having the coherent light and the incoherent light, and destructively interfering the coherent light to null the coherent light with no destructive interference of the incoherent light so as to project the incoherent light without the coherent light.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   BACKGROUND OF THE INVENTION 
   The present invention relates generally to optical apparatus and, more particularly, to a nuller. 
   Extrasolar planets must be imaged directly if their nature is to be better understood. This will be difficult, however, since the bright light from the parent star (or rather its diffracted halo in the imaging apparatus) can easily overwhelm nearby faint sources. Nulling interferometry has been considered a promising technique for reducing a star&#39;s brightness relative to its surroundings, which has the potential to enable the direct detection of extrasolar planets and zodiacal light. The technique is based on the precise cancellation, or nulling, of the starlight received by two separate telescopes, and so the amplitudes, phases, and polarizations of the two on-axis electric fields must all be matched to high accuracy across the wave band of interest. To cancel on-axis starlight to high accuracy, some have sought to combine the electric fields from two telescopes viewing a common star exactly out of phase at all wavelengths across the band of interest. One method for introducing the needed achromatic π-rad phase difference is a geometric flip of the electric-field vector, such as that provided by a rotational shearing interferometer. This requires nanometer-level path-length control which is rather complex to implement. 
   BRIEF SUMMARY OF THE INVENTION 
   Stellar interferometers usually operate in a Michelson (pupil plane) configuration which means they have a nearly zero phased field of view. The star image is nulled because the path lengths from the separate apertures have exactly the same path length plus ½ wavelength. The null is not complete as one moves off axis slightly allowing the light from the nearby planet to be observed. Multiple telescope imaging systems are operated in Fizeau (image plane) configuration which gives a large phased field of view. A traditional stellar nulling system would null the entire image. 
   Embodiments of the present invention are directed to a nuller that employs a relatively simple configuration to null only the radiation from a coherent source allowing all of the image to be viewed. The dynamic coherent nuller is used to significantly reduce or eliminate a monochromatic radiation signal within a polychromatic object field. For example, an astronomical monochromatic laser guide star can be nulled while viewing the polychromatic star field surrounding the coherent source. An interferometry technique is used to produce destructive interference between coherent waves of the coherent light of the monochromatic source without destructive interference of the incoherent light of the polychromatic image. In some embodiments, a simple Mach-Zehnder or Michelson interferometer configuration is used. The technique is applicable to a single aperture telescope as well as a multiple telescope array. The nuller can be used with stellar interferometers to reduce the signal from a star in order to observe nearby planets or exo-zodiacal dust. 
   An aspect of the present invention is directed to a nuller for nulling a coherent light from a light beam having the coherent light from a monochromatic source and an incoherent light from a polychromatic image. The nuller comprises a collimating lens configured to collimate the light beam having the coherent light and the incoherent light. An interferometer is configured to receive the collimated light beam and to produce destructive interference of the coherent light to null the coherent light with no destructive interference of the incoherent light so as to project the incoherent light of the polychromatic image without the coherent light from the monochromatic source. 
   In some embodiments, the interferometer is one of a Mach-Zehnder interferometer and a Michelson interferometer. The interferometer comprises a beam splitter configured to split the light beam into two paths which are recombined, the two paths have a path length difference equal to (n+½) times a wavelength of the coherent light, where n is an integer. The interferometer comprises a movable reflective member in one of the two paths, and the movable reflective member is movable to adjust a path length in the path to achieve the path length difference. 
   Another aspect of the invention is directed to a method of nulling a coherent light from a light beam having the coherent light and an incoherent light. The method comprises collimating the light beam having the coherent light and the incoherent light, and destructively interfering the coherent light to null the coherent light with no destructive interference of the incoherent light so as to project the incoherent light without the coherent light. 
   In some embodiments, destructively interfering the coherent light comprises splitting the light beam into two split light beams along two paths and recombining the light beams from the two paths. The two paths have a path length difference equal to (n+½) times a wavelength of the coherent light, where n is an integer. One of the two paths may include a movable reflective member which is movable to adjust a path length in the path to achieve the path length difference for nulling the coherent light. Alternatively, one of the two paths may include a pair of movable reflective members which are oriented at an angle relative to one another and which are movable together to adjust a path length in the path to achieve the path length difference for nulling the coherent light. The coherent light has a coherence length which is substantially higher than a coherence length of the incoherent light. For example, the coherent light may have a coherence length of at least about 40 μm and the incoherent light may have a coherence length of less than about 20 μm. In a specific embodiment, the coherent light has a coherence length of about 50 μm and the incoherent light has a coherence length of about 10 μm. 
   In accordance with another aspect of the present invention, a method of nulling a coherent light from a light beam having the coherent light and an incoherent light comprises collimating the light beam having the coherent light and the incoherent light. The coherent light has a coherence length which is substantially higher than a coherence length of the incoherent light. The method further comprises splitting the light beam into two split light beams along two paths, and recombining the light beams from the two paths. The two paths have a path length difference equal to (n+½) times a wavelength of the coherent light, where n is an integer. 
   In some embodiments, the coherent light is produced from an astronomical monochromatic laser guide star, and the incoherent light is produced from an image of a polychromatic star field. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified schematic view of a coherent nuller employing a Mach-Zehnder configuration according to an embodiment of the present invention; 
       FIG. 2  is a simplified schematic view of a coherent nuller employing a Michelson configuration according to another embodiment of the present invention; and 
       FIG. 3  is a simplified schematic view of a coherent nuller employing a Michelson configuration in a laboratory experimental setup according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a nuller  10  employing a Mach-Zehnder configuration to null a monochromatic light from a coherent point source. An image is projected onto an image plane  12  from a multiple telescope system  14 . Also projected on the image plane  12  by the telescope system  14  is a monochromatic light from a coherent point source. The monochromatic light may have been projected from the telescope system  14  into the sky to correct for turbulence of the atmosphere. The image and monochromatic light are directed and processed through a Mach-Zehnder interferometer  16 . 
   As shown in  FIG. 1 , the beam containing the image and monochromatic light are collimated by a collimating lens  20 . A first beam splitter  22  splits the beam along two paths. The first path leads a portion of the beam  23  to a pair of reflective members or mirrors  24 ,  26  which reflect that portion of the beam to a second beam splitter  28 . The second path leads the other portion of the beam  29  from the first beam splitter  22  directly to the second beam splitter  28 . The second beam splitter  28  split the two beams  23 ,  29  along two paths. Along the first path, portions of the beams  23 ,  29  are combined into a beam  31  directed through an imaging lens  32  which projects the combined beam  31  to a first image plane  34 . Along the second path, portions of the beams  23 ,  29  are combined into a beam  35  directed through an imaging lens  36  which projects the combined beam  35  to a second image plane  38 . 
   The reflective members  24 ,  26  are movable together in a direction  40 . For instance, the reflective members  24 ,  26  may be attached to a piston which is driven by an actuator or precision stage in the direction  40 . The monochromatic light has a wavelength λ. The position of the reflective members  24 ,  26  are adjusted in the direction  40  to produce a path delay so that the path length difference is n+½ waves (i.e., (n+½)λ). The first splitting of the initial beam into beams  23 ,  29  by the first beam splitter  22  generates a phase shift of the beam  23  by the reflective members  24 ,  26  with respect to the beam  29  due to the path length difference. When the two beams  23 ,  29  are combined and redirected by the second beam splitter  28  to the two image planes  34 ,  38 , the beams  23 ,  29  interfere destructively toward the first image plane  34  for the monochromatic light to null the monochromatic light due to the n+½ wavelength difference, and the beams  23 ,  29  interfere constructively toward the second image plane  38  to show the monochromatic light because the phase shift is n waves. The broadband incoherent beams of the image do not interfere because the path lengths differ by a distance (e.g., the coherence length is typically higher than about 30 μm, such as about 50 μm) that is much greater than the coherence length of the broadband scene (typically less than about 20 μm, such as about 10 μm). Thus, the split images add incoherently on the focal planes  34 ,  38 , while the point sources add coherently to null the monochromatic light on the focal plane  34 . 
     FIG. 2  shows a nuller  60  employing a Michelson configuration to null a monochromatic light from a coherent point source. An image is projected onto an image plane  62  from a telescope system  64 . Also projected on the image plane  62  by the telescope system  64  is a monochromatic light from a coherent point source. The monochromatic light may have been projected from the telescope system  64  into the sky to correct for turbulence of the atmosphere. The image and monochromatic light are directed and processed through a Michelson interferometer  66 . 
   As shown in  FIG. 2 , the beam containing the image and monochromatic light are collimated by a collimating lens  70 . A beam splitter  72  splits the beam along two paths. The first path leads a portion of the beam  73  to a first reflective member or mirror  74 , which reflects that portion of the beam  73  directly back to the beam splitter  72 . The second path leads the other portion of the beam  79  from the beam splitter  72  to a second reflective member or mirror  76 , which reflects that portion of the beam  79  directly back to the beam splitter  72 . The beam splitter  72  combines the reflected beam portions  73 ,  79  and directs the resulting beam  81  through an imaging lens  82  which projects the combined beam  81  to a detector such as a CCD  84 . 
   At least one of the reflective members is movable. For example, the reflective member  76  is movable in a direction  90  by a piston or the like which is driven by an actuator or precision stage. The monochromatic light has a wavelength λ. The position of at least one of the reflective members, in this case the mirror  76 , is adjusted in the direction  90  to produce a path delay so that the path length difference between the two beams  73 ,  79  is n+½ waves (i.e., (n+½)λ). The splitting of the initial beam into beams  73 ,  79  by the beam splitter  72  and the reflection of the beams  73 ,  79  by the reflective members  74 ,  76 , respectively, generate a phase shift between the beams  73 ,  79  due to the path length difference. When the two beams  73 ,  79  are combined into the beam  81  and directed by the beam splitter  72  to the detector  84 , the beam components  73 ,  79  interfere destructively toward the detector  84  for the monochromatic light to null the monochromatic light due to the n+½ wavelength difference. The broadband incoherent beams of the image do not interfere because the path lengths differ by a distance (e.g., the coherence length is typically higher than about 30 μm, such as about 50 μm) that is much greater than the coherence length of the broadband scene (typically less than about 20 μm, such as about 10 μm). Thus, the split images add incoherently on the focal plane at the detector  84 , while the point sources add coherently to null the monochromatic light on the focal plane at the detector  84 . 
   Experiments using laboratory setups have been conducted to verify the methodology.  FIG. 3  shows an experimental apparatus  100  employing a Michelson configuration to null a monochromatic light from a coherent point source. In this embodiment, the coherent point source is a laser diode  102 . An image is projected using a white light  104  onto a transparency  106  which represents an extended scene at an image plane. A beam splitter  108  is used to combine the monochromatic light from the laser diode  102  with the beam from the white light  104  which illuminates the transparency. The image and monochromatic light are collimated and processed through a Michelson interferometer  110 , which may be the same as the Michelson interferometer  66  of FIG.  2 . For convenience, the same reference characters are used for the collimating lens  70 , the beam splitter  72 , the first reflective member  74 , the second reflective member  76 , the imaging lens  82 , and the detector  84 . Of course, the experimental set in another embodiment may employ a different interferometer, such as the Mach-Zehnder interferometer  16  of FIG.  1 . The position of the second reflective member  76  is adjusted to null the monochromatic light from the laser diode  102 . A null of 40:1 peak value ratio is demonstrated. 
   The above-described arrangements of apparatus and methods are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims. For instance, the Mach-Zehnder or Michelson interferometer may be replaced by another interferometry setup. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.